Enhanced Photocatalytic Activity of Ag3VO4 Loaded with Rare-Earth

Ind. Eng. Chem. Res. 2009, 48, 10771–10778

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Enhanced Photocatalytic Activity of Ag3VO4 Loaded with Rare-Earth Elements under Visible-Light Irradiation Hui Xu,† Huaming Li,*,†,‡ Li Xu,† Chundu Wu,† Guangsong Sun,† Yuanguo Xu,† and Jinyu Chu† Schools of Chemistry and Chemical Engineering, Jiangsu UniVersity, Zhenjiang 212013, P. R. China, and Department of Chemistry, Hainan Normal UniVersity, Haikou 571158, P. R. China

Rare-earth-loaded Ag3VO4 catalysts (RE3+-Ag3VO4, RE ) Nd, Sm, and Eu) were prepared by the wetness impregnation technique and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of the samples were evaluated by rhodamine B (RhB) dye degradation under visible-light irradiation. It was found that the presence of rare-earth oxides in the host Ag3VO4 could decrease the band gap and accelerate the separation of photogenerated electron-hole pairs, which led to higher photocatalytic activities. Among the rare-earth-loaded samples, Nd3+-loaded Ag3VO4 catalyst showed the highest photocatalytic activity. The optimum contents of Nd3+, Eu3+, and Sm3+ were found to be 2, 3, and 2 wt %, respectively. It was also found that the photocatalytic degradation of RhB over RE3+-Ag3VO4 (2 wt %) followed pseudo-first-order kinetics. A possible mechanism for RhB photocatalytic degradation over RE3+-Ag3VO4 catalysts is also proposed. 1. Introduction Photocatalytic degradation of organic pollutants has attracted considerable attention owing to its promising role in environmental protection during the past decade. Among semiconductor photocatalysts, TiO2 has proven to be an excellent catalyst for the decomposition of toxic organic contaminants.1,2 However, TiO2 has a large band gap (3.2 eV) and absorbs only UV light, which restricts its wide practical application.3 With the purpose of efficiently utilizing natural sunlight, it is indispensable to design and exploit novel types of visible-light-induced photocatalysts. Recently, it was found that Ag3VO4 with a monoclinic structure exhibits photocatalytic activity for splitting water and decomposing organic pollutants under visible-light irradiation.4,5 Yet, its large-scale application is still limited because of the high electron-hole recombination rate in the photocatalytic reaction. Therefore, to effectively eliminate the recombination of electrons and holes, it is necessary to design methods to solve this important and challenging issue. Surface modification by cocatalyst doping is a typical approach to improving the photocatalytic activity of semiconductors. Many studies in the literature have reported that cocatalysts loaded on TiO2-based photocatalysts play an essential role in increasing the separation rate of photogenerated holes and electrons, such as modifications with metals, nonmetals, and other semiconductor oxides.6-11 Up to now, lanthanide ions have been considered to be the ideal cocatalyst materials, because lanthanide ions can form complexes with various Lewis bases (e.g., acids, amines, aldehydes, alcohols, thiols) through interactions of these functional groups with the f orbitals of the lanthanides.12,13 Ranjit et al.12,13 indicated that the enhanced degradation efficiency over Ln2O3/ TiO2 photocatalysts was attributed to the formation of the Lewis acid-base complex between the lanthanide ions and the substrates on the surface of photocatalyst. Xu et al.14 found that TiO2 doped with seven different rare-earth ions (Gd, La, Ce, Er, Pr, Nd, and Sm) could enhance the photocatalytic activity * To whom correspondence should be addressed. Tel.: +86-51188791800. Fax: +86-511-88791708. E-mail: [email protected]. † Jiangsu University. ‡ Hainan Normal University.

