Charge adsorption induced

Adsorption of a neutral (n ) onto a metal surface leads to a heat of adsorption of Q, as the electrons and nuclei of the neutral and metal attract or repel each other. Partial positive and negative charges are induced on each with the formation of a dipolar field (Figure 7.4). 

Thus, the major conclusions of tiie early studies by Volkenshtein and his colleagues applicable to the theory of the method of semiconductor gas sensors are the following a) chemisorption of particles on a semiconductor surface can be accompanied by a charge transfer between adsorption-induced surface levels and volume bands of adsorbent and b) only a certain fraction of absorbed particles is charged, the fraction being dependent on adsorbate and adsorbent. 

As it has been already mentioned in previous section the process of adsorption of gaseous particles on the surface of semiconductor adsorbent is one of the major and in several cases the most important stage controlling the adsorption-induced change in the value of surface charge resulting in alteration of numerous surface and volume electrophysical characteristics of adsorbent. 

The adsorption-induced charging of the barriers of such adsorbents results in the change in heights of inter-crystalline barriers and transforms the profile of their distribution function. As the model suggests, it is this change in the distribution function of the heights of barriers, that is responsible for adsorption-induced change of such an important characteristic of polycrystal as the differential coefficients of its volt-ampere characteristics or, which is more convenient for our studies,... 

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]. 

The results mentioned together with data outlined in Section 1.11 indicate that adsorption induced change in electric conductivity of sintered and partially reduced oxide is mostly dependent on adsorption related change in concentration of stoichiometric metal atoms which are responsible for dope electric conductivity rather than by charging of the surface of adsorbent due to transformation of radicals of O2 and O". 

In conclusion to this part it seems noteworthy that in contrast to the effect of adsorption of molecular particles on electrophysical properties of oxide semiconductors, the major peculiarity of this effect for such chemically active particles as the simplest free radicals or atoms of simple gases (H2, O2, N2, CI2, etc.) is that they are considerably more chemically active concerning the impurity centres [47]. The latter are responsible for dope conductivity of oxide semiconductors. As for the influence of electric fields on their adsorption due to adsorption-induced surface charge distribution, they are of minor importance which is proved by results of the experiments on assessing field effect on adsorp-... 

The lack of coincidence between CIP and lEP of (hydr)oxides and related materials indicates the presence of strongly adsorbing species, added intentionally or present as impurities. Specific adsorption induces shifts in the lEP on the one hand and in the point of zero proton charge on the other in two opposite directions. When a shift in the CIP and lEP is addressed, the pristine PZC of given material (cf. Tables... 

There is little or no evidence of micelle adsorption in these systems, and, indeed, it is not to be expected owing to the coulombic repulsion between the negatively charged silica or clay and the micelles. Surfactant anion adsorption at positive (cationic) sites followed by further surfactant adsorption induced by lateral attraction between the hydrocarbon tails to form... 

Adsorption-Induced Charge Transfer at Surfaces Metals and Insulators... 

ADSORPTION-INDUCED CHARGE TRANSFER AT SURFACES METALS AND INSULATORS... 

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