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Electrochemical Oxidation and ex-situ STM Observations ofBis(4-dimethylamino-2-dihydroxyphenyl)squaraine Dye Layers on
HOPG Electrodes†
Norihiko Takeda, Michele E. Stawasz, and Bruce A. Parkinson*Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA
Abstract
Electrochemical oxidation of insoluble highly ordered bis(4-dimethylamino-2-dihydroxyphenyl)squaraine (1-1OHSQ) dye layers
adsorbed on highly oriented pyrolytic graphite (HOPG) electrodes was studied in aqueous electrolytes. Staircase cyclic voltammetry
obtained in chloride electrolytes revealed hysteresis characterized by large peak separations (100 – 200 mV) and sharp redox peaks
(fwhm 10 – 60 mV) the shape and potentials of which depended on electrolyte concentrations. Small stochastic reduction peaks
were observed at more negative potentials that are associated with the reduction of small domains of the oxidized 1-1OHSQ layers.
Peak potentials and peak shapes were also dependent on the identity of the electrolyte anion species. The results support a reaction
scheme where electrolyte anions are incorporated into surface confined one-electron oxidized dye molecular layers. From the results
of peak potential shifts, the preference for ion-pairing is estimated to be in the order of: I- > Br- > Cl- > SO4
2– ≈ ClO4- ≈ F-. Ex-situ
STM observations were performed to reveal structural changes upon electrochemical oxidation of 1-1OHSQ layers in LiCl electrolytes.
Two polymorphs with different angles of the long-axis of molecules to the directions of the molecular row were observed for the
oxidized samples. Both polymorphs had smaller packing densities compared to the reduced form of the dye.
Keywords: Surface confined redox processes; Squaraine dye; 2D surface immiscibility; Cyclic voltammetry; Scanning tunneling
microscopy
1. Introduction
Surface confined voltammetry of molecular adsorbates on
highly oriented pyrolytic graphite (HOPG) was first reported
by Brown, Koval, and Anson for the reduction and re-oxidation
of aromatic compounds including phenanthraquinone [1, 2]. The
voltammograms exhibited typical surface confined reversible
behavior. More recent studies of surface confined
electrochemical reactions have reported systems where there is
considerable hysteresis in the voltammograms, with the
oxidation and reduction peaks separated by as much as 250 mV
or even more [3-27]. These reactions usually involve tightly
packed 2D surface phases, such as azobenzene Langmuir-
Blodgett films [3] or methyl viologen cation radicals [4],
although the peak separation in the latter was rather small.
Similar behavior was also observed for conducting 3D phases
such as the organic conducting salts of tetrathiafulvalene (TTF)
[5-8, 13] and 7,7,8,8-tetracyanoquinodimethane (TCNQ) [5-7,
9-13], C60
crystallites [14-23], and liquid crystal perylene
diimides [24], as well as non-conducting solid organometallic
compounds [25-27]. Often the electrochemical reaction of these
phases involves the incorporation of anions or cations into the
oxidized or reduced phase. The large structural changes,
necessary due to the need to accommodate ions in the adsorbed
film or molecular crystal, results in the large hysteresis in the
voltammograms. The barrier to nucleation and growth of the
new phase formed upon oxidation or reduction was used to
explain and model many of the electrochemical results [4, 8,
11-13, 23, 28]. Recently Scholz et al. [29, 30] have pointed out
that the hysteresis in voltammograms may not be a result of
nucleation kinetics but instead is a thermodynamic result due
to a miscibility gap between the two phases. If the oxidized
and reduced surface phases are both insoluble in the electrolyte
* Corresponding author. Tel: +1-970-4910504; fax: +1-970-
4911801.
E-mail address: [email protected] (B.A.
Parkinson).† This paper is a contribution to this special issue honoring Professor
Fred C. Anson, my mentor and friend, who has exhibited the utmost
class and integrity throughout his distinguished career in electrochem-
istry.
Manuscript accepted for publication in Journal of Electroanalytical Chemistry
2 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
and immiscible then there is no mechanism for the equilibration
of the two phases. In the case where a neutral surface phase is
oxidized or reduced and ions need to be incorporated into the
new phase, it is not surprising that the 2D neutral and 2D ionic
phases are immiscible.
We report the investigation of a new surface confined redox
system, involving squaraine dye molecules (1-1OHSQ), that
exhibits remarkable voltammetry on HOPG electrodes.
In addition, we use scanning tunneling microscopy (STM)
to study the actual molecular arrangements of these molecules
on the surface of HOPG in both their oxidized and reduced states.
2. Experimental
2.1 Chemicals
Bis(4-dimethylamino-2-dihydroxyphenyl)squaraine (1-
1OHSQ) was a generous gift from the Lexmark International
Corporation and was used as received. Dye stock solutions
were prepared by dissolving 1-1OHSQ in dichloromethane
(Fischer, Spectranalyzed grade). Actual concentration of dye
solutions were checked by measuring UV-vis absorption spectra
using a HP8452A spectrophotometer (ε= 3.3 × 105 cm-1 M-1 at
636 nm [31]). Lithium chloride, sodium bromide, sodium
fluoride, sodium iodide, and sodium carbonate were Fischer
chemical reagents and hydrochloric acid, potassium chloride,
sodium chloride, and sodium sulfate were Mallinckrodt
analytical reagents, and they were used as received. A 2.0 M
sodium perchlorate stock solution was prepared from diluted
perchloric acid solution and sodium carbonate. Aqueous
electrolyte solutions were prepared using water purified with
Millipore Milli-Q system.
2.2 Electrode preparation
Highly Oriented Pyrolytic Graphite (HOPG) was cut into
small pieces to prepare working electrodes. Basal planes of the
HOPG pieces were cleaved with adhesive tape (Scotch, 3M) to
expose fresh surfaces. Deposition of 1-1OHSQ dye layers on
HOPG substrates was carried out by similar methods employed
in our previous photoelectrochemical study of 1-1OHSQ on tin
disulfide [32]. In this study, pieces of freshly cleaved HOPG
were dipped in 10–12 µM 1-1OHSQ dichloromethane solution
for 2 min. For comparison, dye coated electrodes were also
prepared by dropping a known amount of 1-1OHSQ dye solution
onto a freshly cleaved HOPG substrate followed by evaporation
of solvents in ambient air. Dye stock solutions were filtered
through a membrane filter (0.02 µm pore diameter, Whatman
anodisc 13) before use to remove any undissolved dye particles
or inert particles that could act as nucleation sites.
