M A TE RI A L S CH A RACT ER IZ A TI O N 73 (2 0 1 2 ) 1 2 4–1 2 9
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Electronic structure and magnetic properties of FeWO4 nanocrystals synthesized by the microwave-hydrothermal method M.A.P. Almeidaa , L.S. Cavalcanteb,⁎, C. Morilla-Santosc , P.N. Lisboa Filhoc , A. Beltránd , J. Andrésd , L. Graciad , E. Longoa, b a
INCTMN-DQ-Universidade Federal de São Carlos, São Carlos, P.O. Box 676, 13565‐905, SP, Brazil INCTMN-Universidade Estadual, Paulista, P.O. Box 355, 14801‐907, Araraquara, SP, Brazil c MAv-Universidade Estadual, Paulista, P.O. Box 473, 17033‐360, Bauru, SP, Brazil d Department de Química Física i Analítica, Universitat Jaume I, E-12071 Castelló, Spain b
AR TIC LE D ATA
ABSTR ACT
Article history:
This communication reports that FeWO4 nanocrystals were successfully synthesized by
Received 26 June 2012
the microwave-hydrothermal method at 443 K for 1 h. The structure and shape of these
Received in revised form
nanocrystals were characterized by X-ray diffraction, Rietveld refinement, and transmission
18 August 2012
electron microscopy. The experimental results and first principles calculations were combined
Accepted 21 August 2012
to explain the electronic structure and magnetic properties. Experimental data were obtained by magnetization measurements for different applied magnetic fields. Theoretical calculations
Keywords:
revealed that magnetic properties of FeWO4 nanocrystals can be assigned to two magnetic
Clusters
orderings with parallel or antiparallel spins in adjacent chains. These factors are crucial to
Crystal growth
understanding of competition between ferro- and antiferromagnetic behavior.
Electronic properties
© 2012 Elsevier Inc. All rights reserved.
Magnetic measurements
1.
Introduction
In recent years, wolframite-type tungstates have attracted attention since they are a fascinating class of materials that offers a wide range of technological applications, such as: catalysis [1], optics [2], humidity sensors [3] and magnetic properties [1,4–8]. Iron tungstate (FeWO4) is one of the most promising wolframitetype materials due to its electronic structure and magnetic properties which have been studied in different applications [1,4–6,9]. The electronic structure and magnetic properties of FeWO4 nanocrystals have been only briefly studied, and therefore further studies on the magnetic behavior in FeWO4 are necessary to fully understand its electronic properties. Recently, Stüber et
⁎ Corresponding author. Tel.: + 55 16 3361 5215; fax: + 55 16 3361 8214. E-mail address:
[email protected] (L.S. Cavalcante). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.08.006
al. [10] and Rajagopal et al. [11] reported magnetic properties of FeWO4 and other wolframite-type tungstates such as: MnWO4, NiWO4 and CoWO4. These researchers have indicated that these tungstates have an incomplete 3d shell and are paramagnetic at room temperature; at low temperatures, these complex metal oxides undergo cooperative transitions to antiferromagnetically ordered states [12]. One of the main drawbacks in the study on wolframite-type tungstate is the difficulty in obtaining monophasic samples. Numerous synthetic methods have been developed to obtain FeWO4 crystals such as a solid state reaction, spray pyrolysis, the Czochralski method, the conventional hydrothermal (CH) process, etc. [1,2,7,9,13–15]. However, these synthetic methods
M A TE RI A L S C HA RACT ER I ZA TI O N 73 ( 20 1 2 ) 1 2 4–1 2 9
require long processing times as well as high-temperature synthesis to obtain FeWO4 crystals. To minimize these problems, the microwave-hydrothermal (MH) method was developed by Komarneni et al. [16] and has been used recently as a promising method for the preparation of different nanomaterials with a controlled size and shape [17–20]. The MH method is fast, clean, simple and energetically more efficient than the CH method. Our group has produced different nanocrystals by the MH method as recently reported [21,22] where it is quite efficient because it is possible to synthesize nanocrystalline materials at reduced times and mild processing temperatures as well as the use of environmentally friendly solvents. Moreover, this method enables the reduction of time and temperature processing which facilitates the optimization of experimental factors [23,24]. Therefore, this synthesis method is very promising for the preparation of different tungstates. To the best of our knowledge, the MH method has not been reported in the literature to obtain FeWO4 nanocrystals with magnetic properties. Therefore, in this communication, we report the synthesis of FeWO4 nanocrystals by the MH method. Moreover, these nanocrystals were characterized by X-ray diffraction, Rietveld refinement, and transmission electron microscopy. Finally, an understanding on electronic structure and magnetic properties has been achieved by a combination of first principles calculations and experimental measurements.
