Submicron Quantum Dot Pixel Arrays Based on Electrophoretic Deposition for Light Emitting Diode

Park, Byung-Geon 부산대학교 2023 국내박사

Displays that are indispensable things in our lives, have developed in the direction of getting thinner, higher resolution, and richer colors. For this reason, many researchers has studied the quantum dot light emitting diode (QD-LED) which has high color purity and an ultra-thin structure that does not require a backlight. However, the technology for arraying quantum dots (QDs) for an ultra high resolution of over 2,000 ppi suitable for virtual reality (VR) devices is insufficient. Here in, I demonstrate that a new method to array the QDs in ultra high resolution which has not been reached by any technology and manufactured and evaluated a QD-LED with this QD array. For this purpose, the ligands of QDs were modified to be suitable for EPD and dispersed in water and QDs could be effectively arranged only in the pixel region through the difference in surface energy. By using electrophoretic deposition (EPD), not only a full color array of 2 μm × 6 μm sub-pixels, but also a monochromatic array of 0.5 μm width was made, thereby successfully fabricating the QD array of less than 1 μm. Here, optimal process conditions were derived using Taguchi method to uniformly deposit QDs. In addition to QDs, the zinc oxide (ZnO) layer, which is the electron injection layer (EIL) and electron transport layer (ETL), was also manufactured by EPD to avoid cross talk issues. Thus, from the EIL to the QD layer, only one photo lithography and solution process were used. The fabricated QD-LED of over 2,000 ppi achieved an external quantum efficiency of 6.31%, which showed superior performance than the toluene based conventional QD EL device, demonstrating the superiority of the QD array using EPD. In addition, it has a great meaning by showing high performance even though the resolution was kept differently from the other processes that made large pixels due to the difficulty of the process when fabricating the electroluminescence device. The proposed EPD process enables QD arrays regardless of substrate materials such as transparent substrates, opaque substrates, and flexible substrates, so, it is applicable to various industries like televisions, mobile devices, flexible displays, and transparent displays. In particular, it is expected that the era of realistic VR without screen door effect will be opened by producing of ultra high resolution displays.

Transition metal based nano-catalyst synthesis and electrode manufacturing process for polymer electrolyte membrane water electrolysis

