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Study on photocatalysis and dynamics properties of self-catalysis BiVO4 assembled in porous Sr2MgSi2O7:Eu2+,Dy3+
Journal of Alloys and Compounds 655 (2016) 1e5
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Letter
Study on photocatalysis and dynamics properties of self-catalysis BiVO4 assembled in porous Sr2MgSi2O7:Eu2þ,Dy3þ a b s t r a c t Keywords: Sr2MgSi2O7:Eu2þ,Dy3þ BiVO4 Energy-storage Self-catalysis Dynamics
Energy-storage self-catalysis material that BiVO4 assembled into porous Sr2MgSi2O7:Eu2þ,Dy3þ was successfully synthesized by the impregnation method. Crystalline phases and microstructure of the energy-storage self-catalysis material were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and atomic force microscope. Results indicated that BiVO4 was assembled into Sr2MgSi2O7:Eu2þ,Dy3þ pores physically. The photocatalysis property of the assembled sample was characterized through degrading the 5 ppm Rhodamine B solution and kinetics property was fitted according to the LangmuireHinshelwood formula. Results demonstrated that the assembled compounds exhibited higher photodegradation efficiency than the pure BiVO4 no matter under the light or dark condition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, environmental pollution has increased more and more public concern [1,2], especially in water splitting, CO2 reduction and organic contaminants decomposition aspects [3]. Photocatalysts with high photoactivities have attracted considerable attention. Recently, monoclinic scheelite BiVO4 [4e7] is regard as a promising visible-light-driven photocatalyst material [5] due to its stability, effective sunlight utilization and high photoactivity [4,8]. Monoclinic BiVO4 exhibits band gap energy of 2.4eV, the absorption thresholds of which correspond to 517 nm. But one problem exists that BiVO4 is deprived of photoactivities in darkness. While long-lasting phosphor Sr2MgSi2O7:Eu2þ,Dy3þ can absorb and store energy, then release the energy slowly in darkness. The luminescence mechanism of long-lasting phosphor Sr2MgSi2O7:Eu2þ,Dy3þ can be explained by the hole transfer model. Eu2þ plays the role of luminescence center and Dy3þ as the trap center. When it is illuminated by light source, Eu2þ is excited from the ground state to the excited state and rapidly relaxed to the metastable state (Eu1þ). When the hole which is released by Eu2þ is captured by Dy3þ, Dy3þ is oxidized to Dy4þ. After stoping irradiating, the hole is again released into the valence band by subsequent thermal excitation and recombines with Eu1þ. Currently the studies of Sr2MgSi2O7:Eu2þ,Dy3þ mainly focused on the synthesis techniques, such as solidestate reaction method [9,10], solegel method [11]. Jun Luo et al. [12] synthesized Sr2MgSi2O7:Eu2þ,Dy3þ blue long-lasting phosphor by solidestate reaction method, whose emission peak was located at about 460 nm. Nowadays its afterglow time can last more than 12 h and it has widely used as luminous paints, emergency signs, decoration. But the released energy of Sr2MgSi2O7:Eu2þ,Dy3þ can not be used efficiently in darkness, which is a kind of energy waste. http://dx.doi.org/10.1016/j.jallcom.2015.09.168 0925-8388/© 2015 Elsevier B.V. All rights reserved.
While porous material is widely applied in composite material due to its large specific surface area and high porosity. Feng Gao et al. [13] studied the photoelectric properties of the composite material ZnO encapsulated in mesoporous silica flakes. They found that the composite material has the characteristics of stronger photoluminescence, optical transmission and shorter electrical response time. Hui Yu and Qingzhou Zhai [14] prepared mesoporous SBA-15 molecular sieve to evaluate its application as nimodipine drug delivery system. Results demonstrated that nimodipine can be selectively doped inside the pores of the mesoporous SBA15 molecular sieve and the medical effect of nimodipine was improved. So porous material is fit to acting as carrier in composite material. In order to make full use of the energy stored in Sr2MgSi2O7:Eu2þ,Dy3þ and improve the photocatalytic efficiency of BiVO4, we expect to assemble BiVO4 particles into Sr2MgSi2O7:Eu2þ,Dy3þ pores. In this article, catalyst BiVO4 particles distributed at around 70 nm were successfully assembled into the pores of the porous energy-storage material Sr2MgSi2O7:Eu2þ,Dy3þ, which was used as the energy carrier to provide external energy when the assembled sample was exposed in darkness. Selfcatalytic action happened according to the phenomenon of RhB solution degradation. The photocatalytic property and its dynamics fitting were investigated. 2. Experimental details 2.1. Preparation of compounds The raw materials Bi(NO3)3$5H2O, NH4VO3 and EDTA were purchased from Tianjin Guangfu Technology Development Co. Ltd. All the chemicals were of analytical grade and used as received
2
Letter / Journal of Alloys and Compounds 655 (2016) 1e5
