Photodriven hydrogen evolution by molecular
catalysts using Al
2
O
3
-protected perylene-3,4-
dicarboximide on NiO electrodes†
Rebecca J. Kamire, Marek B. Majewski, William L. Hoffeditz, Brian T. Phelan,
Omar K. Farha, Joseph T. Hupp and Michael R. Wasielewski
*
The design of efficient hydrogen-evolving photocathodes for dye-sensitized photoelectrochemical cells
(DSPECs) requires the incorporation of molecular light absorbing chromophores that are capable of
delivering reducing equivalents to molecular proton reduction catalysts at rates exceeding those of charge
recombination events. Here, we report the functionalization and kinetic analysis of a nanostructured NiO
electrode with a modified perylene-3,4-dicarboximide chromophore (PMI) that is stabilized against
degradation by atomic layer deposition (ALD) of thick insulating Al
2
O
3
layers. Following photoinduced
charge injection into NiO in high yield, films with Al
2
O
3
layers demonstrate longer charge separated
lifetimes as characterized via femtosecond transient absorption spectroscopy and photoelectrochemical
techniques. The photoelectrochemical behavior of the electrodes in the presence of Co(
II) and Ni(II)
molecular proton reduction catalysts is examined, revealing reduction of both catalysts. Under prolonged
irradiation, evolved H
2
is directly observed by gas chromatography supporting the applicability of PMI
embedded in Al
2
O
3
as a photocathode architecture in DSPECs.
Introduction
The harvesting and storage of light energy in the chemical bonds
of liquid and gaseous fuels has been of increasing interest in
recent years.
1–3
One of many promising approaches uses pairs
of dye-sensitized electrodes within photoelectrochemical cells
(DSPECs) to drive molecular catalysts to perform the two half
reactions of water oxidation and hydrogen evolution.
4–6
Incorpo-
ration of both light-driven reactions into one efficient system
presents many challenges and opportunities to understand
photodriven electron transfer events, charge accumulation steps,
catalytic mechanisms, and avoidance of degradation pathways
within the components of DSPECs. Several designs that are
capable of photodriven (or photoassisted) hydrogen evolution
combine molecular catalysts with photosensitized nanomaterials
in solution,
7–11
or photosensitized electrodes,
12–18
or use semi-
conductors as light absorbers.
19–27
Mechanistic investigation of
dye-sensitized photocathode architectures remains particularly
rare,
28
and such work could lead to improvement in the perfor-
mance of devices. Current device designs suffer from degradation
of the molecular light absorbers over time and losses in efficiency
due to charge recombination at rates competitive with charge
accumulation and catalysis. New chromophores based on earth
abundant elements are also desirable, since the molecular
photosensitizers used in the aforementioned device architectures
are almost exclusively ruthenium-based, with the exception of
only a few organic chromophores.
13,15,29
We previously demonstrated that a perylene-3,4-dicarbox-
imide (PMI) chromophore (PMI, Fig. 1) is capable of electron
injection into nanostructured TiO
2
following photoexcitation
and further capable of oxidizing a molecular water oxidation
precatalyst.
30
Studies have also demonstrated the utility of
PMI-based chromophores for photodriven hole injection into
NiO in p-type dye-sensitized solar cells (DSCs),
31–36
which is
encouraging for future work on hydrogen evolving photocath-
odes. One report suggests that no molecular catalyst is
necessary for photodriven hydrogen evolution by a chromo-
phore that includes a PMI moiety on NiO at pH 7,
37
and
a similar PMI-based chromophore has driven hydrogen
evolution by a cubane molybdenum-sulde cluster in acidic
conditions.
29
However, these photosensitizer designs generally
require complex synthesis in order to incorporate a donor–
acceptor character that extends the lifetime of the charge
separated state . PM I can be synthesized from commercially
available perylene-3,4:9,10-tetracarboxydianhydride in only
four steps, and the monoanhydride opens to form the dicar-
boxylate when exposed to metal oxide semiconductors for
convenient electrode f unctio nalizati on.
