104 Acta Chim. Slov. 2016, 63, 104-112 DOI: 10.17344/acsi.2015.1991 Scientific paper Synthesis, Structural and Optical Characterization of Copper and Rare Earth doped CdS Nanoparticles Sonika Khajuria,1 Sumit Sanotra,1 Heena Khajuria,1 Anuraag Singh2 and Haq Nawaz Sheikh1* 1 Department of Chemistry, University of Jammu, Jammu Tawi, 180 006 India 2 Electron Microscope Facility, Department of Anatomy, All India Institute of Medical Sciences (AIIMS), New Delhi-110029 * Corresponding author: E-mail: hnsheikh@rediffmail.com Tel.: +91-191-2493124; Fax: +91-191-2431365 Received: 15-09-2015 Abstract Cadmium sulphide (CdS) nanoparticles doped with Cu2+ and co-doped with rare earth metallic ions (Ln3+) have been synthesized by co-precipitation method. The synthesized nanoparticles were characterized by powder X-ray diffraction studies (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size by dynamic light scattering (DLS), UV-visible spectroscopy (UV), and photoluminescence (PL) studies. It is observed that CdS nanoparticles can be co-doped with copper and rare earth metallic ions without altering XRD pattern as indicated by X-ray diffraction results. SEM and TEM results show that synthesized particles have spherical aggregation. Absorption spectra of all the samples show strong blue shift from bulk CdS. Interesting luminescence pattern with enhanced intensity has been observed for co-doped CdS nanoparticles at room temperature. Keywords: Nanoparticles; CdS nanoparticles; PXRD; absorption; photoluminescence. 1. Introduction In the past few decades, synthesis of nanoscale materials in the field of nanotechnology has attracted attention to due to their unique physical and chemical properties.1-4 Generally their properties are highly dependent on the size and morphology.5,6 Much emphasis has been laid on the synthesis and characterization of semiconductor nanoparticles due to their significant quantum confinement effects. These effects result in significant variation in electrical and optical properties of nanoparticles. Doped nanosized semiconductors were studied due to their potential applications in optoelectronic nanodevices and functional materials.7-10 Doping of materials allows to noticeably modify the properties of materials. Because of their excellent luminescent properties, transition metal and rare earth ions doped semiconductor nanostructures have been the subject of numerous investigations.11-15 The doped nanostructures are known to have unusual physical and chemical properties in comparison with the corresponding bulk materials.16-18 Among chalcogenides, special photoelectrical and chemical properties of CdS (band gap 2.42 eV at 300 K for bulk CdS and 3.25 eV for nanosized CdS19 ) have generated much interest because of its relatively large nonlinear response,20 photocatalytic activity21 and luminescence.22 Both size and surface effects are important in nanosized materials.23 It is possible to design materials of required optical, electrical, magnetic, elastic and chemical properties by controlling both these effects. The nature of the dopant ion strongly influences the luminescent properties of doped nanopartic-les as they play an important role in changing the electronic structure and transition probabilities of the host materials. G. Giribabu and co-workers have reported the structural, optical and magnetic properties of cobalt and aluminium co-doped CdS nanoparticles synthesized by simple co-precipitation method.24 A. Sundaria et al. have synthesized star-shaped copper doped CdS nanoparticles and have studied the relation between shape and antibacterial properties of the nanoparticles.25 Xiaohui Wang et al. have reported the synthesis of lanthanide doped CdS and ZnS quantum dots in natural polysaccharide template and studied their optical properties.26 M. Thambidurai and co- Khajuria et al.: Synthesis, Structural and Optical Characterization Acta Chim. Slov. 2016, 63, 104-112_ 105 workers have studied the structural, optical and electrical properties of cobalt doped CdS Quantum dots. They have carried a systematic study to understand the influence of co-doping on various properties of CdS nanoparticles.27 Magnetic and structural characterization of Ni and Cu co-doped CdS nanoparticles has been carried out by S. Kumar and co-workers.28 P. Yang et al. have reported a new class of luminescent materials by synthesizing ZnS nano-crystals co-activated by transition metals and rare earths.29 Generally, the properties of the dopant are mainly reflected in luminescence of rare-earth doped systems. Incompletely filled 4f shells result in different electronic structure of rare-earth ions as compared to other elements. The 5s2 and 5p6 electron orbitals shield the 4f electrons. A sharp line emission is observed on exciting such doped materials due to intra 4fn shell transition of rare earth ion core which lies in a wide range covering UV, Vis and IR regions of the spectrum.30 There is little work done on luminescence studies related to semiconductor nanopartic-les co-activated with two kinds of (transition and rare earth) metallic ions. A number of techniques have been utilized to synthesize doped nanoparticles such as sol-gel, solid state reaction, co-precipitation etc.31,32 In the present work, doped CdS nanoparticles were synthesised by co-precipitation method as it is inexpensive and more yield producing method. Herein, we report the structural and optical characterization of CdS semiconductor nanoparticles co-doped with Cu2+ and rare earth ions (Ln3+) synthesized by co-precipitation method. 2. Experimental 2. 