as compared to bare TiO2 for nitrite degradation. It has been demonstrated that the photocatalytic activity of Ce3+-TiO2 catalysts in 2-mercaptobenzothiazole (MBT) degradation was significantly enhanced because of a higher adsorption capacity and better separation of electron-hole pairs.15 In the case of the photocatalytic degradation of dyes, TiO2 photocatalysts doped with rare-earth ions (Sm3+, Nd3+, Pr3+) exhibited high photocatalytic activities.16 Rengaraj et al.17 also confirmed that Nd-modified TiO2 showed higher photocatalytic activity than pure TiO2 for the reduction of Cr(VI). Moreover, Parida and Sahu18 indicated that Ln3+-TiO2 (where Ln ) La, Nd, and Pr) containing 0.4 mol % lanthanum exhibited the highest photocatalytic activity in the reduction of hexavalent chromium and degradation of methylene blue. In addition, rare-earth ion-/ oxide-doped non-TiO2 catalysts also show enhanced photocatalytic activities for water splitting and pollutant degradation. Kudo and Kato19 reported that the activities of water splitting over NiO-NaTaO3 photocatalysts were improved by doping of lanthanides (Pr, Nd, Sm, Gd, Tb, and Dy). Anandan et al.20 found that 0.8 wt % La-doped ZnO showed a high relative photonic efficiency and high photocatalytic activity for the degradation of monocrotophos (MCP). All of the above results indicate that rare-earth ions can promote the separation of photoexcited electron and holes during photocatalytic reactions. Therefore, it can be inferred that loading proper rare-earth ions/oxides as cocatalysts should enhance the activities of photocatalysts. In this article, three different rare-earth-loaded Ag3VO4 catalysts (RE3+-Ag3VO4) were prepared through wetness impregnation, and their photocatalytic performance was tested by rhodamine B (RhB) dye degradation under visible-light irradiation. The relationship between the photocatalytic activities and the structural features of the prepared catalysts were investigated through a systematic characterization analysis, including X-ray diffraction (XRD), scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS). The mechanism by which rare-earth ions enhance photocatalytic activities is also discussed.

10.1021/ie900835g CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2009

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2. Experimental Section 2.1. Synthesis of the Different Ag3VO4-based Photocatalysts. Silver vanadate powder was prepared by a precipitation reaction as reported by Hu et al.5 The rare-earthloaded Ag3VO4 catalysts were prepared by impregnating Ag3VO4 with aqueous solutions of the rare-earth nitrates, which were obtained by dissolving the rare-earth oxides (Nd2O3, Eu2O3, and Sm2O3) in concentrated nitric acid. For each rare-earth-loaded catalyst, 0.5 g of Ag3VO4 powder and a suitable rare-earth nitrate solution were put into a ceramic dish. Then, the suspension was stirred using a glass rod during evaporation of the water in a water bath. The obtained powers were calcined at 300 °C for 4 h. The pure Ag3VO4 catalyst was also calcined at 300 °C for 4 h. 2.2. Photocatalyst Characterization. The crystalline phases of the prepared catalysts were analyzed by X-ray diffraction (XRD) on a Bruker D8 diffractometer with Cu KR radiation (λ ) 1.5418 Å) in the range of 2θ ) 10-80°. XPS measurements were performed on an ESCALab MKII spectrometer using Mg KR radiation. The surface morphology and particle size of the samples were determined on a field-emission scanning electron microscope (model JEOL JSM-7001F). The elemental analysis of the photocatalysts was detected by an energy-dispersive X-ray spectrometer attached to the scanning electron microscope. Diffuse reflectance spectra were obtained on a UV-2450 (Shimadzu) instrument in the range of 300-800 nm. BaSO4 was used as the reflectance standard material. 2.3. Photocatalytic Activity. The photocatalytic activities of RE3+-Ag3VO4 catalysts were evaluated by degradation of RhB dye. Experiments were carried out in a Pyrex photocatalytic reactor with two 150-W tungsten halogen lamps as the visiblelight source. For each experiment, 0.075 g of RE3+-Ag3VO4 was added to 150 mL of a solution containing the RhB dye (10 mg L-1). Prior to visible-light irradiation, the suspension was stirred strongly with a magnetic stirrer for 30 min in the dark in order to achieve adsorption/desorption equilibrium. At specific time intervals, 5 mL of the suspension was removed from the Pyrex reaction glass and analyzed after centrifugation. The photocatalytic degradation efficiency (E) of RhB was calculated by the formula C0 - C A0 - A E) × 100% ) × 100% C0 A0 where C0 is the adsorption equilibrium concentration of RhB, C is the concentration of RhB solution at time t, and A0 and A are the corresponding values for the absorbance. Adsorption behavior studies of the catalysts were performed in the dark. A fixed amount of the adsorbent (0.075 g) was added to 150 mL of RhB solution (10 mg L-1) and then strongly stirred with a magnetic stirrer. After 30 min, the absorbance of RhB was determined by spectrophotometer at 553 nm. The adsorption capacity of the catalyst (D) was calculated from the equation D)

