Appl. Chem. Eng., Vol. 31, No. 6, December 2020, 591-595 https://doi.org/10.14478/ace.2020.1083 Review 임창용 *, **, 백새연 * 박소연 * 김하민 * * 경북대학교나노소재공학부, ** 경북대학교미래과학기술융합학과 (2020 년 10 월 26 일접수, 2020 년 11 월 5 일채택 ) Synthesis Strategy for Electrodes and Metal-Organic Frameworks based on Metal Nanoparticle using Flashlight Changyong Yim *, **,, Saeyeon Baek *, Soyeon Park * and Hamin Kim * * School of Nano & Materials Science and Engineering, Kyungpook National University (KNU), Sangju 37224, Korea ** Department of Advanced Science and Technology Convergence, Kyungpook National University (KNU), Sangju 37224, Korea (Received October 26, 2020; Accepted November 5, 2020) 초록 Intensive pulsed light (IPL) 기술은빛을 millisecond 단위의짧은시간에상온, 상압환경에서대상물질에조사하여에너지를전달한다. 이렇게단시간에조사되는특징을가진플래시라이트 (flashlight) 에대한관심의증대로 IPL을이용한금속입자의광소결연구가대표적으로이루어져왔으며, 최근에는 IPL을다양한물질합성에적용한사례가발표되고있다. 본총설논문은지금까지연구되어밝혀진 IPL을활용한다양한물질합성전략들에대한것으로 IPL 기술을이용한물질합성에대한이해를증진시키고자한다. 특히, 금속나노입자의소결을이용한유연전극제작및금속유기골격체 (metal-organic framework, MOF) 합성을다루었다. 전극제작의핵심요소인전극의산화저항성과전기전도도향상을위한과정을다루었고, 금속기판으로부터금속유기골격체를합성하는과정을설명하였다. 이를향후 IPL을이용한전극제작및물질합성응용에관한연구를하는연구자에게이해하기쉽게설명하고자하였다. Abstract Intensive pulsed light (IPL) technique enables energy to be transferred to a target substance in a short time per millisecond at room temperature under an ambient atmosphere. Due to the growing interest in flashlights with excellent functionality among various technologies, light-sintering research on metal particles using IPL has been carried out representatively. Recently, examples of the application of IPL to various material synthesis have been reported. In the present article, various strategies using IPL including the manufacture of flexible electrodes and the synthesis of metal-organic frameworks were discussed. In particular, the process of improving oxidation resistance and electrical conductivity of electrodes, and also the metal-organic framework synthesis from metal surface were explained in detail. We envision that the review article can be of great help to researchers who investigate electrode manufacturing and material synthesis using IPL. Keywords: Intensive pulsed light (IPL), Flash light sintering, Conductive inks, Metal nanoparticles, Metal organic frameworks (MOFs) 1. 서론 1) 플래시라이트 (flashlight) 는흔히카메라를이용한사진촬영시주변을밝게해주는용도로이용되는장치이다. 전문사진관에서는제논램프 (Xenon lamp), 반사판 (reflector), 캐패시터 (capacitor) 로이루어진 Corresponding Author: Kyungpook National University (KNU), School of Nano & Materials Science and Engineering, Sangju 37224, Korea; Kyungpook National University (KNU), Department of Advanced Science and Technology Convergence, Sangju 37224, Korea Tel: +82-54-530-1335 e-mail: cy.yim@knu.ac.kr pissn: 1225-0112 eissn: 2288-4505 @ 2020 The Korean Society of Industrial and Engineering Chemistry. All rights reserved. 