Defect-Engineered Hydroxylated Mesoporous Spinel Oxides as Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions

In this work, defect-rich ordered mesoporous spinel oxides, including CoCo2O4, NiCo2O4, and ZnCo2O4, were developed as bifunctional electrocatalysts toward oxygen reduction and evolution reactions (ORR and OER, respectively). The materials are synthesized via nanocasting and modified by chemical treatment with 0.1 M NaBH4 solution to enhance the defect concentration. The synthesized samples have metal and oxygen divacancies (VCo + VO) as the primary defect sites, as indicated by positron annihilation lifetime spectroscopy (PALS). Cation substitution in the spinel structure induces a higher number of oxygen vacancies. The increased number of surface defects and the synergistic effect between two incorporated metals provide a high activity in both the OER and ORR in the case of NiCo2O4 and ZnCo2O4. Especially, ZnCo2O4 exhibits the highest OER/ORR activity. The defect engineering with 0.1 M NaBH4 solution results in a metal-hydroxylated surface (M-OH) and enhanced the catalytic activity for the post-treated metal oxides in the ORR and OER. This fundamental investigation of the defective structure of the mixed metal oxides offers some useful insights into further development of highly active electrocatalysts through defect engineering methods.

Synergistic effects ofα-Fe2O3nanoparticles and Fe-doping on gas-sensing performance of NiO nanowires and interface mechanism

High surface area nickel oxide nanowires (NiO NWs), Fe-doped NiO NWs andα-Fe₂O₃/Fe-doped NiO NWs were synthesized with nanocasting pathway, and then the morphology, microstructure and components of all samples were characterized with XRD, TEM, EDS, UV-vis spectra and nitrogen adsorption-desorption isotherms. Owing to the uniform mesoporous template, all samples with the same diameter exhibit the similar mesoporous-structures. The loadedα-Fe₂O₃nanoparticles should exist in mesoporous channels between Fe-doped NiO NWs to form heterogeneous contact at the interface of n-typeα-Fe₂O₃nanoparticles and p-type NiO NWs. The gas-sensing results indicate that Fe-dopant andα-Fe₂O₃-loading both improve the gas-sensing performance of NiO NWs sensors.α-Fe₂O₃/Fe-doped NiO NWs sensors presented the highest response to 100 ppm ethanol gas (55.264) compared with Fe-doped NiO NWs (24.617) and NiO NWs sensors (3.189). The donor Fe-dopant increases the ground state resistance and the absorbed oxygen content in air.α-Fe₂O₃nanoparticles in electron depletion region result in the increasing resistance in ethanol gas and decreasing resistance in air. In this way,α-Fe₂O₃/Fe-doped NiO NWs sensor presents the excellent gas-sensing performance due to the formation of heterogeneous contact at the interface.

Nanocasting synthesis of BiFeO3 nanoparticles with enhanced visible-light photocatalytic activity

In this work, monodisperse BiFeO₃ nanoparticles with a particle diameter of 5.5 nm were synthesized by a nanocasting technique using mesoporous silica SBA-15 as a hard template and pre-fabricated metal carboxylates as metal precursors. To the best of our knowledge, the synthesized particles are the smallest BiFeO₃ particles ever prepared by any method. The samples were characterized by X-ray powder diffraction, transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The phase purity of the product depends on the type of carboxylic acid used in the synthesis of the metal precursors, the type of solvent in the wet impregnation process, and the calcination procedure. By using tartaric acid in the synthesis of the metal precursors, acidified 2-methoxyethanol in the wet impregnation process and a calcination procedure with intermediate plateaus, monodisperse 5.5 nm BiFeO₃ nanoparticles were successfully obtained. Furthermore, the nanoparticles were applied in photodegradation reactions of rhodamine B in aqueous solution under visible-light irradiation. Notably, the cast BiFeO₃ nanoparticles demonstrated very high efficiencies and stability under visible-light irradiation, much higher than those of BiFeO₃ nanoparticles synthesized by other synthetic methods. The possible mechanism in the photodegradation process has been deeply discussed on the basis of radical trapping experiments.

