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Control of structural disorder in spinel ceramics derived from layered double hydroxides
Ceramics International xxx (xxxx) xxx–xxx
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Control of structural disorder in spinel ceramics derived from layered double hydroxides Il Rok Jeonga,1, Jun Han Leeb,1, Jaejung Songa, Yoon Seok Ohb,∗∗, Seungho Choa,∗ a b
School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea Department of Physics, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Spinels Layered double hydroxides Structural disorder Ceramics Composites
Spinel oxides are versatile and functional materials with physicochemical properties that are significantly influenced by their structural disorder. The transition from layered double hydroxides (LDHs) to mixed metal oxides (MMOs) containing spinel oxides is a powerful approach to create materials with tailored properties suitable for various applications. Herein, we report the control of structural ordering in spinels related to the crystallinity, cationic inversion, and coordination number of constituent atoms by varying the transformation energy. Transformation from LDHs to spinels with higher applied energy leads to higher crystallinity, lower degree of inversion, and lower ratios of low-coordinated atoms to fully coordinated atoms after treatment with acid. We provide a general framework for controlling structural ordering, that can be applied to other spinels and metal oxides. In addition, LDHs and MMOs possess many chemical and structural degrees of freedom, making it possible to create materials that suit the needs of different applications, such as memory, sensors, catalysis, and energy conversion.
1. Introduction Solid-state structural transformations, including topotactic transitions, are effective strategies to obtain crystalline materials with desirable properties. Nature uses these strategies to reduce free energies in crystal growth [1–3], which has prompted the adoption of this approach for artificial formation of structures that cannot readily be achieved by classical crystal growth [4–6]. A remarkable example of the transformations is the thermal transition from layered double hydroxides (LDHs) to mixed metal oxides (MMOs). LDHs are an important class of ionic lamellar solids [7], which can be prepared by solutionbased methods. In general, compared to physical vapor deposition, solution-based syntheses consume less energy (hence, lower cost) and are easier to scale-up, which is beneficial for practical applications. Unfortunately, it is relatively difficult to control elemental compositions of the products of solution-based syntheses. In contrast, LDHs formed in solution can accommodate a wide range of different cations and anions [8,9], leading to compositional variety and precise control of cation ratios. In addition, LDHs are characterized by the interspersion, rather than the segregation, of cations within hydroxide layers [10,11]. Solid-state structural transformations from LDHs to MMOs
result in nanoscale phase separation with controllable phase sizes and crystallographic orientations within structures of tailored shapes and sizes [12]. Therefore, together with the aforementioned unique features of LDHs as precursors, MMOs derived from LDHs can possess many chemical and structural degrees of freedom, with versatility to fit different application needs [13–20]. Representative components of MMOs derived from LDH precursors are spinel oxides (AB2O4), as the ionic radii of metal cations in LDHs and those of spinel oxides match well. Spinel oxides are a large class of compounds that are isotypic with the mineral spinel MgAl2O4, possessing the distinct feature of cationic inversion [21]. Factors that influence cationic inversion include synthetic conditions, cation site preference in terms of size, covalent bonding effects, and crystal field stabilization energy. Spinels are widely used in industry due to their functional properties as magnetic materials, semiconductors, pigments, catalysts, and refractories [22]. Structural disorder of spinels related to crystallinity, cationic inversion, and coordination numbers of constituent atoms significantly influences the physicochemical properties, including magnetism, carrier transfer, carrier injection, and catalytic activity [23–27]. Thus, control of structural ordering is beneficial to effectively use spinels in a wide range of applications, such as memory
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y.S. Oh), [email protected] (S. Cho). 1 Il Rok Jeong and Jun Han Lee contributed equally to this work. ∗∗
https://doi.org/10.1016/j.ceramint.2019.11.145 Received 29 September 2019; Received in revised form 15 November 2019; Accepted 17 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Il Rok Jeong, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.145
Ceramics International xxx (xxxx) xxx–xxx
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coordination numbers. Structural ordering is tailored during the thermal transition from the LDH by controlling the amount of applied energy for the transformation. We chose ZnFe2O4 (ZFO) as our model spinel oxide because the Zn2+ cation prefers tetrahedral sites (A-sites) in the spinel [21,24]. Hence, ZFO of a normal spinel has antiferromagnetic ordering below T = 10 K [28,29]. However, cationic inversion (i.e., partial occupation of the tetrahedral A-sites and octahedral B-sites by Fe3+ and Zn2+ ions, respectively) induces ferrimagnetic ordering by intersite A–B interactions in ZFO [30–32]. Thus, magnetic properties of ZFO indicate its structural disorder. Zinc iron LDHs (ZnFe-LDHs) were synthesized, and after drying, the LDH powders were calcined for conversion to MMO structures with ZFO and ZnO phases. The amount of applied energy for transformation from the LDH to MMOs was easily tuned by varying calcination temperature, which allowed for sensitive control of structural disorder in spinel oxides, represented by crystallinity, cationic inversion, and ratios of low-coordinate atoms to fully coordinated atoms after selective etching of the other phase (ZnO in this study). Such control of the structural properties plays a crucial role in the applicability of spinels to a wide range of applications.
