工学院大学研究報告第 123 号

平成 29 年 10 月

白金イリジウム線の交流電解研磨における周波数依存性
青 山 宜 樹＊1，高 見 知 秀＊2

Frequency dependence of alternate current electrochemical
etching of platinum/iridium wire
Yoshiki AOYAMA＊1 and Tomohide TAKAMI＊2
Abstract
白金イリジウム線を交流電解研磨したときの交流周波数依存性について調べた．周波数が 100 ヘルツから 1000 ヘ
ルツに増加するにつれて反応速度は増加したが，交流による電極の変化に依存する反応持続時間は減少した．このため，作製された探針の細くなっている部分の長さは長くなり，研磨された部分の根元での抉れは大きくなった．更に 10000
ヘルツになると，アノードとカソードが交互に入れ替わる周期が早くなるため研磨が不十分になり，研磨している探針
表面に塩化白金が不動態として残った．上記の３つの周波数での比較では，1000 ヘルツのときが最も良好な研磨となり，走査トンネル顕微鏡の探針として用いたときに最も良質な像を得ることができた．
Keywords: Platinum Iridium, Scanning Tunneling Microscopy, Electrochemical Etchingfor the etching of PtIr, 20,21) alternate current (AC)1．Introduction

etching with alkali chloride solution is widely used tohas been

get rid of using toxic solution.22-24) Czerepak showed

contributed to the development of surface science andthe detail method of AC tip etching of PtIr wire.25)

nanotechnology. In STM, sharp metal tip is the gate ofZhang and Lian discussed the behavior during the AC

nano-signal and the most important part. Just a cut tip

tip etching.26) Vitus et al. showed the light emission

with diagonals is available to use for STM observation

spectra of sparking during the etching and confirmedwith atomic resolution on an atomically flat surface.

all the elements in the etching process.27) Libioulle et

On the surface with roughness or steps, however, theal. conducted additional micro polishing with H2SO4

tunneling point of the STM tip changes during theaq. solution after the AC tip etching.28) Kupper added

scanning, which result in the ghost image attributed

acetone in the CaCl2 solution to make the hydrogento the multi-tip effect.9) Moreover, the variation of STM

bubble smaller during the AC tip etching.29)

Scanning tunneling microscopy (STM)

1-8)

such as photon STM,10-13) STM induced luminescence,14)
tip-enhanced Raman scattering spectroscopy,

15)

However, the chemical reaction mechanism ofandAC tip etching is still ambiguous because the AC

requires sharp and symmetricfrequency dependence of the etching reaction has not

STM tip. Therefore, the fabrication of atomically sharp

been investigated. Moreover, the mechanism of theand symmetric STM tip is needed for the application

chemical reaction to produce platinum chlorides, theof the STM to the various kinds of the method of

by-products from the AC tip etching of PtIr wire, iselectrochemical STM

16-18)

19)

still not elucidated.

nanoscale surface observation and manipulation.

Platinum-iridium (PtIr) alloy wire is widely used

In this paper, the AC frequency dependence of thefor the STM tip. Although KCN solution can be usedchemical etching of PtIr wire has been reported. The＊1＊2

工学院大学工学部応用化学科 2017 年卒業 Department of Applied Chemistry, Kogakuin University
工学院大学教育推進機構ナノ化学研究室教授 Nanotechnology Chemistry Laboratory, Division of Liberal Arts, Kogakuin University— 53 —

工学院大学研究報告

PtIr tips were produced with the AC electrochemical

was held with an alminum sharp pencil body (Steadtler,

etching at 100, 1000, and 10000 Hz. The produced tips

type 0.3 mm) as a probe holder. Nickel ribbon was

were observed with scanning electron microscopy

fabricated to ring-shape and set in a glass dish in order(SEM), and were evaluated from the observed STM

to apply symmetric electric potential to PtIr wire.

images using the tips. Moreover, 4-pyridinecarboxylic

Figure 1(b) shows the tool to remove bubbles fromacid – pyrazolone was used for the determination

the PtIr wire during the etching process. Donuts-shapeof chlorine gas from the trapped gas during thesilicone rubber surrounding the PtIr wire was set on

etching, and ion chromatography was applied for thethe solution surface. In this way, the bubbles formed

determination of chloric acids in the solution after thearound the PtIr wire were removed from the wireetching.

with the point symmetry.

2．Experimental methods
Platinum iridium (80:20) wire with 0.20 mm diameterand nickel ribbon with 0.20 mm thick and 7.0 mm
wide were purchased from Nilaco Co. Ltd. The water
solvent used in this study was produced with Organo
BB-5B/G-35PA, and the typical specific resistancewas 5 MΩ・cm. Potassium chloride (Yanagishima
Pharmaceutical Co. Ltd., special grade, >99.5%) was
dissolved in the prepared water to produce 1.5 M KCl
solution used for the electrochemical etching.
Figure 1(a) shows the apparatus for the AC

Fig.2

electrochemical etching. The body was formed with a(Color online) The schematics showing the circuit for the
AC etching.

