Study On The Structure Manipulation And Electro-catalytic Properties Of Nanoporous Gold

Due to their unique stable bicontinuous structure with empty channels and solid ligaments,high surface-to-volume ratio and low density,nanoporous materials have the wide potential applications in fields such as electrocatalysis,sensors,actuators and lithium-ion battery and fuel cells.The insert transition metal element Au is the study object in this thesis.We choose the binary Al2Au alloy as the precursor alloy and HCl solution as the dealloying solution.The microstructure,phase composition,electro-catalytic activity,etc,were investigated using X-ray diffractometer(XRD),advanced electrochemical workstation,scanning electron microscopy(SEM),transmission electron microscopy(TEM).The influence of the precorsor ribbon circumferential speeds,magnetic field,and graphene on the formation and catalytic performance of nanoporous gold were mainly discussed.The main research contents can be described as follows:(1)The relationships between the precursor ribbon circumferential speeds and catalytic activity of nanoporous gold for methanol and H2O2Nanoporous gold(np-Au)with a three dimensional(3D)bicontinuous interpenetrating ligament-channel structure has been prepared by dealloying the rapidly solidified Al0.66Au0.34 ribbon precursors with different circumferential speeds(Sc),and the dealloying solution is 5 wt.%HCl solution.The catalytic activiies for the methanol and H2O2 were investigated by CV tests and potentiodynamic polarization tests.The Al0.66Au0.34 precursors have a decreasing lattice constant(a0),a decreasing preferred(220)orientation factor F(220),and an increasing F(111)with increasing Sc,indicating an increase of compressive stress in the ribbons.After the dealloying procedure,np-Au samples have an increasing F(111)and a decreasing F(200)with decreasing Sc,which shows the heredity between precursors and np-Au samples.Meanwhile,the np-Au samples have an increasing a0 and a decreasing pore size with increasing Sc.On the one hand,the content of intermediate phase AlAu in precursor ribbons increases with increasing Sc.The size of ligament/pore of nanoporous gold 0can be refined by the intermediate phase in the dealloying process.On the other hand,the inner compressive stress exists in the precursor ribbons;however,the nanoporous gold samples present tensile stress after dealloying,suggesting that Au atoms in the precursor with a higher Sc need overcome a higher initial compressive pressure in the rearrangement process.The liagment/pore size of nanoporous gold decreases with increasing Sc.Methanol molecules can be oxidized on np-Au surface through two different mechanisms,both are dependent on the F(100)and F(111)as well as the pore size of np-Au samples.And Au(111)face plays an important role in improving the catalytic performance on methanol eletro-oxidation reaction(MOR).Moreover,np-Au exhibits a good linear responding as well as high sensitivity for the concentration of H2O2 in phosphate buffered solutions(PBS).These results indicate that np-Au with a smaller pore size has a higher catalytic activity both for MOR and H2O2 detection.In other words,we can change the pore size of np-Au by adjusting the Sc of the precursors;and then we can enhance the catalytic activity of np-Au.(2)Manipulation the formation and electrochemical properties of nanoporous gold through adding the magnetic fieldNp-Au samples with the 3D bicontinuous interpenetrating ligament-channel structure were prepared by dealloying the rapidly solidified Al0.66Au0.34 ribbon in 0,0.02,and 0.2 T magnetic fields,and the dealloying solution is 5 wt.%HCl solution.The microstructure and phase composition were investigated by XRD,SEMM and TEM.The catalytic activities for methanol and CO were investigated by CV tests,Tafel tests,EIS tests,and CA tests.Np-Au dealloyed in 0.02 T owns the smallest lattice constant a0 and the highest preferred orientation factor F(111)of(111)face,indicating that np-Au dealloyed in 0.02 T bears the largest compression stress.The addition of magnetic field leads to a formation of intermediate phase AlAu and a slow dealloying rate.The morphologies of np-Au dealloyed for 24 h in 0,0.02,and 0.2 T(labeled as DA0,DA1,and DA2,respectively)are the maze structure,the uniform honeycomb-like structure with the small ligament/pore size and the soda crackers-like structure with thick ligaments,respectively;and their phase compositions are coarse nanocrystals,fine nanocrystals + trace amorphous phase,and finer nanocrystals + a few amorphous phase,respectively.The formation of fine nanocrystals is due to the nucleation sites supplied by the intermediate phase AlAu in dealloying.The surface coverage of the redox species Γ*charge-transfer rate constant ks from the CV tests,exchange current density j0 and corrosion potential Ecorr from the Tafel tests and charge transfer resistance Rct from the EIS analysis show that the MOR activity of three samples is in this order:DA1>DA2>DAO,which can be attributed to the crystallographic,thermodynamic and defective reasons.In other words,we can change the microstructure of the dealloyed np-Au by adjusting the magnetic field strength in the dealloying process and then we can enhance the catalytic activity of np-Au.(3)Effect of graphene on the formation of nanoporous gold and nanoporous gold/graphene nanocompositeThe nanocomposite of nanoporous gold and reduced oxidized graphene(NPG/RGO)were prapared by dealloying the Al0.66Au0.34 ribbons and powders,respectively.The catalytic activities for elctro-oxidation of methanol were investigated by CV tests,Tafel tests,and CA tests.The graphene oxide sheets are reduced by H2 generated from the reaction of Al and HCl,and then attached on the porous Au.And the nanoporous gold samples were prepared by dealloying Al0.66Au0.34 ribbons with 0,5,10,15,and 20 ml 2 mg/ml GO solution in 200 ml 10 wt.%HCl solution,which were labeled as S1,S2,S3,S4,and S5,respectively.The lattice constant a0 of dealloyed Au ribbon samples are larger than standard Au,indicating the tensile stress is dominant in the dealloyed Au.And the Au ribbons have an increasing a0 as the GO content increases.There are D and G bands of S2～S5 in the Raman spectrums,which means that the RGO can be effectively attached on NPG structure.The results of TGA indicate that wt%of RGO increases with increasing RO contants.SEM shows that S1 is the typically 3D bicontinuous ligament/channel structure,while the several almost transparent RGO layers inhomogeneous distribute in S2 surface.The thickness of RGO layers increases and the pore size decreases with increasing GO contents.According to the TEM,the phase compositions of S1 and S2 are single crystal with regular crystal face arrangement and polycrystal with a tremendous amount of Au nanocrystals + few RGO crystals,respectively.The real surface area of S2 is largest,indicating it can supply more active sites in electro-oxidation reaction.According to the CV and Tafel results,the catalytic activity for methanol electro-oxidation reaction(MOR)of samples is in the order:S2>S3>S4>S5>S1,which means that the addition of moderate RGO can enhance the catalytic activity of NPG for MOR.The Al0.66Au0.34 powders were ball-milled into micro order powders using the high energy ball-milling.Then nanoporous gold powders were obtained by dealloying the Al0.66Au0.34 powders in 200 ml 10 wt.%HCl solution with addition of 5 ml 2 mg/ml GO solution,which marked as P2.And the blank powders samples(without GO)were marked as P1.The SEM,Raman and TGA results demonstrate that the RGO can be effectively attached on NPG.According to the CV and Tafel results,the catalytic activity for MOR of P2 is higher than P1.Moreover,P2 depicts a higher stability than P1 according to the chronoamperograms(CAs).These consequences supply a new method for us to obtain the nanoprous gold with excellent microstructure and catalytic activity through adding the graphene oxide in dealloying process.