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Doctoral Dissertations University of Connecticut Graduate School
4-15-2019 Design of Transition Metal Oxide and Phosphide Nanomaterials: Their aC talytic Activities Md Mahbubur Rahman Shakil University of Connecticut - Storrs, [email protected]
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Recommended Citation Shakil, Md Mahbubur Rahman, "Design of Transition Metal Oxide and Phosphide Nanomaterials: Their aC talytic Activities" (2019). Doctoral Dissertations. 2096. https://opencommons.uconn.edu/dissertations/2096 Design of Transition Metal Oxide and Phosphide Nanomaterials: Their Catalytic Activities
Md Mahbubur Rahman Shakil, Ph. D.
University of Connecticut, 2019
This thesis is focused on developing transition metal-based oxide and phosphide nanomaterials for catalytic application in energy conversion and the organic transformation reaction. The research projects presented in this thesis are: (1) the design and synthesis of transition metal doped ZnO catalysts for the oxygen reduction reaction (ORR), (2) the development of mesoporous FeP and
CoP materials as hydrogen evolution reaction (HER) catalysts for electrochemical water splitting, and (3) the fabrication of different ZnO morphologies for coumarin synthesis by the Knoevenagel condensation.
The first chapter describes the significance and background of the three research projects. We emphasize the current challenges in developing HER and ORR catalysts for water electrolysis and fuel cell application. Additionally, the importance of coumarins and challenges in their synthesis via the Knoevenagel condensation is included in this chapter.
The second chapter contains the synthesis and characterization of transition metal (Mn, Fe, Co, and Ni) doped ZnO nanocrystals. The goal of this study is to develop ZnO based low-cost ORR catalysts. Single-doped and multi-doped transition-metal ZnO samples are synthesized to investigate the effects of doping in the ZnO structure and their activities for ORR. The ORR Md Mahbubur Rahman Shakil – University of Connecticut, 2019 activity of the ZnO samples is governed primarily by the oxygen vacancies created as a result of the incorporation of dopant elements.
The third chapter is comprised of developing mesoporous FeP and CoP nanomaterials as an efficient HER catalyst. A noble approach, the inverse micelle sol-gel method, is utilized to synthesize mesoporous FeP, Co-FeP, CoP, Ni-CoP, and Ni-CoP/CNT materials. The mesoporosity and HER activity of the materials are monitored with the dopants and CNT support. The HER activity of the catalysts predominantly alters with the charge-transfer capabilities of the materials.
The fourth chapter includes the preparation of different ZnO morphologies as catalysts for coumarin synthesis. The ZnO properties of different morphologies and their catalytic activities for coumarin synthesis by the Knoevenagel condensation reaction are examined. Catalysts synthesized using methanol show the highest activity. The enhanced activity of the ZnO synthesized in methanol is attributed to the combined effects of relatively high surface area, pore volume, and pore sizes of the materials. Design of Transition Metal Oxide and Phosphide
Nanomaterials: Their Catalytic Activities
Md Mahbubur Rahman Shakil
B. Sc., University of Dhaka, 2006
M. S., University of Dhaka, 2008
A Dissertation
Submitted in Partial Fulfilment of the
Requirement for the Degree of
Doctor of Philosophy
at the
University of Connecticut
2019
i
Copyright by
Md Mahbubur Rahman Shakil
2019
ii
APPROVAL PAGE
Doctor of Philosophy Dissertation
Design of Transition Metal Oxide and Phosphide
Nanomaterials: Their Catalytic Activities
Presented by
Md Mahbubur Rahman Shakil, B.Sc., M.S.
Major Advisor…………………………………………………………………………………….. Steven L. Suib
Associate Advisor…………………………………………………………………………………. Alfredo Angeles-Boza
Associate Advisor………………………………………………………………………………….
Gaël Ung
University of Connecticut 2019
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Dedicated to
My Parents
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Acknowledgements
I would like to express my deepest gratitude to my advisor Dr. Steven L. Suib for his guidance, encouragement, and continuous support throughout my graduate studies. I would also like to thank my associate advisors Dr. Alfredo Angeles-Boza, Dr. Jing Zhao, Dr. Gaël Ung, and Dr. Fatma
Selampinar for their advice, support, and encouragement during my graduate career. In addition, I am so grateful to Dr. Howell for her support and encouragement from the first day of my graduate life.
I would like to thank Mrs. Bonnie Suib for encouragement and hospitality. I acknowledge Dr.
Galasso for his help in a project and continuous encouragements about research.
I would like to express my sincere thanks and gratitude to Dr. Seraji for his many many helps and constant inspiration in my entire graduate life. I am also thankful to Drs. Lakhsitha, Sourav, Curt,
Niluka, Tahereh, Miao, and Wei. My appreciation also goes to Andrew, Yang, John, Alireza, Peter,
Shanka, Junkai, Jacob (my undergrad), and all present and past Suib group members.
I am thankful to the Department of Chemistry and Physics for the teaching assistantships in my graduate career. I am grateful to all employees (teaching instructors, lab technicians) of the departments for their help and support. I would like to acknowledge the U.S. Department of Energy for their financial support of my research.
I would like to express my heartiest gratitude to my wife, Habiba Tasnim and our loving son, Talha for being patient with me and their continuous support for everything. Finally, I would like to express my special thanks to my parents for their unconditional and endless support, love and all continuous efforts for me from day one to till today.
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Table of Contents
CHAPTER 1. INTRODUCTION……………………………………………………………….1
1.1 Overview…………………………………………………………………………………....1
1.2 Background of this Research…………………………………………………………….....2
1.2.1 Transition Metal-Doped (Mn, Fe, Co, and Ni) ZnO Catalysts for the Oxygen
Reduction Reaction…………………………………………………………………...2
1.2.2 Mesoporous Transition Metal Phosphides (FeP, CoP, and their TM-doped
analogs) for the Hydrogen Evolution Reaction……………………………………….6
1.2.3 Synthesis of Various Morphologies of ZnO for Coumarin Synthesis via the
Knoevenagel Condensation Reaction…………………………………………...... 12
References……………………………………………………………………………………..14
CHAPTER 2. SINGLE-DOPED AND MULTIDOPED TRANSITION METAL (Mn, Fe, Co, AND Ni) ZnO AND THEIR ELECTROCATALYTIC ACTIVITIES FOR OXYGEN REDUCTION REACTION……………………………………………………………………24
2.1 Overview…………………………………………………………………………………...24
2.2 Introduction………………………………………………………………………………...25
2.3 Experimental Section………………………………………………………………………27
2.3.1 Chemicals…………………………………………………………………………….27
2.3.2 Materials Synthesis…………………………………………………………………...27
2.3.3 Materials Characterization…………………………………………………………...27
2.3.4 Electrochemical Measurements ……………………………………………………...29
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2.4 Results……………………………………………………………………………………...29
2.4.1 XRD Analysis………………………………………………………………………...29
2.4.2 Surface Morphology and Surface Area Analysis……………………………………..31
2.4.3 Chemical Composition Analysis……………………………………………………..34
2.4.4 XPS Analysis…………………………………………………………………………35
2.4.5 EPR Analysis………………………………………………………………………...40
2.4.6 Optical Analysis (Raman and Photoluminescence Spectroscopy)……………………41
2.4.7 ORR Activity Evaluation of the Synthesized ZnO Samples…………………………44
2.5 Discussion …………………………………………………………………………………48
2.5.1 Phase Identification of the Materials………………………………………………….48
2.5.2 Surface Morphology of the Materials………………………………………………...49
2.5.3 Presence of Dopants in the Synthesized Materials……………………………………49
2.5.4 Oxidation States of the Dopants within the Materials………………………………...50
2.5.5 Location and Co-ordination of Dopants within the Materials………………………...52
2.5.6 Phase Purity of Synthesized ZnO Materials and the Presence of Defects……………..52
2.6. Conclusions………………………………………………………………………………...55
References………………………………………………………………………………………56
CHAPTER 3. TRANSITION METAL (Co & Ni) DOPED MESOPOROUS CoP AND FeP NANOMATERIALS FOR THE HYDROGEN EVOLUTION REACTION………………64
3.1 Overview…………………………………………………………………………………...64
3.2 Introduction………………………………………………………………………………...65
3.3 Experimental Section………………………………………………………………………67
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3.3.1 Chemicals…………………………………………………………………………….67
3.3.2 Synthesis of Mesoporous Fe3O4, 5 % Co -Fe3O4 and Co3O4, 5 % Ni-Co3O4 and 5 %
Ni-Co3O4 / CNT)……………………………………………………………………..67
3.3.3 Synthesis of Mesoporous FeP, 5 % Co-FeP and CoP, 5 % Ni-CoP, 5 % Ni-CoP/ CNT
Catalysts…...... 68
3.3.4 Characterization……………………………………………………………………...69
3.3.5 Electrochemical Measurements………………………………………………………70 3.3.5.1 Working Electrode Preparation………………………………………………70 3.3.5.2 Electrochemical Tests………………………………………………………...70 3.4 Results……………………………………………………………………………………...71 3.4.1 Crystalline Structure of Mesoporosities of Oxide Precursors ……………………….71 3.4.2 Characterization of Phosphide Materials…………………………………………….75 3.4.3 HER Performance of the Synthesized Mesoporous Phosphide Materials…………...88 3.4.3 Stability of Mesoporous FeP Catalyst………………………………………………..92 3.5 Discussion………………………………………………………………………………….93 3.6 Conclusions………………………………………………………………………………...98 References………………………………………………………………………………………98
CHAPTER 4. SYNTHESIS OF ZnO WITH DIFFERENT MORPHOLOGIES: THEIR CATALYTIC ACTIVITIES TOWARD COUMARIN SYNTHESIS VIA THE KNOEVENAGEL CONDENSATION REACTION………………………………………..108
4.1 Overview…………………………………………………………………………………108
4.2 Introduction………………………………………………………………………………108
4.3 Experimental Section…………………………………………………………………….110
4.3.1 Materials……………………………………………………………………………110
4.3.2 ZnO Preparation…………………………………………………………………….111
4.3.3 Materials Characterization………………………………………………………….111
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4.3.4 Catalytic Activity Measurements…………………………………………………...112
4.4 Results……………………………………………………………………………………113
4.4.1 Structural Characterization…………………………………………………………113
4.4.2 Morphology and Growth Face Analysis of ZnO Catalysts………………………….115
4.4.3 Optical Analysis……………………………………………………………………120
4.4.4 Textural Properties (Surface Area and the Porosity) Analysis…………………….114
4.4.5 Basicity Analysis of ZnO Catalysts………………………………………………..126
4.4.6 Studies of Catalytic Activity of the Prepared ZnO samples………………………...127
4.4.5.1 Optimal Conditions for Catalytic Reaction…………………………………128
4.4.5.2 Catalytic Performance of the ZnO Samples…………………...... 130
4.4.7 Reusability, Stability, and Heterogeneity of the Catalysts………………………….132
4.5 Discussion………………………………………………………………………………..134
4.5.1 Structure of Synthesized ZnO Samples…………………………………………….134
4.5.2 Catalytic Activity of ZnO in Coumarin Synthesis via Knoevenagel Condensation
Reaction…………………………………………………………………………...136
4.6 Conclusions………………………………………………………………………………138
References……………………………………………………………………………………138
FUTURE WORK……………………………………………………………………………...146
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List of Figures
Figure 1.1. A schematic diagram of a fuel cell…………………………………………………….2
Figure 1.2. Mechanism of HER in acidic media………………………………………………….7
Figure 1.3. Exchange current density as a function of hydrogen adsorption free energy for various
HER catalyst materials. (a) The experimental “volcano plot” for the HER (reprinted from ref.25 with permission.), (b) The theoretical HER volcano plot. The data taken from the ref.21 with permission…………………………………………………………………………………………8
Figure 1.4. (a) Triangular prism structures of phosphide, (b) MnP-type orthorhombic crystal structure of FeP. Data taken from ref.40 with permission………………………………………..10
Figure 1.5. Benzo-α-pyrone (2H-1-benzopyran-2-one) ring of coumarin family………………12
Figure 1.6. Schematic representation of the Knoevenagel condensation reaction………………..13
Figure 2.1. XRD patterns of transition metal (Ni, Co, Fe, and Mn) doped ZnO samples (a) single doped ZnO (b) multi-doped ZnO samples……………………………………………………….30
Figure 2.2. XRD peak for (101) plane of the single doped ZnO materials (left) and multi doped
ZnO materials (right)…………………………………………………………………………….30
Figure 2.3. SEM photograph of (a) undoped ZnO (b) 5% Ni-ZnO (c) 5% Fe-ZnO (d) 5% Co-ZnO
(e) 5% Mn-ZnO…………………………………………………………………………………..32
Figure 2.4. SEM images (a) Undoped ZnO (b) 10 % MD-ZnO (c) 15% MD-ZnO and (d) 20 %
MD-ZnO…………………………………………………………………………………………33
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Figure 2.5. EDX spectra of (a) 20% MD-ZnO, (b) 5% Fe-ZnO, (c) Table of EDS elemental composition of MD-ZnO and (d) Fe-doped ZnO (inset: percentage of elements in single doped
ZnO)……………………………………………………………………………………………...35
Figure 2.6. XPS spectra of synthesized 20% MD-ZnO sample, (a) XPS survey spectrum of 20%
MD-ZnO, (b) High-resolution XPS spectra of Zn 2p peaks, (c) High-resolution XPS spectra of
Ni 2p peaks, (d) High-resolution XPS spectra of Co 2p peaks, (e) High-resolution XPS spectra of
Mn 2p peaks, (f) High-resolution XPS spectra of Fe 2p peaks (g) High-resolution XPS spectra of
O 1s peak. ………………………………………………………………………………………..40
Figure 2.7. The room temperature EPR data of single doped ZnO and multi-doped ZnO (MD ZnO) samples with the hyperfine lines of Mn-ZnO (inset)……………………………………………...41
Figure 2.8. Raman spectra of (a) single doped ZnO, (b) multi-doped ZnO samples……………43
Figure 2.9. Room temperature PL spectra of synthesized samples, (a) Emission spectra of samples
(excited at 325 nm), (b) Excitation spectra (emission wavelength set at 550 nm). Note: WO = without…………………………………………………………………………………………...44
Figure 2.10. LSV curves of (a) single doped ZnO and (b) multi-doped ZnO. Potentials are recorded in volts vs. SCE, and then converted to RHE……………………………………………………45
Figure 2.11. Chronoamperometric i-t responses for 5 % Mn-ZnO compared with Pt/C at potential of -0.25 V vs. RHE. (Note: % retention = retention of initial current (%) at a particular time)……47
Figure 3.1. Wide-angle XRD patterns of synthesized meso oxide samples.……………………...72
Figure 3.2. Low-angle XRD data of (a) meso Co3O4 samples; (b) meso Fe3O4 samples..………72
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Figure 3.3. N2 sorption isotherms of the synthesized oxides (a) meso Co3O4 samples; (b) meso
Fe3O4 samples……………………………………………………………………………………75
Figure 3.4. BJH pore size distributions of synthesized (a) meso Co3O4; (b) meso Fe3O4 samples
……………………………………………………………………………………………………75
Figure 3.5. XRD patterns of (a) meso CoP and meso 5 % Ni-CoP; (b) meso 5% FeP and meso 5
% Co-FeP materials (XRD patterns of meso Co3O4 and meso Fe3O4 are shown to compare with the phosphides phase)……………………………………………………………………………77
Figure 3.6. Low-angle XRD patterns of mesoporous (a) CoP and 5 % Ni-CoP; (b) FeP and 5%
Co-FeP materials…………………………………………………………………………………77
Figure 3.7. N2 sorption isotherms of synthesized (a) meso CoP and meso 5 % Ni-CoP samples;
(b) meso FeP and meso 5 % Co-FeP samples.…………………………………………………...78
Figure 3.8. BJH pore size distributions of synthesized (a) meso CoP and meso Ni-CoP; (b) meso
FeP and meso Co-FeP materials…………………………………………………………………79
Figure 3.9. SEM images of synthesized (a) meso CoP; (b) meso 5 % Ni-CoP; (c) meso 5 % Ni-
CoP / CNT; (d) meso FeP; (e) meso 5 % Co-FeP samples………………………………………80
Figure 3.10. TEM images of (a) meso FeP; (b) HR-TEM image of meso FeP with identified diffraction planes (inset); (c) HAADF, High-angle annular dark-field; and (d), (e) EDX- elemental mapping of Fe, and P, respectively.……….………………………………………….81
Figure 3.11. TEM images of (a) meso 5 mol % Co-FeP; (b) meso 5 mol % Ni-CoP (c) meso CoP materials.…………………………………………………………………………………………82
Figure 3.12. EDX elemental mapping of meso 5 mol % Co-FeP samples. (a) HAADF, High- angle annular dark-field; (b), (c), (d) Elemental mapping of Fe, P, and Co, respectively.………83
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Figure 3.13. EDX elemental mapping of meso 5 mol% Ni-CoP samples. (a) HAADF, High- angle annular dark-field; (b), (c), (d) Elemental mapping of Co, P, and Ni, respectively……….84
Figure 3.14. EDX elemental mapping of meso CoP samples. (a) HAADF, High-angle annular dark-field; (b) and (c) Elemental mapping of Co and P………………………………...……….84
Figure 3.15. (a) XPS survey spectrum of meso FeP sample, (b) High-resolution XPS spectrum of
C 1s (c) High-resolution XPS spectrum of O 1s…………………………………………………85
Figure 3.16. High-resolution XPS spectra of meso FeP: (a) Fe 2p region; (b) P 2p region…….86
Figure 3.17. XPS spectra of meso CoP: (a) Survey spectrum, (b) High-resolution XPS spectrum of O 1s, (c) High-resolution XPS spectrum of Co 2p, (d) High-resolution XPS spectrum of P 2p.
