Charge distribution and Fermi level in bimetallic nanoparticles

Abstract

Upon metal-metal contact, a transfer of electrons will occur between the metals until the Fermi levels in both phases are equal, resulting in a net charge difference across the metal-metal interface. Here, we have examined this contact electrification in bimetallic model systems composed of mixed Au-Ag nanoparticles containing ca. 600 atoms using density functional theory calculations. We present a new model to explain this charge transfer by considering the bimetallic system as a nanocapacitor with a potential difference equal to the work function difference, and with most of the transferred charge located directly at the contact interface. Identical results were obtained by considering surface contacts as well as by employing a continuum model, confirming that this model is general and can be applied to any multimetallic structure regardless of geometry or size (going from nano-to macroscale). Furthermore, the equilibrium Fermi level was found to be strongly dependent on the surface coverage of different metals, enabling the construction of scaling relations. We believe that the charge transfer due to Fermi level equilibration has a profound effect on the catalytic, electrocatalytic and other properties of bimetallic particles. Additionally, bimetallic nanoparticles are expected to have very interesting self-assembly for large superstructures due to the surface charge anisotropy between the two metals.

Contact Potentials, Fermi Level Equilibration, and Surface Charging

“…diameter gold sphere, 16 whereas DFT calculations done with 591 different atom Au−Ag particles show total charge transfer of 5−17 electrons, depending on the arrangement of the atoms. 18 These examples show that the relative amount of the transferred electrons can be very high in nanoscale systems, whereas less significant effects are observed in the macroscopic systems.…”

“…This simple electrostatic model for the contact electrification of Janus particles has been justified by comparison with DFT calculations. 18 The electrons in both phases have the same electrochemical potential. This means that the difference in real potential of electrons in different metals is compensated by a Volta potential difference between the metals.…”

“…18 The separation between the two metals was chosen as 0.353 Å so that the charge transferred matched the value obtained by DFT calculations. 18 The results are shown in Figure 6 with streamlines for the electric field.…”

“…To illustrate this, charge distribution upon contact charging was investigated with the capacitance model. 18 The Laplace equation ∇ 2 ϕ = 0 for the electrostatic potential in vacuum was solved in 2D axis symmetry, both analytically and numerically using COMSOL Multiphysics, to illustrate the potential distribution because of the contact electrification of Ag−Au Janus particles (radius 12.6 Å) with different compositions (see the Supporting Information for details). In particular, the surface fraction of Ag was varied from 5 to 95% ( Figure 5).…”

“…The potential difference between the two metals is directly given by the work function difference, and the amount of transferred charge can be calculated simply from electrostatics. 18 Comparison between numerical simulations of the electrostatics and more comprehensive density functional theory (DFT) calculations showed that the continuum calculations can satisfactorily describe the general charge transfer behavior, but DFT calculations are required to accurately describe the atomic scale charge transfer at the metal−vacuum interface and to resolve the finer details of transferred charge at the metal− metal interface. 18 …”