preparation of monolayer modified electrodes

5

1Preparation of MonolayerModified Electrodes

1.1 Modification of Transparent Conducting Oxide (TCO) Electrodesthrough Silanization and Chemisorption of Small Molecules . . . . . 15Michael Brumbach and Neal R. Armstrong

1.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.1.1.1 The Metal Oxide Electrode and Modification Schemes . . . . . . . . . . 151.1.1.2 Commonly Encountered TCO Electrodes . . . . . . . . . . . . . . . . . . . 161.1.1.3 Variability in TCO Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.1.2 Factors that Control the Surface Composition of the TCO Electrode . 181.1.3 Chemical Modification of TCO Surfaces Using Silane Chemistries . . 191.1.3.1 Introduction to Silane Modification . . . . . . . . . . . . . . . . . . . . . . . 191.1.3.2 Examples of Silane Modification . . . . . . . . . . . . . . . . . . . . . . . . . 211.1.3.3 Silane Modification of Electrodes for Use in Devices . . . . . . . . . . . 221.1.4 Modification of TCO Surfaces through Chemisorption of Small

Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231.1.4.1 Modification using Small Molecules Containing Carboxylic Acid

Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231.1.4.2 Modification Using Phosphonic Acids and Other Chemisorbing

Functionalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261.1.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

1.2 SAMs on Au and Ag Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . 301.2.1 Preparation of Self-assembled Monolayers (SAMs) on Au and Ag . . . 30

Uichi Akiba and Masamichi Fujihira1.2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301.2.1.1.1 Molecular Self-assembly at Surfaces . . . . . . . . . . . . . . . . . . . . . . 301.2.1.1.2 Historical Background on SAMs . . . . . . . . . . . . . . . . . . . . . . . . 301.2.1.2 Preparation Techniques of Substrates Used for Forming SAMs . . . . 341.2.1.2.1 Gold as a Substrate Material . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Encyclopedia of Electrochemistry. Edited by A.J. Bard and M. StratmannVol. 10 Modified Electrodes. Edited by M.Fujihira, I.Rubinstein, and J.F.RuslingCopyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. ISBN: 978-3-527-30402-8

6 1 Preparation of Monolayer Modified Electrodes

1.2.1.2.2 Preparation Method of a Flat Gold Surface on Mica . . . . . . . . . . . . 351.2.1.2.3 Gold Substrate on Silicon Wafer . . . . . . . . . . . . . . . . . . . . . . . . . 361.2.1.2.4 Template-stripping Technique as Another Route for Preparation of

Flat Gold Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 361.2.1.2.5 Single Crystal Bead of Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381.2.1.2.6 Single Crystal Cut of Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381.2.1.2.7 Other Substrates of Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381.2.1.3 Preparation Techniques of Alkanethiol-based SAMs on Gold . . . . . . 381.2.1.3.1 Preparation from Solution Phase . . . . . . . . . . . . . . . . . . . . . . . . 381.2.1.3.2 Electrode Potential Control of SAM Deposition . . . . . . . . . . . . . . . 391.2.1.3.3 SAM Deposition from Aqueous Micellar Solution . . . . . . . . . . . . . 401.2.1.3.4 Preparation from Liquid and Supercritical Carbon Dioxide . . . . . . . 411.2.1.3.5 Preparation from the Gas Phase . . . . . . . . . . . . . . . . . . . . . . . . . 411.2.1.4 Structural Characterization and Chemical Properties of SAMs . . . . . 411.2.1.4.1 Structure of Alkanethiol-based SAMs on Gold . . . . . . . . . . . . . . . 411.2.1.4.2 Close-packed Structure of SAMs on Gold (111) Surface . . . . . . . . . 431.2.1.4.3 Superlattice Structure of SAMs on Gold (111) . . . . . . . . . . . . . . . . 441.2.1.4.4 SAM Surface Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451.2.1.4.5 Characterization of Defects by STM . . . . . . . . . . . . . . . . . . . . . . 451.2.1.4.6 Structural Variation in the Backbone Unit of SAMs . . . . . . . . . . . . 461.2.1.5 Chemistry and Energetics of the Formation of Alkanethiol-based

SAMs on Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481.2.1.5.1 Overall Reaction and Energetics of Chemical Adsorption at the

Gold/Thiol Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481.2.1.5.2 Chemisorption Energetics for Gold/Organosulfurs Interfaces . . . . . 491.2.1.5.3 Stabilization of SAM Structure due to Alkane Chain–Chain Interaction

within the Densely Packed Monolayers . . . . . . . . . . . . . . . . . . . . 491.2.1.5.4 Structural Effects on SAM Formation and Stability . . . . . . . . . . . . 491.2.1.6 Growth Mechanisms of SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . 501.2.1.6.1 General Profiles for SAM Formation Process . . . . . . . . . . . . . . . . 501.2.1.6.2 Growth Studies from the Gas Phase . . . . . . . . . . . . . . . . . . . . . . 511.2.1.6.3 Growth Studies from Solution Phase . . . . . . . . . . . . . . . . . . . . . 511.2.1.6.4 Further Techniques for Investigating the Growth of SAMs on Gold . 511.2.1.7 Patterned SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521.2.1.7.1 Two Approaches for Fabricating Patterned SAMs: the Top-down

Method and the Bottom-up Method . . . . . . . . . . . . . . . . . . . . . . 521.2.1.7.2 Spontaneous Phase Separation . . . . . . . . . . . . . . . . . . . . . . . . . . 531.2.1.7.3 Scanning Probe Lithography Using SAMs . . . . . . . . . . . . . . . . . . 531.2.1.7.4 Dip-pen Nanolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561.2.1.7.5 Microcontact Printing by Use of Organosulfur Inks . . . . . . . . . . . . 561.2.1.7.6 Photolithography for Fabricating Patterned SAMs . . . . . . . . . . . . . 591.2.1.7.7 Nanotransfer Printing of Patterned Gold Thin Films . . . . . . . . . . . 601.2.1.7.8 Other Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

1 Preparation of Monolayer Modified Electrodes 7

1.2.1.8 Advanced Modification Technique of SAMs . . . . . . . . . . . . . . . . . 601.2.1.8.1 Protecting Groups for Unstable Organic Thiols . . . . . . . . . . . . . . . 601.2.1.8.2 Modification Technique of SAMs of Dithiols . . . . . . . . . . . . . . . . 611.2.1.9 Rigid SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611.2.1.9.1 BCO and Adamantane SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . 611.2.1.9.2 Cholesterol SAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621.2.1.9.3 Aromatic SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621.2.1.10 π -Conjugated SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631.2.1.11 Chemistry on SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631.2.1.11.1 Click Reaction on SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631.2.1.11.2 Electrochemistry on Thiol-based SAMs Modified at Electrodes . . . . . 651.2.1.11.3 Bio-SAMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 661.2.1.12 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 681.2.2 Photoelectrochemical Properties of Electrodes Modified with

Self-assembled Monolayers (SAMs) of Various Alkylthiol Derivatives 80Toshihiro Kondo and Kohei Uosaki

1.2.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 801.2.2.2 Photo-induced Electron Transfer at SAM-modified Electrodes . . . . . 811.2.2.3 Photo-induced Electron Transfer at Electrodes Modified with

SAM-covered Nanocluster Layers . . . . . . . . . . . . . . . . . . . . . . . . 871.2.2.4 Electron Transfer Controlled by Photoisomerization at SAM-modified

Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 891.2.2.5 Application of Luminescence from SAM-modified Electrodes . . . . . 931.2.2.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

1.3 SAMs on Hg Electrodes and Si . . . . . . . . . . . . . . . . . . . . . . . . . . 1051.3.1 SAMs on Hg Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Rolando Guidelli1.3.1.1 Introduction and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051.3.1.2 Phospholipid SAMs on Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1051.3.1.2.1 Preparation Methodology and Procedures . . . . . . . . . . . . . . . . . . 1051.3.1.2.2 Structure and Physical Properties of Phospholipid SAMs on Hg . . . 1071.3.1.2.2.1 Differential Capacity of Phospholipid SAMs on Hg . . . . . . . . . . . . 107

Impedance spectroscopy measurements . . . . . . . . . . . . . . . . . . . 1101.3.1.2.2.2 Inhibitive Properties of Phospholipid SAMs on Hg . . . . . . . . . . . . 1131.3.1.2.2.3 Modeling of Phospholipid SAMs on Hg . . . . . . . . . . . . . . . . . . . . 113

The adsorption isotherm of lipids monolayers and the reorientationpeaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

1.3.1.2.3 Incorporation of Lipophilic Molecules in Phospholipid SAMs on Hg 1191.3.1.2.3.1 Electroinactive Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

Electroinactive neutral compounds . . . . . . . . . . . . . . . . . . . . . . . 119


