Simple, robust and resource-friendly synthesis of nano-sized Pt NCs with high purity based on PVP as a structure-directing agent.

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We investigated the formation of Pt nanocubes (NCs) and their electrocatalytic oxygen reduction reaction (ORR) properties and structural stability using two different capping agents, namely, polyvinylpyrrolidone (PVP) and oleylamine (OAm). The mono‐dispersity of the obtained Pt NCs and their interactions with PVP and OAm were analyzed by transmission electron microscopy (TEM), energy dispersive X‐ray spectroscopy (EDX), X‐ray photoelectron spectroscopy (XPS), Fourier‐transformed infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The TEM data show a high mono‐dispersity (82 %) and a large mean particle size (9‐10 nm) for the Pt NCs obtained by the oleylamine‐assisted method compared to those prepared via the PVP‐assisted procedure (68 %, 6–7 nm). FTIR, XPS, and TGA data show that PVP and OAm still remain at the Pt surface, despite washing. Interestingly, the OAm‐capped Pt NCs show significantly higher electrochemically active surface area (ECSA) and ORR activity than the PVP‐capped ones. An accelerated stress protocol, however, reveals that the OAm‐capped NCs possess a poor structural stability during electrochemical cycling. The loss of a defined surface arrangement in the NCs is connected with a transformation into a near‐spherical particle shape. In contrast, the PVP‐capped NCs mainly retain their particle shape due to their strong capping behavior. In addition, we have developed a degradation model for NCs as a function of electrochemical parameters such as upper potential and cycle number. Altogether, we provide fundamental insights into the electronic interactions between capping agent and Pt NCs and the role of the adsorption strength of the capping agent in improving the electrochemical ORR performance as well as the structural stability of shape‐controlled nanoparticles.

Polyvinylpyrrolidone (PVP) polymer is among one of the widely used surfactants to prepare nano-materials with desired particle shape and particle size. The critical challenge is to remove PVP polymer from the metal surface without loss of the surface arrangement and particle agglomeration. Here, we developed a strategy to remove the surfactant PVP which prefers to form a multi-layer shell and thus blocks the catalytically active surface of the Pt nanocubes (6–7 nm). Since PVP is partially soluble in polar solvents, we studied four different solvent mixtures (volume ratio), (i) methanol/ethanol (3:1), (ii) acetone/water (3:1), (iii) ethanol/chloroform (3:1), and (iv) aqueous 0.1 M acetic acid by using transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR) and cyclic voltammetry (CV). Only, the washing process with methanol/ethanol and acetone/water generates Pt nanocubes with almost clean particle surface. Based on our FTIR results, a shift of the carbonyl band in IR spectrum was observed for methanol/ethanol-washed Pt nanocubes, indicating the coordination of the carbonyl oxygen of the PVP to platinum. The electrochemical experiments showed that the surface area of the methanol/ethanol-washed Pt nanocubes was increased by a factor of 14 compared to the unwashed, while an improvement of 11 times was achieved by washing in acetone/water. However, the CV profile still signifies the presence of strongly adsorbed PVP on the Pt surface. To remove the chemisorbed PVP, an electrochemical cleaning including 200 potential cycles between 0.06 and 1.00 V vs. RHE at 200 mV s−1 was applied. The potential cycling reveals the potential-controlled ad/desorption behavior of the PVP at the Pt surface. Altogether, we designed a cleaning procedure for surfactant-capped metal nanoparticles and provide insights into the interactions between the PVP and Pt surface.

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