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

Without the assistance of a WO3 seed layer, a uniform tungsten trioxide hydrate (WO3 center dot 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)(2)C2O4) 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 degrees C were comprised of orthorhombic WO3 center dot H2O and were completely converted to monoclinic WO3 at 180 degrees C. SEM analysis revealed that the thickness of the WO3 center dot H2O nanoplates increased with the increase of growth temperature as well as growth time. Moreover, the formation mechanism of the WO3 center dot H2O plate-like arrays was discussed. It was found that the (NH4)(2)C2O4 played an important role in the formation of vertically aligned plate-like arrays. The thin films calcined at 450 degrees C for 1 h showed fine photocatalytic activities. The plate-like arrays grown on the bare FTO substrate synthesized at 120 degrees 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.