Effects of Phase Purity and Pore Reinforcement on Mechanical Behavior of NU-1000 and Silica-Infiltrated NU-1000 Metal-Organic Frameworks

Metal-organic framework (MOF) materials have shown promise in many applications, ranging from gas storage to absorption and catalysis. Because of the high porosity and low density of many MOFs, densification methods such as pelletization and extrusion are needed for practical use and for commercialization of MOF materials. Therefore, it is important to elucidate the mechanical properties of MOFs and to develop methods of further enhancing their mechanical strength. Here, we demonstrate the influence of phase purity and the presence of a pore-reinforcing component on elastic modulus and yield stress of NU-1000 MOFs through nanoindentation methods and finite element simulation. Three types of NU-1000 single crystals were compared: phase-pure NU-1000 prepared with biphenyl-4-carboxylic acid as a modulator (NU-1000-bip), NU-1000 prepared with benzoic acid as a modulator (NU-1000-ben), which results in an additional, denser impurity phase of NU-901, and NU-1000-bip whose mesopores were infiltrated with silica (SiO_x(OH)_y@NU-1000) by nanocasting methods. By maintaining phase purity and minimizing defects, the elastic modulus could be enhanced by nearly an order of magnitude: phase-pure NU-1000-bip crystals exhibited an elastic modulus of 21 GPa, whereas the value for NU-1000-ben crystals was only 3 GPa. The introduction of silica into the mesopores of NU-1000-bip did not strongly affect the measured elastic modulus (19 GPa) but significantly increased the load at failure from 2000 μN to 3000-4000 μN.

Lotus-Leaf-Inspired Flexible and Tunable Random Laser

We describe herein a flexible and tunable random laser made from a flexible poly(dimethylsiloxane) substrate. The substrate is prepared by casting via soft lithography from a lotus leaf to produce a micropapilla surface structure similar to that of a lotus leaf. The micropapilla provides efficient multiple scattering for the photons generated in the gain medium, and random lasing emerges because photons undergo closed-loop paths by scattering from three equilaterally arranged micropapillae. Given the diverse distribution of microscale features on the soft substrate, the random laser spectrum can be tuned by as much as 26.0 nm by changing the pump position. Furthermore, the random laser can be easily tuned by about 14 nm by flexing it, which modifies the micropapilla density and thereby changes the reabsorption strength of the laser dye. The photostability of the random laser is ensured by sealing the gain medium (i.e., dye solution) in a closed system. The results provide a promising method to realize a variety of laser-based applications such as optical biosensors on chips, microscale structural alteration detectors, flexible wearable devices, and multicolor (even white) random lasers.

An Assembly and Interfacial Templating Route to Carbon Supercapacitors with Simultaneously Tailored Meso- and Microstructures

The development of facile strategies for simultaneously tailoring robust pore hierarchy and integrated microstructures in carbonaceous materials is critical for the efficient multiscale control of fluid, molecular/ionic, and charge transport in applications spanning separations, catalysis, and energy storage. Here, we synthesize three-dimensionally ordered hierarchically porous carbon powders by the assembly of glucose with silica nanoparticle building blocks of sacrificial NP-crystalline templates. Such template-replica coassembly offers an attractive alternative to conventional nanocasting by circumventing the need for sequential template preformation and infiltration-based replication. In addition, interfacial templating leads to hierarchically structured carbons with tunable mesopore volumes (as high as 5.8 cm³/g). Beyond mesostructuring, we identify the template-replica interface as a potentially versatile but generally unexploited handle for tailoring the sp² hybridized carbon content in the porous replicas under mild carbonization conditions and without specific chemical activation or catalytic graphitization. This multiscale (meso-micro) templating offered by a single template expands the potential versatility of nanocasting for the hierarchical structuring of replica materials. Application of the resulting carbons as electrochemical double layer capacitors demonstrates the combined benefit of simultaneously tailored pore hierarchy and tuned microstructures upon ion and charge transport, respectively, yielding supercapacitors achieving specific capacitance as high as 275 F/g in the aqueous electrolyte (H₂SO₄) and retention of 90% up to a current density of 10 A/g.

