Khadijeh Khezri 1,*
1 Student Research Committee, Department of Pharmaceutics, Faculty of Pharmacy, Mazandaran University of Medical Sciences, Sari, Iran
Abstract
In recent years, there has been an increasing interest in the growth of using nanomaterials in different fields. Nanomaterials can propose the main benefits over conventional materials. Many preparation approaches have been developed to fabricate nanomaterials. This mini-review is about the preparation methods of nanomaterials. Top-down and bottom-up methods and their subgroups are reviewed in detail. It can be concluded that achievement knowledge about the different preparation processes of nanomaterials would be helpful for designing superior nanomaterials. A combination of two or more suitable methods may lead to an outstanding view of preparing novel advanced nanomaterials.
Keywords: Nanomaterials, Top-down methods, Bottom-up methods
Recently, nanomaterials attract much attention because of promising applications in various fields. The prefix “nano” has initiated in the last decade and it derives from the ancient Greek word that means very small [1]. According to the International System of Units (SI), it is applied to specify a reduction factor of 109 times. The nanosized and nanometer range mainly can be included 1nm to 100 nm based on different standards [2]. Therefore, nanotechnology can be defined as the science of the very small materials. Indeed, it includes the use and manipulation of matter on a very small scale. According to scientific reports, at this size, the materials work differently and possess a diversity of amazing and exciting properties [3].
Many synthesis methods have been developed to make nanomaterials, such as chemical mechanical attrition, vapor production, gas condensation, sol-gel system and etc. There are various methods for the preparation and synthesis of nanoparticles. But top-down and bottom-up procedures are the most common. The bottom-up synthesis approach has been used to design and arrange atoms, molecules and ions in nanostructures in which the top-down approach has been applied to nanomaterial synthesis of bulk solid for preparing nanoparticles by structural decomposition. Figure 1 indicates the top-down and the bottom-up methods of making nanomaterials [4].
Figure 1. The top-down and the bottom-up procedures for preparation of nanomaterials
The top-down method is a process with reverse engineering that obtaining insight into its compositional sub-systems. A top-down method is mostly organized with using black boxes for easier decomposition to subsystem levels. In fact, this method is an overview of a system that has been formulated with first-level subsystems [5]. However, black boxes maybe accurate enough to validate the model and or may be unsuccessful for clarify elementary mechanisms. In this approach, the formulated system decomposes into smaller segments. A variety of top-down approaches for preparing nanoparticles are as follows: vacuum deposition, gas condensation, chemical vapor deposition (CVD), chemical vapor condensation (CVC), mechanical attrition procedures [5].
2.1. Gas condensation
The first technique for preparing nanoparticles such as nanocrystalline metals and alloys is gas condensation. In this technique, the process of evaporation of metallic or inorganic substances is done by means of thermal evaporation bases such as electron beam evaporation devices and heated refractory crucibles. High pressure of a gas is applied to produce ultra-fine nanoparticles in the range of <100 nm. In this method, evaporated atoms are bombed with remaining gas molecules and are formed ultra-fine nanoparticles. Sources of evaporation used for this process include high energy electron beams, low energy electron beam, resistive heating, inducting heating [6].
Homogenous nucleation in the gas phase forms nanostructured cluster and cluster-growth processes depend on the amount of mixed gas with the atoms. The W, Ta or Mo containers are used to evaporate metals. Evaporation by electron beam technique is used when the metals react with containers. This approach is very slow. The limitations of this method are as follows: dissimilar evaporation rates in an alloy, temperature ranges and a source-precursor incompatibility [6].
In recent years, other sources such as the inert gas atmosphere (He) have been developed to evaporate metals (Fe). Also, laser evaporation or sputtering approaches may be applied as an alternative source for thermal evaporation [7].
Sputtering is a non-thermal technique to yield a diversity of clusters containing Ag, Fe and Si that utilizes an ion beam or glow discharge in low-pressure environment. Recently, alternative sources such as sputtering electron beam heating and plasma approaches have been successfully developed to produce ultrafine particles or clusters [8].
2.2. The vacuum deposition
The vacuum deposition approach is a process in which alloys, elements or substances are evaporated at a pressure of less than 0.1 Pa (1 m Torr) and in vacuum levels of 10 to 0.1 MPa by heat sources and deposited in a vacuum. If the evaporation rate is high (vapor pressure of 1.3 Pa (0.01 Torr)), an acceptable deposition rate is obtained in the vacuum. Vapor phase nucleation can form in the cloud of dense vapor by multi-body interactions of atoms to provide nanoparticles. High deposition degrees and being a low-cost process are the benefits of the vacuum deposition approach. However, the deposition of numerous compounds is problematic. Supersaturated vapor can form nanoparticles longer than the cluster [9].
