2- tin oxides

Tin Oxide. - To our knowledge this system has been studied (with one exception) only by Thornton and Harrison at the University of Nottingham, who have produced seven papers on the subject. I.r. has been used to investigate the surface as a function of the evacuation temperature. Molecular H2O is largely removed at 320 K and fully removed at 473 K. H-bonded -OH groups are present, and the free -OH absorbs at 3640 cm . CO2 yields carbonates and bicarbonates. CO is not adsorbed as such, but forms carbonates by the partial reduction of Sn to Sn . Adsorption of both NH3 and pyridine reveals Lewis basicity only. The adsorption of small organic molecules shows the oxidizing properties of tin oxide as indicated by CO. 

Methanol is oxidized to formate ions, acetone and acetaldehyde to acetate species. An enolic form of adsorbed acetone is seen, which accounts for rapid isotopic exchange of -OH. On O2 adsorption two bands at 1155 and 1020 cm are formed,which are however not assigned. Mixtures of CO plus O2 lead to the obvious formation of carbonates and bicarbonates. CO interaction with presorbed NH3 forms carbamate species. Oxidizing properties of the surface are shown by the reaction of trichloroacetone to trichloroacetate species. Nitriles are, however, hydrolysed to acetimidate species R-CNH . Similar reactions with surface -OH are shown by ethyl and phenyl isocyanate, which form urethanes by reaction with isolated -OH groups, and 1,3-diethylurea with -OH pairs. Reaction with surface -OH is also shown by trimethylchlorosilane, which, however, harshly attacks the surface and forms silicone species.  

Other papers deal with tin oxide as a matrix for transition-metal ions and have already been discussed in Section 2 of this review. Adsorption of NO on H2 reduced samples leads to the reoxidation of the sample (N2 and N2O are formed in the gas phase). On evacuated samples, a chelating NO2 species is formed, together with a dinitrosylic species. 

Tin oxide (Sn04) has found applications in high-temperature conductors, ohmic resistors, transparent thin-film electrodes and gas sensors. 

It crystallizes in the tetragonal rutile structure (see Fig. 5.27) with cell dimensions a = 474 pm and c = 319 pm in the single-crystal form it is known by its mineralogical name, cassiterite. It is a wide band gap semiconductor, with the full valence band derived from the 02p level and the empty conduction band from the Sn 5s level. The band gap at OK is approximately 3.7eY, and therefore pure stoichiometric Sn02 is a good insulator at room temperature when its resistivity is probably of the order of 106Qm. 

In practice both natural and synthetic crystals are oxygen deficient, leading to donor levels approximately 0.1 eY below the bottom of the conduction band and consequently to n-type semiconductivity. Doping the crystal with group V elements also induces n-type semiconductivity the usual dopant is antimony. The ground state electronic configuration of the Sb atom is 5s2p3, and when it 

Song and Kim [287] used a two-microemulsion method for the synthesis of Sn02 particles the combinations were  

Microemulsion 1 O.IM SnCl4/water/AOT/n-heptane Microemulsion 2 l.OM NH40H/water/A0T/n-heptane 

The two reverse microemulsions were mixed for several hours at a constant stirring rate. The particles formed were flocculated by acetone, gathered by centrifugation, washed with n-heptane (the oil phase itself) to remove excess surfactant and dried at 100°C/24h. The particles had a size of 2-3 nm with a narrow size distribution. 

When calcined at 600°C/2 h, the hydroxide precursor particles were converted to phase-pure Sn02 (20-40 nm). 

Cerium (IV) oxide nanoparticles were synthesized by Masui et al [236] by use of a two-microemulsion technique. One of the microemulsions contained polyoxyethylene(lO) octylphenyl ether (OP-10) as the surfactant, n-hexyl alcohol as the co-surfactant, cyclohexane as the continuous phase, and an aqueous solution of cerium nitrate as the droplet phase. The second microemulsion was the same except that the droplet phase was an aqueous ammonia solution. The two were mixed to cause precipitation the particles thus obtained were gathered by centrifugation and washing under sonication with methanol, deionized water and acetone. The final treatment involved freeze-drying and vacuum drying. The mean particle size varied with experimental conditions in the range 2.5-4.0 nm. 

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Tin ll) oxides. Lower tin oxides SnO (white, NH4OH to SnCli solution black, heat on white SnO red), form a complex system. 

Shanthi E, Dutta V, Baneqee A and Chopra K L 1980 Electrical and optical properties of undoped and antimony-doped tin oxide films J. Appi. Rhys. 51 6243-51... 

Figure C1.5.12.(A) Fluorescence decay of a single molecule of cresyl violet on an indium tin oxide (ITO) surface measured by time-correlated single photon counting. The solid line is tire fitted decay, a single exponential of 480 5 ps convolved witli tire instmment response function of 160 ps fwiim. The decay, which is considerably faster tlian tire natural fluorescence lifetime of cresyl violet, is due to electron transfer from tire excited cresyl violet (D ) to tire conduction band or energetically accessible surface electronic states of ITO. (B) Distribution of lifetimes for 40 different single molecules showing a broad distribution of electron transfer rates. Reprinted witli pennission from Lu andXie [1381. Copyright 1997 American Chemical Society.

