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APPROVED: Oliver M. R. Chyan, Major Professor Teresa D. Golden, Committee Member Ruthanne D. Thomas, Chair of the
Department of Chemistry Sandra L. Terrell, Dean of the Robert B.
Toulouse School of Graduate Studies
INTERFACIAL STUDY OF COPPER ELECTRODEPOSITION WITH THE
ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE
Oscar Ulises Ojeda Mota, B.S.
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
May 2005
Ojeda Mota, Oscar Ulises, Interfacial Study of Copper Electrodeposition
with the Electrochemical Quartz Crystal Microbalance (EQCM), Master of
Science (Analytical Chemistry), May 2005, 113 pp., 4 tables, 45 figures,
reference list, 139 references.
The electrochemical quartz crystal microbalance (EQCM) has been
proven an effective mean of monitoring up to nano-scale mass changes related
to electrode potential variations at its surface. The principles of operation are
based on the converse piezoelectric response of quartz crystals to mass
variations on the crystal surface. In this work, principles and operations of the
EQCM and piezo-electrodes are discussed. A conductive oxide, ruthenium oxide
(RuO2) is a promising material to be used as a diffusion barrier for metal
interconnects. Characterization of copper underpotential deposition (UPD) on
ruthenium and RuO2 electrodes by means of electrochemical methods and other
spectroscopic methods is presented.
Copper electrodeposition in platinum and ruthenium substrates is
investigated at pH values higher than zero. In pH=5 solutions, the rise in local pH
caused by the reduction of oxygen leads to the formation of a precipitate,
characterized as posnjakite or basic copper sulfate by means of X-ray electron
spectroscopy and X-ray diffraction. The mechanism of formation is studied by
means of the EQCM, presenting this technique as a powerful in-situ sensing
device.
ii
Copyright 2005
by
Oscar Ulises Ojeda Mota
iii
ACKNOWLEDGMENTS
I would like to express my earnest appreciation and acknowledgements to
all the people that had helped me in completing this goal. To Dr. Oliver M.R.
Chyan who’s many teachings and example I will carry through the rest of my life,
thanks for your dedication. My special acknowledgments go to Dr. Teresa
Golden and Dr. Mohamed El-Bouanani, from whom I obtained invaluable
patience and knowledge when discussing and learning about their fields of
specialization. The financial support form the Welch Foundation and UNT Faculty
Research Grant is greatly appreciated. I am grateful for the support of my fellow
group members, Yibin Zhang, Long Huang and Praveen Reddy, especially to Dr.
Tiruchirapalli Arunagiri and Dr. Raymond Chan, pillars of the group, whose
enthusiasm and constructive discussions helped in the arrival of the work here
presented. The help from the support staff and many graduate and
undergraduate students in the chemistry department is highly appreciated. I
would like to extend my thanks to Dr. Oscar Mendoza and Dr. Miguel Angel
Valenzuela, whose encouragement was paramount to the continuation of my
academic formation. To Dr. Manuel Quevedo, Dr. Rosa Amelia Orozco and Dr.
Alejando Hernández, who showed me the value of good friendship at all times.
Most importantly to my parents, Miguel and Norma, who sparked my interest in
science, and supported my decisions with confidence thought the years, and
Sarah Flores; my gratitude goes to you, for giving me understanding, love and
support, since we started this plan together.
iv
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS .....................................................................................iii LIST OF TABLES ...............................................................................................vi LIST OF FIGURES ............................................................................................vii Chapters 1 INTRODUCTION ..................................................................................... 1
1.1 Piezoelectricity .............................................................................. 1 1.2 The Electrochemical Quartz Crystal Microbalance (EQCM) ......... 5
1.2.1 Frequency-Mass Correlation .............................................. 6 1.2.2 The Equivalent Circuit ...................................................... 11 1.2.3 Applications of the EQCM ................................................ 12
1.3 Electrode Shot Preparation ......................................................... 14 1.4 Electrochemistry Fundamentals .................................................. 19 1.5 X-Ray Photoelectron Spectroscopy ............................................ 25 1.6 Conclusions ................................................................................ 27 1.7 References.................................................................................. 28
2 COPPER UNDERPOTENTIAL DEPOSITION ON RUTHENIUM AND
RUTHENIUM OXIDE ............................................................................. 33 2.1 Introduction ................................................................................. 33 2.2 Experimental ............................................................................... 35 2.3 Results and Discussion............................................................... 37
2.3.1 Copper Deposition on Ruthenium .................................... 37 2.3.2 Copper Deposition on Ruthenium Oxide .......................... 46
2.4 Ongoing Research ...................................................................... 53 2.4 Conclusion .................................................................................. 57 2.5 References.................................................................................. 58
3 FORMATION OF BASIC COPPER SULFATES .................................... 62
3.1 Introduction ................................................................................. 62 3.2 Experimental ............................................................................... 64
v
3.3 Results and Discussion............................................................... 68 3.3.1 Characterization of the Process at Different Stages......... 74 3.3.2 Effect of the Oxygen Reduction Reaction......................... 81 3.3.3 Mechanism of Formation.................................................. 86 3.3.4 Mass Dumping ................................................................. 93
3.4 Conclusion .................................................................................. 93 3.5 References.................................................................................. 95
REFERENCE LIST......................................................................................... 101
vi
LIST OF TABLES
Page
1-1 Applications of the earlier piezoelectric devices....................................... 4 2-1 Thermodynamic parameters for Cu UPD on ruthenium......................... 44 2-2 Thermodynamic parameters for Cu deposition on ruthenium oxide....... 49 3-1 Mass change efficiencies at anodic peaks “A1” and “A2” ...................... 88
vii
LIST OF FIGURES
Page
1-1 A quartz crystal ........................................................................................ 3
1-2 AT type quartz cut.................................................................................... 5
1-3 Employed EQCM setup ........................................................................... 7
1-4 Oscillating crystal ................................................................................... 10
1-5 Equivalent circuit of the oscillating crystal.............................................. 11
1-6 Mold to make an electrode..................................................................... 15
1-7 Soldering the shot and copper wire ....................................................... 16
1-8 Pouring the epoxy into the mold ............................................................ 17
1-9 Steel puck with unpolished electrode..................................................... 18
1-10 Lapping fixture with unpolished electrode inside.................................... 19
1-11 Cyclic voltammetry potential-time profile................................................ 23
1-12 Potential-time profile of a CA experiment............................................... 24
1-13 Photoelectron emission in an XPS analyzer .......................................... 26
2-1 Inter-grain boundary diffusion prevention by the use of RuOx............... 35
2-2 Ru EQCM electrode in background electrolyte ...................................... 38
2-3 Ru electrode in background electrolyte.................................................. 39
2-4 XRD pattern of a Ru electrode............................................................... 40
2-5 CV response of a Ru shot in a 2 mM CuSO4 solution + 0.5 M H2SO4.... 41
2-6 Potential region of peak “A1” obtained in a 2 mM CuSO4+0.5M H2SO4
solution on a Ru electrode ..................................................................... 42
2-7 CV of ruthenium electrode in a higher copper concentration solution.... 45
2-8 Underpotential deposition of Copper from a 10 mM CuSO4+0.5M H2SO4
solution on a Ru electrode ..................................................................... 46
2-9 A) Ru surface, B) The oxidized Ru surface (both 5x)............................. 47
viii
2-10 Background of oxidized Ru substrate in 0.5 MH2SO4 solution ............... 48
2-11 Ruthenium Oxide CV response in a 2 mM CuSO4+0.5MH2SO4 solution ............................................................................................................... 50
2-12 CV in 2 mM CuSO4+0.5 M H2SO4 solution of a thermally produced ruthenium oxide ..................................................................................... 51
2-13 Effect of different oxidation times of an oxidized electrode in the 2 mM CuSO4+0.5MH2SO4 solution .................................................................. 52
2-14 CV profile of a Ru film deposited on a Si wafer substrate, in the 2 mM CuSO4+0.5 M H2SO4 solution ................................................................ 54
2-15 EQCM response of a Ru electrode in 2 mM CuSO4+0.5MH2SO4 solution ............................................................................................................... 55
2-16 EQCM response of Ru in a 5 mM CuSO4 + 0.5 M H2SO4 solution ......... 56
2-17 EQCM response of Ru in a 10 mM CuSO4 + 0.5 M H2SO4 solution ....... 57
3-1 Adaptation to the EQCM’s mount for microscope picture ...................... 67
3-2 Current/mass profiles of a Pt electrode in 0.5M H2SO4 solution; A) Current density, B) Mass change........................................................................ 68
3-3 Current/mass profiles of copper deposition from 2 mM CuSO4 + 0.1M K2SO4 adjusted at different pH values.................................................... 70
3-4 Multiple scanning experiments at pH=5, 2 mMCuSO4+0.1MK2SO4....... 72
3-5 EQCM profile of copper deposition in a Pt. electrode at different .......... 73
3-6 XRD spectra of the precipitate formed, as obtained form the top of a Ru electrode P: Posnjakite .......................................................................... 75
3-7 Optical pictures of the Pt electrode in a 10 mM Cu2+ concentration solution, taken at different stages of the potential sweep....................... 76
3-8 SEM Image of Cu2O crystals on a Ru coated Si wafer .......................... 78
3-9 Cu 2p region of XPS spectrum for compounds formed in 2 mM CuSO4
+0.1 K2SO4 pH 5 solution a) holding at “C2” for 3 min; b) holding at “C1” for 3 min; c) “A2” or precipitate .............................................................. 79
3-10 S 2p XPS spectrum of precipitate .......................................................... 80
3-11 Cyclic voltammetry and EQCM corresponding to the CVs on a Pt electrode in 2 mM CuSO4 +0.1M K2SO4 pH 5 solution with Ar purging . 82
ix
3-12 Background of the Pt electrode in 0.1M K2SO4 pH=5 solution, with different gas purging ............................................................................................ 83
3-13 CV of a Pt electrode in a 5 mM CuSO4+0.1 K2SO4 pH=5 buffer solution ............................................................................................................... 85
3-14 Cycling of a Cu2O loaded platinum surface in the “A2” potential region in 0.1M K2SO4 solution............................................................................... 90
3-15 Potential stepping of a cuprite loaded Pt surface to “A2” potential in K2SO4
electrolyte .............................................................................................. 93
1
CHAPTER 1
INTRODUCTION
1.1 Piezoelectricity
The first experimental demonstration of a connection between the
appearance of macroscopic piezoelectric phenomena and its relationship with
crystallographic structure was published in 1880 by Pierre and Jacques Curie [1].
Their experiment consisted of measurements of surface charges appearing on
specially prepared crystal [2] (quartz, topaz, sugar cane, Rochelle salt) which
were subjected to mechanical stress. In those years, the results were credited to
the Curies' imagination and perseverance, considering that they were obtained
with very simple and common materials, like tinfoil, glue, wire, magnets and a
jeweler's saw.
While this was a scientific curiosity for the next third of a century, in the
scientific circles of the day, this effect was considered quite a "discovery," and
was quickly dubbed as "piezoelectricity" in order to distinguish it from other areas
of materials and electric phenomena such as "contact electricity" (friction
generated static electricity) and "pyroelectricity" (electricity generated from
crystals by heating).
Analysis of the underlying reason of piezoelectricity is important in
understanding the operation of piezoelectric devices. Because some atomic
2
lattice structures have as an essential unit (or "cell") a cubic or rhomboid cage
made of atoms [3], with no center of symmetry. This cage holds a single semi-
mobile ion which has several stable quantum position states inside the cell. The
ion's position state can be caused to shift by either deforming the cage (applied
strain) or by applying an electric field. The coupling between the central ion and
the cage provides the basis for transformation of mechanical strain to internal
electric field shifts and vice versa.
The Curie brothers did not, however, predict that crystals exhibiting the
direct piezoelectric effect (electricity from applied stress) would also exhibit the
converse piezoelectric effect (stress in response to applied electric field). This
property was mathematically deduced from fundamental thermodynamic
principles by Lippman [1] in 1881. The Curies immediately confirmed the
existence of the "converse effect" and continued on to obtain quantitative proof of
the complete reversibility of the mechanical deformations in piezoelectric
materials. This reverse piezoelectric effect is the primary principle of many
devices and sensors [4-9]. The first serious applications work on piezoelectric
devices took place during World War I. In 1917, P. Langevin [10] and French co-
workers began to perfect an ultrasonic submarine detector. Their transducer was
a mosaic of thin quartz crystals glued between two steel plates (the composite
having a resonant frequency of about 50 kHz), mounted in a housing suitable for
submersion. Working on past the end of the war, they achieved their goal of
emitting a high frequency "chirp" underwater and measuring depth by timing the
3
return echo. The strategic importance of their achievement was not overlooked
by any industrial nation, and since that time the development of sonar
transducers, circuits, systems, and materials has never ceased.
