Dopant-Defect Engineering in Strontium Titanate-Based Materials

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Doctoral Dissertations Dissertations and Theses

March 2017

Dopant-defect Engineering in Strontium Titanate-based Materials

Keith Lehuta University of Massachusetts Amherst

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DOPANT-DEFECT ENGINEERING IN STRONTIUM TITANATE-BASED MATERIALS

A Dissertation Presented

KEITH ALLEN LEHUTA

Submitted to the Graduate School of the University of Massachusetts Amherst in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

February 2017

Department of Chemistry

DOPANT-DEFECT ENGINEERING IN STRONTIUM TITANATE-BASED MATERIALS

A Dissertation Presented

KEITH ALLEN LEHUTA

Approved as to style and content by:

______Kevin R. Kittilstved, Chair

______Michael J. Knapp, Member

______Richard W. Vachet, Member

______Dimitrios Maroudas, Member

______Craig T. Martin, Department Head Department of Chemistry

DEDICATION

To my wife, Elizabeth, for her continuing support, love, and encouragement.

ACKNOWLEDGEMENTS

I would like to thank my advisor, Professor Kevin R. Kittilstved for his patience, guidance, and contributions to my research over my time working with him. I am very grateful for the support and opportunities you have granted me in order to grow as a researcher.

I would also like to thank my other committee members, Prof. Michael J. Knapp,

Prof. Richard W. Vachet, and Prof. Dimitrios Maroudas for their advice throughout my dissertation process.

Next I want to thank all of the members of the Kittilstved group that I have had the privilege to work with during my time at UMass Amherst. It is always important to grow as a group and support one another, whether through advice on an experiment, listening to a practice talk, or simply talking about something besides lab work. I want to thank

Dongming Zhou for his help with X-Ray Photoelectron Spectroscopy. I also want to thank the undergraduate students who I have worked along with me, especially Anubhab Haldar and Justin Campbell, who have made contributions to the work herein.

I also want to thank my family for their continued support throughout my life that has gotten me to this point. My parents, John and Mary, always encouraged me to pursue my dreams, taught me the values to be the person I am today, and supported me in any way they could. Also my brother, James, who was always there to look after his little brother and spark my curiosity with the question: “what do you think would happen if we…”

Lastly, I want to thank the friends I’ve made at UMass, who have been a fantastic support system here in Amherst. We’ve had a lot of fun and appreciate every moment.

ABSTRACT

DOPANT-DEFECT ENGINEERING IN STRONTIUM TITANATE-BASED

MATERIALS

FEBRUARY 2017

KEITH ALLEN LEHUTA, B.S., ILLINOIS WESLEYAN UNIVERSITY

Ph.D., UNIVERSITY OF MASSACHUSETTS AMHERST

Directed by: Professor Kevin R. Kittilstved

Strontium titanate is a wide gap oxide perovskite that has been studied for numerous applications. Its potential use as a photocatalyst is limited due to only being able to utilize

UV light. The introduction of metal dopant ions has been shown to alter the band structure to allow visible light photocatalysis, as well as alter the materials properties for other applications. This work will look to better explain the process of transition metal dopant ion incorporation and how the dopant ion can affect the defect chemistry of the material.

The use of dopant specific spectroscopies, such as electron paramagnetic resonance

(EPR) spectroscopy for paramagnetic dopant ions, is critical to fully understand the incorporation dopant ions into the lattice. Through the use of systematic studies and correlating dopant valence state to formation of the materials. This also allows for the determination of the dopant location, i.e. internal, surface, clustered, which can affect the properties further.

Through the use of an adapted NaBH4 reduction method, the valence state of the dopant ion is altered as shown using quantitative EPR spectroscopy and the corresponding changes in the absorption of these reduced materials is shown. The correlation to the dopant

vi ion control and changes in the VO defects charge state to act as charge compensation was studied in order to better understand methodologies for additional control in the photocatalysts.

The incorporation of dopant ions into the Ruddlesden-Popper phase Sr2TiO4, a layered perovskite, is also described herein, including the determination of EPR parameters and dopant specific emission spectroscopy. The changes in these materials as it relates to controlling the valence state of dopant ions and altering the defect chemistry in the lattice are also studied using the NaBH4 reduction.

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TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS…….…………………………………………………………v

ABSTRACT……………………………………………………………………………...vi

LIST OF TABLES……………………………………………………………………...... xi

LIST OF FIGURES….………………………………………………………………...... xii

CHAPTER

1. INTRODUCTION TO THE USE OF STRONTIUM TITANATE BASED

MATERIALS AS PHOTOCATALYSTS…………………………………………….1

1.1. Strontium titanate based materials: An introduction to perovskite and Ruddlesden-Popper phases……………………………………………………….1

1.2. Important factors in photocatalysis……………………………………………….2

1.3. Role of doping in visible light photocatalysis……………………………………5

1.4. The role of charge balance and defects…………………………………………...7

1.5. Advantages of Ruddlesden-Popper phases……………………………………….9

1.6. Focal points for development of SrTiO3 based visible light photocatalysts…….11

1.7. References……………………………………………………………………….12

2. SPECIATION OF Cr (III) IN INTERMEDIATE PHASES DURING THE SOL-GEL

PROCESSING OF Cr-DOPED SrTiO3 POWDERS………………………………...18

2.1. Introduction……………………………………………………………………...18

2.2. Experimental Methods…………………………………………………………..20

2.3. Results and Discussion……………………………………...…………………..21

viii

2.4. Conclusions……………………………………………………………………...36

2.5. Supplementary Information……………………………………………………..37

2.6. References………….……………………………………………………………43

3. REVERSIBLE CONTROL OF THE CHROMIUM VALENCE IN CHEMICALLY

REDUCED Cr-DOPED SrTiO3 BULK POWDERS………………………………...48

3.1. Introduction……………………………………………………………………...48

3.2. Materials and Methods………………………………………………...………..50

3.3. Results and Discussion…………………………………………………...……..52

3.4. Conclusions……………………………………………………………………...64

3.5. Supplementary Information……………………………………………………..64

3.6. References……………………………………………………………………….69

4. SOLID-STATE SYNTHESIS AND LOW-TEMPERATURE CHEMICAL

REDUCTIONS OF SUBSTITUTIONAL Cr-DOPED Sr2TiO4……………………..73

4.1. Introduction……………………………………………………………………...73

4.2. Experimental Methods…...……………………………………………………..75

4.3. Results and Discussion...………………………………………………………..77

4.4. Conclusions……………………………………………………………………...84

4.5. Supplementary Information………………………………………………….….85

4.6. References……………………………………………………………………….87

5. SYNTHESIS AND ANALYSIS OF Mn-DOPED SrTiO3 AND Sr2TiO4…………..91

5.1. Introduction……………………………………………………………………...91

5.2. Materials and Methods...………………………………………………………..92

5.3. Results and Discussion…...……………………………………………………..94

5.4. Conclusions…………………………………………………………………….106

5.5. Supplementary Information……………………………………………………107

5.6. References……………………………………………………………………...110

6. CONCLUSIONS AND FUTURE DIRECTIONS…………………………………114

BIBLIOGRAPHY………………………………………………...…………………….118

LIST OF TABLES

Table Page

4+ 2.1 Molecular symmetries and bond lengths of Ti sites in relevant TiO2 and SrTiO3 phases. Axial and equatorial bond lengths for tetragonal and rhombic sites are differentiated by “ax” and “eq” abbrevations……………………………………………24

2.2 Spin-Hamiltonian parameters for Cr3+ in various oxide lattices……………………..28

3+ 2.3 Calculated atomic percent from XPS analysis for in nominally 5% Cr :SrTiO3 powders as a function of annealing time at 1050 °C…………………………………….32

5.1 Spin-Hamiltonian parameters of Mn in SrTiO3 and Sr2TiO4……………………….100

LIST OF FIGURES

Figure Page

1.1 Structural representation of SrTiO3…………………………………………………...1

1.2 Band structure representation of different materials with potentials compared to those required for photocatalytic water splitting………………………………………………...3

1.3 Schematic representation of the processes in photocatalysis in physical space and in terms of potential. (A) Generation of an electron hole pair. (B) Migration of electron and hole to the surface. (C) Electron/hole recombination, which is the competing process. (D) Reduction and oxidation of protons and water respectively, forming H2 and O2 gases…..4

1.4 Schematic representation of how the incorporation of dopant metals can alter the band structure of SrTiO3 to allow for visible light photocatalysis……………………………...6

1.5 Structural representations of Ruddlesden-Popper phases of the Sr-Ti-O series for n=∞, 1, and 2………………………………………….…………………………………...9

2.1 Powder XRD patterns of nominally 1 % Cr:SrTiO3 samples after calcination (4h at 800 °C) and annealing (4 h at 1050 °C). The bottom region shows the diffraction positions and relative intensities for SrTiO3, R-TiO2, A-TiO2, and SrCO3 from refs [32], [31], [33], and [30], respectively.……………………………………………………...22

2.2 Powder XRD patterns in an expanded region of nominally 1 % Cr:SrTiO3 samples as a function of annealing temperatures of 850 °C (A), 950 °C (B) and 1050 °C (C) for 0, 0.5 h, 3 h, and 6 h. Panel (D) shows the diffraction peaks for the same phases as given in Fig. 1 and Sr2TiO4 [34], Sr3TiO7 [35], and SrO [36]. Note the change in the spacing of 2θ to the left and right of 30.5°……………………………………………………………...23

2.3 (A) Absorption of pure SrTiO3 annealed at 1050 °C (−·−), nominally 1 % Cr:SrTiO3 before (―) and after annealing at 1050 °C (···). (B) Cr3+ R-line emission at 15 K from nominally 1% Cr:SrTiO3 before and after annealing at 1050 °C for 4 h. (C) Temperature dependence of energy difference of the R-line emission to the 15 K R-line position for 1 % powders (closed circles). Vertical arrows in (A) and (B) are used to differentiate the new spectral features in the spectra before and after the annealing steps. Literature absorption and 20.3 K emission spectra (− −) and temperature dependence (open circles in panel C) of the R-lines were collected on single crystals containing 0.01 wt% (A) and 0.005 wt% Cr2O3(B). Data were extracted from refs. [39] and [40] using commercial digitization software (DigitizeIt, v1.5.7)…………………………………………………25

2.4 (A) EPR spectra of 1% Cr:SrTiO3 before annealing at 1050 °C (0h, bottom), after annealing at 1050 °C for 0.5 h (middle), and after annealing at 1050 °C for 4 h (top). 3+ Assignment of resonance fields for transitions in this field range for Cr :SrTiO3 (vertical 3+ 3+ lines), Cr :A-TiO2 (open triangles), and Cr :Sr2TiO4 (open circles). (B) Plot of the

xii integrated intensity of the central EPR feature at g = 1.978 vs. annealing time at 850 °C, 950 °C and 1050 °C……………………………………………………………………...28

2.5 XPS data of 5% Cr:SrTiO3 after calcining at 800 °C for 4 h and annealing at 1050 °C for 0 h (dotted lines), 0.5 h (dashed lines) and 4 h (solid lines). For each sample we show the Cr 2p (A), Ti 2p (B), Sr 3d (C), and O 1s (D) regions of the spectra………………..31

2.6 (A) Experimental and (B,C) simulated EPR spectra of the two major Cr3+ species in the 1 % Cr:SrTiO3 annealed at 1050 °C for 0.5 h. Experimental spectra collected at hν = 9.599 GHz and room temperature. See text for details of the simulation………………..33

2.7 Summary of the sol-gel processing of Cr:SrTiO3 when heated at different temperatures for different times. Phases in parentheses are only detected via EPR spectroscopy………...... 35

2.8 Variable temperature emission of 0.01% Cr:SrTiO3. The sample was excited by the 488 nm line of an Ar+ laser………………………………………………………………37

3+ 2.9 Temperature dependence of the effective magnetic moment of Cr :SrTiO3 after being annealed for 0 h (triangle), 0.5 h (square), or 4 h (circle) at 1050 °C. Effective moments are reported using the nominal Cr3+ concentration. Magnetic susceptibility was measured from 2-300 K on samples in an applied magnetic field of H = 0.5 Tesla (Quantum Design MPMS XL). Diamagnetic corrections for the SrTiO3 host, the gel capsule and kapton tape were tabulated using reported diamagnetic susceptibilities and Pascal’s tables.…………………………………………...... ……..38 2.10 Normalized EPR spectra after annealing at 850 °C (A), 950 °C (B), and 1050 °C (C). All spectra were normalized to a peak height of 1 for the Cr3+ feature at g=1.978…39

2.11 Experimental EPR spectrum of polycrystalline powders of Cr-doped Sr2TiO4 (top) and angular dependence of the simulated EPR spectrum (bottom) with the best agreement between the EPR spectrum and the simulation. The right side of the figure shows the central features on an expanded scale. Spin-Hamiltonian parameters given in textbox. Angular dependence of the resonance fields was calculated using XSophe simulation software (Bruker Biospin)………………………………...………………………..…….40

3+ 2.12 Simulated EPR spectrum of Cr :Sr2TiO4 with varying amount of D-strain from σD = 0 (top) – 30×10−4 cm−1 (bottom)……………………………………………………….41 2.13 EPR intensity ratio of Peak II (|±3/2 ↔ |±1/2 when B⊥z, filled-circles) and Peak III (|±3/2 ↔ |±1/2 when B∥z, open-circles) to the central peak (Peak I: |1/2 ↔ |−1/2). The intensities of the signals was taken as the peak-to-peak difference. The intensities of the |±3/2 ↔ |±1/2 transitions decrease considerably compared to the central |1/2 ↔ |−1/2 transition with small increases in the D-strain (D = 207×10−4 cm−1)……………..42

3+ 2.14 Simulated EPR spectra of Cr in the Sr2TiO4 phase (top) and A-TiO2 phase (bottom). Input values for the simulation are given in Table 2 of the main text...... 42

xiii

2.15 Experimental EPR spectrum of calcined powders (air, 800 °C, 4 h) and simulated 3+ 3+ 3+ spectra of Cr :SrTiO3, Cr :Sr2TiO4, and Cr :A-TiO2. The feature at g = 2.0035 is not 3+ due to any known Cr phase. It is tentatively attributed to a defect in A-TiO2 or SrTiO3 that is rapidly oxidized > 850 °C (see Fig. 2.9)………………………………………….43

3.1 (A) Color photographs of 1% Cr:SrTiO3−δ as-prepared (“ap,” left) and after chemical reduction at 300 °C, 325 °C, 350 °C, 375 °C, 400 °C, and 430 °C (2nd from left to right, respectively). (B) Powder XRD of Cr:STO−ap, Cr:STO−350, and Cr:STO−430. Literature powder X-ray diffraction patterns (gray sticks) and assignments from [23]…53

3.2 Diffuse reflectance spectra of Cr:STO-ap (black) and NaBH4 reduced samples: Cr:STO-325 (blue), Cr:STO-375 (green), and Cr:STO-430 (red). The vertical lines in the “defect-related” VO states are placed at the calculated energies for the electronic transitions from the valance band to the specific VO from ref. [27]……………………...55

3.3 295 K quantitative EPR spectra of 1% Cr:STO−ap and after NaBH4 reduction at 300 °C and 350 °C. Inset shows the relative peak area (squares) and linewidths (fwhm, circles) of the Cr3+ signal in the as-prepared (open symbols) and as a function of reduction temperature (filled symbol)…………………………………………………...56

3.4 Room temperature EPR spectra of 0.1% (nominal) Cr:STO−ap, −325, and −375…..57

3.5 77 K EPR spectra of 0.1% Cr:STO−325, −350, and −375. Enlarged sections in for the Cr:STO−325 and −350 spectra are scaled by 4………………………………………...58

3.6 (A) Cr:STO−350 (left) reoxidized (from 2nd from left to right) at 250 °C, 300 °C, 325 °C, 350 °C, 375 °C, 400 °C, and 450 °C for 30 minutes. Far right is original Cr:STO. (B) Diffuse reflectance and (C) EPR spectra of same samples, Cr:STO−350 (teal, shaded), Cr:STO (black) and reoxidized samples from low to high temperature from dark to light trace color…………………………………………………………………..…………….60

3.7 The (A) EPR and (B) diffuse reflectance spectra of Cr:STO−400 (red trace with shading), Cr:STO−400 after aerobic annealing at 450 °C (dashed red line) and 600 °C (dashed red and yellow line) for 30 min. Inset: color scans of samples……………..…..62

3.8 Schematic model of the NaBH4 reduction on the fermi level (EF) of Cr-doped SrTiO3 with multivalent Cr ions. The positions of the Cr4+/3+ and Cr6+/4+ redox levels are below the bottom of the conduction band which is the Ti4+/3+ redox level that is reached at temperatures above 375 °C……………………………………………………………....63

3.9 Change in the diffuse reflectance spectra from Cr:STO-ap to after reduction at different temperatures. The dashed black line is at zero………………………………....65

3.10 Integrated EPR signal of Cr:STO samples as-prepared and after NaBH4 reduction at different temperatures……………………………………………………………………65

3.11 Room temperature EPR spectrum of Cr:STO-375……………………………...….66

xiv

3.12 EPR spectrum of Cr:STO-350 after reduction for 3 h instead of 30 minutes……....66

3.13 EPR spectroscopy of Cr:STO-325 (A) and Cr:STO-375 (B) at 77 K…………...….67

3.14 Area and full-width half max analysis for quantitative EPR measurements for oxidation of Cr:STO-350 (open markers) at different temperatures. As-prepared values added for reference………………………………………………………………...…….67

3.15 EPR spectra of 0.1% Cr:STO reduced at 400 °C and after oxidation at 450 and 600 °C. Spectra were collected at 77 K. Trace Mn4+ impurity was observed in the reox 600 sample. As the Ti3+ signal decreases in intensity, a feature at g=1.920 is visible which is 3+ consistent with a rhombic Ti center in SrTiO3 [2]……………………………….…….68

3.16 EPR spectra of Cr:STO-400 and after oxidation at 450 and 600 °C taken at 77 K...... 68

3.17 XRD patterns after reoxidation at 450 °C of (A) Cr:STO-350 and (B) Cr:STO-400……………………………………………...…………………………..69

4.1 X-ray powder diffraction of the Sr2TiO4 reaction intermediates and products after the initial 6 h (black) and additional 21 h (red) aerobic heating at 1100 °C. The reference diffraction patterns of SrO, TiO2-R, SrTiO3, Sr2TiO4, and Sr3Ti2O7 are taken from literature.21,37,39–41…..…………………………………………………………………….78

4.2 Room temperature EPR spectra of the central (|−1/2⟩↔|+1/2⟩) transitions of 0.1% Cr:Sr2TiO4−δ powder after annealing at 1100 °C for 6 h (black) and 21 h (red). Simulated spectrum calculated using previously reported16 and refined spin-Hamiltonian parameters (see text)………………………………………………………………………………….79

4.3 (a) Quantitative EPR spectra of 0.1% Cr:Sr2TiO4−δ before (black) and after NaBH4 reduction at 300 °C (blue), 325 °C (green), and 375 °C (red) for 30 min under static vacuum at 4 × 10–4 mbar. Note that the left and right panels show the ±3/2  ±1/2 transitions that are scaled by 50 and have different field ranges than the narrow central +1/2  1/2 transition in the central panel. (b) Calculated angular dependence of the 3 EPR-allowed transitions..…………………………………………………………..…….80

-1 4.4 Emission of 0.1% Cr:Sr2TiO4-δ at 20 K when excited at 20492 cm with a spectral resolution of 2 cm-1. Inset shows the temperature dependence of the emission energy for this sample and Cr:SrTiO3 from chapter 2…………………………………………...…..82

4.5 Normalized diffuse reflectance spectra for 1% Cr:Sr2TiO4 before and after NaBH4 reductions at temperatures between 300 and 400 °C…………………………………….83

4.6 Quantitative EPR of 1% Cr:Sr2TiO4 as prepared and after reductions at temperatures from 300 and 375 °C……………………………………………………………………..85

4.7 Room temperature EPR of 0.1% Cr:Sr2TiO4-δ after NaBH4 reduction at 400 and 425 °C for 30 minutes……………………………………………………………………..….85

4.8 Quantitative EPR results of 0.1% Cr:Sr2TiO4-δ reduced at 375 °C, then reoxidized at different temperatures for 30 minutes in normal atmosphere………………………...….86

4.9 Emission spectra of 0.1% Cr:Sr2TiO4 after reduction at 375 °C at temperatures between 20.2 K and 149.3 K. The sample was excited at 20492 cm-1 and the spectral resolution is 4 cm-1……………………………………………………………………….86

4.10 Normalized diffuse reflectance of 0.1% Cr:Sr2TiO4 as prepared and after chemical reductions at temperatures between 300 and 400 °C…………………………………….87

5.1 Powder XRD patterns of 0.1% Mn-doped SrTiO3 and Sr2TiO4 including literature patterns for comparison from references 34 and 35 for comparison………………….…95

5.2 Room temperature EPR spectrum of 0.1% Mn:SrTiO3………………………….…..96

-1 5.3 Emission spectrum at 22 K of 0.1% Mn:SrTiO3 when excited at 23809 cm ………96

-1 5.4 Emission of 0.1% Mn:Sr2TiO4 excited at 20492 cm . The spectrum was collected at 20.5 K with spectral resolution of 2 cm-1…………………………………………….…..97

-1 5.5 Variable temperature emission spectra of 0.1% Mn:Sr2TiO4 excited at 20492 cm and measured at spectral resolution of 4 cm-1. Inset is a plot of the energy of the ZPL at different temperatures……………………………………………………………...... …..98

5.6 Room temperature X-band EPR spectrum of 0.1% Mn:Sr2TiO4………………...…..99

5.7 Comparison of experimental EPR spectrum of 0.1% Mn:Sr2TiO4 (black) and simulations of spectra representing Mn4+ in the axially compressed B-site (red) and Mn4+ in the B-site of SrTiO3 (blue)…………………………………………………...………100

5.8 Diffuse reflectance spectra of 1% Mn:SrTiO3 (A) and Sr2TiO4 (B)…………..……101

5.9 Quantitative EPR spectra of 0.1% Mn:SrTiO3, as prepared and reduced at 300 and 325°C (A). The EPR spectrum of 0.1% Mn:SrTiO3 after reduction at 350 °C, run in a capillary tube……………………………………………………..……………………..103

5.10 Quantitative EPR spectra of 0.1% Mn:Sr2TiO4 as prepared and after NaBH4 reduction between 300 and 350 °C……………………………………………..………104

5.11 EPR spectrum of 0.1% Mn:Sr2TiO4 after NaBH4 reduction at 400 °C………...…105

5.12 Diffuse reflectance spectra of 1% Mn-doped SrTiO3 (A) and Sr2TiO4 (B) for the as- prepared samples and after being reduced at temperatures between 300 and 400 °C….106

xvi

5.13 Emission of 0.1% Mn:Sr2TiO4 at 20 K, excited at 420 nm………………...……..107

5.14 Room temperature EPR spectrum of 0.1% Mn:Sr2TiO4, zoomed into different regions…………………………………………………………………………………..108

5.15 Comparison of simulated spectra to experimental spectrum of 0.1% Mn:Sr2TiO4 in different regions of magnetic field……………………………...………………………108

5.16 Variable temperature emission of 0.1% Mn:Sr2TiO4 in the temperature range of 19.8-201.5 K………………………………………………………………………..…..109

-1 -1 5.17 Emission of 0.1% Mn:SrTiO3 excited at 25641 cm (A) and 18518 cm (B). Temperature range was 20K to 250K………………………………………….……….109

5.18 Electronic studies of 0.1% Mn:Sr2TiO4. Black trace is diffuse reflectance spectroscopy, red is an excitation scan of the Mn4+ emission at 13889 cm-1, blue is room temperature fluorescence excited at 18519 cm-1……………………………………..…110

5.19 Room temperature EPR spectra of 0.1% Mn:SrTiO3 after chemical reduction at 375 (A) and 400 °C (B). Samples were analyzed in capillary tubes………..…………….….110

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CHAPTER 1 INTRODUCTION TO THE USE OF STRONTIUM TITANATE BASED MATERIALS AS PHOTOCATALYSTS

1.1 Strontium titanate based materials: An introduction to perovskite and Ruddlesden-

Popper phases

Strontium titanate (SrTiO3) is a complex mixed metal oxide compound that falls into the category of perovskites. Simple perovskites have a chemical formula of ABX3 where A and B are cations, with A being larger in ionic radius than B, and X is an anion. In strontium titanate, the A- site is occupied by Sr2+, the B-site by Ti4+, and the anion is O2-.1 The B-site cation is octahedrally coordinated to six O2- anions while the A-site cation sits in the empty space of the octahedron and is pseudo-twelve coordinate (see figure 1.1). Depending on the ions in the perovskite, slight anisotropy can occur in the lattice of the perovskite, but for SrTiO3, the B-site has Oh symmetry.

Figure 1.1 Structural representation of SrTiO3.

The properties of SrTiO3 have made this compound of interest for multiple applications. After

2 the discovery of photocatalytic water splitting of TiO2 by Honda and Fujishima , the similar band structure of SrTiO3 to TiO2 made it a logical material to be studied for use in water splitting. It was

3 demonstrated in 1976 that SrTiO3 could be used to split water to form H2 and O2 gases. The band gap between the primarily O 2p and Ti 3d orbitals which has an energy of 3.2 eV allows for its use as a photocatalyst as will be further discussed below.4

1.2 Important factors in photocatalysis. The idea to use materials to split water to form H2 gas as a potential fuel source stems from the same fundamental processes of photosynthesis utilized in plants, where the use of sunlight to generate a fuel source would allow for a reduction in the use

5 of non-renewable fossil fuel sources. Water is split to form H2 and O2 gases through a redox reaction using the electron/hole pair (e-/h+) as is shown in eq. 1.1.6

− + 2퐻2푂 + 4 푒 + 2ℎ → 2퐻2 + 푂2 eq. 1.1

The two half reactions of this process involve the oxidation of water to form O2 gas and protons, followed by the reduction of protons to generate H2 gas (equations 1.2a and 1.2b respectively).

+ + 2퐻2푂 + 2ℎ → 푂2 + 4퐻 eq. 1.2a

+ − 2퐻 + 2푒 → 퐻2 eq. 1.2b

The necessary potential for these two reactions to occur is +1.23 eV (vs. NHE) and 0.00 eV (vs.

NHE) for the oxidation and reduction respectively.5 In order for a material to be a plausible photocatalyst for total water splitting, the generated electrons must have a negative potential, while the hole must have a oxidation potential greater than +1.23 eV (vs. NHE). Figure 1.2 depicts the oxidation and reduction potentials of some common materials compared to those required for photocatalytic water splitting. In figure 1.2, there are 4 general types of materials: (1) the wide gap metal oxides (SrTiO3 and TiO2) which have appropriate redox potentials, but are sacrificing a large potential difference in at least one half reaction, (2) narrower gap sulfur based quantum dots (CdS and ZnS) that can do both reactions, but have stability issues in water, (3) oxidation catalysts (WO3

2 or Fe2O3) that can only facilitate the oxidation half reaction, or (4) reduction catalysts (Si and GaP) that can generate H2, but won’t oxidize water.

Figure 1.2 Band structure representation of different materials with potentials compared to those required for photocatalytic water splitting.

For materials that have the appropriate redox potentials to split water photocatalytically, there is a 3 step process that must take place (see figure 1.3). The absorption of light generates an e-/h+ pair (1), these charge carriers migrate to the surface of particle (2) where they can react with surface adsorbed species either as an oxidant or reductant (3).5 The efficiency of each step in this process determines the quality of the photocatalysis as well as the minimization of two competing pathways in photocatalysis, e-/h+ recombination or a charge carrier getting trapped prior to reaching the surface.

The separation of the e-/h+ pair and the transport to the surface of a photocatalyst can largely be affected by the physical composition of the material. Factors including crystallinity, size, and the presence of defects can largely influence the performance of materials as photocatalysts. The crystallinity of a photocatalyst largely affects the charge transport in the material due to decreased

5 defect sites which can trap the charge carriers prior to reaching the surface. In TiO2 nanoparticles,

3 it was demonstrated that the degree of crystallinity was more important than size as it related to photocatalytic performance.7 Synthetic processes have largely shown an effect on the crystallinity,

Figure 1.3. Schematic representation of the processes in photocatalysis in physical space and in terms of potential. (A) Generation of an electron hole pair. (B) Migration of electron and hole to the surface. (C) Electron/hole recombination, which is the competing process. (D) Reduction and oxidation of protons and water respectively, forming H2 and O2 gases. particularly calcination as was demonstrated in anatase TiO2, which with increased temperature treatment, showed higher crystallinity and higher photocatalytic performance.8 The crystallinity of nanoparticles synthesized by hydrothermal methods are dependent upon both reaction time and

9 10 pH and the corresponding effects on photocatalytic performance in BiVO4 and ZnWO4 respectively. The combined effects of crystalline growth was monitored in mesoporous SrTiO3 which showed as crystals grew and became more crystalline, photocatalytic performance increased.11

While the effects of crystallinity have been shown to be more critical than particle size, the effects of size on photocatalytic performance cannot be ignored. Smaller particles have an increased surface area per volume, which allows for more surface reaction sites for the redox

4 reactions to occur. In a smaller particle a charge carrier will also have shorter distance to travel to react with water or protons, allowing for this occur before recombination. However as particle size decreases, the number of defect sites increases, causing more sites for the charge carrier to be trapped. A midpoint between smaller size and good crystallinity for each catalyst is important to find in order to maximize surface area and charge transport while limiting recombination and

5 trapping. This was shown in TiO2 particles for the photocatalysis of propyne where the quantum efficiency increased as the size decreased, but only to 6 nm, with smaller sizes decreasing in quantum yield.12 The other major difference that results from a changing size is the absorption properties due to quantum size effects as was demonstrated in NiOx-SrTiO3 catalysts which showed 30 nm particles generated less photocurrent than bulk particles.13

Despite the fact that SrTiO3 can split water to form both H2 and O2 gas, one factor prevents

SrTiO3 from being a viable and marketable photocatalyst, it absorbs only UV light. The ultraviolet portion of the solar spectrum only makes up approximately 4.2% of the sun’s light. If a photocatalyst could utilize visible light, there would be 10-fold more light that could be utilized to split water. While other strategies have been researched to achieve visible light photocatalysis, the incorporation of dopant ions remain an area of high interest as an approach to allow for a single material to more effectively split-water.

