One-step solvothermal synthesis of porous MnFe2O4 nanoflakes and their magnetorheological properties

Journal Pre-proof One-step solvothermal synthesis of porous MnFe2O4 nanoflakes and their magnetorheological properties Guangshuo Wang, Yingzhe Zeng, Fei Zhou, Xin Chen, Yingying Ma, Liyun Zheng, Meixia Li, Yang Sun, Xiaoyan Liu, Huiying Liu, Ruitao Yu PII:

S0925-8388(19)34290-2

DOI:

https://doi.org/10.1016/j.jallcom.2019.153044

Reference:

JALCOM 153044

To appear in:

Journal of Alloys and Compounds

Received Date: 18 September 2019 Revised Date:

13 November 2019

Accepted Date: 14 November 2019

Please cite this article as: G. Wang, Y. Zeng, F. Zhou, X. Chen, Y. Ma, L. Zheng, M. Li, Y. Sun, X. Liu, H. Liu, R. Yu, One-step solvothermal synthesis of porous MnFe2O4 nanoflakes and their magnetorheological properties, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/ j.jallcom.2019.153044. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

One-step solvothermal synthesis of porous MnFe2O4 nanoflakes and their magnetorheological properties Guangshuo Wanga, Yingzhe Zenga, Fei Zhoua, Xin Chena, Yingying Maa,*, Liyun Zhenga, Meixia Lia, Yang Sunb,*, Xiaoyan Liua, Huiying Liua, Ruitao Yuc,d,* a School of Materials Science and Engineering, Hebei University of Engineering, Handan, 056038, China b School of Mechanical and Equipment Engineering, Hebei University of Engineering, Handan, 056038, China c Key Laboratory of Tibetan Medicine Research, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, 810008, China d Qinghai Key Laboratory of Tibetan Medicine, Northwest Institute of Plateau Biology, Chinese Academy of Sciences, Xining, 810008, China *

Correspondence to: [email protected] (YY Ma); [email protected] (Y Sun); [email protected] (RT Yu)

ABSTRACT In this study, porous MnFe2O4 nanoflakes were synthesized by a facile one-step solvothermal method, and the obtained products were employed as new magnetorheological (MR) materials to prepare a well dispersed MR suspension. The synthesized MnFe2O4 nanoflakes were systematically examined using XRD, FE-SEM, TEM, XPS, BET and VSM. The shear stress, yield stress, storage modulus and loss modulus of MR fluid strongly depended on the applied magnetic field strengths, indicating typical MR performances. More importantly, the obtained MR fluid demonstrated enhanced sedimentation stability in the recording time. This phenomenon was mainly attributed to the unique porous lamellar structure and the decreased particle-fluid density mismatch. In conclusion, the prepared MnFe2O4 nanoflakes as novel MR materials will serve as a bright candidate in various technological applications. Keywords: MnFe2O4 nanoflakes; Solvothermal method; Porous structure; Magnetorheological fluid; Sedimentation stability

Introduction Smart materials are designed materials that have flexile properties with desirable characteristics, which can change under surrounding conditions, such as temperature, moisture, light, pH, electric or magnetic fields, have become a central topic in both fundamental research and practical applications because of their impressive properties [1-3]. The magnetorheological (MR) fluids as a general class of smart materials have garnered tremendous interest due to their significant and unique physical, rheological and mechanical properties. The MR fluids are ordinarily consisted of magnetic materials dispersed in a carrier liquid [4]. In the absence of a magnetic field, the MR fluids remain in a disordered liquid condition. Once a magnetic field is exerted, they exhibit a reversible phase change from a Newtonian fluid to a Bingham plastic fluid [2-6]. Therefore, the MR fluids have a great potential for multiple applications involving dampers, brakes, control systems, shock absorbers, polishing devices and artificial muscles [5-9]. However, the widespread applications of MR fluids are seriously limited owing to their poor long-term stability [10, 11]. It has been recognized that sedimentation problem of MR fluids is mainly attributed to the significant density difference between dispersed particles and carrier fluids [10-12]. Like many other systems containing suspended particles, the stability of MR suspensions is thought to be the fundamental problem especially at a high particle concentration. To combat the sedimentation problem, scholars and researchers have concentrated on adjusting the compositions of MR fluids such as decreasing size of the particles, introduction of thixotropic materials and surfactants, and application of viscoplastic fluids as the continuous phases [3, 4, 10, 13-16]. Although carbonyl iron (CI) particles are considered as the most commonly used in MR fluids owing to the high magnetic properties, they usually have a fatal sedimentation problem resulting from their high density. Instead of that material, manganese ferrite (MnFe2O4) nanomaterials are considered to be promising candidates to overcome the existing sedimentation problem due to their lower density and significant magnetic behavior.