2.
Experimental Procedure
2.1.
FeWO4 Nanocrystal Synthesis
The typical procedure for the synthesis of FeWO4 nanostructures was as follows: 1 × 10−3 mol of sodium tungstate (VI) dihydrate [Na2WO4.2H2O, 99.5% purity, Aldrich] and 2 × 10−3 mol of sodium dodecyl sulfate [NaC12H25SO4 (SDS), 99% purity, Aldrich] followed by the addition of 1 × 10−3 mol of ammonium iron (II) sulfate hexahydrate [(NH4)2Fe(SO4)2.6H2O, 99% purity, Aldrich] were dissolved while pH 9 was maintained with NH4OH before processing. In the sequence, 100 mL of this suspension was transferred into a Teflon vessel autoclave without stirring. Then, the reactor of Teflon was sealed and placed inside a domestic microwave system and processed at 443 K for 1 h [23]. The temperature of the system was monitored using an in-vessel temperature sensor (Model CNT-120, INCON Electronic Ltda., São Carlos-SP, Brazil) [24]. The MH reactions were carried out in polyethylene vessels of 150 mL capacity.
2.2.
Characterizations
These FeWO4 nanocrystals were structurally characterized by X-ray diffraction (XRD) patterns using a D/Max-2500PC diffractometer (Rigaku, Japan) with Cu–Kα radiation (λ = 1.5406 Å) in the 2θ range from 10° to 60° in a normal routine with a scanning velocity of 2°/min and from 10 to 110° with a scanning velocity of 1°/min in the Rietveld routine. The shape and size of these FeWO4 nanocrystals were observed by a transmission electron microscopy (TEM) and high resolution (HR)-TEM TECNAI F20 FEI microscope was used, operating at 200 kV. Magnetization versus an applied magnetic field in a zero field
125
cooled (ZFC) and field cooled (FC) measurements was performed using a Quantum Design Magnetic Properties Measurement System (MPMS) XL-5 Superconducting Quantum Interference Device.
2.3. Computational Method and Periodic Model of FeWO4 Crystals First-principles-based density functional theory (DFT) calculations are commonly used to fully understand the crystal structure and its relationship to the magnetic (spin density) and electronic (band gap and density of states) properties of this material. Here we combine insights from DFT simulations with atomic positions and unit cell parameters. All calculations have been carried out with the CRYSTAL09 program [25] in the framework of the DFT with the hybrid functional B3LYP [26,27]. Fe, W and O centers have been described in the scheme 86-411d41G [28], 11d31G [28], and 8-411G [28], respectively. These basis sets are all the effective core potentials. The definition of core and valence electrons for the valence basis set representing the Fe, W, and O centers can be found at the CRYSTAL09 web site [28].
3.
Results and Discussion
3.1.
