To grow and thrive, plants have evolved to perform spellbinding alchemy—with chlorophyll-filled leaves, they capture carbon dioxide (CO2) and sunlight, transforming and storing them as energy-filled sugars. In a bid to slow the snowballing effects of climate change, researchers in sustainable technologies worldwide are eying a similar principle to create ‘clean’ fuels.
Photocatalysts, which harness solar energy to accelerate chemical reactions, are a class of materials at the heart of these green energy-converting systems. In particular, metal oxide-based photocatalysts are gaining traction as a potent, low-cost option to turn CO2 into useful solar fuels like methane and methanol at an industrial scale. However, while they hold immense potential, they need to be optimised prior to wider adoption, say researchers.
“Before metal oxide-based photocatalysts can be feasibly used in industrial applications, we need to enhance their performance significantly,” said Enyi Ye, a Principal Scientist I at A*STAR’s Institute of Materials Research and Engineering (IMRE) and Institute of Sustainability for Chemicals, Energy and Environment (ISCE2). “This requires a more in-depth understanding of the role of oxygen valencies (OV), which would enable superior photocatalyst designs.”
OVs are tiny gaps caused by missing oxygen atoms in the structures of metal oxides. Once thought of as undesirable defects that might hinder catalytic reactions, recent findings suggest that OVs, if carefully placed within a photocatalyst's structure, might in fact enhance its abilities.
To scope out the current state of the art of OV engineering, Ye and colleagues from IMRE and ISCE2 collaborated with researchers from the Hefei National Laboratory for Physical Sciences at the Microscale and the University of Science and Technology of China to comprehensively review the field's challenges, breakthroughs and opportunities.
“OVs influence all the stages of a photocatalyst’s functionality: light harvesting, charge separation and CO2 reduction,” explained Ye. “These defects not only promote the conversion of more CO2 molecules on the catalyst’s surface, but also coax the formation of more desirable compounds that can be used as fuel."
The team highlighted the ‘doping’ of photocatalysts with small amounts of various metals as a common way to promote OV formation in metal oxide structures, enhancing their stability and efficiency versus 'pristine' counterparts.
To drive further progress in photocatalyst design, Ye proposed three avenues to explore. Firstly, depositing or coating nanoparticles on catalyst surfaces may form metal/oxide blends that improve OV formation and CO2 conversion. Secondly, a suite of characterisation tools can be used to gain deeper structural insights into OVs in metal oxides and their effect on CO2 conversion. Lastly, the possibility of OVs being optimal sites for depositing other catalytically active components should be explored, opening the door for more powerful hybrid nanostructures.
Looking ahead, the team plans to explore how adding heat and electricity into the mix can galvanise stronger photocatalytic reactions. “We're also eager to delve deeper into these catalysts at an atomic level and tweak individual atoms to gain greater control over their structure,” said Ye.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).