Aorigin - Aequilibrium × 100% Aorigin

where Aorigin and Aequilibrium are the initial RhB absorbance and the RhB absorbance at equilibrium, respectively. The intermediates and the original dye were detected by highperformance liquid chromatography (HPLC) using an Agilent 1200 instrument with an XDB-C18 column (5 µm, 4.6 × 150 mm) held at 30 °C.

Figure 1. (A) XRD pattern of pure Ag3VO4 and 2 wt % RE3+-Ag3VO4 samples. (B) Diffraction peak positions of the (1j21) and (121) planes in the range of 30-33°.

3. Results and Discussion 3.1. XRD Analysis. Figure 1 shows the XRD patterns of RE3+-loaded Ag3VO4 samples. It can be seen that the RE3+loaded Ag3VO4 samples exhibit the same XRD patterns as does pure Ag3VO4. The crystalline phase of all catalysts is the monoclinic phase (JCPDS no. 43-0542), except for a peak of Ag2O impurity at 32.99°, which was confirmed by Hu and Hu’s report.5 However, pure Ag2O has no photocatalytic activity for dye degradation, so the presence of the Ag2O does not influence the photocatalytic activity of Ag3VO4. A careful comparison of the (1j21) and (121) diffraction peaks (in the range of 2θ ) 30-33°) of the samples is shown in Figure 1B. It can be seen that the peak position of RE3+-doped Ag3VO4 is almost unchanged compared to that of pure Ag3VO4. The lattice parameters calculated according to the XRD results reveal the influence of the rare-earth-ion doping on the crystalline structure of Ag3VO4 (Table 1). It can be seen that there is no evident difference in the lattice parameters a, b, and c between undoped Ag3VO4 and RE3+-loaded Ag3VO4, which suggests that the rareearth ions are not doped into the lattice of Ag3VO4. In fact, typically, during wet impregnation, the additives are not expected to replace atoms in the lattice of the catalyst support. Xu et al.14 reported that rare-earth metal salts can be changed into rare-earth oxides (RE2O3) during the calcination process. Therefore, it can be inferred that the RE3+ ions were dispersed


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3+

Table 1. Physical Characteristics of Pure Ag3VO4 and RE -Loaded Ag3VO4 Catalysts and RhB Equilibrium Adsorption Efficiency (D) before Photocatalytic Reactions lattice parameters photocatalyst

loading amount (wt %)

a (Å)

b (Å)

c (Å)

D (%)

pure Ag3VO4 Nd3+-Ag3VO4 Eu3+-Ag3VO4 Sm3+-Ag3VO4

2 2 2

8.701 8.696 8.729 8.728

6.717 6.715 6.699 6.672

6.510 6.538 6.529 6.515

3.1 13.8 12.3 13.2

on the surface of Ag3VO4 in the form of small RE2O3 clusters. However, no diffraction peaks of rare-earth oxides appeared when the samples were calcined at 300 °C, which might be because the content of rare-earth oxides was below the detection limit of XRD analysis. 3.2. Scanning Electron Microscopy and EDS Analysis. The typical morphologies of the synthesized Ag3VO4 and rareearth-loaded Ag3VO4 photocatalyst samples are shown in Figure 2. The particles in the pure Ag3VO4 powder were spherical shape and had sizes in the range of 50-500 nm. In the micrographs of the RE3+-Ag3VO4 samples, the particles have irregular shapes with smooth surfaces. In addition, many small particles can be clearly observed on the face of the host Ag3VO4, which are speculated to be rare-earth oxides. According to the results of the EDS analysis, rare-earth ions were present in the RE3+Ag3VO4 samples. 3.3. XPS Study. To investigate the chemical states of the samples, XPS measurements were performed on the Nd3+Ag3VO4, Eu3+-Ag3VO4, Sm3+-Ag3VO4, and pure Ag3VO4 catalysts, as shown in Figure 3. The results indicate that Ag, V, and O elements existed on the surface of the pure Ag3VO4 (Figure 3a), whereas Ag, V, O, and Nd were present on the surface of Nd3+-Ag3VO4 (Figure 3b). In Figure 3b, the Ag 3d, V 2p, O 1s, C 1s, and Nd 3d binding energies are at 367.9, 516.5, 530.0, 284.6, and 975.6 eV, respectively. In the case of

