Improving Photocatalytic Hydrogen Production via MOF-derived RuO2/N,S-TiO2 Heterojunctions with High Apparent Quantum Yield

Photocatalytic hydrogen production represents a transformative approach to addressing global energy demands and environmental concerns by utilizing renewable resources such as sunlight and water. This process offers a sustainable alternative to fossil fuels, which are the primary contributors to greenhouse gas emissions and environmental degradation. Despite its potential, photocatalytic hydrogen production faces several significant challenges that impede its widespread adoption and efficiency. One of the primary challenges in photocatalytic hydrogen production is the development of efficient catalysts that can effectively harness sunlight to drive the water-splitting reaction. Titanium dioxide (TiO₂) is a widely studied photocatalyst due to its chemical stability, non-toxicity, and cost-effectiveness. However, TiO₂ suffers from a large band gap that limits its activity to the ultraviolet region of the solar spectrum, which constitutes only a small fraction of the total sunlight. Additionally, TiO₂ experiences a high recombination rate of photogenerated electrons and holes, which reduces its overall photocatalytic efficiency. To overcome these limitations, researchers have explored various strategies, including doping with heteroatoms, defect engineering, and the creation of heterojunctions. Heterojunctions, which involve the combination of two or more semiconductors with complementary properties, have shown great promise in enhancing photocatalytic performance. The synergy between the different materials can lead to improved charge separation, extended light absorption, and enhanced overall efficiency. New study published in Angewandte Chemie International Edition and conducted by Emmanuel Musa, Ankit Yadav, Kyle Smith, Min Soo Jung, William Stickle, Peter Eschbach, Xiulei Ji and led by Professor Kyriakos Stylianou from Oregon State University, researchers aimed to address the challenges of photocatalytic hydrogen production by developing innovative metal-organic framework (MOF)-derived metal oxide heterojunctions. MOFs are a class of materials known for their high surface area, tunable porosity, and structural versatility. By pyrolyzing MOFs, it is possible to create metal oxide heterojunctions that retain the beneficial properties of the parent MOFs while gaining new functionalities.

The primary focus of this study was the development and characterization of a novel heterojunction, RTTA, composed of RuO₂/N,S- TiO₂. The researchers hypothesized that the combination of ruthenium oxide (RuO₂) and nitrogen/sulfur-doped titanium dioxide (N,S- TiO₂) would result in a highly efficient photocatalyst. RuO₂ was chosen for its unique metallic properties and ability to create an internal electric field at the heterojunction interface, which can facilitate charge separation and enhance photocatalytic activity.

The researchers conducted a series of experiments to synthesize, characterize, and evaluate the photocatalytic performance of the MOF-derived heterojunctions, with a focus on the RuO2/N,S- TiO₂ (RTTA-1) system. The initial step involved synthesizing the parent MOFs: Ru-HKUST-1 and MIL-125-NH₂. These were prepared through established procedures. The researchers then pyrolyzed these MOFs in the presence of thiourea to generate mixed metal oxides. Specifically, the heterojunction RTTA-1 was created by calcining a mixture of Ru-HKUST-1/MIL-125-NH2/thiourea, with a mass ratio of 0.05:1:1. This process resulted in the formation of RuO2/N,S- TiO₂ with different RuO2 loadings, ranging from RTTA-1 to RTTA-5. To confirm the successful synthesis of the heterojunctions, the researchers employed powder X-ray diffraction (PXRD), which indicated the coexistence of RuO2 and TiO2 phases. X-ray photoelectron spectroscopy (XPS) further revealed strong interactions between RuO₂ and TiO₂, as evidenced by the shift in binding energy peaks. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) provided insights into the morphology and composition, showing that RTTA-1 consisted of nanoparticles covered with a dense layer of RuO₂.

The photocatalytic activity of the synthesized heterojunctions was evaluated by measuring the hydrogen evolution rates in a suspension of the metal oxides in water with glycerol as a sacrificial reagent. The suspension was irradiated using a 300 W Xenon lamp. Gas chromatography was used to monitor the hydrogen production. RTTA-1 exhibited an impressive hydrogen evolution rate of 10,761 μmol·hr-1·g-1 in the presence of glycerol, significantly outperforming the individual MOF-derived metal oxides. The apparent quantum yield (AQY) for RTTA-1 was determined to be 10.0%, showcasing its superior photocatalytic efficiency. This performance was attributed to the optimal band alignment and synergistic interactions between RuO2 and TiO2, which facilitated efficient charge separation and transfer.

