Fundamental Formation of Three-Dimensional Fe3O4 Microcrystals

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Fundamental Formation of Three-Dimensional Fe3O4 Microcrystals and Practical Application in Anchoring Au as Recoverable Catalyst for Effective Reduction of 4‑Nitrophenol Yue Chen,†,‡ Tong Wu,†,‡ Guoliang Xing,§ Yichuan Kou,†,‡ Boxun Li,∥ Xinying Wang,⊥ Ming Gao,†,‡ Lei Chen,†,‡ Yaxin Wang,†,‡ Jinghai Yang,†,‡ Yang Liu,*,†,‡ Yongjun Zhang,*,†,‡ and Dandan Wang*,# Downloaded via NOTTINGHAM TRENT UNIV on August 12, 2019 at 00:49:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

College of Physics, Jilin Normal University, Siping 136000, China Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, China § Jilin Special Equipment Inspection and Research Institute, Jilin 132013, China ∥ School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China ⊥ School of Engineering and Architecture, Northeast Electric Power University, Jilin, 132012, China # QRA-PFA-Chemical FA, GLOBALFOUNDRIES (Singapore) Pte. Ltd., 60 Woodlands Industrial Park D, Street 2, Singapore 738406, Singapore ‡

S Supporting Information *

ABSTRACT: In this study, the three-dimensional (3D) flowerlike porous Fe3O4 microcrystals were prepared by a self-assembly approach with the assistance of ethylene glycol (EG). The generation mechanism of the 3D flowerlike Fe3O4 microcrystals was revealed through controlling the parameters of the hydrothermal reaction time, the molar mass of the urea, and the calcination temperature. The proposed 3D flowerlike Fe3O4 microcrystals exhibited superparamagnetic behaviors with high saturation magnetization (i.e., up to 73.1 emu· g−1) at room temperature. The Fe3O4−Au magnetic composites (MCs) were further prepared by a seed deposition process, and surface features were revealed by TEM, XRD, XPS, UV−vis, and SQUID techniques. Compared with the Fe3O4 microcrystals themselves, the Au (∼20 nm) covered Fe3O4 microcrystals provided efficient and recyclable catalytic performance (e.g., unprecedented high turnover frequency of 2.874 min−1) for 4-nitrophenol (4-NP). More importantly, the proposed Fe3O4−Au MCs could be used to reduce 4-NP for more than six cycles, elaborating that Fe3O4−Au MCs are promising catalysts in the field of environmental purification.

1. INTRODUCTION Due to high biocompatibility, good stability, and low toxicity, magnetic nanomaterials have attracted intense interest for broad application prospects in biomedical fields such as drug delivery and release, targeted thermal treatment, magnetothermal therapy, magnetic resonance imaging agents, and biosensors.1−6 Among numerous magnetic nanomaterials, Fe3O4 nanocrystals are in the ascendant on account of their unique characteristics. On one hand, the superparamagnetic Fe3O4 nanocrystals possess high saturation magnetization and magnetic susceptibility, which can accomplish rapid separation by using external magnetic fields and efficient recycling.7,8 On the other hand, since Fe3O4 nanocrystals have excellent biocompatibility, it is easy to introduce abundant active functional groups on the surface of Fe3O4 nanocrystals.9,10 Fe3O4 nanocrystals with different morphologies and structures were designed and prepared with plenty of methods. © XXXX American Chemical Society

The morphology of the Fe3O4 nanocrystals mainly includes zero-dimensional (0D) Fe3O4 quantum dots, one-dimensional (1D) Fe3O4 nanowires/nanorods, two-dimensional (2D) Fe3O4 nanoplates/nanoflakes, and three-dimensional (3D) Fe3O4 nanoflowers/microspheres. For instance, Saha and Viswanatha11 prepared Fe3O4 quantum dots by decomposition of ferrous acetate. Ren and co-workers12 synthesized uniform Fe3O4 magnetic nanorods by hydrothermal reaction. Zhang et al.13 obtained Fe3O4 high-quality nanowires through the oxidation−precipitation processes. Fe3O4 nanosheets were also prepared via a facile sonochemical route.14 In our previous work, Fe3O4 nanocubes were achieved by the thermal Received: May 21, 2019 Revised: July 26, 2019 Accepted: July 30, 2019