Figure 3. XPS spectra of Nd3+-Ag3VO4, Eu3+-Ag3VO4, Sm3+-Ag3VO4, and pure Ag3VO4 samples.

the Eu3+-Ag3VO4 and Sm3+-Ag3VO4 catalysts, the appearance of the Ag 3d peak at 367.9 eV, the V 2p peak at 516.7 eV, and the O 1s peak at 530.2 eV confirmed the existence of Ag+, V5+, and O2- (Figure 3c and 3d). The presence of the element C can be ascribed to residual carbon (C 1s at 284.6 eV). Highresolution scanning XPS spectra of Ag 3d, V 2p, O 1s, Nd 3d, Eu 3d, and Sm 3d are shown in Figure 4A-E, respectively. The four products (pure Ag3VO4 and RE3+-Ag3VO4) exhibit similar XPS peaks of Ag 3d, V 2p, and O 1s, as shown in Figure 4A and 4B. As for the XPS spectrum of the Nd 3d in the case of Nd3+-loaded Ag3VO4 catalyst (Figure 4C), the binding energy of around 975.6 eV can be ascribed to Nd2O3. It can be seen that the Eu3+-loaded Ag3VO4 photocatalyst exhibited a characteristic band at 1134.6 eV due to Eu2O3 (Figure 4D), which

Figure 2. SEM images of (A) pure Ag3VO4, (B) Nd3+-Ag3VO4, (C) Sm3+-Ag3VO4, and (D) Eu3+-Ag3VO4.

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Figure 4. XPS spectra of Nd3+-Ag3VO4, Eu3+-Ag3VO4, Sm3+-Ag3VO4, and pure Ag3VO4 catalysts: (A) Ag 3d peak, (B) O 1s and V 2p peaks, (C) Nd 3d peak, (D) Eu 3d peak, and (E) Sm 3d peak.

agrees with the results previously reported in the literature.12 The Sm 3d spectrum of Sm3+-Ag3VO4 shows a sharp peak centered around 1084.0 eV (Figure 4E) that is mainly assigned to the Sm2O3.21 Therefore, combined with the XRD and SEMEDS analyses these results suggest that Nd2O3, Sm2O3, and Eu2O3 were present on the surfaces of the corresponding RE3+loaded Ag3VO4 catalysts. 3.4. DRS Analysis. The DRS spectra of pure Ag3VO4 and RE3+-modified Ag3VO4 samples are shown in Figure 5A. In comparison with those of pure Ag3VO4, the optical absorption edges of the RE3+-Ag3VO4 samples are shifted toward longer wavelength, in the order Nd3+-loaded photocatalyst f Sm3+-loaded photocatalyst f Eu3+-loaded photocatalyst. This red shift in the optical absorption band is due to the presence of the rare-earth

ions, and it can be attributed to a charge-transfer transition between the lanthanide f electrons and the conduction or valence band of the host Ag3VO4. Compared with the other RE3+-Ag3VO4 samples, the Nd3+-Ag3VO4 photocatalyst had the highest absorption intensity and largest red shift, which indicates that the loading of Nd3+ should have the best photoresponse in the visible-light region, as well as the highest photocatalytic activity. The optical band gap of the Ag3VO4-based samples were estimated by the following equation (for direct-band-gap materials)22,23 (Ahν)2 ) hν - Eg where Eg is the band-gap energy. Plots of (Ahν)2 versus Eg of the photocatalsts are shown in Figure 5B. Extrapolating the lines to


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Figure 5. (A) UV-vis diffuse absorption spectra of photocatalysts (A) and estimated band gap of RE3+-Ag3VO4 (B).

