A Universal Lab-on-Salt-Particle Approach to 2D Single-Layer Ordered Mesoporous Materials

Maillard Reaction Inspired Microexplosion toward Fast Synthesis of Two-Dimensional Mesoporous Tin Oxides for Efficient Chemiresistive Gas Sensing

Two-dimensional (2D) mesoporous transition metal oxides are highly desired in various applications, but their fast and low-cost synthesis remains a great challenge. Herein, a Maillard reaction inspired microexplosion approach is applied to rapidly synthesize ultrathin 2D mesoporous tin oxide (mSnO2). During the microexplosion between granular ammonia nitrate with melanoidin at high temperature, the organic species can be carbonized and expanded rapidly due to the instantaneous release of gases, thus producing ultrathin carbonaceous templates with rich functional groups to effectively anchor SnO2 nanoparticles on the surface. The subsequent removal of carbonaceous templates via calcination in air results in the formation of 2D mSnO2 due to the confinement effect of the templates. Pd nanoparticles are controllably deposited on the surface of 2D mSnO2 via in situ reduction, forming ultrathin 2D Pd/mSnO2 nanocomposites with thicknesses of 6-8 nm. Owing to the unique 2D mesoporous structure with rich oxygen defects and highly exposed metal-metal oxide interfaces, 2D Pd/mSnO2 exhibits excellent sensing performance toward acetone with high sensitivity, a short response time, and good selectivity under low working temperature (100 °C). This fast and convenient microexplosion synthesis strategy opens up the possibility of constructing 2D porous functional materials for various applications including high-performance gas sensors.

A Novel Molten Salt Mediated Synthesis of Mesoporous Metal Oxides with High Crystallization

The controlled synthesis of mesoporous metal oxides remains a great challenge because the uncontrolled assembly process and high-temperature crystallization can easily destroy the mesostructure. Herein, we develop a facile, versatile, low-cost, and controllable molten salt assisted assembly strategy to synthesize mesoporous metal oxides (e.g., CeO2, ZrO2, SnO2, Li2TiO3) with high surface area (115-155 m2/g) and uniform mesopore size (3.0 nm). We find this molten salt mediated assembly enables the desolvation of the precursors and forms bare metal ions, enhances their coordination interaction with the surfactant, and promotes their assembly into a mesostructure. Furthermore, the molten salt assisted crystallization process can lower the collision probability of the target metal atom, inhibit its further growth into large crystals, and achieve a well-maintained mesostructure with high crystallization. Furthermore, this method can be expanded to synthesize various structured mesoporous metal oxides, including hollow spheres, nanotubes, and nanosheets by introducing the carbon template. The obtained mesoporous CeO2 microspheres loaded with Cu species exhibit excellent antibacterial performance and superior catalytic activity for the hydrogenation of nitrophenol with high conversion and cycling stability.

2D Ordered Mesoporous Lamellar Hetero-Nanochannels with Asymmetric Wettability for Controllable Ion Transport

Heterogeneous membranes play a crucial role in osmotic energy conversion by effectively reducing concentration polarization. However, most heterogeneous membranes mitigate concentration polarization through an asymmetric charge distribution, resulting in compromised ion selectivity. Herein, hetero-nanochannels with asymmetric wettability composed of 2D mesoporous carbon and graphene oxide are constructed. The asymmetric wettability of the membrane endows it with the ability to suppress the concentration polarization without degrading the ion selectivity, as well as achieving a diode-like ion transport feature. As a result, enhanced osmotic energy harvesting is achieved with a power density of 6.41 W m-2 . This represents a substantial enhancement of 102.80-137.85% when compared to homogeneous 2D membranes, surpassing the performance of the majority of reported 2D membranes. Importantly, the membrane can be further used for high-performance ionic power harvesting by regulating ion transport, exceeding previously reported data by 89.1%.

