English

• 0.   Introduction

  Multifunctional magnetic materials possessing both magnetic properties and additional functions are highly desired and have received great attention recently^[1-4]. Abundant physical and chemical properties such as fluorescence^[5], conductivity^[6], chirality^[7], dielectricity^[8], porosity^[9], redox^[10], and catalysis^[11] have been intelligently integrated into these magnetic materials. From a synthetic point of view, molecular magnetic materials are very promising for the development of multifunctional magnetic materials because of their inorganic-organic character^[12-15]. Besides, compared with the widely studied homogeneous materials which can have a clear structure-function relationship, heterogeneous materials have their advantages in the tunable morphologies, easy synthesis processes, and most of all, controllable selection of the desired composites and functions^{[3, 16-17]}.

  Among the different magnetic properties, spin crossover (SCO) and long-range magnetic ordering (LRMO) are two highly desired functions in the area of molecular magnetism. However, due to the different requirements for these properties, molecular materials with coexisting SCO and LRMO properties are still limited in both the homogenous molecular magnetic materials^[18-21] and the heterogenous hybrids^[22]. For the hybrids, materials containing different Prussian blue analogs (PBAs)^[23] and those containing the Hofmann-like SCO compounds and the NiCr PBAs, namely a thin film heterostructure with Fe(azpy)[Pt(CN)₄]·xH₂O (azpy=4, 4′-azopyridine) and Ni^Ⅱ[Cr^Ⅲ(CN)₆]_0.7·nH₂O^[24], and a nanoparticle hybrid containing Fe(phpy)₂[Ni(CN)₄]·0.5H₂O (phpy=4‑phenylpyridine) and K_0.4Ni_1.0[Cr(CN)₆]_0.8·nH₂O^[25], have been reported. Interestingly, photoinduced magnetization changes in the LRMO network were correlated to the LIESST (light-induced excited spin state trapping) effect of the SCO network and the synergistic photomagnetic effect arises from the coupling of the two lattices^[24-25].

  PB(A)s with a general formula A_xM_y[M′(CN)₆]_z (A=monovalent cation, M and M′=transition metal ions) have attracted intense interest in the field of magnetic materials, especially after the observation of the magnetic ordering in PB below 5.6 K^[26]. By tuning the compositions, a huge number of PB(A)s with different magnetic properties ranging from high T_c up to room temperature^[27], spin crossover^[28], photomagnetism^[29], magnetic pole inversion^[30], and so on have been achieved. The PB(A)s have also been acknowledged as promising materials in the fields of batteries^[31], supercapacitors^[32], proton conductivity^[33], gas adsorption^[34], catalysis^[35], and so on. The research on PB(A) heterostructures has also been widely performed, rendering them very promising for material science^[36], energy and environmental applications^[37], biomedical and theranostic applications^[38], and so on. For example, a series of core-shell nanoparticles with different PB(A)s and other nano-components (PBA1@PBA2^[39], Au@PBA^[40], Au@ PBA2@PBA3^[41], PBA@SiO₂^[42], and so on) have been found to show interesting performances, such as strain-controlled magnetism, electrocatalytic performances, a combination of plasmonic optical and magnetic properties.

  On the other hand, the 1D triazole-bridged Fe(Ⅱ) SCO compounds (FeTrz), pioneered by Kahn^[43], Bousseksou^[44], Coronado^[45], Roubeau^[46], et al., are probably the most widely studied SCO materials. They have been used extensively to prepare multifunctional materials because of their chemical stability, easy synthesis, the existence of thermal hysteresis around room temperature, and facile modifications of multiple properties (color, volume, dielectricity, and so on). For instance, to tune the electrical properties, FeTrz compounds were integrated with various conducting materials, such as the carbon nanotube^[47], nanoparticles and nanowires of Ag and Au^[48-49], conducting organic polymers^[6], ferroelectric polymers^[50], and MoS₂ layers^[45]. The mechanical actuators triggered by external stimuli (temperature, electricity, and light) have been achieved in the SCO composite materials containing FeTrz and different organic polymers such as PMMA^[51], SU-8^[52], and P(VDF-TrFE)^[53]. Luminescent SCO hybrid materials can be also achieved in the Fe(Ⅱ)-triazole silica-coated nanoparticles where different fluorescence groups, such as 3-dansyl^[54], pyrene^[55], Tb³⁺ complex^[56], and Re(Ⅰ) complex^[57], were anchored in the silica shells.