without further purification. The energy-storage self-catalysis material was assembled together by the impregnation method, using energy-storage carrier Sr2MgSi2O7:Eu2þ,Dy3þ and object BiVO4 particles. The carrier porous Sr2MgSi2O7:Eu2þ,Dy3þ powder was previously prepared and suppressed into small wafer whose diameter was 8 mm and thickness 3 mm. The object catalysis material BiVO4 precursor was prepared by hydroethermal method. Bi(NO3)3$5H2O was dissolved in HNO3 (30wt%). NH4VO3 and EDTA-2Na were separately dissolved in distilled water. The solutions were mixed together with the order that put the NH4VO3 solution firstly and then EDTA solution into Bi(NO3)3$5H2O solution, respectively. The mixture was stirred for 5 h into homogeneous precursor and transferred into a 150 mL Teflon-lined stainless autoclave, pH of which was about 1. The carrier was put into the object precursor in the drying closet for several hours, which the reaction schematic diagram was showed in Fig. 1. Finally the compound was dried at 100 C for 12 h, followed by sintering at 400 C for 3 h in air. The sample was obtained after cooling down to room temperature in furnace. In addition, the preparation of unassembled BiVO4 was the same as the above processes, except for the addition of the carrier. 2.2. Characterization of the samples The crystalline phases were investigated using an X-ray diffractometer (Rigaku D/Max-2500) with Cu Ka radiation (l ¼ 0.15406 nm) and scanned from 10 to 70 . The micromorphology were analyzed by scanning electron microscopy (SEM, Hitachi Limited, Japan) and energy dispersive spectroscopy (EDS, Hitachi Limited, Japan). The surface of the sample was scanned with atomic force microscope (AFM, NSF-Es, Switzerland). 2.3. Evaluation of photocatalytic activity The photocatalytic activity of the compounds was evaluated by the degradation of Rhodamine B (RhB) aqueous solution under simulated sun-light irradiation in a quartz photochemical reactor. A 500W Xenon light was used as sun-light source. Compared to the compounds, 0.04 g pure BiVO4 and the compounds were added into 50 mL, 5 ppm RhB solution. Before illumination, the suspensions were maintained vigorous stirring in dark for 30 min to reach the absorptionedesorption equilibrium. Afterwards, the suspensions were irradiated by xenon light while vigorous stirring, at given time intervals, 5 mL of suspension was collected and centrifuged to remove the particles. The concentration of RhB was analyzed by the absorbance at lmax ¼ 554 nm, on a UVevis spectrophotometer (Shanghai Chenguang, 723N).
Fig. 1. Schematic diagram of the assembling process with the impregnation method.
In order to investigate the kinetics during the photocatalysis RhB degradation process, the kinetics fitting was studied according to the LangmuireHinshelwood formula as followed:
ln
C0 ¼ kt Ct
Ct: Reactant concentration after some time, mol/L; t: Reaction time, min; K: Adsorption value of reactants, L/mol. 3. Results and discussions Fig. 2 shows XRD analysis of the assembled compounds, pure BiVO4 and the carrier Sr2MgSi2O7:Eu2þ,Dy3þ. The curve of the compounds implies that both Sr2MgSi2O7:Eu2þ, Dy3þ phase and monoclinic scheelite BiVO4 phase can be identified. The preparation of object BiVO4 was investigated previously with the hydrothermal method. It can be prepared under 140 C for 15 h and latter the object BiVO4 in the carrier pores was calcined at 400 C for 3 h. It was reported by Guoqiang Tan [15] that tz-BiVO4 was formed at low temperature and converted into ms-BiVO4 at higher temperature. Compared to the patterns of pure carrier Sr2MgSi2O7:Eu2þ, Dy3þ (PDF 15-0016) and object BiVO4 (PDF 14-0688), strength of the characteristic peaks decreased rapidly. In our opinion, this phenomenon was due to the different atmosphere the crystal occupied. The pores may restrict the growth of the object. Besides, the small amount of the object was also the reason why its characteristic peaks was weaker. Fig. 3 shows the SEM images and EDS analysis of the assembled compounds. There are significant differences in the morphologies and particle shape of the object BiVO4. Apparently, the object BiVO4 performed regular square slabs, whose size was about 70 nm. And it distributed uniformly in the pores and on the surface of the carrier. The clear structure of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ is revealed by the magnified image shown in the inset of Fig. 3(a), which shows that the carrier was consisted with regular pores distributed at about 400 nm. To verify the introduction of the object BiVO4, the EDS spectra of the partial BiVO4 are shown in Fig. 3(b). Clearly, the particles near the pores was analyzed with Bi, V and O elements. Consequently, it implies that the BiVO4 was introduced into the pores of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ. Fig. 4 shows AFM analysis of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ and the assembled sample. The lamellar structure showed in
Fig. 2. XRD patterns of the assembled samples.
Letter / Journal of Alloys and Compounds 655 (2016) 1e5
3
Fig. 3. SEM images (a) and EDS analysis (b) of the assembled samples.
Fig. 4. AFM analysis of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ (a) and the assembled compounds (b).
