Electrochemistry vol 11 nanosystems electrochemistry

Specialist Periodical Reports

Edited by R G Compton and J D Wadhawan

Electrochemistry
Volume 11
Nanosystems Electrochemistry


Electrochemistry
Volume 11: Nanosystems Electrochemistry



A Specialist Periodical Report

Electrochemistry
Volume 11: Nanosystems
Electrochemistry
A Review of Recent Literature
Editors
Richard G. Compton, University of Oxford, UK
Jay D. Wadhawan, University of Hull, UK
Authors
Mathieu Etienne, Universite´ de Lorraine (UHP Nancy I), France
Jonathan E. Halls, University of Bath, UK
Alexander Kuhn, Universite´ de Bordeaux, France
Gabriel Loget, Universite´ de Bordeaux, France
Emmanuel Maisonhaute, Laboratoire Interfaces et Syste`mes
Electrochimiques, UPMC Univ Paris 06, CNRS, France
Vicente Montiel, Instituto Universitario de Electroquı´mica, Universidad de Alicante,

Spain
Martin Pumera, Nanyang Technological University, Singapore
Carlos M. Sa´nchez-Sa´nchez, Instituto Universitario de Electroquı´mica,
Universidad de Alicante, Spain
Jose´ Solla-Gullo´n, Instituto Universitario de Electroquı´mica, Universidad de
Alicante, Spain
Alain Walcarius, Universite´ de Lorraine (UHP Nancy I), France
Xiao-Shun Zhou, Institute of Physical Chemistry, Zhejiang Normal University,
China


If you buy this title on standing order, you will be given FREE access
to the chapters online. Please contact with proof of
purchase to arrange access to be set up.
Thank you

ISBN: 978-1-84973-401-1
DOI: 10.1039/9781849734820
ISSN: 0305-9979
A catalogue record for this book is available from the British Library
& The Royal Society of Chemistry 2013
All rights reserved
Apart from any fair dealing for the purpose of research or private study for
non-commercial purposes, or criticism or review, as permitted under the terms
of the UK Copyright, Designs and Patents Act, 1988 and the Copyright and
Related Rights Regulations 2003, this publication may not be reproduced,
stored or transmitted, in any form or by any means, without the prior
permission in writing of The Royal Society of Chemistry, or in the case of
reprographic reproduction only in accordance with the terms of the licences
issued by the Copyright Licensing Agency in the UK, or in accordance with the

terms of the licences issued by the appropriate Reproduction Rights
Organization outside the UK. Enquiries concerning reproduction outside the
terms stated here should be sent to The Royal Society of Chemistry at the
address printed on this page.
Published by The Royal Society of Chemistry,
Thomas Graham House, Science Park, Milton Road,
Cambridge CB4 0WF, UK
Registered Charity Number 207890
For further information see our web site at www.rsc.org


Preface
DOI: 10.1039/9781849734820-FP005

We are delighted to introduce this re-launched series of Specialist Periodical
Reports in Electrochemistry, to serve the global community with topical,
critical and tutorial reviews covering the breadth of Electrochemical
Science, Technology and Engineering.
Electrochemistry is the study of charge transfer across an interface, and
finds application and relevance to a plurality of subfields and disciplines
such as energy and environmental science, materials science, physical,
organic, inorganic and analytical chemistries, engineering, earth sciences,
biology and medicine. It is a subject that empowers the engineering of
devices for monitoring the state of our health, for converting the ‘‘free’’
energy from the Sun to workable power that we may consume, and for large
scale systems, for extracting materials we require on a day-to-day basis
from their naturally occurring inorganic minerals. There is even common
ground between stock markets, particularly those dealing with Futures and
Options, with electrochemical systems, notably those involving ‘‘diffusionwith-drift’’ – hydrodynamic electrodes. It is a diverse and engaging subject
that is of major significance in the present world of the ever-increasing

electrification; we have endeavoured to embrace and encompass this cultural and technological expansivity of our subject through this book series,
and capture the essence of this big picture through the artwork that forms
the front cover image of this series – it is Migration by Pia de Richemont/
www.piaderichemont.com; we thank Pia for allowing us to use her work.
This first volume under our editorship, and Volume 12, are concerned
with Nanosystems Electrochemistry – the study of interfacial charge transport when the materials or interfaces are spatially confined to submicron
levels. The use of small electrodes is highly advantageous since the electrical
time constant decreases as the electrode size reduces, allowing for ultrafast
measurements (timescale on the order of ten nanoseconds) to occur. In the
first chapter, Emmanuel Maisonhaute and Xiao-Shun Zhou provide an
overview of Electrochemistry to Record Single Events, introducing the
reader to nanoelectrodes, nanopores, nanogaps, nanoparticle detection,
molecular electronics using single molecules, and the impact of tiny, fast
moving acoustic cavities on electrified interfaces. Carlos Sa´nchez-Sa´nchez,
Jose Solla-Gullo´n and Vincent Montiel take this further in their chapter on
Electrocatalysis at Nanoparticles, where they examine the functional competency of nanoparticle index plane on the electrocatalysis of nanoparticles
with low-co-ordinated surface atoms – an area of immense importance for
fuel cell material science, and move to the study of single nanoparticles on
nanoelectrodes, where they detail the effects of single nanoparticle collisions
through the amplification of electrocatalysis. In moving to apply these
principles to develop innovative devices for the nanoworld, Gabriel Loget
and Alexander Kuhn review the fascinating field of Biopolar Electrochemistry in the Nanosciences, where control of the electrical field between
two electrodes is employed to induce motion and transformation of
Electrochemistry, 2013, 11, v–vi | v

