Bridging Molecule Assisted Organic–Inorganic Interface Coassembly to Rationally Construct Metal Oxide Mesostructures

Mesostructured materials exhibit unique properties and attract great attention in many applications, but it is still challenging to synthesize mesostructured late transition metal oxides (e.g., ZnO and CuO) based on the conventional coassembly of surfactants and corresponding molecular precursors. In this work, a bridging molecule assisted coassembly strategy was developed by using ligand-capped crystalline ZnO and CuO nanocrystals (NCs) as a building block to assemble structure directing agent block copolymers (BCPs). Various mesostructured materials, including mesoporous metal oxide films and striped ellipsoidal particles, were obtained in elaborately controlled synthesis. Particularly, the structure variation under different conditions was systematically investigated by manipulating colloidal NCs–BCPs interface interactions during coassembly. Through calcination treatment to selectively decompose BCPs, a mesoporous metal oxide can be readily obtained. Taking the obtained mesoporous ZnO as an example, it exhibits excellent acetone sensing performance with high sensitivity and superior selectivity under a low working temperature (180 °C), because of the advantages of a high specific surface area (92 m2/g), rich active sites, and the unique NCs assembled framework. This bottom-up NCs–BCPs interface assembly approach can be well expanded to construct other mesostructure systems (e.g., noble metals and metal oxides–metal nanocrystal heterojunctions), serving as a universal methodology for the rational design of functional mesoporous materials with rich structural and compositional diversities

Bottom-Up Construction of Mesoporous Cerium-Doped Titania with Stably Dispersed Pt Nanocluster for Efficient Hydrogen Evolution

Hydrogen generation is one of the crucial technologies to realize sustainable energy development, and the design of advanced catalysts with efficient interfacial sites and fast mass transfer is significant for hydrogen evolution. Herein, an in situ coassembly strategy was proposed to engineer a cerium-doped ordered mesoporous titanium oxide (mpCe/TiO2), of which the abundant oxygen vacancies (Ov) and highly exposed active pore walls contribute to good stability of ultrasmall Pt nanoclusters (NCs, ∼ 1.0 nm in diameter) anchored in the uniform mesopores (ca. 20 nm). Consequently, the tailored mpCe/TiO2 with 0.5 mol % Ce-doping-supported Pt NCs (Pt-mpCe/TiO2-0.5) exhibits superior H2 evolution performance toward the water–gas shift reaction with a 0.73 molH2·s–1·molPt–1 H2 evolution rate at 200 °C, which is almost 6-fold higher than the Pt-mpTiO2 (0.13 molH2·s–1·molPt–1 H2). Density functional theory calculations confirm that the structure of Ce-doped TiO2 with Ce coordinated to six O atoms by substituting Ti atoms is thermodynamically favorable without the deformation of Ti–O bonds. The Ov generated by the six O atom-coordinated Ce doping is highly active for H2O dissociation with an energy barrier of 2.18 eV, which is obviously lower than the 2.37 eV for the control TiO2. In comparison with TiO2, the resultant Ce/TiO2 support acts as a superior electron acceptor for Pt NCs and causes electron deficiency at the Pt/support interface with a 0.17 eV downshift of the Pt d-band center, showing extremely obvious electronic metal–support interaction (EMSI). As a result, abundant and hyperactive Ti3+-Ov(-Ce3+)-Ptδ+ interfacial sites are formed to significantly promote the generation of CO2 and H2 evolution. In addition, the stronger EMSI between Pt NCs and mpCe/TiO2-0.5 than that between Pt and mpTiO2 contributes to the superior self-enhanced catalytic performance during the cyclic test, where the CO conversion at 200 °C increases from 72% for the fresh catalyst to 99% for the used one. These findings reveal the subtle relationship between the mesoporous metal oxide-metal composite catalysts with unique chemical microenvironments and their catalytic performance, which is expected to inspire the design of efficient heterogeneous catalysts

Cementing Mesoporous ZnO with Silica for Controllable and Switchable Gas Sensing Selectivity

Nanostructured ZnO semiconductors as gas sensing materials have attracted great attention due to their high sensitivities, especially to reducing gases. However, ZnO based gas sensors lack controllable sensing selectivity. Herein, for the first time novel silica-cemented mesoporous ZnO materials with different contents of silica, high surface areas, and well-interconnected pores (∼29 nm) are synthesized through the evaporation-induced co-assembly (EICA) approach, and these amorphous ZnO materials exhibit controlled selectivity to ethanol or acetone. Strikingly, pure ZnO is found to exhibit better sensitivity to ethanol than that of acetone, while 2 wt % silica cemented mesoporous ZnO exhibits oppositely a selectively higher response to acetone than that of ethanol. In situ gas chromatograph–mass spectrum (GC-MS) analysis during the sensing process, in combination with intelligent gravimetric analyzer (IGA) measurement, reveals that such a preferential enhancement of acetone sensitivity by silica modification is mainly attributed to the dramatically improved adsorption of polar acetone molecules with a larger dipole moment of 2.88 D on the silica-cemented ZnO materials with higher surface polarity imparted by rich Zn–O–Si–OH bonds, and the acetone sensing process on pure ZnO and silica-cemented ZnO is found to experience a different reaction pathway

Noble Metal Nanoparticles Decorated Metal Oxide Semiconducting Nanowire Arrays Interwoven into 3D Mesoporous Superstructures for Low-Temperature Gas Sensing

