Photocatalysis: Fundamentals and Perspectives: Volume 14 (Energy and Environment Series) - Hardcover

Hardcover

ISBN 10: 1782620419 ISBN 13: 9781782620419

Publisher: Royal Society of Chemistry, 2016

Synopsis

Combining basic concepts with the synthesis of new catalysts, reactor and reaction engineering, this book is a comprehensive resource for researchers.

About this title

About the Authors

Jenny Schneider is a Researcher at the Institute for Technical Chemistry, Gottfried Wilhelm Leibniz University of Hannover, Germany. She is a specialist in time-resolved investigations of photocatalytic processes.

Detlef Bahnemann is a Professor at the Institute for Technical Chemistry, Gottfried Wilhelm Leibniz University of Hannover, Germany, and Director of the Laboratory for Photoactive Nanocomposite Materials at Saint-Petersburg State University, Russia. He has worked in the field of photocatalysis for over 30 years.

Jinhua Ye is Managing Director for the Photo-Catalytic Materials Center (PCMC) at the National Institute for Materials Science, Japan. Her research is dedicated to developing new photocatalytic materials for environment preservation.

Gianluca Li Puma is Professor of Chemical and Environmental Engineering at Loughborough University, UK. He is an expert in reaction and reactor engineering, including photochemical and photocatalytic systems.

Dionysios D. Dionysiou is a Professor in the Environmental Engineering and Science Program, Department of Biomedical, Chemical and Environmental Engineering (DBCEE), University of Cincinnati, USA. He has over 20 years of experience in the field of photocatalysis.

From the Back Cover

Combining the basic concepts of photocatalysis with the synthesis of new catalysts, reactor and reaction engineering, this book provides a comprehensive resource on the topic.

The book introduces the fundamental aspects of photocatalysis including the role of surface chemistry and understanding the chemistry of photocatalytic processes before exploring the theory and experimental studies of charge carrier dynamics. Specific chapters then cover new materials for the degradation of organics; water splitting and CO2 reduction; as well as reactor and reaction engineering. Researchers new to this discipline can learn the first principles, whilst experienced researchers can gain further information about aspects in photocatalysis beyond their area of expertise.

Together with Photocatalysis: Applications, these volumes provide a complete overview to photocatalysis.

From the Inside Flap

Combining the basic concepts of photocatalysis with the synthesis of new catalysts, reactor and reaction engineering, this book provides a comprehensive resource on the topic.

The book introduces the fundamental aspects of photocatalysis including the role of surface chemistry and understanding the chemistry of photocatalytic processes before exploring the theory and experimental studies of charge carrier dynamics. Specific chapters then cover new materials for the degradation of organics; water splitting and CO2 reduction; as well as reactor and reaction engineering. Researchers new to this discipline can learn the first principles, whilst experienced researchers can gain further information about aspects in photocatalysis beyond their area of expertise.

Together with Photocatalysis: Applications, these volumes provide a complete overview to photocatalysis.

Excerpt. © Reprinted by permission. All rights reserved.

Photocatalysis

Fundamentals and Perspectives

By Jenny Schneider, Detlef Bahnemann, Jinhua Ye, Gianluca Li Puma, Dionysios D. Dionysiou

