Dipole Modulation And High-activity Mechanism Of Wide-spectrum Response Cobalt-based Sillenite

Photocatalytic degradation of organic pollutants has become a significant pollutant treatment technology in recent years due to its environmentally friendly,non-toxic and low energy consumption characteristics.However,some of the reported photocatalysts are only responsive to limited wavelengths（l<600 nm）,which cannot sufficiently utilize the solar energy.Most of the developed wide-spectrum-responsive photocatalysts exhibit low activity due to the high carrier recombination rate caused by the insufficient excitation energy and the inappropriate band edge.Thus,it is important to develop new wide-spectrum-response and high-activity photocatalysts for the degradation of organic pollutants.Sillenite photocatalysts have been reported to exhibit wide-spectrum responsive and high structure tolerance.They also showed some activity for pollutants photodegradation.However,there are still some shortcomings such as multiple defects and larger particles for the reported sillenite photocatalysts,limiting their photocatalytic activity to a low level.In this paper,g-Bi₂O₃,as the simplest sillenite,was chosen as the parent material for further modification.The modification was avhieved by the substitution of Bi3+ with different amount of Co3+ at the tetrahedral and/or octahedral sites in g-Bi₂O₃.After the substitution,widespectrum response and large lattice dipole Co-based sillenite photocatalysts were obtained.The series substituted Bi-Co-O photocatalysts is used for the degradation of organic pollutants through photodegradation,photothermal degradation and synergistic persulfate photothermal degradation processes.The influence of the structure on the degradation activity is studied and the degradation mechanism is clarified.The details are show below:（1）A new wide-spectrum response Co-based sillenite photocatalyst,Bi25 Co O40,exhibit a spectrum response up to 750 nm,was prepared by a solvothermal method.The occupation of Bi5+ and Co3+ in Bi25 Co O40 lattice structure was studied by XRD refinement.The composition of Bi25 Co O40 energy band structure and dipole generation were investigated by DFT calculation.The photodegradation results show that the degradation rate of MB and 4-CP on Bi25 Co O40 is2-4 times higher than those of common photocatalysts.The excellent photocatalytic activity of Bi25 Co O40 is orginated from the enhanced oxidation capability and charge separation efficiency induced by the near 2 e V valence band edge and large lattice dipole（30.1 D）,respectively.（2）The Co-based sillenite photocatalyst Bi12 Co O20 was prepared by a hydrothermal method,and the spectrum response range of Bi12 Co O20 is further broadened to 1000 nm.It is found that the Bi12 Co O20 as a photothermal catalyst that can effectively convert and utilize infrared light and achieve efficient utilization of the solar energy.The thermal effect modulating the valence state of Co3+ in the octahedral sites of Bi12 Co O20 was confirmed by in-situ tests.Combined with theoretical calculations,it was found that the valence state changes of Co3+leads to the rearrangement of electrons,resulting in a new dipole between adjacent octahedrons up to 34.6 D.The degradation rate constant of phenol on Bi12 Co O20 reaches 0.12 min-1,and it is 2.7 times higher than that of Bi25 Co O40 under the same degradation condition.The remarkable photothermal activity of Bi12 Co O20 originates from the fact that its dipole is nearly2 times larger than that of Bi25 Co O40,facilitating the separation of photogenerated carriers.Meanwhile,the photothermal effect induced a shift of the valence band potential of Bi12 Co O20,thus accelerating the production of reactive oxygen species.（3）Bi-Co-O sillenite photocatalysts with different Co contents were prepared by a hydrothermal method.The activation of persulfate coupled with photothermal degradation was studied on the prepared Bi-Co-O series photocatalysts.The results showed that the phototheraml degradation rate on Bi12 Co O20 was highest among all the series for phenol degradation.After persulfate was added,the activity was further 1.8 times higher than that of Bi12 Co O20 alone.The system was also evaluated for the degradation of organic pollutants with high concentrations.It is found that efficient degradation of antibiotic pollutants can be achieved from 50 to 200 ppm.The excellent activity originates from the efficient activation of persulfate in the present photothermal/Bi12 Co O20 system,generating oxidative ×SO4-（2.5 V vs NHE）.The efficient activation is condiered to be due to the inhibition of the recombination of photogenerated carriers in Bi12 Co O20,leading the sufficient photogenerated electrons to transfer to the surface to achieve activation of persulfate.