3D Carbon Microspheres with a Maze-Like Structure and Large Mesopore Tunnels Built From Rapid Aerosol-Confined Coherent Salt/Surfactant Templating

Hierarchically porous carbons with tailor-made properties are essential for applications wherein rich active sites and fast mass transfer are required. Herein, a rapid aerosol-confined salt/surfactant templating approach is proposed for synthesizing hierarchically porous carbon microspheres (HPCMs) with a maze-like structure and large mesopore tunnels for high-performance tri-phase catalytic ozonation. The confined assembly in drying microdroplets is crucial for coherent salt (NaCl) and surfactant (F127) dual templating without macroscopic phase separation. The HPCMs possess tunable sizes, a maze-like structure with highly open macropores (0.3-30 µm) templated from NaCl crystal arrays, large intrawall mesopore tunnels (10-45 nm) templated from F127, and rich micropores (surface area >1000 m2 g-1 ) and oxygen heteroatoms originated from NaCl-confined carbonization of phenolic resin. The structure formation mechanism of the HPCMs and several influencing factors on properties are elaborated. The HPCMs exhibit superior performance in gas-liquid-solid tri-phase catalytic ozonation for oxalate degradation, owing to their hierarchical pore structure for fast mass transfer and rich defects and oxygen-containing groups (especially carbonyl) for efficient O3 activation. The reactive oxygen species responsible for oxalate degradation and the influences of several structure parameters on performance are discussed. This work may provide a platform for producing hierarchically porous materials for various applications.

Insights into effects of grain boundary engineering in composite metal oxide catalysts for improving catalytic performance

Volatile Organic Compounds (VOCs) have long been a threat to human health. However, designing economical and efficient transition metal composite oxide catalysts for VOCs purification remains a challenge. Herein, this study demonstrates the enormous potential of grain boundary engineering in facilitating VOCs decomposition over ordered mesoporous composite oxide denoted as 3D-MnxCoy (x, y = 1, 3, 5, 7, 9). Specifically, the three-dimensional (3D) Mn7Co1 catalyst shows 100% ethyl acetate removal efficiency for a continuous airflow containing 1000 ppm ethyl acetate over 60000 h-1 space velocity at 160 °C. Mechanism study suggests that the high catalytic performance originates from the lattice distortion caused by the introduction of heteroatoms, along with the size effect of nanopore walls, which leads to the formation of various grain boundaries on the catalyst surface. The presence of grain boundaries facilitates the generation of oxygen vacancies, thus promoting the migration and activation of oxygen species. Furthermore, the near-atmospheric pressure X-ray photoelectron spectroscopy (NAP- XPS) monitoring results reveal that the bimetallic synergy enhanced by grain boundary accelerates the catalytic reaction rate of VOCs through Mn3++Co3+↔Mn4++Co2+ redox cycle. This study may shed light on the great potential of ordered mesoporous bimetallic oxide catalysts in VOCs pollution control.

Stepwise Monomicelle Assembly for Highly Ordered Mesoporous TiO2 Membranes with Precisely Tailored Mesophase and Porosity

Mesoporous materials with crystalline frameworks have been acknowledged as very attractive materials in various applications. Nevertheless, due to the cracking issue during crystallization and incompatible hydrolysis and assembly, the precise control for crystalline mesoscale membranes is quite infertile. Herein, we presented an ingenious stepwise monomicelle assembly route for the syntheses of highly ordered mesoporous crystalline TiO₂ membranes with delicately controlled mesophase, mesoporosity, and thickness. Such a process involves the preparation of monomicelle hydrogels and follows self-assembly by stepwise solvent evaporation, which enables the sensitive hydrolysis of TiO₂ oligomers and dilatory micelle assembly to be united. In consequence, the fabricated mesoporous TiO₂ membranes exhibit a broad flexibility, including tunable ordered mesophases (worm-like, hexagonal p6mm to body-centered cubic Im3̅m), controlled mesopore sizes (3.0-8.0 nm), and anatase grain sizes (2.3-8.4 nm). Besides, such mesostructured crystalline TiO₂ membranes can be extended to diverse substrates (Ti, Ag, Si, FTO) with tailored thickness. The great mesoporosity of the in situ fabricated mesoscopic membranes also affords excellent pseudocapacitive behavior for sodium ion storage. This study underscores a novel pathway for balancing the interaction of precursors and micelles, which could have implications for synthesizing crystalline mesostructures in higher controllability.

