Catalytic Properties And Surface Modification Of Silicon Nanowire Arrays

Silicon nanowire arrays (SiNWAs) are vertically aligned nanowire arrays withdiameters less than100nm. The physical nature and quantum size effects endowSiNWAs with various excellent properties, including low light reflectanceand adsorption,high catalytic activity and large specific surface area. Therefore, SiNWAs have a widerange of potential applications in the fields of energy, catalysis, environment, and biology.Moreover, SiNWAs are more advantageous in certain applications compared to dispersedSilicon nanowire systems due to reduced dispersion and separation. However, currentefforts concerning structuraltuning, property development, and surface modification ofSiNWAsremain inadequate. This is especially evident in the area of the surfacemodification of SiNWAs, where the majority ofwell-establishedtechniques for the surfacemodification of plane surfaces are not yet attemptedin SiNWAs. It is believed thatSiNWAs, which have inherently superior properties, will exhibit even greater capabilitiesupon the introduction of surface modification. In addition, the use of SiNWAs incatalysis has previously required the use of other metal materials in combination.However, preliminary results indicate that bare SiNWAs or surface-modified SiNWAsalone is sufficient to exhibit excellent catalytic properties. Thus, it is worthwhile toexplore the catalytic property of SiNWAs.In the present study, research was carried out in three main phases: the preparationof SiNWAs, the surface modification of SiNWAs, and the catalysis property of SiNWAs.The main content is as follows: First, the preparation of SiNWAswith different surface treatment method wasstudied and compared. SiNWAs were prepared using metal catalyzed chemical etchingmethod and the surface of SiNWAs was treated with “Piranha”solution (P-SiNWAs) orHF solution (H-SiNWAs), which resulted in a hydrosilated or hydroxylated surface,respectively. The results from scanning electron microscopy (SEM) showed that thelength of Silicon nanowire increased linearly with increasing etching time in a range from4to49μm.The chemical composition of P-SiNWAs and H-SiNWAs was characterizedusing X-ray photoelectron spectroscopy (XPS) and in situ Raman spectroscopy.Theresults from static water contact angle measurements showed that the H-SiNWAs andP-SiNWAs exhibited superhydrophobic andsuperhydrophilic properties, respectively,which is ascribe to the combined effect of surface chemical composition and the topologyof SiNWAs.Secondly, the catalytic property of SiNWAs with different surface treatment wasinvestigated. Proton nuclear magnetic resonance (H-NMR) and UV/Vis absorptionspectroscopy were used to study the catalytic effect of P-SiNWAs and H-SiNWAs in thereduction reaction of P-nitrophenol (PNP) by NaBH4. The results demonstrated that theH-SiNWAs exhibited significant catalytic activity.The conversion rate of PNP catalyzedby H-SiNWAs increased with time and was almost completed within30min. The resultsfrom Raman spectra indicated that Si-H played a significant role in this catalysis process.In addition, the catalytic effect of H-SiNWAs with different length wasexplored and itwas found that the catalytic activity was enhanced with increasing length of nanowire.Finally, the surface modification of SiNWAs using different functional polymers wasstudied. SiNWAs were successfully grafted with poly(2-dimethylaminoethylmethacrylate)(PDMAEMA) and Poly(N-isopropylacrylamide)(PNIPAAm) viasurface-initiated atom transfer radical polymerization (SI-ATRP). The results from watercontact angle measurement showed that the modified SiNWAs by PDMAEMA or PNIPAAm exhibited significantly improved PH-responsiveness or thermoresponsiveness,respectively,compared to plane Silicon substrate modified with correspondingpolymers.Furthermore, poly(ethylene glycol) PEG with different molecular weight andsingle-ended allylation was grafted to H-SiNWAs. The modified SiNWAs showedsuperhydrophilicity and enhancedrepellence towards non-specific protein adsorption. Thenon-specific protein repellence property can be further improved by reducing themolecular weight of PEG grafted to SiNWAs. Moreover, by grafting polyethyleneimine(PEI) to SiNWAs, the water contact angle of the modified SiNWAs is3oand the graftingdensity of PEI can reach to149nmol/cm2with optimized grafting time. The PEI modifiedSiNWAs have potential applications in antibacterial and gene delivery fields.