Deciphering PtNi Nanoparticles Formation:In-situ X-ray spectroscopy Insights into Microstructural Evolution

The combination of Pt-based bimetallic alloy nanocrystals with 3d transition metals exhibits remarkable performance advantages in the hydrogen evolution reaction (HER), making them a key focus in energy conversion applications. The overpotential for the hydrogen evolution reaction (HER) at a current density of 10 mA/cm² is 15.2 mV, demonstrating excellent catalytic performance. This outstanding performance makes PtNi-NP highly valuable for further research.

To better understand the changes in local structures and phase compositions during the synthesis of PtNi bimetallic nanoparticles (PtNi-NP), this research utilizes in-situ techniques such as X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS).

After heating to 400°C, the diffraction peak of Pt (111) shifts to a higher angle and becomes sharper, indicating changes in the lattice structure due to the reduction in lattice spacing during heating. In-situ XANES successfully observed the entire transformation process from PtNi precursor to PtNi-NP. The data show that the valence state of Pt transitions from 4+ to 0+ as the temperature increases, with a significant shift to the metallic state starting at 400°C. Meanwhile, Ni shows minimal changes, indicating that it remains mostly in the metallic state throughout the process.

In EXAFS measurements, Ni bonds are primarily Ni–Ni/Pt (~2.1 Å), and their intensity decreases with increasing temperature, likely due to thermal disorder. On the other hand, Pt bonds undergo a significant change from Pt–Cl (~1.6 Å) to Pt–Pt/Ni (~2.1 Å). The fitting results further indicate that during the heating process, the transformation first favors the formation of Pt–Pt bonds, which persist until 800°C, after which Pt–Ni bonds become dominant.

Fitted data from in-situ XPS further confirm that as the temperature rises, Pt transitions from a 4+ valence state to a metallic state, consistent with the XANES and EXAFS results. These findings demonstrate that in-situ X-ray spectroscopy provides direct and accurate evidence of local structural transformations during the formation of PtNi-NP and highlight the superior catalytic efficiency of PtNi-NP in HER applications.