……………………………………………………………………………………………………86
Figure 3.18. XPS spectra of meso 5 % Ni-CoP: (a) Survey spectrum, (b) High-resolution XPS spectrum of Co 2p, (c) High-resolution XPS spectrum of Ni 2p, (d) High-resolution XPS spectrum of P 2p, (e) High-resolution XPS spectrum of O 1s……………………………………………….87
Figure 3.19. XPS spectra of meso 5 % Co-FeP: (a) Survey spectrum, (b) High -resolution XPS spectrum of Co 2p, (c) High-resolution XPS spectrum of Fe 2p, (d) High-resolution XPS spectrum of P 2p, (e) High-resolution XPS spectrum of O 1s……………………………………………….88
Figure 3.20. LSV curves of the synthesized mesoporous phosphide materials. LSVs were recorded using SCE as a reference electrode. All potentials are then referenced to RHE using the conversion formula (E (vs. RHE) = E (vs. SCE)V +0.261V )………………………………………………...89
Figure 3.21. EIS spectra of the synthesized mesoporous phosphide materials………………….91
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Figure 3.22. Chronopotentiometric response of meso FeP under constant current density of 10
2 mA / cm in 0.5 M H2SO4 solution. (Potential shown in the figure is vs SCE)………………….93
Figure 4.1. XRD patterns of all ZnO samples synthesized in different solvents………………114
Figure 4.2. SEM images of: (a) ZnO in Ethanol, (b) ZnO in Acetonitrile, (c) ZnO in Water, (d)
ZnO in Methanol, (e) ZnO in DEA, (f) ZnO in 1-propanol…………………………………….116
Figure 4.3. TEM images (A-E) of synthesized ZnO samples with different morphologies: (x)
TEM image, (y) HR-TEM image; Inset: FFT and inverse FFT images of the selected area…..119
Figure 4.4. UV-Vis absorption spectra of the prepared ZnO samples………………………...121
Figure 4.5. Raman spectra of the synthesized ZnO samples ………………………………….122
Figure 4.6. The PL spectra of synthesized ZnO samples………………………………………123
Figure 4.7. N2 sorption isotherms of ZnO prepared in different solvents. ………………………125
Figure 4.8. BJH pore size distributions of the prepared ZnO………………………………….125
Figure 4.9. CO2 adsorption curves (at 25 °C) for synthesized ZnO materials…………………127
Figure 4.10. a) Reusability test of the ZnO in Methanol catalyst (reaction conditions: o-hydroxy benzaldehyde (2 mmol), diethyl malonate (2.5 mmol), ZnO (8.2 mg), solvent free, 100° C, 24 h, and a closed vessel reaction. b) PXRD patterns of ZnO in Methanol before and after reuse of the catalyst, c) Test of homogeneity (leaching test/hot filtration test): catalyst was removed after 52
% conversion (reaction conditions: same as reusability test)…………………………………..133
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List of Schemes
Scheme 4.1. Synthesis of Coumarin from o-hydroxybenzaldehyde and Diethyl
Malonate…………………………………………………...... 128
List of Tables
Table 2.1. Lattice Parameters of the Single-Doped ZnO Materials after XRD Pattern
Refinement……………………………………………………………………………………….31
Table 2.2. Lattice Parameters of the Multidoped ZnO Materials after XRD Pattern Refinement...31
Table 2.3. BET Surface area, BJH Pore Diameters, and Total pore Volumes of the Synthesized
Samples………………………………………………………………………………………….34
Table 2.4. The XRF-Estimated Amounts of Dopants in the Synthesized ZnO Samples………….35
Table 2.5. The Calculated Potentials of Synthesized ZnO Catalysts at a Current Density of -3 mA/cm2…………………………………………………………………………………………..45
Table 2.6. The Calculated Current Densities of the Synthesized ZnO Samples at Potentials of 0.5
V vs RHE…………………………………………………………………………………………47
Table 3.1. BET Surface Area, BJH Pore Diameters, and Total Pore Volumes of the Synthesized
Mesoporous Oxide and Phosphide samples………………………………………………………74
Table 3.2. Calculated Overpotentials at Current Density 10 mA / cm2 of the Synthesized
Phosphide Samples……………………………………………………………………………….90
Table 3.3. Calculated Charge-transfer Resistance (Rct) of the Synthesized Phosphide Samples…92
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Table 4.1. Crystallite Sizes of the Synthesized ZnO Samples………………………………...115
Table 4.2. Summary of N2 Sorption Analysis of Prepared ZnO Materials………….…………..126
Table 4.3. Optimization of Coumarin Synthesis reaction (Scheme 4.1)a ……………………….129
Table 4.4. Synthesis of Coumarin from o-Hydroxy Benzaldehyde and Diethyl Malonate (Scheme
4.1)a……………………………………………………………………………………………..131
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CHAPTER 1. INTRODUCTION
1.1 OVERVIEW
Electrochemical water splitting and fuel cells are among the most promising energy technologies to address the future energy crisis and environmental pollution. The hydrogen evolution reaction
(HER), a half reaction, occurs in electrochemical water splitting whereas the oxygen reduction reaction (ORR) half reaction, occurs in fuel cells. These reactions are catalyzed by precious metal- based materials, such as platinum (Pt), Pt-group metals, and their alloys. The use of precious metals hampers the large-scale application of these processes. In this thesis, we have focused on developing earth-abundant transition metal-based oxide and phosphide nanomaterials for catalytic applications in energy conversion reactions (HER, ORR). In addition, synthesis of inexpensive
ZnO based catalysts for coumarin synthesis is included in this research work. First, design and synthesis of transition metal doped ZnO nanocrystals for ORR activities is discussed. Different transition metal dopants and their various levels are used to tune the ORR activity of the materials.
Second, mesoporous iron and cobalt phosphide nanomaterials for HER are synthesized and their physiochemical properties are modified by introducing dopants and carbon supports to enhance
HER activities. Finally, different morphologies of ZnO nanomaterials are obtained using a hydrothermal method for use as efficient catalysts for coumarin synthesis via Knoevenagel condensation.
1
1.2 BACKGROUND OF THIS RESEARCH
1.2.1 Transition Metal-Doped (Mn, Fe, Co, and Ni) ZnO Catalysts for the Oxygen
Reduction Reaction
The growing energy demand, depletion of fossil fuels, and environmental pollution from the burning of traditional fuels has driven researchers to develop alternate energy sources. Among various energy alternatives, a fuel cell is one of the promising means for conversion of the chemical energy of fuels (hydrogen, natural gases, methanol etc.) into electrical energy. This then can be used to power vehicles, space shuttles, industries, buildings, and houses. The use of fuel cells as alternate energy sources has several advantages. For example, fuel cells have a high efficiency and no issues of environmental pollution as well as unlimited sources of reactants.1 Therefore, fuel cells are considered to be used commercially in transportation, stationary, and portable power generation.1,2
Figure 1.1. A schematic diagram of a fuel cell.
2
A schematic diagram of a fuel cell is shown in Figure 1.1. Like other electrochemical cells, there are two types of reactions that occur in fuel cells: oxidation and reduction.3 Oxidation happens at the anode where the fuels (hydrogen, for example) oxidize and produce protons and electrons. The produced protons then move from anode to cathode through the electrolyte of the cell. The electrons, also produced from the oxidation of the fuels, are flown through an external circuit, which produces the electricity. A reduction reaction occurs at the cathode where the hydrogen ions, electrons, and oxygen react to produce water. In this reaction, oxygen is reduced to water. Hence, the oxygen reduction reaction (ORR) is a cathodic half-reaction of a fuel cell.
+ - 2H2 (g) → 4 H (aq) + 4e Anode reaction (1)
+ - O2 (g) + 4 H (aq) + 4e → H2O (l) Cathode reaction (2)
ORR in acidic media occurs via a process involving either the four-electron transfer (4e-)
- 4–8 reduction of O2 to H2O or the two-electron (2e ) transfer reduction of O2 to H2O2. For fuel cell operation, a direct 4e- pathway is preferred to achieve a high efficiency of the cell.5 The 4e- / 2e- pathways of ORR are as follows:
+ - 0 O2(g) + 4H (aq) + 4e ↔ 2H2O(l); E = 1.229 V vs SHE (3)
+ - 0 O2(g) + 2H (aq) + 2e ↔ H2O2(l); E = 0.695 V vs SHE (4)
+ - 0 H2O2(l) + 2H (aq) + 2e ↔ 2H2O(l) E = 1.776 V vs SHE (5a)
2H2O2(l) ↔ 2H2O(l) + O2 (g) (5b)
A full reduction (4e- transfer) of oxygen to water follows either the associative mechanism or dissociative mechanism, depending on where O2 molecules dissociate before the reduction. The associative mechanism involves three different intermediates, *OOH, *O, and *OH whereas the
3 dissociation mechanism involves only *O and *OH intermediates.9 Both mechanisms are described as follow:
The steps involved in the associative mechanism of ORR9 are as follows:
O2 → O2*
+ - O2* + H + e → *OOH
+ - *OOH + H + e →*O + H2O
*O + H+ + e- →*OH
+ - *OH + H + e → H2O
The steps involved in the dissociation mechanism of ORR5,9 are as follows:
O2 → O2*
O2* → 2O*
O* + H+ + e- → *OH
+ - *OH + H + e → H2O
In both mechanisms, adsorption of oxygen on the catalyst is the first step. The binding energy of oxygen on the catalyst surface is very critical efficiency of the ORR process.10 The oxygen binding capability on a catalyst depends on the electronic environment of the surface.
Engineering surface electronic structure and surface atomic arrangement of a catalyst are believed be beneficial for enhancing ORR activity. The adapted electronic structure modifies the adsorption properties of a catalyst which is believed to be the origin of the enhanced ORR activity of a catalyst.10
4
The kinetics of the ORR are very slow, five to six or even more orders-of-magnitude slower than the anodic hydrogen oxidation reaction.2,3 The sluggishness of the ORR reaction is mainly due to the difficulty of adsorption of oxygen gas on the catalyst surfaces, the first electron transfer for ORR, O-O bond cleavage/activation, and desorption of intermediates from the electrode surface.10 Hence, ORR requires very efficient catalysts to adjust the reaction rate to a practical level. Moreover, the reaction demands high amounts of catalyst loading to achieve a desirable reaction rate.1 The state-of-the-art catalyst for ORR is Platinum (Pt) or Pt-based alloy materials.1,11
The scarcity of Pt and the requirements of a high amount of loading makes the use of fuel cells limited for commercial purposes. Therefore, it is required to develop highly active, stable ORR catalyst based on inexpensive earth-abundant materials.
Our goal is to develop ZnO based ORR catalyst materials. ZnO is an inexpensive, simple- synthesis material with a wide-scope of tunable catalyst potential.12–15 Although pure ZnO materials show poor activity as electrocatalysts, our objective is to enhance their electrocatalytic
(ORR) activity. The approach we applied here is manipulation of their atomic structures, especially surface electronic structures of the catalysts. This manipulation of electronic structure is carried out by substitution of the Zn ions in the ZnO lattice with other transition metal (Mn, Fe, Co, and
Ni) ions using a simplified method. Since adsorption of oxygen on the surface of an ORR catalyst is the main barrier, we have created more oxygen vacancies within the ZnO structure by doping with other transition metals, which decrease the adsorption abilities of the catalyst and enhance the
ORR activity.
5
1.2.2 Mesoporous Transition Metal Phosphides (FeP, CoP, and their TM-doped analogs) for the Hydrogen Evolution Reaction
The present dependence on finite fossil fuels has underscored concerns about an impeding energy crisis and current environmental pollution. There has been an increasing demand to replace fossil fuels with a non-polluting and sustainable energy source. Hydrogen (H2) is considered to be a very promising alternative to fossil fuels, since H2 is environmentally non-polluting and has a high energy density.16 Hydrogen can be produced using several methods, including catalytic steam reforming, partial oxidation of oil, and coal gasification. All of these approaches are associated
17 with CO2 emissions. The hydrogen production via electrochemical water splitting, (hydrogen evolution reaction, HER), is regarded as a clean, economical, and sustainable method for large- scale utilization.18 The efficiency of the electrolytic hydrogen generation is dependent on the activity, stability, and affordability of the catalyst used to perform the reaction. Precious metal Pt, and Pt-based materials are known as best catalysts for HER.16,18,19 The high cost of precious metals
(~$300/g of Pt) hampers the large-scale implementation of electrochemical hydrogen production.
Therefore, numerous research have been attempted to develop inexpensive, earth-abundant, and non-noble-metal alternatives.20–22 In particular, various inorganic materials have been explored, including non-noble transition metals (Fe, Co, Ni)23,24 transition metal-based sulfides,21,25–27 selenides,28,29 phosphides,30,31 carbides,18 nitrides,32 and other hybrid materials.33,34 A desired HER electrocatalyst should meet the following standards: (1) efficiency similar to that of noble metals,
(2) stability over an extended period of time, (3) high chemical and electrocatalytic stability over a wide range of pH, (4) low-cost, and (5) a scalable technique for catalyst synthesis. So far, none of the known noble metal-free HER catalysts exhibit all of the afore-mentioned requirements.
6
HER in aqueous solution is assumed to proceed via two steps: (1) the Volmer (discharge) step and (2) the Heyrovsky or Tafel step.21,35,36 In the first step, an electron is transferred to the cathode to capture a proton in the electrolyte, forming an intermediate state of an adsorbed hydrogen (Had) atom on the active site of the catalyst surface. The next step follows either the
Heyrovsky step (also called the electrochemical desorption step) or the Tafel step depending on the Had coverage. In the Heyrovsky step, which occurs at a low Had coverage, the adsorbed hydrogen prefers to couple with a new electron and another proton in the electrolyte to evolve into
H2. The Tafel reaction occurs when the Had coverage is relatively high. In this step, two adjacent
35 adsorbed hydrogen atoms combine to form H2. The reaction pathways in basic solution are similar, except that Had is formed by the dissociation of water in the Volmer step. In both cases, the reaction proceeds through hydrogen atom adsorption at the catalyst surfaces, Had, and thus the
20 rate of the overall reaction is influenced by the free energy of hydrogen adsorption, ∆GH.
Figure 1.2. Mechanism of HER in acidic media.21,36
When ∆GH is large and negative, hydrogen is too strongly bound to the surface. The desorption of hydrogen will then lower the rate of the overall process. Alternatively, if the hydrogen is too weakly bound to the surface (when ∆GH is large and positive), the adsorption step will limit the overall reaction rate. The materials whose hydrogen adsorption energies, ∆GH ≈ 0 will display the
7 highest HER activity.21 Hence, a volcano curve is expected to be obtained by plotting the catalyst activity (HER exchange current) as a function of ∆GH. The theoretical HER volcano plot (Figure
2b) predicts that catalysts with a hydrogen binding energy equal to zero will have the highest activity.37,38 Figure 2a shows an experimentally determined volcano plot of various HER catalysts.
Pt with a slightly negative hydrogen absorption energy, ∆GH, has the highest activity. In addition to the hydrogen adsorption energies, the HER reaction in aqueous environments is recently believed to be controlled by two other factors: (1) the nature of proton donors, (2) the availability of the active sites.35
Figure 1.3. Exchange current density as a function of hydrogen adsorption free energy for various
HER catalyst materials. (a) The experimental “volcano plot” for the HER (reprinted from ref.25 with permission.), (b) The theoretical HER volcano plot. The data taken from the ref.21 with permission.
Since Pt is the state-of-the-art HER catalyst, cost-effective and efficient alternatives to noble metals have been markedly explored.20,22,35 Among them, metal phosphides are considered
8 to be promising for HER. The potential of metal phosphides as HER catalysts was first predicted by Rodriguez’s group using DFT calculations in 2005, where they found the (001) surfaces of Ni2P to be potential active catalysts for HER.39 Their study showed that the number of active sites decreased due to the presence of P elements in Ni2P. This provides the favorable bonding strength with the intermediates and products of the reaction on the Ni2P (001) surfaces. A charge transfer occurs from transition metal to phosphorous atom in the metal phosphides. Consequently, the metal atoms become partially positive and P atoms become partially negatively charged. The P atoms and metal atoms (Ni) then function as proton-acceptor sites and hydride-acceptor sites, respectively. During the HER process, hydrogen is strongly bound to Ni sites, but with the assistance of P, the bound hydrogen can easily be removed from the Ni2P (001) surface. The existence of both sites together on the surface of Ni2P (001) is called an ensemble effect, which facilitates the HER. These factors led to the establishment of metal phosphides as good candidates for HER. These materials have attracted particular attention in the field of HER due to their high mechanical strength, electrical conductivity, and chemical stability. Metal phosphides form face- centered cubic (FCC), hexagonal close-packed (HCP) or simple hexagonal crystal structures. The metal phosphide structures are based on triangular prisms due to the large radius of the phosphorous atoms (0.109 nm). These prismatic building blocks are also common in sulfide structures. However, metal phosphides tend to form an isotropic crystal structure (Figure 3) rather than the layered structures observed in metal sulfides.40 This structural difference allows metal phosphides to have a greater concentration of coordinately unsaturated surface atoms than metal sulfides. For this reason, earth abundant TM-based phosphide materials have recently been reported as promising HER catalysts in acidic media.30,41,42
9
Figure 1.4. (a) Triangular prism structures of phosphide, (b) MnP-type orthorhombic crystal structure of FeP. Data taken from ref.40 with permission.
Among all 3d-transition-metal based phosphides, nanostructured iron phosphide (FeP) and cobalt phosphides (CoP) have recently been reported as promising materials for HER catalysts.43–
46 40 These materials commonly crystallize with Fe2P-type and MnP-type orthorhombic structures.
In transition metal phosphides (TMPs), phosphorous (P) content is important for HER activity of the materials. TMP with higher P content usually shows better HER activity.43 As an example,
44 Fe2P is found to be less HER active than FeP. Despite FeP and CoP materials having good HER performance, these catalysts still suffer from lower efficiency compared to noble metal catalysts, namely platinum. Engineering these materials improves their HER activity. An effective way of improving the activity is to increase the number of exposed active sites by modification of surfaces.20,21,28,47 Mesoporous materials typically possess interesting intrinsic textural features,48 including high surface areas, tunable pore sizes, and good adsorption properties. For this reason, we suggest the synthesis of mesoporous structured materials as a strategy to increase the number of exposed active sites for HER catalysts.
10
In addition to mesoporous structures, doping is another common and effective way to alter the catalyst surfaces and enhance their catalytic performance. Introducing small amounts of transition metal (TM) heteroatoms can improve electrocatalytic activity by more than an order of magnitude.49–51 For example, TM doping in the nickel phosphide system has recently been reported as promising to increase the HER activity in acidic media.52 The activity acceleration of electrocatalysts by TM doping is actually governed by the number of defects (morphological effects) as well as by the number of electrons.17 Among the 3d-transition metals, Co is a relatively abundant TM in the earth’s crust, and CoP has already been proven to be a promising and stable
HER catalyst in acidic media.31,42,53 The ionic radii of Fe3+ (55 pm, ls), Ni3+ (56 pm, ls) and Co3+
(54.5 pm, ls) are very close. Thus, we consider Co and Ni as dopants in FeP and CoP nanomaterials to alter HER activity of the materials.
In TMPs, the electron density is transferred from metal to phosphorous atom due to the high electronegativity of phosphorous.43 This transfer implies a cationic state of the metal and an anionic state of the P atom. Since P has a higher electron density, this species can bind to a proton to form an adsorbed hydrogen, Hads, intermediate. In the case of Co doping in the FeP, Co occupies the Fe sites in the FeP lattice. The electrons are then more likely to be located near the Co atoms due to a slightly higher electronegativity of Co compared to Fe (1.88 > 1.83). Cobalt’s ability to attract electron density leads to the conclusion that Co could also be bound to a proton, providing a stronger proton-binding site and thus promoting HER activity. Similarly, Ni doping in the CoP structure is assumed to provide a stronger interaction with protons and enhance the activity.
The ionic radii of Mn3+ (58 pm, ls), Cr3+ (61.5, ls) and V3+ (64 pm) are closer to the Fe3+ or
Co3+ ions, implying favorable steric considerations if used to replace them in the FeP and CoP lattices. However, the electronegativities of Manganese (1.55), Chromium (1.66) and Vanadium
11
(1.63) are less than those of Fe (1.83) and Co (1.88), making them unlikely to provide the electronic effect that Co doping does in FeP or Ni doping does in CoP. Hence, Co doping in FeP and Ni doping in CoP are assumed to be promising for enhanced HER activity. In addition, catalytically active materials coupled with carbon provide high surface areas, modulated electronic densities, and electronic distributions. These are favorable conditions for improving electrocatalytic performance of the materials. Hence, TM doped CoP/FeP compositing with carbon nanotubes (Ni-
CoP/CNT) could also be considered for development for enhanced HER activity.
The phosphide materials have high electrical conductivity, favorable structural features for
HER activity, and stability in strongly acidic solutions. In this work, mesoporous FeP and CoP have been prepared for HER catalysis. The mesoporous structures of FeP and CoP and the surfaces of the materials were modified by introducing dopants (Co and Ni, respectively) and carbon supports to tune the HER activity of the materials.
1.2.3 Synthesis of Various Morphologies of ZnO for Coumarin Synthesis via the Knoevenagel
Condensation Reaction.
Coumarins are heterocyclic compounds containing a benzo-α-pyrone (2H-1-benzopyran-2-one) nucleus. These compounds have been studied due to their wide range of pharmacological activities, including antidepressants,54 antimicrobials,55 anti-oxidants,56 anti-inflammatories,57 antinociceptives,57 anti-tumor agents,58 antiasthmatics,59 antivirals (including anti-HIV),60 and anti-coagulants.61 In addition, coumarins exhibit interesting optical properties. For examples,
Figure 1.5. Benzo-α-pyrone (2H-1-benzopyran-2-one) ring of coumarin family.
12 coumarins have a wide range of spectral response, high quantum yields and photo-stability. Hence,
It has applications in laser dyes, fluorescent probes, optical recording, and solar collectors.62 Due to the pharmacological and optical properties of coumarins, they have been attractive target to researchers. As a result, synthesis of coumarins by simple and flexible methods has become a great attraction to organic synthetic chemists.
