3. Bottom-up approach: BACK
3.1 Wet Chemical Synthesis of nanomaterials (Sol-gel process):
The sol-gel process, as the name implies, involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The precursors for synthesizing these colloids consist usually of a metal or metalloid element surrounded by various reactive ligands. The starting material is processed to form a dispersible oxide and forms a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the gel produces the oxide.
Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based precursors such as Si (OEt) 4 (tetraethyl orthosilicate, or TEOS). The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides M(OR)z can be described as follows:
MOR + H2O → MOH + ROH (hydrolysis)
Sol-gel method of synthesizing nanomaterials is very popular amongst chemists and is widely employed to prepare oxide materials.
The sol-gel process can be characterized by a series of distinct steps.
Step 1: Formation of different stable solutions of the alkoxide or solvated metal precursor (the sol).
Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged network (the gel) by a polycondensation or polyesterification reaction that results in a dramatic increase in the viscocity of the solution.
Step 3: Aging of the gel (Syneresis), during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores. Ostwald ripening (also referred to as coarsening, is the phenomenon by which smaller particles are consumed by larger particles during the growth process) and phase transformations may occur concurrently with syneresis. The aging process of gels can exceed 7 days and is critical to the prevention of cracks in gels that have been cast.
Step 4: Drying of the gel, when water and other volatile liquids are removed from the gel network. This process is complicated due to fundamental changes in the structure of the gel. The drying process has itself been broken into four distinct steps: (i) the constant rate period, (ii) the critical point, (iii) the falling rate period, (iv)the second falling rate period.
If isolated by thermal evaporation, the resulting monolith is termed a xerogel. If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel.
Step 5: Dehydration, during which surface- bound M-OH groups are removed, there by stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to 8000C.
Step 6: Densification and decomposition of the gels at high temperatures (T>8000C). The pores of the gel network are collapsed, and remaining organic species are volatilized. The typical steps that are involved in sol-gel processing are shown in the schematic diagram below.
Fig: Schematic representation of sol-gel process of synthesis of nanomaterials.
The interest in this synthesis method arises due to the possibility of synthesizing nonmetallic inorganic materials like glasses, glass ceramics or ceramic materials at very low temperatures compared to the high temperature process required by melting glass or firing ceramics.
The major technical difficulties to overcome in developing a successful bottom-up approach is controlling the growth of the particles and then stopping the newly formed particles from agglomerating. Other technical issues are ensuring the reactions are complete so that no unwanted reactant is left on the product and completely removing any growth aids that may have been used in the process. Also production rates of nano powders are very very low by this process. The main advantage is one can get monosized nano particles by any bottom up approach.
3.2 Liquid solid reactions:
Ultrafine particles are produced by precipitation from a solution, the process being dependent on the presence of the desired nuclei. For example, TiO2 powders have been produced with particle sizes in the range 70-300 nm from titanium tetraisopropoxide. The ZnS powders were produced by reaction of aqueous zinc salt solutions with thioacetamide (TAA). Precursor zinc salts were chloride, nitric acid solutions, or zinc salts with noncommon associated ligands (i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate). The 0.05 M cation solutions were heated in a thermal bath maintained at 70° or 80 °C in batches of 100 or 250 ml. Acid was added dropwise to bring it to a pH of 2. The reaction was started by adding the TAA to the zinc salt solution, with the molar ratio of TAA and zinc ions being set to an initial value of either 4 or 8. In intervals, aliquots were collected from the reacting solution.
3. 3. Gas Condensation Processing (GPC)
Fig: Schematic representation of typical set-up for gas condensation synthesis of nanomaterials followed by consolidation in a mechanical press or collection in an appropriate solvent media.
