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

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 (PVS°), or N,N'-bis(3-sulfonatopropyl)-2,2^'-bipyridinium (DQS°), or methyl viologen (MV²⁺). The rates and quantum yields for the formation of the radical PVS^•-anion in both the TiO₂-SiO₂/PVS⁰and the TiO₂-SiO₂/Ru(bpy)₃²⁺/PVS⁰systems 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⁺/DQS°/Pt systems also show a similar ionic strength dependence. Kinetic analysis of the 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.