Using visible light to efficiently decompose carbon dioxide

Carbon dioxide (CO₂) emissions from human activities have risen drastically over the last century and a half and are seen as the primary cause of global warming and abnormal weather patterns. So, there has been considerable research focus, in a number of fields, on lowering our CO₂ emissions and its atmospheric levels. One promising strategy is to chemically break down, or 'reduce,' CO₂ using photocatalysts—compounds that absorb light energy and provide it to reactions, speeding them up. With this strategy, the solar powered reduction of CO₂, where no other artificial source of energy is used, becomes possible, opening doors to a sustainable path to a sustainable future.

A team of scientists led by Drs. Shinji Kawasaki and Yosuke Ishii from Nagoya Institute of Technology, Japan, has been at the forefront of efforts to achieve efficient solar-energy-assisted CO₂ reduction. Their recent breakthrough is published in Nature's Scientific Reports.

Their research began with the need to solve the limited applicability problem of silver iodate (AgIO₃), a photocatalyst that has attracted considerable attention for being useful for the CO₂ reduction reaction. The problem is that AgIO₃ needs much higher energy than that which visible light can provide to function as an efficient photocatalyst; and visible light is the majority of solar radiation.

Scientists have attempted to work around this efficiency problem by combining AgIO₃ with silver iodide (AgI), which can efficiently absorb and utilize visible light. However, AgIO₃-AgI composites have complicated synthesis processes, making their large-scale manufacturing impractical. Further, they don't have structures that offer efficient pathways for the transfer of photoexcited electrons (electrons energized by light absorption) from AgI to AgIO₃, which is key to the composite's catalytic activity.

"We have now developed a new photocatalyst that incorporates single-walled carbon nanotubes (SWCNTs) with AgIO₃ and AgI to form a three-component composite catalyst," says Dr. Kawasaki, "The role of the SWCNTs is multimodal. It solves both the synthesis and the electron transfer pathway problems."

The three-component composite's synthesis process is simple and involves just two steps: 1. Encapsulating iodine molecules within the SWCNT using an electrochemical oxidation method; and 2. Preparing the composite by immersing the resultant of the previous step in an aqueous solution of silver nitrate (AgNO₃).

Spectroscopic observations using the composite showed that during the synthesis process, the encapsulated iodine molecules received charge from the SWCNT and converted into specific ions. These then reacted with AgNO₃ to form AgI and AgIO₃ microcrystals, which, due to the initial positions of the encapsulated iodine molecules, were deposited on all the SWCNTs uniformly. Experimental analysis with simulated solar light revealed that the SWCNTs also acted as the conductive pathway through which photoexcited electrons moved from AgI to AgIO₃, enabling the efficient reduction of CO₂ to carbon monoxide (CO).

The incorporation of SWCNTs also allowed for the composite dispersion to be easily spray-coated on a thin film polymer to yield flexible photocatalytic electrodes that are versatile and can be used in various applications.

Dr. Ishii is hopeful about their photocatalyst's potential. "It can make the solar reduction of industrial CO₂ emissions and atmospheric CO₂ an easy-to-scale and sustainable renewable energy-based solution tackling global warming and climate change, making people's lives safer and healthier," he says.

The next step, the team says, is to explore the possibility of using their photocatalyst for solar hydrogen generation.