APPROVED: Oliver M. R. Chyan, Major Professor Michael G. Richmond, Co-Major Professor William E. Acree, Jr., Chair of the Department of Chemistry James D. Meernik, Acting Dean of the
Toulouse Graduate School
ELECTROCHEMICAL DEPOSTION OF BISMUTH ON RUTHENIUM AND
RUTHENIUM OXIDE SURFACES
Daniel M. Taylor, B.S.
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
May 2012
Taylor, Daniel M. Electrochemical Depostion of Bismuth on Ruthenium and
Ruthenium Oxide Surfaces. Master of Science (Chemistry – Analytical Chemistry), May
2012, 56 pp., 3 tables, 22 illustrations, reference list, 42 titles.
Cyclic voltammetry experiments were performed to compare the
electrodeposition characteristics of bismuth on ruthenium. Two types of electrodes were
used for comparison: a Ru shot electrode (polycrystalline) and a thin film of radio-
frequency sputtered Ru on a Ti/Si(100) support. Experiments were performed in 1mM
Bi(NO3)3/0.5M H2SO4 with switching potentials between -0.25 and 0.55V (vs. KCl sat.
Ag/AgCl) and a 20mV/s scan rate.
Grazing incidence x-ray diffraction (GIXRD) determined the freshly prepared thin
film electrode was hexagonally close-packed. After thermally oxidizing at 600°C for 20
minutes, the thin film adopts the tetragonal structure consistent with RuO2. A hydrated
oxide film (RuOx∙(H2O)y) was made by holding 1.3V on the surface of the film in H2SO4
for 60 seconds and was determined to be amorphous. Underpotential deposition of Bi
was observed on the metallic surfaces and the electrochemically oxidized surface; it
was not observed on the thermal oxide.
ii
Copyright 2012
by
Daniel M. Taylor
iii
ACKNOWLEDGMENTS
I would like to thank my advisor, Dr. Oliver Chyan, whose encouragement and
guidance helped me produce this work. I am very thankful for the unique opportunity to
work and grow professionally under his advisement. A great debt of gratitude is owed to
my colleagues Kyle Yu, Karthik Pillai, Fan Yang and Po-Fu Lin who offered their support
daily inside and out of the lab.
I would like to thank Dr. Richmond and many others within the UNT Chemistry
Department, faculty and staff, for overseeing my continued success in the program and
fostering an excellent learning environment. I would also like to thank those who
generously provided me with the financial support for my work, namely the Welch
Foundation, UNT Faculty Research Fund, and Semiconductor Research Corporation.
There are no words to express how grateful I am to have the unconditional
support of my parents, family and friends whom I have been able to count on for far
longer than I can remember. Thank you all.
iv
TABLE OF CONTENTS
ACKNOWLEDGMENTS .................................................................................................. iii
LIST OF TABLES ............................................................................................................ vi
LIST OF FIGURES ......................................................................................................... vii
CHAPTER 1: INTRODUCTION ....................................................................................... 1
1.1 Basic Electrochemistry .......................................................................................... 1
1.2 Electrode/Solution Interfacial Chemistry ................................................................ 3
1.2.1 Electrochemical Deposition ............................................................................. 5
1.3 Analytical Techniques in Electrochemistry ............................................................. 6
1.3.1 Three Electrode Configuration ........................................................................ 6
1.3.2 Open Circuit Potential ..................................................................................... 7
1.3.3 Cyclic Voltammetry ......................................................................................... 8
1.3.5 Calculating the Surface Coverage ................................................................. 10
1.4 X-Ray Diffraction Theory ..................................................................................... 12
1.5 Preparative Techniques ....................................................................................... 16
1.5.1 Ruthenium Electrode Preparation ................................................................. 16
1.5.2 Physical Cleaning Techniques ...................................................................... 16
1.5.3 Electrochemical Cleaning Techniques .......................................................... 18
1.6 References .......................................................................................................... 18
CHAPTER 2: BISMUTH DEPOSITION ON RUTHENIUM AND RUTHENIUM OXIDE
ELECTRODES ................................................................................................... 20
2.1 Introduction .......................................................................................................... 20
2.2 Materials and Experiment Design ........................................................................ 21
v
2.3 Results and Discussion ....................................................................................... 23
2.3.1 Bismuth Deposition on Electrochemically Cleaned Ruthenium ..................... 23
2.3.2 Effects of Progressive Oxidation of Ruthenium on Bismuth Deposition ........ 26
2.4 Conclusions ......................................................................................................... 34
2.5 References .......................................................................................................... 35
CHAPTER 3 GRAZING INCIDENCE X-RAY DIFFRACTION INVESTIGATIONS OF
ELECTROCHEMICAL AND THERMAL OXIDES FORMED ON RUTHENIUM
THIN FILMS AND THE ELECTROCHEMICAL NATURE OF RUTHENIUM
THERMAL OXIDE IN ACIDIC BISMUTH SOLUTIONS ...................................... 37
3.1 Introduction .......................................................................................................... 37
3.3 Experimental ........................................................................................................ 39
3.3 Results and Discussion ....................................................................................... 42
3.3.1.GIXRD Studies of Ruthenium and Ruthenium Oxide Thin Films ................... 42
3.3.2 Electrodeposition of Bismuth on RuO2 Thin Film Electrodes ......................... 46
3.4 Conclusions ......................................................................................................... 49
3.5 References .......................................................................................................... 51
REFERENCE LIST........................................................................................................ 53
vi
LIST OF TABLES
Page
Table 2.1: Comparison of ΔEp Values ........................................................................... 26
Table 3.1: Comparison of XRD Peak Locations (2θ) and Miller Indices (hkl) ................ 45
Table 3.2: Comparison of Peak Locations and Current Densities (ρQ) of Ruthenium and Ruthenium Oxide Thin Films ......................................................................................... 49
vii
LIST OF FIGURES
Page
Figure 1.1: An electrochemical cell. This setup contains (a) a voltmeter, (b) a salt bridge, and the left and right half-cells, (c) and (d). Note: in practice it is not always necessary to physically separate the half-cells. ................................................................................ 2
Figure 1.2: The electrochemical double layer; where (a) is the bulk metal of an electrode, (b) is the interfacial surface of the electrode, (c) is inner Helmholtz plane and (d) is the outer Helmholtz plane. At (c) the particle is “specifically adsorbed” on (b) and at (d) the ion is still fully solvated.9 .................................................................................. 4
Figure 1.3: Basic three-electrode configuration (left) and experimental setup (right). Includes (a) the potentiostat (controller), a working electrode (b), a reference electrode (c), the counter electrode (d), and finally computer readout system (e). ......................... 7
Figure 1.4: Example of a triangular potential waveform for cyclic voltammetry. .............. 8
Figure 1.5: Cyclic Voltammogram for Fe(CN)63-/ Fe(CN)6
4- Redox Couple.14 .................. 9
Figure 1.6: Close-packed adatoms on an electrode surface. ........................................ 11
Figure 2.1: Images at 5X magnification of a Ru shot electrode (left) and a Ru thin film electrode (right). Below are their respective surfaces after oxidizing at 1.3V for 30sec in 0.5M H2SO4. The geometric areas are 0.368cm2 (shot) and 0.229cm2 (thin film). ........ 21
Figure 2.2: Background scans of Ru electrodes in 0.5M H2SO4 and 20mV/s scan rate. Shot electrode (left) and thin film electrode (right). ........................................................ 23
Figure 2.3: CV voltammograms of 1st, 2nd, and 20th(final) cycles of Ru shot (left) and Ru thin film (right) electrodes in 1mM Bi3+ / 0.5M H2SO4. Scan rate = 20mV/s. .................. 24
Figure 2.4: Charge density comparisons of peaks over course of CV experiment (by segment number). Top graphs (left and right) are for the shot electrode and the bottom (left and right) are for the thin film electrode. ................................................................. 25
Figure 2.5: CV overlay of first scans of Ru shot electrode in 1mM Bi/0.5M H2SO4. Inset (top right) is an expanded view of the peaks in the UPD region. ................................... 28
Figure 2.6: Comparison of anodic charge density versus holding potential for the shot electrode (top) and thin film electrode (bottom). The first (left) and final (right) CV cycles are shown. The line shown in red is the average charge density value excluding the point at 1300mV. ........................................................................................................... 29
Figure 2.7: Overlays of successive CV cycles of the RuOx(H2O)y in 1mM Bi/0.5M H2SO4. The shot electrode is shown at the top and the thin film CVs are below. .......... 30
viii
Figure 2.8: Overlaid CV cycles of anodic UPD current density vs. potential and charge density vs. CV segment number. Data for the shot electrode is at the top and thin film data is below. ................................................................................................................ 31
Figure 2.9: Calculated surface coverage from charge density data. Coverage vs. potential for the shot electrode (top left) and wafer (top right) are shown. Coverage vs. segment number for the oxidized electrodes, shot (bottom left) and wafer (bottom right), are shown as well. ......................................................................................................... 32
Figure 3.1: Denton Vacuum Desktop Pro® confocal DC/RF sputtering instrument (left) for making metal thin films. The inside of the chamber is shown to the right. ................ 38
Figure 3.2: A Lindberg/Blue tube furnace. ..................................................................... 40
Figure 3.3: Oxidized thin film electrodes. Thermal oxide thin film electrodes (top left) and 5X magnified images of open configuration (orange, top right) and closed configuration (purple, bottom left). Bottom left image is a 5X magnification of the electrochemically oxidized film (Chapter 2). .............................................................................................. 41
Figure 3.4: Bragg-Brentano geometry ........................................................................... 42
Figure 3.5: X-ray diffraction spectrum of a pure Ru thin film (95nm) on Ti/Si(100). ....... 43
Figure 3.6: XRD spectra for RuO2 open configuration (top), RuO2 closed configuration and RuOx∙(H2O)y (bottom). ........................................................................................... 44
Figure 3.7: Comparison of CV scans of RuO2 in Bi free 0.5M H2SO4 and in 1mM Bi/0.5M H2SO4. The top left/right are CV scans of the RuO2 formed at 600C in the “open configuration” and middle left/right are for the “closed configuration”. The CV scans at the bottom are for the electrochemical oxide, RuOx(H2O)y. ........................................... 48
1
CHAPTER 1
INTRODUCTION
The first chapter of this work is intended to introduce the reader to the
fundamentals of the advanced techniques used to gather the data necessary for
developing this thesis. My research concerns electrochemical depositions of bismuth on
ruthenium metal and ruthenium oxide electrode surfaces performed in acidic media.
