The characterization and application of p-type semiconducting mesoporous carbon nanofibers

The microstructures of mesoporous carbon nanofibers were characterized by scanning electron microscopy, transmission electron microscopy, nano-Raman, nitrogen adsorption-desorption and optical transmission. They possessed a high specific surface area 840 m² g⁻¹ and a 1.07 eV band gap. All mesoporous carbon nanofiber network can act as the channel material in p-type field-effect transistor devices with field-effect mobilities over 10 cm²/V s. Furthermore, mesoporous carbon nanofiber network exhibits better sensitivity and faster response to NO₂ gas than that of carbon nanotubes, which makes it a promising candidate as poisonous gas sensing nanodevices.     

A low-temperature autoclaving route to synthesize monolithic carbon materials with an ordered mesostructure

Low-temperature autoclaving has been demonstrated to synthesize monolithic carbon materials with an ordered mesostructure by using triblock copolymer F127 as template, and resorcinol/formaldehyde resol as carbon precursor under acidic conditions. Transmission electron microscopy, small angle X-ray scattering, Fourier transform infrared spectroscopy and nitrogen adsorption measurements show that the crack-free carbon monoliths have a 2-D hexagonal pore system, a uniform pore size of ∼5.0 nm and a high surface area of ∼675 m² g⁻¹. The macroscopic morphology can be tuned by changing the diameter of the autoclave. The influence of the synthesis conditions including the autoclaving treatment time and the molar ratio of formaldehyde (F) to resorcinol (R) are discussed. It is found that while the F/R molar ratio ?2 and the autoclaving treatment time ?2 d, highly ordered mesoporous carbon monoliths can be obtained. In comparison, monolithic mesoporous carbon materials prepared through an evaporation-induced self-assembly strategy are partly cracked with a disordered wormhole-like mesostructure, suggesting that low-temperature autoclaving is an efficient way to prepare crack-free monolithic carbon materials with an ordered mesostructure.     

PREPARATION OF ORGANIC MESOPOROUS GEL BY SUPERCRITICAL/FREEZE DRYING

ABSTRACT

Resorcinol-formaldehyde (RF) aerogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and supercritical drying with carbon dioxide. The control of mesoporous structure of the aerogels was studied by changing the amounts of resorcinol, formaldehyde, distilled water, and sodium carbonate (basic catalyst) used in the polycondensation. As a result of characterization by nitrogen adsorption, the mesopore radius of the RF aerogel was controlled in the range of 2.5 – 9.2 nm. After the hydrogels were immersed in excess of t-butanol, RF cryogels were prepared by freeze drying. The cryogels prepared were mesoporous materials with high surface areas > 500 m²/g and large mesopore volumes > 0.8 cm³/g. Although the surface areas and mesopore volumes of RF cryogels were smaller than those of RF aerogels, the cryogels were useful precursors of mesoporous carbons.     

Preparation of ordered mesoporous carbon membranes by a soft-templating method

Preparation of continuous mesoporous carbon membranes without the use of an intermediate inorganic template was achieved using a thermosetting phenolic resin, resorcinol/phloroglucinol/formaldehyde, and a thermally-decomposable organic template, Pluronic F127 (PEO₁₀₆–PPO₇₀–PEO₁₀₆). The coating solution was cast on porous α-alumina supports by dip-coating. Afterwards, decomposition of the organic template and solidification of the carbon precursors are simultaneously performed through a carbonization process. A composite layer of carbon/alumina was formed. The single gas permeation was governed by the Knudsen diffusion mechanism. The membrane exhibited high hydrothermal stability and high alkaline resistance.     

Preparation and characterization of carbon membranes made from poly(phthalazinone ether sulfone ketone)

Carbon membranes were prepared from a novel polymeric precursor of poly(phthalazinone ether sulfone ketone) (PPESK), of which the changes of microstructure and chemical compositions during pyrolysis from 500 °C to 950 °C were monitored by thermal gravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It has been found that the weight loss of the PPESK precursor up to 800 °C is about 43.0 wt%. After the heat treatment, the typical chemical structure of the PPESK precursor disappears, at the same time a graphite-like structure with more aromatic rings is formed. The interlayer spacing (i.e., d value) decreases from 0.471 nm to 0.365 nm as the pyrolysis temperature increases. The gas permeation performance of carbon membranes has been tested using pure single gases including H₂, CO₂, O₂ and N₂. For the carbon membrane obtained by carbonizing the PPESK precursor at 800 °C, the maximum ideal permselectivities for H₂/N₂, CO₂/N₂ and O₂/N₂ gas pairs could reach 278.5, 213.8 and 27.5, respectively.     

