Decamethylferrocene redox chemistry and gold nanowire electrodeposition at salt crystal|electrode|nonpolar organic solvent contacts

Article abstract of DOI:10.1021/om2006112

This report describes exploratory experimental findings for electrochemical processes in nonpolar solvents (hexane, toluene, and dichloroethane). Conventional 3 mm diameter glassy-carbon-disk electrodes are used in contact with a crystalline salt electrolyte (ammonium nitrate) immersed in nonpolar solvents. The insoluble salt is employed as a "surface thin film electrolyte", with humidity causing electrical connection from the working electrode to the SCE counter-reference electrode. The organic solvents are employed without intentionally added electrolyte. Humidity in the nonpolar solvents is shown to be essential for the processes to work. The oxidation of decamethylferrocene is demonstrated as a test organometallic redox system. The electrochemical reduction of Au(III) in toluene (solubilized with tetraoctylammonium bromide, TOABr) is employed to demonstrate and visualize the reaction zone around salt crystal|working electrode contact points. Gold nanowire bundle formation is observed, presumably due to an ordered interfacial surfactant microphase at salt|electrode contact points. The triple phase boundary nature of these processes is discussed, and future applications are suggested.

Full text of DOI:10.1021/om2006112

Article
pubs.acs.org/Organometallics
Decamethylferrocene Redox Chemistry and Gold Nanowire
Electrodeposition at Salt Crystal|Electrode|Nonpolar Organic Solvent
Contacts
John D. Watkins, Christopher E. Hotchen, John M. Mitchels, and Frank Marken*
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
ABSTRACT: This report describes exploratory experimental
findings for electrochemical processes in nonpolar solvents
(hexane, toluene, and dichloroethane). Conventional 3 mm
diameter glassy-carbon-disk electrodes are used in contact with
a crystalline salt electrolyte (ammonium nitrate) immersed in
nonpolar solvents. The insoluble salt is employed as a “surface
thin film electrolyte”, with humidity causing electrical
connection from the working electrode to the SCE counter-
reference electrode. The organic solvents are employed
without intentionally added electrolyte. Humidity in the
nonpolar solvents is shown to be essential for the processes
to work. The oxidation of decamethylferrocene is demon-
strated as a test organometallic redox system. The electro-
chemical reduction of Au(III) in toluene (solubilized with tetraoctylammonium bromide, TOABr) is employed to demonstrate
and visualize the reaction zone around salt crystal|working electrode contact points. Gold nanowire bundle formation is observed,
presumably due to an ordered interfacial surfactant microphase at salt|electrode contact points. The triple phase boundary nature
of these processes is discussed, and future applications are suggested.
INTRODUCTION
■
Electrochemical processes for the conversion of organic or
organometallic redox systems in nonpolar solvents are
important and desirable even in highly nonpolar media such
as fluorohydrocarbons,1 toluene,2 oils,3 and heptane.4 New
methods have been developed, for example, on the basis of
novel nonpolar solvent soluble and sufficiently dissociated
electrolyte salts or ionic liquids,5,6 on the basis of liquid−liquid
triple phase boundary reactor systems7 where electrolytic
conduction is required only in the polar phase,8,9 and on the
basis of particle supported (heterogeneous) electrolyte systems
such as pyridinium-substituted polymer beads.10,11 The last
technology allowed substantial currents to be passed and bulk
product to be generated with a heterogeneous supporting
electrolyte system that is readily recovered by filtration and
reused. In the work presented here, a heterogeneous supporting
electrolyte system is proposed on the basis of insoluble salt
(ammonium nitrate) crystals in contact with the working
electrode surface and surrounded by the nonpolar reagent
media.
Figure 1A shows a schematic drawing with a salt crystal in
contact with a glassy-carbon-electrode surface. In a dry
nonpolar solvent environment there would be no significant
ionic conductivity and therefore electrochemical processes are
difficult to observe. However, when the solvent is saturated
with water before use (mutual solubilities at 298 K are x2 = 6 ×
10−4 or 4.6 mM for water in hexane,12 3 × 10−3 or 28 mM for
water in toluene,13 and 10 × 10−3 or 130 mM for water in
Figure 1. Schematic representation of (A) a salt|nonpolar solvent|
electrode contact and (B) the electrochemical cell with humidified
nonpolar solvent flowing through the salt.
dichloroethane14), the surface of the salt crystals (here
ammonium nitrate) will be equilibrated and coated with a
thin water layer sufficient for ion conductivity to be achieved.
