Mechanism Of Surface Charge And Photogenerated Charge Transfer Induced By Piezoelectric Polarization And Catalytic Production Of Reactive Oxygen Species

Enhanced properties in inducing photogenerated charge transfer and accelerating photocatalytic activities were embodied in ferroelectric semiconductors by virtue of spontaneous polarization.Some abnormal photoelectric effects have also been found in that photovoltaic devices composed of ferroelectric semiconductors.Moreover,the ferroelectric and piezoelectric semiconductors under piezoelectric polarization show obvious comparative advantages in regulating charge carrier transport and conduction,inducing interfacial catalytic reaction and enhancing photocatalytic efficiencies.These phenomena indicated that the electric polarization progresses play a crucial role in modulating the redistribution of surface charges and inducing the effective separation and transfer of bulk photogenerated charges in ferroelectric and piezoelectric semiconductors,which reflect the positive significance of electric polarization in affecting electrical phenomena and increasing catalytic reaction rates.However,for the catalytic reactions elicited by ferroelectric and piezoelectric semiconductors,it is still difficult to completely distinguish the correlation among electric polarization,interfacial charge transfer,and catalytic reaction rate,which seriously restricts the further development of catalysis,surface and interface chemistry and photocatalytic technology.Given this,based on the effects of constructing heterojunctions,loading plasma metal nanoparticles,doping chemical elements and sensitizing carbon quantum dots on the intrinsic characteristics of tetragonal nano-Ba Ti O₃,the synergistic principle of piezoelectric polarization,and surface free charge and photogenerated charge transfer were systematically studied.Besides,the catalytic reaction mechanisms of piezoelectric-photo coupling exciting Ba Ti O₃-based piezoelectric semiconductors to produce reactive oxygen species（ROSs;such as·OH,·O₂^–,¹O₂,H₂O₂）were proposed,and the oxidative degradation performances of these ROSs on organic contaminants in wastewaters were evaluated.The main research contents are as follows:（1）Piezoelectric Zn O/Ba Ti O₃ heterojunctions were prepared by hydrothermal and impregnation methods,and their catalytic activities for the degradation of rhodamine B（Rh B）were comparatively studied under ultrasound activation（piezocatalysis）,UV-Visible（UV-Vis）light irradiation（photocatalysis）,and ultrasound concurrent with UV-Vis light irradiation（piezophotocatalysis）,respectively.The charge transfer path and ROSs generation mechanism in the processes of piezophotocatalysis were analyzed and inferred.It was found that Rh B were degraded slowly with Zn O/Ba Ti O₃ under ultrasound or UV-Vis light irradiation.The apparent rate constant k of quasi first-order reaction was corresponded to 3.5×10^–2 or 5.6×10^–2 min^–1,respectively.However,the degradation efficiency of rhodamine B with Zn O/Ba Ti O₃ under co-irradiation of ultrasound and UV-Vis light was significantly accelerated,and the k increased to 1.2×10^–2 min^–1,suggesting that the piezoelectric-photo coupling effect was helpful to optimize and improve the catalytic oxidation activities of heterostructure Zn O/Ba Ti O₃.Simultaneously,the piezo-,photo-and piezophotocatalytic efficiencies of Zn O/Ba Ti O₃ were higher than those of pure Zn O and Ba Ti O₃,indicating that the electrochemical potential difference existing in the heterojunction region helped to accelerate the separation and transfer of excited charge carriers,and then improve the catalytic reactivities.Finite element piezoelectric analysis showed that the piezopotential difference of Zn O/Ba Ti O₃ under10⁸ Pa external pressure was 414 m V,which was greater than 33 m V of Zn O and 410m V of Ba Ti O₃ at the same simulation conditions,which further confirmed that the cooperation of larger macroscopic polarization intensity and the electrochemical potential difference at the heterojunction interface improve the utilization of excited charge carriers and photogenerated charges,thus resulting in a significant increase in catalytic activities of these materials.The oxidative degradation of Rh B in the present reaction system was mainly caused by the production of·O₂^–and·OH during piezophotocatalysis.