Figure 6. Photocatalytic degradation efficiency of RhB on Ag3VO4 loaded with 2 wt % of different rare-earth elements under visible-light irradiation. Catalyst dosage ) 0.5 g/L, initial concentration of RhB ) 10 mg/L.

the Eg axis, the band-gap energies of Ag3VO4, Nd3+-Ag3VO4, Sm3+-Ag3VO4, and Eu3+-Ag3VO4 were estimated to be 2.34, 2.16, 2.18, and 2.21 eV, respectively. 3.5. Photocatalytic Activities of the Photocatalysts. To investigate the photocatalytic activities of the Ag3VO4-based samples, photocatalytic degradation experiments were carried out using RhB in an aqueous suspension under visible-light irradiation. Figure 6 shows the results of the photocatalytic degradation of RhB over the different rare-earth-loaded Ag3VO4 catalysts (Catalyst dosage ) 0.5 g L-1) with the same doped contents (2 wt %) for 60 min of visible-light irradiation. The three different rare-earth-loaded photocatalysts exhibited higher activities than pure Ag3VO4, indicating the promoting effect of the rare-earth modification. It can also be seen that the photocatalytic degradation efficiency of the Ag3VO4-based samples significantly exceeded that of the commercial photocatalyst P25-TiO2 (Figure 6) and also that the photocatalytic degradation efficiency of RhB over RE3+-Ag3VO4 was significantly higher than that of pure Ag3VO4 under UV-light irradiation. It has been reported in the literature that the photocatalytic activities of catalysts depend on many factors, such as crystallinity, surface properties, absorption properties, and so on.24 In this case, the enhanced photocatalytic activity of RE3+-Ag3VO4 can be explained in the following way: On one hand, the adsorption capacity (D) of the rare-earth-modified samples increased, as shown in Table 1. Generally, a high

adsorption amount would be beneficial to increasing the photocatalytic degradation efficiency of an organic substrate, such as RhB dye.25 On the other hand, it was observed that the presence of rare-earth ions shifts the absorption edge to a longer wavelength in the order Nd3+-Ag3VO4 > Sm3+-Ag3VO4 > Eu3+Ag3VO4 > Ag3VO4 (Figure 5), which is consistent with the order of the photocatalytic performances. A larger red shift indicates that a sample can absorb more photons. Therefore, the red shift in the absorption band is favorable for photocatalytic reaction. This finding is in agreement with the results of Xu et al14 and Fu et al.26 Moreover, for RE3+-loaded Ag3VO4 samples, the presence of lanthanide ions could increase the interfacial charge transfer and inhibit the recombination of electron-hole pairs. As for the rare-earth-modified TiO2 photocatalysts, lanthanide ions could act as effective electron scavengers to trap the conduction-band electrons of TiO2.15 In the presence of lanthanide-loaded Ag3VO4 catalysts, the rare-earth oxides present on the surface of the host Ag3VO4 could act as electron traps, which could enhance electron-hole separation and increase the quantum efficiency. Then, the trapped electrons could be transferred to the adsorbed O2 molecules to generate the O2radical ions, as shown in eqs 1 and 2 RE3+ + e- f RE2+

(1)

RE2+ + O2 f RE3++O2 -

(2)

Therefore, the photogenerated electrons could easily transfer from the interior to the surface of the catalyst, which would promote photocatalytic reaction. In addition, lanthanide ions are known for their ability to form complexes with various Lewis bases through the interaction of these functional groups with the f orbitals of the lanthanides.12,13 The incorporation of lanthanide ions could provide a way to concentrate organic pollutants on the catalyst surface. Therefore, the special role of the lanthanide ions might be another important reason for enhancing the photocatlytic reaction in the RE3+Ag3VO4 system. On the basis of the above analysis, it is suggested that the enhanced photocatalytic activity of rare-earthloaded Ag3VO4 could be a synergetic effect of many factors, including larger adsorption, red shifts to longer wavelengths, and enhanced rates of interfacial charge transfer. In the present study, it is worth mentioning that Nd3+modified Ag3VO4 exhibited the highest photocatalytic activity among all photocatalysts. This can be attributed to the fact that the highest amount of adsorption (Table 1) and the largest red-

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Figure 7. Kinetic fit for the degradation of RhB with pure Ag3VO4 and RE3+-Ag3VO4 catalysts (initial concentration of RhB ) 10 mg/L, catalyst amount ) 0.5 g/L).