Synthesis of resorcinol-formaldehyde/silica composite aerogels and their low-temperature conversion to mesoporous silicon carbide

Resorcinol-formaldehyde/silica composite (RF/SiO₂) gels were acid-catalyzed formed in one pot at 27 °C within 60 min, and then dried to aerogels with supercritical fluid CO₂. After carbonization in nitrogen atmosphere and a magnesiothermic reaction at 700 °C, RF/SiO₂ aerogels were successfully converted to monolithic mesoporous silicon carbide (SiC aerogels). The starting RF/SiO₂ aerogels had an interpenetrating organic/inorganic network and the resulting SiC products preserved monolithic mesoporous morphology similar to the original templates. The as-synthesized SiC aerogels consisted of nanocrystalline β-SiC, possessed a BET surface area of 232 m²/g and showed sufficient mesoporosity. They had a direct band gap of about 3.2 eV (less than that of bulk β-SiC) and showed photoluminescence at room temperature. The mechanisms about the formation of RF/SiO₂ gels and the conversion to SiC aerogels were discussed. Potentially, the reported method can be used to convert many metal or semimetal oxide/carbon composite aerogels to carbide aerogels at relatively low temperature for many catalytic, electronic, photonic and thermal applications.     

Synthesis and characterization of ruthenium-containing ordered mesoporous carbon with high specific surface area

A Ru-containing ordered mesoporous carbon with a high specific surface area of 2186 m²/g was synthesized through evaporation-induced multi-constituent co-assembly method, wherein soluble resol polymer is used as the carbon precursor, silicate oligomers as the inorganic precursor, triblock copolymer as the template, and RuCl₃ · 3H₂O as the Ru precursor. The resultant sample was characterized by X-ray diffraction, nitrogen sorption, transmission electron microscopy and scanning electron microscopy. The results showed that the carbon material exhibited highly ordered mesoporous structure, and the ruthenium particles with sizes of ∼2 nm were uniformly distributed in the carbon matrix. The sample was used to catalyze benzene hydrogenation, which displayed high efficiency for this reaction.     

Effects of carbonisation on pore evolution and gas permeation properties of carbon membranes from Kapton polyimide

Carbon membranes were prepared by carbonisation of Kapton^® polyimide at different temperatures under vacuum and nitrogen flow. Pore structure development of the membranes during carbonisation was studied. Carbonisation temperature was critical in the modification of membrane structure. At the same temperature, the carbon membranes fabricated under nitrogen atmosphere had higher gas permeances than those fabricated under vacuum. During heat treatment, the value of d-spacing for the carbon membranes decreased with increasing temperature, however, vacuum and nitrogen atmosphere had different influences on the changes in the d-spacing. CO₂ adsorption showed that the carbon membranes prepared at 1273 K under vacuum had the highest micropore volume whilst the membranes prepared at 1073 K under vacuum had the highest characteristic adsorption energy. N₂ adsorption showed that the samples obtained at 873 K under vacuum had the highest nitrogen uptake. Mesopores were deemed to be connected through micropores and narrow channels between meso- and/or micropores were supposedly present. The micropores predominantly controlled the transport properties of the carbon membranes. The membrane samples obtained at 1173 K under vacuum yielded ideal separation factors of 558.27, 60.87, 19.69 and 138.53 for He/N₂, CO₂/N₂, O₂/N₂ and CO₂/CH₄, respectively, with permeances of 7.26, 0.79, 0.26, 0.13 and 0.006 mol/(m² s Pa) for He, CO₂, O₂, N₂ and CH₄, respectively.     

Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes for oxygen permeation

An ultrathin La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) hollow fibre membrane for enhanced oxygen permeation flux was fabricated using a wet spinning/sintering method. The membrane exhibits a highly asymmetric structure comprising of a very thin dense outer layer supported by finger-like structures that are fully open on the inner surface. Oxygen permeation measurements were conducted using sweep gas as an operating mode. Effects of operating temperatures and flow rates of the sweep gas on the oxygen permeation fluxes were investigated in details. The highest oxygen permeation flux, i.e. 0.096 cm³/cm² s (5.77 cm³/cm² min) was obtained from the ultrathin hollow fibre membrane at 1323 K (1050 °C) and the sweep gas flow rate of 2.42 cm³/s. The results indicate that the oxygen permeation flux obtained is much higher (4.9-11.2 times) than that obtained from conventional LSCF hollow fibre membranes mainly due to the reduced thickness of the membrane as well as the porous surface on the permeate side. In addition, despite a very thin dense layer, the LSCF hollow fibre membrane possessed a reasonable mechanical strength (113.22 MPa).     

Improvement of mesoporosity of carbon cryogels by ultrasonic irradiation

Mesoporous carbon gels are usually obtained by pyrolyzing resorcinol-formaldehyde (RF) gels, which are synthesized via the sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution followed by drying. However, mesoporous carbon gels cannot be prepared under the conditions of high catalyst concentration or high pH of RF solution even by using supercritical drying or freeze drying. In this work, mesoporosity of carbon cryogels is improved by ultrasonic irradiation to RF solution. It is found that the gelation time of RF solution becomes greatly short by ultrasonic irradiation and that ultrasonic can improve mesoporosity of carbon cryogels prepared at high catalyst concentration (C/W). Although the carbon cryogels prepared from C/W = 80 mol/m³ have no mesopores, the carbon sonogels prepared by ultrasonic irradiation under the same catalyst condition have sharp mesopore size distribution. The utilization of ultrasonic in the preparation of RF gel is an interesting way in improving mesoporosity of carbon gels prepared at high C/W or pH.     

Shape-controlled synthesis of nanocarbons from resorcinol-formaldehyde nanopolymers using surfactant-templated vesicular assemblies

The NaOH-catalyzed polymerization of resorcinol (R) and formaldehyde (F) confined to the vesicular assemblies of cetyltrimethylammonium bromide as a core template mixed with 1,3,5-trimethylbenzene and tert-butanol (tBuOH) as cosurfactants yielded RF polymer/cetyltrimethylammonium composite nanowires and nanospheres depending on the amount of tBuOH. Carbonization of the resulting nanpolymers led to microporous carbon nanowires of 45-240 nm diameter and nanospheres of 260-650 nm diameter. Similar but a little modified reactions successfully resulted in monodispersed carbon nanospheres of ca. 280 nm diameter as well as monodispersed carbon nanowires of ca. 70 nm diameter with a high surface area of 1777 m²/g. The present approach could be further extended to the synthesis of a wide range of carbon nanomaterials by using various surfactants and cosurfactants.     

Activation, characterization and hydrogen storage properties of the mesoporous carbon CMK-3

Activation of mesoporous carbon CMK-3 with CO₂ for hydrogen storage was studied. Huge structure and texture changes emerged for the activated CMK-3 based on the characterization by using XRD, TEM and nitrogen adsorption at 77 K. The ordered mesoporous structure of CMK-3 gradually became disorder and its specific surface area and volume of pores especially micropores were enhanced remarkably. Hydrogen sorption measurement showed that the activation led to an obvious increase of the H₂ sorption capacity of CMK-3. The maximum H₂ uptake of 2.27 wt% at 77 K and 1 bar was obtained for the sample activated at 1223 K for 8 h. The small pores with the diameter smaller than 1 nm contributed greatly to the H₂ uptake, and were confirmed more effective than other pores for hydrogen storage.     

