Adsorption and electrodeposition on SnO2 and WO3 electrodes in 1 M LiClO4/PC. In situ light scattering and in situ atomic force microscopy studies

Article abstract of DOI:10.1149/1.1393607

A novel spectroscopic in situ light scattering technique was used with in situ atomic force microscopy (AFM) to study electrode surfaces subjected to adsorption and electrodeposition. Tin dioxide and tungsten trioxide were used as electrodes in a 1 M LiClO₄/propylene carbonate electrolyte. Both in situ methods showed the same increase in surface roughness immediately after the electrode was immersed in the electrolyte. The onset potential for electrodeposition could be determined; its specific value depended on the film composition as well as on the composition and purity of the electrolyte. A potential step technique revealed a progressive growth of the first electrodeposited layer. The growth mode after fully developed electrodeposition was characterized by a preferential growth of large crystals, evident from light scattering as well as AFM. Our experimental techniques make it possible to determine whether electrodeposition or electrochromism, due to electrochemical insertion of ionic species, dominates the observed modulation of the optical properties. The deposited layer was investigated using infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Although the composition of this layer cannot be stated conclusively, it most likely contains lithium alkyl carbonate species.

Full text of DOI:10.1149/1.1393607

2784
Journal of The Electrochemical Society, 147 (7) 2784-2795 (2000)
S0013-4651(99)08-124-0 CCC: $7.00 © The Electrochemical Society, Inc.
Adsorption and Electrodeposition on SnO2 and WO3 Electrodes
in 1 M LiClO4/PC
In Situ Light Scattering and In Situ Atomic Force Microscopy Studies
J. Isidorsson,a T. Lindström,a C. G. Granqvist,a,*,z and M. Herranenb
aDepartment of Materials Science, bDepartment of Inorganic Chemistry, The Ångström Laboratory, Uppsala University,
SE-751 21 Uppsala, Sweden
A novel spectroscopic in situ light scattering technique was used with in situ atomic force microscopy (AFM) to study electrode
surfaces subjected to adsorption and electrodeposition. Tin dioxide and tungsten trioxide were used as electrodes in a 1 M
LiClO4/propylene carbonate electrolyte. Both in situ methods showed the same increase in surface roughness immediately after the
electrode was immersed in the electrolyte. The onset potential for electrodeposition could be determined; its specific value depend-
ed on the film composition as well as on the composition and purity of the electrolyte. A potential step technique revealed a pro-
gressive growth of the first electrodeposited layer. The growth mode after fully developed electrodeposition was characterized by
a preferential growth of large crystals, evident from light scattering as well as AFM. Our experimental techniques make it possi-
ble to determine whether electrodeposition or electrochromism, due to electrochemical insertion of ionic species, dominates the
observed modulation of the optical properties. The deposited layer was investigated using infrared spectroscopy, X-ray diffraction,
and X-ray photoelectron spectroscopy. Although the composition of this layer cannot be stated conclusively, it most likely contains
lithium alkyl carbonate species.
© 2000 The Electrochemical Society. S0013-4651(99)08-124-0. All rights reserved.
Manuscript submitted August 30, 1999; revised manuscript received February 24, 2000.
potentials, thereby studying the pertinent reactions at different
potentials and charge levels.
Electrochromic electrodes rely on reversible electrochemical
insertion and extraction of ions in a host material, thereby changing
the color of the electrode.1,2 These processes can only be accom-
plished within a well-defined potential range. Practical applications
often employ aprotic electrochemical systems capable of providing
a wide potential window. Outside the potential window, the elec-
trolyte decomposes and precipitates on the electrode. Such a deposit-
ed layer affects the optical behavior of the electrode in a generally
undersirable way. Consequently, it is important to be able to estab-
lish if electrodeposition takes place, as well as to determine the spe-
cific onset potential of this phenomenon.
In this work, we investigate the initial adsorption taking place on
an electrode immersed in an electrolyte, as well as the potential
range that can be used for electrochemical experiments to avoid the
onset of electrodeposition. The methods applied are in situ spectro-
scopic light scattering under electrochemical polarization and in situ
atomic force microscopy (AFM). The examined electrochromic
electrodes are sputter deposited tin dioxide3 (SnO2) and tungsten tri-
oxide4 (WO3) immersed in an anhydrous electrolyte of 1 M LiClO4
dissolved in propylene carbonate (PC).
The section on Theory outlines the approach chosen to describe
the surface morphology, as well as the principle of the light scatter-
ing measurements. The Experimental section summarizes experi-
mental details concerning the preparation and analysis of the SnO2
and WO3 films. The section on Adsorption on the surface considers
the adsorption when the electrode is immersed in the electrolyte, as
detected by light scattering and AFM. The section on Onset of Elec-
trodeposition describes the onset of electrodeposition and the growth
of the deposited layer as seen by light scattering. The section on
Growth of the electrodeposited layer considers the growth of this
layer by analyzing the chronoamperometric response and AFM data.
The composition of the surface layers is discussed in the section on
Film composition. The Conclusions section summarizes the work.
