Thermal desorption spectra of hydrogenated and water treated diamond powders

The thermal desorption mass spectra of hydrogenated and water treated diamond powders were measured in the range from room temperature to 1250°C. After preliminary outgassing up to 1150°C, samples were either hydrogenated under 1 Pa hydrogen at 500–1000°C or treated in water vapor at room temperature to 800°C. Hydrogenated diamond exhibited two desorption peaks due to H₂ at about 900 and 1050°C, while water treated one desorbed mainly H₂ and CO, and small amounts of H₂O and CO₂. Oxidation after hydrogenation and treatment with a mixture of hydrogen and oxygen showed some correlation between adsorption states of hydrogen and oxygen. The maximum amount of hydrogen desorbed (0.81 × 10¹⁵atom/cm²) was only one-third of the valu estimated as full surface coverage.     

Shock-induced melting of diamond powders

Conditions of shock-induced melting of diamond powders are considered. For powders of various porosities, shock Hugoniots in the coordinates (p, u) and (p, T) and the pressures at the beginning and end of equilibrium melting are calculated. Thermal nonequilibrium of the shock compression of the powders is analyzed. Pressures at the onset of melting near pores are calculated. Shock-induced melting of detonation nanodiamond powders is considered. Calculation results show that explosion-induced melting of diamond powders is possible only under energy cumulation conditions.     

Thermal desorption of gases and solvents from graphite and carbon nanotube surfaces

The interaction of 23 gases and solvents with the basal plane of highly oriented pyrolytic graphite (HOPG) and with single-wall carbon nanotube (SWCNT) samples is studied using thermal desorption spectroscopy. Pre-exponential frequency factors used for analysis of desorption traces are obtained from vapor pressure data. Activation energies for desorption at monolayer coverage are determined using the Redhead peak-maximum method. Binding energies of non-polar adsorbates to the HOPG surface are found to scale with the adsorbate polarizability providing clear evidence for the van der Waals character of the interaction. Low coverage desorption temperatures on SWCNT samples are found to be 50-100% higher than on HOPG. Such increase has previously been attributed to physisorption in higher coordinated sites such as grooves on the external SWCNT rope surfaces. Polar adsorbates on the other hand typically desorb at much higher temperatures from SWCNT samples which is here tentatively attributed to stronger interaction with defect sites.     

Plasma modification of diamond surfaces

Because of its extreme hardness, diamond is a very interesting material for many industrial applications. One way to extend the field of diamond application and optimize the process conditions in diamond tool production is to enhance its surface reactivity. Low-pressure oxygen-plasma treatments were applied to equip the diamond surfaces with chemically reactive groups to improve their adhesion properties. XPS and DRIFT spectroscopy investigations showed that oxygen plasmas were suitable to oxidize the diamond surface. Combining derivatization reactions and solvatochromic dye adsorption studies, the introduction of alcoholic groups was confirmed. Besides the surface oxidation, the plasma treatment initiates decomposition of the diamond lattice which is followed by a re-arrangement of the carbon atoms in a graphite-like structure. The planar graphite structure supports the formation of carbonyl groups.     

XPS and UPS in situ study of oxygen thermal desorption from nanocrystalline diamond surface oxidized by different process

The chemistry of oxygen bonding on the diamond surface is a rich area of surface science research. It is well known that different surface terminations lead to strong variation of the material work function. This effect in diamond assumes peculiar consequences. In fact the oxidized diamond surface is hydrophilic, due to the high work function it shows a positive electron affinity and it is non conductive. On the contrary hydrogenation completely changes the orientation of the surface dipoles, the surface becomes hydrophobic, the work function lowers leading to a negative electron affinity. In addition hydrogen induces subsurface carriers which render the diamond surface semiconducting. These distinctive electronic properties make the diamond surface very interesting for the fabrication of surface field effect transistors just playing with the oxygen/hydrogen chemistry. Hydrogenation is generally obtained during the diamond synthesis in plasma reactors. Differently, the diamond surface oxidation may be accomplished with different processes (wet chemistry, plasma, UV irradiation).The realization of electronic devices calls for a complete understanding of the carbon-oxygen interactions, their stability and their influence on the electronic properties of diamond. Aim of this work is to explore the properties of diamond surfaces oxidized with piranha mixture, with O₂ plasma and with UV irradiation in a pure O₂ atmosphere. Each of these oxidized surfaces were annealed in situ at different temperatures and analyzed with photoelectron spectroscopies. Decreases of the oxygen concentration obtained via thermal desorption are then correlated with variations of the electronic properties obtained from UPS analyses.     

ESCA study of oxidized wood surfaces

Results of a preliminary study of the surface of wood exposed to outdoor weathering as well as to UV irradiation showed that ESCA provides valuable information and insight into the manifestation of weathering and photooxidation. From the ESCA spectra, the increase in signal intensities of carbon–oxygen bonds and oxygen–carbon–oxygen bonds (or unsaturated carbon oxygen bond) and oxygen-to-carbon ratio, and the decrease in carbon–carbon and carbon–hydrogen bonds of weathered and UV-irradiated wood surfaces suggested that wood surface was oxidized. Nevertheless, it was a superficial effect. Only a slow oxidation was observed at 100 μm under the exposed wood surfaces. From the oxygen-to-carbon ratio data, it revealed that weathered wood surface was rich in cellulose, poor in lignin. The leached-away degradation products from weathered wood surface accounted for the discrepancy between the ESCA line shapes of UV-irradiated and weathered wood surfaces.     

