Mechanical and Civil Engineering Seminar

I will discuss an atmospheric, cost-effective and scalable flame synthesis method for the growth and doping of metal oxide nanowires and these nanowires (NWs) exhibit superior photoelectrochemical (PEC) performance due to their high crystallinity, great morphology tunability and chemical composition control. First, arrays of tungsten trioxide (WO₃) NWs are synthesized on fluorinated tin oxide (FTO) coated glass substrates by rapid, atmospheric flame vapor deposition, in which a flame oxidizes and evaporates tungsten metal to produce tungsten oxide vapors that condense onto a colder FTO substrate with tunable morphologies of NWs. Importantly, the WO₃ NWs synthesized by flame have higher areal number density and longer length than state-of-the-art WO₃ NW photoanodes grown by chemical vapor deposition and hydrothermal methods, resulting in stronger light absorption and doubled saturation photocurrent. Second, the flame synthesized WO₃ NWs are further coated with thin BiVO₄ shells to form a core/shell heterojunction NW array. The WO₃/BiVO₄ heterojunction NW array utilizes the individual strengths of WO₃ (good electron transport) and BiVO₄ (strong light absorption) and outperforms all other WO₃ or BiVO₄-based photoanodes in the literature, regardless of doping, heterojunction, or catalyst. Finally, we report a novel ex-situ method to codope rutile TiO₂ with (W, C) pair by sequentially annealing tungsten (W)-precursor coated TiO₂ nanowires in flame and CO. The unique advantages of the flame annealing are that the high temperature and heating rate of flame enable rapid diffusion of W into TiO₂ that prevents the damage of TiO₂ nanowire morphology and crystallinity, and the delicate glass substrate. Significantly, this is the first experimental demonstration that the codoped TiO₂:(W, C) nanowires doubles the saturation photocurrent of undoped TiO₂ for PEC.