The Royal Society of Chemistry

Copyright © 2016 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-041-9

Contents

Part 1: Fundamental Aspects of Photocatalysis, 
Chapter 1 Photoelectrochemistry: From Basic Principles to Photocatalysis Laurence M. Peter, 3, 
Chapter 2 Understanding the Chemistry of Photocatalytic Processes Amer Hakki, Jenny Schneider, and Detlef Bahnemann, 29, 
Chapter 3 Current Issues Concerning the Mechanism of Pristine TiO2 Photocatalysis and the Effects on Photonic Crystal Nanostructures Cecilia B. Mendive, Mariano Curti, and Detlef Bahnemann, 51, 
Chapter 4 Specificity in Photocatalysis Yaron Paz, 51, 
Chapter 5 Photoexcitation in Pure and Modified Semiconductor Photocatalysts Gonu Kim, Yiseul Park, Gun-hee Moon, and Wonyong Choi, 110, 
Chapter 6 New Concepts in Photocatalysis Ying Wu and Michael Wark, 129, 
Part 2: Primary Processes in Photocatalysis, 
Chapter 7 Kinetic Processes in the Presence of Photogenerated Charge Carriers Yoshio Nosaka and Atsuko Y. Nosaka, 165, 
Chapter 8 Traps and Interfaces in Photocatalysis: Model Studies on TiO2 Particle Systems Thomas Berger and Oliver Diwald, 
Chapter 9 Interplay Between Physical and Chemical Events in Photoprocesses in Heterogeneous Systems Alexei V Emeline, Vladimir K. Ryabchuk, Vyacheslav N. Kuznetsov, and Nick Serpone, 218, 
Part 3: New Materials, 
Chapter 10 New Materials: Outline Jinhua Ye, 247, 
Chapter 11 New Materials for Degradation of Organics Shuxin Ouyang and Hua Xu, 252, 
Chapter 12 New Materials for Water Splitting Kazuhiko Maeda, 295, 
Chapter 13 New Materials for CO2 Photoreduction Yong Zhou, Wenguang Tu, and Zhigang Zou, 318, 
Part 4: Reactor and Reaction Engineering, 
Chapter 14 Fundamentals of Radiation Transport in Absorbing Scattering Media Orlando M. Alfano, Alberto E. Cassano, Javier Marugan, and Rafael van Grieken, 351, 
Chapter 15 Photocatalytic Reactor Design Javier Marugan, Rafael van Grieken, Alberto E. Cassano, and Orlando M. Alfano, 367, 
Chapter 16 Photocatalytic Reactor Modeling Fiderman Machuca-Martinez, Miguel Angel Mueses, José Colina-Marquez, and Gianluca Li Puma, 388, 
Subject Index, 425, 

CHAPTER 1

Photoelectrochemistry: From Basic Principles to Photocatalysis

LAURENCE M. PETER

1.1 Introduction

The foundations of semiconductor photoelectrochemistry were laid by Gerischer, Pleskov, Memming, Bard and others in the 1960s. Several authoritative texts are available that summarize the basic concepts. At this time, the physics and chemistry of light-driven reactions at semiconductor surfaces were studied extensively using well-defined bulk monocrystalline materials. Interest in semiconductor photoelectrochemistry became more widespread following the 1973 oil crisis, which stimulated an urgent search for alternative energy technologies. During this period, several efficient liquid-junction solar cells were developed that utilized (mainly single crystal) semiconductors in contact with redox electrolytes. Examples of materials that were studied include CdS, CdSe, GaAs, GaP, InP, WSe and MoSe2 (see Morrison for an excellent literature survey for this period). However, problems of long-term stability and high costs led ultimately to a lessening of activity in the area. The possibility of using illuminated semiconductor/electrolyte junctions to split water was also recognized at this time, and the much-cited Nature paper by Fujishima and Honda marked the beginning of a sustained search for stable semiconductors that can split water using visible light. Again, the initial enthusiasm declined when faced with the stringent demands for high efficiency combined with long-term chemical stability.

Semiconductor photoelectrochemistry experienced a renaissance stimulated by the development of mesoporous dye-sensitized solar cells following the 1991 Nature paper of O'Regan and Grätzel. The resulting move away from well-defined single crystal bulk materials to high surface area nanostructured electrodes opened a new field of research, and many of the ideas that had been developed for bulk semiconductor electrodes required re-examination in view of the very different length scales. Nanostructured semiconductor electrodes are now also being utilized for light-driven water splitting and environmental remediation. This historical development has resulted in a convergence of the fields of semiconductor photoelectrochemistry and photocatalysis at semiconductor particles. The objective of this chapter is to review the basic ideas that were developed originally to understand the photoelectrochemical behaviour of bulk semiconductors and to see how these ideas need to be modified when considering nanostructured semiconductor electrodes and dispersed colloidal systems.

1.2 A Brief Summary of Semiconductor Physics

The band model of solids' leads to the diagram shown in Figure 1.1, which is the starting point for the construction of band diagrams for p-n and metal-semiconductor junctions as well as semiconductor-electrolyte junctions. An important quantity shown in Figure 1.1 is the Fermi energy, EF, which is a measure of the free energy of electrons.

The semiconductor in Figure 1.1 is doped n-type by the presence in the crystal lattice of donor atoms that can be ionized at room temperature, releasing electrons to vacant levels in the conduction band. The concentration of electrons in the conduction band under conditions of thermal equilibrium is given by the Fermi-Dirac equation:

[MATHEMATICAL EXPRESSION OMITTED] (1.1a)

where Nc is the density of states in the conduction band. For normal levels of doping (<1018 cm-3), the exponential term in eqn (1.1a) is much larger than unity, so that the electron density can be approximated by the Boltzmann equation:

[MATHEMATICAL EXPRESSION OMITTED] (1.1b)

It follows that the Fermi energy shown in Figure 1.1 indicates the type and level of doping. The higher the n-doping, the closer EF is to the conduction band. In the case of p-type doping, electron acceptors in the crystal lattice accept electrons from the occupied valence band, creating holes. The concentration of holes under thermal equilibrium conditions is given by:

