Preparation and characterisation of single-walled carbon nanotubes functionalised with amines

The amines octadecylamine, 2-aminoanthracene, 1-H,1-H-pentadecafluorooctylamine, 4-perfluorooctylaniline and 2,4-bis(perfluorooctyl)aniline, have been reacted with chemically modified single-walled carbon nanotubes (SWCNTs). The chemical modification consisted on a thermal treatment in air of arc discharge-grown SWCNTs with an optional purification with HNO₃. The selected amines have been allowed to interact with the obtained materials and with those resulting from an additional treatment with thionyl chloride. The resulting materials were characterised with different spectroscopic techniques such as X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, Raman spectroscopy, electron energy loss spectroscopy and fluorescence spectroscopy complemented with the thermogravimetric analysis and transmission electron microscopy. The chemically modified SWCNTs exhibit different solubility properties depending on the amines and the solvents used.     

Chemical preparation and characterization of nitrogen-rich carbon nitride powders

Creamy white powders were obtained by the chemical vapor reaction of carbon tetrachloride and ammonia at 1000 °C, followed by washing with boiling water. The composition of the material was C₃N_5.5O_0.5H_5.4, having a nitrogen content greater than that of the hypothetical hard material C₃N₄. The powders were harder than quartz whose Mohs hardness is 7, but were not as hard as sapphire (Mohs hardness: 9). The hardness could be explained by the C-N single bond observed in the ESCA C1s spectrum. The material washed with boiling water showed photoluminescence, which was mostly bright and white-blue in color to the naked eye.     

X-ray photoelectron spectroscopy of polybithiophene–polystyrene composites

An investigation of the structure, stability, and charge distributions of conducting polybithiophene–polystyrene composite chemically synthetized using X-ray photoelectron spectroscopy (XPS) and electron microprobe analysis is described. XPS results confirm the reduction of the oxidant (Fe³⁺ is reduced to Fe²⁺ and Cu²⁺ to Cu⁺) during the bithiophene polymerization and indicate that the positive charges of doped polybithiophene are preferentially localized on the carbon atoms. Measurements versus ambient atmosphere exposure support a decreasing of the atomic ratio Cl/Fe or Cl/Cu and an increasing of the atomic ratio O/C, which could be responsible for the observed electrical conductivity instability. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1839–1845, 1997     

High resolution XPS characterization of chemical functionalised MWCNTs and SWCNTs

High resolution XPS analysis of chemical functionalised multi-wall carbon nanotubes (MWCNTs) and single wall carbon nanotubes (SWCNT) was done with ESCA300 (overall instrument resolution of 0.35 eV). Information to the degree of functionalisation was ascertained by argon ion bombardment of the samples followed by XPS analysis to detect the functional groups, the percentage atomic concentration of various elements present and whether or not the detected functional groups imposed a chemical shift on the CNT atoms. The results show that true chemical functionalisation was achieved and by argon ion bombardment these functional groups can be altered relative to the C 1s carbon atoms of the CNT. The choice of chemicals used for functionalisation, the techniques employed and the types of nanotubes treated are important factors in chemical characterisation. The carbon atom on the nanotube ring to which the functional group (atom) is bonded, the chirality of the CNT, the electronegativity of the functional group, the bond type and whether the CNT is single-wall or multi-wall, or cut (short) could play a role in determining the chemical shift on the CNTs atoms. These investigations are relevant to chemical functionalisation of carbon nanotubes for various applications for example DNA sensors and other biomedical sensors.     

X-ray photoelectron spectroscopy study of anodically oxidized SIMFUEL surfaces

The surface composition of anodically oxidized SIMFUEL (doped uranium dioxide) has been determined as a function of applied potential over the range −500 to +500 mV (versus SCE). Cathodically cleaned UO₂ specimens were anodically oxidized for 1 h and subsequently analyzed by XPS. Using published binding energies, the U (4f_7/2) and O (1s) peaks were resolved into contributions from U^IV, U^V, U^VI, O^II, OH⁻ and H₂O. It was shown that over the potential range −500 to approximately +50 mV a thin surface layer of U^IV/U^V oxide (UO_2+x) formed. At more positive potentials, a U^VI hydrated oxide (UO₃·yH₂O) was deposited on the electrode surface. At very positive potentials (≥400 mV) the rapid anodic formation and hydrolysis of UO₂²⁺ led to local acidification in pores in the deposited UO₃·yH₂O layer and its subsequent re-dissolution.     

