Preparation And Photocatalytic Pefrormances Of Several Nanofibers Based On Electrospinning

In recent years, one-dimensional （1D） nanostructures have attracted muchinteresting due to their novel properties and intriguing applications. Many advancedtechniques have been developed to fabricate1D nanostructures with well-controlledmorphology and chemical composition. Among a large number of methods,electrospinning seems to be the simplest and most versatile technique for generating1D nanostructures. It is low cost to produce large numbers of different kinds ofnanofiber.There are many oxide semiconductor photocatalysts, but anatase crystalline form ofTiO₂has been intensively used in the photocatalytic studies, owing to its uniqueproperties such as high physical and chemical stability in nature, nontoxicity and lowcost. In addition to crystalline form of TiO₂, structural parameters, such as particle size,specific surface area and morphology, also affect its photocatalytic activity. TiO₂withmesostructures could enhance the photocatalytic performance compared with bulkones, owing to their large specific surface area and high pore volume.1D TiO₂nanostructure has attracted considerable interesting, because of its large surface areasand efficient charge separation. Thus, TiO₂with novel properties and multiplefunctions could be detected by combining mesoporous structure and1D nanostructure.The electrospinning technique has been proven to be a powerful and effective method capable of generating nanofibers with small diameters and high surface-to-volumeratio. We develop a simple method to prepared mesoporous TiO₂nanofibers bycombining a sol-gel method with the electrospinning technique. TiO₂nanofibersprepared by this method have anatase structure. The diameters of fibers are in therange of100-300nm, and the lengths keep several micrometers without fracture. Thenanofibers consist of many TiO₂nanocrystalline particles with the size of25-35nm indiameter. Mesoporous TiO₂nanofibers have the type IV isotherm in nitrogenadsorption-desorption isotherm, and many less ordered worm-like mesopores wereobserved among those nanofibers. We have choosed RhB as a model to evaluate thephotocatalytic activities of mesoporous and nonmesoporous TiO₂nanofibers. Theresults show that mesoporous TiO₂nanofibers have improved photocatalytic activity comparedwith the nonmesoporous TiO₂nanostructures, which could be attributed to themesoporous nanostructures that allow mesoporous TiO₂nanofibers have higherspecific surface area and pore volume in comparison with the nonmesoporous TiO₂nanofibers.TiO₂mainly absorbs UV light, owing to the large band-energy gap （3.2eV foranatase）. This practically eliminates the use of sunlight as an energy source forphotocatalysis. For this reason, improving photocatalytic activity of TiO₂bymodification has become an extensive research topic among scientists in the past fewyears. Doped TiO₂photocatalysts could be activated by visible light, but they alwaysdisplay poor activity and durability. Therefore, it is of great interest to developefficient visible light responsive photocatalysts to extend absorption wavelength rangeinto the visible light region. In semiconducting oxides, the conduction band levels ofthe small band-gap semiconductors are usually low due to the formation of the deepvalence bands by O₂p. This restricts the development of visible-light-driven andstable oxide photocatalysts. From this viewpoint, it is necessary to control the valenceband with orbitals of some elements instead of O₂p. Bismuth is a potential candidatefor such a valence band control element. Bi-contained perovskite oxides often offergreat optical properties for photocatalytic applications. Perovskite-type bismuth ferrite（BiFeO₃） has wide applications as photocatalytic materials owing to the small band-gap energy （².2eV）, and high chemical stability. In the present work, we reporta successful attempt to fabricate BiFeO₃nanofibers by combining sol-gel method withthe electrospinning technique. The diameter of the as-spun fibers is in the range of220to480nm, while their length can reach several micrometers. The energy bandgap of BiFeO₃photocatalyst is determined to be2.19eV for BiFeO₃nanofibers. Suchnarrow band gap is benefit for the efficient utilization of visible light forphotocatalysis. In the system of the simultaneous presence of BiFeO₃nanofibers andH2O₂, about97.3%of RhB was degraded after5h under visible light irradiation.During the synthesis process, it is easy for the mole ratio of Bi/Fe to deviate fromchemical stoichiometric proportion, owing to the partial bismuth volatilization. Thus,they lost the metal atoms at the crank points of the crystal lattice and result in metalvacancies. In order to keep the charge balance, oxygen vacancies should be formedwhich lead to the increase of oxygen absorbed. Therefore, the concentration ofreactive OH become higher when oxygen absorbed increased, which helps to thephotocatalytic reaction. At the same time, the samples have a magnetic behavior.Therefore, this will provide an easy and efficient way to separate BiFeO₃photocatalysts from the suspension system under an external magnetic field.TiO₂shows low photoactivity due to its rapid electron-hole recombination, whichseriously restricts its application as photocatalyst. To this end, several metal oxidesand metal sulfides have been used as catalysts for photodegradation purpose. ZnS isan important II–VI group semiconductor with a band gap of3.65eV at roomtemperature. Furthermore, ZnS is a good photocatalyst owing to its rapid generationof electron-hole pairs and the highly negative reduction potentials of excited electrons.Besides, ZnS is available in abundance and nontoxic similar to TiO₂. In the presentwork, we have prepared TiO₂/ZnS nanofibers by combining electrospinning techniquewith hydrothermal method. By varying the concentration of the reactants during thehydrothermal, TiO₂decorated with different amount of ZnS nanoparticles. Under UVlight irradiation, TiO₂/ZnS heterostructures display the improved photocatalyticactivities in comparison with the pure TiO₂nanofibers. When the samples irradiatedby UV light, the ZnS could be excited and the generated electrons in the ZnS were then migrated to the conduction band of TiO₂. Moreover, due to the high crystallinityof the ZnS, the resistance of electron transport was very low and reduced electron-hole recombination. Consequently, the efficient charge separation increased thelifetime of the charge carriers and enhanced the efficiency of the interfacial chargetransferred to the adsorbed substrates, leading to higher activity of the TiO₂/ZnSphotocatalyst.