Preparation and photocatalytic activity of La3+ and Eu3+ co-doped TiO2 nanoparticles: photo-assisted degradation of methylene blue

JOURNAL OF RARE EARTHS, Vol. 29, No. 8, Aug. 2011, P. 746

Preparation and photocatalytic activity of La3+ and Eu3+ co-doped TiO2 nanoparticles: photo-assisted degradation of methylene blue SHI Huixian (৆᜻䋸)1, ZHANG Tianyong (ᓴ໽∌)1, WANG Hongliang (⥟㑶҂)2 (1. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; 2. Department of Nuclear Medicine, The First Affiliated Hospital of Sun Yat-sen University, Guangzhou 510080, China) Received 24 February 2011; revised 18 March 2011

Abstract: Rare earth ions La3+ and Eu3+ co-doped TiO2 photocatalyst (La-Eu/TiO2) was prepared by sol-gel method, and characterized by various techniques such as X-ray diffraction (XRD), specific surface area and porosity (BET and BJH), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the La-Eu/TiO2 was evaluated by the degradation of methylene blue (MB) under UV light irradiation. The catalyst had a relatively uniform particle diameter distribution in the range of 40–60 nm. When calcining at 600qC, the XRD patterns of La-Eu/TiO2 indicated the anatase phase, while the XPS patterns showed the Ti4+, La3+ and Eu3+ ions existence. The DRS spectra showed red shift in the band-gap transition. The experimental results of MB degradation demonstrated that the photocatalytic activity of La-Eu/TiO2 was significantly enhanced due to better separation of photogenerated electron-hole pairs. Keywords: lanthanum; europium; titania; photocatalytic activity; degradation; methylene blue; rare earths

TiO2 is the most widely used photocatalyst for solar energy conversion, air purification and wastewater treatment. However, the photocatalytic efficiency at the present stage is still very low, due to the fast recombination of photogenerated electron-hole pairs. Thus, the improvement of the photocatalytic efficiency is still a major challenge in the photocatalysis research field until now. Doping ions in TiO2 lattice have been proven to be an efficient route to enhance the photocatalytic activity[1–7]. It has been reported that the photocatalytic activity of TiO2 could be significantly enhanced by doping with rare earth ions, and the doping of rare earth ions could improve the separation efficiency of electron-hole pairs by trapping photogenerated electrons[8–13]. In recent years, numerous studies have been focused on the photocatalytic activities of rare earth ions hosted in crystalline matrices[14–25]. The photocatalysts Sm/TiO2[14,15,17,24], Pr/TiO2[16–18], Er/TiO2[15,18], Eu/TiO2[15,19], Gd/TiO2[18,19], CeO2/TiO2[18,19,23], La/TiO2[15,16,18–20], Dy/TiO2[19], Nd/ TiO2[16–19,25] were prepared via different methods, and their photocatalytic activities were evaluated by the degradation of pesticide, dyes, and other organism, and all of them exhibited higher photocatalytic activity on organic degradation than pure TiO2. Moreover, the simultaneous doping of two kinds of atoms into TiO2 have attracted considerable interest[26–30]. Nanocrystalline catalyst Fe-Eu/TiO2[26] and Fe-Nd/TiO2[27] showed a synergistic effect and highly photocatalytic activity. The absorbance spectra of Eu-N/TiO2 exhibited a significant red shift to visible region, and the

