Semiconductor Nanowires for Photoelectrochemical Water Splitting

Photolysis of water with semiconductor materials has been investigated intensely as a clean and renewable energy resource by storing solar energy in chemical bonds such as hydrogen. One-dimensional (1D) nanostructures such as nanowires can provide several advantages for photoelectrochemical (PEC) water splitting due to their high surface areas and excellent charge transport and collection efficiency. This dissertation discusses various nanowire photoelectrodes for single or dual semiconductor systems, and their linked PEC cells for self-driven water splitting. After an introduction of solar water splitting in the first chapter, the second chapter demonstrates water oxidative activities of hydrothermally grown TiO2 nanowire arrays depending on their length and surface properties. The photocurrents with TiO2 nanowire arrays approach saturation due to their poor charge collection efficiency with longer nanowires despite increased photon absorption efficiency. Epitaxial grains of rutile atomic layer deposition (ALD) shell on TiO2 nanowire increase the photocurrent density by 1.5 times due to improved charge collection efficiency especially in the short wavelength region. Chapter three compares the photocurrent density of the planar Si and Si nanowire arrays coated by anatase ALD TiO2 thin film as a model system of a dual bandgap system. The electroless etched Si nanowire coated by ALD TiO2 (Si EENW/TiO2) shows 2.5 times higher photocurrent density due to lower reflectance and higher surface area. Also, this chapter illustrates that n-Si/n-TiO2 heterojunction is a promising structure for the photoanode application of a dual semiconductor system, since it can enhance the photocurrent density compared to p-Si/n-TiO2 junction with the assistance of bend banding at the interface. Chapter four demonstrates the charge separation and transport of photogenerated electrons and holes within a single asymmetric Si/TiO2 nanowire. Kelvin probe force microscopy measurements show the higher surface potential on the n-TiO2 (photoanode) side relative to the p-Si (photocathode) side under UV illumination as the result of hole accumulation on the TiO2 side and electron accumulation on the Si side which are desirable charge separation for solar water splitting. In chapter five, TiO2 is replaced with single phase InGaN nanowire in a dual bandgap photoanode to show the potential for solar water splitting with high surface area Si/InGaN hierarchical nanowire arrays and InGaN as a possible candidate for visible light absorber. An enhancement of 5 times in photocurrent was observed when the surface area increased from InGaN nanowires on planar Si to InGaN nanowires on Si wires. Chapter six demonstrates a self-driven water splitting device with the p/n PEC cell which consists of a photocathode and a photoanode. The operating photocurrent (Iop) with the p/n PEC cell is enhanced when n-Si/p-Si photovoltage cell was embedded under an n-TiO2 photoanode by utilizing the photovoltage generated by a Si PV cell. Also, the Si nanowire photocathode surface is modified with Pt and TiO2 to increase hydrogen reducing activity and stability which enhances Iop of the p/n PEC cell as well. When Si/TiO2 nanowire photocathode is linked with n-Si/p-Si photovoltage cell embedded TiO2 nanowire photoanode, the p/n PEC cell shows water splitting without bias voltage confirmed with 2:1 ratio of hydrogen:oxygen gas evolution and a 92 % Faradic efficiency. These studies represent a significant step towards realizing the benefit of the advanced 1D nanowire configuration for efficient solar to energy conversion.

Advisor: Yang, Peidong