Maintaining the Structure of Templated Porous Materials for Reactive and High-Temperature Applications

Nanoporous and nanostructured materials are becoming increasingly important for advanced applications involving, for example, bioactive materials, catalytic materials, energy storage and conversion materials, photonic crystals, membranes, and more. As such, they are exposed to a variety of harsh environments and often experience detrimental morphological changes as a result. This article highlights material limitations and recent advances in porous materialsthree-dimensionally ordered macroporous (3DOM) materials in particularunder reactive or high-temperature conditions. Examples include systems where morphological changes are desired and systems that require an increased retention of structure, surface area, and overall material integrity during synthesis and processing. Structural modifications, changes in composition, and alternate synthesis routes are explored and discussed. Improvements in thermal or structural stability have been achieved by the isolation of nanoparticles in porous structures through spatial separation, by confinement in a more thermally stable host, by the application of a protective surface or an adhesive interlayer, by alloy or solid solution formation, and by doping to induce solute drag

Simulation-Aided Design and Synthesis of Hierarchically Porous Membranes

Free-standing silica membranes with hierarchical porosity (ca. 300 nm macropores surrounded by 6–8 nm mesopores) and controllable mesopore architecture were prepared by a dual-templating method, with the structural design aided by mesoscale simulation. To create a two-dimensional, hexagonal macropore array, polymeric colloidal hemisphere arrays were synthesized by a two-step annealing process starting with non-close-packed polystyrene sphere arrays on silicon coated with a sacrificial alumina layer. A silica precursor containing a poly­(ethylene) oxide–poly­(propylene oxide)–poly­(ethylene) oxide (PEO–PPO–PEO) triblock-copolymer surfactant as template for mesopore creation was spin-coated onto the support and aged and then converted into the free-standing membranes by dissolving both templates and the alumina layer. To test the hypothesis that the mesopore architecture may be influenced by confinement of the surfactant-containing precursor solution in the colloidal array and by its interactions with the polymeric colloids, the system was studied theoretically by dissipative particle dynamics (DPD) simulations and experimentally by examining the pore structures of silica membranes via electron microscopy. The DPD simulations demonstrated that, while only tilted columnar structure can be formed through tuning the interaction with the substrate, perfect alignment of 2D hexagonal micelles perpendicular to the plane of the membrane is achievable by confinement between parallel walls that interact preferentially with the hydrophilic components (PEO blocks, silicate, and solvent). The simulations predicted that this alignment could be maintained across a span of up to 10 columns of micelles, the same length scale defined by the colloidal array. In the actual membranes, we manipulated the mesopore alignment by tuning the solvent polarity relative to the polar surface characteristics of the colloidal hemispheres. With methanol as a solvent, columnar mesopores parallel to the substrate were observed; with a methanol–water mixed solvent, individual spherical mesopores were present; and with water as the only solvent, twisted columnar structures were seen

Quenching Performance of Surfactant-Containing and Surfactant-Free Fluorophore-Doped Mesoporous Silica Films for Nitroaromatic Compound Detection

Various surfactant-templated, mesoporous silica thin films containing a phenyl-substituted pyrene fluorophore were prepared and tested as sensors for the nitroaromatic compound 2,4-dinitrotoluene (DNT). The effects of materials parameters on quenching efficiency were evaluated, including the influence of mesopore architecture (wormlike, cubic, or hexagonal mesopores), the presence or absence of the templating surfactant in the mesopores, and the mode of fluorophore incorporation (doping, impregnating, or grafting). Among films with similar components, films with wormlike mesopore architecture exhibited a better quenching performance than those with 2D-hexagonal or 3D-hexagonal mesopore structure. Surfactant-free, fluorophore-bridged films with wormlike mesopores showed the best quenching performance (43% after 5 s and 88% after 60 s), which compares favorably with state-of-the-art sensors based on fluorescent conjugated polymers. Surfactant-containing, fluorophore-doped films with wormlike mesopores were also effectively quenched by DNT, with 39% quenching after 45 s and 94% of quenching after 405 s. It is notable that the surfactant blocks the diffusion of DNT only slightly while it enhances the binding of DNT to the film, boosting the quenching performance

Effects of Integrated Carbon as a Light Absorber on the Coloration of Photonic Crystal-Based Pigments