There are several methods utilized to synthesize coumarins, These are Pechmann reaction,
Perkin reaction, Knoevenagel condensation, Reformatsky, and Wittig reactions.63 Among them, the Knoevenagel condensation reaction is one of the widely employed methods to synthesize coumarins and their derivatives. The Knoevenagel condensation is a modified form of aldol condensation which is commonly used to form C-C bonds in organic synthesis. In this reaction, a nucleophilic addition of an active hydrogen compound occurs to a carbonyl compound. The reaction is followed by removal of one molecule of water (Figure 1.6) to produce an α, β- unsaturated ketone (a conjugated enone).
Figure 1.6. Schematic representation of the Knoevenagel condensation reaction.
In Knoevenagel condensation reactions, E of the active hydrogen compound should be a powerful electron withdrawing group to enable deprotonation to the enolate ion. This reaction usually catalyzed by weak bases.64 For examples, piperidine, pyridine, ammonia, or sodium
13 ethoxide primarily used in organic reaction media as homogenous catalysts. Most of the chemicals used in homogenous catalysis are toxic. Hence, for large-scale synthesis in industry, heterogeneous catalysis is preferable. Moreover, heterogeneous catalysis is more advantageous over homogenous counterparts due to easy separation, low-cost, and reusable nature of the catalyst.
ZnO is an amphoteric oxide and can act as a base in the presence of an active hydrogen with a strong electron withdrawing group. ZnO is abundant and can be easily synthesized with many different pathways. Moreover, ZnO is environmentally benign, inexpensive, and can be prepared with different properties using different methods.12,14,15 ZnO could be an inexpensive, robust, and environmentally safe heterogeneous catalyst for coumarin synthesis via the
Knoevenagel condensation reaction. ZnO prepared by various methods provides different properties14,15, including different morphologies, surface areas, pore sizes, and pore volumes.
These properties are crucial for tailoring the activity of the materials in heterogeneous catalysis.66,
67 We intended to see how the activity of ZnO changes with the change of ZnO textural properties in coumarin synthesis via the Knoevenagel condensation reaction. This study could guide researchers selecting a low-cost, effective catalyst for coumarin synthesis.
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Chemical Synthesis and Biological Activity. Nat. Prod. Rep. 2015, 32 (10), 1472–1507.
(63) Kumar, B. V.; Naik, H. S. B.; Girija, D.; Kumar, B. V. ZnO Nanoparticle as Catalyst for
Efficient Green One-Pot Synthesis of Coumarins through Knoevenagel Condensation. J.
Chem. Sci. 2011, 123 (5), 615–621.
(64) Zhou, J.-F.; Song, Y.-Z.; Lv, J.-S.; Gong, G.-X.; Tu, S. Facile One-Pot, Multicomponent
Synthesis of Pyridines Under Microwave Irradiation. Synth. Commun. 2009, 39
(February), 1443–1450.
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(66) Kim, D.; Huh, Y. Morphology-Dependent Photocatalytic Activities of Hierarchical
Microstructures of ZnO. Mater. Lett. 2011, 65 (14), 2100–2103.
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23
CHAPTER 2. SINGLE-DOPED AND MULTIDOPED TRANSITION METAL (Mn, Fe, Co,
AND Ni) ZnO AND THEIR ELECTROCATALYTIC ACTIVITIES FOR OXYGEN
REDUCTION REACTION
2.1 OVERVIEW
The electrochemical oxygen reduction reaction (ORR) is the limiting half-reaction of fuel cells which is mediated by using platinum (Pt) based catalysts. Hence, the development of low-cost, active ORR catalysts is highly required to make fuel cell technology commercially available. In this report, transition metal (Mn, Fe, Co, and Ni) single and multi-doped (MD) ZnO nanocrystals
(ZNs) were prepared using a simple precipitation method for use as ORR catalysts. The effects of single and multi-doping on the structure, morphology, and properties of the TM doped ZNs were analyzed using XRD, SEM, XRF, XPS, EPR, Raman, and photoluminescence (PL) spectroscopy.
The XRD results reveal that synthesized ZnO samples retained a pure hexagonal wurtzite crystal structure, even at high levels of multi-doping (nominal 20 %). SEM analyses show that the morphology of the prepared ZNs varies with doping elements, doping mode, and the amounts of doping. The observation of peak shifting and peak intensity changes in Raman studies confirm the presence of dopants in ZnO. The PL investigation reveals that the incorporation of dopants into
ZnO structure increases the oxygen vacancies within the materials. The highest oxygen vacancies were present in Mn-doped ZnO and 15 % multi-doped ZnO among the single doped and multi- doped samples, respectively. Linear sweep voltammetry studies conclude that doped ZnO shows enhanced ORR activity compared to the undoped samples. The Mn-doped ZnO and 15 % MD-
ZnO exhibited the highest ORR activity among the prepared single doped and multi-doped ZN samples, respectively. In comparison, single doping showed better ORR activity than the multi-
24 doping system. The enhanced ORR activity of the synthesized ZN materials correlates with the amount of oxygen vacancies present in the samples. The enhanced activity of TM doped ZnO suggests these materials can be used as potential, low-cost electrocatalysts for ORR in fuel cell technology.
2.2 INTRODUCTION
Due to the gradual depletion of fossil-based energy resources, as well as their associated environmental issues, renewable-energy sources such as fuel cells and electrochemical water splitting technologies have received much attention in recent years.1–5 The electrochemical oxygen reduction reaction (ORR) is a key reaction of advanced electrochemical energy conversion technologies, such as fuel cells.6 The ORR happens in the cathodic part of a fuel cell. This reaction is kinetically six fold slower than the anodic reaction of the cell2. Therefore, this process needs to be mediated by highly efficient catalysts. Platinum (Pt) catalysts and their alloys are known to be the best catalysts for ORR.7 However, these are expensive and scarce. Being a very slow reaction,
ORR also demands high loading of the catalyst. The use of precious metal-based catalysts makes fuel cell technology unfavorable for commercialization. Hence, precious-metal-free catalysts based on earth-abundant 3d metals have been vigorously pursued, yet substantial progress is still needed.6,8,9–14 Developing a low-cost ORR catalyst with high activity is still a great challenge.
Although ZnO materials usually show poor activity as electrocatalysts, the Driess and Park group showed that transition-metal-doping in ZnO nanoparticles enhances their electrocatalytic activity.15,16 In their work, the Driess group showed that Co doped ZnO exhibits increased OER activity, and the degree of Co substitution in the ZnO lattices is a crucial factor in determining their activity in OER. The Park group studied a series of metal oxides doped with various transition metals for OER and found that doped oxides exhibit surprisingly higher OER activity than their
25 undoped counterparts. These results were a good indication of transition metal doped ZnO to be promising materials for ORR activity. However, no studies have been reported on the transition- metal-doping effect in ZnO nanomaterials for ORR activity. ZnO materials can be prepared with multiple morphologies17,18 using numerous versatile methods, which gives these materials the advantage of providing a wide scope of activities for various applications.19,20 Moreover, ZnO materials could be a good choice for practical applications since they are chemically stable, inexpensive, and environmentally benign. Our goals were to synthesize transition metal doped
ZnO materials using an easy method and applying them for ORR. We also wanted to look at how structure and properties of materials vary with different dopant elements. The objective of this study was to explore the possibility of the change in ORR activity with different transition metal doping in the ZnO lattice.
In this research, transition metal (Mn, Fe, Co, and Ni) doped ZnO materials were synthesized using the precipitation method. This was a facile, single-step, and one-pot robust method. In this method, the incorporation of transition metals into the ZnO lattice was carried out in two different modes: single doping mode (where dopants were added separately) and multi- doping mode (where all four dopants were added together). The effects of doping on the ZnO crystal structure, morphology, and optical properties were investigated by X-ray diffraction
(XRD), scanning electron microscopy (SEM), and Raman Spectroscopy, respectively. The electrocatalytic performances of the materials were studied using ORR as a model reaction.
Finally, the effect of doping in the ZnO structure-property-activity relationship was investigated in this work.
26
2.3 EXPERIMENTAL SECTION
2.3.1 Chemicals
To synthesize both undoped and doped ZnO nanocrystals, analytical reagent grade zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, ≥ 97.0%), hexamethylenetetramine (HMT), Manganese(II) nitrate tetrahydrate (Mn(NO3)2 · 4H2O, ≥ 97.0%), Nickel(II) nitrate hexahydrate (Ni(NO3)2 · 6H2O ≥
97.0%), Cobalt(II) nitrate hexahydrate (Co(NO3)2 · 6H2O ≥ 98.0%), Iron(III) nitrate nonahydrate
(Fe(NO3)3 · 9H2O ≥ 98.0%) and distilled deionized water (DDW) were used as starting materials.
All of the chemicals were purchased from Sigma-Aldrich and used without further purification.
2.3.2 Materials Synthesis
In a 250 mL beaker, 5.95 g (0.8 M) of Zn(NO3)2 · 6H2O and appropriate amounts of dopant precursors were dissolved in 25 mL of double deionized water (DDW). In a separate beaker, 2.80 g (0.8 M) of HMT was dissolved in 25 mL of DDW. For single doped ZnO, the amount of each metal doping was 5 mol % and for multi-doping samples the amount of each metal was 2.5 mol
%, 3.75 mol %, and 5 mol % for 10 %, 15 %, and 20 % multi-doped ZnO (MD ZnO), respectively.
The aqueous solution of zinc nitrate hexahydrate and dopant precursor was stirred for 10 min with a magnetic stirrer. The HMT solution was then slowly added drop-wise under constant stirring. To grow ZnO, the solution mixture was heated at 90–95 °C in a laboratory oven for 8 hours. After the reaction time elapsed, the precipitate on the bottom of the flask was centrifuged (6500 rpm for
3 minutes) with DDW several times followed by an ethanol wash. The precipitate was dried overnight at 70 °C and the dried samples were then calcined at 300 °C for 3 h.
2.3.3 Materials Characterization
The phase identifications of the powder samples were carried out using a Rigaku Ultima IV powder
X-ray diffractometer with Cu Kα (1.54 Å) radiation, operated at a voltage of 40 kV, and beam
27 current of 44 mA. The surface morphologies and the surface areas of the materials were determined using scanning electron microscopy (SEM) and the Brunauer-Emmett-Teller (BET) methods from
N2 sorption measurements, respectively. The elemental composition of the materials was estimated with X-ray fluorescence (XRF) spectroscopy/energy dispersive X-ray (EDX) methods. The XRF analyses were conducted using a Rigaku ZSX Primus IV sequential wavelength dispersive XRF spectrometer with an X-ray tube, 4 KW Rh. The SEM/EDX data were obtained using FEI Quanta
250 FEG field emission SEM. The phase purity and possible defects of the samples were studied using Raman spectroscopy and photoluminescence (PL) spectroscopy at room temperature.
Raman spectra were recorded using a Renishaw 2000 Raman microscope attached to a charge- coupled device (CCD) camera with an Ar+ laser at 514.4 nm as the excitation source. The spectrometer was calibrated with a silicon wafer before tests. PL studies were performed on the powder samples using a Horiba Fluorolog III Spectrometer with a Xenon lamp (λex = 325 nm).
The electronic state of the elements was determined by using X-ray photoelectron spectroscopy
(XPS). The XPS measurements were carried out using a PHI model 590 Multi-probes system with monochromatic Al Kɑ radiation (λ = 1486.6 eV) as the X-ray source. The pressure of the analysis chamber was 2.0 × 10−8 torr during analysis. The XPS spectra obtained were analyzed and fitted using CasaXPS software (version 2.3.12). All of the spectra were calibrated to the C 1s transition set at 284.6 eV. The coordination environments of the dopant elements were confirmed by obtaining electron paramagnetic resonance (EPR) spectra using a Varian E-3 EPR spectrometer with the following conditions: 9.65 GHz frequency, 1.00 mW power, room temperature, and a modulation amplitude of 18.9 G.
28
2.3.4 Electrochemical Measurements
The electrochemical measurements for the materials were carried out in a conventional three- electrode cell connected to a CHI 660A electrochemical workstation. Linear sweep voltammetry
(LSV) was performed in 0.1 M KOH purged with high purity oxygen for 30 min before starting the electrochemical measurements. A platinum wire and a standard calomel electrode were used as the counter and reference electrodes, respectively. The working electrode consisted of pyrolytic graphite (PG) coated with catalyst ink. The catalyst ink was prepared by mixing 4 mg of catalyst,
1 mg of carbon black, and 85 µL of 5 wt % Nafion solution in 1.0 mL of DDW/ethanol (4:1). The catalyst ink mixture was sonicated for 30 min to ensure proper mixing of the components and then loaded onto the PG electrode (mass loading 0.2 mg / cm2) by the drop casting method. Finally, the working electrode was air-dried at room temperature overnight for the electrochemical measurements.
2.4 RESULTS
2.4.1 XRD Analysis
Figures 2.1 (a) and 2.1 (b) show the XRD patterns of single doped and multi-doped ZnO samples, respectively. The shape and intense peaks indicate that the as-synthesized samples are highly crystalline. All the XRD peaks are indexed as the hexagonal wurtzite crystal structure of
ZnO. There are no trace of impurities or secondary phases present. The XRD results indicate that the incorporation of transition metal ions does not alter the crystal structure. There was a slight peak shifting in all peaks for both the single and multi-doped ZnO materials. A comparison of the
(101) planes is shown in Figure 2.2. For the MD ZnO materials, upon increasing the dopant concentration, the peaks show consistent shifting. The lattice parameters and crystallite sizes of all
29 the samples are provided in Table 2.1 and Table 2.2. The observed peak position values are in good agreement with the reported values for pure ZnO21,22 (JCPDS File Card No.:01-080-0075).
(a) (b)
Figure 2.1. XRD patterns of transition metal (Ni, Co, Fe, and Mn) doped ZnO samples (a) single doped ZnO (b) multi-doped ZnO samples.
Figure 2.2. XRD peak for (101) plane of the single doped ZnO materials (left) and multi doped ZnO materials (right).
30
Table 2.1. Lattice Parameters of the Single-Doped ZnO Materials after XRD Pattern Refinement. sample name lattice parameter, a = b (Å) lattice parameter, c (Å) crystallite Size (Å) Undoped ZnO 3.2501 (8) 5.2057 (2) 359 Mn-ZnO 3.2498 (1) 5.2047 (9) 328 Fe-ZnO 3.2503 (4) 5.2065 (6) 335 Co-ZnO 3.2505 (6) 5.2066 (1) 323 Ni-ZnO 3.2502 (6) 5.2057 (1) 298
Table 2.2. Lattice Parameters of the Multidoped ZnO Materials after XRD Pattern Refinement.
sample name lattice parameter, a = b (Å) lattice parameter, c (Å) crystallite Size (Å) Undoped ZnO 3.2501 (8) 5.2057 (2) 359 10% MD ZnO 3.2501 (2) 5.2062 (4) 337 15% MD ZnO 3.2508 (7) 5.2071 (1) 329 20% MD ZnO 3.2518 (7) 5.2091 (1) 302
2.4.2 Surface morphology and Surface Area Analysis
The surface morphology of the materials was studied using SEM. Figure 2.3 shows the
SEM images of single doped samples and Figure 2.4 shows the morphology of multi-doped ZnO samples with different dopant percentages. The SEM images reveal that both undoped and TM doped ZnO are nanocrystalline materials. Figure 2 demonstrates that the morphologies of the single doped ZnO materials are clearly different from undoped samples. The particle sizes of the single doped ZnO samples are slightly smaller than that of the undoped ones. This change is prominent for the Co doped samples. However, no significant morphology change is observed between Mn-doped and undoped ZnO samples. The alteration of morphology is also seen in single doped ZnO compared with multi-doped ZnO (Figure 3). The morphology of the MD-ZnO samples varies with the percentages of dopants.
31
Figure 2.3. SEM photograph of (a) undoped ZnO (b) 5% Ni-ZnO (c) 5% Fe-ZnO (d) 5% Co-ZnO (e) 5% Mn-ZnO.
32
Figure 2.4. SEM images (a) Undoped ZnO (b) 10 % MD-ZnO (c) 15% MD-ZnO and (d) 20 % MD-ZnO.
The surface area of the synthesized materials was calculated from the N2 sorption isotherms using the BET method. Table 2.3 shows the surface area, pore volume, and pore sizes of the synthesized ZnO materials. According to the pore sizes, all the materials are microporous except
20 % MD-ZnO (pore size ≥ 2 nm). The incorporation of Mn and Ni into ZnO does not significantly alter the surface area. However, the surface area increases with Co and Fe doping in the ZnO system. The surface area of the materials also increases systematically with higher percentage dopants in the case of the multi-doped ZnO system.
33
Table 2.3. BET Surface area, BJH Pore Diameters, and Total pore Volumes of the Synthesized Samples materials* BJH desorption pore total pore volume BET surface area diameter (nm) (cc/g) (m2/g)
Undoped ZnO 2.0 0.04 6
5% Ni-ZnO 2.3 0.02 7
5% Co-ZnO 2.0 0.07 12
5% Fe-ZnO 2.3 0.10 21 5% Mn-ZnO 2.0 0.05 7 10% MD-ZnO 2.3 0.06 14 15% MD-ZnO 2.0 0.08 20
20% MD-ZnO 1.8 0.10 21
*All percentages are mol %
2.4.3 Chemical Composition Analysis
The investigation of the chemical composition of the synthesized ZnO materials was carried out using EDX and XRF analysis. The EDX spectra and the amounts of dopants present in samples are shown in Figure 2.5. In order to calculate the amounts of dopants accurately, XRF studies were also performed. The XRF calculated amounts are shown in Table 2. For the single doping system, the obtained XRF amounts for Mn, Fe, Co, and Ni dopants were 0.01, 2.77, 1.84, and 1.61 mass percentages, respectively. The XRF experimental amounts of dopants in 20 % MD-
ZnO were 0.04, 10.13, 0.88, and 1.12 mass percentages for Mn, Fe, Co, and Ni, respectively. The dopant concentration in the starting materials was 5 mol%. The Table 2.4 demonstrate that the
XRF estimated amounts are always much less than those of the starting materials. The amounts obtained from XRF analysis vary with the size of the dopants and with the mode of doping system
(single and multi-doped).
34
Figure 2.5. EDX spectra of (a) 20% MD-ZnO, (b) 5% Fe-ZnO, (c) Table of EDS elemental composition of MD-ZnO and (d) Fe-doped ZnO (inset: percentage of elements in single doped ZnO).
Table 2.4. The XRF-Estimated Amounts of Dopants in the Synthesized ZnO Samples samples (dopants) nominal Amounts XRF-calculated amounts of dopants (mass %) of dopants (mol %) Mn-ZnO (Mn) 5 0.01 Fe-ZnO (Fe) 5 2.77 Co-ZnO (Co) 5 1.84 Ni-ZnO (Ni) 5 1.61 MD-ZnO (Mn, Fe, Co, 20 (5 mol % each) 0.04 (Mn), 10.13 (Fe), 0.88 (Co), 1.12 (Ni) & Ni)
2.4.4 XPS Analysis
To investigate the chemical states of the elements within the samples, XPS spectra of 20 % MD-
ZnO were measured. The elements Zn, Mn, Fe, Co, Ni, O, and adventitious C were detected in the survey scan. The survey spectrum, shown in Figure 2.6 (a), suggests the incorporation of Mn, Fe,
Co, and Ni in the samples. The high-resolution XPS spectra of Zn 2p, Co 2p, Ni 2p, Mn 2p, Fe 2p
35 and O 1s of the samples are further illustrated in Figures 2.6 (b) - (g), respectively. All measured binding energies are calibrated to the C 1s peak (284.6 eV). Figure 2.6 (b), the high-resolution
XPS spectrum of Zn 2p, shows two strong peaks appearing at 1020.1 eV and 1043.2 eV which are attributed to the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively. The spin-orbital splitting energy for the Zn 2p peaks were around 23.1 eV. The Zn 2p3/2 peak could also be de-convoluted into three peaks centered at 1021.6 eV, 1020.0 eV, and 1018.0 eV. Figure 2.6 (c) shows the high- resolution XPS spectrum of Ni 2p, where Ni 2p3/2 and Ni 2p1/2 peaks are centered at 853.5 eV and
871.5 eV accompanied with satellite peaks observed at 859.5 eV and 878.4 eV.