30
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER)
Center, Northwestern University, Evanston, IL 60208-3113, USA. E-mail:
m-wasielewski@northwestern.edu
† Electronic supplementary information (ESI) available: Experimental details;
additional electrochemical and photoelectrochemical characterization, UV-Vis
spectra, and fsTA results; quantication of evolved hydrogen; and
DFT-computed ground state structure of PMI diester. See DOI:
10.1039/c6sc02477g
Cite this: Chem. Sci.,2017,8,541
Received 5th June 2016
Accepted 17th August 2016
DOI: 10.1039/c6sc02477g
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The application of an Al
2
O
3
tunneling barrier by atomic layer
deposition (ALD) before dye loading on a surface has been
shown to act as an alternative to complex molecular synthesis by
providing the important advantages of slowing back electron
transfer between charges in semiconductors and on chromo-
phores and redox shuttles in DSCs.
16,38–46
In DSPECs, slowing
charge recombination at the interface is similarly advantageous
for efficient charge accumulation and catalysis. We investigate
here the use of an ALD layer of varying thickness deposited aer
dye loading to slow charge recombination between charges in
the dye and semiconductor with those on the catalyst.
However, ALD can also serve an additional purpose impor-
tant in DSPECs. Dye desorption and degradation limit the
performance of electrodes fabricated with PMI, since desorp-
tion in the acidic operating conditions and oxidative or reduc-
tive degradation of organic chromophores under catalytic
conditions are some of the most signicant limiting factors in
the stability of DSPECs.
3,5
We and others have demonstrated
that ALD of either TiO
2
or Al
2
O
3
following dye absorption on
nanostructured semiconductors can prevent desorption of
bound molecules from surfaces in DSCs
44,47–50
and electrodes for
water oxidation.
51,52
In addition, we hypothesize that a thicker
layer of metal oxide surrounding the PMI molecules can protect
the dyes from undesirable degradation side reactions. Meyer
and coworkers have recently observed such a stabilization effect
by Al
2
O
3
ALD layers over Ru-based dyes and water oxidation
catalysts on nanostructured semiconductors.
53
Here we report the ability of PMI bound to nanostructured
NiO electrodes and protected by Al
2
O
3
to drive well-characterized
cobaloxime
54,55
and [Ni(P
R
2
N
R
2
)
2
]
2+
(ref. 56 and 57) hydrogen
evolution electrocatalysts (Fig. 1). Besides their efficacy in other
photodriven systems, these catalysts were selected based on
their electrocatalytic hydrogen production at comparable redox
potentials (Table 1, Fig. S4†). We explore the charge transfer
dynamics and nd that the Al
2
O
3
layers not only stabilize the
organic chromophores against desorption and degradation
but also favor longer charge separated state lifetimes and
light-driven hydrogen evolution.
Experimental
Synthesis and lm preparation
The PMI dye and PMI diester (see structure in ESI†),
30
coba-
loxime catalyst,
58
and nickel catalyst
57
were prepared following
reported procedures. Nanostructured NiO lms on conductive
uorine doped tin oxide (FTO) slides were prepared based on
a literature procedure
59
and stored in a 110
C oven until dye
loading. To dye functionalize, lms were gently shaken in
a 0.45 mM solution of PMI in 1 : 3 toluene : methanol protected
from light overnight, rinsed with CH
2
Cl
2
, and dried under
a nitrogen stream. The NiO|PMI surface was then treated with
0–30 ALD cycles of dimethylaluminum isopropoxide and water
to yield NiO|PMI|Al
2
O
3
lms, as described in the ESI.† Films for
femtosecond transient absorption (fsTA) spectroscopy were
either sealed under nitrogen with glass slides using UV-curable
epoxy in a glove box, or placed in an acidic electrolyte solution
of 0.1 M H
2
SO
4
and 0.1 M Na
2
SO
4
in 1 : 1 H
2
O : MeCN in
gas-tight cuvettes and purged with argon. The lms for photo-
electrochemical experiments were attached to stranded
conductive wire using conductive silver epoxy (CircuitWorks
Chemtronics), which was then covered with non-conductive
epoxy (LOCTITE 9340 Hysol) and cured at 110
C for 10 min.
Optical spectroscopy
Femtosecond transient absorption (fsTA) spectroscopy
experiments were conducted using a reg eneratively amplied
Ti : sapphire laser system with samples translated in two
dimensions and irradiated at 495 nm as previously
described.
