TEM, FTIR and Electrochemistry Study: Desorption of PVP from Pt Nanocubes

Abstract

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.

Halide‐Induced Leaching of Pt Nanoparticles – Manipulation of Particle Size by Controlled Ostwald Ripening

“…Removal of surfactants requires thermal treatments, which can lead to sintering of the NPs and diminish the benefit of colloidal approaches to prepare tailored catalysts . Another possibility for removal of surfactants is the use of ozone and electrochemical treatment ,. However, it has been shown that surfactants are not completely removed by ozone treatments or electrochemical treatments, which can ultimately affect the catalytic performance …”

Effects of Capping Agents on the Oxygen Reduction Reaction Activity and Shape Stability of Pt Nanocubes

“…Both C−N and C=O stretching vibrations originate from the pyrrolidone structure of PVP. The assignment of the absorption peaks of pure PVP has already been reported in our previous work …”

“…However, the weak absorption bands such as C=O and CH 2 /CH still signify the presence of strongly chemisorbed PVP residues on the particle surface. It is noted that the as‐treated PVP shows a C=O stretching band at 1660 cm −1 compared to the pure PVP (1638 cm −1 ) and washed Pt NCs (1635 cm −1 ).The observed blue shift signifies a modification of PVP heat‐treated in EG due protonation or resonance stabilization. Very importantly, for the washed NCs a shift of the carbonyl band to lower wavenumbers (red shift) is detected which is very likely attributed to the coordination of the PVP to the Pt by decrease of the electronic density in the C=O group …”

“…Details of the synthesis of PVP‐capped Pt NCs have recently been reported in our previous works . The original synthetic recipe from Song et al .…”

“…For example, polyvinylpyrrolidone (PVP) is able to strongly adsorb on the metallic surface. Therefore, various cleaning procedures like UV/ozone radiation, polar organic solvent washing, acid washing and oxidative thermal treatment have been applied to remove PVP residues from the surface of Pt nanocubes (NCs). In contrast, oleylamine (OAm) which is well‐known as moderate capping agent can be easily removed after synthesis by soft techniques like solvent washing using acetone, methanol/ethanol and hexane .…”