Temperature-Controlled Transformation of WO3 Nanowires into Active Facets-Exposed Hexagonal Prisms toward Efficient Visible-Light-Driven Water Oxidation.

A unique transformation of WO3 nanowires (NW-WO3) into hexagonal prisms (HP-WO3) was demonstrated by tuning the temperature of the (N2H4)WO3 precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO3 nanocrystals (ca. <100 nm width, 3-5 µm length) with anisotropic growth of monoclinic WO3 crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO3 nanocrystals (ca. 500-1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO3 nanocrystals with a single-crystalline character. The HP-WO3 electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO3 electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO3 was 3.1-fold higher than 15% for the NW-WO3 electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO3 layer with the unidirectional nanowire structure is more efficient than that through the HP-WO3 layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO3 electrode is more efficient than the NW-WO3 electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO3 electrode. The efficient water oxidation reaction at the surface for the HP-WO3 electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO3 to suppress the electron-hole recombination at the surface.

Facile Fabrication of a Highly Crystalline and Well-Interconnected Hematite Nanoparticle Photoanode for Efficient Visible-Light-Driven Water Oxidation.

Facile and scalable fabrication of α-Fe2O3 photoanodes using a precursor solution containing FeIII ions and 1-ethylimidazole (EIm) in methanol was demonstrated to afford a rigidly adhered α-Fe2O3 film with a controllable thickness on a fluorine-doped tin oxide (FTO) substrate. EIm ligation to FeIII ions in the precursor solution brought about high crystallinity of three-dimensionally well-interconnected nanoparticles of α-Fe2O3 upon sintering. This is responsible for the 13.6 times higher photocurrent density (at 1.23 V vs reference hydrogen electrode (RHE)) for photoelectrochemical (PEC) water oxidation on the α-Fe2O3 (w-α-Fe2O3) photoanode prepared with EIm compared with that (w/o-α-Fe2O3) prepared without EIm. The w-α-Fe2O3 photoanode provided the highest charge separation efficiency (ηsep) value of 27% among the state-of-the-art pristine α-Fe2O3 photoanodes, providing incident photon-to-current conversion efficiency (IPCE) of 13% at 420 nm and 1.23 V vs RHE. The superior ηsep for the w-α-Fe2O3 photoanode is attributed to the decreased recombination of the photogenerated charge carriers at the grain boundary between nanoparticles, in addition to the higher number of the catalytically active sites and the efficient bulk charge transport in the film, compared with w/o-α-Fe2O3.