Comparative Study on Formation of Boride Thin Films Deposited by Co-sputtering with Molybdenum*

Ammonia plays a crucial role in our society. Ammonia is used as a hydrogen carrier for energy conversion, combustion source without emitting carbon dioxides, and an important element for plant growth. The Harber-Bosch process, which consumes large amount of energy, still supports the demand for ammonia. In this situation, development of an alternative method for ammonia synthesis is required. In this study, we demonstrated deposition of metal contained borides by co-sputtering method and followed calcination to investigate crystal formation under relatively mild condition. The crystal structure of the deposited film is confirmed by x-ray diffraction, and underlayer of aluminum does not affect crystallization process of aluminum diborides films. In the case of molybdenum is co-sputtered, part of molybdenum is diffused into silicon substrate, resulting in formation of Mo-Si bonds. Suppression of diffusion process would be the key to the formation of highly crystalline borides. Surface morphology was observed by scanning electron microscope and layered-structure is found at the surface of the films. This structure is originated from 2D structure of borides. Additionally, we investigated electrocatalysis of molybdenum-aluminum boride thin film in a nitrogen-saturated electrolyte solution. Linear sweep voltammetry reveals that the film has an electrocatalysis of ammonia synthesis, confirming sufficient current density.