Surface modifications of activated carbon by gamma irradiation

Four commercial activated carbons with different chemical and textural characteristics were modified by gamma irradiation under five different conditions: irradiated in absence of water, in presence of ultrapure water, in ultrapure water at pH = 1.0 and 1000 mg L⁻¹ Cl⁻, in ultrapure water at pH = 7.5 and 1000 mg L⁻¹ Br⁻, and in ultrapure water at pH = 12.5 and 1000 mg L⁻¹ NO₃⁻. Changes in surface chemistry were studied by X-ray photoelectron spectroscopy; pH of point of zero charge, total acidic groups and total basic groups, which were determined by assessment with HCl and NaOH; and textural changes were determined by obtaining the corresponding adsorption isotherms of N₂ and CO₂. Outcomes show that the activated carbon surface chemistry can be modified by gamma irradiation and that the changes depend on the irradiation conditions. Modifications in the sp² hybridization of the surface carbons suggest that the irradiated carbons undergo graphitization. Measurements of structural parameters indicate that the irradiation treatment does not modify the textural properties of the carbons. Finally, studies of pristine and irradiated activated carbons using diffuse reflectance spectroscopy with the Kubelka–Munk function revealed a reduction in band gap energy in the irradiated carbons associated with an increase in sp² hybridization of the carbon atoms.     

Chitosan derived nitrogen-doped microporous carbons for high performance CO2 capture

Nitrogen-doped microporous carbons were fabricated by a simple chemical activation strategy in which chitosan and K₂CO₃ were employed as the precursor and activation agent, respectively. The textural and chemical properties of the porous carbons could be easily tuned by changing the ratio of K₂CO₃/chitosan and activation temperature. Due to their large pore volume, well-defined microporosity and relatively high nitrogen content, these porous carbons were applied as adsorbents for CO₂ capture and demonstrated excellent CO₂ uptake performances. In particular, the sample prepared at 635 °C with K₂CO₃/chitosan ratio = 2 shows a CO₂ uptake as high as 3.86 mmol g⁻¹ at 25 °C, 1 atm. Furthermore, the CO₂ uptake remains almost constant in five consecutive adsorption–desorption cycles, indicating this material has great stability and recyclability as a CO₂ sorbent. In addition, an extraordinary separation selectivity against N₂ (CO₂/N₂ selectivity of ca. 21) was also observed.     

Preparation and carbon dioxide uptake capacity of N-doped porous carbon materials derived from direct carbonization of zeolitic imidazolate framework

The preparation, characterization and CO₂ uptake performance of N-doped porous carbon materials and composites derived from direct carbonization of ZIF-8 under various conditions are presented for the first time. It is found that the carbonization temperature has remarkable effect on the compositions, the textural properties and consequently the CO₂ adsorption capacities of the ZIF-derived porous materials. Changing the carbonization temperature from 600 to 1000 °C, the composites and the resulting porous carbon materials possess a tuneable nitrogen content in the range of 7.1–24.8 wt%, a surface area of 362–1466 m² g⁻¹ and a pore volume of 0.27–0.87 cm³ g⁻¹, where a significant proportion of the porosity is contributed by micropores. These N-doped porous composites and carbons exhibit excellent CO₂ uptake capacities up to 3.8 mmol g⁻¹ at 25 °C and 1 bar with a CO₂ adsorption energy up to 26 kJ mol⁻¹ at higher CO₂ coverages. The average adsorption energy for CO₂ is one of the highest ever reported for any porous carbon materials. Moreover, the influence of textural properties on CO₂ capture performance of the resulting porous adsorbents has been discussed, which may pave the way to further develop higher efficient CO₂ adsorbent materials.     

