materials Article
Synthesis of Macroporous Magnetic Fe3O4 Microparticles Via a Novel Organic Matter Assisted Open-Cell Hollow Sphere Assembly Method Huixia Wang, Ximing Pu, Yaquan Zhou, Xianchun Chen, Xiaoming Liao, Zhongbing Huang and Guangfu Yin * College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China;
[email protected] (H.W.);
[email protected] (X.P.);
[email protected] (Y.Z.);
[email protected] (X.C.);
[email protected] (X.L.);
[email protected] (Z.H.) * Correspondence:
[email protected]; Tel.: +86-28-8541-3003 Received: 30 July 2018; Accepted: 14 August 2018; Published: 23 August 2018
Abstract: Macroporous magnetic Fe3 O4 microparticles, which might act as both drug carriers and magnetocaloric media, were expected to have broad application prospects on magnetocaloric-responsively controlled drug release systems. A kind of macroporous magnetic Fe3 O4 microparticle was prepared by an organic matter assisted open-cell hollow sphere (hollow sphere with holes on shell) assembly method in this study. 1-vinyl-2-pyrrolidinone (NVP) and 2-acrylamido-2-methyl propane sulfonic acid (AMPS) were selected as the template and the binder, respectively. Ferrous ions were specifically bound to carbonyl groups on NVP and were then reduced by NaBH4 . The reduced irons underwent heterogeneous nucleation and grain growth to form Fe0 /Fe3 O4 microspheres consisting of a lot of nano-Fe0 grains, and were then assembled into Fe0 /Fe3 O4 microparticles wrapped by AMPS. Results indicate that NVP binding with ferrous ions can promote a self-polymerization process and the formation of Fe0 /Fe3 O4 microspheres, while AMPS enwrapping around the resultant microspheres can facilitate their assembly into larger aggregates. As a result, macroporous Fe3 O4 microparticles composed of several open-cell hollow Fe3 O4 microspheres can be obtained under a Kirkendall-controlled oxidation. Moreover, these as-prepared macroporous Fe3 O4 microparticles possess a narrow particle size distribution and exhibit ferromagnetism (Ms = 66.14 emu/g, Mr = 6.33 emu/g, and Hc = 105.32 Oe). Our work, described here, would open up a novel synthesis method to assemble macroporous magnetic Fe3 O4 microparticles for potential application in magnetocaloric-responsively controlled drug release systems. Keywords: macroporous Fe3 O4 microparticles; open-cell hollow microsphere; nanoscale Kirkendall effect; 1-vinyl-2-pyrrolidinone; 2-acrylamido-2-methyl propane sulfonic acid
1. Introduction Nowadays, the nanoscale/microscale porous particles of transition metal oxides have attracted great attention due to their specific optical, electrical, and magnetic performances derived from the d-layer orbitals with unfilled valence, as well as their unique absorptivity, penetrability, and chemical activity—resulting from their porous structure [1–4]. Among them, the porous magnetite (ferroferric oxide, Fe3 O4 , one of the transition metal oxides) has been widely applied in catalysis, electrode, microwave absorption, and separation, owing to its low cytotoxicity, adjustable magnetism, high loading capacity, and long circulation [5–8]. Depending on the pore size, porous materials can be classified into microporous (pore diameter < 2 nm), mesoporous (2–50 nm), and macroporous materials (>50 nm). Microporous Materials 2018, 11, 1508; doi:10.3390/ma11091508
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material notes were easy to get and are not the focus of our attention. Mesoporous materials have emerged as the most widely studied porous materials in recent years, and various methods for preparing mesoporous materials have been reported [9–13]. However, there are few studies on macroporous materials, which have larger pore diameters and possess a wider application prospect than mesoporous materials in the field of loading and the transport of large-sized particles, especially organic nanoparticles. For instance, macroporous Fe3 O4 micro/nanoparticles are able to act as carriers for loading and transporting drug-loaded temperature-sensitive micelles in magnetocaloric-responsively controlled drug release systems. On the one hand, macroporous Fe3 O4 micro/nanoparticles would improve the drawbacks of drug loss in micelles, and on the other hand, they would also provide a heat source for the control of temperature-sensitive materials. Hence, the development of macroporous Fe3 O4 micro/nanoparticles is of importance to achieve an effective and controlled release of drugs [14]. At present, the preparations of macroporous materials mostly rely on adopting the colloidal crystal template method. However, the colloidal crystal template is usually obtained by the self-assembly of organic microspheres, and the size of the template is far exceeded by the micron scale, thus leading to the colloidal crystal template method being just as suitable for the preparation of the macroporous bulk materials—rather than the macroporous microparticles [15–17]. Therefore, there is, so far, still a lack of an efficient synthesis approach to prepare macroporous transition metal oxides-based microparticles. In recent years, various hollow nanospheres have been reported in succession [18–20], and some nano or submicron scale hollow Fe3 O4 spheres have been prepared through the template method [21], the hydrothermal method [22], the self-assembly method [23], and the controlled oxidation method [24]. Among these hollow spheres synthesis methods, the controlled oxidation method, based on the nanoscale Kirkendall effect (NKE), has attracted widespread attention because of its controllability in outer and inner diameters of hollow spheres. The Kirkendall controlled oxidation method is involved in two main processes. The first process is the synthesis of Fe0 /Fe3 O4 particles, in which the Fe0 particles are first synthesized and then the Fe0 /Fe3 O4 core-shell particles are formed after the surficial oxidation is finished. The second process is the oxidation and cavitation of Fe0 /Fe3 O4 particles, in which case the Fe atoms in the core preferentially diffuse to the surface at the elevated temperature under the action of NKE, thus resulting in a hollowing Fe3 O4 structure [25–27]. The morphology and size of the Fe0 /Fe3 O4 core-shell particles, formed in the first step, determine the size and pore diameter of the final hollow Fe3 O4 spheres. As the spheres are gradually hollowed during the oxidation process, some holes might appear on the shell to form the open-cell hollow sphere (hollow sphere with holes on the shell). Based on the open-cell hollow nanospheres, a potential strategy of assembling the open-cell hollow microspheres/nanospheres into macroporous microparticles (the aggregate of open-cell hollow microspheres/nanospheres in micron/submicron scale) is proposed in our research. There are two paths to assemble hollow spheres into macroporous aggregates, including the hollowing-first method and the assembling-first method. Some troubles might exist in the direct assembly of the open-cell hollow Fe3 O4 microspheres into macroporous aggregates because the holes on the shell is probably sealed up by the binder, and the aggregation of open-cell hollow Fe3 O4 microspheres is difficult to maintain once the binder is removed through calcination. In contrast, it would be practicable that the Fe0 /Fe3 O4 microspheres are pre-assembled into aggregates and then oxidized and hollowed in consideration of the local fusion of adjacent microspheres, which are the result of the grain growth during the oxidation process that is propitious to the maintenance of macroporous aggregates. In this study, a kind of macroporous magnetic Fe3 O4 microparticles, composed of Fe3 O4 open-cell hollow microspheres with sizes ranging from several hundred nanometers to several microns, were prepared by a novel organic matter assisted open-cell hollow sphere assembly method, in which 1-vinyl-2-pyrrolidinone (NVP) and 2-acrylamido-2-methyl propane sulfonic acid (AMPS) were selected as the template and the binder, respectively. The effects of NVP and AMPS additions on the composition and morphology of the Fe0 /Fe3 O4 microspheres and the corresponding aggregates were explored in
