01 KH (이기영)_총설( ).hwp

Appl. Chem. Eng., Vol. 28, No. 6, December 2017, 601-606 https://doi.org/10.14478/ace.2017.1011 Review 양극산화를이용한산화타이타늄나노튜브구조형성원리 이기영 경북대학교나노소재공학부 (2017 년 1 월 24 일접수, 2017 년 1 월 30 일심사, 2017 년 2 월 20 일채택 ) Principle of Anodic TiO 2 Nanotube Formations Kiyoung Lee School of Nano & Materials Science and Engineering, Kyungpook National University, 2559 Gyeongsang-daero, Sangju, Gyeongbuk 37224, Korea (Received January 24, 2017; Revised January 30, 2017; Accepted February 20, 2017) 초록금속표면처리의대표적인기술인양극산화를통하여일차원나노구조금속산화물을형성할수있다. 여러가지금속산화물중에기능성이뛰어난 TiO 2 에대한관심의증대로 TiO 2 나노튜브를이용한연구가많이이루어지고있다. 본총설논문에서는지금까지연구되어밝혀진 TiO 2 나노튜브가형성원리에대한해설논문으로전기화학적측면에서의양극산화공정에대한이해를통하여나노튜브형성을위한전기적조건, 화학적조건, 물리적조건에대하여다루었다. 특히 TiO 2 나노튜브성장의핵심요소인산화물의형성과에칭의평형관계, 다공성구조의형성원인을다루었다. 나아가전해질조건에따른 TiO 2 나노튜브의형태학적고찰을함으로써향후양극산화를통한 TiO 2 나노튜브응용에관한연구를하는연구자에게이해하기쉽게설명하고자하였다. Abstract One-dimensional nanostructured metal oxide can be formed through an anodic oxidation, which is a typical technique of metal surface treatment. Studies on TiO 2 nanotubes have been widely carried out with increasing interests in TiO 2, which has an excellent functionality among various metal oxides. The present article reviews the principles of formation of TiO 2 nanotubes, which have been studied so far. In particular, the article discussed the equilibrium relationship between the oxide formation and etching, which is a key parameter of TiO 2 nanotube growth, and the formation of the porous structure. Furthermore, morphological considerations of TiO 2 nanotubes according to electrolyte conditions will be explained to the researchers who will study the application of TiO 2 nanotubes formed through the anodic oxidation in the future. Keywords: anodization, TiO 2, nanotubes, metal oxide, anodic nanostructures 1) 1. 서론 양극산화기술 (anodization) 은금속의대표적인표면처리기술로서금속표면에전기화학적방법을통하여산화물을형성하여금속의부식을방지하거나장식용피막처리로이용하기위한기술이다 [1-3]. 양극산화기술은설비비용이저렴하며, 대면적의금속을표면처리하기에용이하여산업에서오래전부터이용되어온기술이다. 이런이유로양극산화는끊임없이연구 개발되어왔다. 최근나노기술이각광받음에따라양극산화기술을이용하여다양한형태의나노구조를형성하는연구가많이이루어지고있다. 