Another growing apphcation that overlaps the electrically functional area is the use of transparent conductive coatings or tin oxide, indium—tin oxide, and similar materials in photovoltaic solar ceUs and various optic electronic apphcations (see Photovoltaic cells). These coatings are deposited by PVD techniques as weU as by spray pyrolysis, which is a CVD process. 

Acetylsucrose [63648-81-7] has been prepared in 40% yield by direct acetylation of sucrose using acetic anhydride in pyridine at 40° C (36). The 6-ester has subsequently been obtained in greater than 90% yield, by way of 4,6-cycHc orthoacetate. Other selective methods for the 6-acylated derivatives include the use of alkyl tin reagents such as dibutyl tin oxide (37) and of dibutyl stannolane derivatives (38). Selective acetylation of sucrose by an enzymic process has also been described. Treatment of sucrose with isopropenyl acetate in pyridine in the presence of Lipase P Amano gave, after chromatography, 6-0-acetylsucrose (33%) and 4/6-di-O-acetylsucrose (8%). The latter compound has been obtained in 47% yield by the prolonged treatment (39). 

Cychc carbonates result from polyols by transesterification using organic carbonates (115). Thus sorbitol and diphenylcarbonate in the presence of dibutyl tin oxide at 140—150°C form sorbitol tricarbonate in quantitative yield (116). 

Spray Pyrolysis. In spray pyrolysis, a chemical solution is sprayed on a hot surface where it is pyrolyzed (decomposed) to give thin films of either elements or, more commonly, compounds (22). Eor example, to deposit CdS, a solution of CdCl plus NH2CSNH2 (thiourea) is sprayed on a hot surface. To deposit Iu202, InCl is dissolved in a solvent and sprayed on a hot surface in air. Materials that can be deposited by spray pyrolysis include electrically conductive tin—oxide and indium/tin oxide (ITO), CdS, Cu—InSe2, and CdSe. Spray pyrolysis is an inexpensive deposition process and can be used on large-area substrates. 

Fuming is also an alternative to roasting in the processing of low grade concentrates (5—25 wt % tin). This procedure yields a tin oxide dust, free of iron, which is again fed back to a conventional smelting furnace. 

In a fire-assay method used at the smelters, a weighed quantity of concentrate is mixed with sodium cyanide in a clay or porcelain cmcible and heated in a muffle furnace at red heat for 20—25 min. The tin oxide is reduced to metal, which is cleaned and weighed. Preliminary digestion of the concentrate with hydrochloric and nitric acids to remove impurities normally precedes the sodium cyanide fusion. 

If tin and sulfur are heated, a vigorous reaction takes place with the formation of tin sulfides. At 100—400°C, hydrogen sulfide reacts with tin, forming stannous sulfide however, at ordinary temperatures no reaction occurs. Stannous sulfide also forms from the reaction of tin with an aqueous solution of sulfur dioxide. Molten tin reacts with phosphoms, forming a phosphide. Aqueous solutions of the hydroxides and carbonates of sodium and potassium, especially when warm, attack tin. Stannates are produced by the action of strong sodium hydroxide and potassium hydroxide solutions on tin. Oxidizing agents, eg, sodium or potassium nitrate or nitrite, are used to prevent the formation of stannites and to promote the reactions. 

Stannic and stannous chloride are best prepared by the reaction of chlorine with tin metal. Stannous salts are generally prepared by double decomposition reactions of stannous chloride, stannous oxide, or stannous hydroxide with the appropriate reagents. MetaUic stannates are prepared either by direct double decomposition or by fusion of stannic oxide with the desired metal hydroxide or carbonate. Approximately 80% of inorganic tin chemicals consumption is accounted for by tin chlorides and tin oxides. 

Stannic Chloride. Stannic chloride is available commercially as anhydrous stannic chloride, SnCl (tin(IV) chloride) stannic chloride pentahydrate, SnCl 5H20 and in proprietary solutions for special appHcations. Anhydrous stannic chloride, a colorless Aiming Hquid, fumes only in moist air, with the subsequent hydrolysis producing finely divided hydrated tin oxide or basic chloride. It is soluble in water, carbon tetrachloride, benzene, toluene, kerosene, gasoline, methanol, and many other organic solvents. With water, it forms a number of hydrates, of which the most important is the pentahydrate. Although stannic chloride is an almost perfect electrical insulator, traces of water make it a weak conductor. 

The inorganic tin compound that has received the most study from a toxicological viewpoint is stannic oxide. Autopsies performed on workers in the tin mining and refining industry, who inhaled tin oxide dust for as long as 20 yr, disclosed no pulmonary fibrosis (57). Inhalation for long periods produces a benign, symptomless pneumoconiosis with no toxic systemic effects (58). 

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