Solar Water Splitting on Low-Dimensional Semiconductor Nanostructures

Solar water splitting;Artificial photosynthesis;III-nitrides;Transition metal dichalcogenides;Low dimensional nanostructures;Semiconductors;Electrical Engineering;Engineering;Electrical and Computer Engineering

Wang, YongjieZhong, Zhaohui ;

University of Michigan

关键词: Solar water splitting;    Artificial photosynthesis;    III-nitrides;    Transition metal dichalcogenides;    Low dimensional nanostructures;    Semiconductors;    Electrical Engineering;    Engineering;    Electrical and Computer Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/149836/yjiewang_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Mimicking the natural photosynthesis process, the direct conversion of solar energy into hydrogen fuel via unassisted water splitting process is one of the key sustainable technologies for future clean, storable, and renewable source of energy. An essential step of artificial photosynthesis is solar water splitting, which can be achieved by either photocatalytic or photoelectrochemical approach. A theoretical solar-to-hydrogen (STH) conversion efficiency ~27% has been predicted for a double-junction photoelectrode consisting of a 1.70 eV top junction and a 1.05 eV bottom junction (e.g. silicon) under AM 1.5G one-sun illumination. To date, however, there has been no demonstration of such efficient and stable device for photocatalytic or photoelectrochemical water splitting, which has been largely limited by the lack of semiconductor photoelectrodes that can operate efficiently and stably under visible light and can be directly integrated onto Si wafers.III-nitride semiconductors, including GaN, InN, and InxGa1-xN compounds, have recently emerged as highly promising materials for solar fuel conversion due to the tunable bandgaps from 0.65 to 3.4 eV by varying the indium compositions, which can cover nearly the entire solar spectrum. Moreover, the band edges of InGaN compounds can straddle the required chemical potentials for water reduction and water oxidation reactions with indium incorporation up to 50% (bandgap ~1.7 eV). In addition, III-nitride semiconductor nanostructures grown by molecular beam epitaxy can exhibit N-termination, which is highly stable in harsh photocatalysis condition, protecting photoelectrode surfaces against photocorrosion, which is one major issue for practical solar water splitting devices and systems. In this dissertation, we investigate the MBE growth, electronic and optical properties, and photocatalytic and photoelectrochemical performance of low-dimensional III-nitride and transition metal dichalcogenide (TMDC) nanostructures. Relatively high STH efficiency (>5%) photocatalytic water splitting has been demonstrated on monolithically integrated multi-band InGaN nanowires grown on nonplanar Si wafers. We also report on the demonstration of an InGaN nanowire photocathode for efficient proton reduction reaction, demonstrating a maximum applied bias photon-to-current efficiency (ABPE) ~4%, which is nearly one order of magnitude higher than the previously reported values for III-nitride photocathodes in solar water splitting. With the incorporation of an InGaN tunnel junction structure, we have achieved an improved unassisted solar water splitting in two-electrode configuration, demonstrating a true STH efficiency ~3.4% with ~300 hours stability, which is the highest efficiency value ever achieved in a single-junction photocathode for unbiased photoelectrochemical water splitting. To achieve solar water splitting with both high efficiency and long-term stability, we have further developed GaN-protected GaInP2/GaAs/Ge triple-junction (GaN/3J) photocathode, which can exhibit an STH efficiency ~12.6%. The monolithic GaN/3J photocathode exhibits relatively good stability (>50 hours) in unassisted solar water splitting, which is the best reported stability for multi-junction photocathodes with >10% STH efficiency for true unassisted solar water splitting. In addition, two-dimensional TMDC materials, e.g. tungsten diselenide (WSe2), have been recently reported as potentially low-cost and bi-functional photocatalysts for solar water splitting. Wafer-scale synthesis of monolayer WSe2 is demonstrated in this work by molecular beam epitaxial growth, which can exhibit multi-functionality in overall solar water splitting, including extraordinary capacities for efficient light harvesting, water oxidation reaction, and proton reduction reaction. Work presented in this thesis provides a new approach for achieving high efficiency and highly stable solar water splitting using the commonly used semiconductors, e.g. silicon and gallium nitride.