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DOI: 10.1021/acs.iecr.9b02777 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Schematic of the Preparation Process and Application of Fe3O4−Au MCs

decomposition method.15 Among various morphologies of Fe3O4 nanocrystals, 3D hierarchical/porous Fe3O4 microcrystals composed of diverse nanoscale building blocks have been paid much attention in catalysis and chemical engineering because 3D hierarchical/porous Fe3O4 microcrystals have a high specific surface area, which can supply ample active sites to combine with a high density of the functional groups.16 In general, the common synthetic route of 3D hierarchical/ porous Fe3O4 microcrystals is self-assembly, which can form ordered aggregates by the spontaneous process. In the majority of cases, the use of surfactants and polymers is necessary to achieve the objectives in the synthetic process. Therefore, most researchers focused on the effect of the surfactants on the morphology of 3D Fe3O4 microcrystals.17 In fact, the growth mechanism of 3D Fe3O4 microcrystals is related to many factors. From the technical and theoretical viewpoints, the synthesis conditions including the temperature and time of calcination and hydrothermal reaction, the additional amount of OH− ions, and reduction agents also play an important role in Fe3O4 microcrystal formation. There is thereby an urgent need, but it is still a significant challenge to reveal the growth mechanism of 3D Fe3O4 microcrystals by adjusting the different experimental parameters. Noble metal Au nanocrystals were widely studied based on their distinct and adjustable optical properties and highly efficient catalytic activities.18−24 Especially the catalytic reduction of 4-nitrophenol (4-NP) by Au nanocrystals with the assistance of reducing agent sodium borohydride has been widely studied.25 Unfortunately, the agglomeration of Au nanocrystals results in the decrease of the catalytic activities. To suppress aggregation of Au nanocrystals, many works focused on adding suitable stabilizers to improve the stabilization of Au nanocrystals.26,27 However, although the introduction of the stabilizers inhibits the agglomeration of Au nanocrystals, the stabilizers instead reduce or even deactivate the catalytic activity of Au nanocrystals because the surface sites of Au nanocrystals are occupied by the stabilizers.28−30 Recently, extensive efforts have been devoted to researching the possibility that Au nanocrystals are uniformly covered on solid supports (such as polymers, carbonaceous materials,

semiconductor materials, or metal oxides).31−35 Among those materials, 3D Fe3O4 magnetic microcrystals are considered the most promising supports to disperse and stabilize Au nanocrystals, not only because Fe3O4 magnetic microcrystals exhibit good magnetic responsiveness to an external magnetic field, but also because 3D Fe3O4 microcrystals can provide more binding sites to combine with Au nanocrystals compared with 0D/1D/2D Fe3O4 nanocrystals. In the present work, 3D flowerlike porous Fe3O4 microcrystals have been prepared by a self-assembly approach. The effects of the hydrothermal reaction time, the molar amount of urea, and the calcination temperature on the morphological properties of the Fe3O4 microcrystals were analyzed, and the corresponding possible mechanism was also proposed. After a polyethylenimine dithiocarbamate (PEI-DTC) bonding layer was modified on the surfaces of the 3D flowerlike porous Fe3O4 microcrystals, the surfaces of Fe3O4 microcrystals were decorated uniformly with Au nanocrystals to achieve Fe3O4− Au magnetic composites (MCs). The Fe3O4−Au MCs were employed to catalyze the reduction of 4-NP with the assistance of NaBH4. Scheme 1 presents the fabrication of the Fe3O4 microcrystals and Fe3O4−Au MCs and the catalytic degradation of 4-NP.

2. EXPERIMENTAL SECTION 2.1. Materials. Methanol, polyethylenimine (PEI), tetrabutylammonium bromide (TBAB), and carbon disulfide (CS2) were received from Aladdin Industrial Co., Ltd. Urea and iron chloride hydrate (FeCl3·6H2O) were acquired from Shanghai Macklin Biochemical Co., Ltd. Ethylene glycol (EG), sodium citrate dihydrate (Na3C6H5O7·2H2O), potassium hydroxide (KOH), NaBH4, 4-nitrophenol (4-NP), and gold(III) chloride hydrate (HAuCl4·3H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received. 2.2. Synthesis of 3D Porous Fe3O4 Microcrystals. To uncover the growth mechanism of 3D porous Fe 3 O 4 microcrystals, 3D porous Fe3O4 microcrystals were prepared by controlling the hydrothermal reaction time, the molar amount of urea, and the calcination temperature, respectively. B


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Industrial & Engineering Chemistry Research

Figure 1. SEM images of iron alkoxide precursors with different hydrothermal reaction times of 5 (a), 10 (b), and 20 min (c). (d) XRD patterns of (c); (inset) photograph of sample corresponding to different hydrothermal reaction time intervals from 0 to 20 min.

