Water Photo- and Electro-Catalysis E-Book

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 883

Veröffentlichungsjahr: 2024

Beschreibung

This book covers different technologies of water (photo)electrocatalysis for HER, OER or HPER, HOR and ORR, e.g., heterogenous and/or homogeneous photocatalysis, electrodes-based photoelectrocatalysis, electrocatalysis and photovoltaic-electrocatalysis. Mechanisms, materials, devices and systems of water (photo)electrocatalysis for energy conversion are intrinsically reviewed. This book also shows the principles are being used across these fields to develop technology with improved functionality and higher operating efficiency.

Leseprobe

Table of Contents

Cover

Table of Contents

Title Page

Copyright

Preface

1 Solar Energy Conversion by Dye‐sensitized Photocatalysis

1.1 Introduction

1.2 Light Absorbers

1.3 Semiconductor Materials

1.4 Dye‐sensitized Photocatalysts in Electrochemical Systems

1.5 Conclusion

References

2 Photocatalytic Hydrogen Production Over CdS‐based Photocatalysts

2.1 Introduction

2.2 Basic Principles for Semiconductor‐based Photocatalytic H

2

Production from Water

2.3 Chemical Additives for H

2

Production Enhancement

2.4 Construction of CdS‐based Heterojunction Photocatalyst to Enhance H

2

Production

2.5 Conclusions and Perspectives

References

3 Photocatalytic Hydrogen Production System

3.1 Introduction

3.2 Fundamentals of Hydrogen Production by Photocatalytic Water Splitting

3.3 Classifications of the Photocatalytic Hydrogen Production System

3.4 Example of Hydrogen Production System by Photocatalytic Water Splitting

3.5 Future Work in Terms of Challenges and Chances

Appendix: Basic Information of Meteorological Stations

References

4 Photoelectrochemical Water Splitting

4.1 Introduction

4.2 Oxide Semiconductor

4.3 Sulfide Semiconductor

4.4 Silicon and III–V Group GaAs, GaN, GaInAs/GaInP/AlInP

4.5 Nitride and Oxynitride Semiconductor

4.6 Dye‐sensitized Photocatalysts

4.7 Strategies for Improving PEC Performance

4.8 Summary

References

5 Photoelectrochemical and Photovoltaic–Electrochemical Water Splitting

5.1 Introduction

5.2 PEC Water Splitting: Theory and Working Principles

5.3 Photoanodes

5.4 Photocathodes

5.5 Tandem Devices

5.6 PV‐EC Water Splitting

5.7 Conclusion

Acknowledgments

References

6 Electrocatalytic Reduction of Carbon Dioxide

6.1 Introduction

6.2 Fundamentals of Electrocatalytic Reduction of CO

2

6.3 Electrolytes

6.4 Catalysts for Electrochemical CO

2

Reduction

6.5 Gas Diffusion Electrode for E‐CO

2

RR

6.6 Summary and Outlook

References

7 Electrocatalytic Nitrogen Reduction with Water

7.1 The Design and Regulation Strategy of Nitrogen Reduction Reaction (NRR) Catalysts

7.2 The Influence of Reaction Microenvironment

7.3 In Situ Characterization Method and Mechanism of Nitrogen Reduction

References

8 Recent Advances in Electrocatalytic Organic Transformations Coupled with H

2

Evolution

8.1 Introduction

8.2 Representative Organic Compounds for Anodic Oxidation

8.3 Representative Anodic Addition Reactions with Nucleophiles and Radicals

8.4 Oxidative Coupling Reactions Coupled with H

2

Production

8.5 Conclusions

Acknowledgments

References

9 The Advancement of Catalysts for Proton‐Exchange Membrane Fuel Cells

9.1 The Introduction of Proton‐Exchange Membrane Fuel Cells

9.2 Proton‐Exchange Membrane Fuel Cells

9.3 The Anode Hydrogen Oxidation Reaction

9.4 The Cathode Oxygen Reduction Reaction

9.5 Conclusions and Remarks

References

10 Advanced X‐ray Absorption Spectroscopy on Electrocatalysts and Photocatalysts

10.1 Introduction

10.2 Synchrotron‐based X‐ray Absorption Spectroscopy

10.3 Energy Generation Systems

10.4 Summary and Future Outlook

Acknowledgments

References

11 Advanced

Operando

/In Situ Spectroscopy Studies on Photocatalysis for Solar Water Splitting

11.1 Introduction

11.2 Basic Principles of Electromagnetic Spectrum

11.3 Pump‐Probe Principle for Spectroscopy Techniques

11.4 Basic Photophysical Processes in Photocatalysts

11.5 Photochemical and Photocatalytic Processes

11.6 Summary and Future Prospects

References

Index

End User License Agreement

List of Tables

Chapter 3

Table 3.1 Design parameters of CPC.

Table 3.2 Parameters of the reaction modules in the system.

Table 3.3 Valve operation during the reaction.

Table 3.4 Operation parameters of the photocatalyst preparation process.

Table 3.5 Simulation model validation parameters.

Table 3.6 Sunshine duration and solar radiation of each season in the system...

Table 3.7 Summary of hydrogen production under different seasons and Rows wi...

Table 3.8 Summary of hydrogen production under different seasons and Rows wi...

Table 3.9 Hydrogen production of different TIRRS in the system’s life cycle....

Table 3.10 Main inventory data for the plant construction stage.

Table 3.11 Main inventory data for the plant operation stage.

Table 3.12 Main inventory data for the plant dismantling stage.

Chapter 6

Table 6.1 Standard electrode potential of half‐reactions during CO

2

reductio...

Chapter 11

Table 11.1

Operando/in situ

spectroscopy techniques in photo(electro)‐ and e...

Table 11.2 Reaction orders of surface reaction on typical photo(electro)cata...

List of Illustrations

Chapter 1

Figure 1.1 Electron transfer processes in a dye‐sensitized photocatalysis sy...

Figure 1.2 Molecular structures of (a) [Ru(bpy)

2

(CN)

2

]

2

Ru(bpy(COO)

2

)

2

2−

...

Figure 1.3 Structures of (a) a Ru trisdiimine complex and (b) a triphenylami...

Figure 1.4 Molecular structures of the (a)

N749

, (b)

DX1

, (c)

GS11

, (d)

GS12

Figure 1.5 (a) Energy diagram of the components and the device performance o...

Figure 1.6 Molecular structures of (a) phenothiazine, (b) BODIPY, (c) the pa...

Figure 1.7 Molecular structures of (a) one of the earliest developed triphen...

Figure 1.8 Molecular structures of (a)

Dye1

and (b)

Dye2

.

Figure 1.9 Molecular structures of (a)

Calix‐3

and (b)

HO‐TPA

. (...

Figure 1.10 Schematic of the “layer‐by‐layer” assembly.

Figure 1.11 Structures of (a) S‐linker, (b) FS‐linker, (c) TP‐linker, (d) FS...

Figure 1.12 Electron transfer processes in a dye‐sensitized O

2

evolution sys...

Figure 1.13 (a) HRTEM image of an ATP/Co‐MoS

x

catalyst. (b) Scheme of the in...

Figure 1.14 Schematic of Z‐scheme water splitting using Ru dye‐sensitized Al

Figure 1.15 Molecular structures of (a)

D1

, (b)

D2

, and (c)

D3

. (d) Calculat...

Figure 1.16 Schematic of water‐splitting dye‐sensitized (a) n‐type and (b) p...

Figure 1.17 Structure of the molecular assembly of the four‐molecular layer‐...

