Zetian Mi1
University of Michigan1
III-nitride nanostructures, e.g., Ga(In)N nanowires, have emerged as one of the most promising material platform for artificial photosynthesis, including the chemical transformation of sunlight, water, and carbon dioxide into clean chemicals and fuels such as hydrogen and hydrocarbons. Recent studies suggested that the energy bandgap of GaN can be tuned across nearly the entire solar spectrum with the incorporation of indium as well as dilute anions (e.g., antimony). Moreover, the conduction and valence band edges of dilute anion III-nitride nanostructures can be tuned to straddle a broad range of redox potentials, including water splitting, carbon dioxide reduction and methane oxidation, which is essentially required for high efficiency artificial photosynthesis. Our recent studies further showed that the surfaces of GaN-based nanostructures can be terminated with nitrogen, not only for their top c-plane but also the lateral non-polar surfaces. Significantly, the lateral nonpolar surfaces can be further transformed to oxynitride, which can lead to improved performance, instead of degradation, during harsh photocatalysis reaction. This unique surface transformation and property has enabled the demonstration of photoelectrochemical solar water splitting with stability ~3,000 hrs without any performance degradation.<br/><br/>Moreover, epitaxial III-nitride nanostructures have shown great promise to overcome the sluggish kinetics related to various surface chemical reactions. As an example, the surfaces of epitaxial III-nitrides were found to enable unique interaction with carbon dioxide and other molecules/chemical species to significantly reduce the energy barrier in some otherwise difficult chemical reactions. With the integration of suitable co-catalysts, the selective production of high value chemicals and fuels can be potentially achieved, which includes the one-step light-driven conversion of carbon dioxide to methane, formic acid, syngas, and methanol with high Faradaic efficiency. Our studies also revealed the synergistic effect of light and thermal energy in significantly enhancing the efficiency, selectivity, and production rate.<br/><br/>III-nitride photocatalyst nanostructures have also enabled a series of achievements of light-driven organic transformations and nitrogen fixation, which are also critical for carbon neutrality and environmental sustainability. For example, by coordinating GaN with Ru nanoparticles, a metal/semiconductor interfacial Schottky junction was formed to effectively extract photoinduced electrons from GaN light absorber, rendering Ru nanoparticles with high electron density for facilitating the extremely inert N≡N bond cleavage with the use of hydrogen, resulting in mild ammonia synthesis powered by sunlight, in stark contrast with the harsh Haber-Bosch process.