Georg Garnweitner

Organic chemistry in inorganic nanomaterials synthesis

Research on the preparation of inorganic nanoparticles and nanostructures has made immense progress in the past few years. The synthesis protocols developed give access to nanomaterials with a wide range of compositions, monodisperse crystallite sizes, sophisticated crystallite shapes, and with complex assembly properties. The objective of this Feature Article is to have a closer look at the organic side of inorganic nanomaterials’ synthesis performed in nonaqueous, but liquid reaction media. Based on selected literature examples, we will discuss the complex role of the organic components in determining the compositional, structural, and morphological characteristics of the inorganic products.

Diluted magnetic semiconductors: Mn/Co-doped ZnO nanorods as case study

The structure and the magnetic properties of 3 and 5 mol% (based on the starting concentrations) Co- and Mn-doped ZnO nanorods, synthesized by a straightforward and experimentally simple nonaqueous sol–gel route based on benzyl alcohol as solvent, have been investigated by various characterization techniques, including X-ray diffraction with Rietveld refinement, high-resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, magnetization measurements and electron paramagnetic resonance. The doped as-synthesized ZnO nanocrystals retain the wurtzite structure with a morphology in the form of nanorods grown along the [001] direction, whose dimensional parameters as well as degree of agglomeration depend on the type and level of doping. The Co-doped ZnO powders are ferromagnetic with a Curie temperature exceeding room temperature. Conversely, the Mn-doped samples show antiferromagnetic correlations with a possible transition to an antiferromagnetic ground state below TN = 10 K. The results suggest that the magnetic ground state is extremely sensitive to the type of dopant, which is in agreement with previous studies.

Phase-controlled synthesis of ZrO2 nanoparticles for highly transparent dielectric thin films

The synthesis of ZrO2 nanoparticles with controlled phase contents of the monoclinic and tetragonal polymorphs is presented. By increasing the reaction temperature the quantity of the tetragonal phase rises from 20% to almost 100%, with additional dependence on the liner material of the reactor. Additionally, it is shown that by a simple post-synthetic chemical modification the nanoparticles can be dispersed in propylene glycol monomethyl ether acetate without the presence of agglomerates. The obtained dispersions can then be employed to fabricate thin film based capacitors via low-temperature solution processing. Hence, the effect of the phase composition on the dielectric properties of the thin films is also elucidated.

Nonaqueous synthesis, assembly and formation mechanisms of metal oxide nanocrystals

Nonaqueous solution routes to metal oxide nanoparticles are a valuable alternative to the well-known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and avoidance of surfactants in order to control the crystal growth. In the first part of this paper we give an overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes developed in our group. 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 behaviour. In general, these routes involve the reaction of metal oxide precursors such as metal halides, alkoxides, or acetylacetonates with benzyl alcohol, benzylamine or carbonyl compounds like ketones and aldehydes. Whereas the reaction between metal halides and benzyl alcohol enables the direct synthesis of crystalline nanoparticles via simple beaker chemistry, the other reaction systems require a solvothermal treatment at temperatures between 200C and 250C. The metal halidebenzyl alcohol system additionally allows for an insitu functionalisation process, where the surface of the nanoparticles can be modified during nanoparticle synthesis in order to tailor the solubility as well as the assembly behaviour. This is an important step towards the use of metal oxides as nanobuilding blocks for the fabrication of structures like nanowires or mesoporous materials. In the second part, various reaction pathways to nanoparticle formation are discussed. Solvothermal processes are not easy to monitor insitu. In order to elucidate possible formation mechanisms, we analyse the reaction solution obtained after synthesis of the nanoparticles as well as after reference experiments with altered reaction conditions. The organic species found in the mixtures allow us to propose possible formation mechanisms. As an important example, we will discuss the formation mechanism of ceria nanoparticles synthesised from cerium(III) isopropoxide and benzyl alcohol, involving a C-C bond formation between the isopropoxy ligand and benzyl alcohol. This reaction pathway was also found to lead to the formation of BaTiO3 nanoparticles. The last part of this paper deals with possible applications of metal oxide nanoparticles, especially with regard to gas sensing devices.

Comparative Study of Ligand Binding during the Postsynthetic Stabilization of Metal Oxide Nanoparticles

In the absence of stabilizers in the reaction medium, the nonaqueous synthesis of metal oxide nanoparticles usually results in agglomerated products. Stabilization is however often possible in a postsynthetic treatment, involving the addition of organic ligands that coordinate to the nanoparticle surface. The ligands are commonly expected to chemisorb via functional groups; however, we have recently shown that also weakly and unspecifically interacting ligands can lead to stabilization. Here, we present detailed investigations on the stabilization, comparing the binding of weakly coordinating ligands to a system with strongly and selectively binding stabilizers and additionally exploring the effect of ligand chain length. Although in all cases stabilization and disintegration of agglomerates to the primary particle level are achieved, strong differences are observed with respect to the processes at the particle surface. Moreover, these processes are shown to be more complex than simple ligand adsorption and need to be understood for proper design and choice of stabilizers.

