Enhanced Photocatalytic Activity of W-Doped and WLa-Codoped TiO 2 Nanomaterials under Simulated Sunlight

W-doped TiO 2 and W-La-codoped TiO 2 nanomaterials were successfully synthesized via the sol-gel method. The products were characterized by X-ray diffraction, UV-vis diffuse reflectance spectrophotometer, transmission electron microscopy, and X-ray photoelectron spectroscopy. The presence of W and La results in significant red shift of absorption edge for TiO 2 -based nanomaterials. The weight ratios of La and W in the composites play important roles in the absorption edge for TiO 2 -based nanomaterials. The photocatalytic activities of both W-doped TiO 2 and W-La-codoped TiO 2 photocatalysts for decolorization of methyl orange solution were evaluated under simulated sunlight irradiation. The results showed that both W-doped and WLa-codoped can effectively improve the photocatalytic behaviors of TiO 2 nanomaterials ascribed to the improved photoinduced charge carriers separation, enhanced light absorption, and large surface area. Furthermore, W-La-codoped TiO 2 exhibited higher photocatalytic activity thanW-doped TiO 2 . Considering their high photocatalytic activity, the doped TiO 2 nanomaterials could be applied in wastewater treatment and environmental purification.


Introduction
Nowadays, in view of the increasing awareness on environmental pollution, application of semiconductor photocatalysis for chemical wastes treatment has been considered as a promising approach and attracted extensive attention [1][2][3][4][5][6][7][8][9][10].A series of semiconductors has been successfully used as photocatalysts, including oxides and sulfides [11][12][13][14][15][16].Among these semiconductors, TiO 2 is especially attractive because of its high photocatalytic activity, excellent chemical stability, nontoxicity, and low cost, making it potentially suitable for conversion of solar energy to electricity or chemical energy [17][18][19][20][21].However, TiO 2 with large energy bandgap of 3.2 eV can be only excited by UV light, which merely occupies about 4% of solar energy.It leads to the less utilization of solar energy and inhibits the practical application of TiO 2 under sunlight.Aiming at solving this shortcoming, many approaches have been successfully reported to make TiO 2based materials respond to visible light, such as composite semiconductors, transition metal doping, and noble metal deposition [22][23][24][25][26][27][28][29].Among these methods, transition metal doping is usually considered to be one of the most effective strategies for improving the photocatalytic activity of TiO 2 under visible light.The construction of doped TiO 2 materials introduces additional electronic states within the bandgap, leading to an extension of light absorption in the visible region.In addition, doped TiO 2 materials increase the lifetime of photoinduced charge carriers by bandgap irradiation, thereby leading to enhancement of photocatalytic efficiency [25].Palmisano has investigated the influence of a series of transition metal ions (Co, Cr, Cu, Fe, Mo, V, and W) on the photocatalytic activity of TiO 2 .The results show that only W-doping distinctly enhances the photocatalytic activity of TiO 2 under visible light irradiation [26].In addition, rare earth metals can also be used to enhance the visible light photocatalytic activity of TiO 2 [30][31][32][33][34].Although single metal ion doped TiO 2 has been widely investigated, the utilization of two-metal-ion codoped TiO 2 as a visible light photocatalyst has not been adequately investigated.
In this paper, both W-doped and W-La-codoped TiO 2 nanomaterials have been successfully synthesized via a solgel method.The results show that the existence of W and La in the nanomaterials can effectively enhance the photocatalytic behaviors of TiO 2 for decolorization of methyl orange solution under simulated sunlight irradiation.It is mainly attributed to the improved photoinduced charge carriers separation, enhanced light absorption, and large surface area.with the concentration of 20 mg⋅L −1 was added to the above solution under vigorous stirring.The solution was aged at room temperature for 5 h and the precursor gel was obtained.Finally, the gel was dried at 100 ∘ C for 6 h, grinded, and calcined at 400 ∘ C for 3 h in air.The weight ratio of W in the catalysts could be easily controlled by adjusting the amount of (NH 4 ) 10 W 12 O 41 solution.The obtained samples with different weight ratio of W to Ti (0, 0.4%, 0.6%, 0.8%, 1%, and 1.2%) were denoted by TiO 2 , 0.4% W/TiO 2 , 0.6% W/TiO 2 , 0.8% W/TiO 2 , 1.0% W/TiO 2 , and 1.2% W/TiO 2 , respectively.

