Synthesis of Nanostructured TiO 2 Photocatalyst with Ultrasonication at Low Temperature

A thin film TiO2 in hierarchical nano-structure with high photocatalytic activities was synthesized in simple steps with ultrasonication. The crystal structure and morphology of the photocatalyst were investigated by X-ray diffraction (XRD) and high-resolution field emission scanning electron microscope (FE-SEM). In the present work, nanostructured TiO2 was directly formed onto a Ti substrate via a solution approach. This nanostructured TiO2 photocatalyst can be reused and will not generate secondary contamination to treated water. The photocatalytic activity of the synthesized TiO2 photocatalyst was evaluated by the degradation of phenol under UVC irradiation in water and was compared with the general sol-gel derived TiO2 films as well as a commercial DP-25 TiO2 coating. It was found that the synthesized nanostructured TiO2has significantly high and stable photocatalytic activity.


Introduction
Titanium dioxide (TiO 2 ) is one of the mostly studied semiconductors for various applications such as photocatalysts [1] [2], photovoltaic cells [3] [4], gas sensors [5], photonic crystals [6] and biomaterials [7].Recently, semiconductors in nanostructure have drawn great interest [8] [9].Some studies have appeared on preparation of TiO 2 nanotubes, nanowires, and especially, nanorods [10]- [12].For example, Sugimoto et al. prepared TiO 2 (anatase) nanoparticles with shapes varying from round-cornered cubes to sharp-edged cubes by introducing amines as shape controllers in their so-called gel-sol method [10].Yang et al. precipitated rod-like rutile from a TiCl 4 /HCl solution exposed to a strong ultrasonic irradiation [11].Pradhan et al. grew anatasenanorods on a WC-Co substrate at 873 K by metalorganic chemical vapor deposition (MOCVD) [12].Nevertherless, these methods, together with the widely used approaches to obtain crystalline TiO 2 [13] [14], have particular drawbacks.TiO 2 prepared through hydrolysis and condensation of titanium alkoxides requires subsequent hightemperature thermal treatment (calcination e.g.) to induce crystallization and to remove the residual alkoxide groups [13] [14].The high-temperature thermal process results in particle agglomeration, grain growth and therefore the reduced specific surface area.Although hydrothermal process, instead of the calcination, has been successfully used to induce crystallization, it requires high pressure and high temperature (usually > 413 K) conditions [10].Well-crystallized TiO 2 can be obtained under mild conditions between ambient temperature to 373 K through the hydrolysis of the titanium (IV) inorganic salts of TiCl 4 [12] [15] [16], TiF 4 [17] and TiOSO 4 [18] [19] in aqueous phase; however, the counter anions of the starting titanium salts would remain in the products and deteriorate the purity of TiO 2 [17] [19].In addition, TiCl 4 is so active that it should be used in a protective atmosphere at low temperature; whileTiF 4 is very expensive.Although MOCVD is an advanced technique, it requires stringent conditions to obtain crystalline TiO 2 nanorods [12].
Hydrogen peroxide (H 2 O 2 ) is an oxidant bringing no secondary contaminations to the reaction system.In an attempt to verify the biocompatibility of titanium, Tengvall et al. studied the titanium peroxide gels derived through interactions between metallic Ti and H 2 O 2 [20].Ayers and Hunt prepared TiO 2 aerogels through the reaction of H 2 O 2 with excess Ti followed by the subsequent solvent exchanges with ethanol and carbon dioxide, and supercritical drying [13].Thermal treatment under argon at 473 K caused rapid decomposition of the aerogel, forming a blue-gray powder consisting of a mixture of rutile and anatase [13].Wang et al. obtained anatase layer on Ti surface by heating (between 573 and 873 K) the amorphous TiO 2 gel formed through oxidizing Ti with 30 wt% H 2 O 2 solution containing 3 mM TaCl 5 at 353 K for 1 h [21].The present authors found that simply ageing this amorphous TiO 2 gel in water at 353 K can also induce crystallization which is in line with Wu et al. study [22].This paper reported the preparation of well-crystallized nano TiO 2 with satisfying homogeneity through direct oxidation of metallic Ti with H 2 O 2 at a low temperature of 353 K. To the authors' knowledge, this is the lowest temperature reported to prepare crystalline TiO 2 nanoflakes.