박윤수 중앙대학교 대학원 2023 국내박사

The inherent properties of nano-catalyst can be engineered into macroscopic structures by capturing the collective electrocatalytic characteristics via film fabrication to realize macroscopic functionality. Constructing a macroscale catalyst layer with high robustness on porous substrate is an essential procedure for implementing electrodes applicable in a commercialized hydrogen generator such as a polymer electrolyte membrane water electrolysis. In addition, it is necessary to explore the key factors of the electrode manufacturing process and the origin of the electrochemical properties of the applied materials to achieving maximized electrode performance. Electrophoretic deposition (EPD) and electrodeposition (ED) are facile fabrication techniques for the nanostructured electrode on the porous substrate. However, further studies are required to fully elucidate the factors that affect catalytic performance of the electrode such as morphological characteristic of catalyst layer, chemical property, and composition of the catalyst. In this study, whole procedure was studied from material synthesis for nano-catalyst to application as a practical water electrolysis system with fundamental aspect. First, through the elucidation of critical EPD factors, facile routes were established for controlling the kinetics of colloidal nanoparticle (NP) deposition and fabrication of strong contacted NP film structure with improved properties. Second, nanostructured metal alloy electrocatalyst with various surface morphologies and compositions were prepared by controlling ED parameters such as deposition potential, time, and electrolyte composition. The compositional structure of the alloy electrocatalyst dependent on the ED potential was realized, which was based on the different reduction potentials of the metal ions. Then, deposition control of electrode fabrication method provided insight into understanding the dynamics of functionalized nano-catalyst, thereby leading to the engineering of electrode for use in advanced devices of water electrolysis. The powerful electrode manufacturing pathway was developed to fabricate high-performance gas diffusion electrode (GDE) for proton exchange membrane water electrolyzer (PEMWE), in which porous nanostructured catalyst layers were deposited on macroporous conductive carbon paper (CP) substrate. Non-precious transition metal-based nano-catalyst, promising cost-efficiency for hydrogen evolution reaction (HER), was prepared as a building block for HER electrode fabrication. Robust contact of the FeP/CP electrode leading to high conductivity was fabricated through EPD with fast deposition kinetics. The multicomponent alloy-based electrode was electrodeposited with a structure in which Cu and Mn of non-active materials, which cause a synergistic effect, were introduced into Co and Ni-based electrocatalysts that act as intrinsic active sites. Both HER electrodes showed excellent HER activity and high stability under 0.5 M H2SO4 (overpotential ≤ 100 mV @10 mA cm−2) was achieved due to its specialized catalyst layer properties such as high catalytic surface area, lowest charge transfer resistance, and electrochemical activation with surface metal leaching. The HER electrodes were realized as a highly efficient and low-cost cathode GDE for PEMWE, exhibiting a high current density of 1.48 A cm-2 (EPD-fabricated FeP/CP electrode) and 2.57 A cm-2 (ED-fabricated CoNiCuMn/CP electrode) at a cell voltage of 2.0 V with excellent long-term stability of 100 hr under current density of 2.0 A cm-2. 나노 촉매가 갖는 고유한 내재적 성질은 박막 공정을 통하여 그 촉매 특성이 집합된 거시적 형태로 구조화될 수 있다. 다공성 기판 상에 견고한 거시적 촉매층을 구축하는 것은 고분자 전해질막 수전해와 같이 상용화된 수소 생산 장치에 적용 가능한 전극을 구현하기 위한 필수적인 과정이다. 또한, 전극 성능을 극대화하기 위해서는 제조 공정 상의 핵심 요소와 재료의 전기화학적 특성의 근원을 규명할 필요가 있다. 전기 영동 증착 및 전해 도금은 다공성 기판에 나노 구조화된 전극을 제조하기 위한 효율적인 공정이다. 그러나 전극 촉매층의 형태학적 특성, 화학적 성질, 촉매 조성 등 전극 특성에 영향을 미치는 주요 요인들을 완전히 밝히기 위해서는 추가적인 연구가 필요하다. 본 연구에서는, 나노 촉매 합성부터 실제 수전해 시스템으로의 응용에 이르기까지의 모든 과정을 근본적인 측면에서 다루었다. 첫번째로, 전기 영동 증착법의 주요 공정 요소에 대한 탐구를 통해 나노 입자 증착 동안의 동역학을 제어하고, 향상된 성능을 갖도록 나노 입자가 밀접하게 접촉되어 있는 나노 입자 기반 박막 구조를 구현하는 공정을 개발하였다. 두번째로, 도금 전위, 시간 및 도금 용액 조성과 같은 전해 도금의 공정 변수를 제어하여 표면 형태 및 조성이 다양한 나노 구조화된 금속 합금 촉매 전극을 제조하였다. 도금 전위에 의존하는 합금 촉매 전극의 조성 구조가 구현되었으며, 이는 금속 이온들의 상이한 환원 전위를 기반으로 하였다. 이어, 본 전극화 공정 연구는 기능성 나노 재료의 역학을 이해할 수 있는 선행 연구로써 기여하여, 수전해 장치에 사용하기 위한 전극을 구현하는 후속 연구로 이어졌다. 다공성의 전도성 카본 페이퍼 기판에 나노 구조화된 촉매층을 형성할 수 있는 전극화 공정이 개발되었고, 본 전극은 양성자 교환막 수전해 장치의 고성능 가스 확산 전극으로 활용되었다. 비용 효율적 수소 발생 촉매인 전이 금속 기반 나노 촉매가 전극 구성을 위한 최소 단위의 조립체로 준비되었다. 견고한 접촉을 형성하여 높은 전도성을 확보한, 카본 페이퍼 상에 인화철 나노 입자가 증착된 구조의 전극은 빠른 증착 동역학이 관여한 전기 영동 증착법을 통해 제조되었다. 다성분계 합금 촉매 기반 전극은 전해 도금법을 통해 고유한 활성 사이트로 작용하는 코발트 및 니켈 기반 촉매에 시너지 효과를 일으키는 비활성 물질인 구리 및 망간을 도입한 구조로 전극화 되었다. 두 종류의 촉매 전극은 0.5 M 황산 전해질에서 매우 우수한 수소 발생 반응 촉매 활성을 달성하였는데 (100 mV @10 mA cm-2 이하의 과전압), 이는 높은 촉매층 표면적, 낮은 전하 전달 저항 및 금속 이온 용출이 관여한 전기화학적 활성화 등과 같은 전극의 특성화된 촉매층 성질에 기인하였다. 개발된 본 전극들은 양성자 교환막 수전해 장치의 고효율 및 저가의 수소극으로써 가스 확산 전극으로 적용되었으며, 2.0 V의 셀 전압 및 90 도의 작동 환경에서 1.48 A cm-2 (전기 영동 증착된 FeP 전극) 및 2.57 A cm-2 (전해 도금된 CoNiCuMn 전극)의 우수한 전류 밀도를 달성하고 2.0 V의 셀 전압 구동에서100 시간의 우수한 장기 안정성을 확보하였다.

