Synthesis of Nanomaterials Based on Solid State Chemistry

SYNTHESIS OF NANOMATERIALS BASED ON SOLID STATE CHEMISTRY N.Z.Lyakhov Institute of Solid State Chemistry and Mechanochemistry SB RAS, 630128, Novosibirsk, Kutateladze str., 18, e-mail: [email protected]

Abstract. An increasing importance of solid state chemistry as the methodological basis for the understanding of a number of phenomena encountered by researchers in the area of nano-sized particles and materials is observed to accompany the development of the "nanotechnological initiative" all over the world. The ideas of heterogeneity, defectiveness, bulk and surface diffusion, nucleation and coalescence, as well as size effects on the reactivity of small particles have always been natural for solid state chemistry and have been used to interpret chemical transformations observed in experiments. However, it is evident today that solid state chemistry may and must be considered as a powerful tool to solve the problems of obtaining various nanomaterials, first of all (but not only) nanopowders, these "bricks" to build nano- structured materials with different functionality. In this situation, The task of governing not only the size but also the shape of particles to be synthesized become of prime importance in solid state chemistry, as frequently required by practice. There are no clearly established regularities in this area, but the number of empirical observations is so large that it would be unreasonable to neglect them in solving the problems of nanomaterial technologies. Below we will make an attempt to illustrate with specific examples the comprehensive advantages of solid state chemistry, including mechanochemistry.

1. Synthesis of nano dispersed materials 1.1. Synthesis of metal nanopowders. In spite of the progress of recent years in superfine grinding, synthesis of nanopowders is mainly directed to chemical methods in all cases when there are strong requirements to purity and to stability of the product composition. In practice, any types of chemical processes are in use: from the classical sol-gel procedure to combustion and explosion. It is unnecessary to say that the consumer properties of powders are strongly dependent not only on the process type but also on the process conditions. The latter circumstance often imposes restrictions on process scaling to the technologically reasonable level. A mean to achieve acceptable results in solid state chemistry is the procedure involving precursors, which often allows one to vary the characteristics of powders within the same process, that is, without breaking too much into the technology itself. The matter concerns the possibility to obtain chemically and structurally identical products using different initial reagents (precursors). For example, oxides can be obtained by thermal decomposition of hydroxides, salts, complex compounds (within a single process), but also they can be obtained from metals by means of oxidation or combustion. Hence, in the market, one can come across a wide range of the products under the same name but with quite different technological characteristics even in the case of approximately the same particle size distribution. In general, this problem has been recognized long ago, but a trend to use such diversity for meeting the requirements of powder consumers appeared in connection with the needs of

1 nanotechnologies. It should be noted that the whole above-indicated approach is sound only in the case if the synthesis of a precursor itself does not present essential technological difficulties; this would ensure a reasonable price of a manufactured powder. Various methods of synthesis and modification of metal and oxide nanoparticles have been developed. An efficient and easily scalable method is reduction of metal salts to the metal state by means of heating in a high-boiling organic liquid (RF Patent No. 2233730, Bulletin No. 22 of 10.08.2004). Wide possibilities are provided by the use of salts, namely carboxylates, specially synthesized as precursors to obtain metals. The simplest carboxylates are formates [1-3].

Fig.1. Electron microscopic photographs of Bi Formate (a,b) and Oxoformate (c,d) obtained at room temperature (left) and at 60 °C (right).

Figure 1 shows how the shape and size of the particles of bismuth formate and bismuth oxoformate change under different synthesis conditions. Only temperature varies, but the morphology of the synthesized compounds (which may be used for reduction) changes substantially. So, varying the length of the carboxylic chain and the conditions under which metal particles are obtained, we may have powdered bismuth, silver, copper, nickel and other metals with different properties, first of all with different size. An example is shown in Fig. 2.

2 Fig. 2. TEM pictures of Bi nanoparticles obtained from Bi Formate (a) and from Bi Caprilate (b) by thermal decomposition process. For comparison the SEM picture of Bi particles obtained by reduction of Bi Stearate in the medium of Benzyl alcohol is presented.

Solid state chemistry in its classical version opens the way to additional control of metal particle size through doping the salts, for example carboxylates, with the ions of other metals. In particular, if we add silver, characterized by a trend to easier reduction, we may strongly increase dispersity of the resulting nanopowder, for example copper or nickel. For instance, doping of copper caprylate with silver ions results in a decrease in the average size of copper particles (from nearly one micrometer) by an order of magnitude (Fig. 3).

