Date of Award
Doctor of Philosophy (PhD)
Photoelectrochemical (PEC) cells enable the conversion of solar energy into storable fuels, which is critical in overcoming the intermittent nature of this largest renewable source. However, the majority of semiconductors used as photoelectrodes in these cells have low conversion efficiencies and/or stabilities. Silicon (Si) is an attractive semiconductor material for photoelectrodes, but the development of efficient Si-based photoanodes is challenging due to their instability in alkaline solutions. Thus, one focus of this dissertation is the design and fabrication of highly stable nickel (Ni)-patterned Si photoanodes through interfacial engineering of the barrier heights. Recently, hot carriers in plasmonic metal nanostructures have also been used to enhance solar energy conversion in photovoltaic and PEC cells. The generation and transfer of hot carriers in a metal/semiconductor heterostructure are not yet well understood. Other goals of this thesis are thus to investigate how the spatial distribution of hot-carrier photochemistry depends on local variations in the electromagnetic field intensity and how the interfacial hot-carrier transfer varies for different geometries of metal/semiconductor interfaces.In the first project, we fabricated high-performance, Si-based metal-insulator-semiconductor (MIS) photoanodes patterned with arrays of Ni catalysts for PEC water oxidation. Using a combination of laser-write lithography and interfacial engineering, we systematically varied the size of the Ni catalysts as well as the difference in barriers heights at the inhomogeneous Schottky junction. Specifically, we introduced a stacked layer of SiOx/Al2O3 that imposes a large difference in the potential barriers of the inhomogeneous junction. We demonstrate for the first time that the pinch-off regime can be successfully achieved even with microscale Ni catalysts. The pinched-off, n-Si/SiOx,RCA/Al2O3/Ni photoanodes produce a high photocurrent density of 30.2 mA cm-2 at 1.23 V vs. RHE (i.e., the reversible hydrogen electrode) and a low onset potential of 0.98 V vs. RHE. In addition to the excellent PEC performance, the photoanodes are robust and exhibit a stable photocurrent for over 48 h in a highly corrosive alkaline solution. The photochemistry of hot carriers is investigated using different configurations of plasmonic metal/semiconductor heterostructures that are fabricated through electron beam lithography and laser-write lithography to precisely control the size and shape of the metal nanostructures. Photodeposition is used to map hot-carrier photochemistry in plasmonic gold (Au)/ titanium oxide (TiO2) heterostructures. The distribution of the reactivity maps obtained from photodeposition agrees with the simulated electromagnetic field intensities. The contact area between Au and Si in n-Si/Au composite electrodes is controlled using metal-assisted chemical etching, followed by characterization using atomic force microscopy and photoelectrochemical measurements. The results reveal that the doping density and crystallographic orientation of the Si substrate affect the interface geometry between the Au and Si during etching, which determines the performance of the n-Si/Au photoanode.
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