Hybrid Method of Electrophoretic and Electrostatic Deposition of Nanomaterials for Multifunctional Surfaces: Development of Antimicrobial and Hydrophobic Nanoparticle Bilayers to Prevent Pathogenic Bacteria Invasion

김지윤 숙명여자대학교 대학원 2020 국내석사

We developed a hybrid method of electrophoretic and electrostatic depositions of nano-materials to produce anti-wetting and anti-bacterial fabrics. Clothing and textiles used for medical and healthcare purposes must be prevented from the contamination of pathogenic bacteria which can infect others through direct contact. Therefore, it is essential to use the functional fabric capable of resisting bacteria to prevent secondary infection. In this work, anti-bacterial fabrics with superhydrophobicity were produced by electrophoretic depositions (EPD) of zinc oxide (ZnO) and polydimethylsiloxane (PDMS) modified silicon dioxide (SiO2) nanoparticles on the polymerically modified fabric surfaces with electrostatic deposition. The produced fabrics have super-hydrophobicity with excellent anti-bacterial performance. The outer layer of SiO2 prevents the direct attachment of bacterial droplets, and the inner layer of ZnO provides the anti-bacterial activity for the residual bacteria on the outer surface of the SiO2 layer. As a result, the fabrics developed can eliminate the possibility of bacterial-attachment and biological-viability, simultaneously, thereby effectively reducing the risk of bacterial infection. Our method can deposit various nanoparticles for the desired functions on non-conductive surfaces such as fabric and paper with precise control of the coating thickness and the nano/micro-structures, unlike the conventional EPD and fabric coating methods. In addition, depending on the electrode size and shape, we can control the area and pattern of functional surfaces; therefore, it has a significant advantage of the mass production of functional fabrics without destroying the original characteristics of the fabrics after functionalization. The hybrid method can be applied to a variety of supplies such as fabrics and filters commonly use in biological contaminations and can give us enormous opportunities to develop new functional commercial products. 병원균에 자주 노출되는 의료진의 의복은 대장균, 포도상구균 및 녹농균과 같은 박테리아로 오염되어 있어 사람들에게 전염될 수 있다. 따라서 박테리아에 의한 2차 감염을 방지하기 위해서는 직물에 항균 기능을 부여하는 것이 필수적이다. 본 연구에서는 기존 일반적인 전기영동 증착법을을 보완하여 비전도성 표면에도 여러 층의 나노입자를 증착하여 복합 기능을 직물에 부여할 수 있는 전기영동증착법을 개발하였다. 본 연구에서는 전기영동증착법을 통해 산화아연 및 폴리디메틸실록산 (PDMS)-이산화규소 나노입자의 이중 층 구조를 갖는 방수, 항균 직물을 제조하였다. PDMS-이산화규소 층은 박테리아가 포함된 액적의 부착을 근본적으로 방지하며, 산화아연 층은 직물에 부착된 잔여 박테리아에 대한 항균력을 제공한다. 개발된 기법은 기존의 직물 코팅 방법 또는 전기영동증착법과 달리 직물, 종이와 같은 비전도성 표면에도 나노 입자 코팅 두께를 세밀하게 제어할 수 있고 다양한 종류의 나노입자를 증착할 수 있다. 또한 전극의 크기와 모양에 따라 직물의 기능성 표면의 면적과 모양을 제어할 수 있다. 따라서 공정 후, 직물의 원래 특성을 파괴하지 않으면서 대량생산이 가능한 이점을 갖는다. 본 연구에서 개발한 전기영동증착법으로 코팅된 항균성 직물은 박테리아의 부착과 생존율을 효과적으로 감소시킬 수 있으므로 박테리아 감염과 관련된 질병의 위험을 효과적으로 낮출 수 있다. 또한 항균성이 요구되는 직물 및 필터뿐만 아니라 의료진의 유니폼, 수술복 및 붕대와 같은 의료용품에 적용할 수 있으며, 사용하는 나노입자의 종류에 따라 다양한 기능성을 부여할 수 있기 때문에 앞으로 전기영동법을 이용한 기능성 소재 개발에 새로운 기회를 제공할 것으로 기대된다.

