Ground state C2 density measurement in carbon plume using laser-induced fluorescence spectroscopy

The temporal evolution and spatial distribution of C₂ molecules produced by laser ablation of a graphite target is studied using optical emission spectroscopy, dynamic imaging and laser-induced fluorescence (LIF) investigations. We observe peculiar bifurcation of carbon plume into two parts; stationary component close to the target surface and a component moving away from the target surface which splits further in two parts as the plume expands. The two distinct plumes are attributed to recombination of carbon species and formation of nanoparticles. The molecular carbon C₂ moves with a faster velocity and dies out at ~ 800 ns whereas the clusters of nanoparticle move with a slower velocity due to their higher mass and can be observed even after 1600 ns. C₂ molecules in the d³Π_g state were probed for laser-induced fluorescence during ablation of graphite using the Swan (0,0) band at 516.5 nm. The fluorescence spectrum and images of fluorescence d³Π_g − a³Π_u(0,1)(λ = 563.5 nm) are recorded using a spectrograph attached to the ICCD camera. To get absolute ground state C₂ density from fluorescence images, the images are calibrated using complimentary absorption experiment. This study qualitatively helps to get optimum conditions for nanoparticle formation using the laser ablation of graphite target and hence deducing optimum conditions for thin film deposition.     

Electrochemistry of acetylide anion and anodic formation of carbon films in a LiCl–KCl–CaCl2–CaC2 melt

The electrochemical deposition of carbon films on a nickel substrate was carried out through anodic oxidation of calcium acetylide dissolved in a LiCl–KCl–CaCl₂ melt at 823 K. Continuous and tenacious carbon films were prepared by a two-stage anodically potentiostatic deposition at a fast rate, and characterized by SEM, Raman spectroscopy, XRD and XPS. The results show the carbon films composed of micron-sized particles with graphitized and amorphous phases containing a mixture of sp³ and sp² carbon. The cyclic voltammetry behavior of acetylide anion on graphite and nickel electrodes indicated that C₂^2 − ions are oxidized more favorably on the nickel substrate due to the anodic depolarization from nickel carburization.     

Submonolayer-Deposition of Mass-Selected Au+ and Au n + (n = 3 and 7) on HOPG and Amorphous Carbon

Ions of gold monomer and clusters emitted from a liquid metal ion source were mass-selected, and deposited on cleaved HOPG (highly oriented pyrolytic graphite) surfaces and on amorphous carbon thin films at room temperature with the impinging energy E _i from 0 to 500 eV. The coverage of deposited ions were 1/100 and 1/1000 monolayers on HOPG surfaces and 1/3 monolayers on carbon films. Scanning tunneling microscopy of the HOPG surfaces deposited with low impinging energy (E _i<50 eV) revealed that large clusters with diameters ranging from 2 to 5 nm and height of 1–2 layers were present instead of isolated monomers and original clusters. When E _i was higher than 100 eV, HOPG surfaces were damaged and only bumpy surfaces were observed by STM. Transmission electron microscopy of Au⁺-deposited carbon films showed the formation of clusters with diameter 0.5–20 nm, depending on the E _i and the time elapsed after deposition.     

Suppression of lithium deposition at sub-zero temperatures on graphite by surface modification

Lithium deposition on graphite anodes is considered as a main reason for failures and safety for lithium ion batteries (LIB). Different amounts of carbon coating on the surface of natural graphite are used in this work to suppress the amount of lithium deposited at − 10 °C. Pulse polarization experiments reveal relative polarization of graphite anodes at various temperatures and show that lithium deposition is accelerated at lowered temperatures. Electrochemical experiments, along with photographs, scanning electron microscopy (SEM) images and ex-situ X-ray diffraction (XRD) data suggest that carbon coating not only suppresses the lithium deposition but also enhances the formation of LiC₆ at − 10 °C. The homogeneous potential profile on the graphite surface attained by the carbon coating explains such an improved low temperature performance, as it allows efficient Solid Electrolyte Interface (SEI) film formation, which is a prerequisite for safety LIB.     

