Spectral Dependence and Wavelength Selectivity in Heterogeneous Photocatalysis. I. Experimental Evidence from the Photocatalyzed Transformation of Phenols

The photocatalyzed transformation of phenol (PhOH) and 4-chlorophenol (ClPhOH) in air-equilibrated TiO 2 dispersions has been used to examine the spectral behavior of the quantum yields Φ (and photonic efficiencies, η) of loss of these two substrates at various wavelengths. Contrary to the band model of semiconductors and conventional wisdom which predict rapid thermalization of photogenerated charge carriers to the lowest energy levels in their respective conduction and valence band states, and consequently to spectrally independent quantum yields, experimental results demonstrate that in fact the quantum yields of loss of PhOH and ClPhOH are spectrally dependent, displaying well resolved band structures at 3.13, 3.21, 3.32, 3.60, 3.81, 4.28 and 4.57 eV for phenol and at 3.16, 3.25, 3.41, 3.59, 3.70 and 4.15 eV for 4-chlorophenol. These energies correlate with absorption and emission band energies of direct and indirect band-to-band transitions reported earlier by Serpone et al. ( J. Phys. Chem ., 1995, 99, 16655) and with theoretical estimates by Daude and co-workers ( Phys. Rev. B, 1977, 15, 3229) for TiO 2 crystals. Both the spectral dependence of Φ and η and the wavelength selectivity of TiO 2 ( S e and S h ) are discussed in terms of theoretical predictions based on the solution of the continuity equation. The implicit emphasis of this study is the need to rethink the predictions based on the conventional band model of semiconductors. Thermalization of hot carriers may not (in some cases) be as rapid as once envisaged.