A. Cheesman - Academia.edu (original) (raw)
Papers by A. Cheesman
The Journal of Physical Chemistry A, 2006
First illustrations of the utility of pulsed quantum cascade lasers for in situ probing of the ch... more First illustrations of the utility of pulsed quantum cascade lasers for in situ probing of the chemistry prevailing in microwave plasma activated hydrocarbon/Ar/H2 gas mixtures used for diamond thin film growth are reported. CH4 and C2H2 molecules, and their interconversion, have been monitored by line-of-sight single pass absorption methods, as a function of process conditions (e.g., choice of input hydrocarbon (CH4 or C2H2), hydrocarbon mole fraction, total gas pressure, and applied microwave power). The observed trends can be rationalized, qualitatively, within the framework of the previously reported modeling of the gas-phase chemistry prevailing in hot filament activated hydrocarbon/H2 gas mixtures (Ashfold et al. Phys. Chem. Chem. Phys. 2001, 3, 3471). Column densities of vibrationally excited C2H2(v5=1) molecules at low input carbon fractions are shown to be far higher than expected on the basis of local thermodynamic equilibrium. The presence of vibrationally excited C2H2 molecules (C2H2(double dagger)) can be attributed to the exothermicity of the C2H3 + H <==> C2H2 + H2 elementary reaction within the overall multistep CH4 --> C2H2 conversion. Diagnostic methods that sample just C2H2(v=0) molecules thus run the risk of underestimating total C2H2 column densities in hydrocarbon/H2 mixtures operated under conditions where the production rate of C2H2(double dagger) molecules exceeds their vibrational relaxation (and thermal equilibration) rates.
Physical Chemistry Chemical Physics, 2005
Density functional theory-based electronic structure computations on small models of the diamond ... more Density functional theory-based electronic structure computations on small models of the diamond {100} surface have enabled prediction of the energetics and activation parameters of a number of plausible mechanistic steps for boron incorporation into, and boron loss from, the growing diamond surface. Initial proving calculations for the carbon-only case show, as in previous work, that the rate-limiting step for diamond growth involves opening of a five-membered ring species, and subsequent closure to form six-membered rings as in bulk diamond. The five-membered ring intermediate arises following 2 x 1 reconstruction of the {100} surface, or at steps on the {111} surface. Diamond growth arises as a result of successful competition between the ring-opening step and a two-carbon loss step, both of which involve significant activation barriers. In the boron case, we find that BH(x) (x = 0-3) species can all bind to radical sites on the diamond {100} surface to form stable adducts. Interconversion between the surface bound BH, species is facile at the H and H2 number densities and temperatures typical for diamond CVD conditions. B incorporation can occur by a ring expansion mechanism, as in the all-carbon case, and by direct insertion of surface bound BH (and B) species into the C-C bond on the diamond {100} surface. BH(x) loss processes identified include release of surface bound BH3 and/or CH2BH species into the gas phase. Both B incorporation into, and B loss from, the diamond {100} surface are deduced to be significantly less energy demanding than the corresponding carbon addition and loss processes.
Journal of Applied Physics, 2002
The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vac... more The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vacuum has been investigated using wavelength and spatially resolved optical emission spectroscopy and Langmuir probes. All lines in the observed optical emission spectra are assignable to ...
Journal of Applied Physics, 2009
CH{sub 4} and C{sub 2}H{sub 2} molecules (and their interconversion) in hydrocarbon/rare gas/H{su... more CH{sub 4} and C{sub 2}H{sub 2} molecules (and their interconversion) in hydrocarbon/rare gas/H{sub 2} gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption ...
Abstract In this study the efficiency of different bare, doped and composite photocatalysts were... more Abstract
In this study the efficiency of different bare, doped and composite photocatalysts were compared, under UV and visible light irradiation in order to show a detailed picture of the relative performance of the best photocatalysts developed in our laboratories and the mostly investigated reference titanias. The syntheses of our photocatalysts were optimized in order to achieve maximum photocatalytic activity under UV and visible light irradiation. Non doped commercial (Aeroxide P25, Aldrich anatase) and synthesized titanias (produced by sol-gel and flame hydrolysis techniques) and nitrogen, iron, iodine doped and silver or gold deposited titanium dioxides were investigated with two model pollutants (phenol and oxalic acid) under identical experimental conditions. The material properties of these selected photocatalysts were thoroughly characterized by X-ray diffraction, diffuse reflectance spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy and BET methods. The highest degradation rate of phenol was determined for the flame made titania sample with relatively low specific surface area (20 m2/g) when UV irradiation was applied. In contrast with that, our nitrogen doped photocatalyst with high specific surface area (139 m2/g) was the best for phenol degradation under visible light irradiation. Although the most efficient oxalic acid mineralization occurred with noble metal photodeposited samples under UV irradiation, this type of modification was detrimental when VIS irradiation is applied. The decomposition rate of oxalic acid was maximum under VIS irradiation using the iron and nitrogen doped photocatalysts. For both substrates and irradiation conditions our best photocatalysts were found to be significantly more active than Aeroxide P25 TiO2. Intermediate studies revealed that phenol degradation resulted in dihydroxy benzene intermediates, such as pyrocatechol and hydroquinone both under UV and visible light irradiation with our TiO2-N photocatalyst. The results of this comparative study could promote the determination of the optimal synthesis conditions of titanium dioxide based photocatalysts for a given organic pollutant in water.