The cation C+, the anion A−, or additional ions such as protons
H+ are likely to contribute to the charge transport (see Figure
1A) and the overall process. In the vicinity of the contact point
of the salt with the electrode surface various types of redox
processes become possible, including processes involving
redox-active reagents dissolved in the nonpolar solvent.
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: July 9, 2011
Published: January 10, 2012
© 2012 American Chemical Society
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In recent reports the use of salt|electrode contact processes
has been demonstrated for the case of salt surface immobilized
redox systems,15 for palladium catalyst coated electrodes,16 and
for textile-coated electrodes.17 In this study it is demonstrated
that ammonium nitrate salt crystals can be used as a “thin film
electrolyte phase” also in a nonpolar solvent such as hexane to
electrically connect the working electrode and a counter-
reference electrode (see Figure 1B). The resulting salt|electrode
contact points allow solvent flow experiments and provide
unique reaction environments for redox processes, involving for
example decamethylferrocene metal complexes, as well as for
bulk metal deposition from a Au(III) precursor solution in
toluene. The key aims of this study are (i) to demonstrate
redox processes in nonpolar solvents such as hexane and
toluene, (ii) to introduce salt|nonpolar solvent|electrode
contacts for electrochemical processes, and (iii) to employ
gold electrodeposition in order to visualize localized activity
and nanoscale pattern formation on electrode surfaces.
here in the form of a nitrate salt (see eq 1).
−
FeCp* (hexane) + NO (salt)
2
3
+
−
−
⇄ FeCp* NO (solid) + e (electrode)
(1)
2
3
The shape of the voltammetric peak responses is symmetric
at lower scan rates (10 mV s−1 and less), and the absence of a
“diffusional tail” is observed for both the oxidation and the
back-reduction. For the oxidation the formation of the product
FeCp2*+NO3−(solid) as a precipitate in the electrochemically
active region of the electrode is likely to lead to electrode
blocking and thin film behavior (vide infra). During the back-
reduction the solid precipitate redissolves. The separation of
the oxidation and back-reduction peaks (hysteresis) is due here
to the formation of the new solid phase.
When the scan rate is increased, the peak currents for both
oxidation and back-reduction increase while the charge under
the voltammetric response decreases with higher scan rate. The
shape of these voltammetric responses can be seen to be
affected by uncompensated iR-drop phenomena with an
estimated resistivity of 70 kΩ (see Figure 2A inset). This
resistivity is likely to be localized predominantly in the salt
electrolyte connecting the working and the counter-reference
electrode. However, contributions from the hexane|salt|
electrode triple phase boundary are also possible (vide infra).
An important parameter in this experiment is the humidity level
in the organic solvent. When the experiment is conducted with
dry hexane, there are no significant currents observed. Also,
when the experiment is conducted in wet solvent without salt,
no significant currents are observed. In contrast, water-
saturated hexane (containing ca. 4.6 mM water12) allows
sufficient ion conduction at the salt|organic solvent interface to
occur.
RESULTS AND DISCUSSION
■
Oxidation of Decamethylferrocene at Salt | Electrode
Contacts. I. Processes in Hexane. Decamethylferrocene is a
well-known one-electron-redox system with good solubility in
nonpolar media such as hexane.18 The redox chemistry of
decamethylferrocene has been studied in the past in solution,19
as a solid immersed in aqueous media,20 and in liquid|liquid
environments.21,22 Here decamethylferrocene is dissolved into
hexane (humidified but without any added electrolyte) and
investigated at salt|glassy-carbon working electrode contacts.
Figure 2A shows a typical set of cyclic voltammograms obtained
II. Comparison of Hexane, Toluene, and Dichloroethane.
In order to explore the reactivity of decamethylferrocene in
other nonpolar solvent media under the same salt|electrode
contact conditions, experiments were performed with toluene
and with dichloroethane. Figure 2B shows typical voltammetric
responses for the oxidation of 2 mM decamethylferrocene in
toluene as a function of scan rate. At lower scan rates peak-
shaped responses are observed with a midpoint potential of ca.
Emid = 0.17 V vs SCE. The process is very similar to that
observed in hexane, and therefore a similar mechanism is
suggested (eq 2).
