Abstract
The electronic structure and charge redistribution of 6s conduction charge and 5d charge in Au and Pt alloys, Au9Pt and AuPt9 have been investigated using a charge compensation model. It is found that, both the Au and Pt 4f binding energy (BE) exhibits a negative shift in the alloys relatively to the pure metal in apparent disagreement with electroneutrality considerations (Au is the most electronegative metallic element); more interestingly, the negative Au 4f BE shift in Au-Pt alloy is in contrast to previous observations for a large number of Au bimetallic systems with more electropositive hosts in which the more electropositive the host„ the more positive the Au 4f BE shift. This anomaly is counter intuitive to electronegativity considerations. This dilemma was resolved by the charge compensation model in which both electronegativity and charge neutrality can be satisfied and the overall charge flow δ, onto Au is small and positive and δ arises from charge flow of 6s conduction charge, Δnc onto Au site, which is partially compensated by the depletion of 6d charge Δnd at the Au site (δ = Δnc+ Δnd ∼0.1 >0). The much larger Coulomb interaction between 4f and 5d than that between 4f and 6s results in positive 4f BE shifts. The Au 4f BE shift in Au-Pt alloys together with 193Au Mössbauer data were used in the charge compensation model analysis which shows that the model is still valid in that the Au 4f shift in Au-Pt alloy arises from mainly conduction charge gain with little depletion of d charge at the Au site. The model also works for Pt. The Au and Pt 5d character in the alloys have been examined with valence band spectra which show both maintain their d characteristic in dilute alloys with Pt d piling up at the Fermi level, and the top of the Au valence band being pushed toward the Fermi level; this is confirmed with DFT densities of state calculations. When Pt is diluted in Au, it gains d charge as evident from the reduction in whiteline intensity at the Pt L3-edge XANES. What emerges from this work is a picture in which the s-d charge compensation in Au bimetallic alloys is triggered by electronegativity difference between Au and the host. For Au-Pt and Au-Pd systems, the difference in electronegativity is very small, conduction charge transfer dominates, and the Au 4f shift is negative whereas in most Au bimetallics, the larger the electronegativity difference, the larger the compensation and the larger the Au 4f shifts.
Highlights
The electronic structure of bimetallic Au systems exhibits some peculiar behaviour leading to an apparent anomaly in the Au 4f binding energy shifts1–4- that is that in Au-metalloid intermetallic065210-2 Wang et al.AIP Advances 8, 065210 (2018)systems, including compounds and random alloys, such as AuAl2, AgGa2, AuTe25 and AuZn6 etc. as well as Au-3d, 4d and 5d metal alloys and compounds, such as Au diluted in 3d transition metals and a series of Au-Cu7,8 and Au-Ag9 alloys, as well as Au-Ti10 and Au-Ta11 metallic compounds, the Au 4f binding energy shift in alloys relative to that of Au metal is always positive, indicating charge depletion at the Au site upon alloying
The slight variation of ∆binding energy (BE) using different photon energies is due to slight chemical inhomogeneity of a random alloy, different cross-section[41] and penetration depths of the X-rays[42] and escape depths of the electrons as well as surface contribution,[43] and it does not affect the direction of the shift and the conclusion of the analysis
Since the 4f partial photoionization cross-section for Au and Pt are nearly identical,[41] this indicates some loss of the majority component during the repeated arc melting or segregation of the minority components towards the surface, (iv) there is a small shoulder at the higher binding energy side of the Pt 4f peak in Au9Pt which is reduced upon sputtering but cannot be removed totally upon repeated sputtering, it is attributed to surface oxide in the cracks in the surface
Summary
The advent of nanotechnology has led to the synthesis of Au-Pt nano systems such as nanoparticles and nanowires which exhibit controllable catalytic behavior.[26,27,28,29,30] Interestingly, it was reported that in Au-Pt bimetallic particles, both the Au and Pt 4f binding energy exhibit a negative shift relative to that of the pure metal, an anomaly among the anomaly of Au 4f binding energy shift in Au bimetallics.[4] 119Au Mossbauer of Au-Pt nanoparticles exhibit a small but measurable positive IS.[30] At first glance, these results show that both Au and Pt gain charge upon alloying; this is not in good accord with electronegativity and electroneutrality considerations.
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