Synergy between theory and experiment in structure resolution of low-dimensional oxides

Abstract

In this paper, I review recent progress in joint theoretical and experimental studies aiming at atomic structure determination of low-dimensional metal oxides. Low-dimensional systems can be generally defined as materials of unusual structure that extend to less than three dimensions. In recent years low-dimensional systems have attracted increasing attention of physicists and chemists, and the interest is expected to rise in the near future. Two- and one-dimensional structures in form of thin oxide films or elongated oxide chains have many potential applications including model supports for heterogeneous catalysts and insulating layers in semiconductor industry. The interest in zero-dimensional gas-phase oxide clusters ranges from astrophysics to studies of elementary steps in catalysis. The key prerequisite for understanding physical and chemical properties of low-dimensional systems is a detailed knowledge of their atomic structures. However, such systems frequently present complex structures to solve. Only in a few cases experimental data can provide some information about possible arrangement of atoms, but data interpretation relies to a large extent on intuition. Therefore, in the recent years quantum chemical calculations became an indispensable tool in structure identification of low-dimensional systems, yet the accuracy of theoretical tools is often limited. The results reviewed here demonstrate that often the only way of an unambiguous atomic structure determination of low-dimensional systems are experimental studies combined with theoretical calculations. Particularly the global optimization methods such as genetic algorithm in combination with the density functional theory prove very useful in automatic structure determination of the observed surface structures and gas-phase clusters.

Introduction

The term low-dimensional systems refers to materials which extend in less than three dimensions. The short dimensions are intermediate between those characteristic of atoms or molecules and those of bulk materials and usually extend to the nanometer scale. Examples of such systems are two-dimensional (2D) sheets, thin films or surface layers, one-dimensional (1D) chains or elongated surface structures and, finally zero-dimensional (0D) atomic and molecular clusters. At this intermediate state, some properties of low-dimensional systems including chemical, catalytical, optical and magnetic ones can be very different from those of their atomic and bulk counterparts.

In recent years, low-dimensional systems have attracted an increasing attention of physicists and chemists, and the interest is expected to rise further in the near future [1], [2], [3], [4], [5], [6], [7], [8]. As an important example, two- and one-dimensional structures play a very important role in electronics. The increasing complexity in microprocessor and integrated circuits means that every 2 years there is a doubling of chip density (Moore’s law). This leads to individual electronic components having dimension less than 50 nm [9], [10], bringing them into the low-dimensional region. Low-dimensional electronic devices exhibit unusual and novel electronic, optical and magnetic properties. This has been shown with the fabrication of quantum well [11] and superlattice devices [12]. Moreover, as materials science makes tremendous progress, more and more substances are being discovered with low-dimensional crystal structures, such as superconducting thin films [13], fullerites (solids with lattices of clusters of 60 or more carbon atoms) and carbon nanotubes [14], to name just a few. Finally, the zero-dimensional clusters, i.e., finite aggregates containing 2–10⁴ particles often exhibit unique physical and chemical phenomena which provide ways and means to explore the land between molecular and condensed matter physics [1], [2], [5], [15], [16], [17], [18], [19], [20], [21], [22]. From the experimental point of view, the remarkable progress in the area of gas-phase cluster physics makes isolated clusters in ultrahigh vacuum amenable to microscopic studies. The availability of high quality experimental data allows the exploration of a wealth of structural, quantum, electronic, electromagnetic, thermodynamic and chemical size effects in finite systems and on surfaces, whose size can be continuously varied. Apart from their fundamental scientific interest, zero-dimensional clusters are also of practical importance. A variety of technological applications rests on cluster physics and chemistry, for example, photography, aerosols and smoke, ceramic coatings formed by cluster deposition [1], [5], [23]. Clusters are also important in astrophysical applications. These include the formation mechanism and properties of cosmic dust and stars (see e.g., [24], [25], [26], [27]). Furthermore, inorganic and organic clusters in the interstellar medium may be responsible for some phenomena in astrophysical spectroscopy, e.g., the ubiquity of some undefined spectral absorption bands [28].

