A small amount of inexpensive molybdenum disulfide can function as a potent photocatalyst, killing nearly all bacteria in water samples within minutes, according to a study (Nat. Nanotechnol. 2016, DOI: 10.1038/nnano.2016.138). Using sunlight and a light-activated catalyst is a simple, low-cost way to rid water of harmful pathogens. But the process is slow, taking up to 48 hours, because typical catalysts used in this application respond primarily to ultraviolet light, which accounts for just 4% of sunlight’s energy. So Yi Cui of Stanford University and colleagues designed a catalyst that efficiently harvests visible light, which represents roughly 50% of solar energy. The team grew vertically aligned sheets of MoS2 three to 10 molecular layers thick and analyzed their properties. Exposing the catalyst in water to visible light stimulates electronic excitations that generate bacteria-killing reactive oxygen species, such as O2•−, singlet oxygen (1O2), OH•, and hydrogen peroxide, along the edges of the sheets. Control tests show that visible-light disinfection using this nanostructured form of MoS2 kills more than 99.999% of bacteria within 20 minutes, far outperforming TiO2, which is a common photocatalyst, and other forms of MoS2.