Chemical Vapor Deposition

The CVD process is a method of chemical vapor reaction which utilizes the evaporation of volatile metal compounds and chemical reactions to form the compounds desired and the rapid cooling in gas atmospheres to result in the formation of nanoparticles/nanocapsules. The nanoparticles/nanocapsules produced by CVD have several advantages, like high purity, high uniformity, small particle size, narrow size distribution, good dispersal, and high chemical reaction activity. CVD is suitable for synthesizing not only nanoparticles/nanocapsules but also fibers/nanotubes of metals, metal compounds, for instance, nitrides, carbides, borides, etc. In accordance with the types of the reactions, CVD can be divided into two kinds of methods: vapor decomposition and vapor synthesis. According to the type of reaction precursor, the CVD can be classified to be the gas-gas reaction one, the gas-solid reaction one, and the gas-liquid reaction one. For the occurrence of the chemical reactions, it is used to utilize the heating and radiation activating the molecules in the reaction systems. The methods for the activation of the molecules include resistant heating, chemical flame heating, laser heating [158-160], plasma heating [161], X-ray radiation, etc.

The vapor decomposition method is a so-called single compound heat decomposition method. The compound is heated to evaporate and decompose, as in the following reaction:

The materials used for the vapor decomposition method are usually the volatile metal compounds, Fe(CO)5, Si(NH)2, SiH4, Si(OH)4, (CH3)4Si, etc., which have high evaporation pressure and high reaction activity. The reaction equations are as follows:

Fe(CO)5(g) ^ Fe(s) + 5CO(g) f 3[Si(NH)2](g) ^ Si3N4(s) + 2NH3(g) f

SiH4(g) ^ Si(s) + 2H2(g) f 2Si(OH)4(g) ^ 2SiO2(s) + 4H2O(g) f (CH3)4Si(g) ^ SiC(s) + 6H2(g) f

The vapor synthesis method usually utilizes the vapor reactions between two (or more than two) different materials to form the nanoparticles/nanocapsules of the new compounds. The common reaction equation is

The materials used for the vapor synthesis method can be SiH4, CH3SiCl, (CH3)4Si, SiCl4, BCl3, TiCl, TiCl4, Til, TiI4, ZrCl4, MCl3, MoO3, WCl6, H2, NH3, CH4, C2H4, etc. The vapor synthesis method is highly flexible because of the combination and the exchange of the different precursors. The following reactions are listed as examples:

3SiH4(g) + 4NH3(g) ^ Si3N4(s) + 12H2(g) f 3SiCl4(g) + 4NH3(g) ^ Si3N4(s) + 12HCl(g) f SiCl4(g) + CH4(g) ^ SiC(s) + 4HCl(g) f

The heating by a resistant furnace is a traditional procedure for chemical vapor reaction, which consists of precursor treatment, the parameter control of the reaction, nucleation and growth, cooling, and condensation. For both the vapor decomposition method and the vapor synthesis method, when the laser heating is used, one needs to choose the materials with high absorption ability of the laser beam as the reaction precursors [158-160]. The mechanism of laser induced chemical vapor reaction for synthesis of nano-particles is that the gases strongly absorb the high energy of the laser beam so that the gas atoms/molecules are heated and activated instantly. The gases reach the high temperature needed for the chemical reactions and accomplish the reactions, nucleation, and growth in a very short time. The nanoparticles of metals Fe, Ni, Al, Ti, Cr, Zr, Mo, and Ta, their oxides, and nitrides have been synthesized [151-154, 158-160].

The different preparation methods of ultrafine powders of high melting point compounds (carbides, borides, nitrides, and oxides) were reviewed [161]. The consolidation behavior of these compounds in the nanocrystalline state was described in detail. Compaction by hot pressing, including high pressure and high temperature, sintering, and high-energy consolidation methods, was analyzed. The microstructure, recrystallization, and mechanical and physical properties of the nanocrystalline carbides, nitrides, and oxides were characterized [161].

Nanoparticles of uranium dicarbide encapsulated in carbon smaller than 100 nm were obtained by chemical reactions at high temperatures [162]. Two types of nanocapsules were identified and characterized. The majority of them had small diffuse kernel surfaces, with dimensions between 5 and 15 nm, surrounded by thick spherical carbon cover. Others, in minor quantity and ranging from 15 to 40 nm, were poly-hedrical and surrounded by several perfect graphite layers oriented parallel to their external surface. The nanocapsules are as chemically inert as graphite.

Pyrolysis of acetylene over quartz plates coated with various metal catalysts resulted in the formation of all-carbon nanostructures [163]. The nanotubes appear to grow as ultrathin tubes with a central hollow core and considerable thickening due to secondary pyrolytic deposition. In some cases, catalytic particles were entrapped within the tubes to form the nanocapsules. The efficiency of the catalysts was evaluated semiquantitatively.