Easy Method for the Transformation of Levulinic Acid into Gamma-Valerolactone Using a Nickel Catalyst Derived from Nanocasted Nickel Oxide

Different nickel catalysts have been tested for the transformation of levulinic acid into γ-valerolactone using an easy hydrothermal method, taking advantage of the properties of the high temperature water. A metallic nickel catalyst derived from NiO synthesized by a nanocasting procedure can achieve a productivity to γ-valerolactone, which is two orders of magnitude higher than that obtained by a commercial nickel catalyst. This nanocasted metallic nickel catalyst has shown bifunctionality as it is capable of activating water as the source for hydrogen and undertaking the further hydrogenation step. In contrast with metallic nickel, nickel oxide has shown to be incapable of transforming levulinic acid into γ-valerolactone.

2D Single Crystal WSe2 and MoSe2 Nanomeshes with Quantifiable High Exposure of Layer Edges from 3D Mesoporous Silica Template

The design and fabrication of layered transition metal chalcogenides with high exposure of crystal layer edges is one of the key paths to achieve distinctive performances in their catalysis and electrochemistry applications. Two-dimensional WSe₂ and MoSe₂ nanomeshes with orderly arranged nanoholes were synthesized by using a mesoporous silica material KIT-6 with three-dimensional mesoporous structure as a hard template via a nanocasting strategy. Each piece of the nanomesh is a single crystal, and its c axis is always perpendicular to the nanomesh plane. The highly porous structure brings these nanomeshes extremely high exposure of layer edges, and the well-defined nanostructure provides an opportunity to quantitatively estimate the specific length of the crystal layer edges for the WSe₂ and MoSe₂ nanomeshes synthesized in this work, which are estimated to be 3.8 × 10¹⁰ and 6.0 × 10¹⁰ m g^-1, respectively. The formation of a 2D sheet-like nanomesh structure inside a 3D confined pore space should be attributed to the synergistic effect from the crystal self-limitation growth that is caused by their layered crystal structures and the space-limitation effect coming from the unique pore structure of the KIT-6 template. The catalytic activities of the nanomeshes in an electrocatalytic hydrogen evolution reaction were also investigated.

Casting Nanoporous Platinum in Metal-Organic Frameworks

Nanocasting based on porous templates is a powerful strategy in accessing materials and structures that are difficult to form by bottom-up syntheses in a controlled fashion. A facile synthetic strategy for casting ordered, nanoporous platinum (NP-Pt) networks with a high degree of control by using metal-organic frameworks (MOFs) as templates is reported here. The Pt precursor is first infiltrated into zirconium-based MOFs and subsequently transformed to 3D metallic networks via a chemical reduction process. It is demonstrated that the dimensions and topologies of the cast NP-Pt networks can be accurately controlled by using different MOFs as templates. The Brunauer-Emmett-Teller surface areas of the NP-Pt networks are estimated to be >100 m² g^-1 and they exhibit excellent catalytic activities in the methanol electrooxidation reaction (MEOR). This new methodology presents an attractive route to prepare well-defined nanoporous materials for diverse applications ranging from energy to sensing and biotechnology.

Hydrogels as Porogens for Nanoporous Inorganic Materials

Organic polymer-hydrogels are known to be capable of directing the nucleation and growth of inorganic materials, such as silica, metal oxides, apatite or metal chalcogenides. This approach can be exploited in the synthesis of materials that exhibit defined nanoporosity. When the organic polymer-based hydrogel is incorporated in the inorganic product, a composite is formed from which the organic component may be selectively removed, yielding nanopores in the inorganic product. Such porogenic impact resembles the concept of using soft or hard templates for porous materials. This micro-review provides a survey of select examples from the literature.

