ZnO surfaces

Morrison, S. R. Measurement of Surface State Energy Levels of One-Equivalent Adsorbates on ZnO. Surface Set. 27 (1971) pp. 586-604. 

The photoelectrochemical behavior of ZnSe-coated CdSe thin Aims (both deposited by vacuum evaporation on Ti) in polysulflde solution has been described by Russak and Reichman [112] and was reported to be similar to MIS-type devices. Specifically, Auger depth profiling showed the ZnSe component of the (ZnSe)CdSe heterostructures to convert to ZnO after heat treatment in air, thus forming a (ZnO)CdSe structure, while the ZnO surface layer was further converted to a ZnS layer by cycling the electrode in polysulfide electrolyte. This electrochemically generated ZnS layer provided an enhanced open-circuit potential compared to CdSe alone. Efficiencies as high as 5.4% under simulated AM2 conditions were recorded for these electrodes. 

The WGS reaction is a reversible reaction, that is, it attains equilibrium with reverse WGS reaction. Thus the fact that the WGS reaction is promoted by H20(a reactant), in turn, implies that the reverse WGS reaction may also be promoted by a reactant, H2 or CO2. In fact the decomposition of the surface formates produced from H2+CO2 is promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions can conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility[63]. 

Fig. 4.30. Variation rate of the electric conductivity of a ZnO film as a function of the intensity of the electron beam bombarding the film, for different times of preliminary exposure of the film to molecular hydrogen at room temperature (/) - ZnO surface free of hydrogen, (2) - the exposure time is 5 min, (i) - the exposure time is 30 min, (4) - the exposure time is 120 min.

The H-atoms originated during the reaction are ionized on the ZnO surface. As a result, the electrical conductivity of the adsorbent increases. 

As for the energy transfer to the subsurface layers of zinc oxide from the singlet oxygen molecules, the transfer should lead to an intn ease in the electrical conductivity of semiconductor either due to ejection of electrons into the conduction band h-om shallow traps [67], or due to the injection of electrons into zinc oxide by excited particles [68]. Effects of this kind were observed in the interaction between a ZnO surface and excited pairs of benzophenone [70], and also in adsorption of singlet oxygen on the surface of ZnO monocrystal in electrolyte [69]. 

It follows from the formula that the experimentally evaluated value of / activation energy on the ZnO surface may be related to the activation energy of oxygen desorption from the zinc oxide surface. This value well agrees with the desorption activation energy measured with the aid of semiconductor detectors in work [109]. 

This supposition is experimentally substantiated by Kupriyanov et al. [160], In this work they investigated the influence of RGMAs upon the electrical conductivity of pure zinc oxide films and films activated by microcrystals of gold. The gold was chosen as the activator because of its chemical inactivity and high lateral mobility. This makes it possible to obtain islet films on a ZnO surface at room temperature, thus avoiding probable metallurgical processes. 

Another methodical trait of the Au/ZnO sensor application to detect metastable atoms of rare gases is the limitation of the range of operating temperatures. When heated to above 500 K, these sensors irreversibly loose their sensitivity to RGMAs. The loss of sensitivity is associated with the coalescence of Au microcrystals applied to a ZnO surface. The causes of this will be discussed later. 

For the results of measurements, see Fig. 5.22. Curve 1 describes the electron emission current as a function of the Au concentration on the ZnO surface at a constant concentration of He atoms in the gaseous phase. This dependence is of extremal nature with a linear rise and a maximum peak at the Au concentration of 0.9-10 atoms/cm. A similar curve having its maximum at the same point resulted for Ne atoms. It can be seen from curve 3 that near the emission current maximum, an... 

Comparing the above-mentioned results with the data of morphological survey of islet films of gold on a ZnO surface [116- 168] leads to an inference that the maximum of curve 1 is associated with changes in the geometric surface of the Au islet film as it grows, while the maximum at curve 2 is connected with changes in the mean size of microcrystals in the islet film. 

The rate of charges accumulation at the traps is in direct proportion to the concentration of vacant traps - <) and to the frequency of interactions between the metallic particles on the ZnO surface and RGMAs, which in turn is proportional to their concentration in the gaseous phase (Afm)- The relaxation of charge at the traps can be described by instant time of relaxation r t,T) that are generally a time and a temperature functions. The equation that describes the process of trap recharging is as follows... 

The blue light absorbed by CdS is subtracted from the white light incident on the surface of the solid. The remaining reflected light is colored, yellow in this case. When the violet light is subtracted from the white light incident on the ZnO surface, the reflected light appears white. 

