Synthesis and Catalytic Activity of Silicon Nanowires Decorated with Noble Metal Nanoparticles

In this paper, a new nanomaterial catalyst for methylene blue reduction was prepared. The design rationale is explained as follows: (1) silicon nanowires (SiNWs) show a number of remarkable advantages, such as easy surface modification with various noble metal nanoparticles (NMNPs), large surface-to-volume ratio, and stability to atmosphere environment. However, metal particles usually have large size and low density because of their aggregation; (2) Sodium dodecyl sulfate (SDS) is used as stabilizer, therefore metal nanoparticle size can be effectively controlled; furthermore, due to the weak-interaction between SiNWs and NMNPs, the catalytic activity may be mantained. Therefore, it is anticipated that the SiNWs can be modified by NMNPs with high density so as to become high efficient catalysts. In details, SiNWs were grown on Si wafers by thermal evaporation of SiO powder at 1350 degrees C for 6 h, under a 300 Ton flowing gas mixture of 5% H-2-Ar. The as-prepared SiNWs were etched 3 min with aqueous HF to remove their surface oxide layer. With SDS as protector, noble metal nanoparticles (NMNPs) were loaded on silicon nanowires (SiNWs) via the redox reaction between Mn+ and HF-treated SiNWs for 5 minutes. In order to gain AuNPs/SiNWs composites with high coverage density and uniform size of AuNPs, experiment parameters were adjusted, such as the SDS/Au molar ratio (200 : 1, 1000 : 1, 1200 : 1), the HAuCl4 concentration (1.0 X 10(-5), 2.5 X 10(-5), 4.0 X 10(-5) mol/L), and reaction temperature (15, 30 degrees C). Meanwhile, Different NMNPs/SiNWs composites, such as PtNPs/SiNWs and AgNPs/SiNWs were also prepared. Finally, AuNPs/SiNWs were used in the reduction of methylene blue in the presence of sodium borohydride. In a representative reduction experiment, 0.1 mL of methylene blue solution (2.5 X 10(-3) mol/L) was mixed with 2.8 mL of Na2CO3/NaHCO3 buffer solution (pH=9.5), and purged with N-2 gas for 5 similar to 6 min to remove all dissolved oxygen. The solution showed a lambda(max) at 664 nm. Then 5 mg the as-prepared catalysts and 0.15 mL 0.01 mol/L of freshly prepared sodium borohydride were added. The progress of the reaction was monitored by a spectrophotometer. As a result, 55% methylene blue was reduced in 60 min, which showed AuNPs/SiNWs had excellent catalytic activity. And it was found that 62% methylene blue was reduced in 100 min, and the reduction was going on even after 160 min reaction. These composites could be separated from reaction solution, thus ensuring recycling of nanocatalysts.