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地心游记2在线观看An internationa l j ourna l of inorganic che m istry
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Volume 40 | Number 37 | 7 October 2011 | Pages 9329–9620
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PAPER
(Zn,Mg)2GeO 4:Mn 2+submicrorods as promising green phosphors for field emission displays:hydrothermal synthesis and luminescence properties
Mengmeng Shang,a ,b Guogang Li,a ,b Dongmei Yang,a ,b Xiaojiao Kang,a ,b Chong Peng,a ,b Ziyong Cheng a and Jun Lin a
Received 15th April 2011,Accepted 8th June 2011DOI:10.1039/c1dt10673b
福州约克外语培训(Zn 1-x -y Mg y )2GeO 4:x Mn 2+(y =0–0.30;x =0–0.035)phosphors with uniform submicrorod morphology were synthesized through a facile hydrothermal process.X-Ray diffraction (XRD),field e
mission scanning electron microscopy (FE-SEM),photoluminescence (PL),and cathodoluminescence (CL)spectroscopy were utilized to characterize the samples.SEM and TEM images indicate that
Zn 2GeO 4:Mn 2+samples consist of submicrorods with lengths around 1–2m m and diameters around 200–250nm,respectively.The possible formation mechanism for Zn 2GeO 4submicrorods has been prented.PL and CL spectroscopic characterizations show that pure Zn 2GeO 4sample shows a blue emission due to defects,while Zn 2GeO 4:Mn 2+phosphors exhibit a green emission corresponding to the characteristic transition of Mn 2+(4T 1→6A 1)under the excitation of UV and low-voltage electron beam.Compared with Zn 2GeO 4:Mn 2+sample prepared by solid-state reaction,Zn 2GeO 4:Mn 2+phosphors obtained by hydrothermal process followed by high temperature annealing show better luminescence properties.In addition,codoping Mg 2+ions into the lattice to substitute for Zn 2+ions can enhance both the PL and CL intensity of Zn 2GeO 4:Mn 2+phosphors.Furthermore,Zn 2GeO 4:Mn 2+phosphors exhibit more saturated green emission than the commercial FEDs phosphor ZnO:Zn,and it is expected that the phosphors are promising for application in field-emission displays.
1.Introduction
Field emission displays (FEDs)have recently gained much attention as they are considered to be the next generation flat panel displays.By employing the same operating principles,FEDs provide the potential of achieving comparable or superior levels of performance to the conventional cathode ray tubes (CRTs).1–5Moreover,FEDs have some fascinating features such as great brightness,wide horizontal and vertical view angles,good contrast ratio,high efficiency with a low power consumption,short respon time,and wide work temperature range.6,7Although LCD has a great market at prent,FEDs have applications in many specific aspects,such as some special military devices besides the general flat display application.It is still in the developing stage and necessary for investigation.In the development of FEDs,it is important to develop phosphors which show high efficiency and good stability under excitation with low-voltage electron (£5kV)and high current density (10–100m A cm -2).8–15Many efficient sulfide-bad compounds such as Y 2O 2S:Eu,Gd 2O 2S:Tb,
a
garlandState Key Laboratory of Rare Earth Resource Utilization,Changchun Institute of Applied Chemistry,Chine Academy of Sciences,Changchun,130022,Jilin,China.E-mail:jlin@ciac.jl;Fax:+86-431-85698041;Tel:+86-431-85262031b
Graduate University of the Chine Academy of Sciences,Beijing,100049,P .R.China
负责人 翻译
SrGa 2S 4:Eu,Zn(Cd)S:Cu,Al,ZnS:Ag,Cl,etc.,have been explored as possible low-voltage phosphors,but sulfide phosphors are easily decompod and emit sulfide gas under electron excitation,subquently causing the cathodes to deteriorate and lowering the luminous efficiency of the phosphors.16,17Therefore,in order to improve the performance of FED devices,it is necessary to find novel phosphors with high luminescence efficiency and good chemical durability.Oxide-bad phosphors are more stable and environmentally friendly in comparison with sulfides.So,oxide-bad phosphors for FEDs have been of great interest due to their excellent light output,color rendering properties and superior stability under electron bombardment.18–25Germanates,as oxide-bad hosts,have potential applications in electronic,photode-tectors,electroluminescent,white LED and catalytic fields.26–28Zinc germanate (Zn 2GeO 4)has been found to be good host material for green emitting by Mn 2+doping with good stability.29,30However,to the best of our knowledge,Zn 2GeO 4:Mn 2+phosphor was usually prepared by high temperature solid-state reaction and few literatures focus on its cathodoluminescence (CL)property.30
Generally,the chemical and physical properties of inorganic materials are related fundamentally to their composition,mor-phology,and size.31Therefore it has become an important rearch issue in re
cent years that synthesis of inorganic materials can be with uniform and specific size and morphology which allow for applications in optics,electrics,magnetics,and optoelectronics.32
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Moreover,one-dimensional (1D)and quasi-one-dimensional (Q-1D)structures (e.g.micro/nano-rods,-tubes and -belts)have attracted considerable academic and industrial interest due to their unique optical,electronic,and magnetic properties.33To date,lots of methods including chemical or physical vapor deposition,lar ablation,arc discharge,vapor-pha transport process,electrospinning and a template bad method have been developed to prepare 1D and Q-1D materials.34However,most of the synthetic methods require either complex steps or expensive and sophisticated apparatus.
