Zinc Oxide, 445,

Zinc oxide has been investigated already in 1912. With the beginning of the semiconductor age after the invention of the transistor [1], systematic investigations of ZnO as a compound semiconductor were performed. In 1960, the good piezoelectric properties of zinc oxide were discovered [2], which led to the first electronic application of zinc oxide as a thin layer for surface acoustic wave devices [3]. 

Currently, research on zinc oxide as a semiconducting material sees a renaissance after intensive research periods in the 1950s and 1970s [4, 5]. The results of these earlier activities were summarized in reviews of Heiland, Mollwo and Stockmann (1959) [6], Hirschwald (1981) [7], and Klingshirn and Haug (1981) [8]. Since about 1990 an enormous increase of the number of publications on ZnO occurred (see Fig. 1.1) and more recent reviews on ZnO have been published [9-11], 

The renewed interested in ZnO as an optoelectronic material has been triggered by reports on p-type conductivity, diluted ferromagnetic properties, thin film oxide field effect transistors, and considerable progress in nanostructure fabrication. All these topics are the subject of a recently published book [11], 

Further information on p-type doping of ZnO is given in other chapters of this book (2 and 7). 

In this book the chemical, structural, optical, electrical, and interface properties of zinc oxide are summarized with special emphasis on the use of ZnO as transparent conductive electrode in thin film solar cells. This application has a number of requirements, which can be fulfilled by ZnO  

Zinc oxide (ZnO) has useful piezoelectric properties. It has an hexagonal structure (wurtzite type) with a density of 5.66 g/cm. It is relatively unstable and decomposes above 1700°C, which is below its melting point (1975°C). It is readily attacked by all common acids and bases. It has limited CVD applications at this time. 

It is deposited by MOCVD, with an alkyl precursor, such as dimethyl zinc and tetrahydrofuran (THF) f l 

The deposition temperature range is 300-500°C, the partial pressure of the alkyl is 0.5-2.5 Torr, and that of THF is 20-80 Torr. ZnO has found applications in piezoelectric devices, transducers, coatings for photoconductive devices, and non-linear resistors (varistors), and overvoltage protectors. 

Zinc oxide (ZnO) can exhibit a wide range of electrical properties depending on minor constituents and sintering conditions. It is also remarkable for the ease with which high-density ceramics can be achieved over a wide range of sintering temperatures and for its excellent resistance to thermal shock. 

Varistor compositions based on ZnO contain a number of dopants at a level of approximately 1 mol.% a typical formulation is (in mol.%) 96.5ZnO-0.5Bi2O3- 

Electrodes usually consist of fired-on silver paint with a small glaze content. It may be necessary to remove a high-resistivity surface layer from the ceramic before silvering. The contacts formed in this way are unlikely to be ohmic, but 

The structure of the ceramic consists of ZnO grains of diameter 10-50 /mi with an intergranular phase varying in thickness between 1 nm and 1 /mi. The intergranular phase, which usually has a high bismuth content, is insulating with a resistivity of the order of 106 Qm. 

Mn2+ and Mn3+ may also behave as acceptors for electrons liberated from oxygen vacancy traps and so prevent them from entering the conduction band. Cobalt and chromium may also contribute to reactions of this type through the coexistence of Co2+ and Co3+ and of Cr3+ and Cr4+ ions, but they appear to be less effective in the absence of manganese. Such mechanisms may lead to the presence of a layer on the ZnO grains that has a higher resistivity than that of the interior, which is an essential part of the model outlined earlier, but the precise mechanism has yet to be determined. 

Zinc oxide, ZnO, is a p-type semiconductor and shows piezoelectric properties which make this material useful for microsensor devices and micromachined actuators [69, 70]. The Al-doped material is used as a transparent electrode [71]. Other applications of ZnO include gas sensors [72], solar cell windows [73] and surface acoustic devices [74]. The organometallic compounds diethylzinc, ZnEt2 [75-77], and dimethylzinc, ZnMe2 [78], are frequently used as precursors for the deposition of zinc oxide. However, these reagents are highly pyrophoric and tend to react prematurely in the presence of water or oxygen. If ZnEt2 is combined with an alcohol in the reaction chamber, stable intermediates, presumably zinc alkoxides and/or alcohol adducts, are formed in the vapor-phase [79]. These compounds are more stable than dialkyl zinc rea- 

Zinc Oxide. - From the classical investigations of Eischens et al. and Kokes et al. it is well known that hydrogen is chemisorbed at room tem- 

in New Horizons in Catalysis , Proceedings 7th Int. Congr. on Catalysis, Tokyo, 1980, ed. T. Seiyama and K. Tanabe, Elsevier, Amsterdam, 1981, Part B, p. 1163. 

