Heterogeneous Photocatalysis E-Book
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Discover the latest research in photocatalysis combined with foundational topics in basic physical and chemical photocatalytic processes
In Heterogeneous Photocatalysis: From Fundamentals to Applications in Energy Conversion and Depollution, distinguished researcher and editor Jennifer Strunk delivers a rigorous discussion of the two main topics in her field—energy conversion and depollution reactions. The book covers topics like water splitting, CO2 reduction, NOx abatement and harmful organics degradation.
In addition to the latest research on these topics, the reference provides readers with fundamental information about elementary physical and chemical processes in photocatalysis that are extremely practical in this interdisciplinary field. It offers an excellent overview of modern heterogeneous photocatalysis and combines concepts from different viewpoints to allow researchers with backgrounds as varied as electrochemistry, material science, and semiconductor physics to begin developing solutions with photocatalysis.
In addition to subjects like metal-free photocatalysts and photocarrier loss pathways in metal oxide absorber materials for photocatalysis explored with time-resolved spectroscopy, readers will also benefit from the inclusion of:
- Thorough introductions to kinetic and thermodynamic considerations for photocatalyst design and the logic, concepts, and methods of the design of reliable studies on photocatalysis
- Detailed explorations of in-situ spectroscopy for mechanistic studies in semiconductor photocatalysis and the principles and limitations of photoelectrochemical fuel generation
- Discussions of photocatalysis, including the heterogeneous catalysis perspective and insights into photocatalysis from computational chemistry
- Treatments of selected aspects of photoreactor engineering and defects in photocatalysis
Perfect for photochemists, physical and catalytic chemists, electrochemists, and materials scientists, Heterogeneous Photocatalysis will also earn a place in the libraries of surface physicists and environmental chemists seeking up-to-date information about energy conversion and depollution reactions.
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Kinetic and Thermodynamic Considerations for Photocatalyst Design
1.1 Introduction
1.2 Mechanistic Aspects of Photochemical Reaction Systems
1.3 Common Parameters of Photochemical Reaction Systems
1.4 Differences Between Photocatalytic and Photosynthetic Reaction Systems
1.5 Conclusion
Acknowledgment
References
2 Design of Reliable Studies on Photocatalysis: Logic, Concepts, and Methods
2.1 Photocatalysis
2.2 Reliability in Scientific Studies
2.3 Methods in Photocatalysis Studies
2.4 Design of Reliable Studies on Photocatalysis
References
3 In Situ Spectroscopy for Mechanistic Studies in Semiconductor Photocatalysis
3.1 Introduction
3.2 Challenges in In Situ and
Operando
Characterization in Photocatalysis
3.3 Overview of Methods and Examples from the Literature
3.4 Outlook and Future Perspectives
References
4 Principles and Limitations of Photoelectrochemical Fuel Generation
4.1 Introduction
4.2 Photoelectrochemical Energy Storage
References
Notes
5 Photocatalysis – The Heterogeneous Catalysis Perspective
5.1 Introduction
5.2 Examples of Relevant Catalytic Properties of Photocatalysts
5.3 Conclusions
References
6 Insights into Photocatalysis from Computational Chemistry
6.1 Introduction
6.2 Computational Descriptors
6.3 Examples of Computational Studies of Photocatalyst Materials
6.4 Conclusion
References
7 Selected Aspects of Photoreactor Engineering
7.1 Fundamentals of Photochemical Reaction Engineering
7.2 Radiation Field and Rate of Reaction
7.3 Light Sources
7.4 Particularities of Different Types of Photocatalysts
7.5 Types of Photoreactors
7.6 Conclusions and Outlook
Symbols and Abbreviations
References
Note
8 Defects in Photocatalysis
8.1 Introduction
8.2 Influence of Defects on the Photocatalytic Performance
8.3 Concluding Remarks
References
9 Photocarrier Loss Pathways in Metal Oxide Absorber Materials for Photocatalysis Explored with Time-Resolved Spectroscopy: The Case of BiVO
4
9.1 Introduction
9.2 Photodynamics of BiVO
4
– Carrier Trapping and Polaron Formation
9.3 Conclusions
References
10 Metal-free Photocatalysts
10.1 Introduction
10.2 Graphitic Carbon Nitrides
10.3 Covalent Organic Frameworks
10.4 Conjugated Polymers
10.5 Conclusions
Acknowledgments
References
11 Photocatalytic Water Splitting: Fundamentals and Current Concepts
11.1 Solar Energy Conversion
11.2 Photocatalyst: Fundamental Concept
11.3 Reporting Protocol
11.4 Photon Absorption
11.5 Exciton Separation
11.6 Carrier Transport
11.7 Electrocatalysis
11.8 Mass Transfer: Electrolyte
11.9 Suppression of Back Reaction
11.10 Photocatalytic Overall Water Splitting: State of the Art
11.11 Concluding Remarks
References
12 Photocatalytic CO
2
Reduction and Beyond
12.1 Introduction
12.2 Photocatalytic Reactions Utilizing CO2
12.3 Summary
References
13 Photocatalytic NO
x
Abatement
13.1 Introduction
13.2 Basic Principle
13.3 Reaction Pathway
13.4 Reaction Kinetics
13.5 Strategies to Improve the Performance
13.6 Strategies to Incorporate the Catalysts into Building Materials
13.7 Results from Field Tests and Simulations
References
14 Photoactive Nanomaterials: Applications in Wastewater Treatment and Their Environmental Fate
14.1 Introduction
14.2 Photoactive Semiconductor Nanomaterials and Their Applications in Wastewater Treatment
14.3 Environmental Fate and Behavior of Photoactive Nanomaterials in Wastewater Treatment Processes
14.4 Environmental Effects of Nanomaterials Toward Wastewater Treatment Processes
14.5 Conclusion
References
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Types of photochemical conversion devices [23].
Chapter 3
Table 3.1 In situ techniques for mechanistic studies in photocatalysis: infor...
Chapter 7
Table 7.1 Properties of suspended and immobilized photocatalysts.
Chapter 10
Table 10.1 Representative conducting polymers, their structures, and conducti...
Chapter 12
Table 12.1 Redox potentials for different photocatalytic reactions (pH = 7) [...
Table 12.2 Possible side reactions in CO
2
utilization.
Table 12.3 A summary of reaction parameters and reaction products for selecte...
List of Illustrations
Chapter 1
Figure 1.1 (a) General photochemical reaction system, including generation o...
Figure 1.2 (a) Photochemical reaction system with separate compartments for ...
Figure 1.3 (a) Photochemical reaction system exhibiting charge transfer sele...
Figure 1.4 (a) Natural photosynthesis as a photochemical reaction system tha...
Figure 1.5 Limiting parameters for all photochemical reaction systems.
