Electrochemical Synthesis Of Gold-Based Nanocrystals In Deep Eutectic Solvents And Their Electrocatalytic Applications

Gold and its alloy nanocrystals（NCs）have been widely used as promising catalysts in various fields,such as sensors,electronics,optics,and fuel cells due to their fascinating physical and chemical properties.The control of the shape,surface structure and size of Au-based nanoparticles（NPs）has a crucial influence on their electrocatalytic properties.Deep eutectic solvents（DESs）,called a new generation of green solvents.Because of their remarkable physicochemical properties,DESs as promising solvents have broad applications in synthesis of nanoparticles,metal-organic frameworks and electrochemistry.In recent years,the shape-controlled synthesis of Au-based nanomaterials in DESs represents an intense research interest.As promising fuel,alcohols have potential applications in direct alcohol fuel cells（DAFCs）,and have also attracted great interest.In addition,electroorganic synthesis has become an established,versatile and environment-friendly alternative to traditional organic synthesis for the oxidation/reduction of organic compounds.Very recently,the application of electrochemical oxidation in electroorganic synthesis has received increasing attention.Based on the above research background,Au and Au-based alloy NCs with different shape,morphologies and properties were successfully prepared in different DESs by simple,green and shape-controlled electrochemical reduction.The effects of different electrodeposition conditions on morphologies,structures,and sizes of the as-prepared Au-based NCs were systematically investigated.Moreover,the as-prepared Au-based NCs were characterized by various characterization methods.Furthermore,the application of the as-prepared Au-based NPs in the electrocatalytic oxidation of alcohols and organic synthesis of electrooxidation were also investigated.The work was supported financially by the National Natural Science Foundation of China（No.21573058）,and the main research contents are as follows:1.Star-like Au NCs were prepared in urea-choline chloride DES（reline）by a shape-controlled one-step electrodeposition method.The effects of various influencing factors（such as the applied potential,electrodeposition temperature,HAuCl₄ concentration and electrodeposition time）,on morphologies,structures,and sizes of prepared Au NCs were systematically investigated by changing the electrodeposition conditions.Moreover,the as-prepared Au nanostars（NSs）were characterized by various characterization methods,and the mechanism of nucleation and growth of Au NSs was analyzed and discussed.Results show that reline acted as not only solvent but also shape-directing agent plays a crucial role in the nucleation and growth of the as-prepared Au NSs.This approach for preparing Au NSs in reline does not need any supporting electrolyte and organic solvent,and thus is more environmentally friendly than water as a medium.In addition,the electrode modified by the as-prepared Au NSs exhibits a good poisoning-resistance ability and a high electrocatalytic activity for ethylene glycol electrooxidation in alkaline media.2.Au NCs with specific dendritic nanostructures were synthesized in ethylene glycol-choline chloride DES（ethaline）using a facile,green and shape-controlled electrodeposition method.The effects of the applied potential on morphologies and structures of prepared Au NCs were studied.The morphology,structure and composition of the as-prepared Au nanodentists（NDs）were characterized by different characterization methods.Ethaline acted as solvent and shape-directing agent plays an important part in the nucleation and growth of the as-prepared Au NDs in the preparation process.This method is more environmentally friendly than water systems.Furthermore,the electrochemical properties of the as-prepared Au NDs modified electrode were also studied.The results show that the as-prepared Au NDs modified electrode exhibits a high electrocatalytic activity and poisoning-resistance for ethanol electrooxidation in alkaline media.3.Au NCs with hierarchical flower nanostructures were synthesized in glycerol-choline chloride DES（glyceline）using a simple,green and shape-controlled electrodeposition method.The effects of the applied potential on morphologies and structures of prepared Au NCs were also studied.The morphology,structure and composition of the as-prepared Au hierarchical nanoflowers（NFs）were characterized by different characterization methods.The results indicate that glyceline plays a dual role of solvent and shape-directing agent in the formation and growth of Au NFs.In addition,the method is also simple and green.Finally,the electrochemical properties of the as-prepared Au NFs modified electrode for the electrocatalytic oxidation of methanol in alkaline media were studied,showing a high electrocatalytic activity and poisoning-resistant ability.4.A green,facile and shape-controlled synthetic strategy was developed to synthesize Au NCs with different morphologies in three DESs（reline,ethaline and glyceline）with different water contents.In this strategy,the content of water was used for the first time as a key factor to control the morphologies and structures of Au NCs.The morphologies,structures and sizes of Au NCs can be tuned by changing the content of water（10¹00%,v/v）.Rsults show that the water content is particularly important to shape-controlled synthesis of nanomaterials in DESs.In addition,the mechanism of the formation and growth of prepared Au NCs was analyzed and discussed.This new strategy for controlling the morphology and structure of prepared nanoparticles by changing the content of water in DESs will also have significance for the shape-controlled synthesis of other noble metal nanocrystals.5.We adopted a unique strategy,i.e.,adding moderate water（10%）into ethaline,so that the DES both maintains its intrinsic states and characteristic and reduces its melting point and viscosity.Using this strategy,a flower-like AuPt alloy nanoparticles（AuPt NFs）were prepared by one-step electrochemical co-reduction method at a low temperature of 30°C and a low potential of-0.30 V（vs.Pt）.The effects of various influencing factors on morphologies,structures,and sizes of prepared Au NCs were systematically investigated by changing the electrodeposition conditions.Moreover,the as-prepared AuPt NFs were characterized by various characterization methods.In this process,the ethaline acted as solvent,shape-directing agent and auxiliary reductant.In addition,this strategy is very simple and environmentally friendly.6.For the first time,we introduced the prepared nanomaterial modified electrode as the anode to electroorganic synthesis.Xanthone（XO）was synthesized easily by electro-oxidation of xanthene（XT）under a constant low potential（0.80 V,vs.Ag/AgCl）and room temperature in air.Moreover,the effects of experimental conditions,such as electro-oxidation method,oxidation potential,anode material and reaction time,and reaction atmosphere,on the electro-oxidation reaction were systematically investigated.Moreover,the synthesis process was milder and more environment-friendly than conventional organic syntheses.Very significantly,we used the potentiostatic method instead of common galvanostatic method that require relatively high applied voltages,and consequently can control the progress of the reaction and avoid side reactions to carry out.Compared with other commercial electrodes,the constructed AuPt NFs modified electrode has better electro-oxidation effect.XO was synthesized in a high isolated yield of 87%after 16 h electro-oxidation.This new synthetic strategy of direct electro-oxidation using a functionalized electrode can extend the scope of electroorganic synthesis and improve the efficiency of the electro-synthesis,and would have broad application prospects in electroorganic synthesis fields.