Characterization.
The samples were examined by X-ray diffraction (XRD) on a Shimadzu XRD-6100 diffractometer with Cu K radiation ( = 0.154 nm), operating at 40 kV and 100 mA.The diffused reflectance UV-visible spectra (DRS) were recorded on a CARY 100 CONC spectrometer.Transmission electron microscopy (TEM) was carried out with a JEOL JEM-2000EX operating at 200 kV.X-ray photoelectron spectra (XPS) were recorded with a VG Scientific ESCALAB250 XPS instrument.The C(1s) level was used as an internal reference at 284.6 eV.Energy dispersive X-ray spectroscopy (EDS) was recorded with Oxford INCA.The N 2 adsorption-desorption was measured by JW-BK222 nitrogen adsorption analyzer.Photoluminescence (PL) spectra were recorded by fluorescence spectroscopy (LS55) with the excitation wavelength of 280 nm.

Photocatalytic Decolorization of MO Solution.
The photocatalytic activities were evaluated by photodecolorization of MO solution with pH = 3.0 at 30 ∘ C. In this case, a 350 W xenon lamp was used as a light source.200 mg of photocatalyst was added to 200 mL of MO solution with the initial concentration of 20 mg ⋅ L −1 and then magnetically stirred in the dark for 20 min to ensure the establishment of an adsorption-desorption equilibrium and uniform distribution.The suspensions were collected every 20 min and centrifuged to separate the photocatalyst particles.The MO concentration was determined by monitoring the absorbance peak at  max = 464 nm using a Shimadzu, UV-1600 (PC) UVvisible spectrophotometer.

Results and Discussion
Figure 1 shows the XRD patterns of the as-synthesized Wdoped TiO 2 and W-La-codoped TiO 2 .It can be seen that all of the diffraction peaks are ascribed to anatase TiO 2 and no peaks indexed to WO 3 and La 2 O 3 are detected, indicating that crystalline WO 3 and La 2 O 3 are not formed in this case.According to previous report [35], W 6+ should be present in substitutional positions of Ti 4+ in the lattice of TiO 2 due to the similar ionic radii between W 6+ and Ti 4+ .However, it seems difficult for La 3+ to be incorporated into the lattice of TiO 2 due to the mismatch of the ionic radii of Ti 4+ and La 3+ , and it may exist in the form of La 2 O 3 which was very small and highly dispersed in the composites [31,36].In addition, the broadened peaks suggest that the samples are composed of nanosized particles.
XPS analysis is performed to identify the chemical composition of the W-La-codoped TiO 2 .The XPS spectrum for 4% La/W/TiO 2 shown in Figure 2(a) displays the corresponding peaks of O1s, Ti2p, C1s, W4f, and La3d, indicating the existence of W and La in the composite.The spectrum in Figure 2(b) shows two peaks locating at 458.9 and 464.4 eV, arising from the spin-orbit splitting of the Ti2p components, namely, Ti2p 3/2 and Ti2p 1/2 , which are in good agreement with those of titanium dioxide [37].Figure 2(c) exhibits the W4f 7/2 core level binding energy peak at 36.7 eV, which is consistent with that of tungsten trioxide [38].The weak peak located at 835 eV in Figure 2(d) is related to La3d components (La3d 5/2 ).For further confirming the existence of W and La, EDS has also been performed (Figure 3).It can be clearly seen that the signals for W are present in W-doped TiO 2 nanomaterials, and the weight ratio of W/Ti is consistent with what we proposed.Similarly, W and La signals also appeared in the EDS spectrum of W-La-codoped TiO 2 , illustrating the existence of W and La.
In order to study the morphology of the obtained samples, TEM is performed.As shown in Figure 4(a), pure TiO 2 are composed of small nanoparticles with the diameter in the Figure 1: XRD patterns of (A) pure TiO   range of 15-30 nm.However, both W-doped TiO 2 and W-La-codoped TiO 2 consist of small nanoparticles, which are uniform with the main diameter of approximately 10 and 8 nm, respectively.It is attributed to the presence of W and La in the composites, which can inhibit the growth of TiO 2 nanoparticles [36,39,40].This is well consistent with the results of XRD studies.In HRTEM image, the fringe spacing of 0.35 nm can be indexed to the (101) plane of anatase TiO 2 , indicating well crystalline TiO 2 nanoparticles are formed in this case.