Figure 1.6 (a) Effect of optical absorbance on apparent quantum efficiency o...
Figure 1.7 (a) Energetics of H
2
evolution from CdSe quantum dots with sulfit...
Figure 1.8 Sensitivity differences of photocatalytic and photosynthetic devi...
Figure 1.9 (a) Dependence of the dye degradation rate on the surface area of...
Figure 1.10 (a) Effect of Mo concentration on the photocurrent in BiVO
4
phot...
Chapter 2
Figure 2.1 Schematic representation of heterogeneous photocatalysis in pheno...
Figure 2.2 Conceptual structure of scientific studies to find/disclose truth...
Figure 2.3 Logic–relation between four propositions, converse, inverse, and ...
Figure 2.4 “Round-coin hypothesis” and its killer card.
Figure 2.5 Examples of measurements of absorbance (a) and Kubelka–Munk funct...
Figure 2.6 (a) Two kinds of plots for determination of bandgap energy (
E
g
) o...
Figure 2.7 Fundamental aspects of action spectrum analyses.
Figure 2.8 Action spectrum analysis for photoinduced reaction of methylene b...
Figure 2.9 Action spectra of methanol dehydrogenation (a) and acetic acid de...
Figure 2.10 Second-order decay of trapped electron-originated photoabsorptio...
Figure 2.11 Mechanism and kinetic equations of peroxy-radical-mediated radic...
Figure 2.12 Light intensity dependence of photocatalytic oxidative decomposi...
Figure 2.13 Light intensity dependence of photocatalytic oxygen evolution fr...
Figure 2.14 Energy-resolved distribution of electron traps (ERDT) and conduc...
Chapter 3
Figure 3.1 (a) Photophysical processes and (b) their approximate timescales ...
Scheme 3.1 In situ techniques for mechanistic studies in photocatalysis. Dar...
Figure 3.2 Schematic diagram of the nanosecond diffuse reflectance laser fla...
Figure 3.3 Plausible mechanisms for water oxidation catalysis on a hematite ...
Figure 3.4 Schematic diagram of an infrared spectroscopy system used for the...
Figure 3.5 A representation of the water-covered SrTiO
3
surface before and a...
Figure 3.6 Schematic of monodentate and bidentate methoxy structures and oxy...
Figure 3.7 Schematic illustration of the operando APXPS (a) and operando hig...
Chapter 4
Figure 4.1 Simplified schematic of the valence band, conduction band, and Fe...
Figure 4.2 Band structures of (a, b) an n-type and (c, d) a p-type semicondu...
Figure 4.3 n-Type semiconductor/electrolyte interface under (a) a positive b...
Figure 4.4 Examples of PEC water splitting device configurations: (a) single...
Figure 4.5 Graphical circuit analysis of a photocathode measurement (solid l...
Figure 4.6 Top: General current–voltage curves for the two half-cell reactio...
Figure 4.7 (a) Single junction limiting efficiencies. Limiting efficiencies ...
Figure 4.8 (a) STF efficiency for CO production as a function of the bandgap...
Figure 4.9 General comparison of maximum STF efficiency as a function of the...
Figure 4.10 Photoelectrochemical performance improvement of a photocathode v...
Chapter 5
Figure 5.1 Schematic representation of a hypothetical catalytic process. Ads...
Figure 5.2 Schematic representation of a catalytic process taking place at a...
Figure 5.3 Free energy profile of the dissociation of nitrogen on a Ru(0001)...
Figure 5.4 Free energy change of several reactions of relevance of solar ene...
Figure 5.5 Visualization of the progression of a photocatalytic (photosynthe...
Figure 5.6 (a) Schematic picture of the location of Rh deposition sites on T...
Figure 5.7 Visualization of the two different regimes in which the performan...
Figure 5.8 Schematic energy band diagram of a contact between a metal and a ...
Figure 5.9 Ligand-to-metal charge transfer of an isolated VO
4
site; the big ...
Figure 5.10 Visualization of the different catalytic and photophysical prope...
Chapter 6
Figure 6.1 Schematic diagrams illustrating the (a) accuracy of standard DFT,...
Figure 6.2 (a) Schematic illustration of the relationships between the energ...
Figure 6.3 (a) Schematic illustration of band bending at the interface betwe...
Figure 6.4 (a) Gibb's free energy profile computed for intermediates in the ...
Figure 6.5 Local Fermi softness
(
S
F
(
r
))
computed for close-packed surfaces o...
Figure 6.6 (a) Illustration of the bandgaps and band edges of various semico...
Figure 6.7 (a) Computed DOS for Pt nanoparticles of various sizes at the
TiO
...
Figure 6.8 (a) Geometry of bulk ZnS (top left), CdS (top right), and
Zn0.5Cd
...
Figure 6.9 (a) Atomic structure of a
(TiO
2
)
[35] nanoparticle with representa...
Figure 6.10 (a) Computed spin density and electronic density of states for (...
Chapter 7
Figure 7.1 Aspects of photochemical reaction engineering. Source: Guba et al...
Figure 7.2 Basic reactor geometries: (a) parallel plate, (b) cylindrical, (c...
Figure 7.3 Basic emission characteristics of light sources: (a) tubular, (b)...
Figure 7.4 Absorption factor
f
λ
(a) and absorption factor
f
λ
norma...
Figure 7.5 Average volumetric rate of photon absorption
for specific wavel...
Figure 7.6 Typical aging behavior of gas discharge lamps and LEDs. Source: S...
Figure 7.7 Emission spectra of (a) a medium pressure mercury lamp; (b) a
ex...
Figure 7.8 (a) Radiant efficiencies, (b) total photon flux, and (c) photon f...
Figure 7.9 Emitted spectra of MDELs at applied microwave powers of 80 W: lam...
Figure 7.10 Cartoons illustrating the (a) external and (b) internal position...
Figure 7.11 Top: (a) Picture of wirelessly powered LEDs encapsulated in a po...
Figure 7.12 Absorption efficiency, defined as the ratio of absorbed photon f...
Figure 7.13 Reflection spectra of several materials: (1) silver, (2) gold, (...
Figure 7.14 Common solar photoreactors: (a) parabolic trough reactor, (b) co...
Figure 7.15 Fundamental reactor configurations for the use of artificial lig...
Chapter 8
Figure 8.1 Defect classification according to their dimensionality including...
Figure 8.2 Analysis depth of common characterization techniques. Source: Agg...
Figure 8.3 Examples for defect characterization: (a) constant height STM ima...
Figure 8.4 Absorption for a defect-free and reduced semiconductor (example
T
...
Figure 8.5 Effect of defects on photocatalytic processes at the example of o...
Figure 8.6 (a) Photocatalytic efficiency for methylene blue degradation usin...
