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Cite This: Chem. Mater. 2019, 31, 2263−2268
Synthesis Mechanism of Magnetite Nanorods Containing Ordered Mesocages Hui Wu,† Dongkyu Lee,† Lemma Teshome Tufa,† Jeonghyo Kim,† and Jaebeom Lee*,‡ †
Department of Cogno-Mechatronics Engineering, Pusan National University, Busan 46241, Republic of Korea Department of Chemistry, Chungnam National University, Daejeon 34134, Republic of Korea
‡
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S Supporting Information *
Scheme 1. Synthesis of MNOMa
O
ne-dimensional (1D) nanoscale mesoporous magnetite, which combines the advantages of 1D architecture and mesoporous properties, has attracted increasing attention owing to its unique physical properties such as a large surface area and anisotropic magnetic properties. It has been employed in various applications, including catalysis,1 energy storage,2 magnetic data storage,3 metamaterial assembly,4 and biomedical applications.5−9 1D magnetic nanomaterials, compared to their spherical counterparts, exhibit anisotropic magnetization characteristics in the coercive force, remanence, and hysteresis shape. The anisotropic nature of the elongated shape allows prior mutual interactions as they are wellorganized and assembled, leading to enhanced anisotropic properties.10 The experimental obstacles in the production of monodispersed mesoporous magnetite nanorods may be attributed to the concurrent manipulation of directional growth and pore production. Thermal decomposition of organometallic precursors at an elevated temperature produces magnetic nanoparticles (NPs) with the highest degree of homogeneous 1D structure.11 However, the complex synthesis mechanism, small synthesis batches, and toxic solvents hinder its application. Template-assisted methods have been employed to fabricate anisotropic nanomaterials but are limited by the fabrication and removal of the template.12−16 It has been recently reported that the use of an anisotropic nanomaterial precursor, i.e. preshaped 1D NP precursors, could be an effective and versatile procedure to produce anisotropic and non-Platonic structures which are difficult to grow directionally.17−19 It has been demonstrated that physical and chemical transformations of the anisotropic nanomaterial precursor have an important role to produce iron-based nanorods.20−23 A typical example is the fabrication of silica/titania hybrid nanowires consisting of a silica core with a linear array of mesocages and titania shell, which are self-assembled in a block copolymer soft template inside a nanoporous hard template.24 However, it is still challenging to fabricate magnetite nanorods with linear arrays of mesopores. In this study, a template-free strategy is employed to fabricate magnetite nanorods containing ordered mesocages (MNOM). The overall scheme of the synthesis and plausible mechanism are shown in Scheme 1. MNOM was synthesized by a two-step reaction. First, βFeOOH nanorods were prepared by hydrolysis of an FeCl3 aqueous solution in the presence of polyethyleneimine (PEI) by heating at 90 °C under magnetic stirring for 4 h. Then, the β-FeOOH nanorods were treated with oleylamine and refluxed © 2019 American Chemical Society
a
(a) Template-free synthesis through a two-step reaction. (b) Schematic of porous structure evolution and phase transformation of the β-FeOOH nanorod precursor in the second step.
at 200 °C for another 4 h under protection of nitrogen gas to produce the mesoporous structure. Oleylamine was selected as both reducing agent and solvent owing to the high boiling point and low reducibility,25 which can contribute to maintain the rod structure. Figure 1a presents the TEM image of as-synthesized βFeOOH nanorods with certain rod morphologies and uniform size distribution with an average length of 60 ± 10 nm and average width of 15 ± 5 nm. PEI as surfactant and structure inducer had an essential role in the formation of the rod-like structure, and the FTIR spectrum of β-FeOOH nanorods was shown in Figure S1. After the phase transformation and precise control of the dissolution and recrystallization, MNOM (Figure 1b) were produced, exhibiting well-defined rod morphology. The phase transition and product were further evaluated by XRD. The peaks in the XRD pattern (Figure 1c) were well-matched with those of a standard sample of tetragonal FeOOH (ICDD Card No. 34-1226), confirming Received: January 20, 2019 Revised: March 24, 2019 Published: March 25, 2019 2263
DOI: 10.1021/acs.chemmater.9b00256 Chem. Mater. 2019, 31, 2263−2268
Communication
Chemistry of Materials
magnetic field. The strong magnetic response of the MNOM concentrated them in a spot within a few seconds, while no response was observed in the FeOOH solution for a few days. A high-resolution TEM (HRTEM) image of MNOM from the selected area in Figure 2a is shown in Figure 2b. The d-
Figure 2. Characterization of the MNOM. (a) TEM image, (b) corresponding HRTEM image in the selected area of panel a, (c) SAED pattern, (d) STEM image, (e) electron microscopy image and STEM/EDS elemental maps of Fe and O in the selected area (Fe map in e1, O map in e2), (f) STEM image and corresponding line EDS, (g) EDS analysis, and (h) FTIR spectrum.