Table I is a guide to the different mechanisms and properties that
have been studied in this work; it specifies the techniques to probe
the characteristics and the corresponding potential ranges, and it
states in what section of this paper the discussion can be found.
Clearly, we have chosen to describe our work starting from the vir-
gin electrode state and then going successively toward lower applied
Theory
Surface morphology.—The morphology of an interface can be
described by a topographic function, involving a large amount of
data, but because of the complex nature of randomness, the interface
is usually simplified using statistical parameters. Typically, both ver-
tical and lateral descriptors are used. Most common are the interface
width, often represented by the root-mean-square (rms) roughness
value ␦, and the lateral correlation length ␰, which gives the average
distance between features in the surface profile within which the sur-
face variations are correlated. The morphology of a surface can also
be described by power spectral density (PSD) functions. The PSD
function g(K), where K is the spatial wave vector with units of in-
verse length, is a measure of both vertical and lateral properties of a
surface. It is found directly from a surface profile by taking the square
of the Fourier transform of the topographical profile z(r). The perti-
nent expression in the two-dimensional case, with r ϭ (x, y), is5-7
2
1
g(K) ϭ lim
z(r) exp(iKиr)dr
∫ ∫
A→ϱ A
2
N
M
dxdy
(nϪ1)(sϪ1) (mϪ1)(pϪ1)
Ϸ
znm exp Ϫ2␲i
ϩ
NM ∑∑
N
M
nϭ1mϭ1
[1]
where the second term in Eq. 1 is valid for a sampled profile with N
and M sample positions in the two directions. The wavenumber K ϵ
K refers to the spatial wavelengths making up the surface profile
| |
z(r). Furthermore, dx ϭ Lx/(N Ϫ 1) and dy ϭ Ly/(M Ϫ 1) with Lx and
Ly referring to the scan length in the two directions, Kx ϭ 2␲s/Ndx
with s ϭ 1, 2, ..., N/2, and Ky ϭ 2␲p/Mdy with p ϭ 1, 2, ..., M/2. This
PSD function has the dimension of (length)4.
The PSD function describes two aspects of the surface rough-
ness: the spread of heights from a mean plane, and the lateral dis-
tance over which these height variations occur, i.e., the different
length scales. This is in contrast to the rms roughness value, which
only gives the average height variation along the profile, and no lat-
eral information. The PSD function is also important in light scat-
tering theories, since it connects the scattering into a certain angle
* Electrochemical Society Active Member.
E-mail: Claes-Goran.Granqvist@angstrom.uu.se
z
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Table I. Outline of contents of sections of this paper.
Mechanism or
Experimental method
property studied
to probe the characteristic
Section in paper
Air
OCP
3.5
Surface roughness
Adsorption on surface
Adsorption on the Surface: Light scattering,
AFM measurement and detailed roughness analysis
AFM, light scattering
AFM
Roughness evolutiona
Adsorption on the Surface: AFM measurement and detailed
roughness analysis
Electrochromic behavior
Spectrophotometry
Onset of Electrodeposition
0.9
Onset of electrodeposition
Initial growth mechanism
Light scattering
Potential step technique
Onset of Electrodeposition
Growth of the Electrodeposited Layer: Potential step technique
Growth of the Electrodeposited Layer: Growth of thick layers
Film Composition
0.8
0.3
Thick layer growtha
Composition
AFM
XPS, IR, XRD
a Denotes a recalculation of the potential range from a Pt to a Li reference electrode.
with the spatial wavelengths of the surface through the first-order
grating equation.5
roughness may be written ␦SC or ␦AFM, depending on the selected
range of length scales.
There is a relation between the PSD function and the rms rough-
ness: the square root of the integral of the PSD function over all
wavenumbers K gives the rms roughness value. For a two-dimen-
sional surface, one has8
Light scattering theory.—Scalar scattering theories14-19 have
been formulated for the total integrated scattering from a random
rough surface. The total integrated scattering is the normalized scat-
tering, i.e., the ratio between the diffuse transmittance Td and the
total transmittance Tt. Carniglia19 showed that this ratio is a measure
of the rms roughness for a single, dielectric interface by the relation
K
max
1
1
∫
2␲
2
␦
ϭ
g(K)d(K) ϭ
Kg(K)dK
[2]
2
(2␲)∫ ∫
␬
K
min
2
Td
2␲(ni cos ␪ Ϫ ns cos ␪r )␦
i
where ␬ is an area in the K plane. The second term is obtained by
turning a two-dimensional spectrum of an isotropically rough sur-
face into a one-dimensional representation by averaging over the
polar angle, i.e., by assuming azimuthal symmetry.
Ϸ
[4]
T
␭
t
where ns is the substrate refractive index, and ␪r is the refracted angle
of the specular beam as given by Snell’s law. It is assumed for Eq. 4
that Td ϩ Ts ϭ Tt. There are no details of the angular dependence of
the light scattered (or any polarization effects) in Eq. 4; only values
for scattering into the entire hemisphere in transmittance are given.
