Preparation of pillared carbon thin films and their size selective electrical response to organic vapor

Thin films of graphite oxide silylated with octyltrichlorosilane were first prepared and they were further silylated with 3-aminopropyltriethoxysilane. Pillared carbon thin films with an interlayer spacing of 1.7 nm were prepared from the pyrolysis of the resulting thin films. Larger propylene carbonate molecules were not intercalated into them. They showed selective electrical response to electron donating and smaller organic molecules. The increase in the electrical resistance upon exposure to organic molecule seems to be larger for those with larger donor numbers.     

Preparation of pillared carbon thin films from the reduction of silylated graphite oxide by UV light irradiation and their size selective electrical response to various molecules

Silylated graphite oxide thin films were reduced by UV light irradiation using a super high pressure Hg lamp at 95 °C. The reduction of silylated graphite oxide was completed within 24 h and a new pillared carbon with an interlayer spacing of 0.81 nm was obtained. Pillared carbons with larger interlayer spacings of about 1.13 nm were also obtained from graphite oxide silylated with octyltrichlorosilane and then with 3-aminopropyltriethoxysilane for 1.5–6 h, when they were reduced by UV light irradiation and heating at 200 °C under vacuum. Selective electrical response during exposure to gaseous vinylene carbonate, acetonitrile, ozone and hydrogen molecules has been achieved by changing the pillar density in the resulting pillared carbon films.     

Graphite oxide intercalation compounds with rotator hexagonal order in the intercalated layers

Graphite oxide intercalation compounds (GOIC), exhibiting a large distance between graphite oxide sheets as well as a long-range order in the organization of guest species, are obtained by intercalation of organic cations containing two long hydrocarbon tails. In particular, graphite oxide layers with interlayer spacing of 0.84 nm, when ionically bonded with cations with two C18 chains, lead to GOIC with interlayer spacing of 3.4 nm and with a hexagonal rotator order in the packing of the long hydrocarbon tails. The intercalation of a second guest species with one or two long hydrocarbon tails not only leads to a further large increase of the interlayer spacing (from 3.4 nm up to 5.8 nm, for guests with C18 alkyl chains) but also, surprisingly, improves the order in the stacking of the layers as well as in the organization of the hydrocarbon chains in the interlayer space. X-ray diffraction measurements on powders and oriented films indicate that these ordered GOIC present intercalate structures with inclined and perpendicular bilayers of guest molecules. Differential scanning calorimetry associated with X-ray diffraction measurements show the occurrence of reversible loss of the hydrocarbon rotator order, which can be associated with significant changes of the interlayer spacing.     

The effect of alkyl chain length on the structure of pillared carbons prepared by the silylation of graphite oxide with alkyltrichlorosilanes

Pillared carbon was obtained from graphite oxide silylated with alkyltrichlorosilanes with various alkyl chain lengths. The interlayer spacings of silylated graphite oxides became maximal when 1.1-1.5 molecules of bound silyl groups were attached per C₈ graphite oxide unit. Pillared carbon with an interlayer spacing of 1.27 nm was obtained only when graphite oxide silylated with methyltrichlorosilane was heated under vacuum above 300 °C. Infrared absorption measurements indicated that the pillars possessed the ladder type silsesquioxane structure. The adjacent graphite oxide layers were connected to each other even before pyrolysis, judging from the observation that n-hexadecylamine was not intercalated. The formation of a ladder type pillar was completed after the oxygen-containing groups were removed from the graphite oxide layers. When the alkyl chain length was longer, the connection of adjacent layers became difficult and pillared carbons were not formed. The surface area of the pillared carbon was very small because the distance between pillars was too small for nitrogen molecules to pass, but it was estimated based on the composition of the pillared carbon.     

In-situ observation of structural change in MWCNTs under high-pressure H2 gas atmosphere

Multi-walled carbon nanotubes having a tube-opened framework (o-MWCNTs) have been pressurized in an atmosphere of hydrogen up to 2.5 GPa and at room temperature using a diamond-anvil cell (DAC). Compression up to 0.57 GPa was accompanied by expansion of the honeycom-lattice structure of graphene sheets in spite of decreasing interlayer distance. A deviation in the in-plane C–C distance was obtained to be 3.8 × 10^− 4 nm at 0.57 GPa. The expansion in the honeycom-lattice structure suggests generation of charge-transfer interaction between graphite and hydrogen. This is one of the proofs that hydrogen is intercalated in the interlayer space of o-MWCNTs.     

