Facile Postsynthesis of N-Doped TiO 2-SBA-15 and Its Photocatalytic Activity

N-doped TiO 2 -SBA-15 (denoted as N-TiO 2 -SBA-15) material has been successfully synthesized by a two-step procedure. Firstly, TiO 2 -SBA-15 was prepared by impregnating tetraisopropyl orthotitanate on SBA-15 and followed by calcination at 550C. In the second step, TiO 2 -SBA-15 was modified by doping nitrogen with the assistance of urea.The resulting material, N-TiO 2 -SBA-15, was characterized by XRD, TEM, SEM,N 2 adsorption/desorption at 77K, DRUV-Vis, and XPS.The results showed that N-TiO 2 -SBA-15 material maintains its ordered hexagonal mesostructure and exhibits the absorption of visible region. The photocatalytic activity of N-TiO 2 -SBA-15 sample was evaluated by the photodegradation of methylene blue under visible light.


Introduction
Heterogeneous photodegradation of organic pollutants with semiconductors has attracted much of the attention of researchers because of its efficiency and promises of economy [1,2].Among the semiconductor materials, TiO 2 has been investigated more widely due to its high photocatalytic efficiency, commercial availability, chemical stability, and environmental friendliness [3,4].However, its technological application seems to be limited by several factors such as relatively low surface area and the requirement of UV light.In order to overcome these limitations, a number of modifications have been tried.
Many efforts have been made on supporting TiO 2 on porous materials for increasing surface area of TiO 2 [5][6][7].Among the various porous materials, SBA-15 silica has attracted great attention as an ideal catalytic support due to its large surface area, adjustable pore size ranging from 5 to 30 nm, ordered frameworks with the wall being in the thickness range of 3.1-6.4nm, and transparency to UV radiation [8].
Beside NH 3 as a significant nitrogen source for the doping, urea has attracted considered attention as an alternative nitrogen source because of its useful characteristics such as environment-friendly and easy operation in doping [43][44][45].The aim of our research study is to use urea as a nitrogen source to synthesize N-doped TiO 2 supported on SBA-15.
A combination of modified TiO 2 and SBA-15 can take advantage of both SBA-15 and TiO 2 .In this paper, we report a facile preparation of N-doped TiO 2 -SBA-15 using urea as a nitrogen source.The as-synthesized material showed visiblelight response and potential application in photodegradation of methylene blue under visible light.

Synthesis.
The mesoporous silica SBA-15 was prepared according to the literature [46].In a typical synthesis, 2 g of P123 was added to 62.5 g of 1.9 M HCl with stirring.3.84 g of TEOS was added after the mixture was heated to 40 ∘ C. The reaction mixture was stirred for 20 h at 40 ∘ C and then aged in an autoclave at 100 ∘ C for 24 h.The solid products were separated by filtration and washed with distilled water for several times.The structure directing agent (P123) was removed by heating the samples at 550 ∘ C for 6 h in the air.The TiO 2 -SBA-15 sample was prepared according to the following procedure.0.5 g of SBA-15 was added to the solution of 0.75 g of Ti(C 3 H 7 O) 4 in 20 mmL of ethanol.After evaporation of the solvent with stirring at 40 ∘ C, the resulting solid was dried at 100 ∘ C for 12 h and then heated at 550 ∘ C for 5 h.The obtained solid was denoted as TiO 2 -SBA-15.For the doping of TiO 2 -SBA-15 with nitrogen, a mixture of TiO 2 -SBA-15 and urea solution with the urea/TiO 2 -SBA-15 ratio of 1 : 3 in weight was prepared.After evaporation of the solvent with stirring at 40 ∘ C, the resulting solid was dried at 100 ∘ C overnight and then calcined at 500 ∘ C for 1 h in the air.The resulting material was denoted as N-TiO 2 -SBA-15.

