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.