Lipophilic ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Peptides and proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

1.3.1.2.3.2 Electroactive Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1261.3.1.3 Thiol SAMs on Hg . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1301.3.1.3.1 Preparation Methodology and Procedures . . . . . . . . . . . . . . . . . . 1311.3.1.3.2 Structure and Physical Properties of Thiol SAMs on Mercury . . . . . 1321.3.1.3.2.1 Surface-sensitive Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 1321.3.1.3.2.2 Electrochemical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

Soluble thiols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134Insoluble thiols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

1.3.1.3.2.3 Total and Free Charge Density on Mercury Coated with Thiol SAMs 1401.3.1.3.2.4 Electron Transfer across Thiol SAMs on Mercury . . . . . . . . . . . . . 1451.3.1.4 Tethered Bilayer Lipid Membranes on Hg . . . . . . . . . . . . . . . . . . 151

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1541.3.2 Modification of Silicon Wafer Surfaces with Small Organic Moieties 157

Taro Yamada1.3.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1571.3.2.2 Handling of Si Specimens for Adsorbate Preparation and Surface

Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1611.3.2.3 Summary of the Latest Researches . . . . . . . . . . . . . . . . . . . . . . . 1621.3.2.3.1 Adsorption of Terminal Olefins on H:Si(111) . . . . . . . . . . . . . . . . 1621.3.2.3.2 Grignard Reaction on Halo- genated Si(111) . . . . . . . . . . . . . . . . . 1651.3.2.3.3 Grignard Reagents and H:Si(111) . . . . . . . . . . . . . . . . . . . . . . . . 168

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

1.4 Langmuir–Blodgett (LB) Films on Electrodes . . . . . . . . . . . . . . . . 1711.4.1 Photoelectrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Masaru Sakomura and Masamichi Fujihira1.4.1.1 History of the Langmuir–Blodgett (LB) Film . . . . . . . . . . . . . . . . 1711.4.1.2 Basic Principles of Surface and Film . . . . . . . . . . . . . . . . . . . . . . 171

Surface Tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171Surface Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173Gibbs and Insoluble Monolayers . . . . . . . . . . . . . . . . . . . . . . . . 174Surface Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174Surface Pressure – area (π − A) Isotherm . . . . . . . . . . . . . . . . . 176

1.4.1.3 Preparation Methods for LB Films . . . . . . . . . . . . . . . . . . . . . . . 177Molecules and Subphase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177LB Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177Equipment for LB Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 179

1.4.1.4 LB Assemblies Designed for Photoelectric Devices . . . . . . . . . . . . 181Energy Transfer in Monolayer Organization . . . . . . . . . . . . . . . . . 181Layer-by-layer Assemblies of Electron Acceptor and Dye Molecules . 183Acceptor/Sensitizer/Donor Multilayer Systems . . . . . . . . . . . . . . . 185LB Assemblies of Synthetic Molecular Photodiodes . . . . . . . . . . . . 187


A–S–D Triad Monolayer Systems . . . . . . . . . . . . . . . . . . . . . . . 189Alternate Multilayered Systems for Scanning Maxwell StressMicroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192A–S–D Triad Monolayer and Second Donor Bilayer Alternate Systems 195LB Assemblies for Simulations of Key Aspects in NaturalPhotosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

1.4.2 Langmuir–Blodgett (LB) Films on Electrodes (B) Electrochemistry . . 203Takashi Nakanishi and Naotoshi Nakashima

1.4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2031.4.2.2 Electrochemistry of LB Films of ‘‘Molecular Wires’’ and

Phthalocyanines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2031.4.2.3 Langmuir Monolayer of Fullerenes at the Air–Water Interface . . . . 2051.4.2.4 Electrochemistry of LB Films of Fullerenes on Electrodes . . . . . . . . 2071.4.2.5 Carbon Nanotube LB Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2081.4.2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

1.5 Electrochemistry of Monolayer Assemblies at the Air/Water Interface 211Marcin Majda

1.5.1 Langmuir and Gibbs Monolayers at the Air/Water Interface . . . . . . 2111.5.2 Horizontal Touch Electrochemical Characterization of Monolayer

Films at the Air/Water Interface . . . . . . . . . . . . . . . . . . . . . . . . . 2121.5.3 Two-dimensional Electrochemistry . . . . . . . . . . . . . . . . . . . . . . . 2181.5.3.1 Fabrication and Characterization of Line Microelectrodes . . . . . . . . 2191.5.3.2 2D Electrochemical Investigations of Langmuir Monolayer Films . . 2211.5.3.2.1 Dynamics of the Lateral Diffusion of Surfactants on the Water Surface 2211.5.3.2.2 Measurements of the LE/G Phase Transitions and their Critical

Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2271.5.3.2.3 Kinetics of Electron Hopping in Langmuir Monolayers . . . . . . . . . 228


1.6 Recent Trends in Chemically Modified sp2 and sp3 Bonded CarbonElectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236Doug Knigge, Pushwinder Kaur, and Greg M. Swain

1.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2361.6.2 Chemically Modified sp2 Bonded Carbon Electrodes . . . . . . . . . . . 2381.6.2.1 Hydrogen-terminated Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 2381.6.2.2 Surfaces Modified by Chemisorbed Molecules . . . . . . . . . . . . . . . 2401.6.2.3 Effect of Surface Modification on the Electrochemical Properties . . . 2431.6.3 Chemically Modified sp3 Bonded Carbon Electrodes . . . . . . . . . . . 2441.6.3.1 CVD Diamond Thin-film Growth . . . . . . . . . . . . . . . . . . . . . . . . 2441.6.3.2 Surface Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247


1.6.3.3 Oxygen-terminated Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 2501.6.3.4 Surfaces Modified by Chemisorbed Molecules . . . . . . . . . . . . . . . 2511.6.3.5 Effect of Surface Modification on the Electrochemical Properties . . . 2571.6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258


1.7 Chemically Modified Oxide Electrodes . . . . . . . . . . . . . . . . . . . . . 261Chimed Ganzorig and Masamichi Fujihira

1.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2611.7.2 Electrode Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2611.7.2.1 General: Transparent Conducting Oxide Films . . . . . . . . . . . . . . . 2611.7.2.2 Cleaning of Substrate Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 2621.7.2.2.1 Cleaning with Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

Rubbing and Immersion Cleaning . . . . . . . . . . . . . . . . . . . . . . . 264Vapor Degreasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264Ultrasonic Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265Spray Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

1.7.2.2.2 Cleaning by Heating and Irradiation . . . . . . . . . . . . . . . . . . . . . . 2651.7.2.2.3 Cleaning by Stripping Lacquer Coatings . . . . . . . . . . . . . . . . . . . 2661.7.2.2.4 Cleaning in an Electrical Discharge . . . . . . . . . . . . . . . . . . . . . . . 2661.7.2.2.5 Cleaning Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2671.7.2.2.6 Cleaning of Organic Glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2671.7.2.3 Chemical Methods of Film Deposition . . . . . . . . . . . . . . . . . . . . 2681.7.2.3.1 Spraying Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2681.7.2.3.2 Chemical Vapor Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2691.7.2.3.3 Sol-gel Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2701.7.2.4 Physical Methods of Film Deposition . . . . . . . . . . . . . . . . . . . . . 2711.7.2.4.1 Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

Postoxidation of Metal Films . . . . . . . . . . . . . . . . . . . . . . . . . . . 271Reactive Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271Activated Reactive Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . 271Direct Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

1.7.2.4.2 Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271Reactive Sputtering of Metallic Targets . . . . . . . . . . . . . . . . . . . . 272Sputtering of Oxide Targets (Direct Sputtering) . . . . . . . . . . . . . . . 272Ion Beam Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272Magnetron Sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273

1.7.2.4.3 Reactive Ion Plating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2731.7.2.4.4 Pulsed Laser Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2741.7.2.4.5 Other Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2741.7.2.5 The Properties of Transparent Conducting Oxide Electrode Surfaces 2741.7.3 Electrode Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2771.7.3.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277


1.7.3.2 SnO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2781.7.3.3 ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2811.7.3.4 ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2851.7.3.5 TiO2 and ZrO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2881.7.3.5.1 Polypyridyl Ru Complexes and Organic Dyes as Sensitizers . . . . . . 2881.7.3.5.2 Short-chain Alkanoic and Benzoic Acids . . . . . . . . . . . . . . . . . . . 2911.7.3.5.3 Long-chain Organic Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2941.7.3.6 Other Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2971.7.4 Organosilanization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3021.7.4.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3021.7.4.2 Wide Bandgap Transparent Conducting Oxide Electrodes . . . . . . . . 3031.7.4.2.1 SnO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3031.7.4.2.2 TiO2 and ZrO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3041.7.4.2.3 ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3051.7.4.2.4 ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3061.7.4.3 Large Bandgap Oxide Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . 3061.7.5 Potential Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3081.7.5.1 Tuning the Work Function of Electrodes . . . . . . . . . . . . . . . . . . . 3091.7.5.2 Correlation of Device Performance with Changes in the Work Function 3121.7.5.3 Other Possible Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3171.7.5.3.1 SnO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3171.7.5.3.2 ITO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3181.7.5.3.3 ZnO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3181.7.5.3.4 TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3181.7.5.3.5 Other Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3191.7.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