Fig. 3. Effect of Silver doping on particle size of Copper: (a – undoped, b – doped with Ag ions).

A similar effect is observed for nickel powder obtained by means of reduction from nickel formate in benzyl alcohol. However, sometimes implementation of this approach in full leads to effects which are difficult to explain.

3 Fig.4. Mixed products of Bi and Ag obtained from Stearates with different proportions.

Figure 4 shows the particles obtained from mixed salts – bismuth and silver stearates. One can see that, independently of the amount of silver added, bismuth and silver form coarse globules assembled from individual nanoparticles. For any composition, independent formation of bismuth and silver nanoparticles occurs. The driving force of such a spectacular aggregation of particles remains the subject for further research. The use of metal nanopowders in various applications is a large and self- dependent problem. Due to their pyrophorous properties (except for silver and precious metals), even their storage requires special efforts. Different organic liquids (for example, hexane or kerosene) are used to protect the powders from atmospheric oxygen. This is why metal nanopowders are sold in the form of more or less concentrated suspensions. From this point of view, the method of metal reduction in benzyl alcohol medium patented by us is well compatible with subsequent technological operations, for example in the production of multilayer ceramic capacitors. It follows from the above considerations that obtaining protected metal powders will be of special importance as time goes by. Nanoparticles of metals, including magnetic ones, can be obtained in nanoreactors, which can be layered double hydroxides [4-5]. In this method, the role of precursor is played by a complexonate of a metal (nickel, cobalt, copper or their combination) intercalated into the interlayer space of the hydroxide. Under heating, metal nanoparticles of almost identical size (though differing from one metal to another) are formed (Fig. 5); they are organically included into a ceramic matrix.

4 Fig. 5. Schematic representation of metal nanoparticle synthesis in layered silicates and SEM

Such a peculiar composite, or some others obtained with a similar procedure, can be of special interest for electronic materials science. However, it is important to note that so small metal particles wrapped into the oxide matrix are not prone to noticeable oxidation even after long-term storage of the composite in the air. The use of mechanochemistry allows one to solve the problem of encapsulating nanoparticles of many metals by combining their synthesis with activated grinding in the presence of amorphous carbon, boron, etc. [6-8]. After a short-time mechanical activation followed by thermal annealing at a moderate temperature, we obtain metal nanoparticles neatly coated with a layer of graphite or boron nitride (Fig. 6).

5 Fig.6. TEM pictures of encapsulated metallic particles with carbon and boron nitride coatings.

These powders are stable to oxidation in the air, too. Potential areas of their application are wider than those of usual metals. Core-shell structures possess specific mechanical properties. They may be used as modifying agents in alloys, plastics or lubricating oil though the practical assimilation of these structures yet comes across insurmountable technological problems. Similar encapsulated silver and bismuth powders have recently been obtained by means of explosion treatment of the corresponding carboxylates. Photographic images shown in Fig. 7 clearly exhibit carbon layers on the surface of silver particles [8].

Fig.7. Silver particles obtained by shock wave treatment, coated with amorphous carbon.

Taking into account the fact that the experience of large-scale synthesis of nanoparticles (for example, nanodiamonds) according to the explosion technology has already been accumulated in the world, the recent results are inspiring about the technological development of encapsulated metals.

1.2. Synthesis of metal oxide nanopowders. Nanopowders of metal oxides, both simple and complex ones, are used either as-obtained or as the construction materials for the production of various ceramics with improved and even unique characteristics. Well-known sol-gel processes have not yet justified hopes for large-scale processes

6 due to the low yield and large amount of liquid toxic wastes. That is why various methods of the synthesis of oxide nanopowders are being patented all over the world in order to decrease expenses for their production. It should be kept in mind that this is a competition with such large-scale production procedures as gas flame, spray drying, SHS, and sometimes even with grinding procedures. A combination of the above-described precursor technique with mechanical activation allows one sometimes to solve the problem of nanooxide synthesis by elegant and simple methods allowing future reasonable scale-up and development of industrial technologies. A simple example is mechanochemical synthesis of zirconium dioxide for the production of fine ceramics having numerous important applications. The process scheme is shown in Fig. 8 [9-10].

