Nanocrystalline TiO2 for Photocatalysis 1588830624

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Encyclopedia of Nanoscience and Nanotechnology

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Nanocrystalline TiO2 for Photocatalysis Hubert Gnaser, Bernd Huber, Christiane Ziegler Universität Kaiserslautern, Kaiserslautern, Germany

CONTENTS 1. Introduction 2. Electronic and Charge-Transfer Processes in Photocatalysis 3. Preparation of Nanostructured Materials and Thin Films 4. Structural Properties of Nanocrystalline TiO2 Films 5. Electrical Properties of Nanocrystalline TiO2 Films 6. Photocatalytic Properties of Nanocrystalline TiO2 7. Photocatalytic Applications of Nanocrystalline TiO2 Glossary References

1. INTRODUCTION The development of novel materials and the assessment of their potential application constitutes a major fraction of today’s scientific reasearch efforts. In fact, there exist various major governmental research and development programs related to nanostructured materials. Furthermore, it is estimated that nanotechnology has grown into a multibillion dollar industry and may become the most dominant single technology of the twenty-first century. To allow for this fact, this encyclopedia [1] encompasses a series of contributions devoted to a very prominent field of current materials research activities, namely, nanoscience and nanotechnology. The importance of these developments is reflected also in a number of recent books and articles reviewing this rapidly evolving field [2–10]. This article focuses on a specific class of such novel nano-scaled materials, nanocrystalline TiO2 , and its photocatalytic properties. The title of this article encompasses three main terms (“(photo)catalysis,” “nanocrystalline,” and “TiO2 ”) which, individually, stand for very important areas of scientific research and of, perhaps even more important, technological applications. Their synergistic combination, as ISBN: 1-58883-062-4/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

indicated by the present theme, has stimulated great hopes in accomplishing thereby achievements with paramount benefits for human beings and the global environment. To outline the present state of that quest is the major goal of this article. “Catalysis” is probably the most familiar of the three terms mentioned. A catalyst is incorporated in essentially everybody’s automobile, with the goal of reducing or even eliminating the engine’s toxic gaseous components by converting them into less harmful (albeit not necessarily benign) substances. As is the case in all catalytic reactions, the catalyst itself is not part of the reaction, but is expected to enhance its rate, that is, the velocity of the transformation from the original components (the “educts” in the chemist’s terminology) into the final ones (the “products”). Hence, a catalyst is an entity that accelerates a chemical reaction without being consumed itself in the process. Without catalysts, various chemical reactions of great importance would proceed too slowly [11]. The economic significance of catalysis is enormous. In the U.S. alone, the annual value of products manufactured with the use of catalysts is roughly in the vicinity of one trillion dollars [12]. Indeed, more than 80% of the industrial chemical processes in use nowadays rely on one or more catalytic reactions [13]. A number of those, including oil refining, petrochemical processing, and the manufacturing of commodity chemicals (olefins, methanol, ethylene glycol, etc.), are already well established. But many others, as will be seen in this contribution, represent challenges requiring the development of entirely new approaches. But apart from their industrial importance, catalytic phenomena effect virtually all aspects of our lives. They are crucial in many processes occurring in living things, where enzymes are the catalysts. They are important in the processing of foods and the production of medicines. The reader may have noticed that we have as yet refrained from specifying the meaning of photocatalysis; which will be one of the major topics of this article. This term refers to a catalytic process that is triggered by illuminating the system by visible light or ultraviolet irradiation. Ideally, that light flux would be the sun’s radiance. Next we shall consider the meaning of “nanocrystalline.” First, it is noted that in today’s science world rather inflationary used, the prefix “nano” refers to a fraction of

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 6: Pages (505–535)

506 one part in one billion (109
and, hence, its correct usage would require it being connected to some kind of unit (e.g., of length, time, energy, mass, etc.). In the present context (and in that of “nanotechnology”), “nano” most often relates to the dimension, that is, the size of an object. Therefore, nanocrystalline in the ensuing discussions will designate particles (of crystalline structure and, primarily, with the chemical composition of titanium dioxide) whose typical sizes are in the range of a few to several nanometers (nm), that is, of the order of the one billionth part of one meter. Obviously, these are extremely tiny objects and can be “seen” and studied only with the help of sophisticated analytical instruments like an electron microscope. At first glance, it may appear that such tiny particles are a rather modern contrivance, but this is probably a premature conclusion. In fact, it is quite firmly established that nm-sized particles (mostly very refractory ones like corundum, diamond, or silicon carbide) are ubiquitous in the universe [14] and that they were already present at the time and the location of the formation of the solar system. This “stardust” originated from stellar outflows and supernova ejecta, which may have occurred eons before the gas and dust condensed into what is now the sun, the earth, and the planets. In fact, this dust has intrigued astronomers since the days of William Herschel who noted, in the 1780’s, the existence of small regions in the sky where there appeared to be a complete absence of stars [15]. These regions are most easily seen against the rich star-fields of the Milky Way. Evidence of the presolar origin of these nanocrystalline particles comes primarily from their isotopic abundance pattern [16], which deviates typically to such an extent from any other known matter that a terrestrial or solar origin is virtually impossible. (Most of these particles that have been investigated were extracted from primitive meteorites in which they were incorporated during the formation stage of the solar system; these did not experience any later modification and, hence, preserved the presolar dust particles unaltered [17].) Only now, some billions of years later, mankind has initiated the manufacture and application of such nanocrystalline materials. Nanostructured materials with crystal sizes in the range of 5–50 nm of a variety of materials, including metals and ceramics, have been artificially synthesized by many different techniques in the past couple of years [2, 3, 5–7]. Such new ultrafine-grained materials have properties that are often significantly different and greatly enhanced as compared to coarser-grained or bulk substances. These favorable changes in properties result generally from their small grain sizes, the large percentage of atoms in grain boundaries and at surfaces, the large surface-to-bulk ratio, and the interaction between individual crystallites. Since these features can be tailored to a considerable extent, during synthesis and processing, such nanophase materials are thought to have great technological potential even beyond their current applications. Let us finally turn to a brief discussion of the third term, “TiO2 ” ( i.e., titanium dioxide). TiO2 has three different crystal structures [18]: rutile, anatase, and brookite; only the former two of them are commonly used in photocatalysis. Like for many other metal oxides (also for titanium oxide) have the respective structural, optical, and electronic properties

Nanocrystalline TiO2 for Photocatalysis

been elucidated through several decades of intense scientific research (for a review see, e.g., [19]); some of them will be referred to in the course of the present overview. The feature probably most important in the present context is the fact that TiO2 is a semiconductor with a bandgap of ∼ 3.2 eV. On the other hand, TiO2 , in its nanocrystalline form, constitutes an enormously important commercial product. In fact, the world production of titanium dioxide white pigments amounts to some 4.5 million tons per annum and the global consumption may be considered a distinct economic indicator. White pigments of TiO2 have average particle sizes of around 200–300 nm, optimized for the scatter of white light, resulting, thereby, in a hiding power. Reducing the crystallite size (to ≤ 100 nm), the reflectance of visible light (vis) decreases and the material becomes more transparent; it is widely employed, for example, in paints, plastics, paper, or pharmaceuticals. Nanocrystalline TiO2 exhibits, in addition, a pronounced absorption of ultraviolet (UV) radiation. Because of this high UV absorption and the concurrent high transparency for visible light, TiO2 particles with a size of