Chapter 2

The Synthetic Strategy for Developing Mesoporous Materials through Nanocasting Route

Rawesh Kumar and Biswajit Chowdhury*

Department of Applied Chemistry; Indian School of Mines, Dhanbad, Jharkhand, India

*Corresponding author: biswajit_chem2003@yahoo.com

Abstract

Nanocasting is a powerful methodology for creating ordered mesoporous materials that are more difficult to synthesize by conventional processes. Synthesis starts with fabrication of inorganic/carbon precursors inside the nanospaces of a mesoporous hard template, which is used as a true mold to produce the mesoporous materials with controllable pore size, morphology of the network. After fabrication inside the nanospaces, the template framework is selectively removed and the mesoporous ordered inorganic/organic material is obtained. In this chapter, a comprehensive literature survey for fabrication of mesoporous materials via mesoporous hard template, e.g., SBA-15, KIT-6, and mesoporous carbon, is presented. First two sections are limited to the basic understanding of nanocasting methodology. The later three sections comprise a review of works based on mesoporous silica/carbon template, which has been nurturing till date. A wide range of studies for synthesis of several inorganic compounds range from metal oxides, metal sulfides, metal carbide phosphate, and ceramics is covered.

Keywords: Nanomaterials, functional metal oxides, mesoporous, nanocasting.

2.1 Introduction to Nanocasting

The fabrication of porous materials, especially the creation of mesoporous materials, has been extensively investigated over the last few years. The worldwide large interest in mesoporous materials is due to its accessibility to extremely complex structures, [1] strong curvatures in solid state chemistry, interesting mass transport facilities, [2] confinement effect [3-5] and error tolerant (tend to reorganize spontaneously) capacity. Mesoporous materials are widely achieved by two processes namely soft templating and hard templating. Soft-templating is a surfactant based methodology where variety of organic surfactant forms mold and around which the inorganic/organic frameworks are weaved. Finally removal of these surfactant results in a cavity which retains the same morphology and structure of the organic surfactant. In soft-templating processes, the sol-gel [3, 4] and evaporation induced self-assembly (EISA) processes [8-11] are typically involved in the synthesis of ordered mesoporous materials. In sol gel process, the assembly of surfactant and inorganic/organic widely depends on matching of charge density between them at interface [5, 6]. In basic medium, assembly of inorganic/organic anions (I-) would match with surfactant cations (S+) through coulomb forces (S+I-) whereas in acid medium, assembly of inorganic/organic cations would match with surfactant anions through coulomb force (S-I+). If both surfactant and inorganic/organic have same type of charge then a bridging counter ions is required to balance the coulombic interaction as (S-X+I-) or (S+X-I+). In strong acid medium, the interaction initiates through coulomb forces as S+X-I+ that gradually transforms to the (IX)- S+ interaction. In EISA processes, a volatile solution of inorganic/organic precursor and surfactant is allowed to form surfactant-inorganic/organic interface. As well as solvent evaporates, precursors hydrolyze and cross-link to each other and highcontent surfactants form liquid-crystal phases. In this way, inorganic/organic materials accumulate around the voids of liquid-crystalline phase and so a mesostructured hybrid is formed. Finally, ordered mesophases are solidified to form a rigid inorganic framework. Afterwards, the surfactants can be removed by calcinations and in this way high ordered mesoporous solids are synthesized. The mesoporous structure that coming out from soft-templating is generally difficult to predict because self-assembly and solvent induced assembly depends upon many parameter like temperature, solvent, concentration, hydrophobic/hydrophilic properties, interface interaction and ionic strength.

