Plasmon-enhanced water splitting on TiO2-passivated GaP photocatalysts

Abstract

Integrating plasmon resonant nanostructures with photocatalytic semiconductors shows great promise for high efficiency photocatalytic water splitting. However, the electrochemical instability of most III–V semiconductors severely limits their applicability in photocatalysis. In this work, we passivate p-type GaP with a thin layer of n-type TiO₂ using atomic layer deposition. The TiO₂ passivation layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. In addition, the TiO₂ passivation layer provides an enhancement in photoconversion efficiency through the formation of a charge separating pn-region. Plasmonic Au nanoparticles deposited on top of the TiO₂-passivated GaP further increases the photoconversion efficiency through local field enhancement. These two enhancement mechanisms are separated by systematically varying the thickness of the TiO₂ layer. Because of the tradeoff between the quickly decaying plasmonic fields and the formation of the pn-charge separation region, an optimum performance is achieved for a TiO₂ thickness of 0.5 nm. Finite difference time domain (FDTD) simulations of the electric field profiles in this photocatalytic heterostructure corroborate these results. The effects of plasmonic enhancement are distinguished from the natural catalytic properties of Au by evaluating similar photocatalytic TiO₂/GaP structures with catalytic, non-plasmonic metals (i.e., Pt) instead of Au. This general approach of passivating narrower band gap semiconductors enables a wider range of materials to be considered for plasmon-enhanced photocatalysis for high efficiency water splitting.