Abstract
The core-shell structured SiO2@Ni-Al layered double hydroxide (LDH) composites were prepared via self-assembly of Ni-Al LDH on the surface of SiO2 spheres. Only coating a layer of ultrathin Ni-Al LDH sheet, the resulting SiO2@Ni-Al LDH composites exhibit significantly enhanced electrorheological (ER) characteristics compared to conventional bare SiO2 spheres. The monodispersed SiO2 spheres with average diameters of 260 nm were synthesized by the hydrolysis of tetraethyl orthosilicate (TEOS), while the shell part, Ni-Al LDH sheet was prepared by the hydrothermal procedure. The morphology of the samples was investigated via scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The structure of the samples was characterized by X-ray diffraction (XRD). The species and distribution of elements in samples were confirmed by X-ray photoelectron spectroscopy (XPS), Energy dispersive analysis of X-ray (EDX) and elemental mapping in STEM. Subsequently, the ER characteristics of the composites dispersed in insulating oil were characterized by a rotational rheometer. The electric field-stimulated rheological performances (yield stress, viscosity, modulus, etc.) were observed under an external electric field, which is different from the Newtonian state in the free electric field.
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Introduction
There are many examples of biological and inorganic materials with complicated and efficient hierarchical morphologies in natural world, such as marine coccolith, radiolarian shell and clay, exhibiting superior performance1,2,3,4,5. To synthesize inorganic materials with natural materials’ structure for the development of catalysis6, electronic7, lithium secondary battery8 and many other applications to meet the challenges of green energy and sustainability, has been one of the hotspots in material chemistry fields. Here, the layered double hydroxides (LDH) are hydrotalcite-like clays with the empirical formula, [MII1-xMIIIx(OH)2]x+[An−]x/n·mH2O, where MII and MIII are divalent and trivalent metals such as Mg, Ni, Co, Al and Fe, An− can be almost any anion with the charge number of n9,10,11. Due to their special and complicated structure and anion exchange property, LDH materials have been applied to many areas including catalysts12,13,14,15,16, anion exchangers17,18,19,20, electrodes for alkaline secondary batteries and supercapacitor21,22, especially for electrorheological (ER) fluids23,24,25. Compared to other clay minerals including smectites, kaolins, Pyrophyllite, talo etc., LDH sheets possess distinctive advantages, such as positive charges on its surface due to the existence of metal ions (Ni+, Al3+, Mg2+ etc.) and negative charges among the interlayers26, which are more sensitive to electric stimulus for the application of ER systems. ER fluids are smart materials that experience continuous and reverse changes in rheological properties in a few milliseconds under an applied electric field27,28,29,30. In general, it involves a dispersed phase such as polyaniline (PANI), polypyrrole (PPy), polystyrene (PS), silica, titania, zeolite, clay or other polarizable or semiconducting particles and a continuous phase, including silicone oil or corn oil, etc.24,31,32,33. The solid-like property emerges when electric field is applied across the ER fluids, with the formation of chain-like or columnar structures aligned to the electric field. Due to the fast response to electric field, controllable mechanical properties and no waste discharge, the ER fluid has been paid more and more attention in an automotive industry such as shock absorbers, engine mounts and clutches34,35.
The inorganic materials (aluminosilicate, zeolite, alumina, TiO2, SiO2 etc.) and the semiconducting polymer (PANI, copolypyrrole, copolyaniline, polyacene quinone radicals etc.) have been widely utilized to prepare ER suspensions due to their low cost and anisotropic morphology36,37,38,39,40,41,42,43,44,45,46,47,48. Recently, much attention has been focused on the layered inorganic materials, such as LDH sheet, graphene or graphene oxide (GO) based hybrids. However, the aggregation of LDH sheet usually greatly weakens the material’s ER properties, thus significantly limits the applications in an automotive industry field. The core-shell structured composites with controllable morphology, orientation and dimensionality have evoked considerable interest because of their remarkable ER properties, low cost and well controlled particle size49,50.
Diverse composite nanoparticles with core-shell structure have been found to show excellent ER properties. PS microspheres coated with GO sheets were prepared using π–π stacking interactions51. PANI-coated anisotropic snowman-like poly(methyl methacrylate) (PMMA) particles were also prepared with an anionic surfactant as ER fluid suspension particles52. Core-shell structured titania/PANI and PS/PANI microspherical composites were synthesized via in situ polymerization in the presence of titanium oxide or PS nanospheres42,53. SiO2/TiO2 particles were prepared by means of a co-precipitation process by Wu and his coworkers54. Monodispersed PMMA microbeads coated with MWNT–NH2 were also prepared via a grafting reaction55. To prepare core-shell structured biomimetic materials, Mirkin and co-workers have explored diatom silica walls as the core template to integrate with inorganic nanoparticles. The nanoparticles formed a near-monolayer along the surface morphology and shape of the diatom template56. And a facile approach related to direct self-assembly of inorganic nanostructures under mild conditions without the utilization of harsh chemical treatments is under explored urgently. Seeking for greener synthetic routes to avoid the use of toxic reagents and realize time-saving has been one of the major concerns of the global vision of world economy.
