Thermally stable hydrogen compounds obtained under high pressure on the basis of carbon nanotubes and nanofibers

Compounds containing 6.3–6.5 wt % H and thermally stable in vacuum up to 500°C were obtained by annealing graphite nanofibers and single-walled carbon nanotubes in hydrogen atmosphere under a pressure of 9 GPa at temperatures up to 45°C. A change in the X-ray diffraction patterns indicates that the crystal lattice of graphite nanofibers swells upon hydrogenation and that the structure is recovered after the removal of hydrogen. It was established by IR spectroscopy that hydrogenation enhances light transmission by nanomaterials in the energy range studied (400–5000 cm^?1) and results in the appearance of absorption bands at 2860–2920 cm^?1 that are characteristic of the C–H stretching vibrations. The removal of about 40% of hydrogen absorbed under pressure fully suppresses the C–H vibrational peaks. The experimental results are evidence of two hydrogen states in the materials at room temperature; a noticeable portion of hydrogen forms C–H bonds, but the most of the hydrogen is situated between the graphene layers or inside the nanotubes.     

Comparative X-ray absorption investigation of fluorinated single-walled carbon nanotubes

The C 1s and F 1s X-ray absorption spectra of pristine and fluorinated single-walled carbon nanotubes with different fluorine contents and nanodiamond as a reference compound have been measured with the aim of characterizing single-walled carbon nanotubes and their products formed upon treatment of the nanotubes with molecular fluorine at a temperature of 190°C. The spectra obtained have been analyzed by thoroughly comparing with the previously measured spectra of highly oriented pyrolytic graphite and fluorinated multiwalled carbon nanotubes and the spectrum of nanodiamond. It has been established that the fluorination of single-walled and multiwalled carbon nanotubes leads to similar results and is characterized by the attachment of fluorine atoms to carbon atoms on the lateral surface of the nanotube with the formation of the σ(C-F) bonds due to the covalent mixing of F 2p and C 2p _z π valence electron states.     

Gas-phase synthesis of nitrogen-containing carbon nanotubes and their electronic properties

Nitrogen-containing carbon nanotubes are synthesized using a gas-phase reaction. The synthesis of nitrogen-doped carbon nanotubes from 100 to 500 Å in diameter is accomplished through pyrolysis of acetonitrile (CH₃CN) at a temperature of 800°C. Cobalt and nickel metallic particles formed upon thermal decomposition of a mixture of maleate salts are used as catalysts. The materials synthesized are investigated by scanning and transmission electron microscopy. Analysis of the x-ray photoelectron spectra demonstrates that the content of nitrogen atoms in three nonequivalent charge states is approximately equal to 3%. A comparison of the CK _α x-ray fluorescence spectrum of the carbon nanotubes synthesized through electric-arc evaporation of graphite and the x-ray fluorescence spectrum of the nitrogen-containing carbon nanotubes prepared by catalytic decomposition of acetonitrile indicates that, in the latter case, the spectrum contains a certain contribution from the sp ³ states of carbon atoms. The temperature dependences of the electrical conductivity for different types of multi-walled carbon nanotubes are compared. The difference observed in the temperature dependences of the electrical conductivity is associated with the presence of additional scattering centers in nitrogen-containing carbon nanotubes.     

Hydrogenation of single-walled carbon nanotubes

Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls, and such C-H bonds can be completely broken by heating to 600 degrees C. We demonstrate approximately 65 +/- 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes, which is equivalent to 5.1 +/- 1.2 wt % hydrogen capacity. We also show that the hydrogenation is a reversible process.     

