This article was originally published and is reproduced here for reference.

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)—in an interview 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 can be formed by numerous molecular pairings, resulting in a wide range of potential structures. “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 strong light absorption, corresponding to semiconductors with a suitably small bandgap and energy levels compatible with water-splitting reactions. The second demands a low chemical barrier at the catalyst surface to facilitate reaction kinetics. The third, which must be achieved concurrently, involves high electron mobility to suppress electron–hole recombination.

The research team addressed these challenges by leveraging Janus-induced intrinsic electric fields while carefully designing heterostructures to achieve favourable band alignment. In parallel, they calculated and optimised parameters related to surface hydrogen generation reactions and electron mobility. All computational work was conducted using the high-performance computing 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.

The study was published under the title “Rational Design 2D Heterobilayers Transition-Metal Dichalcogenide and Their Janus for Efficient Water Splitting” in ACS Applied Energy Materials (Q1 journal). The findings also earned First Prize at the Euréka Scientific Research Student Awards for Nguyễn Trần Gia Bảo, the paper’s 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 findings open new pathways for designing new materials for sustainable green hydrogen production 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.

Mỹ Hạnh – The Science and Development Newspaper