transformation from anatase to rutile was suppressed by doping Eu and N atoms, the photocatalytic activity of dye red X-3B degradation was higher than N-TiO2[28]. The photocatalyst Ce-N/TiO2 exhibited a high photocatalyst activity for nitrobenzene degradation under visible light, and the nitrogen atoms were incorporated into the crystal of titania and could narrow the band gap energy, and the doping cerium atoms existed in the forms of Ce2O3 and dispersed on the surface of TiO2. The improvement of the photocatalytic activity was ascribed to the synergistic effects of the N and Ce co-doping[29]. Sm-N/TiO2 nanocryatalline showed strong visible-light response and high photocatalytic activity for 4-chlorophenol degradation under irradiation by visible-light (400–500 nm)[30]. However, preparation of TiO2 nanoparticles modified by doping two kinds of rare earth ions and the investigation of their photocatalytic behaviors have seldom been reported so far. In this study, La3+ and Eu3+ co-doped TiO2 nano photocatalysts (La-Eu/TiO2) were prepared by sol-gel process, and methylene blue (MB) was used as a probe molecule to evaluate the photocatalytic activity of the catalysts under UV irradiation.

1 Experimental 1.1 Preparation of La-Eu/TiO2 All chemical reagents used in the experiments were ana-

Foundation item: Project supported by the “863 Program” of the Ministry of Science & Technology of China (2006AA06Z348) Corresponding author: ZHANG Tianyong (E-mail: [email protected]; Tel.: +86-22-27406610) DOI: 10.1016/S1002-0721(10)60535-2

SHI Huixian et al., Preparation and photocatalytic activity of La3+ and Eu3+ co-doped TiO2 nanoparticles: photo-assisted …

lytically pure. Tetra-n-butyloxy titanium [Ti(OBu)4], La(NO3)3, Eu(NO3)3, acetic acid, absolute ethanol, nitric acid were supplied by Kewei Chemical Reagents Company, Tianjin, China. La-Eu/TiO2 catalyst was prepared by sol-gel process with the following procedure. 34 ml Ti(OBu)4 was dissolved into 44 ml absolute ethanol with stirring for several minutes, and then 3 ml acetic acid as hydrolysis suppressant was added dropwise to the above solution under stirring for 30 min to give solution A. Solution B containing 44 ml absolute ethanol, 7.2 ml H2O, and rare earth metal salts in the required stoichiometry were slowly added to solution A. The mixture was hydrolyzed at 25 ºC for 30 min under agitation and a transparent sol was obtained. The gelation was finished by aging the sol for 24 h at room temperature. The gel was dried at 60ºC, then ground to powder and calcined 3 h at different temperatures. 1.2 Characterization of La-Eu/TiO2 To determine the crystal phase composition of the prepared La-Eu/TiO2, we carried out X-ray diffraction (XRD) measurements using a PANAalytical Xcpert Por X-ray diffractometer (Holland) with Cu KD radiation. The diffractograms were recorded in the 2T range 20q–100q with steps of 0.017q. The specific surface area (BET method), specific pore volume and average pore diameter (BJH method) of the catalysts were determined by nitrogen adsorption-desorption isotherms using a Quantachrome NOVA-2000 sorption analyzer (America), and the samples were analyzed at 77 K by nitrogen adsorption-desorption. Transmission electron microscopy (TEM) micrographs were obtained by using a Philips Tecnai G2 F20 instrument (Holland). Scanning electron microscopy (SEM) images were obtained by using a TESCAN VEGA TS-5130SB (Czech), light source of electron-beam was tungsten lamp, the voltage was 10–30 kV. The UV-vis diffuse reflectance spectra (DRS) were performed with a Lambda 900 UV-vis spectrophotometer (Perkin-Elmer Co.). The X-ray photoelectron spectroscopy (XPS) analyses of the samples were performed on a PHI-1600 ESCA spectrometer (America).

