Synthesis and Photoelectrochemical Characterization of Tantalum Oxide Nanoparticles and Their Nitrogen Derivatives

University of Nevada, Reno

Synthesis and Photoelectrochemical Characterization of Tantalum Oxide Nanoparticles and their Nitrogen Derivatives

A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical engineering

Vijay Khanal

Dr. Vaidyanathan (Ravi) Subramanian – Advisor August 2019

The Graduate School

We recommend that the dissertation

prepared under our supervision by

VIJAY KHANAL

Entitled

Synthesis and Photoelectrochemical Characterization of Tantalum Oxide Nanoparticles and their nitrogen Derivatives

Be accepted in partial fulfillment of the

Requirements for the degree of

DOCTOR OF PHILOSOPHY

Vaidyanathan (Ravi) Subramanian, Ph.D., Advisor

Dhanesh Chandra, Ph.D., Committee Member

Alan Fuchs, Ph.D., Committee Member

Victor Vasquez, Ph.D., Committee Member

Mario A. Alpuche, Ph.D. Graduate School Representative

David W. Zeh, Ph.D., Dean, Graduate School

August 2019

ABSTRACT

This work presents a surfactant-mediated approach for the synthesis of Ta2O5 nanopar-

2 1 2 1 ticles with a very high surface area (41 m g− vs. 1.6 m g− for commercial equivalent), and their subsequent nitridation to yield two distinct nitrogen derivatives: TaON and

Ta3N5. Excellent photocatalytic efficiency of Ta2O5 toward photodegradation of methylene blue and NOx removal is reported. Ta3N5 shows significant photoconversion of methylene blue attributable to its lowered bandgap and high adsorption capability of MB molecules on its surface. A new method for simplifying the existing synthesis protocols of single phase Tantalum oxynitride (TaON) was devised. Series of photoelectrochemical characterization shows that the newly synthesized TaON can significantly enhance the photoassisted charge generation, separation, and transportation compared to the parent oxide (Ta2O5). ii

DEDICATION

VILAKSHAN

PALLAVI. . . iii

ACKNOWLEDGEMENTS

I could not have succeeded in graduate school if it were not for the community and people surrounding me. First and foremost, I am forever grateful to my advisor Dr.

Vaidyanathan (Ravi) Subramanian for his steadfast support and patience over the past

ﬁve years. He not only has taught me the rigors of research, but also has played a role of a guardian in my life. I feel truly fortunate to have Dr. Ravi as my advisor. Next, thank you to my other committee members for taking the time to be a part of this campaign:

Dr. Dhanesh Chandra, Dr. Victor Vasquez, Dr. Mario Alpuche, and Dr. Alan Fuchs.

Also, I would like to acknowledge and thank Nikhil Dhabarde, one of our group members, for making the lab always lively. All the friends and families of Nepalese Students’

Association (NSA), UNR are always thankful for creating homely environment in the foreign land. Special thanks to Chiranjivi Bhattarai and his family for always being there to support in one way or the other. I can not forget all the hardworking undergraduate co-workers who joined the group in one way or the other during my Ph.D. career. Few names I would like to mention here are: Eric Sotto Harrison, Raghavi Anand, Margaux

Haurat, Cara Dumonte, Brianna Peacock, and Darion Homayoon. I am thankful to all the students and instructors from CHE 374, CHE 232, CHE 440, and CHE 410 during my TA career.

This research could not have been done without the generous support from the depart- ment of Chemical engineering, UNR. I am specially thankful to Dr. Jeff Lacombe, chair iv

our chair, for acknowledging the problems that I faced as an international student. Dr.

Lacombe has played the role of a ‘savior’ in my life, and I will never forget his generosity.

Support from afar is just as valuable as local support. Thank you to my brother

Lekhnath Khanal, for always being there to talk about our family, our career, and most importantly the Python programming! All the beautiful graphics that I have created for this work were possible only after his guidance through bits and pieces of Python.

Prof. Bahnemann, Narmina Balayeva and Carsten Gunnemann (Leibnitz University,

Germany), thank you all for being integral part of the Nox removal project. I could learn so many intricacies about EPR and TAS analysis with you guys, and thanks for sharing your knowledge and expertise in return to the incessant emojis sent over facebook mes- sanger from my side!

My parents Sitaram Khanal and Dhanakala Khanal have been my constant champi- ons and cheerleaders and I would not be the person I am today without them. Down the memory lane, I have vivid memories of their tearful eyes as they waived goodbye to me as I left my country for higher studies. I feel blessed and grateful to let them know that I am about to cross the ﬁnish line!

On top of suffering the inexplicable pain, anxiety, and suffering during her pregnancy period, my wife Laxmi Khanal has raised our two wonderful kids while working full time! She has silently suffered my frustrations, and has joyfully joined in my celebrations

. . . and the only thing I am offering in return to her, now, is: ‘thank you, Laxmi... I love you too!’ v

TABLEOF CONTENTS

Table of Contentsv Page

List of Figures viii

1 Introduction and Motivation1 1.1 Why Semiconductors?...... 3 1.2 Why nanosized semiconductors? ...... 4

2 Synthesis of Tantalum Oxide6 2.1 Why Oxides? ...... 6 2.2 Why Tantalum Oxide? ...... 7 2.3 Synthetic Approach: Why Sol-gel? ...... 8 2.4 Sol-gel Synthesis of Tantalum Oxide...... 9

3 Characterization of Tantalum Oxide 11 3.1 Physical Characterization ...... 12 3.2 Optical Characterization...... 13 3.3 Photoelectrochemical characterization...... 14

4 Examining the Photocatalytic properties of Tantalum Oxide 17 4.1 Photodegradation of Methylene Blue...... 17 4.1.1 Determination of Photocatalytic properties ...... 18 4.1.2 Photocatalysis in comparision to the commerical equivalent . . . . 19 4.2 Photocatalytic performance of Tantalum Oxide compared to generic pollutants and catalysts ...... 20 4.2.1 Competency of Tantalum Oxide against popular catalysts . . . . . 20 4.2.2 Photodegradation of RhB ...... 21 4.3 Loading effects on MBPD performance ...... 22 vi

5 Tantalum Nitride 24 5.1 Synthesis of Tantalum Nitride ...... 24 5.2 Physical Characterization of Tantalum Nitride ...... 25 5.2.1 X-Ray Diffraction Pattern of Tantalum Nitride...... 26 5.2.2 SEM/(HR)TEM/SAED Pattern...... 27 5.2.3 X-Ray Photoelectron Spectroscopy(XPS) of Tantalum oxide and Tantalum Nitride...... 27 5.3 Optical Characterization of Tantalum Nitride ...... 28 5.4 Photoelectrochemical Characterization of Tantalum Nitride...... 30 5.5 Photocatalytic activity of Tantalum nitride...... 31

6 Tantalum oxide Applied for Nox removal 34 6.1 Why NOx Removal?...... 34 6.2 Why employ Tantalum oxide for NOx Removal?...... 36 6.3 Experimental...... 37 6.3.1 Photocatalytic NO removal...... 37 6.3.2 Transient Absorption Spectroscopy (TAS) ...... 39 6.3.3 Electron Paramagnetic Resonance Spectroscopy (EPR) ...... 41 6.4 Evaluation of the photocatalytic property...... 43 6.5 Examining the photoactivity using charge transfer kinetics as a probe . . 46

6.6 EPR study of Ta2O5 catalyst...... 49 6.7 Results of Nox Removal Experiment...... 50 6.7.1 Nox removal under UV illumination ...... 50

6.7.2 Proposed Mechanism for Nox Removal by Ta2O5 ...... 53

7 Tantalum oxynitride: A straightforward synthesis approach 56 7.1 Simplied Synthesis and outreaching Importance ...... 58 7.2 Experimental...... 60 7.3 Physical Characterization ...... 61 7.3.1 XRD of TaON ...... 62 7.3.2 Physical features and crystallinity of the nanoparticles ...... 62 7.4 Optical Characterization...... 63 7.5 Photoelectrochemical Characterization ...... 64

7.6 Comparative Impedance analysis of Ta2O5 and TaON ...... 66 7.7 Control over nitridation ...... 66 7.7.1 Temperature of the bath...... 66 vii

7.7.2 pH of the ammonium hydroxide solution...... 67 7.8 Thermal Stability of the Tantalum Oxynitride...... 68 7.9 Photocatalytic activity of Newly Synthesized Tantalum Oxynitride . . . . 69 7.10 Examining the nitridation protocol with other typical Oxide catalysts . . . 71 7.10.1 Titanium Dioxide (P25) ...... 71 7.10.2 Nitrided Strontium Titanate...... 72 7.10.3 Overarching signiﬁcance of the newly introduced simpliﬁcation in conventional nitridation methods ...... 73

8 Conclusion 75

Bibliography 77 viii

LISTOF FIGURES

FIGURE Page

1.1 Energy Band Model Schematic ...... 3 1.2 Why nanospherical semiconductors are desirable...... 4

2.1 Band Edge Positions of Ta-Based Materials...... 8 2.2 Sol-gel Synthesis in ﬂow-chart...... 10

3.1 XRD, SEM, & TEM images of Ta2O5 ...... 12

3.2 UV-vis spectra and Tauc’s plot for Bandga Estimation of Ta2O5 ...... 13

3.3 J/t, J/V, & EIS response from Ta2O5 ...... 15

4.1 MB Absorbance Spectra over Ta2O5 assisted photocatalysis...... 18

4.2 Photodegradation of MB by Ta2O5: Normalized concentrations plotted against illumination time ...... 19

4.3 Catalyst Loading effects on Photodegradation of RhB by Ta2O5 compared to

TiO2 ...... 21

4.4 Photodegradation of RhB by Ta2O5 compared to TiO2 ...... 22

4.5 Loading effects on MBPD performance by in-house & commercial Ta2O5 . . . 23

5.1 Conventional Synthesis setup of TaON ...... 25

5.2 XRD Patterns of Ta3N5 ...... 26

5.3 SEM/(HR)TEM/SAED images of Ta3N5 ...... 27

5.4 XPS spectra of Ta2O5 and Ta3N5 ...... 28

5.5 UV-vis Absorbance and Tauc’s plot for bandgap estimation of Ta3N5 ...... 29

5.6 Photoelectrochemical characterization of Ta3N5: J/t, J/V, EIS responses . . . . 30

5.7 Ta3N5 assisted Photocatalytic degradation of Methylene Blue compared to the parent oxide ...... 32

6.1 Experimental setup of Ta2O5 assisted Nox Removal...... 38 ix

6.2 TAS spectra of Ta2O5 under 355 nm...... 40

6.3 TAS spectra of Ta2O5 under 355 nm...... 41

6.4 Fractal like Mapping of Ta2O5 TAS spectra ...... 41

6.5 EPR Response of Ta2O5 ...... 42 6.6 NOx Removal Results under UV illumination (λ 355 nm) of Ta O and = 2 5 TiO2P(25)...... 51 6.7 NOx Removal Results under visible light (λ 463 nm) illumination ...... 52 = 6.8 Proposed Mechanism for Ta2O5 assisted NOx Removal...... 53

7.1 Conventional Synthesis setup of TaON ...... 58 7.2 empty ...... 60 7.3 XRD of TaON...... 62 7.4 Physical Characterization of TaON synthesized by new method: XRD/TEM/SAED 63 7.5 Optical Characterization of TaON synthesized by new method: UV-vis absorbance, Tauc’s plots...... 64 7.6 Photoelectrochemical Characterization of TaON synthesized by new method:

J/t, LSV, EIS Compared to parent Ta2O5,...... 65 7.7 Control over nitridation...... 67 7.8 Examining the thermal stability of newly synthesized TaON...... 68 7.9 Examining the photoactivity of newly synthesized TaON...... 70

7.10 Applying new nitridation method to TiO2 ...... 71

7.11 Applying new nitridation method to SrTiO3 ...... 72 1 HAPTER C 1

INTRODUCTIONAND MOTIVATION

s the human population is increasing with increased aspirations, energy de-

mand in the world is deemed only to increase. Since the deposits of traditional A sources of fossilized fuels are ﬁxed in nature and are being constantly depleted, such sources of energy are not too far away from exhaustion[1–4]. More importantly, on

the other hand, the impact that the fossil fuels are leaving behind as they are anthro-

pogenically burnt is adversely affecting the environment. Therefore, it follows that the

use of fossil fuels comes with two-fold problems: (1) energy scarcity (2) environmental

degradation. And solar energy is thought to be one of the promising alternatives toward

solving these problems together. However, the appealingly attractive features of solar

energy such as environmentally friendly, perennial, and free availability are downplayed

by its diffuse and intermittent nature in the earth atmosphere. The diffuse and intermit-

tent nature of solar radiation, in fact, is indispensable for living creatures in the earth[5].

But its use as a source of energy at its present form is not just enough to deliver the

energy demand. Therefore channelizing the available solar energy to our beneﬁt probably

is the only option left behind. To work around the diffuse and intermittent nature of

solar energy, researchers around the world are utilizing a very basic principle of funda- 2

mental Physics: Conservation of energy, which states ‘ Energy can neither be created nor destroyed, but can be transformed from one form to the other!’ [6] Present report also is not different from this guidance. In particular, the viability of high surface area

Tantalum based materials (semiconductors) toward ‘collection and conversion of solar energy’ is explored in this work. Starting from synthesizing high surface area Tantalum pentoxide (or simply Tantalum Oxide, Ta2O5 hereafter) as a photoactive semiconductor, a series of nitrogen derivatives of the material are synthesized, characterized, and examined toward photocatalytic applications. These materials serve as the ‘collectors’ of solar energy facilitating the photocatalytic reactions leading toward mineralization of varioius pollutants. In short, focus of this research is to engineer the crystal structure of

Tantalum-based materials so as to maximize collection and conversion of the solar energy.

