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Influence of Drying on the Characteristics of Zinc Oxide

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Influence of Drying on the Characteristics of Zinc Oxide i “21” — 2009/5/13 — 10:15 — page 248 — #1 i i i 248 C.P. Rezende et al. Influence of Drying on the Characteristics of Zinc Oxide Nanoparticles C.P. Rezende1, J.B. da Silva1;2, N.D.S. Mohallem1∗ 1Laboratorio´ de Materiais Nanoestruturados, Departamento de Qu´ımica, ICEx UFMG and 2Centro de Desenvolvimento da Tecnologia Nuclear – CDTN/CNEN 31270-901 Belo Horizonte-MG, Brazil (Received on 1 July, 2008) The recent growth in the field of porous and nanometric materials prepared by non-conventional processes has stimulated the search of new applications of ZnO nanoparticulate. Zinc oxide is an interesting semiconductor material due to its application on solar cells, gas sensors, ceramics, catalysts, cosmetics and varistors. In this work, the precipitation method was used followed by controlled and freezing drying processes. The materials obtained were thermally treated at various temperatures. The influence of temperature on structural, textural, and morphological properties of the materials was studied by powder X-ray diffraction, infrared spectroscopy, scanning electron microscopy, nitrogen adsorption, and thermal analysis. The characteristics of both materials were compared. Keywords: zinc oxide, drying process, nanoparticle I. INTRODUCTION II. EXPERIMENTAL Zinc nitrate and sodium carbonate were used as precursors Zinc oxide is a versatile material due to properties such of the ZnO particles. In a typical synthesis, Zn(NO3)2.6H2O as: pyroelectricity, semiconducting, piezoelectricity, lumi- were dissolved in deionized water in the molar ratio of 1:5, nescence, and catalytic activity [1 − 2]. Its optical band gap, and mixed for homogenization during 1 h. Na2CO3 were also chemical and thermal stabilities are also very important char- dissolved in deionized water in the molar ratio of 7:10, and acteristic. These properties are essential for electronic and mixing with the zinc solution leading to the formation of a photonic industry due to the wide range of technological white precipitate. The precipitate was washed with deionized applications in devices including acoustic surface, wave fil- water to remove impurities. The obtained powder was sepa- ters, sensors, transparent conductors, light-emitting diodes, rated in two aliquots to be submitted to drying processes. One laser diodes, solar energy conversion [3 − 5], electro-acoustic aliquot (S1) was dried in an oven at 110oC for 48 h and the transducers, photovoltaic devices, catalysts and varistors other (S2) was immersed in liquid nitrogen to freezing and [6 − 8]. Zinc oxide powders are also a commercially impor- then was lyophilized by 78 h at -40 oC and vacuum of 0.7 tant material utilized in paints and rubber [9]. Pa. Both the aliquots were calcined at different temperatures between 200 and 1000oC for 2 hours. The properties of ZnO depend mainly of the crystallite size Simultaneous thermogravimetric and differential ther- and morphology [10]. With the objective to prepare ZnO mal analysis (TG − DTA) measurements were performed nanostructures with desire properties, some methods are used, (TAInstrumentSDT2960). The samples were heated from such as the sol-gel process [11], hydrothermal route, chemi- room temperature up to 1100oC at 10oC.min−1 under airflow. cal vapor deposition, homogeneous precipitation and electro- deposition. Farley et al produced ZnO from zinc acetate as All samples were analyzed by X-ray diffractometry precursor, getting a high quality semiconductor oxide for var- (Rigaku;Geiger f lex3034) with CuKa radiation, 40 kV and ious technical applications [12]. The hydrothermal process 30 mA, time constant of 0.5 s and crystal graphite monochro- was used to obtain different structures of ZnO [13]. ZnO mator. Crystallite size was determined by Scherrer equation nanoparticulate was prepared by solid state reaction at low (D = 0.9l/bcosq, where D is the crystallite diameter, l is the temperature of calcination (500 oC) through thermal decom- radiation wavelength and q the incidence angle) [17]. The position, [14]. The evolution of the microstructure during the value of b was determined from the experimental integral formation of ZnO powder particles obtained by the freeze- peak width with silicon as a standard. Values were corrected drying method was investigated by Marinkovic et al [15]. for instrumental broadening. These syntheses allowed to the preparation of ZnO with a Infrared spectroscopy (Perkin − Elmer;2838) was used variety of morphologies, including nanoparticles, nanowire, to evaluate the chemical modifications of ZnO powders nanorod, nanoribbon, nanoplate, nanotube, multipods, etc