−
FeCp* (toluene) + NO (salt)
2
3
+
−
−
⇄ FeCp* NO (solid) + e (electrode)
(2)
2
3
Figure 2. Cyclic voltammograms (scan rate (i) 10, (ii) 20, (iii) 50, and
(iv) 100 mV s−1) for (A) the oxidation and back-reduction of 2 mM
decamethylferrocene in humidified hexane in contact with NH4NO3|
glassy carbon, (B) as for (A) but in toluene, (C) as for (A) but in
dichloroethane.
The charge under the voltammetric peaks in toluene
(humidified) is similar to that for the hexane process, and the
effect of resistivity appears similar. In contrast, when the
process is investigated in dichloroethane (humidified), a more
reversible voltammetric response is observed (see Figure 2C).
Oxidation and back-reduction peaks are less separated and the
midpoint potential is ca. Emid = −0.02 V vs SCE. The negative
shift in the midpoint potential is likely to be caused by the
for 2 mM decamethylferrocene in hexane (humidified) under
no-flow conditions. A clear oxidation and back-reduction redox
cycle is observed centered at Emid = 1/2(Epox + Epred) = 0.16 V
vs SCE, with a clear separation between the oxidation and the
reduction peaks. The oxidation product, decamethylferroce-
nium cations, is insoluble in hexane and assumed to precipitate
+
higher solubility of the product FeCp*2 in dichloroethane
(DCE), and a modified mechanism is proposed (eq 3).
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FeCp* (DCE) + NO (salt)
Article
−
2
3
+
−
⇄ FeCp* (DCE) + NO (DCE)
2
3
−
+ e (electrode)
(3)
Voltammetric responses in dichloroethane do exhibit a
typical “diffusional tail” in the current, and an analysis of the
charge under the peaks suggests planar diffusion of product
away from the reaction zone (no formation of precipitate; vide
infra). Therefore, in this case a comparison of the experimental
peak current with the theoretically expected peak current is
possible. The theoretical peak current (for planar diffusion to a
flat electrode) as a function of scan rate is given by the
Randles−Sevcik expression23 (see eq 4), where the peak
current Ip is related to the number of transferred electrons per
molecule diffusing to the electrode surface, n, the Faraday
constant, F, the gas constant, R, the absolute temperature, T,
the scan rate, v, the electrode area, A (without taking into
account partial blocking of the electrode by salt crystals; vide
infra), the diffusion coefficient, D, and the concentration of
redox-active material, c.
Figure 3. (A) Cyclic voltammograms (scan rate 10 mV s−1) for the
oxidation of 2 mM decamethylferrocene in humidified (i) hexane, (ii)
toluene, and (iii) dichloroethane obtained at a NH4NO3 salt|glassy-
carbon electrode contact. (B) Cyclic voltammograms (scan rate 10 mV
s−1) for the oxidation of (i) 1 mM, (ii) 2 mM, and (iii) 5 mM
decamethylferrocene in hexane. (C) Cyclic voltammograms (scan rate
10 mV s−1) for the oxidation of 2 mM decamethylferrocene in
dichloroethane in (i) static and (ii) flowing solution at 400 μL min−1.
nFνD
RT
Ip = 0.446nFAc
(4)
For decamethylferrocene in dichloroethane the diffusion
coefficient estimated with the Wilke−Chang expression24 is 0.7
× 10−9 m2 s−1 (at 293 K) and therefore the expected peak
current at 10 mV s−1 scan rate is Ip = 10 μA. The measured peak
current is only 2 μA, suggesting that only approximately 20% of
the area of the glassy-carbon electrode surface is active. The
remaining 80% of the electrode area is either blocked by salt
crystals or is too far from the salt contacts and remains inactive.
The extent of the “active zone” in this experiment is dependent
on the salt crystal size and shape or coverage at the electrode
surface. A summary of the effect of the solvent on the
voltammetric response is shown in Figure 3A. Both the peak-to-
peak separation and the resistivity effect are changed on going
from highly nonpolar (hexane, toluene) to modestly nonpolar
(dichloroethane) solvent systems.
The effect of the decamethylferrocene concentration on the
voltammetric response is shown in Figure 3B for the case of
hexane solutions. The precipitation mechanism is responsible
for a nonlinear correlation, with higher concentrations causing
slightly sharper peaks without an increase in area under the
peak. The shape of the peaks also suggests a decrease in
resistivity upon increasing the decamethylferrocene concen-
tration. This as well as the considerable overpotential for
oxidation at lower concentrations is associated with the
precipitation mechanism, where a higher concentration (or
more correctly a higher activity) may allow a more rapid
precipitation closer to the salt|electrode contact region.