c

The Royal Society of Chemistry 2013

particles contained therein without direct mechanical contact of a material
with an electrode.
The nanostructuring of an electrode is considered in the chapters by
Martin Pumera, and Mathieu Etienne and Alain Walcarius. Pumera reviews
Nanocarbon Electrochemistry, covering graphene, carbon nanotubes and
doped nanodiamond; Etienne and Walcarius provide a pedagogic account
on Electrochemistry within Templated Nanosystems covering the preparation, electrochemistry and applications of metallic nanostructured electrodes, metal oxide and sol-gel derived nanomaterials, and ordered macro- and
mesoporous carbons. Last, Jonathan Halls and Jay Wadhawan provide a
tutorial overview on Electrochemistry within Liquid Nanosystems, where
they examine the effect when the solvent into which an electrode is immersed
contains some form of long-range order – ‘‘liquid nanotechnology’’, with
particular emphasis on the electrochemistry within lyotropic liquid crystals –
quasi-biphasic nanosystems where water may not exist in a bulk state, so
that macroscopic properties such as pH become essentially meaningless
concepts.
We hope you enjoy this volume. It remains for us to thank Merlin
Fox, Alice Toby-Brant, Leanne Marle, Katrina Harding and the rest of the
RSC Publishing team for all their diligent work, and Bruce Gilbert and the
Specialist Periodical Reports Editorial Board for enabling the resurrection
of this book series.
Richard Compton
Oxford University
Jay Wadhawan
University of Hull

vi | Electrochemistry, 2013, 11, v–vi


CONTENTS
Cover

Migration by Pia de Richemont
(www.piaderichemont.com).

Preface
Richard Compton and Jay Wadhawan

v

Electrochemistry to record single events
Xiao-Shun Zhou and Emmanuel Maisonhaute
1 Introduction
2 Individual systems explored with nanoelectrodes
3 Single molecules for molecular electronics
4 A fast moving nanometric interface: the example of
acoustic cavitation
5 Conclusions
Acknowledgements
References

1

Electrocatalysis at nanoparticles
Carlos M. Sa´nchez-Sa´nchez, Jose Solla-Gullo´n and
Vicente Montiel
1 Introduction
2 Electrocatalysis at nanoparticles with low-coordinated
surface atoms
3 Electrocatalytic reactions studied at single nanoparticles
4 Summary
Acknowledgements

References

1
2
7
21
26
27
27

34

34
35
59
65
66
66

Electrochemistry, 2013, 11, vii–viii | vii

c

The Royal Society of Chemistry 2013


Bipolar
Gabriel
1
2

electrochemistry in the nanosciences
Loget and Alexander Kuhn
Introduction
Well-established and macroscopic applications of bipolar
electrochemistry
3 Novel micro- and nano applications of bipolar
electrochemistry
4 Conclusion
References

71
71
82
86
99
99

Nanocarbon electrochemistry
Martin Pumera
1 Introduction
2 Graphene
3 Carbon nanotubes
4 Doped nanodiamonds
5 Concluding remarks
References

104

Electrochemistry within template nanosystems

124

Mathieu Etienne and Alain Walcarius
1 Introduction
2 Metallic nanostructured electrodes
3 Metal oxide and sol-gel-derived nanomaterials on/as
electrodes
4 Ordered macro- and mesoporous carbons
5 Conclusion
References

104
107
114
120
121
122

124
126
141
166
181
182

Electrochemistry within liquid nanosystems

198

Jonathan E. Halls and Jay D. Wadhawan
1 Introduction
2 Assembly of liquid nanosystems
3 Consequences of restricted media
4 Electrical circuit equivalent of lyotropic liquid crystals
5 Electron transfer kinetics within liquid nanosystems
6 Transport within liquid crystal media
7 Applications
8 Conclusions
Acknowledgements
References

198
199
204
207
208
211
227
231
232
232

viii | Electrochemistry, 2013, 11, vii–viii


About the Editors
Richard G Compton is Professor of Chemistry and Aldrichian Praelector at
Oxford University, United Kingdom where he is also Tutor in Chemistry
at St John’s College. Compton has broad interests in both fundamental