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2 Results and discussion 2.1 Characterization of La-Eu/TiO2 2.1.1 Surface area and porosity (BET and BJH) As shown in Fig. 1, the N2 adsorption-desorption isotherms are characteristic of a type Langmuir IV isotherm with a H2 hysteresis loop. The composites exhibited mesoporosity with average pore size approximating to 4.9 nm. The BET specific surface areas, pore volume and pore diameters of the samples are given in Table 1. The specific surface area of the La-Eu/TiO2 calcined at 600 ºC is in the range of 83–95 m2/g, which is larger than naked TiO2 (30.3 m2/g). The relative high surface area of La3+ and Eu3+ doped samples confirmed that the frameworks of TiO2 have better adsorption ability. This may be due to the linkage between the rare earth ions and titanium by oxygen bridge, which effectively enhance specific surface area of TiO2. 2.1.2 XRD analysis The XRD was used to investigate the effect of La3+ and Eu3+ doping on the phase composition of La-Eu/TiO2 nanoparticles. Fig. 2 displays the XRD patterns of different amounts (a) and different calcination temperatures (b) of La-Eu/TiO2. As shown in Fig. 2(a), the peaks at 25.4°, 37.8° and 48.1° elucidate the diffractions of the (101), (004) and (200) anatase-type TiO2, which can be indexed as the phase of anatase (PDF21-1272). The mean that crystallite sizes of catalysts are estimated by applying the Scherrer equation (1): D

kO

(1)

E cos T

Where D is the average crystallize size (nm), Ȝ is the wave-

1.3 Photocatalytic degradation of methylene blue (MB) The photocatalytic activity of La-Eu/TiO2 was evaluated by degradation of MB. The initial concentration of MB was 40 mg/L. The photocatalytic reactor consists of a 160 ml Pyrex glass bottle with a jacket outside and a 125 W high pressure Hg lamp (365 nm) in parallel to the reactor. In all the experiments, the reaction temperature was kept at 25±1qC by a continuous circulation of water in the jacket around the reactor. Reaction suspensions were prepared by adding different amounts of the photocatalyst powers to 50 ml MB aqueous solution under vigorous stirring. Prior to irradiation, all the reaction suspensions were stirred in dark for 20 min to establish an adsorption-desorption equilibrium. The samples were collected at regular intervals of time.

Fig. 1 Nitrogen adsorption-desorption isothermal (a) and the pore size distribution curve calculated from adsorption branch of nitrogen isotherm by BJH method (b) of La-Eu/TiO2 (0.5% La, 1% Eu, calcined at 600qC) Table 1 XRD and N2 adsorption-desorption results for the LaEu/TiO2 catalysts Samples Naked TiO2

Crystallite

SSABET /

Pore

Pore volume/

size/nm

(m2/g)

diameter/nm

(cm3/g)

21.1

30.2

4.3

0.006

La(0.5%)-Eu(0.25%)/TiO2 11.6

83.1

3.8

0.033

La(0.5%)-Eu(0.5%)/TiO2

10.1

93.4

4.9

0.109

La(0.5%)-Eu(1.0%)/TiO2

9.2

95.7

3.8

0.060

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Fig. 2 XRD patterns of the samples (a) La and Eu with different dopant amounts calcined at 600 qC; (b) La-Eu/TiO2 (0.5% La, 1% Eu) at different calcined temperatures 3+

3+

length of the Cu KĮ X-ray radiation (Ȝ=0.15406 nm), k is a coefficient usually taken as 0.94, ȕ is the full width at halfmaximum intensity of the peak observed at 2ș. The crystallite sizes are calculated from diffraction plane (101) of anatase phase, and the results are presented in Table 1. The XRD patterns of La-Eu/TiO2 indicate the anatase phase. The crystalline phase of the titania is an important factor that determines its photocatalytic activity. The anatase phase is more active in the photodegradation of organic compounds. As shown in Fig. 2(b), the peaks intensity of anatase increase with the increase of calcination temperature from 400 to 900 ºC, and the width of (101) plane becomes narrow. The rutile phase appears slightly at 900 qC for La-Eu/TiO2, i.e., the phase transformation starts from ana-

