Microemulsion

The microemulsion method is also useful to synthesize silica nanoparticles. Arriagada and Osseo-Asare reported synthesis of silica nanoparticles by using a reverse microemulsion method. They used decane, ammonium hydroxide, and sodium bis(2- ethylhexyl)sulfosuccinate (AOT) in the formation of reverse microemulsion (Arriagada and Osseo-Asare 1995). Hollow silica nanospheres are also synthesized by using the microemulsion method. The report has demonstrated hollow silica synthesis by using a template of unilamellar vesicles. They synthesized it by hydrolysis and condensation of silicon alkoxides (Hubert et al. 2000). Zoldesi and Imhof have reported the synthesis of monodispersed, spherical nanosilica, silica capsules, and microballons using microemulsion methods. The properties of these different morphological silica particles could be tuned by modifying reaction conditions (Zoldesi and Imhof 2005). Microemulsion templates have recently gained attention due to the ease of control for tuning the particles’ morphology and size. Ossero-Asare also synthesized nanosilica by using water-in-oil reverse microemulsion (Arriagada and Osseo-Asare 1999a). They also proposed a statistical nucleation model, which is based on the relationship of nanosilica size and water-to-sur- factant ratio. Nanosilica was also synthesized by controlling the hydrolysis of TEOS in the NP 5/cyclohexane/ammonium microemulsion solution (Arriagada and Osseo-Asare 1999b). They also showedthe advantage of the microemulsion method incontrolling the size of silica nanoparticles by analyzing the influence of ammonia concentration and the water-to-surfactant ration on the size of nanosilica. Micoemulsion methods are also preferred to synthesize various morphological silica nanoparticles. Tao and Li have demonstrated the synthesis of microskel- etal silica nanosphere by using a reverse microemulsion method (Tao and Li 2005). Nevertheless, this method has limitations in synthesizing nanoparticles due to the use of the aqueous system. Hence, the room temperature ionic liquid (RTIL) method is preferred over a conventional microemulsion method. In this method, low melting points containing a series of organic salts are used. This method has wide applications in catalysis, electrochemical deposition, organic synthesis, and separation (Welton 1999; Mehnert et al. 2002; He et al. 2006). The major advantage of the RTIL is that it can be modified and designed via changing anions and cations. Hence, this microemulsion method has great potential applications. Han et al. have produced silica nanorods by using ionic liquid microemulsion in an aqueous system composed of water, l-butyl-3-methylimid- azolium hexafluorophosphate(bmimPF6), and Triton X-100 (TX-100) (Li et al. 2006). They investigated the importance of bmimPF6 in the synthesis of nanosilica and also showed that this method also shows similar limitations as a conventional method due to the presence of an aqueous system. Recently, nonaqueous ion liquid microemulsion gained interest and a few reports have shown the synthesis of nanosilica. The first report was for the synthesis of hollow silica spheres with the use of a nonaqueous ionic liquid microemulsion method. They prepared nonaqueous microemulsion with benzene, TX-100, and 1-butyl- 3-methylimidazolium tetrafluoroborate (bmimBF4) (Zhao et al. 2008). However, catalysis of TEOS by the microemulsion without the addition of acid or alkali has a slow reaction rate. Hence, a recent report has studied the synthesis of silica nanoparticles via nonaqueous ionic liquid microemulsion method with the addition of acid or alkali. They show that the addition of acid resultedin ellipsoid silica nanostructures and the addition of alkali leads to the formation of hollow silica spheres. In addition, they also demonstrated that the use of acid or alkaliaccel- erated the reaction rate. This microemulsion method has been proposed as a facile method to prepare nanosilica with advantages over a conventional,aqueous microemulsion method.

ESD

For the betterment of the deposition of silica nanoparticles, it is essential to develop methods for micropatterning (Innocenzi et al. 2008). Self-assembly patterning is one of the methods in which modification of wet surface can be processed (Masuda et al. 2005). However, there is a limitation to this wet-based patterning method. The resulting pattern may be compromised due to the surface tension and capillary flow during the drying process. Therefore, dry-based patterning methods are also developed (Lu et al. 2001; Malfatti et al. 2006). These methods include lithography process like electron-beam lithography(Wu et al. 2004) and interferometric lithography (Xia et al. 2007). Lithography methods provide high resolution and uniform thickness. However these drying based patterning processes are costly due to the multiple fabrication steps. Recently, an electrospray deposition (ESD) method has been developed for the micropatterning of silica nanoparticles (Higashi et al. 2014). In this method, a stencil mask is used that is similar to the masks utilized for micropatterning during CVD (Kim et al. 2003; Brugger et al. 2000). ESD is suitable to perform at room temperature and atmospheric pressure that makes it cost-effective. This method has advantages in that it can be utilized for water sensitive and vacuum sensitive materials. In the process of ESD. a TEOS in water/ethanol/HCl is stirred at room temperature for 5 min. TEOS sol is heated to achievea viscosity of 7 mPa-s. In this system, an 8 mm diameter nozzle of the glass syringe is filled with the prepared TEOS sol. In the arrangement of components, the syringe is positioned horizontally at a height of 120 mm. A 0.5 mm thick 400 mm square silicon substrate is placed 40 mm far from the nozzle of the syringe.The silicon substrate is covered with the stencil mask to micropattern the nanosilica.

 
Source
< Prev   CONTENTS   Source   Next >