The Fabrication Of Mn-doped TiO₂Nanotubes By Anodization And Photocatalytic Properties In Visible Light

Titanium dioxide (TiO2) is known to be one of the promising photocatalysis due to itsnontoxicity, high efficiency, low cost and remarkable photoelectronic properties. Themechanism of photocatalysis is based on the photoelectrons activated from valence band (VB)to the conduction band (CB) and holes left in the valence band. The said photoelectrons andholes can then participate into the redox reactions with pollutants or other organic molecularspecies absorbed on the surface of TiO2. However, as a wide intrinsic band gap semiconductor(Eg=3.2eV in theory), only UV light below387nm can be utilized by anatase TiO2. Those UVlight only accounts for7%around of solar energy which largely limit the applications of TiO2under sunlight. In order to extend the spectral response of TiO2to the visible region, manyefforts have been tried in the past decades, including doping TiO2with nonmetal atoms,introducing metal ions into the lattice, coating the surface with Ag or Pt etc..Due to the unique d electronic configuration, some transition metals like Mn can eitheradd impurity levels between CB and VB, or shrink the original bands leading to the shift ofthe absorption edge to the visible light region and improve the photocatalytic properties.However, the common synthesis of Mn-doped TiO2(Mn-TiO2) are sol-gel, magnetronsputtering and hydrothermal.In this research, one-step anodization was firstly utilized to fabricate Mn-TiO2. KMnO4was added into NH4F aqueous electrolyte as dopant and Mn-TiO2nanotubes weresuccessfully synthesized without changing the morphology. The photocatalytic experimentsshowed that Mn-TiO2could significantly improve the photocatalytic properties under visibleirradiation. Comparison with the degradation of38.14%for TiO2,0.1%Mn-TiO2annealed for6h at450℃could decompose96.42%MB in12hours.XRD and Raman were employed to identify the phase and structure of Mn-TiO2, onlyanatase was detected in both pure TiO2and Mn-TiO2, indicating that no Mn oxides weregenerated. The Mn2p and Ti2p levels showed that Mn3+replaced Ti4+in the lattice andchanged the chemical environment so that increase the crystallinity of anatase. The absorptionedge of Mn-TiO2was red shifted therefore the band gap shrank to3.01eV and new absorptionpeaks were observed in visible region, which demonstrated that doping TiO2with Mn could definitely extend the spectral response in visible light. Photocurrent and photoelectroniclifetime tests confirmed that the number of photoelectrons and recombination ofphotoelectron-hole pairs were both enhanced, contributing to the perfect photocatalyticproperties of Mn-TiO2.