  Despite the above-mentioned achievements on the hybrids containing either FeTrz or PB(A)s, composites containing both FeTrz and PB(A)s have never been reported. In these heterostructures, coexisting or synergetic magnetic properties, such as "SCO+LRMO", multi-step SCO, and "SCO+photomagnetism", are anticipated. Herein, we reported the facile preparation and characterization of the first examples of the heterostructures combining the FeTrz and PB materials (FeTrz@PB). A simple and very efficient precipitation transformation strategy was employed for the synthesis of these composites (Scheme 1). This strategy involves the transformation of two materials of different stability and/or solubility and has been used for the preparation of functional materials either in homogeneous or heterogeneous forms. The compositions, morphologies, and magnetic properties of these FeTrz@PB composites were thoroughly characterized. Magnetic measurements revealed the coexisting of the SCO at above room temperature and the LRMO at the low temperature for these materials. These FeTrz@PB composites are the first heterogeneous examples containing both attractive materials (FeTrz and PB(A)s). The very simple synthetic strategy reported here could be used as a general method for more multifunctional hybrid materials.

  Scheme 1

  

  Scheme 1.  Schematic illustration of the preparation process of FeTrz@PB composites

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  1.   Experimental

  1.1   Materials and measurements

  Fe(BF₄)₂·6H₂O (99%) and 1H-1, 2, 4-triazole (Htrz, 99%) were purchased from Aladdin. Ascorbic acid and K₃[Fe(CN)₆] were received from Macklin. Methanol was purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water (H₂O) was obtained from ion-exchange water treatment. All chemicals and solvents were obtained from commercial sources and used without further purification.

  FTIR data were recorded on a Nexus 870 FTIR spectrometer in a range of 4 000-400 cm^-1 (KBr pellets). Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer under Cu Kα radiation (λ=0.154 06 nm) operated at 40 kV and 40 mA in a range of 5°-50°. Dynamic light scattering (DLS) analyses were performed on a DynaPro NanoStar and samples were suspended in ethanol. Scanning electron microscopy (SEM) and energy-dispersion X-ray analysis (EDX) were performed on a Hitachi S-4800 instrument. Transmission electron microscopy (TEM) was performed on a JEOL JEM‑2800 high‑ resolution transmission electron microscope with a beam energy of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on monochromatic radiation (Al Kα) of the PHI5000 Versa Probe spectrometer. Thermogravimetric analysis (TGA) was performed in Al₂O₃ crucibles using a PerkinElmer Thermal Analyser in the temperature range of 28-800 ℃ under a nitrogen atmosphere with a heating rate of 10 ℃·min^-1. Magnetic measurements were performed on a Quantum Design SQUID VSM magnetometer and all the data were corrected for the diamagnetic contribution from the sample holder.

  1.2   Synthesis of [Fe(Htrz)₂(trz)]BF₄

  The [Fe(Htrz)₂(trz)]BF₄ microcrystals were synthesized using the literature method^[61]. The concentration of precursors was controlled to prepare the [Fe(Htrz)₂(trz)]BF₄ micro-particles of suitable size for the growth of the PB nanoparticles on the surface. Generally, a molar ratio of 1∶3 for Fe(BF₄)₂·6H₂O and Htrz was used for the reaction. Fe(BF₄)₂·6H₂O (202.9 mg, 0.3 mol·L^-1) in 2 mL H₂O and a small amount of ascorbic acid were added to Htrz (124.1 mg, 0.9 mol·L^-1) in 2 mL H₂O in a 10 mL test tube. The tube was heated at 45 ℃ for 48 h. After cooling to room temperature, the resulting precipitates were filtered off, washed with methanol (3×5 mL), and dried at 60 ℃ in a vacuum for 12 h. Yield: 77%. IR (KBr, cm^-1): 3 174(s), 3 093(s), 1 496(m), 1 455(w), 1 224(w), 630(m).