tase to rutile, but the anatase phase was dominant. Thus the dopant is expected to play a significant role in the selective crystallization of anatase phase during sol-gel process[31]. Further, TiO2 in the anatase phase was proved to exhibit higher photocatalytic activity than in the rutile phase. The anatase phase also exhibits low rate of recombination in comparison to rutile due to its ten fold greater rate of hole traping[32]. 2.1.3 Microstructure analysis Fig. 3 shows the SEM and HRTEM micrographs of the La-Eu/TiO2 powders calcined at 600 ºC, respectively. SEM image shows that the sample has a relative uniform particle diameter distribution in the range of 40–60 nm. Furthermore, the nanoparticles with mesoporous characteristic can be observed more clearly from the HRTEM. The HRTEM image shows the surface micrographs, where the set of fringes corresponds to the (101) lattice planes of anatase phase. It was proved that anatase phase has already formed in the mesoporous wall, which is consistent with the XRD analysis results. 2.1.4 UV-vis spectra To investigate the optical absorption properties of catalysts, the DRS of naked TiO2 and La-Eu/ TiO2 catalysts in the range of 200–800 nm were measured and the results are shown in Fig. 4. The spectra of La-Eu/ TiO2 show red shift in the band-gap transition as the content of doping ions increases which can be seen in the iconograph of Fig. 4. Red shift of this type can be attributed to the charge-transfer transition between rare earth ions f electrons and TiO2 conduction or valence band[33]. The titania co-doped with La3+ and Eu3+ has absorption bands in the visible-light regions. These peaks are attributed to the 4f electron transitions of La3+ and Eu3+. In addition, it can be noted that the optical absorption intensity in the UV region is also enhanced, in fact, the enhanced absorption in the UV region for rare earth ions doping TiO2 was also reported somewhere[23]. 2.1.5 XPS To evaluate the surface state, the pure TiO2 and La-Eu/TiO2 samples were characterized by XPS. The binding energy of the main peaks pertaining to Ti2p, O1s, La3d and Eu4d were summarized in Table 2. The Ti2p levels of all samples show two peaks at approximately 463.6 and 458.3 eV, which are assigned to Ti2p1/2 and Ti2p3/2 and indicate that the Ti element mainly existed as the chemical

Fig. 3 SEM (a) and HRTEM (b) images of La-Eu/TiO2 (0.5% La, 1% Eu) calcined at 600 qC

SHI Huixian et al., Preparation and photocatalytic activity of La3+ and Eu3+ co-doped TiO2 nanoparticles: photo-assisted …

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rate is as follows:

D

Fig. 4 DRS of TiO2 and La-Eu/TiO2 samples with different dopant amounts calcined at 60qC (1) La(0.5%)-Eu(1.0%)/TiO2; (2) La(0.5%)-Eu(0.5%)/TiO2; (3) La(0.5%)-Eu(0.25%)/TiO2; (4) TiO2

state of Ti4+ on the basis of the principle and instrument handbook of XPS[34]. The Ti2p XPS spectra of catalysts calcined at 600 qC are shown in Fig. 5. The photoelectron peak of O1s at about 529 eV is characteristic of metallic oxides, which is in agreement with O1s electron binding energy arising from lanthanum, europium and titania lattice, the oxygen atoms in the titania matrix make the primary contribution to the spectrum. Typical XPS La3d and Eu4d spectra for La(0.5%)-Eu(1.0%)/TiO2 samples calcined at 600qC are shown in Fig. 6. The La and Eu atoms existed in the form of La3+ and Eu3+ valences in the sample, the main peaks at 834.2 and 135.9 eV are well in accordance with the standard XPS peaks of La3+ and Eu3+.