Gas condensation was the first technique used to synthesize nanocrystalline metals and alloys. In this technique which was pioneered by Gleiter and co-workers a metallic or inorganic material, e.g. a suboxide, is vaporised using thermal evaporation sources such as Joule heated refractory crucibles, electron beam evaporation devices or sputtering sources in an atmosphere of 1-50 mbar He (or another inert gas like Ar, Ne, Kr). Cluster form in the vicinity of the source by homogenous nucleation in the gas phase and grow by coalescence and incorporation of atoms from the gas phase. The cluster or particle size depends critically on the residence time of the particles in the growth regime and can be influenced by the gas pressure, the kind of inert gas, i.e. He, Ar or Kr, and on the evaporation rate/vapour pressure of the evaporating material. With increasing gas pressure, vapour pressure and mass of the inert gas; the average particle size of the nanoparticles increases. A rotating cylindrical device cooled with liquid nitrogen was employed for the particle collection: the nanoparticles in the size range from 2-50 nm are extracted from the gas flow by thermophoretic forces and deposited loosely on the surface of the collection device as a powder of low density and no agglomeration. Subsequenly, the nanoparticles are removed from the surface of the cylinder by means of a scraper in the form of a metallic plate. In addition to this cold finger device several techniques known from aerosol science have now been implemented for the use in gas condensation systems such as corona discharge, etc. These methods allow for the continuous operation of the collection device and are better suited for larger scale synthesis of nanopowders. However, these methods can only be used in a system designed for gas flow, i.e. a dynamic vacuum is generated by means of both continuous pumping and gas inlet via mass flow controller. A major advantage over convectional gas flow is the improved control of the particle sizes. Depending on the flow rate of the He-gas, particle sizes are reduced by 80% and standard deviations by 18%.
Eevaporation can be done from refractory metal crucibles (W, Ta or Mo). If metals with high melting points or metals which react with the crucibles, are to be prepared, sputtering, i.e. for W and Zr, or laser or electron beam evaporation has to be used. Sputtering is a non-thermal process in which surface atoms are physically ejected from the surface by momentum transfer from an energetic bombarding species of atomic/molecular size. Synthesis of alloys or intermetallic compounds by thermal evaporation can only be done in the exceptional cases that the vapour pressures of the constituents elements are similar. As an alternative, sputtering from an alloy or mixed target can be employed. Composite materials such as Cu/Bi or W/Ga have been synthesised by simultaneous evaporation from two separate crucibles onto a rotating collection device. It has been found that excellent intermixing on the scale of the particle size can be obtained.
However, control of the composition of the elements has been difficult and reproducibility is poor. Nanocrystalline oxide powders are formed by controlled postoxidation of primary nanoparticles of a pure metal (e.g. Ti to TiO2) or a suboxide (e.g. ZrO to ZrO2). Although the gas condensation method including the variations has been widely employed to prepare a variety of metallic and ceramic materials, quantities have so far been limited to a laboratory scale. The method is extremely slow. The quantities of metals are below 1 g/day, while quantities of oxides can be as high as 20 g/day for simple oxides such as CeO2 or ZrO2. These quantities are sufficient for materials testing but not for industrial production. However, it should be mentioned that the scale-up of the gas condensation method for industrial production of nanocrystalline oxides by a company called nanophase technologies has been successful.
3.4. Chemical Vapour Condensation (CVC)
Chemical vapor condensation (CVC) was developed in Germany in 1994. It involves pyrolysis of vapors of metal organic precursors in a reduced pressure atmosphere. Particles of ZrO2, Y2O3 and nanowhiskers have been produced by CVC method. As shown schematically in Figure, the evaporative source used in GPC is replaced by a hot wall reactor in the Chemical Vapour Condensation or the CVC process. The original idea of the novel CVC process which is schematically shown below where it was intended to adjust the parameter field during the synthesis in order to suppress film formation and enhance homogeneous nucleation of particles in the gas flow. It is readily found that the residence time of the precursor in the reactor determines if films or particles are formed. In a certain range of residence time both particle and film formation can be obtained. Adjusting the residence time of the precursor molecules by changing the gas flow rate, the pressure difference between the precursor delivery system and the main chamber and the temperature of the hot wall reactor results in the prolific production of nanosized particles of metals and ceramics instead of thin films as in CVD processing. In the simplest form a metalorganic precursor is introduced into the hot zone of the reactor using mass flow controller. For instance, hexamethyldisilazane (CH3)3 Si NHSi (CH3)3 was used to produce SiCxNyOz powder by CVC technique. Besides the increased quantities in this Continuous process compared to GPC it has been demonstrated that a wider range of ceramics including nitrides and carbides can be synthesised. Additionally, more complex oxides such as BaTiO3 or composite structures can be formed as well. In addition to the formation of single phase nanoparticles by CVC of a single precursor the reactor allows the synthesis of
1. Mixtures of nanoparticles of two phases or doped nanoparticles by supplying
two precursors at the front end of the reactor, and
2. Coated nanoparticles, i.e., n-ZrO2 coated with n-Al2 O3 or vice versa, by
Supplying a second precursor at a second stage of the reactor. In this case
nanoparticles which have been formed by homogeneous nucleation are coated
by heterogeneous nucleation in a second stage of the reactor.
