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Surface properties, simultaneous photocatalytic and magnetic activities of Ni2FeVO6 nanoparticles
Applied Surface Science 359 (2015) 259–265
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Surface properties, simultaneous photocatalytic and magnetic activities of Ni2 FeVO6 nanoparticles Xuebin Qiao a , Yanlin Huang b , Han Cheng c , Hyo Jin Seo c,∗ a Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, PR China b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China c Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 August 2015 Received in revised form 8 October 2015 Accepted 16 October 2015 Available online 19 October 2015 Keywords: Semiconductors Nanostructures Photocatalysis Optical properties Surface properties
a b s t r a c t Nickel-ferro-vanadium oxide Ni2 FeVO6 nanoparticles were prepared by the sol–gel film coating and subsequent sintering method. The phase formation was investigated X-ray polycrystalline diffraction (XRD) measurement. The surface characteristics were measured by scanning electron microscope (SEM), transmission electron microscopy (TEM), specific surface area, energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). This vanadate has a narrow band-gap energy of 1.784 eV. The investigations concluded that Ni2 FeVO6 nanoparticles have photocatalytic ability under visible-light irradiation. The ferromagnetic behavior of the nanoparticles was confirmed by the magnetic hysteresis loops. The nanoparticles can be magnetically recoverable after photocatalytic reactions. The photocatalytic activities were discussed on the base of the multivalent cations in crystal lattices. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis of semiconductors have attracted much attention in decades, which are the redox reactions on the surfaces under the irradiation of UV–vis Light. The photocatalysis can be initiated by absorbing photons to create excited electron–hole pairs. Afterwards, some reactive oxidative species (ROS) such as OH• , O2 −• and holes could be directly and indirectly produced, which migrate to the surface and result in redox reactions. The photocatalysis is a potential application in removal of polluted organics, sterilizing most of the bacteria in the environment or the production of hydrogen by photocatalytic water splitting [1]. So far many photocatalysts have been developed with different crystal structures, and most of them containing transition metal ions such as V, Ta, Nb, W, Mo, etc. because these species are effective in obtaining photocatalytic activities [2]. Among them, complex vanadates (CVO) containing diverse vanadium-oxygen polyhedra linkages belongs to a group of useful catalysts used for the oxidation of organic compounds in solutions and splitting of water [1]. The visible-light driven vanadates have attracted growing interest such
∗ Corresponding author. E-mail address: [email protected] (H.J. Seo). http://dx.doi.org/10.1016/j.apsusc.2015.10.112 0169-4332/© 2015 Elsevier B.V. All rights reserved.
as BiVO4 [3–5], Bi0.5 Y0.5 VO4 [6], InVO4 [7], LiNiVO4 [8,9], M3 V2 O8 (M = Mg, Ni, Zn) [10], ␣-AgVO3 [11], BiCu2 VO6 [12], Bi4 V2 O11 [13], Bi23 V4 O44.5 [14], SrBi3 VO8 [15], Bi7 VO13 [16] etc. However, most of photocatalysts have a difficulty in the recycling after its photocatalytic use. Consequently, it is valuable to investigate photocatalysts with magnetic properties by considering its recycling. In this regard, photocatalyst nanoparticles with magnetic abilities have been reported in the most studies, for example, metal oxides coupled nanocomposites [17,18], Fe3 O4 @SiO2 [19,20], ZnFe2 O4 /SrFe12 O19 [21], ␣-Fe2 O3 nanoparticles [22] et al. In present work, Ni2 FeVO6 was selected to detect its possible application as a visible light driven photocatalyst. This motivation depends on its efficient optical absorption and magnetic properties due to the optical-active ions (Ni2+ and VO4 ) and magnetic ions (Fe3+ , Ni2+ ) in the crystal lattices, respectively. Crystal structures of vanadates are constructed by different linkages of vanadium oxygen polyhedral depending on the nature of the counter cations and stoichiometry of the host. Previous work indicates that Ni2 FeVO6 the shows efficient optical absorption in both UV and visible light wavelength region. The structure of Ni2 FeVO6 has been reported in the references. Ni2 FeVO6 crystallizes in the FeTiO3 -type (ilmenite) rhombohedral structure, which contains alternate metal layers forming a two-dimensional magnetic structure [23–27]. In Ni2 FeVO6−ı the EPR signals have been assigned to the high-spin centers or ferromagnetic clusters [28].
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Our aim in this article is to present the synthesis, surface properties, and photocatalytic activity of Ni2 FeVO6 nanoparticles prepared using the sol–gel film coating and subsequent sintering treatment. The crystal phase formation, the detailed surface properties and photoactivities were investigated. The Ni2 FeVO6 nanoparticles show potential application for the photo degradation of methylene blue (MB) under visible-light irradiation. 2. Experimental Ni2 FeVO6 nanoparticles were prepared by the sol–gel film coating and subsequent sintering method. The raw materials are NH4 VO3 , Fe(NO3 )3 ·9H2 O and Ni(NO3 )2 ·6H2 O. Firstly, An appropriate citric acid were added in the clear solutions containing the stoichiometric raw materials to get a full complexation reaction of the Fe3+ , Ni2+ and VO4 3− ions. Then the solutions were neutralized by the ammonium hydroxide (30% wt). Secondly, the solutions were added into some liquid polyvinylalcohol (PVA) to adjust its viscosity. The homogeneous solutions were obtained by continue stirring for about 4 h. Thirdly, the viscous solutions were slowly coated on the glass-substrates to prepare the films. After natural air drying, the dried films can be obtained containing the raw materials. Finally, the films were peeled off from the glass-substrates and then heated at 800 ◦ C for 2 h. The loose powders could be were obtained after the elimination of the organic components during the heating process. XRD patterns of the samples were measured using a Rigaku D/Max diffractometer (40 kV, 30 mA) with Cu K␣ radiation. SEM pictures were recorded to study the surface morphologies. The images were measured using a JEOL JEM-2010F microscope. The XPS measurements were completed on Kratos analytical, ESCA3400, Shimadzu. The photocatalysis experiments were finished in the reactor (500 ml) equipped by an xenon lamp (500 W) providing the visible light irradiation. In this process, a cut-filter with 420 nm was applied to avoid the UV light. An air-flow was supplied in the reactor by a pump. In a typical experiment, the photocatalyst was supplied by 0.05 g by mixing it in the 300 mL methylene blue (10 mg L−1 ) solution. Prior to photocatalysis the system was stayed in dark for 30 min to fulfill the adsorption/desorption equilibrium. At a designed time interval, 5 mL methylene blue solution was extracted to analyze the optical absorption on a UV–vis spectrophotometer. The degradation percentage could be obtained from the formula of [1 − (A/A0 )] × 100%, where A0 is the original optical absorption before the light irradiation; and Ai is that taken after 15 min light irradiation.
Fig. 1. A typical XRD pattern of Ni2 FeVO6 nanoparticles in comparison with the PDF#2 standard card No: 53-0372.
Fig. 2. The typical SEM pictures of Ni2 FeVO6 nanoparticles.
of V2 O5 ·nH2 O sheets. The cations could insert into the layers. The nanorods could be developed with the crystallization process. For example, it has been reported in MnV2 O6 nanorods that the regular nanoplates could be firstly developed with V2 O5 ·nH2 O sheets [29]. After Mn2+ ions were introduced into the sol system, the Mn2+ ions were intercalated into the layered structure and then reacted with V2 O5 ·nH2 O sheets to form aligned MnV2 O6 nanosheets. Fig. 3 is the EDX spectrum to investigate the elemental compositions in the nanoparticles. The specific emission lines show the
3. Results 3.1. Phase formation and surface properties Fig. 1 presents the XRD result of nickel iron vanadium oxide Ni2 FeVO6 nanoparticles compared with the standard card PDF2 No. 53-0372. The experimental pattern was fully indexed to the standard card. The result reveals that the sample crystallized with a pure Ni2 FeVO6 phase. The nanoparticles crystallize in a ˚ b = 8.427 A, ˚ rhombic phase the unit cell parameters of a = 17.316 A, ˚ V = 853.61 A˚ 3 , and Z = 8 [23]. c = 5.982 A, The typical SEM images of Ni2 FeVO6 nanoparticles are shown in Fig. 2. The sample contains closely-packed ball-like units. The particles have a little aggregation, which is easy to disperse under a gentle grind in a mortar or stirring in a solution. As shown in Fig. 2, the sample contains some nanorods, which have length and radius of 500 nm and 50 nm, respectively. In sol–gel preparation of vanadates, the vanadate gels easily form layered-structures made
Fig. 3. The typical EDX spectrum of Ni2 FeVO6 nanoparticles and the element percentage.