Chemical and thermal properties of organoclays derived from highly stable bentonite in sulfuric acid

Generally, acid activation modified the physico-chemical properties of the raw clay minerals. The extent of these modifications depended on the type, origin of the clay minerals and the conditions of the acid activation. In this study, a bentonite exhibited a strong stability toward the acid treatment at 90 °C and at higher acid/clay mineral ratios, with slight depletion of Mg^2 +, Fe^3 + and Al^3 + cations (about 5%). The resulting organo-acid activated clays prepared after a reaction with cetyltrimethylammonium (C16TMA) hydroxide solution, exhibited uptaken amounts of surfactants between 0.80 mmol and 0.7 mmol/g with interlayer spacings of 2.20 nm and 1.80 nm, independently of the initial concentrations of the organic molecules. These organoclays were stable in acidic and basic solutions. However, after heating at 200 °C, the interlayer spacing shrunk due to the degradation of the organic surfactants as indicated by thermogravimetric analysis. The rehydration of the calcined organoclays at temperatures below 200 °C, did not lead to the increase of the basal spacings, due to change in configuration of the C16TMA cations.     

The gram-scale synthesis of carbon onions

The combustion of naphthalene has been found to yield gram-scale quantities of carbon onions that are free of impurities and furthermore without the use of catalysts. X-ray diffraction (XRD) indicates that the interlayer spacing between concentric shells of the carbon onions is not uniform across the particle; rather it decreases from a graphite-like 0.34 nm and approaches a diamond-like 0.29 nm interlayer spacing towards the inner layers. The dispersion in the interlayer spacing is believed to result from differing external pressures exerted on the individual nanometer-sized graphitic membranes during formation of the onions. Electron microscopy techniques such as high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy demonstrate the extensive formation of carbon onions. The HRTEM indicates that the onions consist of 50–54 shells, found to be in good agreement with the XRD data.     

Supercritical CO2 intercalation of polycaprolactone in layered silicates

Supercritical carbon dioxide (scCO₂) intercalation of polycaprolactone (PCL) in layered silicates (clay) is studied. Wide angle X-ray diffraction patterns find that PCL is slightly intercalated in unmodified montmorillonite clay (NaMMT) but considerably intercalated in organic-modified montmorillonite clay (OMMT). The interlayer spacing in OMMT increases considerably from 1.94 nm in OMMT to 3.58 nm in the OMMT/PCL 10/90 sample. PCL8 having molecular weight of 80,000 is harder to intercalate into OMMT than PCL1 having molecular weight of 10,000. Higher scCO₂ pressures at a temperature allow larger intercalations of PCL in OMMT to exhibit larger interlayer spacings in OMMT. The interlayer spacings in OMMT, however, are not clearly found to relate with the CO₂ temperature at a given pressure. TGA data show that OMMT enhances the thermal stability of PCL1, with a higher content of OMMT giving a higher amount of PCL1 residue. DSC data find that the PCL1-intercalated OMMT expedites the melt-crystallization rate of PCL1 from the melt but suppresses the crystallinity of PCL1. Study of Avrami's rate constants k and exponent n finds that the PCL1-intercalated OMMT enhances the isothermal crystallization rate of PCL1 and that the crystal growth dimension is 3 for pure PCL1 but decreases with increasing OMMT content in the blends. Modulus data find that the PCL1-intercalated OMMT is an effective reinforcement for PCL8.     

Intercalation of cetyl trimethylammonium ion into sericite in the solvent of dimethyl sulfoxide

In order to improve the microenvironment between layers of sericite in the manufacture of polymer/sericite nanocomposites, a pre-modified sericite was intercalated by cetyl trimethylammonium ion (CTA⁺) in a dimetzhyl sulfoxide (DMSO) solvent. The relevant experimental conditions, such as intercalating time, temperature, solid content and pH value, were optimized, and the intercalated sericite was characterized by X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy analyses. At an intercalation rate of 94%, the amount of CTA⁺ intercalated in the interlayer space of sericite was 36.3% and an expansion of d₀₀₁ value from 1.00 nm to 5.07 nm was observed. The intercalated CTA⁺ in the interlayer space of sericite adopted a paraffin-type bilayer configuration with a tilting angle of 53.6°. Due to its unique structure, DMSO would be better than water as the solvent for intercalation of organic cations. The intercalated sericite was more suitable to combine with polymer to form polymer/sericite nanocomposites.     

