Fabrication of SrTiO₃/rGO/BiVO₄ Z-Scheme films and selective cocatalyst photodeposition for overall water splitting

국문 초록 (Abstract)

현재 탄소중립 실현을 위해, 그린수소가 핵심 에너지 전환 수단으로 주목 받고있다. 본 연구는 태양광만을 에너지원으로 활용하여 수소를 생산하는 광촉매 기반의 물분해(OWS) 시스템에 초점을 맞추었다. 광촉매 활성을 극대화하기 위해 Z-scheme (SrTiO3/rGO/BiVO4) 이중접합 구조를 설계 및 구현했으며, 실제 모듈 적용을 위한 필름 제작을 연구하였다.
광촉매 활성화를 제한하는 밴드갭 딜레마(Bandgap Dilemma)를 해결하기 위해, 수소생산용 광촉매인 SrTiO3, 산소생산용 광촉매인 BiVO4, 전하 매개체인 rGO로 구성된 Z-scheme 이중접합 구조의 광촉매 시스템을 합성하였다. 특히, 수소발생을 위한 물분해 반응을 촉진하기 위해, 조촉매를 활용하였다. 균일한 혼합을 위한 초음파 처리와 설계된 이동 경로의 전하 거동을 극대화하기 위해 조촉매의 위치 선택적 광증착 과정을 최적화하였으며, 그 결과 단일 광촉매보다 약 2배 높은 수소발생량을 보였다.
필름 형태 합성의 최적화 및 대면적화 확보를 위해 닥터 블레이드(Doctor Blade) 도포 방식을 도입하였다. 이는 드롭 캐스팅(Drop casting), 스핀 코팅(Spin-coating)과 같은 다른 방식들과 비교하였을 때, 광촉매 나노입자의 고유 성질을 유지하면서도 높은 균일성과 안정성을 달성하는데 가장 효과적인 방식인 것을 확인하였다. 그 결과물인 필름은 자체 설계된 모듈 크기에 맞게 안정적으로 합성되었다.
또한, 광촉매의 근본적인 반응 속도론적 한계(kinetic limitations)를 극복하기 위해 고농축 광열(High-Flux Photothermal) 시스템을 적용하였다. 실내 실험 결과, 광 조사 광도가 약 7.3 SUN에 도달했을 때 STH 효율이 비선형적으로 증가하며 10배 이상 급증하는 것을 확인하였다. 이 효과는 실제 태양광을 이용한 100 SUN 야외 실험에서 극대화 되었고, 최고 효율인 1.97 %를 달성하였다. 이 결과는 광 이외에도 반응 시스템의 온도가 130 ℃까지 상승한 것을 통해 열 에너지 효과로 인한 반응속도를 과속화했음을 시사한다.
본 연구는 고효율의 광촉매 실용화 방안을 제시하면서, 효율적이고 대면적으로 확장 가능한 수소 발생 시스템을 실현하는 길을 열어줄 것으로 기대된다.

다국어 초록 (Multilingual Abstract)

