Platinum nanoparticles functionalized with acetylene derivatives: Electronic conductivity and electrocatalytic activity in oxygen reduction

Stable platinum nanoparticles were prepared by the self assembly of acetylene derivatives (1-alkynes, 4-ethylphenylacetylene, and 4-tert-butylphenylacetylene) onto bare Pt colloid surfaces. Transmission electron microscopic measurements showed that the nanoparticles exhibited an average core size of 2.85 ± 0.62 nm. FTIR study showed that with the acetylene ligands adsorbed onto the Pt nanoparticle surface, the CH vibrational stretches disappeared completely, along with a substantial redshift of the CC vibrational stretch, as compared to those of the monomeric ligands. These were ascribed to the breaking of the CH bond and the formation of PtsurfaceH and PtsurfaceC at the metal–ligand interface. The conjugated bonding interactions between the triple bond and Pt were found to lead to extensive intraparticle charge delocalization between the acetylene moieties, and hence unique photoluminescence properties of the nanoparticles. For nanoparticles functionalized with 4-ethylphenylacetylene or 4-tert-butylphenylacetylene, the excitation and emission peak positions showed an apparent redshift as compared to those of 1-alkyne-capped platinum nanoparticles. Electronic conductivity measurements of the nanoparticle solids showed that for the nanoparticles capped with 1-alkynes or 4-ethylphenylacetylene, the temperature dependence of the ensemble conductivity was consistent with that of semiconducting materials, whereas for the 4-tert-butylphenylacetylene-capped nanoparticles, metallic behaviors were observed instead. An apparent discrepancy was also observed in the electrocatalytic reduction of oxygen in alkaline media, where the specific activity was all markedly better than that of commercial Pt/C catalysts, with the best performance by the 4-ethyphenylacetylene-capped Pt nanoparticles. These observations were accounted for by the deliberate manipulation of the electronic structure of the Pt nanoparticles by the organic ligands.