Therefore, a brief discussion of electrochemistry, an overview of electrode/solution
interface chemistry, and an introduction to the instrumentation used to interpret the
nature of electrode surfaces and deposited materials will be given. Relevant mathematic
treatments of data will also be discussed, e.g. calculating monolayer coverage values
using current (i) vs. voltage (E) curves from cyclic voltammetry experiments. The final
section of the introductory chapter covers the preparative techniques used to clean
electrodes for experimentation.
Experimental data and conclusions regarding the deposition of bismuth on
metallic ruthenium/ruthenium oxides, and a discussion of unfinished works and potential
future projects are found in chapters 2 and 3, respectively.
1.1 Basic Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions as they
relate to the flow of electrons in a system. These types of chemical reactions are known
as oxidation-reduction (or redox) reactions, where electrons flow from reductant to
oxidant. The two species must be in some form of contact for a charge-transfer process
(a reaction) to occur. A basic setup would look similar to the diagram above (Figure
1.1), which shows two metallic conductors in their respective half-cells. Connecting the
2
half-cells through a network of charge carrying electrolyte solutions creates a circuit.
When a voltmeter is placed between the two half-cells, one can determine the available
potential energy, E (in Volts, where 1V = 1 Joule/C), in the net electrochemical reaction
that can be used to drive electrons through the circuit. The change in Gibbs free energy
for an electrochemical reaction is related to the electrochemical potential by the
equation:
∆G = -nFE (1.1)
where n is the number of transferred electrons, F is 9.649 X 104 C/mole e- (Faraday
constant), and E is net electrochemical cell potential. It follows that a spontaneous
reaction is one that has a negative value for the change in Gibbs energy.
Electrochemical cells of this type are known as Galvanic cells, or batteries; whereas a
cell that requires energy input to proceed is called an electrolytic cell. As either type of
reaction proceeds, a measure of the number of transferred electrons (also charge in
coulombs, C, as the unit charge 1.602 X 10-19 C) through the circuit can be made in
Figure 1.1: An electrochemical cell. This setup contains (a) a voltmeter, (b) a salt bridge, and the left and right half-cells, (c) and (d). Note: in practice it is not always necessary to physically separate the half-cells.
3
units of current, or Amperes, where 1 Ampere (A) = 1 C/s. Integrating the curve of
current vs. time will yield charge. This can then be used to calculate the extent of the
reaction over time.
A net electrochemical cell reaction is described by the half-cell reactions that
illustrate the chemistry occurring at each electrolyte/conductor interface; chemistry
occurring at the oxidation reaction site, or anode, and the reduction reaction site, or
cathode. When these two reactions are combined and balanced they will then illustrate
the full chemical reaction that occurs in the system. From the Nernst equation, derived
from Le Châtelier’s principle of chemical reactions, the potential energy of each half-cell
is:
E = E°- RTnF
ln(Q) (1.2)
where E° is the standard reduction potential, R is gas constant, T is temperature
(Kelvin), n is number of transferred electrons, F is Faraday constant, and Q is the
reaction quotient (which is dependent on the activities of the reacting species). The
Nernst equation can then be applied to the complete theoretical cell by the equation:
E = E+ - E- (1.3)
where E is the whole cell potential, E+ is the half-cell potential for the cathode and E- is
the half-cell potential of the anode.
1.2 Electrode/Solution Interfacial Chemistry
The ability of an electrode to transfer electrons into or out of the system depends
on several factors including the material’s work function and the surface potential. It is
also a function of the structure of the material both at the surface and within the bulk.1,13
4
There are a number of studies that probe the differences between single crystals and
polycrystalline structures of electrode materials.7 Aside from the processes within an
electrode, the chemistry of interest (e.g. half-cell reactions) often takes place at or near
the interface between the electrode surface and an ionic conductor. The latter is
typically an electrolyte solution that contains the analyte. Hence, some authors refer to
the discipline as interfacial electrochemistry because the theories of both
electrochemistry and surface science are necessary in order to properly model an
electrochemical system.1,13
There are several theoretical models used to describe the interfacial region, a
very simplistic version is as follows. For a metallic conductor (electrode): excess charge
builds up at the surface, a positive charge if the potential with respect to a reference
electrode is driven toward positive voltages, and likewise a negative charge if the
potential is driven towards negative potentials. In this way, an electrochemical interface
is often described as a capacitor since the interfacial potential difference between the
electrode and the solution creates an electric double layer similar to the opposing
Figure 1.2: The electrochemical double layer; where (a) is the bulk metal of an electrode, (b) is the interfacial surface of the electrode, (c) is inner Helmholtz plane and (d) is the outer Helmholtz plane. At (c) the particle is “specifically adsorbed” on (b) and at (d) the ion is still fully solvated.9
5
charges that develop on the two opposing plates of a capacitor (Figure 1.2). The Gouy-
Chapman-Stern model of the interfacial region adds some complexity by putting forth
that the solution-based charges form a gradient across the region. This gradient of
charges forms due to the nature of the forces at work within the region (electrostatic,
etc), which act over short distances. The width of the interfacial region may be only
hundreds of angstroms wide and the build-up of charges decays exponentially with
distance, as there are other thermodynamics that act against the aforementioned
forces.1,13 A mathematical treatment of the GCS model and other models with respect to
the energies and forces in the interfacial region is beyond the scope of this thesis.
1.2.1 Electrochemical Deposition
An electrochemical deposition is one that electrochemists designate as an inner-
sphere electrode reaction, meaning a reacting species must make contact with the
electrode surface at some point during the overall reaction. In the case of depositions of
multivalent ions, a series of charge transfers may occur, possibly at a distance (outer-
sphere charge transfer) before the ion, in a different oxidation state, is deposited on the
surface and fully discharged. In general, for a deposition to occur, the surface/adsorbate
interaction should be energetically favorable and stronger than the forces that may try to
keep the adsorbate solvated rather than deposited.1,9,13 In the case of a reversible
electrochemical deposition, the net reaction is:
M(solv)n+ + ne- M(bulk) (1.4)
and the Nernstian potential of deposition is:
E = E° - 0.5916n
ln 1Mn+
(1.5)
6
There is a special case in which the free energy of adsorption (eqn. 1.1) is lower
than that which is predicted by the Nernst equation. This known as underpotential
deposition (UPD), as the species deposits at a potential more positive than the
Nernstian potential. The phenomenon has been well reported in the literature for a
variety of metal electrode surfaces and deposited materials. Clavilier et al.2,3 have even
reported spontaneous and irreversible depositions may even occur on contact with an
analyte-containing solution, but in the absence of applied potential or current.
1.3 Analytical Techniques in Electrochemistry
1.3.1 Three Electrode Configuration
Up to this point the discussion of an electrochemical cell has only included two
electrodes, a working electrode (metal conductor) and a reference electrode (e.g. SHE),
working in concert to transfer charge in and out of the system. However, since a
reference electrode is intended to remain a static measuring point, the reaction in this
half-cell must remain relatively unchanged. In order to accomplish this, reference
electrodes are generally manufactured as self-contained systems and a high resistance
is set between the reference electrode and the working electrode so virtually no current
flows between them. A new counter electrode is employed to act as the counterbalance
for the working electrode. This counter electrode is now responsible for taking up
charge or releasing charge as needed while the potential is controlled and varied
between the working electrode and the reference electrode. A good counter electrode
has a large surface area, is a good conductor and is chemically stable. For these
7
reasons, platinum foil or coiled platinum wire are commonly used in experiments. Below
is a basic three-electrode setup for electrochemical experimentation (Figure 1.3).
1.3.2 Open Circuit Potential
The open circuit potential (OCP) is the potential difference between the working
electrode and the reference electrode when no appreciable current is allowed to flow
through the system; thus no chemistry is occurring. This measurement, while seemingly
innocuous, can give some significant revelations as to the nature of the surface of the
electrode. For example, when the OCP is taken over time, changes in the potential can
signal spontaneous/gradual adsorption of foreign materials, surface
oxidation/passivation, and even physical breakdown/dissolution of the electrode.1,13
OCP is also a useful reference point for determining proper functioning and cleanliness
of the working surface. Kolb et al.5,9,10,11 have also pointed out that the potential of zero
change (aka OCP) is closely related to the work function of the electrode. Often the
Figure 1.3: Basic three-electrode configuration (left) and experimental setup (right). Includes (a) the potentiostat (controller), a working electrode (b), a reference electrode (c), the counter electrode (d), and finally computer readout system (e).
8
OCP can be a useful starting potential value in cyclic voltammetry experiments.
1.3.3 Cyclic Voltammetry
Voltammetry is one of many electrochemical techniques that are based on
monitoring current as the applied potential is varied in a controlled fashion. In cyclic
voltammetry, the controller/potentiostat is set to vary the potential of the working
electrode surface relative to the reference electrode by employing triangle potential
waveform (Figure 1.4). In this way, the potential at the working electrode is cycled, or
scanned, between the upper and lower limits of a potential window, which are called the
switching potentials. The slope of the waveform is the scan rate, how quickly the
potentiostat cycles between the switching potentials. As mentioned earlier, the current
(either cathodic or anodic) is recorded at the counter electrode.6
There are several important features of a cyclic voltammogram (a plot of current
versus potential) that provide chemists with insights into the nature of the
electrochemical system being studied. An example voltammogram is shown for the
Figure 1.4: Example of a triangular potential waveform for cyclic voltammetry.
9
Fe(CN)63-/Fe(CN)6
4- redox couple (Figure 1.5). As the potential is brought from 0.6 V
towards negative potentials (the forward scan), a cathodic current develops and comes
to a peak at the Nernstian potential as Fe(III) is reduced to Fe(II) at the surface of the
working electrode. As the potential continues negative, the magnitude of the current
peaks and drops as the reaction begins reaching the diffusion limit. This means the
concentration of reactant near the surface is decreasing and the reaction is governed by
the amount of reactant that is able to diffuse into the double layer from the bulk solution.
A similar description can be applied to the reverse scan as the potential is switched to
progress towards positive potentials. For a reaction to be considered reversible the rate
of mass transfer (diffusion of ions toward the surface replacing reacted material) must
be close to equilibrium and the rate of charge transfer must be on this same time scale.
Therefore the peak currents for the anodic and cathodic processes are equal in
magnitude. In the case of a reversible reaction the peak current (Ip) of either the anodic
or cathodic process is governed by:
Ip = 2.69 x 105n3 2⁄ ACD1 2⁄ v1 2⁄ (1.6)
Figure 1.5: Cyclic Voltammogram for Fe(CN)63-/ Fe(CN)6
4- Redox Couple.14
10
where 2.69 x 105 is a collection of constants and has units in C mol-1 V-1/2, n is the
number of electrons transferred, A is the area of the electrode, C is the concentration of
the species, D is the diffusion coefficient of the species and v is the scan rate in V/s.
