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A Novel Method for the Synthesis of ZnS for Use in the Preparation of Phosphors for CRT Devices

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A Novel Method for the Synthesis of ZnS for Use in the Preparation of Phosphors for CRT Devices
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  A Novel Method for the Synthesis of ZnS for Use in thePreparation of Phosphors for CRT Devices D. A. Davies, * ,z J. Silver, *  A. Vecht, *  P. J. Marsh, and J. A. Rose Centre for Phosphor and Display Materials, University of Greenwich, Woolwich SE18 6PF, United Kingdom The decomposition of thiourea dioxide in aqueous solution at elevated temperatures, in the presence of zinc acetate, has been usedto precipitate ZnS. The important reaction pathway for the formation of sulfide ions has been elucidated, and some common ZnSphosphors have been prepared from the precipitate. The preparation of ZnS by this method is extremely simple and does not yieldlarge amounts of liquid or gas containing volatile sulfur species. Thus, this method has been shown to be an excellent method forthe preparation of ZnS phosphors, particularly copper-activated materials, requiring no purification of the reagents, with littleproduction of sulfur-containing waste species, and resulting in small particle size powders without postproduction milling orseparation of the powders. These phosphors have been shown to have exceptional luminescent properties compared to standardcommercial materials.© 2001 The Electrochemical Society.   DOI: 10.1149/1.1400123   All rights reserved.Manuscript submitted February 22, 2001; revised manuscript received May 30, 2001 Available electronically September 11, 2001. Metallosulfides and selenides, due to their useful range of physi-cal and chemical properties, are in common use in industry   includ-ing color, semiconductor, and luminescence  . 1 Metal sulfides in par-ticular have found an important use as phosphors. Zinc andcadmium sulfide were both commonly used in the past, 2 but toxico-logical dictates have lessened the use of Cd to all but a few unavoid-able applications. ZnS is one of the most commonly used host lat-tices for phosphors. Activated with Ag or Ag and Al, it is used as theblue component in color cathode ray tube   CRT   displays. 2 Acti-vated with Cu, Cu and Al, or Cu, Al, and Au, it is used as a greenCRT phosphor. ZnS phosphors are reviewed in Ref. 2.CRT phosphors utilizing ZnS as the host lattice are commonlymade by firing a mixture of ZnS, activator, and flux   to promoteactivator diffusion and crystallization  . 2 In some cases the atmo-sphere under which the firing is carried out is controlled. 1 The re-sulting phosphor is a powder consisting of crystalline particles of different sizes and shapes. For modern high-definition television  HDTV   screens, the phosphor particles are required to be smallerand preferably more regularly shaped than those found in most cur-rent commercial phosphors. 3 It has previously been demonstratedthat high-quality luminescent emission can be achieved using sub-micrometer spherical phosphor particles which are precipitated fromsolution before firing. 4 It has also been demonstrated that small-particle ZnS phosphors   about one-fourth the particle size of com-mercial materials   can yield as good or better luminescence   interms of color and intensity of emission  . 1 As it appears that the bestmorphological and size control is achieved during the precipitationof the unfired ZnS particles, we have extended investigations on thepreparation of ZnS phosphors and have developed a new solutionmethod for the precipitation of ZnS utilizing thiourea dioxide.Thiourea dioxide   TDO, formamidine sulfinic acid   is a well-known chemical used in the paper industry to bleach wood pulp.TDO decomposes at elevated temperature, forming peroxides whichbleach the pulp. 5 Herein the use of the thermal decomposition of aqueous TDOsolutions, in the presence of Zn 2  , as a method for the precipitationof ZnS and its application to phosphor synthesis is reported. Theresults of the characterization of the phosphors prepared and theirluminescent properties are compared to commercial materials andsimilar phosphor samples prepared using the previously reportedmethod for the precipitation of ZnS   aqueous Zn 2  ions precipitatedby the action of sulfur solutions in hydrazine monohydrate  . 