空心纳米球的制备方法及其研究进展

空心纳米球的制备方法及其研究进展

摘要: 空心纳米球作为一种新的纳米结构, 其特有的核-壳空心结构及纳米厚度的壳层使它具有许多优异的物理化学性能, 从而在医学、制药学、材料学、染料工业等领域具有很好的应用前景。本文综述了微乳液聚合法、模板法和由模板法发展而来的L-b-L 自组装法制备无机材料空心纳米球的一般过程及原理, 最后总结了空心纳米球材料的研究进展。

1 引言

探索新的纳米结构已成为近年来物理、化学、材料等领域的研究热点之一。如今已问世的纳米结构有准一维纳米材料包括纳米管、纳米线、纳米棒和纳米电缆等, 而且这些纳米结构材料的制备技术已日趋成熟并逐步实用化。

空心纳米球作为一种新的纳米结构, 其一个明显的特征就是具有很大的内部空间及厚度在纳米尺度范围内的壳层。这种特殊结构使它可作为客体物质的载体, 从而在医学和制药学领域应用范围很广。此外, 空心球的特殊空心结构还使得这种材料与其块体材料相比具有比表面积大、密度小等很多特性, 因此空心纳米球的应用范畴不断扩大, 已扩展到材料科学、染料工业等众多领域。可作为轻质结构材料[ 1] 、隔热、隔声和电绝缘材料[ 2] 、颜料、催化剂载体[ 3] 等。

由于空心纳米球材料的优异性能及广阔应用前景, 其开发研究引起了人们的广泛关注, 现已形成制备空心纳米球的多种方法, 如模板法[ 4, 5] 、吸附技术[ 5] 、喷雾高温分解法[ 6, 7] 、超声化学法[ 8] 、水热法[ 9] 等。用这些方法已成功制备出CdS[ 10] 、ZrO2[ 11] 、金属Ag[ 12, 13] 、TiO2[ 14] 、Si[ 15] 、SnO2[ 1 6] 等多种无机材料空心纳米球,及聚合物空心纳米球, 如PSt [ 17, 18] 、聚甲基丙烯酸甲酯[ 19] 等。

目前关于空心纳米球的报道多局限于空心球的制备, 而对具体制备方法的阐述则比较少。模板法作为最常用的一种制备方法被广泛地用于各种材料的空心纳米球的制备中, 而其在聚合物空心纳米球制备中的应用已有文献综述报道[ 20] , 且技术已相对成熟。因此本文将综述使用微乳液聚合法、模板法和由模板法发展而来的L-b-L 自组装法制备无机材料空心纳米球的一般过程及原理。

2.1 Microemulsion method

Microemul sion technology was applied to produce polymer in the 1980s. Stoffer et al[ 45] fir stly polymerized the methyl methacrylate (MMA) and methacrylate (MA) by microemulsion technology. Since then , the microemul sion technology as a roused widespread concern. And now it has become an important approach to prepare the hollow nanospheres , especially for those that the diameter is small (minimum 10 ～60nm) . The preparation process has three steps[ 46] : firstly ,precur sors of target product s hydrolyze and generate oxide with aquifer or hydroxides on the surface of the droplet of microemul sion ; afterwards , the stable colloidal particles that is produced by polycondensation coat and form the core-shell structure of emul sion and gel ; at last , water or organic solvent are used to separate the product f rom the microemulsion. Then hollow nanospheres can be prepared. The process is shown in Fig. 1.

Fig. 1 Schematic illustration of formation process of hollow microspherical structure with microemulsion method

Park et al[ 47] obtained porous polymeric hollow nanospheresby using W/ O two2phase microemul sion. And the surface st ructure can be cont rolled by regulating the volume fraction of the water phase.

Li et al[ 48] designed a W/ O microemul sion system of Span80-kerosene-water containing nonionic surfactant . After hydrolyzed , tet raethyl orthosilicate ( TEOS) underwent polycondensation on the surface of the surfactant in microemul sion. Then stable hollow spheres with pores were yielded.

PS hollow spheres with tailored dimensions had been accomplished by microemul sion polymerization using different surfactant s by J ang et al[ 49] , the size of which was about 20nm and the thickness of shell was 3nm. They pointed out that the size of the hollow spheres could be cont rolled by means of the characteristics of surfactant s with chain of different length.

The polymeric hollow nanospheres prepared by microemul sion technology have many advantages , such as narrow dist ribution of diameter , simple experimental device , easy operation and the cont rollable size. However , in the process , a great amount of emul sifier s are needed. It even should reach 1/ 10～3/ 10 of the amount of the monomer[ 50] , which would bring some bad effect s and limit it s indust rialization. Therefore , Candau et al[ 51] t ried some ways to reduce the amount of emul sifier s. They polymerized the colloidal particles in the area of phase invertion to increase the concent ration of disper sed phase to the highest and added sodium acrylate to the acrylamide monomer (Aam) to reduce the interfacial energy. They al so found that through changing the proportion of mixed emul sifier , the optimal value of HLB could be obtained. By using the microemul sion with the optimal value of HLB , the usage of emul sifier can be minimized.

2.2 模板法制备空心纳米球

传统的制备空心球的方法主要是利用各种可牺牲性模板, 如聚苯乙烯球[ 11, 14, 21] 与二氧化硅粒子及它们的晶体阵列[ 16] 、液滴[ 10] 、硅球[ 22] 、树脂球[ 23] 、囊泡[ 24] 、微乳液滴[ 25] 等作为核制备空心球, 因此称为模板法。其过程是首先通过物理或化学方法得到核-壳型复合粒子, 然后通过加热、煅烧或溶剂溶解除去核, 得到空心球, 其过程可见图1。