Fabrication and Characterization of Composite TiO2/Carbon Nanofilms with Enhanced Photocatalytic Activity
Licentiate thesis, 2011
The work presented in this thesis is dedicated to fabrication and characterization of an efficient thin film photocatalyst for environmental cleaning and solar fuel applications. Because of its simplicity and reproducible fabrication, the chosen combination of nanometer thin titania and carbon films serves also as a model system for a wider class of photoactive materials.
The films were prepared by physical vapor deposition and annealed in inert atmosphere. They showed the characteristic features of anatase and graphitic-like carbon phase in Raman and XRD measurements. The deduced crystallite size from Raman and XRD revealed qualitatively a smaller titania crystalline size in composite films in comparison to single titania films. In addition, a red shift in optical absorption was observed for composite films.
The performance of the films was tested in the reaction of photocatalytic decomposition of methanol. The results showed that composite films were with superior photocatalytic activity as compared to single titania films of same thickness. The enhanced activity was attributed to the effects of carbon film at its interface with nanocrystalline titania. Accordingly, the photogenerated charge carriers are efficiently separated (the holes move to the film surface and participate in chemical reaction, while the electrons are conducted away through the carbon film). The results also demonstrated a clear dependence of the activity on film thickness, which in turn was related to the limits of the charge carriers free path. Other carbon effect on the observed photoactivity was associated to the surface area, absorption, and the crystalline phase.
The origin of enhanced activity was further analyzed by photoluminescence (PL) measurements. The observed PL intensity for composite films was lower in comparison to single titania films. The origin of lower PL intensity was due to more non-radiative recombination. Time-resolved PL measurements were carried out to investigate the influence of carbon film on the charge carrier dynamics. More specifically, a shorter lifetime was observed for composite films. The shorter lifetime and lower PL intensity was attributed mainly to charge carrier leakage at the interface owing to band level alignment and also to inferior crystallinity (smaller crystallites) but played a minor role. Low temperature. PL measurements revealed no trace of carbon doping in composite films. The observed PL results supported the hypothesis for the enhanced photocatalytic activity.