Chapter 2

Figure 2.1 Basic principles for semiconductor‐based photocatalytic H

2

produc...

Figure 2.2 Schematic diagram of the main processes in semiconductor‐based ph...

Figure 2.3 Schematic diagram of the Schottky junction formed between semicon...

Figure 2.4 (a) TEM image of the Pt‐tipped seeded rods with a length of 27 nm...

Figure 2.5 (a) TEM and (b) HRTEM images of the prepared multi‐armed CdS nano...

Figure 2.6 (a) TEM and (b) HRTEM images of the prepared p‐CdS and Pt/CdS, re...

Figure 2.7 (a) Schematic illustration of the preparation for the Pt‐tipped C...

Figure 2.8 Schematic illustration of the effect of localized electromagnetic...

Figure 2.9 (a) Schematic illustration on the preparation process of Au–Pt–Cd...

Figure 2.10 (a) Schematic illustration on the assembly process for the CdS c...

Figure 2.11 (a) TEM and (b) HRTEM images of the Au@CdS/ZnO photocatalyst. (c...

Figure 2.12 (a) Schematic illustration of the preparation and TEM images and...

Figure 2.13 (a) Schematic diagram of the two different deposition techniques...

Figure 2.14 (a) Comparison of H

2

production rate over Pt–CdS with different ...

Figure 2.15 Comparison of photocatalytic H

2

production rates over different ...

Figure 2.16 (a) Schematic diagram of photocatalytic H

2

evolution via the hol...

Figure 2.17 Calculated free energy toward H

2

production of different co‐cata...

Figure 2.18 (a) Schematic illustration of the preparation of Cd/CdS NRs nano...

Figure 2.19 (a–b) SEM images of hierarchical 3D NiO‐CdS heteroarchitecture a...

Figure 2.20 (a) A schematic diagram of the loading process of MoO

x

clusters ...

Figure 2.21 (a) TEM image of core–shell Co(OH)

2

/CdS photocatalyst. (b) Compa...

Figure 2.22 (a) Schematic diagram of the exfoliation of MoS

2

nanocrystals by...

Figure 2.23 A schematic illustration of energy‐level comparison among CdS, S...

Figure 2.24 (a–b) TEM images of CdS@MoS‐5% composite. (c) Comparison of H

2

e...

Figure 2.25 (a) Time courses of H

2

evolution over MoS

x

/CdS samples and Pt/Cd...

Figure 2.26 A schematic structure and the charge transfer in the MnO

x

@CdS/Co...

Figure 2.27 (a) Comparison of photocatalytic H

2

evolution rates over differe...

Figure 2.28 (a) Comparison of photocatalytic H

2

evolution rates over CdS wit...

Figure 2.29 Three types of band alignment formed in the interface between di...

Figure 2.30 (a) Schematic diagram and (b) HRTEM image of the F‐TiO

2

/CdS‐DETA...

Figure 2.31 TEM images of (a) TiO

2

nanocrystals and (b) CdS nanorods and Pt ...

Figure 2.32 Schematic illustration of the mechanism for enhanced photocataly...

Figure 2.33 A schematic illustration of the wurzite and zinc‐blende CdS crys...

Figure 2.34 (a) Schematic diagram of the formation process of CdS NRPJs cons...

Figure 2.35 (a) Schematic illustrating the preparation process and plausible...

Figure 2.36 (a) HRTEM images and (b) the comparison of photocatalytic H

2

evo...

Figure 2.37 (a) TEM and (b) HRTEM images of the prepared core–shell CdS/ZnS‐...

Figure 2.38 (a) TEM and (b) HRTEM images of the prepared CdS/CdSe heterostru...

Figure 2.39 (a) TEM and (b) EDS elemental mapping images of the durian‐shape...

Figure 2.40 TEM images of the (a) CdS@TaON and (b) GO–CdS@TaON composite. (c...

Figure 2.41 (a) TEM and (b) HRTEM images of the BP−Au−CdS photocatalyst. (c)...

Chapter 3

Figure 3.1 Schematic diagram of photocatalytic hydrogen production.

Figure 3.2 Schematic diagram of the pilot‐scale photocatalytic hydrogen prod...

Figure 3.3 The overview of the photocatalytic hydrogen production system....

Figure 3.4 Physical model diagram of CPC in this system.

Figure 3.5 Experimental data: (a) solar radiation, (b) average hydrogen prod...

Figure 3.6 Hourly hydrogen production curves with sunshine duration: (a) TIR...

Figure 3.7 Life cycle assessment of the pilot plant.

Figure 3.8 System boundary for life cycle assessment of the system.

Figure 3.9 Life cycle assessment result – ADP by CML approach: (a) Construct...

Figure 3.10 Life cycle assessment result – GWP by CML approach: (a) Construc...

Figure 3.11 Life cycle assessment result – ODP by CML approach: (a) Construc...

Figure 3.12 Life cycle assessment results by CML approach: (a) AP, (b) EP, (...

Figure 3.13 Life cycle assessment result – Human health by ReCiPe Endpoint a...

Figure 3.14 Life cycle assessment result – Ecosystems by ReCiPe Endpoint app...

Figure 3.15 Life cycle assessment result – Resources by ReCiPe Endpoint appr...

Figure 3.16 Radiation data of 80 stations in China: (a) Average solar radiat...

Figure 3.17 Life cycle assessment results of 80 stations in China: (a) GWP, ...

Chapter 4

Figure 4.1 Illustrative diagram of a PEC cell for the photo‐electrolysis of ...

Figure 4.2 Schematic diagram of the possible mechanism for the formation of ...

Figure 4.3 Schematic illustration of the synthesis of MW photoanodes using P...

Figure 4.4 (a) Schematic illustration of the tandem water‐splitting device c...

Figure 4.5 (a) Current density–potential curves of planar Sb

2

Se

3

thin‐film‐b...

Figure 4.6 (a) Schematic diagram of the CdS/CdTe/TiO

2

/Ni/NiO

x

photoanode. (b...

Figure 4.7 SEM images of the CdS/CuInS

2

film: (a) top view and (b) cross‐sec...

Figure 4.8 (a) Photocurrent density–voltage curves of CuInS

2

, CuInS

2

/Pt, FeO...

Figure 4.9 (a) Current density–potential and (b) incident photon‐to‐current ...

Figure 4.10 (a) Schematic energy band diagram of B‐Si photocathodes for wate...

Figure 4.11 Linear sweep voltammetry of (a) planar GaN photoanodes and (b) G...

Figure 4.12 (a) Tri‐

s

‐triazine‐based structures of g‐C

3

N

4

Schematic illu...

Figure 4.13 (a) The crystal structure of Ta

3

N

5

(b) Ta

3

N

5

nanorod single ...

Figure 4.14 (a) Multiband photoelectrodes formed by InGaN and GaN. (b) S...

Figure 4.15 Schematic illustration of (a) electrochemical cell composed of p...

Figure 4.16 (a) The hierarchical nanorod bilayer structure possesses light‐t...

Figure 4.17 (a) Schematic diagrams illustrating the Co‐Pi/Co

3

O

4

/Ti:Fe

2

O

3

pho...

Figure 4.18 (a) UV–vis absorption spectra of the red OV

H

‐TiO

2

referring to t...

Figure 4.19 (a) Scheme of an unassisted water‐splitting tandem cell with the...

Figure 4.20 Schematic illustration showing the degradation mechanisms of the...

Figure 4.21 (a) Schematic illustration of Co(CO

3

)

x

OH

y

/BiVO

4

in KBi, F:Co(CO

3

Chapter 5

Figure 5.1 A schematic representation of a PEC cell used for water splitting...

