Composite Hydroxide-Mediated (CHM) Approach

The method is based on a composite hydroxide-mediated (CHM) on the use of composite molten hydroxides as a solvent in chemical reactions at ~ 200 ° C for the installation of a wide range of nanotechnology. This principle is entered synthesis method, and is main factors that affect the study of the morphology and size. And explains the advantages of low temperature synthesis, low pressure and low cost through the installation of functional wires, rods, belts and other nanostructures [6].

2.5  How to use Composite-Hydroxide-Mediated (CHM) technique

While bulk oxides have long been prepared in hydroxide melts at from 350℃  to over 1000℃, [7-8]. the preparation of uniform nanostructures using the CHM method at lower temperature has only arisen in consideration of the eutectic point of composite hydroxides by us in 2006 [9-10]. CHM method is based on chemical reactions of materials in eutectic hydroxide melts at a temperature of 200℃ and ambient pressure in the absence of organic dispersants or capping reagents. Although the melting points of pure sodium hydroxide, potassium hydroxide and lithium hydroxide are above 300 _C (Tm ¼ 323℃ for NaOH, Tm ¼ 404 ℃ for KOH, and Tm ¼ 477 ℃ for LiOH) the eutectic points for particular mixtures of these elements, NaOH/KOH ¼ 51.5:48.5, LiOH/ KOH ¼ 0.31:0.69, and NaOH/LiOH ¼ 0.71:0.29 are only about 165℃,

225℃ and 220℃, respectively. The mixed hydroxides play the role of the reaction medium. Normally, the synthesis process of the CHM method is a one-step process. All of the raw materials with a certain amount of mixed hydroxides are placed within the Teflon vessel at one time. Then, the nanostructures form within the vessel after heating in a furnace at a temperature of  200℃ for several hours or days. The as-produced materials are crystalline with clean surfaces, which are favorable for further investigating their intrinsic properties [11-12]. and surface functionalization.

2.5.1 Why We Don’t Use the Other Synthesis Techniques

All the methods which were written above have their own advantages and disadvantages, most of them involve high vacuum and a high temperature, or a high pressure, or a low temperature with high pressure. The categories and mass of materials prepared by the VAPOUR PHASE PROCESS and JET-DEPOSITION synthesis are limited, which cause the high price of the products. Although MILLING processes can produce large quantities of crystal powders, the diameters of the produced particles are only sub-micrometer, and the morphology of the particles is difficult to control. The SOL-GEL and MICROEMULSION methods are very practical syntheses and low cost methods for nanostructures, but they involve a final calcination or annealing step at high temperature, thus they cannot be considered to be true low-temperature  synthesis methods, and it is difficult to control the particle size and morphology. HYDROTHERMAL and SOLVOTHERMAL syntheses are effective for synthesizing nanomaterials and available for many kinds of materials. However some surfactants and capping agents used in hydrothermal or solvothermal methods are not completely removed from the final products, which can affect some of the physical properties of the crystals. In addition, large amount of organic solvent used in solvothermal synthesis would bring about environmental pollution. Moreover, because of the high pressure involved in hydrothermal and solvothermal syntheses, sophisticated equipment is required. Various syntheses methods are  reported to be capable of producing various nanostructures, but most of the methods are limited to synthesizing one type of or a specific collection of nanostructures [13]. Except a general approach is available for utilizing the hydrothermal route to synthesis a large variety of nanoparticles using fatty acids as structure-directing agents.

2.5.2  Advantages of the CHM Approach

CHM approach and unique advantages.

·         It is a simple approach to install in one step and slow reaction, which makes it possible to control the growth kinetics for the purpose adjust the size and morphology of nano manufactured.

·         This is synthesis at low growth temperature of ~ 200 ° C in the atmosphere, and it does not require expensive equipment or sophisticated, so the technique is likely to be easy to be adopted and the transfer of technological applications.

·         The reaction materials are not expensive and cost the entire synthesis process effectively. High yield and they can be easily expandable to produce on a large scale.

·         Most of nanotechnology as producing high quality single crystals, which are fit for the requirements of many applications.

·         There is no capping reagent on the surfaces of nanotechnology as it is synthesized, and therefore, the clean surfaces can be easily functionalized for various purposes [14].