Spectroscopic evaluation of out-of-plane surface vibration bands from surface functionalization of graphite oxide by fluorination

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp²/sp³ carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ∼743 cm⁻¹ and 482 cm⁻¹, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite – FG). IR studies confirmed the stability of the formed C–F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ∼4–5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C–F species and a defect formation was observed with the evidence of increase of the I_D/I_G ratio from Raman spectra. To this end, understanding the origin of surface C–F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage.     

Interaction between single walled carbon nanotube and 1D crystal in CuX@SWCNT (X = Cl,Br, I) nanostructures

CuX@SWCNT (X = Cl, Br, I) nanostructures were prepared by capillary filling of 1.4–1.6 nm single-walled carbon nanotubes (SWCNT) with copper halides. The structure of CuX@SWCNT (X = Cl, Br, I) represents a distorted two-layer hcp of halogen atoms arranged along the SWCNT. The EXAFS and the high angle angular dark field (HAADF) HRTEM data indicate that Cu is partially coordinated by C. According to the optical absorption, valence band photoemission spectroscopy and work function measurements, a Fermi level (FL) downshift as compared with the initial value for the nanotubes and a corresponding charge transfer from the nanotubes to the 1D crystals is observed for CuX@SWCNT nanostructures. The FL shift increases in the sequence CuI < CuBr < CuCl due to an increase of the electron affinity for the halogen atoms. The XPS data confirm the acceptor effect of copper halides and indicate that metallic and semiconducting nanotubes behave differently. Raman spectroscopy performed under electrochemical charging allowed estimation of the value of charge transfer between the nanotube walls and the intercalated 1D crystal. The X-ray absorption and emission spectra for carbon and copper thresholds revealed a new energy level composed of the carbon 2р_z and copper 3d-orbitals. This indicates the Cu–C bonding, which in line with the structural HAADF HRTEM and EXAFS data.     

Comparative study of the structure and microstructure of PAN-based nano- and micro-carbon fibers

The work presents results on the manufacture and comparative assessment of the structure and microstructure parameters of polyacrylonitrile polymer (PAN)-based carbon nano- and micro-fibers. Using the same polymer solution, PAN nano- and microfibers were obtained. The PAN nanofibers were obtained by electrospinning, and microfibers were spun using the conventional solution-spinning method. The PAN-based fiber precursors were annealed to 1000 °C, 2000 °C and to 2800 °C. Using X-ray diffraction and Raman spectroscopy, the structural and microstructural parameters of both types of carbon fibers were examined. The morphology of PAN nanofibers and carbon nanofibers (CNF) were studied by SEM. Both types of ex-PAN carbon fibers (nano and micro) have similar the c-axis spacing (d₀₀₂) values and crystallite sizes after heat treatment to 2000 °C presenting turbostratic structure. HR-TEM images of low temperature CNF show uniform microstructure with the misoriented small carbon crystallites along the fiber axis. The ratio of the integrated intensities of the D and G peaks for carbon nanofibers after heat treatment at 2000 °C was distinctly higher in comparison to carbon microfibers (CF). After additional annealing the fibers to 2800 °C a better structural ordering show CNF. The crystallite sizes (L_c, L_a) in CNF were distinctly higher in comparison to the crystallites in CF. CF consist of two carbon components, whereas CNF contain three carbon components varying in structural and microstructural parameters. One of carbon phases in CNF was found to have the interlayer spacing close to graphite, i.e. d₀₀₂=0.335 nm.     

The presence of multiple structural components in pyrolytic carbon at the deposition stage

The X-ray diffraction patterns of samples cut from different depths of a 2.54 cm thick plate of pyrolytic carbon showed that this material consists of structural components whose interlayer spacings d₀₀₂ are 0.352, 0.345, 0.341, and 0.338 nm. During deposition thermally activated transformations between components occur sequentially from the 0.352 nm component down to the 0.338 nm component and it is suggested that these components are quasi-equilibrium phases.     

