Researchers at VNUHCM-University of Science (HCMUS) and the Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Japan, have developed a novel material designed to produce green hydrogen using sunlight. This innovation promises to reduce reliance on fossil fuels and lower emissions.
‘Clean energy’ and ‘energy transition’ have become the most aggressively deployed solutions across numerous nations in recent years, aiming to curb greenhouse gas emissions. Among such strategies, hydrogen energy based on water-splitting technology stands as a potent candidate for solving the CO2 emission puzzle, as the process utilises direct sunlight to generate H2 from H2O.
However, clean technologies often come with high production costs, and hydrogen is no exception. “The difficulty remains that photocatalytic materials require photocatalysts with excessive costs, whilst performance has not yet met expectations,” shared Nguyễn Trần Gia Bảo—the first author, a HCMUS student and a member of the research group led by Dr Nguyễn Tuấn Hưng (Frontier Research Institute for Interdisciplinary Sciences, Tohoku University)—with the Science and Development newspaper.
A primary cause for the reduction in photocatalytic efficiency is the electron-hole pair recombination process. “Should this process occur too rapidly, energy dissipation ensues,” Nguyễn Trần Gia Bảo explained.
Consequently, the research team turned attention to the distinct advantages of Janus materials. Named after the Roman god, these materials feature two faces formed from different molecules, such as S-Mo-Se (Sulfur – Molybdenum – Selenium). The asymmetric structure generates an intrinsic electric field, facilitating the spatial separation of electron-hole pairs. Such separation slows the recombination process and enhances water-splitting efficiency within the Janus material.
The challenge arises because Janus materials are formed by varying pairs, meaning a multitude of potential structures exist. “Which pairing proves optimal for photocatalytic water splitting? Our research focused on answering that question through material calculation and simulation,” noted Nguyễn Trần Gia Bảo.
Finding the Missing Piece
In reality, employing Janus materials is not an unprecedented concept. Globally, numerous studies have explored photocatalytic water splitting; concerning Janus materials specifically, theoretical works have shown the substance typically possesses a conduction band higher than the H+/H2 reduction level and a valence band lower than the H2O/O2 oxidation level—factors favourable for the water-splitting reaction.
“Previous research groups have also indicated that the intrinsic electric field of Janus materials can support the water-splitting process,” the team stated. “However, this study concentrates on two-dimensional (2D) Janus materials possessing a heterostructure (comprising two different material layers paired together, where one is a Janus material)—the novelty of this work.”
To conduct the research, the team led by Dr Nguyễn Tuấn Hưng collaborated with the group of Associate Professor Vũ Thị Hạnh Thu at HCMUS to design and pair materials, creating 20 Janus 2D heterostructures. Subsequently, the scientists selected pairs with potential for photocatalytic water splitting based on suitable light absorption criteria and superior charge separation capabilities. Finally, the team conducted an in-depth evaluation to determine suitability for hydrogen generation based on calculations of electron mobility and photochemistry.
“A distinctive feature is that this research does not merely ‘find a good material’, but also establishes simple principles based on geometric structure and electronegativity between material layers to select the optimal heterostructure for photocatalytic water splitting. These principles can help shorten the search time for new materials significantly compared to complex simulation calculations,” a representative of the research group noted.
Building upon a collaboration with the group of Prof. Jing Kong (Massachusetts Institute of Technology – MIT, USA)—who successfully synthesised Janus 2D heterostructures—and the group of Prof. Shengxi Huang (Rice University, USA) regarding the optical properties of the material layer, Dr Nguyễn Tuấn Hưng and team members possessed a solid foundation to investigate Janus 2D heterostructures for photocatalytic water splitting.
Nevertheless, on the path to discovery—a core element of scientific research—challenges always exist. “The most difficult aspect of this study was selecting a material that simultaneously satisfies three conditions for the photocatalytic water-splitting reaction to occur effectively,” Nguyễn Trần Gia Bảo and Dr Nguyễn Tuấn Hưng recalled.
The first condition requires good light absorption (semiconductors with a small bandgap), where energy levels suit the water-splitting reaction. The second condition demands a low chemical barrier on the catalyst surface to facilitate the reaction. The third condition, which must be met concurrently, involves high electron mobility to prevent the recombination of electrons and holes.
Following the research process, the team solved this problem by using Janus to create an intrinsic electric field whilst designing the heterostructure to create favourable energy band alignment. Furthermore, the group calculated and optimised parameters directly related to surface reactions (hydrogen generation) as well as the electron mobility of the material. The entire calculation process took place via the supercomputing system of the research group at HCMUS.
From 20 material pairs, the group identified three most promising heterostructures: WSe2-SWSe, WSe2-TeWSe, and WS2-SMoSe. Among these, WS2-SMoSe achieved the highest solar-to-hydrogen energy conversion efficiency—16.62%. Many other structures also reached between 14% and just under 16.62%. “These efficiencies all surpass the 10% threshold—a figure typically regarded as the benchmark for commercial viability,” the research team shared, noting the promising signals.
With these results, the work appeared in the article “Rational Design 2D Heterobilayers Transition-Metal Dichalcogenide and Their Janus for Efficient Water Splitting” in the journal ACS Applied Energy Materials (Q1). Simultaneously, the findings garnered the First Prize at the Euréka Scientific Research Student Awards for Nguyễn Trần Gia Bảo—the primary author.
“Combining transition-metal dichalcogenides (TMDC) with Janus layers is akin to playing with LEGO, as infinite structures exist for testing. Our method allows for the efficient and accurate identification of the most promising material combinations for water splitting, thereby significantly shortening the time spent searching for material structures,” Nguyễn Trần Gia Bảo envisaged in a statement to Tohoku University.
These new discoveries also help open directions for designing new materials to produce sustainable green hydrogen using sunlight, reducing dependence on fossil fuels and lowering emissions. “Moving forward, the team will continue with experimental work to verify the best materials. Additionally, the research group will further optimise by adjusting the structure, adding layers, or incorporating catalytic supports to increase durability and efficiency, whilst expanding the search to many other 2D material combinations,” Dr Nguyễn Tuấn Hưng stated.
Conventionally, hydrogen receives three colour classifications—green, blue, and grey—corresponding to the production method and carbon storage. ‘Grey’ hydrogen comes from fossil fuels, created through gasification or reforming processes to generate H2, releasing CO2 into the atmosphere. If capturing and burying this CO2 underground becomes part of the process, the product transforms into ‘Blue’ hydrogen. If hydrogen production utilises renewable energy sources such as wind and solar power through water electrolysis, the product creates ‘Green’ hydrogen by default. Hydrogen serves as a crucial element in helping industries achieve carbon neutrality, although the technology remains expensive and in the early stages of development. Industries can use hydrogen as fuel for cars, buses, and heavy trucks, allowing vehicles to emit water vapour instead of carbon gases. Hydrogen also acts as a chemical commonly used in metallurgy, oil refining, semiconductor manufacturing, and cosmetics. Drawing from the experiences of other nations, experts argue that dependence on foreign technology and equipment in the green hydrogen production process will lead to future consequences, such as reliance on chemicals and equipment from suppliers (due to companies keeping technology confidential). Therefore, researching and developing domestic materials will help gradually master hydrogen production technology, whilst paving the way for cooperation with domestic enterprises wishing to invest in developing this sector.
What is Green Hydrogen?
Mỹ Hạnh – The Science and Development Newspaper
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