Decomposition of the Precursor

Many processes can occur when the drop hits the surface of the substrate, such as evaporation of the residual solvent, scattering of the drops and decomposition of salt. Many models exist for the decomposition of the precursor salt. But most authors suggest that the process of CVD gives high-quality films by spray pyrolysis. 

Chen et al. [17] studied the correlation between the morphology of the films and the deposition parameters. They concluded that the most important parameter 

290 and 600 °C. This indicates that the liquid phase of MgC5H702 is partially evaporated. Plain fihns were deposited at temperatures between 350 and 420 °C. Still, some cracks in the film were observed. 

The morphology of the film is highly dependent on the substrate temperature at the time of deposit in spray-pyrolysis. Chen et al. [10] observed four types of morphology dense hhns, dense films with embedded particles, dense film with a porous layer on top and porous morphology. 

Ruiz et al. [19] investigated the influence of gas loading, substrate composition, temperature and time on the deposited film. The most important parameters according to these authors were the drop size and substrate temperature. The behavior of the spray droplet is determined by surface tension and roughness of the substrate to be coated. The roughness of the substrate and porosity of the substrate also significantly influence the film morphology. 

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One of the main challenges in the field is the controlled crystalhzation of the nanoparticles into 3D super-lattices, similar to artificial opals but including much smaller individual particles. For this we have used electrically charged stabihzers (ligands and surfactants). For example, the photochemical decomposition of the precursor [Sn(NMe2)2]2 in the presence of HDA... 

In chemical vapor deposition (CVD or MOCVD), a film of the desired material is prepared by evaporation of volatile precursor molecules which then decompose to give a film deposited on the substrate. The ordering in the film as it grows is dictated by the surface ordering of the substrate, hence the deposition is epitaxial . The necessary decomposition of the precursor molecules can take place either on the surface of the substrate or in the gas phase close to it. 

Zinc telluride, ZnTe, was deposited on quartz, silicon, InAs, and GaSb substrates using Zn[TeSi(SiMe3)3]2 at temperatures between 250 °C and 350 °C. On InAs (orientation not specified) a cubic ZnTe layer was obtained. Problems of stoichiometry are encountered at temperatures below 325 °C because decomposition of the precursor is incomplete, while at higher temperatures (above 350 °C) the deposited ZnTe decomposes into Zn (which evaporates) and involatile elemental tellurium which remains. The results with the analogous cadmium precursor (1.4 torr, 290 °C) indicate that the CdTe films may be of better stoichiometry than those of ZnTe, with XRD results indicating that on a Si substrate the hexagonal phase is predominantly... 

Outside of catalyst preparation, reaction of sucrose with metal nitrates has been used to prepare nanocomposite mixed oxide materials. Wu et al. [46] reported the synthesis of Mg0-Al203 and Y203-Zr02 mixed oxides by reaction of nitrate precursors with sucrose. The resulting powders had smaller particles than those prepared without sucrose. Das [47] used a similar method in the presence of poly vinylalcohol to produce nanocrystalline lead zirconium titanate and metal ferrierites (MFe204, M = Co, Ni, or Zn). The materials prepared using sucrose had smaller crystallites than those made without. Both authors observed an exothermic decomposition of the precursors during calcination. 

Upon heating, decomposition of the precursor to the crystalline SnSe2 xSx semiconductor occurs through the pathway (neglecting any neutral hydrazine that may be in the structure, which would evolve first at low temperature if present) ... 

Even though all three reactors share the same precursor delivery system, each tool offers specific advantages. For example, a cold-wall reactor (reactor B) helps prevent decomposition of the precursor before it reaches the substrate. A pulsed aerosol injection system at low pressure (reactor C) allows the film to grow under better-defined conditions than in a continuous process (reactor A) because of the minimization of undesirable transient effects caused by the high volatility of the solvents used.46 A more detailed description of each of the conditions for film growth, including reactor type, precursor type, delivery method, deposition temperature, growth time, and other parameters are summarized in Table 6.2. Depositions were done on bare and Mo-coated... 