1.3 Role of doping in visible light photocatalysis

The intentional introduction of impurities into a semiconductors in order to alter its properties is called doping. Doping has been used to change many different properties in materials including its magnetic, optical, and electronic properties. The use of dopant ions into wide-gap metal oxides has been shown to be a plausible route to utilize visible light for photocatalysis, allowing for enhancement of materials as photocatalysts. This strategy is effective due to the introduction of

5 new states in the band gap that can allow for visible light absorptions of new charge transfer transitions. As is shown in figure 1.5, the introduction of new energy levels into the band gap can allow for transitions at different energies. With the doping of transition metal ions into SrTiO3, the new states introduced into the band gap will primarily be composed of the 3d orbitals of the dopant ion.

Figure 1.4. Schematic representation of how the incorporation of dopant metals can alter the band structure of SrTiO3 to allow for visible light photocatalysis.

The use of dopant ions for visible light photocatalysis has been demonstrated using many different dopant ions, including good results with noble metals including Rh,14–18 Ru,14,18 Au,19 and Ag.19,20 However, due to availability and cost-effectiveness, most research currently focuses on the incorporation of first row transition metals in SrTiO3 for visible light photocatalysis. While all nine other first row transition metal ions have been doped for the purposes of visible light photocatalysis, Cr5,21,22 and Ni23–26 doping have shown the greatest potential.

One important factor in the use of transition metal ions for visible light photocatalysis is the valence state of the dopant ion. In research on Cr-doped SrTiO3, the valence state of the Cr has been shown to largely affect both photocurrent and the photocatalytic performance. La Mattina et

3+ al. studied Cr-doped SrTiO3 crystals as a proof-of-principle memory device where if Cr was present, a photocurrent could be detected, but no photocurrent was measured for Cr4+ ions in the

27 crystal. The photocatalytic performance of Cr-doped SrTiO3 has been demonstrated to strongly correlate to the valence state, with Cr3+ promoting visible light photocatalysis, but Cr6+ inhibiting

28–32 H2 evolution. This photocatalytic inhibition has been linked to the energy level of the 3d orbitals of the Cr6+ falling just below the conduction band minima, with a reduction potential not suited for the reduction of protons to form H2 gas.

6+ After the determination of Cr inhibiting the photocatalytic performance of Cr-doped SrTiO3, multiple approaches have been utilized in order to remove Cr6+. One of the most common techniques was to incorporate a co-dopant that can balance charges in the lattice, a 3+ dopant into the A-site or a 5+ dopant into the B-site. The use of B-site dopants have included Ta28,33,34, Nb35, and Sb.36 The main A-site dopant to balance charge with Cr3+ that has been used is La.30,37

1.4 The role of charge balance and defects

While the valence state of a transition metal dopant ion is closely related to the potential applications, the role of defects in the materials also must be considered. While defects can be present as a result of synthetic techniques and treatment of materials as will be discussed further in this section, the role of defects as it relates to charge compensation for dopant ions must be considered when making doped SrTiO3 materials. The most commonly observed defects that can

38,39 40,41 act as charge compensation are vacancies, either strontium or oxygen vacancies (VO). The coexistence of dopant ions in multiple valence states, such as a B-site dopant in the 3+ and 5+ in equal amounts and the material remaining charge neutral. This case has been demonstrated in Sb-

36 doped SrTiO3.

3+ 4+ 5+ In the case of Cr doping in SrTiO3, the valence state has been shown as Cr , Cr , Cr , and

Cr6+. Assuming substitutional doping in the B-site for Ti4+, the stoichiometry can be represented as SrTi1-xCrxO3-δ, where x and δ are the Cr and VO concentrations respectively. These values are expressed in Kroger-Vink notation in equations 1.3 and 1.4.

′ × • •• 푥 = [CrTi] + [CrTi] + [CrTi] + [CrTi] eq. 1.3

× • •• 훿 = [푉O ] + [푉O] + [푉O ] eq. 1.4

x The concentration of V O defects arise from synthetic methods, since they don’t affect charge compensation. The correlation of Cr valence state and VO defects can be expressed in terms of the concentration of Cr3+ as is shown in equation 1.5.

′ • •• • •• 2[CrTi] = 2[CrTi] + [CrTi] + 2[푉O] + [푉O ] eq. 1.5

The effects of oxygen vacancies in photocatalysts is complex, but can be both positive and negative towards the performance of these materials. VO defects can positively affect photocatalytic performance by acting as adsorption sites on the surface42 or introducing new

43 energy levels allowing for visible light oxidation reactions. However if the concentration of VO defects are too high, reduced crystallinity lowers photocatalytic activity.44

The use of VO defects in order to impose visible light photocatalysts has been demonstrated

44 45 46 47,48 49 in various metal oxide catalysts including SrTiO3-δ, WO3-δ, BiPO4-δ, ZnO1-δ, and TiO2-δ.

These samples are prepared via different methods that reduce the samples, using reducing agents

44,50 49 like NaBH4 under Ar/H2 blends, hydrogenation under high pressures, or simply heating under inert atmospheres or vacuum.46,47,51 These techniques are similar to some of those that were utilized

3+ to increase Cr concentrations in Cr-doped SrTiO3. In SrTiO3 and TiO2 samples, the introduction

44,49,50 of VO defects resulted in the white samples to transition to gray and eventually black. This color transition is the result of the combination of the formation of VO defects forming new

8 transitions in the visible portion of the spectrum and the formation of localized Ti3+ which shows absorption in the far-visible and near IR regions of the spectrum. The formation of Ti3+ has been

52–54 confirmed in TiO2 via electron paramagnetic resonance spectroscopy (EPR) and observed in

55,56 photo-induced EPR in SrTiO3 single crystals.

1.5 Advantages of Ruddlesden-Popper phases

Ruddlesden-Popper phases are “layered” perovskite systems that have the formula of

57 An+1BnX3n+1. In the case of the Sr-Ti-O Ruddlesden-Popper phases, SrTiO3 is the n=∞ member of the series. The change in the structure of the members of the Ruddlesden-Popper series is the separation of a perovskite layer by a layer of SrO. In the n=1 compound, Sr2TiO4, each layer of

TiO6 octahedra is separated by a layer of SrO as is illustrated in figure 1.5. With increasing values of n, a monolayer of SrO separates growing layers of SrTiO3. Synthesis of these materials has been limited to molten salt reactions at high temperatures (n=1-3)58 and the growth of single crystalline films using molecular beam epitaxy (n=1-5).59 The stability of these phases has been in some contention, which has limited synthesis via other approaches.60,61

Figure 1.5 Structural representations of Ruddlesden-Popper phases of the Sr-Ti-O series for n=∞, 1, and 2.

Interest in Ruddlesden-Popper materials stem largely from two factors, the change in the charge transport properties resulting from the layered structure and the differences in the surface

9 chemistry as it relates to the adsorption of reagents to undergo photocatalytic reactions. The band structure of Sr2TiO4 and Sr3Ti2O7 vary minimally from that of SrTiO3, but the band gap decreases slightly according to theoretical calculations due to an increased bandwidth in the density of states of the conduction band, primarily composed of Ti 3d orbitals.62 A decrease in the conduction band potential would be problematic for H2 production, but experimental results show the band gap

30 absorption for SrTiO3 and Sr2TiO4 at the same energies.

The changes in charge transport properties in layered perovskite materials stem from confinement of photogenerated charge carriers to two dimensions. The charge separation in

Ruddlesden-Popper materials occurs similarly to that in heterostructures such as LaAlO3-SrTiO3, which shows increased mobility depending on each layers thickness.63 Due to the conduction band being primarily composed of 3d orbitals of Ti, the photogenerated electron-hole pair forms in the

29 TiO6 portion of the Ruddlesden-Popper phase and is unable to cross the SrO layer. The role of the charge separation with regards to layered perovskites also relies heavily on the presence of a dipole moment, which was discussed and it’s increased photocatalytic activity that resulted was

64 65 demonstrated in BaTi4O9 and Sr2M2O7 (M=Nb or Ta).

The layered perovskite materials are also believed to have increased photocatalytic performance due to the layering causing more sites on the surface for reactions to occur. In the hydrous layered H2La2/3Ta2O7, the amount of n-butylamine adsorbed increased in the higher layered perovskite.66 Layered perovskites have also been determined to have higher A-site cations at the surface than B-site cations.67,68 The higher A-site surface concentration in combination with the shown migration of the electron and hole to the interlayer of the AO phase of Ruddlesden-

Popper phases combine to contribute to increased photocatalytic performances.29 The surface

10 structure of a material has been demonstrated to greatly affect the photocatalytic performance of a material69 and needs to be considered moving forward.

While studies in undoped Ruddlesden-Popper phases have shown increased photocatalytic performance, few studies on doping these materials have been done. Work on Cr-doped Sr2TiO4

30 has shown visible-light photocatalysis, and higher performance than that of Cr-doped SrTiO3.

Visible light photocatalysis has also been demonstrated in Cr-doped Sr2TiO4 in two other reports, one studying the photochromic effects,31 while the other look at co-doping with La.29 Other reports of doping in the Ruddlesden-Popper phases focus on emission studies including Pr-doped in the n=1 and 2 phases70 and Eu71. The effects on the thermoelectric properties of doping with Er, Y,

Dy, Gd, Eu, Sm, Nd, and La in Sr3Ti2O7 have also been studied, but feature no spectroscopic evidence to confirm doping.72,73

1.6 Focal points for development of SrTiO3 based visible light photocatalysts

While SrTiO3 and its related Ruddlesden-Popper phases have received a large amount of attention in the scientific community for the development of visible light photocatalysts as well as in other material applications, the viability of these materials in catalytic applications has not progressed to the point of marketability. Large variations in the methods for performance testing obfuscates determination of areas that need improvement, as does a lack detailed structure- functional experiments. Attempts solely to increase photocatalytic performance without proper characterization or pinpointing of the electronic and physical alterations that are leading to the changes in performance have left progress stagnant.

The work described herein will look to better understand the incorporation of dopant ions in

SrTiO3 and the n = 1 Ruddlesden-Popper phase, Sr2TiO4. Through the use of dopant-specific characterization techniques such as electron paramagnetic resonance (EPR) spectroscopy and

11 variable temperature emission spectroscopy, the valence state and location of the dopant ion is studied systematically as it is incorporated into the host lattice. The first determination of the spin-

3+ 4+ Hamiltonian parameters of Cr and Mn in Sr2TiO4 was also performed.

A low-temperature NaBH4 reduction was utilized in order to not only control the dopant ion valence states, but also to alter the VO defects in the lattice. The correlation of the dopant ions and the valence state not only provides more information about the charge compensation in the host lattice, but also the alterations to the absorption of the materials which is crucial to understand for photocatalytic applications.

1.7 References

1. Mitchell, R. H., Chakhmouradian, A. R. & Woodward, P. M. Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr(1-2x)Na(x)La(x))TiO3. Phys. Chem. Miner. 27, 583–589 (2000).

2. Fujishima, A. & Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38 (1972).

3. Wrighton, M. S. et al. Strontium Titanate Photoelectrodes. Efficient Photoassisted Electrolysis Of Water At Zero Applied Potential. J. Am. Chem. Soc. 98, 2774–2779 (1976).

4. van Benthem, K., Elsä sser, C. & French, R. H. Bulk electronic structure of SrTiO3: Experiment and theory. J. Appl. Phys. 90, 6156 (2001).

5. Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–78 (2009).

6. Smil, V. Energy: A Beginner’s Guide. (Oneworld Publishers, 2006).

7. Abdul Rashid, S., Othman, S. H., Mohd Ghazi, T. I. & Abdullah, N. Effect of postdeposition heat treatment on the crystallinity, size, and photocatalytic activity of TiO2 nanoparticles produced via chemical vapour deposition. J. Nanomater. 2010, (2010).

8. Inagaki, M., Nonaka, R., Tryba, B. & Morawski, A. W. Dependence Of Photocatalytic Activity Of Anatase Powders On Their Crystallinity. Chemosphere 64, 437–445 (2006).

9. Yu, J. & Kudo, A. Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv. Funct. Mater. 16, 2163–2169 (2006).

10. Lin, J., Lin, J. & Zhu, Y. Controlled synthesis of the ZnWO4 nanostructure and effects on the photocatalytic performance. Inorg. Chem. 46, 8372–8378 (2007).

11. Zhan, H. et al. Correlation between multiple growth stages and photocatalysis of SrTiO3 nanocrystals. J. Phys. Chem. C 119, 3530–3537 (2015).

12. Anpo, M., Shima, T., Kodama, S. & Kubokawa, Y. Photocatalytic Hydrogenation of CH3CCH with H2O on Small-Particle TiO2: Size Quantization Effects and Reaction Intermediates. J. Phys. Chem. 91, 4305–4310 (1987).

13. Townsend, T. K., Browning, N. D. & Osterloh, F. E. Nanoscale Strontium Titanate Photocatalysts for Overall Water Splitting. ACS Nano 6, 7420–7426 (2012).

14. Bae, S. W., Borse, P. H. & Lee, J. S. Dopant dependent band gap tailoring of hydrothermally prepared cubic SrTixM1-xO3 (M=Ru,Rh,Ir,Pt,Pd) nanoparticles as visible light photocatalysts. Appl. Phys. Lett. 92, 104107 (2008).

15. Iwase, A., Ng, Y. H., Ishiguro, Y., Kudo, A. & Amal, R. Reduced graphene oxide as a solid- state electron mediator in Z-scheme photocatalytic water splitting under visible light. J. Am. Chem. Soc. 133, 11054–7 (2011).

16. Kato, H., Sasaki, Y., Shirakura, N. & Kudo, A. Synthesis of highly active rhodium-doped SrTiO3 powders in Z-scheme systems for visible-light-driven photocatalytic overall water splitting. J. Mater. Chem. A 1, 12327 (2013).

17. Shen, P. et al. Photocatalytic activity of hydrogen evolution over Rh doped SrTiO3 prepared by polymerizable complex method. Chem. Eng. J. 223, 200–208 (2013).

18. Konta, R., Ishii, T., Kato, H. & Kudo, A. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation. J. Phys. Chem. B 108, 8992–8995 (2004).

19. Subramanian, V., Roeder, R. K. & Wolf, E. E. Synthesis And UV-Visible-Light Photoactivity of Noble-Metal-SrTiO3 Composites. Ind. Eng. Chem. Res. 45, 2187–2193 (2006).

+ 2+ 20. Irie, H., Maruyama, Y. & Hashimoto, K. Ag and Pb Doped SrTiO3 Photocatalysts. A Correlation Between Band Structure and Photocatalytic Activity. J. Phys. Chem. C 111, 1847–1852 (2007).

21. Yu, H. et al. Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production. J. Mater. Chem. 21, 11347 (2011).

22. Chen, H., Huang, C., Wu, J. C. S. & Lin, S. Theoretical Investigation of the Metal-Doped SrTiO3 Photocatalysts for Water Splitting. J. Phys. Chem. C 116, 7897–7903 (2012).

23. Niishiro, R., Kato, H. & Kudo, A. Nickel and either tantalum or niobium-codoped TiO2 and

SrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueous solutions. Phys. Chem. Chem. Phys. 7, 2241–5 (2005).

24. Kang, H. W., Lim, S. N. & Park, S. Bin. Co-doping schemes to enhance H2 evolution under visible light irradiation over SrTiO3:Ni/M (M = La or Ta) prepared by spray pyrolysis. Int. J. Hydrogen Energy 37, 5540–5549 (2012).

25. Kang, H. W., Lim, S. N. & Park, S. Bin. Effect of tri-doping on H2 evolution under visible light irradiation on SrTiO3:Ni/Ta/La prepared by spray pyrolysis from polymeric precursor. Int. J. Hydrogen Energy 37, 10539–10548 (2012).

26. Jia, A., Liang, X., Su, Z., Zhu, T. & Liu, S. Synthesis and the effect of calcination temperature on the physical-chemical properties and photocatalytic activities of Ni,La codoped SrTiO3. J. Hazard. Mater. 178, 233–42 (2010).

27. La Mattina, F., Bednorz, J. G., Alvarado, S. F., Shengelaya, A. & Keller, H. Detection of charge transfer processes in Cr-doped SrTiO3 single crystals. Appl. Phys. Lett. 93, 22102 (2008).

28. Ishii, T., Kato, H. & Kudo, A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J. Photochem. Photobiol. A Chem. 163, 181–186 (2004).

29. Sun, X. et al. Photocatalytic Hydrogen Production over Chromium Doped Layered Perovskite Sr2TiO4. Inorg. Chem. 54, 7445-7453 (2015).

30. Jia, Y. et al. Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation. J. Mater. Chem. A 1, 7905 (2013).

31. Yi, Z. G., Iwai, H. & Ye, J. H. Photochromism and visible light induced H2 generation in Sr2TiO4:Cr complexes. Appl. Phys. Lett. 96, (2010).

32. Wang, D., Ye, J., Kako, T. & Kimura, T. Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites. J. Phys. Chem. B 110, 15824–30 (2006).

33. Kang, H. W. & Park, S. Bin. H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor. Int. J. Hydrogen Energy 36, 9496–9504 (2011).

34. Shi, J., Shen, S., Chen, Y., Guo, L. & Mao, S. S. Visible light-driven photocatalysis of doped SrTiO3 tubular structure. Opt. Express 20 Suppl 2, A351-9 (2012).

35. Reunchan, P. et al. Theoretical Design Of Highly Active SrTiO3-based Photocatalysts From Codoping Scheme Toward Solar Energy Utilization For Hydrogen Production. J. Mater. Chem. A (2013).

36. Kato, H. & Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106, 5029–5034 (2002).

37. Ouyang, S. et al. Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts. J. Am. Chem. Soc. 134, 1974–7 (2012).

38. Kolodiazhnyi, T. & Petric, A. The Applicability of Sr-deficient n-type SrTiO3 for SOFC Anodes. J. Electroceramics 15, 5–11 (2005).

39. Moos, R. & Hardtl, K. H. Defect Chemistry of Donor-Doped and Undoped Strontium Titanate Ceramics between 1000° and 1400°C. J. Am. Ceram. Soc. 80, 2549–2562 (1997).

40. Muller, K. A., Berlinger, W. & Rubins, R. S. Observation of Two Charged States of A Nickel-Oxygen Vacancy Pair in SrTiO3 by Paramagnetic Resonance. Phys. Rev. 186, 361– 371 (1969).

3+ 41. Li, H. et al. Roles of Cr doping and oxygen vacancies in SrTiO3 photocatalysts with high visible light activity for NO removal. J. Catal. 297, 65–69 (2013).

42. Schaub, R. et al. Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 87, 266104 (2001).

43. Mitra, C., Lin, C., Robertson, J. & Demkov, A. A. Electronic structure of oxygen vacancies in SrTiO3 and LaAlO3. Phys. Rev. B 86, 155105 (2012).

44. Tan, H. et al. Oxygen Vacancy Enhanced Photocatalytic Activity of Pervoskite SrTiO3. ACS Appl. Mater. Interfaces 6, 19184–19190 (2014).

45. Liu, G. et al. Enhancement of visible-light-driven O2 evolution from water oxidation on WO3 treated with hydrogen. J. Catal. 307, 148–152 (2013).

46. Lv, Y., Zhu, Y. & Zhu, Y. Enhanced Photocatalytic Performance for the BiPO4−x Nanorod Induced by Surface Oxygen Vacancy. J. Phys. Chem. C 117, 18520 (2013).

47. Lv, Y. et al. The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1−x photocatalyst. Catal. Sci. Technol. 3, 3136 (2013).

48. Wang, J. et al. Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO. ACS Appl. Mater. Interfaces 4, 4024–4030 (2012).

49. Chen, X., Liu, L., Yu, P. Y. & Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 331, 746–50 (2011).

50. Tan, H. et al. A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity. Nanoscale 6, 10216–23 (2014).

51. Bai, X. et al. Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization. Langmuir 29, 3097–3105 (2013).

52. Zuo, F. et al. Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light. J. Am. Chem. Soc. 132, 11856–11857 (2010).

53. Khomenko, V. M., Langer, K., Rager, H. & Fett, a. Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile. Phys. Chem. Miner. 25, 338–346 (1998).

3+ 4+ 54. Ji, Y. et al. Surface Ti /Ti Redox Shuttle Enhancing Photocatalytic H2 Production in Ultrathin TiO2 Nanosheets/ CdSe Quantum Dots. J. Phys. Chem. C 119, 27053-27059 (2015).

3+ 55. Laguta, V. V. et al. The Photoinduced Ti centre in SrTiO3. J. Phys. Condens. Matter 14, 13813–13825 (2002).

56. Schirmer, O. F. & Muller, K. A. Defects In Neutron-Irradiated Strontium Titanate: Ti3+ ‘Off Center’ on a Sr2+ Site. Phys. Rev. B 7, 2986–2995 (1973).

57. Ruddlesden, S. N. & Popper, P. New compounds of the K2NIF4 type. Acta Crystallogr. 10, 538–539 (1957).

58. Jacob, K. T. & Rajitha, G. Thermodynamic properties of strontium titanates: Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, and SrTiO3. J. Chem. Thermodyn. 43, 51–57 (2011).

59. Haeni, J. H. et al. Epitaxial growth of the first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series. Appl. Phys. Lett. 78, 3292 (2001).

60. Ramadan, A. H. H., Allan, N. L. & De Souza, R. a. Simulation Studies of the Phase Stability of the Srn+1TinO3n+1 Ruddlesden-Popper Phases. J. Am. Ceram. Soc. 96, 2316–2321 (2013).

61. Berbenni, V., Marini, A. & Bruni, G. Effect of Mechanical Activation on The Preparation of SrTiO3 and Sr2TiO4 Ceramics From The Solid State System SrCO3-TiO2. J. Alloys Compd. 329, 230–238 (2001).

62. Reshak, A. H., Auluck, S. & Kityk, I. Electronic Band Structure and Optical Properties of Srn + 1TinO3n + 1 Ruddlesden – Popper Homologous Series. Jap. J. Appl. Phys 47, 5516 (2008).

63. Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. 427, 423–427 (2004).

64. Kohno, M., Ogura, S., Sato, K. & Inoue, Y. Properties of photocatalysts with tunnel structures: Formation of a surface lattice O- radical by the UV irradiation of BaTi4O9 with a pentagonal-prism tunnel structure. Chem. Phys. Lett. 267, 72–76 (1997).

65. Kudo, A., Kato, H. & Nakagawa, S. Water Splitting into H2 and O2 on New Sr2M2O7 (M = Nb and Ta) Photocatalysts with Layered Perovskite Structures: Factors Affecting the Photocatalytic Activity. J. Phys. Chem. B 104, 571–575 (2000).

66. Shimizu, K. I. et al. Photocatalytic water splitting on ni-intercalated ruddlesden-popper tantalate H2La2/3Ta2O7. Chem. Mater. 17, 5161–5166 (2005).

67. Druce, J., Ishihara, T. & Kilner, J. Surface composition of perovskite-type materials studied by Low Energy Ion Scattering (LEIS). Solid State Ionics 262, 893–896 (2014).

68. Jung, W. & Tuller, H. L. Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes. Energy Environ. Sci. 5, 5370 (2012).

69. Nurlaela, E. et al. Critical role of the semiconductor-electrolyte interface in photocatalytic performance for water-splitting reactions using Ta3N5 particles. Chem. Mater. 26, 4812– 4825 (2014).

70. Inaguma, Y., Nagasawa, D. & Katsumata, T. Photoluminescence of Praseodymium-Doped Srn +1TinO3n+1 ( n =1, 2, ∞). Jpn. J. Appl. Phys. 44, 761–764 (2005).

71. Lu, Z., Zhang, L., Wang, L. & Zhang, Q. Concentration Dependence of Luminescent 3+ Properties for Sr2TiO4:Eu Red Phosphor and Its Charge Compensation. J. Nanomater. 698434 (2012).

72. Wang, Y. et al. Enhancement of thermoelectric performance in rare earth-doped Sr3Ti2O7 by symmetry restoration of TiO6 octahedra. J. Electroceramics 24, 76–82 (2010).

73. Sun, R. R. et al. The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7. J. Phys. Chem. Solids 75, 629–637 (2014).

CHAPTER 2

SPECIATION OF Cr (III) IN INTERMEDIATE PHASES DURING THE SOL-GEL PROCESSING OF Cr-DOPED SrTiO3 POWDERS

This chapter has been published as:

Lehuta, K.A. and Kittilstved, K.R.; Speciation of Cr (III) in intermediate phases during the sol- gel processing of Cr-doped SrTiO3 powders. J. Mat. Chem. A, 2014, 2, 6138-6145.

2.1 Introduction

SrTiO3 is a wide-gap oxide-based perovskite semiconductor that displays interesting electronic phenomena, including composition and size-dependent dielectric constant,1 quantum paraelectricity,2 ferroelectricity,3 and superconductivity at heterostructured interfaces.4 With an indirect band gap energy of 3.2 eV, there is strong interest in using SrTiO3 for photocatalysis

5,6 applications due to the conduction band potential being ~0.2 eV more negative than TiO2.

However, the upper limit to the efficiency of inexpensive transparent oxides (SrTiO3, TiO2, ZnO, etc.) as photocatalysts is limited to a maximum of 4 % under solar irradiation.5 To address this limitation, many groups have focused on introducing targeted impurities to decrease the optical band gap of the semiconductor and increase its overlap with the solar spectrum. For typical wide- gap oxide semiconductors the reduction potential is only slightly more negative than 0 V vs normal hydrogen electrode (NHE) at pH=0, while the oxidative potential is >1 V more positive than required to oxidize water (1.1 V vs NHE). Therefore, a large effort has focused on reducing the oxidation potential of carriers in the valence band by doping wide-gap oxide semiconductors with

7–9 p-type defects such as N in TiO2. Some successes toward increasing the reductive potential of oxide photocatalysts have also been demonstrated,10,11 however, visible light sensitization is a requirement to achieve photocatalytic efficiencies greater than 4 % under solar irradation in these materials.

Kudo and co-workers have demonstrated that transition metal ions can also sensitize wide-gap

12–14 3+ photocatalysts with visible light. His group used Cr as a dopant ion in SrTiO3 to

15–18 3+ photocatalytically generate H2 from water using visible light. Cr :SrTiO3 has also shown promise in solid-state memory devices.19 In both applications, it was demonstrated that the oxidation state of Cr is critical to the material performance. For example, the presence of Cr6+ in

3+ 20 the desired Cr :SrTiO3 material is detrimental to its photocatalytic activity. Likewise, the

3+ ferroelectricity of Cr :SrTiO3 can be modulated by laser irradiation or electrically switching the

Cr oxidation state between 3+ (“on”) and 4+ (“off”).21 The ability to identify and control the electronic structures of dopant ions with multiple oxidation states is critical to the implementation of doped inorganic materials for visible-light photocatalysis or electronics applications.

3+ 22 While the electronic structure and functions of Cr :SrTiO3 have been fairly well established, the speciation of dopants and photochemistry of doped intermediate phases that may also be present in undetectable quantities by conventional diffraction and electron microscopy are not well understood. For example, Ruddlesden-Popper (RP) phases23 with the general formula

24 Srn+1TinO3n+1 are known to form during the sol-gel processing of SrTiO3 (n= ∞). The RP phase,

25 Sr2TiO4 (n=1), has a 0.3 eV larger band gap energy compared to SrTiO3 and has shown promise as a thermoelectric material.26 Only a few experimental studies of transition metal dopants in

3+ Sr2TiO4 have been reported including a recent study of heavily-doped (10 %) Cr :Sr2TiO4 that

27 displayed visible-light photocatalytic H2 evolution.

3+ In this study, we focus on the speciation of Cr dopants during the sol-gel synthesis of SrTiO3.

By systematically studying the evolution of the crystallographic phases and the dopant specific spectroscopy of Cr3+, we are able to correlate the electronic structure of Cr3+ with both detected and undetected phases. We identify the speciation of Cr3+ substitutionally-doped at the Ti4+ site of

Sr2TiO4 by EPR spectroscopy for the first time and discuss its implications on mechanism of dopant incorporation in SrTiO3.

2.2 Experimental Methods

2.2.1 Materials. Strontium nitrate (99%+ for analysis), titanium dioxide (98%+, anatase powder,

A-TiO2), and citric acid (99% pure anhydrous) were purchased from Acros Organics. Chromium nitrate nonahydrate (crystalline, certified) and ethylene glycol (lab grade) were purchased from

Fisher Scientific. All chemicals were used as received without further purification.

