Campus Monterrey
School of Engineering and Sciences
Study of the electrodeposition of silver nanoparticles on glassy carbon by galvanostatic control
A thesis presented by
Roberto Moreno Hern´andez
Submitted to the
School of Engineering and Sciences
in partial fulfillment of the requirements for the degree of
Master of Science
in
Nanotechnology
Monterrey, Nuevo Le´on, May, 2020
To my parents, for their unconditional support and motivation. You will always be my moti- vation to go ahead and pursue my dreams.
To my brother V´ıctor, for taking care of me and making my life happy during the course of this investigation.
To my friends, especially those that I made during the master program. Thanks you for the shared moments and your support. Without you, my stay in the master’s program would not be the same
iii
Thanks to Tecnol´ogico de Monterrey for the support on tuition and CONACYT for the support for living, that made possible the completion of my master’s studies.
Thanks to Dr. Marcelo Videa for accepting me in his research group, for his teachings, his recommendations and support.
Thanks to Dr. Alfonso Crespo for his support during the execution of the experiments, as well as the valuable discussion on the results.
Thanks to Dr. Oliver Rodriguez for the support provided and introduce me the bases of electrochemistry in a droplet.
iv
glassy carbon by galvanostatic control by
Roberto Moreno Hern´andez Abstract
Electrodeposition of silver nanoparticles from an ammonium complexated silver electrolytic bath was accomplished using a low-resistance capillary cell. The advantage of the proposed methodology is that the electrodeposition is not limited to a specific geometrical shape of the working electrode and only small volumes of electrolyte are required.
The pulse current technique is discussed considering a fixed charge transfer on a glassy carbon electrode, finding that a pulse of 1.70 mA/cm2 generates a high density of small par- ticles with little variability. It was found that the 92±3% of the total duration of the current pulse is used to achieve the stabilization of the potential, which is associated to the particle growth limited by diffusion of the silver ions. The comparison between the galvanostatic and potentiostatic electrodeposition shows that the galvanostatic control generates smaller parti- cles with higher density compared to the potentiostatic pulse. However, the potentiostatic pulse leads to a better distribution on the surface delimited by the droplet.
The morphological characterization was carried out by Scanning Electron Microscope (SEM) revealing that silver follows a progressive nucleation.
v
1.1 Schematic diagram for the equilibrium of a bulk metal crystal with its own
ionic solution. . . 3
1.2 Two-dimensional projection of overlapping diffusion zones around hemispher- ical nuclei on an electrode surface. . . 10
1.3 Non-dimensional plot for (a) instantaneous and (b) progressive nucleation. . . 11
1.4 Schematic representation on different growth models in metal deposition on foreign substrate depending on the binding energy of Meads on S. . . 12
1.5 Models of surface nuclei representing the interaction with the substrated. . . . 13
1.6 Schematic representation of the steps in the electrodeposition process of metals. 16 1.7 SEM images of (a) AuDs on GC, (b) zoom-in of Au-Ds on GC, (c) and (d) of Ag-AuDs. . . 17
1.8 Top view of the AAO template with a diameter of: (a) 70nm and (b)300nm. SEM images of nanowires obtained in templates with nanopores of (c) smaller and (d) larger diameter under constant current. . . 18
1.9 Schematic of the free droplet cell (top) and capillaries fitted with a silicone rubber gasket (bottom). . . 19
1.10 (a) Schematic of the formation of the micro-rod and (b)optical image of a branching Cu micro-rod. . . 20
2.1 Schematic of a conventional 3 electrode cell. . . 25
2.2 First prototype of three-electrode capillary-base droplet cell . . . 27
2.3 Second electrochemical set up . . . 28
2.4 Third prototype . . . 28
vi
magnets, b)two neodymium magnet and NBR o-ring, c)NBR o-ring. . . 29 2.6 Final prototype. . . 30 2.7 Schematic of the order of the site selection for electrodeposition during a se-
ries of electrodeposition experiments. . . 32 2.8 Schematic representation on the zones analyzed for each droplet . . . 35 2.9 CV for a reversible reaction charge-transfer reaction. The plot is presented
using the United States convention. . . 35 2.10 Schematic diagram of a chronopotentiogram for a reversible system O+e−
R . . . 37 2.11 (a)Equivalent circuit for unspecific impedance ZR: solution impedance, ZC:
double-layer impedance, ZF: faradaic impedance. (b) Equivalent circuit for redox reaction with diffusion. . . 38 3.1 Complex plane impedance diagram for glassy carbon in 10 mM K4[Fe(CN)6
+ 1 M KCl solution in different electrochemical cell . . . 41 3.2 Cyclic voltametry of 10 mM [Fe(CN)6]4− on glassy carbon electrode at scan
rate of 20mV/s. . . 43 3.3 Cyclic voltammograms of glassy carbon electrode in 10mM K4[Fe(CN)6] +
1M KCl electrolytic bath at different scan rates. Each experiment was per- formed at the same spot on the GC electrode, with a 10µL droplet. . . 43 3.4 Cyclic voltammograms of polycrystalline gold electrode at the same condi-
tions of the glassy carbon experiments. . . 44 3.5 Peak intensity versus root of scan rate . . . 45 3.6 CV of GC electrode in 0.5M H2SO4at scan rate of 50mV/s prior to use in the
redox couple and after using in the redox couple . . . 46 3.7 Chronopotentiometry showing the anode and cathodic pulses applied in each
experiment . . . 48 3.8 Polarization curves at different current pulses, maintaining a constant charge
of 45 µC. The minimum potential value depends on the intensity of the current pulse . . . 49
vii
(a) experimental data and (b) theoretical model of Isaev and Grishenkova [1] . 50 3.10 Polarization curves at different current pulses. The red asterisks mark the time
at which a charge of 0.23 µC was transferred in the process. . . 51 3.11 Minimum potential versus current density of (A) experimental data and (B)
theoretical model of Isaev and Grishenkova . . . 52 3.12 Numerical derivative of the electrode potential for each current pulse . . . 53 3.13 SEM micrographs of a silver electrodepositions generated from a pulse of
0.57mA/cm2 for 1.5 seconds. The white dots represent the silver particles . . 53 3.14 Particle size distribution plots (left) and SEM micrograph of silver particles at
the center of the droplet generated by (a) and (b) 0.38 mA cm−2 for 2.25 s, (c) and (d) 0.57 mA cm−2for 1.5 s, (e) and (f) 0.75 mA cm−2 for 1.125 s. . . 55 3.15 Particle size distribution plots (left) and SEM micrograph of silver particles at
the center of the droplet generated by (a) and (b) 1.13 mA cm−2 for 0.75 s, (c) and (d) 1.70 mA cm−2for 0.5 s, (e) and (f) 1.88 mA cm−2for 0.45 s. . . 56 3.16 Particle densities obtained at different current pulses. Vertical lines represent
the standard deviation of all the spots analyzed for each experiment at constant charge transfer. . . 57 3.17 Current density versus particle size: (a) inverse of mean equivalent diame-
ter and (b) inverse of radius according to the theoretical model of Isaev and Grishenkova [2]. . . 58 3.18 Circularity of particles according to the current density . . . 59 3.19 Chronocoulmetry (left) and chronopotentiometry on silver droplets. Both ex-
periments were performed at the same conditions . . . 60 3.20 Numerical derivative of the chronocoulmetry data (lines blue and orange).
