Precursor Solution Chemistry

The precursors most often used in the sol—gel process are hydrolyzed alkoxides in alcohol solutions. A short discussion of the alkoxide chemistry is usefrd to explain their gelling characteristics. 

The alkoxide precvirors are commonly formed as one of a series of homoleptic alkoxides Af(OR) , where n = 1—6. Organic molecules such as alcohols tend to be strong ir electron donors and thus stabilize the highest oxidation state of the metal [36]. The specific synthesis route of an alkoxide is dictated by the electronegativity of the metal. The most common routes for metal alkoxide formation are [37] 

Catalyzed reaction with labile Af-JVRg or M-C bonds for less active metals. 

The electronegative metals usually form unstable alkoxides that tend to polymerize rapidly to form [-M(OR)2-0-] . Alkoxides are easily solubilized in alcohols. Alkoxide precursors must be kept fme of water to avoid hydrolysis. Hydrolysis is the first step in the reaction of alkoxides to form gel networks. This is difficult because alkoxide solutions easily absorb water finm the atmosphere. 

Polymer network solutions can also be formed from aqueous chemical systems. Aqueous metal chelates that have at least one additional carboxyl group as a reaction site can undergo polyesterification with a polyhydroxyl alcohol to form a network [39,40]. Aqueous metal ions can also react with polyaaylic acid and be precipitated as a crosslinked polymer [41]. The poljnnerization mechanism and its rate are important factors in determining the molecular weight of the pol5maers and the density distribution of the microstructure formed. 

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This chapter reviews the general aspects of the CSD method for ferroelectric thin-film preparation, with attention given to precursors, solution chemistry, and process development. An additional focus of the chapter is on the structural evolution of the solution precursor into the crystalline (typically perovskite) state and the impact of precursor chemistry and film fabrication conditions on the transformation process. Lastly, the chapter reviews the advantages and disadvantages of the CSD method and discusses industrial implementation of the technique. 

PZN-PT, and YBa2Cug02 g. For the preparation of PZT thin films, the most frequently used precursors have been lead acetate and 2irconium and titanium alkoxides, especially the propoxides. Short-chain alcohols, such as methanol and propanol, have been used most often as solvents, although there have been several successful investigations of the preparation of PZT films from the methoxyethanol solvent system. The use of acetic acid as a solvent and chemical modifier has also been reported. Whereas PZT thin films with exceUent ferroelectric properties have been prepared by sol-gel deposition, there has been relatively Httle effort directed toward understanding solution chemistry effects on thin-film properties. 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

A method to circumvent the problem of chalcogen excess in the solid is to employ low oxidation state precursors in solution, so that the above collateral reactions will not be in favor thermodynamically. Complexation strategies have been used for this purpose [1, 2]. The most established procedure utilizes thiosulfate or selenosulfate ions in aqueous alkaline solutions, as sulfur and selenium precursors, respectively (there is no analogue telluro-complex). The mechanism of deposition in such solutions has been demonstrated primarily from the viewpoint of chemical rather than electrochemical processes (see Sect. 3.3.1). Facts about the (electro)chemistry of thiosulfate will be addressed in following sections for sulfide compounds (mainly CdS). Well documented is the specific redox and solution chemistry involved in the formulation of selenosulfate plating baths and related deposition results [11, 12]. It is convenient to consider some elements of this chemistry in the present section. 

Because of these precursor modification reactions, the process chemistry of chelate processes is as complex, or more so, than that involved in sol-gel processes.78 However, it is typical for chelate processes that some control of process chemistry is sacrificed in return for more expedient solution preparation. For example, the hour-long (or longer) reflux processes that have been historically used in 2-methoxyethanol based sol-gel processing of ferroelectric films are not used. Rather, the entire solution preparation procedure is generally completed within one hour, with only the initial phase of the procedure being carried out under dry box and inert atmosphere conditions. Once the chelation reaction(s) has occurred, the hydrolysis sensitivity of the precursor solution is reduced to the point where the remaining process chemistry may be carried out under ambient conditions.46... 

An overview of the precursors, process chemistry, and relative advantages and disadvantages of the three principal methods of inorganic electronic thin film preparation is shown in Table 2.1. Generally, sol-gel methods offer the greatest control over the nature of the solution precursor species, but they have involved... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Kato, K. Zheng, C. Dey, S. K. Torii, Y. 1997. Chemistry of the alkoxy-derived precursor solutions for layer-structured perovskite thin films. Int. Ferro. 18(l-4) 225-235. 

Despite facile formation of p-type ZnTe from the precursor, films of ZnTe formed from a drop cast precursor solution had a small grain structure (using conditions that we have so far explored), which limited prospective TFT device performance. Note that we expect a similar chemistry to that for ZnTe to also be operative for selected other metal telluride precursors. In contrast to the ZnTe films, well-crystallized films of In2Te3 have been formed using... 

The practical use of the desorption reaction requires a catalyst for the improvement of the kinetics. The first work on catalyzed alanates at MPI - Miilheim was derived from studies that used transition-metal catalysts for the preparation of MgH2- The NaAlH4 was doped with Ti by solution chemistry techniques whereby nonaqueous liquid solutions or suspensions of NaAlH4 and either TiCl3 or the alkoxide Ti(OBu )4 [titanium(IV) w-butoxide] catalyst precursors were decomposed to precipitate solid Ti-doped NaAlH4 [57, 58]. 

