Structural and optical properties of visible active photocatalytic WO₃ thin films prepared by reactive dc magnetron sputtering

Abstract

Nanostructured tungsten trioxide films were prepared by reactive dc magnetron sputtering at different working pressures P tot 5 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.

Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering

“…25 These trap states were associated with oxygen vacancies in the thin film, and the measured concentrations correlated well with previous approximate values obtained from near infrared (NIR) absorption. 24 An oxygen vacancy concentration of x ≈ 0.005 (determined by an average over three films) was inferred in the more sub-stoichiometric WO 3-x films, prepared at P tot = 10 mTorr, while the less sub-stoichiometric films prepared at P tot = 30 mTorr showed vacancy concentrations of x ≈ 0.001 (determined by an average over four films). 25 It was shown that the films mainly exhibited the monoclinic γ-WO 3 phase with a fraction of the monoclinic ε-phase and a slight strain in films primarily at P tot = 10 mTorr.…”

“…More information about the experimental details can be found elsewhere. 23,24 The O 2 /Ar ratio was kept at 0.43, and the samples were sputtered at a substrate temperature of T s = 553 K, and post-annealed at T a = 673 K for 1 h in air. Samples were sputtered at working pressures, P tot = 10, 15, 20, 25 and 30 mTorr.…”

“…In previous studies we have reported on the physical properties of tungsten oxide films prepared in this manner. [23][24][25] From cyclic voltammetry (CV), a lower limit of the concentration of trap states in the band gap was calculated. 25 These trap states were associated with oxygen vacancies in the thin film, and the measured concentrations correlated well with previous approximate values obtained from near infrared (NIR) absorption.…”

“…23,24 Intermediate polaron absorption in the ε-phase may contribute to the P1 peak in Fig. 2, in particular at low P tot .…”

Irreversible thermochromic response of RF sputtered nanocrystalline BaWO4 films for smart window applications

“…Films deposited at 600 • C are in full deep -blue color and the transmittance of the film reduced to 20% and can be attributed to the release of oxygen during the deposition process. 14,15 With a further increment in substrate temperature up to 800 • C, the transmittance of the film increases to 70%; this shows the evidence of thermochromic nature in BWO films. In addition, there are two regions, where a transparent oscillating region and a region of strong absorption.…”