Lanthanum and boron co-doped BiVO4 with enhanced visible light photocatalytic activity for degradation of methyl orange

JOURNAL OF RARE EARTHS, Vol. 31, No. 9, Sep. 2013, P. 878

Lanthanum and boron co-doped BiVO4 with enhanced visible light photocatalytic activity for degradation of methyl orange WANG Min (⥟ ᬣ)*, CHE Yinsheng (䔺ᆙ⫳), NIU Chao (⠯ 䍙), DANG Mingyan (‫ܮ‬ᯢች), DONG Duo (㨷 ໮) (College of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110165, China) Received 24 April 2013; revised 6 July 2013

Abstract: BiVO4 photocatalysts co-doped with La and B were prepared by sol-gel method using citric acid as chelate. The samples were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brumauer-Emmett-Teller (BET), UV-Vis diffuse reflectance spectra (DRS) and the photocatalytic activity was investigated by photocatalytic degradation of methyl orange (MO). The results showed that boron and lanthanum ions incorporated into the lattice of BiVO4, and co-doping led to more surface oxygen vacancies, high specific surface areas, small crystallite size, narrow band gap and intense light absorbance in visible region. And the doped La(III) ions could help the separation of photogenerated electrons. Compared with BiVO4 and B-BiVO4, the photocatalytic activity of La-B co-doped BiVO4 was remarkably improved due to the synergistic effects of the co-doped ions. The degradation rate of MO in 60 min was 98.4% when La doping content was 0.05 mol.%, which was much higher than that of pure BiVO4(20%) and B-BiVO4(37%). Keywords: La-B-codoped; BiVO4; photocatalyst; visible light; methyl orange solution; rare earths

Semiconductor photocatalytic processes have been widely applied as techniques of destruction of organic pollutants in wastewater and effluents. Of various semiconductors, TiO2 is a good photocatalyst because of its low cost, non-toxicity and high stability in aqueous solutions[1,2]. However, its band gap is considerably large (E= 3.20 eV), thus it can be excited only by ultraviolet light at a wavelength below 387.5 nm, which accounts for only 4% of the incoming solar energy. Visible light occupies the largest portion of the solar spectrum and is an artificial light source. Therefore, the development of a visible-light-driven photocatalyst with high activity would be highly beneficial to the field of photocatalysis. A significant number of novel semiconductor photocatalysts with visible-light response have been developed. BiVO4 is one of the non-titania-based visible light driven semiconductor photocatalysts that has attracted considerable attention in the field. It has three structure types: zircon tetragonal, scheelite monoclinic, and scheelite tetragonal[3]. Previous studies showed that monoclinic BiVO4 is the only semiconductor that exhibits photocatalytic ability under visible light irradiation. Several methods have been reported for the synthesis of BiVO4, such as solid-state reaction[4], co-precipitation processes[5], hydrothermal treatment[6], and sonochemical synthesis. However, the activity of pure BiVO4 is low because of its poor adsorptive performance and the low migration of photogenerated electron-holes pairs[7–9].

Studies on improving the activity of pure BiVO4 have used metals or metal oxides doping, including Co/ BiVO4 [8–10], Pd/BiVO4 [9], Fe/BiVO4 [11], AgO/BiVO4 [12–15], WO3/BiVO4 [16], V2O5/BiVO4 [17], CuO/BiVO4 [18], Bi2O3/ BiVO4 [19], Au/BiVO4 [20], and Eu/ BiVO4 [21]. Metals or metal oxides doping effectively restricts the recombination of photo-generated electron-holes pairs and thus enhances the photocatalytic performance. On the other hand, the nonmetal doping seems to be also a promising method of achieving higher visible-light activity in semiconductors. There are many preparation methods for dopants such as B, N, F, S and halogen atoms[22–25]. Furthermore, co-doped photocatalysts with two or more ions have been investigated and attracted much attention. The photocatalytic activity of photocatalysts co-doped with some ions is better than that of the photocatalysts with single ion, because of the synergistic effect of co-doped ions[26–28]. In the present study, we prepared lanthanum and boron co-doped BiVO4 photocatalyst by sol-gel method, with the aim to explore possible synergistic advantages arising from the simultaneous presence of the dopants. To the best of our knowledge, there are no records on lanthanum and boron co-doped BiVO4. Herein we reported on the synthesis, characterization and application of La-BBiVO4 prepared by sol-gel method using citric acid as chelate. The La-B-BiVO4 samples showed better photocatalytic activity towards degradation of MO solution