Figure 8.7 Changes in reaction selectivity of NO to
N
2
and
O
2
for 1% Fe-dope...
Figure 8.8 (a) Illustration of charge transfer across the α–β phase junction...
Figure 8.9 (a) Example for a negative effect of hydrogenation (20 bar, 24 ho...
Chapter 9
Figure 9.1 Accessible energy scale ranging from meV to keV for time-resolved...
Figure 9.2 Proposed photocarrier localization within the bandgap of BiVO
4
be...
Figure 9.3 Timescales for shallow and deep trapping of electrons and holes i...
Figure 9.4 Femtosecond transient diffuse reflectance transients derived at d...
Figure 9.5 Hole lifetimes for W-doped and W–Ti co-doped BiVO
4
films in N
2
an...
Figure 9.6 Schematic diagram showing polaron transport in pristine and W- or...
Figure 9.7 Reaction scheme for timescales of structural change in BiVO
4
rela...
Figure 9.8 Typical time-resolved conductivity pump-probe principle. In most ...
Figure 9.9 Nanosecond-resolved TRMC decay for undoped and 1% W-doped BiVO
4
t...
Figure 9.10 (a) TRTS photoconductivity transient of BiVO
4
showing early carr...
Figure 9.11 Enhancement of the photoconductivity with hydrogen incorporation...
Figure 9.12 Electronic alignment of the polaron transport and water splittin...
Chapter 10
Figure 10.1 Comparison of the vb and cb positions between inorganic- and org...
Figure 10.2 Heptazine (tri-
s
-triazine, a) and
s
-triazine (b) based structure...
Figure 10.3 Oxidation of alcohols with mpg-C
3
N
4
.
Figure 10.4 Formation of new CC bonds in
N
-aryltetrahydroisoquinolines.
Figure 10.5 Preparation of cyclopentane derivative from 2-bromomalonate.
Figure 10.6 Band positions of g-C
3
N
4
and TiO
2
in comparison to reduction pot...
Figure 10.7 Highly active COFs for the photocatalytic production of hydrogen...
Figure 10.8 Photodegradation of phenol under UV light (a) and under visible ...
Chapter 11
Figure 11.1 An example of the simple math used to calculate the area require...
Figure 11.2 Conceptual schematic of the working principles of a photoelectro...
Figure 11.3 Energy diagram of photocatalytic and photosynthetic reactions.
Figure 11.4 Photon number of AM 1.5G as a function of wavelength, and theore...
Figure 11.5 A conceptual schematic of how the photocatalytic water splitting...
Figure 11.6 Schematic image of the photocatalytic water splitting process. T...
Figure 11.7 Processes and properties associated with photocatalysis at diffe...
Figure 11.8 (a) Absorption coefficient as a function of wavelength measured ...
Figure 11.9 Characteristics of the Frenkel and Mott–Wannier exciton models. ...
Figure 11.10 (a) Hole and (b) electron lifetimes in heavily doped n-type and...
Figure 11.11 Correlation between photocatalytic overall water splitting rate...
Figure 11.12 An example of successful photocatalytic overall water splitting...
Figure 11.13 (a) Scanning electron microscopy images of SrTiO
3
prepared usin...
Figure 11.14 (a) Schematic of a 1 × 1 m water splitting panel. Nine photocat...
Chapter 12
Figure 12.1 Schematic of photocatalytic reduction of CO
2
to energy-bearing p...
Figure 12.2 Schematic diagram of the energy correlation between semiconducto...
Figure 12.3 CO
2
utilization in chemical reactions and relevant reaction enth...
Figure 12.4 Dry reforming equilibrium conversions.
Figure 12.5 Comparison of classical endothermic (a) and photocatalytic (b) r...
Figure 12.6 Schematic of the photocatalytic reduction of CO
2
by CH
4
based on...
Figure 12.7 Schematic of the photocatalytic reduction of CO
2
by CH
4
and H
2
O ...
Chapter 13
Figure 13.1 The typical primary processes occurring at the semiconductor par...
Figure 13.2 The major reaction pathways of NO/NO
2
to nitric acid (HONO
2
) ove...
Figure 13.3 The typically observed reaction rate as a function of the NO con...
Figure 13.4 NO conversion (a) and absolute removed amount of NO (b) for the ...
Figure 13.5 Overview of the different mechanisms employed to make a wide ban...
Figure 13.6 An overview of the different strategies employed to improve the ...
Figure 13.7 Overview over the different strategies to incorporate the photoc...
Chapter 14
Figure 14.1 Schematic illustration on the formation of photogenerated electr...
Figure 14.2 FE-SEM images of (a) cauliflower-like, (b) truncated hexagonal c...
Figure 14.3 (a) SEM image of hierarchically nanostructured α-Fe
2
O
3
hollow sp...
Figure 14.4 Possible exposure routes and transformation of photoactive nanom...
Figure 14.5 Illustration of the possible (a) chemical transformations, (b) p...
Figure 14.6 Phonon- and charge-carrier-driven reaction mechanisms on metals....
Guide
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Heterogeneous Photocatalysis
From Fundamentals to Applications in Energy Conversion and Depollution
Edited byJennifer Strunk
Editor
Prof. Dr. Jennifer StrunkLeibniz Institute for CatalysisDepartment of Heterogeneous PhotocatalysisAlbert-Einstein-Str. 29a18059 RostockGermany
CoverCourtesy of Pawel Naliwajko and Jennifer Strunk;icons © Liudmila Klymenko/Shutterstock
All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
Library of Congress Card No.:applied for
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© 2021 WILEY-VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-527-34464-2ePDF ISBN: 978-3-527-81527-2ePub ISBN: 978-3-527-81526-5oBook ISBN: 978-3-527-81529-6
Preface
It seems so simple: Take a beaker and shine light on it. This is an opinion I have come across occasionally since starting to work in the field of photocatalysis pretty much exactly 10 years ago. Indeed, it may be one way to start. However, if one is active in this research field for a while, it will be unavoidable to face highly complex questions that can only be appropriately addressed with broad interdisciplinary expertise. My own starting point was classical heterogeneous catalysis, but other researchers enter the field of photocatalysis with a background in electrochemistry, material science, or semiconductor physics, to name just a few. I firmly believe that by combination of the different viewpoints, the toolbox of photocatalysis can be fully exploited to develop the solutions for sustainable energy conversion and environmental depollution the world so dearly needs. In this sense, it is the purpose of this book to pave the way toward mutual understanding. Researchers new to the field of photocatalysis can acquire basic knowledge in those research areas still unfamiliar to them. I still remember my early days, when I spent much time searching for literature on electrochemistry relevant for my research. Therefore, I wanted to provide a book where all the most important fundamentals from all fields can be found in one place. Experienced photocatalysis researchers will be able to use this book not only in teaching but also to get an overview over the state of the art in the various target applications. In addition to reviewing the numerous recent scientific articles in their field, all the chapter authors wanted to gain and share new knowledge by combining the various studies into a unified picture. I sincerely hope that new insights and ideas will emerge from this endeavor.
