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.