Preparation Of Cold-based Noble Metal Aerogels With Controlled Surface Facets And Study On Their Facet-dependent Electrocatalytic Performance

Due to their unique physical and chemical properties,noble metallic nanoparticles(NPs)have great application value in catalysis,sensing and electronics.Among them,noble metallic aerogels have become very promising materials in the field of catalysis due to their large surface area and high porosity,and therefore they have attracted the attention of researchers.In heterogeneously catalyzed reactions,the performance of the catalyst is closely related to its terminal surface structure.Over the past few decades,researchers have successfully achieved control of the surface facets of monodisperse nanostructured catalysts,including nanospheres,nanorods,nanowires,and thin films.However,it is difficult to effectively control the surface structure of the three-dimensional(3D)noble metallic aerogels.The improvements in electrocatalytic performance of noble-metallic aerogels are still achieved by improving the known properties of traditional aerogels(high porosity and high surface areas).The surface properties of ligaments(such as lattice strain,defect density,and its correspondence to local surface curvature)in noble-metal aerogels towards electrocatalytic performance are less investigated.Manipulating surface facets of noble-metal aerogels to make it suitable for different catalytic reactions is still at an early stage.In addition,among the designed structures of electrocatalysts,the core-shell structure with an ultrathin shell serves as an ideal catalyst for maximizing the utilization of rarer(or more expensive)shell elements and tuning the electronic structures with synergistic effect to have higher electrocatalytic properties.Therefore,it is very meaningful to design an aerogel electrocatalyst with a core-shell structure.However,there are still significant challenges in experimentally preparing aerogels with core-shell structures.Therefore,it is very promising to develop a simple,green method for preparing aerogels with core-shell structure and to control the crystal planes on the aerogel surface to be accessible for different catalytic reactions.The main research contents of this paper are as follows:(1)Synthesis of gold aerogels with regulated surface facets.We have successfully achieved the regulating of surface planes of 3D Aum-n aerogels by NaCl-induced assembly of citrate-stabilized gold nanoparticles(Au NPs)of two different sizes and further size-dependent localized Ostwald ripening at controlled particle-number ratios,where m and n represent the size of Au NPs,respectively.Based on the analysis of the UV-Vis spectrum,TEM and the color change of the solution of the intermediate products during the assembly process,the possible formation process of 3D Aum-aerogels was discussed.Based on the intrinsic defect of gold nanoparticles and the newly generated defects during fusion,the resulting 3D Aum-n erogels have more abundant defects.Combined with high-resolution TEM images and electrochemical tests,the types and proportions of exposed crystal planes on the three types of Aum-n aerogels prepared from three groups of Au NPs of two different sizes were determined.In addition,thermodynamic models were used to further analyze the possible formation mechanism of the surface crystal planes of the three different Aum-n aerogels.(2)Synthesis of core-shell Au@Pd bimetallic aerogels.The optimal particle-number ratios of two differently-sized gold nanoparticles used for synthesis of the Aum-n aerogels with optimal electrocatalytic performance among the three different Aum-n aerogels were determined through the CV curves under alkaline conditions.3D Aum-ny@PdX aerogels were prepared by addition of optimal amounts of aqueous Na2PdCl4 solution during the formation process of Aum-n aerogels with optimal electrocatalytic performance,where x and y represent the molar fractions of Pd in the shells and Au in the cores,respectively.Due to the epitaxial growth of ultrathin Pd shells on the corresponding Aum-n aerogels,the types of defects and exposed crystalline facets on the surfaces of the corresponding Aum-n aerogels are also hardly affected.Thus,the resulting Aum-n@Pd aerogels bear the same exposed crystalline facets as the corresponding Aum-n aerogels.The characteristics of core-shell structured Aum-ny@Pdx aerogels were further investigated by high resolution TEM images and X-ray photoelectron spectroscopy.Similarly the crystal plane of the Pd shell can be tuned by controlling the crystalline structures of gold substrate.(3)Study on facet-dependent performance of the resulting aerogel electrocatalysts(Aum-n aerogels and Aum-ny@Pdx aerogels).Taking the electrooxidation of small organic molecules(such as methanol and ethanol)by the resulting Aum-n aerogels and Aum-ny@Pd.aerogels as examples,it is found that surface facets of metal aerogels with excellent performance can be regulated to realize preferential surface facets for methanol electro-oxidation and ethanol electro-oxidation,respectively,due to facet-dependent performance.Moreover,their activities and stability are also highly determined by the area ratio of active facets and inactive facets on their surfaces,respectively,and these ratios are varied via the mismatch of sizes of adjacent nanoparticles.Thus,this work not only indicates the realization of the regulation ofthe surface facets of metal aerogels by size-dependent localized Ostwald ripening,but also will open up a new way to improve electrocatalytic performance of 3D metallic aerogels by surface regulation.In addition,compared with commercial Pd/C,it is found that the specific activity of Aum-ny@Pdx aerogels is about 10 times higher than commercial Pd/C.The core-shell Aum-ny@Pdx aerogels exhibited greatly improved Pd utilization efficiency for ethanol oxidation reaction demonstrating the superiority of the core-shell structural aerogels which provides a new possible strategy for the design of core-shell electrocatalysts in the future.