Cation Distribution in ZnCr2O4 Nanocrystals Investigated by X

Effects of synthesis method on cation distribution and optical properties of Co/Cr co-doped MgGa2O4 nanoparticles. Journal of Alloys and Compounds 201...
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Cation Distribution in ZnCr2O4 Nanocrystals Investigated by X‑ray Absorption Fine Structure Spectroscopy Shuangming Chen, Yanfei Wu, Peixin Cui, Wangsheng Chu, Xing Chen,* and Ziyu Wu* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China ABSTRACT: ZnCr2O4 nanocrystals with different sizes were synthesized by a coprecipitation and postcalcined method. X-ray powder diffraction and transmission electron microscopy showed that the nanocrystals annealed at different temperatures have the same crystal structure and their size increases with increasing annealing temperature. Cr and Zn K-edge X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) was used to investigate the change of the inversion parameter vs the annealing temperature. The inversion parameter is found to decrease with increasing annealing temperature. In particular, the sample annealed at 750 °C has a normal spinel structure, like bulk ZnCr2O4. On the contrary, the inversion parameter of the sample annealed at 300 °C can get 0.24. This study of ZnCr2O4 shows for the first time a variation of the inversion degree with annealing temperature, and provides a method to obtain ZnCr2O4 with different inversion parameters.

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INTRODUCTION Spinel compounds, which have a general formula AB2O4, have a wide range of applications as catalyst, ceramic, magnetic, and sensing materials because of their excellent physical and chemical properties.1−4 There are two types of cation sites in the spinel structure: octahedral and tetrahedral. In general, in an ideal spinel, A cations occupy the tetrahedral sites while B cations occupy the octahedral sites. However, most of the spinel compounds deviate from the ideal distribution and the occupation of the two sites is affected by various factors, such as the temperature, ionic size, particle size, cationic charge, electronic state, etc.5−8 Actually, when all of the A cations occupy the octahedral sites and half of the B cations occupy the tetrahedral sites, we have an inverse spinel. However, only some A cations exchange with B cations, obtaining a mixed spinel. On the basis of this, the general spinel formula can be written as (A1−xBx)[AxB2−x]O4, for which the brackets ( ) and [ ] refer to tetrahedral and octahedral sites, respectively. The inversion parameter or the degree of inversion is defined by x.8 Among the spinel compounds, ZnCr2O4 spinel attracted a lot of attention because of their sufficient thermodynamic stability and sensing properties.9 Different methods including the microemulsion,10 the hydrothermal,11 the mechanical activation,12 the high-temperature solid-state reaction,13 the sol− gel, 14 etc., 15,16 have been used to synthesize ZnCr 2 O 4 nanoparticles. Actually, the physical and chemical properties of spinel depend strongly on the distribution of A and B cations in the different crystallographic sites.17,18 In particular, the most important properties of ZnCr2O4 are dominated by the local structure and the cation distribution of Zn and Cr cations. However, probably because its inversion parameter is theoretically closing to zero and because of the limitations of conventional characterization methods, the cation distribution of Zn and Cr in the ZnCr2O4 structure is not well-known.19 X© 2013 American Chemical Society

ray absorption structure (XAS) spectroscopy is a powerful tool to investigate the local structure around the photoabsorber.20 Both EXAFS and XANES provide valuable information. Recently, they have been used to investigate the cation distribution in a similar spinel, such as CuFe2O4,21,22 ZnFe2O4,23,24 MnFe2O4,25,26 and other spinels.27,28 In this manuscript, we present an XAS study on the Zn and Cr distribution in ZnCr2O4 nanoparticles synthesized by a coprecipitation and postcalcined method. Some ZnCr2O4 nanoparticles with an inversion parameter of 0.24 were obtained, and we show also the inversion degree of ZnCr2O4 decreases with the annealing temperature.

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EXPERIMENTAL SECTION Sample Preparation. In a typical procedure, appropriate amounts of starting materials Cr(NO3)3·9H2O (1.6006 g) and Zn(NO3)2·6H2O (0.5950 g) were dissolved in 40 mL of deionized water. In order to obtain a smaller particle size and better particle size distribution,29,30 PVP (1.1114 g) as a stabilizer was also added to this solution. Then, 0.8 g of NaOH was added to the mixed solution under constant magnetic stirring, rapidly forming a dark green suspension. After, the mixture was continuously stirred for 30 min. Next, it was transferred to a stainless steel autoclave with a 50 mL Teflon liner and heated in an oven at 200 °C for 10 h. After the autoclave was air-cooled to room temperature, the supernatant in the autoclave was poured out and the green precipitation was gained by centrifugation and washed several times with distilled water and absolute ethanol. Then, the products were dried in a Received: May 21, 2013 Revised: November 4, 2013 Published: November 4, 2013 25019