Experimental determination of bulk modulus of 14 Å tobermorite using high pressure synchrotron X-ray diffraction

Using a diamond anvil cell, 14 Å tobermorite, a structural analogue of calcium silicate hydrates (C–S–H), was examined by high-pressure synchrotron X-ray diffraction up to 4.8 GPa under hydrostatic conditions. The bulk modulus of 14 Å tobermorite was calculated, K_o = 47 GPa. Comparison of the current results with previous high pressure studies on C–S–H(I) indicates that: (1) the compression behavior of the lattice parameters a and b of 14 Å tobermorite and C–S–H(I) are very similar, implying that both materials may have very similar Ca–O layers, and also implying that an introduction of structural defects into the Ca–O layers may not substantially change in-plane incompressibility of the ab plane of 14 Å tobermorite; and (2) the bulk modulus values of 14 Å tobermorite and C–S–H(I) are dominated by the incompressibility of the lattice parameter c, which is directly related to the interlayer spacing composed of dreierketten silicate chains, interlayer Ca, and water molecules.     

Intercalation of lecithins for preparation of layered nanohybrid materials and adsorption of limonene

The intercalation of biosurfactants (lysolecithin and lecithin) in layered clay mineral supports was investigated to assess the suitability of the resulting nanohybrid materials as flavor and fragrance delivery system. The protonated biosurfactant molecules (pH = 2.3) were intercalated into the Na-montmorillonite, whereas the deprotonated biosurfactants (pH ~ 12) were intercalated into Mg–Al layered double hydroxides. The amount of lysolecithin and lecithin bound to the layered adsorbents was estimated by measuring adsorption isotherms. The basal spacing obtained from X-ray diffraction measurements suggested that the molecules are arranged in parallel with the layers of montmorillonite, whereas in the case of layered double hydroxides, the adsorbed molecules are in a vertical position between the layers. The interaction of layered adsorbents and biosurfactants was further evidenced by infrared spectroscopy. The intercalated montmorillonite and LDH particles were then probed for their ability to intercalate limonene molecules. Only the lysolecithins modified samples adsorbed limonene. The theoretical sizes of molecules and their possible arrangement between the layers were modeled by HyperChem 7.0 molecular calculations to correlate the ability to bind the lecithins in the confined space of the layered materials.     

Synthesis of double-walled carbon nanotubes from coal in hydrogen-free atmosphere

Double-walled carbon nanotubes (DWNTs) were synthesized from coal in large quantity by arc-discharge method in hydrogen-free atmosphere, which were systematically examined using scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The results show that the as-synthesized DWNTs have an outer diameter of 1.0–5.0 nm with an interlayer spacing in the tube walls of ca. 0.41 nm. The possible mechanism involved in the formation process of DWNTs is proposed and discussed in term of the unique chemical composition of coal and the process parameters adopted in the study.     

Caprylate intercalated layered double hydroxide as adsorbent of the linuron, 2,4-DB and metamitron pesticides from aqueous solution

This study was carried out to elucidate the synthesis of organo-layer double hydroxide (LDH) and its capacity to adsorbs the widely applied pesticides linuron, 2,4-DB and metamitron from waters. The adsorbent (LDH-Cap) was synthesized by incorporating organic anion caprylate into magnesium aluminum layered double hydroxide with chloride as interlayer anion (LDH-Cl) via ion exchange. Characterization of the LDH-Cap adsorbent was carried out using powder X-ray diffraction (PXRD), infrared spectroscopy (FT-IR) and thermal analyses (TG and DTA). PXRD patterns indicate that caprylate anion was successfully intercalated in LDH according to the basal spacing d₀₀₃ = 19.2 Å. Adsorption results indicated that these three pesticides were adsorbed on LDH-Cap. The high percentage of initial pesticide amounts removed within the first 30 min (~ 90% of the total amount adsorbed) for linuron and 2,4-DB revealed a rapid adsorption process, while it was more gradual for metamitron. PXRD results suggest that adsorbed 2,4-DB and metamitron were intercalated in the LDH interlayers probably between caprylate chains and the brucite layers. Linuron was probably adsorbed on the external particle surface of LDH. Adsorption kinetic study revealed that the adsorption process followed pseudo-second-order equation. Adsorption data were well fitted to the Freundlich isotherm. The desorption of the pesticides from adsorption products were tested with three different solvents (distilled water, ethanol and acetone) and it was partial in all cases.     