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detail. Moreover, the regulatory mechanisms of NVP and AMPS on the formation and assembly of Fe0 /Fe3 O4 microspheres were preliminary discussed. In addition, the morphology, pore structure, and magnetic performance of macroporous Fe3 O4 aggregates upon the Kirkendall controlled oxidation were also investigated. The results show that the macroporous Fe3 O4 microparticles, in the micron scale, were prepared by the assembly of the Fe3 O4 open-cell hollow microspheres—in the assembling-first way—whilst the organic monomers of NVP and AMPS played a crucial role in the formation of the Fe0 /Fe3 O4 microspheres and their assembly into Fe0 /Fe3 O4 microparticles. 2. Materials and Methods 2.1. Raw Materials and Reagents Two kinds of organic monomers, 1-vinyl-2-pyrrolidinone (C6 H9 NO, NVP) with 99.5% purity that is stabilized with 4-Methoxyphenol (MEHQ), and 2-acrylamido-2-methyl propane sulfonic acid (C7 H13 NO4 S, AMPS) with 98% purity, were purchased from J & K Scientific Ltd., Beijing, China. Trimethylamine N-oxide (C3 H9 NO, TMAO) with 98% purity was the oxygen transfer agent used in the solvothermal reaction and was purchased from Sigma Aldrich Trading Ltd., Burlington, MA, USA. Iron chloride tetrahydrate (FeCl2 ·4H2 O) and sodium borohydride (NaBH4 ), as well as other conventional reagents were analytical reagents without further purification and were purchased from Kelong Chemicals, Chengdu, China. All reagents were used without further purification. 2.2. Synthesis of Fe0 /Fe3 O4 Microparticles In a typical synthesis, 0.05 g AMPS, 25 µL NVP, and 1.243 g iron chloride tetrahydrate (FeCl2 ·4H2 O) was dissolved in 50 mL aqueous under mechanical stirring, followed by 30 min ultrasonic oscillation for complete dissolution. 20 mL 0.54 mol/L NaBH4 aqueous solution was slowly dripped into the FeCl2 solution under vigorously mechanical agitation. After the completion of the dripping, agitation was continued for 30 min. The precipitate was washed with deionized water and ethanol several times, and then vacuum dried at room temperature for 8 h to obtain the Fe0 /Fe3 O4 microparticles marked as P0.05 25 . The superscript indicated the quality of AMPS in grams and the subscripts indicated the volume of NVP in microliters. A series of experiments based on the control variable method were conducted to investigate the effects of the two organic matters on the formation of the Fe0 /Fe3 O4 microparticles. In the first group, the AMPS addition was fixed at 0.05 g, and the NVP additions were determined as 0, 12.5, 25, 50, and 100 µL, respectively. While in the second group, the NVP addition was fixed at 25 µL, and the AMPS additions were selected as 0, 0.01, 0.05, 0.5, and 1 g, respectively. 2.3. Synthesis of Macroporous Fe3 O4 Microparticles 0.16 g of Fe0 /Fe3 O4 microparticles P0.05 25 was dispersed in a mixed solution of 55 mL of hexane and 5 mL of ethanol dissolved with 0.06 g of TMAO. Then, the mixed slurry was decanted into a 100-mL Teflon lined stainless steel autoclave, sealed, and preserved at 180 ◦ C for 4 h. After cooling to room temperature, the precipitate was washed by deionized water and ethanol and dried in a vacuum oven for 8 h (marked as I0.05 25 ). After calcination at 400 ◦ C for 3 h in a nitrogen atmosphere, the macroporous Fe3 O4 microparticles were obtained (marked as F0.05 25 ). 2.4. Characterization The phase composition of prepared powders was determined by X-ray diffractometer (XRD6100, Shimadzu, Kyoto, Japan) with Cu Kα radiation (λ = 0.154 nm) at a scanning rate of 5◦ /min for 2θ ranging from 10◦ to 80◦ and X-ray photoelectron spectroscope (XPS, Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). The existence forms of the two organic matters were analyzed by the Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, Bruker Optics, Ettlingen, Germany) using the KBr method. Morphological observation was carried out by scanning electron microscopy
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(SEM, JSM-7500F, JEOL Ltd., Tokyo, Japan) at a voltage of 15 kV and a low-resolution/high-resolution transmission electron microscopy (TEM/HRTEM, Zeiss Libra 200FE, Carl Zeiss, Jena, Germany) at Materials 2018, 11, x revised file 2 4 of 12 voltage of 200 kV, respectively. Element distribution was characterized by an energy dispersive X-ray Zeiss, Jena,(EDXS, Germany) at voltage of 200 kV,San respectively. Element characterized electron by spectroscope Super-X, EDAX Inc., Diego, CA, USA)distribution attached towas a transmission an energy(TEM, dispersive X-ray spectroscope (EDXS, Super-X, EDAX Inc., San CA,size USA)was attached microscopy Zeiss Libra 200FE, Carl Zeiss, Jena, Germany). TheDiego, particle measured a transmission electron microscopy (TEM, Zeiss 3000E, Libra 200FE, CarlInstruments Zeiss, Jena, Germany). The UK) by ato laser particle size analyzer (LPSA, Mastersizer Malvern Ltd., Malvern, particle size was measured by a laser particle size analyzer (LPSA, Mastersizer 3000E, Malvern using water as the dispersant. Nitrogen adsorption-desorption isotherms were obtained with a Instruments Ltd., Malvern, UK) using water as the dispersant. Nitrogen adsorption-desorption high performance micropore analyzer (Kubo-X1000, Beijing Biaode Electronic Techology Co. Ltd., isotherms were obtained with a high performance micropore analyzer (Kubo-X1000, Beijing Biaode Beijing, China) at 77 K. Hysteresis loop was obtained by vibrating sample magnetometer (VSM, Electronic Techology Co. Ltd., Beijing, China) at 77 K. Hysteresis loop was obtained by vibrating Lakeshore Lake Shore Cryotronics USA) at room temperature. sample 7410, magnetometer (VSM, LakeshoreInc., 7410,Columbus, Lake Shore OH, Cryotronics Inc., Columbus, OH, USA) at room temperature.
3. Results and Discussion 3. Results and Discussion
3.1. Composition and Morphology of Fe0 /Fe3 O4 Microparticles 3.1. Composition and Morphology of Fe0/Fe3O4 Microparticles
The crystal phase composition of the microparticles P0.05 25 , obtained by reduction, The crystal composition of XRD the microparticles P0.05 25, obtained by reduction, was was investigated by phase XRD and XPS. In the pattern (Figure 1a), the diffraction peaks centered at investigated by XRD and XPS. In the XRD pattern (Figure 1a), the diffraction peaks centered at 44.7° ◦ 0 44.7 is indexed to the characteristic peaks (110) of Fe (JCPDS NO. 06-0696), while the peaks centered is indexed to the characteristic peaks (110) of Fe0 (JCPDS NO. 06-0696), while the peaks centered at at 35.5◦ and 62.6◦ are indexed to the characteristic peaks (311) and (440) of Fe3 O4 (JCPDS NO. 88-0866), 35.5° and 62.6° are indexed to the characteristic peaks (311) and (440) of Fe3O4 (JCPDS NO. 88-0866), respectively. The characteristic peaks of Fe2 O3 are absent in the XRD pattern, and there are no specific respectively. The characteristic peaks of Fe2O3 are absent in the XRD pattern, and there are no specific satellite structures of ferric oxide in the resolution Fe 2p spectroscopy (Figure 1b)—verifying satellite structures of ferric oxide in high the high resolution FeXPS 2p XPS spectroscopy (Figure 1b)— the absence γ-Fe O33 or [28,29]. the oxide in the sample is pure Fe3 O4Fe . Moreover, 2 O3 or verifyingofthe absence ofα-Fe γ-Fe22O α-Fe2OTherefore, 3 [28,29]. Therefore, the oxide in the sample is pure 3O4. 0 are much the characteristic of Fe0 are much higher than higher that of than Fe3 Othat that the microparticles Moreover, the peaks characteristic peaks of Fe of Fe3O4, indicating that the 4 , indicating 0 and 0 and a of microparticles are mainly composed large amount of Fe small of Fe3O 4, resulting are mainly composed of a large amountofofa Fe a small amount Fe3 amount O4 , resulting from the surficial 0/Fe3O4 microparticles). 0 from the surficial oxidation in the atmosphere (expressed with Fe oxidation in the atmosphere (expressed with Fe /Fe3 O4 microparticles).
