Surface and structural characterization of multi-walled carbon nanotubes following different oxidative treatments

Six commonly used wet chemical oxidants (HNO₃, KMnO₄, H₂SO₄/HNO₃, (NH₄)₂S₂O₈, H₂O₂, and O₃) were evaluated in terms of their effects on the surface chemistry and structure of MWCNTs using a combination of analytical techniques. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDX) were used to characterize the extent of surface oxidation, while chemical derivatization techniques used in conjunction with XPS allowed the concentration of carboxyl, carbonyl, and hydroxyl groups at the surface to be quantified for each MWCNT sample. Our results indicate that the distribution of oxygen-containing functional groups was insensitive to the reaction conditions (e.g., w/w% of oxidant), but was sensitive to the identity of the oxidant. MWCNTs treated with (NH₄)₂S₂O₈, H₂O₂, and O₃ yielded higher concentrations of carbonyl and hydroxyl functional groups, while more aggressive oxidants (e.g., HNO₃, KMnO₄) formed higher fractional concentrations of carboxyl groups. IR spectroscopy was unable to identify oxygen-containing functional groups present on MWCNTs, while Raman spectra highlighted the frequently ambiguous nature of this technique for measuring CNT structural integrity. TEM was able to provide detailed structural information on oxidized MWCNT, including the extent of sidewall damage for different oxidative treatments.     

Using oxidation to increase the electrical conductivity of carbon nanotube electrodes

Recent studies have demonstrated that significantly low sheet resistance (Rs) (<100 Ω/sq; comparable to ITO) were achieved in single-walled carbon nanotube (SWCNT) films treated with HNO₃ followed by thionyl chloride. Here we show that H₂SO₄ can effectively reduce the Rs of SWCNT electrodes. H₂SO₄ treatment generates defects (COOH and SO₃H functionalities) on SWCNTs and the produced chemical functionalities are beneficial for enhancing the electrical conductivity in SWCNT electrodes. It is plausible that the H₂SO₄p-dopes the SWCNTs and the attachment of chemical functionalities helps to stabilize p-doping owing to their electron-deficient property.     

Effects of carbon nanotube functionalization on the mechanical and thermal properties of epoxy composites

Chemically functionalized multi-walled carbon nanotube (MWCNT)/bisphenol-A glycidol ether epoxy resin/2-ethyl-4-methylimidazole composites were prepared. MWCNTs were first treated by a 3:1 (v/v) mixture of concentrated H₂SO₄/HNO₃, and then triethylenetetramine (TETA) grafting was carried out. X-ray photoelectron spectroscopy analysis proved the effectiveness of H₂SO₄/HNO₃ treatment and confirmed the TETA functionalization mechanism. Chemical functionalization decreases the crystalline content of MWCNTs, however, it did not greatly disrupt their structure. Transmission electron microscopy showed that there was a TETA thin layer on the MWCNT surface, which contributes to the homogenous dispersion of MWCNTs in epoxy matrix and the improvement of the MWCNT-epoxy interfacial interaction. Thus the impact strength, bending strength and thermal conductivity of the composites are enhanced.     

Flattening of oxidized diamond (111) surfaces with H2SO4/H2O2 solutions

Changes in the topography of a diamond (111) surface with atomically flat and wide terraces, caused by immersion in HNO₃/H₂SO₄ and H₂SO₄/H₂O₂ solutions were investigated by atomic force microscopy. We observed surface roughening from the HNO₃/H₂SO₄ treatment, and flattening of the HNO₃/H₂SO₄ treated surface from the H₂SO₄/H₂O₂ treatment. This suggests that the H₂SO₄/H₂O₂ treatment is an effective wet-process for preparing atomically flat oxidized diamond (111) surfaces.     

