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J Appl Electrochem (2010) 40:849–854
DOI 10.1007/s10800-009-0068-z

Author's personal copy

ORIGINAL PAPER

Behavior of hydrogen nanobubbles in alkaline electrolyzed waterand its rinse effect for sulfate ion remained on nickel-plated
surfaceToshikazu Takenouchi

Received: 9 July 2009 / Accepted: 23 December 2009 / Published online: 14 January 2010
Ó Springer Science+Business Media B.V. 2010

Abstract More than ordinary rinsing using pure water,
cathode water obtained by electrolysis of dilute potassium
carbonate aqueous solution (alkaline electrolyzed water:
AEW) exhibits a stronger rinse effect for elimination ofremaining sulfate ions when rinsing nickel-plated surfaces.
This rinse effect was recognized even for AEW that wasused 24 h after it was produced, but not 1 week after.
Behaviors of hydrogen nanobubbles observed by dynamic
light scattering revealed nanobubbles of about 128-nm
diameter even 24 h after generation. The Ostwald ripening
phenomenon was observed. Hydrogen nanobubbles in an
open system changed: some shrank because of ripening,later dissolving in the aqueous solution and disappearing;others showed swelling and expansion. One week later, few
nanobubbles smaller than 300 nm were observed. Rinse
effects by AEW, which are attributable to the actions ofhydrogen nanobubbles generated in AEW, occur because
sulfate ions are cleaned and removed from the nickelplated surface.
Keywords Alkaline electrolyzed water Hydrogen nanobubbles  Rinse  Sulfate ion

1 Introduction
Cathode water—obtained by electrolysis of a diluteelectrolyte aqueous solution—such as alkaline electro-

T. Takenouchi (&)
Research and Development Division, Shinko Electric Industries
Co., Ltd, 36 Kitaowaribe, Nagano-shi 381-0014, Japane-mail: ttakenoh@shinko.co.jp

lyzed water (AEW) removes particulates and surfactantsfrom the surface of a material to be cleaned [1]. It hascleaning performance that is comparable to a degreasingor cleaning method using conventional chemicals, rendering it useful for precision cleaning [2]. The electrolyzed water has high alkalinity. Moreover, hydrogen
bubbles resulting from electrolysis act on the material to
be cleaned and the particulates that are present on its
surface, thereby improving the cleaning effects. In thesemiconductor field, hydrogen-dissolved water (hydrogengas dissolved in ultrapure water) is used to remove
nonionic particulates from silicon wafer surfaces and isused in practical wafer cleaning processes [3]. Ultrasonic irradiation of hydrogen-dissolved water reportedly enhances particulate removal effects by virtue of thefine hydrogen bubbles generated by the ultrasonic waves[4].
The authors, considering that AEW might remove ionic
impurities in addition to degreasing, cleaning, and particulate removal effects, have been promoting its investigation. Empirically, it is known that a Watts bath, primarily
using nickel sulfate, leaves sulfate ions on a nickel-plated
surface to be cleaned. For removal of ionic impurities
including sulfate ions, various techniques are used during
cleaning after plating: repeated cleaning with pure water,
ultrasonic cleaning, or the like. In a previous article, wereported that cleaning with pure water after a nickel-plated
specimen was once immersed into AEW (hereinafter,
rinsing) was able to decrease the amount of sulfate ionsremaining on the nickel-plated surface [5]. Results of thisstudy verified our idea that the rinse effects of AEW are
attributable to hydrogen bubbles generated in AEW, particularly nanosize hydrogen bubbles (hereinafter, hydrogen
nanobubbles).

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850

(2)

2 Experimental
2.1 Reagents, conditions, and apparatus for AEW
generation
Potassium carbonate (analytical reagent grade; Wako Pure
Chemical Industries Ltd.) was used as the electrolyte for
AEW generation, along with an electrolytic apparatus (Espax; Zipcom Co. Ltd.) used for maintaining theconcentration of potassium carbonate in aqueous solution
at 0.01 wt%. Pure water having resistivity of more than
10 MX cm was generated using ion exchange and reverse
osmosis, and passed through a filter having openings of
0.1 lm. The electrolytic apparatus we used is a continuousgenerating-type cylindrical electrolytic compartment consisting of a diaphragm with a neutral membrane filter(Yumicron; GS Yuasa Battery Ltd.) and platinum-coatedtitanium electrodes (plating thickness 1 lm; Tanaka
Kikinzoku Group). The anode and cathode electrode apparent surface areas were, respectively, 922 and 1,063 cm2.
Electrolytic conditions were 50 V voltage, 15 A current,
and a 3 dm3 min-1 flow rate. Figure 1 shows the electrolytic cell. Measurements of the aqueous solution’s pH,
redox potential, and concentration of dissolved hydrogenwere conducted, respectively, using a pH meter (F-22;
Horiba Instruments Ltd.), a redox potential meter (TRX90; Toko Kagaku Co. Ltd.), and a dissolved hydrogen
meter (DH-35A; DKK-TOA Corp.).
2.2 Rinse effect experiments by AEW
Experiments were performed according to the following
steps to verify that AEW rinses away sulfate ions [5]:(1)