Na3C6H5O7·2H2O solution was injected to gold chloride solution dropwise, and it was heated and stirred for an additional 15 min. Au nanocrystal colloids were obtained and cooled to room temperature in a dark environment. 2.4. Synthesis of Fe3O4−Au MCs. Three-dimensional flowerlike porous Fe3O4 microcrystals were chosen to be coated with Au nanocrystal colloids. Fe3O4−Au MCs were synthesized based on previous reports.4,36 In brief, the fabrication of Fe3O4−Au MCs was composed of two steps: the synthesis of Fe3O4@PEI-DTC and the attachment of Au nanocrystal colloids onto the surfaces of Fe3O4@PEI-DTC. 2.5. Evaluation of Fe3O4−Au MCs for Catalytic Reduction of 4-NP. Catalytic reduction of 4-NP was carried out by Fe3O4−Au MCs with the help of excess NaBH4, and the catalytic performance and recyclability of Fe3O4−Au MCs were investigated. In a typical procedure, 4-NP (0.005 mol·L−1, 1 mL) and NaBH4 (0.2 mol·L−1, 1 mL) were added to a cuvette. After that, Fe3O4−Au MCs were added for catalytic reduction. The reaction solution was detected by a UV−vis spectrophotometer every 60 s until the solution became colorless. A magnet was used to separate the used samples to evaluate the repeatability of Fe3O4−Au MCs. After the achieved Fe3O4−Au MCs were rinsed with ethanol and deionized water, the same procedure was repeated for six cycles. 2.6. Characterization. An X-ray diffractometer (XRD; Rigaku D/Max-2500), Mö ssbauer spectroscopy (FAST Comtec Mössbauer system), a scanning electron microscope (SEM; JEOL JSM-7800F), a transmission electron microscope (TEM; JEOL 2100), and X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCALAB 250Xi) were employed to investigate the structures and morphologies of samples.

Part I is to adjust the hydrothermal reaction time of the iron alkoxide precursor. In the first place, FeCl3·6H2O (4.4 mmol), TBAB (10.8 mmol), and urea (80 mmol) were added to 190 mL of EG in a three-necked flask. The mixture was stirred with a magnetic stirrer for 30 min. Then it was refluxed at 195 °C for 0, 5, 10, 15, and 20 min in a flow of nitrogen, respectively. The mixed solution was cooled to room temperature, and precipitate (iron alkoxide precursor) was formed and collected by centrifugation. Part II is to change the molar amount of urea. FeCl3·6H2O (4.4 mmol), TBAB (10.8 mmol), and a proper amount of urea (15, 90, and 270 mmol) were added to 190 mL of EG in the flask. The mixed solution was stirred with a magnetic stirrer for 30 min and then was refluxed at 195 °C in a flow of nitrogen for 20 min. A yellow-green iron alkoxide precursor was obtained. Part III is to change the calcination temperature of the iron alkoxide precursor. FeCl3·6H2O (4.4 mmol), TBAB (10.8 mmol), and urea (80 mmol) were added to 190 mL of EG in the flask. The mixed solution was stirred with a magnetic stirrer for 30 min and then was refluxed at 195 °C in a flow of nitrogen for 20 min. The obtained yellow-green iron alkoxide precursor was centrifuged and rinsed with ethanol. After the iron alkoxide precursor was dried overnight at 60 °C, the iron alkoxide precursor was calcined at 200, 500, and 700 °C for 3 h in a flow of nitrogen, respectively. The sample calcined at 500 °C was named as 3D flowerlike porous Fe3O4 microcrystals. 2.3. Synthesis of Au Nanocrystal Colloids. A 40 mg (1%) sample of Na3C6H5O7·2H2O and 1 mL of 29 mmol·L−1 HAuCl4·3H2O (29 mmol·L−1) were placed into 4 and 109 mL of deionized water, respectively. After they were mixed, the mixture was heated under reflux with stirring to boiling. The C


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Figure 2. SEM images of obtained samples with different molar amounts of urea: 15 (a), 90 (b), and 270 mmol (c).