Synthesis of composite electrode based on graphene and conducting polymer for energy storage

이수정 성균관대학교 일반대학원 2012 국내석사

The rapid development of portable electronics and electrical vehicles has stimulated interest in energy storage device. Supercapacitors, with their high specific power/energy density and long cycle durability, are one of the most important subjects in the high power source field. Recently, advancement of nanoscale binding technique provides an innovative route to prepare conducting based composites with better performance as active material for supercapacitors. Graphene based composites have developed in many area due to extraordinary electric, thermal, and mechanical properties (high thermal/electrical conductivity, large surface area, high mechanical strength). In this study, a simple and fast electrochemical method to p-doped graphite into thin graphene layer was demonstrated. Expanded graphite is doped using various doping agents, and the p-doped graphite is transformed into doped graphene. From well dispersed p-doped graphene is prepared thin graphene layer by applying a voltage to a poly(3,4-ethylenedioxythiophene) (PEDOT) coated electrode. The behavior of the graphene-deposition is confirmed by quartz crystal micro balance (QCM) and Raman spectra as a function of the doping agent. This method is powerful for the fabrication of graphene layers on desired substrates and for applications requiring large areas. Based on electrophoretic deposition of graphene above mentioned, thin multilayer graphene/PEDOT films were prepared by electrochemical deposition. PEDOT was deposited on gold electrodes by cyclic voltammetry, and p-doped graphene, which was introduced form expanded and p-doped graphite, was assembled on the PEDOT coated electrode by chronoamperometry at -1V. The process was repeated as necessary to obtain the desired number of multilayer. Successful deposition of graphene/PEDOT layers was demonstrated by SEM. The maximum specific capacitance of the multilayer film was 154Fg-1 for six graphene layers. Further, the multilayer film exhibited improved electrochemical stability with a retention life of 86% after 1000cycles. These results indicate that the graphene would make a good electrode material for supercapacitors and that the technique described herein should be considered in the design of electrodes which need a large surface area. 본 연구에서는 그라파이트로부터 전기영동을 이용한 그래핀의 증착법 및 이를 이용한 그래핀과 전도성 고분자의 다층복합체를 제작하여 슈퍼커패시터로써의 성능을 연구하였다. 첫째로 그라파이트를 산 처리하여 도핑된 그라파이트를 만들고, 용액상에서 초음파를 가하여 그라파이트의 박리를 통해 도핑된 상태의 그래핀을 얻는다. 여기에 전기 영동법으로 집전체 표면에 균일하게 그래핀 층을 형성하였다. 이러한 방법은 기존의 그래핀 제조 방법인 거름-전사방법에 비하여 공정단계가 단순하고, 필터에 의한 그래핀의 면적 제한이 없으며 전사 시 그래핀의 손상 위험이 없다. 그래핀에 부여된 양전하로 인하여 Raman 분석에서 그래핀의 도핑정도를 확인할 수 있으며 각각의 도핑정도는 산의 종류에 따라 HCl, HNO3, H2SO4의 순서로 크게 나타났다. 그래핀의 도핑정도는 각 도펀트의 산성도에 비례하나, H2SO4의 경우 그래핀 층 내부로의 확산 한계 때문에 다른 도펀트에 비해 낮은 도핑정도를 보였다. 이때 그래핀의 증착은 수정진동자저울을 이용하여 실시간으로 관찰하였으며, 도핑이 많이 되었을수록 단위시간동안 증착되는 그래핀의 양이 많다는 것을 알 수 있다. 이로써 그래핀의 전기영동 증착은 그래핀에 부여되는 양전하의 함량 및 증착시간으로 증착되는 그래핀의 양을 조절할 수 있으며, 집전체에 직접적으로 전극을 형성할 수 있는 장점이 있다. 이러한 전기영동 증착법을 바탕으로 그래핀과 전도성 고분자인 PEDOT의 다층 복합체를 합성하여 슈퍼커패시터 전극으로 활용하였다. 기존의 그래핀과 전도성 고분자의 복합체는 집전체 위에 전극을 형성하기 위하여 바인더가 필수적이나 본 방법은 별도의 바인더가 요구되지 않는다. PEDOT 필름 상에 곧바고 전기영동 증착법으로 그래핀이 균일하게 형성되며, 이를 반복적으로 수행하여 다층구조의 그래핀/ PEDOT 전극을 제조하였다. 이렇게 제작한 전극은 그래핀의 삽입 층수에 따라 전기화학적 방법을 통하여 성능향상을 확인하였고, 이중 그래핀이 6층 삽입된 다층 복합필름의 경우 기존의 PEDOT에 비해 2.7배 향상된 비정전용량을 나타내었다. 이러한 다층 복합필름의 경우, 단순히 그래핀이 분산된 전도성 고분자에 비해 그래핀이 전해액에 노출되기 쉽고, 이로서 그래핀이 전하이동의 다리역할을 함으로써 소자 성능 향상에 기여한 것으로 보인다. 위와 같은 방법은 PEDOT외에 금속산화물 및 다른 전도성 고분자에도 동일하게 적용되며, 에너지 저장장치 외에 전기화학 센서 및 태양전지의 전극제작에도 적용할 수 있다.

