| Photocatalytic technology is one of the most promising methods in the field of pollution control since it is environmental friendly, and could degrade many organics completely under mild reaction conditions. However, traditional TiO2-based photocatalytic materials have some defects, such as low solar energy utilization and degradation efficiency, which seriously hinder their practical application. Therefore, the development of new photocatalytic materials with high efficiency and wide optical absorption range has become one of the key issues in photocatalytic techniques. Silicon (Si), the second most abundant element (29.4%) in the earth crust after oxygen, is widely used in photovoltaic applications due to its low cost and narrow band gap (Eg=～1.12 eV), which is relatively well matched to the solar spectrum. However, Si is easily oxidized in moist air or aqueous solutions and subsequently formed an insulated SiOx layer, which seriously reduces the photoelectrochemical activity of Si. Therefore, improving the stability of Si materials is desiderated in its application on the degradation of pollutants in water. Graphitic carbon nitride (g-C3N4) nanosheets (bandgap,2.7 eV) was utilized as a protective layer of Si nanowires (SiNWs), which can prevent the SiNWs from being passiviated and facilitate the separation of photogenerated eleetron-hole pairs, thus improving the photoelectric conversion and the efficiency of photoelectrocatalysis. The specific research contents and conclusions are as follows:(1) The SiNW arrays were fabricated using Ag-assisted electroless chemical etching method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images indicated that the SiNW arrays were large-scale oriented and uniformly aligned on the surface of the Si wafer. The average length of SiNW was about 30 μm and the diameter of a single SiNW was about 300 nm. The g-C3N4 nanosheets were synthesized using two-stage calcinating method with melamine as the precursor. The SEM images indicated that g-C3N4 nanosheets presented wrinkled and crimped structure. The TEM and atomic force microscopy (AFM) results revealed that the size of g-C3N4 nanosheets was about 4 μm × 8μm, and the average thickness was about 1.1 nm. The SiNW/g-C3N4 composite electrode was fabricated by electrophoresis depositing g-C3N4 nanosheets on the surface of SiNW arrays. The electrophoretic deposition time was the key factor for the photoelectrochemical performance of SiNW/g-C3N4. The samples with the deposition time of 20 s showed the best stability and the highest photocurrent density. (2) Cyclic voltammetry measurements were performed under xenon lamp irradiation to evaluate the stability of the obtained materials. After 10 circles of cyclic voltammetry curves, the photocurrent of SiNW/g-C3N4 composite electrode kept stable. And the photocurrent density was 0.28 mA cm-2, which was 2.33 times as much as that of the naked SiNW array electrode (0.12 mA cm-2). In the subsequent photocurrent-time measurement, the photocurrent density of SiNW/g-C3N4 at 1.0 V in 240 s was 0.12 mA cm-2, which was 6 times as much as that of the naked SiNW array electrode (0.02 mA cm-2). The above photoelectrochemical tests results showed that g-C3N4 nanosheets can not only protect Si from being passiviated, but also enhance the photo-absorption and photocurrent density of Si materials. The results of photoelectrocatalytic degradation of phenol under xenon lamp irradiation illustrated that the removal efficiency of phenol on SiNW/g-C3N4 at 1.0 V was up to 90% in 180 min, and the kinetic constant of SiNW/g-C3N4 was 0.010 min-1, which is 3.33 times as much as that of SiNW electrode. The enhanced photoelectrocatalytic ability of SiNW/g-C3N4 electrode can be attributed to the effective charge separation caused by the built-in electric field at the interface, thus making more photogenerated holes to participate in the oxidative degradation process. |
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