Chemically activated fungi-based porous carbons for hydrogen storage

A set of porous carbons has been prepared by chemical activation of various fungi-based chars with KOH. The resulting carbon materials have high surface areas (1600–2500 m²/g) and pore volumes (0.80–1.56 cm³/g), regardless of the char precursors. The porosities mainly derived from micropores in activated carbons strongly depend on the activation parameters (temperature and KOH amount). All activated carbons have uniform micropores with pore size of 0.8–0.9 nm, but some have a second set of micropores (1.3–1.4 nm pore size), further broadened to 1.9–2.1 nm as a result of increasing either the activation temperature to 750 °C or KOH/char mass ratio to 5/1. These fungi-based porous carbons achieve an excellent H₂ uptake of up to 2.4 wt% at 1 bar and −196 °C, being in agreement with results from other porous carbonaceous adsorbents reported in the literature. At high pressure (ca. 35 bar), the saturated H₂ uptake reaches 4.2–4.7 wt% at −196 °C for these fungi-based porous carbons. The results imply a great potential of these fungi-based porous carbons as H₂ on-board storage media.     

Microporous carbons finely-tuned by cyclic high-pressure low-temperature oxidation and their use in electrochemical capacitors

Microporous carbons with a finely controlled porosity have been prepared from non-porous chars by cyclic oxidation/thermal desorption and further used in supercapacitor electrodes working in organic medium. The described activation method is shown to be effective for at least two types of non-porous carbons derived from sucrose and cellulose. The low temperature oxidation is realized by H₂O₂ at 200 °C and followed by thermal desorption of the surface functional groups at 900 °C under nitrogen flow. The porosity-forming procedure involves 4–5 oxidation/decomposition cycles, thus allowing a gradual adjustment of average pore size to that of ions making up the standard organic electrolyte ?1 mol L^?1 TEA⁺ BF₄^? in acetonitrile. The build-up of pore volume during the initial cycles proceeds essentially through the opening/formation and deepening of narrow micropores (L₀ ≈ 0.8 nm), whereas a slight pore widening appears to be the main outcome of further cycles. Due to the low burn-off of the overall process, the carbons are shown to form much denser coatings (0.71 g cm^?3) than a steam-activated carbon used in industrial supercapacitors (0.52 g cm^?3).     

High selectivity of TiC-CDC for CO2/N2 separation

A series of carbide-derived carbons (CDC) have been prepared starting from TiC and using different chlorine treatment temperatures (500–1200 °C). Contrary to N₂ adsorption measurements at −196 °C, CO₂ adsorption measurements at room temperature and high pressure (up to 1 MPa) together with immersion calorimetry measurements into dichloromethane suggest that the synthesized CDC exhibit a similar porous structure, in terms of narrow pore volume, independently of the temperature of the reactive extraction treatment used (samples synthesized below 1000 °C). Apparently, these carbide-derived carbons exhibit narrow constrictions were CO₂ adsorption under standard conditions (0 °C and atmospheric pressure) is kinetically restricted. The same accounts for a slightly larger molecule as N₂ at a lower adsorption temperature (−196 °C), i.e. textural parameters obtained from N₂ adsorption measurements on CDC must be underestimated. Furthermore, here we show experimentally that nitrogen exhibits an unusual behavior, poor affinity, on these carbide-derived carbons. CH₄ with a slightly larger diameter (0.39 nm) is able to partially access the inner porous structure whereas N₂, with a slightly smaller diameter (0.36 nm), does not. Consequently, these CDC can be envisaged as excellent sorbent for selective CO₂ capture in flue-gas streams.     

Spherical carbons: Synthesis,characterization and activation processes

Spherical carbons have been prepared through hydrothermal treatment of three carbohydrates (glucose, saccharose and cellulose). Preparation variables such as treatment time, treatment temperature and concentration of carbohydrate have been analyzed to obtain spherical carbons. These spherical carbons can be prepared with particle sizes larger than 10 μm, especially from saccharose, and have subsequently been activated using different activation processes (H₃PO₄, NaOH, KOH or physical activation with CO₂) to develop their textural properties. All these spherical carbons maintained their spherical morphology after the activation process, except when KOH/carbon ratios higher than 4/1 were used, which caused partial destruction of the spheres. The spherical activated carbons develop interesting textural properties with the four activating agents employed, reaching surface areas up to 3100 m²/g. Comparison of spherical activated carbons obtained with the different activating agents, taking into account the yields obtained after the activation process, shows that phosphoric acid activation produces spherical activated carbons with higher developed surface areas. Also, the spherical activated carbons present different oxygen groups’ content depending on the activating agent employed (higher surface oxygen groups content for chemical activation than for physical activation).     