Functionalization of Mesoporous Semiconductor Metal Oxides for Gas Sensing: Recent Advances and Emerging Challenges

With the emerging of the Internet of Things, chemiresistive gas sensors have been extensively applied in industrial production, food safety, medical diagnosis, and environment detection, etc. Considerable efforts have been devoted to improving the gas-sensing performance through tailoring the structure, functions, defects and electrical conductivity of sensitive materials. Among the numerous sensitive materials, mesoporous semiconductor metal oxides possess unparalleled properties, including tunable pore size, high specific surface area, abundant metal-oxygen bonds, and rapid mass transfer/diffusion behavior (Knudsen diffusion), which have been regarded as the most potential sensitive materials. Herein, the synthesis strategies for mesoporous metal oxides are overviewed, the classical functionalization techniques of sensitive materials are also systemically summarized as a highlight, including construction of mesoporous structure, regulation of micro-nano structure (i.e., heterojunctions), noble metal sensitization (e.g., Au, Pt, Ag, Pd) and heteroatomic doping (e.g., C, N, Si, S). In addition, the structure-function relationship of sensitive materials has been discussed at molecular-atomic level, especially for the chemical sensitization effect, elucidating the interface adsorption/catalytic mechanism. Moreover, the challenges and perspectives are proposed, which will open a new door for the development of intelligent gas sensor in various applications.

Unusual Role of the Surfactant in the Self-Assembly of Pt Alloy in Ordered Mesoporous Carbon: Tuning the Nanocluster Size

In the one-step self-assembly synthesis of metallic nanocrystals in ordered mesoporous carbon (OMC), the surfactant functionalizes well as the structure directing agent and mesopore template. Interestingly, this work demonstrates another unusual role the surfactant plays: tuning the size of the nanocrystals. Our investigation shows that the decreasing molecular weight of the PS segment of PEO-b-PS leads to sequentially reduced PtRu particle sizes of 4.4, 3.9, and 2.9 nm, while F127 which has a distinctly smaller hydrophobic PO domain with a bending structure in the micelles successfully results in sub-2 nm PtM (M = Ru, Ir, Rh, Pd) nanoclusters in OMC. This well indicates that the nanocluster size is largely decided by the volume of the hydrophobic segment of the surfactant to which the metallic precursor is linked. The smaller the volume, the fewer the precursor molecules are adsorbed, and the smaller the alloy nanoclusters. In the electrocatalytic methanol oxidation reaction, the mass activity of PtRu-1.6/OMC with 1.6 nm PtRu clusters at 0.87 V reaches 1.07 A mg_Pt^-1, which is 2.9 times that of commercial PtRu/C with an average alloy size of 2.7 nm. In principle, a wide range of ultrafine metallic clusters embedded in OMC can be prepared via this route for various applications.

Dynamic Coassembly of Amphiphilic Block Copolymer and Polyoxometalates in Dual Solvent Systems: An Efficient Approach to Heteroatom-Doped Semiconductor Metal Oxides with Controllable Nanostructures