shift in the optical absorption band were exhibited by the Nd3+loaded Ag3VO4 sample (Figure 5). To investigate the kinetics of RhB degradation over the Ag3VO4-based samples, a pseudo-first-order reaction model was applied to describe the experimental data as follows

( )

-ln

C ) kapt C0

where kap is the apparent reaction rate constant, C0 is the adsorption equilibrium concentration of RhB, t is the reaction time, and C is the concentration of RhB at reaction time t. It was found that RhB degradation over RE3+-Ag3VO4 was fitted by pseudo-first-order kinetics, as shown in Figure 7. It can be seen that the lanthanide-loaded samples exhibited higher photocatalytic degradation rates than both pure Ag3VO4 and P25TiO2. The blank experiment indicated that the photodegradation of RhB under visible-light irradiation without catalysts is insignificant. The apparent rate constant of pure Ag3VO4 was found to be 0.0122 min-1. For the Nd3+-loaded Ag3VO4 sample, the degradation rate attained a maximum value of 0.0573 min-1, which is 4.7 and 12.4 times higher than those of pure Ag3VO4 and P25-TiO2, respectively. The apparent reaction rate constants k for the other rare-earth-loaded catalysts were found to be 0.0498 and 0.0402 min-1 for the Eu3+- and Sm3+-loaded samples, respectively. The pseudo-first-order constants and relative coefficients are summarized in Table 2. For all of the rare-earth-loaded samples, there was an optimal dopant content. Figure 8 illustrates the photocatalytic activities of the Nd3+-, Eu3+-, and Sm3+-loaded Ag3VO4 samples with different rare-earth contents, and the optimum contents of Nd3+, Eu3+, and Sm3+ were 2, 3, and 2 wt %, respectively. It can be noted that the photocatalytic activity increased with increasing rare-earth loading up to the optimum values and then decreased when the loading content was further enhanced. The similar results obtained for the TiO2-based materials can be explained in terms of the influence of the space-charge layer thickness.14 Xu et al.14 indicated that there is an optimum concentration of dopant ions to make the thickness of space-charge layer essentially equal to the light penetration depth. In fact, the optimum content of the rare-earth ions in the RE3+-Ag3VO4 samples was found to be related to the recombination rate of photogenerated electrons and holes. When the lanthanide content was below its optimum value, the photodegradation efficiency

Figure 8. Photocatalytic degradation of RhB over RE3+-Ag3VO4 with different rare-earth elements: (A) Nd3+, (B) Sm3+, and (C) Eu3+. Table 2. Kinetic Constants (k) and Relative Coefficient (R2) for the Degradation of RhB under Visible-Light Irradiation photocatalysts

k (×102 min-1)

R2

P25-TiO2 pure Ag3VO4 2 wt % Nd3+-Ag3VO4 2 wt % Eu3+-Ag3VO4 2 wt % Sm3+-Ag3VO4

0.46 1.22 5.73 4.98 4.02

0.981 0.998 0.992 0.978 0.978

of RhB was low because fewer active trapping sites were available. The availability of fewer active sites could be detrimental to the separation of electron-hole pairs. On the


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Figure 9. Changes in UV-visible absorption spectra of RhB (initial concentration of RhB ) 10 mg/L) over different catalysts: (A) pure Ag3VO4 (B) Nd3+-Ag3VO4 (2 wt %), (C) Eu3+-Ag3VO4 (3 wt %), and (D) Sm3+-Ag3VO4 (2 wt %).

