Synthesis of semiconducting and conducting metal oxide aerogels for energy and environmental applications

다국어 초록 (Multilingual Abstract)

Aerogel is a material that contains many mesopores, which contribute to its low density, high specific surface area, and low thermal conductivity. Since their incep-tion in 1931 by Stephen Kistler, silica aerogels have served as the primary represen-tation of aerogels. Silica aerogel is extensively utilized as a heat insulating material because of its low thermal conductivity. Additionally, it is being reported that silica aerogel can also be used for sound insulation, as a catalyst and catalyst support, and as a dielectric material. Despite its excellent properties, silica aerogel has limited ap-plications due to its insulating property. Therefore, many researchers focused on the synthesis of non-insulating aerogels with various composition of metal oxides. Nev-ertheless, the synthesis of various metal oxides using traditional metal alkoxide as a precursor shows a difficulty in creating three-dimensional gelation networks, which in turn impact the inherent properties of the materials due to the low reactivity and difficult to control the reaction kinetics. Use of organic epoxide offers a potential solution to overcome these limitations. This dissertation presents three studies that demonstrate the design, synthesis, analysis, and application of advanced semicon-ducting/conducting metal oxide-based aerogels for energy and environmental appli-cations. These aerogels possess high surface area and numerous pores, which con-tribute to their effectiveness in this field. For the semiconducting metal oxide, SnO2 was used as a wide bandgap photocatalyst with the reduced graphene oxide. Also, doping of fluorine to SnO2 reduced its resistivity and applied as a catalytic support to replace the carbon in the field of water electrolysis.

In the second study, RuOx based aerogels were synthesized using Ru chloride as a precursor. Ni was doped to RuOx to enhance its electrocatalytic hydrogen evolution reaction (HER) activities. Nickel chloride was used as a doping source, and it was possible to achieve homogeneous doping to RuOx aerogel matrix owing to the high solubility and controlled reaction kinetic of epoxide-initiated sol-gel method. Also, RuOx based aerogels was grown on the surface of Ni foam to be nanostructured RuNi alloy aerogel. As a result, the enhancement in HER activities was achieved. The epoxide-initiated sol-gel method allowed to control the stoichiometry of metal oxide for desired composition.

국문 초록 (Abstract)

에어로겔은 많은 meso 기공을 포함하고 있어 낮은 밀도, 높은 비표면적, 낮은 열전도율을 자랑하는 재료이다. 1931년 스티븐 키슬러에 의해 처음 만들어진 이래로, 실리카 에어로겔은 에어로겔의 주된 형태로 인정받아 왔다. 이 실리카 에어로겔은 낮은 열전도율 덕분에 열 절연 재료로 널리 사용되고 있다. 추가적으로, 실리카 에어로겔은 방음 재료로, 촉매와 촉매 지지체로, 그리고 유전체 재료로도 활용될 수 있다고 알려져 있다. 그러나 뛰어난 특성에도 불구하고, 실리카 에어로겔의 활용은 그 절연 특성으로 인해 제한적이다. 따라서 많은 연구자들은 다양한 금속 산화물 조성을 갖는 비절연 에어로겔의 합성에 중점을 두었다. 전통적인 금속 알콕사이드를 전구체로 사용하여 다양한 금속 산화물을 합성하는 과정에서 3 차원 겔화 네트워크를 생성하는 데 어려움이 있으며, 이는 낮은 반응성과 반응 속도를 제어하기 어려움으로 인해 재료의 본래 특성에 영향을 준다. 유기 에폭사이드의 사용은 이러한 한계를 극복할 잠재력을 제공한다. 이 논문은 에너지와 환경 소재로 적용을 위한 반도체/도체 금속 산화물 기반 에어로겔의 설계, 합성, 분석, 그리고 응용을 보여주는 세 가지 연구를 제시한다. 이 에어로겔들은 높은 표면적과 다수의 기공을 가지고 있어, 이 분야에서의 효과성에 기여한다. 반도체 금속 산화물인 SnO2 에어로겔은 큰 밴드갭 광촉매로 사용되었으며, 환원된 그래핀 산화물과 함께 사용되었다. 또한, SnO2에 플루오린을 도핑함으로써 그 저항성을 줄이고, 수전해 분야에서 탄소를 대체할 촉매 지지체로 적용되었다.