Similar discrete and continuous effects have been observed with the C02 Zn0 system.The first effect causes the splitting of the bands due to linearly co-ordinated CO2 (because of the formation of dimeric species) and the discrete shift of the bands due to bidentate carbonates (because of the adsorption of a linear species in an adjacent position) the second effect is responsible for the continuous shift of some bands of the adsorbed carbonates caused by long distance mutual interaction. Unlike the H2 and H2 C0 cases, these effects are suggested as occurring on steps instead of regular faces. 

The interaction of hydrocarbons with the ZnO surface has been extensively studied. In particular (/) cyclopentene-ZnO interaction gives rise to mainly rr-bonded species (//) cyclobutene is adsorbed with rupture of the ring (Hi) allyl-benzene yields adsorbed phenyl-allyl species (CeHsCHCHCH ) (/V) ethynylbenzene is polymerized on the surface under the irradiation of the laser beam (v) 3-phenyl-1-propyne and 1-phenyl-l-propyne give rise 

In the i.r. range usually investigated (0.5-0.1 eV), besides the vibrational contributions of adsorbed species and multiphonon modes, plasmonic contributions associated with free electrons in the conduction band and defect excitations can also contribute, whose relative intensity is strongly influenced by the pretreatment conditions. In particular when the conduction band is extensively populated, the plasmonic modes couple with the vibrational ones (due to both adsorbed species and multiphonon lattice vibrations) causing a dramatic spectral modification and loss of any vibrational detail. 

NanoTek zinc oxide - nanoparticle size zinc oxide manufactured by physical vapor synthesis process Societe des Blancs de Zinc de la Mediterranee, Marseille, France Cachet Or - French process zinc oxide Zinc Corporation of America, Monaca, PA, USA Kadox - French process zinc oxide 

MAJOR PRODUCT APPLICATIONS paints, coatings, crosshnker of rubber, sealants 

Zinc oxide is produced either by the French or by the American process. Both processes are pyrometallurgical techniques in which the metal in a vapor state reacts with oxygen, forming zinc oxide. The difference between the methods is in the raw material used for the synthesis. In the French process, pure metal is evaporated, and the final product is as pure as the metal used for its production. In the American process, zinc vapor is obtained directly from an ore by burning it as a mixture with coal or in an electrothermic process where electric current provides the heat. More recently, a new method, somewhat similar to the French process, was introduced by Nanophase Technologies Corporation who patented a physical vapor synthesis process in which zinc metal is vaporized. The vapor is rapidly cooled in the presence of oxygen, causing nucleation and condensation of nanoparticle size zinc oxide. The particles are non-porous and free of contamination. 

The purest grades of zinc oxide from the French process contain more than 99.99% of ZnO. The purity of zinc oxide is essential in many applications because ZnO is a photochemically active material and impurities may severely affect its properties. Zinc oxide has found many applications due to its photochemical 

Several reasons are behind the widespread use of zinc oxide. Zinc oxide is a popular crosslinker for rubber and for various resins. Zinc oxide is also used as an UV stabilizer and as an additive having biocidal activity. It is frequently used in paints. Zinc oxide also has a relatively high refractive index which makes it an efficient white pigment. 

Generally, the remaining 10% of ruhher compounds have cure systems based mostly on peroxide curatives. However, a small number of compounds based on halogen-ated elastomers (such as polychloroprene) have cure systems based on metal oxides. Also, resin cures are used in special cases to cure some compounds such as curing bladders for tires. 

The two most common activators used with sulfur cure systems are zinc oxide and stearic acid. Virtually all sulfur-cured rubber compounds contain zinc oxide and stearic acid. This means that thousands upon thousands of rubber recipes have these two ingredients. This makes them the most commonly used rubber compounding ingredients, appearing in more different recipes than any other ingredient. 

Zinc oxide is an essential ingredient as an activator in over 90% of all rubber recipes used in commerce. Globally, there was approximately 1 billion pounds of zinc oxide produced in 2010, of which almost one-half was used by the rubber industry. 

