Tubular Superstructures Composed of α‐Fe2O3 Nanoparticles from Pyrolysis of Metal−Organic Frameworks in a Confined Space: Effect on Morphology, Particle Size, and Magnetic Properties

While preparation of metal oxide from metal organic frameworks (MOFs) has been widely studied, crystal growth via thermal decomposition of MOFs in a confined space is rarely investigated. We demonstrate a confinement effect on the crystal growth via pyrolysis of MOFs at high temperature. Iron containing MOF (Fe-MIL-88A) was calcined inside a SiO2. The crystalline phase, particle size, morphology, and magnetic properties of the synthesized iron oxide were characterized; α-Fe2O3 tubular structures that consisted of nanoparticles (around 10 nm) were observed. Studies of the magnetic properties show enhanced magnetization with superparamagnetic behavior. These results indicate that space confinement during the thermal treatment in air at high temperature allows the synthesis of small nanoparticles and the preservation of initial morphology of MOF precursors, which cannot be obtained via heating of MOFs without shell under identical conditions. T preparation of nano or microstructured metal oxides from metal−organic frameworks (MOFs) has attracted a lot of attention owing to the facile modulation of the shapes and the high performance of in a variety of applications. The nanoor micromorphology, particle size, and crystal structure of metal oxides obtained by thermal decomposition of MOFs are strongly affected by the environment during the calcination process. For example, α-Fe2O3 is one of the final structure at high temperature of thermal phase transition of other iron oxides through various Fe2O3 polymorphs including γ-, ε-, βFe2O3. 5 It can be prepared by thermal decomposition of MOFs with a tunable size, shapes, and structures (Scheme 1). Cho et al. reported that both hematite and magnetite structures were created from identical Fe-MIL-88B particles by controlling the calcination temperature, amount of organic ligand, and gas conditions. In addition, Prussian blue microcubes were transformed into iron oxide microboxes at 350 °C, porous microboxes at 550 °C, and hierarchical microboxes at 650 °C. However, it is observed that high calcination temperatures result in destruction of the morphology and enlarged metal oxide particles. It has been difficult to generate morphology-controlled, size-regulated metal oxides from MOFs through thermal decomposition at high temperatures. Thus, the development of a new method for controlling the structure, morphology, and particle size of metal oxides produced by a thermal decomposition process is of interest. Such a method will help to broaden our knowledge of crystal growth and has the potential to open up new applications in this field. Received: April 18, 2017 Revised: July 27, 2017 Published: August 22, 2017 Scheme 1. Schematic Representation of the Preparation of the Tubular α-Fe2O3 Superstructure and α-Fe2O3 Particles from Fe-MIL-88A Precursors Communication pubs.acs.org/crystal © 2017 American Chemical Society 4496 DOI: 10.1021/acs.cgd.7b00547 Cryst. Growth Des. 2017, 17, 4496−4500 Nanoreactors provide a space with a unique chemical and physical environment that is isolated and protected from the surrounding bulk material. The confined interior space, which can be created by metal-based shells, self-assembled organic molecules, or molecular cage templates, can be used as a vessel for chemical reactions. The thermal stability, activity, and selectivity of catalysts have been enhanced by using nanoreactors for catalytic applications. More importantly, the rate of some chemical reactions has been shown to be dramatically accelerated when performed within such systems. Nanoreactors also enable controllable synthesis within an isolated space, which means that nanocrystals can be grown with less interference from the outside environment and other species. Recently, MOF-derived Fe2O3, Co3O4, and ZnO were prepared inside TiO2 and SiO2 shells using a calcination process. Although the synthesis of transition metal oxides from the MOF precursors was successfully achieved inside the shells, the authors were less interested in the fact that it had occurred inside a confined space than in the retention of the original morphology of the MOF and the applications. An understanding of high-temperature crystal growth mechanisms in confined spaces is essential for the development of new synthetic strategies and the creation of nanomaterials with unique crystal structures. To gain such an understanding, it is necessary to elucidate the structure, particle size, morphology, and physical properties of metal oxides that are prepared from MOFs in a confined thermal decomposition process. In this study, we have explored a space-confined calcination strategy to clarify the effects of confinement on the growth of metal oxide crystals derived from MOFs. We demonstrate that high-temperature thermal decomposition in a nanoreactor system not only preserves the initial morphology of the ironcontaining