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The synthesize of lanthanide doped BiVO4 and its enhanced photocatalytic activity
Journal of Molecular Liquids 211 (2015) 25–30
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The synthesize of lanthanide doped BiVO4 and its enhanced photocatalytic activity Xiaoming Gao a,⁎, Zihang Wang a, Xiang Zhai a, Feng Fu a, Wenhong Li b a b
Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an University, Yanan, Shaanxi 716000, China Department of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, China
a r t i c l e
i n f o
Article history: Received 13 May 2015 Received in revised form 29 May 2015 Accepted 21 June 2015 Available online xxxx Keywords: Nd–BiVO4 composite Photocatalysis Phenol Desulfurization Photocatalytic mechanism
a b s t r a c t Lanthanide Nd doped BiVO4 composite with 2D long rod shaped structures was prepared by the facile synthesis method. The corresponding relationship was obtained among loaded content to phase, morphology, and optical absorption property of Nd–BiVO4 composite by UV–vis DRS, XRD, SEM, and EDS. The results shown that Nd loaded did not change the crystallinity of BiVO4 significantly, but enhanced the optical absorption ability for visible light and improved the morphologies and microstructure. The photocatalytic activities of the Nd–BiVO4 composite were evaluated for the photodegradation of phenol and desulfurization of thiophene under visible light irradiation. The results showed that Nd loaded greatly improved the photocatalytic activity of BiVO4, and the content of Nd had an obvious impact on the catalytic activity of BiVO4. The 0.8 wt.% Nd–BiVO4 composite exhibited the best photocatalytic activity in both the photodegradation of phenol and desulfurization of thiophene. Afterwards, the mechanism of enhanced photocatalytic activity of Nd–BiVO4 composite was also investigated. The determination of function of the active free radicals confirmed that hydroxyl free radical •OH was main active radicals during photocatalytic oxidation process. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor material has attracted comprehensive interest due to the photo-degradation of environmental pollutants and production of hydrogen from water under light irradiation [1–5]. However, many semiconductors can be excited only under ultraviolet light irradiation because of its relatively wide band gap and inefficient quantum yield, which hinders its further application in industrial process [6–8]. Up to date, a variety of strategies have been employed to improve the photocatalytic efficiencies of photocatalysts in the visible range by modification with nonmetal/metal element doping [9–11]. Unfortunately, the methods used are not completely controlled. Moreover, these dopants may become recombination centers between photogenerated electrons and holes. Evidently, the expected methods are somewhat limited [12–14]. Therefore, to explore more efficient photocatalyst with visible light responsiveness and thermal stability is urgent and indispensable. In recent, doping with lanthanide ions with 4f electron configurations can significantly enhance the photocatalytic activity of photocatalysts [15,16]. In this aspect, the partly filled 4f electron orbits in rare earth ions can form a new energy level between the valence and conduction band of photocatalysts, which results in the narrowed band gap. For example, Dua et al. synthesized a porous Nd-doped TiO2 monolith by sol–gel method. The result showed that Nd doping could ⁎ Corresponding author. E-mail address: [email protected] (X. Gao).
http://dx.doi.org/10.1016/j.molliq.2015.06.058 0167-7322/© 2015 Elsevier B.V. All rights reserved.
increase the TiO2 surface area and suppress the recombination of photo-produced hole/electron pairs, and Nd-doped TiO2 monolith showed better behavior than the other as-prepared samples and Degussa P25 [17]. Thomas et al. reported that samarium-doped anatase TiO2 with a higher photocatalytic activity was prepared via a low temperature hydrothermal route [18]. Hwang et al. found that the 4f orbits of rare earth ions greatly modified the TiO2 band structures resulting in better photocatalytic behavior [19]. As an important n-type photosensitive semiconductor, BiVO4 has attracted increasing interest because of the photodecomposition of environmental pollutants and generation of hydrogen from water under light irradiation [20–24]. Unfortunately, there are three main drawbacks, which limit the application of BiVO4 photocatalyst [25–28]. Firstly, BiVO4 only exhibits photoabsorption properties from UV to visible light with wavelength shorter than about 450 nm. Secondly, the rapid recombination of photogenerated electron–hole pairs and inefficient quantum yield seriously limits the light energy-conversion efficiency. Lastly, BiVO4 exhibits high dispersion in the solid–liquid system, which results in a difficult for its recovery. Therefore, to improve these drawbacks, especially, to broaden the range of visible light responsiveness and enhance the separation of photogenerated carriers are important in enlarging the efficiency photocatalytic application. Therefore, to further enrich versatile photocatalytic material with visible light catalytic efficiency, we design and fabricate an Nd–BiVO4 composite by doping Nd element to BiVO4. Evidently, the formation of Nd–BiVO4 composite with the efficiency separation and transfer of
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photogenerated carriers is more possible, and well-fabricated Nd–BiVO4 composite could availably restrict the recombination of photogenerated carriers and effectively enhance the quantum yield. Although, there are some reports about lanthanide elements doping BiVO4 [29–33], however, to the best of our knowledge, 2D long rod shaped structures Nd– BiVO4 composite has never been constructed, and the influence of lanthanide sources on the photocatalytic activity of the Nd doped BiVO4 for the degradation of phenol and desulfurization under visible irradiation has never been studied. Hence, in the present work, we synthesized Nd-doped BiVO4 composite. Nd doping can suppress the recombination of photogenerated electron and hole pairs. Phenol was chosen as model pollutants to evaluate the photocatalytic activity under visible light irradiation. Nd doping can enhance the photocatalytic activity evidently. Afterwards, the photocatalytic mechanism of Nd–BiVO4 composite was also discussed based on the calculated energy positions.
separated from the phenol-containing solution by centrifugation. The phenol content in the supernatant was determined using a UV–vis spectrophotometer (Shimadzu, UV-2550, Japan) in the 4-AAP spectrophotometric method. Photocatalytic desulfurization experiment was carried out as follow, thiophene was dissolved in n-octane to form model oil with a concentration of 500 wppm of sulfur. 200 mg of the Nd-doped BiVO4 sample, 50 mL of acetonitrile and 250 mL of model oil were added into a quartz tube to form reaction solution, which prior to illumination is stirred at 140 rpm continuous magnetically in dark for 2 h to assure an adsorption–desorption equilibrium between the Nd-doped BiVO4 sample and the target organic pollutant. Then, the solution was illuminated under a 400 W metal halide with a 400 nm cut-off and stirred magnetically. At a given time interval, the solution was centrifuged for removing the Nd-doped BiVO4 sample. The total sulfur concentration was analyzed in the n-octane phase by Elementar Analyzer (Elementar, Germany).