Fig. 8. Mechanochemical synthesis of Zirconium Dioxide.

The process is as easy as a pie: zirconium chloride is mixed with an alkali; after several minutes of activation (5 minutes only!) the reaction is completed. The products are zirconium dioxide and sodium chloride which is easily washed off with water. The formation of NaCl plays extremely positive part preventing inevitable aggregation of the oxide nanoparticles. As a result, one may obtain zirconium oxide particles as small as about 20 nm in size! Unfortunately, because of the absence of solid chlorides of many metals, the process shown in Fig. 8 cannot be multipurpose, especially when the synthesis of the powder of complex oxides is considered. However, some solutions can be found. Figure 9 shows indium oxide obtained by annealing (at a temperature as low as 250 °C) of the mechanically activated mixture (mechanocomposite) of metallic indium and carbamide peroxide. One of the most important components for microelectronics may be obtained in the fine state (with particle size less than 10 nm) using this simple procedure [11].

7 Fig. 9. TEM picture of Indium Oxide obtained by thermal annealing of a preliminary prepared mechanocomposite of metallic In with Peroxide of Carbamide.

It turned out that a similar approach can be used to synthesize nano-sized complex oxides used in the production of electroceramics. A usual way to synthesize ceramic materials for electronics is diffusion annealing at relatively high temperature (900 оС to 1500 оС). Oxides or carbonates of the metals included into the complex oxide are used in this case as the initial reagents. This procedure is used today to synthesize ferrites, ferro- and piesoelectrics without which it is impossible to imagine the least complicated electronic devices. The product of synthesis is always a cake to be ground and classified at the final stage, in order to obtain powder with the suitable particle size. The traditional technology is energy-consuming and leaves almost no possibilities to manufacture nanopowder. The situation was restrained, at least for a number of interesting compounds based on Barium or Stroncium. In our experiments, we involved the same principle of making an energy-enriched precursor – a mechanocomposite obtained by treating a mixture of Barium (or Strontium) peroxide and the corresponding metal or its oxide (both versions were tested) in an activator mill. These processes are shown schematically in Fig. 10 together with the electron microscopic images of the products.

8 Fig. 10. Mechanochemical preparation of precursors for the low temperature synthesis of complex oxide electroceramics.

In all the cases, nanopowders were obtained by thermal treatment of mechanocomposites at a temperature which is very low for such a system (200 оС – 400 оС). It is important to stress the single-phase character of the resulting compounds, which has become possible due to the low synthesis temperature. For the traditional synthesis from Barium Carbonate and the oxides of Tungsten, Molybdenum, Tantalum, the high volatility of the latter compounds gives a lot of troubles. It is evident that the very possibility of chemical reactions controlled by solid-phase diffusion at such a low temperature has been realized due to the formation of a nanocomposite at the stage of mechanical activation; metal (oxide) nanoparticles in this nanocomposite are indeed submerged into relatively plastic barium peroxide. These assumptions were confirmed by numerous microscopic observations, though it seems impossible yet to provide a quantitative description of reactions in these systems.

2. Important applications of nanopowders. 2.1. Modification of metals and alloys. Dispersion modification of metals and alloys is one of the promising directions in nanotechnologies. Theoretically, improvement of mechanical (and some other) properties of metal alloys was predicted long ago. The foundation is the possibility to grind grains due to adding to the melt metal-insoluble ceramic nanoparticles. However, in practice, there are still no universal methods to introduce a powder into the melt. An obstacle is the low wettability of ceramic powders by metal melts, and especially the large difference between the densities of modifying agents (carbides, oxides, nitrides) and the melt itself. The problem of compatibility of the modifier and the melt seems to be solved by means of preliminary

9 modification of the powder itself. It was noticed that the joint mechanochemical treatment of ceramic powders in the presence of metals not only leads to rapid comminuting of ceramic particles to nano-size but also results in plating their surface with the metal, thus cardinally improving their wettability by metal melts. As a result, the introduction of modifying nanoparticles into the melt becomes much simpler and particles are distributed over the bulk almost uniformly without additional mixing of the melt. Under cooling of the melt, the presence of the heterogeneous admixture with high wettability naturally serves as a crystallization seed thus decreasing the necessary supercooling to several degrees instead of usual 100-200 оС, which eliminates inevitable temperature non-uniformities accompanying crystallization and additionally improves the quality of molds. All these advantages were tested experimentally with copper, cast iron and steel [12-13]. Photographic images of the samples of grey cast iron without additives and with the addition of 0,1% of plated silicon carbide powder are shown in Fig. 11.