Unlike other transition metal oxides, Ta2O5 offers an easier option of incorporating nitrogen atom owing to its relatively straightforward option toward transformation of lattice structure at suitable conditions[7–10]. In particular, the Ta based oxide family can be transformed with relative ease into other class of materials: oxynitrides, and nitrides.

[7] This process is different from composite formation such as by add-on approaches like in doping of a parent material with foreign element. The use of nitrogen is a desirable goal in optoelectronic engineering because (i) inert (non-toxic) nature of nitrogen makes it environmentally friendly (ii) nitrogen is cost-friendly because it is earth abundant element, and (ii) from electronic bandstructure point of view, N 2P orbitals are more negative than O 2P orbitals in reference to normal hydrogen electrode (NHE) and therefore the hybridization of N 2P and O 2P orbitals in the nitrogen incorporated matetrial can raise the valence band maximum edge of the new material thereby lowering the bandgap.[11] Bandgap lowering is important because that’s how the corresponding semiconductor can absorb wider solar spectrum, and hence store more energy. Therefore, 3

the amenability of Ta2O5 aids into the facility of tuning the bandgap - an important parameter of a photocatalyst. This was one of the motivations behind exploring Ta2O5 as an oxide photocatalyst, and its nitrogen derivatives as the bandgap engineered semiconductors.

1.1 Why Semiconductors?

The broad class of inorganic materials can be classiﬁed further into subgroups of metals, semiconductors, and insulators under the electronic band theory. Metals are characterized by partially ﬁlled valence band and therefore they are good conductors of heat and electricity at room temperature. [12]Semiconductors are characterized by a forbidden band in between conduction and valence bands, the gap of in energy being also known as the bandgap. Insulators are similar to semiconductors, but they have wider bandgap.

FIGURE 1.1. In contrast to metals, the valence bands of semiconuctors and insulators are fully occupied (at absolute zero). The bandgap in semiconductors is typically less than 4 eV, allowing for excitation possibility due to the solar radiation.

As indicated in ﬁgure 1.1, the semiconductors are of particular interest because they offer the possibility of excitation with less-energetic photons too owing to their narrow bandgap. In particular, the solar radiation in the earth atmosphere is rich in visible spectrum as opposed to the UV (that translates into lesser availability of high 4

energetic photons of UV compared to the visible spectrum). In the context of solar energy utilization, semiconductors are, therefore, the only interesting materials.

The idea of solar energy utilization in conjuction with photoactive semiconductors is that the photon excited electrons(which are reducing agents) and holes (which are strong oxidizing agents) drive the redox reactions upon solar illumination. [4, 5] To exploit this very optimistic idea, a semiconductor should be a good solar energy absorber, a good charge generator, and an equally good charge transporter. Otherwise the components of driving redox process(electrons and holes) are not efﬁciently available so as to maximize the utilization of incident solar energy. Seen in this context, nanosized particles are preferred to other architectures.

1.2 Why nanosized semiconductors?

It is noticeable that when a bulk material is discretized, the cummulative surface area offered is always higher than the bulk. The larger the area, the greater number of active sites are available for chemical reactions that occur at the surface, and hence greater is the overall yield.

FIGURE 1.2. A cubical geometry (A) and a spherical geometry(B) are compared for the sake of comparing surface areas.

Figure 1.2 illustrates why spherical nanoparticles are desired in practice. As the (3D) geometries are discretized, the cummulative surface area increases in both cases A and 5

B, but the increment in case (B) is larger. The reason is that a spherical geometry offers minimum surface area, and hence one can achieve greater number of discrete units from a spherical object when it is discretized. Mathematically, as

3V lim ( )n1/3 n →∞ R

, cummulative surface area tends to infinity as well. Hence, spherical morphology is desirable while synthesizing nanoparticles [At this point, it should be emphasized that sol-gel synthesis provides high dispersion in the end products, in addition to other practical benefits such as simplicity and cost-effectiveness, usually leading to spherical geometry in the end product. This is the motivation behind adopting sol-gel synthes method for synthesizing high surface area Ta2O5 nanoparticles]. Added benefits of nanosized materials are: (1) Shortened diffusion length Availability of nascent =⇒ charge carriers (2) Increased Life time of charge carriers Reduced recombination (3) =⇒ Quantum Confinement size dependent optoelectronics =⇒ 6 HAPTER C 2

SYNTHESISOF TANTALUM OXIDE

2.1 Why Oxides?

hotocorrosion resistance is one of the most desirable attributes in photocat-

alytic applications. Also, if the material of interest is to be well integrated with P other materials of different electronic conﬁgurations, it should be compatible, in general, with both organics and inorganics. For example, photocatalysts need to be

stable in aqueous solution under intense photonic irradiation otherwise they photocor-

rode and deactivate over time. On top of all these stringent requirements, the surface

area to volume ratio is desirable to be as high as possible so as to expose and exploit

larger cross section (or larger number of active sites) of the material. Crystallinity, on

the other hand, plays crucial role in efﬁcient absorption, generation, separation, and

utilization of electron-hole pairs upon photonic excitation of the photocatalyst. Seen in

all of these contexts, metal oxides are among primary choices of researchers because

they are (1) highly stable against corrosion, (2) well integrable with both organics and

inorganics, (3) exhibit bandgap tunability with earth abundant materials, (4) simpler

synthetic strategies are available for nanosized particles, (5) and being metal oxides, they 7

are normally available in crystalline forms. A better crystallinity with few defects can usually minimize the trapping states and recombination sites, resulting in an increased efﬁciency in the usage of the photogenerated carriers for desired photoreactions.

Oxide nanoparticles with single cations have traditionally been used in solar energy transformation. Processes such as photoelectro chemical water splitting or photoelectrochemical remediation of environment are leading examples of solar energy utilization.

Oxides may be employed i) independently, ii) in combination with another material such as metal or oxide, iii) or in a physically modiﬁed manner (such as 0-D or 1-D geometry aka nanoparticles or nanorods). Improving the light absorbance using doping or by using overlying additives is often used for oxides that only absorb in the UV. Popular single metal large bandgap oxides such as TiO2 or ZnO and formation of their composites with metals, an additional oxide, or chalcogenides demonstrate reasonable photoactivity.

However, formation of such composites can often lead to instability driven by phase separation or oxidation of non-oxides.

2.2 Why Tantalum Oxide?

The multifunctionality of tantalum-based single metal oxides is evident in its dielectric properties, light activated processes, and transportation (waveguides). However, tantalum oxides demonstrate very limited photoactivity attributable to its large bandgap

(3.6–3.9 eV) which makes it photoactive only in the far right of the UV. However, this large bandgap of tantalum oxide works to our advantage since the bandedges straddle the redox potential of water. This feature makes tantalum oxides a viable candidate, if properly optimized to absorb visible light. 8

FIGURE 2.1. The relative positions of the bandedges of the tantalum oxide, oxynitrides (TaOxNy), and nitride with respect to the populaar titanium dioxide TiO2

For example tantalum oxides may be integrated with the most earth abundant and eco-friendly element – nitrogen – to form oxynitrides – TaOxNy [y = 1-x][13]. This singular option allows for tuning the optical bandgap (Eg) over a range of 1.6–4.0 eV solely based on Ta: O: N ratio overarching a majority of the visible domain using eco- friendly elements as building blocks. As shown in ﬁgure 2.1, such phases are capable of visible light-driven photoactivity.

2.3 Synthetic Approach: Why Sol-gel?

As illustrated in ﬁgure 1.2, it is desirable to have large surface area of photocatalysts. For instance in photocatalytic watersplitting, the idea is to make a suspension of photoactive materials followed by illumination. In doing so, the catalyst in the form of powder, is 9

desirable to expose as much active sites as it is possible. And larger surface area is expected to provide larger active sites of the catalyst. In this light, sol-gel is preferred as the easiest, cost-friendly, and easier-to-commercialize approach. More importantly, sol-gel process facilitates phase evolution by negligible agglomeration of the end products by providing high degree of dispersion. Also, a good control over the stoichiometry is also achieved as the phase evolves. Being simple, this method offers economical option of making nanosized materials.

2.4 Sol-gel Synthesis of Tantalum Oxide

There are various synthesis protocols available for synthesizing nanoparticles such as

Sol-gel, hydrothermal and chemical vapor deposition. An advantage of the sol-gel method over other methods is that it produces materials with high surface area with a good control over phase even under very mild conditions. In addition, if the sol-gel method proceeds with a surfactant or structure directing agent, the size of the particles and their distribution can be tightly controlled. The additives such as surfactants, are burnt off during a heat treatment as the ﬁnal step of the process.

The in-house oxide phase of Tantalum Ta2O5 was synthesized by implementing the sol-gel process1. A typical synthesis involves the use of 2.5 g Pluronic(F-127) added with

30 mL dry ethanol. The mixture was stirred until the solute was partially solubilized ( 5 minutes). 2.5 mL of glacial acetic acid was added to this solution followed by a dropwise addition of 1 mL Hcl. The mixture was ultrasonicated for 3 minutes. Then the solution was magnetically stirred for 15 minutes to add 3 mL of Tantalum ethoxide dropwise and then again stirred vigorously for 90 minutes. Finally, the solution was kept at 40 ◦C in air for 15 hours for allowing the gelation process to occur allowing for the formation

1V. Khanal et. al, Editor’s choice: JES 2019 10

of the crystallized precursor. Greenish white grains were recovered and placed in a convective oven at 65 ◦C for 24 hours to burn off the moisture. Subsequently, ramping the temperature up to 700 ◦C over 12 hours slowly burned off the precursor associated carbon content. Fine white crystals (identiﬁed as Ta2O5 as discussed later) were formed as a result after the oxidative annealing. They were crushed using mortar and pestle to collect in the form of ﬁne shining-white powder.

FIGURE 2.2. Step-by-step illustration of Ta2O5 syntheis protocol is shown. In the inset,(I) is followed by (II). 11 HAPTER C 3

CHARACTERIZATIONOF TANTALUM OXIDE

he characterization of the Ta2O5 photocatalyst was performed using several

complementary tools1. A Hitachi S-4700 scanning electron microscope (SEM) Twas used to examine the physical features of the powder. High resolution transmission electron microscopy ((HR)TEM) analysis was performed using a JEOL

2100F instrument equipped with a selected area electron diffraction (SAED) analyzer.

The optical properties of the powder were examined using a UV–vis diffuse absorbance

measurement with a Shimadzu UV 2501PC spectrophotometer. Brunauer Emmett-

Teller spe- 130 ciﬁc surface areas (SBET) of the synthesized materials were determined

using a micromeritics system. X-ray diffraction patterns were taken using a Bruker D8

diffractometer (Cu Kα radiation, 40 mA, 40 kV). X-ray photoelectron spectroscopy (XPS)

measurements were done under a monochromatic Al Kα X-ray source (1486.74 eV, Specs

Focus 500 monochromator). The photoelectrons were detected with a hemispherical

analyzer (Specs Phoibos 100). The binding energies were calibrated to the adventitious

carbon peak.

1V. Khanal et. al, Editor’s Choice: JES 2019 12

3.1 Physical Characterization

The as-obtained greenish white gel was conﬁrmed to be amorphous by X-Ray diffraction measurements as shown in ﬁgure 3.1(A). After the heat treatment in atmosphere, the phase was seen to be transformed to crystalling Ta O with JCPDS #00 018 1304. 2 5 − −

Estimation of the surface area of these nanoparticles (using single point BET measurements)was performed using N2 physisorption. The estimated values for the commercial

2 1 2 1 Ta2O5, (in–house) Ta2O5,and Ta3N5 are 1.6 m g− , 41 m g− respectively. The higher surface area is attributed to the smaller nanoparticles and minimal aggregation.

FIGURE 3.1. (A) XRD pattern of heat treated Ta2O5. The inset image also de- picts the phase transformation from amorphous form of Ta2O5 (the greenish yellow gel being the coloration of the organic-associated matrix) to white crystalline powder of Ta2O5 upon heat treatment at 700 ◦C in open atmosphere. (B) SEM image of commercially available Ta2O5 showing the morphology distinctly different from (C), that of in-house Ta2O5. (D) shows the characteristic d-spacing of Ta2O5 through HRTEM.

The size and shape of the procured and synthesized oxides were initially examined using microscopy. As evident from the Figure 3.1, the commercial samples are non-spherical with nanoparticles starting at 40 nm. There are clusters consisting of 3–4 aggregated nanoparticles that are 100 150 nm observed. The SEM image of the in-house syn- ≈ − thesized after thermal treatment is shown in Figure 3.1(C). In comparison, the size distribution among the in-house nanoparticles is narrow and well controlled. These 13

nanoparticles appear more spherical with distinct boundaries and smaller diameter in the range of 20–30 nm. The nanoparticles appear to be better dispersed and do not show evidence of cluster formation.

3.2 Optical Characterization

The nanoparticle was deposited as ﬁlms on FTO coated glass before determining their optical response. The absorbance spectra of the ﬁlms are shown in Figure 3.2, the Tauc’s plots for bandgap estimation are shown in inset, while the inset image shows photographs of the powder prior to its deposition. Both commercial and in-house Ta2O5 are white and show an onset absorbance of 320 nm. Ta2O5 bandgap are estimated to be in the range of 3.6–3.9 eV based on the onset absorbance and they are consistent with the literature reported observations. This onset is also evidence that the photoactivity is driven predominantly by UV light.

