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Journal of Nanomaterials
Volume 2014, Article ID 943796, 10 pages
http://dx.doi.org/10.1155/2014/943796
Research Article

Enhanced Photocatalytic Activity of W-Doped and W-La-Codoped TiO2 Nanomaterials under Simulated Sunlight

1School of Light Industry and Chemical Engineering, Dalian Polytechnic University, No. 1 Qinggongyuan, Dalian 116034, China
2Instrumental Analysis Center, Dalian Polytechnic University, No. 1 Qinggongyuan, Dalian 116034, China

Received 10 July 2014; Revised 29 August 2014; Accepted 30 August 2014; Published 21 October 2014

Academic Editor: Yuekun Lai

Copyright © 2014 Chenghe Hua et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

W-doped TiO2 and W-La-codoped TiO2 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 TiO2-based nanomaterials. The weight ratios of La and W in the composites play important roles in the absorption edge for TiO2-based nanomaterials. The photocatalytic activities of both W-doped TiO2 and W-La-codoped TiO2 photocatalysts for decolorization of methyl orange solution were evaluated under simulated sunlight irradiation. The results showed that both W-doped and W-La-codoped can effectively improve the photocatalytic behaviors of TiO2 nanomaterials ascribed to the improved photoinduced charge carriers separation, enhanced light absorption, and large surface area. Furthermore, W-La-codoped TiO2 exhibited higher photocatalytic activity than W-doped TiO2. Considering their high photocatalytic activity, the doped TiO2 nanomaterials could be applied in wastewater treatment and environmental purification.

1. 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 [110]. A series of semiconductors has been successfully used as photocatalysts, including oxides and sulfides [1116]. Among these semiconductors, TiO2 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 [1721]. However, TiO2 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 TiO2 under sunlight. Aiming at solving this shortcoming, many approaches have been successfully reported to make TiO2-based materials respond to visible light, such as composite semiconductors, transition metal doping, and noble metal deposition [2229]. Among these methods, transition metal doping is usually considered to be one of the most effective strategies for improving the photocatalytic activity of TiO2 under visible light. The construction of doped TiO2 materials introduces additional electronic states within the bandgap, leading to an extension of light absorption in the visible region. In addition, doped TiO2 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 TiO2. The results show that only W-doping distinctly enhances the photocatalytic activity of TiO2 under visible light irradiation [26]. In addition, rare earth metals can also be used to enhance the visible light photocatalytic activity of TiO2 [3034]. Although single metal ion doped TiO2 has been widely investigated, the utilization of two-metal-ion codoped TiO2 as a visible light photocatalyst has not been adequately investigated.

In this paper, both W-doped and W-La-codoped TiO2 nanomaterials have been successfully synthesized via a sol-gel method. The results show that the existence of W and La in the nanomaterials can effectively enhance the photocatalytic behaviors of TiO2 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.

2. Experimental Section

2.1. Materials

Anhydrous ethanol (C2H5OH, AR), tetrabutyl titanate (TBOT, CP), acetic acid (CH3COOH, AR), ammonium tungstate hydrate , lanthanum nitrate hexahydrate , methyl orange (MO, AR), and nitric acid (HNO3, GR, 65–68 wt%) were used without further purification.

2.2. Preparation of W-Doped TiO2 Nanomaterials

Firstly, a sol-gel solution, denoted by solution A, was prepared by dispersing 17 mL of TBOT and 0.2 mL of HNO3 in 40 mL of C2H5OH with ultrasonic vibration for 30 min. Another solution, denoted by solution B, was prepared by mixing 4 mL of deionized water, 5 mL of CH3COOH, and 20 mL of C2H5OH. Then, solution B was added dropwise into solution A under vigorous stirring. Subsequently, a certain amount of (NH4)10W12O41 solution with the concentration of 20  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 (NH4)10W12O41 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 TiO2, 0.4% W/TiO2, 0.6% W/TiO2, 0.8% W/TiO2, 1.0% W/TiO2, and 1.2% W/TiO2, respectively.

2.3. Preparation of W-La-Codoped TiO2 Nanomaterials

The synthesis of W-La-codoped TiO2 nanomaterials was similar to that of 0.8% W/TiO2 nanomaterials, except that solution B was added to different amount of to make the weight ratio of La to Ti be 1%, 2%, 3%, 4%, and 5%. These samples were denoted by 1% La/W/TiO2, 2% La/W/TiO2, 3% La/W/TiO2, 4% La/W/TiO2, and 5% La/W/TiO2, respectively.

2.4. Characterization

The samples were examined by X-ray diffraction (XRD) on a Shimadzu XRD-6100 diffractometer with Cu Kα radiation ( 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 N2 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.

2.5. Photocatalytic Decolorization of MO Solution

The photocatalytic activities were evaluated by photodecolorization of MO solution with 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  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  nm using a Shimadzu, UV-1600 (PC) UV-visible spectrophotometer.

