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/K4-xHxNb6O17/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/K4-xHxNb6O17 and do not increase the rate of hydrogen evolution. Anionic surface modifiers [TiNbO5]nn-, 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...