Assemblies of polyvinylpyrrolidone-capped tetrahedral and spherical Pt nanoparticles in polyelectrolytes: hydrogen underpotential deposition and electrochemical characterization

Abstract

Polyvinylpyrrolidone (PVP)-capped Pt nanoparticles (NPs) were synthesized in mostly tetrahedral (TH-Pt, 〈edge〉 = 4.3 ± 0.7 nm) or spherical (S-Pt, 〈d〉 = 3.4 ± 0.8 nm) shapes and assembled layer-by-layer in poly(diallyldimethylammonium) chloride on electrodes driven by electrostatic and hydrophobic interactions. The nanostructured Pt electrodes were characterized using hydrogen underpotential deposition (H_upd) in 1 M H₂SO₄. The H_upd charge increased linearly with the PDDA–Pt NP adsorption cycle measured up to 10 cycles revealing a linear incorporation of Pt NPs per cycle, indicative of reproducible surface charge reversal despite the submonolayer NP coverage imaged by TEM on a PDDA layer, and showing the feasibility of charge and mass transport in the thickness of the films. H_upd at both PVP-TH-Pt and PVP-S-Pt occurred in two states, a major weak-adsorption H(W) peak, and a minor strong-adsorption state H(S) appearing as a shoulder. H_upd features and other electrochemical processes at assemblies of PVP-Pt NP in PDDA were compared to assemblies of 2.5 nm polyacrylate-capped Pt NPs in PDDA and to polycrystalline Pt. Results indicated that H(W) adsorption likely occurs on a PVP-modified Pt NP surface without being accompanied by PVP desorption, while H(S) occurs on free (100) sites. The PVP-Pt NPs were resistant to surface oxidation and were stable against usual surface restructuring when scanned into the Pt-oxide potential region as they remained modified with PVP. O₂ evolution was also suppressed by PVP-capping compared to PAC-Pt NPs and polycryst-Pt, but the assemblies were electrocatalytic for hydrogen evolution, hydrogen oxidation, and oxygen reduction. Increasing anodic polarization increased the H(W) charge but without causing a potential shift, indicating absence of PVP decapping or Pt surface restructuring, but possibly some structural polymer rearrangement increasing the accessibility of buried sites for H-adsorption.