Figure 3. SEM images of samples obtained at different calcination temperatures of 200 (a), 500 (b), and 700 °C (c). (d) Corresponding XRD patterns.

reaction times (5, 10, and 20 min) are presented in Figure 1, which can reflect the morphological evolutionary process of iron alkoxide precursors. When the hydrothermal reaction time is 5 min, the sample consists of the about 35 nm nanoparticles (Figure 1a). When it increases to 10 min (Figure 1b), a few nanoparticles aggregate to form microspheres. At the same time, the 3D flowerlike microstructures consisting of nanosheets appear locally, but a large number of nanoparticles still exist. Interestingly, as the hydrothermal reaction time increases to 20 min, the nanoparticles completely disappear and the samples exhibit 3D flowerlike microstructures with a mean size of about 2−3 μm (Figure 1c). The color of the solution changes from orange to yellow-green (as shown in the inset of Figure 1d) when prolonging the reaction time. The XRD pattern of 3D flowerlike iron alkoxide precursors is illustrated in the Figure 1d. The positions of diffraction peaks are in accordance with those of the iron alkoxide precursor reported in the ref 17. Previous reports suggested that the development

Specific surface areas of samples were characterized by the Brunauer−Emmett−Teller (BET) method using nitrogen adsorption−desorption isotherms (Micrometrics ASAP 2020), and pore sizes were achieved by the Barrett−Joyner− Halenda (BJH) method. Ultraviolet−visible (UV−vis) spectra were monitored by a spectrophotometer (Shimadzu UV 3600). Magnetic properties were achieved with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS3).

3. RESULTS AND DISCUSSION 3.1. Influence of Experimental Parameters on Structures and Morphologies of 3D Porous Fe3O4 Microcrystals. To reveal the growth mechanism of 3D porous Fe3O4 microcrystals, a series of experimental conditions were changed, which included hydrothermal reaction time, molar amount of urea, and calcination temperature. SEM images of iron alkoxide precursors with different hydrothermal D


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Industrial & Engineering Chemistry Research of the 3D flowerlike microstructures usually included two stages: rapid nucleation and the aggregation and crystallization of amorphous primary nanoparticles.37 Combined with the results of our experiments, a feasible formation mechanism of the 3D flowerlike microstructures is proposed. First, EG coordinates with iron chloride to form the iron alkoxide and it precipitates to get primary nuclei. Second, the nucleus grows into the nanoparticles, which subsequently aggregate into microspheres. Simultaneously, taking the microspheres as the core of 3D flowerlike microstructures, the 3D flowerlike microstructures begin to appear due to the self-assembly of adjacent microspheres. As the hydrothermal time further increases, all the microspheres turn into fully developed 3D flowerlike microstructures. In addition, the surfaces of each petal of 3D flowerlike iron alkoxide precursors become smooth owing to Ostwald ripening. The importance of the addition quantity of the urea to control the self-assembly process of samples cannot be ignored. Figure 2 presents SEM images of samples with different molar amounts of urea (15, 90, and 270 mmol). When the molar amount of urea is 15 mmol, the shape of the sample is flaky and the self-assembly of the iron alkoxide precursors cannot be observed. When the molar amount of urea increases to 90 mmol, well-structured hierarchical 3D flowerlike iron alkoxide precursors are achieved. When the molar amount of urea further increases to 270 mmol, the morphology of the prepared samples changes from the 3D flowerlike microstructures to 3D ball-flower microstructures. This demonstrates that the urea is essential in adjusting the morphology of the iron alkoxide precursors. EG acts as the linking and reducing agent to coordinate with iron chloride to produce iron alkoxide. In this process, hydrochloric acid is formed as a byproduct, which inhibits the formation of the iron alkoxide. Because urea can provide OH− ions through urea hydrolysis, urea can neutralize the hydrochloric acid and boost the formation of the 3D flowerlike microstructures. When the molar amount of urea is only 15 mmol, the transformation of FeCl3 to iron alkoxide is insufficient and thus the 3D hierarchical microstructures cannot be achieved. The influence of calcination temperature on the crystallization and morphology of iron alkoxide precursors was also investigated. SEM images of the samples prepared at 200, 500, and 700 °C are represented in Figure 3a−c. Figure 3d is the corresponding XRD patterns of the three samples. The SEM images reveal that the sample annealed at 200 °C exhibits a 3D flowerlike shape, and the corresponding XRD patterns show that the structure of the sample has no significant changes, which is named as 3D flowerlike iron alkoxide precursors. As for the sample annealed at 500 °C, though it maintains the 3D flowerlike morphology, close observation reveals that the petal of the 3D flowerlike microstructures is transformed from the smooth and dense structure to the polycrystalline and porous structure due to the decomposition of iron alkoxide precursors in the sintering process. Based on the corresponding XRD pattern of the sample, all the peaks are indexed to the magnetite (JCPDS card no. 85−1436).38 Therefore, this sample annealed at 500 °C is named as 3D flowerlike porous Fe3O4 microcrystals. Figure S1a is the Pawley refinement of the XRD pattern for 3D flowerlike porous Fe3O4 microcrystals. The residual Rwp of the sample is 11.73%, indicating that the 3D flowerlike porous Fe3O4 microcrystals are in accord with the standard magnetite Fe3O4.39 Since magnetite Fe3O4 has a structure similar to that of maghemite γ-Fe2O3, Mössbauer