2.3 Electrochemical measurements
A Teflon-made electrode holder was designed to use HOPG
pieces as working electrodes and to make samples transferable
between electrochemical measurements and scanning probe
microscopy experiments. For electrochemical measurements,
dye deposited HOPG pieces were mounted on a copper plate
with a silver paste (SPI Supplies) inside the holder, through
which electrical contacts were made. Samples were sandwiched
in a two-piece Teflon holder with a flat Latex washer (4 mm
i.d., 0.6 mm thick) with a 4 mm diameter window in one piece.
The holder was held together with four screws to seal all but
the ~0.13 cm2 active area from the electrolyte.
Electrochemical measurements were conducted using a
conventional three-compartment electrochemical cell. A
saturated calomel electrode (SCE) or a sodium-saturated calomel
electrode (SSCE, in the case of perchlorate electrolyte) were
used as reference electrodes and a Pt gauze was used as a counter
electrode. Staircase cyclic voltammograms (SCVs) and staircase
linear sweep voltammograms were taken with an EG&G
Princeton Applied Research 174A polarographic analyzer
controlled by a MacLab interface (AD Instruments) and a
Macintosh computer. The step height was 1 mV and the step
width depended on scan rate (= (step height)/(scan rate)), which
was 100 ms at scan rate of 10 mV s-1. Current signals were
sampled and averaged over the last 25 % of time duration of
each step at 0.1 ms intervals. After immersing the electrodes,
aqueous electrolyte solutions were purged with nitrogen gas
for at least 15 min prior to measurements. All measurements
were conducted at ambient temperature.
2.4 Scanning probe microscopies
Molecular arrangements of both electrochemically oxidized
and reduced (neutral form before electrochemical oxidation)
forms of 1-1OHSQ layers adsorbed on HOPG prepared by the
dipping method were investigated by scanning tunneling
microscopy. STM experiments were performed ex-situ with a
Digital Instruments Nanoscope III scanning tunneling
microscope operating under ambient conditions. Vibration
isolation was provided by a bungee system enclosed in an
environmental chamber. STM tips were cut to a sharp point
mechanically from (80:20) platinum:iridium 0.2 mm diameter
wire (Alpha Aesar). Images of oxidized or reduced 1-1OHSQ
layers were obtained using typical tunneling parameters of –
1.4 V to –1.6 V (sample negative) and 40 pA with the STM
operating in the constant current mode. This range of sample
bias was slightly different than that reported for imaging of 1-
O
O
N NH3C
H3C
CH3
CH3
HO
OH
Bis(4-dimethylamino-2-dihydroxyphenyl)squaraine (1-1OHSQ)
2
N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 3
1OHSQ (not electrochemically treated) under phenyloctane
solvent [33].
STM samples of oxidized 1-1OHSQ on HOPG were
prepared by anodically scanning non-oxidized samples by
staircase linear sweep voltammetry (scan rate 10 mV s-1) after
subjecting to several potential cycles to check their cyclic
voltammetric behavior. Samples were mounted in the STM
within minutes of the electrochemical experiment. Due to the
ex-situ nature of this experimental setup, it was not possible to
investigate the same exact area of the adsorbate surface upon
exchanging the sample between the STM and the
electrochemical cell for CV cycling. Therefore it was necessary
to rely upon statistical sampling of numerous areas of the surface
to ensure that differences observed in its oxidized state (as
compared to the reduced state) were truly due to electrochemical
reorganization, and not just a local orientational difference. To
reduce adsorbate etching presumably caused by capillary force
(described below) the tip was sometimes immersed into a drop
or two of phenyloctane (Aldrich) deposited onto the sample
surface. This procedure improved the resolution and reduced
the noise when imaging the oxidized form of the dye due to its
negligibly small solubility in phenyloctane. Some of the reduced
form of the dye dissolved in the phenyloctane increasing the
image noise due to the higher solubility of the reduced dye in
this solvent. To prevent dissolution of the reduced squaraine
adsorbate into the phenyloctane layer, the phenyloctane was
presaturated with reduced form of 1-1OHSQ. It was then
deposited on the adsorbate surface in the same manner as the
oxidized films and succeeded in improving image quality and
adsorbate layer stability.
Atomic force microscopy was performed with a Digital
Instruments Nanoscope IIIa scanning probe microscope operated
in tapping mode. Conical silicon AFM tips (Ultralevers™, Park
Scientific Instruments) with a spring constant of ~0.26 or ~0.40
N m-1 were used. An HOPG sample with a freshly deposited 1-
1OHSQ layer prepared by the dropping method was fixed on a
steel plate with a silver paste and then mounted on an AFM
scanner (type-J). All measurements were conducted in ambient
air. After investigating several areas by AFM, the sample was
mounted in the Teflon holder in a same manner as described
above to perform electrochemical studies.
3. Results and discussion
3.1 General trends of cyclic voltammograms of 1-1OHSQ
adsorbed on HOPG
Law et al. reported electrochemical properties of squaraine
dyes dissolved in dichloromethane using Pt as a working
electrode and tetrabutylammonium perchlorate (TBAP) as
supporting electrolyte [34]. The cyclic voltammogram of
squaraines showed two reversible waves with ~60 mV peak
separation as expected for a Nernstian diffusion-controlled one-
electron electrochemical reaction [35]. For the 1-1OHSQ dye,
the first and the second formal oxidation potentials were
determined to be +0.41 and +0.91 V vs Ag | AgCl in
dichloromethane, respectively [34].
In the present study staircase cyclic voltammograms (SCVs)
of the 1-1OHSQ dye layers adsorbed on HOPG (1-1OHSQ/
HOPG) electrodes prepared by the dipping method showed
remarkably different behavior from that reported for solution
electrochemistry. General trends of observed voltammogramms
are explained below. Figures 1 (a) and (b) show examples of
cyclic voltammograms of 1-1OHSQ/HOPG electrodes obtained
in 0.1 M LiCl aqueous solution at scan rate of 10 mV s-1. In
each panel voltammograms obtained in the first scan (dotted
lines) and after several cycles (solid lines) are compared for
two different samples prepared by similar procedures. Both
samples revealed sharp oxidation and reduction peaks with a
relatively large peak separation of 140 – 230 mV with no
significant currents between them. Usually the first
voltammetric scans gave considerably different peak potentials
and wave shapes than those from subsequent scans. After the
first scan, main peak potentials were relatively stable but wave
0
0.5µA
0
-0.2 -0.1 0 0.1 0.2 0.3
1µA
(a)
(b)
E / V vs SCE
Fig. 1. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-
trodes (dipping method) in 0.1 M LiCl aqueous solutions. In each panel,
the first scan (dotted line) and the eleventh (a) or tenth (b) scan (solid
line) for two different samples are shown. Scan rate 10 mV s–1. Elec-
trode area 0.13 cm2.