2.3. Chemical Vapor Deposition (CVD) and Chemical Vapor Condensation (CVC)
CVD is a chemical reaction in which a solid precipitate on a heated surface through a gas phase or vapor and activation energy is required to perform CVD reactions. This energy can be supplied in various ways. The activation energy required for thermal CVD and plasma CVD is high temperature and plasma, respectively. The pyrolysis reaction occur in laser CVD when an adsorbent is heated by laser heat energy. The chemical interactions between reactant molecules occurs in photo-laser CVD when induced photon energy of ultraviolet radiation is sufficient to break the chemical bonds. The nanocomposite powders were developed by CVD. The pyrolysis process of metal-organic precursor in chemical vapor condensation (CVC) is performed at low-pressure atmosphere and nanoparticles such as ZrO2, Y2O3 and nano whiskers are produced by this method [10].
2.4. Mechanical Attrition
Unlike other methods stated before, mechanical attrition forms nanomaterials not by cluster assembly but by the structural decomposition as a result of plastic deformation. The powders of Al and β-SiC and ceramic/ceramic nanocomposite WC-14% MgO are ready in a high energy ball mill. The ball milling and rod milling methods have received attention as a powerful tool to synthesize complex materials. Mechanical alloying is a distinctive procedure that can progress at room temperature and can perform on both high energy mills, centrifugal type mill and vibratory type mill, and low energy tumbling mill [11].
It defines as connection systems together to provide more complicated systems, creation them sub-systems of the developing system. In a bottom-up method, the individual base elements of the system are quantified in detail at first. These elements are then pieced together to form subsystems, which are linked again, sometimes on several levels, until a complete top-level system comes to form [8]. In this section, various bottom-up approaches for preparing nano-materials are discussed.
3.1. Chemical Precipitation
In a chemical precipitation technique, the size is in control by precipitation procedure, i.e. avoiding the physical changes and aggregation of tiny crystallites in a liquid. Double layer repulsion of crystallites can control Thermal coagulation and Oswald ripening using non-aqueous solvents at low temperatures to synthesize nanomaterials. Using a surfactant can cause superior separation between the particles formed during the process. The formed nanocrystals are centrifuged, washed and vacuum dried [12].
3.2. Sol-Gel Techniques
This procedure is a veritable tool for network growth of colloidal suspension (sol) and gelatin in the continuous liquid phase (gel) and PH control is main in this approach. In the sol-gel techniques, ions of metal alkoxides and aloxysilanes are the precursor for synthesis of colloids such as silica gels that is formed from tetramethoxysilane (TMOS), and tetraethoxysilanes (TEOS). Alkoxides are insoluble substances in water with the organometallic precursors for silica, titanium, aluminum and etc [13].
3.3. Hydrolysis
During hydrolysis, the replacement of [OR] group with [OH-] group results as water addition. The progress can perform faster by adding a catalyst such as HCl and NH3. Hydrolysis occurs as the oxygen contained in the water attacks the silicon atom to produce siloxane bonds (Si-O-Si) [14].
3.4. Condensation
In condensation process, either water or alcohol can be formed a siloxane bond during the Polymerization process. Monomers, dimers, cyclic tetramers, and high order rings are formed as the final consequence of condensation yields. The degree of hydrolysis is affected by pH, reagent concentration, molar ratio, ageing and drying, which offer a possibility to diverge the structure and possessions of sol-gel derived inorganic systems [15].
3.5. Growth and Agglomeration
As the number of siloxane bonds increases, the molecules join together as a network that forms a gel upon drying off the water and alcohol. Spherical nanoparticles are formed at values of pH of greater than 7, and H2O/Si value ranging from 7 to 5. Above a pH of 7. The solubility of silica increases and silica particles grow in size. Growth stops when the solubility of the smallest and largest particles develops indifferent. Higher temperatures can form larger particles. Some problems and limitations have remained despite developments in both chemical and physical approaches of production. The laser vaporization system has provided several benefits over other heating methods. The target material is forced with a high energy pulsed laser which causes high vaporization and temperature (10,000°C). A high density of plasma (Typical yields are 1014-1015 atoms from the surface area of 0.01 cm2) is produced in 10-8 s that is beneficial for the direct deposition of units [16].
3.6. Electro-deposition
Electro-deposition method can produce mechanically strong and uniform nanostructured films. Most of the progress of nanostructured coating can be applied either by PVD or CVD. Hypersonic plasma particle deposition (HPPD) has been applied for the synthesis and deposition of nanoparticles as an unusual method. The high potential of nanomaterial production and their uses is remained unexplored by now, hence gaining knowledge about the process would come to use in designing superior materials. A blending of enhanced hardness and wear resistance, that are powerfully influenced by grain size, would cause a better coating performance [17].
4. Conclusion
The utstanding properties of nanomaterials lead to the increasing use of them in different areas. Top-down and bottom-up methods and their subgroups were reviewed in this paper. It can be concluded that attainment information about the different preparation processes of nanomaterials would be useful for designing superior nanomaterials. A combination of two or more suitable methods may lead to an outstanding view to prepare novel advanced nanomaterials.
Acknowledgments
The author state that there is no financial support for this study.
Conflict of interest
The author has no conflict of interest in this paper.
References
HOW TO CITE
Khezri, K. (2019). A short view on conventional methods for preparation of nanomaterials. Journal of Advanced Chemical and Pharmaceutical Materials (JACPM), 2(2), 124-128. Retrieved from http://advchempharm.ir/journal/index.php/JACPM/article/view/111
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