Some of the most commonly used household appliances such as
phonogram cartridges and smoke detectors have evolved from the development
of this type of materials. In account for the evolution in the application of
piezoelectric devices, some of the early applications of these piezoelectric
properties [3] are summarized in the following table (table 1-1).
From all the aforementioned materials, quartz (SiO2) has gained its own
special place as application material for piezo systems, since it has a unique set
of mechanical properties that makes it the most suitable material for precision
control devices. Its internal friction is low [3], and its properties can be found
repetitively from one specimen to another. The most important characteristic is
that its properties are very stable with respect to time. A perfect crystal unit of
quartz (crystallographic group no.32) is shown in the next figure [7], showing its
lack of symmetry center.
Figure 1-1 A quartz crystal.
z
x y
4
Table 1-1 Applications of the earlier piezoelectric devices.
TIME PERIOD APPLICATION
1910 – 1940
• Early sonar
• Development of piezoelectric materials (zirconate
titanate).
1940 – 1960
• Phono cartridge.
• Microphones.
• Ceramic audio tone transducers.
1960 – 1980
• Development of new (Barium Titanate) family of
piezomaterials.
• Low fidelity-signal filters.
1980-1990
• Audio buzzers (smoke alarms compatible tone
generators).
• High frequency signal filtering.
When a quartz crystal rod is cut at different angles to its crystallographic
axes, different piezoids are obtained [11]. The most commonly used cuts are the
AT and BT cut quartz crystals (figure 1-2). These particular slices are used in
5
order to minimize the effect of temperature change related to the high frequency
drift. The characteristic frequency vs. temperature of a typical AT –cut quartz
crystal will have a minimum thermal drift at a temperature similar to that of the
experimental conditions (room temperature) [12].
Figure 1-2 AT type quartz cut.
1.2 The Electrochemical Quartz Crystal Microbalance (EQCM)
One of the most recent developments in the field of electrochemical
gravimetric measurements that take full advantage of the piezoelectric and
mechanical properties of quartz is the Electrochemical Quartz Crystal
Microbalance. The microbalance is based on a quartz crystal wafer that is
sandwiched between two electrodes [13]. If one of these AT-Cut quartz slices is
coated with gold or other conductive metal [14], and this metal layers are placed
in contact so as to function as a working electrode of an electrochemical setup,
and connect it to an oscillator circuit design (with a high-gain amplifier in its
feedback loop to allow for oscillation of the crystal in solution), a mass sensing
X y
z
35º 15’
6
device based on the reverse piezoelectric effect of the vibrating quartz wafer will
be created [15].
The term “EQCM” refers to the electrochemical application of the quartz
crystal device, with the “M” (as for “micro”) standing for the detection sensitivity
commonly found when performing experiments with the same. A schematic of
EQCM setup issued in the present work is represented in figure 1-3. The two
parts of a Teflon®* cell (obtained from CH InstrumentsTM†) compress the crystal
wafer placed between two o-rings in the bottom of the cell. The exposed
electrode area of the crystal consists in part of the metal’s key-hole pattern
deposited on top of the quartz wafer. Platinum, gold and blank crystals are
commercially available from different vendors, including International Crystal
Manufacturing (ICMTM‡).
Until fifteen or so years ago, the general belief was that the application of
a sensing device using quartz in shear mode vibration would be hampered by the
limitation of liquids, causing the cease of oscillation by damping effects of the
fluid in contact having a higher viscosity than air. Bruckenstein and Kanazawa
[16, 17] pioneered the application of the use of a quartz resonator in contact with
liquid, opening the field for new areas of discovery with electrochemical insight.
1.2.1. Frequency-Mass Correlation
* E.I. du Pont de Nemours and Company, www.teflon.com † CH Instruments, www.chinstruments.com ‡ International Crystal Manufacturers Corporation, www.icmfg.com
7
Sauerbrey [18] first demonstrated the relationship between a crystal
vibrating and the corresponding changes in mass, by a series of simple
deductions, based on the thickness shear mode of oscillation of a quartz crystal,
depicted as follows [19].
Counter Electrode Reference Electrode
Oscillator
Potentiostat
Working Electrode
Working Electrode
ComputerCell (Side View)
Cell(Top view)
Figure 1-3 Employed EQCM setup.
If we relate the thickness of the crystal to its resonance frequency by:
2/qqt λ= (1)
8
Where tq is the thickness of the slice, and λq is the wavelength of the
elastic wave propagating at the surface. We can write this relation in terms of the
resonant frequency fq and the speed of propagation of the wave as:
2q
vtf = (2)
Therefore, by algebraic manipulation, the change in frequency (dfq)
caused by an infinitesimal change in the crystal thickness can be represented as:
q
q
q
q
tdt
fdf
−= (3)
Where the negative sign indicates that a decrease in the resonance frequency is
caused by an increase in the thickness of the crystal. The relationship between
the mass of the crystal (Mq.) and its density (ρq=2.648 g cm-3) is stated as:
q
qq A
Mt
ρ= (4)
Where A=surface of the quartz area. Substituting (4) in (3) gives us:
q
q
q
f
MdM
fd −
= (5)
The assumption was made that the acoustic properties of the foreign layer
are identical to those of quartz. Thus the foreign layer is treated as a layer of
quartz itself (dMq), uniformly distributed over the crystal surface. One can define
mf and mq as the mass per unit area of the foreign and quartz crystal layer,
correspondingly, and rewrite equation (5) as:
9
q
f
q
f
mm
fd −
= (6)
And for materials of uniform density, we can rearrange to:
Avmf
fqq
q
ρ
22∆−=∆ (7)
The shear wave velocity(vq) is related to the shear modulus of quartz
(µq=2.947x1011 g*cm-1s-2) by the relation:
21
=
q
qqv
ρµ
(8)
Substituting (8) in (7) will give us the final “Sauerbrey’s equation”, where fq
equivalent to f0 is the fundamental frequency of the oscillator (quartz crystal).
( )21
202
qqA
mff
ρµ
∆−=∆ (9)
This more commonly used variation of the equation is:
(10) ∆mC∆f f−=
10
Where Cf is a constant that can be obtained by using either the
fundamental frequency of oscillation of the quartz, or experimentally, by
measuring the fundamental frequency at the beginning of each experiment. For
crystals with a fundamental frequency of oscillation of 7.995 MHz, the calibration
value of 1.4156x10-9 ng per hertz change is obtained.
It is assumed that the transverse velocity of the quartz surface is identical
to that of the adjacent liquid layer. Figure 1-4 illustrates the vibration mode of the
EQCM resonator [20]. Its surface is an antinode, which allows the shear wave
displacement in the x direction. This wave radiates from the surface in the z-axis
into the electrolyte. The amplitude of the shear wave is described by an
exponentially damped cosine function, decaying in water to 1/e of its original
amplitude at ca. 0.25 µm from the surface.
Shear Displacement( x )
Oscillation( z )
Figure 1-4 Oscillating crystal.
11
1.2.2. The Equivalent Circuit
For a better understanding of the variables involved in the EQCM
measurements, an equivalent circuit of the quartz resonator is commonly
introduced. From figure 1-5 [21], we can see that the capacitor (Co) represents
the shunt capacitance that includes the metal electrodes evaporated into the
quartz surface, holder, and leads. The rest of the RCA elements are composed
of the motional arm of the crystal (L1), represented as an induction force caused
by the vibrating mass of the arm, the motional capacitance(C1) reflecting the
quartz elasticity, and the bulk losses within the quartz crystal, represented as a
resistance R1 [22]. We can see from these diagrams [23] that the components of
the crystal oscillator can play an important roll in its overall performance.
Figure 1-5 Equivalent circuit of the oscillating crystal.
wwR1
C11
C0
L1
12
Although it has been shown that thin films, rigidly attached to the resonant
surface, will not cause an increase in the width (impedance) of the oscillation
[24], care should be taken in order to minimize these effects, especially when
studying cases of non rigid films.
1.2.3 Applications of the EQCM
The Sauerbrey relationship has been applied so as to measure minute
mass changes that accompany many processes, like underpotential deposition
(UPD) or the electrodeposition of metal monolayer(s) on a foreign metal
substrate at potentials less negative than that for deposition on the same metal
surface [25]) with an observable difference of mass response caused by the
adsorption-desorption process. Also of new interest has been the study of
electrode morphology changes, ion transport during potential changes in redox
and conductive polymer films. The determination of the point of zero charge
(PZC) has been also studied by means of the EQCM [11].
One of the main advantages of the EQCM technique in the study of
adsorption phenomena is the possibility of obtaining simultaneous voltammetric
and gravimetric response. An equivalent molar mass can be determined
according to Faraday’s law, and get direct information about the electrode
reaction species, measured at the same surface [26]. The conventional treatment
of oxidation-reduction cycling to obtain reproducible voltammetric curves should
13
be treated with caution, since contribution from the electrode roughness (trapped
water molecules) could occur and should me minimized.
Another strategy directed toward the taking advantage of the mass
sensing capabilities of the quartz microbalance, is to coat the surface of a QCM
with a polyurethane template, to effectively increase the affinity and detection
limit for patterned molecules. This effect is used in the so-called “artificial noses”,
particularly used in the fragrance and scents industry [27]. Surface imprinting has
been successfully performed on a molecular level for artificial biocatalysts and
protein imprinting [28, 29]. The polymer self-organizes around the living cells and
forms pits with selective recognition capabilities. The imprinting process takes
place on micrometer thick polyurethane coatings directly on microscope slides or
QCM electrodes. Targeted cells are then transferred to a stamp and are
subsequently pressed on pre-coated QCMs. This procedure generates packed
honeycomb-like imprinted structures on the polymer surface.
The aforementioned method has also been used to sample E. coli
bacteria, tobacco mosaic viruses (TMV) and potassium chloride (KCl) as an
inorganic probe molecule [30]. This has shown increased selectivity and
sensitivity towards analyte target molecules. These devices have improved time
and practical advantages over other silicon chip-based detectors [31], which in
turn can help solve the problem of real time sensing for bioterrorist attacks [32].
Among other uses, the crystal microbalance has also been employed as
an important tool in studying the controlled release of drugs upon potential
14
application [7], as immunosensors [9] and to study surface tension and wetting
velocities [11, 33]. The concept of “massograms” first described by Snook et al.
[34] in which the rates of changes of mass versus electrode potentials are
plotted, were used to effectively demonstrate the power of the EQCM to trace
non-faradic, non-deposition reactions (gas evolution). Electrochemical studies on
the dissolution mechanism of electroplated films [35-37] as well as selective
adsorption of anions [38-41] have also been reported.
1.3. Electrode Shot Preparation
In order to obtain working electrodes of the different materials used in the
present experiment, a procedure that describes the preparation of the mold and
electrode that will be contained is drafted and presented. Metallic shots from
ESPI®§ were used for the fabrication of the electrodes. For a thorough
description on the materials and polishing procedure, refer to the lab’s manual
[42]:
Procedure for making of the mold:
1. Take the MLA pipette and use the vise to secure the pipette tip.
2. With the hacksaw, cut off the non-tapered end of the pipette.
This non-tapered end of the pipette will serve as the mold to make the
electrode.
Electrode preparation:
§ Electronic Space Products Incorporated, www.espi-metals.com
15
1. Hold the mold with the vise, and use the 5/64-inch drill bit to drill a hole in the
middle of the mold.
2. Cut a 6-inch piece of copper wire using the wire cutters, and strip the ends of
the wire. Place your metal shot on a clean and hard surface. Using the
soldering iron, heat the metal shot and the copper wire; place the wire on the
metal shot to solder the two together. The risk here is that the metal and
copper wire will become very hot during the soldering step.
Figure 1-6 Mold to make an electrode.
16
3. Place the mold on a sheet of parafilm®** with the larger uncut opening facing
down, and put the shot inside the center of the mold. Make sure the copper
wire goes through the drilled hole.
4. Prepare the epoxy.
a. Place the petri dish on the analytical balance, and tare it.
Figure 1-7 Soldering the shot and copper wire.
b. In the petri dish, pour 10 grams of resin and 1.4 grams of hardener of the
LECOTM†† epoxy.
c. With the wooden stick, mix the resin and the hardener thoroughly and
gently as you try to minimize the occurrence of bubbles.
** Pechiney Plastic Packaging, Inc. Corporation, www.pechineyplasticpackaging.com †† Leco Corporation, www.leco.com
17
5. Pour the epoxy slowly into the mold (Fig.1-10), and allow it to cure for 24
hours under a wooden box. This will minimize the effects of air currents.
To test if the epoxy has dried, use a toothpick to touch the epoxy that you did
not use. If the toothpick sticks to the epoxy when attempting to be moved, the
epoxy is dry. If however the toothpick moves, the epoxy has not dried; so do
not touch the mold until further testing.
Figure 1-8 Pouring the epoxy into the mold.
6. After making sure the epoxy has dried, remove the plastic mold from the dried
epoxy by making little cuts on the plastic mold with the wire cutters. The
plastic mold should fracture into pieces, making the removal of the plastic
mold extremely easy. At this step you already have an unpolished electrode.