Figure 5(a) shows the UV-vis absorption spectra of the TiO 2 and W-doped TiO 2 nanomaterials with various amounts of W, converted from the corresponding diffuse reflectance spectra.It can be seen that pure TiO 2 nanomaterials only exhibit the fundamental absorption band in the UV light region, while the absorption edge shifts towards visible light region obviously for the W-doped TiO 2 materials.Furthermore, a gradual red shift of absorption edge is noticed along with increasing the amount of W, when the amount of W is smaller than 0.8%.The absorption of the UV light in TiO 2 is ascribed to the excitation of the O2p electron to the Ti3d level [41].The red-shifted absorption edge for W-doped TiO 2 may be associated with the charge transfer between TiO 2 valence or conduction band and W ion level.It implies that the bandgap of TiO 2 can be adjusted by the trapping level, which is formed through doping with appropriate metal ions.It has been reported that the existence of La can also result in the red shift of absorption range of TiO 2 to visible light region [42].As shown in Figure 5(a), it is also found that the absorption edge of 4% La/W/TiO 2 is red-shifted compared with that of 0.8% W/TiO 2 .The bandgaps estimated from the plots of (()ℎ]) 2 versus photon energy ℎ] are presented in Figure 5(b) and it is apparent W-doping and La-doping lead to the decrease of bandgap of TiO 2 from 3.19 to 3.10 eV, indicating that the obtained TiO 2 nanomaterials display the visible light response.They are anticipated to exhibit better photochemical activity under visible light.
In order to further investigate the texture of the obtained composites, N 2 adsorption/desorption measurements are conducted.Figure 6(a) shows the nitrogen adsorptiondesorption isotherms of pure TiO 2 , W-doped TiO 2 , and W-La-codoped TiO 2 nanomaterials.The isotherms are characteristic of type-IV curves, indicating the presence of mesoporosity in the synthesized materials.Figure 6(b) shows the pore size distributions of pure TiO 2 , W-doped TiO 2 , and W-La-codoped TiO 2 .It is clearly seen that the pores in pure TiO 2 have the diameter of about 4 nm, whereas the majority of pores in W-doped TiO 2 and W-La-codoped TiO 2 are around 2 nm.However, it should be noted that some mesopores with the diameter of 5 nm are present in Wdoped TiO 2 .The BET specific surface areas of pure TiO 2 , W-doped TiO 2 , and W-La-codoped TiO 2 are 5.0, 86.3, and 102.7 m 2 ⋅ g −1 .The higher specific surface areas of W-doped TiO 2 and W-La-codoped TiO 2 are ascribed to the smaller particle size.For comparison, P25 is also characterized by N 2 adsorption/desorption measurement.The BET specific surface area is 42.8 m 2 ⋅ g −1 and the main diameter of pores is approximately 2 nm.
