Time-resolved infrared absorption study of SrTiO₃ photocatalysts codoped with rhodium and antimony

In this study, SrTiO₃ photocatalysts doped with Rh and Sb are prepared by hydrothermal synthesis. Doping Rh alone causes optical absorption centered at 580 and 420 nm, attributed to Rh⁴⁺ and Rh³⁺. By adding Sb, the oxidation state of Rh shifts to be 3+ and the absorption at 580 nm disappears. The doped and nondoped photocatalysts exhibit transient infrared absorption at 3000-1000 cm^-1 due to excited electrons, when pumped by 355 or 532 nm light pulses. The absorbance decay is observed in a vacuum as a function of microsecond time delays to deduce the relative rate of electron-hole recombination. The active role of Rh⁴⁺ as a recombination center is evidenced by the photocatalyst doped with Rh alone having the fastest absorbance decay. The decay retards in the presence of Sb to show the limited role of Rh³⁺ in recombination. Steady-state H ₂ production in an aqueous methanol solution is examined in the presence of Pt cocatalyst on the doped SrTiO₃ photocatalysts. By comparing the H₂ production rate with the recombination rate, a common efficiency of electron-to-H₂ conversion is suggested, regardless of different dopant compositions. On the basis of the O₂ production rate observed in a AgNO₃ solution, the hole-to-O ₂ conversion efficiency is suggested to be sensitive to the dopant compositions.