Foundation item: Project supported by Program of National Natural Science Foundation of China for Youth (21207093, 51004072) * Corresponding author: WANG Min (E-mail: [email protected]; Tel.: +86-24-24680345) DOI: 10.1016/S1002-0721(12)60373-1

WANG Min et al., Lanthanum and boron co-doped BiVO4 with enhanced visible light photocatalytic activity for …

than B-BiVO4 and La-BiVO4 under visible light irradiation. The promoting effects of the dopants on photocatalytic activity of BiVO4 were studied. Degradation pathway for decomposition of MO solutions was proposed based on characterization results.

1 Experimental 1.1 Photocatalysts preparation In a typical preparation process, 0.01 mol of Bi(NO3)3·5H2O was dissolved in 50 mL of 10% (w/w) HNO3. Then 0.02 mol of citric acid was added and thus forming solution A. NH4VO3 was dissolved in 80 °C distilled water, and 0.02 mol of citric acid was added to form solution B. Solution A was then added drop-wise into solution B with continual magnetic stirring. The required amount of HBO3 (0.0004 mol) and La(NO3)3 was added to solution C. Under vigorous stirring, the pH of the mixture was adjusted to approximately 6.5 using an ammonia solution. The mixture was stirred at 80 °C until the dark blue sol-gel was obtained and then dried at 80 °C in a drying cabinet for 10 h to produce the BiVO4 precursor. The resulting powder was collected and calcined in air at 500 °C for 5 h, cooled to room temperature, and then crushed to obtain La-B-doped BiVO4 nano-particles. Pure BiVO4 was prepared by the same way in the absence of H3BO3 and La(NO3)3. And B-doped BiVO4 was also prepared by the same way in the absence of La(NO3)3, denoted as B-BiVO4. The lanthanum doping concentration (x%) was chosen as 0.01, 0.02, 0.05 and 0.08, which was the mole percentage of La in BiVO4. The obtained photocatalysts with corresponding La concentration were denoted as xLa-B-BiVO4.

evaluated by the photocatalytic degradation of MO. The experiments were carried out in 250 mL of glass reactor, and a 250 W halogen lamp equipped with a cutoff filter smaller than 420 nm was used as the light source which was placed at about 14 cm from the reactor. In each run, 15 mg catalyst was added into 50 mL MO solution of 15 mg/L. Prior to irradiation, the suspension was kept in the dark under stirring for 60 min to ensure the establishment of an adsorption/desorption equilibrium. At given time intervals, the collected samples, after centrifugation and then were filtered through a 0.45 m Millipore filter to remove catalyst particles. The filtrate concentration was determined by recording the absorbance at 464 nm using a UV-1800 UV-Vis spectrophotometer (Puxi, China).