Inspired by the previous work, here, SiO2@Ni-Al LDH core-shell structure was prepared via self-assembly of Ni-Al LDH on SiO2 sphere as shown in Fig. 1. As the core material, monodispersed SiO2 with diameters of 260 nm were synthesized by the hydrolysis of tetraethyl orthosilicate (TEOS) induced using the catalysis of ammonia in ethanol medium, while the shell part, Ni-Al LDH, was prepared by the hydrothermal method. Subsequently, the ER characteristic of the SiO2@Ni-Al LDH composite dispersed in insulating oil was characterized by a rotational rheometer.
Schematic diagram of the stepwise preparation of electro-responsive SiO2@Ni-Al LDH composites.
Results and Discussion
Generally, the surface of the bare SiO2 sphere in ethanol is negatively charged with a zeta potential value of −15.8 mV, the zeta potential value for Ni-Al LDH sheets is 16.18 mV. Therefore, the electrostatic interactions between the negatively charged SiO2 sphere and positively charged Ni-Al LDH sheet could take place. Due to the low zeta potential, the anionic surfactant was also introduced to enhance the adsorption between SiO2 sphere and Ni-Al LDH sheet. The morphology of Ni-Al LDH sheet is shown in Fig. 2. It can be observed from the SEM and TEM images, Ni-Al LDH sheet is irregular and thin hexagonal platelet with a mean lateral size of 300 nm, while most of the LDH sheets stack with each other.
(A,C) SEM and TEM images of Ni-Al LDH sheets; (B,D) High magnification SEM and TEM images of Ni-Al LDH sheets.
The morphology of SiO2@Ni-Al LDH composites was analyzed through SEM (Fig. 3A,B) and TEM (Fig. 3C,D) images. The SiO2 spheres are found to be monodispersed with a uniform size of ~260 nm. The final SiO2@Ni-Al LDH composites exhibited a roughly spherical morphology with curly Ni-Al LDH sheets coated on the surface. The presence of Ni-Al LDH sheet in the composites was also proved through EDX (Fig. 3M), the weight percentages of Ni and Al elements were calculated to be 7.17% and 0.56% respectively.
(A,B) SEM images (C,D) TEM images of SiO2@Ni-Al LDH composites; (E) Dark-field STEM image of SiO2@Ni-Al LDH composites. The elemental mapping for Al (F), Ni (G), Si (H) and O (L) elements in the core-shell structure of SiO2@Ni-Al LDH composites; and (M) EDX spectrum of SiO2@Ni-Al LDH composites.
A dark-field STEM analysis of SiO2@Ni-Al LDH composites was also used to characterize the distribution of the elements and the result is shown in Fig. 3E. It can be seen that the SiO2@Ni-Al LDH composites have a representative core-shell structure. In addition, element mapping of the identical spheres showed the space distribution of Si, O, Ni, Al. As shown in Fig. 3H,L, the core part is filled with intense Si and O signals. Meanwhile, the Ni, Al elements are mainly located on the shell part, indicating the successful coating of the Ni-Al LDH sheet.
The XRD patterns of SiO2 sphere, Ni-Al LDH sheet and SiO2@Ni-Al LDH composites are shown in Fig. 4B. The diffraction pattern of SiO2 sphere depicts a reflection characteristic of amorphous silica57. After the coating with the Ni-Al LDH sheet, peaks related to 003, 006, 012 and 015 originating from Ni-Al LDH sheet can be observed in addition to those resulted from silica. The composition of SiO2 sphere and SiO2@Ni-Al LDH composites was investigated using the XPS technique. As shown in Fig. 4A, the XPS survey spectrum of SiO2@Ni-Al LDH composites revealed the presence of Si, O, Ni, Al elements, implying the formation of SiO2@Ni-Al LDH composites. Taken together, the results of SEM, TEM, EDX, STEM, XRD and XPS evidenced the successful synthesis of SiO2@Ni-Al LDH composites.
(A) XPS survey spectra of SiO2 sphere and SiO2@Ni–Al LDH composites. (B) XRD patterns of SiO2 sphere, Ni−Al LDH sheet and SiO2@Ni-Al LDH composites, respectively.