1.8 Surface Modification of Chalcogenide Electrodes . . . . . . . . . . . . . . 335Chimed Ganzorig and Masamichi Fujihira

1.8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3351.8.1.1 Surface Treatment of Substrate Chalcogenide Electrodes . . . . . . . . 3371.8.1.2 Adsorption of Acids to Chalcogenides . . . . . . . . . . . . . . . . . . . . . 3371.8.1.3 Adsorption of Organic Sulfur Compounds . . . . . . . . . . . . . . . . . . 3371.8.2 Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3381.8.2.1 Dialkyl Chalcogenide Compounds . . . . . . . . . . . . . . . . . . . . . . . 3381.8.2.2 Thiols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3381.8.2.3 Dithiols, Disulfides, and Other Sulfur Compounds . . . . . . . . . . . . 3391.8.3 Characterization and Applications . . . . . . . . . . . . . . . . . . . . . . . 3421.8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343



1.9 Metal Substrates for Self-assembled Monolayers . . . . . . . . . . . . . . 345Marie Anne Schneeweiss and Israel Rubinstein

1.9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3451.9.2 General Considerations Regarding Substrates . . . . . . . . . . . . . . . 3461.9.2.1 Specific Requirements Imposed by Analysis Techniques . . . . . . . . 3461.9.2.2 Polycrystalline, Textured, Single-crystalline Substrates . . . . . . . . . . 3461.9.2.3 Major Aspects Regarding Substrate Preparation/ Pretreatment . . . . 3461.9.3 Substrates Used for SAM Preparation . . . . . . . . . . . . . . . . . . . . . 3471.9.3.1 Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3471.9.3.1.1 Gold Films on Various Supports . . . . . . . . . . . . . . . . . . . . . . . . 3471.9.3.1.1.1 Evaporated Gold Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Gold films evaporated on mica . . . . . . . . . . . . . . . . . . . . . . . . . . 348Gold films evaporated on glass . . . . . . . . . . . . . . . . . . . . . . . . . . 349Gold films evaporated on silicon . . . . . . . . . . . . . . . . . . . . . . . . . 350Gold films evaporated on miscellaneous supports . . . . . . . . . . . . . 351Annealing procedures for evaporated gold films . . . . . . . . . . . . . . 352Cleaning procedures for evaporated gold samples . . . . . . . . . . . . . 352

1.9.3.1.1.2 Sputtered Gold Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3541.9.3.1.1.3 Electroless Deposited Gold Films . . . . . . . . . . . . . . . . . . . . . . . . 3551.9.3.1.1.4 Template-stripped Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3551.9.3.1.2 Bulk Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3581.9.3.1.2.1 Polycrystalline Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3581.9.3.1.2.2 Commercial Gold Single Crystals . . . . . . . . . . . . . . . . . . . . . . . . 3591.9.3.1.3 Gold Beads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3591.9.3.1.4 Oxidized Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3601.9.3.1.5 Other Gold Substrates for Special Applications . . . . . . . . . . . . . . . 3601.9.3.1.5.1 Gold Powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3601.9.3.1.5.2 Gold Colloids (Nanoparticles) . . . . . . . . . . . . . . . . . . . . . . . . . . 3601.9.3.1.5.3 Transparent Gold Foil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3611.9.3.1.5.4 Ultrathin Gold Island Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3611.9.3.1.5.5 Electroplated Gold Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3611.9.3.1.5.6 Composite Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3611.9.3.1.6 Comparative Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3621.9.3.2 Silver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3631.9.3.2.1 Silver Films on Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3631.9.3.2.2 Bulk Silver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3641.9.3.3 Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3641.9.3.3.1 Copper Films on Supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3641.9.3.3.2 Bulk Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3651.9.3.3.3 Copper Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3651.9.3.4 Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3661.9.3.5 Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3661.9.3.6 Platinum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366


1.9.3.7 Nickel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3671.9.3.8 Titanium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3671.9.3.9 Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3681.9.3.10 Mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

15

1.1Modification of Transparent ConductingOxide (TCO) Electrodes throughSilanization and Chemisorption of SmallMolecules

Michael Brumbach and Neal R. ArmstrongUniversity of Arizona, Tucson, Arizona

1.1.1Introduction

1.1.1.1 The Metal Oxide Electrode andModification SchemesSurface modification of transparent con-ducting oxide (TCO) thin film electrodes(indium–tin oxide (ITO), SnO2, TiO2,ZnO, etc.) has traditionally been directedtoward enhancement of heterogeneouselectron transfer rates of solution speciesfor which the electron transfer process isthermodynamically favored but kineticallyinhibited. The surface-confined moleculeis intended to exhibit rapid and reversibleelectron transfer at the oxide surface to pro-vide rapid mediation of electron transferto a solution species [1]. Surface-confinedmolecules are typically not directly ‘‘wired’’into the conduction or valence band of theoxide, but electron transfer rates are oftenreasonably fast for these species simplybecause they have been brought into close

proximity to the electronically active sitesof the metal oxide. Chemical modificationof the TCO surface can be accom-plished through covalent bond formation(silanization) or through chemisorption ofmolecules containing functional groupssuch as carboxylic acids, phosphonic acids,alkanethiols, and amines.

Initial modification strategies were em-ployed for oxidation of biomolecules,such as ascorbic acid, neurotransmitters,heme proteins, and so forth using var-ious surface-confined molecules or thinfilms of conducting polymers. Modifica-tion schemes have also been used to attachphotoactive dye molecules, such as ph-thalocyanines, porphyrins, and a varietyof organometallic complexes, to oxide sur-faces to enhance redox processes betweensolution species and semiconducting elec-trodes [2–6]. Chemical modification ofconducting oxide surfaces has also beenused to enhance charge injection rates intocondensed phase organic thin film devices(for example, in organic light emittingdiodes (OLEDs) and organic photovoltaics(OPVs)). There is still some debate asto whether the enhancement in deviceperformance is due to enhancements inwettability of the oxide surface toward thenonpolar molecules used in these devices,to changes in work function of the ox-ide surface, and/or to enhancements in

Encyclopedia of Electrochemistry. Edited by A.J. Bard and M. StratmannVol. 10 Modified Electrodes. Edited by M.Fujihira, I.Rubinstein, and J.F.RuslingCopyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. ISBN: 978-3-527-30402-8


intrinsic rates of charge injection [7–13].Many of the same strategies initially usedto enhance solution electron transfer rateson these oxide surfaces appear to posi-tively impact device efficiencies in simpleOLEDs and OPVs [9, 14–18].

1.1.1.2 Commonly Encountered TCOElectrodesThe most commonly encountered TCOelectrodes, typically studied as thin films,are antimony-doped tin oxide (ATO) orfluorine-doped tin oxide (FTO), ITO, ti-tanium oxide (anatase or rutile, TiO2),and zinc oxide (ZnO) [19]. There are alsosome recently reported trinary and ternaryoxides based on modifications of ITO, zinc-indium tin oxide, ZITO or IZTO, for exam-ple, which may become more popular withtime as electrodes for solution-based redoxchemistry and as anodes in devices such asOLEDs and OPVs [7]. These new tailoredcomposition oxides may exhibit higher sta-bility, higher work functions, and/or agreater variety of surface sites with morepossibilities for chemical modification.

An idealized metal oxide surface isshown in Fig. 1, where the metal atomsare in tetrahedral coordination with fouroxygen atoms. This metal oxide, empiricalformula MOx , is stoichiometric, having nodefects or surface hydrolysis products, andcan be considered as a typical platform forunderstanding the modification of mostTCO thin film materials of electrochemicalinterest. TCO films are typically depositedby sputtering, chemical vapor deposition,or pulsed laser deposition, and tend tobe ‘‘microcrystalline’’ whose dominant ex-posed faces tend to be either 〈111〉 or 〈100〉

lattice terminations [19–23]. The structureand electronic properties of several TCOfilms can be summarized as follows:Indium–Tin Oxide (ITO): ITO has thecomplex unit cell of indium oxide (bixbyitelattice) consisting of more than 80 atoms.Indium oxide has two distinct indiumsites in the lattice and as many as fouroxygen species, considering surface hy-drolysis products and oxygen vacancies[24]. Oxygen vacancies and electron-rich in-terstitial tin dopant sites lie close in energyto the conduction band edge so that promo-tion of electrons into the conduction band,at room temperature, is facile [25]. Figure 2shows the proposed band diagram for ITOexhibiting the common features that dic-tate the electrical properties of many TCOmaterials. The upper portion of the va-lence band arises from filled O(2p) orbitals,while the lower part of the conduction bandarises from unfilled metal orbitals (In(5s)orbitals in the case of ITO). Oxygen defectsare induced into the TCO material duringits formation, usually by depositing or an-nealing the thin film in an oxygen-deficientatmosphere, thus creating electron-richsites within a few kT of the conductionband edge. An interstitial dopant, tin, isoften introduced into these In2O3 latticesat concentrations up to 10% (atomic), pro-ducing additional electron-rich sites withina few kT of the conduction band edge. InITO, the donor density can be as high as1020 cm−3 with a sheet resistance as lowas 10 � sq−1. The introduction of oxygendefect sites and other dopants, however,may also increase the chemical reactivityof these oxide surfaces toward hydrolysis

O

OO

OO

OO

OO

OO

OO

O

O

O

M M M M M M MFig. 1 Schematic view of anideal metal oxide surface.