Fig. 4(a) is one of the five common structure which was belonged to Sr2MgSi2O7:Eu2þ,Dy3þ. In the lamellar structure, each four adjacent silica tetrahedrons in the same level are connected together according to their angle. Then it changed into the closed tetrahedron. When some metal ion entry into the crystal lattice, the tetrahedron sheet are linked together and then formed octahedral sheet. Consequently, the interlaced tetrahedron sheet and octahedral sheet result into the lamellar structure. It is also noticed by Kostova M.H [16]. that Photoluminescent layered Y (III) and Tb (III) silicates doped with Ce (III) performed obvious lamellar structure and brilliant luminescent property. The sunken part in Fig. 4(a) is considered as the pore structure which is proper to be a carrier. The AFM analysis of the assembled compounds is shown in Fig. 4(b). Being obviously different with Fig. 4(a), many embossments and hollows can be detected on the surface of the assembled compounds. And the lamellar structure of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ has not been destroyed. Together with the analysis of SEM images, the assembling process caused successful combination of the energystorage material Sr2MgSi2O7:Eu2þ,Dy3þ and the catalysis material BiVO4. In order to investigate the photocatalysis property of the assembled compounds, RhB photodegradation under 500W Xe light irradiation are shown in Fig. 5. Fig. 5(a) and (b) exhibit the UVevis absorption spectra changes of photocatalytic degradation for RhB under simulated Xe light irradiation by pure BiVO4 and the assembled compounds. It is observed that the maximum absorption band decreases slowly at the wavelength of 554 nm with the increase of reaction time. In addition, there is no obvious shift on the maximum absorption band of RhB, which infers that RhB may
suffers a facile cleavage of the whole conjugated chromophore [17]. The UVevis absorption spectra changes of the assembled compound are shown in Fig. 5(b). The maximum absorption band decline sharply, which means that RhB are completely decomposed. Fig.5(c) shows the photodegradation of all catalysts under simulated Xe light irradiation. In order to eliminate the self degradation of RhB, the pure RhB was used as a contrast experiment. The results in Fig. 5(c) clearly showed that the self degradation of RhB can reach 29.1% and that the assembled compounds exhibit higher photodegradation efficiency than the pure BiVO4. After 150 min irradiation, the photodegradation rate has reached up to 94.1% for the assembled sample, surpassing 10.7% than the pure BiVO4. Since the samples were exposed in light condition, therefore, the function of Sr2MgSi2O7:Eu2þ,Dy3þ as an energy storage-release carrier can't be reflected. So the difference of above results indicates that the porous atmosphere provided by the carrier plays an important role in the improvement of photocatalytic activity of the catalysts. The tremendous area beyond to the mesporous carrier Sr2MgSi2O7:Eu2þ,Dy3þ provides more efficient reaction points, which could expand the area of the catalysts and improve the efficiency. Similar to our results, Deng et al. [18] reported the magnetization property of Fe3O4 particles that located in the SiO2 cores was improved obviously. To quantitatively study the reaction kinetics of RhB photodegradation, the LanmuireHinshelwood model is followed, as the equation ln(C0/C) ¼ kt, where k is reaction rate constant, C0 is initial concentration of RhB, C is concentration of RhB at the reaction time t [19]. Fig. 5(d) presents the relationship between ln(C0/C)
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Letter / Journal of Alloys and Compounds 655 (2016) 1e5
Fig. 5. UVevis absorption spectra of photocatalytic degradation for RhB by pure BiVO4 (a) and assembled compound (b); the photodegradation of the samples (c) and the dynamics of RhB photodegradation reaction (ln(C0/C) versus time) for the samples under simulated sun-light irradiation (d).
Fig. 6. The RhB degradation of the samples (a) and the dynamics of RhB photodegradation reaction (ln(C0/C) versus time) for the samples under the dark condition (b).
and irradiation time for catalysts under simulated Xe light irradiation. From Fig. 5(d), the reaction rate constants (k) of the assembled sample is higher than that of pure BiVO4. The result fully illustrates that the porous efficient points provided by the carrier have the superior photocatalytic activities. Fig. 6 shows RhB photodegradation of the pure BiVO4 and the assembled sample with no outside energy which was irradiated under Xe light for 30 min firstly and then exposed in darkness for
150 min. Fig. 6(a) illustrates the photodegradation rate of the two catalysts. It is clear to notice that when exposed in darkness, degradation rate of RhB, the pure BiVO4 and the assembled sample were 9%, 10.4% and 25.7%, respectively. As the contrastive experiment, the degradation performance of RhB was attributed to the loss of its eOH functional group. That the slight difference of the degradation rate of the pure RhB and the pure BiVO4 was to say that catalyst BiVO4 had no effect on degradation of RhB in darkness. It is worth to
Letter / Journal of Alloys and Compounds 655 (2016) 1e5
notice that the BiVO4 which can absorb visible light assembled in the porous energy-storage Sr2MgSi2O7:Eu2þ,Dy3þ which emits 430e510 nm blue light showed degradation in darkness, which was considered that the carrier Sr2MgSi2O7:Eu2þ, Dy3þ provided not only external energy, but more recombination centers probably. In this case, electrons located on the valence band was irradiated to conductive band and an electron hole was left, who jumped to the surface of particle and made full use of eOH functional group to react with the pollutant [20]. Fig. 6(b) presents the relationship between ln(C0/C) and the reaction time t. Obviously the reaction rate constant (k) of the assembled sample are higher than that of the pure BiVO4, which are 0.00197 min1 and 0.00128 min1, respectively. The result infers that assembled sample has the superior photocatalytic activity.