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Fig. 4. The TEMs (a, b), HRTEM image (c), SAED pattern (d) of Ni2 FeVO6 nanoparticles.
EDX signals from the elements V, Ni, Fe, and O in the lattices. The componential ratio of V/Ni(Fe) was in agreement with the stoichiometric value required in the chemical formulae of Ni2 FeVO6 . The TEM images of Ni2 FeVO6 nanoparticles are shown in Fig. 4a and b confirming the morphology of the nanorods. The length and width were estimated to be about 500 nm and 50 nm, respectively. The measured HRTEM images (Fig. 4c) present well-resolved lattice fringes, which show the interplanar spacing d of 0.502 nm corresponding to the (3 1 0) lattice plane in Ni2 FeVO6 nanorods. The well crystallized single-phase structure of Ni2 FeVO6 nanoparticles can be confirmed by SAED pattern in Fig. 4d. The Brunauer–Emmett–Teller (BET) measurement for the surface-area and the pore-size distribution of Ni2 FeVO6 nanoparticles were measured and shown in Fig. 5. The isotherm is in the representative IV pattern a characterization of a hysteresis loop. The adsorption region at P/P0 is close to 1.0 indicating the coexistence of mesopores and macropores in Ni2 FeVO6 nanoparticles. The specific surface area of Ni2 FeVO6 nanoparticles was decided to be 65 m2 g−1 . The pore size distribution is estimated to be about
on 50 nm (inset Fig. 4). The pore size distribution was narrow and monomodal indicating that the sample is composed of the uniform nanoparticles.
Fig. 6 shows the UV–vis absorption spectra of Ni2 FeVO6 nanoparticles, which has the wide optical absorption in the visiblelight region. The spectrum shows that the absorption edge of extends to wavelength 700 nm. This suggests that the sample is qualified for the possible photocatalysis under visible light irradiation. The energy of band-gap (Eg ) was calculated to be 1.784 eV (670 nm) based on the equation of ˛h ∝ (h − Eg )k , where ˛: absorbance, : optical frequency; the electronic transitions could be direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions when k is 1/2, 2, 3/2 or 3, respectively. In the nanorods, k = 1/2 was confirmed to be a best fitting parameter indicating a direct-gap character for the material [30]. As shown insert in Fig. 6, Ni2 FeVO6 powders show Dark brown color. The deep color
Fig. 5. The typical N2 adsorption–desorption isotherms with the corresponding pore-size distribution curve of Ni2 FeVO6 nanoparticles.
Fig. 6. The UV–vis absorption spectrum of Ni2 FeVO6 nanoparticles; inset band-gap calculation and the digital photos of the nanopowders.
3.2. UV–vis optical absorption
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Fig. 8. The repetitive operations of Ni2 FeVO6 nanoparticles for photocatalytic degradation of MB solutions.
optical absorption could be degradated by 90% in 300 min. The results show that Ni2 FeVO6 is a possible efficient photocatalyst. The photo-stability of Ni2 FeVO6 was examined by recycling the photocatalytic experiments. Fig. 8 is three repetitive operations of Ni2 FeVO6 nanoparticles for MB photodegradation. The results show that the photodegradation of MB solutions keeps a stable level. 3.4. Magnetic property
Fig. 7. (a): The UV/vis absorption spectra; and (b): the photodegradation effects of MB solutions by Ni2 FeVO6 nanoparticles, Ni2 FeVO6 + • OH scavenger of the Tert-butyl alcohol (TBA) additive, and P25 on under visible-light irradiation.
is obviously generally attributed to crystal-field transitions in the lattices. 3.3. Photoactivities The MB photocatalytic degradation by of Ni2 FeVO6 nanoparticles under visible light illumination is presented in Fig. 7a. The photodegradation was evaluated by the UV/vis optical adsorption of the MB solutions with the addition of Ni2 FeVO6 nanoparticles. The absorption intensity of becomes weaker and weaker with the increase of the irradiation time indicating the decoloration of the MB solutions by Ni2 FeVO6 . There are not any appearances of new absorption because of the reaction intermediates induced by the photocatalysis. Fig. 7b gathers the effects of Ni2 FeVO6 nanoparticles on the photodegradation of MB solutions under visible light irradiation by monitoring the decrease of UV–vis absorption intensity (max = 665 nm). The desorption/absorption equilibrium can be reached after the 30 min process in dark for Ni2 FeVO6 photocatalyst. A commercial P25 sample was also measured under the same test condition for a comparison. The photodegradation of MB solutions shows few changes in the presence of P25 photocatalyst under visible light irradiation. This can be attributed to its bad photo-absorption of visible light in TiO2 . However, the MB solutions were obviously decolored by Ni2 FeVO6 nanoparticles. The
Fig. 9 shows the magnetic hysteresis loops of the initial Ni2 FeVO6 nanoparticles measured at room temperature in the field sweeping region from −40 to 40 kOe. The Ni2 FeVO6 nanopowders exhibit a hysteresis M–H behavior as expected. This vanadate compound crystallizes in the rhombohedral FeTiO3 -type (ilmenite) structure, which contains the alternate metal (Ni/V) layers forming a two-dimensional magnetic structure [23]. Therefore, the ferromagnetism (FM) behavior of the Ni2 FeVO6 sample can be expected. The saturation magnetization values (Ms) at 2 K and 300 K were measured to be 2.64 and 0.74 emu/g, respectively. The coercive magnetic field value (Hc) at 30 K is 0.48 kOe for Ni2 FeVO6 nanoparticles. In this situation, the ferromagnetism property of the Ni2 FeVO6 sample could be applied for magnetically separating the photocatalyst after its photodegradation of the MB dye solutions. This facilitates the separation of photocatalysts from treated solutions. 4. Discussions The results show that there are simultaneous photocatalysis and magnetism in Ni2 FeVO6 nanoparticles. The photocatalysis is
Fig. 9. Magnetic hysteresis loops of Ni2 FeVO6 nanoparticles at 2 K and 300 K. Inset shows the separation of the samples from solutions under the external magnetic field.
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Fig. 10. The XPS spectra of Fe 2p (a), Ni 2p (b), V 2p (c) and O 1s (d) measured in Ni2 FeVO6 nanoparticles. The dot lines in the figures are the decomposed Gaussian components.
regarded to be the redox reactions on the surfaces of nanoparticles. So the valance changes of the elements in the compounds are concerned in the photocatalytic process. It is well-known that Ni, Fe and V elements could present multiple valences in a compound. The information on oxidation states usually can be provided by the binding energy obtained in XPS measurements. The XPS spectra of the elements measured are shown in Fig. 10 indicating the major peaks from Fe 2p, Ni 2p, V 2p, and O 1s in Ni2 FeVO6 . The Fe 2p3/2 and 2p1/2 XPS peaks with multiplet splitting phenomenon were detected at 710.7 eV and 723.9 eV (Fig. 10a) respectively, corresponding to Fe3+ and Fe2+ [31,32]. The Ni-2p3/2 XPS spectrum in Fig. 10b presents a typical satellite peak at the binding energy about 865 eV, which is usually observed in Ni-containing oxides [33]. The asymmetry can be given two fit giving two BE values, i.e., 854.8 eV and 857.1 eV in Fig. 10b, which are from the contributions of Ni2+ and Ni3+ ions, respectively [34]. This usually suggests the electron exchange (valence degeneracy) between Ni2+ ion pairs. In the same situation, the XPS curve of V 2p (Fig. 10c) also shows an obvious asymmetry profile, indicating the multiply valences states. The curve can be deconvoluted into two Gaussian components at 514.75 eV and 517.85 eV assigned to the V4+ [35] and V5+ [36] on the surface, respectively. As confirmed by XPS measurements, the multivalences of the cations were appeared in Ni2 FeVO6 nanoparticles. The required charge compensation was clarified by the XPS curves of O in the lattices. O 1s XPS (Fig. 10d) has a strong peak at 530.5 eV, which can be regarded as characteristic signal from O 1s in the lattices. The asymmetrical peak is deconvoluted into three Gaussian bands as 1, 2, 3 on the curve. Peak 1 was undoubtedly assigned to O components in VO4 3− groups. While the Peak 1 and 2 should be from the O components such as the defect oxygen and adsorbed oxygen [37]. This reveals that there exists some defects oxygen in the lattices, such as oxygen vacancies created by the multivalent cations in the lattices [38].
