Understanding the Polymorphism of Cobalt Nanoparticles Formed in Electrodeposition- An In Situ XRD Study

An advanced synchrotron-based in situ X-ray diffraction (XRD) technique is successfully developed and employed to track and monitor the formation and phase selection of cobalt (Co) in electrodeposition in real time and verify DFT computational results. The impacts of a number of controlling factors including the pH of the electrolyte and deposition overpotential are systematically studied. The results show that the yielded phase of the electrodeposited Co is controlled by both thermodynamics and kinetics. The low pH low overpotential condition favors the formation of the thermodynamically stable fcc phase. While the high pH high overpotential condition promotes the formation of the metastable hcp phase. The experimental results agree well with the nanometric phase diagram computed with DFT. Layer-by-layer alternative stacking of fcc-hcp polymorphic phases can be facilely fabricated by just varying the overpotential. This work not only offers an effective means to control the phase of electroplating of Co but also presents a new approach to reveal the fundamental insights of the formation of metals under electrochemical reduction driving force.

Automated summary

An advanced synchrotron-based in situ X-ray diffraction (XRD) technique was developed to track the formation and phase selection of cobalt in electrodeposition in real time and confirm DFT computational results. The impacts of pH and deposition overpotential were studied, revealing that the phase of electrodeposited cobalt is controlled by both thermodynamics and kinetics. The experimental results matched well with the phase diagram calculated with DFT, and layer-by-layer alternative stacking of fcc-hcp polymorphic phases could be fabricated by adjusting the overpotential. This work provides an effective means to control the phase of electroplating of cobalt and offers insights into the formation of metals under electrochemical reduction driving force.