We observed the carrier recombination in SrTiO3 single crystals with several crystal faces and Nd doping concentrations using time-resolved photoluminescence and microwave photoconductivity decay methods. The shapes of the decay curves were independent of the excited carrier concentrations, and thus the carriers recombined through the Shockley–Read–Hall and surface recombination processes. From the measurements for different crystal faces, we found that, although the surface recombination is significant for the (100), (110), and (111) faces, they have similar surface recombination velocities of ∼106 cm s−1. Therefore, as photocatalysts, the shape of the materials has a minor effect on the energy conversion efficiency. Based on the dependence of the Nb doping concentration, a high doping concentration significantly enhances the carrier recombination, whereas a moderate Nb doping induces only a slight reduction in the time constants of the carrier decay. In addition, the time constant increases with temperature, and thus moderate Nb doping will have a positive effect on the energy conversion efficiency of SrTiO3 photocatalysts at high temperatures.
We characterized the photocatalytic performance of rutile TiO 2 single crystals with Nb doping (0.01-0.05 wt%) for water splitting and then observed the effects of doping. Although Nb doping induces additional optical absorption below the bandgap energy, it does not contribute to photocatalytic performance. Moreover, Nb doping enhances carrier recombination and donor concentration, resulting in reduction of the hole diffusion length and depletion of the layer width. Furthermore, it reduces the existing holes generated by light for photocatalysis because of the difficulty of charge separation by an internal electric field. In addition, comparison of the theoretically calculated photocurrents indicates that high Nb doping has other negative effects on the photocatalytic performance. These results suggest that, although Nb doping enhances the electrical conduction by increasing the donor concentration, it has negative effects on other important characteristics for photocatalysis. Therefore, Nb doping has optimum concentrations depending on structures of the photoelectrode.
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