Improved Visible Light Photocatalytic Activity for TiO2 Nanomaterials by Codoping with Zinc and Sulfur

S/Zn codoped TiO 2 nanomaterials were synthesized by a sol-gel method. X-ray diffraction, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy, photoluminescence spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the morphology, structure, and optical properties of the prepared samples. The introduction of Zn and S resulted in significant red shift of absorption edge for TiO 2 -based nanomaterials. The photocatalytic activity was evaluated by degrading reactive brilliant red X-3B solution under simulated sunlight irradiation. The results showed S/Zn codoped TiO 2 exhibited higher photocatalytic activity than pure TiO 2 and commercial P25, due to the photosynergistic effect of obvious visible light absorption, efficient separation of photoinduced charge carriers, and large surface area. Moreover, the content of Zn and S in the composites played important roles in photocatalytic activity of TiO 2 -based nanomaterials.


Introduction
Photocatalysis, emerging as a potential technique owing to its valuable utilization of solar energy in environmental purification, has been paid increasing attention.In order to search effective photocatalysts, semiconductor investigation is progressing rapidly.A series of semiconductor materials, such as WO 3 [1], ZnO [2], TiO 2 [3], CdS [4], Bi 2 VO 4 [5], SnO 2 [6], ZrO 2 [7], and BiWO 6 [8], have been used in photocatalysis.Among these semiconductor photocatalysts, anatase TiO 2 is especially attractive because of its nontoxicity, low cost, and long-term stability.However, the efficiency of anatase TiO 2 is far from satisfactory thanks to the high recombination rate of photogenerated electron-hole pairs and wide bandgap of 3.2 eV, which limits the effective use of solar energy.
In recent years, intense works have been devoted to improve the optical response of TiO 2 under visible light irradiation, including doping TiO 2 with metal/nonmetal ions and decorating TiO 2 with metals, semiconductors, and polymers.For example, N-doped TiO 2 [9], In-doped TiO 2 [10], Ag@TiO 2 [11], SnS 2 /TiO 2 [12], and polymer/TiO 2 nanocomposites [13] have been demonstrated to exhibit photoactivity upon visible light irradiation.Among these methods, metal ion doping is especially attractive due to introducing additional electronic states within the bandgap of TiO 2 , thereby extending the range of light absorption to the visible region.However, the kind of doping metals has an influence on photocatalytic activity for TiO 2 -based materials.Di Paola et al. have reported a series of transition metal ions, such as Co, Cr, Cu, Fe, Mo, and V, can accelerate recombination of the photoinduced charge carrier, leading to poor photoactivity for TiO 2 -based materials [14].It is now well accepted that suitable metal ions for doping TiO 2 require not only improvement in light absorption but also high quantum efficiency of photoinduced charge carrier.Some previous reports have demonstrated TiO 2 doped with rare earth metal ions, such as La 3+ , Ce 3+ , Er 3+ , Pr 3+ , Gd 3+ , Nd 3+ , and Sm 3+ , exhibits high visible light photocatalytic activity [15,16].In addition, nonmetal elements, such as N and S, are also effective dopants for improving the photoactivity of TiO 2 .In these cases, they act as electron donors or acceptors in the forbidden band of TiO 2 to induce absorption in the visible region [17,18].Moreover, it is worth noting that codoped TiO 2 with metal and/or nonmetal ions, which could further enhance visible light activity of TiO 2 due to the synergistic effect, has been highlighted [19][20][21].Niu et al. has elucidated Fe/S codoped TiO 2 nanomaterials for the photocatalytic degradation of phenol under visible light irradiation [19].Hamadanian et al. reported In/S codoped TiO 2 and Cr/S codoped TiO 2 present highly improved visible light photocatalytic activity compared to pure TiO 2 [20,21].To the best of our knowledge, the utilization of S/Zn codoped TiO 2 as a visible light photocatalyst has not been adequately investigated.
Herein, we present the fabrication and characterization of a series of S/Zn codoped TiO 2 nanomaterials.Such a modification provides a broadening optical window for effective light harvesting and high quantum efficiency of photogenerated electron-hole pairs.As a result, the nanomaterials are anticipated to exhibit good photocatalytic activities under visible light irradiation.After the reaction mixture was further stirred for 2 h, the solution was aged for 48 h at room temperature and the precursor gel was obtained.Finally, the gel was dried at 100 ∘ C for 6 h and grinded and calcined at 500 ∘ C for 2 h in air at ramp rate of 5 ∘ C/min.For comparison, Zn doped TiO 2 and pure TiO 2 photocatalysts were also prepared under the same condition.