（2）A series of piezoelectric composites consisted of tetragonal Ba Ti O₃nanoparticles chimeric Bi OX(X=Cl,Br,Cl_0.166Br_0.834)nanoplates were synthesized through a two-step hydrothermal method.The tuning effect of Ba Ti O₃-mediated piezoelectric polarization on photogenerated charge transfer of Bi OX was studied,and the similarities and differences among piezo-,photo-and piezophotocatalysis were revealed.The experimental results showed that the narrow band gap of Bi OBr made it easier to be compared with Bi OCl and Cl-Bi OBr excited by simulated sunlight（λ>380 nm）,and the piezoelectric polarization derived from deformed Ba Ti O₃ helped to prevent the recombination of photogenerated charges in Bi OBr.Thus,the piezophotocatalytic reactivities of Bi OBr/Ba Ti O₃ for the production of·OH,·O₂^–,¹O₂were higher than that of Bi OCl/Ba Ti O₃ and Cl-Bi OBr/Ba Ti O₃.At the same time,the Bi OBr/Ba Ti O₃ showed excellent oxidative degradation ability for different organic contaminants in wastewater.In addition,the piezophotocatalytic efficiencies of Bi OX/Ba Ti O₃ were higher than that of pure Bi OX and Ba Ti O₃,and the piezophotocatalytic efficiencies of Bi OX/Ba Ti O₃ were significantly enhanced as compared with the individual piezo-and photocatalysis.The finite element analysis showed that the piezopotential differences generated by pure Ba Ti O₃ and Bi OBr under the external pressure of 10⁸ Pa were 31 and 30 m V,respectively,which were far less than the piezopotential difference of 100 m V generated by Bi OBr/Ba Ti O₃ under the same conditions.This finding further proved that the electrochemical potential gradient and high piezopotential in piezoelectric composites were the critical factors in improving the reactivities of piezophotocatalysis.（3）Focusing on the piezoelectric heterojunction of iodine-doped Bi VO₄/Ba Ti O₃loaded on Ag and Cu nanoparticles（Bi VO₄:I/BTO-Ag and Bi VO₄:I/BTO-Cu）,the mechanism of local surface plasmon resonance coupling piezophototronic effect on catalytic production of ROSs was studied,and a new approach for H₂O₂ synthesis was proposed.The experimental results showed that Bi VO₄:I/BTO-Ag catalyzed the production of·OH and·O₂^–with the highest rates of 371 and 292μmol g^–1 h^–1 during piezophotocatalysis,respectively,followed by Bi VO₄:I/BTO-Cu,and their catalytic rates were higher than their respective piezo-and photocatalysis.The rates of H₂O₂production from aqueous ethanol solution catalyzed by Bi VO₄:I/BTO-Ag and Bi VO₄:I/BTO-Cu under co-irradiation of ultrasound and visible light（λ>420 nm）were 0.3 and 0.2μmol g^–1 h^–1,respectively,which were also greater than ultrasound and visible light-mediated catalytic reaction systems.These phenomena could be ascribed to the loading of Ag and Cu effectively improving the optical response and reduced interfacial charge transfer resistance of Bi VO₄:I/BTO and then enhanced catalytic activity for the production of ROSs through the collaboration of photoexcited electrons and piezoelectric polarization processes.In addition,the rate of piezophotocatalytic production of H₂O₂ is closely related to the type of hole-trapping agent selected in the present reaction system.The results showed that when the reaction solutions contained 1.0 m M Na HCO₃,the yield of H₂O₂ with Bi VO₄:I/BTO-Ag in piezophotocatalysis was much higher than that of the reaction system containing 1.9 m M ethanol,1.0 m M EDTA-2Na or 1.0 m M（NH₄）₂C₂O₄,indicating that the hole trapping agents with high polarities could maximize the utilization of photogenerated electrons and accelerated the production rates of H₂O₂.（4）A heterostructure constructed with Ti O₂-doped Li Nb O₃-type Zn Ti O₃（Zn Ti O₃·Ti O₂）and tetragonal Ba Ti O₃ was synthesized by a hydrothermal method.The reaction mechanism of piezophotocatalytic production of ROSs by synergistic enhancement with this heterojunction modified with Pt or Fe O_x nanoparticles（ZBTO-Pt and ZBTO-Fe O_x）was studied.The analysis of electron band structure showed that the energy difference between the bottom of the conduction band and the top of the valence band of Zn Ti O₃·Ti O₂ and Ba Ti O₃ effectively promoted the migration of photogenerated electrons and holes in ZBTO.The finite element analysis illustrated that the macroscopic polarization intensity produced