Synthesis, Characterization And Visible Light Photocatalytic Activities Of Bi₂MoO₆-and Silver-based Hetero-structural Photocatalysts

Energy crisis and environment pollution are the two of top10problems facing humanityin the21st century. Semiconductor-based photocatalytic technology, belonging to the greenchemistry, shows great potential for solar energy conversion and environmental protection.The traditional photocatalyst of TiO₂, however, can only utilize the ultraviolet light （about4%solar spectrum） and its practical application is limited due to its low quantum efficiency aswell as the difficulty in separation and reuse of the powder catalysts. For this, the explorationof novel, highly efficient visible-light-responded photocatalysts has become one of the mostimportant research areas for environmental pollution control and renewable energy sources. Inaddition, a challenge of the pristine semiconductor faced before its practical application is thepoor quantum yield, resulting from the rapid recombination of photogenerated electrons andholes. In this dissertation, our work is focused on the synthesis, characterization and visiblelight photocatalytic activities of Bi₂MoO₆-based layered semiconductor heterojunctions andsilver-based semiconductor composites.The details are summarized briefly as follows:Bismuth-based semiconductors possess the unique layered structures which can favor theeffective separation and transport of the photoinduced carriers. Furthermore, their structurescan be tailored by intercalation, exfoliation and ion exchange reactions due to the highactivity of the interlamination ions.（1） Hierarchical β-Bi2O3/Bi₂MoO₆heterostructured flower-like microspheres assembledfrom nanoplates with different β-Bi2O3loadings were synthesized through a one-steptemplate-free solvothermal route. Under visible-light illumination （λ>420nm）, over99%ofrhodamine B （RhB） was degraded within90min on the21.9mol%of β-Bi2O3loadedBi2O3/Bi₂MoO₆microspheres. The remarkable enhancement of photocatalytic activity of thehierarchical Bi2O3/Bi₂MoO₆can be attributed mainly to the effective separation of thephotoinduced charge carriers at the interfaces and in the semiconductors. The electrons are themain active species in aqueous solution under visible light irradiation. The Bi2O3/Bi₂MoO₆also displays visible-light photocatalytic activity of the inhibition for the growth of E. coli. Inaddition, the β-Bi2O3in the hierarchical Bi2O3/Bi₂MoO₆microspheres is very stable and thecomposite can be easily recycled by a simple filtration step, thus the second pollution can beeffectively avoided.（2） Bi₂O₃/Bi₂MoO₆heterostructured photocatalysts were successfully prepared by a facile solvothermal process. The microsphere-like Bi₂O₃/Bi₂MoO₆composites exhibitedsuperior visible light photocatalytic activity towards degradation of RhB. The highestdegradation efficiency was observed on the catalyst with Bi/Mo molar ratio of2.88/1, whichcan degrade99%RhB within90min, while only44%RhB was degraded over the pureBi₂MoO₆microspheres and2%over the Bi2O2CO3nanoplatelets. The photo-generated h+VBinthe Bi₂O₃/Bi₂MoO₆photocatalysts turn out to be the dominant active species. Importantly,Bi₂O₃/Bi₂MoO₆displayed visible-light photocatalytic activity for the destruction of E.coli （the percent kill is99.09in60min）. In addition, the Bi₂O₃/Bi₂MoO₆composite wasvery stable during the reaction and can be used repeatedly. These features mean the presentheterostructured photocatalyst can be applied in environmental remediation, and waste waterdisinfection.（3） Heterostructured sesame-biscuit-like Bi₂O₃/Bi₂MoO₆nanocomposites weresuccessfully prepared via a facile anion exchange approach under hydrothermal process withthe graphitic carbon nitride （g-C₃N₄） as the precursor of carbonate anion. TheBi₂O₃/Bi₂MoO₆nanocomposites are based on ca.30-45nm thick single-crystal Bi₂MoO₆nanoplatelets embedded with homogeneously dispersed Bi2O2CO3nanoparticles. Thephotocatalyst prepared by using11wt%g-C₃N₄as the precursor shows the highest activity,which can degrade99%RhB in30min. The degradation rate of the Bi₂O₃/Bi₂MoO₆photocatalyst is more than64times faster than that of using bare Bi₂MoO₆under visible lightirradiation. The dramatically enhanced photocatalytic activity of the Bi₂O₃/Bi₂MoO₆photocatalysts can be attributed to the large heterojunction interface, intrinsically layeredstructure, two-dimensional morphology and effective separation of the photoinduced carriersat the interfaces and in the semiconductors. This method may usher a new phase for thesynthesis of novel and highly efficient Bi2O2CO3-based heterostructures for light-harvestingand energy conversion applications.Additionally, the silver and its derivatives are light-sensitive and slightly soluble inaqueous solution. They will be photo-corroded and their photocatalytic activity deterioratesgradually. By forming heterostructures with other semiconductors, not only the silver crystalscan be effectively protected from dissolution in aqueous solutions, but also their stabilitiesand the photocatalytic performance are enhanced.（4） A fast and energy-saving anion exchange method for the synthesis of Ag₃PO₄microcrystals with various shapes of microspheres, rhombic dodecahedron, truncatedtetragonal bipyramid microbox and self-assembled parallel quadrilateral prism have been successfully attained. And then, Ag₃PO₄nanoparticles were deposited on the surface ofBi₂MoO₆via a following deposition-precipitation technique. The photocatalytic testsdisplayed that Ag₃PO₄/Bi₂MoO₆nanocomposites possessed a much higher degradation rate ofRhB and methylene blue than the Ag₃PO₄nanoparticles and Bi₂MoO₆under visible light. The50mol%Ag₃PO₄-loaded Bi₂MoO₆spheres exhibited the best photocatalytic activity. Theobserved improvement in photocatalytic activity is associated with the extended absorption inthe visible light region, and the effective separation of photogenerated carriers at theAg₃PO₄/Bi₂MoO₆interfaces. Moreover, the tests of radical scavengers confirmed that h+and OH were the main reactive species for the degradation of RhB.（5） Ag/AgBr-grafted g-C₃N₄is fabricated by in situ photoreduction of AgBr/g-C₃N₄hybrids prepared by deposition-precipitation method. The Ag/AgBr/g-C₃N₄hybrids exhibitstrong absorbance in the visible region and near-infrared light due to the surface plasmonresonance absorption of Ag nanocrystals. Compared with bare g-C₃N₄and Ag/AgBrnanoparticles, a28-fold and6-fold enhancement in the degradation rate toward rhodamine Bis observed over Ag/AgBr/g-C₃N₄hybrids under visible light irradiation, respectively. Thegreat enhancement of photocatalytic activity is attributed to the extended absorption in thevisible light region, the effective charge separation due to surface plasmon resonance of Ag0,the light scattering resulted from the non-uniform Ag0nanocrystals and the synergisticenhancement in the ternary Ag/AgBr/g-C₃N₄system. Moreover, the composite can bereclaimed easily by sedimentation without any decrease of its photocatalytic activity. Thisstudy provides new insight into the fabrication of highly efficient and stable g-C₃N₄-basedplasmonic photocatalysts and facilitates its practical application in environmental issues.In summary, construction of heterostructured semiconductor photocatalysts is a promisingway to enhance its visible light harvesting ability, the separation rate of electon-holes andphotocatalytic performance.