Synthesis Of Bismuth Vanadate-based Photocatalytic Materials And Its Properties Research

In recent years,energy shortages and environmental pollution problems have become more and more serious.Semiconductor photocatalytic technology,as a promising advanced oxidation technology,not only can make full use of solar energy to produce hydrogen and oxygen by splitting water,converting solar energy into chemical energy,but also can degrade organic pollutants in wastewater.Therefore,it can effectively alleviate energy shortages and deal with environmental crises,causing semiconductor photocatalytic materials to have become one of the hot areas in the recent researches.However,due to the large band gap energy,the traditional photocatalysts can only response to the UV region which only accounts for about 4% of the total sunlight,greatly restricting its wide application.Therefore,it is very important to design and fabricate novel visible-light-responsive semiconductor photocatalytic materials.Monoclinic bismuth vanadate?m-BiVO₄?,as a typical semiconductor photocatalyst,has attracted considerable attention,due to its suitable band structure,earth-abundant elemental composition,good chemical and thermal stability,non-toxic,environmentally friendly,especially for its narrow band gap.Nevertheless,the conversion efficiency of the pure m-BiVO₄ sample prepared by the traditional method is very low,mainly due to the poor photogenerated electronic mobility,weak adsorptive performance,as well as hard separation and rapid recombination rate of photogenerated charge carrers.Therefore,it is necessary to be modified to improve its photocatalytic activity.Based on the shortcomings of the conventional m-BiVO₄ samples,microspherical BiVO₄ samples were synthesized by the solution combustion method in this paper.In addition,a series of well-defined morphologies of BiVO₄ samples,and oriented along highly active {040} facets of BiVO₄ samples,as well as BiVO₄/Bi₂WO₆ composites,were synthesized by the solvothermal method.The equipments of the two methods are simple,and the morphologies,structures and chemical compositions of the products can be easily controled by changing the reaction conditions.V⁴⁺ and Ce³⁺ self-doped BiVO₄/CeO₂ nanocomposites with high surface areas and porous structures were prepared by the templating approach combined with a sol-gel method.This method provides an effective means for the preparation of nanocomposite metal oxide materials with high surface areas and porous structures.Based on these,main works are carried out as follows:1.The microspherical m-BiVO₄ samples were synthesized by the solution combustion method.The effects of citric acid and urea contents on the structures and morphologies of samples were studied in details.The photocatalytic activity was evaluated by the photocatalytic degradation of rhodamine B.Compared with the bulk BiVO₄ sample prepared by the traditional solid-state method,the effect of different preparation methods on the structure,morphology and catalytic property of the samples was discussed.The results showed that the citric acid and urea contents had significant effects on the morphology of BiVO₄ samples synthesized by the solution combustion method.What's more,compared with the bulk BiVO₄ sample synthesized by the solid-state method,the microspherical BiVO₄ sample synthesized by the solution combustion method exhibited obviously enhanced photocatalytic activity.2.Novel self-assembled m-BiVO₄ samples with red blood cell-like shape,dendrite-like structure and flower-like microsphere were synthesized via a solvothermal approach by using anhydrous citric acid as the chelating agent,and Na₂CO₃ or NH3·H2O as the pH controlling agent.The photocatalytic activity was evaluated by the photocatalytic degradation of methylene blue.The effects of the citric acid and pH controlling agent on the structure,morphology and catalytic property of samples were studied in details.The results showed that the citric acid and pH controlling agent had great influences on the morphologies of samples.Benefiting from the unique morphology,the BiVO₄ sample with the red blood cell-like structure exhibited the best photocatalytic activity,then the flower-like microsphere sample.The dendrite-like BiVO₄ sample exhibited the worst photocatalytic activity.3.A series of hierarchically structured m-BiVO₄ samples with well-defined morphologies and oriented along highly active {040} facets were synthesized via a solvothermal route,without adding any surfactant or template,as well as acid inhibitor.The photocatalytic activity was evaluated by the photocatalytic degradation of methylene blue.The effects of the volume ratio of EG/H2 O,the amount of raw material and the pH value on the crystal structure,morphology and photocatalytic activity of samples were investigated in details.The results showed that the volume ratio of EG/H2 O,the amount of raw material and the pH value played great influences on the morphology and crystal structure of samples.The relative intensities of the diffraction peaks I040/I–121 increased gradually with the decrease of EG content in mixed solvents,while increased and then decreased with the increase in the amount of raw material and the pH value.The photocatalytic results showed that the photocatalytic activities of the synthesized BiVO₄ samples were closely related to relative intensities of the diffraction peaks I040/I–121.In general,the photocatalytic activities of samples enhanced with the increase in the relative intensities of the diffraction peaks I040/I–121.4.Self-assembled hierarchically structured Bi VO₄/Bi₂WO₆ composites with well-defined morphologies were synthesized via a solvothermal method.The photocatalytic activity was evaluated by the photocatalytic degradation of methylene blue.The effect of the V/W molar ratio on the morphology,structure and photocatalytic activity of the BiVO₄/Bi₂WO₆ composite photocatalysts was systematically investigated.The stability of the catalyst was also studied.The reason of the enhanced photocatalytic activity was studied by PL spectra,transient photocurrent responses,EIS spectra and Mott-Schottky plots.In addition,a possible photocatalytic mechanism was proposed on the basis of the calculated energy band structures.The results showed that the BiVO₄/Bi₂WO₆ composites exhibited uniform and three-dimensional microsphercial structures assembled with thin two dimensional nanoplates.The molar ratio of V to W had a significant influence on the morphology,structure,photocatalytic activity and photoelectrochemical properties.When the molar ratio of V to W was 2:1,the BiVO₄/Bi₂WO₆ composite exhibited the best photocatalytic activity and photoelectrochemical properties.What's more,there was no obvious loss in photocatalytic degradation efficiency for MB dye after four times successive recycle test,suggesting that the BiVO₄/Bi₂WO₆ composite photocatalysts had good stability.5.V⁴⁺ and Ce³⁺ self-doped BiVO₄/CeO₂ nanocomposites with high surface areas and porous structures were prepared using the templating approach combined with a sol-gel method,employing MCM-41 as the hard template.The photocatalytic activities were evaluated by the photocatalytic degradation of rhodamine B and methyl orange.The effect of the BiVO₄/CeO₂ molar ratio on the structure,morphology and photocatalytic activity of nanocomposite photocatalysts was systematically investigated.The reason of the enhanced photocatalytic activity was discussed by PL spectra,transient photocurrent responses and EIS spectra.In addition,the possible photocatalytic mechanism was analyzed based on the active species trapping experiments and calculated energy band structures.The results showed that the BiVO₄/CeO₂ molar ratio had a significant influence on the structure,morphology,photocatalytic activity and photoelectrochemical properties.The two phases in the composite samples could influence each other on the crystal structure,causing the formation of oxygen vacancy defects.Moreover,once the CeO₂ was introduced,the surface areas of the composites increased significantly,while the particle sizes decreased obviously.XPS results showed that the self-doped V⁴⁺ and Ce³⁺ ions existed in the nanocomposites.And the existence of V⁴⁺ ions was further confirmed by the ESR spectra.The photocatalytic results showed that the nanocomposite exhibited the best photocatalytic activity and photoelectrochemical properties,when the molar ratio of BiVO₄/CeO₂ was 0.4:0.6.After irradiation with 210 min,the degradation efficiencies of Rh B and MO were reached to 92.68% and 71.95%,respectively.The active species trapping experiments demonstrated that photogenerated holes played the most important role in the photocatalytic system and were the dominant active species.