4 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
shapes and peak currents occasionally changed with repetitive
scanning. In Fig. 1(a), two overlapping oxidation peaks at
+0.129 and +0.159 V vs SCE can be seen in the first scan (dotted
line). In the eleventh scan, shown as a solid line in the figure,
these peaks had merged to become a single peak at +0.137 ±0.002 V vs SCE. A similar behavior appeared in the reverse
negative scan in which a peak at around –0.01 V vs SCE had
evolved into a larger peak. In Fig. 1(b), the oxidation wave in
the first scan had a prewave at the foot of the peak (+0.185 V vs
SCE) which fused with a ~50 mV more negatively shifted peak
at around +0.135 V vs SCE in the following scans. On the
reduction side, a peak at –0.043 V vs SCE split into two
comparable peaks in the tenth scan as shown. Moreover, small
jagged peaks appeared at potentials more negative than the main
peak. In general, similar current spikes often appeared especially
in cathodic scans. Sometimes the magnitude of these currents
was larger than that of the main peak. These current spikes
behaved rather stochastically, that is the potential and peak
heights changed from scan to scan. This phenomenon made
the determination of reduction potentials more ambiguous than
for the oxidation peaks. The small current spikes tended to
disappear after many potential cycles (as seen in Fig. 1).
However, in some cases multiple peaks remained after extensive
cycling.
The total integrations of the anodic and cathodic currents
were approximately the same and were relatively constant with
scan numbers as in Fig. 1. However, in some cases
voltammogram area as well as peak current decreased with
repetitive scanning. It is possible that the oxidized form of the
dye has a slight solubility in aqueous solutions and desorbed
from HOPG electrodes, or that electrochemically inactive areas
could be formed. The total charge passed by electrochemical
oxidation and reduction of 1-1OHSQ/HOPG electrodes differed
from sample to sample although similar procedures were
employed to prepare the samples. This uncertainty is
presumably because the amounts of 1-1OHSQ molecules
adsorbed on HOPG were different, which could be explained
by such conditions as the quality of the HOPG basal plane
cleavage, the withdrawing process of the HOPG from the dye
solutions, or randomness in the nucleation and growth of the
dye layers in the deposition processes.
Some comments about the magnitude of currents obtained
in our SCVs are needed. If nucleation and growth type kinetics
are involved, as suggested below, current transients for each
potential step would peak. Our SCV currents were averaged
over last 25 % of each step. Due to the signal processing method
employed in this study, integrating the currents to determine
charges and surface coverages will underestimate the actual
coverages. Further coulometric studies are needed to provide
more accurate surface coverages. However, it can be safely
said that multilayers of the dye would be formed, at least in
some cases, since the underestimated charges and the STM
results presented below both confirm this.
3.2 Reaction scheme including electrolyte anion incorporation
In the present study, dye modified electrodes were immersed
in aqueous electrolyte solutions in which 1-1OHSQ is not
soluble. Accordingly, the electroactive 1-1OHSQ layers can
be regarded as immobilized on HOPG surfaces. However,
unlike typical surface confined systems including physically
[1, 2] or covalently [36-44] attached redox molecular species,
the present system revealed a large hysteresis in cyclic
voltammograms even at slow scan rates. This indicates large
activation barriers for redox reactions of adsorbed 1-1OHSQ
layers. The large splitting of redox peaks in the voltammograms
is reminiscent of other reported systems such as TTF and TCNQ
salts [5-13], and solid C60
immobilized on electrodes [14-23].
This behavior was often attributed to large structural changes
during redox reactions. Laviron had simulated the voltammetric
shape of surface confined species, and obtained a hysteresis
effect by using strong “interaction parameters” between
adsorbed molecules [45]. As described in the introduction, the
splitting of anodic and cathodic peaks was also explained by
considering the insolubility of the adsorbed redox species and
the immiscibility of the two adsorbed redox phases [29]. In
this case, the Nernst equation cannot be satisfied in a certain
potential range (“miscibility gap”) where the two phases are
immiscible and the redox reaction is suppressed. If the electrode
is further polarized, exceeding the critical potential, the redox
reaction is initiated and a sharp current peak appears. It should
be noted that in this case the potential region halfway between
the anodic and cathodic peaks is not necessarily the value of
the formal potential of the redox reaction [29].
We hypothesize that the redox reaction of 1-1OHSQ
adsorbed on HOPG proceeds as follows. A neutral 1-1OHSQ
molecule is oxidized by one electron to form a positively charged
1-1OHSQ radical cation (reaction (1)), followed by the
association of an electrolyte anion to maintain charge neutrality
(reaction (2)) as given below.
(1-1OHSQ) – e- ⇔ (1-1OHSQ)+ (1)
(1-1OHSQ)+ + X- ⇔ (1-1OHSQ)+X- (2)
X- is the electrolyte anion species and (1-1OHSQ)+ is a one-
electron oxidized 1-1OHSQ molecule. Since these processes
are reversible, re-reduction of the oxidized form of 1-1OHSQ
is accompanied by release of the electrolyte anion from the film.
We assume a one-electron redox reaction because the redox
peaks of immobilized 1-1OHSQ appeared at a similar potential
as the first one-electron oxidation of dissolved 1-1OHSQ in
dichloromethane solution [34] when the same electrolyte anion
(perchlorate) was used (see section 3.4), and the formal potential
of molecular adsorbed species was usually found to be similar
to that of dissolved species [36, 37, 39, 40].
Many of the observations described above can be interpreted
using the reaction scheme represented by reactions (1) and (2).
N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 5
Previous STM studies showed that the squaraine dyes have
strong intermolecular interactions and form tightly packed well
ordered structures on HOPG substrates [33]. A large structural
change of the ordered 1-1OHSQ molecules occurs in order to
include or exclude the electrolyte anions upon oxidation or
reduction (reaction (2)). The reaction could be initiated at
nucleation sites, possibly at defect sites within the packed
molecular layers or at domain edges, then rapidly expand to
entire domains or layers to give the sharp redox peaks in the
voltammograms.