특히 1995년일본의 Masuda 그룹에서정렬도가높은다공성알루미늄산화물을형성함으로써그 Author: Kyungpook National University, School of Nano & Materials Science and Engineering, 2559 Gyeongsang-daero, Sangju, Gyeongbuk 37224, Korea Tel: +82-54-530-1333 e-mail: kiyoung@knu.ac.kr pissn: 1225-0112 eissn: 2288-4505 @ 2017 The Korean Society of Industrial and Engineering Chemistry. All rights reserved. 관심은더욱높아져이와관련된연구가많이수행되어왔다 [2,3]. 정렬도가높은다공성알루미늄산화물은나노선 (nanowire), 나노막대 (nanorod) 등을제조하는템플레이트 (template) 나멤브레인 (membrane) 으로의활용가치가무척이나높게평가되고있다 [4]. 하지만알루미늄산화물은전기적으로절연체의특성을가지고있어보다넓은영역에서의활용에는한계를가지고있다. 이런이유로폭넓은분야로의이용가치를향상시키기위하여다양한금속산화물의나노구조를양극산화를통하여얻으려는노력이오래전부터있어왔다. 가장대표적인물질이이산화티타늄 (TiO 2 ) 으로, TiO 2 는 3.0~3.2 ev를가지는반도체물질로서 1972년 Fujishima와 Honda에의해서광촉매반응에의한물분해에대한연구가발표된이래로꾸준히관심을받아왔다 [6-7]. 특히 1991년 Gratzel에의해염료감응형태양전지 (dye-sensitized solar cells, DSSCs) 가보고된후태양전지에서중요한역할을하는 TiO 2 의나노구조에대한관심은폭발적으로증가하기시작하였다 [8,9]. 양극산화기술을이용한 TiO 2 나노구조형성에대한최초의시도는 601

602 이기영 Figure 1. (a)-(d) Representative cross-sectional SEM image of TiO 2 nanotubes. The anodization was performed in 0.15 M NH 4 F/3 vol% H 2 O containing ethylene glycol electrolyte at 60 V. 1984년 Assefpour-Dezfuly그룹과 1999년 Zwilling그룹에의해서이루어졌다 [10,11]. 이당시불소이온이첨가된크롬산용액에서의양극산화를통하여나노포어 (pore) 형태에가까운나노구조를보여주게된다 [10,11]. 이후 2000년대중반에이르러독일의 Schmuki 그룹과미국의 Grimes 그룹에의하여양극산화를이용한 TiO 2 나노튜브구조형성기술을통한응용은급격하게발전하게된다 [12-16]. 본고에서는이런양극산화기술로형성된 TiO 2 나노튜브구조의형성메커니즘을설명하기위하여양극산화의기본원리에서부터다공성금속산화물형성메커니즘까지정리하였다. 2. 본론 2.1. 양극산화를통한다공성금속산화물구조형성 양극산화는전기화학공정으로산화가일어나는작업전극에관심의대상이되는금속 ( 산화극, + 극 ) 과상대전극으로는 Pt과같은안정적인금속 ( 환원극, -극) 을적절한전해질내에서전기적힘인전압을인가하거나전류를흐르게하여산화물을형성하는공정이다. 양극산화의조건에따라금속산화물은두꺼운장벽형 (barrier type 또는 compact type) 이라일컫는부동태층이나튜브구조나다공성 (porous) 구조라일컫는다공성구조로형성된다 (Figure 1). 이때부동태층또는다공성구조결정에영향을미치는가장중요한인자는전해질의조건, 전류또는전압조건, 온도와같은환경조건인것으로알려져있다. 작업전극인금속에전압 (+ 극 ) 을인가하게되면금속은산화되어전해질내물에있는산소이온과만나금속산화물을형성하거나또는금속이온이전해질로용출되는과정을거치게된다. 반면상대전극 (- 극 ) 에서는전해질내의물이환원되면서수소기체를발생하게된다 (Figure 2(a)). 이때작업전극의금속에서산화물형성또는전해질로용출여부는 Pourbaix diagram에의한각물질의열역학적특성에의하여결정된다. 이런양극산화법을통하여형성되는 TiO 2 나노튜브는오래전부터알려진다공성알루미나형성메커니즘과비슷하다. 우선전기화학반응에의하여전해질에서의형성된 O 2- 이온이 Ti 금속표면에이미형성된자연산화막 (native oxide) 층으로높은전기장에의하여이동 (migration) 하게된다 (high-field mechanism)(figure 2(a)). 