Fabrication of graphitic-C3N4 quantum dots coated silicon nanowire array as a photoelectrode for vigorous degradation of 4-chlorophenol

Abstract

Although Si is a successful photovoltaic material, its application in the photoelectrochemical (PEC) field is limited because an insulated SiO₂ layer always extends quickly from surface to depth once bare Si contacts with an aqueous solution. To inhibit this oxidation passivation, Si nanowires (SiNWs) were coated with graphitic-C₃N₄ quantum dots (g-C₃N₄ QDs) to form SiNWs@g-C₃N₄ QDs, in which g-C₃N₄ QDs acted as a protection layer to isolate Si from water and to improve the photogenerated charge transfer. Observed by SEM and TEM, it was confirmed that dispersive g-C₃N₄ QDs anchored on the surface of SiNWs. PEC performance indicated that the photocurrent of bare SiNWs declined obviously while the photocurrent of SiNWs@g-C₃N₄ QDs was stable. The photocurrent of SiNWs@g-C₃N₄ QDs reached 6.7 mA cm⁻² at −1.5 V (vs. SCE) which was 1.6 times higher than that of pristine SiNWs (4.2 mA cm⁻²). Taking 4-chlorophenol as a target pollutant to investigate the photoelectrocatalytic capability of the SiNWs@g-C₃N₄ QDs, more than 85% of 4-chlorophenol was successfully removed in 120 min, while the value for SiNWs was only 52.0%. The pseudo-first-order kinetic constant of 4-chlorophenol degradation on SiNWs@g-C₃N₄ QDs was 2.3 times as great as that on pristine SiNWs. The improved degradation efficiency benefited from the improved stability as well as the enhanced photo-generated charge transfer and separation driven by the built-in electric field at the interface between g-C₃N₄ QDs and SiNWs. The SiNWs@g-C₃N₄ QDs will also be useful in other research areas such as water splitting, sensors, etc.