spectroscopy is used to determine the phase structure of the 3D flowerlike porous Fe3O4 microcrystals to exclude the possibility of γ-Fe2O3.40 In Figure S1b, two well-resolved sextets with the double six-peak structures of magnetite can be easily identified. One is originated from Fe3+ ions at the tetrahedral A-sites. The other is assigned to mixed valence Fe2+ and Fe3+ at the octahedral B-sites.41 Therefore, Mössbauer analyses further confirm that the obtained 3D flowerlike porous Fe3O4 microcrystals are pure magnetite (Fe3O4). SEM images of the sample calcinated at 700 °C show that it still maintains the 3D flowerlike morphology, as is exhibited in Figure 3c. However, the corresponding XRD results show that, apart from the diffraction peaks of magnetite Fe3O4, the additional diffraction peaks of ferrous oxide FeO (JCPDS card no. 74-1886) and elemental iron Fe (JCPDS card no. 87-0721) can also be indexed, implying the Fe3O4 microcrystals are partially reduced at the calcination temperature higher than 500 °C in the reaction process.42 To gain more insight, the magnetic hysteresis (M−H) loops of the iron alkoxide precursor calcinated at different temperatures (200, 500, and 700 °C) are measured, as shown in Figure S2. The iron alkoxide precursor which is calcined at 200 °C has low magnetism because the phase transition to magnetite Fe3O4 is not complete. The saturation magnetization (Ms) value of the sample calcinated at 500 °C reaches up to 73.1 emu·g−1 owing to the formation of magnetite Fe3O4. However, as the calcination temperature further increases from 500 to 700 °C, the Ms value abnormally decreases to 49.5 emu·g−1 owing to oxidation of a portion of the Fe3O4 at higher temperatures. Figure 4 presents the nitrogen adsorption−desorption isotherms and corresponding BJH desorption pore size

Figure 4. Nitrogen adsorption−desorption isotherms and corresponding pore size distributions (inset) of 3D flowerlike porous Fe3O4 microcrystals.

distribution of 3D flowerlike porous Fe3O4 microcrystals. The isotherm of 3D flowerlike porous Fe3O4 microcrystals displays the features of type H3 hysteresis loops based on BDDT classification, which is characteristic of mesoporous materials and indicates the presence of mesopores in the sample.43 The analyses show that the BET specific surface area of 3D flowerlike porous Fe3O4 microcrystals is 61.54 m2·g−1. In addition, the pore size distribution of the 3D flowerlike porous E


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Figure 5. TEM images and corresponding SAED pattern of as-prepared 3D flowerlike porous Fe3O4 microcrystals (a and b) and Fe3O4−Au MCs (c and d). Insets of (a) and (c) are corresponding HRTEM images. (e) XRD pattern of Fe3O4−Au MCs.

