Study On The Crystal Face, Structure Control And Electrocatalytic Performance Of Pd-based Nanomaterials At Sub-10nm Scale

The surface and structure characteristics of the final products can be effectively controlled by precise regulation of the thermodynamic and kinetic factors affecting the growth of palladium-based nanomaterials,so as to improve their electrocatalytic activity and stability.However,it still remains a great challenge to accurately control the size（smaller than 10 nm）,crystal facets and morphology of palladium-based nanoparticles at the same time.In addition,under the category of small size（smaller than 10 nm）,how to modify and optimize the surface structure of palladium-based nanomaterials,this part of the research is still lacking.Based on the above,this research used a solvent method to synthesize the high-purity sub-5 nm Pd tetrahedrons（Sub-5nm Pd Ths）,investigated the thermodynamic and kinetic factors during its growth to analyze the reason why the crystal plane and structure can be controlled accurately in the small size.Subsequently,we focused on the surface engineering of the Sub-5 nm Pd Ths via the seed-mediated growth method,as a result,the sub-10 nm Pd@Pt core-shell truncated octahedrons（Pd@Pt TOhs）and Pd@Ru core-shell tetrahedrons which Pd tetrahedrons enclosed by Ru（111）facets[Pd@Ru（111）Ths]were successfully synthesized and served as the effective catalysts for oxygen reduction reaction（ORR）and acidic hydrogen evolution reaction（HER）.The main research contents are as follows:1.In this chapter,we synthesized Sub-5 nm Pd Ths with high conversion rate and purity（>98%）by one-pot solvent method,and quantitatively analyzed the effects of various thermodynamic and kinetic factors in the system on the control of crystal growth by combining density functional theory（DFT）-calculation and ultraviolet spectrum.Studies have shown that the product morphology can be effectively controlled by adjusting the dispersion state of additives in this system.When 1,1′-binaphthylene and ammonia(C₂₀H₁₄+2NH₃·H₂O)were used as additives,strong adsorption and relatively fast reduction rate may force it to grow into a thermodynamic control product with low specific surface energy,so as to obtain Sub-5 nm Pd Ths.When the two-NH₂ were bound to C₂₀H₁₄[1,1′-binaphthyl-2,2′-diamine(C₂₀H₁₆N₂)]as additives,relatively weak adsorption and slow reduction rate make it tends to be anisotropic growth mode,forming laciniate Pd nanourchins（Pd LUs）.Compared with Pd LUs（0.843 V）and commercial Pd black（0.830 V）,the half-wave potential（0.864V）of Sub-5nm Pd Ths positively shifted nearly 21 and 34 m V respectively,and had excellent stability at the same time.2.The Sub-5 nm Pd Ths synthesized in the above chapter were used as the crystal seed,and the Pd@Pt TOhs with the size controlled within 10 nm were successfully prepared with Na₂Pt Cl₆ as the precursor,PVP as the stabilizer,AA as the reducing agent by slow injection reduction.Owing to large specific surface area,low precious metal Pt loading,regular shape and controllable material size,the material shows excellent catalytic activity and stability toward ORR under alkaline condition.The electrochemical results showed that the half wave potential（0.886 V）of Pd@Pt TOhs positively shifted about 22 m V compared with that of commercial Pt black catalyst（0.864 V）.After 1000 cycles of accelerated durability tests,the half wave potential of Pd@Pt TOhs only negatively shifted 3 m V,indicating that the synthesized Pd@Pt TOhs exhibited excellent electrocatalytic activity and stability for ORR.In particular,compared with Sub-5 nm Pd Ths,the activity of Pd@Pt TOhs was significantly improved,which proves that optimizing the surface composition and structure of ultrafine Pd tetrahedron at sub 10 nm scale is an effective approach to improve the catalytic performance.3.Similarly,the sub-10 nm Pd@Ru（111）Ths were prepared by using the Sub-5nm Pd Ths as seeds via seed-mediated growth method.The shell thickness of Ru grown on the surface of Pd tetrahedron was only 1.1 nm,and it displayed a typical face centered cubic（fcc）crystal type,which is different from the traditional Ru hexagonal closest packed（hcp）crystal type.The unique crystal surface characteristics of the internal Pd（111）tetrahedron is the key to control the external Ru tending to fcc structural growth.The obtained material exhibited excellent catalytic activity and stability toward HER in the acidic condition.Electrochemical results showed that the overpotential of Pd@Ru（111）Ths was 68 m V at the current density of 10 m A cm^-2,and the Tafel slope was 30 m V dec^-1.After 3000 cycles of accelerated durability tests,the overpotential of Pd@Ru（111）Ths only negatively shifted 3 m V at the current density of 10 m A cm^-2,showing that Pd@Ru（111）Ths had excellent electrocatalytic performance and stability for HER.