Tungsten nonstoichiometric oxides

Many molybdenum and tungsten oxides are known. The simple ones are Mo03, W03, Mo02, and W02. Other, nonstoichiometric, oxides have been characterized and have complicated structures. 

Like molybdenum and tungsten, vanadium forms a variety of nonstoichiometric oxides with the alkali metals and other cations, for example, Cu+, Ag+, Zn+, or Al+, which have bronze-like properties. " These compounds can be made by heating appropriate mixtures of V2O5 and the desired alkali metal carbonate or oxide in platinum vessels under argon at 700-800 °C and then cooling slowly over... 

Tungsten is stable in dry air at room temperature. Oxidation becomes significant at 300°C, rapid at 500°C, and total at 650 C. The final product is a lemon-yellow solid WO3 that gives tungstate solutions in the presence of bases. WO3 may be reduced to WO2 or to various nonstoichiometric oxides WOx (2 < x < 3) characterized by an intense blue or purple color. 

G. L. Frey, A. Rothschild, J. Sloan, R. Rosentsveig, R. Popovitz-Biro, and R. Tenne, Investigations of nonstoichiometric tungsten oxide nanoparticles, J. Solid State Chem. 162(2), 300 (2001). 

Tungsten reacts with oxygen at high temperatures. The finely-divided powder is pyrophoric. But the bulk metal begins to oxidize at about 400°C. The metal oxidizes rapidly when heated in air or oxygen at red heat. Two simple oxides are known, a blue monoclinic dioxide, WO2, and a lemon yellow trioxide, WO3. The trioxide, WO3, is the most stable oxide and the ultimate product of heating the metal in oxygen. Many other oxides also are known, but they are of nonstoichiometric compositions and are unstable. The metal also is oxidized by water vapor at red heat. 

Kim YS, Ha SC, Kim K et al (2005) Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film. Appl Phys Lett 86(21) 213105-1-213105-3... 

Abstract. Nanopowders of nonstoichiometric tungsten oxides were synthesized by method of electric explosion of conductors (EEC). Their electronic and atomic structures were explored by XPS and TEM methods. It was determined that mean size of nanoparticles is d=10-35 nm, their composition corresponds to protonated nonstoichiometric hydrous tungsten oxide W02.9i (OH)o.o9, there is crystalline hydrate phase on the nanoparticles surface. After anneal a content of OH-groups on the surface of nonstoichiometric samples is higher than on the stoichiometric ones. High sensitivity of the hydrogen sensor based on WO2.9r(OH)0.09 at 293 K can be connected with forming of proton conductivity mechanism. 

Nanoparticles of nonstoichiometric tungsten oxides W03 x are promising material to produce active elements for hydrogen sensors. High work temperature that causes degradation processes is a problem of exploitation of gas sensors based on nanoparticles of semiconductor oxides. 

The surface of the nanocrystalline nonstoichiometric tungsten oxide conditioned on air (sample NN1) is almost completely formed of crystalline hydrate W03 (H20). Thus on the W4f-spectra (Fig. 2-3) the main component is comp, e with Ep=36.1 eV, on the Ols-spectra (Fig. 3-3) along with the 02 -states, the contributions from OH-groups (comp, g) and from H20 (comp, m) are present. 

Figure 1. TEM micrograph of nanoparticles.of nonstoichiometric tungsten oxide.

After the anneal on air of the nonstoichiometric tungsten oxide nanopowder the signal from crystalline hydrate WO3 (H20) (comp, e) in the spectrum of W4f-level (sample NN2, Fig. 2-4) disappeared. The component d from W6+-states of the oxide dominated in the spectrum of W4f-level after anneal on air and the component c from W5+-states (EpW4f7/2=34.8 eV) appeared in the low energy region. 

Simultaneous appearing of W5+-, W6 -statcs in the W4f-spectrum are connected with the phase of nonstoichiometric tungsten oxide. Taking into consideration the presence of OH-groups in the sample NN2 and knowing the contributions of W5+-, W6+-states (Table 1) it is possible to determine a coefficient x for matrix Wx5+Wi x6+03 x(0H)x, which equals x=0.09. Thus the sample NN2 can be classified as protonated oxide WO2.9i (OH)0.09. 

At the comparison of the Ols-spectra of the samples MS2 and NN2 it is seen (Fig. 3-2, Fig. 3-4, Table 1), that after anneal on air the OFF/O2 - ratio in the nonstoichiometric sample of tungsten oxide is higher than in the stoichiometric sample. High concentration of the OH -groups on the active element s surface can form a proton conductivity mechanism and cause a high sensitivity of the hydrogen sensor based on W02.9i (OH)0.09 nanoparticles at 293 K. 

The existence of at least nine phases in the molybdenum-oxygen system is well established. Their crystal structures are briefly described and it is shown that they can be classified into four main families dependent on whether they possess a basic structure of rutile type, ReOs type, or MoOs type, or have complex structures where polygonal networks can be distinguished. The known tungsten and mixed molybdenum tungsten oxides fit into this scheme. Because of their complicated formulas many of these compounds may be termed "nonstoichiometric," but variance in composition has not been observed for any of them. 

X > 0.28, this results in metalhc conductivity (Goodenough, 1965 Greenblatt, 1996). Nonstoichiometric sodium tungsten oxide (x < 1) has a distorted perovskite stmcture with unequal W-O bond lengths and tilted WOe octahedra. 

Nanopowders of nonstoichiometric tungsten oxide W03 x were synthesized by EEC-method. According to the TEM data (Fig. 1) the mean size of nanoparticles is 10-35 nm, size distribution is normal (Gaussian). The nanoparticles have spherical shape, well-defined crystalline structure, their agglomeration is almost absent. The macropowders of stoichiometric tungsten oxide W03 was obtained at combustion of metal in oxygen. The macropowders of stoichoimetric (sample MS) and the nanopowders of nonstoichiometric (sample NN) tungsten oxides were examined conditioned on air (samples MSI and NN1 respectively) and annealed on air at 563 K (samples MS2 and NN2 respectively). 

The tungsten-oxygen system is rather complex. Besides the stable stoichiometric binary oxides (WO3, WO2.9, WO2.72, and WO2), and the stoichiomettic tungstates and acids, a variety of nonstoichiometric, fiilly oxidized and reduced compounds exists, according to the scheme in Fig. 4.2. 

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