Improved Charge Separation and Photosensitized H2 Evolution from Water with TiO2 Particles on Colloidal SiO2 Carriers

Article abstract of DOI:10.1021/ja00246a012

Laser flash photolysis and steady-state photolysis studies show that electrostatic interactions have a dramatic influence on the kinetics for charge separation and hydrogen production in aqueous systems (pH 9.8) of 20-nm diameter TiO2-modified SiO2 colloids in various combinations with an electron relay, a photosensitizer, and a Pt catalyst.Either direct excitation of the semiconductor or the photosensitizer Ru(bpy)3²⁺ (bpy= 2,2'-bipyridine), electrostatically adsorbed to the colloid, initiate electron transfer to either the zwitterionic electron relay, N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium (PVS⁰), or N,N'-bis(3-sulfonatopropy l)-2,2'-bipyridinium (DQS⁰), or methyl viologen (MV²⁺).The rates and quantum yields for the formation of the radical PVS^.- anion in both the TiO2-SiO2/PVS⁰ and the TiO2-SiO2/Ru(bpy)3²⁺/PVS⁰ systems decline with increasing ionic strength.The rate and quantum yields for H2 production in both the TiO2-SiO2/DQS⁰/Pt and the TiO2-SiO2/Ru(bpy)3²⁺/DQS⁰/Pt systems also show a similar ionic strength dependence.Kinetic analysis of data infers that repulsion of the reduced zwitterionic relay PVS^.- and DQS^.- from the negatively charged colloidal interface inhibits back electron transfer to both the semiconductor and the surface-attached oxidized photosensitizer Ru(bpy)3³⁺.Formation of the cation MV^.+ radical and its back electron transfer to the semiconductor are rapid and imply that the MV²⁺ electron relay is in close proximity to the colloid.Both the photogenerated valence-band holes and the oxidized photosensitizer Ru(bpy)3³⁺ oxidize surface Ti-O^- groups of TiO2.This redox process has the important effect of recycling the photosensitizer for further reaction.The addition of the superoxide dismutase enzyme to the oxidized TiO2-(SiO2) system regenerates, in part, the activity of the semiconductor to evolve H2 and to release molecular oxygen.