by ZBTO was significantly greater than Zn Ti O₃·Ti O₂ and Ba Ti O₃ under the same external load.Furthermore,the loading of Pt and Fe O_x improved the optical response and reduced the interface charge transfer resistance of heterojunction.Therefore,the yields of·OH catalyzed by ZBTO-Pt and ZBTO-Fe O_x under co-irradiation of ultrasound and simulated sunlight（λ>380 nm）were 48%and 21%higher than those of ZBTO,and the yields of·O₂^–were 11%and 6%higher than those of ZBTO,respectively.Besides,the yields of H₂O₂ catalyzed by ZBTO-Pt and ZBTO-Fe O_x in aqueous ethanol solution during piezophotocatalysis were 189 and 124μM h^–1,respectively,which were higher than those generated in isolated piezo-and photocatalysis.This finding further confirmed that the piezoelectric polarization of ZBTO was conducive to the rapid separation and transfer of photogenerated charge carriers in ZBTO-Pt and ZBTO-Fe O_x.（5）A new method for improving the catalytic activity of Ba Ti O₃ for of synthesizing of H₂O₂ in piezophotocatalytic was proposed.The analyses of electrochemical AC impedance spectroscopy and Mott-Schottky curves showed that Nb-doped Ba Ti O₃（Ba Ti O₃:Nb）not only reduced the charge transfer resistance but also increased the carrier concentration of pristine Ba Ti O₃.As visible-light photosensitizers,carbon quantum dots（CDs）can inject photogenerated electrons into the conduction band of Ba Ti O₃:Nb.At the same time,the piezoelectric polarization initiated by Ba Ti O₃:Nb drove the injected electrons to migrate to the surface of Ba Ti O₃:Nb and then induced the redox reaction between dissolved O₂ and H₂O in the open reaction system to produce H₂O₂.Due to the piezoelectric polarization electric field induced by Ba Ti O₃:Nb under ultrasound activation,the recombination of photogenerated electron-hole pairs was prohibited at the interface between Ba Ti O₃:Nb and CDs,resulting in a high rate of 1360μmol g^–1 h^–1 for the production of H₂O₂under co-irradiation of ultrasound and visible-light（λ>420 nm）.The production rate of H₂O₂ with CDs-sensitized Ba Ti O₃:Nb in piezophotocatalysis was much higher than those of rates produced in piezo-(953μmol g^–1 h^–1)and photocatalysis(234μmol g^–1h^–1).In addition,CDs could optimize the free charge transfer path of the Ba Ti O₃:Nb surface,which was why the piezocatalytic activities of CDs sensitized Ba Ti O₃:Nb for the production of H₂O₂ was higher than that of Ba Ti O₃ and Ba Ti O₃:Nb.（6）Multilayer In₂S₃ nanosheet clusters co-modified by Ba Ti O₃ and Zn S nanoparticles（Zn S/In₂S₃/BTO）were used to explain the structure-activity relationship between the structural composition of multicomponent composites and its piezophotocatalytic performance to produce ROSs.Electronic structure by first-principle calculations indicated that the energy-band shift of Ba Ti O₃,Zn S and In₂S₃ at the three-phase interface helped to transfer excited electrons to the conduction band of In₂S₃.The generated holes converge to the valence band of Ba Ti O₃ or Zn S to inhibit the recombination of excited electron-hole pairs and improving the utilization of excited charge carriers.The finite element analysis showed that the photogenerated charge carriers were easier to transfer between ternary composites due to the theoretical piezopotential difference provided by Zn S/In₂S₃/BTO,was about 1.7 times of that Zn S/In₂S₃,much larger than Zn S/BTO and In₂S₃/BTO.The experimental results showed that the yield of ROSs synthesized by Zn S/In₂S₃/BTO in piezophotocatalysis was higher than that of heterostructures（Zn S/BTO,In₂S₃/BTO and Zn S/In₂S₃）and pure compounds（Zn S,In₂S₃ and BTO）.Moreover,the rate of H₂O₂ formation from aqueous ethanol solution catalyzed by Zn S/In₂S₃/BTO under co-irradiation of ultrasound andλ>400 nm visible light was 1200μmol g^–1 h^–1,higher than ultrasound(800μmol g^–1 h^–1)and visible light(400μmol g^–1 h^–1)irradiation systems.In addition,the cumulative amounts of H₂O₂ synthesized with Zn S/In₂S₃/BTO reached 58μmol after continuous reaction for 5 h in piezophotocatalysis.Their catalytic activities remained unchanged after recycling for6 times.These findings would provide valuable theoretical and practical guidance for applying piezoelectric polarization in modulating surface charge and photogenerated charge transfer and inducing surface redox catalysis.