Luminescent And Photocatalytic Properties Of Zn₂GeO₄ Doped With Several Kinds Of Ions

Zinc germanate(Zn2GeO4)is know as a self-excited blue phosphor and a new type of substrate material for light emitting device because of it’s suitable conductivity and high stability,besides this material also has good photocatalytic properties.In addition,a large number of reports said some ions doping Zn2GeO4 can become a long persistent luminescent material,such as Zn2GeO4:Eu;Zn2GeO4:Mn2+ and so on.The long persistent luminescence was barely seen in the pure Zn2GeO4 phase,However,there have been few reports on the enhancement of long persistent luminescence of Zn2GeO4 host.As we know,the introduction of defects is an effective way to control physical properties and to explore new functions in inorganic crystals,ion doping is regarded as a common method to introduce defects in the process of material synthesis.Considering Ti4+ ion and Si4+ ion have the equivalent charge and similar ionic radius with Ge4+ ion,we intend to introduce some new defects by Ti4+/Si4+doping in Zn2GeO4 host and investigated their luminescent and persistent luminescent properties.In addition,the long persistent luminescent and photocatalytic properties of Zn2GeO4: Mn2+ by the rare earth ion Eu3+ doping are discussed,and the relationship between long afterglow and photocatalytic properties is also discussed.Zn2GeO4 and Zn2Ge1-xO4:xTi4+ powders were synthesized by high temperature solid-state reaction method.X-ray diffraction(XRD)was used to characterize the structure.The photoluminescence(PL)and persistent luminescence properties of Zn2Ge1-xO4:xTi4+phosphors were characterized by the excitation spectra,the emission spectra and the persistent decay curves.The results showed that the Ti4+ doping prolonged the persistent luminescence period of the Zn2GeO4 host.The thermoluminescence(TL)and absorption spectra indicated that a new defect band was formed by Ti4+ doping,Since Ti4+ is necessary to capture electrons and photoreduction to Ti3+ so that the trap band act as an electron trap center.The new defect band,which is located between the conduction band and valence band close to bottom of the conduction band,can capture the electrons to decay the electrons recombining with holes resulting in a prolonged persistent luminescence.Zn2GeO4 and Zn2SixGe1-xO4 powders were synthesized by high temperature solid-statereaction method.The X-ray diffraction(XRD)of all sample showed that the Zn2GeO4 crystal structure did not change by Si4+ ion doping,and the diffraction peaks of Zn2SixGe1-xO4 samples shift to higher diffraction angle with the increase of Si4+ doping concentration,which indicated that Si4+ ions occupy the Ge4+ sites cause the lattice contraction.The excitation spectra and emission spectra of the samples are obviously blue-shifted with the increase of Si4+ ion doping concentration,which may be related to the local distortion of the Ge4+ sites substituted by the Si4+ ion in Zn2GeO4 crystal.The absorption spectra of the samples showed that the band gap of Zn2SixGe1-xO4 is larger with the increase of Si4+ doping concentration.The thermoluminescence spectra indicate that Si4+ ion doping can introduce a new trap level in the Zn2 Six Ge1-xO4 series of samples.These trap centers,which can store and transmit energy and prolong the persistent luminescence time of the samples.Zn2GeO4:Mn2+,Eu3+ and Zn2GeO4:Mn2+ powders were synthesized by high temperature solid-state reaction method.X-ray diffraction(XRD)and scanning electron microscopy(SEM)were used to characterize the structure and morphology of prepared powders,respectively.The photocatalytic properties and long persistent luminescence performance were also measured.The results showed that the Eu3+ doping enhanced the photocatalytic activity and prolonged the persistent luminescence period.Thermoluminescence(TL)measurement showed that the trap concentration in the material was increased with Eu3+ doping.The results of our experiments indicated that the Eu3+doping could form trap centers in Zn2GeO4:Mn2+.The trap centers effectively increase the separation of photo-generated electrons and holes,resulting in an improved photocatalytic activity.Meanwhile,the trap centers can capture the electrons or holes to decay the electrons or holes recombining with their opposite partners resulting in a prolonged persistent luminescence.Therefore,to introduce the trap centers may be a general method to improve the photocatalytic activity and persistent luminescence.