A team of researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) developing a bioinorganic hybrid approach to artificial photosynthesis have achieved another milestone. Having generated quite a buzz with their hybrid system of semiconducting nanowires and bacteria that used electrons to synthesize carbon dioxide into acetate, the team has now developed a hybrid system that produces renewable molecular hydrogen and uses it to synthesize carbon dioxide into methane, the primary constituent of natural gas.

“This study represents another key breakthrough in solar-to-chemical energy conversion efficiency and artificial photosynthesis,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and one of the leaders of this study. “By generating renewable hydrogen and feeding it to microbes for the production of methane, we can now expect an electrical-to-chemical efficiency of better than 50 percent and a solar-to-chemical energy conversion efficiency of 10-percent if our system is coupled with state-of-art solar panel and electrolyzer.”

Yang, who also holds appointments with UC Berkeley and the Kavli Energy NanoScience Institute (Kavli-ENSI) at Berkeley, is one of three corresponding authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS). The paper is titled “A hybrid bioinorganic approach to solar-to-chemical conversion.” The other corresponding authors are Michelle Chang and Christopher Chang. Both also hold joint appointments with Berkeley Lab and UC Berkeley. In addition, Chris Chang is a Howard Hughes Medical Institute (HHMI) investigator. (See below for a full list of the paper’s authors.)

Artificial photosynthesis used to produce renewable molecular hydrogen for synthesizing carbon dioxide into methane. Credit: Berkeley Lab

Photosynthesis is the process by which nature harvests the energy in sunlight and uses it to synthesize carbohydrates from carbon dioxide and water. Carbohyrates are biomolecules that store the chemical energy used by living cells. In the original hybrid artificial photosynthesis system developed by the Berkeley Lab team, an array of silicon and titanium oxide nanowires collected solar energy and delivered electrons to microbes which used them to reduce carbon dioxide into a variety of value-added chemical products. In the new system, solar energy is used to split the water molecule into molecular oxygen and hydrogen. The hydrogen is then transported to microbes that use it to reduce carbon dioxide into one specific chemical product, methane.

“In our latest work, we’ve demonstrated two key advances,” says Chris Chang. “First, our use of renewable hydrogen for carbon dioxide fixation opens up the possibility of using hydrogen that comes from any sustainable energy source, including wind, hydrothermal and nuclear. Second, having demonstrated one promising organism for using renewable hydrogen, we can now, through synthetic biology, expand to other organisms and other value-added chemical products.”

The concept in the two studies is essentially the same – a membrane of semiconductor nanowires that can harness solar energy is populated with bacterium that can feed off this energy and use it to produce a targeted carbon-based chemical. In the new study, the membrane consisted of indium phosphide photocathodes and titanium dioxide photoanodes. Whereas in the first study, the team worked with Sporomusa ovata, an anaerobic bacterium that readily accepts electrons from the surrounding environment to reduce carbon dioxide, in the new study the team populated the membrane with Methanosarcina barkeri, an anaerobic archaeon that reduces carbon dioxide using hydrogen rather than electrons.

“Using hydrogen as the energy carrier rather than electrons makes for a much more efficient process as molecular hydrogen, through its chemical bonds, has a much higherdensity for storing and transporting energy,” says Michelle Chang.

In the newest membrane reported by the Berkeley team, solar energy is absorbed and used to generate hydrogen from water via the hydrogen evolution reaction (HER). The HER is catalyzed by earth-abundant nickel sulfide nanoparticles that operate effectively under biologically compatible conditions. Hydrogen produced in the HER is directly utilized by the Methanosarcina barkeri archaeons in the membrane to produce methane.

“We selected methane as an initial target owing to the ease of product separation, the potential for integration into existing infrastructures for the delivery and use of natural gas, and the fact that direct conversion of carbon dioxide to methane with synthetic catalysts has proven to be a formidable challenge,” says Chris Chang. “Since we still get the majority of our methane from natural gas, a fossil fuel, often from fracking, the ability to generate methane from a renewable hydrogen source is another important advance.”

Adds Yang, “While we were inspired by the process of natural photosynthesis and continue to learn from it, by adding nanotechnology to help improve the efficiency of natural systems we are showing that sometimes we can do even better than nature.”

The problem has been that the vast majority of these atomically thin 2D crystals are unstable in air, so react and decompose before their properties can be determined and their potential applications investigated.

Writing in Nano Letters, the University of Manchester team demonstrate how tailored fabrication methods can make these previously inaccessible materials useful.

By protecting the new reactive crystals with more stable 2D materials, such as graphene, via computer control in a specially designed inert gas chamber environments, these materials can be successfully isolated to a single atomic layer for the first time.

Combining a range of 2D materials in thin stacks give scientists the opportunity to control the properties of the materials, which can allow ‘materials-to-order’ to meet the demands of industry.

High-frequency electronics for satellite communications, and light weight batteries for mobile energy storage are just two of the application areas that could benefit from this research. The breakthrough could allow for many more atomically thin materials to be studied separately as well as serve as building blocks for multilayer devices with such tailored properties.

The team, led by Dr Roman Gorbachev, used their unique fabrication method on two particular two-dimensional crystals that have generated intense scientific interest in the past 12 months but are unstable in air: black phosphorus and niobium diselenide.

The technique the team have pioneered allows the unique characteristics and excellent electronic properties of these air-sensitive 2D crystals to be revealed for the first time.