Graphene Oxide Hole Injection Layer for Organic Light-Emitting Diodes by Using Electrophoretic Coating and Electrical Reduction

Kim, Jungyoon 고려대학교 그린스쿨대학원 2014 국내석사

Many researchers have interest in graphite family such as graphene oxide (GO), reduced graphene oxide (RGO) and Graphene. Among this graphite family, the GO has unique properties which are band gap, high dispersion force in water and high transmittance. These properties are very useful for making devices like Field Effect Transistor (FET), Organic Photovoltaic (OPV), and Organic Light-emitting Diodes (OLED). Since the band gap of GO is too large for these kind of devices, band gap engineering is an essential prerequisite. The band gap gradually narrowed down from GO to RGO, hence it is necessary to control band gap reduction as per requirement of the device. There are some methods for the Band gap engineering. The first one is a chemical method that uses toxic solutions such as hydrazine. Another is a physical method that needs high temperature about 1000℃. Both methods are very toxic, dangerous and inefficient methods. So we have studied about electrical method to reduce GO. Also the fabrication of GO thin film is important issue for adapting GO to devices. Traditionally spin-coating method was used to fabrication GO thin film. The spin-coating method is valuable method. The difficulties in spin coating method are thickness control and limitation of the coated area. Electrophoretic deposition (EPD) method solves the problems which are related with Band gap engineering (reduction) and fabrication (deposition) of the GO thin film. Both phenomena are happened at the same time and respective electrode. On applying voltage reduction of GO occurs at cathode while deposition of GO occurs at anode. Using these phenomena we can get GO film at the anode. Then deposited GO film moved to the cathode position for reduction. We can regulate not only the thickness of GO film by controlling the deposition time but also band gap of GO by controlling the reduction time. Since GO can only be deposited on conductive material, we can deposit GO on ITO that is used as electrode of device. The thin films that coated at the anode and reduced at the cathode have various properties, it is important to clarify the distinctions for adapting to device. These properties of GO and RGO thin films coated by EPD method are demonstrated by Raman spectroscopy, Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (SEM), X-ray Photoelectron spectroscopy (XPS) and transmittance. Finally, we adopt the GO thin film to OLED device as a Hole Injection Layer (HIL) and demonstrated increasing performance of the OLED device. Taken all together, we consider that EPD is simple and useful method for fabricating GO thin film and OLED device.

