Photoelectrochemical Hydrogen Production at Neutral pH Phosphate Buffer Solution Using TiO2 Passivated InAs Nanowire/p-Si Heterostructure Photocathode

Abstract Formation of heterojunctions with other light absorbing semiconductors is one of the attractive approaches to realize efficient silicon-based solar water splitting. As one of the promising candidates for heterojunction with silicon, InAs compound semiconductor has two major advantages of narrow bandgap and high electron mobility. However, the integration of Si with InAs is challenging due to several physical issues. Here, we successfully demonstrate the integration of InAs nanowires (NWs) and p-Si via a catalyst-free nanowire growth method. The presence of InAs NWs effectively improved the photoelectrochemical activities of the Si photocathode in neutral pH condition. The longer NWs are more advantageous for the proton reduction at the photoelectrode/electrolyte interface, since the InAs NWs act like a highway for photogenerated electrons due to their excellent electron mobility. The TiO2 layer grown by atomic layer deposition (ALD) effectively passivated the vulnerable InAs NWs and Pt co-catalyst prevented the photoelectron accumulation on the TiO2 surface, suppressing the self-reduction of TiO2 passivation layer to ensure device stability over 20 hours. The Pt/TiO2/InAs NWs/p-Si photocathode exhibited an onset potential shift of 0.91 V (versus reversible hydrogen electrode) compared to bare p-Si and a photocurrent density as high as 8.6 mA/cm2 at 0V vs. RHE. Furthermore, we precisely analyzed the heterojunction band diagram of the Pt/TiO2/InAs NWs/p-Si photocathode to investigate the band offsets and electronic barriers occurring at each heterojunction interface.