Thermal oxidation of CVD diamond

A thermal oxidation process of diamond films grown by chemical vapor deposition (CVD) has been studied. The oxidation was realized via heating of the CVD films in air. Pristine and oxidized CVD diamond films were analyzed with Raman spectroscopy and scanning electron microscopy (SEM) techniques. Raman spectroscopy revealed substantial changes in the polycrystalline diamond film composition induced by oxidation. A selective oxidation of disordered carbon and small size diamond crystallites was obtained at appropriate temperatures. A model explaining the formation and oxidation of the CVD diamond films containing the micrometer single diamond cores surrounded by the nanocrystalline diamond and disordered carbon has been proposed on the basis of the obtained results.     

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.     

Thermal desorption study of the acetic acid decomposition on clean Ni/Cu(110) alloy surfaces

The decomposition of acetic acid was studied on a clean Ni/Cu(110) alloy single crystal by means of thermal desorption spectroscopy. The primary alloy surface composition employed in this work was 37% Ni and 63% Cu as measured by Auger electron spectroscopy. Acetate was the predominant surface intermediate observed, giving rise to the CO₂ and H₂ decomposition products observed, in analogous fashion to the decomposition of formic acid. Surface carbon residue was also detected and could be driven into the bulk by annealing the sample above 900 °K. The rate constant for the decomposition of the acetate intermediate on the alloy was found to be 10^13.5 exp(?33(kcal/mol)/RT)sec^?1. The autocatalytic decomposition of both carboxylic acids previously observed on a clean Ni(110) surface was totally suppressed. The product distribution from acetic acid observed on both the 37% Ni/65% Cu alloy and the carburized Ni(110) surfaces were very similar, indicating chemical similarities between these two surfaces. At high coverages of the acetate intermediate the activation energy for CO₂ formation increased by 14 kcal/gmol. This effect was attributed to strong attractive interactions in the adsorbate layer.     

Interference colors of oxidized titanium metal surfaces

The range of interference colors that become apparent when thin layers of titanium oxide are electrolytically produced on the surface of the metal are described in terms of simple visual assessments, Munsell designations, and CIELAB color space values. the observed colors change to some extent with the angle of illumination and viewing. the form of the visible reflectance spectra of the surfaces also depends on the optical configuration of the spectrophotometer. Measurements made with an integrating sphere with a 0°/8° geometry provide CIELAB values that are more representative of the appearance of the colors than the values calculated from spectra obtained by 0°/45° diffuse reflectance.     

Thermal damage mechanisms of Si-coated diamond powder based polycrystalline diamond

The polycrystalline diamond (PCD) composites were synthesized from Si-coated diamond or diamond powder mixtures with Si. The characterizations of phase structures, morphologies and thermal stability of PCD were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal gravimetric-differential scanning calorimetry (TG-DSC). The thermal damage mechanisms of PCD were studied under air condition up to 750 ℃ by thermal weight loss, XRD, SEM and wear tester. The results showed that the PCD was successfully synthesized by the Si-coated diamond with a homogeneous structure. The thermal stability and wear resistance of PCD with Si-coated diamond were better than those of PCD with uncoated diamond. It was determined that the chemical thermal damage mechanism with oxidation reaction dominated the damage of the PCD after high temperature annealing and the physical thermal damage mechanism also took effect on the exfoliation of binder and fine diamond grains from PCD.     

Fluorescence quenching effects of nanocrystalline diamond surfaces

Undoped diamond has conductive properties when terminated by hydrogen and exposed to air or aqueous solution. Here, it is shown that nanocrystalline diamond, fabricated with hydrogen termination and deposited on quartz substrates using chemical vapor deposition, significantly quenched the fluorescence of adsorbed, dye-labeled fibrinogen protein in aqueous solutions at near neutral pH. Smaller levels of quenching were observed from oxygen terminated NCD surfaces. We suggest that these near-surface fluorescence quenching effects may arise from surface conductance effects in hydrogen terminated NCD. It is also shown that despite bulk quenching effects, single molecules of fibrinogen could be imaged on nanocrystalline diamond surfaces using epi-fluorescence techniques.     

The temperature-programmed desorption of hydrogen from copper surfaces

The desorption kinetics of H₂ from a Cu/ZnO/Al₂O₃ catalyst for methanol synthesis were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments after various pretreatments of the catalyst. Complete saturation with adsorbed atomic hydrogen was obtained by dosing highly purified H₂ for 1 h at 240 K and at a pressure of 15 bar. The TPD spectra showed symmetric H₂ peaks centered at around 300 K caused by associative desorption of H₂ from Cu metal surface sites. H₂ TPD experiments performed with different initial coverages resulted in peak maxima shifting to higher temperatures with lower initial coverages indicating that the desorption of H₂ from Cu is of second order. The microkinetic analysis of the TPD traces obtained with different heating rates yielded an activation energy of desorption of 78 kJ mol^–1 and a corresponding frequency factor of desorption of 3×10¹¹ s^–1> in good agreement with the kinetic parameters obtained with Cu(111) under UHV conditions.