Chem. Rev. 1992, 92, 411-433
411
Electron Transfer at Semiconductor Electrode-Liquid Electrolyte Interfaces CARL A. KOVAL' and JASON N. HOWARD Depariment of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), Universw of Cohwado, BouMer, Colorado 80309-02 15 Received August 2, 1991 (Revised Manuscript Received February 4, 1992)
Contents I. Introduction/Organization A. Photoelectrochemlstry and Interfacial Electron Transfer B. Basic Model for Electron Transfer at Semiconductor Electrodes C. Organization of the Review 11. Other Revlew Articles A. Pre-1980 B. 1980-1985 C. 1986 to the Present 111. Types of Semlconductor-Electrolyte Interfaces A. Types of Semiconductor Electrodes 1. Slngle Crystals and Epilayers 2. Polycrystalline Electrodes and Thin Films 3. Suspensions of Semiconductor Particulates and Colloids 4. SitaQuantlzed Systems B. Types of Redox Electrolytes 1. Diffusive Redox Systems 2. Adsorbed Redox Systems 3. Immobilized Redox Systems 4. Redox Polymer Films IV. Techniques for Investigating the Semlconductor-Electrolyte Interface A. Classical Techniques 1. Current-Potential Curves 2. Impedance Anaiysls 3. Photovdtage 4. Electrochemlcal Product Analysis 8. Optical Techniques Specific to Semiconductors 1. Photoluminescence 2. Electroluminescence 3. Transient and Tlme-Resolved Measurements 4. Intensity-Modulated Photocurrent spectroscopy 5. Reflectance Methods C. General Spectroscopic Techniques Applied to the SEI 1. I R Techniques 2. Raman Spectroscopy 3. Ellipsometry D. Scanning Probe Methods 1. Laser-Spot Scanning 2. Scanning Tunneling Microscopy 3. Scanning Electrochemical Microscopy
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E
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E. Ex Situ Ultrahigh Vacuum Techniaues 1. Effects of Surface Preparation 2. Adsorbed Species on Semiconductor Surfaces 3. Corrosion and Oxidation Products Aspects of Interfacial Energetics A. Description B. Surface Preparation C. Measurements of Interfacial Energetics 1. Systems with Constant E 2. Systems with Variable D. Surface States 1. Origin and Significance 2. Characterization E. Equivalent Circuits Mechanisms of Interfacial Electron Transfer Reactions A. Processes Related to the Semiconductor Electrode 1. Dark Processes 2. Photoinduced Processes B. Processes Related to the Solution 1. Outer-Sphere ET (Weak Coupling) and Inner-Sphere ET (Strong Coupling) 2. Multielectron Processes 3. Normal and Inverted Energy Regimes 4. Eiectrocataiysis C. Corroslon and Photoetching D. Nonthermalized (Hot) Carriers Rates of Interfacial Electron Transfer Reactions Theory of Interfacial Electron Transfer Reactions A. Difficulties and Theoretical Approaches B. Calculation of Reorganization Energies C. Promising Experimental Approaches Concludlng Remarks Abbreviations and Symbols Acknowledgments References
VII. VIII.
IX. X. XI. XII.
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I . Introductlon/Organlzatlon A. Photoelectrochemlstry and Interfacial
Electron Transfer
Even though the phenomena of electron transfer (ET) across semiconductor-liquid interfaces has been discussed in the literature for over 30 years, the early 1970s saw a dramatic growth in research in this area.14 This growth was largely due to interest in solar energy conversion. During the past 20 years, research related to 0 1992 American Chemical Soclety
412 Chemlcal Reviews. 1992. Vol. 92. No. 3
Koval and Howard
published papers on photoelectrochemistry, our understanding of ET at semiconductor-electrolyte interfaces (SEI) has not advanced as rapidly. Most experiments in this field are still interpreted within the context of a model developed in the 19608 by Gerischand many important questions pertainiig to this model have remained unanswered or only partially answered.I3 One explanation for the slow development of fundamental principles lies in the large number of practical successes that have been achieved in photoelectrochemistry. B. Baslc Model for Electron Transfer at Semlconductor Electrodes Carl A. K m l recelved his B.S. in Chemistry. summa cum laude. lrom Junlata College In 1974 end his W.D. from the CalHcfnia InsHluie of Technology in 1979. He dld postdoctoral research at Rnxn, UtWersky from 1978 to 1980, and joined the facuky in the Department of Chemistry and Biochemistry at the Unlverslty of Colorado (CU). Boulder In 1980 where he is preSenny an Associate Professor. Ha Is also a fellow of the Cooperative Institute for Research In Environmental Sclences (CERES) at CU. He has published approximately 50 papers in the areas of photoelechochemistry. homogeneous and heterogeneous elecbon transfer klnetics. Inorganic complexation reactions. and separatlon proCBSSBS. He has dkected nlne Ph.D. w ses and m M.S. T m wim his present group of nine graduate students. he Is currently doing research in two areas. One area is understanding interfacial electron bansfer pocasses In p h O t o e W " i c a 1 mi$; me o w area is developlng new separation processes that utillze m e w branes. reversible armpleratim reactkms. elecb0chemisby. andlor magnetic fields. He is married (wife's name, Chrlsta) and has a daughter. Sara. and a child due in April. He enjoys living in Colorado. gardening. runnlng. skiing. and hiking In the mountains.
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Jason Homud was bwn and rased in MawchuSBtls. He mcehed h(s B.A. h chemsby h m Bowdon b l e g e in 1987. He I8 compbHng his Ph D. wnh Prof. Carl