Computational and Experimental Studies of Field-Driven Additive Manufacturing Processes

Yun, Zachary University of California, Berkeley ProQuest Disser 2023 해외박사(DDOD)

Industrial coating and painting operations have used electric fields as a driving force for decades to help more of the particles reach and stick to the intended part. More recently, this concept has been explored for use in additive manufacturing (AM) processes as a way to control the microstructure and geometry of particle deposits through processes like Electrospray Deposition and Electrophoretic Deposition (EPD). These AM processes are increasingly used to make components that require specific structures such as colloidal crystals made of micro- and nano- particles, which are then used in applications in microelectronics, sensors, and superlattice materials. So far, most development of the EPD process for colloidal crystals has been performed via experiments, with many measurements taken ex-situ to weigh deposits over time. However these experiments do not directly observe how the particles integrate into the crystal, so the kinetics of the deposit formation are not yet well understood. This work aims to contribute towards EPD and other field-driven AM process development in two ways: 1) by creating a computational framework to directly simulate the particles in these field-driven processes and 2) work towards an experimental platform for in-situ observations of these particles to better understand how they form the deposit.In this dissertation, I present my work towards a computational framework for simulating these field-driven processes and a preliminary experimental setup for in-situ process observation. The simulation model captured the essential physics of the processes, while remaining computationally tractable on limited resources, such as a laptop. I discuss the results from verification studies showing good agreement with analytical theory as the model is developed and then use illustrative examples to explore deposit behavior under different processing conditions. I then present preliminary results from experiments aimed at further expanding observations of EPD. The preliminary results from both the simulation and experimental work show that these methods can contribute to process development and have several avenues for future extensions.

Programable Electrochemical Patterning System using Microelectrode Arrays

임도연 인천대학교 일반대학원 2023 국내석사

Electrophoretic deposition (EPD) of surface modified graphene oxide (GO) on copper substrate for anti-corrosion applications

황민주 성균관대학교 일반대학원 2017 국내석사

Anti-corrosion of metals is a vital issue for many industries requiring novel methods that could delay the corrosion processes. The most recent studies suggest the use of graphene for the control of corrosion due to its uniquely impermeable 2D structure which simultaneously exhibits an exceptional barrier to reactive liquids, salts and acids. Here, p-phenylenediamine modified graphene oxide (PPD-GO) coating for anti-corrosion performance is fabricated on copper by an electrophoretic deposition (EPD) method using the colloids of PPD-GO in ethanol. In order to establish the influence of the EPD parameters on the properties of the deposited films, the EPD kinetics of PPD-GO is investigated as a function of concentration, deposition time and voltage, accompanied by microscopic characterization of the deposited films. The experimental results show that the deposition follows a linear growth law, in good agreement with the predictions of Hamaker’s law. The uniform PPD-GO coating allows the tightly-consolidation, excellent adhesion and highly anti-corrosion performance, which are further developed during thermal treatment. According to the results of the potentiodynamic polarization method, the decrease in corrosion current density and the positive shift in corrosion potential have both demonstrated that PPD-GO coating served as a corrosion inhibitor, protecting copper from NaCl aqueous solution.