Generalized nonaqueous sol-gel synthesis of different transition-metal niobate nanocrystals and analysis of the growth mechanism

A general nonaqueous route for the synthesis of phase-pure transition-metal niobate (InNbO(4), MnNb(2)O(6), and YNbO(4)) nanocrystals was developed based on the one-pot solvothermal reaction of niobium chloride and the corresponding transition-metal acetylacetonates in benzyl alcohol at 200 degrees C. All samples were carefully characterized by XRD, TEM, HRTEM, and energy-dispersive X-ray (EDX) analysis. The crystallization mechanism of these niobate nanocrystals points to a two-step pathway. First, metal hydroxide crystals and amorphous niobium oxide are formed. Second, metal niobate nanocrystals are generated from the intermediates by a dissolution-recrystallization mechanism. The reaction mechanisms, that is, the processes responsible for the oxygen supply for oxide formation, were found to be rather complex and involve niobium-mediated ether elimination as the main pathway, accompanied by solvolysis of the acetylacetonate ligands and benzylation reactions.

Spontaneous water release inducing nucleation during the nonaqueous synthesis of TiO2 nanoparticles

The formation of anatase nanoparticles by reaction of titanium(IV) isopropoxide in benzyl alcohol was studied. In contrast to previous reports on the nonaqueous synthesis, in this system the particle formation occurs within a very limited time span in the course of the synthesis, concurrently to a fast step-type pressure increase within the closed reaction system. By Karl Fischer titration and 1H NMR spectroscopy of both the liquid and the gaseous phase at different stages of the reaction, it is shown that water formation occurs during the pressure increase due to catalytic ether formation from benzyl alcohol. The generated water leads to instant nucleation and fast growth of crystalline nanoparticles, which is traced by powder X-ray diffraction as well as small-angle X-ray scattering and thereby shown to play a crucial role in the particle formation process.

Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates

The design of hierarchically structured nano- and microparticles of different sizes, porosities, surface areas, compositions, and internal structures from nanoparticle building blocks is important for new or enhanced application properties of high-quality products in a variety of industries. Spray-drying processes are well-suited for the design of hierarchical structures of multicomponent products. This structure design using various nanoparticles as building blocks is one of the most important challenges for the future to create products with optimized or completely new properties. Furthermore, the transfer of designed nanomaterials to large-scale products with favorable handling and processing can be achieved. The resultant aggregate structure depends on the utilized nanoparticle building blocks as well as on a large number of process and formulation parameters. In this study, structure formation and segregation phenomena during the spray drying process were investigated to enable the synthesis of tailor-made nanostructures with defined properties. Moreover, a theoretical model of this segregation and structure formation in nanosuspensions is presented using a discrete element method simulation.

Fabrication of transparent polymer-matrix nanocomposites with enhanced mechanical properties from chemically modified ZrO2 nanoparticles

Optically transparent nanocomposites with enhanced mechanical properties were fabricated using stable dispersions of sub 10xa0nm ZrO2 nanoparticles. The ZrO2 dispersions were mixed with a commercially available bisphenol-A-based epoxy resin (RIMR 135i) and cured with a mixture of two amine-based curing agents (RIMH 134 and RIMH 137) after complete solvent removal. The colloidal dispersions of ZrO2 nanoparticles, synthesized through a non-aqueous approach, were obtained through a chemical modification of the ZrO2 nanoparticle surface, employing different organic ligands through simple mixing at room temperature. Successful binding of the ligands to the surface was studied utilizing ATR–FT-IR and thermogravimetric analysis. The homogeneous distribution of the nanoparticles within the matrix was proven by SAXS and the observed high optical transmittance for ZrO2 contents of up to 8xa0wt%. Nanocomposites with a ZrO2 content of only 2xa0wt% showed a significant enhancement of the mechanical properties, e.g., an increase of the tensile strength and Young’s modulus by up to 11.9 and 12.5%, respectively. Also the effect of different surface bound ligands on the mechanical properties is discussed.

Facile surface tailoring of metal oxide nanoparticles via a two-step modification approach

The tailoring of surface properties of metal oxide nanoparticles is highly important to exploit their benefits in an optimal way for diverse applications. For example, in polymer matrix nanocomposites one of the most critical aspects is the interaction of the particles with the matrix, which is determined by the chemistry of the particle surface and can be adjusted by attachment of organic ligands. Whilst many empirical solutions have been presented for specific combinations of particles and matrix, generalized approaches are not available yet. As a versatile and arbitrary method to permanently modify the surface of metal oxide nanoparticles, we present a two-step approach and prove its applicability for the versatile adjustment of surface properties of two types of nanoparticles.