1 research outputs found
Improved Visible Light Harvesting of WO<sub>3</sub> by Incorporation of Sulfur or Iodine: A Tale of Two Impurities
We report the incorporation of sulfur
or iodine into monoclinic
tungsten trioxide (S:WO<sub>3</sub> or I:WO<sub>3</sub> 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 WO<sub>3</sub> precursor solutions. Red
shifts of the absorption spectra were observed with S and I incorporation
(from ∼2.7 to 2.6 and 2.1 eV respectively), likely due to the
formation of intragap impurity bands. S:WO<sub>3</sub> 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:WO<sub>3</sub> films, but showed surface segregation of I in the I:WO<sub>3</sub> 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