Reduction of Carbon Dioxide at Semiconductor Electrodes in the Dark

Reduction of Carbon Dioxide at Semiconductor Electrodes in the Dark 

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

In commenting on the observations of Tinnemans et a/.,84 Augustynski remarked86 that the importance of the marked affinity of the hydrated Ti02 for C02 was apparent from anodic peaks observed on the voltammograms obtained on the reverse sweep after scanning up to a sufficient negative potential at which C02 

In this connection, cyclic voltammetric measurements on the electrochemical reduction of C02 at n-Ti02 and platinized Ti02 film electrodes were reported a little later by Augustynski and co-workers.87 The existence of two electrochemically detectable species resulting from C02 reduction was suggested by anodic peaks on the cyclic voltammograms (Fig. 4). Unfortunately, however, no 

Since indium is one of the most effective metals for electrochemical reduction of C02, n-Ti02 on which indium had been electrodeposited was examined.92 Enhancement of the faradaic efficiency of C02 reduction by one order of magnitude or more compared to that at undoped n-Ti02 was observed, but the product detected was mainly hydrogen with a small amount ( 5% 

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