2 Results and discussion 2.1 XPS analysis The binding energy of La3d at 830.9 eV for 0.05La-BBiVO4 sample (as shown in Fig. 1(a)) suggests that La species are present as La(III) oxidation states[11]. From Fig. 1(b), the Bi4f spectra of BiVO4 consist of two strong symmetrical peaks at Eb=159.1 and 164.4 eV, corresponding to the Bi4f 7/2 and Bi4f 5/2 signals, respectively. And the Bi4f spectra of B-BiVO4 and 0.05La-BBiVO4 also consist of two strong symmetrical peaks at Eb=159.8 and 165.1 eV for B-BiVO4, Eb=159.6 and

1.2 Characterizations and measurements The crystal phases of the samples were analyzed by X-ray diffraction (XRD) with Cu K radiation (model D/max RA, Rigaku, Japan). The accelerating voltage and the applied current were 40 kV and 150 mA, respectively. The morphology of the samples were investigated by a scanning electron microscope (SEM, S-3000N, Hitachi, Japan) coupled with an energy-dispersive X-ray spectrometer (EDX, Oxford Instrument). The BrunauerEmmett-Teller (BET) surface areas of the sample were obtained from nitrogen adsorption-desorption isotherms determined at liquid nitrogen temperature on an automatic analyzer (Autosorb-iQ-MP, Quantachrome, USA). The diffuse reflectance spectra (DRS) were obtained for the drypressed disk samples using a UV-Vis spectrophotometer (TU-1901, Puxi, China) equipped with an integrating sphere assembly using BaSO4 as the reflectance standard. 1.3 Photocatalytic activity tests The activities of the as-synthesized samples were

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Fig. 1 XPS spectra of La3d (a) and Bi4f (b)

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164.9 eV for 0.05La-B-BiVO4, corresponding to the Bi4f7/2 and Bi4f5/2 signals, which were characteristic of the Bi3+ species[13]. From Fig. 2 (a, b and c), the asymmetric V2p3/2 signals are decomposed into two peaks at Eb=516.1 and 516.7 eV for BiVO4 (a), Eb=515.5 and 516.4 eV for B-BiVO4 (b), Eb=515.8 and 516.6 eV for 0.05La-BBiVO4 (c), being attributed to the surface V4+ and V5+ species, respectively[13]. O1s XPS spectra of BiVO4, B-BiVO4 and 0.05La-B-BiVO4 samples are shown in Fig. 3(a), (b) and (c), respectively. With respect to O1s of BiVO4, B-BiVO4 and 0.05La-B-BiVO4 samples, the asymmetric peaks can be fitted with two peaks at different positions. The asymmetric peak centered at 530 eV was decomposed into two components at Eb=529.6 and 531.4 eV for BiVO4, Eb=529.4 and 531.2 eV for BBiVO4, Eb=529.7 and 530.1 eV for 0.05La-B-BiVO4 and these are due to the surface lattice oxygen (O latt) and the adsorbed oxygen (O ads) species[13], respectively. The

surface O ads/O latt molar ratios of samples are 0.14 for B-BiVO4, 0.28 for B-BiVO4 and 0.50 for 0.05La-BBiVO4. From the result, the 0.05La-B-BiVO4 contains larger amount of surface oxygen vacancies, and the presence of oxygen vacancies may be advantageous for the enhancement in photocatalytic performances of the BiVO4, as confirmed by the activity data shown in Section 2.4. B1s XPS spectra of B-BiVO4 and 0.05La-B-BiVO4 samples are shown in Fig. 4(a) and (b), respectively. With respect to B1s of B-BiVO4 and 0.8Eu-B-BiVO4 samples, the asymmetric peaks can be fitted with two peaks at different positions. The asymmetric peak centered was decomposed into two components at Eb=184.2 and 190.8 eV for B-BiVO4, Eb=184.8 and 189.7 eV for 0.05La-B-BiVO4. The peaks at 184.2 and 184.8 eV are assigned to B4C[29] which had no photocatalytic activity. The standard binding energies for B1s in B2O3 or H3BO3 (B–O bond), and in VB2 (V–B bond) are 193.6, 193.0,

Fig. 2 V2p XPS spectra of BiVO4 (a), B-BiVO4 (b) and 0.05La-B-BiVO4 (c)

Fig. 3 O1s XPS spectra of BiVO4 (a), B-BiVO4 (b) and 0.05La-B-BiVO4 (c)