The derivation of the scalar scattering theory assumes small sur-
face slopes, i.e., the ratio of surface rms roughness and correlation
length should be small: ␦/␰ << 1 (or equivalently, ␦ << ␭ and ␰ >> ␭).20
This implies that the local radius of curvature is small and that varia-
tions in reflectance and transmittance due to changes in the local angle
of incidence can be neglected. Furthermore, the surface does not shad-
ow itself for nongrazing incidence, and there is no multiple scattering.
Two limitations are frequently referred to20 for scalar scattering theo-
ry. First, the scalar theory does not consider the polarization in the
incident and scattered light. This is a problem for nonnormal incidence
and large scattering angles. Second, for correlation lengths that are
small compared to the wavelength, the theory fails to predict the scat-
tering at high angles. In this work, these limitations can be neglected
since we carry out the measurements at normal incidence, and the
upper collecting angle of the instrument is restricted to 70Њ.
Note that the minimum and maximum spatial wavenumbers in
Eq. 2, Kmin and Kmax, are functions of the specific method to assess
the surface roughness. All experimental techniques have such limits,
thereby defining the bandwidth of the roughness spectrum obtained.
The bandwidth limits of a profiling instrument are defined by the
scan length and twice the smallest increment in the data point spac-
ing. However, if the profiling probe has a lateral resolution exceed-
ing the data point spacing, the bandwidth limit is twice the lateral
resolution of the probe.9,10 In angular resolved optical measure-
ments, the bandwidth limits are set by the angle intervals covered by
the instrument, i.e., only the lateral wavelengths of the rough surface
that scatter into collection angles of the instrument are possible to
determine. These lateral frequencies are given by the spatial
wavenumber K according to5
2␲
K ϭ
ni(sin ␪ Ϫ sin ␪i )
s
[3]
␭
Here, ␪i (␪s) is the polar angle of the incident (scattered) light beam,
␭ is the wavelength of light, and ni is the refractive index of the
medium wherein the incident light beam propagates. Equation 3 is
the first-order grating equation, where K corresponds to 2␲ times the
grating frequency f. This means that the roughness of a surface can
be considered to be composed of a summation of sinusoidal sur-
faces.11,12 The grating spacing ⌬, in turn, is given by ⌬ ϭ 1/f. As a
consequence, high wavenumbers correspond to small length scales
in the surface profile, and low wavenumbers to large-sized surface
features.
To conclude, irrespective of which method is used for surface
characterization, the bandwidth limits of the instrument, i.e., the
range of length scales contributing to the measured parameter, must
be taken into account.13 Two different measurement techniques are
used in this paper: light scattering and AFM. These two methods
have different bandwidth limits (measured in ␮mϪ1), which are re-
ferred to by the subscripts SC and AFM, respectively. Hence, the rms
Experimental
Film preparation.—Our films were made by reactive magnetron
sputtering of tin or tungsten in a versatile turbomolecular pumped
vacuum deposition unit as described elsewhere.3,4 Sputtering was
performed in an Ar ϩ O2 mixture at pressures ranging from 30 to
40 mTorr. Both types of films were deposited onto unheated 1 mm
thick glass plates precoated with transparent and conducting
In2O3:Sn (known as ITO) with a resistance/square of less than 20 ⍀.
The film thickness d, measured by surface profilometry, ranged from
20 to 520 nm. The deposition rate, obtained by dividing d by the
sputtering time, was between 7 and 18 nm/min.
Electrochemical experiments.—We used a standard three-elec-
trode cell21 with the sample as working electrode and both the refer-
ence electrode and counter electrode made of metallic Li foil. Con-
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2786
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sequently, the potentials in this paper are given with lithium as ref-
erence; the only exception is data taken by AFM, in which a plat-
inum reference was used. The electrolyte was 1 M Merck anhydrous
Selectipur LiClO4 with less than 100 ppm H2O dissolved in Merck
anhydrous Selectipur PC with less than 30 ppm H2O.
The electrochemical experiments were carried out in Ar atmos-
phere in a glove box with less than 1 ppm water (dew point below
Ϫ76ЊC). The electrochemical cell was controlled by an ECO Chemi
Autolab instrument or a TopoMetrix potentiostat. The two basic
electrochemical techniques used were chronopotentiometry and
chronoamperometry.
During a typical experiment of a few hours, the sample was
polarized to low potentials by sweeping the potential with a scan rate
Յ10 mV/s in the cathodic direction. The potential sweep was inter-
rupted when the current reached 0.250 mA, and the sample was held
at constant potential under potentiostatic control until the desired
charge was attained. The applied potential was then reversed in order
to minimize the current, so that the potential was diminished over the
interface while the AFM image was captured.
Sequences of surface images were recorded, starting at OCP and
at growing layer thicknesses corresponding to increasing levels of
charge transfer. Two regimes were studied: first the spontaneously
adsorbed layers at OCP and very low levels of intercalation and, sec-
ond, a regime with a total charge transfer of several C/cm2 ␮m
wherein an electrodeposited layer was studied.