1 Experimental All chemicals used were of analar grade and were used as pur-chased, except thiourea dioxide   99%, Acros  . Activators used wereof 99.99% purity or higher and were added to the Zn solution beforeprecipitation of the unfired sulfide. Characterization of the productsand luminescence testing were carried out using the previously re-ported apparatus, 1 except cathodoluminescence measurements over5000 V which were carried out using a Kimball Physics EMG-12/ EGPS-12 electron gun and firing unit. The electron gun created anilluminated spot of 2 mm diam and the current used was 2   A. ZnSphosphors from ZnS precipitated using sulfur solutions in hydrazinemonohydrate was prepared by the previously reported method. 1 Ac-tivator concentrations were 0.01 mol % Zn 2  concentration unlessotherwise stated. Precipitation of ZnS by the aqueous thermal decomposition of thiourea dioxide .—Zinc acetate   40 g, 0.18 mol   and a suitable ac-tivator salt   metal chloride, acetate or nitrate, typically 1.8  10  5 mol   dissolved in water   4.5 L   was placed on a hot platestirrer in a 5 L beaker covered by a watch glass. The beaker wasthen wrapped in aluminum foil and heated to approximately 90°C.TDO   200 g, 1.83 mol   was then added and the solution was heatedfor 1 h. The resulting white suspension was then filtered undervacuum and the solid was then washed with water   4.5 L  , thenisopropanol   500 mL  , and dried overnight at 130°C. The yield of ZnS was around 90%.  High-temperature firing of ZnS precipitates .—The dried ZnSprecipitates were intimately ground with flux   2 mass % NaCl   andpacked tightly into silica crucibles which were covered with lids.The crucibles were then placed in a muffle furnace at 880°C for 1 hbefore being removed and allowed to cool to ambient temperatures.Once cooled the phosphors were treated as previously describedby washing with dilute acetic acid and then water before drying. 1 Results and Discussion Precipitation of ZnS using the thermal decomposition of aqueousTDO solutions was achieved simply and rapidly in good yield  90%  . The precipitates had broad X-ray powder diffraction pat-terns consistent with cubic ZnS. 6 The precipitation is achieved byadding an excess of TDO   compared to the   Zn 2     to a hot aqueoussolution of ZnOAc 2 . Initially this results in the suspension of thesolid TDO; this suspension clears as the solid TDO dissolves and agas is evolved. Then the onset of the precipitation of ZnS clouds themixture again. The pH of the suspension is slightly acidic and re-mains in the range 5-7 throughout the reaction. Addition of bariumsolutions to the filtrate after ZnS removal results in a white precipi-tate mainly of BaSO 3   identified by comparison with ICPDS data  . 6 A recent study of the aerobic decomposition of TDO in alkali solu- *  Electrochemical Society Active Member. z E-mail: DD05@gre.ac.uk   Journal of The Electrochemical Society ,  148   10   H143-H148   2001  0013-4651/2001/148  10   /H143/6/$7.00 © The Electrochemical Society, Inc. H143  tions showed that the material decomposed by a free radical reactioninto a wide range of products, including the dithionite anion   someof these are presented in Fig. 1  . 7 Addition of base to the startingreaction mixture causes Zn  OH  2  to be precipitated, which is whybase was not used in this work. The production of peroxide   see Fig.1   is the explanation of the evolution of gas seen during the earlystages of the precipitation process.A sample reaction using a less concentrated reaction mixture   3 gTDO   1 g ZnSOAc in 100 mL H 2 O   was examined using gravi-metric and spectroscopic methods. The concentration of TDO wasfollowed using electronic absorption spectroscopy and the mass of ZnS formed was measured gravimetrically. No absorbance fordithionite was observed in any of the spectra recorded(  max  315nm). 