Fabrication And Doping Of Hematite Nanotubc Arrays For Photocatalytic Applications

The photocatalytic technology has received great attention for water splitting anddecomposition of organic pollutant since1972when Fujishima and Honda first discovered TiO2as photonade for photoelectrolysis of water. Among many candidates for photocatalysts, TiO2isthe most intensively studied material for decontamination and disinfection of water, purificationof volatile organic compounds in indoor air, carrying out organic synthesis by solar energy, andeven producing hydrogen as fuel by water photohydrolysis. However, the band gaps for anataseand rutile TiO2being3.2and3.0eV require UV-light for activation. But UV-light in solarradiation as reaching the earth surface is relatively a small fraction （ca.4%）. Iron oxide （α-Fe₂O₃,hematite） is one of promising materials for photocatalytic applications. The band gap energy ofFe₂O₃is2.2eV, which can be activated with visible light, collecting up to40%of the solarspectrum energy. However, the poor conductivity and high electron-hole （e-h） recombinationlimit its practical applications as photocatalyst.To overcome these difficulties of Fe₂O₃photocatalysts, we have synthesized the α-Fe₂O₃nanotube arrays on pure iron foil through electrochemical anodization. The one-dimensional （1D）hematite nanostructures show higher photo activities compared to nanoporous films andnanoparticles for the unique possibility to control the direction and path of the charge carriersthrough quantum confinement. We have studied the effection to the nanotubes of each influencefactors to optimise the preparation. SnO₂was doped into the α-Fe₂O₃nanotube arrays byhydro-thermal method to enhance the photocatalytic ability. SEM, XRD, Electrochemicalworkstation and UV-vis spectroscopy investigation were carried out to study the microstructure,crystallization, photocatalytic ability and photocurrent of the samples. The main works andresults in this paper are listed as follows:1. α-Fe₂O₃nanotube arrays were prepared in organic electrolyte containing F-byanodization. F-content, the temperature of electrolyte, applied potential, water content andanodic oxidation time had obvious influence on the surface morphology of Fe₂O₃nanotubearrays. The results showed that the oxide on the surface of Fe₂O₃nanotube arrays withappropriate F-content, upper temperature of electrolye, higher applied potential, appropriatewater content and prolonged oxidation time can be dissolved and disappeared.2. The mechanism of α-Fe₂O₃nanotube arrays prepared by anodization is similar to that ofTiO2nanotube arrays, but the growth of Fe₂O₃nanotubes are much faster than TiO2nanotubesdue to the bad corrosion resistence of pure Fe. The formation and selective dissolution of Fe₂O₃layers leads to the growth of α-Fe₂O₃nanotubes.3. α-Fe₂O₃/SnO₂heterojunction was synthesized by hydro-thermal method. Both OH-concentration and reaction time affected the SnO₂doping with different mechanism. Withincreasing reaction time, SnO₂deposited on the α-Fe₂O₃nanotubes and covered them. but it cannot deposit if the OH-concentration persist too high. A thin SnO₂layer distributed on the surfaceof the α-Fe₂O₃nanotubes wall uniformly When sample kept for0.5h under a appropriate OH-concentration. There is no obvious SnO₂signal in XRD patterns and UV-Vis absorbancespectrum for a small atomic ratio of Sn about2%. 4. The sample reacted for0.5h showed high photocatalytic property for efficient separationof electron-hole pairs at the α-Fe₂O₃/SnO₂heterojunction interface and it’s three times asα-Fe₂O₃nanotube arrays under visible light illumination. The SnO₂nanorod arrays （reacted for2h） showed better photocatalytic ability under UV light irradiation for the one-dimentional （1D）nanorod structure property. The photocurrent density of the SnO₂nanorod was22μA/cm2, whichis far superior to that of α-Fe₂O₃nanotube arrays （about2μA/cm2）.