Carbon nanocapsules with SiC and Au nanoparticles were produced by thermal decomposition of polyvinyl alcohol at about 500 °C in Ar gas atmosphere [164]. The formation mechanism of nanocapsules was discussed and a structural model for the nanocapsule/SiC interface was proposed. In addition, carbon clusters were formed at the surface of carbon nanocapsules, and carbon onions were produced by electron irradiation of amorphous carbon produced from polyvinyl alcohol. Silver nanoparticles encapsulated within boron nitride nanocages were produced from mixtures of boric acid, urea, and silver nitrate upon reduction at 700 °C in hydrogen [165]. Multilayered polymer nanocapsules were fabricated via sequential adsorption of oppositely charged polyelectrolytes onto gold nanoparticles followed by dissolution of the gold core in cyanide solution [166]. A gold nanoparticle was coated with eight polyelectrolyte layers.

Preparation and layer-by-layer self-assembly of silver nanoparticles capped by graphite oxide nanosheets were carried out [167]. Sodium borohydride reduction of silver ions in aqueous dispersions of exfoliated graphite oxide (GO) resulted in the formation of remarkably monodisperse 10 nm diameter oblate Ag particles that are effectively protected by 0.5 nm thick GO sheets, Ag-GO. Ag-GO could be self-assembled onto poly(diallyldimethylammonium) chloride (PDDA) coated substrate, and subsequent layer-by-layer self-assembly of Ag-GO and PDDA led to well-ordered ultrathin films of S-(PDDA/Ag-GO)„.

The specific features of the catalytic reduction of methyl-viologen by dihydrogen in water in the presence of platinum colloids synthesized by various methods were studied [168]. The colloids prepared by the radiation-chemical reduction of PtCl42- in the presence of polyacrylate or polyphosphate as stabilizers and those prepared by the reduction with dihydro-gen efficiently catalyzed the reaction. The "citrate" colloids synthesized by the reduction of PtCl62- with citric acid were characterized by a prolonged induction period after which these colloids also gained catalytic activity.

A technique for the formation of carbon-encapsulated metal nanoparticles on silicon was developed [169]. Carbon encapsulated NiFe nanoparticles were prepared by high-temperature methane encapsulation of the bare bimetallic particles on alumina [170]. About 6-nm-thick carbon layers encapsulated 10-20 nm diameter NiFe nanoparticles. The NiFe nanoparticles were single crystalline and no carbide was found at the NiFe-C interface. Metallic Pd nano-particles with a diameter of 1-2 nm were anchored on the carbon layers, which created a Pd/NiFeC&C type of catalyst that could be used for liquid phase reactions.

Carbon-encapsulated cobalt nanoparticles were prepared over a range of temperatures and partial pressures by CVD from cyclohexane on prepared substrates consisting of ground powders of SiO2 impregnated with Co(NO3)2 and (NH4)2Mo2O7 [171]. The encapsulated cobalt nanoparticles were freed from the incorporated SiO2 and molybdenum by treating the deposition product with aqueous hydrofluoric acid.

Carbon-coated cobalt nanocapsules were synthesized by CVD with cobalt carbonyl [Co2(CO)8] used as precursor and carbon monoxide (CO) as carrier gas [172]. The characterization and magnetic properties of carbon-coated cobalt nanocapsules were investigated systematically. Figure 5 shows a TEM image and a selected area diffraction (SAD) pattern of the Co(C) nanocapsules synthesized by the CVD process [172]. The decomposition of Co2(CO)8 and CO gas can decrease efficiently the content of the oxygen in nano-capsules. The metal Co nanoparticles completely coated by carbon can resist the dilute acid erosion as well as the oxidation.

An in-situ process was developed to create a dispersed phase of lubricant MoS2 nanoparticles in a matrix of a hard TiN coating [173]. The particle formation at different

(a)

Figure 5. (a) TEM image and (b) selected area diffraction (SAD) pattern of Co(C) nanocapsules with core-shell structure, which are synthesized by chemical vapor condensation process with the cobalt carbonyl (Co2(CO)8) used as precursor and the carbon monoxide (CO) as carrier gas. After [172], Z. H. Wang et al., Carbon, in press. © Elsevier Science.

Figure 5. (a) TEM image and (b) selected area diffraction (SAD) pattern of Co(C) nanocapsules with core-shell structure, which are synthesized by chemical vapor condensation process with the cobalt carbonyl (Co2(CO)8) used as precursor and the carbon monoxide (CO) as carrier gas. After [172], Z. H. Wang et al., Carbon, in press. © Elsevier Science.

total and partial pressures was investigated. High yields of particles with diameters of 30-50 nm were obtained thermally at temperatures > 300 °C and pressures > 5 hPa with MoCl5/H2S as precursors. In the low pressure range, particles were formed by a plasma CVD process using the reaction of elementary sulphur or H2S with MoCl5.

