Shape-controlled Synthesis Rhodium Nanoflowers And Their Electrocatalytic Properties

In recent years,platinum group metal Rh nanomaterials have received considerable attention for its wide applications in many areas,including optical sensors,drug design,catalytic reactions,novel fuel cells and others due to their unique optical,electrical,acoustic,magnetic and thermal properties.Their special properties are closely related to the morphological structure.Therefore,the morphology control synthesis of Rh nanomaterials has attracted much attention.So far,the morphology of reported Rh-based nanomaterials mainly focuses on the shapes of cube,octahedron,hexadecahedron and starfish,and so on.The dominant synthetic approaches include one-step synthesis,hydrothermal reaction and solvothermal methods.The flower-like nanostructures attract wide interest due to their artistic and functional characteristics.In this study,Rh nanomaterials with several new morphologies were prepared by adjusting the type of the reducing agents,reaction temperature,reaction pressure,various stabilizers,the solvent effects and other kinetic effects in nanomaterial growth.The relevant electrocatalysis performance was also investigated.The reaction was performed at 145°C in an oil bath for 3 hours.The products were characterized by Transmission Electron Microscopy?TEM?,powder X-Ray Diffraction?XRD?,Scanning Electron Microscope?SEM?and X-ray photoelectron spectroscopy?XPS?and High Resolution Transmission Electron Microscopy?HRTEM?,respectively.The effects of temperature,reaction time,reducing agent,the amount of AA and PVP on the formation of raspberry Rh nanoparticles were investigated.The forming mechanism was explained.The results shown that AA promoted the nucleation and growth of Rh atoms and the synergistic interaction between AA and PVP played a critical role to produce raspberry Rh nanoparticles.The optimal molar ratio of Na₃RhCl₆:PVP:AA was 1:10:50.The three-dimensional ultrathin nanoflowers formed with Rh nanosheets were synthesized by using Rh?acac?₃ as a precursor,oleylamine?OA?and dimethylformide?DMF?as a co-solvent,oleylamine as a reducing agent as well as a morphological controller by means of solvothermal method without any stabilizers at 200°C for 8.5hours.The products were characterized by TEM,XRD,SEM,XPS and HRTEM.The effects of temperature,reaction time,reducing agent,the amount of AA and PVP on the formation of three-dimensional ultrathin nanosheets were investigated.The forming mechanism was discussed.The results showed that it was critical for the formation of three-dimensional ultrathin nanosheets to control reaction time,temperature,solvent and the kind of precursor.In the reaction system,the optimum volume ratio of DMF:OA was 1:4,the concentration of Rh?acac?₃ was 5 mM.Uniform and well-defined Rh nanonail flowers were successfully prepared by using Rh?acac?₃ as a precursor,mixture of oleylamine and ethylamine as a co-solvent,reductant and morphological controlling agent,PVP as a stabilizer at 200°C for 8hours.One Rh nanonail flower was consisted of 12 nanonails.The nail cap and column of a Rh nanonail flower showed pentagonal cone and pentaprism,respectively.The length and the cross-width of the column were 40 nm and 8 nm,respectively.The products were characterized by TEM,HRTEM,SEM,XRD and XPS.The effects of temperature,reaction time,the kind of solvent,the amount of PVP on the formation of Rh nanonail flowers were investigated.The forming mechanism was preliminarily explored.The results showed that the use of oleylamine and ethylamine was the key for the formation of Rh nano-nail flowers.The optimum condition for the formation of Rh nanonail flowers was as follows:the concentration of Rh?acac?₃ was 5 mM,the molar ratio of Rh?acac?₃:PVP was 1:4,the volume ratio of ethylamine:oleylamine was1:4,solvothermal temperature was 200°C,and the reaction time was 8 h.The forming process of Rh nanonail flowers was as follows:with the progress of the reaction,the dodecahedral nanoparticles were formed firstly serving as seeds.Then,nail caps and nail columns generated.Finally,Rh nanonail flower consisted of 12 nanonails for each one was produced.The electrocatalytic oxidation of formic acid and methanol for the as-synthesized Rh nanonail flowers,3-dimensinal ultrathin Rh nanoflowers,raspberry Rh nanoparticles were investigated,and those of Rh black and small Rh nanoparticles were used as contrasts.The results showed that the electrocatalytic activities of these Rh nanoflowers were improved significantly and dependent upon their morphologies.The order of electrocatalytic activities of these Rh nanoflowers was as follows:Rh nanonail flowers>Ultra-thin Rh nanoflowers>Raspberry-like Rh nanoflowers>Rh black>Rh nanoparticles.The electrocatalytic activities of Rh nanonail flowers,3-dimensinal ultrathin Rh nanoflowers,raspberry Rh nanoparticles were 4.2?3.6 and2.9 times that of Rh black for the electrocatalytic oxidation of formic acid,respectively,while 3.5?3.0 and 2.5 times that of Rh black for the electrocatalytic oxidation of methanol,respectively.