장치를활용하여고출력의밝은빛을방출한다. 일반적으로제논램프기반의플래시라이트는 1~4 kw의파워를갖는 300~900 nm 파장대의백색광 (white light) 을수밀리초 (milli-seconds) 에조사 (irradiation) 하는특징을갖고있다 [1-4]. 이렇듯초단시간의고출력광원을이용하는기술을 intensive pulsed light (IPL) 라한다. 특히유연디스플레이 (flexible display) 제작을비롯한인쇄전자 (printed electronics) 분야에대한기술개발수요가많아지면서, IPL 기술을이용한전극제작연구가활발히이루어지고있다 [5,6]. 일반적으로 IPL을이용한전극제작에는금속나노입자를이용한다. 금속나노입자를원하는전극의형태로패터닝한후에 IPL을조사하면금속나노입자가빛을흡수하여상온, 상압에서온도가순간적으로 591
592 임창용 백새연 박소연 김하민 Figure 1. Schematic illustration of the flashlight system used on a laser-printed PET film before and after flashlight irradiation. Copyright 2017 Springer Nature Customer Service Centre GmbH. 300 이상올라가면서소결 (sintering) 이진행되어전극을형성할수있다 [2,3]. 금이나은과같이전기전도도가우수하면서열적산화에강한금속나노입자들이전극제작에주로이용된다. 금과은은비저항이각각 2.04, 1.78 µω cm로전기전도도가우수하지만, 킬로그램당가격이각각 1000, 500 USD/kg로가격적인측면에서고비용이라는단점이있다. 반면에, 구리는비저항이 1.65 µω cm으로금, 은과같이전기전도도가우수하고가격이 5 USD/kg으로저렴하여저비용전극재료로주목받고있다. 하지만, 구리는고온에서산소와반응하여전도성을상실하는문제가발생한다 [7-9]. 따라서구리를전극재료로이용하기위해서는열적산화를막아주는것이필수적이다. 이를해결하기위해, 구리나노입자를은나노입자, 카본나노튜브 (carbon nanotube, CNT) 등과혼합하여열적산화를억제할수있는전략들이있다 [1,10-13]. 또한 IPL 기술은금속나노입자의소결뿐만아니라, 금속이온과유기리간드의화학결합으로이루어진금속유기골격체 (metal-organic frameworks, MOF) 합성에이용될수있다. MOF는비표면적이높고, 세공의크기조절, 다양한기능기 (functional group) 부여가가능하기때문에기체포집및분리, 기체센서, 촉매등의분야에이용된다 [14-20]. 기존 MOF 합성에이용되는수열합성법과는달리, IPL을 MOF 합성에이용하면금속기판위에균일한 MOF필름을용매없이상온, 상압에서합성할수있다 [21-24]. 본고에서는목표대상에강력한에너지를짧은시간내전달하는 IPL 기술을이용하여금속나노입자기반전극및필름형태의 MOF 합성전략을소개하고자한다. 2. 본론 2.1. IPL 기술을이용한 PET 필름가공 제논램프로부터발생된빛을조사받는물질은고유의색깔및표면특성에따라흡수하는빛의양이달라진다. 물질이빛을흡수해상승하는온도변화의폭은물질의빛흡수율 ( 또는반사율 ) 과열전도도등의특성에따라달라진다. 예를들면, 빛을조사받는물질이실리콘웨이퍼와같이빛을잘흡수할경우대략 1000 까지온도가상승하고, 구리나노입자의경우대략 350 까지상승한다 [25,26]. 반면투명한 PET 필름은제논램프로부터나오는빛의파장중 90% 를투과하는성질을갖고있어제논램프의영향을거의받지않고, 온도또한상승하지않는다 [2,27]. 이러한성질을가진 PET 필름에레이저프린터를이용하여빛을흡수할수있는잉크패턴을만들면, IPL을이용하여빠르게 PET 필름을가공할수있다. IPL을이용한 PET필름가공원리를모식도로정리하였다 (Figure 1). 우선얇은 PET필름위에레이저프린터로잉크를프린트한뒤제논램프를이용해빛을조사한다. 프린트된잉크는색상고유의빛흡수율 Figure 2. Optical microscope images of the black square patterned PET films with various thicknesses (a)~(d) after 4800 W, (e)~(h) after 3400 W, (i)~(l) after 1200 W flashlight exposure. (a), (e), (i) 12.7 µm, (b), (f), (j) 25.4 µm, (c), (g), (k) 76.2 µm, (d), (h), (l) 127 µm. Optical microscope images of the square patterns on PET films with varied colors (m)~(p) after 4800 W flashlight exposure. The scale bar represents 0.25 cm. Copyright 2017 Springer Nature Customer Service Centre GmbH. Figure 3. UV-Vis reflectance spectrum of yellow, red, green, blue and black colors onto PET film. Copyright 2017 Springer Nature Customer Service Centre GmbH. 