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vacuum oven at 90 °C for 5 h. Finally, the products were milled into powders and the resulting powders were divided into four parts, calcined at 300, 450, 600, and 750 °C for 4 h, and labeled as ZC300, ZC450, ZC600, and ZC750, respectively. Sample Characterization. X-ray diffraction was performed on a TTR-III high-power X-ray powder diffractometer employing a scanning rate of 0.02 s−1 in a 2θ range from 20 to 80° with Cu Kα radiation. The morphology and microstructure of samples were investigated using a transmission electron microscope (TEM, JEM-2011), equipped with energy dispersive X-ray spectroscopy (EDS). The Cr and Zn K-edge XAFS spectra of all samples were measured in the transmission mode at the beamline U7C of the National Synchrotron Radiation Facility (NSRL) in Hefei. The storage ring was operated at 0.8 GeV with a maximum current of 250 mA. A double crystal Si(111) monochromator was used for the experiments. The powder samples were homogeneously smeared on the Scotch adhesive tape, and several layers were folded to reach the optimum absorption thickness (Δμd ≈ 1, Δμ is the absorption edge jump and d is the physical thickness of the sample). XAFS Data Analysis. XAFS data were analyzed with the ATHENA and ARTEMIS code of Ravel.31 The energy thresholds were determined as the maxima of the first derivative. Absorption curves were normalized to 1, and the EXAFS signals χ(k) were obtained after the removal of the preedge and post-edge background. The Fourier transform (FT) spectra were obtained as k3χ(k) with a Hanning window in the range 3−15 Å−1 for the Zn K-edge and 3−14.5 Å−1 for the Cr K-edge. Theoretical amplitude and phase-shift functions were calculated with the FEFF8.2 code32 using the structure parameters of the ZnCr2O4.33 Due to the transferability of amplitude and phase shifts, the coordination parameters of unknown samples can be obtained by fitting the experimental curves with theoretical amplitudes and phase shifts. XANES Calculation. Zn K-edge XANES simulations based on self-consistent multiple-scattering (MS) theory were performed using FEFF 8.2 code with the Hedin−Lundqvist (H−L) exchange potential. A 57-atom cluster (radius 0.53 nm) was used to calculate the self-consistent field (SCF) muffin-tin atomic potential, and a 221-atom cluster (radius 0.80 nm) for full-multiple scattering (FMS) XANES calculations was considered. The normal spinel ZnCr2O4 was used as the model structure.

Figure 1. XRD patterns of the samples annealed at different temperatures.

annealing temperature increases. As an example, the microstructure of the ZC450 obtained by HRTEM is shown in the inset of Figure 2b. The periodical lattice fringes demonstrate that these nanoparticles can be considered as good single crystals. The interplanar spacing pointed to by the arrow in the image is 5.03 Å, a value really close to the ideal value of 5.02 Å of the ZnCr2O4 {311} facets. The corresponding energy dispersive spectroscopy (EDS) spectra have been collected, giving a percentage of Zn and Cr of 0.52, in excellent agreement with the theoretical value of 0.5. XANES Analysis. Normalized Zn K-edge XANES spectra of samples ZC300, ZC450, ZC600, and ZC750 are shown in Figure 3 (left side). There are three resolved peaks and a significant shoulder at the white line region. A close look of Figure 3 (see inset in Figure 3, left side) reveals that feature A decreases while feature B increases in amplitude with the increasing annealing temperature. In a similar report for the spinel structure of ZnFe2O4, Stewart et al. have shown that feature A increases while feature B decreases for the Zn K-edge XANES spectrum when the degree of inversion increases.35 It is highly probable that for ZnCr2O4 these changes have the same origin. To support this hypothesis, we performed Zn K-edge XANES simulations for the normal ZnCr2O4 structure (shown on the left side of Figure 4) and for isolated Zn atom substitutionally replacing Cr at octahedral sites (right side of Figure 4). Calculated spectra were shown in Figure 3 (right side of Figure 4). For this calculation, the Z + 1 approximation (Z, atomic number) for the absorber atom is necessary to account for the three main peaks. It can be seen from the figure that the white line of Zn substitutionally replacing Fe has she same position with peak A in the standard ZnCr2O4. We also found that there is no apparent feature in the peak B region in the spectrum of line b. All of this means, as the inversion degree increases, the occupation number of Zn atom in octahedral sites increases; i.e., the amplitude of feature A increases, while that of feature B decreases. Therefore, we can conclude that the inversion degree increases with the decreasing annealing temperature. Furthermore, we also found that the intensity of the white line increases with the decreasing annealing temperature. As we know, the white line peak intensity is related to the symmetry of the central atom local geometrical structure of the photoabsorber. It increases with the occupation number of Zn atom substituted to the octahedral sites. This change also supports the increase of the inversion parameter.

