New catalyst promotes artificial photosynthesis

Scientists have created an oxygen-evolution catalyst that combines with semiconductors for solar water splitting, an advance that assists the conversion of solar energy to chemical energy in the form of hydrogen and oxygen.

When evenly applied to a semiconductor, the film catalyses solar water splitting for energy production and other applications
When evenly applied to a semiconductor, the film catalyses solar water splitting for energy production and other applications

The discovery was made in the lab of Kenton Whitmire, a Rice University professor of chemistry, with assistance from researchers at the University of Houston.

They found that growing a layer of an active catalyst directly on the surface of a light-absorbing nanorod array produced an artificial photosynthesis material that could split water at the full theoretical potential of the light-absorbing semiconductor with sunlight.

Finding a clean renewable source of hydrogen fuel is already the focus of research, but the technology is yet to be commercialised but one promising method is through an oxygen-evolution catalyst that splits water into hydrogen and oxygen.

For their research, the Rice team combined iron, manganese and phosphorus into a precursor that can be deposited directly onto any substrate without damaging it.

To demonstrate the material, the lab placed the precursor into its custom chemical vapour deposition (CVD) furnace and used it to coat an array of light-absorbing, semiconducting titanium dioxide nanorods. The combined material – a photoanode – is claimed to have exhibited excellent stability while reaching a current density of 10mA per square centimetre.

The results appear in two new studies. The first, on the creation of the films, appears in Chemistry: A European Journal. The second, which details the creation of photoanodes, appears in ACS Nano.

Scientists created a catalyst from iron, manganese and phosphorus and coated it evenly onto an array of titanium dioxide nanorods to create a highly efficient photoanode for artificial photosynthesis
Scientists created a catalyst from iron, manganese and phosphorus and coated it evenly onto an array of titanium dioxide nanorods to create a highly efficient photoanode for artificial photosynthesis

Whitmire said the catalyst is grown from a molecular precursor designed to produce it upon decomposition, and the process is scalable. The Rice lab combined iron, manganese and phosphorus (FeMnP) into a molecule that converts to a gas when vacuum is applied. When this gas encounters a hot surface via CVD, it decomposes to coat a surface with the FeMnP catalyst.

The researchers claim their film is “the first heterobimetallic phosphide thin film” created from iron, manganese and phosphorus that starts out as a single precursor. The resulting films are said to contain stable hexagonal arrays of atoms that had only been seen at temperatures above 1,200°C. The Rice films were created at 350°C in 30 minutes.

“Temperatures above 1,200°C destroy the semiconductor array,” Whitmire said in a statement. “But these films can be made at low temperatures, allowing them to evenly coat and interact with the photo absorber and create a hybrid electrode.”

The researchers coated the three-dimensional arrays of titanium dioxide nanorods with the metallic-looking film. The composite material showed potential as a high-surface-area semiconductor for photoelectrochemical cells.

Growing the transition metal coating directly onto the nanorods allows for maximum contact between the two, Whitmire said. “That metallic, conductive interface between the semiconductor and the active catalytic surface is key to the way this device works,” he said.

The film also has ferromagnetic properties, in which the atoms’ magnetic moments align in the same direction. The film has a low Curie temperature, the temperature at which some materials’ magnetic properties need to be induced. That could be useful for magnetic refrigeration, the researchers said.

Having established their technique, Whitmire said it would now be much easier to investigate hybrid catalysts for many applications, including petrochemical production, energy conversion and refrigeration.