Abstract 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 WO2.9 nanowires (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 CO2 route 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.

中文翻译：

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研究从钨衬底火焰合成氧化钨纳米线的氧化生长路线

摘要 氧化钨纳米线是直接从插入逆流扩散火焰中的钨衬底探针的表面合成的，由于流场的准一维性，将形成的形态与局部条件相关联。计算模拟有助于设计与相关化学物质和温度相关的实验的火焰结构。钨基体插入火焰反应区空气侧或燃料侧的火焰结构中，允许评估 H2O（或 CO2）与 O2 的作用，O2 作为反应物物种在所得钨-氧化物纳米结构。此外，甲烷火焰与氢气火焰进行了比较，氢气火焰只有 H2O（没有 CO2）作为产物种类。甲烷和氢气火焰的温度分布有目的地匹配，以比较作为纳米结构生长反应物的火焰产生的化学物质的影响。在气相温度为 20–50 nm，长度 > 10 µm，覆盖密度为 109–1010 cm-2）的单晶、垂直排列和致密的 WO2.9 纳米线甲烷火焰的空气侧为 1720 K。探测位置和火焰种类之间的比较表明，CO2 路线是一种异质的路线，有助于在成核阶段为纳米线的生长提供种子，随后的气固生长发生在其他路线上。对氢火焰燃料侧的探测隔离了 H2O 路线并确认它能够生产氧化钨纳米线，尽管速率和产量非常低。此外，考虑到 H2O 与 W 反应形成气态 W/O 物种的热力学不利性，提出了一种自光催化机制，其中 H2O 在 WOx 表面分解为活性 OH，促进挥发性 W/O 物种的产生，以通过氧化钨纳米线的气固机理。研究了 1280、1500 和 1720 K 的气相温度的影响，由于增加的成核和增加挥发性 W/O 化合物的形成，温度升高对应于更高的产量密度。通过氧化钨纳米线的气固机制促进挥发性 W/O 物质的产生，以实现持续生长。研究了 1280、1500 和 1720 K 的气相温度的影响，由于增加的成核和增加挥发性 W/O 化合物的形成，温度升高对应于更高的产量密度。通过氧化钨纳米线的气固机制促进挥发性 W/O 物质的产生，以实现持续生长。研究了 1280、1500 和 1720 K 的气相温度的影响，由于增加的成核和增加挥发性 W/O 化合物的形成，温度升高对应于更高的产量密度。