The photocatalytic activity of the W-doped TiO 2 nanocrystals is evaluated by photodecolorization of MO solution, which is a typical azo dye and difficult to be degraded.Figure 7(a) presents the adsorption effect of pure TiO 2 , 0.8% W/TiO 2 , and 4% La/W/TiO 2 on MO in the dark.The adsorption equilibrium is reached for all three samples after 20 min.It is obvious that the adsorption capacity of 4% La/W/TiO 2 is superior to those of pure TiO 2 and 0.8% W/TiO 2 , due to the large specific surface area of 4% La/W/TiO 2 .Figure 7(b) presents the photocatalytic performance for the decolorization efficiency of MO solution in the presence of P25, pure TiO 2 , and W-doped TiO 2 nanocrystals.It is clear that all of the W-doped TiO 2 nanomaterials exhibit higher photocatalytic activity than that of pure TiO 2 and P25, which possess the similar porous structures.This is attributed to improved photoinduced charge carriers separation, enhanced light absorption, and large surface area.Under simulated sunlight irradiation, the electrons in the TiO 2 structures can be easily excited from the valence band into the conduction band.Then the excited electrons in the conduction band probably transfer to the W ion states and the holes are left in the valence band [35].Subsequently, the charge carriers transfer to the surface of photocatalyst and initiate the oxidation of MO.In addition, W-doping increases the light absorption owing to the red shift of absorption range to the visible light region.Meanwhile, the surface area is also important for photocatalytic activity.Comparing with the pure TiO 2 and P25, 0.8% W/TiO 2 nanomaterials display a larger surface area, giving birth to more contact area during photocatalystic reaction.All of above advantages make 0.8% W/TiO 2 nanomaterials present higher photocatalytic activity.It should be noted that the decolorization efficiency increases with increasing of the amount of W as long as it is smaller than 0.8%.Further increasing the dopant content results in the decrease of decolorization efficiency.It may be attributed to that W ions begin to act as recombination centers for the photogenerated electrons and holes, when the amount of W is larger than 0.8%.So the decolorization efficiency is dramatically decreased.Figure 7(c) displays the decolorization efficiency of MO over the as-synthesized products under simulated sunlight irradiation.It is obvious that the decolorization efficiency increases to 67.4% along with the increase of La-doping amount to 4%.Similar to W-doped TiO 2 , further increasing the amount of La results in the decrease of decolorization efficiency.And all of the W-La-codoped TiO 2 nanomaterials exhibit higher photocatalytic activity than those of W-doped TiO 2 nanomaterials.
According to previous report [31], La 2 O 3 on the surface of TiO 2 may favor the separation of charge carriers and prolong the life of carriers.In addition, the existence of La 2 O 3 can also increase the amount of hydroxyl radicals on the surface of the photocatalyst, available for oxidation of MO.As a result, the presence of La enhances the photocatalytic activity of W-Lacodoped TiO 2 nanomaterials.The efficient charge separation in the W-doped TiO 2 and W-La-codoped nanomaterials is confirmed by PL spectra.As observed in Figure 7(d), 0.8% W/TiO 2 exhibits much lower emission intensity than that of pure TiO 2 nanomaterials, indicating that the recombination of the photoinduced charge carrier is greatly inhibited in 0.8% W/TiO 2 nanomaterials.It can also be found that W-Lacodoped TiO 2 presents apparent quenching of the W-doped TiO 2 fluorescence, which indicates the suppression of the recombination of electrons and holes.The recycling performance is known as one of the main methods for describing the role of catalysts in practical applications.Figure 8 shows the recycling behaviors of 0.8% 200 300 400 500 600 700 800 Wavelength (nm)  W/TiO 2 and 4% La/W/TiO 2 for decolorization of MO.It is seen that the decolorization efficiency of MO using 0.8% W/TiO 2 and 4% La/W/TiO 2 reaches 45.0% and 67.4% in the first cycle.After five cycles, the decolorization efficiency of MO on 0.8% W/TiO 2 and 4% La/W/TiO 2 is 42.8 and 65.1%, respectively, indicating both 0.8% W/TiO 2 and 4% La/W/TiO 2 possess superior recycling ability.
In order to get a better comparison of the photocatalytic activities of the doped TiO 2 nanomaterials, the kinetic analysis of decolorization of MO is also discussed.As shown in Figure 9, the kinetic linear simulation curves of the photocatalytic decolorization of MO solution in the presence of different photocatalysts show that all the decolorization reactions can be described by first-order reaction dynamics due to the low initial concentration of the MO solution (20 mg ⋅ L −1 in this case).Meanwhile, the slopes of these lines suggest the reaction rate constant of 4% La/W/TiO 2 is larger than any other sample in this case, implying 4% La/W/TiO 2 nanomaterials possess the highest photocatalytic activity.

Conclusions
In summary, the W-doped TiO 2 and W-La-codoped TiO 2 nanomaterials are prepared through a sol-gel method.The presence of W and La results in red shift of absorption edge for TiO 2 nanomaterials.The investigation of photocatalytic activity indicates the W-doped and La-doped can effectively improve the photocatalytic activity of TiO 2 under simulated sunlight irradiation due to improved photoinduced charge carriers separation, enhanced light absorption, and large surface area.Considering their high photocatalytic activity, the doped TiO 2 nanomaterials can be applied in wastewater treatment and environmental purification.