Chapter 6: Interfacial Charge Transfer in Oxygen Deficient
6.11. Visible light Photocatalytic Degradation Studies of MB
6.11.1. Degradation and Reaction Kinetics
Fig. 6.11 depicts the change in absorption intensity of MB in catalyst samples and its reaction kinetics. Fig. 6.11(a-d) represents the UV-vis absorption spectra of MB up to 120 min irradiation using GQD, T16, T16/GQDM and T16/GQD catalysts, respectively.
It is evident from figure that the variation in absorption intensity of MB as a function of irradiation time in T16/GQD is higher than that of other catalyst samples subjected to visible light irradiation up to 120 min. Fig. 6.11(e) shows the concentration ratio (final to initial concentration, C/C0) of MB and MB in different catalyst samples. MB is only 6%
self-catalysed after irradiation of 120 min. Although the GQD catalyst amount is high as compared to the PC studies of GQD discussed in Chapter 5, here MB is degraded to only 31%. This is due to the very poor visible absorption characteristic of GQD, as discussed
Figure 6.10: (a, b) Schematic illustration of structural transformation in TiO2 NPs by ultrasonication that creates additional oxygen vacancy defects in TiO2. (c, d) TiO2-GQD hybrid formation through the possible C-O-Ti bonds aided by ultrasonication of TiO2 NPs and GQDs mixture. The images are simulated in Materials Studio software.
in Chapter 5. On the other hand, the degradation of MB in T16 is increased to the 44%, while it is dramatically high (97%) for T16/GQD hybrid. The photo degradation of MB in T16 is primarily due the effect of band gap narrowing. Note that the band gap narrowing in T16 is the combined effect of strain and oxygen vacancies induced by milling process15, 41. On the other hand, due to the band bending at the heterojunction of T16/GQD hybrid, the degradation efficiency is vastly improved. We believe that the chemisorption of GQDs on TiO2 and possible C-O-Ti bond formation are responsible for the efficient interfacial charge transfer and enhanced PC performance of the hybrid sample. Note that, our degradation studies on the T16/GQDM sample prepared by simple
Figure 6.11: UV-Vis absorption spectra of MB in catalysts: (a) GQD, (b) T16, (c) T16/GQDM and (d) T16/GQD. (e) Change in relative concentration (C/C0) of MB in different catalysts as a function of irradiation time up to 120 min. (c) Plot of ln(C/C0) vs. irradiation time (t) for MB degradation in different catalysts.
stirring showed only 59% degradation in contrast to the 97% degradation in case of T16/GQD. The lower degradation in case of T16/GQDM is believed to be due to the weak van der Waals interaction between the GQD and TiO2 during the stirring. In contrast, during ultrasonication, oxygen vacancy mediated bond formation takes place through the possible C-O-Ti bonding and due to strong interaction between TiO2 and GQD, the efficient charge carrier separation at the interface takes place, which results in high degradation rate. Besides the interfacial charge transfer process, the hot charge carrier injection from GQDs to TiO2 42 is another considerable factor for the difference in the PC of hybrid sample. Considering the T16/GQD sample, GQDs are strongly coupled to the TiO2 as compared to that in T16/GQDM. As a result, the interfacial charge separation and hot carrier generation is higher in T16/GQD as compared to that of T16/GQDM.
Next, we estimated the degradation rate constants for MB in different catalysts.
The kinetics of the PC reaction follows a pseudo first order reaction process, which is expressed by ln(C/C0) = -kt, where ‘k’ is the apparent reaction rate constant, C and C0 are the concentration of MB after irradiation time ‘t’ and initial concentration, respectively.
The rate constants are obtained from the slope of the linear plots. Fig. 6.11(f) shows a plot of ln(C/C0) vs irradiation time for degradation of MB in different catalysts. The symbols represent the experimental data and the line represents the fitted data. The % of MB degradation and rate constants are shown in Fig. 6.12(a). The value in the parenthesis indicates the rate constant. The rate constant for MB and GQD are 0.61×10-3 and 0.26×10-2 min-1, respectively. In case of T16 the rate constant is 0.46×10-2 min-1 and that of T16/GQDM is 1.47×10-2 min-1. The rate constant in T16/GQD is 2.40×10-2 min-1, which is 9.2 fold and 5.2 fold higher than that of the GQD and T16, respectively. Thus, the hybrid sample shows much improved degradation rate constant than the pristine samples. These results signify the influence of interaction of GQDs and TiO2 for enhanced PC degradation of MB.
Further, we studied the reusability of the T16/GQD catalyst, since it has shown the highest efficiency photodegradation of MB as compared to the other catalysts. The relative concentrations of MB in T16/GQD catalyst for three repeated cycles of photocatalysis are shown in Fig. 6.12(b). It’s evident from the data that the hybrid catalyst is quite stable up to three repeated cycles. Furthermore, in order to assess the
structural stability of the catalyst, we recorded the XRD pattern of the samples before and after three cycles of catalysis. Fig. 6.12(c) shows the XRD pattern of T16/GQD before and after catalysis. Note that the anatase TiO2, Ti3O5 phase and GQD reflection planes are marked with different symbols in Fig. 6.12(c). These results indicate that crystalline phase of the TiO2 and GQD is unchanged even after three repeated cycles of photocatalysis. These results are consistent with the TEM results shown in Fig. 2(i). The degradation mechanism and the nature of free radical species involved in the degradation of MB are elucidated in the following section.