Evaluating the Efficiency of a Trace Amount of Zr Dopant on Photocatalytic Activity of TiO 2 in Decolorization of Azo Dye

Zr-doped TiO2 nano photocatalyst obtained with a various trace amount of Zr, by using the sol-gel method. Several spectroscopic techniques applied to characterize the structures and properties of nanophotocatalyst. It founded that the effect of doping Zr ions to the TiO2 lattice led it to improve the surface properties and decreasing the particle size of nano-photocatalyst. Therefore, it causes increasing specific surface area and photocatalytic activity as compared with pure TiO2. The results revealed that there was an inverse relationship between the growth of the crystallite size of the nano photocatalyst and the decline of the molar ratios of Zr. The photocatalytic activity of the nano photocatalyst employed for the decolorization of Congo red as an Azo dye in the aqueous solution under UV irradiation in a batch reactor. The results exhibited that the TiO2/Zr0.0004 brings the best photocatalytic activity.


1-Introduction
One of the most important sources of environmental contamination is dye pollution.Heterogeneous photocatalysis is an interesting method employed for the thorough removal of pollution under solar or artificial light irradiation [1][2].TiO2 based photocatalysis enjoys such advantages as low cost, non-toxicity and chemical stability, making it as a good choice for the efficient oxidization and degradation of dye pollutants [3].It has been shown that photon absorption of anatase TiO2 is only limited to the UV range due to its large band gap (3.2ev) [2].A good way to improve photocatalytic activity is through the incorporation of dopant into TiO2 lattices, introducing additional energy levels and defects into the microstructure of the TiO2 [4].There are different modification techniques to reduce band gap; these include doping with noble metals like Au, Ag and Pt [5][6][7][8][9], non-precious metals such as Fe, Cu, Cr, Mn, W, Ru, Ni, Sb [10][11][12][13][14], oxides such as ZnO,ZrO2,SiO2 [15][16] and non-metallic elements like N, C, S, F and P [17][18][19].Zirconium ions, which act as a kind of dopant in TiO2, have been found to modify the photocatalytic activity and stabilization of anatase phase at high temperatures [20][21][22].Many different methods have been proposed for the preparation of Zr-doped TiO2 nanocomposites; these are such as mechanochemical doping, co-precipitation, chemical vapor deposition and sol-gel methods.However, it must be noted that most of the above-mentioned techniques are time consuming and require expensive equipment.The sol-gel method has the unique advantage that it can synthesize at low temperature, under high purity and easily controlled reaction condition [23][24], its simplicity and possibility of the synthesis of nano photocatalyst at low temperature with high purity [25].Recently, according to properties of Zr, TiO2 was used with specific molar ratios of Zr for different applications [26].
In this research, instead of employing the specific molar ratio of zirconium, investigated the effect of trace amounts of zirconium as a dopant in TiO2 to find the better photocatalytic activity and compare it with pure Titanium dioxide.TiO2/Zr nanocomposites were prepared using different trace molar ratios of Zr.The nanocomposites were prepared using the sol-gel method and the TiO2/Zr nano photocatalyst is characterized by several spectroscopic techniques.Such as Fourier Transform infra-red spectroscopy (FT-IR), X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM), Brunauer-Emmet-Teller (BET) analysis and High Resolution Transmission Electron Microscope (HR-TEM).Then the photocatalytic activity of the synthesized nano photocatalyst was investigated for the decolorization of Congo red in the aqueous solution and under UV irradiation in a batch reactor.

2-2-Preparation of Nanocomposites
TiO2 sample was synthesized by employing the Sol-Gel method.The TiO2 sol was obtained at room temperature and TTIP was utilized to serve as a precursor in some stages.Initially, TTIP was dissolved in absolute ethanol, with molar ratio of TTIP to ethanol being (1:75) and then stirred for 15 min in order to get a precursor solution; following that, 0.1 g HPC was added as a stabilizer, and the mixture was continuously stirred for 15 min to obtain a yellow transparent Sol (Sol 1).To get To Zr doped-TiO2, Zr sol was developed in the following way: First, zirconium tetrachloride was allowed to be dissolved in absolute ethanol (alcoholic solution of ZrCl4), with the molar ratios of Zr to ethanol being (0.0002:1.52, and 0.0004:1.52).It stirred for 5 min and respectively added to the mixture of absolute ethanol, Nitric acid and 0.1 g CTAB (Sol 2).They were added slowly in a dropwise manner during 30 min into the precursor (Sol 1) and then stirred continuously for 15 min to get a transparent Sol.This Sol was aged at room temperature for 48 h in order to develop a Gel.Then, the developed Sol was allowed to be dried in the air and heat-treated for 10 min at 100 •C to remove the water and organic solvents; after that, it was calcinated in an electric furnace at the temperature of 525•C for 4 h.Following the heat treatment method, the samples were naturally cooled.Synthesized samples were coded; the codes specified have been brought in the following Table 1.