The potential value of a reversible process is given by:
∆Ep = Epa- Epc= 2.22RTnF
= 0.057Vn
(1.7)
where Epa is the anodic peak potential, Epc is the cathodic peak potential and n is
number of electrons transferred during the reaction.1,6,13
1.3.5 Calculating the Surface Coverage
By taking the integral of the peaks in a CV plot, transferred charge (in C) can be
extracted and then used to calculate the relative amount of reacted material(s), or in the
case of a deposition, the amount of material(s) adsorbed to the working electrode
surface. The fractional surface coverage is denoted by the variable theta (θ) which is
often described in terms of monolayer (ML) coverage. It was assumed that the adatoms
on the working electrode surface adopt close packing, unless otherwise stated (Figure
1.6). However, this is not always the case; less dense packing arrangements are often
adopted when the radius of the deposit atom is larger than that of the surface atoms in
the electrode.1,12,13 A surface coverage calculation procedure is as follows. For close
packing:
Area of Unit Cell (cm2):
Area (parallelogram, A) = b x h (1.8)
where b and h may be defined as:
11
b = 2 x r (1.9a)
h = √3 x r (1.9b)
and, substituting into (1.5), the final result becomes:
A = 2√3 x r2 (1.10)
One can see from the Figure 1.6 that each unit contains one adatom along with
some dead volume between atoms due to the packing arrangement. We must now
solve for the number of adatom units that will fit within the exposed, working surface
area of the electrode, A′. The solution requires using the relationship between the
charge transferred (extracted from CV data) under during an anodic or cathodic event
and amount of reacted (deposited) material, which is given by the equation:
q = n x F (1.11)
where q is the charge in Coulombs, n is the number of moles of electrons transferred
given by the half-cell reaction(s) under investigation and F is Faraday’s constant, or
9.649 x 104 C/mole e-. For example, if the half-cell reaction being investigated in this
thesis is:
Figure 1.6: Close-packed adatoms on an electrode surface.
12
Bi3++ 3e- Bi(ad) (1.12)
where Bi(ad) is deposited Bi, then the exact number of units deposited (NBi) is:
q(C) x 1 mole e-
96490 C x 1 mole Bi(ad)
3 mole e- x 6.023 x 1023atoms1 mole Bi(ad)
= NBi (1.13)
Since the number of deposited units was calculated, the final step requires
multiplying the number of adatom units (NBi) by the area taken up by each unit
according to their packing arrangement (equation 1.7) to give the total area of the
deposited material. Lastly, dividing the total area of the deposited material by the total
area of the exposed working electrode surface (A′) gives the surface coverage ratio (θ).
It is important to note that the electrode surface is generally not a uniform surface,
especially in the case of polycrystalline materials; often the geometric area of the
working surface is larger than the real electroactive surface. In the literature, the active
underpotential deposition coverage.12
A more direct approach to determining the fractional coverage (θ) can be taken if
the charge needed to deposit one monolayer (qθ = 1) is known. If so:
θ = qp qθ = 1⁄ (1.14)
area of a Ru electrode can be determined accurately by determining the Cu where qp is
the charge under a peak in the CV plot.
1.4 X-Ray Diffraction Theory
X-ray crystallography, a technique used to probe the crystalline nature of atoms
in a substance, is foundationally based on the wave properties of light and the ability of
planes of atoms to scatter light.4,8 When a fixed wavelength of light strikes a plane of
atoms at some incident angle (θ), many scattering events will occur simultaneously. If
13
the planes of atoms have order, light will diffract coherently only at specific θ values
depending on the arrangement of the atoms in the plane that is reflecting the incident
beam of light (Figure 1.7). This is the basis of Bragg’s law which states that the
wavelength of light (λ) is a function of the interplanar spacing (d) and the sine of the
angle between the incident light and the plane of atoms (θ), dubbed the Bragg angle.
This is written mathematically as:
λ = 2d sin θ (1.15)
In the case of an amorphous substance (random arrangements of atoms), reflected light
is scattered at random, leading to more destructive interference than constructive, thus
no specific Bragg diffraction angles exist for these types of materials.
It was discovered by Bravais that there are only 14 possible lattices or
arrangements of the unit cells of atoms with an infinite number of unique planes that can
be drawn through these lattices of atoms. The lattice points on an x, y, z coordinate
system are rewritten in terms of a fractional coordinate system by taking the cell edges
to be a, b, and c, thus the x/a, y/b, and z/c are the fractional coordinates. When the
Figure 1.7: A wavelength of light (λ) reflecting off planes of atoms (hkl).
14
edge of a plane intersects any one of the axes, the value of the fraction goes to infinity.
In x-ray crystallography, a system of Miller indices (hkl) is used where h, k, and l are
now the reciprocals of the fractional coordinates x/a, y/b, and z/c, respectively. This
clears any fractions and make what was infinity, zero.
The Miller indices of the planes can be related to the interplanar spacing, d, by
geometric analysis. For example, in a hexagonal, close-packed lattice
1d2 = 4
3h2+ hk+k2
a2 + l2
c2 (1.16)
where a = distance between cell edges along the “a” axis and c = distance between cell
edges along the “c” axis. By solving Bragg’s law (equation 3.1) for “d” and substituting it
into equation 3.2 we arrive at the equation
sin2 θ = λ2
3a2 h2+hk+k2+ l2
(c a⁄ )2 (1.17)
If the wavelength of incident light is fixed at a value less than 2d (required by Bragg’s
law), then the above equation can be used to calculate the diffraction angles resulting
from any hkl reflection.
From the preceding explanation, it follows that discrete patterns of constructive
and destructive interference emerge when light is diffracted from an ordered lattice of
Figure 1.8: A simple tetragonal lattice (left) and a simple hexagonal lattice (right).4,8
15
atoms. These patterns are unique to each of the Bravais lattices and there are selection
rules that govern the intensities of the diffracted beams of light coming from hkl
reflections in a lattice. The selection rules are related to: the wave properties of the light,
the positions of the planes of atoms, and an atoms ability to scatter light, which is
related to the density of electrons in the atom. The equation that relates the intensity of
the diffracted light to the Miller indices is called the structure factor, F, which is given by
F = ∑ fne2πi(hun+ kvn + lwn)N1 (1.18)
where u, v and w = the fractional coordinates of the atoms in the unit cell, N = number of
atoms in a unit cell, and f = atomic scattering factor (a function of atomic electron
density). When the value of F is zero, the intensity of the beam of light is zero (from
destructive interference) and the hkl reflection is not seen in the diffraction pattern for
the lattice. In crystallography, these are known as forbidden reflections. For example, a
pure powder sample of metallic Ru is known to adopt the hexagonal close-packed
crystal structure (hcp) with unit cell lattice which contains two atoms in the unit cell, one
located at (0, 0, 0) and the other at (1/3, 2/3, 1/2). From the structure factor equation,
the forbidden reflections occur when both of the following rules are satisfied
simultaneously
l = odd (1.19)
h+2k = 3N (1.20)
For the purposes of this thesis, similar mathematic relationships can be
developed for the tetragonal lattice, also called the rutile structure, which is adopted by
ruthenium dioxide (RuO2). These equations are listed below.
Interplanar spacing in a tetragonal lattice
1d2 = h2+k2
a2 + l2
c2 (1.21)
16
Bragg’s law substituted d-spacing equation in a tetragonal lattice
sin2θ = λ2
4a2 h2+k2+ l2
(c a⁄ )2 (1.22)
Structure factor selection rules for tetragonal lattice
none, all reflections possible in primitive unit cells
Cell volume of hexagonal unit cell
V = √32
a2c (1.23)
Cell volume of tetragonal unit cell
V = a2c (1.24)
1.5 Preparative Techniques
1.5.1 Ruthenium Electrode Preparation
Ruthenium (Ru) electrodes constructed from polycrystalline metal shots (99.95%
pure, Electronic Space Products International, Inc., Ashland, OR, www.espi-
metals.com) that have been lapped and polished to a smooth, flat surface and encased
in epoxy resin were previously prepared by a colleague using a procedure detailed
elsewhere.14 A method for preparing Ru(~95nm)/Ti(~5nm)/Si (100) electrodes in our
laboratory is also described elsewhere.[14]
1.5.2 Physical Cleaning Techniques
To prepare the shot electrodes for use, they were first rinsed with 18.2MΩ, ultra-
pure water from a Millipore™ filtration system (Millipore, Inc., Billerica, MA,
www.millipore.com). After drying the electrode under a stream of dry nitrogen gas, the
surface was mechanically polished on a variable speed polisher fitted with a neoprene
17
microporous pad coated with a lubricant/extender (Red Lube Diamond Extender®,
Allied High Tech Products Inc., Rancho Dominguez, CA, www.alliedhightech.com) and
a diamond, polishing suspension (Allied High Tech Products, Inc., Rancho Dominguez,
CA, www.alliedhightech.com). Diamond suspensions with grit sizes of 6, 3, 1 and 0.5µm
were used in that order while polishing the metal to a smooth, mirror finish. Rinsing was
performed before changing between grit sizes and after the final mechanical polishing at
0.5µm. Under most circumstances, rinsing with purified water and lightly polishing with
0.5µm grit is adequate. Removing irreversible, electrochemical oxidation (Chapter II)
generally requires polishing with 3, 1, and 0.5µm grits and adjusting the speed of the
polisher to reveal a fresh metallic Ru surface. Naked-eye and microscope inspection
were performed to ensure a mirror polish was achieved.
A polished electrode was then placed in a beaker containing an 11:1 mixture of
ultra-pure water and a degreaser/cleaner solution (Branson GP Solution®, Branson
Ultrasonics Corporation, Danbury, CT http://www.bransonultrasonics.com/) and the
beaker was placed in a sonication bath. Sonication was performed for approximately
fifteen minutes or longer to ensure the electrode surface was free of diamond girt, other
fixed particles, and any organic residues. The sonicated electrode was finally rinsed
with generous amounts of ultra-pure water. The sonicated electrode was checked once
again under a microscope to ensure a clean surface was obtained. A similar treatment
was performed to clean the surface of the thin film electrodes with the exception that
they were not polished prior to degreasing.
18
1.5.3 Electrochemical Cleaning Techniques
A polished and degreased electrode was placed in a clean beaker that contained
aqueous 0.5M H2SO4 (Supplier) and connected in a three-electrode configuration that
used a platinum foil counter electrode and a saturated Ag/AgCl reference electrode
(0.197V vs SHE). The potentiostat (CH Instruments model 760D or 440A, CH
Instruments, Inc., Austin, TX http://www.chinstruments.com/) was programmed to cycle
between -0.22 and 0.55V for 20 cycles at a rate of 20mV/s. Once a repeatable CV plot
was obtained, the open circuit potential of the working electrode was measured and
checked against previously recorded values (between 0.45 and 0.57V vs sat. Ag/AgCl)
and the electrode deemed fit for experiments.
