Scientists find a way to use visible light to decompose CO2 with high efficiency

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The photoexcited electron from silver iodide (AgI) travels along the carbon nanotube to silver iodate (AgIO3) where carbon dioxide (CO2) is reduced to carbon monoxide (CO) | Image courtesy: Shinji Kawasaki and Yosuke Ishii from Nagoya Institute of Technology

To tackle the challenge of global warming, scientists have been looking into green and sustainable methods of breaking down carbon dioxide in emissions and in the atmosphere. Now, a group of researchers from Nagoya Institute of Technology, Japan, have developed a novel, easy to synthesize composite compound that enables the efficient use of solar energy to reduce carbon dioxide, taking us one step closer to achieving a green economy.

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. Perhaps, humanity’s future is bright after all!