A copper substrate (20 mm 9 20 mm 9 1 mm) was
nickel-plated electrolytically using a Watts bath toyield 1 lm thickness.
anode water
cathode wateranode

390 cmdiaphragm

(3)

(4)
(5)

J Appl Electrochem (2010) 40:849–854

The specimen was immersed into AEW with 0.5 dm3,
dipped in and out in a vertically oscillating fashion
three times, and immersed for a total of 3 s (rinsing
by AEW). Because of the comparative control, thesame operations were performed using pure waterinstead of AEW.
Pure water was introduced at a rate of 2 dm3 min-1;
the specimen was cleaned with this running water fora minute.
Dry nitrogen gas was blown on the specimen fordrying.
The specimen was boiled in ultrapure water. Thenconcentrations of anion and cations contained in theboiling water were determined using ion chromatography (DX-120; Dionex Corp.) based on JIS K0556.
The amount of ionic impurities per unit area of thespecimen was obtained. Ultrapure water with resistivity of not less than 18 MX cm was used. Experiments were performed three times; their values wereaveraged and used for analyses.

The experiments described above were performed using
AEW immediately after generation, AEW 24 h after generation, and AEW 1 week after generation. The results
were compared. Then AEW was stored at room temperature in open conditions.
2.3 Measurements of hydrogen nanobubbles
A dynamic light scattering photometer (FDLS-3000; Otsuka Electronics Co. Ltd.) was used for particle diameterand size distribution measurements of the hydrogen bubbles in AEW. This study used a dynamic light scattering
apparatus with a light source equipped with a semiconductor excitation solid-state laser having 523 nm wavelength [6]. The use of wavelengths shorter than theconventional 780 nm improved the detection sensitivity by
50–100 times. The hydrogen nanobubble behaviors were
therefore more clearly identifiable. Dynamic light scattering measurements were conducted for AEW preserved for
5 h, 24 h, and 1 week after preparation. In the closed
system, the AEW was left in the dark in a stoppered flaskwith no headspace. In the open system, it was left in a nonstoppered flask at room temperature. The closed system
specimen was used after storage for 5 h.

cathode3 Results and discussion
10 0

cm

Φ

0.01 wt %K2CO3 solution

Fig. 1 Schematic diagram of electrolytic cell

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3.1 Characteristics of AEW
Table 1 portrays characteristics of AEW of potassium
carbonate. The redox potential of AEW was remarkably

J Appl Electrochem (2010) 40:849–854

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Table 1 Characteristics of AEW and 0.01 wt% K2CO3 aqueous
solution

pH

11.90

10.50

Redox potentiala/mV versus SHE

-690

370

Dissolved hydrogena/mg dm-3

1.20

0.00

a

Measured in 5 min after electrolysis

140

sulfate ion
chloride ion
potassium ion-20.01 wt% K2CO3
solution

2500 400
160

Residual ions / ng cm

AEW

851

120
100
80
60
40

-3

400

1.4

200

1.2

0

1
0.8

-200
Redox potential

-400

0.6

Dissolved hydrogen

0.4

-600
-800
0.01

0.2
0.1

1

10

100

Dissolved hydrogen / mg dm

Redox potential / mV vs. SHE

20
1.6

0
1000

time / hFig. 2 Change in redox potential and dissolved hydrogen concentration of AEW of 0.01 wt% K2CO3 with elapsed time after electrolysis