Dynamic coassembly of block copolymers (BCPs) with Keggin-type polyoxometalates (POMs) is developed to synthesize heteroatom-doped tungsten oxide with controllable nanostructures, including hollow hemispheres, nanoparticles, and nanowires. The versatile coassembly in dual n-hexane/THF solvent solution enables the fomation of poly(ethylene oxide)-b-polystyrene (PEO-b-PS)/POMs (e.g., silicotungstic acid, H₄SiW₁₂O₄₀) nanocomposites with different morphologies such as spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles, which can be readily converted into diverse nanostructured metal oxides with high surface area and unique properties via in situ thermal-induced structural evolution. For example, uniform silicon-doped WO₃ (Si-WO₃) hollow hemispheres derived from coassembly of PEO-b-PS with H₄SiW₁₂O₄₀ were utilized to fabricate gas sensing devices which exhibit superior gas sensing performance toward acetone, thanks to the selective gas-solid interface catalytic reaction that induces resistance changes of the devices due to the high specific surface areas, abundant oxygen vacancies, and the Si-doping induced metastable ε-phase of WO₃. Furthermore, density functional theory (DFT) calculation reveals the mechanism about the high sensitivity and selectivity of the gas sensors. On the basis of the as-fabricated devices, an integrated gas sensor module was constructed, which is capable of real-time monitoring the environmental acetone concentration and displaying relevant sensing results on a smart phone via Bluetooth communication.

Constructing Unique Mesoporous Carbon Superstructures via Monomicelle Interface Confined Assembly

Constructing hierarchical three-dimensional (3D) mesostructures with unique pore structure, controllable morphology, highly accessible surface area, and appealing functionality remains a great challenge in materials science. Here, we report a monomicelle interface confined assembly approach to fabricate an unprecedented type of 3D mesoporous N-doped carbon superstructure for the first time. In this hierarchical structure, a large hollow locates in the center (∼300 nm in diameter), and an ultrathin monolayer of spherical mesopores (∼22 nm) uniformly distributes on the hollow shells. Meanwhile, a small hole (4.0-4.5 nm) is also created on the interior surface of each small spherical mesopore, enabling the superstructure to be totally interconnected. Vitally, such interconnected porous supraparticles exhibit ultrahigh accessible surface area (685 m² g^-1) and good underwater aerophilicity due to the abundant spherical mesopores. Additionally, the number (70-150) of spherical mesopores, particle size (22 and 42 nm), and shell thickness (4.0-26 nm) of the supraparticles can all be accurately manipulated. Besides this spherical morphology, other configurations involving 3D hollow nanovesicles and 2D nanosheets were also obtained. Finally, we manifest the mesoporous carbon superstructure as an advanced electrocatalytic material with a half-wave potential of 0.82 V (vs RHE), equivalent to the value of the commercial Pt/C electrode, and notable durability for oxygen reduction reaction (ORR).

Hard-templated engineering of versatile 2D amorphous metal oxide nanosheets

Two-dimensional (2D) nanomaterials have received ever-increasing attention and in-depth exploration in multifarious fields on account of their superior mass transfer ability and abundant catalytic-active sites. Especially, the amorphous 2D nanomaterials feature unique properties distinct from atomic crystalline materials. However, the synthesis of high-quality and large-sized amorphous 2D nanomaterials encounters a big challenge. Here, a general and facile synthetic strategy for a series of 2D amorphous metal and nonmetallic oxides nanosheets, including SiO₂, AlOOH, ZrO₂and TiO₂nanosheets, is reported. The versatile 2D amorphous nanomaterials are fabricatedviamanipulating the surface energy of relevant metal alkoxide precursors with liquid feature and controlling the related synthesis parameters to form solid 2D amorphous nanosheets byin situhydrolysis and condensation of precursors. Density functional theory (DFT) calculations reveal the molecular adsorption mechanism of wetting process of precursor infiltrated on solid NaCl substrate, which attributes to the strong interaction between Na-O atom pairs from NaCl and metal alkoxides respectively. Furthermore, taking the 2D Fe-ZrO₂nanomaterials as the catalyst, the excellent catalytic performance for Rhodamine B (RhB) degradation illustrates that these 2D nanomaterials prepared by this method have the characteristics of easy functionalization. This work provides an efficient strategy for nanomaterials functionalization during 2D nanosheets synthetic process and further being applied in catalysis-related field and beyond.

