Nitrogen Bubbles at Pt Nanoelectrodes in a Nonaqueous Medium: Oscillating Behavior and Geometry of Critical Nuclei

Article abstract of DOI:10.1021/acs.analchem.9b05510

Gas bubble evolution is present in many electrochemical and photoelectrochemical processes. We previously reported the formation of individual H₂, N₂, and O₂ nanobubbles generated from electrocatalytic reduction of H⁺ and oxidation of N₂H₄ and H₂O₂, respectively, at Pt nanodisk electrodes in an aqueous solution. All the nanobubbles formed display a dynamic stationary state of a three-phase boundary with an invariant residual current. Here, we test the hypothesis that gas nanobubbles can also be electrogenerated in a nonaqueous medium. Interestingly, we found oscillating bubble behavior corresponding to nucleation, growth, and dissolution in dimethyl sulfoxide and methanol. One possible explanation of the oscillation mechanism is provided by the instable dynamic equilibrium between the gas influx due to supersaturation and outflux due to Laplace pressure. Furthermore, the critical gas concentrations for N₂ nanobubble nucleation are estimated to be 148, 386, 200, and 16 times supersaturation and the contact angles of the critical nuclei to be 164°, 151°, 160°, and 174° in water, dimethyl sulfoxide, ethylene glycol, and methanol, respectively. This is the first report on electrochemical nucleation of gas bubbles in nonaqueous solvents. Our electrochemical gas bubble study based on a nanoelectrode platform has proven to be a prototypical example of single-entity electrochemistry.

Full text of DOI:10.1021/acs.analchem.9b05510

Subscriber access provided by UNIV OF WESTERN ONTARIO
Article
Nitrogen Bubbles at Pt Nanoelectrodes in Non-Aqueous
Medium-Oscillating Behavior and Geometry of Critical Nuclei
Qianjin Chen, Yuwen Liu, Martin A. Edwards, Yulong Liu, and Henry S. White
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b05510 • Publication Date (Web): 13 Apr 2020
Downloaded from pubs.acs.org on April 17, 2020
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 7
Analytical Chemistry
1
2
3
4
5
6
7
8
9
Nitrogen Bubbles at Pt Nanoelectrodes in Non-Aqueous Medium-
Oscillating Behavior and Geometry of Critical Nuclei
Qianjin Chen a*, Yuwen Liu b, Martin A. Edwards c, Yulong Liu a and Henry S White c*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
aKey Lab of Science and Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering
and Biotechnology, Donghua University, Shanghai, 201620, China
bCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, China
cDepartment of Chemistry, University of Utah, Salt Lake City, Utah, 84112, United States
ABSTRACT: Gas bubble evolution are omnipresent in many electrochemical and photoelectrochemical processes. We previously
reported the formation of individual H2, N2 and O2 nanobubbles generated from electrocatalytic reduction of H+, oxidation of N2H4
and H2O2, respectively, at Pt nanodisk electrodes in the aqueous solution. All the nanobubbles formed display a dynamic stationary
state of three phase boundary with an invariant residual current. Here we test the hypothesis that gas nanobubbles can also be
electrogenerated in non-aqueous medium. Interestingly, we found oscillating bubble behavior corresponding to nucleation, growth
and dissolution in dimethylsulfoxide and methanol. One possible explanation of the oscillation mechanism is provided by the instable
dynamic equilibrium between the gas influx due to supersaturation and outflux due to Laplace pressure. Furthermore, the critical gas
concentration for N2 nanobubble nucleation are estimated to be 148, 386, 200 and 16 times supersaturation and the contact angle of
critical nuclei to be 164, 151, 160, and 174o in water, dimethylsulfoxide, ethylene glycol and methanol, respectively. This is the first
report on electrochemical nucleation of gas bubbles in non-aqueous solvents. Our electrochemical gas bubble study based on
nanoelectrode platform has proven to be a prototypical example of single-entity electrochemistry.
probe electrochemistry, a technique utilizing glass nanopipettes
to create nanoscale dimension electrode surface, has been
INTRODUCTION
successfully adopted to probe and quantify the nucleation and
Gas-evolution is important to many electrocatalytic process,
such as chlorine-alkali process,1-2 H2 and O2 production from
water splitting,3-4 and CO2 and N2 from electro-oxidation in fuel
cells (such as methanol and hydrazine).5-6 It is known that the
formed gas bubbles on electrode surface will increase the
Ohmic resistance and reduce electrolysis efficiency.7-8
Management of these interfacial gas bubbles inherently
requires a better understanding of bubble evolution including
nucleation, growth and detachment. Several analytical
techniques have been introduced to study the individual
nanoscale bubble at interface as well as in the liquid phase.9
Atomic force microscopy (AFM) is most popular technique and
provides the earliest evidence on the existence of surface
nanobubbles,10-11 however, it suffers from very limited
temporal resolution, which is critical for bubble dynamics study.