30,60
Further details and the tting procedures are
provided in t he ESI.†
Electrochemical and photoelectrochemical experiments
Electrochemical measurements were performed using a CH
Instruments Model 660A or 750E electrochemical workstation.
Fig. 1 Chemical structures of the PMI photosensitizer and the H
2
evolution catalysts.
Table 1 Steady-state optical properties and redox potentials (V vs. Ag/
AgCl)
E
00
(eV) [H
2
SO
4
] (M) E
ox
(V) E
red1
(V) E
red2
(V)
PMI
a
2.30 0.00 1.34 0.79 1.05
NiL
2
b
— 0.00 — 0.47 0.74
— 0.07 —— 0.45
c
CoL
2
b
— 0.00 — 0.51 —
— 0.10 — 0.49 0.85
c
a
Measurements on PMI diester in CH
2
Cl
2
for optical expts; in 0.1 M
TBAPF
6
in CH
3
CN for redox expts.
28
b
1.0 mM Fc, 0.1 M Na
2
SO
4
in
1:1 H
2
O : MeCN, [H
2
SO
4
] listed (Fig. S4).
c
Catalytic onset, as the
intersection of the line of the catalytic slope with the cyclic
voltammogram trace before acid addition.
542
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Measurements on the dye-functionalized lms employed the
lms as the working electrode, a 3.0 mm diameter glassy carbon
counter electrode, and an Ag/AgCl (3 M NaCl) reference elec-
trode. Light was applied from two white LEDs (100 mW cm
2
incident on the sample per LED) covered with 410 nm long-pass
lters. Photocatalysis experiments were performed in a sealed
cell under argon with a 0.40 V bias applied concurrently with
illumination for two hours. The H
2
in the headspace was
identied following triplicate injections of 300 mL each into
a gas chromatograph. Faradaic e ffi ciencies were calculated
from the amount of H
2
in the headspace and the amount of
current passed during the experiment.
Results and discussion
Steady-state spectroscopy
The lms were characterized by UV-Vis spectroscopy to gain
information on the physical effect of ALD on the dye molecules
(Fig. 2). The absorption spectrum for NiO|PMI lms without
ALD displays a (0,0) vibronic absorption maximum at 506 nm,
a (0,1) band at 485 nm, and a small shoulder around 575 nm.
The low energy shoulder is suggestive of dye aggregation
45,61
but
is absent aer ALD is applied. Thus, we postulate that aggre-
gates should not play a signicant role in the photophysics of
the ALD-treated lms. As the number of cycles of ALD is
increased, the signal broadens until by 30 cycles the vibronic
structure is no longer distinct and the l
max
is red-shied to
522 nm. Increasing to 40 and 50 cycles produces no further
spectral change (Fig. S1†). This spectrum is identical to that of
the corresponding PMI diester spin coated onto glass but not to
that of PMI diester encapsulated in a so and exible PMMA
matrix (Fig. S2†), which suggests that the change in band shape
aer ALD could be due to the imposition of structural rigidity
on PMI enforced by the surrounding Al
2
O
3
. The red-shi is likely
due to an untwisting of the PMI core that is twisted in the free
molecule by the presence of the bulky phenoxy groups.
61
It is
unlikely that the shi results from a change in dielectric envi-
ronment, as we have not observed that effect in previous work.
45,47
In an attempt to predict the Al
2
O
3
thickness with respect to
the PMI molecules, the geometry optimized PMI diester
structure was obtained from DFT calculations (B3LYP/6-31G*)
and shows that the maximum height of PMI from the NiO
surface cannot be more than 17.5
˚
A (Fig. S23 and Table S3†).
The ALD growth rate on at surfaces is ca. 1
˚
A per cycle as
determined by ellipsometry, so it was initially anticipated that
the Al
2
O
3
layer would fully encapsulate PMI by 20 cycles of ALD.
However, we surmise based on the change in band shapes in the
absorption spectra between 20 and 30 cycles of ALD, and no
further changes thereaer, that only by 30 cycles of ALD is the
dye molecule effectively encapsulated in Al
2
O
3
.
The stability of the dye molecules on the surface was also
investigated by UV-Vis spectroscopy aer the ALD process and
aer exposure to ambient light and air over 45 days (Fig. S3†).
