3.10. Reaction kinetics
The photo degradation of the MB and RhB dyes fitted well with the pseudo-first order equation. The equation was considered as −ln(Ct/C0) = kapp(t), where C0 and Ct are the reactant concentrations at time t and Kapp and t are the apparent rate constant and time respectively. Fig. 12(a and b) shows the pseudo-first order kinetic data for the RhB and MB dyes. The Kapp value of BGO5 was found much higher than other composites. These results further confirmed that BGO5 composite is much more effective catalyst material than any other composite material. From the obtained data the rate constant K values of the RhB and MB dyes are listed in Table 1
Fig. 12.
Pseudo first order kinetic plot for: (a) MB and (b) RhB using BiOBr and BGO composites.
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In addition the photocatalytic performance of BGO composite, TOC studies have been measured. Fig. 13 shows the TOC removal for MB and RhB dyes respectively. The RhB dye degradation by BGO5 composite exhibits 65% of TOC removal. For MB dye BGO5 composite shows 60% of TOC removal respectively. But the plain BiOBr exhibited only 35% and 30% of TOC removal for RhB and MB dyes. Thus the effective TOC removal of BGO 5 composite compare with pure BiOBr which further confirms the introduction of GO sheets makes the BGO5 composite exhibit the higher mineralizing ability of dyes due to its high adsorption rate of BGO composite which makes an effective degradation.
Fig. 13.
TOC removal profile for MB and RhB dyes using BiOBr, BGO5 composite under visible light.
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3.11. Photocatalytic mechanism
It is clear from the results that the superior photocatalytic activities of the GO/BiOBr catalyst are mainly due to the enhanced adsorption of the composite because of the incorporation of the graphene material. It is attributed that homogeneous distribution of BiOBr particles on the surface of the GO sheets acts as a reaction centre to facilitate better photocatalytic reaction. By the irradiation of visible light the GO combined with the BiOBr matrix, the electrons flowed easily, to form Scottky barriers between GO and BiOBr matrix. These Scottky barriers [61] play a key role in capturing the electrons from the BiOBr matrix easily [54]. Meanwhile, the BiOBr matrix excited electrons to conduction band (CB) from a valence band (VB), to form electron holes. Finally, the number of electrons in the system was enriched. The generated electrons (e−) reacts with the dissolved oxygen molecules to produce the oxygen peroxide radicals O2*−. The positively charged hole (h+) reacts with H2O to form hydroxyl radicals OH*−[55]. The MB or RhB molecules were degraded by these free radicals and finally converted into CO2and H2O molecules respectively. The enhancement of photocatalytic activity of BiOBr after GO hybridization attributed to the higher separation efficiency of electron hole pairs and inhibition of recombination rate resulting the increasing of number of electron holes participation in the photo degradation process [33] and [56]. The high surface area and numerous functional groups in GO matrix allow better interaction of dye molecules to their surfaces during the photocatalytic process. Finally the π–π* interactions between the GO and contaminant would extract the dye molecule from solution and then concentrated dye molecule near the BiOBr surface and facilitate the process effectively.
GO/BiOBr composite + light → h+VB + e−CB
e−CB + O2 → O2−
h+VB + H2O → OH + H+
O2− (or)•OH− + {RhB or MB} → CO2 + H2O (degraded products)
During visible light irradiation GO/BiOBr composite absorbs visible light and was excited. The electrons (e−) were migrated from the valence band (VB) of BiOBr to the conduction band (CB) leaving the holes (h+) in the VB of BiOBr matrix. The well matched band potential between the GO sheets and BiOBr, will aid the photo generated electrons accumulated on the CB of the BiOBr. This is transferred on to the GO matrix, thereby reducing the electron hole recombination rate. In this photocatalytic pathway the GO sheets function as electron collector as well as transporter to increase the lifetime of the charge carriers [62] and [63].
For better understanding the mechanism of the pathway of the BGO composites is illustrated in graphical abstract.
4. Conclusions
GO/BiOBr photocatalyst was synthesized by a simple solvothermal route. The incorporation of GO in BiOBr matrix was confirmed by the XPS, XRD, EDAX and Raman spectroscopy studies. TEM, SEM analysis reveals that GO was incorporated effectively with the BiOBr matrix .The as synthesized BGO photocatalysts show tremendous improvement in photocatalytic degradation under visible light. The removal efficiency of BGO5 catalyst was found to be nearly 95% for both the dyes, which is twice that of the bare BiOBr composite. Hence, it can be stated that the enhanced photocatalytic efficiency is due to the effective separation of photo-induced electron–hole pairs by the GO sheets. Additionally, the GO also acts as a co-catalyst for BiOBr matrix, because the interfacial contact between the GO and BiOBr was perfect, and made the very fast interfacial charge transfer between these two materials. The degradation time was 30 min as MB dye 45 min and RhB dye, by the BGO5 composite. In conclusion our work provides a new option for the fabrication of bismuth oxy bromide composites, and to produce their excellent photo catalytic performance in visible light, which would be greatly beneficial for the environment that faces a major crisis from pollution throughout the world.