Carbon Neutrality (Net Zero), which aims to eliminate greenhouse gas emissions, the pursuit of Green Hydrogen as a key energy transition vector is paramount. This study focuses on photocatalytic overall water splitting (OWS), a technology that uses only solar energy to produce hydrogen. To maximize the photocatalytic activity, we implemented Z-scheme (SrTiO3 / rGO / BiVO4) structure and investigated film fabrication for practical application.
To resolve the inherent Bandgap Dilemma that limits activation, we synthesized Z-scheme photocatalyst system, which consisted of SrTiO3 for HER and BiVO4 for OER with rGO as an electron mediator. For promoting the overall water splitting reaction for H2 production, cocatalysts were photodeposited. Initial sonication for uniform mixing and site-selective photodeposition of cocatalysts were optimized to maximize engineered charge carrier migration. The H2 production rate approximately 2-fold increased compared to single photocatalyst.
For scalable application, synthesizing photocatalysts into a film is important. Doctor Blade method was the most effective for achieving high uniformity and stability, which preserves properties of photocatalyst nanoparticles, compared to other coating methods such as drop casting or spin-coating. The resulting film was stably synthesized suitable for integration into the custom-designed module.
A High-Flux Photothermal system was employed to overcome the fundamental kinetic limitations of photocatalyst. Indoor experiments identified a threshold intensity around 7.3 SUN where the STH efficiency rose non-linearly, surging by over 10-fold. This effect was maximized in outdoor experiments with 100 SUN concentration condition using sunlight. It reached 1.97 % of STH efficiency. This peak performance showed the thermal energy effect as the temperature reached 130 ℃ in the sealed reaction system.
This study highlights a highly promising pathway for realizing exceptionally efficient and scalable hydrogen production systems.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• Chapter 2. Background 10
• 2.1 Principle of Photocatalytic Water Splitting and Bandgap Dilemma 10
• 2.2 Photocatalytic Overall Water Splitting Research Trends and Limitations 12
• 2.3 Heterojuction: Type-Ⅱ vs Z-scheme 14
• 2.4 Z-scheme Photocatalyst: Component Role 17
• 2.4.1 HER Photocatalyst (PC Ⅰ) 17
• 2.4.2 OER Photocatalyst (PC Ⅱ) 18
• 2.4.3 Electron Mediator 18
• 2.5 System Optimization: Photodepositon and Film 19
• 2.5.1 Selective Co-catalyst Photodeposition 19
• 2.5.2 Film Fabrication 20
• 2.6 Enhancing Efficiency via Reaction Condition Control 21
• 2.6.1 Photothermal Catalysis (Light and Heat) 22
• 2.6.2 Concentrated Sunlight 23
• 2.6.3 Gas phase system 23
• Chapter 3. Experimental 25
• 3.1 Z-scheme Photocatalyst 25
• 3.1.1 SrTiO3:Al (HER Photocatalyst) 25
• 3.1.2 BiVO4:Mo (OER Photocatalyst) 25
• 3.1.3 rGO / BiVO4:Mo (Mediator + OER Photocatalyst) 26
• 3.2 Selective Photodeposition of Cocatalyst 26
• 3.2.1 SrTiO3:Al Single Photocatalyst 26
• 3.2.2 SrTiO3:Al / rGO / BiVO4:Mo Z-scheme Photocatalyst 27
• 3.3 Synthesis of Photocatalyst Film 27
• 3.3.1 SrTiO3:Al Film 27
• 3.3.2 Z-scheme Film 27
• 3.4 Measurement of Hydrogen Production 28
• Chapter 4. Results and Discussion 29
• 4.1 Characterization 29
• 4.1.1 SrTiO3:Al (HER Photocatalyst) 29
• 4.1.2 BiVO4:Mo (OER Photocatalyst) 34
• 4.1.3 rGO / BiVO4:Mo (Mediator + OER Photocatalyst) 36
• 4.2 Hydrogen Production 41
• 4.2.1 SrTiO3:Al Single Photocatalyst 41
• 4.2.2 SrTiO3:Al / rGO / BiVO4:Mo Z-scheme Photocatalyst 47
• 4.3 Film Fabrication 51
• 4.3.1 SrTiO3:Al Photocatalyst Film 51
• 4.3.2 Z-scheme Photocatalyst Film & Module 56
• 4.4 High-Flux Photothermal-Assisted Water Splitting: Synergistic Enhancement of Hydrogen Evolution over SrTiO3:Al Photocatalyst 60
• 4.4.1 Non-linear Enhancement under Simulated Concentrated Light 61
• 4.4.2 Practical Outdoor Application with 100 SUN High-Flux Concentration 65
• 4.4.3 Elucidation of Photothermal Synergistic Mechanism 68
• Chapter 5. Conclusion 74
• References 76