Effects of microwave radiation on electrodeposition processes at tin-doped indium oxide (ITO) electrodes

Article abstract of DOI:10.1016/j.electacta.2009.06.062

In situ microwave activation is investigated for the electrodeposition of a metal (gold) and for a metal oxide (hydrous Ti(IV) oxide) onto tin-doped indium oxide (ITO) film electrodes. It is demonstrated that localized microwave heating of the ITO film ca

Full text of DOI:10.1016/j.electacta.2009.06.062

Electrochimica Acta 54 (2009) 6680–6685
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effects of microwave radiation on electrodeposition processes at tin-doped
indium oxide (ITO) electrodes
Liza Rassaeia, Elena Vigilb, Robert W. Frencha, Mary F. Mahona, Richard G. Comptonc, Frank Markena,∗
a Department of Chemistry, University of Bath, Bath BA2 7AY, UK
b Facultad de Fisica, Insituto de Ciencia y Tecnologia de Materiales, Universidad de La Habana, Ciudad Habana 10400, Cuba
c Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2009
Received in revised form 12 June 2009
Accepted 20 June 2009
Available online 28 June 2009
In situ microwave activation is investigated for the electrodeposition of a metal (gold) and for a metal
oxide (hydrous Ti(IV) oxide) onto tin-doped indium oxide (ITO) film electrodes. It is demonstrated that
localized microwave heating of the ITO film can be exploited to affect electrodeposition processes.
3−/4−
The electrochemically reversible and temperature sensitive one-electron redox system Fe(CN)6
was employed in aqueous solution in order to calibrate the average surface temperature at the ITO film
electrode. In the presence of microwave radiation the average electrode surface temperature reached ca.
363 K whereas under the same conditions the bulk solution temperature reached ca. 313 K. Therefore
localized heating of the ITO film appears to be important.
The rate of electrodeposition of gold from an aqueous 1 mM tetrachloroaurate(III) solution in 0.1 M KCl
(adjusted to pH 2) is enhanced by microwave activation. However, the morphology of deposits remains
un-effected. Hydrous titanium (IV) oxide films were electrodeposited from an aqueous solution of 1 mM
TiCl3 in 0.1 M acetate buffer pH 4.7. Dense films with blocking character were obtained with conventional
heating but a fibrous more open deposit forms in the presence of microwaves.
Keywords:
Microwave activation
Voltammetry
Electrodeposition
Titanium dioxide
Gold
Electroplating
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
study explores the effect of microwave radiation on electrochemical
processes at tin-doped indium oxide (ITO) thin film electrodes. The
Microwave activation in chemistry has been employed success-
fully in a wide range of applications in materials chemistry [1],
in organic [2] or inorganic [3] chemical synthesis, in analytical
chemistry [4], and more recently in analytical electrochemistry [5].
Microwave radiation provides a unique tool to guide energy into
localized reaction zones and to facilitate chemical processes [6]. In
electrochemistry thermal effects are important [7] and localized
heating has been investigated with conventional heating [8], for
Rf-heated “hot wire” electrodes [9], Rf-heated microelectrodes [10]
and channel flow systems [11], laser heating with front [12] or back
[13] exposure, and for microwave heating [14].
Microwave activation experiments in electrochemical sys-
tems have usually been restricted to applications using focused
microwave fields with heating effects localized only at the tip of
microelectrodes [15]. The benefits of this experimental arrange-
ment are fast on–off switching, negligible bulk heating, low power
requirements, and safe operation. However, often electrochemical
processes require larger electrodes and in particular electroplat-
ing processes require extended areas or more complex shapes. This
absorption of microwave radiation by charge carriers in semicon-
ductor or degenerate semiconductor materials is well known [16]
and exploited here to create a localized heating effect. The deposi-
tion of a metal (gold) and the deposition of a metal oxide (hydrous
Ti(IV) oxide) are employed as model cases.
