You can see that if the band gap is too small, there would be no driving force for the electrons and holes to cause a reaction.This presents a dilemma. We need a small band gap to absorb more light, but a larger band gap is required for redox capability. There should be a balance somewhere in the middle, where we can get the best of both worlds.
In this week’s Journal of the American Chemical Society, Ouyang et al. reported that they indeed found an optimum point for one material.They synthesized AgAl1-xGaxO2 with x from 0 to 1. As they increased the proportion of Ga, the band gap became smaller, and the color visibly changed from dull green to orange.
The activity, as measured by the decomposition of iso-propanol under visible light, showed that a peak was reached with AgAl0.6Ga0.4O2, which had 35 times the activity of AgAlO2 and 65 times that of AgGaO2.
This paper showed that it is possible to tune the band structure in order to increase the photocatalytic activity dramatically.
Ouyang, S., & Ye, J. (2011). β-AgAlGaOSolid-Solution Photocatalysts: Continuous Modulation of Electronic Structure toward High-Performance Visible-Light Photoactivity
Journal of the American Chemical Society DOI: 10.1021/ja110691t