Dielectric functions.—The refractive index of the electrolyte was
determined through transmittance measurements and standard thin-
film calculations.22 The calculation was performed for 400 < ␭ < 800
nm, where the extinction coefficient was low, and the absorption
could be neglected. A Cauchy dispersion relation23 was employed
for the refractive index; this is a reasonable assumption for energies
above the bandgap, where only electronic transitions take place. Fig-
ure 1 shows that the refractive index is Ϸ1.4, which is in good agree-
ment with tabulated data.24 The optical constants for the SnO2 and
WO3 electrodes were taken from the literature.25,26 Numerical val-
ues are given in Fig. 1. Note that both of the oxides have refractive
indices of Ϸ2, which is much larger than that for the electrolyte.
Surface properties were evaluated over all 300 ϫ 300 pixel points
in the AFM image. The bandwidth limits, Kmin and Kmax, of the
AFM instrument were calculated5 to be 0.63 and 133 ␮mϪ1 for a
10 ϫ 10 ␮m scan area, and to be 0.31 and 66 ␮mϪ1 for a 20 ϫ
20 ␮m scan area.
In situ light scattering measurements.—A spectroscopic light
scattering experiment was developed to study changes in surface
roughness of the electrodes. The electrodes were optically character-
ized in situ in an electrochemical cell, and the diffuse and specular
transmittance were recorded using a spectroscopic total integrated
scattering (TIS) instrument in the range 330 < ␭ < 970 nm.27 An out-
line of the experimental setup is shown in Fig. 2.28 The electrochem-
ical cell is of closed three-electrode type; it was filled with electrolyte
in the glove box. The working electrode was placed in the focal point
of the light beam in the TIS instrument. The instrument focuses all
light that is forward scattered between 2.5 and 70Њ on one detector,
whereas transmitted light, which leaves the hemispherical mirror
through the specular port, reaches another detector. The design of the
mirror only allows for a very small fraction of the light scattered in
the electrolyte to be focused onto the detector, since the focal depth
is about a few hundred micrometers at most. A lock-in technique was
used for signal acquisition, and the design of the instrument made it
possible to detect diffuse reflectance and transmittance values as low
as 10Ϫ5 of the total signal. A computerized data collection facility in
combination with the two detectors allowed transmission spectra to
be recorded in real time during the experiments.
In situ AFM.—AFM studies of the films were performed using a
TopoMetrix Discoverer TMX 2000 instrument equipped with an
electrochemical cell. Electrical contact to the sample was achieved
by attaching it to the instrument platform with silver glue. A plat-
inum wire was used as the reference electrode, and a platinum ring
served as the counter electrode. All AFM measurements were per-
formed at room temperature (25ЊC) with microfabricated pyramidal
Si3N4 tips (spring constant 0.032 N/m); the applied force was typi-
cally less than 0.5 nN.
In all experiments, the sample was mounted in the empty cell,
and the surface was imaged in air before the complete cell was trans-
ferred to the glove box. The electrolyte was introduced under argon
atmosphere in the glove box. The cell was then transferred to the
microscope, and the surface was imaged at the open-circuit potential
(OCP) before the electrochemical treatment took place.
In the light scattering experiment, the effective rms roughness is
unique for each wavelength and thus only includes length scales in
the roughness profile covered by the TIS instrument at that specific
wavelength. For ␭ ϭ 400 nm and for air as the incident medium,
Kmin and Kmax for the TIS instrument were, respectively, 0.69 and
15 ␮mϪ1, according to Eq. 3; similarly, for the electrolyte as the
incident medium, they were 0.98 and 21 ␮mϪ1, respectively.
Compositional and structural analysis.—The samples for com-
positional and structural analysis were prepared in the glove box in
Figure 2. (a) Outline of the total integrated scattering instrument and (b) the
electrochemical cell: 1, Focusing mirror; 2, electrochemical cell; 3, incident
light beam; 4, specularly transmitted light beam; 5, scattered light cone; 6,
detectors with preamplifiers; 7, lock-in amplifiers; 8, electrochemical inter-
face; 9, computer; 10, sample; 11, window; 12, working electrode; 13,
counter electrode; 14, reference electrode. After Ref. 28.
Figure 1. Spectral refractive indices for the electrolyte and for the SnO2 and
WO3 electrodes.
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an Ar atmosphere. The samples were charged galvanostatically at
Ϸ25 ␮A/cm2 until a potential of 0.3 V vs. Li was reached. They were
then cleaned with tetrahydrofuran (C4H8O) and allowed to dry in the
glove box before the analysis.
tical size as in the image of the electrode in air, and an additional
large-grain distribution that stood out distinctively in the PSD spec-
tra. After polarizing the SnO2 electrode below the OCP, the initial
incomplete substrate coverage is followed by a more extensive cov-
erage of a large-grained adsorbate, as seen in Fig. 4c. A further low-
ering of the potential decreased the roughness values to levels com-
parable to those of the initial surface in air, ␦SC ϭ 10 nm. The rough-
ness values of the AFM images of SnO2 taken at OCP are in excel-
lent agreement with those from the light scattering measurements as
seen in Fig. 3.