Zinc oxide from the French process (indirect process) is the most common method used today. 

Usually ZnO is manufactured from zinc ore (usually sphalerite, ZnFeS). However, from the wet process, feedstocks can be zinc sulfate or zinc chloride. 

This is a white pigment used as a reinforcing filler (which is comparable in volume with zinc oxide) and it is also an excellent heat-resisting filler for silicone rubbers. The rutile form gives a rather creamier, more reflectant colour, which is more stable at high temperatures. They are widely used fillers in the manufacture of white or light colour chemical resistance compounds for the pigmentation industry. 

We first present some general information on the structure of ZnO, and then continue to discuss various types of catalytic process, principally for hydrocarbons. It should become clear to the reader that ZnO provides one of the best characterized examples among oxide catalysts, at least as far as the identity of surface intermediates and the mechanisms of reaction are concerned. 

Zinc oxide may crystallize in either the wurtzite or, more rarely, the zinc blende structure, the former being more stable at lower temperatures. In both structures the zinc and oxygen ions are tetrahedrally co-ordinated to each other. The bonding is intermediate between the completely ionic and the completely covalent, both ions being more polarizable than in a perfect ionic crystal and carrying an effective charge of only 0.5e. As is well known, non-stoicheio-metric ZnO, with excess of Zn, is an n-type semiconductor with a band gap of 3.2 eV. 

Dent and Kokes consider that wurtzite derives from isotropically expanded, hexagonal close-packed layers of oxide ions, with correspondingly expanded zinc layers in which zinc ions occupy one half of the tetrahedral holes between oxide layers. This expansion increases the radius of the trigonal holes in the oxide layers such that, at 0.058 nm, they can almost accommodate a zinc ion. The structure is quite open and consists of straight channels of octahedral sites, each 0.20 nm in diameter, separated by trigonal squeeze points , 0.12 nm in diameter. 

Pak has shown, by a correlation between O—H stretching frequency and the strength of the M—OH bond, that the Bronsted acidity of ZnO is similar to that of MgO and NiO and is much less than that of y-Al20a. A similar conclusion was reached from studies of benzene adsorption and of its effect on hydroxyl stretching frequencies, a for ZnO of 10.5 being reported. In general then we may expect ZnO to display basic/Lewis acid properties in catalytic reactions, as shown by MgO, on which carbanions are readily produced from both alkenes 

ZnO catalysts, as already indicated, are invariably subjected to calcination at high temperature 613 K), either in vacuo or under an inert gas, in order for activity to be developed. Frequently, oxidation is also applied to try to ensure surface/bulk stoicheiometry. Unless explicitly stated to the contrary, it may be assumed in what follows that such activation procedures were adopted. 

Lu and Yeh [174] record an emulsion technique for the synthesis of ZnO. They dissolved zinc acetate in de-ionized water to obtain the aqueous phase, n-heptane was used as the continuous phase, in which a surfactant (Span 80) was added. The two phases in different proportions were mixed continuously for Ih for obtaining homogeneous emulsions. Ammonium hydroxide was added into the emulsions to cause precipitation of zinc. The precipitates were dried and calcined at 700 -1000°C/2 h, which yielded white powders of ZnO. The modal particle size was 0.080 pm, while the mean size was about 0.08-0.09 pm depending on experimental conditions. 

Sager et al. [171] prepared emulsions similar to those described in case of Zr02 [183], i.e. two emulsions (a) Arkopal 40, DiDAB (see above under Zirconium Dioxide Pure and Doped Forms and Derivatives), decane and aqueous solution of zinc nitrate or HMTA for the synthesis of zinc hydroxide. After precipitation, the particles were washed with decane and heated to 200 C. Zinc hydroxide particles (doped with 5 % Mn) thus produced were partially crystallized and had a diameter range of 300-600 nm, peaking at around 400 nm. This was apparently a suitable precursor for the synthesis of ZnO. 

What is the reaction between Cu++ and OH- ions To a little IN CuS04 add 6N NaOH drop by drop, until it is present in excess. 

Explain what successive reactions occur when ammonium hydroxide is added instead of sodium hydroxide. 

To 5 cc. of IN CuSCh add 10 cc. of a molal solution of tartaric acid then add sodium hydroxide solution, as in (1), and compare the results with those in (1) and (2), but do not attempt to ascribe a definite formula to the complex compound formed. 

