NANOPARTICLES AND POWDERS
2.1 INTRODUCTION
2.2 Background, 17
2.2.1 Size and Structure of Nanoparticles, 17 2.2.2 Novel Properties of Nanoparticles, 20 2.3 Case Study 1: Magnesium Oxide Nanoparticles, 21
2.3.1 Introduction, 21
2.3.2 Properties of MgO Nanoparticles, 22 2.3.3 Summary, 26
2.4 Case Study 2: Gold Nanoparticles, 26 2.4.1 Introduction, 26
2.4.2 Properties of Au Nanoparticles, 27 2.4.3 Summary, 31
2.5 Concluding Remarks and Future Outlook, 31 Suggested Further Reading, 32
References, 33 Problems, 34 Answers, 35
2.1 INTRODUCTION
Nanoparticles (NPs) form a new class of materials possessing unique properties that are characteristic of neither the molecular nor the bulk solid-state limits. They have become the focus of considerable fundamental and applied research leading to
Nanoscale Materials in Chemistry, Second Edition. Edited by K. J. Klabunde and R. M. Richards Copyright# 2009 John Wiley & Sons, Inc.
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important technological applications in areas such as heterogeneous catalysis, optical communications, gas sensing, nanoelectronics, and medicine. NPs come in a wide range of sizes and shapes, with varied electronic, optical, and chemical properties.
However, throughout this diversity a universal concept is applicable: the properties of NPs are intimately connected to their nanoscale size and atomic-scale structure.
Understanding these properties requires careful consideration of the nature of bonding both between the constituent atoms of NPs and between atoms and molecules in their environment. In these respects theoretical models have played a central role and have provided interpretations for many experimental observations.
It is useful at the outset to explain some nomenclature. The terms nanoparticles, nanocrystallites, and clusters are often encountered in the literature and are frequently used interchangeably. A particle of matter is normally referred to as an NP if its exten-sion in all three dimenexten-sions is less than 100 nm. To put this size into perspective it is about one thousandth of the width of a human hair. A nanocrystallite is generally understood to possess crystalline order in addition to nanoscale size, although not necessarily the crystal structure characteristic of the corresponding bulk material.
Finally, clusters are particles containing a very small number of atoms such that it is no longer possible to clearly distinguish “bulk” atoms from those at the surface.
There is no universally understood definition but a general rule is a few hundred atoms or smaller.
There is considerable variety in the types of NP systems that have been fabricated and studied. Aside from differences in their size and shape one important variable is their composition. Almost every element in the periodic table, together with various alloys and compounds, can form NPs. They can be metallic, semiconducting, or insu-lating and typically their properties are very different to those of the corresponding bulk material. For example, small metallic NPs behave like insulators as there is a sizable gap near the Fermi energy (see Question 1). Another source of variety is the environment of the NP. While isolated NPs can be produced, for example, by conden-sing a vapor in an inert gas, they are more commonly supported on substrates, collected into powders, or embedded inside another material. For example, supported NPs can be fabricated simply by collecting already formed NPs from a solution gas or by directly depositing atoms or molecules onto a surface (e.g. by evaporation or molecular beam epitaxy).
An important idea that underpins much of nanotechnology is that by controlling composition, size, and structure at the nanoscale one can engineer almost any desired properties. This is particularly true for NPs as demonstrated by their many varied tech-nological applications. They also have considerable fundamental interest as one can study the emergence of bulk properties (i.e. electronic, chemical, structural, thermal, mechanical, and optical) as the number of atoms in the particle increases from one to many thousands. The aim of this chapter is to provide a broad introduction to NP systems, with particular attention paid to structure and its connection to various prop-erties (so-called structure-property relationships). This chapter is divided into three main parts. First in Section 2.2, some general issues concerning the atomic structure of NPs and its dependence on various factors, such as particle size and the presence of a support, are discussed. Associated properties of NP systems, such as chemical
UNIQUE BONDING IN NANOPARTICLES AND POWDERS 16
and electronic, are also described, together with some of their important technological applications. Although Section 2.2 is intentionally fairly general, important issues are explored in more detail by considering two particular NP systems as case studies. These systems are chosen to represent prototypes for ceramic (MgO, Section 2.3) and metallic (Au, Section 2.4) materials, which are very important for numerous applications.
2.2 BACKGROUND
2.2.1 Size and Structure of Nanoparticles
The size and atomic-scale structure of NPs depend on their history; that is, how they are fabricated and the temperature and environments to which they are subsequently exposed. Numerous techniques can be used to produce NPs, for example, evaporation or deposition onto surfaces, wet chemistry synthesis, and gas-phase aggregation.
They are also ubiquitous in nature, for example, as soot particles in the atmosphere and dust in interstellar space, and they are even produced by certain types of bacteria.
