Researchers at AMOLF have found a way of turning calcium carbonate structures, such as a sea urchin skeleton, into perovskite materials, by modifying the composition of the material. The team explained that "the experiment involves no more than dripping two liquids over the calcium carbonate structure. The conversion is complete within a couple of minutes. If you shine a UV lamp on the structure, you can see the conversion taking place in front of your eyes: The sea urchin skeleton, which initially appears blue under the lamp, changes into a bright green structure with each drop".
The researchers estimate that the perovskite microstructures made in this process result in more stable materials. They therefore state that solar cells made from this material should last longer. "In addition, we can produce perovskite structures in every desired color. This means that the material could also be used for LEDs in various applications, such as screens," says the research team.
With the new process, it is possible to convert every calcium carbonate structure into perovskite. This process concerns the controlled conversion of one crystal structure into another, which is a difficult process in chemistry. The ions in calcium carbonate are different from those in perovskite, and the stacking is different as well. The researchers replace all the ions in the calcium carbonate—first, the positively charged calcium ions with lead irons, and then the negatively charged carbonate irons with chloride, for example. Finally, they add another ion, methyl ammonium. This last ingredient gives rise to a new stacking pattern as a result of which perovskite is produced. "The reaction conditions, such as concentration and pH level, must be exactly right, as otherwise the structure falls apart immediately. It took us six months to discover those exact conditions."
The ion exchange method can be used on a wide range of materials. Not only calcium carbonate, but also barium carbonate and strontium carbonate are suitable, and possibly sulfates as well. The AMOLF researchers expect that the reaction can also be expanded to other types of perovskites to make a wide range of applications possible. "We can apply the principles to other materials such as catalysts. In those cases, you want to be able to control the material's surface shape and composition as well."