Mesoporous materials have been extensively studied for various applications due to their high specific surface areas and well-interconnected uniform nanopores. Great attention has been paid to synthesizing stable functional mesoporous metal oxides for catalysis, energy storage and conversion, chemical sensing, and so forth. Heteroatom doping and surface modification of metal oxides are typical routes to improve their performance. However, it still remains challenging to directly and conveniently synthesize mesoporous metal oxides with both a specific functionalized surface and heteroatom-doped framework. Here, we report a one-step multicomponent coassembly to synthesize Pt nanoparticle-decorated Si-doped WO3 nanowires interwoven into 3D mesoporous superstructures (Pt/Si-WO3 NWIMSs) by using amphiphilic poly­(ethylene oxide)-block-polystyrene (PEO-b-PS), Keggin polyoxometalates (H4SiW12O40) and hydrophobic (1,5-cyclooctadiene)­dimethylplatinum­(II) as the as structure-directing agent, tungsten precursor and platinum source, respectively. The Pt/Si-WO3 NWIMSs exhibit a unique mesoporous structure consisting of 3D interwoven Si-doped WO3 nanowires with surfaces homogeneously decorated by Pt nanoparticles. Because of the highly porous structure, excellent transport of carriers in nanowires, and rich WO3/Pt active interfaces, the semiconductor gas sensors based on Pt/Si-WO3 NWIMSs show excellent sensing properties toward ethanol at low temperature (100 °C) with high sensitivity (S = 93 vs 50 ppm), low detection limit (0.5 ppm), fast response–recovery speed (17–7 s), excellent selectivity, and long-term stability

Polymerization-Induced Aggregation Approach toward Uniform Pd Nanoparticle-Decorated Mesoporous SiO₂/WO₃ Microspheres for Hydrogen Sensing

Hydrogen as an important clean energy source with a high energy density has attracted extensive attention in fuel cell vehicles and industrial production. However, considering its flammable and explosive property, gas sensors are desperately desired to efficiently monitor H2 concentration in practical applications. Herein, a facile polymerization-induced aggregation strategy was proposed to synthesize uniform Si-doped mesoporous WO3 (Si-mWO3) microspheres with tunable sizes. The polymerization of the melamine–formaldehyde resin prepolymer (MF prepolymer) in the presence of silicotungstic acid hydrate (abbreviated as H4SiW) leads to uniform MF/H4SiW hybrid microspheres, which can be converted into Si-mWO3 microspheres through a simple thermal decomposition treatment process. In addition, benefiting from the pore confinement effect, monodispersed Pd-decorated Si-mWO3 microspheres (Pd/Si-mWO3) were subsequently synthesized and applied as sensitive materials for the sensing and detection of hydrogen. Owing to the oxygen spillover effect of Pd nanoparticles, Pd/Si-mWO3 enables adsorption of more oxygen anions than pure mWO3. These Pd nanoparticles dispersed on the surface of Si-mWO3 accelerated the dissociation of hydrogen and promoted charge transfer between Pd nanoparticles and WO3 crystal particles, which enhanced the sensing sensitivity toward H2. As a result, the gas sensor based on Pd/Si-mWO3 microspheres exhibited excellent selectivity and sensitivity (Rair/Rgas = 33.5) to 50 ppm H2 at a relatively low operating temperature (210 °C), which was 30 times higher than that of the pure Si-mWO3 sensor. To develop intelligent sensors, a portable sensor module based on Pd/Si-mWO3 in combination with wireless Bluetooth connection was designed, which achieved real-time monitoring of H2 concentration, opening up the possibility for use as intelligent H2 sensors

Polymerization-Induced Colloid Assembly Route to Iron Oxide-Based Mesoporous Microspheres for Gas Sensing and Fenton Catalysis

Iron oxide materials have wide applications due to their special physicochemical properties; however, it is a great challenge to synthesize mesoporous iron oxide-based microspheres conveniently and controllably with high surface area, large pore volume, and interconnected porous structures. Herein, mesoporous α-Fe₂O₃-based microspheres with high porosity are synthesized via a facile polymerization induced colloid assembly method through polymerization of urea–formaldehyde resin (UF resin) and its simultaneously cooperative assembly with Fe­(OH)₃ colloids in an aqueous solution, followed by subsequent thermal treatment. Remarkably, by controlling the cross-linking degree of UF, pure mesoporous α-Fe₂O₃ and α-Fe₂O₃/carbon hybrid microspheres can be synthesized controllably, respectively. They exhibit a uniform spherical morphology with a particle size of around 1.0 μm, well-interconnected mesopores (24.5 and 8.9 nm, respectively), and surface area of 54.4 m²/g (pure mFe₂O₃ microspheres) and 144.7 m²/g (hybrids), respectively. As a result, mesoporous pure α-Fe₂O₃ microspheres exhibited excellent H₂S sensing performance with a good selectivity, high response to low concentration H₂S at 100 °C, and quick response (4 s)/recovery (5 s) dynamics owing to the high surface area, open mesopores, and crystalline structure of the n-type α-Fe₂O₃ semiconductor. Moreover, mesoporous α-Fe₂O₃/carbon hybrid microspheres were used as a novel Fenton-like catalyst for the decomposition of methylene blue in a mild condition and exhibit quick degradation rate, high removal efficiency (∼93% within 35 min), and stable recycling performance owing to the synergetic effect of a high surface area and the carbon-protected α-Fe₂O₃