18
7. Place the stainless steel puck on the heat plate, and heat it for 5 minutes.
Apply a thin layer of clear hot mounting wax on the surface of the steel puck,
and place the unpolished electrode with the backside on the wax. Let it cool.
Figure 1-9 Steel puck with unpolished electrode.
8. Insert the steel puck with the unpolished electrode into the polishing holder or
lapping fixture. Loosen the side screw on the lapping fixture in order to adjust
and select the amount to be polished off.
19
Figure 1-10 Lapping fixture with unpolished electrode inside.
The electrode is now ready to be polished to mirror finish, by using
increasing grit polishing pads, and poly diamond suspensions up to 1 micron
(from Allied®‡‡).
1.4 Electrochemistry Fundamentals
In order to obtain thermodynamic data of the electrochemical reactions
and of the involved compounds, electrochemical measurements were conducted
in a three-way electrode cell. Information related to the energy and entropy of a
system can only be obtained in systems that are very near or at equilibrium.
Electrochemical reactions always involve the passage of current, so it is
easy to let the reaction proceed near to the equilibrium by limiting the passage of
charge per time (current), which is an expression of the reaction rate [43]. The
relationship between the potential dependence on heterogeneous rate constants
‡‡ Allied High Tech Products Incorporated, www.alliedhightech.com
20
resides in the fact that the energy of electrons on the electrode’s surface is
linearly bound with potential. Negative potential changes will destabilize the
energy of electrons, thus increasing the amount of charge available. Positive
potential changes will cause an accumulation of a positive excess charge, and
thus reduce the energy of the electrons [44].
Data acquisition is accomplished by measuring the electrode’s potential-
current proceeding at a working electrode with the use of a reference electrode.
Usually, reference electrodes are chosen to have electroactive species, with a
composition that is not affected by current flow, and connected in the low
impedance circuit of a potentiostat. A flow of hydrogen gas, covering a Pt foil, or
the standard hydrogen electrode (SHE) was among the first reference electrodes
to be used, and it electrode potential was chosen to be the zero reference.
The same current that flows in the working electrode must flow in the
counter or auxiliary electrode. However, experiments are designed in a way that
the processes at the counter electrode are not the rate-determining step. The
purpose of the counter-electrode is this to close the circuit which the working
electrode.
Thermodynamic equilibrium can de described by the Nernst equation,
which make use of the standard potential; an empirical constant that gives
information about the redox-active energy levels in oxidized (Ox) and reduced
(R) species[13]:
21
R
Oxo
aa
nFRTEE ln+= (11)
Although standard potentials are the basic values for all thermodynamic
calculations, in practice, we have to look at potentials with conditional constants.
This means that the constant reflects the specific condition which deviates from
standard conditions.
][][ln
][][ln0'0
ROx
nFRT
ROx
nFRTEE
γγ++= (12)
According to the Nernst equation, one could measure the potential E=Eº at
25ºC and 1 atmosphere of pressure, when the activities of all species are equal
to 1. In solutions with high concentrations, this is not accurate, so the need arises
to measure potentials at concentrations orders of magnitude lower than the
custom used solutions, and extrapolate the linear dependence to activity units. In
this work, solutions in the mili-molar (mM) range were employed.
If we can control the extent and direction of polarization of an
electrochemical reaction by the use of an external power supply, like a
potentiostat, we can also expect to control the speed and direction of the
reaction. These techniques are employed in controlled potential experiments,
were the energy of the electrode’s surface, as measured by its potential, is swept
or stepped towards a conceited value.
22
Cyclic voltammetry (figure 1-11) is by far the most widely used technique
to obtain qualitative information about electrochemical reactions. It consists of a
linear scanning of the working electrode through triangular potential waveform,
with the current response being observed [45]. The rate at which the potential is
scanned through an experiment, is called the scan rate, and the switching
potential, or apex, can be adjusted by the program before each experiment. In
this work, typical values for scan rate consisted of 20 mV/s.
A variation of the CV experiment is the multiple cycle experiment, in which
the same triangular waveform is repeated for a determined number of times.
Typical values consisted of 10 to 20 cycles.
Among other parameters used to gather thermodynamic and kinetic
information on electron-transfer reactions are the anodic and cathodic peak
currents (ia and ic) and the anodic and cathodic peak potentials (Ea and Ec).
Time (λ)
Switching potential
E(λ) E0
23
Figure 1-11 Cyclic voltammetry potential-time profile.
The formal redox potential (Eº’) for a reversible process, is given by the
mean (2
ac EE + ) of the peak potentials. Another characteristic potential parameter
is the separation of the peak potentials ∆Ep. The theoretical value for ∆Ep for a
reversible process is 0.057/n V. However, the measured value for a reversible
process is generally higher due to the presence of defects on the electrode
surface, uncompensated solution resistance and non-linear diffusion and/or a
poor cleaning procedure of the surface.
The peak current in a CV experiment for a reversible process is given by
the Randles-Sevcik equation [14]:
i x n AD Cp = ( . )2 69 10532
12
12υ (13)
Where A is the electrode surface area (cm2), D is the diffusion coefficient (cm2/s),
C is the concentration of the electroactive species in the bulk solution (mol/cm3),
n is the number of electrons transferred and υ is the scan rate. Hence, for a
diffusion limited process, ip is proportional to C and proportional to υ1/2. If the
diffusion coefficient for the redox couple is known ( i.e. potassium ferrous- and
ferri- cyanide), the area can thus be obtained.
24
When coupled with in-situ EQCM measurements, the potentiostat can be
used in such a way so as to hold or “pause” the scanning potential during the
regular CV cycle in order to obtain the specific mass change.
Chronoamperometry is another potential step experiment commonly used
in electrochemistry. The potential is stepped, from an initial potential, usually the
open circuit potential, to a potential desired, for a certain amount of time. This
aims to increase or reduce the electron’s energy so a desired electrochemical
behavior is obtained. Using this method, the nucleation and growth of different
films have been elucidated, and information about the rate of the reaction or
mass efficiency of electrochemical depositions, (when used with the EQCM) can
also be used (Figure 1.12).
Figure 1-12 Potential-time profile of a C.A. experiment.
Ef
E0
Time
25
The electrochemical experiments were performed using a CHI
InstrumentsTM 400 or 440 model time-resolved EQCM potentiostat and controlled
by a personal computer. Custom software was used to record cyclic
voltammograms (CVs) and to record all data from electrochemical experiments.
A platinum (Pt) foil or coiled rod was used as counter electrode, and the
reference electrode consisted of an Ag/AgCl, with a potential difference of
E=0.197 vs. SHE, and a mercury/mercurous sulfate (Hg/HgSO4) reference
electrode (MSE), with a standard potential of 0.64V vs. SHE.
1.5 X-Ray Photoelectron Spectroscopy
Developed in 1940 by Siegban et al. in Uppsala (Sweden), this is a
commonly used technique to elucidate the chemical and structural components
of surfaces. It was originally denominated electron spectroscopy for chemical
analysis (ESCA), and nowadays, is mostly referred as X-Ray photoelectron
spectroscopy (XPS). It uses soft x-ray sources, like Al and Mg Kα hν=1253.6 and
1486.6 respectively) [46] emission, incident to the analyzed surface. Its operation
is based on the photoelectric effect; by energy conservation, the monochromatic
electromagnetic wave will have the effect of kicking out an electron from the
atom’s core energy levels, overcoming its binding energy (figure 1.13).
26
Figure 1-13 Photoelectron emission in an XPS analyzer.
The photoelectron will enter the analyzer, (assuming vacuum conditions
present in the analyzer) with a kinetic energy consisting of:
bke EhE −Φ−= υ (14)
Where hν is the energy of the incident ray, Φ is the work function of the
apparatus, and Eb represents the electron’s binding energy. [47]. The detected
kinetic energy is related to the binding energy of the analyzed species by the use
of the work function of the analyzer, and the energy of the monochromatic beam.
The electron binding energy is characteristic of each analyzed sample, and it will
reflect the oxidation state or the electron affinity of the chemical environment
where the photoelectron was emitted. Higher oxidation states will have a shift
towards higher binding energy values.
hν Vacuum
Core
Ebe
Eke
27
Since energy levels are quantized, the emitted photoelectrons have a
kinetic energy distribution in the form of peaks, associated with electron energy
levels from the each ionized orbital. The intensity of the peaks is directly related
to the amount of species found in the detected volume. The beam can penetrate
only to about 100 nm into the analyzed sample. Calibration is done by comparing
a reference value (Au 4F) with the Eb peak obtained by the instrument (4.5 eV).
The results from XPS analysis in the present work were obtained in a
VG®§§ ESCALAB MK II spectrometer, with an aluminum (Al) Kα emission ray as
source of excitation.
1.6 Conclusions
From this chapter, we can observe the historical evolution of the
piezoelectric resonator as a transducer device, arriving at the development in
electrochemical applications of the quartz crystal microbalance. The principles of
operation of the EQCM as a thickness shear mode oscillator were presented. By
means of the Sauerbrey’s equation, the EQCM technique was introduced as a
powerful mean to trace minute mass changes of interfacial reactions, and its
application as a powerful electrochemical in-situ and sensing device tool is
explored. A brief description on the methodology used in for the preparation of
electrodes was included.
Finally, some fundamental concepts of the most commonly techniques
used in this work, electrochemistry and XPS analysis were described.
§§ Thermo Electron Corportion, www.thermo.com
28
1.7 References
1. Mason, W.P., The Journal of the Acoustical Society, 1981. 70(6): p. 1561-
1566.
2. Ward, M.D. and Buttry, D.A., Science, 1990. 249(4972): p. 1000-1007.
3. Salt, D., Hy-Q Handbook of Quartz Crystal Devices, 1st edition, ed.
Berkshire, J., 1987. Van Nostrand Reinhold, Amsterdam. 229.
4. Deakin, M.R. and Buttry, D.A., Analytical Chemistry, 1989. 61(20): p.
1147A-1154A.
5. Buttry, D.A., Applications of the Quartz Crystal Microbalance, in
Electroanalytical Chemistry: A Series of Advances, ed. Bard, A.J., 1996.
Marcel Dekker: New York. p. 1-85.
6. Marx, K.A., et al., Biosensors and Bioelectronics, 2001. 16: p. 773-782.
7. O'Sullivan, C.K. and Guilbault, G.G., Biosensors and Bioelectronics, 1999.
14: p. 663-670.
8. Rodhal, M.F., Hook, M.F. and Kasemo, B.,Analytical Chemistry, 1996.
63(12): p. 2219-2227.
9. Sakti, S.P., et al., Sensors and Actuators B, 2001. 78: p. 257-262.
10. Ballentine, J., Acoustic Wave Sensors: Theory, Design and Physico-
Chemical Applications, in Acoustic Sensors: Theory, Design and Physico-
Chemical Applications, ed. J. Ballentine. Vol. 1. 1997, San Diego:
Academic Press. 263.
29
11. Hepel, M., Electrode-Solution Interface with the Electrochemical Quartz
Crystal Nanobalance, in Interfacial Electrochemistry: Theory, Experiment
and Application, ed. Wieckowski, A., 1999, Marcel Dekker: Postdam. p.
599-629.
12. Lu, C., Applications of Piezoelectric Quartz Crystal Microbalances, in
Methods and Phenomena: Their application in Science and Technology,
ed. C.C. Lu, A.W., 1984, Elsevier Science: Amsterdam. p. 393.
13. Wang, J., Analytical Electrochemistry. 2nd ed. 2000, New York: Wiley-
VCH. 207.
14. Bard, A.J. and Faulkner, L.R., Electrochemical Methods: Fundamentals
and Applications., 2nd ed. 2001, New York: Jonh Wiley and Sons. 833.
15. Buttry, D.A. and Ward, M.D., Chemical Reviews, 1992. 92(6): p. 1355-
1379.
16. Bruckenstein, S. and Swathirajan, S., Electrochimica Acta, 1985. 30(7): p.
851-855.
17. Kanazawa, K.K. and J.G.G. II, Analytical Chemistry, 1985. 57: p. 1770-
1771.
18. Sauerbrey, G., Zeitschrift fuer Physik, 1959. 155: p. 206-222.
19. Zhang, M., Electrochemical Quartz Crystal Microbalance (EQCM) Studies
of Electrocatalytic Reactions, in Chemistry Department. 1995, University
of Ottawa: Ottawa. p. 262.
30
20. Lee, W.W., White, H.S. and Ward, M.D., Analytical Chemistry, 1993. 65:
p. 3232-3237.
21. Schmitt, R.F., et al., Sensors and Actuators B, 2001. 76: p. 95-102.
22. Tsionsky, V., et al., Looking at the Metal/Solution Interface with the
Electrochemical Quartz Crystal Microbalance: Theory and Experiment., in
Electroanalytical Chemistry, a Series of Advances, ed. A.J. Bard, 2002,
Marcell Dekker: New York.
