Study of excited charge carrier's lifetime for the observed photoluminescence and photocatalytic activity of CdS nanostructures of different shapes

This paper demonstrates the influence of the relaxation time of photoexcited charge species on the photoluminescence and photocatalytic activity for oxidation–reduction reactions by CdS nanostructures of different dimensions. CdS nanospheres (size ∼10 nm) and different aspect ratio (17 and 23) CdS nanorods have been prepared by two different techniques. CdS nanorods formed by autoclaving is found to be more lengthy, wider (length ∼170 nm and width ∼10 nm) and having better crystallinity than CdS nanorods (length ∼126 nm and width ∼5.5 nm) prepared by refluxing under similar conditions. Characterizations have been done by optical absorption, fluorescence emission, time resolved fluorescence decay, scanning and transmission electron microscopy, X-ray diffraction and BET specific surface area measurements. Relaxation lifetime of photoexcited electron–hole pairs is measured to be 20, 24 and 116 μs for CdS nanosphere, shorter and longer CdS nanorod, respectively, seems to be responsible for the observed fluctuation in photoluminescence and photocatalytic activity. The photooxidation rate of salicylic acid (0.5 mM) and photoreduction of p-nitrophenol (0.2 mM) are significantly improved with increasing dimensions of CdS nanorods despite having a comparable surface area (81 and 76 m2 g−1) and CdS nanospheres (18 m2 g−1) exhibit poor photocatalysis. The better delocalization of charge species along the radial as well as longitudinal dimensions of CdS nanorods, higher crystallinity and delayed recombination time facilitate electrons or holes for active participation in the photoinduced reactions, and Au deposition always displayed superior photoactivity.

The photocatalytic activity for salicylic acid oxidation and p-nitrophenol reduction are found to be enhanced with the increasing (20–116 μs) relaxation time (measured by time resolved fluorescence decay) of photogenerated electron–hole pairs of irradiated CdS nanospheres (10–12 nm) and nanorods of increasing length and width ca. 126 nm × 5.5 nm and 170 nm × 10 nm, respectively. Better delocalization of e−–h+ along the lengthy CdS nanorod can lead to a remarkable decrease in e−/h+ recombination probability, hence the improved photoactivity.Figure optionsDownload high-quality image (215 K)Download as PowerPoint slideHighlights
► Excited charge carrier's life increases (20–116 μs) from CdS sphere to nanorod.
► Photoluminescence decreased with increasing lifetime of excited charge species.
► Photooxidation and reduction rate improve with excited charge pair relaxation time.
► Au deposition over CdS notably retards the electron–hole pair recombination rate.
► Increasing CdS length (126–170 nm) enhances (24–116 μs) charge relaxation.