3. Results and Discussion

Figure 1 shows the XRD patterns of the as-synthesized W-doped TiO2 and W-La-codoped TiO2. It can be seen that all of the diffraction peaks are ascribed to anatase TiO2 and no peaks indexed to WO3 and La2O3 are detected, indicating that crystalline WO3 and La2O3 are not formed in this case. According to previous report [35], W6+ should be present in substitutional positions of Ti4+ in the lattice of TiO2 due to the similar ionic radii between W6+ and Ti4+. However, it seems difficult for La3+ to be incorporated into the lattice of TiO2 due to the mismatch of the ionic radii of Ti4+ and La3+, and it may exist in the form of La2O3 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.

fig1
Figure 1: XRD patterns of (A) pure TiO2, (B) 0.4% W/TiO2, (C) 0.6% W/TiO2, (D) 0.8% W/TiO2, (E) 1.0% W/TiO2, and (F) 1.2% W/TiO2 in (a) and (A) 1% La/W/TiO2, (B) 2% La/W/TiO2, (C) 3% La/W/TiO2, (D) 4% La/W/TiO2, and (E) 5% La/W/TiO2 in (b).

XPS analysis is performed to identify the chemical composition of the W-La-codoped TiO2. The XPS spectrum for 4% La/W/TiO2 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, Ti2p3/2 and Ti2p1/2, which are in good agreement with those of titanium dioxide [37]. Figure 2(c) exhibits the W4f7/2 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 (La3d5/2). 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 TiO2 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 TiO2, illustrating the existence of W and La.

fig2
Figure 2: The XPS core level binding energy in the 4% La/W/TiO2 nanomaterials. (a) An overview XPS spectrum of the sample; (b) Ti2p spectrum; (c) W4f spectrum; and (d) La5/2d spectrum.
fig3
Figure 3: EDS spectra of the (a) 0.8% W/TiO2 and (b) 4% La/W/TiO2 nanomaterials, respectively.

In order to study the morphology of the obtained samples, TEM is performed. As shown in Figure 4(a), pure TiO2 are composed of small nanoparticles with the diameter in the range of 15–30 nm. However, both W-doped TiO2 and W-La-codoped TiO2 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 TiO2 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 TiO2, indicating well crystalline TiO2 nanoparticles are formed in this case.

fig4
Figure 4: TEM images of (a) pure TiO2, (b) 0.8% W/TiO2, and (c) 4% La/W/TiO2 nanomaterials. (d) The high-resolution TEM (HRTEM) image of 4% La/W/TiO2 nanomaterials.

Figure 5(a) shows the UV-vis absorption spectra of the TiO2 and W-doped TiO2 nanomaterials with various amounts of W, converted from the corresponding diffuse reflectance spectra. It can be seen that pure TiO2 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 TiO2 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 TiO2 is ascribed to the excitation of the O2p electron to the Ti3d level [41]. The red-shifted absorption edge for W-doped TiO2 may be associated with the charge transfer between TiO2 valence or conduction band and W ion level. It implies that the bandgap of TiO2 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 TiO2 to visible light region [42]. As shown in Figure 5(a), it is also found that the absorption edge of 4% La/W/TiO2 is red-shifted compared with that of 0.8% W/TiO2. The bandgaps estimated from the plots of ()2 versus photon energy are presented in Figure 5(b) and it is apparent W-doping and La-doping lead to the decrease of bandgap of TiO2 from 3.19 to 3.10 eV, indicating that the obtained TiO2 nanomaterials display the visible light response. They are anticipated to exhibit better photochemical activity under visible light.

fig5
Figure 5: (a) UV-vis absorption spectra of the pure TiO2, W-doped TiO2 with different W content and 4% La/W/TiO2 nanomaterials and (b) versus plots of (A) 4% La/W/TiO2, (B) 0.8% W/TiO2, and (C) pure TiO2 nanomaterials.

In order to further investigate the texture of the obtained composites, N2 adsorption/desorption measurements are conducted. Figure 6(a) shows the nitrogen adsorption-desorption isotherms of pure TiO2, W-doped TiO2, and W-La-codoped TiO2 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 TiO2, W-doped TiO2, and W-La-codoped TiO2. It is clearly seen that the pores in pure TiO2 have the diameter of about 4 nm, whereas the majority of pores in W-doped TiO2 and W-La-codoped TiO2 are around 2 nm. However, it should be noted that some mesopores with the diameter of 5 nm are present in W-doped TiO2. The BET specific surface areas of pure TiO2, W-doped TiO2, and W-La-codoped TiO2 are 5.0, 86.3, and 102.7 . The higher specific surface areas of W-doped TiO2 and W-La-codoped TiO2 are ascribed to the smaller particle size. For comparison, P25 is also characterized by N2 adsorption/desorption measurement. The BET specific surface area is 42.8  and the main diameter of pores is approximately 2 nm.

fig6
Figure 6: N2 adsorption-desorption (a) isotherms and (b) pore size distribution plot.

