Synthesis and Photocatalytic Activity of Mo-Doped TiO2 Nanoparticles

The undoped and Mo-doped TiO 2 nanoparticles were synthesized by sol-gel method. The as-prepared samples were characterized by X-ray diffraction (XRD), diffuse reflectance UV-visible absorption spectra (UV-vis DRS), X-ray photoelectron spectra (XPS), and transmission electronmicroscopy (TEM).Thephotocatalytic activitywas evaluated by photocatalytic degradation ofmethylene blue under irradiation of a 500W xenon lamp and natural solar light outdoor. Effects of calcination temperatures and Mo doping amounts on crystal phase, crystallite size, lattice distortion, and optical properties were investigated.The results showed thatmost of Mo took the place of Ti in the crystal lattice of TiO 2 , which inhibited the growth of crystallite size, suppressed the transformation from anatase to rutile, and led to lattice distortion of TiO 2 . Mo doping narrowed the band gap (from 3.05 eV of TiO 2 to 2.73 eV of TiMo 0.02 O) and efficiently increased the optical absorption in visible region. Mo doping was shown to be an efficient method for degradation of methylene blue under visible light, especially under solar light. When the calcination temperature was 550C and the Mo doping amount was 2.0%, the Mo-doped TiO 2 sample exhibited the highest photocatalytic activity.


Introduction
Titanium dioxide (TiO 2 ) has been considered as one of good photocatalysts due to its excellent properties such as low cost, nontoxicity, chemical stability, and high photocatalytic activity [1][2][3][4][5][6].However, TiO 2 only becomes active under irradiation with ultraviolet (UV) light (3-4% of the solar energy) because of its wide band gap (3.2 eV), and visible portion (approximately 45%) cannot get used effectively [7].Therefore, it is a critical issue to reduce the band gap of TiO 2 for making it photosensitive to visible light.The enhancement of optical absorption in visible region will improve the photocatalytic efficiency of TiO 2 , which may promote the utilization of the solar light.In the last decade, great efforts have been made to modify the band gap of TiO 2 .These results show that metal ion doping is one of the effective ways.At present, the investigations about doping elements mostly focus on transition metal ions doping [8][9][10][11].Transition metal ions modify microstructures and electronic structures of TiO 2 and increase its photocatalytic efficiency.Molybdenum (Mo) is a transition metal and its doping into TiO 2 can shift the absorption edge towards visible region, increase the absorption under both UV and visible light, and enhance the photocatalytic activity of TiO 2 [12][13][14][15].However, due to the large number of possible variations, it is not simple to find out the optimum doping amount and calcination temperature at the same time.Besides, obtaining optimum photocatalysts to function under solar light is very meaningful.
In this work, we successfully prepared Mo-doped TiO 2 nanoparticles by sol-gel method.The effects of doping amount and calcination temperature on the photocatalytic activity of photocatalysts were studied.Structure characteristics characterization and analysis of as-prepared samples were studied by XRD, UV-vis DRS, XPS, and TEM.By degradation of methylene blue, we investigated the optimal doping amount and calcination temperature of Mo-doped

Preparation of Photocatalysts.
Mo-doped TiO 2 nanoparticles were synthesized by sol-gel method.Under continuous stirring, different amounts of ammonium molybdate were previously dissolved in a mixture solution consisting of 48.2 mL ethanol, 6 mL deionized water, and 0.6 mL nitric acid to form the mixture solution A. Then solution A was added dropwise into the mixture solution B containing 21.3 mL tetrabutyl titanate and 48.2 mL ethanol.The obtained homogeneous solution was magnetically stirred continuously for 1 h to form a gel and subsequently aged at room temperature for 24 h.The gel was then dried in an oven at 60 ∘ C until a dry gel was obtained.The dry gel was calcined in a muffle furnace at 300, 450, 550, and 650 ∘ C, respectively, to obtain Mo-doped TiO 2 nanoparticles.For comparison, the samples of pure TiO 2 and Mo-doped TiO 2 were prepared by similar procedures.Atomic ratios of Mo in the samples were 0.5%, 1.0%, 2.0%, and 3.5%, respectively.The as-prepared samples were denoted as MT( ), where hereafter  represented the atomic ratio of Mo/Ti (%) and  represented calcination temperature ( ∘ C).The undoped TiO 2 was denoted as T() and used as a reference.

