| Persistent luminescent materials, which show long-persistent luminescence, are based on the transient storage of energy in the form of trapped electrons and holes. This gives rise to a visible-light emission lasting for minutes or hours by the slow release and radiative recombination of the charge carriers. As a type of wide band gap (4.5 eV) semiconductor, ZnGa2O4 phosphor has attracted enormous attentions due to its many possible applications for night vision display, field emission displays and electroluminescent devices due to its chemically and mechanically stable structure, which may be sustained under harsh environments. Recently, ZnGa2O4 is also reported to exhibit excellent performance in water and air purification due to its high photocatalytic activity. The highly dispersed bottom of conduction band is considered to promote the mobility of photo-generated electrons and benefit the high photocatalytic activity of ZnGa2O4. Thus, we discussed the inner relations between persistent luminescence and photocatalytic properties by doping ions in the ZnGa2O4 host. A promising approach via controlling the doping ions and luminescence properties can be proposed to develop the photocatalytic activity of photocatalysts. The main contents of this paper are as follows:The long persistent phosphors ZnGa2O4, ZnGa1.8Ge0.1O3.9, Zn0.9Ga2Ge0.1O4.1 were prepared by a high temperature solid state reaction. The results from X-ray diffraction (XRD) shows that the doping of Ge4+ions has no significant influence on the crystal structure of ZnGa2O4. The XRD patterns and Raman data indicate that Zn0.9Ga2Ge0.1O4.1 contains small content of Ga2O3. All phosphors have a broad-band emission of ZnGa2O4 host peaking at 450 nm and Zn0.9Ga2Ge0.1O4.1 phosphor has another emission band at 505 nm. The band at 505 nm is because that when Ga3+ion is located on a Zn2+site in the spinel structure, the interactions of p orbitals in Ga3+ion with the orbitals of the six oxygen ligands in the distorted octahedral configuration will lead to the splitting of 3d orbitals and to the 505 nm emission band. When Ga2O3 is excessive and substituting gallium for a zinc site, luminescence properties and photocatalytic efficiency will be improved. The doping of Ge4+produced more trap energy levels, then more photo-generated electrons and holes can be captured by traps, which can suppress the recombination efficiency of photo-generated electrons and holes, so as to prolong the lifetime of photo-generated electrons and holes and thus prolong the persistent luminescence period. A part of hoto-generated electrons escape from the traps and react with oxygen to generate peroxide (·O2-) and hydroxyl radicals (· OH), peroxide and hydroxyl radicals possess strong oxidizability, they can degradate most of organic pollutants into water and carbon dioxide, that process is called photocatalysis. Prolonged lifetime of photo-generated electrons is conductive to generate more peroxide and hydroxyl radicals, thus the photocatalytic performance of the sample can be enhanced.The long persistent phosphors ZnGa2O4:x% Cr3+(0,0.5,0.7,1.0, and 1.1) were prepared by the high temperature solid state reaction. The structure, photoluminescence, afterglow and fluorescence decay properties were studied. The XRD analysis show that all samples have similar structure which is cubic spinel structure. This blue afterglow emission band of ZnGa2O4 have a big overlaps with the absorption of Cr3+. What is more, the host blue afterglow luminescence disappeared when doped with Cr3+ ion and only the near-infrared (NIR) afterglow luminescence of Cr3+ ion can be observed. The excitation spectrum contain three main bands, the band at 300 nm,410 nm and 550 nm, respectively. Under 300 nm excitation, which is the absorption wavelength of ZnGa2O4, ZnGa2O4:Cr3+ phosphor exhibited the most excellent long-lasting luminescence properties. Although Cr3+ ions show intense absorption at the region of 350 nm-600 nm, when excited by 410 nm and 550 nm, ZnGa2O4:0.5%Cr3+ phosphor exhibited faster luminescence decay process and almost no long-lasting luminescence properties, respectively. The ratio of the NIR emission intensity (698 nm) to the blue (505 nm) increases with increasing Cr3+ concentrations when only ZnGa2O4 is optically excited. The fluorescence lifetime of ZnGa2O4 at 505 nm become shorter with the increase of the Cr3+ concentration. Thus the NIR long-lasting luminescence in ZnGa2O4:Cr3+ was proved comes from the persistent energy transfer from ZnGa2O4 to Cr3+.The long persistent phosphors ZnGa2O4:x% Cr3+ (0,0.1,0.3,0.5,0.7,1.0, and 1.5) were prepared by the high temperature solid state reaction. XRD patterns of ZnGa2O4 and ZnGa2O4:Cr3+ are assigned to the ZnGa2O4 spinel phase and no characteristic peaks for the dopants have been observed. The morphologies of the as-synthesized ZnGa2O4 and ZnGa2O4:0.5%Cr3+ are demonstrated by the SEM images the as-prepared ZnGa2O4 and ZnGa2O4:0.5%Cr3+ samples show same irregular shapes. The average particle sizes of ZnGa2O4 and ZnGa2O4:0.5%Cr3+ are about 0.78 mm and 0.80 mm, respectively. Photocatalytic activity test shows that photo-degradation efficiency of ZnGa2O4 is highly suppressed via doping with Cr3+. As the two samples have the similar morphologies, shapes and particle sizes, the reason is discussed in detail mainly on the basis of defects. From luminescence properties, persistent luminescence decay curves and TL curves, it can be concluded that doped Cr3+ ions act as recombination centers but not as trap centers, and that the amount of trapped electrons/holes in ZnGa2O4 is almost seven times higher than that of ZnGa2O4:Cr3+. The results about persistent luminescence are consistent with that of photocatalytic performance, and the suppression of persistent luminescence intensity of ZnGa2O4:Cr3+ comes from less trapped electrons/holes in ZnGa2O4:Cr3+. Photo-generated electrons play important roles in photocatalysis, the amount and lifetime of photo-generated electrons can highly influence the photocatalytic activity of photocatalyst. The entrained recombination centers, which can highly reduce the amount and lifetime of photo-generated electron-hole pairs, could be the cause of suppressed photocatalytic activity in ZnGa2O4:Cr3+. |
