Full-spectrum Photocatalytic And Dark Catalytic Mechanism Of Defect And Build-in Electric Field Mediated Z-scheme Heterojunction

In order to address the issues of low full-spectrum absorption stability,reaction driving force stability,and dark catalytic reaction stability of defectmediated Z-scheme heterojunction,a series of defects,build-in electric field(BIEF)and metal-modified Z-scheme heterojunctions were prepared via UV light irradiation,hydrothermal,solvothermal and photodeposition methods in this work.The effects of defect construction on the optical,chemical,photoelectrochemical properties and carrier dynamics processes of the prepared heterojunctions were investigated by the advanced characterization techniques combined with the theoretical calculations.The activity and stability of the photocatalytic and dark catalytic reactions of defects,BIEF,and metal-modified Z-scheme heterojunctions were evaluated by degrading organic pollutants and removing NO.Meanwhile,the structure-function relationship between defects,BIEF,and metal modification and the stability of the prepared heterojunctions was established.This work might provide a theoretical foundation for the design and preparation of the Z-scheme heterojunction with full-spectrum stable photocatalytic and long-lasting dark catalytic activity.The conclusions are listed as follows:(1)The influences of interfacial build-in electric field(IBIEF)and build-in polarization electric field(BIPEF)on the type of g-C3N4-x/Zn2SnO4N/ZnO heterojunction and the dynamics of the photogenerated carriers are investigated.The photogenerated electrons of Zn2SnO4N jump with the N impurity level as the springboard due to the formation of the BIPEF induced by the interstitial N atoms.The IBIEFs from g-C3N4-x/Zn2SnO4N to Zn2SnO4N/ZnO are constructed owing to the interfacial polarization charge transfer(PCT)from g-C3N4-x/Zn2SnO4N to Zn2SnO4N/ZnO.Ⅱ-Ⅰ-type heterojunction is transformed into the double Z-scheme heterojunction driven by the IBIEFs.Moreover,the formation of the interfacial chemical bonds not only promotes the charge transfer,but also broadens the photoresponse range.Therefore,the photogenerated carriers of the prepared heterojunction show the higher separation efficiency and the stronger redox ability,resulting in the enhanced UV-visible photocatalytic activity.Under the simulated sunlight irradiation,the degradation rate of the prepared heterojunction to rhodamine B(RhB)is 1.78 and 8.61 times higher than that of g-C3N4-x and Zn2SnO4N/ZnO,respectively.Under the visible light irradiation,NO removal rate of the obtained heterojunction is 1.27 and 8.85 times higher than that of g-C3N4-x and Zn2SnO4N/ZnO,respectively.(2)The influences of the dual defects of nitrogen vacancies and oxygen vacancies,and IBIEF on the formation mechanism,light absorption characteristics and the migration paths of the photogenerated carriers of W18O49/g-C3N4-x and gC3N4-xBiOCl/WO2.92 heterojunctions are studied.The exciton dissociation is promoted due to the existence of nitrogen vacancies.The IBIEFs from g-C3N4-x to W18O49,and g-C3N4-x/BiOCl to BiOCl/WO2.92 are constructed due to the PCT from g-C3N4-x to W18O49,and g-C3N4-x/BiOCl to BiOCl/WO2.92,which drive the conversion from Ⅱ(Ⅱ-Ⅱ)-type to the(double)Z-scheme heterojunctions.The obtained heterojunctions show the full-spectrum absorption characteristics owing to the localized surface plasmon resonance(LSPR)effect of oxygen vacancies.The charge separation efficiency of the heterojunctions is markedly improved due to the exciton dissociation effect of nitrogen vacancies,the near-field enhancement effect of oxygen vacancies,and the passivated surface defects induced by the heterogeneous interface.The oxygen vacancies in W18O49 and WO2.92 can capture the photogenerated electrons to stabilize the free charge density,resulting in the continuous production of the LSPR high-energy hot electrons which can directly participate in the photocatalytic reactions.Therefore,the prepared heterojunctions exhibit the significantly boosted full-spectrum driven photocatalytic activity.NO removal rate over W18O49/g-C3N4-x heterojunction can reach 83.55%under the simulated sunlight irradiation,which is 6.15 and 1.41 times higher than that of W18O49 and g-C3N4-x.Moreover,RhB can be decomposed by the W18O49/g-C3N4-x heterojunction under the NIR light irradiation.Under the visible light irradiation,NO removal rate by g-C3N4-x/BiOCl/WO2.92 heterojunction can reach 68.70%,which is 1.88 and 3.25 times higher than that of g-C3N4-x and BiOCl/WO2.92.At the same time,the generation of the toxic NO2 is markedly suppressed.