EFFECT OF DIFFERENT PRECURSORS IN THE CHEMICAL SYNTHESIS OF ZnO NANOCRYSTALS

Size: px

Start display at page:

Download "EFFECT OF DIFFERENT PRECURSORS IN THE CHEMICAL SYNTHESIS OF ZnO NANOCRYSTALS"

Cornelius McCarthy
5 years ago
Views:

1 EFFECT OF DIFFERENT PRECURSORS IN THE CHEMICAL SYNTHESIS OF ZnO NANOCRYSTALS M. Gusatti *1, G. S. Barroso 1, C. E. M. Campos 2, D. A. R. Souza 1, J. A. Rosário 1, R. B. Lima 1, L. A. Silva 1, H. G. Riella 1, N. C. Kuhnen 1 1 Departamento de Engenharia Química e Engenharia de Alimentos, Programa de Pós-Graduação em Engenharia Química, Universidade Federal de Santa Catarina, Campus Universitário, Florianópolis, Santa Catarina, Caixa Postal 476, , Brazil 2 Departamento de Física, Universidade Federal de Santa Catarina, Campus Universitário, Florianópolis, Santa Catarina, , Brazil ABSTRACT This work aims to evaluate the effect of ZnCl 2 and Zn(NO 3 ) 2.6H 2 O precursors in the synthesis of ZnO nanocrystals. The materials were obtained at a temperature of 90 C by a simple solochemical route. The resulting samples were characterized with respect to the determination of the formed phases, particle size and morphology, using the techniques of X-ray diffraction (XRD) and transmission electron microscopy (TEM). These characterization techniques confirmed that the sample obtained with Zn(NO 3 ) 2.6H 2 O has hexagonal crystal structure of ZnO and dimensions in the nanoscale. However, the material formed with ZnCl 2 was composed of a mixture of the ZnO phase and another correspondent to the Zn 5 (OH) 8 Cl 2.H 2 O phase. For both precursors, the predominant morphology of the obtained ZnO nanocrystals is rod like structure. Keywords: nanocrystalline materials, zinc oxide, solochemical method. INTRODUCTION Nanotechnology is one of the areas of knowledge that most attract the attention of researchers worldwide because of the many innovations created by reducing the size of the materials to the nanoscale. Such innovations include very peculiar properties, different even from the material itself on a larger scale. A material is nanometric when its structural components have at least one dimension in the nanometer scale. Due to their extraordinary mechanical, electrical, magnetic, optical and chemical properties, zinc oxide is one of the most studied materials in nanotechnology. ZnO 1395

2 has hexagonal wurtzite structure, lattice parameters equal to = Å and c = Å and belongs to space group P6 3 mc 1. This material stands out among semiconductors for its large band gap (3.37 ev) associated with a high exciton binding energy (60 mev) 2-3. Reducing the size of the ZnO to the nanoscale significantly changes its properties because they depend on the size, orientation and morphology of the particles 4. This material has many technological applications such as opto-electronic devices, catalysts, cosmetics, gas sensors, varistors and pigments 5-8. The synthesis of ZnO nanostructures can be accomplished by physical and chemical routes. However, chemical methods are more suitable for the production of ZnO on an industrial scale 9 due to low cost and efficiency in obtaining nanostructures with uniform size and morphology 10. Among the chemical methods, the solochemical technique stands out for its simplicity, quickness and low cost in the production of ZnO nanocrystals with high quality. Moreover, this method uses milder reaction conditions than those employed by most chemical methods proposed in the literature 11. The method involves a reaction between a heated alkaline solution and another containing the precursor at room temperature. Therefore, a decomposition reaction of the reactants is initiated causing the immediate formation of ZnO nanocrystals 12. In this work, different materials were obtained by solochemical processing, at a temperature of 90 C, using the precursor solutions of zinc chloride (ZnCl 2 ) and zinc nitrate hexahydrate (Zn(NO 3 ) 2.6H 2 O) at a concentration of 0.7 mol.l -1. The crystal structure of the samples was identified by X-ray diffraction. The size and morphology of particles were determined by transmission electron microscopy. EXPERIMENTAL PROCEDURE In this study, samples were prepared with different precursor solutions (Zn(NO 3 ) 2.6H 2 O and ZnCl 2 ) at a concentration of 0.7 mol.l -1 mixed with an 1.0 M solution of sodium hydroxide (NaOH). These reagents are of analytical grade and were used without further purification. The experimental arrangement and procedure for the obtainment of samples by solochemical processing is simple and identical for both precursors. For the production of the samples were used basically a reactor, a separation funnel and a 1396