Figure 1. Electron microscopy and physical characterizations of the synthesized β-FeOOH nanorod precursors and MNOM. TEM images of (a) β-FeOOH and (b) MNOM. (c) XRD patterns of β-FeOOH nanorods and MNOM. (d) Fe 2p XPS of the β-FeOOH and MNOM. (e) Magnetic hysteresis loops of the MNOM and β-FeOOH nanorods; the inset shows a magnified view of the low-field region. (f) Photograph of the dispersed β-FeOOH and MNOM under an external magnetic field (after 10 s).
spacing is approximately 0.149 nm, which agrees well with the lattice spacing of the (440) planes in the cubic phase of Fe3O4. Moreover, the selected-area electron diffraction (SAED) pattern of the nanorods in the selected area (Figure 2c) exhibits five distinguishable crystal planes of (440), (511), (400), (311), and (220). It indicates that the precursors of βFeOOH were fully converted to the cubic-phase Fe3O4 through the chemical transformation after the reaction over 4 h. The SAED pattern shows that the prepared sample is polycrystalline with a cubic inverse spinel crystal structure, which agrees with the XRD results. Figure 2d shows a scanning transmission electron microscopy (STEM) image of the nanorods. The product exhibits a rod-like shape with mesocages (dark spots in the STEM images marked with yellow arrows) distributed along the axial direction of the nanorod. Each cavity had a quasi-spherical shape with an average radius of 3 nm. Figure 2e1, e2 show STEM elemental maps of Fe and O in the selected area of Figure 2e. Empty or low-concentration areas of Fe and O elements were observed in the nanorod (outlined by the yellow arrows in the image and circular areas in the maps), which can be attributed to the mesocage structure in the nanorod. In addition, energydispersive spectra (EDS) line scans were employed to investigate the element distributions. The tendency of the intensity of Fe element matched with that of the O element; Fe and O were homogeneously distributed in the nanorod. The intensities of the Fe and O elements around 20 nm in Figure 2f are relatively low, which further confirms the presence of
the purity of the product and the absence of other crystalline phases. The XRD patterns of the product obtained from the second step were also recorded (Figure 1c), which matched well with those of a standard sample of cubic Fe3O4 (ICDD Card No. 19-0629). To differentiate the Fe3O4 phase from γ-Fe2O3 phase, the final product was further analyzed by XPS. The Fe 2p XPS spectra of the final product are presented in Figure 1d. The photoelectron peaks of Fe 2p3/2 and Fe 2p1/2 appear at approximately 710.6 and 724.4 eV, respectively, which strongly indicated the existence of Fe3+ oxidation state; they agreed well with the reference data.26 For β-FeOOH (Figure 1d), the peaks of Fe 2p3/2 and Fe 2p1/2 appear at approximately 711.6 and 724.2 eV, respectively. Furthermore, a small satellite peak of the Fe 2p spectrum is observed at 720 eV, which is different from that of Fe3O4; it was consistent with the reported structure of β-FeOOH.27 This further verifies the phase transformation of the β-FeOOH nanorods. Figure 1e shows the magnetic behaviors of the two products. The measured specific saturation magnetization (Ms) of the MNOM was 22.5 emu/g. Additionally, a hysteresis loop was observed in the lowfield region (inset in Figure 1e), which indicated the paramagnetic behavior of the as-prepared MNOM. The hysteresis loop of β-FeOOH shows a typical antiferromagnetic behavior.28 Figure 1f shows a photograph of the dispersed βFeOOH in water and MNOM in hexane under an external 2264
DOI: 10.1021/acs.chemmater.9b00256 Chem. Mater. 2019, 31, 2263−2268
Communication
Chemistry of Materials
shows inhomogeneous NPs with diverse shapes. They are considerably different from the MNOM , as it is hard to control the conversion process and maintain the structure of the nanorods when the reaction temperature is close to the phase transition point. Subsequently, the morphology variations in the nanorods with the reaction time at 120 °C were monitored. The reaction was naturally quenched to 25 °C at different time points (4 and 48 h); the corresponding intermediates were analyzed by TEM, as presented in Figure S4a, b. The pores inside the nanorods with disordered distributions demonstrated that the reaction time was an essential factor to maintain the structures of the nanorods. The results confirmed that the product obtained at 200 °C with the ordered mesocage structure (OMS) consisted of β-FeOOH (Figure S5). This implies that only a physical transformation occurred before the formation of the OMS. As the reaction temperature was maintained at 200 °C for 4 h, the precursor was totally transformed into Fe3O4, and the OMS was retained, which can be attributed to the moderate reducibility of oleylamine. It is of importance to reveal the exact formation mechanism of the OMS during the synthesis. In this regard, the effect of water molecules was studied by drying the precursor of βFeOOH at different temperatures. First, the precursor was dried at 150 °C for 1 h. Then, the temperature- and timedependent structure evolutions of β-FeOOH were characterized by TEM. The temperature-dependent TEM images (Figure S7) show that the OMS did not appear even when the temperature increased to 230 °C. The time-dependent TEM image in Figure S8 does not show an OMS after the reflux in oleylamine at 200 °C for 4 h. Meanwhile, notice that there is no change in morphology, size, or phase composition of FeOOH obtained at 200 °C, which confirms that the coalescence of small pores is accompanied by the material transport. On the basis of the above experimental results, it is plausible that the loss of water molecules