XRD Patterns and Rietveld Refinement Analysis
Fig. 1(a) and (b) show XRD patterns and the Rietveld refinement plot for FeWO4 nanocrystals obtained by MH at 443 K for 1 h, respectively. All XRD patterns can be assigned to a monoclinic structure belonging to the (P2/c) space group with unit cell parameters: a = 4.6801 Å, b = 5.6975 Å, c = 4.9841 Å, α = γ = 90° and β = 89.881° which is in agreement with the respective JPCDS-PDF card file No. 46‐1446 (see Fig. 1(a)). Results obtained from the Rietveld refinement method with the GSAS program [29] show good agreement between observed XRD patterns and theoretical results (Fig. 1(b)). Moreover, profiles of XRD patterns experimentally observed and theoretically calculated by Rietveld refinement indicate small differences on the intensity scale near zero as illustrated by the (IObserved − ICalculated) line [30]. No secondary phases were detected which verifies that the products are of very high purity and are single phase. In addition, the intense and sharp diffraction peaks suggest that the as-synthesized products have a high degree of crystallization. An analysis and comparison of these data prove that the corresponding values are consistent with results reported by Rajagopal et al. [13] as well as previous values obtained by other researchers [1,15]. Structural and geometrical parameters obtained from Rietveld refinement data from XRD patterns are shown in Table 1. In this table, the fit parameters (Rwp, Rp, Rb, and χ2) suggest that refinement results are very reliable. Note that there are considerable variations in atomic positions related to the oxygen atoms, while the iron and tungsten atoms keep their fixed positions within the structure. These results indicate structural distortions on the octahedral [FeO6] and [WO6] clusters into the monoclinic lattice which can be induced by different levels of coupling between the microwave radiation and the magnetic lattice.
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M A TE RI A L S CH A RACT ER IZ A TI O N 73 (2 0 1 2 ) 1 2 4–1 2 9
Fig. 1 – (a) XRD patterns and (b) Rietveld refinement plots for FeWO4 nanocrystals synthesized by the MH at 443 K for 1 h with SDS 0.002 mol.
3.2.
Representation of the FeWO4 Supercells
A schematic representation for the monoclinic structure of FeWO4 nanocrystals with four unit cells modeled from the Rietveld refinement data and projected spin density map for these four unit cells are illustrated in Fig. 2(a) and (b), respectively.
In each structure, we have two visible molecules per unit cell (Z = 2) where every metal atom is surrounded by six oxygen atoms forming octahedral [FeO6] and [WO6] clusters, and zigzag chains of octahedral [FeO6] clusters are aligned along the c-axis (Fig. 2(a)). Two non-equivalent positions for oxygen atoms are characteristic of FeWO4 crystals with a wolframite-type monoclinic structure. There are two W atoms and one Fe atom in the nearest arrangement of O1 atoms while one W atom and two iron atoms are among the nearest neighbors of O2 atoms (see Table 1). To understand the magnetic behavior of the FeWO4 system, we employed the periodic supercell technique by using a 2 ×1 × 1 unit cell doubled along the a direction. This model comprises four Fe atoms with a 3d6 electronic configuration. To our knowledge, to date this approach has not been applied to this investigative system. We explored three different spin states for each Fe atom: a high spin (quintuplet), a low spin (triplet) and a closed shell (singlet); the high spin state is the most stable. For this quintuplet spin state at each Fe, three magnetic states have been calculated by fixing the total spin S. The first state is comparable to the antiferromagnetic state (AF1), S = 0, with the ferromagnetic ordering of spins at Fe atoms within zigzag chains but with the antiferromagnetic alignment between adjacent chains (see Fig. 2(a)). Note that the AF1 state requires the doubling of the crystallographic unit cell along the a-axis direction as a magnetic unit cell. There is also another possibility for the AF state (AF2) with