Synthesis And Photocatalytic Properties Of Bismuth Oxide Based Composite Materials

In recent years,environmental pollution has become more and more severe.Especially in China,as a large country with population and industrial manufacturing,environmental pollution has seriously restricted the sustainable development of social economy.Semiconductor photocatalysis technology utilizes solar energy to generate electron-hole pairs and degrades organic pollutants.Since the conception was introduced to the public firstly in the 1970s,this technology has been a research hotspot in the field of environment restoration.TiO₂,as a typical photocatalytic material,its frequency response is in the ultraviolet region?the band gap is greater than 3eV?,and has no photocatalytic activity in the visible region,which greatly limits the application of TiO₂ in photocatalysis field.Bismuth oxides have attracted the many attention from researchers,due to its nontoxicity and harmlessness to human,low cost and narrow band gap.However,monophase bismuth oxide has a high recombination rate of photo-generated charges which restrict its application in the field of pollution remediation.To find other semiconductors with appropriate band structure matching for bismuth oxides is an effective strategy to inhibit recombination rate.In this paper,the photocatalytic activities of two bismuth oxides,?-Bi₂O₃ and Bi₄O₇,are improved by heterogeneous composite structures.The main work is as follows:Pentavalent bismuthates are common precursors for the synthesis of other bismuth based semiconductor materials.For the first time,the basic pure pentavalent bismuthates?sodium bismuthate dihydrate,potassium bismuthate?were prepared by persulfate oxidation?oil bath?.Since persulfate is solid powder with stable physicochemical properties,no volatility and low cost.Compared with traditional synthetic method by sodium hypochlorite?chlorine gas flow into concentrated alkali solution?or bromine water,it not only does no harm to human and environment but also more simple and controllable.And that is very suitable for laboratory preparation and industrial mass production.Sodium bismuthate dihydrate is used as a precursor to synthesize?-Bi₂O₃/?BiO?₄?OH?₂CO₃ composite materials in one pot by adjusting the volume ratio of water to ethanol in the hydrothermal system.During the hydrothermal process,the pentavalent bismuth ions oxidize ethanol to form carbonate CO₃^2-.The basic bismuth carbonate consisted of a two-dimensional nanosheet with the size of micron and the thickness were about 100 to 200 nm.And the?-Bi₂O₃ consisted of nanoparticles with an average size of nanometer.The volume ratio of water to ethanol in the hydrothermal system plays an important part in the formation of basic bismuth carbonate,and an excessive or too small ethanol/water volume ratio is unfavorable to prepare basic bismuth carbonate.The heterogeneous composite structure formed by basic bismuth carbonate and?bismuth oxide would promote the separation of photogenerated electron hole pairs and improve the degradation rate of the organic pollutant Rhodamine B.The results from capture agents experiment show that the photogenerated hole?h⁺?plays a major role in the photocatalysis,while the superoxide radical?·O₂^-?plays a secondary role in the reaction process.We prepared the mixed-valence bismuth oxide Bi₄O₇ successfully by using potassium bismuthate as a hydrothermal precursor,which reduced the hydrothermal temperature from 220°C?saturated vapor pressure 3.3 MPa?to 140°C?saturated vapor pressure 0.36 MPa?.XRD results show that the pure phase Bi₄O₇ could be formed from 140°C to 180°C when potassium bismuthate was used as a precursor.Moreover,compared with sodium bismuthate dihydrate as a precursor,the initial pH value of hydrothermal system could be ranging from 3 to 11,and there would be no other mixed-valence bismuth oxides in the final products.SEM shows that the as-prepared Bi₄O₇ consisted of a regular sheet-like rod structure with no agglomeration,and its length is 0.5-2?m and the thickness is around 50-100 nm.Tetragonal BiOCl nanosheets were formed by a hydrochloric acid etching method at room temperature.The heterogeneous structure consisted of Bi₄O₇ and BiOCl can promote the separation of photogenerated charges and enhance the photocatalytic activities of pure phase Bi₄O₇ on the degradation of organic pollutants.