Evaluation of Two Methods for Determining Shell Thicknesses of Core–Shell Nanoparticles by X-ray Photoelectron Spectroscopy

Abstract

We evaluated two methods for determining shell thicknesses of core-shell nanoparticles (NPs) by X-ray photoelectron spectroscopy (XPS). One of these methods had been developed for determining thicknesses of films on a planar substrate while the other was developed specifically for NPs. Our evaluations were based on simulated Cu 2p3/2 spectra from Cu-core/Cu-shell NPs with a wide range of core diameters and shell thicknesses. Copper was chosen for our tests because elastic-scattering effects for Cu 2p3/2 photoelectrons excited by Al Kα X-rays are known to be strong. Elastic scattering could also be switched off in our simulations so that the two methods could be evaluated in the limit of no elastic scattering. We found that the first method, based on both core and shell photoelectron intensities, was unsatisfactory for all conditions. The second method, based on an empirical equation for NPs developed by Shard, also utilized both core and shell photoelectron intensities and was found to be satisfactory for all conditions. The average deviation between shell thicknesses derived from the Shard equation and the true values was −4.1 % when elastic scattering was switched on and −2.2 % when elastic scattering was switched off. If elastic scattering was switched on, the effective attenuation length for a Cu film on a planar substrate was the appropriate length parameter while the inelastic mean free path was the appropriate parameter when elastic scattering was switched off.

Comparisons of Analytical Approaches for Determining Shell Thicknesses of Core–Shell Nanoparticles by X-ray Photoelectron Spectroscopy

“…Shard assumed that the fourth EAL, L 2 ,b , could be estimated from L 2, b = L 1, a / BC .. We note that eq. (1d) was shown incorrectly in our previous paper 17 but the numerical results therein are correct.…”

“…We consider here two approaches for determining shell thicknesses of core-shell NPs by XPS. First, we extend our earlier evaluation 17 of the Shard 8 equation for measuring shell thicknesses from ratios of photoelectron intensities from the core and shell materials of the NP. Second, we address a necessary simplification generally made in the development of analytical models describing photoelectron intensities from NPs of different sizes and with different shell thicknesses.…”

“…Our previous evaluation 17 of the Shard equation was performed for Cu-core/Cu-shell NPs and for this idealized case, B = C = 1. We will consider here core-shell material combinations with more extreme values of B and C .…”

Influence of different cleaning processes on the surface chemistry of gold nanoparticles

“…These results support the conclusion that the citrate displacement is not uniquely dependent on the weak Au-COO bond strength (2 kcal/mol) with respect to the strong Au-S bond strength (about 40 kcal/mol). 70,71 In fact, previous studies 36,52 have shown that the structure of the adsorbed citrate molecules plays a fundamental role in the thiols-citrate exchange. Furthermore, in the case of alkanethiols, the coverage was found to be independent on the thiol's chain length.…”

“…However, CLS, DLS, and SEM analyses indicate that nanoparticles are aggregating after the third and the fourth centrifugation cycle (Figs. S1 and S2), 71 when changes in distribution index (DI) and poly-distribution index (PDI) are recorded. Moreover, a slight decrease in pH FIG.…”