lower and the dissolved hydrogen concentration was higher
than that of the aqueous solution of potassium carbonate, which was not subjected to electrolysis. According to
the Nernst equation, the pH and hydrogen generation
potential are expressed as EH = -RT/2F lnPH2 - 0.059
pH. At 25 °C and atmospheric pressure, it is expressed as
EH = -0.059 pH; results of the measurement nearly satisfied the relational expression. Furthermore, Figure 2
portrays temporal changes in the redox potential of AEW
and the concentration of dissolved hydrogen. Figure 2
shows that although a slight change in redox potential isnoticed until 4 h after generation, it increases rapidly at 5–6 h and thereafter. The reduced concentration of dissolvedhydrogen causes this change.
3.2 Amount of sulfate ions remaining on the surfaceof specimen
The amount of sulfate ions remaining on the surface of thespecimen is depicted in Fig. 3. The quantitative limit ofsulfate ions shown by ion chromatography is 0.1 lg dm-3.
The sulfate ions remaining on the specimen surface
immediately after plating were 2,500 ng cm-2. That value
was 40 ng cm-2 after cleaning by rinsing with AEW andsubsequent rinsing with pure water. It was 120 ng cm-2
when pure water was used for rinsing. Although sulfate
ions are liable to remain for cases in which all cleaning0
Nothing

AEW just AEW kept AEW kept Deionizedprepared for 24 h for a week
water

Fig. 3 Amount of residual ions on electroplated nickel specimen
rinsed with AEW. Specimen was treated nothing, rinsed with AEW
just prepared, AEW kept for 24 h, AEW kept for a week, anddeionized water only

after plating is performed with pure water, when rinsing
with AEW was performed, fewer sulfate ions adhered,
thereby exhibiting the rinse effect. The result of similar
experiments using materials 24 h after AEW generation
was 60 ng cm-2. However, the use of AEW 1 week aftergeneration yielded identical results to those obtained when
the surface had been cleaned solely with pure water. Thisphenomenon is likely to be associated with the hydrogen
nanobubbles in AEW, as described later. For chloride ions,no difference was found in either case. Although theremaining sulfate ions were fewer—although chloride ionsare rinsed more easily than sulfate ions—some infinitesimal amount of the ions invariably remains. Existingpotassium ion amounts were originally small and were notdetected after rinsing.
Takahashi [7] identified that air nanobubbles in thewater are stable for a long period. Similarly, it is considered that hydrogen nanobubbles exist stably for a long
period. The fact that the rinse effect appeared even 24 hafter AEW generation suggests that hydrogen nanobubbles
exist in a stable manner. However, the rinse effect disappears within AEW 1 week after its generation, implyingthat hydrogen nanobubbles contributing to the rinse effecthad disappeared.
3.3 Behavior of hydrogen nanobubbles in AEW
Regarding water electrolysis, water molecules are reduced,
producing hydrogen molecules on the electrode surface.
Hydrogen bubbles are generated from the hydrogen molecules. The hydrogen bubbles then combine to producehydrogen nanobubbles. Therefore, most nanobubbles are ofhydrogen.

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852
25

25

A

D

20

frequency / %frequency / %Fig. 4 Distribution of hydrogen
bubble sizes in AEW. Measured
after electrolysis; a: 5 h, closed
system, average size (ave.)
160 nm, b: 24 h, closed system,
ave. 184 nm, c: 24 h, open
system, ave. 13, 128, and
1349 nm, d: 1 week, closed
system, ave. 750 nm, e: 1 week,
open system, ave. 735 nm

J Appl Electrochem (2010) 40:849–854

15
10
5

20
15
10
5

0

0
10

1

100

1000

10000

1

10

1000

10000

diameter / nmdiameter / nm
25

25

B

E

20

frequency / %frequency / %

100

15
10

20
15
10
5

5

0

0
1

10

100

1000

10000

diameter / nm

1

10

100

1000

10000

diameter / nm

frequency / %

25

C
20
15
10
5
0
1

10

100

1000

10000

diameter / nm

The particle size distribution of hydrogen nanobubbles
in the closed system 5 h after AEW generation is portrayedin Fig. 4a. The average particle diameter was 160 nm. The
dissolved hydrogen concentration was 1.20 mg dm-3.
Figure 4b presents the distribution in the closed system
24 h later: the particle size distribution was 100–300 nm;
the average particle diameter was 184 nm; and the dissolved hydrogen concentration was 0.98 mg dm-3. At thesame time, 5–10 lm microbubbles were also generated,probably because of generation of micrometer-size bubbles
as a result of binding of some hydrogen nanobubbles. In theclosed system, nanobubbles grew slightly and were stable,
although the dissolved hydrogen concentration decreasedslightly. Figure 4c portrays the distribution in the open
system observed 24 h later. Three peaks were observed:
their average particle diameters were, respectively, 13, 128,
and 1349 nm. This phenomenon illustrates Ostwald
ripening. Regarding hydrogen nanobubbles’ changes inparticle diameter in the open system, some shrank becauseof ripening, dissolving in the aqueous solution and disappearing, but others expanded [6]. The dissolved hydrogen