The optical microscopy is a nonintrusive and direct method
with balanced temporal and spatial resolutions. Fluorescence
microscopy has been utilized to monitor the dynamic nucleation
and growth of gas nanobubbles from solvent exchange12 and
electrocatalytic water splitting.13 Super-resolution dark-field
microscopy14-15 and surface plasmon resonance microscopy16-
bubble behavior on Pt surface.20-21
Previously, we developed nanoelectrode method for the
electrogeneration of individual H2, N2 and O2 nanobubbles,
where gas molecules are electrogenerated by reduction or
oxidation of a species yielding gas product. For example, an
individual N2 nanobubble can be formed at a Pt nanoelectrode
by hydrazine oxidation, creating a highly N2 gas supersaturated
solution around the electrode surface, leading to a bubble
nucleation.22 Similarly, an individual H2 nanobubble can be
formed at Pt, Au and Pd nanoelectrodes by electroreduction of
proton,23-26 while an individual O2 nanobubble can be generated
by oxidation of water25 or hydrogen peroxide.27 Among all
these electrogenerated gas bubbles, the nucleation is indicated
by a sharp current drop to a small but nonzero current in the
voltammetry. The gas nanobubble blocks almost all the
electrode surface and the detected residual current stays
invariant when the potential is further scanned, suggesting a
dynamic equilibrium between the gas dissolution and re-
generation.28 Based on this nanoelectrode platform, the critical
27
concentration necessary for gas nucleation,22-23,
bubble
nucleation rate,29-30 as well as the geometry of critical bubble
18
are also appropriate for the visualization of interfacial
nuclei31 have been successfully measured.
nanobubble dynamics. These studies allow the evaluation of
catalytic activity of individual nanocatalysts and provides new
ideas to elucidate heterogeneous nucleation process of
nanobubbles. On the other hand, advanced electrochemical
methods have excellent temporal resolution and current
sensitivity for single gas nanobubbles. Ying and Long et al.
employed the patch clamp technique to measure the formation
and growth time of single H2 nanobubble from NaBH4
hydrolysis through a nanopore confinement.19 The scanning
Noted that for all the previous electrochemical study of
nanobubbles, the solution medium is limited to water. We
speculated that the metal electrode surface must be sufficiently
hydrophobic for the gas nanobubble to remain tightly pinned.
Manipulation of gas/electrode/solution interaction could obtain
very different surface energetics that possibly lead to different
bubble dynamics. Herein, based on this nanoelectrode platform,
we reported an interesting bubble oscillation behavior in non-
ACS Paragon Plus Environment

Analytical Chemistry
aqueous solution and measured the critical concentration for
nucleation, and the nanoscopic geometry of critical nuclei at
single nucleation level. Specifically, N2 bubble from hydrazine
oxidation was chosen for the case study because the mechanism
Page 2 of 7
with automatic dispenser (Attension Theta Lite Tensiometer) with 2
μL liquid. The profile of the droplet was recorded using a video
camera as shown in Figure S2.
1
2
3
4
5
6
7
8
9
and kinetics of the electrode reaction on Pt have been RESULTS AND DISCUSSIONS
previously investigated32-34 and anhydrous hydrazine is
The prepared Pt nanoelectrode was first used for the control
commercially available and directly used for solution
preparation. The non-aqueous solvents chosen need to be
miscible with anhydrous hydrazine and relatively
electrochemically inert within the potential window for
hydrazine oxidation in the experiment. Selected solvents
physical properties are listed in Table 1.
experiments of H2 bubble study in 0.5 M H2SO4 and N2 bubble
study in 1.0 M N2H4 aqueous solution. Typical cyclic
voltammetry is shown in Figure 1a and b and is very
reproducible over tens of cycles. A peak current of 16.8 nA for
H2 bubble nucleation and 5.8 nA for N2 bubble nucleation, as
well as the essentially stable residual current after bubble
formation for a 24 nm radius Pt nanoelectrode is observed, in
agreement with our previous results.22-23
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table
1.
Physical
properties
of
selected
solventsa
Solvent
H2O
EG
DMSO
MeOH
Viscosity
(mPa S)
0.890
16.06
1.987
42.92
1.1736
0.544
22.17
5.7835
Surface tension
(mN/m)
72.06
0.66
48.02
0.6135
N2 solubility
(mM)
N2 diffusivity
(×10-5 cm2/s)
1.9
0.11b
0.85b
3.11b
aAll data taken from Handbook of Chemistry and Physics at 25 C,
o
except where otherwise referenced.37
Figure 1. Typical cyclic voltammetry indicating bubble formation at a
24 nm radius Pt disk electrode recorded at a sweep rate of 100 mV/s.
(a) H2 bubble formation in 0.50 M H2SO4 aqueous solution with four
consecutive cycles. (b) N2 bubble formation in 1.0 M N2H4 aqueous
bDiffusivity is calculated from Stokes Einstein equation.