Plasma enhanced CVD was used to synthesize Fe nano-particles in an amorphous boron carbide matrix [174]. The nanoparticles ranged in size from approximately 0.7 to 4.5 nm. The size of the nanoparticles was proportional to the density of the Fe precursor (ferrocene) in the vapor.

Anatase titania nanoparticles were prepared using pyrol-ysis of titanium tetrabutoxide in oxygen-free and oxygen containing atmospheres by metal organic chemical vapor deposition (MOCVD) [175]. Influence of oxygen on the properties of titania nanoparticles was investigated. With increasing oxygen flow rates, the average grain sizes of the nanoparticles decreased and the particle size distributions became uniform. Oxygen exerted great influence on the nucleation rate of the nanoparticles and reaction kinetics occurred in the reactor. The formation of titania nano-particles by MOCVD was not a growth controlled process but was a nucleation rate controlled process.

Cobalt nanoparticles encapsulated in carbon shells were synthesized by catalytic CVD in high yield by reducing with a H2/CH4 gas mixture a Mg09Co0 jO solid solution impregnated MgO catalyst [176]. The carbon encapsulated Co nanoparticles had a narrow distribution of diameters within the range of 5-15 nm. They were made of fcc Co which is very stable to air oxidation and the magnetic properties confirmed that Co was present in the metallic state.

A modification of the conventional inert gas condensation apparatus was developed for making nanostructured powders, wherein an evaporative source was replaced by a chemical source [177]. CVC combines CVD together with cooling condensation in evaporation. This process was used to synthesize loosely agglomerated amorphous nanoparticles of SiCxNyOz, starting from hexamethyldisilazane as precursor compound [177]. The density, surface area, particle size, and composition of as-synthesized nanoparticles of SiCxNyOz were modified by changing the synthesis temperature and carrier gas. The phase and morphology of as-synthesized powders were modified by heat treatment.

A concept was introduced of synthesis of nonagglomer-ated nanoparticles by rapid condensation from the vapor phase in a reduced pressure environment [45]. The source of the material was an evaporative source, which is ideally suited for low vapor pressure and low melting point metals. A variation in this process was developed, in which the source of the nanophase material was a metalorganic precursor [178-180]. The process, called combustion flame chemical vapor condensation (CF-CVC), is a modification of the original CVC process which involves pyrolysis of chemical precursors in the gas phase. The key parameters are gas phase residence time, temperature of the hot-wall reactor, and precursor concentration in the carrier gas. Careful selection of flow parameters resulted in powders that were only loosely agglomerated, significantly enhancing their usefulness in commercial applications. The nanopowders had a narrow particle size distribution with a mean particle size controllable between 5 and 50 nm. The CVC processing unit is an effective nanoparticle generator that is suitable for many different types of materials, like SiC, Si3N4, Al2O3, TiO2, ZrO2, and other refractory compounds. They extended the processing capabilities to include a flat flame combustor unit which is particularly suited to synthesis of oxide phases either as powders, films, coatings, or freestanding forms.

The ductile behavior of nanophase yttria doped zirconia ceramics was investigated during low-temperature deformation experiments [181]. Ceramics were produced by following a standard processing route of mechanical compaction of the dispersion mixed nanoparticles synthesized by inert gas condensation or CVC and pressureless sintering. The influence of initial grain size and porosity on strain and strain rate was a topic of interest as well as the microstructural evolution during deformation.

The effect of the ratio of O2/He flow rate in the reactor on the characteristics of nanosized TiO2 powder synthesized by CVC was investigated under fixed conditions of supersaturation ratio, collision rate, and residence time [182]. As the ratio of O2/He flow rate increased, the particle size of TiO2 powder almost remained unchanged but the agglomeration of nanoparticles enlarged in which the degree of agglomeration was defined as the ratio of particle size to crystallite size.

A vapor phase synthesis process was developed, in which vapors of chemical precursors were pyrolyzed in a low pressure flat flame [183, 184]. By controlling the time-temperature history of the particles in the hot zone of the flame, high surface area nanopowders of oxides that have primary and secondary (aggregate) nanoscale particle sizes were produced. The synthesis process, scalability issues, powder properties, and areas of application were described.

Magnetic nanoparticles of Fe and Co were synthesized by CVC using the precursors of iron carbonyl [Fe(CO)5] and cobalt carbonyl as the sources under a flowing helium atmosphere [185-187]. Typical particle sizes were on the order of 5 to 13 nm with uniform dispersion. A correlation between the process parameters of CVC and the resulting microstructure of the nanoparticles was investigated. Average particle size increased with increasing the decomposition temperature of the precursor.

A series of iron-cobalt alloyed (Fe1-xCox, x < 50 wt%) nanoparticles with core/shell structure was prepared by CVC [188, 189]. The relation between the process parameters and the resulting characterization and microstructure and the crystalline structure after oxidation at different temperatures were studied. The particles had a mean size of 5-25 nm, which consisted of metallic cores and oxide shells. The nano-particle size increased with increasing gas flow rate and decomposition temperature.

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