Photocatalytic activity enhancing for TiO₂ photocatalyst by doping with La

WEN Chen (文  晨), DENG Hua (邓  桦), TIAN Jun-ying (田俊莹), ZHANG Ji-mei (张继梅)

Institute of Material and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China

Received 10 April 2006; accepted 25 April 2006

Abstract: La doped nanocrystalline TiO₂ photocatalyst was developed by sol-gel method. The prepared La-TiO₂ photocatalysts with anatase phases were characterized by X-ray diffractometry (XRD), UV-Vis absorption spectroscopy, and photoluminescence spectra (PL). The photocatalytic activity was evaluated by the photocatalytic degradation of phenol in solution under sunlight irradiation. The results show that the crystallinity of anatase is improved by La doping. Moreover, La not only suppresses phase transition from anatase to rutile but also exhibits an absorption in the λ≥ 400 nm range. The photocatalytic activity of La-doped TiO₂ photocatalysts exceeds that of pure TiO₂ photocatalyst prepared by the same method when the molar ratio of La to Ti is kept at 0.3%.

Keywords: La doping; TiO₂ photocatalyst; photocatalytic activity; sunlight

1 Introduction

The photoassisted catalytic degradation of organic pollutants in water and wastewater employing semiconductors as photocatalysts has been a promising method[1, 2]. Many photocatalysts have been attempted for the degradation of pollutants in wastewater. Among the semiconductors employed, anatase phase of TiO₂ is the most preferable material for the photocatalytic process due to its high photosensitivity, non-toxic nature, large bandgap and stability[3]. Despite the positive attributes of TiO₂, there are a few drawbacks associated with its use: 1) charge carrier recombination occurs within nanoseconds [4]; 2) the band edge absorption threshold does not allow the utilization of visible light [5]. To circumvent these particular limitations, a number of strategies have been proposed to improve the light absorption features and lengthen the carrier life time characteristics of TiO_2. It has been shown that the photocatalytic activity of a catalyst can be influenced by its crystal structure, surface area, size distribution, porosity, band gap and surface hydroxyl group density [6]. In recent years, many attempts to dope transitional metals into TiO₂ have been made in order to absorb light in the visible region of the solar spectrum [7]. According to the pioneering works of ASAHI et al[8], some groups demonstrated the substitution of anion atoms such as fluorine (F), carbon (C) and sulfur (S) for oxygen [9-11]. DOMEN et al[12] reported the La and N codoped TiO₂ as visible light photocatalysts. Therefore, the synergistic effects of rare earth elements such as La and Y in TiO₂ will enhance the photocatalytic properties.

In this study, we studied the prepared process and characterization of the La-doped TiO₂, the catalytic effects of the La-TiO₂ particles on the degradation of phenol were also demonstrated under a simulated sunlight irradiation.

2 Experimental

All reagents were standard grade and used without further purification. Tetrabutylorthotitanate was used as a titanium source. La(NO₃)₃ as a precursor of the dopant was dissolved in 10 mL deionized water. The synthesis of the La-TiO₂ particles is as follows. The mixture containing 10 mL tetrabutylorthotitanate and 40 mL absolute ethyl alcohol was agitated for 30 min and was added dropwise to a mixted solution containing 10 mL La(NO₃)₃-H₂O, 10 mL absolute ethylalcohol and 2 mL HNO₃ with vigorous stirring for 2 h at room temperature. The molar ratios of La to Ti, which hereafter was designated as R_La, were 0.1%, 0.3% and 0.4%. After being aged for 5 h, the samples were dried at 70 ℃ for 48 h to vaporize water and alcohol and then ground to fine particles to obtain xerogel samples. The xerogel samples were calcined at 773, 873 and 973 K in air for 2 h, respectively. The pure TiO₂ without any doping was obtained also using the same procedure.

The X-ray diffraction (XRD) patterns were obtained on a Burker D8 GADD X-ray diffractometer using Cu K_α radiation. A UV-Vis spectrophotometer (JASCO V-570) was used to obtain the absorption spectra of samples. A Hitachi F-2500 fluorescence spectrophotometer (PL) was used to investigate the photogenerated electrons and holes information about the surface of samples.

A total of 100 mL phenol solution (20 mg/L) was put into an open reactor and air was bubbled at a flow rate of 100 mL/min through the solution in the dark for 30 min at 30 ℃. The La-TiO₂ particles used for each experiment were kept at 1.0 g / L. After the adsorption of phenol on the particles reached equilibrium, the solution was irradiated with a 250 W metal halide lamp as a simulated sunlight source. The intensity of light entered the phenol solution measured with a UV-Vis radiometer was (13.4±10) mW/cm² for 310-550 nm. The concentration of phenol was monitored by a Waters 1525 HPLC system. The photocatalytic activity of pure TiO₂ particles was also examined under the same conditions.

3 Results and discussion

The crystalline phase of each sample was determined by powder XRD, and the calcination temperature on phase changes of the as-prepared pure TiO₂ and La-TiO₂ are shown in Fig.1. It can be seen that, in Fig.1(a), as the calcination temperature increases, the phase changes form pure anatase 773 K to mixed anatase /rutile (873 K and 973 K), but at 973 K, rutile is the main phase when doping La in TiO₂. The peak intensities of anatase increase and the width of the (101) plane diffraction peak of anatase becomes narrower, indicating that the as-prepared La-doped TiO₂ samples (773 K and 873 K) have a main anatase structures. As shown in Fig.1(b), as the calcination temperature increases, the prepared pure TiO₂ are an anatase /rutile mixture (773  and 873 K), and the rutile phase starts to appear at 973 K. However, the diffraction peak (La₂O₃) of La doping in the XRD patterns is not observed, probably owing to a lower La doping content (0.3 %). The fact also implies that the La₂O₃ is well dispersed within TiO₂ phase. In addition, it is also clear that the temperature of phase transition in the La-doped TiO₂ is increased compared with that of pure TiO₂.