이렇게자연산화막을통하여이동된 O 2- 은자연산화막과금속의경 (a) (b) Figure 2. (a)-(b) Schematic diagrams showing growth of oxide by field aided transport of mobile ions through the oxide layers in the absence and presence of fluoride ions: fluoride migration leads to accumulation at the metal/oxide interface. 계면에서용출되는금속이온과결합하여새로운산화막을형성할수있게되어있다. 이때이온의이동은인가되는전류에영향을받으며그식은다음과같이표현된다 [17]. I = Aexp (BF) = Aexp (B U/d) (1) I : 양극산화시전류 F : 산화막에인가된전기장 ( U/d) U : 산화막에인가되는전압 d : 산화막두께 A, B : 실험상수 d = f U (2) f : 산화막성장인자 식 (1) 에서보는바와같이전해질에의하여용출되지않는산화막의두께가증가함에따라산화막에인가되는전기장이작아지게되어결국산화막성장에는한계를갖는다. 이때산화막의두께는식 (2) 와같이정의되며 TiO 2 의경우그값은 2.5 nm/v을갖는다. 이는산화막이계속해서성장하기위해서는산화막이용출되는과정과새로생성되는과정이 평형 을이루어야함을의미한다. 다시말해높은전기장에의한산화막형성반응과경쟁반응인화학적산화막용해반응이정상상태에이르러야양극산화에의한산화물형성이꾸준히일어나게된다 [18]. TiO 2 의경우, 전기화학반응에의하여생성된산화물을용해시키는이온으로불소이온이이용되며이때의화학반응식은다음을따른다. Ti + 2H 2 O TiO 2 + 4H + + 4e - (3) TiO 2 + 6F - [TiF 6 ] 2- + 2H 2 O (4) 반응식 (4) 에서보는바와같이불소이온에의하여 Ti 4+ 가 [TiF 6 ] 2- 를형성하며전해질로용출된다 (Figure 2(b)). 만약에불소이온이과량으로반응하게되면산화막은모두용출되어결국전기화학적에칭반응에이르게된다. 또한용해반응에의하여용출된 Ti 4+ 이온은전해질의전도도를상승시켜양극산화로형성된TiO 2 의나노구조에영향을 공업화학, 제 28 권제 6 호, 2017

양극산화를이용한산화타이타늄나노튜브구조형성원리 603 (a) Figure 4. (a)-(c) Schematic diagrams showing (a) growth of TiO 2 nanotubes by anodization. (d), (c) Representation of the formation of a tubular morphology from an originally porous morphology by selective dissolution of the fluoride-rich layer and preferential etching by H 2 O in the electrolyte. (d)-(e) Repetitive SEM images of (d) porous and (e) tubular surface morphology. (f) SEM image of bottom of anodized TiO 2 nanotubes. (b) Figure 3. (a) Concentrations of Ti ion with the iterative number of anodization (b) Variation of the tube length and the solution conductivity with the iterative number of anodization. Reproduced with permission from[19]. Copyright 2009 The Korean Physics Society. 미치기도한다 (Figure 3)[19]. 양극산화반응에의한일차원다공성산화물형성에대한이해는양극산화초기에형성되는압축응력 (compressive stress) 에의한곡면형성에서시작된다고알려져왔다 (Figure 4(a))[20,21]. 양극산화초기에가해지는응력의대표적인원인은금속이산화하여금속산화물로되면서부피팽창이일어나기때문이다. 이때의부피비를 Pilling- Bedworth ratio라한다 [20,21]. 또한양극산화반응중인가되는전압에의한전기변형력 (electrostrictive force) 에의하여그응력은더욱강해진다 [22]. 