One often distinguishes between two types of NP structures: those of low potential energy, which are close to thermodynamic equilibrium, and those of higher potential energy, which are formed by kinetically limited processes. It is often possible to transform the latter into the former by suitable thermal annealing; however, kinetically controlled structures can be preferable for applications if they exhibit structural features with desirable properties. The most common approach to theoretically mod-eling NPs is to determine the atomic configuration that has the lowest potential energy. Even though in reality NPs are characterized by a statistical distribution of structures, such models are often useful as they can be indicative of more general trends.
The distinguishing feature of NPs, irrespective of their detailed structure, is that they possess a large surface area relative to their volume and a large fraction of atoms that are under-coordinated compared to the bulk. These surface atoms are responsible for many of the unique properties of NPs and some insights into their structure can be gained by considering macroscopic surfaces. The orientation of a crystalline surface is conveniently defined in terms of Miller indices of the form ijk (see Reference 1 for example). In many cases the atomic structure of a given surface is as would be expected if one cut the bulk crystal in two and separated the resulting halves. In such cases there is usually only a small relaxation of atoms near the surface from their bulk positions due to the perturbation associated with missing bonds. On the other hand, some surfaces reconstruct more significantly [e.g.
Si(111) or Au(111)] to adopt atomic structures with translational periodicity different to the bulk. To assess the thermodynamic stability of different surface structures one can compare surface formation energies, gijk, defined as the cost in energy associated with forming a unit area of a surface (ijk).
Perhaps the simplest way one can imagine constructing an NP is to cut it from the bulk crystalline lattice by forming a polyhedron out of various surface planes. This is
2.2 BACKGROUND 17
the philosophy behind the Wulff construction (2, 3) which gives the particle shape that minimizes the potential energy given various surface energies, gijk. This geometrical procedure is illustrated in two dimensions in Figure 2.1a. The approach can also be applied to NPs that are supported upon a substrate. In this situation there is often a preferred NP orientation such that atoms maximize their adhesion to the substrate.
The adhesion energy, gad(per unit area), can be defined and used in an analogous way to surface energies. This is known as a Wulff – Kaichew construction and is illustrated in Figure 2.1b (4). A further application of this method is to situations
Figure 2.1 (a) An illustration of the Wulff construction in two dimensions. Vectors are drawn in directions [ij] with their length proportional to the surface energy gij. Additional lines (or planes in 3D) are constructed to lie at the end of each vector and are oriented perpendicular to them. The polygon (polyhedron) formed by the intersection of the lines (planes) is the particle shape that minimizes the potential energy. (b) For NPs supported on a substrate the adhesion energy, gad, reduces the extension of the vector directed towards the surface (Wulff – Kaichew construction). The strength of the adhesion energy relative to the surface energy determines the wetting of the NP on the surface. (c) In an atmosphere or liquid molecules may interact with the NP and modify its surface energies. As a consequence the shape that minimizes the potential energy can change.
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where molecules (in the ambient atmosphere) can interact with facets of the particle modifying the energy of surfaces. In this case one can replace the surface energies with surface free energies which are a function of molecular concentrations and temperature. As a consequence the equilibrium shape of the NP will depend on its environment (e.g. see Fig. 2.1c). These effects have been studied experimentally using, for example, transmission electron microscopy (TEM) to observe the shape change of Pt NPs caused by H2S (5).
The NP shape predicted by the Wulff construction does not depend on its size because it is assumed that its facets are sufficiently large that interfaces between them, that is, edges and vertices, make a negligible contribution to the total energy.
It is also assumed that the NP possesses the same crystalline structure as the bulk material. However, for particles smaller than about 10 nm the number of surface atoms can represent a significant fraction of the total number of atoms and in this case the NP may lower its energy by adopting structures very different to the bulk.
The thermodynamically favored structure results from a delicate balance between volume and surface energy contributions and determining this structure is a complex problem. NPs can, in principle, be imaged on an atomic level using experimental tech-niques, such as x-ray diffraction, TEM, and scanning tunneling microscopy (STM).
However, in practice this is very difficult and a large part of the information we have about the atomic structure of NPs comes from theory.
Numerous computational methods have been developed to find atomic configur-ations that minimize the potential energy. Techniques such as simulated annealing, Monte Carlo and genetic algorithms (6) are designed to explore the atomic con-figuration space sufficiently such that the global minimum of the potential energy surface can be found. Empirical interatomic potentials parameterized on the basis of experimentally determined properties are often used for these calculations as the com-putational cost of quantum mechanical calculations is in many cases prohibitive.
Examples include the embedded atom model for metals, the Lennard – Jones potential for noble gases, and the shell model for metal oxide systems. Detailed investigations into the dependence of structure on the number of atoms in the NP have been made for a wide range of systems (see Reference 7 for a review).