Figure 1. pattern (a) and(a) Fe 2p XPSFehigh spectroscopy (b)spectroscopy of Fe /Fe3O4 microparticles Figure 1. XRD XRD pattern and 2presolution XPS high resolution (b) of Fe0P/Fe253. O4 0.05 microparticles P 25 . The morphology and microstructure of P0.0525 were explored by SEM, TEM, and LPSA. The SEM photograph (Figure 2a) reveals that the microparticles are obtained by the aggregation of the The morphology P0.05 25 were explored by SEM, TEM, LPSA. submicron spheres and withmicrostructure a rough surface.ofMoreover, a TEM photograph (Figure 2b)and shows that The the SEM photograph (Figure 2a) reveals that the in microparticles are obtained the aggregation of theto submicron single submicron sphere is wrapped the floccules that could be by organic AMPS, according the laterwith analysis. With the element distribution EDXS element mapping images, in Figure spheres a rough surface. Moreover, a TEMfrom photograph (Figure 2b) shows thatshown the single submicron 2c, is wewrapped can see that the floccules submicronthat spheres have a distinctAMPS, core-shell structure, which interiorWith of the sphere in the could be organic according toin the laterthe analysis. the sphere is rich in the Fe element, while the surface has more O elements. We believe that the core element distribution from EDXS element mapping images, shown in Figure 2c, we can see that the material of the core-shell structure is mainly Fe0, while the shell material is composed of Fe3O4 and submicron spheres have a distinct core-shell structure, in which the interior of the sphere is rich organic matter. Simultaneously, a HRTEM image (Figure 2d) of the materials surface, taken from the in the Fe element, while the surface has more O elements. We believe that the core material of the black square area in Figure 2b, 0further shows that the surface of the submicron sphere is amorphous core-shell structure isby mainly , while the shell material is composed of Fe organic matter. (the area indicated the redFearrow), whilst interspersed with a large number of3 O Fe43Oand 4 nanocrystals Simultaneously, a HRTEM image (Figure 2d) of the materials surface, taken from the black issquare (the area indicated by the blue arrow). The interplanar crystal spacing of the nanocrystals areadetermined in Figure 2b, thattothe surface the3Osubmicron sphere is amorphous (the area to befurther 0.48 nm,shows belonging (111) Planesof of Fe 4. Furthermore, the LPSA analysis results (Figureby 2e)the also shows that whilst there are two particle size concentrated ± 0.05 μm and indicated red arrow), interspersed withdistributions a large number of Fe3 Oat4 0.17 nanocrystals (the area 1.50 ± by 0.50the μm, respectively. Combining withcrystal the results of the and TEM observations, it is to be indicated blue arrow). The interplanar spacing of SEM the nanocrystals is determined that the small Planes amountofofFesubmicron particles (about 0.17 analysis μm) are results the unaggregated 0.48 confirmed nm, belonging to (111) O . Furthermore, the LPSA (Figure 2e) also 0
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0.05
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shows that there are two particle size distributions concentrated at 0.17 ± 0.05 µm and 1.50 ± 0.50 µm, Materials Combining 2018, 11, x revisedwith file 2 the results of the SEM and TEM observations, it is confirmed 5 of 12 that the respectively. 0 small amount of submicron particles (about 0.17 µm) are the unaggregated Fe /Fe3 O4 microspheres, Fe0/Fe3O4 microspheres, while the dominant micron particles (about 1.5 μm) are the aggregates of while the dominant micron particles (about 1.5 µm) are the aggregates of microspheres. All these microspheres. these Materials 2018, 11, xAll revised file 2results demonstrate that some submicron Fe0/Fe3O4 microspheres could 5 of 12 0 /Fe O microspheres could assemble into the micron results assemble demonstrate that some submicron Fe 3 4 microparticles) 4 into the micron aggregates (i.e., Fe0/Fe3O under the assistance of organic 0/Fe Fe 3O4 microspheres, while the dominant micron (about 1.5 μm) are the aggregates of aggregates (i.e., Fe0 /Fe3 O4 microparticles) under theparticles assistance of organic matters. matters. microspheres. All these results demonstrate that some submicron Fe0/Fe3O4 microspheres could assemble into the micron aggregates (i.e., Fe0/Fe3O4 microparticles) under the assistance of organic matters.
Figure 2. SEM (a); TEM (b); mapping images (c); and HRTEM (d) photographs (black arrow:
Figure background; 2. SEM (a); TEM (b); mapping images (c); and HRTEM (d) photographs (black arrow: red arrow: amorphous; blue arrow: crystal grains); and particle size distribution by 0/Fe3O4 amorphous; 0.0525. arrow: crystal grains); and particle size distribution by background; red arrow: LPSA (e) of Fe microparticles Pblue LPSA (e) of Fe2.0 /Fe P0.05images Figure SEM3 O(a); TEM (b); mapping (c); and HRTEM (d) photographs (black arrow: 4 microparticles 25 . 3.2. Effects of Organic Matters on Fe0/Fe 3O4 arrow: Microparticles background; red arrow: amorphous; blue crystal grains); and particle size distribution by 25. LPSA (e) of Fe /Fe3O4 microparticles P Microparticles 3.2. Effects of Matters on based Fe0 /Feon A Organic series of experiments 3 Othe 4 control variable method were conducted to investigate the 0
0.05
effects of the NVP and AMPS organic matters on the formation of the Fe0/Fe3O4 microparticle. The
Effects Organic Matters on Fe 3O4control Microparticles A 3.2. series of of experiments based on0/Fe the variable method were conducted to investigate crystal compositions of the Fe0/Fe3O4 microparticles (P0.050, P0.0512.5, P0.0525, P0.0550, and P0.05100) which were the effects of the NVP and AMPS organic matters on formation of thefixed Fe0to/Fe A series under of experiments the control were conducted investigate the 3 Omg, 4 microparticle. synthesized different based NVP on additions, butvariable with the themethod addition of AMPS at 50 were 0 0.05 0.05 0.05 0.05 0.05 0 effects of theby NVP and matters on the(Figure formation of the Fe /Fe microparticle. The The crystal compositions ofAMPS the Feorganic /Fe (P3a), similar , 3PO , P phase determined XRD. According to the XRD patterns to P0.05 25,4 the of P 0, P 12.5 25crystal 50 , and 100 ) 3O 4 microparticles 0 0.05 0.05 0.05 0.05 0.05 0 crystal compositions of Fe /Fe3to O4Fe microparticles (P the 0, P increase 12.5, P of 25,NVP Paddition 50addition, , and Pof AMPS 100 ) which were other synthesized samples can allunder bethe indexed and Fe 3O4. With the intensity ofat 50 mg, which were different NVP additions, but with the fixed synthesized under different NVPdecrease additions, but addition of AMPS fixed at 50 0.05 were the diffraction gradually thewith Fe0 the phase diffraction peaks, especially at 44.7°, were determined bypeaks XRD. According to theand XRD patterns (Figure 3a), similar to P mg, 25 , the crystal 0.0525, the crystal phase of determined by XRD. According to the XRD patterns (Figure 3a), similar to P became wider, indicating that the growth of crystalline grains could be inhibited by NVP so that the phase of other samples can all be to indexed to Fe0 and Fe the increase of NVP addition, 3 O4 . With 0/Fe other samplesofcan indexed Fe0 and decreases Fe3O4. With thethe increase of NVP addition, the intensity of crystallinity theall Febe 3O4 microparticle with increase of NVP addition. 0 phase the intensity of the diffraction peaks gradually decrease and the Fe diffraction especially 0 the diffraction peaks gradually decrease and the Fe phase diffraction peaks, especiallypeaks, at 44.7°, ◦ at 44.7 became , became wider, indicating that the growth of crystalline couldbybeNVP inhibited by NVP so wider, indicating that the growth of crystalline grains couldgrains be inhibited so that the 0/Fe3O of the microparticle decreases with the increase of NVP addition. O microparticle decreases with the increase of NVP addition. that thecrystallinity crystallinity of Fe the Fe04/Fe 3 4
Figure 3. XRD patterns (a), SEM images (b–f) of Fe0/Fe3O4 microparticles synthesized under different NVP addition with a fixed AMPS addition of 0.05 g: (b) P0.050, (c) P0.0512.5, (d) P0.0525, (e) P0.0550, and (f) P0.05100, respectively. Figure 3. XRD patterns (a), SEM images (b–f) of Fe0/Fe3O4 microparticles synthesized under different Figure NVP 3. XRD patterns SEM images (b–f)ofof0.05 Fe0g:/Fe O0.05 0.0525, (e) P0.0550,under 4 0microparticles addition with (a), a fixed AMPS addition (b)3 P , (c) P0.0512.5, (d) Psynthesized and (f)different 0.05 0.05 0.05 NVP addition with a fixed AMPS addition of 0.05 g: (b) P 0 , (c) P 12.5 , (d) P 25 , (e) P0.05 50 , P0.05100, respectively.
and (f) P0.05 100 , respectively.
