Decoding Flat-Al₁₃ Structural Evolution in the Sol-Gel Process toward Aluminum Hydroxide Formation

Understanding the molecular mechanisms underlying the sol-gel transition of aluminum-based systems remains a long-standing challenge due to complex species transformations and aggregation dynamics. In this study, we uncover the critical role of the flat-Al₁₃ cluster, which is formed via pH-driven isomerization of ε-Al₁₃, in directing the structural evolution over the sol-gel transition and in guiding low-temperature crystallization. Comprehensive spectral characterizations reveal a two-stage transformation, that is, the conversion of the ε-Al₁₃ cluster to the flat-Al₁₃ cluster, followed by their oriented aggregation, ultimately forming a gel system characterized by global disorder coexisting with local ordering. Notably, gels enriched with flat-Al₁₃ exhibit significantly enhanced crystallinity and transform into layered aluminum hydroxide at temperatures lower than those of conventional systems. This energy-efficient pathway stems from the structural templating effect of flat-Al₁₃, which enables topological condensation to bypass the high-energy barrier of classical nucleation. These findings elucidate the structure-directing role of flat-Al₁₃ in bridging molecular-scale transformations to macroscopic material properties, establishing a mechanistic framework for the design of hierarchically structured aluminum-based materials through controlled sol-gel dynamics.