Spectral Selective Solar Light Enhanced Photocatalysis: TiO2/TiAlN Bilayer Films

Abstract

We demonstrate that spectral selective photocatalytic multilayer films can be tailored such that they can harness the full solar spectrum for enhanced photocatalytic gas-phase oxidation of acetaldehyde. Thin films of anatase TiO 2 were deposited on a thin solar absorber TiAlN film to fabricate bilayer TiO 2 /TiAlN films by dc magnetron sputtering on aluminium substrates. The structural and optical properties of the films were characterized by X-ray diffraction and Raman spectroscopy. The reaction rate and quantum yield for acetaldehyde removal was measured and an almost tenfold enhancement of the quantum yield was observed for the TiO 2 /TiAlN films compared with the single TiO 2 film, on par with enhancements achieved with new heterojunction photocatalysts. The results were interpreted by a temperature-induced change of the reaction kinetics. Absorption of simulated solar light illumination resulted in a temperature increase of the TIAlN film that was estimated to be at most 126 K. We show that a concomitant temperature increase of the top layer TiO 2 by 100 K shifts the water gassurface equilibrium from multilayer to submonolayer coverage. We propose that this is the main reason for the observed enhancement of the photocatalytic activity, whereby gas phase molecules may come in direct contact with free surface sites instead of having to diffuse through a thin water film. The implications of the results for judicious control of temperature and relative humidity for efficient gas-phase photocatalysis and exploitation of selective solar absorbing films are discussed.

Synergistic TiO2/VO2 Window Coating with Thermochromism, Enhanced Luminous Transmittance, and Photocatalytic Activity

“…The importance of hydroxyls on the surface have been shown by UHV studies, wherein the lack of hydroxyls on the photocatalyst surface is a limiting factor for the photocatalytic degradation. 36,85 This effect of the hydroxyls was corroborated in separate measurements done at t s = 80 C using a slightly humid airflow with a relative humidity of 0.8% (corresponding to a water coverage of about 0.02 monolayers at steady-state conditions 49 ) at 80 C, which, as expected, 49,84 yielded higher photocatalytic activity than for measurements in dry air (Table S2).…”

“…Specifically, we show that the temperature rise due to the absorption of light, in particular near-infrared light, in VO 2 produces a very marked enhancement of the photocatalytic reactivity of TiO 2 at the same time as antireflection enhances the glazing's luminous performance, thus realizing and implementing our previously reported concept for boosting the reactivity of a photocatalyst by purposely adjusting its temperature and water coverage at the surface. 49…”

“…At intermediate conditions, a balance can be obtained between free adsorption sites, O 2 coverage, and water coverage on the surface. 49,84 Thus a TiO 2 surface at 80 C is expected to be considerably dehydrated compared to the case of such a surface at ambient temperature, which accounts for the lower reaction rate. The importance of hydroxyls on the surface have been shown by UHV studies, wherein the lack of hydroxyls on the photocatalyst surface is a limiting factor for the photocatalytic degradation.…”