Microwave-Assisted Self-Organization of Colloidal Particles Inside Water-in-Oil Emulsions

Commercial amidine polystyrene microspheres were self-organized to obtain colloidal microclusters from water-in-oil emulsion droplets as confining geometries. For demulsification process, microwave was irradiated to remove water droplets selectively resulting in the shrinkage of water droplets which induces inward capillary pressure for the particle self-assembly. The amidine polystyrene clusters were coated with silica nanospheres or titania nanoparticles by co-organizing the mixed particle suspensions inside water droplets by microwave heating. Titania-coated polystyrene clusters were calcined to produce hollow macroporous titania powders. Finally, sulfate-coated polystyrene microspheres were self-assembled with silica nanoparticles to generate polystyrene/silica composite clusters by microwave irradiation method.     

Colloidal crystals formed by aqueous suspensions of monodispersed silica, polystyrene, and poly(methyl methacrylate) colloidal spheres

Colloidal crystals consisted of silica, polystyrene, and poly(methyl methacrylate) monodispersed suspensions; deionized sufficiently in water at the same condition; were formed; and their properties were compared changing sphere diameter and volume fraction systematically. The size of these colloidal crystals was maximized at their critical sphere concentration irrespective of their sphere size. The Bragg peak wavelengths of these colloidal crystals were uniquely determined only by the sphere diameter and volume fraction for all kinds of colloidal spheres used in this work. The larger the sphere volume fraction, the larger the crystal growth rates, and there were no significant differences among the colloidal spheres. The rigidity of colloidal crystals increased in proportion to the number density of spheres. Consequently, the crystallization mechanism and properties of colloidal crystals formed by these spheres are not dependent on the kind of spheres, but they are dependent only on the sphere diameter and number density.     

垂直排列法组装胶体晶体的最新进展

光子晶体是一种具有光子带隙的新型功能材料.利用胶体粒子自组装三维光子晶体由于其制备过程比较经济、简单,从而为很多人所关注.目前报道的方法已有多种.其中垂直排列法的简便易行使得其受到了广泛的研究,但另一方面这种方法本身的缺点也限制了它的使用范围.针对这种情况,很多研究机构提出了他们的改进方法.本文简要概述了在这一方面的最新进展,并且在本实验室已能够制备任意单分散、均一尺寸二氧化硅粒子的基础上,采用恒温快速蒸发自组装法得到了高质量的胶体晶体排列.     

Synthesis of porous carbon balls from spherical colloidal crystal templates

Spherical inverse opal (IO) porous carbon was produced utilizing silica colloidal crystal spheres as templates. The spherical colloidal crystals were obtained through the self-assembly of monodisperse particles inside an emulsion droplet with confined geometry. The templates were inverted using a carbon precursor, phenol-formaldehyde (PF) resol. We demonstrated a two-step synthesis involving the subsequent infiltration of the PF resol precursor into the spherical colloidal crystal template and a one-step synthesis using a silica colloidal solution containing dissolved PF resol. In the former case, the sizes of the IO carbon balls were controlled by the size of the colloidal crystal templates, and diameters of a few micrometers up to 50 μm were obtained. The average diameter of the macropores created by the silica particles was 230 nm. Moreover, meso-/macroporous IO carbon balls were created using block-copolymer templates in the PF resol. In the one-step synthesis, the concentration of PF resol in the colloidal solution controlled the diameter of the IO carbon balls. IO balls smaller than 3 μm were obtained from the direct addition of 5% PF resol. The one-step synthesis produced rather irregular porous structures reflecting the less ordered crystallization processes inside the spherical colloidal crystals. Nitrogen adsorption and cyclic voltammetry measurements were conducted to measure the specific area and electroactive surface area of the IO carbon balls. The specific area of the mesopores-incorporated IO carbon balls was 1.3 times higher than that of bare IO carbon balls. Accordingly, the meso-/macroporous porous carbon balls exhibited higher electrocatalytic properties than the macroporous carbon balls.     

