Hybrid system that mimics natural photosynthesisA new study has made a breakthrough in artificial photosynthesis using captured carbon dioxide that could lead to the production of chemicals and fuels in a completely renewable way. A team from the Lawrence Berkeley National Laboratory and the University of California (UC) Berkeley has developed a system able to capture CO2 emissions before they are vented into the atmosphere, using solar energy to convert it into chemical products such as biodegradable plastics, pharmaceutical drugs and liquid fuels.
To achieve this solar-powered green chemistry, the study, as reported in Nano Letters [Liu et al. Nano Lett. (2015) DOI: 10.1021/acs.nanolett.5b01254], used a hybrid system of semiconducting nanowires and bacteria that can mimic the natural photosynthetic process of plants when they take advantage of the energy from sunlight to synthesize carbohydrates from carbon dioxide and water. The artificial photosynthetic system synthesizes carbon dioxide and water into acetate, the main building block for biosynthesis.
With atmospheric carbon dioxide at its highest level, techniques for sequestering carbon before it escapes into the atmosphere have become crucial. Although the storage of the captured carbon has been a problem until now, this new system – with its four main components of harvesting solar energy, generating reducing equivalents, reducing CO2 to biosynthetic intermediates and producing value-added chemicals – manages to put such carbon dioxide to good use.
"We decided to look in nature and decided on certain bacteria that efficiently reduce CO2 to acetate, which can be used to make more complex and valuable carbon compounds.”Peidong Yang
The team used an artificial forest of nanowire heterostructures made up of silicon and titanium oxide nanowires. When sunlight is absorbed, photo-excited electron–hole pairs are generated in the silicon and titanium oxide nanowires. The photo-generated electrons in the silicon are passed onto bacteria for the CO2 reduction, and the photo-generated holes in the titanium oxide split water molecules to make oxygen. Once the forest has been established, it is populated with microbial populations that produce enzymes that selectively catalyze the reduction of carbon dioxide.
Here, they used Sporomusa ovata, as is easily accepts electrons directly from the surrounding environment and uses them to reduce carbon dioxide. Once the carbon dioxide has been reduced to acetate, E.coli that have been genetically engineered synthesize targeted chemical products. As one of the study leaders, Peidong Yang, said “we decided to look in nature and decided on certain bacteria that efficiently reduce CO2 to acetate, which can be used to make more complex and valuable carbon compounds.”
It was important for the system that requirements for light-capture efficiency and catalytic activity were separate using the nanowire/bacteria hybrid technology, as it improved solar energy conversion efficiency under simulated sunlight to about the same as that of a leaf.