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Controlled synthesis of [Fe(pyridine)2Ni(CN)4] nanostructures and their shape-dependent spin-crossover properties
Journal of Magnetism and Magnetic Materials 496 (2020) 165938
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Research articles
Controlled synthesis of [Fe(pyridine)2Ni(CN)4] nanostructures and their shape-dependent spin-crossover properties
T
Yuqi Yanga,b, Xiaoping Shenb, Hongbo Zhoub, Leiming Langa,⁎, Guoxing Zhub, Zhenyuan Jib a b
Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang University, Nanjing 211171, PR China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
ARTICLE INFO
ABSTRACT
Keywords: Coordination polymer Nanostructures Preparation Spin-crossover
Ultrathin nanosheets of the spin crossover (SCO) coordination polymer [Fe(py)2Ni(CN)4] (py = pyridine) have been achieved through a reverse micelle route. The nanosheets possess regular square shape with a thickness less than 10 nm and a main width of 600–800 nm. Through controlling the synthetic parameters, the nanostructures of [Fe(py)2Ni(CN)4] can be tuned to be square plate-, cube- and box-like shapes, respectively. The possible formation mechanism for these diversiform nanocrystals was proposed. Magnetic measurements and differential scanning calorimetry analyses revealed an interesting morphology-dependent SCO behavior of [Fe(py)2Ni (CN)4]. The hysteresis width decreases in the order of nanoplates (ca. 22 K), nanocubes (ca. 16 K), nanoboxes (ca. 4 K) and nanosheets (ca. 2 K). The enthalpy (ΔH) and entropy (ΔS) variations associated with the spin transitions of nanosheets and nanoboxes are significantly lower than those of nanocubes and nanoplates. Thus, the nanosheets and nanoboxes exhibit stronger nano-effect and their SCO properties are largely weakened owing to the large specific surface areas resulting from their unique morphologies.
1. Introduction Spin crossover (SCO) phenomenon is one of the most interesting examples of bistability used in sensors or memory devices for switching [1], information storage [2] and contrast agent [3]. Under external stimulation or perturbations (such as temperature, pressure and light irradiation), SCO systems are switchable between the low-spin (LS) and high-spin (HS) states, which causes remarkable changes in their magnetic, optical, and dielectric properties [1,2]. Furthermore, hysteresis accompanies the first order spin transition (ST) when these changes can be performed cooperatively in special cases [1,2]. Among various SCO systems, iron (II)-based complexes are particularly well known [1,2,4–11]. Many iron(II)-based (with d6 configuration) complexes exhibit a spin change from a diamagnetic LS electronic state [S = 0, 1A1 (t2g6)] to a paramagnetic HS one [S = 2, 5T2 (t2g4eg2)] upon heating, and thus they often show hysteresis behavior at a certain temperature range, which endows the materials with a memory effect. With the development of nanoscience, some progress has recently been made in the preparation of SCO micro-/nanostructures [12–17]. It is expected that miniaturization of SCO materials to nanoscale will bring some new physical and chemical apperception owing to the enhanced surface and interface effect. For instance, the nanocrystals of a three-dimensional (3D) SCO network of [FeII(pz)PtII (CN)4] ⁎
(pz = pyrazine) display a size-dependent spin transition behavior [12,15], while the nanoparticles of a one-dimensional (1D) SCO polymer of [Fe(Htrz)2(trz)](BF4) (Htrz = 1,2,4-1H-triazole) show a different behavior with almost no dependence of the abruptness of the transition on the size reduction [13]. It is obvious that the research on nanostructured SCO complexes is still in its very early stage. Due to the difficulty in morphological synthesis of SCO materials, the reported SCO nano-objects are mainly limited in irregular particles and film [12–17], and the shape-dependent spin transition behaviour is scarcely investigated until now. Therefore, it is highly desired to fabricate SCO nanostructures with controllable and multifarious morphologies. In recent decade, two-dimensional (2D) nanomaterials have aroused enormous interest in the field of materials research due to its unique structure as well as the structure-dependent physical and chemical properties. It has been found that the ultrathin nanosheets can be achieved generally from the layered inorganic compounds [18]. However, although a great number of two-dimensional (2D) layered coordination polymers have been synthesized so far, the ultrathin nanosheets derived from these organic-inorganic hybrids are still in its infant stage [19]. It is well known that [Fe(py)2Ni(CN)4] (py = pyridine) is a SCO coordination polymer with a typical 2D layered structure [20–22]. Recently, Kitagawa et al. fabricated nanometer-sized thin film of [Fe(py)2Ni(CN)4] by using a layer-by-layer method [23]. In this
Corresponding author. E-mail address: [email protected] (L. Lang).
https://doi.org/10.1016/j.jmmm.2019.165938 Received 12 March 2019; Received in revised form 28 August 2019; Accepted 30 September 2019 Available online 03 October 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
2. Experimental section
(c)
(422) (711)
Intensity (a.u.)
(d)
(200) (001) (20-1) (11-1) (111) (31-1) (400) (020) (311) (021) (401) (51-1)
paper, we demonstrate the successful preparation of [Fe(py)2Ni(CN)4] ultrathin nanosheets with a thickness less than 10 nm. Through adjusting the experimental parameters, the morphology of [Fe(py)2Ni (CN)4] nanostructures can also be easily tuned into square plate-, cubeand box-like shapes, respectively. The SCO properties of these [Fe (py)2Ni(CN)4] nanostructures are revealed to be drastically influenced by the morphology of the particles.