Photocatalytic Degradation Experiment.
The photocatalytic activity experiment of prepared nanoparticles was conducted in a quartz glass (Φ70 * 80 mm).In each experiment, 0.2 g photocatalyst was added to 400 mL of 20 mg⋅L −1 methylene blue solution.Two kinds of light source were used: a 500 W xenon lamp and natural solar light outdoor (July 24, 2014; N43.88 ∘ , E125.32 ∘ ).Every 30 min, 5 mL suspension was sampled, centrifuged, and tested by UNICO 2100 visible spectrophotometer at 664 nm.

Results and Discussion
3.1.XRD Analysis.Figure 1(a) shows XRD patterns of samples calcined at 550 ∘ C with different Mo doping amounts.
The XRD peaks at 2 = 25.6 ∘ (101) and 2 = 27.7 ∘ (110) were often taken as the characteristic peaks of anatase and rutile crystal phase, respectively [16].The intensities of anatase peaks increased and the width of peaks became broader with Mo doping amount increasing.
The phase contents of the samples were calculated by where  A is the fraction of anatase phase and  A and  R are the intensities of the anatase (101) and rutile (110) diffraction peaks, respectively [17].The crystallite sizes were calculated with Scherrer formula  = / cos  where  is the average crystallite size in angstroms,  is a dimensionless constant (0.89 here),  is the wavelength of the X-ray radiation (Cu K = 0.15406 nm),  is the full width at half maximum (FWHM).The lattice distortions were attained from  = /4 in which  is lattice distortion and  is the diffraction angle [18].Results were listed in Table 1.
It could be seen that crystallite size decreased and anatase TiO 2 increased with Mo doping amount increasing.Conclusions could be derived that the Mo doping inhibited the growth of crystallite size and suppressed the transformation from anatase to rutile of TiO 2 .It was noted that there was no detected MoO 3 phase.This might be ascribed to the incorporation of Mo 6+ ion into the TiO 2 lattice.The ionic radius of Mo 6+ is 0.062 nm, and that of Ti 4+ is 0.068 nm [19].Because of the similarity in their ionic sizes, Mo could easily be incorporated into the TiO 2 lattice, resulting in a narrower energy gap, which could be observed in the following UVvis spectroscopy.Another consequence was that Mo doping increased lattice distortion of samples, which was confirmed by data in Table 1.Generally, smaller crystallite was attributed to greater lattice distortion caused by larger doping amount, which would also enhance the concentration of lattice defects and thus precipitate carrier recombination [20].This implied that excessive doping, though more carriers could be generated due to smaller energy gap, would exert a negative effect on photocatalytic properties.
Figure 1(b) shows XRD patterns of samples with different calcination temperatures.With calcination temperature increasing, the diffraction peaks became sharper and stronger due to the growth of anatase crystallites.Obviously, the phase transformation from anatase to rutile occurred between 550 ∘ C and 650 ∘ C. The average crystallite sizes at 300, 450,   [20].To MT(3.5 550), however, the absorption edge moved inversely to the shorter wavelength range.This phenomenon was a typical consequence of quantum effect and could be explained as follows: as crystallite size fell into nanoscale, the movement of electrons would be confined more intensively, resulting in the differentiation near the Fermi level and broaden the band gap [22].The band gap was calculated by   = 1240/ where   (eV) is the band gap and  (nm) is the wavelength of the absorption edge in the spectrum [23].The wavelength of the absorption edge and the calculated band gap of samples were listed in Table 2.It could be seen that when Mo doping amount increased from 0 to 2.0%, the band gap decreased from 3.05 eV to 2.73 eV.But with Mo doping amount further increasing, the band gap increased to 2.79 eV, which corresponded to the blue shift of absorption edge as discussed above.Therefore, it could be concluded that an appropriate amount of Mo could effectively shorten the energy of TiO 2 while excessive Mo might have the opposite effect.