FIGURE 3.2. Tauc’s relation: αhν A(E hν)n is used for bandgap estimation = g − and n 1 is taken assuming direct bandgap. = 2 14

3.3 Photoelectrochemical characterization

For photoeletrochemical measurements, the powdered samples of Ta3N5 was mixed thoroughly with terpinol and made a ﬁne slurry suspension. Typically, 0.1 g of photocatalyst was taken in a glass vial, mixed with 0.2 mL of terpinol, and magnetically

2 rd stirred for 30 minutes. Single drop of thus obtained slurry was casted on 3 of the length of from ITO coated glass slides (ITO coated glass plates were obtained Pilkington

Ford, OH) using 1 mL plastic pipette, and the slide was slanted (15°) so that the slurry self-transforms into a uniform ﬁlm by ﬂowing under gravity. These ITO slides were dried der an inert (N2) atmosphere prior to use in photoelectrochemical at 80 ◦C for 30 minutes and were annealed at 350 ◦C for 4 h under inert (N2) atmosphere prior to use in photoelectrochemical measurement.

The photoelectrochemical measurements were performed using a potentitostat/galvanostat

(Autolab PGStat series). The measurements were recorded for ﬁlms of the photocatalyst prepared by depositing an aqueous suspension of the photocatalyst slurry on conducting

(FTO) glass slide followed by N2 annealing at 350 ◦C. Chronoamperometry, Linear sweep voltammetry, and measurements for bandgap estimation (tauc plots) were obtained using

0.2 M aqueous NaOH.A Xe lamp was used as a light source with a CuSO4 ﬁlter for high

UV cut off. A Pt mesh was used as the counter electrode and Ag/AgCl was used as a reference [Filling Electrolyte(In the reference electrode): 3.4 M KCl, electrolyte used:

0.2M NaOH]

The analysis of photoelectrochemical or PEC data offer valuable information into the separation and transport mechanisms of charges photogenerated in the catalysts upon illumination. This approach allows to track both hole and electrons upon generation in the catalysts and can be correlated with the photocatalytic activity. The chronoamperometry or j/t responses of the commercial and in-house synthesized Ta2O5 obtained with a Pt wire as the counter electrode in a 3-electrode PEC cell are shown in Figure 3.3(A). The 15

electrolyte was alkaline to facilitate the removal of holes using hydroxyl ions [OH – + h+ OH ]. The multiple on-off cycles show that the response in both films are light −−−→ · triggered and the reproducible nature of these responses indicate that the films formed on the conducting glass slides are reproducible. The inset of the Figures 5a and 5b shows the photographs of the films after the PEC measurements, indicating they are stable (do not delaminate).

FIGURE 3.3. (A) The chronoamperometry (J/t) responses of (a) in-house, and (b) commercial Ta2O5. [Operating conditions: Pt counter electrode, leak-free Ag/AgCl (in 0.2M NaOH) as the reference electrode and sample coated ITO slide as working electrode. Light intensity: 80 mW/cm2, applied potential: no external bias was applied in the three electrode system except the intrinsic potential developed in the ﬁlm upon illumination, estimated ﬁlm thickness: 1 2µm ≈ −

The multiple on-off cycles show that the response responses indicate that the ﬁlms formed on the conducting glass slides are reproducible. The in-house Ta2O5 shows a

2 differential photocurrent (illuminated dark) of 1.3 µA/cm while the commer cial Ta2O5 response is approximately 0.85 µA/cm2. This increase in photocurrent by 50% with the ≈ in-house Ta2O5 could be attributed to the difference in the physical dimensions of the nanoparticles and/or the improved photoactivity of this ﬁlm, but has to be veriﬁed using alternative approaches.

Linear sweep voltammogram are different than the current time (chronoamperome- 16

try) characteristics as they provide insight into the relative position of “apparent flatband potentials” for the film and thereby help qualitatively distinguish the charge separation in the films. The magnitude of the photocurrent obtained in the 3- electrode setup is shown to be higher by 10% than the in-house Ta O compared to the commercial ≈ 2 5 equivalent. The zero current potential is a measure of the apparent flatband potential in the film and is often estimated to provide an insight into the band bending in particulate films. A more negative shift is indicative of charges separating effectively and is thus indicative of a better charge separation compared to the 0.48 V vs. Ag/AgCl − the in-house film shows 90 mV negative shift and becoming available for transport or redox activity. At -0.57 Vand commercial Ta2O5 respectively. Such responses from well controlled nanoparticulate films of oxides are reported with other oxides such as TiO2.

The ﬁgure 3.3 (C) shows the results of the electrochemical impedance analysis (EIS,

Nyquist Plots) of commercial Ta2O5, in-house Ta2O5. All measurements in illumination

2 by 80 mWcm− Xenon lamp, Pt counter electrode, leak-free Ag/AgCl (in 0.2M NaOH) as the reference electrode and sample coated ITO slide as working electrode. The basis for performing the impedance analysis on the tantalum-based photocatalysts is to gain insights into ﬁlm properties such as the n-/p-characteristics and a qualitative estimate of the extent of charge separation. The Ta2O5 in-house sample shows a low radius of curvature compared to the commercial sample indicating that charge separation is better with the in-house samples. 17 HAPTER C 4

EXAMININGTHE PHOTOCATALYTICPROPERTIESOF TANTALUM OXIDE

t is usual to test the practical viability of nanoparticles by testing them as photo-

catalysts for environmental as well as energy applications. In particular, Ta2O5 Iis reported to show environmental applications such as photocatalytic oxidation of dyes [9, 14–16]. Further, from a greenhouse gas mitigation standpoint, Ta2O5 is also reported to aid with CO2 reduction[17]. Just as with other oxides, the photoactivity of tantalum-based oxides are critically inﬂuenced by the method used to synthesize them including the additives used in the synthesis and the shape in which they may evolve

[18, 19]. Therefore, it is pertinet to test the photocatalysis of newly synthesized Ta2O5 to see if the response tallies with the photoelectrochemical responses.

4.1 Photodegradation of Methylene Blue

Methylene Blue (MB) was chosen as a probe molecule to test the photocatalytic properties. Owing to its mild toxicity and well pronounced absorbance in the visible spectrum, it is customary to test the photodegradation effciency of a photocatalyst ﬁrst by choosing MB as a probe molecule. MB is a multipurpose organic colored dye used in textile industries and is considered a representative model pollutant whose photodegradation can be tracked using spectroscopy to determine the activity of the photocatalyst. 18

4.1.1 Determination of Photocatalytic properties

For photocatalysis studies, a suspension of the synthesized material was prepared with various dyes independently in water. Ta2O5 or Ta3N5 and 200 mL of 28.6 µM Methylene Blue solution under aerated conditions were used. The sample solution was sonicated for 3 minutes before it was transferred to a jacketed borosilicate reactor and was left in dark for 1.5 hours under constant magnetic stirring so as to achieve adsorption-desorption equilibration. Then the solution was illuminated using a 480 W solar simulator lamp from the top for determining the activity of the materials as a photocatalyst. The change in concentration of the dyes upon illumination (and hence the photoactivity of the catalyst samples) was tracked by sampling 3 mL of the solution every 30 minutes for four times (total illumination period: 2 hours), centrifuging it, and eventually measuring the optical absorbance with UV-vis spectrophotometer.[Insert it somewhere: 3 mL aliquots were sampled and ﬁltered to remove the solid phase] Typical absorbance pattern over illumination period are shown in ﬁgure 4.1.

FIGURE 4.1. (A) A set of absorbance spectra of the dye, methylene blue, MB, in the presence of the in-house synthesized Ta2O5 photocatalyst indicating a gradual decrease in the presence of Uv-vis illumination

The absorbance data was then corresponded with instantaneous concentration of the dye based upon Beer-Lambert’s law. Since Beer-Lambert’s law suggests A(t) C(t) [where A(0) = C(0) A(t) is the absorbance at any time ‘t’, A(0) being the peak absorbance corresponding to the dye MB concentration prior to illumination; C(t) and C(0) are the corresponding concentrations of MB], the instantaneous concentration was normalized by the initial 19

concentration and was plotted against time.

As the bandgap of Ta2O5 was determined to be 3.66 eV, the material’s photoactivity should be further pronounced under UV-illumination under theoretical considerations. To see if this is correct in practice, the same experiment was performed under high pressure Hg light (rich in UV output). And the corresponding plots are shown in figure 4.2. As can be seen from figures 4.1 and 4.2, the photocatalytic response by Ta2O5 is primarily driven by UV illumination. The solar simulator has 5% UV portion, so there is some activity ≈ under solar simulator (figure 4.1) too. This result is in contrast to the response under UV illumination (Overall conversion: 30% under solar simulator, 60%). Additionally, the ≈ ≈ response of in-house Ta2O5 is better than the commercial equivalent.

4.1.2 Photocatalysis in comparision to the commerical equivalent

FIGURE 4.2. 0.1 g in-house Ta2O5 assisted photodegradation of 28.6 µM MB under 480 W Hg lamp. Compared to the photlysis of MB in the light alone, the effect of addition of Ta2O5 is pronounced by enhanced degradation almost by doubling the overall conversion, attributable to the photocatalytic performance of Ta2O5. 20

Figure 4.2 also shows the change in the absorbance of the dye solution at various time before and after illumination. Compared to the control experiments (catalyst-free conditions) a higher dye conversion is observed attributable to the degradation initiated in the presence of the Ta2O5. In the presence of the commercial and inhouse Ta2O5 a signiﬁcant decrease in the peak absorbance is observed over a period of 2 hours of continuous illumination. The commercial Ta2O5 of 0.1 g loading shows a 31% decrease in the dye concentration. In contrast, the in-house Ta2O5 shows a 58% decrease over the same period, with the same loading. Since MB is not regarded as an “ideal pollutant”, the activity of the commercial and inhouse Ta2O5 was further compared toward the degradation of Rhoadamine B (RhB). The degradation of 200 mL 28.6 µM, under aerated conditions using a 480 W solar simulator at the end of 3.5 hours 80% respectively using commercial and in-house Ta2O5 suggesting (1.5 hour dark + 2 hours illumination) was observed to be 45% and the in-house Ta2O5 is far superior. The improvement in the dye degradation with the in-house catalyst can be attributed, to the smaller particle size of the in-house Ta2O5 as evident from the SEM and (HR)TEM analyses.

4.2 Photocatalytic performance of Tantalum Oxide compared to generic pollutants and catalysts

4.2.1 Competency of Tantalum Oxide against popular catalysts

It was noticed that Ta2O5 is far superior in terms of MB adsorption compared to TiO2. The photodegradation of MB was examined at various catalyst loadings, and the adsorption of MB on Ta2O5 was seen to reach to competitive conversion compared to TiO2 at 0.15 g. It is to be noted that the conversion of MB stems primarily from larger adsorption of

MB molecules on Ta2O5 surface. Given the difference in bandstructure, difference in photocatalytic responses is to be expected. The results are shown in ﬁgure 4.3. 21

FIGURE 4.3. Performance of Ta2O5 as compared with TiO2 toward conversion of 28 µM Rhodamine B tends to competitive yields at higher catalyst loadings. (Ads: Adsorption, PC: Photocatalysis)

4.2.2 Photodegradation of RhB

As methylene blue is debated as to whether it is the ‘ideal’ probe molecule, photocatalysis of Ta2O5 was examined with a rather recalcitrant molecule Rhodamine B (RhB). Also, the photocatalysis was compared with the traditionally popular photocatalyst TiO2. TiO2 and Ta2O5 belong to the same oxie family, but their bandstructure is different: TiO2 has bandgap of 3.2 eV, while Ta2O5 has 3.66 eV. Therefore, both the catalysts are primarily excited under UV illumination. Therefore, the photocatalysis was performed under UV rich 176 W. The normalized concentrations against time are plotted in ﬁgure 4.4. 22

FIGURE 4.4. Performance of 0.05 g Ta2O5 compared withTiO2 toward photodegradation of 28 µM Rhodamine B.

As seen in ﬁgure 4.4, the conversion of RhB is the result of adsorption followed by photocatalytic degradation. In terms of adsorption, they are similar, but the photocatalytic degradation is different. This difference can be attributed to the difference in bandgap structure. Acidic swing pH( 2, using nitric acid) of RhB solution was seen to = facilitate adsorption of RhB molecules on both of the catalyst surfaces as the same was not noticeable in neutral pH conditions.

4.3 Loading effects on MBPD performance

Ta3N5 produced by thermally reduced process was tested as a photocatalyst for the degradation of MB. The result shown in ﬁgure 9A and B indicate three key observations. Firstly, the Ta3N5 photodegrades the MB successfully. Secondly, as the loading increases the conversion increases. In fact, a degradation of 80% and 95% is noted with ≈ > Ta3N5 at a loading of 0.1 g and 0.15g. Comparing these results with the performance of Ta2O5 a higher activity with Ta3N5 at different loadings is noted. It is noteworthy to mention here that at a loading of 0.15 g Ta3N5 the conversion observed is higher than what was observed with Ta2O5 even at higher loading of 0.3g.Finally, the dark 23

absorbance phase during equilibration is remarkably different for the two photocatalysts.

Considering the fact that the Ta3N5 are bigger than the Ta2O5 (see figure 2 and 3), interestingly enough a significant drop in absorbance of the dye is noted during dark equilibration. This is indicative of a very effective absorbance of the dye on the Ta3N5 surface than the Ta2O5 (see figure 7). A bar graph similar to the plot for Ta2O5, comparing the performance of Ta3N5 at two loadings is shown in supporting information SI2 for figure 9). The absorbance data was linearized and plotted Us in a logarithmic scale involving the concentration ratio vs. time. The linearization shows that the photocatalytic degradation process follows a lumped parameter kinetic model represented using a power law expression of the form r k[MB]n where k and n represent he rate − MB = constant and order of the reaction.[29, 38, 39] The plot itself is shown in SI3 for the various photocatalysts at different loading and the rate constants are summarized in Table I for the various photocatalysts. The rate constant is observed to increase in the order kc (Ta O ) kin house(Ta O5) k(Ta N ) at any given loading and also follows − 2 5 << − 2 < 3 5 the same trend at different loadings indicating the superiority of the activity of the in-house synthesized photocatalysts. This approach to represent dye degradation has have been reported elsewhere. [29, 40] It is to be noted that rigorous analysis of the photodegradation is the required, and is often the next step implemented [41, 42] to decouple the effects of various contributions such as light interactions, catalyst loading, adsorption effects etc. It is not within the scope of this present work.