In general, the shear stress of the bare silica particles based suspensions are not very sensitive to the external electric field stimulus due to their weak ER activity58. Therefore, coating a layer of ultrathin Ni-Al LDH sheet, is an effective way to enhance the ER activity of the bare SiO2 spheres. Here, the prepared SiO2@Ni-Al LDH composites exhibited improved ER performance compared to the conventional bare SiO2 particles, which have been described below:
The shear stress-shear rate behaviour of the SiO2@Ni-Al LDH composites (10 wt% particle concentration) was tested using a rotational rheometer equipped with a high voltage generator subjected to different electric field strengths. As shown in Fig. 5A, the shear stress increased linearly with the increasing shear rate without an external electric field, which is consistent with a Newtonian fluid behaviour59. However, when a high electric field was applied, the ER fluid exhibited a large increase in shear stress because of the formation of chain-like structure. This performance was similar to a traditional Bingham fluid. The shear stress of bare SiO2 particle based ER fluid exhibited slightly changes with the increasing of electric field strength shown in Fig. 6A, however, pure Ni-Al LDH based ER fluid resulted in electric short at even 1.05 kV/mm due to its relatively higher electric conductivity (Fig. 6B). Figure 5B shows the shear viscosity of SiO2@Ni-Al LDH composites as a function of the shear rate. The ER fluid of SiO2@Ni-Al LDH composites showed Newtonian fluid characteristics without an electric field and typical shear thinning behaviour60 under different electric fields.
Flow curves of shear stress (A) and viscosity (B) vs. shear rate for the SiO2@Ni-Al LDH composites (particle concentration, 10 wt%)-based ER fluid.
Flow curves of shear stress vs. shear rate for the single SiO2 particle (A) and Ni−Al LDH (B) (concentration, 10 wt%)-based ER fluid.
Furthermore, the dynamic yield stress was plotted as a function of electric fields in log-log scale shown in Fig. 7. The correlation between the yield stress (τy) and electric fields (E) is fitted by the equation τy ∝ Em, where m equals to 1.5, which is consistent with the conduction model. The conductivity mismatch between the particles and the liquid medium is considered to be the critical factor to result in the aligned array along the field direction in ER system.
Yield stress as a function of electric field strength for 10 wt % SiO2@Ni-Al LDH composite based ER fluid.
The viscoelastic properties of the ER fluids was examined by the oscillation measurements61. As shown in Fig. 8A, the linear viscoelastic range was clearly shown in the amplitude sweep. In addition, the storage modulus (G′) was larger than the loss modulus (G″) under an electric field, indicating a higher degree of solid-like behaviour displayed by the ER fluid. When the strain amplitude was beyond a critical value, both G′ and G″ decreased sharply because of the breakdown of the structure of the ER fluid.
(A) Amplitude sweep for SiO2@Ni-Al LDH composites (particle concentration, 10 wt%)-based ER fluid at 6.28 rad s−1 of angular frequency. (B) Frequency sweep for the SiO2@Ni-Al LDH composites (particle concentration, 10 wt%)-based ER fluid with a strain amplitude of 0.01% (G′: closed symbols; G″: open symbols).
Figure 8B shows the G′ and G″ in the frequency sweep for the SiO2@Ni-Al LDH ER fluid in the linear viscoelastic region under different electric fields. The suspension exhibited solid-like behaviour, in which G′ was substantially larger than G″ and G′ remained almost constant over a broad frequency range. The larger G′ with increasing electric field strength reflects the higher ER effect41.
In order to verify the sensitivity and the stability of the ER fluid to an electric field, the steady shear flow was operated at a fixed shear rate of 1 s−1 under applied square voltage pulse (t = 20 s). The shear stress under various electric field strengths is shown in Fig. 9. When exposed to electric field, the shear stress of the ER fluids jumped to higher level from the zero-field shear stress and dropped to zero-field level again right away when the electric field is removed. This phenomenon indicates the fast and reversible transition in the as-prepared ER fluid corresponding to the electric field.
(A) Shear stress of SiO2/Ni-Al LDH composites based ER fluids at a fixed shear rate of 1 s−1 in the electric field with a square voltage pulse (t = 20 s).
Methods
All chemical regents were used as received from commercial sources without further purification. Ultrapure water was made by the Flom ultrapure water system and used in all experimental and washing processes.
