Yinhai Wang

Suppression of photocatalysis and long-lasting luminescence in ZnGa2O4 by Cr3+ doping

ZnGa2O4 powder, synthesized by the solid state method, exhibited efficient photocatalytic activity for rhodamine B (RhB) degradation under mercury lamp illumination. However, the photocatalytic activity of ZnGa2O4 was highly suppressed when doped with Cr3+ ions. We discussed the mechanism of photocatalysis based on the photoluminescence properties of ZnGa2O4 and ZnGa2O4:Cr3+, and the blue fluorescence lifetimes of host ZnGa2O4 powders with different Cr3+ concentrations were also measured. The results indicated that the doped Cr3+ ions act as recombination centers that can highly reduce the amount and lifetime of the electron–hole pairs, thus reducing the photocatalytic activity of ZnGa2O4. The thermoluminescence (TL) curves of ZnGa2O4 and ZnGa2O4:Cr3+ showed that the amount of trapped electrons/holes in ZnGa2O4 is almost seven times higher than that of ZnGa2O4:Cr3+. The suppressed long-lasting luminescence intensity and photocatalytic activity of ZnGa2O4:Cr3+ were supposed to have come from the decrease of trapped electrons/holes and shortened lifetimes of electron–hole pairs. Possible mechanisms for long-lasting luminescence and photocatalysis of ZnGa2O4 coupled with photoluminescence mechanisms of ZnGa2O4:Cr3+ were also proposed.

Persistent luminescent and photocatalytic properties of ZnxGa2O3+x (0.8 ≤ x ≤ 1) phosphors

The ZnxGa2O3+x (x = 1, 0.95, 0.9, 0.85, 0.8) phosphors were synthesised successfully via high temperature solid-state reaction. The photoluminescent, persistent luminescent and photocatalytic properties of the phosphors had been studied systematically. The results indicated that their excitation and emission spectra were similar to those of ZnGa2O4 phosphors and all of them have excellent persistent luminescent and photocatalytic properties. The optical properties were changed with the ratio of the Zn2+/Ga3+ and the Zn0.85Ga2O3.85 phosphor showed the best persistent luminescent and photocatalytic properties. The Zn0.85Ga2O3.85 phosphor can be effectively activated by an ultraviolet lamp and ultraviolet excitation can lead to 10 min of persistent green emission. Moreover, the Zn0.85Ga2O3.85 could provide more defect energy levels which can act as photogenerated electron traps and maintain the electron–hole pairs for a longer period, enhancing the persistent luminescent and photocatalytic performance.

A Facile Approach to Prepare Black TiO2 with Oxygen Vacancy for Enhancing Photocatalytic Activity

Black TiO2 has triggered worldwide research interest due to its excellent photocatalytic properties. However, the understanding of its structure–property relationships and a more effective, facile and versatile method to produce it remain great challenges. We have developed a facile approach to synthesize black TiO2 nanoparticles with significantly improved light absorption in the visible and infrared regions. The experimental results show that oxygen vacancies are the major factors responsible for black coloration. More importantly, our black TiO2 nanoparticles have no Ti3+ ions. These oxygen vacancies could introduce localized states in the bandgap and act as trap centers, significantly decreasing the electron–hole recombination. The photocatalytic decomposition of both rhodamine B and methylene blue demonstrated that, under ultraviolet light irradiation, better photocatalytic performance is achieved with our black TiO2 nanoparticles than with commercial TiO2 nanoparticles.

Enhanced persistent luminescence and photocatalytic properties of Ga 2 O 3 :Cr 3+ by In 3+ doping

Ga2O3:Cr3+, In3+ phosphors, synthesized via a high temperature solid state reaction, exhibit photocatalytic activity and persistent luminescence. With substituting In3+ for Ga3+ in Ga2O3:Cr3+, the duration of near-infrared (NIR) persistent luminescence was prolonged obviously at room temperature under 254 nm ultraviolet (UV) excitation and the photocatalytic activity was highly improved. The emission and excitation spectra indicated that In3+ doping has no obvious effect on peak positions of Ga2O3:Cr3+. The thermoluminescence (TL) curves showed that a new suitable trap was introduced into Ga2O3:Cr3+ by In3+ doping. It was considered that photocatalytic activity and persistent luminescence properties are highly associated. What’s more, the new trap plays an important role for capturing photo-generated electrons or holes, which is highly responsible for the high separation of photo-generated electron-hole pairs and could improve the persistent luminescence and photocatalytic properties of Ga2O3:Cr3+ effectively.

Influence of co-doping Si ions on persistent luminescence of ZnGa 2 O 4 : Cr 3+ red phosphors

The impact of the addition of some amount of SiO2 in the ZnGa2O4:Cr3+ phosphors have been studied onto its persistent luminescent performance. The ZnGa2O4:Cr3+ phosphors with different Si4+ concentrations have been synthesized by using conventional solid-state reaction method. The X-ray diffractive patterns, photoluminescence (PL), thermoluminescence (TL) and afterglow decay have been measured and analyzed. The experimental results indicated that all the phosphors with different Si4+ codopant have the characteristic emission of Cr3+ and the co-doped Si4+ intensifies emission of N2 line and R lines. Furthermore the persistent luminescence was improved in both intensities and decay rates, in which the phosphor with 1mol% Si4+ has the best TL and the appropriate trap depth leading to the good persistent performance.