1.6 References
1. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, NY, 2001.
2. Clavilier, J.; Feliu, J. M.; Aldaz, A. An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes. J. Electroanal. Chem. 1988, 243, 419-433.
3. Clavilier, J.; Feliu, J. M.; Fernandez-Vega, A.; Aldaz, A. Electrochemical behaviour of irreversibly adsorbed bismuth on Pt (100) with different degrees of crystalline surface order. J. Electroanal. Chem. 1989, 269, 175-189.
4. Cullity, B. D. Elements of X-ray Diffraction; Cohen, Morris, Ed.; Addison-Wesley Metallurgy Series; Addison-Wesley Publishing Company, Inc.: Reading, MA, 1956.
5. Gerischer, H.; Kolb, D. M.; Pazasnyski, M. Chemisorption of metal atoms on metal surfaces in correlation to work function differences. Surf. Sci. 1974, 43, 662-666.
6. Harris, D. C. Quantitative Chemical Analysis, 5th ed.; W. H. Freeman and Company: New York, NY, 1999.
19
7. Herrero, E.; Buller, L. J.; Abruna, H. D. Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials. Chem. Rev. (Washington, D. C.) 2001, 101, 1897-1930.
8. Jenkins, R.; Snyder, R. Introduction to X-Ray Powder Diffractometry, 1st ed.; Wiley: New York, NY, 1996
9. Kolb, D. M. An atomistic view of electrochemistry. Surf. Sci. 2002, 500, 722-740.
10. Kolb, D. M.; Gerischer, H. Further aspects concerning the correlation between underpotential deposition and work function differences. Surf. Sci. 1975, 51, 323-327.
11. Kolb, D. M.; Przasnyski, M.; Gerischer, H. Underpotential deposition of metals and work function differences. J. Electroanal. Chem. 1974, 54, 25-38.
12. Quiroz, M. A.; Meas, Y.; Lamy-Pitara, E.; Barbier, J. Characterization of a ruthenium electrode by underpotential deposition of copper. J. Electroanal. Chem. 1983, 157, 165-174.
13. Schmickler, W. Interfacial Electrochemistry; Oxford University Press, Inc.: New York, NY, 1996.
14. Yu, K. K. Study of Copper Electrodeposition on Ruthenium Oxide Surfaces and Bimetallic Corrosion of Copper/Ruthenium in Gallic Acid Solution, University of North Texas, Denton, TX, 2007.
20
CHAPTER 2
BISMUTH DEPOSITION ON RUTHENIUM AND RUTHENIUM OXIDE ELECTRODES
2.1 Introduction
The electrodeposition of bismuth is a well-studied topic in electrochemistry.
Bismuth depositions have been characterized on a variety of metallic surfaces such as
gold (Au), platinum (Pt), and palladium (Pd) as well as semiconductor surfaces like
gallium arsenide (GaAs), and also glassy carbon surfaces.2,15,19-22 In 1988, M. A.
Quiroz, et al.17 first characterized the electrodeposition of Bi on a polycrystalline Ru
electrode in sulfuric acid. This work stands as the only publication to date (to this
author’s knowledge) regarding the Bi/Ru electrochemical system. Despite this, bismuth
electrodeposits have practical uses such as an electrode surface modification for
detecting lead in solution and also for use in electrochromic devices (e.g. flat panel
computer screens).1,5,10
It is the intention of this work to further develop our understanding of nature of
the Bi/Ru electrochemical system. This chapter will provide further insight into the
bulk/UPD electrodeposition characteristics of Bi on Ru by using CV analysis to compare
two different types of ruthenium substrates. The surfaces being compared are: 1) a
polycrystalline ruthenium shot electrode, and 2) a RF sputtered ruthenium thin film
electrode on a titanium/silicon(100) support. Experiments detailed later (Chapter 3) will
show that the sputtered thin film is also polycrystalline. Within our own laboratory, it has
been shown that surface oxidation on ruthenium can have an impact on Cu
electrodeposition. A similar study was performed with the Bi/Ru system and is
21
presented within this chapter as well.
2.2 Experimental
A solution containing 1mM Bi3+ (from Bi(NO)3, Fisher Scientific) and 0.5M H2SO4
(96.2%, Mallinckrodt Baker, Inc.) was prepared in pretreated glassware and with ultra-
pure water (18.2MΩ, Millipore). All glassware used was treated for the removal of
metal/organic contaminants by boiling in a nitric acid bath followed by rinsing with
ultrapure water. The bismuth solution was sonicated prior to use to ensure it was well
Figure 2.1: Images at 5X magnification of a Ru shot electrode (left) and a Ru thin film electrode (right). Below are their respective surfaces after oxidizing at 1.3V for 30sec in 0.5M H2SO4. The geometric areas are 0.368cm2 (shot) and 0.229cm2 (thin film).
22
mixed; the solution was not sparged or purged. Unless otherwise specified, all
experiments were carried out under lab ambient conditions.
A three electrode was setup similar to one depicted in Chapter 1 (Figure 1.3) was
using a CH Instruments potentiostat (model 760D or 440A) with a clean, platinum foil
counter electrode, a Ag/AgCl reference electrode (KCl saturated, 0.197V vs. SHE) and
a ruthenium working electrode (shot or film). Voltage will be reported in reference to the
saturated Ag/AgCl electrode unless otherwise stated. All ruthenium electrodes were
prepared for experimental use by the instructions listed in Chapter 1 (Section 1.4) with
the exception that the Ru thin film electrodes were not polished prior to use. Detailed
instructions for fabricating a Ru shot electrode and thin film sputter deposition
procedures are described elsewhere.22
The geometric working area of the shot electrode was measured with precision
calipers and calculated to be approximately 0.368cm2. For the film electrode, a working
area was created by employing Kapton® Tape (E. I. du Pont de Nemours and
Company, Inc., Wilmington, DE) that has been hole-punched with a cork borer and
carefully applied to the clean metal surface (Figure 2.1). The working area was
determined to be 0.229cm2. Electrical contact to the electrodes was made with alligator
clips. In the case of the film electrode, one turn of copper tape was wound around the
teeth to prevent damage to the surface while ensuring a good connection. As
mentioned, prior to collecting experimental data, the electrodes were cycled twenty
times within the experimental potential window (-0.22 to 0.55V) swept from positive to
negative potentials.
23
2.3 Results and Discussion
2.3.1 Bismuth Deposition on Electrochemically Cleaned Ruthenium
Figure 2.2 shows a side by side comparison of background scans from the two
electrode surfaces. These voltammograms match what has been seen elsewhere in the
literature for polycrystalline ruthenium in H2SO4.7,8,14 Each shows a distinct reduction
peak in the first cathodic sweep, -0.023V for the shot and the shoulder near -0.15V for
the film, followed by a sharp increase in current indicating the onset of hydrogen
evolution. The preceding cathodic peak is the result of the reduction of surface oxidation
that has formed while the electrodes were left exposed to air between experiments. It
can be seen that a more stable oxide forms on the surface of the thin film electrode after
air exposure as it requires a more negative reduction potential to be reduced.7,8,16-7 This
Figure 2.2: Background scans of Ru electrodes in 0.5M H2SO4 and 20mV/s scan rate. Shot electrode (left) and thin film electrode (right).
24
may be due to the difference in the apparent smoothness of the surfaces (Figure 2.1).
The grain sizes in the film are smaller which may make them more prone to attack by
oxygen and may facilitate subsequent reorganization of an oxide layer to a more stable
surface structure. The thin film electrode voltammogram shows the hydrogen
adsorption/desorption peaks found around -0.2V and -0.15V, respectively, slowly
diminish over time. Again, since the oxide reduction and hydrogen adsorption processes
of Ru overlap, available sites for hydrogen deposition on the thin film may be impacted
to a far greater degree due to the small grain sizes of the surface crystals. This warrants
further study on the influence of surface grain sizes on the electrochemical reactivity of
ruthenium. Ultimately, the voltammograms show stable surfaces can be achieved after
only 20 cycles, which is congruent with what has been previously reported.16
Figure 2.3: CV voltammograms of 1st, 2nd, and 20th(final) cycles of Ru shot (left) and Ru thin film (right) electrodes in 1mM Bi3+ / 0.5M H2SO4. Scan rate = 20mV/s.
25
Cyclic voltammetry data for each electrode in a 1mM Bi3+/0.5M H2SO4 solution
was collected between -0.22 to 0.55V for 20 cycles with a scan rate of 20mV/s (Figure
2.3). It was observed that these CV cycles show a similar shape and peak placement as
what was reported by Quiroz et al.18 The area under a peak within a curve of current
versus potential yields the value of the charge transferred during that event. Figure 2.4
shows how the charge density, ρQ (in C/cm2), of each event changes with successive
potential cycles (up to 20). Since the potential is swept from positive to negative and
Figure 2.4: Charge density comparisons of peaks over course of CV experiment (by segment number). Top graphs (left and right) are for the shot electrode and the bottom (left and right) are for the thin film electrode.
26
back, odd segments are cathodic (negative current) and even segments are anodic
(positive current); two segments equal one full cycle. The ratio of charge density
(ρQa/ρQc) for the bulk Bi deposition is roughly 2, while charge densities for Bi UPD shows
a steady decrease in cathodic current density until it nearly overlaps with the anodic
charge density. The UPD stripping current density deviates around an average value of
7.4 x 10-4 C/cm2 (shot) and 7.46 x 10-4 C/cm2 (thin film).
Table 2.1 details/compares the peak placements of the bulk and underpotential
deposition of bismuth within the same segments used for the charge density
comparisons from Figure 2.4. Recall from equation 1.7, for a reversible electrochemical
process, ΔEp = |Epa – Epc| = 0.057V/n(moles e-) where n = 3 for Bi3+/Bi(ad) system. Thus,
the value of ΔEp must be less than or equal to 0.020V for a reversible bismuth
electrodeposition process. The values presented in Table 2.1 show that this process is
irreversible (aka quasi-reversible) at both ruthenium surfaces.