If zinc sulphate in solution is treated with sodium bicarbonate, pure zinc carbonate is precipitated, because a sodium bicarbonate solution contains but a minute quantity of OH ions. On the other hand, a sodium carbonate solution, in consequence of hydrolysis, contains a considerable quantity of OH ions, and thus it furnishes both the C03 and OH ions necessary for the formation of basic zinc carbonate. 

Basic zinc carbonate is decomposed by heat into zinc oxide and carbon dioxide. 

ZnO exhibits many unusual properties including uniaxial piezoelectric response and n-type semiconductor characteristics. 

The electrochemically deposited single crystalline ZnO nanowires can be applied in LEDs [147, 148]. ZnO nanowire films can be embedded in an insulating spin-coated polystyrene layer. The spin-coating parameters are carefully fine-tuned to completely fill out the space between the ZnO nanowires and produce only a very thin coverage of the nanowire tips. The polystyrene layer thickness at the tips can be further reduced by the plasma etching treatment to make the n-type ZnO tip junctions outside. A top 

It is generally believed that nanocrystalline MgO should possess superior mechanical properties, especially of strength and hardness, in comparison with its microcrystalline counterparts. If this hypothesis were true, it has never attracted any scientific evidence, and to the present authors knowledge there is no information currently available on the mechanical properties of pure nanocrystalline MgO. 

ZnO normally has the hexagonal (wurtzite) crystal structure with lattice parameters a = 3.25 A and c = 5.12A (space group P63mc). The Zn atoms are tetrahedraDy coordinated to four O atoms, where the Zn d-electrons hybridize with the oxygen p-electrons. Layers occupied by zinc atoms alternate with layers occupied by oxygen atoms [94]. Whilst a bond between the Zn and O atoms exhibits covalent characteristic in the c-direction, it is mostly ionic in the o-direchon [95] consequently, ZnO single crystals have highly anisotropic properties. 

ZnO nanocrystals may have dtEFerent stmctures, depending on the method of preparation. For example, nanoparticles formed by the oxidation of zinc vapor have the zinc-blende structure when smaller than 20 nm, and form tetrapod-hke crystals on further growth [96]. ZnO particles prepared via the flash evaporation method have a cubic crystal structure [97]. 

The physical properties of ZnO crystals depend heavily on the concentration of native defects caused by deviations from the stoichiometric composition. A review on this subject is available in Ref. [104]. 

TaUe 1.4 Basic physical properties of single-crystal ZnO. 

Pigment White 7 formula ZnS Pigment White 5 formula ZnS/BaSO, 

Reactive hydroxyl (HO ) and hydroperoxy (HOj) radicals may play an important role in TiOj photocatalysed degradation of polymers [502, 1626, 2193]. Both these radicals can abstract hydrogen from the polymer chain to give a polymer alkyl radical (P ), which would undergo main chain cleavage via /S-scission and/or addition of oxygen to form polymer peroxy radicals (POO ) (cf section 6.1). 

It has also been proposed that Ti02 pigments can have a photocatalytic effect on the decomposition of polymeric hydroperoxides (POOH), probably by an energy transfer process [2093]  

The photochemistry of solid ZnO is complex. Under UV irradiation an exciton (electron (e )/hole (p ) pair) is formed, which further reacts with ZnO by the following reactions [145]  

The species Oj has been detected, by ESR spectroscopy, in several oxide pigments ZnO, MgO and Ti02 [1375]. 

---

Rinmann s green, ZnCojO. A spinel formed when cobalt nitrate solution is placed on zinc oxide and the mixture heated to redness. The green colour forms a delicate test for Zn. 

Gases which are high in FIjS are subject to a de-sulphurisation process in which H2S is converted into elemental sulphur or a metal sulphide. There are a number of processes based on absorption in contactors, adsorption (to a surface) in molecular sieves or chemical reaction (e.g. with zinc oxide). 

As an example, consider the reduction of zinc oxide to zinc by the reaction ... 

Consider the reduction of zinc oxide, by carbon monoxide. The equations are ... 

Hence for the reduction of zinc oxide by carbon monoxide we have. 

The complete reduction of zinc oxide is favoured by a small value of K. i.e. when log,u A, > log,o X,. Figure 3.5 shows plots of logio Xi, and logio X2 against 1/T where the two graphs intersect log)o X for the reduction process is zero and hence X = 1. 