XPS Analysis.
The XPS spectra of T(550) and MT(2.0 550) were measured.As shown in Figure 3(a), the peaks located at binding energy of 458.4 eV and 463.9 eV corresponded to Ti2p 3/2 and Ti2p 1/2 of TiO 2 , respectively, which  were consistent with the values of Ti 4+ in TiO 2 lattice [24].
For MT(2.0 550), the binding energies of Ti2p 3/2 and Ti2p 1/2 were 458.55 eV and 464.25 eV, respectively; a few right shifts were caused by Mo doping, which might be an indication that molybdenum atoms indeed substituted titanium atoms in the lattice.Mo3d 5/2 and Mo3d 3/2 peaks of MT(2.0 550) are shown in Figure 3(b).The peaks located at 232.6 eV and 235.7 eV corresponded to the feature of Mo 6+ , while peaks located at 231.8 eV and 234.8 eV corresponded to Mo 5+ [25].No Mo 4+ peak was observed, indicating that the main valances of molybdenum in the samples were +6 and +5.From the ratio of peak area, it could be obtained that the atomic percentage of Mo 6+ and Mo 5+ would be 72.2% and 27.8%, respectively.That was as follows: most doped Mo ions existed as Mo 6+ ions in TiO 2 lattice, but a small part of Mo ions existed as Mo 5+ ions.The presence of Mo 5+ ions signified no adequate oxygen in TiO 2 lattice to support Mo being as complemented oxidation state of Mo 6+ ions.So, the existence of Mo 5+ ions also implied that MT(2.0 550) was in oxygen deficiency state (as one titanium atom needed two O atoms but one molybdenum atom needed three O atoms).The surface deficiency of O could be complemented by adsorbing more oxygen, which was beneficial to photocatalytic degradation.
XPS spectrum of the O1s of MT(2.0 550) is given in Figure 3(c).The O1s spectrum could be decomposed into two peaks.The peak at 529.7 eV was assigned to crystal lattice oxygen (O  ), while the peak located at 531.5 eV could be attributed to adsorbed oxygen (O  ) [26].The O  was mainly attributed to the contribution of Ti-O in TiO 2 crystal lattice, and the O  was ascribed to lattice distortion as well as porous structure brought about by Mo doping.
According to the valance band (VB) spectra in Figure 3(d), the VB maxima of T(550) and MT(2.0 550) were 3.04 eV and 2.73 eV, respectively.It indicated that Mo doping could narrow the band gap and extend the absorption edge of TiO 2 towards visible light, which was consistent with the UV-vis spectroscopy.
3.4.TEM Analysis.Figure 4 showed the microstructures of T(550) and MT(2.0 550).It could be seen that MT(2.0 550) showed a better dispersion than T(550).All the samples consisted of highly crystalline and compact nanoparticles.For MT(2.0 550), a fine particulate morphology in porous structure could be observed.Figures 4(a) and 4(b) revealed that there were different kinds of crystalline TiO 2 .In Figure 4(b), it could be seen that the measured lattice spacing of T(550) was 0.32 nm, which was in coincidence with the spacing distance of (110) plane of rutile TiO 2 [27].In Figure 4(d), it is indicated that in the MT(2.0 550) there existed a lattice spacing (0.35 nm) which was anatase TiO 2 [27].So both the XRD and TEM observations presented the coincident results and showed that MT(2.0 550) was nanocrystalline anatase TiO 2 .These results also provided evidence that the Mo doping suppressed the formation of the rutile TiO 2 and inhibited the agglomeration of TiO 2 .
3.5.Photocatalytic Activity.Figure 5 shows photocatalytic degradation of as-prepared photocatalysts with different Mo doping amounts.It could be seen that with Mo doping amount increasing from 0 to 2.0%, the degradation rate of methylene blue increased by 36%, which indicated that Mo was doped into TiO 2 lattice and enhanced the photocatalytic activity.However, with Mo doping amount further increasing to 3.5%, the degradation efficiency decreased, which was still better than that of T(550), MT(0.5 550), and MT(1.0 550) samples.This could be ascribed to the fact that the concentration of holes on the valence band increased with Mo 6+ ions increasing and excessive Mo 6+ ions would cause the carriers recombination.As a result, the recombination of generated electron/hole pair exceeded the carrier transition to the surface of photocatalysts.Once the concentration of Mo was beyond an optimum quantity, Mo 6+ ions role as a carrier recombination center would counteract its role of trapping carriers and prolonging carrier lifetime [28].So a decrease of photocatalytic activity was observed.
Figure 6 shows the results of photocatalytic degradation of samples with different calcination temperatures.It indicated that calcination temperature had great influence on the structure of photocatalyst.With the calcination temperature increasing from 300 ∘ C to 550 ∘ C, the degradation effect increased.However, when the calcination temperature reached 650 ∘ C, the degradation effect decreased, which might be related to the crystallite size and the ratio of anatase and rutile TiO 2 in the samples.High temperature caused particle agglomeration and decreased the surface   3)).area of photocatalysts, which could also affect the efficiency.According to Figures 5 and 6, conclusions could be derived that the optimum Mo doping amount and the calcination temperature were 2.0% and 550 ∘ C, respectively.
Due to different band gap corresponding to different excitation wavelengths, light utilization of photocatalysts on different wave bands was not the same.Two kinds of light source were used in the experiments, respectively.The results are shown in Figure 7.It could be observed that MT(2.0 550) had a better performance than T(550) under both solar light and xenon lamp.It might be ascribed to the fact that Mo ions could capture the photogenerated carriers to prolong the lifetime of carriers or quicken the separation of carriers [29], which enhanced the photocatalytic activity and improved the utilization of solar light.Mo-doped TiO 2 photochemical catalysis was shown to be an efficient method for degradation of methylene blue under visible light, especially under solar light.