Accordingly in this paper,we synthesized Zn 2GeO 4:Mn 2+submicrorods by a facile hydrothermal method without any surfactant assistance.The morphological evolution and the growth mechanism for the synthesized Zn 2GeO 4:Mn 2+submicrorods have been studied.The photoluminescence (PL)and cathodolumi-nescence (CL)properties have been investigated in detail.We also compared the CL intensity of Zn 2GeO 4:Mn 2+prepared by hydrothermal process (marked as Zn 2GeO 4:Mn 2
+-HT)with that of Zn 2GeO 4:Mn 2+prepared by solid-state reaction (marked as Zn 2GeO 4:Mn 2+-SS).In addition,Mg 2+ions were codoped in Zn 2GeO 4:0.04Mn 2+sample in order to check its influence on the crystal structure and luminescence properties.
2.
Experimental ction
2.1.
Chemicals and materials
GeO 2(99.999%)was purchad from Sinopharm Chemical Reagent Co.,Ltd.,and other chemicals were purchad from Beijing Chemical Corporation.All chemicals were of analytical grade reagents and were ud directly without further purification.2.2.
Preparation
The Zn 2GeO 4:Mn 2+submicrorods were synthesized by a sim-ple hydrothermal method without any surfactant assistance.In a typical synthetic procedure,GeO 2,Zn(CH 3COO)2·H 2O and Mn(CH
3COO)2·4H 2O were added to 40mL of deionized water.The doping concentration of Mn 2+is 0–3.5mol%of Zn 2+in Zn 2GeO 4.The molar ratio of GeO 2/[Zn(CH 3COO)2·2H 2O +Mn(CH 3COO)2·4H 2O]is 1:1.The mixture was stirred for 30min.Then NaOH solution (5mol L -1)was introduced dropwi to the vigorously stirred solution to adjust pH to 8.After additional agitation for 20min,the as-obtained white colloidal precipitate was transferred to a 50mL autoclave,aled,and heated at 140◦C for 24h,then cooled naturally to room temperature.The products were collected by centrifugation,washed veral times with ethanol and deionized water.Finally,the products were dried at 80◦C for 12h.The Mg 2+ions codoped Zn 2GeO 4:Mn 2+submicrorods were prepared by the same procedure above,ex-cept that a certain amount of Mg(CH 3COO)2·4H 2O together with Zn(CH 3COO)2·2H 2O were ud as the starting material.In addition,the stoichiometry amounts of ZnO,GeO 2,and Mn(CH 3COO)2·4H 2O were mixed in an agate mortar,adequately triturated and calcined in air at 1200◦C for 4h to synthesize Zn 2GeO 4:Mn 2+-SS for comparison.To distinguish clearly,the products obtained by hydrothermal process and solid-state reac-tion were marked as Zn 2GeO 4:Mn 2+-HT and Zn 2GeO 4:Mn 2+-SS,respectively.