(600) (511) (402) (222)
Journal of Magnetism and Magnetic Materials 496 (2020) 165938
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(b) (a)
2.1. Synthesis
Standard pattern
All chemicals used in the study were of analytical grade, and used without further purification. Deionized water was used in all the experiments. Preparation of [Fe(py)2Ni(CN)4] nanosheets: In a typical procedure for synthesizing [Fe(py)2Ni(CN)4] square nanosheets, a solution was firstly prepared with 2.46 g (5.55 mmol) of sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) and 44 mL of octane. Then, an aqueous solution containing 67.4 mg (0.2 mmol) of Fe(BF4)2·6H2O and 64 mg (0.8 mmol) of pyridine in 2 mL of water was carefully added into the above NaAOT solution. The obtained mixture was stirred until the formation of a light yellow water-in-oil microemulsion (w = [H2O]/ [NaAOT] = 20). With the same NaAOT solution, another slightly opaque water-in-oil microemulsion was prepared from K2[Ni(CN)4] aqueous solution containing 48.2 mg (0.2 mmol) of K2[Ni(CN)4] and 2 mL of water. The obtained two microemulsions were quickly mixed and stirred at 50 °C for 24 h. The formed precipitates (square nanosheets of [Fe(py)2Ni(CN)4]) were then separated by centrifugation, washed five times with ethanol, and dried in air at ambient temperature. [Fe(py)2Ni(CN)4] nanostructures with other shapes were synthesized following the same protocol used for the nanosheets, but with different experimental parameters. If the reaction was carried out at 50 °C for 10 h, square nanoplates with bigger thickness were obtained. If the amounts of NaAOT and pyridine used in the synthesis were increased to 4.94 g (11.1 mmol) and 134.8 mg (1.685 mmol), respectively (w = [H2O]/[NaAOT] = 10), and the reaction temperature was increased to 60 °C, [Fe(py)2Ni(CN)4] nanoboxes and nanocubes could be obtained with reaction time of 24 and 10 h, respectively. The synthesis parameters for different morphological nanocrystals of [Fe(py)2Ni (CN)4] are listed in Table 1.
10
20
30
2θ (degree)
40
50
Fig. 1. XRD patterns of the [Fe(py)2Ni(CN)4] products with different morphologies: (a) nanosheets, (b) nanoplates, (c) nanoboxes, (d) nanocubes.
MPMS-XL superconducting quantum interference device (SQUID) magnetometer. Thermogravimetry (TG) measurements were carried out using a NETZSCH STA449C thermal analyzer under nitrogen atmosphere in the temperature range of 25–1000 °C with a heating rate is 10 °C/min. Differential scanning calorimetry (DSC) analyses were performed using a differential scanning calorimeter (NETZSCH DSC 204 F1) in the temperature range of −150 to 0 °C under a flow of dry nitrogen gas. 3. Results and discussion The coordination polymer [Fe(py)2Ni(CN)4] crystallizes in monoclinic system and shows cyanide-bridged 2D layered structure in bc plane [20,21]. The crystal phases of the obtained products were examined by XRD measurement. Fig. 1 shows the XRD patterns of the products. For comparison, the standard XRD pattern simulated from the single crystal structure data is also given [20,21]. It can be seen that all the diffraction peaks of the products can be well-matched to those of the standard pattern, and no peaks from impurities were detected, suggesting the high purity of the products. For nanosheets and nanoplates, the relative intensities of (2 0 0), (4 0 0) and (6 0 0) peaks have been dramatically improved, indicating that the base surfaces of the nanosheets and nanoplates are enclosed with {1 0 0} planes, which is consistent with the intrinsic layer structure packing along a direction in [Fe(py)2Ni(CN)4] crystal. In addition, the obvious broadening of the diffraction peaks suggests the small particle size of these products. The IR spectra (Fig. S1, ESI†) of all the products show similar absorption bands: a strong peak at 2159 cm−1 ascribed to the C^N stretching vibration, the peaks at 3082, 3020 cm−1 ascribed to the CeH stretching vibration, the peaks at around 1601, 1486, 1446, 1039 and 1011 cm−l attributable to the stretching vibrations of pyridine ring, the peaks at around 1219, 1153, 1069 cm−l attributable to the CeH inplane bending vibrations, while the CeH out-of-plane bending vibrations at about 946 and 690 cm−l. The bands at 628 and 752 cm−l can be ascribed to the pyridine ring in-plane bending and out-of-plane bending vibrations, respectively. All these peaks are well consistent with those reported in the literature [24]. The TG analyses exhibit that
2.2. Characterization The morphology and the size of the products were examined by field emission scanning electron microscopy (FE-SEM; JSM-7001F) and transmission electron microscopy (TEM; JEM-2100). Samples for TEM observation were prepared by dropping the products on a carboncoated copper grid after ultrasonically dispersed in absolute ethanol and allowed to dry in air before analysis. The phase structure of the products was characterized by X-ray diffraction (XRD; Bruker D8 Advance diffractometer) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 4° min−1. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr pellets. The magnetic measurements were conducted on a Quantum Design Table 1 Synthesis parameters for different morphological nanocrystals of [Fe(py)2Ni(CN)4]. Samples
Py/mmol
AOT/mmol
Temp/°C
Time/h
Size/nm
Nanosheets Nanoplates Nanoboxes Nanocubes
0.8 0.8 1.685 1.685
5.55 5.55 11.1 11.1
50 50 60 60
24 10 24 10
Width: 600–800 thickness: 9 Width: 250 thickness: 40 115 110
2
Journal of Magnetism and Magnetic Materials 496 (2020) 165938
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Fig. 2. (a,b) SEM and (c,d) TEM images of the [Fe(py)2Ni(CN)4] nanosheets. The inset of (c) shows the width distribution of the nanosheets.