The black dash line represent the galvanostatic pulse. . . 61 3.21 SEM micrographs of silver particles synthesized by: (a) potential pulse of -
0.3V for 0.5s, (b) current pulse of 100µA for 896ms, (c) potential pulse of -0.3V for 0.25s and (d) current pulse of 100µA for 635ms . . . 62
viii
1.1 Conditions and techniques used for silver electrodeposition . . . 21 1.2 Statistical information of the experiments of each electrodeposition method. . 22 2.1 Parameters used for the silver electrodeposition. The electrode area is 0.05 cm2 33 3.1 Electrochemical parameters of impedance for GC electrode in 10 µL droplet
10 mM K4[Fe(CN)6] + 1M KCl solution . . . 41 3.2 Potentital difference for each scan rate . . . 44 3.3 Potential difference for each scan rate on glassy carbon electrode from from
different electrochemical cells. . . 47 3.4 Statistical information of the experiments for each electrodeposition method . 61 A.1 Acronyms . . . 66
ix
Abstract v
List of Figures viii
List of Tables ix
1 Introduction 1
1.1 Fundamentals of electrocrystallization . . . 1
1.2 Electrochemical Supersaturation and Undersaturation . . . 2
1.3 Nucleation . . . 4
1.3.1 Nucleation rate . . . 7
1.3.2 Instantaneous and progressive nucleation . . . 8
1.4 Electrochemical crystal growth . . . 11
1.4.1 Forms of electrochemical growth . . . 12
1.4.2 Nucleation and growth under controlled current . . . 12
1.5 Electrochemical synthesis of nanoparticles . . . 15
1.5.1 Underpotential deposition . . . 17
1.5.2 Template-assisted electrodeposition . . . 17
1.5.3 Droplet electrodeposition . . . 18
1.5.4 Electrodeposition of silver . . . 20
1.5.5 Potentiostatic and galvanostatic electrodeposition . . . 22
1.6 Hypothesis . . . 22
1.7 Objectives . . . 23
x
2.1 Electrochemical cell . . . 24
2.2 Electrochemical cell design . . . 27
2.2.1 Electrolytic bath . . . 30
2.3 Experiment description . . . 31
2.3.1 Ferrocynanide-Ferricyanide experiments . . . 31
2.3.2 Silver electrodepositions . . . 32
2.4 Characterization techniques . . . 34
2.4.1 Scanning Electron Microscope . . . 34
2.4.2 Cyclic voltammetry (CV) . . . 35
2.4.3 Chronopotentiometry . . . 37
2.4.4 Electrochemical Impedance Spectroscopy . . . 37
3 Results and discussion 40 3.1 Validation of the cell design . . . 40
3.1.1 EIS measurements . . . 40
3.1.2 Cyclic voltammetry results . . . 42
3.2 Silver electrodepositions . . . 47
3.2.1 Pulse current experiments at constant charge . . . 48
3.2.2 Potentiostatic and galvanostatic electrodeposition . . . 59
4 Conclusions 63
A Abbreviations and acronyms 66
Bibliography 73
xi
Introduction
The formation of a new crystalline phase by means of electrodeposition is determined by the mechanisms of nucleation and growth. In this chapter theoretical aspects regarding the nucleation and growth processes during the electrodeposition of metals are addressed. Sub- sequently, the basic aspects of the electrodeposition method are described, as well as the techniques used for the synthesis of silver nanoparticles.
1.1 Fundamentals of electrocrystallization
The term electrocrystallization was coined by Fisher in the 1940s to describe a crystalliza- tion process in which mass transfer is accompanied by charge transfer [3]. However, the first contributions were made by Faraday by stating the laws of electrolysis, which were estab- lished experimentally by means of electrodeposition of Sn, Pb and Sb on Pt, being the study of electrocrystallization the oldest part of experimental electrochemistry [4].
The fundamentals aspects of electrocrystallization of metals are directly related to the problems of nucleation and growth [5]. The competition between growth and nucleation determines the granularity of the deposit. The higher the nucleation rate during deposition, the finer are the crystal grains of the deposit. On the other hand, the forms of the growing crystals determine the general appearance and structure of the deposit [6].
Basic thermodynamic concepts of nucleation and crystal growth were formulated in 1878 by Gibbs in his study ‘On the equilibrium of Heterogeneous Systems’. Later in 1926,
Volmer used a statistical thermodynamic approach to derive a formula for the work of for- mation in the case of three- and two-dimensional nuclei [7]. These concepts were later de- veloped by Kossel, Stranski, and Kaischew and Bercker-D¨oring by introducing statistical and molecular-kinetic approaches[6].
The first observations of the form of electrodeposits under a microscope were recorded in 1905 by Huntington, and the first X-ray examinations were made by Glocker and Kaupp [4]. The discovery of Scanning Tunneling Microscope (STM) and Atomic Force Microscope (AFM) offered new possibilities for in situ studies of the electrocrystallization phenomena down to an atomic level [8].
1.2 Electrochemical Supersaturation and Undersaturation
Lets us consider an electrochemical system consisting of a electrolyte solution of metal ions (Mez) with a valence z and an electrochemical potential ˜µs,∞an infinitely large metal crystal of the same material with an electrochemical potential ˜µc,∞ and an inert foreign substrate, which can be used as a working electrode and whose Galvani potential φwe,∞can be varied by means of an external source. For the purpose of this consideration we assume that the working electrode is polarized to the Galvani potential φc,∞. The temperature T is kept constant. The metal ions Mez of the electrolyte may absorb on the foreign substrate forming adatoms with an electrochemical potential ˜µad,∞.
The state of thermodynamic equilibrium in such system, can be described through the equality of the electrochemical potentials
˜
µs,∞ = ˜µc,∞= ˜µad,∞ (1.1)
where
˜
µs,∞ = µ0s+ kBT ln(as,∞) + zeφs,∞ (1.2)
˜
µc,∞ = µ0c+ zeφc,∞ (1.3)
˜
µad,∞ = µ0ad+ kBT ln(aad,∞) + zeφc,∞ (1.4)
In equations 1.2–1.4 µ0s, µ0c and µ0ad are the standard state chemical potentials of the three species, e is the elementary electric charge and kBis the Boltzmann constant
Making use of the equality ˜µs,∞= ˜µc,∞one obtains the Nernst equation
E∞= E0+kT
ze ln(as,∞) (1.5)
which gives the equilibrium potential E∞ = φc,∞− φs,∞ of a bulk metal crystal dipped in a solution of its ions. The standard potential E0is definied as E0 = (µ0s− µ0c)/ze.
Figure 1.1: Schematic diagram for the equilibrium of a bulk metal crystal with its own ionic solution. Taken from [8]
In order to initiate either the growth of the bulk crystal or the formation of nuclei on the inert foreign substrate it is necessary to supersaturate the parent phase, the electrolyte solution.