2. Experimental section 2.1. Preparation of Nd-doped BiVO4 sample
3. Result and discussion
All the reagents were analytical grade. The typical preparation process of Nd-doped BiVO4 sample is showed in the following way. 0.1 mol Bi(NO3)3·5H2O and 0.1 mol NH4VO4 were dissolved in nitric acid and sodium hydroxide solution, respectively. 0.02 g of sodium dodecyl benzene sulfonate was added in each solution. The resulted two solutions were combined together and the mixed solution was stirred for 30 min vigorously. Subsequently, the pH value of the solution was adjusted to 7 with ammonia, into which appropriate amounts of Nd(NO3)3·6H2O was added and the resulted solution was stirred for another 30 min, and then transferred and sealed in a 100 mL Teflon liner stainless vessel, which was heated at 180 °C for 12 h. After slow cooling to room temperature, the precipitate was filtered off, washed with distilled water and absolute ethyl alcohol. Afterwards, the precipitate was dried at 80 °C in air for 8 h. To investigate the effects of the amounts of Nd on the photocatalytic performances of BiVO4, the added amounts of Nd3+ were varied from 0 wt.% to 1.0 wt.%, and the resulting samples were named as 0 wt.% Nd-doped BiVO4, 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4, 0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nddoped BiVO4.
3.1. XRD analyses of Nd-doped BiVO4 sample
2.2. Characterization of Nd-doped BiVO4 sample The phase and composition of the Nd-doped BiVO4 sample were identified by X-ray diffraction (XRD) using monochromatized Cu Kα radiation under 40 kV and 100 mA and with the 2θ ranging from 10° to 80° (Shimadzu XRD-7000). The morphologies and microstructures of the Nd2O3-BiVO4 sample were analyzed by the scanning electron microscope (SEM) (JEOL JSM-6700F). The UV–vis diffuse reflectance spectrum (DRS) of the Nd-doped BiVO4 sample was recorded with an UV–vis spectrophotometer (Shimadzu UV-2550) using an integratingsphere accessory, BaSO4 was used as a reflectance standard. The chemical state of constituent elements of the Nd-doped BiVO4 sample was analyzed by EDS (Bulker Q73).
The XRD patterns of Nd-doped BiVO4 samples are shown in Fig. 1, and the lattice constants are shown in Table 1. The diffraction peaks at 2θ of 18.6°, 28.9°, 30.5°, 34.4°, 35.3, 39.4°, 42.3°, 46.1°, 46.6°, 47.3°, 53.3°, 58.3°, and 59.9° can be indexed to monoclinic BiVO4 (JCPDS NO. 14–0688; Pnca (61) spaces; a = 5.214 Å, b = 5.084 Å, c = 11.706 Å), corresponding to the indices of (101), (013), (112), (200), (020), (211), (105), (123), (204), (024), (301), (303) and (224) planes, respectively [29]. Moreover, the diffraction peaks of Nd element and other impurities are not observed in these Nd-doped BiVO4 samples. It is indicated that Nd elements are not present, although the electronegativity and ionic radius of Nd3 + ions (0.099 nm) are approximately equal to those of Bi2+ ions (0.1110 nm) in BiVO4, there is still a chance for Nd3+ to incorporate into the BiVO4 lattice. From Table 1, the lattice constants of pure BiVO4 are nearly identical compared to that of Nddoped BiVO4 samples, which indicates that doped Nd do not change the crystal type of BiVO4, and Nd3+ only disperse on the surface of the BiVO4 rather than replace the atom backbone to enter the internal lattice of BiVO4. It should be noted that the XRD peaks of the crystal plane (112), (004) and (301) for all Nd-doped BiVO4 were shifted to a low diffraction angle, attributed to doping with Nd3+ into BiVO4 to enlarge lattice parameter of the crystal.
2.3. The visible light photocatalytic properties of Nd-doped BiVO4 sample Photocatalytic property of the Nd-doped BiVO4 sample was evaluated by degradation of the phenol-containing wastewater at ambient temperature. A 400 W metal halide with a 400 nm cut-off filter was used as the visible light source. 250 mL of a phenol-containing solution of a desired concentration (10 mg L−1) was added to a quartz tube that contained 200 mg of the Nd-doped BiVO4 sample at a constant temperature of 25 °C. The tubes were agitated at 140 rpm continuous magnetically in the dark for 2 h to assure the adsorption–desorption equilibrium between the Nd-doped BiVO4 sample and the target organic pollutant. After certain contact time, the Nd-doped BiVO4 sample was
Fig. 1. XRD patterns of Nd-doped BiVO4 sample with doped contents of (a) 1.0 wt.%, (b) 0.8 wt.%, (c) 0.6 wt.%, (d) 0.3 wt.% and (e) 0 wt.%.
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Table 1 The lattice constants of pure BiVO4 and Nd-doped BiVO4 series samples. Samples
Lattice constants a(Å)
b(Å)
c(Å)
Pure monoclinic phase BiVO4 0.3 wt.% Nd-doped BiVO4 0.6 wt.% Nd-doped BiVO4 0.8 wt.% Nd-doped BiVO4 1.0 wt.% Nd-doped BiVO4
5.214 5.216 5.219 5.220 5.221
5.084 5.089 5.091 5.092 5.092
11.706 11.709 11.712 11.713 11.714
3.2. The UV–vis diffuse reflectance spectra of Nd-doped BiVO4 sample The optical absorption property of a semiconductor is an important factor in determining its photocatalytic performance. However, the optical absorption property is frequently deemed to be relevant to the electronic structure feature [30]. The optical properties of the Nddoped BiVO4 samples are shown in Fig. 2. According to the inset of Fig. 2, the samples show strong absorption in visible light region in addition to that in the UV light region, it is implied the possibility of photocatalytic performance for this samples under visible light irradiation. In the inset of Fig. 2, based on the equation Ahν = c (hν − Eg)n [31], the band gaps of the samples are estimated to be 2.28 eV, 2.24 eV, 2.18 eV, 2.15 eV and 2.12 eV from the onset of the absorption edges corresponding to the 0 wt.% Nd-doped BiVO4, 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4, 0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nddoped BiVO4, respectively. These data clearly demonstrate that the electronic structures of BiVO4 are changed by the amount of doped Nd used in the hydrothermal synthesis. The variations in the electronic structures lead to different degrees of delocalization of photogenerated electron–hole pairs, which therefore results in different mobilities of photogenerated holes. Such differences may be attributed to the slight changes of lattice parameters of BiVO4. A worthy phenomenon is that the band gap transition is shifted to visible range for these Nd-doped BiVO4 samples.
