The pioneering work of Honda and Fujishima in 1972, which showed that anodic current flows when the n-type semiconductor TiO2 is irradiated with UV light, initiated the extensive study of semiconductors as photocatalysts. Each semiconductor has a band gap–the energy space in between the valence band and the conduction band where no electron states can exist–that allows for the photocatalysis of electrochemical reactions. By exciting an electron from the valence band to the conduction band and leaving behind a positively charged hole, a photon with energy greater than or equal to that of the semiconductor’s band gap can facilitate a redox reaction.
Specifically, semiconductors can photocatalyze the stoichiometric splitting of water into hydrogen and oxygen gases. Various materials, as shown below, have been shown to photocatalyze the splitting of water, but many are expensive, inefficient, or prone to photodecomposition. Because predicting and characterizing new candidate materials is difficult and time consuming, the Bocarsly Lab has developed a simple yet elegant assay to determine whether a semiconductor might be an effective photocatalytic water-splitter.
Along with decomposition, the issue of electron-hole recombination–when a photoexcited electron jumps from the valence band into the conduction band then falls back down to the valence band, recombining with a photogenerated hole–significantly reduces quantum yields of semiconductor photocatalysts. By using (a) triethanolamine (TEOA), a sacrificial electron donor, to mitigate electron-hole recombination and (b) Pt co-catalysts to localize charge on the material’s surface, we were able to optimize H2 evolution for known water-splitting semiconductors such as GaP and TiO2 at different pH values. Using the same optimized values, we can now quickly screen materials by irradiating (with both 365 and 394 nm LEDs) septum-sealed vials containing individual semiconductors and the optimized solution mixture. The reduction of water into hydrogen gas is detected using gas chromatography, with successful materials noted for further characterization and study.
<sup>1</sup>Fujishima, A., & Honda, K. (1972). Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 238(5358), 37–38. Retrieved from http://dx.doi.org/10.1038/238037a0