The Co 2p XPS high-resolution spectrum is shown in Figure 2.6 (d). The observed peaks centered at 778.8 eV and 794.8 eV are ascribed to Co 2p3/2 and Co 2p1/2, respectively. The spin- orbital splitting energy of these peaks is 16.0 eV. The high-resolution XPS Mn 2p region (Figure
2.6 (e)) shows four deconvoluted peaks at 663.1 eV, 655.6 eV, 651.9 eV, and 640.5 eV. The peaks at binding energies of 651.9 eV, 640.5 eV are assigned to Mn 2p1/2 and Mn 2p3/2, respectively. Two other peaks observed at 663.1 eV, 655.6 eV are ascribed to Zn L3MM and Zn L2MM Auger peaks, respectively. Figure 2.6 (f) exhibits high-resolution Fe 2p XPS spectra. The obtained peak can be deconvoluted into five peaks at 723.6 eV, 718.4 eV, 714.9 eV, 710.6 eV and 703.6 eV. The deconvoluted peaks at 723.6 eV and 710.6 eV are attributed to Fe 2p1/2 and Fe 2p3/2, respectively.
The observed peaks at binding energies of 718.4 eV, 714.9 eV, and 703.6 eV are assigned as overlapping Auger peaks for CoL3MM, NiL3MM, and CoL2MM, respectively. The XPS spectrum of O 1s is shown in Figure 2.6 (g). The asymmetry of the O 1s peak indicates the presence of multi- component oxygen species in the sample. The de-convoluted O 1s peak can be fitted with four components centered at 528.3 eV, 529.9 eV, 531.5 eV, and 532.9 eV, respectively.
36
(a)
(b)
37
(c)
(d)
38
(e)
(f)
39
(g)
Figure 2.6. XPS spectra of synthesized 20% MD-ZnO sample, (a) XPS survey spectrum of 20% MD-ZnO, (b) High-resolution XPS spectra of Zn 2p peaks, (c) High-resolution XPS spectra of Ni 2p peaks, (d) High-resolution XPS spectra of Co 2p peaks, (e) High-resolution XPS spectra of Mn 2p peaks, (f) High-resolution XPS spectra of Fe 2p peaks (g) High-resolution XPS spectra of O 1s peak.
2.4.5 EPR Analysis
EPR spectroscopy was performed to examine the coordination symmetries and locations of the dopants in the ZnO lattice. The room-temperature EPR spectra of the prepared ZnO samples are illustrated in Figure 2.7 with the corresponding g-values of the samples. The EPR spectra of Co-
ZnO, Fe-doped, and MD-ZnO show extremely broad signals whereas those of Mn-ZnO exhibit hyperfine spectra with six clearly observable lines. The separation between hyperfine lines in the
EPR spectrum of the Mn-ZnO sample is 78.7 Gauss.
40
Figure 2.7. The room temperature EPR data of single doped ZnO and multi-doped ZnO (MD ZnO) samples with the hyperfine lines of Mn-ZnO (inset).
2.4.6 Optical Analysis (Raman and Photoluminescence Spectroscopy)
The phase purity and the possible defects in the synthesized materials were studied using Raman and photoluminescence (PL) spectroscopy. Dopants may enter into lattice points or segregate out of crystals and stay within the materials as amorphous phase. Raman spectroscopy was also performed to determine these effects of dopants in the ZnO structure. Figure 2.8 shows the Raman spectra of ZnO, single doped ZnO, and multi-doped ZnO nanostructures in the range of 200 to 800 cm-1. Both undoped and doped ZnO have a predominant peak at 437 cm-1 along with other relatively weaker peaks at 333 cm-1, 378 cm-1, and 574 cm-1. Although most of the Raman peaks in the doped ZnO match well with undoped ZnO, the peak positions and peak intensities alter with dopants. The intensity of the peak at 437 cm-1 increases when ZnO is doped with Mn and Ni,
41 whereas the peak intensity decreases for Fe and Co doping in single doped ZnO samples. The peak at 437 cm-1 for the Fe and Co doped samples also red-shifted. In the case of multi-doping of ZnO, the intensity of the Raman peak at 437 cm-1 reduces but that of the other peaks become stronger compared to the undoped ZnO samples. The peaks also blue shifted with various amounts of TM doping.
(a)
42
(b)
Figure 2.8. Raman spectra of (a) single doped ZnO, (b) multi-doped ZnO samples.
The room temperature PL spectra were also studied to investigate the presence of defects in the materials. Figure 2.9 (a) shows emission spectra of the materials when they were excited at
325 nm and Figure 2.9 (b) shows the excitation spectra of the materials when the emission wavelength was set at 550 nm. A typical PL spectrum of ZnO contains a UV band (at around 391 nm), a violet band (at around 417 nm), a blue band (at around 478 nm) and a green band (at around
523 nm). These data suggest that both the undoped and TM doped ZnO materials show fairly similar excitation spectra and emission spectra. However, the peak intensities for the doped ZnO materials are smaller compared to undoped ZnO. The PL peak intensity of Mn-ZnO (Figure 2.9
(a)) is the highest among the single doped ZnO systems whereas that of 15% multi-doped ZnO is the highest among the multi-doped ZnO samples.
43
Undoped ZnO Ni-ZnO (a) Co-ZnO Fe-ZnO Mn-ZnO 15 % MD
MD WO Fe
Intensity (a. u.)
450 500 550 600 650 700 Wavelength (nm)
Undoped ZnO Ni-ZnO (b) Co-ZnO Fe-ZnO Mn-ZnO 15 % MD
MD WO Fe
Intensity (a.u.)
300 350 400 450 500 Wavelength (nm)
Figure 2.9. Room temperature PL spectra of synthesized samples, (a) Emission spectra of samples (excited at 325 nm), (b) Excitation spectra (emission wavelength set at 550 nm). Note: WO = without.
2.4.7 ORR Activity Evaluation of the Synthesized ZnO Samples
To understand the relationship between structure and electrocatalytic activities of the synthesized materials, LSV studies were carried out using the materials for ORR (as a model reaction). Figures
44
2.10 (a) and (b) show the LSV curves of the single doped and multi-doped ZnO catalysts, respectively. The LSV investigation of Pt (20%) / C was also reported to compare the performance of the synthesized materials. The ORR performance of the ZnO samples was evaluated by calculating the required potential to produce a current density of -3 mA / cm2. The closer the potentials (to 1.23 Volts vs. RHE), the better the ORR performance of the materials.
Figure 2.10. LSV curves of (a) single doped ZnO and (b) multi-doped ZnO. Potentials are recorded in volts vs. SCE, and then converted to RHE.
Table 2.5: The Calculated Potentials of Synthesized ZnO Catalysts at a Current Density of - 3 mA / cm2
potentials (V) vs. RHE Single-doped multidoped pure ZnO Pt (20 %) / C ZnO ZnO Mn-ZnO 0.64 10 % MD ZnO N/A Fe-ZnO 0.59 15 % MD ZnO 0.59 0 .84 N/A Co-ZnO 0.55 20 % MD ZnO N/A Ni-ZnO 0.54 N/A = Not applicable
45
The estimated potentials at a current density of -3 mA / cm2 for all synthesized ZnO samples are shown in Table 2.5 and Figure 2.10. The single doped Mn-ZnO, Fe-ZnO, Co-ZnO, and Ni-
ZnO catalysts require the potentials of 0.64, 0.59, 0.55, and 0.54 V (vs RHE), respectively to produce a benchmark current density (-3 mA / cm2) for ORR. On the other hand, the pure ZnO was not able to produce the benchmark current even at a higher negative potential (0.4 V vs RHE).
Hence, the ORR activity of the single doped samples is in the order of pure ZnO < Ni-ZnO < Co-
ZnO < Fe-ZnO < Mn-ZnO. For the multi-doped ZnO samples, only 15 % MD ZnO produced the benchmark -3 mA / cm2 current density at applied potential of 0.59 V, whereas the current density of the 10 % and 20% MD ZnO at a potential of 0.59 V is much lower than 15 % MD samples. This indicates poor performance in ORR. In addition, the ORR performances of synthesized catalysts are compared by calculating current densities at a potential of 0.5 V vs. RHE (since numerical values of the produced current can be calculated at this potential), which is shown in Table 2.6.
All the synthesized TM doped ZnO samples (both single and multi-doped) exhibit better ORR activity than their undoped counterparts. Remarkably, the Mn-ZnO and 15 % MD ZnO show the highest ORR activities among the single doped and multi-doped ZnO, respectively. In comparison with multi-doped ZnO, single doped ZnO samples demonstrate higher ORR performance in 0.1
KOH solution.
Table 2.6: The Calculated Current Densities of the Synthesized ZnO Samples at Potentials of 0.5 V vs RHE
Current densities (mA / cm2) Pure Single doped Multi-doped Pt (20 %) / C ZnO ZnO ZnO Mn-ZnO 5.90 10 % MD ZnO 2.31 Fe-ZnO 3.64 15 % MD ZnO 3.67 9.22 1.73 Co-ZnO 3.22 20 % MD ZnO 2.55 Ni-ZnO 3.14
46
The stability of the 5% Mn-ZnO catalyst was studied using chronoamperometric current-time
(i-t) responses at a constant potential of -0.25 V vs. RHE. The stability of Pt/C was also investigated for comparison using the same conditions. The obtained i-t curves are shown in Figure
2.11. The catalyst 5% Mn-ZnO loses 47 % of its initial activity after 5 hours whereas Pt/C loses only 12 %. At 20 h, Pt/C and 5% Mn-ZnO lose 20% and 73 % of their initial activity (current), respectively.
Figure 2.11. Chronoamperometric i-t responses for 5 % Mn-ZnO compared with Pt/C at potential of -0.25 V vs. RHE. (Note: % retention = retention of initial current (%) at a particular time)
47
2.5 DISCUSSION
In this study, the effects of different transition metal (Mn, Fe, Co, and Ni) doping on structural and optical properties of ZnO were investigated. The single and multi-system transition metal doping effects on ZnO properties were also studied. Finally, the relationship between the structure and properties of ZnO with transition metal dopants (single and multi) towards ORR activity were explored in this work.
This study shows that the incorporation of transition metals into the ZnO lattice enhances the ORR activity of the catalysts. The activity enhancement varies with different transition metal dopants and the doping mode, whether single doping or multi-doping. The enhanced ORR activity of the materials is governed by oxygen vacancies created through the introduction of transition metals into the ZnO lattice.
2.5.1 Phase Identification of the Materials
All synthesized materials are pure phased wurtzite structures of ZnO confirmed by XRD patterns and Raman spectra. The XRD results also indicate that the incorporation of the transition metal ions in the ZnO system does not alter the crystal structure of the materials. This may be due to the incorporation of dopants into the ZnO lattice sites. The strong intensity and narrow width of the diffraction peaks are indications of good crystallinity within the synthesized materials. The observed change in peak position, peak intensity, lattice parameters, and crystallite sizes with retention of phase purity for doped ZnO samples is a clear evidence of incorporation of dopants into the framework of ZnO structure. These results may be attributed to the lattice distortion caused by substitution of Zn ions with dopant ions having different ionic radii. The presence of impurities
(dopants ions) may slow down the nucleation and the growth of crystals which explains the
48 reduction of crystallite sizes of the doped samples. These results are in good agreement with reports by other researchers.23–27
2.5.2 Surface Morphology of the Materials
The surface morphology of the ZnO materials changes with different dopants and doping modes.
The alteration of surface morphologies and particle sizes with various metal dopants (Figures 3 and 4) are good indications of the presence of dopants and their effects in the ZnO materials. The variation of morphology with the incorporation of dopants may be due to the change of the growth mechanism in the presence of dopants.28 The dopants and the Zn ion have different electronegativities, which may play a crucial role in changing the growth mechanism of the crystals.29 The BET surface areas of the synthesized ZnO materials are altered with the incorporation of the dopants. This change in surface area correlates with the average particle sizes of the materials. The introduction of dopants results in a reduction of the particles sizes,30 which is responsible for the increased surface area of the materials.
2.5.3 Presence of Dopants in the Synthesized Materials
The XRF results show that the detected amounts of dopants are much less than the original doping concentrations. The transition metal dopants (Mn, Fe, Co, and Ni,) in the ZnO lattice presumably replace the Zn atoms from ZnO.31,32, 33,34 The Zn sites in ZnO lattice are tetrahedrally coordinated.
The dopants Mn, Fe, Co, and Ni prefer octahedral coordination sites in the oxide structures35.
Hence, the Zn lattice sites are an unfavorable environment for the dopant ions. This might be a possible reason for the lower amount of loading of the dopants in the doped ZnO samples. In addition, the relative amounts of dopants in ZnO are also closely connected to the ionic sizes of dopants. The ionic size of Mn2+ (0.66 Å, hs) is higher than that of Zn2+ (0.6 Å) which might lead to less amounts of Mn loading in the lattice. On the other hand, the ionic sizes of Fe, Co, and Ni
49
(0.49 Å, 0.58 Å, and 0.55A, hs, respectively), are relatively smaller than Zn which might favor a relatively higher amount of loading than Mn in the ZnO. Hence, the ionic sizes and coordination environments both play critical roles in the amount of loading for dopants into the ZnO system.
2.5.4 Oxidation States of the Dopants within the Materials
The high-resolution XPS spectra were measured to determine the oxidation states of the elements present in the samples. The binding energy positions and spin-orbital splitting constant are indications of the oxidation states of the elements. The observed spin-orbital splitting energy of the Zn 2p peaks (23.1 eV) in the XPS spectrum confirms the presence of the 2+ oxidation state of
36 Zn in the ZnO materials. The de-convoluted Zn 2p3/2 peaks at around 1020.0 eV and 1018.0 eV are due to the presence of Zn in the ZnO structure. Another de-convoluted Zn 2p3/2 peak at 1021.6 eV is assigned to the variation of broken Zn-O bonds in the ZnO structure. The presence of the broken Zn-O bonds provides evidence of successful substitution of transition metals in the ZnO structure. These results are consistent with other reports.32
In the case of the Ni 2p peak, the energy difference between Ni 2p3/2 and Ni 2p1/2 core levels is 18.0 eV which is close to the value of 17.9 eV for NiO [NIST data]. Additionally, the observed Ni 2p3/2 peak position (853.5 eV) is close to that of Ni (853.8 eV) in NiO and is also
36 quite different than that of metallic Ni (852.7 eV) and Ni (856.7 eV) in Ni2O3. The binding energy positions of Ni 2p peaks along with two characteristic satellite peaks and the spin-orbital splitting constant provide evidence for a Ni2+ state. The calculated spin-orbital splitting energy of
Co 2p3/2 and Co 2p1/2 XPS peaks is 16.0 eV, which is close to that of CoO (15.50 eV) and higher
37 than that of Co metal (15.05 eV). The Co 2p3/2 and Co 2p1/2 peak positions and their splitting constant demonstrate the presence of Co2+ rather than metallic Co or Co3+. The characteristic
50 satellite peaks for Co2+ are missing. This may be due to the low intensity of the Co 2p XPS spectrum. The presence of a low-intensity, sharp peak positioned at 640.5 eV corresponds to the
2+ 23,38, Mn 2p 3/2, confirming that the Mn in ZnO is mainly present in the chemical state of Mn ions.
39 These results rule out the possibility of formation of metallic Mn or any other Mn oxide structures such as Mn304, Mn2O3, and MnO, which have a Mn 2p 3/2 signal centered at 639.0 eV,
641.2 eV, 641.6 eV, and 642.2 eV respectively.38 The low XPS signal intensity obtained for Mn
2p may be due to the low levels of Mn dopant used. The binding energy position of Fe 2p peaks centered at 723.6 eV (Fe 2p1/2) and 710.6 (Fe 2p3/2) and spin orbital splitting (13.0 eV) provide the
3+ 40,41 evidence that Fe exists as Fe in the ZnO structure. In addition, the Fe 2p3/2 peak position
(710.6 eV) is different than the metallic Fe peak (706.7 eV), ruling out the possibility of metallic
Fe.42 Since the binding energy difference between the Fe2+ and Fe3+ ion is small, it is hard to clearly resolve the core-level XPS spectra in oxide matrices by de-convolution.32 The presence of the Fe3+ ion is further confirmed by EPR studies.
The de-convoluted O 1s peaks provide important information about lattice oxygen and oxygen vacancies. Among the four de-convoluted O 1s peaks, the peak at about 528.3 eV is attributed to the C-O bonding due to the adsorbed carbonate species on the surface of the materials.
The peak at 529.9 eV is associated with lattice oxygen. The peak appearing at 531.5 eV is assigned to the oxygen vacancies within the ZnO lattice. The peak at 532.9 eV is ascribed to interstitial O atoms or surface oxygen in the form of hydroxyl or adsorbed H2O or absorbed O2. The shift of binding energy of oxygen core peaks from standard values further indicates the presence of dopants in the sample.
51
2.5.5 Location and Co-ordination of Dopants within the Materials
The location and coordination environments of the dopants in the ZnO lattice were determined by measuring EPR spectra of the samples. The Co-ZnO and Fe-ZnO samples show characteristic EPR spectra with g-values of 2.336 and 2.000, respectively. For the Co-ZnO sample, the g-value matches very well with reported data,43, 44 indicating Co2+ ions are located at Zn2+ ion sites in the
ZnO lattice. Fe-doped ZnO samples with g-values of 2.000 show good agreement with reported g values for the iron doped ZnO, having Fe3+ ions in the Zn2+ lattice sites45. These results indicate that both the Co2+ ions and Fe3+ ions are in tetrahedral coordination environments within the crystals of ZnO. The presence of hyperfine lines in the EPR spectrum of Mn-ZnO sample is a clear indication of the existence of the Mn2+ state in the sample.21 The hyperfine splitting constant (A=
78.7 Gauss) is strong evidence of the 2+ state and tetrahedral coordination of the Mn in the ZnO lattice. The oxidation states observed by EPR studies are consistent with the XPS results.