Please allow me to express my deepest gratitude to all my international expert colleagues who contributed their valuable specialist expertise to successfully cover a wide range of topics. And no less, I would like to thank everyone at Wiley-VCH for making this book possible.
Rostock, 18 October 2020
Jennifer Strunk
1Kinetic and Thermodynamic Considerations for Photocatalyst Design
Frank E. Osterloh
University of California, Department of Chemistry, One Shields Avenue, Davis, CA, 95616, USA
1.1 Introduction
Photochemical processes play a central role on Earth. While natural photosynthesis powers our biosphere and economic growth (through the use of photosynthesis-derived fossil fuels), additional photochemical processes are involved in shaping the “photogeochemistry” of our planet [1]. Light reactions play a role in the creation and function of the ozone ultraviolet (UV) filter in the atmosphere, the degradation of plant materials, man-made chemicals and plastics, and even in the chemical conversion of Earth-abundant minerals.
The potential of photochemical processes for technical applications was first demonstrated by A.E. Becquerel in 1839 when he discovered the photovoltaic effect. Interestingly, it took over a century before this knowledge was applied to practical photovoltaic cells [2]. In 1968, Gerischer's discovery of the dye sensitization effect at illuminated semiconductor surfaces [3] paved the way for Grätzel's construction of the first dye-sensitized photovoltaic cell 1991 [3] and also inspired for the production of hydrogen fuel from illuminated TiO2 photoanodes [4, 5].
Since then, the interest in photochemical reactions for environmental remediation [6–10] and for the production of sustainable fuels has gained steadily [11–17]. In 2018, over 6000 articles were published with the term photocatalytic or photocatalyst in the title. This is about 60 times as many as published on this topic in 1991 when Grätzel's dye-sensitized solar cells made headlines. In contrast, the number of papers published on photosynthesis has been relatively steady in the past three decades, with approximately 1000 publications per year.
In the science community, photochemical reaction systems are typically referred to as “photocatalysts,” or as “photosynthetic systems” or sometimes as devices for “artificial photosynthesis.” Interestingly, there is no strong differentiation between these terms. For example, the International Union of Pure and Applied Chemistry (IUPAC) defines a “photocatalyst” as a “Catalyst able to produce, upon absorption of light, chemical transformations of the reaction partners. The excited state of the photocatalyst repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each cycle of such interactions.” [18] This definition makes no distinction between reactions that deposit photochemical energy in the products and reactions that do not.
According to Nozik [19] and Bard [20], excitonic reactions can be divided into photosynthetic and photocatalytic processes, depending on the thermodynamics of the associated reaction: [21].
“Photoelectrolytic cells … can be classified as photosynthetic or photocatalytic. In the former case, radiant energy provides a Gibbs energy to drive a reaction such as H2O + H2 + ½ O2, and electrical or thermal energy may be later recovered by allowing the reverse, spontaneous reaction to proceed. In a photocatalytic cell the photon absorption promotes a reaction with ΔG < 0 so there is no net storage of chemical energy, but the radiant energy speeds up a slow reaction.” [22]
As we will show here, this distinction between photocatalytic and photosynthetic devices becomes very significant to the understanding of their function and also to their optimization. An overview of the fundamental processes in photochemical reaction systems is presented in the following sections.
1.2 Mechanistic Aspects of Photochemical Reaction Systems
Photochemical (excitonic) reaction systems generally rely on the creation and transfer of charge carriers to induce the transformation of reagents in the vicinity of the light absorber. Usually, the process begins with the absorption of one or several photons (step 1), as shown in Figure 1.1a. This generates photoelectrons and holes, which subsequently react with reagents (step 2) to produce products. These products may interact with the photocatalyst repeatedly to undergo further transformations or they may react to form the starting materials again.
A very important aspect of a photochemical reaction system is the energy balance of the overall process. Two outcomes are possible, theoretically. In the first one, the products have a greater free energy than the reagents and the Gibbs free energy change for the process is positive, ΔG > 0. An example for this kind of reaction is the photochemical water splitting reaction that produces hydrogen and oxygen. This process has a reaction free energy change of +237 kJ per mol of water, i.e. it is highly endergonic, as intended for a fuel forming reaction.
In the second outcome, the products have a lower combined Gibbs free energy content than the reagents and the overall process is exergonic, ΔG < 0. An example of the second type is the photochemical oxidation of organic matter into carbon dioxide and water, which is highly exergonic, because of the formation of CO2(ΔGF = −394.4 kJ mol−1) and H2O (ΔGF = −228.6 kJ mol−1). For example, the Gibbs free energy change for the combustion of propane is −2.074 MJ mol−1.
Figure 1.1 (a) General photochemical reaction system, including generation of photochemical charge carriers (1), electrochemical forward reactions (2), and backward (3) reactions. (b) Energetics of photochemical reactions. Source: Osterloh [23]. © 2017, American Chemical Society.
The difference in energetics for these processes has an important consequence on the design of the reaction system. If the forward process is endergonic, there is also a need to prevent the reverse thermodynamically favored reaction. This is shown in Figure 1.1b. Because the reaction products are at higher Gibbs free energy than the reagents, they may reform the starting materials, either by direct reaction or with the aid of the “photocatalyst.” This limitation does not apply to exergonic processes, which cannot be reversed without additional energy input. Therefore, the ability to prevent the reverse, thermodynamically favored reaction is an important attribute of photochemical reaction systems that promote endergonic reactions.
Thus, in analogy to photoelectrochemical systems (see above), photochemical reaction systems can be classified as either photosynthetic when they promote endergonic (fuel forming) reactions or as photocatalytic when they promote exergonic, thermodynamically favored reactions. The efficiency of a fuel producing device is normally assessed with the energy efficiency of the process, i.e. the amount of photochemical energy stored in the reaction products [24]. For a photocatalytic device, on the other hand, the apparent quantum efficiency and the product selectivity are more suitable for assessing performance.
Because they have differing functions, it is expected that design of photosynthetic and photocatalytic devices will also be different. In this regard, it is useful to analyze a general photochemical process with specific emphasis of the ways for this reaction to become reversed. Let the photochemical reaction system in Figure 1.1a be that of the endergonic process I where an oxidized reagent ROX and a reduced reagent RRED are being converted into a reduced product PRED and an oxidized product POX. For natural photosynthesis, for example, ROX = CO2 and RRED = H2O and PRED = {CHOH} (sugar fragment) and POX = O2.
