Photocatalytic activities of sulfur doped SrTiO 3 under simulated solar irradiation

S-doped SrTiO3 was synthesized by the solid state reaction method between S and SrTiO3 power under the N2 flow. The effect of temperature, calcination time and S-content on the formation and photocatalytic activity of SrTiO3 were investigated. The morphology and properties of obtained powders were characterized by XRD, UV-DRS, UV-VIS, SEM, BET. The photocatalytic activities of S-doped SrTiO3 was also investigated through the decomposition of methylene blue. As a result, the 10 %S-doped SrTiO3 contributed to the decrease of band gap energy to 2.73 eV and enhanced the photocatalytic activity for methylene blue degradation of 74.5 % after 180 min irradiation.


INTRODUCTION
Strontium titanate (SrTiO 3 ) is one of important materials which has applications in photocatalysis and electronics industry and has attracted much attention from both fundamental and practical viewpoints [1].As an efficient photocatalyst, strontium titanate (SrTiO 3 ) has been widely researched for the degradation of various organic contaminants, such as dyes and other organic compounds, contributing to solve the environmental problems or for water splitting to produce clean energy [2].However, there has been the drawback of pure SrTiO 3 which just could respond to UV light due to its large energy gap (3.0-3.2eV), and thus, more than 95% solar light would be wasted [3].
In recent years, some groups have carried out the studies on the doping sulfur into TiO 2 lattice to red shift the absorption edge.It was found that sulfur is more efficient for improving the photocatalytic activity under visible light region [4].For example, Zhou Zhiqiang et al [5] prepared S-doped nanosized TiO 2 .The asprepared S-doped TiO 2 nanosized possessed strong absorption for visible light of 400-650 nm, and showed high photocatalytic activity for decomposition of methylene blue under irradiation of visible light.Besides,Mohamad et al. [6] demonstrated that the visible light responsible sulfur-doped TiO 2 samples (STN) was successfully synthesized.The results indicated that the amount of sulfur doping could enhance the photocurrent.These STN samples are interesting candidates to drive photochemical reactions, such as water reduction (H 2 production) and oxidation of pollutants.Furthermore, Teruhisa Ohno et al. [7] succeeded in preparing S-doped TiO 2 photocatalysts which shows relatively high photocatalytic activity under visible light at wavelengths longer than 500 nm, may have a wide range of applications.
Regarding to efficiency of doping of sulfur, Teruhisa Ohno et al [8] modified SrTiO 3 by doping S and C that improved the photocatalytic activity of the doped SrTiO 3 for oxidation of 2propanol.Under a wide range of light irradiation (at wavelengths longer than 350 nm) the photocatalytic activity levels of C, S cationcodoped SrTiO 3 were about two times higher than those of pure SrTiO 3 .
In this research, the modified SrTiO 3 by sulfur photocatalytic material were synthesized and characterized using analytical techniques such as XRD, SEM, UV-vis (DRS).Finally, their photocatalytic activities were evaluated by studying the degradation of methylene blue under visible light irradiation.

Synthesis of SrTiO 3
The SrTiO 3 powder was prepared by solgel method.The process was as follows: 0.015 mol of Ti(OC 4 H 9 ) 4 was dissolved in 120 mL H 2 O 2 and 60 mL NH 3 , the mixture was stirred at room temperature until the solution became clear.Then, the solution was added into 100 mL citric acid solution, following by 0.015 mole Sr(NO 3 ) 2 (the mole ratio of Sr 2+ : CA is 1:3).
Next, 0.9311 g ethylene glycol was added into the solution for esterification.The resulting solution was heated at 80 -90 o C for 5-6 hours to form a gelation.The gel was dried at 150 o C in 2 hours and then ground, named as raw sample, calcined at different temperatures and duration.

Synthesis of S-doped SrTiO 3
The modification of SrTiO 3 by sulfur was carried out by griding mixture of 800 o C calcined STO and S power for 2h with variuos weight percentage of 5, 10, 20.The obtained mixture was calcined under the N 2 flow at 400-600 o C for 2h.This process synthesized the S doped STO at various S contents namely 5%, 10% and 20%.

The characterization of products
X-ray diffraction (XRD) patterns of Sdoped SrTiO 3 and undoped powders were using monochromatic high intensity CuK α radiations (λ = 0.15418 nm) at the scanning rate of 0.03 o /s and in the scanning range from 20 o to 75 o .Specific surface area using Brunauer-Emmett-Teller (BET) analysis was obtained by nitrogen adsorption-desorption isotherms at 77 o K after degassing the sample at 300 o C for 2 hours under nitrogen gas, using Quantachrome NOVA 1000e.The band gap energy of samples was determined by diffuse reflectance spectra (DRS) from 300 to 800 nm, scanning step was 2 nm, at 400 nm/min speed, using Solid UV-vis JASCO Corp equipment.A total organic carbon (TOC) was used for the determination of MB as TOC content.