EXPERIMENTAL SECTION
solution
with
four
consecutive
cycles.
Materials. Sulfuric acid (98%) and anhydrous hydrazine (N2H4,
98%) were purchased from Sigma-Aldrich and used as received.
All aqueous solutions were prepared using deionized water
Electrogeneration of N2 nanobubble from hydrazine
oxidation in nonaqueous solvent were observed in EG, DMSO
and MeOH. As shown in Figure 2a, c and e, at low hydrazine
concentrations, the current shows a quasi-sigmoidal wave,
increasing with increasing overpotential. Compared to the well-
defined mass transport limited current response in aqueous
solution as demonstrated in earlier report,22 the voltammogram
in non-aqueous solution is rather “kinetically” controlled,
probably due to the absence of extra charge carrier/supporting
electrolyte in solution.32, 39 The current is proportional to the
hydrazine concentrations, confirming that the possible Faradic
current contribution from EG or MeOH oxidation is negligible
compared to hydrazine oxidation. Overall, the generated N2 gas
molecules at electrode surface in this scenario is not sufficient
to stimulate a stochastic nucleation. As hydrazine concentration
further increases to 2.0 M in EG, 3.0 M in DMSO and 1.0 M in
MeOH, respectively, as shown in Figure 2b, d and f, precipitous
current drop corresponding to bubble nucleation and growth is
observed. Specifically, the hydrazine oxidation current
decreases from 1.07 nA to a residual current of 0.1 nA at a
potential of 0.20 V vs SCE with 2.0 M N2H4 in EG, while the
current drops from 7.8 nA to 0.5 nA at 0.22 V with 3.0 M N2H4
in DMSO, and from 5.5 nA to 2.5 nA at 0.33 V with 1.0 M N2H4
in MeOH, respectively. Moreover, the peak current
corresponding to bubble nucleation is found essentially
invariant with increasing hydrazine concentration (e.g., 1.07,
0.92 and 0.87 nA for 2.0, 3.0 and 5.0 M N2H4 in ethylene glycol,
respectively) and the peak potential shifts to lower
overpotential due to higher transport flux at higher hydrazine
concentration. At low scan rates, the concentration of dissolved
gas around the nanoelectrode is maintained at a steady state due
to the rapid mass transport at nanoelectrodes (see Figure S3 for
scan rate independence of voltammograms). Thus the surface
concentration of dissolved gas prior to nucleation is directly
proportional to the current by:
(18.2
M
Ω
·cm).
Non-aqueous solvents including
dimethylsulfoxide, acetonitrile, dimethylformamide, methanol,
ethanol, ethylene glycol, acetone, tetrahydrofuran, 1,4-dioxane,
nitromethane and dichloromethane were purchased from Fisher
scientific. Ferrocene (98%, Sigma-Aldrich) was purified twice
by sublimation. In all the solution prepared, no additional
electrolyte was added.
Nanoelectrode Fabrication. Pt nanodisk electrodes were fabricated
according to previously reported procedures.38 Details can be found
in the literature as well as our previous reports.23-24 A 25 μm diameter
Pt wire (Goodfellow Corp. 99.99%) was first electrochemically
sharpened, then carefully aligned along the glass capillaries (Dagan
Corp. o.d. = 1.65 mm, i.d. = 1.10 mm) and sealed within the glass in
H2 flame. The radii of the nanodisk electrodes, a, were determined
from the voltammetric steady-state diffusion-limited current, ilim, for
the oxidation of ferrocene (Fc → Fc+ + e−) dissolved in acetonitrile
(CH3CN)
containing
0.10
M
tetrabutylammonium
hexafluorophosphate (TBAPF6), using the equation:
ilim  4nFDFcCFca
(1)
where DFc (2.4 × 10−5 cm2/s) and CFc are the diffusion coefficient and
the bulk concentration of Fc, respectively, and n is the number of
electrons transferred per molecule (= 1 for Fc oxidation).
Experimental steady-state voltammograms for measuring the
electrode radii are presented in Figure S1 in the Supporting
Information.
Electrochemical Measurement. A Dagan Cornerstone Chem-
Clamp potentiostat and a Pine RDE4 (used as waveform generator)
were interfaced to a computer through a PCI data acquisition card
(National Instruments) to collect i−V data. A Ag/AgCl (in 3 M NaCl)
electrode and a saturated calomel electrode (SCE) were used as the
counter/reference electrode in a two-electrode cell configuration.
Macroscopic Contact Angle Measurement. Macroscopic contact
angle of liquid droplet on pre-cleaned bare Pt surface without
potential bias was measured by an optical contact angle goniometer
i
Csurf

(2)
4nFDa
where n is the number of electrons transferred per gas molecule
ACS Paragon Plus Environment

Page 3 of 7
Analytical Chemistry
(= 4 for N2), F is Faraday’s constant, D is the diffusion methanol in Figure 2f, the current after nanobubble formation
coefficient of gas molecule in the solution (see Table 1), and a displays rigorous variations and such noisy becomes significant
1
2
is the radius of nanoelectrode.
for 3.0 M N2H4 in MeOH.
