Thermal Stability and Segregation Processes in Self-Assembled Size-Selected Au_xFe_1-x Nanoparticles Deposited on TiO₂(110): Composition Effects

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Abstract

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 diblock-copolymer encapsulation. Upon stepwise annealing from 300 to 1060 °C, a remarkable thermal stability of the Au−Fe NPs was observed, maintaining their original spatial arrangement on the TiO2 surface up to 900 °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 °C - 800 °C, and for Au0.2Fe0.8 NPs at 800 °C, while a phase mixture of bcc Fe and Au−Fe alloy was observed for the Au0.8Fe0.2 system at 800 °C-900 °C. For all samples the segregation of Au atoms toward the NP surface was detected upon high temperature annealing (800 °C) in vacuum. Nearly complete Au desorption was observed by XPS at 900 °C for Au0.2Fe0.8 NPs, at 1000 °C for Au0.5Fe0.5 NPs, and at 1060 °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 °C, an Fe signal was detected at this temperature for the Au-rich samples (Au0.8Fe0.2 and Au0.5Fe0.5).

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