Conclusions
Undoped and Mo-doped TiO 2 nanoparticles were successfully synthesized by sol-gel method.XRD results showed that crystallite size decreased and anatase TiO 2 increased with Mo doping amount increasing.Mo doping inhibited the growth of crystallite size and suppressed the transformation from anatase to rutile of TiO 2 .For Mo-doped TiO 2 , the phase transformation from anatase to rutile occurred between 550 ∘ C and 650 ∘ C. The crystallite size increased with temperature increasing, which might be caused by particle agglomeration under high temperature.UV-vis DRS indicated the optical absorption edges of Mo-doped samples shifted to longer wavelength regions.Ti2p, Mo3d, and O1s of Mo-doped TiO 2 were detected by XPS, which suggested that molybdenum atom was doped into TiO 2 and most doped Mo ions existed as Mo 6+ ions in TiO 2 lattice.All the samples consisted of highly crystalline and compact nanoparticles.Mo-doped TiO 2 photochemical catalysis was shown to be an efficient method for degradation of methylene blue under visible light, especially under solar light.When the calcination temperature was 550 ∘ C and the Mo doping amount was 2.0%, the Mo-doped TiO 2 sample exhibited the highest photocatalytic activity.

Figure 2 :
Figure 2: UV-visible absorption of samples with different Mo doping amounts.

Figure 5 :Figure 6 :
Figure 5: Photocatalytic degradation of samples with different Mo doping amounts.

Table 1 :
Crystallite parameters of TiO 2 with different Mo doping amounts.

Table 2 :
The band gap energies of samples with different Mo doping amounts.Figure2shows the UV-visible absorption spectra of samples with different Mo doping amounts.It showed that Mo doping caused a notable red shift of the absorption edge, which was beneficial to the photocatalytic activity of Mo-doped TiO 2 nanoparticles.By offering more valence electrons which could be incited easily into free carriers by photons, Mo doping introduced a donor level under the conduction band of TiO 2 , leading to a narrower band gap [21]were 5.3, 8.9, 16.6, and 25.7 nm, respectively.It could be observed that the crystallite size increased with temperature increasing, which might be caused by particle agglomeration under high temperature[21].3.2.UV-Vis DRS Analysis.

Table 3 :
Solar light intensity in the experiment.