  1.3   Synthesis of FeTrz@PB composites

  [Fe(Htrz)₂(trz)]BF₄ (1 g, 2.87 mmol) and K₃[Fe(CN)₆] (1.316 g, 4 mmol) were ultrasonically dispersed into 100 mL H₂O in a flask and then stirred constantly at RT under the N₂ atmosphere. The mixture turned blue slowly. Then, 12.5 mL of the mixture was taken out from the flask in the subsequent reaction time of 1, 3, 5, 8, 12, 24, 36, and 48 h, respectively. These mixtures were centrifuged at 6 000 r·min^-1 for 4 min to give the FeTrz@PB powders, which were washed five times with 15 mL H₂O to remove the residual K₃[Fe(CN)₆] solution. The resulting blue precipitates were then collected and dried at 60 ℃ in a vacuum for 12 h. The characterization of these products will be described below.

  2.   Results and discussion

  2.1   Synthesis

  The FeTrz@PB composites can be synthesized through the simple precipitation transformation method and the preparation details are illustrated in Scheme 1. This synthetic method involves the transformation from one material to a new one of lower solubility. The tendency of the transformation reaction is proportional to the difference in the solubility of the two species^[58]. For example, the cation and/or ligand exchange reactions in the post-synthetic treatments of the metal-organic frameworks (MOFs) have been very successful in the preparation of new MOFs and/or core-shell hybrids, such as structures of MOF@PB(A) with catalytic oxygen evolution ability^[59]. For the materials studied here, it is well-known that PB is a very stable coordination framework of very low solubility in water with the K_sp constant around 10^{-41 [60]}. Therefore, adding the [Fe(CN)₆]^3- to the reaction media leads to the dissolution of the FeTrz rods and the formation of the PB nanoparticle on the surface of the FeTrz rods, which can be simply regarded as the source of the Fe²⁺ center. Thanks to the very intense blue color of PB (known as an excellent blue pigment^[60]), the solution in the bottle turns blue quickly, indicating the formation of PB nanoparticles. From our reaction, we can see that one of the advantages of the precipitation transformation method is its simplicity: it can take place at room temperature and is very easy to implement.

  As for the FeTrz starting material, different phases can be obtained by controlling the synthesis temperature and precursor concentrations. These different phases and even the same phase with different sizes and morphologies might lead to different SCO behaviors^[61-62]. To have better control of the final morphologies of the prepared composites, the phase Ⅰ material of FeTrz having a micro-rod morphology was preferred and prepared by controlling the concentration of the precursors and reaction temperature^[62]. In our reaction condition, the microcrystals of FeTrz were long micro-rods with lengths of about 5 μm and diameters of about 1 μm (Fig.S1, Supporting information). The PXRD patterns of these materials as shown in Fig.S2 confirmed that this material crystallized in the phase Ⅰ structure with high phase purity. These rods had very smooth surfaces and were suitable for the growth of the PB nanoparticles by precipitation transformation. Furthermore, to better reveal the growth process and the morphology changes of the FeTrz@PB composites, the reaction time of the transformation was controlled from 1 to 48 h by taking out 1/8 portion of the reaction solution each time at 1, 3, 5, 8, 12, 24, 36, and 48 h. The resulting FeTrz@PB composites were then collected. As can be seen from the optical photos of the FeTrz@PB products (Fig.S3), the longer reaction time led to a darker blue color, indicating more transformation reaction and a greater proportion of the PB component in the FeTrz@PB composites. Note that the pictures of the FeTrz material at various stages under the same experimental condition were also taken and the pink color indicates the stability of the FeTrz material (Fig.S4). In addition, as shown below, the obtained FeTrz@PB composites have been fully characterized by SEM, TEM, PXRD, FTIR, XPS, EDX, and TGA.

