Control of ordering and structure in soft templated mesoporous carbon films by use of selective solvent additives

Unveiling the mechanism of methylcellulose-templated synthesis of Al2O3 microspheres with organic solvents as swelling agents in microchannel

Herein, we report a systematic investigation of the preparation of large-pore-volume Al₂O₃ microspheres using a complex synthesis system with methylcellulose (MC) as the template and gelation initiator and organic solvents as the swelling agent and carrier medium under the flow characteristics of a coaxial microchannel. The adsorption of MC micelles on boehmite colloidal nanoparticles (NPs) was proven and determined by interfacial tension measurements, dynamic light scattering, and cryogenic transmission electron microscopy. Isothermal titration calorimetry demonstrated that the adsorption process was caused by nonspecific hydrophobicity; one binding site was involved, and the affinity constant was 1060 M^-1. When the MC:NPs mass ratio exceeded 0.1, the template-NP bridged each other to form large aggregates, thereby forming large mesopores and enhancing the gelation speed. Alkanes, alcohols, and amines were applied to further enhance the porosity, and the swelling capacities were investigated experimentally and theoretically. Amines were efficient swelling agents owing to their excellent ability to swell MC micelles and insert into the acid colloid network. The coaxial microchannel was subjected to molding; this process significantly influenced the morphology and textural properties owing to the internal circulation during droplet formation. When trihexylamine with suitable steric hindrance, alkalinity, and polarity was used as the swelling agent, the microspheres exhibited an optimal specific surface area of 403 m²/g and a pore volume of 1.85 cm³/g.

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

Colloidal bimetallic platinum–ruthenium nanoparticles in ordered mesoporous carbon films as highly active electrocatalysts for the hydrogen evolution reaction

Ordered mesoporous carbon films with high surface area, good electrical conductivity and an improved distribution of NPs with tunable composition show high electrocatalytic activity in HER.

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.

Control of Pore Size in Ordered Mesoporous Carbon-Silica by Hansen Solubility Parameters of Swelling Agent

The cooperative assembly of functional precursors with block copolymers (BCPs) is a powerful, general route to fabricate ordered mesoporous materials, but the precise tuning of the mesopore size generally requires trial and error to obtain the correct BCP template or appropriate swelling agent. Here, we demonstrate the ability to effectively modulate both expansion and contraction of the ordered cylindrical mesopores relative to those obtained from cooperatively assembled Pluronic F127, resol, and tetraethylorthosilicate. The two key physical parameters for the swelling agents are their hydrophobicity, as quantified by the octanol-water partition coefficient (K_ow), and Hansen solubility parameters that describe the interactions of the solvent with the different components of the BCP template. Four low volatility solvents are examined that span a wide K_ow with up to 90 wt % solvent relative to the Pluronic F127. Glycerol triacetate (K_ow < 1) can decrease the average mesopore size from 5.9 to 4.8 nm due to segmental screening of the interactions in the Pluronic F127 to decrease chain stretching at intermediate loadings. A modest increase in mesopore size to 8.1 nm can be achieved with trimethylbenzene (TMB, K_ow = 3.42). Dioctyl phthalate (DOP), which is slightly more hydrophobic (K_ow = 8.1), is more effective than TMB at expanding the pore size (maximum: 13.5 nm) without loss of ordered structure. A more hydrophobic solvent, tris (2-ethylhexyl) trimellitate (K_ow = 12.5), is less effective at increasing the pore size (maximum: 8.2 nm). The Hansen solubility parameters for DOP most closely match those of the hydrophobic segment in the Pluronic F217 template. We attribute this similarity, which is related to the solvent quality, to the improved efficacy of DOP in increasing the pore size. These results illustrate that both the Hansen solubility parameters (relative to the hydrophobic segment of the template) and relative hydrophobicity of the swelling agent determine the obtainable pore sizes in cooperatively assembled ordered mesoporous materials.

Controlling the Pore Size of Mesoporous Carbon Thin Films through Thermal and Solvent Annealing

Herein an approach to controlling the pore size of mesoporous carbon thin films from metal-free polyacrylonitrile-containing block copolymers is described. A high-molecular-weight poly(acrylonitrile-block-methyl methacrylate) (PAN-b-PMMA) is synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The authors systematically investigate the self-assembly behavior of PAN-b-PMMA thin films during thermal and solvent annealing, as well as the pore size of mesoporous carbon thin films after pyrolysis. The as-spin-coated PAN-b-PMMA is microphase-separated into uniformly spaced globular nanostructures, and these globular nanostructures evolve into various morphologies after thermal or solvent annealing. Surprisingly, through thermal annealing and subsequent pyrolysis of PAN-b-PMMA into mesoporous carbon thin films, the pore size and center-to-center spacing increase significantly with thermal annealing temperature, different from most block copolymers. In addition, the choice of solvent in solvent annealing strongly influences the block copolymer nanostructure and the pore size of mesoporous carbon thin films. The discoveries herein provide a simple strategy to control the pore size of mesoporous carbon thin films by tuning thermal or solvent annealing conditions, instead of synthesizing a series of block copolymers of various molecular weights and compositions.