Nonpolar solvent contributions to the resistivity in the triple
phase boundary region seem possible under these conditions.
Next, the effect of solvent flow on the voltammetric
responses is investigated. A syringe pump was employed for a
flow of 400 μL min−1 of solvent through the salt layer (see
Figure 1B). Perhaps surprisingly, the flow did not affect the
voltammetric responses for hexane and toluene (not shown).
Peak-shaped oxidation and back-reduction responses were
observed in both cases. Therefore, the flow-induced diffusion
layer thickness change was ineffective in preventing the
precipitation process. However, for the dichloroethane system
an increase in current and a change from peak-shaped to
sigmoidally shaped current response is seen (Figure 3C).
Liquid flowing out from the cell exhibits a green color, as
expected for the oxidized decamethylferrocenium. These
observations confirm the proposed mechanism in eq 3.
Reduction of Au(III) to Gold Nanowires at Salt|
Electrode Contacts. In order to investigate the reaction
zone formed when NH4NO3 salt crystals are in contact with the
glassy-carbon electrode surface, additional experiments were
conducted employing a gold electrodeposition approach. In
order to visualize the electrochemically active zone for salt|
electrode contact points, a deposition from a toluene solution
of AuIIICl4− was chosen. Tetraoctylammonium bromide as a
phase transfer reagent was employed to transfer tetrachlor-
oaurate with a final concentration of 1 mM AuCl4− and 5 mM
tetraoctylammonium bromide in toluene. Voltammetric
responses (not shown) suggest highly resistive conditions. In
order to achieve a gold deposition current of ca. 10 μA, a
potential of −3 V vs SCE was applied and the deposition
continued for 30 min.
SEM imaging of the resulting electrode surface after rinsing
(with acetone and water) and drying shows a characteristic
pattern with the outline of salt crystals (ca. 10−100 μm in size)
clearly visible (see Figure 4B). Energy-dispersive X-ray analysis
confirmed the presence of a gold deposit (not shown). Perhaps
surprisingly, the gold deposits exhibit a fine structure. Closer
inspection (see Figure 4D) shows bundles of nanowires with
individual filaments of less than 10 nm diameter. The
mechanism is similar to that for the recently discovered
surfactant template induced gold nanowire formation25 and
related to the nanostructure electrodeposition in lyotropic
media pioneered by Attard et al.26 The process can be
explained on the basis of a liquid crystal phase domain formed
in the vicinity of the salt crystals in the presence of humidity
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Figure 4. SEM images for gold deposits formed at −3 V vs SCE at NH4NO3|electrode contact points with ca. 10 μA deposition current and 30 min
deposition time in 1 mM AuCl4− and 5 mM tetraoctylammonium bromide in toluene (see the Experimental Section).
and tetraoctylammonium bromide (eq 5).
cene has been demonstrated and investigated as a mechanism
involving precipitation in highly nonpolar media. The shape of
voltammetric responses (static and with solvent flow) changes
considerably when precipitation is avoided in dichloroethane
solvent. The visualization of the triple phase boundary reaction
zone was possible with a gold electrodeposition from toluene
solution. Gold nanowire formation was observed in the
presence of a surfactant template based on tetraoctylammo-
nium bromide and humidity at the salt|electrode contact.
In the future, the effects of the salt crystal grain size, single-
crystal salt contacts with electrodes, and the nature of the salt
on the electrochemical process (e.g., localized pH control or
direct supply of reagents) need to be studied in more detail.
New types of electrochemical processes will be possible in
media such as nonpolar oils, with a considerable range of
applications.
AuCl4−(toluene) + 3e−(electrode)
⇄ Au(nanowire) + 4Cl−(toluene)
(5)
The reaction zone is predominantly localized around regions
where salt crystals contacted the electrode surface. Lines of
good contact can be observed, and these indicate where the
ammonium nitrate crystals interacted with the glassy carbon.
The estimate of ca. 20% active electrode area (from
voltammetry data; vide supra) can now be rationalized with
this image (Figure 4B). Active electrode zone information from
the gold deposit in conjunction with an assumed average
diffusion path of ca. δ = (DRT/vF)1/2 = 42 μm (see parameters
in eq 4) suggests that a substantial part (more than 20%) of the
solution phase close to the electrode surface can be accessed.