The fact that the first scan showed a more positive oxidation
peak than successive scans (Fig. 1(b)) indicates that larger
overpotentials are needed to incorporate anions into the 1-
1OHSQ layers prior to reconstruction, as was observed for
another system [19]. Also, the release of solvent which might
have been trapped during the dye deposition processes
(dichloromethane in the present case) could occur after the initial
scan to alter the voltammograms. Furthermore, there could be
time dependent factors, especially for cathodic processes, if
nucleation and growth type kinetics are involved. The multiple
current peaks observed in staircase cyclic voltammograms could
be related to the existence of multiple domains of 1-1OHSQ
layers with different nucleation rates for anion association or
expulsion. The appearance and disappearance of the multiple
current peaks could then be the result of the formation and
annealing of small domains into larger domains with repetitive
cycling. It has been shown that the nucleation rate limits the
formation of ordered liquid crystal layers into small domains
created by controlled-size pits on HOPG [46]. Large areas of
HOPG were quickly covered with ordered layers since the
probability of forming the critical nuclei on some part of the
surface was high [46]. Different polytypes of packed 1-1OHSQ
domains with different formal potentials or overpotentials could
also account for the multiple peaks. The large double peaks
observed in Fig. 1(b) may be the result of the two polymorphs
found in STM studies discussed later.
3.3 Effect of electrolyte concentration
If the above-mentioned reaction scheme is occurring, the
properties of the electrolyte will affect the voltammetric behavior
of 1-1OHSQ layers on HOPG. The effect of electrolyte
concentration was investigated by using three different
concentrations of aqueous LiCl, NaCl, and KCl solutions as
supporting electrolytes (0.01 M, 0.1 M, and 1 M). Dye modified
electrodes were prepared by the same dipping method as in the
preceding section. Changes in the electrolyte cation from lithium
to sodium to potassium had only a small influence on the
voltammetric behavior, as long as the concentrations of the
electrolytes were equal. This is reasonable because reaction
(2) does not involve electrolyte cations but anions. The
electrolyte concentrations did affect some features of
voltammograms. Staircase cyclic voltammograms obtained in
0.01, 0.1, and 1 M of LiCl aqueous solutions after several cycles
are compared in Fig. 2 (scan rate 10 mV s-1). The same 1-
1OHSQ/HOPG sample was used to obtain all of these
voltammograms. It can be seen that both the oxidation peak
potential (Epox) and the reduction peak potential (Ep
red) depend
on the concentration of the supporting electrolyte. Furthermore,
peaks became sharper as the electrolyte concentrations
increased. Values of full width at half maximum (fwhm) of
oxidation and reduction peaks in Fig. 2 are 48 and 42 mV in
0.01 M, 23 and 34 mV in 0.1 M and 9 and 11 mV in 1 M
solutions, respectively.
Oxidation and reduction peak potentials for different
concentrations of LiCl, NaCl, and KCl, determined at scan rate
of 10 mV s-1 after the voltammograms are stabilized by repeated
scanning, are plotted in Fig. 3. In some cases multiple peaks
remained after many potential cycles. The potential of the main
peaks with the largest areas were considered to be the peak
potentials. Regression lines shown in the figure were least-
squares fits to the data obtained for each electrolyte. In each
case, the potentials shifted negatively with increasing
concentrations. Slopes of lines were –70 mV (Epox) and –47
mV (Epred) for LiCl, –70 mV (Ep
ox) and –27 mV (Epred) for NaCl,
and –71 mV (Epox) and –34 mV (Ep
red) for KCl per decade change
of concentration. The magnitude of the potential shift was larger
for Epox than for Ep
red in each case. As a result the peak separation
between the oxidation peak and the reduction peak (Epox – Ep
red)
tended to become larger as the concentration of the electrolyte
was decreased. The different potential shifts could be related
to the differences in the oxidation and reduction processes (for
(a)
(b)
(c)
E / V vs SCE
0
0
2µA
0
-0.2 -0.1 0 0.1 0.2 0.3 0.4
Fig. 2. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-
trodes (dipping method) in 0.01 M (a), 0.1 M (b), and 1 M (c) LiCl
aqueous solutions. Scan rate 10 mV s–1. Electrode area 0.13 cm2.
6 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
polarization [48]. However, in the system where a large
hysteresis is observed, the occurrence of a sharp peak, much
narrower than the ideal Nernstian system itself, could be
explained by sudden redox reactions after passing the
“miscibility gap” as described in the preceding section [29]. In
such a case, the peak width is also influenced by redox or ion
transfer kinetics. Broadening of the voltammetric peak with
decreasing electrolyte concentration observed in this study might
be due to slower kinetics such as slower diffusion of electrolyte
anions into or out of the 1-1OHSQ layers.
3.4 Effect of electrolyte anion species
Voltammetric behavior of 1-1OHSQ deposited on HOPG
by the dipping method was found to be affected dramatically
by the electrolyte anion. Figure 4 compares cyclic
voltammograms of 1-1OHSQ on HOPG in 0.1 M of NaCl, NaBr,
and NaI aqueous electrolytes (scan rate 10 mV s-1). Again, sharp
redox peaks with a large hysteresis were observed. The peak
potentials were significantly shifted to more negative potentials
in bromide (~150 mV) and iodide (~370 mV) electrolytes when
(a)
(b)
(c)
E / V vs SCE
0
2µA
0
0.4µA
0
-0.4 -0.2 0 0.2
0.2µA
Fig. 4. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-
trodes (dipping method) in 0.1 M NaCl (a), NaBr (b), and NaI (c) aque-
ous solutions. Scan rate 10 mV s–1. Electrode area 0.13 cm2.
Cl– concentration / M
Pea
k po
tent
ial E
p / V
vs
SC
E
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.01 0.1 1
Fig. 3. Plots of peak potentials for the oxidation (Ep
ox, open symbols)
and reduction (Ep
red, closed symbols) of 1-1 OHSQ dye layers on HOPG
electrodes as a function of supporting electrolyte concentration. Sup-
porting elecrolyte: lithium chloride (circles), sodium chloride (triangles),
and potassium chloride (inverse triangles). Scan rate 10 mV s–1. Lines
are least-squares fits to the data.
instance, the asymmetric nature of anion inclusion and
exclusion). On average, there is a 53 mV shift of peak potentials
with a 10-fold increase or decrease of the electrolyte
concentration, close to a ~59 mV shift predicted for Nernstian
systems. A similar dependence of peak potentials on electrolyte
ion concentration was observed for other surface confined
systems such as redox-functionalized self-assembled
monolayers (SAMs) on gold [40-42, 44] and the solid
compounds attached to graphite substrates [8, 12, 20], where
electrolyte ions were involved in redox reactions. The 53 mV
value closer to the 59 mV Nernstein value for a one-electron
process further supporting our assumption that the surface
oxidation/reduction process is a one electron process. A ~30
mV shift would be observed if two anions were involved in
reaction (2).
