Thermodynamics of Pd (‐Au) nanoparticle‐titania interface studied by aberration‐corrected TEM

Abstract

Over the last decade, supported metallic nanoparticles (NPs) have garnered continuous interests across many fields of research due to their novel physico‐chemical properties which are, among others, shape‐dependent. Though many synthesis schemes are being developed to generate a variety of NP shapes, understanding why a NP adopts a given shape is still challenging due to the intricate influences of thermodynamic, kinetic and energetic factors. In this contribution, we first report on the structural properties of Au‐Pd nanoalloys supported on rutile titania, which can be considered as model catalysts. Then, using a recently proposed scheme that combines TEM imaging of single nanoparticles and a generalized Wulff‐Kaishew theorem [1], the interface and triple‐line energies of the Au‐Pd NP‐titania system are determined experimentally and studied as a function of particle composition and epitaxial relationship. Bimetallic Au‐Pd nanoalloys with well‐controlled composition were grown on well‐defined rutile titania nanorods by pulsed laser deposition. Titania with rod‐like shape and narrow size distribution was prepared using a two‐step hydrothermal procedure developed by Li and Afanasiev [2]. The nanorods preferentially expose (110) facets. Bimetallic Au‐Pd nanoalloys with well‐controlled composition were grown on these nanorods by alternate ablation of two monometallic Au and Pd targets in a UHV chamber. During particle nucleation and growth, the rods were kept at a temperature of 300°C, the nominal thickness of deposited metal was 1 nm. For ultra‐high resolution TEM imaging and X‐ray spectroscopy, a JEOL ARM 200F microscope was used. This microscope combines a cold field emission gun and a CEOS hexapole spherical aberration corrector (CEOS GmbH) to compensate for the spherical aberration of the objective lens. The microscope was operated at 80 kV to limit beam damage. Bimetallic nanoparticles with Au, Pd, Au 38 Pd 62 and Au 57 Pd 43 stoichiometries were synthesized. Their composition was precisely determined by EDX analyses of assemblies of particles. Single‐particle imaging of the as‐synthesized samples showed the formation of well‐separated NPs with size range 2‐8 nm. As a result of the poor epitaxy between the metallic NPs and their support, most NPs displayed droplet‐like morphology with ill‐defined crystalline structure. Wherever a higher degree of epitaxy prevailed, Au‐Pd NPs in the shape of truncated octahedra and having a completely disordered fcc structure (random alloy) were observed. Various epitaxial relationships were identified between the nanoparticles and the titania support, with the two dominant and previously unreported relationships being Au‐Pd(111)//Rutile(110)[1‐1‐1] and Au‐Pd(100)//Rutile(110)[1‐10] (Figure 1). With the precise equilibrium morphology of the NPs known, the interface and triple‐line energies of the metal/oxide systems were determined by combining particle size measurements in atomically‐resolved projected TEM images acquired parallel to the metal‐oxide interface and a generalized Wulff‐Kaishew theorem derived from Sivaramakrishnan et al . [4] (Figure 2). This theorem takes into account the influence of triple‐line energy on nanoparticle equilibrium shape. Interface and triple‐line energies were investigated as a function of particle composition and epitaxy. For any given epitaxial relationship, the relative amplitude of the NP truncation at the interface is found to increase linearly with particle size, i.e . the bigger the NP, the more it wets the oxide surface. On the rutile support, analysis of Pd, Au 38 Pd 62 and Au 57 Pd 43 NPs in epitaxial relationship Au‐Pd(111)//Rutile(110)[1‐1‐1] shows clearly that the interface and triple‐line energies are strongly influenced by particle composition and epitaxy. The value of the interface energy of the bimetallic Au‐Pd NPs γ i,Au‐Pd is about 1 J m ‐2 , which is about two times that of the monometallic Pd NPs, respectively (γ i,Pd = 0.5 ± 0.1 J m ‐2 ). As for the triple‐line energy, it is 0.8 ± 0.2 J m ‐2 for the monometallic Pd nanoparticles. This value is about four times the average triple‐line energy measured in Au‐Pd NPs.