The carbonyl platinum anions, [Pt3(CO)6]2, (n = 1-6,10) were first synthesized and characterized by Chini and coworkers1 3. They obtained these compounds by reaction of Pt(IV) or Pt(II) salts at room temperature with bases such as sodium hydroxide or sodium acetate under a carbon monoxide atmosphere. The product composition is quite sensitive to the Pt-base ratio, reaction time, and reaction conditions. As a consequence of this sensitivity, product mixtures with An = 1 are usually obtained, which are separable only with difficulty by fractional crystallization. Interest in this series of compounds for (a) their unique redox solution chemistry, (b) their use as precursors for higher nuclearity carbonyl platinum anions,4 and (c) their use as precursors for novel supported Pt catalysts5 8 prompted efforts to develop... 

The precursor in a sol-gel preparation can either be a metal salt/alkoxidc dissolved in an appropriate solvent or a stable colloidal suspension of preformed sols. Metal alkoxidcs have been the most extensively used because they are commercially available in high purity and their solution chemistry has been documented [1-3]. At its simplest level, sol gel chemistry... 

Solution chemistry to form a gel type of precursor type of solvent pH (acid/base content) water content precursor concentration temperature... 

Solution chemistry precursor solvent nitric acid content water content precursor concentration temperature Titanium n-butoxide (Ti(C4H90)4) methanol 0.125 mol/mol of precursor 4 mol/mol of precursor 0.625 mmol/mL of methanol room temperature Zirconium n-propoxide (Zr(CjH70)4) n-propanol 0.83 mol/mol of precursor 2 mol/mol of precursor 1.0 mmol/mL of n-propanol room temperature... 

In the sol-gel preparation of supported metals, a metal precursor is usually added directly to the solution prior to gelling. Regardless of whether the metal precursor participates in hydrolysis and/or condensation, it will become part of the network as the gel forms. Thus, any parameters that are important in solution chemistry (Table 1) could affect the properties of the metal upon activation. An example is the work of Zou and Gonzalez [39] cited in Section 2.I.4.3.A. When these authors used water content as a variable to change the pore size distribution of a series of Pt/Si02 catalysts, they found that the particle size distribution of reduced Pt (in the form of crystallites) is also dependent on the hydrolysis ratio. The average Pt particle size nearly doubles (from about 1.7 to 3nm) as the hydrolysis ratio increases from 10 to 60. As noted earlier, the stability of these catalysts, in terms of the resistance of Pt particles towards sintering, is a function of how well the pore diameter and particle size match. 

The ability to introduce several components into solution during the sol-gel step makes this approach especially attractive for the preparation of multicomponent oxides and bimetallic catalysts. Of course, the solution chemistry becomes more complex with additional components. But many of the concepts that we have discussed, such as matching relative precursor reactivity and changing the microstructure of the gel network, remain valid in principle. Furthermore, a promoter or an active species can be introduced the same way. There have been recent reports on the onc-step preparation of zirconia-sulfate aerogels [45] and Li/MgO catalysts [46], These samples arc active in the isomerization of n-butane and the oxidative coupling of methane, respectively. 

The next stage in processing ceramic films from solutions is the development of structure. There are two types of structure development to consider (1) the structure of the primary aggregate or polymer, and (2) the structure that develops upon gelation. It is possible to control both t5T)es of structure development through the chemistry of the precursor solutions. The control of structure is essential to the final microstructure of the dried gel. 

Another method that has been developed for the preparation of thicker layers involves the use of diol solvents, such as 1,3-propanediol. Phillips et al. and Tu et al. have explored this route most thoroughly, and it has also been utilized by manufacturers. While the method has not been as intensely investigated as the 2-methoxyethanol process, a number of spectroscopic investigations have been carried out to study the details of the reaction chemistry. Coating thickness per deposited layer is 0.5 pm, compared to 0.1 pm for layers prepared by sol-gel methods using more common solvents, or by chelate or MOD processes. The manufacture of 1 pm films is therefore significantly less labor intensive. Lastly, others have prepared suspensions from sol-gel precursor solutions and powders... 

Conversion of the as-deposited film into the crystalline state has been carried out by a variety of methods. The most typical approach is a two-step heat treatment process involving separate low-temperature pyrolysis ( 300 to 350°C) and high-temperature ( 550 to 750°C) crystallization anneals. The times and temperatures utilized depend upon precursor chemistry, film composition, and layer thickness. At the laboratory scale, the pyrolysis step is most often carried out by simply placing the film on a hot plate that has been preset to the desired temperature. Nearly always, pyrolysis conditions are chosen based on the thermal decomposition behavior of powders derived from the same solution chemistry. Thermal gravimetric analysis (TGA) is normally employed for these studies, and while this approach seems less than ideal, it has proved reasonably effective. A few investigators have studied organic pyrolysis in thin films by Fourier transform infrared spectroscopy (FTIR) using reflectance techniques. - This approach allows for an in situ determination of film pyrolysis behavior. 

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