Particulate forms of semiconductors may one day capture energy from sunlight and use it directly to generate fuels, thanks to a study that uncovered the role crystal facets play in photocatalysis.
Large semiconductor sheets are widely used in photovoltaics to harvest sunlight and produce electricity. The process depends on preventing positive and negative charges, generated via photoexcitation, from recombining and neutralizing the excitation. Similarly, light captured by inexpensive semiconductor powders can be used directly to drive reactions, for example to split ocean water to make hydrogen. A key challenge in making that photocatalytic process work efficiently is keeping the charges separated on these tiny particles.
Peng Chen and Xianwen Mao of Cornell University wondered whether small differences in the electronic properties of various crystal faces, or facets, of the semiconductor particles could help do the job.
Specifically, the team knew that the amount of energy needed to remove an electron from a surface varies slightly from facet to facet. That difference leads to one type of facet accumulating more positive or negative charge than a different type. But do the edge regions between adjacent facets have unique properties, and can those junctions be tuned to boost charge separation and photocatalytic performance? The team showed that the answer to both questions is yes (Nat. Mater. 2021, DOI: 10.1038/s41563-021-01161-6).
To study those issues, Mao and Chen studied particles of bismuth vanadate, a semiconductor and a promising photoanode material for water oxidation, one of the steps of water splitting. They prepared BiVO4 particles with two types of exposed facets. The facets differ slightly in electronic properties and composition. One facet is richer in O2- ions, the other is richer in Bi3+ ions.
In earlier work, the team devised a fluorescence microscopy method with single-molecule resolution. Now they used the method to selectively map BiVO4 surface reactions induced by photogenerated positive charges, or holes, and electrons. They monitored the reactions and the associated photogenerated current as water oxidation occurred in a photoelectrochemical cell.
One key finding is that the light-stimulated process sets up micrometer-sized surface regions that exhibit enhanced photocurrent. The team showed through control tests that the junctions between facets drive the shape of these catalytically active regions.
By analyzing nitrogen-doped and undoped particles, the team showed that doping, which alters the concentration of charged species, changes the shape and size of the junction regions and enhances photoactivity.
The discovery reported here regarding the properties of transitional zones between adjacent facets “is quite interesting and novel,” says Michael V. Mirkin, an electrochemist at Queens College, City University of New York. “This is high-quality work that opens new avenues of research,” he adds.
University of Oregon’s Shannon Boettcher, a specialist in photoelectrochemistry, agrees. He says that other researchers are likely to follow up, for example, by depositing catalysts at specific surface sites to speed up the kinetics of making hydrogen and oxygen and to further improve charge separation. Water splitting photocatalyst technology is still far from commercialization, Boettcher says. Yet “this work could ultimately lead to systems with much higher efficiency that are synthesized at scale using common chemical methods.”