Fig. 3. (a, b) SEM and (c, d) TEM images of the nanoboxes. The inset of (d) shows the size distribution of the nanoboxes.
all the products with different morphologies have similar TG properties (Fig. S2). With increasing temperature, all the products can keep stable until ca 210 °C without obvious weight loss, suggesting that no solvent molecules (crystal water) exist in these samples. While above 210 °C, they start to decompose with two weight loss steps in the temperature ranges of 210–350 °C and 420–540 °C, which could be ascribed to the
loss of pyridine and the destroying of cyanometallate framework, respectively. Fig. 2(a) and (b) show the typical SEM images of the as-synthesized nanosheets with different magnifications. It can be seen that the product consists of a large amount of ultrathin nanosheets, which possess regular quasi-square shape with a thickness of ca. 9 nm and a main 3
Journal of Magnetism and Magnetic Materials 496 (2020) 165938
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width of 600–800 nm. This result indicates that large-scale square ultrathin nanosheets of [Fe(py)2Ni(CN)4] can be successfully prepared through this approach. Several nanosheets with obvious curling can also be observed in Fig. 2(a) and (b), suggesting the fairly good flexibility of the nanosheets. Fig. 2(c) and (d) show the TEM images of the nanosheets. The morphology and size obtained from the TEM observation are in good agreement with that of SEM. The square nanosheets have smooth surfaces. Especially, the nanosheets show so low contrast that they are almost transparent, which suggests their thin thickness. It should be noted that besides the square sheets with a width of 600–800 nm, some smaller square sheets with a width down to 150–300 nm can also be observed (as denoted by the arrows in Fig. 2c). Furthermore, these smaller square sheets show relatively higher contrast, that is, they may have bigger thickness. The inset of Fig. 2c shows the width distribution of the nanosheets. Although some superposition of the nanosheets is observed, these nanosheets are highly dispersed and no agglomeration occurs. It is quite difficult to determine the surface crystal plane of the nanosheets by TEM technique because the high content of organic component in [Fe(py)2Ni(CN)4] makes the products easily decompose under the irradiation of electron beams during selected-area electron diffraction (SAED) and/or high resolution TEM observation. However, based on the XRD results shown above, the base surfaces of the square nanosheets can be determined to be {1 0 0} planes. When the feeding amounts of NaAOT and pyridine were increased respectively to about two times of those for the synthesis of square nanosheets, [Fe(py)2Ni(CN)4] hollow nanoboxes were obtained with high yield. Fig. 3a and b show SEM images of the as-synthesized nanoboxes. The nanoboxes have well-defined tetragonal morphology and relatively uniform size. From Fig. 3b, the hollow interior of the nanoboxes can be clearly observed with a wall thickness of ca. 15 nm. Further evidence for the hollow structural feature of the nanoboxes is from TEM observation (Fig. 3c and d). The strong contrast difference between the dark edges and brighter centers indicates the hollow structure. The size distribution of the nanoboxes is shown in the inset of Fig. 3d. The mean edge length of the nanoboxes is about 115 nm, and the side-length of the cube-like cavities is in the range of 60–90 nm with an average size of about 70 nm. Besides the great influence of feeding amounts of NaAOT and pyridine on the products, the morphology of [Fe(py)2Ni(CN)4] nanocrystals is also dependent on reaction time. If decreasing the reaction time of the above two systems, thicker quasi-square nanosheets but with smaller width (referred to as nanoplates), and solid quasi-cube particles (nanocubes) can be obtained, respectively. Fig. 4a shows a typical SEM image of the obtained nanocubes. It can be seen that the crystals have a regular cube-like shape with sharp edges of a mean size of 110 nm. Fig. 4b shows a SEM image of the nanoplates. The nanoplates possess square-like shape with a mean width of 250 nm and a thickness of ca. 40 nm. From above experimental results, it seems that with increasing
reaction time, the nanoplates would become thinner and bigger; and the nanocubes would become hollow. These phenomena may originate from the different stability of crystal planes. With longer reaction time, the crystals will dissolve from the instable crystal plane. In the case of nanoplates, the dissolved components further grew on the stable planes forming ultrathin nanosheets, while the nanocubes were evolved into hollow nanoboxes via a dissolution-recrystallization process [25]. The nanosheets and the nanoplates could be considered as a variant of nanocubes with bigger side length and smaller thickness. Based on the XRD results and the intrinsic crystal structure of [Fe(py)2Ni(CN)4], we tentatively propose the possible surface crystal plane and growth direction of these nanostructures. As shown in Fig. 5, the difference among the nanosheets, nanoplates and nanocubes can be essentially ascribed to the different growth rate along [1 0 0] direction. The nanosheets were terminated mainly with {1 0 0} planes due to the lowest growth rate along [1 0 0] direction, while the nanocubes with the highest [1 0 0] growth rate were enclosed by {1 0 0}, {0 1 0} and {0 0 1} planes. These nanocrystals with various morphologies could exhibit different surface activity due to their different bare crystal planes. Because the shape of nanoparticles has a strong influence on their properties [26,27], we studied the magnetic properties of the obtained [Fe(py)2Ni(CN)4] nanocrystals with different morphologies. Fig. 6 shows the temperature dependence of χMT (where χM stands for the molar magnetic susceptibility) curves for the four [Fe(py)2Ni(CN)4] products. At 300 K, χMT is equal to 3.73 cm3 K mol−1, 3.58 cm3 K mol−1, 3.49 cm3 K mol−1 and 3.42 cm3 K mol−1 for the nanoplates, nanocubes, nanosheets and nanoboxes, respectively. The χMT values fall into the range expected for one high spin (HS) state (S = 2) of FeII [10]. The inconsistent room temperature χMT values for the four nanocrystals imply the different proportion of residual low spin (LS) fraction at this temperature. On lowering the temperature, the χMT values basically maintain temperature-independent until the vicinity of SCO behaviors (Tc↓ = 187, 180, 190 and 174 K for nanoplates, nanocubes, nanosheets and nanoboxes, respectively) where the χMT values abruptly decrease within a few kelvins, as observed for the typical SCO behaviors of FeII complexes [4–11]. Below the Tc↓, the χMT values sharply decrease but become again almost temperature independent for the nanoplates and nanocubes, which indicate that most iron(II) ions have undergone spin transition from FeII (HS) to FeII (LS). However, for nanosheets and nanoboxes, the χMT values still show a gradual decrease after the SCO transition. At the very low temperature (less than20 K), the χMT values decrease rapidly again but far from zero, which can be ascribed to the zero field splitting (ZFS) of residual HS fraction of FeII in the nanocrystals. In the warming mode, the characteristic Tc↑ temperatures are 210, 196, 192 and 180 K for nanoplates, nanocubes, nanosheets and nanoboxes, respectively. The magnetic susceptibilities for nanosheets and nanoboxes almost match the cooling mode with a tiny hysteresis loop, but nanoplates and nanocubes exhibit similar large SCO
Fig. 4. SEM images of (a) nanocubes and (b) nanoplates. 4
Journal of Magnetism and Magnetic Materials 496 (2020) 165938
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Fig. 5. Illustration of the layered crystal structure of [Fe(py)2Ni(CN)4] and the relations between the nanocrystal morphologies and the [1 0 0] growth direction as well as surface crystal planes.