Figure 2 shows the circuit for the AC etching. The

plastic kit (Tamiya universal joint), and the PtIr wire

AC wave was produced with a function synthesizer (NF(a)

Electronic Instruments 1915) and it was amplified witha bipolar power supply (NF Electronic Instruments
BP4610). The voltage and current during the etchingwere monitored with digital multimeters (Advantest
R6451A and SANWA PC720M), and the AC wave
was observed with a digital storage oscilloscope (A&D
AD5142D) via an isolation transformer to get rid of theshort of the circuit to ground.
After the PtIr wire was set to the holder and the(b)circuit was prepared, 20 mL of 1.5 M KCl solution
was poured into the dish with the circle nickel ribbon
counter electrode, and the PrIr wire was set in such a
way that the wire dipped 5 mm into the solution. Next,
7 V and 100 Hz AC pulse for 1 s was applied to the
PtIr wire to wash the surface of the wire. Confirmed
the bubbles formed around the PtIr wire, the wire waswithdrawn from the solution once. Then the wire was

Fig.1(Color online) The apparatus for the AC electrochemical
etching (a) and the tool to remove bubbles from the PtIr
wire during the etching process (b)dipped in the solution again for 0.5 mm. After setting
the AC voltage to 20 V, the AC voltage was applied
to the PtIr wire to start the etching. Figure 3 shows— 54 —

白金イリジウム線の交流電解研磨における周波数依存性

a typical example, in case of 100 Hz, of the etchingcurrent sequence during the etching. The etching
started at 0.16 A and the current fluctuated between
20 and 190 s. The current fluctuation was due to the
bubbles around the PtIr wire during the etching, andthe bubbles caused the decrease of contacting areaof the wire to the solution. The large spike at 140 s
was due to the large bubble formed around the wire.
The frequency of the formation of large bubble could

Fig.4

be reduced by using donuts-shape silicone rubberTime sequence of the applied
pulse for washing the etched tip.

surrounding the PtIr wire as shown in Fig. 1(b). After
190 s, the volume of the formed bubbles and the

3．Results and discussion

corresponding current fluctuation was reduced. The
falling at 230 s is the point to stop etching because

First, AC etching at 1 Hz was tried in vain. Just DC

the junction between the wire and the solution got

etching of the following reactions occurred alternately,
Platinum anode: 2Cl− → Cl2↑ + 2e− small. It is noted that the amplitude of the sound got−

small and the frequency of the sound got higher at the(1)−

Nickel cathode: 2H2O + 2e → 2OH + H2↑ 
−

(2)

−

point. After the etching, 3 V and 100 Hz AC pulse for

Platinum cathode: 2H2O + 2e → 2OH + H2↑  (3)1 s, as shown in Fig. 4, was applied to the etched PtIr

Nickel anode: Ni → Ni2+ + 2e− (4)

tip to wash the tip surface and remove the platinum

The following reaction occurs from the reactions (3)chlorides formed on the tip surface. Last, the tip wasand (4),Ni2+ + 2OH− → Ni(OH)2↓ 

rinsed with distilled water for 3 times. The etching(5)

AC frequencies were 100, 1000, and 10000 Hz in thisand the most of the formed Cl2 gas were dissolved instudy. The washing AC frequency before and after

the solution to produce HClO,

the etching was unified to 100 Hz at any tip producing
processes.

Cl2 + H2O → HCl + HClO (6)No etching occurred at 1 Hz; overnight experimentcaused the dried up of the solution to produce thepowder mixture of KCl, KOH, and Ni(OH) 2 . The
formation of Ni(OH)2 can be confirmed with the lightgreen color powder and the light green color powder
can be dissolved in ammonium solution to show blue
color of the solution, i.e., the complex [Ni(NH3)6]2+ wasformed.
Table 1 shows the etching time and initial current
at the etching AC frequencies of 100, 1000, and 10000
Hz. Though the etching rate increased from 100 Hz

Fig.3

Typical example of the etching current sequence during
the etching. Frequency: 100 Hz.

to 1000 Hz and the etching time got short, the etchingrate decreased from 1000 Hz to 10000 Hz and theetching time got long.

The prepared PtIr tips were observed with SEM(JEOL JSM6380A) and used for STM observation

Table 1(JEOL JSPM5200). No additional metal deposition was

The etching time and initial current at each etching AC
frequency.

Frequency / Hz

100

1000

10000

No smoothing process was conducted to the obtained

Etching time / s

240±3090±20

270±30

STM images.

Initial current / A

0.150±0.050

0.200±0.050

0.300±0.100

conducted to the PtIr tips for the SEM observation.

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工学院大学研究報告

The AC etching was proceeded with the alternate

100 Hzreactions of oxidation reaction at anode, and hydrogengas was formed reaction at cathode, resulting in thesurface corrosion and removal of surface productsby hydrogen gas bubble. The AC frequency is theperiodicity of the alternation of the reactions. It
is noted that the PtIr wire also vibrated with thefrequency of the applied AC voltage and the spreading
rate of black platinum chloride powder in the solution
was higher at 1000 Hz than at 100 Hz.