RESULTS AND DISCUSSION
The XRD patterns of different calcined temperature STO are shown in Figure 1(a).As can be seen, at the 600 o C there is the formation of the dominant peaks at 2 = 32.5, 39.9, 46.6, 57.9, 77.08 o correlated to the indexed peaks in SrTiO 3 JCPDS card number of 35-0734.Moreover, the intensity of peaks increases with the increase of calcination temperature resulting in the rise of crystallinity.
The influence of calcination times (0.5, 1, 3, 6 h) on the purity phase and crystallinity of STO was also investigated through the X-Ray diffraction represented in Figure 1(b).It is clearly that the structure of STO was formed after 30 min of calcination due to the appearance of peaks at 2 = 32.5, 39.9, 46.6, 57.9, 77.Therefore, it can be concluded that the solid solution of S-doped SrTiO 3 was successfully prepared.
From the table 1, there is the presence of a small amount of S in all doped samples and this amount increased with the increase of initial ratio between S : SrTiO 3 .According to the calculation from the SEM-EDX resuls, the ratio of the total amount of (%O + %S) to the Sr is approximately equal 3:1 indicating that S was dispersed and just partially subtituted for oxygen in the STO structure.As the results, the formulars for the doped SrTiO 3 with 5%, 10% 20% of S is SrTiO 2.99 S 0.01 (A1), SrTiO 2.97 S 0.03 (A2), SrTiO 2.71 S 0.29 (A3), respectively  The morphologies of the S-doped SrTiO 3 powder are represented in Figure 3.In general, the particles sizes are in range of 80-150 nm and there is the agglomeration among particles.
Besides, the particle size of S-doped STO are smaller (around 80-100 nm) and less agglomeration than that of undoped sample.
Moreover, it seems that the degree of agglomeration increase with increasing the S content.Therefore, the specific surface area of samples was increases with the increasing of sulfur contents, as shown in Table 2. UV-vis diffuse reflectance spectra of Sdoped STO with the different sulfur contents is shown at Fig. 4 and the band gap energy is also calculated using the formular of E g = 1240/ λ (eV) [9].As can be seen, the photo-absorption of S-STO in the visible region increases with the increase of dopant content.Besides, the results from the Table .3indicated that the band gap energy decrease gradually from 3.2 eV (SrTiO 3 ) to 3.04 eV (A1) and 2.75 eV (A2).

CONCLUSION
This study prepared the S-doped SrTiO 3 applying for the decomposition of Methylence Blue.The XRD, BET, SEM, UV-Vis results indicated the obtained S-doped SrTiO 3 was single phase and had the spherical shape, the specific surface area also increased with the increase of S content.Regarding to the photocatalytic activity, The SrTiO 2.97 S 0.03 (A2) could get the highest rate of decomposition in the comparison with the remaining samples.In conclusion, the presence of small amount of S content could decrease the band gap energy and enhance the photocatalytic activity of SrTiO 3 through the decomposition of MB.
Photocatalytic activity of the material was evaluated by determining the decomposition efficiency of MB in isothermal condition room temperature.Each experiment consisted of 0.1 g catalyst, which was dispersed into 200 mL solution of 10 ppm methylene blue, the solution was kept in darkness for 1 hours in order to reach absorption/desorption equilibrium.Then, the mixture was lightened by 195W Compact light (simulated solar), the wavelength (λ) of which was from 390 to 750 nm.During the process, the solution was stirred constantly and cooled to room temperature by water-jacket system.Every period of time, approximately 3-5 mL solution was taken out and filtered using GC (PTFE 0.45 µm).The filtered solutions were then determined concentration of excess of methylene blue by Spectro 2000-RS, respectively, with maximum absorption wavelength of 664 nm.Then those excess solutions were brought back to reactor to maintain the volume.Blank sample, which had

Figure 1 .
Figure 1.The XRD patterns of SrTiO 3 at different temperatures (a) and times (b).

Figure 4 .
Figure 4. UV-vis diffuses reflectance spectra of (a) SrTiO 3 , (A1), (A2) (A3) samples.The photocatalytic activity of STO at various calcined temperatures is represented in Figure 5(a).The yield of decomposition of MB increase with the calcined temperature, from 25% (at 600 o C) to 45% (at 800 o C) after 180 min irradiation.However, the activity of 900 o C calcined samples is lower than that of 800 o C calcined STO.It can be explained that the

Figure 5 .Figure 6 .
Figure 5.The MB decomposition of STO with (a) various calcined temperature and (b) calcined duration

Table 1 .
The EDX results of S-doped SrTiO 3 with the various S contents.

Table 2 .
Specific surface area of S-doped