and applied electrochemistry and electroanalysis including nanochemical
aspects. He has published more than 1100 papers (h = 67; Web of Science,
July 2012), 6 books and numerous patents. The 2nd edition of his graduate
textbook Understanding Voltammetry (with C E Banks) was published in
late 2010 by Imperial College Press.
He is CAS Visiting Professor at the Institute of Physical Sciences,
Hefei and a Lifelong Honorary Professor at Sichuan University. He holds
Honorary Doctorates from the Estonian Agricultural University and
Kharkov National University of Radioelectronics (Ukraine) and is a Fellow
of the RSC and of the ISE. He is the Founding Editor and Editor-in-Chief
of the journal Electrochemistry Communications (current IF=4.86) published by Elsevier.
Scientist ranking (Essential Science Indicators, ISI, June, 2012): # 120 of
7849 top 1% Scientists in Chemistry; #133 of 7180 top 1% Scientists in
Engineering; 1444 of 70037 top 1% Scientists (all fields).
Jay D. Wadhawan (age 34) is Senior Lecturer in Electrochemical Science,
Technology & Engineering at University of Hull, where he represents the
Faculty of Science at Senate. He is Vice-Chair in Molecular Electrochemistry at the International Society of Electrochemistry and Research CoChampion in Electrocatalysis for Carbon Capture and Utilisation for the
CO2CHEM EPSRC Grand Challenge Network. He has acted as a Tutorial
Lecturer at Universidade Federale de Alagoas, Brazil, Visiting Professor at
Universite´ de Bordeaux 1, France, and Visiting Professor at Universite´ Paris
Diderot, France. He is an Associate Member of University of York.

Electrochemistry, 2013, 11, ix–ix | ix

c

The Royal Society of Chemistry 2013

Electrochemistry to record single events
Xiao-Shun Zhoua and Emmanuel Maisonhaute*b
DOI: 10.1039/9781849734820-00001

1

Introduction

From the nineteenth’s century pioneers such as Faraday, electrochemists
have been tracking or exploiting the consequences of charge transfer. In the
early 20th century, the polarography-derived methods allowed the first
microelectroanalysis and the rise of the mechanism notion. In molecular
electrochemistry, very complex paths could be (and still are) elucidated by
cyclic voltammetry.1 The main advantage of electrochemical techniques is
that the diffusion rate can be adjusted and thus used as a reference toward
the timescale of the event to be studied. The present temporal resolution is
about 10 ns, which amounts to disturb the concentration profiles near the
electrode only over about one nanometre.2–4 Concomitantly, for sensor or
more fundamental applications, the size of the electrodes is decreased gradually to attain presently a few nanometres.5
Nevertheless, whatever the approach, the thermodynamic and kinetic
information derived from these approaches still represent an average over a
large number charge transfer events from individual structures. There
would be of course a great interest to get the possibility to track individual
electron transfer in electrochemical systems, as has been realised by solidstate physicists in very specific systems, but it is presently impossible in
electrochemistry.6
In solution, spectroscopic tools have been the first to demonstrate that a
signal coming from individual molecules could be collected.7,8 Here, the
high number of excitation/fluorescence cycles are occurring while a molecule travels through the confocal volume of a microscope furnishes a
measurable flux of photons.9 More information such as fate due to chemical
reactions or photobleaching can be obtained relying on correlation

measurements.10,11
In electrochemistry, for amplifiers used in classical electronics, increasing
the gain induces a decrease in the bandwidth (in the simplest theory the
gain  bandwidth product is constant). The quantity to consider is thus the
minimum number of electrons that can be detected.12 With the best commercial systems available at present, this number is about 1000, which
already allows to monitor the activity of solely a few tens of enzymes acting
on a nanoelectrode.13 One physical reason for such limitation is that to be
detected one electron should induce a significant perturbation on one
observable, for example the current flowing through a transistor. The
electrochemical limitation is that the double layer capacity at electrode/
solution interface induces a noise.14 This is one of the reasons for the rise of
ultramicroelectrodes. Unfortunately, stray capacitance takes over when
a

Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical
Chemistry, Zhejiang Normal University, Jinhua, Zhejiang 321004, China
b
UPMC Univ Paris 06, CNRS, UPR15, Laboratoire Interfaces et Syste`mes Electrochimiques,
4 place Jussieu, F-75005 Paris, France. E-mail:

Electrochemistry, 2013, 11, 1–33 | 1

c

The Royal Society of Chemistry 2013


interfacial capacitance reaches subpicofarad value, a limit easily attained
corresponding to electrode diameter around 1 mm in standard conditions.
Thus, at present only multiple electron transfers have been detected in

electrochemical systems, and reaching the single electron is still a great
challenge.5 This may induce the same revolution than when individual
photons could be detected in photochemistry. Last, but not least, when
individual events are probed, some special precautions should be taken
when analysing the data, and statistical analysis is mandatory.
There are however still several scientific purposes to get information from
individual systems. Indeed, the statistical analysis gives access to the fluctuations of the system, which is another way to access the information using
the fluctuation-dissipation relationship. Second, several populations may be
identified from the average information, which helps rationalising the global
response observed onto large systems, for example in view of optimising
catalytic systems. This review will highlight recent developments where
electrochemistry is demonstrated to be a great tool to investigate single
systems behaviour. The first section will be devoted to individual systems
dispersed in solution and detected individually at small electrodes. Next, we
will focus onto the contribution of electrochemistry in break junction
experiments, thus with a focus onto molecular electronics. Last, we will
underline that a good temporal resolution allows visualisation of nanometric interfaces evolution with the example of acoustic cavitation.
2