23. Ehahourn, H., et al., Analytical Chemistry, 2002. 74(5): p. 1119-1127.
24. Tsionsky, V., et al., Journal of Electroanalytical Chemistry, 2002. 525-525:
p. 110-119.
25. Herrero, E., Buller, L.J. and Abruña, H.D., Chemical Reviews, 2001.
101(7): p. 1897-1930.
26. Bard, A.J. and Ru-Ren, F., Journal of Physical Chemistry B, 2002. 106: p.
279-287.
27. Tomic, O., Ulmer, H. and Haugen, J.E., Analytica Chimica Acta, 2002.
220450: p. 1-13.
28. Hayden, O., et al., Sensors and Actuators B, 2003. 91: p. 316-319.
29. Hayden, O. and Dickert, F.L., Advanced Materials, 2001. 13(19): p. 1480-
1483.
30. Dickert, F.L., et al., Sensors and Actuators B, 2001. 76: p. 295-298.
31. Iqbal, S.S., et al., Biosensors and Bioelectronics, 2000. 15: p. 549-578.
32. Dickert, F.L., et al., Sensors and Actuators B, 2003. 95: p. 20-24.
31
33. Komplin, G.C. and Pietro, W.J., Sensors and Actuators B, 1996. 30: p.
173-178.
34. Snook, G.A., Bond, A.M. and Fletcher, S., Journal of Electroanalytical
Chemistry, 2002. 526: p. 1-9.
35. Itagaki, M., Tagaki, M. and Watanbe, K., Corrosion Science, 1996. 38(7):
p. 1109-1125.
36. Méndez, P.F., et al., Electrochimica Acta, 2004. In press.
37. Uchida, H., Hiei, M. and M. Watanabe, M., Journal of Electroanalytical
Chemistry, 1998. 452: p. 97-106.
38. Galliano, F., Olsson, C.-O.A. and Landolt, D., Journal of the
Electrochemical Society, 2003. 150(11): p. B504-B511.
39. Uchida, H., et al. Quartz Crystal Microbalance Study of the Specific
Adsorption of Halide Ions on Highly Ordered Au(111). in Sixth
International Symposium on Electrodes. 1996: The Electrochemical
Society.
40. Doblhofer, K., et al., Journal of the Electrochemical Society, 2003.
150(10): p. C657-C664.
41. Watanabe, M., Uchida, H. and Ikeda, N. Journal of Electroanalytical
Chemistry, 1995. 380: p. 255-260.
42. Flores, S., Making an Electrode. 2003: Denton, Tx. p. 19.
43. Scholz, F., Electroanalytical Methods. 1st edition. 2002, Berlin, New York:
Springer. 331.
32
44. Faulkner, L.R., Electrochemical Characterization of Chemical Systems, in
Physical Methods in Modern Chemical Analysis. 1983, San Diego, CA
Academic Press:.
45. Grosser, D.K., Cyclic Voltammetry: Simulation and Analysis of Reaction
Mechanism. 1993, New York: Wiley-VCH.
46. Nascente, P.A.P., Journal of Molecular Catalysis A: Chemical, 2005. 228:
p. 145-150.
47. Ebel, M.F., Journal of Electron Spectroscopy and Related Phenomena,
1976. 8: p. 213-224.
33
CHAPTER 2
COPPER UNDERPOTENTIAL DEPOSITION ON RUTHENIUM AND
RUTHENIUM OXIDE
2.1. Introduction As second row transition metal, ruthenium is a metal largely immune to
attack of atmosphere. It has a melting point of 2334ºC, and a boiling point of
4150ºC. The density of the solid is 12370 kg·m-3. It has a resistivity value of 6.8
µΩcm, and its hexagonal closed packed structure has a cell size of 449.19 X
310.66 pm [1]. Ruthenium (Ru) has been investigated as a possible catalyst for
the oxidation of carbon monoxide [2-4] formic acid and methanol, as a fuel cell
catalyst [5-8].
Although ruthenium is insoluble in hot acids, including aqua regia, it reacts
readily with oxygen under heating (800ºC) to form Ru2O. Further oxidation will
yield ruthenium (VIII) oxide (RuO4), which is a toxic gas [9]. Ruthenium (IV) oxide
34
has a rutile structure, with available electrons in the d orbitals that enable it to
exhibit metallic properties, (e.g. enhanced conductivity) [10]. Values for the
resistivity of RuO2 have been reported to be as low as 35 µΩcm [1]. This material
has been used as a dimensionally stable anode for the chlor-alkali industry [11].
Due to its high capacitance (700 F/g), ruthenium (IV) oxide is currently
explored as a promising material for electrode capacitors [12-15], and studied as
a diffusion barrier for oxygen [16-18] and metal gate devices [19].
Under potential deposition (UPD), this is, the electrodeposition of metal
monolayer(s) on a foreign substrate, at potential less negative than that for
deposition on the same metal surface [20], has been the subject of thorough
study, largely driven by the fundamental possibility of understanding the
deposition process in general [21-25], and the observation of different
electrochemical behavior caused by the strong interaction of the adsorbed atoms
and its corresponding substrate [26-30].
35
Figure 2-1 Inter-grain boundary diffusion prevention by the use of RuOx.
Due to its immiscibility with copper, ruthenium and ruthenium (IV) oxide
can be explored as barrier materials for copper (Cu) interconnects in integrated
circuits [31, 32]. In the work presented herein, Underpotential deposition of Cu on
Ru and Ru oxide is explored. Ruthenium oxide can be used as a possible “plug”
or intergrain boundary diffusion barrier that can in turn be applied to the sub-65
nm microelectronics industry (figure 2-1).
2.2. Experimental
Hemispherical Ru shots with an average diameter of 4 mm were made
into disk electrodes as described earlier (see: Chapter 1). Prior to every data
collection series (electrochemical and x-ray diffraction), the electrodes were
polished to a fine grid, using silicon carbide pads with to a 1µm mirror finish. High
TaN
RuOx as Diffusion Plug Cu Seed
Ta
Proposed
Cu
Current
Cu ILD ILD
(Ru, RuO2)
No RuOx
ILD
C C
R R
36
purity copper sulfate (Aldrich®*) and sulfuric acid (Mallinckrodt®†) were used to
make all electrolyte solutions in ultra-pure water (18.2 MΩ, Millipore®‡). All
electrolyte solutions were pre-purged with argon gas and maintained under argon
(Ar) atmosphere.
In the case of reported Ru EQCM current-mass response, deposition of
films were done in a high vacuum chamber (1X10-8 Torr base pressure) at 200W
for 120 minutes, using a ruthenium target (Kurt J. Lesker®§, 99.95% purity) with
Ar as carrier gas.
Electrochemical investigations were performed using CHI 440 (CH
InstrumentsTM**) potentiostat/galvanostat. A conventional three-electrode cell with
a Platinum (Pt) foil as counter electrode and a saturated mercury/mercurous
sulfate (Hg/HgSO4 or MSE) with a potential shift of 0.647 V vs. the standard
hydrogen electrode (SHE), or a silver/silver chloride (Ag/AgCl 3M, 0.197 V vs.
SHE) as reference electrodes were employed. All potentials here referred,
including the potentials presented in the cyclic voltammetry (CV) curves, are
reported versus the MSE reference electrode.
The EQCM cell was placed inside a glass encasement, placed on top of a
granite base. Flowing water from a recirculation pump, connected to the glass
* Sigma Aldrich Corporation, www.sigmaaldrich.com † Mallinkrodt Incorporated, www.mallinkrodt.com ‡ Millipore Corporation, www.millipore.com § Kurt J. Lesker Corporation, www.lesker.com ** Ch Instruments, www.chintrsuments.com
37
encasement, was kept at 26ºC. This served to maintain a stable temperature
throughout each experiment.
X-ray diffraction analysis (XRD) was done using a Siemens®†† model d-
500 diffractometer using CuKα1 (λ=0.15406 nm) and CuKα2 (λ=0.154438nm)
radiation. The diffractometer consists of a Bragg-Brentano [33] configuration
(θ:2θ coupled), 2θ ranging from 30° to 100°. The high voltage generator was set
to a potential of 35 kV and 24mA. Additional x-ray photoelectron spectroscopy
(XPS) characterization was performed on a VG®‡‡ ESCALAB MK II
spectrometer, with Al Kα as x-ray source.
2.3 Results and Discussion
2.3.1 Copper Deposition on Ruthenium
Figure 2-2 shows the cyclic voltammetric curves of a ruthenium film in the
background electrolyte (05 M H2SO4), studied by means of the electrochemical
quartz crystal microbalance (EQCM). The presence of hydrogen evolution and
desorption of absorbed hydrogen can be observed (region I). The anodic region
of surface oxidation (region II) is overlapped with the former region [34, 35].
At more anodic potentials, the presence of a reversible form of ruthenium
oxide, or early adsorption of oxygen region (region III) is observed, which is
†† Siemens AG, www.siemens.com ‡‡ Thermo Electron Corporation, www.thermo.com
38
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-1000-800-600-400-200
0200400600
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-80-70-60-50-40-30-20-10
0
IIIIII
Cur
rent
Den
sity
(µA/
cm2 ) 0.5 M H2SO4 / Ru
III
II
I
∆ M
ass
(ng)
V vs MSE
Figure 2-2 Ru EQCM electrode in background electrolyte.
suggested by different authors [36-38]. In this work, a clear difference in the Cu
plating behavior was found between the freshly polished ruthenium electrodes,
and electrodes polarized at higher potentials (0.85 V).
The anodically polarized films had higher capacitance as compared
39
to the Ru electrode shots, (figure 2-3). XRD characterization of a Ru electrode
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-100.0
-80.0
-60.0
-40.0
-20.0
0.0
20.0
Cur
rent
Den
sity
(µA/
cm2 )
V vs MSE
0.5M H2SO4/Ru
Figure 2-3 Ru electrode in background electrolyte.
40
20 30 40 50 60 70 80 90 100 110
0
200
400
600
800
1000
1200
(202)(004)
(201)(112)(103)
(110)(102)
(101)
(002)
(110)Inte
nsity
(Ab.
Uni
ts)
2θ
Figure 2-4 XRD pattern of a Ru electrode.
(figure 2-4) helped confirm that the 2θ value for the experimental patterns is
within the range of 0.01° to 0.04°, as compared to that of the reference pattern of
ruthenium (see: card PDF#06-063).
Figure 2-5 shows the result of progressive vertex cycling of a Ru electrode
in 2mM CuSO4 + 0.5 M H2SO4 solution. The voltammetry started from the open
circuit potential value (OCP) close to 0.0 V vs. MSE, and scanned negatively
towards the -0.65V vs. MSE potential limit, at 20mV/s. No electroactivity was
41
observed when scanning towards positive potentials close to the OCP value.
Two predominant anodic peaks can be observed, one close to the nernstian
potential value of -0.4 V vs. MSE, and another one close to the potential value of
-0.26 V vs. MSE.
The relationship between the appearance of the first cathodic wave C1, at
-0.36 V, and the more anodic desorption peak A2, at -0.26V can be established
from these experiments. The peak current height of A1 increases as the cathodic
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-400-200
0200400600
8001000120014001600
Vertex 1 (-0.35 V) Vertex 2 (-0.4 V) Vertex 3 (-0.45 V) Vertex 4 (-0.5 V) Vertex 5 (-0.55 V) Vertex 6 (-0.65 V)
C2C1
A1
A2
Curr
ent D
ensi
ty (µ
A/cm
2 )
V vs MSE
Figure 2-5 CV response of a Ru shot in a 2 mM CuSO4 solution + 0.5 M H2SO4.
42
vertex is extended to more negative values viz. the behavior observed for the
peak “A2”, reaching an almost constant height when scanned beyond -0.4V.
The sharp increase in anodic current with increased cathodic potential
related to the peak “A1” is characteristic of bulk electrolysis of the deposited
metal, or over-potential deposition (OPD) desorption peak. A more detailed look
at the potential region of “A2”, unveils a well-defined diffusion limited peak, with a
maximum ca. -0.26 V (figure 2-6).
-0.4 -0.2 0.0 0.2-200.0
-150.0
-100.0
-50.0
0.0
50.0
100.0
Curr
ent (
µA/c
m2 )
V vs. MSE
Figure 2-6 Potential region of peak “A1”, obtained in a 2 mM CuSO4+0.5M H2SO4
solution on a Ru electrode.
43
A single crystal Ru (0001) face accommodates ca. 1.65X1015 atoms/cm2.
Nonetheless, assuming that a monolayer of copper, undergoes an epitaxial
growth following the hexagonal structure of the ruthenium substrate at a
coverage comprised of 1 X 1015 atoms/cm2 for a polycrystalline ruthenium
surface[39, 40], the copper coverage can be calculated from the measured
charge under the anodic UPD peak, Avogadro’s number (6.022X1023 atoms/mol)
and faraday’s law [41]:
nFNQ = (1)
Where, Q is the measured charge, (C), n is the number of electrons transferred,
F is the Faraday’s constant (assumed as 96484.6 C/mol), and N the number of
moles involved in the electrochemical process.