Effect of different oxidation treatments on the chemical structure and properties of commercial coal tar pitch

Different oxidation treatment was used for the increase of the softening point of a commercial coal tar pitch. H₂SO₄, HNO₃, H₂O₂ and air are selected as treatment reagents. These preliminary investigations show that the oxidation treatment of commercial coal tar pitch with different reagents at 160 °C and heat treatment to 250 °C causes considerable changes in the chemical composition of obtained pitches. This leads to increase of TI and QI fraction, and results in considerable increase in the softening point of the pitches. The yield of modified pitches is considerable in the case of treatment with H₂SO₄, H₂O₂, and HNO₃ and lesser in the case of air blowing. The data obtained also indicate some differences in the composition and softening point of pitches obtained after modification with different reagents. These differences could influence the applicability of the obtained pitches in the various areas of carbon material production.     

Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites

The electrical conductivity of oxidized multiwalled carbon nanotubes (MWNT)/epoxy composites is investigated with respect to the chemical treatment of the MWNT. The oxidation is carried out by refluxing the as-received MWNT in concentrated HNO₃ and H₂O₂/NH₄OH solutions, respectively, under several different treatment conditions. The oxidized MWNT are negatively charged and functionalized with carboxylic groups by both solutions. The MWNT oxidized under severe conditions are well purified, but their crystalline structures are partially damaged. It is recognized that the damage to the MWNT has considerable influence on the electrical properties of the MWNT composites, causing the electrical conductivity to be lowered at a low content of MWNT and the percolation threshold to be raised. The MWNT oxidized by the mixture of H₂O₂ and NH₄OH solution provides epoxy composites with a higher conductivity than those produced with the MWNT oxidized by nitric acid over the whole range of MWNT, independently of the oxidation conditions.     

Thermal decomposition of ammonium sulphate

The thermal decomposition and vaporisation of ammonium sulphate, (NH₄)₂SO₄, is shown to take place via two distinct sets of reactions. In the first, ammonium pyrosulphate, (NH₄)₂S₂O₇, is the primary condensed phase product: 2(NH₄)₂SO₄ ← (NH₄)₂S₂O₇+2NH₃+H₂O The second stage concerns the decomposition of the pyrosulphate. Ammonia, sulphur dioxide, nitrogen and water are the major products, the dominant reaction being 3(NH₄)₂S₂O₇ ← 2NH₃+6SO₂+2N₂+9H₂O     

Effects of pretreatment of natural graphite by oxidative solutions on its electrochemical performance as anode material

Modification of natural graphite has recently moved into the focus of methods to prepare anode materials for lithium ion batteries. Here we report a treatment method for the modification of a common natural graphite employing oxidative solutions. Four solutions based on H₂O₂, Ce(SO₄)₂, HNO₃ and (NH₄)₂S₂O₈ were employed; their effects were investigated with X-ray photoelectron spectroscopy, thermogravimmetric and differential thermal analysis, high resolution electron microscopy and measurement of electrochemical capacity. All oxidants resulted in marked improvements of reversible capacity, columbic efficiency in the first cycle and cycling behavior due to elimination of some imperfections with high activities towards lithium such as carbon chains, edge carbon atoms and sp³-hybridized carbon atoms, creation of more micropores/nano-channels, modification of the surface of natural graphite with a coverage of a dense layer of oxides, and improvement in stability of graphite structure. Effects of the oxidative solutions on electrochemical performance of natural graphite are compared.     

Modified activated carbons for catalytic wet air oxidation of phenol

This study aims at testing several activated carbons for the catalytic wet air oxidation (CWAO) of phenol solutions. Two commercial activated carbons were used both as received and modified by treatment with either HNO₃, (NH₄)₂S₂O₈, or H₂O₂ and by demineralisation with HCl. The activated carbons were characterised by measuring their surface area, distribution of surface functional groups and phenol adsorption capacity. The parent and treated activated carbons were then checked for CWAO using a trickle bed at 140 °C and 2 bar of oxygen partial pressure. The treatments increase the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Only (NH₄)₂S₂O₈ treatment yields a significant decrease in surface area. CWAO tests show that catalytic activity mainly depends on the origin of the activated carbon. The modifications generally had a low impact on phenol conversion, which correlates somewhat with the increase in the acidity of the carbons. Characterisation of the used activated carbon evidences that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals play a major role in the CWAO over activated carbon.     

