Semiconductor-Solution Interfaces From First Principles

Open Access
- Author:
- Campbell, Quinn Thomas
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 24, 2019
- Committee Members:
- Ismaila Dabo, Dissertation Advisor/Co-Advisor
Ismaila Dabo, Committee Chair/Co-Chair
Susan B Sinnott, Committee Member
Michael John Janik, Committee Member
Adri van Duin, Outside Member - Keywords:
- first principles
semiconductor interfaces
water splitting
high throughput
computational
silicon - Abstract:
- Hydrogen is a promising alternative energy source for lowering carbon emissions from transportation. However, current methods for generating hydrogen produce carbon in the synthesis process. A carbon free alternative for hydrogen generation involves immersing a semiconductor in water, where solar energy absorbed by the semiconductor can be directly transferred to liquid-phase chemical reactions for splitting water molecules into hydrogen and oxygen gases. While many promising semiconductor--solution based devices have been made, they are limited in their performance by relatively low efficiencies due to the complex interactions between water molecules and the semiconductor surface that lead to passivation and poor charge photogeneration ability. First principles computational studies, particularly utilizing density functional theory, can accelerate the process of finding new materials for hydrogen generation by predicting the quantum mechanical properties of relevant materials with limited experimental data. To predict new photoelectrodes for hydrogen generation we examined > 66,000 materials using first-principles capabilities beyond conventional electronic-structure methods to predict accurate optical and electronic properties of semiconductors. We also assessed the ability to synthesize the proposed photocatalysts by an extensive survey of the experimental literature. Using this approach, we predicted 28 promising materials for hydrogen production, many of which have not been previously reported. We then worked with experimental collaborators to synthesize and characterize 15 of these materials, demonstrating hydrogen production in many, but not all of the materials. We attributed the lack of hydrogen production in some of the predicted materials to undesirable reactions taking place at the surface of the semiconductor exposed to water. Previous first principles work has generally only addressed the semiconductor--solution interface at neutral interfaces due to computational limitations in incorporating long range electrostatic interactions into the atomic length scale of the interface. In this dissertation, we develop a novel method to model the bulk semiconductor as a continuum placed in equilibrium with a quantum mechanical semiconductor-solution interface, allowing us to predict interfacial characteristics as a function of applied voltage. We used this method to predict important properties of the semiconductor--solution interface such as the stable termination, equilibrium Schottky barrier, which provides the driving force for charge separation, and the charge--pinning fraction, which identifies the degree to which surface adsorbates can affect the charge separation ability and thus efficiency of a semiconductor electrode. We applied this method to silicon--water interfaces, characterizing the physical mechanisms limiting silicon's performance as a photoelectrode for water splitting. Finally, we implemented an automated algorithm to efficiently examine the surface stability and Schottky barriers of the layered Aurivillius oxides Bi2WO6 and Bi2MoO6, predicting which layer of the oxide will terminate the surface as a function of voltage and pH. We identify that high magnitude Schottky barriers are stable over a wider potential range of the Bi2WO6 (100) surface than the (010) interface, helping to provide a molecular interpretation for experimental findings of the superior activity of the (100) interface. The results of this work offer a computationally tractable model for both predicting new semiconductor photoelectrodes for water splitting and exploring atomic-level interactions at semiconductor--solution interfaces from first principles. The proposed methodology can easily be extended to other interfacial systems, including semiconductor--metal, semiconductor--semiconductor, and semiconductor--insulator interfaces, allowing for accurate simulations of a wide variety of electronic devices.