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RESULTS AND DISCUSSION XRD and TEM Characterization. XRD patterns of samples annealed at different temperatures are shown in Figure 1. Neglecting the broadening of XRD peaks for the smaller particle samples ZC300, all XRD patterns of the annealed samples are consistent with the crystalline ZnCr2O4 (PDF = 82−1532). No additional peaks are observed in the patterns of the annealed samples, ruling out the presence of impurities. The sharpening and the increasing intensities of the diffraction peaks vs the annealing temperature point out an increasing size of the crystallite. Representative TEM images of ZnCr2O4 nanocrystals annealed at different temperatures are shown in Figure 2. The sample ZC300 is composed by nanoparticles without defined grain boundaries probably due to residual surfactants on the surface. Sigmund et al. have reported that the PVP is completely removed at 450 °C.34 The particle distribution becomes worse and the crystallinity improves when the 25020

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Figure 2. TEM images of the samples (a) ZC300, (b) ZC450, (c) ZC600, and (d) ZC750. The inset in panel b shows the corresponding HRTEM image and EDS spectrum.

Figure 3. Normalized XANES spectra of the Zn K-edge (left side); the simulation result of the Zn K-edge XANES spectrum of normal ZnCr2O4 (a) and isolated Zn atom substitutionally replacing Cr at octahedral sites in ZnCr2O4 structure (b) (right side).

very low.24 The increase in the intensity of the pre-edge peak can be attributed to the enhancement of the orbital p−d mixing, allowed in a tetrahedral symmetry but forbidden in the octahedral one. Thus, the increase of the pre-edge peak intensity reflects the increase of the occupation number of Cr atom in tetrahedral sites shown as follows: ZC300 > ZC450 > ZC600 > ZC750. In addition, the decrease in the amplitude of

Figure 5 shows the normalized Cr K-edge XANES spectra. As we can see from the inset in the upper left and upper right corner, the pre-edge peak C decreases while the white line peak increases in amplitude with the increasing annealing temperature. As we know, the pre-edge peak is due to the 1s to 3d quadrupole transitions (formally electric dipole forbidden), though the intensity of the quadrupole transition is generally 25021

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Figure 4. Structural models for XANES simulations: normal ZnCr2O4 structure (left side) and disordered ZnCr2O4 structure with one Zn atom exchange site with one Cr atom (right side).

the white line also points out the increase of the inversion degree. All data are well consistent with the analysis results of the Zn K-edge. EXAFS Analysis. The EXAFS signals of the Zn and Cr Kedge with k3-weight are compared in Figure 6. For the Zn−K and Cr−K edges, the oscillations are similar among samples ZC300, ZC450, ZC600, and ZC7500, implying that ZC300, ZC450, ZC600, and ZC750 have a very similar structure around Zn and Cr. The corresponding Fourier transforms of Zn and Cr K-edges are compared in Figure 7. As we know, in the bulk ZnCr2O4, the Zn atoms are tetra-coordinated by four oxygens at 1.97 Å and 12 Cr atoms are at 3.45 Å as second neighbors. On the other hand, Cr atoms are octal-coordinated by 6 O at 1.99 Å and 6 Cr atoms at 2.94 Å as second neighbors. Obviously, compared to the first coordination peak, the second coordination peak is more sensitive to the inversion degree. Taking a close look at the Zn K-edge at Figure 7 (left side), we may point out that, except for the two normal intensive peaks centered around 1.6 and 3.1 Å, an additional relatively weaker

Figure 5. Normalized XANES spectra of the Cr K-edge.

Figure 6. The k3-weighted EXAFS signals for the Zn (left side) and Cr K-edge (right side). 25022

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Figure 7. The Fourier transform EXAFS signals for the Zn (left side) and Cr K-edge (right side).