To disclose more details, high temporal resolution current
time traces at 20 kHz sampling rate corresponding to Figure 2d
with 3.0 M N2H4 in DMSO is presented in Figure 3a. For
comparison, a parallel current time trace with 3.0 M N2H4 in
ethylene glycol is shown in Figure 3b where no oscillation is
associated with N2 nanobubble. Zoom-ins of bubble nucleation
and partial current oscillations in DMSO are shown in Figure
3c. Initially, the N2 molecules are continuously generated from
3
4
5
6
7
8
9
hydrazine oxidation (state ①). Once surf is sufficiently high,
CN
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
stochastic nucleation of a bubble occurs and a small bubble
nuclei forms at the electrode surface (state ②). Due to
thermodynamically favorable bubble growth, the bubble
quickly covers almost the whole electrode surface (state ③).
Afterwards, the bubble continues to expand with increasing
contact angle from gas side (state ④), until the bubble breaks
and dissolves into the solution. The growing of gas bubble with
increasing volume and inner contact angle, but with invariant
residual current was previously predicted by molecular
simulations.40 This gas nucleation, growth and dissolution cycle
occurs repeatedly as long as high supersaturation can be
achieved. Corresponding schematic of bubble dynamic is
shown in Figure 3d. Eventually, when the potential scans to less
than 0.10 V vs SCE at 16.45s in Figure 3a, the N2 generation
from hydrazine oxidation can no longer balance the bubble and
it suddenly shrinks and dissolves (in less than 1 ms) without
subsequent bubble nucleation. At this moment, the electrode
surface is the same as state ① prior to nucleation. Additional
current time trace with bubble oscillation and their zoom-ins for
3.0 M N2H4 in MeOH are provided in the Supporting
Information (Figure S6).
While Figure 2d and Figure S7 in the Supporting Information
clearly shows the bubble oscillation behaviour is related with
N2H4 concentration, studies at different scan rate in Figure 4a-
4d display the oscillation behaviour diminishes with increasing
scan rates. At scan rate as high as 1000 mV/s, only the bubble
nucleation and dissolution is observed. At low scan rate of 40
mV/s, Figure 4a shows periodic bubble oscillation with the
peak current proportional to scan potential. This correlation is
confirmed by experiments of bubble oscillation at constant
potential applied (Figure 4e-i). While the oscillation frequency
increases slightly from 1.36 s-1 at 0.05 V to 1.64 s-1 at 0.60 V
vs SCE, and the average bubble nucleation and growth time
decreases with increasing potential (Figure S8). Considering
that optical techniques for such nanobubbles dynamics at
nanoelectrodes is extremely challenging, if not possible, we
further consider the noise spectrum of the current oscillation
from Figure 4e-h. Figure S9 shows the power spectral densities
of current signal at different potentials. A main peak at
approximately 1.5 Hz indicates the periodic bubble release at
nanoelectrode. As the potential increases, this peak shifts to
higher frequency with lower power densities, indicating faster
bubble oscillation due to faster gas generation, consistent with
previous results of the overpotential fluctuation at a gas
evolving microelectrode with constant current density.41
Figure 2. Typical cyclic voltammetry of hydrazine oxidation at a 24
nm radius Pt disk electrode recorded at a sweep rate of 100 mV/s. (a, b)
N2H4 in ethylene glycol, (c, d) N2H4 in DMSO, (e, f) N2H4 in methanol
with concentrations as indicated. The arrows indicate the direction of
forward and backward scan.
A current overshoot prior to current drop is often observed in
nonaqueous solvents, as indicated in Figure 2b, d and f. Careful
investigation shows such a current overshoot is also general for
N2 bubble nucleation in aqueous solution when high temporal
resolution data sampling was used (Figure S4). We speculate
such current overshoot origins from the Faradic current due to
enhanced hydrazine mass transport at the nanoelectrode
accompanying the phase transition (see Figure S5). We take the
value prior to the overshoot as the critical current for bubble
nucleation quantification. Upon bubble formation, an
essentially invariant residual current is observed with N2H4 in
EG in Figure 2b, suggesting very similar gas nanobubble
stabilization as in the aqueous solution. For the cyclic
voltammetry in DMSO as shown in Figure 2d, the current
responses show oscillating current after bubble is formed. As
the potential scans back to negative, the small residual current
suddenly increases, indicating the nanobubble is rapidly
dissolved and electrode surface is fully re-exposed. As for
ACS Paragon Plus Environment

Analytical Chemistry
Page 4 of 7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. High temporal resolution current traces of voltammetric responses at scan rate of 100 mV/s and sampling rate of 20 kHz (a) in 3.0 M
N2H4 DMSO solution with bubble oscillation, (b) in 3.0 M N2H4 ethylene glycol solution without current oscillation. The red and blue curves stand
for the current and potential, respectively. (c) Zoom-ins of four current oscillation peaks from (a) as indicated by different symbols (▲, bubble
nucleation,▼ and ◆, two typical oscillation, ★, bubble dissolution). (d) Schematics for different states of bubble, ①bare electrode, ②bubble
nucleation, ③growth and ④expanding.
currents as a function of potential (i). All the measured are recorded at
a sampling rate of 20 kHz.