The first EELS experiments were reported by Propst and Piper in 1967 and concerned the adsorption of H2, N2, CO, H20 on the (100) surface of tungsten [49]. Ibach studied the energy losses of electrons to phonons in ZnO surfaces around 1970 [50] and continued to develop the technique for studying adsorbates on metal surfaces [51,52]. In the 1980s EELS grew further into an extremely important and generally accepted tool in surface science. 

The v(OD) peak at 2706 cm 1 on an OD-covered ZnO surface attributable to linear OD groups on two-coordinated Zn ions, decreased by reaction with CO at 473 K and accompanied with the appearance of uLS(OCO) and vs(OCO) peaks for surface bidentate formats (DCOO ) at 1586 and 1342 cm1, respectively, suggesting that the OD groups react with CO to produce the bidentate formates. The formates (DCOO-) react with the D atoms of bridge (2682 cm ) or threefold-hollow (2669 cm-1) OD groups at 573 K as monitored by FT-IR, evolving D2, COz, D20, and CO in the gas phase. 

The WGS reaction is a reversible reaction that is, the WGS reaction attains equilibrium with the reverse WGS reaction. Thus, the fact that the WGS reaction is promoted by H20 (a reactant), in turn implies that the reverse WGS reaction may also be promoted by a reactant, H2 or C02. In fact, the decomposition of the surface formates produced from H2+C02 was promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility. The activation energy for the decomposition of the formates (produced from H20+CO) in vacuum is 155 kJ/mol, and the activation energy for the decomposition of the formates (produced from H2+C02) in vacuum is 171 kJ/mol. The selectivity for the decomposition of the formates produced from H20+ CO at 533 K is 74% for H20 + CO and 26% for H2+C02, while the selectivity for the decomposition of the formates produced from H2+C02 at 533 K is 71% for H2+C02 and 29% for H20+C0 as shown in Scheme 8.3. The drastic difference in selectivity is not presently understood. It is clear, however, that this should not be ascribed to the difference of the bonding feature in the zinc formate species because v(CH), vav(OCO), and v/OCO) for both bidentate formates produced from H20+C0 and H2+C02 show nearly the same frequencies. Note that the origin (HzO+CO or H2+C02) from which the formate is produced is remembered as a main decomposition path under vacuum, while the origin is forgotten by coadsorbed H20. 

The behavior of a ZnO surface toward [Fe3(CO)i2] [9] is very similar to that of the surface of MgO or AI2O3. The surface OH groups behave as nucleophiles toward coordinated CO, generating the anion [HFe3(CO)n]T which can be extracted from the surface with [(Ph3P)2N]Cl dissolved in CH2CI2. Yields of the isolated products were not reported. 

The films (both ZnO and ZnO Al) were wurizite stracture with a preferential texturing (c-axis -L substrate). No AI2O3 was found in the XRD spectra, suggesting either dispersal of the A1 in the ZnO matrix or its presence as very tiny crystals of (hydr)oxide on the ZnO surface. TEM measurements showed an average grain size of 25 nm (ZnO) and 45 nm (ZnO Al). 

Both stereoisomers were formed, implying a loss of stereochemical integrity during the formation of the second carbon-carbon bond. When the reaction was conducted on ZnO, surface-related processes affected both the rate and stereochemistry. The effect of various quenchers could be explained as competitive adsorption at active sites, with or without interference with electron transfer. A reaction scheme involving formation of dimer, both in the adsorbed state and in solution, was proposed, the former route being the more important On CdS, the reaction could sometimes be induced in the dark as well because of the presence of acceptor-iike surface states. Neither particle size, surface area, nor crystal structure appeared to significantly influence the dimerization observations parallel to those found in the CdS photoinduced dimerization of N-vinylcarbazole... 

The ZnO surface is made more organophilic by coating it with oil and propionic acid. The ZnO is often deaerated and sometimes pelletized or granulated to improve handling properties. 

Figure 28.8 Comparison between GaP and ZnO surfaces. [From Ref. 48, reprinted with permission.]...

Because of these slow processes, a ZnO surface remembers for some time whether and where it has been illuminated. The memory keeps the properties of the surface caused by illumination for some time after the illumination. This has been observed with respect to sorption and catalysis 41 >42), surface potential, as just discussed, and conductivity. It was been observed also on other semiconductors, e.g. the exchange of oxygen isotopes and oxygen sorption on illuminated MgO 43>- The memory effect has been treated extensively by the electronic theory of catalysis 41.44-45). 

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