Adsorption acceptor molecules

CNTs can be easily doped by noncovalent means via molecular adsorption, an aspect that has been considerably exploited to develop ultrasensitive field effect transistor sensors [88-91]. However, substitutional doping with B and N to confer p and n character to the CNTs has also been carried out [92]. Such doped systems can be more susceptible to react with donors or acceptors molecules (depending on the doping) allowing the chemically reactivity to increase. 

Note that there are alternative considerations of luminescence quenching not taking into account the adsorption of quencher molecules on the surface of nanoparticles [41, 42]. They evaluate nonlinear equations, which are the same to equation (2.19). However below we show that in aqueous solutions it is necessary include the surface properties of CdS and adsorption of electron acceptor molecules. 

I. Adsorption of Electron-Donor and Electron-Acceptor Molecules. 245... 

Dynamic electron transfer is observed when the adsorption between the charge carrier acceptor species and the semiconductor particle is weak. As such, charge carrier acceptor molecules have to diffuse from the homogeneous solution bulk to the surface of the semiconductor particle in order to accept the photogenerated electrons/holes. As described in Section 9.2.1, photogenerated carriers typically recombine on the sub-nanosecond time scale thus, it is necessary to accumulate one of the charge carriers... 

The results characterizing MgO can be summarized as follows Adsorption of electron-acceptor molecules such as NB in vacuo onto a MgO powder prepared by thermal outgassing in vacuo exhibited an EPR signal that could be assigned to NB or the corresponding negative radical ions A. The concentration of the radicals was monitored as a function of the outgassing temperature of MgO in vacuo. Twm maxima were found in the... 

Two well-known snrface stoichiometric photochemical reactions can be identified (i) the photostimnlated adsorption of O2 (reduction of acceptor molecules), and (ii) the photostimnlated adsorption of H2 (oxidation of donor molecules) on a metal-oxide surface. Both result in a new state of the heterogeneous system with charged species adsorbed on the solid. If these two processes occurred simultaneously they would yield a reaction identifiable as the photocatalysed oxidation of hydrogen to water. Nonetheless, snch a simple mechanism gives bnt a small indication of the real processes that take place on solids and at interfaces of heterogeneous systems. We examine these cases later after a discnssion of the nature of solids and a description of the photochemical/photophysical events taking place in these complex materials. 

A non-photocatalytic reaction occurring on the surface of an irradiated wide bandgap metal oxide such as ZrOa can also affect the process of photoinduced formation of Zr F- and V-type colour centres. The effect of such reactions is seen as the influence of photostimulated adsorption on the photocolouration of the metal-oxide specimen. Photoadsorption of electron donor molecules leads to an increase of electron colour centres, whereas photoadsorption of electron acceptor molecules leads to an increase of hole colour centres. Monitoring the photocolouration of a metal-oxide specimen by DRS spectroscopy during surface photochemical reactions can provide a further opportunity to evaluate whether the reactions are photocatalytic. 

Figure 5.2. Scheme of space-charge buildup at an n-type semiconductor surface upon adsorption of electron acceptor molecules. 

The principal methods for the immobilization of chemical receptors are (1) physical adsorption to a solid surface, (2) chemical adsorption (covalent attachment) to the surface, (3) affinity binding to physically or chemically boimd species, and (4) entrapment within a matrix. Since physical adsorption relies on relatively weak forces (van der Waals, ionic, solvation, donor/acceptor), molecules placed in this way may detach over time and/or exhibit nommiform biological activity becanse of a distribution of surface orientations/conformations. However, this method is clearly the simplest of the four and therefore often finds use. An example is the popular enzyme-linked immunosorbent assay (ELISA) used in medical diagnostics. 

Cations at the surface possess Lewis acidity, i.e. they behave as electron acceptors. The oxygen ions behave as proton acceptors and are thus Bronsted bases. This has consequences for adsorption, as we will see. According to Bronsted s concept of basicity, species capable of accepting a proton are called a base, while a Bronsted acid is a proton donor. In Lewis concept, every species that can accept an electron is an acid, while electron donors, such as molecules possessing electron lone pairs, are bases. Hence a Lewis base is in practice equivalent to a Bronsted base. However, the concepts of acidity are markedly different. 

We used polycrystalline films of ZnO and Sn02 as adsorbents. The films were deposited from the water suspension of respective oxides on quartz substrates. These substrates contained initially sintered contacts made of platinum paste. The gap between contacts was of about lO" cm. All samples were initially heated in air during one hour at T 500 C. We used purified molecular oxygen an acceptor particle gas. H and Zn atoms as well as molecules of CO were used as donor particles. We monitored both the kinetics of the change of ohmic electric conductivity and the tangent of inclination angle of pre-relaxation VAC caused by adsorption of above gases and the dependence of stationary values of characteristics in question as functions of concentrations of active particles. 

It has been proven by experiment that there are donor acceptor atoms and molecules of absorbate and their classification as belonging to one or another type is controlled not only by their chemical nature but by the nature of adsorbent as well (see, for instance [18, 21, 203-205]). From the standpoint of the electron theory of chemisorption it became possible to explain the effect of electron adsorption [206] as well as phenomenon of luminescence of radical recombination during chemisorption [207]. The experimental proof was given to the capability of changing of one form of chemisorption into another during change in the value of the Fermi level in adsorbent [208]. 

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