Figure 6. High resolution XPS spectra of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs: Au 4f (a) and Fe 2p (b).

formation of the pure phase of the magnetite.49 In contrast to the 3D flowerlike porous Fe3O4 microcrystals, the Fe3O4−Au MCs looks like a black pompon, as shown in Figure 5c. The reason is that surfaces of 3D flowerlike porous Fe3O4 microcrystals are equably coated with Au nanocrystals. The Au nanocrystals appear black and the 3D flowerlike porous Fe3O4 microcrystals are light-colored, given that the elemental gold has a higher electron density than the Fe3O4.50 The mean particle size of Au nanocrystals is around 20 nm (Figure S4).51 The HRTEM image of Figure 5c demonstrates that the characteristic spacings of 0.255 and 0.237 nm can be indexed to the (311) crystal plane of Fe3O4 and the (111) crystal plane of Au, respectively. To further verify the presence of Au in Fe3O4−Au MCs, XRD analysis in Figure 5e is performed on the Fe3O4−Au MCs. Apart from the diffraction peaks of pure Fe3O4, the four addition peaks appearing at 38.4, 44.5, 64.7, and 77.5° correspond to characteristic diffraction of gold [JCPDS card no. 04-0784].26 The SAED pattern of the Fe3O4−Au MCs in Figure 5d represents that the diffraction rings are indexed to the diffractions of Fe3O4 and Au nanocrystals.52 Therefore, TEM, HRTEM, XRD, and SAED results make us conclude that the Fe3O4−Au MCs consist of Fe3O4 microcrystals and Au nanocrystals.

Fe3O4 microcrystals (inset of Figure 4) is 2−148 nm and the average value is about 3.72 nm.44 On the contrary, as can be seen from Figure S3, the adsorption and desorption isotherms of the 3D flowerlike iron alkoxide precursors are analogous to type II isotherms.45 Unlike type H3, type II isotherms are representative for nonporous or macroporous materials.46 This result further confirms that high temperature annealing over 500 °C can lead to the formation of the mesopores owing to the decomposition of organic species in the process of sintering.47 3.2. Structural and Magnetic Properties of 3D Flowerlike Porous Fe3O4 Microcrystals and Fe3O4−Au MCs. Figure 5 shows TEM images of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs. It is easily indicated from the TEM that the proposed Fe3O4 microcrystals have a porous structure composed of the interconnected nanoparticles. High resolution TEM (HRTEM) was employed to investigate the crystal structure. The interplanar spacing of 0.255 nm matches with the (311) plane of Fe3O4 in Figure 5a.48 The selected area electron diffraction (SAED) pattern (Figure 5b) for 3D flowerlike porous Fe3O4 microcrystals consists of (220), (311), (400), (422), (511), and (440) diffraction rings from magnetite, which strongly confirms F


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Industrial & Engineering Chemistry Research The chemical states of 3D flowerlike porous Fe 3 O 4 microcrystals and Fe3O4−Au MCs were studied by XPS. The C 1s peak at 284.8 eV was used as a charge correction reference.53,54 As shown in Figure S5, the XPS survey scans present the coexistence of Fe, O, C, and Au peaks. The Au 4f XPS spectrum for Fe3O4−Au MCs in Figure 6a displays that the bimodal peaks locating at 84.18 and 87.83 eV are assigned to Au 4f7/2 and Au 4f5/2, respectively.55,56 The XPS spectrum of Fe 2p for the 3D flowerlike porous Fe3O4 microcrystals (Figure 6b) shows that two intense peaks at 711.08 and 725.08 eV are in agreement with Fe 2p3/2 and Fe 2p1/2, respectively.57 Compared with the peak position of Fe 2p3/2 for 3D flowerlike porous Fe3O4 microcrystals, the peak position of Fe 2p3/2 of Fe3O4−Au MCs shifts from 711.08 to 710.58 eV, which may rely on electronic interaction between Au and Fe3O4.58 In addition, it needs to be mentioned that Fe 2p peak intensity is weakened with the addition of the Au nanocrystals. A possible explanation is that the XPS intensity is in direct proportion to atomic concentration and the atomic sensitivity factor.53 The UV−vis absorption spectra of 3D flowerlike porous Fe3O4 microcrystals, Au nanocrystal colloids, and Fe3O4−Au MCs are presented in Figure 7. As shown, the absorbance band