5 However, the formation of the dithionite anion wasnot necessarily arrested, as its decomposition by hydroxide ions toform sulfite and sulfide ions has been documented in the literature 8 and is outlined in Fig. 1. The top reaction in Fig. 1 is not written asa balanced equation due to the complexity of the reaction pathwaysand the range of possible product which can be formed dependingon the reaction conditions. The precipitation of BaSO 3  on addition Figure 1.  Some of the reactions occurring during the precipitation of ZnS. Figure 2.  SEM images of the ZnS:Ag phosphors.  Figure 3.  CL emission spectra of the ZnS:Ag phosphors. Table I. Particle size data for phosphors studied in this investi-gation. PhosphorMean particlesize   m  Standarddeviation   m  ZnS:Ag  TDO   3.10 2.83ZnS:ag  HH/S   5.24 4.21ZnS:Ag  STD   6.42 3.90ZnS:Cu  TDO   0.005 mol %   1.79 0.80ZnS:Cu  TDO   0.01 mol %   2.31 2.95ZnS:Cu  TDO   0.03 mol %   2.17 3.68ZnS:Cu  HH/S   8.458 8.55ZnS:Cu  STD   4.731 4.65ZnS:Cu,Al  TDO   2.282 0.60ZnS:Cu,Al  STD   6.080 6.22ZnS:Cu,Al,Au  TDO   2.425 1.78ZnS:Cu,Al,Au  STD   8.273 7.97  Journal of The Electrochemical Society ,  148   10   H143-H148   2001  H144  of a barium solution to the filtrate suggests that the dithionite may bedecomposing rapidly compared to its rate of formation.These observations explain   i   the formation of sulfide ion andthus the precipitation of the ZnS;   ii   the requirement for an excessof TDO due to the stoichiometry of the process;   iii   the nonquan-titative yield as the high   SO 32    holds some of the Zn 2  in solutionand the rate of formation of sulfide ion decreases, resulting in aninsufficient   S 2    to precipitate the zinc ions;   i v   why BaSO 3  isprecipitated when barium solution is added to the filtrate; and   v  why no sulfurous waste gases are evolved in the reaction. Thus it ishere suggested that the reactions outlined, though not precisely themechanism, do explain the precipitation of ZnS by the aqueous ther-mal decomposition of thiourea dioxide and confirm its application tocleaner sulfide synthesis.Two other observations are   i   Low yields of ZnS result fromcarrying out the precipitation under anaerobic conditions.   The lit-erature reports that the formation of ditionite during the anaerobicdecomposition of TDO is limited by the amount of oxygen dissolvedin the starting mixture, which explains this observation. 7   ii   Use of ZnCl 2  in place of the acetate results in a decreased yield of ZnS. Thestarting mixture is more acidic than that formed with the acetate andmay reduce the   S 2    formed.For the preparation of phosphors the addition of metal dopants tothe ZnS is required. Previously this has been achieved by the addi-tion of metal salts to the zinc solution used; in this work a similarapproach was used. High-temperature firings of the precipitateswere carried out using a sodium chloride flux to promote crystalli-zation of the precipitates. The resulting phosphor powders werecompared to commercial materials, and similar phosphors were pre-pared by a previously reported method. 1 The results of the investi-gations are discussed. All the phosphor materials were shown to becrystalline ZnS   mainly in the cubic phase, but with a small compo-nent of the hexagonal phase present   from powder X-ray diffraction  XRD   patterns. 6 The low concentration of the hexagonal phasepresent made calculation of its concentration unreliable.  ZnS:Ag phosphors .—Zinc sulfide activated with silver preparedusing TDO   ZnS:Ag  TDO   was compared with a similar phosphorprepared by the previously reported method 1  using sulfur solutionsin hydrazine monohydrate   ZnS:Ag  HH/S   and a commercial phos-phor   ZnS:Ag  STD  . Scanning electron microscopy   SEM   imagesof the phosphors are presented in Fig. 2 and particle size data inTable I. It can be seen that both the ZnS:Ag  TDO   and HH/S phos-phors have smaller particle sizes than the commercial sample. The Figure 4.  Plot of voltage  vs.  luminance under CL conditions for the ZnS:Agphosphors. Figure 5.  