두 번째 연구에서는 루테늄 클로라이드를 전구체로 사용하여 RuOx 기반 에어로겔이 합성되었다. Ni가 RuOx에 도핑되어 그 전기화학적 수소 발생 반응(Hydrogen Evolution Reaction, HER) 활성을 향상시켰다. 니켈 클로라이드가 도핑 소스로 사용되었으며, 에폭사이드 개시 졸-겔 방법을 통한 높은 용해도와 제어된 반응 속도 덕분에 RuOx 에어로겔 매트릭스에 균일한 도핑이 가능했다. 또한, RuOx 기반 에어로겔은 Ni 폼 표면에 성장하여 나노구조화 된 RuNi 합금 에어로겔이 되었다. 결과적으로, HER 활성의 향상이 달성되었다. 에폭사이드 개시 졸-겔 방법을 통해 원하는 구성의 금속 산화물의 화학양론을 제어할 수 있었다.

목차 (Table of Contents)

• TABLE OF CONTENTS i
• List of Figures v
• List of Tables xvi
• ABSTRACT xviii
• CHAPTER 1 1
• Introduction 1
• CHAPTER 2 5
• Theoretical Background 5
• 2.1 Aerogel 5
• 2.1.1 Gelation chemistry 6
• 2.1.2 Hydrolytic polycondensation and addition condensation 7
• 2.1.3 Epoxide-initiated sol-gel method 9
• 2.1.4 Supercritical drying 12
• 2.2 Semiconducting metal oxide-based aerogel 13
• 2.3 Conducting metal oxide-based aerogel 18
• 2.4 Photocatalytic water purification 22
• 2.5 Electrocatalytic hydrogen evolution reaction (HER) 25
• CHAPTER 3 28
• Experimental Apparatus and Procedures 28
• 3.1 Preparation of oxide aerogel 28
• 3.2 Characterization techniques of aerogels 30
• 3.2.1 Scanning Electron Microscopy (SEM) 30
• 3.2.2 Transmission Electron Microscope(TEM) & Energy Dispersive X–ray Analysis (EDX) 30
• 3.2.3 Brunauer-Emmett-Teller (BET) & Barrett–Joyner–Halenda (BJH) 31
• 3.2.4 X-Ray Diffraction (XRD) 31
• 3.2.5 X-ray photoelectron spectroscopy (XPS) 31
• 3.2.6 Fourier Transform Infrared (FT-IR) Spectroscopy 32
• 3.2.7 Ultraviolet diffuse reflectance spectra (UV-DRS) 32
• 3.2.8 Photoluminescence (PL) 33
• 3.2.9 X-ray absorption near edge structure (XANES) 33
• 3.2.10 Inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometer (ICP-MS) 33
• 3.2.11 Photocatalytic properties 34
• 3.2.12 Electrochemical measurement 35
• CHAPTER 4 36
• Synthesis of semiconducting metal oxide aerogels by epoxide-initiated sol-gel method 36
• 4.1 Epoxide-initiated sol-gel method for semiconducting aerogels 36
• 4.2 Synthesis of SnO2 aerogel and its nanocomposite with graphene oxide 36
• 4.2.1 Experimental detail 37
• 4.2.2 Results and discussions 38
• 4.3 Synthesis of F-doped SnO2 aerogel with high electrical conductivity 51
• 4.3.1 Experimental detail 55
• 4.3.2 Results and discussion 56
• 4.4 Conclusion 72
• CHAPTER 5 77
• Synthesis of conducting metal oxide aerogels by epoxide-initiated sol-gel method 77
• 5.1 Epoxide-initiated sol-gel method for conducting aerogels 77
• 5.2 Synthesis of Ni-doped RuOx aerogel using propylene oxide 78
• 5.2.1 Experimental detail 81
• 5.2.2 Results and discussions 84
• 5.3 Synthesis of Partially oxidized inter-doped RuNi alloy aerogel 94
• 5.3.1 Experimental detail 94
• 5.3.2 Results and discussion 97
• 5.4 Conclusion 113
• CHAPTER 6 116
• Highly porous metal oxide aerogels for photo/electrocatalytic applications 116
• 6.1 Introduction to metal oxide aerogels on Energy and Environmental applications 116
• 6.2 Semiconducting metal oxide aerogels for photocatalytic applications 117
• 6.2.1 Experimental detail 119
• 6.2.2 Results and discussions 120
• 6.3 Semiconducting metal oxide aerogels for electrocatalytic HER 125
• 6.3.1 Experimental detail 127
• 6.3.2 Results and discussion 130
• 6.4 Electrical conducting Ni-doped Ru-based aerogels for electrocatalytic HER 150
• 6.4.1 Experimental detail 151
• 6.4.2 Results and discussions 152
• 6.5 Electrical conducting RuNi alloy-based aerogels for electrocatalytic HER 157
• 6.5.1 Experimental detail 158
• 6.5.2 Results and discussions 159
• 6.6 Conclusion 172
• CHAPTER 7 180
• Overall Conclusions 180
• ABSTRACT IN KOREAN 228