Figure 5.2 (a) Conduction band (CB) and valence band (VB) positions for impo...

Figure 5.3 Doping strategies used in TiO

2

photoanodes for enhancing photocur...

Figure 5.4 (a) Schematic of the Si MIS photoanode. (b) J–V curves of the Si ...

Figure 5.5 Schematic representation of three different electrodeposition rou...

Figure 5.6 Removal of surface defects and improving photovoltage of hematite...

Figure 5.7 (a)

J

–

V

response under simulated one‐sun air mass 1.5G chopped il...

Figure 5.8 (a) Schematic of the Si I‐MIS photocathode. (b)

J

–

V

curves, (c) c...

Figure 5.9 (a) Cross‐sectional SEM image of TiO

2

/CdS/Sb

2

Se

3

on Au/FTO substr...

Figure 5.10 Unassisted all‐oxide solar water splitting. (a) Illustration of ...

Figure 5.11 (a) Schematic of the experimental setup and electrode geometry o...

Figure 5.12 (a) The PV‐electrolysis system consisting of a triple‐junction s...

Figure 5.13 (a) Schematic diagram of the dual perovskite NiFe electrode tand...

Figure 5.14 (a) Simplified schematic energy diagram of the integrated perovs...

Figure 5.15 Schematic of PV‐driven electrochemical water splitting system in...

Figure 5.16 (a) The

J

–

V

curves of the perovskite–organic tandem solar cell u...

Figure 5.17 Schematic diagram of PV‐electrolysis being conducted with the DS...

Chapter 6

Figure 6.1 A schematic of E‐CO

2

RR.

Figure 6.2 The possible reaction pathways of E‐CO

2

RR from CO

2

to (a) C

1

, (b)...

Figure 6.3 Comparison of electrolyte classification employed for E‐CO

2

RR....

Figure 6.4 Schematic illustration of the capture of CO

2

molecules and format...

Figure 6.5 Main product category of metal catalysts for E‐CO

2

RR shown in a c...

Figure 6.6 (a) Structural models of nitrogen atoms with different chemical e...

Figure 6.7 Linear sweep voltammetry curves (a) and OH adsorption activities ...

Figure 6.8 (a) FT‐EXAFS fitting line of Cu–N

2

/GN. (b) FE

CO

of Cu–N

4

/GN‐700, ...

Figure 6.9 (A) Linear scan voltammetry curves in the CO

2

‐saturated 0.25 M KH...

Figure 6.10 (a) Schematic illustration of proposed one‐pot tandem catalytic ...

Figure 6.11 Schematic illustration of the construction of a carbon‐based GDE...

Figure 6.12 Schematic diagram of the carbon‐based GDE for E‐CO

2

RR.

Figure 6.13 (a1–a3) Schematic illustration of different methods of the prepa...

Figure 6.14 Schematic illustration of flow cell.

Figure 6.15 Illustration of the ion transport by different membranes of AEM,...

Figure 6.16 Schematic illustration of membrane electrode assembly cell.

Chapter 7

Figure 7.1 (a) Schematic of the atomic orbital of BC

3

for binding N

2

. (b) Fr...

Figure 7.2 (a) Atomistic structure scheme showing the reaction pathway of th...

Figure 7.3 (a) N

2

adsorption geometry on PCN with nitrogen vacancy. (b, c) T...

Figure 7.4 Calculated free‐energy diagrams of NRR and HER on (a) Pd/NC, (b) ...

Figure 7.5 Structural similarities of Fe−S−Mo bonding. Ground‐state structur...

Figure 7.6 (a) Schematic view of the three‐phase reactor for electrochemical...

Figure 7.7 (a) The free‐energy change (Δ

G

*NNH

) required to form *NNH. (b) Δ

G

Figure 7.8 (a) Illustration of the formation of hydronium ion, and the hydro...

Figure 7.9 (a) The schematic local catalytic micro‐environments of the enzym...

Figure 7.10 Schematic diagram illustrating the nitrogen reduction pathway on...

Figure 7.11 (a) FTIR spectra during the first segment from 0.4 to −0.4 V on ...

Figure 7.12 Online DEMS investigation. (a) Schematic illustration of the DEM...

Figure 7.13 In situ EC‐STM images over the Cu (100) surface in 0.1 M HF with...

Figure 7.14 In situ XAS and Raman measurements under electrolysis. (a and b)...

Figure 7.15 Schematic of an H‐type cell. It shows all the details we emphasi...

Figure 7.16 Flow diagram of recommended protocols to rigorously conduct NRR ...

Figure 7.17 Identification and elimination of NO

x

contaminations. (a) False‐...

Figure 7.18 Rigorous protocol for performing ENR. It is a concoction of expe...

Chapter 8

Figure 8.1 Electrochemical oxidative alkoxysulfonylation of alkenes and its ...

Figure 8.2 (a) Proposed mechanism of the anodic alcohol oxidation catalyzed ...

Figure 8.3 Proposed mechanistic steps for the anodic triethylamine oxidation...

Figure 8.4 Anodic decarboxylative trifluoromethylation of α, β‐unsaturated c...

Figure 8.5 Proposed mechanism for the C–C cross‐coupling between phenols and...

Figure 8.6 Kolbe electrolysis for the cyclization of 2‐arylbenzoic Acid.

Figure 8.7 Proposed mechanism for the oxidative phosphonylation of C(sp

2

)–H ...

Chapter 9

Figure 9.1 The working mechanism of typical types of fuel cells.

Figure 9.2 (a) the schematic shows the structure of a typical PEMFC, and (b)...

Figure 9.3 (a) STEM‐EDS mapping of IrRu@ZIF‐8; (b) HAADF‐STEM image of IrRu–...

Figure 9.4 the volcano plot of TMs for ORR activity related to the calculate...

Figure 9.5 The simulation of expected (a) specific activity and (b) mass act...

Figure 9.6 (a) Comparison of limiting kinetic currents; and (b) the catalyti...

Figure 9.7 (a) The schematic graph shows the procedure to fabricate Ir‐SAC; ...

Figure 9.8 The volcano plots of the catalytic properties and d‐band center, ...

Figure 9.9 (a) Pt

20

Pd

n

Cu

80−

n

alloy NPs triangle plots in terms of Pt, ...

Figure 9.10 (a) HRTEM images of Pt

72

Cu

28

nanowires [83]. (b) STEM‐EDS mappin...

Figure 9.11 Representative (a) HAADF‐STEM image, (b) TEM image, (c) TEM‐EDX,...

Figure 9.12 (a) HAADF‐STEM image of an individual nanocage. (b) STEM‐EDS...

Figure 9.13 (A) Atomic‐resolution ADF‐STEM image of Pt

3

Co/C‐700 after Richar...

Figure 9.14 The

E

1/2

of various M–N–C catalysts evaluated in acidic electrol...

Figure 9.15 (a) The scheme shows the synthesis strategy for FeN

4

/HOPC‐C‐1000...

Figure 9.16 (a) Schematic for synthesis of TPI@Z8(SiO

2

)‐650‐C, (b) TEM image...

Chapter 10

Figure 10.1 (a–j) Synchrotron X‐ray spectroscopies associated with various p...

Figure 10.2 (a) Electrochemical cell assembly for

in situ

XAS; (a) Si

3

N

4

win...

Figure 10.3 (a) Gas cell for the study of water vapor–metal surface.(b–d...

Figure 10.4 (a, b)

In situ

gas cell with a high transmission rate for studyi...

Figure 10.5 (a) Fe K‐edge, (b) Co K‐edge, and (c) Ni K‐edge XANES spectra; F...