Three-dimensionally ordered macroporous (3DOM) materials prepared by colloidal crystal templating are examples of photonic crystals that can exhibit structural color. The color intensity can vary widely, from a pale, nearly white opalescence to vivid, brilliantly metallic colors. Such variations are observed even for 3DOM materials of a single nominal composition that exhibit virtually identical structural order in scanning electron micrographs and are prepared from the same colloidal crystal templates. In this study we investigate the cause of the variations in color intensity for 3DOM ZrO₂ systems, considering both the role of zirconia grains in the skeleton of the photonic crystal and the presence or absence of carbonaceous components in the material. Such components act as broad spectral light absorbers and are introduced either directly in the synthesis through the precursor and the polymeric template or by postsynthesis addition and carbonization of sucrose solutions. We conclude that grain-size effects do not play a significant role but that the carbon content in 3DOM ZrO₂ provides direct control over the intensity of structural color in these photonic pigment materials

Paper-Based All-Solid-State Ion-Sensing Platform with a Solid Contact Comprising Colloid-Imprinted Mesoporous Carbon and a Redox Buffer

We report the design, structure, and performance of a planar paper-based ion-sensing platform that utilizes colloid-imprinted mesoporous (CIM) carbon as a solid contact, with a redox buffer as the internal reference. This device contains an all-solid-state ion-selective electrode and an all-solid-state reference electrode that are integrated into the paper substrate with a symmetrical cell design. To ensure calibration-free sensor operation, each interfacial potential within the device is well-defined by the use of a redox buffer added to the sensing and reference membranes that controls the interfacial potentials at the CIM carbon/sensing membrane and CIM carbon/reference membrane interfaces. Two types of redox buffers were evaluated for this purpose, i.e., one based on the tetrakis­(pentafluorophenyl)­borate salts of cobalt­(II/III) tris­(4,4′-dinonyl-2,2′-bipyridyl) and one consisting of 7,7,8,8-tetracyanoquinodimethane and its corresponding anion radical. The feasibility of the design was demonstrated with aqueous KCl solutions. By design, the device only needs one droplet of sample, and it does not need any supply reagents or sensor pretreatment (i.e., conditioning and calibration) to function

Ion-Selective Electrodes with Colloid-Imprinted Mesoporous Carbon as Solid Contact

A new type of solid-contact ion-selective electrode (SC-ISE) has been developed that uses colloid-imprinted mesoporous (CIM) carbon with 24 nm diameter, interconnected mesopores as the intermediate layer between a gold electrode and an ionophore-doped ISE membrane. For a demonstration, valinomycin was used as K⁺ ionophore, and a good Nernstian response with a slope of 59.5 mV/decade in the range from 10^–5.2 to 10^–1.0 M was observed. The high purity, low content of redox-active surface functional groups and intrinsic hydrophobic characteristics of CIM carbon prepared from mesophase pitch lead to outstanding performance of these sensors, with excellent resistance to the formation of a water layer and no interference caused by light, O₂, and CO₂. When a redox couple is introduced as an internal reference species, calibration-free SC-ISEs can be made with a standard deviation of <i>E</i>° as low as 0.7 mV. Moreover, the interconnected mesopore structure of ISE membrane-infused CIM carbon facilitates both ion and electron conduction and provides a large interfacial area with good ion-to-electron transduction. Because of the large double layer capacitance of CIM carbon, CIM carbon-based SC-ISEs exhibit excellent potential stability, as shown by chronopotentiometry and continuous potentiometric measurements. The capacitance of these electrodes as determined by chronopotentiometry is 1.0 mF, and the emf drift over 70 h is as low as 1.3 μV/h, making these electrodes the most stable SC-ISEs reported so far

Effect of Ion Identity on Capacitance and Ion-to-Electron Transduction in Ion-Selective Electrodes with Nanographite and Carbon Nanotube Solid Contacts

The use of large surface area carbon materials as transducers in solid-contact ion-selective electrodes (ISEs) has become widespread. Desirable qualities of ISEs, such as a small long-term drift, have been associated with a high capacitance that arises from the formation of an electrical double layer at the interface of the large surface area carbon material and the ion-selective membrane. The capacitive properties of these ISEs have been observed using a variety of techniques, but the effects of the ions present in the ion-selective membrane on the measured value of the capacitance have not been studied in detail. Here, it is shown that changes in the size and concentration of the ions in the ion-selective membrane as well as the polarity of the polymeric matrix result in capacitances that can vary by up to several hundred percent. These data illustrate that the interpretation of comparatively small differences in capacitance for different types of solid contacts is not meaningful unless the composition of the ion-selective membrane is taken into account

