Tungsten oxide coatings from the aerosol-assisted chemical vapor deposition of W(OAr)6 (Ar=C6H5, C6H4F-4, C6H3F2-3,4);: photocatalytically active γ-WO3 films

Tungsten hexaaryloxide complexes of formula W(OAr)(6) (Ar = C6H5, 1; C6H4F-4, 2; C6H3F2-3,4, 3) have been synthesized by the reaction of W(O)Cl-4 and ArOH in 22-28% yield. The complexes were characterized by H-1, C-13, F-19, and W-183 NMR (1, delta = -474.8; 2, delta = -416.3; 3, delta = -446.3), melting point (1, 95 degreesC; 2, 101 degreesC; 3, 105 degreesC), IR, microanalysis, and single-crystal X-ray diffraction (octahedral coordination). Aerosol-assisted chemical vapor deposition of W(OAr)6 using acetone and toluene solvents and substrate temperatures from 300 to 500 degreesC produced blue tungsten oxide films on glass substrates. The films were analyzed by glancing angle powder diffraction and shown to be amorphous at 300 degreesC, but consisted of crystalline WO3-x at higher deposition temperature. Annealing the films from 1 at 550 degreesC in air for 30 min produced yellow gamma-WO3 with preferred growth in the [020] direction. Raman analysis of the as-formed films showed WO3-x with evidence for W-V-W-VI paired ions. Annealed samples produced Raman patterns for gamma-WO3 (807, 715, 273, and 133 cm(-1)). The films were adhesive to the substrate, conformal, passed the Scotch tape test, and gave good coverage. Typical growth rates were 20-50 nm min(-1). SEM showed uniform films consistent with an island growth mechanism. Energy dispersive analysis confirmed the presence of only tungsten and oxygen. XPS showed that the as-formed films were WO3-x (x approximate to 0.22) with binding energy shifts for W 4f(5/2) at 37.2 and 4f(7/2) 34.4 eV and for oxygen Is at 532.6 eV. The as-formed films had an absorption maxima at 320-360 nm (dependent on film thickness) and an onset for absorption at 400 nm. Reflectance/transmission measurements produced interference fringes that enabled a calculation of film thickness. In general the films showed good transmission from 500 to 1000 nm (60%) and some reflectivity (ca. 20%) over the same region. After annealing, the absorption edge was pushed more into the visible and the films showed evidence for some scattering associated with haze. The blue WO3-x films showed negligible photocatalytic activity. The annealed, yellow WO3 films were active photocatalysts and readily destroyed an overlayer of stearic acid.