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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Repositório Institucional da Universidade de Aveiro J. MATER. CHEM., 1994, 4(10), 1611-1617 161 1 Preparation of Zinc Oxide and Zinc Sulfide Powders by Controlled Precipitation from Aqueous Solution Tito Trindade," Julio D. Pedrosa de Jesus*a and Paul O'Brienb a Department of Chemistry, University of Aveiro, 3800 Aveiro, Portugal Department of Chemistry, Queen Mary and Westfield College, Mile End Road, London, UK El 4NS ZnO and ZnS powders were prepared from aqueous solutions of zinc salts in the presence of ethylenediamine or triethanolamine. The morphology of the powders was analysed by scanning electron microscopy and infrared spectroscopy. The effect of the experimental conditions upon the size and shape of the particles is described with a special emphasis on the role of the organic ligand. There is considerable current interest in developing micro- ion which, on hydrolysis, leads to precipitation. Well defined crystalline powders suitable for use as precursors for ceramic particles of iron oxides," nickel and cobalt both as metal or materials. This work forms part of various efforts to produce oxide" have been prepared by such methods. If an appropriate high-performance ceramics of homogeneous composition and chalcogenide source is added, such methods can be used to microstructure, as such materials should have improved prepare sulfides and selenides.8 Needle-like crystals iof ZnO reliability as compared to conventional materials.' The electri- were obtained by the hydrothermal decomposition of aqueous cal properties of zinc oxide are also markedly affected by the solutions of Na,Zn-EDTA', and spherical particles of ZnO characteristics of the precursors used to prepare an individual were prepared using a spray pyrolysis method.', The con- sample, these properties are crucial in defining the suitability trolled preparation of ZnS particles has been achieved l4 using of zinc oxide for use in varistor^.^-^ Zinc sulfide is another thioacetamide as a source of sulfide, but with no other added example of a material for which the control of particle organic ligand. formation may be of importance as this material is used in This paper reports the synthesis and characterizlition of luminescence devices and pigments6 ZnO and ZnS powders by precipitation from homogeneous Chemical methods have been applied to the synthesis of solution, and the effect of experimental conditions on the powdered materials, and submicrometre particles of narrow morphology of the products is discussed. The work is part of size distribution, high purity and uniform morphology have a programme aimed at collecting systematic information been achie~ed.'.~.~Although technological interest in such about the role of chelating agents in the control of powder controlled synthesis is recent, many natural systems synthesize morphologies which should help to underpin the mechanism biominerals with remarkable properties.' initially proposed by LaMer and DinegarI5 for the fo-rmation Matijevic' showed that well defined hydrated metal oxide and shape control of colloidal particles. particles, as well as other inorganic solids, can be precipitated from homogeneous aqueous solution by several methods including the thermal treatment of solutions of coordination Experimental complexes. The method is based on forming a complex Chemicals sufficiently stable to prevent spontaneous precipitation at room temperature, i.e. masking the metal ion with a suitable The following chemicals were used without further purifi- ligand. Raising the temperature promotes the dissociation of cation: zinc nitrate hexahydrate, Zn(NO,), 6H,O ( Merck); the complex and allows the controlled generation of free metal zinc sulfate heptahydrate, ZnS0,.7H20 (Merck ); zinc chloride, ZnC1, (Merck); zinc acetate dihydra,te, Zn (CH,C02), .2H20(Merck); sodium hydroxide, NaOH pellets Table 1 Experimental conditions for the preparation of ZnO samples (Pronalab); nitric acid 65%, HNO, (Merckj; sodium chloride, using zinc nitrate at pH 13.0 NaCl (Merck); sodium nitrate, NaNO, (Merck); sodinm sulf- sample aging temp./"C aging time/min organic ligand ate decahydrate, Na,SO,.lOH,O (Merck); sodium acetate, NaCH,CO, (Merck); ammonia, NH, (Merck); ethanolamine, A 150 120 en NH2CH2CH20H, (Merck); triethanolamine (tea), B 150 120 tea N(CH2CH20H), (Merck); diethylamine, NH(C€€,CH,), C 100 120 en (Merck); ethylenediamine (en), NH,CH,CH,NH, ( Merck); D 100 120 tea diethylenetriamine (dien), NH(CH2CH,NH2), (BD H); tri- E 100 120 - ethylenetetramine (trien), NH,CH,CH,NHCH,CH,NH- Table 2 Experimental parameters used in the prepartion of ZnO powders ligand pH (20 "C) anion" time/min T/"C I(NaNO,)/niol kg- tea 10.0 NO3- 5 70 0.3 1 en 11.0 so42- 30 80 0.56 - 12.0 c1- 60 90 0.8 1 13.0 CH3C02- 120 100 1.06 - 240 150b not controlled " The respective zinc salt was used and an excess of the sodium salt (0.025 or 0.05 mol) was added to 50 cm3 of solution. Hydrothermal conditions. 