coordination with terminal solvent molecules. From this point of view, it is reasonable to conclude that the particles with higher specific surface area will suffer from not only weakening cooperativity and diminishing SCO domain, but also decreasing quantities of active SCO FeII atoms, eventually leading to the observed morphology/size dependent SCO behaviors. It is worth noting that the nanosheets exhibit extraordinarily high Tc↓ value, possibly owing to their unique morphology. Furthermore, it should be mentioned that size distribution will also affect the SCO features, especially the shape of hysteresis loop [12,14]. Narrow size distribution will be advantageous to produce rectangleshape hysteresis loops. In particular, the relatively large size distribution of the nanosheets could also contribute to the continuous character of the spin transition [15]. For further investigating the SCO behaviors of these [Fe(py)2Ni (CN)4] nanocrystals with different shapes, low temperature differential scanning calorimetry (DSC) analyses were adopted to evaluate the enthalpy (ΔH) and entropy (ΔS) variations associated with the spin transitions (Fig. 7 and Table 2). It can be seen from Table 2 that the values of ΔH and ΔS for all four nanocrystals are much smaller than those reported for the bulk materials, and the values for nanosheets and
Fig. 6. χMT vs T plots for [Fe(py)2Ni(CN)4] nanocrystals with different morphologies: (a) nanoplates, (b) nanocubes, (c) nanosheets, (d) nanoboxes.
hysteresis loops (width of hysteresis loops: 22 K for nanoplates and 16 K for nanocubes) as that observed for bulk crystals [21,22], indicating that nanosheets and nanoboxes show much better reversibility of the spin transition processes than nanoplates and nanocubes. Obviously, the SCO behaviors of [Fe(py)2Ni(CN)4] (critical temperature, hysteresis width, and residual HS/LS state sites) strongly depend on the morphology of the nanocrystals. For instance, the hysteresis width decreases from nanoplates (ca. 22 K) to nanocubes (ca. 16 K), nanoboxes (ca. 4 K) and nanosheets (ca. 2 K). Monte Carlo simulations for cubic and spherical SCO nanoparticles had revealed that particles with larger size exhibit more significant hysteresis width [28], which was also confirmed by the recently reported SCO nanoparticles based on FeII [12,14,15], in which residual HS/LS state sites increase as the particle size decreases. Similarly, in our case, the room temperature χMT values for the four nanocrystals decrease but the low temperature ones increase when the morphology changes from nanoplates to nanoboxes (Fig. 6). In fact, the intrinsic factor that affects the SCO features is mainly the specific surface area of the particles. The nanosheets with ultrathin 2D structure and nanoboxes with hollow structure obviously possess larger surface area than nanoplates and nanocubes, so the former particles behave weaker cooperativity and smaller SCO domain, leading to narrower hysteresis. As far as the residual HS/LS state is concerned, FeII ions located at the surface of the nanocrystals probably do not exhibit SCO due to coordinative defects that are coordinative unsaturation like incomplete {FeNx} coordination and/or
Fig. 7. DSC curves of the [Fe(py)2Ni(CN)4] samples with different morphologies: (a) nanosheets, (b) nanoplates, (c) nanoboxes, (d) nanocubes. 5
Journal of Magnetism and Magnetic Materials 496 (2020) 165938
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References
Table 2 Spin transition temperatures of four nanocrystals in the heating (Tc↑) and cooling (Tc↓) modes together with the enthalpy and entropy variations associated with the spin transition. Samples
Nano-sheets Nano-plates Nano-boxes Nano-cubes Bulk crystals [22] a b
Tc↑/K
Tc↓/K
[a]
[b]
[a]
[b]
191.7 210.4 180.4 196.2 210
191.7 209.9 186.8 201.5 209
189.7 187.2 174.5 179.7 194
185.5 189.2 174.2 175.4 191
ΔHab/kJ mol−1
ΔSb/J K−1 mol−1
1.77 3.63 1.16 4.59 14.9
9.35 18.16 6.38 24.35 74
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From magnetic susceptibility data. From DSC data.
nanoboxes are smaller than those for nanoplates and nanocubes. The results suggest that the values of ΔH and ΔS decrease with increasing specific surface area of the particles. It is reasonable because the incomplete {FeNx} coordination sites lying at the surface of nanocrystals would not undergo the spin crossover transition, the fraction of the molecules related to the spin crossover transition decreases with increasing surface area, and as a result, the smaller values of ΔH and ΔS would be obtained for these nanocrystals as compared with the bulk crystals. The fact that nanoboxes and nanosheets with bigger surface area show smaller ΔH and ΔS values than nanocubes and nanoplates further supports this opinion. Therefore, the nano-effect of the nanoplates and nanocubes is relatively weak due to their relatively small specific surface area. In contrast, the nanosheets and nanoboxes show significant nano-effect and their SCO properties are largely weakened owing to the large specific surface areas resulting from their unique morphologies. In addition, the characteristic temperatures for the spin transition obtained from DSC measurements are basically consistent with those extracted from magnetic data (Table 2). 4. Conclusions In conclusion, novel ultrathin nanosheets and hollow nanoboxes of the SCO coordination polymer [Fe(py)2Ni(CN)4] have been successfully prepared through a reverse micelle technique. The synthetic parameters such as reaction time and reagent concentration have a great influence on the morphologies of the resulting nanostructures. The SCO properties of [Fe(py)2Ni(CN)4] are highly dependent on the shape of the nanostructures. We believe that this result provides not only the importance of gathering deeper insight into the correlation between SCO properties and particle shape in Fe(II)-based coordination polymers, but also a novel organic-inorganic hybrid nanomaterial, which might find attractive applications in next-generation flexible and transparent nanodevices. Conflict of interest There are no conflicts to declare. Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Nos. 51072071 and 21875091), the Natural Science Foundation of Jiangsu Province (No. BK20171295) and Nanjing Xiaozhuang University Research Project (No. 2018NXY23). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165938.