2.3.2 Effects of Progressive Oxidation of Ruthenium on Bismuth Deposition
In experiments using x-ray emission spectroscopy, Mitchell, et al. reported the
composition of the oxide found on a ruthenium surface in 1M H2SO4 in the potential
range of 0.03 – 1.1V (or 0.01 – 0.9V vs KCl sat. Ag/AgCl) may be an amorphous, non-
stoichiometric hydrated RuO. At higher potentials (1.1 – 1.5V vs SHE), they report that a
conversion of the hydrated RuO to hydrated RuO2 (also amorphous) takes place by the
following mechanism16:
RuOx∙(H2O)y RuO(x+δ)∙(H2O)(y-δ) + 2δH+ + 2δe- (2.1)
Table 2.1
Comparison of ΔEp Values
27
Shot Electrode Wafer Electrode
Bulk Underpotential Bulk Underpotential
Cycle EA EC ΔEp EA EC ΔEp EA EC ΔEp EA EC ΔEp
1 0.024 -0.065 0.089 0.356 0.191 0.165 0.020 -0.041 0.061 0.357 0.203 0.154
2 0.025 -0.062 0.087 0.358 0.204 0.154 0.021 -0.037 0.058 0.357 0.207 0.150
3 0.025 -0.062 0.087 0.357 0.204 0.153 0.021 -0.036 0.057 0.357 0.205 0.152
4 0.025 -0.061 0.086 0.359 0.203 0.156 0.021 -0.036 0.057 0.358 0.205 0.153
5 0.025 -0.060 0.085 0.358 0.202 0.156 0.021 -0.036 0.057 0.361 0.205 0.156
6 0.025 -0.060 0.085 0.357 0.203 0.154 0.021 -0.036 0.057 0.363 0.204 0.159
7 0.025 -0.060 0.085 0.358 0.203 0.155 0.021 -0.036 0.057 0.363 0.205 0.158
8 0.025 -0.060 0.085 0.358 0.203 0.155 0.020 -0.036 0.056 0.361 0.205 0.156
9 0.025 -0.060 0.085 0.359 0.203 0.156 0.020 -0.036 0.056 0.361 0.205 0.156
10 0.025 -0.060 0.085 0.359 0.202 0.157 0.020 -0.036 0.056 0.363 0.205 0.158
13 0.025 -0.060 0.085 0.36 0.203 0.157 0.020 -0.036 0.056 0.366 0.206 0.160
15 0.025 -0.061 0.086 0.361 0.203 0.158 0.021 -0.036 0.057 0.367 0.207 0.160
17 0.025 -0.061 0.086 0.366 0.203 0.163 0.020 -0.036 0.056 0.369 0.209 0.160
20 0.025 -0.062 0.087 0.364 0.205 0.159 0.020 -0.036 0.056 0.367 0.209 0.158
Quiroz, et al.17 proposed that Ru2O and Ru2O3 are thermodynamically feasible, though
Mitchell et al.16 show these oxides are likely not present.
To study the effects of the surface oxidation state of ruthenium on
electrochemical deposits of bismuth, a series of potentials (550, 650, 750, 850, 950,
1100, 1300mV vs KCl sat. Ag/AgCl) were held on the electrode for 30 seconds in Bi free
0.5M H2SO4 to initiate oxidation. When a potential of 1.3V was held on the electrode
28
surface, oxygen evolution can be seen and the surface changes from a metallic
appearance to a dull, golden brown/grey color. This is presumably a consequence of
equation 2.1. After holding the oxidizing potentials, the electrode was transferred to
clean beaker containing a 1mM Bi/0.5M H2SO4 solution. Cyclic voltammetry data was
collected for 20 cycles between -0.25V and 0.55V at a scan rate of 20mV/s.
Figure 2.5 shows the result of overlaying the data collected on the shot electrode
surface in the initial cycle (segments 1 and 2 of 20). When the holding potential is
moved toward greater positive potentials, the cathodic Bi UPD peak current in the first
scan shifts to more negative potentials. At 1.1V, the cathodic UPD peak appears
completely coupled with the onset of bulk deposition and where before there is a slight
suppression of hydrogen evolution, a sharp downturn in current around -0.23V onset of
the reaction.
Multiple reduction/deposition events appear to overlap in the first scans as the
Figure 2.5: CV overlay of first scans of Ru shot electrode in 1mM Bi/0.5M H2SO4. Inset (top right) is an expanded view of the peaks in the UPD region.
29
holding potential of oxidation was increased. What is clear in the subsequent anodic
scans is the oxidation peaks are shifting to slightly more positive potentials (from 0.36 to
0.41V). The UPD stripping peaks show that even as the deposition peaks are shifted
more negatively, underpotential deposition appears to be a process that is well
connected
Figure 2.6: Comparison of anodic charge density versus holding potential for the shot electrode (top) and thin film electrode (bottom). The first (left) and final (right) CV cycles are shown. The line shown in red is the average charge density value excluding the point at 1300mV.
30
to the bulk deposition despite the crowding of events in the cathodic scans. Figure 2.6
shows a comparison of charge density required to strip the bismuth UPD from each
electrode surface. It can be seen that after 20 cycles, the surface oxidation, deposition,
and stripping processes stabilize. Overlays of the final CV cycles show a near overlap
with the exception of the CV from the irreversible, hydrated oxide formed at 1.3V. On
this surface, a 2 to 2.5-fold increase over the average anodic UPD current density can
be seen by the end of 20 cycles for the shot and thin film, respectively.
Figure 2.7: Overlays of successive CV cycles of the RuOx(H2O)y in 1mM Bi/0.5M H2SO4. The shot electrode is shown at the top and the thin film CVs are below.
31
Figure 2.7 shows the unique features of Bi deposition the irreversible, hydrated
oxide of ruthenium. Both surfaces show that Bi deposition, most notably the
underpotential deposition peaks, grows with each cycle; something that was not
observed at any other oxidizing potential. Figure 2.8 shows the trend for the UPD
stripping current density from initial to final CV cycle is more apparent for the thin film
(nearly 8-fold increase) than for the shot electrode (almost 3-fold increase). This may be
Figure 2.8: Overlaid CV cycles of anodic UPD current density vs. potential and charge density vs. CV segment number. Data for the shot electrode is at the top and thin film data is below.
32
a result of the differences in the uniformity of coverage of the hydrated oxide. From the
magnified images of the surfaces (Figure 2.1), the oxide appears more uniformly
distributed on the thin film electrode as there are large grains on the shot electrode that
still retain their metallic appearance even after oxidation. Chernyi et al.3 reported that
adsorption of hydrogen and oxygen by ruthenium is dependent the crystallographic
orientation of the surface of the electrode. Orientations with densely packed crystal
faces show higher rates of oxygen and hydrogen adsorption.3 However, a later
Figure 2.9: Calculated surface coverage from charge density data. Coverage vs. potential for the shot electrode (top left) and wafer (top right) are shown. Coverage vs. segment number for the oxidized electrodes, shot (bottom left) and wafer (bottom right), are shown as well.
33
publication states that the sweep rates used in the former study were too high to
adequately resolve the processes.16 This fact may not be important as the
electrochemical oxide in this study was produced by holding a static potential on the
surface. Ultimately, since there are features on the metal surface of the shot electrode
that do not appear to be oxidized, these areas may account for the initial, larger value of
the UPD stripping charge density when compared to the thin film electrode. With
progressive scanning, the new sites on both may become available for Bi UPD. Since
the thin film electrode is more uniformly covered by oxidation and because of possible
grain size effects on surface modification, it is reasonable that this surface shows the
most change over the course of potential cycling.
Kolb et al. have reported the potential shift of UPD, ΔEUPD, is a function of the
difference in the work functions for the deposited metal and the metal surface. By their
analysis the relationship follows the trend:
ΔEUPD = (0.5V/eV) x (ΦM – ΦS) (2.2)
where ΦM is the work function of the electrode metal and ΦS is the work function of the
adatom being deposited. The work function of the electrode is dependent on the
crystalline structure of the metal and it is also a function of surface coverage by foreign
adatoms. For the Bi/Ru electrochemical system the theoretical value is:
ΔEUPD = (0.5V/eV) x (4.71eV – 4.34eV) = 0.185V (2.3)
Kolb as points out that polycrystalline surface can show faceting (i.e. reorganization of
the crystalline surface) under the influence of potential cycling and these
reorganizations can also be affected by incorporation of oxygen into the surface crystal
lattices (i.e. oxidation).6,11-3 All of these reasons, among others, may account for the
observed differences in Bi UPD on the shot electrode and the wafer electrode.
34
2.4 Conclusions
In the background scans prior to experimentation, the thin film appeared to be
more prone to air oxidation under lab ambient conditions as the first cathodic CV
segments show the peak for oxide reduction is found at -0.15V vs -0.023V for the shot
electrode (Figure 2.1). Results of a comparison of the fresh ruthenium shot electrode
and thin film electrode surfaces shows that the CV data captured in an acidic bismuth
solution is in agreement with what has been previously reported. A comparison of ΔEp
values shows that the bulk and underpotential deposition of bismuth on ruthenium is
irreversible (Table 2.1).
Experiments show that Bi UPD and bulk deposition are adversely affected by the
progressive oxidation of the underlying ruthenium electrode surface. As the oxidizing
potential becomes more positive, the UPD peak of Bi is moved more negative in
potential and at 1.1V the peak is completely coupled with the peak for bulk deposition.
However, after 20 cycles the reduction, deposition, and stripping processes seem to
equilibrate (Figure 2.6). It was also noted that the onset of hydrogen evolution is
suppressed by the presence of Bi on Ru. Voltammograms for the deposition events on
the hydrated, irreversible oxide of ruthenium formed at 1.3V show a pronounced change
over time. This may arise from the differences in the crystallographic nature between
the shot electrode and sputtered thin film electrode surface. Large patches of
unoxidized ruthenium remain after holding 1.3V on the shot electrode surface, while the
thin film shows a more uniform coverage.
35
2.5 References
1. Arduini, F.; Calvo, J. Q.; Palleschi, G.; Moscone, D.; Amine, A. Bismuth-modified electrodes for lead detection. Trends Anal. Chem. 2010, 29, 1295-1304.
2. Ashok K., V. A quantitative treatment of chemisorption of metal atoms on metal surfaces based on the underpotential electrodeposition of metals. Surf. Sci. 1974, 46, 282-286.
3. Chernyi, V. V.; Zuikova, V. S.; Vasil'ev, Y. B.; Gryaznov, V. M.; Gorina, N. B.; Bagotskii, V. S. Effect of crystallographic orientation on the adsorption and electrochemical properties of ruthenium. Elektrokhimiya 1972, 8, 1341-1345.
4. Conway, B. E. Electrochemical oxide film formation at noble metals as a surface-chemical process. Prog. Surf. Sci. 1995, 49, 331-452.
5. De, O.,Silvio C.; De, M.,Luis C.; Curvelo, A. A. d. S.; Torresi, R. M. An Organic Aqueous Gel as Electrolyte for Application in Electrochromic Devices Based in Bismuth Electrodeposition. J. Electrochem. Soc. 2003, 150, E578-E582.
6. Gerischer, H.; Kolb, D. M.; Pazasnyski, M. Chemisorption of metal atoms on metal surfaces in correlation to work function differences. Surf. Sci. 1974, 43, 662-666.
