Chemical Synthesis Of Nanomaterials

There is a growing realization that the knowledge of fundamental molecular interactions and processes (=chemistry) is of central importance for the progress of nanoscience, which deals with matter at a scale where the properties are dominated by molecular behaviors, including single molecules and molecular assemblies. Despite clear practical implications of nanocrystalline materials, there exists a deficit in the ability to produce materials with desired intrinsic properties and a precise control over size, shape, and composition [35]. This situation is distressing, especially in view of the fact that most of the recent advances in nanomaterials are based on novel materials with extraordinary properties (e.g., fullerenes, carbon nanotubes, quantum dots, 1-D oxide wires, etc.) as well as new processing routes (e.g., aerosol nucleation, mechanochemistry, templated growth in meso-porous matrices, hydrothermal methods, etc.).

Chemistry has profoundly influenced the emergence of advanced materials through the development and application of novel chemical routes for the synthesis of nanosized powders and nanostructured films. The "nano-movement" in materials chemistry is partly triggered by the observation that the predefined metal stoichiometry and reaction chemistry of molecular precursors can enforce a molecular-level homogeneity in the obtained materials [36-38]. In solid-state processing, the solid precursors (such as metal oxides, carbonates, and salts) must be brought into contact by grinding and mixing and subsequently heat-treated at high temperatures to facilitate the diffusion of atoms or ions in the reaction mixture. Since the diffusion process depends on the temperature of the reaction and grain interfaces, the mixing and grinding steps are usually repeated throughout the heat treatments to prepare fresh surfaces for further reactions. When compared with solid-state processing, the diffusion of matter in the vapor or liquid phase is typically many orders of magnitude higher. Moreover, the chemical reactions allow a controlled interaction of atoms or molecules to form uniformly dispersed solid particles. The atomic-scale mixing of the phase-forming elements results in a faster nucleation of the product at low temperature, which makes kinetic control over the process possible. As a result it is possible to suppress the preferred formation of thermodynamically favored substances and to perform a phase-selective synthesis of metastable compounds.

Although the concept of synthesizing materials from molecules has long been suggested as a mild approach (the so-called soft chemistry methods), the interest, for a long period of time, was to apply these routes to known materials. It is only recently that attention has been shifted to the synthesis of new materials with novel compositions. To decipher how molecular interactions give rise to complexity, it is chemical synthesis that can provide new compounds to manipulate molecular interactions and thereby control the process of material formation as well as the properties of the resulting materials. The chemical techniques use direct, often very simple, chemical reactions to produce the solid of interest. In addition, chemical processing allows control of chemical structure, morphology, and dimension required on several scales of magnitude. For instance, with the manipulation of the reaction kinetics so as to encourage particle nucleation over particle growth, ultrafine particles can be obtained. The nucleation kinetics are easily regulated by varying the amounts or ratios of the reactants or reaction time, for example, by massively exceeding the supersaturation ratio in precipitation reactions or by increasing the amount of water added in an alkoxide-based sol-gel process to drive hydrolysis at the expense of condensation reactions. Furthermore, the reaction kinetics can also be influenced by adjustment of the thermal energy input or thermal energy distribution in the reaction vessel. The enhanced degree of mixing achieved by ultrasonic irradiation can accelerate reaction kinetics by two orders of magnitude and yield nanometric particles [39]. Sonic and ultrasonic agitation methods are used to create small bubbles, in a reaction pool, to accelerate reaction kinetics by cavitation. This method, known as sonochemical synthesis, relies on the phenomenon of acoustic cavitation (a point where the intensity of ultrasound exceeds the intramolecular forces, and, as a result, the molecular structure collapses and a cavity is formed) that results from enormous stresses (500-1000 atm) and elevated temperatures (~3000 K) induced in the reaction medium by the application of ultrasound (20 kHz to 10 MHz). Another example of accelerated chemical reaction strategy is the straightforward synthesis of hexagonal gallium nitride (GaN) nanocrystals by detonation of gallium azides [40, 41].

In basic chemistry terms, the starting materials (input) of a chemical reaction are the reactants and the material (output) to which the reactants are converted, the product. In most of the chemical methods in practice, the starting materials are commonly available reagents, which are close to the desired composition but need a heat-treatment step to attain the final composition. For example, the calcination of CaCO3 eliminates CO2 and results in the formation of CaO. Similarly, firing Al(OH)3 drives off water to produce Al2O3. In such cases, where the "precursor" is an infusible solid, the size of the particles in the precursor will impose a minimum limit for the size of the particles in the final calcined material. However, particles can grow larger because of fusion (necking) that takes place at elevated temperatures. Further problems associated with such powders are (i) that particles are rarely round (spherical powders pack better), (ii) the presence of dense agglomerates of much smaller crystallites formed during firing and crystallization, and (iii) the external contamination of particles during milling or grinding steps that are necessary to break down the larger agglomerates. For these reasons, minimizing the calcination temperature and steps is a prerequisite in the preparation of nanocrystalline materials by conventional chemical techniques. An elegant way to limit the particle size at the precursor stage is to use well-defined molecules that can be seen as "molecular fragments" of the solidstate structure. A number of methods have been devised to control the final particle size by performing the reactions in spatially confined reaction pools, for example, hydrolysis in aqueous reaction pools separated by an intervening liquid (water-in-oil micro(nano)emulsion) or pyrolysis of small droplets of precursor solution separated from each other by a gaseous phase (aerosol). The methods are not straightforward in predicting the final particle size because of chemical exchange and agglomeration between pools in the microemulsion method, whereas coalescence and drying of droplets in the case of aerosol technique lead to a size distribution and chemical inhomogeneities in the final material. In addition, the chemical reaction occurring within the droplets and the nature of different chemical intermediates can further complicate the synthesis. Since no positional control is offered, the different compounds present in the reaction mixture randomly collide to form various intermediate species with metal ratios unfavorable for obtaining a single-phase material. As a consequence, phase separation and element segregation are present at the nanometer scale, although the global stoichiometry of the product corresponds to the desired composition. This is a common observation in the conventional material synthesis procedures (e.g., solid-state reactions, coprecipitations, combustion reactions, etc.) where the intrinsic chemical behaviors (e.g., different hydrolysis rates of the reactants in solution-phase reactions or different vapor pressure or thermal stability in the gas-phase reactions) of the different components present in a reaction mixture make the stoichiometry of the target material highly susceptible to inaccuracies. For instance, a simple mixing of two chemical precursors A and B, to produce the binary system AB (multisource precursor, MSP), may produce a visibly homogeneous mixture, but the molecular-level scenario can be different, and, despite a correct global stoichiometry, the precursor "cocktail" may have element segregation and non-ideal A:B ratios at the nanometer scale that may be carried forward to the end product (Scheme 1).

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