As investigated above, although the Ni, Fe, and V keep the dominated valence states of +2, +3 and +5 in Ni2 FeVO6 , there are still minor heterovalences of +3, +2, and +4, respectively. And these minor valences, which formed during the preparation process, mainly exist on the surface of the particles, have not an influence on the crystal phase stability, which can be confirmed by XRD and SAED patterns. The mixed valences are great beneficial to the photocatalysis. The redox reaction could be more easily taken place due to the mixed valances of the cation ions as shown in Fig. 11. M denotes the Ni2+ , Fe2+ , or V4+ ions, and n is the present valance. Mn ions could be created when the Mn−1 traps an electron as shown in Eq. (1). Mn + e− → Mn−1
(1)
Fig. 11. The schematic diagram on describing the effects of multiple valences of cations Mn (M = Ni2+ , Fe2+ , or V4+ ions) on photocatalytic process in Ni2 FeVO6 nanoparticles.
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The oxygen molecule could capture the trapped electron to form a superoxide radical in Eq. (2), which can perform as an oxidant for the photodegradation of organic components [39]. Mn−1 + O2adsorption → Mn + O2 −•
(2)
Mn
ion acts as the hole-trap, it can be further oxidized into a If Mn+1 ions ion. The hole could be changed into a hydroxyl anion by creating a hydroxyl radical shown in the equation; or it could be also transformed to the dye molecule nearby to form a dye anion: Mn + h+ → Mn+1
(3)
Mn+1 + OH− → Mn + • OH
(4)
Mn+1 + dyeadsorption → Mn + dye+
(5)
The proposed mechanism could enhance the charge transfer process taking place in the interface. The detrapped electrons/holes (charge carriers) can be beneficial for the generation of highly oxidative free radicals such as hydroxyl and super oxide radicals [39]. As suggested in the above reactions, hydroxyl radicals (• OH) usually act as the major active species during the photocatalytic oxidation reaction [40]. Usually (• OH) produced in semiconductor photocatalysts in aqueous solution under light irradiation could be confirmed by some measurements such as photoluminescence (PL) technique using coumarin (COU) as a probe molecule [40], or addition of • OH scavenger Tert-butyl alcohol (TBA) [41] in the process of the photocatalysis. In this work, we selected TBA as an effective scavenger to confirm the existence of the hydroxyl radicals. As shown in Fig. 7b, after 2 mL TBA was added to the solutions during the photocatalysis process, the MB photodegradation became weak under the light irradiation. This effect could be driven by the contribution of • OH radicals. The hydroxyl radicals are the major active species responsible for the MB photodegradation, which could oxidize the adsorbed pollutants MB on the surfaces of Ni2 FeVO6 . • OH
+ MB → oxidation − products
(6)
5. Conclusions The photocatalyst Ni2 FeVO6 nanorods were developed by the sol–gel film coating and subsequent sintering method. The nanoparticles developed in a length of 500 nm and a radius of 50 nm. The detailed surface properties were reported. The BET specific surface area and the pore size are 65 m2 g−1 and 50 nm, respectively. Ni2 FeVO6 nanorods have efficient optical absorption characterized by a direct allowed electronic structure of a band gap 1.784 eV. The methylene blue solutions can be photodegraded by Ni2 FeVO6 nanoparticles under the irradiation of the visible light ( > 420 nm) irradiation. The mixed valances for the cations of V, Ni, and Fe are suggested to be beneficial for its photocatalysis. It is interesting to note that the sample exhibits ferromagnetic behavior. Ni2 FeVO6 could be more superior in the separation than the conventional photocatalysts because it can be recovered and recycled readily. Furthermore, the Ni2 FeVO6 nanoparticles could also have possible applications as photoelectric semiconductor and novel magnetoelectric devices. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2013R1A1A2009154), and supported by National Natural Science Foundation of China (No. 61405081) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
References [1] R. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal. 35 (2014) 989–1007. [2] S. Girish Kumar, K.S.R. Koteswara Rao, Tungsten-based nanomaterials (WO3 & Bi2 WO6 ): modifications related to charge carrier transfer mechanisms and photocatalytic applications, Appl. Surf. Sci. 355 (2015) 939–958. [3] E. Aguilera-Ruiz, U.M. García-Pérez, M. de la Garza-Galván, P. Zambrano-Robledo, B. Bermúdez-Reyes, J. Peral, Efficiency of Cu2 O/BiVO4 particles prepared with a new soft procedure on the degradation of dyes under visible-light irradiation, Appl. Surf. Sci. 328 (2015) 361–367. [4] J.H. Li, W. Zhao, Y. Guo, Z.B. Wei, M.S. Han, H. He, S.G. Yang, C. Sun, Facile synthesis and high activity of novel BiVO4 /FeVO4 heterojunction photocatalyst for degradation of metronidazole, Appl. Surf. Sci. 351 (2015) 270–279. [5] S.Q. Wei, J.Q. Li, L. Liu, J. Shi, Z.C. Shao, Photocatalytic effect of iron corrosion products on reduction of hexavalent chromium by organic acids, J. Taiwan Inst. Chem. Eng. 45 (2014) 2659–2663. [6] W. Chen, B.B. Yang, Q.T. Yu, L.Q. Mao, Z.Y. Fan, Q.Z. Wang, W.F. Shangguan, Effect of Rh oxide as a cocatalyst over Bi0.5 Y0.5 VO4 on photocatalytic overall water splitting, Appl. Surf. Sci. 355 (2015) 1069–1074. [7] D. Ma, Y.X. Zhang, M.C. Gao, Y.J. Xin, J. Wu, N. Bao, RGO/InVO4 hollowed-out nanofibers: electrospinning synthesis and its application in photocatalysis, Appl. Surf. Sci. 353 (2015) 118–126. [8] X. Qiao, Y. Huang, H.J. Seo, Optical property and visible-light-driven photocatalytic activity of inverse spinel LiNiVO4 nanoparticles prepared by Pechini method, Appl. Surf. Sci. 321 (2014) 488–494. [9] H.M. Liu, R. Nakamura, Y. Nakato, A visible-light responsive photocatalyst, BiZn2 VO6 , for efficient oxygen photoevolution from aqueous particulate suspensions, Electrochem. Solid-State Lett. 9 (2006) 187–190. [10] D. Wang, J.W. Tang, Z.G. Zou, J.H. Ye, Photophysical and photocatalytic properties of a new series of visible-light-driven photocatalysts M3 V2 O8 (M = Mg, Ni, Zn), Chem. Mater. 