The great versatility of semiconductor materials and the possibility of generation of electrons, holes, hydroxyl radicals, and/or superoxide radicals have increased the applicability of photoelectrocatalysis dramatically in the contemporary world. Photoelectrocatalysis takes advantage of the heterogeneous photocatalytic process by applying a biased potential on a photoelectrode in which the catalyst is supported. This configuration allows more effectiveness of the separation of photogenerated charges due to light irradiation with energy being higher compared to that of the band gap energy of the semiconductor, which thereby leads to an increase in the lifetime of the electron-hole pairs. This work presents a compiled and critical review of photoelectrocatalysis, trends and future prospects of the technique applied in environmental protection studies, hydrogen generation, and water disinfection. Special attention will be focused on the applications of TiO 2 and the production of nanometric morphologies with a great improvement in the photocatalyst properties useful for the degradation of organic pollutants, the reduction of inorganic contaminants, the conversion of CO 2 , microorganism inactivation, and water splitting for hydrogen generation.

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This work describes, the decoration of Ti/TiO 2 nanotubes by nanoparticles of ZIF-8 (zeolite imidazole framework-8) grown using a layer-by-layer process. Morphological and crystallographic analyses showed that the TiO 2 nanotubes were coated with ZIF-8 nanoparticles around 50 nm in size. Curves of I ph vs. E showed that the incorporation of ZIF-8 at Ti/TiO 2 electrodes increased the photocurrent and that the values were dramatically increased in solution saturated with CO 2. The CO 2 adsorbed on the ZIF-8 formed stable carbamates, as demonstrated by spectroscopic and voltammetric assays. Photoelectrocatalytic reduction of CO 2 at Ti/TiO 2 NT-ZIF-8 electrodes resulted in formation of up to 10 mmol L −1 of ethanol and 0.7 mmol L −1 of methanol in 0.1 mol L −1 Na 2 SO 4 , at E app of +0.1 V, under UV-vis irradiation at room temperature. Our findings open up new applications of metal-organic frameworks (MOFs) in photoelectrocatalysis for the highly efficient preconcentration and conversion of CO 2 in aqueous media at ambient temperature.

The role of CuO nanoparticles decorating TiO 2 nanotubes (TNT) thin film photoanodes in the behavior of photoelectrocatalytic (PEC) cells for water splitting reaction is investigated. CuO is present mainly as small nanoparticles of few nanometer decorating the internal walls of the TiO 2 nanotubes. Their presence improves i) the photocurrent behavior, ii) the H 2 generation rate by water splitting in a full PEC device (without application of a bias) and iii) the solar-to-hydrogen (STH) efficiency. The increase is about 20% with respect to parent TNT photoanodes using open spectrum light from a solar simulator and about 50% increase using AM 1.5G filtered light from a solar simulator. An STH efficiency over 2% in the full PEC cell is observed in the best conditions. IPCE (incident photon to current conversion efficiency) measurements clearly evidence that the presence of CuO nanoparticles induce an enhanced IPCE in the 300-340 nm region. The increase in the performances in water splitting is mainly associated to the transient generation of a p-n junction between the Cu x O nanoparticles and TNT upon illumination, which enhances photocurrent density by promoting charge separation.

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h i g h l i g h t s Photoelectrochemical CO 2 reduction at an n-p Ti/TiO 2 /CuO electrode. Formation of methanol, ethanol, and acetone as reduction products. Temperature, supporting electrolyte and potential influence in CO 2 reduction. A charge transfer and reaction scheme are presented to account for the product evolution.

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