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

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Synthesis and Photoelectrochemical Characterization of Tantalum Oxide Nanoparticles and Their Nitrogen Derivatives University of Nevada, Reno Synthesis and Photoelectrochemical Characterization of Tantalum Oxide Nanoparticles and their Nitrogen Derivatives A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical engineering by Vijay Khanal Dr. Vaidyanathan (Ravi) Subramanian – Advisor August 2019 The Graduate School We recommend that the dissertation prepared under our supervision by VIJAY KHANAL Entitled Synthesis and Photoelectrochemical Characterization of Tantalum Oxide Nanoparticles and their nitrogen Derivatives Be accepted in partial fulfillment of the Requirements for the degree of DOCTOR OF PHILOSOPHY Vaidyanathan (Ravi) Subramanian, Ph.D., Advisor Dhanesh Chandra, Ph.D., Committee Member Alan Fuchs, Ph.D., Committee Member Victor Vasquez, Ph.D., Committee Member Mario A. Alpuche, Ph.D. Graduate School Representative David W. Zeh, Ph.D., Dean, Graduate School August 2019 i ABSTRACT This work presents a surfactant-mediated approach for the synthesis of Ta2O5 nanopar- 2 1 2 1 ticles with a very high surface area (41 m g− vs. 1.6 m g− for commercial equivalent), and their subsequent nitridation to yield two distinct nitrogen derivatives: TaON and Ta3N5. Excellent photocatalytic efficiency of Ta2O5 toward photodegradation of methy- lene blue and NOx removal is reported. Ta3N5 shows significant photoconversion of methylene blue attributable to its lowered bandgap and high adsorption capability of MB molecules on its surface. A new method for simplifying the existing synthesis protocols of single phase Tantalum oxynitride (TaON) was devised. Series of photoelectrochemical characterization shows that the newly synthesized TaON can significantly enhance the photoassisted charge generation, separation, and transportation compared to the parent oxide (Ta2O5). ii DEDICATION VILAKSHAN & PALLAVI. iii ACKNOWLEDGEMENTS I could not have succeeded in graduate school if it were not for the community and people surrounding me. First and foremost, I am forever grateful to my advisor Dr. Vaidyanathan (Ravi) Subramanian for his steadfast support and patience over the past five years. He not only has taught me the rigors of research, but also has played a role of a guardian in my life. I feel truly fortunate to have Dr. Ravi as my advisor. Next, thank you to my other committee members for taking the time to be a part of this campaign: Dr. Dhanesh Chandra, Dr. Victor Vasquez, Dr. Mario Alpuche, and Dr. Alan Fuchs. Also, I would like to acknowledge and thank Nikhil Dhabarde, one of our group mem- bers, for making the lab always lively. All the friends and families of Nepalese Students’ Association (NSA), UNR are always thankful for creating homely environment in the foreign land. Special thanks to Chiranjivi Bhattarai and his family for always being there to support in one way or the other. I can not forget all the hardworking undergraduate co-workers who joined the group in one way or the other during my Ph.D. career. Few names I would like to mention here are: Eric Sotto Harrison, Raghavi Anand, Margaux Haurat, Cara Dumonte, Brianna Peacock, and Darion Homayoon. I am thankful to all the students and instructors from CHE 374, CHE 232, CHE 440, and CHE 410 during my TA career. This research could not have been done without the generous support from the depart- ment of Chemical engineering, UNR. I am specially thankful to Dr. Jeff Lacombe, chair iv our chair, for acknowledging the problems that I faced as an international student. Dr. Lacombe has played the role of a ‘savior’ in my life, and I will never forget his generosity. Support from afar is just as valuable as local support. Thank you to my brother Lekhnath Khanal, for always being there to talk about our family, our career, and most importantly the Python programming! All the beautiful graphics that I have created for this work were possible only after his guidance through bits and pieces of Python. Prof. Bahnemann, Narmina Balayeva and Carsten Gunnemann (Leibnitz University, Germany), thank you all for being integral part of the Nox removal project. I could learn so many intricacies about EPR and TAS analysis with you guys, and thanks for sharing your knowledge and expertise in return to the incessant emojis sent over facebook mes- sanger from my side! My parents Sitaram Khanal and Dhanakala Khanal have been my constant champi- ons and cheerleaders and I would not be the person I am today without them. Down the memory lane, I have vivid memories of their tearful eyes as they waived goodbye to me as I left my country for higher studies. I feel blessed and grateful to let them know that I am about to cross the finish line! On top of suffering the inexplicable pain, anxiety, and suffering during her pregnancy period, my wife Laxmi Khanal has raised our two wonderful kids while working full time! She has silently suffered my frustrations, and has joyfully joined in my celebrations . and the only thing I am offering in return to her, now, is: ‘thank you, Laxmi... I love you too!’ v TABLE OF CONTENTS Table of Contentsv Page List of Figures viii 1 Introduction and Motivation1 1.1 Why Semiconductors?................................ 