2. Experimental details
2.1. Chemical reagents
Chemicals were obtained commercially in analytical grade and
used without further purification. Tetrachloroauric(III) acid, potas-
sium hydroxide, K4(Fe(CN)6, K3Fe(CN)6, and potassium chloride
were purchased from Aldrich. A 15% solution of TiCl3 in aqueous 10%
hydrochloric acid was obtained from Merck. Deionized and filtered
water of resistivity not less than 18 Mꢀ cm (at 20 ◦C) was taken from
an Elga water purification system. Argon (BOC, UK) was employed
for de-aeration of the electrolyte solutions.
2.2. Instrumentation
∗
Electrochemical experiments were performed in a conven-
tional three-electrode flow cell system using a micro-Autolab III
Corresponding author. Tel.: +44 1225 383694; fax: +44 1225 386231.
E-mail address: F.Marken@bath.ac.uk (F. Marken).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.06.062

L. Rassaei et al. / Electrochimica Acta 54 (2009) 6680–6685
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Fig. 2. Plot of (i) the average electrode temperature, Telectrode (filled squares, esti-
mated from the equilibrium potential in aqueous solution of 5 mM Fe(CN)63−/5 mM
Fe(CN)6 in 0.1 M KCl) and (ii) the cell outlet temperature (empty squares) vs. the
applied microwave intensity (as magnetron anode current).
Fig. 1. Schematic drawing of the custom-made 10 cm3 electrochemical glass cell (b)
placed through a port (a) into the multi-mode microwave system. A flow system
driven with a peristaltic pump (28 cm3 min−1) was employed to pump electrolyte
solution from a reservoir through the reference connection (d), through the cell,
and out through the counter electrode (e) into the collection vessel. The working
electrode (c) with an ITO slide attached (f) was placed from the top into the cell.
4−
netron anode current) and a calibration graph determined (see
Fig. 2(i)). During microwave experiments a considerable level of
bulk heating was observed and in order to distinguish bulk heat-
ing from electrode surface heating also the outlet temperature of
electrolyte solution leaving the electrochemical cell was monitored
(see plot in Fig. 2(ii)).
system (Eco Chemie, NL). A saturated calomel electrode (SCE,
Radiometer) was used as the reference electrode (upstream) and
a platinum gauze electrode as the counter electrode (downstream).
The working electrode was prepared from ITO-coated glass (tin-
doped indium oxide films sputter coated onto glass, active area
10 mm × 10 mm, resistivity 15 ꢀ per square, ITO film thickness ca.
80 nm) obtained from Image Optics Components Ltd. (Basildon,
Essex, UK). The ITO electrodes were rinsed with acetone and water,
heat treated in a tube furnace (Elite Thermal Systems Ltd.) for 1 h
at 500 ◦C, and re-equilibrated to ambient conditions before use.
Voltammograms were recorded in staircase mode (0.67 mV step
potential). The morphology of deposited mesoporous Au or TiO2
films were characterized by scanning electron microscopy (SEM)
with a JEOL JSM6480LV system. Samples were gold sputter coated
prior the experiments. Powder diffraction data were recorded on a
Bruker D8 diffractometer fitted with Goebal mirrors and a 0.2 mm
beam slit.
2.4. Electrodeposition procedures
2.4.1. Gold electrodeposition
Gold(III) chloride hydrate was dissolved in 0.1 M KCl in deionized
water and then the pH was adjusted to approximately 2 by addition
of 1 M HCl. For continuous gold deposition, a potential of 0.4 V vs.
SCE was applied for a duration of 900 s.
2.4.2. Hydrous Ti(IV) oxide electrodeposition
The electrodeposition solution was an aqueous 1 mM TiCl3
prepared by dilution of a 15% solution of TiCl3 in aqueous 10%
hydrochloric acid. The pH of the solution was adjusted to 4.7 by
addition of 0.1 M sodium acetate solution. The Ti(III) solution was
kept under an atmosphere of argon in order to prevent the oxida-
tion to Ti(IV) by oxygen. Fresh solutions were prepared to minimise
the formation of colloid. Voltammetric scans were recorded (vide
infra) and anodic deposition was observed over the entire potential
range. Therefore the deposition of hydrous Ti(IV) oxide was carried
out by continuously recorded cyclic voltammograms with a scan
rate of 50 mV s−1 over a −0.7 to +1.2 V vs. SCE potential range and
for a specified number of potential cycles.
