工学院大学研究報告第123号　平成29年10月

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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

Frequencydependenceofalternatecurrentelectrochemicaletchingofplatinum/iridiumwire

YoshikiAOYAMA＊1andTomohideTAKAMI＊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:PlatinumIridium,ScanningTunnelingMicroscopy,ElectrochemicalEtching

1．Introduction

　Scanning tunnelingmicroscopy (STM)1-8) hasbeencontributedtothedevelopmentofsurfacescienceandnanotechnology.InSTM,sharpmetaltipisthegateofnano-signalandthemostimportantpart.JustacuttipwithdiagonalsisavailabletouseforSTMobservationwithatomicresolutiononanatomically flat surface.Onthesurfacewithroughnessorsteps,however,thetunnelingpointof theSTMtipchangesduring thescanning,whichresult in theghost imageattributedtothemulti-tipeffect.9)Moreover,thevariationofSTMsuchasphotonSTM,10-13)STMinducedluminescence,14)tip-enhancedRaman scattering spectroscopy,15) andelectrochemicalSTM16-18)requiressharpandsymmetricSTMtip.Therefore,thefabricationofatomicallysharpandsymmetricSTMtip isneededfortheapplicationof theSTM to thevariouskinds of themethod ofnanoscalesurfaceobservationandmanipulation.19)

　Platinum-iridium (PtIr) alloywire iswidelyusedfortheSTMtip.AlthoughKCNsolutioncanbeused

for the etching of PtIr,20,21) alternate current (AC)etchingwithalkalichloridesolution iswidelyusedtoget ridofusing toxic solution.22-24)CzerepakshowedthedetailmethodofAC tip etchingofPtIrwire.25)ZhangandLiandiscussedthebehaviorduringtheACtipetching.26)Vitus et al. showed the light emissionspectraofsparkingduringtheetchingandconfirmedall theelements in theetchingprocess.27)Libioulleet al. conductedadditionalmicropolishingwithH2SO4

aq. solutionafter theACtipetching.28)Kupperaddedacetone in theCaCl2 solution tomake thehydrogenbubblesmallerduringtheACtipetching.29)

　However, the chemical reaction mechanism ofAC tip etching is still ambiguousbecause theACfrequencydependenceoftheetchingreactionhasnotbeen investigated.Moreover, themechanismof thechemical reaction toproduceplatinumchlorides, theby-products fromtheACtipetchingofPtIrwire, isstillnotelucidated.　Inthispaper, theACfrequencydependenceof thechemicaletchingofPtIrwirehasbeenreported.The

＊1工学院大学工学部応用化学科2017年卒業　DepartmentofAppliedChemistry,KogakuinUniversity＊2工学院大学教育推進機構ナノ化学研究室教授　NanotechnologyChemistryLaboratory,DivisionofLiberalArts,KogakuinUniversity

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PtIrtipswereproducedwiththeACelectrochemicaletchingat100,1000,and10000Hz.Theproducedtipswere observedwith scanning electronmicroscopy(SEM), andwereevaluated fromtheobservedSTMimagesusingthetips.Moreover,4-pyridinecarboxylicacid – pyrazolonewasused for the determinationof chlorine gas from the trapped gas during theetching,and ionchromatographywasapplied for thedeterminationofchloricacidsinthesolutionaftertheetching.

2．Experimental methods

　Platinumiridium(80:20)wirewith0.20mmdiameterandnickel ribbonwith 0.20mm thick and 7.0mmwidewerepurchasedfromNilacoCo.Ltd.ThewatersolventusedinthisstudywasproducedwithOrganoBB-5B/G-35PA, and the typical specific resistancewas 5 MΩ・cm. Potassium chloride (YanagishimaPharmaceuticalCo.Ltd., specialgrade,>99.5%)wasdissolvedinthepreparedwatertoproduce1.5MKClsolutionusedfortheelectrochemicaletching.　Figure 1(a) shows the apparatus for the ACelectrochemicaletching.Thebodywasformedwithaplastickit (Tamiyauniversal joint),andthePtIrwire

washeldwithanalminumsharppencilbody(Steadtler,type0.3mm)as aprobeholder.Nickel ribbonwasfabricatedtoring-shapeandsetinaglassdishinordertoapplysymmetricelectricpotentialtoPtIrwire.　Figure1(b)showsthetool toremovebubbles fromthePtIrwireduringtheetchingprocess.Donuts-shapesiliconerubbersurroundingthePtIrwirewassetonthesolutionsurface. In thisway, thebubbles formedaround thePtIrwirewere removed from thewirewiththepointsymmetry.

