Electronic and geometric structures of Pt / Au nanoparticles and their electrocatalytic activity

학위논문 (박사)-- 서울대학교 대학원 : 화학생물공학부(에너지환경 화학융합기술전공), 2014. 2. 성영은.Due to being high efficient and environmentally friendly, proton exchange membrane fuel cells are one of the most promising next generation energy conversion devices to power the energy demands of the future. Substantial efforts are devoted to enhance the sluggish oxygen reduction reaction (ORR) at cathode compared to feasible hydrogen oxidation reaction at anode. The development of DFT-based theoretical understanding of the mechanisms of ORR yielded promising electrocatalyst materials. Pt is the most efficient ORR electrocatalyst due to the moderate intensity with adsorbate. To enhance the activity further, the oxygen species adsorption intensity should be lowered about 0.2 eV by DFT analysis. The Pt/Au structure was predicted that the lower activity owing to the lattice strain effects. The Au which atomic size is larger than Pt, induced the tensile strength and made the Pt d-band center up-shift with low degree of orbital overlap. High d-band center correlates to the high chemisorption energy and low ORR activity. Due to these expectations, few researches were conducted. In this study, the Pt/Au nanoparticle systems were applied to the ORR electrocatalyst beyond the previous unfavorable expectation and single crystal studies. The Au@Pt core-shell nanosized electrocatalysts and AuPt alloy nanoparticles were synthesized. Delicate investigations of nanoparticle structure were conducted using high resolution transmission electron microscopy and x-ray diffraction. X-ray absorption fine structures and photo emission spectroscopy also conducted at synchrotron facilities to confirm the inter-atomic distances, information related to the coordination numbers and electronic structures. From the CO stripping experiments, Pt-CO chemisorption energy was larger than that of the pure Pt until certain Pt surface compositions. However, the deposition of Pt increased, the Pt-CO chemisorption energy is lower than pure Pt which was controversy to previous predictions which the Au induced tensile strain to Pt with high chemisorption energy to CO and oxygen related species. To explain the experimental results, elucidations with quantum mechanics were performed. The chemisorption energy between metal and CO and oxygen species was interpreted two termscenter of d-band structures and orbital repulsion terms. The center of d-band positions was related to the band width. By changing the local environment, the d-band structure was changed due to the changing the orbital overlap between near atoms. Orbital repulsion terms were related to the adsorbate– metal orbital coupling matrix. The shorter the bond induced the more orbital overlap, the adsorbate anti-bonding orbital filled with the interaction between metal s,p orbitals. Due to the different electronegativity between Au and Pt, orbital repulsion should also be considered. Au has higher electronegativity compared to the Pt, induced the charge redistribution from Pt to Au. The lower electron in Pt induced the shorter bond distance with adsorbate, made the repulsion. The two parameters were mixed in Au/Pt systems. In the case of low Pt deposition to surface, the strain induced d-band center overwhelmed the effect of orbital repulsion terms. As amount of surface Pt increased, the strain factors decreased. Orbital repulsion factors were no longer the minor parameter at certain points. In the case of AuPt alloy nanoparticles, their flow of electron redistribution was different to core-shell structures. Considering the electronic structures, the core-shell structures were good candidates for ORR electrocatalysts. The ORR activity trends also followed with the CO stripping results. The different results between previous single crystal studies and current nanoparticle case were explained with the characteristics of nanoparticles. In the case of nanoparticles, their surface atoms have low coordinated which induces the surface atom contraction. Due to the surface contraction effect, the tensile strength was little effects on the Au@Pt system in nanoparticles. To confirm the nanosized atomic contraction effects, the surface coordination number was controlled using sonochemical method. The coordination number control was confirmed using extended x-ray absorption fine structures. The fitting parameter was shown as low coordination number, smaller inter-atomic distances and high Debye waller factors. Hydrogen oxidation reaction was also confirmed the surface low coordination number after sononchemical methods. After the coordination number controls, the Pt deposited to the Au nanoparticles. The lattice parameter of Pt was investigated using Rietveld refinement. The Pt deposited with low coordination numbered Au had low lattice parameter. Controlling the surface coordination number of core nanoparticle, strain could be controlled. The Au@Pt_0.5_S had 1.7 times higher ORR activity compared to without sononchemical method and 2.5 times higher activity compared to Pt nanoparticles. From these researches, the possible factors related to the adsorbate chemisorption energy were consideredcenter of d-band structures and orbital repulsion under the quantum mechanics. The strain effects could be controlled with change of coordination number of nanoparticles. These considerations can be applied to the strategies to design the nanoscale electrocatalysts.Abstract i List of Tables vii List of Figures viii Chapter 1. Introduction 1 1.1. Electrochemistry and Fuel cells 1 1.1.1. Electrochemistry kinetics 1 1.1.2. Proton Exchange Membrane Fuel Cell (PEMFC) 3 1.2. Oxygen reduction reaction 6 1.2.1. Oxygen Reduction Reaction (ORR) 6 1.2.2. Electronic and geometric structures and related to the ORR activity. 10 1.2.3 Research trends of Au/Pt systems for oxygen reduction reaction 15 1.3. Objectives of this dissertation 18 Chapter 2.Experimental 21 2.1. Preparation Stage: 21 2.1.1. Chemicals and Materials 21 2.1.2. Synthesis of AuPt (alloy) nanoparticles 21 2.1.3. Synthesis of Au@Pt (core-shell) nanoparticles 22 2.1.4. Physical characterization 23 2.1.5. Electrochemical measurements 26 2.1.6. CO stripping and oxygen reduction reactions 27 Chapter 3. Results and Discussion 29 3.1. Electronic and geometric structures of Au/Pt nanoparticles 29 3.1.1. Synthesis and morphology 29 3.1.2. Structure characterizations 40 3.1.3. Electrochemical measurements 57 3.1.4. Oxygen reduction reactions 83 3.2. How to improve the ORR activity in Au/Pt systems? 87 3.2.1. Difference between single crystal and nanoparticles 87 3.2.2. Surface coordination number control and strain effects 89 Chapter 4.Conclusions 108 References 112 국문초록 126Docto