  2.2   SEM and TEM images of FeTrz@PB

  The SEM images revealed that the morphologies of the synthesized FeTrz@PB composites gradually changed upon the reaction times (Fig. 1). When the reaction time was 1 h, no obvious PB nanoparticles can be observed on the surface of the FeTrz rods from SEM (Fig. 1a). However, compared with the initial FeTrz micro-rods, the surface roughness of the microcrystals increased obviously. This observation, together with the blue color of the resulting product, indicated the start of the precipitation transformation process. When the reaction time was increased to 3 h, a small number of small nanocubes of the average size of about 58 nm were observed on the surface of the micro‑rods (Fig. 1b), directly confirming the partial conversion from the pre-synthesized FeTrz micro-rods to the PB nanoparticles. At this time, the PB nanoparticles were mostly isolated on the surface of the FeTrz rods, showing barely any aggregation of the particles. Further increase in the reaction time led to the darker color of the product, the increased average size of the PB particles, more aggregations of the PB particles, and increased surface coverage. In addition, we noticed that at 8 h, the etching of the surface of FeTrz rods was obvious and the average size of PB reached ca. 116 nm (Fig. 1d). Longer reaction times lead to bigger PB particles and the slow fade of the FeTrz micro-rods. For example, at 36 h, the smooth rod morphology of FeTrz was barely observable (Fig. 1g) and part of the generated PB particles was observed to be separated from the micro-rods. Finally, when the reaction time was up to 48 h, the products seemed to contain only the large PB particles with an average size of about 321 nm, while no images of the FeTrz rods could be observed, suggesting the completeness of the transformation reaction (Fig.S5). Meanwhile, the sizes and morphologies of these FeTrz@PB composites with different reaction times were also confirmed by the TEM images (Fig. 2). Especially, in the TEM images, a small amount of small PB particles of ca. 17 nm could also be observed for the sample at 1 h (inset of Fig. 2a). Clearly, the FeTrz rods could still be seen in the TEM images after 36 h reaction time, while most of the products were PB particles after 48 h, which also confirmed the completeness of the reaction. In addition, we want to emphasize that compared with the physical mixture of the two materials, the homogeneity of these heterostructural composites synthesized here is much better. The controlled precipitation transformation method ensures that the PB nanoparticles are uniformly distributed on the surface of the rods of Fe-Trz. However, for the physical mixture, the particles of each material are separated from each other with limited homogeneity.

  Figure 1

  

  Figure 1.  SEM images of FeTrz@PB with the reaction time of (a) 1 h, (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 36 h, and (h) 48 h

  The average size of PB particles is shown in the figures.

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  Figure 2

  

  Figure 2.  TEM images of FeTrz@PB with the reaction time of (a) 1 h (Inset: high-resolution image), (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 36 h, and (h) 48 h

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  2.3   PXRD of FeTrz@PB

  Simultaneously, the PXRD patterns of the FeTrz@PB composites at different reaction times were collected to follow the formation of the heterostructures (Fig. 3). As shown in Fig. 3, the PXRD patterns of the products of reaction time from 1 to 24 h didn′t show obvious changes compared to the patterns of the pure FeTrz material. This seemly inconsistency might be due to the low quantity and small size of the PB nanoparticles and the strong diffraction of the FeTrz microcrystals. However, for the product of 36 h, the intensities of the diffraction peaks of the FeTrz at 10°, 11°, 18°, 19°, 23°, and 25° significantly decreased, while new peaks attributed to PB (at 17.3°, 24.5°, 39.5°, and 43.4°) appeared^[63]. It is noteworthy that at 48 h only the diffraction peaks of PB remained, which is consistent with the SEM and TEM results and confirms the complete transformation from FeTrz to PB. In addition, to gain a better view of the gradual changes of the two peaks, PXRD patterns were further collected for the samples with reaction times from 24 to 48 h (Fig.S6). A gradual decrease in the intensity of the peaks at 10°, 11°, 18°, 19°, 23°, and 25° was observed for the products up to 40 h. At 42 h, no peaks of the FeTrz material can be observed anymore. These observations further confirmed the slow consumption of the FeTrz component.