Au-TiO2-Loaded Cubic g-C3N4 Nanohybrids for Photocatalytic and Volatile Organic Amine Sensing Applications

A green and efficient approach for efficient nanohybrid photocatalysts in extending the light response to the visible spectrum is a hot research topic in sustainable energy technologies. In this work, novel Au-TiO₂@m-CN nanocomposite was synthesized using hard template of cubic ordered mesoporous KIT-6 via the nanocasting process. The as-prepared Au-TiO₂@m-CN nanohybrids exhibit enhanced photocatalytic activities with improved stability and reusability using methyl orange dye. The enhanced photocatalytic performance is a result of the conjugated effect of catalytic active Au and TiO₂ nanoparticles supported on highly efficient visible light sensitizer, graphitic carbon nitride (m-CN or g-C₃N₄), and ordered mesoporous morphology. Besides, the sensing performance of Au-TiO₂@m-CN nanohybrids was also tested for the detection of amine gases, wherein a significant response was reported for triethylamine at low operating temperatures. This study reveals a simple and scalable methodology to design and develop next generation of layered mesoporous materials for multifunctional applications.

Volatile Organic Compound Gas-Sensing Properties of Bimodal Porous α-Fe2O3 with Ultrahigh Sensitivity and Fast Response

Porous solid with multimodal pore size distribution provides plenty of advantages including large specific surface area and superior mass transportation to achieve high gas-sensing performances. In this study, α-Fe₂O₃ nanoparticles with bimodal porous structures were prepared successfully through a nanocasting pathway, adopting the bicontinuous 3D cubic symmetry mesoporous silica KIT-6 as the hard template. Its structure and morphology were characterized by X-ray diffraction, nitrogen adsorption-desorption, transmission electron microscopy, and so on. Furthermore, the gas sensor fabricated from this material exhibited excellent gas-sensing performance to several volatile organic compounds (acetone, ethyl acetate, isopropyl alcohol, n-butanol, ethanol, and methanol), such as ultrahigh sensitivity, rapid response speed (less than 10 s) and recovery time, good reproducibility, as well as stability. These would be associated with the desirable pore structure of the material, facilitating the molecules diffusion toward the entire sensing surface, and providing more active sensing sites for analytical gas.

Unraveling the Origin of Magnetism in Mesoporous Cu-Doped SnO₂ Magnetic Semiconductors

The origin of magnetism in wide-gap semiconductors doped with non-ferromagnetic 3d transition metals still remains intriguing. In this article, insights in the magnetic properties of ordered mesoporous Cu-doped SnO₂ powders, prepared by hard-templating, have been unraveled. Whereas, both oxygen vacancies and Fe-based impurity phases could be a plausible explanation for the observed room temperature ferromagnetism, the low temperature magnetism is mainly and unambiguously arising from the nanoscale nature of the formed antiferromagnetic CuO, which results in a net magnetization that is reminiscent of ferromagnetic behavior. This is ascribed to uncompensated spins and shape-mediated spin canting effects. The reduced blocking temperature, which resides between 30 and 5 K, and traces of vertical shifts in the hysteresis loops confirm size effects in CuO. The mesoporous nature of the system with a large surface-to-volume ratio likely promotes the occurrence of uncompensated spins, spin canting, and spin frustration, offering new prospects in the use of magnetic semiconductors for energy-efficient spintronics.

Catalytic Behavior of Chromium Oxide Supported on Nanocasting-Prepared Mesoporous Alumina in Dehydrogenation of Propane

Mesoporous alumina with narrow pore size distribution centered in the range of 4.4–5.0 nm and with a specific surface area as high as 270 m²·g⁻¹ was prepared via the nanocasting approach using a CMK-3 carbon replica as a hard template. Based on this support, a series of catalysts containing 1, 5, 10, 20 and 30 wt % of chromium was prepared by incipient wetness impregnation, characterized, and studied in the dehydrogenation of propane to propene (PDH). Cr species in three oxidation states—Cr(III), Cr(V) and Cr(VI)—were found on the oxidized surface of the catalysts. The concentration of these species varied with the total Cr loading. Temperature-programmed reduction (H₂-TPR) and UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) studies revealed that Cr(VI) species dominated at the lowest Cr content. An increase in the Cr loading resulted in an appearance of an increasing amount of Cr(III) oxide. UV-Vis-DRS measurements performed in situ during the PDH process showed that at the beginning of the catalytic test Cr(VI) species were reduced to Cr(III) redox species. A crucial role of the redox species in the PDH process over the catalysts with the low Cr content was confirmed. The stability test for the catalyst containing 20 wt % of Cr showed that this sample exhibited the reproducible catalytic performance after the first four regeneration–dehydrogenation cycles. Moreover, this catalyst had higher resistance on deactivation during the PDH process as compared to the reference catalyst with the same Cr loading, but was supported on commercially available alumina.