Colloidal crystal microarrays and two-dimensional superstructures: a versatile approach for patterned surface assembly

A simple, fast, and robust approach to colloidal assembly on patterned surfaces was developed. The approach involves the rapid settling and dewetting of suspensions of spherical colloids on lithographically templated surfaces. Using this method, we can quickly and easily fabricate close-packed colloidal crystal microarrays of both silica and polystyrene spheres that range in size from 500 nm to 4.5 microm. The microarrays tend to induce the formation of monolayer colloidal crystals, which can be interconnected and removed from the templates as free-standing colloidal crystal slabs. The same approach can also be used to assemble two-dimensional colloidal crystal superlattices that can adopt a variety of structures. Graphite, kagome, body-centered cubic, open hexagonal, tetragonal, and linear chain structures can all be quickly accessed by adjusting the ratio of the sphere diameter to the template diameter.     

Homogeneous and heterogeneous binary colloidal clusters formed by evaporation-induced self-assembly inside droplets

In this paper, we report the preparation of binary clusters of colloidal particles with different sizes or species into complex structures using oil-in-water emulsion droplets as confining geometries. First, polystyrene or silica particles with bimodal size distribution were packed densely by evaporation-induced self-assembly inside oil-in-water emulsion droplets. The configurations of larger particles inside the droplets minimize the second moment of the particle locations for the ratio of large to small particle sizes less than 3. Also, the configurations of bimodal clusters were predicted by using a surface evolver simulation, and the simulation predictions were compared with the experimental results. In addition, heterogeneous colloidal clusters were produced by emulsifying the binary mixture suspension of polystyrene and silica particles in aqueous medium followed by evaporating the oil phase. A density gradient centrifugation was applied to fractionate the asymmetric binary dimers comprised of PS and silica microspheres.     

Synthesis of spherical polymer and titania photonic crystallites

The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 microm in diameter and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.     

Effects of liquid bridge between colloidal spheres and evaporation temperature on fabrication of colloidal multilayers

For the application of colloidal crystal films as "photonic band gap" materials, their domain size and thickness are significant. The substrate withdrawing speed, the colloidal suspension volume fraction, and the colloidal suspension temperature have been studied for the domain size and thickness controls of colloidal crystals in this study. Stable dispersions of monodispersed polystyrene spheres with a diameter of 245 nm were synthesized according to a general emulsion polymerization for colloidal crystal films. By experimental results and the theoretical relationship between the number of layers and other parameters, we could know that the water bridge between colloidal spheres (which is formed by capillary force) influences the number of colloidal crystal layers significantly.     

High-speed fabrication of patterned colloidal photonic structures in centrifugal microfluidic chips

In this paper, we report a rapid and facile method for fabricating colloidal photonic crystals inside microchannels of radially symmetric microfluidic chips which were made using soft-lithography. As the suspension of monodisperse silica or polystyrene latex spheres was driven to flow through the channels under the action of centrifugal force, the colloidal spheres were quickly assembled into face centered cubic arrangement which had a few photonic stop bands. The soft-microfluidic channels and cells confined the colloidal crystals into designed patterns. The optical reflectance was modulated by the refractive-index mismatch between the colloidal particles and the solvent in the interstices between the particles. Therefore, the present microfluidic chips with built-in colloidal photonic crystals can be used as in-situ optofluidic microsensors for high throughput screening or light filters in integrated adaptive optical devices.     

Microwave-assisted self-organization of colloidal particles in confining aqueous droplets

Monodisperse aqueous emulsion droplets encapsulating colloidal particles were produced in the oil phase, and controlled microwave irradiation of the aqueous drop phase created spherical colloidal crystals by so-called evaporation-induced self-organization of the colloidal particles. Unlike usual colloidal crystals, colloidal crystals in spherical symmetry (or photonic balls) possessed photonic band gaps for the normal incident light independent of the position all over the spherical surface. While the consolidation of colloidal particles in emulsion droplets in an oven took several hours, the present microwave-assisted evaporation could reduce the time for complete evaporation to a few tens of minutes. Under the microwave irradiation, the aqueous phase in emulsions was superheated selectively and the evaporation rate of water could be controlled easily by adjusting the microwave intensity. The result showed that the packing quality of colloidal crystals obtained by the microwave-assisted self-organization was good enough to show photonic band gap characteristics. The reflectance of our photonic balls responded precisely to any change in physical properties including the size of colloidal particles, refractive index mismatch, and angle of the incident beam. In particular, for polymeric particles, the photonic band gap could be tuned by the intensity of microwave irradiation, and the reflection color was red-shifted with stronger microwave irradiation. Finally, for better photonic band gap properties, inverted photonic balls were prepared by using the spherical colloidal crystals as sacrificial templates.     