7. Hadži-Jordanov, S.; Angerstein-Kozlowska, H.; Conway, B. E. Surface oxidation and H deposition at ruthenium electrodes: Resolution of component processes in potential-sweep experiments. J. Electroanal. Chem. 1975, 60, 359-362.
8. Hadži-Jordanov, S.; Angerstein-Kozlowska, H.; Vukovic, M.; Conway, B. E. The state of electrodeposited hydrogen at ruthenium electrodes. J. Phys. Chem. 1977, 81, 2271-2279.
10. Howard, B. M.; Ziegler, J. P.; McKee, M.; Tornberg, N.; Caudy, K. Reversible electrodeposition of bismuth thin films for flat panel display applications. Proc. - Electrochem. Soc. 1993, 93-26, 353-61.
11. Kolb, D. M. An atomistic view of electrochemistry. Surf. Sci. 2002, 500, 722-740. 12. Kolb, D. M.; Gerischer, H. Further aspects concerning the correlation between
underpotential deposition and work function differences. Surf. Sci. 1975, 51, 323-327.
13. Kolb, D. M.; Przasnyski, M.; Gerischer, H. Underpotential deposition of metals and work function differences. J. Electroanal. Chem. 1974, 54, 25-38.
14. Łosiewicz, B.; Martin, M.; Lebouin, C.; Lasia, A. Kinetics of hydrogen underpotential deposition at ruthenium in acidic solutions. J. Electroanal. Chem. 2010, 649, 198-205.
36
15. Martins, M. E.; Galindo, M. C.; Arvia, A. J. Bismuth electrodeposition on polycrystalline gold electrodes in acid solutions. An. Quim. 1990, 86, 327-6.
16. Michell, D.; Rand, D. A. J.; Woods, R. A study of ruthenium electrodes by cyclic voltammetry and X-ray emission spectroscopy. J. Electro. Chem. 1978, 89, 11-27.
17. Quiroz, M. A.; Meas, Y.; Lamy-Pitara, E.; Barbier, J. Characterization of a ruthenium electrode by underpotential deposition of copper. J. Electroanal. Chem. 1983, 157, 165-174.
18. Quiroz, M. A.; Salgado, L.; Meas, Y. Arrangement of bismuth on a ruthenium electrode. Electrochim. Acta 1988, 33, 435-437.
19. Salie, G.; Bartels, K. Partial charge transfer and adsorption at metal electrodes. The underpotential deposition of Hg(I), Tl(I), Bi(III) and Cu(II) on polycrystalline gold-electrodes. Electrochim. Acta 1994, 39, 1057-1065.
20. Salles, M. O.; Ruas, d. S.,Ana Paula; Naozuka, J.; Vitoriano, d. O., Pedro; Bertotti, M. Bismuth modified gold microelectrode for Pb(II) determination in wine using alkaline medium. Electroanalysis 2009, 21, 1439-1442.
21. Vereecken, P. M.; Searson, P. C. Electrochemical deposition of Bi on GaAs. Proc. - Electrochem. Soc. 2001, 2000-29, 431-440.
22. Yang, M.; Hu, Z. Electrodeposition of bismuth onto glassy carbon electrodes from nitrate solutions. J. Electroanal. Chem. 2005, 583, 46-55.
23. Yu, K. K. Study of Copper Electrodeposition on Ruthenium Oxide Surfaces and Bimetallic Corrosion of Copper/Ruthenium in Gallic Acid Solution, University of North Texas, Denton, TX, 2007
37
CHAPTER 3
GRAZING INCIDENCE X-RAY DIFFRACTION INVESTIGATIONS OF
ELECTROCHEMICAL AND THERMAL OXIDES FORMED ON RUTHENIUM THIN
FILMS AND THE ELECTROCHEMICAL NATURE OF RUTHENIUM THERMAL OXIDE
IN ACIDIC BISMUTH SOLUTIONS
3.1 Introduction
It is well understood that the properties of a material, both at the surface and in
the bulk, are directly linked to its crystal structure among other things. For example a
metal’s work function (its ability to transport electrons from within the bulk to the
surface) is directly related to crystallographic orientation and in electrochemical
systems, linked to the surface potential of an electrode. It has been well documented
that electrodes made from single crystal orientations of the same metal (e.g.Pt or Au)
have very different electrochemical behaviors by comparison.4 Kolb et al. have shown
that that the work functions of the electrode material and surface adatoms are linked to
the phenomenon of underpotential deposition; a deposition event that occurs at more
positive potentials than that of the bulk.3, 6-8 Many common surface science techniques
are being adapted for use in observing and tracking surface changes brought about by
the forefront of modern surface/materials science. For this reason, it is often called
interfacial electrochemistry.2,9,10-1
38
electrochemical reactions. This has thrust modern electrochemistry to
The techniques used to make metal electrodes can have a significant impact on
the final structure of the electrode material. For example, magnetron sputter deposition
of metals can create films of much smaller grain sizes when compared to vacuum arc
melting, which is used to make much larger metal pellets (aka shots) that are cooled
from a molten liquid state. In the preceding chapter, bismuth electrodeposition
experiments were performed on polycrystalline electrode made from a pure ruthenium
shot and a thin film electrode made from RF sputter deposition of pure ruthenium on a
Ti/Si(100) support (Figure 3.1). In this chapter, preliminary studies of the
crystallographic nature of the surfaces of ruthenium and ruthenium oxide thin films using
grazing incidence x-ray diffraction will be presented. For the first time, to this author’s
knowledge, the electrodeposition of bismuth on a thermally produced thin film of RuO2
will be presented.
Figure 3.1: Denton Vacuum Desktop Pro® confocal DC/RF sputtering instrument (left) for making metal thin films. The inside of the chamber is shown to the right.
39
3.3 Experimental
Ru/Ti/Si(100) wafers prepared as described elsewhere were carefully fractured
along the [111] directions until smaller square and rectangular shapes with an average
area of 1 cm2 were made.12 One thin metal surface was used as the
reference/background for XRD spectra, while the other electrodes were used in
electrochemistry experiments described later. A procedure similar to one detailed in
Chapter 2 was used to create a circular working surface on each film (area = 0.229cm2)
just before use as an electrode (Figure 3.3).
The thermal oxide of ruthenium, RuO2, was formed on freshly cleaned surface
(Chapter 1.4) and can be prepared as follows. A cut Ru/Ti/Si(100) wafer sample was
placed on a cleaned and degreased quartz boat, and the sample/boat were pushed into
the left side of the glass tube of a Lindberg/Blue tube furnace (Figure 3.2). The glass
tube is fitted with a ground glass plug that may be left off (called the “open
configuration”) or banded in place (the “closed configuration”) depending on the
conditions desired during operation. On the left side of the glass tube there is also a
stem which may be connected to a gas line for controlled the environment in the tube.
The furnace controls were set for a 600°C holding temperature and a heating ramp of
40°C/min from room temperature. The samples were subjected to 600°C for 20mins(not
including ramp time). In the closed configuration a steady flow of O2 (>99.5%, Air
Liquide) was introduced during heating. After heat treatment, the quartz boat/sample
was removed from the heater with a hooked glass rod and left to cool for another
20mins or until they could be handled with Teflon tweezers.
X-ray diffraction spectra were collected by a Rigaku Ultima III XRD equipped with
40
a thin film sample stage using Cu Kα radiation (λ = 1.54178 Å) generated with 44kV bias
across the tube filament. The geometric setup of the instrument was a parallel beam
modified, Bragg-Bretano configuration (Figure 3) with incident beam angles (ω) of either
0.5 or 1.0 degrees. Data was recorded over a range of diffracted beam angles (in 2θ)
between 26 - 100° for the background and 26 - 58° for the experimental samples. A
5º/min scan rate was used for collecting the spectra of the control sample’s surface (RF
sputtered metallic Ru), while 1º/min scan rate was used for the thermal oxide and
hydrated oxide surfaces. The incident beam was corrected using 5º Soller slits and a
divergent slit width of 2mm and also 5mm. The sample was placed in the center of the
stage and automatically aligned by computer control to correct for the stage height and
position of the sample and properly fix the incident angle. The diffracted beam was
filtered by 5º Soller slits and a receiving slit width was set to 5mm.
A standard three electrode configuration was used for studying Bi deposition on
the oxidized ruthenium surfaces (Figure 1.3). This setup was controlled by a CH
Instruments model 440A potentiostat. The thin films served as the working electrodes, a
Figure 3.2: A Lindberg/Blue tube furnace.
41
platinum foil was used as a counter electrode and potential will be controlled against a
Ag/AgCl reference electrode (KCl saturated, 0.197V vs. SHE). All potential values will
be given according to the saturated Ag/AgCl reference electrode used and experiments
were carried out in lab ambient conditions unless otherwise stated. Background cyclic
voltammograms and Bi electrodepostion studies were performed in a manner similar to
those detailed in Chapter 2 of this thesis.
Figure 3.3: Oxidized thin film electrodes. Thermal oxide thin film electrodes (top left) and 5X magnified images of open configuration (orange, top right) and closed configuration (purple, bottom left). Bottom left image is a 5X magnification of the electrochemically oxidized film (Chapter 2).
42
3.3 Results and Discussion
3.3.1.GIXRD Studies of Ruthenium and Ruthenium Oxide Thin Films
After subjecting the Ru thin film to 600°C for twenty minutes, it was observed that
the film surface that resulted from heating in the open configuration showed an
orange/rust/metallic color, while the film surface subjected to heating in the closed
configuration was a dull purple color. The color seems to initial suggest that these two
oxides may have either a different composition or adopt different crystalline orientations.
Microscopic images of the thermal oxide surfaces show visible signs of roughening
when compared to the electrochemically formed oxide, likely a result of incorporating
oxygen into the surface structure (Figure 3.3).
A powder sample of pure metallic Ru is known to adopt a hexagonal close-
packed structure (a =2.7058Å and c = 4.2819Å, Cu Kα1 = 1.5405Å). However, it has
been shown the anhydrous thermal oxide, RuO2, adopts a tetragonal crystal structure,
known as the rutile structure (a = 3.1071Å and c = 4.4994Å, Cu Kα1 = 1.5405Å). Mitchell
et. al. reported that x-ray emission data shows that an amorphous, hydrated ruthenium
oxide (RuOx·(H2O)y) is formed while cycling the potential to 1.5V (vs. SHE) in a 1M
Figure 3.4: Bragg-Brentano geometry
43
H2SO4 solution.9 Also, Wen et. al. have reported similar results using x-ray diffraction to
show that the hydrated ruthenium oxide is amorphous when it is deposited on a Ti
substrate from an acidic solution of RuCl3, 0.01M HCl and 0.1M KCl.11
Analyzing the surface of a RF sputtered thin film of pure Ru on a Ti/Si(100)
support with grazing x-ray diffraction confirms the hexagonal structure of Ru metal is
adopted(Figure 3.5). The peak locations of the diffracted beams show very good
agreement with the standard provided by the Jade PDF card database for Ru(0)
(PDF#06-0663). However, upon inspection of the background spectra, the peak
intensity values compare only fairly well. Not only can this be due to the decreased
intensity inherent with thin film XRD work (incident beam penetrates less deeply, thus
less planes of atoms are diffracting the beam), but also a large contribution could be
due to a decrease in crystallite grain size. The most intense peak in the standard
Figure 3.5: X-ray diffraction spectrum of a pure Ru thin film (95nm) on Ti/Si(100).