2-3-Characterization of Nanocomposites
FT-IR spectra were got as KBr pellets, with the wave number range of 400-4000 cm -1 (Thermo Nicolet Nexus 870FT-IR spectroscopy).Phase identification of the nanocomposites was carried out using XRD from SCIFERT-3003 PTS with CuKa radiation, in the range of 0 to 100 (2θ), at room temperature.SEM (HITACHI S-4160) investigated the morphology and microanalysis of the nanocomposites.Varian UV-Vis spectrophotometer (Cary UV-Vis 100) was employed for the determination of the degradation concentration.The BET measurement was got by utilizing (Quantachrome Nova 2200).The nanostructure of the samples was observed by drawing on TEM (Philips EM 208 electron microscopy).

2-4-Photocatalytic Activity Measurement
Photocatalytic degradation experiments were conducted in a cubic wooden reactor.Six (4 W) UVC Lamp (Osram) was used to serve as a light source; they were installed inside the reactor (Figure 1).At first, 0.01 g of the catalyst was added to a 25ml aqueous solution of Congo red, which had an initial concentration of 5 ppm.

3-1-FT-IR Spectroscopy
FT-IR spectra of the TiO2 and TiO2/Zr nanophotocatalyst with various molar ratios of Zr were obtaines and the results are shown in (Figure 2) in the wave number range from 4000 to 400 cm -1 .

3-2-SEM
Surface morphology of the synthesized nano photocatalyst was characterized; their SEM were carried out and the results were shown in seen in (Figure 3).SEM pictures show the effect of metal doping on particle size and morphology.The average particle size of each samples in the X-axis [(a) {35.16 nm}(b){27.34}(c){ 23.44 nm}] was reported.SEM revealed that the particle morphology of TZ4 was the best.
In this equation, ßis the full-width at half maximum (FWHM) of the peak, k is 0.89, λ is 0.154060 nm for CuKα, and d is the average particle size.

3-4-Photocatalytic Activity
The effect of different amounts of dopant on decolorization of Congo red (5 ppm) was observed in the aqueous solution under UV irradiation, results shown in (Fig. 5).As can be seen, with the photocatalytic activity in samples 1-3, the absorbance of Congo red would be reached to 0 after different times, with (T{145 min}, TZ2{90 min}, and TZ4 {75 min} of irradiation.TZ4 could be an effective photocatalyst.

3-5-BET-Surface Area Analysis
The data related to the BET-surface of all samples can be seen in table 3. The data revealed that by enhancing the molar ratio of Zr, the surface area was increased.

3-6-TEM
The nanostructure of the samples was observed by TEM (Philips EM 208 electron microscopy); the results are shown in (Figure 6).As can be seen, when the molar ratio of Zr was increased the structures were not aggregated.

4-Conclusion
The Zr-doped TiO2 nano photocatalyst was organized with a trace amount of Zr applying the Sol-Gel process.The calcination temperature at 525 ± 5 C was used.According to results, the TZ4 catalyst was obtained the best results in photodegradation and 0.004 mol ratio of Zr is the best value of dopant.The SEM images showed that the sample TZ4 was well-ordered, with the good size distribution of particles; therefore, it could be applied for photocatalytic dye decolorization.Furthermore, it was observed that this sample had no aggregation, as compared with other samples and with pure TiO2.It should be noted that for photocatalytic activity, the particle size of the photocatalyst should be homogeneous and important.The results of XRD revealed that in all Zr-Doped samples, the particle size was decreased.The photocatalytic activity of the synthesized nanocomposite was studied for the decolorization of Congo red as an Azo dye, thereby confirming that photocatalytic activity of TZ4 nanocatalyst was the best.The data obtained by BET showed that by increasing the molar ratio of Zr, the surface area was increased.Results of TEM revealed that the TZ4 had good particle size and dispersion without any aggregation.Therefore, a sample of TZ4 could be regarded to have the best photocatalytic activity in comparison to other samples; also, the absorbance of Congo red solution reached to 0 after 75 min of irradiation.