adversely affected the formation of the ordered mesoporous structure, which can be related to the dissolution and recrystallization during the heating. To quantify the content of water molecules, a thermogravimetric analysis (TGA) was carried out to characterize the as-prepared products. The TGA curves in Figure S9 show a small weight loss of approximately 2% at 150 °C, which is smaller than that of 7.5% obtained at 25 °C. This suggests that the content of water molecules is crucial for the formation of the OMS. The metastable nature of β-FeOOH and hydrophobic environment created by oleylamine led to the diffusion and penetration of physically adsorbed water molecules into the interior parts of the nanocrystals under the high temperature. Additionally, dissolution and recrystallization occurred during the heating, accompanied by competing evaporation of water molecules and migration. In the process of regrowth, PEI provided an alkaline environment and promoted the recrystallization, while the water molecules acted as a soft template leading to the emergence of micropores owing to the Ostwald ripening.29,30 The water molecules were confined to certain areas to form the pores inside the nanorods. Furthermore, the corresponding HRTEM image (Figures S5−7) at the solid part show stronger and clearer lattice fringe. Meanwhile the intensity of FFT pattern of the solid part (inserted in HRTEM image) is obviously clearer and stronger than that of the cavity part, which demonstrates that the solid part has a crystallinity higher than that of the cavity part.31 With the progress of the reaction, the micropores
mesocages. The EDS results in Figure 2g show that the nanorods were completely converted from β-FeOOH to MNOM. The Fourier-transform infrared (FTIR) spectrum confirms the surface coatings of the MNOM, verifying the binding of oleylamine to Fe3O4 (Figure 2h). The peak at 3741 cm−1 corresponds to the N−H stretching mode of the primary amine, while the peaks at 1455, 1112, and 878 cm−1 were attributed to the NH2 scissoring, C−N stretching, and NH2 wagging modes, respectively. The typical stretching of the N− H bond indicated the existence of PEI. To study the morphological change and phase transition, the reaction was carried out at different reaction temperatures, and the corresponding intermediates were analyzed by TEM images (Figure 3). Notice that the sizes of pores become
Figure 3. Temperature-dependent structure evolution of β-FeOOH, treated and refluxed in oleylamine: (a) 120, (b) 150, (c) 170, (d) 190, (e) 200, (f) 210, (g) 220, (h) 230, and (i) 240 °C. All of the scale bars correspond to 50 nm.
bigger as the reaction temperature increases. Then, the size and distribution of the pores are almost fixed as the temperature reaches 200 °C. These pores have quasi-spherical shape with average radius of 3 nm. The conversion rapidly proceeded when the reaction temperature was heated to 190 °C. Small pores appeared and grew with the increase in the temperature. When the temperature reaches 200 °C, the nanorods appeared to be remarkably porous. Small pores coalesced into several isolated larger pores, which were distributed orderly along the axial direction of the nanorod. It is worth noting that the overall morphology of the ordered mesocages only slightly changed upon the increase in the temperature in the range of 200−230 °C. When the reaction temperature was finally heated to 240 °C, the larger pores finally resulted in structural collapse of the nanorods due to the phase transition, which is consistent with the differential scanning calorimetry (DSC) data, as shown in Figure S2. Another investigation of the phase transition was carried out, with an incubation time of 4 h. When the reaction temperature was heated to 230 °C and was maintained for 4 h, the corresponding TEM image in Figure S3 2265
DOI: 10.1021/acs.chemmater.9b00256 Chem. Mater. 2019, 31, 2263−2268
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Chemistry of Materials
attributed to the different exposed crystal planes and difference in the surface-area-to-volume ratio.33−36 The CV curves of the MNOM at different scan rates (1−30 mV/s) are shown in Figure 4b, signifying that the surface of the electrode was homogeneously electroactive at the lower and higher scan rates. In addition, no significant changes are observed between the CV profiles at different scan rates, suggesting the consistent electrochemical stability of MNOM.37 In summary, we proposed a robust two-step synthesis method of MNOM. Temperature- and process-time-dependent reducing protocols were applied to the anisotropic nanomaterial precursor during the second step of the reaction. We demonstrated that the use of the anisotropic nanomaterial precursor to design and synthesize unique nanostructures could be an effective and versatile approach to produce anisotropic and non-Platonic structures, which are difficult to grow directly. Plausible dissolution and recrystallization mechanisms were proposed. The competing evaporation of water molecules and migration were crucial factors in the formation of the unique OMS. Other factors besides temperature and process time which can facilitate dissolution and recrystallization may provide possible ways to obtain such ordered mesocages. With the combination of 1D and mesoporous properties, these MNOM should have a great potential for magnetic particle-based medical imaging and therapeutic applications. Furthermore, the electrochemical measurement indicated that the MNOM exhibited superior electrochemical properties compared with the NPs. The proposed synthesis strategy may be extended to other similar transition metal oxyhydroxides to grow novel OMS.
