Compositional tuning of gas-phase synthesized Pd-Cu nanoparticles

Abstract

Bimetallic nanoparticles have gained significant attention in catalysis as potential alternatives to expensive catalysts based on noble metals. In this study, we investigate the compositional tuning of Pd-Cu bimetallic nanoparticles using a physical synthesis method called spark ablation. By utilizing pure and alloyed electrodes in different configurations, we demonstrate the ability to tailor the chemical composition of nanoparticles within the range of approximately 80 : 20 at% to 40 : 60 at% (Pd : Cu), measured using X-ray fluorescence (XRF) and transmission electron microscopy energy dispersive X-ray spectroscopy (TEM-EDXS). Time-resolved XRF measurements revealed a shift in composition throughout the ablation process, potentially influenced by material transfer between electrodes. Powder X-ray diffraction confirmed the predominantly fcc phase of the nanoparticles while high-resolution TEM and scanning TEM-EDXS confirmed the mixing of Pd and Cu within individual nanoparticles. X-ray photoelectron and absorption spectroscopy were used to analyze the outermost atomic layers of the nanoparticles, which is highly important for catalytic applications. Such comprehensive analyses offer insights into the formation and structure of bimetallic nanoparticles and pave the way for the development of efficient and affordable catalysts for various applications.

Fig. 1. Experimental set-up for the spark discharge generator.

Fig. 2. Compositions of nanoparticles generated using different electrode configurations measured by TEM-EDXS and XRF. PXRD compositions were estimated from the linear correlation given by Zen's law (Fig. 3 and Table SI1†). Occurring crystal structure types (fcc or ordered bcc (CsCl-type)) are given in the top panel. Shadowed region marks the difference in the measured composition from the different techniques.

Fig. 3. Time-resolved XRF measurements of the composition of the nanoparticles as generated from a Pd anode and a Cu cathode (“Pd + Cu”) and from a Cu anode and a Pd cathode (“Cu + Pd”).

Fig. 4. Mean volume V̄ atom versus Pd content for Pd–Cu binaries and elemental Pd and Cu, respectively. Phases are labeled by their Pearson symbol (see Table SI3†). For this study (open symbols), unit cell volume (from Rietveld refinement of PXRD data) was divided by the number of atoms per unit cell and plotted versus compositions measured by XRF (red open symbols) and TEM-EDXS (blue open symbols). The dotted blue line connects samples with the same electrode composition. The broken line denotes behavior as expected from Zen's law.

Fig. 5. HRTEM (a)–(c) and STEM line scans (d)–(f) of nanoparticles generated with a Pd anode and Cu cathode (a) and (d), PdCu anode and cathode (b) and (e), and a PdCu anode and Cu cathode (c) and (f).

Fig. 6. (a) Cu 2p_3/2 core level spectra and (b) Pd 3d_5/2 core level spectra of PdCu nanoparticles generated using alloyed electrodes. Three different photon energies were used for each sample to generate photoelectrons with a kinetic energy of 100 eV (red), 300 eV (green), and 500 eV (blue) in order to probe different depths in the samples. The dashed lines show the reported and measured peak values. A Shirley background profile has been applied and subtracted for all spectra.

Fig. 7. NEXAFS spectra for PdCu nanoparticles generated by alloyed electrodes. Partial electron yield (PEY) and total electron yield (TEY), both normalized to incoming intensity, with pre-edge intensity set to zero and normalized to the maximum intensity.