Adsorbate effects electrical conductance

In sections 1.7- 1.9 we have examined effects of surface charging in semiconductor adsorbent on electrophysical characteristics of the adsorbent. Although we did not go into details with respect to the crystalline origin of adsorbent, the consideration of effect of adsorption on electric conductivity of surface-adjacent layer led to conclusion that we considered monocrystalline samples. 

Above theoretical analysis of adsorption effects on electric conductivity and VAC profiles in polycrystalline semiconductor adsorbent with accounting for its barrier disorder indicate that the value and kinetics of change in o(t) and yS(t) during adsorption of both acceptors and donors sharply differ from those predicted by theory both for the case of ideal monocrystal and for polycrystal considered from the standpoint of bicrystal model. 

The effect of adsorption on concentration of the surface and volume interstitial defects in semiconductor adsorbent and resultant change in electric conductivity... 

The comparison of experimental data on adsorption of various particles on different adsorbents indicate that absorbate reaction capacity plays a substantial role in effects of influence of adsorption on electric conductivity of oxide semiconductors. For instance, the activation energy of adsorption of molecular oxygen on ZnO is about 8 kcal/mole [83] and molecular hydrogen - 30 kcal/mole [185]. Due to such high activation energy of adsorption of molecular hydrogen at temperatures of adsorbent lower than 100 C (in contrast to O2) practically does not influence the electric conductivity of oxides. The molecular nitrogen and... 

As it has been mention in preceding section, the vast effect of the mechanism of adsorption-caused change in electrophysical characteristics of adsorbent is provided by availability of defects [32]. However, various admixtures play similarly important role on effects of properties of oxides including the sensitivity of their electrophysical properties to adsorption [4, 5]. Small amounts of admixtures (of the order of 0.5 -1 mol.-%) can both increase the sensitivity of oxide for instance to oxygen (addition of Y2O3 to calcium oxide over pressure interval lO -10 Torr [189]) and decrease it (addition of Ga203 to ZnO [190]), or can result in insensitivity of electric conductivity on the pressure of the gas in question (as it is the case with respect to O2 while adding 0.5 -1 mol.-% of lithium to NiO [190]). 

The latter conclusion is reliably confirmed by experimental results [40] in which the studies of effect of the structure on the character of adsorption change in electric conductivity of monocrystal or partially reduced polycrystalline ZnO adsorbents were conducted. The comparative studies of the character and the value of response of electric conductivity in both types of adsorbents on adsorption of various atoms and molecular particles led the authors to conclusion on identical origin of both the mechanisms of electric conductivity and mechanisms of its adsorption induced change. 

The effect of adsorption on electric conductivity of sintered polycrystalline adsorbents... 

In case of the use of polycrystalline adsorbent subject to high temperature oxidation and characterized by almost stoichiometric content of the surface its electric conductivity can be linked with penetration of the current carriers through high intercrystalline barriers. The effect of adsorption on electric conductivity of adsorbents of such type is mainly manifested through the change of heights in intercrystalline barriers controlled by the value of the surface charge. 

We should note that this effect of the hydrogen on electric conductivity is related neither to reduction of oxide nor to the volume dissolution of hydrogen in oxide. The first option was ruled out due to initial heating of adsorbent in hydrogen atmosphere up to establishing the stationary value of electric conductivity at the temperature exceeding the maximum temperature of experiment (Theati 400 420 C). The con-... 

Thus, the model proposed explains the effect of CO on electric conductivity of several oxides only in case when oxygen is present in ambient volume which was observed in numerous experiments. Accordingly, the fact of existence of relatively narrow temperature interval in which an adsorbent is sensitive to CO becomes clear. This can be linked with the fact that if the operational temperature To is small the reaction products (in case of CO this is CO2) cannot get desorbed (see expression (2.80)), i.e. regeneration of the centers of oxygen adsorption is not feasible. If Tq is very high both adsorption of oxygen and reducing gas should be ruled out. 

As it has been shown in Section 1.11 a comparison of experimental data on effects of adsorption of various particles on electric conductivity of oxide adsorbents results in conclusion that the dominant role in this... 

Thus, we have considered in detail various theoretical models of effect of adsorption of molecular, atom and radical particles on electric conductivity of semiconductor adsorbents of various crystalline types. Special attention has been paid to sintered and partially reduced oxide adsorbents characterized by the bridge type of intercrystalline contacts with the dominant content of bridges of open type because of wide domain of application of this very type of adsorbents as sensitive elements used in our physical and chemical studies. 

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

The results obtained in above experiments confirm the removal of chemisorbed particles in the process of immersion of the film with preliminary chemisorbed radicals in a liquid acetone. Note that at low pressures of acetone, the CHa-radicals absorbed on ZnO film could be removed only by heating the film to the temperature of 200 - 250°C. Moreover, if the film with adsorbed radicals is immersed in a nonpolar liquid (hexane, benzene, dioxane), or vapours of such a liquid are condensed on the surface of the film, then the effect of removal of chemisorbed radicals does not take place, as is seen from the absence of variation of electric conductivity of the ZnO film after it is immersed in liquid and methyl radicals are adsorbed anew onto its surface. We explain the null effect in this case by suggesting that the radicals adsorbed on the surface of the ZnO film in the first experiment remained intact after immersion in a nonpolar liquid and blocked all surface activity of the adsorbent (zinc oxide). 

---