Torr H2 Research Articles

Intercalation-etching of graphene on Pt(111) in H2 and O2 observed by in-situ low energy electron microscopy

Graphene layers are often exposed to gaseous environments in their synthesis and application processes, and interactions of graphene surfaces with molecules particularly H2 and O2 are of great importance in their physico-chemical properties. In this work, etching of graphene overlayers on Pt(111) in H2 and O2 atmospheres were investigated by in-situ low energy electron microscopy. Significant graphene etching was observed in 10-5 Torr H2 above 1023 K, which occurs simultaneously at graphene island edges and interiors with a determined reaction barrier at 5.7 eV. The similar etching phenomena were found in 10-7 Torr O2 above 973 K, while only island edges were reacted between 823 and 923 K. We suggest that etching of graphene edges is facilitated by Pt-aided hydrogenation or oxidation of edge carbon atoms while intercalation-etching is attributed to etching at the interiors at high temperatures. The different findings with etching in O2 and H2 depend on competitive adsorption, desorption, and diffusion processes of O and H atoms on Pt surface, as well as intercalation at the graphene/Pt interface.

Structures and Reactivities of Tin Oxide on Pt(111) Studied by Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS)

The reduction of disordered and ordered tin oxide monolayers and multilayers on Pt(111) by H2 was studied in situ by ambient pressure X-ray photoelectron spectroscopy. The disordered tin oxide monolayer was highly reactive and reduced by 5.5 × 10−7 Torr H2 at 295 K. However, the ordered monolayer was much less reactive, and nearly inert even in 2.2 Torr H2 at 295 K. Reduction of the tin oxide monolayer occurred only when the sample temperature increased to above 600 K in 5.5 × 10−7 Torr H2 or when the sample temperature was 450 K in 2.2 Torr H2. Disordered and ordered multilayers were found to be much less reactive by comparison to the disordered monolayer film. The reduction activity of these ordered and disordered Sn oxide films varied dramatically (by a factor of 107), demonstrating the structural flexibility of Sn oxides and the key interplay of structure and reactivity.

Hydrogen Intercalation of Graphene and Boron Nitride Monolayers Grown on Pt(111)

H2 atmosphere is often involved in growth and application of two-dimensional (2D) atomic crystals, and it is of great importance to understand interaction of the 2D materials with H2 molecules. Here, a full graphene layer and a full hexagonal boron nitride (h-BN) layer grown on Pt(111) were exposed to H2 atmosphere, which were investigated by in situ near ambient pressure X-ray photoelectron spectroscopy and quasi in situ ultraviolet photoelectron spectroscopy. We confirm the occurrence of hydrogen intercalation of the graphene and h-BN overlayers in ambient pressure H2. The hydrogen intercalation in 0.1 Torr H2 at room temperature and hydrogen desorption in 0.1 Torr H2 at 200 °C are fully reversible on the graphene/Pt(111) and h-BN/Pt(111) surfaces. Furthermore, hydrogen desorption on the graphene/Pt(111) and h-BN/Pt(111) surfaces was found to happen at lower temperature than that on the Pt(111) surface due to the graphene and h-BN cover effect.

Simultaneous Monitoring of Surface and Gas Phase Species during Hydrogenation of Acetylene over Pt(111) by Polarization-Dependent Infrared Spectroscopy

Acetylene hydrogenation was monitored at ambient pressure with polarization-dependent reflection absorption infrared spectroscopy (RAIRS), which permitted gas phase and surface species to be simultaneously monitored as C2H2(g) was converted first to C2H4(g) and then to C2H6(g). Experiments in which an acetylene-covered surface was hydrogenated with 1.0 × 10–2 Torr H2 between 120 and 300 K indicated that vinyl is the intermediate species to ethylene formation and that the addition of one H to acetylene is the rate-limiting step of the reaction. At a C2H2(g)/H2(g) ratio of 1:100, the reaction was monitored from 300 to 370 K and separately in a constant pressure and constant temperature reaction at 370 K. Ethylidyne and di-σ-ethylene were observed on the surface in both reactions and were found to be spectator species in the hydrogenation of ethylene to ethane. A minor hydrogenation pathway involves a third species, which is best assigned to an ethylidene intermediate. A small coverage of π-ethylene was also...

