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1 Available online at International Journal of Nanomaterials and Biostructures Universal Research Publications. All rights reserved Original Article Optical properties of capping agents mediated lead oxide nanoparticles via facile hydrothermal process S. Gnanam*, V. Rajendran Department of Physics, Presidency College, Chennai , Tamilnadu, India. Telephone no: , Fax: Corresponding author address: Abstract Received 14 September 2011; accepted 03 October 2011 Capping agents such as citric acid, EDTA and PEG mediated lead oxide nanoparticles were synthesized by using lead nitrate and oxalic acid as precursors via hydrothermal process at 450 C. The obtained nanoparticles were investigated using XRD, FTIR, SEM, TEM, UV-vis absorption and PL emission spectroscopy. The spherical-like morphology was observed from TEM studies with size in the range from 25-30nm. The quantum confinement effect was illustrated by the blue-shift in the optical absorption. The PL emission spectrum revealed a strong blue emission band centered at 442nm, which might be related with oxygen vacancies. A rocky-stone, slab and spherical with some small rods-like morphologies of the prepared lead oxide samples were observed in the SEM studies Universal Research Publications. All rights reserved Key words: Lead oxide; nanoparticles; hydrothermal process; capping agents; optical properties Introduction Nanostructured materials have attracted the attention of researchers not only by their unique chemical and physical properties but also by their potential application in many fields, which has stimulated the search for new synthetic methods for these materials. These materials consist of small grains with sizes below 100 nm. Semiconductor nanocrystals have attracted a great deal of attention from researchers for both their fundamental size-dependent optoelectronic properties and their wide range of applications [1]. Lead element has a lot of oxide forms including PbO (, and amorphous), Pb 2 O 3, Pb 3 O 4, PbO 2 (, and amorphous) [2]. Due to their unique properties, lead oxides have wide applications such as network-modifiers in luminescent glassy materials [3], pigments [4], gas sensors [5], paints [6], storage batteries [7, 8] and nanoscale electronic devices [9]. Among lead oxides, lead dioxide has been studied more than other forms. Bervas et al. synthesized the nanosized - PbO 2 and - PbO 2 at room temperature by a low-cost solution technique involving the chemical oxidation of lead nitrate with ammonium persulfate in concentrated alkaline medium [10]. Konstantinov et al. used the spray pyrolysis method for the synthesis of nano-structured lead oxide. They studied the effect of various spray pyrolysis parameters, such as temperature, solution concentration and solution flow rate on the morphology, crystallization process, crystal size and specific surface area [11]. Several wet-chemical processes have been developed to synthesize lead oxide Among them, hydrothermal synthesis has been a major focus in the preparative investigations due to the advantages over other techniques, such as less-cost, mild temperature, potential controllability over size and morphology, which play key roles in tailoring the properties of nanomaterials. Under hydrothermal conditions, many starting materials can undergo quite unexpected reactions, which are often accompanied with the formation of nanoscopic morphologies that are not accessible by traditional routes. In the present work, we tried to introduce a new easy method for preparing nanostructured lead oxide. In this method, the capping agents like citric acid, EDTA and PEG was used as a useful tool to prevent from agglomeration and growth of nano-particles, and also, it was used as a structure director additive. The 12