We now consider possible reasons contributing to the
oscillating behavior of nanobubles observed in DMSO and
MeOH. Conventional gas nanobubbles pinned on the surface
have been found to have extraordinary long lifetime and
42-44
unusual small contact angles (from gas side).10,
Gas
supersaturation and contact line pinning are proposed to explain
the stabilization of nanobubbles.45-47 Lohse and Zhang
speculated the contact angle of a surface nanobubble is set by
the equilibrium between gas influx due to oversaturation and
gas outflux due to the Laplace pressure.46 At equilibrium,
sinθe=ζL/Lc, where L is the nanobubble contact length with
critical contact length of Lc=4γ/P0, and ζ is the oversaturation.
In the case of nanobubble electrogenration in water as shown in
Figure 1, upon nucleation, the bubble pinned on the electrode
will grow aggressively and the contact angle (from gas side)
increases. When more and more electrode surface are covered
by the bubble, the electrode edge available for gas
electrogeneration will decrease, causing a reduction of gas
influx into the gas bubble. Due to strong contact line pinning of
gas nanobubble on hydrophobic electrode surface, the dynamic
equilibrium between the gas influx and outflux can always be
sustained. However, in the case of non-aqueous medium, the
pinning of gas nanobubble on electrode can be much weaker
due to the very different surface energetics. This weaker gas
nanobubbling pinning on electrode surface consequently causes
nanobubble shape instability and easier break of the equilibrium
between gas influx and outflux, eventually leading to the bubble
oscillation behavior. Note that in our previous study of H2
nanobubble from proton reduction in aqueous solution in the
presence of surfactants (Triton X-100, TEGME and CTAB), a
similar current oscillation was not observed.23 We speculate
that while the surfactant molecules will certainly decrease
liquid surface tension and lead to a different surface energetics
at the three phase interface, the surfactant molecules adsorbed
at the liquid/gas interface will also play a role to mechanically
strengthen the nanobubble shape and kinetically slow down
Figure 4. Typical cyclic voltammetry of N2 bubble oscillation at a 24
nm radius Pt disk electrode in 5.0 M N2H4 in DMSO solution as a
function of different scan rate (a-d) 40, 200, 400, and 1000 mV/s.
Typical current time traces of bubble oscillation at different potential
applied (e-h) 0.05, 0.20, 0.40 and 0.60 V vs SCE. Oscillation peak
ACS Paragon Plus Environment

Page 5 of 7
Analytical Chemistry
Figure 5. (a) Typical i-V responses of Pt nanoelectrodes with radii of
diffusive outflux of gas from nanobubble, both of which can
stabilize a surface nanobubble. We believe such shape
instability is an overall consequence of several complicated
surface processes including gas bubble wetting on
electrode/glass surface interface, electrochemical reaction at
three phase boundary, and gas diffusion around surface bubble.
Possible nanobubble coalescence process prior to bubble
detachment cannot be excluded. Systematic evaluation of this
current oscillatory behaviour in experimental conditions is
certainly further needed in order to identify the detailed
mechanism. Eventually, unlike in DMSO and MeOH, the
oscillating bubbles in EG is not typical as shown in Figure 2b.
We speculate this is due to the extremely high viscosity of EG,
which could kinetically trap an oscillating bubble dynamics.
5, 17, 21 and 55 nm in 5.0 M N2H4 DMSO solution at a scan rate of 100
mV/s. Potential window was limited in order to clarify the peak shape
1
2
3
4
5
6
7
8
p
voltammetric response. (b) Nanobubble peak current, , (c) critical N2
inb
concentration for bubble nucleation calculated from eq 2., Ccrit.,and (d)
critical supersaturation for bubble nucleation, S (Ccrit. /C0), based on the
solubility from Table 1 as a function of electrode radius in different
solvent mediums. Error bars are based on the voltammetric response
from multiple measurement with the same electrode. The lines and
colored bands are drawn to guide the eye.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 2. Contact angle from liquid side in a macroscopic liquid
droplet, oscillating bubble behaviour, critical gas concentration, Ccrit.
,
critical supersaturation, S, for bubble nucleation, contact angle from
liquid side and radius of curvature of critical nuclei in the selected
solvents. No obvious correlation between the macroscopic and
nanoscopic angles is seen.
Many more non-aqueous solvents have been tested (Table S1
for the whole list), however, the well-defined peak shaped
voltammogram corresponding to bubble nucleation are not
always observed, due to possible solvent electrooxidation and
bubble instability. We limit our electrochemical analysis of N2
gas bubble to the four different solvents as listed in Table 1.