Narrowing of peak widths with increasing electrolyte
concentrations was also commonly observed in NaCl and KCl
electrolytes as well as in LiCl electrolyte. The fwhm of a one-
electron redox voltammetric peak was predicted to be 90.6 mV
(at 25 ˚C) for an ideal Nernstian surface confined redox systems
[35, 37]. Theoretical treatments including effects of electrolyte
concentration on voltammetric shape and peak potentials have
been developed considering the potential distribution across the
redox-functionalized SAMs [47, 48]. It is interesting to note
that recently Ohtani et al. explained the potential shift of sharply
peaked voltammograms by considering strong ion-pair
formation with the surface confined redox-active molecules,
which increased the potential drop and required a smaller
N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 7
compared to the chloride electrolytes of the same concentration.
The differences in the magnitude of the observed currents were
probably due to the different surface coverages of 1-1OHSQ
on HOPG that resulted from each sample preparation.
In Fig. 5, cyclic voltammograms obtained in 0.1 M NaF
and 0.1 M Na2SO
4 aqueous electrolytes after several potential
cycles (scan rate 10 mV s-1) are shown. In these electrolytes,
broader redox peaks appeared at much more positive potentials
than those in the halide electrolytes of the same concentration.
In both cases, the anodic peak in the first scan appeared more
positive than that of subsequent scans shown in Fig. 5, whose
behavior was also observed in halide electrolytes. In addition,
the anodic peak observed in the first scan was much larger than
the cathodic peak. There could be some irreversibility for the
incorporation of fluoride or sulfate anions into 1-1OHSQ layers.
Voltammograms obtained with five successive scans in 0.1 M
NaClO4 aqueous electrolyte (scan rate 10 mV s-1) are shown in
Fig. 6. Note that potentials are given versus SSCE reference
electrode in this figure. Redox peaks appeared in a similar
potential range to those observed in fluoride and sulfate
electrolytes. In Fig. 6, significant decreases of the peak current
with repetitive scans can be seen. Many dye molecules are
more soluble as perchlorate salts and we suspect loss of the
oxidized dye due to a small aqueous solubility. Voltammograms
obtained in fluoride, sulfate, and perchlorate electrolytes also
resemble each other since measurable currents appear between
the two peaks. Peaks were also somewhat broader than those
observed in chloride, bromide, and iodide electrolytes. The
miscibility gap concept explains the very low currents between
peaks in the halide electrolytes and the different shapes and
peak separations of the voltammograms of adsorbed 1-1OHSQ
redox reaction. One requirement for the miscibility gap is that
the two phases cannot equilibrate through the solution, thus any
solubility of the oxidized or reduced phase would allow for this
equilibration.
All the results presented above confirm the importance of
the electrolyte anions in the redox reaction of 1-1OHSQ on
HOPG supporting the applicability of the reaction scheme
proposed above (reactions (1) and (2)). The order of oxidation
and reduction peak positions observed in 0.1 M electrolytes was
I- < Br- < Cl- < SO4
2– ≈ ClO4- ≈ F- for the anion species
investigated. As suggested in studies on other surface confined
redox reactions involving electrolyte ions [39-44, 48], shifts of
redox peak potentials could be interpreted by differences in the
degree of association (association constant) of electrolyte anions
with the oxidized form of the 1-1OHSQ molecules. Preferential
ion-pairing of positively charged molecular layers with anions
would shift the formal potential of the redox reaction to more
negative potentials. The dramatic shift of the redox potential
observed in this study (~600 mV difference between I- and F-
electrolytes) suggests that the nature of electrolyte anions plays
an important role in the thermodynamics of the present redox
system. Factors such as hydrodynamic radii of anions and free
E / V vs SCE
0
0.3µA
0
-0.1 0 0.1 0.2 0.3 0.4 0.5
0.4µA
(a)
(b)
Fig. 5. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-
trodes (dipping method) obtained in 0.1 M NaF (a), and Na2SO4 (b)
aqueous electrolytes. Scan rate 10mV s–1. Electrode area 0.13 cm2.
E / V vs SSCE
Cur
rent
/ µA
-0.4
-0.2
0
0.2
0.4
0.6
-0.1 0 0.1 0.2 0.3 0.4 0.5
Fig. 6. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-
trode (dipping method) obtained in 0.1 M NaClO4 aqueous electrolyte.
Scan rate 10mV s–1. Allows indicate direction of decreasing currents
with repetitive cycling. Electrode area 0.13 cm2.
8 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
energy differences between hydrated and dye-associated anions
would also affect the ion-pair formation. Following the observed
redox potentials, the order of affinity of anions would be I- >
Br- > Cl- > SO4
2– ≈ ClO4- ≈ F-. The fact that similar peak
potentials are obtained in sulfate, perchlorate, and fluoride
suggests that interactions of these anions with the oxidized 1-
1OHSQ layers are similar.
3.5 Effect of scan rates
An example of a scan rate dependence for staircase cyclic
voltammograms of 1-1OHSQ on HOPG prepared by the dipping
method and measured in 0.1 M KCl aqueous electrolyte is shown
in Fig. 7. Scan rates were varied from 10 to 100 mV s-1 in panel
(a) and from 1 to 10 mV s-1 in panel (b) of the figure. Both the
oxidation and reduction currents depended on scan rates but
were not linearly proportional as expected for a typical surface
immobilized redox species [35, 37]. Also, peak potentials and
fwhm of peaks often depended on scan rates, as can be seen in
Fig. 7. As the scan rate was increased, redox peaks became
broader and the peak separation also became larger.
Furthermore, tails after current peaks tended to become
significant especially at higher scan rates. It is probable that
diffusion of anions, either from the electrolyte or the solid film
would affect the current and ultimately limit the overall reaction
rate. The current sampling of the staircase current may also be
a factor in interpreting peak height with such sharp peak currents.
Further investigation is needed to clarify these issues.