2.5.6 Phase Purity of Synthesized ZnO Materials and the Presence of Defects
4 ZnO with a wurtzite crystal structure belongs to the C 6v (P63mc) symmetry group which has two formula units per primitive cell. The group theoretical calculation predicts a total of eight zone- center optical phonons, which can be represented by irreducible representations as follows:
46 A1+E1+2B1+2E2 . The A1 mode is polarized parallel to the C-axis and the E1 mode is polarized perpendicular to the C-axis. Both A1 and E1 modes are polar and split into transverse optical (TO) and longitudinal optical (LO) phonons, with all being Raman and infrared active.47 The non-polar
48 E2 (High) modes are Raman active, while the B1 modes are silent . The non-polar E2 mode has two frequencies: E2 (high) associated with the motion of oxygen atoms and E2 (low) associated
49 with a single crystal. The E2 (high) mode is a characteristic Raman active mode for the wurtzite
52
47,50 hexagonal phase of ZnO. The E1 (LO) mode is associated with impurities and the formation of defects such as oxygen vacancies.49 All spectra show the most prominent peak at 437 cm-1 which
47,49,50,23 is attributed to the E2 (high) mode of ZnO. This confirms the TM doped ZnO (both single doped and multi-doped) have wurtzite hexagonal crystal structures. The band at around 574 cm-1
23 - is ascribed to the LO mode of A1 or E1 symmetry. The increased intensity of the peak at 574 cm
1 for doped ZnO samples confirms the increase of defects such as oxygen vacancies through the incorporation of dopants.49 The peak intensity at 574 cm-1 is the highest for the Mn-doped and 15
% multi-doped ZnO samples (15 % MD-ZnO). This means that these two samples are associated with the highest amount of oxygen vacancies. The Raman band at around 378 cm-1 is attributed to
46 −1 the A1 (TO) mode. The peak at around 333 cm is assigned to E2 (high) – E2 (low) which is a second-order vibrational mode caused by multi phonon processes.47 This peak of the Mn-doped sample is sharper than other samples, suggesting that most of the Mn-ZnO nanocrystals are single crystals. All these observations confirm the incorporation of dopants in the ZnO lattice as well as the presence of defects (especially oxygen vacancies) within the materials. The absence of any additional peaks in Raman spectra confirm the phase purity of doped ZnO, and also rules out the existence of dopants as secondary phases. Hence, dopants occupy the Zn lattice sites of the ZnO crystal. These results are in complete agreement with reported results.31,33,51,52
The effect of TM doping in the creation of defects in the ZnO lattice is further confirmed by the emission spectra. The ZnO crystal defects are altered by incorporation of TMs into the crystals. This effect is monitored by the change of intensity in the peak at around 550 nm, which is related to the local defects of the crystal. The higher the peak intensity at around 550 nm, the higher the local defects in the materials. The decrease of peak intensity in the emission spectra upon TM doping indicates that the doping reduces the local defects of the materials. These data
53 suggest that the undoped ZnO materials have higher levels of local defects compared to all other synthesized materials. The dopants presumably occupy the Zn vacancies (VZn) and oxygen vacancies (Vo) of the crystal which lead to the reduction of VZn in the materials. The VZn reduction causes doped ZnO samples to have lower local defects and hence a lower peak at 550 nm.
However, there is no reduction of such vacancies (VZn) occurring in the case of pure ZnO, resulting in the highest levels of local defects compared to all other samples. The lowering of the peak intensity is observed for all doped ZnO materials. Mn-ZnO has the highest peak intensity at around
550 nm, meaning the highest local defects among all synthesized single TM doped ZnO. The amount of dopant present is related to the ionic sizes of the elements involved. The ionic size of
Mn2+ (0.66 Å, hs) is larger than Zn2+ (0.60 Å) whereas other ions Fe3+ (0.49 Å, hs), Ni2+ (0.55 Å), and Co2+ (0.58 Å, hs) are smaller than Zn2+. Due to the larger ionic size, Mn2+ ions are less likely to be in Zn sites compared to other dopant ions. Hence, less amounts of VZn could be filled in the case of Mn doping in ZnO, resulting in the higher levels of local defects in Mn-ZnO compared to all other single doped samples. In addition to the reduction of VZn for the Mn-ZnO, there is an increase in oxygen vacancies (which is evident from the increase intensity of the Raman peak at
574 cm-1) observed in its Raman spectra (Figure 2.8). However, it isn’t certain how the amount of oxygen vacancies increased in the case of Mn-ZnO as well as 15 % MD ZnO.
The ORR reaction in heterogeneous catalysis proceeds through adsorption of oxygen on the catalyst surface.53,54 Oxygen must be bound to the catalyst surface at the beginning of the reaction. A good ORR catalyst should have high oxygen adsorption capabilities.13 The presence of oxygen vacancies in the catalyst surface is very important for ORR. The oxygen vacancies on the catalyst surface act as oxygen binding centers (active centers) toward the reacting oxygen. The larger the amount of oxygen binding centers, the higher the ORR activity of the catalyst. Different
54 transition metals introduce different levels of oxygen vacancies which results in the variation of
ORR activity for the materials. This could be a major cause for all doped samples to have higher
ORR performances than pure ZnO. In addition, the relative amounts of vacancies created by the introduction of dopants might affect the conductivity of the materials, causing significant variation in the electrocatalytic performances.55,56 According to the Raman analyses (Figure 2.8), the Mn-
ZnO and 15 % MD-ZnO have the highest levels of oxygen vacancies, which explains the higher
ORR activities of these materials comparatively.
The morphology, particle sizes and shapes, and surface areas of the materials play vital roles in the electrocatalytic activities. Although the morphologies of pure ZnO and Mn-ZnO are very similar (Figure 2.3), they behave differently toward ORR activities. These observations confirm that morphology is not a major factor in ORR activities of the synthesized materials. The estimated surface areas (Table 2.3) of pure ZnO, Mn-ZnO, and Ni-ZnO are very close (6, 7, and 7 m2/g), but their ORR activities for ORR are very different. This confirms that there is no direct correlation between observed ORR activities and the surface areas of the synthesized materials.
2.6. CONCLUSIONS
TM doped ZnO nanocrystals were synthesized successfully using the simple precipitation method.
The crystal structure, morphology, composition, and optical properties were investigated. XRD and Raman spectroscopy reveal that both pure and doped ZnO crystals possess hexagonal
(wurtzite) crystal structures even at the highest levels (20% nominal) of doping. SEM images reveal that the morphology of the materials changes with the dopant elements and doping systems.
EDX and XPS survey spectra confirm the presence of dopants within the synthesized ZnO materials. The high-resolution XPS and EPR spectra suggest the oxidation states of Zn2+, Mn2+,
55
Fe3+, Ni2+, and Co2+ ions in the materials. The EPR results confirm that all dopant elements occupy the tetrahedral coordination sites (Td) of the ZnO matrix. The LSV studies show that all synthesized
TM doped ZnO nanocrystals have enhanced ORR activity compared to the undoped counterpart.
The Mn-doped ZnO exhibits the highest ORR activity among all of the single doped ZnO ZNs and the 15 % MD-ZnO is the highest among MD-ZnO samples in ORR activity. PL analysis demonstrates that transition metal doping in ZnO leads to the formation of oxygen vacancies compared to pure ZnO samples. Mn doping and 15% multi-doping provide the highest amount of oxygen vacancies which is responsible for the enhanced ORR activities of the synthesized materials.
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63
CHAPTER 3. TRANSITION METAL (Co & Ni) DOPED MESOPOROUS CoP AND FeP
NANOMATERIALS FOR THE HYDROGEN EVOLUTION REACTION
3.1 OVERVIEW
The electrochemical hydrogen evolution reaction (HER) offers a simple and environment-friendly means to produce hydrogen fuel from water. Use of platinum (Pt) as a catalyst is the main obstacle for the technology to be economically viable. Herein, we synthesized mesoporous phosphide materials: FeP, 5 mol % Co-FeP, CoP, 5 mol % Ni-CoP, and 5 mol % Ni-CoP/CNT. The materials were prepared using a phosphidation process of their corresponding mesoporous oxides synthesized via an inverse-micelle templated method. The HER activity of the materials was determined using linear sweep voltammetry (LSV) analyses in 0.5 M H2SO4 solution. The LSV revealed that the synthesized mesoporous phosphides showed enhanced HER activity. The HER activity of the prepared samples shows increasing activity as follows: Meso 5 mol % Ni-CoP /
CNT < Meso CoP < Meso 5 mol % Co-FeP < Meso 5 mol % Ni-CoP < Meso FeP. Ni doping in the mesoporous CoP structure enhanced HER activity whereas Co doping in FeP system decreased
HER. The enhanced HER performances of the synthesized mesoporous phosphides are governed by the highly interconnected porous networks, crystallinity, large charge-transfer rates, and high surface areas of the materials. The chronopotentiometry analyses showed that the most active mesoporous FeP material is stable up to 28 h under the experimental acidic conditions (0.5 M
H2SO4).
64
3.2 INTRODUCTION
1,2 Hydrogen (H2), is a clean and sustainable energy alternative to fossil fuels and is considered to
3,4 be a promising energy source to address the future energy crisis. H2 fuel can be produced using the electrochemical water splitting reaction that involves two half-reactions: one occurs at the
+ - anode and the other at the cathode. The hydrogen evolution reaction (HER), 2H (aq) + 2e = H2(g), is the cathode half reaction of the water splitting reaction, which produces H2 fuel from water.
Electrochemical water splitting based on polymer exchange membrane (PEM) technology operates under strong acidic conditions. Abundant nickel-based alloys have been commercially used as
HER catalysts.5,6 However, nickel-based materials are not stable in acidic environments. In addition to stability, an effective HER catalyst should produce hydrogen efficiently with a low overpotential.5,7,8 The most effective HER catalysts are mainly Pt and Pt-group metals,9,10 which suffer from high cost and consequently cannot be used in large-scale operations. Hence, it is urgent to develop non-precious metal-based, highly active, and stable catalysts for HER. This has led the research community to find active, stable, and noble metal-free HER catalysts. Substantial progress has been made in the preparation and exploration of novel materials for HER
11–21 electrocatalysis over the last decade.
Recently, 3d-transition metal (such as Fe, Co, and Ni) based phosphides have been prepared and found to be promising as HER catalyst under acidic conditions.22–30 Different synthetic approaches have been proposed to obtain 3D- transition metal phosphides (TMPs) as efficient HER catalysts.5,6,29 The TMPs are stable in strongly acidic and basic media. Among all transition metal phosphides, iron (Fe) phosphide is particularly important because iron is the cheapest and one of the most abundant (5 %) transition metal elements in the earth’s crust. Iron phosphides crystallize with mainly Fe2P-type (Fe2P) and MnP-type structures (FeP). Among
65 different metal phosphide structures (FeP, Fe2P, Fe3P for example), higher phosphorous content structures (FeP, for example) are observed to be more active.31 Nanostructured iron phosphides
(FeP) have recently been reported as promising materials for HER activity.3,6,24,32 The efficiency of these FeP catalysts is still lower than that of noble metal catalysts like platinum. Thus, the electrocatalytic activity of phosphide materials requires further enhancement. The electrocatalytic reactions occur on the surface of the catalysts. Surface modification is one of the common ways to enhance the catalytic activity of a catalyst. There are a number of ways to modify the surfaces of a catalyst. For instances, preparing porous structures or engineering surface electronic structures by doping with other elements are effective methods.11,15,33–36 Mesoporous materials typically possess interesting textural properties37 such as high surface areas, tunable pore sizes, and high absorption capabilities which are favorable for increasing catalytic activities. In addition, small amounts of doping in the metal phosphide structure can enhance electrocatalytic activities by an order of magnitude.38–40
Despite the remarkable HER activity of metal phosphides, controlling the surface structure and composition to increase the efficiency of the catalysts remains challenging. In this work, we developed mesoporous structures of FeP and CoP materials utilizing the inverse- micelle templated method41 for the first time. The effects of (Co and Ni) doping in the structure, properties, and HER activities of FeP and CoP materials are studied. In addition, we synthesized mesoporous Ni doped
CoP materials on carbon nanotube (CNT) supports (5 % Ni-CoP/CNT) to investigate the effects of CNT supports on the structure, properties and activities (HER) of these materials in the HER.
The mesoporous phosphide structures were synthesized via a one-step phosphidation of their mesoporous oxides with NaH2PO2 in argon atmosphere. Mesoporous oxide precursors were prepared using the conventional inverse-micelle templated method proposed by the Suib group.41
66
3.3. EXPERIMENTAL SECTION
3.3.1 Chemicals
Iron(III) nitrate nonahydrate (Fe(NO3)3 · 9H2O ≥ 98.0 %), cobalt(II) nitrate hexahydrate
(Co(NO3)2 · 6H2O ≥ 98.0 %), nickel(II) nitrate hexahydrate (Ni(NO3)2 · 6H2O ≥ 97.0 %), a surfactant poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)
(PEG20 -PPG70 -PEG20, Pluronic P123), nitric acid (HNO3 ≥ 70 %), 1-butanol (≥ 99.4 %), 20 wt %
Pt / C, sodium hypophosphite monohydrate (NaH2PO2 ≥ 99 %), Nafion perfluorinated ion exchange resin, (5 wt % solution in lower aliphatic alcohols/ H2O mix) were purchased from
Sigma-Aldrich. All chemicals were reagent-grade and used without any further purification.
3.3.2 Synthesis of Mesoporous Fe3O4, 5 % Co-Fe3O4 and Co3O4, 5 % Ni-Co3O4 and 5 % Ni-
Co3O4 / CNT
Mesoporous metal oxides (Fe3O4 / Co3O4 and Co doped Fe3O4/ Ni doped Co3O4) were synthesized
41 by the sol-gel-based inverse micelle method (UCT method). In a typical synthesis of Fe3O4, 0.01 mol (4.04 g) of iron(III) nitrate nonahydrate was dissolved in 0.12 mol (8.90 g) of butanol-1 in a
400 mL beaker. Mixtures of 0.019 mol of (1.20 g) nitric acid and 2.04 × 10-04 mol of (1.18 g) P123 were added to the beaker, which was mixed at room temperature with magnetic stirring. After about 30 min of stirring a clear gel was obtained, which was then put in an oven under air at 95 °C for 3 h. The obtained powder was washed twice and centrifuged 5 times with ethanol. The washed powder was dried in a vacuum oven overnight. The dried powder was then subjected to a heating cycle of 150 °C 12 h + 250 °C 3 h + 350 °C 3 h. Samples were naturally cooled down after each heating cycle and all heating treatments were done under an air atmosphere. To synthesize 5 mol
67
% Co doped Fe3O4 (Co-Fe3O4), a certain amount of cobalt(II) nitrate hexahydrate was added to the iron precursor at the start of the experiment and the rest of the steps were kept unaltered.
The mesoporous Co3O4, 5 mol % Ni doped Co3O4 (5 % Ni-Co3O4) and 5 mol % Ni doped Co3O4 on carbon nanotube (5 % Ni-Co3O4 / CNT) were also synthesized using the same steps discussed above. To prepare mesoporous Co3O4, 0.017 mol (5.00 g) of cobalt(II) nitrate hexahydrate was dissolved in 0.33 mol (17.00 g) of butanol-1 in a 400 mL beaker. Next 0.038 mol (2.40 g) of nitric acid and 4.31 × 10-04 mol (2.50 g) of P123 were added to the beaker, which was mixed at room temperature with magnetic stirring. After about 30 min of stirring, the obtained clear gel was put in an oven under air at 120 °C for 3.5 h. The powder obtained was then washed, centrifuged, and dried. The heating cycle 150 °C 12 h + 250 °C 3 h + 350 °C 3 h. was applied to get the final product. To prepare 5 % Ni-Co3O4 and 5 % Ni-Co3O4/CNT, a certain amount of nickel(II) nitrate hexahydrate was added to precursor solution and the remaining process was kept the same. For 5
% Ni-Co3O4/CNT samples, the metal oxide to carbon ratio was (1: 4).
3.3.3 Synthesis of Mesoporous FeP, 5 % Co-FeP and CoP, 5 % Ni-CoP, 5 % Ni-CoP/CNT
Catalysts
Mesoporous metal phosphides were obtained from the corresponding metal oxides by a low- temperature phosphidation method. In a typical synthesis of FeP, 60 mg of Fe3O4 and 114 mg of
NaH2PO2 were placed at two separate positions in a quartz tube reactor under an inert argon (Ar) atmosphere. NaH2PO2 was placed at the upstream side of the furnace. Subsequently, the samples were heated at 300 ºC for 2 h with a heating speed of 2 ºC/min and an Ar flow rate of 40 sccm.
The materials were collected as a final product after being cooled to ambient temperature. The same procedure was repeated to prepare mesoporous 5 % Co-FeP, CoP, 5 % Ni-CoP, and 5 % Ni-
CoP/CNT catalysts.
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3.3.4 Characterization
X-ray diffraction (XRD) was performed on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 1.5406 Å) at an operating voltage of 40 kV and a beam current of 44 mA. Both wide-
° ° ° ° angle (2θ = 5 - 75 ) and low-angle (2θ = 0.5 - 5 ) XRD data were collected. N2 sorption analyses were conducted using a Quantachrome Autosorb IQ instrument. All samples were degassed under vacuum at 120 °C for 3 h prior to the measurements to remove surface adsorbed species. The
Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods were used to calculate the surface areas and the pore size distributions, respectively. Scanning electron microscopy (SEM) analyses were done on an FEI Nova NanoSEM 450 to study the particle morphology and the composition of the materials. High-resolution transmission electron microscopy (HR-TEM) analysis was done with an FEI Talos F200X microscope operating at 200 kV to confirm the crystalline phase and mesoporosity of the synthesized samples. To analyze the oxidation states of present elements, X-ray photoelectron spectroscopy (XPS) measurements were done on a PHI model Quantaum 2000 spectrometer with scanning ESCA multiprobes (Φ Physical
Electronics Industries Inc.), using Al Kα radiation (λ = 1486.6 eV) as the radiation source. The spectra were recorded in the fixed analyzer transmission mode with pass energies of 187.85 eV and 29.35 eV for recording survey and high-resolution spectra, respectively. The powder samples were pressed on a double-sided carbon tape mounted on an Al coupon pinned to a sample stage with a washer and screw, then placed in the analysis chamber. The obtained XPS spectra were analyzed and fitted using CasaXPS software (version 2.3.16). Sample charging effects were eliminated by correcting the observed spectra (Figure 7b) with the C 1s binding energy (BE) value of 284.8 eV.
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3.3.5 Electrochemical Measurements
3.3.5.1 Working Electrode Preparation
The working electrode was made using pyrolytic graphite (PG) and catalyst ink. PG was first polished with 600 grit SiC paper in a wet condition and sonicated 2 min with ethanol to achieve a smooth and clear surface. The catalyst ink was prepared by dispersing 5 mg of active material in
1 mL of 4:1 v/v water and ethanol mixed solvent, followed by adding 50 µL of 5 wt % Nafion solution. The mixture was then sonicated for 40 - 60 minutes to form a homogenous ink. 20 µL of the ink was loaded onto the PG surface (5 mm in diameter). The working electrodes were air-dried overnight prior to the tests.
3.3.5.2 Electrochemical Tests
The electrochemical tests were conducted on a CHI 760E electrochemical work station using a conventional three-electrode cell. The cell consists of a saturated calomel electrode (SCE) as the reference, a platinum wire as the counter, and a modified PG working electrode. Measurements were carried out at room temperature (22 ± 2 °C) using 0.5 M H2SO4 as the electrolyte. Before the measurements, the electrolyte was degassed by bubbling argon gas for 30 min. The electrocatalytic performances of the materials were tested using linear sweep voltammetry (LSV) with a rotating disk electrode (rotating speed was 1600 rpm/ min). LSV was recorded within the potential range of -0.1 to -0.7 V (vs SCE) with a scan rate of 5 mVs-1. Electrochemical impedance spectroscopy (EIS) data were recorded at -0.45 V (vs SCE) using the frequency range of 0.1 Hz to
100 KHz with an AC voltage of 5 mV amplitude. The stability of the materials was examined by chronopotentiometric responses with the current density set at 10 mA cm-2 and the potential at a range of -0. 40 to -0.55 V (vs SCE). The electrochemical data of industrially used catalyst Pt (20
%, w/w) / C were taken to compare the results. All recorded LSVs were iR compensated.
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3.4. RESULTS
3.4.1 Crystalline Structure and Mesoporosities of Oxide precursors
The crystalline structures of the synthesized oxide precursors were studied by XRD analyses. The
XRD patterns of synthesized oxides (Fe3O4, Co-Fe3O4, Co3O4, and Ni-Co3O4/CNT) are shown in
Figure 3.1. The XRD data show that both Fe3O4 and 5 % Co-Fe3O4 adopt the pure magnetite
(Fe3O4) crystalline phase and Co3O4, Ni-Co3O4, and Ni-Co3O4/CNT crystallize with a pure Co3O4 structure. The peak position and peak intensities matched with the standard PDF (Powder
Diffraction File) card of magnetite (19-0629) phase and Co3O4 (34-1003) phase, respectively. Co doping in the meso Fe3O4 structure and Ni doping in the meso Co3O4 structure did not alter the crystalline structures of the materials. Ni-Co3O4/CNT samples show an additional peak at around
2θ = 26.2 (deg.), which was ascribed to carbon nanotubes.
71
Figure 3.1. Wide-angle XRD patterns of synthesized meso oxide samples.
