Photocatalytic WO3/TiO2 nanowires: WO3 polymorphs influencing the atomic layer deposition of TiO2

Abstract

50-70 nm hexagonal (h-) and 70-90 nm monoclinic (m-) WO3 nanoparticles (NPs) were prepared by controlled annealing of (NH4)xWO3 in air at 470 and 600 °C, respectively. In addition, 5-10 nm thick and several micrometer long h-WO3 nanowires (NWs) were obtained by microwave hydrothermal synthesis at 160 °C with Na2WO4, HCl and (NH4)2SO4 as starting materials. TiO2 was deposited on h-WO3 NWs by atomic layer deposition (ALD) at 300 °C using Ti(iOPr)4 and H2O as precursors. The as-prepared materials were studied by TG/DTA-MS, XRD, Raman, SEM-EDX, TEM, ellipsometry, UV-Vis, and their photocatalytic activity was also tested by the photodecomposition of aqueous methyl orange. Our study is the first evidence of diverse ALD nucleation on various WO3 polymorphs, since on h-WO3 NWs TiO2 nucleated only as particles, whereas on m-WO3 conformal TiO2 film was formed, explained by the different surface OH coverage of h- and m-WO3. The h-WO3 NWs had significantly higher photocatalytic activity compared to h-WO3 NPs, and similar performance as m-WO3 NPs. By adding TiO2 to h-WO3 NWs by ALD method, the photocatalytic performance increased by 65%, showing clearly the uniqueness of ALD to obtain superior oxide composite photocatalysts.