2.2.2 Synthesis of Cr-doped SrTiO3 powders. Bulk Cr-doped SrTiO3 was prepared using a modified Pechini-type sol-gel method.28 Cr doping is given as a percentage of the nominal substitution of Ti4+ (B-site) and reported as x × 100% where x = [Cr]/([Cr]+[Ti]). A typical synthesis involved dissolving a mixture of (Cr(NO3)3)x:(TiO2)1−x (where x = 0 – 0.05) and Sr(NO3)2 powders (20 mmol each) in 50 mL of 0.54 M citric acid solution. Ethylene glycol (10 mL) was added and the mixture was heated at 140 °C to form a sol. Further heating at 215 °C drove off the excess ethylene glycol. The sol was then transferred to porcelain crucibles and calcined in air for

4 h at 800 °C (Lindberg Blue Mini-Mite tube furnace). Samples were annealed in air up to 1050

°C for up to 6 h unless otherwise specified.

2.2.3 Physical Characterization. Powder X-ray diffraction patterns were collected on a

PANalytical X'Pert Material Research Diffractometer. Diffuse reflectance spectra were collected with an integrating sphere (Ocean Optics ISP-REF) coupled by fiber optics to a CCD-based spectrophotometer (Ocean Optics USB2000+ VIS-NIR). Room temperature X-band EPR spectra were collected on powders using the perpendicular mode of a dual mode resonator cavity (Bruker

Elexsys E-500 with ER-4116 cavity). Variable temperature emission spectra were collected at 4

20 cm−1 spectral resolution with a Fourier transform spectrometer (Varian 670) equipped with a PMT detector (Hamamatsu R116) and excited with the 488 nm line of an Ar+ laser (JDSU 2218).

Powdered samples for emission were fixed to a copper plate using conductive silver paint

(Leitsilber 200, Ted Pella) that was placed in a closed-cycle helium optical cryostat (Advanced

Research Systems HC-2). X-ray photoelectron spectroscopy (XPS) was measured using a Physical

Electronics Inc., 5000 Series Spectrophotometer. Data was analyzed using Physical Electronics

Multipak v6.

2.2.4 Cr3+ ground state electronic structure and EPR simulations. Cr3+ has a S = 3/2 ground

4 state with no orbital angular momentum ( A2g in Oh point symmetry). The degeneracy of the mS =

|±1/2 and |±3/2 spin sublevels is lifted by the combination of a low- symmetry crystal field and

2nd-order spin-orbit coupling. The axial and rhombic components to the zero-field splitting (ZFS) are parameterized in the spin Hamiltonian (eqn (2.1)) by the parameters D and E, respectively.

2 2 2 퐻 = 퐷(푆̂푧 − S(S + 1)/3) + 퐸(푆̂푥 − 푆̂푦) + 퐴(푰 ∙ 푺) + 품훽퐵 (2.1)

The electron-nuclear hyperfine coupling from 53Cr nuclear spins (I=3/2, 9.5% relative abundance) is parameterized by A; the Landé factor is given by the g tensor, which is slightly anisotropic

(gz≠gx,y) in an axial crystal field (D≠0), β is the bohr magneton; and B is the external magnetic field.

The simulations of the EPR spectra, angular dependencies of the resonance fields, and D-strain

(standard deviation in D) were carried out using XSophe29 in accordance with eqn (2.1).

2.3 Results and Discussion

2.3.1 Phase behavior during calcination and annealing. Figure 2.1 shows the powder XRD pattern for nominally 1% Cr:SrTiO3 after calcination of the sol-gel precursors at 800 °C for 4 h

21 followed by annealing at 1050 °C for an additional 4 h. After the calcination step, the diffraction pattern contains peaks that correspond to SrTiO3 and additional peaks that are assigned to A-TiO2

24,30 and SrCO3. These impurity phases were subsequently removed upon further annealing in air at 1050 °C for longer than 4 h resulting in primarily SrTiO3 with some rutile (R-TiO2) secondary phase at 2θ = 27.47°.31

Fig. 2.1 Powder XRD patterns of nominally 1 % Cr:SrTiO3 samples after calcination (4h at 800 °C) and annealing (4 h at 1050 °C). The bottom region shows the diffraction positions and relative intensities for SrTiO3, R-TiO2, A-TiO2, and SrCO3 from refs [32], [31], [33], and [30], respectively.

The temporal evolution of the conversion from precursor oxides and carbonates to SrTiO3 at temperatures of 850 °C, 950 °C and 1050 °C for up to 6 h was monitored by powder diffraction

(see Figure 2.2). The diffraction patterns for the sample annealed at 850 °C for up to 6 h in Figure

2.2A shows gradual disappearance of the SrCO3 peaks at 2θ ≈ 25.5° and 26° concomitant with an increase in the SrTiO3 peak at 2θ ≈ 32.4°. The A-TiO2 peak at 2θ ≈ 25.5° does not change in relative intensity to the SrTiO3 peak after 3 h at 850 °C. The feature at 2θ ≈ 29.7° is the (200) peak

30 of SrCO3.

Increasing the annealing temperature to 950 °C shows (Figure 2.2B) complete removal of all precursor phases after 6 h and a gradual increase in the SrTiO3 peak (2θ ≈ 32.4°) occurs. However,

22 increasing the temperature to 1050 °C as shown in Figure 2.2C did not result in accelerated formation of SrTiO3 as expected. After 30 minutes an intense peak at 2θ ≈ 31.2° and a shoulder at

~31.7° emerges that we assign to the mixed perovskite-rock salt compounds Sr2TiO4 and Sr3TiO7, respectively.23,34,35 These RP phases were not observed at lower temperature annealing steps

(Figure 2.2A-B).

Fig. 2.2 Powder XRD patterns in an expanded region of nominally 1 % Cr:SrTiO3 samples as a function of annealing temperatures of 850 °C (A), 950 °C (B) and 1050 °C (C) for 0, 0.5 h, 3 h, and 6 h. Panel (D) shows the diffraction peaks for the same phases as given in Fig. 1 and Sr2TiO4 [34], Sr3TiO7 [35], and SrO [36]. Note the change in the spacing of 2θ to the left and right of 30.5°.

4+ Table 2.1. Molecular symmetries and bond lengths of Ti sites in relevant TiO2 and SrTiO3 phases. Axial and equatorial bond lengths for tetragonal and rhombic sites are differentiated by “ax” and “eq” abbrevations.

4+ Lattice Ti R(Ti-O), Å Oax-Ti-Oeq Oeq-Ti-Oeq Ref. SrTiO3 Oh 1.9517 90° 90° [30] ax=1.9152 Sr TiO D 90° 90° [36] 2 4 4h eq=1.9420 ax=1.9157 Sr Ti O D 90° 90° [35] 3 2 7 4h eq=1.9500 ax=1.8740 A-TiO D 75.9°, 104.1° 90° [29] 2 2d eq=1.9229 ax=1.9805 R-TiO D 90° 81.17°, 98.83° [33] 2 2h eq=1.9470

The closest peak from other possible phases is TiO2 in the brookite structure with intense diffraction peaks at 2θ = 25.4°, 25.7°, and 30.9°, however, this phase is unstable above 750 °C.37,38

With longer annealing times at 1050 °C, we observe the disappearance of these metastable phases and the appearance of R-TiO2 at 2θ = 27.5° in addition to SrTiO3 (see also Figure 2.1).

4+ 23 The molecular symmetry of Ti in the RP phase Sr2nTinO3n+1 (where n = 1 and 2) is D4h, which

32 4+ is very different from SrTiO3 (Oh, see Table 2.1). Additional Ti site symmetries of the different

31,33 4+ titanium oxide phases include D2d in A-TiO2 and D2h in R-TiO2. The variations of the Ti symmetry manifests itself in the EPR spectroscopy of the samples annealed at 1050 °C (vide infra).

2.3.2 Ligand Field Electronic Structure. The powder diffraction patterns shown in Figures 2.1 and 2.2 provide no information about the location or oxidation state of the Cr ions. We therefore employed dopant-specific probes to determine the local environment of the Cr. Figure 2.3A shows the absorption spectra converted from the diffuse reflectance using the Kubelka-Munk function41 for nominally 1% Cr:SrTiO3 powders after the calcination (800 °C for 4 h) and after the high- temperature annealing (1050 °C for 4 h). The absorption threshold for the band gap of SrTiO3 occurs at ~25650 cm−1 as shown for an undoped sample in Figure 2.3A. After the calcination step,

Fig. 2.3 (A) Absorption of pure SrTiO3 annealed at 1050 °C (−·−), nominally 1 % Cr:SrTiO3 before (―) and after annealing at 1050 °C (···). (B) Cr3+ R-line emission at 15 K from nominally 1% Cr:SrTiO3 before and after annealing at 1050 °C for 4 h. (C) Temperature dependence of energy difference of the R-line emission to the 15 K R-line position for 1 % powders (closed circles). Vertical arrows in (A) and (B) are used to differentiate the new spectral features in the spectra before and after the annealing steps. Literature absorption and 20.3 K emission spectra (− −) and temperature dependence (open circles in panel C) of the R-lines were collected on single crystals containing 0.01 wt% (A) and 0.005 wt% Cr2O3(B). Data were extracted from refs. [39] and [40] using commercial digitization software (DigitizeIt, v1.5.7). the sample absorbs some visible light, but after annealing at 1050 °C for 4 h, the sub-bandgap transition at ~20000 cm−1 increases in intensity. This transition has been previously assigned in dilute single crystals to a Ti3+-related charge transfer based on the observations that it does not

3+ 39 sensitize the Cr emission and is similar to a broad transition in n-type SrTiO3 that was annealed under high vacuum.42 The extinction coefficient per Cr ion is estimated at ε ≈ 5000 – 6000 L mol−1

−1 3+ cm based on the concentration of Cr , the thickness of the SrTiO3 single crystal, and reported optical density.39 This molar extinction coefficient is 2-3 orders of magnitude too large to be assigned to a spin-allowed ligand-field transition for Cr3+ in a pseudo-octahedral symmetry.

3+ 4 4 4 4 Excitation scans of the Cr emission in single crystals place the A2→ T2 and A2→ T1 transitions at 16300 cm−1 and 21300 cm−1.39

Figure 2.3B shows the ~15 K emission spectra of the two Cr3+-doped samples shown in Figure

2.3A. For comparison, Figure 2.3B includes the literature emission spectrum from a 0.005 wt %

3+ 39 2 4 3+ Cr :SrTiO3 single crystal. The E→ A2 (R-line) emission of the annealed sample Cr in SrTiO3 occurs at 12600 cm−1 with weaker vibronic fine structure extending ~700 cm−1 to the red of the zero-phonon line.39,43 Both the R-line emission and the vibronic fine structure are absent in the powder sample after calcination at 800 °C. In contrast, a sharp feature at ~12600 cm−1 was observed in the sample after the 1050 °C anneal for 4 h. This sharp feature was only observed below 165 K. The broad feature red-shifted from the R-line emission is assigned to vibronic sidebands. The increased inhomogeneous line width of the R-lines due to higher Cr3+ concentrations in our study does not allow us to resolve the splitting of the 2E excited state as

3+ 39 observed previously on a sample with only 0.005 wt % Cr :SrTiO3. This also decreases the relative intensity of the R-line compared to the vibronic sidebands in our sample.

The temperature dependence of the Cr3+ average R-line emission energy was also measured and

18 2 4 compared to the reported behavior in Cr:SrTiO3 single crystals. The energy of the E→ A2 emission line increases with increasing temperature (+45 cm−1 from 4 K to 295 K) unlike Cr3+ in

−1 18 MgO or Al2O3 (−8.4 and −6.6 cm , respectively). The energy difference between 15 K and 165

−1 K in the 1% Cr:SrTiO3 powder (shown in Figure 2.3C) was +32.5 cm . This result is in agreement with the single crystal data where the shift was +34 cm−1 over this temperature range. The temperature dependence confirms strong electronic coupling between the SrTiO3 soft phonons and

40 2 4 the dopant ion. The energy and temperature dependence of the E→ A2 emission with associated vibronic fine structure provides unambiguous evidence for the successful doping of Cr3+ at the

4+ substitutional site of Ti in SrTiO3.

3+ 2.3.3 EPR Spectroscopy of Cr During SrTiO3 Annealing. The ground state electronic structure of Cr3+ is sensitive to the geometric restraints imposed by its crystalline environment. We

26 employed EPR spectroscopy as a probe of the Cr3+ speciation during the systematic variation of the annealing time and temperature. Figure 2.4 shows the EPR spectra of Cr3+ after the calcination step at 800 °C for 4 h (labelled time “0 h at 1050 °C”), and after annealing at 1050 °C for 0.5 h and 4 h. For all samples, a sharp feature at g≈1.978 is present which is typical for Cr3+ in a pseudo- octahedral ligand field environment composed of oxo ligands (see Table 2.2). Additional weaker peaks surrounding the intense central EPR signal in all the spectra shown in Figure 2.4 originate from the nuclear-electron hyperfine interaction of 53Cr and from small axial zero-field splittings

4 3+ of the A2 ground term of Cr .

Figures 2.2 and 2.3 provided clear evidence that SrTiO3 was the major phase present and that

3+ 4+ Cr was substituting for Ti sites in SrTiO3 by emission spectroscopy after annealing the samples at 1050 °C for >3 h. Figure 2.4A shows a EPR spectrum of Cr3+ in this phase displays a peak centered at g=1.978 with 4 evenly spaced weaker transitions that are caused by the electron-nuclear hyperfine interaction of 53Cr ions. The experimental hyperfine splitting constant, |A| = 16.2 × 10−4

−1 3+ cm , is in excellent agreement with the recently reported value for internally-doped Cr :SrTiO3

(see Table 2.2).44

The 1% Cr:SrTiO3 that was annealed for only 0.5 h at 1050 °C, however, displays multiple EPR signals that we assign to a superposition of two Cr3+ species occupying cubic and axial sites. The

3+ 4+ specific assignments are: (1) Cr substituted SrTiO3 (cubic Ti ) with the same EPR spectrum as

3+ 4+ shown in the 4 h, 1050 °C annealed sample; and (2) Cr in an axial Ti site of Sr2TiO4. Even though both RP phases have axially-compressed Ti4+ sites (see Table 2.1),23,34,35 the latter assignment is based on the relative intensity of the Sr2TiO4 phase compared to Sr3TiO7 in the powder XRD diffraction pattern (see Figure 2.2C). Simulations of the EPR spectrum confirm the axial site symmetry of the Cr3+ (vide infra).

Fig. 2.4 (A) EPR spectra of 1% Cr:SrTiO3 before annealing at 1050 °C (0h, bottom), after annealing at 1050 °C for 0.5 h (middle), and after annealing at 1050 °C for 4 h (top). Assignment 3+ 3+ of resonance fields for transitions in this field range for Cr :SrTiO3 (vertical lines), Cr :A-TiO2 3+ (open triangles), and Cr :Sr2TiO4 (open circles). (B) Plot of the integrated intensity of the central EPR feature at g = 1.978 vs. annealing time at 850 °C, 950 °C and 1050 °C. Table 2.2 Spin-Hamiltonian parameters for Cr3+ in various oxide lattices.

Host D |E| g|| g A Refs. A-TiO2 −372 0 1.9714 1.9728 16.0 [45,46] R-TiO2 5500 2700 1.97 1.97 15 [47] SrTiO3 - - 1.978 1.978 −16.2 [45] # Sr2TiO4* 207 0 1.9803 1.9793 −16.2 The units of D, E, and A are 10-4 cm-1. The sign of D and A is only given if reported.*The sign of # D cannot be determined, however, if D>0, then g||>g⊥ and if D<0, then g||

The EPR spectrum of the calcined sample (0 h at 1050 °C) is much broader than both annealed samples with the exception of the sharp signal near g≈1.978. Although the spectrum is broad, we

28 know from Figures 2.1 and 2.2 that the major phases are A-TiO2, SrCO3 and SrTiO3. We can

3+ identify Cr :A-TiO2 in the calcined sample based on the simulated resonance positions shown as open triangles in Figure 2.4. In addition, the strong axial signal in the short annealed sample that

3+ we assign to Cr :Sr2TiO4 is also present in the calcined sample despite the fact that it is not detected by powder XRD. The feature at ~342 mT (g = 2.0035) is in a similar region to a trapped

48 oxygen vacancy in TiO2. The rapid disappearance of this feature upon annealing at all elevated temperatures (see supplementary information) is consistent with this assignment.

The inset of Figure 2.4 (panel B) shows the integrated peak area of the central EPR peak at

3+ g≈1.978 attributed to Cr :SrTiO3 as a function of annealing time at 850 °C, 950 °C, 1050 °C

(spectra at 850 °C and 950 °C shown in supplementary information). At 850 °C and 950 °C the peak area increases gradually with the 950 °C rate being greater. However, this trend does not hold for the 1050 °C anneals, which display a small increase in intensity with a large increase in area between 2 h and 3h. The first step is correlated with both the formation of the Sr2TiO4 phase

(Figure 2.2) and the axial EPR spectrum (Figure 2.4), which is only observed in samples heated between 0.5 h and 2 h at 1050 °C. The rapid increase in EPR intensity between 2h and 3h followed by a gradual increase in intensity up to 6 h is also correlated to the removal of the Sr2TiO4 phase and formation of SrTiO3.

In an attempt to rationalize the temporal dependence of the EPR intensities shown in Figure

2.4B, we measured the magnetic susceptibility of the same samples in Figure 2.4A. The results are shown in the supplementary information for 1% CrSrTiO3 after annealing for 0, 0.5, and 4 h at

1050 °C. There is a clear decrease in the effective magnetic moment of the sample after the 0.5 h annealing step at 1050 °C. After 4 hours of annealing at 1050 °C, the effective magnetic moment increases significantly compared to the original sample. The susceptibility is consistent with the

29 observed EPR intensity versus annealing time at 1050 °C shown in Figure 2.4 (inset).

2.3.4 Surface Characterization. The susceptibility and EPR results are consistent with one or two

3+ scenarios: (1) clustering of Cr in Sr2TiO4, which would result in a lower magnetic moment due to strong antiferromagnetic coupling between oxo-bridged Cr3+ ions; or (2) the Cr ions exist in

6+ 3+ more than one oxidation state in Sr2TiO4 (including Cr with S=0), which are reduced to Cr after conversion of this phase to SrTiO3. In order to identify additional Cr in other oxidation states, in particular 6+ and 4+ based on their importance in modulating the photocatalytic or electronic properties as discussed earlier, we utilized X-ray photoelectron spectroscopy (XPS) to characterize the chemical environments of the powder surfaces.

XPS was collected on 5 % Cr:SrTiO3 powders after calcination at 800 °C, and after annealing at

1050 °C for 0.5 h and 4 h. The samples were analyzed in the Cr 2p, Ti 2p, Sr 3d, and O 1s regions

3+ as shown in Figure 2.5. Analysis of the Cr 2p spectra shows the presence of Cr in SrTiO3 based on the binding energy matching the previously reported value,49 however, we were unable to resolve any Cr4+ or Cr6+ features due to the poor signal- to-noise in the spectrum. The Ti 2p energy also matches the previously reported value, and the shift to lower energy with increased annealing time likely corresponds to the removal of TiO2 since the binding energy is higher than SrTiO3

(458.9 vs. 457.5 eV).50,51 Similarly, analysis of the Sr 3d shows a redshift in the binding energy

52 due to the removal of SrCO3.

Fig. 2.5 XPS data of 5% Cr:SrTiO3 after calcining at 800 °C for 4 h and annealing at 1050 °C for 0 h (doltted lines), 0.5 h (dashed lines) and 4 h (solid lines). For each sample we show the Cr 2p (A), Ti 2p (B), Sr 3d (C), and O 1s (D) regions of the spectra. The results of the elemental surface composition for the three samples is shown in Table 2.3.

The overall intensity of the Cr 2p does not change significantly, however, the increase in Ti 2p, Sr

3d and O 1s intensities results in a decrease of the relative amount of Cr at the particle surface with increasing annealing time. The decrease in Cr at the surface combined with the increasing EPR intensity and effective magnetic moment is consistent with increased Cr3+ diffusion and incorporation into SrTiO3 as the intermediate phases are heated to higher temperatures for longer times. The changing values for the atomic percentages of Sr and Ti near the surface suggest

3+ diffusion of Cr as well as the removal of the SrCO3 and TiO2 impurity phases, and decomposition of the RP phases to form SrTiO3.

3+ Table 2.3. Calculated atomic percent from XPS analysis for in nominally 5% Cr :SrTiO3 powders as a function of annealing time at 1050 °C. Time Cr Sr Ti O 0 h 4.7 16.5 18.9 59.9 0.5 h 3.8 24.2 12.1 59.9 4 h 2.8 25.2 15.8 56.2

2.3.5 Simulation of EPR Spectra. Figure 2.6A shows the experimental spectrum of interest on a wider magnetic field range. In addition to the central split peak shown in Figure 2.4 (Peak I), there are also broad and weak peaks (Peaks II and III) separated by over 100 G from the central peaks that decrease in intensity as they move away from the central peaks. These are not seen in the

3+ sample annealed at 1050 °C for 4 h with the majority of Cr in SrTiO3 (not shown). The energy separations and relative intensities of Peaks II and III are inconsistent with the hyperfine

53 53 3+ interaction with Cr, which is expected to be similar to Cr in SrTiO3 (see Table 2.2).

To support our assignment of the axial Cr3+ species in the EPR spectrum shown in Figure 2.4

3+ (0.5 h at 1050 °C) and 2.6A, we simulated the EPR spectrum of Cr in both cubic (SrTiO3) and tetragonal (Sr2TiO4) sites by increasing the strength of the axial component to the ZFS, D. The simulated spectrum for Cr:SrTiO3 shown in Figure 2.6B was calculated using just the isotropic g and A values given in Table 2.2.45 It is clear from Figure 2.6B that the sharp, central peak and 53Cr

3+ hyperfine transitions in Figure 2.6A is due to Cr in SrTiO3.

Fig. 2.6 (A) Experimental and (B,C) simulated EPR spectra of the two major Cr3+ species in the 1 % Cr:SrTiO3 annealed at l1050 °C for 0.5 h. Experimental spectra collected at hν = 9.599 GHz and room temperature. See text for details of the simulation.

3+ The simulated spectrum for Cr :Sr2TiO4 is shown in Figure 2.6C. Through trial-and-error iterations, the best agreement to experiment was found when |D|=0.0207 cm−1 with a slight increase and anisotropy of the g-values from g=1.978 in SrTiO3 (see Table 2.2 and supplementary information). The assignment of the transitions between the |MS = |−3/2, |−1/2, |1/2 and |3/2 sublevels follows: Peak I = |1/2|−1/2; Peak II = |±3/2|±1/2 when B ⊥ z; and Peak III =

|±3/2|±1/2 when B ∥ z. The fine structure of the central Peak I shown in Figure 2.4 is similar

3+ to the EPR spectrum of the tetragonally-distorted Cr in PbTiO3 nanopowders reported by Erdem and co-workers.53

The relative intensities for Peaks II and III to Peak I were higher in the simulated spectrum compared to the experimental spectrum (Figure 2.6A). However, these transitions broaden and weaken relative to the split central line with the addition of D-strain to the EPR simulations (see supplementary information). We estimate the D-strain is ca. σD ≈ 0.0030 cm−1, which is consistent

33 with related tetragonally-distorted Cr3+ compounds and nanopowders.53 The origin of the observed

D-strain is assigned to inhomogeneous broadening of the local Cr3+ environment by high Cr3+

53 concentration in Sr2TiO4. Contributions to the simulated spectrum from the hyperfine splitting Cr

−4 −1 were included in our simulations, but fixed at the values for SrTiO3 (A = −16.2×10 cm ).

3+ We also compared the simulated EPR spectrum of Cr in A-TiO2 against our experimental spectrum in Figure 2.6A, since the reported D=−0.0372(5) cm−1 is similar to our value of

|D|=0.0207 cm−1.45,46 This results in an increase of the separation between Peaks II and III by about

3+ almost a factor of 2 compared to Cr in Sr2TiO4 (see supplementary information). The additional difference in g-values results in less agreement between Figure 2.6A and the simulated spectrum

3+ for Cr in A-TiO2.

Based on the simulations and identification of Sr2TiO4 in the powder diffraction of this sample

3+ (Figure 2.2), we assign the axial EPR spectrum to Cr :Sr2TiO4. This EPR signal is also observed at other temperatures, however, the Sr2TiO4 content is below the sensitivity of powder XRD (see

Figure 2.2 and supplementary information). Variable-temperature EPR studies are currently

3+ underway to further characterize the electronic structures of Cr and other dopant ions in Sr2TiO4.

3+ 2.3.6 Mechanism of Cr incorporation into SrTiO3.The temperature dependence of the various phases before and after the calcination step as detected by powder diffraction (Figures 2.1 and 2.2) and Cr3+ speciation by EPR spectroscopy (Figures 2.4, 2.6, and supplementary information) are summarized in Scheme 2.1. The crystalline phases detected by powder XRD after the calcination step are reported in Scheme 2.1 without parentheses. Positive confirmation of Cr3+ doping by EPR spectroscopy in phases detected by powder XRD are designated by the “Cr3+:” prefix and italicized. The sensitivity of EPR spectroscopy also allowed us to identify Cr3+ EPR signals in

34 phases that were not detected by powder XRD. EPR-only phases are given inside of parentheses in figure 2.7.

Figure 2.7. Summary of the sol-gel processing of Cr:SrTiO3 when heated at different temperatures for different times. Phases in parentheses are only detected via EPR spectroscopy.

To summarize, after calcination in air at 800 °C for 4 h, we identify only 3 phases from powder

3+ diffraction (SrCO3, A-TiO2, and SrTiO3). EPR spectroscopy confirms doping of Cr incorporation into two of the phases detected by powder XRD (A-TiO2 and SrTiO3), but also in Sr2TiO4 that is undetected by powder XRD (see supplementary information). These same phases and EPR signals persist even after annealing at 850 °C for an additional 6 h. At 950 °C, we observe the same phases after 0.5 h. However, after 6 h of heating the major phase by powder diffraction is SrTiO3 and EPR

3+ spectroscopy confirms Cr in SrTiO3 and A-TiO2. Annealing at 1050 °C for 0.5 h results in

3+ R-TiO2, SrTiO3, and Sr2TiO4 phases by powder XRD and Cr in SrTiO3 and Sr2TiO4 as discussed

3+ earlier. After 6 h the R-TiO2 and SrTiO3 remain and only Cr :SrTiO3 is observed by EPR spectroscopy.

The reaction of SrCO3 and TiO2 (anatase or rutile) in the solid-state to form SrTiO3 under similar

54,55 high-temperature aerobic annealing is known. The calcined powder contains SrCO3 and

3+ 3+ Cr :A-TiO2, which can react at 1050 °C to produce Cr :SrTiO3 and CO2 according to eqn (2.2).

3+ 3+ SrCO3 + Cr :A-TiO2 → Cr :SrTiO3 + CO2(g) (2.2)

3+ The calcined powder also contains Cr :Sr2TiO4, which can form via eqn (2.3).

3+ 3+ 2SrCO3 + Cr :A-TiO2 → Cr :Sr2TiO4 + 2CO2(g) (2.3)

3+ 3+ Conversion of Cr :Sr2TiO4 can then either decompose to form Cr :SrTiO3 and SrO (eqn (2.4a))

3+ 24 or react with doped or undoped A-TiO2 to form Cr :SrTiO3 (eqn (2.4b)).

3+ 3+ Cr :Sr2TiO4 → Cr :SrTiO3 + SrO (2.4a)

3+ 3+ Cr :Sr2TiO4 + TiO2 → 2(Cr :SrTiO3) (2.4b)

We do not observe formation of SrO by powder XRD during the course of our study. However, we still cannot distinguish between eqns (2.3a) and (2.3b) based on the limited sensitivity of powder XRD.

3+ 5+ Mechanisms of Cr substitution into SrTiO3 involving surface Cr speciation prior to

3+ 37,56 internalization (as suggested for the Cr -TiO2 system ) can be ruled out based on EPR spectroscopy and from the annealing conditions being in air. We therefore propose that the

3+ mechanism of Cr substitution into SrTiO3 in this study proceeds according to eqns (2.2) and

(2.4). The role of eqn (2.3) is considered negligible based on the ~1:1 Sr:Ti ratio in the sol.

2.4 Conclusions

Through the combination of structural characterization and dopant-specific probes, the substitution

3+ 4+ of Cr at the Ti site in the RP phase Sr2TiO4 was confirmed by EPR spectroscopy. Simulations

3+ of Cr :Sr2TiO4 spectra within the framework of the S = 3/2 spin Hamiltonian gave best agreement

−1 for |D|=0.0207 cm , g||=1.9803, and g⊥=1.9793. The Sr2TiO4 phase was identified by powder

XRD only in samples that were annealed at 1050 °C between 0.5 h to 3 h. Annealing the powders

36 at 1050 °C for at least 4 h resulted in disappearance of most intermediate phases and the Cr3+ ions

4+ are located at the Ti site in SrTiO3 based on (1) EPR spectroscopy, (2) agreement with the

2 4 emission energy of the E→ A2 transition obtained on single crystals, and (3) the temperature-

2 4 dependent shift of the E→ A2 emission energy.

The chemistry and photophysics of the RP phases with targeted transition metal dopants is largely unexplored. Studies of the visible-light photocatalytic activity and characterization of Cr3+ and other transition metal dopants in Sr2TiO4 are currently underway. This work has demonstrated the importance in understanding the speciation and substitution mechanism of dopant ions in multi- functional materials where the dopant oxidation state is critical to material performance.