Characterization. X-ray diffraction (XRD) patterns for
the samples were measured on a Bruker D8 Advance diffractometer using CuK radiation ( = 1.5406Å).Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were recorded on JEOL JEM-2100F and JEOL 5410, respectively.N 2 adsorption-desorption isotherms were obtained on ASAP 2010 at 77 K. Before the measurement, the samples were degassed at 100 ∘ C for 6 h.Diffuse-reflectance UV-Vis spectra were investigated on a Sinco S-4100 spectrometer.X-ray photoelectron spectroscopy (XPS) spectra were recorded on a VG Microtech Multilab ESCA 3000 spectrometer.
For the photocatalytic test, 100 mg of the catalyst was suspended in 50 mL of 100 mg/L methylene blue (MB) aqueous solution, and then the mixture was stirred for 2 h in dark to obtain the equilibrium adsorption.The reaction mixture was exposed to visible light from a 300 w lamp with a UV cutoff filter (>420 nm).After a given irradiation time, about 4 mL of the mixture was withdrawn and the catalysts were separated by centrifugation.The selected solutions were measured using a UV-Vis spectrophotometer (Jenway 6800).However, the peak intensity, especially for (110) and ( 200) reflections, significantly decreases with the graft of TiO 2 and the doping of nitrogen.This is probably due to decreasing scatter contrast between the silica framework and TiO 2 inside the pores [47,48].From the 2 values of the (100) reflections, the hexagonal lattice parameters are calculated and reported in Table 1.

