HYDROTHERMALLY SYNTHESIZED NANOPARTICLES OF ZINC PEROXIDE ANDTHEIRSTRUCTURALDESCRIPTION

The aitn of this work is to present sorne results of the synthesis of nanoparticles of zinc oxide by rneans of a hydrothermal rnethod. Also, a description of the structural cbaracteristics of these nanoparticles will be presented. Toe nanoparticles can be used as the supported active pbase for pbotocatalytic applications, as has been recen ti y publisbed in tbe literature. Tbe bydrotbermal synthesis is based in debydrated zinc acetate as tbe precursor using ternperatures of 100 oC. Toe structural analysis of the obtained nanoparticles has been performed using X-ray diffraction tecbniques with a subsequent Rietveld refinernent ofthe pattern. Tbis allows obtaining refined values oftbe cell pararneters and tbe approxirnate crystallite sizes (10-20 nrn) and also gives an insigbt on the rnorpbology ofthe nanoparticle (truncated cube). Sorne high resolution electron rnicroscope irnages are also presented and tbeir structural results are cornpared with those obtained through the X-ray diffraction studies.


l. Introduction
Recently, there has been increasing interest in zinc peroxide (ZnO 2 ) and zinc oxide (ZnO), attracting the attention of many researchers because they have demonstrated to be very useful semiconductor materials with applications in rubber industry [1,2], in the high-tech plastic processing [3], as an oxidant in explosives and pyrotechnic mixtures [4]. Zinc peroxide can also be useful in the cosmetic [5,6] and pharmaceutical and therapeutic applications [6][7][8][9]. Stable nanoparticles of zinc peroxide can additionally be used as precursor for preparation ofZnO nanoparticles [10][11][12][13]. It is found that ZnO2 nanoparticles decompose into ZnO at about 230 ºC and is stable up to 36 GPa at ambient temperature. With a great potential in applications, ZnO nanocrystals can be used as catalyst and photocatalyst [11,[14][15][16][17] and also in solar cells [18]. ZnO could be added to zeolites in order to improve their catalytic properties. Nanocrystals, consisting of small crystallites of diameter < 100 nm, often have novel physical and chemical properties differing from those of the corresponding bulle materials [ 19]. The unique properties of nanocrystalline materials open up the general question of how crystallite size and shape affect the structural stability, and numerous efforts have been focused on controlling the sizes and shapes of inorganic nanocrystals, since they modify the surface area of the crystallites generally improving, among others, their catalytic properties. As mentioned above, they can be useful in many fields but little work has been done in sorne areas [9,20]. Therefore, it is of great importance to produce high-quality ZnO/ZnO nanocrystalline materials for basic research as well as industrial and high-technology applications [21,22]. Divers methods have been used in the preparation of zinc peroxide and zinc oxide, chemical and physical methods such as the hydrothermal one, laser ablation, sol-gel method precipitation in alcoholic medium, polyol synthesis, etc. In addition, othermethods have been applied for the synthesis of ZnO nanostructures such as sputtering, molecular beam epitaxy, hydrothermal and electrodeposition [23][24][25][26][27]. In this work, nanocrystalline powders of zinc peroxide (ZnO2), a precursor of zinc oxide (ZnO), have been prepared by means ofa simple and low cost hydrothermal process of synthesis from zinc acetate dehydrates. These powders were structurally characterized having as propose to know their crystal size as well as their crystal shape. The powder samples were characterized mainly for running their X-ray diffraction pattems with a good statistic in order to calculate their crystallographic parameters by refining the pattem. Conditions of preparation and thermal treatment are related with the microstructure including nanocrystal size and shape. A pure cubic phase ZnO2 was identified and after it was refined by using the Rietveld method to calculate sorne microstructural features of the nanocrystalline powders.

