Hollow mesoporous silica spheres synthesized with cationic and anionic mixed surfactant as templates

Highlights

• Hollow mesoporous silica spheres (HMSSs) of 200–300 nm have been synthesized according to a modified Stober method at room temperature.

• CTAB/SDS mixed surfactant was adopted as dual templates.

• The possible formation mechanism of HMSSs using mixed surfactant templates has been discussed according to vesicle template approach.

Abstract

Hollow mesoporous silica spheres (HMSSs) with ordered mesoporous and uniform size distribution of 200–300 nm were synthesized using mixed surfactant of cationic cetyltrimethylammonium bromide (CTAB) and anionic sodium dodecyl sulfate (SDS) as templates at room temperature. Our work provides a facile approach to synthesize hollow mesoporous silica spheres by a modified Stober method. The results showed that the BET surface area is high up to 951 m² g⁻¹ and the average pores diameter is 2.4 nm. The possible formation mechanism of HMSSs using mixed surfactant templates was discussed by vesicle template approach.

Introduction

Mesoporous silicas with uniform large pore between 2 and 50 nm and high surface area have attracted much attention since the pioneering work by the scientists at the Mobil Corporation in 1992 [1], [2]. Hollow silica sphere with ordered mesopores is one of the most attractive structures which has been applied in many important research fields, such as catalysis, drug delivery, and medical imaging, owing to its uniform pore size, high surface area, large hollow interiors, abundant Si–OH active bonds, and good biocompatibility [3], [4], [5].

Two main methods have been proposed to obtain hollow mesoporous silica spheres including hard-template and soft-template approach. As for the hard-template method, the mesoporous silica shell must be deposited on the sacrificial hard-templates like the uniform monodispersed polymer microspheres [6] or inorganic beads [7] and then the cores should be removed. Although the hard-template method offers advantages in providing a well controllable size distribution and hollow structures dimensions, the removal of template cores usually requires some toxic solvents. Moreover, the preparation and removal procedures of the sacrificial templates are generally complicated, uneconomic, and time consuming [8].

Compared with the hard-template method, the soft-template as core could dissolve in the solution or evaporate from particle interior, leaving an empty cavity and making this route more facile, inexpensive and environment friendly. Up to now, various self-aggregation structure of micelles [9], vesicles [10], [11], and oil-in-water emulsions [12] have been adopted for the preparation of hollow cavity in combination with surfactants as the soft-template. However, these template routes usually need some special surfactants or waste large amount of expensive surfactants as sacrificial reagent and the synthesis of the soft-templates themselves may require complex procedures [13], [14], [15]. Therefore, it is highly valuable to develop a facile and effective strategy for preparing the mesoporous hollow spheres with ordered mesoporous structures.

Modified Stober method is a facile method to produce silica spheres under certain mild conditions [16]. According to this method, we attempted to introduce mixed anionic–cationic surfactant as dual soft-template which will theoretically provide silica sphere with hollow mesoporous structures. Moreover, the anionic surfactants have been applied widely in industrial fields and the techniques for their synthesis and purification turn out to be more mature and cheap [17]. Meanwhile, the dual template method has potential application in obtaining other novel functional materials.

Herein, we demonstrated the preparation of hollow silica spheres with ordered mesopores by SDS/CTAB dual templates approach at room temperature. In our case, the hollow mesoporous silica spheres formed through a special assembly behavior of the SDS/CTAB templates [18], [19] in a facile condition. The as-prepared hollow mesoporous silica spheres were characterized by XRD, TEM and nitrogen adsorption measurements, and the possible formation mechanism was discussed by vesicle approach.