Pd-Ru core-shell nanoparticles with tunable shell thickness for active and stable oxygen evolution performance

Keyword ruthenium, oxygen evolution, ruthenium nanoparticles, core-shell nanoparticles, electrocatalysis

Lucy Gloag, Tania M. Benedetti, Soshan Cheong, Richard F. Webster, Christopher E. Marjo, J. Justin Gooding, Richard D. Tilley*

Electronic Supplementary Material (ESI) for Nanoscale.This journal is © The Royal Society of Chemistry 2018

Experimental methodsPd nanoparticles:The 4.7 ± 0.4 nm Pd seed nanoparticles were prepared using a procedure modified from Hyeon and co-workers,[19] and Yu and co-workers.[20] 0.05 g palladium acetylacetonate ([Pd(acac)2], Aldrich, 99%) was dissolved in 5 mL oleylamine (Aldrich, 70% oleylamine) and in a 25 mL three-necked flask equipped with a stir bar. The solution was heated under Ar to 100 °C and kept at this temperature for 30 minutes. 0.25 mL trioctylphosphine (Aldrich, 97 %) was then injected into the solution. The temperature was further raised to 250 °C at a heating rate of 5-11 °C min-1 and maintained for another 30 minutes. After cooling down to room temperature, the solution was washed once with equal amount of methanol for 20 minutes at 4000 rpm. The washed product was stored in 2 mL of mesitylene. (Aldrich, 98 %).Pd-Ru core-shell nanoparticles:0.01 g ruthenium acetylacetonate ([Ru(acac)3], Aldrich, >99%) and 86 µL oleylamine were dispersed in 1 mL mesitylene and dissolved by sonication at 60 oC. A volume of Pd nanoparticle solution (100 – 623 µL) was further purified by centrifugation in a 1:1 mixture toluene:methanol for 10 min at 14000 rpm, repeated 5 times. The Ru(acac)3 solution was added to the purified Pd seeds and further sonicated for 5 min. The mixture was transferred to a Fischer-Porter bottle, which was then evacuated from air and filled to 2 bar H2 gas. The Fischer-Porter bottle was placed in an oil bath at 130 oC for 16 h. Nanoparticles were collected by centrifugation with equal amounts of ethanol at 14000 rpm for 10 min.Catalyst preparation: Pd-Ru core-shell nanoparticles and Vulcan XC (Fuel cell store) carbon were sonicated for 2 h in 20 mL hexane to give a 10wtRu%. The catalyst was collected by centrifugation and washed with acetone once. Surface surfactant was removed by heating the carbon loaded nanoparticles at 185 oC for 5 h in air. Commercial Ru/C (20 wt%, 2.5-3.5 nm diameter) were obtained from Fuel Cell Store. Mass loading was characterized by EDX analysis of the carbon loaded nanoparticles on a Hitachi S3400 scanning electron microscope.Characterization:TEM samples were prepared by drop casting a solution of nanoparticles in toluene on a carbon coated copper grid and dried under ambient conditions. The low resolution TEM analysis was performed on a Phillips CM200 microscope operated at 200 kV. The high resolution TEM, STEM and EDX analysis were performed on a JEOL JEM-F200 operated at 200 kV. This microscope is equiped with a 2k x 2k CCD camera, ADF and BF detectors for STEM imaging and a JEOL 100 mm2 SDD EDX detector. STEM images were obtained with a covergence angle of 8.2 mrad, a bright field collection angle of 10 mrad and an ADF inner collection angle of 55 mrad. EDX results analysed using Pathfinder software have been filtered to remove background counts and Bremsstrahlung radiation. XRD characterization was performed on an Empyrean-II powder diffractometer (PANalytical, Netherlands) fitted with a 10 mm slit, recorded between 30o and 70o. X-ray photoelectron spectroscopy (XPS) was conducted on a ESCALAB250Xi (Thermo, United Kingdom). The sample was mounted onto clean aluminium foil, then measured using a 500 μm diameter beam of monochromatic Al Kα radiation (photon energy = 1486.6 eV) at a pass energy of 20 eV. The core level binding energies (BEs) were aligned with respect to the C 1s BE of 285.0 eV. Peak fitting was performed on the Thermo Avantage software.

Electrochemical measurements: The working electrode was prepared by drop casting a solution containing the nanoparticles on carbon onto a glassy carbon RDE. A 2.5 mg mL-1 solution of nanoparticles was prepared by dispersing 1 mg catalyst in a mixture containing 75 % water, 24 % isopropanol and 1 % Nafion (5 wt % in isopropanol). The mixture was sonicated for 15 min to form a homogeneous nanoparticle ink. The ink was deposited onto a glassy carbon rotating disk electrode (RDE) to give a loading of 12.8 µg cm-1. The electrode was dried at 60 oC in a vacuum oven for 5 min. The electrolyte used was 0.1 M HClO4 (pH = 1). The electrochemical measurements were performed using a µAutolab potentiostat with a Ag/AgCl (3 M KCl) and a Pt mesh as the reference and counter electrodes, respectively. All potentials are reported

against the reversible hydrogen electrode (RHE) by adding 0.294 V to the measured accordingly to the measured potential difference between the Ag/AgCl (3 M KCl) and a RHE prior to the experiments. The electrocatalytic activity of the nanoparticles was studied by cyclic voltammetry (CV) in the potential range from 1.00 to 1.53 V (vs. RHE) at a scan rate of 50 mV s-1 in 0.1 M HClO4 as electrolyte while rotating the working RDE at 1600 rpm. The electrochemically active surface area was calculated from Cu under-deposition by running CV scans in the potential range 1.00 to 1.53 V (vs. RHE) in a 0.1 M solution of CuSO4.

Figure S1. a) Low resolution TEM image of the 4.7 ± 0.4 nm Pd nanoparticles used as seeds. b) Histogram of the size distribution of Pd nanoparticles generated from measurement of 500 nanoparticles.

Figure S2. EDX spectra of the Pd-Ru nanoparticles with a) 0.3 nm, b) 0.5 nm, c) 0.8 nm, and d) 1.2 nm shells.

Figure S3: High resolution TEM images of the Pd-Ru nanoparticles synthesized with Pd:Ru ratios of a) 0.3 nm, b) 0.5 nm and c) 0.8 nm shells.

Figure S4: X-ray diffraction patterns for the Pd-Ru core-shell nanoparticles with 0.3 nm (top), 0.5 nm (middle) and 0.8 nm (bottom) shells matching with reference patterns for fcc-Pd (green) and hcp-Ru (purple).

Figure S5: Low resolution TEM images of Pd-Ru core-shell nanoparticles loaded onto carbon with 10 wtRu% for electrocatalysis. The nanoparticles shown have a) 0.3 nm, b) 0.5 nm, c) 0.8 nm, and d) 1.2 nm shells.

Figure S6: High-angle annular dark field (HAADF), and STEM-EDX maps of Pd (green) and Ru (purple) for Pd-Ru core-shell nanoparticles with a) 0.3 nm, b) 0.5 nm and c) 0.8 nm shells.

Figure S7: X-ray photoelectron spectroscopy analysis of Pd-Ru core-shell nanoparticles with 0.5 nm shells indicating 3d binding energies for Ru (3d5/2 279.9 eV) and Pd (3d3/2 340.2 eV, 3d5/2 335.0 eV).