Mechanism of WS2 Nanotube Formation Revealed by in Situ/ex Situ Imaging

Multiwall WS2 nanotubes have been synthesized from W18O49 nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the “surface-inward” mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS2 nanotubes are obtained. In the present work, an upgraded in situ SEM μReactor with H2 and H2S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named “receding oxide core”, which complements the “surface-inward” model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS2 layers via the surface-inward reaction. At this point, the diffusion of H2S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H2S gas, forming fresh WS2 layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 °C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with contemporary insight.十余年来，科研人员已可大规模从W₁₈O₄₉纳米晶须合成多层二硫化钨（WS₂）纳米管。当前已确立的生长模型基于“表面向内”机制：即通过与硫化氢（H₂S）的高温反应在纳米晶须表面启动，氧化物向硫化物的转化向内推进，最终得到具有空心核结构的多层二硫化钨纳米管。本研究搭建了搭载氢气（H₂）与硫化氢气源的升级款原位扫描电子显微镜微反应器（in situ SEM μReactor），以详细解析其生长机制。我们观测到一种可补充“表面向内”模型的、迄今未见报道的生长机制——“后退氧化物核”机制，并对其开展了动力学评估。初始阶段，纳米晶须通过表面向内反应被数层二硫化钨钝化；此时硫化氢透过已形成的外层的扩散变得极为迟缓，表面向内反应也显著放缓。随后，钨亚氧化物核在靠近其尖端的核内部发生各向异性挥发，核内的氧化物蒸气发生部分向外扩散，局部形成随反应时间不断扩张的空腔。与此同时，氧化物蒸气与腔内的硫化氢发生反应，在初生纳米管的空腔中生成新的二硫化钨层。当反应温度高于900℃时，后退氧化物核模式的生长速率会随之升高。此外，我们还在常压流动反应器中开展了纳米管生长实验，结果表明所提出的“后退氧化物核”生长模型在常规反应参数下同样适用。本研究结合已有生长模型与最新研究认知，全面阐明了二硫化钨纳米管的生长机制。