Self-Assembly of Ir-Based Nanosheets with Ordered Interlayer Space for Enhanced Electrocatalytic Water Oxidation

Iridium (Ir)-based electrocatalysts are widely explored as benchmarks for acidic oxygen evolution reactions (OERs). However, further enhancing their catalytic activity remains challenging due to the difficulty in identifying active species and unfavorable architectures. In this work, we synthesized ultrathin Ir-IrO_x/C nanosheets with ordered interlayer space for enhanced OER by a nanoconfined self-assembly strategy, employing block copolymer formed stable end-merged lamellar micelles. The interlayer distance of the prepared Ir-IrO_x/C nanosheets was well controlled at ∼20 nm and Ir-IrO_x nanoparticles (∼2 nm) were uniformly distributed within the nanosheets. Importantly, the fabricated Ir-IrO_x/C electrocatalysts display one of the lowest overpotential (η) of 198 mV at 10 mA cm^-2_geo during OER in an acid medium, benefiting from their features of mixed-valence states, rich electrophilic oxygen species (O^(II-δ)-), and favorable mesostructured architectures. Both experimental and computational results reveal that the mixed valence and O^(II-δ)- moieties of the 2D mesoporous Ir-IrO_x/C catalysts with a shortened Ir-O^(II-δ)- bond (1.91 Å) is the key active species for the enhancement of OER by balancing the adsorption free energy of oxygen-containing intermediates. This strategy thus opens an avenue for designing high performance 2D ordered mesoporous electrocatalysts through a nanoconfined self-assembly strategy for water oxidation and beyond.

Mesoporous perovskite titanates via hydrothermal conversion

We demonstrate the successful hydrothermal conversion of mesoporous TiO₂ to mesoporous perovskite SrTiO₃. This method allows for control of pore size distribution and can be readily applied for the preparation of other mesoporous titanates such as BaTiO₃ and Li₂TiO₃. Such high-surface perovskites have potential in high-temperature applications due to their thermal stability.

Interfacial Assembly and Applications of Functional Mesoporous Materials

Functional mesoporous materials have gained tremendous attention due to their distinctive properties and potential applications. In recent decades, the self-assembly of micelles and framework precursors into mesostructures on the liquid-solid, liquid-liquid, and gas-liquid interface has been explored in the construction of functional mesoporous materials with diverse compositions, morphologies, mesostructures, and pore sizes. Compared with the one-phase solution synthetic approach, the introduction of a two-phase interface in the synthetic system changes self-assembly behaviors between micelles and framework species, leading to the possibility for the on-demand fabrication of unique mesoporous architectures. In addition, controlling the interfacial tension is critical to manipulate the self-assembly process for precise synthesis. In particular, recent breakthroughs based on the concept of the "monomicelles" assembly mechanism are very promising and interesting for the synthesis of functional mesoporous materials with the precise control. In this review, we highlight the synthetic strategies, principles, and interface engineering at the macroscale, microscale, and nanoscale for oriented interfacial assembly of functional mesoporous materials over the past 10 years. The potential applications in various fields, including adsorption, separation, sensors, catalysis, energy storage, solar cells, and biomedicine, are discussed. Finally, we also propose the remaining challenges, possible directions, and opportunities in this field for the future outlook.

Self-Hybrid Transition Metal Oxide Nanosheets Synthesized by a Facile Programmable and Scalable Carbonate-Template Method

2D transition metal oxides (TMO) nanosheets have attracted considerable attention in both fundamental research and practical applications. Herein, a convenient programmable and scalable carbonate crystals templating synthesis is developed to produce high-quality self-hybrid TMO nanosheets (Si-WO_{3- x} , Ta_x O_y , Mn_x O_y ) and their respective polymetallic oxide hybrid nanosheets with tunable composition, low-cost and high-yield. Taking tungsten oxide nanosheets as example, silicotungstic acid precursor is in situ converted into tungsten oxide nanosheets like scales on the surface of calcium carbonate crystals through the simple soaking-drying-calcination process, and after selectively dissolving calcium carbonate by etching, the dispersive tungsten oxide nanosheets with unique self-hybrid Si-doped h-WO₃ /ε-WO₃ /WO₂ compositions are obtained, which show excellent acetone gas-sensing performances at low temperatures. This carbonate-template method opens up the possibility to economically produce various functional TMO nanosheets with specific compositions for diverse applications.

