TiO₂ Surface Functionalization to Control the Density of States

Abstract

Surface functionalization of mesoporous nanocrystalline (anatase) TiO2 thin films with decyltriethoxysilane, octyltriethoxylsilane, hexyltriethoxysilane, decylphosphonic acid, undecanoic acid, and hemin was accomplished by room temperature reactions in toluene, acetonitrile, or DMSO. Surface functionalization was verified by attenuated total reflection infrared spectroscopy (ATR-FTIR) and the integrated density of unfilled TiO2 states (DOS) were probed by spectroelectrochemical, reactivity, and excited-state injection yield measurements. With the exception of hexyltriethoxysilane, all surface functionalizations were found to shift the DOS positive on an electrochemical scale (away from the vacuum level) in 0.1 M tetrabutylammonium ion containing electrolyte. The magnitude of the effect was found to be dependent on the surface coverage. The potential onset of the unfilled TiO2 states was not affected by functionalization in 0.1 M lithium ion containing electrolyte but the DOS at more negative potentials was significantly decreased. The 532 nm sensitized injection yield with Ru(dcb)(bpy)2(PF6), where dcb is 4,4′-(COOH)2-2,2′-bipyridine and bpy is bipyridine, was 0.89 ± 0.09 for all surface functionalizations. An enhancement of the open circuit photovoltage in regenerative solar cells with 0.5 M LiI/0.05 M I2 was measured after surface functionalization, and an analysis of this data with the diode equation indicated decreased rates for I3− reduction by factors of 7−330. The second-order rate constant for the reduction of carbon tetrachloride by electrochemically reduced TiO2 that had been surface functionalized with decytriethoxysilane, 0.21 ± 0.01 M−1 s−1, was decreased relative to an unfunctionalized TiO2 thin film, 1.02 ± 0.03 M−1 s−1, behavior attributed to the ability of the functionalized surface to prevent close encounters with electron acceptors