Individual systems explored with nanoelectrodes

2.1 Redox cycling
The first way to detect single molecules is to provoke a redox cycling of the
same entity between two electrodes polarised so as to induce reduction on
the first and oxidation on the second. The same molecule then causes a large
number of electron transfer within the response time of the electronics. The
pioneer approach of Bard consisted in fully insulating a Pt/Ir tip with
Apiezon wax, and then provoke a controlled crash so as to get an
electrode shape about such as the one depicted in Fig. 1.15–17 This tip was
approached towards an electrode so as to produce a gentle crash that opens

a hole of 10 to 20 nm diameter. In this peculiar utilisation of the Scanning
Electrochemical Microscope,18 the electrode was thus slightly recessed in

Fig. 1 Single molecule detection with the SECM. Redox cycling occurs between the tip and
substrate. Reprinted with permission from Ref. 15.

2 | Electrochemistry, 2013, 11, 1–33


the insulator, as can be assessed by performing an approach curve on an
insulating substrate (or an electrode polarised so as to avoid redox cycling).
Then, at a constant distance of about 2 nm with positive feedback, the
current is monitored. A simple calculation for a 2 mM solution of a redox
probe and a volume delimited by a 20 nm diameter tip recessed by 10 nm
gives that there should be only a few molecules processing the feedback. The
time corresponding to travel from the UME to the substrate is then about
50 nanoseconds. If one electron is exchanged, this gives a current of about
1.6 pA per molecule (considering 100 ns for one complete cycle and taking
D=10 À5 cm2 sÀ1), which is a measurable current. A quantified change in
the current reflects the arriving or departure of limited number (and often a
single) of molecules between the electrodes. This configuration has been
revisited recently by Mirkin.19
This pioneer experiment is however not adapted for further analytical
tools developments. Indeed, in the SECM configuration, drifts may occur
substantially so that the precise distance (already not measured directly)
may shift. Moreover, the Apiezon wax coating of the electrode is not
reproducible. Recently, the Lemay group has revisited this concept but
relying on devices produced by nanolithography methods. This led to the
invention of electrochemical correlation spectroscopy.20,21 In these systems
(see Fig. 2) the gap width is perfectly controlled and is about 50–100

nanometres. Furthermore, they can be stored easily and the chromium layer
protecting the electrodes can be etched just before the experiment. The
redox cycling can be clearly visualized on both electrodes as fluctuations of
the same amplitude but opposite signs of the currents. In order to get more
information, the power spectral density (or equivalently the autocorrelation
function) of the signal can be analysed. Additional fluctuations were then
observed and attributed to adsorption events. There is no doubt that this
approach is very promising. Further refinements and modification of electrode surfaces will even induce more specificity and sensitivity in the near
future.
Another promising alternative developed by Demaille et al. is to attach
the molecules on the tip or on the surface with a polymer linker (Fig. 3).22–25

Fig. 2 (a) Concept of the device for electrochemical correlation spectroscopy. (b) SEM image
of the device. (c) Current fluctuations for both electrodes in a 1.2 mM Fc(MeOH)2 solution in
water (black curve) and a 1.0 mM solution in acetonitrile (red curve). Fluctuations in opposite
directions are observed. Reprinted with permission from Ref. 20.

Electrochemistry, 2013, 11, 1–33 | 3


Fig. 3 Left: when the AFM/SECM tip is far from the surface, the oscillation amplitude is large
and there is no redox cycling. Right: when the tip is close, the amplitude diminishes and redox
cycling starts. Reprinted with permission from Ref. 23.

There is a sufficient chain flexibility so that redox cycling occurs efficiently.
Here, the authors control the distance by performing simultaneously an
AFM measurement. At present, a few hundreds of molecules can be detected,
and further developments may allow reaching the single molecule level. A key
point to unlock is to favour the redox cycling. Biological discrimination
between protein arrays has already been performed by this approach.

2.2 Molecules flowing through a nanopore
Another trick is to rely on conductivity measurements. The measurable
current corresponds then to an ionic flux. In the conductivity cell, a
nanometric constriction is present, so that the major part of the potential
drop occurs in this region. As a consequence, any large molecule that can
obstruct the ionic transport through the pore will be detected. This strategy
has been nicely pioneered by several groups to study transport through a
biomimetic membrane.26–34 The White group first made nanoelectrodes,
and further etched the metal to produce a hole having a few tens to a few
hundreds nanometres. Then, a bilayer is deposited so as to fully cover the
hole. In this configuration, the resistance is extremely high if there is no
leakage. A great advantage of relying onto low-diameter membrane is that
they are very stable compared to other systems, and also induce less noise.
Then, insertion of a-hemolysin, an ion-channel protein, in the membrane
can be detected by molecule as current steps (Fig. 4).26 Last, but not least,
when a single protein is inserted, and that one compartment contains DNA
(a negatively charged backbone), each DNA strand that passes through the
protein is detected. The current focus is to reach a submolecular recognition, ideally in view of DNA sequencing.35–37 The problem to unravel in this
approach is then to take into account the random walk and global motion
of the DNA bases so as to avoid to count the same base several times. For
uncharged particles (vesicles or nanoparticles), the same method can be
used, but the translocation of the structure needs to be induced by a pressure difference between the two compartments.38–40 Other phenomena can
be studied with similar setup, for example protein unfolding as a function of
temperature, that limits incorporation of the protein in the membrane.41
4 | Electrochemistry, 2013, 11, 1–33


Fig. 4 Single strand DNA passing through an a-hemolysin inserted in a lipid bilayer created in
a nanopore. Reprinted with permission from Ref. 33.