　Figure2showsthecircuitfortheACetching.TheACwavewasproducedwithafunctionsynthesizer(NFElectronicInstruments1915)anditwasamplifiedwithabipolarpower supply (NFElectronic InstrumentsBP4610).Thevoltageandcurrentduringtheetchingweremonitoredwithdigitalmultimeters (AdvantestR6451A andSANWAPC720M), and theACwavewasobservedwithadigitalstorageoscilloscope(A&DAD5142D)viaanisolationtransformertogetridoftheshortofthecircuittoground.　After thePtIrwirewasset to theholderandthecircuitwasprepared, 20mLof 1.5MKCl solutionwaspouredintothedishwiththecirclenickelribboncounterelectrode,andthePrIrwirewassetinsuchawaythatthewiredipped5mmintothesolution.Next,7Vand100HzACpulse for1swasappliedto thePtIrwiretowashthesurfaceof thewire.ConfirmedthebubblesformedaroundthePtIrwire,thewirewaswithdrawnfromthesolutiononce.Thenthewirewasdippedinthesolutionagainfor0.5mm.AftersettingtheACvoltageto20 V, theACvoltagewasappliedtothePtIrwiretostart theetching.Figure3shows

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)

Fig.2 (Color online) The schematics showing the circuit for the AC etching.

(a)

(b)

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

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a typical example, in caseof 100Hz, of theetchingcurrent sequenceduring the etching.The etchingstartedat0.16Aandthecurrentfluctuatedbetween20and190s.ThecurrentfluctuationwasduetothebubblesaroundthePtIrwireduringtheetching,andthebubbles caused thedecreaseof contactingareaof thewire to thesolution.The largespikeat140swasduetothe largebubbleformedaroundthewire.Thefrequencyoftheformationof largebubblecouldbe reducedbyusingdonuts-shape silicone rubbersurroundingthePtIrwireasshowninFig.1(b).After190 s, the volume of the formed bubbles and thecorrespondingcurrent fluctuationwasreduced.Thefallingat230s is thepoint to stopetchingbecausethe junctionbetween thewireand the solutiongotsmall. It isnotedthattheamplitudeofthesoundgotsmallandthefrequencyofthesoundgothigheratthepoint.Aftertheetching,3Vand100HzACpulsefor1s,asshowninFig.4,wasappliedtotheetchedPtIrtip towashthetipsurfaceandremovetheplatinumchlorides formedonthetipsurface.Last, thetipwasrinsedwithdistilledwater for3 times.TheetchingACfrequencieswere100,1000,and10000Hz in thisstudy.ThewashingAC frequencybeforeandaftertheetchingwasunifiedto100Hzatanytipproducingprocesses.

　ThepreparedPtIr tipswereobservedwithSEM(JEOL JSM6380A) and used for STMobservation(JEOLJSPM5200).Noadditionalmetaldepositionwasconducted to thePtIr tips for theSEMobservation.NosmoothingprocesswasconductedtotheobtainedSTMimages.

