Effects of Surfactants on the Reactivity and Durability of Shape-Controlled Pt Nanoparticles for the ORR
Authors: Oezaslan Mehtap , Safo Isaac Adjei 
Published: February 2020

In the last 10 years, a broad tool of synthetic routes has been developed to prepare shape-controlled metal nanoparticles for the electrocatalytic application, e.g. oxygen reduction reaction (ORR).[1-2] They have the great potential to reduce or to close the knowledge gap between extended and well-ordered surfaces and industrially used metal nanoparticles. However, surfactants and capping agents are still required to control the growth, orientation and size of the seeds to form nano-scaled single crystals with high monodispersity. Hence, the critical challenge is to remove the adsorbed surfactant from the surface of the shape-controlled nanoparticles without any structural loss.
In this work, we prepared Pt nanocubes with an edge length of 6 nm for the ORR by using different surfactants/capping agents like polyvinylpyrrolidone (PVP), oleylamine etc.. The electronic and structural interactions between the adsorbed surfactant/capping agent and the metallic surface were studied by using high resolution (scanning) transmission electron microscopy (HR-(S)TEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), thermogravimetric analysis (TGA) and fourier-transform infrared spectroscopy (FTIR). Afterwards, several conditioning strategies like electrochemical cycling, thermal annealing and chemical treatment were applied to increase the number of exposed surface Pt atoms for the ORR and to investigate their effect on the structural stability of the Pt nanocubes. The effectiveness of the conditioning processes was compared with each other. Based on these results, a matrix was developed to correlate the electrochemical parameters (scan rate, potential range, cycle number, temperature, pH value, electrolyte, etc.) with the changes of the shape structure and the catalytic properties for the ORR. In case of PVP-adsorbed Pt nanocubes, the electrochemical cycling up to 0.8 V in acidic media increases the number of exposed surface Pt atoms, while the electrochemical treatment in alkaline media produces some residuals of oxidized PVP that can stronger block the surface of the nanocubes by re-adsorption.[3]
After successful cleaning process, the catalytic properties and the structural durability of the Pt nanocubes were identified during an accelerated degradation protocol. According to the thermodynamic aspect, a structural transformation of the nanocubes to spherical nanoparticles was observed. The non-stabilized corner and edge surface atoms exhibit higher surface mobility and lead to the smoothing of the particle shape. This transformation process critically depends on the upper potential, cycle number and scan rate.
Our goal is to better understand the interactions between surfactant and metallic surface for the development of cleaning protocols and to fill the knowledge gap between single crystal surfaces and nano-scaled polycrystalline nanoparticles for the ORR.

Reference

[1] S. Guo, E. Wang, Nano Today
2011, 6, 240-264.
[2] Z. Peng, H. Yang, Nano Today
2009, 4, 143-164.
[3] I. A. Safo, M. Oezaslan, Electrochimica Acta
2017, just accepted.