2. Experimental details Sample preparation: Zinc sulfate heptahydrate (ZnSO4·7H2O, ≥99.0%, Sigma-Aldrich), iron sulfate heptahydrate (FeSO4·7H2O, ≥99.0%, Sigma-Aldrich), and sodium hydroxide (NaOH, ≥98%, Sigma-Aldrich) were used. Hydrochloric acid solution (HCl, 35–37%, Samchun Chemicals) was used for acid treatments. All chemicals were used without further purification. To prepare zinc iron layered double hydroxides (ZnFe-LDHs), ZnSO4·7H2O and FeSO4·7H2O were dissolved in 50 mL deionized water to obtain a 0.44 M ZnSO4/0.22 M FeSO4 aqueous solution (Solution A). NaOH was dissolved in 50 mL deionized water to obtain a 0.82 M NaOH aqueous solution (Solution B). Solutions A and B were then simultaneously poured into 50 mL deionized water and stirred (500 rpm) at room temperature (25 °C) for 48 h. Precipitates were gathered and washed with deionized water using a centrifuge and vortex mixer. The powders were then collected and dried at 60 °C for 24 h. To prepare
Fig. 1. (a) X-ray diffraction θ-2θ scan and (b) scanning electron microscopy images of zinc iron layered double hydroxides synthesized from reaction at room temperature and ambient pressure.
storage, sensors, catalysis, and energy conversion. In this study, we demonstrate control of structural ordering parameters of spinel oxides, including crystallinity, cationic inversion, and
Fig. 2. X-ray diffraction θ-2θ scans and scanning electron microscopy images of mixed metal oxides prepared by calcination of zinc iron layered double hydroxide at (a and d) 300 °C, (b and e) 500 °C, and (c and f) 900 °C. 2
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should be suppressed in solution. Pure ZnFe-LDH powders were synthesized at room temperature and ambient pressure. Fig. 1a shows the X-ray diffraction (XRD) pattern (θ-2θ scan) of powders synthesized using these mild conditions. The peaks are assigned to a typical LDH phase. Scanning electron microscopy (SEM) revealed that the morphology of ZnFe-LDH powders is sheet-like (Fig. 1b). Energy-dispersive X-ray spectroscopy (EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) were used to confirm the expected 2:1 Zn2+/Fe3+ ratio of ZnFe-LDHs based on the molar ratio used during synthesis. For comparison, we performed the same synthesis of ZnFeLDHs at higher temperatures (90 °C and 150 °C), which resulted in mixed phases rather than pure ZnFe-LDHs (Fig. S1 in the Supporting Information). Therefore, we determined that pure ZnFe-LDHs could be successfully synthesized under ambient conditions without superfluous energy or manipulation, which is beneficial toward large-scale preparation for practical applications. Solid-state structural transformations occurred within the ZnFe-LDH structures by applying thermal energy. A θ-2θ XRD scan of the ZnFeLDH powders that underwent calcination at 300 °C (Fig. 2a) shows that the XRD peaks from the LDH phase disappear, indicating that the layered structures were not retained and that the crystalline structure changed. The broad peak at ~13° 2θ originates from the XRD sample holder (Fig. S2 in the Supporting Information). The noticeable peak centered at ~35° 2θ is assigned to (311) of spinel ZFO, indicating low crystallinity of the MMOs. Increasing the transformation energy by performing calcination of pristine LDHs at 500 °C resulted in spinel ZFO and hexagonal ZnO diffraction peaks (Fig. 2b). The MMOs prepared from calcination at 300 and 500 °C inherited the original sheet-like morphology of pristine ZnFe-LDHs (Figs. 1b, 2d and 2e). We also conducted transformations using even higher calcination temperatures (900 °C and 1200 °C), further increasing the transformation energies. The corresponding XRD patterns (Fig. 2c and Fig. S3a in the Supporting Information) show that as the calcination temperature increases, the MMOs produce sharper diffraction peaks, indicative of higher crystallinity in both phases. Higher calcination temperatures led to larger grain sizes, with clearly observed 4-fold and 6-fold symmetric polyhedron facets (Fig. 2f and Fig. S3b in the Supporting Information). In addition to XRD, we investigated the site disorder of cations in the transformed spinel oxides by measuring their magnetic properties, whereby we systematically studied the transformation energy dependent magnetic properties (Fig. 3). As shown in Fig. 3a, with decreasing temperature, both zero-field cooling (ZFC) and field cooling (FC) magnetic susceptibilities of the pristine ZnFe-LDH powders monotonically increased without displaying any magnetic anomaly down to 3 K. The magnetic hysteresis (M(H)) curve of the ZnFe-LDHs exhibits paramagnetic behavior at 3 K (Fig. 3b). In contrast, MMOs prepared by calcination of the ZnFe-LDHs at 300 °C exhibit a magnetic anomaly at 10.7 K in the ZFC temperature dependent magnetic susceptibility (χDC(T)) measurement, divergence of ZFC and FC χDC(T)s, and clear magnetic hysteresis with remanent magnetization (Mr) in M(H). These behaviors indicate that MMOs calcined at 300 °C possess ferrimagnetic long-range ordering below 10.7 K, even though at 300 °C the applied transformation energy is not enough to fully induce crystallized positional ordering (Fig. 2a). It is noteworthy that χDC of the FC χDC(T) increased without a local maximum as temperature decreased, which can be attributed to the coexistence of ferrimagnetic/antiferromagnetic and paramagnetic phases that remain due to insufficient transformation energy. MMOs calcined at 500 °C exhibit a large deviation between FC and ZFC χDC(T)s and enhanced Mr (Fig. 3a and b), as a result of improved crystallinity (Fig. 2a and b). However, the transformation energy from 500 °C calcination was still insufficient, and a portion of the Zn2+ cations occupied the octahedral B-sites of the spinel phase. In the case of 900 °C calcination (Fig. 2c), the crystallinity of ZFO significantly improved. Nevertheless, Mr and the deviation between FC and ZFC χDC(T) are comparable with those of the 500 °C case (Fig. 3a and b). Increasing
Fig. 3. (a) Temperature dependence of magnetization with zero-field cooling (ZFC) and field cooling (FC). (b) Magnetic hysteresis (M–H) curves at 3 K of pristine zinc iron layered double hydroxide and mixed metal oxides prepared by calcination of zinc iron layered double hydroxide at 300 °C, 500 °C, 900 °C, and 1200 °C.