3.3. Morphologies and microstructure of Nd-doped BiVO4 sample 2D long rod shaped hierarchical structures are observed in the SEM image of 0.8 wt.% Nd-doped BiVO4 composite (Fig. 3). 2D long rod shaped structures 0.8 wt.% Nd-doped BiVO4 composite can be formed as follow, irregular short rod micro-particle are produced by bulk and
Fig. 3. The SEM patterns of 0.8 wt.% Nd-doped BiVO4 sample.
flake shaped nanoparticles in the initial stage of synthesis. Afterwards, 2D long rod shaped structure particles are formed by the selfassembling of short rod micro-particle in a one dimensional fashion along with [001] direction. During the self-assembly process, the crystal growth can be described in terms of Ostwald ripening, which involved the growth of larger particles at the expense of the smaller ones driven by the tendency of the solid phase in the system to adjust it to achieve a minimum total surface free energy. It can be found that 1D long rod shaped structure particles appear to be high crystallinity and display mainly thin particles with borders of several hundred nanometers. To determine Nd content of 0.8 wt.% Nd-doped BiVO4 sample, the EDS of samples is carried out (Fig. 4). From EDS analysis (Fig. 4), Bi, V, O and Nd elements existed in the samples, which further confirm that the sample is composed from BiVO4 and Nd. From insert table in Fig. 4, Nd content of experimental value is also close to the theoretical calculated value in Nd-doped BiVO4 composite, and the Nd content of 0.8 wt.% Nd-doped BiVO4 composite is determined to be 0.78 wt.%. The XPS spectra of pure BiVO4 and 0.8 wt.% Nd-doped BiVO4 sample is illustrated in Fig. 5, which indicated that Nd-doped BiVO4 were successfully prepared. The position of the Nd3d5/2 peak at 983.6 eV, which is very close to the standard binding energy of Nd3d5/2 for Nd2O3 [17], indicates that Nd2O3 was deposited on the BiVO4 surface during the hydrothermal process. 3.4. The visible light photocatalytic properties of Nd-doped BiVO4 sample
Fig. 2. The UV–vis diffuse reflectance spectra of the Nd-doped BiVO4 sample and the inset shows the relationship between (Ahv)2 and photon energy of Nd-doped BiVO4 samples. (a) 0 wt.% Nd-doped BiVO4, (b) 0.2 wt.% Nd-doped BiVO4, (c) 0.6 wt.% Nd-doped BiVO4, (d) 0.8 wt.% Nd-doped BiVO4 and (e) 1.0 wt.% Nd-doped BiVO4.
The photocatalytic property of the Nd-doped BiVO4 sample is evaluated by comparing the degradation efficiency of phenol under 400 W metal halide with a cut-off filter to cut off the light below 400 nm. The change of phenol concentration vs. illumination time is shown in Fig. 6. From Fig. 6, it can be seen that the degradation efficiency of phenol over BiVO4 sample can reach 15.12% after being illuminated for 120 min. While doped metal Nd to BiVO4, the degradation efficiency enhances obviously, and the degradation rate of phenol is 61.34%, 69.58%, 73.12%, 90.82%, 80.54% for 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4, 0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nd-doped BiVO4 composite after being illuminated for 120 min, respectively. It indicates that phenol can be degraded more efficiently by Nd-doped BiVO4 composite than pure BiVO4. On analyzing the change of phenol concentration in Fig. 7, the result obeys the pseudofirst order kinetics model, i.e., ln(c / c0) = k t, where c and c0 are the phenol concentrations at time t and 0, respectively, and k is the pseudofirst order rate constant. Evidently, the degradation process of phenol follow with the pseudofirst order kinetics with R2 = 0.9788, 0.9927, 0.9963, 0.9971, 0.9977 and 0.9918 for 0 wt.% Nddoped BiVO4, 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4,
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Fig. 4. The EDS analysis: Nd content in 0.8 wt.% Nd-doped BiVO4 sample by theoretical calculation and experimental analysis.
0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nd-doped BiVO4, respectively. However, for high Nd doped amount, the nonlinearity is caused by the increase in surface to volume ratio of Nd-doped BiVO4, leading to the increase in exciton formation. This formation could also be increased by longer irradiation time. The apparent rate constant k is 1.5 × 10−3, 7.8 × 10−3, 10 × 10−3, 11.4 × 10−3, 14.2 × 10−3 and 21 × 10−3, respectively, revealing the significant effect of Nd-doped BiVO4 on the degradation process, and the apparent rate constant is increased with the increase in the Nd concentrations. Especially, 0.8 wt.% Nd-doped BiVO4 exhibits the highest photocatalytic activity in phenol degradation. It is evident that the photocatalytic activity of the 0.8 wt.% Nd-doped BiVO4 composite is about 14 times higher than that of 0 wt.% Nddoped BiVO4. To further discover the performance of the series of Nd–BiVO4, an amount of thiophene was a dissolved in n-octane to form a model oil solution with a definite concentration of sulfur compound, and the photocatalytic desulfurization experiment of Nd–BiVO4 was carried out under visible light irradiation, the result is shown in Fig. 8. It can be seen from Fig. 8 that thiophene shows high stability under visible illumination. However, the photocatalytic desulfurization efficiency is increased obviously when BiVO4 are used as photocatalyst, and after being illuminated for 210 min under visible light, the photocatalytic desulfurization effect are 53.3%. When the content of loaded Nd is lower than 0.8 wt.%, the photocatalytic desulfurization effect of Nd–BiVO4 is enhanced with the
Fig. 5. The XPS spectra of BiVO4 and 0.8 wt.% Nd-doped BiVO4 sample.
increase of content of loaded Nd. However, when the content of loaded Nd is higher than 0.8 wt.%, the photocatalytic desulfurization effect of Nd–BiVO4 decreases with the increase of content of loaded Nd. Moreover, the photocatalytic desulfurization effect increases with the content of loaded Nd, and reaches the best effect when BiVO4 is loaded with 0.8 wt.% Nd, the desulfurization rate of model oil can reach 90.8% after being illuminated for 210 min under visible light. It well known that the photocatalytic behavior is closely related to the efficiency of the photogenerated electron–hole separation and the diffusion from the inner regions to the surface of the grains. Therefore, the enhanced photocatalytic activities of the Nd–BiVO4 can be explained by improvement of the charge separation, which is highly dependent on the content of loaded Nd. The stability of the photocatalyst is important for its application. To demonstrate the potential applicability of 0.8 wt.% Nd-doped BiVO4, circulating runs in the degradation of phenol were carried out under visible light irradiation. As shown in Fig. 9, 0.8 wt.% Nd-doped BiVO4 shows relatively stable performance under repeated use with constant degradation rate. After five recycles for the degradation of phenol, the photocatalyst did not exhibit any significant loss of activity.
Fig. 6. The photocatalytic degradation efficiency of phenol by series catalysts. (a) Blank, (b) 0 wt.% Nd-doped BiVO4, (c) 0.2 wt.% Nd-doped BiVO4, (d) 0.6 wt.% Nd-doped BiVO4, (e) 0.8 wt.% Nd-doped BiVO4 and (f) 1.0 wt.% Nd-doped BiVO4.
X. Gao et al. / Journal of Molecular Liquids 211 (2015) 25–30
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Fig. 9. Cycling runs in the photodegradation of phenol by 0.8 wt.% Nd-doped BiVO4. Fig. 7. Kinetic linear simulation curve (y = ln(c0/c),x = t) of photodegradation of series catalysts. (a) 0 wt.% Nd-doped BiVO4, (b) 0.2 wt.% Nd-doped BiVO4, (c) 0.6 wt.% Nddoped BiVO4, (d) 0.8 wt.% Nd-doped BiVO4 and (e) 1.0 wt.% Nd-doped BiVO4.