Characterization. X-ray powder diffraction (XRD) analysis was measured by a Shimadzu XRD-6100 diffractometer
with Cu K radiation, operating at 40 kV and 100 mA.The diffraction data was recorded for 2 values between 20 ∘ and 80 ∘ and the scanning rate was 8 ∘ /min.Ultravioletvisible (UV-vis) diffuse reflection spectroscopy (DRS) was recorded on a spectrophotometer (Lambda 35, Perkin-Elmer Corporation, USA) with an intergrating sphere and 2 nm spectral resolutions at room temperature in the range of 200-800 nm, and BaSO 4 was chosen as a reference.The morphology of the products was observed using transmission electron microscopy (TEM, JEM-2000EX JEOL, Japan), operating at 200 kV.X-ray photoelectron spectroscopy (XPS) measurements were taken with a VG ESCALAB 250 spectrometer equipped with a monochromatic Al K radiation source (1486.6 eV), and C 1s (284.6 eV) was used as a reference.The photoluminescence (PL) measurements were carried out using a Perkin-Elmer LS 55 fluorescence spectrophotometer.BET surface areas of the samples were measured using JW-BK222 specific surface area analyzer.Energy dispersive Xray spectroscopy (EDS, Oxford INCA) was also used for elemental analysis of the samples.The Fourier transformed infrared spectroscopy (FTIR) was performed on a Perkin-Elmer Spectrum 10 FTIR spectrophotometer.

Measurement of Photocatalytic Activity.
The photocatalytic activity of the as-prepared samples was evaluated by measuring the degradation rate of X-3B solution at room temperature and comparing the results with the activity of the commercial P25.The photocatalytic reaction was carried out using a magnetically stirred quartz reactor at 30 ∘ C. For a typical photocatalytic experiment, 200 mg of photocatalyst was added to 200 mL X-3B solution with an initial concentration of 20 and 40 mg/L in quartz reactor and stirred in the dark for 20 min to get an adsorption-desorption equilibrium before being placed under a 350 W xenon lamp and irradiated with continuous stirring.The suspensions were collected every 20 min and centrifuged immediately to separate the photocatalyst particles.The X-3B concentration was determined by monitoring the absorbance at  max = 537 nm by using a 721B spectrophotometer.

Results and Discussion
3.1.XRD Analysis.The phase structure and crystallinity of Zn doped TiO 2 and S/Zn codoped TiO 2 samples were investigated by XRD.As shown in Figure 1, the strong diffraction peaks correspond well to the peaks of anatase TiO 2 for all the samples.No peaks assigned to ZnO and ZnS were detected.It is probably due to the low Zn and S content in the samples.It should be noted that the full width at half maximum peak of doped TiO 2 is larger than that of undoped TiO 2 , suggesting that the particle size of doped TiO 2 is smaller than that of undoped TiO 2 .shows two peaks locating at 458.0 and 464.1 eV, which are attributed to the spin-orbit splitting of Ti 2p 3/2 and Ti 2p 1/2 , respectively, in good agreement with those of titanium (IV) dioxide [22].As shown in Figure 3(c), the binding energy values of Zn 2p 3/2 and Zn 2p 1/2 are observed at 1021.5 and 1044.7 eV, indicating that the Zn element could be in +II state bonding with oxygen [23].From Figure 3(d), it could be observed that the S 2p peak is located at about 169 eV.This peak can be deconvoluted into two overlapping peaks at 168.3 and 169.4 eV, which is attributed to the S (+VI).The S (+VI) may be adsorbed on the surface of TiO 2 in the form of SO 4 2− ions [17].This can be demonstrated by IR characterization.As shown in Figure 4, the peaks at 1142 and 1055 cm −1 , ascribed to SO 4 2− coordinating to Ti 4+ and forming Ti-O-S bond [20], are observed on the spectrum for S/Zn codoped TiO 2 .Based on the above observation, Zn and S elements should be doped into TiO 2 by the sol-gel method.

