Structural evolution of AuPt and AuPd nanoparticles fabricated by microwave assisted synthesis: A comparative study

Abstract

Bimetallic nanoparticics (NPs. particularly Au/Pd and Au/Pt, have attracted extensive attention due to their wide-spread application in catalysis, optoelectronics and energy recuperation (lJ Here we have attempted the thbrication of Au/Pt and AuPd bimetallic Ni's by an energy-efficient cco-fnendly microwave methodology The microwave-assisted reactions enable considerabis large product yields over conventional colloidal methods due to ía) almost two-fold increased reaction kinetics, (b) localized superheaung at reaction sites and rapid nyc of initial tcmperature(2) Au NI'y sizes 20 3 nm) are fabricated in the first step followed by the reduction of IPdCI:tNlli):J or (K2PICI'.IIn tctracthylcne glycol at Itt' 'C for 2 mm Controlling and undertandmng the atomic structure and elemental distributions of these NPs arc crucial for their optimitcd performances So, we address the fundamental question of the most likely arrangement of Au and Pd or Pt atoms in these bimetallic Ni's prepared under similar conditions by complementary characterizations using UV-Vts spectroscopy. X-ray diffraction (XRD) and LranSnussion electron microscopY (TEM) The IJV-Vis spectroscops reveals the forniation of an alloy shell The extent of depression of the plasmon peak of Au and its bhme-shift reveals substantial deposition of Pd atoms on an Au core and significant alloying in comparison to Au/Pt NPs XRD reveals the gradual shift of the difliaetmon peak from the position of Au to the position of Pd or Pt with change in composition XRD supports the formation of a thick alloy shell in these NPs, However, the TEM images reveal a very interesting resuk With increase in Pt concentration, the size of the dispersed Ni's decreases from 20 ± 3 nm to about 16 nm (a I nm) and there is evolution of a bimodal particle size distribution with small particles about 1-2 nm diameters On the contrary, with increasing Pd concentration, the particle size of the dispersed particles increases to about 32 nm (a I nm). This discrepancy of particle size evolution for the two systems arises due to the differences in surface energies (Pt> Pd > Au atoms). Pt atoms tend to diffuse towatds the core with the formation of Au nano-islands winch eventually segregates leading to a reduction in particle size and bimodal distribution. At higher conceniration of Pt, Pt and Au atoms tend to nucleate separately also contribute to the bimodal distribution. While for Au/Pd NPs, we have an Au core with an alloyed shell having higher Pd concentration. This is further supported by experimental evidence by selective etching and dissolution of Au by potassium-iodide solution Furthermore, the Au/Pd bimetallic NPs are found to possess better catalytic activities in the reduction of 4-nitrophenol to 4-aminophenol than Au/Pt and monometaltic NPs.