Preparation And Optical Properties Of Nano-ZnAl₂O₄

As a kind of important materials with multifunctions, spinel oxides are widely used as ceramic, photocatalytic and optoelectronic materials due to their excellent thermal,mechanical and optical properties. Therefore, it is significant for researchers to synthesize the nanoparticles of spinel oxides with good properties. In order to investigate the physical and chemical properties of spinel ZnAl2O4 nanoparticles,high-dispersed ZnAl2O4 nanoparticles are prepared by a modified polyacrylamide gel route and their physical and chemical properties are investigated in this thesis. The contents mainly referred to are given as follows:(1) ZnAlO nanoparticles with different ratios of Zn to Al are prepared by using citric acid as the chelating agent and changing the atomic ratios of Zn to Al. The effects of the different atomic ratios of Zn to Al on the purity of phase and optical properties are investigated. The results reveal that the pure ZnAl2O4 phase nanoparticles can be obtained by calcining xerogel with the mole ratio of Zn:Al=1:2 at 600 ℃. In addition,for Zn:Al=1:1.8, ZnAl2O4 nanoparticles can be obtained after sintering at 900 ℃ or higher temperatures. However, the powder prepared by sintering xerogel with the mole ratio of Zn:Al=1:1 even at 1000 ℃ still contains a partial amount of ZnO. For the atomic ratios of Zn:Al=1:1.8 or Zn:Al=1:2, The nanoparticle size of as-prepared samples increases with the increasing calcining temperature. A blue emission peak at around 469 nm is observed with the excitation wavelength of 352 nm. And the changes of luminescence intensity also occur with the increase of calcining temperature. On the basis of experimental results, the chelating mechanisms between the chelating agent and zinc and aluminum ions and luminescence mechanism are investigated.(2) Nano-ZnAl2O4 is prepared by using citric acid as the chelating agent and different aluminum sources or gel collapse prevention agents. The effects of the aluminum sources or gel collapse prevention agents on the purity of phase and optical properties are investigated. The investigations demonstrate that different aluminum sources or gel collapse prevention agents significantly influence on the purity of phase,morphology, band gap and optical properties of the nano-ZnAl2O4. Pure phase zinc aluminate can be obtained by using Al2(SO4)3·18H2O or AlCl3·6H2O as the aluminum source, while a small amount of impurity exists in the sample obtained by usingAl(NO3)3·9H2O as the aluminum source. It is meaningful that the S=O bonds appear at high temperature due to the introduction of SO42-. Therefore, the zinc aluminate with high purity can be obtained at higher calcining temperature. On the basis of experimental results, the luminescence phenomena of different samples are explained.The luminescence spectra demonstrate that the all ZnAl2O4 samples prepared by using different aluminum sources possess an emission peak at 469 nm under the irradiation of the ultraviolet light of 352 nm. However, the sample synthesized by using Al(NO3)3·9H2O as aluminum source also possess an emission peak located between 373 and 406 nm.(3) Nano-ZnAl2O4 is prepared by using different chelating agents. The effects of the chelating agents on the purity of phase, surface morphology, band gap and optical properties are investigated. The studies indicate that different chelating agents significantly influence on the purity of phase, morphology, band gap and optical properties of the nano-ZnAl2O4. Due to the difference of chelating abilities of chelating agents, the single phase ZnAl2O4 nanoparticles can be obtained by using tartaric acid as the chelating agent. And its energy band gap is 3.93 eV. However, no pure phase ZnAl2O4 nanoparticles can be obtained by using oxalic acid, salicylic acid or glycine as the chelating agent. And their energy band gaps are 3.21, 3.21 and 3.16 eV, respectively.The luminescence spectra demonstrate that the products synthesized by using oxalic acid, salicylic acid or glycine as the chelating agent don’t only possess an emission peak at 469 nm but also a weak emission peak located between 377 and 405 nm under the excitation of the ultraviolet light with the wavelength of 352 nm. In addition, their luminescence intensities are distinctly different. It can be known by the experimental results that the chelating abilities of chelating agents are not equal. Thus, it is of important significance that other oxide nanoparticles are prepared by using different chelating agents.