heated at different temperatures. The morphology of the [16 − 18]. composites was evaluated by scanning electron microscopy (JEOL − JSM840A). The textural characteristics of the sam- In this work the precipitation process was used to ob- ples were determined through nitrogen gas adsorption (Au- tain ZnO particles by reaction between aqueous solutions of tosorb - Quantachrome Nova 1200) at liquid nitrogen tem- Zn(NO3)2 and Na2CO3. The textural, microstructural, and perature. Nitrogen gas was used with a 22-point adsorption- morphological characteristics were investigated to establish desorption cycle. The samples were outgassed at 200oC for 3 the correlation between the powder characteristics with the h before each analysis. Specific surface areas and total pore drying type and the calcination temperature. volumes were obtained by the application of the Brunauer- Emmett-Teller (BET) equation and the BJH method. The particle size of the powders was evaluated using the equation: d(nm) = 6=S · r, where 6 is the form factor for spherical or ∗Electronic address: [email protected] cubic particles, S is the superficial area in m2 g−1and r is the i i i i i “21” — 2009/5/13 — 10:15 — page 249 — #2 i i i Brazilian Journal of Physics, vol. 39, no. 1A, April, 2009 249 true density in g cm−3. with the reaction below: Zn(NO3)2:6H2O + Na2CO3 ! ZnCO3 + 2NaNO3 + 6H2O ZnCO3 ! ZnO +CO2 " III. RESULTS AND DISCUSSION Figures 2a and 2b show XRD patterns of S1 and S2 sam- ples calcined at various temperatures. The characteristic TG curve for the S1 sample (Fig:1a) shows three stages o of weigh loss accompanied by two endothermic and two peaks of the ZnO phase increase in intensity above 200 C with the calcination temperature. The S1 sample presented exothermic peaks in DTA curve. The first endothermic event o between 100 and 200oC can be associated to removal of sur- peaks characteristics of ZnO phase at 200 C and by-products face water of the sample with weight loss of 18%. The weight such as NaNO3 and ZnCO3(4 %), which decomposed grad- o ually with increasing in the temperature. The pure phase of lost of 2% in the range of 210-380 C, accompanied by one en- o dothermic and one exothermic peak, and of 2 % in the range ZnO was formed below 800 C. o The S2 sample shows peaks of ZnO phase with 21% of by- of 410-800 C is due to the nitrate and CO3 decomposition. o The broad and intense exothermic peak could be associated products such as ZnCO3 and NaNO3 between 200 and 600 C, which decomposed with the increase in the temperature, orig- with the formation of the pure ZnO phase. The obtained o residue is around 78% (pure ZnO phase). inating pure phase of ZnO above 800 C. o (a) 100 1,2 o o o o 95 1,0 o o o 1000 C o 0,8 90 0,6 o 85 0,4 800 C 80 0,2 o 0,0 600 C Intensity ( a. u.) (a. Intensity Weight (%) Weight 75 -0,2 400 oC Temperature Difference (ºC/mg) Difference Temperature 70 -0,4 TG 200 oC -0,6 x 65 ____ DTA (a) -0,8 60 70 60 50 40 30 20 10 0 200 400 600 800 1000 1200 2 θ (degrees) Temperature (ºC) o (b) 100 TG 1,2 o o ____ DTA (b) o 1,0 o 1000 C 95 o o o o 0,8 90 0,6 85 0,4 800 oC 80 0,2 0,0 600 oC Weight (%) Weight 75 Intensity ( a. u.) (a. Intensity -0,2 Temperature Difference (ºC/mg) Difference Temperature o 70 -0,4 400 C v o 65 -0,6 v v x v x x 200 C -0,8 60 70 60 50 40 30 20 10 0 200 400 600 800 1000 1200 2 θ (degrees) Temperature (ºC) FIG. 2: X-ray diffraction patterns of o = ZnO, x = NaNO3 and FIG. 1: TG/DTA curves of ZnO previously dried at 110oC, (a) con- v = ZnCO3 powders treated at different temperatures a) prepared trolled and (b) freeze drying. by controlled drying (S1) and b) prepared by freeze drying (S2). The S2 sample (Fig:1b) lost 14% weight in the range of 50- Figure 3 shows infrared spectra of the ZnO samples (S1 180oC due to surface water removal related to an endothermic and S2) treated at various temperatures. The spectra show event. The other two endothermic events between 180-250oC bands (3500 cm−1) assigned to OH− stretching vibrations of o and 580-820 C were attributed to loss of sodium nitrate (11 crystalline and adsorbed H2O. Two bands were observed at −1 %) and carbon dioxide (10 %). These events are accompanied 1512 and 1365 cm , corresponding to n3 frequency of car- by endothermic peaks in DTA curves. A broad exothermic bonate groups (C – O and C = O). In the spectra of S1 sam- event appears between 800 and 930oC, probably due to the ple, these bands disappear after treatment at 600oC and in S2 formation of the pure ZnO phase. In this case, the residue spectra only after 800oC. Bands at 1050 cm−1, 835 cm−1 and −1 obtained is around 65 %. We can describe the ZnO formation 703 cm relate to n1, n2; and n4 frequencies of the carbonate i i i i i “21” — 2009/5/13 — 10:15 — page 250 — #3 i i i 250 C.P.
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