Study On The Preparation And Modification Of BiVO₄ With Highly Exposed Reactive Facets And Photocatalytic Performance

The photocatalytic technology has been proved to be a suitable method for nitrogen oxides(NOx)removal and photocatalytic water splitting for H2 or O2 production.To achieve this objective,the major challenge is the efficient and stable photocatalysts.BiVO4,as a visible light driven semiconductor photocatalyst,has recently attracted much attention due to its excellent photocatalytic performance,high stability,easily recycled and cheap and so on.This paper is committed to the investigation of the photocatalytic performance of the removal of NO and decomposition of water to H2 or O2,and then study the BiVO4-based photocatalysts preparation,modification and its photocatalytic mechanism.It forcus on the formaton mechanism of the modified BiVO4-based materials and its corresponding photocataytic reaction mechanism.Firstly,a surfactant-free solvothermal method was developed for the controlled synthesis of 3D BiVO4 by easily adjusting the pH value.This work focused on the evolution process of the different morphologies and the major factors influcing the photocatalytic H2O2 for NO oxidation.The characterization results indicated that the pH could adjust the BiVO4(010)facets,thus affecting the morphology.Herein,the(010)facets could provide more BiV4 active center,which induced more free electron participating in the photocatalytic reaction process,thus enhancing the photocatalytic activity.The results showed that the photocatalytic oxidation of NO performance was positively correlated with the exposure ratio of BiV04(010)facets.The flowerlike morphology(pH=8)with a more(010)reactive crystal plane exhibited higher photoactivity than those of other samples.Moreover,the unique morphology helped dissolve the oxidation products NO3-,in the H2O2 system,greatly reduced the accumulation on the surface of photocatalysts and the cover of(010)facets and further improved the photocatalytic stability.Secondly,graphene-decorated 3D flower-like BiVO4(BiVO4/rGO)photocatalyst was synthesized.And the photocatalytic oxidation of NO performance increased from 48.5%to 65%.It could be found in the characterization analysis that the GO facilitated the preferential crystal growth of the 3D BiVO4 photocatalyst along its(010)facets.Meanwhile,GO was reduced to high conductive rGO and then covered on the surface of BiVO4.The synergy effect facilitied the effective separation of photogenerated electrons-hole pairs,thus promoting the photocatalytic oxidation of NO activity.In addition,the material balance of N element was conducted to comfirm the main product of photocatalytic H2O2 oxidation of NO was NO3-theoretically and experimently.And the active species in the photocatalytic reaction systemt was detected via adding the corresponding capture agents and the order was:·OH>·O2->h+.Experiments results showed that the high(010)facets exposure ratio could induce the separation of photogenerated electronic-hole pairs,and then improve the photocatalytic activity.Therefore,it was necessary to directly synthesize the BiVO4 sample with high(010)facets exposure.The decanedron BiVO4 with preferred exposure of(010)crystal facet was successfcilly prepared,and the metal Ag was deposited on the(010)crystal facet by the photodeposition method,which was due to the(010)electrons accumulation facets characteristic.Subsequently,the flocculent g-C3N4 with the suitable band gap structure was covered on the surface of Ag/BiVO4(010)to prepare the g-C3N4@Ag/BiVO4(010)hybrid photocatalyst.The hybrid photocatalyst showed great photocatalytic oxidation performance not only for the photocatalytic H2O2 for NO oxidation but also for water splitting to produce O2.Through the systematic characterization tests and in-depth analysis of mechanism,we found that the photoinduced charge carrier separation and transfer in the ternary g-C3N4@Ag/BiVO4(010)system followed the Z-scheme mechanism.Then,more free electrons and holes were produced in the system,and thus improving the photocatalytic performance.Thereafter,metal Pt and metal oxide Co3O4 were loaded on the surface of BiVO4(010)and(110)crystal plane,respectively,by the photodeposition method,thus it could also confirm that the(110)crystal facets were the holes accumulation facets.The study found that the photocatalytic performance not only for NO oxidation activity but also for water splitting to O2 of double crystal facets deposition photocatalyst of Pt(010)/Co3O4(110)/BiVO4 was superior to that of single crystal facets deposition of Pt(010)/BiVO4 or Co3O4(110)/BiVO4.And i-t and EIS results also showed that the photogenerated charge carriers of double crystal facets deposition sample were much larger than that of the single crystal facets deposition materials,which was the main factor that affecting the photocatalytic activity.Finally,based on the above research,it was necessary to find a H2-photocatalyst combining electron mediator rGO and the O2-photocatalyst(Co3O4(110)-BiVO4)to form a Z-scheme composite material for photocataljytic overall water splitting to H2 and O2.Due to the simple synthesis and strong adsorption capacity of ZnIn2S4,it could be used as a H2-photocatalyst to self-assembly the Pt-ZnIn2S4/rGO/Co3O4-BiVO4(110)Z-scheme photocatalyst in the photocatalytic process.The results showed that H2 and O2 could be obtained under the visible light/H2O/Z-scheme photocatalyst system,and the amount were as high as 294.3 μmol/g and 143.4 μmol/g in 12 h,respectively.At the same time,the role of Co3O4 and rGO for the separation rate of photogenerated charge carriers was also investigated.It was found that the proportion of Co3O4 and rGO directly effect the the balance of the number of free electrons and holes,and further influnced the photocatalytic activity and the ratio of H2 and O2.