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 TiO 2 . 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 TiO 2 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.
Researches on the synthesis and characterization of nanomaterial are in booming development nowadays since the initial definition of Richard P. Feynman in 1959 [
4-Chlorophenol (4-CP) was purchased from Sigma. Acid Orange 7 (AO7) was purchased from Acros Organic. Benzoquinone and hydroquinone were purchased from Fluka. They were all used without further purifications. Methanol (HPLC grade), formic acid (≥95%), acetone and trichloroethylene were purchased from Sigma Aldrich. Stock solutions containing the desired concentrations of 4-chlorophenol (4-CP), hydroquinone (HQ) and benzoquinone (BQ) were prepared in Milli-Q water.
To fabricate anodic TiO2 nanotube layers, we used Ti foil (Goodfellow, 99.6% purity) with a thickness of 25 µm. The Ti foil was degreased by successive sonification in trichloroethylene, acetone, and methanol, followed by rinsing with de-ionized water and blown dry with nitrogen. Anodization was carried out at room temperature (20˚C) in 0.4 wt% HF aqueous solution with the anodizing voltage maintained at 20 V [
The surface topography characterization of the anodized Ti foil was performed using a Zeiss Supra 55 VP scanning electron microscope (SEM) with secondary emission and in lens detectors. The accelerating voltage and the working distance were 3 kV and 5 mm, respectively.
High temperature studies were performed for 12 h at 700˚C in air using a Setaram TGDTA 92-1600 micro thermobalance for mass gain and a high temperature Anton PAAR HTK 1200 chamber with integrated sample spinner in a Philips X’pert MPD diffractometer for X-ray diffraction studies. The annealing condition (12 h at 700˚C) was chosen to promote the complete formation of rutile crystalline phase which is considered to be the most thermodynamically stable bulk phase in comparison with other possible TiO2 crystalline phases which are respectively anatase and brookite (obtained at lower annealing temperatures) as underlined by several authors [
For the photocatalysis studies, the irradiations were carried out in monochromatic parallel beam in 1 cm (path length) quartz cell. The light source was a mercury lamp (200 W) equipped with an Oriel monochromator. The monochromatic irradiation was set at wavelength 365 nm (
The photo-catalytic decomposition of 4-CP solution was monitored by the decrease of the solution’s absorbance at 280 nm (maximum absorption band of the 4-CP solution), using a Waters HPLC system. The HPLC system was equipped with a diode array (type 996) UV-Vis detector, an automatic injector (type 717), two pumps (type 600). To investigate the degradation of 4-CP under UV irradiation (365 nm), experiments were performed using a reverse phase Agilent column (Eclipse XDB C8, 250 mm × 4.6 mm, 5 µm). For analyses using HPLC, the elution was accomplished by water with formic acid (0.3%) and methanol (65/25, v/v) with flow rate of 1.0 mL/min and the injection volume was 30 µL.
Photodegradation of AO7 was also used as a probe to assess the photo-activity of the TiO2 layers. AO7 con-
taining azo bond is a model molecule commonly used to perform photocatalytic tests to simulate azo dyes wastewater pollutants coming from industries. Photo-catalytic experiments were conducted in aqueous solution of AO7 (from Acros Organics, also called Orange II) with a concentration of 5.0 × 10−5 mol・L−1, placed in a cylindrical glass reactor equipped with a magnetic stirrer. The glass reactor was irradiated with polychromatic fluorescent UV lamps (Philips TDL 8 W) (total optical power, 1.3 W), in a configuration providing about 0.35 mW/cm2 at the sample surface. The photodegradation kinetics was recorded by assaying the AO7 solution submitted to different UV irradiation time using a Perkin Elmer lambda 35 UV spectrophotometer. Quartz glass cells with an optical pathway of 1 cm were used. De-ionized water was taken as reference. The photodegradation of the dye was followed by monitoring the decrease of the solution’s absorbance at 483 nm (strong absorption band of the Acid Orange 7).
Three anodized and three bulk samples (total surface areas 3 cm2: and thicknesses: 25 µm) were tested by thermogravimetry to clearly observe the anodization effect on oxidation behaviour at 700˚C for 12 h.
