Chimie-Douce Route to Tungsten Oxide Hydrate: Comprehensive Insight into the Formation Mechanism by Liquid-Phase Electron Microscopy Combined with Complementary Techniques

The chimie-douce route has emerged as an efficient strategy for synthesizing inorganic and organic–inorganic hybrid materials under mild conditions, enabling controls over material size, structure, and functionality. While sol–gel chemistry has been extensively studies to form silicate phases in-depth studies on tungsten oxide gel growth remain scarce. Here, we take advantage of recent advances in the development of in situ techniques and methodologies, particularly in the field of the liquid-phase transmission electron microscopy (LP-TEM), to shed light on the formation of these transition metal oxide-based materials. The starting tungstic acid solution consists of the polyanion decatungstate [W10O324–], which is the most stable polyanionic tungstate species at acidic pH, as confirmed by 183W liquid nuclear magnetic resonance (NMR) spectroscopy and synchrotron small-angle X-ray scattering (SAXS). SAXS and LP-TEM revealed the presence of several types of intermediate structures, in particular specific nanometric clusters rich in tungsten, referred to as tungsten-rich aggregates [Wrich-agg], which play a key role in the nucleation and growth of the gel. The observed first-order kinetics, measured by dynamic light scattering (DLS) and 17O NMR, is attributed to the consumption of [Wrich-agg] into intermediate bigger cluster aggregates [WOA] with sizes between 20 and 30 nm. Upon prolonged aging (typically after 24 h), SAXS and ex situ TEM measurements showed the formation of a dense tungsten oxide gel network with a fractal dimension of Df ≈ 2, characteristic of reaction-limited cluster–cluster aggregation (RLCCA). From a methodological perspective, this correlative approach, combining several multiscale techniques, including microscopy, scattering, diffraction, and spectroscopy, offers insights into the growth mechanism of materials obtained via a “chimie-douce” process.

温和化学（chimie-douce）路线已成为一种在温和条件下合成无机及有机-无机杂化材料的高效策略，可实现对材料尺寸、结构与功能的精准调控。尽管溶胶-凝胶化学已被广泛用于硅酸盐相的制备，但针对氧化钨凝胶生长的深入研究仍较为匮乏。本研究借助原位技术与方法学领域的最新进展，尤其是液相透射电子显微镜（LP-TEM）技术，为揭示这类过渡金属氧化物基材料的形成机制提供了新视角。初始钨酸溶液中的多阴离子物种为十钨酸根[W10O32^4–]，该物种是酸性pH条件下最稳定的钨多阴离子，这一点已通过183W液相核磁共振（NMR）光谱与同步辐射小角X射线散射（SAXS）得以证实。SAXS与LP-TEM观测发现，体系中存在多种中间结构，尤其是一类富含钨的特定纳米级团簇——即富钨聚集体[Wrich-agg]，其在凝胶的成核与生长过程中发挥关键作用。通过动态光散射（DLS）与17O NMR测得的一级动力学行为，可归因于[Wrich-agg]转化为尺寸介于20~30 nm的更大尺寸中间团簇聚集体[WOA]。经过长时间陈化（通常24小时后），SAXS与非原位TEM测试结果显示，体系形成了分形维数Df≈2的致密氧化钨凝胶网络，这符合反应限制型团簇-团簇聚集（RLCCA）的特征。从方法学角度而言，这种结合显微成像、散射、衍射与光谱学等多尺度技术的关联表征方法，可为阐明温和化学路线制备材料的生长机制提供重要参考。