Nanomaterials: The Power of Absence

The sun delivers vast amounts of energy to the Earth daily, yet only a fraction is currently utilized economically. One promising method to convert solar energy into storable energy is splitting water (H₂O) into hydrogen (H₂) and oxygen (O₂). “This chemical reaction holds enormous future potential: H₂ as a climate-friendly energy carrier, water as an abundant resource, and no harmful side reactions, just a few advantages. At the same time, photoelectrochemical water splitting is considered one of the ultimate challenges in the energy and catalysis research community,” explains Emiliano Cortés, Professor of Experimental Physics at LMU. The biggest hurdle: The materials used so far are not efficient enough to make the technology competitive on a large scale. Now, an international, interdisciplinary research team led by Prof. Min Liu from Central South University (CSU) at Changsha, China, and Prof. Cortés at LMU, one of the leading PIs of the e-conversion Excellence Cluster, has gained crucial insights into improving a key step – charge separation. The findings were recently published in Nature Catalysis.

Keeping charges separated for longer

The researchers focused on metal oxides – solid materials composed of a transition metal and oxygen. The team investigated a series of oxides – tungsten, titanium, and bismuth – which are among the most promising material classes for photoelectrochemical water splitting due to their semiconductor and light-absorbing properties. A key step in the process is charge separation induced by solar energy. When sunlight hits a semiconducting material, charge carriers (negatively charged electrons) jump to a higher energy level, leaving an energetic gap – creating a positively charged “hole” at the lower energy level. These separated charges are then available for water splitting, enabling the production of hydrogen and oxygen. “One major challenge is that electrons and holes can quickly recombine, making them unavailable for the desired reaction,” says Cortés. “To prevent this and maintain charge separation as long as possible, we deliberately introduce metal defects by removing a fraction of these atoms.” The researchers were already aware that oxygen vacancies can strongly influence material performance so they went the other way around.

More mobility through metal defects

The researchers aimed to create metal vacancies instead of focusing on oxygen vacancies. The idea: Increase the mobility of the holes to reduce the chance of recombination with electrons. “We know that excited electrons are already highly mobile, whereas holes tend to be static or very slow and heavy,” explains the LMU physicist. “By making them more mobile, they have a better chance of participating in water splitting rather than recombining.” The team’s measurements confirmed that this strategy works surprisingly well. In tungsten oxide, for example, the mobility of positive charge carriers (holes) increased by 430 percent due to metal vacancies. This greater mobility extended the lifetime of electron-hole pairs to twelve nanoseconds, compared to just five nanoseconds without metal vacancies. For photoelectrochemical reactions, this timeframe is sufficient and significantly improves material efficiency. Additionally, the material became more stable overall. “With our strategy, I think we have found a universal approach to improving the inherently low hole mobility in metal oxides,” explains Jun Wang, lead author of the study and a researcher in the team of Prof. Min Liu at CSU. “These findings provide valuable guidance for developing devices for photoelectrochemical water splitting based on fast charge separation.”

Valuable impulses for solar water splitting

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This breakthrough was made possible by the close collaboration of interdisciplinary scientists. A highlight in this sense was the research stay of an LMU PhD student — funded through an e-conversion exchange grant – at the collaborating Central South University in Changsha, China. There, state-of-the-art measurement techniques enabled precise investigation of the mechanisms behind metal vacancies. “I am very grateful for the experiences I gained during my four-month research stay in Prof. Liu’s team. It was an exciting project where I connected with outstanding experts. Our work provides valuable insights for designing materials for solar water splitting,” says Yicui Kang, a PhD student in Cortés’ research group.

Photoelectrochemical water splitting is a key process for sustainable hydrogen production. “Our results advance the usage of sun, beyond solar cells and electricity, showing that also solar fuels – like solar hydrogen from water – could be soon among us, replacing fossil fuels” emphasizes Cortés. “Furthermore, our research results provide important impulses for applications in several fields, including optoelectronics, energy, environment, and catalysis.”

Jun Wang, Kang Liu, Wanru Liao et al.: Metal vacancies in semiconductor oxides enhance hole mobility for efficient photoelectrochemical water splitting. Nature Catalysis 2025