Sample Preparation and Characterization
Ti plates (99.9% in purity, Titan Engineering, Singapore) with sizes of 12.5 × 15 × 1 cm 3 were pickled in the mixture of HF, HNO 3 and H 2 O by volume ration of 1:3:6 at ambient temperature.After ultrasonically cleaning in deionized water, each piece of the Ti plate was soaked in a 50 ml H 2 O 2 solution with different concentrations.The reactants were kept at 353 K with ultrasonication for a certain time in water bath.Then, the Ti plate was taken out and gently rinsed with deionized water and dried in oven.Ultrasonication and drying time, as well as concentration of H 2 O 2 were tested systematically to study their influence on the photoctalysts activity.The morphology of as-prepared photocatalyst was observed via field emission scanning electron microscopy (FE-SEM, JEOL 6340F, Japan).Samples were characterized on X-ray diffraction (XRD, Bruker, Germany) with Cu Κα1 radiation (λ = 1.5406Å).Surface elemental analysis was conducted on energy-dispersive X-ray detector equipped on the FESEM.

Evaluation of Photocatalytic Activity
The photocatalytic activity of the prepared Ti plates was conducted in a glass reactor in a blackbox at room temperature.The light source was a 9 W UVC spot light source.A piece of prepared TiO 2 plate (4.5 × 5 cm 2 ) was immersed into 100 mL of phenol solution (100 mg/L) for certain time.The reaction solution was magnetically stirred continuously during the photocatalytic reaction.Temperature variation throughout the reaction is less than 3 K.Phenol concentration change was analyzed by a high performance liquid phase chromatography (HPLC, LC-20AD, Shimadzu, Japan) with UV detector at a wavelength of 272 nm, using a 250 mm C15 RP column with particle size at 5 µm.The mobile phase employed was methanol/water (60/40) with flow rate at1 mL/min.Parallel experiments were carried out to ensure the reproducibility.The adsorption of phenol on the TiO 2 plate was assessed by the dark condition reaction.The photocatalytic activity of the Ti plate was defined as follows: where C 0 is the phenol initial concentration and C t is the concentration at reaction time (t).

Morphology and Surface Structure
The morphology of the aged Ti plate is shown in The porosity and surface-to-volume ratios of the sample were greatly enhanced by these primary nanostructures, which could induce efficient diffusion and increased mass transfer of organic pollutant molecules within the microfibers architecture.The EDX spectra shows that the TiO 2 nanoflakes were elementally composed of only pure Ti and O with no extraneous elements from intermediates observed in the final anatase product.
Figure 2 shows the FESEM images of the samples prepared with and without ultrasonication.The nanostructure and density of the thin film prepared in water bath with ultrasonication is obviously finer than that obtained without ultrasonication.It could be attributed to the higher heating uniformity, lower temperature variation and higher heat transfer speed introduced by ultrasonication.
The as-prepared Ti plates were either dried in air or aged in oven at 80˚C for 8 h, respectively.The XRD patterns of the completed products in Figure 3 shows that substantial crystallization of TiO 2 in an atasephase took place in the aging process, indicating that aging is not only helpful for surface stabilization, but also for the atomic diffusion and crystallization.215) are all corresponding to the lattice plane of pureanatase phase.The absence of peaks at 2θ = 27.5˚(110) and 30.8˚ (121) corresponding to rutile and brookite phase confirms the synthesized TiO 2 was in pure anatasephase [23].The aged TiO 2 shows a wide (101) peak with more background disturbance indicating that the amorphous nature of the synthesized nanomaterials.However, no obvious peak was obtained for TiO 2 prepared without ultrosonication.The crystallite sizes are calculated from the broadening of the anatase (101) peaks using Scherer's formula.D = Kλ/(βcosθ) where D is the crystallite size (nm), K is the shape constant (0.9), k is the wave length of Cu Κα1 radiation (1.5406 Å), θ is the diffraction angle (˚) and β is the full width at half maximum peak height.The crystallite size of the air dried and aged TiO 2 is calculated as 14.8 and 4.65 nm, respectively.The result confirms that aging process helps crystallization of synthesized TiO 2 nanoflakes.From the above investigation, it is clear that low-temperature aging of samples promoted the crystallinity of anatase formed and enhanced polymorph stability.Based on these results, the effect of H 2 O 2 concentration was further studied.The oxidization reactions were conducted at 80˚C using four different concentrations (5%, 10%, 20% and 35%) of H 2 O 2 .Figure 4 illustrates the SEM images of the surface morphology of the four samples.The uniformity and density of the grown nanoflakes are increased with increasing H 2 O 2 concentration.