[7]
[8]
[9]
[10]
[11]
[12]
4. Conclusion [13]
Energy-storage self-catalysis material has been successfully prepared by the impregnation method. XRD results demonstrate phases of Sr2MgSi2O7:Eu2þ,Dy3þ and BiVO4 comparing to each pure sample. The crystallinity of both the carrier and the object are influenced after assembled together. The morphology of cubic-like catalyst can be found inside and outside the pores. EDS analysis shows that the object was composed with Bi, V and O elements. The existence of the energy-storage carrier is detrimental to the higher degradation rate of the assembled sample reacted in darkness. Finally, the assembled sample provides an efficient way to achieve the self-catalysis action of the catalyst BiVO4 in darkness and degradation of RhB solution. Acknowledgments The authors gratefully acknowledge the support from the Natural Science Foundation of Hebei Province (Grant No. 2014209204) and the Program for Top-notch Young Talents in university of Hebei Province (Grant No. BJ20142030). References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301e310. [2] S. Chala, K. Wetchakun, S. Phanichant, B. Inceesungvorn, N. Wetchakun, Enhanced visible-light-response photocatalytic degradationof methylene blue on Fe-loaded BiVO4 photocatalyst, J. Alloys Compd. 597 (2014) 129e135. n, G. Colo n, Heterostructured Er3þ doped BiVO4 with exceptional [3] S. Obrego photocatalytic performance by cooperative electronic and luminescence sensitization mechanism, Appl. Catal. B 158e159 (2014) 242e249. [4] M.Z. Xie, Y.J. Feng, X.D. Fu, L.A. Peng, L.Q. Jing, Phosphate-bridged TiO2-BiVO4 nanocomposites with exceptional visible activities for photocatalytic water splitting, J. Alloys Compd. 631 (2015) 120e124. [5] Y.Y. Luo, G.Q. Tan, G.H. Dong, H.J. Ren, A. Xia, Effects of structure, morphology, and up-conversion on Nd-doped BiVO4 system with high photocatalytic activity, Ceram. Int. 41 (2015) 3259e3268. [6] J. Huang, G.Q. Tan, L.L. Zhang, H.J. Ren, A. Xia, C.C. Zhao, Enhanced
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photocatalytic activity of tetragonal BiVO4: influenced by rare earth ion Yb3þ, Mater. Lett. 133 (2014) 20e23. B.X. Lei, L.L. Zeng, P. Zhang, Z.F. Sun, W. Sun, X.X. Zhang, Hydrothermal synthesis and photocatalytic properties of visible-light induced BiVO4 with different morphologies, Adv. Powder Technol. 25 (2014) 946e951. A.J.E. Rettie, H.C. Lee, L.G. Marshall, J.-F. Lin, C. Capan, J. Lindemuth, J.S. McCloy, J. Zhou, A.J. Bard, C.B. Mullins, Combined charge carrier transport and photoelectrochemical characterization of BiVO4 single crystals: intrinsic behavior of a complex metal oxide, J. Am. Chem. Soc. 135 (2013) 11389e11396. Y.Q. Li, Y.H. Wang, X.H. Xu, Y. Gong, Effects of non-stoichiometry on crystallinity, photoluminescence and afterglow properties of Sr2MgSi2O7:Eu2þ,Dy3þ phosphors, J. Lumin. 129 (2009) 1230e1234. H.Y. Wu, Y.H. Hu, L. Chen, X.J. Wang, Investigation on the enhancement and the suppression of persistent luminescence of Re3þdoped Sr2EuMgSi2O7(Re ¼ Dy, Yb), J. Alloys Compd. 509 (2011) 4304e4307. X.J. Geng, Y.J. Chen, G.M. Qiu, L.J. Xiao, C.H. Yan, Y.B. Sun, Synthesis of long afterglow photoluminescent materials Sr2MgSi2O7:Eu2þ,Dy3þ by solegel method, J. Rare Earths 23 (2005) 292e294. J. Luo, Q. Gao, K.Y. Zhang, M.Q. Ge, J.L. Liu, Structure and luminescent properties of luminous polypropylene fiber based on Sr2MgSi2O7:Eu2þ,Dy3þ, J. Rare Earths 32 (2014) 696e701. F. Gao, N. Chino, S.P. Naik, Y. Sasaki, T. Okubo, Photoelectric properties of nano-ZnO fabricated in mesoporous silica film, Mater. Lett. 61 (2007) 3179e3184. H. Yu, Q.Z. Zhai, Mesoporous SBA-15 molecular sieve as a carrier for controlled release of nimodipine, Microporous Mesoporous Mater. 