Unconventional soft-templating strategies for mesoporous metal oxides: beyond evaporation-induced self-assembly and calcination-based conversion - Polymer Journal

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Mesoporous metal oxides (MMOs) have been widely used in energy storage, catalysis, and sensing applications owing to their large surface areas, short ion diffusion paths, and tunable physicochemical properties [1,2,3,4,5,6,7,8,9]. To benefit from these structural advantages, various synthetic methods have been developed to construct mesoporous architectures in metal oxide systems. Templating-based methods can be classified into hard- and soft-templating methods. Hard-templating methods rely on prefabricated sacrificial scaffolds, which involve multistep processing and, as a result, limit structural and compositional flexibility [10,11,12]. These limitations have motivated the widespread use of soft-templating methods, in which amphiphilic block copolymers (BCPs) or surfactants act as structure-directing agents [13,14,15,16,17,18,19]. Compared with hard-templating approaches, soft-templating approaches eliminate the need for prefabricated sacrificial templates, enabling greater flexibility in controlling composition, pore architecture, and film geometry.

Among soft-templating approaches, evaporation-induced self-assembly (EISA), which is based on the coassembly of BCPs and inorganic precursors, is the most widely used platform for synthesizing MMOs [20,21,22]. In a typical EISA process, solvent evaporation induces microphase separation of BCPs, during which metal oxide precursors selectively interact with hydrophilic blocks to create organic-inorganic mesostructured composites. These composites are subsequently converted to MMOs by high-temperature thermal treatment, during which the polymer templates are removed and inorganic frameworks are synthesized through precursor condensation and crystallization. On the basis of this platform, a wide range of approaches have been developed to precisely control the composition and morphology of MMOs [23,24,25].

Despite the substantial progress, conventional EISA-based syntheses face intrinsic challenges at each processing stage, primarily stemming from their reliance on solvent evaporation for self-assembly and high-temperature processing for conversion. In the self-assembly step, structure formation relies heavily on the use and evaporation of organic solvents, which are often toxic or highly volatile, resulting in the challenges associated with environmental burden, operator health, and process sustainability, which hinder industrial implementation [26,27,28,29,30,31,32]. In the conversion step, the high-temperature treatments required for template removal and the formation of inorganic frameworks via condensation or crystallization result in substantial energy consumption, reduce the applicability of metal oxides that crystallize rapidly even at relatively low temperatures, and essentially reduce their compatibility with flexible or polymeric substrates, thereby limiting material versatility and direct film formation for flexible and device-oriented applications [23, 24, 33,34,35,36,37,38,39,40].

To mitigate these limitations, unconventional soft-templating approaches have recently been explored. In this study, we summarize the basic concepts of soft-templated MMO synthesis and discuss recent unconventional methods by highlighting (i) emerging self-assembly approaches that reduce the dependence on solvent evaporation and expand accessible processing pathways and (ii) alternative conversion processes that reduce energy and substrate compatibility constraints. We summarize the remaining challenges and future directions that soft-templating methods must address for greater applicability and practical implementation.