Borophene and Carbon Nitride Nanosheets for Energy Storage Applications by Sean Patrick Grixti A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Department of Materials Science and Engineering University of Toronto © Copyright 2018 by Sean Patrick Grixti Abstract Borophene and Carbon Nitride Nanosheets for Energy Storage Applications Sean Patrick Grixti Master of Applied Science Department of Materials Science and Engineering University of Toronto 2018 Borophene phases are a set of two-dimensional (2D) allotropes of boron that are excellent candidates as cathode materials in lithium sulphur (Li-S) batteries. Ab initio calculations were performed to determine stripped borophene and β12-borophene’s applicability as Li-S battery cathode materials. The calculations suggest β12-borophene would sufficiently inhibit the dissolution of lithium polysulphides into the electrolyte, preventing the “shuttle effect”. 2D carbon nitrides including tg-C3N4, hg-C3N4, C2N, and C3N were investigated as potential photocatalyst for solar water splitting, through first principles calculations. Band engineering via n-type, p-type, and isoelectronic doping was demonstrated for tuning the electronic structure, optimizing solar absorption and band alignment for photocatalysis. Among the dopants, Si and Ge dopants can narrow the band gap of carbon nitrides and increase their optical absorption in the visible spectrum. This work expands on the surface chemistry of borophene and comments on the complexities of substitutional doping with group IV elements. ii Dedication To Cody. iii Acknowledgements I would like to thank my supervisor Dr. Chandra Veer Singh for the guidance and encouragement I received throughout my graduate studies. I would also like to thank Dr. Sankha Mukherjee and Dr. Meysam Makaremi for all the training and mentorship, from which made this thesis possible. Furthermore, I would like to extend my gratitude the entire CME Lab for their friendship and continuous support throughout my research. The work presented here would not have been possible with the funding provided by the Department of Materials Science and Engineering, and the Natural Sciences and Engineering Research Council of Canada (NSERC). I would also like to extend my thanks to Compute Canada and the SciNet HPC Consortium for the computational resources used in this work. iv Contents Acknowledgements .............................................................................................................. iv Table of Contents .................................................................................................................. v List of Tables ...................................................................................................................... vii List of Figures .................................................................................................................... viii List of Abbreviations .......................................................................................................... xi List of Symbols ................................................................................................................... xii 1. Introduction ..................................................................................................................... 1 1.1. Background ............................................................................................................................ 1 1.2. Lithium Sulphur Batteries ..................................................................................................... 3 1.3. Photocatalytic Hydrogen Production ..................................................................................... 6 1.4. Thesis Motivation .................................................................................................................. 9 1.5. Thesis Objectives ................................................................................................................. 10 1.6. Thesis Organization ............................................................................................................. 11 2. Computational Methods ............................................................................................... 12 2.1. Density Functional Theory .................................................................................................. 12 2.1.1. Born-Oppenheimer Approximation ..................................................................... 13 2.1.2. Hartree-Fock Method ........................................................................................... 15 2.1.3. Hohenberg-Kohn theorems and Kohn-Sham equations ....................................... 16 2.1.4. Bloch’s Theorem ................................................................................................. 19 2.1.5. Pseudopotentials .................................................................................................. 20 3. Borophene as a Lithium Sulphur Battery Cathode Material .................................. 22 3.1. Introduction ......................................................................................................................... 22 3.2. Literature Review ................................................................................................................ 23 3.2.1. 2D materials for Li-S battery cathodes................................................................ 23 3.2.2. Borophene ........................................................................................................... 25 3.3. Computational Details ......................................................................................................... 26 3.4. Structural Characteristics of Borophene .............................................................................. 28 3.4.1. β12-Borophene ..................................................................................................... 28 3.4.2. Stripped Borophene ............................................................................................. 29 3.5. Adsorption of Li2Sx on the Borophene Surfaces ................................................................. 29 3.6. Electronic Characterisitics of β12-Borophene with Bound Li2Sx Species ........................... 39 v 3.7. Diffusion of Li2Sx Species Across β12-Borophene .............................................................. 41 3.8. Theoretical Capacitance of β12-Borophene .......................................................................... 43 3.9. Conclusions ......................................................................................................................... 44 4. Band Engineering of Carbon Nitride Nanosheets for Solar Water Splitting .......... 45 4.1. Introduction ......................................................................................................................... 45 4.2. Literature Review ................................................................................................................ 46 4.2.1. hg-C3N4 ................................................................................................................ 46 4.2.2. tg-C3N4 ................................................................................................................. 46 4.2.3. C2N ....................................................................................................................... 47 4.2.4. C3N ...................................................................................................................... 47 4.3. Computational Methods ...................................................................................................... 48 4.4. Structural Characteristics of C2N, C3N, hg-C3N4, and tg-C3N4 .......................................... 49 4.5. Structural and Electronic Characteristic of Doped-Carbon Nitride Nanosheets ................ 50 4.6. Light Absorbance of Doped-Carbon Nitride Nanosheets ................................................... 56 4.7. Conclusions ........................................................................................................................ 59 5. Conclusions .................................................................................................................... 61 5.1. Summary .............................................................................................................................. 61 5.2. Future Work ......................................................................................................................... 62 Bibliography vi List of Tables 1.1 Energy density of various fuels ............................................................................ 7 3.1 Binding energies and corresponding shortest distance of Li2Sx compounds on the β12-borophene surface ............................................................................... 34 3.2 Binding energies and corresponding shortest distance of Li2Sx compounds on the stripped borophene surface ...................................................................... 34 3.3 Bader charge analysis of each Li2Sx species, with average electronic charge gain for S and Li shown .......................................................................... 34 3.4 Computed diffusion energy barriers across the x and y directions of the β12-borophene surface .......................................................................................... 42 4.1 Structural