X-ray photoelectron spectroscopy study of sodium reactions in carbon cathode blocks of aluminium oxide reduction cells

Aluminium oxide reduction was performed in a laboratory electrolysis cell with different industrial carbon cathode blocks (semi-graphitic, graphitic, and graphitized blocks). During electrolysis, sodium species migrate from the bath into the carbon cathode. Consequences of this migration include expansion of the blocks—the so-called sodium swelling—that may lead to failure of the cell. Characterisation of the blocks by XPS indicated that in addition to ionic sodium species (e.g. NaF and NaHCO₃), two different types of metallic sodium were present in the cathodes. One type of metallic sodium is associated with a degradation of the graphitic structure, suggesting that this sodium is intercalated between the graphene layers, whereas the other type of metallic sodium was most probably present in micropores. Both types of metallic sodium were detected in semi-graphitic blocks while only the “micropore” sodium was found in graphitic and graphitized blocks. The metallic sodium was remarkably stable in the laboratory atmosphere, probably due to the fact that, after electrolysis, the entire porosity of the carbon cathode is filled with penetrated bath. This limits the access of oxygen and humidity to the metallic sodium.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

Detection of low concentration oxygen containing functional groups on activated carbon fiber surfaces through fluorescent labeling

Covalent fluorescent labeling of surface species (FLOSS) was used to detect relatively low concentrations of surface functional groups (OH, COOH and CHO) on activated carbon fiber surfaces. The chromophores were attached to the surface through a reaction specific to each type of surface functional group. FLOSS indicated the presence of 8.7 × 10¹¹ COOH groups/cm² and 1.3 × 10¹² CHO groups/cm² on the ACF 25 fiber surface. Neither the infrared spectrum nor the X-ray photoelectron spectrum showed evidence of the existence of those low concentration groups. The concentration of OH groups on the fiber surface was lower than the detection limit (∼10¹⁰/cm²) of FLOSS under the present conditions. The FLOSS results for CHO and COOH groups were compared with the concentrations determined by Boehm titration (3.11 × 10¹³/cm² for CHO and 1.05 × 10¹³/cm² for COOH). The limited accessibility of the ACF surface to relatively large chromophores is one of the main reasons for the discrepancy between these two methods. FLOSS detects only exposed functional groups as opposed to functional groups hidden in small pores. This apparent limitation, however, highlights the surface sensitivity and specificity of FLOSS technique.     

X-ray absorption near edge structure and X-ray photoelectron spectroscopy studies of chloride in passive oxide films

X-ray absorption near edge structure (XANES), utilizing both electron yield and X-ray fluorescence detectors, and X-ray photoelectron spectroscopy (XPS) were used to follow chloride uptake by oxide-covered aluminum in 0.1 M NaCl solutions. The aluminum samples were polarized at selected potentials below (less positive than) the pitting potential. The electron yield XANES and XPS showed multiple peaks. The XPS chloride spectra showed two distinct sets of doublets. One doublet is related to chloride on the surface and the second is related to chloride incorporated in the oxide film. The XANES results also showed two peaks which are attributed to chloride on the surface and in the bulk of the oxide.     

X-ray absorption near-edge structure and photoelectron spectroscopy of single-walled carbon nanotubes modified by a HBr solution

X-ray absorption near-edge structure (XANES) spectroscopy and photoelectron spectroscopy (PES) have been used to investigate single-walled carbon nanotubes (SWNTs) modified by immersion in a HBr solution at room temperature. After treatment XANES spectra of SWNTs show a new pronounced feature, which has been assigned to new bonds between the sidewall of the SWNTs and Br atoms. This investigation demonstrates the unique capabilities of the XANES spectroscopy as a tool to achieve structural and bonding information of carbon nanotubes induced by chemical processes.     