The coverage for the peak in figure 2.6 is close to that of a monolayer
(0.84). Thus assignment of this peak to a UPD process is made. The change in
free energy between underpotentially deposited copper and its ruthenium
substrate can be calculated from equation (2) [32].
pEnFG ∆=∆ (2)
Where ∆Ep is the potential difference between bulk and UPD anodic peaks n is
the number of electrons transferred, and F is Faraday’s constant.
The ruthenium electrode scanned to bulk cathodic vertex (-0.65 V), shows
a ∆Ep value of 120 mV. This is an indication of the stronger binding between
copper and ruthenium, as compared to copper deposited into its own lattice (bulk
deposition).
44
The complete coverage (monolayers) of the copper UPD on ruthenium
peak, as function of the cathodic peak potential, is presented in table 2-1, as well
as free energy values obtained form OPD-UPD peak to peak separation.
Table 2-1 Thermodynamic parameters for Cu UPD on Ruthenium.
Cathodic Vertex/V ∆Ep/V Charge /C Monolayers
(ML) ∆G/kJ/mol
-0.35 2.39E-05 0.13 -0.40 1.52E-04 0.84 -0.45 2.12E-04 1.17 -0.50 0.17 1.86E-04 1.03 33.00-0.55 0.15 1.74E-04 0.96 29.14-0.65 0.12 1.85E-04 1.02 23.35
An increase in the concentration of copper ions available in solution
shifted the potential more positively, as expected for a nernstian behavior. Figure
2-7 shows the CV for a ruthenium electrode in a 10 mM CuSO4 + 0.5 M H2SO4
solution. Although obscured by the bulk desorption peak of copper, the UPD
desorption peak, maintained almost a similar current height as in a 2 mM Cu2+
concentration solution, indicating similar coverage (figure2-8) , and its peak
potentials shifted more positively, (ca. 22 mV).
45
The nature of the surface limited growth of the UPD process is
demonstrated by the constant current peak height at the higher copper
concentration, indicating a constant copper coverage at this potential.
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-2.0
0.0
2.0
4.0
6.0
8.010 mM CuSO4 + 0.5 M H2SO4
Cur
rent
(mA/
cm2 )
V vs MSE
Figure 2-7 CV of ruthenium electrode in a higher copper concentration solution.
46
-0.4 -0.2 0.0 0.2
-150.0
-100.0
-50.0
0.0
50.0C
urre
nt (µ
A/cm
2 )
V vs. MSE
Figure 2-8 Underpotential deposition of copper from a 10 mM CuSO4+0.5M
H2SO4 solution on a Ru electrode.
2.3.2 Copper Deposition on Ruthenium Oxide
Chronoamperometry (see: chapter 1) was used in order to attain oxidation
of the Ru substrate. The technique performed involved a potential step program
designed to hold the ruthenium electrode at a polarization potential of 0.85 V vs.
MSE, for three minutes. Copious bubbles ascribed to the presence of oxygen
evolution were observed during the potential step, and a steep change in color of
47
the ruthenium electrode, from metallic bright to blue-black color, could be noted
on its surface (figure 2-9), from the optical microscope pictures, taken at 5X
magnification. The OCP value increased to 0.3 V, indicating a change in the
electrochemical state of the obtained surface oxide.
Figure 2-9 A) Ru surface, B) oxidized Ru surface (both 5x).
A mixed proton-electron conductor, ruthenium oxides obtained by
electrochemical means in aqueous solutions are known to be of the hydrated
form ruthenium oxides, or ruthenium hydroxide [37, 41-43]. The presence of a
larger surface area of hydrous ruthenium oxide is deduced by the increase in the
overall capacitance of the electrode, which can be observed from the CV
recorded in 0.5 M H2SO4 (figure 2-10).
A B
48
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
-500
-400
-300
-200
-100
0
100
200
300
Curr
ent (
µA/c
m2 )
V vs MSE
Figure 2-10 Background of oxidized Ru substrate in 0.5 MH2SO4 solution.
Upon potential cycling in the 2 mM CuSO4 +0.5M H2SO4, two anodic
desorption peaks are also observed, one at -0.4 V, and the other close to -0.24 V
(figure 2-11). Expanding the cathodic vertex to negative potentials (-0.65 V)
demonstrated the presence of a constant peak height of the second anodic peak.
This behavior is similar to the electrochemical behavior of the UPD desorption
peak present in ruthenium. The change in coverage, as well as the
49
thermodynamic relation between the free energy and the OPD-UPD peak to peak
separation is summarized in the following table (table 2.2)
Table 2-2 Thermodynamic parameters for Cu deposition on ruthenium oxide.
Cathodic Vertex/V ∆Ep/V Charge /C
Monolayers (ML) ∆G/kJ/mol
-0.35 6.65E-05 1.10
-0.4 7.18E-05 1.20
-0.45 8.34E-05 1.38
-0.5 0.206 8.80E-05 1.46 39.76
-0.55 0.204 1.00E-04 1.66 39.37
-0.65 0.177 1.06E-04 1.67 37.73
The peak to peak separation has increased to a value higher than 170
mV. Measurement of the peak to peak separation for the OPD-UPD diffusion
limited processes indicates that the free energy has increased to 14 kJ/mol. This
is due to the stronger affinity of the deposited copper layer to the oxide covered
ruthenium substrate than to the bare ruthenium electrode. Usually, the presence
of underpotential deposition of a foreign metal adlayer onto a different substrate
is explained in the terms of the difference in the workfunction (φ) of the deposited
metal [44]. That is; the metal with the lower workfunction number will deposit
underpotentially onto a metal with higher workfunction number [45].
In this case, a 2D structure of copper will be formed on an oxidized
ruthenium substrate, (higher workfunction value than ruthenium).
50
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
-1000
0
1000
2000
3000 Vertex 1 (-0.35 V) Vertex 2 (-0.4 V) Vertex 3 (-0.45 V) Vertex 4 (-0.5 V) Vertex 5 (-0.55 V) Vertex 6 (-0.65 V)
A2
A1
C2 C1
Cur
rent
Den
sity
(µA/
cm2 )
V vs MSE
Figure 2-11 Ruthenium oxide CV response in a 2 mM CuSO4+0.5MH2SO4
solution.
Scan rate dependant studies performed up to 800 mV/s, showed visible
UPD adsorption and desorption peaks and provided additional confirmation of
the strong affinity of the sub-monolayer layer copper deposited on top of the
ruthenium oxide substrate.
51
X-ray photoelectron spectroscopy (XPS, see: chapter 1) characterization
of a ruthenium oxide electrode, scanned to the UPD potential of -0.45, confirmed
the presence of Cu on both ruthenium, and ruthenium oxide electrodes.
The electrodes were analyzed ex-situ after a short time, trying to avoid
any oxidation of the copper layer caused be the atmosphere [32].
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1-80.0
-60.0
-40.0
-20.0
0.0
20.0
40.0
Cur
rent
(µA)
V vs. MSE
Figure 2-12 CV in 2 mM CuSO4+0.5MH2SO4 solution of a thermally produced
ruthenium oxide.
It is important to remark that the presence of Cu UPD has been observed
only on the electrochemically formed ruthenium oxide; cyclic voltammetry with
52
copper containing solutions of the same concentration (2 mM), performed on
thermally annealed ruthenium shots under O2, revealed the absence of UPD
cathodic and anodic peaks. Thus, the presence of the hydrated oxide layer is
indicated as an essential requirement for copper UPD deposition on ruthenium
oxide (figure 2-12).
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1
-500
0
500
1000
1500
2000
5 seconds oxidation 30 second oxidation 3 minutes oxidation
Cur
rent
(µA/
cm2 )
V vs. MSE
Figure 2-13 Effect of different oxidation times of an oxidized electrode in the
2 mM CuSO4+0.5MH2SO4 solution.
53
Thermodynamically, the energy required to attain such a behavior is
dominated by the polarization potential of the electrode, not by the oxidation time.
This was confirmed by experiments and can be observed by looking at the CV of
the ruthenium oxide prepared by doing a potential step to 0.85 V for different
times (5s, 30s and 3 minutes) in figure 2-13. The current vs. potential profile for
the short oxidation pulse (5s.) is similar to that of the longer (i.e. thicker) hydrous
ruthenium oxide, obtained at 3 minutes polarization potential, at the same
potential value.
2.4 Ongoing research
Characterization of Ru films on different electrodes has been a continuous
endeavor. With the aim of attaining a better understanding of the
electrochemistry of copper electroplating on ruthenium substrates, CVs have
been performed on prepared ruthenium films. The used silicon substrates were
hydrogen terminated by HF etching, following the standard (RCA) cleaning
procedure, to assure cleanness of the surface. Figure 2-14 shows the result of
cyclic voltammetry of a Cu containing solution in a ruthenium film deposition on a
silicon wafer via magnetron sputtering at 100 Watts for 2 hr. (120 nm thickness)
under an Ar plasma pressure of 20 mTorr. There is no visible differentiation
between bulk and UPD desorption peaks. Thus an effect on the electrochemical
behavior of the copper deposition process due to morphological effects on the
ruthenium sputtered film’s surface is suspected.
54
-0.8 -0.6 -0.4 -0.2 0.0 0.2-400
-300
-200
-100
0
100
200C
urre
nt (µ
A)
V vs. MSE
Figure 2-14 CV profile of a Ru film deposited on a Si wafer substrate in a 2 mM
CuSO4+0.5MH2SO4 solution.
Experiments were also performed with EQCM crystals, deposited via
vacuum sputtering with a ruthenium film of ca. 90 nm thicknesses (2-15). Un-
bonded and un-mounted crystals, of 8.0 Mhz base frequency, with no underlying
adhesive metal, were employed, in order to avoid any contamination or substrate
effect on the electrochemical behavior of the system.
55
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
-2000
-1000
0
1000
2000
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4-100
0100200300400Cu
rren
t Den
sity
(µA/
cm2 )
∆ M
ass
(ng)
V vs. MSE
Figure 2-15 EQCM response of a Ru electrode in a 2 mM CuSO4+0.5M H2SO4
solution.
This has also been observed at higher copper concentrations cyclic
voltammetry experiments on Ru EQCM crystals (figure 2-16). In a 10 mM CuSO4
+ 0.5 M H2SO4 solution potential cycling shows two distinct desorption peaks
(figure 2-17). However, the mass change associated with the desorption peak at
-0.2 V (44 ML) indicates bulk desorption process, and it is comparable to the
coverage obtained by charge measurement (40 ML), therefore ruling out the sole
presence of UPD deposition of copper.
56
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2-100
0100200300400500600
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
-2000-1000
0100020003000
∆ M
ass
(ng)
V vs. MSE
Cur
rent
Den
sity
(µA/
cm2 )
Figure 2-16 EQCM response of Ru in a 5 mM CuSO4 + 0.5 M H2SO4 solution.
Since the underpotential deposition of metal adlayers is a process strongly
influenced by the chemical and physical structure of the substrate metal, future
work can focus on the study of the morphology of the films and its relationship
with UPD behavior.
57
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2-4000
-2000
0
2000
4000
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
0
400
800
1200
1600Curr
ent D
ensi
ty (µ
A/cm
2 )∆
Mas
s (n
g)
V vs MSE
Figure 2-17 EQCM response of Ru in a 10 mM CuSO4 + 0.5 M H2SO4 solution.
2.4 Conclusion
The electrochemical characterization of copper underpotential deposition
on ruthenium and ruthenium oxide electrodes by means of electrochemical
methods are presented. Spectroscopic (XRD, XPS) and gravimetric (EQCM)
techniques helped characterized different surface states of the presented
ruthenium electrodes.
58
Distinctive diffusion limited peaks at potentials more positive than the
nernstian equilibrium potential are observed in both ruthenium and ruthenium
oxide. Increasing the negative potential vertex to -0.65 produced a surface
coverage of ca. 1 ML. Annealing of Ru electrodes at high temperature suggested
the requirement of a hydrous ruthenium layer for the presence of UPD behavior.
The technological aspects of underpotential deposition are discussed,
highlighting the use of ruthenium oxide surface, as a material for applications in
the microelectronics industry.
2.5. References
1. Seddon, E.A. and Seddon, K.R. The Chemistry of Ruthenium. in Topics in
Inorganic and General Chemistry, ed. R.J.H. Clark. Vol. 19. 1955,
Amsterdam: Elsevier Science Publishers S.V. 1373.
2. Bracchini, C., et al., Catalysis Today, 2000. 55: p. 45-49.
3. Wang, J.X., et al., Journal of Physical Chemistry B, 2001. 2001: p. 2809-
2814.
4. Lin, W.F., P.A. Christensen, and A. Hamnet, Journal of Physical
Chemistry B, 2000. 104: p. 6642-6652.