3 magnetic stirrer with controled temperature and agitation. The primary solution was prepared by dissolving the precursor (Zn(NO 3 ) 2.6H 2 O and ZnCl 2 ) in 100 ml of deionized water at room temperature. The alkaline solution was obtained by dissolving sodium hydroxide in 100 ml of deionized water. This alkaline solution was placed in the reactor and heated to 90 C under constant stirring. At this temperature, the primary solution is added slowly into the reactor for 1 h under vigorous stirring. After the addition of the precursor solution, the suspension formed in the reactor was kept for over two hours under vigorous stirring at the temperature of 90 C. At the end of the reaction, the formed material was filtered, washed several times with deionized water and dried in a vacuum oven at 65 C for a few hours. The material characterization was performed by X-ray diffraction, using a diffractometer brand PanAnalytical X Pert PRO Multi-Purpose with radiation Cu Kα (λ = Å) operating at 40 kv and 30 ma of electric current. The 2θ variation was employed with a 0.05 degrees step and a time step of 1 second. The morphology and particle size of the products obtained were analyzed by transmission electron microscopy using a JEOL JEM 1011 microscope operating at 100 kv. RESULTS AND DISCUSSION The crystal structure of the sample formed at 90 C by chemical reaction between NaOH and Zn(NO 3 ) 2.6H 2 O was examined by XRD (Fig. 1). In the same figure, the diffraction pattern of ZnO supplied by ICSD database (Card No ) is shown for comparison. The diffractogram of the sample reveals that all peaks correspond to the characteristic peaks of hexagonal wurtzite structure of ZnO with space group P6 3 mc and lattice parameters a = 3.25 Å and c = 5.20 Å, as reported in the ICSD database. The absence of extra peaks, which could be related to impurities, indicates that the ZnO samples produced with Zn(NO 3 ) 2.6H 2 O are of high quality. Thus, the experimental diffraction patterns confirm that the proposed route using Zn(NO 3 ) 2.6H 2 O precursor is suitable for production of ZnO material. 1397

4 Figure 1. XRD pattern of the sample prepared at 90 C with Zn(NO 3 ) 2.6H 2 O by solochemical processing. The ICSD card No is also shown for comparison. The crystal structure of the sample formed at 90 C by chemical reaction between NaOH and ZnCl 2 was analyzed by XRD (Fig. 2). The diffractogram shows that the products formed exhibit the characteristic diffraction peaks of ZnO with hexagonal wurtzite structure (space group P6 3 mc). These peaks are probably related to the crystalline planes (100), (101), (110) and (103) of ZnO. However, most diffraction peaks of the samples are not explained by the diffraction pattern of ZnO. The positions of these peaks coincide well with those of the hexagonal phase of the crystal Zn 5 (OH) 8 Cl 2.H 2 O with space group R-3mH and lattice parameters a = 6.34 Å and c = Å (ICSD, Card No ). According to previous studies, depending on the concentration of precursor solution, the nanostructures prepared by chemical or electrochemical methods may exhibit in their composition ZnO particles mixed with other phases such as Zn(OH) 2 and Zn 5 (OH) 8 Cl 2.H 2 O The presence of these phases in the final composition of the material indicates that the conversion of reactants in ZnO desired product was not complete. 1398

5 Figure 2. XRD patterns of (a) ICSD card No and (b) ICSD card No (c) XRD pattern of the sample prepared at 90 C with ZnCl 2 by solochemical processing. Studies indicate that the compound Zn 5 (OH) 8 Cl 2.H 2 O is formed when the Zn 2+ concentration is higher than 0.01 M 15, which is in agreement with the results obtained in this study, that show the Zn 5 (OH) 8 Cl 2.H 2 O produced with a 0.7 M solution of ZnCl 2. Furthermore, the formation of this compound can be caused also by the low reaction temperature used in this preparation procedure. The literature shows that the Zn 5 (OH) 8 Cl 2.H 2 O can be completely decomposed into ZnO when calcinated at a temperature equal or higher than 500 C 4. The technique of transmission electron microscopy was used to examine the morphological characteristics of the product synthesized at 90 C with Zn(NO 3 ) 2.6H 2 O (Fig. 3). The image shows the presence of short nanoprisms and nanorods. These particles have an average length of nm and an average diameter of nm. The sample synthesized under these conditions has particles with uniform size. 1399

material synthesized with ZnCl 2 at 90 C (Fig. 4).

