Chemisorption conductance

Since the pioneering work of Rohrer and Binning,77 scanning tunelling microscopy (STM) has been used to image atomic-scale features of electrically conductive surfaces under ultra-high-vacuum but also at atmospheric pressure and in aqueous electrochemical environments. The ability of STM to image chemisorption and surface reconstruction is well... 

Figure 3.51. CO chemisorption by pulse response of a reduced 5 wt% Pt/Al203 a. Thermal Conductivity Detector (TCD) signals after the CO pulses, b. Cumulative amount of CO chemisorbed. The monolayer capacity is 0.06 mmol/g Pt, corresponding with a dispersion of 24%.

The percolation model of adsorption response outlined in this section is based on assumption of existence of a broad spread between heights of inter-crystalline energy barriers in polycrystals. This assumption is valid for numerous polycrystalline semiconductors [145, 146] and for oxides of various metals in particular. The latter are characterized by practically stoichiometric content of surface-adjacent layers. It will be shown in the next chapter that these are these oxides that are characterized by chemisorption-caused response in their electrophysical parameters mainly generated by adsorption charging of adsorbent surface [32, 52, 155]. The availability of broad spread in heights of inter-crystalline barriers in above polycrystallites was experimentally proved by various techniques. These are direct measurements of the drop of potentials on probe contacts during mapping microcrystal pattern [145] and the studies of the value of exponential factor of ohmic electric conductivity of the material which was L/l times lower than the expected one in case of identical... 

In paper [141] it was shown that the change in electric conductivity in case of chemisorption of various alkyl radicals on the same oxide is notably dependent on chemical nature of free radicals. In this case the arrangement of simplest radicals in the order of decreasing degree of effect of electric conductivity of ZnO given in [132, 186] will be the following ... 

We should point out that up to now we have considered only polycrystals characterized by an a priori surface area depleted in principal charge carriers. For instance, chemisorption of acceptor particles which is accompanied by transition-free electrons from conductivity band to adsorption induced SS is described in this case in terms of the theory of depleted layer [31]. This model is applicable fairly well to describe properties of zinc oxide which is oxidized in air and is characterized by the content of surface adjacent layers which is close to the stoichiometric one [30]. 

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

In our view the final verification was given to this conclusion in paper [66] in which simultaneous O2 adsorption on partially reduced ZnO and resultant change in electric conductivity was studied. It was established in this paper that the energies of activation of chemisorption and that of the change of electric conductivity fully coincide. The latter is plausible only in case when localization of free electron on SS is not linked with penetration through the surface energy barrier which is inherent to the model of the surface-adjacent depleted layer. 

Let us dwell now on the issue of non-dissociative form of low temperature chemisorption of H2 proposed in [89]. As it has been mentioned above absolutely different behavior of electric conductivity of adsorbent... 

The opposite change in electric conductivity of adsorbent occurs during adsorption of such active particles as atoms of hydrogen and atoms of metals [115, 124,125]. The similar result is obtained during radiolysis of hydrocarbons [126] due to formation and chemisorption of H-atoms. Both the rate of adsorption caused change in electric conductivity and the value of its stationary values are determined in this cases by all the processes accompanying chemisorption [127],... 

This radicals do not escape from the surface (this is indicated by a semiconductor microdetector located near the adsorbent surface) undergoing chemisorption on the same semiconductor adsorbent Him. Thus, they caused a decrease in the electric conductivity of the adsorbent sensor, similarly to the case where free radicals arrived to the film surface from the outside (for example, from the gas phase). Note that in these cases, the role of semiconductor oxide films is twofold. First, they play a part of adsorbents, and photoprocesses occur on their surfaces. Second, they are used as sensors of the active particles produced on the same surface through photolysis of the adsorbed molecular layer. 

Figure 4.8. displays oscillograms of evolution of the electric conductivity of the ZnO film in the process of catalytic dehydration of isopropyl alcohol at various temperatures of the catalyzer and equal portions of alcohol (5-10-2 Torr) admitted into the reaction cell. Experimental curves 1-4 are bell-shaped. We suppose that this fact is associated with two circumstances. On one hand, alcohol vapors dissociate on the oxide film producing hydrogen atoms. The jump in electric conductivity is caused by chemisorption of these hydrogen atoms on the film which plays a part of the sensor in this case. Chi the other hand, the drop in electric conductivity is caused by complete dissociation of the admitted portion of alcohol ( depletion of the source of hydrogen atoms) and by... 

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... 

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