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0/Fe The 3O4 microparticles, synthesized under different NVP additions, were 0 /Fe Themorphologies morphologiesofofFeFe 3 O4 microparticles, synthesized under different NVP additions, observed by SEM as shown in Figure 3b–f. The 3b–f. Fe0/Fe3The O4 microparticles prepared without NVP addition were observed by SEM as shown in Figure Fe0 /Fe3 O4 microparticles prepared without mainly exists in the form of floccules, and can only be observed with a few particles (Figure 3b). After NVP addition mainly exists in the form of floccules, and can only be observed with a few particles introducing 12.5 μL NVP (Figure 3c), a large amount of submicron spheres, with an average diameter (Figure 3b). After introducing 12.5 µL NVP (Figure 3c), a large amount of submicron spheres, with an of about diameter 300 nm appear, these submicron spheres assemble intoassemble larger micron aggregates average of about and 300 nm appear, and these submicron spheres into larger micron wrapped by some floccules of the polymerized AMPS. With the increase of NVP addition, the size of aggregates wrapped by some floccules of the polymerized AMPS. With the increase of NVP addition, microspheres gradually increases, and in particular, it enlarges even to 1 μm when the NVP addition the size of microspheres gradually increases, and in particular, it enlarges even to 1 µm when the NVP increases to 50 μL to (Figure Thereafter, the sizethe of size the microspheres has nohas change even even if theif addition increases 50 µL3d,e). (Figure 3d,e). Thereafter, of the microspheres no change NVP addition is as high as 100 μL, however, a large amount of irregular sheets appeared in the NVP addition is as high as 100 µL, however, a large amount of irregular sheets appeared inthe the aggregates, aggregates,which whichcan canbe beinferred inferredto tobe bethe theexcess excessNVP NVP (Figure (Figure 3f). 3f). 0 125) Figure 3O4 microparticles (P0250 , P0.01250.01 , P0.05, 25 ,0.05 P0.525, ,PP0.5 Figure4a 4ashows showsthe theXRD XRDpatterns patternsof of the the Fe Fe0/Fe /Fe 3 O4 microparticles (P 25 , P 25 P 25 25 , synthesized underunder different AMPS additions while the the NVP addition is fixed at at 2525 μL. P1 25 ) synthesized different AMPS additions while NVP addition is fixed µL.As Ascan canbe be clearly crystal phase of the Fe0/Fe O4 microparticles can be indexed to Fe0 and Fe03O4, but the clearlyseen, seen,the the crystal phase of the Fe03/Fe 3 O4 microparticles can be indexed to Fe and Fe3 O4 , 0 crystallinity of Fe /Fe3of O4Fe microparticles is gradually withalong the increase AMPS 0 /Fe O microparticles but the crystallinity is enhanced gradually along enhanced with theofincrease 3 4 0/Fe3O4 additions, implying that the existence of AMPS is conducive to the assembly of the Fe of AMPS additions, implying that the existence of AMPS is conducive to the assembly of the microspheres. Fe0 /Fe O microspheres.
3
4
Figure 4. XRD patterns (a); SEM photographs (b–f) of Fe00/Fe3O4 microparticles synthesized under Figure 4. XRD patterns (a); SEM photographs (b–f) of Fe /Fe3 O4 microparticles synthesized under different AMPS additions with a fixed NVP addition of 25 μL: (b)0P025; (c) 0.01 P0.0125; (d) P0.0525; (e) P0.525; different AMPS additions with a fixed NVP addition of 25 µL: (b) P 25 ; (c) P 25 ; (d) P0.05 25 ; (e) P0.5 25 ; 1 and (f) P 25 , respectively. and (f) P1 25 , respectively.
Figure 4b–f presents the SEM images of Fe0/Fe3O4 microparticles (P025, P0.0125, P0.0525, P0.525, P125). In 0 /Fe O microparticles (P0 , P0.01 , P0.05 , P0.5 , Figure of 4b–f presents the SEM precipitates images of Feare 3 4 25 components 25 25 about 25 the absence AMPS, the prepared stacked loosely by blocky of 1 ). In the absence of AMPS, the prepared precipitates are stacked loosely by blocky components of P 25 100 nm in size (Figure 4b). Then, some irregular aggregates composed of microspheres with a size of about100 100nm nmappear in sizeafter (Figure 4b). 0.01 Then, some irregular aggregates composed with about adding g of AMPS, (Figure 4c). With the increaseofofmicrospheres AMPS additions, a size of about 100 nm appear after adding 0.01 g of AMPS, (Figure 4c). With the increase of AMPS the microsphere gradually increases to 500 nm in size and is wrapped by some floccules, meanwhile, additions, thegradually microsphere gradually increases to 500 nm in4d,e). size and is wrapped byaddition some floccules, the aggregate becomes denser and larger (Figure Under the AMPS of 1 g, meanwhile, the aggregate gradually becomes denser and larger (Figure 4d,e). Under the AMPS the microspheres are aggregated more seriously and even the rod-like aggregates that are more than addition of 1 g, the microspheres are aggregated more seriously and even the rod-like aggregates that 10 μm in size can be observed (Figure 4f). are more than 10 µm in size can be observed The above experimental results indicate (Figure that the4f). two organic monomers played different roles The above experimental results indicate that the two organic monomers different in regulating the formation of the Fe0/Fe3O4 microparticles, that is, the NVPplayed is conducive toroles the 0 in regulating the formation of the Fe /Fe O microparticles, that is, the NVP is conducive to the 0 0 3 4 formation of the Fe /Fe3O4 microsphere, while the AMPS facilitates the assembly of the Fe /Fe3O 4 0 /Fe O microsphere, while the AMPS facilitates the assembly of the Fe0 /Fe O formation of the Fe 0 3 4 3 4 microsphere into aggregates (Fe /Fe3O4 microparticles). microsphere into aggregates (Fe0 /Fe3 O4 microparticles).
