Templating nanostructures by mesoporous materials with an emphasis on room temperature and cryogenic TEM studies
Introduction
The ever-growing interest in the development of nanostructured materials is attributed to the possibility of tailoring their properties in order to suit the needs of a wide variety of applications, which include catalysis [1•], electronic and optical devices such as transistors, light emitting devices and lasers [2•], [3•]. The dimension and the size distribution of the nanostructured materials are the most important parameters in tuning the properties for these applications. The most prominent nanostructures are nanoparticles [1•], nanowires [2•], nanorods [4], nanoribbons [5] and nanotubes [6]. Several approaches used in their preparation include polymer controlled growth [7], liquid crystal or micellar template approach [8•], [9], solvothermal approach [10], laser assisted catalytic growth [11•], chemical approach [12•] and porous template approach [13], [14•], [15•]. Porous materials are excellent templates owing to the ease in the preparation of well-ordered and uniform sized nanomaterials on a large scale. Furthermore, the facile preparation of porous templates combined with relatively low cost renders them very attractive.
Two prominent porous templates are currently used, namely, anodic alumina membrane and mesoporous silica templates [14•], [16•]. Porous alumina is fabricated electrochemically through anodic oxidation of aluminum, yielding highly ordered arrays of pores starting at several hundreds down to several tens of nanometers in size. The depths of the pores are uniquely limited by the anodization time, thus very deep pores can be obtained, in the order of millimetres. The interested readers on porous alumina templates may refer to an excellent review reported in the literature [16•].
The focus of this article is limited to a review on the preparation of nanostructures templated by ordered mesoporous silica (OMS) having a well-defined, discrete, pore shape and size of 2–50nm, emphasizing the role of TEM as an important structural characterization tool. Various families of OMS have been reported so far including Mobil Composition of Matter, MCM [17], Santa Barbara, SBA [18], Hybrid Mesoporous Material, HMM [19], [20] and Folded Sheet Mesoporous, FSM [21]. These mesoporous materials, having a discrete pore size, are ideal templates for preparing various inorganic nanostructures including nanoparticles, isolated nanowires or ordered arrays of nanowires and nanotubes.
Most commonly used methods employed in the insertion of guest species into the pores of OMS are co-condensation [22], wet impregnation [14•], [15•], [23•] and ion exchange process [24]. After the nanostructures' formation, the OMS is removed by washing with HF or NaOH, resulting in template-free nanostructured materials. This review presents the significant progress in the fabrication of nanostructures templated by OMS either as a powder or as thin film. Ordered mesoporous carbon (OMC) templated by OMS is being dealt as a separate sub-section since it is a widely researched material holding technological importance in several areas such as purification of water, adsorption, electrochemistry, energy storage or catalytic supports in fuel cells electrodes [25•].
The use of transmission electron microscopy (TEM), as an indispensable characterization tool for the investigation of nanostructures, is emphasized along the entire review with some illustrations. TEM is a powerful direct imaging technique, which enables to probe the structure of materials at the nanometer scale, providing information on the shapes, sizes, crystallinity and organization of the observed objects. Conventionally, a two-dimensional (2D) image is obtained from an inherently 3D sample. However, an unambiguous interpretation of the 2D images may be difficult as the structural features of the 3D sample are projected and may overlap in the 2D image. The development of 3D imaging techniques in recent years is therefore advantageous to both material scientists and biologists seeking structural information at a high-resolution level [26••]. An example highlighting the importance of 3D imaging technique for the study of mesoporous-templated nanostructures is given in Section 2.1.
Section snippets
Metals
The importance of metal nanostructures has been underlined not only in catalysis but also in miniaturized optical and electronic devices [1•], [27]. Earlier efforts to prepare metal nanostructures include wet impregnation or ion-exchange [28•], [29]. These methods require repeated reaction and/or long time treatment with the guest precursor. Therefore, in situ reduction methods have been developed in recent years [30•], [31], [32]. Selective functionalization of the external and internal
Nanostructured materials templated by OMS thin films
In most of the initial studies on templated nanostructures, OMS is used in a powder form. However, from the device point of view, it is vital to grow nanostructures within the OMS pore channels in a thin film geometry, as the guests are not only separated by large gap (dielectric) SiO2 but also regularly aligned with respect to each other [45]. Indeed, a recent report reveals the preparation of ultra-high density CdS nanowire arrays templated by SBA-15 mesoporous thin films (MTFs) using
Conclusion
Templating by OMS is a promising and powerful method for preparing one dimensional nanostructured materials. Metal nanoparticles and nanowires such as Pt, Pd and Au supported by OMS have shown enhanced catalytic activity and better selectivity than conventional catalysts of the same material. Preparation of ordered mesoporous carbon such as CMK-3 and CMK-5 with high reproducibility in their properties demonstrates the success of OMS as excellent templates. The importance of TEM in structural
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