17 (2005) 5177–5182. [11] R. Konta, H. Kato, H. Kobayashi, A. Kudo, Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates, Phys. Chem. Chem. Phys. 5 (2003) 3061–3065. [12] H.M. Liu, R. Nakamura, Y. Nakato, Bismuth–copper vanadate BiCu2 VO6 as a novel photocatalyst for efficient visible-light-driven oxygen evolution, ChemPhysChem 6 (2005) 2499–2502. [13] Y. Lu, Y. Pu, J. Wang, C. Qin, C. Chen, H.J. Seo, On structure and methylene blue degradation activity of an Aurivillius-type photocatalyst of Bi4 V2 O11 nanoparticles, Appl. Surf. Sci. 347 (2015) 719–726. [14] A. Watanabe, Y. Kitami, An outline of the structure of oxide-ion conductors Bi23 V4−4x P4x O44.5 (0 ≤ x ≤ 1), Solid State Ion. 113 (1998) 601–606. [15] Y.F. Pu, J. Wang, Y.L. Huang, C.L. Chen, S.I. Kim, H.J. Seo, Visible-light-induced degradation of methylene blue by SrBi3 VO8 nanoparticles, J. Am. Ceram. Soc. 98 (2015) 2528–2533. [16] Y.F. Pu, Y.Z. Li, Y.L. Huang, S.I. Kim, P.Q. Cai, H.J. Seo, Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7 VO13 nanoparticles, Mater. Lett. 141 (2015) 73–75. [17] A.S. Ganeshraja, A.S. Clara, K. Rajkumar, Y. Wang, Y. Wang, J. Wang, K. Anbalagan, Simple hydrothermal synthesis of metal oxides coupled nanocomposites: structural, optical, magnetic and photocatalytic studies, Appl. Surf. Sci. 353 (2015) 553–563. [18] M. Scarisoreanu, I. Morjan, C.T. Fleaca, I.P. Morjan, A.M. Niculescu, E. Dutu, A. Badoi, R. Birjega, C. Luculescu, E. Vasile, V. Danciu, G. Filoti, Synthesis and optical properties of TiO2 -based magnetic nanocomposites, Appl. Surf. Sci. 336 (2015) 335–342. [19] T. Xin, M. Ma, H. Zhang, J. Gu, S. Wang, M. Liu, Q. Zhang, A facile approach for the synthesis of magnetic separable Fe3 O4 @TiO2 , core–shell nanocomposites as highly recyclable photocatalysts, Appl. Surf. Sci. 288 (2014) 51–59. [20] Y. Hou, J. Ma, T. Wang, Q. Fu, Phosphotungstic acid supported on magnetic core-shell nanoparticles with high photocatalytic activity, Mater. Sci. Semicond. Process. 39 (2015) 229–234. [21] T. Xie, L. Xu, C. Liu, Y. Wang, Magnetic composite ZnFe2 O4 /SrFe12 O19 : preparation, characterization, and photocatalytic activity under visible light, Appl. Surf. Sci. 273 (2013) 684–691. [22] J. Zhou, Z. Zhang, B. Cheng, J. Yu, Glycine-assisted hydrothermal synthesis and adsorption properties of crosslinked porous ␣-Fe2 O3 nanomaterials for p-nitrophenol, Chem. Eng. J. 211–212 (2012) 153–160. [23] N. Guskos, V. Likodimos, S. Los, W. Kempinski, J. Stankowski, M. Wabia, J. Typek, A. Blonska-Tabero, P. Tabero, I. Rychlowska-Himmel, EPR study of Ni2 FeVO6−ı , Phys. B: Condens. Matter 284 (2000) 1456–1458. ´ [24] L. Kolpakowa, J. Pietrzak, J.N. Latosinska, P. Pawlicki, Magnetism in Ni2 FeVO6−ı , J. Magn. Magn. Mater. 140 (1995) 1583–1584. [25] L. Kolpakova, J. Pietrzak, V. Starodub, The nature of magnetic anisotropy in ilmenite NiFe0.5 V0.5 O3−ı , J. Magn. Magn. Mater. 160 (1996) 374–375. ´ [26] P. Pawlicki, L. Kolpakova, A. Maryanowska, J.N. Latosinka, J. Pietrzak, A spin model for magnetization in the ilmenite NiFe0.5 V0.5 O3−ı , J. Magn. Magn. Mater. 160 (1996) 372–373. [27] I. Jacyna-Onyszkiewicz, T. Mydlarz, N.P. Tishchenko, Crystal and magnetic properties of Cu1−x Nix (FeV)0.5 O3 systems, J. Magn. Magn. Mater. 160 (1996) 376–377. ˙ [28] N. Guskos, J. Typek, G. Zolnierkiewicz, A. Blonska-Tabero, S. Los, W. Kempinski, Effects of thermal treatment on magnetic properties of Ni2 FeV3 O11−ı compound, Mater. Sci. 24 (2006) 983–986.
X. Qiao et al. / Applied Surface Science 359 (2015) 259–265 [29] S. Lei, K. Tang, Y. Jin, C. Chen, Preparation of aligned MnV2 O6 nanorods and their anodic performance for lithium secondary battery use, Nanotechnology 18 (2007) 175605–175611. [30] P. Li, Y. Zhou, W.G. Tu, Q. Liu, S.C. Yan, Z.G. Zou, Direct growth of Fe2 V4 O13 nanoribbons on a stainless-steel mesh for visible-light photoreduction of CO2 into renewable hydrocarbon fuel and degradation of gaseous isopropyl alcohol, ChemPlusChem 78 (2013) 274–278. [31] S.L. Liu, D.D. Tao, L.N. Zhang, Cellulose scaffold: a green template for the controlling synthesis of magnetic inorganic nanoparticles, Powder Technol. 217 (2012) 502–509. [32] P. Ghods, O.B. Isgor, J.R. Brown, F. Bensebaa, D. Kingston, XPS depth profiling, study on the passive oxide film of carbon steel in saturated calcium hydroxide solution and the effect of chloride on the film properties, Appl. Surf. Sci. 257 (2011) 4669–4677. [33] A.F. Carley, S.D. Jackson, J.N. O’Shea, M.W. Roberts, The formation and characterisation of Ni3+ – an X-ray photoelectron spectroscopic investigation of potassium-doped Ni(1 1 0)–O, Surf. Sci. 440 (1999) 868–874. [34] I. Predaa, R.J.O. Mossanekb, M. Abbateb, L. Alvarezc, J. Méndezc, A. Gutiérreza, L. Sorianoa, Surface contributions to the XPS spectra of nanostructured NiO deposited on HOPG, Surf. Sci. 606 (2012) 1426–1430.
265
[35] W.T. Draai, G. Blasse, Energy transfer from vanadate to rare-earth ions in calcium sulphate, Phys. Status Solidi (a) 21 (1974) 569–579. [36] J. Mendialdua, R. Casanova, Y. Barbaux, XPS studies of V2 O5 , V6 O13 , VO2 and V2 O3 , J. Electron. Spectrosc. Relat. Phenom. 71 (1995) 249–261. [37] W. Zhu, W. Zhu, X. Feng, Z. Wu, Z. Man, On the annealing mechanism in PbWO4 crystals, Phys. B: Condens. Matter 324 (2002) 53–58. [38] Q.H. Wu, A. Thissen, W. Jaegermann, M.L. Liu, Photoelectron spectroscopy study of oxygen vacancy on vanadium oxides surface, Appl. Surf. Sci. 236 (2004) 473–478. [39] L.G. Devi, N. Kottam, S.G. Kumar, K.E. Rajashekhar, Preparation, characterization and enhanced photocatalytic activity of Ni2+ doped titania under solar light, Cent. Eur. J. Chem. 8 (2010) 142–148. [40] Q. Xiang, J. Yu, P.K. Wong, Quantitative characterization of hydroxyl radicals produced by various photocatalysts, J. Colloid Interface Sci. 357 (2011) 163–167. [41] Y. Hu, D. Li, F. Sun, H. Wang, Y. Weng, W. Xiong, Y. Shao, One-pot template-free synthesis of heterophase BiVO4 microspheres with enhanced photocatalytic activity, RSC Adv. 5 (2015) 54882–54889.
