Microstructure Tuning and Photocatalytic Activity of Several Bismuth-Based Photocatalysts

Author:Zhang Yang Yang

Supervisor:liu he zhou guo yi ping


Degree Year:2016





Photocatalysis is an ideal approach to solving the increasingly sever environmental and energy crises.The development of non-toxic,cost-effective,stable,versatile and highly efficient novel photocatalysts has been the key issue remains to be solved.Traditional TiO2 can meet nearly all the requirements except for high efficiency owing to its wide band gap and poor light harvesting ability.Although many technical means have been adopted to improve its photoactivity,advances in breaking its inherent shortage are still limited.Recently,bismuth-based visible light driven photocatalysts are receiving more and more attention due to their potentials in avoiding the deficiency of TiO2 and at the same time reserving its advantages.Among which,Bi2Fe4O9 and Bi2S3 are two photocatalysts with ultralow band gaps(Eg<2 eV)and abilities to harvest solar energy covering the whole visible light range.Bi2O3 and BiVO4 with band gaps around 2.4 eV are also two important visible light responsive photocatalysts capable of utilizing a large amount of visible light.In comparison with Bi2O2CO3,Bi2WO6,Bi2MoO6,BiOX(X=Cl,Br,I)and other widely investigated Bi-containing photocatalysts,Bi2Fe4O9,Bi2S3,Bi2O3 and BiVO4 are more desirable owing to their exclusive of heavy metals,noble metals and unstable harmful halogen.Aiming at the weaknesses of four specifically selected Bi-based photocatalysts,methods including morphology optimization,doping,semiconductor coupling,graphene loading,crystal structure and electronic structure controlling are employed to improve their overall photocatalytic performances and reveal their interior physical characteristics and mechanisms behind properties improvement.Primary conclusions are summarized as below:(1)An n-type mono-dispersed cuboid-shaped Bi2Fe4O9 semiconductor is synthesized via a hydrothermal method in concentrated NaOH solution.It is found that the cuboids are preferentially grown along[001]direction with85%active side facets(110)and(110)parallel to it.The UV-Visible-Near infrared absorption spectrum shows that besides two broad absorption edges in visible spectrum region,remarkable near-infrared absorption is also observed,indicating Bi2Fe4O9 is a promising semiconductor capable of utilizing all solar band energy.Hence,steady and distinct photocurrents are measured to be 0.35μA/cm2 and 7μA/cm2 under near-infrared irradiation and visible-light,respectively.First principle calculation is used to reveal the electronic structure of Bi2Fe4O9 and the derived band gap is 1.23 eV,which agrees well with our experimental value of 1.29 eV.The calculation results also show that Bi2Fe4O9 is an indirect bandgap semiconductor which is contrary to previous results.Besides,the extra absorption peak at around 700 nm in the UV-Visible-Near infrared spectrum should be attributed to the intervalence charge transfer between Fe3+and Fe 2+.(2)Monodispersed monoclinic BiVO4 nanorods grown along[010]direction were prepared using a one-step low temperature hydrothermal method in the presence of the low-cost,nontoxic sodium oleate served as chelating agent.The BiVO4 nanorods with diameters of 15-25 nm possess a huge specific surface area as large as 28.2 m2/g.Meanwhile,the specific[010]growth direction is capable of facilitating electron-hole separation efficiency by accumulating electrons on{010}facets.Plentiful V4+species,which can bring oxygen vacancies,endow BiVO4 nanorods with strong absorption of Rhodamine B.The visible-light-driven photocatalytic activity of the BiVO4 nanorods after heat treatment is 88 times higher than P25 and suprior to the as-prepared BiVO4nanorods owing to the optimized charge-carrier transport characteristics resulted from elevated crystallinity and decreased V4+species.(3)Based on as-prepared V4+self-doped BiVO4 nanorods,rGO wrapped BVO/rGO composite nanorods are successfully obtained via room-temperature self-assembling followed by a two-step reducing process.The thickness of rGO layers is less than 10 nm.After rGO coating,the transient photocurrent values,photocatalytic oxygen evolution efficiencies and photodegradation properties are all obviously enhanced.Specifically,the2 wt.%rGO covered V4+-BVO/rGO composite nanorod with a photocurrent of 117μA/cm2,oxygen production rate of 119.5μmol/h and rate constant of 0.488/h for degrading phenol is the best photocatalyst.However,an excess of wrapped rGO is detrimental to the harvest of solar light and the OHgeneration process by hindering the reaction between photogenerated holes and H2O molecules.(4)BiO2CH3 nanoplate,a new organometallic crystal,with octagonal morphology is successfully prepared using methanol solvothermal method and further employed as template to fabricate porousβ-Bi2O3 octagonal nanoplates with a thickness of less than50 nm.Then a series of hierarchical octagonal haystack-likeβ-Bi2O3/Bi2S3 hybrids with large heterointerface and small bandgaps(1.36-1.44 eV)are constructed.It is found that the photoactivities of allβ-Bi2O3/Bi2S3 heterostructures are continuously enhanced with growing test cycles,owing to the formation of Bi2O3-xSx solid solution,in which the built-in electric filed induced by lattice distortion can brings about higher e-/h+separation efficiency.The gradually increasing photocatalytic activity can reach to an up limit in about 5 photocatalytic cycles and maintain at this level.β-Bi2O3/Bi2S3 heterostructure loaded with 20 mole%of Bi2S3 is demonstrated to be the best photocatalyst and capable of fully decomposing Rh B in 30 min.This research systematically investigated the influence of morphology optimization,doping,semiconductor coupling,graphene loading,crystal structure and electronic structure controlling on improving photoactivities and provide new ideas and methods for the development of other novel environmental,cheap,stable and highly efficient photocatalysts.