If the normal carbonate is used, the basic carbonate or white lead, Pb(OH),. 2PbCO,. is precipitated. The basic carbonate was used extensively as a base in paints but is now less common, having been largely replaced by either titanium dioxide or zinc oxide. Paints made with white lead are not only poisonous but blacken in urban atmospheres due to the formation of lead sulphide and it is hardly surprising that their use is declining. 

It catalyses the decomposition of potassium chlorate(V). Mixed with zinc oxide, it is used as a catalyst in the manufacture of methanol. It is used as a pigment, being very resistant to weathering. 

Zinc oxide or zinc white is used in paints, but more preferable, because of its better covering power, is lithopone (a mixture of zinc sulphide and barium sulphate). Both paints have the advantage over white lead that they do not blacken in air (due to hydrogen sulphide). Zinc dust and also zinc chromate are constituents of... 

Zinc carbonate and zinc oxide are constituents of calamine lotion. Zinc oxide, an antiseptic, is present in zinc ointment and in cosmetic powders. 

In a 2 litre bolt-head flask, equipped with an efficient mechanical stirrer, place 60-5 g. (50 ml.) of pure nitrobenzene and a solution of 30 g. of ammonium chloride in 1 litre of water. Stir vigorously and add 75 g. of a good quality zinc powder (about 90 per cent, purity) in small portions over a period of 5 minutes. The main reaction occurs about 5 minutes after the addition and the temperature rises. When the temperature reaches about 65°, add enough ice to the weU-stirred mixture to reduce the temperature to 50-55°. Filter the solution through a Buchner funnel twenty minutes after the first portion of zinc powder was introduced wash the zinc oxide residues with 600-700 ml. of boiling water. 

This polymer is vulcanised by heating with metalhc oxides, such as zinc oxide the reaction may involve the formation of ether bridges ... 

Zinc is also used extensively to galvanize other metals such as iron to prevent corrosion. Zinc oxide is a unique and very useful material for modern civilization. It is widely used in the manufacture of paints, rubber products, cosmetics, pharmaceuticals, floor coverings, plastics, printing inks, soap, storage batteries, textiles, electrical equipment, and other products. Lithopone, a mixture of zinc sulfide and barium sulfate, is an important pigment. 

Zinc is not considered to be toxic, but when freshly formed ZnO is inhaled a disorder known as the oxide shakes or zinc chills sometimes occurs. Where zinc oxide is encountered, recommendations include providing good ventilation to avoid concentration exceeding 5 mg/ms, (time-weighted over an 8-hour exposure, 40-hour work week). 

Zincite, see Zinc oxide Zincosite, see Zinc sulfate Zincspar, see Zinc carbonate Zirconia, see Zirconium oxide... 

Comparison of specific surface of anatase and zinc oxide determined by electron microscopy A ) and by nitrogen adsorption A )... 

Leaded tin bronze Leaded tin bronzes Leaded yellow brass Leaded zinc oxide Lead 2-ethylhexanoate... 

Zinc oxide beds Zinc oxide eugenol Zinc oxides Zinc-oxygen cell... 

The alkalized zinc oxide—chromia process developed by SEHT was tested on a commercial scale between 1982 and 1987 in a renovated high pressure methanol synthesis plant in Italy. This plant produced 15,000 t/yr of methanol containing approximately 30% higher alcohols. A demonstration plant for the lEP copper—cobalt oxide process was built in China with a capacity of 670 t/yr, but other higher alcohol synthesis processes have been tested only at bench or pilot-plant scale (23). 

Isobutyl alcohol [78-83-1] forms a substantial fraction of the butanols produced by higher alcohol synthesis over modified copper—zinc oxide-based catalysts. Conceivably, separation of this alcohol and dehydration affords an alternative route to isobutjiene [115-11 -7] for methyl /-butyl ether [1624-04-4] (MTBE) production. MTBE is a rapidly growing constituent of reformulated gasoline, but its growth is likely to be limited by available suppHes of isobutylene. Thus higher alcohol synthesis provides a process capable of supplying all of the raw materials required for manufacture of this key fuel oxygenate (24) (see Ethers). 

In the United States there was Httie interest in solvent processing of coals. A method to reduce the sulfur content of coal extracts by beating with sodium hydroxide and zinc oxide was, however, patented in 1940 (116). In the 1960s the technical feasibiHty of a coal deashing process was studied (117),... 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

---