Studies Of Formation Of Silicon Nanowires By Electroreduction Of Porous Ni/SiO₂ And NiO/SiO₂ Blocks In Molten Salts

Silicon nano wires (SiNWs) are one of the most important 1D semiconductors, which have become a critical research field in nanomaterials and nanotechnology. In this thesis, SiNWs with straight morphology were prepared via the direct electroreduction of porous Ni/SiO2 or NiO/SiO2 blocks. The hot-pressing process was studied in order to fabricate porous Ni/SiO2 blocks, together with the analysis on the process parameters of the preparation of straight SiNWs. Moreover, the characterization on the structure of synthesized SiNWs was conducted by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), high-magnification transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Further analysis on SiNWs was also implemented, in terms of photoluminescence (PL) and electrochemical properties. Based on the study on the interaction between porous Ni/SiO2 blocks and the melt and on the electrolysis process, the metal-catalyzed electrochemical nucleation and growth mechanism was proposed.SiO2 powders were mixed with nickel powders, and then hot pressed (compacting pressure:10 Mpa, hot-pressing temperature:950℃ or 1200℃, dwell time:75min) to form porous Ni/SiO2 blocks. It should be noted that after hot-pressing at 950℃, SiO2 in the blocks remained granule morphology, which is beneficial for the preparation of SiNWs. Large quantities of SiNWs with straight morphology and a wire diameter distribution of 80-250nm were successfully produced by the electrochemical reduction of porous Ni/SiO2 (mole ratio of SiO2 to Ni is 10) blocks at 1.5 V against graphite crucible as the anode in molten CaCl2 at 900℃ for 5h. It is confirmed that nickel-silicon droplets were found on the tips of the SiNWs.Further study indicated that after immersing in molten CaCl2 at 900℃, the dissolution of nickel and SiO2 in the porous blocks to the melt hardly occurred, neither did the reaction between the materials in the blocks and the melt. However, micro-structure of porous Ni/SiO2 blocks immersed in the melt did not change. Results from FESEM showed that the morphology of SiO2 changed from particle to porous network and exhibited partially or wholly melted form, while nickel particles were coated by the melted SiO2 and connected to the SiO2 network. The reduction occurred initially at the contacting interline between the Mo wire and the block in molten CaCl2, and then proceeded radically at the dynamic Ni(Mo, NiSix)/melted SiO2/CaCl2 three-phase interface with the passage of time on the surface of the block and in 3-D dimensions from the outside into the interior of the block.Cyclic voltammetry was performed in the three-electrode manner in molten CaCl2 with Mo-Ni/SiO2 contacting electrode as the working electrode, graphite rod as the counter electrode, and a self-made Ag/AgCl electrode as the reference electrode. Results showed that partially or wholly melted SiO2 at the Ni/SiO2/CaCl2 three-phase interline was first reduced and formed Si-Ni alloy with nickel particles. Cross-sectional FESEM image of a block after 1-min electrolysis revealed that the prepared SiNWs had straight morphology and droplets at their tips with the diameter close to that of Nanowires. Those droplets led the growth of SiNWs along the path of silica network.Based on the results above, with consideration of the mechanism reported before, the growth mechanism of nanowire was proposed which can be explained as follows. Upon immersing the Mo-Ni/SiO2 contacting electrode into molten CaCl2 at 900℃, nano-silica was partially or wholly melted to form network structure connecting with nickel particles. At the initial stage, liquid nickel silicides were formed at the Ni/SiO2/CaCl2 three-phase interface, and then nickel-silicon droplets were produced. The droplets served as new conducting sites, at which the silica was reduced to silicon. Further reduction of silica at this site and continuous dissolution of silicon into the droplets led to the supersaturation of silicon, eventually leading to the oriented growth of SiNWs.SiNWs were also produced by the electrochemical reduction of porous NiO/SiO2 bolcks (The mole ratio of SiO2 to NiO in the blocks is 10) at 1.5V in molten CaCl2 at 900℃ for 3h. NiO additives could promote the growth of straight SiNWs with diameters ranging from 60 to 150nm. The produced SiNWs were analyzed by HRTEM, SAED and EDS. The results showed that the nanowire had outer amorphous layers with thickness about 2.5-3.1nm and single crystal silicon core. NiO was reduced to nickel during the electrolysis process, and the Ni/SiO2/CaCl2 three-phase interline could be formed at which nano-sized nickel-silicon droplets were produced. The droplets could seed the growth of straight SiNWs.