In September 2015, chemistry professor Peidong Yang was notified that he had won a prestigious MacArthur Fellows Program “genius” award.
Says Yang, “It was an amazing experience to win a MacArthur fellowship. I think it has to do with the nature of the award, which makes it very accessible to the public. The media interest has been almost overwhelming.”
According to the foundation, the fellowships recognize “talented individuals who have shown extraordinary originality and dedication in their creative pursuits and a marked capacity for self-direction.” Yang’s research accomplishments in the months since the award are confirmation of the foundation’s confidence in his scientific creativity.
Yang, 44, was born in Suzhou, China, near Shanghai. He attended the University of Science and Technology of China (USTC) in Hefei from 1988 to 1993, earning a B.A. in the university’s fiveyear chemistry degree program.
In 1993 Yang moved on to the research group of Charles Lieber at Harvard University, where he received his doctorate in 1997. Yang then came to California for a postdoc with Galen Stucky at UC Santa Barbara. There he studied nanostructures and high-surface-area silica. He joined the Berkeley faculty in 1999.
Since his arrival at Berkeley, Yang’s research has blossomed, and he now oversees the country’s leading research group in nanowire fabrication. These nanowires are long thin structures less than 100 nanometers in diameter, typically made of a semiconductor material. They have applications from fiber-optic communications and inexpensive solar cells to waste heat recovery and the production of carbon-neutral fuels.
For Yang, the MacArthur genius award is an affirmation of the importance of a task he set for himself 12 years ago—to create an artificial photosynthesis system using nanotechnology, catalysts and robust inorganic and physical chemistry.
Yang is seeking the direct conversion of sunlight to fuels, a process that stores the sun’s energy in chemical bonds. This is known as artificial photosynthesis because it mimics the natural version, which allows plants to store solar energy as glucose and other simple carbohydrates.
Yang intends not only to mimic photosynthesis, but also to improve it. In order to grow, plants must take glucose and convert it to its polymer, cellulose. To assure growth even in the shade or on cloudy days, photosynthesis is optimized to work in low light levels, which wastes the majority of photons that fall upon the plants in bright sunlight.
Explains Yang, “Artificial photosynthesis devices would not have the same constraints that plants face. A robust synthetic system built with inorganic and biological components can be optimized to extract energy from all the photons that hit it. And because the system doesn’t have to grow like a plant, it can use almost all the photons to make fuel and other chemicals.”
Fortunately, planet Earth has photons to spare. The sunlit side of the Earth is constantly illuminated with more than 100,000 trillion watts of solar energy.If we could harness just a small part of that flow of energy, 17 trillion watts, we could meet the current demand of humankind.
While photovoltaic panels and windmills produce renewable energy without creating carbon dioxide, we still lack ways to reliably store their electrical energy for when the sun doesn’t shine and the wind doesn’t blow.
In addition, during the 20th century, humankind embarked on a building project of unprecedented scale—the infrastructure to produce, transport and burn liquid hydrocarbon fuels. It would be unwise to turn our backs on this infrastructure while at the same time attempting to meet the needs of our planet’s growing population.
Artificial photosynthesis is the breakthrough that could bring the two pieces of the problem together—carbon-neutral “drop-in” fuels that could utilize existing infrastructure. But it is turning out to be a hard problem. “Evolution has had millions of years to perfect photosynthesis, while my lab has only been working on the problem for 10 to 12 years,” Yang points out. “We’ve made great progress, but we have a long way to go.”
That goal moved much closer to reality in 2013 when the Yang lab constructed an integrated nanowire device to split water into hydrogen and oxygen. In Yang’s device, water splitting is done by a forest of silicon nanowire “tree trunks” with smaller nanowire titanium dioxide “branches.” The branches and trunks are coated with nanoparticle catalysts that make the reactions run more efficiently (see illustration).
Says Yang, “The efficiency of the system is still low, less than one percent, about the same as natural photosynthesis. But the beauty of this system is that we can test different semiconductor compo- nents and we can change the architecture of the system by varying the structure of the nanowires.
“However,” Yang adds, “water splitting is only half of the story in photosynthesis. The other half is carbon dioxide reduction, which breaks the carbon-oxygen double bonds and frees the carbon atoms to be used to produce simple compounds like sugar and fuels. Finding an efficient and selective catalyst for CO2 reduction is one of the biggest challenges in artificial photosynthesis.”
CO2 reduction is a tough problem for natural photosynthesis, too. Plants rely on a slow, nonselective enzyme known as Rubisco. Yang has sought a robust, selective, fast-acting inorganic analogue for Rubisco, and in 2014 his lab made an important first step.
In that year, the Yang lab created a new system for building nanoparticles that allowed researchers to hunt more effectively for the illusive CO2 reduction catalyst. Explains Yang, “We don’t really have enough fundamental understanding to design an outstanding electrocatalyst from first principles. We are exploring the problem with gold-copper bimetallic nanoparticles of different compositions, which are well-defined platforms for learning more about catalytic activity.” Through this effort the Yang group discovered a nanocatalyst with a composition of Au3Cu that has the highest known mass activity for the electrochemical conversion of CO2 to CO.
But overall progress on CO2 reduction electrocatalysts has been slow, and Yang wasn’t going to let the lack of a good inorganic catalyst impede his progress. Instead, he began working on hybrid inorganic/ biological solutions. He knew that some single-cell organisms can reduce CO2 very effectively.
In 2008, he started a program studying how bacteria would interact with semiconductor nanostructures. After demonstrating that these bacteria can happily interact with high-surface-area semiconductor nanostructures, Yang spent a few years perfecting his lab’s techniques and then teamed up with College of Chemistry chemical biologists Michelle and Chris Chang. Together they began to create hybrid systems to make carbon compounds. This approach quickly began to bear fruit, and the year 2015 saw three major breakthroughs.