Typically one can identify ranges in size over which various structural types pre-vail, with the Wulff construction being valid only for very large particles. To compare the stability of NPs as a function of size a useful quantity is the binding energy divided by the number of atoms it contains. It is often found that NPs containing certain
“magic” numbers of atoms are particularly stable. These magic numbers usually form a sequence corresponding to truncated morphologies with few low coordinated atoms. Experimentally, magic number sequences can be observed as peaks in the mass spectra of NPs (8). For small clusters, containing of the order of tens of atoms or less, structures result from a complex interplay between geometric and electronic effects.
The configurations that have the lowest energy are often quite unusual, involving linear, planar, or even tubular arrangements of atoms. These structures are often easily perturbed by molecules that may adsorb on them or by nearby surfaces.
To summarize, large NPs with low surface to volume ratio have bulk-like crystal-line structures and are terminated by low energy surfaces (the Wulff construction).
2.2 BACKGROUND 19
Smaller NPs can adopt a range of different geometrical structures due to a competition between bulk and surface energies, while the structure of very small clusters can be dominated by electronic effects.
2.2.2 Novel Properties of Nanoparticles
At finite temperature NPs can behave quite differently to macroscopic systems. For example, melting temperature is normally an intensive property but for NPs it decreases with decreasing particle size (e.g. for metals, see Reference 9 and Questions 2 and 3). Melting starts at the surface and then propagates into the interior and the dependence on particle size can be understood as a result of reduced atomic coordination. At finite temperature the structure of NPs can change dynamically as atoms diffuse between facets and are exchanged between the NP and the support.
As a result of such processes a collection of NPs may undergo Ostwald ripening (where larger NPs grow at the expense of smaller ones) or sintering (where mobile clusters coalesce). Therefore, atomic diffusion can be an efficient mechanism for particles to move and to change their size and shape (10), which on the macroscale are attributes normally associated with liquids rather than solids.
The interaction of molecules with NPs is of interest for a wide range of applications but has been studied most intensively for heterogeneous catalysis. Examples of sys-tems employed for catalysis include transition metals, noble metals, and metal oxides NPs for reactions such as oxidation, hydrocarbon reforming, and hydrogenation.
While the precise nature of activity in many systems is still debated, it has long been recognized that atoms with reduced coordination, such as those found at surfaces, have modified chemical properties (e.g. molecular adsorption energies) and can be active sites for various reactions. Therefore, NPs are attractive catalysts, in part, simply because of their high surface area. They contain atoms with a range of different coordination numbers, for example, corresponding to atoms in closely packed facets, edges, and vertices. Moreover, the concentration of these chemically active sites can, in principle, be controlled by engineering the size and shape of NPs. A further advan-tage of the high surface area to mass ratio of NPs is that less active material is needed to produce a working catalyst (see Question 4). This is commercially very important because many materials that are used are very expensive; for example, Pt is used in automotive catalysts.
While the increased surface area of NPs is a relatively simple consequence of reducing their size, NPs also exhibit nontrivial size effects that can be important for chemical reactivity. For example, the electronic charge associated with atoms or ions can be different in small NPs and electronic structure can be modified due to confinement of electronic states. Another important factor in determining chemical activity is the interaction between NPs and their support, which can involve a number of interrelated effects.
1. Geometrical: The structure of NPs is influenced by the support and they may be strained.
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2. Electronic: As a result of NP-support bonding or electrostatic effects electronic structure can be modified and in some cases there can be charge transfer either from or to the support.
3. Chemical: For example, the NP may be partially oxidized or reduced on interaction with the support.
Besides their unique chemistry NPs exhibit many other properties that find a myriad of technological applications. For example, TiO2NPs are used in coatings for self-cleaning glass due to their photocatalytic properties and also in invisible sun creams. The luminescence spectra of semiconductor NPs are very sensitive to particle size, a fact related to quantum confinement of electronic states, and have high quantum yields making then suitable for light-emitting devices. Metallic NPs, such as Au and Ag, also have size- and shape-dependent optical spectra connected to the excitation of plasmon resonances and have applications in medicine and photonics (see Section 2.4.2). The magnetic properties of NPs are also important for high density data storage applications. Magnetic moments can be enhanced on low coordinated atoms at the surface and materials that are normally nonmagnetic, for example, Au, can become magnetic when in the form of small clusters. These are just a few examples that indicate the wide-ranging properties and applications of NPs but there are many more that have not been mentioned. In the next two sections the morphology and associ-ated properties of MgO and Au NPs will be discussed in more detail.
2.3 CASE STUDY 1: MAGNESIUM OXIDE NANOPARTICLES