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)(2)Ru(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 H-2 and I-3(-), 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 I-3(-) 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](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 spec troscopy was used to monitor the formation and disappearance of I-3(-) ions with 1/K4-xHxNb6O17 and PSS/1/K4-xHxNb6O17. The rate constant for the back electron transfer reaction between conduction band electrons and I-3(-) ions decreases from 3.17(+/-0.03) x 10(7) to 3.01(+/-0.02) x 10(6) M(-1) s(-1) upon adsorption of PSS.