Eco-Friendly Quantum Dots Hit Record Solar Hydrogen Output With 1:1 Sulfur-Selenium Mix

Hydrogen produced directly from sunlight and water has long been one of the most tantalizing prizes in renewable energy research, but the materials used to capture that sunlight have often carried an uncomfortable caveat: toxicity. Now, a team at the Daegu Gyeongbuk Institute of Science and Technology (DGIST), working with collaborators at Konkuk University, has shown that a deceptively simple compositional adjustment can push heavy-metal-free quantum dots to a record-setting performance level for solar hydrogen production. By balancing sulfur and selenium atoms in equal measure within copper-indium-sulfur-selenium quantum dots, the researchers dramatically suppressed the atomic defects that have historically crippled the efficiency of environmentally friendly semiconductor nanocrystals.

Photoelectrochemical water splitting is the process at the heart of this advance. In such a system, a photoanode absorbs sunlight and uses the resulting energetic charge carriers to tear water molecules apart, releasing hydrogen at a cathode. Quantum dots, nanoscale semiconductor crystals just a few nanometers across, are attractive candidates for the light-absorbing layer because their bandgaps can be tuned by size and composition, and because they absorb light with exceptional strength. Yet the highest-performing quantum dots in the literature have almost invariably depended on cadmium or lead, toxic heavy metals whose regulated status has hindered any realistic path toward large-scale deployment of quantum-dot-based solar fuels.

Eco-friendly alternatives drawn from the I–III–VI semiconductor family, such as copper indium sulfide and copper indium selenide, have emerged as appealing substitutes. Their chemistry, however, is far more complicated than that of binary cadmium chalcogenides. With three cation species and a chalcogenide component to juggle, these multinary crystals are prone to forming native defects, and the most damaging among them are anion vacancies, empty sites where a sulfur or selenium atom should sit. These vacancies act as traps that capture charge carriers before they can contribute to water splitting, degrading both photocurrent and stability. Controlling their formation without sacrificing the environmental advantages of the material has therefore been one of the field’s most stubborn challenges.

The DGIST-led team approached the problem from the composition itself. They synthesized a series of five CuIn(S₁₋ₓSeₓ)₂ quantum dot compositions spanning the full range from pure sulfide to pure selenide, carefully holding particle size and cation stoichiometry constant so that any differences in performance could be attributed to the sulfur-to-selenium ratio alone. Their hypothesis was that a carefully chosen mixed-chalcogenide composition could relieve strain in the crystal lattice and thereby reduce the thermodynamic drive to form anion vacancies. The work, published on September 1, 2026, in the open-access journal eScience (DOI: 10.1016/j.esci.2025.100518), reveals that the optimal point lies precisely at an equimolar balance.

Structural characterization told the story with unusual clarity. Transmission electron microscopy confirmed that all five compositions formed uniform particles of approximately 4.3 nanometers, ruling out size effects. X-ray diffraction showed that the equimolar composition, CuIn(S₀.₅Se₀.₅)₂, exhibited the lowest tetragonal lattice distortion of the series, an indication of minimal crystal strain. Extended X-ray absorption fine structure (EXAFS) analysis added a second layer of evidence: the 1:1 composition displayed higher coordination numbers for both copper and indium bonds, meaning that a larger fraction of the cation sites had their full complement of neighboring chalcogen atoms and fewer vacant anion sites existed in the lattice.

Electron paramagnetic resonance (EPR) spectroscopy provided the most direct confirmation of the defect argument. The technique, which is sensitive to unpaired electrons associated with vacancy defects, showed that the equimolar quantum dots contained the lowest concentrations of both sulfur and selenium vacancies of any composition in the series. The consequence of this cleaner crystal structure was visible in the material’s electronic properties. Time-resolved photoluminescence measurements revealed significantly extended carrier lifetimes, and the hole concentration in the optimized dots nearly doubled compared with the pure sulfide quantum dots. In photoelectrochemical terms, more carriers being generated, surviving longer, and reaching the interface translates directly into more efficient water splitting.

When the team assembled the optimized quantum dots into working devices, the results were striking. The researchers sensitized titanium dioxide (TiO₂) photoanodes with the CuIn(S₀.₅Se₀.₅)₂ quantum dots, protected the assembly with combined zinc sulfide (ZnS) and silicon dioxide (SiO₂) passivation layers, and measured a photocurrent density of 15.1 milliamperes per square centimeter at 0.6 volts versus the reversible hydrogen electrode. That figure stands as a record for heavy-metal-free quantum dot systems, bringing the eco-friendly platform into territory previously dominated by cadmium- and lead-based materials. In the photoelectrochemical cell, photogenerated electrons travel to the cathode for hydrogen evolution while holes are consumed by the electrolyte, and the engineered dots shuttle that charge with unprecedented efficiency for a heavy-metal-free system.

Efficiency alone, however, does not make a practical solar fuel technology, and the second half of the team’s result addresses durability. The dual passivation strategy, layering both ZnS and SiO₂ over the quantum dot sensitizer, proved critical for keeping the device alive under operating conditions. The passivated photoanodes maintained Faradaic efficiency above 80 percent for more than six hours of continuous operation, demonstrating that the catalytic activity of the quantum dots survives prolonged illumination and water exposure. For any photoelectrochemical system contemplating industrial relevance, sustained operational stability is as decisive a metric as peak photocurrent, and the passivation scheme offers a concrete, transferable pathway to achieving it.

The research team emphasized that the most exciting aspect of the study is how a seemingly simple adjustment, balancing sulfur and selenium at the atomic scale, resolves what has long been considered the Achilles’ heel of eco-friendly quantum dots. According to the authors, the approach turns an intrinsic weakness of multinary chemistry into a strength, proving that these materials can perform on par with their toxic counterparts without compromising environmental safety. Beyond the performance benchmark itself, they argued, the work establishes a fundamental design principle for defect engineering in multinary quantum dots, one that could accelerate the transition toward sustainable hydrogen energy technologies by giving materials designers a compositional lever to pull rather than relying on post-synthetic fixes.

The implications reach well beyond hydrogen production. Composition-driven defect control of the kind demonstrated here could benefit quantum-dot solar cells, photodetectors, and light-emitting devices, all of which depend on minimizing trap states to preserve charge transport or emission quality. The ability to achieve high efficiency without cadmium or lead opens the door for large-scale deployment in renewable energy infrastructure, particularly in regions pursuing decentralized hydrogen production systems where ease of manufacturing and environmental safety carry extra weight. The research was supported by the National Research Foundation of Korea, the Ministry of Trade, Industry and Energy, and the Korea Institute for Advancement of Technology. As the solar fuels community continues its search for materials that are simultaneously efficient, stable, and safe, this study suggests that sometimes the answer lies not in inventing new compounds but in finding the precise atomic ratio at which an existing one stops sabotaging itself.

Subject of Research: Defect-engineered CuIn(S1-xSex)2 quantum dots for photoelectrochemical solar hydrogen production

Article Title: Tuning sulfur and selenium mix unlocks record solar hydrogen from eco-friendly quantum dots

Article References: Tuning sulfur and selenium mix unlocks record solar hydrogen from eco-friendly quantum dots. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum dots, solar hydrogen, photoelectrochemical water splitting, anion vacancies, chalcogenide, CuIn(S,Se)2, defect engineering, heavy-metal-free, photocurrent, passivation, DGIST, eScience