This means to increase its electrochemical potential to the value ˜µslarger than that of the bulk new phase, the metal crystal (˜µs > ˜µc,∞). Then its difference ∆˜µ = ˜µs− ˜µc,∞ > 0 defines the electrochemical supersaturation, which is the thermodynamic driving force for the phase transition. In the opposite case, when ∆˜µ = ˜µs − ˜µc,∞ < 0, it defines the electrochemical undersaturation, which, if attained would cause the electrochemical dissolution of the bulk crystal. Thus the solid line in figure 1.1 indicates the stability limits of the infinitely large metal crystal.
The general formula for ∆˜µ that is most frequently used is
∆˜µ = zeη (1.6) where η is the electrochemical overpotential defined as
η = E∞− E (1.7)
or as
η = kBT ze ln
as as,∞
(1.8) Equation 1.7 and 1.8 show that the parent phase can be supersaturated with respect to the bulk crystal in two different ways. The first possibility is to keep the activities of the metal ions constant at their equilibrium value, i.e as,∞, and to polarize the working electrode to a potential more negative than the equilibrium potential. The second possibility to achieve supersaturation of the parent phase is to increase the solution activity to a value larger than the equilibrium value, keeping constant the electrode potential. These processes are marked in figure 1.1 as processes (1) → (P ) and (2) → (P ), respoectively.
The two quantities, η, and ∆˜µ give a measure of the deviation from the state of stable thermodynamic equilibrium. Still, the mere fact that the parent phase is supersaturated does not mean that a phase transition should necessarily occur.
1.3 Nucleation
In the formation and growth of a cluster, two processes are of fundamental importance: (1) the arrival or adsorption of atoms (adatoms) at the surface and, (2) the motion of these adsorbed atoms on the surface. An adatom on the surface of a perfect crystal stays on its surface as an adatom temporarily since its binding energy to the crystal is small.[9]. Therefore, the formation of a new phase, as required in the initial stages of adsorption, is kinetically limited by the specificity of the Gibbs formation energy dependence of a cluster of the new phase on its size N , N being the number of atoms forming the cluster [6].
The Gibbs formation energy, ∆G(N ), of a cluster of N atoms contains two terms.
∆G(N ) = −N ze|η| + Φ(N ) (1.9)
where the first term is related to the transfer of N ions from solution to the crystal phase, and the second term, Φ(N ) represents the excess free energy of the cluster surface due to the creation of new interfaces when a nucleus appears on the electrode surface.
For a 3D nucleation, two cases are considered depending on the size of the cluster. For small clusters, Φ(N ) relates to an increase of the Gibbs energy and can proceed as an energy fluctuation process only. For larger clusters, the first term increases faster than the second with increasing N , and the ∆G(N ) function takes a negative slope. The cluster with size NC where the function ∆G(N ) has its maximum, is called the critical cluster or the nucleus of the new phase. With the maximum condition, d∆G(N )/dN = 0, the probability for further growth of the nuclei is equal to that of its dissolution.
Consider a crystalline cluster of size N formed on a substrate. Its total surface energy is given by
Φ =X
i6=j∗
σiAi+ Aj∗(σj − β) (1.10)
where A represent the area in contact with the substrate, the sub-index i and j∗represent the crystal plane and the contact face respectively, σj∗ is the specific interface energy and β is the adhesion energy. Therefore, the Gibbs formation energy can be redefined as
∆G(N ) = −N ∆ze|η| +X
i6=j∗
σiAi+ Aj∗(σj − β) (1.11)
A differentiation with respect to N is only possible if a relation between Ai, Aj∗ and N exists. This is the case when a given arbitrary geometrical form is considered. The surface area of any given 3D geometrical form is related to its volume by A3 = BV2, where B is a constant depending on the geometry. With A3 = BV2 and V = VmN , the maximum of
∆G(N ) is found at
NC = 8BVm2σ3
27(ze|η|)3 (1.12)
and with the value of NC
∆GC = 4BVm2σ3
27(ze|η|)2 (1.13)
where Vm is the volume occupied by one atom in the crystal lattice and
σ = P
i6=j∗σiAi+ Aj∗(σj − β)
A (1.14)
is the average specific surface energy, which does not depend on the size of the crystal.
For a 2D nucleation, the excess energy Φ(N ) is connected to the formation of the steps edges: Φ =P
iεiδLi, where εi is the specific edge energy and Li is the side length of the 2D crystal.
For any given conservative geometrical form, the length of the peripheral sites Li of a 2D cluster, i.e., the perimeter P = P
iLi is related to its area A by P2 = 4bA. The surface area A, is related to N by A = sN with s the area occupied by one atom on the surface of the cluster so thatP
iLi = 2√ bsN
If εiis constant for all sides considered, or if an averaged value, ε is taken for the specific edge energy,
ε = P
iεiLi
P (1.15)
Therefore, Φ(N ) can be calculated from the perimeter P and given as a function of N : Φ = εP = 2ε√
bsN . With this relation, differentiation of equation 1.9 gives:
NC = bsε2
(ze|η|)2 (1.16)
and
GC = bsε2
ze|η| (1.17)
In both cases, 3D and 2D, NCstrongly depends on the overpotential. The critical nucleus
size can be given by the inscribed radius pC and can be found as a function of |η|:
pC,3D = 2Vmσ
ze|η| and pC,2D = sε
ze|η| (1.18)
This relations are a generalized form of the Gibbs-Thompson (Lord Kelvin) equation applied to the electrochemical case.
1.3.1 Nucleation rate
Classical approach
The nucleation rate J is a probability connected with the energy of formation of the critical cluster, ∆GC which, owing to the creation of the new crystal/solution and crystal/substrate interfaces, is always positive. The probability of a fluctuation connected with an increase of the Gibbs energy ∆G of a system is given in the case of nucleation by
J = k exp
−∆GC kBT
(1.19) where the preexpotential factor A only depends on the supersaturation. It can be applied to 2D and 3D nucleation process. Using equations 1.13 and 1.17 one obtains
J = k3Dexp
− 4BVm2σ3 27(ze|η|)2kBT
(1.20)
J = k2Dexp
− bsε2 ze|η|kBT
(1.21) where k3Dand k2Dcan be treated as constants. However, they contain overpotential-dependent factors as the Zeldovich factor, Γ, with an insignificant overpotential contribution to J and the attachment probability, ω, of one atom to the nucleus, converting the atomic assembly from a cluster in labile thermodynamic equilibrium to a cluster capable of spontaneous growth.
Atomistic approach
In this approach, the value of the formation energy, ∆G(N ), can be calculated using binding energies ψi where ψi is the binding energy of an atom in position i to the cluster, including the interaction between the atom and the substrate.