Ordered Large-Pore Mesoporous Cr2O3 with Ultrathin Framework for Formaldehyde Sensing

A series of ordered mesoporous chromium oxides (Cr₂O₃) were synthesized by first replicating bicontinuous cubic Ia3d mesoporous silica (KIT-6), then a controlled mesostructural transformation from Ia3d to I4₁32 symmetry during the replication from KIT-6 to Cr₂O₃ was achieved by reducing the pore size and interconnectivities of KIT-6, accompanied with an increase in pore size from 3 to 12 nm and a decrease in framework thickness from 8.6 to 5 nm of the resultant Cr₂O₃ replicas. The gas-sensing behavior of the Cr₂O₃ replicas toward formaldehyde (HCHO) was systematically investigated. Ordered mesoporous Cr₂O₃ with both large accessible pores (12 nm) and an ultrathin framework (5 nm) exhibits the best sensing performance, with a response (R_gas/R_air = 119) toward 9 ppm of HCHO 4.4 times higher than that (R_gas/R_air = 27) of its counterpart with small pores and a thick framework. Moreover, it possesses excellent selectivity for detecting HCHO over other interference gases such as CO, benzene, toluene, p-xylene, NH₃, H₂S, and moisture. The significantly enhanced sensing performance of ordered large-pore mesoporous Cr₂O₃ with ultrathin framework suggests its great potential for the selective detection of HCHO.

Nanocasting of Periodic Mesoporous Materials as an Effective Strategy to Prepare Mixed Phases of Titania

Mesoporous titanium dioxide materials were prepared using a nanocasting technique involving silica SBA-15 as the hard-template. At an optimal loading of titanium precursor, the hexagonal periodic array of pores in SBA-15 was retained. The phases of titanium dioxide could be easily varied by the number of impregnation cycles and the nature of titanium alkoxide employed. Low number of impregnation cycles produced mixed phases of anatase and TiO₂(B). The mesoporous TiO₂ materials were tested for solar hydrogen production, and the material consisting of 98% anatase and 2% TiO₂(B) exhibited the highest yield of hydrogen from the photocatalytic splitting of water. The periodicity of the pores was an important factor that influenced the photocatalytic activity. This study indicates that mixed phases of titania containing ordered array of pores can be prepared by using the nanocasting strategy.

Synthesis of Ordered Mesoporous CuO/CeO₂ Composite Frameworks as Anode Catalysts for Water Oxidation

Cerium-rich metal oxide materials have recently emerged as promising candidates for the photocatalytic oxygen evolution reaction (OER). In this article, we report the synthesis of ordered mesoporous CuO/CeO₂ composite frameworks with different contents of copper(II) oxide and demonstrate their activity for photocatalytic O₂ production via UV-Vis light-driven oxidation of water. Mesoporous CuO/CeO₂ materials have been successfully prepared by a nanocasting route, using mesoporous silica as a rigid template. X-ray diffraction, electron transmission microscopy and N₂ porosimetry characterization of the as-prepared products reveal a mesoporous structure composed of parallel arranged nanorods, with a large surface area and a narrow pore size distribution. The molecular structure and optical properties of the composite materials were investigated with Raman and UV-Vis/NIR diffuse reflectance spectroscopy. Catalytic results indicated that incorporation of CuO clusters in the CeO₂ lattice improved the photochemical properties. As a result, the CuO/CeO₂ composite catalyst containing ~38 wt % CuO reaches a high O₂ evolution rate of ~19.6 µmol·h-1 (or 392 µmol·h-1·g-1) with an apparent quantum efficiency of 17.6% at λ = 365 ± 10 nm. This OER activity compares favorably with that obtained from the non-porous CuO/CeO₂ counterpart (~1.3 µmol·h-1) and pure mesoporous CeO₂ (~1 µmol·h-1).