Hydrogen sensing behavior of Pt-coated mesoporous anodic titania

We report on excellent hydrogen sensing properties of a Pt/TiO2 system based on mesoporous titania grown by plasma electrolytic oxidation of a Ti metal substrate. The Pt phase is deposited onto the titania surface as an electrically continuous Pt nanomesh layer with an e-beam process. Resistive sensor response between the Pt and Ti terminals of the resultant structure is studied at room temperature for different partial pressures of H2 gas (0.003–3 Torr) and different source voltages (0.5–2.0 V). A maximum resistance effect, reaching four orders of the magnitude for 3 Torr H2 in 160 Torr O2 is obtained at the bias voltage 1.4 V, and a strong effect is detected for only 0.003 Torr, or 20 ppm H2 concentration. For voltages in the range 1.0–1.4 V, electrical conductance of the sample is about a linear function of the H2 partial pressure in the gas phase. Influence of the background atmosphere (O2, N2 and vacuum) and the effect of bias reversal were investigated also, leading to a conclusion that the sensing mechanism can be related to ionic transport in the TiO2 layer.

Designed catalysts from Pt nanoparticles supported on macroporous oxides for selective isomerization of n-hexane.

Selective isomerization toward branched hydrocarbons is an important catalytic process in oil refining to obtain high-octane gasoline with minimal content of aromatic compounds. Colloidal Pt nanoparticles with controlled sizes of 1.7, 2.7, and 5.5 nm were deposited onto ordered macroporous oxides of SiO2, Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2 to investigate Pt size- and support-dependent catalytic selectivity in n-hexane isomerization. Among the macroporous oxides, Nb2O5 and Ta2O5 exhibited the highest product selectivity, yielding predominantly branched C6 isomers, including 2- or 3-methylpentane, as desired products of n-hexane isomerization (140 Torr n-hexane and 620 Torr H2 at 360 °C). In situ characterizations including X-ray diffraction and ambient-pressure X-ray photoelectron spectroscopy showed that the crystal structures of the oxides in Pt/oxide catalysts were not changed during the reaction and oxidation states of Nb2O5 were maintained under both H2 and O2 conditions. Fourier transform infrared spectra of pyridine adsorbed on the oxides showed that Lewis sites were the dominant acidic site of the oxides. Macroporous Nb2O5 and Ta2O5 were identified to play key roles in the selective isomerization by charge transfer at Pt-oxide interfaces. The selectivity was revealed to be Pt size-dependent, with improved isomer production as Pt sizes increased from 1.7 to 5.5 nm. When 5.5 nm Pt nanoparticles were supported on Nb2O5 or Ta2O5, the selectivity toward branched C6 isomers was further increased, reaching ca. 97% with a minimum content of benzene, due to the combined effects of the Pt size and the strong metal-support interaction.

Separation of hot electron current component induced by hydrogen oxidation on resistively heated Pt/n-GaP Schottky nanostructures

Studies of chemically induced hot electron flow over Schottky barriers in catalytic planar nanostructures provide a direct insight into underlying charge transfer processes involved in chemical energy dissipation at solid surfaces. A systematic approach is described here to separate the hot electron and thermal current contributions to the total generated current based on in-situ resistive heating of cathode nanolayer of the Schottky structure. The method is applicable at high pressures in the gas phase. Analysis of the current induced by H2 oxidation to H2O on Pt/n-GaP nanostructure is performed for surface temperatures in the range of 453–513 K, and 120 Torr oxyhydrogen environment with 15 Torr H2. All the current components grow monotonously with temperature, while relative fraction of the hot electron current decreases with temperature from 85 to 52%.

Reforming of C6 Hydrocarbons Over Model Pt Nanoparticle Catalysts

Size-controlled model Pt nanoparticle catalysts, synthesized by colloidal chemistry, were used to study the hydrogenative reforming of three C6 hydrocarbons in mixtures with 5:1 excess of H2: methylcyclopentane, n-hexane and 2-methylpentane. We found a strong particle size dependence on the distribution of different reaction products for the hydrogenolysis of methylcyclopentane. The reactions of 50 Torr methylcyclopentane in 250 Torr H2 at 320 °C, using 1.5 and 3.0 nm Pt nanoparticles produced predominantly C6 isomers, especially 2-methylpentane, whereas 5.2 and 11.3 nm Pt nanoparticles were more selective for the formation of benzene. For the hydrogenolysis of n-hexane and 2-methylpentane, strong particle size effects on the turnover rates were observed. Hexane and 2-methylpentane reacted up to an order of magnitude slower over 3.0 nm Pt than over the other particle sizes. At 360 °C the isomerization reactions were more selective than the other reaction pathways over 3.0 nm Pt, which also yielded relatively less benzene.