The excess energy is given by the difference of the binding energy of the cluster includ- ing its interaction with the substrateP ψiand that of n atoms in the bulk of the crystal nψkink, where nψkink is the binding energy of a kink atom (equal to the average binding energy of an atom in the bulk of the crystal). The excess energy nψkink−P ψi is connected with the unsaturated bonds of the atoms on the surface of the cluster and can be identified as a surface energy. Internal strain in the cluster can be included in the calculation of the psii values for every atom individually, or can be extracted from the sum as a property of the ensemble of n atoms in the form εn, where ε is the average strain energy per cluster atom. Then
J = K exp
−ncψkink−P ψi
kBT
exp (nc+ β∗)ze|η|
kBT
exp
− ncε kBT
(1.22)
where β∗denotes the charge transfer coefficient. The value of internal strain ε can be changed by changing the starting potential, especially in the underpotential deposition (UPD) region.
1.3.2 Instantaneous and progressive nucleation
The nucleation law for a uniform probability with time t of conversion of a site on the metal electrode into nuclei is given by
N = N0[1 − exp(−Ant)] (1.23)
where N0 is the total number of sites (the maximum possible number of nuclei on the unit surface) and Anis the nucleation rate constant. This equation has two limiting cases for the initial stages of nucleation (low t value). The first, for large nucleation constant An, the equation reduces to:
N ≈ N0 (1.24) indicating that all electrodes sites are converted to nuclei instantaneously. Thus, this is referred to as instantaneous nucleation.
In the second case, for small Anand short t, the exponential term in equation 1.23 can be represented as a linear approximation
N ≈ AnN0t (1.25)
In this case, the number of nuclei N is a function of time t and the nucleation is called progressive. It is possible to distinguish between these two modes of nucleation experimen- tally, such as the use of potentiostatic current-time transients.
As an example, diffusion-controlled hemispherical growth will be described. Hills, Schiffirin, and Thompson [10] calculated the current of N0 individual nuclei that growth in- dependently of each other. The expression for the potentiostatic time transient are
i = zF π(2Dc)3/2 M ρ
1/2
N0t1/2 (1.26)
for instantaneous nucleation and
i = 2
3zF π(2Dc)3/2 M ρ
1/2
N0Ant3/2 (1.27)
for progressive nucleation, where M and ρ are the molecular weight and the density of the de- positing metal, c and D is the concentration and diffusion coefficient of the electro-depositing specie.
These equations account for the growth of single hemispherical nucleus, i.e. each nu- cleus grows independently of each other; therefore, it is necessary to consider the mutual interplay of nucleation and growth process that occurs during multiple nucleation.
Scharifker and Hills develop a theory that deals with the potentiostatic current tran- sients for 3D nucleation with diffusion-controlled growth. According to this theory, during the growth stage of the deposit, the nuclei develop diffusion zones around themselves. The
area of the diffusion zone projected on the surface can be described by the equation
S = πkDt (1.28)
where k is a numerical constant determined by the conditions of the experiment.
Figure 1.2: Two-dimensional projection of overlapping diffusion zones around hemispherical nuclei on an electrode surface. Taken from[11]
An equation given by Avrami can be used to relate the real surface coverage with differ- ent diffusion zones θ to the surface coverage
θ(t) = exp[−θext(t)] (1.29)
θext being the extended fractional area that would be covered by nuclei or by nucleation exclusion zones if none overlaps. If the growth of the diffusion zones is unlimited, then θext = N πkDt. Conservation of mass requires that the amount of material entering the diffusion zones is equal to the amount being incorporated into the growing nuclei and the current density to the whole electrode surface is therefore
i = zF c D πt
1/2
[1 − exp(N πkDt)] (1.30)
for instantaneous nucleation. Comparison of equation 1.30 at t → 0 with equation 1.26 leads to
k = 8πcM ρ
(1.31)
The equation for the current density as a function time for progressive nucleation is
i = zF c D πt
1/2
1 − exp
−An
2 πk0Dt2
(1.32)
Comparison of equation 1.32 at t → 0 with equation 1.27 gives
k0 = 4 3
8πcM ρ
1/2
(1.33)
Equations 1.30 and 1.32 are used to produce non-dimensional plots to distinguish between these two process in potentiostatic experiments.
Figure 1.3: Non-dimensional plot for (a) instantaneous and (b) progressive nucleation. Taken from [11]
1.4 Electrochemical crystal growth
Three different growth modes (Volmer-Weber, Frank-van der Merwe, and Stranki-Krastanov) can be distinguished, depending on the vertical bending energy between a metal adatom Meads
on a foreign substrate S, and on the crystallographic Me-S misfit as schematically illustrated in figure 1.4
In case of weak adhesion between the substrate and the deposit, 3D metal clusters are formed at supersaturation ˜µ3D > 0, according to the classical Volmer-Weber mechanism. At stronger adhesion, 2D nuclei may form and grow on the foreign substrate at supersaturation
˜
µ2D > 0 following a layer by layer growth mechanism as predicted by Frank and van der
(a) Volmer-Weber (3D island formation) (b) Frank–van der Merwe (layer by layer formation)
(c) Stranski–Krastanow mechanism
Figure 1.4: Schematic representation on different growth models in metal deposition on for- eign substrate depending on the binding energy of Meadson S. Adapted from [12].
Merwe. Depending on the specific substrate-deposit lattice mismatch this type of crystal growth take places only during the deposition of the first few monolayers, which are internally strained. With increasing the thickness of the 2D deposit the influence of the foreign substrate diminishes, the internal strain increases and the process of metal deposition may continue via the formation of 3D nuclei on the top of the two-dimensional monolayers according to the Stranski-Krastanow growth mechanism [13].
1.4.1 Forms of electrochemical growth
When the charge-transfer step in an electrodeposition reaction is fast, the rate of nuclei is determined by either of two steps: (1) the lattice incorporation step or (2) the diffusion of electrodepositing ions in the cluster. Four simple models of nuclei are usually considered: (a) a two-dimensional (2D) cylinder, (b) a three-dimensional (3D) hemisphere, (c) a right-circular cone, and (d) a truncated four-sided pyramid
1.4.2 Nucleation and growth under controlled current
The galvanostatic conditions are quite rarely used to study the initial stages of electrochemical phase formation. Under these conditions, nucleation and growth of the new phase proceed
Figure 1.5: Models of surface nuclei representing the interaction with the substrated.Taken from [9]
at variable supersaturation, which significantly complicates the analysis of this process[1].
The main difficulties appear to arise when the results of a nucleation experiment have to be interpreted.
Isaev and Grishenkova [2, 14] proposed a theoretical model to describe the three-dimensional formation of nuclei and their growth, controlled by the bulk diffusion of ions to the cluster surface. In this model it is assumed that the cluster of the new phase have a hemispherical shape.
The equation for the current balance at the electrode surface can be written as follows:
iT = ic+ if +XIg
s (1.34)
In equation 1.34
ic= Cddη
dt (1.35)
if = zedΓ
dt (1.36)
where icis the current density flowing into the double-layer charging process, if is the current density flowing into accumulation of single adatoms, Ig is the growth current of the cluster, s
is the electrode surface area, Cdis the specific capacity of the double electric layer, Γ is the single adatom (monomer) concentration. Here, the summation of Ig is carried out over all N supercritical clusters at the electrode surface.
