Improved Visible Light Harvesting of WO3 by Incorporation of Sulfur or Iodine: A Tale of Two Impurities

We report the incorporation of sulfur or iodine into monoclinic tungsten trioxide (S:WO3 or I:WO3 respectively), with the aim to improve its visible light-harvesting ability. Films were synthesized by spray pyrolysis with either ammonium sulfide or iodide added to the aqueous WO3 precursor solutions. Red shifts of the absorption spectra were observed with S and I incorporation (from similar to 2.7 to 2.6 and 2.1 eV respectively), likely due to the formation of intragap impurity bands. S:WO3 samples exhibited enhanced photoelectrochemical (PEC) performance at low S concentrations, but this quickly deteriorated with increasing S content. Incident photon conversion efficiency (IPCE) data showed that this initial improvement was driven by improved collection efficiency at longer wavelengths. Conversely, photocurrent decreased at all levels of I addition. IPCE measurements for these films showed only a marginal increase in efficiency at longer wavelengths, indicating that the extra absorbed photons did not contribute significantly to the photocurrent. Time of flight-secondary ion mass spectrometry (ToF-SIMS) depth profiling revealed a uniform distribution of S throughout the S:WO3 films, but showed surface segregation of I in the I:WO3 samples. Raman and X-ray photoelectron spectrometry (XPS) showed that S and I substituted for oxygen, but in the case of S, other pathways such as interstitial incorporation and cation substitution could not be ruled out. The complexities of intentionally adding nonmetal impurities to metal oxide systems are highlighted in the context of the existing body of literature.