https://www.scientificamerican.com/article/scientists-sculpt-nanoparticle-shells-with-light/

For the first time, researchers have used light to control the shape of nanoparticles and create micron-size hollow shells from crystals of cuprous oxide (copper and oxygen). Such particles could have future applications as a low-cost catalyst to help pull excess carbon dioxide from the air, a way to improve microscopic imaging and more, says Bryce Sadtler, a chemist at Washington University in St. Louis and senior author of a study on the new method, published last October in Chemistry of Materials.

The hollowing process involves visible light, an alkaline solution and a source of voltage, Sadtler explains. Illuminating a cuprous oxide microcrystal excites its electrons, which join with copper ions to form regular copper atoms. No longer bound to oxygen, these atoms are free to jump to the particle’s surface and form a copper metal coating that shields parts of the underlying crystal from the solution.