44
powder sample of Ru(0) is from the refracted beam at a Bragg angle (2θ) of 44.00 (I(f) =
100) which is Ru(101). The second most intense peak is found at 38.38 (I(f) = 40) for
Ru(100) and the third at 42.15 (I(f) = 35) for Ru(002). However, in the sputtered
Ru/Ti/Si(100) sample, the predominant reflection is Ru(002) followed by Ru(101) and
Ru(100) at 81.7% and 25.3% of maximum intensity, respectively. This appears to be a
considerable departure from the powder standard and more work will be needed to
Figure 3.6: XRD spectra for RuO2 open configuration (top), RuO2 closed configuration and RuOx∙(H2O)y (bottom).
45
properly characterize this phenomenon. Table 3.1 shows the peak locations for all
experimental surfaces.
The XRD spectra collected on the thermal oxide surfaces show both thin film
samples (open and closed configuration) adopt the rutile structure and peaks are
comparable to those of the powder standard for RuO2 (Jade PDFdatabase, #00-040-
1290(RDB)). However, the each of the spectrums show there may be a mixed
metal/metal-oxide surface, as peaks of Ru hcp are also seen (Figure 3.6). This is very
apparent in the spectrum for the thermal oxide surface formed under the open tube
configuration (orange surface). This suggests that only a very thin film of oxide was
grown under these conditions. Jelenkovic and Tong reported a similar phenomenon.5
The dull purple surface (closed tube configuration in pure O2 atmosphere) shows far
less hcp Ru mixing on the surface, which is likely a result of Ru’s high affinity for oxygen
and the increased concentration in the atmosphere above the surface. Therefore, the
XRD spectrum for this oxide surface suggests a thicker layer of RuO2 has formed.
It was also found that the electrochemically formed hydrated oxide surface
created by simply holding an oxidizing potential (1.3V vs. Ag/AgCl sat. KCl) on a fresh
metallic Ru surface was amorphous (Figure 3.6). Wen et al.11 reported that since only
diffraction peaks for the Ti substrate are seen in the XRD data, the hydrated oxide layer
must not have an ordered crystallographic orientation. Despite the fact that the method
for forming the hydrated oxide layer in this study is different, a similar logic can be
applied to the XRD results in this report as only the Ru(hcp) peaks are seen.
Table 3.1 Comparison of XRD Peak Locations (2θ) and Miller Indices (hkl)
46
RF Sputtered Ru and Hydrated Oxide Thermal Oxide
Background Ru(hcp) RuOx(H2O)y† Open Configuration Closed Configuration
hkl 2θ hkl 2θ hkl 2θ hkl 2θ
(100) 38.3 (100) 38.6 (110) 28.2 (110) 28.05
(002) 42.2 (002) 42.2 (101) 35.25 (101) 35.2
(101) 44.1 (101) 44.15 Ru(100) 38.8 (200) 40.05
(102) 58.3 (200) 40.05 (111) 40.2
(110) 69.4 (111) 40.2 (210) (44.99)‡
(103) 77.95 Ru(002) 42.5 (211) 54.35
(112) 84.8 Ru(101) 44.3
(201) 85.85 (210) (44.99)‡
(211) 54.5
† Miller indices for this oxide are those of the underlying Ru(hcp) as RuOx(H2O)y is believed to be amorphous
‡ Reflection not observed. In the pure powder, this reflection has an intensity that is 1% of maximum.
3.3.2 Electrodeposition of Bismuth on RuO2 Thin Film Electrodes
After it was confirmed by GIXRD that the thin films of sputter deposited Ru on
Ti/Si(100) adopt the rutile structure by heat treatment, they were prepared for use as
electrodes. A three electrode was setup as described earlier and CV experiments were
carried out in a 1mM Bi/0.5M H2SO4 solution. A comparison of the voltammograms for
47
each thermally oxidized thin film surface is shown in Figure 3.7 along with the
voltammogram for the hydrated, irreversible oxide of ruthenium.
Using Kolb’s equation,8 we can use the values of the work functions reported in
the literature for RuO2 and Bi to predict the value of ΔEUPD which is:
ΔEUPD = 0.5V/eV (5.15eV – 4.34eV) = 0.405V (3.1)
This equation predicts that the Bi/RuO2 should exhibit a stronger interaction than that of
the Bi/Ru couple (ΔΦM/S = 0.37eV, ΔEUPD = 0.185V). However, despite this prediction,
the CV scans of Bi deposition on the oxidized surface do not show Bi UPD (Figure 3.7).
More interesting, the GIXRD results show that the thermal oxide surface produced in
the open configuration (left open to ambient conditions) showed signs of having some
mixing of Ru and RuO2 near the surface, though it seems to have no effect on the UPD
characteristics of Bi. The CV results seem to suggest that the surface is that of a thin
layer of RuO2 only and the results from the GIXRD can be interpreted as consequence
of the penetration of the x-ray beam into the Ru layers below that of the oxide. Kolb et
al. state that the work function of a surface is coverage dependent and once a thin layer
of material is deposited, the work function shifts from that of the substrate to that of the
deposited material.8 This does not fully explain the absence of the UPD peaks as
predicted by the differences in work function and more work will need to be done to
understand this phenomenon.
In the first cycle of the background scan for the orange thermal oxide surface,
two peaks were observed in the cathodic region, a broad peak from 0.20 to 0.50V
48
(0.371V at peak current) and another at -0.120V, followed by the onset of hydrogen
evolution. In the reverse scan, the peak at -0.064V corresponds to the peak in the
cathodic (-0.12V) and a broad peak was observed from 0.20 to 0.50V (0.358V at peak
current). The charge densities associated with the cathodic peaks are 7.336µC/cm2
(0.371V) and 12.82µC/cm2 (-0.12V), while the charge densities for the anodic peaks are
4.939µC/cm2 (-0.064V) and 10.38µC/cm2 (0.358V). By the conclusion of cycling (20th
cycle), the cathodic peaks have shifted in potential to 0.369V (broad peak,
10.99µC/cm2) and -0.028V (7.485µC/ cm2) and the anodic peaks have shifted to 0.052V
(13.86µC/cm2) and 0.371 (broad peak, 7.598µC/cm2). It is not immediately clear what
Figure 3.7: Comparison of CV scans of RuO2 in Bi free 0.5M H2SO4 and in 1mM Bi/0.5M H2SO4. The top left/right are CV scans of the RuO2 formed at 600C in the “open configuration” and middle left/right are for the “closed configuration”. The CV scans at the bottom are for the electrochemical oxide, RuOx(H2O)y.
49
Table 3.2 Comparison of Peak Locations and Current Densities (ρQ) of Ruthenium and Ruthenium Oxide Thin Films
Surface Process (Bulk) Ep (V) ρQ (mC/cm2) ΔEp (V)
RuO2 (open) Cathodic -0.116 1.878E-03 0.15
Anodic 0.034 3.606E-03
RuO2 (closed) Cathodic -0.113 1.387E-03 0.14
Anodic 0.027 2.821E-03
RuOx(nH2O)y Cathodic -0.043 6.480E-04 0.061
Anodic 0.018 2.718E-03
Ru Cathodic -0.036 1.429E-03 0.056
Anodic 0.02 3.172E-03
process causes the peaks seen in this background. More work will need to be done to
properly characterize them. These peaks were not observed in the background scan of
the purple thermal oxide surface.
A summary of the peak locations and charge densities for the oxide surfaces in
acidic bismuth solution are found in Table 3.2. The current densities of bulk deposition
for all of the thin film surfaces are of the same order of magnitude, though the ΔEp data
shows that this process is more irreversible on the thermal oxide of ruthenium.
3.4 Conclusions
It has been shown that radio frequency sputtered thin films (~95nm) of pure Ru
50
on a Ti/Si(100) supportare indeed polycrystalline with much smaller grain sizes than
those formed by vacuum arc melting (pelleting). Heat treatment of these films at 600°C
for 20 minutes revealed that when the atmosphere within the tube is not controlled (lab
ambient, open configuration) an orange/rust/metallic surface develops. However, when
the tube furnace is purged and filled with a steady stream of pure O2 gas (closed
configuration), a dull grayish-purple surface is formed. Microscopic images of both
surfaces reveal that the surface roughness changes considerably during the heat
treatment. GIXRD results showed that both heat treated films contain layers of RuO2 on
the surface. When the surface was heat treated in the open configuration, GIXRD
initially shows that the RuO2 is either mixed with layers of Ru(hcp) or the oxide layer is
very thin since the beam penetrates to the Ru(hcp) subsurface. However, GIXRD
results show that surface layer formed during heat treatment in the closed configuration
is almost completely RuO2. This is similar to what has been reported elsewhere in the
literature.5
While more work is needed here, it has been shown for the first time that the bulk
deposition of bismuth is possible on a thermally formed RuO2 surface. Despite the
predicted value of ΔEUPD from Kolb’s equation, there was no evidence in the CV results
of the underpotential deposition of bismuth on RuO2. However, the charge densities for
the bulk deposition process on the oxide surfaces are on the same order of magnitude
as those for the hydrated oxide surface (RuOx(nH2O)y) and the electrochemically
cleaned, ruthenium thin film surface. The increase in ΔEp for the bulk deposition of Bi on
the thermal oxide is most likely due the increase in the work function difference from
0.37eV (ΔΦRu/Bi) to 0.81eV (ΔΦRuO2/Bi), which predicts a much stronger interaction
between Bi and RuO2. More work will need to be done to understand the differences in
51
the background scans between the orange thermal oxide surface and the purple
thermal oxide surface.
It is unclear why the first cycle of the RuO2 (orange) shows twin anodic peaks for
the bulk deposition of Bi. This anomaly is not seen in later scans. This could mean that
there are either two overlapping stripping processes occurring around this potential, or
there are RuO2 and Ru sites performing chemistry, which would support the GIXRD
results showing possible mixing of layers.
3.5 References
1. Conway, B. E. Electrochemical oxide film formation at noble metals as a surface-chemical process. Prog. Surf. Sci. 1995, 49, 331-452.
