Deposition of metallic powders from aqueous solutions without an external current source is presented. Metallic powders can successfully be produced via galvanic displacement reaction or by electroless deposition from homogenous aqueous solutions or slurries. Formation of powders such as Cu, Ni, Co, Ag, Pd, and Au from homogenous solutions using an appropriate reducing agent or Ag via galvanic displacement reaction was demonstrated. The hydrolysis of metallic ions is a crucial step in the deposition metallic powders via electroless deposition from homogenous solutions. To confirm that the hydrolysis phenomenon is essential in the electroless deposition of metallic powders, oxides, such as Ag 2 O, Cu 2 O, and CuO, were suspended in water. It was clearly demonstrated that these oxides can be successfully reduced with an appropriate reducing agent, leading to the precipitation of metallic powders.
Deposition of bismuth powders was investigated in this work. Bi powders can be obtained from heterogeneous systems e.g., via galvanic displacement reaction by an immersion of aluminum metal into Bi(III) complexed solutions. It was shown that, on the aluminum substrate, Bi powders can be obtained from acid, close to neutral or from alkaline solutions. Bi particles obtained via galvanic deposition are predominantly dendritic. To avoid the incorporation of Bi -oxides into the final product, it is recommended that the deposition via galvanic displacement reaction from K[BiI 4 ] or complexed Bi(III)-citrate-EDTA solutions on Al substrate is carried out in the acidic solutions in order to suppress the hydrolysis of Bi(III) ions. Alternatively, bismuth powders can successfully be produced from homogenous aqueous systems using Bi(III) complexed solutions as a source of Bi and an alkaline Sn(II) solution acting as a reducing agent of Bi(III) ions. The average particle size was about 0.25 μm for Bi powders produced from Bi(III) -citrate complexed solution using alkaline Sn(II) as a reducing agent. For Bi powders produced from K[BiI 4 ] solutions with Sn(II) as a reducing agent, the average particle size was estimated at about 0.65 μm.
Electrodeposition and electroless deposition of metallic powders were comparatively investigated. Electrodeposition of copper is accompanied with the simultaneous hydrogen evolution, which significantly influences the morphology of Cu powder. At lower overpotentials, branched dendrites were produced. At higher overpotentials honeycomb-like deposits of copper were obtained. Formation of silver powders was characterized by the comparison of the exchange and limiting current densities. Instantaneous growth of dendrites starts at low overpotential due to large exchange current density in silver nitrate solution. Formation of powders such as Ni, Co, Ag, Pd and Au from homogenous solutions using an appropriate reducing agent or via galvanic displacement reaction was demonstrated. The hydrolysis of metallic ions is crucial in the deposition metallic powders via electroless deposition from homogenous solutions. Oxides, such as Ag 2 O, Cu 2 O and CuO, suspended in water can successfully be reduced with an appropriate reducing agent, leading to the precipitation of metallic powders.
Galvanic processes on silicon surfaces in alkaline fluoride-free solutions containing Cu(II) ions were investigated in this work. Deposition of copper (I) oxide (Cu 2 O) and copper metal at pH 14 onto silicon surfaces at room or elevated temperatures has been demonstrated. This deposition does not require the presence of a reducing agent in the solution. The results clearly show that Cu(II) ions can be reduced to Cu(I) or Cu (0) Metallization of silicon with copper, silver and gold can be realized using electrochemical methods such as electrodeposition or electroless deposition from aqueous solutions. These methods are very attractive since they are simple and relatively easy to scale up. Practical difficulties arise due to the fact that at the surface of silicon wafers, oxide films of SiO 2 are present. SiO 2 films are non-conductive, and consequently, they must be removed prior to direct metallization. Although the metallization of silicon substrates can be achieved via autocatalytic deposition through surface activation with SnCl 2 /PdCl 2 catalysts, 1 this process would not remove the SiO 2 film leading to unacceptable electronic device performance. In order to remove the SiO 2 film and to deposit metals such as copper or silver from aqueous solutions directly onto silicon, pre-treatment steps are required during processing. These pre-treatment steps may involve fluoride-containing solutions, 2 which have deleterious effects on the environment. The direct deposition of copper, silver or gold from aqueous fluoride-containing solutions onto silicon substrates via the galvanic displacement reaction can be quite successful.3-6 According to recent investigations, silver can be successfully deposited onto silicon substrates from fluoride-free aqueous solutions.7 Reports on the deposition of copper onto silicon substrates via the galvanic displacement reaction from fluoride-free aqueous solutions have not been found in the literature. In an attempt to further explore the behavior of silicon in fluoride free solutions without a presence of reducing agents in the aqueous phase, the present communication describes preliminary observations on galvanic reactions on silicon substrates using Cu(II) containing alkaline fluoride-free aqueous solutions.Experimental p-(B doped) and n-(P doped) single crystal (100) silicon wafers were used as substrates in the present work. As received, silicon substrates were very carefully washed with 3:1 (H 2 SO 4 -H 2 O 2 ) solution and then with water and ethanol. After the cleaning procedure, silicon substrates were immersed into respective proprietary fluoridefree solutions containing Cu(II) ions. Immersions were performed at pH 14 using a CuC 4 H 4 O 6 (copper tartrate) complexed solution. The pH of this solution was adjusted with NaOH. The concentration of Cu(II) ions in alkaline solutions was 3.54 g/L. The immersion solutions in the present work were used as prepared. Consequently it should be assumed that O 2 was present in the solutions during the immersion.The specific immersion ...
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