Experimental
In a typical synthesis 585.4 mg ofzinc acetate dihydrate [Zn(CH 3 COO)z-2HP, 99.6%, Baker] was dissolved in 80 ml of ultrapure deionized water (E-pure Bamstead system, 18 MO cm) under vigorous stirring until to obtain a transparent solution. After, 4 ml ofhydrogen peroxide (H 2 O 2 sol. 30%, Baker) was added. Then, the solution was heated up to 100 ºC, and the hydrothermal reaction was conducted through 1 O h. Once the solution was cooled to room temperature, the powder material was separated from the liquid phase by centrifugation and washed with deionized water several times. Structural and morphological characterization was performed by X-ray diffraction (XRD, Bruker Advance D-8) and transmission electron rnicroscopy (TEM, Philips Tecnai F20, 200 kV, point resolutionof2.3 Á).
The X-ray powder diffraction pattem of ZnO2 sample was recorded in 0----B configuration at room temperature using CuKaradiation (1.5418 A). Diffraction intensitywas measuredin the 2 ofl0º -110º; setting 0.02º and4.0 s as step width and step counting time, respectively. The crystalline structure of the nanoparticles was refined through Rietveld method by means ofFULLPROF98 code [28]. The diffraction peaks were fitted using pseudo-Voigt functions, the modified TCH (Thompson-Cox-Hastings) one [29], containing the refinable parameters related to average crystallite size and shape [30,31 ]. Sixteen variables were refined, including scale factor, refinable atom fractional coordinates, lattice parameters, isotropic thermal displacements, crystallite shape and size parameters, and values of strains. The modeled crystal structure was based in the available crystallographic data for cubic-ZnO 2 phase [32]. Finally, on using spherical harmonics functions, representative shape of the nanocrystallites was proposed. TEM specimens were prepared by dispersing and subsequent drying a drop of solution on a copper grid (3 mm in diameter) covered with an amorphous carbon film. For comparison HREM simulated images have been obtained using the electron diffraction dynamical theory (multislice approach) [33] and sorne insights on the structural characteristic of nanometric ZnO2 particle is withdrawn. Figure 1 shows the experimental and refined X-ray pattems of analyzed sample. The XRD pattem reveals the single phase and crystal structure of cubic-ZnO 2 nanoparticles (Space group: Pa-3, No. 205), all diffraction peak positions match with those reported in the JCPDS card No. 13-0311 for cubic-ZnO 2 powders. Since no appreciable difference of relative XRD peak intensities between our diffraction pattem and such reported for powder samples, it is suggested that neither texture or preferential orientation were induced in preparing the crystalline samples for XRD; also that none preferential growth is present in the nanopowders. No diffraction peak attributed to wurtzite-ZnO can be observed in XRD pattem of as-synthesized sample. These results, together with the slight broadening observed in the reflections, suggest that the synthesized sample is constituted by a collection ofZnO 2 nanocrystals, which was confirmed by TEM analysis, as it will be shown below. Toe marks shown below indicate the XRD peak position of cubic ZnO2 structure. Table 1 shows the refined parameters calculated form Rietveld method. After the refinement procedure, the obtained values were: 4.89605 (7) A for the cubic lattice parameter a ( =b=c ), 15 .4( 6) nm for the average crystallite size and 18 % for the maximum microstrain. The fitting or quality of the refinement was measured by, among others, the residual function Rwp, which in these crystalline samples was around 8.9%.  (2) With these microstructural results it is possible to draw a model structure of the cubic unit cell utilized to perform the refinement. Figure 2 shows the model of the cubic unit cell with the obtained parameters after refinement. This model, as known in the literature [32], is constituted by an array ofirregular octahedra formed by a Zn ion at the center surrounded by four O ions located one at each comer of the polyhedron. lt is important to mention that the atom positions undergo only a very small change with respect to the theoretical structure. On using the microstructural results summarized in table I, it also was possible to model the hydrothermally grown ZnO2 nanoparticles, i. e., by using the known cubic cell model but with the precise parameters obtained in the synthesis and the refinement. Refinement procedure includes sorne parameters which can be used to simulate the shape and size ofthe crystallites which constitutes the powder sample. The reported size ofthe crystallite is an average of its diameter in all the planar directions in the crystallite. Shape of the particle is drawn with our software elaborated in the free programming text editor medit; it was made starting from a spherical model, the zero order, and modifying it from the inclusion of spherical harmonic of higher order. Series of spherical harmonics functions are applied to rebuild the shape of such representative nanocrystallite (Figure 3). lt can be observed that the final shape of the simulated crystallite could be related with a truncated cube, where the surface planes are the (100) and the truncated ones are the (111 ). lt is possible to appreciate relative similitude between the simulated morphology and the observed in TEM images.  . Insert 4d shows the supercell used for the simulation. It is interesting to observe that the nanocrystals, nearly spherical, have a faceted morphology however; this morphology could be related with the morphology ofthe proposed shape ofthe ZnO 2 nanocrystals ( Figure 3). However, a more detailed HR-TEM analysis ofthe set ofsingle ZnO2 nanocrystals was not possible to do.

Results and discussion
It will be necessary a study about the catalytic response of this material, however we can advance that the shape but especially the size in the nanometric regime, both features, increase the surface area and this should favor their catalytic properties.

Conclusions
The hydrothermal chemical route is a facile, low cost and reliable method to synthesize high purity cubic-ZnO 2 nanoparticles, with faceted crystals and mean diameter of around 15 nm. Simulated morphology, structural properties and the unit cell parameters of synthesized cubic-ZnO 2 nanoparticles have been calculated using the Rietveld method. U se of this method in the characterization of these materials is the main aim of our work at this point. The obtained results are nearly consistent with those observed by means of transmission electron microscopy (TEM). We believed that, according with the cited literature, our ZnO 2 nanopowder can be easily used as precursor material to synthesize ZnO nanoparticles with similar morphology and size. This kind of morphologies and this nanosize offer a higher area for the catalytic activity of the material. Our work is right now addressed to study the catalytic behavior ofthe zinc nano-oxides specially by impregnating zeolites.