Preface

Following an initiative by the Multidisciplinary Digital Publishing Institute (MDPI), a series of articles on heterogeneous photocatalysis were published in Molecules from September 2014 to August 2015 after rigorous peer-review. These articles are freely accessible online. Nevertheless, it was thought that a printed book gathering them in an organized manner will be very useful. The book format allows one to browse through the articles in a much easier way. Anybody in a laboratory can have the printed book at hand for consulting at any time. Attention of potential readers to the existence of a book can be drawn readily in libraries and online. A book is also more appropriate for storage than a pile of copies!

In the present case, another argument in favor of the book format was that the articles as a whole provide a timely picture of heterogeneous photocatalysis. This picture includes both retrospective analyses and proper examples of diverse aspects of the current research, so that this book presents an outstanding overview of the field. I would also like to emphasize that such an ensemble of as well-known authors has rarely been brought together in previous books on this topic. Undoubtedly, this will allow the community of senior scientists and students to possess a book of great significance for a low price.

This book contains a total of thirty-one articles among which six are feature articles and seven are reviews. It is structured in four sections. The contents of each section are summarized hereafter.

As detailed in its title, **Section 1** covers many basic aspects of photocatalysis. The feature article by T. Egerton addresses a fundamental aspect that is too rarely considered in sufficient detail, viz. the photon absorption which is obviously critical for photocatalytic reactions and depends on several parameters that are not always taken into account properly. Combined with the review from the Y. Paz's group on the too frequent use of dyes for evaluating photocatalytic efficiency, this feature article also shows the importance of "good practices" in photocatalysis for valid comparisons of experimental results. A review from J. Zhao's group draws the attention on the great interest of employing O-labeled dioxygen and water to unravel the photocatalytic basic mechanisms of the hydroxylation or ring-opening of benzene derivatives and the decarboxylation of aliphatic acids. Three other papers concern the appropriate use of spectroscopic techniques to obtain information on the fundamental steps in photocatalysis; both the review from Y. Nosaka's group and the article from V. Brezova's group draw attention on the issues and illustrate the interest of using electron paramagnetic resonance to detect some active species and to determine their role in photocatalytic mechanisms and pathways, while the article from K. Gray's group emphasizes the pertinence of the use of infrared spectroscopy to investigate the interactions of molecules with photocatalysts, as shown by the examples of H2O and CO2.

**Section 2** devoted to photocatalysts contains the highest number of articles in this book, which reflects the importance of the material aspect. Unsurprisingly, with the exception of three, these articles deal with TiO2 as the unique or principal photo-active component of the photocatalysts. The feature article from P. Kelly's and J. Verran's groups presents the advantages of magnetron sputtering as a versatile, flexible coating method (including of thermally sensitive materials) that allows easy doping and can be industrialized. The effects of the texture and the structure of pure TiO2 upon the rates of photocatalytic reactions are discussed in a short review by P. Pichat relative to nanotubes and an article from B. Ohtani's group relative to anatase particles differing by shape and ratios of exposed facets; these reports show the complexity of the issue. Several articles refer to the combination of TiO2 with another non-semiconducting adsorbent (zeolites, a clay, silica and carbon) or with WO3 in attempts to increase the photocatalytic efficiency. The effects have been assessed in a variety of photocatalytic reactions principally in the gas phase (H. Yamashita's, D. Kozlov's and A. Yasumori's groups), and also in the aqueous phase (C. Langford's group) or in a liquid organic phase (H. Zhu's group), and interpretations for these effects are suggested. Section 2 also reports on the use of semiconductors other than TiO2, the main goal being to explore the potentialities of materials more sensitive to visible-light irradiation for solar applications. A comprehensive review (from Z. Chen's group with 132 references) deals with perovskite-based photocatalysts and two articles concern WO3 (R. Amal's group) or KTaO3, CdS, and MoS2 either pristine or as composites (A. Zaleska's group). These reports provide the state-of-the-art and insights into the potential development of these semiconductors in photocatalysis.