3.6 STM imaging of reduced and oxidized forms of 1-1OHSQ
Dialkylamino squaraines have been found to be highly
surface active and readily amenable to investigation by STM
[33]. It would be useful to summarize here several observations
obtained in an earlier study.1 Squaraine molecules appeared in
most STM images as football-shaped areas of bright contrast
measuring 15.0 ± 1.5 Å in length, 2.5 ± 1.0 Å in width, and
from 0.2 to 1.5 Å in relative height. Occasionally two bright
spots within the football-shaped region were resolved, each
measuring ~3 Å in length, with a peak to peak distance of about
8 Å (see Figure 4 in reference 33). The measured lengths of the
football shapes correspond with the size of the chromophore of
the molecule (from the nitrogen at one end of the molecule to
the nitrogen at the other end). The width of the football
corresponds well with the width of a phenyl group, not including
the oxygen atoms and the hydroxyls. An explanation for why
the oxygens and hydroxyls are not imaged has been presented
elsewhere [33]. The methyl groups in 1-1OHSQ (and longer
alkyl tails for other dialkylamino squaraines) have not been
clearly resolved. Nevertheless, the length, width, and relative
height of the bright football-shaped areas correspond well with
a model of the molecule lying with the chromophore flat on the
HOPG surface. STM measured heights of 1–2 Å are typical for
aromatic systems adsorbed lying flat on the substrate surface
[49]. Since the methyl tails are not resolved, packing of the
bright chromophores must be examined quantitatively to deduce
the orientation of the methyl tails.
Different ordered structures, or plymorphs, for the same
squaraine molecules were frequently observed in the previous
study [33]. Longer-tailed squaraines tended to produce more
stable adsorbate layers with row structures while the herring-
bone structures formed by the shorter-tailed squaraines seemed
less stable since they were more easily stripped-off of the
substrate surface by the rastering motion of the tip [33]. This
was often observed in this investigation of the layers of 1-
1OHSQ deposited from dichloromethane solutions. The
dichloromethane solution concentrations used within this study
often produced multiple layers of 1-1OHSQ on HOPG. Layer
E / V vs SCE
0
50µA
0
-0.2 -0.1 0 0.1 0.2 0.3
25µA
(a)
(b)
Fig. 7. Staircase cyclic voltammograms of 1-1 OHSQ/HOPG elec-
trode (dipping method) in 0.1 M KCl aqueous electrolyte obtained at
various scan rates of 10, 20, 50, and 100 mV s–1 (a) and 1, 2, 5, and 10
mV s–1 (b). Allows indicate direction of increasing scan rates. Elec-
trode area 0.13 cm2.
1 In an earlier study [33], molecules were primarily adsorbed from
phenyloctane solution directly on the HOPG surface. It should be also
noted that 1-1OHSQ was denoted 1-1 SQ in Ref. [33].
N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 9
heights were measured to be from 1.5 to 4.5 Å in thickness (~
one to three layers). The initial STM images showed that the
surface layers of 1-1OHSQ were smooth with domain
boundaries barely visible as fine fractures. A few minutes after
engagement the rastering motion of the tip began to remove
material from the domain boundaries. The removal of the
adsorbate layer also caused the STM image to become noisy
and streaky, presumably due to the interference of the desorbed
dye molecules with the tunneling junction. Both oxidized and
reduced forms of the dye were susceptible to tip erosion.
At smaller scan sizes (<100 nm) the adsorbate layers were
more resistant to tip-induced erosion and were less noisy as
long as no missing molecules defects or domain boundaries were
present. The duration of imaging a particular area was still
dictated by the removal of molecules in that area. At the smaller
scan sizes it was apparent that removal of additional molecules
was occurring at rectangular defects composed of a few (2-3)
missing molecules. These missing-molecule sites usually
occurred within a single molecular row. As the defect grew,
molecules from adjacent rows were also removed. The process
of defect nucleation and growth of holes in the layer can be
seen in Fig. 8. Continual scanning caused the defects to grow
until neighboring holes coalesced. Capillary forces and tip
collisions with domain or defect edges are presumed to be
responsible for the removal of the molecules from the surface.
As mentioned above, layers of reduced 1-1OHSQ appeared
smooth except for step edges and missing molecule defects,
which often revealed the presence of multiple layers (Fig. 9A).
At larger scan sizes the domain intersections are barely visible
except where erosion has occurred between the domains. A
few times small islands of molecules adsorbed on top of the
smooth mono or multiple layers were imaged. These islands
were 60 – 70 nm in length, 10 – 20 nm in width, and 2 – 3nm in
height. They tended to orient with their long axis at 30° or 60°
angles to each another and along crystallographic directions, as
can be seen in Fig. 9B, where some oriented islands are parallel
to a graphite step edge. At smaller scan sizes (<300 nm)
molecular lamella were observed. Widths of the bright stripes
were typically measured to be ~14 Å, a distance which
corresponds to the length of the chromophore of the 1-1OHSQ
molecule. The lamellar domains often showed rectangular
missing molecule defects. Some experiments, in which multiple
layers of 1-1OHSQ were adsorbed on the graphite surface,
exhibited entire missing rows (see Fig. 9C). These row defects
were measured to be one or more layers deep.
Figure 10 (right) shows a 15 nm × 15 nm image of the row
structure observed for the reduced 1-1OHSQ on HOPG. The
chromophores of the molecules appear as football-like shapes
measuring ~14 Å in length. The distance between chromophores
within the same row was measured to be ~10 Å, while the
distance between chromophores in adjacent rows was ~ 24 Å.
The angle of the long axis of the molecule to the direction of
the row was measured to be ~47°. Dialkylaminophenyl moieties
in squaraines have an electron-donating character while the
central four-membered ring has an electron-accepting character
[50]. The 47° angle of molecule to the row aligns adjacent
molecules within the same row such that their donor and acceptor
moieties interact (see the molecular model proposed for this
structure in Fig. 10). The donor-acceptor interaction between
adjacent molecules within the same molecular row has been
observed for other dialkylamino hydroxyphenyl squaraines and
we believe is the dominant interaction driving the ordering of
these squaraines on HOPG [33]. This structure is similar to the
row structure observed for 1-1OHSQ deposited onto HOPG
from phenyloctane solutions. A smearing of the electron density
between adjacent molecules (in the same row) was occasionally
observed and produced a bright zig-zag pattern within the row
(see the left image in Fig. 10). A relatively blunt tip that smears
Fig. 8. Series of 50 nm × 50 nm STM images of reduced 1-1 OHSQ adsorbed in rows on HOPG showing the time-lapsed erosion of the adsorbate
layer. Image (A) shows a number of missing-molecule defects which appear as dark rectangles. Image (B) was taken 1.5 minutes after image (A) and
shows the growth and coalescence of several of the defects. Arrows correlate the defects from image (A) which have grown and coalesced in image
(B). Image (C) was taken 1.5 minutes after image (B) and shows the erosion of the adsorbate layer in the upper right. The direction of thermal drift
is shown in the bottom right corner of the figure.
