This brief review aims at analyzing the use of dyestuffs for evaluating the photocatalytic properties of novel photocatalysts. It is shown that the use of dyes as predictors for photocatalytic activity has its roots in the pre visible-light activity era, when the aim was to treat effluents streams containing hazardous dyes. The main conclusion of this review is that, in general, dyes are inappropriate as model compounds for the evaluation of photocatalytic activity of novel photocatalysts claimed to operate under visible light. Their main advantage, the ability to use UV-Vis spectroscopy, is severely limited by a variety of factors, most of which are related to the presence of other species. The presence of a second mechanism, sensitization, diminishes the generality required from a model contaminant used for testing a novel photocatalyst. While it is recommended not to use dyes for general testing of novel photocatalysts, it is still understandable that a model system consisting of a dye and a semiconductor can be of large importance if the degradation of a specific dye is the main aim of the research, or, alternatively, if the abilities of a specific dye to induce the degradation of a different type of contaminant are under study.
The last four decades have shown a remarkable increase in scientific interest in photocatalysis as a tool for tackling the world's energy and waste problems. The apparent similarity between photocatalytic water splitting and photocatalytic degradation of pollutants, which have been studied so far by two different scientific communities, raises the question regarding the extent to which one may utilize knowledge obtained in one field for the benefit of the other. This review examines the common features and differences between the two areas. The main similarities stem from the common dependence on the absorption of photons and on subsequent charge-carrier dynamics. The main dissimilarities are linked to thermodynamics, the type of reactants and end products, and to the role of adsorption and desorption. At present the fundamental differences between storing energy and using it to solve environmental issues affect practical solutions. Yet, easy transfer of knowledge, research resources, and personnel between the two is not only possible but should be encouraged.
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