Beyond limits! Plasma-Activated β-FeOOH/CQD-Coated WO₃ Nanoplates Unlock Superior Photoelectrochemical Efficiency

The push to move away from fossil fuels and embrace cleaner energy sources has never been more urgent. With the growing demand for sustainable solutions, researchers have been working hard to develop better ways to generate renewable energy. One promising approach is solar-driven photoelectrochemical (PEC) water splitting, a process that uses sunlight to split water molecules into hydrogen and oxygen. This method has the potential to create green hydrogen, a clean fuel that could significantly cut down carbon emissions and help meet the world’s increasing energy needs. However, despite its potential, PEC water splitting has struggled to become a commercially viable solution. The main limitation is low efficiency and the high amount of energy required to produce hydrogen effectively. For PEC technology to be useful on a large scale, the materials used—especially the photoanodes—need to be both highly efficient and durable. They should be able to absorb plenty of sunlight, separate charge carriers efficiently, and maintain stability over time. Among the materials studied for this purpose, tungsten trioxide (WO₃) has stood out because of its strong chemical stability, well-suited band structure, and excellent electron mobility. That being said, WO₃ is not perfect and has some weaknesses that limit its ability to perform well in real-world applications.

One major issue with WO₃ is its indirect bandgap (~2.53 eV), which means it primarily absorbs ultraviolet light while wasting most of the visible light in the solar spectrum. Since visible light makes up a huge portion of the sunlight reaching Earth, this is a major drawback. Another big problem is charge recombination—the process where electrons and holes cancel each other out before they can be used for the reaction. This drastically reduces efficiency. On top of that, WO₃ has slow charge transfer kinetics, meaning it struggles to move charges where they need to go in the reaction. This slows down the oxygen evolution reaction, which is an important step in PEC water splitting. To tackle these challenges, researchers have tried many different modifications, such as doping, heterostructures, surface treatments, and nanostructuring. But most of these solutions only address one or two issues at a time, leaving other problems unresolved. That is where a new study, published in the Journal of Materials Chemistry A by Jui-Teng Lee, Zhi-Cheng Yan, Kuan-Han Lin, Po-Hsuan Hsiao, Pin-Chao Liao, Ying-Chih Pu, and led by Professor Chia-Yun Chen from National Cheng Kung University in Taiwan, comes in. They developed a dual-modification strategy that significantly boosts WO₃’s performance in PEC applications. Their approach combines three key enhancements: First, they coated WO₃ nanoplates with β-FeOOH (iron oxyhydroxide) nanoclusters, forming a core-shell heterostructure that improves charge separation and carrier transport. Second, they used argon plasma treatment, a method that creates oxygen vacancies on the material’s surface. These vacancies act as active sites that enhance hole transport and catalytic efficiency. Finally, they incorporated carbon quantum dots (CQDs)—tiny nanomaterials known for their unique electronic properties. The CQDs expanded WO₃’s light absorption range, allowing it to utilize more of the visible spectrum while improving overall charge dynamics. The researchers started by carefully crafting WO₃ nanoplates through a hydrothermal method, tweaking reaction conditions to get a smooth, uniform film on fluorine-doped tin oxide (FTO) substrates. When they examined the material’s structure, they confirmed that the nanoplates had a large surface area—an important factor for boosting efficiency. However, when they ran initial tests under light, they quickly saw the usual problems: high charge recombination and poor absorption of visible light, which meant the material was not performing as well as it could. Clearly, more work was needed.