An efficient strategy for the purification of cloth-like single walled carbon nanotube soot produced by arc discharge

High purity single walled carbon nanotubes (SWCNTs) were prepared from arc discharge produced cloth-like soot by a new purification strategy, in which liquid oxidation and steam oxidation were combined with a freeze-drying process to remove the metallic and carbonaceous impurities. The process gives a product of >98% purity, which is acquired from a gram-scale dirty raw soot with an overall yield of ∼75% of the SWCNTs. The purity of the samples was characterized by thermogravimetric analysis, scanning and transmission electron microscopy, Raman and Vis-NIR spectroscopy, and magnetometry. A highly pure SWCNT sample with relative purity of 170.4% and I_G/I_D value of 78.92 is achieved. Experiments showed that HNO₃/HCl refluxing combined with freeze-drying is the key process that renders the crude SWCNTs hydrophilic with a large surface area, and thus remarkably increases the efficiency of the steam treatment to remove most of the carbonaceous impurities.     

Quantitative characterization of acidic groups on acid-treated multi-walled carbon nanotubes using 1-aminopyrene as a fluorescent probe

Acidic functional groups produced on the surface of acid-treated multi-walled carbon nanotubes (MWCNTs) were quantified by fluorescence measurements using 1-aminopyrene (1-AP) as an in situ probe molecule. The 1-AP cation-like bands were observed on the HNO₃/H₂SO₄ mixture-treated MWCNT surfaces because the 1-AP molecule was tightly immobilized by the hydrogen bonding interaction between its amino group and the Brönsted-acidic groups on the MWCNT surface. The fluorescence measurement allowed us to confirm the Langmuir-type adsorption of 1-AP on the functional groups of the MWCNTs, and estimate their amount of the functional groups and adsorption equilibrium constant. A longer acid treatment caused the chemical modification to generate higher amounts of the Brönsted-acidic functional groups and improve the adsorption ability on the MWCNT surface. About 2% of carbon in the MWCNTs was oxidized by the 24 h acid treatment. This value corresponded to 15–22% of carbon in the surface layer.     

Surface modification and adsorption of eucalyptus wood-based activated carbons: Effects of oxidation treatment, carbon porous structure and activation method

The incorporation of oxygen functional groups onto the surface of eucalyptus activated carbon and its surface chemistry were investigated as a function of oxidation conditions, carbon porous properties and carbon preparation method. Under all treatment conditions of increasing time, temperature and oxidant concentration, liquid oxidation with HNO₃, H₂O₂ and (NH₄)₂S₂O₈ and air oxidation led to the increase of acidic group concentration, with carboxylic acid showing the largest percentage increase and air oxidation at the maximum allowable temperature of 350 °C produced the maximum content of both carboxylic acid and total acidic group. Nitric acid oxidation of chemically activated carbon produced higher total acidic content but a lower amount of carboxylic acid compared to the oxidized carbon from physical activation. The increased contents of acidic groups on oxidized carbons greatly enhanced the adsorption capacity of water vapor and heavy metal ions.     

Thermally induced variation in redox chemical bonding structures of single-walled carbon nanotubes exposed to hydrazine vapor

The effect of hydrazine (N₂H₄) vapor on the properties of single-walled carbon nanotube (SWCNT) networks was investigated by sheet resistance measurement, scanning electron microscopy, Raman spectroscopy, ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS). Our results show that, even after an auxiliary thermal desorption treatment at 80 °C, the n-doping effect on our SWCNTs caused by N₂H₄ vapor still persistently remained. Further analysis on the XPS data suggests that a reactive chemical species, nitrene (NH), generated during thermal decomposition of N₂H₄, could react with SWCNTs by cycloaddition to form cyclic nitrogen-containing aziridine structures on SWCNTs. Our results also show that the formed nitrogen-containing bonding structures were thermally metastable and could be significantly eliminated upon further annealing at 350 °C. Moreover, it was found that the N₂H₄ vapor treatment could introduce nitroso groups and carbonyl groups, but not carboxyl groups, to our pristine SWCNTs. The mild oxidation could be attributed to the HNO₂ and H₂O₂ produced from the reactions of NH and N₂H₄ with oxygen, respectively, when a N₂H₄ treatment was performed in air.     