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Research articles
Controlled synthesis of [Fe(pyridine)2Ni(CN)4] nanostructures and their shape-dependent spin-crossover properties
T
Yuqi Yanga,b, Xiaoping Shenb, Hongbo Zhoub, Leiming Langa,⁎, Guoxing Zhub, Zhenyuan Jib a b
Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang University, Nanjing 211171, PR China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
ARTICLE INFO
ABSTRACT
Keywords: Coordination polymer Nanostructures Preparation Spin-crossover
Ultrathin nanosheets of the spin crossover (SCO) coordination polymer [Fe(py)2Ni(CN)4] (py = pyridine) have been achieved through a reverse micelle route. The nanosheets possess regular square shape with a thickness less than 10 nm and a main width of 600–800 nm. Through controlling the synthetic parameters, the nanostructures of [Fe(py)2Ni(CN)4] can be tuned to be square plate-, cube- and box-like shapes, respectively. The possible formation mechanism for these diversiform nanocrystals was proposed. Magnetic measurements and differential scanning calorimetry analyses revealed an interesting morphology-dependent SCO behavior of [Fe(py)2Ni (CN)4]. The hysteresis width decreases in the order of nanoplates (ca. 22 K), nanocubes (ca. 16 K), nanoboxes (ca. 4 K) and nanosheets (ca. 2 K). The enthalpy (ΔH) and entropy (ΔS) variations associated with the spin transitions of nanosheets and nanoboxes are significantly lower than those of nanocubes and nanoplates. Thus, the nanosheets and nanoboxes exhibit stronger nano-effect and their SCO properties are largely weakened owing to the large specific surface areas resulting from their unique morphologies.
1. Introduction Spin crossover (SCO) phenomenon is one of the most interesting examples of bistability used in sensors or memory devices for switching [1], information storage [2] and contrast agent [3]. Under external stimulation or perturbations (such as temperature, pressure and light irradiation), SCO systems are switchable between the low-spin (LS) and high-spin (HS) states, which causes remarkable changes in their magnetic, optical, and dielectric properties [1,2]. Furthermore, hysteresis accompanies the first order spin transition (ST) when these changes can be performed cooperatively in special cases [1,2]. Among various SCO systems, iron (II)-based complexes are particularly well known [1,2,4–11]. Many iron(II)-based (with d6 configuration) complexes exhibit a spin change from a diamagnetic LS electronic state [S = 0, 1A1 (t2g6)] to a paramagnetic HS one [S = 2, 5T2 (t2g4eg2)] upon heating, and thus they often show hysteresis behavior at a certain temperature range, which endows the materials with a memory effect. With the development of nanoscience, some progress has recently been made in the preparation of SCO micro-/nanostructures [12–17]. It is expected that miniaturization of SCO materials to nanoscale will bring some new physical and chemical apperception owing to the enhanced surface and interface effect. For instance, the nanocrystals of a three-dimensional (3D) SCO network of [FeII(pz)PtII (CN)4] ⁎
(pz = pyrazine) display a size-dependent spin transition behavior [12,15], while the nanoparticles of a one-dimensional (1D) SCO polymer of [Fe(Htrz)2(trz)](BF4) (Htrz = 1,2,4-1H-triazole) show a different behavior with almost no dependence of the abruptness of the transition on the size reduction [13]. It is obvious that the research on nanostructured SCO complexes is still in its very early stage. Due to the difficulty in morphological synthesis of SCO materials, the reported SCO nano-objects are mainly limited in irregular particles and film [12–17], and the shape-dependent spin transition behaviour is scarcely investigated until now. Therefore, it is highly desired to fabricate SCO nanostructures with controllable and multifarious morphologies. In recent decade, two-dimensional (2D) nanomaterials have aroused enormous interest in the field of materials research due to its unique structure as well as the structure-dependent physical and chemical properties. It has been found that the ultrathin nanosheets can be achieved generally from the layered inorganic compounds [18]. However, although a great number of two-dimensional (2D) layered coordination polymers have been synthesized so far, the ultrathin nanosheets derived from these organic-inorganic hybrids are still in its infant stage [19]. It is well known that [Fe(py)2Ni(CN)4] (py = pyridine) is a SCO coordination polymer with a typical 2D layered structure [20–22]. Recently, Kitagawa et al. fabricated nanometer-sized thin film of [Fe(py)2Ni(CN)4] by using a layer-by-layer method [23]. In this
Corresponding author. E-mail address: [email protected] (L. Lang).
https://doi.org/10.1016/j.jmmm.2019.165938 Received 12 March 2019; Received in revised form 28 August 2019; Accepted 30 September 2019 Available online 03 October 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
2. Experimental section
(c)
(422) (711)
Intensity (a.u.)
(d)
(200) (001) (20-1) (11-1) (111) (31-1) (400) (020) (311) (021) (401) (51-1)
paper, we demonstrate the successful preparation of [Fe(py)2Ni(CN)4] ultrathin nanosheets with a thickness less than 10 nm. Through adjusting the experimental parameters, the morphology of [Fe(py)2Ni (CN)4] nanostructures can also be easily tuned into square plate-, cubeand box-like shapes, respectively. The SCO properties of these [Fe (py)2Ni(CN)4] nanostructures are revealed to be drastically influenced by the morphology of the particles.
