US researchers advance artificial photosynthesis

U.S. researchers have created a hybrid system of semiconducting nanowires and bacteria to mimic the natural photosynthetic process, an advancement they believe could be a potentially game-changing breakthrough.

The system is designed to capture carbon dioxide (CO2) emissions before they are vented into the atmosphere and then, powered by solar energy, convert that greenhouse gas into chemical products, including biodegradable plastics, pharmaceutical drugs and hopefully liquid fuels.

"We believe our system is a revolutionary leap forward in the field of artificial photosynthesis," said Peidong Yang, a chemist with the Materials Sciences Division of the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory and one of the leaders of this study, in a press release.

Yang also holds appointments with University of California at Berkeley.

In the natural photosynthetic process, plants use the energy in sunlight to synthesize carbohydrates from carbon dioxide and water.

As described in the latest issue of the journal Nano Letters, the artificial photosynthetic system, built by Yang and his colleagues with the Berkeley Lab and UC Berkeley, synthesizes the combination of carbon dioxide and water into acetate, the most common building block today for biosynthesis.

"In natural photosynthesis, leaves harvest solar energy and carbon dioxide is reduced and combined with water for the synthesis of molecular products that form biomass," said Christopher Chang, an expert in catalysts for carbon-neutral energy conversions and another leader of the study.

"In our system," Chang explained, "nanowires harvest solar energy and deliver electrons to bacteria, where carbon dioxide is reduced and combined with water for the synthesis of a variety of targeted, value-added chemical products."

"Our system represents an emerging alliance between the fields of materials sciences and biology, where opportunities to make new functional devices can mix and match components of each discipline, " said Michelle Chang, an expert in biosynthesis who led the project together with the two other scientists.

The system starts with an "artificial forest" of nanowire heterostructures, consisting of silicon and titanium oxide nanowires. "Our artificial forest is similar to the chloroplasts in green plants," Yang said. "When sunlight is absorbed, photo- excited electron hole pairs are generated in the silicon and titanium oxide nanowires, which absorb different regions of the solar spectrum. The photo-generated electrons in the silicon will be passed onto bacteria for the CO2 reduction while the photo- generated holes in the titanium oxide split water molecules to make oxygen."

Once the forest of nanowire arrays is established, it is populated with microbial populations that produce enzymes known to selectively catalyze the reduction of carbon dioxide. And once the carbon dioxide has been reduced to acetate or some other biosynthetic intermediate, genetically engineered E.coli are used to synthesize targeted chemical products.

A key to the success of the artificial photosynthesis system is the separation of the demanding requirements for light-capture efficiency and catalytic activity that is made possible by the nanowire/bacteria hybrid technology. With this approach, the Berkeley team achieved a solar energy conversion efficiency of up to 0.38-percent for about 200 hours under simulated sunlight, which is about the same as that of a leaf.

"We are currently working on our second generation system which has a solar-to-chemical conversion efficiency of three-percent," Yang said. "Once we can reach a conversion efficiency of 10- percent in a cost effective manner, the technology should be commercially viable."

"Our system has the potential to fundamentally change the chemical and oil industry in that we can produce chemicals and fuels in a totally renewable way, rather than extracting them from deep below the ground," he said.