To explore the impact of RuO₂ loading on the photocatalytic performance, the researchers varied the amount of Ru-HKUST-1 in the precursor mixture, creating heterojunctions with different RuO2 contents. The findings revealed that RTTA-1, with the lowest RuO₂ content, exhibited the highest hydrogen evolution rate. As the RuO2 content increased (from RTTA-2 to RTTA-5), the hydrogen production rates decreased. This decline was attributed to the creation of charge recombination centers due to excessive RuO₂, which hindered the photocatalytic efficiency.

In addition to RTTA, the researchers synthesized two other heterojunctions: ZTTA-1 (ZnO/N,S- TiO₂) and ITTA-1 (In2O3/N,S- TiO₂). These were generated using a similar pyrolysis approach with ZIF-8 and In-TBAPy as precursors. While ZTTA-1 and ITTA-1 also showed enhanced photocatalytic activity compared to their individual components, their performance was inferior to RTTA-1. ZTTA-1 and ITTA-1 achieved apparent quantum yields of 0.7% and 0.3%, respectively, indicating that the RuO2/TiO2 combination in RTTA-1 was more effective for hydrogen production.

The stability and reusability of RTTA-1 were assessed through repeated photocatalytic experiments. RTTA-1 consistently generated hydrogen at a rate of over 10,000 μmol·hr-1·g-1 across multiple cycles. PXRD analysis of the recovered catalyst confirmed that its structural integrity was maintained, demonstrating its durability. This stability was further validated by the absence of significant degradation in photocatalytic activity or crystalline structure after repeated use. To demonstrate the practical applicability of RTTA-1, the researchers conducted hydrogen evolution experiments using tap water and river water samples. The performance of RTTA-1 remained robust, producing hydrogen at rates of 8,190 μmol·hr-1·g-1 in tap water and 6,390 μmol·hr-1·g-1 in river water. These results highlighted the catalyst’s effectiveness in real-world conditions, despite the presence of impurities and dissolved ions. The electrochemical impedance spectroscopy (EIS) measurements revealed that RTTA-1 exhibited the lowest charge transfer resistance, indicating efficient interfacial charge transfer. The diffuse reflectance UV-Vis spectroscopy (DRS) showed that RTTA-1 had an absorption edge around 385 nm, extending into the near-infrared region due to the presence of RuO2. This broadened light absorption range contributed to the enhanced photocatalytic performance. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) studies further supported the efficient charge separation and reduced recombination in RTTA-1.

The significance of this study lies in its demonstration of how MOF-derived metal oxide heterojunctions can significantly enhance the efficiency of photocatalytic hydrogen production. The innovative approach of using metal-organic frameworks (MOFs) as precursors to create metal oxide heterojunctions offers several advantages over traditional methods. These advantages include improved structural properties, such as higher surface area, better porosity, and optimal band alignment, all of which contribute to enhanced photocatalytic performance.  The RTTA-1 heterojunction exhibited an impressive hydrogen evolution rate and apparent quantum yield, significantly outperforming traditional photocatalysts. This efficiency can lead to more effective and scalable hydrogen production processes, which are crucial for developing sustainable energy systems. By utilizing ruthenium oxide (RuO₂), which is more cost-effective than platinum (Pt) and other precious metals commonly used in photocatalysis, this study offers a pathway to more affordable hydrogen production technologies. This can lower the overall cost of producing hydrogen fuel, making it more competitive with fossil fuels. The demonstrated stability and reusability of the RTTA-1 heterojunction in various water sources, including tap and river water, highlight its potential for real-world applications. This durability ensures that the photocatalyst can maintain high performance over extended periods, reducing the need for frequent replacement and lowering maintenance costs. The synthesis method involving MOF pyrolysis is relatively straightforward and can be scaled up for industrial applications. This scalability is essential for transitioning from laboratory research to large-scale hydrogen production facilities.  By providing a clean and renewable method for hydrogen production, this study contributes to reducing reliance on fossil fuels and lowering greenhouse gas emissions. This aligns with global efforts to combat climate change and transition to a sustainable energy future.  The approach of using MOFs as precursors to create various heterojunctions (e.g., ZTTA-1, ITTA-1) demonstrates the versatility of this method. It opens up possibilities for exploring other MOF-derived heterojunctions with different metal oxides and doping elements to further enhance photocatalytic performance.