Nanomeshes with Sub-10 nm Pores by Glycerol-Triggered 2D Assembly in Liquid Phases for Fast and Selective Membranes

Nanomeshes having ultrathin thicknesses and penetrating nanopores promise fast diffusion and precise selectivity and are highly desired in diffusion-involved processes such as separation. Herein, we report a liquid-phase two-dimensional (2D) assembly strategy to synthesize phenolic and carbonaceous nanomeshes with sub-10 nm pores and thicknesses. The synthesis is enabled simply by introducing glycerol in the thermopolymerization of resol/polyether micelles dispersed in ethanol. Experimental and simulation results reveal that glycerol's strong ability to form hydrogen bonds constrain the motion of the micelles, directing them to pack and merge exclusively in the lateral direction. Upon removal of polyether, we obtain phenolic nanomeshes with lateral sizes up to hundreds of micrometers, which can be further converted to carbonaceous nanomeshes. As a proof of concept, we use stacked phenolic and carbonaceous nanomeshes as separation membranes. They show superior permselectivity to nanosized solutes with permeance ∼2-110 times higher than that of other membranes.

General and Efficient Synthesis of Two-Dimensional Monolayer Mesoporous Materials with Diverse Framework Compositions

Two-dimensional (2D) mesoporous materials have received substantial research interest due to their highly exposed active sites and unusual nanoconfinement effect. However, controllable and efficient synthesis of 2D mesoporous materials and investigation of their intrinsic properties have remained quite rare. Herein, a general and effective surface-limited cooperative assembly (SLCA) method enabled by leveling precursor solutions on KCl crystals via centrifugation is employed to conveniently synthesize two-dimensional (2D) monolayer mesoporous materials with different compositions. This novel strategy is performed in a manner similar to spin coating, not only enabling generation of ultrathin mesostructured composite film on KCl particles and recycling excessive precursor solution but also providing favorable solvent annealing environment for the film to form ordered mesostructures. Taking monolayer mesoporous Ce_0.8Zr_0.2O₂ solid solutions as a sample, they display ultrathin nanosheet morphology with a thickness of ∼20 nm, highly open porous structure, and easily accessible active sites of surface superoxide species. Upon decoration of 2D mesoporous Ce_0.8Zr_0.2O₂ nanosheets with Pt nanoparticles, the obtained catalyst exhibits superior catalytic activity and stability toward CO oxidation with a low onset temperature of 30 °C and a 100% conversion temperature of 95 °C, which are 35-70 °C lower than those for their counterpart materials, namely, three-dimensional (3D) mesoporous Pt/Ce_0.8Zr_0.2O₂. Moreover, their TOF_Pt value is ∼11.3 times higher than that of 3D mesoporous Pt/Ce_0.8Zr_0.2O₂. Characterizations based on various techniques indicate that such an outstanding catalytic performance is due to the ultrashort distance (20 nm) of mass diffusion, highly exposed active sites, rich surface-chemisorbed oxygen, and the synergistic effect between the Ce_0.8Zr_0.2O₂ matrix and Pt species.

Facile synthesis of a mesoporous TiO2 film templated by a block copolymer for photocatalytic applications

Large-sized mesoporous TiO2 through a polymeric micelle assembly approach for photocatalysts.

Mechanochemical redox: a calcination-free process to support CoMnOx catalysts

A calcination-free process (mechanochemical redox loading method) to load CoMnO_x onto various supports for efficient and stable CO oxidation.

Self-assembly of block copolymers towards mesoporous materials for energy storage and conversion systems

This paper reviews the progress in the field of block copolymer-templated mesoporous materials, including synthetic methods, morphological and pore size control and their potential applications in energy storage and conversion devices.