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0 3.3. Mechanism of NVP andand AMPS on Fe 4 3Microparticles 3.3.Regulatory Regulatory Mechanism of NVP AMPS on/Fe Fe30O /Fe O4 Microparticles As mentioned above, above,the the organic monomers and AMPS play roles crucial roles in the As mentioned organic monomers NVPNVP and AMPS play crucial in the formation 0 /Fe O microspheres and the assembly of the Fe0 /Fe O microspheres into formation of the Fe 3 4 3 4 of the Fe0/Fe3O4 microspheres and the assembly of the Fe0/Fe3O4 microspheres into Fe0/Fe3O4 0 /Fe O microparticles. Fe 3 4 microparticles. NVP NVPisisaawater-soluble water-solublevinyl vinylmonomer monomerand andisisprone proneto topolymerization polymerizationto topolyvinylpyrrolidone polyvinylpyrrolidone (PVP). The highly charged O on the carbonyl of NVP and its polymer PVP molecules (PVP). The highly charged O on the carbonyl of NVP and its polymer PVP moleculescan canchemically chemically bind bind to tothe themetal metalions ionsin inthe thesolution solution[30]. [30].Also, Also,as asaawater-soluble water-solublevinyl vinyl monomer, monomer, AMPS AMPS isiseasily easily polymerized aqueous media, media,and andthe thepolymeric polymericproduct product exhibits high temperature endurance polymerized in in aqueous exhibits high temperature endurance and and hydrolysis resistance. While there also exists a carbonyl group in chemical structures, the AMPS hydrolysis resistance. While there also exists a carbonyl group in chemical structures, the AMPS molecule moleculeisishard hardto tobind bindto tometal metalions ionsdue dueto tothe thestrong strongsalt saltresistance resistanceresulting resultingfrom fromthe thesalt-insensitive salt-insensitive sulfonic acid group [31]. sulfonic acid group [31]. When When the the iron iron chloride chloride tetrahydrate, tetrahydrate, NVP, NVP, and and AMPS AMPS were were dissolved dissolved together together in in aqueous aqueous solution, the ferrous ions were preferentially coordinated with the carbonyl (C=O) on NVP, meanwhile solution, the ferrous ions were preferentially coordinated with the carbonyl (C=O) on NVP, the NVP molecules gradually Subsequently, the coordinated ferrous ionsferrous were meanwhile the NVPwere molecules werepolymerized. gradually polymerized. Subsequently, the coordinated reduced byreduced sodiumby borohydride, leading toleading the heterogeneous nucleation and the and growth of the ions were sodium borohydride, to the heterogeneous nucleation the growth 0 grain. Because of the binding effect of NVP molecules, the Fe0 grain growth was greatly restrained. Fe of the Fe0 grain. Because of the binding effect of NVP molecules, the Fe0 grain growth was greatly 0 Upon the polymerization of NVP, a lot ofofnano-Fe to0NVP, could be aggregated together restrained. Upon the polymerization NVP, agrains, lot of bound nano-Fe grains, bound to NVP, could be 0 into submicron Fe microspheres mingled with a NVP mingled polymer.with On the other hand, the aggregated together into submicron Fe0 microspheres a NVP polymer. Ongradually the other polymerized AMPS were wrapped outside thewrapped Fe0 microspheres, a number of the submicron hand, the gradually polymerized AMPS were outside theand Fe0 microspheres, and a number 0 0 Fe microspheres could be assembled into the Fe microparticles in the micron scale during the further 0 0 of the submicron Fe microspheres could be assembled into the Fe microparticles in the micron scale polymerization of AMPS. In addition, the slight oxidation might take place on the surface during the further polymerization of AMPS. In addition, the slight oxidation might take place of onthe the 0 microspheres in the atmosphere so as to obtain Fe0 /Fe O microparticles. This regulatory formed Fe 3 4 0 0 surface of the formed Fe microspheres in the atmosphere so as to obtain Fe /Fe3O4 microparticles. mechanism is consistent with the experimental about results the effects NVP andofAMPS on This regulatory mechanism is consistent with the results experimental aboutofthe effects NVP and 0 /Fe O microparticles. Therefore, the proposed regulatory mechanism of NVP and AMPS on Fe AMPS3 on4 Fe0/Fe3O4 microparticles. Therefore, the proposed regulatory mechanism of NVP and the formation Fe0 /Fe3 O microparticles is illustrated in Figure in 5. Figure 5. 0/Fe3O4 microparticles AMPS on the of formation of4 Fe is illustrated
Figure5.5. Diagrammatic Diagrammatic sketch sketch for for speculative speculative regulatory regulatory mechanism mechanism of of NVP NVP and and AMPS AMPS on on the the Figure 0 formationof ofFe Fe0 /Fe /Fe3O 4 microparticles. formation 3 O4 microparticles. 0/Fe3O4 microparticles Figure 6 shows the infrared absorption spectra of AMPS, NVP, and Fe 0 /Fe Figure 60.05shows the infrared absorption spectra of AMPS, NVP, and Fe O4 microparticles including P 0, P025, and P0.0525 synthesized under different additions of NVP and 3AMPS. Clearly, the including P0.05 0 , P0 25 , and P0.05 25 synthesized under different additions of NVP and AMPS. Clearly, −1 0.05 C–N absorption band of NVP at 1285 cm ,−which is absent in the spectrum of P 0, but appears in the0 C–N absorption band of NVP at 1285 cm 1 , which is absent in the spectrum of P0.05 0 , but appears 25. Similarly, the characteristic bands of AMPS, such as S=O symmetric stretching P 250 and P0.050.05 in P 25 and P 25 . Similarly, the characteristic bands of AMPS, such as S=O symmetric stretching vibration band and C–N stretching vibration band at 1211 cm−1 − and 1157 cm−1, are observed in the 1 and −1 ,not vibration band0 and C–N stretching vibration band at 1211 cm 1157 cm are not observed spectrum of P 25, but appear in P0.050 and P0.0525. These results agree with the experiments, whether the NVP and AMPS were added in or not.
of P 25 (NVP added only) and P 25 (both NVP and AMPS added). It is well known that the reductant NaBH4 used in our synthesis exhibits the characteristic of selectively reducing ketones. The unprotected carbonyl groups on AMPS and NVP were likely to be reduced to hydroxyl groups in the preparation of the Fe0/Fe3O4 microparticles. Actually, the coordinated ferrous ions can protect these carbonyl groups from reduction. The carbonyl groups in AMPS were completely reduced by NaBH4, Materials 2018, 11, 1508 8 of 12 while the carbonyl groups in NVP bound to Fe2+ had avoided the reduction. However, the absorption band belonging to C=O is also weakened due to its coordination with ferrous ions. These results also 0.05 . of prove that NVP is directly regulate formation the Fe0/Fe3O4 microspheres by binding to in the spectrum of able P0 25to , but appear in P0.05the 0 and P 25 These results agree with the experiments, ferrous ions, and and AMPS is involved in theinregulatory whether the NVP AMPS were added or not. aggregation of the Fe0/Fe3O4 microparticles.
