ReviewNanomaterials and supercritical fluids
Introduction
There is a great interest in the preparation and application of nanometer size materials since they can exhibit new properties of industrial interest. Which are the matter properties that can show dramatic changes at nanoscale range? Mainly the properties related to the ratio between surface and volume: at nanoscale, surface properties become relevant with respect to volume properties. For example, surface molecules can impart high hardness to metals and higher energy to propellants and explosives; electronic devices and pharmaceuticals with improved performance can also be produced.
Different interpretations of the dimensions that set the boundary between normal materials and nanomaterials have been proposed. In this work, we assume that a nanoproduct should have at least one dimension smaller than 200Ā nm; though, more restrictive definitions have been proposed that set the upper limit at 100Ā nm. Nanoparticles, nanofilms and nanowires are nanometric along three, two and one dimension, respectively. In the case of nanostructured materials, at least one of the components has nanometric dimensions.
The various processes that have been proposed to obtain nanomaterials follow two main approaches: top down and bottom up. Top-down is characterized by the production of nanoproducts departing from normal size materials; i.e., reducing the dimensions of the original material; for example, using special size reduction techniques (Fig. 1). Bottom-up approach is related to the āsynthesisā of nanosized materials, starting from the molecular scale (Fig. 1); for example, the formation of particles by precipitation from a fluid phase.
Supercritical fluids (SCFs) have also been proposed as media to produce nanomaterials. The properties that make supercritical fluids particularly attractive, as a rule, are gas-like diffusivities, the continuously tunable solvent power/selectivity and the possibility of complete elimination at the end of the process. Particularly, the mix of gas-like and liquid-like properties can be useful in many applications related to nanotechnologies.
The most widely used supercritical fluid is carbon dioxide (CO2), that is cheap and non polluting, and whose critical parameters are simple to be obtained in an industrial apparatus. However, ammonia, alcohols, light hydrocarbons and water have been proposed, among the others, for nanomaterials production at supercritical conditions.
Among all the nanoproducts that can be envisaged, two main areas have been explored using supercritical fluids: nanoparticles and nanostructured materials. Nanoparticles cover a wide range of applications; it will be possible to produce explosives with a higher potential; i.e., approaching the ideal detonation; coloring matter with brighter colors; toners with a higher resolution; polymers and biopolymers with improved functional and structural properties. Moreover, pharmaceutical products can be designed that have enhanced pharmaceutical activity or that use different delivery routes and/or overcome human body internal barriers. Metals, metal oxides and ceramic compounds at nanodimensions can exhibit unusual strength and/or can be used as fillers in nanostructured materials. Composite nanospheres and nanocapsules can be used, for example, in pharmaceutical applications for controlled and sustained release of drugs. The production of nanowires, nanofilms and nanotubes has also been considered in supercritical fluid assisted processes.
Nanostructured polymers can be generated in form of nanocellular foams and membranes. For example, nanocomposite polymers can be obtained modificating the host polymer properties using nanofillers (nanoparticles, nanoclays). However, nanostructured polymers will not be treated in this work since they have been the subject of a recent excellent review [1].
Several supercritical based techniques have been proposed in the literature for the production of micro and nano materials, since several processes can operate in the micronic or in the nanometric domain depending on the operating conditions and on the process arrangement. For what concerns micrometric and sub-micrometric particles generation by supercritical assisted processes, some good reviews are available in literature [2], [3], [4], [5], [6].
A large number of papers in the literature claims the production of nanomaterials by supercritical fluids assisted processes; but, the dimensions of materials described are in several cases more properly in the range of sub-micronic products. Therefore, in this work we performed a first selection of the papers to be discussed on the basis of the ānanoā definition previously proposed; i.e., papers related to materials with characteristic dimensions larger than 200Ā nm have not been considered. A critical analysis is proposed that highlights the most relevant positive and negative characteristic of each process and the kind of nanomaterial that can be produced.
Section snippets
Nanoparticles generation
A possible general classification of SCF based nanoparticles generation techniques can be proposed according to the role played by the SCF in the process. Indeed, SCFs have been proposed as solvents, solutes, anti-solvents and reaction media.
Nanofibers, nanowires and nanotubes
Nanowires can have a particular relevance as building blocks for the realization of nanoscale structures, for example, in the microelectronic industry in which they can act as both devices and electrical contact. For this reason, the production of semiconductor nanowires has been particularly investigated. Several techniques have been proposed, including laser ablation, liquid crystals templating methods and vaporāliquidāsolid growth processes [114]. These routes, typically produce disordered
Nanocomposite materials
The production of composite nanomaterials can be of interest in several applications: particularly for the controlled release of pharmaceuticals, medical devices, semiconductors and superconductors, microelectronic applications and barrier materials (gas barriers, oxygen barriers, food packaging).
Conclusions and perspectives
A large quantity of SCF based processes that were successful in producing nanomaterials has been found in the literature at laboratory scale. They can be divided in new processes and (more commonly) adaptation of existing processes to the use of SCFs. In all cases, using SCFs, more flexible and/or simplified processes have produced and with a reduction of the environmental impact. The final result is that nanomaterials with potentially better performances have been obtained using improved
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