Structural and optical properties of visible active photocatalytic WO_3 thin films prepared by reactive dc magnetron sputtering

摘要

Nanostructured tungsten trioxide films were prepared by reactive dc magnetron sputtering at different working pressures P_(tot) = 1-4 Pa. The films were characterized by scanning electron microscopy, x-ray diffraction, Rutherford backscattering spectroscopy, Raman spectroscopy, and ultraviolet-visible spectrophotometry. The films were found to exhibit predominantly monoclinic structures and have similar band gap, E_g ≈ 2.8 eV, with a pronounced Urbach tail extending down to 2.5 eV. At low P_(tot), strained film structures formed, which were slightly reduced and showed polaron absorption in the near-infrared region. The photodegradation rate of stearic acid was found to correlate with the stoichiometry and polaron absorption. This is explained by a recombination mechanism, whereby photoexcited electron-hole pairs recombine with polaron states in the band gap. The quantum yield decreased by 50% for photon energies close to E_g due to photoexcitations to band gap states lying below the O_2 affinity level.