Fig. 11. Microphotographs of grey cast iron as received (left) and modified with 0.1% of nanopowder (SiC).

Higher uniformity of a grain on the fracture surface may be observed almost by naked eye and is confirmed with electron microscopic images (Fig.12).

10 Fig. 12. SEM pictures of broken samples of grey cast iron without additives (upper row) and modified with 0.1% (bottom left) or 0.4% (bottom right) of nanopowder.

Almost all the strength characteristics improve. An additional positive effect is an increase in the corrosion stability of the modified grey cast iron in comparison with the usual one. Similar observations were made for shock destruction and rupture tests with the samples of stainless steel; a 2-fold increase in the corrosion stability in seawater was also established (Fig.13 and 14).

11 Fig. 13. Shock destruction of stainless steel samples without additives (left) and modified with 0.1% of nanopowder.

Fig. 14. Corrosion resistance of stainless steel in see water.

It is important to stress that the positive effect of nanodispersion modifying has an optimum versus the concentration of an additive, as a rule, at a level of 0,05 -0,1 mass %. This is a very small amount; generally speaking, it is at the level of the accuracy of determination of the main non-metallic components of cast iron and steel (for example, carbon). In other words, nanodispersion modifying does not affect the

12 composite status of alloys having a substantial effect only on their internal structure (grain size, carbon distribution, etc.). 2.2. Mechanocomposites for curative cosmetics. Layered silicates (talc, kaolin), which are usual components of cosmetics, mechanochemically interact with biologically active compounds forming nanocomposites with chemically grafted organic compounds. For example, Chitosan Succinate (a derivative of Succinic acid) combines with the silicate releasing chitosan, which forms particles with a size as small as 5 nm on the surface of the silicate. Of course, this chitosan sample would possess increased biological activity supplementing the beneficial influence of Succinic acid on skin. The method allows one to fix a number of useful organic acids, salts, alcohols on silicates, thus combining their action for the purpose of achieving a definite curative effect corresponding to the specific kind of skin (Fig. 15).

Fig. 15. Formation of chemically bonded biologically active substances on layered silicates.

It is possible to prepare similar mechanocomposites with some medical substances, which opens the way to new medicinal forms of known medicines, including those acting through skin. 2.3. Cathode materials for Lithium-ion accumulators. The basic material for Lithium- ion based electrical current sources (accumulators for mobile phones, portable computers, etc.) is Lithium Cobaltate LiCoO2. Its main disadvantage is a decrease in capacity characteristics under cycling, in other words, from one charging procedure to another. This classical material was modified by nano-coating with nanoparticles of other oxides for the purpose of improving the electrochemical characteristics of Lithium Cobaltate and increasing its stability against cycling [14]. As a matter of fact, this work was crowned with the development of a new cathode material with surprisingly high cycling stability. The capacity of the battery with the new cathodes did not decrease during about a hundred of cycles, it even somewhat increased (Fig.16).

13 Fig. 16. Charge – discharge cycling of Li-ion element with surface modified cathode material.

2.4. Titanium Diboride in Copper matrix. With the help of mechanical activation of a mixture of Copper, Titanium and Boron powders, it is possible to start up the interaction of Boron with Titanium in the combustion regime directly in the Copper matrix [15]. The product is a nanocomposite with high concentration of combustion product, i.e., Titanium Diboride (Fig. 17).

Fig. 17. Microphotograph of Titanium Diboride nanoparticles formed in copper matrix as a result of SHS process.

This composite possesses high electrical conductivity and surpasses all the known composites (of copper-tungsten type) in corrosion stability. The material will find application for electrodes in the devices of emergency shutdown of electricity supply networks and possibly in arc plasmatrons. Tests are being carried out. An increase of the electrode lifetime by about an order of magnitude is expected.

14 The framework of the present paper does not allow a more detailed consideration of other potential applications of nanopowders. This is a rapidly developing area, and we are to meet gratifying surprises in future.

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