FIGURE 4.5. Linearized plots of normalized concentrations of MB after photodegradation with different loading of various catalysts 24 HAPTER C 5

TANTALUM NITRIDE

s discussed in the introduction section, Tantalum Oxide can be obtained in

the fully nitrided form with relative ease than other metal nitrides1. Such a A nitrided form is desirable because (i) nitrogen yields uplift of valence band so as to lower the bandgap of the material (ii) Ta3N5 straddles the redox potential of

water [20–22]. Conventional synthesis of Ta3N5 involving high temperature nitridation

of Ta2O5 is employed and characterized by a series of photoelectrochemical methods.

Also, the photocatalytic activity of Ta3N5 is compared in contrast with the parent oxide

Ta2O5.

5.1 Synthesis of Tantalum Nitride

Previously procured crystals of Ta2O5 were used as the starting precursor for the syn-

thesis of Ta3N5. A representative step involved placing the white crystals in a high

temperature furnace to perform nitridation (Ammonia ﬂow Rate 1000 mL/min) in the

1v. Khanal et.al., Editors’ Choice: JES 2019 25

presence of a continuous ammonia flow at 850 ◦C. It was noted that the ammonia flowrate affected the coloration of the final product. For photoelectrochemical measurements, the powdered samples of Ta3N5 were mixed thoroughly with terpinol and made as fine slurry suspension. 0.1 g of photocatalyst was taken in a glass vial, mixed with 0.2 mL of terpinol, and magnetically stirred for 30 minutes. The ITO coated glass slides (ITO coated glass plates were obtained from Pilkington Ford, OH). Single drop of thus obtained slurry was

2 rd casted on ( 3 ) of the length of ITO using 1 mL plastic pipette, and the slide was slanted (15° angle ) so that the slurry self-transforms into a uniform ﬁlm by ﬂowing under gravity.

These ITO slides were dried at 80 ◦C for 30 minutes prior to use in photoelectrochemical measurements and were annealed at 350 ◦C for 4 h under an inert (N2) atmosphere.

Typical synthetic steps are shown in the ﬁgure 5.1

1 FIGURE 5.1. Dry ammonia(ﬂowrate: 1000 mLh− ) is passed through Ta2O5 powder being treated at high temperatures 850 ◦C for 15 hours, and brilliant red colored Ta3N5 powder (conﬁrmed later by series of characterizations) is achieved

5.2 Physical Characterization of Tantalum Nitride

The characterization ofthe photocatalysts was performed using several complementary tools. A Hitachi S-4700 scanning electron microscope (SEM) was used to examine the physical features of the powder. High resolution transmission electron microscopy

((HR)TEM) analysis was performed using a JEOL 2100F instrument equipped with a selected area electron diffraction (SAED) analyzer. The optical properties of the powder were examined using a UV-vis diffuse absorbance measurement with a Shimadzu 26

UV 2501PC spectrophotometer. Brunauer-Emmett- Teller speciﬁc surface areas (SBET) of the synthesized materials were determined using a micromeritics system. X-ray diffraction patterns were taken using a Bruker D8 diffractometer (Cu Kα radiation,

40 mA, 40 kV). X-ray photoelectron spectroscopy (XPS) measurements were done under a monochromatic Al Kα X-ray source (1486.74 eV, Specs Focus 500 monochromator). The photoelectrons were detected with a hemispherical analyzer (Specs Phoibos 100). The binding energies were calibrated to the adventitious carbon peak.

5.2.1 X-Ray Diffraction Pattern of Tantalum Nitride

First, X-Ray diffraction was performed on the as-nitrided powder. As shown in ﬁgure 5.2, the XRD response changes from Ta2O5. As conﬁrmed from JCPDS# [ ], the changed XRD patterns correspond to single phase Ta3N5.

FIGURE 5.2. As can be seen, the crystal structure of Ta2O5 transforms into Ta3N5 upon nitridation at high temperatures. Important Miller indices (hkl) are indicated to corresponding peaks. 27

5.2.2 SEM/(HR)TEM/SAED Pattern

The morphology of the procured and synthesized oxides were examined using microscopy.

Interestingly, the reductive treatment shows an insignificant impact on the physical features of the oxides. The post treatment (nitridation process) dimensions of the nanoparticles are mostly in the 30 – 40 nm range as indicated in Figure 5.3. There is also no evidence of clustering of the nanoparticles. Given this size of the nanoparticles, the approach used to prepare the films using a binder, and the amount of the material taken as a slurry, the particulate films formed is expected to evolve with film thickness upto a few microns.

FIGURE 5.3. Reductive treatment has not signiﬁcanttly changed the morphology, except that the particles are grown in size. The SAED pattern conﬁrms that Ta3N5 is still polycrystalline.

5.2.3 X-Ray Photoelectron Spectroscopy(XPS) of Tantalum

oxide and Tantalum Nitride

X-ray photoelectron spectroscopy (XPS) measurements were performed in order to investigate the chemical nature of the samples. Core-level spectra for the Ta2O5 and

Ta3N5 samples are shown in Figure 5.4. Ta2O5 samples only show a peak at 404.9 eV (Ta

4p 3 ), and no peak for nitrogen. The Ta 4p 3 peak shifts to lower binding energy for the 2 2

Ta3N5 samples, which is attributed to the lower electronegativity of N as compared to O.

In addition, the Ta3N5 samples show peaks at binding energies of 396.7 and 394.8 eV, 28

which can be assigned to N 1s.

FIGURE 5.4. Ta 4p 3 and N 1s core-level X-ray photoelectron spectra of Ta2O5 2 and Ta3N5 nanoparticles

5.3 Optical Characterization of Tantalum Nitride

The nanoparticles were deposited as films on FTO coated glass before determining their optical response. The absorbance spectra of the films are shown in Figure 5.5, the Tauc’s plots for bandgap estimation are shown as inset in the same figure. Also, the photographs of the powders prior to their deposition are shown. 29

FIGURE 5.5. UV-vis absorbance spectra for Ta2O5 (as-synthesized, after thermal treatment) and Ta3N5 are shown. The corresponding Tauc’s plots for bandgap estimation are shown as in inset. The Tauc’s plot of the in-house synthesized material indicates Ta O has bandgap (E ) 3.66 eV, while 2 5 g = for Ta N , E 1.68 eV 3 5 g =

As can be seen from the optical absorbance plots, the Ta3N5 shows a redshift as far as 630 nm with the onset, broadening the absorbance, indicative of a much smaller bandgap. The experimental determination of the bandgap was performed using Tauc plot analysis. Tauc’s relation: αhν A(E hν)n; where n 1 . The reason for choosing = g − = 2 n 1 is that the the linear part was achieved for n 1 . A bandgap of 3.66 eV and 1.68 = 2 = 2 eV for Ta2O5 and Ta3N5 respectively is estimated as indicated in the inset. (Note that, the approximate and relative bandedge positions are reported based on illuminated 30

conditions). The coloration is indicative of the incorporation of nitrogen in the Ta2O5. It is to be noted that thermal reduction protocol is applied on Ta2O5 for this reason, i.e. to broaden the absorbance response from UV to include visible light.

5.4 Photoelectrochemical Characterization of

Tantalum Nitride

FIGURE 5.6. (a) The chronoamperometry (j/t), (b) the linear sweep voltametry (J/V), and (c) electrochemical impedance spectroscopy (EIS) responses of In-house Ta2O5 and Ta3N5. [Operating conditions: Pt counter electrode, leak-free Ag/AgCl (in 0.2M NaOH) as the reference electrode and sample 2 coated ITO slide as working electrode. Light intensity: 80 mWcm− , applied potential: no external bias was applied in the three electrode system except the self-developed intrinsic potential in the ﬁlm upon illumination, estimated ﬁlm thickness: 1–2 µm

As seen in ﬁgure 5.6(a), the photocurrent density due to Ta3N5 is seen to increase to

2 2 2 µA/cm from 0.5 µA/cm as given by in-house Ta2O5.

The zero current potential is a measure of the apparent flatband potential in the film and is often estimated to provide an insight into the band bending in particulate films. A more negative shift is indicative of charges separating effectively. As seen from figure

5.6(b), the Ta3N5 shows the largest negative shift, indicating that Ta3N5 facilitates the most effective charge separation among three classes of materials. Hence, the efﬁciency 31

of charge separation in these materials can be qualitatively ordered as: commercial

Ta O In-house Ta O (In-house) Ta N . 2 5< 2 5 < 3 5

The plot of the impedance response for the Ta2O5 and Ta3N5 photocatalyst ﬁlms is shown in Figure 5.6(c). The Ta2O5 in-house sample shows a low radius of curvature compared to the commercial sample indicating that charge separation is better with the in-house samples. Finally, the application of the nitridation process to Ta2O5 to form

Ta3N5 is beneficial as it shows the lowest radius of curvature indicating the most effective transport among the three photoactive films. Thus, the series of PEC measurements indicate that i) the in-house Ta2O5 and Ta3N5 demonstrate photocurrent generation, ii) reproducible and stable response, and iii) the Ta3N5 demonstrates a distinguishable enhancement in the photocurrent response compared to the Ta2O5 films which can be attributed to the better charge separation, transport across the film thickness, and collection at the underlying FTO plate.

5.5 Photocatalytic activity of Tantalum nitride

The photocatalytic activity of Ta3N5 produced by thermal reduction process was assessed by tracking the decrease in dye absorbance in the visible spectrum. Figure 5.7 shows the changes to the dye absorbance at 664 nm in the absence and presence of catalysts, since photolytic conversion of MB is reported [23]. As evident in Figure 5.7, MB undergoes photolytic conversion up to 20% over 2 hours of continuous UV-vis illumination. 32

FIGURE 5.7. The photodegradation of methylene blue or MB was performed using UV-visible light after 1.5 hours of equilibration under dark to ensure adsorption (A-D). 0.1 g of commercial Ta2O5(C) and in-house synthesized Ta2O5(I) & Ta3N5 were used in the photocatalyst with a solar simulator (SS) as the light source [Light: 480 W solar simulator, distance between the reactor and light source: 30 cm

Figure 5.7 also shows the change in the absorbance of the dye solution at various time before and after illumination. Compared to the control experiments (catalyst- free conditions) a higher dye conversion is observed attributable to the degradation initiated in the presence of the Ta3N5. During equilibration, the dark absorbance region is remarkably different for the two photocatalysts: Ta2O5 & Ta3N5. Considering the fact that the Ta3N5 particles are bigger than the Ta2O5 leading to lesser surface area

2 1 ( 17 m g− ), interestingly enough a signiﬁcant drop in absorbance of the dye is noted ≈ 33

during dark equilibration with Ta3N5. This is indicative of a very effective adsorption of the dye on the Ta3N5 surface than the Ta2O5. Preliminary estimates of the fractional conversion of the dye using Ta3N5 was determined to be 90%. The role of the signiﬁcantly higher adsorption of MB molecules on Ta3N5 surface than Ta2O5 surface in promoting photocatalysis remains to be fully understood. 34 HAPTER C 6

TANTALUMOXIDE APPLIEDFOR NOX REMOVAL

6.1 Why NOx Removal?

ndustrial revolution might have given tremendous advancements to humankind,

but the world is still suffering its consequences. Air pollution is one of the major I leading environmental issues among various forms of such consequences. Various chemical efﬂuents such as dust, toxic gases, greenhouse gases (GHG), and metal salts are known to constitute air pollution[24]. Among them, nitric oxide (NO) is important to be considered, because NO can (i) lead to various health issues such DNA mutation and suppression of blood-oxygen content, (ii) cause acid rain, and (iii) exacerbate greenhouse effect inviting ozone hole formation. Nitric oxide (NO) is produced primarily by combustion of fossil fuels[25–29]. Anthropogenically, such combustion is caused during operation of automobiles, industries, aircrafts, and in internal combustion processes. Another way of NO production is natural such as in the decomposition of organic substances in soil, ocean, volcanic activities, and earthquakes. As a function of ambient emission conditions, varying oxidation states of nitrogen, and oxygen give rise to NOx (a collective nomen- 35

clature for thus produced gaseous products eg. nitric oxide (NO), nitrous oxide N2O, nitrogen dioxide (NO2), dinitrogen trioxide N2O3, dinitrogen tetroxide (N2O4), dinitrogen pentoxide (N O ))[26, 28]asek2013Removal. NO is the leading constituent ( 95% by 2 5 ≈ vol.) among all these products, and therefore is of primary interest in pursuit of NOx removal.

The photocatalytic detoxiﬁcation of air pollutants using particulate heterogeneous photocatalysts has drawn attention as these materials can utilize the freely available solar energy to effectively decrease the concentration of such toxic substances. [26, 30]A solid foundation for economic, as well as benign air pollution control for such an approach was established after Fujishima and Honda demonstrated that the conversion of radiant energy to electrical or chemical energy was possible using a catalytic approach. [31]

Their demonstration of the decomposition of water into hydrogen and oxygen with Pt and TiO2 electrodes was later extended by Bard into particulate based heterogeneous catalysis. [32] The particulate systems, in addition, being simpler and less expensive to design and utilize, are of particular interest because they offer added benefits such as a) high specific surface area to volume ratios compared to their bulk counterparts allowing for more effective utilization of active sites, b) efficient charge carrier generation and utilization because of nanosized-inspired enhancement in lifetime of photogenerated charge careers, c) amenability to dispersion making colloidal suspension, which can facilitate light transmission more readily than powders, and d) quantum size effect, arising from the nanoscale, which gives rise to effective utilization of light. [33] However, the practical application of such nanosized photocatalysts is limited by the fact that the concentration of gaseous nitrogen oxides in ambient air is very low ( 10 200ppb) and ≈ − it interferes with the charge carrier diffusion restrictions in the catalysts to lower the overall efficiency. [34] Therefore, it is imperative to develop highly efficient photocatalysts 36

that can achieve practical control over air pollution.