2 presence of various capping agent modifies the structural, morphological and optical properties of the lead oxide Experimental Procedure All the chemical reagents are analytical grade and without further purification before utilization. Lead (II) nitrate (0.1M) was dissolved in 100ml of distilled water to form a transparent solution. 0.2M of oxalic acid dissolved in 100ml of distilled water was added dropwise to the above solution under magnetic stirring. Then, 5ml polyethylene glycol (PEG) was added to the above solution. The solution was stirred for about 12hrs and a white precipitate was isolated which was washed with water and ethanol several times to remove impurities. The white precipitate of lead oxalate was dried at 120 C for 24hrs. Finally, the dark orange colored lead oxide nanoparticles were obtained, when the white precipitate was calcined at 450 C for 2h. The same procedure was followed by the preparation of Ethylenediaminetetraacetic acid (EDTA) and citric acid capped lead oxide The reaction equation of the synthesis of lead oxalate and lead oxide are shown in equation (1). Pb(NO 3 ) 2 (aq) + H 2 C 2 O 4.2H2O (aq) [Pb(O 4 C 2 )] (s) + 2HNO 3 (aq)+2h 2 O (aq) (1) [Pb(O 4 C 2 )] (s) PbO + CO 2 + CO The XRD pattern of the calcined lead oxide nanoparticles was recorded by using a powder X-ray diffractometer (Schimadzu model: XRD 6000 using CuKα (λ=0.154 nm) radiation, with a diffraction angle between 20 and 70º. The Fourier transform infrared (FTIR) spectra of the samples were recorded using a Bruker IF3 66V spectrophotometer by the KBr pellet method in the wave number range of cm 1. Scanning Electron Microscopy (SEM) studies were carried out on JEOL, JSM Transmission electron microscope (TEM) image was taken using an H-800 TEM (Hitachi, Japan) with an accelerating voltage of 100kV. UV-Vis absorption spectrum for the samples recorded using a Varian Cary 5E spectrophotometer in the range of nm for the powder samples in nujol mode. The photoluminescence (PL) emission spectra of the samples were recorded by Fluoromax- 4 spectrofluorometer with a Xe lamp as the excitation light source. Results and discussion Fig.1 shows the XRD pattern of lead oxide nanoparticles using different capping agents. Lead oxalate nanoparticles were observed by using citric acid as a capping agent as shown in Fig.1a. Large particles with smooth surfaces are noted as the citric acid capped sample. On the other hand, XRD diffraction peak of the powder was broad and had low intensity in the case of EDTA and PEG as capping agent (Fig.1b and 1c). Comparing the 2 peaks at Fig.1 Powder XRD pattern of (a) citric acid (b) EDTA and (c) PEG mediated lead oxide these patterns with standard XRD cards shows that the samples include -PbO, -PbO and small amount of Pb 3 O 4. XRD analysis by using of standards showed that the amount of Pb 3 O 4 is lower than 10%. As it is seen from Fig.1c, lead oxide powder has more and phases than other samples. By using Scherer s equation, the average crystal size of the particle was calculated to be 35nm for the EDTA capped sample and 28nm for the PEG assisted powder sample calcined at 450 C. The size of the obtained particles decreased with the capping agent. Fig.2 FTIR spectrum of PEG mediated lead oxide Fig.2 shows the PEG mediated lead oxide nanoparticles calcined at 450 C for 2h. The peaks around 1610 and 3433cm -1 correspond to the bending vibrations of absorbed molecular water and the stretching vibrations of OH groups respectively. In the IR spectrum of lead oxalate, no peak appears at 1384 cm 1, which is a typical absorbance of nitrate 13

3 Fig.3 SEM images of (a) citric acid (b) EDTA and (c) PEG mediated lead oxide Fig.4 TEM image of PEG mediated lead oxide anions, indicating the lack of nitrate anions in the lead oxalate. The peak appears at 663cm 1, which is assigned to the Pb O bond. Fig. 3 shows the effect of three various capping agents on morphology of lead oxide Fig.3a shows the rocky-stone like morphology of citric acid assisted lead oxide They have some spherical-like particles, which aggregated to form larger rock-like particles. Considering the reaction of the chelating agent (EDTA), we suggest that oxalate anions coordinated to the Pb ions in Pb(NO 3 ) 2 inhibit the agglomeration of metal cations and decrease the particle size of the PbO nanoparticles (slab-like morphology) as shown in Fig.3b. However, spherical and some small rod-like particles were observed from the PEG mediated lead oxide nanoparticles with the particles size in the range of ~30nm. The lower rate of particle growth can help to form controlled uniform structure with more porosity as shown in Fig.3c. It is shown that among the effective additives for synthesis of lead oxides, citric acid, EDTA and PEG makes more useful and considerable positive effects on product morphology and particle size. PEG played an important role in determining the morphology of products. Among the capping agents, PEG mediated Lead oxide can synthesize in excellent and more porous nano-structure. Therefore, PEG