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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, February 2014., Cataloged from PDF version of thesis. "October 2013.", Includes bibliographical references (pages 90-109)., This thesis focuses on the design of novel inorganic water-splitting photocatalysts for solar applications using first principles computations. Water-splitting photocatalysts are materials that can photo-catalyze the water-splitting reaction under certain conditions. They provide an alternative way to capture and store the energy from the sun. Currently, the energy conversion efficiency of photocatalytic devices under solar illumination and in pure water (pH=7) is still far from the commercialization target. The design of new photocatalysts with better potentials is the key to solve this problem. We have first developed a so-called three-step method to compute the relative position of a semiconductor's conduction band (valence band) vs. the H₂/H₂O (O₂/H₂O) level in solution from first principles. The merits of the method have been highlighted, and the performance of the method has been tested and compared with the performance of other methods. We conclude that the three-step method provides the desired accuracy for high throughput screening at an acceptable computational cost. We have designed a three-tier first principles high throughput screening system to identify new water-splitting photocatalysts by examining the phase stability, band gap and band edge positions of the candidate compounds. We construct the screening system by integrating the three-step method together with other previously developed methods in our group. We use the system to screen about 3000 different materials. Through the screening, most of the known water-splitting photocatalysts have been reproduced and, more importantly, sixteen new promising candidates have been proposed. Properties of these new candidates have been analyzed and compared to those of the known photocatalysts. Some particularly promising ones are highlighted. Ti₃O₃N₂ is one of the identified candidates from the high throughput screening, and is particularly interesting as it has good phase stability, a low band gap and sui, by Yabi Wu., Ph. D.

In this paper, we present a first principles high throughput screening system to search for new water-splitting photocatalysts. We use the approach to screen through nitrides and oxynitrides. Most of the known photocatalytic materials in the screened chemical space are reproduced. In addition, sixteen new materials are suggested by the screening approach as promising photocatalysts, including three binary nitrides, two ternary oxynitrides and eleven quaternary oxynitrides., United States. Dept. of Energy (contract DE-FG02-96ER4557), National Science Foundation (U.S.) (TeraGrid resources under Grant No. TG-DMR970008S), Pittsburgh Supercomputing Center, University of Texas at Austin. Texas Advanced Computing Center, Eni-MIT Solar Frontiers Center

We investigated the cathodic and anodic limits of six room-temperature ionic liquids (ILs) formed from a combination of two common cations, 1-butyl-3-methylimidazolium (BMIM) and N,N-propylmethylpyrrolidinium (P13), and three common anions, PF6, BF4, and bis(trifluoromethylsulfonyl)imide (TFSI), using an approach that combines molecular dynamics (MD) simulations and density functional theory (DFT) calculations. All interion interactions were taken into account by explicitly modeling the entire liquid structure using classical MD, followed by DFT computations of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies. The relative cathodic and anodic limits of BMIM PF6, BMIM BF4, BMIM TFSI, and P13 TFSI obtained from our approach are in fairly good agreement with existing experimental data. From our DFT calculations, we also obtained the cation- and anion-projected density of states (DOS), which provide information on the likely species contributing to reductive and oxidative decomposition. Our predictions support Howlett et al.’s earlier finding(1) that the TFSI anion is less stable than the P13 cation against reduction. In addition, our results provide surprising evidence of possible cation anodic instability; we predict the aromatic BMIM cation to be less stable against oxidation than the respective anions in BMIM PF6 and BMIM BF4, and the P13 cation to be less stable against oxidation than the PF6 anion in P13 PF6. We also present a comparison of the predictions of our approach with that of simpler approximations based on in vacuo or polarizable continuum model calculations., DuPont MIT Alliance

The ability to predict a semiconductor's band edge positions in solution is important for the design of water-splitting photocatalyst materials. In this paper, we introduce a first-principles method to compute the conduction-band minima of semiconductors relative to the water H2O/H2 [H subscript 2 O / H subscript 2] level using density functional theory with semilocal functionals and classical molecular dynamics. We test the method on six well known photocatalyst materials: TiO2 [Ti O subscript 2], WO3 [W O subscript 3], CdS, ZnSe, GaAs, and GaP. The predicted band edge positions are within 0.34 eV of the experimental data, with a mean absolute error of 0.19 eV., Eni S.p.A. (Firm), Eni-MIT Solar Frontiers Center, Chesonis Family Foundation, MIT Solar Revolution Project, National Science Foundation (U. S.) (Grant no. TG-DMR970008S)