Synthesis of carbon-rich hafnia thin films by pulsed laser deposition

Carbon-rich ceramics are an emerging class of materials with attractive high-temperature properties, including resistance to crystallization, dense microstructure, and low porosity. We explored direct synthesis of carbon-rich hafnia, which is known to form as a compact interlayer in the oxide scales of oxidized hafnium carbide. The material was synthesized by pulsed laser deposition, using pure HfO₂ targets in C₂H₂ background gas at low pressures. Stable films up to 700 nm thick and with high molar fractions (～0.1–0.45) of carbon were obtained. The predominant chemical bonding of Hf and O atoms is that of oxygen-deficient HfO₂, while carbon is present in elemental or hydrogenated forms. Annealing at 600 °C leads to loss of most of the hydrogen from the films, which is accompanied by enhanced sp² bonding of carbon. The films have amorphous, compact, and finely grained microstructure. Carbon molar fractions higher than ～0.2 inhibit microcrystallinity to at least 600 °C.     

Graphene nanochains and nanoislands in the layers of room-temperature fluorinated graphite

Intercalated compound of graphite fluoride with n-heptane has been synthesized at room temperature using a multi-stage process including fluorination by a gaseous BrF₃ and a set of intercalant exchange reactions. It was found that composition of the compound is CF_0.40(C₇H₁₆)_0.04 and the guest molecules interact with the graphite fluoride layers through the van der Waals forces. Since the distance between the filled layers is 1.04 nm and the unfilled layers are separated by ∼0.60 nm, the obtained compound can be considered as a stack of the fluorinated graphenes. These fluorinated graphenes are large in area making it possible to study local destruction of the π conjugated system on the basal plane. It was shown that fluorine atoms form short chains, while non-fluorinated sp² carbon atoms are organized in very narrow ribbons and aromatic areas with a size smaller than 3 nm. These π electron nanochains and nanoislands preserved after the fluorination process are likely responsible for the value of the energy gap of the compound of ∼2.5 eV. Variation in the size and the shape of π electron regions within the fluorinated graphene layers could be a way for tuning the electronic and optical characteristics of the graphene-based materials.     

Diameter dependence of interwall separation and strain in multiwalled carbon nanotubes probed by X-ray diffraction and Raman scattering studies

We have made a systematic study on the diameter dependent spectral features in X-ray diffraction (XRD) and Raman scattering studies of multiwalled carbon nanotubes (MWCNTs) of various diameters in the range 5?100 nm. High resolution transmission electron microscopy (HRTEM) imaging reveals a systematic decrease in the interwall separation from 3.8 Å down to 3.2 Å as the diameter of nanotubes increases from 5.8 nm to 63.2 nm. Analysis of the XRD patterns shows an exponential decrease in d₀₀₂ interlayer spacing with increasing tube diameter, in close agreement with the HRTEM results. Further, XRD profile line width shows inverse diameter dependence and exponential increase in intensity as the diameter of the MWCNTs increases. Raman spectra of different diameter nanotubes show different evolutions of metallic and semiconducting components in the G-band, as found from spectral deconvolution. The frequency and full width at half maximum (FWHM) of the semiconducting component of the G^? band gradually decreases as the tube diameter increases. Ratio of intensities of G^? band to D-band first shows a sharp fall as the tube diameter increases from 7 nm to 15 nm and then slowly increases with increasing diameter. On the other hand, G′ mode frequency shows large up shift when average diameter is increased from 7 nm to 15 nm and then saturates for higher diameter tubes. Analysis of Raman and XRD data reveals that the lowest diameter (7 nm) MWCNTs have features similar to those of the single walled nanotubes, while the spectral features are distinctly different for higher diameter MWCNTs due to the interaction among tube walls that is very significant for large diameter MWCNTs. Observed diameter dependence of the spectral features is explained in terms of nanotube curvature and atomic vibrations involving interaction among the walls in MWCNTs. The present study demonstrates the power of XRD for nondestructive evaluation of diameter distribution and interwall separation in MWCNTs with wide range of diameters.