Size and Shape Dependence on Pt Nanoparticles for the Methylcyclopentane/Hydrogen Ring Opening/Ring Enlargement Reaction

Monodisperse Pt nanoparticles (NPs) with well-controlled sizes in the range between 1.5 and 10.8 nm, and shapes of octahedron, cube, truncated octahedron and spheres (~6 nm) were synthesized employing the polyol reduction strategy with polyvinylpyrrolidone (PVP) as the capping agent. We characterized the as-synthesized Pt nanoparticles using transmission electron microscopy (TEM), high resolution TEM, sum frequency generation vibrational spectroscopy (SFGVS) using ethylene/H2 reaction as the surface probe, and the catalytic ethylene/H2 reaction by means of measuring surface concentration of Pt. The nanoparticles were supported in mesoporous silica (SBA-15 or MCF-17), and their catalytic reactivity was evaluated for the methylcyclopentane (MCP)/H2 ring opening/ring enlargement reaction using 10 torr MCP and 50 torr H2 at temperatures between 160 and 300 °C. We found a strong correlation between the particle shape and the catalytic activity and product distribution for the MCP/H2 reaction on Pt. At temperatures below 240 °C, 6.3 nm Pt octahedra yielded hexane, 6.2 nm Pt truncated octahedra and 5.2 nm Pt spheres produced 2-methylpentane. In contrast, 6.8 nm Pt cubes led to the formation of cracking products (i.e. C1–C5) under similar conditions. We also detected a weak size dependence of the catalytic activity and selectivity for the MCP/H2 reaction on Pt. 1.5 nm Pt particles produced 2-methylpentane for the whole temperature range studied and the larger Pt NPs produced mainly benzene at temperatures above 240 °C.

Dependence of Gas-Phase Crotonaldehyde Hydrogenation Selectivity and Activity on the Size of Pt Nanoparticles (1.7–7.1 nm) Supported on SBA-15

The selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by alcohol reduction of a Pt salt in the presence of poly(vinylpyrrolidone) (PVP), followed by incorporation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. With an increase in particle size from 1.7 nm to 7.1 nm, the selectivity towards crotyl alcohol increases from 13.7% to 33.9% (8 Torr crotonaldehyde, 160 Torr H2 and 353 K). The turnover frequency increases from 2.1 × 10−2 s−1 to 4.8 × 10−2 s−1 with increasing particle size. Additionally, the decarbonylation pathway to form propene and CO is enhanced over smaller nanoparticles. The apparent activation energy remains constant (~16 kcal mol−1 for the formation of butyraldehyde and ~8 kcal mol−1 for the formation of crotyl alcohol) as a function of particle size as does the reaction order in H2, which is unity. In the presence of 130–260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of −1 to −1.4 for crotyl alcohol and butyraldehyde. Hydrogen reduction at 673–723 K results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with respect to the organic capping agent, PVP.

Influence of Particle Size on Reaction Selectivity in Cyclohexene Hydrogenation and Dehydrogenation over Silica-Supported Monodisperse Pt Particles

The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C6H10, 200–600 Torr H2, and 273–650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H2 (H2: C6H10 ratio = 20:60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (<423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (>573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.

Reactions of methylcyclopentane on Rh–Pt catalyst prepared by underpotential deposition of Rh on Pt/SiO2

Rh was deposited on to a well-characterized 3.1% Pt/SiO2 (InCat-1) parent catalyst by underpotential deposition method to obtain a model Rh–Pt bimetallic catalyst. TEM and EDS was used to determine its mean particle size and bulk composition: the particles of ca. 3 nm contained ca. 60% Pt and 40% Rh. The Rh–Pt catalyst was tested in methylcyclopentane (MCP) reaction between 513 K and 603 K and 60–480 Torr H2 pressure (with 10 Torr MCP). The parent Pt/SiO2 as well as a 5% Rh/SiO2 catalyst were also studied for comparison. Four subsequent treatments with O2 and H2 up to T = 673 K were applied on the bimetallic catalyst before the catalytic runs. The overall activity showed positive hydrogen order on all samples, bimetallic Rh–Pt resulting in the lowest TOF values. Ring opening and hydrogenolysis products, as well as unsaturated hydrocarbons were formed from MCP. The selectivity of ring opening products and fragments over Rh–Pt catalyst was between the values observed on Pt and Rh, while the selectivity towards benzene formation was highest on the bimetallic sample, especially at higher temperatures. “Selective” ring opening occurred on all samples, resulting mostly in 2 and 3-methylpentane and less hexane. Different pretreatments with H2 and O2 affected slightly the dispersion values and the catalytic behavior of Rh–Pt sample. The selectivities of the Rh–Pt catalyst being between the values observed for Pt/SiO2 and Rh/SiO2 indicates that the sample studied represented a real bimetallic catalyst, resembling both components and exhibiting at the same time, new properties in addition to those, characteristic of Pt or Rh.

