Synthesis And Characterization Of Nano-heterostructure Photocatalysts

In recent years, there has been growing interest in composite nanostructuresintegrated with individual building blocks, such as magnetomicelles, compositesemiconducting nanocrystals, semiconductor-metal composite nanostructures, and p-njunctions. The heteronanostructures have applications in many areas, includingsemiconductors, water splitting, Li-ion batteries, especially as photocatalysts.Traditional methods for photocatalyst recovery such as coagulation, flocculation, andsedimentation are quite complex and expensive. A composite photocatalyst with astrong magnetic compound and a photocatalyst could be effectively recovered byapplying an external magnetic field, which simplifies the recycling procedures.Heterojunction structures with appropriate band gaps could promote the charge pairseparation and interfacial charge transfer efficiency, and then effectively enhance thephotoactivity. In this paper, we prepare magnetic heterostructure photocatalysts ofFe3O4/FeWO4、γ-Fe2O3/WO30.5H2O and urchinlike Cu2O/CuO hybrid nanostructureswith large surface area. The details are as follows:1. Magnetic n-type semiconductor Fe3O4nanoparticle and p-typesemiconductor FeWO4nanowire heterostructures are successfully synthesized withoutany surfactants or templates via a facile one-step hydrothermal process. The TEMresults indicate that the crystalline FeWO4nanowires grow along the a-axis. Theaverage width of the nanowires is about10nm and the length of the nanowires isfrom200to500nm. The n-type Fe3O4nanoparticles grow on the surface of the p-typeFeWO4nanowires to form the p-n heterojunction. Magnetic measurements indicatedthe coexistence of ferrimagnetic behavior of Fe3O4and weak antiferromagneticbehavior of FeWO4. By UV-Visible analyses, the band gap of the Fe3O4/FeWO4magnetic photocatalyst is calculated to be2.50eV, indicating that the Fe3O4/FeWO4 heterojunction has a suitable band gap for photocatalytic degradation of organicpollutants under visible light irradiation. The degradation of methylene blue (MB)under UV-Visible light irradiation is studied as a model experiment to evaluate thecatalytic activity of the Fe3O4/FeWO4heterostructure p-n junctions. Thedecomposition efficiency is97.1%after one hour UV-Visible irradiation. After thedegradation process, this magnetic photocatalyst can be easily recovered from thesolution using a permanent magnet and redispersed by removing the magnet.2. A facile hydrothermal method is developed to synthesize magneticheterostructure of γ-Fe2O3/WO30.5H2O. The Fe3O4polyhedron is obtained withtwo-phase method first and then react with sodium tungstate to formγ-Fe2O3/WO30.5H2O hybrid nanostructure. During the loading process, Fe3O4changed to γ-Fe2O3and composite with WO30.5H2O to form heterostructure. The pHvalue has direct influence on the product. When the pH value is less than4, there is animpurity phase in the product. The WO30.5H2O has a dissolutin in alkalineenvironment when the pH value is larger than5. The γ-Fe2O3/WO30.5H2O areobtained with the pH value between4to5. The n-type WO30.5H2O grow on thesurface of n-type γ-Fe2O3to form n-n heterojunction. By UV-Visible analyses, themagnetic photocatalyst has a suitable band gap for photocatalytic degradation oforganic pollutants under UV-Visible light irradiation. The sample exhibits a removalefficiency of100for RhB in100minutes. After the degradation process,γ-Fe2O3/WO30.5H2O can be easily separated from the suspension by applying anexternal magnetic field.3. The Cu2O/CuO hybrid nanostructures are prepared in a hydrothermal method.The Cu2O nanospheres are obtained by reflux method first, and then oxidationoccurred in the template of nanospheres to form Cu2O/CuO hybrid nanostructure.Different drying conditions have direct influences on the product, Cu2O nanosphereswith smooth surface can obtained under the room temperature, while the Cu2O/CuOheterostructure is formed at60C in an oven with the Cu2O nanospheres as thetemplate. The Cu2O/CuO hybrid nanostructures is urchinlike mophology, inside of theball is CuO and the surface and thorn is composed by Cu2O. N2adsorption-desorptionisotherm curve are conducted to investigate the specific surface area. Our resultssuggest that the urchinlike morphology have more accessible surface area comparing with that of the nanospheres and show a good adsorption capacity of organic dye. Thephotocatalytic properties of the composite structure are examined using methyleneorange, the degrading efficiency can reach90.7%in160minutes, which is higherthan that of Cu2O nanospheres.