David N. Belton

In situ characterization of diamond nucleation and growth

Filament‐assisted chemical vapor deposition (CVD) diamond film growth on Si(100) was studied using x‐ray photoelectron spectroscopy (XPS) to examine the sample at selected intervals during the nucleation and growth processes. The sample was transferred under vacuum from the growth chamber to the attached XPS analysis chamber without exposure to air. Before growth XPS showed that the Si sample is covered by a layer of SiO2 and carbonaceous residue; however, after 15 min of growth both of these substances are removed and replaced by a distinct SiC layer [Si(2p)=100.3 eV and C(1s)=282.7 eV].

Loss of epitaxy during diamond film growth on ordered Ni(100)

Filament assisted diamond film growth was studied on a clean and highly oriented Ni(100) substrate using x‐ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), x‐ray excited Auger electron spectroscopy (XAES), and low‐energy electron diffraction (LEED) to examine the sample at selected intervals in the growth process. The sample was transferred between the growth chamber and the ultrahigh vacuum analytical chamber without exposure to air. Scanning electron microscopy (SEM) was also used to characterize the sample; however, this required exposing the sample to air. Prior to growth the Ni(100) crystal was cleaned and ordered with a combination of Ar‐ion bombardment, oxygen/hydrogen treatments, and annealing. After 2 min growth, microcrystalline graphite islands formed on top of a c(2×2) carbon layer. Later, LEED showed a graphite surface with very poor azimuthal orientation. This surface developed into a disordered nondiamond surface that we assign as glassy carbon. Next, EELS and...

Electron spectroscopic identification of carbon species formed during diamond growth

Carbon deposits formed during filament assisted diamond film growth were characterized with x‐ray photoelectron spectroscopy (XPS) and electron energy‐loss spectroscopy (EELS). The samples were transferred between the growth chamber and the ultrahigh vacuum analytical chamber without exposure to air. These results are presented with an emphasis on data analysis for carbon chemical state identification. By comparison of the C(1s) binding energies to a highly oriented pyrolytic graphite standard, we are able to distinguish pure carbon species, such as diamond and graphite, from compounds, such as SiC and CxHy. For substrates with Z≤30, the loss electrons from the C(1s) core level are free from overlap with the substrate core levels; therefore, EELS spectra can be obtained by x‐ray excitation of the C(1s) level. These EELS data can then be used as a fingerprint for distinguishing between diamond, graphite, and carbides. For samples with mixed deposits of diamond and graphite, or carbide and diamond, curve fi...

Effect of hydrothermal aging on oxygen storage/release and activity in a commercial automotive catalyst

Abstract We have studied the effect of hydrothermal aging on the oxygen storage/release and activity in a commercial automotive catalyst containing Pt, Rh, Ni, and Ce. X-ray photoelectron spectroscopy (XPS) was used to measure changes in the oxidation state of the Ce and Ni present in the washcoat of the catalyst after various oxidation and reduction treatments and rate measurements of the carbon monoxide oxidation reaction were used to determine the reaction kinetics and activity. The catalyst was tested fresh and after hydrothermal aging at 1000°C under cyclic redox conditions. To determine which Ce species were present on the surface, we fit the XPS Ce(3d) data with combinations of spectra from standard materials (CeO2 and Ce2O3). Based on our measurements we conclude that aging the catalyst causes a loss of oxygen storage capacity due to sintering of the ceria particles which reduces the ceria/noble metal interaction and does not allow the Ce to cycle between oxidation states. In addition, hydrothermal aging causes the loss of oxygen storage of the Ni due to the irreversible formation of NiAl2O4. For the fresh catalyst, the observed kinetics for the uncycled carbon monoxide oxidation reaction show all of the signatures attributable to a catalyst with a high degree of ceria/noble metal interaction (complete suppression of the carbon monoxide inhibition effect, decreased sensitivity of the reaction rate to gas-phase oxygen concentration, and decreased apparent activation energy). Changes in the kinetics upon aging are consistent with a loss of ceria/noble metal contact area due to sintering of the ceria particles.

Diamond formation on platinum

We have made in situ mass spectral measurements and x‐ray photoelectron spectroscopy (XPS) measurements at the surface of a diamond film growing on a platinum substrate. We measured the concentrations of CH4, C2H4, and C2H2 and detected additional species with from three to ten carbon atoms. The gas‐phase chemical kinetics controlling the concentrations of the C1 and C2 species was modeled, and agreement between the calculated and measured concentrations was good. The presence or absence of the platinum foil had no effect on the measured concentrations, showing that heterogeneous chemistry on platinum did not affect the gas‐phase environment. XPS spectra were taken during the course of the diamond growth without exposing the platinum foil to air. After exposure to a room‐temperature CH4/H2 mixture but before any growth, the platinum surface was mostly covered with graphitic carbon. Once growth was initiated the graphitic layer was gradually replaced by 1–3 monolayers of hydrocarbon material, which did not...

