One great example of nanomaterials that can address environmental problems is photocatalytic water splitting, which produces hydrogen gas through a chemical reaction that consumes only water and sunlight. This eco-friendly hydrogen can power zero-emissions fuel cells found in cars and a number of other emerging clean technologies. The goal is to replace conventional methods of manufacturing hydrogen, which generally consume fossil fuels and/or large amounts of electricity.
In photocatalysis, materials like titanium dioxide (TiO2) nanoparticles catalyze water splitting by absorbing light and transferring the light’s energy to nearby water molecules. In turn, the water breaks apart into its constituent elements, hydrogen and oxygen. Because of the absorption properties of TiO2, artificially generated ultraviolet light is required for the reaction to proceed efficiently. However, in a recent publication in Science, a group of Berkeley Lab researchers have shown that a slightly modified version of TiO2 nanoparticles can split water under natural sunlight.
Led by mechanical engineering professor Samuel Mao, the researchers tuned the properties of the TiO2 using a process they call “disorder engineering”, so that the particles absorb much more sunlight than they ordinarily would. While the group performed extensive tests to measure the improvement, one very simple test can be done by eye: conventional TiO2 nanoparticles are white (they reflect visible light) while Mao’s modified nanoparticles are black (they absorb visible light).
Past efforts from Mao’s group to improve the solar absorption of TiO2 nanoparticles include introducing small quantities of other elements into the particles, as well as coating the particles with a material that is better at absorbing sunlight. Their recent breakthrough is somewhat simpler: roughening the nanoparticle surface. The group achieved a controlled amount of surface disorder by hydrogenating conventional nanoparticles, i.e., exposing them to hydrogen gas at high pressure for several days, and the resulting disorder-engineered nanoparticles show greater photocatalytic activity under sunlight.
As the technology for solar electricity generation continues to improve, it is interesting to consider alternative ways to harness the sun’s energy, such as photocatalysis. Chemical manufacturing is just one field where solar-driven processes could replace electricity- or fuel-driven ones; some of my other favorites include passive solar buildings and bioluminescent street lights. If nothing else, these alternatives to photovoltaics offer researchers tons of room for creativity and outside-the-box thinking. Case in point: Mao’s project is one of the first I have seen in which engineering that results in disorder is actually a good thing.
Chen X, Liu L, Yu PY, & Mao SS (2011). Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science (New York, N.Y.), 331 (6018), 746-50 PMID: 21252313