BiVO has emerged as one of the most promising photoanode materials for solar water splitting. The breakthrough is the result of the successful identification of the performance limiting factors in the material:
Poor catalytic activity
Slow electron transport (low carrier separation efficiency).
The first problem is solved by coupling water oxidation co-catalysts on the surface of BiVO, while the latter is overcome by introducing dopants, such as W and Mo. We recently showed that this can be further improved by introducing a gradient in the W dopant concentration in BiVO, which has resulted in carrier separation efficiencies as high as 80%. As a result, water oxidation catalysis and bulk carrier recombination no longer limit the photocurrent of BiVO. Instead, modest light absorption—especially for photons with energy close to the bandgap—becomes the main limitation.
To illustrate this, the amount of light absorbed in our best W:BiVO samples is ~5 mA/cm, compared to a theoretical maximum of ~7.5 mA/cm (assuming that all photons with an energy higher than the bandgap are absorbed and collected). This means that a significant increase of the photocurrent (up to 2.5 mA/cm) can be achieved when the light absorption is improved.
In this work, we explore the application of core-shell nanoparticles on the surface of 100 nm-thick spray-deposited BiVO films as a route to enhance the optical absorption of the films by near-field plasmonic enhancement. Similar to the work of Thomann et al., SiO is used as the shell layer to avoid surface recombination. However, here, Ag is used as a core material because: (i) Ag is less expensive than Au, and (ii) the plasmon frequency of Ag can be tailored between 400 to 550 nm, which is a better match with the absorption spectrum of BiVO. Under back-side AM1.5 illumination, we achieve a ~2.5 fold photocurrent improvement when decorating BiVO with Ag/SiO core-shell nanoparticles. We show that this enhancement can be attributed to two factors introduced by the core-shell nanoparticles. The first factor is related to the improved optical absorption in the layer, caused by both the optical near-field enhancement and improved light scattering (also called far-field effects) induced by the excitation of localized surface plasmon resonance in Ag nanoparticles. In addition, a significant non-optical enhancement is also observed, which is tentatively attributed to the reduction of surface recombination.
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