At a minimum, this conversion must involve steps II–IV, where step II is the absorption of one or several photons to produce one or several electrons and holes with a lifetime sufficient to react in steps III and IV. Step III uses the photoelectron to reduce ROX to PRED and step IV uses the photohole to oxidize RRED to POX.
(I)
R
OX
+ R
RED
→ P
RED
+ P
OX
Δ
G
> 0
(II)
Cat +
hν
→ e
−
+ h
+
(III)
R
OX
+ e
−
→ P
RED
(IV)
R
RED
+ h
+
→
P
OX
After step IV, the photochemical reaction cycle can begin anew with the absorption of more photons.
Once products have been formed in sufficient quantity, the thermodynamically favored backreaction becomes increasingly favorable. It may proceed via reaction paths V–VII. If the products of the reaction are kinetically labile, they may react directly with each other to reform the original reagents, according to step V. This would be possible if the products are free radicals or radical intermediates, for example, superoxide (O2−) or partially reduced carbon compounds (e.g. methyl radical). Such radicals often form as intermediates in multistep photochemical conversion reactions, such as methanol synthesis from carbon dioxide.
However, when the products are kinetically inert against direct reaction with each other (for example, H2/O2 from the water splitting reaction), the reverse reaction must occur via steps VI or VII. Here, the reduced product (e.g. H2) might get oxidized by additional photoholes in the light absorber, or the oxidized product (e.g. O2) might get reduced by additional photoelectrons.
(V)
P
RED
+ P
OX
→
R
OX
+ R
RED
Δ
G
< 0
(direct backreaction)
(VI)
P
RED
+ h
+
→ R
OX
(oxidation of reduced product)
(VII)
P
OX
+ e
−
→ R
RED
(reduction of oxidized product)
These backreactions can occur when the products of the photosynthetic reaction come into contact with hole and electron-donating sites at the light absorber, as shown in Figure 1.1a. These reactions must be prevented in order for the endergonic reaction to proceed in the forward direction. No such strict requirement exists for exergonic processes, for example, photocatalytic dye degradation, because there is no thermodynamic driving force for this process to reverse.
In principle, there are two different strategies to prevent reactions V–VII from occurring. In the first one (Figure 1.2), the backreaction is prevented by spatially separating the half reactions III and IV of the overall photochemical process from each other. This involves separating the products of the forward reaction and separating the charge carriers involved in them. Because the half reactions occur in different areas of the photochemical reaction system, any cross talk between them is prevented. This can be achieved by creating a physical barrier between reducing and oxidizing sites on the light absorber that prevents mixing of the products and reactions between them. The barrier action also extends to the photochemical charge carriers, which need to be separated so that they can reach the spatially distinct reaction sites of the light absorber.
The photoelectrochemical water splitting cell in Figure 1.2b is an example of this kind of reaction system. Anodic and cathodic reactions occur in separate compartments in the device, and electrons and holes are separated by the photoanode. No backreaction can occur because no electrons are available at the anodic side, and no holes at the cathode, and because H2 and O2 are physically separated in the two half cells. However, to allow the coupled electrochemical reactions to proceed, charge carriers and charge compensating ions must be able to transfer between the compartments.
Figure 1.2 (a) Photochemical reaction system with separate compartments for anodic and cathodic processes. A barrier prevents mixture of reagents and products and of positive and negative charge carriers. (b) Photoelectrochemical water splitting cell as an example for this type of reaction system. Charge carriers are separated by the electric field at the solid–liquid junction of the photoanode. A membrane prevents mixing of H2 and O2 but allows transfer of hydroxide and/or protons and electrons and holes. Source: Osterloh [23]. © 2017, American Chemical Society.
A fundamentally different way to prevent the backreaction is shown in Figure 1.3a. Here, neither products nor charge carriers are separated, and all reactions occur inside of a single reaction compartment. Backreactions VI and VII are prevented by endowing the photochemical reaction sites with charge transfer selectivity. This charge transfer selectivity ensures that electrons can only be given to ROX but not to POX and holes can only be transferred to RRED but not to PRED. Because the products are not separated, this type of reaction system can only be used if the products are sufficiently inert and will not react with each other according to reaction V.
The water splitting photocatalyst shown in Figure 1.3b is an example of a reaction system that exhibits electrochemical selectivity. Water oxidation and reduction take place on different sites on the photocatalyst, and product selectivity is achieved by preventing oxygen reduction with a Cr2O3 layer on top of the proton reduction site. No such precaution needs to be taken at the anodic reaction site, which is naturally selective for water oxidation because hydrogen oxidation is slow on most electrodes. Hydrogen and oxygen coevolution in the same sample space is possible because under the conditions of the water splitting reaction, the gas mixture is metastable with regard to the spontaneous formation of water. However, a spark can promote an explosion releasing the stored free energy. This means the gases will need to be separated in order to utilize them as fuels.
The design constraints specified in Figures 1.2 and 1.3 do not apply to exergonic photochemical reaction systems, such as a dye degradation photocatalyst. The photocatalysts for such exergonic reactions are not susceptible to catalyzing the backreaction. Therefore, photocatalysts can operate on the basis of the general scheme in Figure 1.1a without the strict requirement for spatial separation of half reactions or charge transfer selectivity.
Figure 1.3 (a) Photochemical reaction system exhibiting charge transfer selectivity for anodic and cathodic processes that prevents oxidation of reduced product and reduction of oxidized products. (b) Example of a particle-based water splitting system for the production of H2 and O2 in a single compartment. Selective proton reduction is achieved by encasing the proton reduction site in a Cr2O3 shell that prevents photoreduction of oxygen. Source: Osterloh [23]. © 2017, American Chemical Society.
As Table 1.1 shows, most existing photochemical reaction systems can be classified as exergonic, or as endergonic with charge selectivity or endergonic with spatial separation of products and charge carriers. The list includes various types of photochemical water splitting systems [43, 44], which can be designed as spatially separated or as charge-selective devices. For example, the electrochemical cell with photovoltaic input would be a spatially separate photosynthetic device because carriers are separated in the solar cell and delivered to separate electrodes. On the other hand, a tandem (or z-scheme) of light-activated water oxidizing and water reducing particles in the same compartment would require electrochemical selectivity because both products (H2 and O2) are generated in the same reaction compartment.
The literature also contains many examples of suspended photocatalysts that produce H2 or O2 in the presence of a sacrificial donor or acceptor, which act as electron sources or sinks, respectively [45, 46]. Often, the addition of these reagents turns an endergonic process into an exergonic one, which means that no photochemical energy is deposited into the products. Examples are the photocatalytic water oxidation in the presence of Ce4+ as an extremely strong electron acceptor (Eo = +1.72 V) or water reduction in the presence of sulfite (Eo = −0.93 V in basic solution) or methanol. However, when the sacrificial donor is a mild reducing agent, the overall process may still be endergonic. For example, H2 evolution (0.0 V RHE) from a SrTiO3:Rh/Pt/[Fe(CN)6]3−/4− system (+0.36 V RHE) is endergonic because there is a net gain of 0.36 eV per transferred electron [47]. In photoelectrosynthetic cells, the sacrificial reagent is substituted by a voltage bias. This bias needs to be considered in calculating the photosynthetic efficiency of the system [44].
