Metal Oxide Nanoparticles in Organic Solvents

Research on nanoparticles, including synthesis, characterization of the structural, chemical and physical properties, assembly into 1-, 2- and 3-dimensional architectures extending over several lengths scales and with hierarchical construction principles, and application in various fields of technology, represents a fundamental cornerstone of nanoscience and nanotechnology. Many different synthesis techniques gave access to nanomaterials with a wide range of compositions, well-defined and uniform crystallite sizes, extraordinary and unprecedented crystallite shapes, and complex assembly properties. Although gas-phase processes are successfully employed for the low-cost production of large quantities of nanopowders [7, 12, 25], it seems that liquid-phase syntheses are more flexible with regard to the controlled variation of structural, compositional, and morphological features of the final nanomaterials. Liquidphase routes include coprecipitation, hydrolytic as well as nonhydrolytic solgel processes, hydrothermal or solvothermal methods, template synthesis and biomimetic approaches [3]. However, often the synthesis protocol for a targeted material involves not just one, but a combination of several of these methods.

The most widely used synthetic technique for bulk metal oxides has been the ceramic method, which is based on the direct reaction of powder mixtures. These reactions are completely controlled by the diffusion of the atomic or ionic species through the reactants and products. To bring the reaction partners sufficiently close together and to provide high mobility, these solid state processes require high temperature and small particle sizes. Although the harsh reaction conditions only lead to thermodynamically stable phases, preventing the formation of metastable solids, these approaches gave access to a large number of new solid compounds, enabling the development of structureproperties relationships. However, in comparison to organic chemistry, where highly sophisticated synthetic pathways are employed to make and break chemical bonds in a controlled way, the ceramic method is a rather crude approach. It is therefore no surprise that for the size- and shape-controlled synthesis of nanoparticles especially liquid-phase routes represent the most promising alternatives. In contrast to solid-state processes, but analogous to organic chemistry, “chimie douce” approaches offer the possibility to control the reaction pathways on a molecular level during the transformation of the precursor species to the final product, enabling the synthesis of nanoparticles with well-defined and uniform crystal morphologies and with superior purity and homogeneity [12].

In 1993, Murray et al. published the synthesis of monodisperse CdX (X=S, Se, Te) nanocrystallites in molten trioctylphosphine oxide (TOPO) [90]. This work provided the basis for the so-called hot-injection method, which involves the injection of a room-temperature (“cold”) solution of precursor molecules into a hot solvent in the presence of surfactants [87] (Figure 3.1).

The use of coordinating organic solvents represents an alternative to surfactants, especially in cases where the accessibility of the particle surface as well as the amount of organic impurities are important parameters [42]. In comparison to the synthesis of metal oxides in the presence of surfactants the solvent-controlled approaches are on the one hand considerably simpler (the starting reaction mixture generally just consists of a metal oxide precursor and a common organic solvent), and on the other hand the synthesis temperature is lower, typically in the range of 50 to 200°C. However, without any doubts the main advantage of surfactant-free synthesis methods lies in the improvement of product purity. Surface-adsorbed surfactants not only influence the toxicity of nanoparticles, but also lower the accessibility of the nanoparticle surface in catalytic and sensing applications. These problems are circumvented in nanopowders obtained by surfactant-free routes (cf. also Chapter 2.4).

Controlled assembly and positioning of nanoparticles in desired locations and across extended length scales is a key step in the design of integrated materials with advanced functions. The motivation to use nanoparticles as building blocks lies in the fact that size alone is not the defining parameter of many nanoscale-derived effects [28]. Also the interaction between the nanoparticles as a consequence of the spatial arrangement plays a role in determining the properties, opening-up unique and exciting opportunities for the creation of novel materials with unprecedented structures [70, 4]. Consequently, the future challenge of Nanoscience lies in the shift of focus from the size- and shape-controlled synthesis of nanoparticles towards the exploration of collective properties associated with their geometrical organization. On the way to the development of a chemistry of organized matter [16], one can follow basically two strategies (although in some cases this division gets blurred): i) synthesis of nanoparticles and assembly into supraparticulate architectures in one step, and ii) synthesis and arrangement into 1-, 2- or 3- dimensional arrays in two consecutive steps.

In order to relate the physical properties to the size, shape and crystallinity of nanoobjects an accurate and detailed characterization has to be performed. The determination of the size and shape distribution of nanometer size particles can be addressed with several techniques like for example: analytical ultracentrifugation (AUC) [3], light scattering techniques, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [50, 51, 52, 19, 21] etc. The most common tools for the structural characterization of nanoobjects are high resolution TEM (HRTEM) and diffraction techniques such as electron diffraction (ED) and powder X-ray diffraction (XRD). HRTEM permits to directly visualize the atomic columns of a single particle and to determine its structure and possible structural defects. However, this method is not statistically applicable to a large amount of particles. Powder XRD measurements are able to overcome this limitation and provide a global information about the crystallinity of a sample, thus making it a perfect complementary technique to HRTEM. This chapter mainly focuses on the characterization of inorganic nanoobjects by electron microscopy and diffraction techniques. Additional useful and widely used characterization techniques such as Fourier transform infrared (FT-IR) and solid state nuclear magnetic resonance (SSNMR) will also be discussed in a particular example.

The unique characteristics of transition metal oxides make them the most diverse class of materials, with properties covering almost all aspects of materials science and solid state physics [82]. Metal oxides experience the same trend as many other advanced materials: The miniaturization of functional devices in emerging technologies such as gas sensing, catalysis, energy storage and conversion and electroceramics demands for the production of these materials with the highest possible purity, small crystallite size, narrow particle size distributions and well-defined particle morphology and chemical composition. The high scientific as well as technological interest in nanoparticles lies in the fact that the dependence of the chemical and physical properties of nanocrystalline solids on particle size and shape provides a powerful tool to tailor the properties of a material. In this chapter we shall introduce some physical properties of selected metal oxide nanoparticles synthesized in organic solvents. The chapter is organized in the following sections: i) 8.2 magnetic properties of intrinsic magnetic oxides (e.g. ferrites) and oxides doped with paramagnetic ions (e.g. manganese doped zinc oxide), ii) 8.3 photoluminescence properties of rare earth and semiconductor metal oxides, iii) 8.4 (photo)catalytic properties, iv) 8.5 gas sensing devices fabricated using metal oxide nanoparticles as active component for the detection of various gases and v) 8.6 the possible use of metal oxide nanoparticles in medical applications such as magnetic resonance imaging and cancer treatment.

In the last few years, the number of synthesis approaches to metal oxide nanoparticles and nanostructures reported in literature almost exploded. They gave access to a large and rapidly growing collection of oxide-based nanoparticles with a wide range of compositions, monodisperse or well-defined crystallite sizes, sophisticated crystallite shapes, and with complex assembly properties. In contrast to aqueous systems, in which smallest changes in the experimental conditions result in alteration of the products, nonaqueous procedures are very robust within one system. Therefore, most of these processes are highly reproducible, easy to scale-up to gram quantities (or more) and applicable to a broad family of metal oxides. Consequently, the nonaqueous routes summarized in this book, whether they do or do not involve the use of surfactants, offer a unique opportunity not only to chemists, but also to physicists, materials scientists, and engineers to find the appropriate synthesis method for a targeted material with the desired properties.