Organic Reaction Pathways in the Nonaqueous Synthesis of Metal Oxide Nanoparticles

Abstract

Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.

Nonaqueous Sol–Gel Routes to Metal Oxide Nanoparticles

“…7,32,33 Potential solvents are oxygen-containing organic solvents such as alcohols, ketones, or aldehydes as well as oxygenfree solvents like amines or nitriles with short alkyl chains. Even "inert" solvents such as toluene or mesitylene can be used.…”

“…38,39 If halide impurities in the final oxidic product are a problem, synthesis routes based on the reaction of metal acetates, acetylacetonates, or alkoxides with alcohols or amines represent a halide-free alternative. 7 The glycothermal method involving the reaction of metal alkoxides or acetylacetonates in 1,4-butanediol is a versatile approach to various metal oxides. 40 Another powerful reaction system is based on the use of metal alkoxides and benzyl alcohol, which can be applied to a wide range of binary and multi-metal oxides.…”

“…This aspect is of utmost significance considering the fundamental role of organic reaction pathways in these synthesis approaches. 7 Parallel to the formation of the inorganic nanoparticles, also the initial organic species (i.e., solvent and organic constituent of the precursor) undergo transformation reactions that are often based on elementary organic chemistry principles. Identification and quantification of these organic byproducts provide valuable information about the chemical reaction mechanisms leading to the nanoparticles.…”

“…7,8 Nonaqueous processes can be divided into two general methodologies, namely, surfactant-and solvent-controlled preparation routes. This Account provides a short over-view of these two strategies, highlighting advantages and limitations as well as elaborating some general reaction principles based on selected examples from the literature.…”

Low temperature crystallisation of mesoporous TiO2

“…In contrast to the conventional hydrolytic sol-gel (HSG) chemistry, non-hydrolytic sol-gel (NHSG) synthesis routes allow the formation of TiO 2 nanocrystals in solution at quasiroom temperature 27,28 . This has fostered a variety of studies that aim to manufacture mesoporous, nanocrystalline TiO 2 at low processing temperatures 29,30 .…”