Thermal Stability and Segregation Processes in Self-Assembled Size-Selected AuxFe1-x Nanoparticles Deposited on TiO2(110): Composition Effects

In-situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements have been performed to investigate the formation and thermal stability of mono- and bimetallic AuxFe1-x (x = 1, 0.8, 0.5, 0.2, 0) nanoparticles (NPs) supported on TiO2(110). Nearly hexagonal arrangements of size-selected Au, Fe, and Au-Fe NPs with well-defined interparticle distances have been achieved by diblockcopolymer encapsulation. Upon stepwise annealing from 300 to 1060 degrees C, a remarkable thermal stability of the Au-Fe NPs was observed, maintaining their original spatial arrangement on the TiO2 surface up to 900 degrees C. A majority phase of a gold-iron alloy (solid solution) was achieved for our Au0.5Fe0.5 NPs in the temperature range of 700 degrees C - 800 degrees C, and for Au0.2Fe0.8 N.Ps at 800 degrees C, while a phase mixture of bcc Fe and Au-Fc alloy was observed for the Au0.2Fe0.2 System at 800 degrees C-900 degrees C. For all samples the segregation of Au atoms toward the NP surface was detected upon high temperature annealing (800 degrees C) in vacuum. Nearly complete Au desorption was observed by XPS at 900 degrees C for Au0.2Fe0.8 NPs, at 1000 degrees C for Au0.5Fe0.5 NPs, and at 1060 degrees C for Au0.8Fe0.2 NPs. The enhanced thermal stability of Au in the Au0.8Fe0.2 NPs is believed to be related to the formation of core(Fe)/shell(Au) structures. Furthermore, contrary to the case of pure Fe or Fe-rich NPs where nearly complete Fe desorption or Fe diffusion into TiO2 was observed at 1000 degrees C, an Fe signal was detected at this temperature for the Au-rich samples (Au0.8Fe0.2 and Au0.5Fe0.5).