1. Introduction

Assembly of anisotropic particles have aroused wide research interest owing to the potential applications in various novel optic devices ^[1], catalysis ^[2], chemical biological sensing, field effect transistor ^[3], and etc. Typically, Wang et al. ^[4] reported the selfassembly of CdSe-CdS semiconductor nanorods by shape and structural anisotropy, producing multiple well-defined supercrystalline domains. Chen et al. ^[5] achieved the kagome lattice by designing a building block with the orthogonal self-adjusted coordination number on the particle's surface. Ding et al. ^[6] fabricated3Dcolloidal crystals from ellipsoidal g-Fe₂O₃-SiO₂ coreshell particles by the convective self-assembly in an external magnetic field ^[7].Ye etal. ^[8] investigated a class ofhighly faceted planar lanthanide fluoride nanocrystals nanoplates through interface design ^[9] induced particle orderly assembly. We developed an effective approach to achieve the well-ordered assembly based on the droplet template and interface assembly, which produces one-step formation of anistropic assembly from cake-shaped, and flower-shaped particles, the assemblies showed the special broad reflection signal and improved photo-limit behavior ^[10]. However, there is a challenge for the fabrication of pattern assembly from anisotropic particles. Recently, Su et al.^[11] developed an effective approach for the pattern fabrication from small molecular, monodispersed particles based on groove- structured template by sandwich structure. Wang et al.optimized the approach for the pattern assembly of monodispersed particles by combined effect of superhydrophobic template and superhydrophilic substrate^[12].Which provides an important insight for the pattern anisotropic assembly. Herein, we extended the method for the fabrication of the pattern assembly from the anisotropic rod-like silica particles based on the sandwich structure consisting of the superhydrophobic template and superhdyrophilic substrate, which resulted in the orientated arrangement of the anisotropic rod-like silica particles toward the template direction, producing an important insight for the creation of novel function materials from anisotropic particles.

2. Experimental

Synthesis of rod-like silica: Rod-like silica is synthesized by hydrolysis of tetraethylorthosilicate ^[13]. Firstly, 0.8 g hexadecyl- trimethylammonium bromide is dissolved in 140mL deionized water in 500 mL conical flask at room temperature (20 ℃), and then 4.5 mL ammonium hydroxide was added into it after 15 min. Followed by the magnetic stirring of 15 min to make sure all the homogeneous mixing of the solution, 2.4 mL tetraethoxysilane was added in the system drop by drop with stirring speed at 600 r/min for 5 h. Finally, white power was obtained after the particles were separated by filter and calcinated at 550 ℃. Different kinds of particles were obtained when controlling the amount of ammonium (Fig. S1 in Supporting information). The length/diameter ratio gradually changes as the different amount of ammonium added. The results of statistical are shown in Fig. S2 in Supporting information. The component of the sample is characterized by XRD in Fig. S3 in Supporting information.

Orientated assembly of rod-like silica based on sandwich approach:A heptadecafluoro-1, 1, 2, 2-tetradecyltrimethoxysilane modified groove-structured silicon substrate with width of 5 mm, gap of 20 mm, and height of 20 mm was held horizontally. Then rod-like silica suspension (5 wt% in water, with 20 mL SDBS (0.2 mg/mL)) was carefully dropped onto the template and covered by a flat substrate, yielding a sandwich assembly. After the assembly system was kept at 20 ℃ for 12 h, linear-like assemblies were achieved from rod-like silica. It is noted that controlling the gap between the chosen substrate and the groove-structured template at ca. 20 mm is crucial for obtaining a continuous liquid stripe. The process of other pattern assembly was similar except changing the kind of templates, the optic microscopy images of template are shown in Fig. S4 in Supporting information.

Characterization: SEM images were obtained from a Thermo- 7500F high-resolution field emission scanning electron microscope.The X-ray diffraction measurement was carried out on a Rigaku D/ max-2500 X-ray diffractometer. The particles were separated from dispersion by HC-3518 centrifugal machine. The particles were calcined at 550 ℃ for 9 h. The optic photos were obtained by BX51 Upright Metallurgical Microscope.