Ao

 At Ao

u 100%

(2)

Where D is the decolor rate, Ao the initial absorbance of MB and At the absorbance of MB after ‘t’ minutes. 2.2.1 Photocatalytic degradation and dark adsorption It is observed from Fig. 7 that in the absence of catalyst, the degradation of MB solution is only 2.0% after 2 h UV light irradiation. Meanwhile, the dark adsorption of MB solution is negligible. It is found that about 100% of MB solution degradation takes place by taking 40 mg/L MB solution over 0.2 g catalyst, under UV light irradiation. 2.2.2 Effect of La3+ and Eu3+ doping amount In order to evaluate the photocatalytic activity of La-Eu/TiO2, MB degradation test was carried out by varying dopant amount in La-Eu/TiO2 (Fig. 8). The photocatalytic degradation effiTable 2 XPS binding energy values for the TiO2 and La-Eu/ TiO2 samples calcined at 600qC Binding energy/eV Samples

Ti2p

O lattice

La3d

Eu4d

1/2

3/2

463.60

458.10

529.40





La(0.5%)-Eu(0.25%)/TiO2 463.74

458.32

529.49

834.24

135.99

La(0.5%)-Eu(0.5%)/TiO2 463.97

458.47

529.72

834.72

135.96

La(0.5%)-Eu(1.0%)/TiO2 463.99

458.50

529.49

834.99

135.99

Naked TiO2

2.2 Photocatalytic degradation of MB When photocatalytic reaction is conducted in aqueous medium, the holes are effectively scavenged by the water and generated hydroxyl radical (˜OH), which is strong and unselected oxidant species in respect of totally oxidative degradation for organic substrates. Both holes and hydroxyl radicals have been proposed as the oxidizing species responsible for the degradation of the organic substrates[16]. The photocatalytic activity of La-Eu/TiO2 was determined by degradation of MB aqueous solution. Concentration changes of MB solution were measured using a UV-vis spectrometer at 665 nm (Ȝmax). The activity of catalyst is evaluated by the decolor rate (D) of the samples. The equation of the decolor

Fig. 5 Ti2p XPS spectra of the La-Eu/TiO2 catalysts calcined at 600 qC (1) La(0.5%)-Eu(0.25%)/TiO2; (2) La(0.5%)-Eu(0.5%)/TiO2; (3) La(0.5%)-Eu(1.0%)/TiO2

Fig. 6 Typical XPS O1s, La3d and Eu4d spectra for La (0.5%)-Eu (1.0%)/TiO2 samples calcined at 600qC

750

Fig. 7 Effect of photocatalytic degradation and dark adsorption ([MB]o 40 mg/L, 0.2 g La(0.5%)-Eu(1.0%)/TiO2 calcined at 600qC)

Fig. 8 Effect of La3+ and Eu3+ dopant amount in 1 mol TiO2 ([MB]o 40 mg/L, catalyst calcined at 600 qC, dosage 0.2 g/50 ml)

ciency of La-Eu/TiO2 is higher than naked TiO2 and La-TiO2. The degradation efficiency increase with increasing Eu3+ concentration and a 100% degradation efficiency was obtained with sample containing 0.005 mol La3+ and 0.01 mol Eu3+ in 1 mol TiO2. The crystalline phase of the titania is an important factor that determines its photcatalytic activity. The anatase phase is more active in the photodegradation of organic compounds than the rutile phase. The catalysts containing rare earth ions exist in the anatanse phase whereas the non-doped TiO2 catalyst contains rutile phase. Another factor that could influence the photocatalytic activity of the catalyst is the surface area of the catalysts. The average specific surface area of La-Eu/TiO2 is 95 m2/g while that of unmodified TiO2 is only 30 m2/g. In fact, the ionic radius of La3+ (0.1061 nm) and Eu3+ (0.0947 nm) are bigger than that of Ti4+ (0.0605 nm). Therefore, it is difficult for La3+ and Eu3+ to enter into the lattice of TiO2. From the XPS spectra we can find that the La and Eu atoms existed in the form of La3+ and Eu3+ valences in the sample surface, so it might be due to the substitution of La3+ and Eu3+ atoms in the lattice of La2O3 and Eu2O3 by Ti4+. Ti–O–La and Ti–O–Eu bands could be formed at the interface of catalyst, a charge imbalance would occur. Therefore, both formation of Ti–O–La and Ti-O-Eu bond and charge

