Synthesis of Undoped ZnO Nanoparticles by Spray

Advances in Science and Technology Vol. 45 (2006) pp. 237-241 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland

Synthesis of Undoped ZnO Nanoparticles by Spray Pyrolysis C. Rossignol1, M. Verelst1 J. Dexpert-Ghys1 and S. Rul2 1

CEMES - CNRS - Université Toulouse III, 29 J. Marvig, BP 94347, 31055 Toulouse, France 2 Marion Technologies, Cap Delta - Parc Technologique Delta Sud, 09340 Verniolle, France [email protected], [email protected], [email protected], [email protected] Keywords: ZnO, nanoparticles, spray-pyrolysis Abstract. During the past few years, a pilot-scale spray pyrolysis set-up able to produce large quantities of submicronic powders (1kg/day) has been assembled in our laboratory. During the process, droplets of a precursor solution are dried and decomposed to the required compound. The presence of an additional soluble flux in the precursor solution permits to obtain agglomerate - free nanoparticles after washing the product. Therefore, pure zinc oxide nanoparticles have been successfully synthesized by adding lithium or sodium nitrates to the initial zinc nitrate solution. The nanoparticles have been characterized by X-ray diffraction, field-emission scanning electron microscopy, laser scattering size analyzer. The influence of the precursor solution composition and of the operating parameters on the morphology and the average size of ZnO nanoparticles is discussed.

Introduction Spray-pyrolysis is a processing technique often employed to synthesize oxide microor nano-powders: some very nice examples have been recently reviewed by K. Okuyama et I.W Lenggoro [1]. In our group, a laboratory scale set-up has been employed for the synthesis of sub-micronic mixed oxide powders [2, 3, 4]. A pilot-scale set-up has also been assembled and the processing parameters of this set-up have been employed to undertake the modelisation of the physico-chemical processes occurring during the synthesis [5, 6]. In the present work we describe the synthesis of sub-micronic and nanometric zinc oxide powders with the lab-scale and the pilot-scale set-ups. The sub-micronic powders were obtained from zinc nitrate and the nanometric ones from zinc – lithium (or sodium) nitrates mixtures. We discuss our observations considering the different stages of the process, namely: the evaporation of the solvent (essentially water), the concomitant metals nitrates precipitation, and the successive decompositions (metal nitrates to metal oxides) and crystallization-densification. Experimental In the laboratory scale (LS) set-up, the spray is generated by ultrasonic vibrations of a piezoelectric pellet (2.4 MHz) immersed in a solution of appropriate salts. The droplets are collected by an air stream (0.3 m3/h) and drawn into a drying area maintained at 100-120°C, then a decomposition-densification area where the temperature may be adjusted up to 1200°C. Collection of the dried powders is achieved by an electrostatic collector. The residence time of the droplets in the hot zone is 1 sec. In the pilot scale (PS) set-up, an ARECO spray generator is employed; it comprises four pellets vibrating at 1.6 MHz. The air flow is fixed at 6 m3/h. The drying column is vertical and the decomposition/densification is horizontal, both have 20cm internal diameter, and 1.8m long. The heating system is composed of juxtaposed electrical furnaces of 12 kW of All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 193.49.32.252-30/11/06,09:57:09)

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total power. Thermocouples are put on the outer wall of the reactor and allow adjusting the temperature up to 250°C in the drying zone and up to 1000°C in the decomposition/densification zone. The solid phase is separated from the vapor phase through a Donaldson - UMA40 unit, equipped with polyester filtering bags. The residence time of the droplets in the hot zone is 10 sec. The final powders are characterized by their X-Ray diffraction recorded on a Seifert C3000 diffractometer. The crystallite size of ZnO particles is calculated from the half-width of diffraction peaks using Scherrer's equation. The powder size distribution is performed by laser scattering on a Master-Sizer S (Malvern) in water suspension after ultrasonification in order to obtain well dispersed suspensions. The morphology is observed by a field-emission scanning electron microscope (Jeol JSM 6700F). Results Experiments at the laboratory scale. The synthesis conditions, i.e. the precursor solution and the temperature of the decomposition area are reported in Table 1, as well as the crystalline phases identified.