In our previous study, we adopted graphene oxide (GO) nanosheets as flexible substrates to load MnFe2O4 nanoparticles, and the obtained magnetic MnFe2O4/GO nanocomposites were used as the dispersed particles and assessed the application potential in MR fluids [17]. The results showed that the sedimentation stability of MR fluid was greatly improved, which can be attributed to the unique two-dimensional structure and reduced density mismatch. Inspired by the success of MnFe2O4/GO-based MR fluid, the present study aimed to synthesize two-dimensional MnFe2O4 nanoflakes by a facile one-step solvothermal strategy and examined their application in MR fluids. It is expected that the emerging MnFe2O4 nanoflakes are thought to be promising candidates for MR fluids to resist the existing sedimentation problems owing to their low particle density, porous characteristics, special nanoflake structure and high specific surface area.

Experimental Synthesis of MnFe2O4 nanoflakes Iron trichloride hexahydrate (FeCl3·6H2O), manganese (II) chloride tetrahydrate (MnCl2·4H2O), hexamethylenetetramine (HMT) were purchased from Sigma Aldrich (Shanghai) Trading Co., Ltd. The MnFe2O4 nanoflakes were prepared via a one-step solvothermal method. In a typical synthesis, 0.32 g of FeCl3·6H2O, 0.12 g of MnCl2·4H2O and 1.5 g of HMT were added to 80 mL of ethylene glycol under vigorous magnetic stirring. The formed uniform mixture solution was sealed in a 100 mL Teflon-lined stainless-steel autoclave, and then heated at 200oC for 12 h. After the autoclave was cooled to room temperature, the resultant precursors were collected by centrifugation, washed with alcohol several times and dried in a vacuum oven at 60oC for 24 h. Subsequently, the samples were calcined at 450oC for 5 h under nitrogen atmosphere to obtain MnFe2O4 nanoflakes. Preparation of MR fluids Commercial carbonyl iron (CI) particles were supplied by Jiangyou Hebao Nanomaterial Co., Ltd. The densities of CI particles and MnFe2O4 nanoflakes were measured by a pycnometer method, and the values were determined to be 7.81 g/cm3 and 4.36 g/cm3, respectively. To prepare two types

of MR suspensions, the CI particles and MnFe2O4 nanoflakes were respectively dispersed in 500 cS silicone oil with 30 wt% of particle concentration. Characterization The identification of crystal phase structure was made by a Bruker D8 Advance powder X-ray diffraction (XRD) system using Cu/K-α radiation. The morphology and microstructure were fully characterized by a ZEISS MERLIN Compact field emission scanning electron microscopy (FE-SEM) and a Tecnai G2 F20 transmission electron microscopy (TEM). The chemical analysis was performed with a ThermoFisher Scientific KAlpha X-ray photoelectron spectroscopy (XPS). The Brunauer Emmett Teller (BET) surface area was determined using a Quantachrome NOVA 4000e instrument and the pore size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method based on the adsorption isotherm data. The magnetic properties were investigated at room temperature using a Lakeshore 7304 vibrating sample magnetometer (VSM) from -2387 kA/m to 2387 kA/m. The MR measurements were conducted on an Anton Paar Physica MCR301 rheometer equipped with a MR device (MRD180, parallel-plate system PP 20, gap distance is 1 mm). The sedimentation experiments of MR fluids were performed by placing the same volume amounts of two MR fluids into measuring cuvettes. The sedimentation ratio was expressed as the height percentage of the concentration suspension relative to the initial suspension.

