sp2-Hybridized Carbon-Containing Block Copolymer Templated Synthesis of Mesoporous Semiconducting Metal Oxides with Excellent Gas Sensing Property

In recent years, rational design of ordered mesoporous metal oxides, especially metal oxide semiconductors with adjustable pore architecture and framework compositions, has aroused extensive research interest owing to their unique electronic structures, long-range ordered porous framework, uniform mesopore size, and high specific surface area. Research on mesoporous materials has been booming in the past 30 years, and many synthesis methods have been developed, such as templating methods based on amphiphilic copolymers as soft templates or mesoporous carbon/silica as hard templates, respectively. Soft-templating synthesis has been considered as one of the most efficient and flexible methods in designing ordered mesoporous materials through the controllable interfacial induced coassembly process. However, most commercial amphiphilic copolymers, such as poly(ethylene oxide)- b-poly(propylene oxide) based Pluronic-type ones, suffer the drawback of poor thermal stability, because they are too easy to be decomposed even in inert atmosphere. Therefore, they are difficult to support the structures of mesoporous metal oxides under high calcination temperatures (>400 °C). To solve this challenge, we designed new amphiphilic block copolymers with high content of sp2-hybridized carbon in the hydrophobic segments that were relatively stable and could be in situ converted into residual carbon to support the mesoporous structure, via living free radical polymerization. We developed a variety of novel synthesis methods based on sp2-hybridized carbon-containing block copolymer, such as ligand-assisted assembly and resol-assisted assembly strategies, achieving a controllable and versatile synthesis of mesoporous semiconducting metal oxides with excellent gas sensing performance. In this Account, we first outline the features of sp2-hybridized carbon-containing block copolymers synthesized via living free radical polymerization, particularly their pyrolysis behavior in converting into residual carbon. Combining the solvent evaporation induced coassembly and the carbon-supported crystallization strategies, we realized the rational design of various ordered mesoporous semiconducting metal oxides (e.g., WO3, SnO2, Co3O4, In2O3, TiO2, ZnO) and the regulation of their architectural features. To overcome the fast hydrolysis rate of metal precursors and weak interaction between block copolymers and metal precursors, we developed efficient ligand-assisted (e.g., acetylacetone and acetic acid) coassembly and resol-assisted coassembly methods to retard hydrolysis behavior and enhance the interaction via hydrogen bonds, covalent bonds, electrostatic interactions, etc. We also highlight the applications of these ordered mesoporous semiconducting metal oxides of both n-type and p-type in gas sensing fields, and they show tremendous sensing performance due to their abundant active sites on electron depletion layer and rapid gas diffusion via accessible pore channels. Finally, on the basis of the classic surface-electron depletion layer model, we elucidated in depth the surface catalytic reactions between the target gas molecules and the activated species (e.g., the adsorbed oxygen species) in the surface of mesoporous metal oxides during sensing process. These newly developed soft-templating synthesis methods that rely on sp2-hybridized carbon-containing block copolymers will open a new avenue for the design and application of ordered mesoporous semiconducting metal oxides in various fields.