Elsevier

Surface Science

Volume 609, March 2013, Pages 161-166
Surface Science

Strain-release mechanisms in bimetallic core–shell nanoparticles as revealed by Cs-corrected STEM

https://doi.org/10.1016/j.susc.2012.12.001Get rights and content

Abstract

Lattice mismatch in a bimetallic core–shell nanoparticle will cause strain in the epitaxial shell layer, and if it reaches the critical layer thickness misfit dislocations will appear in order to release the increasing strain. These defects are relevant since they will directly impact the atomic and electronic structures thereby changing the physical and chemical properties of the nanoparticles. Here we report the direct observation and evolution through aberration-corrected scanning transmission electron microscopy of dislocations in AuPd core–shell nanoparticles. Our results show that first Shockley partial dislocations (SPD) combined with stacking faults (SF) appear at the last Pd layer; then, as the shell grows the SPDs and SFs appear at the interface and combine with misfit dislocations, which finally diffuse to the free surfaces due to the alloying of Au into the Pd shell. The critical layer thickness was found to be at least 50% greater than in thin films, confirming that shell growth on nanoparticles can sustain more strain due to the tridimensional nature of the nanoparticles.

Highlights

► Atomic resolution imaging of the interfaces of AuPd nanoparticles. ► Systematic study of the growth of Pd shells over Au nanoparticles. ► Shockley partial dislocations, edge dislocations and Au migration into the shell release the interfacial strain.

Introduction

Strain engineering has been used to tailor the energy band-edge properties for enhanced charge carrier transport [1], [2] and photoemission properties [3] in semiconductor heterostructure materials. In catalysis, bimetallic core–shell nanoparticles have raised a lot of interest in different fields due to their enhanced properties compared to their monometallic counterparts [4], [5], [6]. Core–shell nanoparticles present an ideal system to control the strain of the shell metal by changing the core metal and morphology and the shell thickness. The electronic structure of the surface atoms can be tuned by the presence of core and shell metals thereby tuning their chemical and physical properties [7], [8]. Both, epitaxial and non-epitaxial shells have been synthesized for different applications [9], [10], [11], [12], [13]; however, epitaxial growth has not been thoroughly investigated in core–shell nanoparticles. Several works have studied the effect of elastic strain in the catalytic activity of metal films [14], [15], [16], [17] and core–shell nanoparticles [4], [6], [18], [19], [20], [21], [22] as well as a substrate for surface-enhanced Raman scattering [23]. Therefore, it is necessary to study in detail the interface of these heterostructures to understand the growth of the shell and the appearance of defects that will alter the properties of such materials.

Strasser et al. demonstrated the increased reactivity of Pt in dealloyed Pt–Cu nanoparticles for fuel cell applications, which was the difference in lattice between the Pt–Cu core and the Pt shell [4]. Their results showed an optimal compressive strain for the oxygen-reduction reaction, which if tuned properly, would make a very efficient catalyst. Filhol et al. have shown that the catalytic activity can be enhanced by the presence of 4 monolayers of Pd on NiO(110) because of the local strain present in the top Pd layer [24]. This effect was also observed for Pd layers grown on Au seeds in different reactions [25], [26], [27], [28].

Despite the fact that epitaxial Au–Pd growth in films is well understood [29], [30], [31], [32], the tridimensionality of the nanoparticles changes the strain distribution in the shell, which will change the behavior of the Pd shell changing the strain-release mechanisms compared to thin films.

In this paper, we report a detailed analysis of the epitaxial growth of Pd on Au truncated-octahedral seeds, mainly composed by large {111} surfaces and small {100} surfaces. Lattice mismatch between Au and Pd is about 4.7%, which will lead to the nucleation of dislocations in order for the Pd layers to continue growing. We varied the Pd shell thickness by changing the Pd precursor volume, resulting in shells from 3 layers thick up to 10 nm thick and observed the evolution of the interface by Cs-corrected scanning transmission electron microscopy (STEM). Our results revealed that the strain release mechanism changes from the formation of Shockley partial dislocations (SPD) and stacking faults (SF) to the introduction of misfit dislocations, which will later disappear by diffusion of Au atoms into the Pd shell. These dislocations accommodate strain in the Pd layer changing the physical and chemical properties of the surface of the nanoparticles, hence, it is important to understand the defects that appear at the nanoscale in order to further control the properties of the nanoparticles.

Section snippets

Experimental section

All the reagents used: gold(III) chloride trihydrate (HAuCl4·3H2O), potassium tetrachloropalladate (K2PdCl4), sodium borohydride (NaBH4), l-ascorbic acid (AA, 99%), hexadecyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich and used as received. Deionized water was used throughout the entire experiment.

Results and discussion

Our syntheses yielded in average 80% of nanocubes and 20% triangular shapes; however, here we will focus on the evolution of the AuPd interface in the concave nanocubes; then were further characterized by Cs-corrected STEM characterization at several volumes of Pd to observe the progressive growth of the shell and observe strain release mechanisms.

Fig. 1a shows a AuPd core–shell nanoparticle (10 μl of Pd precursor) along a [011] zone axis where it is barely possible to observe the Pd layers

Conclusions

Our experimental results revealed new insights on the formation of AuPd core–shell nanoparticles. Some works have shown experimental results on the structure of core–shell nanoparticles; however, their observations were limited and did not follow any trends in these structures [10], [32], [38], [39]. Here we show the strain release mechanisms that take place as a Pd shell grows over a Au nanoparticle. SPDs and SFs at the Pd surfaces are the first mechanism to release lattice mismatch strain up

Acknowledgments

This work was supported by grants from the National Center for Research Resources (5 G12RR013646-12) and the National Institute on Minority Health and Health Disparities (G12MD007591) from the National Institutes of Health. The authors would like to acknowledge the NSF for the support with grants DMR-1103730, “Alloys at the Nanoscale: The Case of Nanoparticles Second Phase” and PREM: NSF PREM grant # DMR 0934218; “Oxide and Metal Nanoparticles — The Interface Between Life Sciences and Physical

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