The changes in visible absorption peak of Naphthol blue black (600 nm) are plotted with respect to irradiation time in Figure 5B. (UV−visible spectra of dye degradation with TiO2 and TiO2/RGO are presented in the Supporting Information, Figure S4.) When RGO is present in the solution, the decrease in absorbance initially (first 10 min) is similar to the one without RGO. (Similar degradation rates were also observed for methylene blue, which completely degrades within 5 min under similar conditions. Supporting Information, Figure S5.) However, the later part of dye degradation in Figure 5B proceeds at a slower rate in the presence of TiO2/RGO than with TiO2 alone. The competition between RGO and the dye toward OH• radicals inhibits the rate of dye degradation when the dye concentration becomes lower. The slower rate of absorbance change at longer times in Figure 5B shows the dye molecules and RGO/f-PAH intermediates compete for OH•
radicals. This competitive oxidation reaction is analogous to the competition between RGO and SCN− for OH• radical oxidation, which resulted in lower (SCN)2•− quantum yield in the flash photolysis experiments (Figure 3B). In particular, the total disappearance of the RGO color after completion of the irradiation confirms the degradation of RGO during the photocatalysis. Furthermore, blank UV irradiation experiments carried out with TiO2/RGO composite (in absence of the dye) also confirm RGO susceptibility to oxidation (see Figure S6, Supporting Information).
The changes in visible absorption peak of Naphthol blue black (600 nm) are plotted with respect to irradiation time in Figure 5B. (UV−visible spectra of dye degradation with TiO2 and TiO2/RGO are presented in the Supporting Information, Figure S4.) When RGO is present in the solution, the decrease in absorbance initially (first 10 min) is similar to the one without RGO. (Similar degradation rates were also observed for methylene blue, which completely degrades within 5 min under similar conditions. Supporting Information, Figure S5.) However, the later part of dye degradation in Figure 5B proceeds at a slower rate in the presence of TiO2/RGO than with TiO2 alone. The competition between RGO and the dye toward OH• radicals inhibits the rate of dye degradation when the dye concentration becomes lower. The slower rate of absorbance change at longer times in Figure 5B shows the dye molecules and RGO/f-PAH intermediates compete for OH•radicals. This competitive oxidation reaction is analogous to the competition between RGO and SCN− for OH• radical oxidation, which resulted in lower (SCN)2•− quantum yield in the flash photolysis experiments (Figure 3B). In particular, the total disappearance of the RGO color after completion of the irradiation confirms the degradation of RGO during the photocatalysis. Furthermore, blank UV irradiation experiments carried out with TiO2/RGO composite (in absence of the dye) also confirm RGO susceptibility to oxidation (see Figure S6, Supporting Information).
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