3 1.2 Why nanosized semiconductors? ......................... 4 2 Synthesis of Tantalum Oxide6 2.1 Why Oxides? ..................................... 6 2.2 Why Tantalum Oxide? ............................... 7 2.3 Synthetic Approach: Why Sol-gel? ........................ 8 2.4 Sol-gel Synthesis of Tantalum Oxide....................... 9 3 Characterization of Tantalum Oxide 11 3.1 Physical Characterization ............................. 12 3.2 Optical Characterization.............................. 13 3.3 Photoelectrochemical characterization...................... 14 4 Examining the Photocatalytic properties of Tantalum Oxide 17 4.1 Photodegradation of Methylene Blue....................... 17 4.1.1 Determination of Photocatalytic properties .............. 18 4.1.2 Photocatalysis in comparision to the commerical equivalent . 19 4.2 Photocatalytic performance of Tantalum Oxide compared to generic pollu- tants and catalysts ................................. 20 4.2.1 Competency of Tantalum Oxide against popular catalysts . 20 4.2.2 Photodegradation of RhB ......................... 21 4.3 Loading effects on MBPD performance ..................... 22 vi 5 Tantalum Nitride 24 5.1 Synthesis of Tantalum Nitride .......................... 24 5.2 Physical Characterization of Tantalum Nitride ................ 25 5.2.1 X-Ray Diffraction Pattern of Tantalum Nitride............ 26 5.2.2 SEM/(HR)TEM/SAED Pattern...................... 27 5.2.3 X-Ray Photoelectron Spectroscopy(XPS) of Tantalum oxide and Tantalum Nitride.............................. 27 5.3 Optical Characterization of Tantalum Nitride ................. 28 5.4 Photoelectrochemical Characterization of Tantalum Nitride......... 30 5.5 Photocatalytic activity of Tantalum nitride................... 31 6 Tantalum oxide Applied for Nox removal 34 6.1 Why NOx Removal?................................. 34 6.2 Why employ Tantalum oxide for NOx Removal?................ 36 6.3 Experimental..................................... 37 6.3.1 Photocatalytic NO removal........................ 37 6.3.2 Transient Absorption Spectroscopy (TAS) ............... 39 6.3.3 Electron Paramagnetic Resonance Spectroscopy (EPR) . 41 6.4 Evaluation of the photocatalytic property.................... 43 6.5 Examining the photoactivity using charge transfer kinetics as a probe . 46 6.6 EPR study of Ta2O5 catalyst............................ 49 6.7 Results of Nox Removal Experiment....................... 50 6.7.1 Nox removal under UV illumination .................. 50 6.7.2 Proposed Mechanism for Nox Removal by Ta2O5 . 53 7 Tantalum oxynitride: A straightforward synthesis approach 56 7.1 Simplied Synthesis and outreaching Importance ............... 58 7.2 Experimental..................................... 60 7.3 Physical Characterization ............................. 61 7.3.1 XRD of TaON ................................ 62 7.3.2 Physical features and crystallinity of the nanoparticles . 62 7.4 Optical Characterization.............................. 63 7.5 Photoelectrochemical Characterization ..................... 64 7.6 Comparative Impedance analysis of Ta2O5 and TaON ............ 66 7.7 Control over nitridation .............................. 66 7.7.1 Temperature of the bath.......................... 66 vii 7.7.2 pH of the ammonium hydroxide solution................ 67 7.8 Thermal Stability of the Tantalum Oxynitride................. 68 7.9 Photocatalytic activity of Newly Synthesized Tantalum Oxynitride . 69 7.10 Examining the nitridation protocol with other typical Oxide catalysts . 71 7.10.1 Titanium Dioxide (P25) .......................... 71 7.10.2 Nitrided Strontium Titanate....................... 72 7.10.3 Overarching significance of the newly introduced simplification in conventional nitridation methods .................... 73 8 Conclusion 75 Bibliography 77 viii LIST OF FIGURES FIGURE Page 1.1 Energy Band Model Schematic ............................ 3 1.2 Why nanospherical semiconductors are desirable................. 4 2.1 Band Edge Positions of Ta-Based Materials..................... 8 2.2 Sol-gel Synthesis in flow-chart............................. 10 3.1 XRD, SEM, & TEM images of Ta2O5 ......................... 12 3.2 UV-vis spectra and Tauc’s plot for Bandga Estimation of Ta2O5 . 13 3.3 J/t, J/V, & EIS response from Ta2O5 ......................... 15 4.1 MB Absorbance Spectra over Ta2O5 assisted photocatalysis..........
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