Characterization of fluorinated multiwalled carbon nanotubes by x-ray absorption spectroscopy

The C 1s and F 1s x-ray absorption spectra of fluorinated multiwalled carbon nanotubes with different fluorine contents and reference compounds (highly oriented pyrolytic graphite crystals and “white” graphite fluoride) were measured using the equipment of the Russian-German beamline at the BESSY II storage ring with a high energy resolution. The spectra obtained were analyzed with the aim of characterizing multiwalled carbon nanotubes and their products formed upon treatment of the nanotubes with fluorine at a temperature of 420°C. It was established that, within the probing depth (～15 nm) of carbon nanotubes, the process of fluorination occurs uniformly and does not depend on the fluorine concentration. The interaction of fluorine atoms with multiwalled carbon nanotubes in this case proceeds through the covalent attachment of fluorine atoms to graphene layers of the graphite skeleton and is accompanied by a change in the hybridization of the 2s and 2p valence electron states of the carbon atom from the trigonal (sp ²) to tetrahedral (sp ³) hybridization.     

多壁碳纳米管束储氢机理的X射线吸收谱研究

采用X射线吸收精细结构光谱探索性地研究了多壁碳纳米管束.在多壁碳纳米管束不同入射角的X射线吸收精细结构光谱中，观察到C—H σ^*共振峰强度随入射角的变化而发生变化.在常温常压下出现C—H键可能与多壁碳纳米管束中存在缺陷有关，缺陷数量越大C—H σ^*共振峰的强度越大.光谱中C—C π^*和C—C σ^*共振峰强度的变化趋势都不同于C—H σ^*共振峰，这有力地证明了在常温常压条件下氢原子是吸附在多壁碳纳米 关键词： X射线吸收精细结构光谱 碳纳米管 储氢 化学吸附     

Nitrogen 1s electron binding energy assignment in carbon nitride thin films with different structures

Carbon nitride thin films deposited by dc unbalanced magnetron sputtering have been analyzed by high-resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS data show that N 1s binding states depend on substrate temperature (T_s). By comparison with the Raman spectra, N 1s binding states are assigned in which nitrogen atoms are mainly bound to sp² and sp³ carbon atoms at T_s = 100°C, whereas at T_s = 500°C nitrogen atoms are mainly bonded to sp², sp³ and sp¹ carbon atoms.     

A preservation study of carbon nanotubes in alumina-based nanocomposites via Raman spectroscopy and nuclear magnetic resonance

Raman spectroscopy was used to study the preservation of the carbon nanotube structure in nanotube-reinforced alumina nanocomposites consolidated via spark plasma sintering (SPS). A series of Raman spectroscopy experiments was used to identify the thermal breakdown temperature of single-walled carbon nanotubes (SWCNTs) embedded in nanocrystalline alumina. It was found that the carbon nanotube structure remains intact after sintering at 1150 °C, but almost completely breaks down by 1350 °C after only 5 min. Also, ²⁷Al nuclear magnetic resonance (NMR) was used to study the chemical and structural effects of high-energy ball milling (HEBM) and SPS consolidation on pure alumina and SWCNT-alumina nanocomposites. HEBM does not change the mixed coordination number of the as-received alumina, but slight peak shifts indicate residual stresses. No Al₄C₃ was detected in any of the consolidated samples – even up to 1550 °C for 10 min. Thus, it is concluded that consolidation of carbon nanotube-reinforced composites should be completed at temperatures below ∼1250 °C in order to preserve the carbon nanotube structure. PACS 61.18.Fs; 61.46.Fg; 61.82.Rx; 62.25.+g; 76.60.-k     

Prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on surfaces of single-walled carbon nanotubes along their axes can lead to a modification of cylindrical carbon cores of these single-walled carbon nanotubes to carbon cores that have a nearly prismatic shape (prismatic modification). In faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. It has been established that the main characteristics of the single-walled carbon nanotubes thus modified are rather close to the corresponding characteristics of the related isostructural polymer conjugated systems (such as cis-polyenes, polyphenylenes, poly(periacenes), or polyphenantrenes). Fragments of model nanotubes of the (n, n) and (n, 0) types that contain up to 360 carbon atoms and their derivatives doped with fluorine atoms have been calculated using the semiempirical parametric method 3.     