Table 1.1 Types of photochemical conversion devices [23].
Process
Device
Endergonic, Δ
G
> 0 (photosynthetic) spatially separate
Endergonic, Δ
G
> 0 (photosynthetic), electrochemically selective
Exergonic, Δ
G
< 0 (photocatalytic)
Water photoelectrolysis: H
2
O → H
2
+ ½O
2
; Δ
G
> 0
Electrochemical cell with photovoltaic power input [
25
,
26
]
X
Photoelectrochemical cell with photocathode or photoanode or both
[27]
X
Suspension or film of single absorber particles
[28]
X
Suspension or film of tandem absorber particles in one compartment [
29
,
30
]
X
Tandem absorber particles in two compartments [
31
,
32
]
X
H
2
evolution from proton reduction with reducing agent (sacrificial donor): H
2
O + Red → H
2
+ Ox; Δ
G
< 0
Particle suspension/solution
[33]
X
O
2
evolution from water with chemical oxidizer (sacrificial acceptor): H
2
O + Ox → O
2
+ Red; Δ
G
< 0
Particle suspension/solution
[34]
X
Natural photosynthesis: 6 CO
2
+ 6 H
2
O → C
6
H
12
O
6
+ O
2
; Δ
G
> 0
Cyanobacteria, algae, plants
[35]
X
X
CO
2
reduction with reducing agent (sacrificial donor): CO
2
+ Red + H
2
O → CH
x
O
y
+ Ox; Δ
G
< 0
Particle suspension/solution [
36
,
37
]
X
Oxidative degradation of dyes: dye + O
2
→ Ox; Δ
G
< 0
Particle suspension/solution [
38
,
39
]
X
Oxidation of NO
x
: 2 NO + 3/2 O
2
+ H
2
O → 2 HNO
3
; Δ
G
< 0
Particle suspension
[40]
X
Nitrogen fixation with reducing agent Red + N
2
+ H
2
O → Ox + NH
3
; Δ
G
< 0
Particle suspension/solution
[41]
X
Oxidative C–H activation R
3
C–H + O
2
→ products + H
2
O; Δ
G
< 0
Particle suspension/solution
[42]
X
C–C and C–N coupling
Particle suspension/solution
X
Source: Adapted from https://pubs.acs.org/doi/abs/10.1021%2Facsenergylett.6b00665.
Figure 1.4 (a) Natural photosynthesis as a photochemical reaction system that employs separation of half reactions in addition to electrochemical selectivity. Source: Osterloh [23]. © 2017, American Chemical Society. (b) Plant cells with visible chloroplasts (from a moss, Plagiomnium affine). Source: Kristian Peters (https://en.wikipedia.org/wiki/Photosynthesis).
Difficult reactions with inert reagents (synthesis of ammonia from N2 and protons) [42] or with complex reaction mechanism (CO2 reduction to methane) [48–50] and in C–H activation or C–C or C–N coupling reactions often require a strong thermodynamic bias in the form of a powerful reducing agent or an applied voltage bias to achieve the desired outcome [51–53]. However, this applied bias may change the nature of the overall process from an energy-creating to an energy-consuming one, which cannot be tolerated if the goal is the production of fuels, such as methane or hydrogen.
Photochemical reactions that use oxygen as an abundant electron sink are also often thermodynamically favored. Examples of this type are the oxidative activation of methane or many processes for environmental remediation (degradation of organic compounds and nitrogen oxides, NOx) [6, 9, 40, 54, 55] These reactions generate highly reactive hydroxyl and superoxide radicals that can abstract hydrogen atoms from hydrocarbons, causing various follow up reactions, and complete mineralization in some cases. Because of the oxidizing character of O2 (Eo = +1.23 V vs. reversible hydrogen electrode [RHE]), the thermodynamics are usually downhill and the backreaction is not a significant problem.
Clearly, natural photosynthesis in Table 1.1 belongs to the class of endergonic reactions. The process is carried out by plants and phytoplankton (algae and cyanobacteria). These photosynthetic cells utilize both chemical selectivity and spatial separation of the half reactions to achieve its function [35]. As can be seen schematically in Figure 1.4, plant cells have two separate compartments for the carbon-reducing half reaction (Calvin cycle) and the light-driven water oxidation reaction. The connection between the subsystems is achieved through the electron transport chain and various channels that move protons, ADP/ATP, and NADP/NADPH across the cell.
The problem of natural photosynthesis is that the permeability of the cell walls allows oxygen to enter into the Calvin cycle where it can be reduced preferentially over CO2. This oxygen reduction reaction is the first step of the backreaction of photosynthesis. Plants have learned to suppress it by a process called photorespiration [56, 57]. Here, enzymes are employed to remove oxygen reduction products from the Calvin cycle and to improve the selectivity toward glucose. Additionally, water oxidation at the oxygen evolving complex is very selective. This shows that natural photosynthesis employs charge transfer selectivity together with spatial separation of the half reactions.
1.3 Common Parameters of Photochemical Reaction Systems
The above classification of photochemical reaction systems allows a more detailed description of the parameters that limit each system type. All systems have in common that they rely on the absorption of photons, the generation of charge carriers, and the reaction of these carriers with reagents. These functions are enhanced by large absorption coefficients, long excited state/carrier lifetimes, and fast electrochemical reaction kinetics (low overpotentials), as summarized in Figure 1.5. The importance of these parameters is supported by experimental evidence.
For example, it is well known that the photochemical water splitting rate of the GaN:ZnO particle catalyst is a function of the absorption coefficient of the material [58]. As shown in Figure 1.6a, photons with energies near the bandgap energy of the material are only weakly absorbed causing a low apparent quantum efficiency for water splitting. Higher photon energies can excite the material more effectively causing a higher apparent quantum efficiency. A similar dependence on the absorption coefficient has been demonstrated for molecular hydrogen evolution catalysts [59], for Rh-doped SrTiO3 nanocrystals [47], for various photoelectrodes [60, 61], and for photovoltaic devices [62, 63]. Also, for natural photosynthesis (Figure 1.6b), the carbon fixation rate P is empirically linked to the absorption coefficient of chlorophyll αChl, the chlorophyll concentration C, the photosynthetic quantum efficiency Φm, and the irradiance E [56, 64].