10 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
the electron densities from two adjacent aminophenyl moieties
(represented by the circled area in Fig. 10) might be responsible
for this pattern. Unit cell values for both phenyloctane-deposited
and dichloromethane-deposited 1-1OHSQ are shown in Table
1. Only the most commonly observed polymorph deposited
from phenyloctane from reference 33 (90˚ herringbone) was
included in Table 1. This polymorph was not observed in the
experiments reported herein and the phenyloctane-deposited row
polymorph reported herein was not observed previously. None
of the polymorphs (neither reduced nor oxidized) observed for
the dichloromethane deposition reported herein were observed
in reference 33.
Adsorbed 1-1OHSQ that had been electrochemically
oxidized in 0.1 M LiCl aqueous electrolyte showed multilayers
that were not as smooth as those formed by the reduced form of
the dye. As can be seen in Fig. 11A, the oxidized dye layers are
more broken and defective. Domain boundaries are more
obvious and are often rounded in shape as opposed to the
predominantly faceted domains of the reduced form. Domains
are also smaller, typically measuring 100–300 nm in diameter.
At smaller scan sizes lamella are visible that appear to be similar
to those of the reduced form (Fig. 11B). Rectangular missing
molecule defects are also visible. Repeated scanning eventually
expands these defects and erodes away the adsorbate layer again
in a similar fashion to the erosion of the reduced dye layers.
Missing row defects like those observed in the reduced form of
the dye were found in only a very small percentage of the
experiments.
Upon closer inspection of the lamella it was found that two
different row structure polymorphs exist for oxidized 1-1OHSQ.
Polymorphs are commonly observed in 2D surface structures
of squaraine molecules [33]. The unit cells measured for these
polymorphs can be found in Table 1. Figure 12 shows molecular
resolution images and proposed molecular models for both of
these row structure polymorphs. Both polymorphs exhibit
similar distances for both adjacent molecules within the same
row and molecules in adjacent rows. The angles of the molecules
within the row to the direction of the row, however, are different
for each of the polymorphs. Polymorph 1 exhibits a ~67° angle
of the molecules to the direction of the row. This angle does
not offset adjacent molecules from one another enough to
achieve “optimum” donor-acceptor interaction. In addition, the
partially-negatively charged oxygen atoms in the square core
are still relatively close. Repulsion between these two charged
moieties may be mitigated by possible hydrogen-bonding
between the partially charged oxygen and the hydroxyl group
of the molecule adjacent to it. Polymorph 1 was observed in
only ~25% of the experiments. Polymorph 2 exhibits a ~46°angle much like the reduced dye row structure. This angle offsets
adjacent molecules within the same row such that their donor
and acceptor moieties line up much more effectively, with a
minimum of Oδ- – Oδ- repulsion. The only difference between
the oxidized polymorph 2 structure and the reduced structure is
the distance between molecules in adjacent rows. This distance
Fig. 9. (A) 500 nm × 500 nm STM image showing smooth layers of
reduced 1-1 OHSQ on HOPG (dipping method). Multiple layers are
evident at defect sites. (B) 1.5 µm × 1.5 µm scan STM image of reduced
1-1 OHSQ monolayer adsorbed on HOPG terraces. Small islands
oriented at 30° or 60° to one another and parallel to HOPG step edge
are visible. (C) Several missing rows are evident in this 42 nm × 42 nm
STM image. Missing rows were not observed in every experiment and
were only observed in multiple layer adsorption.
N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 11
Fig. 10. A 15 nm × 15 nm scan STM image (right) showing the row orientation of reduced 1-1 OHSQ. Individual squaraine chromophores are visible
as bright football-like shapes. The black box represents the unit cell, which has been enlarged and projected onto the proposed molecular model
(bottom) for comparison. Donor-acceptor interactions between adjacent molecules within the same row appear to be the dominant interaction driving
this order. A 5 nm × 5 nm imgae (left) shows a zig-zag contrast which is sometimes observed for the reduced form of 1-1 OHSQ. The black ovals
overlayed on both the left image and the model illustrate the coupling of electron density between adjacent molecules within the same row which
might lead to such a zig-zag structure.
etatS tnevloS c hpromyloP a Å/ b Å/ α ˚/ Å/aeraraluceloM 2 elucelom 1-
DER enatcolynehP enobgnirreh˚09 d 2±52 2±52 2±09 91±651
DER enatcolynehP woR e 1±12 1±01 4±53 61±012
DER HC 2 lC 2 woR 6.0±2.42 5.0±9.9 3.1±5.64 9±042
XO HC 2 lC 2 wor1epyT 8.0±4.02 4.0±3.9 0.4±8.76 8±091
XO HC 2 lC 2 wor2epyT 0.1±6.02 0.1±6.9 0.4±2.64 51±002
Table 1
Unit cell parameters for reduced a (RED) and oxidized b (OX) forms of 1-1OHSQ layers adsorbed on HOPG as determined by STM
a Before electrochemical treatments.b Electrochemically oxidized in 0.1 M LiCl aqueous electrolyte; chloride ions were presumably incorporated as counter ions.c Solvents used for deposition of 1-1OHSQ.d Most commonly observed polymorph in the previous STM study (Ref. [33]), but was not observed again in this study. Note that 1-1OHSQ
was denoted as 1-1 SQ in Ref. [33].e Row polymorph observed for the reduced form of 1-1OHSQ deposited from phenyloctane (same manner as in Ref. [33]) in this study was
not observed in Ref. [33].
12 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
is ~1 Å longer in the reduced form, resulting in the reduced
structure having a slightly smaller packing density. This is
counterintuitive since it would be expected that the oxidized
structure would be slightly larger to account for the presence of
the counterions within the adsorbate layer. This contraction of
the adsorbate structure may explain the increased observance
of missing molecule defects in the oxidized adsorbate as
compared to the reduced. Also the double peaks sometimes
observed in the SCV (Fig. 1(b)) may be due to different
polymorphs having slightly different redox properties.