3．Results and discussion

　First,ACetchingat1Hzwastriedinvain.JustDCetchingofthefollowingreactionsoccurredalternately,　　Platinumanode:2Cl−→Cl2↑+2e− (1)　　Nickelcathode:2H2O+2e−→2OH−+H2↑ (2)　　Platinumcathode:2H2O+2e−→2OH−+H2↑ (3)　　Nickelanode:Ni→Ni2++2e− (4)The followingreactionoccurs fromthereactions (3)and(4),　　Ni2++2OH−→Ni(OH)2↓ (5)andthemostoftheformedCl2gasweredissolvedinthesolutiontoproduceHClO,　　Cl2+H2O→HCl+HClO (6)Noetchingoccurredat1Hz; overnightexperimentcaused thedriedupof the solution toproduce thepowder mixture of KCl, KOH, and Ni(OH)2. TheformationofNi(OH)2canbeconfirmedwith the lightgreencolorpowderandthe lightgreencolorpowdercanbedissolved inammoniumsolutiontoshowbluecolorof thesolution, i.e., thecomplex [Ni(NH3)6]2+wasformed.　Table1showstheetchingtimeand initialcurrentattheetchingACfrequenciesof100,1000,and10000Hz.Thoughtheetchingrate increased from100Hzto1000Hzandtheetchingtimegotshort,theetchingratedecreased from1000Hz to 10000Hz and theetchingtimegotlong.

Table 1 The etching time and initial current at each etching AC frequency.

Fig.3 Typical example of the etching current sequence during the etching. Frequency: 100 Hz.

Fig.4 Time sequence of the applied pulse for washing the etched tip.

Frequency/Hz 100 1000 10000

Etchingtime/s 240±30 90±20 270±30

Initialcurrent/A 0.150±0.050 0.200±0.050 0.300±0.100

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　TheACetchingwasproceededwith thealternatereactionsofoxidationreactionatanode,andhydrogengaswas formedreactionatcathode, resulting in thesurface corrosion and removal of surfaceproductsbyhydrogengasbubble.TheAC frequency is theperiodicity of the alternation of the reactions. Itis noted that thePtIrwire alsovibratedwith thefrequencyoftheappliedACvoltageandthespreadingrateofblackplatinumchloridepowderinthesolutionwashigherat1000Hzthanat100Hz.　The etching current I could bewritten in thefollowingequation,　　

(7)

whereVistheappliedACvoltage,Ristheresistanceof the solution, f is theAC frequency,andC is thecapacitance. It isderived fromtheequation (7) thatthe initialcurrent increaseswith the increaseof thefrequency,which is inagreementwith thevalues inTable1.　From the above results, it is predicted that thedifferenceof theetching timeat100and1000Hz isattributed to (i) thesupplyof reactive fleshsolutiontotheetchingwireduetothestirringofthesolutionfromthehighfrequencyvibrationofthewireand(ii)the increaseof theetchingcurrentwiththe increaseofthefrequencyaccordingtotheequation(7).　TheSEMimagesoftheproducedPtIrtipsateachfrequencyareshowninFig.5.Thesurfaceroughnesswasthelargestat100Hzetching.Itisnotedthattheroughnessalso related to theapplyingACvoltage.Zhang and Lian reported that the lower voltagecausedtheincreaseofetchingtime,26)andSørensenet al.reportedthatthehighervoltagecausedthesurfaceroughnessandthinnessof theetchedtip.30)Actually,comparedtheproducedtipat1000Hzwiththatat100Hz,theshankwaslongandtheembankment,concaveshapeattherootoftheetchedpart,waslarge,whichwere confirmedby opticalmicroscope, due to theincreaseof theetchingrate.On theotherhand, thedurationtimeof thecorrosivereactionwas longerat100Hzthanat1000Hz,whichresultsinthetipwithno flakeat the topand the short shank. In caseof10000Hz, thetipsurfacewascoveredwithplatinumchloridescaused thechargeup in theSEM images.

The tip topwas roundand the shankwas shorter.Theamountofproducedblackpowderofplatinumchlorideswaslittle,asshowninFig.6,whichindicatesthechangeinreactionmechanism.　It is predicted from the residues of the etching,as shown inFig. 6, that the etching reactionwasthesameat100Hzand1000Hz,whereasdifferentat 10000Hz from lower frequencies.Therefore, theresidueswere investigatedwith4-pyridinecarboxylicacid –pyrazolonemethodusingHitachiU-5100 forthedeterminationof chlorinegas from the trappedgas during the etching, and ion chromatography

100 Hz

1000 Hz

10000 Hz

Fig.5 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.

