  Narrow-bandgap III-V semiconductor nanowires (NWs) with a suitable band
structure and strong light-trapping ability are ideal for high-efficiency
low-cost solar water-splitting systems. However, due to their nanoscale
dimension, they suffer more severe corrosion by the electrolyte solution than
the thin-film counterparts. Thus, short-term durability is the major obstacle
for using these NWs for practical water splitting applications. Here, we
demonstrated for the first time that a thin layer (~7 nm thick) of compact
TiO$_2$ deposited by atomic layer deposition can provide robust protection to
III-V NWs. The protected GaAs NWs maintain 91.4% of its photoluminescence
intensity after 14 months of storage in ambient atmosphere, which suggests the
TiO$_2$ layer is pinhole-free. Working as a photocathode for water splitting,
they exhibited a 45% larger photocurrent density compared with un-protected
counterparts and a high Faraday efficiency of 91%, and can also maintain a
record-long highly-stable performance among narrow-bandgap III-V NW
photoelectrodes; after 67 hours photoelectrochemical stability test reaction in
strong acid electrolyte solution (pH = 1), they show no apparent indication of
corrosion, which is in stark contrast to the un-protected NWs that are fully
failed after 35-hours. These findings provide an effective way to enhance both
stability and performance of III-V NW based photoelectrodes, which are highly
important for practical applications in solar-energy-based water splitting
systems.