123 (2009) 298e305. Y.Y. Luo, G.Q. Tan, G.H. Dong, L.L. Zhang, J. Huang, W. Yang, C.C. Zhao, H.J. Ren, Structure transformation of Sm3þ doped BiVO4 with high photocatalytic activity under simulated sun-light, Appl. Surf. Sci. 324 (2015) 505e511. M.H. Kostova, R.A.S. Ferreira, D. Ananias, L.D. Carlos, J. Rocha, Photoluminescent layered Y (III) and Tb (III) silicates doped with Ce (III), J. Phys. Chem. B 110 (2006) 15312e15316. rez, A. Martínez-de la Cruz, S. Sepúlveda-Guzma n, J. Peral, LowU.M. García-Pe temperature synthesis of BiVO4 powders by Pluronic-assisted hydrothermalmethod: effect of the surfactant and temperature on the morphology and structural control, Ceram. Int. 40 (2014) 4631e4638. Y.H. Deng, D.W. Oi, C.H. Deng, X.M. Zhang, D.Y. Zhao, Superparamagnetic high-magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Ceram. Soc. 130 (2008) 28e29. P. Ju, P. Wang, B. Li, H. Fan, S.Y. Ai, D. Zhang, Y. Wang, A novel calcined Bi2WO6/BiVO4 heterojunction photocatalyst with highly enhanced photocatalytic activity, Chem. Eng. J. 236 (2014) 430e437. N. Arconada, A. Duran, S. Suarez, R. Portela, J.M. Coronado, B. Sanchez, Y. Castro, Synthesis and photocatalytic properties of dense and porous TiO2anatase thin films prepared by solegel, Appl. Catal. B 86 (2009) 1e7.
Yanzhi Meng, Yi Shen*, Luyao Hou, Guifu Zuo, Xiaoli Wei, Xiaomin Wang College of Materials Science and Engineer, North China University of Science and Technology, Tangshan, Hebei, 063009, PR China Fengfeng Li Qing Gong College, North China University of Science and Technology, 063000, PR China * Corresponding author. E-mail address: [email protected] (Y. Shen).
27 May 2015 Available online 21 September 2015
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Letter
Study on photocatalysis and dynamics properties of self-catalysis BiVO4 assembled in porous Sr2MgSi2O7:Eu2þ,Dy3þ a b s t r a c t Keywords: Sr2MgSi2O7:Eu2þ,Dy3þ BiVO4 Energy-storage Self-catalysis Dynamics
Energy-storage self-catalysis material that BiVO4 assembled into porous Sr2MgSi2O7:Eu2þ,Dy3þ was successfully synthesized by the impregnation method. Crystalline phases and microstructure of the energy-storage self-catalysis material were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and atomic force microscope. Results indicated that BiVO4 was assembled into Sr2MgSi2O7:Eu2þ,Dy3þ pores physically. The photocatalysis property of the assembled sample was characterized through degrading the 5 ppm Rhodamine B solution and kinetics property was fitted according to the LangmuireHinshelwood formula. Results demonstrated that the assembled compounds exhibited higher photodegradation efficiency than the pure BiVO4 no matter under the light or dark condition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, environmental pollution has increased more and more public concern [1,2], especially in water splitting, CO2 reduction and organic contaminants decomposition aspects [3]. Photocatalysts with high photoactivities have attracted considerable attention. Recently, monoclinic scheelite BiVO4 [4e7] is regard as a promising visible-light-driven photocatalyst material [5] due to its stability, effective sunlight utilization and high photoactivity [4,8]. Monoclinic BiVO4 exhibits band gap energy of 2.4eV, the absorption thresholds of which correspond to 517 nm. But one problem exists that BiVO4 is deprived of photoactivities in darkness. While long-lasting phosphor Sr2MgSi2O7:Eu2þ,Dy3þ can absorb and store energy, then release the energy slowly in darkness. The luminescence mechanism of long-lasting phosphor Sr2MgSi2O7:Eu2þ,Dy3þ can be explained by the hole transfer model. Eu2þ plays the role of luminescence center and Dy3þ as the trap center. When it is illuminated by light source, Eu2þ is excited from the ground state to the excited state and rapidly relaxed to the metastable state (Eu1þ). When the hole which is released by Eu2þ is captured by Dy3þ, Dy3þ is oxidized to Dy4þ. After stoping irradiating, the hole is again released into the valence band by subsequent thermal excitation and recombines with Eu1þ. Currently the studies of Sr2MgSi2O7:Eu2þ,Dy3þ mainly focused on the synthesis techniques, such as solidestate reaction method [9,10], solegel method [11]. Jun Luo et al. [12] synthesized Sr2MgSi2O7:Eu2þ,Dy3þ blue long-lasting phosphor by solidestate reaction method, whose emission peak was located at about 460 nm. Nowadays its afterglow time can last more than 12 h and it has widely used as luminous paints, emergency signs, decoration. But the released energy of Sr2MgSi2O7:Eu2þ,Dy3þ can not be used efficiently in darkness, which is a kind of energy waste. http://dx.doi.org/10.1016/j.jallcom.2015.09.168 0925-8388/© 2015 Elsevier B.V. All rights reserved.