2.3.Characterization
X-ray powder diffraction (XRD)measurements were performed on a D8Focus diffractometer (Bruker)
at a scanning rate of 12◦min -1in the 2q range from 10to 80◦,with Cu-K a radiation (l =0.15405nm).The morphology and composition of the samples were inspected using a scanning electron microscope (SEM,S-4800,Hitachi)equipped with an energy-dispersive X-ray spectrum (EDX,JEOL JXA-840).Transmission electron microscopy (TEM)was performed using FEI Tecnai G2S-Twin with a field emission gun operating at 200kV .Images were acquired digitally on a Gatan multiple CCD camera.The photoluminescence (PL)excitation and emission spectra were recorded with a Hitachi F-7000spectrophotometer equipped with a 150W xenon lamp as the excitation source.Luminescence lifetimes were measured with a Lecroy Wave Runner 6100digital oscilloscope (1GHz)using a tunable lar (pul width =4ns,gate =50ns)as the excitation (Continuum Sunlite OPO).The CL measurements were carried out in an ultra-high-vacuum chamber (<10-8Torr),where the samples were excited by an electron beam at a range of 2.5–5kV with different filament currents,and the spectra were recorded by using an F-7000spectrophotometer.printer
3.Results and discussion
3.1.Pha structure
Fig.1A shows the XRD pattern of the Zn 2GeO 4:0.04Mn 2+sample.All the diffraction peaks of the Zn
2GeO 4:0.04Mn 2+sample can be assigned to a pure rhombohedral Zn 2GeO 4pha (JCPDS No.11-0687).No other impurity pha can be detected,indicating that the Mn 2+ions are completely dissolved in the Zn 2GeO
4
Fig.1(A)XRD pattern of Zn 2GeO 4:0.04Mn 2+.The standard data of Zn 2GeO 4(JCPDS No.11-0687)as reference;(B)crystal structure of Zn 2GeO 4with a unit cell.
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host lattice by replacing the Zn 2+ions.The crystal structure of Zn 2GeO 4is shown in Fig.1B.Zn 2GeO 4is a binary compound
oxide consisting of ZnO and GeO 2with the space group R 3
¯and has a phenacite structure with lattice constants of a =b =bust
14.231A
˚and c =9.53A ˚.The coordination number of Zn,Ge,O atoms is 4,4,3,respectively.30
Due to the comparable ionic radii of Mg 2+and Zn 2+ions,Mg 2+ions can be codoped into the Zn 2GeO 4:0.04Mn 2+sample (Zn 1.96-1.96y Mg 1.96y GeO 4:0.04Mn 2+)in order to check its influence on the crystal structure and luminescence properties.The effect of Mg 2+ions codoping on the structure of Zn 2GeO 4is prented in Fig.2A.The spectrum of Zn 2GeO 4:0.04Mn 2+is plotted as a reference.The XRD patterns clearly imply that Mg 2+ions can substitute for Zn 2+ions in Zn 2GeO 4host and form a single pha for y =0.05and 0.10.The energy-dispersive X-ray spec-trum (EDX)(Fig.2B)of sample Zn 1.96-1.96y Mg 1.96y GeO 4:0.04Mn 2+(y =0.10)further confirms the prence of Zn,Mg,Mn,Ge,O from the sample.The EDX spectrum and XRD patterns together validate that Mg 2+ions have been dissolved into the Zn 2GeO 4host lattice.But for y =0.20or higher,pha gregation commences and the peaks other than that of Zn 2GeO 4are identified to be of orthorhombic Mg 2GeO 4pha.As the crystal structures of Zn 2GeO 4(rhombohedral)and Mg 2GeO 4(orthorhombic)and the ionic radii of Zn 2+(0.60pm)and Mg 2+(0.57pm)for
sofa的音标four
Fig.2(A)XRD patterns of Zn 1.96-1.96y Mg 1.96y GeO 4:0.04Mn 2+(y =0,0.05,0.10,0.20,0.30),*reprents peaks of Mg 2GeO 4pha;(B)EDX spectrum of Zn 1.96-1.96y Mg 1.96y GeO 4:0.04Mn 2+(y =0.1)sample.
coordination are different,substitution of Zn 2+ions with Mg 2+ions does not occur for all concentrations and results in pha paration.35The variation of the lattice constants with Mg 2+ions concentration was calculated from the obrved XRD data and is shown in Fig.3.The c -axis length shows contraction with incread Mg 2+ions concentration and a-axis length shows elongation with increasing Mg 2+ions concentration in Fig.3A.
This is becau the c -axis length of Zn 2GeO 4is 9.53A
˚and that of Mg 2GeO 4is 4.91A
˚.However,cell volume (Fig.3B)increas continuously with increasing Mg 2+ions concentration,which is due to an elongation in the a
parameter.
Fig.3Variation of a /c -axis (A)and cell volume (B)with Mg 2+concen-tration in Zn 1.96-1.96y Mg 1.96
buzyy GeO 4:0.04Mn 2+.