mixed metal oxides (MMOs), the ZnFe-LDH powders were placed in alumina boats and then transferred to a quartz tube in the furnace and calcined at 300, 500, 900, or 1200 °C for 2 h with heating rates of 20 °C min−1. For selective dissolution of ZnO in MMOs, 0.2 mL HCl was added to 20 mL deionized water. The MMO powders were stirred for 2 min in the acidic solution. The resulting powders were filtered, washed with deionized water, and dried at 60 °C for 24 h. Characterization: The crystallinity of the samples was investigated by X-ray diffraction (XRD) using an X-ray diffractometer (D8 ADVANCE, Bruker AXS) with Cu Kα radiation (λ = 1.5406 Å). Morphologies of the samples were investigated using field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi High-Technologies). A transmission electron microscope (TEM; FEI Tecnai G2 F20 X-Twin TEM system operated at 200 kV) was used to evaluate the structural properties of the samples. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was conducted using a Varian 700-ES spectrometer. X-ray photoelectron spectroscopy (XPS) was performed with a ThermoFisher K-alpha system using a monochromatic Al Kα X-ray source. Magnetization versus temperature and magnetic field data were obtained using a vibrating sample magnetometer (Physical Property Measurement System, Quantum Design). 3. Results and discussion ZnFe-LDHs were used as precursors for MMOs composed of ZFO and ZnO. In order to prepare pure ZnFe-LDHs, the formation of other phases 3
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Fig. 4. (a) X-ray diffraction θ-2θ scan, (b and c) transmission electron microscopy images, (d) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, (e–g) energy-dispersive X-ray spectroscopy (EDS) elemental maps of acid-treated mixed metal oxides (MMOs) prepared by calcination of zinc iron layered double hydroxides (ZnFe-LDHs) at 500 °C, (h) magnetization versus temperature (M–T) curves with zero-field cooling (ZFC) and field cooling (FC), (i) magnetic hysteresis (M–H) curves at 3 K of MMOs prepared by calcination of ZnFe-LDHs at 500 °C and 900 °C before and after acid treatments, (j) Xray diffraction θ-2θ scan, and (k) scanning electron microscopy image of acid-treated MMOs prepared by calcination of ZnFe-LDHs at 900 °C.
other MMO phase(s). The ZnO phase was selectively etched to form ZFO nanoparticles with high surface-to-volume ratios and hence a large concentration of low-coordinate atoms per unit volume. The XRD θ-2θ scan pattern of acid-treated powders calcined at 500 °C shows that the diffraction peaks from the ZnO phase disappear and only peaks assigned to the ZFO phase remain (Fig. 4a), indicative of the selective dissolution of ZnO. Transmission electron microscopy (TEM) reveals that nanoparticles were formed after treatment with acid (Fig. 4b). The average particle diameter is 6.87 nm, thus confirming their very high surface-tovolume ratios. The nanoparticles are highly crystalline with a lattice spacing of 0.30 nm, corresponding to the distance between the (220) planes in the ZFO crystal lattice (Fig. 4c). Fig. 4d shows a high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) image of a porous structure composed of nanoparticles. EDS elemental maps show that Zn (Fig. 4e), Fe (Fig. 4f), and O (Fig. 4g) are distributed homogeneously within the structure. Based on the cation ratio of pristine LDH and MMO powders (determined using EDS and ICP-OES), a 1:1 ratio by mass of ZFO and ZnO in the MMOs was
the transformation energy to 1200 °C produced not only very high crystallinity of the spinel (Fig. S3a in the Supporting Information), but also preferential occupation of the tetrahedral A-sites by Zn2+ cations, which led to normal spinel regions and hence increase in fraction of the antiferromagnetic phase. Therefore, Mr decreased. In order to confirm the valence state of iron cations, X-ray photoelectron spectroscopy (XPS) measurements were performed (Fig. S4 in Supporting Information). For all samples, the positions of the peaks in the Fe 2p spectra are consistent with the Fe3+ oxidation state [33]. Furthermore, the positions and shapes of the peaks in the Fe 2p spectra are similar, indicating that changes in magnetization were not due to different valence states of iron cations. The coordination numbers of constituent atoms can also significantly affect the various physicochemical properties of spinel oxides, such as magnetic, electronic, ionic, catalytic, and mechanical properties [26,34–38]. In this study, we also demonstrate control of the ratio between low-coordinate atoms of spinels to fully coordinated atoms by varying the transformation energy and selective dissolution of
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Fig. 5. Schematic illustration of the transformation from layered double hydroxide to mixed metal oxide ceramic containing spinel with controlled structural ordering. Spinel crystal grain sizes also represent degrees of crystallinity.