2.5 Supplementary Information

Fig. 2.8. Variable temperature emission of 0.01% Cr:SrTiO3. The sample was excited by the 488 nm line of an Ar+ laser.

3+ Fig. 2.9 Temperature dependence of the effective magnetic moment of Cr :SrTiO3 after being annealed for 0 h (triangle), 0.5 h (square), or 4 h (circle) at 1050 °C. Effective moments are reported using the nominal Cr3+ concentration. Magnetic susceptibility was measured from 2-300 K on samples in an applied magnetic field of H = 0.5 Tesla (Quantum Design MPMS XL). Diamagnetic 57 58 corrections for the SrTiO3 host, the gel capsule and kapton tape were tabulated using reported diamagnetic susceptibilities and Pascal’s tables.59

Fig. 2.10 Normalized EPR spectra after annealing at 850 °C (A), 950 °C (B), and 1050 °C (C). All spectra were normalized to a peak height of 1 for the Cr3+ feature at g=1.978.

Fig. 2.11 Experimental EPR spectrum of polycrystalline powders of Cr-doped Sr2TiO4 (top) and angular dependence of the simulated EPR spectrum (bottom) with the best agreement between the EPR spectrum and the simulation. The right side of the figure shows the central features on an expanded scale. Spin-Hamiltonian parameters given in textbox. Angular dependence of the resonance fields was calculated using XSophe simulation software (Bruker Biospin).

3+ Fig. 2.12 Simulated EPR spectrum of Cr :Sr2TiO4 with varying amount of D-strain from σD = 0 (top) – 30×10−4 cm−1 (bottom).

Fig. 2.13 EPR intensity ratio of Peak II (|±3/2 ↔ |±1/2 when B⊥z, filled-circles) and Peak III (|±3/2 ↔ |±1/2 when B∥z, open-circles) to the central peak (Peak I: |1/2 ↔ |−1/2). The intensities of the signals was taken as the peak-to-peak difference. The intensities of the |±3/2 ↔ |±1/2 transitions decrease considerably compared to the central |1/2 ↔ |−1/2 transition with small increases in the D-strain (D = 207×10−4 cm−1).

3+ Fig. 2.14 Simulated EPR spectra of Cr in the Sr2TiO4 phase (top) and A-TiO2 phase (bottom). Input values for the simulation are given in Table 2 of the main text.

Fig. 2.15 Experimental EPR spectrum of calcined powders (air, 800 °C, 4 h) and simulated spectra 3+ 3+ 3+ of Cr :SrTiO3, Cr :Sr2TiO4, and Cr :A-TiO2. The feature at g = 2.0035 is not due to any known 3+ Cr phase. It is tentatively attributed to a defect in A-TiO2 or SrTiO3 that is rapidly oxidized > 850 °C (see Fig. 2.9).

2.6 References

1. Beier, C. W., Cuevas, M. a. & Brutchey, R. L. Low-temperature synthesis of solid-solution BaxSr1−xTiO3 nanocrystals. J. Mater. Chem. 20, 5074 (2010). 2. Müller, K. A. & Burkard, H. SrTiO3: An intrinsic quantum paraelectric below 4 K. Phys.

Rev. B 19, 3593–3602 (1979).

3. Guo, Y., Liu, H., Yu, D. & Liu, J.-M. Ferroelectricity and superparamagnetism in Sr/Ti nonstoichiometric SrTiO3. Phys. Rev. B 85, 104108 (2012).

4. Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. 427, 423–427 (2004).

5. Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–78 (2009).

6. Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 20, 35–54 (2008).

7. Sato, S. Photocatalytic activity of NOx-doped TiO2 in the visible light region. Chem. Phys. Lett. 123, 126–128 (1986).

8. Di Valentin, C., Pacchioni, G., Selloni, A., Livraghi, S. & Giamello, E. Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations. J. Phys. Chem. B 109, 11414–11419 (2005).

9. Gole, J. L., Stout, J. D., Burda, C., Lou, Y. & Chen, X. Highly Efficient Formation of Visible Light Tunable TiO2-xNx Photocatalysts and Their Transformation at the Nanoscale. J. Phys. Chem. B 108, 1230–1240 (2004).

10. Etacheri, V., Roshan, R. & Kumar, V. Mg-doped ZnO nanoparticles for efficient sunlight- driven photocatalysis. ACS Appl. Mater. Interfaces 4, 2717–2725 (2012).

11. Cohn, A. W., Kittilstved, K. R. & Gamelin, D. R. Tuning the potentials of ‘extra’ electrons in colloidal n-type ZnO nanocrystals via Mg2+ substitution. J. Am. Chem. Soc. 134, 7937– 7943 (2012).

12. Kato, H., Asakura, K. & Kudo, A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J. Am. Chem. Soc. 125, 3082–9 (2003).

13. Sasaki, Y., Nemoto, H., Saito, K. & Kudo, A. Solar Water Splitting Using Powdered Photocatalysts Driven by Z-Schematic Interparticle Electron Transfer without an Electron Mediator. J. Phys. Chem. C 113, 17536–17542 (2009).

14. Iwase, A., Ng, Y. H., Ishiguro, Y., Kudo, A. & Amal, R. Reduced graphene oxide as a solid- state electron mediator in Z-scheme photocatalytic water splitting under visible light. J. Am. Chem. Soc. 133, 11054–7 (2011).

15. Yu, H. et al. Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production. J. Mater. Chem. 21, 11347 (2011).

3+ 16. Li, H. et al. Roles of Cr doping and oxygen vacancies in SrTiO3 photocatalysts with high visible light activity for NO removal. J. Catal. 297, 65–69 (2013).

17. Ouyang, S. et al. Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts. J. Am. Chem. Soc. 134, 1974–7 (2012).

18. Reunchan, P. et al. Theoretical Design Of Highly Active SrTiO3-based Photocatalysts From

Codoping Scheme Toward Solar Energy Utilization For Hydrogen Production. J. Mater. Chem. A (2013).

19. Janousch, M. et al. Role of Oxygen Vacancies in Cr-Doped SrTiO3 for Resistance-Change Memory. Adv. Mater. 19, 2232–2235 (2007).

20. Wang, D., Ye, J., Kako, T. & Kimura, T. Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites. J. Phys. Chem. B 110, 15824–30 (2006).

21. La Mattina, F., Bednorz, J. G., Alvarado, S. F., Shengelaya, A. & Keller, H. Detection of charge transfer processes in Cr-doped SrTiO3 single crystals. Appl. Phys. Lett. 93, 22102 (2008).

22. Muller, K. A. & Kool, T. W. Properties of Perovskites and Other Oxides. (World Scientific, 2010).

23. Ruddlesden, S. N. & Popper, P. New compounds of the K2NIF4 type. Acta Crystallogr. 10, 538–539 (1957).

24. Kutty, T. R. N. & Nag, A. Role of interface states associated with transitional nanophase 3+ 3+ precipitates in the photoluminescence enhancement of SrTiO3:Pr , Al J. Mater. Chem. 13, 2271 (2003).

25. Eng, H. W., Barnes, P. W., Auer, B. M. & Woodward, P. M. Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family. J. Solid State Chem. 175, 94–109 (2003).

26. Schimizu, T. & Yamaguchi, T. Band offset design with quantum-well gate insulating structures. Appl. Phys. Lett. 85, 1167 (2004).

27. Yi, Z. G., Iwai, H. & Ye, J. H. Photochromism and visible light induced H2 generation in Sr2TiO4:Cr complexes. Appl. Phys. Lett. 96, (2010).

28. Galinetto, P. et al. Magnetic and Structural Studies in Co- and Mn-implanted SrTiO3 Single Crystals. Ferroelectrics 368, 120–130 (2008).

29. Hanson, G. R. et al. XSophe-Sophe-XeprView®. A computer simulation software suite (v. 1.1.3) for the analysis of continuous wave EPR spectra. J. Inorg. Biochem. 98, 903–916 (2004).

30. Pannhorst, W. & Lohn, J. Zur Kristallstruktur von Strontianit, SrCO3. Zeitschrift fur Krist. 131, 455 (1970).

31. Swope, R. J., Smyth, J. R. & Larson, A. C. H in rutile-type compounds: I. Single-crystal neutron and X-ray diffraction study of H in rutile. Am. Mineral. 80, 448–453 (1995).

32. Yamanaka, T., Hirai, N. & Komatsu, Y. Structure change of Ca1-xSrxTiO3 perovskite with composition and pressure. Am. Mineral. 87, 1183–1189 (2002).

33. Parker, R. L. Zeitschrift fur Krist. Krist. Krist. Kryst. 59, 1 (1924).

34. Lukaszewicz, K. Rocz. Chem. 33, 239 (1959).

35. Ruddlesden, S. N. & Popper, P. The compound Sr3Ti2O7 and its structure. Acta Crystallogr.

11, 54–55 (1958).

36. Gerlach, W. Die Gitterstruktur der Erdalkalioxyde. Zeitschrift fur Phys. 9, 184–192 (1922).

37. Amorelli, A., Evans, J. C. & Rowlands, C. C. Electron paramagnetic resonance study of transition-metal-ion impregnated brookite titanium dioxide powders. J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases 85, 4031 (1989).

38. Wyckoff, R. W. G. Crystal Structures 1. (Interscience Publishers, 1963).

39. Stokowski, S. E. & Schawlow, A. L. Spectroscopic Studies of SrTiO3 Using Impurity-Ion Probes. Phys. Rev. 178, 457–464 (1969).

3+ 40. Stokowski, S. E. & Schawlow, A. L. Dielectric-Related Optical Line Shifts in SrTiO3:Cr . Phys. Rev. 178, 464–470 (1969).

41. Kubelka, P. & Munk, F. N. Zeits. f. tech. Phys. 12, 593 (1931).

42. Baer, W. S. Free-carrier absorption in reduced SrTiO3. Phys. Rev. 144, 734–738 (1966).

43. Trepakov, V. a et al. SrTiO3:Cr nanocrystalline powders: size effects and optical properties. J. Phys. Condens. Matter 21, 375303 (2009).

3+ 44. Azamat, D. V. et al. Pulse-electron paramagnetic resonance of Cr centers in SrTiO3. J. Appl. Phys. 113, 174106 (2013).

45. Muller, K. A. in Proceedings of the First International Conference I, Paramagnetic Resonance (ed. Low, W.) 17 (Academic Press, 1963).

46. Grunin, V. S., Daytran, G. D., Ioffe, V. A. & Latrina, I. B. Fiz. Tverd. Tela 17, 2174–2176 (1975).

47. Gerritsen, H. J., Harrison, S. E., Lewis, H. R. & Wittke, J. P. Fine Structure, Hyperfine 3+ Structure, and Relaxation Times of Cr in TiO2 (Rutile). Phys. Rev. Lett. 2, 153–155 (1959).

48. Zou, X. et al. Facile synthesis of thermal- and photostable titania with paramagnetic oxygen vacancies for visible-light photocatalysis. Chem. - A Eur. J. 19, 2866–2873 (2013).

49. Kato, H. & Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106, 5029–5034 (2002).

50. Sulaeman, U., Yin, S. & Sato, T. Solvothermal synthesis and photocatalytic properties of chromium-doped SrTiO3 nanoparticles. Appl. Catal. B Environ. 105, 206–210 (2011).

51. Greenlief, C. M. & White, J. M. An XPS Investigation of TiO2 Thin Films on Polycrystalline Pt. J. Phys. Chem. 89, 5025–5028 (1985).

52. Bhaskar, S., Allgeyer, D. & Smythe, J. A. Depth profiling of dielectric SrTiO3 thin films by angle-resolved x-ray photoelectron spectroscopy. Appl. Phys. Lett. 89, 254103 (2006).

53. Erdem, E., Böttcher, R., Gläsel, H. J. & Hartmann, E. Size effects in chromium-doped PbTiO3 nanopowders observed by multi-frequency EPR. Magn. Reson. Chem. 43, 174–182

(2005).

54. Tagawa, H., Kimura, K., Fujino, T. & Ouchi, K. Denki Kagakuoyobi Kogyo Butsuri Kagaku 52, 154–159 (1984).

55. Tagawa, H. & Igarashi, K. Reaction of Strontium Carbonate with Anatase and Rutile. J. Am. Ceram. Soc. 69, 310–314 (1986).

56. Egerton, T. a et al. An EPR study of diffusion of chromium into rutile. Phys. Chem. Chem. Phys. 2, 3275–3281 (2000).

57. Tregenn-Piggot, P. L. W. Magprop (part of the NIST DAVE software suite). (2008). 58. Garcia, M. A. et al. Sources of experimental errors in the observation of nanoscale magnetism. J. Appl. Phys. 105, (2009).

59. Bain, G. A. & Berry, J. F. Diamagnetic Corrections and Pascal’s Constants. J. Chem. Educ. 85, 532 (2008).

CHAPTER 3

REVERSIBLE CONTROL OF THE CHROMIUM VALENCE IN CHEMICALLY REDUCED Cr-DOPED SrTiO3 BULK POWDERS

This chapter has been published as:

Lehuta, K.A. and Kittilstved, K.R.; Reversible control of the chromium valence in chemically reduced Cr-doped SrTiO3 bulk powders. Dalton Trans., 2016, 45, 10034-10041.

3.1 Introduction

The electronic and photochemical properties of metal oxide semiconductors is intimately correlated with the oxygen stoichiometry of the material.1 When there is a significant oxygen deficiency in the lattice, the nature and location of the oxygen-related defect can have a profound influence on the (photo-)catalytic activity of the metal oxide. The oxygen-

2–5 related defect can be an oxygen vacancy (VO) located either at the surface or within the lattice,3 or a reactive oxygen species, such as superoxide ions or hydroxyl radicals.2,4 For example, surface VO’s have been recently demonstrated to act as sites for substrate or gas adsorption that react with electron-hole pairs in BiPO4−x and ZnO generated by UV photons.2–4 Interest in extending the range of metal oxide photocatalysts into the visible region of the solar spectrum has increased following the report of a low-temperature chemical reduction route that introduces oxygen defects into the surface layers of nanoscale

5 TiO2−δ (anatase-type, δ > 0). Since that report there have been similar studies showing

6 enhanced photocatalytic activities with visible photons for chemically-reduced SrTiO3−δ,

3,7 8 ZnO1−δ, and WO3−δ.

An alternative mean to introduce lattice defects in metal oxide semiconductors is

II IV through chemical substitution with aliovalent dopants. For SrTiO3 (STO), an A B O3-type perovskite, typical donor dopants include La3+ that substitute exclusively at the A-site and

Nb5+ with substitution exclusively at the B-site of the lattice. In order to retain overall charge neutrality when these types of ions replace a host cation, the lattice typically creates

Ti4+ vacancies to compensate, which results in higher electrical conductivities in doped

9 SrTiO3−δ. Transition metal ions can behave as both acceptor and donor defects in SrTiO3−δ and also increase the visible-light absorption of the lattice. One prototypical example of this is Cr-doped in SrTiO3 (Cr:STO), where Cr has been detected with 3+, 4+, 5+ and 6+

4+ 10–15 valences when substituted at the Ti -site. Cr-doped SrTiO3 is a multi-functional material that can photocatalytically reduce water to H2 using visible light and modulate the

10–12 ferroelectric response of SrTiO3−δ by laser irradiation. The laser-induced switching of the resistivity in Cr-doped SrTiO3−δ by La Mattina, et al. unambiguously demonstrated that

3+ 4+ this effect is correlated with the relative concentrations of Cr and Cr in the SrTiO3−δ

10 6+ lattice. Additionally, Cr in SrTiO3 has been shown to be detrimental to the photocatalytic activity of Cr3+:STO.11–13

6+ One successful strategy to reduce the concentration of Cr in SrTiO3 is through co- doping strategies with donor defects such as Sb5+ at the B-site.13,16,17 High temperature

12,18 5,19,20 3+ vacuum and H2 annealing have also been used to increase concentration of Cr in

10,11 STO and introduce VO into metal oxides. Photoreduction has also been used to control

6 the valance state of dopant ions in SrTiO3. Tan, et al. utilized the NaBH4 chemical reduction process in pure STO powders to enhance the visible light photocatalytic activity.

Through high-resolution TEM studies they concluded that the process results in a

SrTiO3/SrTiO3−∂ core/shell type heterostructure where the shell is largely distorted due to surface reduction.

6,21 In this study, we systematically modify this NaBH4 reduction method to control the oxidation state and electronic structures of redox-active chromium dopants in SrTiO3−δ bulk powders. We observe significant changes to the electronic structure of Cr-doped

SrTiO3−δ as a function of reduction temperature. Variable-temperature electron paramagnetic resonance (EPR) and diffuse reflectance (DR) spectroscopy allows us to directly probe and identify the nature of the dopants and defects present. Specifically, after annealing equimolar mixtures of NaBH4 and Cr-doped SrTiO3−δ at temperatures below 375

°C and static pressures of ~10−4 mbar, we observe changes in the visible absorption that is attributed to VO and the valence state of Cr ions. Using this method we are able to control

3+ the Cr content by over an order-of-magnitude in the SrTiO3 lattice after chemical reduction at 375 °C. Increasing the chemical reduction temperature above 375 °C also results in lattice reduction evidenced by near-IR absorption and EPR signatures of localized

Ti3+ defects. These chemical perturbations are quantitatively reversible upon reoxidation of the samples in air. However, the reoxidation temperature required to achieve the electronic structure of the as-prepared samples is dependent on the type of lattice defect that is

3+ generated (i.e., VO at low temperatures versus Ti and VO at higher temperatures).

3.2 Materials and Methods

3.2.1 Materials. TiO2 (99.9% nanopowder, anatase, 32 nm APS powder, Alfa Aesar),

Sr(NO3)2 (99+%, for analysis, Acros Organics), Cr(NO3)3∙9H2O (crystalline, certified,

Fisher Scientific), NaBH4 (≥98%, white powder, MP Biomedical), MgO (Fisher Science

Education), and ethanol (190 proof, ACS/USP grade, Pharmco-Aaper) were used as received.

3.2.2 Synthesis of Cr:SrTiO3 (Cr:STO). SrTi1−xCrxO3−δ (abbreviated as Cr:STO) was synthesized using a conventional solid state reaction, where x is the nominal concentration of Cr and δ represents the concentration of oxygen vacancies (VO). All nominal Cr dopant concentrations are 1% unless otherwise noted. Sr(NO3)2, Cr(NO3)3∙9H2O, and TiO2 were mixed in the desired stoichiometric ratio and ground with a mortar and pestle for ca. 10 min to achieve uniform consistency. The mixture was then transferred to a porcelain crucible and heated for 6 hours at 1000 °C. The sample was then reground after cooling and heated for another 15 h at 1000 °C.

3.2.3 NaBH4 Reduction and Reoxidation. Chemical reductions of Cr:STO were carried out using a modified literature method.6 Briefly, Cr:STO powders were ground in a 1:1 mole ratio with NaBH4 and placed in a porcelain crucible that was placed at the center of a tube furnace inside of a custom 1” diameter quartz insert with fittings and adapters to allow dynamic or static pumping with a turbomolecular pump (Pfeiffer HiCube 80 Eco pumping station with a Pfeiffer HiPace 80 turbo pump). The samples were heated at various temperatures under a static vacuum of ~10−4 mbar for 30 min. After cooling to room temperature the samples were collected and alternately washed with deionized water and ethanol and collected by centrifugation. Samples were dried in an oven at 100 °C for 2 h.

Chemically reduced samples are referred to as Cr:STO−T where T is the reduction temperature. Reoxidation of chemically reduced Cr:STO−T samples was done by heating the powders in a crucible in air with the tube furnace at various temperatures.

3.2.4 Physical Characterization. Powder X-ray diffraction patterns were collected using a Bruker AXS-D8 powder diffractometer. X-band EPR spectra were measured on powders using the perpendicular mode of a dual-mode resonator cavity with a quartz finger dewar

51 insert (Bruker Elexsys E-500 with ER-4116 cavity). Finely-ground powder samples were loaded into 4mm quartz EPR tubes for EPR measurements (Wilmad-Glass). To ensure the

EPR spectra could be compared quantitatively between samples, we added enough sample to ensure that the height of the samples exceeded the cavity height.22 Diffuse reflectance spectra were collected with an integrating sphere (Ocean Optics ISP-REF) coupled by fiber optics to a CCD-based spectrophotometer (Ocean Optics USB2000+ UV-Vis). The optical density between the absorption minimum and the absorption at 340 nm was adjusted by mixing the powders with MgO to dilute the sample.

3.3 Results and Discussion

3.3.1 Low-temperature chemical reductions. Fig. 3.1A shows the effect of NaBH4 reduction on the physical appearance of the Cr:STO samples as a function of reduction temperature. The as-prepared Cr:STO sample is reddish brown in color that gradually changes with increasing reduction temperature to yellowish at 325 °C, light green at 350

°C, and ultimately black above 430 °C (higher temperature reductions not shown). Tan and co-workers showed that the color of nominally pure SrTiO3 after chemical reduction under flowing Ar gas (not vacuum) also gradually changes, but only from white to gray to black and over a lower temperature range from 300 °C to 375 °C.6 Despite the change in appearance of Cr:STO after chemical reduction there is no change to the structure of the lattice as shown by the powder X-ray diffraction (XRD) patterns of representative Cr:STO samples in Fig. 3.1B. The powder XRD patterns of the as-prepared (Cr:STO−ap) and both reduced (Cr:STO−350 and Cr:STO−430) samples do not exhibit any secondary phases nor any change in crystal structure resulting from the low-temperature chemical reduction, which is consistent with prior studies on the chemical reduction of undoped STO.6 Based

52 on the lack of structural change upon reduction, we utilized spectroscopic techniques to identify the origin of the changes to the Cr:STO electronic structure.

Fig. 3.1 (A) Color photographs of 1% Cr:SrTiO3−δ as-prepared (“ap,” left) and after chemical reduction at 300 °C, 325 °C, 350 °C, 375 °C, 400 °C, and 430 °C (2nd from left to right, respectively). (B) Powder XRD of Cr:STO−ap, Cr:STO−350, and Cr:STO−430. Literature powder X-ray diffraction patterns (gray sticks) and assignments from [23].

The diffuse-reflectance spectra of the Cr:STO powders as a function of reduction temperature is shown in Fig. 3.2. The Cr:STO−ap sample displays a broad absorption feature in the visible centered at about 510 nm that tails into the near-infrared. Despite being the approximate energy of the weak spin-allowed Cr3+ d−d transitions, this band is instead attributed to electronic transitions involving oxygen vacancies (VO) defects introduced during synthesis since this peak has also been observed in SrTiO3 after vacuum annealing.24–26 The energies of the electronic transitions from the valence band to either

x • •• 27 VO, VO, and VO has been calculated to occur 539, 510, and 454 nm respectively, suggesting

x • the broad peak in the Cr:STO−ap sample involves both VO and VO centers. Recent

•• calculations also suggest this transition involves a VO defect, but with the two excess electrons trapped at two distinct Ti3+ sites (the so-called “double polaron” model).28

Upon chemical reduction, this feature decreases in intensity and is barely visible in

Cr:STO−325 (see supplementary information for diffuse-reflectance spectra of Cr:STO at all reduction temperatures). The spectrum of Cr:STO−325 displays a weak and broad feature centered at ~430 nm, which has also previously been assigned to oxygen vacancies6,18 and is similar in energy to the calculated energy of the transition from the

•• 27 valence band to VO defects. This feature may also be present in the Cr:STO−ap sample, but is occluded by the 510 nm absorption. The decrease in the intensity of the peak at 510 nm and the emergence of a more pronounced peak at 430 nm suggests that there may be

• •• conversion of VO to VO during the chemical reduction in the Cr:STO−325 spectrum. This transition has also been assigned to an electron transfer transition from Cr3+  Ti4+

(conduction band).13,29,30

Samples that were reduced at temperature >350 °C also display further decrease in the absorption feature at 430 nm and the appearance of an absorption feature in the near-IR. The onset of this new near-IR absorption in Cr:STO−375 and Cr:STO−430 extends throughout the visible to the SrTiO3 band gap. We assign the near-IR absorption to the formation of Ti3+ lattice defects and oxygen vacancies based on prior assignments of chemically reduced and vacuum annealed samples of

6,21,31 undoped SrTiO3 and TiO2. The increase in the relative intensity of the broad absorption in the near-IR suggests that at higher reduction temperatures, more lattice reduction occurs resulting in increased Ti3+ in the samples. The disappearance of the Cr3+  Ti4+ charge transfer transition is also consistent the Fermi level near or above the bottom of the conduction band. Both Ti3+ and

Cr3+ are EPR-active ions and we measured EPR spectra of the samples to determine the presence of these centers.

Fig. 3.2 Diffuse reflectance spectra of Cr:STO-ap (black) and NaBH4 reduced samples: Cr:STO-325 (blue), Cr:STO-375 (green), and Cr:STO-430 (red). The vertical lines in the “defect- related” VO states are placed at the calculated energies for the electronic transitions from the valance band to the specific VO from ref. [27].

Fig. 3.3 shows the room-temperature EPR spectra at X-band frequencies of

Cr:STO−ap, Cr:STO−300 and Cr:STO−350 under quantitative conditions (see supplementary information for details). The EPR spectrum of the Cr:STO−ap sample is relatively narrow with a full-width at half-maximum (fwhm) of Γ = 7.8 G at the central peak and spectral parameters that are in agreement with the previously reported EPR spectrum for Cr:STO single crystals (g-value = 1.978 and A = −16.2 × 10−4 cm−1 from the

53Cr isotope with 9.5% relative abundance and I = 3/2).24,32,33 Upon chemical reduction at only 300 °C, there is a significant increase in both the spectral intensity and linewidth of the Cr3+ signal (Γ = 22 G). The area of the integrated EPR spectrum of Cr:STO−300 is 7 times greater than Cr:STO−ap. The integrated peak area and fwhm (Γ = 36 G) continue to increase in the spectrum of Cr:STO-350 (see Fig. 3.3 inset).

Fig. 3.3 295 K quantitative EPR spectra of 1% Cr:STO−ap and after NaBH4 reduction at 300 °C and 350 °C. Inset shows the relative peak area (squares) and linewidths (fwhm, circles) of the Cr3+ signal in the as-prepared (open symbols) and as a function of reduction temperature (filled symbol).

We attribute the increase in integrated Cr3+ intensity to chemical reduction of EPR- silent Cr4+ or Cr6+ located at Ti4+-sites at these low temperatures. The increase in the linewidth of the Cr3+ signal with increasing reduction temperatures up to 350 °C is also consistent with increased dipole coupling between Cr3+ ions and inhomogeneous broadening. Quantitative analysis of samples reduced at 375 °C and higher temperatures could not be accomplished due to decreased cavity Q-values which could be the result of increased absorption of the microwaves. These samples show significant broadening compared to Cr:STO−350 and the 53Cr hyperfine can no longer be observed (see supplementary information). While increasing the reduction time at 350 °C does continue to increase the Cr3+ concentration, it is not as significant a perturbation as increasing the reduction temperature. For instance, the EPR spectrum of Cr:STO−375 after 30 minutes exhibits a Cr3+ signal with a linewidth nearly twice as large as a sample reduced at 350 °C for 3 hours (see supplementary information).

Fig. 3.4 Room temperature EPR spectra of 0.1% (nominal) Cr:STO−ap, −325, and −375.

To reduce the broadening of the Cr3+ signal we lowered the Cr3+ content by an order of magnitude to 0.1% in SrTiO3. The room temperature EPR spectra of Cr:STO−ap,

Cr:STO−325 and Cr:STO−375 with 0.1% nominal Cr mole fraction are shown in Fig. 3.4.

At a reduction temperature of 325 °C, the EPR spectrum is dominated by isolated substitutional Cr3+ ions. Upon increasing the reduction temperature to 375 °C, we see broadening of the EPR signal and a pronounced feature at g = 2.004, which has previously

− 34 been assigned to the EPR-active superoxide anion (O2 ) adsorbed onto the SrTiO3 surface.

− The generation of the surface O2 can occur by the transfer of an electron from a surface

• 3+ VO or Ti to adsorbed O2. This EPR signal is much weaker, but also observed, in the 325

°C reduced sample, but likely occluded by the Cr3+ signal in the 1% samples shown in Fig.

3+ 3.3. At room temperature, the Ti EPR signal in SrTiO3 is negligible, but can be detected at cryogenic temperatures.35 Therefore, EPR measurements were collected at 77 K to reveal possible Ti3+ lattice defects.

g-value 2.0 5 2.0 0 1.9 5 1.9 0 3+ no Ti

325 °C

u .

a 3+

i s

n 350 °C

R T = 77 K

P E

d Cr

a m

r 3+ o Ti N 375 °C

3300 3400 3500 3600 Field / G Fig. 3.5 77 K EPR spectra of 0.1% Cr:STO−325, −350, and −375. Enlarged sections in for the Cr:STO−325 and −350 spectra are scaled by 4.

Fig. 3.5 shows the 77 K EPR spectra of Cr:STO−325, Cr:STO−350 and

Cr:STO−375 (all 0.1% nominal Cr mole fraction). An asymmetric EPR signal centered at higher magnetic fields than the Cr3+ signal is readily observed that increases in intensity with increasing reduction temperature. The transition is broad and overlaps with the sharp

3+ 3+ Cr signal, but has a g-value near 1.97, which agrees well with the gx,y resonance of Ti

31,36,37 3+ in TiO2 (gx,y=1.975 and gz=1.944) and the photoinduced Ti signal from UV-

35,38 illuminated SrTiO3 single crystals. This signal is not visible in the 1% Cr samples due to inhomogeneous broadening of the lines (see supplementary information). Noticeably

– absent is the strong O2 resonance at 77 K. We tentatively assign this to electron trapping on localized, but shallow, Ti3+ sites at low temperature. At room temperature, the Ti3+ can

• transfer the electron to the conduction band and/or VO and react with surface adsorbed O2.