Table 1: Experimental conditions. Decomposition zone temperatures TD, and solutions compositions in mol.L-1 of metal nitrates in (90% water + 10% ethanol). "Washed" refer to the samples after washing in water. Name of the identified crystalline phases and average crystallite size.

/

Li

Sample

TD(°C)

[Zn(NO3)2] (mol.L-1)

[LiNO3]/ [NaNO3]

SP SP-Li-0.9

SP-Na-0.75-2

600 600 washed 600 washed 600 washed 600 washed 600 washed 600 washed 650

0.6 0.1 / 0.25 / 0.5 / 0.1 / 0.25 / 0.5 / 0.25

/ 0.9 / 0.75 / 0.5 / 0.9 / 0.75 / 0.5 / 0.75

SP-Na-0.75-3

700

0.2

0.75

SP-Li-0.75 SP-Li-0.5 SP-Na-0.9 SP-Na-0.75

Na SP-Na-0.5

XRD phase ZnO ZnO+Li2CO3 ZnO ZnO+Li2CO3 ZnO ZnO+Li2CO3 ZnO ZnO+NaNO3 ZnO ZnO+NaNO3 ZnO ZnO+NaNO3 ZnO ZnO+NaNO3+NaOH ZnO+NaNO3+NaOH +Na2Zn3(CO3)4 ?

Crystallite size (nm) 20±2 23±2 25±3 27±3 19±2 20±2 22±2

Starting from pure zinc nitrate, a pure zincite ZnO phase is obtained by pyrolysis at 600°C. Figure 1 (left) is a SEM imaging of the powder. The overall spherical shape of the droplets is maintained in the solid phase. Each sphere is made by a complex arrangement of several crystals that look more or less like platelets. These spheres are not hollow spheres, since we don't observe a specific variation of the density along one radius, the spheres cannot be described properly as denses ones but rather as "roses des sables". The average particle diameter from laser size analysis after regular sonification is 1.8µm. These spherical particles

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can to some extent be broken into smaller aggregates after being submitted 60 minutes to higher power sonification (average diameter 0.85µm). Starting from mixed metallic nitrates solutions, the as-sprayed dry solids are composed of zinc oxide and one or more additional phases (Table 1). The principle of the method is to choose the additional metallic salts, here lithium or sodium nitrate, so that the final phase(s) containing these metals will be removed by washing whereas insoluble zinc oxide particles will be isolated from the liquid after centrifugation. Actually one may conclude from Table 1 that this is done for all experimental conditions with the lithium nitrate and for three of the tested conditions with the nitrate salt. At higher decomposition temperatures, it appears that a mixed oxide (tentatively identified as Na2Zn3(CO3)4 ) is produced, so complete removing of this component by washing is impossible. The as-sprayed powders are made of rather big vesicles (center in Fig. 1). After aqueous washing and drying on the contrary much smaller particles are observed as shown in Fig. 1, right. An example of the X-Ray diffraction data for sample SP-Li-0.75 as-sprayed and after washing is given in Fig. 2. The ZnO particles exhibit a rather spherical morphology and a rather narrow sizes distribution that has been determined by counting on the SEM images to be centered around 85nm for the sample shown in Fig.1. Each particle is well crystallized zinc oxide, but still is made of several monocrystalline domains, the size of which is on average 25±3 nm.

Fig. 1: SEM images. Left : sample SP (cursor=1µm). Center : sample SP-Li-0.75 (cursor=10µm). Right : sample SP-Li-0.75 washed (cursor = 100nm)

Fig. 2: X-Ray diffractograms of sample SP-Li-0.75 before and after washing;

