Niemeyer, Dirk; (2001) The gas sensitive material Cr (2-x) Ti (x) O3. Doctoral thesis (Ph.D), The gas sensitive material Cr (2-x) Ti (x) O3..

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Abstract

Chromium titanium oxide (CTO) is a recently discovered gas sensor material that is capable of detecting small concentrations of toxic or flammable gases in air with stability of performance over the short and long-term and minor influences of variations of humidity. CTO is the first new material to be successfully commercialised in large-volume manufacture for sensing of hydrocarbons, VOCs, hydrogen and carbon monoxide at low (ppm) concentrations in air since the introduction of SnO2 for this purpose in the 1960s. The main focus of this thesis lies in the systematic investigation of this material in order to develop a detailed theoretical understanding of the gas response mechanism. CTO was prepared by solid-state reaction of Cr2O3 and TiO2 in air at 1000C. It crystallises in a solid solution with the general formula Cr2-xTixO3. The phase limit is at x ~ 0.3-0.4. Above the phase limit a 2-phase mixture with CrTiO3 is found. Substitution of Ti strongly decreases the electrical conductivity of the porous bodies studied. This effect, and the surface segregation of Ti, controls the gas sensor behaviour. Atomistic simulations have been performed on the (0001) and (1012) face to assess defect models for pure- and titanium doped Cr2O3. In the absence of titanium, one stable defect is a CrVI - Vcr" pair, which segregates to the (0001) surface and contributes to the relatively high p-type conductivity shown by finely porous bodies of Cr2O3 at elevated temperature; with titanium addition, a stable defect, segregated on both of the investigated surfaces, is the complex (TiIV)3Vcr". The proportion of surface CrVI is decreased. Surface studies of Cr2O3 have been performed extensively in the literature because it adsorbs oxygen and catalyses combustion of hydrocarbons. Whilst Cr2O3 shows a good combustion rate of CO to CO2 but only a small gas response, titanium doped Cr2O3 behaves the opposite way. X-ray photoelectron spectroscopy of the chromium 2p3/2 core states show a multiplet splitting, proposed to originate from local magnetic moments at the Cr site. A CrVI state is also detected. Substituting titanium decreases the proportion of surface CrVI and reduces the splitting of the Cr 2p3/2 state. A reappearance of the splitting at higher Ti concentrations was observed, speculatively attributed to ordering of the surface structure. Although currently used solely in oxygen-containing environments CTO can also detect gases in a nitrogen atmosphere. Decreasing oxygen partial pressure caused a large decline in conductivity that is explained by removal of oxygen surface acceptor states. Whilst carbon monoxide and ethanol react with the few remaining acceptor states causing a large gas response, H2S sulphates the surface. The stable gas response to H2S is caused by reversible adsorption of H2S molecules on a CrIII site, forming a new surface trap state. Replacing titanium in CTO by vanadium, iron, magnesium or manganese resulted in a significantly reduced gas response to carbon monoxide. These elements did not form a solid solution with the Cr2O3 lattice. However, if introduced as additional elements in CTO manganese and iron dopants crystallised as MnCr2O4 and (Cr0.4Fe0.6)2O3 impurity phases and reduced the amount of chromium. A significantly reduced response to water vapour relative to the response to carbon monoxide was observed with these materials.

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