This figure shows that both anodized and bulk samples exhibit parabolic oxidation rates due to the formation of a protective oxide layer near the surface (theses oxidation curves are characteristic of a diffusion of species limited by the growing oxide layer).
different mass gain curve evolutions obtained by thermogravimetry in the case of anodized and bulk references samples.
In-situ high temperature X-ray diffraction analyses (T = 700˚C in air) were performed, using the CuKα1 = 1.5406 Å radiation, every 30 minutes for 12 hours on both anodized and bulk titanium samples to observe the initial nucleation stage of new compounds induced by the heat treatment.
In-situ high temperature X-ray diffraction analyses performed between 2 h and 5 h (
X-ray diffraction analyses performed between 4 h 30 and 7 h 30 (
In-situ high temperature X-ray diffraction analyses performed on titanium anodized sample during the first 2 h 30 of annealing are given in
X-ray diffraction analyses obtained before heat treatment and thermogravimetric results suggest that anodization process promotes in fact the formation of an amorphous oxide layer on nanotube surface as underlined by several works on anodized titanium [
It is important to note that the high intensity of the rutile X-ray diffraction peak is due to the crystallisation of the amorphous layer all along nanotube surfaces (anodization promoting a highest surface area compared to bulk sample) rather than the growth of a thicker oxide layer than those of bulk sample. After one hour of annealing, rutile diffraction peak becomes the highest peak observed on X-ray diffractogram whereas no intensity evolution is observed for the main anatase diffraction peak. These results also suggest that anatase crystallites are mainly localized at the oxide layer-metal interface under the rutile external layer. In-situ high temperature X-ray diffraction analyses show the continuous increase of the main rutile characteristic peak for the first 5 h of annealing (
all along the heat treatment process.
The photo-degradation experiments of AO7 in the presence of TiO2 nanotubes under different conditions are summarized in
The
Upon irradiation of 4-CP with nanotube layer of TiO2, the organic compound disappearance was observed together with the formation of 2 by-products: P1 and P2. The two of them have been formally identified by injecting commercial compounds. The first of them (P1) was identified as hydroxyquinone, and the second (P2) as benzoquinone. The
benzoquinone (BQ) versus irradiation time. The both photoproduct are apparently formed with a period of induction (after 2.5 h of irradiation) and accumulated with irradiation time up to a maximum of 4.5 µM for BQ and 3.5 µM for HQ after 35 h. Its concentrations account for over 34% of 4-chlorophenol degradation.
At the same time, another byproduct is also detected but no possible to quantify. This product is expected to be the hydroxybenzoquinone (HQB) as described in literature [
Taking into account the identification of the byproducts the envisaged pathway of photocatalytic decomposition of 4-chlorophenol is illustrated in
This study allows us to follow successfully the oxidation behaviour of titanium dioxide nanotube layers under the annealing treatment. In-situ high temperature X-ray diffraction first reveals, on anodized samples, the building of a substantial amorphous oxide layer, and then the simultaneous growths of anatase and rutile phases occur. Ultimately, 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). The initial amorphous oxide layer is responsible for the lower mass gain recorded on the anodized samples compared to bulk material. The external rutile layer, detected after a longer annealing time, induces the parabolic shapes of the mass gain curves.
The photodegradation study of acid orange 7 using annealed nanotubes layer attests the photocatalytic activity of the annealed samples. A complete disappearance of the organic dye after 25 h of irradiation is recorded. Concerning the photodegradation of 4-chlorophenol, two by-products hydroxyquinone (HQ) and benzoquinone (BQ) are identified by HPLC analysis after a 2.5 h period of induction. Taking into account the chemical structures of these compounds, this analytical result seems to confirm the reaction pathway often found without experimental evidence in the literature. These results are a contribution to a better understanding of the different crystallization steps of titanium dioxide nanotube layers submitted to the annealing treatment. This work opens the way to the optimization of annealing parameters in order to obtain stable nanostructured layers required to counter corrosion in the field of titanium nanostructured prostheses.
Marie Siampiringue,Christophe Massard,Eric Caudron,Yves Sibaud,Mohammed Sarakha,1 1,Komla Oscar Awitor, (2016) Impact of Annealing Treatment on the Behaviour of Titanium Dioxide Nanotube Layers. Journal of Biomaterials and Nanobiotechnology,07,142-153. doi: 10.4236/jbnb.2016.73015