Formation and hydrogenation of ethylidene on the Pt(111) surface

Abstract Hydrocarbon intermediates formed in the course of ethylene adsorption and decomposition on Pt(1 1 1) were investigated using reflection absorption infrared spectroscopy (RAIRS) and mass spectrometry. The ethylidene (CHCH3) species is identified on the basis of RAIRS peaks at 2960 and 1387 cm−1 that are assigned to the CH3 asymmetric stretch and the CH3 symmetric bend, respectively. It is found that the highest ethylidene coverage can be produced by ethylene exposure to the surface at temperatures around 280 K. Ethylidene undergoes rapid hydrogenation upon exposure to 2 × 10−7 Torr of H2 at 250 K to produce gas-phase ethane, as detected by a mass spectrometer. The results indicate that ethylidene could be an intermediate in ethylene hydrogenation on transition metal surfaces near room temperature.

Plasma enhanced chemical vapour deposition of hydrogenated amorphous silicon at atmospheric pressure

Amorphous hydrogenated silicon films were grown using an atmospheric pressure helium and hydrogen plasma with silane added downstream of the source. A maximum deposition rate of 120±12 Å min−1 was recorded at a substrate temperature of 450°C, 6.3 Torr H2, 0.3 Torr SiH4, 778 Torr He, 32.8 W cm−3, and an electrode-to-substrate spacing of 6.0 mm. The deposition rate increased rapidly with the silane and hydrogen partial pressures, up to 0.1 and 7.0 Torr, respectively, then remained constant thereafter. By contrast, the deposition rate decreased exponentially as the electrode-to-substrate distance was increased from 5.0 to 10.5 mm. The total hydrogen content of the films ranged from 2.5 to 8.0± 1.0 at%. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

In-Situ Observation of π-Allyl c-C6H9 Intermediate during High-Pressure Cyclohexene Catalytic Reactions on Pt(111) Using Sum Frequency Generation Vibrational Spectroscopy

Surface vibrational sum frequency generation (SFG) has been used to identify surface species in situ during catalytic reactions of 1.5 Torr cyclohexene (C6H10) in the presence and absence of 15 Torr H2 on Pt(111) crystal surfaces. Various surface species were observed during the reactions in the range 303−483 K. Upon adsorption at 303 K in the absence of H2, cyclohexene is dehydrogenated to form 1,4-cyclohexadiene (C6H8). At 323 K, this species converts to π-allyl c-C6H9 intermediate. At temperature higher than 423 K, benzene coexists with C6H9 on the surface. Similar results were obtained in the presence of 15 Torr H2 except that in this circumstance 1,3-cyclohexadiene coexists with 1,4- cyclohexadiene at 303 K, and with C6H9 at temperature higher than 403 K. This study reports, for the first time, the existence of π-allyl c-C6H9 as a major surface species in the presence of high-pressure cyclohexene and H2 on Pt(111). In addition, adsorbed 1,3-cyclohexadiene has been observed as a reactive intermediate in the process of dehydrogenation to benzene. Finally, it has been found that excess hydrogen increases the disordering and decreases the surface coverage of the C6H9 on the surface.

Oxovanadium(IV), Nickel(II) and Palladium(II) Complexes of Tridentate Salicylaldiminates Derived from 2,4-Di-ter-butyl-6-aminophenol

