Investigation Of Light Energy/mechanical Energy Co-driven Synergistic Catalysis For Elimination Of Organic Pollutants

Semiconductor photocatalytic technology can directly convert solar energy into chemical energy for energy storage and environmental remediation.Nevertheless,due to the insufficient utilization of sunlight and the serious recombination of photogenerated carriers,the photocatalytic activity is still low.In addition to solar energy,mechanical energy（e.g.,sound,water flow,vibration）is a common form of energy in nature.And the conversion of mechanical energy to chemical energy can be realized by using the piezoelectric effect of piezoelectric materials.The mechanical force applied to the piezoelectric material can induce positive and negative polarization charges on both sides of the catalyst surface,which attract free carriers to the surface of the catalyst to participate in the redox reaction.Meanwhile,the piezoelectric field formed by these polarization charges can be used as the driving force for effectively improving transport behavior of electrons and holes both in bulk and on surface of photocatalysts.In this thesis,with a view to the physical properties of the materials,we screened and fabricated piezoelectric materials with visible light response,and investigated the photopiezocatalytic performance for degradating organic pollutants through coupling the solar and mechanical energy.The related work is as follows:1.BiOBr nanosheets were synthesized by a direct precipitation method.The photopiezocatalytic efficiency for the degradation of dyes was investigated by using a 9 W LED as a light source and an ultrasonic cleaner to provide periodic ultrasonic vibration.After 120 min of simultaneous illumination and ultrasonic vibration,the catalytic decomposition rate of rhodamine B（Rh B）by BiOBr catalyst was significantly increased,which was 5.62 and 11.0 times higher than that of individual photocatalysis and piezocatalysis,respectively.This catalyst has high structural stability and can be recycled for reuse,as well as presents good activity for degrading different organic pollutants.On the basis of the piezoelectric effect and experimental results,a possible photopiezocatalytic mechanism was proposed.The internal piezoelectric field induced by ultrasound promoted the migration of photogenerated electrons and holes to the opposite surface of the catalyst,and inhibited the recombination of charge carriers,thus improving the catalytic performance.2.The photocatalyst of Bi₂WO₆ with intrinsic ferroelectricity was synthesized through a simple hydrothermal method.The photopiezocatalytic performance of Bi2WO6 was investigated using Rh B as a model.Through coupling the illumination by a 9 W LED and the ultrasonic vibration,the nanoflower-like Bi2WO6 composed of ultrathin nanosheets showed a much more enhanced photopiezocatalysis performance for purification of Rh B.Its degradation kinetic constant was 17.5 and 2.10 times higher than those of individual photocatalysis and piezocatalysis,respectively.Furthermore,the high mineralization efficiency and the good durability of the Bi2WO6 catalyst were demonstrated.The results shown that ultrasoundinduced piezoelectric field in Bi2WO6 drove photo-generated electrons（e-）and holes（h+）to diffuse along opposite directions,consequently promoting the separation efficiency of charge carriers.Hydroxyl radical（·OH）and superoxide radical（·O2-）were the main active species in photopiezocatalysis,which were verified by the quenching of active oxygen species and the EPR experiment.3.The pyroelectric BiOIO₃ nanoparticles were prepared by simple hydrothermal method.We simultaneously used photo-irradiation and ultrasound-vibration to activate and separate free charge carriers in the BiOIO3 catalyst,and then investigated its synergetic photopiezocatalysis for oxidatively decomposing a variety of organic pollutants,including dyes and dichlorophenol.Through coupling the xenon lamp irradiation and ultrasonic vibration,BiOIO3 achieved high photopiezocatalytic efficiency for the degradation of Rh B.The degradation kinetic constant of photopiezocatalysis by BiOIO3 reached 7.33×10-2 min-1,which is 4.97 and 4.33 times higher than those of individual photocatalysis and piezocatalysis,respectively.BiOIO3 with non-centrosymmetric structure could generate piezoelectric field under ultrasonic vibration,which promoted the efficient separation of photo-generated carriers and thus greatly improved its photopiezocatalytic performance.h+ and ·O2-were the main active species in photopiezocatalysis,which were verified by the quenching of active oxygen species experiment,and a possible degradation pathway of Rh B was proposed based on the determined intermediates of LC-MS.4.The ultrathin two-dimensional（2D）g-C₃N₄ was obtained by a modified thermal polycondensation approach.Physicochemical properties shown that the 2D g-C3N4 nanosheet possesses ultrathin structure,curled edge and a significantly piezoelectric response.The photopiezocatalytic performance of 2D g-C3N4 was investigated for 2,4-dichlorophenol（2,4-DCP）degradation.Experimental results showed that under the excitation of the ultrasonic vibration and the irradiation of xenon lamp,the degradation kinetic constant of 2,4-DCP by 2D g-C3N4 reached 5.72×10-2 min-1,which was 2.07 and 2.15 times higher than those of individual photocatalysis and piezocatalysis,respectively.Furthermore,the 2D g-C3N4 also has good stability,reusability and superior photopiezocatalytic performance for the degradation of other chlorophenols（CPs）.·OH and ·O2-were the main active species in photopiezocatalysis,which were verified by the quenching of active oxygen species and the EPR experiment,and a possible degradation pathway of 2,4-DCP in photopiezocatalysis was proposed based on the determined intermediates of LC-MS.