Abstract
In this work, we study the influence of the annealing treatment on the behaviour of titanium dioxide nanotube layers. The heat treatment protocol is actually the key parameter to induce stable oxide layers and needs to be better understood. Nanotube layers were prepared by electrochemical anodization of Ti foil in 0.4 wt% hydrofluoric acid solution during 20 minutes and then annealed in air atmosphere. In-situ X-ray diffraction analysis, coupled with thermogravimetry, gives us an inside on the oxidation behaviour of titanium dioxide nanotube layers compared to bulk reference samples. Structural studies were performed at 700°C for 12 h in order to follow the time consequences on the oxidation of the material, in sufficient stability conditions. In-situ XRD brought to light that the amorphous oxide layer induced by anodization is responsible for the simultaneous growths of anatase and rutile phase during the first 30 minutes of annealing while the bulk sample oxidation leads to the nucleation of a small amount of anatase TiO2. The initial amorphous oxide layer created by anodization is also responsible for the delay in crystallization compared to the bulk sample. Thermogravimetric analysis exhibits parabolic shape of the mass gain for both anodized and bulk sample; this kinetics is caused by the formation of a rutile external protective layer, as depicted by the associated in-situ XRD diffractograms. We recorded that titanium dioxide nanotube layers exhibit a lower mean mass gain than the bulk, because of the presence of an initial amorphous oxide layer on anodized samples. In-situ XRD results also provide accurate information concerning the sub-layers behavior during the annealing treatment for the bulk and nanostructured layer. Anatase crystallites are mainly localized at the interface oxide layer-metal and the rutile is at the external interface. Sample surface topography was characterized using scanning electron microscopy (SEM). As a probe of the photoactivity of the annealed TiO2 nanotube layers, degradation of an acid orange 7 (AO7) dye solution and 4-chlorophenol under UV irradiation (at 365 nm) were performed. Such titanium dioxide nanotube layers show an efficient photocatalytic activity and the analytical results confirm the degradation mechanism of the 4-chlorophenol reported elsewhere.
Highlights
Researches on the synthesis and characterization of nanomaterial are in booming development nowadays since the initial definition of Richard P
We investigated the photocatalytic activity of our TiO2 nanotube layers by monitoring the degradation of an acid orange 7 (AO7) dye solution and 4-chlorophenol under UV irradiation
Initial nucleation stage of rutile and anatase takes place, for the anodized samples, with their normal crystallographic growth orientations, contrary to the heating of the bulk reference sample where nucleation follows the preferential orientation of laminated titanium bulk (002)
Summary
Researches on the synthesis and characterization of nanomaterial are in booming development nowadays since the initial definition of Richard P. After electrochemical anodization of Ti foils, amorphous titanium dioxide nanotube layers are obtained [10] This amorphous structure is too disordered, induces a lack of electronic properties and is not convenient for the applications. In order to better understand the crystallization behaviour of titanium oxide nanotube arrays under annealing, in-situ X-ray diffraction is an efficient analytical tool [14]. We use in-situ X-ray diffraction analysis, coupled with thermogravimetry, to study the oxidation behaviours of titanium dioxide nanotube layers compared to bulk reference samples. These investigations are in extension of our already published work [15], concerning the evaluation of the photocatalytic activity versus the annealing temperature. Analytical monitoring of the by-products of 4-chlorophenol degradation confirms the degradation mechanism reported previously [17]
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