Visible light photolysis of hydrogen iodide using sensitized layered metal oxide semiconductors: The role of surface chemical modification in controlling back electron transfer reactions

The internally platinized wide bandgap semiconductor K₄Nb₆O₁₇ can be sensitized by [(bpy)₂Ru(4-(2,2′-bipyrid-4-yl)-phenylphosphonic acid](PF₆)₂ (1). In aqueous iodide solutions at pH 2, the visible light photolysis of HI, to form H₂ and I₃^-, is catalyzed by 1/K_4-xH_xNb₆O₁₇/Pt. The strong bond between the surface and the phosphonate group of 1 allows one to adsorb other surface species, which decrease the rate of the back electron transfer reaction between conduction band electrons and I₃^- ions. Methylphosphonic acid and undecylphosphonic acid do not form good surface monolayers on 1/K_4-xH_xNb₆O₁₇ and do not increase the rate of hydrogen evolution. Anionic surface modifiers [TiNbO₅]_n^n-, derived from exfoliation of KTiNbO₅, and poly(styrenesulfonate), PSS, increase the initial hydrogen evolution rate by factors of 3 and 5, respectively. In the latter case, the initial quantum yield for HI photolysis is ca. 3%. Transient diffuse reflectance spectroscopy was used to monitor the formation and disappearance of I₃^- ions with 1/K_4-xH_xNb₆O₁₇ and PSS/ 1/K_4-xH_xNb₆O₁₇. The rate constant for the back electron transfer reaction between conduction band electrons and I₃^- ions decreases from 3.17 (±0.03) × 10⁷ to 3.01(±0.02) × 10⁶ M^-1 s^-1 upon adsorption of PSS.