Results and discussion Fig. 1 illustrates the XRD pattern of MnFe2O4 nanoflakes. The characteristic diffraction peaks are observed at 2θ = 18.4°, 30.2°, 35.5°, 43.2°, 53.6°, 57.1° and 62.7°, which can be assigned to the (111), (220), (311), (400), (422), (511) and (440) crystal planes, respectively. The positions and intensities of all the identified peaks match well with the data for spinel MnFe2O4 (JCPDS card No. 74-2403). No other characteristic peaks from any impurity are found in the XRD pattern, indicating that high phase purity of MnFe2O4 was formed by the solvothermal reaction. In addition, the average crystallite size of MnFe2O4 nanoflakes can be calculated using the Scherrer’s equation as follows:

D=

kλ β cosθ

(1)

where, D is the average diameter of crystals, k is the shape factor, λ is the wavelength of X-ray diffraction (λ = 0.154 nm), β is the peak-width at half maximum intensity, and θ is the Bragg angle (degree). Therefore, the average crystallite size of MnFe2O4 nanoflakes is determined to be about 15.5 nm based on the diffraction peak of (311). The surface morphology of MnFe2O4 nanoflakes was characterized by FE-SEM and TEM, as shown in Fig. 2 and Fig. 3, respectively. The FE-SEM images of the as-synthesized MnFe2O4 are presented in Fig. 2. It is clearly observed that the MnFe2O4 particles have a well-defined flake-like morphology with a lateral size of several microns. The FE-SEM image with high-magnification (Fig. 2b) reveals that there are some slight wrinkles and crimps at the edge of the coarse MnFe2O4 flakes. The thickness of MnFe2O4 nanoflakes is determined to be about 21.4 nm through measuring the crimped flakes. The representative TEM images of MnFe2O4 nanoflakes are depicted in Fig. 3, from which the obtained MnFe2O4 is present as a two-dimensional layered-like structure. The lateral size of MnFe2O4 nanoflakes is estimated to be 950 nm. The enlarged TEM image reveals that the formed MnFe2O4 nanoflakes are composed by a number of interconnect nanocrystals. The measured lattice spacing of 0.297 nm accords with the (220) interplanar distances of spinel phase MnFe2O4, which is consistent with the results of XRD characterization. The chemical composition and valence state of the as-synthesized MnFe2O4 nanoflakes were analyzed by XPS measurements as illustrated in Fig. 4. The XPS survey spectrum of MnFe2O4 nanoflakes (Fig. 4a) shows that the binding peaks assigned to O KLL, Fe LMM, Mn 2p, Fe 2p and O 1s are obviously displayed. The high-resolution core level XPS spectra of Mn 2p, Fe 2p and O 1s were further carried out. For the Mn 2p spectrum (Fig. 4b), two strong peaks at 640.9 eV and 652.9 eV can be assigned to the Mn 2p3/2 and Mn 2p1/2, respectively, confirming the existence of Mn2+ in the obtained MnFe2O4 products [18, 19]. Two peaks at 710.9 eV and 724.2 eV are observed in the Fe 2p spectrum (Fig. 4c), corresponding to the Fe 2p3/2 and Fe 2p1/2 of Fe3+, respectively [3]. The fitting

