What Happens as Various Elements Are Mixed within NPs?

Abstract: Nanoparticles (NPs) containing multi-elements in a single phase have recently attracted much attention as new types of nanomaterials such as catalysts. In particular, alloys containing more than five elements at almost equal ratios called high entropy alloys (HEAs) have been intensively studied in these five years. We synthesized new HEA NPs and studied their catalytic properties based on their electronic structures.^1-6 For example, platinum-group metal HEA NPs (PGM-HEA NPs) show very high electrocatalytic activity toward hydrogen evolution reaction (HER), which is higher than a commercial Pt catalyst.³ Although it is well known that HER activity of metal catalysts has a volcano-type relationship with their d-band center because the d-band center and hydrogen adsorption energy of catalysts show a linear relationship, very interestingly, the HER activity of PGM-HEA NPs cannot be explained by this theory. This might originate from the very complex surface atomic arrangements and the diverseness of the local density of states (LDOS) of the surface sites. Furthermore, we also synthesized HEA NPs consisting of all eight noble-metal-group elements (NM-HEA NPs). We revealed that the LDOS of every surface atom in NM-HEA is different.² Some atoms of the same constituent element in HEA NPs have different LDOS profiles, whereas atoms of other elements can have similar LDOS profiles. This may provide an interesting result that the NM-HEA NPs showed 10.8 times higher activity for HER than Pt/C. In addition to alloys, we recently succeeded in synthesizing denary perovskite HE oxide.⁷ In this presentation, we will discuss the possibilities of these multi-element nanoparticles.