Study On The Evolution Of Au(111) Facetduring The Electrocatalytic Oxidation Of Thiourea

The importance of thiourea-assistant gold leaching lies on its environment-friendly properties for the leaching of gold mine of which the production is increased every year in China.However,detailed mechanism of gold leaching by thiourea is rarely reported especially in the field of the interaction between thiourea and gold at mesoscopic level.Using in-situelectrochemical atomic force microscopy,scanning electrochemical workstation,scanning electron microscopy and energy spectrum,and plasma spectral analysis test methods,we studied the Au(111)crystal surface morphology evolution in process of thiourea electro catalytic oxidation.Additionally,the density functional theory was applied to simulate the experimental phenomena.The effect of thiourea on the crystal structure of Au(111)was studied in different electric potential ranges.Referencing with Pt electrod,the open circuit potentialof the reaction between gold and thiourea solution was-0.14 V(vs.Ptwire).In open circuit potentialrange of gold dissolution by thiourea,the Au(111)crystal surface is first dissolved layer by layer,and then transformed into a wave-dissolution.The causes of layer-dissolution and wave-dissolution is explained as follows.The coordination number of gold atoms in defect edges and steps is low,so the gold atom is the active site.The whole crystal surface is dissolved at a faster rate because of the initial number of crystal defects too much.Due to the space steric effect and the diffusion effect,the defects are not transitive in the dissolution of the crystal surface.With the dissolution of the crystal surface,the number of defects decreased gradually in crystal surface.The active site on the crystal surface is only the gold atoms on the steps.The dissolution of the crystal surface occurs only on the step and the dissolution wave is formed along the steps.Above open circuit potentialrange of gold dissolution by thiourea,due to the influence of applied potential,thiourea and gold atoms on the surface of Au(111)are easier to bond.No matter the steps,the defect edges,or the gold atoms on the terraces,it can react with thiourea,and the Au(111)crystal surface shows an undifferentiated dissolution rapidly.Slightly under open circuit potential range of gold dissolution by thiourea,the bonding between thiourea and gold atoms on Au(111)crystal surface is inhibited becsuse of the influence of applied potential.Even the gold atoms with higher activity at the steps and the edge of the defect cannot be combined with thiourea,which leads to the dissolution of the Au(111)surface being suppressed.Au(CS(NH2)2)2+could be reduced to gold atoms far below open circuit potentialrange,and deposited on the Au(111)crystal surface.Due to the double influence of space steric and active position of Au(111)surface,Au(111)crystal surface appears multiple layered deposition simultaneously.Using the density functional theory,the interaction between thiourea and gold atoms of Au(111)crystal surface was simulatedunder different electric fields.When the external electric field is removed,the gold atoms on the terrace of the crystal surface do not react with thiourea,while the gold atoms on the steps react with thiourea.The external electric field can accelerate the reaction between thiourea and gold atoms on Au(111)surface.When the electric field reaches,the gold atoms on the terrce of the crystal surface can also be dissolved by thiourea.The simulation results are in good consistent with the experimental results.In addition,effects of the AFM probe and the impurities on the dissolution of Au(111)facet in thiourea solution was studied,respectively.During the scanning process,the AFM probe weakened the bound of the gold atoms in the scanned region and accelerated the dissolution of Au(111)crystal surface while the probe was used to exert the mechanical force on the scanned gold atoms.Meanwhile,the impurity on the Au(111)crystal surface decreases the binding energy barrier between thiourea and gold atoms,causing the dissolution of the Au(111)crystal surface to form layer-dissolution from wave-dissolution.