was selected as suitable additive for synthesizing of lead oxide. Fig.4 shows the TEM image of PEG mediated lead oxide The nanoscale particles are believed to have coalesced into larger particles with any grain boundary elimination. It can be observed that a PEG molecule traps the Pb 2+ ions and reduces the rate of particle growth. The spherical-like morphology was obtained with the particle size range from 25-30nm, which is in agreement with the XRD results. Fig.5 shows the UV-vis absorbance spectrum of PEG mediated lead oxide The absorption coefficient of the mixed particles ( -PbO, -PbO and small amount of Pb 3 O 4 ) are enhanced in the entire recorded spectral 14

4 Fig.5 UV absorption spectrum of PEG mediated lead oxide Fig.6 PL emission spectrum of PEG mediated lead oxide range from 200 to 800 nm. The extended absorption wavelength towards the visible-light region and the increased absorption intensity indicated increased formation of electron hole pairs on the lead oxide surface. These particles depicted broad shoulders between nm and nm at high [Pb 2+ ] and may be assigned as excitonic bands. It can be seen that the absorption has a weak broad peak at about 242 nm. Hence, the absorption edge of the products obtained by us exhibits a large blue-shift, which is attributed to the quantum confinement of charge carriers in the nanomicrocrystals [12]. Lead oxide nanocrytallites have a unique property of luminescence characteristics, whose specific emission wavelengths mainly depend on the nature of the semiconductors, the physical dimensions and the chemical environments, bringing on great application potentialities in the fields of optoelectronic (light emitting) devices and biosensors. Fig.6 shows the PL emission spectrum of PEG mediated lead oxide The emission spectrum revealed a blue emission band centered at 442nm, which might be related with oxygen vacancies. Thus, the blue emission could also be attributed to recombination of electrons in the conduction band with deep doubly ionized oxygen vacancies. Emission bands in the range of blue emission are usually related to the transition of electrons from the conduction band edge to holes, trapped at interstitial Pb 2+ sites [13]. Particularly, the peak position at 467nm is sharp; it is due to the characteristics of the surface traps present in the nanoparticles [14]. It shows that the appearance of emission bands in varying wavelength. Also supports formation of clusters with varying size. Hence, the particles might have varying sizes with different band gap values. Moreover, the PL spectrum have weak green and orange-red emission band peaking around 545 and 657nm, respectively. It was therefore reasonable to believe that the green light emission from the lead oxide nanoparticles in our work could be attributed to the single ionized oxygen vacancy. The peak in the orangered wavelength range may be attributed to oxygen interstitials in lead oxide nanoparticles [15]. Conclusion The present study reports the preparation and characterization of capping agent mediated lead oxide nanoparticles via hydrothermal route. The spherical-like morphology of PEG mediated lead oxide nanoparticles was observed with size in the range from 25-30nm. Absorption spectrum revealed that the extended absorption wavelength towards the visible-light region and the increased absorption intensity indicated increased formation of electron hole pairs on the lead oxide surface. The PL study reveals the blue and green emissions due to the defects originated during the synthesis and no evidence of near band edge UV emissions was found. The present lead oxide product may be useful for constructing a special type of optoelectronic device that emits a wide range of visible light. We anticipate that this method would provide a platform for the fabrication of semiconductor nanocrystallites, which would have potential applications in fabricating devices with optical, electrical and magnetic properties. Acknowledgement The authors are grateful to the University Grant Commission for extending financial support to carry out this work. References 1. Y.L. Wu, A.I.K. Tok, F.Y.C. Boey, X.T. Zeng, X.H. Zhang, Appl.Surface Sci. 253 (2007) M.H. Cao, C.W. Hu, G. Peng, Y.J. Qi, E. Wang, J. Am. Chem. Soc. 125 (2003) A. Thulasiramudu, S. Buddhudu, Spectrochim. Acta A 66 (2007) T.L. Blair, J. Power Sources 73 (1998)

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