DRS Analysis.
The photoabsorption abilities of the samples were probed with UV-visible (UV-vis) diffuse reflectance spectroscopy (DRS).As shown in Figure 6, because of the interband electronic transitions, the onset of the absorption spectrum of pure TiO 2 represents a strong and broad absorption feature in the UV region.Furthermore, in comparison   Since the surface areas of photocatalysts play important roles in the photocatalytic activity, N 2 adsorption/desorption analysis for pure TiO 2 , ZT-4, and SZT-3 has also been performed (Table 1).This observation indicates that both ZT-4 and SZT-3 have large surface area, which is beneficial to dye adsorption during photocatalytic process.On the basis of DRS, PL, and N 2 adsorption/desorption studies, the improved photocatalytic activity for the modified TiO 2 can be attributed to the photosynergistic effect of obvious visible light absorption, efficient separation of photoinduced charge carriers, and large surface area.In addition, Zn doping can also significantly decrease the amount of trap states, facilitating electron transport and charge separation [23].However, it should be noted that Zn doping can also act as the recombination centers for the photogenerated electrons and holes, inhibiting the photocatalytic activity.Thus, the optimal dopant amount of Zn is 4%.Further introducing S to the Zn doped TiO 2 , the recombination of the photogenerated charge carriers can be restrained by S element, which can act as a hole trap [20].Therefore, the charge carriers with a long lifetime make the sample exhibit higher photocatalytic activity.In addition, doping with both Zn and S increases the light absorption owing to extending the absorption to the visible range.In combination with the above results, it could be concluded that the photocatalytic activities of modified TiO 2 catalysts under simulated sunlight were greatly enhanced compared with those of pure TiO 2 , which could be ascribed to the photosynergistic effect of obvious visible light absorption, efficient separation of photoinduced charge carriers, and larger specific surface area.
Figure 9 shows the UV-vis absorption spectra of X-3B solution before and after simulated sunlight irradiation for different exposure periods with SZT-3.The characteristic absorption band of X-3B at 537 nm diminished quickly and became quite weak after 180 min under simulated sunlight irradiation.

Conclusions
In summary, S/Zn codoped TiO 2 nanomaterials were prepared by a sol-gel method.This method allows for the successful doping of Zn and S into anatase TiO 2 .The absorption spectrum of S/Zn codoped TiO 2 nanomaterials displays a red-shift to visible light region.The results show that S/Zn codoped TiO 2 possesses better photocatalytic activity than Zn doped TiO 2 and P25, due to enhanced separation efficiency of photoinduced electron-hole pairs and light absorption.Considering their high photocatalytic activity, the S/Zn codoped TiO 2 nanomaterials are promising photocatalysts for application in degradation of dye wastewater.
EDS analysis was performed to better understand chemical composition of the obtained materials.EDS spectra of ZT-4 and SZT-3 are shown in Figure2.It can be clearly seen that the signals for Zn are present in Zn doped TiO 2 nanomaterials; Zn and S signals also appear in the EDS spectrum of S/Zn codoped TiO 2 , illustrating the existence of Zn and S. For further confirming the existence of Zn and S, XPS has also been performed.

Figure 3 (
a) displays the XPS survey spectrum of the same sample.The main elements are Ti, Zn, O, and S with sharp

3. 3 .
TEM Analysis.The textures of undoped TiO 2 , Zn doped TiO 2 , and S/Zn codoped TiO 2 were characterized by TEM.As shown in Figure 5(a), undoped TiO 2 are composed of small nanoparticles with the diameter in the range of 15-30 nm.However, both Zn doped TiO 2 and S/Zn codoped TiO 2 consist of small nanoparticles, which are uniform with the main diameter of approximately 14 and 11 nm, respectively.It is attributed to the presence of Zn and S in the composites, which can inhibit the growth of TiO 2 nanoparticles [20, 23].The d-spacing of the lattice fringe (Figure 5(d)) is measured to be 0.35 nm, corresponding to the (101) plane of anatase TiO 2 , indicating well crystalline TiO 2 nanoparticles are formed in this case.

Table 1 :
[25]ific surface area of the samples determined by BET equation.Zn doped TiO 2 presents much lower PL intensity than pure TiO 2 , implying that Zn doping suppresses the recombination of electrons and holes.This result is in accordance with Xie's report, in which Zn doped TiO 2 not only promotes the charge separation but also delays the charge recombination[23].Furthermore, the S/Zn codoped TiO 2 has a greater decline in PL intensity, indicating that S/Zn codoped TiO 2 nanomaterials have higher separation efficiency of the photoinduced electrons and holes compared to Zn doped TiO 2 .It is ascribed to the fact that oxygen vacancies and/or defects, generated by S introduction, can capture the photoinduced electrons[25].
2 , P25, and SZT-3, respectively.It is obvious that SZT-3 shows the highest photocatalytic activity, while pure TiO 2 presents the lowest photocatalytic activity.In this case, the degradation rate of X-3B solution with SZT-3 could reach 74.6%, whereas those of ZT-4 and pure TiO 2 are 61.2% and 43.6%, respectively.