The Journal of Physical Chemistry A, 2006
First illustrations of the utility of pulsed quantum cascade lasers for in situ probing of the ch... more First illustrations of the utility of pulsed quantum cascade lasers for in situ probing of the chemistry prevailing in microwave plasma activated hydrocarbon/Ar/H2 gas mixtures used for diamond thin film growth are reported. CH4 and C2H2 molecules, and their interconversion, have been monitored by line-of-sight single pass absorption methods, as a function of process conditions (e.g., choice of input hydrocarbon (CH4 or C2H2), hydrocarbon mole fraction, total gas pressure, and applied microwave power). The observed trends can be rationalized, qualitatively, within the framework of the previously reported modeling of the gas-phase chemistry prevailing in hot filament activated hydrocarbon/H2 gas mixtures (Ashfold et al. Phys. Chem. Chem. Phys. 2001, 3, 3471). Column densities of vibrationally excited C2H2(v5=1) molecules at low input carbon fractions are shown to be far higher than expected on the basis of local thermodynamic equilibrium. The presence of vibrationally excited C2H2 molecules (C2H2(double dagger)) can be attributed to the exothermicity of the C2H3 + H <==> C2H2 + H2 elementary reaction within the overall multistep CH4 --> C2H2 conversion. Diagnostic methods that sample just C2H2(v=0) molecules thus run the risk of underestimating total C2H2 column densities in hydrocarbon/H2 mixtures operated under conditions where the production rate of C2H2(double dagger) molecules exceeds their vibrational relaxation (and thermal equilibration) rates.
Physical Chemistry Chemical Physics, 2005
Density functional theory-based electronic structure computations on small models of the diamond ... more Density functional theory-based electronic structure computations on small models of the diamond {100} surface have enabled prediction of the energetics and activation parameters of a number of plausible mechanistic steps for boron incorporation into, and boron loss from, the growing diamond surface. Initial proving calculations for the carbon-only case show, as in previous work, that the rate-limiting step for diamond growth involves opening of a five-membered ring species, and subsequent closure to form six-membered rings as in bulk diamond. The five-membered ring intermediate arises following 2 x 1 reconstruction of the {100} surface, or at steps on the {111} surface. Diamond growth arises as a result of successful competition between the ring-opening step and a two-carbon loss step, both of which involve significant activation barriers. In the boron case, we find that BH(x) (x = 0-3) species can all bind to radical sites on the diamond {100} surface to form stable adducts. Interconversion between the surface bound BH, species is facile at the H and H2 number densities and temperatures typical for diamond CVD conditions. B incorporation can occur by a ring expansion mechanism, as in the all-carbon case, and by direct insertion of surface bound BH (and B) species into the C-C bond on the diamond {100} surface. BH(x) loss processes identified include release of surface bound BH3 and/or CH2BH species into the gas phase. Both B incorporation into, and B loss from, the diamond {100} surface are deduced to be significantly less energy demanding than the corresponding carbon addition and loss processes.
Journal of Applied Physics, 2002
The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vac... more The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vacuum has been investigated using wavelength and spatially resolved optical emission spectroscopy and Langmuir probes. All lines in the observed optical emission spectra are assignable to ...
Journal of Applied Physics, 2009
CH{sub 4} and C{sub 2}H{sub 2} molecules (and their interconversion) in hydrocarbon/rare gas/H{su... more CH{sub 4} and C{sub 2}H{sub 2} molecules (and their interconversion) in hydrocarbon/rare gas/H{sub 2} gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption ...
Abstract In this study the efficiency of different bare, doped and composite photocatalysts were... more Abstract
In this study the efficiency of different bare, doped and composite photocatalysts were compared, under UV and visible light irradiation in order to show a detailed picture of the relative performance of the best photocatalysts developed in our laboratories and the mostly investigated reference titanias. The syntheses of our photocatalysts were optimized in order to achieve maximum photocatalytic activity under UV and visible light irradiation. Non doped commercial (Aeroxide P25, Aldrich anatase) and synthesized titanias (produced by sol-gel and flame hydrolysis techniques) and nitrogen, iron, iodine doped and silver or gold deposited titanium dioxides were investigated with two model pollutants (phenol and oxalic acid) under identical experimental conditions. The material properties of these selected photocatalysts were thoroughly characterized by X-ray diffraction, diffuse reflectance spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy and BET methods. The highest degradation rate of phenol was determined for the flame made titania sample with relatively low specific surface area (20 m2/g) when UV irradiation was applied. In contrast with that, our nitrogen doped photocatalyst with high specific surface area (139 m2/g) was the best for phenol degradation under visible light irradiation. Although the most efficient oxalic acid mineralization occurred with noble metal photodeposited samples under UV irradiation, this type of modification was detrimental when VIS irradiation is applied. The decomposition rate of oxalic acid was maximum under VIS irradiation using the iron and nitrogen doped photocatalysts. For both substrates and irradiation conditions our best photocatalysts were found to be significantly more active than Aeroxide P25 TiO2. Intermediate studies revealed that phenol degradation resulted in dihydroxy benzene intermediates, such as pyrocatechol and hydroquinone both under UV and visible light irradiation with our TiO2-N photocatalyst. The results of this comparative study could promote the determination of the optimal synthesis conditions of titanium dioxide based photocatalysts for a given organic pollutant in water.