While porous material is widely applied in composite material due to its large specific surface area and high porosity. Feng Gao et al. [13] studied the photoelectric properties of the composite material ZnO encapsulated in mesoporous silica flakes. They found that the composite material has the characteristics of stronger photoluminescence, optical transmission and shorter electrical response time. Hui Yu and Qingzhou Zhai [14] prepared mesoporous SBA-15 molecular sieve to evaluate its application as nimodipine drug delivery system. Results demonstrated that nimodipine can be selectively doped inside the pores of the mesoporous SBA15 molecular sieve and the medical effect of nimodipine was improved. So porous material is fit to acting as carrier in composite material. In order to make full use of the energy stored in Sr2MgSi2O7:Eu2þ,Dy3þ and improve the photocatalytic efficiency of BiVO4, we expect to assemble BiVO4 particles into Sr2MgSi2O7:Eu2þ,Dy3þ pores. In this article, catalyst BiVO4 particles distributed at around 70 nm were successfully assembled into the pores of the porous energy-storage material Sr2MgSi2O7:Eu2þ,Dy3þ, which was used as the energy carrier to provide external energy when the assembled sample was exposed in darkness. Selfcatalytic action happened according to the phenomenon of RhB solution degradation. The photocatalytic property and its dynamics fitting were investigated. 2. Experimental details 2.1. Preparation of compounds The raw materials Bi(NO3)3$5H2O, NH4VO3 and EDTA were purchased from Tianjin Guangfu Technology Development Co. Ltd. All the chemicals were of analytical grade and used as received
2
Letter / Journal of Alloys and Compounds 655 (2016) 1e5
without further purification. The energy-storage self-catalysis material was assembled together by the impregnation method, using energy-storage carrier Sr2MgSi2O7:Eu2þ,Dy3þ and object BiVO4 particles. The carrier porous Sr2MgSi2O7:Eu2þ,Dy3þ powder was previously prepared and suppressed into small wafer whose diameter was 8 mm and thickness 3 mm. The object catalysis material BiVO4 precursor was prepared by hydroethermal method. Bi(NO3)3$5H2O was dissolved in HNO3 (30wt%). NH4VO3 and EDTA-2Na were separately dissolved in distilled water. The solutions were mixed together with the order that put the NH4VO3 solution firstly and then EDTA solution into Bi(NO3)3$5H2O solution, respectively. The mixture was stirred for 5 h into homogeneous precursor and transferred into a 150 mL Teflon-lined stainless autoclave, pH of which was about 1. The carrier was put into the object precursor in the drying closet for several hours, which the reaction schematic diagram was showed in Fig. 1. Finally the compound was dried at 100 C for 12 h, followed by sintering at 400 C for 3 h in air. The sample was obtained after cooling down to room temperature in furnace. In addition, the preparation of unassembled BiVO4 was the same as the above processes, except for the addition of the carrier. 2.2. Characterization of the samples The crystalline phases were investigated using an X-ray diffractometer (Rigaku D/Max-2500) with Cu Ka radiation (l ¼ 0.15406 nm) and scanned from 10 to 70 . The micromorphology were analyzed by scanning electron microscopy (SEM, Hitachi Limited, Japan) and energy dispersive spectroscopy (EDS, Hitachi Limited, Japan). The surface of the sample was scanned with atomic force microscope (AFM, NSF-Es, Switzerland). 2.3. Evaluation of photocatalytic activity The photocatalytic activity of the compounds was evaluated by the degradation of Rhodamine B (RhB) aqueous solution under simulated sun-light irradiation in a quartz photochemical reactor. A 500W Xenon light was used as sun-light source. Compared to the compounds, 0.04 g pure BiVO4 and the compounds were added into 50 mL, 5 ppm RhB solution. Before illumination, the suspensions were maintained vigorous stirring in dark for 30 min to reach the absorptionedesorption equilibrium. Afterwards, the suspensions were irradiated by xenon light while vigorous stirring, at given time intervals, 5 mL of suspension was collected and centrifuged to remove the particles. The concentration of RhB was analyzed by the absorbance at lmax ¼ 554 nm, on a UVevis spectrophotometer (Shanghai Chenguang, 723N).
Fig. 1. Schematic diagram of the assembling process with the impregnation method.