Abstract Several new binuclear nickel(II), oxovanadium(IV) and palladium(II) complexes (MLX) of tridentate salicylaldimines (LXH2) obtained from substituted salicylaldehydes and 2,4-di-tert-butyl-6-aminophenol, as well as mononuclear nickel(II) and oxovanadium(IV), MQx nH2O, complexes with hydrogenated LX H2 ligands (QXH2) have been prepared and their spectroscopic, magnetic and catalytic activity (for PdLx) have been investigated. NiLx and VOLx complexes when dissolved in pyridine, take up three or two pyridine molecules to form six coordinated complexes, respectively. The Ni(II) and VO(IV) complexes obtained from QXH2 can be formu­lated as NiQx-3H2O and VOQx H2O. All VO(IV) complexes have been characterized by ESR parameters. It has been found that some PdLx complexes without any preliminary activation, in EtOH, THF and DMF exhibit high catalytic activity in the hydrogenation of nitrobenzene or cyclohexene at room temperature and at 30 -40 °C under 760 torr H2. When this reaction has been carried out in the ESR cavity at room temperature the triplet of 12 line multiplets due to the nitroxyl type coordinated radical (g = 2.013, AN = 10 G, AH = 0.8 G) was detected.

Synthesis and Decomposition of Formate on a Cu/SiO2 Catalyst: Comparison to Cu(111)

The kinetics of formate synthesis from CO2 and H2 and from formate decomposition on Cu/SiO2 were studied by temperature-programmed decomposition experiments and compared with those reported for Cu(111), Cu(110), and Cu(100) surfaces under similar reaction conditions. The initial rate of the formate synthesis from 380 Torr CO2/380 Torr H2 at 323–353 K over the Cu/SiO2 catalyst was in good agreement with those on Cu(111) and Cu(110). At 353 K, for example, the initial rate was measured to be 5.3×10−4 molecules site−1 s−1. The apparent activation energy was determined to be 58.8 kJ mol−1, which was also comparable with those obtained for Cu(111) (54.6–56.6 kJ mol−1), Cu(110) (59.8 kJ mol−1), and Cu(100) (55.6 kJ mol−1). The decomposition rate of formate was first-order in formate coverage, and the rate constant was 1.13×10−4 s−1 at 384 K. The activation energy and the pre-exponential factor were determined to be 115.7 kJ mol−1 and 5.38×1011 s−1, respectively, in good agreement with those obtained for Cu(111), 107.9–112.8 kJ mol−1 and (1.87–4.02)×1011 s−1, rather than those for Cu(110) (145.2 kJ mol−1 and 1.22×1016 s−1) and Cu(100) (130–155.0 kJ mol−1). The promotional effect of H2 upon the decomposition of formate was observed on the Cu/SiO2 catalyst as observed for Cu(111) but not for Cu(110). The decomposition rate of formate on Cu/SiO2 was promoted by a factor of 6 at an H2 pressure of 457 Torr. These kinetic results clearly indicate that the surface of Cu particles supported on SiO2 comprises Cu(111) planes. Equilibrium formate coverage on Cu/SiO2 during the hydrogenation of CO2 was well reproduced by the calculated coverage based on the kinetics of the formate synthesis and the formate decomposition. However, the equilibrium formate coverage on Cu/SiO2 was greater than that measured for Cu(111) under the same reaction conditions because the hydrogen-promoting decomposition rate on Cu/SiO2 is less than that on Cu(111). This is explained by a decrease in hydrogen coverage on Cu surfaces of the Cu/SiO2 catalyst caused by the spillover of hydrogen atoms from Cu onto the SiO2 surface. Thus, the effect of SiO2 was observed in the hydrogen-promoting decomposition kinetics on Cu.

Deposition of diamond on graphite and carbon felt from graphite heated in hydrogen at low pressure