A high CO2 permselective mesoporous silica/carbon composite membrane for CO2 separation

Ordered mesoporous silica/carbon composite membranes with a high CO₂ permeability and selectivity were designed and prepared by incorporating SBA-15 or MCM-48 particles into polymeric precursors followed by heat treatment. The as-made composite membranes were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and N₂ adsorption, of which the gas separation performance in terms of gas permeability and selectivity were evaluated using the single gas (CO₂, N₂, CH₄) and gas mixtures (CO₂/N₂ and CO₂/CH₄, 50/50 mol.%). In comparison to the pure carbon membranes and microporous zeolite/C composite membranes, the as-made mesoporous silica/C composite membranes, and the MCM-48/C composite membrane in particular, exhibit an outstanding CO₂ gas permeability and selectivity for the separation of CO₂/CH₄ and CO₂/N₂ gas pairs owing to the smaller gas diffusive resistance through the membrane and additional gas permeation channels created by the incorporation of mesoporous silicas in carbon membrane matrix. The channel shape and dimension of mesoporous silicas are key parameters for governing the gas permeability of the as-made composite membranes. The gas separation mechanism and the functions of porous materials incorporated inside the composite membranes are addressed.     

Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol-formaldehyde polymer as carbon precursor

LiFePO₄/C composites were synthesized by two methods using home-made amorphous nano-FePO₄ as the iron precursor and soluble starch, sucrose, citric acid, and resorcinol-formaldehyde (RF) polymer as four carbon precursors, respectively. The crystalline structures, morphologies, compositions, electrochemical performances of the prepared powders were investigated with XRD, TEM, Raman, and cyclic voltammogram method. The results showed that employing soluble starch and sucrose as the carbon precursors resulted in a deficient carbon coating on the surface of LiFePO₄ particle, but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO₄ particle, and the corresponding thicknesses of the uniform carbon films are 2.5 nm and 4.5 nm, respectively. When RF polymer was used as the carbon precursor, the material showed the highest initial discharge capacity (138.4 mAh g^− 1 at 0.2 C at room temperature) and the best rate performance among the four materials.     

Facile in situ template synthesis of sulfonated polyimide/mesoporous silica hybrid proton exchange membrane for direct methanol fuel cells

Highly conductive and hydration retentive organic-inorganic hybrid proton exchange membranes for direct methanol fuel cells were synthesized by in situ sol-gel generation of mesoporous silica (mSiO₂) in sulfonated polyimide (SPI) via self-assembly route of organic surfactant templates for the tuning of the architecture of silica. The microstructure and properties of the resulting hybrid membranes were extensively characterized. The mesopores of about 3 nm in silica dispersion phase were formed in the SPI matrix. The existence of the mesoporous structure of silica improved the thermal stability, water-uptake and proton conductivity as well as methanol resistance of the hybrid membranes. The hybrid membrane with 30 wt.% mSiO₂ exhibited the water-uptake of 44.8% at 25 °C, and proton conductivity of 0.214 S cm⁻¹ at 80 °C at RH = 100%, while pure SPI exhibited the values of 40.6% and 0.179 S cm⁻¹ in the same test conditions, respectively. The results suggested that the highly hydrophilic character of Si-OH groups and the large surface area of mSiO₂ should contribute to the improvement of the water-uptake, meanwhile the mesoporous channels may supply the continuous proton conductive pathway in the hybrid membranes.     

Easy synthesis of ordered mesoporous carbon containing nickel nanoparticles by a low temperature hydrothermal method

Ordered mesoporous carbons (OMCs) with embedded metallic nickel (Ni) nanoparticles have been directly synthesized by a simple and low temperature (50 °C) hydrothermal method. The synthesis involved the use of a triblock copolymer Pluronic F127 as the mesostructure directing agent, resorcinol (R) and formaldehyde (F) as carbon precursors, and Ni(NO₃)₂·6H₂O as nickel source. It consisted in the self-assembly of F127, Ni²⁺ salt and RF polymer in an acidic medium and further carbonization, where the Ni²⁺ was captured by the network of F127/RF and further reduced into metallic Ni nanoparticles. The resultant Ni/carbon materials were characterised by X-ray diffraction, thermogravimetric analysis, transmission electron microscopy and nitrogen sorption. Ni/carbon materials with a highly ordered mesostructure were obtained using equal moles of resorcinol and formaldehyde molar ratio (R/F = 1/1), whereas an excess amount of formaldehyde (R/F = 1/2) was found to not form an ordered carbon structure. The results showed that nickel particles, with sizes of ∼10–50 nm, were homogeneously dispersed in the carbon matrices, while the pore mesostructure remained intact. The homogeneous Ni/carbon composites synthesized by this easy hydrothermal route have been demonstrated to be effective molecular adsorbents for magnetic separation.     