Another process named "hard templating" seems to be one of the most promising synthetic pathways to create porous materials, especially if materials with ordered porosity are the goal. Hard templates are molds in which ordered hollow cavity (pores) can be filled by precursors solution and allowed these to polymerize inside, i.e., MCM-41, SBA-15, KIT-6, etc. After digesting the hard template, ordered porous material having one-to-one replica can be created. In case of microporous materials, it is not easy to achieve a one-to-one replica due to the lack of understanding for the template function in micropores. In case of ordered mesoporous, the synthesis indeed corresponds to a direct templating mechanism, where a relatively precise replica of the template is created. This replication process can be so perfect that one is tempted to use the term "nanocasting or casting process on the nanometer scale" to describe this process. It implies that the template is actually used as a true mold to produce the mesoporous materials with controllable pore size, morphology, distribution, properties and composition of the network. The application of the nanocasting technique to the fabrication of inorganic/carbon compounds implies that the fabrication of these products take place in the nanospaces of mesoporous hard template. After the synthesis of the material, the template framework is selectively removed and the inorganic/organic ordered mesoporous product is obtained. Due to the fact that the synthesis takes place in a confined nanospace, the sintering of the particles is restricted and the preparation of high surface area nanostructures or nanoparticles is achieved. Moreover, this synthetic strategy clearly suggests that the structure of the synthesized compounds can be tailored depending on the pore characteristics of the selected template. Mesoporous structures that coming out from hard template where fixed nanoscale pore architectures can be easily predicted. The pore walls of the replicas coming from hard templates can range from amorphous (such as carbon), to semi-crystalline (such as TiC) as well as single-crystalline (in most cases) whereas it is mostly amorphous in case of soft templating.

2.2 Steps of Nanocasting

Basically, the nanocasting route comprises in three steps: (i) Infiltration or introduction of precursor, (ii) The casting step or heat treatment under a controlled atmosphere of the impregnated template to convert the infiltrated precursor into the inorganic material, and (iii) Template removal by dissolution (i.e., silica) or by oxidation at high temperatures (i.e., carbon). A generalized scheme of nanocasting for getting mesoporous structures is summarized in Figure 2.1.

Figure 2.1 A generalize scheme of nanocasting for getting mesoporous structures.

2.2.1 Infiltration

During nanocasting infiltration step is governed by four sub-steps, named (1) Selection of precursors, (2) Selection of solvent, (3) Selection of template, and (4) Infiltration of precursor solution/gas into the pores of template.

2.2.1.1 Selection of Precursors

Selection of metal precursor has of prime importance in nanocast mesoporous metal oxide preparation. The weak acid salts like acetate, citrates and oxalates have low volume yield as well as high coordination ability which inhibit transportation of metal ion. Metal precursor should have high volume yield as heteropoly acid (e.g., phosphotungstic acid or phosphomolybdic acid) has 90% volume yield. But unfortunately apart from tungsten and molybdenum, this kind of heteropolyacid is rare. Nitrate metal precursors are easily decomposable and have highest mesoscopic regularity. In spite of its low volume yield, these salts are mostly reported due to low formation energy and fast transportation of ion inside the pore of template, e.g., Co3O4 [7].

The metal precursors; which have lower melting point than its decomposition temperature, can be directly grinded with template and further left for heating treatment. With rise in temperature above melting point, metal precursor melts and then impregnates into the mesochannels of the silica template through capillary force [15-17]. As for example to prepare nanocasted mesoporous NiO, Ni(NO3)2.6H2O metal precursor is taken which have melting point (56.7°C) that is much lower than its decomposition temperature (>110°C). At a temperature higher than 57°C, the liquid Ni (NO3)2.6H2O easily moves into the mesopores channels of silica [15, 8]. This method is known as solid-liquid method. Metal precursor which have higher melting point should impregnate with solvent assistant.

Some group select low cost metal chloride precursor for infiltration into the mesopores channel. In this particular case, ammonia exposure is required before calcinations for conversion of metal chloride to metal hydroxide [9, 10]. A solution of cupper nitrate infiltration into the mesochannels generally results into poor solution infiltration. So, again here ammonia exposure is required to make a good infiltration into mesochannels [11].

2.2.1.2 Selection of Solvent

Apart from above discussed, solvent cooperation with metal precursor is required for improving pore occlusion. The volatile solvent should have high dissolving activity, low coordination ability and weak interaction with silica surface (hydrogen bonding interaction). The highly soluble precursor (in suitable solvent) is...