Figure 10.6 (a) Fe K‐edge XANES for LDHs, W‐LDHs, and CoFePi. Inset displays...

Figure 10.7 (a) FT‐EXAFS (

k

2

‐weighted) of Pt foil, PtO

2

, and Mo

2

TiC

2

T

x

–Pt

SA

....

Figure 10.8 (a) C K‐edge and N K‐edge XANES spectra of CNN, BDCNN, and self‐...

Figure 10.9 (a) C K‐edge XANES and (b) N K‐edge XANES spectra for BCN, UCN, ...

Figure 10.10 XAS at (a) C K‐edge and (b) N K‐edge for g‐C

3

N

4

, g‐C

3

N

4

/Ag, and...

Figure 10.11 (a)Ni K‐edge XANES spectra of PCNNi‐3 and PCN. Inset in (a) sho...

Figure 10.12 XAS spectra of (a) Fe L‐edge, (b) Ti L‐edge, and (c), (d) O K‐e...

Figure 10.13 (a–d) O K‐edge STXM images and optical density images. (e–h) Po...

Chapter 11

Figure 11.1 Electromagnetic spectrum.

Figure 11.2 Schematical illustration of the principle of pump‐probe techniqu...

Figure 11.3 The Jablonski diagram, a schematic of the transition of electron...

Figure 11.4 Timescale of charge/carrier and lattice processes in photo‐/elec...

Figure 11.5 (a) Schematic diagram of an optical pump terahertz probe spectro...

Figure 11.6 (a) Schematic of fs‐TAM.(b) Experimental and simulated fs‐TA...

Figure 11.7 (a) Schematic of a TAS apparatus. (b) A typical transient absorp...

Figure 11.8 Signal assignment of photoelectron and photohole. (a) Transient ...

Figure 11.9 (a) The comparison of time‐resolved optical and mid‐infrared tra...

Figure 11.10 Photoinduced absorption spectroscopy setup.

Figure 11.11 (a) Light‐intensity‐dependent PIA spectra and (b) photocurrent ...

Figure 11.12 Photoinduced absorption kinetics in Rh:SrTiO

3

(a) and La,Rh:SrT...

Figure 11.13 Schematic of SECA spectroscopy equipment.

Figure 11.14 (a) Oxidative charging of CoCat at pH 7 followed by freeze‐quen...

Figure 11.15 (a) Electrode potential and absorption at 600 nm for a CoCat el...

Figure 11.16 Schematic diagram of a PL spectroscopy equipment.

Figure 11.17 Two‐dimensional time‐resolved PL spectra and decay‐associated s...

Figure 11.18 (a) Steady‐state PL spectra (top panel) and TRPL decay curves f...

Figure 11.19 The principle of Michelson interferometer used in FTIR spectrom...

Figure 11.20 (a) Hydroxyl radical‐coupling mechanism on Ga

2

O

3

‐based photocat...

Figure 11.21 Ultrafast IR detection of intermediate. (a) SrTiO

3

single cryst...

Figure 11.22 (a) Raman scattering effect. (b) An

operando/in situ

electroche...

Figure 11.23

Operando

‐Raman spectra of birnessite (a) and birnessite/bixbyit...

Figure 11.24 (a) A typical X‐ray absorption spectrum of Pt foil at the Pt L

3

Figure 11.25 Structural characterizations of mononuclear manganese. (a) Chem...

Figure 11.26 (a) Steady‐state

operando

O K‐edge XAS of IrO

x

at applied poten...

Figure 11.27 (a)

Operando

electrochemical APXPS: Ir 4f XPS spectra of iridiu...

Guide

Cover

Table of Contents

Title Page

Copyright

Preface

Begin Reading

Index

End User License Agreement

Pages

iii

iv

xiii

xiv

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

270

271

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308

309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346

347

348

349

350

351

352

353

354

355

356

357

358

359

360

361

363

364

365

366

367

368

369

370

371

372

373

374

375

376

377

378

379

380

381

382

383

384

385

386

387

388

389

390

391

392

393

394

395

397

398

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436

437

438

439

440

441

442

443

444

445

446

447

449

450

451

452

453

454

455

456

457

458

459

Water Photo- and Electro-Catalysis

Mechanisms, Materials, Devices, and Systems

Edited by Shaohua Shen and Shuangyin Wang

Editors

Prof. Shaohua ShenXi’an Jiaotong UniversityNo. 28 Xianning West StreetXi’an 710049China

Prof. Shuangyin WangHunan UniversityNo. 2 Lushan RoadChangsha 410082China

Cover Image: © Andriy Onufriyenko/Getty Images

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

British Library Cataloguing‐in‐Publication DataA catalogue record for this book is available from the British Library.

Bibliographic information published by the Deutsche NationalbibliothekThe Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.

© 2024 WILEY‐VCH GmbH, Boschstraße 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: 978‐3‐527‐34835‐0ePDF ISBN: 978‐3‐527‐83099‐2ePub ISBN: 978‐3‐527‐83101‐2oBook ISBN: 978‐3‐527‐83100‐5

Preface

I believe that water will one day be employed as a fuel, that hydrogen and oxygen that constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable.

—Jules Verne

Water, consisting of hydrogen and oxygen, could act as a sustainable reactant or product to participate in various chemical reactions along with clean energy conversion. As driven by solar energy or thus‐generated electricity, water photo‐ and electro‐catalysis for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been considered as an effective approach to green hydrogen production. In reverse electrocatalysis processes, hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) inevitably happen in a fuel cell, which has been considered as a promising technology for green electricity generation. It should be further noted that, by acting as the reactant for electrocatalysis, water could drive the reactions of CO2 (CO2RR) and N2 (NRR) reduction to produce value‐added chemicals and fuels (e.g. hydrocarbons and NH3), and even realize organic transformations coupled with H2 production. Thus, water photo‐ and electro‐catalysis is becoming an increasingly important field while being considered as an important way to solve the global problems of energy shortage and environmental pollution, and is indeed the technology of the twenty‐first century by significantly impacting on human activity,

This book aims to bring together the latest developments in water photo‐ and electro‐catalysis with a focus on its impact on the energy agenda, which splits broadly into different technologies including photocatalysis, photoelectrocatalysis, electrocatalysis, and photovoltaic–electrocatalysis for HER, OER, HOR, ORR, CO2RR, NRR, and organic transformations. This book shows how the underlying principles are being used across these fields to develop technology with improved functionality and high operating efficiency in terms of water‐involved energy conversion reactions. It consists of 11 chapters and introduces different technologies of water photo‐ and electro‐catalysis for energy conversion. The basic mechanisms, emerging materials, devices, and systems of water photo‐ and electro‐catalysis for energy conversion are intrinsically linked, by covering: (i) the fundamentals, materials, and systems of semiconductor‐based photocatalytic water splitting; (ii) the mechanisms, photoelectrode/catalyst materials, and devices of photoelectrocatalytic and photovoltaic‐electrocatalytic water splitting; (iii) the fundamentals, materials, and devices for water electrocatalysis, including HER, OER, HOR, and ORR; (iv) the materials and devices for electrocatalytic CO2RR, NRR, and organic transformations; (v) the advanced characterizations on water photo‐ and electro‐catalysis.

In the past years, exciting breakthroughs in materials and nanotechnology have stimulated a huge amount of renewed interest in this field, although some critical issues of materials, like poor sunlight absorption, poor electric conductivity, and retarded water electrocatalysis kinetics, have thwarted many earlier efforts to reach the “Holy Grail” of photo‐ and electro‐catalysis for energy conversion with economic and ecological sustainability. While we cannot even hope to approach completeness of such technologies in a single book, we nevertheless hope that both scientists and engineers, experts and newcomers in this field find some useful information and details here those can help their academic research and industrial development.

