Aligned Macropores Research Articles

Aligned porous Chitosan/Ti3C2 MXenes monolith as a high-performance fixed-bed separation medium toward globular albumin separation

Designing and tuning macropores into aligned structure is of considerable importance in promoting mass transfer, improving chromatographic performance and enhancing column efficiency. Herein, we have synthesized an innovative aligned porous Chitosan/Ti3C2 MXenes monolith via a facile, versatile and reproducible unidirectional “freeze-casting” method. Typically, the accumulation of Ti3C2 MXenes directs the formation of aligned structure during the unidirectional freezing process and thus aligned macropores are formed by a subsequent freeze-drying process. By various physical characterization, the distribution of aligned macropores, micropores and specific surface area are determined to be 21 μm, 2.53 nm and 103.09 m2·g−1, respectively. In addition, monolith bed permeability is calculated to be 1.94 × 10-12 m2, approximately 10 times than conventional monolith. Batch adsorptive experiments confirm that adsorptive capacities to bovine serum albumin and bovine hemoglobin are remarkably elevated on account of the high affinity of proteins onto those imbedding Ti3C2 MXenes. Meanwhile, continuous mass transfer equation describes the aligned macropores facilitate the film mass transfer factor ([kLa]f) in a separation process thus driving the global mass transfer factor ([kLa]g). It is expected that the aligned porous Chitosan/Ti3C2 MXenes monolith promises great potential for a continuous separation of proteins when a wholesale production is required.

Magnetically derived fabrication of aligned porous poly(sulfonated polystyrene-co.-chitosan) monolith as a high efficient adsorbent for metal ions separation

Designing and tuning towards the porous structure of adsorbents is of great importance for promoting the adsorptive performance. Herein, a magnetic-induced copolymerization method is applied to generate an aligned porous structure in the aligned porous poly(SPS-co.-CTS) monolith. Typically, the aligned macropores primarily derives from the equilibrium of the magnetic dipolar force to the thermodynamic Brown force by tuning the applied magnetostatic field intensity to 25 mT. Due to the aligned macroporous structure, the monolith features with low back pressure, low mass transfer resistance and high adsorptive capacity. The hydrophilicity of the monolith surface is further improved by capping chitosan activated by amino onto its porous surface via the Hinsber reaction. Detail analyses are performed by a series of physical characterizations, by which exhibit the aligned macropores centering at 5.18 μm, specific surface reaching 54 m2 g−1 and large porosity. Finally, batch adsorption experiments confirm that the adorptive behaviors to Cu2+ ions and Co2+ ions submit to the Langmuir model and Pseudo-second order adsorption kinetics. Most intriguing, the film mass transfer ([kLa]f) has been stabilized due to the aligned macropores, thus facilitating the porous diffusion ([kLa]d) and improving the chromatographic performance of the monolith. All results demonstrate that the aligned porous poly(SPS-co.-CTS) monolith promises broad applications in the field of wastewater remediation.

Fabrication of hierarchically porous silicon carbonitride monolith using branched polysilazane and freeze casting method without crosslinking step: Effect of polymer additive to aligned macropore structure

AbstractA hierarchically porous SiCN monolith with aligned macropores was prepared using a branch‐structured polysilazane, silsesquiazane (SSQZ), as a precursor and polymer additive, through the freeze casting method. As polymer additives, polystyrene (PS) and poly(methyl methacrylate), which have different compatibilities with SSQZ, were used to confirm the effect on the pore formation. In addition, the pore formability according to the molecular weight was investigated using PSs with different molecular weights. When the SSQZ and high‐molecular‐weight PS (HPS) were used, a porous SiCN with micro/meso/macropore structure could be fabricated because of the structural stability of the SSQZ and high molecular weight of HPS. The hierarchically porous SiCN monolith was successfully manufactured using SSQZ and HPS with 639 m2/g of specific surface area, 0.36 cm3/g of pore volume, and 2.1 nm of average pore size. The hierarchically porous SiCN monolith with high HPS content has improved dimensional stability to show its compressive fracture energy.