Nanopores can also be produced by advanced lithography methods.35,42–45
One aim is to get the thinnest possible hole to be more sensitive to the
submolecular variations. Graphene has been proposed as a ‘‘one-atom’’ (or
at least a few for multilayer graphene) thick membrane.46–49 To refine the
analysis, a tunnel junction can be incorporated in the hole, requiring a fourelectrode configuration.50–52 A full analysis of the electrical cross talk has
been recently proposed by Albrecht.53 A specific receptor can also be
incorporated in the pore to enhance the selectivity.54 The temporal resolution of these devices is very good, and recent progress is allowed to reach a
sub-microsecond resolution.55
2.3 Detection of nanoparticles
The exponential interest onto nanoparticles chemistry and electrochemistry
has recently also been focused onto individual behaviour quantification.56
We artificially split this section in electractive and electrocatalytic structures.
2.3.1 Electrocatalytic nanostructures. Recently, the Bard group has
produced an innovative method based on SECM to screen the catalytic
activity of individual clusters.57–61 Using microdispensers, nanostructures of
different composition can be deposited on an electrode. The reactant to test
is then locally generated on the SECM tip in the Tip Generation-Substrate
Collection mode. The efficiencies of the different catalysts is directly
visualized in the resulting image (Fig. 5). This method is particularly efficient to probe multicomponent systems since a single experiment can help
choosing between a large range of compositions.
Electrochemistry, 2013, 11, 1–33 | 5


Fig. 5 Formic acid oxidation SECM images of a Pd-Co electrocatalyst array at (a) À 0.1 V,
(b) 0.0 V and (c) 0.1 V in 0.1 M KHCO3 saturated with CO2. The tip substrate distance
was 50 mm. The scan rate was 250 mm s À 1. Tip potential À1.9 V. Reprinted with permission
from Ref. 60.

Another currently active field consists to monitor nanoparticle collisions
with an electrode by the intermediate of the electrocatalytic reaction they

provoke.57,58,60,62–65 Here, as soon as the nanoparticle contacts the electrode, the reaction starts, which amplifies greatly the current. Due to the
large turnover, particles as small as 3 nm diameter could be detected.
Moreover, the nanoparticles may stick on the electrode or have several
collisions before travelling back to the bulk depending on the electrode
treatment.64–65 In the first case, a stepwise behaviour is observed, while a
blip response is observed in the second case. Diffusion and probability to
collide to an active site have been considered in an analytical theory.
Random walk simulations also helped the interpretation.
2.3.2 Electroactive nanoparticles. However, when nanoparticles are
large enough to incorporate enough redox centers, they may be individually
detected directly. This has been pioneered by the Compton group, starting
with silver components.66 Here, the number of electrons available is easily
computed knowing the radius and density of the system. In Fig. 6, the blips
correspond to individual particle disintegration to Ag þ. For each event,
integration of charge gives the radius. When concentration is small enough
(in the picomolar/nanomolar range), so that each event can occur separately, this experiment can help determining the size dispersity or following
the coalescence of these nanoparticles in solution. This has been recently
extended to non destructive measurements by tagging the nanoparticles
with redox centers.67–69
6 | Electrochemistry, 2013, 11, 1–33


Fig. 6 Chronoamperometric profiles showing oxidative Faradaic collisions of Ag nanoparticles in citrate solution. Reprinted with permission from Ref. 66.

3

Single molecules for molecular electronics

The dream of molecular electronics would be to build an entire signal
processing unit only with molecules. But to be competitive with top down

methodologies, several problems need to be unravelled. First, the molecular
design should allow a function. It has been now demonstrated that molecules indeed are able to treat electrical, optical or even magnetic information. This has been demonstrated for long relying onto experiments with
collection of molecules. Another problem to encompass will be to organise
individually each structure. In this aspect it is now common to get organisation of some nanoobjects over several hundreds of nanometers (the
limitation often comes from the substrate) but usually the same entity is
assembled.70–73 Moreover, these organisations often rely on weak interactions and more robust structures need to be produced for practical
applications.
The first property to be tested was the ability to propagate a signal, i.e. to
act as a molecular wire. This can be performed by placing an electron donor
and an acceptor at each end of a linear entity and performing a photochemical activation. For example, this was thoroughly studied in DNA by
Barton.74–76
In electrochemistry, the donor or the acceptor can be replaced by an
electrode. Then the measured quantity is the heterogeneous rate constant
kET. Several experimental protocols are available to deduce this parameter.
First, transient methods such as chronoamperometry and ultrafast voltammetry are rather direct.77,78 However, a great care should be paid
to take into account ohmic losses and the inherent time constant of the
electrode.4 Usually, when rather fast systems are measured, ultramicroelectrodes should be used to encompass these problems.12,79–81 The
second approach was pioneered by Feldberg et al. and consists in illuminating an electrode so as to produce a temperature jump.82 The local
Electrochemistry, 2013, 11, 1–33 | 7