two peaks of O 1s in Fig. 4d at 530.1 eV and 531.1 eV are attributed to the lattice oxygen from MnFe2O4 and surface-adsorbed oxygen, respectively [20]. Therefore, the XPS results further confirm that the MnFe2O4 nanoflakes with high phase purity are successfully synthesized in the solvothermal reaction. Nitrogen adsorption/desorption isotherms of MnFe2O4 nanoflakes and their pore size distribution are depicted in Fig. 5. The resulting isotherms (Fig. 5a) can be identified as a typical type-IV isotherm with H3 hysteresis, indicating the presence of mesoporous structure. The samples have a high specific surface area and the value is determined to be 92.86 m2/g by the BET equation. Fig. 5b shows the pore distribution plots by BJH analysis of the adsorption branches. It is clearly observed that the calculated pore size distribution is narrow, indicating that the MnFe2O4 nanoflakes possess a uniform mesoporous structure. The average pore diameter and total pore volume are determined to be 12.62 nm and 0.293 cm3/g, respectively. In order to examine the magnetic properties, magnetization curves of MnFe2O4 nanoflakes were recorded in a magnetic field ranging from -2387 kA/m to 2387 kA/m as shown in Fig. 6. The inset shows enlarged hysteresis loops at low applied fields. The ignorable remanence (Mr) and coercivity (Hc) confirm the typical superparamagnetic characteristic of the prepared MnFe2O4 nanoflakes at room temperature, which is suitable for MR applications. This is, the magnetic particles can suspend stably in the suspension and keep no any residual magnetism by removing the external magnetic field [21, 22]. In addition, the value of saturation magnetization (Ms) for the MnFe2O4 nanoflakes is about 58.8 Am2/kg, which is sufficiently high to guarantee the application of MnFe2O4 nanoflakes in MR fluids. The as-prepared MnFe2O4 nanoflakes were dispersed into silicone oil to prepare MR fluid with a particle concentration of 30 wt%. The MR properties of MnFe2O4 nanoflakes based MR fluid were measured by a rotational rheometer under varied applied magnetic fields. Fig. 7 shows the flow curves of MR fluid. The curves describe the relationship between the shear stress and shear rate and

the effect of magnetic field strength on shear stress. One can see that the shear stress in the absence of magnetic field increases almost linearly with increasing shear rate, suggesting a typical Newtonian viscous fluid behavior. Under the action of an external magnetic field, the dipole-dipole interactions among the adjacent polarized nanoflakes give rise to the yield stress [23]. It needs to be pointed out that the values of shear stress under magnetic fields are found to be larger than those without magnetic fields. In addition, the shear stress of MR fluid increases with the increasing strength of magnetic field due to the enhanced dipole-dipole interactions [4, 9]. The dynamic yield stress of the MR fluid under the corresponding magnetic field strength was estimated based on the Casson fluid model, which can be written mathematically as:

√ =

+

for τ > τy

where τ is the shear stress, τy represents the yield stress, ηc is the Casson’s viscosity, and

(3) defines

the shear rate. The experimental curves of shear stress-shear rate under different magnetic field strengths were fitted using Casson model, and the detailed rheological parameters are summarized in Table 1. Fig. 8 presents the obtained yield stress as a function of magnetic field strength. It is obviously found that the yield stress is proportional to magnetic field strength, which suggests that the interactions between MnFe2O4 nanoflakes are enhanced owing to the increase of magnetic field strengths [24]. In addition, a power law is used to describe the dependence of dynamic yield stress on magnetic field strength, which is expressed as follows:

τy ∝Hm

(3)

where the exponent m can be obtained by fitting the yield stresses against magnetic field strength in a log-log model. The slope of the plot was calculated to be approximately 1.0. According to the magnetic polarization model, the slope of the curve of yield stress versus magnetic field strength is known to be 2.0 at low magnetic field strengths, while the slope occurs to be 1.5 at intermediate field strengths as the magnetic field saturation dominates near particle-particle contacts. The transition of slope from 2.0 to 1.5 has been reported in CI particles-based MR fluids [14-16, 25-27]. In addition,