Fig. 4 B1s XPS spectra of B-BiVO4 (a) and 0.05La-B-BiVO4 (b)

WANG Min et al., Lanthanum and boron co-doped BiVO4 with enhanced visible light photocatalytic activity for …

188.3–190.1 eV[29] respectively. So, there is no doubt that the boron atoms were not bonded by means of B–V–B bond or B–O bond. It may be concluded that a part of B might be doped in BiVO4 lattice and replaced the O to form the O–Bi–B bond. But the binding energy of B1s decreased after lanthanum doping. It may be because that the doped La3+ ions substituted part of Bi3+ ions and form O–La–B band and the electron cloud density around B decreased because the electro-negativity of La3+ (1.1 Pauling electro-negativity scale) was less than that of Bi3+ (1.9 Pauling electro-negativity scale). The XPS results show that both boron and europium are successfully doped into the BiVO4 lattice. 2.2 XRD and SEM analysis The crystalline phases of B-BiVO4 and B-La co-doped BiVO4 samples were analyzed by XRD. Fig. 5 gives the XRD patterns of the different samples. The properties of the prepared photocatalysts are summarized in Table 1. As shown in Fig. 5, the diffraction peaks of all samples are ascribed to the monoclinic BiVO4 (JCPDS cards No. 75-1866). Furthermore, no peaks other than monoclinic BiVO4 were detected. One reason is that the concentration of doping species was so low that it cannot be detected by XRD. By using Bragg’s law (2dsin=), the crystal sizes of all samples calculated for (121) peak are shown in Table 1. As can be seen from Table 1, the crystallite sizes of B and La co-doped BiVO4 are slightly lower than that of B doped BiVO4, which indicates the occurrence of a slight lattice distortion in the structure of BiVO4. The dimension decrease in crystallite size may be caused by the substitution of the part of the Bi3+ (r=0.120 nm) site by La3+ (r=0.115 nm). The surface morphologies and particle sizes of pure BiVO4, B-BiVO4 and La-BiVO4 were observed by SEM. A spherical structure is observed for all samples, and the particles are uniform in size. There is a little difference in the morphologies and particle shapes of the samples between pure BiVO4 and B-BiVO4. After lanthanum dop-

Fig. 5 XRD patterns of different BiVO4 samples (1) B-BiVO4; (2) 0.01La-B-BiVO4; (3) 0.02La-B-BiVO4; (4) 0.05La-B-BiVO4; (5) 0.08La-B-BiVO4

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Table 1 Some selected properties of BiVO4 Sample

XRD analysis 3

SBET/

Band gap

2

D/nm

V/nm

(m /g)