(3)The effects of oxygen vacancies and metallic plasmon Bi on the full-solarspectrum absorption stability,full-spectrum responsive catalytic activity and cycle stability of g-C3N4-xBi/Bi2O2(CO3)1-x(Br,I)x heterojunction are investigated.The prepared heterojunction shows the significantly enhanced visible-NIR light absorption due to the synergistic LSPR effects of oxygen vacancies and plasma Bi.The formation of oxygen vacancies in Bi2O2(CO3)1-x(Br,I)x is promoted owing to the PCT from Bi/Bi2O2(CO3)1-x(Br,I)x to g-C3N4-x.The increase in the concentration of oxygen vacancies not only improves the adsorption ability of the heterojunction to tetracycline(TC),but also increases the separation efficiency of the hot carriers of plasma Bi via capturing hot electrons by oxygen vacancies.The main role of metal Bi is to suppress the inactivation of oxygen vacancies while induces the conversion from Ⅱ-type heterojunction to Z-scheme heterojunction as an electron mediator.Moreover,the energy band structures of Bi/Bi2O2(CO3)1-x(Br,I)x and g-C3N4-x are downshifted and upshifted respectively induced by the PCT,causing the stronger redox ability of the photogenerated carriers.Hence,the obtained heterojunction shows the remarkably enhanced and stable full-spectrum driven photocatalytic activity.Under the visible light irradiation,76.73%of NO can be removed by the heterojunction,which is 2.10 and 1.36 times higher than that of g-C3N4-x and Bi/Bi2O2(CO3)1-x(Br,I)x,and the generation of the toxic NO2 is significantly inhibited.Under the NIR light irradiation,the degradation efficiency of the prepared heterojunction to TC is 2.61 and 1.27 times higher than that of g-C3N4-x and Bi/Bi2O2(CO3)1-x(Br,I)x,respectively.(4)The influences of defect and PCT on the full-spectrum absorption characteristics and stability,energy band structure and full-spectrum catalytic and dark catalytic activities of BiO2-x/Bi2O2.75 and Zn2SnO4/BiO2-x/Bi2O2.75 heterojunctions are studied.The PCT from Bi2O2.75 to BiO2-x results in the energy band structures of BiO2-x and Bi2O2.75 rise and fall,respectively,and Ⅰ-type heterojunction is transformed into Ⅱ-type heterojunction.The BIPEF in Bi2O2.75 is constructed due to the separation of the positively and negatively charged centers induced by oxygen vacancies,which drives the conversion from Ⅱ-type to Zscheme heterojunction.BiO2-x/Bi2O2.75 shows the enhanced full-spectrum driven photocatalytic activity due to the LSPR effect of oxygen vacancies and Z-scheme mechanism.Under the simulated sunlight and the NIR light irradiation,the degradation rates of the heterojunction to RhB are 10.22 and 2.24 times higher than that of BiO2-x.Both ·O2-and ·OH radicals can be generated by the oxygen reduction reaction driven by the localized electrons at oxygen vacancy sites,resulting in that the organic pollutants can be decomposed by BiO2-x/Bi2O2.75 heterojunction in dark.The introduction of the positively charged Zn2SnO4 promotes the PCT from BiO2-xBi2O2.75 to Zn2SnO4,leading to the upshifted and downshifted band structures of Zn2SnO4 and Bi2O2.75,and the transformation fromⅡ-Ⅰ-type to Ⅱ-Ⅱ-type heterojunction.The Ⅱ-Ⅱ-type heterojunction is further converted into the double Z-scheme heterojunction driven by the IBIEFs induced by the electrostatic attraction and the potential difference.Moreover,the formation of oxygen vacancies was promoted due to the PCT,leading to the stable LSPR effect and full-spectrum light absorption.Zn2SnO4/BiO2-xBi2O2.75 heterojunction shows the boosted full-spectrum driven reactivity owing to the optimized band structures and the double Z-scheme mechanism.NO removal rate of the heterojunction is 1.25 times higher than that of BiO2-x/Bi2O2.75 under the visible light irradiation.The degradation rate to RhB is 18.75 and 3.13 times higher than that of Zn2SnO4 and BiO2-x/Bi2O2.75 under the NIR light irradiation.Moreover,the higher concentrations of ·O2-and ·OH radicals can be produced in dark due to the increased oxygen vacancies concentration.(5)The energy storage mechanisms,and the enhancement mechanisms of full-spectrum catalytic and long-lasting dark catalytic activities of Ag/NaBiO3 and Ag/BiO2-xBi2O2.75 heterojunctions are investigated.Ag/NaBiO3 shows the single electron storage mechanism,and the electron storage concentration of Ag/NaBiO3 is increased from 20.83 to 72.57 μmol·g-1 due to the synergistic effect of Ag and oxygen vacancies,resulting in the enhanced and long-lasting dark catalytic activity.Furthermore,under the visible light irradiation,NO removal rate of Ag/NaBiO3 is increased from 35.94%to 49.70%due to the adsorption and pre-oxidation of NO at Ag sites while the NO2 conversion rate is decreased from 51.43%to 26.62%.Ag/BiO2-x/Bi2O2.75 shows the unique electron-hole storage mechanism.Ag is directionally deposited on the surface of BiO2-x due to the Z-scheme charge transfer mechanism.Electrons and holes are stored in Ag and bismuth vacancy sites in Bi2O2.75,respectively.The reverse recombination of electrons and holes is effectively inhibited due to the formation of the Schottky junction and Ag-Bi/AgO bonds at the Ag/BiO2-x interface,resulting in that the storage concentrations of electrons and holes are increased from 24.94 and 0.0042 μmol·g-1 to 86.55 and 0.0481 μmol·g-1,respectively.In comparison with Ag/NaBiO3,Ag/BiO2-x/Bi2O2.75 exhibits the enhanced dark catalytic activity.The adsorption and activation of the reactants are promoted due to the formation of the electron-hole mechanism.The electron transition and the interfacial charge transfer are simultaneously improved due to the synergistic LSPR and propagating SPR(PSPR)effects between Ag and oxygen vacancies,so that the concentrations of the photogenerated holes and active oxygen species are increased.NO on the surface of catalyst is firstly activated to N2O22-and NO+ by trapping electrons and holes,which are further oxidized to NO2-/NO3-.NO oxidation pathway of NO→NO2→NO2-/NO3-is changed to NO→NO+/N2O22-→NO2-/NO3-due to the formation of electron-hole storage mechanism,and the generation of NO2 is inhibited at the source.Under the simulated sunlight irradiation,84.82%of NO can be removed by Ag/BiO2x/Bi2O2.75,which is 1.61 times higher than that of BiO2-x/Bi2O2.75.