The creation of periodic nanoarchitectures with rich compositions presents a significant challenge in materials science. Traditional methods, relying on direct host-guest interactions like covalent bonds, hydrogen bonding, π-π stacking, and Coulombic interactions, often require stringent synthesis parameters and lack flexibility. While the assembly of acidic POMs and copolymers using Coulombic attraction has shown promise in synthesizing mMOs, this approach is limited in its synthesis conditions. This research aims to address these limitations by developing a general and versatile method for creating mMOs with rich compositions and diverse pore structures. The importance of this research lies in its potential to enable the design of advanced materials with tailored properties for various applications, including catalysis and sensing. The creation of a flexible and widely applicable synthesis method for mesoporous metal oxides addresses a significant gap in existing methodologies, pushing the boundaries of materials design and opening avenues for innovative applications.

Previous research has explored various methods for creating mesoporous materials, including those that utilize direct host-guest interactions between templates and precursors. However, these methods often suffer from limitations in terms of flexibility, control over synthesis parameters, and the range of achievable compositions. The use of polyoxometalates (POMs) in the synthesis of mMOs has shown some promise, particularly using Coulombic interactions between acidic POMs and protonated copolymers. However, this approach has limitations in terms of the range of synthesis conditions under which it is effective. This literature review highlights the need for a more general and versatile approach to creating mMOs with diverse compositions and structures, paving the way for the innovative "solvent-pair surfactants" method presented in this study. Existing methods, while contributing valuable insights, fall short in providing a universal solution for the synthesis of mMOs with the desired complexity and control.

This study introduces a novel "solvent-pair surfactants" enabled assembly (SPEA) method. The key innovation is the use of a binary solvent system, such as DMF/H₂O, which forms molecule-like complexes. These complexes act as "solvent-pair surfactants", dissolving non-acidic POMs in the binary solvent. The hydrophilic and DMF-phobic nature of the POMs enables their dissolution, and their interaction with the hydrophilic PEO segments of spherical PEO-b-PS micelles promotes ordered POMs/AB copolymers mesostructures. Solvent evaporation and subsequent thermal treatment converts POMs into highly crystalline metal oxides, while counter-cations participate in the process, forming heteroatom-doped or alkali metal-intercalated mMOs. The study employs various characterization techniques, including FESEM, TEM, XRD, XPS, N₂ adsorption-desorption, FTIR, and DFT calculations to analyze the structure, composition, and properties of the synthesized mMOs. The synthesis of mN-WO₃ using (NH₄)₆H₂W₁₂O₄₀ and PEO₁₁₄-b-PS₂₀₀ in a DMF/H₂O system is detailed as a proof-of-concept, demonstrating the successful formation of ordered mesostructures with uniform spherical pores, iso-oriented nanocrystalline walls, and a homogeneous N-doped framework. Gas sensing tests are conducted to evaluate the performance of the obtained mN-WO₃. The generality of the SPEA approach is explored by employing different precursors (POMs or molecular metal salts) and AB copolymers, showcasing the flexibility of the method in designing mMOs with adjustable pore sizes and compositions. Detailed experimental procedures, including chemical and materials specifications, the synthesis of ordered mesoporous nitrogen-doped WO₃ (mN-WO₃), and characterization and measurement methods (FESEM, TEM, XRD, XPS, N₂ adsorption-desorption, FTIR, etc.), are included in the supplementary information.

The study successfully demonstrates the SPEA method for synthesizing mMOs, achieving highly ordered mesostructures with tunable properties. The use of a binary solvent system forms molecule-like complexes that act as "solvent-pair surfactants", enabling the dissolution of non-acidic POMs and facilitating their assembly with AB copolymers. The synthesized mN-WO₃ exhibits an fcc arranged mesoporous structure with uniform pores (12.1–33.2 nm), iso-oriented nanocrystalline walls, and a homogeneous N-doped framework. Characterizations confirm the uniform distribution of elements, high thermal stability, and the formation of ferroelectric ε-WO₃ due to nitrogen doping. The N-doping leads to a reduction in the band gap, improving the material's properties. Importantly, mN-WO₃ demonstrates superior gas-sensing performance towards acetone, exhibiting high sensitivity, selectivity, and fast response-recovery dynamics. The SPEA method is shown to be generalizable, adaptable to various POMs, and AB copolymers, resulting in mMOs with various pore sizes, structures (spherical, cylindrical, lamellar), and compositions (heteroatom-doped, composite, noble metal-loaded). The ability to tune the pore size (12.1-33.2 nm) by adjusting the PS segments of the PEO-b-PS copolymer is a key finding, showcasing the method's versatility in tailoring material properties. The superior gas sensing performance of mN-WO₃, particularly its high sensitivity even at ppb-level acetone detection, surpasses that of the control sample (mWO₃), highlighting the benefits of the SPEA-synthesized material.

The findings demonstrate the efficacy of the SPEA method in addressing the limitations of traditional mMO synthesis techniques. The indirect interaction between POMs and AB copolymers, mediated by the "solvent-pair surfactants", allows for a facile and flexible approach to designing mMOs with tailored properties. The superior gas-sensing performance of mN-WO₃ highlights the potential of the SPEA method in producing advanced materials for various applications. The ability to tune the pore size and composition expands the possibilities for designing mMOs with specific functionalities. The use of DFT calculations to understand the interaction between the solvent pairs and the formation of the "solvent-pair surfactants" provides crucial mechanistic insights. The generality of the SPEA method opens up avenues for exploring a wider range of materials and functionalities, contributing significantly to the field of mesoporous material synthesis. The observed superior gas sensing performance compared to traditionally synthesized mWO₃ underscores the advantages of the SPEA method in producing high-performance materials.

This study successfully demonstrates the "solvent-pair surfactants" enabled assembly (SPEA) method as a versatile and general approach to synthesize functional mesoporous metal oxides (mMOs). The method's flexibility in controlling pore size and composition, along with the superior performance of the synthesized mN-WO₃ in acetone sensing, highlights its potential for designing advanced materials. Future research could explore the application of SPEA to other classes of inorganic precursors and block copolymers, further expanding the library of accessible mMOs and exploring new applications. Investigating the scalability and industrial applicability of the SPEA method would be crucial for wider implementation.

While the SPEA method demonstrates significant potential, certain limitations should be considered. The current study primarily focuses on a limited set of POMs and AB copolymers. Further research is needed to fully explore the method's compatibility with a wider range of materials. A detailed understanding of the dynamic equilibrium involved in the self-assembly process and its influence on the final structure is necessary. The scalability and cost-effectiveness of the method need further investigation to assess its industrial feasibility.