We would like to appreciate the efforts from all the contributors and also the financial support from the National Natural Science Foundation of China (52225606, 51888103).

3 August 2023Xi’an, China

Shaohua Shen, PhDXi’an Jiaotong University, China

Shuangyin Wang, PhDHunan University, China

1Solar Energy Conversion by Dye‐sensitized Photocatalysis

Shunta Nishioka and Kazuhiko Maeda

School of Science, Tokyo Institute of Technology, Department of Chemistry, Tokyo 152-8550, Japan

1.1 Introduction

Dye sensitization enables the generation of charge carriers in a wide‐bandgap semiconductor under irradiation by visible light that cannot be absorbed by the semiconductor. Dye‐sensitized photocatalysis (DSP) was first proposed by Gerischer in 1972 [1] and was later demonstrated by Grätzel et al. [2]. Because of its potential applications in solar energy conversion, DSP has been studied for decades, especially for H2 evolution via water splitting [3–5]. The DSP H2‐evolution system consists of two building blocks—a light absorber and a semiconductor material (Figure 1.1)—and the H2 evolution reaction proceeds as follows: First, the photosensitizer absorbs light and is excited (1). The excited dye injects an electron into a semiconductor (2). The injected electron is consumed via a proton‐reduction reaction on the semiconductor’s surface, and H2 is evolved (3). The oxidized light absorber generated by the electron injection returns to the ground state by accepting an electron from a reductant (4). Unfortunately, some undesirable reactions can occur during this reaction scheme (5–7). To improve the overall efficiency of this system, researchers have devoted extensive effort to promoting the forward reactions and impeding the backward reactions.

In this chapter, we present the strategies for improving DSP systems by separating each building block, pointing out the important factors that influence the DSP performance. We survey recent achievements in the DSP field, especially those related to water‐splitting systems, including electrochemical systems, and discuss how various factors can be controlled to improve the performance of dye‐sensitized systems.

1.2 Light Absorbers

The development of photosensitizers has been rapidly promoted with the growth of the dye‐sensitized solar cell (DSSC) field, which has been pioneered by Tsubomura and coworkers [6] and by O’Regan and Grätzel [7]. The practical application of dye‐sensitized photovoltaic cells became realistic with the development of a trinuclear Ru complex that possesses two cyano‐bridges and four carboxyl‐anchoring groups (Figure 1.2a) [8]. A substantial achievement in the DSSC field is the exploitation of the N3 dye (Figure 1.2b); the electron‐injection quantum yield (QY) from N3 into TiO2 has reached almost unity, with a solar‐to‐electric conversion efficiency of 10% under AM1.5G illumination [9]. The structure of N3 is very similar to the anchoring unit of the trinuclear dye in that N3 has four carboxyl‐anchoring moieties and two isothiocyanato ligands. The next excellent dye developed was N719 (Figure 1.2c), which gave a power conversion efficiency greater than 9.18% under AM1.5G. This value is still high even now, although the original work was published 20 years ago [10, 11]. These highly efficient dyes for DSSCs, however, have not been well utilized in DSP systems because of the difference in the DSSC and DSP catalytic cycles. In the case of DSSCs, the injected electrons in a semiconductor (e.g. TiO2) should migrate quickly to reach the counter electrode through an external circuit. Because of the rapid collection of the injected electrons, acceleration of the electron‐injection process by strong coupling between the dye and the semiconductor is effective for DSSC systems. In the case of DSP, however, a surface catalytic reaction involving multi‐electron transfer (e.g. proton reduction) would become the rate‐determining step, whose time scale is at least four orders of magnitude slower than that of the excited‐charge‐carrier transfer processes [5]. This is distinct from DSSCs, which are operated by single electron transfer processes. Accelerating the electron injection leads to an increase in the standby electrons in the conduction band for the catalytic reaction, which is not effective for DSP systems. On the contrary, the strong coupling may promote undesirable back electron transfer from the conductive substrate to the oxidized dye [12]. Therefore, the dyes developed for DSSCs need to be modified for use in DSP systems. In this section, new dyes developed for efficient DSSCs are introduced and then dye sensitizers optimized for DSP are discussed.

Figure 1.1 Electron transfer processes in a dye‐sensitized photocatalysis system. C.B., conduction band; V.B., valence band; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; D, electron donor; D+: oxidized electron donor; A, electron acceptor; A−, reduced electron acceptor. Solid and broken arrows represent forward and backward electron transfers, respectively.

Before moving to the details, we here explain the basic molecular design of photosensitizers. One of the most studied classes of dyes is metal complexes, which have been used in pioneering research in DSP systems [2, 3, 5] and DSSCs [4, 7–11]. Recent advances in the DSP for H2 evolution have included the development of Ru [13], Zn [14], and Ir [15] complexes, and these complexes are still mainstream materials used in the DSP field because their photo‐ and physicochemical properties are chemically controllable. Visible‐light absorption by metal complexes used in dye‐sensitized systems originates mainly from metal‐to‐ligand charge transfer (MLCT). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are distributed around the metal center and the ligands, respectively. There are three important properties for an effective sensitizer: (i) wide and strong absorption in the visible region, (ii) an efficient excited‐charge‐carrier transfer cycle, and (iii) high stability. Shifting the HOMO and LUMO levels varies the light‐absorption properties of dyes. Efficient excited‐charge‐carrier transfer is achieved through vectorial electron transfer. Increased stability is attained via strong adsorption onto a semiconductor.

Figure 1.2 Molecular structures of (a) [Ru(bpy)2(CN)2]2Ru(bpy(COO)2)22− (bpy = 2,2′‐bipyridine), (b) N3, and (c) N719. TBA: tetrabutylammonium.

All three of the aforementioned important properties for designed dye molecules can be achieved through adaptation of the ligands of the metal complex. A tris(bipyridine)ruthenium(II) derivative is a good example for explaining the molecular design of metal complexes (Figure 1.3a). For a dye to adsorb onto a semiconductor, at least one of the bipyridine ligands should be functionalized with an anchoring moiety, which should be located at the position closest to the semiconductor substrate. In such a case, the LUMO of the complex is distributed at the functionalized bipyridine ligand, which enhances electron injection from the excited‐state complex into the semiconductor. Therefore, if the other peripheral ligand(s) possess a higher energy than the anchoring ligand, the excited electrons would gather on the anchoring ligand, thereby accelerating the electron injection. In addition, because the electron density of the metal center should be increased to ensure wide visible‐light absorption, the peripheral ligand(s) should demonstrate electron‐donating behavior. Triphenylamine, which exhibits strong electron‐donating ability, was introduced onto a Ru complex as a secondary electron donor unit [16]. Although a triphenylamine‐based dye had been studied previously [17], excellent dyes that possess a triphenylamine moiety as an electron donor were developed in the same period [18–20]. The triphenylamine‐based dyes have a donor–π–acceptor (D–π–A) conjugated structure, which consists of carbon–carbon double‐bonded π‐bridges and a cyano electron‐acceptor group (Figure 1.3b). The D–π–A structure enables vectorial charge transfer, which is one of the aforementioned important characteristics of dye‐sensitized systems. One of the triphenylamine‐based dyes gave an overall DSSC efficiency of 5.3%, which is similar to the efficiency of N719 (7.7%). This advancement triggered the rapid development of organic dyes, which are now also used for DSP H2 evolution. DSP for H2 evolution has been studied using triphenylamine [21, 22], an organoboron complex [23], coumarin [24], perylene [25], calixarene [26], and tetrathiafulvalene [27], as building blocks for photosensitizers. During the development of these photosensitizers, numerous factors for improving dye‐sensitized systems have been revealed. We here discuss these factors, along with some examples of dyes demonstrating the effect of each factor.

