Efficient visible light photocatalysis and tunable photoluminescence from orientation controlled mesoporous Si nanowires

Abstract

Herein, orientation controlled growth of single crystalline mesoporous Si nanowires (NWs) by Ag assisted etching of Si(100) and Si(111) wafers and its visible light photocatalysis efficiencies have been studied. By choosing different orientations of the starting Si wafer and concentration of etchants, we have tuned the morphology of Si NWs and achieved straight, bent, kinked and zigzag Si NWs. High resolution FESEM and TEM imaging reveals that the Si NWs are decorated with ultra-small Si nanocrystals (NCs) of arbitrary shape due to the lateral etching of the NWs. A strong broad-band visible to near-infrared (NIR) photoluminescence (PL) in the range 1.4–2.4 eV is observed from these Si NWs/NCs at room temperature. Our studies reveal that the quantum confinement of carriers in Si NCs and the nonbridging oxygen hole center (NBOHC) defects in the SiO_x layer both contribute to the broad visible-NIR PL. In addition to the efficient PL emission, the metal assisted chemical etching (MACE) grown Si NWs exhibit excellent photocatalytic degradation of methylene blue under visible light illumination. The Si–H bonds present on the surface of the Si NWs/NCs facilitate the photocatalytic activity by efficient separation of the photogenerated e–h pairs. Our studies reveal that the MACE grown vertically aligned Si NWs behave as a stable photocatalyst for up to five cycles of catalysis. It is found that with the increase in the concentration of HF during etching, PL intensity systematically decreases while the photocatalytic efficiency increases with the HF concentration due to the Si–H bonds. Our results open up the possibility of using Si NWs arrays in white light display applications as well as organic waste treatment and hydrogen production by photocatalytic water splitting.