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 = C 6 H 5, 1; C 6 H 4 F-4, 2; C 6 H 3 F 2 -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 1 H, 13 C, 19 F, and 183 W NMR ( 1, δ = −474.8; 2, δ = −416.3; 3, δ = −446.3), melting point ( 1, 95 °C; 2, 101 °C; 3, 105 °C), 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 °C produced blue tungsten oxide films on glass substrates. The films were analyzed by glancing angle powder diffraction and shown to be amorphous at 300 °C, but consisted of crystalline WO 3 - x at higher deposition temperature. Annealing the films from 1 at 550 °C in air for 30 min produced yellow γ-WO 3 with preferred growth in the [020] direction. Raman analysis of the as-formed films showed WO 3 - x with evidence for W V −W VI paired ions. Annealed samples produced Raman patterns for γ-WO 3 (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 WO 3 - x ( x ≈ 0.22) with binding energy shifts for W 4f 5/2 at 37.2 and 4f 7/2 34.4 eV and for oxygen 1s 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 WO 3 - x films showed negligible photocatalytic activity. The annealed, yellow WO 3 films were active photocatalysts and readily destroyed an overlayer of stearic acid.