changing the amount of applied energy, morphology and structural disorder of spinels can be significantly altered, which in turn influences their physicochemical properties. Although ZFO is used as a specific example here, we provide a general framework for controlling structural ordering that can be applied to other spinels and ceramics. Furthermore, such controlled transformation can also maximize synergistic composite effects between coupled spinels and the other component(s).
expected. The mass of the powders before and after acid treatment indicates ~50% reduction in mass after selective dissolution of the ZnO phase in the MMOs, thus confirming the initial 1:1 mass ratio. The ratio of low-coordinate atoms to fully coordinated atoms can influence the magnetic ordering [21–23,26–28]. Magnetization values significantly increased after selective ZnO etching, resulting in formation of ZFO nanoparticles with high surface-to-volume ratios (Fig. 4h and i). This magnetization enhancement can be attributed to deviation from ideal normal spinel ordering of ZFO on the surface of ZFO nanoparticles, where a significant portion of low-coordinate cations are located. Such structures with high surface-to-volume ratios are beneficial for catalysis and energy storage or conversion applications. To confirm the effect of low-coordinate atoms, the magnetic properties of the ZFO powders with much lower surface-to-volume ratios were also measured. As shown by XRD and SEM (Fig. 2), MMOs prepared by calcination at 900 °C exhibit far larger ZFO and ZnO grains than those calcined at 500 °C. In addition, faceted crystals with distinct (4-fold and 6-fold) symmetries can clearly be seen, indicating that surfaces of cubic ZFO and hexagonal ZnO were already exposed before acid treatments (Fig. 2c). Thus, in contrast to the MMOs prepared by calcination at 500 °C, no significant difference between magnetization of ZFO before and after selective dissolution of ZnO was expected for MMOs calcined at 900 °C. After acid treatment of the MMOs calcined at 900 °C, the ZnO peaks disappear, leaving only ZFO peaks in the XRD pattern (Fig. 4j), indicating selective dissolution of ZnO. Likewise, SEM also revealed the disappearance of crystals with 6-fold symmetry after selective dissolution of ZnO (Fig. 4k). The ZFO particles had dimensions of hundreds of nanometers, which is two orders of magnitude larger than those calcined at 500 °C. ZFO exhibits similar magnetization before and after selective dissolution of ZnO as expected, confirming the low-coordinate cation effect on spinel magnetic properties (Fig. 4h and i).
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We acknowledge the 2019 Research Fund (1.190036.01) of UNIST (Ulsan National Institute of Science & Technology) for financial support. This work was also supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean government (MSIP; Ministry of Science, ICT & Future Planning; No. NRF-2017R1A4A1015323, NRF-2018R1C1B6002342, NRF2019R1H1A1079794, and NRF-2019M1A2A2065612). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.11.145. References [1] J. Lee, J. Yang, S.G. Kwon, et al., Nonclassical nucleation and growth of inorganic nanoparticles, Nat. Rev. Mater. 1 (2016) 16. [2] S. Weiner, L. Addadi, Crystallization pathways in biomineralization, Annual Review of Materials Research vol. 41, Annual Reviews, Palo Alto, 2011, pp. 21–40. [3] A. Navrotsky, Energetic clues to pathways to biomineralization: precursors, clusters, and nanoparticles, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 12096–12101. [4] H.Z. Zhang, J.F. Banfield, Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2, J. Phys. Chem. B 104 (2000) 3481–3487. [5] H.G. Yang, C.H. Sun, S.Z. Qiao, et al., Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638. [6] X.G. Han, Q. Kuang, M.S. Jin, et al., Synthesis of titania nanosheets with a high percentage of exposed (001) facets and related photocatalytic properties, J. Am. Chem. Soc. 131 (2009) 3152. [7] V. Rives, Layered Double Hydroxides: Present and Future, Nova Publishers, 2001. [8] A.I. Khan, D. O'Hare, Intercalation chemistry of layered double hydroxides: recent developments and applications, J. Mater. Chem. 12 (2002) 3191–3198. [9] P.J. Sideris, U.G. Nielsen, Z.H. Gan, et al., Mg/Al ordering in layered double
4. Conclusions In summary, pure ZnFe-LDH precursors were prepared at room temperature and ambient pressure. The LDH to MMO spinel transformations were conducted by controlled heat treatments. As shown in the schematic illustration (Fig. 5), structural ordering of spinel oxides related to crystallinity, cationic inversion, and coordination numbers of constituent atoms after acid treatment was controlled by simply varying the transformation energy. Transformation from LDHs to spinel oxides with lower applied energies led to more structural disorder (i.e., lower crystallinity and higher degree of inversion) and higher ratios of lowcoordinate atoms to fully coordinated atoms after acid treatment. Upon 5
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Control of structural disorder in spinel ceramics derived from layered double hydroxides Il Rok Jeonga,1, Jun Han Leeb,1, Jaejung Songa, Yoon Seok Ohb,∗∗, Seungho Choa,∗ a b
School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea Department of Physics, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 44919, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Spinels Layered double hydroxides Structural disorder Ceramics Composites
