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Investigating oxidation growth routes in the flame synthesis of tungsten-oxide nanowires from tungsten substrates

机译：研究从钨基材火焰合成氧化钨纳米线的氧化生长途径

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Tungsten-oxide nanowires are synthesized directly from the surface of tungsten substrate probes inserted into counter-flow diffusion-flames to correlate as-formed morphologies with local conditions because of the quasi-one-dimensionality of the flow field. Computational simulations aid in designing the flame structure for the experiments with respect to relevant chemical species and temperature. The tungsten substrates are inserted into the flame structure on either the air side or fuel side of the flame reaction zone, permitting evaluation of the roles of H2O (or CO2) versus O2, which serve as reactant species in the growth of the resulting tungsten-oxide nanostructures. Furthermore, methane flames are compared with hydrogen flames, which only have H2O (and no CO2) as product species. The temperature profiles of the methane and hydrogen flames are purposefully matched to compare the effect of chemical species produced by the flame which serve as reactants for nanostructure growth. Single-crystalline, well-vertically-aligned, and dense WO2.9nanowires (diameters of 20–50 nm, lengths of > 10 µm, and coverage density of 109–1010 cm−2) are obtained at a gas-phase temperature of 1720 K on the air-side of the methane flame. Comparisons among the probed locations and flame species indicate that the CO2route is a heterogeneous one that helps in seeding the growth of nanowires at the nucleation stage, with subsequent vapor–solid growth occurring from other routes. Probing on the fuel side of the hydrogen flame isolates the H2O route and confirms that it is able to produce tungsten-oxide nanowires, albeit at a very reduced rate and yield. Moreover, given the thermodynamic unfavorability of H2O reaction with W to form gaseous W/O species, a self-photocatalytic mechanism is proposed where H2O decomposes to reactive OH on the surface of WOx, facilitating production of volatile W/O species for continued growth by the vapor–solid mechanism for the tungsten-oxide nanowires. The effect of gas-phase temperatures of 1280, 1500, and 1720 K are examined, with increasing temperatures corresponding to higher yield density because of increased nucleation and augmented formation of volatile W/O compounds.
机译：氧化钨纳米线是直接从插入逆流扩散火焰中的钨基探针表面合成的，以将形成的形态与局部条件相关，这是由于流场的近似一维性。计算模拟有助于针对相关化学物种和温度设计用于实验的火焰结构。将钨基材插入火焰反应区的空气侧或燃料侧的火焰结构中，从而可以评估H2O（或CO2）与O2的作用，O2在生成的钨中起反应物的作用。氧化物纳米结构。此外，将甲烷火焰与氢火焰进行了比较，后者仅以H2O（无CO2）为产物。甲烷和氢气火焰的温度曲线经过有意匹配，以比较火焰产生的化学物质的作用，这些化学物质可作为纳米结构生长的反应物。在1720的气相温度下获得了单晶，垂直取向良好且致密的WO2.9纳米线（直径20–50 nm，长度> 10 µm，覆盖密度109-1010 cm-2）。甲烷火焰空气侧的K。探测位置和火焰种类之间的比较表明，CO2路径是一种异质路径，有助于在成核阶段播种纳米线的生长，随后其他路径中也会发生气固生长。在氢火焰的燃料侧进行探查可将H2O路径隔离开来，并确认它能够生产氧化钨纳米线，尽管其速率和产率都大大降低。此外，考虑到H2O与W反应形成气体W / O物种的热力学不利因素，提出了一种自光催化机制，其中H2O分解为WOx表面上的反应性OH，促进了挥发性W / O物种的产生，可通过氧化钨纳米线的气固机理。考察了1280、1500和1720 K气相温度的影响，随着成核度的增加和挥发性W / O化合物形成的增加，温度升高对应于更高的产率密度。

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