0 /Fe 0/Fe3 4 microparticles Figure6.6. FTIR FTIRspectra spectraof ofAMPS, AMPS,NVP, NVP,and andFeFe adding different different Figure synthesized by adding 3 O4 0.05 0 0.05 0.05 0 25 25 organicmatters: matters: AMPS, AMPS, NVP, NVP,PP 00,, PP 2525, ,and from thethe bottom up.up. organic andPP0.05 from bottom
3.4. Morphology and Magnetism of Macroporous Fe3O4 1711 Microparticles It is worth noting that the absorption band at about cm−1 , belonging to C=O, disappears in 0/Fe3aOrelatively the spectrum of P0.05 0 (AMPS added only), but appears with weak intensity in the spectra After undergoing the solvothermal reaction, the Fe 4 microparticles were converted to the 0 0.05 of (NVP added only) and Pby organic NVP and(Figure AMPS7a). added). is well the reductant 25 (both matters FeP3O25 4 microparticles wrapped It canItbe seen known that thethat hollow structure NaBH of selectively reducing ketones. The unprotected 4 used in our of microsphere is synthesis basically exhibits formed the andcharacteristic the microspheres are still wrapped by a floccule layer of carbonyl groups on AMPS and NVP were likely to be reduced to hydroxyl groups in the preparation of organic matters. 0 /Fe O microparticles. Actually, the coordinated ferrous ions can protect these carbonyl groups the FeThe 3 4 macroporous Fe3O4 microparticles were obtained after calcination to remove the organic matters from reduction. The carbonyl groups AMPS were completely reduced by NaBH while the carbonyl under a protective atmosphere. Theinphase composition and morphology of the4 ,macroporous Fe3O4 2+ groups in NVPwere bound to Fe had by avoided the reduction. However, theto absorption band belonging to microparticles characterized XRD, TEM, and LPSA. According the XRD pattern (Figure 7b), C=O is also weakened due to its coordination with ferrous ions. These results also prove that NVP the well-resolved diffraction peaks are well indexed to Fe3O4 (JCPDS NO. 88-0866), and the crystal is able toadirectly regulate theNitrogen formation of the Fe0 /Fe3 O4 microspheres by binding to ferrous ions, exhibits high crystallinity. adsorption-desorption curves (Figure 7c), a type IV isotherm, 0 and AMPS in the regulatory of the /Fe3 O4 microparticles. show that is theinvolved Fe3O4 has a stacked poreaggregation structure and theFemulti-point BET specific surface area is
estimated to 29.23 m2/g, and the BJH cumulative total pore volume (d > 2 nm) is 0.16 cc/g. As for the 3.4. Morphology and Magnetism of Macroporous Fe3 O4 Microparticles specific porous structure, it can be further and clearly seen from the TEM image (Figure 7d), in which After undergoing solvothermal reaction, the Fea0 /Fe weremicrospheres. converted to the macroporous Fe3O4the microparticle is assembled from number of open-cell hollow 3 O4 microparticles TheFe pore of the Fe3O4 microparticle is mainly distributed in 300–500 nm, be while the porosity of the the microparticles wrapped by organic matters (Figure 7a). It can seen that the hollow 3 O4size Fe3O4 microparticle can is bebasically reasonably attributed both the open-cell of by microspheres and structure of microsphere formed and thetomicrospheres are stillhollow wrapped a floccule layer theorganic gaps between of matters.the microspheres, as shown in Figure 7d. The HRTEM image of Fe3O4 nanoparticles (the The rightmacroporous middle insertFe in3 O Figure 7d), from the shell of open-cell hollow sphere by the black square were obtained after calcination to remove the organic 4 microparticles area, shows that the interplanar crystal The spacing is about 0.30 nm, corresponding to the (220) plane of matters under a protective atmosphere. phase composition and morphology of macroporous Fe33O44. microparticles In addition, the main polycrystalline diffraction rings (marked as 1,to2,the 3, XRD 4, 5),pattern which Fe were characterized by XRD, TEM, and LPSA. According correspond to the (220), (311), (400), (511), and planes Fe3O4NO. , respectively, canthe be (Figure 7b), the well-resolved diffraction peaks are(440) wellcrystal indexed to Fe3of O4the (JCPDS 88-0866), and clearly exhibits observedain the selected area electron pattern (the right bottom insert7c), in Figure crystal high crystallinity. Nitrogendiffraction adsorption-desorption curves (Figure a type 7d) IV from the red round area—this is consistent with structure the XRD and results. The particleBET size specific distribution by isotherm, show that the Fe3 O4 has a stacked pore the multi-point surface LPSA 7e) to reveals theand average size cumulative of the Fe3O4 total microparticles is about ± 0.50 μmcc/g. with area is (Figure estimated 29.23that m2 /g, the BJH pore volume (d > 1.50 2 nm) is 0.16 As for the specific porous structure, it can be further and clearly seen from the TEM image (Figure 7d), in which the macroporous Fe3 O4 microparticle is assembled from a number of open-cell hollow microspheres. The pore size of the Fe3 O4 microparticle is mainly distributed in 300–500 nm, while the porosity of the Fe3 O4 microparticle can be reasonably attributed to both the open-cell hollow of microspheres and the gaps between the microspheres, as shown in Figure 7d. The HRTEM image of Fe3 O4 nanoparticles (the right middle insert in Figure 7d), from the shell of open-cell hollow sphere by the black square area, shows that the interplanar crystal spacing is about 0.30 nm, corresponding
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to the (220) plane of Fe3 O4 . In addition, the main polycrystalline diffraction rings (marked as 1, 2, 3, 4, 5), which correspond to the (220), (311), (400), (511), and (440) crystal planes of the Fe3 O4 , respectively, can be clearly observed in the selected area electron diffraction pattern (the right bottom insert in Figure 7d) from the red round area—this is consistent with the XRD results. The particle size Materials 2018, 11, x revised file 2 distribution by LPSA (Figure 7e) reveals that the average size of the Fe3 O4 microparticles9 ofis12about 1.50 ± 0.50 µm with a PDI 0.218, indicating that the prepared macroporous Fe3 O4 microparticles have a PDI 0.218, indicating that the prepared macroporous Fe3O4 microparticles have a good dispersibility a good dispersibility in the phase andinless agglomeration in application. in the aqueous phase andaqueous less agglomeration application.
Figure 7. TEM image of I0.0525 before calcination (a); XRD pattern (b); nitrogen adsorption-desorption
Figure 7. TEM image of I0.05 25 before calcination (a); XRD pattern (b); nitrogen adsorption-desorption curves (c); TEM image (d); particle size distribution (e); and magnetic hysteresis loop (f) of curves (c); TEM image (d); particle size0.05 distribution (e); and magnetic hysteresis loop (f) of macroporous macroporous Fe3O4 microparticles F 25 after calcination. 0.05 Fe3 O4 microparticles F 25 after calcination.
The magnetic hysteresis loop by VSM (Figure 7f) verifies that the prepared macroporous Fe3O4 microparticles ferromagnetism with a high magnetization (Msmacroporous = 66.14 emu/g),Fe O The magnetic exhibits hysteresis loop by VSM (Figure 7f)saturation verifies that the prepared 3 4 remnant magnetization (Mr = 6.33 emu/g), and coercivity (Hc = 105.32 Oe). Such a microparticles exhibits ferromagnetism with a high saturation magnetization (Msferromagnetic = 66.14 emu/g), macroporous material is expected to be applied as a drug-loaded material and to provide a heat remnant magnetization (Mr = 6.33 emu/g), and coercivity (Hc = 105.32 Oe). Such a ferromagnetic source for the temperature-sensitive material by applying an alternating magnetic field. macroporous material0.05is expected to be applied as a drug-loaded material and to provide a heat source Similar to F 25, after subsequent NKE controlled oxidation, Fe0/Fe3O4 microparticles with for the temperature-sensitive material by applying an alternating magnetic field. different diameters of spheres will be converted into macroporous Fe3O4 particles with different pore 0.05 Similar F 25sizes. , after oxidation, Fe0 /Fe 4 microparticles sizes andtoparticle Thesubsequent macroporousNKE Fe3O4controlled particles with such a large pore3 Ostructure and with ferromagnetic different diameters of spheres will be converted into macroporous Fe O particles with properties may provide a good carrier and heat source for the magnetocaloric3 4 responsively controlled release system. different pore sizes and drug particle sizes. The macroporous Fe3 O4 particles with such a large
pore structure and ferromagnetic properties may provide a good carrier and heat source for the 3.5. A Preliminary Discussion on Holing Mechanism of Open-Cell Hollow Microsphere magnetocaloric-responsively controlled drug release system. More interestingly, many open-cell hollow microspheres appeared after undergoing the
3.5. Asolvothermal Preliminaryreaction Discussion Holing Mechanism Open-Cell Hollow andon calcination (F0.0525). Thisofphenomenon would Microsphere be of great significance to the controllable synthesis of various hollow microspheres. On the basis of a comprehensive analysis on
More interestingly, many open-cell hollow microspheres appeared after undergoing the the synthesis process, probable holing mechanisms of open-cell hollow microspheres would be solvothermal reaction and calcination (F0.05 25 ). This phenomenon would be of great significance preliminarily suggested. to the controllable various hollow microspheres. On to thethe basis of a comprehensive analysis One of thesynthesis probableofholing mechanisms could be related thermal stress effect. In the on the synthesis process, probable holing mechanisms of open-cell hollow microspheres would Kirkendall controlled oxidation process, the contacted microspheres would be locally fused together be preliminarily due to thesuggested. sustained grain growth in the adjacent region to form the dumbbell-like or bead-like One of microspheres the probable(shown holinginmechanisms could be of related to the thermal stress effect. hollow Figure 8a). The neck the dumbbell-like or bead-like hollowIn the microsphere was prone to fracture duethe to contacted the thermal stress on thewould interface elevated Kirkendall controlled oxidation process, microspheres be under locallyanfused together temperature, and a hole was left on the shell. This holing process might occur during either due to the sustained grain growth in the adjacent region to form the dumbbell-like or bead-likethe hollow solvothermal reaction or the calcination process. microspheres (shown in Figure 8a). The neck of the dumbbell-like or bead-like hollow microsphere Another probable holing mechanism could be the surface tension mechanism. As mentioned earlier, a large amount of NVP was mingled in the obtained Fe0/Fe3O4 microspheres, and remained in the cavity during the cavitation process. At a high temperature, the vaporized NVP could escape
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out of the microsphere through the looser part of the shell, leaving behind an initial hole there. Due Materials 2018, 11, 1508 10 of 12 to the high surface energy resulting from the small radius of curvature at the edge of the initial hole, the surficial atoms could continuously diffuse inward, the shell driven by the surface tension, and was prone to fracture due to the thermal stressThis on the interface elevated in temperature, and a thus the hole could be gradually enlarged. holing way under mainlyanoccurred the calcination hole wasSome left on the shell. Thisbeing holing processare might occur during 8b, either solvothermal reaction or process. probable holes enlarged shown in Figure andthe a typical open-cell hollow 0.05 the calcination process. microsphere is shown in Figure 8c. All these images in Figure 8 are taken from sample F 25.