전기 증착법에 의한 에너지 변환 장치용 백금 촉매 부착 3차원 그래핀 구조체의 합성 및 특성 분석

정대승 전북대학교 일반대학원 2014 국내박사

그래핀 전극제조 시 발생하는 자체 뭉침현상을 최소화하고 비활성 폴리머 바인더의 사용을 없애기 위하여 금속 및 탄소 기재 표면에 3차원 그래핀 구조체를 형성하였다. 또한 이를 담지체로 사용한 고분자전해질 연료전지용 백금촉매를 제조함으로써 성능 및 내구성의 향상을 도모하였다. 수퍼캐퍼시터용 전극의 성능을 향상시키기 위하여 전기영동증착법을 이용하여 니켈폼 기재 표면에 3차원 그래핀 구조체를 형성하였으며 증착 시 전해액의 온도를 변화시킴으로써 증착속도 및 비정전용량 (specific capacitance)의 최적화를 시도하였다. 전기영동증착법을 통하여 그래핀 옥사이드의 환원 및 3차원 그래핀 구조체의 형성을 동시에 달성할 수 있었다. 또한, 전기영동증착 시 전해질의 온도는 그래핀 옥사이드의 환원화 정도, 증착속도, 전극의 기공구조 형성에 중요한 인자로 작용함을 알 수 있었다. 니켈폼에 형성된 그래핀 입체 구조의 특성을 파악하기 위하여 FT-IR, XRD, FE-SEM, Raman 분석을 진행하였다. 또한FE-SEM 이미지를 통하여 증착온도 및 시간이 증가함에 따라 그래핀 입체 구조가 점점 조밀하여지는 것을 확인하였다. 그래핀 입체 구조 니켈 전극의 전기화학특성은 3전극시스템을 이용하여 측정하였으며, 순환전압전류법 (cyclic voltametry), 충방전테스트, 임피던스 등의 분석기법을 사용하였다. 전극의 비정전용량은 증착온도 80℃에서 151 F g-1으로 가장 높게 나타났으며 그래핀 옥사이드의 환원정도가 급격히 증가하는80℃ 이상의 온도에서는 감소하는 경향을 보였다. 상기의 결과를 토대로 전기영동증착 시 온도를 제어함으로써 다양한 3차원 그래핀 구조체 전극제조가 가능함을 확인하였다. 고분자전해질연료전지용 촉매의 내구성을 향상시키기 위하여 카본 페이퍼 표면에 전기영동증착법으로 3차원 그래핀 구조체를 형성하고 이를 백금 촉매의 담지체로 사용하였다. 실험에 사용된 3차원 그래핀 구조체는 상온에서 펄스 전류를 인가함으로써 제조하였다. XPS, Raman, XRD, FE-SEM등의 결과를 통하여 부분적으로 환원된 다층의 그래핀이 자기 조립함으로써 무수히 많은 마이크로 크기의 기공을 형성하고 있음을 알 수 있었다. 제조된 3차원 그래핀 구조체를 작업전극으로 사용하고 나피온 (Nafion)이 포함된 백금용액을 전해질로 사용하여 5~10 나노미터 크기의 백금입자를 전기증착하는데 성공하였다. 가속내구 시험 전 3차원 그래핀 구조체 백금촉매의 전기화학활성면적(ECSA)은 상용 Pt/C 촉매와 거의 비슷한 값을 나타내었다. 0.4V~1.0V(Ag/AgCl 전극 대비)의 전압범위에서 1000사이클 동안 진행된 가속내구시험 후 3차원 그래핀 구조체 백금 촉매의 전기화학활성 면적의 감소는 상용 Pt/C대비 2.5배 적게 나타났다. TEM 및 XPS를 이용하여 내구성 향상의 원인을 분석하였으며 내구성 테스트 후 백금입자가 거의 떨어져나간 Pt/C 와는 달리 3차원 그래핀 구조체 백금촉매는 입자의 뭉침이 있었으나 떨어짐은 거의 관찰되지 않았다. 이는 그래핀을 담지체가 카본부식에 상대적으로 안정하기 때문으로 판단되며 XPS 분석 결과 내구성 전 후의 C/O ratio의 차이가 Pt/C는 6.8인 반면 3차원 그래핀 구조체 백금 촉매는 1.2로 나타나 카본 부식의 영향이 적음을 확인하였다. 상기의 결과를 토대로 나피온 안정화제가 백금입자크기 및 분산성 제어에 효과가 있음을 확인하였으며 3차원 그래핀 구조체를 담지체로 사용할 경우, 백금촉매의 내구성 향상에 기여함을 알 수 있었다. Synthesis, analytical characterization, performance and durability of 3-dimensional graphene structure (3DGS) and its platinum catalyst was investigated. For advanced graphene based supercapacitor electrode, we synthesized 3DGS on Ni foam (3DGS/Ni) by temperature controlled electrophoretic deposition method. The simultaneous reduction of graphene oxide (GO) along with the formation of 3DGS was achieved through this one step process. During the deposition process, temperature of the electrolyte played an important role to control the rate of deposition, degree of reduction and morphology of 3DGS. Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FE-SEM), and Raman spectroscopy were used to characterize the 3DGS. The FE-SEM image shows that 3D networks of graphene nanostructure are more developed with increasing temperature. Electrochemical properties of 3DGS/Ni were characterized in aqueous 6 M KOH by cyclic voltammetry, charge discharge technique and electrochemical impedance spectroscopy (EIS). 3DGS/Ni electrode deposited at 80 °C for 20 min has the highest specific capacitance value of 151 F/g among tested samples. Further increase in temperature results in the decrease in the specific capacitance because of the reduction of GO. The results suggested that the temperature controlled electrophoretic deposition is a promising route to customize the properties of graphene based electrode. For enhanced catalytic performance and durability of proton exchange membrane fuel cell (PEMFC), Pt decorated 3DGS (Pt-3DGS) was investigated. Highly dispersed 5-10nm sized Pt particles were formed on 3DGS by electrochemical deposition in the Nafion stabilized Pt colloid. The Nafion stabilizer considerably decreased the growth rate of Pt nanoparticles (PtNPs) depending on deposition time. Catalytic performance of the Pt-3DGS evaluated by specific electrochemical surface area (SECSA) was similar to that of commercial Pt/C at the 40% Pt content. Durability of Pt-3DGS was compared with that of Pt/C by the loss of SECSA after 1000 of potential cycling in the range of carbon corrosion. The loss of SECSA of Pt-3DGS was revealed 2.5 times lower than that of Pt/C. The change of morphology and degree of carbon corrosion before and after the durability test was characterized by transmission electron microscope (TEM) and XPS analysis. From the results it was concluded that 3DGS is an effective carbon support to improve the durability of PEMFC catalyst due to its carbon corrosion resistance.