Role of nitrogen in evolution of sp2/sp3 bonding and optical band gap in hydrogenated carbon nitride

Drastic changes in the bonding are found in amorphous hydrogenated carbon nitride (a-CNx:H) film as a function of nitrogen concentration (or N/C ratio). The total C-sp³ fraction and hardness shows a sharp decrease (at N/C = 0.40) whereas optical band gap and resistivity shows a gradual increase as nitrogen concentration increases from 0.07 to 0.58. Raman spectrum of a-CNx:H film is fitted with both Gaussian (integrated intensity ratios are used instead of the height ratios of the Lorentzian (D mode)) and Breit–Wigner–Fano (BWF, G Mode) method for a comparative study of optical properties and crystalline size of the graphite domain. Visible Raman (488 nm) spectroscopy finds that the in-plane crystalline size of graphite domains (L_a) is increased (from 34 to 38 Å) with nitrogen incorporation. Optical band gap of a-CNx:H solid measured by means of ellipsometry differs from the one obtained from Raman spectroscopy. In addition, we propose a simple extension of the existing band gap model to obtain the optical band gap of a-CNx:H film from Raman spectrum. Our estimation agrees well with the experimental value.     

Fabrication of highly conductive Ru/a-CNx:H composite films by anode deposit

Ruthenium(0) composite hydrogenated amorphous carbon nitride (Ru/a-CN_x:H) films were deposition on single crystal silicon (1 0 0) substrate by electrochemical deposition technique with acetonitrile as carbon source, and Ru₃(CO)₁₂ as dopant. In the deposited progress, the Si (1 0 0) acted as anode. The relative atomic ratio of Ru/N/C was about 0.28/0.33/1, and Ru nanocrystalline particles about 8 nm were homogeneously dispersed into the amorphous carbon matrix. After doping Ru into a-CN_x:H films, the conductivity of the films were evidently improved and the resistivity drastically decrease from 10⁸ Ω cm to about 100 Ω cm.     

Mobility and aggregation of free clusters soft landed on amorphous and crystalline carbon substrates

Neutral antimony clusters produced by a gas aggregation source have been deposited at room temperature on thin films of amorphous carbon and cleavage surfaces (0001) of graphite. Antimony islands generated from different mean size distributions of preformed clusters Sb_n(n = 4, 90, 150, 250, 600, 2000) have been investigated by transmission electron microscopy. Only compact islands have been observed on amorphous carbon, whereas an evolution from compact to dendritic shapes occurs on graphite substrate as the mean size of the deposited clusters increases. For clusters containing more than 150 atoms the dendritic islands exhibit a fractal character whose dimensional analysis yields a fractal dimension of 1.63 ± 0.07. The different models for island growth are discussed in the light of these results.     

Spontaneous passivity of amorphous bulk Ni–Cr–Ta–Mo–Nb–P alloys in concentrated hydrochloric acids

Rod-shaped amorphous bulk Ni–Cr–Mo-22 at.%Ta-14 at.%Nb–P alloys resistant to concentrated hydrochloric acids were prepared by copper-mold casting. Alloys of amorphous single phase and mixture of nanocrystalline phases in the amorphous matrix were all spontaneously passive in 6 and 12 M HCl and were immune to corrosion in 6 M HCl, although the corrosion weight loss was detected for heterogeneous alloys in 12 M HCl. Spontaneous passivation is due to presence of stable air-formed films in which chromium was particularly concentrated in addition to enrichment of tantalum and niobium. The angle resolved X-ray photoelectron spectroscopy revealed that chromium and molybdenum are rich in the inner part of the film. The major molybdenum species is in the tetravalent state, although penta- and hexavalent state molybdenum is also included. The high corrosion resistance was interpreted in terms of the high stability of the outer triple oxyhydroxide, Cr_1−x−yTa_xNb_yO_z(OH)_3+2x+2y−2z, and the effective diffusion barrier of the inner Mo⁴⁺ and Cr³⁺ oxide layer. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on Electrochemical/Chemical Reactivity of Metastable Materials, Warsaw, 17th-21st September, 2007.     