Spectroscopic characterization of contaminants and interaction with gases in single-walled carbon nanotubes

Several spectroscopic techniques have been used to investigate the presence of contaminants in a commercial purified single-walled carbon nanotube (SWCNT) bucky paper, to determine their cleaning procedure in ultra-high-vacuum conditions and to study how impurities influence the interaction between SWCNTs and gas phase molecules. Nickel catalyst particles and sodium-containing species, likely a residual of the surfactant bath, were fully removed only after prolonged (>2 h) annealing at 1270 ± 30 K. Other impurity elements (S and Si) remain in the material as localised clusters that do not interact with the SWCNTs and do not interfere with their properties.A dramatic difference was observed when the Na-contaminated or the Na-free nanotubes interacted with molecular oxygen. O₂ adsorption was strongly altered by the Na traces, which simulated an intense sample oxidation causing a modification of the tube electronic properties. On the contrary, for the Na-free sample the lack of adsorbed oxygen and the stability of the C1s core level after large O₂ doses demonstrated the absence of any chemical bond between SWCNTs and O₂. Similarly, exposures to N₂, H₂O and CO do not have influence on the electronic properties of SWCNTs. Instead, a sizeable effect on the electronic spectra was observed for SO₂, NO and NO₂ adsorption. The sensitivity of the SWCNT electronic spectra to ppb quantities of nitrogen oxides and sulfur oxide undoubtedly foresees applications in the field of toxic gas sensing.     

Evaluation of mild acid oxidation treatments for MWCNT functionalization

Acidic oxidation methods have been widely reported as an effective method to purify and functionalize the surface of carbon nanotubes (CNTs). Although effective, the strong acids typically employed and the high sonication power used to disperse the nanotubes in the solution frequently cause nanotube damage, limiting their great potential as mechanical and electrical reinforcements. This work examines the use of HNO₃, H₂SO₄ and H₂O₂ at relatively low concentrations, short treatment times and low sonication power, in an attempt to achieve experimental conditions which efficiently functionalize the surface of multiwalled CNTs minimizing nanotube damage. A low power sonochemical treatment employing 3.0 M HNO₃ for 2 h followed by 2 h of identical treatment with H₂O₂ proved to be the most effective for this aim.     

Kinetic modeling and dynamic simulation for the catalytic wet air oxidation of aqueous ammonia to molecular nitrogen

A kinetic model for the catalytic wet air oxidation of aqueous ammonia over Ru/TiO₂ catalyst was developed considering the consecutive reaction steps as follows: (i) formation of active oxygen sites O* by the dissociative adsorption of aqueous O₂ on the catalyst, (ii) oxidation of aqueous NH₃ by the reaction with three O* sites to produce HNO₂, (iii) aqueous phase dissociation of HNO₂ into H⁺ and NO ₂ ^? , (iv) formation of NH ₄ ⁺ by the association of NH₃ with the HNO₂-dissociated H⁺, (v) formation of N₂ by the aqueous phase reaction between NO ₂ ^? and NH ₄ ⁺ , (vi) formation of NO₃ by the reaction of NO ₂ ^? with an O* site. For each reaction step, a rate equation was derived and its kinetic parameters were optimized by experimental data fitting. Activation energies for the reactions (ii), (v), and (vi) were 123.1, 76.7, and 54.5 kJ/mol, respectively, suggesting that the oxidation reaction of aqueous NH₃ to HNO₂ was a ratedetermining step. From the simulation using the kinetic parameters determined, the initial pH adjustment of the ammonia solution proved to be critical for determining the oxidation product selectivity between desirable N₂ and undesirable NO ₃ ^? as well as the degree of oxidation conversion of ammonia.     