equilibrium is then disturbed, and the faradaic impedance discharges in the
double layer capacitance. This produces a small shift of the equilibrium
potential that can be measured by an appropriate follower. The main
advantage is that since there is no net current flux, ohmic losses are absent.
This is however a very indirect method and several assumptions about heat
dissipation in the assembly cannot be verified.
These different methods confirmed that most often the current decays
exponentially with the electrode distance, kET being given by the following
equation:

kET ¼ k0 expð Àbd Þ

ð1Þ

The ideal molecular wire would then have a very small b. This indeed
occurs with conjugated backbones whereas for alkyl chains b is on the order
of 10 nmÀ1. Departures from this law stem from a modification of the
reorganisation energy with the distance or a change in the coupling element
due to geometrical distortions. For long molecular wires, or when several
redox centers are present, electron hopping needs also to be considered.
These macroscopic experiments kept the dream alive, but the properties
need to be tested at the individual level. To that respect, electrochemical
concepts are useful, either to produce devices onto which the molecules can
be studied, or to use the reference electrode as a gate to trigger the current
inside the nanogap. These two ideas are depicted sequentially below.
3.1 Electrochemistry to make nanogaps
3.1.1 Electromigration. The electromigration technique was first
reported by Park et al. in 1999.83 The nanogap is achieved by applying large
current densities on a metal nanowire, which is usually prepared by electron-beam lithography and shadow evaporation. Due to the high electric
field, the metal atoms diffuse in the direction of the electron flow, which
causes the eventual rupture of the thin metal wire (Fig. 7). A gap width
about 1–2 nm is formed and subsequent dipping in a solution allows
molecule to bridge the gap, i.e. to be connected simultaneously to the two
electrodes.84–89

Fig. 7 Field-emission scanning electron image of a representative gold nanowire (a) before
and (b) after the breaking procedure. The nanowire consists of thin (B10 nm) and thick
(B90 nm) gold regions. In the images, diffuse white lines separate these two regions.
(c) Representative conductance trace obtained during a nanowire breaking procedure measured
in a four-probe configuration schematically shown in the inset. The nanowire is broken

by ramping a bias voltage through a 100 O series resistor at a rate of 30 mV/s. Reprinted
with permission from Ref. 83.

8 | Electrochemistry, 2013, 11, 1–33


The disadvantage of this method is that it is difficult to get a fixed and
reproducible distance between the electrodes. Moreover, nanoparticles may
be formed in the gap, leading to an important uncertainty in the results.90–91
3.1.2 Electrodeposition and etching. The electrodeposition and etching
technique92–94 is useful to obtain a pair of electrodes with a defined nanogap. Typically, a pair of metallic electrodes with large gap (usually a few
mm) are defined by standard lithography techniques, and a drop of solution
containing the target metal ion is placed onto the large gap. Then, a
potential can be applied on the electrode in order to deposit or etch the
metal. The distance can be adjusted from several angstroms to several
nanometers by monitoring the gap conductance with time as shown in
Fig. 8.95–97 During the process, tunnelling current94,98 or high-frequency
impedance99,100 can be measured and used as the feedback to stop the
deposition at a desired distance. Another possibility is to place a resistor
in series with the junction to allow a self termination of the procedure.101
I-V behavior of molecules with thiols were carried out by such kind of
nanogap.92,93,97
The main drawback of this method is however that the gap cannot be
modified once the device is formed. Moreover, starting from a gap of several
tens of micrometers, the dendritic growth of the electrodes produces very
fragile devices. Improvements of the solution composition and applied
potentials may nevertheless solve this issue for mostly used metals (gold and

Fig. 8 Fabrication of nanoelectrodes consists of two main steps: (a) Electrodes with large
separation are fabricated by conventional lithography. (b) Metal is electrodeposited onto the

electrodes, reducing their separation. Vdc controls electrodeposition while Vac is used to
monitor the conductance and thus the separation between the electrodes. Reversing Vdc allows
material to be removed rather than deposited. SEM images of the electrodes after electrodeposition (e,f). Reprinted with permission from Ref. 95.