Minimal dye degradation was observed during the ALD treat-
ment, based on the change in shape of the absorption curves,
but following ALD, dye degradation does occur over time.
Degradation decreases with increasing Al
2
O
3
thickness so that
by 30 cycles degradation is negligible over the 45 day period
investigated, which is promising for use of ALD-treated NiO|dye
lms in devices and further supports the assertion that 30 cycles
of ALD is sufficient to cover PMI.
Electrochemistry
In order to examine the energetic driving force for the desired
photodriven charge transfer steps, we compare the redox
properties of PMI as determined in our previous work with
those of the catalysts and with the NiO valence band (Table 1).
30
The NiO valence band is sufficiently low in energy that it is
accessible for hole injection from PMI even at low pH (0.52 V vs.
Ag/AgCl),
12,62,63
and fast hole injection from similar PMI-based
chromophores has been observed previously.
64
The rst reduc-
tion potentials, which are reversible in the absence of acid and
semi-reversible in its presence, and the onsets of catalysis for
NiL
2
and CoL
2
were measured by cyclic voltammetry and are
reported in Fig. S4† and summarized in Table 1. The rst
reduction potential of PMI, E
red
¼0.79 V vs. Ag/AgCl, is
sufficiently negative to reduce either catalyst by one electron. In
the presence of 0.1 M H
2
SO
4
, the catalytic onset for NiL
2
is
milder than the rst reduction without acid, so PMI should be
able to initiate catalysis. The CoL
2
catalyst, however, has
a catalytic onset of 0.85 V vs. Ag/AgCl, so catalysis without
applied bias, if possible, is expected to be less efficient than for
NiL
2
.
Transient absorption spectroscopy
We have previously reported the fsTA spectroscopic character-
ization of the excited state decay of the singlet excited state of
PMI (
1
*PMI)onAl
2
O
3
, where neither electron nor hole injection
into the semiconductor is energetically accessible.
30
The
1
*PMI
state that is formed upon excitation of Al
2
O
3
|PMI (dry) lms is
characterized by a ground state bleach and stimulated emission
signal at 440 to 600 nm and an excited state absorption with
a maximum centered at 675 nm (species A, Fig. S7A†). The
subsequent decay of
1
*PMI along a number of pathways
including excimer formation
65
is approximated using a species-
associated model in which
1
*PMI (species A) decays to species
Fig. 2 Normalized UV-Vis absorption spectra of NiO|PMI films treated
with 0–30 cycles of Al
2
O
3
ALD. The NiO background is subtracted.
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B, which decays to species C, which decays to the ground state
(GS). While the spectra for “species B” and “species C” contain
contributions from the several species formed along these decay
pathways, excimer can be identied by a loss of stimulated
emission and red-shi and broadening of the absorptive signal
in species C (Fig. S7A†).
NiO|PMI lms were then studied by fsTA spectroscopy and,
by comparison to the results for Al
2
O
3
|PMI, photodriven hole
injection into NiO and subsequent charge recombination were
identied for both dry lms and lms in acidic solution (Fig. 3
and S6–S9†). The lack of signicant stimulated emission and
excimer signal in the time-resolved spectra for all NiO|PMI
samples indicates that nearly quantitative hole injection into
NiO occurs very rapidly. The
1
*PMI spectra (Fig. 3B, species A)
and rate of hole injection were identied by global tting to an A
/ B / C / D / GS model, where hole injection (A / B) ts
to 0.5 ps for all samples and is likely multiexponential with an
even faster component occurring within the instrument
response time (Table S1†).
For the NiO|PMI (dry) lm, an excimer population is
observed spectrally as a shoulder at 560–600 nm in species B–D
(Fig. S9†) and kinetically as a fast rate of decay at 655 nm
(Fig. 3C). The shape of the 0.5 ps spectrum for the lm without
ALD, with a very blue-shied x-intercept of about 575 nm,
indicates that the excimer population is large and begins to
form within the instrument response time. The A / B lifetime
of 0.6 0.3 ps is not signicantly different from the A / B
lifetimes for the other samples, so excimer formation likely
occurs on a similar timescale and competitively with hole
injection. Given the large driving force for hole injection from
1
*PMI discussed above, it is likely that hole injection also occurs
from the excimer state. A much smaller excimer population is
also observed in the 500 ps and 5000 ps spectra at 550–600 nm
for lms without ALD under solvent conditions. The low exci-
mer intensity in these samples likely results from dye interac-
tion with solvent molecules weakening the interchromophore
coupling. No excimer is observed for any of the lms with 30
cycles of ALD, which indicates that the disaggregation induced
by ALD eliminates excimer formation so that hole injection
becomes the strongly dominant process.