에따라빛을흡수하게되고, 흡수된빛은열로전환된다. 프린트된영역의온도가상승해 PET 필름의녹는점을넘는다면 PET 필름이프린트된모양에따라가공되는원리이다 [28]. 레이저프린터를이용해 PET필름위검정, 파랑, 초록, 빨강, 노랑색잉크로프린트후제논램프로빛을조사하면, 각잉크의색깔에따라 PET필름의가공정도가다름을알수있다 (Figure 2). 이를 UV vis spectrometer를이용해프린트된잉크의빛의파장에따른반사율을측정하였다 (Figure 3). 이를통해 PET필름의가공은프린트된잉크의반사율에의존적임을알수있다. PET필름가공에사용된검정색잉크와같이제논램프에서조사된빛을충분히흡수할수있는물질이라면, IPL기술과접목해필름가공, 나노재료합성, 금속입자소결등다양한분야에응용될수있음을알수있다 [1,9,23,28,29]. 2.2. IPL 기술을이용한구리 - 은 - 그래핀나노입자기반전극제작 일반적으로구리나노입자는값이싸고전기전도도가우수하지만쉽게산화되어전도성을잃어버리는특징이있다 [7-9]. 따라서구리나노입자가전극으로이용되려면열적산화를막아주는것이필수적이다. 구리전극의열적산화를막기위해구리 / 은나노입자혼합물, 구리 /CNT 나노입자혼합물등이이용되었다 [10-13]. 하지만, 이러한방식은개별나노입자들을합성하여이용해야한다는단점이있다. 이와달리, 질산은과구리나노입자간의 galvanic replacement reaction을이용하면단순한화학반응만으로구리표면을은으로코팅할수있고, 공업화학, 제 31 권제 6 호, 2020
593 Figure 4. Schematic of flash light system on F-CuAg films and optical images of F-CuAg films. Copyright 2016 The American Chemical Society. 이에 IPL을이용하여열적산화를억제할수있는전도성하이브리드구리 / 은나노입자기반의전극을제작할수있다 (Figure 4)[9]. Galvanic replacement reaction을진행하기위해서는구리나노입자표면에존재하는얇은산화층을제거해야한다. 일반적으로산 (acid) 은금속의산화층을제거하는데이용된다. 특히, 포름산을이용하여구리나노입자의산화막을제거하면구리포름산염이생성되고, 이는 IPL 조사를통해다시구리로환원되는장점을갖고있다. 이에대한자세한메커니즘은다음의화학반응식을따른다 [30,31]. Figure 5. (a) Photographs of GnP in formic acid and water (left: right after sonication; right: after 1 h). (b) Electrical resistance of formic acid-and water-treated GnP before and after flash. (c) Schematic illustration of the effect of flashlight and formic acid on Gnp. Copyright 2017 The American Chemical Society. (1) (2) (3) 산화층이제거된구리나노입자표면에질산은을처리하면, galvanic replacement reaction을통해표면이질산은으로코팅된다. 이는추가적인에너지가필요하지않은자발적인반응 (spontaneous reaction) 이며, 이를통해고온에서구리나노입자의산화저항성을향상시킬수있다 [9]. 구리- 은나노입자기반의전극을 PDMS와같은유연기판에서이용하면, 기판을늘렸을때전극에크랙이발생하여손상을입는다 [1]. 따라서나노입자사이를연결해줄수있는 GnP (graphene nanoplatelet) 와같은전도성의가교물질 (Bridge materials) 과의혼합이필수적이다 [32,33]. 일반적으로 GnP는소수성 (hydrophobic) 으로물분자와쉽게결합되지않는다. 이러한특징을가진 GnP를포름산처리하면친수성 (hydrophilic) 으로변화하여용매내에서잘분산되므로산화저항성을향상시킬수있다 [Figure 5(a)]. 하지만, 이러한과정에서 GnP에손상이가해져전기전도성을잃게된다. 이는산화된 GnP에 IPL을조사하여다시전기전도성이향상시켜해결이가능하며 Figure 5(b) 를통해알수있다. GnP의자세한메커니즘은 Figure 5(c) 에나타내었고, Figure 6을통해구리-은- 그래핀나노입자의열적산화가해결되고전기전도도가향상되는것을알수있다. 이를통해산을이용하면 GnP를잘분산시켜열적산화를해결하고, IPL기술을이용하여간단하며저비용공정으로전기전도도를향상시킨구리- 은-그래핀나노입자기반전극을제작할수있어, 향후의류의웨어러블히터에적용되어실제응용분야에도사용될것으로전망한다 [1]. Figure 6. (a) Electrical resistance of F-C (black square), F-C with CNT (blue inverted triangle), F-CG (red circle), and F-CSG (green triangle) samples after heat treatment at 180. (b) Magnified graph of (a). Copyright 2017 The American Chemical Society. 