The Control Synthesis Of Nanosilica And CdS@SiO₂ Core-shell Nanomaterials In Triton X-100 Reverse Microemulsion

Recently, micro or nano- silica particles has drawn more and more attention for their potential applications in catalysis, material science, electronic, ceramic, chromatogram absorbents, drug delivery, thermal insulators and daily life etc. Especially, silica is nontoxic, chemically inert, optically transparent and easy to functionalize, so it can combine with other nanomaterials to form composite materials with special characters and applications. The nanoparticles encapsulated with silica are the highlight of the colloid and material science field. The encapsulated semiconductor with improved optical, electrical characters of chemistry and physics, and also the modification process for silica shell will increase its dissolving ability and enlarge the application field.As a novel liquid phase synthesis medium, reverse microemulsion has been used to obtain various nanoparticles. Recently, silica rod and silica based shell core composites have been synthesized applying this method. Especially, the synthesis of core nanoparticles and the silica shell can be progressed in a micro reactor, which avoids the pre-modification of the core nanoparticles and make the operating steps simple. Also, through choosing reaction parameters R ([H₂O]/[Surfactant]), h ([H₂O]/[alkyl silicate]) and reagent concentrations, the size, size distribution of the silica and core-shell nanoparticles, the number of encapsulated core, and the shell thickness can be controlled effectively. Based on the previous study, we synthesized silica nanoparticles and CdS@SiO₂ core-shell nanoparticles in the reverse microemulsion composed with nonionic surfactant Triton X-100. Some valuable results have been obtained, which are expected to offer some suggestions to the synthesis of nanoparticles and core-shell nanoparticles in the reverse microemulsion. There are four chapters in the paper which are introduced in the following:1. OutlineThe control of the size, size distribution for the silica particles through changing R ([H₂O]/[Surfactant]), h ([H₂O]/[alkyl silicate]), the kind of surfactant and oil and co-surfactant have been discussed in detail; besides, the control of the morphology and micro-structure of silica has also been introduced through changing the composition of reverse microemulsion, surfactant and oil kind. The reverse microemulsion synthesis method can effectively control the size and size distribution of core-shell nanoparticles, which also need not multiple steps. So this method has become the first choice in the synthesis of various spherical nanocrystalline based core-shell nanoparticles in the 21st century. Though there is a long way to obtain the core-shell nanoparticles with only a nanocrystalline with accurate and universal method, the control of core numbers pf partial kinds of nanocrystalline encapsulated in silica shell have been realized through modifying reaction parameters such as reagent concentrations and the addition of other additives.2. The characterization of Triton X-100 based reverse microemulsionand the choices for the reaction parametersFirstly, the phase diagram of reverse microemulsion for the synthesis of silica was drawn. In the absence of alcohols, the W/O phase region is very small. When the mass fraction of TX-100 in the cyclohexane is nearly to 20%, the reverse microemulsion can solubilize considerable water phase. When ammonia is used as water phase, the W/O phase region will be enlarged when the thrice distilled water is replaced. As the ammonia concentration increases, the W/O phase region will enlarge a little gradually. In the presence of alcohols, the W/O phase region will enlarge very obviously. In general, the W/O phase region will enlarge as the increase of alcohol chains. When the mass fraction of blend (surfactant and alcohol) is between 56-80%, the W/O phase region of reverse microemulsion contained n-hexanol is larger than the reverse microemulsion contained n-pentanol. Especially, when the mass fraction of blend is higher (80-90%), the system will transit to O/W microemulsion with an intergradation of bicontinuous microemulsion, and the appearance of the system is always transparent.Based on the analysis of phase diagram and other factors, the concentration of TX-100 is chosen as 0.3mol/L. In the absence of alcohols, R values are 0.5,1,1.5 and 2. In the presence of alcohol, we choose the ammonia of 14.2% as water phase, and R values are 2, 4,6, 8, 10,12 and 14. In the absence of alcohols, the conductivity value is very small. In particular, when R is 0.5, because of the monitor limit, the electrical conductance meter can not measure the small value, so the conductivity is 0. For all the ammonia concentrations, the conductivity increases as the increase of R at first, then abruptly increases when R is 4, which indicates the happen of phase separation. At the same R value, the conductivity will increase as the increase of ammonia concentration. In the presence of alcohols, the profile of the change of conductivity is very interesting. As the increase of R, after a steady and obvious increase, the conductivity begins to decrease. When R is less than 12 and as the alcohol chain increase, the conductivity decreases, which is caused by the increase of interface strength as the increase of alcohol chain. When R is more than 12, the reverse microemulsion contained n-octanol owns the biggest conductivity value, which is because that the system containing n-ocatnol is most approaching to phase transformation.As to the reverse micelle without alcohols, as the R increase to phase separation, the maximum adsorption wavelength (λ_max) of methylene blue will increase continuously, so there is no free water in the reverse micelle. In the presence of alcohols, as R increases to 6, the λ_max will increase gradually. Then the λ_max will keep at a certain value, which implies the unchanging water environment of the reverse micelle. So when R is 6, the free water begins to appear.Compared with the thrice distilled water, the replacement of CdCl₂ solution will not bring obvious influence for the region of the reverse microemulsion. As the increase of CdCl₂ solution, the reverse microemulsion region will enlarge a little. On the base of considering experiment conditions thoroughly, we choose the mass fraction of surfactant as 40%, and the initial mass fraction of CdCl₂ solution is 8%. The influences of initial water phase fraction for the characters of the obtained particles were studied, and the initial water phase fractions were 4%, 8% and 10% respectively. 