Tin(II)-Based Metal-Organic Frameworks Enabling Efficient, Selective Reduction of CO2 to Formate under Visible Light.

Certain metal complexes are known as high-performance CO2 reduction photocatalysts driven by visible light. However, most of them rely on rare, precious metals as principal components, and integrating the functions of light absorption and catalysis into a single molecular unit based on abundant metals remains a challenge. Metal-organic frameworks (MOFs), which can be regarded as intermediate compounds between molecules and inorganic solids, are potential platforms for the construction of a simple photocatalytic system composed only of Earth-abundant nontoxic elements. In this work, we report that a tin-based MOF enables the conversion of CO2 into formic acid with a record high apparent quantum yield (9.8 % at 400 nm) and >99 % selectivity without the need for any additional photosensitizer or catalyst. This work highlights a new MOF with strong potential for photocatalytic CO2 reduction driven by solar energy.

Fluorine-Assisted Low-Temperature Synthesis of GaN:ZnO-Related Solid Solutions with Visible-Light Photoresponse.

Wurtzite-structured Ga1-xZnx(N,O,F) was successfully synthesized by nitridation of mixtures of a Ga-containing oxide and ZnF2. The addition of ZnF2 lowered the nitridation temperature for the synthesis of Ga1-xZnx(N,O,F) to 823 K, even when bulk ZnGa2O4 was used as a paired precursor. This lowering of the synthesis temperature was ascribed to the enhancement of nitridation through the addition of fluorine. The low-temperature nitridation achieved by the addition of fluorine suppressed the volatilization of Zn compared with that during the synthesis of a GaN:ZnO solid solution by a conventional high-temperature ammonolysis reaction. The higher concentration of Zn, as well as the higher N concentration in Ga1-xZnx(N,O,F) achieved through the fluorine-assisted nitridation, led to a redshift of the absorption edge of Ga1-xZnx(N,O,F) to 560 nm compared with that of GaN:ZnO synthesized by the conventional ammonolysis reaction. The visible-light absorption of Ga1-xZnx(N,O,F) can be used to drive the photoelectrochemical oxidation of water.

A bifunctional lead-iron oxyfluoride, PbFeO2F, that functions as a visible-light-responsive photoanode and an electrocatalyst for water oxidation.

The oxyfluoride PbFeO2F was investigated as a photoanode material and as an electrocatalyst for water oxidation. PbFeO2F powder, which was synthesized by a high-pressure method and had an estimated bandgap of 2.1 eV, was deposited onto a fluorine-doped tin oxide (FTO) substrate. Mott-Schottky plot measurements for the PbFeO2F/FTO electrode showed n-type semiconductivity of PbFeO2F, with a flat-band potential of +0.53 ± 0.05 V vs. reversible hydrogen electrode (RHE). The PbFeO2F/FTO electrode, which was modified with a conductive TiO2 layer and a cobalt phosphate water-oxidation cocatalyst, showed a clear anodic photocurrent in aqueous K3PO4 solution under visible-light irradiation (λ < 600 nm). The PbFeO2F/FTO electrode without any modification functioned as a stable water-oxidation electrocatalyst to form O2 with a faradaic efficiency of close to unity. This study demonstrates that PbFeO2F is a bifunctional material, serving as a water-oxidation photoanode under a wide range of visible-light wavelengths and as an electrocatalyst that operates at a relatively low overpotential for water oxidation.