Metal Oxide Coated Porous Silicon Electrode Fabricated by Anodized Liquid Phase Depositions

We fabricated titanium oxide/PSi nanocomposite by anodic liquid phase deposition (LPD) method. This method is soft-solution process for preparation of metal oxide thin film via hydrolysis of metal fluoro-complex with assist of F^- scavenger (e.g. H₃BO₃, SiO₂ and Al). Titanium oxide is superhydrophilic material and changes little volume during Li insertion and extraction, therefore this composite is expected to be high efficiency anode material. PSi was formed by etching of silicon wafer(p-type, 3-4 mΩ•cm) with 60 mA/cm² in HF(47 %):EtOH (1:1, in vol) for 10 min[3] and  immersed in (NH₄)₂TiF₆-H₃BO₃ solution (LPD-H₃BO₃), thermaloxidized and immersed in (NH₄)₂TiF₆ solution (LPD-SiO₂) or anodized in H₂TiF₆-EtOH solution (LPD-anodized Si). Each sample was characterized by SEM-EDX, XPS, N₂-isotherm and electrochemical measurement. The deposition amount of titanium was quantified by ICP-AES.The diameter of PSi was ca.7.4 nm and titanium oxide was deposited on all samples. However, the fine structure of LPD-H₃BO₃ was removed by covering of titanium oxide. After depositon of Ti oxide, the Ti2P binding energy decreased during the LPD reaction measured by XPS.  This result suggests titanium oxide deposition derived from removal of fluorine from metal fluoro-complex via hydrolysis with H₃BO₃, SiO₂ or anodized Si. Anodic electrodepostion of titanium oxide on PSi was performed at 0.3 V vs Ag/AgCl, and the deposition amount; ca. 3x10^-6 mol m^-2 as shown in Fig.1.