Rutile particles doped by Cr and Sb with Sb/Cr ratios greater than unity were active for photocatalytic O2 evolution from an aqueous silver nitrate solution under visible light irradiation. Two vibrational bands appeared in Raman scattering resonant to the electronic absorption and were assigned to symmetric breathing modes of the CrO6 octahedron and neighboring TiO6 octahedra, on the assumption that Cr dopant atoms exactly replaced Ti cations in rutile. Electrons excited with 355-nm and 532-nm light pulses showed absorption of mid-IR light, which was traced as a function of the time delay in a microsecond domain. By optimizing the Sb/Cr ratio, electron-hole recombination was retarded compared with that in non-doped rutile. Chromium dopants when accompanied with antimony dopants are proposed to trap the charge carriers without much enhancement of the recombination. The optimized Sb/Cr ratios were common for both the visible light-induced O2 production and the retarded recombination, suggesting that the charge carriers of the retarded recombination are used to produce O2.
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