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Surface properties, simultaneous photocatalytic and magnetic activities of Ni2 FeVO6 nanoparticles Xuebin Qiao a , Yanlin Huang b , Han Cheng c , Hyo Jin Seo c,∗ a Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, PR China b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China c Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering, Pukyong National University, Busan 608-737, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 August 2015 Received in revised form 8 October 2015 Accepted 16 October 2015 Available online 19 October 2015 Keywords: Semiconductors Nanostructures Photocatalysis Optical properties Surface properties
a b s t r a c t Nickel-ferro-vanadium oxide Ni2 FeVO6 nanoparticles were prepared by the sol–gel film coating and subsequent sintering method. The phase formation was investigated X-ray polycrystalline diffraction (XRD) measurement. The surface characteristics were measured by scanning electron microscope (SEM), transmission electron microscopy (TEM), specific surface area, energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). This vanadate has a narrow band-gap energy of 1.784 eV. The investigations concluded that Ni2 FeVO6 nanoparticles have photocatalytic ability under visible-light irradiation. The ferromagnetic behavior of the nanoparticles was confirmed by the magnetic hysteresis loops. The nanoparticles can be magnetically recoverable after photocatalytic reactions. The photocatalytic activities were discussed on the base of the multivalent cations in crystal lattices. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis of semiconductors have attracted much attention in decades, which are the redox reactions on the surfaces under the irradiation of UV–vis Light. The photocatalysis can be initiated by absorbing photons to create excited electron–hole pairs. Afterwards, some reactive oxidative species (ROS) such as OH• , O2 −• and holes could be directly and indirectly produced, which migrate to the surface and result in redox reactions. The photocatalysis is a potential application in removal of polluted organics, sterilizing most of the bacteria in the environment or the production of hydrogen by photocatalytic water splitting [1]. So far many photocatalysts have been developed with different crystal structures, and most of them containing transition metal ions such as V, Ta, Nb, W, Mo, etc. because these species are effective in obtaining photocatalytic activities [2]. Among them, complex vanadates (CVO) containing diverse vanadium-oxygen polyhedra linkages belongs to a group of useful catalysts used for the oxidation of organic compounds in solutions and splitting of water [1]. The visible-light driven vanadates have attracted growing interest such
∗ Corresponding author. E-mail address: [email protected] (H.J. Seo). http://dx.doi.org/10.1016/j.apsusc.2015.10.112 0169-4332/© 2015 Elsevier B.V. All rights reserved.
as BiVO4 [3–5], Bi0.5 Y0.5 VO4 [6], InVO4 [7], LiNiVO4 [8,9], M3 V2 O8 (M = Mg, Ni, Zn) [10], ␣-AgVO3 [11], BiCu2 VO6 [12], Bi4 V2 O11 [13], Bi23 V4 O44.5 [14], SrBi3 VO8 [15], Bi7 VO13 [16] etc. However, most of photocatalysts have a difficulty in the recycling after its photocatalytic use. Consequently, it is valuable to investigate photocatalysts with magnetic properties by considering its recycling. In this regard, photocatalyst nanoparticles with magnetic abilities have been reported in the most studies, for example, metal oxides coupled nanocomposites [17,18], Fe3 O4 @SiO2 [19,20], ZnFe2 O4 /SrFe12 O19 [21], ␣-Fe2 O3 nanoparticles [22] et al. In present work, Ni2 FeVO6 was selected to detect its possible application as a visible light driven photocatalyst. This motivation depends on its efficient optical absorption and magnetic properties due to the optical-active ions (Ni2+ and VO4 ) and magnetic ions (Fe3+ , Ni2+ ) in the crystal lattices, respectively. Crystal structures of vanadates are constructed by different linkages of vanadium oxygen polyhedral depending on the nature of the counter cations and stoichiometry of the host. Previous work indicates that Ni2 FeVO6 the shows efficient optical absorption in both UV and visible light wavelength region. The structure of Ni2 FeVO6 has been reported in the references. Ni2 FeVO6 crystallizes in the FeTiO3 -type (ilmenite) rhombohedral structure, which contains alternate metal layers forming a two-dimensional magnetic structure [23–27]. In Ni2 FeVO6−ı the EPR signals have been assigned to the high-spin centers or ferromagnetic clusters [28].
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Our aim in this article is to present the synthesis, surface properties, and photocatalytic activity of Ni2 FeVO6 nanoparticles prepared using the sol–gel film coating and subsequent sintering treatment. The crystal phase formation, the detailed surface properties and photoactivities were investigated. The Ni2 FeVO6 nanoparticles show potential application for the photo degradation of methylene blue (MB) under visible-light irradiation. 2. Experimental Ni2 FeVO6 nanoparticles were prepared by the sol–gel film coating and subsequent sintering method. The raw materials are NH4 VO3 , Fe(NO3 )3 ·9H2 O and Ni(NO3 )2 ·6H2 O. Firstly, An appropriate citric acid were added in the clear solutions containing the stoichiometric raw materials to get a full complexation reaction of the Fe3+ , Ni2+ and VO4 3− ions. Then the solutions were neutralized by the ammonium hydroxide (30% wt). Secondly, the solutions were added into some liquid polyvinylalcohol (PVA) to adjust its viscosity. The homogeneous solutions were obtained by continue stirring for about 4 h. Thirdly, the viscous solutions were slowly coated on the glass-substrates to prepare the films. After natural air drying, the dried films can be obtained containing the raw materials. Finally, the films were peeled off from the glass-substrates and then heated at 800 ◦ C for 2 h. The loose powders could be were obtained after the elimination of the organic components during the heating process. XRD patterns of the samples were measured using a Rigaku D/Max diffractometer (40 kV, 30 mA) with Cu K␣ radiation. SEM pictures were recorded to study the surface morphologies. The images were measured using a JEOL JEM-2010F microscope. The XPS measurements were completed on Kratos analytical, ESCA3400, Shimadzu. The photocatalysis experiments were finished in the reactor (500 ml) equipped by an xenon lamp (500 W) providing the visible light irradiation. In this process, a cut-filter with 420 nm was applied to avoid the UV light. An air-flow was supplied in the reactor by a pump. In a typical experiment, the photocatalyst was supplied by 0.05 g by mixing it in the 300 mL methylene blue (10 mg L−1 ) solution. Prior to photocatalysis the system was stayed in dark for 30 min to fulfill the adsorption/desorption equilibrium. At a designed time interval, 5 mL methylene blue solution was extracted to analyze the optical absorption on a UV–vis spectrophotometer. The degradation percentage could be obtained from the formula of [1 − (A/A0 )] × 100%, where A0 is the original optical absorption before the light irradiation; and Ai is that taken after 15 min light irradiation.
Fig. 1. A typical XRD pattern of Ni2 FeVO6 nanoparticles in comparison with the PDF#2 standard card No: 53-0372.
Fig. 2. The typical SEM pictures of Ni2 FeVO6 nanoparticles.
of V2 O5 ·nH2 O sheets. The cations could insert into the layers. The nanorods could be developed with the crystallization process. For example, it has been reported in MnV2 O6 nanorods that the regular nanoplates could be firstly developed with V2 O5 ·nH2 O sheets [29]. After Mn2+ ions were introduced into the sol system, the Mn2+ ions were intercalated into the layered structure and then reacted with V2 O5 ·nH2 O sheets to form aligned MnV2 O6 nanosheets. Fig. 3 is the EDX spectrum to investigate the elemental compositions in the nanoparticles. The specific emission lines show the
3. Results 3.1. Phase formation and surface properties Fig. 1 presents the XRD result of nickel iron vanadium oxide Ni2 FeVO6 nanoparticles compared with the standard card PDF2 No. 53-0372. The experimental pattern was fully indexed to the standard card. The result reveals that the sample crystallized with a pure Ni2 FeVO6 phase. The nanoparticles crystallize in a ˚ b = 8.427 A, ˚ rhombic phase the unit cell parameters of a = 17.316 A, ˚ V = 853.61 A˚ 3 , and Z = 8 [23]. c = 5.982 A, The typical SEM images of Ni2 FeVO6 nanoparticles are shown in Fig. 2. The sample contains closely-packed ball-like units. The particles have a little aggregation, which is easy to disperse under a gentle grind in a mortar or stirring in a solution. As shown in Fig. 2, the sample contains some nanorods, which have length and radius of 500 nm and 50 nm, respectively. In sol–gel preparation of vanadates, the vanadate gels easily form layered-structures made
Fig. 3. The typical EDX spectrum of Ni2 FeVO6 nanoparticles and the element percentage.
