Depletion layer controls photocatalytic hydrogen evolution with p-type gallium phosphide particles

Abstract

p-Type gallium phosphide (p-GaP) is an established photocathode material for hydrogen evolution, however, photocatalytic hydrogen evolution from p-GaP photocatalysts only proceeds with very low activity. The reason for the low activity of p-GaP, and of other p-type semiconducting photocatalysts, is presently unknown. To better understand this limitation, we have investigated the photocatalytic H₂ evolution activity and photovoltage generation of p-type GaP microparticles in the presence of sacrificial electron donors. Sub-micrometer sized particles with a Zn acceptor concentration of 5.5 × 10¹⁷ cm⁻³ were prepared by grinding a commercial p-type Zn:GaP wafer. According to surface photovoltage spectra, the p-GaP particles have an effective bandgap of 1.9 eV and generate a positive photovoltage of 0.41 V, due to movement of the holes in the space charge layer of the particles. After modification with a Ni₂P cocatalyst, p-GaP particles catalyze H₂ evolution under visible light (>400 nm, 400 mW cm⁻²) in the presence of various sacrificial electron donors. The highest hydrogen evolution rate of 13.5 μmol h⁻¹ was achieved with 0.05 M KI, followed by 3.6 μmol h⁻¹ with 0.05 M K₄[Fe(CN)₆] and 0.5 μmol h⁻¹ with 0.05 M Na₂SO₃. Rates are inversely correlated with the standard reduction potential of the donors (more reducing donors give lower rates). This can be explained on the basis of the depletion layer model at the p-GaP/electrolyte interface which directs photoelectrons away from the Ni₂P cocatalyst and toward the sacrificial donors and GaP surface states. SPS measurements in the presence of the sacrificial reagents estimate the donor-dependent potential drop across the depletion layer as 0.25–0.45 V. This correlates well with logarithmic H₂ evolution rates, confirming that the depletion layer limits the forward reaction. This model explains why p-type semiconductors have a much lower photocatalytic hydrogen evolution activity than n-type semiconductors. The SPS measurements also confirm electron trapping on the p-GaP surface as a reason for the slow deactivation of the photocatalysts.