1.1 Modification of TCO Electrodes through Silanization and Chemisorption of Small Molecules 17

and other components of the vacuum de-position system. The reactivity of the oxidesurface has important consequences fortheir further modification and enhance-ment of electron transfer rates, which havebeen extensively studied as functions ofsurface pretreatment [8, 24, 26, 27].

Tin Oxide (SnO2): Tin oxide has a tetrag-onal rutile structure with a unit cell ofsix atoms. The electrical conductivity ofn-doped SnO2 is due to the presence of oxy-gen vacancies, interstitial tin (in excess), oradded dopants such as fluorine, chlorine,or antimony. Carrier concentrations canbe increased to approximately 1020 cm−3

through doping from the intrinsic con-centration of approximately 1018 cm−3 forSnO2 [19].

Titanium Oxide (TiO2): Titanium oxideexists in three different crystallographicstructures, rutile, anatase, and brookite.The most commonly studied material isrutile, being the most stable phase, al-though many studies have been conductedon the anatase phase as well [28]. Stoi-chiometric rutile contains Ti4+ in six-foldcoordination with oxygen staggered byTi4+ in five-fold coordination. Oxygenatoms at the surface are often bridgingand only two-fold coordinated [23]. Theband gap of rutile is 3.0 eV, and 3.2 eVfor anatase, making titanium oxide a use-ful material for photocatalysis [29–31].Photoelectrochemical cells based on sin-tered nanoparticulate arrays of anatase or

Fig. 2 Schematic view of the energylevels in ITO showing the componentsof the valence band and conductionband regions of the oxide, the presenceof oxygen defect sites and interstitial tindopant sites that contribute to electrondensity near the conduction band edge,rendering the oxide conductive at roomtemperature (after Ref. 25).

Ene

rgy

In: 3d10

In: 5s

In: 5p

0.03 eV

O2−: 2p6

Sn3+: 5s1Vo: s2

EF

thin film TiO2 electrodes have been quitesuccessful [32, 33]. Both anatase and ru-tile forms of TiO2 are readily n-dopedby creation of oxygen vacancies leavingelectron-rich Ti3+ and Ti2+ states in theband gap, with electron energies suffi-ciently close to the conduction band edgeso as to provide for reasonable room tem-perature conductivity.

1.1.1.3 Variability in TCO ElectrodesTCO thin films can exhibit tremendousvariability in transparency, microstructure(and surface roughness), surface compo-sition, conductivity, chemical stability athigh current densities (in OLEDs, OPVs,and chemical sensors), and in their chem-ical compatibility with contacting organiclayers. Variability is often noticeable fromproduction batch to production batch andwithin batches of the same metal oxidematerial [34]. Surface pretreatment condi-tions have also been shown to dramaticallyimpact the electrochemical, physical, andphotophysical properties of metal oxidefilms [8, 9, 24, 26, 27, 35]. Modification


of these surfaces through silanization andchemisorption would appear to be a meansfor improving the stability of these ox-ide surfaces, enhancing their compatibilitywith nonpolar solvents and condensedphase materials, and optimizing rates ofinterfacial electron transfer.

1.1.2Factors that Control the SurfaceComposition of the TCO Electrode

The structure shown in Fig. 1, represent-ing an ideal metal oxide surface, providesa useful platform for understanding metaloxide surfaces. A further, and more re-alistic, modification to the model wouldaccount for the presence of surface hydrol-ysis products as shown in Fig. 3. Surfacehydrolysis critically affects the chemicalmodification of the TCO surface. Mod-erate hydrolysis may only break bridgingoxygen-metal bonds leaving surface hy-droxyl groups. More extensive hydrolysiscan lead to fully hydroxylated metal speciesthat may remain physisorbed. The pres-ence of dopants and/or oxygen vacancies,as shown in Fig. 4, can also affect the metaloxide surface by creating sites that can re-act with adventitious impurities presentduring, or after, deposition.

Varying degrees of hydroxylation anddefect density, as well as the preferentialmigration of dopants to the near surfaceregion, contribute to the high degree ofheterogeneity in electron transfer ratesand chemical compatibility commonly

observed with oxide electrodes. Hydrolysisproducts on ITO remain strongly ph-ysisorbed (precipitated) to the electrodesurface at greater than monolayer cover-age, constituting a primary factor for theappearance of electrical ‘‘dead spots’’ onthe oxide surface [24, 36, 37]. The so-called‘‘dead spots’’ are regions exhibiting poorelectroactivity or no electroactivity.

Surface hydrolysis of metal oxide sur-faces can be examined by comparison tosolution equilibrium constants for hydrol-ysis and solubility. Indium oxide has afavorable equilibrium constant for hydrol-ysis [38–40]:

In2O3 + 3H2O ←−→ 2In(OH)3

K = 4.3 × 101 (1)

Whereas, the fully hydrolyzed In moiety,In(OH)3, has a very low solubility product.

In(OH)3 ←−→ In3+ + 3OH−

Ksp = 1.3 × 10−37 (2)

There is also the possibility for incompletehydrolysis of an indium surface oxidecreating the intermediate hydroxylatedspecies, InOOH:

In2O3 + H2O ←−→ 2InOOH (3)

This type of surface site may also ariseby the nucleophilic attack of water onexposed oxide defects, which serve as sitesfor dissociative adsorption. In contrast,hydrolysis of tin oxide is significantly less

O O O

O

O

OOOOOOO

O O

M

O

M

O

M

O O

M M M M

HO

OHMOH

H H HHH

O OHHH Fig. 3 Schematic view of an oxide

surface that has undergone hydrolysis,producing hydroxylated sites, and evenphysisorbed metal hydroxide,monomers or polymers.


Fig. 4 Schematic view of thenear surface region of the oxidethin film with oxygen vacancies,producing electron-rich metalsites and dopant sites of lowerstoichiometry, and also creatingelectron-rich sites.

O O

M

O

D

O

O

D

O

O

M

O

O

M

O

M

O

O

M

O O+n +n

+x +n

favored than for indium oxide, at any pH[38, 39]:

SnO2 + 2H2O ←−→ Sn(OH)4

K = 5.6 × 10−10 (4)

While the solubility of tin hydroxide ishigher than for indium hydroxide:

Sn(OH)2 ←−→ Sn2+ + 2(OH−)

K = 1.4 × 10−28 (5)

Sn(OH)4 ←−→ Sn4+ + 4(OH−)

K = 7.2 × 10−14 (6)

The typical doped SnO2 thin film cantherefore be expected to show monolayer(but not significantly higher) coverages ofhydroxide species. Hydroxide species inexcess of monolayer coverage would beexpected to be more easily removed fromthe oxide surface through pretreatmentsteps.

For titanium oxides and zinc oxides, sim-ilar hydrolysis and dissolution processesmust be considered [39, 41]:

Zn(OH)2 ←−→ Zn2+ + 2OH−

Ksp = 3 × 10−16 (7)

Ti(OH)4 ←−→ Ti4+ + 4OH−

Ksp = 7.94 × 10−54 (8)

Chemical modification of these oxides,therefore, needs to take into account theroutes to formation and loss of surfacehydroxide species playing a key role in

determining the surface coverage of themodifier. Rates of electron transfer/chargeinjection at the modified oxide surfaceare subsequently affected. Despite the un-certainties in oxide surface composition,several modification protocols have beendeveloped over the last thirty years, whichyield improved electrochemical perfor-mance of the TCO thin film. Two strategieshave continued to be successful at modi-fying metal oxide electrodes; silanization,covalent bond formation to metal hydrox-ides, and chemisorption, using functionalgroups known to either hydrogen bond toM–OH sites and/or to coordinate to metalcation sites.