Figure 8. SEM local magnification images of macroporous Fe3O4 microparticles0.05 F0.0525: dumbbell-like Figure 8. SEM local magnification images of macroporous Fe3 O4 microparticles F 25 : dumbbell-like or or bead-like hollow microspheres (a); expanding holes (b); and typical open-cell hollow microsphere bead-like hollow microspheres (a); expanding holes (b); and typical open-cell hollow microsphere (c). (c).
Another probable mechanism could be the surface tension mechanism. mentioned Certainly, the aboveholing suggestions should be supported by sufficient experimentalAsresults, and 0 /Fe O microspheres, and remained in earlier, a large amount of NVP was mingled in the obtained Fe 3 4 further theoretical calculations and experimental verifications are still ongoing in our laboratory. the cavity during the cavitation process. At a high temperature, the vaporized NVP could escape out the microsphere through the looser part of the shell, leaving behind an initial hole there. Due to 4.ofConclusions the high surface energy resulting from the small radius of curvature at the edge of the initial hole, In this study, a kind of macroporous magnetic Fe3O4 microparticles composed of the aggregates the surficial atoms could continuously diffuse inward, the shell driven by the surface tension, and thus of Fe3O4 open-cell hollow microspheres was successfully prepared by a novel organic matter assisted the hole could be gradually enlarged. This holing way mainly occurred in the calcination process. open-cell hollow sphere assembly method. With the help of the specific affinity of ferrous ions to Some probable holes being enlarged are shown in Figure 8b, and a typical open-cell hollow microsphere carbonyl groups on NVP and the polymerization of organic monomers, the reduced irons underwent is shown in Figure 8c. All these images in Figure 8 are taken from sample F0.05 25 . 0 heterogeneous nucleation and grain growth to form the Fe /Fe3O4 microspheres consisting of a lot of Certainly, the above suggestions should be supported by sufficient experimental results, nano-Fe0 grains. Then, they were assembled into Fe0/Fe3O4 microparticles wrapped by organic and further theoretical calculations and experimental verifications are still ongoing in our laboratory. matters. The NVP can promote the formation of the Fe0/Fe3O4 microsphere via bonding to ferrous ions and self-polymerizing, while the AMPS can facilitate the assembly of the Fe0/Fe3O4 microsphere 4. Conclusions into larger aggregates (Fe0/Fe3O4 microparticles) via enwrapping around the microsphere during the In this study, a kind of macroporous magnetic Fe3 O4 microparticles composed of the aggregates polymerization. After the Kirkendall controlled oxidation had finished, the Fe0/Fe3O4 microparticles of Fe3 O4 open-cell hollow microspheres was successfully prepared by a novel organic matter assisted were converted to the macroporous Fe3O4 microparticles (the aggregates of a number of the open-cell hollow sphere assembly method. With the help of the specific affinity of ferrous ions to hollow/open-cell hollow Fe3O4 microsphere). The unique porous structure of the macroporous Fe3O4 carbonyl groups on NVP and the polymerization of organic monomers, the reduced irons underwent microparticle is mainly derived from both the open-cell hollow 0of microspheres and the gaps between heterogeneous nucleation and grain growth to form the Fe /Fe3 O4 microspheres consisting of a the microspheres. While the research on open-cell hollow microspheres was very preliminary and lot of nano-Fe0 grains. Then, they were assembled into Fe0 /Fe3 O4 microparticles wrapped by the holing mechanism should also be further studied, the strategy0 to assemble the open-cell hollow organic matters. The NVP can promote the formation of the Fe /Fe3 O4 microsphere via bonding Fe3O4 microspheres into the macroporous microparticles has been verified as feasible. Additionally, to ferrous ions and self-polymerizing, while the AMPS can facilitate the assembly of the Fe0 /Fe3 O4 the prepared macroporous magnetic Fe 3O4 microparticles were expected to have broad application microsphere into larger aggregates (Fe0 /Fe O4 microparticles) via enwrapping around the microsphere prospects on magnetocaloric-responsively 3controlled drug release systems. during the polymerization. After the Kirkendall controlled oxidation had finished, the Fe0 /Fe3 O4 microparticles were converted to the macroporous Fe3 Oconcept; of a number 4 microparticles Author Contributions: H.W. and G.Y. proposed experimental H.W., X.P.,(the Y.Z.aggregates and X.C. performed the of the hollow/open-cell hollow Fe O microsphere). The unique porous structure of the macroporous 3 4 and Z.H. analyze experimental data; G.Y. and X.L. reviewed and experiments; H.W. wrote the paper; H.W. Fe3 O4the microparticle is mainly derived from both the open-cell hollow of microspheres and the gaps edited paper. between the microspheres. While the research on open-cell hollow microspheres was very preliminary Funding: This research was funded by National Natural Science Foundation of China grant number 51372157 andthe theKey holing mechanism should also be further studied, the strategy to assemble the open-cell hollow and Research and Development Program of Sichuan, China grant number 2017SZ0047. Fe3 O4 microspheres into the macroporous microparticles has been verified as feasible. Additionally, Acknowledgments: The authors are very much grateful to Shupei Sun, Xiaobei Huang and other researchers on the prepared macroporous magnetic Fe3 O4 microparticles were expected to have broad application the experimental help in the study. prospects on magnetocaloric-responsively controlled drug release systems. Conflicts of Interest: The authors declare no conflict of interest.
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Author Contributions: H.W. and G.Y. proposed experimental concept; H.W., X.P., Y.Z. and X.C. performed the experiments; H.W. wrote the paper; H.W. and Z.H. analyze experimental data; G.Y. and X.L. reviewed and edited the paper. Funding: This research was funded by National Natural Science Foundation of China grant number 51372157 and the Key Research and Development Program of Sichuan, China grant number 2017SZ0047. Acknowledgments: The authors are very much grateful to Shupei Sun, Xiaobei Huang and other researchers on the experimental help in the study. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
4. 5. 6.
7. 8. 9.
10. 11.
12.
13.
14. 15.
16. 17.