Magnetic porous carbons with high adsorption capacity synthesized by a microwave-enhanced high temperature ionothermal method from a Fe-based metal-organic framework

Magnetic porous carbons with high surface areas were easily synthesized from a Fe-based metal-organic framework (MOF) by a novel microwave-enhanced high temperature ionothermal method. By choosing a Fe-based MOF called MIL-100(Fe) as both a Fe and C precursor and a porous template, and furfuryl alcohol as a second precursor, a series of γ-Fe₂O₃/C composites with strong magnetism were prepared in 3 min by a microwave-enhanced high temperature ionothermal method. Structure, morphology and magnetic property, as well as porosity of the products, were carefully studied by powder X-ray diffraction, X-ray photoelectron spectroscopy, the BET surface area method, thermogravimetry, vibrating sample magnetometry, scanning electron microscopy, and high resolution transmission electron microscopy. The obtained γ-Fe₂O₃/C composites possess both high surface areas and magnetic characteristics. Their adsorption properties were preliminarily tested by the adsorptive removal of methylene blue from aqueous solution. The results suggest that such magnetic carbon composite exhibited high adsorption capacity (303.95 mg g⁻¹) and fast adsorption kinetics, as well as a perfect magnetic separation performance (M_s = 4.12–19.54 emu g⁻¹), for the MB removal from aqueous solution.     

Hydrothermal and conventional H3PO4 activation of two natural bio-fibers

Two phosphoric acid activation procedures; Activation after Hydrothermal Impregnation (recently published) and Activation after Incipient Wetness Impregnation instead of conventional impregnation are analyzed in two natural bio-fiber precursors: banana pseudostem and coconut fiber matting. Both procedures are compared analyzing, in both precursors, the influence that variables such as H₃PO₄/precursor ratio, activation temperature and impregnation time have on the resulting activated carbons (ACs) properties. The work also pays special attention to the mesoporosity development and the application of these ACs to adsorb gasoline vapors.Both H₃PO₄ activation procedures develop activated carbons having suitable activation yields and porosity developments, giving the Activation after Incipient Wetness Impregnation method better results than the Activation after Hydrothermal Impregnation. Both natural bio-fibers are good precursors, rendering the coconut fiber matting better results than the banana pseudostem. The variables studied affect the porosity development, being precursor and H₃PO₄/precursor ratio the variables that most affect. By a suitable selection of these variables, activated carbons having high adsorption capacities (BET above 2500 m² g^?1 and micropore volume above 1.00 cm³ g^?1) and well developed mesoporosity (reaching 1.41 cm³ g^?1), can be prepared. Most of the samples prepared perform very well for adsorbing gasoline vapors, showing a linear relationship with their resulting volumes.     

Preparation of sulfur-doped microporous carbons for the storage of hydrogen and carbon dioxide

Structurally well ordered, sulfur-doped microporous carbon materials have been successfully prepared by a nanocasting method using zeolite EMC-2 as a hard template. The carbon materials exhibited well-resolved diffraction peaks in powder XRD patterns and ordered micropore channels in TEM images. Adjusting the synthesis conditions, carbons possess a tunable sulfur content in the range of 1.3–6.6 wt.%, a surface area of 729–1627 m² g^?1 and a pore volume of 0.60–0.90 cm³ g^?1. A significant proportion of the porosity in the carbons (up to 82% and 63% for surface area and pore volume, respectively) is contributed by micropores. The sulfur-doped microporous carbons exhibit isosteric heat of hydrogen adsorption up to 9.2 kJ mol^?1 and a high hydrogen uptake density of 14.3 × 10^?3 mmol m^?2 at ?196 °C and 20 bar, one of the highest ever observed for nanoporous carbons. They also show a high CO₂ adsorption energy up to 59 kJ mol^?1 at lower coverages (with 22 kJ mol^?1 at higher CO₂ coverages), the highest ever reported for any porous carbon materials and one of the highest amongst all the porous materials. These findings suggest that S-doped microporous carbons are potential promising adsorbents for hydrogen and CO₂.     

