Titania Nanoflower Thin Films For Photocatalysis Applications

Fabrication of titania thin films with highly ordered nanostructures is one of the most efficient way to improve the photocatalytic activity due to the significantly improved specific surface area.In our group,thin films of porous titania and quasi-aligned titania nanorods,which exhibited excellent photocatalytic activity,have been deposited on metallic Ti substrates through direct oxidation of the substrate with hydrogen peroxide at a low temperature of 353 K.The reactants bring no additional containments to the resultants.Meanwhile,such a low-temperature approach ultilizes no expensive reactants or those sensitive to atmoshphere,reqires no templates,and is quite simple.The resultant titania thin film is pure,well-crystallized,uniform,and aheres tightly to the substrate.This paper reports our new attempts to fabricate titania nanoflower thin films as well as phase-pure single-crystalline rutile nanorod thin film,through the additon of hexamethylenetetramine(HMT)and concentrated nitric acid to the above-mentioned Ti-H₂O₂ reactants and also through controlling carefully the fabrication parameters. The structure and photon-induced property of the achieved titania thin films were investigated in detail with FE-SEM,HR-TEM,AFM,XRD,UV-Vis DRS,Raman spectra,and XPS.The titania films were also utilized to assist photodegradation of Rhodamine B(RhB)in water.Follows are the main results obtained:1.Titania thin films with a dual structure,that is,flower-like rutile aggregates sitting on top of an anatase intermediate layer,were synthesized by oxidizing metallic Ti plates with the H₂O₂-HMT-HNO₃ solution at 353 K for 60～72 h.A porous titania layer was deposited on the Ti substrate before 48 h.After 60 h,the flower-like rutile aggregates were formed.The resultant intermediate layer consisted mainly of anatase, with a little amorphous titania.The subsequent thermal treatment at 723 K improved the crystallinity.2.The mechanism for low-temperature growth of the titania nanoftower thin films was determined as follows:The reaction between Ti and H₂O₂ resulted in a hydrated amorphous titania layer on the substrates through oxidizing metallic Ti plates by hyderogen peroxide;on the other hand,under the present acidic environment,some precipitated titania dissolved back to the solution,releasing Ti(â…£) ions to the solution.Once the Ti(â…£)ions in the solution reached a critical concentration,tiny titania nanorods with diameters of ca.6 nm,which were single-crystalline and oriented preferably along the[001]direction,precipitated on the porous layer via heterogeneous nucleation;at the same time,similar tiny rutile nanorods precipitated from the solution via homogeneous nucleation,which then aligned to the nanorod grown previously from the substrate to form a bundle and eventually fused through an "oriented attachment" mechanism to achieve a larger nanorods(the main trunk).With the proceeding of the reaction,other single-crystalline futile nanorods deposited on the main trunk via the secondary nucleation and grew through the similar oriented attachment mechanism,which led eventually to the nanoflower structure.3.The second nucleation demanded a relatively high drying force.While no porous titania layer existed on the Ti substrate or the porous titania layer was quite thin and aligned densely,a rutile nanorod monolayer precipitated on the substrate instead of the nanoflowers,which was attributed to the relatively high nucleation rate for the primary nucleation and an insufficient drying force for the socondary nucleation.4.For the titania nanoflower thin film,the thickness of the intermediate anatase layer could be tuned through immerseing metallic Ti plates,which were oxidized previously by hydrogen peroxide for various durations for 1～48 h,into a precursor, which was the resultant solution from the reaction between Ti and the H₂O₂-HMT-HNO₃ solution at 353 K for 48 h.The indirect band gap of the achieved titania nanoflower film increased with increasing thickness of the anatase intermediate layer,and the ability of the film to assist photodegradation of RhB in water improved correspondingly.5.Utilizing the resultant solution from the reaction between Ti and the H₂O₂-HMT-HNO₃ solution at 353 K for 24～60 h,single-crystalline rutile nanorod monolayers deposited on the as-pickled metallic Ti plates or those oxidized by hydrogen peroxide at 353 K within 30 min.The average size of of the rutile nanorod was ca.30～120 nm in diameter and 120～210 nm in length.The deposited rutile nanorods grew preferably along[001]-axis,the alignement of which was affected heavily by the orientation of the Ti grains on the substrates.Deposition duration was the the main factor to control the size of the nanorods.The indirect band gap of the rutile nanorods was ca.2.64 eV,which was significantly lower than the bulk one.6.The evaluation of the various nano-structure titania thin films to assist photodegradation of RhB in water showed that the nanorods and the nanoflowers were beneficial to the photocatalytic activity.The single-crystalline rutile nanorod films were suggested to exhibit better performance in the photodegradation reaction than the rutile nanoparticles isolated from the commercial P25 titania nanoparticles. The BET specific surface area of the titania nanoflower thin film kept at a high value of 77.8 m²/g after heating at 723 K,and its photocatalysis efficiency almost tripled that of the P25 film with identical weight.The enhanced photocatalytic activity was believed to result from the specific morphology of the top nanoflower layer,as well as its synergistic effect with the underneath anatase layer.