1.1.3Chemical Modification of TCO SurfacesUsing Silane Chemistries

1.1.3.1 Introduction to Silane ModificationSurface modification of TCO thin filmswith monofunctional, difunctional, andtrifunctional organosilanes arose initiallyfrom protocols developed in modifyingsilica surfaces [42]. The covalent attach-ment of functional groups to the oxidesurface through silanization involves thecoupling of the functionalized silane tothe metal oxide at hydroxylated sites,through a metal–oxygen–silicon bondednetwork with the release of one or more‘‘leaving groups’’ as shown in Fig. 5. ‘‘Leav-ing groups’’ are generally chlorides oralkoxides (methoxide or ethoxide) yield-ing HCl or the corresponding alcohol asby-products.


O O

M

OH

OO

M

O

O

O

M

O

HSi

O

M

O

O

M

O

O

R

Si

R

Si

O

M

O

O

M

O

O

ROMe OMe OMe

HH

MeO MeOMeO

R–Si(OMe)3 + HO–M R–Si–O–M + MeOH

OMe

OMe

Fig. 5 Schematic view of silanemodification of an oxidesurface, using (as an example) atri-methoxy silane. Reactionwith only one surface hydroxidesite is shown, releasingmethanol as the product.Unreacted hydroxide sitesremain, as do unreacted alkoxygroups on the silane, which canfurther cross-link underappropriate conditions.

In the absence of water, silane modifierscan form multiple bonds to the surface cre-ating a clawlike multidentate attachment,Fig. 6(a) [43]. Silanes with multiple leavinggroups, however, will generally cross-linkduring the surface attachment process,or afterward during storage in high hu-mid environments, to form a polymeric,hydrolytically stable layer, as shown inFig. 6(b). The functional group (R) can bean alkyl or aromatic group terminatingwith a redox active molecule (ferrocene,

viologen, porphyrin, phthalocyanine, etc.)or can simply be tailored to control thewettability of the TCO surface.

It is desirable to start with a surfacethat has been mildly hydroxylated forsurface modification, as opposed to thestoichiometric surface shown in Fig. 1,so that there are sufficient reactive sitesfor silanization. If the redox chemistry ofan attached molecule is to be optimized,it is also desirable to work with mono-functional silanes. In this case, only one

H

O

O

M

O

O

M

R

Si

O

O

O

H

M

O

M

O O

R

SiOMe

H

O

O

M

O

O

M

R

Si

O

M

O

OO

OO

MeO

O

R

SiO

H

O

O

O

M

O

M

O

O

M

O

R

Si

O

M

O O

M

O

OR

Si

OR

OSi

M

O

O O

M

OO

HOH

OO

HH

O

R

SiOMe

H

O

O

H

(a)

(b)

Fig. 6 Schematic view of cross-linkinga silane surface modifier as it passesthrough the (a) multidentate/brushphase to the (b) fully cross-linked phase,with relatively few unreacted M−OHgroups left on the oxide surface.


attachment site is created, thereby leavingthe tethered redox group in close proxim-ity to the electrode surface and promotingreasonable rates of electron transfer [1].Control of surface hydroxylation, however,given the tendency of oxides to exten-sively hydrolyze, is problematic and affectsthe coverage of redox active groups thatcan be achieved through silanization reac-tions. Silane formation on SnO2 surfaces,where hydrolysis can be more easily con-trolled appears to be a more advantageousstrategy.

Silanization is generally performed inrefluxing toluene or benzene with low con-centrations of the desired silane. Silanesof the form X(R2)2SiR1 have only onereactive bond and can therefore makeonly one bond with the metal oxide sur-face (see Fig. 5). X represents a chloride,methoxy, ethoxy, or other ‘‘leaving group.’’R2 groups are typically small substitutentssuch as methyl groups, but can be changedto something larger such as phenyl groupsto provide steric hindrance and blockingunsilanized surface sites. Large side chainswould, however, also sterically reduce thenumber of silane molecules bound to theoxide. The energy barrier for the surfacebonded silane to rehydroxylate, and themetal oxide to ‘‘heal,’’ with a bridging oxy-gen, is extremely low. Therefore, thesebrushlike phases are generally unstableand nearly all of the brushlike phasesreported in the literature are producedwith triply reactive silanes reacted un-der highly controlled dry conditions suchthat multiple bonds can be made withthe surface, as shown in Fig. 6(a). Accord-ing to Kirkov, there are 2.23 × 1015 Sn−Osites cm2 on SnO2 [44, 45], and similarsite densities can be expected for ITO andTiO2. Since not all sites will be uniformlyreactive, a maximum surface coverageis approximately 10−10 moles cm−2 for a

brushlike silanized layer with tridentateattachment.

1.1.3.2 Examples of Silane ModificationTrimethoxysilane molecules will cova-lently attach to ITO following a refluxof the electrode in a 10% (silane:toluene)solution for approximately 30 min [37]. Ex-cess silane can be removed by sonicationfor 10 min in pure toluene. Water con-tact angle measurements on such modifiedsurfaces immediately show increased con-tact angles, consistent with the addition ofsilane molecules with nonpolar function-ality to the surface. On ITO, the contactangle can change from less than 30◦ be-fore modification, to angles higher than65◦ after modification, and even higher ifcross-linking of the silane occurs [37, 46].X-ray photoelectron spectroscopy (XPS) isalso used to verify the presence of siliconon the surface. Shepard and Armstronghave measured the electrode interfacial ca-pacitance at tin oxide electrodes modifiedwith a series of silanes [47–50]. The ca-pacitance studies showed the sensitivity ofthe electrical properties of modified sur-faces to variations in reaction conditionsduring silanization, and to the types offunctional groups attached to these sur-faces.

Untereker et al. used several meth-ods for creating clawlike brush phasesilane layers on SnO2, TiO2, andglass [43]. SnO2 was modified with γ -aminopropyl-triethoxysilane, 3-(2-(amino-ethylamino)propyltrimethoxysilane), andβ-trichlorosilyl-2-ethylpyridine by expos-ing the electrodes to 2–10% organosi-lane solutions in refluxing benzene orxylene under N2 for up to 12 h. Theelectrodes were washed with dry ben-zene. A similar procedure was used forsilanizing SnO2 and TiO2 at room tem-perature with lower silane concentrations.


Another brief exposure was performedwith 1% organosilane in benzene at 6 ◦Cunder argon for 10 s. A more aggres-sive silanization procedure was used tomodify glass with neat organosilane in asealed tube at 90 ◦C for 12 h. The authorsevaluated their films via XPS to deter-mine thickness and approximate surfacecoverages.

Finklea and Murray accomplished thesilanization of single crystal TiO2 fol-lowing a 5 min exposure of the elec-trodes to 10% silane solutions in ei-ther toluene or benzene. Excess silanewas removed with fresh solvent, whilemethanol was found to reduce the silanecoverage [51]. The silanes evaluated inthese studies were 3-(2-aminoethylamino)-propyltrimethoxysilane, methyltrichlorosi-lane, and dimethyl-dichlorosilane. XPSsurface analysis showed the presence ofsome cross-linking but could not conclu-sively characterize the homogeneity of themodified surface. Their photocurrent re-sults indicated that the M−O−Si bondis oxidatively stable to photo-generatedholes and, therefore, may provide a use-ful means of modifying TiO2 particles foruse in dye sensitized solar cells DSSCs. Re-dox active and photoelectrochemically ac-tive metal phthalocyanines were attachedto γ -aminopropyltriethoxysilane-modifiedSnO2. Tetrasulfonated cobalt and copperphthalocyanines (CoTSPc and CuTSPc)were modified with thionyl chloride, toprovide for attachment to the silanized sur-face via formation of sulfonamide linkages[47, 50].

Phenyl and diphenyl silane-modifiedITO surfaces have been used to im-prove the physical compatibility with Lang-muir–Blodgett thin films of phthalocya-nine assemblies, where the direct electrontransfer of the assembly with the ITO sur-face was of concern [46].

C60 has been tethered to ITO ina self-assembled network by exposingthe ITO to basic conditions followedby a reflux in high concentrations of(MeO)3Si(CH2)3NH2 in benzene for halfa day [52]. The electrodes were then mul-tiply rinsed before refluxing in a 1-mMsolution of C60 in benzene for as long as2 days. This scheme resulted in an ITO-silane-N-C60 tethered network and allowedfor the formation of an organized C60

layer on the ITO surface. Quartz and glasswere similarly modified for spectroscopiccomparisons. Nearly, all monolayer cover-age of C60 was determined electrochem-ically, 1.7 × 10−10 mols cm−2. Reductionof C60 in these films is shifted morenegatively, indicating a direct interactionbetween the underlying silyl-amine layerand C60.