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Fig. 4. The TEMs (a, b), HRTEM image (c), SAED pattern (d) of Ni2 FeVO6 nanoparticles.
EDX signals from the elements V, Ni, Fe, and O in the lattices. The componential ratio of V/Ni(Fe) was in agreement with the stoichiometric value required in the chemical formulae of Ni2 FeVO6 . The TEM images of Ni2 FeVO6 nanoparticles are shown in Fig. 4a and b confirming the morphology of the nanorods. The length and width were estimated to be about 500 nm and 50 nm, respectively. The measured HRTEM images (Fig. 4c) present well-resolved lattice fringes, which show the interplanar spacing d of 0.502 nm corresponding to the (3 1 0) lattice plane in Ni2 FeVO6 nanorods. The well crystallized single-phase structure of Ni2 FeVO6 nanoparticles can be confirmed by SAED pattern in Fig. 4d. The Brunauer–Emmett–Teller (BET) measurement for the surface-area and the pore-size distribution of Ni2 FeVO6 nanoparticles were measured and shown in Fig. 5. The isotherm is in the representative IV pattern a characterization of a hysteresis loop. The adsorption region at P/P0 is close to 1.0 indicating the coexistence of mesopores and macropores in Ni2 FeVO6 nanoparticles. The specific surface area of Ni2 FeVO6 nanoparticles was decided to be 65 m2 g−1 . The pore size distribution is estimated to be about
on 50 nm (inset Fig. 4). The pore size distribution was narrow and monomodal indicating that the sample is composed of the uniform nanoparticles.
Fig. 6 shows the UV–vis absorption spectra of Ni2 FeVO6 nanoparticles, which has the wide optical absorption in the visiblelight region. The spectrum shows that the absorption edge of extends to wavelength 700 nm. This suggests that the sample is qualified for the possible photocatalysis under visible light irradiation. The energy of band-gap (Eg ) was calculated to be 1.784 eV (670 nm) based on the equation of ˛h ∝ (h − Eg )k , where ˛: absorbance, : optical frequency; the electronic transitions could be direct allowed, indirect allowed, direct forbidden and indirect forbidden transitions when k is 1/2, 2, 3/2 or 3, respectively. In the nanorods, k = 1/2 was confirmed to be a best fitting parameter indicating a direct-gap character for the material [30]. As shown insert in Fig. 6, Ni2 FeVO6 powders show Dark brown color. The deep color
Fig. 5. The typical N2 adsorption–desorption isotherms with the corresponding pore-size distribution curve of Ni2 FeVO6 nanoparticles.
Fig. 6. The UV–vis absorption spectrum of Ni2 FeVO6 nanoparticles; inset band-gap calculation and the digital photos of the nanopowders.
3.2. UV–vis optical absorption
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Fig. 8. The repetitive operations of Ni2 FeVO6 nanoparticles for photocatalytic degradation of MB solutions.
optical absorption could be degradated by 90% in 300 min. The results show that Ni2 FeVO6 is a possible efficient photocatalyst. The photo-stability of Ni2 FeVO6 was examined by recycling the photocatalytic experiments. Fig. 8 is three repetitive operations of Ni2 FeVO6 nanoparticles for MB photodegradation. The results show that the photodegradation of MB solutions keeps a stable level. 3.4. Magnetic property
Fig. 7. (a): The UV/vis absorption spectra; and (b): the photodegradation effects of MB solutions by Ni2 FeVO6 nanoparticles, Ni2 FeVO6 + • OH scavenger of the Tert-butyl alcohol (TBA) additive, and P25 on under visible-light irradiation.
is obviously generally attributed to crystal-field transitions in the lattices. 3.3. Photoactivities The MB photocatalytic degradation by of Ni2 FeVO6 nanoparticles under visible light illumination is presented in Fig. 7a. The photodegradation was evaluated by the UV/vis optical adsorption of the MB solutions with the addition of Ni2 FeVO6 nanoparticles. The absorption intensity of becomes weaker and weaker with the increase of the irradiation time indicating the decoloration of the MB solutions by Ni2 FeVO6 . There are not any appearances of new absorption because of the reaction intermediates induced by the photocatalysis. Fig. 7b gathers the effects of Ni2 FeVO6 nanoparticles on the photodegradation of MB solutions under visible light irradiation by monitoring the decrease of UV–vis absorption intensity (max = 665 nm). The desorption/absorption equilibrium can be reached after the 30 min process in dark for Ni2 FeVO6 photocatalyst. A commercial P25 sample was also measured under the same test condition for a comparison. The photodegradation of MB solutions shows few changes in the presence of P25 photocatalyst under visible light irradiation. This can be attributed to its bad photo-absorption of visible light in TiO2 . However, the MB solutions were obviously decolored by Ni2 FeVO6 nanoparticles. The
Fig. 9 shows the magnetic hysteresis loops of the initial Ni2 FeVO6 nanoparticles measured at room temperature in the field sweeping region from −40 to 40 kOe. The Ni2 FeVO6 nanopowders exhibit a hysteresis M–H behavior as expected. This vanadate compound crystallizes in the rhombohedral FeTiO3 -type (ilmenite) structure, which contains the alternate metal (Ni/V) layers forming a two-dimensional magnetic structure [23]. Therefore, the ferromagnetism (FM) behavior of the Ni2 FeVO6 sample can be expected. The saturation magnetization values (Ms) at 2 K and 300 K were measured to be 2.64 and 0.74 emu/g, respectively. The coercive magnetic field value (Hc) at 30 K is 0.48 kOe for Ni2 FeVO6 nanoparticles. In this situation, the ferromagnetism property of the Ni2 FeVO6 sample could be applied for magnetically separating the photocatalyst after its photodegradation of the MB dye solutions. This facilitates the separation of photocatalysts from treated solutions. 4. Discussions The results show that there are simultaneous photocatalysis and magnetism in Ni2 FeVO6 nanoparticles. The photocatalysis is
Fig. 9. Magnetic hysteresis loops of Ni2 FeVO6 nanoparticles at 2 K and 300 K. Inset shows the separation of the samples from solutions under the external magnetic field.
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Fig. 10. The XPS spectra of Fe 2p (a), Ni 2p (b), V 2p (c) and O 1s (d) measured in Ni2 FeVO6 nanoparticles. The dot lines in the figures are the decomposed Gaussian components.
regarded to be the redox reactions on the surfaces of nanoparticles. So the valance changes of the elements in the compounds are concerned in the photocatalytic process. It is well-known that Ni, Fe and V elements could present multiple valences in a compound. The information on oxidation states usually can be provided by the binding energy obtained in XPS measurements. The XPS spectra of the elements measured are shown in Fig. 10 indicating the major peaks from Fe 2p, Ni 2p, V 2p, and O 1s in Ni2 FeVO6 . The Fe 2p3/2 and 2p1/2 XPS peaks with multiplet splitting phenomenon were detected at 710.7 eV and 723.9 eV (Fig. 10a) respectively, corresponding to Fe3+ and Fe2+ [31,32]. The Ni-2p3/2 XPS spectrum in Fig. 10b presents a typical satellite peak at the binding energy about 865 eV, which is usually observed in Ni-containing oxides [33]. The asymmetry can be given two fit giving two BE values, i.e., 854.8 eV and 857.1 eV in Fig. 10b, which are from the contributions of Ni2+ and Ni3+ ions, respectively [34]. This usually suggests the electron exchange (valence degeneracy) between Ni2+ ion pairs. In the same situation, the XPS curve of V 2p (Fig. 10c) also shows an obvious asymmetry profile, indicating the multiply valences states. The curve can be deconvoluted into two Gaussian components at 514.75 eV and 517.85 eV assigned to the V4+ [35] and V5+ [36] on the surface, respectively. As confirmed by XPS measurements, the multivalences of the cations were appeared in Ni2 FeVO6 nanoparticles. The required charge compensation was clarified by the XPS curves of O in the lattices. O 1s XPS (Fig. 10d) has a strong peak at 530.5 eV, which can be regarded as characteristic signal from O 1s in the lattices. The asymmetrical peak is deconvoluted into three Gaussian bands as 1, 2, 3 on the curve. Peak 1 was undoubtedly assigned to O components in VO4 3− groups. While the Peak 1 and 2 should be from the O components such as the defect oxygen and adsorbed oxygen [37]. This reveals that there exists some defects oxygen in the lattices, such as oxygen vacancies created by the multivalent cations in the lattices [38].