For microwave activation experiments,
a
multi-mode
microwave oven (Panasonic NN-3456, 2.45 GHz) with modi-
fied smooth power supply, a water energy sink, and a port for
the electrochemical cell was used [17]. The microwave intensity
was controlled via the anode current of the magnetron. A fast
flowing electrolyte solution circulated with a peristaltic pump
(Watson–Marlow, 28 cm3 min−1
) ensured stable temperature
conditions during experiments. The working electrode (Fig. 1) con-
sisted of an ITO slide with 1 cm2 exposed surface area connected
to a 200 ␮m diameter insulated copper wire with a silver epoxy
contact. Special care is required when inserting metal objects into
microwave systems and experiments should always be conducted at
low power. Before and during operation, the system was monitored
for leaking microwave radiation with an Apollo radiation meter.
For comparison additional experiments were conducted in a
thermostated water bath (Haake) employing the same electro-
chemical cell but with no flow to allow temperature equilibration.
3. Results and discussion
3.1. Microwave-induced effects on the electrochemical
3−/4−
reduction/oxidation of Fe(CN)6
(ITO) electrode
at a tin-doped indium oxide
The effect of applied microwave radiation on the mass transport
and temperature conditions at solution-immersed metal electrodes
with a diameter of less than 1 mm has been studied [5] and the size
of the electrode has been shown to strongly affect local heating
effects [19]. The volume of liquid directly affected by the focused
microwave field at the tip of the metal electrode scales approxi-
matelywithr3 (whereristheelectroderadius)andthereforeenergy
requirements are hugely dependent on electrode diameter. With an
electrode diameter bigger than 1 mm, the level of microwave power
required to induce thermal effects is very high and the focusing
effect induced by the metal electrode becomes insufficient when
compared to the bulk liquid heating effect for aqueous electrolyte
media. However, for thin film electrodes with high surface area
2.3. Temperature calibration procedure
The average temperature at the ITO electrode surface, Telectrode
,
was determined based on the shift in equilibrium potential for an
4−
aqueous solution containing 5 mM Fe(CN)63− and 5 mM Fe(CN)6
in0.1 M KCl. First theequilibriumpotentialwasmonitoredasafunc-
tion of temperature during conventional heating in a thermostated
cell giving a temperature coefficient dE/dT = −1.54 mV K−1 as
reported in the literature [18]. The shift in equilibrium potential was
then measured as a function of applied microwave power (mag-

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L. Rassaei et al. / Electrochimica Acta 54 (2009) 6680–6685
by the peristaltic pump system which is required to maintain uni-
form temperature conditions during experiments.
Upon application of microwave radiation to the electrochemical
system, a gradual increase of currents with microwave intensity
is observed (see Fig. 3D). Fig. 3C shows a typical cyclic voltam-
mogram obtained with continuous microwave heating. The peak
features have disappeared and a well-defined steady state current
response is observed with a characteristic shift in the reversible
potential towards more negative values (due to the increase in tem-
perature, vide supra). The response time of the electrode system to
microwave pulses provides a measure of the zone of liquid that is
heated. For small electrodes very short temperature rise times are
obtained [17] but for larger electrodes a much slower response is
expected. Fig. 3B shows a voltammogram recorded in the presence
of 4 s microwave pulses. The decay of current during the “off” period
does not reach the room temperature current limit and the rise
in current during the “on” period does not reach the limiting cur-
rent observed under continuous microwave radiation (see Fig. 3C).