2.3. IPL 기술을이용한금속유기골격체 (MOF) 합성 위와같은 IPL 기술은금속나노입자소결뿐만아니라다공성물질인 MOF 합성에도적용할수있다 [23]. 일반적으로 MOF 합성은금속이온과유기리간드가용해된용매를약 100~200 의온도에서반응시키는수열합성법을이용한다 [24,34-36]. 이와달리 IPL 기술을 MOF 합성에적용하면용매없이상온, 상압환경에서합성이가능하다는장점을가진다 [21,22]. IPL을유기리간드가도포된금속기판위에조사하면, 금속기판이빛을흡수하여열을발생시키고, 이를에너지원으로이용하여금속기판에서용출된금속이온과유기리간드가결합하여 MOF를합성할수있다 [23]. 구리금속의경우결합에너지가 932.61 ev로, 916.5 ev의결합에너지를갖는구리수산화물구조체에비해높다 [37]. 따라서구리금속은금속이온을용출하기위해구리수산화물구조체보다더많은양의에너지를필요로한다. 이에따라금속이온용출을용이하게하고자금속기판을산화처리하여금속수산화물기판을얻었다 [Figure 7(a)-(c)]. 이렇게얻어진금속수산화물기판에유기리간드를도포한뒤 IPL을조사하면기판이빛을흡수하여표면에 MOF 필름의형성을가능하게한다. 그형성과정은 SEM, XRD를통해확인할수있다 [Figure 7(d)-(g)]. Figure 8과같이 IPL을다양한유기리간드가도포된금속기판에조 Appl. Chem. Eng., Vol. 31, No. 6, 2020
594 임창용 백새연 박소연 김하민 때필름형태의 MOF가형성됨을알수있었다. 본총설에서 IPL 기술을이용한다양한물질합성전략을소개함으로써, IPL 기술의높은응용가능성을전망한다. References Figure 7. SEM images of (a) Cu-clad, (b) Cu(OH) 2 nanowires, and XRD patterns of (c) Cu-clad (black) and Cu(OH) 2 nanowires on Cu-clad substrate (red). SEM images of Cu-BTC with 0.5 mm BTC as a function of varying number of IPL, (d) Cu-BTC with 5 pulses, (e) Cu-BTC with 10 pulses, (f) Cu-BTC with 15 pulses, and XRD patterns of (g) Cu-BTC films with 0.5 mm BTC as a function of varying number of IPL. Copyright 2018 The American Chemical Society. Figure 8. Schematic illustration of the two-step method for MOF film fabrication by IPL method. Copyright 2018 The American Chemical Society. 사하면 Cu-BTC, Cu-BDC, ZIF-8, MOF-5 등과같은여러종류의 MOF 구조체를합성할수있다 [23]. 이처럼 IPL 기술을 MOF 합성에이용하면, 균일한 MOF 필름을상온, 상압에서단시간에제작할수있어, 향후기체분리막대량생산공정에응용이가능할것으로전망한다. 3. 결론 본고에서는 IPL 기술의기본메커니즘을서술하고, 이기술을접목하여금속나노입자소결을통한전극제작및필름형태의다공성재료합성전략에대해정리하였다. IPL 기술을이용하여산화저항성이향상된전극을제작하기위해구리- 은잉크를제조한결과, 구리와은의 galvanic replacement reaction을통해구리나노입자의열적산화가억제되는것을알수있었다. 포름산처리로인해친수성을띄는 GnP는용매에첨가했을때나노입자간가교역할을수행해분산을용이하게하고, IPL과결합하면전기전도성이향상되는장점을가진다. 이를이용해구리- 은-GnP 잉크를합성하여 IPL로부터에너지를조사하면, 기판의손상없이유연한전극제작이가능함을알게되었다. 또한, IPL 기술을다공성물질인 MOF 합성에이용하기위해금속이온의용출이쉬운금속수산화물기판을얻고 IPL을조사한결과, 충분한양의빛에너지를유기리간드가도포된금속기판에조사했을 1. C. Yim, Z. A. Kockerbeck, S. B. Jo, and S. S. Park, Hybrid copper-silver-graphene nanoplatelet conductive inks on PDMS for oxidation resistance under intensive pulsed light, ACS Appl. Mater. Interfaces, 9, 37160-37165 (2017). 2. S. H. Park, W. H. Chung, and H. S. Kim, Temperature changes of copper nanoparticle ink during flash light sintering, J. Mater. Process Technol., 214, 2730-2738 (2014). 3. H. S. Kim, S. R. Dhage, D. E. Shim, and H. T. Hahn, Intense pulsed light sintering of copper nanoink for printed electronics, Appl. Phys., 97, 791-798 (2009). 