energies/eV

BiVO4

49.52

309.21

2.09

2.29

B-BiVO4

51.08

310.4

3.59

2.11

0.01La-B-BiVO4

50.49

309.57

3.70

2.09

0.02La-B-BiVO4

50.45

309.86

3.77

2.08

0.05La-B-BiVO4

49.87

309.25

4.11

2.05

0.08La-B-BiVO4

45.42

308.93

4.55

2.02

ing, the sizes of some particles decrease which means that La doping can inhabit the particles growth, which is consisted with the results of XRD. The composition of 0.05La-B-BiVO4 sample is determined by energy dispersive X-ray spectroscopy (EDX) experiments. As shown in Fig. 6(d), the obvious signals for O, Bi and V were observed in the spectra together with weak signals of lanthanum, which means the lanthanum was doped into BiVO4 successfully. The BET specific surface areas of the as-prepared samples were measured using a nitrogen adsorption BET method (shown in Table 1). The specific areas of the La-B co-doped samples are slightly higher than that of B-BiVO4. 2.3 UV-Vis diffused reflectance spectra (DRS) UV-Vis diffuse reflectance spectra of La-B co-doped BiVO4 samples are shown in Fig. 6. It can be seen that the absorptions of boron doped and B, La co-doped BiVO4 were enhanced in visible light. The absorption of boron and lanthanum co-doped BiVO4 strengthens as the concentration of lanthanum doping increases. Lanthanum ion doping will produce lattice deformation and form vacancy, which will probably result in impurity state in the band gap of BiVO4. The existence of impurity state can narrow the band gap and improve the absorption[30]. Therefore, with the increasing of lanthanum doping concentration, the absorption is strengthened in the visible region. It is generally accepted that the photocatalytic activity of photocatalyst is determined by the adsorption ability of light, separation efficiency of charges and transfer rate to substrate. Consequently, the boron and lanthanum co-doping can provide prerequisite for the improvement of photocatalytic activity since the absorption ability of visible light for BiVO4 is strengthened. The band gaps (Eg, eV) for the different samples were calculated from the UV-Vis spectra using the equation Eg=1240/. As shown in the spectra, the absorption onsets located at different wavelengths and their corresponding band gap energies are summarized in Table 1. The band gap energy of B-BiVO4 catalysts is 2.11 eV, which is lower than that of pure BiVO4 (2.29 eV). It is indicated that B doping can decrease the band gap energy of BiVO4. With La3+ ions introduced into BiVO4, the

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Fig. 6 SEM images of pure BiVO4 (a), B-BiVO4 (b), 0.05La-BBiVO4 (c), and EDX of 0.05La-B-BiVO4 (d)

absorption curve of La-B co-doped BiVO4 is enhanced more, extending to 612 nm. The band gap is 2.09, 2.08, 2.05, and 2.02 eV for 0.01La-B-BiVO4, 0.02La-B-BiVO4, 0.05La-B-BiVO4, and 0.08La-B-BiVO4, respectively, decreased by increasing the lanthanum doping amounts.

dopant is 0.05 mol.%. And with increased La dosage, the photocatalytic activity increases when La dosage is below 0.05%. However, when La dosage is over 0.05%, the photocatalytic activity decreases with increased La dosage. The reasons of higher photocatalytic activity of lanthanum and boron co-doped BiVO4 may be described as follows: (1) The average crystal size of La-B-BiVO4 is smaller than those of B-BiVO4 and BiVO4 samples as shown in Table 1, which makes the co-doped samples have larger specific surface area and more active centers, and it is beneficial to the phtotocatalytic reaction. To interpret the enhanced photocatalytic activity of the doped BiVO4, a set of adsorption experiments were carried out to evaluate the extent of saturated adsorption. The saturated adsorption of methyl orange was 8.8, 10.1, 13.1, 15.2, 17.8, and 20.8% for BiVO4, B-BiVO4, 0.01La-BBiVO4, 0.02La-B-BiVO4, 0.05La-B-BiVO4, and 0.08LaB-BiVO4, respectively. It indicates that all doped catalysts show stronger adsorption capacities than pure BiVO4. It is well known that the photocatalytic activity is related with the adsorption of methyl orange and the improvement of the interfacial charge transfer reaction. Interfacial charge transfer is possible only when the donor or acceptor is pre-adsorbed before the photocatalytic reaction. When the dosage of lanthanum exceeded 0.05%, although the adsorption of methyl orange increased, the photocatalytic decreased. (2) Because of the special 4f electron configuration, the lanthanum ions doped in BiVO4 broadens response range of visible light (as shown in Fig. 7) and can improve the conversion efficiency of photoelectrons to the visible light range, which results in the improvement of photocatalytic activity. (3) La doping makes the B-BiVO4 contain higher amount of surface oxygen vacancies verified in XPS results, and the presence of oxygen vacancies may be advantageous for the enhancement in photocatalytic performances of the BiVO4. (4) The most important reason is that lanthanum and boron have synergistic effects on improving the photocatalytic activity under visible light irradiation. The