The WO3 film, reported in Fig. 5, had an initial surface rms
roughness of ␦SC ϭ 12 nm in air. The surface turned smoother and
displayed an rms value of ␦SC ϭ 7 nm after immersion in the elec-
trolyte. The grain distribution is approximately of the same lateral
and vertical size as in the image of the electrode in air, and the large
grains seen on the SnO2 film were not observed on the WO3 film.
Polarization to Ϫ1.9 V vs. Pt yielded no change in rms roughness,
Infrared (IR) absorption spectroscopy was used to provide com-
positional analysis. We employed a double-beam Perkin Elmer 983
spectrophotometer in the wavelength range of 500 to 4000 cmϪ1
.
The samples were transferred to the spectrophotometer, and the sam-
ple was exposed to air for less than a minute before it was mounted
and subjected to purging. The IR spectra, recorded at 65Њ angle of
incidence with p-polarized light, were recorded within 5 min.
X-ray diffraction (XRD) at 1Њ glancing incidence, using a Siemens
D5000 unit operated with Cu K radiation, was employed to study the
crystal structure of the deposited layer. The samples were transferred
fresh from the glove box to the diffractometer and were scanned over-
night in air.
X-ray photoelectron spectroscopy (XPS) was used to investigate
the composition of the deposited layer. The samples were mounted
on a holder in the glove box, and transfer to the XPS apparatus
(Perkin Elmer PHI 5500 ESCA) took place without exposing the
sample to the ambient.
␦SC ϭ 7 nm. However, the observed rms roughness after immersion
in the electrolyte did not conform with the light scattering values
presented in Fig. 3. The difference between the adsorption on SnO2
and WO3 is further discussed in the sections on Growth of the Elec-
trodeposited Layer and Film Composition.
Adsorption on the Surface
Onset of Electrodeposition
Light scattering.—A change in the optical response was ob-
served during the first hour after the electrolyte was injected into the
electrochemical cell. The light scattering increased by a factor of
around 10 as the cell was filled with electrolyte. This effect appeared
to be instantaneous, and the dynamics could not be studied in detail
since the filling of the cell took approximately 5 s. Quantitative val-
ues of the increase in roughness may be estimated from Eq. 4, as-
suming the refractive index of the rough surface layer to be that of
either SnO2 or WO3. This is a reasonable simplification since the
refractive index of the adsorbed species is expected to be of the same
order as that of the thin film,29 and since the rms roughness only de-
pends on the difference in refractive indices, according to Eq. 4.19
Equation 4 is derived for a semi-infinite rough substrate. In our
case, we have a finite substrate thickness, and although the back sur-
face is smooth, there may be multiple reflections in the glass sub-
strate increasing the scattering. For this work, the influence of mul-
tiple reflections in the recorded spectra can be ignored. Figure 3
shows evaluations of the rms roughness from the light scattering
measurements, i.e., ␦SC, obtained with the simplifying arguments
just mentioned. Clearly, the roughness, which was 4-9 nm initially
for 400 < ␭ < 800 nm, increased to values between 24 and 40 nm
when the electrolyte was introduced. The results are similar for the
SnO2 and WO3 surfaces.
Electrodeposition was studied in situ and in real time using the
TIS instrument. A 250 nm thick SnO2 film immersed in the electro-
lyte was charged with a current of 1.5 ␮A/cm2. The results are pre-
sented in Fig. 6-8. All the data have been adjusted to compensate for
the influence of the transmission of the electrolyte. Figure 6 shows
the evolution of the working electrode potential and the measured
optical response at ␭ ϭ 650 nm during a light scattering experiment.
At charge levels (integrated current) less than 100 mC/cm2␮m, the
potential decreases due to Li intercalation.2,3,30 The intercalation
causes some electrochromic coloration, manifesting itself as a slight-
ly decreased transmittance. The potential reaches a plateau at about
1.25 V, indicating a phase transition,21 and a valence state change was
observed by Mössbauer spectroscopy.31
The potential drops slowly until it reaches 0.88 V, while the
charge increases to 890 mC/cm2 ␮m. The normalized light scattering
stays almost constant during this charging. Above 890 mC/cm2 ␮m,
the potential increases, i.e., the potential creates a local minimum,
AFM measurement and detailed roughness analysis.—AFM data
showed a significant increase in the surface roughness when the
SnO2 electrode was immersed in the electrolyte, in contrast to the
WO3 electrode, which became smoother. The recorded AFM data
were Fourier transformed according to Eq. 1, and PSD functions
were calculated. Figures 4 and 5 show data for the SnO2 and WO3
electrodes, respectively, taken in air, at the OCP after immersion in
electrolyte, and after polarizing the sample to Ϫ1.6 or Ϫ1.9 V vs. Pt.
In both figures, ␦AFM denotes the rms roughness from an AFM meas-
urement at a particular scan size, whereas ␦SC is the effective rms
roughness (calculated by Eq. 3) from the same AFM measurement
including the bandwidth limits of the TIS instrument at ␭ ϭ 400 nm.
