光子超颗粒丁醇相自组装研究

Self-assembly of the photonic supraparticles in n-butanolJian Dong, Weiping Qian*Department of Biological Science and Medical Engineering, Southeast University,Nanjing 210096, People's Republic of ChinaE-mail: wqian@Abstract：Photonic supraparticles were sassembled by growing colloidal crystals in aqueous droplets suspended in n-butanol. The droplet products were highly ordered and smooth particle assemblies, which diffract light and have remarkable structural stability. The method allows control of particle size and shape from spheres through balls to toroids. The results provide the way to controllable formation of a wide variety of photonic supraparticles.Keywords: Photonic supraparticle, Self-assembly, N-butanol1 IntroductionMany properties of materials possessing a three-dimensional periodicity of the dielectric constant have been attracting increasing attention, because of their potential applications as photonic band gap materials, chemical sensors, optical filters, and switches.[1-6] Monodisperse colloidal spheres are often used to assemble the close-packed arrays in two or three-dimensions, which usually be used as templates for advanced materials. Various techniques, including sedimentation, convective, and electric field, have been developed for photonic crystalline film with single layer or tens to hundreds layers. Furthermore, much significant work has been carried out for the fabrication of photonic crystals with various crystal structures and shapes for more applications. Using droplets of colloidal suspension as templates is one of the self-assembly techniques of the monodispersion colloidal spheres which can rearrange when removing the solvent water. By evaporating the aqueous droplets suspended on fluorinated oil, Velev had fabricated the microstructured particles, such as balls, flattened disks, and toroids.[7] By evaporating the aqueous droplets produced by ink jet, Moon had fabricated hemispherical supraparticles.[8] By evaporating the aqueous droplets produced by electrospray, Yang had also fabricated the photonic ball.[9] Herein, we present a simple method for fabrication suprapartilces including sphere, hemisphere, disk and toroid, In which procedure, the droplets of the colloidal suspension were also used as templates, they were sunk in the n-butanol in vessel before they were dried by pervasion of the water in droplets into the ambient solvent, n-butanol.2 ExperimentalThe aqueous suspensions of the 280nm silica spheres used in our experiments were purchased from Nissan Chemical Corporation, Japan. Before used, the silica aqueous suspensions were purified to remove the impurities by the strong electric field dialysis.The rest ion concentration is about 10-6 M. The vessels employed in our experiments were fabricated by the Teflon materials. The hydrophobic films of the vessels were coated by the vaporization of the toluene solvents of PMMA or PS.To form the photonic ball, 0.5 μL of the silica aqueous suspensions diluted to 40%, 30%, 20, 10%, 5% and 2.5%, were dropped into a solution in a n-butanol/tetrachloromethane mixture (1:1, v/v). The former is the upper layer, and the latter is the bottom layer. The photonic ball will be formed after 4 hours. The non-ball supraparticles were obtained by dropping 0.5 μl of the silica aqueous suspensions diluted to 40%, 30%, 20, 10%, 5% and 2.5% into the vessels loaded with n-butanol for the hydrophobic film on the inner wall. About 30 min latter, the non-ball supraparticles will be formed.Opticle images were obtained using a digital camera (Nikon) under natural light. Scanning electric micrographs were taken as following: the specimens were mounted onto SEM stubs, and then sputter coated with gold for 1 min for SEM analysis at 15 kv.3 Results and DiscussionFigure 1 Optical images of the photonic balls with different sizeFigure 1 shows optical images of the photonic balls. The colors of the balls denote the silica spheres aggregated to form the ball are long range ordering which meet the Bragg reflection. And the color of the balls depends on the size of the silica spheres, for example, the red balls shown in figure 1 are composed of 280 nm silica spheres. The size of the balls is controlled by the colloids volume fraction of colloidal suspensions.Figure 2 The SEM images of silica spheres on the surface (A and B) and in theinner (C) of the photonic ballFigure 2 shows the ordered arrays of the silica spheres on the surface and the inner of the photonic balls by scanning electric microscopic (SEM). Figure 2a shows the surface of the ball is composed of large number of polygons shown as dash lines, which are the hexagonal close-packed ordered silica spheres arrays, i.e. the ball at macroscopic scale is an ultra-polyhedron at microscopic scale. The large polygonal plate contains thousands of spheres, and small polygonal plate contains hundreds of spheres, there also exist local disorder arrays of dozens of spheres at the interfaces