p-Type gallium phosphide (p-GaP) is an established photocathode material for hydrogen evolution, however, photocatalytic hydrogen evolution from p-GaP photocatalysts generally proceeds with very low activity. This can be understood on the basis of the space charge layer model presented here.

The space charge region (SCR) is a majority carrier-depleted region in a semiconductor. The width of this region and the size of the potential drop across it control photochemical charge separation under illumination, as relevant to solar energy conversion with photocatalyst particles, for example. For photocatalysts, the space charge region is often difficult to evaluate due to the small dimensions of the particles. Here, we show that surface photovoltage spectroscopy (SPS) can be used to observe the SCR in nano- and microparticles of strontium titanate (SrTiO3), aluminum-doped strontium titanate, and gallium arsenide (GaAs) on gold substrates. Depending on particle film thickness and thermal annealing conditions, we observe negative or positive photovoltages corresponding to majority or minority carrier diffusion toward the gold substrate. The direction of charge transport is controlled by the potential barrier across the depletion layer at the gold-particle interface. From the inverted photovoltage signal, potential barriers are estimated between 0.005 and 0.189 V and SCR widths between 1.3 and 2.8 μm. The observed barriers are 10–100 times smaller than the theory for an ideal Schottky junction, and the observed SCR widths are up to 100 times larger. The difference can be attributed to trapping of majority carriers in surface states arising from broken bonds and disorder, in the case of GaAs, or from chemisorbed oxygen, in the case of SrTiO3. Trapping of majority carriers in surface states (Fermi level pinning) also diminishes the effective carrier concentrations in the particles, explaining the larger SCR width in the films. These findings showcase the importance of surface states for charge separation in photocatalyst particles and their films.