(a) (b)
Figure 3.2. Low-angle XRD data of (a) meso Co3O4 samples; (b) meso Fe3O4 samples.
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The mesoporous nature of the oxide materials was determined by low-angle XRD and N2 sorption analyses. The low-angle XRD data of the Co3O4 and Fe3O4 samples are shown in Figure
3.2. All of the synthesized oxides show the characteristic low-angle diffraction peak. The peak intensity of 5% Ni-Co3O4 and 5 % Ni-Co3O4 /CNT samples are slightly weaker than that of pure
Co3O4. However, the peak intensity of the 5 % Co-Fe3O4 sample is stronger than that of the pure
Fe3O4 samples. The peak positions for the synthesized materials are shifted towards the higher angles. This shifting is highest for the 5 % Co-Fe3O4 sample.
N2 sorption isotherms of the oxide materials (Co3O4, Fe3O4, and their doped analogs) are shown in Figure 3.3. The Figure shows that the recorded isotherms are type IV and have a hysteresis loop. This type IV isotherm with a hysteresis loop further confirms the mesoporous structure of the synthesized oxide materials. The BJH pore size distributions of the synthesized mesoporous oxides are shown in Figure 3.4. The pore sizes of synthesized oxide materials are distributed mostly within the range of 3-10 nm. The calculated average pore diameters of the synthesized mesoporous oxides are in the range of 3.4 nm to 5.6 nm (Table 3.1).
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Table 3.1. BET Surface Area, BJH Pore Diameters, and Total Pore Volumes of the
Synthesized Mesoporous (meso) Oxide and Phosphide Samples
total pore volume BET surface material meso materials pore diameter (nm) 3 2 (cm /g) area (m /g) type
Co3O4 4.9 0.33 125
5 % Ni-Co3O4 5.6 0.34 151
5 % Ni-Co3O4/ CNT 3.8 0.23 106
Oxides Fe3O4 3.4 0.18 151
5 % Co-Fe3O4 3.8 0.19 122
CoP 6.5 0.18 52
5 % Ni-CoP 5.6 0.25 80
5 % Ni-CoP/CNT 5.6 0.21 69
FeP 3.8 0.10 45 Phosphides
5 % Co-FeP 5.6 0.08 31
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(a) (b)
Figure 3.3. N2 sorption isotherms of the synthesized oxides (a) meso Co3O4 samples; (b) meso
Fe3O4 samples.
(a) (b)
Figure 3.4. BJH pore size distributions of synthesized (a) meso Co3O4; (b) meso Fe3O4 samples.
3.4.2 Characterization of Phosphide Materials
The crystalline phases of the synthesized phosphides are studied using XRD. Figures 3.5a and 3.5b show the wide-angle XRD patterns for the mesoporous CoP and FeP materials along with their doped analogs, respectively. The XRD data obtained for the mesoporous CoP samples revealed
75 that most of the diffraction lines match with CoP (JCPDS 03-065-2593). However, only one low- intense peak, positioned at 2θ = 40.8°, indexed as the Co2P phase. Ni doping in the CoP sample did not change the structure but the crystallinity of the materials decreased as indicated by the reduction of intensity of the diffraction peaks. A diffraction line appearing at 2θ = 44.5° could be indexed as the cobalt-dicobalt(III) oxide phase (JCPDS 01-080-1537). In the case of the 5 % Ni-
CoP/CNT samples, the intensity of the diffraction lines decreases further, indicating the poor crystalline nature of the sample. Figure 3.5b indicates that the recorded XRD peaks match mostly with the FeP phase (JCPDS 03-065-2595). There are also a few diffraction lines in good agreement with the Fe2P crystalline phase (JCPDS 01-085-1727). Co doping in the FeP materials did not alter the structure of the materials. The XRD data also reveal that both the undoped and doped FeP samples are low crystalline materials. The low-angle XRD data of the synthesized phosphide materials (Figure 3.6) reveal that only the 5 % Ni-CoP / CNT sample has a clear diffraction line positioned at 2θ = 0.85°. For the other samples, only a very broad low-intense peak (in the case of
FeP and 5 % Co-FeP) or only a shoulder (in the case of CoP and 5% Ni-CoP) can be seen. The mesoporous structure of the synthesized phosphide materials was further confirmed by the N2 sorption studies.
N2 sorption isotherms and pore size distributions of the synthesized phosphide materials are shown in Figures 3.7 and 3.8, respectively. All CoP and FeP materials and their doped analogs show the characteristics of type IV adsorption isotherms with type III hysteresis loops. These results indicate that the synthesized phosphide materials preserve the mesostructure nature after phosphidation of the precursor oxides. Like the meso oxides, prepared meso phosphides have narrow pore size distributions (Figure 3.8). The average pore sizes of the phosphide samples lie
76 between the range of 3.8 to 6.5 nm (Table 3.1), which further reveals the mesoporous structure of the synthesized phosphide catalysts.
(a) (b)
Figure 3.5 XRD patterns of (a) meso CoP and meso 5 % Ni-CoP; (b) meso 5% FeP and meso 5
% Co-FeP materials (XRD patterns of meso Co3O4 and meso Fe3O4 are shown to compare with the phosphides phase).
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Figure 3.6. Low-angle XRD patterns of mesoporous (a) CoP and 5 % Ni-CoP; (b) FeP and 5%
Co-FeP materials.
Figure 3.7. N2 sorption isotherms of synthesized (a) meso CoP and meso 5 % Ni-CoP samples;
(b) meso FeP and meso 5 % Co-FeP samples.
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Figure 3.8. BJH pore size distributions of synthesized (a) meso CoP, meso 5 % Ni-CoP, and meso 5 % Ni-CoP/CNT; (b) meso FeP and meso 5 % Co-FeP materials.
The SEM images of the synthesized mesoporous phosphide samples are shown in Figure
3.9. SEM analyses show that CoP and its doped analogs have nanoparticle morphology.
Nanoparticles aggregate to form big spherical porous chunk structures. In the 5 % Ni-CoP/CNT materials, nanoparticles accumulate between the carbon nanotubes. FeP and 5 % Co-FeP show mostly nanorod-like morphologies where rods are aggregated with each other and with other nanoparticles. The morphologies of 5 % Co-FeP are more regular in nature and the shapes of the particles are uniform. The obtained TEM images of synthesized phosphides (Figures 3.10 and
3.11) show that materials are porous in nature. HR-TEM image of meso FeP (Figure 3.10b) demonstrates that the pores are in the mesopore range and formed by the nanosized particles. The calculated lattice fringes from the HR-TEM analyses indicate the presence of metallic iron and iron oxide phases in the prepared FeP material.
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Figure 3.9. SEM images of synthesized (a) meso CoP; (b) meso 5 % Ni-CoP; (c) meso 5 % Ni-
CoP / CNT; (d) meso FeP; (e) meso 5 % Co-FeP samples.
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Figure 3.10. TEM images of (a) meso FeP; (b) HR-TEM image of meso FeP with identified diffraction planes (inset); (c) HAADF, High-angle annular dark-field; and (d), (e) EDX- elemental mapping of Fe, and P, respectively.
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Figure 3.11. TEM images of (a) meso 5 mol % Co-FeP; (b) meso 5 mol % Ni-CoP (c) meso CoP materials.
82
Figure 3.12. EDX elemental mapping of meso 5 mol % Co-FeP samples. (a) HAADF, High- angle annular dark-field; (b), (c), (d) Elemental mapping of Fe, P, and Co, respectively.
83
Figure 3.13. EDX elemental mapping of meso 5 mol% Ni-CoP samples. (a) HAADF, High- angle annular dark-field; (b), (c), (d) Elemental mapping of Co, P, and Ni, respectively.
Figure 3.14. EDX elemental mapping of meso CoP samples. (a) HAADF, High-angle annular dark-field; (b) and (c) Elemental mapping of Co and P.
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The surface composition and oxidation states of the synthesized mesoporous phosphide samples were studied using XPS. The XPS spectra of meso FeP sample are shown in Figures 3.14 and 3.15. (XPS spectra of meso CoP, meso 5 % Ni-CoP, and meso 5 % Co-FeP samples are shown in Figures 3.16, 3.17, and 3.18, respectively). The survey spectrum reveals the presence of Fe, P,
C, and O elements in the FeP sample surface. The high-resolution XPS spectra of Fe 2p and P 2p regions are shown in Figures 8a and 8b. In the deconvoluted Fe 2p spectrum, there are five sets of peaks observed. There are doublets of peaks positioned at 707.3 eV and 720.2 eV, 709.5 eV and
723.4 eV, 710.2 and 724.1 eV, 711.3 and 724.8, and 712.6 and 725.6 eV. For the high-resolution
P 2p deconvoluted spectrum, a set of peaks is observed at 129.7 eV and 130.6 eV. The second set of peaks position at 133.7 eV and 134.6 eV.
Figure 3.15. (a) XPS survey spectrum of meso FeP sample, (b) High-resolution XPS spectrum of
C 1s (c) High-resolution XPS spectrum of O 1s.
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Figure 3.16. High-resolution XPS spectra of meso FeP: (a) Fe 2p region; (b) P 2p region.
Figure 3.17. XPS spectra of meso CoP: (a) Survey spectrum, (b) High-resolution XPS spectrum of O 1s, (c) High-resolution XPS spectrum of Co 2p, (d) High-resolution XPS spectrum of P 2p.
86
Figure 3.18. XPS spectra of meso 5 % Ni-CoP: (a) Survey spectrum, (b) High-resolution XPS spectrum of Co 2p, (c) High-resolution XPS spectrum of Ni 2p, (d) High-resolution XPS spectrum of P 2p, (e) High-resolution XPS spectrum of O 1s.
87
Figure 3.19. XPS spectra of meso 5 % Co-FeP: (a) Survey spectrum, (b) High -resolution XPS spectrum of Co 2p, (c) High-resolution XPS spectrum of Fe 2p, (d) High-resolution XPS spectrum of P 2p, (e) High-resolution XPS spectrum of O 1s.
4.4.3 HER Performance of the Synthesized Mesoporous Phosphide Materials
The HER activity of the catalysts was estimated using the linear sweep voltammetry (LSV) method. The recorded LSVs are shown in Figure 3.20. The overpotentials for HER at a current density of 10 mA / cm2 are calculated from the LSVs and are tabulated in Table 3.2. The HER activity of the materials is expressed in terms of overpotential required to achieve the benchmark current (current density of 10 mA / cm2). Hence, the lower the overpotentials, the higher the activities. The calculated overpotential data (Table 3.2) reveal that the HER activity order (low to high) of synthesized mesoporous phosphide materials is meso 5 % Ni-CoP / CNT < meso CoP < meso 5 % Co-FeP < meso 5 % Ni-CoP < meso FeP. Nickel doping in the meso CoP increases the
88
HER activity, whereas Co doping in the FeP mesostructure decreases the performance of the materials.
Figure 3.20. LSV curves of the synthesized mesoporous phosphide materials. LSVs were recorded using SCE as a reference electrode. All potentials are then referenced to RHE using the conversion formula, E (vs. RHE) = E (vs. SCE) V +0.261 V.
89
Table 3.2: Calculated Overpotentials at Current Density 10 mA / cm2 of the Synthesized
Phosphide Samples.
materials calculated overpotentials, mV (vs RHE)
Pt/C - 56
meso FeP - 174
meso 5 % Ni-CoP - 196
meso 5% Co-FeP - 228
meso CoP - 244
meso 5 % Ni-CoP/CNT - 259
The overpotential of 196 mV (vs RHE) for mesoporous 5 % Ni-CoP is a 48 mV positive shift from pure meso CoP, showing an increased HER activity compared to meso CoP. A negative overpotential shift of 54 mV is observed for 5% Co-FeP compared to pure FeP showing a decreased HER activity. The meso 5 % Ni-CoP/CNT sample produces a benchmark current (10 mV/ cm2) with an overpotential of 259 mV (highest overpotential among all other synthesized materials). Among the synthesized meso phosphide materials, FeP is most efficient for HER. The overpotentials (174 mV) for meso FeP and meso 5% Ni-CoP (196 mv) to achieve the 10 mA / cm2 current density are significantly lower than reported catalysts such as CoP microspheres (226 mV),42 CoP nanoparticles (203 mV and 221 mV),43,44 MoP nanoparticles (246 mV),45 FeP
22 46 nanosheet (240 mV) and WSe2 ( 230 mv). The HER performances of CoP (244mV), 5% Co-
FeP (288 mV), and meso 5% Ni-CoP/CNT (259 mV) are also excellent compared to reported data
47 (NiS2 = 301 mV and MoS2= 510 mV). The HER performance of our best catalyst (meso FeP) is
90 inferior to some recently developed materials such as FeP powder (150 mV),48 FeP nanoparticles
(154 mV)6, however, synthesized meso FeP is still comparable to those materials.
To understand the HER reaction kinetics, electrochemical impedance spectroscopy (EIS) was conducted. The charge-transfer resistance (Rct) was calculated from the diameter of the semicircle of the Nyquist plot of the EIS data. The Nyquist plots and the estimated Rct for the synthesized phosphide materials are shown in Figure 3.21 and Table 3.3, respectively. The EIS data show that Rct values are directly correlated to the HER activity of the materials. The highest
HER active FeP material has the smallest Rct (22.5 Ω) at a constant potential of -0.45 V vs. SCE.
The activity decreases as the Rct of the materials increases.
Figure 3.21. EIS spectra of the synthesized mesoporous phosphide materials.
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Table 3.3: Calculated Charge-transfer Resistance (Rct) of the Synthesized Phosphide
Samples.
materials calculated charge-transfer resistance (Ω)
Pt/C 10.5
meso FeP 22.5
meso 5 % Ni-CoP 25.4
meso CoP 30.2
meso 5 % Ni-CoP / CNT 34.6
3.4.4 Stability of Mesoporous FeP Catalyst
The stability of the meso FeP materials was investigated using chronopotentiometric voltage-time responses (v-t) of the catalyst. The v-t curve of the FeP materials is shown in Figure
3.22. The synthesized meso FeP materials maintain a very good stability with no significant loss of its initial activity after 28 h. The results show considerable stability of the synthesized mesoporous FeP material. There are some spikes observed in the v-t curve indicating activity losses at those times. However, the activity goes back to its original value after a short moment.
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Figure 3.22. Chronopotentiometric response of meso FeP under constant current density of 10
2 mA / cm in 0.5 M H2SO4 solution. (Potential shown in the figure is vs SCE)
3.5 DISCUSSION
In this study, mesoporous transition metal (Fe, Co) phosphide samples were obtained using phosphidation of mesoporous oxide materials synthesized via the inverse micelle-templated method. The effects of transition metal doping (Co & Ni) in the FeP and CoP mesostructures
(respectively) and CNT supports in 5 % Ni-CoP were inspected. The electrocatalytic activity (HER activity) of the synthesized materials was also examined. The experimental data suggest that the
HER performance of the materials was governed mostly by the charge-transport capabilities, mesoporous nature, and surface areas of the synthesized materials.
93
The mesoporous materials are typically characterized by the presence of the low-angle
XRD peaks and Type IV sorption isotherms. Our synthesized oxide materials (Co3O4, Fe3O4 and their doped analogs) showed both the characteristic low-angle peaks and Type IV adsorption isotherms, confirming their mesoporosities. Although the final materials (phosphide samples) failed to show low-angle XRD peaks, except CoP material that has a low intensity XRD peak in the low-angle region, the mesoporous nature of the synthesized materials was confirmed by N2 sorption analyses. This analysis show that all synthesized phosphide materials have type IV adsorption isotherms with a hysteresis loop (Figure 3.7), a characteristic of mesoporous materials.
In addition, BJH pore size distribution (Figure 3.8) confirms the pore sizes of the materials which were within a narrow mesopore range (Table 3.1). The mesoporous structures of the phosphide materials are further confirmed by TEM analyses (Figures 3.10 and 3.11). In addition to the mesoporous structure, the presence of dopants and their distributions in the phosphide structure were confirmed by the elemental mapping analyses (Figures 3.12 and 3.13) and XPS spectra
(Figures 3.17 and 3.18). The surface areas of the meso phosphide samples are much smaller than the corresponding oxide samples. For instance, the surface areas of mesoporous Co3O4 are 125 m2/g whereas that of the corresponding phosphide materials are 52 m2/g. The differences in surface areas of the mesoporous oxide and phosphide samples are co-related to their pore volumes. The applied heating cycle for the transition of oxide phase into phosphide phase may cause aggregation of phosphide particles, resulting in the lowering of total pores and pore volumes of the samples.
The reduction of surface areas of the phosphide materials may be attributed to the reduction of the total pore volumes. Additionally, the crystalline phases of the synthesized phosphide materials are very different than that of their oxide precursors. This could also be responsible for the discrepancy
49 in surface areas of phosphides and oxides. The incorporation of Ni doping in meso Co3O4
94 increases the surface areas whereas Co doping decreases the surface area of meso Fe3O4. Song et al. reported a similar increasing trend of surface area due to incorporation of Ni and Mn into meso
35 Co3O4 structures. Variation of surface morphology explains the change in the surface areas of our materials. However, the surface area of meso 5 % Ni -Co3O4 decreases when synthesized with
CNTs. This may be due to aggregation of 5 % Ni -Co3O4 oxide particles in the presence of CNT, which was evident from the SEM images shown in Figure 3.9.
The oxygen signal in the XPS survey spectra may be due to surface oxidation of the meso phosphide materials, the presence of an oxide phase from the precursor, or simply the adsorption of water on the surface of the materials50. The high-resolution XPS spectra of O 1s region (Figures
3.15c, 3.17b, 3.18e, and 3.19e) support this claim showing XPS peaks for lattice oxygen and absorbed oxygen. It is a common phenomenon to observe a thin layer of oxides on the surfaces of many non-oxide materials. In the high-resolution spectrum of Fe 2p region (Figure 3.16a), the deconvoluted doublet of peaks centered at 707.3 eV and 720.2 eV, 709.5 eV and 723.4 eV, 710.2 eV and 724.1 eV, 711.3 eV and 724.8 eV, and 712.7 eV and 725.6 eV are attributed to Fe 2p3/2 and Fe 2p1/2 of metallic Fe, Fe2P, FeO, FeP, and Fe2O3, respectively. The observed peaks (at 711.3
3+ eV and 724.8 eV for Fe 2p3/2 and Fe 2p1/2, respectively) of Fe in FeP are consistent with reported binding energies.9 The possible surface oxidation and incomplete phosphidation of oxide precursors explain the appearance of Fe-O bond features in high-resolution Fe 2p spectra. The presence of oxide layers on the surface of FeP are reported in the literature.6,50–52 This explains the presence of O1s XPS peak for the other samples (Figures 3.17b, 3.18e, and 3.19e) as well as M-O bonding features in the high-resolution M 2p XPS spectra (Figures 3.17c, 3.18b, and 3.19b).The metallic Fe may be produced from the reduction of oxides (Fe3O4) either by Ar+ ion sputtering of
XPS or by the presence of PH3 in the reaction system which also explains the presence of metallic
95
Fe peak in XPS spectrum of meso FeP sample (which also explains the presence of metallic Fe peak in XPS spectrum of meso 5 % Co-FeP samples, Figure 3.19c). The XPS results indicating the presence of metallic Fe, Fe2P, FeO, and Fe2O3 phases in mesoporous FeP sample are in complete agreement with the TEM data (Figure 3.10). The XRD data of phosphide samples also provided an indication of the presence of the Fe2P phase. The XPS peaks appearing at 129.7 eV
3- and 130.6 eV are ascribed to P 2p3/2 and P 2p1/2 of the P ion in FeP, which is in good agreement with reported data6,53(this peak also confirms the presence of P3- ion in other samples, Figures
3.17d, 3.18d, and 3.19d ). Another set of peaks appearing at 133.7 eV and 134.6 eV are assigned to the P-O bonding structure of oxide forms, such as phosphate species present on the surface3
(this explains the presence of XPS peak at 134 eV for the other samples, Figures 3.17d, 3.18, and
3.19). These results confirm the formation of FeP. The BE of Fe 2p in FeP (711.3 eV) exhibits a positive shift from that of metallic Fe (707.3 eV) while the BE of P 2p (129.7 eV) exhibits a negative shift from that of elemental P (130.2 eV). 3,21,54 These data suggest that the Fe and P have a partial positive (δ+) and negative (δ-) charge, respectively, in FeP due to electron transfer from
Fe to P.55,56 The negatively charged atoms (P) and positively charged Fe atoms functioned as proton-acceptor sites and hydride-acceptor sites, respectively, which is a favorable condition for
HER.55 The catalytic mechanism of FeP may be similar to hydrogenase and metal complex HER catalysts. In hydrogenase, pendent bases proximate to the metal centers act as active sites,57 where hydrogen evolution occurs.58–60 Synthesized FeP materials also have pendent bases P (δ-) in close proximity to the metal center Fe (δ+), which are believed to be active centers for HER and thus are promoting the HER process.3,5,21,26,55 This explains why transition metal phosphide materials are intrinsically highly HER active.
