Metal-oxide photochemistry, photophysics and modelling

Among coundess photoinduced processes, we focused on those photochemical processes that occur at the interface of solid/liquid and solid/gas heterogeneous systems, and on their relationship(s) with photophysical processes in solids. 

Two well-known snrface stoichiometric photochemical reactions can be identified (i) the photostimnlated adsorption of O2 (reduction of acceptor molecules), and (ii) the photostimnlated adsorption of H2 (oxidation of donor molecules) on a metal-oxide surface. Both result in a new state of the heterogeneous system with charged species adsorbed on the solid. If these two processes occurred simultaneously they would yield a reaction identifiable as the photocatalysed oxidation of hydrogen to water. Nonetheless, snch a simple mechanism gives bnt a small indication of the real processes that take place on solids and at interfaces of heterogeneous systems. We examine these cases later after a discnssion of the nature of solids and a description of the photochemical/photophysical events taking place in these complex materials. 

Despite advanced methods of crystal growth, sohds still contain lattice defects at concentrations around lO -lO cm, and typical concentrations of defects and impurities in commercial samples are around 10 cm . The latter concentration is usually much greater than the concentration of photogenerated free charge carriers in solids under moderate photoexcitation. Consequently, defects are expected to play an important role in photoexcitation and relaxation processes in heterogeneous systems. In fact, defects create a local distortion of the periodic potential in the solid s lattice. 

This leads to the appearance of local energy levels within the bandgap, which themselves lead to changes in the optical and electronic properties of solids. 

Following the work of Siline and Trukchin (1985) for the simplest case of centres with one trapped carrier, the kinetics of their formation typically follow an exponential growth and approach the steady-state concentration of photoinduced defects as given by 

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