  Figure 3

  

  Figure 3.  PXRD patterns of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  2.4   FTIR of FeTrz@PB

  Although the formation of the FeTrz composites could not be detected in the PXRD patterns when the reaction time was shorter than 24 h, the transformation process from FeTrz to PB could be followed by the FTIR spectra (Fig. 4) as the C≡N (Fe^Ⅱ—C≡N—Fe^Ⅲ) stretching vibration at 2 081 cm^-1 is very strong and characteristic for the PB component. Interestingly, for the material with a reaction time of 1 h, even if the PB nanoparticles could not be detected by SEM and PXRD, a small IR peak at 2 081 cm^-1 confirmed the formation of PB, which is consistent with the TEM result. Furthermore, as the reaction time increased, the intensity of the C≡N peak increased together with the gradual decrease of the peaks ascribed to FeTrz (1 455 and 1 496 cm^-1 for the ring stretching of the coordinated triazole ligands, and 630 cm^-1 for the out-of-plane vibration of triazole), indicating the ongoing transformation process. Finally, at 48 h, the peaks ascribed to FeTrz disappeared completely. These features are in good accordance with the above results.

  Figure 4

  

  Figure 4.  FTIR spectra of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  2.5   XPS, EDX, and TGA of FeTrz@PB

  Furthermore, the chemical composition of the FeTrz@PB composite (at 36 h) has been confirmed by XPS measurement (Fig. 5). As shown in Fig.S7, the peaks at 284.7 and 398.9 eV are attributed to C1s and N1s of FeTrz and FeTrz@PB (36 h) composite, respectively. The two characteristic peaks at 707.8 and 720.3 eV are attributed to Fe2p_3/2 and Fe2p_1/2 of Fe²⁺ from FeTrz, respectively. Similarly, two peaks at 711.1 eV (Fe2p_3/2) and 724.7 eV (Fe2p_1/2) can be assigned to the Fe³⁺ of PB nanoparticles. Therefore, the XPS spectra of the FeTrz@PB composite are consistent with the existence of both FeTrz and PB composites in the heterostructure^[64]. In addition, the major elemental composition of FeTrz@PB composites was confirmed by EDX and elemental mapping images (Fig.S8-S10). Compared with the EDX image of FeTrz (Fig.S9), new characteristic peaks ascribed to K⁺ can be observed in all the FeTrz@PB hybrids. This result indicates that the generated PB phase in the composites should contain the K⁺ ions and have a composition of K_xFe^Ⅲ[Fe^Ⅱ(CN)₆]_(3+x)/4 rather than Fe₄^Ⅲ[Fe^Ⅱ(CN)₆]₃. For the final product at 48 h with only the PB phase, the composition of the phase can be estimated to be K_0.53Fe^Ⅲ[Fe^Ⅱ(CN)₆]_0.88·1.7H₂O based on the ratio of K to Fe from EDX and water content from the TGA data (vide post).

  Figure 5

  

  Figure 5.  XPS spectra of (a) full range scan of FeTrz and FeTrz@PB (36 h), (b) Fe2p region of FeTrz, and (c) Fe2p region of FeTrz@PB (36 h)

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  To check the thermal stability of these FeTrz@PB composites, TGA was performed on these heterostructures from room temperature to 800 ℃. The TGA results generally agreed with the above results and showed that the FeTrz@PB components were thermally stable up to about 350 ℃. As shown in Fig. 6, the weight loss started at room temperature upon heating for the parent FeTrz and all the composites. However, the weight loss was quite small before 350 ℃, especially for the composite with a short reaction time. This weight loss can be attributed to the removal of the crystallized water molecules in these materials. Since the PB(A)s are known to be porous and contain some water molecules^[65], the weight losses before 350 ℃ gradually increased for the composites with longer reaction time, consistent with the increasing portion of the PB phase in the FeTrz@PB hybrids. After the removal of the guest molecules, we noticed that the weight losses for the composites with shorter reaction times were significantly large. This should be due to the higher thermal stability of the 3D cyano-bridged PB network compared to the 1D triazole-bridged FeTrz network. At 800 ℃, the residue weight of the composites also increased along with the longer reaction time. This is also consistent with the increasing amount of the PB component in the FeTrz@PB hybrids with longer reaction times since the elemental percent of the iron is higher in PB than in FeTrz. In addition, using the TGA and EDX results, the compositions of the products of different reaction times were estimated and listed in Table S1.

