Additive-free And Template-free Electrochemical Approach To Control The Surface Morphology And Crystal Orientation Of Platinum Micro-/Nanoparticles And Studies On Related Electrocatalytic Performance

Pt-based noble metals have been widely used in a wide range of applications such as the energy conversion and environment protection due to their excellent catalytic performance.However,the scarcity of Pt source and its high cost strongly hinder the large-scale applications.Therefore,developing the Pt catalysts with high catalytic activity and reducing the Pt loading is of great significance for the commercial application of Pt catalysts for energy conversion and environment protection.Currently,studies on the Pt-based noble metals have mainly focused on the preparation of Pt micro-/nanoparticles with controlled surface morphology and crystalline orientation.There have been many preparation methods for controlling the surface morphology and the crystalline orientation,such as seed growth methods,microemulsion methods,hydrothermal/solvothermal synthesis,and templating methods.However,most of the preparation methods require the use of additives and templates,and the subsequent processes for removing the additives and templates may decrease the performance of Pt catalysts.Therefore,in this work,the surfactant-free and template-free electrochemical methods,such as galvanostatic method,cyclic voltammetry and square-wave pulse,were applied for the preparing Pt micro-/nanoparticles with controlled surface morphology and crystalline orientation on various typical conducting substrates such as the indium tin oxide?ITO?,glass carbon?GC?and TiO₂ nanotube arrays.The effect of electrochemical process parameters on the surface morphology and crystalline orientation of Pt catalysts was systematically studied and the corresponding formation mechanisms were discussed.The effects of the surface morphology and crystalline orientation on the electrocatalytic performance of Pt catalysts were systemically studied through the ammonia oxidation or methanol oxidation as the typical model reactions.The main research content was as follows:1.The flower-like Pt particles were prepared using galvanostatic electrodeposition method,and the effect of the electrodeposition current density on the surface morphology of Pt particles was studied.The formation mechanism of the Pt particles with various surface morphologies was discussed based on the nucleation and kinetics of crystal growth.The performance of corresponding Pt catalysts for ammonia electro-oxidation was investigated.The results showed that the Pt particles exhibited spherical,flower-like and sheet-like morphologies in turns as the current density increased from 0.2 to 1.0 mA cm^-2,and all possessed face-centered-cubic?fcc?polycrystalline structure.The effect of the electrodeposition current density on the surface morphology of Pt particles is realized through affecting the key processes during the whole electrodeposition process including the electrochemical reduction,liquid phase diffusion and atomic surface diffusion.The flower-like Pt particles had much higher mass-specific activity(0.063 mA?g^-1)for ammonia oxidation than that the spherical Pt particles(0.022 mA?g^-1).The improved catalytic activity of flower-like Pt particles is not only from the increased electrochemically active surface area,but also from the high specific activity resulted from the presence of large amount of defect atoms at the tips and edges of the flower-like Pt particles.2.A series of Pt micro-/nanoparticles with controlled and complex surface morphologies were prepared through cyclic voltammetry method and by adjusting the low limit potential E_L and sweep rate v.The results showed that the surface morphology of Pt micro-/nanoparticles evolved from smooth spherical to complicated 3D morphologies with the E_L shifting negatively from 0 to-0.6 V?SCE?when the sweep rate was 0.05 V s^-1.As the E_L was-0.1 V?SCE?,the sweep rate has no influence on the surface morphology of Pt particles,while the E_L was-0.6 V?SCE?,the surface morphology of Pt particles in turns exhibited flake-like,flower-like,prickly spherical and cauliflower morphologies with the sweep rate increasing from 0.05 to 20 V s^-1.Electrochemical measurements showed that the mass activity of Pt catalyst with smooth spherical morphology prepared at E_L of-0.1V?SCE?for ammonia oxidation was 0.018⁰.021 mA?g^-1,while the mass activity of Pt catalysts with complicated surface morphology prepared at E_L of-0.6 V?SCE?for ammonia oxidation(0.044⁰.052 mA?g^-1)was much higher than the former.3.Highly dispersed Pt nanosheets were prepared by square-wave pulse electrodeposition method.The result showed that the morphology of deposited Pt particles