Zinc oxide crystals

Zinc hydroxy double salts are layered materials similar to layered double hydroxides which show intercrystalline reactivity and incorporate organic compounds between layers.337 Hydroxy double salts of high crystallinity can be obtained by reacting ZnO with organic metal salts in water. Zinc oxide crystals could then be prepared by thermal treatment of hydroxy zinc acetate.338... 

In the preceding example it was deliberately assumed that the zinc oxide crystal used as starting material was perfect and thus, in particular, of stoichiometric composition. This was done solely in order to show that the crystal became an impurity semiconductor following the adsorption process otherwise the reasoning did not involve in any way the behavior of the bulk of the crystal, and the mechanism of the adsorption process did not depend upon the existence or nonexistence of semiconductivity prior to the uptake of hydrogen. If this were a general situation, further examination of the effect of semiconductivity on chemisorption and catalysis would hardly seem profitable. However cases where semiconductivity may play a direct role can easily be imagined. 

Let us consider again the surface of a zinc oxide crystal but suppose now that the underlying crystal is nonstoichiometric and contains excess zinc atoms in interstitial lattice positions. A fraction of these defects is ionized and their valence electrons move quasifreely through the crystal. A molecule such as nitrous oxide, adsorbed on the surface, can make use... 

Zincite is usually colored red or orange by manganese impurities. Photographs of zincite are shown in Fig. 1.2. Zinc oxide crystals exhibit several typical surface orientations. The most important surfaces are the (0001) and (0001) (basal plane), (1010) and (1120) (prism planes) and (1121) (pyramidal plane) crystal faces. In principle, the (0001) planes are terminated by Zn atoms only, while the (0001) surfaces are terminated by oxygen atoms only. However, this simple picture does not hold in reality (see description of the surface structure in Sect. 4.2.1 of this book). Nevertheless, the etching behavior is noticeably different for these two surfaces [17] (see also Chap. 8). 

Fig. 3. Mott-Schottky plots of capacitance vs bias for two typical zinc oxide crystals. The dotted lines represent absolute theoretical predictions in the absence of surface states.

Bauer, W. (1977). Experimental studies on the spectral sensitization of photoconductivity on zinc oxide crystals. J. Phys. Chem. Solids 38 463-467. 

In the manufacture of zinc white by the French process, metallic zinc is melted, evaporated and oxidized in the vapor state to zinc oxide with air. If the starting material is insufficiently pure, the zinc has to be purified by evaporation and condensation prior to the oxidation step. The size and shape of the zinc oxide crystals can be controlled by the oxidation conditions. After combustion the ZnO is precipitated from the ZnO/air mixture in settling chambers, in which classification of the zinc oxide particles takes place according to their size. 

Petermann G., Tributsch H. and Bogomolni R. (1972), Luminescence of zinc oxide crystals controlled by electrode potential and electrochemical reactions , J. Chem. Phys. 57, 1026-1032. 

Zinc oxides have diverse applications in various industrial [3] and other types of processes[4-5]. It is essentially a dhydrogenating catalyst on which dehydration can also occur. The catalytic activity of zinc oxide is shown to be correlated to catalyst structure [6], although it is also reported that ZnO catalyst is structure insensitive [2], implying that every surface of exposed zinc oxide is equally active. The ZnO structure is shown to correspond to expanded hexagonal close packing with zinc ions filling half the tetrahedral holes[l], Akhter et al. [7] have identified three natural faces of zinc oxide crystals. 

The concentration of ionized zinc interstitials in the zinc oxide crystal lattice thereby reflect the polarization by the oxygen reduction. 

Crisler DF, Cupal JJ, Moore AR. Dielectric, piezoelectric, and electromechanical coupling constants of zinc oxide crystals. Proc IEEE 1968 56 225-6. 

Zinc oxide crystals can exhibit strong piezoelectric properties. Normally recognized as an n-type semiconductor, it has a resistivity less than 103 ohm-cm. When doped with lithium, resistivity rises to 1012 ohm-cm and it exhibits piezoelectricity about four times that of quartz. 

Unfortunately, while the leaf-like copper basic sulphate crystals and the rod-like zinc oxide crystals had uniform particle shapes, in both cases, the particle sizes varied appreciably, as evident from Figures 7 and 8. For this reason, experimental programmes are continuing with a view to identifying the reaction conditions which give uniformly-sized crystals. 

Figure 8 Scanning electron micrograph showing the size range distribution of the rod-like zinc oxide crystals formed by forced hydrolysis of dilute zinc nitrate solutions at 523K...

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