The isolation of graphene in 2004 by a University of Manchester team lead by Sir Andre Geim and Sir Kostya Novoselov led to the discovery of a range of 2D materials, each with specific properties and qualities.

Dr Gorbachev said: “This is an important breakthrough in the area of 2D materials research, as it allows us to dramatically increase the variety of materials that we can experiment with using our expanding 2D crystal toolbox.

“The more materials we have to play with, the greater potential there is for creating applications that could revolutionise the way we live.” Sir Andre Geim added.

The scientists whose job it is to test the limits of what nature–specifically chemistry– will allow to exist, just set up shop on some new real estate on the Periodic Table. Using a method they invented for joining disparate elemental layers into a stable material with uniform, predictable properties, Drexel University researchers are testing an array of new combinations that may vastly expand the options available to create faster, smaller, more efficient energy storage, advanced electronics and wear-resistant materials.

Led by postdoctoral researcher Babak Anasori, PhD, a team from Drexel’s Department of Materials Science and Engineering created the material-making method, that can sandwich 2-D sheets of elements that otherwise couldn’t be combined in a stable way. And they proved its effectiveness by creating two entirely new, layered two-dimensional materials using molybdenum, titanium and carbon.

“By ‘sandwiching’ one or two atomic layers of a transition metal like titanium, between monoatomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced,” Anasori said. “It was impossible to produce a 2-D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesize them.”

The discovery, which was recently published in the journal ACS Nano, is significant because it represents a new way of combining elemental materials to form the building blocks of energy storage technology–such as batteries, capacitors and supercapacitors, as well as superstrong composites–like the ones used in phone cases and body armor. Each new combination of atom-thick layers presents new properties and researchers suspect that one, or more, of these new materials will exhibit energy storage and durability properties so disproportional to its size that it could revolutionize technology in the future.

“While it’s hard to say, at this point, exactly what will become of these new families of 2-D materials we’ve discovered, it is safe to say that this discovery enables the field of materials science and nanotechnology to move into an uncharted territory,” Anasori said.

Mastering materials

Combining two-dimensional sheets of elements in an organized way to produce new materials has been the goal of Drexel nanomaterials researchers for more than a decade. Imposing this sort of organization at the atomic level is no easy task.

“Due to their structure and electric charge, certain elements just don’t ‘like’ to be combined,” Anasori said. “It’s like trying to stack magnets with the poles facing the same direction–you’re not going to be very successful and you’re going to be picking up a lot of flying magnets.”

But Drexel researchers came up with a clever way to circumvent this chemistry challenge. It starts with a material called a MAX phase, which was discovered by Distinguished Professor Michel W. Barsoum, PhD, head of the MAX/MXene Research Group, more than two decades ago. A MAX phase is like the primordial ooze that generated the first organisms–all the elements of the finished product are in the MAX phase, waiting for the researchers to impose some order.

That order was imposed by Michel W. Barsoum, PhD and Yury Gogotsi, PhD, Distinguished University and Trustee Chair professor in the College of Engineering and head of the Drexel Nanomaterials Group, when they first created a stable, two-dimensional, layered material called MXene in 2011.

To create MXenes, the researchers selectively extract layers of aluminum atoms from a block of MAX phase by etching them out with an acid.

“Think of MXene synthesis like separating layers of wood by dunking a plywood sheet into a chemical that dissolves the glue,” Anasori said. “By putting a MAX phase in acid, we have been able to selectively etch away certain layers and turn the MAX phase into many thin 2-D sheets, which we call MXenes.”

As far as energy storage materials go, MXenes were a revelation. Prior to their discovery, graphene, which is a single sheet of carbon atoms, was the first two-dimensional material to be touted for its potential energy storage capabilities. But, as it was made up of only one element, carbon, graphene was difficult to modify in form and therefore had limited energy storage capabilities. The new MXenes have surfaces that can store more energy.

An elemental impasse

Four years later, the researchers have worked their way through the section of the Periodic Table with elements called “transition metals,” producing MAX phases and etching them into MXenes of various compositions all the while testing their energy storage properties.

Anasori’s discovery comes at a time when the group has encountered an obstacle on its progress through the table of elements.

“We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes,” Anasori said. “By adding titanium to the mix we managed to make an ordered molybdenum MAX phase, where the titanium atoms are in center and the molybdenum on the outside.

The next frontier

Now, with the help of theoretical calculations done by researchers at the FIRST Energy Frontier Research Center at the Oak Ridge National Laboratory, Drexel’s team knows that, in principle, it can use this method to make as many as 25 new materials with combinations of transition metals, such as molybdenum and titanium, that previously wouldn’t have been attempted.

“Having the possibility to layer different elements at the thinnest form of material known to the scientific community leads to exciting new structures and allows unprecedented control over materials properties,” Barsoum said. “This new layering method gives researchers an unimaginable number of possibilities for tuning materials’ properties for a variety of high-tech applications.”

Anasori plans to make more materials by replacing titanium with other metals, such as vanadium, niobium, and tantalum, which could unearth a vein of new physical properties that support energy storage and other applications.

“This level of structural complexity, or layering, in 2-D materials has the potential to lead to many new structures with unique control over their properties,” Gogotsi said. “We see possible applications in thermoelectrics, batteries, catalysis, solar cells, electronic devices, structural composites and many other fields, enabling a new level of engineering on the atomic scale.”