Microstructure Tuning Of N-TiO₂-based Photocatalysts And Their Environmental Purification Activities

Photocatalytic technology has attracted lots of attention because it can convert photo-energy into chemical-energy under room temperature and ambient pressure for environmental purification.The key of the photocatalytic technology is photocatalyst.Nitrogen-doped titanium dioxide(N-TiO2)can be excited by visible light and became one of the focuses in this field.However,the light absorption range and photo-induced carriers' separation efficiency should be further improved and enhanced.Herein,metal ions(Mn+)doping were used to reduce the formation energy of the metal nitrogen co-doped TiO2(M,N)-TiO2,thereby modulating the dopant concentration and chemical states of N.The effect of Mo6+doping on electronic structure and crystal structure of N-TiO2 has been investigated systematically.To further enhance the photo-generated charge carriers' separation,CdS and MOS2 were also selected to couple with N-TiO2.The degradation of model gaseous pollutant(benzene)by N-TiO2 was studied,and the adsorption-desorption model was modified.The synergistic purification of phenol-Cr(VI)was also investigated.First-principle calculation and experimental results showed that doping of Nb5+,Mo6+and W6+could reduce the formation energy of(M,N)-TiO2.The N dopant concentration was increased from 0.84%to 1.12%,1.38%and 1.20%,respectively.And it could also promote the change from interstitial N doping to substitution N doping.On the contrary,the doping of Co2+and Fe3+increase the formation energy of(M,N)-TiO2,which is unfavorable for the doping of N into TiO2 lattice.The N dopant concentration decreased from 0.84%to 0.69%and 0.73%,respectively.The effects of Mo6+ doping on the electronic structure and crystal structure of N-TiO2 were studied systematically.Mo6+doping introduced Mo-d states below the conduction band minimum,which improves the optical absorption of TiO2.And the ratio of {001}/{101} crystal plane energy was also decreased from 1.92 to 0.91,resulting in the decrease in growth rate of {001},exposing {001} planes and enhancing the formation of {001}/{101} surface heterojunctions.After Mo6+doping,the photo-induced electrons migrate to {001} plane while the photo-induced holes transfer to the main exposed {101} plane,which enhancing the carriers'separation.The(Mo,N)co-doped TiO2(Mo,N)-TiO2 was prepared by hydrothermal method and was used to degrade benzene under visible light irradiation.After irradiated for 150 min,the degradation ratio of benzene reached 70.9%with a reaction rate constant of 0.0125 min-1,which was 2.4 times of N-TiO2(0.0065 min-1).The photocatalytic performance of N-TiO2 was improved by coupling CdS and MoS2 to form heterojunctions.TEM results showed that N-TiO2 nanoparticles were coupled with flaky CdS to formed heterojunction structures.The optical absorption and carriers'separation of N-TiO2 were improved simultaneously by coupling with CdS.When Cd concentration was 2%,the N-TiO2/CdS exhibited the best photocatalytic performance,which was 3 times that of N-TiO2.N-TiO2/MoS2 heterojunction was prepared in order to improve its chemical stability,the degradation efficiency of N-TiO2/MoS2 was 3.3 times that of N-TiO2 and 1.1 times that of N-Ti02/CdS.We investigated the photocatalytic degradation of benzene,and modified the traditional Langmuir-Hinshelwood(LH)kinetic model.The benzene degradation intermediates were measured by GC/MS,and the degradation pathway was proposed:benzene?phenols?small molecular aliphatic compounds?CO2 and H2O.In kinetic study,it was found that the thermodynamic adsorption constant KL obtained through LH model decreased with the increase in light intensity at a constant reaction temperature.The apparent adsorption constant KA was introduced to modify the LH model and M-LH model was obtained.The M-LH model was used to estimate the benzene concentration vs.different photocatalytic degradation time,which was experimentally verified.We also investigated photocatalytic purification performance of N-TiO2 on the phenol-Cr(?)coexistence system.The synergistic purification enhancement effect has been found.The phenol-Cr(?)system was used to simulate the polluted water.It was found that the light energy utilization ratio in the coexistence system is 3.3%,which was much higher than that in a single phenol system(0.93%)and single Cr(VI)system(0.0053%).The results of radical capturing experiments showed that phenol and Cr(VI)reacted with the oxidizing species(h+,·OH)and reducing species(e-,·O2-),respectively.The coexistence of phenol and Cr(VI)avoided the recombination of photo-generated carriers and resulting in the synergistic purification enhancement effect.We also built second-order kinetic model to investigate the photocatalytic degradation reaction of phenol.