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

A unique transformation of WO 3 nanowires (NW-WO 3 ) into hexagonal prisms (HP-WO 3 ) was demonstrated by tuning the temperature of the (N 2 H 4 )WO 3 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-WO 3 nanocrystals ( ca. <100 nm width, 3–5 μm length) with anisotropic growth of monoclinic WO 3 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-WO 3 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 WO 3 nanocrystals with a single-crystalline character. The HP-WO 3 electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO 3 electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO 3 was 3.1-fold higher than 15% for the NW-WO 3 electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO 3 layer with the unidirectional nanowire structure is more efficient than that through the HP-WO 3 layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO 3 electrode is more efficient than the NW-WO 3 electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO 3 electrode. The efficient water oxidation reaction at the surface for the HP-WO 3 electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO 3 to suppress the electron–hole recombination at the surface.