It is only for the length scales covered by the TIS measurement at
␭ ϭ 400 nm that the spatial wavenumbers overlap with the ones from
the AFM measurements so that a direct comparison can be made
between the rms roughness from the TIS instrument and the AFM.
The SnO2 film, reported in Fig. 4, had an initial surface rms
roughness of ␦SC Ϯ 6 nm in air. The surface turned rougher and dis-
played a rms value of ␦SC ϭ 22 nm after immersion in the elec-
trolyte. Two distributions of grain sizes were detected for the SnO2
surface in contact with the electrolyte, as apparent from Fig. 4b.
They comprise a small-grain distribution of the same lateral and ver-
Figure 3. Spectrally dependent rms roughness for SnO2 and WO3 surfaces
before and after immersion in electrolyte.
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Figure 4. PSD functions from 20 ϫ 20 ␮m AFM images of SnO2. Part (a)
refers to the sample in air, part (b) was taken at OCP after immersion in elec-
trolyte, and part (c) after polarization to U ϭ Ϫ1.6 V vs. Pt. Quantitative val-
ues of the roughness, defined in the main text, are given in the figure.
Figure 5. PSD functions from 10 ϫ 10 ␮m AFM images of WO3. Part (a)
refers to the sample in air, part (b) was taken at OCP after immersion in elec-
trolyte, and part (c) after polarization to U ϭ Ϫ1.9 V vs. Pt. Quantitative val-
ues of the roughness, defined in the main text, are given in the figure.
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transmittance is weakly decreased. A higher charging makes the
transmittance decrease more rapidly, especially in the blue part of
the spectrum, and the film turns visibly brown. A steep decrease in
transmittance is discerned around 890 mC/cm2 ␮m in Fig. 7, and the
normalized scattering in Fig. 8 increases sharply at this level of in-
serted charge. This is where the electrodeposition is initialized.
The behavior of a 380 nm thick WO3 film charged with a current
of 1.5 ␮A differs from the result of the SnO2 films, as apparent from
Fig. 9-11. After charging with less than 50 mC/cm2 ␮m, the transmit-
tance at the wavelength 650 nm is close to zero; see Fig. 9. Figure 10
shows that the transmittance in the red part of the spectrum is close to
zero, while the transmittance in the blue region still is significant. This
is a signature of the well-documented electrochromism in WO3 and
can be explained in terms of small polaron absorption.1 The spectral
properties change as the intercalation progresses; at 300 mC/cm2 ␮m,
the film attains a brownish appearance. It is conceivable that a sub-
stantial number of the available intercalation sites are occupied so that
site saturation prevails,32 and at 300 mC/cm2 ␮m, the intercalation
level is estimated to be Li1.3WO3, well above the upper limit of 0.5 in
the site-saturation model. At 540 mC/cm2 ␮m the potential comes to
a local minimum, as apparent from Fig. 9, indicating a phase forma-
tion or phase transformation. Above 540 mC/cm2 ␮m, the transmit-
tance goes rapidly to zero. There is an accompanying growth of the
normalized light scattering, clearly seen in Fig. 11. These data give
evidence for electrodeposition.
Figure 6. Working electrode potential (solid curve), total transmittance (dot-
ted curve), and normalized scattering multiplied by a factor of 5 (dash-dotted
curve) vs. charge for an SnO2 film. Optical data were taken at a wavelength
of 650 nm. Electrodeposition causes a rapid increase of the light scattering at
Ϸ890 mC/cm2 ␮m.
The light scattering spectra of SnO2 and WO3 differ substantial-
ly at the onset of electrodeposition. The data for WO3 are very dis-
tinct at the point of electrodeposition, which indicates that the nucle-
ation rate is very high, so high that the crystals grew to a size de-
tectable even with long wavelengths within a 20 min interval be-
tween two successive spectral measurements. However, the scatter-
ing spectra of SnO2 show a smooth onset that can be detected first at
which indicates a phase formation. At this point, the sample starts to
scatter light, thereby giving clear evidence for electrodeposition.
Figure 7 illustrates the evolution of the specular transmittance
spectra in the 330 < ␭ < 970 nm range for the SnO2 film in Fig. 6,
and Fig. 8 gives corresponding information on the normalized scat-
tering. Initially, during the first charging up to 70 mC/cm2 ␮m, the
Figure 7. Specular transmittance of an SnO2 electrode as a function of wavelength and charge.
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2790
Journal of The Electrochemical Society, 147 (7) 2784-2795 (2000)
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Figure 8. Normalized scattering of an SnO2 electrode as a function of wavelength and charge. To gain clarity, the axes for wavelength and charge are not the
same as in Fig. 7.
nucleation and growth of crystals are slower on SnO2 than on WO3.
