GaP/GaNP Heterojunctions for Efficient Solar-Driven Water Oxidation

The growth and characterization of an n-GaP/i-GaNP/p⁺ -GaP thin film heterojunction synthesized using a gas-source molecular beam epitaxy (MBE) method, and its application for efficient solar-driven water oxidation is reported. The TiO₂ /Ni passivated n-GaP/i-GaNP/p⁺ -GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO₂ /Ni-coated n-GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm^-2 , leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm^-2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm^-2 for the coated n-GaP and n-GaP/i-GaNP/p⁺ -GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon-to-current efficiency (ABPE) of 1.9% while the ABPE of the coated n-GaP sample is almost zero. Furthermore, the coated n-GaP/i-GaNP/p⁺ -GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon-to-current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low-bias performance and broad absorption of the wide-bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.