Solar Water Oxidation by an InGaN Nanowire Photoanode with a Bandgap of 1.7 eV

Abstract

The performance of overall solar water splitting has been largely limited by the half-reaction of water oxidation. Here, we report a 1.7 eV bandgap InGaN nanowire photoanode for efficient solar water oxidation. It produces a low onset potential of 0.1 V versus a reversible hydrogen electrode (RHE) and a high photocurrent density of 5.2 mA/cm 2 at a potential as low as 0.6 V versus RHE. The photoanode yields a half-cell solar energy conversion efficiency up to 3.6%, a record for a single-photon photoanode to our knowledge. Furthermore, in the presence of hole scavengers, the photocurrent density of the InGaN photoanode reaches 21.2 mA/cm 2 at 1.23 V versus RHE, which approaches the theoretical limit for a 1.7 eV InGaN absorber. The InGaN nanowire photoanode may serve as an ideal top cell in a photoelectrochemical tandem device when stacked with a 0.9−1.2 eV bandgap bottom cell, which can potentially deliver solar-tohydrogen efficiency over 25%.

A Single-Junction Cathodic Approach for Stable Unassisted Solar Water Splitting

“…Recently, III-nitride semiconductor nanostructures, e.g., InGaN, have shown extraordinary potential for direct solar water splitting. [21][22][23][24] By varying the alloy compositions, their energy band gaps can be continuously tuned from ultraviolet, through visible, to the near-infrared. 25 Significantly, the energy band edge positions of InGaN can straddle water redox potentials for indium compositions up to 40%-50%, which corresponds to an energy band gap of $1.7 eV.…”

“…25 In spite of these promises, however, the realization of a single-junction InGaN photoelectrode that can drive photoelectrochemical solar water splitting stably and efficiently without any external bias has remained elusive. 21,22,26,27 The underlying challenges include the corrosion and oxidation of the exposed Ga or In atoms, which, together with the presence of surface states and defects, lead to surface recombination and material degradation during harsh photocatalytic reaction. In addition, conventional InGaN materials grown on Si generally exhibit very large densities of defects and dislocations, [28][29][30] which makes it difficult for the efficient collection of photo-generated holes through the underlying Si wafer while simultaneously achieving efficient extraction of photo-generated electrons at the InGaN/ electrolyte interface.…”

InGaN/GaN Multiple Quantum Well Photoanode Modified with Cobalt Oxide for Water Oxidation

“…[13][14][15][16][17] For instance, a InGaN NW photoanode modified with Iridium oxide (IrO2) co-catalyst exhibited a high photocurrent density under illumination but corroded rapidly in solution. 18 Very recently, Co3O4 nanoislands were shown to reduce the onset potential and improve the stability of a GaN NW photoanode 19 . However, the nano-islands lead to an incomplete coverage of the catalyst over the GaN surface and hence limited stability in a strong alkaline electrolyte.…”

Engineering III–V Semiconductor Nanowires for Device Applications

“…Among them, 1D NWs attracted great attention due to their excellent crystallinity, light trapping ability, direct charge transport paths, high surface to volume ratio. In particular, compound III–V semiconductor NWs, including InGa x N 1− x alloys, InP, GaP, and GaAs have been widely investigated for PEC applications.…”