1.1.3.3 Silane Modification of Electrodesfor Use in DevicesPerfect cross-linking of silane moleculesis unlikely in most circumstances due toa mismatch in the lattice spacing betweenthe cross-linked silane layer and the un-derlying metal oxide substrate. A more ac-curate picture of a silanized surface wouldconsist of some silane molecules attachedvia multidentate binding interspersed withcross-linked neighboring silanes as shownin Fig. 6(b). Extensive cross-linking of sur-face silanes may provide a stable modifiedsurface; however, it also appears to impedeelectron transfer and, for optimization ofsolution redox processes, may not be de-sirable [1, 46].

Marks and coworkers have shown howextensively cross-linked silane monolayersand multilayers can be used to stabilize anelectrode and regulate charge injection in acondensed OLED phase [14, 53–55]. Theirapproach has been to start with silanesfunctionalized with various redox active


groups and to terminate by functionalgroups, which can subsequently be con-verted to new reactive silanes, to producea layer-by-layer ‘‘self-limiting’’ chemistry.Multilayer formation can be well con-trolled, leading to desired film thicknessesand orientation of the charge transport-ing core molecules [14]. In general, thesemodified electrodes show significantly im-proved performance as anodes in OLEDs,apparently due to the fact that hole injec-tion into the tri-arylamine hole transportlayer (which is the core molecule beingcross-linked by these silanes), is impededafter modification. This allows for a morebalanced injection of both holes and elec-trons in the device and results in theemissive state forming in the middle ofthe thin film device [14].

Chlorosilanes are often used to aid inthe extensive cross-linking that is desiredfor the modification of anodes for usein OLED devices. Chlorosilanes liberateHCl as a by-product of silane attachment,which can degrade the TCO surface if lowconcentrations of base are not added forneutralization. No significant etching ofthe TCO anode surface has been noted, andsurface analysis of the modified electrodegenerally indicates no entrapped chlorine[14, 51–53].

Silane modification of ITO and SnO2electrodes has also been used to createnearly perfect ‘‘blocking’’ electrodes, suchas thin films where rates of electron trans-fer are intentionally kept low, so as to pro-vide for measurement in changes in inter-facial potential of an additional overlayer,such as a bilayer lipid membrane, duringion transport, pH changes, and so forth.Hillebrandt and Tanaka have evaluated theeffects of octyltrimethoxysilane (OTMS),octadecyltrimethoxysilane (ODTMS), andoctadecyltrichlorosilane (OTS) on ITOfor applications to lipid-membrane-based

biological sensors [56–58]. The authorsfound that the ITO electrodes can be ef-fectively blocked from electron transfer bysilanization. Silanization results in moreuniform films suitable for the biologicalsensor platform, with ionic diffusion oc-curring almost entirely from pinholes inthe film. Silanization was accomplished bysonicating ITO substrates in 5 vol% solu-tions of the silane in dry toluene with 0.5vol% n-butylamine, followed by 30 min ofincubation. Temperatures were held be-low 293 K for ODTMS and below 280 Kfor OTMS. Sonication in dry toluene for2 min removed excess silane. Markovichand Mandler also derivatized ITO withODTMS but derivatization was performedover a seven-day period [59, 60].

The long alkyl chain length and stabilityof cross-linked silanized surfaces alsoallows for their use as masks for patterningand etching TCO substrates. ODTMS hasbeen used as a mask for patterning ITO[61]. Luscombe et al. have used perfluoro-decyltrichlorosilane as a mask for etchresists for lithographic processing of ITOsurfaces [62].

1.1.4Modification of TCO Surfaces throughChemisorption of Small Molecules

1.1.4.1 Modification using SmallMolecules Containing Carboxylic AcidFunctionalityModification of electrode surfaces throughchemisorption has been more in recent re-ports than silanization, and its discussionhere is intended to contrast the strategiesused for both types of surface modifica-tion. Chemisorption of small moleculesto the TCO surface allows for robustmodifications occurring over short dis-tances and with relatively strong interactiveforces (�H = ca. 200 kJ mol−1) through


combinations of electrostatic, H-bonding,and metal ion coordination. Chemisorp-tion processes generally have a high ac-tivation barrier for adsorption, may beirreversible, and can ‘‘self-limit’’ at mono-layer coverage. Owing to the amphotericnature of metal oxide surfaces, chemisorp-tion of Lewis acids such as carboxylicacids, aliphatic and aromatic amines, andphosphonic acids are generally successful.Chemisorption typically anchors individ-ual molecules in close proximity to metalcationic sites and may also compete forsites with surface hydrolysis products. Pre-vious work has examined chemisorptionto ITO, SnO2, and TiO2 surfaces, withmost of the studies reporting adsorptionof either carboxylic or phosphonic acids orsalts.

Carboxylate functionalities have beenused repeatedly and successfully as ananchoring group for molecular chemisorp-tion to oxide electrodes proceeding throughthe formation of an ester linkage to themetal with the loss of water, or throughhydrogen-bonding interactions to surfacehydroxyl groups. Possible interactions forcarboxylic groups are shown in Fig. 7.Binding constants for the carboxylic group

for ITO and ATO have been determined tobe as high as 8 × 104 M−1 [34].

Chemisorption is often as successful assilanization at obtaining monolayer cov-erages of small molecule modifiers. Asan example, Davis and Murray attemptedto couple iron porphyrins to amine-terminated silanized SnO2 surfaces. Theauthors found that the surface coveragewas independent of the apparent extent ofsurface silanization [63]. Silanized and un-silanized electrodes were soaked in 5 mMporphyrin dissolved in DMF, THF, or pyri-dine solutions overnight. Porphyrins withpendant carboxylic acid groups producedhigher surface coverages (determinedby coulometric analysis of the voltam-metry of the surface-confined species)on bare, unmodified SnO2 than seenon SnO2 electrodes modified with 3-(2-aminoethylamino)propyltrimethoxysilane.

Zotti and coworkers adsorbed sev-eral carboxylated ferrocene deriva-tives on ITO: ferrocene-carboxylic acid(Fc(COOH)), ferrocene-dicarboxylic acid(Fc(COOH)2), and ferrocenylheptanoicacid Fc(CH2)6COOH [64]. Fc(COOH) wasadsorbed from 1-mM solutions of themolecule in 5/95 ethanol/hexane for as

(a)

(b) (c)

O

C

R

O

O

C

RO

O

CR O

OH

OH

MM M M

O

O

M

O

M

OO

M

O O

O

OO

OH

OOOOHHH

O

M

OO

R

O

M

O

HH

O

OO

O

M

O

R

O

O

R

OO

O

H

O

M

O

O O

MM

Fig. 7 Schematic views of the possibleinteraction modes of carboxylic acidswith TCO surfaces.(a) ‘‘Metal–ester’’-like interactions;(b) ‘‘bridging’’ coordination with metalion sites (created by formation ofoxygen vacancies in the lattice);(c) ‘‘chelating’’ interaction – onecarboxylate per open metal site [30].


long as 16 h, followed by a rinse withacetonitrile. Higher solution concentra-tions were found to yield higher surfacecoverage, following a Langmuir adsorptionisotherm, with saturation coverage (ca. 1 ×10−10 mols cm−2) occurring at approx-imately 10 mM solution concentration.The saturation coverage, however, was25% of the expected monolayer coverage(4 × 10−10 mols cm−2) based on the geo-metric area [65]. Fc(COOH)2 was adsorbedfrom ethanol, while Fc[(CH2)6COOH] wasadsorbed from hexane. Adsorption of thesemolecules appears to be kinetically lim-ited. Recent work has shown that timefor adsorption is required to remove hy-drolysis products (e.g. In(OH)3) or othernonelectroactive species adsorbed to theITO surface [66].

Fc(COOH)2 has been used success-fully to increase electron transfer ratesand for enhancing the performance oforganic light emitting devices and or-ganic photovoltaics [24, 37]. In additionto modifications of electron transfer rates,carboxylic acid-modified small moleculeshave been used to introduce dipole fieldsat the surface of TCO substrates. Forexample, benzoic acid derivatives havebeen used to modify the interface dipoleat ITO surfaces, attached to the ox-ide surface via formation of a Lang-muir–Blodgett thin film. Changes inwork function closely relate to changes

in the dipole induced by the derivatizedmodifiers [67].

Dye molecules possessing carboxylatefunctionalities have been extensively ex-plored as sensitizers of semiconductingoxide electrodes toward visible light inphotoelectrochemical cells [4, 32, 68]. Dyesensitization was first explored as a meansof enhancing the photocatalytic propertiesof TiO2 [69–72]. Spitler and coworkersinvestigated dye sensitization of ZnO elec-trodes using carboxylic acid functionalizeddyes [4–6]. The authors found that the pho-tocurrent produced by dye sensitized TiO2

and ZnO crystals closely resembled the ab-sorption spectrum of the dye, indicatingthat the dye is in close proximity to thecrystal and, upon excitation, injects chargeinto the semiconductor from energy levelsthat are relatively unperturbed.