The photocatalytic activity of the W-doped TiO2 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 TiO2, 0.8% W/TiO2, and 4% La/W/TiO2 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/TiO2 is superior to those of pure TiO2 and 0.8% W/TiO2, due to the large specific surface area of 4% La/W/TiO2. Figure 7(b) presents the photocatalytic performance for the decolorization efficiency of MO solution in the presence of P25, pure TiO2, and W-doped TiO2 nanocrystals. It is clear that all of the W-doped TiO2 nanomaterials exhibit higher photocatalytic activity than that of pure TiO2 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 TiO2 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 TiO2 and P25, 0.8% W/TiO2 nanomaterials display a larger surface area, giving birth to more contact area during photocatalystic reaction. All of above advantages make 0.8% W/TiO2 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 TiO2, further increasing the amount of La results in the decrease of decolorization efficiency. And all of the W-La-codoped TiO2 nanomaterials exhibit higher photocatalytic activity than those of W-doped TiO2 nanomaterials. According to previous report [31], La2O3 on the surface of TiO2 may favor the separation of charge carriers and prolong the life of carriers. In addition, the existence of La2O3 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-La-codoped TiO2 nanomaterials. The efficient charge separation in the W-doped TiO2 and W-La-codoped nanomaterials is confirmed by PL spectra. As observed in Figure 7(d), 0.8% W/TiO2 exhibits much lower emission intensity than that of pure TiO2 nanomaterials, indicating that the recombination of the photoinduced charge carrier is greatly inhibited in 0.8% W/TiO2 nanomaterials. It can also be found that W-La-codoped TiO2 presents apparent quenching of the W-doped TiO2 fluorescence, which indicates the suppression of the recombination of electrons and holes.

fig7
Figure 7: (a) Decolorization profiles of MO in the presence of (A) pure TiO2, (B) 0.8% W/TiO2, and (C) 4% La/W/TiO2 in the dark. Photocatalytic decolorization curves of MO over the catalysts under simulated sunlight irradiation: (A) blank, (B) P25, (C) pure TiO2, (D) 0.4% W/TiO2, (E) 0.6% W/TiO2, (F) 0.8% W/TiO2, (G) 1.0% W/TiO2, and (H) 1.2% W/TiO2 in (b) and (A) 0.8% W/TiO2, (B) 1% La/W/TiO2, (C) 2% La/W/TiO2, (D) 3% La/W/TiO2, (E) 4% La/W/TiO2, and (F) 5% La/W/TiO2 in (c). (d) PL spectra of the (A) pure TiO2, (B) 0.8% W/TiO2, and (C) 4% La/W/TiO2 nanomaterials.

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% W/TiO2 and 4% La/W/TiO2 for decolorization of MO. It is seen that the decolorization efficiency of MO using 0.8% W/TiO2 and 4% La/W/TiO2 reaches 45.0% and 67.4% in the first cycle. After five cycles, the decolorization efficiency of MO on 0.8% W/TiO2 and 4% La/W/TiO2 is 42.8 and 65.1%, respectively, indicating both 0.8% W/TiO2 and 4% La/W/TiO2 possess superior recycling ability.

fig8
Figure 8: (A) Decolorization curves of MO over (a) 0.8% W/TiO2 and (b) 4% La/W/TiO2 nanomaterials for reusing 5 times.

In order to get a better comparison of the photocatalytic activities of the doped TiO2 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  in this case). Meanwhile, the slopes of these lines suggest the reaction rate constant of 4% La/W/TiO2 is larger than any other sample in this case, implying 4% La/W/TiO2 nanomaterials possess the highest photocatalytic activity.

fig9
Figure 9: Kinetic linear simulation curves of MO photocatalytic decolorization over the different photocatalysts under simulated sunlight irradiation: (A) p25, (B) pure TiO2, (C) 0.4% W/TiO2, (D) 0.6% W/TiO2, (E) 0.8% W/TiO2, (F) 1.0% W/TiO2, and (G) 1.2% W/TiO2 in (a) and (A) 0.8% W/TiO2, (B) 1% La/W/TiO2, (C) 2% La/W/TiO2, (D) 3% La/W/TiO2, (E) 4% La/W/TiO2, and (F) 5% La/W/TiO2 in (b). and are the MO concentration when adsorption equilibrium is reached and at time , respectively.

4. Conclusions

In summary, the W-doped TiO2 and W-La-codoped TiO2 nanomaterials are prepared through a sol-gel method. The presence of W and La results in red shift of absorption edge for TiO2 nanomaterials. The investigation of photocatalytic activity indicates the W-doped and La-doped can effectively improve the photocatalytic activity of TiO2 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 TiO2 nanomaterials can be applied in wastewater treatment and environmental purification.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The research was supported by Program for Key Science & Technology Platform (191) in Universities of Liaoning Province, the National Natural Science Foundation of China (Grant no. 21476033), and Cultivation Program for Excellent Talents of Science and Technology Department of Liaoning Province (no. 201402610).

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