Study On Electrodeposition Process,Mechanism And Application Of Conductive Metal

Gold coating is extensively employed in electronic industry,aerospace and decoration for its excellent corrosion resistance,malleability,electrical and thermal conductivities.Cyanide,which is used on a large scale in industrial gold plating production,poses a great threat to the environment,operators and social safety in its production,transportation and use.Carrying out research on the mechanism of gold electrodeposition and developing novel cyanide-free gold electrodeposition technology is in line with the common requirements of the country,society,environment and enterprises,which has important theoretical and practical significance.In this paper,the effects of chloride ions on the bath stability,coating performance of gold sulfite electrodeposition process was studied in detail firstly by ultravioletvisible absorption spectroscopy,electrochemical experiments,electrochemical surface enhanced Raman spectroscopy（EC-SERS）and other techniques.On the basis of clarifying the beneficial effects of chloride ions for the first time,a novel cyanide-free gold electrodeposition process directly using commercial chloroauric acid as the main salt was developed.The process parameters were obtained by Hull cell test and the gold coatings were characterized by scanning electron microscope（SEM）,gloss meter,Xray diffraction（XRD）and other techniques.Based on the novel cyanide-free gold electrodeposition process,the surface/interface process of gold electroreduction,sulfite electrode reaction and the action mechanism of hydroxyethylidene diphosphonic acid（HEDP）are studied by electrochemical experiments,EC-SERS,and density functional theory（DFT）calculations.The main results are as follows:（1）The effects of chloride ions on cyanide-free gold electrodepositionIt is very convenient using commercial chloroauric acid as the main salt of gold sulfite electrodeposition bath.UV-vis absorption spectroscopy show that chloride ions improve the stability of gold sulfite solution and delay the disproportionation reaction of Au（I）.Cyclic voltammetry curves show that chloride ions accelerate the cathodic reduction of Au（I）.SEM results show that chloride ions refine the rod-shaped gold grains.Chloride ions are beneficial for gold sulfite electrodeposition.Besides,the structural transformation and electrode reactions of sulfite on gold electrode surface is illuminated particularly.EC-SERS spectra show that with the potential shifts negatively,sulfite adsorbed on the gold surface first experiences a structural transformation(SO₃^2-→S₂O₅^2-),then following an electroreduction(S2O52→S₂O₃^2-).The cathodic reduction of Au（Ⅰ）suppresses the electroreduction of S₂O₅^2-to S₂O₃^2-.Chloride ions promote the cathodic reduction of Au（Ⅰ）and therefore the electroreduction of S₂O₅^2-can be suppressed at a more negative potential.（2）Electrodeposition and characterization of gold coatingThe novel cyanide-free gold electrodeposition process was developed by Hull cell experiments.The bath composition and operating conditions are HAuCl₄ 2 g/L（calculated as gold）,Na₂SO₃ 30 g/L,hydroxyethylidene diphosphonic acid（HEDP）10 g/L,addictive-2-Mercaptobenzothiazole（MBT）10 mg/L,addictive-ISS 20 mg/L,temperature 40～50℃ pH 8.5～9.5 and current density 0.1～0.8 A/dm2.The current efficiency of the process can reach up to 75%and decreases with the increase of the current density,which is in line with the current efficiency characteristics of complex electrodeposition.The dispersion ability and covering ability of the bath are 69.0%and 100%,respectively.The gold coating is pure with a face-centered cubic mixed crystal structure.The resistivity of the coating is 2.5×10^-8 Ω·m,which is very close to that of pure gold(2.4×10^-8 Ω·m).The particles of the gold coating gradually increase with the increase of the coating thickness meanwhile the appearance becomes red and foggy.（3）Control of sulfite dimerization at solid-liquid interfaceThe solid-water interface process for sodium sulfite solution and sodium gold sulfite solution is illustrated at the molecular level for the first time.In the gold electrode-sodium sulfite solution interface,at least two layer of SO₃^2-ions are adsorbed on the gold surface.With the potential shifts negatively,the length of the adsorbed Au-O bond increases from 0.2316 nm（-0.1 V vs.Hg/HgO）to 0.2373 nm（-0.1 V vs.Hg/HgO）,indicating that the adsorption strength of the first layer sulfite decreases gradually.And then the first layer sulfite dimerizes with the second layer SO₃^2-into S₂O₅^2-.Last,S₂O₅^2-is reduced to S₂O₃^2-.When HEDP is introduced to the gold electrode-sodium sulfite solution interface,the secondary layer SO₃^2-is replaced by HEDP and the first layer SO₃^2-is directly desorbed with the weakening of the adsorption strength.This control phenomenon is more intuitive at the gold electrode-gold sodium sulfite solution interface.At a relatively negative potential（minus-0.9 V）,following the electroreduction of Au（SO3）23-,S₂O₅^2-is stably adsorbed on the cathodic surface.At the interface of gold electrode-the mix solution of gold sulfite and HEDP,sulfite dimerization is controlled by the introduced HEDP and therefore SO₃^2-is stably adsorbed on the cathodic surface at a relatively negative potential.At the same time,the varieties of dissociated and reduced gold complexes are relatively single,and the grains of the gold coating are more uniform.Electrochemical experiments show that the control of sulfite dimerization by HEDP promotes and accelerates the cathodic reduction of gold complex ions.