Figure 8. Zn K-edge EXAFS spectra (left) and Fourier transform (right) fit of the ZC450 sample, compared with the best fit curve (dotted line).

method as described in ref 39. The only adjustable parameter is the fractional occupancy of Zn atoms in tetrahedral and octahedral sites. All other parameters are calculated by means of the FEFF 8.2 code. As an example, the best fitting curves of the Zn K-edge for the sample ZC450 are shown in Figure 8. The inversion degree is about 24, 18, 5, and 0% for ZC300, ZC450, ZC600, and ZC750, respectively. That means approximately 24% of Zn2+ ions substitute to octahedral sites for the sample annealed at 300 °C. Conversely, the sample annealed at 750 °C has a normal spinel structure, like bulk ZnCr2O4. The present determination of the inversion degree could be eventually improved by measuring highly homogeneous samples within the standard errors on the coordination numbers of about 10%. No matter the theoretical predictions or the previous reports, the inversion parameter of the ZnCr2O4 is close to zero because the Cr3+ has a large excess of octahedral crystal field stabilization energy and it is extremely difficult for Zn2+ cations to exchange with Cr3+ cations.19 However, in our work, the inversion degree of the sample ZC300 could be 24% probably due to the specific synthesis route and nanosize effect. The hydrothermal process has led to a good homogeneity of the precursor, and the annealing process can accelerate the cation diffusion. The size effect has been proved to have an important role in the cation distribution of the spinel nanoparticles.8,18,23 In fact, the distribution of both Zn2+ and Cr3+ in the ZC300 is not very stable due to the high percentage of atoms located on the surface of the nanoparticle. When annealed at 450 °C, the

peak D (see arrow) appears at about 2.6 Å, which should come from the inversion effect. Stewart et al. have observed a similar change from the inversion effect in the Zn K-edge of ZnFe2O4.35 When Zn atoms substitute to octahedral sites, a new shell of Cr atoms would appear at about 2.94 Å from central Zn, resulting in a new peak. It is clearly shown in the inset of Figure 7 (left side) that peak D decreases in amplitude with the increasing annealing temperature, indicating the decreasing inversion degree. For the Cr K-edge, we show in Figure 7 (see the inset on the right side) that the distance between the central atom and the second coordinated shell peak becomes longer with the decreasing annealing temperature. This behavior can be explained again by the inversion effect. A similar effect can also be found in coprecipitated ZnFe2O4.36 As Cr atoms substitute to tetrahedral sites from octahedral sites, the coordination number of Cr/Zn centered at about 3.45 Å increases, while the coordination number of Cr/ Zn centered at about 2.94 Å decreases. As a result, the second coordinated shell atoms becomes more distant. Therefore, all the EXAFS analysis demonstrates that the inversion degree increases with the decreasing annealing temperature, consistent with the XANES analysis. As shown above, the EXAFS analysis of the Zn K-edge is more straightforward compared to that of Cr K-edge spectra.37,38 In order to estimate the inversion parameter, a least-squares fitting was performed over the r-space range 0.5− 3.6 Å for the Fourier transform of the Zn K-edge, following the 25023