Spinel oxides are versatile and functional materials with physicochemical properties that are significantly influenced by their structural disorder. The transition from layered double hydroxides (LDHs) to mixed metal oxides (MMOs) containing spinel oxides is a powerful approach to create materials with tailored properties suitable for various applications. Herein, we report the control of structural ordering in spinels related to the crystallinity, cationic inversion, and coordination number of constituent atoms by varying the transformation energy. Transformation from LDHs to spinels with higher applied energy leads to higher crystallinity, lower degree of inversion, and lower ratios of low-coordinated atoms to fully coordinated atoms after treatment with acid. We provide a general framework for controlling structural ordering, that can be applied to other spinels and metal oxides. In addition, LDHs and MMOs possess many chemical and structural degrees of freedom, making it possible to create materials that suit the needs of different applications, such as memory, sensors, catalysis, and energy conversion.
1. Introduction Solid-state structural transformations, including topotactic transitions, are effective strategies to obtain crystalline materials with desirable properties. Nature uses these strategies to reduce free energies in crystal growth [1–3], which has prompted the adoption of this approach for artificial formation of structures that cannot readily be achieved by classical crystal growth [4–6]. A remarkable example of the transformations is the thermal transition from layered double hydroxides (LDHs) to mixed metal oxides (MMOs). LDHs are an important class of ionic lamellar solids [7], which can be prepared by solutionbased methods. In general, compared to physical vapor deposition, solution-based syntheses consume less energy (hence, lower cost) and are easier to scale-up, which is beneficial for practical applications. Unfortunately, it is relatively difficult to control elemental compositions of the products of solution-based syntheses. In contrast, LDHs formed in solution can accommodate a wide range of different cations and anions [8,9], leading to compositional variety and precise control of cation ratios. In addition, LDHs are characterized by the interspersion, rather than the segregation, of cations within hydroxide layers [10,11]. Solid-state structural transformations from LDHs to MMOs
result in nanoscale phase separation with controllable phase sizes and crystallographic orientations within structures of tailored shapes and sizes [12]. Therefore, together with the aforementioned unique features of LDHs as precursors, MMOs derived from LDHs can possess many chemical and structural degrees of freedom, with versatility to fit different application needs [13–20]. Representative components of MMOs derived from LDH precursors are spinel oxides (AB2O4), as the ionic radii of metal cations in LDHs and those of spinel oxides match well. Spinel oxides are a large class of compounds that are isotypic with the mineral spinel MgAl2O4, possessing the distinct feature of cationic inversion [21]. Factors that influence cationic inversion include synthetic conditions, cation site preference in terms of size, covalent bonding effects, and crystal field stabilization energy. Spinels are widely used in industry due to their functional properties as magnetic materials, semiconductors, pigments, catalysts, and refractories [22]. Structural disorder of spinels related to crystallinity, cationic inversion, and coordination numbers of constituent atoms significantly influences the physicochemical properties, including magnetism, carrier transfer, carrier injection, and catalytic activity [23–27]. Thus, control of structural ordering is beneficial to effectively use spinels in a wide range of applications, such as memory
∗
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y.S. Oh), [email protected] (S. Cho). 1 Il Rok Jeong and Jun Han Lee contributed equally to this work. ∗∗
https://doi.org/10.1016/j.ceramint.2019.11.145 Received 29 September 2019; Received in revised form 15 November 2019; Accepted 17 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Il Rok Jeong, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.145
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coordination numbers. Structural ordering is tailored during the thermal transition from the LDH by controlling the amount of applied energy for the transformation. We chose ZnFe2O4 (ZFO) as our model spinel oxide because the Zn2+ cation prefers tetrahedral sites (A-sites) in the spinel [21,24]. Hence, ZFO of a normal spinel has antiferromagnetic ordering below T = 10 K [28,29]. However, cationic inversion (i.e., partial occupation of the tetrahedral A-sites and octahedral B-sites by Fe3+ and Zn2+ ions, respectively) induces ferrimagnetic ordering by intersite A–B interactions in ZFO [30–32]. Thus, magnetic properties of ZFO indicate its structural disorder. Zinc iron LDHs (ZnFe-LDHs) were synthesized, and after drying, the LDH powders were calcined for conversion to MMO structures with ZFO and ZnO phases. The amount of applied energy for transformation from the LDH to MMOs was easily tuned by varying calcination temperature, which allowed for sensitive control of structural disorder in spinel oxides, represented by crystallinity, cationic inversion, and ratios of low-coordinate atoms to fully coordinated atoms after selective etching of the other phase (ZnO in this study). Such control of the structural properties plays a crucial role in the applicability of spinels to a wide range of applications.
