The Effects of Native Oxides on the Selective Photo-Reduction Property of Cu Nanoparticles Induced by the Localized Surface Plasmon

Copper nanoparticles (CuNPs) in air are easily oxidized into CuNPs@Cu2O core-shell structure. The localized surface plasmon resonance (LSPR) of CuNPs can be damped seriously by the Cu2O shell. The effect of this native Cu2O layer on the LSPR-induced selective photocatalysis properties of CuNPs is often ignored. In this paper, this effect was explored by comparison of the evolution of the photo-reduction efficiency (PRE) with the nanoparticle size (d) of the as-deposited CuNPs to that of AgNPs via monitoring the transformation from 4-nitrothiophenol (PNTP) to 4,4 trdimercaptoazobenzene (DMAB) using surface enhanced Raman scattering. It was found that the PRE of the as-deposited CuNPs increases first and then decreases with increasing d, much different from that of AgNPs: increases with increasing d. Based on the transmission electron microscope images, X-ray photoelectron spectroscopy, and by monitoring the PRE stability in air of the as-deposited CuNPs, the mechanisms of the PRE evolution with d and the laser illumination time (t) were suggested. Cu2O plays a dominated role when the nanoparticles' sizes were small. Both Cu2O and CuNPs play roles when the nanoparticles' sizes were large, which brought in the low PRE of the as-deposited CuNPs.