Fabrication of binary colloidal crystals and non-close-packed structures by a sequential self-assembly method

Binary colloidal films of polystyrene (PS) spheres and silica spheres were fabricated with a sequential growth method using differently sized colloidal particles. In particular, we demonstrate the structures formed by a silica monolayer growing on top of a PS monolayer and a silica multilayer growing on top of a PS monolayer. By removal of the bottom PS layers, non-close-packed hexagonal, pentagonal, and square silica arrays were obtained at the original silica/PS interface. The possible formation mechanism of the non-close-packed structure was discussed, which may be used to explain how 3D colloidal crystals grow on patterned substrates.     

Tunable optical stop band of silica shell photonic crystals

In this study, optical stop band of three-dimensional silica shell photonic crystals is tuned by adjusting inner diameter and shell thickness of hollow silica spheres. The silica shell photonic crystals are fabricated by sintering crystalline arrays, which are assembled from polystyrene/silica core–shell spheres by an improved vertical deposition method. The inner diameter and the shell thickness are controlled by diameter of polystyrene spheres and concentration of tetraethoxysilane. The results of transmission spectra show that there is an evident red shift of optical stop band as the inner diameter and the shell thickness increase. The red shift of optical stop band is due to variations in the inter-planar spacing and the effective refractive index of silica shell photonic crystals, which result from the variations of the inner diameter and the shell thickness of hollow silica spheres.     

磺化聚苯乙烯胶体晶的红外光谱分析

单分散乳胶体系可以通过自组装形成有序的胶体晶结构 ,在窄波段光过滤器[1] 、生物医学传感器[2 ] 、智能化学传感器[3] 等领域具有重要的应用价值 .最近 ,人们以此有序结构作为模板 ,制备了有序孔材料[4～ 7] .另外 ,此有序结构在仿生学如模拟蛋白石等有序结构等方面也具有重要意义[8] .但是 ,一般的单分散体系如聚苯乙烯体系所形成的有序结构都属于硬性材料 ,缺乏对外场的响应特性 .此外 ,微球表面没有功能性基团 ,很难与其它物质兼容 ,这使得其作为模板合成其它复合材料的潜力大大降低 .本实验室通过对聚合物胶体晶进行改性 ,使其成为功…     

聚苯乙烯光子晶体的制备及其在传感中的应用

以基于毛细作用的垂直沉积法将单分散的二氧化硅胶体微球自组装成光子晶体．在二氧化硅光子晶体的多孔结构里填充聚苯乙烯甲苯溶液,经甲苯挥发,通过氢氟酸处理去除二氧化硅模板,制备出精美的聚苯乙烯光子晶体．研究表明：保留了模板有序多孔结构的聚苯乙烯能被用来作为敏感膜,这使得其在基于折射率变化的传感应用中具有潜在的价值．     

柔软的球状和长方体状氧化硅中空结构的制备研究

采用自组装形成的芘纳米结构作为模板,成功地制备了柔软的球状和长方体状氧化硅中空结构.当不同量的芘在十六烷基三甲基溴化铵(CTAB)溶液中自组装时,产生的自组装结构展现出明显的从球状到长方体状的形貌变化.这些结构被用作氧化硅前驱体溶胶-凝胶反应的模板,获得了球状和长方体状氧化硅/芘复合结构.通过乙醇除去模板后,生成了柔软的球状(直径约为400nm)和长方体状(长为0.5—2.5μm)的氧化硅中空结构.这些结果展现了采用有机纳米结构作为模板来合成无机中空结构的优势:合成简便、结构多样以及结构形貌的灵活可控.     