3.3.2 Reoxidation of reduced Cr:STO. The concentration of VO observed by diffuse reflectance (Fig. 3.2) and EPR (Fig. 3.4) appears to correlate with the increase in Cr3+ and

3+ Ti content after the NaBH4 reductions to maintain charge neutrality. To test the reversibility of this scenario we measured the EPR and diffuse-reflectance spectra of the powders after aerobic annealing at different temperatures. Fig. 3.6 shows the change in color and spectral properties of Cr:STO−350 reduced samples after reoxidation for 30 min at various temperatures. Reoxidation of Cr:STO−350 at 250 °C caused minimal change to the color of the sample as shown in Fig. 3.6A. However, as the temperature is increased to

375 °C, the green color of the sample reverts back to a reddish color similar to the original

Cr:STO−ap sample. Reoxidation above 400 and 450 °C reverts the Cr:STO−350 sample back to the same reddish color as the Cr:STO−ap.

The gradual trend with reoxidation of the sample color is also observed in the diffuse-reflectance spectra as shown in Fig. 3.6B. After reduction at 350 °C, the diffuse reflectance exhibits absorption in the near-IR from Ti3+ defects and the sub-bandgap region between 400-500 nm from VO defects. With increasing oxidation temperature between 250-

450 °C there is a gradual increase in the defect absorption centered at ~510 nm and a decrease in the near-IR absorption. At reoxidation temperatures above 400 °C there are no major changes to the diffuse-reflectance and the features revert to the Cr:STO−ap spectra.

Fig. 3.6C shows the changes to the EPR spectra of the same Cr:STO−350 samples after reoxidation. As shown in Fig. 3, the EPR spectrum of the Cr:STO−350 sample displays a broad Cr3+ resonance at ~3470 G (g = 1.978) with weakly visible hyperfine lines.

The spectra are normalized to emphasize the narrowing of the EPR linewidth with increasing reoxidation temperature. With increasing reoxidation temperature, the spectrum narrows significantly and approaches the linewidth of the Cr:STO−ap sample. This narrowing of the linewidth is the result of decreased dipole coupling between Cr3+ and

59 nearby spins. The spectra show a nearly quantitatively reversible Cr3+ signal after reoxidizing the sample at 450 °C for 30 min (see supplementary information).

Fig. 3.6 (A) Cr:STO−350 (left) reoxidized (from 2nd from left to right) at 250 °C, 300 °C, 325 °C, 350 °C, 375 °C, 400 °C, and 450 °C for 30 minutes. Far right is original Cr:STO. (B) Diffuse reflectance and (C) EPR spectra of same samples, Cr:STO−350 (teal, shaded), Cr:STO (black) and reoxidized samples from low to high temperature from dark to light trace color.

The results shown in Fig. 3.6 are consistent with oxidation kinetics of a reduced lattice defect with a single activation barrier following Arrhenius-type behavior. We tentatively assign this process to the oxidation of substitutional Cr3+. Temperature-dependent oxidation

60 kinetics are currently underway to confirm this assignment and determine the magnitude of this activation barrier.

3.3.3 High-temperature chemical reductions. In addition to the systematic changes observed during the oxidation of Cr:STO−350 powders, we also studied Cr:STO−400 where lattice reduction is readily detected by its black color and near-IR absorption as discussed above (see Fig. 3.1). Fig. 3.7 shows the changes in the physical appearance, EPR, and diffuse reflectance spectra after oxidation of Cr:STO−400 at 450 °C and 600 °C for 30 min. In contrast to the full reversibility of the Cr:STO−350 sample after reoxidation at 450

°C, there is no change in the appearance or diffuse reflectance spectra of the Cr:STO−400 sample after aerobic annealing at 450 °C. However, there is a significant reduction in the linewidth of the Cr3+ EPR signal of the Cr:STO−400 sample after oxidation at 450 °C that approaches the narrow EPR spectrum of the Cr:STO−ap sample. These results demonstrate that substitutional Cr3+ ions can be efficiently oxidized at 450 °C, but this temperature is insufficient to oxidize Ti3+ (lattice) defects. Only after oxidizing the sample at higher temperatures (600 °C) do all of the spectral changes and appearance of the sample revert to those of the Cr:STO−ap sample.

The reoxidation behavior of both the Cr:STO−350 and Cr:STO−400 samples suggest that the activation barriers to reduce lattice Ti4+ ions is greater than Cr4+,6+ dopants and reoxidize Ti3+ defects is greater than Cr3+ dopants. An alternative explanation that does not require reduction of possible charge-compensating Cr6+ defects is that reduction at the

(A)

. u

. Reduced Reoxidized

y t

s n

e 400 °C 450 °C 600 °C

o N

3300 3400 3500 3600 Field / G

(B)

. u

. as-prepared

400 °C (reduced) e

c reoxida on temperature

n a

b 450 °C

r o

s 600 °C

d V

e O 3+

z i

l Ti

o N

400 500 600 700 800 Wavelength / nm Fig. 3.7 The (A) EPR and (B) diffuse reflectance spectra of Cr:STO−400 (red trace with shading), Cr:STO−400 after aerobic annealing at 450 °C (dashed red line) and 600 °C (dashed red and yellow line) for 30 min. Inset: color scans of samples. lower temperature forms oxygen vacancies primarily located at or near the surface of the material and at higher temperatures these oxygen vacancies can diffuse into the subsurface layers. This scenario is supported by previous theoretical work that determined it is

39 energetically favorable for internal VO to form if surface VO are already present. It therefore follows that as the VO concentration increases to compensate for the increasing

Ti3+ and Cr3+ concentrations to maintain charge neutrality during chemical reduction, then the probability of forming internal VO must increase. The VO defects at the surface are more readily oxidized than internal VO, which is consistent with the reversibility of the changes

62 to Cr:STO-350 under a similar temperature range. However, this temperature range is insufficient to reoxidize the internal VO defects in Cr:STO−400.

The overall reduction and reoxidation behavior is represented by Figure 3.8, which summarizes the band structure, approximate Fermi level and relevant redox levels of

Cr:STO. The electronic structure of Cr:STO−ap is our starting point and we place the Fermi level in a position that is consistent with the EPR and diffuse reflectance spectra shown in

Figs 3.2 and 3.3. Upon reduction at 350 °C, the Fermi level is shifted to just below the conduction band edge, but above the reduction potentials of the both higher Cr oxidation

x • states and the VO/VO. This is corroborated by diffuse-reflectance and EPR spectroscopies

3+ that show a blueshift of the VO absorption and an increase in the Cr concentration respectively (Figs 3.2 and 3.3). The chemical reduction at 400 °C shifts the Fermi level into the conduction band which results in the additional formation of Ti3+ in the STO powders.

The presence of Ti3+ was confirmed via diffuse reflectance and EPR spectroscopy (Figs 3.2 and 3.5).

Fig. 3.8 Schematic model of the NaBH4 reduction on the fermi level (EF) of Cr-doped SrTiO3 with multivalent Cr ions. The positions of the Cr4+/3+ and Cr6+/4+ redox levels are below the bottom of the conduction band which is the Ti4+/3+ redox level that is reached at temperatures above 375 °C.

3.4 Conclusions

A mild chemical reduction strategy using NaBH4 has been adapted to control the Cr oxidation state and electronic structure of SrTiO3 bulk powders. We demonstrate that reduction temperatures at or below ~375 °C increases the Cr3+ concentration and reduces the VO-related electronic transition in the visible region. Reduction temperatures above 375

°C produces Ti3+ lattice defects that are readily observed by both diffuse reflectance and 77

K EPR spectroscopies with 0.1% doping levels. Concomitant to the changes in valence state of Cr and Ti in STO with reduction at high-temperatures is a corresponding change in the nature of the VO determined by diffuse reflectance spectroscopy.

The changes to the electronic structure of Cr:STO−350 are fully reversible after reoxidation at 450 °C for only 30 min. However, the Cr:STO−400 sample shows reversibility of only the Cr and VO changes after reoxidation at 450 °C with negligible change to the concentration of Ti3+ lattice defects. To fully reoxidize the Ti3+ defects in the

Cr:STO−400 sample elevated temperatures of 600 °C are required. This chemical perturbation has interesting prospects for controlling the defect structures of doped perovskites and other oxide semiconductors in addition to nominally undoped perovskites for photocatalysis applications. Control over the rich and complex defect chemistry could also be extended to test carrier-mediated and charge-transfer ferromagnetism in transition- metal doped oxides with native n-type defects.40

3.5 Supplementary Information

3.5.1 Quantitative EPR Measurements. For quantitative measurements of EPR, the following protocol was used as described in [22]. All sample volumes were matched in the EPR tubes in order to ensure the same pathlength of absorption during the measurement. Samples were ground

64 to ensure that no clumping of powders would affect the amount of sample in each tube. The use of a finger dewar ensured that each sample was in the same spot relative to the microwave frequency.

All parameters for the measurement were held constant during measurement, and minor adjustments to the frequency were made in order to keep the quality factor constant between different samples.

Fig 3.9 Change in the diffuse reflectance spectra from Cr:STO-ap to after reduction at different temperatures. The dashed black line is at zero.

R fwhm

d 350 °C G = 36 G

r g

e 325 °C G = 30 G

n I 300 °C G = 21 G

as-prepared G = 7.8 G 3200 3300 3400 3500 3600 3700 3800 Field / G

Fig 3.10 Integrated EPR signal of Cr:STO samples as-prepared and after NaBH4 reduction at different temperatures.

Fig 3.11 Room temperature EPR spectrum of Cr:STO-375.

Fig 3.12 EPR spectrum of Cr:STO-350 after reduction for 3 h instead of 30 minutes.

Fig 3.13 EPR spectroscopy of Cr:STO-325 (A) and Cr:STO-375 (B) at 77 K.

Fig 3.14 Area and full-width half max analysis for quantitative EPR measurements for oxidation of Cr:STO-350 (open markers) at different temperatures. As-prepared values added for reference.

Fig 3.15 EPR spectra of 0.1% Cr:STO reduced at 400 °C and after oxidation at 450 and 600 °C. Spectra were collected at 77 K. Trace Mn4+ impurity was observed in the reox 600 sample. As the Ti3+ signal decreases in intensity, a feature at g=1.920 is visible which is consistent with a rhombic 3+ 38 Ti center in SrTiO3.

Fig 3.16 EPR spectra of Cr:STO-400 and after oxidation at 450 and 600 °C taken at 77 K.

Fig 3.17 XRD patterns after reoxidation at 450 °C of (A) Cr:STO-350 and (B) Cr:STO- 400.

3.6 References

1. Marshall, M. S. J., Becerra-Toledo, A. E., Marks, L. D. & Castell, M. R. in Defects at Oxide Surfaces (eds. Jupille, J. & Thornton, G.) (Springer International Publishing, 2015).

2. Bai, X. et al. Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization. Langmuir 29, 3097–3105 (2013).

3. Lv, Y. et al. The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1−x photocatalyst. Catal. Sci. Technol. 3, 3136 (2013).

4. Lv, Y., Zhu, Y. & Zhu, Y. Enhanced Photocatalytic Performance for the BiPO4−x Nanorod Induced by Surface Oxygen Vacancy. J. Phys. Chem. C 117, 18520 (2013).

5. Chen, X., Liu, L., Yu, P. Y. & Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 331, 746–50 (2011).

6. Tan, H. et al. Oxygen Vacancy Enhanced Photocatalytic Activity of Pervoskite SrTiO3. ACS Appl. Mater. Interfaces 6, 19184–19190 (2014).

7. Wang, J. et al. Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO. ACS Appl. Mater. Interfaces 4, 4024–4030 (2012).

8. Liu, G. et al. Enhancement of visible-light-driven O2 evolution from water oxidation on WO3 treated with hydrogen. J. Catal. 307, 148–152 (2013).

9. Sarath Kumar, S. R., Abutaha, a. I., Hedhili, M. N. & Alshareef, H. N. Effect of oxygen

vacancy distribution on the thermoelectric properties of La-doped SrTiO3 epitaxial thin films. J. Appl. Phys. 112, 114104 (2012).

10. La Mattina, F., Bednorz, J. G., Alvarado, S. F., Shengelaya, A. & Keller, H. Detection of charge transfer processes in Cr-doped SrTiO3 single crystals. Appl. Phys. Lett. 93, 22102 (2008).

11. Wang, D., Ye, J., Kako, T. & Kimura, T. Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites. J. Phys. Chem. B 110, 15824–30 (2006).

12. Ishii, T., Kato, H. & Kudo, A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J. Photochem. Photobiol. A Chem. 163, 181–186 (2004).

13. Kato, H. & Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106, 5029–5034 (2002).

14. Glasbeek, M., de Jong, H. J. & Koopmans, W. E. Nonlinear Jahn-Teller coupling and local 5+ dynamics of SrTiO3:Cr near the structural phase transition point. Chem. Phys. Lett. 66, 203–206 (1979).

5+ 15. Müller, K. A., Blazey, K. W. & Kool, T. W. Tetrahedrally coordinated Cr in SrTiO3. Solid State Commun. 85, 381–384 (1993).

16. Ouyang, S. et al. Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts. J. Am. Chem. Soc. 134, 1974–7 (2012).

17. Kang, H. W. & Park, S. Bin. H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor. Int. J. Hydrogen Energy 36, 9496–9504 (2011).

18. Rice, W. D. et al. Persistent optically induced magnetism in oxygen-deficient strontium titanate. Nat. Mater. 13, 481–7 (2014).

19. Janousch, M. et al. Role of Oxygen Vacancies in Cr-Doped SrTiO3 for Resistance-Change Memory. Adv. Mater. 19, 2232–2235 (2007).

20. Alvarado, S. F., La Mattina, F. & Bednorz, J. G. Electroluminescence in SrTiO3:Cr single- crystal nonvolatile memory cells. Appl. Phys. A Mater. Sci. Process. 89, 85–89 (2007).

21. Tan, H. et al. A facile and versatile method for preparation of colored TiO2 with enhanced solar-driven photocatalytic activity. Nanoscale 6, 10216–23 (2014).

22. Eaton, G. R., Eaton, S. S., Barr, D. P. & Weber, R. T. Quantitative EPR. (Springer, 2010).

23. Mitchell, R. H., Chakhmouradian, A. R. & Woodward, P. M. Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr(1-2x)Na(x)La(x))TiO3. Phys. Chem. Miner. 27, 583–589 (2000).

24. Lehuta, K. A. & Kittilstved, K. R. Speciation of Cr(iii) in intermediate phases during the sol–gel processing of Cr-doped SrTiO3 powders. J. Mater. Chem. A 2, 6138 (2014).

25. Stokowski, S. E. & Schawlow, A. L. Spectroscopic Studies of SrTiO3 Using Impurity-Ion Probes. Phys. Rev. 178, 457–464 (1969).

26. Baer, W. S. Free-carrier absorption in reduced SrTiO3. Phys. Rev. 144, 734–738 (1966).

27. Mitra, C., Lin, C., Robertson, J. & Demkov, A. A. Electronic structure of oxygen vacancies in SrTiO3 and LaAlO3. Phys. Rev. B 86, 155105 (2012).

28. Hao, X., Wang, Z., Schmid, M., Diebold, U. & Franchini, C. Coexistence of trapped and free excess electrons in SrTiO3. Phys. Rev. B - Condens. Matter Mater. Phys. 91, 1–6 (2015).

29. Sulaeman, U., Yin, S. & Sato, T. Solvothermal synthesis and photocatalytic properties of chromium-doped SrTiO3 nanoparticles. Appl. Catal. B Environ. 105, 206–210 (2011).

30. Liu, J. W., Chen, G., Li, Z. H. & Zhang, Z. G. Electronic structure and visible light photocatalysis water splitting property of chromium-doped SrTiO3. J. Solid State Chem. 179, 3704–3708 (2006).

31. Zuo, F. et al. Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light. J. Am. Chem. Soc. 132, 11856–11857 (2010).

32. Muller, K. A. in Proceedings of the First International Conference I, Paramagnetic Resonance (ed. Low, W.) 17 (Academic Press, 1963).

3+ 33. Azamat, D. V. et al. Pulse-electron paramagnetic resonance of Cr centers in SrTiO3. J. Appl. Phys. 113, 174106 (2013).

34. Bykov, I. et al. Intrinsic and impurity defects in chromium-doped SrTiO3 nanopowders: EPR and NMR study. Phys. Status Solidi 250, 821–824 (2013).

3+ 35. Laguta, V. V. et al. The Photoinduced Ti centre in SrTiO3. J. Phys. Condens. Matter 14, 13813–13825 (2002).

36. Khomenko, V. M., Langer, K., Rager, H. & Fett, A. Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile. Phys. Chem. Miner. 25, 338–346 (1998).

3+ 4+ 37. Ji, Y. et al. Surface Ti /Ti Redox Shuttle Enhancing Photocatalytic H2 Production in Ultrathin TiO2 Nanosheets/ CdSe Quantum Dots. J. Phys. Chem. C (2015).

38. Schirmer, O. F. & Muller, K. A. Defects In Neutron-Irradiated Strontium Titanate: Ti3+ ‘Off Center’ on a Sr2+ Site. Phys. Rev. B 7, 2986–2995 (1973).

39. Jeschke, H. O., Shen, J. & Valenti, R. Localized versus itinerant states created by multiple oxygen vacancies in SrTiO3. New J. Phys. 17, 6 (2014).

40. Bhattacharya, A. & May, S. J. Magnetic Oxide Heterostructures. Annu. Rev. Mater. Res. 44, 65–90 (2014).

CHAPTER 4

SOLID-STATE SYNTHESIS AND LOW-TEMPERATURE CHEMICAL REDUCTIONS OF SUBSTITUTIONAL Cr-DOPED Sr2TiO4

4.1 Introduction

The use of wide-gap semiconductor oxides such as TiO2 and SrTiO3 as photocatalysts has been an appealing route for renewable energy applications since TiO2 and SrTiO3 were shown to

1,2 split water using a photoelectrochemical cell. Despite having similar band gaps (Eg = 3.2 eV),

SrTiO3 has a more negative reduction potential for an electron in its conduction band (~0.2 eV)

3,4 compared to TiO2, however, this advantage is diminished since both oxides require UV photons.

One strategy that has shown promise for reducing the band gap energy of wide-gap oxides into the visible region is by substitutional doping of transition metal ions such as Ru,5,6 Rh,7–9 Cr,10–14 and

15 Ni at the B-site of SrTiO3. However, there are additional complications that arise from the introduction of transition metal ions with rich redox properties into oxide lattices with equally complex defect chemistries. Recently, the layered perovskites have received renewed attention as a means to confine photogenerated charge carriers within only two dimensions of the perovskite.16–

20 21 SrTiO3 is the infinite member of the Ruddlesden-Popper series with the general formula

Srn+1TinO3n+1 or (SrO)(SrTiO3)n where rock-salt SrO layers insulate the SrTiO3 layers with

4− tetragonally-compressed [TiO6] octahedra. However, the utilization of dopant ions into the

Srn+1TinO3n+1 Ruddlesden-Popper series to introduce sub-bandgap excitation with visible light has been limited.17–19,22

Dopant ions such as Cr can coexist within SrTiO3 with multiple oxidation states including

3+, 4+, 5+, and 6+. The 3+, 5+ and 6+ oxidation states can be compensated by other Cr oxidation states or oxygen vacancies according to eq 4.1.

× × ′ ∙ ∙ ′ ∙∙ ′′ CrTi + OO ⇌ 푥(CrTi + 푉O) + 푦(CrTi + 푉O) + 푧(CrTi + 푉O ) (eq. 4.1)

Visible-light photocatalysis has been correlated to increased amounts of Cr3+ in the lattice.[ref] However, undesired higher oxidation states such as Cr6+ act as efficient trap sites for the photoexcited conduction band electron and thus decreases the visible light photocatalytic

7,10,17–19,23 activity in Cr-doped SrTiO3. In Sr2TiO4, the Cr oxidation state has been altered by either co-doping with La17,18 or exposure to UV light for extended periods of time.19 A common approach to eliminate these higher Cr oxidation states and recover visible-light photocatalytic activity is the co-doping of donor defects such as Nb5+ or Sb5+.14,15,24,25 Recently, we demonstrated an alternative approach that does not require co-doping and instead Cr6+ ions (and potentially Cr4+) are reduced

3+ to Cr through a low-temperature chemical reduction method using NaBH4. In addition to increasing the Cr concentration, the chemical reduction under vacuum also altered the oxygen defect chemistry, which has been shown in oxides to act as adsorption sites that allow the

26–28 photocatalytic reaction to occur. A similar chemical reduction method of TiO2 and SrTiO3 requires higher temperatures to produce localized Ti3+ defects that have been shown to promote hydrogen evolution under visible light.29,30

3+ Previous work on the sol-gel synthesis of Cr:SrTiO3−δ powders revealed presence of Cr

31 in Sr2TiO4. Attempts to synthesize phase pure Cr:Sr2TiO4 via the sol-gel processing yields a maximum of only ~68% phase purity via analysis of diffraction patterns using Maud for a dopant concentration of 0.1% (data not shown). This study will utilize the NaBH4 reduction method in order to systematically alter the Cr oxidation state and oxygen vacancies (VO) in nearly phase pure

Sr2TiO4 powders prepared directly via solid state methods. Electron paramagnetic resonance

(EPR) spectroscopy is used to quantify the relative concentration of substitutional Cr3+ and Ti3+ defects in the Sr2TiO4 lattice under mild reduction and oxidation conditions that do not decompose this metastable phase. We detect a significant increase in the EPR signal of the axial Cr3+ ion in

Sr2TiO4 with minimal change in the linewidth up to a reduction temperature of Tred = 375 °C. These

31 results are qualitatively similar to our previous results of Cr in SrTiO3 under similar chemical

3+ reduction conditions. However, at Tred = 350 °C, we observe a significant enhancement of the Cr signal in Sr2TiO4 by a factor of ~55 compared to only ~12 in SrTiO3.

4.2 Experimental Methods

4.2.1 Materials. TiO2 (99.9%, nanopowder, anatase, 32 nm APS powder, Alfa Aesar), SrO

(Sigma Aldrich, 99.9%, trace metals basis), Cr(NO3)3∙9H2O (crystalline, certified, Fisher

Scientific), NaBH4 (≥98%, white powder, MP Biomedical), MgO (Fisher Science Education), and ethanol (190 proof, ACS/USP grade, Pharmco-Aaper) were used without further purification.

4.2.2 Synthesis of Cr doped Sr2TiO4. Sr2Ti1−xCrxO4−δ powders were made using the solid state method, where x is the nominal concentration of Cr (x = 0.001 to 0.1) and δ is the concentration of oxygen vacancies (VO). In a typical synthesis, SrO, TiO2, and Cr(NO3)3∙9H2O were mixed in the desired stoichiometry and ground using a mortar and pestle until uniform consistency was achieved (~5 min). The mixture was transferred to a crucible and heated for 6 hours at 1100 °C in a tube furnace (Mini-Mite Lindberg BlueM). The crucible was removed from the tube furnace and the product was ground thoroughly again for ~5 min after cooling and then annealed again for an additional 15 hours at 1100 °C.

4.2.3 NaBH4 Reductions. Chemical reductions of Cr:Sr2TiO4−δ were performed using the

31 method previously described for Cr:SrTiO3−δ powders. Briefly, Cr:Sr2TiO4−δ powders were ground in a 1:1 mole ratio with NaBH4 and placed in a crucible that was inserted into a 1” diameter quartz insert with vacuum connections in the tube furnace. The samples were heated under a static vacuum (10−4 mbar pressure) at different temperatures up to 375 °C. After cooling under vacuum,

75 samples were alternately washed with deionized water and ethanol and collected via centrifugation. Samples were then dried at 100 °C for 2 h and products were characterized.

4.2.4 Physical Characterization. Powder X-ray diffraction patterns were collected using a PAN-alytical X’Pert Material Research Diffractometer. Rietveld refinements of the phases were done using FullProf and Match3! (Crystal Impact GbR) generated input files. Emission spectra were collected at 4 cm−1 spectral resolution with a Fourier transform spectrometer (Varian 670) equipped with a PMT detector (Hamamatsu R116) and excited with the 488 nm line of an Ar+ laser

(JDSU 2218). Samples for emission were prepared by fixing loose powder to a copper plate using conductive silver paint (Leitsilber 200, Ted Pella) that was mounted in a closed-cycle helium optical cryostat (Advanced Research Systems HC-2). Diffuse reflectance spectra were collected using an integrating sphere (Ocean Optics ISP-REF) coupled by fiber optic cables to a CCD-based spectrophotometer (Ocean Optics USB2000+ UV-Vis). Signal responses were transformed to absorption using the Kubelka-Munk relation.32 in order to semi-quantitatively compare the diffuse reflectance spectra we diluted the samples with enough MgO so that the Kubelka-Munk response at 310 nm was the same.

Room temperature X-band EPR spectra were measured on powders in the perpendicular mode of a dual resonator cavity (Bruker Elexsys E-500 with ER-4116 cavity). EPR spectra were

4 analyzed using the simplified S = 3/2 spin Hamiltonian that describes the A2 ground term of pseudo-octahedral Cr3+ given by eq 2.

2 퐻̂ = 퐷[푆̂푧 − 1/3(푆(푆 + 1))] + 퐴(퐼 ∙ 푆) + 𝑔휇B퐵 (2) where D is the axial component to the zero-field splitting parameter, A is the hyperfine splitting coefficient, I is the nuclear spin (9.5% relative abundance 53Cr, I = 3/2), g is the Lande g-factor,

µB is the bohr magneton, and B is the external magnetic field strength. The rhombic component to

76 the ZFS is not observed and therefore removed from (1). The tetragonal compression of the B-site of Sr2TiO4 does result in a non-zero D parameter and g-anisotropy (gxy ≠ gz). The D parameter can be determined experimentally by variable-temperature magnetism or EPR measurements. We alternatively estimated the sign of D by ligand field theoretical calculation of the EPR parameters within the angular overlap model using both AOMX and LigField computer programs. The trends in the g-values and D parameters were calculated with a standalone computer program written by

Philip L.W. Tregenna-Piggott that utilizes the calculated magnetic field dependence on the energies of the lowest spin-orbit states from LigField after employing the magnetic moment

33–35 operator, 휇̂ = (퐿̂ + 2푆̂)휇B퐻.

4.3 Results and Discussion

The powder XRD pattern of 0.1% Cr:Sr2TiO4-δ after 6 h via the solid state method at 1100

°C is shown in Figure 1 and matches with many of the peaks of Sr2TiO4. However, the presence of weak diffraction peaks between 34°-43° indicate the presence of SrO, SrTiO3, and TiO2 (rutile-

36–38 type, TiO2-R) impurities in the sample. Noticeably absent is any peak from the n=2 RP phase,

39 Sr3Ti2O7. After additional regrinding of the powder and heating at 1100 °C for 15 hours both

SrTiO3 and TiO2-R phases are not detected within the limit of detection in the diffractometer, but

SrO is still clearly present as an unreacted precursor. Rietveld refinement of the 21 h annealed product is composed of primarily Sr2TiO4 (94%) compared to the initial 6 h intermediate (~73%).

Other minor phases after 21 h are SrTiO3 (3.2%), SrO (1.7%), TiO2-R (0.6%), and Sr3Ti2O7 (0.6%).

The Rietveld refinements of the intermediate and final products are given in supporting information.

1 1

1 1 R1 1 1 1 R1 S 1 1 1 S R R∞R S 1R 1 1S

) exp. (6 h) .

u 1 1

(

y 1

t 1

i 1 s 1 1 S S 1 S 1 n 1 1 1 11 1S

e exp. (21 h)

t n I Sr3Ti2O7 (2)

Sr2TiO4 (1)

SrTiO3 (∞)

TiO2 (R) SrO (S) 20 25 30 35 40 45 50 55 60 2q (°) S Figure 4.1. X-ray powder diffraction1 of the Sr2TiO4 reaction intermediates and products after the initial 6 h (black) and additional 21 h (red) aerobic heating at 1100 °C. The reference diffraction 21,37,39–41 patterns of SrO, TiO2-R, SrTiO3, Sr2TiO4, and Sr3Ti2O7 are taken from literature.