Figure 5a shows the typical voltammetric response of N2 bubble
formation from hydrazine oxidation in DMSO with different
radius of Pt nanoelectrodes. Regardless the current oscillations,
the peak feature and peak current values are very reproducible
(see Figure S3 for multiple scan cycles of cyclic voltammetry
in different solvents). Figure 5b summaries the peak current
Solvent
H2O
58
EG
DMSO
31
MeOH
20
θ of liquid
droplet (deg.)
Oscillation
behavior
44
No
Rarelya
236±12
386±20
151±2
2.5
Yes
Yes
Ccrit. (mM)
98±5
148±8
164±1
9.7
234±5
200±4
160±2
4.3
91±6
16±1
174±2
27.7
S (Ccrit. /C0)
θ of critical
p
nucleus (deg.)b
value for N2 bubble nucleation, , as a function of electrode
inb
rcurv. (nm)
radius in four different solvents, and a linear relationship based
on equation 2 is adopted. The N2 molecule diffusivity in the
non-aqueous solvent is estimated from their liquid viscosity
aTypical example and features is shown in Figure S11 and S12.
bEstimation from voltammetric measurement on individual
nanoelectrodes.
based on Stokes Einstein equation (Table 1), while
in water
DN
2
is taken as 1.9 × 10-5 cm2/s.48 From the slope of linear fit (0.301
± 0.009 for water, 0.040 ± 0.002 for EG, 0.462 ± 0.032 for
MeOH and 0.326 ± 0.007 for DMSO) in Figure 5b, the critical
surface concentration of N2, Ccrit, for bubble nucleation, were
estimated to be 98 ± 5 mM, 236 ± 12 mM, 234 ± 5 mM, and
91 ± 6 mM, in water, EG, DMSO, and MeOH, respectively.
In classical nucleation theory for a bubble, the rate of
nucleation, J, is expressed as49
16 3()
(3)
J  J0 exp(
)
3kBT(Pgas  P )2
0
These overall concentration corresponds to an ~148, ~386, where J0 is the pre-exponential factor; γ is the surface tension
~200, and ~16 times supersaturation of the N2 gas saturation of solvent-gas interface; Φ(θ)=(2-cosθ)(1+cosθ)2/4 is the
value at room temperature and atmospheric pressure for each geometric factor and is a function of the contact angle θ from
solvent as summarized in Table 2. Meanwhile, the critical liquid side. Pgas is the pressure of the gas inside the critical
concentration and supersaturation values for individual nucleus and P0 is the ambient pressure. Based on the
nanoelectrodes are also calculated and presented in Figure 5c distribution of nucleation peak currents from independent
and 5d. Results are consistent from linear fit. For the three non- voltammetric measurement, the pre-exponential factor, J0, and
aqueous solvents, DMSO, EG and MeOH, we found the the properties of single critical nuclei such as contact angle, θ,
supersaturation for N2 bubble nucleation is correlated with can be extracted based on a previous developed statistical
liquid surface tension while water is excepted (Figure S13, model.31 With known critical nuclei internal pressure Pgas and
Supporting Information).
assuming an equilibrium between the solution and the bubble
nuclei, the radii curvature of the critical nuclei, is calculated
from the Young-Laplace equation, rcurv=2 γ/(Pgas-P0). Details of
θ and rcurv of critical nuclei pertaining to bubble nucleation in
different solvents is summarized in Table 2. Interestingly, the
estimated nanoscopic contact angles from liquid side of critical
nuclei in the non-aqueous solvents (151±2o in EG, 160±2o in
DMSO, 174±2o in MeOH) are very close to the value in water
(164±1 o), but are much higher than the macroscopic contact
(58o in H2O, 44o in EG, 31o in DMSO, 20o in MeOH). These
large contact angles of N2 nanobubbles from liquid side are
consistent with previous small contact angle measured from
Tapping mode AFM for surface gas nanobubbles.50 More
interestingly, in our study, the trend of calculated θ of critical
nucleus in the four different solvents agrees remarkably well
with that of the measured gas oversaturation (Ccrit. /C0-1) based
on the model previously proposed by Lohse and Zhang, where
the equilibrium contact angle of a surface nanobubble is
described by sinθe=ζL/Lc.46
ACS Paragon Plus Environment

Analytical Chemistry
Page 6 of 7
8.
Faber, M. S.; Dziedzic, R.; Lukowski, M. A.; Kaiser, N.
CONCLUSIONS
1
2
3
4
5
6
7
8
S.; Ding, Q.; Jin, S., J. Am. Chem. Soc. 2014, 136, 10053-10061.
In summary, we demonstrated the electrogeneration of single
N2 nanobubbles in different solvent medium. In contrast to the
dynamic stationary state of nanobubbles found in aqueous
solution, which is indicated by the invariant residual current,
the oscillating behavior corresponding to bubble nucleation,
growth and dissolution cycle is observed in DMSO and MeOH.