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Experiments at the pilot scale. Synthesis performed with zinc nitrate alone at TD = 600°C results in ZnO and several other phases coming from the incomplete decomposition of the nitrate : Zn1(NO3)(OH)(H2O), Zn3(NO3)4(OH)2, Zn5(NO3)8(OH)2(H2O)2. The synthesis performed at TD = 650°C still exhibits the last one at a very weak amount (estimated a few% with respect to ZnO. For scaling up the synthesis of ZnO nanopowders, the addition of sodium nitrate is preferred to the lithium analogous for cost reasons. But here also we still encounter problems that can be deduced from Table 2 from which it appears that the conditions necessary to obtain a mixture of insoluble ZnO and of one or several soluble phase have till now not been achieved. Table 2: Experimental conditions. Decomposition zone temperatures TD, and solutions compositions in mol.L-1 of metal nitrates in (90% water + 10% ethanol). Name of the identified crystalline phases Zn1 : Zn1(NO3)(OH)(H2O), Zn3 : Zn3(NO3)4(OH)2, Zn5 : Zn5(NO3)8(OH)2(H2O)2. Na2Zn3 : Na2Zn3(CO3)4 Test

TD

SP1 SP2 SP-Na_1 SP-Na_2 SP-Na_3

600 650 600 650 700

[Zn(NO3)2] (mol.L-1) 0.6 0.6 0.25 0.25 0.25

[NaNO3] / / 0.75 0.75 0.75

XRD ZnO + Zn1 + Zn3 + Zn5 ZnO + Zn5 ZnO + Zn5 + NaNO3 ZnO + Zn5 + NaNO3 + NaOH ZnO + Zn5 + NaNO3 + NaOH + Na2Zn3 ?

Discussion and perspectives. This work proves that the synthesis of nanometric powders of ZnO based on a spray-pyrolysis technique is possible by spraying mixed solutions of nitrates that decompose into multiphase dried powders during the process. If the precursor solution composition is properly chosen and the synthesis parameters well controlled, the insoluble nanometric zincite powder may be separated from the secondary (unwanted) soluble phases by washing. In order to get the best compromise between the average size of zinc oxide particles and the overall yield of the experiment to produce reasonable quantities of nanometric powder, several operating conditions have been tested at the laboratory scale. The best results have been achieved by spraying an initial solution made of 0.25 Zn(NO3)2 / 0.75 Li(NO3) and decomposing it at TD = 600°C. Good results are also obtained with 0.25 Zn(NO3)2 / 0.75 Na(NO3) at TD = 600°C, which seems a better solution if one considers the cost of the process. This synthesis method is similar to the so-called SAD (Salt Assisted process) described in [1], it appears nevertheless that each chemical system must be described and considered individually if one wants to understand better the rather complex phenomena that occur during the stages of solvent evaporation, precipitations of the various components, and decomposition into multiphasic samples of simple oxides or of mixed oxides. This must be done to control the operating parameters especially when considering a possible scaling-up of the whole process. Our perspective is to progress in this direction, then to evaluate the properties of the ZnO nanopowders prepared by the liquid phase spray-pyrolysis route with the properties of the same powders obtained via gaz phase reactions at higher temperatures. Acknowledgements. This work has been supported by the french ANR RNMPPRONANOX) and by the Midi-Pyrénées Council. The authors greatly acknowledge Drs Brigitte Caussat and Nicolas Reuge for helpful discussions.

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References [1] K. Okuyama and I.W. Lengorro: Chem. Eng. Science 58 (2003), p.537 [2] D. Veilleux, N. Barthelemy, J.C. Trombe and M. Verelst: J. of Materials Science 36 (2001), p. 2245 [3] S. Alavi, B. Caussat, J.P. Couderc, J. Dexpert-Ghys, N. Joffin, D. Neumeyer and M. Verelst: Advances in Science and Technology, 10th International Ceramics Congress, CIMTEC 2002, p. 417 [4] N. Joffin, J. Dexpert-Ghys, M. Verelst, G. Baret and A. Garcia: J. of Luminescence 113, 249-257 (2005) [5] N. Joffin: "Synthèse par pyrolyse d’aérosol et caractérisation de luminophores : Y2O3 :Eu3+ et Zn2SiO4 :Mn2+ pour applications dans les panneaux à plasma. " Thesis, INP Toulouse (france), september 2004. [6] N. Reuge, B. Caussat, N. Joffin, J. Dexpert-Ghys and M. Verelst: Proceedings of Partec 2004, Nuremberg, Germany (2004). [7]N. Reuge, N. Joffin, J. Dexpert-Ghys, M. Verelst, H. Dexpert and B. Caussat: submitted.