Electrochemistry, 2013, 11, 1–33 | 9


copper). This justifies the actual interest for the two other methods depicted
below.
3.1.3 Electrochemical mechanically controllable break junction (ECMCBJ). Electrically connected individual molecules were first performed
relying on the mechanically controllable break junction.102 The great stability of this device allows getting very reproducible I-V curves onto single
molecules.103
In this setup, a stepper or piezoelectric motor pushes a nanometric constriction made with a metallic wire so as to break it and leave a gap compatible with a molecular size (Fig. 9). This gap can be closed and reopened
with an excellent precision because there is an important demultiplication of
the distance. Typically, a movement of 1 mm of the piezo achieves an
elongation of less than one nm of the gap.
In this context, the Tian group has demonstrated that electrochemistry
offers a good alternative to make very reproducible nanogaps. In an original
configuration, a metal can be deposited onto the electrodes of the MCBJ
taking electric double layer as feedback.104–107 Normally, a pair of goldelectrodes with micrometer-scale-separation is patterned on a substrate.
One is used as working electrode while the other is taken as reference. A
third gold wire is used as counter electrode.105 Gold atoms are deposited
onto the working electrode by imposing a constant negative current while
recording its potential versus the reference. When the potential difference
drops to zero, this indicates that the electrodes are connected (Fig. 10). This
procedure presents several advantages. First, the connection between electrodes is well-controlled, and can be adapted using further bending of
substrate. Second, different kind of metals can be deposited easily by
changing the metal ions in solutions.105 The EC-MCBJ has also been
extended to the conductance measurement of single molecules.108,109
Nevertheless, the main drawback is that at present the contacts cannot be

isolated so that measuring the conductance in an electrolyte medium is
difficult due to the faradaic discharge of the solvent or impurities, especially
for molecular junctions with a small conductance. In order to fulfil this
constraint, only the very end of the electrodes should be left in contact with
the solution. A critical issue is then to coat only the end of the electrodes
while preserving the mechanical properties of the device.
Another problem to solve with the MCBJ is that large separations are
difficult to achieve, which restricts the studies to rather short molecules.
3.1.4 Electrochemical scanning tunneling microscopy break junction (ECSTMBJ). The above drawbacks have been unravelled for a long time in
Metallic wire

Count support
Piezo
Fig. 9 Schematic view of the MCBJ mounting.

10 | Electrochemistry, 2013, 11, 1–33


Fig. 10 Schematic illustration of (a) electrodeposition setup with potential-difference feedback
control on MCBJ. (b) SEM image of pair electrodes with contact caused by the electrodepositon (c) the principle of EC-MCBJ: from an original electrode pair with a large gap, the
metal atoms are deposited onto the electrode pair to form the contact. The metal atomic wire
are formed by the stretching the nanogap. After rupture of the contact, a molecule can be
inserted into the gap. Reprinted with permission from Ref. 105.

studies relying on the Scanning Tunneling Microscopy. Indeed, from the
beginning of this technique, electrochemists are aware that a STM tip can be
insulated so as to perform imaging in conductive liquids such as water. The
most simple technique is to pass the tip through melted polyethylene or
Apiezon wax. Retraction of the polymer while curing liberates only a
few nm2. A second approach consists in using an electrophoretic painting.

This technique also solves the problem of measuring long molecules since
the tip can pull rather far away from the substrate to adjust the gap.
The STM-BJ technique was initially implemented by Tao et al. in 2003.110
An STM tip is approached toward a surface so as to provoke a controlled
crash on the substrate, corresponding to a few atoms contact. Then, the tip
is retracted at a constant velocity. While a nanoconstriction is liberated,
some molecules may bridge both electrodes. Current-distance curves display
clear steps corresponding to a diminishing number of molecules in the
junction.111–115 The same year, Nichols et al. proposed a very interesting
alternative consisting in avoiding the crash.116 A small density layer is
produced so that molecules are quite mobile. From a defined setpoint, the
tip is retracted a few nanometers away from the surface (Fig. 11).117,118 A
variation of the method consists in disabling the feedback for some time
while recording the current. Current jumps are observed when molecules
fluctuate and link both electrodes.
The great advantage of this innovation is that a large number of junctions
can be tested in a relatively short time. Statistical analysis through histograms can then be performed so as to detect the most likely currents that are
Electrochemistry, 2013, 11, 1–33 | 11


(a)

(b)

(c)

(d)

Fig. 11 Schematic illustration of the molecular wires formed by Nichol’s method. (a) A self
assembled monolayer with a low coverage of the molecule is formed on the Au(111) surface.

The setpoint current is increased to bring the tip very close to the surface. (b) One end of the
molecule may attach to the Au STM tip, and the tip is then retracted from the surface while
recording the current. (c) The molecular junction is broken at sufficiently large tip sample
displacements. (d) Current decay curves for a clean Au(111) substrate (A) and HS-6V6-SH on
Au(111) in air (B). Reprinted with permission from Ref. 117.

displayed as peaks in the analysis. A refinement of the method consists in
building 2D histograms, taking the one atom contact rupture or a given
conductance value as a reference for the distance.119,120 This helped a lot
while deciphering different contact geometries.
However, such kind of traditional STMBJ methods may suffer from
difficulties to form molecular junctions with metals other than Au, and
especially for metals with complex electronic structure that may be easily
corroded. To solve this issue, Zhou et al. developed an electrochemical
jump-to-contact STM-BJ approach (ECSTM-BJ) for conductance of
atomic-sized wires121 and single molecular junctions with different metallic
electrodes,122 which is initially used to make nanoclusters.123–125 The procedure for molecular conductance measurement includes the subsequent
operations: firstly, the target metal is electrodeposited onto the tip with
STM feedback enabled. The tip is then withdrawn from the surface with
STM feedback disabled for about several tens of nanometers (Fig. 12a).
Secondly, the tip is driven toward the surface until the tip current reaches a
preset value, A preset voltage pulse is then superimposed onto the z-piezo
of an STM scanner, which results in a further tip advancement of 1.5 nm or
more towards the surface. During this step, atoms of the deposited metal
on the tip transfer to the substrate to form a metallic contact. This is followed by stretching of the tip out of the contact at a typical speed of
20 nm sÀ1 to form atomic-size wire of the deposited metal (Fig. 12b). The
conductance of such nanowires is measured simultaneously.121 Thirdly, the
(deposited metal)–molecule–(deposited metal) junctions are formed upon
breaking of the atomic-size metal wires (Fig. 12c). The current is recorded
during the stretching of the tip at a sampling frequency of 20 kHz to obtain