collapsed due to the interinfiltration among water, thus leading to the formation of isolated large pores with quasispherical shapes inside the nanorods in the process of dissolution and recrystallization of β-FeOOH. Finally, the OMS with 2−4 large holes formed. Nitrogen adsorption−desorption isotherms were also measured to study the structure evolution of β-FeOOH, which was refluxed in oleylamine (Figures S10−S13). Compared with the specific surface area of all samples, it can be seen that the specific surface areas were gradually decreased due to the emerging of small pores. The value of MNOM is 58.4 m2g−1 and that of Fe3O4 nanoplates (Figure S3) is 30.5 m2g−1. On the basis of the above results, improving the specific surface area of precursor, for example reducing the size of precursor of FeOOH, can be a possible way to increase the specific surface area of MNOM. Meanwhile, to analyze the electrochemical properties of MNOM, cyclic voltammetry (CV) was employed in a three-electrode system using 1 M Na2SO3 as the electrolyte. Figure 4a shows the CV curves of
Figure 4. Electroactivity analysis of the synthesized nanoparticles. (a) Cyclic voltammograms of the MNOM and Fe3O4 nanoplates coated on fluorine-doped tin oxide (FTO) electrodes in 1 M Na2SO3 at a scan rate of 100 mV/s and (b) MNOM coated on an FTO electrode in 1 M Na2SO3 at different scan rates (1−30 mV/s).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b00256.
MNOM electrodes in a potential range of −1.2 to 0 V at a scan rate of 100 mV/s. The shapes of the voltammograms obtained for the Fe3O4 nanorod electrodes are different from the rectangular shape of the electrical double-layer capacitor. The origin of the pseudocapacitive behavior is attributed to the reversible redox reaction of the Fe2+/Fe3+ redox couple.32 The well-defined cathodic and anodic peaks in the potential range of −1.2 to 0 V (vs Ag/AgCl) demonstrate the prominent electrochemical properties of MNOM. The reduction peaks (at −1.1 and −0.9 V) correspond to the reductions in electrochemical conversions of Fe3+/Fe2+ and Fe2+/Fe0, while the oxidation peak (at −0.43 V) is related with stepwise phase changes from Fe0 to Fe3+, which are consistent with reported results for Fe3O4 nanorod and NP electrodes.33,34 To compare the electrochemical activity of MNOM, a CV measurement of Fe3O4 nanoplates (Figure S3) was carried out at the same scan rate (Figure 4a). Regardless of their morphology and surface area, the Fe3O4 structures exhibited electrochemical activities. However, some differences are observed between them in the peak potential and peak current intensities. Peak potential could be an effective parameter to estimate electroactivity of a material; that is, a compound with lower oxidation peak potential means higher electroactivity.35 As proved in Figure 2b and Figure S3, the exposed crystal planes of MNOM and Fe3O4 nanoplate are (440) and (111), respectively. The surface energy of the (111) crystal planes in cubic Fe3O4 is lowest, which indicates (440) facet has higher surface energy and is more electroactive.36 Therefore, the MNOMs are more electroactive than the Fe3O4 nanoplates, which might be
Materials synthesis, characterization data, and additional details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hui Wu: 0000-0002-0691-6349 Dongkyu Lee: 0000-0003-3812-9961 Lemma Teshome Tufa: 0000-0001-6929-8464 Jeonghyo Kim: 0000-0003-3363-0714 Jaebeom Lee: 0000-0002-4563-2883 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the National Research Foundation (NRF) of Korea, grant-funded by the Korean Government (MSIP) (NRF-2016R1A2B4012072 and NRF2017R1A4A1015627) and by the International Research and Development Program of the NRF of Korea, funded by the Ministry of Science and Information and Communication Technology of Korea (NRF-2017K1A3A1A30084348). 2266
DOI: 10.1021/acs.chemmater.9b00256 Chem. Mater. 2019, 31, 2263−2268
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