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Electro- and photochemical CO 2 reduction (CO 2 R) is the quintessence of modern-day sustainable research. We report our studies on the electro- and photoinduced interfacial charge transfer occurring in a nanocrystalline mesoporous TiO 2 film and two TiO 2 /iron porphyrin hybrid films ( meso -aryl- and β-pyrrole-substituted porphyrins, respectively) under CO 2 R conditions. We used transient absorption spectroscopy (TAS) to demonstrate that, under 355 nm laser excitation and an applied voltage bias (0 to -0.8 V vs Ag/AgCl), the TiO 2 film exhibited a diminution in the transient absorption (at -0.5 V by 35%), as well as a reduction of the lifetime of the photogenerated electrons (at -0.5 V by 50%) when the experiments were conducted under a CO 2 atmosphere changing from inert N 2 . The TiO 2 /iron porphyrin films showed faster charge recombination kinetics, featuring 100-fold faster transient signal decays than that of the TiO 2 film. The electro-, photo-, and photoelectrochemical CO 2 R performance of the TiO 2 and TiO 2 /iron porphyrin films are evaluated within the bias range of -0.5 to -1.8 V vs Ag/AgCl. The bare TiO 2 film produced CO and CH 4 as well as H 2 , depending on the applied voltage bias. In contrast, the TiO 2 /iron porphyrin films showed the exclusive formation of CO (100% selectivity) under identical conditions. During the CO 2 R, a gain in the overpotential values is obtained under light irradiation conditions. This finding was indicative of a direct transfer of the photogenerated electrons from the film to absorbed CO 2 molecules and an observed decrease in the decay of the TAS signals. In the TiO 2 /iron porphyrin films, we identified the interfacial charge recombination processes between the oxidized iron porphyrin and the electrons of the TiO 2 conduction band. These competitive processes are considered to be responsible for the diminution of direct charge transfer between the film and the adsorbed CO 2 molecules, explaining the moderate performances of the hybrid films for the CO 2 R.

The photocatalytic and electrocatalytic conversion of CO 2 has the potential to provide valuable products, such as chemicals or fuels of interest, at low cost while maintaining a circular carbon cycle. In this context, carbon dots possess optical and electrochemical properties that make them suitable candidates to participate in the reaction, either as a single component or forming part of more elaborate catalytic systems. In this review, we describe several strategies where the carbon dots participate, both with amorphous and graphitic structures, in the photocatalysis or electrochemical catalysis of CO 2 to provide different carbon-containing products of interest. The role of the carbon dots is analyzed as a function of their redox and light absorption characteristics and their complementarity with other known catalytic systems. Moreover, detailed information about synthetic procedures is also reviewed.