Choi’s group investigated the magnetorheological characteristics of CI microparticles with different shapes in detail, and they found a transition of slope from 1.5 to 1.0 for flake-shaped CI-based MR fluid [28]. The slope was 1.5 in the magnetic field strength of 43 to 171 kA/m, and then the slope turned to be 1.0 when the magnetic field intensity was high up to 171 kA/m. For the MnFe2O4 nanoflakes based MR fluid in this study, the slope was determined to be about 1.0 from Fig. 8, which probably due to the complete saturation of MR fluid under the applied magnetic field strengths. To evaluate the real-time responses of the prepared MR fluid, the shear flow curves were recorded at a constant shear rate of 1/s under different imposed magnetic fields. The Fig. 9 describes the changes of shear stress with time at the alternate switch on and off moments of magnetic fields. It was clearly observed that the shear stress increased simultaneously by the application of an external magnetic field, indicating that the solid chains were emerged rapidly in the MR fluid. In contrast, the shear stress reduced quickly back to their original values once the magnetic field was removed. The phenomenon indicates that the developed MR fluid has reversible and reproducible responses upon the manipulation of magnetic field. The dynamic viscoelastic properties of MnFe2O4 nanoflakes based MR fluid were examined with oscillation measurements. The storage modulus (G') and loss modulus (G'') were recorded and plotted as a function of strain at a fixed angular frequency of 10 rad/s, as shown in Fig. 10 and Fig 11, respectively. The values of G' are observed to be higher than those of G'' at the same magnetic field strength, indicating the formation of a stable elastic chain-like structure of magnetic nanoflakes in the MR fluid. When the applied strain is in the low-deformation region, the structures of the MR fluid are undisturbed and the G' and G'' exhibit a flat plateau shape, which is considered in the so-called linear viscoelastic (LVE) region [11]. When the strain is higher than critical value, the MR fluid is known in the nonlinear viscoelastic region, the produced chain-like structures in the MR fluid become unstable with increasing magnetic fields. In addition, the values of G' and G'' decrease

gradually with further increasing strain, which can be assigned to the irreversible changes in the strong chain-like structures [29, 30]. The frequency sweep measurements were performed at a fixed strain of 0.01% to investigate the relationship of viscoelastic properties and angular frequency, as shown in Fig. 12 and Fig. 13. At a fixed magnetic field strength, both the G' and G'' keep constant across the whole operating frequency, which can be attributed to the formation of strong solid-like structures in the MR fluid [31]. However, it is worthy to note that both the G' and G'' increase with the increasing magnetic field strengths, which can be explained by the enhancement of particle-particle interactions. Note that the strong magnetic polarization between particles maintains the existed chain-like structures stable in the MR suspension even at a high strain, which is contrasted with the dynamic behavior of electrorheological (ER) fluids [32, 33]. The sedimentation stability of suspended particles is considered as a basic parameter in evaluating MR fluids. Fig. 14 shows the curves of sedimentation ratio versus time of MnFe2O4 nanoflakes and CI particles-based MR fluids. Compared with CI-based MR fluid, MnFe2O4 nanoflakes based MR fluid settles down slower in the same testing duration, indicating a better sedimentation stability. On one hand, the unique properties of as-prepared MnFe2O4 nanoflakes including porous characteristic, large surface area (92.86 m2/g) and special lamellar structure make them act as the parachutes to increase the sedimentation stability. On the other hand, the density of MnFe2O4 nanoflakes is measured to be 4.36 g/cm3, which is greatly smaller than that of commercial CI particles (7.81 g/cm3). The equation describing the sedimentation law is as follows [34, 35],

V (φ , d ) =

ρ p − ρc × g × d 2 ⋅ 18 × v × ρ c 

[1 − φ ]

4.6φ  1 + 3  (1 − φ ) 

(4)

where V is the particle migration velocity (m/s), ρp is the particle density (kg/m3), ρc indicates the density of liquid medium (kg/m3), ν is the kinematic viscosity of liquid medium, g is the gravity

constant, d is the particle diameter, and φ represents the volume fraction. According to the expression, the particle migration velocity is proportional to the difference in particle density and liquid medium density. Therefore, the sedimentation stability of MnFe2O4 nanoflakes based MR fluid is enhanced due to the decreased density mismatch between MnFe2O4 nanoflakes and silicone oil. In summary, the enhanced sedimentation stability of MnFe2O4 nanoflakes based MR fluid can be attributed to the porous nanoflake structure with high surface area and the reduced particle-fluid density mismatch.