A mechanism for growth on diamond (110) from acetylene

We have proposed a detailed chemical kinetics mechanism for the addition of C2H2 to a (110) diamond surface, which is the fastest growing face. The model contains no adjustable parameters and is based on the hypothesis that diamond surface chemistry may be understood in analogy with gas‐phase hydrocarbon chemistry. We calculated a growth rate of 0.03 μm/h, which gives order‐of‐magnitude agreement with experiments and suggests we have a feasible mechanism for growth on (110) surfaces.

Studies of the oxygen release reaction in the platinum-ceria-zirconia system

Abstract The relative rates of the CO+oxygen storage material (OSM)⇒CO2 reaction (RCO2) and the amounts of rate enhancement obtained upon Pt promotion were examined for ceria and ceria–zirconia OSM. The effect of Pt surface area on RCO2 was de-coupled from metal–oxide surface area by pre-sintering the oxides prior to Pt deposition. We find that RCO2 is linearly dependent on Pt area over Pt/CeO2, but over Pt/Ce0.75Zr0.25O2, the rate is independent of Pt surface area above a threshold surface area. Although Pt sinters more readily on the Ce0.75Zr0.25O2 support, the dispersion effect is more than compensated by the enhanced availability of “bulk” O2− within the Ce0.75Zr0.25O2 particles. Furthermore, this study demonstrates that, on a unit surface area basis, the ceria–zirconia support is at least two times more active for the oxygen release reaction than the pure ceria support when the materials are slightly reduced. It is also shown that O2− diffusion is not rate-limiting for the ceria reduction reaction when the reaction is carried out in the kinetically limited regime at 500°C with CO as the reductant.

Chemisorption geometry of NO on Rh(111) by X-ray photoelectron diffraction

Abstract The adlayer structure of (2 × 2)-3NO on Rh(111) has been investigated by high-energy, scanned-angle X-ray photoelectron diffraction (XPD) in conjunction with single scattering cluster theory and R -factor analysis. In addition, a plausible adlayer registry, consistent with previously published HREELS data, is deduced by means of physical reasoning. The resulting structural model consists of NO bound to atop, three-fold hollow fcc, and three-fold hollow hcp sites through the N atom with an NO bond length of 1.15 A. NO molecules at all three sites within the adlayer are oriented normal to the surface. The Z coordinates ( Z being defined as perpendicular to the surface) of atop and hollow-site NO molecules relative to the top layer of Rh atoms differ by 0.5 A, with hollow-site NO being bound more closely to the substrate. This structure differs considerably from one proposed previously by a LEED I - V analysis in both the site occupancy and the spacing of NO molecules within the adlayer.

Nucleation of chemically vapor deposited diamond on platinum and nickel substrates

Abstract Diamond growth on platinum and nickel substrates, both scratched and unscratched, was examined over a wide variety of growth conditions in an effort to understand the nucleation mechanism of diamond. The experiments used a combination of X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction to examine the time evolution of surface carbon species during diamond growth. Samples were transferred under vacuum from the filament-assisted diamond growth chamber to an attached ultrahigh vacuum analytical chamber. Our results indicate that diamond nucleates on platinum and nickel by pre-deposition of graphitic carbon precursors. Defect sites in these graphite deposits contain the nucleation sites for diamond. These nucleation sites are sensitive to the presence of gas phase oxygen species; therefore, addition of oxygen to the gas phase suppresses diamond nucleation by elimination of the nucleation sites but does not suppress growth of existing diamond. Our results demonstrate at least one mechanism whereby diamond can nucleate on metallic substrates such as platinum and nickel.

Effect of Rh particle size on CO desorption from Rh/alumina model catalysts

Abstract The adsorption of CO on small Rh particles on oxidized Al(100) was studied using temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). The desorption data for CO were obtained for two Rh Al 2 O 3 samples as well as for Rh(111). The supported Rh model catalysts were prepared from thermal decomposition of [Rh(CO2Cl]2 on an oxidized Al(100) substrate. By varying the substrate temperature and the amount of deposited Rh, samples were prepared with average Rh particles sizes of 20 and 70 A. TPD data from the 70 A Rh particles were similar to that from Rh(111), with a major peak at 500 K and a shoulder at 400 K for a saturation CO exposure. TPD from the small particles was very different from Rh(111) and the larger particles, showing a single broad peak centered at 415 K. The data show a particle size effect for the desorption on CO from supported Rh. Redhead analysis of the data for first-order desorption gave an activation energy of 30 kcal/mol for all of the samples at low CO coverages. A more detailed analysis using peak widths and peak temperatures was also performed. These calculations gave a very low activation energy, 19.8 kcal/mol, and preexponential, 1×10 8 cm 2 s for the small particles. These very low values were interpreted to mean that multiple desorption states, mobile precursors, or coverage dependent activation energies were affecting the data from the small particles. Calculations of first-order desorption rates showed that desorption states different than those on Rh(111) are the dominant species on the small particles at high CO coverages.