3.5. Photocatalytic mechanism
þ
Nd ‐ BiVO4 þ hv→Nd ‐ BiVO4 h
The BiVO4 is easily excited and corresponding photogenerated electrons and holes are generated under visible-light irradiation. Meanwhile, Nd3 + ions are most likely located at the surface of the nanocrystalline structure, facilitating separation of charge carriers more efficiently, which prolongs the lifetime of carriers and inhibits the recombination of photogenerated electrons and holes pairs [32]. 3þ
þ e‐ →Nd
3þ
þ h →Nd
Nd Nd
2þ
þ
4þ
hydroxyl free radical •OH directly. Based on the above discussion, the involved reactions of photocatalytic degradation of phenol over Nd–BiVO4 can be simply described as follows:
ð1Þ ð2Þ
þ
Nd ‐ BiVO4 h
þ
Nd ‐ BiVO4 h
þ Nd ‐ BiVO4 ðe‐ Þ
ð3Þ
þ organic pollutant→ Nd ‐ BiVO4 þ Products
ð4Þ
þ H2 O→H2 O þ •OH þ Hþ
ð5Þ
•OH þ organic pollutant→ Products
ð6Þ
O2 þ Nd ‐ BiVO4 ðe‐ Þ→Nd ‐ BiVO4 þ O2 ‐ •
ð7Þ
O2 ‐ • þ organic pollutant→Products:
ð8Þ
On the one hand, the photogenerated holes can act as directly an oxidant, on the other hand, the photogenerated holes can react with OH−/H2O existed in the system to generate hydroxyl free radical •OH. In such a way, the photoinduced electrons and holes can be efficiently separated and the recombination of electron–hole pairs can be controlled, and the decomposition of phenol can be finished by the oxidaand tion of photo-generated hole, superoxide radical anion O−• 2
In order to determine the function of the active free radicals, the benzoquinone (BQ), ethylene diamine tetraacetic acid(EDTA) and isopropanol (IPA) is used to capture the superoxide radical anion O−• 2 , photo-generated hole h+ and hydroxyl free radical •OH by a simple electron transfer mechanism, respectively [33–35]. The degradation effect of phenol over 0.8 wt.% Nd–BiVO4 under different scavengers is shown in Fig. 10. Without any scavengers, the degradation rate of phenol after being illuminated for 120 min is 90.82% (Fig. 6). From Fig. 10,
Fig. 8. The photocatalytic desulfurization by series catalysts. (a) Blank, (b) 0 wt.% Nddoped BiVO4, (c) 0.2 wt.% Nd-doped BiVO4, (d) 0.6 wt.% Nd-doped BiVO4, (e) 0.8 wt.% Nd-doped BiVO4 and (f) 1.0 wt.% Nd-doped BiVO4.
Fig. 10. The degradation effect of phenol over 0.8 wt.% Nd–BiVO4 under different scavengers. a: Adding IPA, b: adding BQ, and c: adding EDTA.
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when adding the EDTA as a scavengers of h+, the degradation ratio of phenol decreases obviously, indicating that the superoxide radical anion h+ play an important role in the photocatalytic reaction. While adding the BQ as a scavengers of O−• 2 , the degradation ratio is also decreased evidently, illustrating that hydroxyl free radical O−• 2 is also an important oxidant in the photocatalytic degradation process. As the addition of IPA leads to the most inhibition of the phenol degradation, suggesting that •OH plays the most important role in the photocatalytic degradation process. Based on the above analysis, it can be concluded that the degradation of phenol is driven mainly by •OH. Meanwhile, + O−• 2 and h also enhance the photocatalytic degradation process. 4. Conclusion 2D long rod shaped structures Nd–BiVO4 composite with different morphologies were prepared by the facile synthesis method, and the corresponding relationship was obtained among loaded content to phase, morphology, and optical absorption property by UV–vis-DRS, XRD, SEM, and EDS. The results show that Nd loaded did not change the crystallinity of BiVO4 significantly, but enhanced the optical absorption ability for visible light. The photocatalytic results indicated that the photocatalytic activity of Nd–BiVO4 composite was greatly enhanced compared with pure BiVO4, and the loaded content of Nd3+ had an impact on the catalytic activity of Nd–BiVO4 composite. The enhanced photocatalytic activity can be attributed to the efficient separation and preventing of the recombination of electron–hole pairs. Therefore, the facile process using Nd–BiVO4 composite could be an efficient strategy for the destruction of environmental pollutants and photocatalytic oxidative desulfurization of fuel oil. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 21406188), and the Industrial Public Relation Project of Department of Science & Technology of Shaanxi (grant no. 2014K10-04). References [1] W. Wu, C.Z. Jiang, V.A.L. Roy, Recent progress in magnetic iron oxide-semiconductor composite nanomaterials as promising photocatalysts, Nanoscale 7 (2015) 38–58. [2] H.J. Zhang, G.H. Chen, D.W. Bahnemann, Photoelectrocatalytic materials for environmental applications, J. Mater. Chem. 19 (2009) 5089–5121. [3] S.O. Obare, T. Ito, G.J. Meyer, Controlling reduction potentials of semiconductorsupported molecular catalysts for environmental remediation of organohalide pollutants, Environ. Sci. Technol. 39 (2005) 6266–6272. [4] X.L. Hu, G.S. Li, J.C. Yu, Design, fabrication, and modification of nanostructured semiconductor materials for environmental and energy applications, Langmuir 26 (2010) 3031–3039. [5] U. Banin, Y.B. Shahar, K. Vinokurov, Hybrid semiconductor–metal nanoparticles: from architecture to function, Chem. Mater. 26 (2014) 97–110. [6] X.M. Feng, G.Q. Hu, J.Q. Hu, Solution-phase synthesis of metal and/or semiconductor homojunction/heterojunction nanomaterials, Nanoscale 3 (2011) 2099–2117. [7] Y.Q. Qu, X.F. Duan, Progress, challenge and perspective of heterogeneous photocatalysts, Chem. Soc. Rev. 42 (2013) 2568–2580. [8] L. Zhang, D.A. Blom, H. Wang, Au–Cu2O core-shell nanoparticles: a hybrid metal– semiconductor heteronanostructure with geometrically tunable optical properties, Chem. Mater. 23 (2011) 4587–4598. [9] W.J. Ong, L.L. Tan, S.P. Chai, S.T. Yong, A.R. Mohamed, Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization, Nanoscale 6 (2014) 1946–2008. [10] Y.C. Huang, H.B. Li, M.S. Balogun, W.Y. Liu, Y.X. Tong, X.H. Lu, H.B. Ji, Oxygen vacancy induced bismuth oxyiodide with remarkably increased visible-light absorption and superior photocatalytic performance, ACS Appl. Mater. Interfaces 6 (2014) 22920–22927.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
The synthesize of lanthanide doped BiVO4 and its enhanced photocatalytic activity Xiaoming Gao a,⁎, Zihang Wang a, Xiang Zhai a, Feng Fu a, Wenhong Li b a b
Department of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan'an University, Yanan, Shaanxi 716000, China Department of Chemical Engineering, Northwest University, Xi'an, Shaanxi 710069, China
a r t i c l e
i n f o
Article history: Received 13 May 2015 Received in revised form 29 May 2015 Accepted 21 June 2015 Available online xxxx Keywords: Nd–BiVO4 composite Photocatalysis Phenol Desulfurization Photocatalytic mechanism