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

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usingShimadzuLC-20ADsp for thedeterminationofchloricacids in thesolutionafter theetching.ThesemeasurementswereaskedtoA-kit,Inc.Unfortunately,thechlorinegaswasunderthedetectionlimitof50volppmbecausemostoftheproducedchlorinegasweredissolvedinthesolutionandchangedintoHClO.Itwasalsoconfirmedfromthefiringoftheresiduegasthattheresiduegaswasmostlyhydrogen.On theotherhand, theresults from ionchromatography indicatedthattheconcentrationoftheproducedhypochloriteionintheresiduesolutionwas45mg/Lat100Hzand0.1mg/Lat10000Hz,whichalso indicatesthatdifferentreactionoccurredat10000Hz.　ThepreparedtipswereusedfortheobservationofgraphitesamplewithSTM.Accordingtothepurposeof theevaluationof theSTMtips, atomicresolutionimaging isnotnecessarybecause itcanbeavailableusing justa cutPtIr tip.Therefore, the imagingatlargescale,500nmsquare,wasconductedtoobservesteps in the image. Figure 7 shows the obtainedSTMimageswiththetipspreparedat100,1000,and10000Hz.Using the tippreparedat100Hz, thoughthemorphologyof thegraphitestepsurfacecanberecognized, thescanning tip frequently touch to thesample, caused the shanking in the image and theshiftingof the step lines.Using the tippreparedat1000Hz,observedstepswereclearandthestepedgewassharpcomparedwiththeimageat100Hz.Usingthetippreparedat10000Hz,thescanningtipcontactwith the samplemany timesand the imagecannotbeusedforthesampleevaluation.Amongtheimagesshown inFig.7, thetippreparedat1000Hzshowedthebestimage.

　Finally, theetchingmechanismateach frequencywasdiscussed, as shown inFig, 8. Sørensen et al.proposed theACetchingmechanism.Chloride ionsattachtothePtIrwireandthehydrogengasbubbleandheatdrivenflowclimbingupthePtIrwirecaused

Fig.6 (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.

100 Hz 100 Hz

Fig.7 (Color onl ine) Scanning tunnel ing microscope images of graphite with steps. 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. Tunneling current: 0.02 nA.

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the formation of Pt(IV)Cl62− at thewire surface.30)Actually, at thePtIrwireelectrode, theproductionof platinum chlorides and chlorine gas occurredwhen theelectrode is anode, and theproductionofhydrogengasoccurredwhentheelectrodeiscathode.The ionchromatographymeasurementat10000Hzindicates that theabovealternate reactionsdidnotoccurefficiently,whichcausedtheresidualplatinumchlorides on the tip surface.As discussed above,theACetchingwasproceededwith the alternateswitching of the anodic and cathodic reactions ofoxidationandhydrogenbubbleformation,respectively.At10000Hz,thedurationtimeofthehydrogenbubbleformationwasnotenough,resulting intheresidueofplatinumchloridesonthewiresurfaceandtheresiduehindered theanodicreaction,which is inagreementwiththeSEMobservation.

4．Conclusion and Remarks

　WehaveobservedthefrequencydependenceoftheACetching forplatinum iridiumwire for theuseoftheSTMtip.Thereactionrateincreasedfrom100Hzto1000Hz,andtheshank lengthof theobtainedtipwaslongandtheembankmentwasdeep.At10000Hz,therapidswitchingofanodicandcathodicreactionsresulted inthe insufficientetchingandtheresidueofplatinumchloridesonthetipsurface. It isconcludedfromtheSTMobservationthatthePtIrtippreparedat1000Hzshowsthebestimagecomparedwiththoseat100and10000Hz.　Thegascollectionprocesswasnotenoughtodetectchlorine gas in this study. Further improvementaimedforthegascollectionmayelucidatethereactionprocess indetail.Weare alsopreparing the easierand low-cost apparatususinga conventionalguitaramplifierfortheACetching.

Acknowledgment

　ThisworkwassupportedbyJSPSKAKENHIGrantNumber15K04678.

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