JOURNAL OF RARE EARTHS, Vol. 29, No. 8, Aug. 2011

balance might affect the photocatalytic activity of La-Eu/ TiO2 catalyst; so more OH  would be adsorbed onto the surface for charge balance. These OH  on the surface can accept holes generated by light irradiation to form hydroxyl radicals, which oxidize adsorbed substrates[35]. So modification of the surface state of the catalyst might cause effective separation of electron-hole pairs. As the content of doping ions increases, the surface barrier becomes higher, and the space charge region becomes narrower. The electron-hole pairs within the region are efficiently separated by the large electric field before recombination, which led to the higher photocatalytic activity. 2.2.3 Effect of calcination temperature To study the effect of catalyst structure on photodegradation, the samples should be calcined at different temperatures that show different crystal structures. It is shown in Fig. 9 that the highest degradation was achieved with the sample calcined at 600qC, the XRD demonstrates that the peak intensity of anatase increases and the width of (101) plane becomes narrow. The anatase structure has a better photocatalytic activity. The catalytic activity of La-Eu/TiO2 declines when calaining temperature was raised to 900qC, the rutile structure formed, but the decolor rate of MB can still reach 91% after irradiation for 120 min. So doping La3+ and Eu3+ can enhance the stability of the catalyst. 2.2.4 Effect of pH value of MB solution pH value is an important variable in the evaluation of aqueous phase mediated photocatalytic degradation reactions. It influences adsorption and dissociation of the substrate, catalyst surface charge, oxidation potential of the valence band and other physico-chemical properties. The effect of pH value of MB degradation is illustrated in Fig. 10. It can be seen that the highest photocatalytic activity can be reached when the pH of MB solution is 11, the high degradation rate in the alkaline pH condition due to the MB is a cationic dye, the catalyst surface has negative charge in the alkaline condition, which benefit absorption of MB. 2.2.5 Effect of catalyst dosage The effect of the dosage of La-Eu/TiO2 on MB degradation is illustrated in Fig. 11. It is expected that MB degradation increases with increasing the

Fig. 9 Effect of calcination temperature of La-Eu/TiO2 catalyst ([MB]o 40 mg/L, La(0.5%)-Eu(1.0%)/TiO2 dosage 0.2 g/50 ml)

SHI Huixian et al., Preparation and photocatalytic activity of La3+ and Eu3+ co-doped TiO2 nanoparticles: photo-assisted …

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References:

Fig. 10 Effect of pH value of MB ([MB]o 40 mg/L, La(0.5%)-Eu(1.0%)/TiO2 0.2 g, calcined at 600qC



Fig. 11 Effect of catalyst dosage ([MB]o 40 mg/L, La(0.5%)-Eu(1.0%)/TiO2, calcined at 600qC

dosage of La-Eu/TiO2, and reaches a maximum of 100% at a dosage of 0.2 g/50 ml. However, a further increase of catalyst dosage slightly reduces the photodegradation efficiency. It is known that the photodegradation rate of the solution is affected by not only the active sites but also the photo-absorption of the catalyst. Adequate dosage of the photocatalyst increases the generation rate of electron-hole pairs to enhance photodegradation, but overload of the photocatalysts will decrease the light penetration by the catalyst suspension and reduce the degradation rate.

3 Conclusions La-Eu/TiO2 photocatalyst was prepared by sol-gel method. The presence of Ti–O–La and Ti–O–Eu bands could inhibit the transformation of anatase phase to rutile phase in TiO2 lattice. The DRS spectra of La-Eu/TiO2 showed red shift in the band-gap transition. The overall photocatalytic activity for MB degradation under UV light irradiation was significantly enhanced by co-doping La3+ and Eu3+, and the photocatalytic activity of La-Eu/TiO2 was higher than naked TiO2. The increase in photocatalytic activity of the La-Eu/TiO2 was due to increasing the BET surface area, red shift, decreased crystallite size and prevention from electron-hole recombination.

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