To describe the diffusion to the electrode surface, which is free of the clusters, we use Fick’s equation with the following boundary conditions
∂c
∂t = D∂2c
∂x2 (1.37)
c(x, 0) = c(∞, t) = c0 (1.38)
∂c
∂x
x=0
= if(t) zeD
This equation can be solved via Laplace transformation in the form
cs= c0− 1 ze√
πD Z t
0
if(τ )dτ
√t − τ (1.39)
where cs is the concentration of depositing ions at the electrode surface, co is the bulk con- centration of depositing ions. Since the growth of the clusters is assumed to be controlled by diffusion of ions in the electrolyte bulk to the surface, then the equation for the current density of the single cluster hast the form
ig = i0 csr
c0 exp[αf (η − ηp)] − exp[βf (ηp− η)]
(1.40)
where i0 is the exchange current density at the electrolyte/cluster interface; α and β are the transfer coefficient (α + β = 1); ηp is the overpotential at witch the cluster of radius r exists in unstable equilibrium with the electrolyte, ηp = 2σν/zer, σ is the surface tension of the electrolyte/cluster interface; ν is the volume of one atom of new phase. The radius of the critical cluster is equal to rc= 2σν/zeη.
The concentration csris determined by conditions of hemispherical diffusion; in station- ary approximation, we obtain
ig = zeDc0− csr
r (1.41)
The case of pure diffusion control (i0 ig) can be obtained by combining equations 1.40 and 1.41
ig = ze ν
dr
dt = zec0D
r [1 − exp(ηp− η)] (1.42)
To describe the nucleation kinetics, the classical electrochemical nucleation is used.
Therefore, the dependence of the nucleation rate J (t) on the overpotential and the total num- ber N (t) of supercritical clusters formed on the electrode is described by the Volmer equation as
J (t) = K1exp
− K2
η2(t)
(1.43)
N (t) = Z t
0
J (τ )dτ (1.44)
where K1 and K2 are nucleation constants. The nucleation constantn K1 includes the Zel- dovich factor, the frequency at which ions join to the critical cluster and concentration of Z1
monomers, taking part in the nucleation process. This nucleation constants can be determined experimentally by different methods.
1.5 Electrochemical synthesis of nanoparticles
Electrochemical deposition, also know as electrodeposition, is a method for the synthesis of a wide range of nanomaterials that involves the reduction of metallic ions that are gen- erated from the electrolyte and converted into the deposited metallic ions in a cathodic sur- face [15]. Diverse nanostructures such as nanorods, nanowires, nanotubes, nanosheets, den- dritic nanostructures, and composite nanostructures are fabricated easily by electrochemical synthesis[16]. The main advantage of this method is that the nanoparticle gets directly at- tached to the substrate, and in comparison to other techniques, the particle size, crystallo- graphic orientation, mass, thickness, and morphology of the nanostructures materials can be controlled by adjusting the operation conditions and electrolytic bath [17].
This method involves deposition of a metal or alloy coating over a conductive surface
(cathode) by means of electrolysis from a well-formulated electrolyte know as bath, which can be an aqueous solution of a simple salt or a complex salt type. In simple salt solutions, metal ions are presented in bulk solution as hydrated ions; the metal ion when hydrated is represented as M(H2O)z+x where x is the number of water molecules in the primary hydration sheath. The reactions involved in the discharge process of ions under the influence of an electric field are the transport of hydrated ions towards the cathode, the alignment of water molecules in the Helmholtz layer, discharge followed by adsorption of the ions at the cathode surface, surface diffusion and the incorporation of adatoms into the crystal lattice at the growth point[18].
Figure 1.6: Schematic representation of the steps in the electrodeposition process of metals.
Taken from [18].
According to Karatutlu and coworkers[19], electrodeposition can be performed into the following three techniques: (1) potentiostatic; (2) galvanostatic; and (3) pulse plating. In the potentiostatic technique a potential is kept constant while the current density response is recorded as a function of time. In the galvanostatic technique the electrode potential is changed under a constant current. In the pulse plating technique, the potential or current is alternated swiftly between two different values, this result in a series of pulses of equal amplitude duration and polarity [20].
1.5.1 Underpotential deposition
UPD is the deposition of a metal monolayer on a foreign substrate at potentials that can be significantly less negative than for deposition on the same metal surface as the adsorbate [21].
It is possible to deposit a wide variety of metals by UDP, typically in electrodes of gold, silver and platinum.
Lai et al.[22], fabricated a bimetallic substrate for Surface-enhanced Raman spectroscopy (SERS) application. This substrate consists of electrodepositing gold on a glassy carbon (GC) surface from an aqueous solution containing 1 mM HAuCl4, 0.5M H2SO4and 0.1mM cysteine by applying square-wave potential pulses, resulting in a dendritic structure (AuDs). After- wards, silver atoms were electrodeposited in the UPD deposition regime to form monolayers on the gold structure, as shown in figure 1.7.
Figure 1.7: SEM images of (a) AuDs on GC, (b) zoom-in of Au-Ds on GC, (c) and (d) of Ag-AuDs. Taken from [22]
1.5.2 Template-assisted electrodeposition
Template-assisted electrodeposition is a technique for synthesizing metallic nanoparticles with controlled shape and size. Arrays of nanostructured materials with specific arrange- ment can be prepared by this method, employing either an active or restrictive template as a
cathode in an electrochemical cell [23].
Wang et al. [24] fabricated organic semiconductor nanowire arrays on anodic aluminium oxide (AAO) template by using AgTCNQ (TCNQ: 7,7,8,8-tetracyanoquinodimethane). The AAO template possesses nanopore arrays with uniform length, diameter and interspacing dis- tance, which can confine the growth of nanostructures in the nanopore array. Under constant voltage, discontinuous Ag nanowires are achieved. Only at low constant current densities or by decreasing the concentration of the electrolyte can nanowires be formed. This is because the reduction process slows down under these conditions, allowing the electrolyte to diffuse into the nanopores.
Figure 1.8: Top view of the AAO template with a diameter of: (a) 70nm and (b)300nm.
SEM images of nanowires obtained in templates with nanopores of (c) smaller and (d) larger diameter under constant current. Adapted from [24]
1.5.3 Droplet electrodeposition
Capillary-based droplet cells are versatile tools for spatially resolved electrochemical investi- gation of metallic surfaces. Besides local resolution, this method has several advantages[25]:
• The sample needs no preparations. Investigations can start immediately
• Only the investigated surface is wetted. The other parts of the sample remain virgin
• The mechanical stress applied to the sample is small
• The shape of the sample can be random. Only small areas with a diameter of some 10 µm must be flat.
• Only small amounts of electrolyte are needed
• Flow-through concepts are possible
This method consists of an electrolyte droplet dispensed and positioned on the surface by a capillary. The droplet between the mouth of the capillary and the surface is held by its own surface tension. The wetted circular area forms the working electrode (WE) and the capillary (made of glass or any other material) contains reference and counter electrode (CE).