The photocatalytic oxidation of NO over particulate systems was ﬁrst reported by

Hori et al. for TiO2.[30] Seen remarkable from an atmospheric chemistry standpoint, researchers have attempted NOx removal in real-scale as well as in lab-scale simulated environments. [35, 36] In addition to above mentioned beneﬁts of nano-particulate systems, nanosized Tantalum oxide (Ta2O5) is one of the promising materials for gas phase pollution abatement, because of its high adsorption quality and acidic nature in amorphous forms. [7, 37] Ta2O5 is a multi-functional material in that its high dielectric constant has found widespread commercial application in electronics, its unique band structure has inspired researchers to explore its viability in clean fuel generation from solar water splitting [2, 15, 38] as well as in environmental remediation such as liquid/gas based pollution abatement. [14, 37, 39–42] There is no information reported about

Ta2O5 assisted photocatalytic NOx removal. Further, Ta-based oxides offer the option to selectively tune bandgap with nitrogen inclusion to facilitate full visible spectrum absorbance that can aid with photocatalysis in a manner that no other catalyst can.

Therefore, it is imperative to understand the light – structure – catalytic property relations of Ta2O5. Therefore, we have studied the NOx removal by Ta2O5 nanoparticles as standalone photocatalysts. Recently, excellent photoelectrochemical response and robust stability under UV-vis illumination was shown for Ta2O5 nanoparticles. [7]

6.2 Why employ Tantalum oxide for NOx Removal?

Tantalum oxide (Ta2O5) in its nanoparticulate form is one of the promising materials for gas phase pollution abatement because of its high adsorption quality and acidic nature in amorphous forms in addition to above mentioned beneﬁts of nano-particulate systems. Ta2O5 is a multifunctional material in that its high dielectric constant[43] 37

has found widespread commercial application in electronics, and its unique bandstructure has inspired researchers to explore its viability in clean fuel generation from solar watersplitting, and environmental remediation such as in liquid/gas based pollution abatement[14, 16]. Still, very little is reported about Ta2O5 assisted photocatalytic NOx removal. Further, Ta-based oxides offers the option to selectively tune bandgap with nitrogen inclusion to facilitate full visible spectrum absorbance that can aid with photocatalysis in a manner that no other catalyst can[7, 44–47]. Therefore, it is imperative to understand the applied light – structure-catalytic property relations of Ta2O5. Therefore, the NOx removal by Ta2O5 nanoparticles as standalone photocatalysts was studied.

6.3 Experimental

The catalyst used, Ta2O5, was prepared by the same protocol discussed in chapter 1. For typical use in Nox removal applications, the Ta2O5 powder was tightly pressed into a polymethyl methacrylate (PMMA) holder with a (4.5 4.5) cm2size, yielding a geometric × 2 2 sample surface area of 20.25 cm and preilluminated with UV light (365 nm, 1 mWcm− ,

Philips CLEO 100 W-R) for three days in order to decompose the organic residual from the surface of the photocatalysts. The gas bottle containing 50 ppm of nitrogen oxide NO as the model pollutant in a mixture of nitrogen gas N2 was employed to the experimental device. The experimental setup is shown in ﬁgure 6.1.

6.3.1 Photocatalytic NO removal

The activity of the Ta2O5 photocatalysts was evaluated by performing the photodegradation of NO under UV illumination at room temperature using a system illustrated in

SI 2 (photograph of setup provided). A chemiluminescence analyzer was employed for the detection of NO in accordance with the ISO standard 22197-1.31 The powder was 38

tightly pressed into a polymethyl methacrylate (PMMA) holder with a (4.3 4.3cm2) size, × yielding a geometric sample surface area of 20.25cm2. The oxide was pre-illuminated

2 with UV light (365 nm, 1 mWcm− , Philips CLEO 100 W-R) for three days to decompose organic residual from the surface of the photocatalysts. A mixture of 50 ppm NO and

NO2 (aka NOx) was introduced into the setup. The setup consisted of three gas flow controllers (Bruker instrument) for adjusting the wet and dry air flow and the NO concentration, a gas-washing bottle as humidifier (50% humidity), a temperature and a relative humidity sensor, a photoreactor made of PMMA covered with borosilicate glass, a light source (Philips CLEO 15 W, λ 365nm for UV irradiation), and a max = chemiluminescence analyzer (Horiba APNA 360). At the beginning of the experiment, the gas concentration was adjusted to 1 ppm and the equilibrium was attained in the dark in the bypass mode. Subsequently, the device was switched to the reactor mode (gas stream runs over the photocatalyst). This allowed for by adsorption-desorption to occur

2 for half an hour in the dark. The samples were illuminated with light (1 mWcm− ) in the reactor mode for 2 h and the degradation performance of the catalysts was evaluated.

At the end of the illumination, the light was switched off and the system was kept in reactor mode until the initial concentration is attained. The gas removal ratio (η) was

(C0 C) calculated using the formula: η − 100%, where C and C0 are the concentration = C0 × of the appropriate gases in the outlet and inlet steam, respectively.

FIGURE 6.1. NOx Removal

The device consisted of three gas flow controllers (Bruker instrument) for adjusting the wet and dry air flow and the NO concentration, a gas-washing bottle as humidifier 39

(50% humidity), a temperature and a relative humidity sensor, a photoreactor made of PMMA covered with borosilicate glass, a light source (Philips CLEO 15 W, λ max = 365 nm for UV irradiation), and a chemiluminescence analyzer (Horiba APNA 360). At the beginning of the experiment, the gas concentration was adjusted to 1 ppm and the equilibrium was attained in the dark in the bypass mode. Subsequently, the device was switched to the reactor mode (gas stream runs over the photocatalyst) by following adsorption-desorption equilibrium for half an hour in the darkness. The samples were

2 illuminated with UV light (1 mWcm− ) in the reactor mode for 2 h and the degradation performance of the catalysts was detected. At the end of the photocatalytic experiment, the light was switched off again and the system was kept in reactor mode until attaining the initial concentration. The gas removal ratio (η) was calculated according to η = (C C) 0− 100%, where C and Co are the concentration of the appropriate gases in the C0 × outlet and inlet steam, respectively.

6.3.2 Transient Absorption Spectroscopy (TAS)

For the transient absorption spectroscopy measurements the samples were placed in a

ﬂat quartz cell and ﬂushed for 30 minutes with either N2 or a mixture of N2 and MeOH.

The measurements were performed using an Applied Photophysics LKS 80 Laser Flash

Photolysis Spectrometer with a pulsed Nd:YAG Laser (Quantel, Brilliant B) in diffuse reﬂectance mode. The third harmonic of the laser emitting a wavelength of 355 nm with

6 ns pulses with an average energy of 3 mJ was used for measurements. The light emitted by the laser was directly guided to the sample, as well as the analyzing light (Xenon lamp, pulsed, Osram XBO, 150 W). The laser was focused with a steering prism, while the xenon lamp is guided by mirrors and lenses to the sample. After the reﬂection at the surface off the sample the light was collected by a lens and reﬂected by a mirror through a lens to a monochromator and then to a detector (Hamatsu PMT R928), which relates to 40

a 50 Ω resistance to an oscilloscope. In the photomultiplier the collected light is directly transferred into a current. By applying a voltage to the photomultiplier, the light level is adjusted to 100 mV for each detection wavelength. The oscilloscope itself detects a change in the voltage during the measurement, whereby the decay of the transient signal was observed. Finally, the values of the change of the reﬂectance ∆J was obtained by the following equation

Abs J0 J ∆J 1 10− − (6.1) = − = J0

The absorbance values, Abs, which depend on the reflected light before (J0) and the laser excitation (J), were directly calculated by the software of the system. It is important to note that a linear relation between the concentration of the charge carriers and the change in the reflectance is given until the change in the reflectance is less than 10 %.32

For each wavelength the measurements were based on a time scale of 10 µs, 15 shots were averaged and the data points were reduced to 200. The spectra were recorded from

400 nmto 750 nm over a step of 25 nm.

FIGURE 6.2. Transient absorption spectra of Ta2O5 powder at different times after the excitation with 355 nm in N2 atmosphere. (B) Transient absorption spectra of Ta2O5 powder at different times after the excitation with 355 nm in deaerated methanol atmosphere. 41

FIGURE 6.3. (A) Transient absorption spectra of Ta2O5 powder at 0.1 µs after the excitation in N2 atmosphere and in deaerated methanol atmosphere. (B) Calculated differences in the change of the reﬂectance in deaerated methanol atmosphere and at N2 atmosphere in 0.1 µs after the excitation.

FIGURE 6.4. Fitting of the transient absorption signals at a wavelength of 650 nm with fractal-like kinetics in (a) N2 and in (b) methanol atmosphere

6.3.3 Electron Paramagnetic Resonance Spectroscopy (EPR)

A MiniScope X-band EPR spectrometer (MS400 Magnettech GmbH) was employed for the investigation of the photogenerated charge species. 0.1 g of the catalyst powder was 42

placed in a quartz tube. The tube was placed in a liquid nitrogen ﬁlled Dewar in order to perform the experiment at 77 K. The EPR ﬁeld frequency was set over a range of

9.42-9.44 GHz. The acquisition parameters were as following: center ﬁeld: 337 mT, sweep time: 60 s, number of points: 4096, number of scans:1, modulation amplitude:0.2 mT , power:10 mW, gain = 5.

FIGURE 6.5. Electron Paramagnetic Resonance (EPR) spectra obtained from Ta2O5 nanoparticles. All experiments were performed at 77K to lessen thermal agitation of photoexcited electrons and holes hence reducing the rate of recombination.

In-situ EPR spectra of the as-synthesized Ta2O5 were measured to investigate the

peaks observed at different values of g (Lande-splitting factor) conﬁrm that there are different charged species being generated upon illumination. In particular, strong EPR signals around g 2.007 are in good agreement with the results observed by Yu et.al. = [48] Similarly, the peaks observed at g 1.97 can be associated with the formation of = +4 Ta species which correspond to the trapped electrons on the surface of Ta2O5. It is interesting to note that Ta+4 may be the reduced form of Ta+5 after the reaction with photogenerated electrons. The weak signal induced at the region of 334.089 mT(g = 2.01) can be addressed to Ta+5 – /Ta+4 - OH species, which can be ascribed to the hole trap of a subsurface oxygen anion radical. Also, the peak observed at g 2.05 can = be associated with the trapped holes. It is known that oxygen vacancy defects form during the generation of Ta+4 species. [? ] Therefore, there are surface oxygen vacancies being created upon illumination of Ta2O5. Thus, the EPR analysis conﬁrms that the root distinct response toward UV-illumination as shown by Ta2O5 crystal lattice lies in the population of charge carriers in the trap sites. Such trap sites in oxide materials originate from surface oxygen vacancies as evident in related materials. [49, 50] This observation is very important for the understanding of Ta2O5 as a photocatalyst, because such oxygen vacancies are well known to modulate the electronic band structure of the material in such a way that surface oxygen vacancies provide active sites for strong chemisorption of gas molecules with lowered activation energy, thereby affecting the photocatalytic activity and product selectivity

6.4 Evaluation of the photocatalytic property

After acquiring the relative equilibrium, where the concentration of NO is 1 ppm,

NO (C C C ) is 1 ppm and NO is 0 ppm in dark the sample starts X NOx = NO − NO2 2 to be illuminated. During illumination, the concentration of NO and NOx decreased due to conversion to the nitrates as a ﬁnal product, whereas some NO2 gases are gen- 44

erating which is byproduct of the oxidation of nitrogen oxide. The change of the gas concentrations of NO and NOx were calculated according to the following equations [36]:

∆C C C (6.2) NO = NO(Dark) − NO(illumination) ∆C C C (6.3) NOx = NOx(Dark) − NOx(illumination)

The general reaction for the decomposition of NO is represented by the following equations:

OH− h+ OH• (6.4) + −−−→

NO OH• HNO (HNO H+ NO −) (6.5) + ads −−−→ 2 2 ←−→ aq + 2

HNO OH− NO H O (6.6) 2 + ads −−−→ 2 + 2

NO OH• HNO (6.7) 2 + ads −−−→ 3

NO OH• NO• (6.8) 2 + ads −−−→ 3

In comparison with the commercial TiO2 (Aeroxide P25), one of the traditionally best-known materials for photocatalytic NO degradation, Ta2O5 shows better activity and stability (ﬁg. 3(A), 3(B)) under UV-illumination (λ 300 400nm), Intensity = = − 2 1 mWcm− ). It is interesting to note that in spite of a lower surface area (S (Ta O ) BET 2 5 = 2 1 2 1 41.2 m g− , S (TiO ) 50 m g− ), and higher bandgap (Eg Ta O = 3.66 eV, Eg (TiO ) BET 2 = 2 5 2

= 3.06 eV), Ta2O5 shows a qualitatively higher NO conversion when compared to TiO2