이런응력의발생은양극산화초기에서발생하게되며, 실험을통하여응력의발생이전기장에영향을받게된다는것을밝혀내었다 [22]. 이렇게양극산화초기에형성된국부적인곡면에는전기장이집중됨으로써전기화학반응에의한산화막형성 / 용출이가속화될뿐아니라응력에의한곡면형성이더욱잘일어나게된다 [3]. 이에따른다공성금속산화물의정렬도는더욱높아진다. 이에대한실험적결과를일본의 Ono 그룹에서발표하였다 [22]. 그결과를보면, 높은전류 ( 전기장 ) 에서양극산화를수행하게되면다공성금속산화물의성장속도가증가할뿐아니라파괴전류 ( 전위 ) 근처에서는산화물의최대팽창압력에의한이상적인육각구조형다공성금속산화물을형성하게된다 [22]. 이렇게양극산화의초기단계에형성된곡면들로부터반응이진행됨에따라산화막의밀어내는힘이강해지게된다. 결국서로밀어내는힘에의하여금속산화물의일차원다공성구조를가지게된다 (Figure 4(b)). 이에대한실험적증명은 Thompson과 Habazaki에의하여증명되었다 [23]. 이실험은알루미늄 (Al) 층사이에텅스텐 (W) 금속층을증착시킨후양극산화과정을투과전자현미경으로관찰하였다. 이실험에서양극산화가진행됨에따라일차원의다공성알루미나구조가형성되고이에따라텅스텐층이응력에의하여위로상승하는모습을볼수있다 [23]. 2.2. 양극산화를통한 TiO 2 나노튜브구조위와같은양극산화과정을통한다공성금속산화물구조형성모델은 AlO 2, TiO 2 등과같은대부분의금속산화물에적용이가능하다. 다만양극산화공정을통해형성된다공성 AlO 2 은공극을둘러싼산화물벽 ( 단위공극셀 ) 간에간격이없는다공성구조 (porous 구조 ) (Figure 4(d)) 이고 TiO 2 의경우는산화물벽사이에일정한간격이있어이른바튜브 (tube) 구조를갖는다 (Figure 4(e)) [24]. 이런튜브구조는다공 (porous) 성구조에서부터형성이된다. 다공성 (porous) 구조의셀경계면이적절한전기화학적에칭반응에의하여분리됨에따라튜브구조를갖게되는것이다 (Figure 4(b)-(c)). TiO 2 나노튜브의경우양극산화를통하여산화물이형성되는과정에서산소이온의이동 (migration) 속도와상대적으로이온의크기가작은불소이온이경쟁을하게된다 [25]. 불소이온이금속과산화물의경계면에축적이되어이른바 fluoride rich layer를형성한다 (Figure 4(a)). 이렇게축적된불소이온은위에서언급한다공성구조형성메커니즘에의하여위쪽으로움직여 Ti-F 복합물로써단위공극셀경계면을둘러싸게된다 (Figure 4(b)). 이 Ti-F 복합물은물에쉽게용해되는데이런용해과정에의하여공극단위셀에간격이생겨결국튜브구조를형성하게된다 (Figure 4(c))[26]. 이때튜브의위쪽부분이오랜에칭시간에의하여선명한튜브구조를관찰할수있는반면튜브의아래쪽부분은아직도육각구조의다공성구조를유지하게된다 (Figure 4(f))[27]. Appl. Chem. Eng., Vol. 28, No. 6, 2017

604 이기영 만아니라 Ta, Zr, Hf, Fe, Nb, Co, V 등다른금속에서도동일한원리에의하여튜브구조가형성이됨을발견함으로써양극산화에대한심도있는연구가계속적으로수행되고있다 [30-43]. Figure 5. (a)-(c) Repetitive SEM images of TiO 2 (a) nanosponge, (b) nanochannel, and (c) fishbone structures. (d) Pore diameter and nanostructures of TiO 2 as a function of the applied potential. The anodization was performed in 10 wt% K 2 HPO 4 in glycerol at 180. Reproduced with permission from[46] and[47]. Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Copyright 2011 The Royal Society of Chemistry. 이와같은기본원리외에도전해질의조성에따라 TiO 2 나노튜브의미세형태는변화하게된다. 전해질의조성과형태적변화에따라크게세가지세대로구분한다. 서론에서언급한바와같이초창기의나노튜브, 이른바 1세대나노튜브는산성의수용액 ( 크롬산또는황산 ) 에불소이온 (HF) 이첨가된전해질에서의양극산화를통하여나노튜브를형성하였다 [10-13]. 