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The use of operando spectroscopic tools in heterogeneous photocatalysis has experienced a strong drive in recent years. When using operando tools we must, however, consider carefully the fraction of the system to be probed in an in situ cell, that is, the volume under the simultaneous influence of excitation light and the reactants. This is not an obvious task for certain spectroscopies, particularly for those attempting to characterize the physico-chemical state of the catalysts. Herein we provide an analysis of this point applied to spectroscopies such as XAS (X-ray absorption spectroscopy) and complete the analysis of operando tools with those aimed to characterize the surface of the photocatalysts (XPS; X-ray photoelectron spectroscopy) as well as those spectroscopies, such as infrared or Raman, focusing on the study of the reactantÀcatalyst interface.

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Photo-catalysis is a research field with broad applications in terms of potential technological applications related to energy production and managing, environmental protection, and chemical synthesis fields. A global goal, common to all of these fields, is to generate photo-catalytic materials able to use a renewable energy source such as the sun. As most active photocatalysts such as titanium oxides are essentially UV absorbers, they need to be upgraded in order to achieve the fruitful use of the whole solar spectrum, from UV to infrared wavelengths. A lot of different strategies have been pursued to reach this goal. Here, we selected representative examples of the most successful ones. We mainly highlighted doping and composite systems as those with higher potential in this quest. For each of these two approaches, we highlight the different possibilities explored in the literature. For doping of the main photocatalysts, we consider the use of metal and non-metals oriented to modify the band gap energy as well as to create specific localized electronic states. We also described selected cases of using up-conversion doping cations. For composite systems, we described the use of binary and ternary systems. In addition to a main photo-catalyst, these systems contain low band gap, up-conversion or plasmonic semiconductors, plasmonic and non-plasmonic metals and polymers.

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