a b s t r a c t Lanthanide Nd doped BiVO4 composite with 2D long rod shaped structures was prepared by the facile synthesis method. The corresponding relationship was obtained among loaded content to phase, morphology, and optical absorption property of Nd–BiVO4 composite by UV–vis DRS, XRD, SEM, and EDS. The results shown that Nd loaded did not change the crystallinity of BiVO4 significantly, but enhanced the optical absorption ability for visible light and improved the morphologies and microstructure. The photocatalytic activities of the Nd–BiVO4 composite were evaluated for the photodegradation of phenol and desulfurization of thiophene under visible light irradiation. The results showed that Nd loaded greatly improved the photocatalytic activity of BiVO4, and the content of Nd had an obvious impact on the catalytic activity of BiVO4. The 0.8 wt.% Nd–BiVO4 composite exhibited the best photocatalytic activity in both the photodegradation of phenol and desulfurization of thiophene. Afterwards, the mechanism of enhanced photocatalytic activity of Nd–BiVO4 composite was also investigated. The determination of function of the active free radicals confirmed that hydroxyl free radical •OH was main active radicals during photocatalytic oxidation process. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor material has attracted comprehensive interest due to the photo-degradation of environmental pollutants and production of hydrogen from water under light irradiation [1–5]. However, many semiconductors can be excited only under ultraviolet light irradiation because of its relatively wide band gap and inefficient quantum yield, which hinders its further application in industrial process [6–8]. Up to date, a variety of strategies have been employed to improve the photocatalytic efficiencies of photocatalysts in the visible range by modification with nonmetal/metal element doping [9–11]. Unfortunately, the methods used are not completely controlled. Moreover, these dopants may become recombination centers between photogenerated electrons and holes. Evidently, the expected methods are somewhat limited [12–14]. Therefore, to explore more efficient photocatalyst with visible light responsiveness and thermal stability is urgent and indispensable. In recent, doping with lanthanide ions with 4f electron configurations can significantly enhance the photocatalytic activity of photocatalysts [15,16]. In this aspect, the partly filled 4f electron orbits in rare earth ions can form a new energy level between the valence and conduction band of photocatalysts, which results in the narrowed band gap. For example, Dua et al. synthesized a porous Nd-doped TiO2 monolith by sol–gel method. The result showed that Nd doping could ⁎ Corresponding author. E-mail address: [email protected] (X. Gao).
http://dx.doi.org/10.1016/j.molliq.2015.06.058 0167-7322/© 2015 Elsevier B.V. All rights reserved.
increase the TiO2 surface area and suppress the recombination of photo-produced hole/electron pairs, and Nd-doped TiO2 monolith showed better behavior than the other as-prepared samples and Degussa P25 [17]. Thomas et al. reported that samarium-doped anatase TiO2 with a higher photocatalytic activity was prepared via a low temperature hydrothermal route [18]. Hwang et al. found that the 4f orbits of rare earth ions greatly modified the TiO2 band structures resulting in better photocatalytic behavior [19]. As an important n-type photosensitive semiconductor, BiVO4 has attracted increasing interest because of the photodecomposition of environmental pollutants and generation of hydrogen from water under light irradiation [20–24]. Unfortunately, there are three main drawbacks, which limit the application of BiVO4 photocatalyst [25–28]. Firstly, BiVO4 only exhibits photoabsorption properties from UV to visible light with wavelength shorter than about 450 nm. Secondly, the rapid recombination of photogenerated electron–hole pairs and inefficient quantum yield seriously limits the light energy-conversion efficiency. Lastly, BiVO4 exhibits high dispersion in the solid–liquid system, which results in a difficult for its recovery. Therefore, to improve these drawbacks, especially, to broaden the range of visible light responsiveness and enhance the separation of photogenerated carriers are important in enlarging the efficiency photocatalytic application. Therefore, to further enrich versatile photocatalytic material with visible light catalytic efficiency, we design and fabricate an Nd–BiVO4 composite by doping Nd element to BiVO4. Evidently, the formation of Nd–BiVO4 composite with the efficiency separation and transfer of
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photogenerated carriers is more possible, and well-fabricated Nd–BiVO4 composite could availably restrict the recombination of photogenerated carriers and effectively enhance the quantum yield. Although, there are some reports about lanthanide elements doping BiVO4 [29–33], however, to the best of our knowledge, 2D long rod shaped structures Nd– BiVO4 composite has never been constructed, and the influence of lanthanide sources on the photocatalytic activity of the Nd doped BiVO4 for the degradation of phenol and desulfurization under visible irradiation has never been studied. Hence, in the present work, we synthesized Nd-doped BiVO4 composite. Nd doping can suppress the recombination of photogenerated electron and hole pairs. Phenol was chosen as model pollutants to evaluate the photocatalytic activity under visible light irradiation. Nd doping can enhance the photocatalytic activity evidently. Afterwards, the photocatalytic mechanism of Nd–BiVO4 composite was also discussed based on the calculated energy positions.
separated from the phenol-containing solution by centrifugation. The phenol content in the supernatant was determined using a UV–vis spectrophotometer (Shimadzu, UV-2550, Japan) in the 4-AAP spectrophotometric method. Photocatalytic desulfurization experiment was carried out as follow, thiophene was dissolved in n-octane to form model oil with a concentration of 500 wppm of sulfur. 200 mg of the Nd-doped BiVO4 sample, 50 mL of acetonitrile and 250 mL of model oil were added into a quartz tube to form reaction solution, which prior to illumination is stirred at 140 rpm continuous magnetically in dark for 2 h to assure an adsorption–desorption equilibrium between the Nd-doped BiVO4 sample and the target organic pollutant. Then, the solution was illuminated under a 400 W metal halide with a 400 nm cut-off and stirred magnetically. At a given time interval, the solution was centrifuged for removing the Nd-doped BiVO4 sample. The total sulfur concentration was analyzed in the n-octane phase by Elementar Analyzer (Elementar, Germany).