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(7) Zhang, Z. J.; Wang, Z. L.; Chakoumakos, B. C.; Yin, J. S. Temperature Dependence of Cation Distribution and Oxidation State in Magnetic Mn-Fe Ferrite Nanocrystals. J. Am. Chem. Soc. 1998, 120, 1800−1804. (8) Sreeja, V.; Smitha, T. S.; Nand, D.; Ajithkumar, T. G.; Joy, P. A. Size Dependent Coordination Behavior and Cation Distribution in MgAl2O4 Nanoparticles from 27Al Solid State NMR Studies. J. Phys. Chem. C 2008, 112, 14737−14744. (9) Mancic, L.; Marinkovic, Z.; Vulic, P.; Moral, C.; Milosevic, O. Morphology, Structure and Nonstoichiometry of ZnCr2O4 Nanophased Powder. Sensors 2003, 3, 415−423. (10) Niu, X. S.; Du, W. P.; Du, W. M. Preparation and Gas Sensing Properties of ZnM2O4 (M=Fe, Co, Cr). Sens. Actuators, B 2004, 99, 405−415. (11) Peng, C.; Gao, L. Optical and Photocatalytic Properties of Spinel ZnCr2O4 Nanoparticles Synthesized by a Hydrothermal Route. J. Am. Ceram. Soc. 2008, 91, 2388−2390. (12) Marinkovic, Z. V.; Mancic, L.; Vulic, P.; Milosevic, O. The Influence of Mechanical Activation on the Stoichmetry and Defect Structure of A Sintered ZnO-Cr2O3 System. Mater. Sci. Forum 2004, 453, 456−461. (13) Levy, S.; Diella, D.; Pavese, V.; Dapiaggi, A.; Sani, M. AP-V Equation of State, Thermal Expansion, and P-T Stability of Synthetic (ZnCr2O4 Spinel). Am. Mineral. 2005, 90, 1157−1167. (14) Yazdanbakhsh, M.; Khosravi, I.; Goharshadi, E. K.; Youssefi, A. Fabrication of Nanospinel ZnCr2O4 using Sol-gel Method and Its Application on Removal of azo dye from Aqueous Solution. J. Hazardous Mater. 2010, 184, 684−689. (15) Chandran, R. G.; Patil, K. C. A Rapid Method to Prepare Crystalline Fine Particle Chromite Powders. Mater. Lett. 1992, 12, 437−450. (16) Marinkovic, Z. V.; Mancic, L.; Maric, R.; Milosevic, O. Microstructure Characterization of Mechanically Activated ZnOCr2O3 System. J. Eur. Ceram. Soc 2001, 21, 2051−2067. (17) Schreyeck, L.; Wlosik, A.; Fuzellier, H. Influence of the Synthesis Route on MgAl2O4 Spinel Properties. J. Mater. Chem. 2011, 11, 483−486. (18) Duan, X. L.; Yuan, D. R.; Yu, F. P. Cation Distribution in Codoped ZnAl2O4 Nanoparticles Studied by X-ray Photoelectron Spectroscopy and 27Al Solid-state NMR Spectroscopy. Inorg. Chem. 2011, 50, 5460−5467. (19) O’Neill, H. St. C.; Dollase, W. A. Crystal Structures and Cation Distribution in Simple Spinels from Powder XRD Structural Refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the Temperature Dependence of the Cation Distribution in ZnAl2O4. Phys. Chem. Miner. 1994, 20, 541−555. (20) Benfatto, M.; Natoli, C. R.; Bianconi, A.; Garcia, J.; Marcelli, A.; Fanfoni, M.; Davoli, I. Multiple-Scattering Regime and Higher-Order Correlations in X-ray-absorption Spectra of Liguid Solutions. Phys. Rev. B 1986, 34, 5774−5781. (21) Krishnan, V.; Selvan, R. K.; Augustin, C. O.; Gedanken, A.; Bertagnolli, H. EXAFS and XANES Investigation of CuFe 2O4 nanoparticles and CuFe2O4-MO2 (M=Sn, Ce) Nanocomposites. J. Phys. Chem. C 2007, 111, 16724−16733. (22) Selvan, R. K.; Krishnan, V.; Augustin, C. O.; Bertagnolli, H.; Kim, C. S.; Gedanken, A. Investigations on the Structural, Morphological, Electrical, and Magnetic Properties of CuFe2O4-NiO Nanocomposites. Chem. Mater. 2008, 20, 429−439. (23) Carta, D.; Marras, C.; Loche, D.; Mountjoy, G.; Ahmed, S. I.; Corrias, A. An X-ray Absorption Spectroscopy Study of the Inversion Degree in Zinc Ferrite Nanocrystals Dispersed on A Highly Porous Silica Aerogel Matrix. J. Chem. Phys. 2013, 138, 054702. (24) Akhtar, M. J.; Nadeem, M.; Javaid, S.; Atif, M. Cation Distribution in Nanocrystalline ZnFe2O4 Investigated using X-ray Absorption Fine Structure Spectroscopy. J. Phys.: Condens. Matter 2009, 21, 405303. (25) Carta, D.; Casula, M. F.; Mountjoyz, G.; Corrias, A. Formation and Cation Distribution in Supported Manganese Ferrite Nano-

PVP decomposed completely and ZnCr2O4 nanoparticles grew bigger. During this annealing process, the activation barrier caused by the exchange of Cr3+ and Zn2+ will be overcome to obtain the cation redistribution. Increasing the annealing temperature, ZnCr2O4 particles become larger and the inversion degree deceases until it coincides with that of the bulk ZnCr2O4.

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CONCLUSION ZnCr2O4 nanoparticles with different degrees of inversion were obtained by a coprecipitation and postcalcined method. Some ZnCr2O4 nanoparticles with an inversion parameter of 0.24 were obtained when the annealing temperature was 300 °C. Details of the cation distribution between the octahedral and tetrahedral sites for the ZnCr2O4 sample annealed at different temperatures were determined using XANES and EXAFS spectroscopy. XANES points out that both the pre-edge peak at the Cr K-edge and the post-edge features at the Zn K-edge are both strongly dependent with the inversion degree. A detailed EXAFS analysis indicates the configuration of the second coordination shell atoms around the Zn and Cr atoms are also related to the inversion degree. Both the EXAFS and XANES analyses demonstrate that the inversion parameter decreases with increasing annealing temperature.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the National Basic Research Program of China (2012CB825800), the Science Fund for Creative Research Groups (11321503), the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-N42), and the National Natural Science Funds (11179031, 11179006, 11275227).

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