In some cases, a gasket ring can be prepared at the mouth of the capillary to enclose the electrolyte droplet, as shown in figure 1.9. This configuration leads to very small ohmic drop due to the small WE-CE distance and enables large currents densities up to 100 A cm−2 for short times[26]. The capillary can be equipped with a XYZ-stage together with a electrolyte supply to scan the surface or make local modifications. In scanning mode the droplet is slowly shifted across the surface.
Figure 1.9: Schematic of the free droplet cell (top) and capillaries fitted with a silicone rubber gasket (bottom). Taken from [26]
Sakairi and his coworkers[27] used a coaxial capillary solution flow drop cell to make
Cu micro-rods by galvanostatic electrodeposition. The concept is similar to that presented by Lohrengel et al. in which the capillary tube is continuously supplied with electrolyte. The substrate is mounted in a pulse stage controlled by computer-XYZ. As shown in figure 1.10 this system is capable to make branches over the micro-rod.
Figure 1.10: (a) Schematic of the formation of the micro-rod and (b)optical image of a branch- ing Cu micro-rod. Adapted from [27]
1.5.4 Electrodeposition of silver
Table 1.1 presents some electrolyte compositions and techniques used to make silver elec- trodepositions. In general, the methodology used for the synthesis of silver nanoparticles is to perform a cyclic voltammetry (CV) to obtain information on the kinetics of silver reduction and oxidation processes, which are dependent on the substrate used and the composition of the electrolyte. Through an optimization process, the best experimental parameters are estab- lished to synthesize the silver nanoparticles for the desired application through potentiostatic techniques: chronoamperometry (CA) or double pulse potentiostatic (DPP).
Table 1.1: Conditions and techniques used for silver electrodeposition
Substrate Electrolyte Technique Structure Ref.
Glassy Carbon
AgNO3
1M NH4OH 1M KNO3
CV, CA Small clusters [28]
Carbon electrodes
10mM AgNO3 1M NH4OH 1M KNO3
CV, CA Not specified [29]
Glassy Carbon
2.2mM AgNO3 1M KCl
NH4OH (pH=11)
CV, CA Not specified [30]
Glassy Carbon
20mM AgNO3
PEG-400/H2O (1:1, v:v)
CA at -0.8V vs quasi silver
reference electrode
Nanodentrites [31]
Glassy carbon modified with ZnO film
1mM AgNO3
0.1M KNO3 DPP Nanoparticles
(NP) [32]
Glassy Carbon 5mM AgNO3 0.1M KNO3
CV, CA at -0.15 V
vs Ag| AgCl Dendritic [33]
Glassy Carbon 20mM AgBF4
in [BMIm][BF4]
CV, CA at -0.80V vs quasi silver reference electrode
Microparticles (100-300nm) [33]
Indium Tin Oxide (ITO)
0.05mM AgNO3 0.2mM sodium citrate
0.1M KNO3
CA, DPP NP [34]
ITO
0.05mM AgNO3 0.2mM sodium citrate
0.1M KNO3
DPP Nanoflowers [35]
Highly Oriented Pyrolitc Graphite
1mM AgClO4 0.5M HClO4
Picolinic acid
CV, CA or DPP Dendritic [36]
Glassy Carbon
AgNO3 in Britton-Robison (pH=2.0)
CA NP [37]
1.5.5 Potentiostatic and galvanostatic electrodeposition
Some advantages of galvanostatic electrodeposition compared to potentiostatic electrodepo- sition are discussed below.
Martinez et al. [38], performed nickel electrodepositions by two different techniques:
galvanostatic and double potentiostatic pulse. Their results show that the galvanostatic method presents a greater control in the generation of nickel deposits, because a higher density of smaller particles are obtained compared with the potentiostatic method. A small summary of their statistical analysis is presented in table 1.2
Table 1.2: Statistical information of the experiments of each electrodeposition method. Taken from [38]
Electrodepostion
method Density of particles Mean equivalent diameter of particles
Standard deviation of equivalent diameter of particles
Potentiostatic (DPP) 0.43/µm2 205nm 82nm
Galvanostatic 0.83/µm2 162nm 53nm
Ali et al. [39], made a comparison study of the potentiostatic and galvanostatic elec- trodeposition of manganese oxide for supercapacitor application which crucially depends on the particle size, surface area and porosity. Both techniques were applied for 30 min. They conclude that deposits synthesized by galvanostatic method is more suitable to be applied as supercapacitor electrode, due to the smaller crystalline size, layered structure with less com- pact nanosheets, higher surface area and wider band gap as compared with the potentiostatic technique.
1.6 Hypothesis
The interaction between the substrate surface and the deposit plays a crucial role in determin- ing nucleation and growth processes. On the other hand, electrochemical reactions are greatly affected by the electrode area, ohmic drop and mass transport.
The size reduction of electrochemical systems brings down the ohmic drop and improves mass transport, where the size of the electrode determines the spatial resolution of the deposit.
As the area decreases, smaller current densities are achievable. Therefore a greater efficiency may be obtained in the number and quality of the deposits, which are essential for the de- scription of the nucleation and growth processes in electrocrystallization in systems such as silver.
1.7 Objectives
As it has been stated above, the nucleation and growth processes during electrodeposition strongly depend on the overpotential. Said variable is a fixed value in potentiostatic exper- iments, influencing the morphological characteristics of the deposit. On the other hand, the galvanostatic control during the electrocrystallization allows the system to freely adjust the appropriate potential value at the surface of the electrode, following the conditions of the sub- strate and the electrolyte to accomodate the imposed electron transfer kinetics, which allows an efficient control on the size of the metallic particles as well as on the density of deposited particles.
In this work, the systematic study of polarization curves through galvanostatic control is proposed, taking as reference the theoretical model proposed by Isaev and Grishenkova.
This research is based on the use of a low-resistance capillary cell, specifically built for this purpose, which may allow greater efficiency in the production of experimental data as the ba- sis for the description and understanding of electrochemical nucleation and growth processes.
The results acquired will provide sufficient experimental data to draw relations between the experimental settings and the morphological characteristics of the deposits using silver as a model.
Methodology
This chapter will describe the procedures and materials used for the construction of the droplet-capillary cell of three-electrodes, as well as its optimization process. Subsequently, the description of the experiments carried out and their purpose is presented. Lastly, a brief introduction to the electroanalytical techniques used as well the characterization techniques is presented.
2.1 Electrochemical cell
An electrochemical cell is the system used to study the electrochemical process. The classic system is composed of two electrodes immersed in the electrolyte. Depending on the opera- tion mode, a convention is required in order to identify the function of each electrode and the reactions that occur on them. Therefore, the electrode where the oxidation reaction occurs is called the anode, and the place where the reduction reaction occurs is called the cathode [40].
The cell reaction will only occur spontaneously if the free energy change associated with the net cell reaction is negative
∆G = −nF (Ecathodic− Eanodic) (2.1)
If the free energy of reaction is positive, it will be necessary to supply energy by applying a potential between the two electrodes. Hence the total cell voltage U , required to bring about
24
chemical change is given by
U (i) = U0(i) ± ηCE(i) ± iR ± ηWE(i) (2.2)
where U0(i) is the theoretical cell voltage at i = 0, ηCE(i) is the change of potential across the electrode/solution interface of the Counter Electrode (CE) and ηWE(i) is the change of potential across the electrode/solution interface at the Working Electrode (WE) the term iR represent the voltage drop across the solution resistance.
