Hydrothermal synthesis and photoelectrochemical properties of vertically aligned tungsten trioxide (hydrate) plate-like arrays fabricated directly on FTO substrates

Abstract

Without the assistance of a WO₃ seed layer, a uniform tungsten trioxide hydrate (WO₃·H₂O) plate-like array film was grown directly on bare fluorine-doped tin oxide (FTO) glass by a simple hydrothermal method at mild temperature using ammonium oxalate ((NH₄)₂C₂O₄) as a structure-directing agent. The dependence of the crystal structure and morphology on the growth temperature and growth time in the as-prepared samples was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD studies showed that the as-prepared thin films obtained below 150 °C were comprised of orthorhombic WO₃·H₂O and were completely converted to monoclinic WO₃ at 180 °C. SEM analysis revealed that the thickness of the WO₃·H₂O nanoplates increased with the increase of growth temperature as well as growth time. Moreover, the formation mechanism of the WO₃·H₂O plate-like arrays was discussed. It was found that the (NH₄)₂C₂O₄ played an important role in the formation of vertically aligned plate-like arrays. The thin films calcined at 450 °C for 1 h showed fine photocatalytic activities. The plate-like arrays grown on the bare FTO substrate synthesized at 120 °C for 12 h exhibited the best photocatalytic activity, which generated an anodic photocurrent of 4.13 mA cm⁻² at 1.6 V (vs. Ag/AgCl) under illumination of a 500 W Xe lamp in 0.5 M H₂SO₄ electrolyte.