Conclusions We adopted a facile solvothermal method to synthesize porous MnFe2O4 nanoflakes and then used them as novel dispersed phase to prepare MR fluids. The MR properties and the sedimentation stability of the as-prepared MR fluid were examined in detail. The results showed that MnFe2O4 nanoflakes possessed a two-dimensional and porous structure with a lateral size of 950 nm. They exhibited a high specific surface area of 92.86 m2/g and a typical superparamagnetic behavior with saturation magnetization of 58.8 Am2/kg, which indicated that the obtained MnFe2O4 nanoflakes were suitable for MR applications. The rheological results showed that the prepared MnFe2O4 nanoflakes based MR fluid demonstrated typical MR performances under different magnetic field strengths. What’s more, the obtained MR fluid provided enhanced sedimentation stability due to the unique two-dimensional structure and the reduced particle-fluid density mismatch. Our present results are not only helpful to understand the MR mechanisms, but also provide a promising method to develop novel MR fluids with an optimum response to different magnetic stimuli.

Acknowledgments This study was financially supported by Science and Technology Research Development Program of Handan (No. 1721211051-2) and National Natural Science Foundation of China (No. 51601053).

References

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Figures and Captions Figure 1. The XRD pattern of MnFe2O4 nanoflakes. Figure 2. The SEM images of MnFe2O4 nanoflakes with different magnifications. Figure 3. The TEM images of MnFe2O4 nanoflakes with different magnifications. Figure 4. The XPS survey (a), Mn 2p (b), Fe 2p (c) and O 1s (d) spectra of MnFe2O4 nanoflakes. Figure 5. Nitrogen adsorption-desorption isotherms (a) and corresponding pore size distributions (b) of MnFe2O4 nanoflakes.

Figure 6. Magnetization curves of MnFe2O4 nanoflakes obtained at room temperature. The inset shows enlarged hysteresis loops at low applied fields.

Figure 7. The shear stress vs. shear rate for MnFe2O4 nanoflakes based MR fluid. Figure 8. The yield stress vs. magnetic field intensity for MnFe2O4 nanoflakes based MR fluid. Figure 9. The shear stress vs. time at a constant shear rate of 1/s under a square wave magnetic field. Figure 10. The storage modulus vs. strain for MnFe2O4 nanoflakes based MR fluid. Figure 11. The loss modulus vs. strain for MnFe2O4 nanoflakes based MR fluid. Figure. 12. The storage modulus vs. angular frequency for MnFe2O4 nanoflakes based MR fluid. Figure 13. The loss modulus vs. angular frequency for MnFe2O4 nanoflakes based MR fluid. Figure 14. Sedimentation curves of CI particles and MnFe2O4 nanoflakes based MR fluids.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11

Fig. 12

Fig. 13

Fig. 14

Table 1 Rheological parameters of MR fluid fitted with Casson model Casson fluid model √ =

Magnetic field

+

intensity (kA/m)

Yield stress τy (Pa)

Casson’s viscosity ηc (Pa·s)

Adj. R-Square (R2)

0

16.06

1.65

0.97443

82

88.49

1.39

0.95253

142

109.31

1.43

0.94121

186

120.92

1.54

0.94063

220

129.03

1.69

0.94299

250

136.99

1.84

0.94829

Highlights: ► Synthesis of porous MnFe2O4 nanoflakes by a facile solvothermal method. ► The MnFe2O4 nanoflakes had high specific surface area and moderate saturation magnetization. ► The MnFe2O4 nanoflakes-based MR fluid exhibited enhanced magnetorheological properties and good sedimentation stability.

Author Contributions Guangshuo Wang: Resources; Writing-Original Draft; Supervision; Writing-Review & Editing Yingzhe Zeng; Investigation; Visualization Fei Zhou: Validation; Visualization Xin Chen: Formal analysis Yingying Ma: Conceptualization; Investigation; Supervision Liyun Zheng: Software; Writing-Review & Editing Meixia Li: Writing-Review & Editing Yang Sun: Supervision; Formal analysis Xiaoyan Liu: Data Curation Huiying Liu: Methodology; Investigation Ruitao Yu: Supervision

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.