Enhanced photocatalytic activity of titania nanotubes by ligand-free Au nanoparticle loading

In the last years titania nanotubes arrays (TNTAs) have attracted increasing attention in photocatalysis due to their unique properties, not least the possibility of a direct production of supported TNTAs by anodic oxidation of metallic titanium foils. Besides, loading of TiO2-based photocatalysts with Au nanoparticles (NPs) has been extensively investigated so as to improve the photocatalytic efficiency. In order to combine the advantages of TNTAs with Au NPs decoration and to overcome the problems derived from current deposition methods this work proposes a simple and scalable synthetic protocol able to load size-controlled (<10 nm) and ligand-free Au NPs onto vertically aligned anatase TNTAs. Metal-supported TNTAs were produced by titanium foils anodization in a solution of 0.5 wt.% NaF/1M Na2SO4. The samples were then thermally annealed in air at 400°C for 2 h in order to achieve the crystallization of the obtained amorphous TiO2. Ligand-free Au NPs were produced by the Metal Vapour Synthesis technique. Au vapours generated at 10-4 mBar by resistive heating of an alumina crucible filled with gold pellets were co-condensed at liquid nitrogen temperature (-196°C) with acetone. The Au NPs were loaded onto the TNTAs surface by simply dipping the samples into the resulting acetone NPs suspension. The photocatalytic activity was assessed by measuring the toluene degradation in air using a continuous-flow stirred photoreactor operated at constant toluene concentration. The obtained Au loadings were strictly related to the sample dipping time into the ligand-free Au NPs suspension. The morphological features of Au/TNTAs composites revealed a highly homogeneous Au NPs dispersion without the particle coalescence often reported using colloidal deposition methodologies. The photocatalytic activity of the pristine batch of bare TNTAs samples was quite homogeneous and a marked increase of activity after Au NPs deposition was observed for the 3.3 and 7.4 µg cm-2 Au loading samples. At lower Au loading (<2 µg cm-2) the reaction rate was comparable to the untreated sample whereas at higher loading (>10 µg cm-2) a pronounced deactivation was observed. Using the described method the Au loading was finely tuned ranging from 1.5 to 11.6 ?g cm-2 by controlling the support/Au dipping time without significant change of Au NPs mean size. No further thermal or chemical post-treatments were necessary. The Au loadings strongly influenced the photocatalytic activities and the best performance results in a three times enhancement of the bare TNTAs activity obtained with a sample containing 3.3 ?g cm-2 of Au. The proposed protocol demonstrated a remarkable enhancement of the photocatalytic activity of metal-supported TNTAs at room temperature without any post-deposition process.