Heavy metal-adsorption on micas and clay minerals studied by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to study the adsorption of Cs-, Ba-, Cu-, Zn-, and Pb-ions on the external surfaces of various, well characterized 2:1 layer silicates (micas and illites).Before studying metal adsorption, it was necessary to determine the charge magnitude of the adsorption surface. This was done for chemically well-characterized micas (margarite, muscovite, sericite). The XPS analyses showed that the depth of analysis is about 15 Å. As a result it was possible to measure the surface- and interlayer ions on both sides of the outermost 2:1 layer. In determining the layer charges, the following strategy was used. The outer surface cations were replaced by Ba²⁺, giving, for ideal margarite an interlayer cation (Ca²⁺)/surface cation (Ba²⁺) ratio of 2:1 and in the case of muscovite a K⁺/Ba²⁺ ratio of 4:1. Deviations from these ratios indicate an asymmetry of layer charge in the outer sheet. Using the margarite, muscovite and sericite as standards, surface charge determination of a number of micas, illites, and I/S clays could be carried out by XPS.The properties of the metal-ions (charge, ionic radius, ionic potential), as well as layer charge characteristics of the clay, including surface charge magnitude and point of origin from tetrahedral or octahedral substitution, are factors which influence adsorption selectivity [Sposito, G., 1989. Surface reactions in natural aqueous colloidal solutions, G. Chimia, 43, 169–176]. The selection of previously well-characterized minerals, margarite, muscovite, celadonite, illite, montmorillonite, and beidellite for XPS study made it possible to relate these factors to heavy metal adsorption by the clay minerals.The results show that Cu²⁺ and Zn²⁺ are adsorbed as monovalent ions, presumably as (CuOH)¹⁺ and (ZnOH)¹⁺ hydroxy surface-complexes, due to their high ionic potential. Saturating the mica series with equimolar pairs of Cu–Zn and Cu–Pb, the ratios of Cu/Zn and Cu/Pb increase systematically with external surface charge. The higher the surface charge, the more selective is the exchange process for Cu with respect to Zn or Pb. Increasing external surface charge parallels increasing tetrahedral charge, which indicates that selectivity takes place at points of tetrahedral negativity on the crystallite surface, whereas octrahedral charge plays little role in the selective adsorption process.     

X-ray scattering and spectroscopy studies on diesel soot from oxygenated fuel under various engine load conditions

Diesel soot from reference diesel fuel and oxygenated fuel under idle and load engine conditions was investigated with X-ray scattering and X-ray carbon K-edge absorption spectroscopy. Up to five characteristic size ranges were found. Idle soot was generally found to have larger primary particles and aggregates but smaller crystallites, than load soot. Load soot has a higher degree of crystallinity than idle soot. Adding oxygenates to diesel fuel enhanced differences in the characteristics of diesel soot, or even reversed them. Aromaticity of idle soot from oxygenated diesel fuel was significantly larger than from the corresponding load soot. Carbon near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was applied to gather information about the presence of relative amounts of carbon double bonds (CC, CO) and carbon single bonds (C-H, C-OH, COOH). Using scanning X-ray transmission microspectroscopy (STXM), the relative amounts of these carbon bond states were shown to vary spatially over distances approximately 50 to 100 nm. The results from the X-ray techniques are supported by thermo-gravimetry analysis and high-resolution transmission electron microscopy.     

sp/sp characterization of carbon materials from first-principles calculations: X-ray photoelectron versus high energy electron energy-loss spectroscopy techniques

We have performed X-ray photoelectron spectroscopy (XPS) and high energy electron energy-loss spectroscopy (HEELS) calculations from first principles on a series of Monte Carlo generated amorphous carbon materials and have used a technique which separates the π* and σ* components of the energy-loss near-edge structure spectra of carbon materials on the basis of the ab initio electronic structure calculations of graphite to determine the sp³ fraction of the carbon systems. While the XPS technique is found to probe the local coordination geometry, the sp³ fractions resulting from the HEELS technique are found to be in very good agreement with those based on the π-orbital axis vector analysis which accounts for the effects of non-planarity in 3-coordinated systems.     

Analysis of hydrothermally-treated and weathered coals by X-ray photoelectron spectroscopy (XPS)

Analysis of hydrothermally-treated and weathered coals by X-ray photoelectron spectroscopy (XPS) was carried out, and the XPS C(1s) and N(1s) spectra obtained were curve-resolved into four peaks (C-C/CC/C-H, C-O, CO, and O-CO) and three peaks (pyridinic-N, pyrrolic-N, and quaternary-N), respectively. Upon hydrothermal treatment, the amount of carbon-oxygen forms decreased; while the ratio of pyridinic-N increased and quaternary-N decreased. On the other hand, some bituminous coals were subjected to natural weathering and laboratory oxidation, which gave opposite results compared to the hydrothermal treatment. The changes in the carbon-oxygen and organic nitrogen forms were discussed in terms of the effect of hydrothermal treatment and weathering (oxidation). Also, the XPS analysis of various kinds of coals (43 SS coals) was carried out, and the amounts of carbon-oxygen and organic nitrogen forms were discussed in terms of coal rank (carbon content).     

