It is well known that TiO2 nanofiber is an efficient UV-driven photocatalyst [34]. Anchoring of CQDs on the surface of TiO2 indeed opens a new way to the sufficient use of solar
light. The photocatalytic behavior of CQDs/TiO2 composite
nanofibers towards degradation of methylene blue (MB) dye
under solar light irradiation is shown in Fig. 5A TiO2
nanofibers decomposed nearly about 71% MB dyes in
95 min, while CQDs/TiO2 composite nanofibers decomposed
almost all MB dyes within the same time. The significant
improvement of the photocatalytic activity of CQDs/TiO2
composite nanofibers towards degradation of MB dye under
visible light irradiation is due to their wider region light
response than pristine TiO2 nanofibers. Moreover, the anchor-ing of CQDs into TiO2 nanofibers can reduce the recombina-tion of the photogenerated electrons and holes resulting in
enhanced photocatalytic activity. In order to evaluate the self-degradation of MB dye under the solar light, the experiment
was carried out without using catalyst and the self-degradation
was found fairly constant. The reusability of the introduced
composite was also evaluated in this work. Fig. 5B shows the
reusability of CQDs/TiO2 composite nanofibers under visible
light irradiation where three successive cyclic tests were carried out by taking the same sample (25 mg) in 25 ml MB
(10 ppm). From the figure, it is clear that photocatalytic
activity of CQDs/TiO2 composite nanofibers towards degrada-tion of MB dye is almost the same for the first two cycles.
However, the decolorization rate was found to be decreased
slightly in the third cycle. The blockage of active sites in the
photocatalyst may be the cause of less degradation of MB in
the successive cycles [34,35]. The mechanism of the photo-catalytic activity of the CQDs/TiO2 composite nanofibers has
been already discussed in literature [24]. When solar light is
irradiated to CQDs/TiO2 composite nanofibers, CQDs absorb
visible light, and then emit shorter wavelength due to the up
converted property. The shorter wavelengths then excite TiO2
nanofibers to produce electron-hole pair. Here, CQDs act as
photogenerated electrons collectors and transporters. These
electron–hole pairs formed after the excitement of electron
were trapped by surface hydroxyl groups (or H2O) at the
catalyst surface to yield OH radicals. Meanwhile, the dissolved oxygen molecules reacted with the excited electrons
to yield superoxide radical anions, O
2 , which on protonation
generated the hydroperoxy radicals, HO
2, producing hydroxyl
radical OH
, which was a strong oxidizing agent to decompose
the organic dyes. The hydroxyl radical is responsible for the
dye degradation.
A
It is well known that TiO2 nanofiber is an efficient UV-driven photocatalyst [34]. Anchoring of CQDs on the surface of TiO2 indeed opens a new way to the sufficient use of solarlight. The photocatalytic behavior of CQDs/TiO2 compositenanofibers towards degradation of methylene blue (MB) dyeunder solar light irradiation is shown in Fig. 5A TiO2nanofibers decomposed nearly about 71% MB dyes in95 min, while CQDs/TiO2 composite nanofibers decomposedalmost all MB dyes within the same time. The significantimprovement of the photocatalytic activity of CQDs/TiO2composite nanofibers towards degradation of MB dye undervisible light irradiation is due to their wider region lightresponse than pristine TiO2 nanofibers. Moreover, the anchor-ing of CQDs into TiO2 nanofibers can reduce the recombina-tion of the photogenerated electrons and holes resulting inenhanced photocatalytic activity. In order to evaluate the self-degradation of MB dye under the solar light, the experimentwas carried out without using catalyst and the self-degradationwas found fairly constant. The reusability of the introducedcomposite was also evaluated in this work. Fig. 5B shows thereusability of CQDs/TiO2 composite nanofibers under visiblelight irradiation where three successive cyclic tests were carried out by taking the same sample (25 mg) in 25 ml MB(10 ppm). From the figure, it is clear that photocatalyticactivity of CQDs/TiO2 composite nanofibers towards degrada-tion of MB dye is almost the same for the first two cycles.However, the decolorization rate was found to be decreasedslightly in the third cycle. The blockage of active sites in thephotocatalyst may be the cause of less degradation of MB inthe successive cycles [34,35]. The mechanism of the photo-catalytic activity of the CQDs/TiO2 composite nanofibers hasbeen already discussed in literature [24]. When solar light isirradiated to CQDs/TiO2 composite nanofibers, CQDs absorbvisible light, and then emit shorter wavelength due to the upconverted property. The shorter wavelengths then excite TiO2nanofibers to produce electron-hole pair. Here, CQDs act asphotogenerated electrons collectors and transporters. Theseelectron–hole pairs formed after the excitement of electronwere trapped by surface hydroxyl groups (or H2O) at thecatalyst surface to yield OH radicals. Meanwhile, the dissolved oxygen molecules reacted with the excited electronsto yield superoxide radical anions, O2 , which on protonationgenerated the hydroperoxy radicals, HO2, producing hydroxylradical OH, which was a strong oxidizing agent to decomposethe organic dyes. The hydroxyl radical is responsible for thedye degradation.A
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