Dielectric properties of layered perovskite nano-sheets by electrophoretic deposition according to post treatments

Lee, Young-Shin 고려대학교 대학원 2017 국내석사

Sr2Nb3O10 (SNO) nanosheets were obtained by exfoliating from a perovskite layered structure of HSr2Nb3O10 (HSNO). The nanosheets were deposited on substrates simultaneously with unintended Tetrabutylammonium cations (TBA+) by electrophoretic deposition (EPD). To eliminate TBA+ from the SNO dielectric nanosheet thin films, the films were exposed to ultraviolet (UV). Since UV exposure cannot decompose TBA+ completely, thermal annealing was additionally conducted employing different furnaces at various atmospheres. A reduction of lattice constant was found to occur after the UV treatment and thermal treatment, indicating that TBA cations were decomposing in the interlayer of nanosheets. XRD was employed to determine the lattice constant, and FT-IR spectra analysis depicted that the organic materials were eliminated through the post-deposition treatments. The optimum process conditions for enhancing dielectric properties were UV exposure for 15 hrs and subsequent thermal annealing in a furnace for 1 hr at 600 oC regardless of atmospheres. The dielectric constants of films further annealed by the box furnace were increased from 14 to 32 at 1 MHz whereas loss (tan δ) fell from 5 % to 2 % at 1 MHz. Taken together, addition of thermal treatment strongly affected to decompose TBA+. The achieved SNO dielectric nanosheet thin films can be easily fabricated by EPD and has great dielectric properties.