2. Experimental details Sample preparation: Zinc sulfate heptahydrate (ZnSO4·7H2O, ≥99.0%, Sigma-Aldrich), iron sulfate heptahydrate (FeSO4·7H2O, ≥99.0%, Sigma-Aldrich), and sodium hydroxide (NaOH, ≥98%, Sigma-Aldrich) were used. Hydrochloric acid solution (HCl, 35–37%, Samchun Chemicals) was used for acid treatments. All chemicals were used without further purification. To prepare zinc iron layered double hydroxides (ZnFe-LDHs), ZnSO4·7H2O and FeSO4·7H2O were dissolved in 50 mL deionized water to obtain a 0.44 M ZnSO4/0.22 M FeSO4 aqueous solution (Solution A). NaOH was dissolved in 50 mL deionized water to obtain a 0.82 M NaOH aqueous solution (Solution B). Solutions A and B were then simultaneously poured into 50 mL deionized water and stirred (500 rpm) at room temperature (25 °C) for 48 h. Precipitates were gathered and washed with deionized water using a centrifuge and vortex mixer. The powders were then collected and dried at 60 °C for 24 h. To prepare
Fig. 1. (a) X-ray diffraction θ-2θ scan and (b) scanning electron microscopy images of zinc iron layered double hydroxides synthesized from reaction at room temperature and ambient pressure.
storage, sensors, catalysis, and energy conversion. In this study, we demonstrate control of structural ordering parameters of spinel oxides, including crystallinity, cationic inversion, and
Fig. 2. X-ray diffraction θ-2θ scans and scanning electron microscopy images of mixed metal oxides prepared by calcination of zinc iron layered double hydroxide at (a and d) 300 °C, (b and e) 500 °C, and (c and f) 900 °C. 2
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should be suppressed in solution. Pure ZnFe-LDH powders were synthesized at room temperature and ambient pressure. Fig. 1a shows the X-ray diffraction (XRD) pattern (θ-2θ scan) of powders synthesized using these mild conditions. The peaks are assigned to a typical LDH phase. Scanning electron microscopy (SEM) revealed that the morphology of ZnFe-LDH powders is sheet-like (Fig. 1b). Energy-dispersive X-ray spectroscopy (EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) were used to confirm the expected 2:1 Zn2+/Fe3+ ratio of ZnFe-LDHs based on the molar ratio used during synthesis. For comparison, we performed the same synthesis of ZnFeLDHs at higher temperatures (90 °C and 150 °C), which resulted in mixed phases rather than pure ZnFe-LDHs (Fig. S1 in the Supporting Information). Therefore, we determined that pure ZnFe-LDHs could be successfully synthesized under ambient conditions without superfluous energy or manipulation, which is beneficial toward large-scale preparation for practical applications. Solid-state structural transformations occurred within the ZnFe-LDH structures by applying thermal energy. A θ-2θ XRD scan of the ZnFeLDH powders that underwent calcination at 300 °C (Fig. 2a) shows that the XRD peaks from the LDH phase disappear, indicating that the layered structures were not retained and that the crystalline structure changed. The broad peak at ~13° 2θ originates from the XRD sample holder (Fig. S2 in the Supporting Information). The noticeable peak centered at ~35° 2θ is assigned to (311) of spinel ZFO, indicating low crystallinity of the MMOs. Increasing the transformation energy by performing calcination of pristine LDHs at 500 °C resulted in spinel ZFO and hexagonal ZnO diffraction peaks (Fig. 2b). The MMOs prepared from calcination at 300 and 500 °C inherited the original sheet-like morphology of pristine ZnFe-LDHs (Figs. 1b, 2d and 2e). We also conducted transformations using even higher calcination temperatures (900 °C and 1200 °C), further increasing the transformation energies. The corresponding XRD patterns (Fig. 2c and Fig. S3a in the Supporting Information) show that as the calcination temperature increases, the MMOs produce sharper diffraction peaks, indicative of higher crystallinity in both phases. Higher calcination temperatures led to larger grain sizes, with clearly observed 4-fold and 6-fold symmetric polyhedron facets (Fig. 2f and Fig. S3b in the Supporting Information). In addition to XRD, we investigated the site disorder of cations in the transformed spinel oxides by measuring their magnetic properties, whereby we systematically studied the transformation energy dependent magnetic properties (Fig. 3). As shown in Fig. 3a, with decreasing temperature, both zero-field cooling (ZFC) and field cooling (FC) magnetic susceptibilities of the pristine ZnFe-LDH powders monotonically increased without displaying any magnetic anomaly down to 3 K. The magnetic hysteresis (M(H)) curve of the ZnFe-LDHs exhibits paramagnetic behavior at 3 K (Fig. 3b). In contrast, MMOs prepared by calcination of the ZnFe-LDHs at 300 °C exhibit a magnetic anomaly at 10.7 K in the ZFC temperature dependent magnetic susceptibility (χDC(T)) measurement, divergence of ZFC and FC χDC(T)s, and clear magnetic hysteresis with remanent magnetization (Mr) in M(H). These behaviors indicate that MMOs calcined at 300 °C possess ferrimagnetic long-range ordering below 10.7 K, even though at 300 °C the applied transformation energy is not enough to fully induce crystallized positional ordering (Fig. 2a). It is noteworthy that χDC of the FC χDC(T) increased without a local maximum as temperature decreased, which can be attributed to the coexistence of ferrimagnetic/antiferromagnetic and paramagnetic phases that remain due to insufficient transformation energy. MMOs calcined at 500 °C exhibit a large deviation between FC and ZFC χDC(T)s and enhanced Mr (Fig. 3a and b), as a result of improved crystallinity (Fig. 2a and b). However, the transformation energy from 500 °C calcination was still insufficient, and a portion of the Zn2+ cations occupied the octahedral B-sites of the spinel phase. In the case of 900 °C calcination (Fig. 2c), the crystallinity of ZFO significantly improved. Nevertheless, Mr and the deviation between FC and ZFC χDC(T) are comparable with those of the 500 °C case (Fig. 3a and b). Increasing
Fig. 3. (a) Temperature dependence of magnetization with zero-field cooling (ZFC) and field cooling (FC). (b) Magnetic hysteresis (M–H) curves at 3 K of pristine zinc iron layered double hydroxide and mixed metal oxides prepared by calcination of zinc iron layered double hydroxide at 300 °C, 500 °C, 900 °C, and 1200 °C.