Figure 8. M−H loops of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs. (inset) Photograph of Fe3O4−Au MCs in deionized water before and after separation by magnet.

of Figure 8, although the Ms value of the Fe3O4−Au MCs decreases, the Fe3O4−Au MCs still have fast magnetic response and may be collected from the deionized water by a magnet, which implies that they can be recyclable. Figure S6 displays zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs in the 10−300 K temperature range. The magnetization of FC curves of the two samples is increased as temperature decreases. In addition, ZFC curves increase to a maximum at 178 and 166 K (the blocking temperature) for 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs, respectively. The slight decrease of the blocking temperature after coating with Au nanocrystals is closely related to the decrease of the dipolar interactions between the magnetic cores.61,62 3.3. Catalytic Performance of Fe3O4−Au MCs for 4NP. 4-NP as the probe molecule with help of NaBH4 was employed to evaluate the catalytic performance of Fe3O4−Au MCs. The UV−vis peak position of 4-NP is at 317 nm in Figure S7. Once NaBH4 solution was added, the peak was red shifted to 400 nm owing to the conversion of 4-NP to 4aminopyridine.63 Figure 9a shows the time-dependent UV−vis absorption spectra for reduction of 4-NP catalyzed by Fe3O4− Au MCs with assistance of excess NaBH4. When the Fe3O4− Au MCs are added into the 4-NP, the intensity of the UV−vis absorption spectra located at 400 nm decreases, which corresponds to the decrease of 4-NP with time. Meanwhile, a new peak emerges and increases at 300 nm originating from the formation of 4-aminopyridine (4-AP). The peak located at 400 nm disappears in 2 min, indicating that all of the 4-NP is converted to the 4-AP. In addition, the catalytic performance of the 3D flowerlike porous Fe3O4 microcrystals and Fe3O4− Au MCs is compared. As can be observed from Figure 9b, the reduction process from 4-NP to 4-AP would not occur spontaneously when the 3D flowerlike porous Fe3O4 microcrystals were used as catalysts even within 8 min. This makes it clear that Au nanocrystals on the surfaces of 3D flowerlike porous Fe3O4 microcrystals are indispensable to the reduction reaction of 4-NP. The linear correlation of ln(C/C0) versus reaction time for the reduction of 4-NP catalyzed by Fe3O4− Au MCs is presented in the inset of Figure 9b. The results

Figure 7. UV−vis absorption spectra of 3D flowerlike porous Fe3O4 microcrystals, Au nanocrystal colloids, and Fe3O4−Au MCs.

is wide from 350 to 750 nm, which confirmed that the Fe3O4 microcrystals are black. The absorbance for Au nanocrystal colloids that originated from the plasmon resonance band of Au is located at 520 nm. The maximum absorption peak of the Fe3O4−Au MCs in UV−vis absorption red shifts from 520 to 531 nm with respect to the Au nanocrystal colloids.59 This red shift results from the strong interaction between adjacent Au nanocrystals on the surfaces of the 3D flowerlike porous Fe3O4 microcrystals.48 M−H loops of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs are exhibited in Figure 8. The coercivity and the remanence of the two samples are almost invisible, indicating that they show superparamagnetism at room temperature.60 The Ms values of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs are 73.1 and 58.42 emu·g−1. The reduction of the Ms value may arise from enhancement of the mass ratio of Au nanocrystals to Fe3O4 and the diamagnetism of Au nanocrystals on the surfaces of 3D flowerlike porous Fe3O4 microcrystals.59 As shown in the inset G


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Figure 9. (a) Time-dependent UV−vis absorption spectra of 4-NP reduced by NaBH4 using Fe3O4−Au MCs as the catalysts. (b) C/C0 versus reaction time for reduction of 4-NP catalyzed by 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs. (inset) ln(C/C0) versus reaction time for reduction of 4-NP catalyzed by Fe3O4−Au MCs. (c) Reusability of Fe3O4−Au MCs for catalytic reduction of 4-NP.