4D 콘텐츠에서 시각적 장면과 합치되는 동작 효과 연구

DoctorFour-dimensional (4D) content is becoming increasingly prevalent with the development of immersive technologies. 4D content provides audiovisual content with physical effects called 4D effects, such as seat motions, vibrations, flashes, and scents in synchronization. Motion effects, which refer to the motions of a user’s seat, are most frequently used among these 4D effects. Congruent—in other words harmonious or appropriate—motion effects should be designed with respect to the audiovisual elements of content because motion effects are inserted into 4D content as additional drivers to enhance user immersion. However, no empirical study has investigated the factors that affect congruency between motion effects and audiovisual content or the effects of congruency on user’s 4D content experience. This research aims to design congruent motion effects and identify the relationships with other subjective factors of the 4D content experience. To achieve the objectives, influencing factors on congruency and effects of congruency on other experiential factors were investigated by a literature survey. The direction and tilted position of a seat’s motion were selected as features of motion effects that should be studied by priority. Design strategies of motion effects, which were four direction strategies and four position strategies, were derived by considering the relative motion of a seat and a visual object on a screen. Two empirical studies were conducted to identify congruency affected by the design strategies and effects of congruency on the other experiential factors. The first empirical study was conducted using a contextless visual scene, and the second study was conducted using contextual visual scenes. The first experiment found that the direction of motion effects more greatly affects congruency than the tilted position. Participants felt the highest congruency when their seat tilted forward/backward (pitch down/up) and right/left (roll right/left) according to the upward/downward and right/left visual motions in a contextless scene, respectively. The participants could discriminate the degree of congruency by the tilted position if the direction of the motion effects matched the visual motions. Congruency had a significant negative relationship with workload and complexity of visual impression, but it had no significant relationship with dynamicity of visual impression. In the second experiment, the participants felt the highest congruency when the seat tilted forward/backward (pitch down/up) and right/left (roll right/left) according to the upward/downward and right/left visual motions, respectively, in all contextual scenes, which is the same result of the first experiment. However, the participants also felt the highest congruency when the seat tilted backward/forward (pitch up/down) and right/left (roll right/left) according to the upward/downward and right/left visual motions when contextual scenes are filmed horizontally with a ground of a virtual environment. Congruency had a positive