We speculate such nanobubble dynamics is probably due to
very different contact line pinning of gas nanobubble on
electrode surface in the non-aqueous solvents, relative to water.
In future work, we will develop this bubble oscillation at
nanoelectrodes as a means of revealing the nanoscopic
interaction at the gas/liquid/electrode three phase boundary.
Based on our nanoeletrode platform for electronucleation, the
critical concentration for N2 bubble nucleation is estimated to
be 98, 236, 234 and 91 mM in water, ethylene glycol, DMSO
and methanol, respectively, corresponding to 148, 386, 200 and
16 times the saturation concentration. Our unique study of
single N2 nanobubbles also allows quantitative characterization
of the geometry of critical nuclei including the contact angle
and radius of curvature. Overall, our electrochemical driven gas
nanobubble study based on nanoelectrode has proven to be a
prototypical example of single-entity electrochemistry.51
9.
10.
Lohse, D.; Zhang, X., Rev. Mod. Phys. 2015, 87, 981-1035.
Lou, S.-T.; Ouyang, Z.-Q.; Zhang, Y.; Li, X.-J.; Hu, J.; Li,
M.-Q.; Yang, F.-J., J. Vac. Sci. Technol., B 2000, 18, 2573-2575.
11. Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K.,
Langmuir 2000, 16, 6377-6380.
12.
174501.
13.
B., Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 5878-5883.
14. Li, S.; Du, Y.; He, T.; Shen, Y.; Bai, C.; Ning, F.; Hu, X.;
Wang, W.; Xi, S.; Zhou, X., J. Am. Chem. Soc. 2017, 139, 14277-
Chan, C. U.; Ohl, C.-D., Phys. Rev. Lett. 2012, 109,
Hao, R.; Fan, Y.; Howard, M. D.; Vaughan, J. C.; Zhang,
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
14284.
15.
Y.; Zhou, X., J. Phys. Chem. Lett. 2018, 9, 5630-5635.
16. Fang, Y.; Li, Z.; Jiang, Y.; Wang, X.; Chen, H.-Y.; Tao,
N.; Wang, W., Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 10566-
10571.
Zhang, T.; Li, S.; Du, Y.; He, T.; Shen, Y.; Bai, C.; Huang,
17.
Chen, J.; Zhou, K.; Wang, Y.; Gao, J.; Yuan, T.; Pang, J.;
Tang, S.; Chen, H.-Y.; Wang, W., Proc. Natl. Acad. Sci. U. S. A.
2019, 201903259.
18.
Chem. 2019, 91, 4665-4671.
19.
Anal. Chem. 2018, 90, 12352-12355.
20.
Electrochim. Acta 2018, 283, 1773-1778.
21.
10, 3887-3892.
22.
J. Am. Chem. Soc. 2015, 137, 12064-12069.
23.
S., J. Phys. Chem. Lett. 2014, 5, 3539-3544.
24.
25.
4581.
26.
122, 15421-15426.
27.
H. S., J. Phys. Chem. Lett. 2017, 8, 2450-2454.
28.
H. S., Langmuir 2017, 33, 1845-1853.
29.
Am. Chem. Soc. 2018, 140, 4047-4053.
30.
Lohse, D.; Edwards, M. A.; White, H. S., Langmuir 2018, 34, 7309-
7318.
31.
13, 6330-6340.
32.
Chem. Rev. 2009, 109, 2209-2244.
33.
Koper, M. T. M.; Unwin, P. R., Anal. Chem. 2015, 87, 5782-5789.
34.
2017, 139, 16813-16821.
35.
Ref. Data 1984, 13, 563-600.
36.
37.
Boca Raton, FL, 2016-2017.
38.
Bergren, A. J.; Jones, R. M.; White, R. J.; Ervin, E. N.; Cauley, C.
C.; White, H. S., Anal. Chem. 2007, 79, 4778-4787.
39.
2nd ed.; Wiley-VCH: 2010.
40.
Molinero, V., J. Am. Chem. Soc. 2019, 141, 10801-10811.
Wang, Y.; Chen, J.; Jiang, Y.; Wang, X.; Wang, W., Anal.
Hu, Y.-X.; Ying, Y.-L.; Gao, R.; Yu, R.-J.; Long, Y.-T.,
ASSOCIATED COTENT
Supporting Information. The Supporting Information is available
free of charge via the Internet at http://pubs.acs.org.
Estimation of nanoelectrode radii, droplet contact angle at Pt plate,
effect of scan rate on the voltammograms in different solvent media,
current overshoot for gas bubble formation in aqueous solution,
possible origin of the current overshoot, potential dependent current
oscillation, Zoom-ins of current oscillation in methanol, current
oscillation in DMSO, current oscillation in ethylene glycol, list of
common solvent media tested in the study. Supersaturation for
bubble nucleation in different solvent media
Perera, R. T.; Arcadia, C. E.; Rosenstein, J. K.,
Wang, Y.; Gordon, E.; Ren, H., J. Phys. Chem. Lett. 2019,
Chen, Q.; Wiedenroth, H. S.; German, S. R.; White, H. S.,
Chen, Q.; Luo, L.; Faraji, H.; Feldberg, S. W.; White, H.