Synthesis and characterization of Ti and N binary‐doped ɑ‐C films deposited by pulse cathode arc with ionic source assistant

Using ionic source assistant, Ti and N co‐doped amorphous C (α‐C:N:Ti) thin films were prepared by pulse cathode arc technique. Microstructure, composition, elemental distribution, morphology, and mechanical properties of α‐C:N:Ti films were investigated in dependence of nitrogen source, pulse frequency, and target current by Raman spectroscopy, X‐ray diffraction, scanning electron microscopy, X‐ray photoelectron spectroscopy, atomic force microscopy, nanoindentation, and surface profilometer. The results show the presence of titanium carbide and nitride in a‐C:N:Ti films. The α‐C:N⁺:Ti film (6 Hz, 60 A) shows the smaller size and the higher disordering degree of Csp² clusters. The α‐C:N⁺:Ti films present smoother surface and smaller particle size than for α‐C:N₂:Ti films. N ions facilitate the formation of N‐sp³C bonds in the α‐C:N⁺:Ti films, and α‐C:N⁺:Ti (10 Hz, 80 A) film possesses the more graphite‐like N bonds. Higher hardness and lower residual stress present in the α‐C:N₂:Ti (10 Hz, 80 A) film.     

Magnetic,optical and electrical properties of Mn-doped CuCrO<Subscript>2</Subscript> thin films prepared by chemical solution deposition method

CuCrO₂ and CuCrO₂:Mn thin films were prepared on sapphire substrates by chemical solution deposition method. The effects of the annealing temperatures and Mn concentration on the structural, electrical and optical properties were investigated. The X-ray diffraction measurement was used to confirm the c-axis orientation of CuCrO₂ and CuCrO₂:Mn thin films. The maximum transmittances of the films in the visible region are about 65% with direct band gaps of 3.25 eV. All films showed the p-type conduction and semiconductor behavior. The electrical conductivity decreases rapidly with the increase of Mn content, the maximum of the electrical conductivity of 1.35 × 10⁻² S cm⁻¹ is CuCrO₂ film deposited at 600 °C temperature in 10⁻³ Torr vacuum, which is about four orders of magnitude higher than that of the Mn-doped CuCrO₂ thin film. The energy band of the samples is constructed based on the grain-boundary scattering in order to investigate the conduction mechanism. Moreover, the samples exhibit a clear ferromagnetism, which was likely ascribed to originating from the double-exchange interaction between the Mn³⁺ and Cr³⁺ ions.     

Preparation of <Emphasis Type="Italic">p</Emphasis>-type conducting transparent CuCrO<Subscript>2</Subscript> and CuAl<Subscript>0.5</Subscript>Cr<Subscript>0.5</Subscript>O<Subscript>2</Subscript> thin films by sol–gel processing

CuCrO₂ and CuAl_0.5Cr_0.5O₂ thin films were prepared by sol–gel processing and subsequent two-step annealing in air and inert gas atmosphere. Phase pure films with delafossite structure were obtained by adjusting the respective temperatures. The related phase development strongly affects the optical and electrical performance, giving leeway for optimization. The resulting CuCrO₂ (16 Ωcm, transmittance 21%) and CuAl_0.5Cr_0.5O₂ (11 Ωcm, transmittance 49%) films showed p-type conductivity by their positive Seebeck coefficients. The microstructure of the systems was characterized by scanning and transmission electron microscopy and correlated to the growth of different crystalline phases during the annealing steps. Thereby, crystal thermodynamics also affects the respective film performance, alleviating delafossite formation from the amorphous phase.     

Graphite multi-micro-disc based mercury film electrode: Comparison of experimental and theoretical anodic stripping results

an ensemble of properly distanced micro mercury film electrodes (MMFE) was used in cyclic and anodic stripping voltammetry. the experimental results were compared with the anodic stripping theory, and the agreement was found to be satisfactory. The MMFE peaks (calculated per unit area) were higher, thinner and shifted towards more negative potentials compared with the large area mercury film electrode (LAMFE) peaks.The initial graphite electrode consisted of 65 independent micro-discs forming a circle, and was prepared from carbon fibres 4.66 μm in radius. The graphite multi-micro-disc electrode was quantitatively checked in a Fe(CN)₆^3? solution under both chronoamperometric and voltammetric conditions. The deposition and oxidation of mercury is discussed also.     

