Abstract
Fe-N-S-tri-doped photocatalysts were synthesized by one step in the presence of ammonium ferrous sulfate. The resulting materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible diffuse reflection spectrum (UV-Vis DRS). XPS analysis indicated that Fe (III) and S6+ were incorporated into the lattice of through substituting titanium atoms, and N might coexist in the forms of substitutional N (O-Ti-N) and interstitial N (Ti-O-N) in tridoped . XRD results showed that tri-doping with Fe, N, and S elements could effectively retard the phase transformation of from anatase to rutile and growth of crystallite size. DRS results revealed that the light absorbance edge of in visible region was greatly improved by tri-doping with Fe, N, and S elements. Further, the photocatalytic activity of the as-synthesized samples was evaluated by the degradation of phenol under visible light irradiation. It was found that Fe-N-S-tri-doped catalyst exhibited higher visible light photocatalytic activity than that of pure and P25 , which was mainly attributed to the small crystallite size, intense light absorbance in visible region, and narrow bandgap energy.
1. Introduction
Since the compounds in wastewater were treated by photocatalytic oxidation in 1976 by Carey et al. [1], TiO2 nanomaterial has been considered as a promising photocatalyst in the degradation of organic or inorganic pollutants due to its inexpensiveness, nontoxicity, photostability and strong oxidation ability. However, TiO2 can only be excited by UV light, which only occupies a small part of the solar spectrum [2]. In order to improve the utilization of solar energy, a great deal of efforts has been made, which include dye sensitization, coupling of TiO2 with a narrow band gap semiconductor, noble metal deposition, and doping of TiO2 with foreign ions [3–6]. Among which, doping of TiO2 with foreign ions has been considered as an effective and feasible approach to enhance the photoresponse and photocatalytic activity. Very recently, it was reported that the doping of TiO2 with two or three elements could further improve the light absorbance in visible region and photocatalytic activity [7–12]. From then on, many attempts have been carried out.
In this study, Fe-N-S-tridoped TiO2 photocatalysts were synthesized by one step in the presence of ammonium ferrous sulfate. Fe (III) and S6+ were incorporated into the lattice of TiO2 through substituting titanium atoms, and N might coexist in the forms of substitutional N (O-Ti-N) and interstitial N (Ti-O-N) in tridoped TiO2. Fe-N-S-tridoped TiO2 catalyst exhibited a higher visible light photocatalytic activity for the degradation of RhB than that of pure TiO2 and P25 TiO2.
2. Experimental
2.1. Synthesis of Materials
The Fe-N-S-tridoped TiO2 photocatalysts were synthesized through sol-gel method in the presence of ammonium ferrous sulfate. Firstly, 10 mL of tetrabutyl titanate was mixed with 40 mL of absolute ethanol. Then, the titanate-ethanol solution was added dropwise into another solution, which consisted of 10 mL of absolute ethanol, 12 mL of dilute nitric acid (1 : 5, volume ratio between concentration nitric acid and deionized water) and the desired amount of ammonium ferrous sulfate under vigorously stirring to carry out hydrolysis. Subsequently, the mixed solution was continuously stirred for 2 h at room temperature. After the resulting sol was aged for 6 h and dried for 36 h at 80°C, the TiO2 precursor was obtained. Finally, Fe-N-S-tridoped TiO2 catalysts were successfully obtained by calcining the TiO2 precursor at 350°C for 4 h in an oven with a heating rate of 3°C·min−1. For comparison, pure TiO2 was synthesized under otherwise the identical conditions in the absence of ammonium ferrous sulfate.
2.2. Characterization of Materials
X-ray powder diffraction (XRD) patterns were performed on a Bruker D8 advance powder X-ray diffractometer with Cu Kα radiation ( nm). X-ray photoelectron spectroscopy (XPS) conducted using a PHI-5700 ESCA system was employed to characterize the chemical states of tridoped ferrum, nitrogen, and sulfur atoms in the as-synthesized samples. All the binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV). The UV-visible diffuse reflectance spectra (DRS) of the samples were recorded on a UV-2550 UV-visible spectrophotometer with an integrating sphere attachment. The analyzed wavelength range was 300~700 nm, and BaSO4 was used as the reflectance standard.
2.3. Evaluation of Photocatalytic Activity
The photodegradation experiments were performed in a self-made photoreactor containing 20 mL of 50 mg·L−1 phenol and 20 mg of catalysts. A 350 W xenon arc lamp equipped with an UV cutoff filter ( nm) was used as the visible light source. Prior to irradiation, the suspension was stirred in the dark for 60 min to establish the equilibrium of adsorption-desorption. At given time intervals, the phenol concentration was analyzed using the colorimetric method of 4-aminoantipyrineby with an UV-visible spectrophotometer (UV-2550) at the wavelength of 510 nm after centrifugation and filtration.