Synthesis of SiO2 sphere and Ni-Al LDH sheets
The TEOS (≥28%, 4.37 g) was firstly added into ethanol (≥99.5%, 50 mL), then the mixture of 2.23 g H2O, 7.7 mL ammonium (28%) and 40 ml ethanol was dropwise added into the above solution. After 20 hours′ hydrolysis reaction, the mixture was separated in a high speed centrifugal machine to collect the precipitation. Then, the white SiO2 sphere was dispersed in ultrapure water and centrifuged to collect the white precipitate, this process was repeated until the supernatant became clear. The sample was further dried under high vacuum to obtain the pure SiO2 spheres. For the preparation of Ni-Al LDH sheet, certain amounts of Ni(NO3)2·6 H2O (99%, 1.308 g), Al(NO3)3·9 H2O (99%, 0.8622 g) and urea (99%, 0.9468 g) were added to 150 mL ultrapure water. Then the mixture was kept in an autoclave pressure vessel at 95 °C for 24 h. Finally, the solution were centrifuged and washed several times with ultrapure water. The clean Ni-Al LDH sheets were dried at 75 °C for 12 h in a vacuum oven. Before using, the Ni-Al LDH sheet dispersed in ethanol was broken using ultrasonic waves for 6 h.
Preparation of SiO2@Ni-Al LDH composites by self-assembled process
SiO2@Ni-Al LDH composites were prepared by electrostatic adsorption between SiO2 sphere and Ni-Al LDH sheet. SiO2 sphere (0.1 g) was dispersed in 20 ml ethanol by ultrasonication for 15 min. The sodium dodecyl benzene sulfonate (SDBS) (98%, 0.01 g) was then added into the above solution, followed by magnetic stirring for 1 h. After centrifugation, the supernatant was discarded. The precipitation was dispersed in 20 ml ethanol under sonication. Dispersion of Ni-Al LDH sheet in ethanol (0.0044 g Ni-Al LDH sheet, 5 ml ethanol) was added into the above solution. The resulting mixture was stirred for 5 h and then centrifuged. The precipitate was collected and then dried at 60 °C for 12 h to afford SiO2@Ni-Al LDH composites.
Characterization
Field-emission scanning electron microscopy (FE-SEM) (Model JSM-2010, JEOL) was used to observe the morphology of Ni-Al LDH sheet and SiO2@Ni-Al LDH composites. Transmission electron microscopy (TEM) (Model H-800, Hitachi, Tokyo, Japan) was performed to analyze the morphology of Ni-Al LDH sheets and SiO2@Ni-Al LDH composites. Scanning transmission electron microscopy (STEM) (Model JEM 2100 F, JEOL) was carried out to characterize the distribution of the elements. X-ray photoelectron spectroscopy (XPS) tests were measured using an ESCALAB MK-II electron spectrometer with an Al Kα X-ray source. X-ray diffraction (XRD) (Model DMax/rA, Rigaku, Japan) was performed using a rotating anode X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm) at 40 kV. The ER characteristics were examined using a rotational rheometer (Physica MCR301, Anton Paar) equipped with a coaxial cylinder.
Conclusions
The SiO2@Ni-Al LDH composites were successfully fabricated via self-assembly of Ni-Al LDH sheet on SiO2 sphere based on electrostatic interactions. SEM, TEM, EDX, STEM, XRD and XPS analyses were carried out to confirm the successful preparation of the core-shell SiO2@Ni-Al LDH composites. The SiO2@Ni-Al LDH composites displayed an enhanced ER characteristic compared to bare SiO2 suspension when dispersed in silicone oil, highlighting the potential applications of the SiO2@Ni–Al LDH composites in an automotive industry such as shock absorbers, engine mounts and clutches.
Additional Information
How to cite this article: Ji, X. et al. Self-assembly preparation of SiO2@Ni-Al layered double hydroxide composites and their enhanced electrorheological characteristics. Sci. Rep. 5, 18367; doi: 10.1038/srep18367 (2015).
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (51173087) and Qingdao (12-1-4-2-2-jch) and Taishan Scholars Program and for financial support.
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X.J., W.Z. and J.L. conceived and designed the experiments. X.J., W.Z., L.S. and Y.T. executed the experiments. X.J., W. Z., L.S., Y.T. and J.L. analyzed the results. Y.T. provided the equipment to measure the rheological characteristics. X.J., W.Z., Y.T. and J.L. composed the paper. All authors reviewed the manuscript.
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Ji, X., Zhang, W., Shan, L. et al. Self-assembly preparation of SiO2@Ni-Al layered double hydroxide composites and their enhanced electrorheological characteristics. Sci Rep 5, 18367 (2016). https://doi.org/10.1038/srep18367
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DOI: https://doi.org/10.1038/srep18367
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