S = 0 when spins at Fe atoms in the same chain have an antiparallel arrangement; in this case, no cell doubling is required since there are two Fe atoms per crystallographic unit cell. The ferromagnetic (FM) state, S = 8 with all spins at Fe atoms ordered collinearly and the nonmagnetic (NM) state were also calculated. AF1 is the most stable state where Fe atoms belonging to adjacent chains interact antiferromagnetically at a distance of 4.68 Å along the a-direction. AF1 lays 1.1, 1.6 and 11.4 eV lower in energy than the FM, AF2 and NM states, respectively. For AF1, calculations show that the magnetic moment on each Fe atom is 3.65 μB (~4 μB). The Mülliken population analysis was used to estimate total atomic charges. They are equal to +1.73 e for Fe, + 3.18 e for W, − 1.02 e for O1 and −1.24 e for O2. These values reveal that both Fe\O and W\O bonds have a mixed ionic-covalent character. The value around + 3 for W, instead of + 6 for the purely ionic one indicates a back charge
Table 1 – Results of Rietveld refinement for FeWO4 nanocrystals with monoclinic structure and space group (P2/c). Lattice parameters a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
4.680(1) V (Å3) 132.90
5.697(5) ρ (g/cm3) 7.589
4.984(1)
90.00
89.881
90.00
Rwp = 10.32%; Rp = 8.23%; Rb = 7.79%; and χ2 = 1.26 Atomic positions Atom
x
y
z
Occupancy
Multiplicity
Fe (2f) W (2e) O1 (4g) O2 (4g)
0.500(0) 0.000(0) 0.226(5) 0.272(1)
0.669(1) 0.176(5) 0.145(4) 0.352(6)
0.250(0) 0.250(0) 0.581(6) 0.143(6)
1 1 1 1
2 2 4 4
M A TE RI A L S C HA RACT ER I ZA TI O N 73 ( 20 1 2 ) 1 2 4–1 2 9
a
c a b
Fe W O
127
TEM images shown in Fig. 3(a) reveal that several nanocrystals have different sizes ranging from 10 nm to 50 nm. The dissolution of this anionic surfactant releases (Na+) positive ions and polar heads (R\SO4−) as counter negative ions. These negative heads have more ability to bond with the Fe2+ ions in the solution. However, the short carbon chain is not large enough to avoid the electrostatic interactions of Fe2+ ions with the WO42− ions during the MH processing. Therefore, the purpose of using SDS is to avoid the rapid growth of crystals. Moreover, the inset in Fig. 3 illustrates the selected area electron diffraction pattern for these FeWO4 nanocrystals which have a polycrystalline nature and can be perfectly indexed to a wolframite-type monoclinic structure. Fig. 3(b) shows the HR-TEM image for FeWO4 nanocrystals with interplanar distances of 0.36 and 0.47 nm, which are related to (110) and
b
Fig. 2 – (a) Schematic representation for monoclinic structure of FeWO4 with four unit cells illustrating the distorted octahedral [FeO6]/[WO6] clusters and (b) projected spin density map of the most stable AF1 configuration on the plane defined by four Fe atoms, two of them belonging to the same chain and the other two are located in the adjacent chain. The red and the blue colors are associated with α and β spins, respectively.
transfer process from the nearest oxygen atom, O1 and O2. This process is responsible for the strong distortions in octahedral [WO6] clusters which are associated with a secondorder Jahn–Teller effect [31]. As a result, there are three different lengths for both W\O bonds (W\O2: 1.70 Å, W\O1: 1.97 Å and W\O1: 2.28 Å) and Fe\O bonds (Fe\O1: 1.86 Å, Fe\O2: 2.16 Å, and Fe\O2: 2.23 Å). An analysis of the results shows that Fe atoms belonging to adjacent chains have α and β spin density distributions (Fig. 2(b)).
3.3.
TEM, HR-TEM and SAED Analysis
Fig. 3(a) and (b) illustrates the TEM images and HR-TEM images of FeWO4 nanocrystals, respectively.
Fig. 3 – (a) TEM images of the FeWO4 nanocrystals. Inset show a SAED pattern; (b) HRTEM image of nanocrystals. Inset shows an OA mechanism between two nanocrystals.
128
M A TE RI A L S CH A RACT ER IZ A TI O N 73 (2 0 1 2 ) 1 2 4–1 2 9
(100) planes, respectively. An analysis of these images reveals that nanocrystals have grown by an oriented attachment (OA) process where large nanostructures are formed from adjacent small nanostructures with a preferential growth in the [100] direction [32].
3.4.