5. Waszczuk, P., et al., Electrochimica Acta, 2002. 47: p. 3637-3652.
6. Waszczuk, P., et al., Journal of Catalysis, 2001. 203: p. 1-6.
7. Wieckowski, A. and Kuk, S.T., Journal of Power Sources, 2005. 141: p. 1-
7.
8. Williams, J.O. and Mahmood, T. Application of Surface Sciences, 1980. 6.
59
9. Trasatti, S. and Buzzanca, G. Journal of Electroanalytical Chemistry and
Interfacial Electrochemistry, 1971. 29(1971): p. A1-A5.
10. Rao, C.N.R. and B. Raveau, Transition Metal Oxides Structure, Properties
and Synthesis of Ceramic Oxides. 2nd. ed. 1998, New York: Wiley-VCH.
11. Trasatti, S., Electrochimica Acta, 1984. 29(11): p. 1503-1512.
12. Kim, H.-K., et al., Thin Solid Films, 2005. 475(54-57).
13. Balek, V., et al., Colloidal and Interface Science, 2003. 260: p. 70-74.
14. Long, J., W., et al., Langmuir, 1999. 15: p. 780-785.
15. Kim, H.W. and C.-J. Kang, Microelectronic Engineering, 2003. 69: p. 89-
96.
16. Giauque, P.H. and Nicolet, M.-A., Journal of Applied Physics, 2003. 93(8):
p. 4576-4583.
17. Burke, L.D. and Healy, J.F., Journal of Electroanalytical Chemistry, 1981.
124: p. 327-332.
18. Chang, C.C. and Wen, T.C., Journal of applied electrochemistry, 1997. 27:
p. 353-363.
19. Zhong, H., Ru-based gate electrodes for advanced dual-metal gate CMOS
devices., in Analytical Chemistry. 2001, North Carolina State University:
Raleigh. p. 257.
20. Herrero, E., Buller, L.J. and Abruña, H.D. , Chemical Reviews, 2001.
101(7): p. 1897-1930.
21. Garcia, S., et al., Electrochimica Acta, 1998. 43(19-20): p. 3007-3019.
60
22. Mendoza-Huizar, L.H., Robles, J. and Palomar, P., Journal of
Electroanalytical Chemistry, 2003. 545: p. 39-45.
23. Rooryck, V., et al., Journal of Electroanalytical Chemistry, 2000. 482: p.
93-01.
24. Swathirajan, S. and Bruckenstein, S., Electrochimica Acta, 1983. 28(7): p.
865-877.
25. Mendez, P.F., et al., Electrochimica Acta, 2004. In Press.
26. Michalitsch, R., Palmer, B.J. and Laibinis, P.E. , Langmuir, 2000. 16: p.
6533-6540.
27. Krznaric, D. and Goricnik, T., Langmuir, 2001. 17: p. 4347-4351.
28. Santos, M.C. and Machado, S.A.S., Journal of Electroanalytical
Chemistry, 2004. 567: p. 203-210.
29. Santos, M.C. and Machado, S.A.S., Electrochimica Acta, 2004. In Press.
30. Nicic, I., et al., Journal of Physical Chemistry B, 2002. 106: p. 12247-
12252.
31. Chyan, O.A., Tiruchirapalli N.; Ponnuswamy, Thomas., Journal of the
Electrochemical Society, 2003. 150(5): p. C347-C350.
32. Zhang, Y., et al., Electrochemical and Solid-State Letters, 2004. 7(9): p.
C107-C110.
33. Cullity, B.D., Elements of X-Ray Diffraction. 3rd ed. 2001, Upper Saddle
River: Prentice-Hall.
61
34. Angerstein-Kozlowska, H., et al., Journal of Electroanalytical Chemistry,
1979. 100: p. 417-446.
35. Hadzi-Jordanov, S., Angerstein-Kozlowska, H. and Conway, B.E.,
Electroanalytical Chemistry and Interfacial Electrochemistry, 1975. 60: p.
359-362.
36. Vukovic, M., Valla, T. and Milun, M. Journal of Electroanalytical Chemistry,
1993. 356: p. 81-90.
37. Horvat-Radosevic, V., Kvastek, K. and Vukovic, M. Journal of
Electroanalytical Chemistry, 1999. 463: p. 29-44.
38. Michell, D.R.,and Woods, R., Journal of Electroanalytical Chemistry, 1978.
89: p. 11-27.
39. Quiroz, M.A. and Meas, Y., Journal of Electroanalytical Chemistry, 1983.
157: p. 165-174.
40. Gonzales-Tejera, M.J. and Nguyen Van Huong, C., Journal of
Electroanalytical Chemistry, 1988. 244: p. 249-259.
41. Conway, B.E., Electrochemical Supercapacitors. First. ed. 1999, New
York: Kluwer-Plenum.
42. Prakash, J. and Joachin, H., Electrochimica Acta, 2000. 45: p. 2289-2296.
43. Vukovic, M. and Dunja, H., Journal of Electroanalytical Chemistry, 1999.
474: p. 167-173.
44. Chrzanowski, W. and Wieckowski, A., Langmuir, 1997. 13: p. 5974-5978.
45. Hartmann, A.J., et al., Applied Physics A, 2000. 70(239-242).
62
CHAPTER 3
FORMATION OF BASIC COPPER SULFATES
3.1 Introduction
The formation of “patinas” or corrosive protective layers [1] is a common
process known to happen after extensive atmospheric exposure of bronzes or
other metallic alloys, usually present in urban areas and historical places, or in
the form of statues, sculptures, façades, and other building objects [2, 3].
Atmospheric corrosion is fostered by the adsorption of water, and in the
case of copper, or copper containing alloys, the incorporation of impurities,
mainly sulfates, leads to the formation of basic copper sulfates [4, 5], like
brochantite (Cu4(OH)6SO4), antlerite (Cu3(OH)4SO4) and posnjakite
(Cu4(OH)6SO4.2H2O).
The chemistry of formation of the basic copper sulfates has been reviewed
by different authors [4, 6-8]. Experimentally, the processes in which salts are
obtained vary from titration [4] to aging to naturally produced patina examination.
The general conclusion is that a cuprous oxide (Cu2O) or cuprite is the initial
species present in the layer [1].
The study of copper plating is of technological relevance, since its wide
use in power line connections, common household electrical appliances and
63
more recently, the electrochemical deposition of copper as an important process
for the IC industry [9]. It has a high conductivity (1.7 µΩ.cm) and it has completely
64
replaced aluminum (Al) as the preferred interconnects material used in microchip
manufacturing [10, 11] . At low pH values, copper plating is the most common
reaction [9, 11-14]. Electroplating at pH values higher than 6.8, in which Cu2O
plating [15-19] is the favored process [8] has also been widely investigated.
Besides the above two common areas of investigation, little is known in
the literature about copper electrodeposition in different pH values, hence the
importance of this study.
In this chapter, the mechanism of formation of posnjakite, observed by the
formation of a precipitate at the end of a cyclic voltammetric cycle is confirmed by
the results obtained with the use of the electrochemical quartz crystal
microbalance (EQCM), x-ray photoelectron spectroscopy (XPS), x-ray diffraction
(XRD) and optical microscopies.
3.2 Experimental
A 400 series model electrochemical analyzer from CH InstrumentsTM*, was
used for the quartz crystal microbalance experimentation. The time resolved
instrument can simultaneously read the frequency change of the crystal along
with the surface’s current change. We used platinum coated crystals. In this way,
the instrument is an effective tool for in-situ monitoring of current-mass related
changes of the electrode’s surface. The oscillating frequency of the crystal is
subtracted from a reference oscillator, vibrating at 7.995 MHz, so the difference
is reported. This is called reciprocal method, and it gives the advantage of
* CH Instruments Inc. www.chinstruments.com
65
precise measurement at a fast rate. In this configuration, the crystal itself,
bonded to a metallic mount, is seated on the bottom of a Teflon®† cell, with its
surface facing the body of the solution. The exposed electrode area is 0.206 cm2,
and it is used as the working electrode in a conventional three way electrode cell.
The platinum (Pt) covered electrodes had a thickness of 1000 Ǻ, with a
titanium underlying layer, 100 Ǻ thick, deposited on an AT cut quartz crystal
obtained from International Crystal Manufacturing‡ (ICM®). A Pt rod
(diameter=0.00508 cm) was submerged in the solution’s body to act as a counter
electrode. A mercury/mercury sulfate (Hg/HgSO4) or MSE reference electrode
was employed for all the measurements.
By means of the Sauerbrey’s equation [20], the change in frequency is
related to the change in mass:
mCf ∆−=∆ (1)
The constant C is determined by various properties of the crystal [21, 22] (i.e.
thickness, fundamental frequency, density, etc...). In the case where crystals with
a fundamental frequency of oscillation of 7.995 the value of 1.4156x10-9 ng per
hertz change is used, as it is the case of the presented study.
In order to get the cyclic voltammetry profiles (CV), the surface of the
electrode was scanned from a value close to the open circuit potential (OCP) of
the Pt surface (0.1 V) to the copper plating region (between -.65 and -0.7 V vs.
† E.I. du Pont de Nemours and Company, www.teflon.com ‡ International Crystal Manufacturing, www.icmfg.com
66
MSE). Potassium sulfate, obtained from Alfa Aesar®§ and copper sulfate
pentahydrate 99.999% pure, from Aldrich®** were used in the solutions, all of
them prepared with ultra pure Millipore®†† water (18.0 MΩ). X-ray diffraction
characterization was done on a Bragg-Brentano Siemens®‡‡ D-500
diffractometer, with a Cu Kα X-ray as source, controlled by a personal computer.
The experiments were run in the 2-68° 2θ angle range, with a step size of
0.05° and 1 second dwell time. XPS data were collected using a VG ESCALAB
MKIITM§§ spectrometer with an Mg Ka X-ray as the excitation source. Optical
images were recorded using a microscope (Nikon® ME600L***)
Acquisition of optical images from the electrode’s surface demanded the
modification of the EQCM crystal mount available from the manufacturer. This
mount impeded the use of the optical microscope’s high magnification objectives,
as it collided against the metallic edge of the lens. In order to overcome this
limitation, two modifications were made; first, the crystal was removed from its
original contact pins. For this step, unbounded crystals were purchased so there
was no originally present epoxy material to me removed. The crystal was then
bonded to Teflon® coated stainless steel wire, which was used as extension.
The crystal was protected with a Teflon® tape mask along the process so
its surface remained unscratched. In order to obtain an electrical connection § Alfa Aesar, www.alfa.com ** Sigma-Aldrich Corporation, www..sigma-aldrich.com †† Millipore Corporation, www.millipore.com ‡‡ Siemens A.G., www.siemens.com §§ Thermo Electron Corporation, www.thermo.com *** Nikon Corporation, www.nikon.com
67
between the electrodes deposited on top of the quartz crystal and the stainless
steel wire, Tra-Duct 29202, from Tra-Con®††† silver epoxy was used. Curing was
achieved by keeping it for 5 minutes at 160 ˚C. The crystal was then placed on
top of one of the o-rings used in the Teflon® cell, so as to align the central
(electrode) area with the center of the cell.
After this was accomplished, extension wires were fixed into position by
using a mass of silicon glue which molded itself inside the cell’s confinement
space (Figure 3-1).This allowed the microscope’s lens to be moved freely along
the way, without displacing the electrode from the objective.
Figure 3-1 Adaptation to the EQCM’s mount for microscope picture.
††† National Starch and Chemicals, www.tra-con.com
Electrode Cell Base Extension
68
3.3 Results and Discussion
Copper electroplating experiments have been performed in Ruthenium
and Ruthenium oxide surfaces [23, 24]. In order to systematically study its
electroplating behavior at different values of pH, the acidity of the plating bath
was increase from its common value of zero (0.5 M H2SO4) to higher pH values.
An electrochemically clean platinum surface was obtained by multiple
cycling, between -0.695V and 0.8V vs. MSE (figure 3-2).
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-10.0-5.00.05.0
10.015.020.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-200.0-150.0-100.0-50.0
0.050.0
B
∆Mas
s (n
g)
V vs MSE
A
Cur
rent
Den
sity
(µAm
p/cm
2 )
Figure 3-2 Current/mass profiles of a Pt electrode in 0.5M H2SO4 solution; A)
current density, B) mass change.
69
As a way to study this phenomenon by means of the EQCM, parallel
experiments were performed using the platinum coated quartz crystal wafers.
Plating baths with pH values of 0, 2, 3 and 4 were prepared by adjusting their
acidity with a 3M NaOH solution.
Scanning towards the cathodic apex, towards the hydrogen adsorption
and evolution region (-0.4V), a mass loss is observed via EQCM measurement.