This confirms the absorption coefficient as a limiting parameter across a set of inorganic, molecular, and biological photochemical reaction systems.
The second important parameter in Figure 1.5 is the lifetime τ (or decay time) of the photochemical charge carriers. This time is defined as the point when 1/e of the photochemical charge carriers has disappeared [63]. The value can be measured using microwave conductivity [65] or with transient absorption spectroscopy [66] by following the concentrations of excess electrons and holes after turning off the light. The longer τ, the greater the chance for the carriers to make their way to the reagents and the greater the photochemical reactivity.
The importance of the carrier lifetime is best established for photovoltaic devices, where it is well documented that short lifetimes caused by carrier recombination diminish the energy conversion efficiency of the device [62]. A similar correlation has also been confirmed for photoelectrochemical cells [66–68], inorganic and [69–72] molecular photocatalysts [73–76], and natural photosynthesis [77, 78].
Figure 1.5 Limiting parameters for all photochemical reaction systems.
Figure 1.6 (a) Effect of optical absorbance on apparent quantum efficiency of photochemical water splitting with GaN:ZnO. Source: Maeda et al. [58]. © 2006, Springer Nature. (b) Variable light absorption in algae suspensions. Source: With permission from Kayla Rude and Professor Annaliese Franz, Department of Chemistry, University of California, Davis, USA.
Lastly, as can be inferred from Figure 1.1, the photochemical activity depends on the kinetics of charge transfer. The rate constant for a simple electron transfer reaction is described by the Butler–Volmer equation [48, 79–84]. The constant kf depends on the free energy of activation and on the thermodynamic driving force for the process, as defined by the reduction potential E of the electron donor and the standard reduction potential E0 of the electron acceptor.
This is illustrated in Figure 1.7 for charge transfer from an illuminated CdSe quantum dot to protons. The larger the bandgap and the more reducing the CdSe conduction band edge, the faster the proton reduction rate. This is an additional kinetic aspect of the general thermodynamic requirement that only those semiconductors whose conduction and valence band edges straddle the water reduction and oxidation potentials can promote the overall water splitting reaction [87].
Cocatalysts added to the light absorbers are known to speed up complex redox reactions such as the four-electron water oxidation [88] or the eight-electron CO2 reduction to methane [50]. The cocatalyst changes the mechanism for the redox reaction, thereby altering the value of the activation free energy for charge transfer. This is illustrated in Figure 1.7b for water oxidation at an illuminated BiVO4 surface with or without added Co–Pi catalyst (Co–Pi stands for cobalt phosphate). The presence of Co–Pi leads to higher photocurrents and a lower overpotential for water oxidation. Additionally, the plot demonstrates the effect of the electrochemical driving force on the photocurrent. As the applied anodic bias E becomes more positive, the rate constant for electron transfer increases exponentially, as seen in the Butler–Volmer equation.
Figure 1.7 (a) Energetics of H2 evolution from CdSe quantum dots with sulfite as a sacrificial electron donor. The conduction band edge becomes more reducing with decreasing nanocrystal size, promoting the proton reduction reaction. Source: Zhao et al. [85]. © 2013, American Chemical Society. (b) Effect of Co–Pi cocatalyst on photoelectrochemical water oxidation with BiVO4. Source: Zhong et al. [86]. © 2011, American Chemical Society.
Instead of a voltage bias, particle-based photochemical reaction systems often employ a chemical bias in the form of sacrificial reagents. Electron-donating reagents (hydrosulfide, methanol, and H2O2) make the oxidation reaction thermodynamically and kinetically more favorable because they form stable reaction products (disulfide, CO, or O2, respectively) in fast reactions. This increases the photon conversion rate of suspended photocatalysts [46, 87, 89, 90] and the anodic photocurrent from Fe2O3[91], BiVO4[92], and WO3 photoelectrodes [93]. The addition of fast electron acceptors such as AgNO3[94], NaIO4[95], or methylviologen dichloride similarly increases the cathodic photocurrent or the oxygen evolution rate of suspended BiVO4[34], Fe2O3, [96] or NiO [97] photocatalysts.
Figure 1.8 Sensitivity differences of photocatalytic and photosynthetic devices. Source: Adapted from Osterloh [23].
1.4 Differences Between Photocatalytic and Photosynthetic Reaction Systems
Under conditions of optimal light absorption coefficients, long carrier lifetimes, and high electrochemical rate constants, photocatalysts and photosynthetic systems are limited by a set of complementary parameters. These are shown schematically in Figure 1.8.
Because they do not have to suppress the thermodynamically favored backreaction, photocatalysts are generally expected to perform better if their specific surface area is increased. This is a direct result of the fact that the heterogeneous charge transfer rate is an extensive property, which scales with the contact area between the charge donor and acceptor. This is well established for electrochemical reactions in general [79, 98].
Also, there are many examples in the literature that confirm the role of the specific surface area on reaction rate. For example, it is found that the photocatalytic hydrogen evolution rate of CdS [99], g-C3N4 [14, 100], KCa2Nb3O10, and TiO2[101], in the presence of sacrificial electron donor increases with surface area. The same also applies to the photocatalytic dye degradation reaction with suspended metal oxide particles (Figure 1.9b) [102]. Under conditions of small specific surface area, the reactivity is limited by access of the reagents to the photocatalyst surface. This limitation is less applicable when the surface area is large, and other factors become limiting. However, it should be noted that dye decolorization experiments are also affected by the dye adsorption ability to the photocatalyst, competitive dye light absorption, and by the degradation mechanism, which varies from dye to dye, with solution pH, and with the photocatalyst [38, 102].
Figure 1.9 (a) Dependence of the dye degradation rate on the surface area of the photocatalyst. Source: Bae et al. [102]. © 2014, Elsevier. (b) Charge separation at the TiO2-dye–electrolyte interface. Source: Gratzel [103]. © 2005, American Chemical Society.
Similarly, it is found that the photocatalytic oxygen evolution rate for Fe2O3 [96, 104] and WO3 [105, 106] increases with surface area. As a result, the highest activities are often found for nanostructured photoelectrodes of these materials. However, there are exceptions to the surface area – activity correlation. For example, quantum size effects in CdSe particles change the electronic structure of the material as the crystal dimension is reduced [85]. This modifies the electron transfer rate constant and the light absorption coefficient, as discussed above. Often, it is also found that larger surface area causes a reduction in the electron–hole lifetimes, which will reduce the photocatalytic activity. This is usually a result of mid bandgap defects present at the surface of the photocatalyst [72].