Luo, L.; White, H. S., Langmuir 2013, 29, 11169-11175.
Chen, Q.; Luo, L.; White, H. S., Langmuir 2015, 31, 4573-
Chen, Q.; Ranaweera, R.; Luo, L., J. Phys. Chem. C 2018,
AUTHOR INFORMATION
Ren, H.; German, S. R.; Edwards, M. A.; Chen, Q.; White,
Corresponding Author
* E-mail: qianjinchen@dhu.edu.cn
* E-mail: white@chem.utah.edu
Liu, Y.; Edwards, M. A.; German, S. R.; Chen, Q.; White,
German, S. R.; Edwards, M. A.; Ren, H.; White, H. S., J.
Soto, Á. M.; German, S. R.; Ren, H.; van der Meer, D.;
ACKNOWLEDGMENT
This work was supported by the Office of Naval Research Award
No. N00014-16-1-2541. Q.C. acknowledges to the National Science
Foundation of China (NSFC-21804018) and Shanghai
(19ZR1470800), as well as the start-up funds from Donghua
University for support.
Edwards, M. A.; White, H. S.; Ren, H., ACS Nano 2019,
Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. M.,
Chen, C.-H.; Jacobse, L.; McKelvey, K.; Lai, S. C. S.;
REFERENCES
1.
Karlsson, R. K. B.; Cornell, A., Chem. Rev. 2016, 116,
Bentley, C. L.; Kang, M.; Unwin, P. R., J. Am. Chem. Soc.
2982-3028.
2.
Moreno-Hernandez, I. A.; Brunschwig, B. S.; Lewis, N. S.,
Battino, R.; Rettich, T. R.; Tominaga, T., J. Phys. Chem.
Energy Enviorn. Sci. 2019, 12, 1241-1248.
3.
I.; Nørskov, J. K.; Jaramillo, T. F., Science 2017, 355, eaad4998.
4. Jovanovič, P.; Stojanovski, K.; Bele, M.; Dražić, G.;
Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff,
Dymond, J. H., J. Phys. Chem. 1967, 71, 1829-1831.
Handbook of Chemistry and Physics. 97th ed.; CRC Press:
Koderman Podboršek, G.; Suhadolnik, L.; Gaberšček, M.; Hodnik,
N., Anal. Chem. 2019, 91, 10353-10356.
Zhang, B.; Galusha, J.; Shiozawa, P. G.; Wang, G.;
5.
Wasmus, S.; Küver, A., J. Electroanal. Chem. 1999, 461,
14-31.
6.
Kosuke, I., Electrochemistry in Nonaqueous Solutions.
Lu, Z.; Sun, M.; Xu, T.; Li, Y.; Xu, W.; Chang, Z.; Ding,
Y.; Sun, X.; Jiang, L., Adv. Mater. 2015, 27, 2361-2366.
Perez Sirkin, Y. A.; Gadea, E. D.; Scherlis, D. A.;
7.
Harrison, J. A.; Kuhn, A. T., Surface Technol. 1983, 19,
249-259.
ACS Paragon Plus Environment

Page 7 of 7
Analytical Chemistry
41.
Möbius, M. E., Langmuir 2014, 30, 13065-13074.
42. Zhang, X.; Chan, D. Y. C.; Wang, D.; Maeda, N.,
Langmuir 2013, 29, 1017-1023.
Fernández, D.; Maurer, P.; Martine, M.; Coey, J. M. D.;
1
2
3
4
5
6
7
8
43.
44.
Craig, V. S. J., Soft Matter 2011, 7, 40-48.
Alheshibri, M.; Qian, J.; Jehannin, M.; Craig, V. S. J.,
Langmuir 2016, 32, 11086-11100.
45.
46.
Liu, Y.; Zhang, X., J. Chem. Phys. 2014, 141, 134702.
Lohse, D.; Zhang, X., Phys. Rev. E 2015, 91, 031003.
47.
Tan, B. H.; An, H.; Ohl, C.-D., Phys. Rev. Lett. 2017, 118,
054501.
48.
9
Cussler, E. L., Diffusion: Mass Transfer in Fluid Systems.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2nd ed. ed.; Cambridge University Press: New York, 1997.
49.
50.
Blander, M.; Katz, J. L., AIChE J. 1975, 21, 833-848.
An, H.; Liu, G.; Atkin, R.; Craig, V. S. J., ACS Nano 2015,
9, 7596-7607.
51. Lemay, S. G., ACS Nano 2019, 13, 6141-6144.
Table of Content
ACS Paragon Plus Environment