Regeneration of activated carbon after contact with sulfuric acid solution

This study deal's with the feasible use of a commercial activated carbon in the uptake of H₂SO₄ from aqueous solution and with the regeneration of the spent product. Thermogravimetry TG and FT‐IR spectroscopy are used in the analysis of samples. The activated carbon is a very effective material for the uptake of H₂SO₄. Using a 9.0 mol dm⁻³ H₂SO₄ solution, the mass increase is 37.8 wt%. From the sample obtained, the H₂SO₄ can be removed largely either by heating at 250 °C for 2 h in a N₂ atmosphere or by washing thoroughly with distilled water for 24 h. The mass loss in both cases amounts to 33.6 wt%. The FT‐IR spectroscopy results indicate that the surface chemistry of the carbon is not affected, noticeably, at least, after its contact with the H₂SO₄ solution. The behavior of H₂SO₄ toward carbon is compared with that of HNO₃. © 2000 Society of Chemical Industry     

Sensitivity of single wall carbon nanotubes to oxidative processing: structural modification, intercalation and functionalisation

The effect of oxidation on modification of single wall carbon nanotubes (SWCNTs) through successive purification steps has been studied. The efficient elimination of metal impurities has been followed by induced coupled plasma spectroscopy. Upon acid treatment, Raman spectroscopy clearly proofed that HNO₃ molecules were intercalated into the bundles of SWCNTs. At the same time, SWCNTs also have suffered a high degree of degradation and defects were introduced. The subsequent thermal processes led to the removal of further defect carbon materials and to the almost complete de-intercalation of the HNO₃ molecules. Changes in the structure of the SWCNT bundles have been observed by transmission electron microscopy. While bundles tend to separate upon acid treatment, after the complete purification process, the remaining SWCNTs tend to form thick bundles again. The existence of functional groups in the raw single wall carbon nanotubes material and their modification and almost complete removal after the final annealing step has been studied by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and temperature programmed desorption. Nitrogen adsorption isotherms analysed according to Brunauer-Emmet-Teller showed important changes in the pore volume and surface area through the purification steps.     

Structural modulation of cage-like mesoporous KIT-5 silica by post-synthesis treatments with ammonia and/or sulfuric acid

The post-synthesis structural modulation of cage-like mesoporous KIT-5 silica by the treatments with the ammonia solution of NH₄OH and/or the aqueous solution of H₂SO₄ was studied. While the NH₄OH-treated KIT-5 silicas generally have smaller unit cell parameters, smaller mesopore cage sizes and lower micropore volumes, the H₂SO₄ treatment gave materials with larger cage-like mesopores and pore entrances but very low micropore volumes. The effects of consecutive treatments with NH₄OH and H₂SO₄ on the structural properties of the treated KIT-5 silicas were also investigated.     

A comparison between oxidation of activated carbon by electrochemical and chemical treatments

The anodic oxidation of a granular activated carbon (GAC) in NaCl solution has been studied. The influence of the electrocatalyst-anode material, applied current and time of treatment on both the surface chemistry and porous texture properties of the GAC has been analyzed. For comparison purposes, the same GAC has been treated with three of the classical chemical oxidants: HNO₃, H₂O₂ and (NH₄)₂S₂O₈ at different concentrations and for different times. Results show that the anodic treatment in NaCl causes a remarkable oxidation of the AC without modifying significantly its textural properties. TPD profiles and the linear dependence of the amount of CO- and CO₂-evolution against the oxidation level denotes that surface oxygen groups of similar nature and composition are formed anodically, regardless of the anode material. The achieved oxidation degree depends on the different ability of each anode for the electrochemical generation of highly oxidizing chlorine species, and it increases progressively with the applied current and the time of treatment. In general, for similar treatment times, the anodic treatment in NaCl can produce oxidation degrees much higher than the chemical treatment with (NH₄)₂S₂O₈, which has been found to be the most oxidative chemical studied in this work.