contrary, when the rare-earth content was over its optimum, the rare-earth oxides covering the surface of Ag3VO4 could act as recombination centers, thereby decreasing the photocatalytic activities of the samples. Therefore, the presence of the optimum rare-earth content should be due to a balance between the enhancement in the number of active trapping sites favoring the effective inhibition of the recombination of electron and holes and fewer trapped parts leading to a lower capacity for the separation of interfacial charge transfer. The temporal evolution of the absorption spectral changes during the photocatalytic degradation of RhB over the pure Ag3VO4 and RE3+-Ag3VO4 samples under visible-light irradiation are shown in Figure 9. It can be seen that photocatalytic degradation of RHB was quite slow and that the RhB could not be sharply degraded in the presence of pure Ag3VO4 after 60 min (Figure 9A). On the contrary, in the case of the RE3+modified samples, the characteristic absorption band of RhB decreased rapidly. The main absorption band almost completely disappeared after 60 min, as shown in Figure 9B-D. The color of the suspension changed from red to colorless (Figure 9B). In the HPLC chromatogram analysis (Figure 10), the peak with a retention time of 5.4 min, denoted peak a, is attributed to the initial RhB dye, and the other peaks correspond to intermediates. It can be seen that peak a disappeared quickly for the Eu3+Ag3VO4 catalyst. After 180 min of visible-light irradiation, the HPLC peaks of the RhB dye and all intermediates disappeared, indicating that the RhB and intermediates were completely degraded by the Eu3+-Ag3VO4 catalyst.

Figure 10. HPLC chromatograms of RhB solution at different irradiation intervals for the photocatalytic degradation process (in the role of Eu3+Ag3VO4).

Other organic dyes such as methyl orange (MO) and methylene blue (MB) were also quickly degraded by RE3+Ag3VO4 catalysts under visible-light irradiation. The results exhibited enhanced photocatalytic activities after the introduction of rare-earth ions.

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4. Conclusions 3+

RE -loaded Ag3VO4 samples were found to exhibit higher photocatalytic activities than pure Ag3VO4 because of the synergetic effect of their larger adsorption ability, red shifts to longer wavelengths, and specific characteristics of lanthanide ions. Nd3+-loaded Ag3VO4 showed the highest activity among all of the rare-earth-loaded samples studied. The optimum contents of Nd3+, Eu3+, and Sm3+ were found to be 2, 3, and 2 wt %, respectively. It was also found that the photocatalytic degradation of RhB over RE3+-Ag3VO4 (2 wt %) obeyed pseudo-first-order kinetics. The photocatalytic degradation efficiency of MB and MO dyes were also enhanced by introduction of rare-earth ions, indicating that the RE3+-Ag3VO4 catalysts could be applied to photocatalytic degradation of other organic pollutants as well. Acknowledgment The authors sincerely appreciate the financial support of this work from the Doctoral Innovation Fund of Jiangsu (CX08B142Z) and the National Nature Science Foundation of China (Nos. 20876071 and 20676057). Literature Cited (1) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced reactivity of titanium dioxide. Prog. Solid State Chem. 2004, 32, 33. (2) Rekoske, J. E.; Barteau, M. A. Kinetics and selectivity of 2-propanol conversion on oxidized anatase TiO2. J. Catal. 1997, 165, 57. (3) Ambrus, Z.; Bala´zs, N.; Alapi, T.; Wittmann, G.; Sipos, P.; Dombi, A.; Mogyoro´si, K. Synthesis, structure and photocatalytic properties of Fe(III)-doped TiO2 prepared from TiCl3. Appl. Catal. B: EnViron. 2008, 81, 27. (4) Konta, R.; Kato, H.; Kobayashi, H.; Kudo, A. Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates. Phys. Chem. Chem. Phys. 2003, 5, 3061. (5) Hu, X.; Hu, C. Preparation and visible-light photocatalytic activity of Ag3VO4 powders. J. Solid State Chem. 2007, 180, 725. (6) Choi, W.; Termin, A.; Hoffmann, M. R. The role of metal ion dopants in quantum-sized TiO2: Correlation between photoreactivity and charge carrier recombination dynamics. J. Phys. Chem. 1994, 98, 13669. (7) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visiblelight photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269. (8) Thompson, T. L.; Yates, J. T., Jr. Surface science studies of the photoactivation of TiO2sNew photochemical processes. Chem. ReV. 2006, 106, 4428. (9) Chen, X.; Mao, S. S. Titanium dioxide nanomaterials: Synthesis, properties, modifications, and applications. Chem. ReV. 2007, 107, 2891. (10) Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Band gap narrowing of titanium dioxide by sulfur doping. Appl. Phys. Lett. 2002, 81, 454.

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ReceiVed for reView May 21, 2009 ReVised manuscript receiVed August 8, 2009 Accepted September 17, 2009 IE900835G

Enhanced Photocatalytic Activity of Ag3VO4 Loaded with Rare-Earth

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