Rao, A.C.N.R. Transition metal oxides. Ann. Rev. Phys. Chem. 1989, 40, 291–326. [CrossRef] Yuan, C.; Wu, H.B.; Xie, Y.; Lou, X.W.D. Mixed transition-metal oxides: Design, synthesis, and energy-related applications. Angew. Chem. Int. Ed. 2014, 53, 1488–1504. [CrossRef] [PubMed] Lee, S.H.; Yu, S.; Lee, J.E.; Jin, A.; Lee, D.J.; Lee, N.; Jo, H.; Shin, K.; Ahn, T.; Kim, Y.; et al. Self-assembled Fe3 O4 nanoparticle clusters as high-performance anodes for lithium ion batteries via geometric confinement. Nano Lett. 2013, 13, 4249–4256. [CrossRef] [PubMed] Kalinin, S.V.; Spaldin, N.A. Functional ion defects in transition metal oxides. Science 2013, 341, 858–859. [CrossRef] [PubMed] Zhu, M.; Diao, G. Synthesis of porous Fe3 O4 nanospheres and its application for the catalytic degradation of xylenol orange. J. Phys. Chem. C 2011, 115, 18923–18934. [CrossRef] Xuan, S.; Wang, F.; Lai, J.M.Y.; Sham, K.W.Y.; Wang, Y.J.; Lee, S.; Yu, J.C.; Cheng, C.H.K.; Leung, K.C. Synthesis of biocompatible, mesoporous Fe3 O4 nano/microspheres with large surface area for magnetic resonance imaging and therapeutic applications. ACS Appl. Mater. Interfaces 2011, 3, 237–244. [CrossRef] [PubMed] Xu, J.; Zhu, Y. Monodisperse Fe3 O4 and γ-Fe2 O3 magnetic mesoporous microspheres as anode materials for lithium-ion batteries. ACS Appl. Mater. Interfaces 2012, 4, 4752–4757. [CrossRef] [PubMed] Xin, X.; Wei, Q.; Yang, J.; Yan, L.; Feng, R.; Chen, G.; Du, B.; Li, H. Highly efficient removal of heavy metal ions by amine-functionalized mesoporous Fe3 O4 nanoparticles. Chem. Eng. J. 2012, 184, 132–140. [CrossRef] Dickinson, C.; Zhou, W.; Hodgkins, R.P.; Shi, Y.; Zhao, D.; He, H. Formation mechanism of porous single-crystal Cr2 O3 and Co3 O4 templated by mesoporous silica. Am. Chem. Soc. 2006, 18, 3088–3095. [CrossRef] Yuan, C.; Gao, B.; Su, L.; Zhang, X. Interface synthesis of mesoporous MnO2 and its electrochemical capacitive behaviors. J. Colloid. Interfaces Sci. 2008, 322, 545–550. [CrossRef] [PubMed] Jiang, T.; Poyraz, A.S.; Iyer, A.; Zhang, Y.; Luo, Z.; Zhong, W.; Miao, R.; El-Sawy, A.M.; Guild, C.J.; Sun, Y.; et al. Synthesis of mesoporous iron oxides by an inverse micelle method and their application in the degradation of orange II under visible light at neutral pH. J. Phys. Chem. C 2015, 119, 10454–10468. [CrossRef] Yu, X.; Xu, R.; Gao, C.; Luo, T.; Jia, Y.; Liu, J.; Huang, X. Novel 3D hierarchical cotton-candy-like CuO: Surfactant-free solvothermal synthesis and application in As(III) removal. ACS Appl. Mater. Interfaces 2012, 4, 1954–1962. [CrossRef] [PubMed] Pellicer, E.; Cabo, M.; López-Ortega, A.; Estrader, M.; Yedra, L.; Estradé, S.; Peiró, F.; Saghi, Z.; Midgley, P.; Rossinyol, E.; et al. Controlled 3D-coating of the pores of highly ordered mesoporous antiferromagnetic Co3 O4 replicas with ferrimagnetic Fex Co3−x O4 nanolayers. Nanoscale 2013, 5, 5561–5567. [CrossRef] [PubMed] Cheng, K.; Peng, S.; Xu, C.; Sun, S. Porous hollow Fe3 O4 nanoparticles for targeted delivery and controlled release of cisplatin. J. Am. Chem. Soc. 2009, 131, 10637–10644. [CrossRef] [PubMed] Kim, J.C.; Kim, Y.N.; Hur, N.H.; Kim, W.S.; Kang, Y.G. Highly ordered macroporous magnetic materials prepared by electrodeposition through colloidal template. Phys. Status Solidi B Basic Res. 2004, 241, 1585–1588. [CrossRef] Imhof, A.; Pine, D.J. Ordered macroporous materials by emulsion templating. Nature 1997, 389, 948–951. [CrossRef] Cong, H.; Yu, B. Fabrication of superparamagnetic macroporous Fe3 O4 and its derivates using colloidal crystals as templates. J. Colloid. Interfaces Sci. 2011, 353, 131–136. [CrossRef] [PubMed]
Materials 2018, 11, 1508
18. 19.
20. 21.
22.
23. 24. 25. 26. 27. 28.
29. 30. 31.
12 of 12
Wang, S.B.; Wang, X.Q.; Zhang, H.L.; Zhang, W.B. Hollow CuO microspheres with open nanoholes: Fabrication and photocatalytic properties. J. Alloys Compd. 2016, 685, 22–27. [CrossRef] Li, Z.; Xie, Y.; Xiong, Y.; Zhang, R. A novel non-template solution approach to fabricate ZnO hollow spheres with a coordination polymer as a reactant Electronic Supplementary Information (ESI) available: X-ray photoelectron spectra and energy-dispersive X-ray analysis of the products. New J. Chem. 2003, 27, 1518. [CrossRef] Yin, Y.D.; Rioux, R.M.; Erdonmez, C.K.; Hughes, S.; Somorjai, G.A.; Alivisatos, A.P. Formation of hollow nanocrystals through the nanoscale Kirkendall Effect. Science 2004, 304, 711–714. [CrossRef] [PubMed] Luo, B.; Xu, S.; Ma, W.; Wang, W.; Wang, S.; Guo, J.; Yang, W.; Hu, J.; Wang, C.-C. Fabrication of magnetite hollow porous nanocrystal shells as a drug carrier for paclitaxel. J. Mater. Chem. 2010, 20, 7107–7113. [CrossRef] Cao, S.; Zhu, Y.; Ma, M.; Li, L.; Zhang, L. Hierarchically nanostructured magnetic hollow spheres of Fe3 O4 and γ -Fe2 O3 : Preparation and potential application in drug delivery. J. Phys. Chem. C 2008, 112, 1851–1856. [CrossRef] Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R.A. Magnetic nanocomposite particles and hollow spheres constructed by a sequential layering approach. Chem. Mater. 2001, 13, 109–116. [CrossRef] Peng, S.; Sun, S. Synthesis and characterization of monodisperse hollow Fe3 O4 nanoparticles. Angew. Chem. Int. Ed. 2007, 46, 4155–4158. [CrossRef] [PubMed] Wang, W.; Dahl, M.; Yin, Y. Hollow nanocrystals through the nanoscale kirkendall effect. Chem. Mater. 2013, 25, 1179–1189. [CrossRef] Leibbrandt, G.W.R.; Hoogers, G.; Habraken, F.H.P.M. Thin oxide film growth on Fe (100). Phys. Rev. Lett. 1992, 68, 1947–1950. [CrossRef] [PubMed] Railsback, J.G.; Johnston-Peck, A.C.; Wang, J.; Tracy, J.B. Size-dependent nanoscale kirkendall effect during the oxidation of nickel nanoparticles. ACS Nano 2010, 4, 1913–1920. [CrossRef] [PubMed] Ren, Y.; Wu, H.; Lu, M.; Chen, Y.; Zhu, C.; Gao, P.; Cao, M.; Li, C.; Ouyang, Q. Quaternary nanocomposites consisting of graphene, Fe3 O4 @Fe core@shell, and ZnO nanoparticles: Synthesis and excellent electromagnetic absorption properties. ACS Appl. Mater. Interfaces 2012, 4, 6436–6442. [CrossRef] [PubMed] Fujii, T.; de Groot, F.; Sawatzky, G.A.; Voogt, F.C.; Hibma, T.; Okada, K. In Situ XPS analysis of various iron oxide films grown by NO2-assisted molecular-beam epitaxy. Phys. Rev. B 1999, 59, 3195–3202. [CrossRef] Hiral, H.; Wakabayashi, H.; Momiyama, M. Preparation of polymer-protected colloidal dispersions of copper. Bull. Chem. Soc. Jpn. 1986, 59, 367–372. [CrossRef] Liu, X.; Jiang, W.; Gou, S. Synthesis and clay stabilization of a water-soluble copolymer based on acrylamide, modular β-cyclodextrin and AMPS. J. Appl. Polym. Sci. 2013, 128, 3398–3404. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
