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TUTORIAL


Toward molecular design of oxide precursors for advanced materials

Liliane G. Hubert-Pfalzgraf
IRC, Université de Lyon1 - 69626 Villeurbanne Cedex (France)

 

I. GENERAL FEATURES

I.1. What are the properties of the precursors relevant for solution routes?

Basic requirements are purity, high yield and thus selective synthetic routes, easy handling and storage, non-toxicity. Suitable physical properties are solubility - generally in non-aqueous media- for sol-gel applications or "wet MOCVD", volatility and thus liquids or solids with low melting points for conventional MOCVD. In terms of transformation into the materials, high ceramic yields and controlled conversion are desired. This implies to control the hydrolysis rates, having access to a suitable rheology (stable sols, homogeneous gels, viscosity...) especially for coatings and, for the materials required in their crystalline form, low temperatures of crystallization. Special chemical functionalities such as donor-acceptor or polymerisable sites may be necessary for special properties such as NLO (non linear optics), hybrids or photopatterning. Multicomponent oxides such as electroceramics can be obtained from mixtures of precursors or from molecules in which two metals M and M’ are chemically associated in a "single source" precursor. The formulation of the latter should match the stoichiometry between the metals required by the material.3,4. Control of hydrolysis, of surface properties such as hydrophobicity, of microstructure (porosity, homogeneity) of the final material, formation of gels for coatings or embedding, possibility to accede to hybrid materials and thus to organic-inorganic arrays with covalent bonding1, 4, 5 are other desired properties .

I.2. Comparison of the different types of oxide precursors

A rough comparison of the basic physical and chemical properties of various oxide sources is given below (for classical alkoxide ligands OR such as ethoxides (OEt), propoxides (OPr), butoxides (OBu),…):

Solubility (in organic solvents)

M(OR)n > M(b-dik)n > M(O2CR)n

Volatility: 

M(b-dik)n > M(OR)n >> M(O2CR)n

Tailoring:

M(OR)n >> M(b-dik)n > M(O2CR)n

Facility to form stable heterometalic species :

M(OR)n > M(O2CR)n > M(b-dik)n

Hydrolysis (and handling): 

M(OR)n >> M(O2CR)> > M(b-dik)n

 

Metal alkoxides can meet solubility as well as volatility requirements and thus be applied in MOCVD processes, especially if they are liquids. Their high ability to cross-link in the presence of water requires storage under inert atmosphere. The high reactivity (lability) of the metal-alkoxide bond makes them useful starting compounds for a variety of heteroleptic species (ie species with different types of ligands) such as M(OR)n-xZx (Z = b-dik or O2CR) (see eq 1).

1.2.2 Carboxylates6a

Acetates M(O2CMe)n are commercially available often as hydrates (Li, Pb, La, …). They can be obtained anhydrous by heating with acetic anhydride or with 2-methoxyethanol (in situ for the latter). These carboxylates are generally insoluble in organic solvents due to the trend of those ligands to act, as shown by Fig 1, as bridging or bridging-chelating ones, thus giving oligomers or polymers [M(O2CR)n]m where m stands for the degree of association or molecular complexity or nuclearity).

Fig. 1: Most common coordination modes of carboxylate ligands

They might be dissolved in the presence of metal alkoxides (see III). 2-Ethylhexanoates M(O2CCHEtnBu)n are the carboxylates with the smallest number of carbon atoms which are soluble in organic media for most elements. A large number of carboxylate derivatives are available for aluminium. Formate Al(O2CH)3(H2O) and carboxylate-alumoxanes [Al(O)x(OH)y(O2CR)z]m could be prepared from inexpensive feedstocks namely gibsite or boehmite.7 Carboxylic acid exchange reactions applied to the formate gave derivatives with tailored solubility properties, spinnable and displaying polymerisable sites. Liquid carboxylates8 were also prepared for a number of metals (Al, Mg, Ba, Ca, Y) by reacting 2-[2-methoxyethoxy]ethoxyacetic acid (MEEA) with acetates, hydroxides or carbonates. Carboxylates are generally non volatile (although volatility was observed with bulky R groups such as tertiobutyl as for instance for bismuth).

1.2.3 Metal b-diketonates6b

Metal b-diketonates [M(RCOCHCOR')n]m are mostly used in material science for their volatility properties. Most of the b-diketonates are monomeric (m = 1 due to achelating behaviour of the ligand) but association (m>1) is the rule for divalent, large elements such as alkaline earth metals (Ca, Sr, Ba). Their volatility makes them attractive for CVD techniques and they act as source of oxide or of metals (especially for late transition metals). The volatility is governed by the bulk of the R and R' groups as well as the nature of the metal which will determine the degree of association m. b-diketonates form easily adducts M(b-dik)nLx with neutral molecules L mostly with nitrogen or oxygen donor sites such as water, alcohols, ethers, amines. Formation of adducts reduces association but stability problems in the vapor phase must be considered.9 Acetylacetonates (R = R' = CH3 = Me) which provide the best ceramic yield are generally used for solution routes. Tetramethylheptanedionates (R = R' = tBu) are precursors of choice for CVD uses.

 
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