Photocatalytic Properties of Columnar Nanostructured TiO 2 Films Fabricated by Sputtering Ti and Subsequent Annealing

Columnar nanostructured TiO2 films were prepared by sputtering Ti target in pure argon with glancing angle deposition (GLAD) and subsequent annealing at 400◦C for different hours in air. Compared with sputtering TiO2 target directly, sputtering Ti target can be carried out under much lower base pressure, which contributes to obtaining discrete columnar nanostructures. In the present study, TiO2 films obtained by annealing Ti films for different hours all kept discrete columnar structures as the Ti films deposited in GLAD regime. The longer the annealing time was, the better the phase transition accomplished from Ti to TiO2 (a mixture of rutile and anatase), and the better it crystallized. In addition, those TiO2 films performed photocatalytic decolorization effectively and showed a law changing over annealing time under UV light irradiation towards methyl orange, which demonstrated the potential applications for treatment of effluent.


Introduction
With the development of agriculture and industry, the global environmental problems are becoming more and more serious and have drawn more and more attention [1,2].The increasing environmental problems create a great demand for stable and environmentally friendly materials, which can perform efficient photocatalytic decomposition of hazardous substances before their emission to the environment.Photocatalytic degradation technique plays an important role to solve the organic pollution problems because it combines with solar energy and in perfect agreement with the requirement of sustainable processes development [3][4][5].This technique can degrade organic pollutants into harmless inorganic substances such as CO 2 and H 2 O under moderate conditions.
As an efficient photocatalyst, extensive research has been performed on titanium dioxide (TiO 2 ) along with its photocatalytic applications for effluent [6,7].Titanium dioxide (TiO 2 ) is nontoxic, chemically stabile and possesses a unique combination of optical and photochemical properties [8][9][10].The mechanism of TiO 2 photocatalytic oxidation is to offer a highly reactive, nonspecific oxidant, namely hydroxyl radical ( • OH) which is capable of destroying, wide range of organic pollutants nonselectively and quickly in wastewater [11][12][13].
A great amount of literature published for TiO 2 photocatalytic oxidation indicates the use of TiO 2 powders [14], but the TiO 2 powders have some practical problems such as immobilization and recycling which requires costly separation procedures after used.The columnar nanostructured TiO 2 film by GLAD [15,16] in this paper has no problem with immobilization and recycling and has a relatively larger surface area compared with flat film which contributes to the photocatalytic efficiency.
This study investigated columnar nanostructured TiO 2 films annealed for different hours to get the law how the morphology, crystal structure, and photocatalytic properties change over annealing time at 400 • C in air.

Experimental
Ti columnar structure was obtained by GLAD using magnetron sputtering and subsequently annealed under appropriate conditions to achieve TiO 2 columnar structure.And  then some performance tests were carried out to get the morphology, crystal structure, and photocatalytic properties.
The main technology used in this paper to acquire columnar structure is glancing angle deposition (GLAD).The columnar microstructure exhibiting a high degree of porosity was obtained as a result of the shadow effect [17][18][19][20].The schematic diagram of GLAD in the magnetron sputtering system is showed in Figure 1.
The sputtering of Ti in pure Ar was performed at low pressure (about 0.11 Pa).The distance between the Ti target and the Si substrate centers was about 11 cm.The purity of the Ti target was 99.99% (diameter of 60 mm and thickness of 3 mm).The Si substrates were 3-inchs monocrystalline ([100]) wafers with low resistivity (0.02 Ωcm).The deposition angle between the substrate normal and the incident flux was fixed at 80 • (θ 1 = 25 • and θ 2 = 55 • ) for all depositions in the GLAD regime and was held constant in each experiment.Ar gas flow was kept 10 sccm during the deposition process which continued 90 minutes with a deposition power of 201.6 W.
Four groups of samples (columnar nanostructured Ti films) with size of 13 mm × 10 mm were annealed at 400 • C in air for 1, 2, 3, and 4 hours, respectively, in quartz tube furnace to be oxidized and crystallized.Then columnar nanostructured films of a mixture of rutile and anatase were obtained.
Characterization was conducted using XRD, SEM, and UV-vis spectrophotometer.
XRD measurements were performed for structural characterization.The parameters of a diffractometer (U = 45 kV, I = 200 mA) were the same for all samples.The surface morphologies of the nanostructured films were observed by SEM.The photocatalytic activities of TiO 2 nanostructures were characterized by photocatalytic decomposition of methyl orange under UV light irradiation.