6 Figure 3. TEM image of the ZnO produced at 90 C using Zn(NO 3 ) 2.6H 2 O. The technique of transmission electron microscopy was used to examine the size and morphology of the material synthesized with ZnCl 2 at 90 C (Fig. 4). In the TEM image, can be clearly observed the presence of nanorods, which is one of the ZnO typical morphologies. These nanostructures have an average particle diameter of about 23 nm. 1400

7 CONCLUSIONS Figure 4. TEM images of the material produced at 90 C with ZnCl 2. In this work, ZnO nanocrystals were prepared by an economical and simple solochemical technique using aqueous solutions of zinc nitrate hexahydrate and sodium hydroxide at 90 C. The ZnO products formed by this method have high quality. The X-ray diffraction results confirmed the synthesis process efficiency, showing that the ZnO crystals are single crystalline with hexagonal wurtzite structure. The image from transmission electron microscopy showed that the powder has the nanometric prism-like and rod-like morphologies. The XRD results indicated that the use of ZnCl 2 precursor alters the composition of the final product, forming ZnO nanocrystals mixed with Zn 5 (OH) 8 Cl 2.H 2 O crystals. The ZnO nanostructures obtained have rod-like morphology. The average diameter of ZnO particles produced at 90 C is of approximately 23 nm. Therefore, the precursor used represents a large influence in obtaining pure ZnO nanostructures and in the efficiency of the solochemical technique. ACKNOWLEDGMENT The authors would like to acknowledge the Central Laboratory of Electron Microscopy (LCME) and X-ray Diffraction Laboratory (LDRX) of UFSC by TEM and XRD measurements. REFERENCES 1. Liu R, Vertegel A A, Bohannan E W, Sorenson T A, Switzer J A. Epitaxial Electrodeposition of Zinc Oxide Nanopillars on Single-Crystal Gold. Chem. Mater. 2001; 13: Kubota J, Haga K, Kashiwaba Y, Watanabe H, Zhang B P, Segawa Y. Characteristics of ZnO whiskers prepared from organic-zinc. Appl. Surf. Sci. 2003; 216:

8 3. Fu Y-S, Du X-W, Sun J, Song Y-F, Liu J. Single-crystal ZnO cup based on hydrothermal decomposition route. J. Phys. Chem. 2007; 111: Duan J-X, Wang H, Huang X-T. Synthesis and characterization of ZnO ellipsoidlike nanostructures. Chin. J. Chem. Phys. 2007; 20: Liu Y, Zhou J, Larbot A, Persin M. Preparation and characterization of nano-zinc oxide. J. Mater. Process. Tech. 2007; 189: Viswanatha R, Sapra S, Satpati B, Satyam P V, Dev B N, Sarma D D. Understanding the quantum size effects in ZnO nanocrystals. J. Mater. Chem. 2004; 14: Kanade K G, Kale B B, Aiyer R C, Das B K. Effect of solvents on the synthesis of nano-size zinc oxide and its properties. Mater. Res. Bull. 2006; 41: Duffy G M, Pillai S C, Mccormack D E. A novel processing route for the production of nanoparticulate zinc oxide using an isophthalate precursor. Smart Mater. Struct. 2007; 16: Wu C, Qiao X, Chen J, Wang H, Tan F, Li S. A novel chemical route to prepare ZnO nanoparticles. Materials Letters. 2006; 60: Hu Y, Chen H J. Preparation and characterization of nanocrystalline ZnO particles from a hydrothermal process. J. Nanopart. Res. 2008; 10: Gusatti M, Rosario J A, Barroso G S, Campos C E M, Riella H G, Kuhnen N C. Synthesis of ZnO nanostructures in low reaction temperature. Chemical Engineering Transactions. 2009; 17: Vaezi M R, Sadrnezhaad S K. Nanopowder synthesis of zinc oxide via solochemical processing. Materials & Design. 2007; 28: Chatterjee A P, Mitra P, Mukhopadhyay A K. Chemically deposited zinc oxide thin film gas sensor. J. Mater. Sci. 1999; 34: Rakhshani A E. Boron-doped ZnO films grown by successive chemical solution deposition. J. Phys. D: Appl. Phys. 2008; 41: Pradhan D, Leung K T. Controlled growth of two-dimensional and onedimensional ZnO nanostructures on indium tin oxide coated glass by direct electrodeposition. Langmuir. 2008; 24:

CHAPTER 6. BLUE GREEN AND UV EMITTING ZnO NANOPARTICLES SYNTHESIZED THROUGH A NON AQUEOUS ROUTE

CHAPTER 6. BLUE GREEN AND UV EMITTING ZnO NANOPARTICLES SYNTHESIZED THROUGH A NON AQUEOUS ROUTE 71 CHAPTER 6 BLUE GREEN AND UV EMITTING ZnO NANOPARTICLES SYNTHESIZED THROUGH A NON AQUEOUS ROUTE 6.1 INTRODUCTION Several techniques such as chemical vapour deposition, electrochemical deposition, thermal