The photocatalytic performance of Ta and Rh co-doped TiO2 tuned by the average dopant valence

Highlights: • Effects of dopant valence on the photocatalytic performance was investigated. • New strategy was worked out to investigate effects of dopant valence. • The dopant valence was changed but the ionic radius keeps constant. • Average valence of Rh and Ta in Ti_0.96Ta_0.04−xRh_xO₂ was changed multiply. • The optimal photocatalyst was obtained as the average valence approaches +4. - Abstract: In order to reveal the effects of dopant average valence on the photocatalytic performance, Ta and Rh co-doped Ti_0.96Ta_0.04−xRh_xO₂ (x = 0, 0.008, 0.016, 0.02, 0.024, 0.032, and 0.04) samples were crystallized at 450 °C and 800 °C, respectively; then the 800 °C crystallized x = 0.02 sample was subsequently reduced in the H₂/N₂ mixture gases at 600, 700, 800, and 900 °C. Under different processing conditions, Ta remains the valence of +5, while the valence of Rh varies. Rh exhibits Rh³⁺ and Rh⁴⁺ in the 450 °C crystallized samples and is completely oxidized to Rh⁴⁺ in the 800 °C crystallized samples. In the reduced samples, Rh exists in the forms of Rh, Rh³⁺ and Rh⁴⁺. The optimal photocatalyst is obtained as the ratio of Rh³⁺/Rh⁴⁺ is 2.11 or the average valence of doped Ta and Rh is closest to the +4 valence of substituted Ti.