3. Results and Discussion
3.1. XRD Analysis
Figure 1 showed the XRD patterns of the as-synthesized TiO2 samples. It can be seen that pure TiO2 contained anatase (JCPDS, no. 21–1272), rutile (JCPDS, no. 21–1276) and brookite (JCPDS, no. 29–1360) with anatase phase in the majority according to their peak intensities. However, Fe-N-S-tridoped-TiO2 consisted of anatase as the unique phase (JCPDS, no. 21–1272). Generally, brookite is a transitional phase from anatase to rutile during the calcinating process. Thus, it can be induced that tri-doping with Fe, N, and S elements could effectively retard the phase transformation of TiO2 from anatase to rutile. In addition, no XRD peaks related to the dopants were detected. One reason was that the concentration of the dopants was so low that it cannot be detected by XRD. The other was that the dopants were incorporated into the lattice of TiO2 by substituting oxygen and titanium atoms or located in the interstitial sites. Further, the average crystallite sizes of the as-synthesized TiO2 samples can be calculated by applying the Debye-Scherrer formula [13] on the anatase (101) diffraction peaks. After calculating, the crystallite sizes were found to be 10 and 5 nm for pure and Fe-N-S-tridoped TiO2, respectively. Thus, we can conclude that tri-doping with Fe, N, and S elements could effectively retard the phase transformation of TiO2 from anatase to rutile and growth of crystallite sizes.
3.2. XPS Analysis
In order to investigate the chemical states of dopants in Fe-N-S-tridoped TiO2 photocatalyst, high-resolution XPS of Fe2p, N1s, and S2p were measured and shown in Figure 2. As shown in Figure 2(a), two peaks at 710.9 and 723.9 eV seen from the Fe2p core-level XPS spectrum were assigned to Fe2p3/2 and Fe2p1/2 photoelectrons [14, 15], respectively, demonstrating that Fe was incorporated into the lattice of TiO2 through substituting the lattice titanium atoms as the form of Fe (III). As seen from Figure 2(b), two peaks at binding energies of 399.6 and 401.6 eV were observed. The first major peak was attributed to the substitutional N in the O-Ti-N structure [10, 16], indicating that some lattice O atoms were substituted by N atoms. The latter peak was attributed to the presence of interstitial N state as the characteristic of Ti-O-N in the doped TiO2 sample [17, 18]. As seen from Figure 2(c), a single S2p peak located at 168.8 eV was observed, which was attributed to the presence of S6+, suggesting that S was incorporated into the lattice of TiO2 through substituting titanium atoms [19, 20].
(a)
(b)
(c)
3.3. DRS Analysis
The optical properties of as-synthesized TiO2 samples were investigated by UV-visible diffuse reflectance spectra and shown in Figure 3. As seen from Figure 3, in both pure and Fe-N-S-tridoped TiO2 samples, the onset of absorption edge was observed at 388 nm, corresponding to the band gap energy of 3.20 eV from anatase TiO2. However, the absorbance edge of Fe-N-S-tridoped TiO2 was greatly red-shifted to visible light region. As shown in [21], the typical absorbance edge of N-doped TiO2 was ranged from 380 to 500 nm due to the formation of localized N2p level above the top of valence band edge. Thus, in this study, the enlarged light absorbance in the range of 380~500 nm was attributed to the N doping, while the enhanced absorbance in the range of 500~700 nm was possibly due to the synergistic contribution from the tridoped species. Thus, it is reasonable that the Fe, N, and S elements were indeed incorporated into the lattice of TiO2, leading to the difference in the crystal and electronic properties of the tridoped TiO2.
3.4. Photocatalytic Activity
The degradation of phenol was used to evaluate the photocatalytic activities of the as-synthesized TiO2 samples. The photocatalytic activity of P25 TiO2 was conducted as a reference. The experimental results were shown in Figure 4. Obviously, as seen from Figure 4, Fe-N-S-tridoped TiO2 exhibited higher visible light photocatalytic activity than that of pure TiO2 and P25 TiO2, for which 85.9% of phenol can be degraded after 2 h of visible light irradiation. The enhanced photocatalytic activity was mainly attributed to small crystallite size, intense light absorbance in visible region and narrow band gap energy.
4. Conclusions
In summary, Fe-N-S-tridoped TiO2 photocatalyst was successfully synthesized by one step in the presence of ammonium ferrous sulfate. Fe (III) and S6+ were simultaneously incorporated into the lattice of TiO2 through substituting titanium atoms, and N might coexist in the forms of substitutional N (N-O-Ti) and interstitial N (O-Ti-N) in tridoped TiO2. In addition, tri-doping with Fe, N, and S elements could effectively retard the phase transformation of TiO2 from anatase to rutile and growth of crystallite size. The light absorbance edge of TiO2 was greatly improved by tri-codoping with Fe, N, and S elements. The as-synthesized Fe-N-S-tridoped TiO2 presented higher visible light photocatalytic activity for the degradation of phenol than that of pure TiO2 and P25 TiO2. The enhanced photocatalytic activity was mainly attributed to the small crystallite size, intense light absorbance in visible region, and narrow band gap energy.
Acknowledgments
The authors wish to gratefully acknowledge the financial support by National Natural Science Foundation of China for Youth (21106035) and Youth Scholar Backbone Supporting Plan Project for general colleges and universities of Heilongjiang province (1151G034).