of polygons. Figure 2b shows a polygon in the ball. Figure 2c shows a SEM image of the inner section of the photonic balls, which exhibits a tetragonal close-packed ordered arrays structure of silica spheres. This phenomenon is rarely observed in the closely packed colloidal crystals. In Velev’ technique, the colloidal spheres, as the arrays on the surface, are hexagonal close-packed ordering arrays in the inner of the photonic balls. In other methods, the hexagonal close-packed arrays are also predominant. The arrays of the colloidal spheres on the surface and in the inner of the photonic bandgap crystals are the same as what happened in our materials.Figure 3 Optical images of the non-ball photonic crystals.Figure 3 shows the optical images of the non-ball photonic crystals. Figure 3ashows photonic balls with a pit in their surface, and figure 3b shows photonic toroids with a pit. Figure 3c shows photonic toroids with a bigger pit than that shown in figure 3b. These different shape supraparticles were obtained by controlling the colloid volume fraction.It is known that n-Butanol is miscible with all common organic solvents. Its density is 0.81g/cm3 at 20 °C. It is immiscibe with water, the solubility of water in n-butanol at 20 °C is 20.1% in mass fraction. Using the property of large solubility of water in n-butanol, the colloidal suspensions sunk in n-butanol can be dried, and the colloidal spheres can be rearranged in the templates of droplets. Tetrachloromethane is insoluble in water but miscible in N-Butanol. Its density is 1.59 g/cm3 at 20 °C. When tetrachloromethane was added to n-Butanol, n-butanol is on the top while tetrachloromethane is sunk to the bottom of the vessel. There is a region of density gradient between them which is formed by pervasion of them. Dropped the colloidal droplets in the above solvent, they will be sunk at the site where the density is equal to themselves. At the initial site, the density of the ambient solvent is lower, and the fraction of the n-butanol is major, which result in the rapider pervasion of the water in the droplets to the ambient solvents. The density of the droplet increased with the pervasion process. Then the droplet sank to higher density region, where the fraction of n-butanol is minority, which results in the slower pervasion of the solvent water. When the fraction of the colloids droplets increased to the crystallization threshold about 50%, the droplets sank at the site of the ambient solvent where the density is about 1.51 g/cm3. At the site of the fraction of n-butanol is about 1%, the pervasion process is very slow which is beneficial for the crystallization of the colloidal spheres. The formed supraparticle is ball, and the initial volume fraction of colloid of the droplets has influence both on the shape and the size of the formed supraparticles. If only one solvent n-butanol load in the vessel the inner wall of which coated a PMMA or PS film, the added droplets would sink at the bottom of the vessel and contact with the PMMA or PS film. If the initial volume fraction of the silica spheres in suspensions is above 40%, the shapes of the formed supraparticles are hemispherical, and if low 40% and above 20%, the shapes are also hemispherical, but on the surface of the hemispheres, there is a pit the deep of which is varied with the initial volume fraction of the silica spheres. If the initial volume fraction of the silica spheres is lower than 10%, the pit would evolve to a hole, and the supraparticles would form a toroid with the size of which varied with the initial volume fraction of the silica spheres. the size of which varied with the initial volume fraction of the silica spheres. In previous work, various techniques have successfully developed to fabricate the colloidal supraparticles. In Velev’ technique, between the droplets and their surrounding there exist two interfaces including one liquid-air interface and one liquid-liquid interface. And no addition of surfactants, the fabricated supraparticles cannot form the toroids. In Yang’ technique, there exist homogenous liquid-air interface between the droplets and their surrounding, but the shape of the fabricated supraparticles only have one, ball. In Moon’ technique, there also exist two interfaces.Although the influences of two interfaces on the surface of the droplets and the existing of surfactants in the droplets on the longer range ordering have not been reported, we present a smart technique resolving the above problems.4 ConclusionsIn conclusion, we have shown a convenient route to fabricate various-shaped and sized colloidal supraparticles, viz., droplet templates and different solvents. Supraparticles of shapes of sphere, disk and toroid were obtained through the solvent of the colloidal suspension droplets removing to n-butanol. The size of them is controlled by the colloids volume fraction of colloidal suspensions. 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