Clean graphene surface through high temperature annealing

The cleanliness of the surface of graphene is important for its proper functioning in devices and sensors. Impurities including residual poly(methyl methacrylate) (PMMA) and hydrocarbon contaminants can alter its electronic and chemical properties. In this study, we used two surface-sensitive techniques, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to monitor the chemical composition of the surface of graphene after washing it with acetone and annealing at high temperatures. The concentration of residual PMMA and hydrocarbon contaminants decreased as the annealing temperature increased. The atomic ratio of sp³ carbons to sp² carbons of a clean graphene surface determined using XPS can be used to estimate the amounts of sp³ defects in graphene. ToF-SIMS spectra indicate that residual PMMA was removed from the surface of graphene at 400 °C, while hydrocarbon contaminants required a higher temperature of 500 °C to remove. In ToF-SIMS spectra obtained at 500 °C, the characteristic ions for graphene, which are related to cleavage of ring structure, include C_x⁺ (x = 1, 2, 3…), C_xH⁺ and C_xH₂⁺ as well as C_x⁻ and C_xH⁻. For the first time, we developed a process to produce a very clean graphene surface which was verified by ToF-SIMS and XPS analyses.     

Substrate temperature effects on the electron field emission properties of nitrogen doped ultra-nanocrystalline diamond

For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N₂ in Ar/CH₄ plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm^− 1 Ω^− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm² to 15 mA/cm². The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp³-bonded carbons into sp²-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.     

Hydrogen storage in CO2-activated amorphous nanofibers and their monoliths

Amorphous carbon nanofibers (CNFs), produced by the polymer blend technique, are activated by CO₂ (ACNFs). Monoliths are synthesized from the precursor and from some ACNFs. Morphology and textural properties of these materials are studied. When compared with other activating agents (steam and alkaline hydroxides), CO₂ activation renders suitable yields and, contrarily to most other precursors, turns out to be advantageous for developing and controlling their narrow microporosity (<0.7 nm), V_DR(CO₂). The obtained ACNFs have a high compressibility and, consequently, a high packing density under mechanical pressure which can also be maintained upon monolith synthesis. H₂ adsorption is measured at two different conditions (77 K/0.11 MPa, and 298 K/20 MPa) and compared with other activated carbons. Under both conditions, H₂ uptake depends on the narrow microporosity of the prepared ACNFs. Interestingly, at room temperature these ACNFs perform better than other activated carbons, despite their lower porosity developments. At 298 K they reach a H₂ adsorption capacity as high as 1.3 wt.%, and a remarkable value of 1 wt.% in its mechanically resistant monolith form.     

Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions

Activated carbons were prepared from sodium lignosulfonate by phosphoric acid activation at carbonization temperatures of 400–1000 °C. The resulting materials were characterized with regard to their surface area, pore volume, pore size distribution, distribution of surface groups and ability to adsorb copper ions. Activated carbons were characterized by nitrogen adsorption, scanning electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analyses. The results indicate that with increasing carbonization temperature, the surface area decreased from 770 m²/g at 400 °C to 180 m²/g at 700 °C and increased at higher temperatures to 1370 m²/g at 1000 °C. The phosphorus content peaked at 11% for carbon obtained by carbonization at 800 °C. Potentiometric titration revealed the acidic character of all the phosphoric acid-activated carbons, which were found to have total concentrations of surface groups of up to 3.3 mmol/g. The carbons showed a high adsorption capacity for copper ions even at pH values as low as 2.     