mixed metal oxides (MMOs), the ZnFe-LDH powders were placed in alumina boats and then transferred to a quartz tube in the furnace and calcined at 300, 500, 900, or 1200 °C for 2 h with heating rates of 20 °C min−1. For selective dissolution of ZnO in MMOs, 0.2 mL HCl was added to 20 mL deionized water. The MMO powders were stirred for 2 min in the acidic solution. The resulting powders were filtered, washed with deionized water, and dried at 60 °C for 24 h. Characterization: The crystallinity of the samples was investigated by X-ray diffraction (XRD) using an X-ray diffractometer (D8 ADVANCE, Bruker AXS) with Cu Kα radiation (λ = 1.5406 Å). Morphologies of the samples were investigated using field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi High-Technologies). A transmission electron microscope (TEM; FEI Tecnai G2 F20 X-Twin TEM system operated at 200 kV) was used to evaluate the structural properties of the samples. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was conducted using a Varian 700-ES spectrometer. X-ray photoelectron spectroscopy (XPS) was performed with a ThermoFisher K-alpha system using a monochromatic Al Kα X-ray source. Magnetization versus temperature and magnetic field data were obtained using a vibrating sample magnetometer (Physical Property Measurement System, Quantum Design). 3. Results and discussion ZnFe-LDHs were used as precursors for MMOs composed of ZFO and ZnO. In order to prepare pure ZnFe-LDHs, the formation of other phases 3
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Fig. 4. (a) X-ray diffraction θ-2θ scan, (b and c) transmission electron microscopy images, (d) high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, (e–g) energy-dispersive X-ray spectroscopy (EDS) elemental maps of acid-treated mixed metal oxides (MMOs) prepared by calcination of zinc iron layered double hydroxides (ZnFe-LDHs) at 500 °C, (h) magnetization versus temperature (M–T) curves with zero-field cooling (ZFC) and field cooling (FC), (i) magnetic hysteresis (M–H) curves at 3 K of MMOs prepared by calcination of ZnFe-LDHs at 500 °C and 900 °C before and after acid treatments, (j) Xray diffraction θ-2θ scan, and (k) scanning electron microscopy image of acid-treated MMOs prepared by calcination of ZnFe-LDHs at 900 °C.
other MMO phase(s). The ZnO phase was selectively etched to form ZFO nanoparticles with high surface-to-volume ratios and hence a large concentration of low-coordinate atoms per unit volume. The XRD θ-2θ scan pattern of acid-treated powders calcined at 500 °C shows that the diffraction peaks from the ZnO phase disappear and only peaks assigned to the ZFO phase remain (Fig. 4a), indicative of the selective dissolution of ZnO. Transmission electron microscopy (TEM) reveals that nanoparticles were formed after treatment with acid (Fig. 4b). The average particle diameter is 6.87 nm, thus confirming their very high surface-tovolume ratios. The nanoparticles are highly crystalline with a lattice spacing of 0.30 nm, corresponding to the distance between the (220) planes in the ZFO crystal lattice (Fig. 4c). Fig. 4d shows a high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) image of a porous structure composed of nanoparticles. EDS elemental maps show that Zn (Fig. 4e), Fe (Fig. 4f), and O (Fig. 4g) are distributed homogeneously within the structure. Based on the cation ratio of pristine LDH and MMO powders (determined using EDS and ICP-OES), a 1:1 ratio by mass of ZFO and ZnO in the MMOs was
the transformation energy to 1200 °C produced not only very high crystallinity of the spinel (Fig. S3a in the Supporting Information), but also preferential occupation of the tetrahedral A-sites by Zn2+ cations, which led to normal spinel regions and hence increase in fraction of the antiferromagnetic phase. Therefore, Mr decreased. In order to confirm the valence state of iron cations, X-ray photoelectron spectroscopy (XPS) measurements were performed (Fig. S4 in Supporting Information). For all samples, the positions of the peaks in the Fe 2p spectra are consistent with the Fe3+ oxidation state [33]. Furthermore, the positions and shapes of the peaks in the Fe 2p spectra are similar, indicating that changes in magnetization were not due to different valence states of iron cations. The coordination numbers of constituent atoms can also significantly affect the various physicochemical properties of spinel oxides, such as magnetic, electronic, ionic, catalytic, and mechanical properties [26,34–38]. In this study, we also demonstrate control of the ratio between low-coordinate atoms of spinels to fully coordinated atoms by varying the transformation energy and selective dissolution of
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Fig. 5. Schematic illustration of the transformation from layered double hydroxide to mixed metal oxide ceramic containing spinel with controlled structural ordering. Spinel crystal grain sizes also represent degrees of crystallinity.