이때의나노튜브의길이는 500 nm 이내의짧은나노튜브만을성장할수있다. 이는산성이라는조건하에서의 TiO 2 의에칭속도가빨라지기때문인것으로여겨지고있다 [26]. 빠른에칭속도는나노튜브의정렬도가떨어지게하기도한다. 또한튜브형성과정에서튜브외벽면의불소화합물층의에칭속도와튜브의성장속도의차이를가지게됨으로써표면이매끈하지않고울퉁불퉁한이른바 ripple이형성되는모습을볼수있게된다 [26]. 이에대한개선을통하여다음세대나노튜브에대한연구가진행되었다. 가장큰발견은용액의 ph와나노튜브길이간의상관관계이다 [28]. 기존의 HF와같은산성전해질이아닌 NH 4 F 또는 NaF염을이용한중성의수용액전해질내에서의양극산화를통하여산화물의에칭속도를늦춤으로써나노튜브의길이를수마이크로미터 (2~3 µm) 까지증가시켜이른바 2세대나노튜브를얻게되었다 [28]. 이후수용액전해질이아닌 DMSO, 글리세롤, 에틸렌글리콜등과같은유기용매전해질내에서의양극산화를함으로써에칭속도를조절하는연구가수행되었다. 유기용매전해질에서의양극산화를통하여튜브의길이를수십에서수백마이크로미터까지증가시킬수있을뿐아니라튜브외벽이매끈한형태로형성되는 3세대나노튜브를제조하는데성공하게된다. 뿐만아니라그정렬도도더욱높아져거의완벽한육각벌집형태의나노튜브구조로형성할수있게되었다 [29]. 이런 3세대의나노튜브가발견됨으로써 TiO 2 나노튜브의응용범위가더욱확장되었을뿐아니라 ph, 온도, 전류, 전압, 물의농도등에따른나노튜브형성에미치는영향에대한다양한연구를하는데발판을마련했다. 나아가불소이온을포함한전해질에서의양극산화를통하여튜브형태의금속산화물나노구조를형성하는것은 Ti 뿐 2.3. 양극산화를통한그밖의 TiO 2 나노구조 양극산화를이용한 TiO 2 나노구조형성은불소이온뿐아니라다른염소이온, 아질산염이온등이포함된전해질에서도가능하다 [44,45]. 이런나노구조는정렬도가높은나노튜브와다르게여러나노선또는정렬도낮은나노튜브들이묶음의형태로나타나는것을볼수가있다 [44]. 이런나노구조의형성은파괴전압 (breakdown potential) 이상에서산화막이깨지면서 (breakdown) 형성되는것으로알려져있다. 하지만이에대한심도있는이론적연구및메커니즘은아직명확하게규명되지는않았다. 또다른양극산화를통한 TiO 2 나노구조형성은 160 이상의높은온도에서인산염이첨가된글리세롤전해질내에서이루어진다 [46-48]. 이와같은양극산화방법에서는인가전압에따라나노스폰지, 나노채널, 생선뼈구조로 3가지형태의나노구조를형성하게된다 (Figure 5)[46-48]. 고온인산염 -글리세롤전해질에서의양극산화는불소이온이첨가된전해질내에서형성된산화물과달리결정성을가지고있다는특징이있다. 이는높은온도와전기장에의해서금속표면에국부적으로온도가급격하게상승하기때문인것으로알려져있다 [49]. 또한, 고온인산염-글리세롤전해질에서의양극산화는금속과산화물계면에 fluoride rich layer가존재하지않는다. 이런두가지요인에의하여나노스폰지, 나노채널, 생선뼈구조는나노튜브구조보다기계적으로안정한특성을가지고있다. 3. 결론 본논문은양극산화공정을통하여형성되는금속산화물의일차원나노구조의형성원리에대하여정리하였다. 특히 TiO 2 나노튜브의형성원리는이른바다공성알루미나 (AAO) 의형성원리와동일하다. 즉전기화학반응에서높은전기장에서의산화막형성, 소성흐름 (plastic flow) 에의한다공성금속산화물형성을기본으로한다. 다만 Ti의경우불소이온이첨가된전해질에서정렬도가높은다공성금속산화물이형성됨에따라육각벌집형태의다공성구조가아닌튜브의형상을가지게되는것이다. 불소이온이외의염소, 아질산염, 인산염등이포함된전해질내에서의 Ti 양극산화를통하여나노튜브이외의다른여러가지형태의금속산화물나노구조를형성시킬수있다. 감 이논문은 2016학년도경북대학교신임교수정착연구비에의하여연구되었음. 사 References 1. S. Kim, J. Lim, and J. Choi, Preparation of polymer nonotubes/ nanowires by using inorganic porous templates, Polym. Sci. Technol., 17, 742 (2006). 2. H. Masuda and K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumi- 공업화학, 제 28 권제 6 호, 2017

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