Title

Surface and bulk characterization of metal-impregnated activated carbon: Synthesis of thin films used for detection of halogenated hydrocarbons

Date of Completion

January 1997

Keywords

Chemistry, Analytical|Chemistry, Inorganic|Physics, Condensed Matter

Degree

Ph.D.

Abstract

This thesis concerns surface and bulk characterization of activated carbons; synthesis and characterization of manganese oxide, and tin oxide thin films; and applications for chemical gas sensors to detect halogenated hydrocarbons.^ Plain and metal (Cu, Zn, and Mo) loaded activated carbons were characterized by several surface sensitive techniques to determine surface and bulk properties. Surface area was reduced after metal impregnation. Impregnated metals existed mainly as metal carbonates and carboxylates. Activated carbons showed strong hydrophobic interactions. Modification (selective etching and introduction of some new functional groups) of activated carbon surfaces was done with c-HNO$\sb3$ (c = concentrated).^ Manganese oxide films were prepared by sol-gel methods (spin, dip, and spatula coatings) and by physical vapor deposition (PVD) followed by thermal oxidation. Crystallinity, surface morphology, and chemical composition were dependent upon preparation methods. The thinnest films were made by spin coating. More crystalline and even films were obtained by spatula coating. Manganese oxide films made by PVD thermal oxidation showed photoeffects under different radiation and sensing behavior towards CHCl$\sb3.$^ Tin oxide thin films were prepared by PVD thermal oxidation and spray pyrolysis. Surface morphology and crystallinity depend on preparation methods. Thermal oxidation conditions control the properties of the films such as film conductivity, crystallinity, and transmittance. Auger electron spectroscopy and X-ray photoelectron spectroscopy were used to determine O/Sn ratios on the surfaces of the films. Tin oxide films made by spray pyrolysis and PVD thermal oxidation with temperature programming showed good sensing behavior towards $\rm CH\sb2Cl\sb2.$^ Thermally oxidized tin oxide thin films with temperature programming were used for the detection of chlorinated methanes $\rm(CH\sb2Cl\sb2,\ CHCl\sb3,$ and CCl$\sb4).$ The films showed the best sensing behavior at 300$\sp\circ$C. The films showed different sensing behavior to different gases. The films were chemically selective to chlorinated methanes and stable for more than three weeks at operating temperature. The films showed fast gas sensing ($<$40 sec) and low detection limits ($<$4 ppm CCl$\sb4).$ Surface available oxygen is critical for gas sensing. The detection mechanism of $\rm CH\sb2Cl\sb2$ and CHCl$\sb3$ seem to be different from that with CCl$\sb4.$ Some of the detected products were CO$\sb2$ and chloride anion. ^