As investigated above, although the Ni, Fe, and V keep the dominated valence states of +2, +3 and +5 in Ni2 FeVO6 , there are still minor heterovalences of +3, +2, and +4, respectively. And these minor valences, which formed during the preparation process, mainly exist on the surface of the particles, have not an influence on the crystal phase stability, which can be confirmed by XRD and SAED patterns. The mixed valences are great beneficial to the photocatalysis. The redox reaction could be more easily taken place due to the mixed valances of the cation ions as shown in Fig. 11. M denotes the Ni2+ , Fe2+ , or V4+ ions, and n is the present valance. Mn ions could be created when the Mn−1 traps an electron as shown in Eq. (1). Mn + e− → Mn−1
(1)
Fig. 11. The schematic diagram on describing the effects of multiple valences of cations Mn (M = Ni2+ , Fe2+ , or V4+ ions) on photocatalytic process in Ni2 FeVO6 nanoparticles.
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The oxygen molecule could capture the trapped electron to form a superoxide radical in Eq. (2), which can perform as an oxidant for the photodegradation of organic components [39]. Mn−1 + O2adsorption → Mn + O2 −•
(2)
Mn
ion acts as the hole-trap, it can be further oxidized into a If Mn+1 ions ion. The hole could be changed into a hydroxyl anion by creating a hydroxyl radical shown in the equation; or it could be also transformed to the dye molecule nearby to form a dye anion: Mn + h+ → Mn+1
(3)
Mn+1 + OH− → Mn + • OH
(4)
Mn+1 + dyeadsorption → Mn + dye+
(5)
The proposed mechanism could enhance the charge transfer process taking place in the interface. The detrapped electrons/holes (charge carriers) can be beneficial for the generation of highly oxidative free radicals such as hydroxyl and super oxide radicals [39]. As suggested in the above reactions, hydroxyl radicals (• OH) usually act as the major active species during the photocatalytic oxidation reaction [40]. Usually (• OH) produced in semiconductor photocatalysts in aqueous solution under light irradiation could be confirmed by some measurements such as photoluminescence (PL) technique using coumarin (COU) as a probe molecule [40], or addition of • OH scavenger Tert-butyl alcohol (TBA) [41] in the process of the photocatalysis. In this work, we selected TBA as an effective scavenger to confirm the existence of the hydroxyl radicals. As shown in Fig. 7b, after 2 mL TBA was added to the solutions during the photocatalysis process, the MB photodegradation became weak under the light irradiation. This effect could be driven by the contribution of • OH radicals. The hydroxyl radicals are the major active species responsible for the MB photodegradation, which could oxidize the adsorbed pollutants MB on the surfaces of Ni2 FeVO6 . • OH
+ MB → oxidation − products
(6)
5. Conclusions The photocatalyst Ni2 FeVO6 nanorods were developed by the sol–gel film coating and subsequent sintering method. The nanoparticles developed in a length of 500 nm and a radius of 50 nm. The detailed surface properties were reported. The BET specific surface area and the pore size are 65 m2 g−1 and 50 nm, respectively. Ni2 FeVO6 nanorods have efficient optical absorption characterized by a direct allowed electronic structure of a band gap 1.784 eV. The methylene blue solutions can be photodegraded by Ni2 FeVO6 nanoparticles under the irradiation of the visible light ( > 420 nm) irradiation. The mixed valances for the cations of V, Ni, and Fe are suggested to be beneficial for its photocatalysis. It is interesting to note that the sample exhibits ferromagnetic behavior. Ni2 FeVO6 could be more superior in the separation than the conventional photocatalysts because it can be recovered and recycled readily. Furthermore, the Ni2 FeVO6 nanoparticles could also have possible applications as photoelectric semiconductor and novel magnetoelectric devices. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2013R1A1A2009154), and supported by National Natural Science Foundation of China (No. 61405081) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), China.
References [1] R. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal. 35 (2014) 989–1007. [2] S. Girish Kumar, K.S.R. Koteswara Rao, Tungsten-based nanomaterials (WO3 & Bi2 WO6 ): modifications related to charge carrier transfer mechanisms and photocatalytic applications, Appl. Surf. Sci. 355 (2015) 939–958. [3] E. Aguilera-Ruiz, U.M. García-Pérez, M. de la Garza-Galván, P. Zambrano-Robledo, B. Bermúdez-Reyes, J. Peral, Efficiency of Cu2 O/BiVO4 particles prepared with a new soft procedure on the degradation of dyes under visible-light irradiation, Appl. Surf. Sci. 328 (2015) 361–367. [4] J.H. Li, W. Zhao, Y. Guo, Z.B. Wei, M.S. Han, H. He, S.G. Yang, C. Sun, Facile synthesis and high activity of novel BiVO4 /FeVO4 heterojunction photocatalyst for degradation of metronidazole, Appl. Surf. Sci. 351 (2015) 270–279. [5] S.Q. Wei, J.Q. Li, L. Liu, J. Shi, Z.C. Shao, Photocatalytic effect of iron corrosion products on reduction of hexavalent chromium by organic acids, J. Taiwan Inst. Chem. Eng. 45 (2014) 2659–2663. [6] W. Chen, B.B. Yang, Q.T. Yu, L.Q. Mao, Z.Y. Fan, Q.Z. Wang, W.F. Shangguan, Effect of Rh oxide as a cocatalyst over Bi0.5 Y0.5 VO4 on photocatalytic overall water splitting, Appl. Surf. Sci. 355 (2015) 1069–1074. [7] D. Ma, Y.X. Zhang, M.C. Gao, Y.J. Xin, J. Wu, N. Bao, RGO/InVO4 hollowed-out nanofibers: electrospinning synthesis and its application in photocatalysis, Appl. Surf. Sci. 353 (2015) 118–126. [8] X. Qiao, Y. Huang, H.J. Seo, Optical property and visible-light-driven photocatalytic activity of inverse spinel LiNiVO4 nanoparticles prepared by Pechini method, Appl. Surf. Sci. 321 (2014) 488–494. [9] H.M. Liu, R. Nakamura, Y. Nakato, A visible-light responsive photocatalyst, BiZn2 VO6 , for efficient oxygen photoevolution from aqueous particulate suspensions, Electrochem. Solid-State Lett. 9 (2006) 187–190. [10] D. Wang, J.W. Tang, Z.G. Zou, J.H. Ye, Photophysical and photocatalytic properties of a new series of visible-light-driven photocatalysts M3 V2 O8 (M = Mg, Ni, Zn), Chem. Mater. 