Coming to the comparison of the and 0-2 series, the possibilities of isomerization and/or decomposition of the precursor lacunary complexes and, presumably, of their metal ion-substituted derivatives, make it necessary to insure first that no fast conversion occurs, in particular from the a structure to the 2 one. Figure 13(a) compares the cyclic voltammograms of the two lacunary complexes in the pH 3 medium. The main difference appears on the third redox system a single two-electron, reversible wave is obtained for the complex in contrast, the corresponding system for the U2 isomer is clearly constituted by two. 

The use of precursor synthesis techniques as described above is driven by the fact that decomposition of the precursor material into the catalyst often results in catalysts that have activity or selectivity superior to that of preferred products. The amine thiomolybdate decomposition described above results in MoS2 catalysts with surface areas exceeding several hundred square meters per gram (27). The HDS activity increases correspondingly and unpromoted catalysts have activities approaching those of promoted systems. 

A similar swelling effect to that observed with AsF3 is seen for the loss of die fluoroxylene, formed during the initial decomposition of the precursor polymer to form predominantly cw-polyacetylene515). The diffusion coefficient is initially 4 x 10 9 cm2 s-1 and decreases to 10 11 cm2 s 1 as the polymer deswells, as shown in Fig. 20. In this case the swelling is not reversible. 

From the reported data, only MgO has been synthesized with high surface areas (usually between 130 and 300m2g 1) whereas for CaO, SrO and BaO only very low surface areas have been achieved ( 60, 10 and 2 m2 g 1, respectively).[37 39] For the preparation of high-surface-area MgO, the thermal decomposition of magnesium salts such as oxalates, hydroxycarbonates, sulfates and preferentially Mg(OH)2 has been frequently selected. The surface area is strongly dependent on the nature of the precursor salt,[40 41] the method and conditions of its prepara-tion[42] as well as the conditions of the thermal decomposition of the precursor salt.[43]... 

Munoz-Paez and Koningsberger use a combination of TPR, MS and EXAFS to study the decomposition of [Pt (NH3)4](OH )2 impregnated on y-Al203. They report the decomposition of the precursor to Pt(NH3)20 during drying in He at 120°C and a partial reduction of the precursor to metallic Pt when reduced at 180 and 200°C. 

Fig. 3. Ca3Fe2Mn07 obtained by the topotactic reduction of Ca,Fe2MnOs. The latter is prepared by decomposition of the precursor carbonate, Ca2Fe4/3Mn2/3(C03)4.

As there are no suitable organometallic precursors commercially available, initial work dealt with the synthesis of such a precursor [4]. 2,5-Bis(rbutyl)-2,5-diaza-l-germa-cyclopentane is a monomeric solid with a melting point of 45 °C and a sufficient vapour pressure of 0.40 mbar at 40 °C to allow its introduction into the CVD reactor. For the details about the synthesis and properties of this precursor we refer to a recent paper [4]. The present work deals with the investigation of the thermal decomposition of the precursor, the deposition of amorphous germanium (a-Ge) and the characterization of the deposited thin films. Finally some data should try to give some understanding about the deposition mechanism. 

Fig. 3. ESCA study of the decomposition of the precursor at the given temperatures...

Palladium nanoparticles vfith a size of a few nanometers supported on carbon are widely used as catalysts, for instance in three-way automotive exhaust catalysts and fuel cells, and can easily be prepared by impregnation of a porous support body with a precursor solution, followed by drying, decomposition of the precursor and, if necessary, reduction. It is well-known that the activity and selectivity of these catalysts for hydrogenation reactions depend on the palladium dispersion for particles sizes in the range 1-10 nm. It is, hence, not surprising that the interaction of Pd with hydrogen, and the infiuence of nanosizing, have been widely studied. 

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