(P25). Such a response and the stability of the sample was conﬁrmed after multiple on-off cycles and measurements with the photocatalysts. The photocatalytic conversion of NOx over Ta2O5 was observed to be 16.5%, slightly lower than the degradation value of NO at 18.8% (the formation of the nitrogen dioxide NO2, as a byproduct: 1.5%, is subtracted from NOx conversion).[34] Furthermore, it is observed that the performance of Ta2O5 is remarkable in that it leads to negligible NO2 (as indicated earlier, NO2 is a more toxic gas than NO). This activity of Ta2O5 is particularly promising because TiO2 P25, in contrast, mostly converts the initial NO gas into the toxic NO2 intermediate (7.6 %) An 45

interesting observation was made as to the photocatalytic activity of Ta2O5 under visible light using LED (ﬁg. 4(A)). It shows negligible NO conversion under visible light, which appears to be naturally against its bandgap structure, because a material with a direct bandgap of 3.66 eV cannot be excited by visible light (λ 1240 388nm). TiO onset = 3.66 = 2 also shows a similar response (ﬁg. 4(B)), which was documented earlier by Bahnemann and coworkers. The visible light response of TiO2 was explained on the basis of weak absorption bands in the visible region of the spectra obtained for TiO2-NO complexes.[34]

Though insigniﬁcant conversion is noted, the importance of this observation lies on the fact that even diffuse light (e.g., indoor illumination) can trigger decontamination of the

NOx and thus help with regulating indoor air pollutants over an extended period. The photonic efﬁciency (χ) for the reference (P25) and the Ta2O5 catalysts was calculated using the following equation [14]

(CDark CLight).P.NA.h.c ζ V. − (6.9) = I.λ.A.R.T

1 Where V is a laminar volume ﬂow (3 Lmin− ), the change of gas concentration is C Dark − 1 1 CLight, R is the gas constant (8.314 JK− mol− ), T is the temperature (K), p the pressure

2 (Pa), I is the irradiation intensity (mWcm− ), λ is the average wavelength (nm), NA is

23 1 34 the Avogadro constant (6.0221 10 mol− ), h is the Planck constant (6.6261 10− Js) × × and c is the speed of light (2.991 108 ms). The photonic efﬁciencies and conversion × percentages are summarized in the following table. Ta2O5 shows a 2-fold increase in the conversion of NO compared to the commercial TiO2 in the presence of UV light. The photoactivity of the two oxides are similar under visible light illumination 46

6.5 Examining the photoactivity using charge

transfer kinetics as a probe

The excitation of Ta2O5 with light that exceeds the bandgap, leads to the generation of electron-hole pairs (eq.6.10). It is expected that a part of the generated charge carriers recombine (eq. 6.11) directly and another part is trapped. In the case of TiO2 it was shown that within 10 ns after the excitation 90 % of the generated charge carriers recombine.

[51] A similar response with Ta2O5 can be expected wherein, the trapping of electrons leads to the formation of Ta4+ ions (eq. 6.12). [52] Photogenerated holes can also be trapped at terminal oxygen ions or hydroxyl radicals (equantions 6.14& 6.13). [ 53] The trapped photogenerated charge carriers can either recombine with one another (eq. 6.11) or take part in other reactions.

Ta O e −• h + (6.10) 2 5 −−−→ CB + VB

e− h+• Heat (6.11) + −−−→ 5 4 Ta+ e − Ta+ (6.12) + CB −−−→ 2 O − h+ O−• (6.13) + −−−→

OH− h+ OH• (6.14) + −−−→

However, it is important to understand the kinetics of the process. These kinetic parameters can be systematically investigated using transient absorption spectroscopy.

In ﬁgure 6.2 (A) the transient absorption spectra of Ta2O5 after sub-bandgap excitation at different time intervals under N2, is shown. Since no electrons can be excited across the bandgap with a 355 nm laser pulse due to the Ta2O5 band structure, the electrons are expected to be excited within the bandgap possibly to the trap states within the gap. Two maxima can be observed at 650 nm and at 475 nm. Related studies focused 47

on Ta-based oxides have indicated similar transient absorption maximum. A maxima at 650 nm forLiTaO3, 40 and in another example by Schneider et al. at 650 nm and at

430 nm for Ba5Ta4O15. The presence of such maxima can be attributed to the trapped electrons at the localized trap states. However, due to lack of any chemical additive for hole scavenging, it is expected that a significant number of these trapped electrons are lost to holes via recombination. Spectroscopic measurements in deaerated methanol were therefore performed to determine the true extent of charge carrier separation in the range between 400 nm and 750 nm. Since, methanol acts as hole scavenger, the addition of methanol has an influence on the lifetime of the charge carriers. The hypothesis is that at a certain wavelength most likely electrons absorb, the transient absorption signal will increase after the addition of methanol compared to N2, while for holes the opposite can be expected. Figure 6.2 (B) shows the transient absorption spectra of Ta2O5 in deaerated methanol. The maxima at 475 nm and 650 nm are no longer present after the addition of methanol. A possible explanation for maxima-free spectra is that photogenerated electrons and holes can absorb to a varying degree over the spectral range. Further, compared to the measurements in N2 a higher transient absorption can be observed in deaerated methanol over the entire wavelength range for 0.1 µs after the excitation, as shown in figure 6.2(A). The enhancement can be attributed to the effective separation of the electrons within the bandgap due to the alcohol-assisted hole scavenging. This observation agrees very well with an earlier reported result for other Ta-based oxides wherein the observation of a maxima at 650 nm and 430 nm allude to a similar behavior caused by photogenerated electron absorbance. Next, to identify where the largest contribution of holes is located, although electrons absorb mostly at all investigated wavelengths, analysis at the difference of the changes in reflectance at the wavelengths in N2 atmosphere and in deaerated methanol atmosphere is considered. The difference 48

is calculated using the expression:

∆J ∆J(λ, MeOH) ∆J(λ, N ) (6.15) = − 2

Figure 6.3(B) contains the calculated differences in the change of the reﬂectance in

N2 and methanol at 0.1 µs after the excitation. It is noted that the value of ∆J at lower wavelengths is lower than at higher wavelengths and this difference can be approximated by a linear relationship. The larger of a difference of the values at both atmospheres allude to a higher contribution of electrons while at lower wavelengths, the contribution of holes progressively increases. To estimate the kinetics of the electrons the transient absorption signals at a wavelength of 650 nm in N2 atmosphere and in deaerated methanol were analyzed. The ﬁtting of the transient signals is performed using the fractal-like kinetics model (eq. 6.16), where A is the height of the initial signal, h the fractal parameter, and k2,f the fractal recombination constant. [54] The fractal parameter, which is a measure for the oxide physical characterstic dimension, is zero in a 3D homogenous medium. [54]

A(1-h) ∆J (6.16) (1 h) = (1-h)+A k t − · 2,f · The transients at a wavelength of 650 nm and the corresponding ﬁtting curves are shown in N2 and in deaerated methanol in ﬁgure 6.4. All data points from 0.1 µs to

9 µs were considered for the ﬁtting and the calculated values are reported in Table 6.1.

By comparing the fractal recombination constants in N2 and in deaerated methanol, it can be seen that the addition of methanol lowers the recombination constant. It is because, methanol reacts rapidly with photogenerated holes and thus the lifetime of the electrons is increased. The lower fractal recombination constant in deaerated methanol also validates that mainly photogenerated electrons absorb at this wavelength. Taking into account the error of the ﬁtting curve the fractal parameter h does not change after the addition of methanol. Because the fractal parameter is related to the structure and to 49

the dimension of the investigated material, [54] no change of the parameter should occur after the addition of methanol. Thus, the model of fractal-like kinetics can be applied very well to the analysis of the transient absorption signals of Ta2O5 in various media.

The observed fractal parameter of Ta2O5 is similar to the fractal parameter of PC105

(Crystal, anatase, = 0.62, particle diameter = 15-20 nm) shown in the literature. [54]

Table 6.1: Determined parameters of the fractal-like kinetics ﬁt function at a wavelength of 650 nm in N2 and in methanol atmosphere.

Fractal parameters N2 Atmosphere Methanol Atmosphere A/a.u. 0.061 0.016 0.095 0.007 ± ± k /a.u. 1227.15 922.81 771.14 303.44 2,f ± ± h 0.64 0.06 0.61 0.03 ± ±

Though intra bandgap excitation is not expected in the photocatalysts studied here, long living species can exist. They can be detected using transient absorption measurements. Also, the maxima in the spectra of Ta2O5 compares well with those of other tantalum-based oxides reported in the literature. Thus, it can be concluded that even with sub bandgap excitation long living photogenerated charge carriers can be created and their participation in redox reactions can be controlled.

6.6 EPR study of Ta2O5 catalyst

In-situ EPR spectra of the as-synthesized Ta2O5 were measured to investigate the

Ta+4/Ta+5 species and potential oxygen vacancy defects created in the catalyst. As shown in ﬁgure 6.5 , the EPR signals become stronger under illuminated conditions compared to the dark, and the intensity changes as a function of the illumination time. Various peaks observed at different values of g (Lande-splitting factor) conﬁrm that there are different charged species being generated upon illumination. In particular, strong EPR signals around g 2.007 are in good agreement with the results observed by Yu et. al. Similarly, = 50

the peaks observed at g 1.97 can be associated with the formation of Ta+4 species which = correspond to the trapped electrons on the surface of Ta2O5. It is interesting to note that

Ta+4 may be the reduced form of Ta+5 after the reaction with photogenerated electrons.

The weak signal induced at the region of 334.089 mT(g 2.01) can be addressed to Ta+5 - = O/Ta+4 - OH species, which can be ascribed to the hole trap of a subsurface oxygen anion radical. Also, the peak observed at g 2.05 can be associated with the trapped holes. It = is known that oxygen vacancy defects form during the generation of Ta+4 species. [? ]

Therefore, there are surface oxygen vacancies being created upon illumination of Ta2O5.

Thus, the EPR analysis conﬁrms that the root distinct response toward UV-illumination as shown by Ta2O5 crystal lattice lies in the population of charge carriers in the trap sites.

Such trap sites in oxide materials originate from surface oxygen vacancies as evident in related materials. [49, 50] This observation is very important for the understanding of

Ta2O5 as a photocatalyst, because such oxygen vacancies are well known to modulate the electronic band structure of the material in such a way that surface oxygen vacancies provide active sites for strong chemisorption of gas molecules with lowered activation energy, thereby affecting the photocatalytic activity and product selectivity. [49, 54? ]

6.7 Results of Nox Removal Experiment

6.7.1 Nox removal under UV illumination

In comparison with the commercial TiO2 (Aeroxide P25 powder with 20% rutile and 80%

2 1 anatase crystal phase and 50 m g− speciﬁc surface area), one of the traditionally popular materials for photocatalytic NO degradation, Ta2O5 shows better activity and stability

1 ﬁgure 6.6.Typically, Ta2O5, having a lower surface area than TiO2 (SBET(Ta2O5) =

2 1 2 1 41 m g− ,(SBET(TiO2) = 1.6 m g− ), shows a 9-fold higher NOx conversion in compari- 1Work in progress with colab. with prof. Detlef Bahnemann and group, Leibnitz University, Germany 51

son to TiO2 (P25). Additionally, the stability of the sample was proven while performing the measurement two times apart from visible light measurements.

FIGURE 6.6. Ta2O5 offers excellent stability and efﬁciency toward Nox removal

The performance of Ta2O5 is more remarkable in that it forms a negligible amount

2 of NO2 and NO2 is a much more toxic gas than NO . In contrast, TiO2 (P25) mostly converts the initial NO gas into the NO2 intermediate, whereas a drastically low amount of NO gas was completely converted to the nitrates. An interesting observation was made as to the photocatalytic activity of Ta2O5 under visible light ﬁgure 6.7. Ta2O5 shows a small amount of nitrogen oxide conversion under visible light, which appears to be naturally against its bandgap structure, because a material with a bandgap of 3.66 eV cannot be excited by visible light.

2Experiments conductoed by Narmina Balayeva, one of the contributors of the manuscript in progress on this work 52

FIGURE 6.7. Ta2O5 shows visible light activity toward Nox removal, that appears to contradict its bandstructure

Similar phenomenon is documented before by Bahnemann et al. where TiO2 was shown to have a visible light activity for the degradation of NO.[36] This observation has been explained on the basis of theoretical calculations that disclose the presence of weak absorption bands in the visible region of the absorption spectra of the TiO2-NO complexes. The NO conversion and photonic efﬁciencies shown by Ta2O5 and TiO2 are summarized in the table3 6.2.

Table 6.2: Efﬁciency of Ta2O5 compared with TiO2 toward Nox removal under UV and visible illumination Samples NO Conversion(% Photonic Efﬁciency(%) TiO2 (P25- UV) 8.8 0.3 Ta2O5 (UV) 18.6 0.64 TiO2 (vis) 4.0 0.01 Ta2O5 (vis) 4.2 0.01

3credit to Narmina Balayeva, Leibnitz University, Germany) 53

6.7.2 Proposed Mechanism for Nox Removal by Ta2O5

FIGURE 6.8. (A) oxygen vacancies created in Ta2O5 during sol-gel synthesis are proposed to introduce shallow trap sites near its conduction band, hence facilitating visible light activity. (B) The shallow valence band (VB) edge of TiO2 is shown in contrast to the deep VB edge of Ta2O5.

The observed enhancement in NOx removal by as-synthesized Ta2O5 in comparision to

TiO2 can be attributed to synergistic effects of the following:

• Differences in valence band edge positions: Taking normal hydrogen elec-

trode(NHE) as the reference, the valence band maximum (VBM) edge of Ta2O5 lies

more deeper than that of TiO2 (+3.3 for Ta2O5 vs +2.7 for n-TiO2). Consequently,

the holes generated upon the optical excitation of Ta2O5 are stronger oxidizing

agents than those in TiO2. Therefore, the oxidation of NOx is pronounced. Similar

observations were made by Dong et.al. [55] for graphitic carbon nitride (g-C3N4) 54

, where they found that the less positive valence band-edge position could not

adequately oxidize NO, thereby generating NO2 in a large amount.