This indicates the increasing coverage of a lighter Hydrogen layer. Upon scan
reversal, a mass gain is noted, indicating the absorption of water and/or anion
molecules, which occupy the sites previously covered by hydrogen. As the
potential reaches the anodic vertex, the onset of oxide formation can be
observed (0.1V), with a mass gain associated. The return to the original
electrode’s mass value, when the scanning returned to the original potential
value demonstrated a small amount of mass loss occurring due to cycling
between this potential limits [25, 26].
Following the electrochemical cleaning, copper was plated at different pH
values, from a 2 mM CuSO4 + 0.1M K2SO4 solution. The CV’s and corresponding
mass responses are shown in figure 3-3.
The positive current or anodic peak, located at the reverse scan, is related
to copper desorption, or copper stripping. The diffusion limited peaks have
potentials of -0.38 to -0.35, 0.34, -0.33 V. along with the increase in pH. The
oxygen reduction peak present at 0V at pH 0 and 2, shifts anodically to -014 and
-0.2 at higher (3, 4) pH.
70
From the EQCM response, we can see distinct features along the
potential scan. Starting from the OCP value of 0.1V, going towards the negative
side, the mass accumulation sharply increases after the reaction:
02 2 CueCu →+ −+ (2)
has reached its nernstian value of -0.45V vs. MSE. Mass increase continues
even after the potential sweep is reversed, with the negative current indicating a
continuum in the reduction reaction [27].
-0.8 -0.6 -0.4 -0.2 0.0 0.2
0100200300400500600
-0.8 -0.6 -0.4 -0.2 0.0 0.2-1000-500
0500
1000150020002500
pH 0 pH 2 pH 3 pH 4
∆Mas
s (n
g)
V vs MSE
pH 0 pH 2 pH 3 pH 4
Curr
ent D
ensi
ty (µ
Amp/
cm2 )
Figure 3-3 Current/mass profiles of copper deposition from 2 mM CuSO4 + 0.1M
K2SO4 adjusted at different pH values.
71
The onset of the single anodic peak present in all scans marks the
initiation of the reverse reaction, as noted also from the mass decrease. The
onset of this peaks moves anodically with increasing pH value. After the anodic
current peak is over, the slope of the mass decrease is reduced, to finally end up
in a mass value similar to the initial one, indicating that at the pH values below 5,
the deposition of copper is a highly reversible process. The associated charge of
the anodic peak corresponds well with the value of mass loss, for its potential
range. The average efficiency of the deposition process, as a measure of the
cathodic charge, versus the mass gain is ca. 60% for all the pH values
presented. Nonetheless, the efficiency of amount of mass loss, as read from the
EQCM, compared to the mass loss obtained from integration of the anodic
desorption peak, is above 90%.
Figure 3-4 shows the results of a typical CV of a 2mM concentration
copper plating solution at pH 5. New features are present in the both the cathodic
and anodic scans. Peak labeled “C1” or a “pre-peak” to the copper plating peak
“C2” appears. The peak are related to the anodic peaks “A1” (copper stripping)
and “A2”. This is confirmed by the multiple scanning experiments in this solution
at the same potential limits. It is interesting to note that the large mass
accumulation present at the end of each cycle, triggered by the appearance of
“A2”, is not present at other pH values. This mass accumulation eliminates the
presence of active centers in the Pt surface for under potential deposition of
72
copper, like it has been observed at lower pH values [28]. On the reverse scan, a
proportional mass desorption or “dumping” appears at potentials close to -0.42V.
The area of the peak “C1” increases with the number of cycles, and
eventually obscures the peak “C2”.
-0.8 -0.6 -0.4 -0.2 0.0 0.2
-800-400
0400800
12001600
-0.8 -0.6 -0.4 -0.2 0.0 0.2
0
800
1600
2400
3200
A2
A1
C2 C1
Cur
rent
Den
sity
(µA/
cm2 )
34
21
∆Mas
s (n
g)
V vs. MSE
Figure 3-4 Multiple scanning experiments at pH=5, 2mMCuSO4+0.1MK2SO4,
Upon changing the concentration of the Cu2+ ions present in solution, to
5mM and 10 mM, there’s an increase in the total mass accumulated at the
73
surface up to the peak “A1”, while the amount of mass at “A2” doesn’t increase
as should correspond to the increase in copper from the solution (figure 3-5).
-0.8 -0.6 -0.4 -0.2 0.0 0.2
0
500
1000
1500
2000
2500-0.8 -0.6 -0.4 -0.2 0.0 0.2-4000
-2000
0
2000
4000
6000
8000
∆ m
ass
(ng)
V Vs MSE
2mM 5mM 10mM
Cur
rent
Den
sity
(µA/
cm2 )
Figure 3-5 EQCM profile of copper deposition in a Pt electrode at different concentrations.
74
The ratio of cathodic versus anodic charge transfer throughout a complete
cycle is always greater than the unity.
Potential holding at the end of the cycle for different times didn’t show any
effect on its dissolution or retention of the precipitate formed. Once the CV
stopped, the mass of the electrode surface, as recorded by the frequency
change, slowly diminished towards the original electrode’s weight, as seen by an
increase in frequency, indicating dissolution of the precipitate formed.
3.3.1. Characterization of the Process at Different Stages
XRD and XPS techniques were used to characterize the mechanism of
copper plating from a 2mM solution, at different stages of the CV performed on
Ru electrodes. These characterization methods are still useful, since the features
appearing at the CV of the Pt surface were reproduced in the Ru electrode as
well. The XRD diffractogram (figure 3-6) shows the planes characteristic of
posnjakite [1, 29] Cu4SO4(OH)6.H2O, a form of basic copper sulfate (PDF#43-
0670, 83-1410). Although this is not the most stable form of the salt [30] it is
noted as a hydrated form of brochantite. This undergoes dehydration, and a
more stable form brochantite is formed [31]. The latter is known to be the product
of corrosion in statues, roof tops and bronzes present in urban areas. Studies of
this salt in marine and natural environments have also been reported [32].
75
10 20 30 40 50 60 700
100
200
300
400
500
600
700
800
P (431)
P (-223)
P (002)P (001)
Ru (100)
Inte
nsity
(A.U
)
2θ
Figure 3-6 XRD spectra of the precipitate formed, as obtained form the top of a
Ru electrode P: posnjakite.
In order to have a visual survey of the surface of the electrode at the
different stages of the cyclic voltammetric profile, images from holding at different
points were taken. Figure 3-7 shows the resulted optical pictures at 100X
magnification taken at different stages of the CV profile of a 10 mM CuSO4
solution with 10 mM Cu2+ concentration. The images are taken in the same place
76
Figure 3-7 Optical pictures of the Pt electrode in a 10 mM Cu2+ concentration
solution, taken at different stages of the potential sweep.
A
B
C
D
E
-0.8 -0.6 -0.4 -0.2 0.0 0.2
0
500
1000
1500
2000
2500-0.8 -0.6 -0.4 -0.2 0.0 0.2
-4000
-2000
0
2000
4000
6000
8000
C2
A1
A2 Apex
C1
∆Mas
s (n
g)
V vs MSE
C2
A1
Vertex
Valley
A2 ApexValley
Vertex
C1
Cur
rent
Den
sity
(µA/
cm2 )
A) Clean Electrode B) 3 min. holding at “C1” C) 3 min. holding at “C1”, stop at Vertex D) 3 min. holding at “C1”, stop at Valley E) 3 min. holding at “C1”, stop at “A2” Apex
77
of the electrode’s surface each time, and were obtained after holding at “C1”
peak (-0.4 V) for three minutes, and halting the sweep at four different positions,
labeled B, C, D, and E, and corresponding to holding at “C1”, the vertex of the
cathodic region (-0.7V), the plateau or valley between the first and second anodic
peaks, and finally, at the apex of the decreased current height peak 2,
respectively.
The image corresponding to the apex of peak ”A2” is displayed in dark
field, to show the features of the blue-white precipitate characterized as
posnjakite [33].
At “C1”, we can observe the number of reddish particles, characteristic of
Cu2O, a brick red electron deficient p-type semiconductor [34-36]. SEM (figure 3-
8) pictures of the film deposited on ruthenium wafer slides unveiled a triangular
and squared shape of the crystals ca. 200 nm wide. This observation of color and
shape, are in accordance to those of Lee et al. [37], who reported the formation
of Cu2O(111) by electroplating from a pH 4.7 solution. Another characteristic of
the deposited particles at “C1” is that their size did not increase with the
concentration of Cu2+ in solution. Only their density (number of particles at a
given surface area) increases.
78
Figure 3-8 SEM image of Cu2O crystals on a Ru coated Si wafer.
X-ray photoelectron spectroscopy of the Ru electrode held for three
minutes at different potentials is presented in figure 3-9, where the energy region
for electrons emitted from the Cu 2p orbital is shown [38]. This confirms that at
peak “C2”, Cu is electrochemically deposited (A).
Indeed the value of the peak at 932.32 eV corresponds to Cu 2p3/2
photoelectrons. When the electrode is held at C1 for three minutes (B), the peak
has shifted to a higher binding energy (932.8 eV), confirming the formation of
Cu2O. Spectra collected from the precipitate indicates the presence of a satellite
79
Figure 3-9 Cu 2p region of XPS spectrum for compounds formed in 2 mM CuSO4
+0.1 K2SO4 pH 5 solution a) holding at “C2” for 3 min; b) holding at “C1” for 3
min; c) “A2” or precipitate.
peak at 941.62 eV, characteristic of CuO corresponding to the formation of Cu2+
oxide and hydroxide species at the surface of the electrode [39]. A peak at
934.29 eV can be assigned to the presence of Cu2+ ions [40] that form upon
970 960 950 940 930 92035
40
45
50
55
60
932.8
c
a
b
934.29
952.21
941.62
932.32In
tens
ity (A
.U.*1
04 )
Binding Energy (eV)
80
dehydration of posnjakite under vacuum. This is an indication of Cu species
under the chemical effect of its sulfate surroundings.
175 170 165 160 155 150
16.6
16.8
17.0
17.2
17.4
In
tens
ity (A
.U.*1
04 )
Binding Energy (eV)
Figure 3-10 S 2p XPS spectrum of precipitate.
Oxygen 1s region spectra, obtained by “C1” holding, with peaks at
530.2eV and 532.2eV confirmed the presence of two chemical states for oxygen
species that can be ascribed to surface oxygen adsorbed from the environment,
and oxygen present in cuprous oxide. Confirmation of the presence of sulfate, in
the formation of posnjakite CuSO4(OH)6.H2O can be found in the spectra of the S
81
2p region of the electrode at the final “A2” state of the potential sweep, by a peak
at 168.53eV (figure 3-10).
3.3.2 Effect of the Oxygen Reduction Reaction
The effect of oxygen removal by use of Ar purge was investigated in a set
of experiments presented in figure 3-11. In these experiments, an aerated
solution was exposed to a flow of Ar from the top of the electrochemical cell, and
CVs were recorded at subsequent time intervals of 30 minutes. By a change in
the oxygen partial pressure gradient above the solution, solvated oxygen
molecules gradually diffused away from it. The observed amount of mass formed
on top of the electrode at the end of each scan decreased with each successive
scan, that is, with the increased time of exposure to Ar gas.
The initial mass accumulation of 6ug, at “A2”, as observed from the in-situ
EQCM experiments, gradually reduces to less than 0.5µg. This is an indication of
the correlation of the peak “C1” with “A2” and it also indicates that the presence
of O2 can be the key component in the formation of the precipitate.
Indeed, through the effect of oxygen reduction, with an onset present at -
0.1 V, is present at “C1” potential (figure 3-12), has been known to have a
definite effect on the pH of the solution.
82
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2-100
0100200300400500600700 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
-800
-400
0
400
800
1200
1600
2000
mas
s ch
ange
(ng)
V vs MSE
C2 C1
A2
A1
Cur
rent
Den
sity
(µA/
cm2 )
1st. CV 2nd. CV 3rd. CV 4th. CV
Figure 3-11 Cyclic voltammetry and EQCM corresponding to the CVs on Pt
electrode in 2 mM CuSO4 +0.1M K2SO4 pH 5 solutions with Ar purging.
83
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-50
-40
-30
-20
-10
0
10
N2 Purge Air Exposed O2 Purge
Cur
rent
Den
sity
(µA/
cm2 )
V vs. MSE
Figure 3-12 Background of the Pt electrode in 0.1M K2SO4 pH=5 solution, with
different gas purging.
Oxygen reduction is a cathodic reaction fostering OH- production [4, 41]:
−− →++ OHeOHO g 442 2)(2 (3)
In acidic solutions, reaction (2) is the thermodynamically favored reaction.
In order to attain formation of Cu2O [8, 42]:
OHOCuOHeCu 222 222 +→++ −+ (4)
84
an increase in the pH values to values higher than 5 should be present. There
has been different studies [7, 43-46] indicating that the pH value of a solution can
increase, from neutral to a value of 9 to 12, upon the effect of this reaction. Thus,
with the presence of the oxygen reduction reaction (ORR), an increase in the
local pH value, in a region distancing 25-30 µm from the surface of the electrode
can be expected, to values close to pH=13, as reported in the literature [8, 47].