(600) (511) (402) (222)
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(b) (a)
2.1. Synthesis
Standard pattern
All chemicals used in the study were of analytical grade, and used without further purification. Deionized water was used in all the experiments. Preparation of [Fe(py)2Ni(CN)4] nanosheets: In a typical procedure for synthesizing [Fe(py)2Ni(CN)4] square nanosheets, a solution was firstly prepared with 2.46 g (5.55 mmol) of sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) and 44 mL of octane. Then, an aqueous solution containing 67.4 mg (0.2 mmol) of Fe(BF4)2·6H2O and 64 mg (0.8 mmol) of pyridine in 2 mL of water was carefully added into the above NaAOT solution. The obtained mixture was stirred until the formation of a light yellow water-in-oil microemulsion (w = [H2O]/ [NaAOT] = 20). With the same NaAOT solution, another slightly opaque water-in-oil microemulsion was prepared from K2[Ni(CN)4] aqueous solution containing 48.2 mg (0.2 mmol) of K2[Ni(CN)4] and 2 mL of water. The obtained two microemulsions were quickly mixed and stirred at 50 °C for 24 h. The formed precipitates (square nanosheets of [Fe(py)2Ni(CN)4]) were then separated by centrifugation, washed five times with ethanol, and dried in air at ambient temperature. [Fe(py)2Ni(CN)4] nanostructures with other shapes were synthesized following the same protocol used for the nanosheets, but with different experimental parameters. If the reaction was carried out at 50 °C for 10 h, square nanoplates with bigger thickness were obtained. If the amounts of NaAOT and pyridine used in the synthesis were increased to 4.94 g (11.1 mmol) and 134.8 mg (1.685 mmol), respectively (w = [H2O]/[NaAOT] = 10), and the reaction temperature was increased to 60 °C, [Fe(py)2Ni(CN)4] nanoboxes and nanocubes could be obtained with reaction time of 24 and 10 h, respectively. The synthesis parameters for different morphological nanocrystals of [Fe(py)2Ni (CN)4] are listed in Table 1.
10
20
30
2θ (degree)
40
50
Fig. 1. XRD patterns of the [Fe(py)2Ni(CN)4] products with different morphologies: (a) nanosheets, (b) nanoplates, (c) nanoboxes, (d) nanocubes.
MPMS-XL superconducting quantum interference device (SQUID) magnetometer. Thermogravimetry (TG) measurements were carried out using a NETZSCH STA449C thermal analyzer under nitrogen atmosphere in the temperature range of 25–1000 °C with a heating rate is 10 °C/min. Differential scanning calorimetry (DSC) analyses were performed using a differential scanning calorimeter (NETZSCH DSC 204 F1) in the temperature range of −150 to 0 °C under a flow of dry nitrogen gas. 3. Results and discussion The coordination polymer [Fe(py)2Ni(CN)4] crystallizes in monoclinic system and shows cyanide-bridged 2D layered structure in bc plane [20,21]. The crystal phases of the obtained products were examined by XRD measurement. Fig. 1 shows the XRD patterns of the products. For comparison, the standard XRD pattern simulated from the single crystal structure data is also given [20,21]. It can be seen that all the diffraction peaks of the products can be well-matched to those of the standard pattern, and no peaks from impurities were detected, suggesting the high purity of the products. For nanosheets and nanoplates, the relative intensities of (2 0 0), (4 0 0) and (6 0 0) peaks have been dramatically improved, indicating that the base surfaces of the nanosheets and nanoplates are enclosed with {1 0 0} planes, which is consistent with the intrinsic layer structure packing along a direction in [Fe(py)2Ni(CN)4] crystal. In addition, the obvious broadening of the diffraction peaks suggests the small particle size of these products. The IR spectra (Fig. S1, ESI†) of all the products show similar absorption bands: a strong peak at 2159 cm−1 ascribed to the C^N stretching vibration, the peaks at 3082, 3020 cm−1 ascribed to the CeH stretching vibration, the peaks at around 1601, 1486, 1446, 1039 and 1011 cm−l attributable to the stretching vibrations of pyridine ring, the peaks at around 1219, 1153, 1069 cm−l attributable to the CeH inplane bending vibrations, while the CeH out-of-plane bending vibrations at about 946 and 690 cm−l. The bands at 628 and 752 cm−l can be ascribed to the pyridine ring in-plane bending and out-of-plane bending vibrations, respectively. All these peaks are well consistent with those reported in the literature [24]. The TG analyses exhibit that
2.2. Characterization The morphology and the size of the products were examined by field emission scanning electron microscopy (FE-SEM; JSM-7001F) and transmission electron microscopy (TEM; JEM-2100). Samples for TEM observation were prepared by dropping the products on a carboncoated copper grid after ultrasonically dispersed in absolute ethanol and allowed to dry in air before analysis. The phase structure of the products was characterized by X-ray diffraction (XRD; Bruker D8 Advance diffractometer) with Cu Kα radiation (λ = 1.5406 Å) at a scanning rate of 4° min−1. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr pellets. The magnetic measurements were conducted on a Quantum Design Table 1 Synthesis parameters for different morphological nanocrystals of [Fe(py)2Ni(CN)4]. Samples
Py/mmol
AOT/mmol
Temp/°C
Time/h
Size/nm
Nanosheets Nanoplates Nanoboxes Nanocubes
0.8 0.8 1.685 1.685
5.55 5.55 11.1 11.1
50 50 60 60
24 10 24 10
Width: 600–800 thickness: 9 Width: 250 thickness: 40 115 110
2
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Fig. 2. (a,b) SEM and (c,d) TEM images of the [Fe(py)2Ni(CN)4] nanosheets. The inset of (c) shows the width distribution of the nanosheets.
Fig. 3. (a, b) SEM and (c, d) TEM images of the nanoboxes. The inset of (d) shows the size distribution of the nanoboxes.