Separation of CO2/CH4 through carbon molecular sieve membranes derived from P84 polyimide

The separation of CO₂/CH₄ separation is industrially important especially for natural gas processing. In the past decades, polymeric membranes separation technology has been widely adopted for CO₂/CH₄ separation. However, polymeric membranes are suffering from plasticization by condensable CO₂ molecules. Thus, carbon molecular sieve membranes (CMSMs) with excellent separation performance and stability appear to be a promising candidate for CO₂/CH₄ separation. A commercially available polyimide, P84 has been chosen as a precursor in preparing carbon membranes for this study. P84 displays a very high selectivity among the polyimides. The carbonization process was carried out at 550–800 °C under vacuum environment. WAXD and density measurements were performed to characterize the morphology of carbon membranes. The permeation properties of single and equimolar binary gas mixture through carbon membranes were measured and analyzed. The highest selectivity was attained by carbon membranes pyrolyzed at 800 °C, where the pyrolysis temperatures significantly affected the permeation properties of carbon membranes. A comparison of permeation properties among carbon membranes derived from four commercially available polyimides showed that the P84 carbon membranes exhibited the highest separation efficiency for CO₂/CH₄ separation. The pure gas measurement underestimated the separation efficiency of carbon membranes, due to the restricted diffusion of non-adsorbable gas by adsorbable component in binary mixture.     

Polyvinylidene fluoride and polyetherimide hollow fiber membranes for CO2 stripping in membrane contactor

Porous polyvinylidene fluoride (PVDF) and polyetherimide (PEI) hollow fiber membranes incorporating polyethylene glycol (PEG) were prepared via spinning process for CO₂ membrane stripping. CO₂ loaded diethanolamine solution was used as liquid absorbent while N₂ was used as a strip gas. The characterization study of the fibers was carried out in terms of permeation test, contact angle measurement and liquid entry pressure (wetting pressure). Performance study via membrane contactor stripping was carried out at specific operating condition. The experimental results showed that PVDF membrane have high gas permeation, effective surface porosity and contact angle despite having lower liquid entry pressure in comparison with PEI membrane. PVDF-PEG membrane showed the highest stripping flux of 4.0 × 10⁻² mol m⁻² s⁻¹ at 0.7 ms⁻¹ compared to that of PEI membrane. Although the stripping flux for PEI-PEG membranes was slightly lower than PVDF membrane (e.g. 3.5 × 10⁻² mol m⁻² s⁻¹ at liquid velocity of 0.85 ms⁻¹), the membrane wetting pressure of PEI membrane is higher than hydrophobic PVDF membrane. Long term performance of both membranes showed severe flux reduction but started to level-off after 30 h of operation.     

Nanoscale fine-tuning of porosity of carbide-derived carbon prepared from molybdenum carbide

Micro- and mesoporous carbide-derived carbon (CDC) was synthesised from molybdenum carbide (Mo₂C) powder by gas phase chlorination in the temperature range from 400 to 1200 °C. Analysis of XRD results show that C(Mo₂C), chlorinated at 1200 °C, consist mainly on graphitic crystallites of mean size, L_a = 9 nm and L_c = 7.5 nm. The first-order Raman spectra showed the graphite-like absorption peak at ∼1587 cm⁻¹ and the disorder-induced (D) peak at ∼1348 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1855 m² g⁻¹ and total pore volume up to 1.399 cm³ g⁻¹ were obtained. Sorption measurements showed the presence of both micro- and mesopores after chlorination at 400-900 °C and only mesopores after chlorination at 1000°-1200 °C. Stepwise formation of micro- and mesopores was achieved and the peak pore size can be shifted from 0.8 nm up to 4 nm by increasing the chlorination temperature.