In the presented experiment, ORR gradually decreased as the dissolved
O2 was removed from the solution. Eventually, the trace amounts of oxygen were
not enough to cause an increase in the pH, thus preventing the electrochemical
processes at “C1” and “A2” from occurring. The effect of pH increase by the ORR
could be tested if the OH- ion could be maintained constant by the use of a
solution with a constant pH value.
Figure 3-13 shows the CV in a buffered 5mM CuSO4+0.1MK2SO4 solution,
adjusted at pH=5 by using acetic acid and sodium hydroxide. As expected, the
change in the local pH is not enough for the formation of diffusion limited peaks
“C1” or “A2”. Only current maximums “C2” and “A1” are present. Furthermore, no
mass is found to precipitate in the surface of the electrode.
85
-0.8 -0.6 -0.4 -0.2 0.0 0.2
0100200300400500
-0.8 -0.6 -0.4 -0.2 0.0 0.2-1000-500
0500
1000150020002500
∆ M
ass
(ng)
V vs MSE
Cur
rent
Den
sity
(µA/
cm2 )
5mM CuSO4 + 0.1 M K2SO4Buffered Solution
Figure 3-13 CV of a Pt electrode in a 5 mM CuSO4+0.1 K2SO4 pH=5 buffer
solution.
XPS data (Cu 2p region) obtained from by holding the Ru electrode at
C1(-0.44V) in 2 mM CuSO4 +0.1M K2SO4 pH 5 solution with different gas purging
confirmed an increased presence of Cu2O in oxygen purged solutions as
compared to non-purged solutions. The sample obtained during N2 flow
produced the least amount of oxide. In acidic solutions, the concentration
gradient from the bulk solution to the surface of the electrode causes a
neutralization of the same, preventing Cu2O to form. This is why the formation of
this oxide is seldom reported [37] in acidic solutions. At pH=5 solutions, the
86
formation of Cu2O is promoted by the local increase in pH [45] caused by the
ORR. This reaction can also be accounted to the previous observation of the
cathodic versus anodic charge rate, being greater than the unity.
3.3.3. Mechanism of Formation
At this point, firm proposals of the electrochemical mechanism can be
asserted. At the potential point “C1”, formation of Cu2O occurs due to the
availability of Cu2+ species in solution, through equation 4, coupled with an
increase in pH through the ORR (equation 3).
Although it has been suggested that Cu(0) can be formed at “C2” by the
reduction of cuprite (Cu2O) or tenorite (CuO) by different authors [48, 49], this is
not likely happening in our case, as an increase in the “C1” (copper oxide
formation) should be accompanied by an increase in the peak at “C2”. In our
case, a broader and bigger C1 peak virtually obscures the presence of the peak
at “C2” in 2 mM Cu2+ solutions, and diminishes the peak height at higher
concentrations; therefore the reduction of Cu from copper ions available in
solution (equation 2) is the most likely process occurring at this stage.
One possible cause for the discrepancy from the results here obtained is
that previous studies focus on a copper electrode in basic solutions viz. Cu
plating on top of a platinum or ruthenium surface, therefore surface changes in a
foreign substrate (Pt/Ru) are not taken into account.
Upon sweep reversal, in the reverse anodic scan peaks, “A1” and “A2” are
observed (figure 3.4). Gewirth and his group [49] have studied the oxide
87
formation in basic solutions. With the aide of a silicon nitride tip, force microscopy
studies suggested the physisorption of hydroxide species in potentials before the
peak “A1”. Their work established that at “A1” the formation of insoluble Cu2O
(via Cu oxidation) in competition with soluble Cu(I) and Cu(II) species (Cu(I)2O22-
and CuO22- (aq)) is reported:
−− ++→+ eOHOCuOHCu 220
21
21 (5)
This implies that a mass increase may be anticipated in the potential
region corresponding to peak A1, due to the change in the formula weight of the
deposited species, Nonetheless, a mass loss, as read by the EQCM is present in
our experimental conditions. Table 3-1 shows the EQCM results obtained from
potential cycling at different Cu2+ ions concentration. To obtain the mass
efficiency, the total charge, measured as the area under each corresponding
peak, was used to obtain the theoretical number of moles for a two electron
process, from the anodic dissolution of copper, was assumed:
−+ +→ eCuCu 220 (6)
Thus:
Mt - Theoretical Mass, as obtained from the number of moles, from Faraday’s
Law.
Ma – Actual mass read from the frequency change in the EQCM.
Ef - The efficiency of mass gain or loss: (Ma/Mt)* 100%
88
Table 3-1 Mass change efficiencies at anodic peaks “A1” and “A2”.
A1 A2
Mt/µg Ma/µg Ef/% Mt/µg Ma/µg Ef/%
2mM CuSO4 0.0308 0.0301 97.7 0.70 1.38 197
5mM CuSO4 0.713 0.582 81.5 0.49 1.56 320
10mM CuSO4 1.68 1.31 78.2 0.49 1.56 320
The value of dissociated mass and the related charge transfer are
comparable for the case of 2 mM CuSO4 solution. So the anodic peak current
“A1” is assigned to the oxidation of Cu species to Cu2+ ions. The difference of the
mass read versus the theoretical mass change is assigned to the presence of
insoluble species, like as reported in literature, and is due to the difference in pH
of our study. We suggested the difference between literature and our study was
caused by pH difference of the solution. It can also be noted (table 3-1) that there
is a marked decrease in the efficiency of this process, with increasing
concentration. It has been reported that Cu oxidation to Cu2+ ions follows a two
step process [50]:
−+ +→ eCuCu 10 (7)
−++ +→ eCuCu 21 (8)
At lower concentration (2 mM) Cu ions can easily diffuse away from the
electrode surface. As this concentration is increased, the amount of ions will be
increased proportionally, as related to the amount of Cu deposited in peak “C1”.
This will effectively limit the speed at which the oxidized ions diffuse away from
89
the surface, and the production Cu oxidation will be the Cu+ ion, favored
thermodynamically [50].
The Anodic peak “A2” is associated with the oxidation of Cu ions and
Cu2O to divalent species, Cu(OH)2 being the most probable one, as agreed in
curret literature [14].
−++ ++→+ eOHCuHOCu 222 22
2 (9)
22 )(2 OHCuOHCu →+ −+ (10)
The source of the Cu2O particles comes from the previously discussed
reaction occurring at “C1”. Whenever there’s a high concentration of Cu2O
species available on the surface, anodic peak A2 will be present, and it will
increase proportionally with “C1” peak area (figure 3-4).
The need for Cu2O species for the formation of peak “A2” was confirmed
by the experiment shown in figure 3-14. Cu2O pre-loading, confirmed by optical
microscopy, produced by holding the Pt electrode’s surface at C1 (-0.4V) for
three minutes in a 10 mM [Cu2+] ph=5 solution, was oxidized slowly by cycling
through the “A2” potential region, without passing through the A1 peak potential,
in a 0.1 M K2SO4 solution. An observable peak with a potential located at the
potential corresponding to peak’s “A2” location (-0.18 V) is observed, triggering a
mass gain process through the rest of the cycling. As observed from XRD
characterization, posnjakite is formed on top of the electrode at the end of the
potential cycling (table 3.1).
90
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2-60.0-40.0-20.0
0.020.040.060.080.0
100.0
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2
0200400600800
10001200
Cur
rent
Den
sity
(µA/
cm2 )
∆ M
ass
(ng)
V vs. MSE
Figure 3-14 Cycling of a Cu2O loaded platinum surface in the “A2” potential
region in 0.1M K2SO4 solution.
The mass accumulation via EQCM sensing at different copper ion
concentrations confirms it. Therefore, it is proposed that at “A2”, precipitation of
posnjakite resulted from an increase in pH of the solution that can be assigned to
the anodic oxidation of cuprite, (equation 9), and the cathodic reduction of
oxygen [51].
91
Availability of sulfate and water molecules in the vicinity of the electrode
surface [52] will lead to the following chemical reaction:
OHOHSOCuOHSOCuOHCu 264422
42
2 )()(3 •→+++ −+ (11)
The conditions present in the electrode surface had reproduced the
mechanism occurring in nature [4, 51]; upon formation of cuprite in the
atmosphere, the product of its oxidation Cu(OH)2, will react in the presence of
trace amounts of sulfates in the available water (rain, moisture). So in our
potential sweep experiments, the following consecutive conditions were met that
led to the formation of artificial patinas: first, cuprite was formed at C1(-0.4V) in
cathodic range (equation 4). Then, Cu2+ ion species were made available
through the deposition process present at “A1” potential was formed when the
scanning through peak “A2” (equation 6). Oxygen reduction reaction, occurring
on the electrode’s surface causing an increase in the available OH- ions leads to
the formation of copper hydroxide Cu(OH)2, on top of the Cu2O layer [53,48],
(equation 10). The precipitate observed at the end of the cycle consisted of
posnjakite formed by the chemical reaction between Cu(OH)2 and Cu2+/SO42-
(equation 11) [54].
The theoretical mass change obtained in table 3.1 is the mass gain of
posnjakite based on the equation 11, the charge calculation using equation 9. In
three reported concentrations of copper (2 mM, 5 mM and 10 mM Cu2+), the
efficiency of posnjakite formation is higher than 100%. This can by explained by
the fact that in the regular CV, the copious amount of copper ions available,
92
obtained by scanning through peak “A1”, and from the diffusion of copper ions
present in the solution, that will be readily available to favor the thermodynamic
formation of posnjakite, thus causing an efficiency higher than the theoretical.
The fact that the formation of posnjakite does not increase proportionally
with the increase in Cu ions concentrations can be expected, since the SO4- ions
available in solution from the potassium, sulfate electrolyte K2SO4, and required
for the formation of the basic copper sulfate, was kept at a constant 0.1M, pH=5
adjusted. The only increase in sulfate concentration came from the sulfate ions
associated with the copper sulfate pentahydrate salt. An important feature is that
the formation of posnjakite occurs only after peak potential A2 has been reached.
This is because Cu2O oxidation will contribute to the increase in pH of the area
(equation 9).
The importance of Cu2O formation can be depicted by the experiment
presented in figure 3.15. After holding for 3 minutes at “C1” potential, in a 5 mM
solution, after which it was evacuated from the cell. Electrolyte containing
solution 0.1 M K2SO4 was used to cover the surface of the electrode and
potentially stepped form the negative open circuit potential value of -0.34 V vs.
MSE, to the “A2” potential (-0.18 V vs. MSE) through the use of the chrono-
amperometry technique (CA), in the electrolyte solution. The results of this
experiment are in agreement with the proposed mechanism.
93
0 10 20 30 40 50 60
0700
1400210028003500
0 10 20 30 40 50 60
0
700
1400
2100
2800
3500Cur
rent
Den
sity
(µA/
cm2 )
∆ M
ass
(ng)
V vs. MSE
Figure 3-15 Potential stepping of a cuprite loaded Pt surface to A2 potential in
K2SO4 electrolyte.
3.3.4. Mass Dumping.
In mass change of EQCM data shown as figure 3-15, a sudden mass loss,
or “dumping” was observed at –0.42V in cathodic region starting from 2nd cycle.
This behavior is also seen in the scans performed in higher copper concentration
solutions. A small change in the pH can cause a large effect in the change on the
Cu concentration; Fitzgerald [4] has concluded that this change in local pH,
94
accompanied by a change in sulfate ions concentration near the electrode
surface can cause brochantite to reduce to tenorite. Therefore, two possibilities
can led to the desorption; either a sudden dissociation of posnjakite, caused by
the departure of surface species present in posnjakite (SO42-, Cu2+, OH-) in the
observed potentials, or due to the reduction of posnjakite, which although it has
been reported [53, 55], it would imply the reduction of copper oxide species [56,
57], which its formation is known to be highly irreversible [35].
3.4 Conclusion
In this paper, the study of copper electrodeposition on platinum in solutions
adjusted to different pH values with 0.1M K2SO4 as supporting electrolyte is
reported. In pH=5 solutions, the rise in local pH caused by the reduction of
oxygen leads to the formation of posnjakite or basic copper sulfate. This behavior
is also observed in solutions with higher copper concentration. The mechanism is
understood with characterization made through the use of XPS, XRD and EQCM
at different stages of the electrode reaction, and parallel experiments using Ru
electrodes. The scope of the in-situ EQCM is proven as a powerful tool to explain
this mechanism.
The effect of local pH increase caused by cuprous oxide oxidation and
oxygen reduction is presented. It is also noted that the rate of cathodic to anodic
rate is always grater than the unity. This emphasizes the role of the ORR
reaction in the proposed mechanism. Since some of the OH- ions produced
during the scan are neutralized back to water molecules and maintain the original
95
value of pH at the end of the scan, posnjakite is not observed in solutions with
basicity greater than pH=5.
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