Preparation Of Porous Silicon And Its Application In The Detection Of Heavy Metal Ions

Recently, heavy metal pollution has becomes increasingly serious with the rapidly growing economy, which has directly affects ours living environment and threaten people’s life and health. Thus, the method with the advantage of fast and effective detection of metal ions concentration in solution must be expected, which is very important significance for human health and environmental safety.In this paper, nano-technology and organic synthesis technology have been jointly exploited. A fluorescent sensor was proposed based on fluorescence quenching feature of porous silicon, and the electrochemical metal ions sensors were developed using the organic group functionalized porous silicon, which shows selective preconcentration or detection for some metal ions. The main research contents are as follows.Porous silicon (PS) was prepared by anodizing highly doped p-type silicon in the solution of H2O/ethanol/HF. The effects of key fabrication parameters (HF concentration, etching time and current density) on the nanostructure of PS were carefully investigated. According to the experimental results, a more full-fledged model was developed to explain the crack behaviors on PS surface. The PL spectra blue shift of the sample with higher porosity is confirmed by HRTEM results that the higher porosity results in smaller Si nanocrystals. A linear model (λPL/nm=620.3-0.595P, R=0.905) was established to describe the correlation between PL peak positions and porosity of PS.Porous silicon nanowires (PSNWs) were fabricated by the metal-assisted chemical etching (MACE) method, the effects of fabrication parameters on the nanostructure and photoluminescence of silicon nanowires (SiNWs) were investigated. The simple pre-oxidization process is firstly used to treat the starting silicon wafer, and then PSNWs are successfully fabricated from the moderately doped wafer by one-step MACE technology in HF/AgNO3system. According to the experiment results, a model was proposed to explain the formation mechanism of porous SiNWs by etching the oxidized starting silicon. PSNWs were also fabricated by the’one-pot procedure’(MACE method in the HF/H2O2/AgNO3system. The experimental results indicate that porous structure can be introduced by the addition of H2O2. When H2O2concentration is0.1mol/L, numerous almost perpendicular pore channels can be observed in the etched silicon. A self-electrophoresis mode driven by H2O2reduction is proposed to explain the PSNWs formation.The porous silicon sample with strong and stable PL emission was selseted by comparing the PL properties of porous silicon fabricated under different conditions. The effects of various metal ions species (Cd2+、Co2+、Cr3+、Mn2+、Ni2+、Pb2+、Zn2+、Cu2+) on PL and surface chemisity of PS were studied. The results indicate that the copper ions with higher redox potential can deposite on PS surface, which leads the surface oxidization and reduction of SiHx group and causes the quenching of PL. The relationship between the PL intensity and copper ions concentration was established, the PL intensity of PS is decreased as the description of the equation of IPL=1269.6-15[CCu2+] when copper ions concentration is between5×10-7～50×10-7M.3-aminopropyltriethoxysilanes (APTES) and thiosemicarbazide derivative (TSCD) modified porous silicon electrode (PSE) has been respectively used for the detection of Ag+and Pb2+in aqueous solution. Under the optimal experimental conditions, the cathode peak currents of APTES-PSE were increased with Ag+concentrations over the range from1×10-3mol L-1to1×10-7mol L-1and the linear equation of｜Ipc｜(μA cm-2)=118.28+15.15×1gCAg+(CAg+/mol L-1)(R2=0.97173) was established. A multi-step graft route was designed for anchoring TSCD groups on porous silicon (PS) surface. The results display that the anodic peak current density increase with the increasing Pb2+concentration over the range of1×10-6mol L-1～1×10-3mol L-1and which has a good fitting with a second order exponential function, I=-156exp(-1562[Pb2+])-260exp(-105[Pb2+])+446(R2=0.998).The cleavable groups (benzimidazoledithi, BDT) were grafted on the PS surface by a stepwise covalent process. The pre-enrichment efficiency of BDT-PS was investigated at the different pH for the different metal species (Cd2+、Cu2+、Hg2+、Pb2+and Co2+). The results show that the BDT-PS has sensitive pre-enrichment for Cd ions (enrichment efficiency>95%). The bond length, the gaps of HOMO and LOMO and energy decrease before and after the chelation of coordination compounds (BDT-M2+) were calculated by Gaussian09. The results indicate that the ligand BDT has sensitive chelating ability for different metal ions (Cu2+>Pb2+>Cd2+>Hg2+>Co2+). The disulfide bonds in un-chelated BDT can be cleaved by glutathione (GSH) and leading the2-mercaptobenzimidazole (MBI) loss. The new disulfide bonds would form with the addition of GSH and the MBI-M2+was replaced by glutathione derivative (GSHD). But, compared with MBI-Cu2+、MBI-Pb2+、 MBI-Hg2+and MBI-Co2+, only MBI-Cd2+has a poor chelating with the resulting GSHD and can enter into GSH solution, which leads the BDT-PS shows a sensitive pre-enrichment proprety for Cd2+.