• Defect sites: Oxygen vacancy defects in Ta2O5 crystals, form a new shallow defect

energy level between the valence band maximum (VBM) and conduction band

minimum (CBM) of Ta2O5, thereby effectively narrowing down the bandgap and

hence harvesting a wider solar spectrum. The defects provide available energy

states for electrons that may be excited within Ta2O5 lattice even with reduced

photonic energy, thereby enhancing solar energy utilization.Such a phenomenon

is already reported in the NOx removal with visible light illumination of other

oxides such as TiO2. Also, it is noteworthy here that in ﬁgure 3.2, the UV-vis

absorption spectrum of Ta2O5 indicates a gradual increment in the absorbance for

wavelengths in visible region until it meets a rapid increment in the UV-region. This

observation is in sharp contrast with TiO2, because TiO2 has a sharp absorbance

onset at around 400 nm. This difference, instead of being just an experimental error,

might be an early hint of differences in the bandstructure modulation stemming

from defect formation in the crystal lattices. And this might explain why Ta2O5

shows visible light activity toward NOx removal.

• Surface Chemistry: The exceptional stability of Ta2O5, as seen toward NOx re-

moval under UV irradiation, may be explained by assuming that the NOx molecules

adhered on the Ta2O5 surface are instantly oxidized, followed by the subsequent

replenishment of fresh molecules in the same active sites. Electrostatic interaction

between the parent NOx molecules, the intermediates, the products and surface

charges on Ta2O5 can reveal further insights into the potential effects of adsorption,

which is beyond the subject of this discussion

+4 +5 • Effect of crystal facets: The higher photocatalytic activity of the Ta /Ta in Ta2O5

matrix may also be ascribed to the trapped electrons and holes at different crystal 55

facets of the matrix. As explained by Zuo et. al. for TiO2 crystallographic plane

() determine the distribution density of the Ti+4/Ti+3 sites which inﬂuences

trapped electron distribution. It is reasonable to expect such contributions impact

the Ta2O5 as well. The distribution of the electrons increases the opportunities to

initiate surface reactions, thus enhancing the photocatalytic activity and should be

examined systematically.

• Effective adsorption: It is reported that the surface oxygen vacancies in oxides

function as critical adsorption sites to promote catalytic reactions. The surface

oxygen vacancies as inferred from EPR analysis in this study, are proposed to

provide better anchorage to the NOx molecules increasing the available active sites

of oxidation on the catalyst surface. The exceptional stability of Ta2O5 as seen to-

ward NOx removal under UV irradiation, could be attributed to the NOx molecules

adhering on the Ta2O5 surface and instantly oxidized, followed by the subsequent

replenishment of fresh molecules at the same active sites. The improved conversion

of NOx on Ta2O5 with minimal NO2 may be attributed to the enhance adsorption of

the NOx on this photocatalyst, relative to TiO2. Electrostatic interaction between

the parent NOx molecules, the intermediates, the products and surface charges on

Ta2O5 can reveal further insights into the potential effects of adsorption, which is

beyond the subject of this discussion 56 HAPTER C 7

TANTALUMOXYNITRIDE:A STRAIGHTFORWARD

SYNTHESISAPPROACH

itrogen derivatives of Ta2O5 are of particular interest because (i) all of the

constituents of TaOxNy(y 1 x) are non-toxic, (ii) they give an option to = − N design color-tuned (as a function of O:N ratio) materials, whose wide-ranged application can be realized in advanced ceramics and inorganic pigments through highly

photoactive semiconductors [25, 56], (iii) they exhibit high stability in aqueous solutions

over the broad range of pH compared to other (oxy)nitrides [9] and (iii) they correctly

bracket the redox potential of water and therefore are promising candidates for the use

of photocatalytic overall watersplitting [1, 2, 44, 45, 57]. Of these oxynitrides, TaON

is of further interest because (i) nitrogen being less electronegative than oxygen, the

contribution from oxygen in holding the electron cloud in TaON is stronger than the

contribution from nitrogen in Ta3N5, and therefore TaON suffers lesser self-oxidation

than Ta3N5 (experimentally, this fact is evidenced in aqueous medium by Hara et.al.,

where they have reported lesser susceptibility of TaON toward hydrolysis than Ta3N5, 57

and the stability of TaON under illumination is reported by Hitoki et. al.[Check this citation, this is just a place holder] [49]), (ii) The deeper valence band maximum (VBM) edge of hybridized O 2P and N 2P orbitals in TaON than the shallow VBM edge due solely to the N 2P orbitals in Ta3N5 have higher oxidation capability as compared to

Ta3N5, and (iii) TaON offers high degree of adsorption and amenability toward surface modiﬁcation leading thereby to enhanced photocatalytic applications such as in realizing overall watersplitting via Z-scheme [44, 58, 59]

Synthesis of TaON, however, is not an obvious chemistry. In particular, the strict requirement of ﬁnding an optimal ratio of Oxygen to nitrogen as demanded by the most stable phase of TaON imposes challenges during its synthesis [49]. Traditionally, the most common route of synthesizing (oxy)nitrides has been the nitridation of corresponding oxides at high temperatures [60–65]. In typical synthesis protocols, the oxide phase

(Ta2O5 for the synthesis of TaON) is treated at elevated temperatures (850 ◦C) as a

ﬂowing stream of nitrogen source is introduced. Dry ammonia, the most common nitrogen source in such synthesis routes, is shown to produce the fully reduced form of Ta2O5

(Ta3N5)[7, 22] while moist ammonia is reported to thermodynamically support the yield of TaON [61, 65, 66]. While the use of gaseous ammonia is not always a ﬁrst choice neither in research facilities nor in commercial plants [67][check this citation.

This is just a placeholder.]Ojha2010ProblemReview due to the gas-phase-associated hazards, additional challenges are encountered due to the requirement of a precise ﬂow of water vapor (together with the ﬂow of dry ammonia) to produce single phase TaON[68].

Therefore, the traditionally popular methods of synthesizing TaON are prone to restrict absolute reproducibility of TaON [65, 68, 69]. Furthermore, defects are intrinsically produced in a material during synthesis and are shown to interfere, and enhance in some instances, with photocatalytic activities [70–73]. Seen in this context also, it is desirable 58

to devise and/or improve on existing nitridation techniques because such defects are borne as a function of multiple number of physico-chemical parameters that themselves are the functions of speciﬁc synthetic routes.

7.1 Simplied Synthesis and outreaching Importance

Ammonia (NH3) is traditionally used as the nitrogen source in the synthesis of (oxy)nitrides.

The idea these methods employ is that NH3 breaks off at high temperatures to give nascent Nitrogen that replaces the Oxygen atoms in the Ta2O5 lattices when the latter is treated at high temperatures ( 850 ◦C. A typical synthesis setup is shown in the ≈ following ﬁgure 7.1.

FIGURE 7.1. Dry NH3 is humidiﬁed later with water vapor so as to treat Ta2O5 at high temperatures (850 ◦C)

NH H O(Vapor) 3+ 2 Ta2O5 TaOxNy −−−−−−−−−−−−→850◦C 59

In such protocols, the water vapor is used to prevent the complete reduction of the Ta2O5 to form Ta3N5. Water vapor is empirically shown to favor the synthesis of

TaON[60, 65, 74]. However, the precise control of two gas/vapor phases sustained for more than an hour invites practical issues with synthesis of TaON.

Preliminary results1 of a simpliﬁed and extremely reproducible method for synthesizing single phase TaON are reported in this discussion. The primary idea presented here is to replace the complex inlet of different phases in traditional synthesis protocols.

Inspired from the fact that the gaseous phase of ammonia (NH3) reacts reproducibly with water (H2O) to give liquid phase NH4OH, it was hypothesized and successfully veriﬁed that the complex nitrogen source used in traditional methods (dry ammonia together with water vapor) can be replaced by a simpler-to-handle liquid phase of NH4OH. The nitridation rate was controlled as a function of two easily controllable parameters: pH of

NH4OH solution and the temperature of heating bath. As the reaction between NH3 and

H2O is reversible, the liquid phase NH4OH gives off a mixture of ammonia gas and water vapor upon raising its temperature, and it serves as the reliable source of humidiﬁed nitrogen as required in the synthesis of TaON. The highlights of this technique are: (i) robustly reproducible method of single phase TaON synthesis that is least associated with hazards and is well suited for adoption in laboratory as well as in commercial facilities, and (iii) cost-effective method because of the simpler design that replaces

ﬂowmeters with a constant temperature bath, and costly anhydrous ammonia gas with cost-friendly ammonium hydroxide. Further experimental details of this new approach are illustrated through a schematic in ﬁgure 7.2.

1Manuscript under progress. . . 60

FIGURE 7.2. Schematic diagram of the ammonolysis setup: (1) NH4OH:H2O (v/v) = 150 :50; (2) Ethylene Glycol: Constant temperature bath; (3) Heating Plate (320 ◦C turned on after the tube furnace reaches 700 ◦C; (4) inlet control valve (Turned on together with the heating plate); (5) inlet carrier tube (slanted on purpose); (6) Excess Vapor trap; (7) Tube furnace (800 ◦C); (8) a thermocouple & 0.5 g Ta2O5 sample containing glass boat; (9) Backﬂow (from exhaust tank) preventer; (10) Exhaust Tank.

7.2 Experimental

In a typical synthesis of TaON, Ta2O5 nanoparticles as synthesized and discussed in previous chapters was used as the precursor material. Strong Ammonium hydroxide

(Fisher Scientiﬁc) was used as the nitrogen source. Deionized water was used for all dilution purposes. A starting solution of NH4OH and H2O in the ratio 150 : 50(v/v) was taken in a round bottomed (RB) ﬂask, and was heated under constant temperature bath of ethylene glycol. 0.5 g of Ta2O5 was used as the oxide precursor that was placed in a 61

customized borosilicate glass-boat in a tube furnace (Thermo Scientific). The tube furnace was equipped with a programmable temperature sensor. The exhaust of the RB flask containing the nitrogen source was connected to another empty RB flask as shown in

figure 7.2 whose exhaust was connected to the tube furnace. The purpose of introducing this empty RB flask was to absorb unwanted water moisture during the passage of the humidified nitrogen source. The humidified nitrogen source travelled all the way to the exhaust (10) through the oxide precursor ( Ta2O5) put inside the tube furnace at temperature 800 ◦C before meeting another empty RB flask. The unreacted ammonia gas got mixed with the water in the reservoir to form liquid ammonium hydroxide (NH3 +

H O NH OH) and hence prevented release of the excess ammonia in the open 2 −−−→ 4 atmosphere.

7.3 Physical Characterization

The characterization of the photocatalysts was performed using several complementary tools. High resolution transmission electron microscopy ((HR)TEM) analysis was performed using a JEOL 2100F instrument equipped with a selected area electron diffraction (SAED) analyzer. The optical properties of the powder were examined using a

UV–vis diffuse absorbance measurement with a Shimadzu UV 2501PC spectrophotometer. Brunauer Emmett- Teller speciﬁc surface areas (SBET) of the synthesized materials were determined using a micromeritics system. X-ray diffraction patterns were taken using a Bruker D8 diffractometer (Cu Kα radiation, 40 mA, 40 kV). 62

7.3.1 XRD of TaON

FIGURE 7.3. Phase structure changes from Ta2O5 to TaON

7.3.2 Physical features and crystallinity of the nanoparticles

TEM analysis is important because a smaller size of the in-house synthesized samples can be indicative of availability of more surface, which is consistent with a surfactant- assisted method used for synthesis. The TEM and HRTEM analysis of the in-house synthesized thermally annealed samples before and after treatment under ammonia flow is shown in figure7.4. All annealed samples show distinct evidence of crystallinity as indicated in the HRTEM. Furthermore, the presence of the geometric diffraction patterns in the SAED confirms thermally induced crystallization. The spacing are marked in the 63

figure and suggest that the samples are TaON. This is further confirmed by their x-ray diffraction patterns as shown in the same figure.

FIGURE 7.4. High temperature nitridation of Ta2O5 has not signiﬁcantly changed the morphology. The SAED pattern conﬁrms that the newly obtained sample is polycrystalline, and is TaON

7.4 Optical Characterization

The nanoparticles were deposited as films on FTO coated glass before determining their optical response. The absorbance spectra of the films and its onset estimate are shown in figures 7.5 while the inset shows photographs of the powder prior to their deposition.

The TaON is yellow in color and shows an onset absorbance of around 580 nm. As can be seen, the absorbance pattern lies intermediate to the two extreme phases Ta2O5 and

Ta3N5. It is interesting to note that the color of the materials is also an indication of the difference in absorbance patterns and onsets. This onset is also an evidence that the photoactivity is now driven by visible light. 64

FIGURE 7.5. The absorbance edge lies intermediate to the two extreme phases: Ta2O5 and Ta3N5

7.5 Photoelectrochemical Characterization

The analysis of photoelectrochemical or PEC data offer valuable information into the separation and transport mechanisms of charges photogenerated in the catalysts upon illumination. This approach allows to track both hole and electrons upon generation in the catalysts and can be correlated with the photocatalytic activity. The chronoamperometry or j/t responses of the as-synthesized TaON was obtained with a Pt wire as the counter electrode in a 3-electrode PEC cell. The electrolyte was alkaline to facilitate the removal of holes using hydroxyl ions OH – + h+ OH . The multiple on-off cycles show that the −−−→ · 65

response in both films are light triggered and the reproducible nature of these responses indicate that the films formed on the conducting glass slides are reproducible. The films after the PEC measurements indicated that indicated that they are stable (do not delam-

2 inate). The TaON photocurrent (illuminated – dark) of 8 µA/cm while the parent Ta2O5 response is approximately 0.85 µA/cm2. This increase 8- fold increase in photocurrent ≈ by the TaON could be attributed to the difference in the improved photoactivity of this

ﬁlm, but has to be veriﬁed using alternative approaches (discussed below).