1612 J. MATER. CHEM., 1994, VOL. 4 Table 3 Results from precipitation experiments of aqueous solutions of zinc nitrate and organic ligands at room temperature and pH 12.0 ligand" solid ammonia zinc hydroxide diethylamine zinc hydroxide monoethanolamine zinc hydroxide triethanolamine none ethylenediamine none diethylenetriamine none triethylenetetramine none nitrilotriacetic acid none EDTA none a [ZnZ+]=0.02mol dmP3;[ligand]: [Zn'+]=5: 1. Table 4 Results from precipitation experiments of aqueous solutions of zinc nitrate and organic ligands at 100 and 150 "C, aging time 2 h and pH 12.0 ligand" solid triethanolamine ZnO ethylenediamine ZnO diethylenetriamine none triethylenetetramine none nitrilotriacetic acid none EDTA none [Zn2+]=0.02mol dmP3;[ligand]: [Zn2+]=5:l. -6.00 pm (a) Fig. 2 SEM images of ZnO obtained at 100 'C in the presence of (a) ethylenediamine; (b) triethanolamine CH,CH, (Riedel-de-Haen); EDTA, ( H02CCH2)2- NCH2CH,N(CH2C0,H), (Carlo Erba); nitrilotriacetic acid trisodium salt, N(CH,COONa)3 (ICN); 2-mercaptoethanol, HSCH2CH20H (Merck); o-phthaldialdehyde. C,H,( CH0)2 (BDH); thioacetamide, CH,CSNH, (Merck). Sodium hydroxide solutions were prepared (1 and 4mol dmP3)using boiled deionized water. Nitric acid solutions of the same molarity were prepared by dilution of the concen- trated acid. Stock aqueous solutions of zinc (0.04mol dm-3) and ligand (0.2mol dmP3)were prepared. All the solutions were filtered using 0.45 pm Millipore membranes before use. H Determination of en was performed by the colorimetric 1 .O Pm method described by Hihara et using a Varian DMS 300 spectrometer. The stock solutions were diluted 1000-fold for the determination. Particle Preparation In a typical screening procedure equal volumes of ligand and Zn" stock solutions were mixed and the pH adjusted to 12.0. The final mixture was left to stand with stirring, at room temperature, for at least 1 day. Thermal treatment of the solutions was performed only for those which did not give a spontaneous precipitate at room temperature. Heating pro- cedures included refluxing (e.g. for 2 h) or heating the solutions in PTFE reaction bombs, in an oven (e.g. 2 h at 150 C). Another procedure involved the addition of sodium hydrox- ide (4mol dmP3) to a solution of the zinc salt (0.02mol -1 .o pm dm-3). Initially a precipitate formed which dissolved at pH> 13.5. This solution was filtered and then refluxed; a Fig. 1 SEM images of ZnO obtained by the hydrothermal treatment precipitate then formed. The precipitates formed by both (150'C) in the presence of: (a) ethylenediamine; (b)triethanolamine procedures were removed by filtration with 0.45 pm Millipore J. MATER. CHEM., 1994, VOL. 4 1613 Fig. 3 SEM images of ZnO after the milling of the powders: (a) sample A; (b)sample B; (c) sample C; and (d) sample D Particle Characterization The X-ray analysis of the powders was performed using a Philips PW 1729 generator using nickel-filtered Cu-Ka radi- ation. JEOL JSM-35C and Hitachi S-4100 scanning electron microscopes were used. The materials were prepared for microscopy by resuspension in water followed hy ultra- sonication. Aliquots of the suspensions were then kept on high-purity aluminium rods at room temperature in a desiccator over calcium chloride until the liquid had evapor- ated. The infrared spectra were recorded with a Mattson Polaris FTIR spectrometer using pellets of 1 mg of the sample and 150 mg of spectroscopic-grade KBr. 6.00pm Results and Discussion Fig. 4 SEM image of ZnO obtained at 100°C in the absence of organic ligand Preparation of ZnO and ZnS Powders Preliminary studies of the thermal and hydrothermal treat- ment of aqueous solutions of zinc in the presence 1i)f a wide variety of ligands have been reported.17 The resulth of these membranes, washed with deionized water and dried in a studies suggest that en and tea are the most promising metal- desiccator over silica gel at room temperature. ion buffers for a detailed study. The various powders obtained were analysed (see Table 1 Using similar systems but with different experimcntal con- for sample identification). The effect of other experimental ditions Matijevic and co-workers have obtained’ ’,’’ ZnO parameters associated with the preparation of the ZnO particles with morphological characteristics similar to those powders was studied by systematically varying the reaction described in our preliminary report.17 However, in the present conditions (see Table 2). paper the influence of a wider range of experimental conditions ZnS powders were prepared using procedures similar to on the powder characteristics are reported. those used in the preparation of ZnO. Thioacetamide was The addition of an organic ligand to a solution of a zinc used to generate the sulfide ions. salt may prevent or inhibit the precipitation of zinc hydroxide. 1614 J. MATER. CHEM., 1994, VOL. 4 Fig.

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