Following hole injection, the resulting signal is characterized
by a ground state bleach centered at 525 nm and an absorptive
feature with a l
max
around 665 nm and is consistent with PMIc
(Fig. 3A).
64,66
This signal decays with little change in shape as
recombination with holes in NiO occurs (species B–D, Fig. 3B).
Recombination is non-exponential, as expected for dye-sensitized
semiconductors,
64,67
and could not be adequately t with fewer
than three rates of decay. Consequently, the resulting t carries
a large degree of uncertainty, with values varying only slightly
from those used as articial starting values when initiating the
t. A comparison of the normalized kinetic traces at 665 nm
provides more meaningful information (Fig. 3C). Recombina-
tion is slowed by the presence of the acidic solution for lms
with 0ALD, as the PMI molecules experience a more polar
environment in solution and thus a lower energy PMIc
state.
68
This nding agrees with a previous study that found
PMI-sensitized NiO to lie in the Marcus normal region.
64
Thus,
dye molecules encased in Al
2
O
3
, which experience an environ-
ment of intermediate polarity, display recombination rates
between those for 0ALD lms with and without solution
present. Recombination is only minimally impeded by solution
for lms with 30ALD because the polarity of the dye environ-
ment remains unchanged. This nding indicates that the ALD
layer should also shield PMI from catalyst molecules in solu-
tion, and slow charge recombination between the reduced
catalyst and the surface, as long as the layer is thin enough for
initial electron transfer through the layer to the catalyst to
occur. However, catalyst reduction could not be directly iden-
tied from the fsTA experiments because samples with either
NiL
2
or CoL
2
in solution had indistinguishable rates of decay
from samples without catalyst (Fig. S10†). We conclude that,
while PMI successfully injects holes into NiO in high yield and
ALD favors injection over excimer formation, the majority of the
electrons on PMI molecules recombine with holes in NiO before
Fig. 3 (A) fsTA spectra of NiO|PMI|30ALD in 0.1 M H
2
SO
4
+ 0.1 M
Na
2
SO
4
in 1 : 1 H
2
O : MeCN under argon following excitation with
a 495 nm laser pulse. (B) Species-associated spectra obtained by
a global fit of the fsTA spectra to an A / B / C / D / ground state
(GS) model. (C) Normalized single wavelength traces at 655 nm from
fsTA results of Al
2
O
3
|PMI (black), NiO|PMI (red), and NiO|PMI|30ALD
(orange) dry under nitrogen and NiO|PMI (green) and NiO|PMI|30ALD
(blue) in the electrolyte conditions as above.
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catalyst diffusion to the surface and subsequent catalyst
reduction can occur. All reported photocathodes based on
sensitized NiO, including those with surface-bound catalyst, are
similarly limited by fast recombination.
12–18,28
We turned to
photoelectrochemical techniques to probe the fate of any
charge separated states that are suffi ciently long-lived to drive
photocatalysis.
Photoelectrochemistry: evidence of hydrogen evolution
Following conrmation that the desired hole injection occurs
from PMI into NiO, photoelectrochemical techniques were used
to probe the ability of PMI to reduce NiL
2
and CoL
2
catalysts and
drive hydrogen evolution. Fig. 4 displays results for linear sweep
voltammetry (LSV) experiments on NiO|PMI with 30 cycles of
Al
2
O
3
ALD (NiO|PMI|30ALD) in an acidic 1 : 1 H
2
O : MeCN
solution (5 mV s
1
from 0.00 to 0.55 V, 10 s light on/off cycles).
In the absence of PMI, photocurrent is negligible (blue), but
when PMI is present, the photocurrent is enhanced (black).
When the light is turned on, an initial strong photocurrent that
rapidly decays results from reduction of the PMI molecules on
the surface and local capacitance effects
53
and hole injection
that is more rapid than dye regeneration.
