Bulk Doping of Metal Oxides
Doping of semiconductor MOX by various additives is one of the main methods for improving operating parameters of gas sensors including the increase of gas sensitivity and selectivity, the reduce of the operating temperature, and enhancement of the respon
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Bulk Doping of Metal Oxides
23.1
General Approach
The doping of semiconductor MOX by various additives is one of the main methods for improving gas sensitivity and selectivity, reducing the operating temperature, enhancing the response rate, etc. (Yamazoe et al. 1983; Yamazoe 1991; Korotcenkov et al. 2005, 2007; Gas’kov and Rumyantseva 2009). Impurities also need to be added to conventional binary MOXs to stabilize their (meso)porous nanocrystal morphology (Taurino et al. 2003). For doping various impurities such as catalytically active noble metals (Pd, Pt, Au, Ag, Rh), transition metals (Fe, Co, Cu, etc.), nonmetals (Se), alkaline earth metals (Ca, Ba, Sr, Mg), metalloids (B, Si), etc., can be used. Some of these additives serve as “accelerators” or “catalysts” while others serve as “inhibitors” of various processes. However, it should be noted that the major doping additives are noble and transition metals. The additives, which are conventionally utilized to modify the properties of the gas-sensing MOXs, are summarized in Table 23.1. Here we consider mainly indium and tin oxides as the most studied gas-sensing materials. More detailed reviews of the effects of doping on MOX gas sensors are available elsewhere (Kohl 1990; Korotcenkov 2005, 2007; Miller et al. 2006). Doping additives are brought to the bulk semiconductor as, for example, interstitials, or are placed on the surface of metal oxides. The bulk doping is done at the stage of metal oxide synthesis or deposition, while surface doping (modification) can be done following sensing layer deposition. As a rule, the surface additives form small clusters located on the surface of much larger grains of the MOX. The distribution of these small dopant particles on the surface is assumed to be more or less homogeneous. High dispersion of the bulk catalyst over the semiconductor support is also essential to obtain good performance of the conductometric gas sensors. Using ceramic and thick-film technologies, for example, noble-metal catalysts can be incorporated into an MOX by: (1) impregnating the pristine MOX powder with a noble-metal chloride such as PtCl4 and PdCl2 solution, followed by drying and calcination (Matsushima et al. 1992); (2) mixing the pristine MOX powder with a colloid of noble metal (Nakao 1995); and (3) chemical bonding of noble-metal complexes such as PdCl42− with surface hydroxyls at the pristine MOX in solution (Kaji et al. 1980). The impurity can also be introduced via sputtering of a thin intermediate layer (Gutierrer et al. 1993; Sayago et al. 1995). In this case, the profile of additive concentrations over the MOX structure is driven by temperature and time of annealing. Specific profiles of additive concentration can be created by using ion implantation techniques via adjusting the density of the ion current and the time of implantation (Sulz et al. 1993; Rosenfeld et al. 1993; Rastomjee et al. 1996). It should be noted that the effect of each doping material is complex and is not well studied. For instance, a surface catalyst may incr
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