current vs. distance curve. By repeating the whole process at new positions
on the surface, usually in square arrays, a large number of conductance
traces are collected to construct a conductance histogram. This approach
can be extended easily to several metals such as Pd, Zn and Fe for systematic investigation of the intrinsic properties of the molecule–electrode
contact.
12 | Electrochemistry, 2013, 11, 1–33


(a)

(b)

(c)

(d) 79.2

(e)
Counts

66.0

G/nS

52.8
39.6

0.0

13.2 26.4 39.6 52.8 66.0 79.2
G/nS

26.4
13.2
0.0
0.0

0.4

0.8
d/nm

1.2

1.6

Fig. 12 Schematic illustration of the ECSTM-BJ approach for conductance measurement of
single molecular junctions with different metallic electrodes (a–c). Single molecule conductance
measurements of succinic acid using Ag as the electrode: (d) typical conductance traces, inset of
(d) conductance histogram and (e) STM image (200 Â 200 nm2) of a 10 Â 10 array of Ag clusters
simultaneously generated with the conductance curves. Reprinted with permission from Ref. 122.

3.2 Electrochemistry within the junction
In molecular electronics, a key path is to make transistors, i.e. to control the
current flowing between two electrodes (the drain and the source) by a third
electrode called a gate (Fig. 13). At present, it remains however very difficult
to make three terminal contacts126 with a molecular resolution.
In an electrochemical approach, electrolyte may thus be used as the gate,
all potentials being referred to a common reference electrode placed further
in the solution. This was early considered by Kuznetsov and Ulstrup while
performing scanning tunneling spectroscopy over electroactive monolayers.

This pioneer theoretical development has been already reviewed recently,127
and we here simply recall the key points graphically displayed in Fig. 14.
The two electrodes are considered as electron reservoirs into which the
electrons are distributed according to a Fermi-Dirac distribution. In the
gap, the redox levels of the oxidized (Ox) and reduced (Red) states fluctuate
with typical amplitudes given by the reorganisation energy of the environment (including internal and solvent coordinates) around the redox center.
These levels are electronically coupled to the electrodes to allow electron
hopping that could be adiabatic (strong coupling) or non-adiabatic (weak
coupling). When hopping occurs from the left electrode to the molecule in
Ox state, relaxation of the molecular level to Red state of molecule
occurs.128 Whether in the relaxation this level passes through both Fermi
levels, the electronic coupling is greatly enhanced so that several other
electrons may tunnel directly between both electrodes. A current maximum
Electrochemistry, 2013, 11, 1–33 | 13


Drain

Source
Gate

Fig. 13 Schematic of a transistor structure with source, drain and gate.

εox,eq

EF1
Intermediate level
2λ
EF2
Electrode 1

Electrode 2
εred,eq
Fig. 14 Energy levels distribution. Gray solid line: level distribution in the electrodes.
gray dash line: Energy levels at equilibrium. Black dash line: intermediate level corresponding
to out of equilibrium situation that can transiently efficiently couple both electrodes.

is predicted when the sample potential gets close to the standard potential.
Small deviations due to the potential profile in the junction may be
expected.
The same group has published seminal experimental illustrations in full
agreement with this theory. For example, comparing a fast redox couple
(osmium center) with a slow one (cobalt center) the current enhancement of
complex containing Os is much larger than that of Co, which could be
correlated to the rate constant of the electron transfer, as shown in
Fig. 15.128–131 Precise estimation of electron transfer kinetics in the case of
the fast couple was however difficult in spite of its very high value. Another
very nice illustration consisted in observing a huge contrast for azurin, a
protein bearing a Cu redox core.132,133
A recent interesting observation linked to this framework was published
recently by Wandlowski et al.134 A gold surface modified with a ferrocene
self assembled monolayer was submitted to several oxidation/reduction
cycles. This produced a roughened substrate with Au nanoclusters about
2.4 nm diameter. When performing scanning tunnelling spectroscopy onto
that sample, the authors observed a quantized charging behaviour with
seven narrow peaks in the ITip – ES curve (Fig. 16). Particularly, the peaks
positioned near standard potential of redox molecule are typically higher
than those at more negative or positive potentials.134 This has not been
theoretically fully explained, but suggests that enhancement of the tunnelling current in the redox-active tunnelling junction near the standard
14 | Electrochemistry, 2013, 11, 1–33