The first success started with the bacterium Sporomusa ovata. This organism is both an electrotroph and an acetogen—it uses electrons to reduce carbon dioxide to acetate, a simple organic compound with one carbon-carbon bond. Yang and colleagues integrated the S. ovata bacteria directly into a nanowire photovoltaic device that, like solar panels, produces electricity from sunlight. The microbes nestled securely between individual nanowires.
When illuminated, the hybrid device absorbed photons, creating energetic electrons that the S. ovata bacteria used to reduce CO2. The end product, acetate, is a precursor to biofuels and other valuable chemicals.
But Yang and colleagues didn’t stop there. They next turned their attention to Methanosarcina barkeri, a single-cell organism that is a member of the domain of Archaea, the close cousins of bacteria. When fed both hydrogen and carbon dioxide, this organism can reduce CO2 to methane, the main component of natural gas.
Based on their previous experiments and the Yang lab’s expertise with nanoscale semiconductors and catalysts, Yang and colleagues constructed a light-driven device that split water into oxygen and hydrogen and fed the hydrogen, along with CO2, to a solution containing M. barkeri organisms. The microbes churned out methane.
“This study represents another breakthrough in solar-to-chemical energy conversion efficiency and artificial photosynthesis,” Yang explains. “And it is a system that can be scaled up. If we start with state-of-the-art solar panels and commercial electrolyzers, we can convert sunlight to hydrogen with almost 20 percent efficiency.
“By feeding this renewable hydrogen to microbes for the production of methane, we currently get a hydrogen-to-methane conversion efficiency of better than 50 percent. Putting the two together, the overall sunlight-to-methane energy conversion efficiency is about 10 percent—much higher than that of natural photosynthesis.”
Yang’s third and most recent breakthrough takes the integration of the inorganic and the biological to a new level. Instead of putting microbes into inorganic light-harvesting devices, why not have bacteria grow their own semiconductor light-harvesting nanoparticles?
Although at first it sounds like science fiction, this is precisely what the Yang lab has accomplished. Yang’s first breakthrough in early 2015 started with the bacteria S. ovata. Yang is now working with Moorella thermoacetica, which is also both an electrotroph and an acetogen—it uses electrons to reduce CO2 to acetates, in particular hydrogen acetate (better known as acetic acid).
To produce cadmium sulfide nanoparticles, Yang grew M. thermoacetica in a glucose culture supplemented with cysteine, a sulfur-containing amino acid, and cadmium nitrate, Cd(NO3)2. Cadmium sulfide (CdS) nanoparticles precipitated and became embedded on the cell surface.
Cadmium sulfide is a well-known semiconductor that enables the microbe to conduct artificial photosynthesis. When light strikes the CdS nanoparticles, it creates electron/hole pairs, just like in a conventional semiconductor. The electrons produced by the nanoparticles reduce CO2 to acetic acid via what biochemists call the Wood-Ljungdahl pathway within the bacteria, while the holes are quenched by grabbing electrons from cysteine molecules and oxidizing them to the related disulfide compound, cystine.
“About 90 percent of the reduced CO2 goes to producing acetic acid, while 10 percent is used for cell growth. The bacterial cells continue to reproduce, suggesting the possibility of a completely self-reproducing hybrid organism sustained purely by solar energy, assuming we can find a way to stabilize the concentration of cysteine and cadmium ions in the system.”
Evolution has provided Yang with a bacterium that can grow its own semiconductor nanoparticle light harvesters. But there is no reason to stop there. Genetic modification of yeast and bacteria has turned them into miniature factories for producing critical drugs like human insulin. These same techniques can also be applied to model organisms like E. coli to allow them to synthesize light-harvesting nanoparticles.
In addition, bioengineered metabolic pathways could produce a variety of chemicals and fuels. Such organisms would be miniature solar-powered chemical and fuel factories, but ones based on principles very different from natural photosynthesis.
“More importantly,” Yang adds, “by working with these bacteria, we can actually learn the inner machinery of CO2 reduction chemistry within them, and hopefully one day we can design and synthesize in the lab a robust inorganic electrocatalyst that would have the same activity and selectivity as the bacteria.”
“Human population has surpassed seven billion,” Yang says, “and will hit 10 billion by around the year 2050. By then we’ll need to supply energy at a rate approaching 40 trillion watts. We won’t be able to meet that challenge with fossil fuels without affecting the health and quality of life on the whole planet.”
“Artificial photosynthesis has the potential to be a real alternative to fossil fuels. While we were inspired by the process of natural photosynthesis and continue to learn from it, by using nanotechnology to help improve the efficiency of natural systems we are showing that sometimes we can do even better than nature.”
The Yang lab has constructed an integrated nanowire device that splits water into hydrogen and oxygen. Water splitting is done by a forest of silicon nanowire “tree trunks” with smaller nanowire titanium dioxide “branches.” The branches and trunks are coated with nanoparticle catalysts that make the reactions run more efficiently. When immersed in solution and illuminated with sunlight, energetic photons in the ultraviolet range create electron/hole pairs in the TiO2 branches. The positively charged holes migrate through the semiconductor material to its surface, where they encounter water molecules. The holes strip electrons from water molecules, freeing O2 gas and allowing protons to pass into the solution.
Meanwhile photons in the visible range create electron/hole pairs in the silicon trunk of the nanowire. Here it is the electrons that migrate to the nanowire surface, where they encounter the protons in the solution, reducing them to H2 gas.