show that the reduction of 4-NP catalyzed by Fe3O4−Au MCs obeys a pseudo-first-order kinetic model and the reaction rate constant (k) is 1.819 min−1. To assess the reusability of Fe3O4−Au MCs, Fe3O4−Au MCs are separated and collected using the magnet. The same reaction is repeated for six cycles, and the catalytic activities remain almost constant, as shown in Figure 9c. The slight decrease in the catalytic activities may be due to the loss of Fe3O4−Au MCs during the recycling process.64,65 Turnover frequency (TOF) is another essential indicator for studying catalytic activity for catalysts. The TOF value of catalytic reduction of 4-NP by Fe3O4−Au MCs is determined to be 2.874 min−1. As summarized in Table 1, compared with

the previously reported catalysts,66−73 the proposed catalysts (Fe3O4−Au MCs) exhibit a much higher TOF value. Based on these results, we draw a conclusion that Fe3O4−Au MCs, as high-efficiency catalysts for convenient separation and reusability, have a very good practical application prospect in the catalytic degradation of organic pollutants.

4. CONCLUSIONS In summary, the 3D flowerlike porous Fe3O4 microcrystals were successfully obtained with a heat treatment route and a subsequent calcining process. The hydrothermal reaction time and the molar amount of urea are very important in the formation of the iron alkoxide precursors, and the possible growth mechanism is proposed. The obtained 3D flowerlike porous Fe3O4 microcrystals have an isotherm with the features of type H3 hysteresis loops, demonstrating the existence of the mesopores in the sample. Fe3O4−Au MCs are formed by attaching Au nanocrystals to the surface of the 3D flowerlike porous Fe3O4 microcrystals, which has good magnetic responsiveness. The catalytic reduction of 4-NP by Fe3O4− Au MCs obeys pseudo-first-order kinetics and the TOF value is 2.874 min−1 by calculation, which is superior for most catalysts reported. The Fe3O4−Au MCs exhibit excellence catalytic activity for 4-NP. Moreover, the Fe3O4−Au MCs can be reused six times without reducing their catalytic activities. Given these advantages, we believe that these Fe3O4−Au MCs can be broadly used as recyclable catalysts in environmental applications.

Table 1. Comparison of Catalytic Capacities of Various Catalysts for Catalytic Reduction of 4-NP catalyst and its amount

ka (min−1)

TOFb (min−1)

ref

Au NPs/chitosan/Fe3O4 CNs Fe3O4@PBLG@Au NPs PAM/PPY/GO−Ag PCP@Au−Ag Pt@Ag NPs FAC-0.5 Fe3O4/Ag@NFC Fe3O4−Au MCs

2.826 0.126  1.26 0.172 0.355 2.100 1.980 1.819

2.100 1.817 0.89 2.557 2.167 0.088 0.540 2.610 2.874

66 67 68 69 70 71 72 73 this work

a

Kinetic rate constant. bTurnover frequency (TOF) = (mol of reacted organic substrate/mol of noble metal) × reaction time (min−1). H


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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.iecr.9b02777.

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Pawley refinement of XRD pattern and Mössbauer spectrum of 3D flowerlike porous Fe3O4 microcrystals; M−H loops of iron alkoxide precursor calcinated at 200, 500, and 700 °C; nitrogen adsorption−desorption isotherms of 3D flowerlike iron alkoxide precursors; TEM images and particle size distribution of Au nanocrystals; XPS survey spectra of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs; ZFC and FC curves of 3D flowerlike porous Fe3O4 microcrystals and Fe3O4−Au MCs; UV−vis absorption spectra of 4-NP before and after adding NaBH4 solution (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 434 3294566. Fax: +86 434 3294566 (Y.L.). *E-mail: [email protected]. (Y.Z). *E-mail: [email protected] or DANDAN.WANG@ globalfoundries.com (D.W.). ORCID

Lei Chen: 0000-0003-2616-2190 Jinghai Yang: 0000-0001-8409-6035 Yang Liu: 0000-0003-1485-8764 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21676115, 51609100, 61675090, and 61575080); and the Thirteenth Five-Year Program for Science and Technology of Education Department of Jilin Province (Item Nos. JJKH20191018KJ and JJKH20191022KJ).

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Fundamental Formation of Three-Dimensional Fe3O4 Microcrystals

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