Hollow silica spheres

Fig. 4 a, b. TEM micrographs of hollow silica spheres produced by calcining PS particles coated with (a) one and (b) three Si02 nanoparticle/PDADMAC layer pairs at 450°C. The wall thickness of the hollow capsules is approximately three times greater for those shown in (b) compared with those shown in (a), c, d Cross-sections of the hollow silica spheres of the same composition as those shown in (b). The hollow silica spheres retain the spherical shape of the original PS particle templates (see Fig. 3). (Adapted from [22,62] by permission of the American Association for the Advancement of Science and the American Chemical Society)...

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

PbS hollow nanospheres with diameters of 80-250 nm have been synthesized by a surfactant-assisted sonochemical route from Pb(CH3COO)2, thioacetamide, and sodium dodecylbenzenesulfonate [3]. 200-400 nm hollow silica spheres with mesoporous walls were prepared by the application of ultrasound to a mixture of nonionic polyoxyethylene surfactant and tetraethylorthosilicate. The presence of surface-active agents during ultrasonic synthesis was proved to be effective because surfactants can act as soft templates as well as structure directing agents for the assembly and subsequent... 

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

Fig. 11 Representation of the coating of polymer coUoids with silica using poly-L-lysine as com-patibiUzer, with TEM image of the resulting hollow silica spheres. Scale bar. 100 nm Adapted from [82] with permission of American Chemical Society...

Guo, X., Liu, X., Xu, B., and Dou, T. 2009. Synthesis and eharacterization of earbon sphere-sUica core-shell structure and hollow silica spheres. Colloids Surf. A Physicochem. Eng. Aspects 345 141-146. 

Jang KY, Kim K, Upadhye RS (1990) Hollow silica spheres of controlled size and porosity by sol-gel processing. J Vac Sci Technol A 8 1732-1735... 

Figure 9.17. Scanning (a, c) and high-resolution (b, d) micrographs of hollow silica spheres synthesized with the TMB/PEO-PPO-PEO/silica emulsion system at 80...

Silica Han and coworkers [52] used the water/TX-100/[bmim][PFJ microemulsions with IL as continuous phase to fabricate silica microrods. Porous silica microrods with nanosized pores were prepared in this microemulsion. Zheng and coworkers [53] synthesized the hollow silica spheres in benzene/TX-100/[bmim]... 

BFJ microemulsion. When TEOS was added to the microemulsion, it dissolved in the benzene core at the same time, some [bmim] [BF ] molecules were located in the palisade layers of the microemulsion. Subsequently,TEOS molecules at the interface of the core were hydrolyzed and polymerized, because the [bmim][BF ] molecules in the palisade layer could probably be employed as Lewis acid. So, the SiO polymerized and grew thickly around the interface of the benzene cores, forming the hollow silica spheres after calcination. Furthermore, Zheng and coworkers [54] synthesized silica products with two different morphologies using nonaqueous IL microemulsion droplets as templates. By adjusting the reaction conditions, ellipsoidal nanoparticles were formed under acidic condition, while hollow silica spheres were obtained under alkaline condition. 

The preparation of porous hollow silica nanoparticles for a drug delivery applications was described by Chen et al. The authors used the time release profile of cefradine as a test example. Caruso et al. " prepared hollow silica spheres and silica-polymer layered composite spheres. A controlled pore structure remained after decomposition of the organic material and possible applications in medical and pharmaceutical applications are described in their publication. Another drug release example was described by Chamay et al., ° who incorporated ibuprofen, a well... 

Figure 18 The microscopic picture of hollow silica spheres of 57 pm in diameter dispersed in corn oil under an electric field 750 V/mm. (a) The static structure under the electric field without shearing (b) The structure under a strain less than the critical value. The top electrode is moving to the right side (c) The structure is under a shear rate 5 s. The top electrode is moving to the right side. Reproduced with permission from D. J. Klingcnbcrg and C. F. Zukoski IV, Langmuir, 6(1990)15.

It is well known that the reflection can be reduced by lowering the refractive index of the top layo". However, there was no method to realize the lower refractive index layCT than that of SiOa layer. By a new concept for designing the material, that is,embedding fine hollow silica spheres in SiOj-fluoride matrix, instead of Si02 matrix, the very low refractive index has been achieved. 

The hard template method is as effective as the soft template method in the synthesis of hollow silica spheres. PS particles are the most widely used template [21]. The general procedure involves two steps the formation of core-shell particles and the subsequent removal of the template, either by selective dissolution or by... 

Figure 11.2 A scheme for the formation of and larger (c) amounts of ammonia result in hollow silica spheres using PS template Spon- no dissolution of the core and deterioration of taneous dissolution of the core occurs with a the core before shell formation, respectively, suitable amount of ammonia (b). Lower (a) (Reproduced with permission from Ref. [26].)...

Figure 11.4 (a) A scheme for the formation of hollow silica spheres by simultaneous dissolution and redeposition of siiica. TEM images of (b) Stober siiica spheres and (c and d)... 

Figure 11.7 (a) A scheme for the preparation d) SEM images and (e and f) TEM images of of hollow mesoporous silica spheres via spon- hollow silica spheres. (Reproduced with taneous self-transformation, (b) TEM image of permission from Ref. [44], Copyright 2013, as-made mesostructured silica spheres, (c and American Chemical Society.)... 

Wan, Y. and Yu, S.-H. (2008) Polyelectrolyte controlled large-scale synthesis of hollow silica spheres with tunable sizes and wall thicknesses. /. Phys. Chem. C. 112, 3641 3647. 

Zhang, A., Zhang, Y., Xing, N., Hou, K., and Guo, X. (2009) Hollow silica spheres with a novel mesoporous shell perforated vertically by hexagonally arrayed cylindrical nanochannels. Chem. Mater,... 

Formation of hollow silica spheres with ordered mesoporous structure by treatment with dimethyl carbonate for selective decomposition of mesoporous silica core. Micropor. Mesopor. Mater.,... 

Fig. 7 a Transmission electron microscopy images of core-sheU polystyrene-siUca colloids after the polystyrene core was removed by heating the samples at 500 °C for 3.5 h. Inset TEM image of a shell at high magnification. The dark area representing the shell shows uniform thickness, b A typical force curve of a hollow silica sphere with a shell of 50 nm and a diameter of 1.9 p.m. The open spheres denote the approach, and the filled spheres denote the retract part of the curve. Hardly any hysteresis is visible, i.e., the deformation is elastic, c Deformation of hollow silica spheres as a function of applied load. The straight line shows a linear fit to the data... 

Zhang L, D Acunzi M, Kappl M, Auernhammer GK, VoUmer D, van Kats CM, van Blaaderen A (2009) Hollow silica spheres synthesis and mechanical properties. Langmuir 25(5) 2711... 

---