all the products with different morphologies have similar TG properties (Fig. S2). With increasing temperature, all the products can keep stable until ca 210 °C without obvious weight loss, suggesting that no solvent molecules (crystal water) exist in these samples. While above 210 °C, they start to decompose with two weight loss steps in the temperature ranges of 210–350 °C and 420–540 °C, which could be ascribed to the
loss of pyridine and the destroying of cyanometallate framework, respectively. Fig. 2(a) and (b) show the typical SEM images of the as-synthesized nanosheets with different magnifications. It can be seen that the product consists of a large amount of ultrathin nanosheets, which possess regular quasi-square shape with a thickness of ca. 9 nm and a main 3
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width of 600–800 nm. This result indicates that large-scale square ultrathin nanosheets of [Fe(py)2Ni(CN)4] can be successfully prepared through this approach. Several nanosheets with obvious curling can also be observed in Fig. 2(a) and (b), suggesting the fairly good flexibility of the nanosheets. Fig. 2(c) and (d) show the TEM images of the nanosheets. The morphology and size obtained from the TEM observation are in good agreement with that of SEM. The square nanosheets have smooth surfaces. Especially, the nanosheets show so low contrast that they are almost transparent, which suggests their thin thickness. It should be noted that besides the square sheets with a width of 600–800 nm, some smaller square sheets with a width down to 150–300 nm can also be observed (as denoted by the arrows in Fig. 2c). Furthermore, these smaller square sheets show relatively higher contrast, that is, they may have bigger thickness. The inset of Fig. 2c shows the width distribution of the nanosheets. Although some superposition of the nanosheets is observed, these nanosheets are highly dispersed and no agglomeration occurs. It is quite difficult to determine the surface crystal plane of the nanosheets by TEM technique because the high content of organic component in [Fe(py)2Ni(CN)4] makes the products easily decompose under the irradiation of electron beams during selected-area electron diffraction (SAED) and/or high resolution TEM observation. However, based on the XRD results shown above, the base surfaces of the square nanosheets can be determined to be {1 0 0} planes. When the feeding amounts of NaAOT and pyridine were increased respectively to about two times of those for the synthesis of square nanosheets, [Fe(py)2Ni(CN)4] hollow nanoboxes were obtained with high yield. Fig. 3a and b show SEM images of the as-synthesized nanoboxes. The nanoboxes have well-defined tetragonal morphology and relatively uniform size. From Fig. 3b, the hollow interior of the nanoboxes can be clearly observed with a wall thickness of ca. 15 nm. Further evidence for the hollow structural feature of the nanoboxes is from TEM observation (Fig. 3c and d). The strong contrast difference between the dark edges and brighter centers indicates the hollow structure. The size distribution of the nanoboxes is shown in the inset of Fig. 3d. The mean edge length of the nanoboxes is about 115 nm, and the side-length of the cube-like cavities is in the range of 60–90 nm with an average size of about 70 nm. Besides the great influence of feeding amounts of NaAOT and pyridine on the products, the morphology of [Fe(py)2Ni(CN)4] nanocrystals is also dependent on reaction time. If decreasing the reaction time of the above two systems, thicker quasi-square nanosheets but with smaller width (referred to as nanoplates), and solid quasi-cube particles (nanocubes) can be obtained, respectively. Fig. 4a shows a typical SEM image of the obtained nanocubes. It can be seen that the crystals have a regular cube-like shape with sharp edges of a mean size of 110 nm. Fig. 4b shows a SEM image of the nanoplates. The nanoplates possess square-like shape with a mean width of 250 nm and a thickness of ca. 40 nm. From above experimental results, it seems that with increasing
reaction time, the nanoplates would become thinner and bigger; and the nanocubes would become hollow. These phenomena may originate from the different stability of crystal planes. With longer reaction time, the crystals will dissolve from the instable crystal plane. In the case of nanoplates, the dissolved components further grew on the stable planes forming ultrathin nanosheets, while the nanocubes were evolved into hollow nanoboxes via a dissolution-recrystallization process [25]. The nanosheets and the nanoplates could be considered as a variant of nanocubes with bigger side length and smaller thickness. Based on the XRD results and the intrinsic crystal structure of [Fe(py)2Ni(CN)4], we tentatively propose the possible surface crystal plane and growth direction of these nanostructures. As shown in Fig. 5, the difference among the nanosheets, nanoplates and nanocubes can be essentially ascribed to the different growth rate along [1 0 0] direction. The nanosheets were terminated mainly with {1 0 0} planes due to the lowest growth rate along [1 0 0] direction, while the nanocubes with the highest [1 0 0] growth rate were enclosed by {1 0 0}, {0 1 0} and {0 0 1} planes. These nanocrystals with various morphologies could exhibit different surface activity due to their different bare crystal planes. Because the shape of nanoparticles has a strong influence on their properties [26,27], we studied the magnetic properties of the obtained [Fe(py)2Ni(CN)4] nanocrystals with different morphologies. Fig. 6 shows the temperature dependence of χMT (where χM stands for the molar magnetic susceptibility) curves for the four [Fe(py)2Ni(CN)4] products. At 300 K, χMT is equal to 3.73 cm3 K mol−1, 3.58 cm3 K mol−1, 3.49 cm3 K mol−1 and 3.42 cm3 K mol−1 for the nanoplates, nanocubes, nanosheets and nanoboxes, respectively. The χMT values fall into the range expected for one high spin (HS) state (S = 2) of FeII [10]. The inconsistent room temperature χMT values for the four nanocrystals imply the different proportion of residual low spin (LS) fraction at this temperature. On lowering the temperature, the χMT values basically maintain temperature-independent until the vicinity of SCO behaviors (Tc↓ = 187, 180, 190 and 174 K for nanoplates, nanocubes, nanosheets and nanoboxes, respectively) where the χMT values abruptly decrease within a few kelvins, as observed for the typical SCO behaviors of FeII complexes [4–11]. Below the Tc↓, the χMT values sharply decrease but become again almost temperature independent for the nanoplates and nanocubes, which indicate that most iron(II) ions have undergone spin transition from FeII (HS) to FeII (LS). However, for nanosheets and nanoboxes, the χMT values still show a gradual decrease after the SCO transition. At the very low temperature (less than20 K), the χMT values decrease rapidly again but far from zero, which can be ascribed to the zero field splitting (ZFS) of residual HS fraction of FeII in the nanocrystals. In the warming mode, the characteristic Tc↑ temperatures are 210, 196, 192 and 180 K for nanoplates, nanocubes, nanosheets and nanoboxes, respectively. The magnetic susceptibilities for nanosheets and nanoboxes almost match the cooling mode with a tiny hysteresis loop, but nanoplates and nanocubes exhibit similar large SCO
Fig. 4. SEM images of (a) nanocubes and (b) nanoplates. 4
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Fig. 5. Illustration of the layered crystal structure of [Fe(py)2Ni(CN)4] and the relations between the nanocrystal morphologies and the [1 0 0] growth direction as well as surface crystal planes.
