Synthesis Of UiO-66-NH₂ And BiOX Based Heterogeneous Architectures As Visible-Light Photocatalysts For Cr（Ⅵ）Reduction

The extensive industrial use of hexavalent chromium（Cr（Ⅵ））has resulted into the contamination of soil and water.This presents a serious threat to the environment and human life due to its strong oxidizing power,high solubility and considerable mobility in environment along with the membranes of living organisms.Unlike Cr（Ⅵ）,Cr（Ⅲ）is relatively less toxic and can be easily precipitated and removed as a solid waste like chromium hydroxide,Cr（OH）₃.Therefore,the conversion of toxic Cr（Ⅵ）to eco-friendly Cr（Ⅲ）is the need of hour for the safety of our society and environmental purity.Among various approaches,photocatalytic reduction of Cr（Ⅵ）to Cr（Ⅲ）is the most straight forward and green method for the removal of toxic Cr（Ⅵ）from underground or industrial wastewater because of its low cost and high efficiency.In recent years,although a number of efforts have been devoted to design photocatalysts for efficient Cr（Ⅵ）reduction under visible light irradiation yet these have to face poor reduction efficiency and slow reduction rate in the most of cases.The current research work presented in this thesis is a part of continuous urgency to develop new visible-light active photocatalysts with the stable and high photocatalytic ability for the Cr（Ⅵ）reduction.In the first chapter,the recent reports have been summarized on Cr（Ⅵ）reduction by using UiO-66-NH₂and its composite under visible light.The second chapter of the thesis describes the construction of a series of core-shell hetero-structures UiO-66-NH₂@Bi OCl-UTN’s（UTNs=Ultra-thin nanoplates）by using pre-synthesized porous UiO-66-NH₂ as a core.Bismuth oxychloride（Bi OCl-UTN’s）was grown onto porous UiO-66-NH₂ core via a facile in-situ synthetic process at room temperature.More interestingly,Bi OCl-UTN’s（4～5 nm）are uniformly wrapped on the surface of UiO-66-NH₂as it could be well ascertained by transmission electron microscopy（TEM）.Lattice fringes of the Bi OCl-UTNs on UiO-66-NH₂-NPs were 2.75（?）.Similarly,encapsulated materials having a distinctive layered structure with an interlayer spacing of 7.38（?）and indexing to the{110}plane of Bi OCl-UTN’s.UiO-66-NH₂@Bi OCl-UTN’s exhibited unique characteristics such as boosted visible light absorption（λ>420 nm）,well crystalline lattice,large surface area(511 m²g^-1)and microporous structure(0.443 cm³g^-1)along with very high thermal and chemical stability.The synthesized composite photocatalyst showed a noticeable photocatalytic reduction of Cr（Ⅵ）as compared to the pristine components because of improved electron-hole separation and suppressed charge recombination.Interestingly,UiO-66-NH₂@Bi OCl-UTN’s showed outstanding regeneration ability,without any obvious decline in adsorption capacity even after four cycles.In chapter three,we constructed Bi OCl_xBr_1-xhetero junction with varying Cl-to-Br ratios in the presence of polyvinyl pyrrolidone（PVP,M_W=10000）successfully,a quite different from the predicted UiO-66-NH₂@Bi OCl-UTN’s core-shell.Furthermore,unlike the predicted high temperature（100-180 ^oC）methods,a simple room temperature process was followed.The results exhibited that Bi OCl_0.8Br_0.2has a well crystalline lattice,large surface area(147 m²g^-1),microporous structure(0.377 cm³g^-1)and high chemical stability.Such amazing properties permitted the quick separation and transfer of photoexcited carriers subsequent with renowned photocatalytic activity for removal of Cr（Ⅵ）.Results revealed that Bi OCl_0.8Br_0.2 is much more active than any other synthesized composite.The visible-light catalytic activity performance of this composite could be attributed to its strong absorption in the visible light region and low electron-hole pair recombination.Moreover,among different scavengers,the chief active species during the photocatalysis process are electrons and^·O₂^-.In chapter four,a series of UiO-66-NH₂@Zn In₂S₄ composites with variable contents of UiO-66-NH₂ was fabricated via a facile solvo-thermal method.The photocatalytic activity was evaluated for the reduction of Cr（Ⅵ）under the influence of visible light（λ>420 nm）.The results revealed that Zn In₂S₄ nanosheets were dispersed on the surface of UiO-66-NH₂.The enhanced photocatalytic activity of 20UN@ZIS（UN=UiO-66-NH₂,ZIS=Zn In₂S₄）was attributed to the well-matched band structure and intimate contact faces between both components which led to effective transfer and separation of the electron along with hole pairs.The prepared composite exhibited excellent stability in the photocatalytic reduction of Cr（Ⅵ）.It is anticipated that the prepared composites containing Bismuth oxyhalide（Bi OX）and porous based UiO-66-NH₂ hetero-structures materials can provide a new pathway to enhance their properties toward the heavy metal ions reduction and other potential applications relating to environmental remediation or energy conversion.