The onset of electrodeposition was detected at 0.87 Ϯ 0.03 V vs. Li
for both the SnO2 and the WO3 electrodes.
short wavelengths. With a high total charge, the total signal is also
very small, i.e., the total transmittance is low, and thus the relative
error in the normalized scattering is larger. We conclude that the
Growth of the Electrodeposited Layer
The light scattering technique described above is a novel method,
and the application of AFM in electrochemistry has only recently
become routine. We have also adopted more traditional techniques in
electrochemistry to obtain complementary information. One tech-
nique able to detect film growth on a surface relies on the applica-
tion of a potential step. This technique has been applied, and we
show a response that can be assigned to a film formation on the sur-
face. AFM was used to study the initial adsorption on the surface,
and it has also been applied to study the dynamics of the film growth
at a stage when the film has reached a substantial thickness.
Potential step technique.—A potential step technique was
applied to all samples. The chronoamperometric response was stud-
ied after steps from a potential Uhigh ϭ 4.5 V to Ulow ϭ 1.5 V vs. Li.
The potential was held for 60 s at each step, and each step was
repeated twice. In the next cycle, Ulow was decreased by 0.1 V to
Ulow ϭ 1.4 V. After a sequence of steps, the sample was relaxed at
the OCP for 2 min before continuing to the next potential. Sequences
of this kind were repeated until steps between Uhigh ϭ 4.5 V and
U
low ϭ 0.6 V were reached. Figure 12 shows the current response of
a 20 nm thick SnO2 film with an area of Ϸ0.8 cm2. At Ulow ϭ 1.2 V,
the current decreases monotonically, while for Ulow ϭ 1.1 V and yet
lower potentials, there is a short-time increase in the current.
The thinnest SnO2 films showed a potential response that can be
modeled with a theory for nucleation and growth of monolayers on
a surface. Current time (I-t) transients, measured in the potential step
Figure 9. Working electrode potential (solid curve), total transmittance (dot-
ted curve), and normalized scattering multiplied by a factor of 30 (dash-dot-
ted curve) vs. charge for a WO3 film. Optical data were taken at a wavelength
of 650 nm. Electrodeposition causes a rapid increase of the light scattering at
Ϸ540 mC/cm2 ␮m.
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Figure 10. Specular transmittance of a WO3 electrode as a function of wavelength and charge.
experiments described above, were fitted to an equation21 for growth
of a monolayer according to
where Im is the current at the maximum, reached after a time tm. The
equation corresponds to a progressive nucleation of growth centers.
Figure 13 shows I-t transients of the film reported in Fig. 12 with
the current peak value normalized to unity. The agreement between
the experimental data points and the curves calculated from Eq. 5 for
progressive nucleation and growth of two-dimensional precipitates is
good. However, the area under the curve with Ulow ϭ 1.1 V in Fig. 13
2 t3 Ϫ tm3
I
t2
(
)
ϭ
exp Ϫ
[5]
tm2
3tm3
Im
Figure 11. Normalized scattering of a WO3 electrode as a function of wavelength and charge. To gain clarity, the axes for wavelength and charge are not the
same as in Fig. 10.
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of 10 ϫ 10 ␮m. The deposit initially forms a homogeneous layer.
Then, at certain sites distributed several micrometers apart, some
crystallites start to grow. This is seen in the PSD diagrams as an
increased intensity at low spatial wavenumbers, i.e., at long length
scales in the surface profile. In a final growth stage, the background,
i.e., the area between these large bumps, becomes rougher. The lat-
ter effect is detected in the PSD plots as increased intensity at short
wavelengths or, equivalently, at high wavenumbers. Thus, the
growth process is preferential in thick layers, where favored sites
produce large crystals growing faster than the background.
Figure 15 shows the growth of precipitates on a WO3 film. This
time we used 20 ϫ 20 ␮m squares. It is apparent from the second
frame that an additional scan of 10 ϫ 10 ␮m is clearly visible in the
20 ϫ 20 ␮m image. Hence, the precipitate did not adhere to the WO3
film as well as it did to the SnO2 film, and the movement of the tip
influenced the surface. In both Fig. 14 and 15, ␦AFM denotes the rms
roughness from the AFM measurement including the bandwidth lim-
its at that particular scan size.
Film Composition
Figure 12. Current vs. time for potential steps applied to a 20 nm thick SnO2
film. The potential steps are from 4.5 V vs. Li to the values indicated in the
figure.
We studied the composition and structure of the precipitate to
more fully understand the onset of the deposition from the elec-
trolyte. The precipitate is formed in a very complex system; a good
review on the subject is given by Aurbach.35 The emphasis in our
work is on the two in situ methods; the compositional and structural
investigation is used only to support those results. Specifically, we
used three ex situ methods: IR absorption spectroscopy, XRD, and
XPS. We prepared samples by polarizing the SnO2 and WO3 films to
a potential as low as 0.3 V vs. Li. This is 0.5 V lower than the onset
potential for the deposition, and our technique gave us a thick
enough layer for analysis with the three mentioned techniques.
corresponds to a charge of about 6 mC/cm2. As the charge yielding a
monolayer of a deposit ought to be significantly smaller than this
charge (the deposition of a monolayer of lithium metal by reduction
of Liϩ would require a charge of only Ϸ0.2 mC/cm2), it is likely that
the transient in Fig. 13 is associated with the deposition of a number
of subsequent monolayers. Although the magnitudes of the tm (and to
some extent also the Im) values depended on the history of the elec-
trodes, these variations did not seem to influence the nucleation and
growth mechanism, and the fits to Eq. 5 are convincing.
