| With the persistent deterioration of ecological environment,heterogeneous photocatalysis has been applied to deal with a variety of environmental problems,especially for water and air purification.TiO2 as a photocatalyst has many advantages such as excellent photocatalytic activity,low cost,chemical stability,good mechanical properties,and non-toxicity.The crystal type of TiO2 is anatase,brookite and rutile.Among them,the anatase is referred to be the most photoactive phase.TiO2 can work well only under UV light with wavelength shorter than 400 nm,which occupies only 3～4%of solar energy.Great efforts have been devoted to tuning the band gap and extending the response of TiO2 to visible light region.Various metals have been used as dopants to tune TiO2 band gap and reduce the electron and hole pair recombination.Many methods have been used to prepare thin films,among which magnetron sputtering method provides many advantages in reproducibility,high mechanical durability,strong adhesion and uniformity,industrial process applicable to large-area deposition,easy-controlling film structure and composition.For future application,thin films are more necessary than powder because powder always requires a post-filtration procedure for recycle.Based on the above mentioned,we are going to use magnetron sputtering method to prepare TiO2 thin films with modified structure for improved photocatalytic activity.The main research contents are shown as follow:(1)TiO2 films were deposited by magnetron sputtering with different N2/Ar mixture gas.The crystal texture,surface morphology,surface composition,energy gap and optical properties,photocatalytic activity of the films have been investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM),X-ray photoelectron spectrometer(XPS),and UV-visible spectrophotometer,respectively.The results show that the as-deposited TiO2 film mainly possesses anatase structure at room temperature with pure Ar gas.The introduction of nitrogen gas into the sputtering system can intensively retard TiO2 crystallization,alter crystallization preferred orientation and lead to nitrogen doping.However,the N doped TiO2 film prepared on the substrate with 673K or more has obvious XRD peaks.The XRD patterns and the XPS spectra indicate that N3-ions are incorporated into the TiO2 lattice,which induces band gap narrowing and improved photocatalytic activity under visible light for the prepared TiO2 thin film.(2)TiO2 films were deposited by DC magnetron co-sputtering with Ar gas.Additionally,the TiO2 thin film incorporated with Cu ion has been explored as well.The increase of incorporated Cu concentration leads to a gradual change in film structure from anatase to rutile phase,which is ascribed to the possible incorporation of Cu1+ in sites previously occupied by Ti4+.TiO2 films with dominated rutile structure can be obtained by modest Cu-incoporated concentration(about 20%).The TiO2 film with high levels of Cu concentration is amorphous.So,the crystallization of TiO2 thin films can be retarded by high level Cu incorporation.Additionally,with the increase of Cu-incorporated concentration,the absorption band of TiO2 film shows a gradually significant shift in visible light wavelength region and the optical band gap displays a systematic decrease obviously.Band structures and density of states have been analyzed based on the density functional theory(DFT),which explains the trend in film structure change and striking gap-narrowing with Cu incorporation.The presence of Cu results in important modifications in the electronic states,which are mainly related to the Cu-3d orbitals in the gap and in the vicinity of the valence band edges for TiO2.(3)TiO2 rutile/anatase heterostructure thin films with varying rutile thickness have been in-situ synthesized via DC magnetron sputtering with Ar gas at room temperature.The crystal texture,surface morphology,energy gap and optical properties of the films have been investigated by XRD,grazing incidence XRD,Raman spectroscopy,SEM,and UV-visible spectrophotometer,which indicates that the rutile/anatase heterostructure films are successfully fabricated.The further degradation experiments display that the photocatalytic activity can be dramatically affected by the thickness of the outmost rutile layer and the 100 nm thickness exhibits the best performance in all of the TiO2 thin films.With the increase of the outmost rutile layer,the optical band gap of TiO2 film displays a systematic decrease slightly.However,the change in photocatalytic activity does not coincide with that in the band gap.The photoresponse and electrochemical properties of the thin films have been characterized to understand the mechanism of the varied photocatalytic activity.The reason that heterostructure exhibited different photocatalytic activity can be explained as follows.First,the efficiency of light absorption for ruile TiO2-Cu may be higher than that for anatase TiO2 owing to the narrower band gap for the former than that for the latter.The outmost-rutile in the heterostructure film can contribute more photoinduced charge carriers to the photodegradation reaction than single anatase layer.Secondly,owing to the efficient separation of the photogenerated electrons and holes in the heterostructure,the total number of photoinduced charge carriers can be much more in the heterostructure with optimized outmost layer thickness than that of single anatase layer,and that of photoinduced charge carriers that can participate in the photocatalytic process increases greatly as well. |
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