2.4 Photocatalytic activity Photodegradation of MO was employed to evaluate the photocatalytic activities of the as-prepared samples. Fig. 6(a) shows the degradation rate of MO over pure BiVO4, B-doped BiVO4 and La-B-doped BiVO4 samples under visible light irradiation for 50 min. The blank test demonstrates that the degradation of MO is extremely slow without photocatalyst. Compared with pure BiVO4, the MO photocatalytic degradation rate of B-doped sample is higher. When a small amount of lanthanum is introduced into B-BiVO4, the activities of the prepared samples are enhanced. It indicates that the La-B-BiVO4 exhibits the best photocatalytic activity. The photo-degradation rate of MO can reach to 98% or so when La

Fig. 7 UV-Vis absorption spectra of some samples (1) Pure BiVO4; (2) B-BiVO4; (3) 0.01La-B-BiVO4; (4) 0.02La-B-BiVO4; (5) 0.05La-B-BiVO4; (6) 0.08La-B-BiVO4

WANG Min et al., Lanthanum and boron co-doped BiVO4 with enhanced visible light photocatalytic activity for …

doped boron replaces the oxygen atoms resident within the BiVO4 lattice and creates an impurity level which is just above the valence band of BiVO4. Simultaneously, the doped lanthanum ions act as shallow electron-trapping centers. Under the visible light irradiation, photons from visible light irradiation were utilized to generate electrons and holes. The electrons were excited from the B impurity level to the conduction bands, the O2 adsorbed on the surface captured the electrons to form O2–, which will react with the organic materials. Meanwhile, the B species could trap the part of photogenerated holes[22]. Thus, the recombination of photogenerated electrons and holes is suppressed. As a consequence, the quantum efficiency of the photocatalytic reaction catalyzed by co-doped BiVO4 is improved. While lanthanum dopant content exceeds 0.05 mol.%, La(III) becomes the recombination centers of the photoinduced electrons and holes, which is detrimental to photocatalytic reaction. In order to know the kinetics reaction of MO photodegradation by doping samples, relationship between ln(C0/C) (C0 and C are MO initial concentration and concentration after photo-degradation, respectively) and photodegradation time are measured (Fig. 8(b)). According to Fig. 8(b), since ln(C0/C) versus t exhibits nearly linear relation, which corresponds to Langmuir-Hinshlwood

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reduced equation[31], MO photo-degradation nearly follows the first order reaction kinetics model: ln(C0/C)=k1t, where C0 and C are the initial and remnant MO concentrations, respectively; k1 is the apparent rate constant, t is the reaction time. The apparent rate constant obtained from various catalyst samples with different europium contents are listed in the inset of Fig. 8(b). The larger the constant, the faster the reaction rate will be. That is, compared with the single doping, synergistic effect of La and B co-doping may accelerate the photo-degradation reaction.

3 Conclusions The La-B-BiVO4 photocatalysts were successfully prepared by sol-gel method using citric acid as chelate. It was found that lanthanum(III) ions and boron species were successfully doped into the crystal lattice of BiVO4. The La and B co-doped BiVO4 photocatalyst showed more surface oxygen vacancies, high specific surface areas, small crystallite size, narrow band gap and intense light absorbance in visible region, which contributed to their high photocatalytic activity for the degradation of methyl orange under visible irradiation. The synergistic effects of lanthanum and boron might efficiently promote the separation of photogenerated holes and electrons, and were the main reasons for high photodegradation of MO under visible light irradiation.

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Fig. 8 MO degradation under visible light illumination for 50 min in the presence of B-BiVO4 with various europium doping, pure BiVO4 and without photocatalyst (a) and ln(C0/C) versus time for B-BiVO4 doping concentration (b) (1) Blank; (2) Pure BiVO4; (3) B-BiVO4; (4) 0.01La-B-BiVO4; (5) 0.02La-B-BiVO4; (6) 0.05La-B-BiVO4; (7) 0.08La-B-BiVO4

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