Deciphering dynamic surface of PtRu alloy nanocatalysts to revisit their synergistic effects during the electrooxidation

Correlating dynamic structural transformation of catalysts with the surface intermediate species under operating conditions is critical for updating the understanding of structure–performance relationships.Here,we probe the electrochemical potentialdependent surface structures and adsorbed intermediates on PtRu binary alloy nanocatalysts to revisit its synergistic mechanisms for CO electrooxidation enhanced activity.In-situ spectral characteristics by using modified shell-isolated nanoparticle-enhanced Raman spectroscopy,show that in acidic solution,when the potential is positively scanned from 0.1 V to 1.5 V relative to reversible hydrogen electrode(RHE),the surface of the alloy catalyst evolves from metallic PtRu to adsorbed oxygen gradually covering and accumulating on Ru sites(denoted as PtRuO_(x),x≤2),forming segregated RuO_(2),and finally forming a threedimensional oxide layer(denoted as 3D PtRuO_(4)).Moreover,molecular evidence associated with periodic density functional theory calculations reveals that electronic effects promote ruthenium to become more oxidizable and oxophilic.In particular,we found here that ^(*)O and ^(*)OH at surface RuO_(x) sites are highly efficient CO oxidation active species in comparison to the same entities adsorbed on metallic Ru sites.This work sheds light on the complex surface dynamic process of practical Pt-based binary nanocatalysts and improves the understanding of synergistic mechanism for the development of fuel cell devices.

Optical response and SERS properties of individual large scale supracrystals made of small silver nanocrystals

There is a considerable interest in producing and understanding the optical and spectroscopic properties of ordered nanoparticle assemblies. Herein, we describe and interpret the optical absorbance and Raman properties of 5.9 nm ± 0.3 nm diameter silver nanocrystals coated with dodecanethiol and organized in highly ordered 3D superlattices of different heights. Each superlattice was studied individually, which allowed to elaborate a model based on Maxwell-Garnett theory to reproduce qualitatively the height and wavelength dependence of the absorbance. Importantly, because of their small size compared to that of traditional nanoparticles used in Surface Enhanced Raman Spectroscopy （SERS）, the large 3D distribution of hot spots generated by the silver superlattices allowed to easily obtain SERS spectra of the surrounding ligands despite their intrinsic low Raman cross section. Accordingly, traces of thiophenol could be detected very easily.