effect on presence, dynamicity of visual impression, valence, and immersion, whereas it had a negative effect on mental load and complexity of visual impression. However, no significant effect was found on physical load. A Congruency–Immersion Model (CIM) was developed to describe how congruency affects immersion. Findings show that congruency can affect immersion directly or indirectly through several experiential factors. The significance of this study is that it can strengthen the basis for empirical studies related to congruency of motion effects in 4D content. This research provides the following contributions. First, the results can be used by 4D effects producers when they design motion effects of commercial 4D content and by algorithm developers of automatic authoring tool of motion effects. Second, the results show that congruency is an important factor of immersive experience and help understand how congruency affects immersion.몰입형 기술의 발전으로 4차원 (4D) 콘텐츠가 점점 더 보편화되고 있다. 4D 콘텐츠는 시청각 콘텐츠에 의자의 움직임 및 진동, 섬광, 향기 등을 포함하는 4D 효과가 동기화되어 더해진 콘텐츠를 의미한다. 다양한 효과들 중에서 콘텐츠 사용자가 앉아 있는 의자의 움직임 효과를 일컫는 동작 효과가 4D 효과로써 자주 활용된다. 동작 효과는 4D 콘텐츠에 사용자의 몰입도를 높이기 위한 추가적인 동인으로 삽입되는 부가적인 요소이기 때문에 콘텐츠의 시청각 요소에 합치되는 동작 효과가 설계되어 사용자들에게 제공될 필요가 있다. 그러나 동작 효과와 시청각 콘텐츠 사이의 합치성 또는 사용자의 4D 콘텐츠 경험에 대한 합치성의 영향을 조사하는 체계적이고 깊이 있는 경험적 연구를 찾기 힘든 실정이다. 본 연구는 시각 장면에서 나타나는 물체의 움직임과 합치되는 동작 효과를 조사하고 합치성과 4D 콘텐츠 경험의 다른 주관적 요소와의 관계를 식별하는 것을 목표로 한다. 목표를 달성하기 위해 합치성에 영향을 미칠 수 있는 자극의 특징들과 합치성이 영향을 미칠 수 있는 경험적 요소들을 문헌 조사를 통해 파악하였다. 의자 움직임의 방향과 기울어진 위치는 우선적으로 연구되어야 할 동작 효과의 물리적인 특징으로 선택되었다. 이를 기반으로 화면 속 물체의 움직임과 의자의 움직임의 상대적인 운동 방향과 위치를 고려하여, 동작 효과의 설계 전략인 네 가지 방향 전략과 네 가지 위치 전략을 도출하였다. 동작 효과의 설계 전략에 의한 합치성과 합치성이 다른 경험적 요소에 미치는 영향을 조사하기 위해 두 가지 경험적 연구를 수행하였다. 첫 번째 연구는 맥락이 없는 시각적 장면에 대하여 실험을 수행하였으며 두 번째 연구는 맥락이 포함된 시각적 장면에 대하여 실험을 수행하였다. 첫 번째 실험을 통해 동작 효과의 방향이 기울어진 위치보다 합치성에 더 큰 영향을 미친다는 것을 발견하였다. 실험 참여자들은 맥락이 없는 장면에서 위/아래 및 오른쪽/왼쪽 시각 물체의 운동에 따라 좌석이 각각 앞/뒤 및 오른쪽/왼쪽으로 기울었을 때 가장 높은 합치성을 느꼈다. 동작 효과의 방향에 의해 시각 물체의 운동과 합치되는 경우, 실험 참여자들은 의자의 기울어진 위치에 따라 합치성의 상대적인 차이를 구별할 수 있었다. 하지만, 동작 효과의 방향이 시각 물체의 운동과 합치되지 않는 경우 의자의 기울어진 위치와 상관없이 모두 동일하게 낮은 합치성을 느꼈다. 합치성은 관찰 작업 부하 및 시각적 인상의 복잡성과 유의미한 부정적인 관계가 존재했지만, 시각적 인상의 역동성과는 유의미한 관계가 없었다. 두 번째 실험에서 실험 참여자들은 맥락이 포함된 모든 장면에서 위/아래 및 오른쪽/왼쪽 시각 물체의 운동에 따라 좌석이 각각 앞/뒤 및 오른쪽/왼쪽으로 기울었을 때 가장 높은 합치성을 느꼈다. 이는 첫 번째 실험과 동일한 결과이다. 하지만 실험 참여자들은 가상 환경 속 지면과 수평이 되는 방향으로 촬영된 장면에서 좌석의 앞/뒤 방향이 반대인 경우에도 가장 높은 합치성을 느꼈다. 합치성은 감정가, 시각적 역동성, 존재감 (혹은 실재감), 몰입감에 유의하게 긍정적인 영향을 미쳤으며, 정신적 부하와 시각적 복잡성에는 유의하게 부정적인 영향을 미쳤다. 하지만 신체적 부하에는 유의한 영향을 미치지 않는 것으로 나타났다. 합치성이 콘텐츠 몰입에 미치는 영향을 설명하기 위해 Congruency-Immersion Model (CIM)을 개발하였다. 이를 통해 합치성은 주로 직접적으로 몰입에 영향을 미치거나, 존재감을 매개하여 합치성에 간접적으로 영향을 미친다는 것으로 나타났다. 몰입은 동작 효과의 합치성 뿐 아니라, 존재감 및 감정가에 직접적인 영향을 받는 것으로 나타났다. 본 연구는 4D 콘텐츠에서 동작 효과의 합치성과 관련된 경험적 연구의 토대를 마련하였다는 점에서 큰 의의가 있다. 본 연구의 결과는 4D 효과 제작자가 4D 콘텐츠의 동작 효과를 설계하거나 동작 효과의 자동 저작 알고리즘을 개발할 때 사용자를 이해하기 위한 기초 자료로써 활용될 수 있다. 그리고 합치성이 몰입 경험의 중요한 요소이며, 합치성이 몰입에 미치는 영향을 심층적으로 이해하는데 높은 기여를 할 것으로 판단된다. 궁극적으로는 4D 콘텐츠 사용자들의 콘텐츠 경험 만족도를 증진시킬 것이라 예상된다

주거공간에서의 3차원 핸드제스처 인터페이스에 대한 사용자 요구사항

22kc