Novel Ordered Mesoporous Carbon Based Sulfonic Acid as an Efficient Catalyst in the Selective Dehydration of Fructose into 5-HMF: the Role of Solvent and Surface Chemistry

Novel ionic liquid derived ordered mesoporous carbons functionalized with sulfonic acid groups IOMC-ArSO3H and GIOMC-ArSO3H were prepared, characterized, and examined in the dehydration reaction of fructose into 5-hydroxymethylfurfural (5-HMF) both in aqueous and nonaqueous systems. To study and correlate the surface properties of these carbocatalysts and some other SBA-15 typed solid acids with 5-HMF yield, hydrophilicity index (H-index) were employed in the fructose dehydration. Our study systematically declared that almost a criterion may be expected for application of solid acids in which by increasing H-index value up to 0.8 the HMF yield enhances accordingly. More increase in H-index up to 1.3 did not change the HMF yield profoundly. Although, it has been shown that the catalyst with larger H-index (∼1.3) resulted in higher activity both in aqueous and 2-propanol systems, during the recycling process deactivation occurs because of more water uptake and the catalysts with optimum amount of H-index (∼0.8) is more robust in the dehydration of fructose.

Synthesis of Mesoporous Metal Oxides by Structure Replication: Thermal Analysis of Metal Nitrates in Porous Carbon Matrices

A variety of metal nitrates were filled into the pores of an ordered mesoporous CMK-3 carbon matrix by solution-based impregnation. Thermal conversion of the metal nitrates into the respective metal oxides, and subsequent removal of the carbon matrix by thermal combustion, provides a versatile means to prepare mesoporous metal oxides (so-called nanocasting). This study aims to monitor the thermally induced processes by thermogravimetric analysis (TGA), coupled with mass ion detection (MS). The highly dispersed metal nitrates in the pores of the carbon matrix tend to react to the respective metal oxides at lower temperature than reported in the literature for pure, i.e., carbon-free, metal nitrates. The subsequent thermal combustion of the CMK-3 carbon matrix also occurs at lower temperature, which is explained by a catalytic effect of the metal oxides present in the pores. This catalytic effect is particularly strong for oxides of redox active metals, such as transition group VII and VIII metals (Mn, Fe, Co, Ni), Cu, and Ce.

Template based precursor route for the synthesis of CuInSe2 nanorod arrays for potential solar cell applications

Polycrystalline CuInSe2 (CISe) nanorods are promising for the fabrication of highly efficient active layers in solar cells. In this work we report on a nanocasting approach, which uses track-etched polycarbonate films as hard templates for obtaining three-dimensionally (3D) arranged CISe nanorod arrays. Copper and indium ketoacidoximato complexes and selenourea were employed as molecular precursors. Arrays of parallel isolated cylindrical pores of 100 nm nominal diameter and 5 μm length were used for the infiltration of the precursor solution under inert atmosphere, followed by drying, thermal conversion into a preceramic 'green body', a subsequent dissolution of the template, and a final thermal treatment at 450 °C. The nanorods that where synthesised in this way have dimensions equal to the pore sizes of the template. Investigation of the CuInSe2 nanorod samples by spectroscopic and diffraction methods confirmed a high purity and crystallinity, and a stoichiometric composition of the CISe ternary semiconductor compound.

Fe-doped MnxOy with hierarchical porosity as a high-performance lithium-ion battery anode

Fe-doped Mnx Oy with hierarchical porosity is prepared from a nanocasting technique using amine-functionalized bromomethylated poly (2,6-dimethyl-1,4-phenylene oxide) (BPPO) membranes as the sacrificial template. The synergistic coupling of a percolating macroporous network, uniformly distributed mesopores, and optimal iron doping is used to improve the electronic and ionic wirings of manganese oxides for Li(+) storage via the conversion reaction. Very impressive Li(+) storage capabilities are shown.