The actual position of the chloride ion within the adsorbate
unit cell is not known at this time. A speculative position is
shown in the molecular models of Fig. 12. Four factors were
taken into account in the placing the chloride at this position:
(1) the most probable site for the oxidation of the squaraine
molecule is the amino groups; (2) the delocalization of this
partial positive charge is most likely over the amino nitrogen
and the phenyl carbon bonded to it; (3) the close proximity of
positive charged parts of adjacent rows to one another; and (4)
the electrochemical data which show that the ion is most likely
associated 1:1 with a 2D phase of oxidized dye molecules rather
than being diffusely associated with the oxidized layer. It is
possible that the counterion resides slightly above the plane of
the adsorbate between the amino groups of adjacent rows nested
above the methyl groups as is if they were the bottom of a
dimethylamino solvation cage around the chloride ion. There
is also a possibility that water or dichloromethane solvent
molecules are incorporated in these structures and are not
resolved in the STM images. Loss of solvent molecules upon
oxidation may also account for the smaller unit cell areas for
the oxidized molecules.
3.7 Effect of morphologies on electrochemistry of 1-1OHSQ
deposits on HOPG
It has been shown that deposition of 1-1OHSQ molecules
from dichloromethane solution by the dipping method produced
electrochemically active 1-1OHSQ layers on HOPG.
Alternatively, deposition could be accomplished by dropping a
known amount of dye solution onto HOPG as described in the
experimental section (approximately 1 ML amount, ~0.8 × 10-
10 mol cm-2 obtained from STM results). However, electrodes
fabricated in this way revealed little or no well-defined redox
peaks in cyclic voltammograms tested in 0.1 M LiCl aqueous
solution. We attribute this to a morphological effect of 1-1OHSQ
molecules on voltammetric behavior as discussed below.
Figure 13 shows a 5 µm × 5 µm scan AFM image of 1-
1OHSQ on HOPG prepared by the dropping method. Many
bright areas attributable to deposited 1-1OHSQ aggregates or
small crystallites can be seen. These structures have typical
heights of several nanometers but sometimes heights exceed
10 nm. The aggregates appear to align along specific directions.
Similar tall aggregate structures of 1-1OHSQ were also observed
on the basal planes of SnS2 single crystals when a similar
deposition method was used [32]. We speculate that the 3D
aggregates have difficulty in incorporating the electrolyte anions
deep into their bulk structure even though a bias positive enough
to oxidize the free molecule was applied. In other words, the
potential drop developed between the electrode surface and the
dye in the resistive 3D aggregates was insufficient to drive the
reaction. A similar logic is true for electrodes coated with
polymer films containing redox active groups that cannot be
permeated by counterions [37].
However, samples prepared by the dipping method showed
electrochemical activity even when the surface coverage of 1-
1OHSQ was estimated to be several layers (judging from total
area of voltammetric peaks and STM observations of the
samples). In some cases a diffusion tail on the SCV peaks did
indicate that diffusion was operating at high coverages (see Fig.
7(a)). The photoelectrochemical behavior of the 3D aggregates
of 1-1OHSQ was shown to be much different than for
monolayers of this dye [32]. The red shift in the photocurrent
Fig. 11. (A) 500 nm × 500 nm STM image showing broken layers of
oxidized 1-1 OHSQ. Rounded domain boundaries are clearly visible.
(B) 3D representation of a 98 × 98 nm scan STM image of an oxidized
layer showing lamella and rectangular defects.
N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye 13
Fig. 12. (A) 20 nm × 20 nm scan STM image showing polymorph 1 type row structure. Oxidized chromophores are evident as bright football-like
shapes. The unit cell is represented by the black box overlayed on the image and projected onto the proposed molecular model. Donor-acceptor
interactions as well as hydrogen-bonding appear to drive this polytype. Possible positions of the chloride counterions are included within the model.
(B) 28 nm × 28 nm scan showing oxidized polymorph 2 type row structure. In this image oxidized chromophores appear as bright dog bone-like
shapes. The slightly more acute angle of the molecular long axis to the direction of the row for this polymorph allows adjacent donor and acceptor
moieties to come into closer proximity.
14 N. Takeda et al. / Electrochemical Oxidation and ex-situ STM Observations of Squaraine Dye
action spectrum for the tall aggregates is indicative of different
molecule-molecule interactions than in the monolayer.
Incorporation of the anions into these 3D structures is necessary
for the oxidation of the 1-1OHSQ molecules but the 3D structure
may have no mechanism for ion conduction. Thus, the
electrochemically inactive aggregates could have different
aggregate structures than the active layers.
4. Conclusions
The potential for electrochemical oxidation of 1-1OHSQ
dye layers adsorbed on the basal plane HOPG electrodes in
aqueous electrolytes has been shown to be strongly dependent
on the properties of the electrolyte anions. Staircase cyclic
voltammograms (SCVs) of these electrodes revealed very sharp
peaks and a large hysteresis for oxidation and reduction of the
dye layers. The large rearrangement of the adsorbed dye layers,
due to the incorporation of the electrolyte anions upon oxidation,
is responsible for the large hysteresis in the voltammograms.
Whether the hysteresis is explained by the miscibility gap
model [29] or if a nucleation model can explain the results is
still an open question. Nucleation does appear to be important
especially in the reduction of the oxidized dye layers. We
attribute the many current spikes in the SCVs to the reduction
of different domains of oxidized 1-1OHSQ. STM images
showed the presence of many domains in both the oxidized and
reduced forms. The increased stability of the ionic 2D lattice
perhaps is responsible for the difficulty of reducing it and
expelling the anions. If a nucleation event was limiting the re-
reduction of the layers it would be expected that the smaller
domains would be less likely to have a nucleation event and
would require a larger electrochemical potential to initiate
reduction. In most experiments most of the charge was present
in the initial reduction peak and after repeated scanning the small
current spikes would eventually disappear. We attribute this to
annealing the small domains into larger domains during the
continual oxidation-reduction cycles.
STM images also provided information on the molecular
arrangements of both the oxidized and reduced forms of dye
layers. Differences observed between the two phases could be
the results of structural changes associated with anion
incorporation caused by electrochemical oxidation. However,
definitive proof of anion incorporation and their location was
not obtained from the present STM study. Further investigation
of redox processes of surface confined ordered molecular layers
is needed to work out the detailed mechanism of the surface
transformation. AFM and STM experiments done in-situ in an
electrochemical cell will be able to shed light on these processes
as well as the mechanism of transforming the 2D reduced dye
structures into the 2D oxidized structures.
Acknowledgments
We would like to thank the Lexmark International
Corporation for providing squaraine dyes used in this study.
This work was supported by the Department of Energy Office
of Basic Energy Sciences under contract DE-F603-96ER14625.
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