Therefore the response time of the electrode system is in the order
of 10 s and in order to reach thermal equilibrium (with electrolyte
flowing through the cell) 10–20 s are required.
The delayed response of the electrode system to microwave
pulses is an indication for a heating effect due to both (i) direct
microwave heating of the ITO electrode film in contact with the
solution phase and (ii) heating of the bulk electrolyte solution dur-
ing transit through the cell. The relative contribution from these
two heating mechanisms can be assessed by determining the bulk
liquid temperature at the outlet of the electrochemical cell. This
“outlet temperature calibration” measurement is contrasted to the
“electrode surface temperature calibration” in the plot in Fig. 2. The
bulk temperature reaches approximately 313 K when the electrode
surface temperature is approximately 363 K. This difference sug-
gests that substantial localized heating occurs due to absorption of
microwave energy in the ITO film electrode.
3.2. Microwave-induced effects on the electrochemical deposition
of gold at a tin-doped indium oxide (ITO) electrode
Electrodeposition processes can be applied for a wide range of
materials including metals [20], metal oxides [21], polymers [22],
orsemiconductors[23]. Oftenwetelectrodepositionofferspractical
advantages over high vacuum or gas phase deposition in industrial
processes. Control over electrodeposition processes is desirable to
improve film quality and deposition rate. Here, microwave activa-
tion is applied first to a metal deposition process and then to a metal
oxide deposition in order to assess effects.
Fig. 3. Cyclic voltammograms (scan rate 10 mV s−1) for the oxidation/reduction of
3−
1 mM Fe(CN)64−/1 mM Fe(CN)6 at a 1 cm2 ITO electrode in aqueous 0.1 M KCl (A)
in the absence, and (B) in the presence of 4 s pulsed and (C) continuous microwave
radiation (magnetron anode current 52 mA corresponding to an average temper-
ature of 83 ◦C). (D) Plot of the mass transport controlled limiting currents for the
4−
3−
oxidation of Fe(CN)6 and for the reduction of Fe(CN)6 vs. temperature (in the
presence of microwaves, see calibration plot in this figure).
The electrodeposition of gold is a well known process employed
for the formation of catalytic nanoparticles [24], for paired elec-
trode junctions [25], and to form nanostructures [26]. There are
several types of gold plating baths [27] but here a simple dilute
hydrochloric acid medium is chosen to investigate gold electrode-
position in the presence of microwave radiation.
Gold deposits are grown on the surface of an ITO substrate
electrode in the absence and in the presence of microwave radi-
ation. Fig. 4A shows data from cyclic voltammetry experiments for
electrodeposition of gold from aqueous 1 mM tetrachloroaurate(III)
solution in 0.1 M KCl (adjusted to pH 2). The deposition of gold com-
mences at ca. 0.4 V vs. SCE. A reduction peak is observed and the
current levels off. After reversal of the scan direction a typical nucle-
ation loop is observed and the gold stripping process commences at
ca. 0.8 V vs. SCE. Increasing the temperature to 343 K (see Fig. 4A(ii))
causes an increase in peak and deposition current and an earlier
onset of the stripping process. The nucleation of the gold deposit
appears to be only weakly affected.
localized microwave-induced temperature effects may be possible
due to the direct interaction of the conducting or semiconducting
film with microwave radiation. Here, a thin tin-doped indium oxide
(or ITO) layer (ca. 80 nm conducting ITO film thickness by SEM) on a
glass slide is employed with an exposed electrode area of ca. 1 cm2.
Initially, the effect of microwave radiation on temperature and mass
3−/4−
transport are assessed by employing the one-electron Fe(CN)6
redox system (see Eq. (1)).
Fe(CN)63−(aq) + e− ꢀ Fe(CN)64−(aq)
(1)
3−/4−
The oxidation and reduction of Fe(CN)6
in aqueous 0.1 M
KCl is shown in Fig. 3. In the absence of microwave radiation (see
Fig. 3A) well-defined current peaks are observed even at a scan rate
of 10 mV s−1 with a midpoint potential Emid = Epox + Epred/2 of ca.