3. The size and morphology control of the nanosilica synthesized inthe W/O system of Triton X-100+(n-alcohol)/cyclohexane/ammoniaTX-100 was chosen as surfactant, and cyclohexane was oil phase, ammonia was water phase and catalyst. Nanosilica was synthesized in the reverse microemulsion, and the influences of R, ammonia concentration and alcohol chain length (5, 6, 8) for the particle size and size distribution were studied; in the absence of agitation, fiber-like silica was synthesized through modifying some parameters, and possible mechanism was discussed.In the absence of alcohols, the size of spherical silica firstly increases steeply and then presents unaltered or falling trends as the increase of R. The number-average size of the particles is between 15 and 60 nm. In the presence of alcohols, the number-average size of the particles is between 40 and 80 nm, the size variety is inverse with the condition without alcohols. In the absence of alcohols, the size distribution is less than 12%, In the presence of alcohols, the σ is between 4 and 16%, and firstly increases and then displays unchanged, and then increases again as the advent of phase separation.In the absence of alcohols, when R is 0.5, the size of the spherical particles gradually increases as the increase of ammonia concentration; but when R is more than 1, the minimum of the particle size appears at 8.94% of ammonia concentration.In the absence of alcohols, the spherical morphology of the particles is not very regular. As the increase of R, the surface smoothness and dispersibility of the particles are both improved, and the condition is more obvious at low ammonia concentration. When the co-surfactant alcohol is added, the regularity of the spherical particles is improved well. Also, the surface smoothness and dispersibility of the particles are very well. But the spherical particles become more irregular as the approach of the phase separation caused by the increase of R. When the alcohol is n-octanol, the regularity of the spherical particles was the worst compared with the conditions with other alcohols as co-surfactant.In the absence of alcohols, agitation will not bring obvious influences for the size distribution at low R. However, the condition is converse when R is high and the strong agitation help for the monodispersibility of the spherical particles. When there are not alcohols and agitation, R is 0.5 and ammonia concentration is 14.2%, fiber-like silica was obtained; the absence of alcohol, agitation and certain R and ammonia concentration are all the necessary conditions for the formation of fiber-like silica.4. The controllable synthesis and characterization of CdS@SiO₂ core-shell nanoparticles in the W/O system of TritonX-100+n-hexanol/cyclohexane/waterCdS nanocrystalline and the following coat of the silica shell are achieved in a micro-reactor which is the reverse microemulsion composed of TX-100, n-hexanol and water phase, and the effects of various parameters on the CdS@SiO₂ nanoparticles and CdS nanocrystalline were investigated in detail.The core-shell particles with more than one CdS crystalline were obtained when the volume ratio of CdCl₂ and TAA solution increases to 2:3/5. The monodispersibility of the particles is good, but spherical morphology is not regular. The CdS nanocrystalline is cubic structure from the results of X-ray powder diffraction (XRD). But the size of CdS crystalline is too small through the calculation from XRD spectrum, which indicates the size obtained from the XRD results is not exact. The coated nanocrystalline presents quantum effects which exhibit in special adsorption and emission spectrum. The number-average size of uncoated nanocrystalline is not related to the volume ration of CdCl₂ and TAA solution, and the volume ration of ammonia and TEOS solution, and no quantum effect is found. The coating of silica on the CdS nanocrystalline makes the size of CdS nanocrystalline a little bigger compared with the uncoated CdS nanocrystalline. The intensity of photoluminescence (PL) spectrum of the coated CdS nanocrystalline becomes a little stronger with uncoated nanocrystalline, but the captured emission intensity becomes stronger very obvious which suggests the increase of surface defects.The influences of reaction time on the core-shell nanoparticles are unobvious, and the coated nanocrystalline form is basically not different which can be proved by the uniform results of TEM, XRD, UV-vis and PL. When the reaction undergoes 6 h, the size distribution is the biggest. The results of PL spectrum suggest the increase of reaction time favors the reduction of surface defects of CdS nanocrystalline.When the initial mass fraction of water phase is lesser, the aggregation of core-shell particles is rather obvious. And as the increase of the initial water phase mass fraction, the size and morphology of the particles don't show obvious change. The coated and uncoated CdS nanocrystalline both displays quantum effects in UV-vis spectrum, but the PL spectrum only shows the difference of intensity.When the concentrations of CdCl₂ and TAA were respectively 0.1 mol/L and 0.01mol/L, a kind of interesting core-shell structure was obtained. Apart from the CdS nanocrystalline locating in the silica interior, the CdS nanocrystalline located at the silica particles surface. More interestingly, the size of coated CdS nanocrystalline and uncoated don not present differences. The results of the PL spectrum indicate the coating of silica shell increases the surface defects of silica, and there exists two kinds of captured states.Based on the analysis and contrast of the uncoated CdS nanocrystalline and the corresponding core-shell nanoparticles, the mechanism of the "raisin bun" type of core-shell nanoparticles was discussed primarily. The formation of core-shell structure, which is more than one CdS nanocrystalline located in the center of the silica particles, is caused by the electrostatic interaction of medium hydrolyzed TEOS monomer and the CdS nanocrysatlline.