The observation of photocurrent fromdye sensitized planar semiconductorsled to the evolution of the DSSC,on the basis of extremely high sur-face area nanoparticulate oxides, usu-ally TiO2. The most successful dye forsensitization, N3, cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′dicarboxylato) ruthe-nium (II), employs several carboxylic acidgroups for adsorption to TiO2, ZnO, SnO2,and/or ITO from an ethanol or acetoni-trile solution over a several-hour period[32]. Electron injection from the excitedstate of this dye to the conduction band

Fig. 8 Schematic views of possiblehydrogen-bonding interactions ofprotonated carboxylic acids to an idealTCO surface. (a) Interaction modewhere the protonated carboxylic acidH-bonds to a bridging oxygen. Ahydroxylated metal site also H-bonds tothe second carboxylic oxygen. (b) Asingle hydrogen bond is also plausible.

MM

O

O

M

O O

OO

H

OO

C

R

H

O

H

O

O

C

R

O

M

O

M

O O

OOH

O

HO

O

H

(a) (b)


of the oxide is believed to occur on afemtosecond timescale [73, 74]. Bindingconstants for the carboxylated dyes assum-ing an ester linkage are 8 × 104 M−1 onSnO2 from chloroform [73]. A similar car-boxylated ruthenium complex has beenevaluated on ATO, ITO, TiO2, and SiO2[34]. The authors argue for ester linkageson all substrates except silica. On silica,adsorption occurs through a chelating car-boxylato link with a second carboxylategroup hydrogen bonding to the surface.Hydrogen-bonding interactions for proto-nated carboxylic groups to TCO surfacescan also be envisioned as shown in Fig. 8.Notice that physisorption can occur tometal oxide surfaces, which are not hydrox-ylated, by formation of hydrogen bonds tobridging oxygens.

1.1.4.2 Modification Using PhosphonicAcids and Other ChemisorbingFunctionalitiesPhosphonic acids are known to chemisorbstrongly to oxides such as ITO and havebeen used to help produce lithograph-ically patterned ITO surfaces [75, 76].The schematic view of the adsorption ofsuch materials is shown in Fig. 9. Adsorp-tion of hexylferrocene phosphonic acid(Fc[(CH2)6PO(OH)2]) has been investi-gated by Vercelli et al. from ethanol tosilanized and unsilanized ITO surfaces

[77]. A maximum surface coverage wasobtained at 4.2 × 10−10 mols cm−2, ap-proximately four times the surface cover-age obtained with the analogous carboxy-lated ferrocene. Cyclic voltammetry (CV)of the adsorbed (Fc[(CH2)6PO(OH)2]) ex-hibits a symmetric voltammogram aboutthe x-axis (applied potential axis). Thehigh degree of symmetry suggests fast, re-versible electron transfer at the interface.Additionally, the modifying moleculeswere found to be very robust to CV cycling.

An amine derivatized ferrocenemolecule, Fc[CH2N(CH3)2], was used tomodify ITO, although less successfullythan the carboxylic derivatives [64].Modification with amines tends to createa less robust modification, where themolecules can be easily removed withsolvents. CV cycling can also leadto the loss of amine modifiers. Thisphenomenon is clearly a result of theweaker Lewis base characteristics ofthe amine for the Lewis acid siteson the metal oxide surface. Han andcoworkers, however, were able to designa dense self-assembled monolayer of 1,12-diaminododecane from methanol ontoITO after 60 h of immersion [78]. Theterminating amine group allowed for thesubsequent addition of a monolayer ofphosphomolybdic acid to provide redoxactive centers. The diaminododecane, at

M

O

M

O

M

O O

OH

POH

R

O OO

OH

OH

O

M

O

M

O O

OH

P

O

OH

R

O

M

O O

O

M

O

M

O O

HO

P

OO

R

O

M

O

(a) (b) (c)

–H2O –H2O

Fig. 9 Schematic views of the possible interaction modes of phosphonic acids with TCOsurfaces; (a) monodentate attachment (b) unidentate attachment and (c) multidentateattachment.


6 × 10−10 mols cm−2, provided a platformfor one-third as much redox activeacid. This protocol may allow for themodification of oxide surfaces throughchemisorption to obtain surfaces similarto those obtained through silanization;however, the robustness of this platformwas not evaluated.

Cyanuric chloride has been used tomodify oxide and graphite electrodes [79].The linkage, an ether bond through surfacehydroxyls, is short and strong, allowing fora robust surface and enhanced electrontransfer between the surface and thearomatic terminal group.

Another modification scheme of note isthe selective addition of tin-phenoxides onan ITO surface, a process developed bySchwartz and coworkers for the modifica-tion of the effective work function of ITOsurfaces [13, 80]. Because of the low con-centration of tin sites on such surfaces,this modification scheme, which does notmodify exposed indium sites, places thefunctional groups at some distance fromeach other, a type of modification notafforded by other modification schemesbased on covalent bond formation, orchemisorption.

Heme proteins have also been adsorbedto TCO surfaces as ‘‘modifiers’’ of theirelectrochemical properties. These adsorp-tion processes are undoubtedly a combi-nation of electrostatic forces, H-bonding,and metal ion coordination in the TCOsurface, and produce reasonably robustand active modifying layers. Cytochrome C

(cyt C) has been the most extensively stud-ied adsorbed heme protein. Hawkridgeand coworkers have examined the elec-trochemical behavior at gold, platinum,and metal oxide electrodes [81]. Electrontransfer rates as high as 10−2 cm s−1

were obtainable at ITO electrodes. Bowdenand coworkers have determined that the

electron transfer rates are associated witha conformational change of the moleculein the adsorbed state at the electrode sur-face [82, 83]. Above a certain scan rate,the conformational change can be over-come. More recently, Runge and Saavedrashowed that cyt C forms a sufficientlystable adsorbed state such that it can be mi-crocontact printed on ITO surfaces, whileretaining high electron transfer rates [84].

1.1.5Conclusions

The modification of TCO surfaces canclearly be used to enhance the electro-chemical performance of oxide electrodes.There are, however, issues yet to beresolved regarding the initial surface com-position of the oxide, especially for ITO,which prevent realization of the full elec-trochemical and electronic potential ofthese electrodes. In some cases, the mod-ification chemistries produce a surface,which is sufficiently robust to be usedin various sensor platforms or condensedphase devices. However, it is not yetclear whether long-term stability can beachieved in those cases where the oxideis exposed to solutions that also promotethe hydrolysis of the oxide unless an ex-tremely strong covalently bonded network,or chemisorption interaction can be pro-duced. These modification strategies willcontinue to evolve with the increasing needfor viable interfaces between electroactivematerials and the metal oxide electrode.

Acknowledgments

Work cited in this review from ourlaboratories was supported in part bythe National Science Foundation (Chem-istry); the Office of Naval Research; the


Department of Energy, National Renew-able Energy Laboratories Program, Photo-voltaics Beyond the Horizon; the Depart-ment of Energy, Basic Energy Sciences,and the National Institutes of Health.

References

1. R. W. Murray, Molecular design of electrodesurfaces in Techniques of Chemistry (Ed.:R. W. Murray), John Wiley & Sons, NewYork, 1992, pp. 1–48, Vol. 22.

2. F. Decker, J. Melsheimer, H. Gerischer, Isr.J. Chem. 1982, 22, 195–198.

3. K. Kalyanasundaram, M. Gratzel, Photosen-sitization and Photocatalysis Using Inorganicand Organometallic Compounds, Kluwer Aca-demic Publishers, Boston, 1993.

4. M. A. Ryan, M. T. Spitler, Langmuir 1988, 4,861–867.

5. M. T. Spitler, M. Calvin, J. Chem. Phys. 1977,66, 4294–4305.

6. M. T. Spitler, M. Calvin, J. Chem. Phys. 1977,67, 5193–5200.

7. J. Cui, A. Wang, N. L. Edleman et al., Adv.Mater. 2001, 13, 1476.

8. J. S. Kim, F. Cacialli, R. Friend, Thin SolidFilms 2003, 445, 358–366.

9. J. S. Kim, R. H. Friend, F. Cacialli, Synth.Met. 2000, 111–112, 369–372.

10. R. W. Murray, Acc. Chem. Res. 1980, 13,135–141.

11. J. J. Pesek, M. T. Matyska, Interface Sci. 1997,5, 103–117.

12. K. D. Snell, A. G. Keenan, Chem. Soc. Rev.1979, 8, 259–282.

13. A. R. Span, E. L. Bruner, S. L. Bernasek et al.,Langmuir 2001, 17, 948–952.

14. J. E. Malinsky, G. E. Jabbour, S. E. Shaheenet al., Adv. Mater. 1999, 11, 227–231.

15. R. A. Hatton, S. R. Day, M. A. Chesters et al.,Thin Solid Films 2001, 394, 292–297.

16. C. Ganzorig, K. J. Kwak, K. Yagi et al., Appl.Phys. Lett. 2001, 79, 272–274.

17. S. Besbes, A. Ltaief, K. Reybier et al., Synth.Met. 2003, 138, 197–200.

18. P. K. H. Ho, M. Granstrom, R. H. Friendet al., Adv. Mater. 1998, 10, 769–774.

19. H. L. Hartnagel, A. L. Dawar, A. K. Jainet al., Semiconducting Transparent Thin Films,

Institute of Physics Publishing, Philadelphia,1995.

