Photoelectrochemistry with TiO2 Aerogels

Abstract

One way of understanding light-induced charge separation processes on semiconducting substrates is to arrive at a parametrization that correlates material properties with output parameters. The Gartner equation and its descendants [1] are classic examples in which we try to correlate output parameters of photovoltaic devices, such as short circuit current and open circuit voltage, with material parameters such as minority carriers’ diffusion length and absorption coefficient These four parameters, and many similar ones, are parameters of the collective system. In the general language of complex systems, they are emerging properties of the aggregates - in our case, in its simplest presentation - of the single crystal substrates. Photoelectrochemistry on semiconducting substrates has now reached the point where we need to develop parametrization that spans different aggregation modes. Figure 1 demonstrates the central issue; we show there five different modes of aggregation. The first mode is a powder of isolated nanocrystalline grains such as Degussa P25. The particles are well defined but the interparticle space is constantly changing. Interparticle transport can take place only through the surrounding electrolyte. The second mode is represented by TiO2 aerogels in which the fundamental units remain the nanocrystallites but they now aggregate to establish connectivity. In this mode the surface area and most of the bulk properties are similar to ones obtained with the powders but the interparticle pore space is well defined and interparticle transport can take place not only through the electrolyte but also between particles. The next mode takes the form of nanocrystalline electrodes in which the morphology is similar to the aerogels but is deposited on a substrate.