16 We previously assigned an axial signal in the EPR spectrum of sol-gel derived Cr:SrTiO3

3+ 4+ to Cr substitution into the tetragonally-compressed Ti site of Sr2TiO4. The EPR spectra of the

3+ 0.1% Cr :Sr2TiO4 after the two annealing stages is shown in Figure 2. The spectra exhibit clear axial splitting from a non-zero D value and g-anisotropy as observed previously. The EPR spectrum after 6 h of heating at 1100 °C resembles the previously reported spectra with 1% Cr levels. The increase in the phase purity after the additional heating step results in observable changes in the EPR spectrum. First, the features assigned to anisotropy of the g-values of the

|−1/2⟩↔|+1/2⟩ transitions shown in Figure 2 narrow significantly after the second heating. The

−4 −1 previous spin-Hamiltonian parameters were refined to D = −201 × 10 cm , g|| = 1.9803, g⊥ =

1.9793, A = −16.2 × 10−4 cm−1. The compression along the 001 direction of the substitutional Cr3+ dopant results in a negative D-value and g⊥ > g|| based on ligand field calculations with the angular overlap model (see SI). The low Cr3+ content is also the main reason why the signal-to-noise ratio was too low to observe transitions between the |±3/2 and ±1/2 spinors that were observed previously.16

The central feature in Figure 4.2 after the additional 15 h heating step at 1100 °C also becomes narrower compared to the 6 h initial heating. This observation could originate from reduced dipole-dipole coupling between nearby Cr3+ ions either by diffusion of Cr3+ ions in the lattice away from each other or by oxidation of Cr3+ to higher valence states during the prolonged aerobic annealing. Based on the increased Sr2TiO4 phase purity observed by powder XRD, the latter is expected to be the cause in the change in the Cr3+ EPR lineshape in agreement with recent

31 3+ observations by us in Cr :SrTiO3-δ. It is noteworthy that we do not observe any EPR signals at

5+ 1 room temperature from Cr (d , S = 1/2) that has a slightly lower expected g-value (in SrTiO3 g =

1.95).42 Therefore, the scenario where Cr3+ is oxidized to the EPR-silent Cr4+ or Cr6+ ions could explain the change in the EPR signal shape. To confirm this, chemical reductions were performed at varying temperature under vacuum and monitored the EPR intensity and lineshape of the Cr3+ signal in order, which we recently demonstrated is an effective strategy to reduce higher oxidation

3+ 31 states of Cr to Cr in SrTiO3.

Figure 4.2. Room temperature EPR spectra of the central (|−1/2⟩↔|+1/2⟩) transitions of 0.1% Cr:Sr2TiO4−δ powder after annealing at 1100 °C for 6 h (black) and 21 h (red). Simulated spectrum calculated using previously reported16 and refined spin-Hamiltonian parameters (see text).

Figure 4.3 shows the quantitative EPR spectra for 0.1% Cr:Sr2TiO4−δ reduced using NaBH4 at temperatures between 300 °C - 375 °C. The intensity of the Cr3+ EPR signal in the axially compressed B-site of Sr2TiO4-δ increases by a factor of 15 after the 375 °C reduction. In addition,

79 the transitions between the ±3/2 to ±1/2 spinors are readily observable after the reduction due to the increase in Cr3+ signal. This increase in signal intensity supports that higher oxidation states of

Cr, which are EPR silent Cr4+ or Cr6+, are present in the as-prepared sample and subsequently reduced through this low-temperature chemical reaction with NaBH4. The spectra also have minimal broadening or change in lineshape after the chemical reduction that we attribute to the low starting concentration that reduces Cr3+ dipole interactions. These interactions are more pronounced in the 1% doped samples, particularly after being reduced at 375 °C (see supplementary information).

Figure 4.3. (a) Quantitative EPR spectra of 0.1% Cr:Sr2TiO4−δ before (black) and after NaBH4 reduction at 300 °C (blue), 325 °C (green), and 375 °C (red) for 30 min under static vacuum at 4 × 10–4 mbar. Note that the left and right panels show the ±3/2  ±1/2 transitions that are scaled by 50 and have different field ranges than the narrow central +1/2  1/2 transition in the central panel. (b) Calculated angular dependence of the 3 EPR-allowed transitions. Chemical reductions above 375 °C could not be studied quantitatively due to increased microwave absorption by the sample. Reduction of the sample at 400 °C results in a significantly broadened EPR spectrum (see supplementary information) and the appearance of a broad absorption in the near-IR region by diffuse-reflectance (vide infra). Both observations are consistent with creation of Ti3+ lattice defects in addition to the increased Cr3+ ion concentration.

Reoxidation of reduced Cr:SrTiO3-δ powders reported recently requires higher oxidation

80 temperatures to revert the EPR intensities back to their as-prepared values. In contrast, the Cr3+

EPR signal after reduction at 375 °C reverts quantitatively back to the as-prepared values after reoxidation of the sample in air at 375 °C (see supplementary information). This result suggests very similar activation energies for the redox reactions of Sr2TiO4-δ compared to SrTiO3-δ.

3+ The NaBH4 annealing of 0.1% Cr:Sr2TiO4 at 375 °C increases the substitutional Cr EPR signal in the lattice, allowing for confirmation of the spin Hamiltonian parameters that has been

3+ 16 previously hypothesized for Cr in the B-site of Sr2TiO4. The comparison of the experimental and simulated spectrum is shown in figure 3B and are in good agreement. This confirms that Cr3+

−4 −1 doped Sr2TiO4−δ has gz = 1.9803 and gxy = 1.9793 and that D = ±207 × 10 cm . The g-anisotropy

3+ 3+ and non-zero value of D that are not present when Cr is doped into SrTiO3 arise from the Cr being in the axially compressed B-site of Sr2TiO4. The magnitude of D is comparable to that of

3+ −4 Cr in the anatase form of TiO2 which also has an axially compressed Ti site (D = −372×10

−1 43 3+ cm ), but is significantly lower than that of Cr in the rutile phase of TiO2 which is equatorially compressed (5500 × 10−4 cm−1).44 Peaks associated with the hyperfine coupling due to 53Cr (I =

3/2, 9.5% relative abundance) are visible as in the spectrum of Cr:Sr2TiO4-δ after reduction at 375

°C, allowing confirmation of A = −16.2 x 10−4 cm−1.

The dopant-lattice interaction for Cr3+ dopant ions can also be studied by looking at the

2 4 E A2 emission at low temperatures (see figure 4.4). The R-line emission for 0.1% Cr:Sr2TiO4-

-1 δ after chemical reduction at 375 °C is observed at 12593 cm at 20 K, and much like bulk

2 16 Cr:SrTiO3, the linewidth was not narrow enough to discern the splitting of the E. Emission of the as-prepared 0.1% Cr:Sr2TiO4 could not barely be observed above the noise level of the

-1 spectrometer (data not shown). The R-line intensity is 10 cm lower than that of Cr:SrTiO3 and the structure of the vibronic sidebands is very similar. The shift in energy arises due to the reduction

81 of site symmetry in Sr2TiO4. The temperature dependence of the R-line emission mirrors that of

Cr:SrTiO3 suggesting that the interaction between the lattice phonons and the dopant ion are similar in the two lattices (see supplementary information for temperature dependent shift).16,45,46

-1 Figure 4.4. Emission of 0.1% Cr:Sr2TiO4-δ at 20 K when excited at 20492 cm with a spectral resolution of 2 cm-1. Inset shows the temperature dependence of the emission energy for this sample and Cr:SrTiO3 from chapter 2.

The as-prepared 1% Cr:Sr2TiO4-δ was purple in color, a shift compared to that of Cr:SrTiO3

(red-brown) and was deeper in color. The corresponding absorption of the as-prepared sample

31 showed broader visible light absorption than did Cr:SrTiO3 prior to chemical reductions. The absorption band extends in the visible light region is visible from 450 nm to about 730 nm (from

450 to 630 nm in SrTiO3). The band in Cr:SrTiO3 arises from a charge transfer transition between the valence band and oxygen vacancy defect states (VO), which was supported by theoretical

47,48 calculations. Previous studies on Sr2TiO4 suggest the primary changes in band structure correspond to the conduction band energy,49 which means the peak location of the charge transfer band may vary slightly, but a second peak is more likely the cause of the extended visible light absorption. This new peak likely does not result from a transition involving the Cr dopant energy levels given its strong presence in the as-prepared 0.1% Cr:Sr2TiO4-δ spectrum (see supplementary information).

Figure 4.5. Normalized diffuse reflectance spectra for 1% Cr:Sr2TiO4 before and after NaBH4 reductions at temperatures between 300 and 400 °C.

The diffuse reflectance spectra in figure 4.5 and supplementary information show distinct

3+ shifts in the absorption of Cr:Sr2TiO4 prior to the formation of Ti in the lattice. At the lowest reduction temperature (300 °C) a decrease in the absorption intensity of the lowest energy peak

(centered around 675 nm) is observed, before the whole broad peak decreases as can be seen at a reduction temperature of 325 °C. In samples reduced at 350 and 375 °C, a new band begins to grow in between 360 and 500 nm. A small amount of Ti3+ (NIR) absorption can be observed in the sample reduced 375 °C, but dominates the spectrum at a reduction temperature of 400 °C. The near-edge absorption band is also visible in the highest reduction temperature, but is less pronounced due to the large visible light absorption.

These changes in the reflectance spectra as a result of reduction temperature are very similar to those of Cr:SrTiO3-δ, with decreasing absorption in the visible region prior to the formation of Ti3+ and a large visible to near IR absorption in the samples containing Ti3+. Due to the reduction of the lower energy absorption peak initially and the similarity to Cr:SrTiO3, this

83 broadened absorption in the as-prepared sample can be attributed to a charge transfer transition

31 3+ between the valence band and a neutrally charged VO defect. As the Cr concentration increases and more charge compensation is needed to prevent charge accumulation to occur in the sample these neutral defects become ionized. The decrease in the intensity of the 450 to 600 nm range that occurs in samples reduced between 325 and 375 °C, along with the increasing intensity of the near edge absorption corresponds to the shift from singly ionized VO defects to doubly ionized VO

3+ defects. Finally as Ti forms, new VO in different both singly and doubly ionized charge states can form accounting for the visible absorption changes, while the Ti3+ accounts for the NIR absorption.

4.4 Conclusions

In this work, the study of Cr:Sr2TiO4 was performed including the confirmation of the spin

−4 −1 −4 Hamiltonian parameters with D = −201 × 10 cm , g|| = 1.9803, g⊥ = 1.9793, A = −16.2 × 10

−1 2 4 3+ - cm . The E→ A2 emission of the Cr in Sr2TiO4 was also studied, with approximately a 10 cm

1 shift in its energy compared to that of SrTiO3, but the emission in bothlattices have very similar temperature dependence. This shows that the interaction between the lattice phonons and the dopant ions are similar in both SrTiO3 and Sr2TiO4.

3+ Using NaBH4 chemical reductions, an ability to the control the Cr concentration was demonstrated as well as altering the VO concentration in the samples. Both of these changes were

3+ similar to that of Cr:SrTiO3, but the formation of Ti required an increased reduction temperature in Sr2TiO4 due to the higher energy associated with the primarily Ti 3d composed conduction band in the axially compressed material.

4.5 Supplementary Information

Figure 4.6. Quantitative EPR of 1% Cr:Sr2TiO4 as prepared and after reductions at temperatures from 300 and 375 °C.

Figure 4.7. Room temperature EPR of 0.1% Cr:Sr2TiO4-δ after NaBH4 reduction at 400 and 425 °C for 30 minutes.

Figure 4.8. Quantitative EPR results of 0.1% Cr:Sr2TiO4-δ reduced at 375 °C, then reoxidized at different temperatures for 30 minutes in normal atmosphere.

Figure 4.9. Emission spectra of 0.1% Cr:Sr2TiO4 after reduction at 375 °C at temperatures between 20.2 K and 149.3 K. The sample was excited at 20492 cm-1 and the spectral resolution is 4 cm-1.

Figure 4.10. Normalized diffuse reflectance of 0.1% Cr:Sr2TiO4 as prepared and after chemical reductions at temperatures between 300 and 400 °C.

4.6 References 1. Fujishima, A. & Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38 (1972).

2. Wrighton, M. S. et al. Strontium Titanate Photoelectrodes. Efficient Photoassisted Electrolysis Of Water At Zero Applied Potential. J. Am. Chem. Soc. 98, 2774–2779 (1976).

3. Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 20, 35–54 (2008).

4. Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–78 (2009).

5. Hara, S. et al. Hydrogen and Oxygen Evolution Photocatalysts Synthesized from Strontium Titanate by Controlled Doping and Their Performance in Two-Step Overall Water Splitting under Visible Light. J. Phys. Chem. C 116, 17458–17463 (2012).

6. Bae, S. W., Borse, P. H. & Lee, J. S. Dopant dependent band gap tailoring of hydrothermally prepared cubic SrTixM1−xO3 (M=Ru,Rh,Ir,Pt,Pd) nanoparticles as visible light photocatalysts. Appl. Phys. Lett. 92, 104107 (2008).

7. Kato, H., Sasaki, Y., Shirakura, N. & Kudo, A. Synthesis of highly active rhodium-doped SrTiO3 powders in Z-scheme systems for visible-light-driven photocatalytic overall water splitting. J. Mater. Chem. A 1, 12327 (2013).

8. Sasaki, Y., Nemoto, H., Saito, K. & Kudo, A. Solar Water Splitting Using Powdered Photocatalysts Driven by Z-Schematic Interparticle Electron Transfer without an Electron Mediator. J. Phys. Chem. C 113, 17536–17542 (2009).

9. Wang, J., Zhao, J. & Osterloh, F. E. Environmental Science Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy. Energy Environ. Sci. 8, 2970–2976 (2015).

10. Ishii, T., Kato, H. & Kudo, A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J. Photochem. Photobiol. A Chem. 163, 181–186 (2004).

11. Li, P., Ouyang, S., Xi, G., Kako, T. & Ye, J. The Effects of Crystal Structure and Electronic Structure on Photocatalytic H2 Evolution and CO2 Reduction over Two Phases of Perovskite-Structured NaNbO3. J. Phys. Chem. C 116, 7621–7628 (2012).

3+ 12. Li, H. et al. Roles of Cr doping and oxygen vacancies in SrTiO3 photocatalysts with high visible light activity for NO removal. J. Catal. 297, 65–69 (2013).

13. Ouyang, S. et al. Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts. J. Am. Chem. Soc. 134, 1974–7 (2012).

14. Reunchan, P. et al. Theoretical Design Of Highly Active SrTiO3-based Photocatalysts From Codoping Scheme Toward Solar Energy Utilization For Hydrogen Production. J. Mater. Chem. A (2013).

15. Niishiro, R., Kato, H. & Kudo, A. Nickel and either tantalum or niobium-codoped TiO2 and SrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueous solutions. Phys. Chem. Chem. Phys. 7, 2241–5 (2005).

16. Lehuta, K. A. & Kittilstved, K. R. Speciation of Cr(iii) in intermediate phases during the sol–gel processing of Cr-doped SrTiO3 powders. J. Mater. Chem. A 2, 6138 (2014).

17. Sun, X. et al. Photocatalytic Hydrogen Production over Chromium Doped Layered Perovskite Sr2TiO4. Inorg. Chem. 150714142222000 (2015).

18. Jia, Y. et al. Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation. J. Mater. Chem. A 1, 7905 (2013).

19. Yi, Z. G., Iwai, H. & Ye, J. H. Photochromism and visible light induced H2 generation in Sr2TiO4:Cr complexes. Appl. Phys. Lett. 96, (2010).

20. Miseki, Y., Kato, H. & Kudo, A. Water splitting into H2 and O2 over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure. Energy Environ. Sci. 2, 306 (2009).

21. Ruddlesden, S. N. & Popper, P. New compounds of the K2NIF4 type. Acta Crystallogr. 10, 538–539 (1957).

22. Jeyalakshmi, V. et al. Visible light driven reduction of carbon dioxide with water on modified Sr3Ti2O7 catalysts. RSC Adv. 5, 5958–5966 (2015).

23. Wang, D., Ye, J., Kako, T. & Kimura, T. Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites. J. Phys. Chem. B 110, 15824–30 (2006).

24. Biasi, R. S. De & Janeiro, R. De. Influence of Chromium Concentration on the Electron 3+ Magnetic Resonance Linewidth of Cr in SrTiO3. Mater. Res. 15, 473–476 (2012).

25. Kato, H. & Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106, 5029–5034 (2002).

26. Bai, X. et al. Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization. Langmuir 29, 3097–3105 (2013).

27. Lv, Y., Zhu, Y. & Zhu, Y. Enhanced Photocatalytic Performance for the BiPO4−x Nanorod Induced by Surface Oxygen Vacancy. J. Phys. Chem. C 117, 18520 (2013).

28. Lv, Y. et al. The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1−x photocatalyst. Catal. Sci. Technol. 3, 3136 (2013).

29. Chen, X., Liu, L., Yu, P. Y. & Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 331, 746–50 (2011).

30. Tan, H. et al. Oxygen Vacancy Enhanced Photocatalytic Activity of Pervoskite SrTiO3. ACS Appl. Mater. Interfaces 6, 19184–19190 (2014).

31. Lehuta, K. A. & Kittilstved, K. R. Reversible control of the chromium valence in chemically reduced Cr-doped SrTiO3 bulk powders. Dalt. Trans. 45, 10034–10041 (2016).

32. Kubelka, P. & Munk, F. N. Zeits. f. tech. Phys. 12, 593 (1931).

33. Adamsky, H., Schonherr, T. & Atanasov, M. in Comprehensive Coordination Chemistry II 661–664 (2003).

34. Tregenn-Piggot, P. L. W. dandgtensorcalc.sav.

35. Mabbs, F. E. & Machin, D. J. Magnetism and Transition Metal Complexes. (Chapman & Hall, 1973).

36. Mitchell, R. H., Chakhmouradian, A. R. & Woodward, P. M. Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr(1-2x)Na(x)La(x))TiO3. Phys.

Chem. Miner. 27, 583–589 (2000).

37. Primak, W., Kaufman, H. & Ward, R. X-Ray Diffraction Studies of Systems Involved in the Preparation of Alkaline Earth Sulfide and Selenide Phosphors 1. J. Am. Chem. Soc. 70, 2043–2046 (1948).

38. Parker, R. L. No Title. Zeitschrift fur Krist. Krist. Krist. Kryst. 59, 1 (1924).

39. Lukaszewicz, K. No Title. Rocz. Chem. 33, 239 (1959).

40. Swope, R. J., Smyth, J. R. & Larson, A. C. H in rutile-type compounds: I. Single-crystal neutron and X-ray diffraction study of H in rutile. Am. Mineral. 80, 448–453 (1995).

41. Miwa, K., Kagomiya, I., Ohsato, H., Sakai, H. & Maeda, Y. Electrical properties of the Sr2Ru1-xTixO4 solid solutions. J. Eur. Ceram. Soc. 27, 4287–4290 (2007).

42. Badalyan, a. G. et al. EPR study of charge compensation of chromium centers in the strontium titanate crystal. Phys. Solid State 55, 1454–1458 (2013).

43. Muller, K. A. in Proceedings of the First International Conference I, Paramagnetic Resonance (ed. Low, W.) 17 (Academic Press, 1963).

44. Gerritsen, H. J., Harrison, S. E., Lewis, H. R. & Wittke, J. P. Fine Structure, Hyperfine 3+ Structure, and Relaxation Times of Cr in TiO2 (Rutile). Phys. Rev. Lett. 2, 153–155 (1959).

3+ 45. Stokowski, S. E. & Schawlow, A. L. Dielectric-Related Optical Line Shifts in SrTiO3:Cr . Phys. Rev. 178, 464–470 (1969).

46. Trepakov, V. a et al. SrTiO3:Cr nanocrystalline powders: size effects and optical properties. J. Phys. Condens. Matter 21, 375303 (2009).

47. Stokowski, S. E. & Schawlow, A. L. Spectroscopic Studies of SrTiO3 Using Impurity-Ion Probes. Phys. Rev. 178, 457–464 (1969).

48. Mitra, C., Lin, C., Robertson, J. & Demkov, A. A. Electronic structure of oxygen vacancies in SrTiO3 and LaAlO3. Phys. Rev. B 86, 155105 (2012).

49. Reshak, A. H., Auluck, S. & Kityk, I. Electronic Band Structure and Optical Properties of Srn+1TinO3n+1 Ruddlesden – Popper Homologous Series. Jap. J. Appl. Phys 47, 5516 (2008).

CHAPTER 5

SYNTHESIS AND ANALYSIS OF Mn-DOPED SrTiO3 AND Sr2TiO4

5.1 Introduction

The use of SrTiO3 materials as photocatalysts has been explored since Wrighton et al.

1 demonstrated the ability to split water to form H2 and O2 gases from the material in 1976. The potential of SrTiO3 is limited by the utilization of UV-light for generation of the electron/hole pair.2 The utilization of visible light for photocatalysis would allow efficiencies greater than 4%.

The incorporation of transition metal ions to sensitize wide-gap photocatalysts has proven a successful technique to allow visible light photocatalysis.3–5 Much of the attention of first-row transition metal doping in SrTiO3 has focused on the incorporation of Cr in the lattice to induce visible light photocatalysis.6–9 Due to the similar ionic radii between Cr3+ and Ti4+, minimal effect

10 6+ on the lattice is observed. The main issue with Cr:SrTiO3 for photocatalysis is that Cr inhibits photocatalysis, requiring the use of a co-dopant11,12,8 or reduction technique13–16 to increase Cr3+ concentration.

Mn4+ is isoelectronic with Cr3+, but has received little attention as a dopant ion for photocatalysis applications17,18 whereas the interest in manganese oxides as catalysts and

19 photocatalysts are high. Primary interest in Mn:SrTiO3 has been for its altered magnetic

2+ 3+ 4+ properties. The dopant ions in SrTiO3 have been found as Mn ,Mn , and Mn via electron paramagnetic resonance (EPR)20–26 and X-ray photoelectron spectroscopies (XPS).18,27 Only in one report with catalytic degradation was the valence state of Mn studied, showing a combination of Mn3+ and Mn4+ in the sample.18

This work will study the effects of using a chemical reduction technique on Mn-doped SrTiO3 to control the valence state of the Mn and alter the defect chemistry of the lattice. Mn-doped

28 Sr2TiO4, the n=1 Ruddlesden-Popper phase, will also be studied for the first time. Interest in the

Ruddlesden-Popper phase has stemmed from altered surface chemistry and charge transport

29–32 properties in the layered material, as the TiO6 octahedra are separated by SrO rich sections.

5.2 Materials and Methods

5.2.1 Materials. TiO2 (99.9% nanopowder, anatase, 32 nm APS powder, Alfa Aesar), Sr(NO3)2

(99+%, for analysis, Acros Organics), Mn(NO3)2∙4H2O (analytical grade, Acros Organics), NaBH4

(≥98%, white powder, MP Biomedical), MgO (Fisher Science Education), and ethanol (190 proof,

ACS/USP grade, Pharmco-Aaper) were used as received.

5.2.2 Synthesis of Mn-doped SrTiO3 and Sr2TiO4. SrTi1-xMnxO3-δ and Sr2Ti1-xMnxO4-δ powders

(abbreviated as either Mn:SrTiO3 or Mn:Sr2TiO4) were synthesized using a solid state reaction, with x representing the nominal concentration of Mn. The concentration of oxygen vacancies is represented by δ. Samples were synthesized with x values of either 0.01 or 0.001. Sr(NO3)2,

Mn(NO3)2∙5H2O, and TiO2 were mixed in desired stoichiometry and ground with a mortar and pestle for about 10 minutes. The mixture was transferred to a porcelain crucible, heated for 6 hours, cooled, ground again, then heated for another 15 hours. Reaction temperatures were 1000 °C and

1100 °C for SrTiO3 and Sr2TiO4 powders respectively.

5.2.3 NaBH4 reductions. Chemical reductions of the powders were carried out using as was

16 previously described for Cr:SrTiO3 powders. For each reduction, the powder was ground in a 1:1 mole ratio with NaBH4 and placed in a porcelain crucible in the middle of a tube furnace fitted with a custom 1” diameter quartz insert with fittings and adapters to allow for static or dynamic pumping with a turbomolecular pump (Pfeiffer HiCube 80 Eco pumping station with a Pfeiffer

HiPace 80 turbo pump). The samples were heated at various temperatures ranging from 300-425

°C under a static vacuum of about 10-4 mbar for 30 minutes. Samples were cooled under vacuum to room temperature and washed alternately with deionized water and ethanol. Centrifugation was used between washes to collect the reduced samples. After washing samples were dried in an oven at 100 °C. Chemically reduced samples will be referred to as either Mn:SrTiO3-T or Mn:Sr2TiO4-

T where T is the temperature of the reduction.

5.2.4 Physical Characterization. Powder X-ray diffraction patterns were collected on a PAN- alytical X’Pert Material Research Diffractometer. X-band Electron paramagnetic resonance were collected at room temperature were collected on powders (in 4 mm quartz EPR tubes, Wilmad-

Glass) in perpendicular mode of a dual mode resonator cavity (Bruker Elexsys E-500 with ER-

4116 cavity). Quantitative EPR measurements were performed using identical sample conditions

(sample position, amount, Q-factor, and all settings).33 A quartz finger dewar insert was used to ensure matching sample position between samples. Samples run in capillary tubes were run using

Kimble Products Kimak-51 quartz tubes (0.9-1.1 x 100 mm dimensions) that were sealed using

Bruker Biospin X-sealant. The capillary tube was then placed in the normal EPR tube for analysis.

In order to compensate for the decreased sample amount, the power for these samples was increased to 20 mW (5 mW for others). All emission measurements were performed on powders that were fixed to a copper plate using conductive silver paint (Leitsilber 200, Ted Pella). Low temperature samples were cooled in a closed-cycle helium optical cryostat (Advanced Research

Systems HC-2) and measured with a Fourier transform spectrometer (Varian 670) equipped with a PMT detector (Hamamatsu R116) and excited using the 488 nm line of an Ar+ laser (JDSU 2218).

Emission measured with excitation wavelengths other than 488 nm, lifetime measurements, and excitation scans were measured using a Cary Eclipse luminescence and photoluminescence

93 spectrophotometer. Diffuse reflectance spectra were collected with an integrating sphere (Ocean

Optics ISP-REF) coupled by fiber optics to a CCD-based spectrophotometer (Ocean Optics

USB2000+ VIS-NIR). The optical density between the absorption minimum and the absorption at

340 nm was adjusted by mixed the powders with MgO to dilute the sample.

4+ 4 5.2.5 EPR Simulations. Mn has a S=3/2 ground state with no orbital angular momentum ( A2g in Oh symmetry) with a spin-Hamiltonian shown in equation 5.1

1 퐻 = 품훽퐵푆 + 퐴(푰 ∙ 푺) + 푫 (푆̂2 − (푆2 − 푆)) + 푬(푆̂2 − 푆̂2) (5.1) 푧 3 푥 푦

D and E represent the axial and rhombic components of the zero-field splitting (ZFS) respectively. The electron-nuclear hyperfine coupling from 55Mn (I=5/2, 100% natural abundance) is parameterized by A, the Landé factor is given by g tensor, with slight anisotropy (gz ≠ gx,y) when

D ≠ 0. Β is the Bohr magneton and B is the external magnetic field.

The simulations of EPR spectra and angular dependencies of the resonance fields were calculated using EasySpin in accordance with equation 1. In order to match the simulated spectra, the frequency of the experimental data was determined based on the Cr3+ impurity incorporated into SrTiO3 and Sr2TiO4 (g=1.978).

5.3 Results and Discussion

5.3.1 Structural Characterization of As-Prepared Samples. Powder X-ray diffraction (XRD) was used to confirm the purity of the prepared powder samples. As can be seen in figure 5.1,

SrTiO3 and Sr2TiO4 were synthesized using the solid state reaction in high purities at temperatures of 1000 °C and 1100 °C respectively as was shown in chapters 3 and 4. The diffraction patterns were compared to previously reported patterns to confirm purity. The Sr2TiO4 does contain two impurity peaks corresponding to SrO at 2θ values 30° and 34°.

Figure 5.1 Powder XRD patterns of 0.1% Mn-doped SrTiO3 and Sr2TiO4 including literature patterns for comparison from references 34 and 35 for comparison.

5.3.2 Analysis of Mn in SrTiO3. EPR spectroscopy was utilized to determine the valence state of the Mn dopant ions in the SrTiO3 lattice. As can be seen in figure 5.2, a sextet of peaks, arising from the nuclear hyperfine of 55Mn (100% abundance, I=5/2). A small impurity peak of Cr contaminant can be observed as well (marked with *) and can be used as a standard to determine the frequency. The g-value for the Mn:SrTiO3 was calculated to be 1.995 with an A-value of 71.12

-4 -1  10 cm . Mn dopant ions in SrTiO3 have previously been characterized via EPR spectroscopy in both 2+ and 4+ valence states at the B-site20,21,24,25,36 and 2+ valence state at the A-site.26 Mn3+

20,22 coupled to VO defects have also been observed in SrTiO3 crystals, but due to being S=2. The g

4+ -4 and A values in this sample are consistent with those of Mn in SrTiO3 (g=1.994, A=69.4 10 cm-1).36

3+ Figure 5.2. Room temperature EPR spectrum of 0.1% Mn:SrTiO3. Star marks Cr impurity in sample.

The use of emission spectroscopy can also confirm the valence state of the Mn-dopant ions.

The low temperature emission spectrum of Mn:SrTiO3 is shown in figure 5.3. The zero phonon

2 4 4+ -1 line (ZPL) of the E A2 emission of Mn can be seen at 13830 cm , consistent with single

-1 37 crystal Mn:SrTiO3 which was reported to emit at 13827 cm . The strong intensity of the phonon

4+ sidebands are consistent with previous reports, further confirming that Mn dopes SrTiO3 as Mn via the solid state reaction at 1000 °C.

-1 Figure 5.3 Emission spectrum at 22 K of 0.1% Mn:SrTiO3 when excited at 23809 cm .

