Photoelectrochemical water splitting offers a clean path to produce hydrogen fuel directly from sunlight and water. Quantum dots - nanoscale semiconductor crystals - are highly promising for this application due to their tunable bandgaps and strong light absorption. However, most high-performance quantum dots rely on toxic heavy metals such as cadmium or lead, limiting their practical deployment.
Eco-friendly alternatives based on I-III-VI semiconductors like copper indium sulfide and copper indium selenide have emerged, but their complex multinary chemistry often introduces native defects - particularly anion vacancies - that trap charge carriers and degrade performance. Based on these challenges, there is an urgent need to develop strategies that can precisely control defect formation in these materials without compromising their environmental benefits.
To address this need, researchers at the Daegu Gyeongbuk Institute of Science and Technology (DGIST), in collaboration with scientists at Konkuk University, devised a composition-driven strategy that systematically adjusts the sulfur-to-selenium ratio in copper-indium-sulfur-selenium quantum dots to modulate anion vacancy concentrations while keeping particle size and cation stoichiometry unchanged. The work published (DOI: 10.1016/j.esci.2025.100518) on September 1, 2026, in the open-access journal eScience (Volume 6, Issue 5, Article 100518), reveals that an equimolar sulfur-selenium balance yields the lowest vacancy density - a finding that prompted the team to systematically investigate how this structural ordering influences optoelectronic properties and photoelectrochemical performance.
The researchers synthesized five compositions of CuIn(S1-xSex)2 quantum dots with sulfur-to-selenium ratios ranging from pure sulfide to pure selenide. Transmission electron microscopy confirmed uniform particle sizes of approximately 4.3 nanometers across all compositions, while X-ray diffraction revealed that the equimolar composition - CuIn(S0.5Se0.5)2 - exhibited the lowest tetragonal lattice distortion, indicating minimal crystal strain. Extended X-ray absorption fine structure (EXAFS) analysis showed higher coordination numbers for both copper and indium bonds in the 1:1 composition, pointing to fewer vacant anion sites.
Electron paramagnetic resonance (EPR) spectroscopy directly confirmed this, showing the lowest concentration of sulfur and selenium vacancies at the equimolar ratio. These structural improvements translated into substantially increased hole concentration - nearly doubling that of the pure sulfide quantum dots - and significantly extended carrier lifetimes, as revealed by time-resolved photoluminescence measurements.
When integrated into titanium dioxide (TiO2)-based photoanodes with zinc sulfide (ZnS) and silicon dioxide (SiO2) passivation layers, the optimized quantum dots delivered a photocurrent density of 15.1 mA cm-² at 0.6 V versus the reversible hydrogen electrode (RHE) - a record for heavy-metal-free quantum dot systems. Beyond setting this performance benchmark, the dual passivation strategy proved equally critical for durability: it maintained faradaic (Faradaic) efficiency above 80% for over six hours of continuous operation, demonstrating that the material can sustain its catalytic activity under prolonged working conditions.
The authors said that the study's most exciting outcome is how a seemingly simple adjustment - balancing sulfur and selenium at the atomic scale - can resolve what has long been considered the Achilles' heel of eco-friendly quantum dots. They explained that their approach turns an intrinsic weakness into a strength, proving that these materials can perform on par with their toxic counterparts without compromising environmental safety. The team emphasized that the work goes beyond just setting a performance record; it establishes a fundamental design principle for defect engineering in multinary quantum dots that could accelerate the transition toward sustainable hydrogen energy technologies.
This advance addresses a critical barrier to commercializing quantum-dot-based solar hydrogen production. The ability to achieve high efficiency without toxic heavy metals opens the door for large-scale deployment in renewable energy infrastructure, particularly in regions seeking to build decentralized hydrogen production systems. The ZnS/SiO2 dual passivation layer also provides a practical pathway to long-term operational stability - a key requirement for industrial adoption. Beyond hydrogen production, the composition-driven defect control strategy could extend to other optoelectronic applications, including quantum-dot solar cells, photodetectors, and light-emitting devices, where defect management is equally critical. The research was supported by the National Research Foundation of Korea (NRF), the Ministry of Trade, Industry and Energy (MOTIE), and the Korea Institute for Advancement of Technology (KIAT).