The objectives of this research were to deposit diamond on graphite and carbon felt using our new closedchamber method [1], and to determine the influence of centrifugation on this deposition. It has long been known that hydrogen reacts with graphite at elevated temperatures to form a variety of hydrocarbon products. Balooch and Olander [2] published a definitive molecular beam study of surface reaction mechanisms. They were not able to detect a reaction with molecular hydrogen at temperatures up to 2200 K. Atomic hydrogen produced only methane from 400 to 800 K, molecular hydrogen between 800 and 1000 K, and acetylene from 1000 to 2200 K. The reaction rate was much higher on the prism plane than on the basal plane. They concluded that both surface adsorption and bulk diffusion of hydrogen atoms occurred. Angus and co-workers provided experimental and theoretical evidence that diamond can be caused to nucleate on the edge of the basal planes of graphite by adsorption of atomic hydrogen in the presence of hydrocarbon gases [3–6]. On the other hand, it was claimed that diamond grew on graphite by chemical vapor deposition only if the graphite had been pre-coated with chromium, nickel or titanium [7]. Details of the surface pre-treatment were not given for diamond grown on graphite using a H2+CH4 microwave plasma [8, 9]. In another study, the surface of carbon fibers containing embedded diamond particles were converted to diamond by exposure to a hydrogen microwave plasma [10]. At 50 torr hydrogen pressure severe etching occurred with little diamond formation, while a ∼1 μm diamond coating was formed with 100 torr H2. The same authors coated carbon fibers with diamond in a H2+CH4 plasma at a rate of ∼0.4 μm/h, although considerable etching of the underlying fiber preceded the growth [11]. The fibers had been pre-treated in an ultrasonic bath consisting of 0.25 μm diamond paste in a methanol slurry. There was little difference in the behavior of fibers prepared from pitch and from polyacrylonitrile. Recently, we described a simple new technique for deposition of diamond on a wide variety of surfaces, with and without centrifugation to accelerate deposition [1]. A graphite rod is heated electrically to approximately 2000 ◦C in the presence of ∼0.1 atm H2 in a closed chamber. The desired substrate is placed nearby, and is heated by the graphite rod. The present paper deals with deposition of diamond on graphite and carbon felt without any prior contact with diamond powder. For experiments aimed at deposition onto graphite, 1.5× 15× 20 mm slabs were cut from U-120 graphite (Fig. 1) from the Ultra Carbon Division of Carbone of America. These were cleaned in an ultrasonic bath, alternating between ethanol and acetone, and then dried in air. Six experiments were performed using a 25× 2× 2 mm graphite rod placed 2.5 mm above the substrate. The chamber was alternately evacuated to ∼1 torr and backfilled with∼1000 torr H2 several times. Then it was filled with hydrogen gas to a pressure ranging from 30 to 60 torr, and sealed. Electric current was passed through the graphite heating rod, raising the substrate temperature to ∼740 ◦C (measured by a K-type thermocouple touching its under side) for 1 h. Figs 2 and 3 show typical results, with faceted diamond crystals randomly distributed over the graphite. Tape adhesion tests removed neither the diamonds nor any graphite. Two of the six experiments were performed at an acceleration of twice earth’s gravity (2g) on the Clarkson materials processing centrifuge HIRB [12]. Without centrifugation, the diamond crystals were on about 50% of the 3 cm2 surface of the graphite substrate, while crystals were on about 90% with centrifugation. The size of the crystals was about 55% larger when deposition was carried out at 2g under the same conditions as without centrifugation. Longer deposition times should yield complete coverage of graphite by polycrystalline diamond.

Solution Delivery for Copper CVD Using Cu(HFAC)2 Reduction

ABSTRACTWe have developed a solution delivery technique for performing copper CVD using the reduction of Cu(hfac)2 [where H(hfac) = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionel. We have obtained deposition rates of up to 3.6 mg cm−2 hr−1 (ca. 60 nm min−1) for a deposition temperature of 300 °C and reactor conditions of 40 Torr H2, 12 Torr isopropanol, and 1 Torr Cu(hfac)2. The increased rates are several times faster than growth rates observed using conventional Cu(hfac)2 sublimation with pure H2 as the carrier gas. We compare growth rates and film microstructure using TiN- and WNx-coated substrates. We also give preliminary results showing how the partial pressures of H2, i-PrOH, and Cu(hfac)2 each influence the deposition rate.

FTIR and Mass-Spectrometric Measurements of the Rate Constant for the C6H5 + H2 Reaction

The rate constant for the C6H5 + H2 → C6H6 + H reaction has been measured by pyrolysis/Fourier transform infrared spectrometry (P/FTIRS) in the temperature range of 548−607 K and by pulsed-laser photolysis/mass spectrometry (PLP/MS) in the temperature range of 701−1017 K. By P/FTIRS, the reaction was studied by measuring time-resolved concentration profiles of the reactant (C6H5NO) and the product (C6H6) using highly diluted mixtures of C6H5NO in H2 (with or without Ar dilution). In PLP/MS experiments, the C6H5 radical was generated by the photolysis of C6H5COCH3 at 193 nm in the presence of several Torr of H2. The C6H5 + H2 rate constant was determined by the absolute yields of C6H6 and C6H5CH3 products. The results of these two spectrometric measurements agree closely with our theoretically predicted expression, k = 5.72 × 104T2.43 exp (−3159/T) cm3/(mol·s) and with that of a shock-tube study by Troe and co-workers (ref 20) in the temperature range of 1050−1450 K. Preliminary kinetic data on the CH3 + C...