Hierarchical InGaN Nanowires for High-Efficiency Solar Water Splitting

Abstract

Solar water splitting has become a promising way to alleviate supply instability of solar energy by directly storing energy in the form of hydrogen fuel. Maximizing efficiency for photoelectrochemical (PEC) water splitting requires (i) a tunable bandgap that captures the solar spectrum, (ii) an energy band edge that spans the water redox potential, and (iii) high quantum efficiency [1,2]. Although a variety of materials such as Si, Ta 3 N 5 , and BiVO 4 have been studied as photoelectrodes, most materials have not yet fulfilled these requirements. For instance, the onset potential of Si and Ta 3 N 5 is too large so charge carriers cannot be sufficiently generated under sun illumination and BiVO 4 has a wide 2.4 eV bandgap that limits the efficient utilization of the solar spectrum [1,2]. The InGaN ternary system is an optimal photoelectrode for efficient solar hydrogen production (Fig. 1a). The bandgap (E g) of InGaN is direct and tunable from 3.4 (GaN) to 0.65 eV (InN) for indium compositions up to ~50%, allowing optimal use of the entire solar spectrum (E g  1.7 eV), which could potentially enable a solar-to-hydrogen efficiency over 25% [1,2,3]. However, creating high-performance InGaN photoelectrodes is difficult as In-rich crystals are highly strained causing phase segregation and subsequent performance degradation.