20. R. Hengerer, B. Bolliger, M. Erbudak et al.,Surf. Sci. 2000, 460, 162–169.

21. L.-Q. Wang, D. R. Baer, M. H. Engelhardet al., Surf. Sci. 1995, 344, 237–250.

22. L.-Q. Wang, K. F. Ferris, A. N. Shultz et al.,Surf. Sci. 1997, 380, 352–364.

23. U. Diebold, Appl. Phys. A. 2003, 76, 681–687.24. C. Donley, D. Dunphy, D. Paine et al., Lang-

muir 2002, 18, 450–457.25. J. C. C. Fan, J. B. Goodenough, J. Appl. Phys.

1977, 48, 3524–3530.26. N. D. Popovich, S. S. Wong, S. Ufer et al., J.

Electrochem. Soc. 2003, 150, H255–H259.27. N. D. Popovich, B. K. Yen, S. S. Wong, Lang-

muir 2003, 19, 1324–1329.28. M. Lazzeri, A. Vittadini, A. Selloni, Phys. Rev.

B: Condens. Matter 2001, 63, 155409.29. M. R. Hoffmann, S. T. Martin, W. Choi et al.,

Chem. Rev. 1995, 95, 69–96.30. K. Kalyanasundaram, M. Gratzel, Coord.

Chem. Rev. 1998, 177, 347–414.31. M. Fujihira, Y. Satoh, T. Osa, J. Electroanal.

Chem. 1981, 126, 277–281.32. M. K. Nazeeruddin, R. Humphry-Baker,

P. Liska et al., J. Phys. Chem., B 2003, 107,8981–8987.

33. M. K. Nazeeruddin, M. Gratzel, J. Pho-tochem. Photobiol., A 2001, 145, 79–86.

34. T. J. Meyer, G. J. Meyer, B. W. Pfennig et al.,Inorg. Chem. 1994, 33, 3952–3964.

35. J. S. Kim, F. Cacialli, M. Granstrom et al.,Synth. Met. 1999, 101, 111–112.

36. Y. H. Liau, N. F. Scherer, K. Rhodes, J. Phys.Chem., B. 2001, 105, 3282–3288.

37. C. L. Donley, Interfaces in Organic Elec-tronic Devices: Surface Characterization andModification and Their Effect on Microstruc-ture in Molecular Assemblies, PhD in Chem-istry. 2003, University of Arizona, Tucson.

38. C. F. Baes Jr., R. E. Mesmer, The Hydrolysisof Cations, John Wiley & Sons, New York,1976.

39. D. C. Harris, Quantitative Chemical Analysis,W. H. Freeman and Company, New York,1999.

40. W. F. Linke, Solubilities: Inorganic and Metal-Organic Compounds, D. Van Nostrand Co.,Inc., New York, 1958.

41. W. F. Linke, Solubilities: Inorganic and Metal-Organic Compounds, American ChemicalSociety, Washington D.C., 1965.


42. J. J. Kirkland, J. J. B. Adams, M. A. vanStraten et al., Anal. Chem. 1998, 70,4344–4352.

43. D. F. Untereker, J. C. Lennox, L. M. Wieret al., J. Electroanal. Chem. 1977, 81, 309–318.

44. P. Kirkov, Electrochim. Acta 1972, 17,519–532.

45. P. Kirkov, Electrochim. Acta 1972, 17,533–547.

46. C. Donley, D. Dunphy, W. J. Doherty et al.,Molecules as Components in Electronic Devices,American Chemical Society, Washington,2003.

47. N. R. Armstrong, V. R. Shepard, J. Elec-troanal. Chem. 1980, 115, 253–265.

48. N. R. Armstrong, A. W. C. Lin, M. Fujihiraet al., Anal. Chem. 1976, 48, 741–750.

49. N. R. Armstrong, V. R. Shepard, J. Phys.Chem. 1981, 85, 2965–2970.

50. V. R. Shepard, N. R. Armstrong, J. Phys.Chem. 1979, 83, 1268–1276.

51. H. O. Finklea, R. W. Murray, J. Phys. Chem.1979, 83, 353–358.

52. K. Chen, W. B. Caldwell, C. A. Mirkin, J. Am.Chem. Soc. 1993, 115, 1193–1194.

53. J. Cui, Q. L. Huang, J. C. G. Veinot et al.,Langmuir 2002, 18, 9958–9970.

54. Q. Huang, G. Evmenenko, P. Dutta et al., J.Am. Chem. Soc. 2003, 125, 14704–14705.

55. W. Li, Q. Wang, J. Cui et al., Adv. Mater.1999, 11, 730–734.

56. H. Hillebrandt, M. Tanaka, J. Phys. Chem., B2001, 105, 4270–4276.

57. H. Hillebrandt, M. Tanaka, E. Sackmann, J.Phys. Chem., B 2002, 106, 477–486.

58. H. Hillebrandt, G. Wiegand, M. Tanakaet al., Langmuir 1999, 15, 8451–8459.

59. I. Markovich, D. Mandler, J. Electroanal.Chem. 2000, 484, 194–202.

60. I. Markovich, D. Mandler, J. Electroanal.Chem. 2001, 500, 453–460.

61. N. L. Jeon, K. Finnie, K. Branshaw et al.,Langmuir 1997, 13, 3382–3391.

62. C. K. Luscombe, H. W. Li, W. T. S. Hucket al., Langmuir 2003, 19, 5273–5278.

63. D. G. Davis, R. W. Murray, Anal. Chem.1977, 49, 194–198.

64. G. Zotti, G. Schiavon, S. Zecchin et al., Lang-muir 1998, 14, 1728–1733.

65. J. Y. Gui, D. A. Stern, F. Lu et al., J. Elec-troanal. Chem. 1991, 305, 37–55.

66. C. Carter, M. T. Brumbach, C. Donley et al.,manuscript in preparation.

67. M. Carrara, F. Nuesch, L. Zuppiroli, Synth.Met. 2001, 121, 1633–1634.

68. W.-P. Tai, Sol. Energy Mater. 2003, 76, 65–73.69. T. Hoffmann, D. Wrobel, J. Mol. Struct. 1998,

450, 155–161.70. P. R. Moses, R. W. Murray, J. Am. Chem. Soc.

1976, 98, 7435–7436.71. P. Cuendet, M. Gratzel, M. L. Pelaprat, J.

Electroanal. Chem. 1984, 181, 173–185.72. P. Cuendet, M. Gratzel, Bioelectrochem.

Bioenerg. 1986, 16, 125–133.73. A. Hagfeldt, M. Gratzel, Chem. Rev. 1995, 95,

49–68.74. A. Hagfeldt, S.-E. Lindquist, M. Gratzel, Sol.

Energy Mater. 1994, 32, 245–257.75. T. L. Breen, P. M. Fryer, R. W. Nunes et al.,

Langmuir 2002, 18, 194–197.76. T. B. Carmichael, S. J. Vella, A. Afzali, Lang-

muir 2004, 20, 5593–5598.77. B. Vercelli, G. Zotti, G. Schiavon et al., Lang-

muir 2003, 19, 9351–9356.78. S.-Y. Oh, Y.-J. Yun, D.-Y. Kim et al., Lang-

muir 1999, 15, 4690–4692.79. A. M. Yacynych, T. Kuwana, Anal. Chem.

1978, 50, 640–645.80. E. L. Bruner, N. Koch, A. R. Span et al., J.

Am. Chem. Soc. 2002, 124, 3192–3193.81. E. F. Bowden, F. M. Hawkridge, H. N.

Blount, J. Electroanal. Chem. 1984, 161,355–376.

82. J. L. Willit, E. F. Bowden, J. Electroanal.Chem. 1987, 221, 265–274.

83. A. El Kasmi, M. C. Leopold, R. Galligan et al.,Electrochem. Commun. 2002, 4, 177–181.

84. A. F. Runge, S. S. Saavedra, Langmuir 2003,19, 9418–9424.

preparation of monolayer modified electrodes

Documents