Composite Silver/Titania Photocatalysts for Visible Light Water Splitting: The Role of Silver Surface Plasmons.

Abstract

Photocatalytic conversion of water to hydrogen and oxygen is a promising avenue for the production of fuels from solar energy. While many semiconductors can absorb solar photons and catalyze the production of hydrogen and oxygen from water, the process typically produces hydrogen at low rates because of inherent deficiencies of most semiconductors.

We have demonstrated the design and evaluation of a composite photocatalyst comprising Ag nanocubes and nitrogen-doped titania (N-titania), which exhibits 10-fold enhancement in the water splitting rate under visible illumination compared to N-titania only. While several mechanisms may be important for different systems, in the present work we have demonstrated that the enhancement of semiconductor activity was due to the excitation of the metal surface plasmon resonance (SPR). The SPR enhances the local electric field intensity around the metal nanoparticles, which increases the rate of charge carrier formation in the semiconductor. Specially, it was demonstrated that the spatially non-homogeneous SPR-enhanced fields selectively increase the production of charge carriers near the semiconductor surface, partially alleviating the problem of charge carrier recombination in the semiconductor bulk.

We have also developed predictive models that aid in the analysis and design of composite plasmonic metal/semiconductor photocatalysts. The composite photocatalyst performance is dependent on the optical properties of individual components. We have provided a framework to identify and predict the optimal construction of composite photocatalysts based on the optical properties of the constituent building blocks. It was also shown that the geometric arrangement of the building blocks within the composite photocatalysts was a critical variable. To address this we developed a model to evaluate the effect of distance between metal and semiconductor, which begins to shed light on the optimum geometric arrangement of the building blocks within the composites.

While we focused on titania-based photocatalysts, the mechanistic understanding and predictive models presented here allows us to transfer the principles to other semiconductors. This, coupled with novel nanoparticle synthesis strategies, allows us to identify and synthesize composite photocatalysts to maximize interaction with any semiconductor under solar illumination, with the ultimate goal of producing highly active photocatalysts for the solar production of hydrogen from water.