changing the amount of applied energy, morphology and structural disorder of spinels can be significantly altered, which in turn influences their physicochemical properties. Although ZFO is used as a specific example here, we provide a general framework for controlling structural ordering that can be applied to other spinels and ceramics. Furthermore, such controlled transformation can also maximize synergistic composite effects between coupled spinels and the other component(s).
expected. The mass of the powders before and after acid treatment indicates ~50% reduction in mass after selective dissolution of the ZnO phase in the MMOs, thus confirming the initial 1:1 mass ratio. The ratio of low-coordinate atoms to fully coordinated atoms can influence the magnetic ordering [21–23,26–28]. Magnetization values significantly increased after selective ZnO etching, resulting in formation of ZFO nanoparticles with high surface-to-volume ratios (Fig. 4h and i). This magnetization enhancement can be attributed to deviation from ideal normal spinel ordering of ZFO on the surface of ZFO nanoparticles, where a significant portion of low-coordinate cations are located. Such structures with high surface-to-volume ratios are beneficial for catalysis and energy storage or conversion applications. To confirm the effect of low-coordinate atoms, the magnetic properties of the ZFO powders with much lower surface-to-volume ratios were also measured. As shown by XRD and SEM (Fig. 2), MMOs prepared by calcination at 900 °C exhibit far larger ZFO and ZnO grains than those calcined at 500 °C. In addition, faceted crystals with distinct (4-fold and 6-fold) symmetries can clearly be seen, indicating that surfaces of cubic ZFO and hexagonal ZnO were already exposed before acid treatments (Fig. 2c). Thus, in contrast to the MMOs prepared by calcination at 500 °C, no significant difference between magnetization of ZFO before and after selective dissolution of ZnO was expected for MMOs calcined at 900 °C. After acid treatment of the MMOs calcined at 900 °C, the ZnO peaks disappear, leaving only ZFO peaks in the XRD pattern (Fig. 4j), indicating selective dissolution of ZnO. Likewise, SEM also revealed the disappearance of crystals with 6-fold symmetry after selective dissolution of ZnO (Fig. 4k). The ZFO particles had dimensions of hundreds of nanometers, which is two orders of magnitude larger than those calcined at 500 °C. ZFO exhibits similar magnetization before and after selective dissolution of ZnO as expected, confirming the low-coordinate cation effect on spinel magnetic properties (Fig. 4h and i).
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We acknowledge the 2019 Research Fund (1.190036.01) of UNIST (Ulsan National Institute of Science & Technology) for financial support. This work was also supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean government (MSIP; Ministry of Science, ICT & Future Planning; No. NRF-2017R1A4A1015323, NRF-2018R1C1B6002342, NRF2019R1H1A1079794, and NRF-2019M1A2A2065612). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.11.145. References [1] J. Lee, J. Yang, S.G. Kwon, et al., Nonclassical nucleation and growth of inorganic nanoparticles, Nat. Rev. Mater. 1 (2016) 16. [2] S. Weiner, L. Addadi, Crystallization pathways in biomineralization, Annual Review of Materials Research vol. 41, Annual Reviews, Palo Alto, 2011, pp. 21–40. [3] A. Navrotsky, Energetic clues to pathways to biomineralization: precursors, clusters, and nanoparticles, Proc. Natl. Acad. Sci. U.S.A. 101 (2004) 12096–12101. [4] H.Z. Zhang, J.F. Banfield, Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2, J. Phys. Chem. B 104 (2000) 3481–3487. [5] H.G. Yang, C.H. Sun, S.Z. Qiao, et al., Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638. [6] X.G. Han, Q. Kuang, M.S. Jin, et al., Synthesis of titania nanosheets with a high percentage of exposed (001) facets and related photocatalytic properties, J. Am. Chem. Soc. 131 (2009) 3152. [7] V. Rives, Layered Double Hydroxides: Present and Future, Nova Publishers, 2001. [8] A.I. Khan, D. O'Hare, Intercalation chemistry of layered double hydroxides: recent developments and applications, J. Mater. Chem. 12 (2002) 3191–3198. [9] P.J. Sideris, U.G. Nielsen, Z.H. Gan, et al., Mg/Al ordering in layered double
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