The influence of demineralisation and ammoxidation on the adsorption properties of an activated carbon prepared from a Polish lignite

The preparation of cheap nitrogen-enriched materials with large adsorptive capacities and selectivity towards volatile organic compounds remains a challenge. Ammoxidation has been used to prepare nitrogen-enriched activated carbons using a demineralised Polish lignite. The lignite samples were demineralised by two different methods before nitrogen-enrichment by ammoxidation and physical activation in steam. The surface chemistry was investigated by elemental analysis, Boehm titration, infrared and XPS spectroscopies and adsorptive properties by a linear solvation energy relationship approach. Results show a quasi-total demineralisation and a higher reactivity towards nitrogen for the demineralised samples. The BET surface is also higher than for the non-demineralised lignite. Active carbons previously ammoxidated and demineralised are more interesting in terms of selective removal of gaseous pollutants.     

Structural and electrochemical characterization of fullerene-based surfaces of C60 mono- or bis-adducts grafted onto self-assembled monolayers

Single layers of C₆₀ mono- and bis-adducts have been obtained by functionalizing an acid-terminated self-assembled monolayer (SAM) of thiols on gold. X-ray photoelectron spectroscopy demonstrated the grafting onto the SAM by covalent bonding, via the formation of an amide bond, while cyclic voltammetry and electrochemical impedance spectroscopy provided information on the redox properties of the C₆₀-films, as well as on structural characteristics.     

Nitrogen in aramid-based activated carbon fibers by TPD, XPS and XANES

Activated carbon fibers were prepared from Nomex® [poly(m-phenylene isophthalamide)] by either H₃PO₄ activation, H₃PO₄-CO₂ activation, or simply CO₂ or steam activation. These treatments converted amide groups from the polymer precursor into complex and heterogeneously distributed nitrogen functionalities. TPD, XPS and XANES were used to study the effects of these treatments on the local bonding environment around nitrogen in the resulting carbons. These analytical techniques showed that nitrogen atoms are present in the 6-membered rings located at the edges of condensed polyaromatic systems as pyridine-like sp² nitrogen (N1 or N2) or in the interior, where nitrogen replaces one carbon atom and is bonded to three carbon neighbors (N3). The occurrence of a species (N2) hypothetically related to a pyridinic cycle bearing oxygen substituents or intracyclic oxygen atoms could be correlated with the degree of oxidation of the carbon surface. Assuming that a relative N3 increase is indicative of aromatization and that the reverse, correlated with a N2 increase, is indicative of surface oxidative denitrogenation, the ratio between these nitrogen species revealed that aromatization and oxidative denitrogenation processes occur sequentially or simultaneously to different extents according to the type of carbon activation and to the burn-off degree. Physical activation involves thermal aromatization reactions during the carbonization stage and the subsequent isothermal activation one. In this second activation stage, co-occurring thermal oxidation reactions lead to a less intense denitrogenation during CO₂ activation than during steam activation. H₃PO₄ activation induces the largest nitrogen retention in the final product in a double process of aromatization and “auto-activation” producing a moderate oxidative attack of nitrogen. However, an increase of the H₃PO₄ ratio fostered the oxidation of the carbon surface and consequently enhanced nitrogen gasification during the subsequent activation.     

Impact of Mn2+-Si4+ co-substitution on the electronic structure of Zn0.3Mn0.7Fe2O4 ferrites studied by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) has been employed to explore the electronic structure of Zn_0.3Mn_0.7+xSi_xFe_2-2xO₄ (x = 0.0–0.3) ferrite series. The Si2p XPS spectra insinuated the presence of Si ions in the +4 valence state. The elemental Si⁰ and suboxide SiO_x are present in the system, the former showing an increase and the latter a decrease in atomic percentage upon Mn–Si substitution. It is also inferred that a fraction of Si⁰ might be residing at the grain boundaries; however, more studies are required to substantiate this. The Fe2p XPS spectra stipulate that ferrous and ferric ions co-occur in the system. The ferrous ions occupy the octahedral sites while the ferric ions dwell on both the octahedral and the tetrahedral sites. The O1s spectra indicate a remarkable increase in the oxygen defects with increasing Mn–Si substitution (x). The Mn2p XPS data indicate that the Mn⁺² states show an overall increasing tendency with increasing Mn–Si concentration. Also, the Mn⁺⁴/Mn⁺³ ratio shows an increment with an increase in Mn–Si substitution.