0.075 V vs. SCE (see dotted line in Fig. 3). The limiting currents for
oxidation (at more positive potentials) and for reduction (at more
negative potential) are 90 and −75 ␮A, respectively, indicating that
convection in the flow cell occurs close to the electrode surface. The
“noise” pattern superimposed on the voltammetric signal is caused
Under microwave conditions flow of solution through the elec-
trochemical cell is imposed with a peristaltic pump system in order

L. Rassaei et al. / Electrochimica Acta 54 (2009) 6680–6685
6683
Fig. 4. Cyclic voltammograms (scan rate 0.02 V s−1) obtained for the reduction and
re-oxidation of 1 mM AuCl4− in aqueous 0.1 M KCl at pH 2 (pH adjusted with HCl) at
an ITO-coated glass electrode (1 cm2) (A) in a thermal bath without flow (i) at 298 K
and (ii) at 343 K and (B) in the presence of microwave radiation (with 28 cm3 min−1
flow, magnetron current (i) 0 mA and (ii) 28 mA). (C) Plot of the peak current for the
−
reduction of AuCl4 (i) in a thermal bath without flow and (ii) in the presence of
microwave with 28 cm3 min−1 flow.
to maintain a stable temperature. As a result currents for the gold
deposition process are enhanced already at room temperature (see
Fig. 4B(i)). In the presence of microwave radiation a further increase
in current is observed (see Fig. 4B(ii)). The effects are summarized
in Fig. 4C which shows the approximately linear increase in gold
deposition current with temperature.
Fig. 5 shows typical SEM images of gold deposits grown onto ITO
electrodes at different temperatures and in absence or presence of
microwave radiation. It can be seen that the morphology of gold
deposits is very similar under all conditions. The effect of the tem-
perature is to increase the density of the gold deposit (due to an
increased rate of nucleation) but the effect is not very strong and
comparable to the effect introduced by flow.
In summary, the electrodeposition of gold onto ITO can be
enhanced by microwave radiation and the density of the deposit
can be increased. However, the effects on the morphology of the
gold deposit are small.
−
Fig. 5. SEM images of gold deposits grown from a solution of 1 mM AuCl4 in 0.1 M
KCl (adjusted to pH 2 with HCl) onto ITO electrode substrates (A) at room tempera-
ture in stagnant solution and (B and C) in the presence of microwave radiation (flow
28 cm3 min−1, 52 mA anode current corresponding to 363 K).
3.3. Microwave-induced effects on the electrochemical deposition
of hydrous titanium oxide at a tin-doped indium oxide (ITO)
electrode
chemical deposition allows thickness and morphology control by
varying the electroplating parameters [36]. The effect of microwave
radiation on the chemical (electroless) solution deposition of TiO2
has been reported by Vigil and co-workers [37–39].
Titanium dioxide, TiO2, is widely known as an important elec-
trode material in semiconductors [28], for photocatalysis [29], solar
cells [30], and biosensors [31]. Most of these useful functions
depend mainly on the composition, configuration, and structure of
TiO2. Many methods have been used to prepare TiO2 thin films such
as magnetron sputtering deposition [32], sol–gel [33], electrode-
position [34], and chemical vapour deposition [35]. Among these
methods, electrodeposition of TiO2 is attractive because electro-
A literature strategy for the electrochemical formation of TiO2 at
electrode surfaces is to employ a TiCl3 precursor which is soluble in
aqueous media and which upon oxidation at the electrode surface
rapidly hydrolyses into a hydrous oxide deposit [40]. Kavan et al.