Figure 1.3 Structures of (a) a Ru trisdiimine complex and (b) a triphenylamine‐based organic sensitizer. The bottom of figure (a) shows a schematic of the electron‐transfer processes of a Ru complex–semiconductor hybrid material. LUMO+: an unoccupied molecular orbital with an energy level higher than that of the LUMO.

1.2.1 Extending the Light Absorption Spectra of Dyes

Extending the absorption spectrum of a dye is a common approach to efficiently utilizing sunlight for solar energy conversion. The strategy for extending the absorption of a dye appears simple: shift the LUMO downward and/or the HOMO upward. The reality, however, is not straightforward because two requirements must be met [28]. First, an excited‐state dye must inject an electron into the semiconductor. Second, a sensitizer needs to oxidize a reductant or electron mediator to regenerate the ground state. Extensive efforts have been devoted to developing new photosensitizers that enable the more efficient utilization of solar energy while still achieving these two requirements.

Figure 1.4 Molecular structures of the (a) N749, (b) DX1, (c) GS11, (d) GS12, and (e) GS13 dyes.

Grätzel’s group synthesized a panchromatic dye (black dye), N749 (Figure 1.4a), using a tridentate terpyridine derivative ligand and three thiocyanato ligands [28]. The development of such a panchromatic dye is the ideal approach under the conventional strategy. The absorption spectrum of the sensitizer was extended by the introduction of three thiocyanato ligands that shift the ruthenium(II) t2g orbitals upward. The LUMO level was kept at a more negative potential than the conduction band of TiO2, and the HOMO was located at a sufficiently positive potential relative to the redox potential of the reductant (i.e. iodide). Recently, Segawa and coworkers developed a phosphine‐coordinated Ru‐complex sensitizer, DX1 (Figure 1.4b) [29]. Their approach to extending the absorption of the dye is unconventional in that it involves a spin‐forbidden transition. Figure 1.5a shows the absorption and emission spectra as functions of the energy diagrams of DX1, along with the spectrum of a black dye. In the case of DX1, the singlet–triplet transition is emphasized clearly, and this extension of the light‐absorption spectrum improved the power conversion efficiency in a DSSC system.

Figure 1.5 (a) Energy diagram of the components and the device performance of the sensitizers. Absorption (solid line) and emission (dashed line) spectra vs. the energy diagrams of BD (black dye, N749, left) and DX1 (right). In the singlet‐to‐singlet transition, the electron transition from the S0 to the T1 excited states causes energy loss via spin‐exchange energy (top).

Source: Reproduced with permission from Kinoshita et al. [29]; © 2013, Springer Nature.

(b) Schematic energy levels of GS11, GS12, GS13, and DX1.

Source: Adapted with permission from Swetha et al. [13]; © 2015, American Chemical Society.

DX1 and its derivatives have been investigated in a DSP for H2 evolution [13]. Four phosphine‐coordinated Ru complexes (Figure 1.4b–e) were synthesized; their energy diagrams are shown in Figure 1.5b. In all cases, the LUMO level was sufficiently negative for excited‐electron injection into the conduction band of TiO2. These four panchromatic photosensitizers were applied to the photocatalytic H2‐evolution reaction on Pt‐modified TiO2, where triethanolamine (TEOA) was used as a sacrificial electron donor. GS12 showed the highest H2‐evolution activity, and the apparent quantum yield (AQY) for H2 evolution under irradiation by a 400 W Hg lamp reached 5.16%. This value is 5.5 times greater than that of N719 under the same conditions. The activity increased in the order GS12 > GS11 > DX1 > GS13, and this trend obviously reflects the LUMO energy level. These results indicate that the DSP performance for the H2‐evolution reaction can be enhanced by negatively shifting the oxidation potential of the dye in the excited state. The use of a panchromatic dye in a dye‐sensitized H2‐evolution system improved the photocatalytic activity, and an investigation of the DX1 derivatives revealed that regulation of the LUMO energy level is an important consideration in the molecular design of sensitizers.

1.2.2 Enhancement of the Absorption Coefficient of Dyes

Increasing the molar extinction coefficient of dyes is a straightforward method of increasing the solar energy conversion efficiency in a dye‐sensitization system. Making the π‐chromophore more rigid is a common tactic to increase the molar extinction coefficient. Rigid molecules can suppress the rotational disorder and enhance the delocalization capacity of π‐electrons [30]. At the same time, however, increasing the rigidity of molecules promotes aggregation, and undesirable aggregation often adversely affects the energy conversion performance via, e.g. competitive nonradiative quenching and a hypsochromic shift of the dye [31]. As an alternative approach to enhancing the molar extinction coefficient, Ning et al. proposed incorporating an additional electron donor unit into the dye to form a starburst 2D–π–A conjugate [32]. This approach is promising for increasing light‐harvesting performance; however, the number of reported starburst 2D–π–A structures is limited because of their complicated synthetic pathways. To address this problem, organoboron complexes have been investigated as relatively small and simple π‐chromophore units.

The approach of incorporating organoboron complexes has been applied to a phenothiazine‐based dye [30]. Phenothiazine (Figure 1.6a) is one of the most extensively studied electron‐donor components and exhibits strong electron‐donating character because of its heterocyclic structure containing S and N [33]. Because of its strong electron‐donating ability, an early phenothiazine‐based dye achieved a solar‐energy‐to‐electricity conversion efficiency comparable to that of N3 dye [34]. Further improving the efficiency of the phenothiazine‐based dye system is difficult because of its low molar extinction coefficient. Given this background, the incorporation of organoboron complexes into phenothiazine‐based dyes is attractive. In particular, 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY, Figure 1.6b) dyes exhibit high molar absorptivity and sharp fluorescence peaks, along with a high QY [35]. A series of BODIPY derivatives, pyridomethene–BF2 complexes (Figure 1.6c), exhibit a very large extinction coefficient (5 × 104 ≤ ε ≤ 1.4 × 105 M−1 cm−1). Among the emission quantum efficiencies of the derivatives, the highest was similar to that of N719[30]. To further improve BODIPY‐sensitized systems, Erten‐Ela et al. developed a dibenzo‐BODIPY dye (Figure 1.6d) using a conventional strategy, with the objective of extending the π‐conjugation and enabling longer‐wavelength absorption [36]. The dibenzo‐BODIPY was combined with phenothiazine (Figure 1.6e), and the absorption reached the near‐infrared (NIR) region. The efficiency of BODIPY‐sensitized solar cell systems eventually overtook that of N719 systems [36]. The dibenzo‐BODIPY and phenothiazine combined dye has been applied to the H2‐evolution reaction on Pt‐modified hierarchical porous TiO2, where ascorbic acid (AA) was used as a sacrificial electron donor [37]. The turnover number of the dye for H2 evolution reached 11,100 under visible‐light irradiation (λ > 400 nm) at an intensity of 100 mW cm−2. The organoboron‐phenothiazine dye improved the visible‐light absorption performance and demonstrated high H2 evolution activity in metal‐free organic dyes. That is, a binary electron donor system comprising a small and simple π‐chromophore unit incorporated into an electron‐donor building block functioned well as a dye‐sensitized photocatalyst system.

