Synergistic Effect of Porosity and Gradient Doping in Efficient Solar Water Oxidation of Catalyst-Free Gradient Mo:BiVO4

In this paper, the synergistic effect of porosity and gradient of Mo doping in BiVO 4 photoanodes for improving charge separation and solar water oxidation performance is reported. A simple solution-based, three-step fabrication route was adopted using a layer-by-layer assembling technique. A water oxidation photocurrent of ∼1.73 mA cm -2 at 1.23 V vs reversible hydrogen electrode in neutral pH was achieved without using any sacrificial agent or electrocatalyst. The gradient Mo doping was found to enhance charge separation efficiency, which was verified through a shift in the water oxidation onset potential cathodically to ∼200 mV. In addition, these results were further confirmed by a higher open-circuit photovoltage and flat band potential investigations. This was attributed to the surface energetics played by gradient Mo doping that served as the driving force in reducing the onset potential for water oxidation. The coupled effect of enhanced light absorption and charge separation was revealed by monitoring the difference in decoupling the water oxidation efficiencies of porous and planar Mo:BiVO 4 photoanodes. This study demonstrated an improvement in the catalytic and charge separation efficiency of Mo:BiVO 4 photoanodes due to the introduction of porous structured homojunctions in a gradient manner. The simple synthesis approach adopted in the present study can be utilized and scaled up in making efficient photoanodes for competent solar water oxidation cells.