123

concentration was 0.30 mg dm-3. Figure 4d presents the
distribution in the closed system 1 week later. Nanobubbles of around 184 nm, as shown in Fig. 4b (24 h after,
closed system) had almost disappeared. The distribution in
350–1,500 nm (average particle diameter 750 nm)
increased. Figure 4e depicts the distribution in the open
system 1 week later. Nanobubbles of around 128 nm, as
observed in C (24 h after, open system), were very few.
The distribution of bubbles of 300–1,200 nm particle
diameter (average 735 nm) increased. It is readily apparent
from Fig. 4 that the frequency of hydrogen nanobubbles
24 h later is 30–40% less than that 5 h later. The sulfate
ions remaining for AEW kept for 24 h were 1.5 times more
numerous than those in AEW that had just been prepared(Fig. 3). Based on this result, the presence of hydrogen
nanobubbles is inferred to cause rinse effects.
Kikuchi et al. identified that, for hydrogen in AEW,
dissolved hydrogen is dissolved in molecular form at morethan the saturated concentration. Furthermore, fine hydrogen bubbles (hydrogen nanobubbles) coexist in colloidal
form [8]. In their experimental conditions, hydrogen

J Appl Electrochem (2010) 40:849–854

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nanobubbles disappeared in 3 h. Additionally, its stability
depends on the current density and ionic strength [9]. In
this study, experiments were conducted under higher current density and lower electrolyte concentration conditions than the electrolysis conditions used in the study by
Kikuchi et al. Therefore, we presume that hydrogen
nanobubbles are likely to exist more stably than in the caseinvestigated by Kikuchi et al. The reason for the stable
presence of hydrogen nanobubbles remains unknown, but
elucidation of this mechanism is an important future task.
The experiments described herein demonstrated thathydrogen nanobubbles in AEW exist even 24 h later inclosed systems at sizes of 100–300 nm. They shrink in
open systems and have a distribution peak around 128 nm.
The nanobubbles exist in stable fashion in either case.
Their rinse effects are reasonably explainable consideringthat the action of nanobubbles smaller than 300 nm is
associated with cleaning action by AEW.
Previously, the authors measured hydrogen bubbles thatwere present in AEW and their particle size distribution
using dynamic light scattering with a storage type electrolytic apparatus. Consequently, hydrogen bubbles of
micrometer and nanometer size were observed. Hydrogen
bubbles mutually join over time to become large bubbles;
they then flow outside the liquid and disappear in about 1 h[5]. Although the stability of hydrogen bubbles depends onelectrolytic conditions and the electrolyte type, finehydrogen bubbles reportedly disappear in about 1–6 h [10].
Therefore, AEW should be used immediately after its
generation. Although optimization of electrolytic conditions for the stable existence of hydrogen nanobubbles in
AEW is difficult, the specifications used for this study
supported storage of AEW containing hydrogen nanobubbles in a comparatively stable fashion for up to about 24 h.
3.4 Rinse effect of AEW for sulfate ionsRegarding the mechanism of particle removal from a siliconwafer using hydrogen-dissolved water, it is considered that(i) hydrogen bubbles in the aqueous solution adhere to theparticulates and act as a physical peeling force, lifting themoff the silicon wafer, and that (ii) hydrogen radicals bond to
the uppermost surface of the wafer and render the surface
inactive [4]. Moreover, because the zeta-potential on thesurfaces of silicon wafer and particulates have higher negative zeta-potential in the alkaline aqueous solution, theremoved particulates repel each other by electric repulsion
force, repelling the silicon wafer also. Therefore, they do
not mutually readhere [11]. This concept is derived from
Lange’s theory [12]. The rinse effect by AEW can also be
considered as attributable to actions by hydrogen bubbles
similarly to the mechanism for particulate removal. Previously, the authors demonstrated that AEW is an aqueous

