High-index core–shell Ni–Pt nanoparticles as oxygen reduction electrocatalysts Leteba, G.M., Mitchell, D.R.G., Levecque, P.B.J., Macheli, L., van Steen, E. and Lang, C.I. 1 Department of Chemical Engineering, Catalysis Institute, University of Cape Town, South Africa; 2 School of Engineering, Macquarie University, Australia, South Africa, 3 University of South Africa, Department of Civil and Chemical Engineering, College of Science, Engineering and Technology (CSET), South Africa Introduction: Understanding the structure-activity-composition relationships in heterogeneous catalysis allows for the logical design of nanostructures for a certain reaction process.1 Optimized synthetic techniques for generating Pt-based alloy nanoparticles are in high demand2. Surfactants are acknowledged to play roles in influencing the size and morphology of nanoparticles.4 Multicomponent surfactant mixtures are used to offer varied growth rates in different crystallographic orientations. Kinetic control and selective adhesion of surfactant molecules on growing crystals can direct nanoparticle growth into structures with promising catalytic properties.5 Herein we demonstrate a facile, and direct co-thermal decomposition synthetic approach to form unusual core− shell Ni−Pt nanoparticles. In the systems investigated, we evaluate the effects of ternary mixtures of hydrophobic surfactants, which contain an array of distinct functional groups, on morphological evolution of PtNi colloids. The synthesized alloys particles displayed demonstrated remarkable catalytic activity toward ORR in terms of both mass and intrinsic specific activities when compared to commercially available Pt/C electrocatalysts. Materials and Methods: In a typical synthesis, Ni(Ac)2.4H2O (0.25 mmol) and H2PtCl6 (8 wt. % in H20, 0.25 mmol), oleylamine (OAm, 20 ml), octadecylamine (ODA, 2.4g) and oleic acid (OLEA, 20 ml) were dissolved in 1-octadecene (1-OD, 25 ml). The resulting solution was then heated at 150°C for 5–10 minutes under vigorous magnetic stirring in a beaker and transferred into a round-bottom flask. The reduction of the metal precursor salts was induced by both the high temperature and the surfactants. The effects of surfactant variation on the structural evolution of these nanoparticles were investigated by replacing OLEA with trioctylamine (TOA, 20 ml) and dioctylamine (DOA, 20 ml) while keeping the other reaction parameters constant. Subsequently, the nanoparticles were extracted from the synthesis media by adding excess ethanol. After settling, the excess organic solvents were decanted, and the particles were cleaned by re-suspending in an ethanol/acetone mixture of equal volumes. This process was repeated five times. The black product was finally recovered. Results and discussion: A mixture of surfactants results in surface functionalization, which controls the morphological evolution of nanoparticles. The nanoparticles exhibited complex chemical growth zoning, rich in Pt geometric topologies, which varied as a function of surfactant mixture. Compositional mapping, using high-angle annular darkfield scanning transmission electron microscopy, coupled with energy dispersive X-ray spectroscopy, highlights core−shell structures ( ∼ 1 nm Pt coating thickness) with edge−vertex Pt-enrichment and Ni-rich faces. These core−shell Ni−Pt nanoparticles demonstrated enhanced activities toward the oxygen reduction reaction (ORR) compared to commercial Pt/C, even after extended potential cycles (5000). Our synthetic approach, which utilizes the surfactants’ array of distinct functional groups, offers new avenues toward the formation of concentric core−shell structures with multifaceted topologies. Significance: Surfactants vary the morphology of the resulting alloy References
1. Wang, C. et al . ACS Catal. 2011 , 1 (10), 1355−1359. 2. Wu, J. al . Acc. Chem. Res. 2013 , 46 (8), 1848−1857. 3. Chen, C. al. Science 2014 , 343 (6177), 1339− 1343 4. Leteba, G. M. et. al. Nanomaterials 2018 , 8 (7), 462. 5. Yin, Y. Alivisatos, A.P. Nature 2005, 437 (7059), 664−670
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