Photocatalytic Degradation of Phenol
In order to evaluate the photocatalytic activity of the prepared photocatalyst, photocatalytic oxidation of phenol was carried out using catalysts prepared with different conditions as shown in Figure 5. Dark condition experiment with photocatalyst was carried out to examine the adsorption of phenol molecules on the catalyst surface.The control experiment was carried out with nophotocatalyst but with light only to evaluate the photolysis of phenol.Three replicates of all experiments were performed at identical conditions to perform error analysis.
Figure 5 shows that only a modest removal of phenol occurred in the control experiment (<20%) due to direct photolysis.While under dark condition with the synthesized photocatalyst, it is clear that limited amount of phenol was removed by adsorption.Comparing between samples prepared with/without ultrasonication, after 6 hours reaction, 94% and 72% of phenol was removed, respectively, which is in line with the surface morphology analysis.
Figure 6 shows the result of photocatalytic degradation of phenol with the air-dried and aged photocatalysts.The aged photocatalyst shows much better photocatalytic activity than that of the air-dried photocatalyst.The result can be attributed to the difference in UV light utilization by these two synthesized photocatalysts.Similar  results have been reported by other researchers on TiO 2 photocatalysts [24].In general, photocatalytic activity depends on the efficiency of utilization of the incident radiation absorbed by the catalyst [25].Due to amorphous nature of the air-dried TiO 2 photocatalyst, the scattering of incident UV light energy was more than the absorbed energy (results in decreased formation of electron and hole pairs) as compared to that of the aged catalyst, which may be resulted in decreased photocatalytic activity.

Conclusion
In summary, a one-step method involving an in situ oxidation process to synthesize highly crystallized, anatase TiO 2 photocatalysts with excellent photocatalytic activity has been developed.From practical point of view, the simple-to-operate strategy is favored by large scale production.The prepared nanostructured TiO 2 photocatalyst can be easily reused and will not generate secondary contamination to treated water.Fabrication conditions such as ultrosonication and post-synthesis aging can effectively improve the surface catalyst crystallization as well as the nanostructure formation, so that improving photocatalytic activity.Based on the catalytic performance of the synthesized nano-TiO 2 , it is feasible to be commercialized as a fixed-bed reactor for industrial application.

Figure 1 .
Uniform photocatalyst thin layers were generated via H 2 O 2 oxidization on both sides of the Ti plates (Figure 1(a)).Figure 1(b) shows densely packed microfibers with uniformly distributed nanoflakes which look like sea corals.High magnification FESEM images in Figure 1(c) and Figure 1(d) show the surface layer consisted of well-aligned nanoflakes with tens of nm in width and hundreds of nm in length.The XRD pattern in Figure 3 shows that this surface layer is composed of well-crystallized anatase phase TiO 2 .As shown in Figure 1(d) of the cross-sectional FESEM image, the grown thin film has a depth of ca.0.2 -0.3 µm.

Figure 3 .
Figure 3. XRD patterns of the obtained TiO 2 nanoflakes.(a) Dried in air; (b) Aged at 80˚C for 8 h.

Figure 4 (
d) demonstrates the largest density and contents of TiO 2 was obtained by 35 wt% H 2 O 2 .

Figure 6 .
Figure 6.Effect from aging process on the photocatalytic activity (PA) of the synthesized catalyst.