2. Experimental section 2.1. Preparation of Nd-doped BiVO4 sample
3. Result and discussion
All the reagents were analytical grade. The typical preparation process of Nd-doped BiVO4 sample is showed in the following way. 0.1 mol Bi(NO3)3·5H2O and 0.1 mol NH4VO4 were dissolved in nitric acid and sodium hydroxide solution, respectively. 0.02 g of sodium dodecyl benzene sulfonate was added in each solution. The resulted two solutions were combined together and the mixed solution was stirred for 30 min vigorously. Subsequently, the pH value of the solution was adjusted to 7 with ammonia, into which appropriate amounts of Nd(NO3)3·6H2O was added and the resulted solution was stirred for another 30 min, and then transferred and sealed in a 100 mL Teflon liner stainless vessel, which was heated at 180 °C for 12 h. After slow cooling to room temperature, the precipitate was filtered off, washed with distilled water and absolute ethyl alcohol. Afterwards, the precipitate was dried at 80 °C in air for 8 h. To investigate the effects of the amounts of Nd on the photocatalytic performances of BiVO4, the added amounts of Nd3+ were varied from 0 wt.% to 1.0 wt.%, and the resulting samples were named as 0 wt.% Nd-doped BiVO4, 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4, 0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nddoped BiVO4.
3.1. XRD analyses of Nd-doped BiVO4 sample
2.2. Characterization of Nd-doped BiVO4 sample The phase and composition of the Nd-doped BiVO4 sample were identified by X-ray diffraction (XRD) using monochromatized Cu Kα radiation under 40 kV and 100 mA and with the 2θ ranging from 10° to 80° (Shimadzu XRD-7000). The morphologies and microstructures of the Nd2O3-BiVO4 sample were analyzed by the scanning electron microscope (SEM) (JEOL JSM-6700F). The UV–vis diffuse reflectance spectrum (DRS) of the Nd-doped BiVO4 sample was recorded with an UV–vis spectrophotometer (Shimadzu UV-2550) using an integratingsphere accessory, BaSO4 was used as a reflectance standard. The chemical state of constituent elements of the Nd-doped BiVO4 sample was analyzed by EDS (Bulker Q73).
The XRD patterns of Nd-doped BiVO4 samples are shown in Fig. 1, and the lattice constants are shown in Table 1. The diffraction peaks at 2θ of 18.6°, 28.9°, 30.5°, 34.4°, 35.3, 39.4°, 42.3°, 46.1°, 46.6°, 47.3°, 53.3°, 58.3°, and 59.9° can be indexed to monoclinic BiVO4 (JCPDS NO. 14–0688; Pnca (61) spaces; a = 5.214 Å, b = 5.084 Å, c = 11.706 Å), corresponding to the indices of (101), (013), (112), (200), (020), (211), (105), (123), (204), (024), (301), (303) and (224) planes, respectively [29]. Moreover, the diffraction peaks of Nd element and other impurities are not observed in these Nd-doped BiVO4 samples. It is indicated that Nd elements are not present, although the electronegativity and ionic radius of Nd3 + ions (0.099 nm) are approximately equal to those of Bi2+ ions (0.1110 nm) in BiVO4, there is still a chance for Nd3+ to incorporate into the BiVO4 lattice. From Table 1, the lattice constants of pure BiVO4 are nearly identical compared to that of Nddoped BiVO4 samples, which indicates that doped Nd do not change the crystal type of BiVO4, and Nd3+ only disperse on the surface of the BiVO4 rather than replace the atom backbone to enter the internal lattice of BiVO4. It should be noted that the XRD peaks of the crystal plane (112), (004) and (301) for all Nd-doped BiVO4 were shifted to a low diffraction angle, attributed to doping with Nd3+ into BiVO4 to enlarge lattice parameter of the crystal.
2.3. The visible light photocatalytic properties of Nd-doped BiVO4 sample Photocatalytic property of the Nd-doped BiVO4 sample was evaluated by degradation of the phenol-containing wastewater at ambient temperature. A 400 W metal halide with a 400 nm cut-off filter was used as the visible light source. 250 mL of a phenol-containing solution of a desired concentration (10 mg L−1) was added to a quartz tube that contained 200 mg of the Nd-doped BiVO4 sample at a constant temperature of 25 °C. The tubes were agitated at 140 rpm continuous magnetically in the dark for 2 h to assure the adsorption–desorption equilibrium between the Nd-doped BiVO4 sample and the target organic pollutant. After certain contact time, the Nd-doped BiVO4 sample was
Fig. 1. XRD patterns of Nd-doped BiVO4 sample with doped contents of (a) 1.0 wt.%, (b) 0.8 wt.%, (c) 0.6 wt.%, (d) 0.3 wt.% and (e) 0 wt.%.
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Table 1 The lattice constants of pure BiVO4 and Nd-doped BiVO4 series samples. Samples
Lattice constants a(Å)
b(Å)
c(Å)
Pure monoclinic phase BiVO4 0.3 wt.% Nd-doped BiVO4 0.6 wt.% Nd-doped BiVO4 0.8 wt.% Nd-doped BiVO4 1.0 wt.% Nd-doped BiVO4
5.214 5.216 5.219 5.220 5.221
5.084 5.089 5.091 5.092 5.092
11.706 11.709 11.712 11.713 11.714
3.2. The UV–vis diffuse reflectance spectra of Nd-doped BiVO4 sample The optical absorption property of a semiconductor is an important factor in determining its photocatalytic performance. However, the optical absorption property is frequently deemed to be relevant to the electronic structure feature [30]. The optical properties of the Nddoped BiVO4 samples are shown in Fig. 2. According to the inset of Fig. 2, the samples show strong absorption in visible light region in addition to that in the UV light region, it is implied the possibility of photocatalytic performance for this samples under visible light irradiation. In the inset of Fig. 2, based on the equation Ahν = c (hν − Eg)n [31], the band gaps of the samples are estimated to be 2.28 eV, 2.24 eV, 2.18 eV, 2.15 eV and 2.12 eV from the onset of the absorption edges corresponding to the 0 wt.% Nd-doped BiVO4, 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4, 0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nddoped BiVO4, respectively. These data clearly demonstrate that the electronic structures of BiVO4 are changed by the amount of doped Nd used in the hydrothermal synthesis. The variations in the electronic structures lead to different degrees of delocalization of photogenerated electron–hole pairs, which therefore results in different mobilities of photogenerated holes. Such differences may be attributed to the slight changes of lattice parameters of BiVO4. A worthy phenomenon is that the band gap transition is shifted to visible range for these Nd-doped BiVO4 samples.