To improve how charge carriers moved through the material, they introduced β-FeOOH nanoclusters onto the WO₃ surface using a solution impregnation technique. By testing different FeOOH concentrations, they found that 5.0 mM was the sweet spot, forming a well-distributed core-shell structure. High-resolution electron microscopy revealed that these nanoclusters stuck well to the WO₃, creating ideal contact points that helped separate charges more efficiently. Additional spectroscopic tests showed Fe²⁺/Fe³⁺ states, which confirmed that the electronic environment had improved, making it easier for holes to move through the material. When they put the FeOOH-modified WO₃ to the test under simulated sunlight, the results spoke for themselves: the photocurrent density nearly doubled which proved that this simple modification significantly reduced charge recombination. Afterward the authors used argon plasma treatment to create oxygen vacancies to increase catalytic activity and generate additional active sites. X-ray photoelectron spectroscopy showed a significant spike in oxygen vacancy density after plasma exposure, with the best results appearing after 120 seconds of treatment. The presence of these vacancies had an immediate impact, creating a more favorable band bending effect at the photoanode/electrolyte interface, which was confirmed using ultraviolet photoelectron spectroscopy. This shift in energy levels made it easier for photogenerated holes to travel efficiently, ultimately lowering the required voltage to drive the reaction. Additional electrochemical impedance spectroscopy tests backed this up, showing a major drop in charge transfer resistance, proving that the material had become much better at moving charges. Compared to FeOOH-modified WO₃ alone, the plasma-treated samples saw a 30% boost in photocurrent, highlighting the clear advantage of vacancy engineering. Wanting to push the material even further, the team added CQDs to the plasma-treated FeOOH/WO₃ system. These tiny nanostructures, created using an ultrasonic method with glucose precursors, were carefully deposited onto the photoanodes using a spin-coating technique. When they zoomed in with electron microscopy, they saw that the CQDs formed a well-dispersed layer, mimicking the staggered arrangement of pine-cone scales—a natural design that optimizes charge transport. Optical tests revealed that the presence of CQDs expanded the material’s light absorption into the visible range, taking advantage of their unique electronic properties. When they tested the CQD-modified photoanodes, the results were impressive: a photocurrent density of 2.18 mA/cm² at 1.23 V vs. RHE, more than three times the performance of unmodified WO₃. Open-circuit voltage measurements showed a cathodic shift of 230 mV, proving that CQDs were actively helping with charge transfer and reducing recombination losses. Further testing using incident photon-to-current conversion efficiency (IPCE) measurements provided deeper insights. The unmodified WO₃ sample had a low IPCE of under 10% in the visible range, confirming its weak ability to harness sunlight. The FeOOH-modified samples did better, but the real breakthrough came with the CQD-functionalized photoanodes. These maintained an average IPCE of 11.49% in the 400–550 nm range and peaked at 85.6% at 310 nm, proving that CQDs made a dramatic difference in how much light the material could use. To check how well the modified photoanodes would hold up over time, the team ran stability tests under continuous light exposure. The CQD-enhanced, plasma-treated FeOOH/WO₃ photoanodes held on to 98% of their original photocurrent after an hour, showing excellent durability. Additional faradaic efficiency measurements confirmed that 88% of the photogenerated holes contributed to oxygen evolution, meaning very little energy was wasted. The combination of high efficiency, long-term stability, and simple, scalable synthesis methods positioned this material as a strong candidate for future solar-driven hydrogen production.

To put their work in perspective, the team compared their results to other WO₃-based photoanodes. They looked at WO₃/MoS₂, WO₃/CuWO₄, WO₃/BiVO₄, and several other advanced designs. The comparison confirmed that their dual-modification strategy outperformed most existing solutions under identical conditions. The combination of FeOOH, plasma treatment, and CQDs created a level of PEC activity that had not been seen before in WO₃-based systems. Not only does this research provide a new pathway for improving PEC efficiency, but it also lays the groundwork for future breakthroughs in solar-to-hydrogen conversion technologies. In conclusion, the research work of Professor Chia-Yun Chen and colleagues is a significant advancement for more efficient photoanodes that can power solar-driven hydrogen production with the use of FeOOH heterostructures, oxygen vacancies induced by plasma treatment, and CQDs, the authors created a powerful system that boosts charge separation, improves carrier movement, and expands light absorption—all key ingredients for making PEC performance significantly better.