1230 J. Phys. Chem. B, Vol. 102, No. 7, 1998
Bond et al.
were HPLC grade (Mallinckrodt, Paris, KY). All electrolytes
were of either electrochemical or analytical reagent grade.
Autoscan system (20 KV accelerator voltage) was used for the
SEM measurements.
In Situ Optical Microscopy. To observe changes occurring
during the redox reactions of [Na(diglyme)2][V(CO)6], a video
Electrochemical Apparatus and Conditions. Cyclic vol-
tammetric and chronoamperometric measurements were ob-
tained with a Cypress CS 1090 system (Cypress Systems,
Lawrence, KS). Multiple scan cyclic voltammograms were
recorded on a BAS 100A (Bioanalytical Systems, West Lafay-
ette, IN) electrochemical analyzer. Simultaneous cyclic (stair-
case) voltammetric and mass balance experiments were under-
taken with an ECQCM consisting of an Elchema (Elchema,
Postdam, NY) model EQCN-701 nanobalance and model PS-
1
3
imaging system consisting of a Nikon Epiphot Inverted
Metallurgical Microscope fitted with a special long-working-
distance objective lens, a CCD TV camera, and an Imaging
Technology PCVision frame grabber was incorporated into an
electrochemical apparatus controlled by a BAS 100A electro-
chemical analyzer. For these experiments, a special electro-
chemical cell with a 20-mm diameter quartz window bottom
was used in which the distance between the working electrode
and the quartz window could be adjusted to ≈0.1 mm.
2
05 potentiostat. The system was controlled with a 486 PC
running VOLTSCAN software (Intellect Software, Postdam,
NY).
Results and Discussion
All experiments were undertaken at ambient temperatures of
-
20 ( 1 °C, and solutions were deoxygenated using high purity
A. Reactivity of [Na(diglyme) ][V(CO) ] and V(CO)6
2
6
nitrogen (BOC Gases, Chatswood, NSW, Australia). Simulation
of the solution-phase cyclic voltammetric responses was achieved
with the computer package DIGISIM Version 2.1 (Bioanalytical
Systems, West Lafayette, IN).
with Oxygen and Water. In the absence of oxygen and water,
the voltammetry of [Na(diglyme)2][V(CO)6] in acetonitrile and
acetone solutions (0.1 M Bu4NPF6) was well defined as
1
2
previously reported. Under the conditions of cyclic voltam-
-
1
metry (scan rates between 10 and 1000 mV s ), a reversible
one-electron redox couple (eq 1) is observed at platinum (Pt),
gold (Au) and carbon (C) electrodes for the oxidation of
The reference electrode for aqueous and acetone experiments
was Ag/AgCl (3 M KCl). This reference electrode was
3-/4-
calibrated against the [Fe(CN)6]
KCl and the reversible half-wave potential E 1/2 was 0.270 (
.002 V versus Ag/AgCl (3 M KCl). All potentials recorded
couple in aqueous 1 M
-
+
r
[V(CO)6] at a half-wave potential of -0.35 V versus Fc /Fc
+
in acetonitrile and at -0.36 V versus Fc /Fc in acetone.
0
The value of the reversible half-wave potential implies that
in aqueous (electrolyte) media are reported relative to this
chemical system. In acetone, the potentials are quoted relative
-
[
V(CO)6] is likely to be air sensitive, and, in fact, a small
+
percentage of V(CO)6 is always present in the solid. However,
oxidation of the solid [Na(diglyme)2][V(CO)6] by molecular
to that of the ferrocenium/ferrocene (Fc /Fc) couple. The
+
reference electrode used for experiments in acetonitrile was Ag /
-
1
oxygen is a very slow process. A strong band at 1842 cm in
the infrared (IR) spectrum of the solid (Nujol mull)12 and the
observation of the expected migration current from steady-state
voltammetry at a Pt microdisk electrode in the absence of
Ag (10 mM AgNO3 with 0.1 M Bu4NPF6 in acetonitrile). This
+
reference electrode also was calibrated against the Fc /Fc
couple, and all potentials in acetonitrile are reported relative to
ferrocene. The auxiliary electrode was a platinum wire.
1
4
electrolyte confirm that the majority of the solid sample is in
The range of working electrodes included a platinum disk
electrode (1.6 mm diameter), a gold disk electrode (1.6 mm
diameter), a glassy carbon disk electrode (3.0 mm diameter), a
basal plane pyrolytic graphite disk electrode (5.0 mm diameter),
and a platinum microdisk electrode (9.8 µm diameter). The
working electrode for the ECQCM measurements was one side
of a 13-mm diameter AT-cut quartz crystal (Bright Star Crystals,
Rowville, Victoria, Australia) that had gold disks (5.0 mm
diameter) vapor deposited on each side and that oscillated at a
frequency of 10 ( 0.05 MHz. The calibration of the ECQCM
was as previously described.8 The [Na(diglyme)2][V(CO)6] was
readily attached to the electrode by rubbing the electrode surface
onto the solid compound. The electrode containing the me-
chanically attached solid was then placed into the aqueous
-
the anionic [V(CO)6] form.
The rate of reaction of [V(CO)6] with oxygen in solution
-
also was found to be slow and of no significance on the
voltammetric time scale. However, the presence of oxygen in
solution causes the couple to become irreversible, due to the
rapid reaction of oxygen with V(CO)6 to give unidentified
products. Consequently, thorough deoxygenation of solutions
was maintained in all experiments.
0
/-
Because the voltammetry of the [V(CO)6] system was to
be studied at an aqueous (electrolyte) interface, the effect of
added water to solutions also was investigated. Figure 1a shows
-
the solution voltammetry of [V(CO)6] in acetone alone,
whereas Figures 1b-d show that the addition of water signifi-
o/-
cantly decreases the chemical reversibility of the [V(CO)6]
(
electrolyte) medium in order to obtain the solid-state voltam-
mograms.
Fourier Transform Infrared Spectroscopy. The Fourier
transform infrared (FTIR) spectrum for the [Na(diglyme)2]-
V(CO)6] was recorded on a Perkin-Elmer 1600 series FTIR
couple. This result indicates that V(CO)6 in solution reacts with
water as well as with oxygen. Figure 2 shows the voltammo-
-
grams at various scan rates of a solution of [V(CO)6] in acetone
with 10% added water. The couple is reversible only at scan
[
-1
rates >1000 mV s . Because Figure 1 shows the reaction rate
spectrometer using a Nujol mull. For gas-phase FTIR measure-
ments, the cell contained NaCl windows, and spectra were
of V(CO)6 with water is dependent on the water concentration,
15
the system was modeled with the DIGISIM computer program
-
1
obtained with a resolution of 1 cm using a BIORAD FTS-60
spectrometer.
as a pseudo-first-order EC reaction according to eqs 2a and 2b:
-
-
Scanning Electron Microscopy. Samples of [Na(diglyme)2]-
V(CO)6] for examination by scanning electron microscopy
SEM) were prepared by rubbing a freshly cleaved basal plane
graphite plate (5 × 5 × 1 mm) onto a small amount of the
carbonyl compound on a filter paper. The surface-modified
carbon plate was fixed with double-sided sticky tape onto a stub
and gold plated in a Belzers sputter-coating unit. An ETEC
(
E)
[V(CO) ] h V(CO) + e
(2a)
(2b)
6
6
[
(
k[H O]
2
(C)
V(CO)6
8 products
The value of the pseudo-first-order rate constant, k[H O] was
measured. The second-order rate constant, k′ was calculated
2