The Preparation And Study Of Functional Metal Film On Silicon Electrode

In this thesis, the functional nano-films of Ag, Au, Pd, Pt, Ru, Ni and Co on p-Si(100) and porous silicon (PS) were prepared by electrochemical techniques. Thepromising application in electrochemical catalysis and microreactor were discussed.The morphologies of films were characterized with Atomic Microscope (AFM) andScanning Electron Microscopic (SEM), while' its compositions were analyzed byElectron Dispersive Spectroscopy (EDS). The properties of films and mechanism ofmetal deposition were investigatedby electrochemical technologies, such as CyclicVoltammeter (CV), Linear Sweep Voltammeter (LSV), Open Circuit Potential～time(Ocp～t), Amperometric i～t, A.C. Impedance (IMP) and Differential PulseVoltammeter (DPV). The main issues of this thesis were described as follows:1. The anodic properties of p-Si (100). Metal ion reduction accompanies anodicdissolution of silicon, so the anodic dissolution behaviors of p-Si (100) wereinvestigated firstly by LSV and IMP in HF and NH₄F solution separately. Two currentpeaks J₁ and J₃ occurred on i～V curves which separately mean direct dissolution(dissolution valence is 2) and indirect dissolution (dissolution valence is 4) of silicon.The mixed potentials of redox reaction in this experiment are less than 2.0 V andbelow the potential of J₁, so the dissolution valence of silicon is 2 during metaldeposition. A single capacitive loop arising from space charge layer was occurred onNyquist plots at measured frequencies, which showed that the dissolution of siliconwas controlled by electrochemical reaction.2. The preparation of porous silicon (PS). A macro-porous silicon template withporous size ofΦ2.5μm×1.7μm fabricated by anodizing p-Si(100) in HF (40ï¼…):CH₃CH₂OH (absolute)=1:1(volume ratio) bath at current density of 40 mA·cm^-2 for30 min and then immersed into 10ï¼…HF solution for 8 h to remove the khaki surfacelayer for baring the bright bottom. Fluorescence showed that the emission wavelength was 441 nm under the excitated wavelength of 386 nm, and the fluorescence intensitywas 900 cd with 2.5 nm narrow gap.3. The preparation of metal films on p-Si (100). The Ag, Au, Pd and Pt films werefabricated by immersion deposition on p-Si (100) in HF bathcontaining noble metalsalt. Experiments revealed that the nucleation of Ag and Pd were instantaneous whileAu and Pt were progressive, and the nuclei growth obeyed 3D island model. Metaldeposition mechanism was mainly local-cell corrosion type, the reduction of metal ionand oxidation of silicon occurred not at same location, electron produced by oxidationof silicon can transfer through silicon substrate and pre-deposits. At same conditions,the stripping peaks of Pd deposits on p-Si (100) are higher one order than that ofothers. Ni films were prepared on Pd-activated p-Si (100) by electroless deposition inNiSO₄+NH₂NH₂ plating bath, and the thermodynamic and kinetic properties ofelectroless deposition were modeling by Ocp～t and Amperometric i～t.4. The preparation of metal films on PS. Ag, Au, Pd and Pt films were prepared on PSby immersion deposition in acidic HF bath, while Ni, Co and Ru films were depositedon PS in alkaline NH₄F bath. The different deposition behaviors were dependent onpH of solution, mixed potential of redox couple, dissolution rate of silicon, complexformation and equilibrium potential of metal ion. The equilibrium potential ofNi²⁺/Ni, Co²⁺/Co and Ru³⁺/Ru are more negative than H⁺/H₂ in HF solution, so thehydrogen evolution disturbed the deposition of metal. But in NH₄F solution, theequilibrium potential of H⁺/H₂ was more negative, the dissolution rate of silicon wasincreased; and the metal complex M(NH₄)_xⁿ⁺ was formed, so those that can notreduced in HF bath can deposit on silicon in NH₄F bath.5. A thick Ni template with thickness of 200μm was fabricated by electroplating afterpre-deposited 15μm Ni layer by electroless deposition on PS. And then peeled offfrom PS by ultrasonic after dipping in 10ï¼…HF solution for 10 min. SEM morphologies showed that Ni deposits filled the pores completely, however, theadhesion between Ni layer and PS substrate was low. Ni template exhibits manysmooth orbicular microchannels on its interface and presents promising application infabricating microheater.6. Position-selective metal deposition on PS. Experiments showed that noble metalpreferentially deposited on pore borders than bottoms of PS. The ratio is about 4.6 forAg deposits at 10 s, while the morphologies of PS almost remained original; Au, Pdand Pt had similar cases. This character can be used to fabricate low-cost microreactorcompositive with catalyzer. Micro-porous materials of Ni, Co and Ru with higherspecific surface area were fabricated while the macro-pores of PS were filled withdeposits. Those materials present promising application in industrial catalyzer.