Fabrication And Photoelectrocatalytic Dechlorination Performance Of Hierarchical Silicon Photoelectrode Modified By Ag Nanoparticles

Photocatalytic technology is considered to be one of the most promising new technologies in environmental pollution control. This technology uses photon-induced holes or hydroxyl radicals (·OH) which have high oxidation ability to degrade toxic and harmful pollutants. Its advantages also include the potential of solar energy utilization, environmentally friendly and mineralization capability for most environmental pollutants. TiO2 as the representative of the commonly used photocatalysts shows some shortcomings. The band gap of TiO2 is 3.2 eV causing that TiO2 only absorbs the UV-light which is only 5% of sunlight. And the high recombination rate of photo-generated electrons and holes leads to the low quantum efficiency of TiO2. The above mentioned disadvantages of TiO2 make it difficult to efficiently utilize the solar energy and restrict its application in photocatalytic techniques. Therefore, broadening the absorption spectra and inhibiting the recombination of photo-generated charges are two important research aspects for photocatalytic technology.Silicon, with the band gap of 1.12 eV, can absorb relatively wide spectral light with wavelength expanded to 1100 nm. Moreover, structuring heterojuction is an effective method to improve the separation efficiency of photo-generated charges. Therefore, designing a new silicon heterojunction photocatalytic material is expected to overcome the above problems.The major problem of using silicon nanomaterials in the field of pollution control is that silicon is easily reacted with water or oxygen, leading to the oxidation and then passivation of silicon. How to reduce reflection is also the key point to improve the light conversion efficiency of silicon. In order to overcome the above mentioned problems, silicon nanomaterial with hierarchical structure was fabricated to reduce the reflection of silicon, and Ag nanoparticles were deposited on the surface of silicon to improve the separation efficiency of photo-generated charges and prevent the contact between silicon and water or oxygen. This work can realize the efficient utilization of solar energy and promote the development of photocatalytic technology in practical application. The specific research contents are as follows:(1) The hierarchical Si-nanowires-grafted Si micropillar array (SiNW/SiMP) was fabricated using metal-assisted electroless chemical etching method. Scanning electron microscopy (SEM) images indicated that the silicon nanowire (SiNW) were large-scale oriented and uniformly aligned on the surface of the silicon micropillar (SiMP). The results of linear cyclic voltammetry curves showed that the SiNW/SiMP with 25 min of etching displayed photocurrent of approximately-31 mA-cm-2 at-1 V (vs. SCE), which was higher than that of the samples etched for 5,10,15,20 and 30 min. Cyclic voltammetry measurements were performed under xenon lamp irradiation to evaluate the stability of the obtained materials. It was shown that the SiNW/SiMP has photocurrent attenuation phenomenon and the photocurrent is unstable due to the instability of silicon in aqueous solution. The silicon can react with water or dissolved oxygen to generate SiO2. The results of photoelectrocatalytic removal of 4-chlorphenol illustrated that when using SiNW/SiMP as photocathode, the removal efficiency is 85%, which was 3.3 and 1.5 times higher than those of photocatalysis process and electrolysis process, respectively. And the removal efficiency of 4-chlorphenol was 85% when using SiNW/SiMP as photocathode, which was 1.4 times higher than that of SiMP(60%). And the removal efficiency of 4-chlorphenol of SiNW/SiMP was 1.4 times higher than that of SiMP(85% vs 60%).(2) Ag nanoparicles was deposited on the surface of SiNW/SiMP (Ag/SiNW/SiMP) by using electroless chemical etching method to inhibit the passivation of SiNW/SiMP. SEM images, X-ray diffraction patterns and energy dispersive X-ray spectroscopy indicated that Ag nanoparticles successfully deposited on the surface of SiNW/SiMP. The results of photoelectrochemical measurements showed that the photocurrent attenuation of Ag/SiNW/SiMP samples was restrained with the prolonging of deposition time of Agwhile photocurrent of samples increase firstly and decrease later. The stable photocurrent of Ag/SiNW/SiMP with 3 min deposition time was-37.5 mA-cm'2. When the deposition time was more than 3 min, the photocurrent was stable, but it decreased because there are too many Ag nanoparticles hindered the optical absorption of Ag/SiNW/SiMP.(3) 4-chlorphenol was the target pollution to evaluate the photoelectrocatalytic dehalogenation performance of Ag/SiNW/SiMP. Ag/SiNW/SiMP with 3 min deposition time exhibited good repeatability after six consecutive running. The results of photoelectrocatalytic dechlorination of 4-chlorphenol illustrated that when using Ag/SiNW/SiMP-3 as photocathode, the highest dechlorination efficiency of 95% was exhibited, which was 1.4 and 2.7 times higher than those of photocatalysis process and electrolysis process, respectively. And the kinetic constant of dechlorination was 0.104 min'1, which was 3.2 times and 8 times higher than those of SiNW/SiMP-25 (0.033 min-1) and SiMP (0.013 min-1), respectively. The enhancement of photoelectrocatalytic ability of Ag/SiNW/SiMP electrode can be attributed to the effective charge separation caused by the built-in electric field at the interface between Si and Ag, thus making more photogenerated carriers to participate in the dechlorination process and improving the photoelectrocatalytic dechlorination activity of Ag/SiNW/SiMP.In conclusion, Ag nanoparticles can protect the Ag/SiNW/SiMP from passivation. Potential built-in electric field formed at the interface between Ag and Si can improve the separation of photogenerated electrons and holes, and the SiNW/SiMP can reduce the light reflection. All of those are helpful to make full use of solar energy to decompose environmental pollutions. This work helps to promote the practical application of photocatalytic technology.