Enhancement of CO2 adsorption on phenolic resin-based mesoporous carbons by KOH activation

Carbons with high surface area and large volume of ultramicropores were synthesized for CO₂ adsorption. First, mesoporous carbons were produced by soft-templating method using triblock copolymer Pluronic F127 as a structure directing agent and formaldehyde and either phloroglucinol or resorcinol as carbon precursors. The resulting carbons were mainly mesoporous with well-developed surface area, large total pore volume, and only moderate CO₂ uptake. To improve CO₂ adsorption, these carbons were subjected to KOH activation to enhance their microporosity. Activated carbons showed 2–3-fold increase in the specific surface area, resulting from substantial development of microporosity (3–5-fold increase in the micropore volume). KOH activation resulted in enhanced CO₂ adsorption at 760 mmHg pressure: 4.4 mmol g⁻¹ at 25 °C, and 7 mmol g⁻¹ at 0 °C. This substantial increase in the CO₂ uptake was achieved due to the development of ultramicroporosity, which was shown to be beneficial for CO₂ physisorption at low pressures. The resulting materials were investigated using low-temperature nitrogen physisorption, CO₂ sorption, and small-angle powder X-ray diffraction. High CO₂ uptake and good cyclability (without noticeable loss in CO₂ uptake after five runs) render ultramicroporous carbons as efficient CO₂ adsorbents at ambient conditions.     

Structural,optical, and electrical properties of conducting p-type transparent Cu–Cr–O thin films

Cu–Cr–O films were prepared by DC magnetron co-sputtering using Cu and Cr targets on quartz substrates. The films were then annealed at temperatures ranging from 400 °C to 900 °C for 2 h under a controlled Ar atmosphere. The as-deposited and 400 °C-annealed films were amorphous, semi-transparent, and insulated. After annealing at 500 °C, the Cu–Cr–O films contained a mixture of monoclinic CuO and spinel CuCr₂O₄ phases. Annealing at 600 °C led to the formation of delafossite CuCrO₂ phases. When the annealing was further increased to temperatures above 700 °C, the films exhibited a pure delafossite CuCrO₂ phase. The crystallinity and grain size also increased with the annealing temperature. The formation of the delafossite CuCrO₂ phase during post-annealing processing was in good agreement with thermodynamics. The optimum conductivity and transparency were achieved for the film annealed at approximately 700 °C with a figure of merit of 1.51×10⁻⁸ Ω⁻¹ (i.e., electrical resistivity of up to 5.13 Ω-cm and visible light transmittance of up to 58.3%). The lower formation temperature and superior properties of CuCrO₂ found in this study indicated the higher potential of this material for practical applications compared to CuAlO₂.     

Carbons with narrow pore size distribution prepared by simultaneous carbonization and self-activation of tobacco stems and their application to supercapacitors

Highly microporous carbons with narrow pore size distribution have been prepared by simultaneous carbonization and self-activation of tobacco wastes at temperatures ranging from 600 to 1000 °C. The efficiency of porosity development, without pores broadening, is attributed to well-distributed alkalis at the molecular level in the tobacco precursor. With Burley tobacco, the BET specific surface area and average micropore size L₀ increased up to 800 °C (Burley 800), where the values reached maxima of 1749 m² g⁻¹ and 1.2 nm, respectively. At temperatures higher than 800 °C, annealing of the materials dominates and provokes a decrease of S_BET and L₀. Burley carbons were implemented in supercapacitors using 1 mol L⁻¹ aqueous Li₂SO₄ or 1 mol L⁻¹ TEABF₄ in acetonitrile. In both electrolytes, the capacitance of Burley carbons followed the same trend as S_BET and L₀. Burley 800 demonstrated outstanding capacitance values of 167 F g⁻¹ (at 0.8 V limit) and 141 F g⁻¹ (at 2.3 V limit) in 1 mol L⁻¹ aqueous Li₂SO₄ and 1 mol L⁻¹ TEABF₄, respectively. Such values, about 50% higher as compared to commercially available carbons, are attributed to the narrow pore size distribution of this carbon with a maximum of pores around 1.2 nm close to the size of solvated ions in these electrolytes.     