Thermo-sensitive colloidal crystals of silica spheres in the presence of large spheres with poly (N-isopropyl acrylamide) shells

Thermo-sensitive colloidal crystals are prepared simply by mixing colloidal silica spheres and large thermo-sensitive gel spheres. The thermo-reversible change in the lattice spacing of colloidal crystals of monodisperse silica spheres (CS82, 103 nm in diameter) depends on the size of the admixed temperature-sensitive gel spheres. For spheres with sizes less and greater than that of the silica spheres, the lattice spacing upon temperature increase above the lower critical solution temperature of poly(N-isopropyl acrylamide) decreases (cf. Okubo et al. Langmuir 18:6783, 2002) and increases, respectively. A mechanism, which is able to explain these experimental findings, is proposed. Moreover, crystal growth rates and the rigidities of the thermo-sensitive colloidal crystals are studied.     

Qualitative and quantitative analysis of crystallographic defects present in 2D colloidal sphere arrays

In this work, we present a study of the typical spontaneous defects present in self-assembled colloidal monolayers grown from polystyrene and silica microspheres. The quality of two-dimensional crystals from different colloidal suspensions of beads around 1 μm in diameter has been studied qualitatively and quantitatively, evaluated in 2D hexagonal arrays at different scales through Fourier analysis of SEM images and optical characterization. The crystallographic defects are identified to better understand their origin and their effects on the crystal quality, as well as to find the best conditions colloidal suspensions must fulfill to achieve optimal quality samples.     

Rigidity of colloidal crystals of silica spheres modified with polymers on their surfaces in organic solvents

Rigidity (G) of colloidal crystals in organic solvents of acetonitrile and nitrobenzene has been measured by reflection spectroscopy in sedimentation equilibrium. The colloidal spheres used are the silica spheres (136 nm in diameter) modified on their surfaces with polymers, poly(maleic anhydride-co-styrene) [P(MA-ST)], poly(methyl methacrylate) (PMMA), or polystyrene (PST). Log G increases linearly with the slope of unity as log N (number density of colloidal spheres) increases. The mean values of the b-factor, which is the fluctuation parameter in crystal lattices and should be smaller than 0.1 according to the Lindeman's rule, are 0.045±0.003, 0.039±0.007, and 0.038±0.003 for P(MA-ST)/SiO₂, PMMA/SiO₂, and PST/SiO₂, respectively. These values are larger than that of colloidal crystals of mother silica spheres in the deionized aqueous suspension, 0.028. These results support the important role of the excluded volume effects from the polymer layers formed around the silica surfaces. However, contribution of the excluded volume effects from the electrical double layers formed around the spheres in the organic solvents is also effective in the colloidal crystallization. Electronic Publication     

二氧化硅胶体晶体及其为模板的多孔材料

用Stober法合成了粒径在35～750nm范围内的单分散的SiO₂粒子,考察了投料比对粒径的影响.研究了所制备的SiO₂胶体晶体的结构和反射光谱.利用两种不同粒径的SiO₂粒子作为模板和模板填充物,分别制备了SiO₂和重氮树脂的多孔材料.     

Synthesis and crystallization of hybrid spherical colloids composed of polystyrene cores and silica shells

The St?ber method has been adopted to prepare hybrid core-shell particles by coating the surfaces of monodisperse polystyrene beads with uniform silica shells. Polystyrene beads with diameters in the range of 0.1-1.0 microm have been successfully demonstrated for use with this process, and the thickness of the silica coating could be controlled in the range of 50-150 nm by adjusting the concentration of tetraethoxysilane, the deposition time, or both. The morphology and surface smoothness of the deposited silica were found to strongly depend on a number of parameters such as the surface functional groups on the polymer beads, the pH value of the medium, and the deposition time. Hollow spheres made of silica could be obtained by selectively removing the polymer cores via calcination in air at an elevated temperature or by wet etching with toluene. These core-shell colloids were also explored as building blocks to fabricate long-range ordered lattices (or colloidal crystals) that exhibited stop bands different from those assembled from spherical colloids purely made of either polystyrene or silica.