|Abstract:||The attachment of organic molecules known as functional groups to the surfaces of metals and semiconductors is called surface functionalization. It is a popular approach for tuning the underlying material’s properties such as its surface chemistry, band gap, and chemical stability. Surface functionalization is used in diverse fields such as molecular electronics, low-dimensional materials, biochemistry, catalysis, and photoelectrochemistry.
Surface functionalization is becoming increasingly popular in the field of photoelectrochemical (PEC) water-splitting, a promising carbon-free approach to produce hydrogen from liquid water. PEC water- splitting cells consist of at least one semiconductor photoelectrode in contact with an aqueous electrolyte. Surface functionalization has been shown to be an attractive strategy for tuning the photoelectrode’s surface properties, enabling high-efficiency PEC devices. Experimental investigations of functionalized photoelectrode surfaces are quite extensive, showing that a diverse set of photoelectrode properties like stability, barrier height, surface chemistry, and catalytic activity may be modified. However, first-principles computational studies typically ignore or approximate properties of the functionalized photoelectrode such as substrate doping and the presence of an electrolyte. Therefore, a systematic theoretical understanding of the effect of functional groups on the photoelectrode’s surface properties is still lacking, resulting in a roadblock in using rational design to further improve PEC device performance.
This thesis presents four illustrative studies wherein first-principles density functional theory (DFT), finite-element device modeling, and first-principles molecular dynamics (FPMD) are used to elucidate the role of adsorbates and organic functional groups in optimizing the photoelectrode’s surface properties and consequently, in improving PEC device performance or modeling. These studies illustrate how molecule- surface interactions may be used to improve the surface chemistry of photocatalysts, increase the barrier height of functionalized photocathodes, improve the modeling and prediction of doped semiconductor surface properties via charge transfer doping, and elucidate the interaction of functionalized polar and non-polar photoelectrode surfaces with liquid water. In each study, the key results are explained in terms of the local chemistry and electrostatics of the photoelectrode surface, and the functional groups or adsorbates. Therefore, the design principles obtained for surface functionalization as well as the computational techniques may potentially be extended to applications beyond PEC water-splitting.