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3D Branched Nanowire Photoelectrodes for High-Efficiency Solar Water Splitting and H2 Production
Keywords: Branched Nanowire , Solar Water Splitting, H2 production
Hydrogen is believed to be a clean energy source to overcome the environmental challenges with a zero carbon dioxide emission. However, over 90% of hydrogen is produced from fossil fuel or biomass, the use of which is believed to be the major cause of irreversible environmental consequences, including global warming and air pollution. Mass-production of hydrogen from another clean energy is the essence of benefiting the environment and technology, such as photoelectrochemical cells (PECs), that utilize solar energy to break down water, have received great attentions recently. With the development of nanotechnology, semiconductor photoelectrodes offer light trapping, enhanced charge generation, separation and collection, and thus improved overall performance. Herein, we report a photoelectrode with high water splitting efficiency based on branched nanowires. The 3D branched nanowire heterostructure is composed of a Si nanowire core and metal oxide nanowire branches, which are all synthesized from cost-effective low temperature solution based methods. Si nanowire array from wet-etching showed nearly 95% absorptance in the visible range and the addition of ZnO nanowires branches on Si nanowire sidewalls from solution hydrothermal growth showed improved light absorption over 97%. P-type and n-type Si substrates were both investigated exploring the potential application as a photocathode and photoanode. Our studies on the length of nanowires showed the increased dark current with longer nanowires due to the increased surface area and thus surface states. Compared to previously reported studies of ZnO nanowires on transparent conducting electrodes (ITO glass), the use of n-type plain Si to form n-n junction suppressed the dark cathodic current and improve the current saturation at high biases. It was also observed that longer Si nanowire cores gave larger anodic photocurrent response and recombination current. Interestingly, longer ZnO nanowire branches improved the cathodic photocurrent response as well as the anodic photocurrent. However, similar to the case of ZnO nanowires on plain Si substrates, the use of ZnO nanowires improved the current saturation effect, which is believed due to the improved minority carrier generation rate from the n-n junctions. Our studies show that the low temperature, large scale, and solution phase synthesized branched nanowire photoelectrodes enhance light absorption and effective charge separation in electrodes, increase the surface area and curvature for efficient chemical reaction and gas evolution, and thus allow an improved light utilization to achieve higher carrier generation/conversion and overall H2 production efficiency.
Ke Sun, Graduate Student
University of California - San Diego
La Jolla, CA

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