NSRRC Activity Report 2023

016 NSRRC ACTIVITY REPORT 2023 Fig. 2 : Dropwise synthesis of dendritic solid-solution HEA nanocrystals. (a) Number of moles of Pd(II), Pt(II), Rh(III), Ir(IV), and Ru(III) ions remaining in the reaction solution, (b) the instantaneous percentages of Pd, Pt, Rh, Ir, and Ru atoms generated, and (c) the instantaneous entropy of mixing ∆ S mix as a function of reaction time. (d) TEM and (e) low-magnification HAADF–STEM images of dendritic PdPtRhIrRu nanocrystals. (f) HAADF–STEM image of an individual dendritic PdPtRhIrRu nanocrystal. (g) Atomic-resolution HAADF–STEM image taken from the region marked by the box in Fig. 2(f). (h,i) Atomic-resolution HAADF–STEM images of PdPtRhIrRu nanocrystals showing the low- coordination defect sites on the nanocrystals. (j) XRD pattern of dendritic PdPtRhIrRu nanocrystals. (k) Low- and (l) high-magnification EDS maps of Pd, Pt, Rh, Ir, and Ru elements in the dendritic PdPtRhIrRu nanocrystals. (m,n) XPS spectra of the dendritic PdPtRhIrRu nanocrystals in the regions of 55 to 80 eV and 270 to 350 eV. [Reproduced from Ref. 3] TEM (HAADF–STEM), energy dispersive spectroscopy (EDS) mapping, X-ray photoemission spectroscopy (XPS), in situ and ex situ X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy. They also carried out electrocatalytic performance studies and density functional theory (DFT) calculations to identify the best method. The dendritic HEA nanocrystals made through method (c) with a defect-rich surface showed the best catalytic activity and durability for hydrogen-evolution and hydrogen-oxidation reactions (HER and HOR). Since it is known that nanodendritic materials display high catalytic activities, the authors found a method to grow dendritic HEA nanocrystals by changing the ratio between the atom deposition rate ( R dep ) and the surface diffusion rate ( R diff ). 4 They could change the rates by reducing the time duration between drops to τ = 11.4 s in the dropwise synthesis method (c) from τ = 68.4 s in method (b). This led to R dep > R diff and facilitated the formation of dendritic nanocrystals in method (c).