17 (2005) 5177–5182. [11] R. Konta, H. Kato, H. Kobayashi, A. Kudo, Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates, Phys. Chem. Chem. Phys. 5 (2003) 3061–3065. [12] H.M. Liu, R. Nakamura, Y. Nakato, Bismuth–copper vanadate BiCu2 VO6 as a novel photocatalyst for efficient visible-light-driven oxygen evolution, ChemPhysChem 6 (2005) 2499–2502. [13] Y. Lu, Y. Pu, J. Wang, C. Qin, C. Chen, H.J. Seo, On structure and methylene blue degradation activity of an Aurivillius-type photocatalyst of Bi4 V2 O11 nanoparticles, Appl. Surf. Sci. 347 (2015) 719–726. [14] A. Watanabe, Y. Kitami, An outline of the structure of oxide-ion conductors Bi23 V4−4x P4x O44.5 (0 ≤ x ≤ 1), Solid State Ion. 113 (1998) 601–606. [15] Y.F. Pu, J. Wang, Y.L. Huang, C.L. Chen, S.I. Kim, H.J. Seo, Visible-light-induced degradation of methylene blue by SrBi3 VO8 nanoparticles, J. Am. Ceram. Soc. 98 (2015) 2528–2533. [16] Y.F. Pu, Y.Z. Li, Y.L. Huang, S.I. Kim, P.Q. Cai, H.J. Seo, Visible light-induced degradation of methylene blue by photocatalyst of bismuth layered Bi7 VO13 nanoparticles, Mater. Lett. 141 (2015) 73–75. [17] A.S. Ganeshraja, A.S. Clara, K. Rajkumar, Y. Wang, Y. Wang, J. Wang, K. Anbalagan, Simple hydrothermal synthesis of metal oxides coupled nanocomposites: structural, optical, magnetic and photocatalytic studies, Appl. Surf. Sci. 353 (2015) 553–563. [18] M. Scarisoreanu, I. Morjan, C.T. Fleaca, I.P. Morjan, A.M. Niculescu, E. Dutu, A. Badoi, R. Birjega, C. Luculescu, E. Vasile, V. Danciu, G. Filoti, Synthesis and optical properties of TiO2 -based magnetic nanocomposites, Appl. Surf. Sci. 336 (2015) 335–342. [19] T. Xin, M. Ma, H. Zhang, J. Gu, S. Wang, M. Liu, Q. Zhang, A facile approach for the synthesis of magnetic separable Fe3 O4 @TiO2 , core–shell nanocomposites as highly recyclable photocatalysts, Appl. Surf. Sci. 288 (2014) 51–59. [20] Y. Hou, J. Ma, T. Wang, Q. Fu, Phosphotungstic acid supported on magnetic core-shell nanoparticles with high photocatalytic activity, Mater. Sci. Semicond. Process. 39 (2015) 229–234. [21] T. Xie, L. Xu, C. Liu, Y. Wang, Magnetic composite ZnFe2 O4 /SrFe12 O19 : preparation, characterization, and photocatalytic activity under visible light, Appl. Surf. Sci. 273 (2013) 684–691. [22] J. Zhou, Z. Zhang, B. Cheng, J. Yu, Glycine-assisted hydrothermal synthesis and adsorption properties of crosslinked porous ␣-Fe2 O3 nanomaterials for p-nitrophenol, Chem. Eng. J. 211–212 (2012) 153–160. [23] N. Guskos, V. Likodimos, S. Los, W. Kempinski, J. Stankowski, M. Wabia, J. Typek, A. Blonska-Tabero, P. Tabero, I. Rychlowska-Himmel, EPR study of Ni2 FeVO6−ı , Phys. B: Condens. Matter 284 (2000) 1456–1458. ´ [24] L. Kolpakowa, J. Pietrzak, J.N. Latosinska, P. Pawlicki, Magnetism in Ni2 FeVO6−ı , J. Magn. Magn. Mater. 140 (1995) 1583–1584. [25] L. Kolpakova, J. Pietrzak, V. Starodub, The nature of magnetic anisotropy in ilmenite NiFe0.5 V0.5 O3−ı , J. Magn. Magn. Mater. 160 (1996) 374–375. ´ [26] P. Pawlicki, L. Kolpakova, A. Maryanowska, J.N. Latosinka, J. Pietrzak, A spin model for magnetization in the ilmenite NiFe0.5 V0.5 O3−ı , J. Magn. Magn. Mater. 160 (1996) 372–373. [27] I. Jacyna-Onyszkiewicz, T. Mydlarz, N.P. Tishchenko, Crystal and magnetic properties of Cu1−x Nix (FeV)0.5 O3 systems, J. Magn. Magn. Mater. 160 (1996) 376–377. ˙ [28] N. Guskos, J. Typek, G. Zolnierkiewicz, A. Blonska-Tabero, S. Los, W. Kempinski, Effects of thermal treatment on magnetic properties of Ni2 FeV3 O11−ı compound, Mater. Sci. 24 (2006) 983–986.
X. Qiao et al. / Applied Surface Science 359 (2015) 259–265 [29] S. Lei, K. Tang, Y. Jin, C. Chen, Preparation of aligned MnV2 O6 nanorods and their anodic performance for lithium secondary battery use, Nanotechnology 18 (2007) 175605–175611. [30] P. Li, Y. Zhou, W.G. Tu, Q. Liu, S.C. Yan, Z.G. Zou, Direct growth of Fe2 V4 O13 nanoribbons on a stainless-steel mesh for visible-light photoreduction of CO2 into renewable hydrocarbon fuel and degradation of gaseous isopropyl alcohol, ChemPlusChem 78 (2013) 274–278. [31] S.L. Liu, D.D. Tao, L.N. Zhang, Cellulose scaffold: a green template for the controlling synthesis of magnetic inorganic nanoparticles, Powder Technol. 217 (2012) 502–509. [32] P. Ghods, O.B. Isgor, J.R. Brown, F. Bensebaa, D. Kingston, XPS depth profiling, study on the passive oxide film of carbon steel in saturated calcium hydroxide solution and the effect of chloride on the film properties, Appl. Surf. Sci. 257 (2011) 4669–4677. [33] A.F. Carley, S.D. Jackson, J.N. O’Shea, M.W. Roberts, The formation and characterisation of Ni3+ – an X-ray photoelectron spectroscopic investigation of potassium-doped Ni(1 1 0)–O, Surf. Sci. 440 (1999) 868–874. [34] I. Predaa, R.J.O. Mossanekb, M. Abbateb, L. Alvarezc, J. Méndezc, A. Gutiérreza, L. Sorianoa, Surface contributions to the XPS spectra of nanostructured NiO deposited on HOPG, Surf. Sci. 606 (2012) 1426–1430.
265
[35] W.T. Draai, G. Blasse, Energy transfer from vanadate to rare-earth ions in calcium sulphate, Phys. Status Solidi (a) 21 (1974) 569–579. [36] J. Mendialdua, R. Casanova, Y. Barbaux, XPS studies of V2 O5 , V6 O13 , VO2 and V2 O3 , J. Electron. Spectrosc. Relat. Phenom. 71 (1995) 249–261. [37] W. Zhu, W. Zhu, X. Feng, Z. Wu, Z. Man, On the annealing mechanism in PbWO4 crystals, Phys. B: Condens. Matter 324 (2002) 53–58. [38] Q.H. Wu, A. Thissen, W. Jaegermann, M.L. Liu, Photoelectron spectroscopy study of oxygen vacancy on vanadium oxides surface, Appl. Surf. Sci. 236 (2004) 473–478. [39] L.G. Devi, N. Kottam, S.G. Kumar, K.E. Rajashekhar, Preparation, characterization and enhanced photocatalytic activity of Ni2+ doped titania under solar light, Cent. Eur. J. Chem. 8 (2010) 142–148. [40] Q. Xiang, J. Yu, P.K. Wong, Quantitative characterization of hydroxyl radicals produced by various photocatalysts, J. Colloid Interface Sci. 357 (2011) 163–167. [41] Y. Hu, D. Li, F. Sun, H. Wang, Y. Weng, W. Xiong, Y. Shao, One-pot template-free synthesis of heterophase BiVO4 microspheres with enhanced photocatalytic activity, RSC Adv. 5 (2015) 54882–54889.