Electrode blocking due to salt crystals appears therefore likely
to be the main reason for suppressed currents in data presented
in Figure 2C.
Potential applications of the localized or patterned electro-
deposition of gold nanowires could be in sensor and electronic
component fabrication. The ammonium nitrate salt crystals
could be replaced by other surface ion conductor materials to
allow more precise control of the patterning process and of the
conditions at the contact point. Gold could be replaced by
other types of metal or nonmetal precursors. Potential future
applications of the nonpolar solvent|salt|electrode interface
could also be in flow sensing (e.g., in oils, nonpolar media,
supercritical or gas media).
EXPERIMENTAL SECTION
■
Chemical Reagents. Ammonium nitrate (≥99%, Sigma Aldrich),
gold(III) trichloride (99.999%, Aldrich), hydrochloric acid (reagent
grade, Fisher Scientific), tetraoctylammonium bromide (Aldrich,
98%), decamethylferrocene (Aldrich, 97%), 1,2-dichloroethane (Al-
drich, GC grade, 99%), toluene (Sigma Aldrich, HPLC grade), and
hexane (Fisher, HPLC grade) were obtained and used without further
purification. Filtered and demineralized water was taken from a
Millipore water purification system with not less than 18 MΩ cm
resistivity.
Instrumentation. For voltammetric studies a microAutolab III
potentiostat system (EcoChemie, Utrecht, The Netherlands) was
employed with a KCl-saturated calomel (SCE) counter/reference
electrode (Radiometer, Copenhagen, Denmark). The working
electrode was a 3 mm diameter glassy-carbon disk (BAS). The surface
morphology and topology of the films were observed using a JEOL
JSM6480LV scanning electron microscope and a JEOL JSM6301F
field emission scanning electron microscope. All experiments were
conducted at 22 2 °C.
CONCLUSIONS
■
It has been shown that electrochemistry in highly nonpolar
solvent media such as hexane is possible at salt|solvent|electrode
triple phase boundary contact points in the presence of
sufficient levels of humidity. The oxidation of decamethylferro-
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Procedure I: Salt Cell Experiments. To prepare the electro-
chemical cell, ca. 1 g of ammonium nitrate was ground using a pestle
and mortar into a finely divided powder (10−100 μm particles). This
powder was then loaded into the cell onto the counter-reference (see
Figure 1B) from the working electrode side. The working electrode
was then gently screwed down to compact the powder into a solid
cake with a good electrical contact between the powder and both the
working and counter/reference electrode. The organic solvent
(hexane, toluene, or dichloromethane, pre-equilibrated with water)
containing a redox probe was then flowed through the cell from the
side with a syringe pump. Experiments were conducted without flow
or with typically 400 μL min−1 solution flow rate.
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Procedure II: Salt Cell Gold Electrodeposition Experiments.
Gold deposition experiments were carried out using a 1 mM solution
of tetrachloroaurate/5 mM tetraoctylammonium bromide in toluene
under static conditions. The solution was prepared following a recipe
by Brust et al.27 A 3 mL volume of a solution of 30 mM of hydrogen
chloride and 30 mM gold(III) trichloride was added to 8 mL of a
solution of 50 mM tetraoctylammonium bromide in toluene. The
mixture was shaken and left to equilibrate for several minutes until the
aqueous layer (bottom) became decolorized and the toluene layer
(upper) became bright red. The toluene layer was recovered by syringe
and diluted with 80 mL of toluene to give a solution with ca. 1 mM
Au(III) and ca. 5 mM tetraoctylammonium bromide. The deposition
potential in the salt cell was chosen to provide roughly 10 μA of
current (ca. −3 V vs SCE). The electrodeposition of gold was carried
out for 30 min (under no-flow stationary conditions), after which the
electrode was removed, thoroughly rinsed with acetone and distilled
water, and dried. Gold deposits were seen by optical microscopy and
further confirmed by EDS and SEM imaging methods.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: F.Marken@bath.ac.uk.
ACKNOWLEDGMENTS
■
Charles Y. Cummings is gratefully acknowledged for help with
SEM imaging. J.D.W. thanks the EPSRC for support through a
DTA studentship.
DEDICATION
■
Dedicated to the memory of Gordon Stone, an inspiring
chemist, leader, and pioneer.
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