3.2.Morphology and formation mechanism
SEM and TEM were ud to characterize the morphology and crystal structure of the sample.Fig.4illustrates the SEM,TEM,and,HRTEM images of the as-prepared Zn 2GeO 4:0.04Mn 2+sample.From the low-and high-magnification SEM micro-graphs in Fig.4A and B,we can clearly e that the as-obtained Zn 2GeO 4:0.04Mn 2+sample consists of submicrorods with smooth surface and perfect prism structure.The length of the submicrorods is approximately 1–2m m,and the diameter ranges from 200to 250nm.In addition,the breakage of the submicrorods indicates that Zn 2GeO 4submicrorods are compod of some small nanorod bundles.Fig.4C shows a typical TEM image of the Zn 2GeO 4:0.04Mn 2+submicrorods.It can further confirm the rod-like morphology of the Zn 2GeO 4:0.04Mn 2+
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Fig.4SEM and TEM images of Zn 2GeO 4:0.04Mn 2+submicrorods:(A,B)Low-and high-magnification SEM images;(C)TEM image,SAED (int in part C);(D)HRTEM image.
sample.The lected area electron diffraction (SAED)pattern (int in Fig.4C)indicates that Zn 2GeO 4submicrorod is sing-crystalline with a growth axis in the [110]crystallographic direc-tion.The HRTEM image (Fig.4D)shows well-resolved lattice fringes with an interplanar distance of 0.27nm corresponding to the (410)d spacing of the rhombohedral Zn 2GeO 4structure.Moreover,in order to compare with the morphology of Zn 2GeO 4-SS,the Zn 2GeO 4-HT samples were subjected to further heat treatment at different temperatures (500–1000◦C).Fig.5shows the SEM images of Zn 2GeO 4:0.04Mn 2+-HT sample after heat treatment at different temperatures:500◦C (A),700◦C (B),1000◦C (C)for 3h and that of Zn 2GeO 4:0.04Mn 2+-SS sample (D).It can be en from Fig.5A and B that the Zn 2GeO 4:0.04Mn 2+samples inherit their original morphologies after heat treatment at 500◦C and 700◦C and the size of submicrorods does not change significantly in comparison with the Zn 2GeO 4:0.04Mn 2+submicrorods obtained directly from the hydrothermal process.After heat treatment at 1000◦C (Fig.5C),part of the submicrorods broke into small nanorods.However,the Zn 2GeO 4:0.04Mn 2+-SS sample (Fig.5D)is compod of uneven
particles.
Fig.5SEM images of Zn 2GeO 4:0.04Mn 2+submicrorods with different postcalcination temperatures:500◦C (A),700◦C (B),1000◦C (C)and Zn 2GeO 4:Mn 2+-SS (D).
For Mg 2+ions codoped Zn 2GeO 4:0.04Mn 2+samples,the mor-phologies have been changed to some extent.The SEM images of the samples Zn 1.96-1.96y Mg 1.96y GeO 4:0.04Mn 2+(y =0.05–0.20)are prented in Fig.6.It should be mentioned that the low codoping amount of Mg 2+ions did not change the morphology of the Zn 2GeO 4:0.04Mn 2+samples.However,the surface of the submicrorods becomes rougher and there are more cohesive substances in the obtained products with incread Mg 2+ions substitution concentration,especially Mg 2+ions substitution up to y =0.20,which can be clearly en from the enlarged FE-SEM micrographs in Fig.6B,D,and E.With the Mg 2+concentration incread,the pha gregation commences and therefore the morphology of products
changes.网络在线英语培训
Fig.6SEM images of the samples Zn 1.96-1.96y Mg 1.96y GeO 4:0.04Mn 2+:(A,B)y =0.05;(C,D)y =0.10;(E,F)y =0.20.
To understand the formation of the Zn 2GeO 4:Mn 2+submicro-rods,we carried out time-dependent shape evolution experiments during which samples were collected after different periods of hydrothermal treatment.At an early stage (15min and 1h),the supersaturated solution leads to the nucleation of Zn 2GeO 4and tiny Zn 2GeO 4particles are formed from the solution (Fig.7A and B).The sample collected 10h later (Fig.7C)shows approximately submicrorods with length of 1m m.As the reaction proceeded (Fig.7D),the smooth submicrorods with perfect prism structure developed.On the basis of the experimental results,the formation mechanism for the Zn 2GeO 4:Mn 2+submicrorods is speculated to be as follows:(1)In the synthetic process,GeO 2is hydrolyzed to form GeO 32-;28(2)When the concentrations of Zn 2+and GeO 32-reach the supersaturation degree of Zn 2GeO 4,small Zn 2GeO 4nuclei form according to the following equation:2Zn 2++GeO 32-+2OH -=Zn 2GeO 4+H 2O.(3)The nuclei further grow along the c -axis to produce Zn 2GeO 4nanorods.With prolonged reaction time,Zn 2GeO 4nanorods will have the tendency to aggregate together,side by side,to form a bundle,resulting in a submicrorod-like
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