Enhanced photocatalytic activity and persistent luminescence in Zn2GeO4:Mn2+ by Eu3+ doping

Zn2GeO4:Mn2+,Eu3+ and Zn2GeO4:Mn2+ powders were synthesized by a high-temperature solid-state reaction. X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structures and morphologies of the synthesized powders, respectively. The photocatalytic properties and long persistent luminescence performance were improved by Eu3+ doping. Thermoluminescent (TL) curves showed that the trap concentration in the material was increased with Eu3+ doping, which formed trap centers in Zn2GeO4:Mn2+. The trap centers can capture the electrons or holes and subsequently increase the separation of photogenerated electrons and holes by suppressing the recombination of captured electrons and holes; thus, resulting in an improved photocatalytic activity and a prolonged persistent luminescence. The present strategy may be used as a general method to improve the photocatalytic activity and persistent luminescence.

Enhancing persistent luminescence and photocatalytic properties in Ti as a trap center in ZnGa2O4

ZnGa2O4 and ZnGa2O4:Ti phosphors were synthesized by the solid state method, and their persistent luminescence and photocatalytic properties were investigated in detail. The results of this study showed that Ti4+ doping improved the persistent luminescence properties. Thermoluminescence measurements demonstrated that the trap concentration considerably increased upon incorporation of Ti4+ ions into the ZnGa2O4 lattice. The traps generated by Ti doping were responsible for improvement in persistent luminescence. Furthermore, photocatalytic activity tests showed that the Ti-doped ZnGa2O4 phosphor exhibited much higher photocatalytic activity than the ZnGa2O4 host. UV–Vis diffuse reflectance spectra demonstrated that Ti-doped ZnGa2O4 shown a higher UV absorption efficiency. A comparison between the density of states of ZnGa2O4:Ti and ZnGa2O4 revealed that the bottom of the conduction band was modified by Ti doping. Hence, it could be concluded that Ti doping enhanced light harvest capability to generate more electron–hole pairs, and acted as a trap center by decreasing the recombination of photogenerated electrons and holes, resulting in the enhancement of both persistent luminescence and photocatalytic activity.

Long-lasting luminescence in ZnGa2O4: Cr3+ through persistent energy transfer

Cr3+-doped zinc gallate (ZnGa2O4) near-infrared (NIR) phosphors were synthesized via a high temperature solid state method. The luminescence properties of the phosphors were studied systematically. A significant spectra overlap between the emission of ZnGa2O4 and the absorption of Cr3+ was observed and 300 nm excitation exhibited the most excellent long-lasting luminescence properties among the three main excitation bands. Luminescence intensity was changed with the ratio of Ga3+/Cr3+ and the blue host emission of ZnGa2O4 was suppressed when doping Cr3+ into ZnGa2O4. The fluorescence decay curves of blue emission of ZnGa2O4 with different Cr3+ doping concentrations indicated that the lifetime of ZnGa2O4 at 505 nm become shorter with the increase of the Cr3+ concentration. Herein, a possible mechanism of long-lasting luminescence in ZnGa2O4: Cr3+ was proposed that the NIR long-lasting luminescence in ZnGa2O4: Cr3+ comes from the persistent energy transfer from ZnGa2O4 to Cr3+.

Persistently luminescent and photocatalytic properties of ZnGa2O4 phosphors

The phosphors ZnGa2O4 were synthesized via high temperature solid-state reaction. The crystal structure, photoluminescence, persistent luminescence, and photocatalytic properties of ZnGa2O4 were studied in detail. The x-ray diffraction patterns showed that some remaining phases of ZnO and β-Ga2O3 appeared with the excess amount of ZnO and Ga2O3, respectively. The results of the Raman spectra indicated that the first order Raman active modes of ZnGa2O4 were attributed to O2− ions and Zn2+ ions in tetrahedral sites. The phosphors exhibited a broad-band emission around 430 nm, which could be ascribed to the Ga–O transition of regular octahedral sites in the spinel lattice of ZnGa2O4. It also exhibited the emission peak around 430 nm shift to longer wavelength with the amount of the excess ZnO. The persistent luminescence of ZnGa2O4 could be observed for 10 min by naked eyes at room temperature under 254 nm ultraviolet (UV) excitation. In addition, photocatalytic activity test showed that ZnGa2O4 exhibited excellent photocatalytic activity for the degradation of Rhodamine B by the UV irradiation. It was indicated that the traps played an important role in trapping the electrons or holes to decrease the combination of the holes or electrons producing by the irradiation.

Synthesis and photoluminescence properties of multicolor tunable GdNbO4: Tb3+, Eu3+ phosphors based on energy transfer

A color-tunable phosphor based on Tb3+/Eu3+ co-doped GdNbO4 were synthesized by a traditional solid-state reaction method. X-ray powder diffraction (XRD), diffuse reflectance spectra, photoluminescence spectra and decay curves were utilized to characterize the as-prepared phosphors. XRD result indicated that various concentrations Tb3+/Eu3+ single-doped and co-doped phosphors were well indexed to the pure GdNbO4 phase. The GdNbO4 host was proved to be a self-activated phosphor with broad absorption range from 200 nm to 325 nm. When Tb3+ ions were added into the host lattice, the energy transferring from host to Tb3+ was identified. And the broad absorption in the UV region was changed and enhanced. Therefore, we selected Tb3+ as the sensitizer ion, and adjusted red component from Eu3+ to control the emission color. The energy transfer from Tb3+ to Eu3+ was confirmed based on the luminescence spectra and decay curves. Furthermore, the energy transmission mechanism was deduced to be the dipole–quadrupole interaction. On the whole, the obtained GdNbO4, GdNbO4:Tb3+, and GdNbO4:Tb3+, Eu3+ phosphors may have potential application in the UV white-light-emitting diodes (w-LEDs) and display devices.