In order to investigate the kinetics during the photocatalysis RhB degradation process, the kinetics fitting was studied according to the LangmuireHinshelwood formula as followed:
ln
C0 ¼ kt Ct
Ct: Reactant concentration after some time, mol/L; t: Reaction time, min; K: Adsorption value of reactants, L/mol. 3. Results and discussions Fig. 2 shows XRD analysis of the assembled compounds, pure BiVO4 and the carrier Sr2MgSi2O7:Eu2þ,Dy3þ. The curve of the compounds implies that both Sr2MgSi2O7:Eu2þ, Dy3þ phase and monoclinic scheelite BiVO4 phase can be identified. The preparation of object BiVO4 was investigated previously with the hydrothermal method. It can be prepared under 140 C for 15 h and latter the object BiVO4 in the carrier pores was calcined at 400 C for 3 h. It was reported by Guoqiang Tan [15] that tz-BiVO4 was formed at low temperature and converted into ms-BiVO4 at higher temperature. Compared to the patterns of pure carrier Sr2MgSi2O7:Eu2þ, Dy3þ (PDF 15-0016) and object BiVO4 (PDF 14-0688), strength of the characteristic peaks decreased rapidly. In our opinion, this phenomenon was due to the different atmosphere the crystal occupied. The pores may restrict the growth of the object. Besides, the small amount of the object was also the reason why its characteristic peaks was weaker. Fig. 3 shows the SEM images and EDS analysis of the assembled compounds. There are significant differences in the morphologies and particle shape of the object BiVO4. Apparently, the object BiVO4 performed regular square slabs, whose size was about 70 nm. And it distributed uniformly in the pores and on the surface of the carrier. The clear structure of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ is revealed by the magnified image shown in the inset of Fig. 3(a), which shows that the carrier was consisted with regular pores distributed at about 400 nm. To verify the introduction of the object BiVO4, the EDS spectra of the partial BiVO4 are shown in Fig. 3(b). Clearly, the particles near the pores was analyzed with Bi, V and O elements. Consequently, it implies that the BiVO4 was introduced into the pores of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ. Fig. 4 shows AFM analysis of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ and the assembled sample. The lamellar structure showed in
Fig. 2. XRD patterns of the assembled samples.
Letter / Journal of Alloys and Compounds 655 (2016) 1e5
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Fig. 3. SEM images (a) and EDS analysis (b) of the assembled samples.
Fig. 4. AFM analysis of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ (a) and the assembled compounds (b).
Fig. 4(a) is one of the five common structure which was belonged to Sr2MgSi2O7:Eu2þ,Dy3þ. In the lamellar structure, each four adjacent silica tetrahedrons in the same level are connected together according to their angle. Then it changed into the closed tetrahedron. When some metal ion entry into the crystal lattice, the tetrahedron sheet are linked together and then formed octahedral sheet. Consequently, the interlaced tetrahedron sheet and octahedral sheet result into the lamellar structure. It is also noticed by Kostova M.H [16]. that Photoluminescent layered Y (III) and Tb (III) silicates doped with Ce (III) performed obvious lamellar structure and brilliant luminescent property. The sunken part in Fig. 4(a) is considered as the pore structure which is proper to be a carrier. The AFM analysis of the assembled compounds is shown in Fig. 4(b). Being obviously different with Fig. 4(a), many embossments and hollows can be detected on the surface of the assembled compounds. And the lamellar structure of the carrier Sr2MgSi2O7:Eu2þ,Dy3þ has not been destroyed. Together with the analysis of SEM images, the assembling process caused successful combination of the energystorage material Sr2MgSi2O7:Eu2þ,Dy3þ and the catalysis material BiVO4. In order to investigate the photocatalysis property of the assembled compounds, RhB photodegradation under 500W Xe light irradiation are shown in Fig. 5. Fig. 5(a) and (b) exhibit the UVevis absorption spectra changes of photocatalytic degradation for RhB under simulated Xe light irradiation by pure BiVO4 and the assembled compounds. It is observed that the maximum absorption band decreases slowly at the wavelength of 554 nm with the increase of reaction time. In addition, there is no obvious shift on the maximum absorption band of RhB, which infers that RhB may
suffers a facile cleavage of the whole conjugated chromophore [17]. The UVevis absorption spectra changes of the assembled compound are shown in Fig. 5(b). The maximum absorption band decline sharply, which means that RhB are completely decomposed. Fig.5(c) shows the photodegradation of all catalysts under simulated Xe light irradiation. In order to eliminate the self degradation of RhB, the pure RhB was used as a contrast experiment. The results in Fig. 5(c) clearly showed that the self degradation of RhB can reach 29.1% and that the assembled compounds exhibit higher photodegradation efficiency than the pure BiVO4. After 150 min irradiation, the photodegradation rate has reached up to 94.1% for the assembled sample, surpassing 10.7% than the pure BiVO4. Since the samples were exposed in light condition, therefore, the function of Sr2MgSi2O7:Eu2þ,Dy3þ as an energy storage-release carrier can't be reflected. So the difference of above results indicates that the porous atmosphere provided by the carrier plays an important role in the improvement of photocatalytic activity of the catalysts. The tremendous area beyond to the mesporous carrier Sr2MgSi2O7:Eu2þ,Dy3þ provides more efficient reaction points, which could expand the area of the catalysts and improve the efficiency. Similar to our results, Deng et al. [18] reported the magnetization property of Fe3O4 particles that located in the SiO2 cores was improved obviously. To quantitatively study the reaction kinetics of RhB photodegradation, the LanmuireHinshelwood model is followed, as the equation ln(C0/C) ¼ kt, where k is reaction rate constant, C0 is initial concentration of RhB, C is concentration of RhB at the reaction time t [19]. Fig. 5(d) presents the relationship between ln(C0/C)
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Letter / Journal of Alloys and Compounds 655 (2016) 1e5
Fig. 5. UVevis absorption spectra of photocatalytic degradation for RhB by pure BiVO4 (a) and assembled compound (b); the photodegradation of the samples (c) and the dynamics of RhB photodegradation reaction (ln(C0/C) versus time) for the samples under simulated sun-light irradiation (d).