Ideally, the I-U response is intended to be characteristic of the processes that occurs at one of the electrodes (the WE). For this purpose, we seek to minimize the change of potential at the CE. To achieve this, the use of a CE with a surface area larger than that of the WE so that no serious polarization of the CE can occur. In practice, the surface of the CE has to be at least five times that of the WE. In addition, a third electrode, the Reference Electrode (RE), is placed close to the WE. In that way, any change in the potential of the WE is compared with the RE, which carries practically no current. Thereby, the change in potential between the RE and the WE is equal to the change in potential of the WE. Therefore, the value of the WE potential is given by
U (i) = U0(i = 0) ± iR ± ηWE(i) (2.3)
Figure 2.1: Schematic of a conventional 3 electrode cell. Adapted from [40]
Three-electrode cells are the most commonly used set up in electrochemical studies.
The WE is where the reaction of interest occur. The CE serves to maintain the current flow in
the circuit formed with the WE. According to Caton [41], an ideal reference electrode (RE) should possess the following properties:
• Have a stable potential
• Meet the demands of a charge transfer imposed by the measuring instrument without changing its potential i.e. be non-polarizable
• Return to its fixed reference potential after accidental polarization
• Obey the Nernst equation for some species in solution
• If the electrode is of the second kind, the solid compound must be only sparingly soluble in the electrolyte
Some common reference electrodes are listed bellow:
Silver-silver chloride
AgCl(s)+ e− Ag(s)+ Cl−(aq) (2.4)
Saturated Calomel Electrode (SCE)
Hg2Cl2+ 2e− 2Hg(liq)+ 2Cl−(aq) (2.5)
Mercury Sulfate Reference Electrode (MSRE)
Hg2SO4(s)+ 2e− 2Hg(liq)+ SO−4(aq) (2.6)
The choice of the RE will depend on the experimental conditions, among other on the current applied, the nature and composition of the electrolyte (aqueous or non-aqueous solutions) and temperature.
By reducing the size of the electrochemical cell, it becomes challenging to incorporate a good reference electrode. For such situations it is convenient to use a pseudo-reference electrode (PRE). Some examples are platinum or silver or Ag|AgCl wires. The essential
difference between a real RE and a PRE is the lack of thermodynamic equilibrium. The ad- vantages of the use of pseudo-reference electrodes are their simplicity, small ohmic resistance and abscence of junction potential [42].
2.2 Electrochemical cell design
Electrodeposition experiments were carried out in a capillary-based droplet cell, aided with a manual linear xz stage. For all the prototypes, the WE is a slab of glassy carbon (GC), held by a metallic clip. Figure 2.2 shows an electrochemical system consisting on a capillary tube
Figure 2.2: First prototype of three-electrode capillary-base droplet cell
of 0.6 mm internal diameter and 1.4 mm outer diameter. A platinum wire (0.25 mm diameter) was used as a CE and placed at the lower end of the capillary tube. A silver wire (0.5 mm diameter) was used as pseudo-reference electrode. Half of this arrangement was inserted into a thicker capillary tube to avoid damaging the system during handling.
The second arrangement, shown in figure 2.3 consisted of a platinum mesh-gauze (25 mm
× 25 mm) as a CE and a silver wire as a pseudo-reference electrode. To avoid a short circuit due to physical contact between these electrodes, the silver wire is inserted inside a capillary
(a) Second prototype (b) Close-up of the hole made on the capillary
Figure 2.3: Second electrochemical set up
tube and the CE is wrapped in the capillary tube. This arrangement is placed inside a second tube which is connected with a micropipette tip. Because the electrodes are separated by the capillary tube, a small hole was made on the surface of the capillary to ensure the contact via electrolyte of the silver wire and the platinum mesh-gauze.
Figure 2.4: Third prototype
The third electrochemical cell (figure 2.4) consisted of a platinum wire 4.0 cm long and 0.25 mm in diameter as a CE and a silver wire 11.0 cm long and 0.50 mm diameter as a PRE.
A Teflon cylinder 80 mm in diameter was used to seal the bottom end of a glass tube. A 1 mm diameter hole was drilled in the center of the Teflon rod, in which a 0.6 mm internal diameter capillary tube was inserted. A 120 mm diameter Teflon bar was used to seal the opposite end of the glass tube. Two holes of 1mm diameter each were made to insert the previously mentioned electrodes. To maintain the seal, the electrodes were wrapped in Parafilm to a thickness of 1mm.
Figure 2.5: Fourth prototype with drop confined by: a)Teflon bar and two neodymium mag- nets, b)two neodymium magnet and NBR o-ring, c)NBR o-ring.
The prototype cell shown in figure 2.5 was mainly designed to avoid the evaporation process. The cell consisted of a 22 mm long capillary tube. A silver wire of 0.5 mm diam- eter was coiled at the mouth of the capillary tube and a platinum wire was placed inside the capillary tube. The capillary was filled with a micropipette and inserted into the confinement system with a manual positioner. Different materials for the confinement system were tested:
(a) a rectangular shaped 20 mm×10 mm×2 mm N35 grade neodymium magnet was placed below the wooden platform. A hole was drilled in a 3 mm diameter Teflon rod, which was lined up with another ring-shaped N35 grade neodymium magnet with 3 mm internal diame- ter, 10 mm external diameter. To avoid the displacement of these two pieces, the magnet was
wrapped in Parafilm. 4 µL droplets were placed inside the Teflon bar, (b) the Teflon bar was substituted for a nitrile butadiene rubber (NBR) o-ring 1/8 inches internal diameter and 1/4 inches external diameter, (c) only the NBR o-ring was used. In all cases only four experiments per piece were performed.
(a) Experimental set up (b) Schematic of the electrochemical cell
Figure 2.6: Final prototype.
The capillary-base droplet cell shown in figure 2.6 consists of two platinum wires 20.0 mm long and 0.25 mm in diameter, one of which is inserted into the capillary tube fulfilling the function of CE and the other wire placed around the tip of the capillary tube (internal diame- ter of 0.48 mm) serving as PRE, which is held at the tip of the capillary tube with a piece of Parafilm. The external diameter of the system including the platinum wire is 2.6 mm. Close to 7 µL are needed to fill the electrochemical cell and 3 µL to form the droplet. This electro- chemical cell is held in a self-closing clamp secured with two NBR o-rings.
2.2.1 Electrolytic bath
For silver electrodepositions, the salt used as precursor was silver nitrate (AgNO3) from Fisher Scientific in a concentration of 10 mM in aqueous solution. To enhance the conductivity, potassium nitrate (KNO3) was used as support electrolyte at 1 M (Baker Chemical Co) and ammonium hydroxide (NH4OH) at 1.6 M to bring the pH to 11 (CTR Scientific).