96
The HER activity of the synthesized materials increases in the order (low to high) of meso
5 % Ni-CoP / CNT < meso CoP < meso 5 % Co-FeP < meso 5 % Ni-CoP < meso FeP. The electron transport efficiency of the synthesized materials follows the same order. The HER activity of the phosphide materials is reported to be directly correlated to the BET surface area.61 However, no direct relationships between the HER activity and the BET surface area of the mesoporous phosphide materials are observed in this study. Hence, these results may be dominated by the charge-transfer rates of materials. The presence of metallic Fe in mesoporous FeP may contribute to lower its electron transfer resistance and increase the electron transfer kinetics, which might be a reason of showing enhanced HER activity than other materials.
Introduction of carbon nanotubes (CNTs) typically enhance the electrocatalytic HER activity of phosphide materials.49 However, the HER performance of the meso 5 % Ni-CoP / CNT was significantly lower compared to meso 5 % Ni-CoP. Typically, high surface area of a material provides more accessible active sites, which make catalysts more efficient. In this case, introduction of CNT in the meso 5 % Ni-CoP resulted in a 14% reduction of its surface area. This suggests that the meso 5 % Ni-CoP/CNT sample has a smaller number of active sites compared to meso 5 % Ni-CoP, which may be responsible for lower activity of meso 5 % Ni-CoP/CNT compared to meso 5 % Ni-CoP. Our mesoporous FeP catalysts are stable up to at least 28 h in 0.5
M H2SO4 solution, which is evidenced by no significant loss of activity in the chronopotentiametric test. The observed spikes in the v-t diagram are due to the generated hydrogen bubbles on the surface of the electrode, which temporarily block the active sites of the catalyst. The activity goes back to its original position when the bubbles burst.
Hence, the intrinsic properties (highly interconnected porous networks), large charge- transfer rates, and high surface areas, all contributed to excellent HER catalytic performances of
97 the synthesized mesoporous phosphide materials. This makes them promising materials for application in hydrogen generation through electrochemical water splitting.
3.6 CONCLUSIONS
Mesoporous FeP, 5 % Co -FeP, CoP, 5 % Ni-CoP, and 5 % Ni-CoP/CNT have been successfully synthesized via the inverse-micelle templated method followed by low-temperature phosphidation.
XRD, N2 sorption, and HR-TEM analyses confirmed the mesostructures of the oxides. The mesoporosity was conserved during the phosphidation process to synthesize the corresponding phosphides, the final products. LSV analysis reveals that the synthesized mesoporous phosphides show enhanced HER activity in an acidic (0.5 M H2SO4) environment. Ni doping in the mesoporous CoP systems increases HER activity whereas Co doping in the FeP system causes a decrease. Highly interconnected porous networks, high surface areas, crystallinity, and large charge-transfer rates of the materials contributed to enhanced HER catalytic. The meso FeP shows the best HER activity among all synthesized materials. The superior HER activity of meso FeP compared to other synthesized phosphides is believed to be mostly due to the low charge-transfer resistance of the materials, which enhances the electron-transfer kinetics of the reaction. The synthesized highly active mesoporous FeP material shows long-time stability (up to 28 h) under the acidic environment of 0.5 M H2SO4.
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CHAPTER 4. SYNTHESES OF ZnO WITH DIFFERENT MORPHOLOGIES: THEIR
CATALYTIC ACTIVITIES TOWARD COUMARIN SYNTHESIS VIA THE
KNOEVENAGEL CONDENSATION REACTION
4.1 OVERVIEW
Heterogeneous catalysts are preferred in fine chemical industries due to their easy recovery and reusability. Here, we report an easily scalable method of ZnO catalysts for coumarin synthesis.
Nanocrystalline ZnO particles with diverse morphologies and crystallite sizes were prepared using different solvents. The change in morphologies results in changes in bandgaps, defects, basicity, and textural properties (surface areas, pore volumes, and pore sizes). The catalytic performances of the synthesized ZnO materials were tested using coumarin synthesis via the Knoevenagel condensation. The catalyst synthesized using methanol shows the highest activity and selectivity
(conversion 74 %, selectivity 94%) with a TON of 14.69. The increased activity of the ZnO synthesized in methanol is attributed to the combined effects of moderate basicity and relatively high textural properties of the sample.
4.2 INTRODUCTION
Coumarins are heterocyclic compounds having a benzo-α-pyrone moiety in their structure. These compounds have been widely studied because of their importance as antidepressants,1 antimicrobials,2 anti-oxidants,3 anti-inflammatories,4 antinociceptives,4 anti-tumor agents,5 antiasthmatics,6 antivirals (including anti-HIV),7 and anti-coagulants.8 There are several methods utilized to prepare coumarins. Some examples are Knoevenagel condensation,9,10 Perkin reaction,11 Pechmann reaction,12,13 and Wittig reactions.14 Among various available methods, the
108
Knoevenagel condensation is the most useful method to synthesize coumarins and their derivatives.10 The Knoevenagel condensation is usually catalyzed by weak acids and bases,15,16 most of those homogeneous catalysts are toxic and harmful chemicals. In addition, homogeneous catalysts are often very hard to separate from the reactants and products. On the other hand, heterogeneous catalysts are easy to separate from the reaction, reusable in nature, inexpensive, and non-toxic. Hence, heterogeneous catalysts are preferred in industry.
A variety of heterogeneous catalysts have been used for the Knoevenagel condensation reaction.10,17–23 Besides these, the Al-Lohedan Group has recently developed melamine immobilized MCM–41,16 urea functionalized SBA–15,24 and the Hamid Group synthesized DL-
Alanine functionalized MCM–4125 mesoporous catalysts for the Knoevenagel condensation.
These catalysts exhibit good catalytic activities with recyclability for the reaction. However, synthetic processes of those catalysts are complex and tedious. ZnO, being basic and abundant, has the potential to be a promising heterogeneous catalyst for coumarin synthesis. ZnO is an environmentally benign and inexpensive catalyst. Moreover, the properties of ZnO can easily be altered26,27,28 as well as activities. Thus, ZnO is considered to be a potential replacement of existing heterogeneous catalysts. For instance, the Kumar Group reported the use of ZnO nanoparticles in coumarin synthesis,29 they focused on establishing a general synthetic strategy for various coumarins using microwave derived ZnO nanoparticles.
The properties of ZnO depend on various morphologies,26,28 which change with synthesis processes27,28. Among various properties, textural properties (morphologies, surface areas, and surface defects) are crucial for tailoring the activity of the materials in heterogeneous catalysis.30,31
Hence, the investigation of catalytic activity of ZnO with their morphologies and other surface properties (surface areas, surface basicity) are very interesting. Xu et al. (2009) previously
109 synthesized different morphologies of ZnO with different solvents and investigated the photocatalytic activities of the materials.26 They discussed the influence of solvents in obtaining different ZnO morphologies and the photodegradation activity of the obtained materials. Since the crystallographic morphology of ZnO depends on the precursor, solvent, pH, and time and temperature of a reaction,32–34 in this study a modified reaction system (in terms of precursors, solvents, and reaction time) are utilized to tailor ZnO morphology, crystalline quality, and textural properties. The correlation between the catalytic activities for coumarin synthesis and structural properties of synthesized ZnO is also explored. This study could be significant in selecting and designing an effective catalyst for coumarin synthesis based on low-cost ZnO materials.
Herein, an easy, scalable preparation of ZnO with various morphologies using a modified solvothermal method is described. The diverse morphologies of ZnO were obtained using different solvents in the reaction. The crystalline qualities, textural properties, surface basicity, and defects of the prepared ZnO samples were investigated. The variation of catalytic activity of ZnO samples with morphologies was examined using the Knoevenagel condensation as a model reaction. The correlation between the activity of ZnO with surface properties was explored.
4.3 EXPERIMENTAL SECTION
4.3.1 Materials
Zinc acetate dihydrate (Zn (CH3COO)2 · 2H2O, ACS reagent, 98 %), commercial ZnO powder
(ACS reagent, 99.0 %), diethylamine (DEA) (40 % w/w aqueous solution), 1-propanol
(Anhydrous, 99.7 %) were purchased from Sigma-Aldrich. Sodium Hydroxide (ACS reagent,
98 %), methanol (HPLC grade, 99.9 %), acetonitrile (LC/MS reagent) was purchased from J.T.
Baker. Ethanol (ACS grade, 99.98 %) was purchased from Pharmco-Aaper.
110
4.3.2 ZnO Preparation
About 1.097 g (0.1 M) Zn (CH3COO)2 · 2H2O and 0.400 g (0.4 M) NaOH were taken into two separate beakers. Zn (CH3COO)2 · 2H2O was dissolved in 50 mL of water whereas NaOH was dissolved in 25 mL of water (distilled de-ionized, DDI). The both solutions were slowly mixed together with stirring to prepare ZnO in Water catalyst. The reaction mixture was heated in a 100 mL Teflon-lined stainless-steel autoclave for 6 h. After the reaction, the mixture was cooled down and the precipitates were collected from the reaction mixture by centrifuge with de-ionized water
(five times) and ethanol (once). The collected precipitates were then dried in a laboratory oven at
60 °C for 12 h. The same procedure was repeated using different solvents: methanol, ethanol, 1- propanol, DEA, and acetonitrile, to prepare ZnO in Methanol, ZnO in Ethanol, ZnO in Propanol,
ZnO in DEA, and ZnO in Acetonitrile, respectively.
4.3.3 Materials Characterization
X-ray powder diffraction (PXRD) were done using a Rigaku Ultima IV powder X-ray diffractometer with Cu Kα radiation (λ=1.54178 Å) to study the structures of the materials. Images of the materials’ surface were recorded using an FEI Quanta 250 FEG field emission scanning electron microscope. Transmission electron microscopy (TEM) analysis was carried out with an
FEI Talos F200X microscope to investigate the exposed faces of the catalysts. The optical properties, crystallinity, and defects of the materials were studied using UV-visible absorption spectroscopy, Raman spectroscopy, and room temperature photoluminescence (PL) spectroscopy.
To analyze with UV-visible absorption spectroscopy, dispersions of samples in double deionized water (DDW) were used. UV-visible absorption measurements were conducted using a
SHIMADZU UV 2450 spectrophotometer. PL studies were carried out directly on the pelletized solid sample using a Horiba Fluorolog III PL instrument with an excitation wavelength of 325 nm.
111
Raman analysis was performed using a Renishaw Raman System 2000 Ramanscope with an Ar+ ion laser (514.5 nm). A NOVA 2000e, Quantachrome, USA was used for N2 sorption analysis using N2 gas as the adsorbate at 77 K (-196 °C). To calculate the specific surface area of the materials, the Brunauer-Emmett-Teller (BET) method was utilized whereas the Barrett-Joyner-
Halenda (BJH) desorption method was used for the pore size distributions and pore volumes. To investigate the basicity of the materials, room temperature CO2 adsorption experiments were performed using a Quantachrome Autosorb-1-1C automated adsorption. All samples were degassed at 150 °C for 3 h before running the sorption analysis.
4.3.4 Catalytic Activity Measurements
To investigate the effects of morphology on the catalytic activity of the ZnO samples, a coumarin synthesis reaction was carried out. The synthesis of ethyl 2-oxo-2H-chromene-3-carboxylate
(coumarin) from o-hydroxy benzaldehyde and diethyl malonate via the Knoevenagel condensation reaction was utilized as a model reaction. In a typical coumarin synthesis, the reactants o-hydroxy benzaldehyde (2 mmol) and diethyl malonate (2.5 mmol), catalyst ZnO (8.2 mg) were put in a closed reactor. The reactor with this mixture was then heated at 100°C for 24 h with vigorous stirring. When the reaction time elapsed, the product was collected from the system by filtration.
The products were analyzed using an Agilent Technologies gas chromatography/ mass spectrometry (GC/MS) instrument. GC/MS instrument consists of a 7820 GC system and a 5975 series MSD mass detector. The products were separated from the reactants in the GC system using a non-polar cross-lined methyl siloxane column with a dimension of 12 in × 0.200 mm × 0.33 μm.
The conversion was calculated from the concentration of reactants and products. The selectivity was estimated on the basis of the coumarin and 2-propenoic acid as the only products.
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4.4 RESULTS
4.4.1 Structural Characterization
Structural characterization of ZnO nanocrystals was done using PXRD techniques. X-ray diffraction patterns (Figure 4.1) indicate that all of the ZnO samples have a hexagonal crystal structure. All observed lines in the XRD data were indexed as ZnO with a hexagonal crystal structure. No additional peaks from other phases were observed. The positions of the diffraction lines and their relative intensities matched with the standard JCPDS card (36-1451) for the ZnO hexagonal crystal structure. The relative intensities of (002) planes of the ZnO sample synthesized in ethanol (ZnO in Ethanol) are slightly different from the JCPDS file. The XRD lines for the ZnO prepared with acetonitrile (ZnO in Acetonitrile), 1-propanol (ZnO in 1-propanol), and DEA (ZnO in DEA) are relatively sharp whereas that of ZnO prepared in water (ZnO in Water), methanol
(ZnO in Methanol), and ZnO in Ethanol are relatively broader. These line broadenings indicate that those materials have smaller crystallite sizes. The calculated crystallite sizes (calculated using the Scherrer equation) of the samples are presented in Table 1. The crystallite sizes of the ZnO samples prepared in different solvents varied from 10 nm (smallest) to 56 nm (largest). The calculated crystallite sizes support the XRD line broadening data shown in Figure 4.1.
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Figure 4.1. XRD patterns of all ZnO samples synthesized in different solvents.
114
Table 4.1. Crystallite Sizes of the Synthesized ZnO Samples
calculated crystallite size synthesized ZnO in various solvents (nm)
ZnO in Ethanol 10
ZnO in Methanol 12
ZnO in Water 31
ZnO in DEA 38
ZnO in Acetonitrile 45
ZnO in 1-propanol 56
Comm ZnO (commercial ZnO) 81
4.4.2 Morphology and Growth Face Analysis of ZnO Catalysts
The SEM images (shown in Figure 4.2) were taken to analyze the morphology of the prepared
ZnO samples. The studies reveal that all synthesized ZnO are nanocrystalline. The ZnO in Ethanol exhibits a sand-like surface morphology. The sand-like particles are interconnected, having a nano- size finger-like shape. When ZnO was synthesized with methanol (ZnO in Methanol), a flat but rough morphology was observed having fairly uniform spherical particles. The particles were attached to each other and formed a compacted surface of spherical particles. The sizes and shapes of the particles were discernible. The particles sizes vary from 10 nm to 15 nm. ZnO synthesized using DEA (ZnO in DEA) as a solvent displays a flower like morphology. The flower was made
115 up of flakes oriented in different angles and directions like flower petals. The lengths of the flakes range from 0.2 - 1.5 µm. The ZnO in 1-propanol also forms a flake-like morphology. These flakes combine to form a large aggregation. The aggregation of flakes is irregular making the surface very rough and bumpy. The size of the flakes was fairly uniform and in the range of 80 - 100 nm.
The ZnO in Acetonitrile presents the same morphology as ZnO in DEA. However, the size of the flakes varies from 50 - 550 nm. The ZnO in Water has a granular morphology. The granules are well-separated from each other. The size of the granules varies from 100 - 500 nm. The granules seem to be made of fused particles of size 10 - 20 nm. ZnO in Methanol displayed a spherical particle morphology.
Figure 4.2. SEM images of: (a) ZnO in Ethanol, (b) ZnO in Acetonitrile, (c) ZnO in Water, (d)
ZnO in Methanol, (e) ZnO in DEA, (f) ZnO in 1-propanol.
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The growth faces or most exposed faces of ZnO materials were studied by TEM analysis.
The TEM/HR-TEM data of the ZnO materials are presented in Figure 4.3.
117
118
Figure 4.3. TEM images (A-E) of synthesized ZnO samples with different morphologies: (x)
TEM image, (y) HR-TEM image; Inset: FFT and inverse FFT images of the selected area.
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TEM analysis shows that exposed planes of the materials vary with the morphology. The most exposed planes for ZnO in Ethanol, ZnO in Acetonitrile, ZnO in Water, ZnO in Methanol, ZnO in DEA, and ZnO in 1-propanol are (101), (102), (100), (002), (100), and (002).
4.4.3 Optical Analysis
The band gaps, crystallinity, and defects of the ZnO samples were examined using UV-vis absorption, Raman, and PL spectroscopies. Figure 4.4 shows UV-vis absorption data for the prepared ZnO samples. A typical absorption band positioning from 360 to 387 nm was found common for all samples, indicating the absorption of ZnO26. The positions of the absorption peak changed with the solvents. The ZnO in Methanol exhibits an absorption band at around 365 nm.
A blue shift (from 362 nm to 353 nm) compared to the ZnO in Methanol was observed for the ZnO in Ethanol samples. The ZnO in Acetonitrile, ZnO in DEA, ZnO in 1-propanol, and ZnO in Water demonstrate a red shift relative to the ZnO in Methanol. The red shift (from 362 nm to 377 nm) is the highest for ZnO in Water followed by the ZnO in 1-propanol, ZnO in Acetonitrile, and ZnO in
DEA. In addition, the ZnO in Methanol and ZnO in Ethanol exhibit a higher absorption than the
ZnO in Acetonitrile, ZnO in DEA, and ZnO in 1-propanol.
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Figure 4.4. UV-Vis absorption spectra of the prepared ZnO samples.
Raman spectroscopy was used to determine the crystallinities and the purity of the crystal phases.
Figure 4.5 shows the data obtained from the Raman spectroscopy analyses. All the spectra exhibit three prominent peaks at 331, 438, 583, and 379 cm-1 (very week).
121
Figure 4.5. Raman spectra of the synthesized ZnO samples.
PL spectroscopy was employed further to study any possible structural defects of the synthesized materials. Figure 4.6 presents the PL spectra of the prepared ZnO nanocrystals. All samples (except ZnO in Ethanol and ZnO in Acetonitrile) show a strong peak positioning at 360 to 380 nm indicating UV emission peak. The samples ZnO in Water and ZnO in DEA demonstrate a broad peak positioning at 520 to 550 nm (green emission peak). The ZnO in DEA exhibit a clear blue shift (from 378.6 to 364.4 nm) while the ZnO in Water show a red shift (from 378.6 to 383.4
122 nm) as compared with ZnO in Methanol. The intensities of the UV emission for the samples in
ZnO in Water and ZnO in DEA were weaker than the green emission.
Figure 4.6. The PL spectra of synthesized ZnO materials.
4.4.4 Textural Properties (Surface Area and Porosity) Analysis
The textural properties of the synthesized materials were determined by N2 sorption analysis.