Transients similar to those in Fig. 13 were only seen in very thin
SnO2 films (20 nm), but we see no reason why the same nucleation
and growth mechanism should not pertain to the thicker films as
well. For the thicker SnO2 films, and the WO3 films for which the
intercalation is a dominant process, we applied a model describing
the release of ions from a layer on the surface and subsequent migra-
tion of these ions into the film to explain the behavior.33,34
IR absorption spectroscopy.—Measurements in the IR by use of
a double-beam Perkin Elmer 983 spectrophotometer yielded a low
reflectance, especially above 1500 cmϪ1. This is due not only to
adsorption but also to light scattering from surface roughness. The
particles seen in the AFM images reported in Fig. 14 and 15 have lat-
eral sizes up to 5 ␮m and vertical peak-to-peak values of about
1 ␮m. Hence, a significant light scattering is expected in the IR
wavelength range.
The surfaces of the deposited layers were very reactive, and ex-
posure of the samples to air for a day yielded a significant change of
the spectra, whose absorption bands became more prominent. We
studied the absorption spectra for fresh samples (exposed to air only
for some minutes) as well as for aged samples stored for a day in air.
The fresh films yielded measured absorption bands indicating the
presence of lithium alkyl carbonate species, ROCO2Li, where R is
an alkyl group.36 In the aged films, the measured absorption bands
indicated the presence of Li2CO3.36,37 Hence, based on these ex situ
measurements, we conclude that lithium alkyl carbonate species are
one of the products in the freshly deposited layer, and that lithium
carbonate is one of the products in the aged layer.
Growth of thick layers.—The subsequent growth of a thick
deposited layer was studied by AFM. Figure 14 shows film growth
during electrodeposition on an SnO2 electrode. We used a scan size
XRD.—An X-ray diffractogram of an electrodeposited layer
showed clear evidence for the ITO layer on the substrate. No struc-
tures due to the SnO2 or WO3 films were seen, and hence these films
can be characterized as amorphous.2,3 The deposited layer did not
show any specific X-ray features. This is consistent with an inho-
mogeneous composition.
XPS.—Our XPS spectra showed an indistinct composition, and it
appears that there is no unique surface layer formed during the depo-
sition. The layer on the WO3 electrode had the overall composition
31 atom % Li, 29 atom % C, and 39 atom % O, while the layer on the
SnO2 electrode showed 21 atom % Li, 40 atom % C, and 36 atom %
O. There were traces of Cl on both electrodes, of some Ca and W on
the WO3 electrode, and of some F and Sn on the SnO2 electrode. We
noted a small variation in the composition between the bumps and the
flat surface (cf. Fig. 14 and 15), and the bumps contained less Li.
Figure 13. Time-dependent current during electrodeposition on a 20 nm
thick SnO2 electrode as obtained from measurements (symbols) and from a
theory for progressive nucleation of growth centers (solid lines).
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Journal of The Electrochemical Society, 147 (7) 2784-2795 (2000)
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Figure 14. Evolution of morphology during electrodeposition on an SnO2 film. AFM images of size 10 ϫ 10 ␮m are shown on the right, and corresponding
PSD functions are shown on the left. The data were recorded for the shown magnitudes of charge transfer.
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2794
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Figure 15. Evolution of morphology during electrodeposition on a WO3 film. AFM images of size 20 ϫ 20 ␮m are shown on the right, and corresponding PSD
functions are shown on the left. The data were recorded for the shown magnitudes of charge transfer.
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Journal of The Electrochemical Society, 147 (7) 2784-2795 (2000)
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Conclusions
The main objective of this investigation was to study adsorption
and electrodeposition on SnO2 and WO3 electrodes with an in situ
light scattering technique and with in situ AFM. We established the
potential range that can be used for electrochemical experiments and
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Both light scattering and AFM showed the same increase in sur-
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no large grains were seen in the AFM. The first small signs of depo-
sition were detected with a potential step technique at a potential of
1.1 V vs. Li. This coincides38 with the potential where reactions
including H2O occur, and we believe that small amounts of water
present in the electrolyte were reduced here.
The onset of a massive decomposition of the electrolyte, and the
resulting electrodeposition, were detected at 0.87 Ϯ 0.03 V vs. Li for
both the SnO2 and the WO3 electrodes. The specific value of the
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The investigation of the composition of the deposited layer is not
conclusive. However, IR spectroscopy with p-polarized light indi-
cates that ROCO2Li is a likely component in the compound formed
during deposition. IR spectra as well as XPS data led to the conclu-
sion that the SnO2 and WO3 electrodes promoted slight differences
in the growth and composition of the surface layer.
Uppsala University assisted in meeting the publication costs of this
article.
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