Abstract

Abstract

The photocatalytic activity of Au modified titanium
dioxide was evaluated in the photodegradation of
the azo dye Acid Red 1 (AR1) under 254 nm
irradiation. Noble metal nanoparticles were
deposited on TiO2 either through depositionprecipitation
(DP), or by immobilisation of preformed
metallic sols (polyvinylalcohol (PVA)/NaBH4 or
tetrakis(hydroxymethyl)phosphonium chloride
(THPC)/NaOH systems). Gold nanoparticles on the
photocatalyst surface had dimensions of around 3-4
nm in diameter, as determined by HRTEM analysis,
and exhibited visible light plasmon absorption.
THPC Au/TiO2 appears to be the most photoactive
amongst the photocatalysts with a 1 wt.% Au
loading, while among THPC samples with different
Au loadings (0.5-20.0 wt.%) the maximum
photoactivity was attained with 5 wt.% Au/TiO2.
The higher AR1 photodegradation rate observed on
Au/TiO2 at basic pH can be related to the higher
concentration of hydroxyl anions at the interface:
these are able to effectively scavenge
photoproduced valence band holes, possibly in
competition with Au0 oxidation to Au+.

  Photocatalytic activities of titanium dioxide (TiO₂)
films deposited by rf sputtering were investigated from view points of their
internal stress. TiO₂ films were deposited on fused quartz glass or
100 µm thick micro-sheet glass substrates at room temperature, 200 or 400°C
under various total gas pressures (P_tot) of 0.3~5.0 Pa with
oxygen flow ratio [O₂/(O₂+Ar)] of 60% using a Ti metal
target. Photocatalytic activity was evaluated by photodecomposition of
acetaldehyde (CH₃CHO) under UV illumination (black light lamp, 0.4
mW/cm²). Compressive internal stress was estimated by cantilever
method using the micro-sheet glass, which clearly decreased from -2.1 to -0.1
GPa with the increase in the P_tot from 0.3 to 3.0 Pa. The
films with the compressive stress less than -0.5 GPa performed the
photocatalytic activity. Furthermore, compressive or tensile stress was applied
by external force on the TiO₂ films deposited on the curved micro
sheet glasses by flattening these substrates after the deposition. The
photodecomposition activity of the films with the slight compressive stress
improved clearly, whereas the one of the films with the tensile stress
degraded.

  If TiO₂ particles are used as components of paint
without any surface modification, binding resin of the paint will be easily
decomposed by the photocatalytic activity of the particles. In this work, plasma
polymerization of octamethylcyclotetrasiloxane as a siloxane monomer is tried to
form thin layer stable to the photocatalytic activity on the surface of
TiO₂ particles. The plasma-polymerized layer containing Si-O and Si-C
bonds is formed on the surface of the particles and shows stability to the
photocatalytic activity of TiO₂. The particles surface-modified with
the plasma polymerization exhibit visible-light activity. The visible-light
activity is originated from carbon doping which brings about in the particles
during the plasma polymerization, and is thermally stable to be maintained after
annealing at 673 K.

Titanium oxide nanotubes are prepared and treated with Au (Au/nanotube sample) and Pt (Pt/nanotube sample), and the photoactivity of these catalysts compared to a standard Degussa P25 photocatalyst is investigated. The samples were analyzed using X-ray diffraction, field emission gun scanning transmission electron microscopy (STEM). Both high-resolution TEM images and high-angle annular dark-field (HAAD) images were recorded for the specimens. Oxidation of acetaldehyde was used to test the efficiency of the catalysts. Nanotube samples showed better photoactivity than the standard P25, because the P25 titania deactivates quickly. Enhanced reactivity of the nanotube is related to surface charge polarity developed on outer and inner surfaces due to the difference in overlap of oxygen anions that resulted from curving of octahedral sheets. A tentative and qualitative surface polarity model is proposed for enhancing electron-hole pair separation. The inner surface benefits reduction; whereas, the outer surface benefits oxidation reactions. Both the metal identity and the size of the metal particles in the nanotubes affected the photocatalytic activity. Specifically, the addition of platinum increased the activity significantly, and increased the total yield. The addition of gold had lesser impact compared to the platinum. Formation of Pt large nanoparticles on the nanotube surfaces reduces the oxidation reactivity.

Abstract

Heterogeneous photocatalysis could be alternative remediation technology for water since it does not need the addition of any chemicals and it is suitable for treating low concentrations of pollutant. Although the TiO2 Degussa P 25 is most used photocatalyst its photonic efficiency still low and its recovery from water is considered as an awkward process. In this study the effect of zirconium addition to titanium was investigated. Ti/Zr-pillared montmorillonites have been prepared from natural bentonite and characterized by UV-Vis DRS and X-ray diffraction. The photocatalytic activities have been tested for the removal of 4-chlorophenol and dichloroacetic acid in water. The influence of preparation conditions and the calcination method, on these activities has been investigated. It was found that the photocatalytic activities increase by addition of zirconium in pillaring process and the calcination by Microwaves (MW) improves the photocatalytic activities.

Abstract