FIGURE 7.6. Charge generation, separation, and transport in TaON is seen to be signiﬁcantly improved compared to the parent oxide phase Ta2O5

The linear sweep voltammogram or j/V plot of the films are shown in figure 7.6. The zero current potential is a measure of the apparent flat band potential in the film and is often estimated to provide an insight into the band bending in particulate films. A more negative shift is indicative of charges separating effectively and becoming available for transport or redox activity. At -0.96V vs Ag/Agcl, TaON shows almost 40 mV of negative shift compared to the parent oxide Ta2O5 is thus indicative of a better charge separation compared to the parent Ta2O5. 66

7.6 Comparative Impedance analysis of Ta2O5 and

TaON

The basis for performing the impedance analysis on the tantalum-based photocatalysts is to gain insights into ﬁlm properties such as the n-/p-characteristics and a qualitative estimate of the extent of charge separation. The plot of the impedance response for the

TaON is shown in ﬁgure 7.6. Firstly, the positive slope evident from all of the samples indicate that the Ta2O5 and TaON demonstrate n-type characteristics. Secondly, the

Ta2O5 in-house sample shows a low radius of curvature compared to the commercial sample indicating that charge seperation is better with the in-house samples. Finally, the application of the nitridation process to Ta2O5 to form TaON is beneficial as it shows the lowest radius of curvature indicating the most effective transport among the three photoactive films. Thus, the series of PEC measurements indicate that i) the in-house Ta2O5 and TaON demonstrate photocurrent generation, ii) reproducible and stable response, and iii) the TaON demonstrates a distinguishable enhancement in the photocurrent response compared to the Ta2O5 films which can be attributed to the better charge separation, transport across the film thickness, and collection at the underlying

FTO plate.

7.7 Control over nitridation

7.7.1 Temperature of the bath

The nitridation was controlled by two independent parameters temperature of the bath, and the pH of the solution. The temperature of the constant temperature bath can be tuned so as to control the evaporation rate of ammonium hydroxide and hence the inﬂux of ammonia and the water vapor. Higher temperature of the bath would ramp the ﬂow 67

rate up as well exhausting the nitrogen source too early before proper nitridation occurs.

Too low temperature of the bath would limit the influx, on the other hand. Therefore, considering the boiling point of water 100 ◦C and that of ammonium hydroxide to be 25 ◦C and the requirement of maintaining the continuous supply of nitrogen source through bubbling the exhaust gas through the water reservoir an optimal temperature of the bath was to be decided. After a preliminary assessment, the constant temperature of the bath fixed at 320 ◦C was good enough to drive the required continuous flux for a period of an hour.

7.7.2 pH of the ammonium hydroxide solution

The pH of the NH4OH solution in above process is an important parameter because the more the pH of the solution, the richer is the inﬂux in terms of nitrogen content.

Therefore, the ammonolysis was performed at various pH conditions as shown in ﬁgure

7.7.

FIGURE 7.7. Phase evolution of Ta2O5 toward TaON as a function of pH of the NH4OH solution. 68

7.8 Thermal Stability of the Tantalum Oxynitride

As can be seen in 7.7, the phase of Ta2O5 transforms towards TaON as the pH is increased from lower to higher values. In a typical experiment, a pH of 13 was seen to be optimal. Also, a control experiment with the ammonium hydroxide alone also was performed keeping the other parameters same, which yielded a mixed phase of Ta3N5 and

TaON, and the coloration turned toward reddish from clean yellow. Such mixed phases are reported in literature for similar attempts of ammonolyis. During the experiments, it was observed that the attempts of ammonolysis of Ta2O5 for longer durations starting with the same volume of the mixture (NH4OH:H2O=150:50(v/v)) resulted in the phase reversal. For shorter duration of nitridation, one could receive brilliant yello colored

TaON while for the longer periods (keeping the nitrogen source volume ﬁxed) the phase would lose its color. Upon XRD tests,it was conﬁrmed that the new white color of the product corresponds with the starting oxide precursor Ta2O5.

FIGURE 7.8. Phase evolution of Ta2O5 toward TaON as a function of pH of the NH4OH solution.

It is well known that oxides can withstand high temperatures. But do the nitrogen 69

derivatives of Ta2O5 also show the same resilience toward high temperature oxidative treatments? The answer was available in the above observation. Analyzing the above observation, it can be seen that the nitridation ceases after a certain period depending on the starting volume of nitrogen source in the particular experiment. Essentially, only the high temperature heat treatment is performed instead of nitridation. Also, the resulting white color suggests that the nitrided compound loses its nitrogen over the high temperature oxidative treatments. Later on, with XRD analysis, it was conﬁrmed that the oxidized sample was indeed Ta2O5. Additionally, it was observed that the fully reduced form of Ta2O5 (Ta3N5, formed with the dry ammonia as the nitrogen source) also transforms back to Ta2O5 when treated at high temperatures. Therefore, it can be concluded that the nitrogen derivatives of Tantalum oxide are not stable at high temperatures. The results are summarized in ﬁgure 7.8

7.9 Photocatalytic activity of Newly Synthesized

Tantalum Oxynitride

The photocatalytic activity of TaON produced by newly devised synthetic protocol was assessed by tracking the decrease in dye absorbance in the visible spectrum. Figure

7.9 shows the changes to the dye absorbance at 664 nm in the absence and presence of catalysts, since photolytic conversion of MB is reported [23]. As evident in Figure 7.9,

TaON shows around 80% conversion, while its parent oxide Ta2O5 shows around 30% conversion. Furthermore, the effect of light on Ta2O5 and TaON are clearly seen different.

The rate at which the concentration of MB falls is faster in case of TaON compared to the parent oxide (Ta2O5) after the light is turned on, which can be attributed to the visible light driven photocatalysis due to the lowered bandgap of TaON. 70

FIGURE 7.9. Photodegradation of 28.6 µM MB under 480 W Solar simulator lamp. Photocatalyst: 0.075 g TaON.

Figure 7.9 also shows the change in the absorbance of the dye solution at various time before and after illumination. Compared to the control experiments (catalyst- free conditions) a higher dye conversion is observed attributable to the degradation initiated in the presence of the TaON. During equilibration, the dark absorbance region is remarkably different for the two photocatalysts: Ta2O5 & TaON. Considering the fact that the TaON particles are bigger than the Ta2O5, interestingly enough a drop in absorbance of the dye is noted during dark equilibration with TaON. This is indicative of a very effective adsorption of the dye on the TaON surface than in the Ta2O5 particles.

Preliminary estimates of the fractional conversion of the dye using TaON was determined to be 80%. The role of the signiﬁcantly adsorption of MB molecules on TaON surface than

Ta2O5 surface in promoting photocatalysis remains to be fully understood, while the faster kinetics of the MB degradation can be attributed to the better charge generation owing to the lowered bandgap, and the better charge separation and transport compared to Ta2O5. 71

7.10 Examining the nitridation protocol with other

typical Oxide catalysts

The amenability of Ta2O5 toward easier incorporation of nitrogen into its crystal and lattice transformation was demonstrated in contrast to other popular catalysts such as

TiO2 and SrTiO3. when tried for nitridation under the same protocol, they underwent a slight change in color but the absorbance pattern did not signiﬁcantly change.

7.10.1 Titanium Dioxide (P25)

As shown in ﬁgure 7.10, TiO2 exhibited slight change in absorbance pattern. Slight yellowish color also was observed, but the XRD patterns remained the same.

FIGURE 7.10. Absorbance edge of TiO2 is not changed, but absorbance gets more steeper.

The change in absorbance pattern of TiO2 may be ascribed to the incorporation of some nitrogen atoms in the vacancy sites of the TiO2 lattice. Such nitrogen atoms, in the form of dopants, can introduce extra shallow energy states near conduction band.

Upon photon excitation of TiO2, the electrons can make easier transitions to those states 72

causing slight red-shift in the absorption edge. Similar observations are observed in nitrogen doped TiO2 in literature too [75].

Bandgap estimation from Tauc’s plot analysis showed an approximately 15% lowering in the bandgap of the material.

7.10.2 Nitrided Strontium Titanate

As shown in ﬁgure 7.11, SrTiO3 exhibited slight change in absorbance pattern. Slight bluish color also was observed, but the change in XRD patterns remained unnoticeable .

FIGURE 7.11. No signiﬁcant change in the absorbance pattern observed in SrTiO3 after the nitridation attempt.

The change in absorbance pattern of SrTiO3 may be ascribed to the incorporation of some nitrogen atoms in the vacancy sites of the SrTiO3 lattice. Such nitrogen atoms, in the form of dopants, can introduce extra shallow energy states near conduction band.

Upon photonic excitation of SrTiO3, the electrons can make energetic transitions to those states causing slight red-shift in the absorption edge. Similar observations are reported 73

in nitrogen doped SrTiO3 in literature too[76].

Bandgap estimation from Tauc’s plot analysis showed no noticeable change in bandgap lowering.

7.10.3 Overarching signiﬁcance of the newly introduced

simpliﬁcation in conventional nitridation methods

Partial nitridation can lead to TaOXNY, where x y or x y. In particular, the strict <> = requirement of ﬁnding an optimal ratio of oxygen to nitrogen as demanded by the most stable phase of TaON imposes challenges during its synthesis.[49] Traditionally, the common route of synthesizing TaON has been the gas phase nitridation of corresponding oxides at high temperature. Ammonia, the most common nitrogen source used in such synthesis routes, is shown to produce the fully reduced form of Ta2O5 (i.e.

Ta3N5)[7, 77, 78] while moist ammonia is reported to thermodynamically support the yield of TaON.[60, 61] Challenge of controlling the precise flowrate of multiple fluids in such approaches often leads to the formation of various mixed (oxy)nitrides. On the other hand, the use of gaseous ammonia is not always a first choice - neither in research facilities nor in commercial plants due to the associated hazards.48 Additional challenges are encountered due to the requirement of a precise flow of water vapor (together with the flow of dry ammonia) to produce single phase TaON. [74]Therefore, the traditionally popular methods of synthesizing TaON are prone to restrict absolute reproducibility of

TaON.[69, 74]

A simpliﬁed and reproducible method for synthesizing single phase TaON is reported in this discussion (ﬁgure 7.2). The primary idea presented here is to replace the complex inlet of multiple phases in traditional synthesis protocols. Inspired from the fact that 74

the gaseous phase of ammonia (NH3) reacts reproducibly with water (H2O) to give liquid phase ammonium hydroxide (NH4OH), we hypothesized and successfully veriﬁed that the complex nitrogen source used in traditional methods (dry ammonia together with water vapor) can be replaced by a simpler-to-handle liquid phase of NH4OH. In such an approach, the nitridation rate can be controlled as a function of two easily controllable parameters: pH of NH4OH solution and the temperature of heating bath. As the reaction between NH3 and H2O is reversible, the liquid phase NH4OH gives off a mixture of ammonia gas and water vapor when heated, and it serves as the reliable source of humidiﬁed nitrogen as required in the conventional synthesis of TaON. The highlights of this technique are: (i) robustly reproducible method of single phase TaON synthesis that is least associated with gaseous hazards, (ii) well suited for adoption in laboratory as well as in commercial facilities, and (iii) cost-effective miniaturized method that replaces

ﬂowmeters with a constant temperature bath, and costly anhydrous ammonia gas with cost-friendly ammonium hydroxide. 75 HAPTER C 8

CONCLUSION

The synthesis of high surface area Ta2O5 nanoparticles using a surfactant assisted process and its subsequent nitridation to yield two distinct nitrogen derivatives: TaON and Ta3N5 has been successfully demonstrated. The physical features, phase evolution, and optical properties was systematically analyzed and indicates that tight size control in all classes of as-synthesized materials can be achieved. Ta2O5, in particular, exhibits signiﬁcant enhancement in photocatalytic performance toward liquid-phase as well as gas-phase pollutants compared to the commerical equivalent. Typically, Ta2O5 shows excellent stability, higher overall conversion, and doubled photonic efﬁciency toward NOx

(Oxides of nitrogen, representative air pollutants) removal compared to the archetypical catalyst TiO2 (P25)

Series of photoelectrochemical characterization conﬁrmed that the incorporation of nitrogen is beneﬁcial in terms of extending the optical absorption edge of the nitrogen derivatives. Preliminary results indicate that the fully reduced nitrogen derivative of

Ta2O5 - Ta3N5 may be used as a photocatalyst for photo-oxidative reactions owing to its promising adsoprtion and photocatalytic activity toward various representative dye 76

molecules.

A straightforward simpliﬁcation in the conventionally existing methods of synthesizing TaON was successfully demonstrated. The method reproducibly yields single phase TaON and shows signiﬁcant enhancement in photoelectrochemical responses. In particular, more than 100-fold improvement in charge generation compared to the parent

Ta2O5 is observed, and qualitative assessments in the charge separation and transport translates into similar results. The newly tested and veriﬁed synthesis protocol aids not only in the simpliﬁcation, but also lowers the associated hazards while making the whole process cost-competitive. The highlights of this technique can be summarized as (i) robustly reproducible method of single phase TaON synthesis (ii) free of hazards associated with gas phase control of NH3, (iii) well suited for adoption in laboratory as well as in commercial facilities, and (iv) cost-effective method because of the simpler design that miniaturizes the setup and replaces costly anhydrous ammonia gas with cost-friendly ammonium hydroxide. 77

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