Fabrication and characterization of aligned macroporous monolith for high-performance protein chromatography

In the present study, a freeze casting method combined with particle accumulation was applied to fabricate the aligned macroporous monolith for high-performance protein chromatography. For the preparation, the reactive colloids were first prepared by using glycidyl methacrylate and ethylene glycol dimethacrylate as monomers. Subsequently, these colloids accumulated regularly and polymerized into the aligned macroporous monolith. The aligned porous structure of the monolith was characterized by SEM, mercury intrusion, and flow hydrodynamics. The results revealed that the generated monolith was possessed of aligned macropores in size of about 10μm and high column permeability. Finally, after being modified with sulfonated groups, the monolith was evaluated for its chromatographic performance. It demonstrated that the aligned macropores endowed the monolith with excellent adsorption capacity and high column efficiency.

Depth detection and fabrication of porous silicon without the stress

The stress in porous Si could affect the integration of porous Si devices with microelectronic integrated circuits based on silicon. In this paper, aligned macropore arrays without the stress were fabricated by an electrochemical etching procedure under a control voltage. The several etching stages of aligned macropore arrays were examined by microscopic technique, which indicates the variation in morphology from pore nuclei to aligned macropores with branching pores, and finally to aligned macropores without branching pores. Due to the difference of Raman peak positions between porous Si layer and substrate (unetched layer), furthermore, z-scan of Raman spectroscopy which is a nondestructive tool can be used to detect the etching depth. The difference should be ascribed to the quantum confinement of phonons in the porous Si.

Ultrathin-layer chromatography nanostructures modified by atomic layer deposition

Stationary phase morphology and surface chemistry dictate the properties of ultrathin-layer chromatography (UTLC) media and interactions with analytes in sample mixtures. In this paper, we combined two powerful thin film deposition techniques to create composite chromatography nanomaterials. Glancing angle deposition (GLAD) produces high surface area columnar microstructures with aligned macropores well-suited for UTLC. Atomic layer deposition (ALD) enables precise fabrication of conformal, nanometer-scale coatings that can tune surfaces of these UTLC films. We coated ∼5μm thick GLAD SiO2 UTLC media with <10nm thick ALD metal oxides (Al2O3, ZrO2, and ZnO) to decouple surface chemistry from the underlying GLAD scaffold microstructure. The effects of ALD coatings on GLAD UTLC media were investigated using transmission electron microscopy (TEM), gas adsorption porosimetry, and lipophilic dye separations. The results collectively show that the most significant changes occur over the first few nanometers of ALD coating. They further demonstrate independent control of film microstructure and surface characteristics. ALD coatings can enhance complex GLAD microstructures to engineer new composite nanomaterials potentially useful in analytical chromatography.

Porous alumina/zirconia layered composites with unidirectional pore channels processed using a tertiary-butyl alcohol-based freeze casting

Symmetric three-layer porous alumina/zirconia composites with controlled “designer” pore structure have been prepared by a tertiary-butyl alcohol-based freeze casting and sintering at 1400–1500°C in air. Unidirectional aligned macropore channels were developed over a long range by controlling the solidification direction of tertiary-butyl alcohol solvent; in this case, they were surrounded by more dense structured walls. In layered composite, the bottom layer consisted of small sized pore channels (∼11μm in diameter) compared with the middle and the top layer, due to the comparatively rapid velocity of the TBA crystal growth during solidification. The compressive strength (63–376MPa) of the sintered porous layered composite remarkably increased as the porosity decreased (64–32%).

Imprinting macropore arrays into mesoporous silica monoliths

Mesoporous silica monoliths containing arrays of aligned macropores oriented perpendicular to their surfaces were prepared by non-destructive imprinting of sol/gels with polymer nanorod arrays. Crack-free monoliths with areas of up to 2.5 cm2 were obtained by a combination of slow drying under ambient conditions and optimized freeze-drying.

Fabrication of porous ceramic composites with improved compressive strength from coal fly ash

A freeze–gel casting method has been proposed to fabricate porous mullite–alumina composite with controlled ‘designer’ pore structure and improved compressive strength. In this work, a tertiary butyl alcohol–coal fly ash slurry system with appropriate addition of Al2O3 was used. Unidirectional aligned macropore channels were developed by controlling the solidification direction of the tertiary butyl alcohol solvent. Simultaneously, microsized pores formed in the walls of the pore channels; consequently, the inner section of the walls of the pore channels exhibited a more porous structure than the outer walls. The compressive strength of the sintered porous composite increased as the porosity decreased, i.e. the low porosity gave a high compressive strength. After sintering at 1500°C with 40 wt-% solid loading, the mullite–alumina composite with a porosity of 61·2% exhibited a compressive strength of ∼64·3 MPa.