Anti-prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on the surfaces of single-walled carbon nanotubes along their cylindrical axes leads to a modification of cylindrical carbon skeletons of these single-walled carbon nanotubes into carbon skeletons that have a nearly “anti-prismatic” shape (anti-prismatic modifications). In the faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. Model fragments of nanotubes of the (n, 0) type that contain up to 360 carbon atoms and their derivatives with regularly adsorbed fluorine atoms on the graphene surface have been calculated using the semiempirical PM3 method. It has been found that the main properties of the single-walled carbon nanotubes modified by the above method are determined by the character of the conjugation of the electrons in isolated carbon tracks, which is close to the character of the conjugation of the electrons in the initial single-walled carbon nanotubes.     

Iron-zirconium oxide catalysts for the hydrogenation of carbon monoxide: In situ studies by iron-57 Mössbauer spectroscopy

Some unsupported iron-zirconium oxide catalysts have been prepared by the calcination in air of precipitates containing 15 mole % iron. The catalyst formed at 500°C was shown by powder X-ray diffraction to consist of a non-equilibriated solid solution of iron(III) in a tetragonal or cubic zirconium dioxide structure whereas the catalyst formed at 1000°C was found to contain a zirconium-doped α-iron(III) oxide, or a magnetically ordered iron-zirconium oxide, in combination with an iron-containing monoclinic polymorph of zirconium dioxide. The⁵⁷Fe Mössbauer spectra recorded in situ following the pretreatment of the solids in nitrogen, carbon monoxide and hydrogen showed that little change is induced in the catalysts under such conditions. The⁵⁷Fe Mössbauer spectra also showed that the pretreated catalysts were unchanged by exposure to a 1:1 mixture of carbon monoxide and hydrogen at 270°C and 1 atmosphere pressure but were partially converted to iron carbide when used for the hydrogenation of carbon monoxide at 330°C and at 20 atmospheres pressure. The hydrocarbon product distribution showed Schulz-Flory α-values of 0.73 to 0.76 which were higher than the α-values obtained from pure iron catalysts which had been prepared and pretreated in a similar fashion. The⁵⁷Fe Mössbauer spectra and the results of the catalytic evaluation may be associated with an interaction between zirconium(IV) and the electron-rich atoms of the reactant carbon monoxide.     

Dynamics of single-wall carbon nanotube synthesis by laser vaporization

The key spatial and temporal scales for single-wall carbon nanotube (SWNT) synthesis by laser vaporization at high temperatures are investigated with laser-induced luminescence imaging and spectroscopy. Graphite/(Ni, Co) targets are ablated under typical synthesis conditions with a Nd:YAG laser at 1000 °C in a 2-in. quartz tube reactor in flowing 500-Torr Ar. The plume of ejected material is followed for several seconds after ablation using combined imaging and spectroscopy of Co atoms, C₂ and C₃ molecules, and clusters. The ablation plume expands in stages during the first 200 7s after ablation and displays a self-focusing behavior. Interaction of the plume with the background gas forms a vortex ring which segregates and confines the vaporized material within a ~1-cm³ volume for several seconds. Using time-resolved spectroscopy and spectroscopic imaging, the time for conversion of atomic and molecular species to clusters was measured for both carbon (200 7s) and cobalt (2 ms) at 1000 °C. This rapid conversion of carbon to nanoparticles, combined with transmission electron microscopy analysis of the collected deposits, indicate that nanotube growth occurs over several seconds in a plume of mixed nanoparticles. By adjusting the time spent by the plume within the high-temperature zone using these in situ diagnostics, single-walled nanotubes of controlled (~100 nm) length were grown and the first estimate of a growth rate on single laser shots (0.2 7m/s) was obtained.     

Spectroscopic study of plasmons in ion-irradiated single-walled carbon nanotubes

The electronic structure of single-walled carbon nanotubes was experimentally investigated using x-ray photoelectron spectroscopy, reflection electron energy-loss spectroscopy, and Auger electron spectroscopy. A shake-up satellite structure observed near the C 1s core-level lines in the x-ray photoelectron spectra at high binding energies in the range 284–330 eV due to excitation of π and π + σ plasmons was studied. The effect of irradiation by 1-keV argon ions on the shape of the spectra was analyzed. The shape of the C 1s satellite spectra was found to be sensitive to Ar⁺ irradiation in the electron energy loss range 10–40 eV corresponding to excitation of π + σ plasmons. Auger spectroscopy revealed the presence of argon on the surface of ion-irradiated samples. The argon content increased to ～4 at. % with increasing irradiation dose. An analysis of the results obtained and their comparison with the data available in the literature led to a qualitative conclusion that the bond angles of the carbon atoms making up the walls of single-walled carbon nanotubes are distorted at sites exposed to Ar⁺ irradiation.     