Magnetic Properties Analysis
Magnetization versus applied magnetic field in zero field cooled (ZFC) and field cooled (FC) measurements was made. Thermal variations of magnetization measured for different applied magnetic fields at 10 Oe, 100 Oe, 1 KOe and 10 KOe and 50 KOe for ZFC and FC are depicted in Fig. 4. During the ZFC/FC cycle, the sample is first cooled in the absence of an external field and then warmed under an applied field until it reaches the paramagnetic state (ZFC). The sample is cooled again under the same applied field as before (FC). An analysis shows two types of profiles for ZFC and FC processes: at 10 Oe, 100 Oe and 1 KOe (see upper part of Fig. 4); at 10 KOe and 50 KOe (see lower part of Fig. 4). For 10 Oe, 100 Oe and 1 KOe, both FC and ZFC profiles exhibit a magnetic transition in the range of 51–55 K; below this range, a different behavior occurs which suggests both FM and AF states. On the other hand, an important feature can be observed concerning a shoulder followed by a pronounced slope of the ZFC magnetization at a low temperature. Moreover, the bulk magnetic dependence of samples for different applied magnetic fields (see Fig. 4) confirms both ferromagnetic (FM) and antiferromagnetic (AFM) responses. In this figure, ZFC and FC curves show a pronounced irreversibility response which indicates a competitive FM and AFM sublattice. The main contribution of the AFM sub-lattice with a Nèel critical temperature TN close to 54 K that it is suppressed as the magnetic field increases. This value of TN is much lower than previously reported elsewhere [1,9]. This decrease in the magnetic transition temperature TN can be directly associated with size effects and shape of the prepared samples [33]. Furthermore, at lower temperatures, a weaker ferromagnetism can also be observed due to inner plane sub-lattice.
Using experimental geometry, we obtained an indirect band gap from Γ (0,0,0) to A (½,½,0) of 1.65 eV which is the direct gap at Γ of 1.76 eV. This value is similar to the 1.78 eV value found by Rajagopal et al. [11] using full potential linearized augmented plane wave calculations. To further illustrate the origin behind magnetism in FeWO4, an analysis of the band structure and partial density of states reveals that the upper valence band is composed mainly of Fe 3d orbitals (mostly dxy and dyz) and to a lesser extent by O 2p orbitals with very little contribution from W 5d orbitals. The predominant contribution into the lower region of the conduction band is due to the W 5d states and also the O 2p and Fe 3d orbitals to a minor extent.
4.
Conclusions
In summary, FeWO4 nanocrystals were synthesized by the MH method for the first time. These nanocrystals were characterized by means of XRD patterns, Rietveld refinement data and TEM/ HRTEM images. XRD patterns and structural refinement indicated that the FeWO4 nanocrystals are monophasic with wolframite-type monoclinic structure. HR-TEM images revealed that FeWO4 nanocrystals grow by the OA mechanism along the [100] direction. On the basis of first-principles quantum mechanical calculations based on the DFT in the B3LYP level and temperature dependence measurements for different applied magnetic fields, we found that the main magnetic properties of FeWO4 nanocrystals are the result of competitive phenomena between the inner planes of FM and an AFM coupling between two successive planes having an antiparallel alignment of spins. Future research will focus on the development of an efficient route using the MH method to achieve the shape-controlled synthesis of FeWO4 nanocrystals with enhanced magnetic/ electrical properties which may motivate potential applications of FeWO4 nanocrystals. For the theoretical study, a clear area of research is the full optimization of the geometry for different magnetic states at a more accurate and costly computational level which is currently under development.
Acknowledgments
Magnetization (10-5emu/Oe)
4.0 FeWO4
3.5 3.0 FC
2.5
10 Oe 100 Oe 1 kOe 10 kOe 50 kOe
2.0 1.5 1.0 0.5
The authors acknowledge the financial support from the following Brazilian research financing institutions: FAPESP (No. 2009/53189-8), CNPq (159710/2011-1) and CAPES. This work is also supported by Generalitat Valenciana through the Prometeo/ 2009/053 project, Ministerio de Ciencia e Innovación for project CTQ2009-14541-C02, Spanish MALTA-Consolider Ingenio 2010 Program (Project CSD2007-00045), Programa de Cooperación Científica con Iberoamerica (Brazil), Ministerio de Educación (PHB2009-0065-PC). A. B. acknowledges a research grant from Generalitat Valenciana BEST/2011.
ZFC
0.0 0
25
50
75 100 125 150 175 200 225 250 275 300
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Temperature (K) Fig. 4 – Temperature dependence of the ZFC and FC magnetizations for FeWO4 nanocrystals measured under various applied fields.
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