During our recent efforts to grow polycrystalline diamond thin films on glassy carbon substrates using microwave-assisted chemical vapor deposition, we have reproducibly observed that a freestanding, polycrystalline diamond film is formed and can be lifted from the surface easily.The glassy carbon surface, on which the diamond films form, is gray in color (as opposed to the normal mirrorlike finish), rough, and hard as the surface is somewhat abrasion resistant after the growth. We postulate that growth of diamond on glassy carbon involves: (I) initial hydrogenation of the graphite edge plane sites forming a diamondlike surface, (ii) nucleation of diamond microparticles on the hydrogenated edge plane sites, and (iii) coalescence of the microparticles into a continuous film. The poor adhesion of diamond to the glassy carbon surface, as grown using our conditions, may result from a combination of growth condition and surface microstructural effects. Results from the characterization of a freestanding film by scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry are presented.
A novel method for determining the rates of adsorption of gaseous adsorbates on granular adsorbents in the millisecond time regime is described. Mixtures of the adsorbate and nitrogen are passed through a solenoid valve and 100 mg of the adsorbent for periods of (typically) 100 ms. The concentration of the adsorbate passing into a low-volume, long path length gas cell is measured with an ultra-rapid-scanning Fourier transform infrared spectrometer capable of gathering 200 mid-infrared spectra per second with 6-cm(-1) resolution. The pressure of the gas entering the cell is measured simultaneously with a capacitance manometer. A dynamic mathematical model was developed to analyze and describe the results in terms of a Langmuir isotherm. The success of this approach is demonstrated by the estimation of the rate of adsorption of vapor-phase acetaldehyde on aminopropylsilylated granular silica gel.
Enhancements of a model capable of precisely defining rates of adsorption in terms of a Langmuir isotherm and determined from infrared and pressure data collected in the millisecond time regime are described in detail. Rates of adsorption are determined for formaldehyde and acetaldehyde exposed to bare and aminopropylsilyl-derivatized silica gel. The model fits the collected data by varying the number of adsorption sites and the adsorption and desorption rate constants and determines the best fit based on the measured IR absorbance, partial pressure, exposure backpressure, number of adsorption sites, and time. The rate of adsorption onto irregular-shaped silica gel was faster than the rate onto the corresponding spherical silica gel. Formaldehyde was adsorbed more efficiently than acetaldehyde on APS silica gel than on bare silica gel, whereas acetaldehyde was adsorbed more efficiently than formaldehyde on underivatized silica, indicating the possibility that formaldehyde is adsorbed through the formation of a Schiff base.
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