1000 HzThe etching current I could be written in thefollowing equation,
(7)

where V is the applied AC voltage, R is the resistanceof the solution, f is the AC frequency, and C is thecapacitance. It is derived from the equation (7) thatthe initial current increases with the increase of thefrequency, which is in agreement with the values in
10000 Hz

Table 1.
From the above results, it is predicted that thedifference of the etching time at 100 and 1000 Hz is
attributed to (i) the supply of reactive flesh solution
to the etching wire due to the stirring of the solution
from the high frequency vibration of the wire and (ii)the increase of the etching current with the increaseof the frequency according to the equation (7).
The SEM images of the produced PtIr tips at eachfrequency are shown in Fig. 5. The surface roughness

Fig.5

was the largest at 100 Hz etching. It is noted that theroughness also related to the applying AC voltage.
Zhang and Lian reported that the lower voltage

Scanning electron microscope images of prepared
PtIr tips at the etching AC frequencies of 100,
1000, and 10000 Hz, respectively. Magnifications:
250x at left column and 3000x at right column.

caused the increase of etching time,26) and Sørensen etal. reported that the higher voltage caused the surface
30)

The tip top was round and the shank was shorter.

Actually,

The amount of produced black powder of platinumcompared the produced tip at 1000 Hz with that at 100

chlorides was little, as shown in Fig. 6, which indicatesHz, the shank was long and the embankment, concave

the change in reaction mechanism.

roughness and thinness of the etched tip.

shape at the root of the etched part, was large, which

It is predicted from the residues of the etching,were confirmed by optical microscope, due to theas shown in Fig. 6, that the etching reaction wasincrease of the etching rate. On the other hand, thethe same at 100 Hz and 1000 Hz, whereas different

duration time of the corrosive reaction was longer at

at 10000 Hz from lower frequencies. Therefore, the

100 Hz than at 1000 Hz, which results in the tip withresidues were investigated with 4-pyridinecarboxylic

no flake at the top and the short shank. In case ofacid – pyrazolone method using Hitachi U-5100 for

10000 Hz, the tip surface was covered with platinum

the determination of chlorine gas from the trappedchlorides caused the charge up in the SEM images.

gas during the etching, and ion chromatography— 56 —

白金イリジウム線の交流電解研磨における周波数依存性
100 HzFig.6

100 Hz(Color online) Photos of the solutions after AC etching;
100 Hz (left) and 10000 Hz (right). Same solution as
100 Hz was obtained at 1000 Hz.

using Shimadzu LC-20ADsp for the determination ofchloric acids in the solution after the etching. These
measurements were asked to A-kit, Inc. Unfortunately,
the chlorine gas was under the detection limit of 50 volppm because most of the produced chlorine gas weredissolved in the solution and changed into HClO. It wasalso confirmed from the firing of the residue gas thatthe residue gas was mostly hydrogen. On the other
hand, the results from ion chromatography indicatedthat the concentration of the produced hypochlorite ion
in the residue solution was 45 mg/L at 100 Hz and 0.1
mg/L at 10000 Hz, which also indicates that different
reaction occurred at 10000 Hz.
The prepared tips were used for the observation ofgraphite sample with STM. According to the purposeof the evaluation of the STM tips, atomic resolution
imaging is not necessary because it can be available
using just a cut PtIr tip. Therefore, the imaging atlarge scale, 500 nm square, was conducted to observe
steps in the image. Figure 7 shows the obtained
STM images with the tips prepared at 100, 1000, and10000 Hz. Using the tip prepared at 100 Hz, though

Fig.7

the morphology of the graphite step surface can berecognized, the scanning tip frequently touch to thesample, caused the shanking in the image and theshifting of the step lines. Using the tip prepared at(Color online) Scanning tunnelingmicroscope images of graphite withsteps. The AC frequencies used for the
prepared PtIr tips: 100 Hz (upper), 1000
Hz (middle), and 10000 Hz (lower).
Sample bias voltage: 0.95 V. Tunnelingcurrent: 0.02 nA.

1000 Hz, observed steps were clear and the step edge
was sharp compared with the image at 100 Hz. Using
the tip prepared at 10000 Hz, the scanning tip contact

Finally, the etching mechanism at each frequency

with the sample many times and the image cannot

was discussed, as shown in Fig, 8. Sørensen et al.

be used for the sample evaluation. Among the images

proposed the AC etching mechanism. Chloride ionsshown in Fig. 7, the tip prepared at 1000 Hz showedattach to the PtIr wire and the hydrogen gas bubble

the best image.

and heat driven flow climbing up the PtIr wire caused— 57 —

工学院大学研究報告

Acknowledgment
This work was supported by JSPS KAKENHI Grant
Number 15K04678.

References
Fig.8(Color online) Schematics showing the mechanism of AC
etching of platinum iridium wire.

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4．Conclusion and Remarks
We have observed the frequency dependence of the
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The gas collection process was not enough to detect
chlorine gas in this study. Further improvement
aimed for the gas collection may elucidate the reaction

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process in detail. We are also preparing the easierand low-cost apparatus using a conventional guitar
amplifier for the AC etching.

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