[41] and Manivannan et al. [42] suggested the use of a hydrochloric
acid medium for the electrodeposition of hydrous TiO2. However,
at elevated temperature a solution of TiCl3 in aqueous HCl pH 2
is etching the ITO electrode film and therefore more mild condi-

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L. Rassaei et al. / Electrochimica Acta 54 (2009) 6680–6685
tions had to be investigated. A solution of TiCl3 in acetate buffer
pH 4.7 was chosen. Cyclic voltammograms in Fig. 6A show typical
responses observed during the deposition process. Initially, a pos-
itive current is observed (see Fig. 6A(i)) over the entire potential
range. The positive current is associated with the oxidation of Ti(III)
to Ti(IV) andthe current graduallydiminishes duetofilm deposition
at the electrode surface. At more negative potentials at ca. −0.5 V vs.
SCE a typical reversible TiO2 film response is observed [43] and this
voltammetric response is increasing with deposition time thereby
indicating growth of a titanium oxide film (see Fig. 6A(ii)). At ele-
vated temperatures (353 K, see Fig. 6B(i)) the deposition process
is faster and the decay of the anodic deposition current is more
rapid. After 10 consecutive potential cycles the current is almost
zero due to the formation of the insulating TiO2 film. The film mate-
rial without calcination is X-ray amorphous and therefore likely
to contain highly hydrated TiO2. The voltammetric response for
the TiO2 deposit at negative potential is irreversible (consistent
with enhanced hydrogen evolution) at elevated temperatures (see
Fig. 6B).
In the presence of microwaves and under flow conditions a sim-
ilar anodic deposition process can be observed (see Fig. 6C). The
deposition current gradually diminishes although even after 10
consecutive potential cycles there is still a clear deposition current
observed.
SEM images of the deposits are shown in Fig. 7. Under con-
ventional deposition conditions a film deposit with granular
appearance is observed and crazing occurs in particular for thicker
deposits (see Fig. 7B). Films deposited under microwave condi-
tions exhibit a different morphology with fibrous components
and a much more open and porous structure. This more open
structure explains the continuing deposition with less blocking
Fig. 6. Multi-cycle voltammograms (scan rate 0.05 V s−1, (i) first potential cycle, (ii)
10th potential cycle) obtained for the oxidation of TiCl3 in aqueous 0.1 M acetate
buffer at pH 4.7 at an ITO film electrode (area 1 cm2) (A) in a thermostated bath
at room temperature, (B) in a thermostatted bath at 353 K, and (C) in the pres-
ence of microwave radiation (flow 28 cm3min−1, magnetron anode current 52 mA
corresponding to Telectrode = 353 K).
Fig. 7. SEM images showing (A) a TiO2 film deposited in stagnant solution at 293 K, (B) a TiO2 film deposited in stagnant solution at 363 K, and (C and D) a TiO2 film deposited
in flowing solution in the presence of microwaves (300 mA magnetron anode current corresponding to 353 K).

L. Rassaei et al. / Electrochimica Acta 54 (2009) 6680–6685
6685
under microwave conditions. The formation of the fibrous com-
ponents may be in part caused by the enhanced deposition of
colloidal titanium oxide material which could be enhanced by local
dieletrophoretic effects.
The hydrous titanium oxide films grown by electrodeposition
are X-ray amorphous and only after calcination at 500 ◦C can the
TiO2 (anatase) structure be clearly observed in the XRD pattern.
Unfortunately, the fibrous morphology of the deposit is destroyed
by heating already at 300 ◦C and the resulting films after calcination
are porous but more uniform (not shown). The crazing effect (see
Fig. 6B) which is associated with density changes during calcination
canbeavoidedwhenusingmicrowaveactivationduringdeposition.
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It has been shown that microwave enhanced electrodeposition
processes are observed when ITO-coated glass slides are employed
as electrodes. A flowing electrolyte system is necessary to stabilise
temperature gradients within the electrochemical cell and average
electrode temperatures of ca. 363 K were obtained. For both gold
and hydrous titanium oxide deposition, enhanced deposition rates
were observed. For the deposition of hydrous titanium oxide also
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future a wider range of thin film electrodes could be studied under
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Acknowledgement
The authors would like to thank the EPSRC for financial support
(EP/F025726/1).
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