The dilemma of the band gap

ResearchBlogging.org

In a photocatalyst, the electronic band gap is one of the most important properties.
The size of the band gap determines the range of light that it can absorb. To excite an electron across a large band gap, a photon with a large energy required. Large energy means a smaller wavelength. In general, metal oxides usually possess band gaps of 3.0 eV or larger, which means that they only absorb UV light (wavelength < 400 nm), and not visible light (wavelength between 400 and 800 nm). Since most of the energy in sunlight is in the visible region, materials that can absorb visible light, i.e. materials with smaller band gaps, are required.
At the same time, the position of the band levels are also important. In order for electrons in the conduction band to cause a desired reduction reaction, the level of the conduction band must be more negative than the reaction’s redox potential. Likewise, the valence band must lie at a more positive level than the oxidation reaction’s potential, in order for the hole to cause the reaction.This may sound confusing, but this diagram should make it clearer.

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.

Figure from S. Ouyang and J. Ye, J. Am. Chem. Soc. 2011

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.

Reference:

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

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