  Figure 6

  

  Figure 6.  TGA curves of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  2.6   Magnetic properties

  The magnetic properties of the parent FeTrz and the FeTrz@PB composites were first investigated by measuring the variable-temperature magnetic susceptibilities data from 300 to 400 K with a 5 K·min^-1 sweep rate (Fig. 7). The data were calculated according to the estimated molecular weight of the composites (Table S1). At 400 K, the χ_MT values for all samples were around 3.3 cm³·mol^-1·K, while the χ_MT values at 300 K gradually increased with the increasing reaction time (the χ_MT values were 0.55, 0.62, 0.90, 1.11, 1.35, 1.65, 2.43, 2.75, and 3.37 cm³·mol^-1·K for the composites of reaction time of 0, 1, 3, 5, 8, 12, 24, 36, and 48 h, respectively). Hysteretic SCO transition in the temperature range of about 350 to 400 K with a loop width of about 30 K could be observed for all the samples except for the one with the reaction time of 40 h, which is the fully converted PB phase. We found that the SCO transition temperatures didn′t show obvious changes although the heights of the loops decreased gradually. Although it′s known that the SCO behavior of FeTrz is influenced by the particle size^[61], we think the change in the SCO properties of these composites is mainly related to the gradually decreasing FeTrz portions. From the SEM and TEM images, it can be seen that the morphology of the FeTrz rods was maintained well during the 1-24 h period and the size change of the FeTrz particles was rather small. Similar SCO properties were thus observed for these composites. As for the Fetrz@PB composite of 36 h reaction time, a small amount of FeTrz material still existed as shown in the PXRD pattern. Due to the non-uniformity of the reaction (different sizes of the rods of the FeTrz starting material, different conversion ratios of FeTrz to PB), there should be some large-sized FeTrz particles left in the composite, which we believe is responsible for the observed squareness of the hysteresis loop.

  Figure 7

  

  Figure 7.  Temperature-dependent χ_MT curves of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  In addition, to probe the LRMO of the FeTrz@PB composites, zero-field-cooled (ZFC) and field-cooled (FC) magnetizations were measured with an applied magnetic field of 10 Oe from 2 to 14 K (Fig. 8). Upon cooling from 14 K, the ZFC and FC magnetization curves rose sharply below 6 K and diverged from each other at (5.6±0.5) K, clearly indicating a magnetic phase transition from the paramagnetic phase to a long-range ordered phase. This ordering temperature agrees well with the critical temperature of the typical PB material. The magnetic susceptibilities of the ZFC/FC curves increased gradually for the hybrids of longer reaction time, consistent with the increased amount of the PB components. However, different from the situation observed above the transition temperature for the SCO transition stayed almost the same for all the composites, the T_c values for the LRMO increased slightly and the divergency temperatures for the ZFC/FC curves moved to higher temperatures along with the increasing reaction time (Fig. 8). For example, for the sample of 1 h reaction time, the divergency temperature was 5.6 K, while it was 6.0 K for the sample of 36 h reaction time. This observation might be related to the particle size of the generated PB nanoparticles. As has been shown in the SEM and TEM images of the composites, the size of the formed nanoparticles for the composites of shorter reaction time was considerably smaller than those of the longer time. The size-dependent control of the magnetic properties of the PB nanoparticles has been well documented and the T_c values were found to decrease with the particle size^[66-67].