5.3.3 Characterization of Mn in Sr2TiO4. Low temperature emission was performed on 0.1%

-1 -1 Mn:Sr2TiO4 using laser excitation (λex=20492 cm ) using a 2 cm spectral resolution. The

4+ -1 presence of Mn in Sr2TiO4 is confirmed by the emission between 14000 and 12000 cm . The

2 4 -1 -1 ZPL of the E A2 emission was observed at an energy of 13902 cm , about 70 cm higher than that of Mn:SrTiO3. While the higher resolution reveals more vibronic sidebands, the most intense phonon mode does not change positions like the ZPL did when going from SrTiO3 to Sr2TiO4 (see supplementary information). The change in the ZPL energy results from the reduction of site

3+ symmetry in Mn:Sr2TiO4 (D4h vs Oh) and the shift is larger than that of Cr in both lattices (10 cm-1) due to the size differences between Mn4+ and Ti3+ compared to Cr3+ and Ti4+.

-1 Figure 5.4: Emission of 0.1% Mn:Sr2TiO4 excited at 20492 cm . The spectrum was collected at 20.5 K with spectral resolution of 2 cm-1.

The temperature dependence of the ZPL associated with a temperature dependence on the dielectric permittivity was measured and a shift of -23.4 cm-1 when the temperature was increased to 77.2 K shown in Figure 5.4. The shift in energy between 20 and 77 K is much larger than the

-1 37,38 value measured on single crystalline Mn:SrTiO3 (-9.4 cm ), implying that the temperature dependence of the dielectric constant is larger in Sr2TiO4. At temperatures higher than 77 K, the

ZPL and sidebands broaden to the extent where the features become indistinguishable. A hot band

97 grows in at ≈14300 cm-1 as the temperature increases, contributing to the obfuscation of the ZPL above 77 K. The growth of this band as well as the broadening and disambiguation of the phonon sidebands with increasing temperature can be seen in the supplementary information (Figure 5.16).

This trend is independent of excitation energy and the decrease in the intensity of the phonon modes and the increase of hot band emission can be seen when excited at 25641 and 18518 cm-1

(see supplementary information, Figure 5.17).

-1 Figure 5.5: Variable temperature emission spectra of 0.1% Mn:Sr2TiO4 excited at 20492 cm and measured at spectral resolution of 4 cm-1. Inset is a plot of the energy of the ZPL at different temperatures.

4+ In order to confirm Mn dopant incorporation in Sr2TiO4, room temperature EPR spectroscopy was utilized. Figure 5.6 shows the EPR spectrum of 0.1% Mn:Sr2TiO4 at room temperature, which shows 7 distinct sextets of peaks and an additional set of peaks that doesn’t show a distinct sextet (see supplementary information for zoomed in regions of EPR spectrum).

The position of the furthest set of paired peaks is consistent to peaks observed single-crystal EPR

4+ 3+ of reduced Mn :SrTiO3 corresponding to peaks that had been previously assigned as Mn coupled to oxygen vacancies when the field was parallel to the [100] face, giving rise to an axial

23 4+ signal. The central sextet at g=1.994 resembles Mn in SrTiO3 based on the similarity of g value

98 and the nuclear hyperfine coupling constant, 71.35 x 10-4 cm-1.36 The change in the EPR spectrum

4+ 3+ between Mn in Sr2TiO4 and SrTiO3 should follow similar behavior to that of Cr in Sr2TiO4.

3+ The changes in the EPR spectrum of Cr in Sr2TiO4 compared to SrTiO3 were the result of the addition of an axial component of the zero field splitting as well as the some slight g-anisotropy.39

These changes are the result of the axial compression that occurs at the B-site of Sr2TiO4. The

4+ same changes to Mn in SrTiO3 would result in 7 sextets of peaks, similar to what is observed in

3+ 0.1% Mn:Sr2TiO4, but with different values. Based on the results of Cr simulations, the set of peaks between 30 and 100 mT would be unaccounted for.

Figure 5.6. Room temperature X-band EPR spectrum of 0.1% Mn:Sr2TiO4.

Using EasySpin, simulations were used by trial and error to approximate the spin-

4+ Hamiltonian parameters for Mn in Sr2TiO4 as described in Equation 1. Figure 5.7 shows the

4+ experimental spectrum for Mn:Sr2TiO4 and the simulated spectra of Mn in the Oh site and in the axially compressed D4h site. As can be seen from this figure, the combination of the two simulated spectra accurately represent the peak location and relative intensity for the experimental section.

The fine structure of the peaks are also matched by the simulated spectra as can be seen in the supplementary information (Figure 5.15). Since all peaks are explained within the experimental

99 spectrum, including the peaks between 30 and 100 mT, by the combination of the two Mn4+

2+ simulated spectra, there is no Mn doped into Sr2TiO4 as prepared.

Figure 5.7. Comparison of experimental EPR spectrum of 0.1% Mn:Sr2TiO4 (black) and simulations of spectra representing Mn4+ in the axially compressed B-site (red) and Mn4+ in the B- site of SrTiO3 (blue).

Table 5.1 Spin-Hamiltonian parameters of Mn in SrTiO3 and Sr2TiO4 Valence Site Symmetry D g∥ g⊥ A Ref. 2+ Oh - 2.005 2.005 78.16 24

4+ Oh -0.65 1.994 1.994 69.81 36, this work

# 4+ D4h 1112 1.9945 1.9850 71.31 This work All values for D and A are given in 10-4 cm-1. No value of E was reported and was 0 in the simulation. For the simulations, spectral broadening was achieved by using D- and E-strain with values of 16.68 and 13.34  10-4 cm-1, respectively. H-Strain was also used with values of 10  10-4 cm-1 in the X, Y, and Z direction. #: The sign of D was not experimentally determined, but if D is positive, g∥ > g⊥, if D is negative g∥ < g⊥.

The spin-Hamiltonian parameters for Mn (II and IV) in the B-site of SrTiO3 from previous reports and the values for Mn:Sr2TiO4 from the simulations are shown in table 5.1. The significant change in the parameters is the large increase in the D component of the ZFS due to the axial

100

4+ compression of the B-site. Unlike Cr:SrTiO3, there is a small ZFS in Mn :SrTiO3 due to the discrepancy in the ion size between Mn4+ and Ti4+ (0.51 Å vs. 0.62 Å).36 This also explains the

4+ -4 -1 -4 large value for D in Mn :Sr2TiO4 (1112 x 10 cm ) compared to that of Cr:Sr2TiO4 (207 x 10 cm-1).39

5.3.4 Electronic properties of Mn-doped SrTiO3 and Sr2TiO4. The diffuse reflectance spectra of 1% Mn-doped SrTiO3 Sr2TiO4 is shown in figure 5.8 with the band edge being extended to

-1 approximately 21000 cm after doping. The Mn:SrTiO3 spectrum shows broad absorption between 28,000 cm-1 and 16,000 cm-1, with a slightly pronounced peak centered at 18,200 cm-1.

This peak is also present in the spectrum for 1% Mn:Sr2TiO4, although more pronounced. There is also the presence of a pronounced band at 25,200 cm-1 in the broad near edge absorption. Single

-1 4 4 crystal Mn:SrTiO3 showed a peak at 17,985 cm which was attributed to the A2→ T2 transition

Figure 5.8. Diffuse reflectance spectra of 1% Mn:SrTiO3 (A) and Sr2TiO4 (B).

101 of Mn4+.25 The peaks in both samples in the 18,200 cm-1 region could have contributions from this

37 feature, as well as charge transfer transitions that have been observed in single crystal Cr:SrTiO3

40 -1 and n-type SrTiO3 that was annealed under high vacuum at 18,500 cm . The energy of this feature shifts to higher energy in 0.1% Mn:Sr2TiO4, suggesting greater involvement of charge transfer transition and less pronounced spin allowed d-d transition absorption of Mn4+ (see supplementary information). The excitation scan of the room temperature emission of 0.1%

4 4 Mn:Sr2TiO4 confirms the contribution of the A2→ T2 transition in the absorption feature.

5.3.5 Changes in Mn:SrTiO3 and Sr2TiO4 resulting from chemical reductions. The ability to control the valence state of Mn-dopant ions in addition to the defect correlation to the materials.

Through the use of EPR and diffuse reflectance spectroscopies, these two can be better understood.

The EPR spectra of 0.1% Mn:SrTiO3 after chemical reductions at different temperatures are shown

16 in figure 5.9. Unlike the trend observed in Cr:SrTiO3, the changes in Mn:SrTiO3 as observed by

EPR spectroscopy were less pronounced at lower reduction temperatures. There is an initial increase in the Mn4+ EPR signal after reduction at 400 °C before a decrease at 425 °C, lower than the as-prepared sample. In the Mn-doped samples, however, quantitative EPR couldn’t be run on samples reduced at 350 °C or above. The reduced Q-factor prevented the ability of the standard

EPR tubes to be utilized. The lower pathlength of capillary tubes allowed for these samples to be run, but at a lower signal intensity.

The EPR spectrum of 0.1% Mn:SrTiO3 after reduction at 350 °C is shown in panel B of figure 5.9. The presence of Mn4+ can still be observed, but a new sextet of peaks is also visible.

The new set of peaks has a g-value of 2.002 and an A-value of 78.96  10-4 cm-1, consistent with

Mn2+ in a Ti4+ site.24 At reduction temperatures at and above 375 °C a broad signal appears in the

102

Figure 5.9. Quantitative EPR spectra of 0.1% Mn:SrTiO3, as prepared and reduced at 300 and 325°C (A). The EPR spectrum of 0.1% Mn:SrTiO3 after reduction at 350 °C, run in a capillary tube. spectrum, centered around g=1.97 (see supplementary information). The asymmetric shape of the signal as well as the g-value, and the NIR absorption in diffuse reflectance (vide infra) lead to the assignment of Ti3+.41–43 The Mn2+ signal is still visible in this sample, but no Mn4+ peaks are visible in the spectrum. This signal becomes the only visible signal in the sample after reduction at 400

°C (see supplementary information).

Quantitative EPR studies on 0.1% Mn:Sr2TiO4 is shown in figure 5.10. There is a slight increase in the intensity of the Mn4+ signal at low temperature reductions (300 and 325 °C) before a decrease in the signal intensity when reduced at 350 °C. As was the case in Cr:SrTiO3 and

Cr:Sr2TiO4 (chapters 3 and 4), the temperature at which the reduction of the Q-factor occurs preventing quantitative EPR is higher in Sr2TiO4 than SrTiO3. In both Mn-doped samples, this

103

Figure 5.10. Quantitative EPR spectra of 0.1% Mn:Sr2TiO4 as prepared and after NaBH4 reduction between 300 and 350 °C. temperature was lower than in their Cr-doped counterparts. EPR of the NaBH4 reductions at 375 and 400 °C of Mn:Sr2TiO4 (figure 5.11 and supplementary information) show a new set of peaks centered at 119.4 mT (gapparent=5.75). The primary center set of peaks has a g-value of 2.002 and an A-value of 79.03  10-4 cm-1. The approximate hyperfine coupling constant of the set of peaks at 119.4 mT (assuming the g-value is approximately 2) is 74.46  10-4 cm-1, approximately halfway

2+ 4+ 4+ between Mn and Mn in SrTiO3 and higher than Mn in Sr2TiO4. Due to the hyperfine coupling constant, as well as similarity to previous reports,22,23 this new set of peaks is assumed to be the result of Mn3+ coupled to a doubly ionized oxygen vacancy. There is also an additional set of peaks just to higher field than the large set of peaks that could be the result of another Mn3+ species or

Mn2+, but is difficult to discern due to the low intensity.

104

Figure 5.11 EPR spectrum of 0.1% Mn:Sr2TiO4 after NaBH4 reduction at 400 °C.

The use of diffuse reflectance to see the shift of the absorption spectra with reduction temperature for both lattices is shown in figure 5.12. The spectra of SrTiO3 change little from the as-prepared sample until the drastic decrease in intensity of the near-edge absorption after being reduced at 350 °C. This shift occurs over at the same temperatures as the appearance of Mn2+ in the sample. Sr2TiO4, undergoes a gradual decrease in the decrease in the intensity of both defined peaks gradually with increasing reduction temperature up to 350 °C. These change in Sr2TiO4 are

3+ consistent with the changes in Cr :Sr2TiO4 (chapter 4), which corresponded to a shift in the VO charge states as the Fermi level of the material increases as described in chapter 3. The minimal shift in the absorption of SrTiO3 after reductions at 300 and 325 °C is indicative of fewer VO defects incorporated in the as-prepared sample.

In samples reduced at or above 350 °C for SrTiO3 and 375 °C for Sr2TiO4, new absorption peaks from the band edge to the NIR form and increase in intensity with increasing reduction temperature. This is indicative of Ti3+ formation and correlates to a darkening of color to gray and eventually black. More defined transitions are visible than what was evident in Cr-doped SrTiO3 and Sr2TiO4 and correspond to charge transfer transitions between defect states and the conduction

105 band or in the case of the lower energy transition (about 16500 cm-1), intraband transitions conduction band.

Figure 5.12. Diffuse reflectance spectra of 1% Mn-doped SrTiO3 (A) and Sr2TiO4 (B) for the as- prepared samples and after being reduced at temperatures between 300 and 400 °C.

5.4 Conclusions

The properties of Mn-doped SrTiO3 and Sr2TiO4 as prepared by a solid state reaction was studied and it was shown via EPR and emission spectroscopies that Mn4+ is the primary valence state of the as-prepared samples. Using simulations, the spin Hamiltonian parameters for Mn4+ in the axially compressed B-site of Sr2TiO4 were determined for the first time, with g∥=1.9945,

-4 -1 -4 -1 -1 g⊥=1.9850, D=1112  10 cm , and A=71.31  10 cm . There was about a 70 cm shift in the

2 4 4+ emission line of the spin-forbidden E→ A2 between Mn in SrTiO3 and Sr2TiO4.

106

The effects of low-temperature chemical reductions on Mn in each lattice were also studied.

EPR spectroscopy showed evidence of reduction from Mn4+ to Mn2+ at temperatures as low as 325

°C, but become increasingly more evident as the reduction temperature is raised to 375 °C. While the Mn2+ signal was distinctly visible, most of the Mn is in the 3+ valence state given the low relative intensity for a S=5/2 system. The changes in Mn:Sr2TiO4 are less evident in the sense that

Mn2+ is not clearly visible at any reduction temperature, but a new set of peaks attributed to Mn3+-

VO is visible in reduction temperatures above 375 °C. The shifts in the absorption of the materials with different reduction temperatures are similar to those of Cr-doped SrTiO3 and Sr2TiO4 (see chapters 3 and 4), but the higher temperature reductions of Mn:Sr2TiO4 reveal a large peak that is present at 16500 cm-1 and attributed to intraband transitions within the conduction band.

5.5 Supplementary Information

Figure 5.13 Emission of 0.1% Mn:Sr2TiO4 at 20 K, excited at 420 nm.

107

Figure 5.14 Room temperature EPR spectrum of 0.1% Mn:Sr2TiO4, zoomed into different regions.

Figure 5.15 Comparison of simulated spectra to experimental spectrum of 0.1% Mn:Sr2TiO4 in different regions of magnetic field.

108

Figure 5.16 Variable temperature emission of 0.1% Mn:Sr2TiO4 in the temperature range of 19.8- 201.5 K.

-1 -1 Figure 5.17 Emission of 0.1% Mn:SrTiO3 excited at 25641 cm (A) and 18518 cm (B). Temperature range was 20K to 250K.

109

Figure 5.18 Electronic studies of 0.1% Mn:Sr2TiO4. Black trace is diffuse reflectance spectroscopy, red is an excitation scan of the Mn4+ emission at 13889 cm-1, blue is room temperature fluorescence excited at 18519 cm-1.

Figure 5.19 Room temperature EPR spectra of 0.1% Mn:SrTiO3 after chemical reduction at 375 (A) and 400 °C (B). Samples were analyzed in capillary tubes.

5.6 References

1. Wrighton, M. S. et al. Strontium Titanate Photoelectrodes. Efficient Photoassisted Electrolysis Of Water At Zero Applied Potential. J. Am. Chem. Soc. 98, 2774–2779 (1976).

2. Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–78 (2009).

3. Iwase, A., Ng, Y. H., Ishiguro, Y., Kudo, A. & Amal, R. Reduced graphene oxide as a solid- state electron mediator in Z-scheme photocatalytic water splitting under visible light. J. Am.

110

Chem. Soc. 133, 11054–7 (2011).

4. Kato, H., Asakura, K. & Kudo, A. Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J. Am. Chem. Soc. 125, 3082–9 (2003).

5. Sasaki, Y., Nemoto, H., Saito, K. & Kudo, A. Solar Water Splitting Using Powdered Photocatalysts Driven by Z-Schematic Interparticle Electron Transfer without an Electron Mediator. J. Phys. Chem. C 113, 17536–17542 (2009).

6. Yu, H. et al. Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production. J. Mater. Chem. 21, 11347 (2011).

7. Reunchan, P. et al. Theoretical Design Of Highly Active SrTiO3-based Photocatalysts From Codoping Scheme Toward Solar Energy Utilization For Hydrogen Production. J. Mater. Chem. A (2013).

8. Ouyang, S. et al. Surface-Alkalinization-Induced Enhancement of Photocatalytic H2 Evolution over SrTiO3-Based Photocatalysts. J. Am. Chem. Soc. 134, 1974–7 (2012).

3+ 9. Li, H. et al. Roles of Cr doping and oxygen vacancies in SrTiO3 photocatalysts with high visible light activity for NO removal. J. Catal. 297, 65–69 (2013).

10. Badalyan, A. G. et al. EPR study of charge compensation of chromium centers in the strontium titanate crystal. Phys. Solid State 55, 1454–1458 (2013).

11. Kang, H. W. & Park, S. Bin. H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor. Int. J. Hydrogen Energy 36, 9496–9504 (2011).

12. Kato, H. & Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106, 5029–5034 (2002).

13. Chen, X., Liu, L., Yu, P. Y. & Mao, S. S. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 331, 746–50 (2011).

14. Janousch, M. et al. Role of Oxygen Vacancies in Cr-Doped SrTiO3 for Resistance-Change Memory. Adv. Mater. 19, 2232–2235 (2007).

15. Alvarado, S. F., La Mattina, F. & Bednorz, J. G. Electroluminescence in SrTiO3:Cr single- crystal nonvolatile memory cells. Appl. Phys. A Mater. Sci. Process. 89, 85–89 (2007).

16. Lehuta, K. A. & Kittilstved, K. R. Reversible control of the chromium valence in chemically reduced Cr-doped SrTiO3 bulk powders. Dalt. Trans. 45, 10034–10041 (2016).

111

17. Konta, R., Ishii, T., Kato, H. & Kudo, A. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation. J. Phys. Chem. B 108, 8992–8995 (2004).

18. Sun, X. & Lin, J. Synergetic effects of thermal and photo-catalysis in purification of dye water over SrTi1-xMnxO3 solid solutions. J. Phys. Chem. C 113, 4970–4975 (2009).

19. Ristig, S., Cibura, N. & Strunk, J. Manganese Oxides in Heterogeneous (Photo)Catalysis: Possibilities and Challenges. Green 5, 23–41 (2015).

20. Serway, R. A., Berlinger, W., Muller, K. A. & Collins, R. W. Electron Paramagnetic Resonance of Three Manganese Centers in Reduced SrTiO3. Phys. Rev. B 16, 4761–4768 (1977). 21. Azzoni, C. B. et al. Magnetic evidence of different environments of manganese ions in Mn- substituted strontium titanate. Solid State Commun. 114, 617–622 (2000).

22. Azamat, D. V. et al. High-frequency electron paramagnetic resonance investigation of Mn3+ centers in SrTiO3. J. Phys. Chem. Solids 73, 822–826 (2012).

23. Azamat, D. V. et al. Electron paramagnetic resonance studies of manganese centers in 3+ 4+ SrTiO3: Non-Kramers Mn ions and spin-spin coupled Mn dimers. J. Appl. Phys. 111, 104119 (2012).

24. Choudhury, D., Pal, B., Sharma, A., Bhat, S. V & Sarma, D. D. Magnetization in electron- and Mn-doped SrTiO3. Sci. Rep. 3, 1433 (2013).

25. Badalyan, a G. et al. Impurity centers and host microstructure in weakly doped SrTiO3:Mn crystals: new findings. J. Phys. Conf. Ser. 93, 12012 (2007).

26. Laguta, V. V et al. Electron spin resonance investigation of Mn2+ ions and their dynamics in manganese doped SrTiO3. Phys. Rev. B 76, 54104 (2007).

27. Rath, M. K., Ahn, B.-G., Choi, B.-H., Ji, M.-J. & Lee, K.-T. Effects of manganese substitution at the B-site of lanthanum-rich strontium titanate anodes on fuel cell performance and catalytic activity. Ceram. Int. 39, 6343–6353 (2013).

28. Ruddlesden, S. N. & Popper, P. New compounds of the K2NIF4 type. Acta Crystallogr. 10, 538–539 (1957).

29. Sun, X. et al. Photocatalytic Hydrogen Production over Chromium Doped Layered Perovskite Sr2TiO4. Inorg. Chem. 150714142222000 (2015).

30. Jia, Y. et al. Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation. J. Mater. Chem. A 1, 7905 (2013).

31. Yi, Z. G., Iwai, H. & Ye, J. H. Photochromism and visible light induced H2 generation in Sr2TiO4:Cr complexes. Appl. Phys. Lett. 96, (2010).

112

32. Miseki, Y., Kato, H. & Kudo, A. Water splitting into H2 and O2 over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure. Energy Environ. Sci. 2, 306 (2009).

33. Eaton, G. R., Eaton, S. S., Barr, D. P. & Weber, R. T. Quantitative EPR. (Springer, 2010).

34. Mitchell, R. H., Chakhmouradian, A. R. & Woodward, P. M. Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr(1-2x)Na(x)La(x))TiO3. Phys. Chem. Miner. 27, 583–589 (2000).

35. Lukaszewicz, K. Rocz. Chem. 33, 239 (1959).

36. Muller, K. A. Electron Paramagnetic Resonance Of Manganese IV In SrTiO3. Phys. Rev. Lett. 2, 341–343 (1959).

37. Stokowski, S. E. & Schawlow, A. L. Spectroscopic Studies of SrTiO3 Using Impurity-Ion Probes. Phys. Rev. 178, 457–464 (1969).

38. Bryknar, Z., Trepakov, V., Potucek, Z. & Jastrabik, L. Luminescence spectra of 4+ SrTiO3:Mn . J. Lumin. 87–89, 605–607 (2000).

39. Lehuta, K. A. & Kittilstved, K. R. Speciation of Cr(iii) in intermediate phases during the sol–gel processing of Cr-doped SrTiO3 powders. J. Mater. Chem. A 2, 6138 (2014).

40. Baer, W. S. Free-carrier absorption in reduced SrTiO3. Phys. Rev. 144, 734–738 (1966).

41. Zuo, F. et al. Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light. J. Am. Chem. Soc. 132, 11856–11857 (2010).

42. Khomenko, V. M., Langer, K., Rager, H. & Fett, a. Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile. Phys. Chem. Miner. 25, 338–346 (1998).

3+ 4+ 43. Ji, Y. et al. Surface Ti /Ti Redox Shuttle Enhancing Photocatalytic H2 Production in Ultrathin TiO 2 Nanosheets/ CdSe Quantum Dots. J. Phys. Chem. C (2015).

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CHAPTER 6

CONCLUSIONS AND FUTURE DIRECTIONS

The incorporation of dopant ions and defects in oxide perovskites such as SrTiO3 and the

Ruddlesden-Popper phase Sr2TiO4 has been of interest in the scientific community for years due to the potential control of the materials properties in order to tailor it for specific applications. This work has focused on the use of systematic approaches to better understand the incorporation of dopant ions in SrTiO3 based materials and the ability to further control the dopant ions and the corresponding defects within the material.

The incorporation of Cr-dopant ions in SrTiO3 using a traditional sol-gel method was studied. Through the use of EPR spectroscopy, the Cr3+ dopant ions were tracked during the formation of SrTiO3 via this technique. The dopant ions were distributed throughout not only in

SrTiO3 as it formed, but also the intermediate phases that were present during formation. This

3+ included the incorporation of Cr in Sr2TiO4, the SrO rich, Ruddlesden-Popper phase, allowing for an approximation of the spin-Hamiltonian parameters for the first time. The use of the dopant specific probes also allowed the detection of secondary phases that were present in the sample, but in concentrations that were below the detection limit of XRD. Phase pure Cr:SrTiO3 was attainable through the use of this method after being calcined at 800 °C and annealing at 1050 °C for 4 hours as was confirmed by XRD, EPR spectroscopy, and variable temperature emission spectroscopy.

Through the use of a solid-state synthetic approach, nearly phase pure Cr:Sr2TiO4 was synthesized. This allowed for the confirmation of the previously predicted spin-Hamiltonian

−4 −1 −4 −1 2 4 parameters: D = −201 × 10 cm , g|| = 1.9803, g⊥ = 1.9793, A = −16.2 × 10 cm . The E→ A2

3+ 3+ emission of Cr in Sr2TiO4 showed a similar temperature dependence as Cr in Sr2TiO4, but had a slight difference in energy due to the axially compression of the site.

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The use of an altered NaBH4 chemical reduction at relatively low temperatures under static

3+ vacuums allowed for further control of the Cr valence state. In both Sr2TiO4 and SrTiO3, Cr concentrations increased with increasing reduction temperature prior to the formation of Ti3+ within the lattice. Diffuse reflectance spectroscopy also helped provide information pertaining to the defect chemistry that occurred as the dopant ions were reduced. An apparent increase in the ionization of the oxygen vacancies was observed at lower temperatures, prior the formation of new

3+ VO defects as the reduction temperature increased to the point of Ti formation. This process was shown to be reversible at fairly similar temperatures prior to Ti3+ formation, but required significantly higher temperatures to reverse the process afterwards. This is further confirmation of the formation of new VO defects, except the new defects form in the lattice of the material, which is less readily re-oxidized.

The incorporation of Cr into the lattice and ability to control the valence state have previously been shown to be critical for visible-light photocatalysis. The study of the dopant ion incorporation and the corresponding VO defect incorporation in the Ruddlesden-Popper phase

Sr2TiO4 shows that the visible-light photocatalysis of the materials should behave similarly. The effect of the altered surface chemistry and the layering of the material will be the only significant difference between the two materials. Since the defect chemistry behaves similarly during the

3+ controlled NaBH4 reduction, Cr:Sr2TiO4 with increased Cr and surface VO defects should be a more effective visible-light photocatalyst than Cr:SrTiO3.

Mn-doped SrTiO3 and Sr2TiO4 was also studied to look at a dopant ion which can be isoelectronic to the beneficial Cr3+ as incorporated, without the need for charge compensation. In both lattices, Mn doped primarily as Mn4+ as prepared by the solid state method. The spin-

4+ Hamiltonian parameters for Mn in Sr2TiO4 were also determined for the first time, with

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-4 -1 -4 -1 g∥=1.9945, g⊥=1.9850, D=1112  10 cm , and A=71.31  10 cm . Chemical reductions on Mn- doped SrTiO3-based powders required higher temperatures to observe a significant change in the

Mn EPR spectra, but did show the reduction of Mn4+ to Mn2+, with a majority of the dopant ions assumed to be Mn3+ after reduction. The diffuse reflectance spectra showed more significant peaks in the NIR region when Ti3+ formed which is attributed to the intraband transitions as well as the

Mn2+/Ti3+ associated transitions.

While Mn:SrTiO3 has been primarily studied for its magnetic properties, it hasn’t been explored much for the purposes of visible-light photocatalysis. The incorporation of Mn4+ in both

3+ 4+ SrTiO3 and Sr2TiO4 should mean that it is a viable visible-light photocatalyst since Cr and Mn are iso-electronic. The lack of need for charge compensation with Mn4+ doping means more phase pure metal oxide lattices can be incorporated, which should be advantageous in nanomaterials. The advantages of surface VO defects and their effects on the surface adsorption process would not persist in the Mn-doped samples however.

While the study of bulk powders has provided insight towards the doping of both SrTiO3 and Sr2TiO4, as well as the corresponding defects in the lattice, this model system is expected to have some variation from nanomaterials which would have some altered properties, especially if the nanomaterials are colloidal. Nanomaterials of doped Ruddlesden-Popper phases have not been attained with reliability, and the synthesis and characterization of these materials would prove insightful towards the surface chemistry as it relates to the photocatalytic active sites. If these materials were made colloidal, the potential for not only studying the dopant-defect interactions after reductions on thin films would be possible, but also more quantitative absorption studies would be attainable.

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Sr2TiO4 and SrTiO3 serve as the two ends of a spectrum structurally in the sense of a mono- layered material and non-layered material. The synthesis of transition metal doped Sr3Ti2O7 and

Sr4Ti3O10 (n=2 and 3) would provide further understanding on the charge transport properties and alterations in charge separation for photocatalysis. These materials are considered fairly unstable, but through the use of altered synthetic approaches and changing the reaction parameters, could be achievable.

Another logical step in better understanding the potential applicability of Ruddlesden-

Popper phases of SrTiO3 would be to look to co-doping. The use of an A-site and B-site dopant could lead to further charge separation or intensify the charge carrier concentration as the charge carriers are not expected to cross over the different layers of the material in Ruddlesden-Popper phases. The use of co-doping as charge compensation in as-prepared materials, i.e. a 5+ dopant and a 3+ dopant in the Ti4+ site, should increase the concentration of both dopant ions being present in the expected valence state and would also reduce the amount of defects in the lattice in the as- prepared samples.

