Localized surface plasmon resonance (LSPR) at the surface of noble metal nanoparticles (NPs), such as, copper, silver and gold, has been intensively investigated in the past decade because they demonstrated enhancement of photochemical reactions and highly sensitive spectroscopies. Recently, the LSPR-induced acceleration of electrocatalytic ethanol oxidation reaction was reported by Wang and co-workers using Pd-Au hetero-nanoparticles, in which photogenerated holes in Au NPs could effectively oxidize ethanol in a solution.1
Platinum is one of the most active catalysts for various reactions in a practical application. However, since Pt NPs hardly exhibit LSPR peak in visible-light region, their application to LSPR-induced catalytic reaction has been little investigated. In this study, we try to enhance the catalytic activity of Pt with LSPR of octahedral Au NPs. An electrochemical technique is used for preparing Pt shell layer on octahedral Au NPs, resulting in the formation of Au@Pt core-shell NPs. The photoexcitation of LSPR peaks of octahedral Au core in Au@Pt NPs can modify the catalytic activity of Pt shell layer for oxygen reduction reaction (ORR) as a model reaction.
Experimental
Octahedral Au NPs were prepared by reduction of HAuCl4 in diethylene glycol containing poly(diallyldimethylammonium chloride) at 230 °C for 1 h. The underpotential deposition (UPD) of Cu monolayer onto octahedral Au NPs immobilized on an HOPG electrode was carried out with constant potential application at +0.3 V vs. RHE in an aqueous solution containing 1.0 mmol dm-3 CuSO4 and 0.1 mol dm-3 H2SO4. Thus-obtained Cu-deposited Au NPs on HOPG substrate immersed in aqueous solution containing 0.5 mmol dm-3 K2PtCl4 for 10 min to replace Cu monolayer by Pt, resulting in the core-shell structure of octahedral Au@Pt NPs.
Results and discussions
Prepared Au NPs (> 90%) had an octahedral shape with sharp edges and triangle flat surface of {111} facets. The average edge length was determined to be 65 ± 8 nm by SEM measurement. In the extinction spectrum, a sharp LSPR peak appeared at ca. 600 nm, the peak width of which was relatively narrow (FWHM > 90 nm) probably due to the uniformity of shape and size of octahedral Au NPs.
The Pt shell were prepared by the galvanic replacement of Cu UPD layer on Au surface with Pt. The electrochemical surface areas of Pt shell and Au NPs were determined with cyclic voltammetry in a 0.1 mol dm-3 H2SO4 aqueous solution. The deposition of Pt layer on Au NPs decreased a peak current assignable to the reduction of Au oxide layer (ca. 1.15 V vs. RHE), while the peaks originating from reduction of Pt oxide layer and H2 desorption on Pt surface were newly observed for Au@Pt NPs at 0.65 and 0.05 V vs. RHE, respectively. These results indicated that the surface of octahedral Au NPs was partially coated with the Pt shell layer. The Pt surface coverage was successfully estimated to be enlarged from 85 to 94% with an increase in the Pt deposition cycles from 1 to 3. TEM measurement revealed that the average edge length of octahedral NPs (original: 65 nm) increased to 66.5 nm and 69 nm by the Pt deposition cycles of 1 and 3, respectively, indicating that the thin Pt layer of ca. 1-2 nm in thickness was formed on Au surface.
Hydrodynamic voltammograms for ORR were measured with or without visible-light irradiation, using a ring-disk-rotating electrode loaded with the octahedral Au@Pt NPs (1 cycle of Pt deposition). A cathodic current for ORR was observed at more negative potential than ca. +0.95 V vs. RHE. The current density at +0.5 V vs. RHE was enhanced from -2.6 mA cm-2 to -3.1 mA cm-2 by the visible-light irradiation. The photocurrent action spectrum of the Au@Pt NPs agreed well with their extinction spectra. These results suggested that the LSPR excitation of the Au core in Au@Pt NPs could enhance the electrocatalytic activity of the Pt shell layer for ORR.
Reference
1. Wnag, Q., et al., Power Sou., 2016, 316, 29.