Figures 4.7 and 4.8 present N2 sorption isotherms and BJH pore size distributions for the materials, respectively. Figure 4.7 demonstrates that the synthesized ZnO samples have Type IV isotherms
123 followed by a hysteresis loop. The obtained sorption isotherms for the synthesized ZnO materials were similar to those reported for the ZnO prepared by other synthetic methods.35,36 The hysteresis loops of the ZnO in Acetonitrile, ZnO in Ethanol, and ZnO in Methanol were somewhat larger compared with that of the ZnO in DEA, ZnO in H2O, and ZnO in 1-propanol. The hysteresis loop of the commercial ZnO was the smallest among all of the materials under this study.
These observations confirm that ZnO materials synthesized in ZnO in Ethanol, ZnO in
Methanol, and ZnO in Acetonitrile were porous and ZnO synthesized in other solvents (ZnO in
DEA, ZnO in Water, ZnO in 1-propanol, and commercial ZnO) were relatively less porous or almost non-porous. The hysteresis loops for all the materials occur at a range of high relative pressure of 0.80 to near to their saturation point. But, for commercial ZnO, a hysteresis loop occurs from 0.5 to 0.95 relative pressure. The hysteresis loops of ZnO in DEA are consistent with the reported values.35,36
The pore size distributions (Figure 4.8) of the prepared materials were broad and most of them are centered at around 30 nm. However, these distributions for commercial ZnO, ZnO in
DEA and ZnO in Water were so broad that there are no observable centers. The BET data (Table
4.2) illustrate that the order of materials that have relatively high surface area is as follows: ZnO in Ethanol (61 m2/g) > ZnO in Acetonitrile > (53 m2/g) > ZnO in Methanol (35 m2/g). Besides surface area, pore diameters and pore volumes of the materials change significantly with solvents.
The ZnO in Methanol has the highest pore diameter (29 nm) followed by ZnO in Ethanol (16.8 nm).
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Figure 4.7. N2 sorption isotherms of ZnO prepared in different solvents.
Figure 4.8. BJH desorption pore size distributions of the prepared ZnO.
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Table 4.2. Summary of N2 Sorption Analysis of Prepared ZnO Materials materials pore diameter (nm) pore volume (cm3 / g) surface area (m2 / g)
ZnO in Methanol 29 0.40 35
ZnO in Ethanol 16.8 0.42 61
ZnO in Water 1.2 0.07 7
ZnO in DEA 4.2 0.07 3
ZnO in Acetonitrile 16.6 0.84 53
ZnO in 1-propanol 1.8 0.10 10
Commercial ZnO 3.2 0.02 3
4.4.5 Basicity Analysis of ZnO Catalysts
The basicity of the ZnO catalyst was estimated from CO2 adsorption analyses. The CO2 adsorption curves for the synthesized ZnO catalysts are shown in Figure 4.9. CO2 adsorption capability depends on the basic sites present in a material. The catalyst with high basicity (having high basic sites) adsorbs high amounts of CO2. The basicity order of the ZnO samples based on the CO2 uptake (Figure 4.9) is as follows: ZnO in 1-propanol > ZnO in Acetonitrile > ZnO in Methanol >
ZnO in Ethanol > ZnO in Water > Comm ZnO > ZnO in DEA.
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Figure 4.9. CO2 adsorption curves (at 25 °C) for synthesized ZnO materials.
4.4.6 Studies of Catalytic Activity of the Prepared ZnO Samples
To investigate the effects of different morphologies (and their associated changes in texture, defects, band gaps, and basicity of ZnO materials) on catalytic activity, coumarin synthesis reactions (using the Knoevenagel condensation) were studied using synthesized ZnO samples as catalysts. Scheme 1 shows the schematic illustration of the catalytic reaction. The catalytic activity of the materials toward coumarin synthesis was determined from the calculated conversion (%) and selectivity (%) of the reactants and products.
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Scheme 4.1. Synthesis of Coumarin from o-hydroxybenzaldehyde and Diethyl Malonate
4.4.6.1 Optimal Conditions for Catalytic Reaction
To find the optimal reaction conditions, a number of control experiments were performed using
ZnO in Methanol as a catalyst. Table 4.3 shows the results of the experiments. The conversion (%) of the coumarin synthesis fluctuates with different temperatures, heating times, and catalyst loadings. The conversion (%) is favored at high temperatures (entries 3-5). However, the high temperature disfavors the selectivity (entries 4,5) of coumarin. The optimal catalyst concentration for the coumarin synthesis reaction was 8.2 mg (5 mol %, entry 8). If the catalyst amount is lower
(entry 11) or higher (entries 12, 13) than that of 8.2 mg, the conversion decreases. There is zero conversion (%) when the reaction was run without catalyst (ZnO) even for 24 h of reaction time
(entry 10). The conversion increased with increasing the reaction time until 24 h (entries 2, 6-8) and after that goes down (entry 9). An experiment was performed using ethanol (5 mL) as a solvent, and the conversion (%) of the reaction is lower than that of the solvent-free condition
(entry 14d). Therefore, the optimal conditions for the coumarin synthesis using ZnO in Methanol catalyst were a temperature of 100°C, with a reaction time of 24 h, with no solvent, and with 8.2 mg of ZnO catalyst.
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Table 4.3. Optimization of the Coumarin Synthesis Reaction (Scheme 4.1) a
Entry reaction time catalyst amount conversionb selectivityb TONc
temperature (h) (mg) (%) (%) of coumarin
(° C)
1 80 3 8.2 3 58 0.60
2 100 3 8.2 8 ˃ 99 1.59
3 120 3 8.2 17 96 3.38
4 140 3 8.2 43 92 8.53
5 160 3 8.2 46 65 9.13
6 100 9 8.2 45 97 8.91
7 100 18 8.2 65 92 12.90
8 100 24 8.2 74 94 14.69
9 100 30 8.2 62 90 12.31
10 100 24 No catalyst 0 0 0.00
11 100 24 4.1 24 95 9.53
12 100 24 16.4 34 93 3.37
13 100 24 32.8 31 93 1.53
14d 100 24 8.2 61 95 12.10 aReaction conditions: o-hydroxy benzaldehyde (2 mmol), ZnO (synthesized in methanol), diethyl malonate (2.5 mmol), solvent free, and a closed vessel reaction. bConversion and bselectivity were estimated from the concentration of the reactants and products detected using GC/MS.c TON = moles of o-hydroxy benzaldehyde converted per mole of catalyst. d Ethanol was used as a solvent.
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4.4.6.2 Catalytic Performance of the ZnO Samples
The catalytic performance of the prepared mater for coumarin synthesis is presented in Table 4.4.
ZnO in Methanol and ZnO in Ethanol exhibit 74 % and 70 % conversion (respectively) with a 94
% selectivity towards coumarin. The ZnO in Water, ZnO in DEA, and ZnO in Acetonitrile exhibit a lower conversion (in the range of 35-49%) with a good selectivity (96%) compared to ZnO in
Methanol and ZnO in Ethanol. In comparison, the commercial ZnO shows a conversion of 51 %, which is lower than ZnO in Methanol and ZnO in Ethanol but higher than ZnO in Water and ZnO in DEA. All these data indicate that the catalytic activity toward coumarin synthesis (via the
Knoevenagel condensation) varies based on the ZnO morphologies.
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Table 4.4. Synthesis of Coumarin from o-Hydroxy Benzaldehyde and Diethyl Malonate
(Scheme 4.1)a
b conversionb selectivity (%) % Yield of catalysts TONc TOFd (%) of coumarin Coumarin
ZnO in Methanol 74 94 14.69 0.612 70
ZnO in Ethanol 70 94 13.90 0.579 66
ZnO in 1-propanol 75 60 14.90 0.620 45
ZnO in Water 35 96 6.95 0.288 34
ZnO in DEA 35 96 6.95 0.288 34
ZnO in Acetonitrile 49 96 9.15 0.381 47
Comm ZnO 51 95 9.51 0.396 48 aReaction conditions: o-hydroxy benzaldehyde (2 mmol), diethyl malonate (2.5 mmol),
ZnO (8.2 mg), solvent free, 100° C, 24 h, and a closed vessel reaction. bConversion and bselectivity calculated from the concentration of the reactants and products detected by
GC/MS. c TON = moles of o-hydroxy benzaldehyde converted per mole of catalyst. dTOF=
TON/reaction time (h). % Yield = % (conversion × selectivity)/100.
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4.4.7 Reusability, Stability, and Heterogeneity of the Catalysts
The reusable nature of the ZnO in Methanol (the best catalyst) was studied by recycling the catalyst several times for the reaction (Scheme 1). After each cycle, the catalyst was regenerated from the mixture of reactants and products by filtration, washing (with ethanol using a centrifuge) and drying. The catalyst was then heated at 400 °C for 4 h to reactivate the catalyst’s reacting sites.
About 20 % loss of the conversion was observed after the fourth cycle of the reaction (Figure
4.10a). The catalyst stability was investigated using PXRD data obtained before and after the reuse of the catalyst (Figure 4.10b). The similarity in the obtained XRD patterns indicates the structural stability of the catalyst at least until the fourth cycle of the reaction. In addition, the catalyst leaching was checked for testing the heterogeneity of ZnO. The conversion of the reactants remains constant (Figure 4.10c) even after filtering the catalyst off from the reaction medium at a conversion of 52 %. All these results implying good stability, remarkable reusability, and heterogeneity of the ZnO in Water catalyst.
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Figure 4.10. a) Reusability test of the ZnO in Methanol catalyst (reaction conditions: o-hydroxy benzaldehyde (2 mmol), diethyl malonate (2.5 mmol), ZnO (8.2 mg), solvent free, 100° C, 24 h, and a closed vessel reaction. b) PXRD patterns of ZnO in Methanol before and after reuse of the catalyst, c) Test of homogeneity (leaching test/hot filtration test): catalyst was removed after 52
% conversion (reaction conditions: same as reusability test).
133
4.5 DISCUSSION
In this research, various morphologies of ZnO were prepared using different solvents via a solvothermal method. Synthesized ZnO materials were characterized to study structural modulation (primarily in surface, crystallinity, vacancies) that occurs with the solvents. The effects of the structural modifications of ZnO in catalytic activities for coumarin synthesis via
Knoevenagel condensation were investigated. ZnO in Methanol and ZnO in Ethanol show 74% and 70% conversation (respectively) with a 94% selectivity. The activity of ZnO in Methanol and
ZnO in Ethanol in coumarin synthesis is best compared to other synthesized ZnO and commercial
ZnO. ZnO with different morphologies provides the materials with different exposed-faces, textures, and basicity, and all these can affect the catalytic activities. The catalytic activity of synthesized ZnO materials is mostly governed by the combined effects of textural properties and basicity.
4.5.1 Structure of Synthesized ZnO Samples
The presence of no additional peaks in XRD and Raman data is evidence of pure hexagonal crystal structures of the prepared ZnO samples. Peak intensities and narrow widths of XRD peaks are clear evidence of good crystallinity.37 The observed peak broadening for ZnO in Methanol and
ZnO in Ethanol may be due to slow growth of ZnO crystals during synthesis, which is related to the reduction of crystallite sizes for corresponding ZnO samples. The morphologies of ZnO are also related to the nucleation process and the kinetics of crystal growth.26 ZnO nucleation and growth kinetics are influenced by the pH of the reaction mixture.26,32,38 Introducing different solvents in the reaction changes the pH, which may affect the nucleation and growth rate of ZnO resulting in different morphologies. Crystal growth rates of ZnO also depend on internal factors such as intermolecular bonding preferences and dislocations, as well as external factors such as
134 precursor, temperature, pressure, concentration, and interface-solvent interaction. These factors may also effect the morphology of the ZnO samples.26,38 In addition, solvent polarity and saturated vapor pressure have a profound effect in nucleation and preferred orientation of crystals.26,38–40
Hence, solvents with different boiling points and polarities can produce different vapor pressures and interface-solvent interactions during synthesis that might affect the growth kinetics of ZnO crystals, leading to different morphologies. The morphology and crystallinity are connected to the band gap.41 The observed UV absorption peak shifts for different ZnO samples may be due to changes in the band gaps of the materials which can influence the catalytic activity ZnO materials.
42,43,44 The representation of ZnO optical phonons are reported as A1+E1+2B1+2E2. The observed
-1 strong peak at 438 cm in the Raman spectra is assigned to the E2 (high) mode, which characterizes
43,44 -1 the wurtzite structure of ZnO. The peak positioning around 583 cm is ascribed to A1 (LO) /
31,45 E1 (LO) modes of ZnO, indicating the presence of oxygen vacancies. Hence, ZnO in 1-propanol has good crystallinity and a smaller number of defects (oxygen vacancies). The peaks positioned
-1 around 331 and 379 cm are associated with E2 (high) - E2 (low) and A1 (TO) modes. However, two other common Raman peaks for ZnO samples that usually appear at around 410, and 657 cm-
1 43,46 , which are ascribed to E1 (TO) and A2 (LO) + E2 (low) mode (respectively), are missing here.
The observed strong emission band around 360-380 nm (3.4 -3.3 eV) in PL spectra of ZnO samples (except for ZnO in Ethanol and ZnO in Acetonitrile) confirms the wide band-gaps of ZnO materials. The origin of UV emission in the ZnO is believed to be due to the recombination of excited electrons and holes.43,47,48 The missing of the UV emission peak for ZnO in Ethanol sample may be due to low crystallinity whereas that of ZnO in Acetonitrile is under investigation. The blue shift of UV emission peak for the ZnO in DEA was indicative of a band gap increase whereas the red shift for the ZnO in Water could be due to a band gap decrease. Low crystallinity of ZnO
135 in water is also evident from XRD data that may result in a narrow band-gap. Consequently, the
UV emission band of ZnO in Water shifted toward a higher wavelength. The emergence of a broad and weak emission band (for the ZnO in Water and ZnO in DEA) in the green region at around
520-550 nm may be due to intrinsic defects of the crystals.31,39 The phenomenon of the green emission of the ZnO due to the intrinsic defects (zinc vacancy, oxygen vacancy, zinc interstitials, oxygen interstitials, and antisite oxygen) has been reported earlier.31,33,49–51 The amounts of intrinsic defects and green emission intensity of ZnO crystals are correlated with each other.39 The variation of the morphology of ZnO is responsible for different levels of defects present in the ZnO samples.39
4.5.2 Catalytic Activity of ZnO in Coumarin Synthesis via the Knoevenagel Condensation
Reaction.
The order of catalytic activity in terms of yield (conversion × selectivity) is as follows: ZnO in
Methanol > ZnO in Ethanol > ZnO in Acetonitrile > Comm ZnO> ZnO in 1-propanol > ZnO in
DEA or ZnO in Water (Table 4.4). Although all the synthesized ZnO materials crystallize with the pure hexagonal wurtzite crystal phase, they vary in their different textural and structural properties due to synthesis with different solvents. For example, crystallite sizes of the synthesized samples fluctuate in the order of ZnO in Ethanol < ZnO in Methanol < ZnO in Water < ZnO in DEA< ZnO in Acetonitrile < ZnO in 1-propanol < Comm ZnO. However, the activity is not correlated to the crystallite sizes (Table 4.1) of the materials.
In some cases of heterogeneous catalysis, the surface area is critical in influencing the catalytic performance. The high surface area of a catalyst is associated with a high number of active sites, which enhances the catalytic activity of the materials. By comparing surface areas and the % yield of coumarin, the surface areas of the ZnO samples are found to be crucial and playing
136 a vital role in the catalytic activities. However, the catalytic activities (in terms of yield = conversion × selectivity) are relatively high in the case of ZnO in Methanol and ZnO in Ethanol, which have relatively high surface areas, pore volumes, and pore diameters. This observation suggests that high activities of the synthesized ZnO catalysts are due to the relatively high textural properties: surface area, pore volume, and pore sizes of the catalysts.
There are two factors that primarily control the basicity of a material. These are the negative charges on the anions and the coordination of the anions. The coordination of anions is affected by the imperfections/defects in the materials.52 Hence, surface basicity changes depending on the defects of the materials. Therefore, the observed basicity of the ZnO catalysts may be due to different intrinsic defects, which are governed by the different morphologies. The variation of exposed planes (TEM data) with morphology may also contribute to the number of basic sites of the ZnO catalysts. Additional studies are required to correlate the basicity of the catalyst to the amount of defects/ types of defect present in the materials. According to the literature, the
Knoevenagel condensation is usually catalyzed by mild acids and bases.53,54 Xu, at. el. suggests that the Knoevenagel condensation prefers the weak strength of acids and bases.20 Hence, the basicity/acidity strength of catalyst has a significant role for the reaction. The intermediate basicity of ZnO in Methanol may favor the coumarin formation. However, further studies are required to find the exact correlation of the observed activity (Table 4.4) and basicity (Figure 4.9).
137
4.6 CONCLUSIONS
ZnO morphologies are synthesized by changing solvents with the solvothermal method.
Crystallinities of the prepared ZnO samples vary with morphologies that lead to changes in band gaps and intrinsic defects (oxygen vacancies) of the ZnO materials. Catalytic activity of the ZnO samples for coumarin synthesis via the Knoevenagel condensation varies with morphologies.
Different morphologies of ZnO provide different textural properties and basicity of the catalyst, which govern the activity of the materials. ZnO synthesized using methanol shows the highest activity in terms of yield with a TON of 14.69. The enhanced activity of the ZnO in Methanol is attributed to the combined effects of its textural properties (relatively high surface area, pore volume, and pore size) and intermediate basic strength.
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FUTURE WORK
Development of inexpensive, active catalyst materials for energy applications and organic molecule syntheses is a matter of great interest. The nanomaterials obtained by the novel design and synthesis in this research show promising activity for ORR, HER, and synthetic organic reactions. As a future perspective, materials could be (1) further optimized to alter physicochemical properties (surface areas, pore sizes, defects, adsorption) responsible for the catalytic activity, (2) synthesized by coupling with other materials (to make more robust), and (3) utilized for other heterogeneous catalysis reactions.
Among the synthesized transition metal doped ZnO materials, Mn-ZnO shows the best
ORR performance. The activity could be tuned by varying Mn percentage, or by preparing mesoporous hybrid materials through dispersion on carbon nanotubes, carbon cloths, and graphene support. The compositing with carbon materials provides the full advantages of conductivity and high surface area to enhance ORR activity. Besides these, ZnO is a multifunctional material and could be utilized in photocatalysis, sensors, solar cells, field emission, and UV lasers.
The HER activity of synthesized transition phosphide materials is very promising. Ni doping in CoP shows a positive change HER activity, and different levels of Ni dopant could be tested to optimize the dopants amounts. More cation substitution (V, Mn, and Co) could be studied.
In addition, anion doping exhibits a positive influence in HER activity of TMP materials. S and
Se doped FeP materials could be prepared and tested for the HER performance. FeP coupled with carbon materials provides a high surface area, modulated electronic density, and electronic distribution to improve the HER performance and stability of the catalyst.
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Since the presence of P limits the electron delocalization in metal phosphides, this can modify the band gap of the materials. This indicates that metal phosphides with appropriate P content may have the potential to generate hydrogen by photocatalytic reaction. Also, electrocatalysts with high HER activity have been used as co-catalysts to reduce the overpotential of the surface of the semiconductors. These are good hints to design and develop novel photocatalysts based on transition metal phosphides for photochemical water splitting.
The development of bifunctional (for both HER and OER) electrocatalysts based on earth- abundant materials is highly desired to make the overall water splitting a cost-effective process.
Highly active bifunctional materials can be produced by combining highly active HER and OER catalysts during synthesis. TMPs are highly HER active and their oxidized species, such as oxides and phosphates are highly OER active. Mesoporous Fe oxide could be partially oxidized with a phosphorous source to produce a Fe phosphide/phosphate hybrid material as a bifunctional catalyst
(presumably active for both HER and OER).
In summary, this work highlights the design and synthesis of transition metal based on low- cost, stable oxide and phosphide nanomaterials for energy conversion and organic synthetic reaction applications. The structure-activity relationship in electrocatalysis and organic synthesis is emphasized in the presented work. Novel design and synthesis of these materials will have uses in energy applications, as well as other versatile uses such as photocatalysis, optoelectronics, spintronics, and the development of sensor and memory devices.
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