TBA-based freeze/gel casting of porous hydroxyapatite scaffolds

Porous ceramic scaffolds with a controlled “designer” pore structure have been prepared by the freeze/gel casting route using a TBA-based hydroxyapatite slurry system with 20–40 wt.% solid content. The products were characterized in terms of sintered microstructure, together with physical and mechanical properties. After sintering at 1050–1250 °C, the advantages of freeze casting and gel casting appeared in the pore structure and compressive strength of the ceramics, i.e., unidirectional aligned macro-pore channels developed by controlling the solidification direction of the TBA solvent used in the freeze casting together with small diameter (micron sized) isolated pores formed in the dense outer walls of the pore channels when processed by gel casting. The sintered porosity and pore size generally resulted in a high solid loading giving low porosity and small pore size, this leading to higher compressive strengths. The scaffolds obtained exhibited an average porosity and compressive strength in the range 41.9–79.3% and 35.1–2.7 MPa, respectively, depending on the processing conditions used.

Studies on the growth of ice crystal templates during the synthesis of a monolithic silica microhoneycomb using the ice templating method

Porous materials with a wide variety of functions can be obtained through sol-gel synthesis. Recently, we found that sol-gel based materials can be molded into a monolithic microhoneycomb structure by simply freezing their parent hydrogels unidirectionally. The main feature of the monoliths obtained through this method, which we named the Ice Templating Method, is that they have straight and aligned macropores, the sizes of which are in the micrometer range. As these macropores are the traces of the ice crystals which are formed during freezing and which practically act as the template, the sizes as well as the shape of them depend on how the template ice crystals are formed and how they grow. Therefore in this work, the growth behavior of the ice crystals formed during the unidirectional freezing of a silica hydrogel was examined and the influences of this growth behavior on the properties of the resulting monoliths were verified.

Novel N−F-Codoped TiO2 Inverse Opal with a Hierarchical Meso-/Macroporous Structure: Synthesis, Characterization, and Photocatalysis

We reported a new simple route to fabricate N−F-codoped titania inverse opal (IO) films with a hierarchical meso-/macroporous structure, in which the IO structure and the N−F codoping play a role as physical and chemical modifications for titania, respectively. Both modifications are simultaneously achieved via a liquid phase deposition process using a silica colloidal crystal as a template for the first time. The average size of a mesopore in the TiO2 frameworks is 3.2 nm, and that of an interpenetrating aligned macropore is adjusted in the range of 201−315 nm. The N−F-codoped TiO2 IO films exhibit 6.6−7.4 times the methylene blue visible-light photodegradation rate comparing with N−F-codoped TiO2 films without the IO structure. This high visible-light activity was mainly ascribed to a synergetic effect of the hierarchical meso-/macroporous structure and N−F codoping. Furthermore, it was found that the multiple scattering effect of the macroporous structure is more pronounced than the slow photon effect of IOs for facilitating visible-light photocatalysis in our system from the comparison with nonperiodic macroporous N−F-codoped TiO2. The repeated cycling tests revealed that samples showed stable photocatalytic activity.

Porous mullite composite with controlled pore structure processed using a freeze casting of TBA-based coal fly ash slurries

Porous ceramic composite with controlled “designer” pore structure has been prepared by a freeze casting route using a tertiary-butyl alcohol (TBA)/coal fly slurry system. Unidirectional aligned macropore channels were developed by controlling the solidification direction. Simultaneously, small sized dendrite-like pores formed in the outer walls of the pore channels. The compressive strength of the sintered porous composite increased as the porosity decreased, i.e. low porosity gave a high compressive strength. After sintering at 1500 °C, the mullite composite with a porosity of 67% exhibited a compressive strength of ∼23 MPa.

Fabrication of Porous Clay Materials with Aligned Pore Structures by Freeze-Drying

Porous clay materials with columnar type pore channels were fabricated from water-based clay slurry and a specially designed mold by using a freeze casting technique. Ice was stimulated to grow vertical direction, and aligned macro pores were formed to the same direction. The phase separation between the ice and clay particles might be responsible for the formation of the columnar type pores. It appears that it is possible to control the size and morphology of pores by modifying a freezing mold, refrigerant temperature, and the weight fraction of clay in slurry.