Ion Doping And Heat Treatment To Control The Crystal Structure And Upconversion Luminescence Properties Of Titanium Dioxide

Potential applications of rare earth ion doped TiO₂nanocrystals in bio-tissue imaging,fuel-sensitized solar cells and other fields have attracted considerable interest of researchers.The research work of this article is as follows:Using tetrabutyl titanate as a titanium source,anhydrous ethanol as a solvent,concentrated nitric acid as a stabilizer and catalyst,glacial acetic acid as an inhibitor.The rare earth nitrate and alkali metal nitrate are doped as spectral regulators.TiO₂sol was obtained by sol-gel method and hydrolyzed by alcohol salt.Rare earth doped TiO₂nanocrystals were obtained after high temperature calcination.The structure and properties of nanocrystals were characterized by XRD,SEM,Raman spectroscopy,infrared spectroscopy,absorption spectroscopy and upconversion spectroscopy.To obtain high efficiency up-conversion red radiation materials,the crystal structure and UCL properties of TiO₂:Er³⁺,Yb³⁺nanocrystals can be regulated by altering the concentration of Yb³⁺ions and calcination temperature.The intensive red radiation of TiO₂:Er³⁺,Yb³⁺nanocrystals is attributed to the strong cross relaxation.The higher phonon energy of TiO₂can induce stronger non radiative transition,faciliate cross relaxation,which contributes to the red light upconversion radiation.Efficient upconversion green emission of rutile TiO₂:Er³⁺,Yb³⁺nanocrystals is achieved by controlling the annealing temperature and Li⁺ion concentration.For green upconversion radiation,there invole 3 main nonlinear transitions:(1)ESA:⁴I_11/2(Er)+a photon(980 nm)?⁴F_7/2,(2)ET:²I_11/2(Er)+²F_5/2(Yb)?⁴F_7/2(Er)+²F_7/2(Yb),and(3)ET:⁴I_11/2(Er)+⁴I_11/2(Er)?⁴F_7/2(Er)+⁴I_15/2(Er).These processes can stimulate Er³⁺from the ⁴I_11/2to the ⁴F_7/2level.Because of the short lifetime of the ⁴F_7/2level,the Er³⁺at the⁴F_7/2level non-radiatively decay to the lower²H_11/2and⁴S_3/2levels rapidly.The green radiation of 550 nm is generated by the ⁴S_3/2to ⁴I_15/2radiation transition process.The crystal structure and UCL properties of TiO₂:Ho³⁺-Yb³⁺nanocrystals are regulated via altering the concentrations of Ho³⁺,Yb³⁺,and Li⁺,and calcination temperature.The lattice structure of TiO₂nanocrystals transforms from the anatase phase to the rutile phase with the increase of the calcination temperature.Ho³⁺and Yb³⁺block the phase transition of TiO₂,whereas Li⁺weaken this blocking effect.It is showed that the TiO₂:Ho³⁺-Yb³⁺nanocrystals doped with 6.5 mol%Li⁺manifest the optimal upconversion luminescence property when annealed at 700°C.The green and red luminescence intensities of the nanocrystal sample with the optimal luminescence are approximately 24 and 10 times higher,respectively,than that of the sample without the Li⁺.The enhancement effect of Li⁺on upconversion luminescence of nanomaterials was studied.The charge imbalance caused by the substitution of Ti⁴⁺with Ho³⁺and Yb³⁺can be compensated by Ti⁴⁺?Ho³⁺(Yb³⁺)+Li⁺.Results show that the maximum luminescence intensity can be obtained when the charge imbalance between Ho³⁺(Yb³⁺)and Ti⁴⁺is fully compensated by Li⁺doping.The results show that Li⁺enter the lattice and occupy the interstitial sites.