  Figure 8

  

  Figure 8.  ZFC and FC magnetizations of FeTrz@PB with an applied dc (direct current) field of 10 Oe

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  From the above magnetic analyses of the FeTrz@PB composites, we can come to a safe conclusion that these composites have the coexistence of the SCO behavior at temperatures above room temperature and LRMO at low temperatures. Furthermore, important magnetic parameters, such as the apparent HS fraction and height of the thermal hysteresis loops for the SCO transition, the intensity of the magnetization at low temperatures, and the critical temperature of the LRMO, can be tuned by simply controlling the reaction time. As has been mentioned in the Introduction part, although the combination of the SCO property and LRMO has been reported in many heterostructures containing the Hofmann‑type materials and the PBAs^{[24-25, 38, 68]} and some homogeneous molecule magnetic materials as listed in Table S2^[18-21], there is no report for the heterostructural materials containing both the Fe-Trz and PBA materials. In this regard, the composites synthesized here are very promising bifunctional magnetic materials.

  Furthermore, although SCO and LRMO are the two properties combined in these FeTrz@PB composites reported here, the precipitation transformation strategy and the facile synthetic method are promising for the preparation of new multifunctional materials. By selecting two or more different materials, it is possible to controllably construct the heterostructural composites such as SCO@PB(A) and SCO1@SCO2 with different magnetic properties such as SCO+photomagnetism, SCO+high T_c, and two-step or multi-step SCO multistable materials. For example, although considerable attention has been paid to the preparation of two-step or multi‑step SCO materials and many interesting examples have been reported^[69], the syntheses of such materials mostly rely on serendipitous molecular self-assembly with poor controllability. However, following the strategy as demonstrated here, one SCO material (SCO1) with high solubility can be partially transferred to the other SCO material (SCO2) leading to the anticipated SCO1@SCO2 composites of core-shell or hybrid heterostructures. By choosing SCO materials of different transition temperatures and controlling the reaction conditions such as the concentration of the reactants and the reaction time, the transition temperatures and the step heights for each step are tunable in principle. In addition, due to the low solubility of PB(A)s, magnetic materials containing other divalent ions can also be used as the starting materials, leading to the preparation of other multifunctional magnetic materials through this precipitation transformation strategy. Therefore, although the work here only reports the coexistence of SCO and LRMO because these two magnetic behaviors occur at very different temperatures (360 and 5 K), interesting performances are possible in the above-mentioned new heterostructural composites. Furthermore, as the heterostructural composites are homogenous particles containing both materials, these studies could even be performed on individual single nanoparticles. Studies are pursued along this line together with our collaborators as an ongoing study following the reported work^[70].

  3.   Conclusions

  In summary, bifunctional FeTrz@PB heterogeneous composites were successfully synthesized through a facile precipitation transformation method. The morphologies and compositions of the FeTrz@PB composites and the size of PB nanoparticles can be finely tuned by the reaction time. Magnetic measurements revealed the coexisting of the SCO at above room temperature and the LRMO at the low temperature of these materials. These FeTrz@PB composites are the first heterogeneous examples containing both FeTrz and PB(A)s. This simple and efficient precipitation transformation synthetic strategy may provide a general method to prepare more multifunctional hybrid materials with promising magnetic properties and other functions. Efforts along this line are currently underway in our lab.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Schematic illustration of the preparation process of FeTrz@PB composites

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  Figure 1  SEM images of FeTrz@PB with the reaction time of (a) 1 h, (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 36 h, and (h) 48 h

  The average size of PB particles is shown in the figures.

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  Figure 2  TEM images of FeTrz@PB with the reaction time of (a) 1 h (Inset: high-resolution image), (b) 3 h, (c) 5 h, (d) 8 h, (e) 12 h, (f) 24 h, (g) 36 h, and (h) 48 h

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  Figure 3  PXRD patterns of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  Figure 4  FTIR spectra of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  Figure 5  XPS spectra of (a) full range scan of FeTrz and FeTrz@PB (36 h), (b) Fe2p region of FeTrz, and (c) Fe2p region of FeTrz@PB (36 h)

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  Figure 6  TGA curves of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  Figure 7  Temperature-dependent χ_MT curves of FeTrz and FeTrz@PB with a reaction time of 1-48 h

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  Figure 8  ZFC and FC magnetizations of FeTrz@PB with an applied dc (direct current) field of 10 Oe

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