Fig. 6. The RhB degradation of the samples (a) and the dynamics of RhB photodegradation reaction (ln(C0/C) versus time) for the samples under the dark condition (b).
and irradiation time for catalysts under simulated Xe light irradiation. From Fig. 5(d), the reaction rate constants (k) of the assembled sample is higher than that of pure BiVO4. The result fully illustrates that the porous efficient points provided by the carrier have the superior photocatalytic activities. Fig. 6 shows RhB photodegradation of the pure BiVO4 and the assembled sample with no outside energy which was irradiated under Xe light for 30 min firstly and then exposed in darkness for
150 min. Fig. 6(a) illustrates the photodegradation rate of the two catalysts. It is clear to notice that when exposed in darkness, degradation rate of RhB, the pure BiVO4 and the assembled sample were 9%, 10.4% and 25.7%, respectively. As the contrastive experiment, the degradation performance of RhB was attributed to the loss of its eOH functional group. That the slight difference of the degradation rate of the pure RhB and the pure BiVO4 was to say that catalyst BiVO4 had no effect on degradation of RhB in darkness. It is worth to
Letter / Journal of Alloys and Compounds 655 (2016) 1e5
notice that the BiVO4 which can absorb visible light assembled in the porous energy-storage Sr2MgSi2O7:Eu2þ,Dy3þ which emits 430e510 nm blue light showed degradation in darkness, which was considered that the carrier Sr2MgSi2O7:Eu2þ, Dy3þ provided not only external energy, but more recombination centers probably. In this case, electrons located on the valence band was irradiated to conductive band and an electron hole was left, who jumped to the surface of particle and made full use of eOH functional group to react with the pollutant [20]. Fig. 6(b) presents the relationship between ln(C0/C) and the reaction time t. Obviously the reaction rate constant (k) of the assembled sample are higher than that of the pure BiVO4, which are 0.00197 min1 and 0.00128 min1, respectively. The result infers that assembled sample has the superior photocatalytic activity.
[7]
[8]
[9]
[10]
[11]
[12]
4. Conclusion [13]
Energy-storage self-catalysis material has been successfully prepared by the impregnation method. XRD results demonstrate phases of Sr2MgSi2O7:Eu2þ,Dy3þ and BiVO4 comparing to each pure sample. The crystallinity of both the carrier and the object are influenced after assembled together. The morphology of cubic-like catalyst can be found inside and outside the pores. EDS analysis shows that the object was composed with Bi, V and O elements. The existence of the energy-storage carrier is detrimental to the higher degradation rate of the assembled sample reacted in darkness. Finally, the assembled sample provides an efficient way to achieve the self-catalysis action of the catalyst BiVO4 in darkness and degradation of RhB solution. Acknowledgments The authors gratefully acknowledge the support from the Natural Science Foundation of Hebei Province (Grant No. 2014209204) and the Program for Top-notch Young Talents in university of Hebei Province (Grant No. BJ20142030). References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301e310. [2] S. Chala, K. Wetchakun, S. Phanichant, B. Inceesungvorn, N. Wetchakun, Enhanced visible-light-response photocatalytic degradationof methylene blue on Fe-loaded BiVO4 photocatalyst, J. Alloys Compd. 597 (2014) 129e135. n, G. Colo n, Heterostructured Er3þ doped BiVO4 with exceptional [3] S. Obrego photocatalytic performance by cooperative electronic and luminescence sensitization mechanism, Appl. Catal. B 158e159 (2014) 242e249. [4] M.Z. Xie, Y.J. Feng, X.D. Fu, L.A. Peng, L.Q. Jing, Phosphate-bridged TiO2-BiVO4 nanocomposites with exceptional visible activities for photocatalytic water splitting, J. Alloys Compd. 631 (2015) 120e124. [5] Y.Y. Luo, G.Q. Tan, G.H. Dong, H.J. Ren, A. Xia, Effects of structure, morphology, and up-conversion on Nd-doped BiVO4 system with high photocatalytic activity, Ceram. Int. 41 (2015) 3259e3268. [6] J. Huang, G.Q. Tan, L.L. Zhang, H.J. Ren, A. Xia, C.C. Zhao, Enhanced
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Yanzhi Meng, Yi Shen*, Luyao Hou, Guifu Zuo, Xiaoli Wei, Xiaomin Wang College of Materials Science and Engineer, North China University of Science and Technology, Tangshan, Hebei, 063009, PR China Fengfeng Li Qing Gong College, North China University of Science and Technology, 063000, PR China * Corresponding author. E-mail address: [email protected] (Y. Shen).
27 May 2015 Available online 21 September 2015