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BIBLIOGRAPHY

Abdul Rashid, S., Othman, S. H., Mohd Ghazi, T. I. & Abdullah, N. Effect of postdeposition heat treatment on the crystallinity, size, and photocatalytic activity of TiO2 nanoparticles produced via chemical vapour deposition. J. Nanomater. 2010, (2010).

Adamsky, H., Schonherr, T. & Atanasov, M. in Comprehensive Coordination Chemistry II 661– 664 (2003).

Alvarado, S. F., La Mattina, F. & Bednorz, J. G. Electroluminescence in SrTiO3:Cr single-crystal nonvolatile memory cells. Appl. Phys. A Mater. Sci. Process. 89, 85–89 (2007).

Amorelli, A., Evans, J. C. & Rowlands, C. C. Electron paramagnetic resonance study of transition- metal-ion impregnated brookite titanium dioxide powders. J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases 85, 4031 (1989).

Anpo, M., Shima, T., Kodama, S. & Kubokawa, Y. Photocatalytic Hydrogenation of CH3CCH with H2O on Small-Particle TiO2: Size Quantization Effects and Reaction Intermediates. J. Phys. Chem. 91, 4305–4310 (1987).

Azamat, D. V. et al. Electron paramagnetic resonance studies of manganese centers in SrTiO3: Non-Kramers Mn3+ ions and spin-spin coupled Mn4+ dimers. J. Appl. Phys. 111, 104119 (2012).

Azamat, D. V. et al. High-frequency electron paramagnetic resonance investigation of Mn3+ centers in SrTiO3. J. Phys. Chem. Solids 73, 822–826 (2012).

3+ Azamat, D. V. et al. Pulse-electron paramagnetic resonance of Cr centers in SrTiO3. J. Appl. Phys. 113, 174106 (2013).

Azzoni, C. B. et al. Magnetic evidence of different environments of manganese ions in Mn- substituted strontium titanate. Solid State Commun. 114, 617–622 (2000).

Badalyan, a G. et al. Impurity centers and host microstructure in weakly doped SrTiO3:Mn crystals: new findings. J. Phys. Conf. Ser. 93, 12012 (2007).

Badalyan, a. G. et al. EPR study of charge compensation of chromium centers in the strontium titanate crystal. Phys. Solid State 55, 1454–1458 (2013).

Bae, S. W., Borse, P. H. & Lee, J. S. Dopant dependent band gap tailoring of hydrothermally prepared cubic SrTixM1−xO3 (M=Ru,Rh,Ir,Pt,Pd) nanoparticles as visible light photocatalysts. Appl. Phys. Lett. 92, 104107 (2008).

Baer, W. S. Free-carrier absorption in reduced SrTiO3. Phys. Rev. 144, 734–738 (1966).

Bai, X. et al. Performance enhancement of ZnO photocatalyst via synergic effect of surface oxygen defect and graphene hybridization. Langmuir 29, 3097–3105 (2013).

118

Bain, G. A. & Berry, J. F. Diamagnetic Corrections and Pascal’s Constants. J. Chem. Educ. 85, 532 (2008).

Beier, C. W., Cuevas, M. a. & Brutchey, R. L. Low-temperature synthesis of solid-solution BaxSr1−xTiO3 nanocrystals. J. Mater. Chem. 20, 5074 (2010).

Berbenni, V., Marini, A. & Bruni, G. Effect of Mechanical Activation on The Preparation of SrTiO3 and Sr2TiO4 Ceramics From The Solid State System SrCO3-TiO2. J. Alloys Compd. 329, 230–238 (2001).

Bhaskar, S., Allgeyer, D. & Smythe, J. a. Depth profiling of dielectric SrTiO3 thin films by angle- resolved x-ray photoelectron spectroscopy. Appl. Phys. Lett. 89, 254103 (2006).

Bhattacharya, A. & May, S. J. Magnetic Oxide Heterostructures. Annu. Rev. Mater. Res. 44, 65– 90 (2014).

Biasi, R. S. De & Janeiro, R. De. Influence of Chromium Concentration on the Electron Magnetic 3+ Resonance Linewidth of Cr in SrTiO3. Mater. Res. 15, 473–476 (2012).

Bryknar, Z., Trepakov, V., Potucek, Z. & Jastrabik, L. Luminescence spectra of SrTiO:Mn4+. J. Lumin. 87–89, 605–607 (2000).

Bykov, I. et al. Intrinsic and impurity defects in chromium-doped SrTiO3 nanopowders: EPR and NMR study. Phys. Status Solidi 250, 821–824 (2013).

Chen, H., Huang, C., Wu, J. C. S. & Lin, S. Theoretical Investigation of the Metal-Doped SrTiO3 Photocatalysts for Water Splitting. J. Phys. Chem. C 116, 7897–7903 (2012).

Choudhury, D., Pal, B., Sharma, A., Bhat, S. V & Sarma, D. D. Magnetization in electron- and Mn-doped SrTiO3. Sci. Rep. 3, 1433 (2013).

Cohn, A. W., Kittilstved, K. R. & Gamelin, D. R. Tuning the potentials of ‘extra’ electrons in colloidal n-type ZnO nanocrystals via Mg2+ substitution. J. Am. Chem. Soc. 134, 7937–7943 (2012).

Di Valentin, C., Pacchioni, G., Selloni, A., Livraghi, S. & Giamello, E. Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations. J. Phys. Chem. B 109, 11414–11419 (2005).

Druce, J., Ishihara, T. & Kilner, J. Surface composition of perovskite-type materials studied by Low Energy Ion Scattering (LEIS). Solid State Ionics 262, 893–896 (2014).

Eaton, G. R., Eaton, S. S., Barr, D. P. & Weber, R. T. Quantitative EPR. (Springer, 2010).

Egerton, T. a et al. An EPR study of diffusion of chromium into rutile. Phys. Chem. Chem. Phys. 2, 3275–3281 (2000).

119

Eng, H. W., Barnes, P. W., Auer, B. M. & Woodward, P. M. Investigations of the electronic structure of d0 transition metal oxides belonging to the perovskite family. J. Solid State Chem. 175, 94–109 (2003).

Erdem, E., Böttcher, R., Gläsel, H. J. & Hartmann, E. Size effects in chromium-doped PbTiO3 nanopowders observed by multi-frequency EPR. Magn. Reson. Chem. 43, 174–182 (2005).

Etacheri, V., Roshan, R. & Kumar, V. Mg-doped ZnO nanoparticles for efficient sunlight-driven photocatalysis. ACS Appl. Mater. Interfaces 4, 2717–2725 (2012).

Fujishima, A. & Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 238, 37–38 (1972).

Galinetto, P. et al. Magnetic and Structural Studies in Co- and Mn-implanted SrTiO3 Single Crystals. Ferroelectrics 368, 120–130 (2008).

Garcia, M. A. et al. Sources of experimental errors in the observation of nanoscale magnetism. J. Appl. Phys. 105, (2009).

Gerlach, W. Die Gitterstruktur der Erdalkalioxyde. Zeitschrift fur Phys. 9, 184–192 (1922).

Gerritsen, H. J., Harrison, S. E., Lewis, H. R. & Wittke, J. P. Fine Structure, Hyperfine Structure, 3+ and Relaxation Times of Cr in TiO2 (Rutile). Phys. Rev. Lett. 2, 153–155 (1959).

Glasbeek, M., de Jong, H. J. & Koopmans, W. E. Nonlinear Jahn-Teller coupling and local 5+ dynamics of SrTiO3:Cr near the structural phase transition point. Chem. Phys. Lett. 66, 203–206 (1979).

Gole, J. L., Stout, J. D., Burda, C., Lou, Y. & Chen, X. Highly Efficient Formation of Visible Light Tunable TiO2-xNx Photocatalysts and Their Transformation at the Nanoscale. J. Phys. Chem. B 108, 1230–1240 (2004).

Greenlief, C. M. & White, J. M. An XPS Investigation of TiO2 Thin Films on Polycrystalline Pt. J. Phys. Chem. 89, 5025–5028 (1985).

Grunin, V. S., Daytran, G. D., Ioffe, V. A. & Latrina, I. B. Fiz. Tverd. Tela 17, 2174–2176 (1975).

Guo, Y., Liu, H., Yu, D. & Liu, J.-M. Ferroelectricity and superparamagnetism in Sr/Ti nonstoichiometric SrTiO3. Phys. Rev. B 85, 104108 (2012).

Haeni, J. H. et al. Epitaxial growth of the first five members of the Srn+1TinO3n+1 Ruddlesden– Popper homologous series. Appl. Phys. Lett. 78, 3292 (2001).

Hanson, G. R. et al. XSophe-Sophe-XeprView®. A computer simulation software suite (v. 1.1.3) for the analysis of continuous wave EPR spectra. J. Inorg. Biochem. 98, 903–916 (2004).

Hao, X., Wang, Z., Schmid, M., Diebold, U. & Franchini, C. Coexistence of trapped and free excess electrons in SrTiO3. Phys. Rev. B - Condens. Matter Mater. Phys. 91, 1–6 (2015).

120

Hara, S. et al. Hydrogen and Oxygen Evolution Photocatalysts Synthesized from Strontium Titanate by Controlled Doping and Their Performance in Two-Step Overall Water Splitting under Visible Light. J. Phys. Chem. C 116, 17458–17463 (2012).

Inagaki, M., Nonaka, R., Tryba, B. & Morawski, A. W. Dependence Of Photocatalytic Activity Of Anatase Powders On Their Crystallinity. Chemosphere 64, 437–445 (2006).

Inaguma, Y., Nagasawa, D. & Katsumata, T. Photoluminescence of Praseodymium-Doped Srn+1TinO3n+1 ( n =1, 2, ∞). Jpn. J. Appl. Phys. 44, 761–764 (2005).

+ 2+ Irie, H., Maruyama, Y. & Hashimoto, K. Ag and Pb Doped SrTiO3 Photocatalysts . A Correlation Between Band Structure and Photocatalytic Activity. J. Phys. Chem. C 111, 1847–1852 (2007).

Ishii, T., Kato, H. & Kudo, A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J. Photochem. Photobiol. A Chem. 163, 181–186 (2004).

Iwase, A., Ng, Y. H., Ishiguro, Y., Kudo, A. & Amal, R. Reduced graphene oxide as a solid-state electron mediator in Z-scheme photocatalytic water splitting under visible light. J. Am. Chem. Soc. 133, 11054–7 (2011).

Jacob, K. T. & Rajitha, G. Thermodynamic properties of strontium titanates: Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, and SrTiO3. J. Chem. Thermodyn. 43, 51–57 (2011).

Janousch, M. et al. Role of Oxygen Vacancies in Cr-Doped SrTiO3 for Resistance-Change Memory. Adv. Mater. 19, 2232–2235 (2007).

Jeschke, H. O., Shen, J. & Valenti, R. Localized versus itinerant states created by multiple oxygen vacancies in SrTiO3. New J. Phys. 17, 6 (2014).

Jeyalakshmi, V. et al. Visible light driven reduction of carbon dioxide with water on modified Sr3Ti2O7 catalysts. RSC Adv. 5, 5958–5966 (2015).

3+ 4+ Ji, Y. et al. Surface Ti /Ti Redox Shuttle Enhancing Photocatalytic H 2 Production in Ultrathin TiO 2 Nanosheets/ CdSe Quantum Dots. J. Phys. Chem. C (2015).

Jia, A., Liang, X., Su, Z., Zhu, T. & Liu, S. Synthesis and the effect of calcination temperature on the physical-chemical properties and photocatalytic activities of Ni,La codoped SrTiO3. J. Hazard. Mater. 178, 233–42 (2010).

Jia, Y. et al. Composite Sr2TiO4/SrTiO3(La,Cr) heterojunction based photocatalyst for hydrogen production under visible light irradiation. J. Mater. Chem. A 1, 7905 (2013).

Jung, W. & Tuller, H. L. Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes. Energy Environ. Sci. 5, 5370 (2012).

121

Kang, H. W. & Park, S. Bin. H2 evolution under visible light irradiation from aqueous methanol solution on SrTiO3:Cr/Ta prepared by spray pyrolysis from polymeric precursor. Int. J. Hydrogen Energy 36, 9496–9504 (2011).

Kang, H. W., Lim, S. N. & Park, S. Bin. Co-doping schemes to enhance H2 evolution under visible light irradiation over SrTiO3:Ni/M (M = La or Ta) prepared by spray pyrolysis. Int. J. Hydrogen Energy 37, 5540–5549 (2012).

Kato, H. & Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 106, 5029–5034 (2002).

Kato, H., Asakura, K. & Kudo, A. Highly efficient water splitting into H2 and O2 over lanthanum- doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure. J. Am. Chem. Soc. 125, 3082–9 (2003).

Kato, H., Sasaki, Y., Shirakura, N. & Kudo, A. Synthesis of highly active rhodium-doped SrTiO3 powders in Z-scheme systems for visible-light-driven photocatalytic overall water splitting. J. Mater. Chem. A 1, 12327 (2013).

Khomenko, V. M., Langer, K., Rager, H. & Fett, a. Electronic absorption by Ti3+ ions and electron delocalization in synthetic blue rutile. Phys. Chem. Miner. 25, 338–346 (1998).

Kohno, M., Ogura, S., Sato, K. & Inoue, Y. Properties of photocatalysts with tunnel structures: Formation of a surface lattice O- radical by the UV irradiation of BaTi4O9 with a pentagonal-prism tunnel structure. Chem. Phys. Lett. 267, 72–76 (1997).

Kolodiazhnyi, T. & Petric, A. The Applicability of Sr-deficient n -type SrTiO3 for SOFC Anodes. J. Electroceramics 15, 5–11 (2005).

Konta, R., Ishii, T., Kato, H. & Kudo, A. Photocatalytic Activities of Noble Metal Ion Doped SrTiO3 under Visible Light Irradiation. J. Phys. Chem. B 108, 8992–8995 (2004).

Kubelka, P. & Munk, F. N. Zeits. f. tech. Phys. 12, 593 (1931).

Kudo, A. & Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 38, 253–78 (2009).

Kudo, A., Kato, H. & Nakagawa, S. Water Splitting into H2 and O2 on New Sr2M2O7 (M = Nb and Ta) Photocatalysts with Layered Perovskite Structures: Factors Affecting the Photocatalytic Activity. J. Phys. Chem. B 104, 571–575 (2000).

122

Kutty, T. R. N. & Nag, A. Role of interface states associated with transitional nanophase 3+ 3+ precipitates in the photoluminescence enhancement of SrTiO3:Pr , Al J. Mater. Chem. 13, 2271 (2003).

La Mattina, F., Bednorz, J. G., Alvarado, S. F., Shengelaya, A. & Keller, H. Detection of charge transfer processes in Cr-doped SrTiO3 single crystals. Appl. Phys. Lett. 93, 22102 (2008).

Laguta, V. V et al. Electron spin resonance investigation of Mn2+ ions and their dynamics in manganese doped SrTiO3. Phys. Rev. B 76, 54104 (2007).

3+ Laguta, V. V. et al. The Photoinduced Ti centre in SrTiO3. J. Phys. Condens. Matter 14, 13813– 13825 (2002).

Lehuta, K. A. & Kittilstved, K. R. Reversible control of the chromium valence in chemically reduced Cr-doped SrTiO3 bulk powders. Dalt. Trans. 45, 10034–10041 (2016).

Lehuta, K. A. & Kittilstved, K. R. Speciation of Cr(iii) in intermediate phases during the sol–gel processing of Cr-doped SrTiO3 powders. J. Mater. Chem. A 2, 6138 (2014).

3+ Li, H. et al. Roles of Cr doping and oxygen vacancies in SrTiO3 photocatalysts with high visible light activity for NO removal. J. Catal. 297, 65–69 (2013).

Li, P., Ouyang, S., Xi, G., Kako, T. & Ye, J. The Effects of Crystal Structure and Electronic Structure on Photocatalytic H2 Evolution and CO2 Reduction over Two Phases of Perovskite-Structured NaNbO3. J. Phys. Chem. C 116, 7621–7628 (2012).

Lin, J., Lin, J. & Zhu, Y. Controlled synthesis of the ZnWO4 nanostructure and effects on the photocatalytic performance. Inorg. Chem. 46, 8372–8378 (2007).

Liu, G. et al. Enhancement of visible-light-driven O2 evolution from water oxidation on WO3 treated with hydrogen. J. Catal. 307, 148–152 (2013).

Liu, J. W., Chen, G., Li, Z. H. & Zhang, Z. G. Electronic structure and visible light photocatalysis water splitting property of chromium-doped SrTiO3. J. Solid State Chem. 179, 3704–3708 (2006).

Lu, Z., Zhang, L., Wang, L. & Zhang, Q. Concentration Dependence of Luminescent Properties 3+ for Sr2TiO4:Eu Red Phosphor and Its Charge Compensation. J. Nanomater. 698434 (2012).

Lukaszewicz, K. Rocz. Chem. 33, 239 (1959).

Lv, Y. et al. The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1−x photocatalyst. Catal. Sci. Technol. 3, 3136 (2013).

Lv, Y., Zhu, Y. & Zhu, Y. Enhanced Photocatalytic Performance for the BiPO4−x Nanorod Induced by Surface Oxygen Vacancy. J. Phys. Chem. C 117, 18520 (2013).

123

Mabbs, F. E. & Machin, D. J. Magnetism and Transition Metal Complexes. (Chapman & Hall, 1973).

Marshall, M. S. J., Becerra-Toledo, A. E., Marks, L. D. & Castell, M. R. in Defects at Oxide Surfaces (eds. Jupille, J. & Thornton, G.) (Springer International Publishing, 2015).

Miseki, Y., Kato, H. & Kudo, A. Water splitting into H2 and O2 over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure. Energy Environ. Sci. 2, 306 (2009).

Mitchell, R. H., Chakhmouradian, A. R. & Woodward, P. M. Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr(1-2x)Na(x)La(x))TiO3. Phys. Chem. Miner. 27, 583–589 (2000).

Mitra, C., Lin, C., Robertson, J. & Demkov, A. A. Electronic structure of oxygen vacancies in SrTiO3 and LaAlO3. Phys. Rev. B 86, 155105 (2012).

Miwa, K., Kagomiya, I., Ohsato, H., Sakai, H. & Maeda, Y. Electrical properties of the Sr2Ru1- xTixO4 solid solutions. J. Eur. Ceram. Soc. 27, 4287–4290 (2007).

Moos, R. & Hardtl, K. H. Defect Chemistry of Donor-Doped and Undoped Strontium Titanate Ceramics between 1000° and 1400°C. J. Am. Ceram. Soc. 80, 2549–2562 (1997).

Müller, K. A. & Burkard, H. SrTiO3 : An intrinsic quantum paraelectric below 4 K. Phys. Rev. B 19, 3593–3602 (1979).

Muller, K. A. & Kool, T. W. Properties of Perovskites and Other Oxides. (World Scientific, 2010).

Muller, K. A. Electron Paramagnetic Resonance Of Manganese IV In SrTiO3. Phys. Rev. Lett. 2, 341–343 (1959).

Muller, K. A. in Proceedings of the First International Conference I, Paramagnetic Resonance (ed. Low, W.) 17 (Academic Press, 1963).

Muller, K. A., Berlinger, W. & Rubins, R. S. Observation of Two Charged States of A Nickel- Oxygen Vacancy Pair in SrTiO3 by Paramagnetic Resonance. Phys. Rev. 186, 361–371 (1969).

5+ Müller, K. A., Blazey, K. W. & Kool, T. W. Tetrahedrally coordinated Cr in SrTiO3. Solid State Commun. 85, 381–384 (1993).

Niishiro, R., Kato, H. & Kudo, A. Nickel and either tantalum or niobium-codoped TiO2 and SrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueous solutions. Phys. Chem. Chem. Phys. 7, 2241–5 (2005).

124

Nurlaela, E. et al. Critical role of the semiconductor-electrolyte interface in photocatalytic performance for water-splitting reactions using Ta3N5 particles. Chem. Mater. 26, 4812– 4825 (2014).

Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423–427 (2004).

Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 20, 35–54 (2008).

Ouyang, S. et al. Surface-Alkalinization-Induced Enhancement of Photocatalytic H(2) Evolution over SrTiO3 Based Photocatalysts. J. Am. Chem. Soc. 134, 1974–7 (2012).

Pannhorst, W. & Lohn, J. Zur Kristallstruktur von Strontianit, SrCO3. Zeitschrift fur Krist. 131, 455 (1970).

Parker, R. L. Zeitschrift fur Krist. Krist. Krist. Kryst. 59, 1 (1924).

Primak, W., Kaufman, H. & Ward, R. X-Ray Diffraction Studies of Systems Involved in the Preparation of Alkaline Earth Sulfide and Selenide Phosphors 1. J. Am. Chem. Soc. 70, 2043–2046 (1948).

Ramadan, A. H. H., Allan, N. L. & De Souza, R. a. Simulation Studies of the Phase Stability of the Srn+1TinO3n+1 Ruddlesden-Popper Phases. J. Am. Ceram. Soc. 96, 2316–2321 (2013).

Rath, M. K., Ahn, B.-G., Choi, B.-H., Ji, M.-J. & Lee, K.-T. Effects of manganese substitution at the B-site of lanthanum-rich strontium titanate anodes on fuel cell performance and catalytic activity. Ceram. Int. 39, 6343–6353 (2013).

Reshak, A. H., Auluck, S. & Kityk, I. Electronic Band Structure and Optical Properties of Srn+1TinO3n+1 Ruddlesden – Popper Homologous Series. Jap. J. Appl. Phys 47, 5516 (2008).

Reunchan, P. et al. Theoretical Design Of Highly Active SrTiO3-based Photocatalysts From Codoping Scheme Toward Solar Energy Utilization For Hydrogen Production. J. Mater. Chem. A (2013). doi:10.1039/b000000x

Rice, W. D. et al. Persistent optically induced magnetism in oxygen-deficient strontium titanate. Nat. Mater. 13, 481–7 (2014).

Ristig, S., Cibura, N. & Strunk, J. Manganese Oxides in Heterogeneous (Photo)Catalysis: Possibilities and Challenges. Green 5, 23–41 (2015).

Ruddlesden, S. N. & Popper, P. New compounds of the K2NIF4 type. Acta Crystallogr. 10, 538– 539 (1957).

Ruddlesden, S. N. & Popper, P. The compound Sr3Ti2O7 and its structure. Acta Crystallogr. 11, 54–55 (1958).

125

Sarath Kumar, S. R., Abutaha, a. I., Hedhili, M. N. & Alshareef, H. N. Effect of oxygen vacancy distribution on the thermoelectric properties of La-doped SrTiO3 epitaxial thin films. J. Appl. Phys. 112, 114104 (2012).

Sasaki, Y., Nemoto, H., Saito, K. & Kudo, A. Solar Water Splitting Using Powdered Photocatalysts Driven by Z-Schematic Interparticle Electron Transfer without an Electron Mediator. J. Phys. Chem. C 113, 17536–17542 (2009).

Sato, S. Photocatalytic activity of NOx-doped TiO2 in the visible light region. Chem. Phys. Lett. 123, 126–128 (1986).

Schaub, R. et al. Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2(110). Phys. Rev. Lett. 87, 266104 (2001).

Schimizu, T. & Yamaguchi, T. Band offset design with quantum-well gate insulating structures. Appl. Phys. Lett. 85, 1167 (2004).

Schirmer, O. F. & Muller, K. A. Defects In Neutron-Irradiated Strontium Titanate: Ti3+ ‘Off Center’ on a Sr2+ Site. Phys. Rev. B 7, 2986–2995 (1973).

Serway, R. A., Berlinger, W., Muller, K. A. & Collins, R. W. Electron Paramagnetic Resonance of Three Manganese Centers in Reduced SrTiO3. Phys. Rev. B 16, 4761–4768 (1977).

Shen, P. et al. Photocatalytic activity of hydrogen evolution over Rh doped SrTiO3 prepared by polymerizable complex method. Chem. Eng. J. 223, 200–208 (2013).

Shi, J., Shen, S., Chen, Y., Guo, L. & Mao, S. S. Visible light-driven photocatalysis of doped SrTiO3 tubular structure. Opt. Express 20 Suppl 2, A351-9 (2012).

Shimizu, K. I. et al. Photocatalytic water splitting on ni-intercalated ruddlesden-popper tantalate H2La2/3Ta2O7. Chem. Mater. 17, 5161–5166 (2005).

Smil, V. Energy: A Beginner’s Guide. (Oneworld Publishers, 2006).

3+ Stokowski, S. E. & Schawlow, A. L. Dielectric-Related Optical Line Shifts in SrTiO3:Cr . Phys. Rev. 178, 464–470 (1969).

Stokowski, S. E. & Schawlow, A. L. Spectroscopic Studies of SrTiO3 Using Impurity-Ion Probes. Phys. Rev. 178, 457–464 (1969).

Subramanian, V., Roeder, R. K. & Wolf, E. E. Synthesis And UV-Visible-Light Photoactivity of Noble-Metal-SrTiO3 Composites. Ind. Eng. Chem. Res. 45, 2187–2193 (2006).

Sulaeman, U., Yin, S. & Sato, T. Solvothermal synthesis and photocatalytic properties of chromium-doped SrTiO3 nanoparticles. Appl. Catal. B Environ. 105, 206–210 (2011).

Sun, R. R. et al. The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7. J. Phys. Chem. Solids 75, 629–637 (2014).

126

Sun, X. & Lin, J. Synergetic effects of thermal and photo-catalysis in purification of dye water over SrTi1-xMnxO3 solid solutions. J. Phys. Chem. C 113, 4970–4975 (2009).

Sun, X. et al. Photocatalytic Hydrogen Production over Chromium Doped Layered Perovskite Sr2TiO4. Inorg. Chem. 54, 7445-7453 (2015). doi:10.1021/acs.inorgchem.5b01042

Swope, R. J., Smyth, J. R. & Larson, A. C. H in rutile-type compounds: I. Single-crystal neutron and X-ray diffraction study of H in rutile. Am. Mineral. 80, 448–453 (1995).

Tagawa, H. & Igarashi, K. Reaction of Strontium Carbonate with Anatase and Rutile. J. Am. Ceram. Soc. 69, 310–314 (1986).

Tagawa, H., Kimura, K., Fujino, T. & Ouchi, K. Denki Kagakuoyobi Kogyo Butsuri Kagaku 52, 154–159 (1984).

Tan, H. et al. A facile and versatile method for preparation of colored TiO2 with enhanced solar- driven photocatalytic activity. Nanoscale 6, 10216–23 (2014).

Tan, H. et al. Oxygen Vacancy Enhanced Photocatalytic Activity of Pervoskite SrTiO3. ACS Appl. Mater. Interfaces 6, 19184–19190 (2014).

Townsend, T. K., Browning, N. D. & Osterloh, F. E. Nanoscale Strontium Titanate Photocatalysts for Overall Water Splitting. ACS Nano 6, 7420–7426 (2012).

Tregenn-Piggot, P. L. W. dandgtensorcalc.sav.

Tregenn-Piggot, P. L. W. Magprop (part of the NIST DAVE software suite). (2008).

Trepakov, V. a et al. SrTiO3:Cr nanocrystalline powders: size effects and optical properties. J. Phys. Condens. Matter 21, 375303 (2009). van Benthem, K., Elsä sser, C. & French, R. H. Bulk electronic structure of SrTiO3: Experiment and theory. J. Appl. Phys. 90, 6156 (2001).

Wang, D., Ye, J., Kako, T. & Kimura, T. Photophysical and photocatalytic properties of SrTiO3 doped with Cr cations on different sites. J. Phys. Chem. B 110, 15824–30 (2006).

Wang, J. et al. Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO. ACS Appl. Mater. Interfaces 4, 4024–4030 (2012).

Wang, J., Zhao, J. & Osterloh, F. E. Environmental Science Photochemical charge transfer observed in nanoscale hydrogen evolving photocatalysts using surface photovoltage spectroscopy. Energy Environ. Sci. 8, 2970–2976 (2015).

Wang, Y. et al. Enhancement of thermoelectric performance in rare earth-doped Sr3Ti2O7 by symmetry restoration of TiO6 octahedra. J. Electroceramics 24, 76–82 (2010). Wrighton, M. S. et al. Strontium Titanate Photoelectrodes. Efficient Photoassisted Electrolysis Of Water At Zero Applied Potential. J. Am. Chem. Soc. 98, 2774–2779 (1976).

127

Wyckoff, R. W. G. Crystal Structures 1. (Interscience Publishers, 1963).

Yamanaka, T., Hirai, N. & Komatsu, Y. Structure change of Ca1-xSrxTiO3 perovskite with composition and pressure. Am. Mineral. 87, 1183–1189 (2002).

Yi, Z. G., Iwai, H. & Ye, J. H. Photochromism and visible light induced H2 generation in Sr2TiO4:Cr complexes. Appl. Phys. Lett. 96, (2010).

Yu, H. et al. Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production. J. Mater. Chem. 21, 11347 (2011).

Yu, J. & Kudo, A. Effects of structural variation on the photocatalytic performance of hydrothermally synthesized BiVO4. Adv. Funct. Mater. 16, 2163–2169 (2006).

Zhan, H. et al. Correlation between multiple growth stages and photocatalysis of SrTiO3 nanocrystals. J. Phys. Chem. C 119, 3530–3537 (2015).

Zou, X. et al. Facile synthesis of thermal- and photostable titania with paramagnetic oxygen vacancies for visible-light photocatalysis. Chem. - A Eur. J. 19, 2866–2873 (2013).

Zuo, F. et al. Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light. J. Am. Chem. Soc. 132, 11856–11857 (2010).

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