Chapter 2 Semiconductor Electrochemistry

Single nanoparticle photoelectrochemistry: What is next?

Semiconductor photoelectrochemistry is a fascinating field that deals with the chemistry and physics of photodriven reactions at solid/liquid interfaces. The interdisciplinary field attracts (electro)chemists, materials scientists, spectroscopists, and theorists to study fundamental and applied problems such as carrier dynamics at illuminated electrode/electrolyte interfaces and solar energy conversion to electricity or chemical fuels. In the pursuit of practical photoelectrochemical energy conversion systems, researchers are exploring inexpensive, solution-processed semiconductor nanomaterials as light absorbers. Harnessing the enormous potential of nanomaterials for energy conversion applications requires a fundamental understanding of charge carrier generation, separation, transport, and interfacial charge transfer at heterogeneous nanoscale interfaces. Our current understanding of these processes is derived mainly from ensemble-average measurements of nanoparticle electrodes that report on the average behavior of trillions of nanoparticles. Ensemble-average measurements conceal how nanoparticle heterogeneity (e.g., differences in particle size, shape, and surface structure) contributes to the overall photoelectrochemical response. This perspective article focuses on the emerging area of single particle photoelectrochemistry, which has opened up an exciting new frontier: direct investigations of photodriven reactions on individual nanomaterials, with the ability to elucidate the role of particle-dependent properties on the photoelectrochemical behavior. Here, we (1) review the basic principles of photoelectrochemical cells, (2) point out the potential advantages and differences between bulk and nanoelectrodes, (3) introduce approaches to single nanoparticle photoelectrochemistry and highlight key findings, and (4) provide our perspective on future research directions.

Photoelectrocatalytic disinfection of E. coli suspensions by iron doped TiO2

Photoelectrocatalytic disinfection of E. coli by an iron doped TiO(2) sol-gel electrode is shown to be more efficient than disinfection by the corresponding undoped electrode. Thus, the improvements in photocatalytic efficiency associated with selective doping have been combined with the electric field enhancement associated with the application of a small positive potential to a UV irradiated titanium dioxide electrode. The optimum disinfection rate corresponds to the replacement of approximately 0.1% of the Ti atoms by Fe. The enhanced disinfection associated with iron doping is surprising because iron doping decreases the photocurrent, and photocurrent is generally taken to be a good indicator of photoelectrocatalytic efficiency. As the level of iron is increased, the character of the current-voltage curve changes and the enhancement of photocurrent associated with methanol addition decreases. This suggests that iron reduces the surface recombination which in the absence of iron is reduced by methanol. Therefore the enhanced photocatalysis is interpreted as due to iron reducing surface recombination, by trapping electrons. It is proposed that at low iron levels the photo-generated electrons are trapped at surface Fe(III) centres and that consequently, because the electron-hole recombination rate is reduced, the number of holes available for hydroxyl radical formation is increased. It is also proposed that at higher iron levels, the disinfection rate falls because electron hole recombination at iron centres in the lattice reduces the number of holes which reach the surface. Our conclusion that the optimum electrode performance is a balance between surface and bulk effects is consistent with the proposal, of earlier authors for photocatalytic reactions, that the optimum dopant level depends on the TiO(2).