SEM images of the ZnS:Cu phosphors. Table II. CIE coordinates for phosphors studied in this investi-gation. Phosphor CIE-  x  CIE-  y ZnS:Ag   TDO   0.162 0.111ZnS:Ag   HH/S   0.155 0.071ZnS:Ag   STD   0.149 0.062ZnS:Cu   TDO   0.005 mol %   0.273 0.493ZnS:Cu   TDO   0.01 mol %   0.299 0.570ZnS:Cu   TDO   0.03 mol %   0.241 0.418ZnS:Cu   HH/S   0.262 0.471ZnS:Cu   STD   0.277 0.560ZnS:Cu,Al   TDO   0.278 0.552ZnS:Cu,Al   STD   0.297 0.614ZnS:Cu,Al,Au   TDO   0.284 0.563ZnS:Cu,Al,Au   STD   0.291 0.610  Journal of The Electrochemical Society ,  148   10   H143-H148   2001   H145  ZnS:Ag  TDO   phosphor has the smallest particle size of all threematerials. This is an excellent result as no milling or particle sizinghad been performed on this phosphor.The phosphors were examined under cathodoluminescent excita-tion   CL  . The emission spectra are presented in Fig. 3 and the CIEcoordinates in Table II. It can be seen in Fig. 3 that the emissionspectrum of the ZnS:Ag  HH/S   is an excellent match for the com-mercial material.   It is in fact a slight improvement in terms of colorand intensity on previously reported samples. 1   This is confirmed bythe CIE coordinates in Table II which are the same to within theexperimental error of chromaticity calculation from the emissionspectra. The spectrum of the ZnS:Ag  TDO   tails into the greenemission region compared to ZnS:Ag  HH/S   and STD, causing theCIE coordinates to be a poor match for the commercial sample.Figure 4 presents a plot of voltage against luminance under CLconditions for all the ZnS:Ag phosphors. The ZnS:Ag  TDO   mate-rial appears to have the most intense emission; however, this is dueto the difference in its emission color compared to the other mate-rials. The ZnS:Ag  HH/S   and STD show similarly intense emis-sions, confirming that the use of sulfur solutions in hydrazine mono-hydrate is an excellent method for the preparation of ZnS:Agphosphors. The HH/S phosphor offers a wider gamut of color thanthe TDO material, in a color display device, and this is probablymore important than the smaller particle size offered by the TDOphosphor. However, for a monocolor application requiring a highinformation density on display, the smaller particle size of the TDOphosphor may be advantageous. Figure 6.  CL emission spectra of the ZnS:Cu phosphors. Figure 7.  Plot of voltage  vs.  luminance under CL conditions for the ZnS:Cuphosphors. Figure 8.  CL emission spectra of the ZnS:Cu   TDO   phosphors with a rangeof copper concentrations. Figure 9.  Plot of voltage  vs.  luminance under CL conditions for the ZnS:Cu  TDO   phosphors with a range of copper concentrations.  Journal of The Electrochemical Society ,  148   10   H143-H148   2001  H146   ZnS:Cu phosphors .—A range of ZnS:Cu   0.005 mol % Cu  phosphors, made by similar methods to those discussed for ZnS:Ag,were investigated using the same techniques. SEM images of thematerials are presented in Fig. 5 and the particle size data in Table I.As for ZnS:Ag the TDO material has the smallest particle size,which is smaller than the commercial material. Since the phosphorhas not been milled or the particles separated, this reiterates that theaqueous thermal decomposition of TDO is an excellent, facile, andclean method for the preparation of ZnS phosphors with smallerparticle sizes than current commercial materials.The CL emission spectra of the materials are displayed in Fig. 6.Their CIE coordinates in Table II and Fig. 7 present a plot of voltageagainst luminance under CL conditions. The emission spectra of theZnS:Cu  HH/S   and TDO materials exhibit a second emission bandaround 460 nm   caused by self-activated emission from ZnS:Cl  , 2 which is larger than that of the commercial material. This means thatthe copper concentration in the phosphors is less than the optimumconcentration for ZnS:Cu. 2 Figure 7 presents the intensity of emis-sion of the ZnS:Cu  TDO  . This is comparable with that of the com-mercial material, while that of the ZnS:Cu  HH/S   is lower than thatof the other two materials. Figure 10.  CL emission spectra of the ZnS:Cu,Al phosphors. Figure 11.  CL emission spectra of the ZnS:Cu,Al,Au phosphors. Figure 12.  Plot of voltage  vs.  luminance under CL conditions for theZnS:Cu,Al,Au phosphors. Figure 13.  Plot of voltage  vs.  luminance under CL conditions for theZnS:Cu,Al,Au phosphors.  Journal of The Electrochemical Society ,  148   10   H143-H148   2001   H147
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