coordination with terminal solvent molecules. From this point of view, it is reasonable to conclude that the particles with higher specific surface area will suffer from not only weakening cooperativity and diminishing SCO domain, but also decreasing quantities of active SCO FeII atoms, eventually leading to the observed morphology/size dependent SCO behaviors. It is worth noting that the nanosheets exhibit extraordinarily high Tc↓ value, possibly owing to their unique morphology. Furthermore, it should be mentioned that size distribution will also affect the SCO features, especially the shape of hysteresis loop [12,14]. Narrow size distribution will be advantageous to produce rectangleshape hysteresis loops. In particular, the relatively large size distribution of the nanosheets could also contribute to the continuous character of the spin transition [15]. For further investigating the SCO behaviors of these [Fe(py)2Ni (CN)4] nanocrystals with different shapes, low temperature differential scanning calorimetry (DSC) analyses were adopted to evaluate the enthalpy (ΔH) and entropy (ΔS) variations associated with the spin transitions (Fig. 7 and Table 2). It can be seen from Table 2 that the values of ΔH and ΔS for all four nanocrystals are much smaller than those reported for the bulk materials, and the values for nanosheets and
Fig. 6. χMT vs T plots for [Fe(py)2Ni(CN)4] nanocrystals with different morphologies: (a) nanoplates, (b) nanocubes, (c) nanosheets, (d) nanoboxes.
hysteresis loops (width of hysteresis loops: 22 K for nanoplates and 16 K for nanocubes) as that observed for bulk crystals [21,22], indicating that nanosheets and nanoboxes show much better reversibility of the spin transition processes than nanoplates and nanocubes. Obviously, the SCO behaviors of [Fe(py)2Ni(CN)4] (critical temperature, hysteresis width, and residual HS/LS state sites) strongly depend on the morphology of the nanocrystals. For instance, the hysteresis width decreases from nanoplates (ca. 22 K) to nanocubes (ca. 16 K), nanoboxes (ca. 4 K) and nanosheets (ca. 2 K). Monte Carlo simulations for cubic and spherical SCO nanoparticles had revealed that particles with larger size exhibit more significant hysteresis width [28], which was also confirmed by the recently reported SCO nanoparticles based on FeII [12,14,15], in which residual HS/LS state sites increase as the particle size decreases. Similarly, in our case, the room temperature χMT values for the four nanocrystals decrease but the low temperature ones increase when the morphology changes from nanoplates to nanoboxes (Fig. 6). In fact, the intrinsic factor that affects the SCO features is mainly the specific surface area of the particles. The nanosheets with ultrathin 2D structure and nanoboxes with hollow structure obviously possess larger surface area than nanoplates and nanocubes, so the former particles behave weaker cooperativity and smaller SCO domain, leading to narrower hysteresis. As far as the residual HS/LS state is concerned, FeII ions located at the surface of the nanocrystals probably do not exhibit SCO due to coordinative defects that are coordinative unsaturation like incomplete {FeNx} coordination and/or
Fig. 7. DSC curves of the [Fe(py)2Ni(CN)4] samples with different morphologies: (a) nanosheets, (b) nanoplates, (c) nanoboxes, (d) nanocubes. 5
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References
Table 2 Spin transition temperatures of four nanocrystals in the heating (Tc↑) and cooling (Tc↓) modes together with the enthalpy and entropy variations associated with the spin transition. Samples
Nano-sheets Nano-plates Nano-boxes Nano-cubes Bulk crystals [22] a b
Tc↑/K
Tc↓/K
[a]
[b]
[a]
[b]
191.7 210.4 180.4 196.2 210
191.7 209.9 186.8 201.5 209
189.7 187.2 174.5 179.7 194
185.5 189.2 174.2 175.4 191
ΔHab/kJ mol−1
ΔSb/J K−1 mol−1
1.77 3.63 1.16 4.59 14.9
9.35 18.16 6.38 24.35 74
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From magnetic susceptibility data. From DSC data.
nanoboxes are smaller than those for nanoplates and nanocubes. The results suggest that the values of ΔH and ΔS decrease with increasing specific surface area of the particles. It is reasonable because the incomplete {FeNx} coordination sites lying at the surface of nanocrystals would not undergo the spin crossover transition, the fraction of the molecules related to the spin crossover transition decreases with increasing surface area, and as a result, the smaller values of ΔH and ΔS would be obtained for these nanocrystals as compared with the bulk crystals. The fact that nanoboxes and nanosheets with bigger surface area show smaller ΔH and ΔS values than nanocubes and nanoplates further supports this opinion. Therefore, the nano-effect of the nanoplates and nanocubes is relatively weak due to their relatively small specific surface area. In contrast, the nanosheets and nanoboxes show significant nano-effect and their SCO properties are largely weakened owing to the large specific surface areas resulting from their unique morphologies. In addition, the characteristic temperatures for the spin transition obtained from DSC measurements are basically consistent with those extracted from magnetic data (Table 2). 4. Conclusions In conclusion, novel ultrathin nanosheets and hollow nanoboxes of the SCO coordination polymer [Fe(py)2Ni(CN)4] have been successfully prepared through a reverse micelle technique. The synthetic parameters such as reaction time and reagent concentration have a great influence on the morphologies of the resulting nanostructures. The SCO properties of [Fe(py)2Ni(CN)4] are highly dependent on the shape of the nanostructures. We believe that this result provides not only the importance of gathering deeper insight into the correlation between SCO properties and particle shape in Fe(II)-based coordination polymers, but also a novel organic-inorganic hybrid nanomaterial, which might find attractive applications in next-generation flexible and transparent nanodevices. Conflict of interest There are no conflicts to declare. Acknowledgements We are grateful for financial support from the National Natural Science Foundation of China (Nos. 51072071 and 21875091), the Natural Science Foundation of Jiangsu Province (No. BK20171295) and Nanjing Xiaozhuang University Research Project (No. 2018NXY23). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165938.
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