3.3. Morphologies and microstructure of Nd-doped BiVO4 sample 2D long rod shaped hierarchical structures are observed in the SEM image of 0.8 wt.% Nd-doped BiVO4 composite (Fig. 3). 2D long rod shaped structures 0.8 wt.% Nd-doped BiVO4 composite can be formed as follow, irregular short rod micro-particle are produced by bulk and
Fig. 3. The SEM patterns of 0.8 wt.% Nd-doped BiVO4 sample.
flake shaped nanoparticles in the initial stage of synthesis. Afterwards, 2D long rod shaped structure particles are formed by the selfassembling of short rod micro-particle in a one dimensional fashion along with [001] direction. During the self-assembly process, the crystal growth can be described in terms of Ostwald ripening, which involved the growth of larger particles at the expense of the smaller ones driven by the tendency of the solid phase in the system to adjust it to achieve a minimum total surface free energy. It can be found that 1D long rod shaped structure particles appear to be high crystallinity and display mainly thin particles with borders of several hundred nanometers. To determine Nd content of 0.8 wt.% Nd-doped BiVO4 sample, the EDS of samples is carried out (Fig. 4). From EDS analysis (Fig. 4), Bi, V, O and Nd elements existed in the samples, which further confirm that the sample is composed from BiVO4 and Nd. From insert table in Fig. 4, Nd content of experimental value is also close to the theoretical calculated value in Nd-doped BiVO4 composite, and the Nd content of 0.8 wt.% Nd-doped BiVO4 composite is determined to be 0.78 wt.%. The XPS spectra of pure BiVO4 and 0.8 wt.% Nd-doped BiVO4 sample is illustrated in Fig. 5, which indicated that Nd-doped BiVO4 were successfully prepared. The position of the Nd3d5/2 peak at 983.6 eV, which is very close to the standard binding energy of Nd3d5/2 for Nd2O3 [17], indicates that Nd2O3 was deposited on the BiVO4 surface during the hydrothermal process. 3.4. The visible light photocatalytic properties of Nd-doped BiVO4 sample
Fig. 2. The UV–vis diffuse reflectance spectra of the Nd-doped BiVO4 sample and the inset shows the relationship between (Ahv)2 and photon energy of Nd-doped BiVO4 samples. (a) 0 wt.% Nd-doped BiVO4, (b) 0.2 wt.% Nd-doped BiVO4, (c) 0.6 wt.% Nd-doped BiVO4, (d) 0.8 wt.% Nd-doped BiVO4 and (e) 1.0 wt.% Nd-doped BiVO4.
The photocatalytic property of the Nd-doped BiVO4 sample is evaluated by comparing the degradation efficiency of phenol under 400 W metal halide with a cut-off filter to cut off the light below 400 nm. The change of phenol concentration vs. illumination time is shown in Fig. 6. From Fig. 6, it can be seen that the degradation efficiency of phenol over BiVO4 sample can reach 15.12% after being illuminated for 120 min. While doped metal Nd to BiVO4, the degradation efficiency enhances obviously, and the degradation rate of phenol is 61.34%, 69.58%, 73.12%, 90.82%, 80.54% for 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4, 0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nd-doped BiVO4 composite after being illuminated for 120 min, respectively. It indicates that phenol can be degraded more efficiently by Nd-doped BiVO4 composite than pure BiVO4. On analyzing the change of phenol concentration in Fig. 7, the result obeys the pseudofirst order kinetics model, i.e., ln(c / c0) = k t, where c and c0 are the phenol concentrations at time t and 0, respectively, and k is the pseudofirst order rate constant. Evidently, the degradation process of phenol follow with the pseudofirst order kinetics with R2 = 0.9788, 0.9927, 0.9963, 0.9971, 0.9977 and 0.9918 for 0 wt.% Nddoped BiVO4, 0.2 wt.% Nd-doped BiVO4, 0.6 wt.% Nd-doped BiVO4,
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Fig. 4. The EDS analysis: Nd content in 0.8 wt.% Nd-doped BiVO4 sample by theoretical calculation and experimental analysis.
0.8 wt.% Nd-doped BiVO4 and 1.0 wt.% Nd-doped BiVO4, respectively. However, for high Nd doped amount, the nonlinearity is caused by the increase in surface to volume ratio of Nd-doped BiVO4, leading to the increase in exciton formation. This formation could also be increased by longer irradiation time. The apparent rate constant k is 1.5 × 10−3, 7.8 × 10−3, 10 × 10−3, 11.4 × 10−3, 14.2 × 10−3 and 21 × 10−3, respectively, revealing the significant effect of Nd-doped BiVO4 on the degradation process, and the apparent rate constant is increased with the increase in the Nd concentrations. Especially, 0.8 wt.% Nd-doped BiVO4 exhibits the highest photocatalytic activity in phenol degradation. It is evident that the photocatalytic activity of the 0.8 wt.% Nd-doped BiVO4 composite is about 14 times higher than that of 0 wt.% Nddoped BiVO4. To further discover the performance of the series of Nd–BiVO4, an amount of thiophene was a dissolved in n-octane to form a model oil solution with a definite concentration of sulfur compound, and the photocatalytic desulfurization experiment of Nd–BiVO4 was carried out under visible light irradiation, the result is shown in Fig. 8. It can be seen from Fig. 8 that thiophene shows high stability under visible illumination. However, the photocatalytic desulfurization efficiency is increased obviously when BiVO4 are used as photocatalyst, and after being illuminated for 210 min under visible light, the photocatalytic desulfurization effect are 53.3%. When the content of loaded Nd is lower than 0.8 wt.%, the photocatalytic desulfurization effect of Nd–BiVO4 is enhanced with the
Fig. 5. The XPS spectra of BiVO4 and 0.8 wt.% Nd-doped BiVO4 sample.
increase of content of loaded Nd. However, when the content of loaded Nd is higher than 0.8 wt.%, the photocatalytic desulfurization effect of Nd–BiVO4 decreases with the increase of content of loaded Nd. Moreover, the photocatalytic desulfurization effect increases with the content of loaded Nd, and reaches the best effect when BiVO4 is loaded with 0.8 wt.% Nd, the desulfurization rate of model oil can reach 90.8% after being illuminated for 210 min under visible light. It well known that the photocatalytic behavior is closely related to the efficiency of the photogenerated electron–hole separation and the diffusion from the inner regions to the surface of the grains. Therefore, the enhanced photocatalytic activities of the Nd–BiVO4 can be explained by improvement of the charge separation, which is highly dependent on the content of loaded Nd. The stability of the photocatalyst is important for its application. To demonstrate the potential applicability of 0.8 wt.% Nd-doped BiVO4, circulating runs in the degradation of phenol were carried out under visible light irradiation. As shown in Fig. 9, 0.8 wt.% Nd-doped BiVO4 shows relatively stable performance under repeated use with constant degradation rate. After five recycles for the degradation of phenol, the photocatalyst did not exhibit any significant loss of activity.
Fig. 6. The photocatalytic degradation efficiency of phenol by series catalysts. (a) Blank, (b) 0 wt.% Nd-doped BiVO4, (c) 0.2 wt.% Nd-doped BiVO4, (d) 0.6 wt.% Nd-doped BiVO4, (e) 0.8 wt.% Nd-doped BiVO4 and (f) 1.0 wt.% Nd-doped BiVO4.
X. Gao et al. / Journal of Molecular Liquids 211 (2015) 25–30
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Fig. 9. Cycling runs in the photodegradation of phenol by 0.8 wt.% Nd-doped BiVO4. Fig. 7. Kinetic linear simulation curve (y = ln(c0/c),x = t) of photodegradation of series catalysts. (a) 0 wt.% Nd-doped BiVO4, (b) 0.2 wt.% Nd-doped BiVO4, (c) 0.6 wt.% Nddoped BiVO4, (d) 0.8 wt.% Nd-doped BiVO4 and (e) 1.0 wt.% Nd-doped BiVO4.