Results and Discussion
As shown in Figure 2, the Ti film by GLAD in the experiment has oblique aligned columnar nanostructure with a high degree of porosity and large surface area.XRD spectra in Figure 3 demonstrate that there is nothing else but Ti [20].SEM images of the TiO 2 films annealed for 1, 2, 3, and 4 hours, respectively, are presented in Figure 4 (cross-sections and surface morphologies).There are almost no differences in discrete columnar morphologies between them [18,21].They all keep discrete columnar structures as the Ti films deposited by GLAD.
The diffraction patterns of the TiO 2 films show that peaks correspond to the known diffraction maxima of the anatase and rutile phase as marked in Figure 5.The columnar structure accomplished the phase transition from Ti to a mixture of rutile and anatase while keeping its discreetness [12].XRD shows that the longer the annealing time is, the better it crystallizes.The average crystallite size D of rutile was calculated by Scherrer's equation using the full width at half-maximum of the XRD peaks of R (110).The sizes of the rutile grains in the TiO 2 films annealed for different hours are all about 10 nm.
Each TiO 2 sample was placed in the center of a small beaker with 5 mL diluted methyl orange (about 10 μmol/L) in it, and one beaker without TiO 2 sample but methyl orange was prepared for comparison.The photocatalytic degradation was performed under 500 W UV lamp for two hours.The concentration change of aqueous methyl orange is obtained from transmittance spectrum measured by a UVvis spectrophotometer as shown in Figure 6.
Transmittances of methyl orange at 465 nm (a) with sample annealed for 4 hours after 2-hour UV radiation, (b) with sample annealed for 3 hours after 2-hour UV radiation, (c) with sample annealed for 2 hours after 2-hour UV radiation, (d) with sample annealed for 1 hour after 2-hour UV radiation, (e) without sample after 2-hour UV radiation, and (f) without sample and no UV radiation (origin methyl orange) are 71.0%,68.3%, 66.0%, 64.0%, 61.0%, and 55.5%, respectively.According to Beer-Lambert law, absorbance and concentration of an absorbing species have a linear relationship, and the relation between A (absorbance) and T (transmittance) is A = −log T; the degradation rates of methyl orange (a), (b), (c), (d), and (e) are 41.9%, 35.2%, 29.4%, 24.3%, and 16.0%, respectively [12].The degradation rate increases from 16.0% to 41.9% due to the photocatalytic activity of TiO 2 nanostructures [13].Furthermore, the degradation rate increases over annealing time as showed in Figure 7.

Conclusions
The columnar structure accomplished the phase transition from Ti to a mixture of rutile and anatase while keeping its discreteness after annealed at 400 • C in air.The longer the annealing time is, the better the phase transition accomplishes from Ti to TiO 2 (a mixture of rutile and anatase), and the better it crystallizes.Those TiO 2 films all perform photocatalytic decolorization effectively and reusably under UV light irradiation towards methyl orange.The degradation rate increases with increasing annealing time and increases from 16.0% to 41.9% due to the photocatalytic activity of the obtained TiO 2 nanostructures.

2 Advances 2 Figure 1 :
Figure 1: Schematic diagram of the GLAD technique in the magnetron sputtering system.

Figure 2 :Figure 3 :
Figure 2: Cross-section and surface morphology of columnar nanostructured Ti film by GLAD.