Fig: A schematic of a typical CVC reactor
Because CVC processing is continuous, the production capabilities are much larger than in GPC processing. Quantities in excess of 20 g/hr have been readily produced with a small scale laboratory reactor. A further expansion can be envisaged by simply enlarging the diameter of the hot wall reactor and the mass flow through the reactor. The microstructure of nanoparticles as well as the properties of materials obtained by CVC has been identical to GPC prepared powders.
3.5. Laser ablation:
Laser ablation has been extensively used for the preparation of nanoparticles and particulate films. In this process a laser beam is used as the primary excitation source of ablation for generating clusters directly from a solid sample in a wide variety of applications. The possibility for preparing nanoparticulate web-like structures over large sample area is of particular interest in view of their novel properties that can be applied to new technological applications as shown by El-Shall and his collaborators.
Laser vaporization cluster beams were introduced by smalley and coworkers to overcome the limitations of oven sources. In this method, a high energy pulsed laser with an intensity flux exceeding 107 W/cm3 is focused on a target containing the material to be made into clusters. The resulting plasma causes highly efficient vaporization since with current, pulsed lasers one can easily generate temperatures at the target material greater than 104 K. This high temperature vaporizes all known substances so quickly that the rest of the source can operate at room temperature. Typical yields are 1014-1015 atoms from a surface area of 0.01 cm2 in a 10-8 s pulse. The local atomic vapor density can exceed 1018 atom/cm3 (equivalent to 100 Torr pressure) in the microseconds following the laser pulse. The hot metal vapor is entrained in a pulsed flow of carrier gas (typically He) and expanded through a nozzle into a vacuum. The cool, high-density helium flowing over the target serves as a buffer gas in which clusters of the target material form, thermalize to near room temperature and then cool to a few K in the subsequent supersonic expansion. A typical laser vaporization source is shown in figure below.
In a recent investigation utilising a novel atomization system, (LINA-SPARK™), LSA, based on laser spark atomization of solids has been developed that seems to be very versatile for different materials. Briefly, the LSA is capable of evaporating material at a rate of about 20 μg/s from a solid target under argon atmosphere. The small dimensions of the particles and the possibility to form thick films make the LSA quite an efficient tool for the production of ceramic particles and coatings and also an ablation source for analytical applications such as the coupling to induced coupled plasma emission spectrometry, ICP, the formation of the nanoparticles has been explained following a liquefaction process which generates an aerosol, followed by the cooling/solidification of the droplets which results in the formation of fog. The general dynamics of both the aerosol and the fog favours the aggregation process and micrometer-sized fractal-like particles are formed. The laser spark atomizer can be used to produce highly mesoporous thick films and the porosity can be modified by the carrier gas flow rate thus enabling for a control of the microstructure of the coatings which make these nanoparticulate thick films suitable candidates for application in membrane technology, catalysis and lithium ion batteries. ZrO2 and SnO2 nanoparticulate thick films were also synthesized successfully using this process with quite identical microstructure. Synthesis of other materials such as lithium manganate, silicon and carbon has also been carried out by this technique.