MOFs, but also restricts the growth of the hematite particles. Tubular α-Fe2O3 structures were created through calcination in a confined space, and their magnetization was enhanced compared with that of α-Fe2O3 clusters prepared without the protective external shell. Fe-MIL-88A is highly flexible iron-containing MOFs that can be prepared by reaction of Fe with fumaric acid (MIL stands for Materials from Institue Lavoisier). Rod-shaped Fe-MIL88A was obtained by using a modified version of the previously reported reaction that iron precursors and sodium fumarate ligands in deionized water. The morphology and size distribution of the prepared Fe-MIL-88A particles were observed by scanning electron microscopy (SEM), and the rod-shaped Fe-MIL-88A particles are shown in Figure 1a. The particle size of MOFs is strongly influenced by the nucleation rate. The powder X-ray diffraction (XRD) pattern of the prepared Fe-MIL-88A particles, shown in Figure S1, contains strong (101) and (002) peaks at 2θ = 11° and 13°, which agree with the reported MIL-88A crystal structure. Fe-MIL-88A particles were calcined under air at 800 °C for 1 h and cooled naturally to room temperature. The color of the MOF particles changed from orange to blood-red. Calcination of iron-containing MOFs led to the formation of the hematite (α-Fe2O3) structure. The XRD data (Figure S2) for the iron oxide particles in the calcined Fe-MIL-88A match well with the structure of bulk α-Fe2O3 crystals, which have a rhombohedral structure (International Centre for Diffraction Data (ICDD) No. 04-006-6579). High-resolution transmission electron microscopy (HRTEM) and SEM images of the morphology of the prepared α-Fe2O3 clusters are shown in Figure 1b. The original morphology of the MOF particles is collapsed and the length of the individual particles is over 100 nm. Each particles have irregular spherical shapes and are connected to give clusters through oriented attachment of their surfaces. This process occurs on calcination and the mechanism involved the decomposition of organic ligand releasing gases and the agglomeration of metal oxide particles. Next, Fe-MIL-88A particles were coated with SiO2 by hydrolysis of tetraethyl orthosilicate with ammonium hydroxide in deionized water at 80 °C for 2 h. Figure S3 shows the SEM images of SiO2-coated Fe-MIL-88A particles, which retained their hexagonal rod shape. Next, the SiO2-coated Fe-MIL-88A particles were calcined under air at 800 °C for 1 h, and the shape and size distributions of the prepared particles (α-Fe2O3@SiO2) were observed by SEM and HRTEM (Figures 2 and S4). The SEM and HRTEM images reveal that the hexagonal rod shape was preserved and that the SiO2 shell prevented the aggregation of particles during calcination. The microscope images also indicate that the αFe2O3@SiO2 particles had a hollow structure. The observed void can be explained by the “gas push” effect that results from the oxidation of organic ligands and the Ostwald ripening process. The weak (104) and (110) peaks at 2θ = 33.18° and 35.3° in the XRD pattern (Figure S5) match the rhombohedral α-Fe2O3 structure (ICDD No. 04-006-6579). An energy dispersive spectroscopy (EDS) mapping analysis was conducted to confirm the presence of both Si and Fe species in the synthesized α-Fe2O3@SiO2 particles. The EDS mapping analysis is displayed in Figure 2d,e, and the colored regions indicate the Si and Fe atoms. This result confirms that both Si and Fe atoms were present in the hollow α-Fe2O3@ SiO2 particles. Nitrogen gas sorption measurement of the prepared αFe2O3@SiO2 gave a Brunauer−Emmett−Teller (BET) surface Figure 1. (a) Scanning electron microscopy image and (b) SEM and TEM images of α-Fe2O3 derived from Fe-MIL-88A. Crystal Growth & Design Communication DOI: 10.1021/acs.cgd.7b00547 Cryst. Growth Des. 2017, 17, 4496−4500 4497 area of 123.61 m g−1 and an average Barrett−Joyner−Halenda (BJH) pore diameter of 5.26 nm were observed (Figure S8). It is suggested that released gas by thermal oxidation of MOFs leads to the formation of mesopores. To investigate the effect of space confinement on the nanoparticle growth during calcination, the SiO2 shell was removed with sodium hydroxide solution. XRD analysis of the iron oxide after removal of the SiO2 shell compared well with the rhombohedral hematite structure (ICDD No. 04-0066579). The morphology of these α-Fe2O3 particles was verified by TEM (Figures 3 and S6). Interestingly, the tubular α-Fe2O3 are formed containing large numbers of connected particles. In addition, the size of the individual particles generated in the confined space within the tubes was smaller (less than 10 nm) than that of the clustered α-Fe2O3 obtained from naked FeMIL-88A. These results indicate that the SiO2 shell restricts the agglomeration of metal oxide particles via the Ostwald ripening process and prevents the collapse of the initial 1D morphology at high temperatures. The calcination of uncoated Fe-MIL-88A was conducted at 700 °C for 1 h and 800 °C for 6 h (powder XRD patterns are shown in Figure S9). The