Ni2+

853

SO42-

hydrogennano bubble

OH-

Fig. 5 Schematic diagram of sulfate ions removal by hydrogen
nanobubbles in AEW

solution having negative zeta-potential including nanosize
and micrometer-size hydrogen bubbles [5]. Therefore,hydrogen generated from electrolysis is present in AEW as
negatively charged colloidal bubbles. Hydroxide ions arereportedly adsorbed onto the surface of negatively charged
hydrogen bubbles [9]. Air nanobubbles also have higher
negative zeta-potential, and hydroxide ions gather on asliding plane of electric double layer at the interface
between air nanobubbles and water [7]. Presumably, when anickel-plated surface is rinsed with AEW, selective
replacement reactions attributable to the ion exchange
reaction between hydroxide ions being adsorbed onto thesurface of hydrogen nanobubbles occur, along with sulfate
ions being adsorbed onto the nickel surface. Consequently,
sulfate ions on the nickel surface are desorbed. Desorbed
sulfate ions do not readhere onto the nickel surface, probably because of repulsion with similarly negatively charged
hydrogen nanobubbles. They are therefore rinsed efficiently. The material to be cleaned is eventually purified.
Figure 5 shows an estimated model of this replacement
reaction.
According to the Pourbaix diagram [13], nickel ispositioned in the corrosive region when in a strong alkaline
solution. However, measurements of nickel soaked in
AEW for 1 month and nickel ion concentration in the
aqueous solution detected no nickel ions. Apparently,
AEW dissolves nickel only slightly. In other words, dissolution of the nickel uppermost surface does not eliminate
sulfate ions. Desorption eliminates the ions through interaction with hydrogen nanobubbles.
3.5 Specific adsorption of sulfate ionsSulfate ions are known to adhere to nickel-plated layer
surfaces. Even rinsing with pure water does not remove
them easily. A silicon wafer cleaned using a sulfuric acid/123Author's personal copy854hydrogen peroxide mixture showed a similar phenomenon.
Reportedly, the complete removal of the sulfate ionsremaining on the wafer is extremely difficult using ultrapure water alone [14, 15]. To date, the specific adsorptionof sulfate ions on the electrode surface remains unclear.
Results of recent studies show that sulfate ions specificallyadsorb onto the surfaces of some metals and influencesubsequent crystal growth. Results of analyses conducted
at a large radiation facility (Super Photon ring 8 GeV;
Spring-8) indicate that oxygen atoms of the sulfate ionsform a closely packed structure surrounding the copper (I)ions. The distance between the two oxygen atoms iscomparable to that of the hydrogen bond in the plane of theclosely packed oxygen atoms when copper is deposited ona gold electrode using a copper sulfate aqueous solution[16]. In that study, the sulfate ions adsorbed to Cu underpotential deposition (UPD) and were deposited on aAu(111) electrode in a sulfuric acid aqueous solution [17].
The specific adsorption of the sulfate ions on an Ir(111)electrode was also observed [18]. Consequently, although
the sulfate ions had not been considered previously as achemical species that easily underwent specific adsorption,
the phenomenon by which the sulfate ions were specificallyadsorbed onto an electrode showed a significant influence.
These results imply that sulfate ions are adsorbed easily
onto the nickel-plated surface. Presumably, during nickel
deposition, the nickel ions deposit in the structure surrounded by the oxygen atoms of the sulfate ions. Then thesulfate ions, retaining covalent bonds, are adsorbed onto
the nickel surface. They cannot be removed easily by
subsequent cleaning treatment.

4 Conclusion
The authors have inferred that rinse effects of sulfate ionsremaining on the nickel-plated surface by AEW are
attributable to the hydrogen nanobubbles in AEW. We
measured the particle diameter and size distribution ofhydrogen nanobubbles in AEW using dynamic light scattering and explored possible relations between their
behaviors and the rinse effects. The following findings
were obtained.
(1)

Actually, AEW, when used for cleaning of the nickelplated surface, has a rinse effect that is better capableof reducing the amount of remaining sulfate ions thancleaning with pure water alone. This rinse effect was
confirmed for AEW even 24 h after generation. No
rinse effect was noticed with AEW 1 week aftergeneration.

123

(2)

(3)

J Appl Electrochem (2010) 40:849–854

Confirmation of hydrogen nanobubbles in AEW using
dynamic light scattering revealed that nanobubbles ofabout 128 nm diameter were present in a stablefashion even 24 h after their generation. However,1 week later, few nanobubbles smaller than 300 nm
diameter were observed.
The authors consider that a rinse effect occurs
because of hydrogen nanobubbles generated in
AEW. The AEW cleans and removes sulfate ionsefficiently from the nickel-plated surface because
sulfate ions replace hydroxide ions on the hydrogen
nanobubble surfaces. The sulfate ions are adsorbed
selectively onto the hydrogen nanobubble surfaces.

Acknowledgments The authors wish to thank Mr. Mitsuaki Hashimoto and Mr. Kazushi Sasa of Otsuka Electronics Co. Ltd. for theirassistance with measurement of the particle size and size distributionof hydrogen nanobubbles. We also wish to thank Dr. Shinichi Wakabayashi of the Nagano Techno Foundation for his valuable advice
related to the specific adsorption of sulfate ions.

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