69
It is followed by
a plateau where hole injection and dye regeneration are in
equilibrium. With either NiL
2
or CoL
2
but without acid, the
photocurrent is enhanced in a wave-like feature around the rst
reduction of each catalyst (0.3 to 0.5 V) (Fig. S11 and S12†),
which suggests that photoassisted catalyst reduction from PMI
occurs. Upon addition of both acid and 0.5 mM NiL
2
or 0.5 mM
CoL
2
to the solution (Fig. 4, red), the photocurrent traces are not
simply the sum of those observed for the lms with only H
2
SO
4
and those with only catalyst. Instead, the lms display a further
enhanced photocurrent that increases at increasingly negative
potentials and lacks capacitive features. The stronger photo-
current at more negative biases reects the higher yield of
reduced catalyst resulting from slowed hole recombination at
these potentials. The lack of capacitive features indicates that
PMI is rapidly regenerated by the catalyst following hole injec-
tion into NiO at any applied potential within the range. These
changes together indicate that the second reduction of each
catalyst and subsequent turnover to produce hydrogen
occur.
12,53,69
Films with 0–20 cycles of Al
2
O
3
ALD and either catalyst also
display characteristics of proton reduction for experiments run
on duplicate batches of lms (Fig. S13–S15†). The shapes of the
photocurrent responses for lms exposed to either catalyst
display no sharp capacitive peak when the light is turned on and
higher intensity at more negative bias as observed for the lms
with 30 cycles of ALD. Some of the lms, however, displayed
a decrease in absolute photocurrent density in the presence of
catalyst relative to those without. The experiments without and
with catalyst were performed consecutively on the same lms
for ease of comparison. Dye degradation and/or desorption at
strongly negative potentials,
70
such as those used in the LSV
experiments, result in less photocurrent on the second sweep for
most lms even without catalyst addition (data not shown). In
contrast, lms that have only been exposed to a 0.40 vs.
Ag/AgCl bias, even with catalyst, do not display the photocurrent
loss because the PMI molecules are much more stable at these
mild potentials (Fig. 6B, S15 and S16†). Gratifyingly, photocur-
rent is stable over a 10 minute experiment for NiO|PMI|0–30ALD
lms with either catalyst at this potential (Fig. S19†). Alterna-
tively, degradation at more negative potentials decreases as we
increase the number of cycles of Al
2
O
3
. The color of the 30ALD
lms is preserved throughout the LSV experiment, unlike lms
with fewer ALD layers, and the photocurrent density on the
second sweep is higher relative to the rst sweep with increasing
ALD layers (Fig. S13†). Thus, we postulate that the shapes of the
photocurrent traces suggest successful hydrogen evolution
occurs despite a lack of enhanced photocurrent in the presence
of catalyst in some instances, and the loss of photocurrent due to
degradation can be avoided on our experimental timescale by
applying mild potentials and by applying sufficient Al
2
O
3
coverage by ALD. We note that Meyer et al. have also observed
a decrease in absolute photocurrent for their TiO
2
|dye|ALD|
catalyst|ALD samples but were able to assign a change in
photocurrent signal shape to an enhanced rate of oxidative
catalysis.
53
Photoelectrochemistry: analysis of rates
In light of the preceding discussion, we propose that it is most
relevant to compare photocurrent response shapes to under-
stand the differences in charge transfer dynamics between
samples. These shapes are quantied using the ratio of photo-
current density at the end of a 10 s illumination period during
the LSV experiment to that at the beginning (I
0.10 V
/I
0.05 V
or
I
light off
/I
light on
, Fig. 5). Small ratios for the lms without catalyst
reect the strong capacitive current spike, whereas larger
current ratios for catalyst-containing samples reect less
Fig. 4 Linear sweep voltammetry measurements with 10 s light on/off
cycles on a NiO working electrode with 0.5 mM NiL
2
(blue) and on
a single NiO|PMI|30ALD working electrode without catalyst (black) and
with 0.5 mM NiL
2
or 0.5 mM CoL
2
(red) (conditions: 0.1 M H
2
SO
4
and
0.1 M Na
2
SO
4
in 1 : 1 H
2
O : MeCN, 5 mV s
1
).
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