Controlling Microstructural Evolution in Pechini Gels through the Interplay between Precursor Complexation, Step-Growth Polymerization, and Template Confinement

The mechanisms driving microstructure formation in template-confined Pechini-type gel systems involving solid solutions of cerium oxide with alkaline earth metals are investigated. Three-dimensionally ordered macroporous microspheres and more extended bicontinuous networks with hierarchical porosity are synthesized directly from a Pechini sol–gel precursor within a colloidal crystal template. The type of morphology generated is related to the mechanisms of phase separation in the precursor, namely, nucleation and growth vs spinodal decomposition. These mechanisms are, in turn, determined by the citric acid concentration in the initial precursor solution and by electrostatic interactions of the precursor with the polymeric template. Microspheres generated by the nucleation-and-growth pathway can be produced between 1–3 μm in size, with polydispersities below 15%. They retain the ordered porous network left by removal of the template. The number of nucleation sites (i.e., oligomers and longer chains of complexed metal) is dependent on the reactant imbalance between metal–citrate complexes and ethylene glycol, as predicted by step-growth polymerization statistics. This method expands existing phase-separation techniques currently exploited in metal alkoxide systems to the production of microstructure in ceramic oxides

Receptor-Based Detection of 2,4-Dinitrotoluene Using Modified Three-Dimensionally Ordered Macroporous Carbon Electrodes

Detection of explosives, such as 2,4,6-trinitrotoluene (TNT), is becoming increasingly important. Here, 2,4-dinitrotoluene (DNT, a common analogue of TNT) is detected electrochemically. A receptor based electrode for the detection of DNT was prepared by modifying the surface of the walls of three-dimensionally ordered macroporous (3DOM) carbon. Nitrophenyl groups were first attached by the electrochemical reduction of 4-nitrobenzenediazonium ions, followed by potentiostatic reduction to aminophenyl groups. Chemical functionalization reactions were then performed to synthesize the receptor, which contains two urea groups, and a terminal primary amine. Detection of DNT using cyclic voltammetry was impeded by a large background current that resulted from the capacitance of 3DOM carbon. Detection by square wave voltammetry eliminated the background current and improved the detection limit. Unfunctionalized 3DOM carbon electrodes showed no response to DNT, whereas the receptor-modified electrodes responded to DNT with a detection limit of 10 μM. Detection of DNT was possible even in the presence of interferents such as nitrobenzene

Titania–Carbon Nanocomposite Anodes for Lithium Ion Batteries Effects of Confined Growth and Phase Synergism

As lithium-ion batteries (LIB) see increasing use in areas beyond consumer electronics, such as the transportation sector, research has been directed at improving LIBs to better suit these applications. Of particular interest are materials and methods to increase Li⁺ capacity at various charge/discharge rates, to improve retention of Li⁺ capacity from cycle-to-cycle, and to enhance various safety aspects of electrode synthesis, cell construction, and end use. This work focuses on the synthesis and testing of three-dimensionally ordered macroporous (3DOM) TiO₂/C LIB anode materials prepared using low toxicity precursors, including ammonium citratoperoxotitanate­(IV) and sucrose, which provide high capacities for reversible Li⁺ insertion/extraction. When the composites are pyrolyzed at 700 °C, the carbon phase restricts sintering of TiO₂ crystallites and keeps the size of these crystallites below 5 nm. Slightly larger crystallites are produced at higher temperatures, alongside a titanium oxycarbide phase. The composites exhibit excellent capacities as LIB anodes at low to moderate charge/discharge rates (in the window from 1 to 3 V vs Li/Li⁺). Composites pyrolyzed at 700 °C retain over 200 mAh/g TiO₂ of capacity after 100 cycles at a C/2 rate (C = 335 mA/g), and do not suffer from extensive cycle-to-cycle capacity fading. A substantial improvement of overall capacities, especially at high rates, is attained by cycling the composite anodes in a wider voltage window (0.05 to 3 V vs Li/Li⁺), which allows for Li⁺ intercalation into carbon. At currents of 1500 mA/g of active material, over 200 mAh/g of capacity is retained. Other structural aspects of the composites are discussed, including how rutile TiO₂ is found in these composites at sizes below the thermodynamic stability limit in the pure phase