An iron-57 Mössbauer spectroscopy and EXAFS study of the hydrogen pretreatment of titania-supported iron-ruthenium catalysts

The changes in composition and structure which are induced in a titania-supported iron-ruthernium catalyst following treatment in hydrogen have been investigated in situ by⁵⁷Fe Mössbauer spectroscopy and by EXAFS. The results show that ruthenium dioxide is readily reduced at temperatures below ca. 500°C to ca. 20 Å clusters of metallic ruthenium whilst α-Fe₂O₃ is partially reduced at 130°C to Fe²⁺ and Fe⁰. The Fe³⁺ which is formed by the reoxidation of Fe²⁺ under the reducing conditions at 500°C segregates to the interface of the bimetallic phase and the titania support. It is suggested that continued treatment at 700°C produces a high dispersion of iron which is coordinated to oxygen atoms of the support. The ca. 20 Å clusters of metallic ruthenium may be envisaged as being anchored to the support via iron-ruthenium bonds     

First-principle study of interaction of molecular hydrogen with BC<Subscript>3</Subscript> composite single-walled nanotube

The physisorption of molecular hydrogen in BC₃ composite single-walled nanotube, investigated using density functional theory, was compared with single-walled carbon nanotube. Both external and internal adsorption sites of these two nanotubes have been studied with the hydrogen molecular axis oriented parallel to the nanotube wall. The calculated results show that: ([see full textsee full text]) the physisorption energies of a H₂ molecule are larger for BC₃(8,0) composite nanotube than for C(8,0) nanotube at all adsorption sites examined. ([see full textsee full text]) For these two nanotubes, the physisorption energies are larger for hydrogen bound inside the nanotubes than for adsorption outside the nanotubes. The different behavior between these two nanotubes is explained by the contour plots of electron density and charge-density difference of them. The present computations suggest that BC₃ nanotube may be a better candidate for hydrogen storage than carbon nanotube.     

The GaP(001) surface and the adsorption of Cs

GaP(001) cleaned by argon-ion bombardment and annealed at 500°C showed the Ga-stabilized GaP(001)(4 × 2) structure. Only treatment in 10^?5 Torr PH₃ at 500°C gave the P-stabilized GaP(001)(1 × 2) structure. The AES peak ratio $\text{P}\text{Ga}$ is 2 for the (4 × 2) and 3.5 for the (1 × 2) structure. Cs adsorbs with a sticking probability of unity up to 5 × 10¹⁴ Cs atoms cm^?2 and a lower one at higher coverages. The photoemission measured with uv light of 3660 Å showed a maximum at the coverage of 5 × 10¹⁴ atoms cm^?2. Cs adsorbs amorphously at room temperature, but heat treatment gives ordered structures, which are thought to be reconstructed GaP(001) structures induced by Cs. The LEED patterns showed the GaP(001)(1 × 2) Cs structure formed at 180°C for 10 h with a Cs coverage of 5 × 10¹⁴ atoms cm^?2, the GaP(001)(1 × 4) Cs formed at 210°C for 10 hours with a Cs coverage of 2.7 × 10¹⁴ atoms cm^?2, the GaP(001)(7 × 1) and the high temperature GaP(001)(1 × 4), the latter two with very low Cs content. Desorption measurements show three stability regions: (a) between 25–150°C for coverages greater than 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.2 eV; (b) between 180–200°C with a coverage of 5 × 10¹⁴ atoms cm^?2, and an activation energy of 1.8 eV; (c) between 210–400°C with a coverage of 2.7 × 10¹⁴ atoms cm^?2, and an activation energy of 2.5 eV.     