3.5. Photocatalytic mechanism
þ
Nd ‐ BiVO4 þ hv→Nd ‐ BiVO4 h
The BiVO4 is easily excited and corresponding photogenerated electrons and holes are generated under visible-light irradiation. Meanwhile, Nd3 + ions are most likely located at the surface of the nanocrystalline structure, facilitating separation of charge carriers more efficiently, which prolongs the lifetime of carriers and inhibits the recombination of photogenerated electrons and holes pairs [32]. 3þ
þ e‐ →Nd
3þ
þ h →Nd
Nd Nd
2þ
þ
4þ
hydroxyl free radical •OH directly. Based on the above discussion, the involved reactions of photocatalytic degradation of phenol over Nd–BiVO4 can be simply described as follows:
ð1Þ ð2Þ
þ
Nd ‐ BiVO4 h
þ
Nd ‐ BiVO4 h
þ Nd ‐ BiVO4 ðe‐ Þ
ð3Þ
þ organic pollutant→ Nd ‐ BiVO4 þ Products
ð4Þ
þ H2 O→H2 O þ •OH þ Hþ
ð5Þ
•OH þ organic pollutant→ Products
ð6Þ
O2 þ Nd ‐ BiVO4 ðe‐ Þ→Nd ‐ BiVO4 þ O2 ‐ •
ð7Þ
O2 ‐ • þ organic pollutant→Products:
ð8Þ
On the one hand, the photogenerated holes can act as directly an oxidant, on the other hand, the photogenerated holes can react with OH−/H2O existed in the system to generate hydroxyl free radical •OH. In such a way, the photoinduced electrons and holes can be efficiently separated and the recombination of electron–hole pairs can be controlled, and the decomposition of phenol can be finished by the oxidaand tion of photo-generated hole, superoxide radical anion O−• 2
In order to determine the function of the active free radicals, the benzoquinone (BQ), ethylene diamine tetraacetic acid(EDTA) and isopropanol (IPA) is used to capture the superoxide radical anion O−• 2 , photo-generated hole h+ and hydroxyl free radical •OH by a simple electron transfer mechanism, respectively [33–35]. The degradation effect of phenol over 0.8 wt.% Nd–BiVO4 under different scavengers is shown in Fig. 10. Without any scavengers, the degradation rate of phenol after being illuminated for 120 min is 90.82% (Fig. 6). From Fig. 10,
Fig. 8. The photocatalytic desulfurization by series catalysts. (a) Blank, (b) 0 wt.% Nddoped BiVO4, (c) 0.2 wt.% Nd-doped BiVO4, (d) 0.6 wt.% Nd-doped BiVO4, (e) 0.8 wt.% Nd-doped BiVO4 and (f) 1.0 wt.% Nd-doped BiVO4.
Fig. 10. The degradation effect of phenol over 0.8 wt.% Nd–BiVO4 under different scavengers. a: Adding IPA, b: adding BQ, and c: adding EDTA.
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when adding the EDTA as a scavengers of h+, the degradation ratio of phenol decreases obviously, indicating that the superoxide radical anion h+ play an important role in the photocatalytic reaction. While adding the BQ as a scavengers of O−• 2 , the degradation ratio is also decreased evidently, illustrating that hydroxyl free radical O−• 2 is also an important oxidant in the photocatalytic degradation process. As the addition of IPA leads to the most inhibition of the phenol degradation, suggesting that •OH plays the most important role in the photocatalytic degradation process. Based on the above analysis, it can be concluded that the degradation of phenol is driven mainly by •OH. Meanwhile, + O−• 2 and h also enhance the photocatalytic degradation process. 4. Conclusion 2D long rod shaped structures Nd–BiVO4 composite with different morphologies were prepared by the facile synthesis method, and the corresponding relationship was obtained among loaded content to phase, morphology, and optical absorption property by UV–vis-DRS, XRD, SEM, and EDS. The results show that Nd loaded did not change the crystallinity of BiVO4 significantly, but enhanced the optical absorption ability for visible light. The photocatalytic results indicated that the photocatalytic activity of Nd–BiVO4 composite was greatly enhanced compared with pure BiVO4, and the loaded content of Nd3+ had an impact on the catalytic activity of Nd–BiVO4 composite. The enhanced photocatalytic activity can be attributed to the efficient separation and preventing of the recombination of electron–hole pairs. Therefore, the facile process using Nd–BiVO4 composite could be an efficient strategy for the destruction of environmental pollutants and photocatalytic oxidative desulfurization of fuel oil. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 21406188), and the Industrial Public Relation Project of Department of Science & Technology of Shaanxi (grant no. 2014K10-04). References [1] W. Wu, C.Z. Jiang, V.A.L. Roy, Recent progress in magnetic iron oxide-semiconductor composite nanomaterials as promising photocatalysts, Nanoscale 7 (2015) 38–58. [2] H.J. Zhang, G.H. Chen, D.W. Bahnemann, Photoelectrocatalytic materials for environmental applications, J. Mater. Chem. 19 (2009) 5089–5121. [3] S.O. Obare, T. Ito, G.J. Meyer, Controlling reduction potentials of semiconductorsupported molecular catalysts for environmental remediation of organohalide pollutants, Environ. Sci. Technol. 39 (2005) 6266–6272. [4] X.L. Hu, G.S. Li, J.C. Yu, Design, fabrication, and modification of nanostructured semiconductor materials for environmental and energy applications, Langmuir 26 (2010) 3031–3039. [5] U. Banin, Y.B. Shahar, K. Vinokurov, Hybrid semiconductor–metal nanoparticles: from architecture to function, Chem. Mater. 26 (2014) 97–110. [6] X.M. Feng, G.Q. Hu, J.Q. Hu, Solution-phase synthesis of metal and/or semiconductor homojunction/heterojunction nanomaterials, Nanoscale 3 (2011) 2099–2117. [7] Y.Q. Qu, X.F. Duan, Progress, challenge and perspective of heterogeneous photocatalysts, Chem. Soc. Rev. 42 (2013) 2568–2580. [8] L. Zhang, D.A. Blom, H. Wang, Au–Cu2O core-shell nanoparticles: a hybrid metal– semiconductor heteronanostructure with geometrically tunable optical properties, Chem. Mater. 23 (2011) 4587–4598. [9] W.J. Ong, L.L. Tan, S.P. Chai, S.T. Yong, A.R. Mohamed, Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization, Nanoscale 6 (2014) 1946–2008. [10] Y.C. Huang, H.B. Li, M.S. Balogun, W.Y. Liu, Y.X. Tong, X.H. Lu, H.B. Ji, Oxygen vacancy induced bismuth oxyiodide with remarkably increased visible-light absorption and superior photocatalytic performance, ACS Appl. Mater. Interfaces 6 (2014) 22920–22927.
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