For ferrocyanide redox studies, the salt used was potassium ferrocyanide (K4Fe(CN)6) in
a concentration of 10 mM (Baker Chemical Co) with 1 M potassium chloride (CTR Scientific) as supporting electrolyte. On the other hand, the activation of the GC was perform on sulfuric acid (H2SO4) (Fermont), in a concentration of 500 mM.
2.3 Experiment description
The surface of the GC was polished with 0.3 and 0.1µm alumina/water suspension on a polish- ing cloth for 15 minutes and rinsed in deionized water with a resistivity of 18.2 MΩ cm after each polishing process. Afterwards, the GC was immersed in acetone, isopropanol, ethanol an deionized water for 10 minutes each in a Fisher Scientific FS20 Ultrasonic Cleaner, then the GC was rinsed with deionized water. Before each experiment, a conventional three electrode electrochemical cell was used to perform the activation of the GC. This cell consisted in a platinum coil as a CE and a commercial MSRE. A three-hole Teflon cap was used to keep the electrodes fixed, in addition to facilitating their immersion in the electrolytic bath. The electrode surface was electrochemically pretreated (activated) by polarizing the GC by cyclic voltammetry, with 20 cycles in 0.5 M H2SO4in the potential range of −0.8 to 0.8 V vs MSRE at a scan rate of 200 mV/s and 20 cycles at 100 mV/s.
A polycristalline gold electrode (provided by MTL-MIT and produced by Scott Poesse) was electrochemical cleaned by cyclic voltammetry in 0.5 M H2SO4at a scan rate of 100 mV/s for 15 cycles and 50 mV/s for 15 cycles.
All the electrolytes were bubbled with nitrogen gas for 5 min before measurements. The experiments were carried out in a CHI Instruments 760D potentiostat/galvanostat equipped with a Faraday cage.
For the droplet system, a home-made linear stage with two stepper motors Nema 17 was used to facilitate the manipulation of the droplets on the WE.
2.3.1 Ferrocynanide-Ferricyanide experiments
The ferrocyanide solution was used to validate the functionality of the cell construction and to observe effects introduced by the geometry of the different electrochemical cells proposed.
CV experiments were performed in the ranges of [−0.1 V to 0.6 V] vs. silver wire, at different
scan rates.
2.3.2 Silver electrodepositions
The methodology used consisted of assembling the cell shown in figure 2.6. Using a mi- cropipette, the capillary tube was filled from the tip until the electrolyte rose to the opposite end of the capillary. Approximately 7 µL was needed to fill the capillary tube. Likewise, the micropipette was used to control the volume of each droplet. The process consisted in ex- tracting 3 µL from the silver solution and getting close to the tip of the capillary tube, which was previously mounted in the manual positioner over the surface of the electrode to form the droplet. Subsequently, the WE is slowly raised towards the droplet until a contact is generated.
After performing the local electrodeposition, the WE descends to separate from the capillary tip, causing the droplet to remain on the electrode surface. Subsequently, a new 3 µL droplet was placed again with the micropipette at a new site chosen to carry out the next experiment, repeating the described process. The pattern followed is shown in figure 2.7
Figure 2.7: Schematic of the order of the site selection for electrodeposition during a series of electrodeposition experiments.
Cyclic voltammetry
A pre-conditioning polarization at 0.3 V vs Ag wire was applied for 30 seconds. CV was performed in the range of −0.6 V to 0.6 V vs Ag wire at a scan rate of 20 mV/s. The pur- pose of this experiment was to determine the appropriate range to work with galvanostatic
electrodeposition.
Pulse current experiments at constant transferred charge
Different current intensities were applied, at pulse duration was adjusted to maintain a con- stant charge transfer of 45 µC. The parameters used are shown in the table 2.3.2. These exper- iments are aimed to evaluate the efficiency of each process, that is, which process generates the largest amount of particles over unit area. It should be noted that surface imperfections, such as cavities or local variations in the roughness of the electrode, can alter the results introducing a variable that cannot be controlled but must be considered.
Table 2.1: Parameters used for the silver electrodeposition. The electrode area is 0.05 cm2 Applied current
density (mA/cm2) Time elapse (ms)
0.19 4500
0.38 2250
0.57 1500
0.75 1125
0.94 900
1.13 750
1.41 600
1.70 500
1.88 450
Comparison between potentiostatic and galvanostatic pulse
To verify the efficiency of proposed electrochemical cell under the galvanostatic control, and to achieve the production of more uniform deposits than the potentiostatic control, the chrono- coulometry technique (potentiostatic), which records the cumulative charge to a constant po- tential, was compared to the chronopotentiometry technique (constant current). Two experi- ments were performed at a constant potential of −0.3 V vs Pt wire, with durations of 0.5 and 0.25 seconds, which is enough to generate small particles, since longer pulses tend to generate larger particles. Also, experiments at a constant current of 100 µA and pulse times of 0.896 s
and 0.635 s were applied to maintain the charge transferred in each applied pulse at constant potential.
2.4 Characterization techniques
2.4.1 Scanning Electron Microscope
The deposits were inspected using a Phenom World Pro X Scanning Electron Microscope equipped with an Energy Dispersive Spectrophotometer (EDS) for elemental analysis. Elec- trons can be accelerated through the vacuum chamber at 5kV and 10kV for high resolution images and 15kV for higher resolution images. This SEM has a backscattered electron de- tector (BSD) to generate the digital images of the sample. This detector allows to identify samples according to the atomic number, therefore a better contrast (Z contrast) is achieved when the difference in the atomic mass between the sample and the deposits is larger.
EDS is a technique that analyzes X-rays generated by the bombardment of the sample by the electron beam. Together with elemental identification software, it allows the elemental analysis of any point in the sample.
Inspection of each silver electrodeposition consisted of analyzing three different areas, as shown in figure 2.8, one at the center of the droplet and the other two correspond to the inner ring of the droplet (between the center and the edge of the droplet). For particle distribution, ImageJ software was used to obtain information about the area and the circularity for each particle. On the other hand, a MATLAB script was used to calculate the mean equivalent diameter, the standard deviation and the mean circularity for each micrograph obtained at 20000X and 45000X magnifications, which have an approximate area of 179.56 µm2 and 36 µm2 respectively. Due to the non-uniform distribution of the silver particles, the values reported correspond to the average of the analyzed sites for each electrodeposition, where the standard deviation correspond to the morphological differences of the particles according to their position.
Figure 2.8: Schematic representation on the zones analyzed for each droplet
2.4.2 Cyclic voltammetry (CV)
CV is an electrochemical based on a linear variation of applied the potential to the working electrode (WE) starting at an initial value E1. After reaching a potential E2, the sweep is reversed and the potential returns linearly to E1. The current response is plotted as a function of the applied potential, as shown in figure 2.9.
Figure 2.9: CV for a reversible reaction charge-transfer reaction. The plot is presented using the United States convention. Taken from [43]
Consider a simple redox process of the form O + e− R. As the potential is scanned positively, the concentration of R is steadily depleted near the electrode as it is oxidized to O.
At the potential at which an anodic peak current (iac) is observed, the current is dictated by the delivery of additional R via diffusion from the bulk solution. The volume of solution at