2. Foelske, A.; Barbieri, O.; Hahn, M.; Kotz, R. An X-Ray Photoelectron Spectroscopy Study of Hydrous Ruthenium Oxide Powders with Various Water Contents for Supercapacitors. Electrochem. Solid-State Lett. 2006, 9, A268-A272.
3. Gerischer, H.; Kolb, D. M.; Pazasnyski, M. Chemisorption of metal atoms on metal surfaces in correlation to work function differences. Surf. Sci. 1974, 43, 662-666.
4. Herrero, E.; Buller, L. J.; Abruna, H. D. Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials. Chem. Rev. (Washington, D. C.) 2001, 101, 1897-1930.
5. Jelenkovic, E. V.; Tong, K. Y. Thermally grown ruthenium oxide thin films. J. Vac. Sci. Technol. , B: Microelectron. Nanometer Struct. --Process. , Meas. , Phenom. 2004, 22, 2319-2325.
6. Kolb, D. M. An atomistic view of electrochemistry. Surf. Sci. 2002, 500, 722-740. 7. Kolb, D. M.; Gerischer, H. Further aspects concerning the correlation between
underpotential deposition and work function differences. Surf. Sci. 1975, 51, 323-327.
8. Kolb, D. M.; Przasnyski, M.; Gerischer, H. Underpotential deposition of metals and work function differences. J. Electroanal. Chem. 1974, 54, 25-38.
9. Michell, D.; Rand, D. A. J.; Woods, R. A study of ruthenium electrodes by cyclic voltammetry and X-ray emission spectroscopy. J. Electro. Chem. 1978, 89, 11-27.
52
10. Tucker, C. W., Jr. Structure types in chemisorption found by LEED [low-energy electron diffraction]. Surf. Sci. 1972, 31, 172-179.
11. Wen, J.; Zhou, Z. Pseudocapacitance characterization of hydrous ruthenium oxide prepared via cyclic voltammetric deposition. Mater. Chem. Phys. 2006, 98, 442-446.
12. Yu, K. K. Study of Copper Electrodeposition on Ruthenium Oxide Surfaces and Bimetallic Corrosion of Copper/Ruthenium in Gallic Acid Solution, University of North Texas, Denton, TX, 2007
53
REFERENCE LIST
Arduini, F.; Calvo, J. Q.; Palleschi, G.; Moscone, D.; Amine, A. Bismuth-modified electrodes for lead detection. Trends Anal. Chem. 2010, 29, 1295-1304.
Ashok K., V. A quantitative treatment of chemisorption of metal atoms on metal surfaces based on the underpotential electrodeposition of metals. Surf. Sci. 1974, 46, 282-286.
Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley & Sons, Inc.: New York, NY, 2001.
Chernyi, V. V.; Zuikova, V. S.; Vasil'ev, Y. B.; Gryaznov, V. M.; Gorina, N. B.; Bagotskii, V. S. Effect of crystallographic orientation on the adsorption and electrochemical properties of ruthenium. Elektrokhimiya 1972, 8, 1341-1345.
Clavilier, J.; Feliu, J. M.; Aldaz, A. An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes. J. Electroanal. Chem. 1988, 243, 419-433.
Clavilier, J.; Feliu, J. M.; Fernandez-Vega, A.; Aldaz, A. Electrochemical behaviour of irreversibly adsorbed bismuth on Pt (100) with different degrees of crystalline surface order. J. Electroanal. Chem. 1989, 269, 175-189.
Conway, B. E. Electrochemical oxide film formation at noble metals as a surface-chemical process. Prog. Surf. Sci. 1995, 49, 331-452.
CRC Handbook of Chemistry and Physics, 85th ed.; Boca Raton, FL, 2004.
De, O.,Silvio C.; De, M.,Luis C.; Curvelo, A. A. d. S.; Torresi, R. M. An Organic Aqueous Gel as Electrolyte for Application in Electrochromic Devices Based in Bismuth Electrodeposition. J. Electrochem. Soc. 2003, 150, E578-E582.
Foelske, A.; Barbieri, O.; Hahn, M.; Kotz, R. An X-Ray Photoelectron Spectroscopy Study of Hydrous Ruthenium Oxide Powders with Various Water Contents for Supercapacitors. Electrochem. Solid-State Lett. 2006, 9, A268-A272.
Gerischer, H.; Kolb, D. M.; Pazasnyski, M. Chemisorption of metal atoms on metal surfaces in correlation to work function differences. Surf. Sci. 1974, 43, 662-666.
Hadži-Jordanov, S.; Angerstein-Kozlowska, H.; Conway, B. E. Surface oxidation and H deposition at ruthenium electrodes: Resolution of component processes in potential-sweep experiments. J. Electroanal. Chem. 1975, 60, 359-362.
Hadži-Jordanov, S.; Angerstein-Kozlowska, H.; Vukovic, M.; Conway, B. E. The state of electrodeposited hydrogen at ruthenium electrodes. J. Phys. Chem. 1977, 81, 2271-2279.
54
Harris, D. C. Quantitative Chemical Analysis, 5th ed.; W. H. Freeman and Company: New York, NY, 1999.
Herrero, E.; Buller, L. J.; Abruna, H. D. Underpotential Deposition at Single Crystal Surfaces of Au, Pt, Ag and Other Materials. Chem. Rev. (Washington, D. C.) 2001, 101, 1897-1930.
Howard, B. M.; Ziegler, J. P.; McKee, M.; Tornberg, N.; Caudy, K. Reversible
electrodeposition of bismuth thin films for flat panel display applications. Proc. - Electrochem. Soc. 1993, 93-26, 353-61.
Jelenkovic, E. V.; Tong, K. Y. Thermally grown ruthenium oxide thin films. J. Vac. Sci.
Technol. , B: Microelectron. Nanometer Struct. --Process. , Meas. , Phenom. 2004, 22, 2319-2325.
Kolb, D. M. An atomistic view of electrochemistry. Surf. Sci. 2002, 500, 722-740. Kolb, D. M.; Gerischer, H. Further aspects concerning the correlation between
underpotential deposition and work function differences. Surf. Sci. 1975, 51, 323-327.
Kolb, D. M.; Przasnyski, M.; Gerischer, H. Underpotential deposition of metals and work function differences. J. Electroanal. Chem. 1974, 54, 25-38.
Leiva, E. P. M. Thermodynamic derivation and model calculations of the metal underpotential dependence on electron work function differences. J. Electroanal. Chem. 1993, 350, 1-14.
López, M. B.; Estiú, G. L.; Castro, E. A.; Arvia, A. J. Theoretical study of underpotential deposition of foreign metal atoms on Pt(111) single-crystal cluster surfaces: Part I. The stability of the different metal underpotential-deposition layer structures as a function of surface coverage and applied electric potential. Journal of Molecular Structure: THEOCHEM 1990, 210, 353-364.
Łosiewicz, B.; Martin, M.; Lebouin, C.; Lasia, A. Kinetics of hydrogen underpotential deposition at ruthenium in acidic solutions. J. Electroanal. Chem. 2010, 649, 198-205.
Machado, S. A. S.; Tanaka, A. A.; Gonzalez, E. R. Underpotential deposition of lead on polycrystalline platinum and its influence on the oxygen reduction reaction. Electrochim. Acta 1994, 39, 2591-2597.
Martins, M. E.; Galindo, M. C.; Arvia, A. J. Bismuth electrodeposition on polycrystalline gold electrodes in acid solutions. An. Quim. 1990, 86, 327-6.
Michell, D.; Rand, D. A. J.; Woods, R. A study of ruthenium electrodes by cyclic voltammetry and X-ray emission spectroscopy. J. Electro. Chem. 1978, 89, 11-27.
55
Quiroz, M. A.; Meas, Y.; Lamy-Pitara, E.; Barbier, J. Characterization of a ruthenium electrode by underpotential deposition of copper. J. Electroanal. Chem. 1983, 157, 165-174.
Quiroz, M. A.; Salgado, L.; Meas, Y. Arrangement of bismuth on a ruthenium electrode. Electrochim. Acta 1988, 33, 435-437.
Salie, G.; Bartels, K. Partial charge transfer and adsorption at metal electrodes. The underpotential deposition of Hg(I), Tl(I), Bi(III) and Cu(II) on polycrystalline gold-electrodes. Electrochim. Acta 1994, 39, 1057-1065.
Sánchez, C.; Leiva, E. Underpotential versus overpotential deposition: a first-principles calculation. J. Electroanal. Chem. 1998, 458, 183-189.
Sánchez, C.; Leiva, E. P. M. Cu underpotential deposition on Au(111) and Au(100). Can this be explained in terms of the energetics of the Cu/Au system? Electrochim. Acta 1999, 45, 691-697.
Sánchez, C. G.; Del Pópolo, M. G.; Leiva, E. P. M. An embedded atom approach to underpotential deposition phenomena. Surf. Sci. 1999, 421, 59-72.
Salles, M. O.; Ruas, d. S.,Ana Paula; Naozuka, J.; Vitoriano, d. O., Pedro; Bertotti, M. Bismuth modified gold microelectrode for Pb(II) determination in wine using alkaline medium. Electroanalysis 2009, 21, 1439-1442.
Schmickler, W. Interfacial Electrochemistry; Oxford University Press, Inc.: New York, NY, 1996.
Stucki, S. Underpotential deposition of metal monolayers as a microprobe for studying composite metal surfaces. J. Electroanal. Chem. 1977, 78, 31-44.
Swathirajan, S.; Bruckenstein, S. Thermodynamics and kinetics of underpotential deposition of metal monolayers on polycrystalline substrates. Electrochim.
Tang, Y.; Furtak, T. E. Study of underpotential deposition of metals using the quartz crystal microbalance. Electrochim. Acta 1991, 36, 1873-1877.
Tucker, C. W., Jr. Structure types in chemisorption found by LEED [low-energy electron diffraction]. Surf. Sci. 1972, 31, 172-179.
Vereecken, P. M.; Searson, P. C. Electrochemical deposition of Bi on GaAs. Proc. - Electrochem. Soc. 2001, 2000-29, 431-440.
Wen, J.; Zhou, Z. Pseudocapacitance characterization of hydrous ruthenium oxide prepared via cyclic voltammetric deposition. Mater. Chem. Phys. 2006, 98, 442-446.
56
Yang, M.; Hu, Z. Electrodeposition of bismuth onto glassy carbon electrodes from nitrate solutions. J Electroanal Chem 2005, 583, 46-55.
Yu, K. K. Study of Copper Electrodeposition on Ruthenium Oxide Surfaces and Bimetallic Corrosion of Copper/Ruthenium in Gallic Acid Solution, University of North Texas, Denton, TX, 2007.