sharp diffraction peaks (104) and (110) at 2θ = 33.18° and 35.3° are characteristic of hematite. All the peaks are identical and become narrower with increasing calcination temperature and time. According to the TEM images in Figure S10, the shapes of the particles are irregularly spherical having an average size over ∼100 nm. As the calcination temperature and time increase, the length of irregularly spherical structured particles increased. On the other hand, XRD patterns of SiO2 coated Fe-MIL88A calcined at 700 °C for 1 h show an amorphous phase. With increasing calcination temperature, the diffraction peaks (104) and (110) at 2θ = 33.18° and 35.3° are observed. In addition, The XRD peak intensities increased with extending the reaction time. Interestingly, additional new peaks at 2θ = 34° and 37° appeared with the increased calcination time at 800 °C. It is suggested that further investigation is needed to identify of the new peaks. From the 104 peaks, using Scherrer’s equation the average crystalline size of the α-Fe2O3 prepared at 800 °C for 1 h and for 6 h are 22 and 10.5 nm, respectively. It can be seen from the TEM images that the morphology of α-Fe2O3 prepared via pyrolysis at 700 °C inside the SiO2 shell is a rod-like structure consisting of small particles. As shown in Figure S10, a large number of very small subunits with a size around 20 nm interconnected tubular structure after calcination at 800 °C. As the calcination time increases, morphology of hematite is disordered. In addition, the length of subunit particles of samples is around 5−20 nm. These observations indicate that assembled structures and crystal phases of α-Fe2O3 confined inside SiO2 shell are affected by the temperature and reaction time. In contrast, the average size of unit particles is dominated by the confinement effect. Thermogravimetric analysis (TGA) was conducted for the Fe-MIL-88A and Fe-MIL-88A@SiO2 particles (Figure S7) to obtain information about their thermal decomposition and to understand the crystallization of the iron oxide structure during the calcination. The TGA curve for Fe-MIL-88A shows a dramatic weight loss at 300 °C and complete decomposition above 430 °C. The possible decomposition mechanism is reported as removal of the water below 100 °C and decomposition of fumarate ligand starting above 200 °C. Subsequently, the weight loss is not observed through 400 °C. In contrast, a slow weight loss was observed for Fe-MIL88A@SiO2, which was complete above 510 °C. We postulate that the rates of heat transfer and gas evolution were reduced in Fe-MIL-88A@SiO2 because external SiO2 shell acted as a heat barrier and mesoporous SiO2 shell limits the amount of gas released. Plots of the magnetization versus the applied magnetic field for the hematite clusters and tubes are displayed in Figure 4. The magnetization of the α-Fe2O3 cluster is 0.86 emu g −1, similar to that of bulk α-Fe2O3 materials (0.3 emu g −1), and shows weak ferromagnetic behavior with a high coercivity of 2775 Oe. In contrast, the magnetization of tubular α-Fe2O3@ SiO2 before and after removal of the SiO2 shell were 10.1 and 10.6 emu g−1, respectively (Figure 4, Figure S10); these values are markedly higher than those of the α-Fe2O3 cluster and the bulk species. The higher magnetization of tubular α-Fe2O3 can be explained by the dependence of the magnetic behavior of hematite on particle size. It is reported that the magnetization Figure 2. (a) Scanning electron microscopy, (b) transmission electron microscopy, and (c) scanning transmission electron microscopy images (STEM) of hollow α-Fe2O3@SiO2. Energy dispersive spectroscopy mapping of (d) Fe and (e) Si of hollow α-Fe2O3@SiO2. Figure 3. (a,b) Transmission electron microscopy images, (c) scanning electron microscopy image, and (d) powder X-ray diffraction pattern of the α-Fe2O3 tubes after removal of the SiO2 shell. Crystal Growth & Design Communication DOI: 10.1021/acs.cgd.7b00547 Cryst. Growth Des. 2017, 17, 4496−4500 4498 of subnano α-Fe2O3 (under 10 nm) particles is much higher than the magnetization of bulk hematite. The increased magnetization of smaller nanoparticles is explained by the surface spin disorder of subnanosized α-Fe2O3 particles more easily aligned in the direction than core spins via the applied magnetic field. In addition, the M−H curve after removal of SiO2 shows zero coercivity, which indicates superparamagnetic behavior. It is also reported that superparamagnetic behavior can be observed above the blocking temperature when the hematite particle size is small enough. Unexpectedly, the coercivity of tubular α-Fe2O3@SiO2 (1141 Oe) is larger than that of SiO2 without α-Fe2O3. We postulated that the magnetic dipole−dipole interaction resulted in increased coercivity. This observation indicates that further study is needed to elucidate what factors increase the coercivity of this system. Thus, the increased magnetization and superparamagnetic behavior of α-Fe2O3 derived from a MOF encased in a SiO2 are strongly indicative of the formation of very small primary