Based on the surface area argument, the largest activity is expected for molecular photocatalysts that have the largest contact area with regard to the reagents. A good proof for this statement is iodide to tri-iodide oxidation in dye sensitized solar cells (Figure 1.9b). For such systems, the quantum efficiency is approaching 100%, i.e. nearly every photon results in oxidation of iodide and electron injection into the conductive support [107]. This remarkable behavior is due to the fact that the molecular dye is perfectly juxtaposed between the iodide/tri-iodide redox couple and the conductive support. The quantum efficiency of molecular systems drops significantly for more difficult reactions, such as water oxidation [108] or for hydrogen evolution [76, 109]. This is because for these multielectron charge transfer reactions, the electrochemical kinetics of the conversion reaction become limiting [110, 111].
Next, we turn the discussion to photosynthetic devices that promote endergonic reactions such as the water splitting reaction (Figure 1.8). In contrast to photocatalytic reaction systems, these devices must also be able to suppress the reverse of the photosynthetic reaction. Photosynthetic devices that rely on spatial separation of the half reactions accomplish this task usually with a solid–solid (buried) or solid–liquid junction [112, 113]. For suspended systems, dipoles in the lattice [114, 115] or at the surface of the absorber also play a role in charge separation [116, 117]. Charge separation can become the limiting factor of a photosynthetic device because it depends on both the effectiveness of the junction and also on the mobility of the charge carriers that are moving across it [62, 63, 118].
This limitation is the reason for the often lower photoelectrochemical performance of early transition metal oxides (such as Fe2O3) [119] when compared to main group element semiconductors (such as silicon) [120, 121]. The latter have a much larger mobility of their charge carriers. For transition metal oxides, doping with electron donors is often employed to improve the mobility of the charge carriers. This is shown in Figure 1.10a for a BiVO4 water oxidation photoanode [61]. Addition of molybdenum or tungsten ions makes the material more n-type and allows extraction of majority carriers at the back contact [123]. For high Mo concentrations, defect recombination effects from the introduction of the Mo states begin to dominate. A similar behavior is seen for Si- and Ti-doped Fe2O3 photoanodes [124, 125].
Because charge separation is added as a functional requirement, the simple correlation of activity and the specific surface area is no longer valid in photosynthetic devices, and the nanostructuring approach is no longer effective [17]. For example, in SrTiO3/Ni particles for the endergonic overall water splitting reaction, it is observed that product rates do not increase with smaller particle sizes and larger surface area. Instead, as the particles get smaller, separation of electrons and holes becomes more difficult [126]. Similarly, silicon nanowire arrays designed for hydrogen evolution from water were found to produce lower photocurrents and to require higher bias voltages than their planar silicon analogs (Figure 1.10b) [122]. The lower performance of the high surface area nanorods is likely due to decreased electron hole separation resulting from the greater junction area [62].
Figure 1.10 (a) Effect of Mo concentration on the photocurrent in BiVO4 photoanodes for water oxidation. Source: Adapted from Seabold et al. [61]. © 2014, Royal Society of Chemistry. (b) Dependence of the proton reduction photocurrent and onset voltage on the surface morphology of the Si photoelectrode. Source: Reproduced from Boettcher et al. [122]. © 2011, American Chemical Society.
A reduction in performance can also result when ion/mass transport connected to the Faradaic reaction is slow. This applies to two-compartment water splitting cells, where protons from the oxygen evolving half reaction need to reach the cathode for hydrogen evolution. Experimentally, it is found here that electrolyte cycling decreases diffusion potentials and increases the performance of the cell [32, 127]. The same consideration applies to natural photosynthesis. Under light-saturated conditions, the carbon fixation rate can be limited by either electron or mass transport in the Calvin cycle [57]. Additionally, transport of CO2 to the reaction center can become rate limiting, which limits plant growth in dense forests without air circulation [56, 128].
These transport limitations are relaxed for photosynthetic devices that operate with electrochemical selectivity instead of spatial separation of the half reactions. Reducing charge transport pathways is particularly effective for transition metal oxides because of their lower carrier mobility and because their depletion regions are often on the nanoscale [119]. Also, in suspension systems, an effective charge separation mechanism is often missing because of the symmetry of the suspended particles and the difficulty of adding junctions and cocatalysts in a spatially controlled fashion [31]. Therefore, for suspended particle systems, electrochemical selectivity for fuel-generating reactions is often achieved with cocatalysts that are added to the surface of the light absorber. For example, Maeda et al. showed in 2006 that the water splitting performance of GaN:ZnO particles could be increased by integrating the absorber with a Rh/Cr2O3 cocatalyst for hydrogen evolution [129]. This cocatalyst was able to reduce protons but prevent the reduction of oxygen, which is the first step in the backreaction to water. The selectivity relies on the ability of the Cr2O3 layer to prevent access of molecular O2 to the buried proton reduction site. Later evolutions of the catalyst use a Rh2−yCryOx composite that can be deposited onto the light absorber using an impregnation route [29, 130–134]. Similarly, in 2007, Kudo and coworkers demonstrated improved hydrogen evolution selectivity of Pt cocatalysts in the presence of [Fe(SO4)(H2O)5]+ and [Fe(OH)(H2O)5]2+ complexes [135] or in the presence of carbonates [136] and iodide [137]. These reagents are believed to alter the Pt surface toward decreased oxygen reduction. The selective charge transfer strategy works for the water splitting reaction because in the absence of a suitable catalyst, hydrogen and oxygen are metastable with regard to water formation. However, additional energy is required to separate products, which can reduce the overall activity of the system [31].
1.5 Conclusion
The above analysis reveals that photochemical reaction systems can be classified into three categories, according to their function and operational principles:
Photocatalytic devices that promote exergonic processes
Photosynthetic devices that promote endergonic (fuel forming) processes by separating the half reactions
Photosynthetic devices that promote endergonic (fuel forming) processes by using charge transfer selectivity
All of these systems rely on the generation and transfer of photochemical charge carriers. Therefore, the activity of all of them increases with the light absorption coefficient, the excited state lifetime, and the electrochemical reaction kinetics. Because photocatalytic devices do not need to suppress the backreaction, their activity scales with the specific surface area. This means that better photocatalysts can be created by increasing the surface area or by decreasing the size of the light absorber. This does not apply to photosynthetic systems with spatially separate half reactions, which under conditions of optimized light absorption, carrier lifetimes, and charge transfer are limited by charge separation and charge and mass transport. Such devices can be improved by increasing the effectiveness of the junctions across the device and by raising the charge carrier mobility and mass transfer rates of reagents and reaction intermediates. On the other hand, the activity of photosynthetic systems that rely on charge transfer selectivity may be limited by this very selectivity. Such devices can be improved through modification of the interfaces and through engineering of more selective charge transfer cocatalysts.
Acknowledgment
Funding Sources
This material is based on the work supported by the National Science Foundation for financial support (CHE 1900136) and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0015329.
The opinions are those of the author and do not necessarily reflect the views of the funding agencies.
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