Figure 1.6 Molecular structures of (a) phenothiazine, (b) BODIPY, (c) the parent pyridomethene–BF2 complex, (d) dibenzo‐BODIPY dye, and (e) a phenothiazine dye with dibenzo‐BODIPY incorporated.

1.2.3 Molecular Design for Efficient Excited‐Charge‐Carrier Separation and Injection

Efficient charge‐carrier separation and injection into a semiconductor strongly affects the efficiency of a dye‐sensitized system. As previously described, the visible‐light absorption of dyes relies predominantly on a charge‐transfer (CT) transition between donor/acceptor units. The donor unit should be physically separated from the semiconductor to suppress back electron transfer from the semiconductor and to facilitate the reaction with a reductant in the reaction solution. The acceptor unit should be physically close to the semiconductor to enable the immediate transfer of an excited electron into the semiconductor. To improve the energy conversion efficiency through the molecular design of dyes, tracing the history of the development of sensitizer molecules and understanding the roles of the building blocks of dye molecules would be beneficial. Here, a triphenylamine‐based dye, which is one of the most studied organic photosensitizers, is used as an example and the history of the development of dyes is described.

Triphenylamine has been studied extensively as an electron‐donor unit since the early 2000s [20]. In the earliest study of triphenylamine as a donor unit, dyes consisting of carboxylic and cyano moieties as electron‐donor and ‐acceptor groups, respectively, were developed (Figure 1.7a) [18]. Because the carbon–carbon double‐bonded π‐bridge in these dyes is problematic in terms of the synthesis and stability of the molecule, a thiophene moiety (Figure 1.7b) was incorporated as a new π‐conjugation unit to address these problems in a coumarin dye (Figure 1.7c) [38]. Combining these moieties, Hadberg et al. developed one of the simplest‐structured triphenylamine‐based dyes, named D5 dye (Figure 1.7d), which gave an energy conversion efficiency comparable to that of N719[39]. The para‐position of the triphenylamine unit was subsequently substituted to increase the electron‐donating character (Figure 1.7e) [40]. Indeno[1,2‐b]thiophene (Figure 1.7f,g) [41] and cyclopentadithiophene (Figure 1.7h,i) [42] units were used to expand the π‐conjugation system. A 2,3‐diphenylquinoxaline unit (Figure 1.7j,k) [43] was introduced not only as an acceptor unit but also as a building block to limit intermolecular aggregation. The use of these good building blocks (electron donor, electron acceptor, and π‐conjugation units) led to the development of a triphenylamine‐based D–π–A‐structured dye, LEG4 (Figure 1.7l), and its derivatives [44].

Figure 1.7 Molecular structures of (a) one of the earliest developed triphenylamine‐based dyes, (b) the thiophene moiety, (c) a thiophene‐bridged coumarin dye (NKX‐2677), (d) dye D5, (e) dye D35, (f) the indeno[1,2‐b]thiophene moiety, (g) an indeno[1,2‐b]thiophene‐bridged dye (JK‐225), (h) the cyclopentadithiophene moiety, (i) a cyclopentadithiophene‐bridged dye (C218), (j) a 2,3‐diphenylquinoxaline unit, (k) a 2,3‐diphenylquinoxaline‐incorporated dye (IQ4), (l) LEG4, (m) WS‐2, (n) LS‐1, (o) S5, (p) SD1, (q) SD2, and (r) SD3.

Further improvement of D–π–A‐structured dyes was realized through a new molecular design: the donor–acceptor–π–acceptor (D–A–π–A) structure [45]. To explain the effects of electron–acceptor insertion between the electron‐donor and π‐bridge, we here discuss two dyes (WS‐2 and LS‐1, Figure 1.7m,n, respectively) as examples. They possess an indoline‐based electron donor, a thienyl π‐linker, and a cyanoacrylic acid electron acceptor. The D–A–π–A dye WS‐2 has benzothiadiazole (BTD) as an additional electron acceptor. The incorporation of an electron‐acceptor unit between the electron‐donor and π‐conjugation units influences the electronic state of the molecule, as confirmed by a density functional theory simulation of the BTD‐inserted D–A–π–A dye WS‐2. The HOMO and LUMO distributions both clearly overlap the orbitals of the BTD unit, which is beneficial for the electron transition. Comparing these two sensitizers reveals four advantages of the WS‐2 dye: (i) strong light‐harvesting ability because of the bathochromic shift of the absorption associated with CT, (ii) the appearance of an additional absorption band attributed to a secondary frontier orbital transition, (iii) suppression of the hypsochromic shift that accompanies adsorption onto a TiO2 film, and (iv) improvement of the stability for light absorption because of the decrease in the LUMO level. The substantial improvement of the light‐absorption capability as a result of the formation of the D–A–π–A structure contributed to a high power conversion efficiency of 9.04% [46].

This new strategy led to the development of a derivative of the LEG4 dye (S5, Figure 1.7o) [47], along with dyes with a different π‐bridge, SD1–3 (Figure 1.7p–r) [21]. The SD dye series was used in both DSSC and H2‐evolution DSP systems. Interestingly, the performance of the dyes was varied dramatically in the different systems. In the DSSC system, in which [Co(bpy)3]3+/2+ was used as an electron mediator, SD1 exhibited the highest power conversion efficiency and the order of efficiency was SD1 > SD3 > SD2. These results are attributed to SD1 reducing charge recombination as a result of its large torsional angle and large driving force for regenerating the dye ground state. In the DSP system, by contrast, SD2 demonstrated the highest H2‐evolution activity on Pt/TiO2, where AA was used as an electron donor. Spectroscopic and photoelectrochemical studies revealed that the fastest electron injection into Pt/TiO2 and the lowest charge‐carrier transfer resistance were achieved in the SD2 system. The highest performance of SD2 toward H2 evolution is likely a consequence of its good electron transfer and electron–hole separation process. SD1, which exhibited the highest efficiency in the DSSC system, exhibited the lowest activity toward the H2‐evolution photocatalytic reaction. This different tendency is attributed to differences in the reaction conditions. The electrons injected into TiO2 migrate to the electrode under an electrical bias in a DSSC system and are consumed by the surface H2‐evolution reaction in a DSP system. The different reaction solutions (organic solvent vs. aqueous solution) can render the dye hydrophilic or hydrophobic. The electron donor, which is the electron mediator and sacrificial reductant in each system, should influence the efficiency of the oxidation reaction. The importance of the hydrophilicity and the reaction with a reductant will be discussed in the next section.

1.2.4 Molecular Design for Facilitating the Regeneration of the Ground State

Accelerating the reduction reaction of a sensitizer in the oxidized state to regenerate the ground state is important for improving the stability of a dye because the oxidized state of a dye is decomposed via light absorption of the oxidized state itself. To investigate the influence of the electron‐donor unit structure, in which the LUMO is distributed, for the regeneration reaction of the sensitizer, Bartolini et al. carried out H2‐evolution reactions using triphenylamine‐based D–A–π–A dyes with and without functionalization by bulky hydrophilic substituents (Figure 1.8) [22]. Dye1 has no special substituents on the terminal of triphenylamine, whereas Dye2 has four bis(ethylene glycol) monomethyl ether (BEG) chains. The BEG chains improve the hydrophilicity of the sensitizer and also behave as a steric, bulky moiety. The H2‐evolution reactions were conducted in aqueous solutions containing TEOA or AA as a sacrificial electron donor (SED), and the activities were exactly opposite depending on the SED used. When TEOA was used as the SED, the H2‐evolution activity of the Dye1 system was twofold greater than that of the Dye2 system. By contrast, when AA was used as the SED, the Dye2 system exhibited twofold greater activity than the Dye1 system. The amount of H2 evolved when AA was used was eighteen‐fold greater than that when aqueous TEOA solution was used. The large influence of the SEDs and the hydrophilic substituent on the activities can be explained in terms of the interaction between them. In the TEOA system, because no noticeable interaction occurs between the BEG chains and TEOA, such a steric bulky substituent would inhibit the access of TEOA to the electron‐donor unit (triphenylamine). The suppression effect dramatically decreases the efficiency for regenerating the dye ground state, leading to very low activity for H2 evolution. In the AA system, however, BEG chains would strongly interact with the highly polar SED, thereby accelerating the regeneration of the sensitizer ground state. That is, the hydrophilic substituent suppresses the oxidation reaction of TEOA but promotes that of AA. Therefore, a large difference in activity between the two SEDs was observed in the Dye2 system.