[MATHEMATICAL EXPRESSION OMITTED] (1.2a)

and for normal doping levels:

[MATHEMATICAL EXPRESSION OMITTED] (1.2b)

It follows that the Fermi level in p-type semiconductors lies close to the valence band. Regardless of the type of doping, the product of the equilibrium concentrations of electrons and holes is given by the law of mass balance:

[MATHEMATICAL EXPRESSION OMITTED] (1.3)

where ni is the intrinsic electron density for the undoped case where electrons and holes are only produced by thermal excitation of electrons across the gap, so that n = p.

Notably, EF is equivalent to the electrochemical potential of electrons. This means that it is a free energy that depends not only on concentration (via the temperature x entropy term in the Gibbs free energy G = U + PV-TS) but also on electrical potential. By contrast, Ec and Ev are internal energy terms that correspond to the standard states for electrons and holes, respectively. For a lucid discussion of the thermodynamics of the Fermi energy, the reader is referred to the excellent book by Würfel.

1.3 Conventional Semiconductor Photoelectrodes

Early work on semiconductor electrodes focussed on the elemental semiconductors silicon and germanium. However, these elements are unstable in contact with water, forming surface oxide or hydroxide layers that complicate their electrochemical behaviour, so that progress was slow. The first semiconductor electrode that was found to behave almost ideally in aqueous solution was monocrystalline n-type ZnO, which can be prepared by chemical etching to expose well-defined stable polar faces with a low density of defects. Even today, ZnO remains one of the best-behaved semiconductor electrodes in terms of the potential distribution across the solid/electrolyte interface. The reason for this is that it can be prepared with a very low density of surface states. These are states associated with defects or impurities that can store electronic charge, altering the potential distribution. Indeed, if the density of surface states is very high, semiconductor electrodes may behave more like metal electrodes. It is important to realize that non-ideal behaviour is unfortunately the norm for most semiconductor electrodes, even if they are monocrystalline. Care is therefore needed in applying some of the simple models developed in the next sections.

1.3.1 Potential and Charge Distribution Across the Semiconductor-Inert Electrolyte Junction

Here we review briefly some essential concepts that are applicable in the case of an ideal semiconductor surface (i.e. one free of surface states) that is immersed in an electrolyte. The situation where a semiconductor is in contact with an inert electrolyte (i.e. one not containing any redox system) in the dark is the simplest to describe. We therefore begin by considering an n-type electrode like ZnO in contact with an electrolyte with a well-defined pH (for a p-type semiconductor, the signs of the charges and consequently the direction of band bending are reversed). The potential of the electrode can be controlled with respect to a reference electrode using a potentiostat, and if no interfacial electron transfer takes place, the electrode behaves as a capacitor. A convenient reference point for describing the potential and charge distribution across the junction is the flat-band potential, Ufb. This is the electrode potential at which there is no excess charge on the semiconductor side of the junction. This means that the number of electrons is exactly matched by the number of ionized donor atoms (remember that n-type doping is achieved by adding donor atoms that can ionize readily to supply electrons to the conduction band, leaving immobile donor ions D+ in the lattice). Because there is no excess charge in the semiconductor, there is no also electrical field. Now if the potential is made more positive than Ufb, electrons are withdrawn from the electrode (depletion condition), leaving a positive space charge region consisting of the ionized donor atoms. The width of the space charge region, Wsc, depends on the doping density, Nd, the relative permittivity, e, and the potential drop across the space charge region ?fsc:

[MATHEMATICAL EXPRESSION OMITTED]

The existence of a uniform positive space charge generates an electric field that varies linearly with distance from x = Wsc to x = 0 (the surface) (Figure 1.2). This linear variation of electric field corresponds to a variation of electrical potential that is parabolic (i.e. proportional to x2). This variation of potential across the space charge region changes the energy of electrons and holes and is reflected in the band bending shown in Figure 1.2.

The positive charge in the space charge region in the semiconductor is balanced by a net charge of opposite sign in the electrolyte created by rearrangement of ions. For concentrated electrolytes, this charge is effectively located at the outer Helmholtz plane of the electrical double layer. The overall charge distribution across the semiconductor-electrolyte junction that results is illustrated in Figure 1.2. It can be seen that the majority of the potential drop across the semiconductor-electrolyte interface occurs in the space charge region. For example, in the case of a doping density of 1017 cm-3 and a band bending of 0.5 eV, >99% of the potential drop across the interface occurs in the space charge region (it is important to note that the situation is different for semiconductors with surface states that can store electronic charge - see Section 1.5). The Fermi energy is constant across the space charge region and current flow is negligible. Since the electron concentration is given by eqn (1.1), it can be seen that the equilibrium electron concentration at the surface is orders of magnitude lower than in the bulk of the semiconductor, hence the term depletion.

