Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis

Abstract

The present work focuses on understanding the heterojunction formation of graphene quantum dots (GQDs) and oxygen deficient TiO2 nanoparticle hybrid system and its enhanced photocatalytic activity under visible light illumination. We explain the formation of TiO2-GQD heterojunction through the bonding between oxygen vacancy sites in TiO2 and in-plane oxygen functional (epoxy) groups in GQDs possibly via COTi bonds. Our FTIR, XPS and Raman results lend support to the proposed mechanism of heterojunction formation. In the TiO2/GQD hybrid, the Raman Eg(1) peak of anatase TiO2 is blue shifted indicating the strong interaction between the GQD and TiO2. The heterojunction formation was simulated through the density functional theory (DFT) calculation to obtain the optical spectrum on the hybrid between oxygen deficient TiO2 and oxygen functionalized GQDs. Interestingly, the calculated results for the hybrid structure show strong optical absorption in the visible to near infrared region, which is in close agreement with the experimental results. The TiO2-GQD heterojunction exhibits enhanced photocatalytic degradation (97%) of MB due to the facile interfacial charge separation, as revealed from the steady state and time resolved photoluminescence studies. Interestingly, the photoluminescence intensity of the TiO2-GQD heterojunction was partially quenched indicating the electron transfer from GQDs to TiO2. The degradation rate constant (first order) for TiO2-GQD hybrid is 5.2 times higher than that of the TiO2. Free radical scavenger test revealed that OH radical played a major role in MB degradation as compared to O2°− radical. These results are significant for the development of metal free catalysts based on carbon nano-materials for ensuing optoelectronic, energy and environmental applications.