Superior carbon-based CO2 adsorbents prepared from poplar anthers

A series of renewable nitrogen-containing granular porous carbons with developed porosities and controlled surface chemical properties were prepared from poplar anthers. The preparation conditions such as pre-carbonization and activation temperatures and KOH amount significantly influence the structures and chemical compositions of the porous carbons, the CO₂ adsorption capacities of which are highly dependent on their pore structures, surface areas, nitrogen contents and adsorption conditions. The sample with developed microporosity, especially with the pores between 0.43 and 1 nm and high nitrogen content shows high CO₂ adsorption capacity at 1 bar and 25 °C. In contrast, when the adsorption pressure is higher than 5 bar, its CO₂ adsorption capacity is dominated by its surface area, and more accurately by its pore volume. Irrespective of this, if the pressure was decreased to 0.1 bar, its CO₂ capture ability is closely correlated to its nitrogen content but not to its porosity. By optimizing the preparation conditions, a porous carbon with a surface area of 3322 m² g⁻¹ and a CO₂ adsorption capacity as high as 51.3 mmol g⁻¹ at 50 bar and 25 °C was prepared.     

Electrochemical hydrogen storage in activated carbons with different pore structures derived from certain lignocellulose materials

Activated carbons exhibiting high hydrogen electrosorption were produced from selected precursors: coconut shells, blackthorn stones, cellulose and lignin. The influence of the carbonization condition and activation with KOH on their structural parameters and, hence, hydrogen electrosorption ability, was investigated. A positive effect of fast, aggressive activation was demonstrated for precursors subjected to initial carbonization at moderately high temperature (700 °C). The highest hydrogen electrosorption, allowing storage of electrical charge of 625 mA h g^?1, an equivalent of hydrogen gas storage of 2.31 wt.%, was obtained for blackthorn stones after 15 min activation at 950 °C and carbonizate/KOH ratio of 1/5. Further prolongation of the activation process is disadvantageous as it leads to the burn-off of the sample and the increase of the micropore diameter and, finally, reduction of hydrogen sorption ability. This loss cannot be fully compensated by the secondary pores created afterwards by the burn-off with K₂CO₃. The investigations confirmed that an efficient hydrogen electrosorption could be obtained by using active carbons made of cheap and easily available natural, plant-derived precursors, instead of special carbonaceous materials produced by complicated methods.     

Li2ZnTi3O8 based High κ LTCC tapes for improved thermal management in hybrid circuit applications

A low temperature co-fired ceramic based on Li₂ZnTi₃O₈ (LZT), that possess auspicious thermal and dielectric properties is reported. In order to achieve the low sintering temperature suitable for LTCC applications (875 °C), 1 wt% of 20:Li₂O-20: MgO-20: ZnO-20:B₂O₃-20: SiO₂ (LMZBS) glass was added to LZT ceramics. The post-milled powder had an average particle size of 450 nm with an effective surface area of 0.812 m²g⁻¹. A well dispersed tape casting slurry was prepared using xylene/ethanol mixture as solvent and fish oil as dispersant. The crystal structure and microstructure of the tapes were analyzed through XRD and scanning electron microscopy (SEM). The microwave dielectric properties of the green as well as sintered tapes were measured at different frequencies (5, 10 and 15 GHz). The Li₂ZnTi₃O₈+1 wt% LMZBS has shown excellent thermal conductivity of 5.8 W/mK, thermal expansivity (11.97 ppm/°C) closer to silver electrode, low temperature coefficient of dielectric constant (−29 ppm/°C) and ultralow dielectric losses (tanδ~10⁻⁴).