strongly depended on the lasting time of the low limit pulse t_l.As the t_l decreased from 1 s to 0.01 s,the prepared Pt catalyst in turns exhibited flower-like,highly dispersed nanosheets,prickly spherical,cauliflower and spherical morphologies,and all possessed the face-centered-cubic?fcc?polycrystalline structure.The results of methanol electro-oxidation measurements showed that the flower-like Pt particles and highly dispersed Pt nanosheets have higher mass activity(0.39 and 0.45 mA?g^-1)than the Pt particles with smooth spherical morphology(0.12 mA?g^-1).4.The Pt catalyst with flower-like morphology was prepared by the electrodeposition on the surface of TiO₂ nanotube arrays prepared using anodic oxidation technique.The catalytic activity of Pt/TiO₂ nanotubes for ammonia oxidation was studied with or without the irradiation of ultraviolet light.The results showed that the specific activity of the Pt/TiO₂ nanotubes catalyst for ammonia oxidation was enhanced by 39%with the irradiation of ultraviolet light than without that.The improved performance of the Pt/TiO₂nanotubes for ammonia oxidation is attributed to the photo-generated holes in the TiO₂which could oxidize ammonia molecules during UV illumination.5.The Pt/Ni bimetal catalyst with flower-like morphology were prepared based the flower-like Ni particles after Pt replacement for 20 min.The surface morphology of Pt/Ni bimetallic catalyst strongly depended on the time of Pt replacement reaction.Electrochemical measurements showed that the flower-like Pt/Ni bimetallic catalyst?after Pt replacement for 20 min?have higher mass activity(0.113 mA?g^-1)for ammonia oxidation,which is 3 times higher than that of the Pt/Ni bimetal catalyst after Pt replacement for 60 min and commercial Pt/C(0.036 mA?g^-1),and 5 times higher than that of the spherical Pt/Ni bimetallic catalysts.The enhanced mass activity could be attributed to the presence of large amounts of the Pt atoms at the top and edge locations on the surface of flower-like particles,which possessed low coordination and high activity.6.Pt nanoparticles with?100?preferential orientation were prepared by adjusting the electrodeposition current density.The effect of the deposition current density on the degree of Pt?100?preferential orientation was studied and the corresponding formation mechanism was discussed.The catalytic activity of the prepared Pt catalysts was characterized by ammonia electro-oxidation.The results showed that the value of h₁/h₂ from H₂SO₄ CVs increased from 0.85 to 1.3 as the electrodeposition current density increased from 0.2 to 10mA cm^-2,suggesting the increase of the exposed Pt?100?sites.Irreversible adsorption of Ge and the deconvolution of hydrogen desorption further indicated that the degree of?100?preferential orientation increased with the increase of the electrodeposition current density.The specific activity of Pt catalyst for ammonia oxidation markedly increased with the increasing preferential degree of Pt?100?preferential orientation,and the enhanced catalytic activity was mainly attributed to the Pt?100?active sites.7.Cubic Pt particles were prepared by square-wave pulse electrodeposition,and the productivity of cubic particles was above 70%.The size dispersion was narrow,which was about 174 nm.Both hydrogen desorption and irreversible adsorption of Ge experiments suggested the formation of Pt particles with preferential Pt?100?crystalline orientation.As a result,the specific activity of cubic Pt particles was 4.6 times higher than the polycrystalline Pt.The primary innovations in the present work include:1.Pt particles with various complicated surface morphologies were prepared by the additive-free and template-free galvanostatic electrodeposition and cyclic voltammetry method.There are lots of surface tips and edges on the Pt surface.Due to the presence of large amount of defect Pt atoms with low coordination and high activity at the top and edge locations on the surface,the specific activity of prepared Pt catalysts can be markedly enhanced.2.The rough Pt nanoparticles with?100?preferential crystalline orientation and cubic Pt particles enclosed by?100?facets were prepared by the additive-free and template-free galvanostatic electrodeposition and square-wave electrodeposition,respectively.The obtained Pt catalysts exhibit high specific activity for the electrochemical reactions which are sensitive to the Pt?100?sites.3.Based on the high specific activity of flower-like Pt particles,the Pt/TiO₂composite photoelectrocatalysts with flower-like Pt particles and the flower-like Pt/Ni bimetallic catalysts with low Pt loading were designed and prepared.This can further improve the electrocatalytic activity of Pt-based electrocatalysts and reduced the Pt loading.