Thursday, October 31, 2013
Mechanical and Civil Engineering Seminar
Flame Synthesized Metal Oxide Nanowires as Effective Photoanodes for Photoelectrochemical Water-Splitting
Xiaolin Zheng, Assistant Professor, Department of Mechanical Engineering, Stanford University
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.