The main motivation for studies of low-dimensional metal oxide systems summarized here is their role as models for solid catalysts [29]. Heterogeneous catalysis plays an important role in numerous practical applications [30], [31], for example, for producing fuels, from gasoline for traditional combustion engines up to hydrogen in fuel cells. A major application is the prevention of pollution: car exhausts, power plants and industrial installations introduce noxious gases in the atmosphere, which can be removed by means of catalysts. They are used in the chemical, petrochemical as well as the food industry, in the latter case, for example, for the production of margarine. As a final example, catalysts are also used to combat soil contamination and for manufacturing fertilizers.

Industrial, high surface area solid catalysts are very complex materials, which present very often ill-defined surfaces and wide size-distributions of the crystallites of the various phases present. This is a consequence of the preparation techniques [30], [31] of the catalysts (impregnation, co-precipitation, etc.) which offer only poor control over the catalyst surface composition. This complicates the task of clearly identifying the influence of the underlying microscopic structure of the surface on the catalytic performance. Moreover, the capabilities of surface analytical tools like X-ray photoelectron spectroscopy (XPS) or Auger-electron spectroscopy (AES) can only be applied in a limited way [32]. Other methods like atomic force microscopy (AFM) and scanning tunneling microscopy (STM) cannot be applied at all to typical heterogeneous catalysts because of their high surface roughness [33], [34], [35], [36]. Even the interpretation of the results of surface-sensitive techniques such as XPS is complicated by the presence of surface contaminants and by the heterogeneity of the catalyst surface [32], [37], [38].

Therefore, a model heterogeneous catalyst is used for the purpose of understanding how the real system works [3], [29], [39], [40], [41], [42], [43]. The purpose is to investigate a complex phenomenon that is not accessible and cannot be quantified or explained when working with real complex catalysts. In other words, a reductionist approach is applied to deconvolute the observations made with the real system. The purpose is not to explore a new method of catalyst preparation with the aim of practical applications. The goal of making and studying a model catalyst is to understand a catalytic phenomenon. This understanding can then be applied to design, synthesis or improvement of new or existing complex catalysts.

The simplest model catalyst is a small gas-phase aggregate of catalytically active species [18], [19], [22], [42], [44], [45], [46], [47], [48]. Studies of isolated gas-phase clusters give access to detailed knowledge of energetics and kinetics of processes and reactions at a strictly molecular level. In the recent years, mass-spectrometric experiments with advanced techniques have been exploited to provide useful insight into the elementary steps of various catalytic reactions and to characterize reactive intermediates that have previously not been within reach of condensed-phase techniques [27], [42], [44], [45], [47], [48], [49], [50]. Obviously, the reactivity of isolated charged clusters and molecules will, in general, be much higher than corresponding active species in the condensed phase. Therefore, gas-phase studies are not applied to investigate the precise mechanisms, energetics, and kinetics operating in applied catalysis, but rather to provide a conceptual framework and an efficient means to obtain direct insight into reactivity patterns, the importance of aspects of electronic structure, and the nature of crucial intermediates. Moreover, the results of gas-phase experiments may be used as a valuable source for benchmarking of approximate theoretical methods which, in turn, can later be applied for computational investigations of more realistic model catalysts.

The main disadvantage of gas-phase studies on isolated reactants is the lack of the intrinsic heterogeneity present in applied catalysis. Nanoclusters of catalytically active species supported on ultrathin oxide films on metallic surfaces allow, unlike isolated gas-phase systems, a study of size and support effects in heterogeneous catalysis [39], [43], [51], [52], [53], [54], [55], [56], [57], [58]. The structure, the electronic properties and reactivity of these supported model catalysts can be studied, in situ, by a large number of surface science techniques. The use of oxide films provides a versatile strategy for the study of model systems in catalysis. The thickness of the oxide film may be used as a control parameter on one hand, to design and investigate materials in their own right whose properties may open interesting routes for catalyst design and, on the other hand, as models of dispersed catalysts on bulk oxide surfaces.

Availability of well defined model catalysts and a variety of experimental techniques complemented by computational investigations offers a hierarchical route for studying applied catalysis. First, the details of kinetics and energetics of elementary bond-making and bond-breaking processes relevant for the catalytic system in question can be studied using gas-phase models of catalytically active species. The type, size, and the composition of the reacting species can be varied providing information about, for example, reactivity patterns. Next, the role and influence of a supporting material, usually a metal oxide, can be elucidated investigating supported model catalysts under well defined conditions. As the thickness and structure of the supporting material and catalytic conditions can be varied, this approach can to some extent bridge the gap between model and applied catalysis.