Figure 1.8 Molecular structures of (a) Dye1 and (b) Dye2.

The effects of hydrophobicity and hydrophilicity have also been studied in other sensitizer systems. In a carbazole‐based dye system whose electron‐donor unit is similar to triphenylamine, long alkoxy chains were found to improve the hydrophobicity of the dye and to increase the H2‐evolution activity in an aqueous TEOA solution [48]. Enhancing the hydrophobicity suppressed undesirable charge recombination and contributed to an improvement of the photocatalytic activity. Modification of phenothiazine with glucose through a triazole ring unit improved the hydrophilicity, thereby improving the photocatalytic H2‐evolution performance when TEOA was used as an electron donor [49]. The improvement of the activity was due to acceleration of the reaction with a reductant in aqueous solution and to suppression of intermolecular quenching, the latter of which was induced by insertion of a sterically bulky moiety. The effect of wetness around dye molecules on the excited electron‐transfer process was investigated using Ru complexes [50]. Protons in water adsorbed onto the substrate oxide (TiO2) tended to assemble around the dye molecules under dry conditions but not under wet conditions, causing instability in the oxidized form of the photosensitizer generated by electron injection. Destabilization of the oxidized dye decreased the efficiency of the electron injection.

1.2.5 Improving Stability by Forming a Strong Connection Between a Dye and a Semiconductor

A dye excited by light absorption can be desorbed from the substrate surface and/or decomposed because of its instability. The desorption and decomposition result in deactivation of the dye‐sensitized system. As described in the Introduction, a dye sensitizer follows a cycle involving photoexcitation, electron injection, and regeneration. Fast regeneration of the ground state should improve the efficiency because deactivation is suppressed, as discussed in Section 1.2.4. Similarly, the excited state, which is more unstable than the oxidized form, needs to be transformed immediately to the oxidized state to improve the stability of the system; that is, fast electron injection into a semiconductor will improve the durability of a dye‐sensitized system. Robust adsorption of a dye onto a semiconductor suppresses desorption of the dye and simultaneously accelerates the electron injection via strong interaction between the sensitizer and the semiconductor. The electron‐injection rate strongly depends on the electronic coupling between the density of states in the semiconductor and the electron‐donating orbital of the sensitizer. In the N3–TiO2 system, ultrafast electron injection was observed (∼50 fs) because of the relatively high density of states of TiO2 and favorable coupling with the electron‐donating orbital (the π* orbital of the carboxyl‐anchoring group of the substituted bipyridine of N3) [51]. Therefore, strong bonding between a dye and a semiconductor is a promising way to improve the longevity of a dye‐sensitized system from the viewpoint of not only desorption but also decomposition.

A representative anchoring group used in DSSCs is the carboxyl moiety; however, the carboxyl moiety is prone to desorption in aqueous solution. A phosphonic acid moiety improves the stability in weakly acidic conditions, whereas most carboxylate‐anchoring groups are desorbed at pH ≈ 6 [52]. Because metal complex dyes, especially those based on Ru(II) trisdiimine complexes, enable chemical functionalization with various ligands, the effect of the number of anchoring groups on the dyes’ desorption stability and DSSC performance has been studied [52]. However, introducing multiple anchoring groups into an organic molecular dye is synthetically difficult, although the literature contains numerous reports related to multiple branching and anchoring metal‐free molecular dyes [53].

A calix[4]arene‐based organic dye, which is easy to synthesize and has multiple anchoring groups, was developed (Figure 1.9a) [54]. Calix[n]arene ([n]: number of units) is a ring oligomer constructed by several units that consist of methylene‐substituted phenols. The four phenol units are combined with a π‐conjugation unit and constitute an electron‐acceptor unit. Therefore, the calix[4]arene‐based dye has four D–π–A structures in one molecule. All the calix[4]arene‐based dyes show a cone conformation (Figure 1.9c) with an electron‐acceptor unit at the top and anchoring groups at the bottom of the cone. When this sensitizer was used as a DSSC device, the system operated for 500 hours with no degradation in performance and showed high stability under light irradiation. A calix[4]arene derivative, HO‐TPA, was effective for DSP for the H2‐evolution and CO2‐reduction reactions (Figure 1.9b) [55]. Stable H2‐evolution activity was observed for 75 hours. The HO‐TPA dye has calix[4]arene and triphenylamine moieties as electron‐accepting and ‐donating units, respectively. The donor–acceptor units are conjugated by an oligothiophene moiety. This sensitizer has four –OH anchoring groups at the calix[4]arene ring, which is the conical top, and the unique structure is beneficial for suppressing dye aggregation and forming strong bonds with the substrate surface.

Figure 1.9 Molecular structures of (a) Calix‐3 and (b) HO‐TPA. (c) The optimized geometry of the Calix‐3 dye was mimicked through molecular modeling with the GAUSSIAN 03 package.

Source: Reproduced with permission from Tan et al. [54]; © 2015, John Wiley & Sons, Inc.

Figure 1.10 Schematic of the “layer‐by‐layer” assembly.

1.2.6 New Insights Based on the Light Harvesting of a Dye‐sensitized Photocatalyst System

Dyes have been developed not only through the traditional strategy (as described in Section 1.2.1) but also through unconventional approaches based on a novel concept. To suppress dye aggregation, Manfredi et al. introduced a coadsorbent that does not function as a light absorber [56]. The H2‐evolution photocatalytic activity was doubled when the coadsorbent was adopted because it suppressed undesirable interaction among dye molecules. A direct electronic transition induced by visible light from a simple, small dye (an azoquinoline carboxylic acid) into a semiconductor (ZnO) was proposed [57]. This transition was enabled by the formation of a novel electronic state because of strong coupling between the dye and ZnO, and the transition could be used for a visible‐light H2‐evolution reaction.

To effectively utilize the limited semiconductor surface, Mallouk and coworkers used a “layer‐by‐layer” (LBL) assembly approach [58]. This approach introduces multiple redox‐active and/or chromophores onto a metal‐oxide surface by forming phosphonate/Zr4+ coordination linkages (Figure 1.10) [59]. A bilayer assembly composed of two layers of Ru(II) complexes exhibited an approximately twofold increase in absorbance compared with a monolayer complex film. The assembly system was successfully applied in a photocatalytic H2‐evolution system [60]. A double‐layer assembly photocatalyst, which consists of two sensitizer (Ru(II) trisdiimine complex) layers and two Zr4+ layers, achieved ∼1% of AQY for H2 evolution under λ = 470 nm irradiation in an I3−/I− redox system. The greater activity of the double‐layer catalyst arose from the suppression of the back reaction (I3− + 2e− → 2I−) as a result of the steric hindrance of the double layer. The adsorbed Zr4+