天然气和二氧化碳转化制合成气的研究 Ⅴ．甲烷脱氢积炭反应特征

采用ＴＧ、ＸＲＤ、ＳＥＭ、ＥＤＡＸ和脉冲色谱技术，研究了Ｎｉ／Ａｌ２Ｏ３和Ｎｉ／ＡＲＭ催化剂的甲烷脱氢积炭反应特征。结果指出，甲烷脱氢反应的积炭行为与催化剂上镍的分散状态有关。Ｎｉ－２催化剂上Ｎｉ的分散度小，晶粒大，甲烷脱氢形成的炭丝较长，主要以石墨型炭游离存在：而Ｎｉ／ＡＲＭ催化剂上Ｎｉ的分散度大，镍晶粒小，甲烷脱氢形成的炭丝较短，主要覆盖在催化剂活性中心表面。甲烷脱氢主要产生无定型炭和石墨型炭，其中无定型炭可以被ＣＯ２部分消除。在催化剂制备时，通过提高镍在催化剂表面的分散度，减小镍的晶粒大小，不仅可以提高催化剂的活性，而且可以提高ＣＯ２对积炭的消炭性能。     

Studies on Carbon Deposition on Hexaaluminate LaNiAl11O19 Catalysts during CO2 Reforming of Methane

The amount of carbon deposited on hexaaluminate LaNiAl₁₁O₁₉ catalyst in CH₄ decomposition and CO₂ reforming of methane was determined by means of thermogravimetric analysis (TGA). The properties of carbon formed on the catalysts were characterized by X-ray photoelectron spectroscopy (XPS), temperature-programmed CO₂ reaction (TPR-CO₂), and temperature-programmed oxidation (TPO) techniques. The experimental results showed that hexaaluminate LaNiAl₁₁O₁₉ catalyst possessed high resistance to carbon deposition in CO₂ reforming of methane to synthesis gas at high temperatures, and CO₂ played an important role in eliminating carbon during the reaction. At least two forms of the deposited carbon, graphite and carbide, were produced during methane reforming with CO₂.     

Physicochemical and catalytic properties of sol gel-prepared copper-chromium oxides

Nanostructured copper-chromium oxides were prepared by the sol–gel process (SG) and were characterised by elemental analysis, thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and by their activity in methane combustion. A comparative study was made with copper chromites commercial catalysts. The as-synthesised copper chromites sample exhibited higher specific surface area (248 m² g⁻¹) with respect to commercial solids (42 m² g⁻¹). The surface quantitative analysis evidenced a Cr⁶⁺ enrichment for the SG catalyst (Cr⁶⁺/Cr³⁺=0.56) with respect to commercial sample (0.39), while the ratio of copper species Cu²⁺/(Cu° + Cu⁺) was the same in both solids. Catalytic activity of SG solids in methane combustion was found to be comparable to that of Pt/Al₂O₃ and superior to that of commercial copper chromites tested under the same conditions.     

Molecular Design of Fluorocarbon Film Architecture by Pulsed Plasma Enhanced and Pyrolytic Chemical Vapor Deposition

Pulsed plasma enhanced chemical vapor deposition (pulsed PECVD) and pyrolytic chemical vapor deposition (pyrolyric CVD) of fluorocarbon films from hexafluoropropylene oxide (HFPO) have demonstrated the ability to molecularly design film architecture. Film structures ranging from highly amorphous crosslinked matrices to linear perfluoroalkyl chain crystallites can be established by reducing the modulation frequency of plasma discharge in plasma activated deposition and by eventually shifting mechanistically from an electrically activated to a thermally activated process. X-ray photoelectron spectroscopy (XPS) showed CF₂ content increasing from 39–65 mol%. Fourier transform infrared spectroscopy (FTIR) showed an increasing resolution between the symmetric and asymmetric CF₂ stretches, and a reduction in the intensity of the amorphous PTFE and CF₃ bands. High-resolution solid-state ¹⁹F nuclear magnetic resonance spectroscopy (NMR) revealed an increasing CF₂CF₂CF₂ character, with the pyrolytic CVD film much like bulk poly(tetrafluoroethylene) (PTFE). X-ray diffraction (XRD) patterns evidenced an increase in crystallinity, with the pyrolytic CVD film showing a characteristic peak at 2 = 18° representing the (100) plane of the hexagonal structure of crystalline PTFE above 19°C.     

Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η-C5H5)2Cr2(CS)2(CO)3 in contrast to (η-C5H5)2Cr2(CO)5

The cyclopentadienylchromium carbonyl thiocarbonyls Cp₂Cr₂(CS)₂(CO)_n (n = 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The lowest energy Cp₂Cr₂(CS)₂(CO)₄ structure can be derived from the experimentally characterized unbridged Cp₂Cr₂(CO)₆ structure by replacing the two terminal carbonyl groups furthest from the Cr-Cr bond with two terminal CS groups. The two lowest energy Cp₂Cr₂(CS)₂(CO)₃ structures have a single four-electron donor η²-μ-CS group and a formal Cr-Cr single bond of length ∼3.1 Å. In contrast to the carbonyl analogue Cp₂Cr₂(CO)₅ these Cp₂Cr₂(CS)₂(CO)₃ structures are viable with respect to disproportionation into Cp₂Cr₂(CS)₂(CO)₄ and Cp₂Cr₂(CS)₂(CO)₂ and thus are promising synthetic targets. The lowest energy Cp₂Cr₂(CS)₂(CO)₂ structures have all two-electron donor CO and CS groups and short CrCr distances around ∼2.3 Å suggesting the formal triple bonds required to give the chromium atoms the favored 18-electron configurations. These Cp₂Cr₂(CS)₂(CO)₂ structures are closely related to the known structure for Cp₂Cr₂(CO)₄. In addition, several doubly bridged structures with four-electron donor η²-μ-CS bridges are found for Cp₂Cr₂(CS)₂(CO)₂ at higher energies. The global minimum Cp₂Cr₂(CS)₂(CO) structure is a triply bridged triplet with a CrCr triple bond (2.299 Å by BP86). A higher energy singlet Cp₂Cr₂(CS)₂(CO) structure has a shorter Cr-Cr distance of 2.197 Å (BP86) suggesting the formal quadruple bond required to give each chromium atom the favored 18-electron configuration.     

Electrochemically Driven Transformation of Amorphous Carbons to Crystalline Graphite Nanoflakes: A Facile and Mild Graphitization Method

Although, in the carbon family, graphite is the most thermodynamically stable allotrope, conversion of other carbon allotropes, even amorphous carbons, into graphite is extremely hard. We report a simple electrochemical route for the graphitization of amorphous carbons through cathodic polarization in molten CaCl₂ at temperatures of about 1100 K, which generates porous graphite comprising petaloid nanoflakes. This nanostructured graphite allows fast and reversible intercalation/deintercalation of anions, promising a superior cathode material for batteries. In a Pyr₁₄TFSI ionic liquid, it exhibits a specific discharge capacity of 65 and 116 mAh g⁻¹ at a rate of 1800 mA g⁻¹ when charged to 5.0 and 5.25 V vs. Li/Li⁺, respectively. The capacity remains fairly stable during cycling and decreases by only about 8 % when the charge/discharge rate is increased to 10000 mA g⁻¹ during cycling between 2.25 and 5.0 V.     

Spontaneous alloying of gold clusters into nanometer-sized antimony clusters

Alloying behavior of gold into nm-sized amorphous antimony (a-Sb) clusters has been studied by transmission electron microscopy (TEM), employing gold clusters in contact with a-Sb clusters. In order to produce gold clusters on individual a-Sb clusters, a-Sb clusters on an amorphous carbon film were cooled down to 96 K, and gold was then condenced on the film. When gold clusters in contact with a-Sb clusters are gradually heated from 96 to 290 K, dissolution of gold into a-Sb clusters sets in around 200K and clusters of a-(Sb-Au) alloys are produced. With increasing annealing temperture, more gold is absorbed into individual a-Sb clusters, and when the gold concentration in a-(Sb-Au) clusters reaches to the stoichiometric composition of AuSb₂, these amorphous clusters crystallize into AuSb2 clusters. The crystallization temperature decreases with decreasing size of initial a-Sb clusters.     

Formation of the Ni–CrOx/MgO and Ni/MgO Catalysts for Carbon Dioxide Reforming of Methane

Temperature-programmed reduction by hydrogen, temperature-programmed desorption of O₂, local X-ray spectral analysis, and scanning electron microscopy are used to study redox processes occurring on the Ni–Cr₂O₃/MgO and Ni/MgO catalysts for carbon dioxide reforming of methane. The reduction of Ni/MgO leads to the formation of nickel clusters distributed over the surface of MgO. During the reduction of NiO–Cr₂O₃/MgO, chromates are transformed into chromites, and then nickel is formed by the reduction of spinel NiCr₂O₄. Reoxidation leads to the oxidized structures NiO, NiCr₂O₄, and NiCrO₄.