Hydrogen-induced optical properties of FC/Pd/Mg films: Roles of grain size and grain boundary

Nanomodification is an effective method to solve the thermodynamic and kinetics limitation of Magnesium (Mg)-based materials, which shows promising application prospects in hydrogen energy field. However, the role of the grain size of pure Mg on the hydrogen-induced performance of the hydrogen sensitive thin film under cyclic hydrogen loading/unloading process at room temperature has rarely been studied systematically. To study the relationship between the structure of Mg layer and the hydrogen-induced optical performance of fluorocarbon (FC)/Pd/Mg films, a series of Mg with different internal structures were prepared by changing the velocity of sputtered atoms under different sputtering powers. The FC/Pd/Mg (40 W) film with fine nanostructure showed faster hydrogenation/dehydrogenation kinetics as well as a larger optical conversion range, which can be attributed to the large population of grain boundaries with high grain boundary energy and more hydrogen diffusion path. As sputtering power gradually increased from 40 to 300 W, the grain inside films grew larger. The FC/Pd/Mg (300 W) film had more columnar-like regions inside and less grain boundaries with lower energy contributing to slower hydrogen absorption/desorption kinetics and lower optical conversion range.