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Self-assembly preparation of SiO2@Ni-Al layered double hydroxide composites and their enhanced electrorheological characteristics

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.

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 clutches 34,35 .
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 interactions 51 . PANI-coated anisotropic snowman-like poly(methyl methacrylate) (PMMA) particles were also prepared with an anionic surfactant as ER fluid suspension particles 52 . Core-shell structured titania/PANI and PS/PANI microspherical composites were synthesized via in situ polymerization in the presence of titanium oxide or PS nanospheres 42,53 . SiO 2 /TiO 2 particles were prepared by means of a co-precipitation process by Wu and his coworkers 54 . Monodispersed PMMA microbeads coated with MWNT-NH 2 were also prepared via a grafting reaction 55 . 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 template 56 . 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, SiO 2 @Ni-Al LDH core-shell structure was prepared via self-assembly of Ni-Al LDH on SiO 2 sphere as shown in Fig. 1. As the core material, monodispersed SiO 2 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 SiO 2 @Ni-Al LDH composite dispersed in insulating oil was characterized by a rotational rheometer.

Results and Discussion
Generally, the surface of the bare SiO 2 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 SiO 2 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 SiO 2 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.
The morphology of SiO 2 @Ni-Al LDH composites was analyzed through SEM (Fig. 3A,B) and TEM (Fig. 3C,D) images. The SiO 2 spheres are found to be monodispersed with a uniform size of ~ 260 nm. The final SiO 2 @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 dark-field STEM analysis of SiO 2 @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 SiO 2 @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 SiO 2 sphere, Ni-Al LDH sheet, and SiO 2 @Ni-Al LDH composites are shown in Fig. 4B. The diffraction pattern of SiO 2 sphere depicts a reflection characteristic of amorphous silica 57 . 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 SiO 2 sphere and SiO 2 @Ni-Al LDH composites was investigated using the XPS technique. As shown in Fig. 4A, the XPS survey spectrum of SiO 2 @Ni-Al LDH composites revealed the presence of Si, O, Ni, Al elements, implying the formation of SiO 2 @Ni-Al LDH composites. Taken together, the results of SEM, TEM, EDX, STEM, XRD and XPS evidenced the successful synthesis of SiO 2 @ Ni-Al LDH composites.
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 activity 58 . Therefore, coating a layer of ultrathin Ni-Al LDH sheet, is an effective way to enhance the ER activity of the bare SiO 2 spheres. Here, the prepared SiO 2 @Ni-Al LDH composites exhibited improved ER performance compared to the conventional bare SiO 2 particles, which have been described below: The shear stress-shear rate behaviour of the SiO 2 @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 behaviour 59 . 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 SiO 2 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 SiO 2 @Ni-Al LDH composites as a function of the shear rate. The  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 (τ y ) and electric fields (E) is fitted by the equation τ y ∝ E m , 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.
The viscoelastic properties of the ER fluids was examined by the oscillation measurements 61 . 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. Figure 8B shows the G′ and G″ in the frequency sweep for the SiO 2 @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 effect 41 .
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.

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 SiO 2 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 H 2 O, 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 SiO 2 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 SiO 2 spheres. For the preparation of Ni-Al LDH sheet, certain amounts of Ni(NO 3 ) 2 ·6 H 2 O (99%, 1.308 g ), Al(NO 3 ) 3 ·9 H 2 O (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 SiO 2 @Ni-Al LDH composites by self-assembled process. SiO 2 @Ni-Al LDH composites were prepared by electrostatic adsorption between SiO 2 sphere and Ni-Al LDH sheet. SiO 2 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 SiO 2 @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 SiO 2 @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 SiO 2 @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 SiO 2 @Ni-Al LDH composites were successfully fabricated via self-assembly of Ni-Al LDH sheet on SiO 2 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 SiO 2 @Ni-Al LDH composites. The SiO 2 @Ni-Al LDH composites displayed an enhanced ER characteristic compared to bare SiO 2 suspension when dispersed in silicone oil, highlighting the potential applications of the SiO 2 @Ni-Al LDH composites in an automotive industry such as shock absorbers, engine mounts and clutches.