Results and Discussion
The large-angle X-ray diffraction for N-TiO 2 -SBA-15 was also studied to characterize the structure of TiO 2 grafted on SBA-15 (Figure 1(b)).Also shown are the patterns of SBA-15 and TiO 2 -SBA-15.All patterns are similar in shape and show a broad peak centered at about 25 ∘ that may be due to amorphous SiO 2 of SBA-15.TiO 2 cannot be detected by The periodicity in the mesostructure of the three materials is confirmed directly by transmission electron microscopy (Figure 2).The TEM micrographs display that both the perpendicular and parallel channels relative to the longitudinal axis are observed, which confirms that the modified SBA-15 materials possess ordered, one-dimensional pore structure similar to that of the pure SBA-15.However, a gradual decrease in structural ordering from SBA-15 to TiO 2 -SBA-15 to N-TiO 2 -SBA-15 can be seen from the images of channels in Figure 2. A raw estimation from the TEM images presents that the pore-pore distances match the lattice parameters calculated from XRD data.The morphology of rope-like domains with a uniform size was obtained for the materials SBA-15 and TiO 2 -SBA-15 (Figures 3(a images of TiO 2 -SBA-15 that TiO 2 nanoparticles disperse homogeneously on SBA-15 silica.This is further supported by the large-angle XRD patterns of TiO 2 -SBA-15 without any peaks as mentioned above.
The N 2 adsorption/desorption isotherms at 77 K of the calcined SBA-15 and the modified SBA-15 samples are shown in Figure 4. Type IV IUPAC isotherms, characteristic of  capillary condensation in mesopores, are found for all samples.However, their shapes are different with a capillary condensation of N 2 occurring over a slightly lower P/P 0 range when going from SBA-15 to TiO 2 -SBA-15 to N-TiO 2 -SBA-15.The pore-size distribution of the materials is shown in the inset of Figure 4.It can be seen that pore diameters of about 7.5, 6.6, and 6.0 nm for SBA-15, TiO 2 -SBA-15, and N-TiO 2 -SBA-15, respectively, were obtained.A decrease in specific surface area and pore size was observed in order of the following materials: SBA-15, TiO 2 -SBA-15, and N-TiO 2 -SBA-15.However, the wall thickness increased in the same order of the materials.These results can be attributed to the fact that TiO 2 particles are attached to the wall of SBA-15 in TiO 2 -SBA-15 and some TiO 2 particles are exfoliated from the wall to block the pores in N-TiO 2 -SBA-15 when TiO 2 -SBA-15 reacted with urea.The summary data on structural, textural properties of the three materials obtained from the N 2 adsorption/desorption isotherms are shown in Table 1.
The doping of TiO 2 on SBA-15 with nitrogen was further supported by the diffuse reflectance UV-Vis spectroscopy.Figure 5 shows that TiO 2 -SBA-15 exhibits a strong absorption in UV region corresponding to the band to band transition.Compared with TiO 2 -SBA-15, the sample N-TiO 2 -SBA-15 possesses a tailing absorption extending out to approximately 600 nm, which is a typical absorption feature of N-doped TiO 2 .The absorption in visible light for the sample N-TiO 2 -SBA-15 may come from the narrowing band.The band gap value was calculated from the diffuse reflectance measurements to be 2.89 eV.
Chemical states of the doped nitrogen in N-TiO 2 -SBA-15 were investigated by XPS.It can be seen from Figure 6(a) that a shape peak around 397.6 eV and two shoulders around 395.5 and 399.2 eV, respectively, for N 1s were found.After fitting the peaks, three peaks at 395.5, 397.6, and 398.1 eV were observed.The binding energy at 395.5 eV can come from replacing the oxygen in the crystal lattice of TiO 2 by nitrogen.The peak at 397.6 eV is attributed to the nitrogen anion incorporated in the TiO 2 in N-Ti-O bond.And the strong peak at 398.1 eV can be assigned to the nitrogen in the Ti-O-N site of the N-TiO 2 matrix.These results are consistent with the previous reports [49][50][51] and will be supported further by the XPS data of Ti 2p and O 1s below.The N content of 3.37% in mol for N-TiO 2 -SBA-15 was obtained from the XPS data.Compared to Ti-N, a relatively higher binding energy at 397.6 eV for N-Ti-O may be explained by the high electronegativity of oxygen leading to the reduction of electron density on the nitrogen [52].This is further supported by the result of XPS spectrum for Ti 2p (Figure 6(b)).Two XPS peaks at 456.6 eV and 462.5 eV corresponding to Ti 2p 3/2 and Ti 2p 1/2 , respectively, were obtained.The Ti 2p 2/3 peak for pure TiO 2 should be at 458.8 eV [53].A decrease in the binding energy of Ti 2p 3/2 after nitrogen doping may come from the electronic interaction of Ti with anions in N-TiO 2 , which is different from that of TiO 2 .The lower electronegativity of nitrogen compared to oxygen may lead to a transfer of partial electron from the N to the Ti.Therefore, the electron density around the anion decreases, resulting in the increase in electron density around the cation [54].This indicates the substitution of oxygen atoms in TiO 2 by nitrogen atoms.The incorporation of nitrogen in TiO 2 can be further confirmed by the binding energy of O 1s.As shown in Figure 6(c), the peak at 530.5 eV may be attributed to the presence of Ti-O-N bonds [52].

Photocatalytic Test.
The photocatalytic activity of the samples was determined by the degradation of methylene blue in water under visible light.Figure 7 shows the variation of methylene blue concentration (C/C 0 ) with irradiation time over the two catalysts.For the N-TiO 2 -SBA-15 catalyst, it can be seen that the concentration of methylene blue decreases rapidly in the initial three hours and slowly in the later period.Methylene blue can be effectively degraded in the presence of N-TiO 2 -SBA-15, and the degradation efficiency reaches about 90% in 6 h irradiation.For comparison, the photodegradation of methylene blue for the sample TiO 2 -SBA-15 under the same conditions is also shown in Figure 7.An insignificant catalytic activity of about 3-5% is obtained for the sample without nitrogen doping.

Conclusion
The visible-light-sensitive nitrogen-doped TiO 2 -SBA-15 material was synthesized by a two-step process.TiO 2 nanoparticles with homogeneous dispersion on the SBA-15 silica were treated with urea to form nitrogen-doped TiO 2 -SBA-15 material.The absorption of visible light for the material comes from the partial substitution of the oxygen in TiO 2 by nitrogen atoms.The nitrogen-doped TiO 2 -SBA-15 material exhibits a good photocatalytic activity for the degradation of methylene blue in the aqueous solution under visible light.