Hydrothermal synthesis of graphene grafted titania/titanate nanosheets for photocatalytic degradation of 4-chlorophenol: Solar-light-driven photocatalytic activity and computational chemistry analysis

Graphene grafted titania/titanate nanosheets (G/TNS) were synthesized through a one-step hydrothermal treatment. The 2.0 wt% grafted composite material appears as flower-like nanoscale sheets, and contains Ti crystalline phases of both anatase and sodium titanate. Graphene acted as a two-dimensional template for growth of Ti-nanosheets while inhibiting transformation of TiO2 into titanate during the hydrothermal reaction. G/TNS with 2.0 wt% graphene showed the highest photocatalytic degradation rate for 4-chlorophenol, and >99.2% of removal was achieved at 120 min. The pseudo-first order rate constant (k(1)) was determined to be 0.041 min(-1), which is similar to 8 times higher than that of anatase and similar to 21 times than unmodified TNS. The findings indicate that the grafted graphene greatly promotes the material response to visible light because: (1) it facilitates rapid transfer of photo-excited electrons, thus inhibiting recombination of the h(+)-e(-) pairs, and (2) narrowed band gap energy leading to enhanced visible light absorption. Evidently, the 2-D sheet-like structures are conducive to high electron transfer efficiency and high solar-light-driven photocatalytic activity. The center dot OH radicals were found to be the primary reactive oxygen species for 4-CP degradation. Density functional theory (DFT) analysis indicates that the sites on 4-CP with high Fukui index (f(-)) can be easily attacked by center dot OH, and the theoretical calculation results were consistent with experimentally identified 4-CP degradation pathway. In addition, G/TNS showed good reusability and >90% of 4-CP still could be removed even after 5 reuse cycles. The new composite material is promising for photocatalytic degradation of persistent organic pollutants under solar light.