Structure Evolution of Ordered Mesoporous Carbons Induced by Water Content of Mixed Solvents Water/Ethanol

In this work, mesostructure evolution of ordered mesoporous carbons (OMCs) from the 2-D hexagonal (space group p6mm) to the discontinuous cubic [Formula: see text], then towards the face-centered cubic lattice [Formula: see text], and finally, to the simple cubic Pm3n is achieved by simply adjusting the cosolvent water content of the mixed solvents water/ethanol in the presence of a reverse nonionic triblock copolymer and low molecular resin by evaporation-induced self-assembly method. Experimental results demonstrate that both the cosolvent and the reverse triblock copolymer play a key role in the mesophase transitions of OMCs. Furthermore, the OMCs with Pm3n symmetry are reported for the first time. Finally, the mechanism of mesostructure transition was discussed and proposed.

Role of Amphiphilic Block Copolymer Composition on Pore Characteristics of Micelle-Templated Mesoporous Cobalt Oxide Films

Block copolymer templating is a versatile approach for the generation of well-defined porosity in a wide variety of framework chemistries. Here, we systematically investigate how the composition of a poly(methoxy poly[ethylene glycol] methacrylate)-block-poly(butyl acrylate) (PMPEG-PBA) template impacts the pore characteristics of mesoporous cobalt oxide films. Three templates with a constant PMPEG segment length and different hydrophilic block volume fractions of 17%, 51%, and 68% for the PMPEG-PBA are cooperatively assembled with cobalt nitrate hexahydrate and citric acid. Irrespective of template composition, a spherical nanostructure is templated and elliptical mesostructures are obtained on calcination due to uniaxial contraction of the film. The average pore size increases from 11.4 ± 2.8 to 48.5 ± 4.3 nm as the length of the PBA segment increases as determined from AFM. For all three templates examined, a maximum in porosity (∼35% in all cases) and surface area is obtained when the precursor solids contain 35-45 wt % PMPEG-PBA. This invariance suggests that the total polymer content drives the structure through interfacial assembly. The composition for maximizing porosity and surface area with the micelle-templating approach results from a general decrease in porosity with increasing cobalt nitrate hexahydrate content and the increasing mechanical integrity of the framework to resist collapse during template removal/crystallization as the cobalt nitrate hexahydrate content increases. Unlike typical evaporation induced self-assembly with sol-gel chemistry, the hydrophilic/hydrophobic composition of the block copolymer template is not a critical component to the mesostructure developed with micelle-templating using metal nitrate-citric acid as the precursor.

Nanocasting of carbon films with interdigitated bimodal three-dimensionally ordered mesopores by template-replica coassembly

Carbon films with interdigitated bimodal three-dimensionally ordered mesoporosity (ib3DOm) are realized by a scalable nanoreplication process that removes the common need plaguing hard-templating strategies for multistep prefabrication of porous sacrificial templates. Specifically, evaporation-induced convective codeposition of size-tunable (ca. 20-50 nm) silica nanoparticles with a surrogate molecular carbon precursor (glucose), followed by carbonization and template etching, leads to remarkably ordered, crack-free mesoporous carbon films of tunable thickness (ca. 100-1000 nm) and pore size. Association of the molecular carbon precursor with the assembling pore forming particles is found to transition the system among three distinct film morphologies (collapsed, ib3DOm C, disordered), thereby establishing a pseudophase behavior controlled by silica solids content and incipient glucose concentration. Namely, a parametric window wherein ib3DOm C films can be realized is identified, with a diffuse lower phase boundary associated with collapsing carbon films, and a more distinct order-to-disorder transition encountered at higher glucose concentrations. Mechanistic insight suggests that glucose association with the lysine-silica nanoparticle surface modulates the lattice spacing, d, of the periodically ordered mesopores in the coassembled films, with the onset of the order-to-disorder transition occurring at a critical normalized lattice spacing, dc/D ∼ 1.16. This appears to apply across the phase space associated with D = 50 nm silica particles and to translate among other phase spaces associated with smaller particles (e.g., 30 nm). We briefly demonstrate the robustness of the codeposition process for realizing ib3DOm C films on rough FTO glass substrates and show that, in this form, these materials hold potential as low-cost alternatives to costly platinum electrodes for dye-sensitized solar cells.

Nanoporous nonwoven fibril-like morphology by cooperative self-assembly of poly(ethylene oxide)-block-poly(ethyl acrylate)-block-polystyrene and phenolic resin

Cooperative self-assembly of block copolymers with (in)organic precursors effectively generates ordered nanoporous films, but the porosity is typically limited by the need for a continuous (in)organic phase. Here, a network of homogeneous fibrous nanostructures (≈20 nm diameter cylinders) having high porosity (≈ 60%) is fabricated by cooperative self-assembly of a phenolic resin oligomer (resol) with a novel, nonfrustrated, ABC amphiphilic triblock copolymer template, poly(ethylene oxide)-block-poly(ethyl acrylate)-block-polystyrene (PEO-b-PEA-b-PS), via a thermally induced self-assembly process. Due to the high glass transition temperature (Tg) of the PS segments, the self-assembly behavior is kinetically hindered as a result of competing effects associated with the ordering of the self-assembled system and the cross-linking of resol that suppresses segmental mobility. The balance in these competing processes reproducibly yields a disordered fibril network with a uniform fibril diameter. This nonequilibrium morphology is dependent on the PEO-b-PEA-b-PS to resol ratio with an evolution from a relatively open fibrous structure to an apparent poorly ordered mixed lamellae-cylinder morphology. Pyrolysis of these former films at elevated temperatures yields a highly porous carbon film with the fibril morphology preserved through the carbonization process. These results illustrate a simple method to fabricate thin films and coatings with a well-defined fiber network that could be promising materials for energy and separation applications.