The articles gathered in **Section 3** focus especially on the relevance of photocatalysis for decontamination, unlike those in Section 2 that concentrate on the materials used in photocatalysis, even though these materials were most often also tested for the removal of pollutants. One feature article from S. Hay's and S. Suib's groups discusses the viability of photocatalysis on the basis of field experiments and the authors conclude that the photocatalyst lifetime needs to be extended for cost-effective purification of indoor air. The other feature article from group led by J. Gimenez addresses the question of the environmental impact of water photocatalytic treatment with an original comparison with other advanced oxidation processes, particularly the photo-Fenton process. A detailed review (169 references) from groups led by J. Byrne is devoted to the state-of-the-art inactivation of microorganisms in water and on surfaces and considers the perspectives. Another review by S. Horikoshi and N. Serpone demonstrates that concomitant use of microwave and photocatalysis can be beneficial and the origins of the improvement are discussed. Two articles present results of field experiments for the photocatalytic treatment of either outdoor air under solar light (by N. Negishi and T. Sano) or indoor air in combination with a plasma reactor (from groups led by T. Ochiai and A. Fujishima) by use of newly conceived demonstration devices. Two other articles report on the potential use of photocatalysis to tackle two serious environmental problems, viz. the removal from water of toxic cations in mixture (from K. O'Shea's and D. Dionysiou's groups) or endocrine disruptors (J. Colina's and G. Li Puma's groups).

Whereas the potentialities of photocatalysis for decontamination are well-established, the viability of photocatalysis regarding the production of energy and chemical compounds is still debated because these processes are based on uphill (or endergonic) reactions. These potentialities and the hurdles to their development are reviewed and illustrated in **Section 4**. One very detailed (370 references) feature article by J. Highfield critically reviews advances in photocatalysis and photoelectrocatalysis for producing H2 from water, reforming bio-oxygenates, and synthesizing organic fuels through CO2 reduction. Another feature article from A. Albini's group elaborates comprehensively on the potential development of photocatalysis and photochemistry in organic synthesis, which would offer the advantage of operating under mild conditions. An article by A. Morawski's group indicates the formation, in both aerobic and anaerobic conditions, of H2 and C1-C3 alkanes from C1-C2 alcohols, acetic acid or glucose, though with low efficiency, over TiO2 modified with 20 wt% Fe. A general article from P. Lianos' group describes in detail under which conditions a photoelectrochemical cell comprising a TiO2 photoanode (which can be made sensitive to visible light) and a carbon black cathode with Pt particles can produce electricity or H2 from fuels. Photoelectrochemical water splitting is considered in two other articles from either the material view or the theoretical view. One from J. Krysa's group underlines the importance for the efficiency not only of the type of semiconductor (TiO2 or Fe2O3) but also of the way it is elaborated. In the other article from groups led by S. Yanagida, simulation of adsorbed H2O clusters via density functional theory is shown to allow one to interpret the photoelectrochemical mechanism of water oxidation using nanocrystalline TiO2 electrodes.

In conclusion, this book will be helpful to the beginners who would like to learn more about heterogeneous photocatalysis and its issues, as well as to the senior scientists who will find reviews and articles allowing them to refresh or update their knowledge of some aspects of this multidisciplinary field.

I sincerely thank the contributors for their response to my solicitation. Initially, none of them knew who would respond positively and they were just confident in me for being able to accomplish this venture. I think they do not regret their decision, given the exceptional group of eminent experts who authored these articles published in Molecules and now gathered in this book. Naturally, I thank them, above all those who wrote long feature articles and reviews, for their time. I am also very grateful to the many reviewers who accepted to evaluate the manuscripts and to write constructive comments. Obviously, hearty thanks are also due to Ms. Ran Dang, Managing Editor at MDPI, with whom I always had excellent and efficient email relationships, and to the Assistant Editors who helped me with high competence to speed up the reviewing process.

> Pierre Pichat *Guest Editor*  CNRS/Ecole Centrale de Lyon, France

Chapter 1: Fundamentals: Photon Absorption, Active Species, Mechanisms, Reaction Pathways, Efficiency Evaluation