The variation of the space charge (Qsc) with the potential drop (?fsc) across the space charge region defines the space charge capacitance, Csc = dQsc/d?fsc. The dependence of Csc on ?fsc is given by the Mott-Schottky equation (here for an n-type electrode):

[MATHEMATICAL EXPRESSION OMITTED] (1.5)

The space charge capacitance appears in series with the Helmholtz capacitance, which is determined by the permittivity and width of the Helmholtz layer:

[MATHEMATICAL EXPRESSION OMITTED] (1.6)

The capacitance of the Helmholtz layer is expected to lie in the range 20-100 µF cm-2. These values are usually much higher than Csc under depletion conditions, and so for low or moderately doped semiconductors it is often assumed that changes in electrode potential appear mainly across the space charge region. In this case, ?fsc in eqn (1.5) can be replaced by U - Ufb, where U is the applied potential. However, for many of the materials that are currently of interest, for example in the context of water splitting, this approximation may not be valid for several reasons. Firstly, Csc may be comparable with CH if the material is doped to levels above 1018 cm-3, as is often the case for non-stoichiometric oxides. Secondly, the derivation of eqn (1.5) involves an assumption that no electronic charge is stored at the surface of the semiconductor (no surface states).

Capacitance-voltage measurements are widely used to determine the flat band potential and doping density of semiconductors. Plots of 1/Cscversus electrode potential are expected to be straight lines with slopes that are inversely proportional to the doping density (cf. eqn (1.5)). If CH [much greater than] Csc, the intercept gives the value of Ufb - kBT/q. However, if the doping density is high, a correction needs to be made for the Helmholtz capacitance. This is particularly important for materials with very high relative permittivity (e.g. rutile).

The potential drop across the Helmholtz layer is also sensitive to the ionic surface charge on the semiconductor. Acid/base equilibria for surface groups on oxide semiconductors give rise to a Nernstian pH dependence of the flat band potential, which shifts negative by 59 mV per pH unit at room temperature:

Ufb = Ufb (pH0) - 0.059pH (1.7)

The flat band potential of chalcogenide semiconductors (e.g. CdS) depends in a similar way on the concentration of chalcogenide ion (e.g. HS-) in the solution as well as on pH, although the pH dependence is not Nernstian. Surface charge is also important in the case of colloidal semiconductors since it influences the position of the valence and conduction bands relative to solution redox levels. The surface dipole potential needs to be taken into account when relating flat band potentials (and hence the band energies) to the absolute energies shown in Figure 1.1. In this context, it is often assumed that the surface dipole associated with ionic species on the surface is negligible at the pH of zero charge, pHpzc. Further details can be found in a comprehensive review by Xu and Schoonen.

1.3.2 The Semiconductor-Redox Electrolyte Junction

If a semiconductor is immersed in an electrolyte containing a redox couple, electronic equilibrium can be established between the two phases by electron transfer across the interface. The principles that govern the formation of such a semiconductor-redox electrolyte junction are the same as those that are applied to describe the junctions between n and p-type semiconductors and between semiconductors and metals. Electron exchange between the semiconductor and the electrolyte involves the reaction (for simplicity we consider a one-electron process):

O + e- = R (1.8)

Since the electrochemical potential or Fermi level of electrons in both phases must be the same at equilibrium, the equilibrium condition for reaction (1.8) can be written in terms of the electrochemical potentials of the reactants and products:

[MATHEMATICAL EXPRESSION OMITTED] (1.9)

It follows that [bar.µ]n, the electrochemical potential of electrons in the semiconductor, is equal to the difference between the electrochemical potentials of R and O. Since the electrochemical potential of electrons is equivalent to the Fermi energy, this allows us to define a redox Fermi energy, EF,redox. EF,redox depends on the standard reduction potential (Uo) of the O/R redox couple and on the concentrations of O and R (Co, CR).

(Continues...)

Excerpted from Photocatalysis by Jenny Schneider, Detlef Bahnemann, Jinhua Ye, Gianluca Li Puma, Dionysios D. Dionysiou. Copyright © 2016 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2016
• Language: English
• ISBN 10: 1782620419
• ISBN 13: 9781782620419
• Binding: Hardcover
• Edition number: 1
• Number of pages: 452
• Editor: Jenny Schneider