Fig.2 shows the effect of R_La on phase structures of TiO₂ particles. With increasing R_La in the system, the peak intensities of anatase increase, and La-doped TiO₂ contains only anatase phase over the calcinations temperature of 773 K for 2 h. Interestingly, R_La=0.3% TiO₂ sample has a smaller particles size than other samples because the width of the (101) plane diffraction peak of anatase (2θ=25.4?) becomes wider. In view of these facts, it is easy to understand the improvement of the crystallinity of anatase of TiO₂ particles by La

Fig.1 XRD patterns of TiO₂ samples calcined at 773 K, 873 K and 973 K for 2 h: (a) R_La = 0.3%TiO₂; (b) Pure TiO₂

Fig.2 XRD patterns of TiO₂ samples with different R_La calcined at 773 K for 2 h

doping. TiO₂ contains three polymorphs: anatase, rutile, and brookite. Each phase exhibits different properties and has different applications.

Anatase with lower agglomeration and smaller

particles size usually shows higher photocatalytic activity than that of rutile or brookite TiO₂ since rutile is normally prepared by calcination of anatase at high temperatures. The results presented here clearly show that the phase and the phase composition of TiO₂ can be controlled by simply doping with La in the synthesis, making it very useful to the study of the photocatalytic activity of TiO₂ particles. In addition, these results also imply that the La-doping not only suppresses the formation of any impurity phase but also prevents phase transition of anatase to rutile.

The optical absorption spectra of the samples are shown in Fig.3. Compared to the undoped TiO₂, La doping expands the wavelength response range of TiO₂ to visible range. The absorption of La-doped TiO₂ in visible light region is stronger than that of pure TiO₂, which is probably caused by the color of the La-doped TiO₂.

Fig. 3 UV-Vis reflection spectra of pure(a) and R_La=0.3%La doped(b) TiO₂ samples calcined at 773 K for 2 h

Fig.4 indicates the fluorescence spectra (PL) of pure and La-doped TiO₂ samples. The spectra of samples are somewhat different, that is, a small decrease in the fluorescence of R_La = 0.3% TiO₂ particles were observed by comparing the spectrum of pure TiO₂. This indicates that an appropriate amount of La doping may slow the radiative recombination process of photogenerated electrons and holes in TiO₂. This may be due to the introduction of new defect sites (or recombination centers) that enhance the recombination of photogenerated electrons and holes. Our PL measurement results confirm that the higher the photocatalytic activity, the lower the intensity of PL spectrum is.

Fig.4 PL spectra (λ_ex = 250 nm) of TiO₂ samples with R_La=0.3%(a) and pure TiO₂(b) calcined at 773 K for 2 h

Fig.5 depicts the results of the degradation of phenol solution photocatalyzed by pure and R_La=0.3% TiO₂ samples at various calcination temperatures. It is apparent that the R_La=0.3% TiO₂ (773 K) shows significantly higher photocatalytic activity compared to other samples and pure TiO₂, it achieves a degradation of about 71 % for the phenol solution, but R_La=0.3% TiO₂ (873 K) and pure TiO₂ (773 K) degrades only 59 % and 31 % in 4 h. However, R_La=0.3% TiO₂ (973 K) shows a poor photocatalytic activity which is probably due to the presence of rutile phase.

Fig.5 Effects of La-doped TiO₂ (R_La=0.3%) and pure TiO₂ samples with various calcination temperatures on degradation of phenol under conditions of bubbling air and sunlight, phenol concentration 20 mg/L, reaction temperature 30±2 ℃ and catalyst amount 1.0 g/L

   The degradation of phenol by the La-doped TiO₂ particles was compared with that using pure TiO₂ under the conditions of bubbling air and sunlight. The experimental results are shown in Fig.6. It is clear that the La-doped TiO₂ particles have higher photocatalytic activity for the degradation of phenol than pure TiO₂ and the degradation rate follows the order: R_La= 0.3% TiO₂ ＞R_La=0.4% TiO₂＞R_La=0.1% TiO₂ ＞pure TiO₂. This is due to the presence of enhanced anatase phase caused by La doping because anatase TiO₂ is believed to exhibit the highest photocatalytic activity. Our experiment results are in good agreement with the XRD, UV-Vis and PL analyses.

Fig.6 Effects of La-doped TiO₂ and pure TiO₂ particles with various R_La on degradation of phenol under conditions of bubbling air and sunlight, phenol concentration 20 mg/L, reaction temperature (30±2) ℃ and catalyst amount 1.0 g/L

4 Conclusions

La-doped TiO₂ nanoparticles with pure anatase and mixed anatase/rutile phases have been prepared. The crystallinity of anatase is improved by La doping. Moreover, with increasing R_La, La not only suppresses the formation of brookite phase but also prevents the phase transition from anatase to rutile. The La-doped TiO₂ samples also show effective absorption in the visible light range and enhance the photocatalytic activity for the degradation of phenol solution under a simulated sunlight irradiation.

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(Edited by YUAN Sai-qian)

Foundation item: Project (02009) supported by the National High Technology Research Plan of China

Corresponding author: WEN Chen; Tel: +86-22-81333894; E-mail: wenchen@tjpu.edu.cn