Density and thermodynamics of hydrogen adsorbed on the surface of single-walled carbon nanotubes

A method is proposed for calculating the adsorption of hydrogen in single-walled carbon nanotubes. This method involves solving the Schrödinger equation for a particle (hydrogen molecule) moving in a potential generated by the surrounding hydrogen molecules and atoms forming the wall of the carbon nanotube. The interaction potential for hydrogen molecules is taken in the form of the Silvera-Goldman empirical potential, which adequately describes the experimental data on the interaction between H₂ molecules (including the van der Waals interaction). The interaction of hydrogen molecules with carbon atoms is included in the calculation through the Lennard-Jones potential. The free energy at a nonzero temperature is calculated with allowance made for the phonon contribution, which, in turn, makes it possible to take into account the correlations in the mutual arrangement of the neighboring molecules. The dependences of the total energy, the free energy, and the Gibbs thermodynamic potential on the applied pressure P and temperature T are calculated for adsorbed hydrogen molecules. These dependences are obtained for the first time with due regard for the quantum effects. The pressure and temperature dependences of the hydrogen density m(P, T) are also constructed for the first time.     

Flexible polyethylene terephthalate (PET) electrodes based on single-walled carbon nanotubes (SWCNTs) for supercapacitor application

Flexible polyethylene terephthalate (PET) electrodes based on pristine single-walled carbon nanotubes (SWCNTs) and acid-treated single-walled carbon nanotubes (A-SWCNTs) were prepared by spray coating technique. Flexible A-SWCNTs electrodes showed enhanced electrochemical properties compared to the pristine SWCNTs electrodes. The electrochemical properties of the flexible A-SWCNTs electrodes were optimized with various types of aqueous electrolytes including sulfuric acid (H₂SO₄), sodium sulfate (Na₂SO₄), potassium chloride (KCl), sodium hydroxide (NaOH), and potassium hydroxide (KOH). The electrochemical performance of the A-SWCNTs electrodes as a function of bending to 30° were evaluated using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) measurements in 1 M H₂SO₄. The specific capacitance value of the unbent A-SWCNTs electrode was 67 F g^?1, which decreased to 63 F g^?1 (94% retention) after 1000 GCD cycles. Interestingly, the specific capacitance of the unbent A-SWCNTs electrode with application of the 1000 GCD cycles was retained even after 500 bending to 30° with 6000 GCD cycles.     

EXAFS and EPR studies of inclusion compounds of the trans-[Cu(en)2(H2O)2]2+ in cucurbit[8]uril

This work describes EXAFS and EPR studies of inclusion compounds of the trans-[Cu(en)₂(H₂O)₂]²⁺ complex in the macrocyclic cavitand CB[8] at different stages of heat treatment in the hydrogen atmosphere in a temperature range of 200–330°C. The structure and composition of the nearest environment of copper atoms are characterized, and the interatomic distances and coordination numbers are determined. It is shown that the structure of the copper complex inside the cavitand CB[8] remains unchanged at the first stage of complex preparation and upon heating up to 280°C in hydrogen atmosphere. The copper environment corresponds to four nitrogen atoms and two oxygen atoms. Further temperature treatment at 330°C causes decomposition of the complex inside the cavitand without the formation of copper clusters.     

Revealing distortion of carbon nanotube walls via angle-resolved X-ray spectroscopy

Arrays of aligned single-walled carbon nanotubes (SWCNTs) produced by supergrowth method were studied by scanning electron microscopy (SEM) and angle-resolved near-edge X-ray absorption fine structure spectroscopy, which defined that nanotube disorder is 10–13° and 23–27°, respectively. The latter value was confirmed by X-ray fluorescent spectroscopy. The difference in the obtained angular deviations was attributed to distortion of the SWCNT walls, because the X-ray spectroscopy methods are sensitive to a local environment of probing atoms, while the SEM examines the nanotubes at a substantially larger length scale. Significant distortion (20–24°) of SWCNT walls could be related to the defects introduced during the growth process.