3. Results and discussion

Fig. 1 schemes the whole formation process of linear-like pattern from rod-like silica particles based on sandwich approach. In this case, the liquid membrane containing nanoparticles can be confined in a fixed space, the designed silicon column template could control the rupture of liquid membrane, induced liquid membrane volatilize at the place. As the solution evaporated, the volatilization rate of the three-phase contact line (TCL)^[14] is the fastest, so the particles are driven and assembled by the liquid membrane. In this case, the suitable gap is crucial for the formation of the pattern assembly, which affect the way of the liquid membrane rupture, leading to the different assembly way. It is noted that the combined effect of superhydrophobic template and the superhydrophilic substrate play an important role on the well- ordered assembly which provides an effective platform for a well- ordered latex assembly owing to its slow evaporation time and additional assembly force from the sliding TCL during the solvent evaporation process ^[12]. Fig. 2 presents optic image and SEM images of the as-fabricated linear assembly. Fig. 2a is the optic images of linear pattern assembled from rod-like silica latex (15 wt%) and the groove-structured silicon template with width of 5 mm, gap of 20 mm, and height of 20 mm. It is found that the width of the arrays is the same with the template size. SEM images in Fig. 2b show the detailed assembly from the groove-structured template, a homogenous line-type structure is observed. Otherwise, the line width can be effectively modulated by varying the concentration of the latex. The line width changes from 2 mm, 1 mm to 100nm when varying the latex concentration from 15 wt%, 8 wt%, to 2 wt%. It is clearly observed that most of rod-like particles can be orientatedly arranged by the direction of the pattern of line although few of particles can be arranged in orderless state.

To understand what affects the orientated assembly of the anisotropic particles, we analyzed the whole assembly process of the particles under the interaction of the sandwich structure in Fig. 3. Three crucial steps are responsible for the process. The first one is the confined effect ^[11], which is resulted from the combined interaction of the superhydrophobic template and the superhydrophilic substrate as shown in Fig. 3b₁. The superhydrophobic template causes the difficult dispersion of the latex suspension on the substrate, while the suerphydrophilic substrate facilitates the effectively spread of the latex suspension onto the substrate, forming the triangle assembly structure as shown in Fig. 1c. All of these lead to the location of the liquid suspension between the space of the template and substrate. The second is break of the liquid film during the solvent evaporation procedure, the receding of TCL aroused from solvent evaporation causes the rupture of the liquid film at the weak linkage region between the superhydrophobic template and superhydrophilic substrate. The final step is the formation of the linear assembly with triangle-shaped structure between the template and substrate. As regarding of the orientated assemblyofthe rod-like silica, it can be attributed to the largest driven force formed in this direction (F2) and the force between particles (F1), which leading to the preferable orientated assembly of the anisotropic particles by retraction direction of the TCL. Wherein, the F2 is aroused from the retraction force of receding TCL, while the F1 is produced from the interactions of particles.

To clearly clarify the orientated assembly of the rod-like particles based on sandwich approach, template with complex geometry such as ring-, square-like was used in this system in Fig. 4. It was found that clear pattern formed based on the various template. More importantly, the rod-like silica keeps the orientated assembly by the pattern direction even if varying the pattern mode.

4. Conclusion

We demonstrate a facile fabrication of the pattern assembly from rod-like silica particles based on sandwich approach from superhydrophobic template and the superhydrophilic substrate. The rod-like silica particles can get an orientated assembly based on the assistance of the different templates. This mainly is aroused from the driven effect of the retraction TCL based on the template assistance. It will provide an effective approach for the orientated assembly of the anisotropic particles, is of great significance for the development of the novel optic devices.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/jxclet.2016.06.004.

Acknowledgment The authors acknowledge the financial support by the National Nature Sciences Foundation (Nos. 51373183, 91127029, and 21074139) and 973 program (No. 2013CB933000) and Key Research Program of the Chinese Academy of Sciences.