4. W. S. Han, J. M. Hong, H. S. Kim, and Y. W. Song., Multi-pulsed white light sintering of printed Cu nanoinks, Nanotechnology, 22, 395705 (2011). 5. M. Singh, H. M. Haverinen, P. Dhagat, and G. E. Jabbour., Inkjet printing-process and its applications, Adv. Mater., 22, 673-685 (2010). 6. J. Song and H. Zeng, Transparent electrodes printed with nanocrystal inks for flexible smart devices, Angew. Chem. Int. Ed., 54, 9760-9774 (2015). 7. X. Xu, X. Luo, H. Zhuang, W. Li, and B. Zhang, Electroless silver coating on fine copper powder and its effects on oxidation resistance, Mater. Lett., 57, 3987-3991 (2003). 8. A. Yabuki and S. Tanaka, Oxidation behavior of copper nanoparticles at low temperature, Mater. Res. Bull., 46, 2323-2327 (2011). 9. C. Yim, A. Sandwell, and S. S. Park, Hybrid copper-silver conductive tracks for enhanced oxidation resistance under flash light sintering, ACS Appl. Mater. Interfaces, 8, 22369-22373 (2016). 10. M. Grouchko, A. Kamyshny, and S. Magdassi, Formation of air-stable copper-silver core-shell nanoparticles for inkjet printing, J. Mater. Chem., 19, 3057-3062 (2009). 11. J. Song, J. Li, J. Xu, and H. Zeng, Superstable transparent conductive Cu@Cu4Ni nanowire elastomer composites against oxidation, bending, stretching, and twisting for flexible and stretchable optoelectronics, Nano Lett., 14, 6298-6305 (2014). 12. H. J. Hwang, S. J. Joo, and H. S. Kim, Copper nanoparticle/multiwalled carbon nanotube composite films with high electrical conductivity and fatigue resistance fabricated via flash light sintering, ACS Appl. Mater. Interfaces, 7, 25413-25423 (2015). 13. I. Kim, Y. Kim, K. Woo, E.-H. Ryu, K.-Y. Yon, G. Cao, and J. Moon, Synthesis of oxidation-resistant core-shell copper nanoparticles, RSC Adv., 3, 15169-15177 (2013). 14. C. Yim and S. Jeon, Direct synthesis of Cu-BDC frameworks on a quartz crystal microresonator and their application to studies of n-hexane adsorption, RSC Adv., 5, 67454-67458 (2015). 15. T. F. Baumann, Metal-organic frameworks: Literature survey and recommendation of potential sorbent materials, Lawrence Livermore National Laboratory, TR-430112, Doi:10.2172/1012427 (2011). 16. D. Britt, D. Tranchemontagne, and O. M. Yaghi, Metal-organic frameworks with high capacity and selectivity for harmful gases, Proc. Nat. Acad. Sci., 105, 11623-11627 (2008). 17. O. Abuzalat, D. Wong, M. Elsayed, S. Park, and S. Kim, Sonochemical fabrication of Cu(II) and Zn(II) metal-organic framework 공업화학, 제 31 권제 6 호, 2020
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