1. Materials and Measurements All the reagents were of analytical grade and were used without further purification. Cadmium acetate dihy-drate (Cd(CH3COO)2 • 2H2O), copper(II) chloride dihy-drate (CuCl2 • 2H2O), thioacetamide (TAA) and rare earth metal salts, gadolinium(III) chloride hydrate (GdCl3 • H2O), terbium(III) chloride (TbCl3 • 6H2O) hexahydrate and erbium(III) chloride hexahydrate (ErCl3 • 6H2O) were purchased from Sigma Aldrich. Ethylene glycol (EG) was purchased from Alfa-Aesar. Doubly-distilled water was used for preparing aqueous solutions. PXRD (Powder X-ray Diffraction) patterns were recorded from 10° to 90° on Rigaku Miniflex diffractometer using monochromatic Cu^a radiations (The Woodlands, TX, USA). Scanning electron micrographs (SEM) were collected on Jeol T-300 scanning electron microscope with gold coating (Tokyo, Japan). Transmission electron micrographs (TEM) were collected on TECNAI 200 kV TEM (Fei, Electron Optics). The particle size was determined by DLS technique using Zetasizer Nano ZS-90 (Malvern Instruments Ltd., Worcestershire, UK). The photoluminescence excitation and emission spectra were recorded at room temperature using Agilent Cary Eclipse Fluorescence Spectrophotometer equipped with a Xenon lamp as an excitation source. 2. 2. Synthesis The undoped CdS, CdS doped with Cu2+ and co-doped with Cu2+-Gd3+, Cu2+-Er3+, Cu2+-Tb3+ nanoparticles were synthesized by chemical precipitation method in de-ionized water in air atmosphere. The precipitation of undo-ped/doped CdS nanoparticles was performed starting with 0.1 M homogeneous solutions each of Cd(CH3COO)2 • 2H2O (0.66 g), thioacetamide TAA, (0.188 g), CuC^ • 2Hp (0.43 g), GdCl3 • H2O (0.658 g), ErCl3 • 6H2O (0.954 g), TbCl3 • 6H2O (0.932 g) each in 25 mL of distilled water. The temperature of each solution was maintained 80 °C and pH was kept at 2 to avoid the decomposition of TAA at premature stage. For the synthesis of undoped CdS nanoparticles, aqueous solution of TAA was added to the solution of Cd(CH3COO)2 • 2H2O dropwise while stirring on magnetic stirrer. The reaction temperature was maintained at 80 °C throughout. Stirring was done for 5 hours at constant temperature and the reaction was terminated by rapidly cooling the reaction mixture in an ice bath. The as-precipitated un-doped sample was centrifuged at 3000 rpm for 30 min., washed with water and isopropyl alcohol several times and then dried at 70 °C for 2 hours. To synthesize Cu2+ doped CdS nanoparticles, 0.1 M aqueous solutions of Cd(CH3COO)2 • 2H2O (0.66 g) and CuCl2 • 6H2O (0.43 g) were mixed and stirred for 30 minutes. To this, 0.1 M solution of TAA was added dropwise while stirring with reaction temperature maintained at 80 °C. All reaction conditions and procedure adopted were same as above. In a typical synthesis for Cu2+ and rare earth metallic ions co-doped CdS nanoparticles, 0.1 M aqueous solutions of Cd(CH3COO)2 • 2H2O, CuCl2 • 2H2O and rare earth metallic ions were mixed and stirred for 30 minutes. To this mixture, 0.1 M solution of TAA was added dropwise while stirring with reaction temperature maintained at 80 °C. The rest of procedure and reaction conditions were kept similar as for above synthesized nano-particles. 3. Results and Discussion 3. 1. PXRD Measurements The structural characterization of the nanoparticles has been carried out by X-ray diffraction technique using CuKa radiation. PXRD patterns of synthesized undoped and doped CdS nanoparticles are shown in Figure 1. The synthesized particles produce highly intense X-ray reflections in their corresponding PXRD pattern indicating that all the compounds are crystalline in nature. All PXRD pat- Khajuria et al.: Synthesis, Structural and Optical Characterization 106 Acta Chim. Slov. 2016, 63, 104-112 0 20 30 40 50 60 70 2 Tbeta (degree) Figure 1. PXRD patterns for (a) undoped CdS (b) CdS:Cu2+ (c) CdS:Cu2+Gd3+ (d) CdS:Cu2+Er3+ (e) CdS:Cu2+Tb3+ terns show (100), (002), (101), (102), (110), (103) and (112) planes of hexagonal phase of CdS corresponding to JCPDS File No. 41-1049. All the peaks in the diffraction pattern are characteristic of CdS suggesting that incorporation of dopant ion in the sample does not introduce appreciable changes in the crystal structure of CdS. This shows that the hexagonal structure is not modified by the addition of different rare earth metals into the matrix. However, a small deviation in full width at half maxima of diffraction peaks was observed by the addition of doping ions, which may be due to a small variation in the size of metal ions. Various parameters such as interplanar spacing (d in A), crystallite size (D in nm), microstrain (e), dislocation density (p in 1015 m/m3) and distortion parameter (g) along the most intense peak were calculated using following well known equations and are shown in Table 1. The various equations used are, D = 0.9X/pcos8; d = X/2sin8; e = Pcos8/4; p = 1/D2 and g = p/tan8 where X = 0.15418 nm for Cu-^a, P is FWHM (Full Width at Half Maximum) of diffraction peaks and 8 is diffraction angle. The variation in particle size occurs due to the interaction between the dopants and grain boundaries and this result in change in grain boundary energy. This leads to the stabilization of the surfaces/grain boundaries and variation in particle size. Also, it is clear from above equations that microstrain values decrease with increase in the crystallite size.33 In order to distinguish the effects of crystallite size and strain on induced broadening, Williamson-Hall plots of XRD profile have been drawn.34 The crystallite size and strain can be obtained from the intercept at y-axis and the slope of line respectively. Phkl cos8 = kX/D + 4esin8 (1) where P is FWHM in radian, D is the crystallite size in nm, e is the strain, X is X-ray wavelength in nanometers. The calculated crystallite sizes are in the range of 46-75 nm. 3. 2. SEM and TEM Analysis The morphology of isolated pure and doped CdS na-noparticles was investigated by scanning and transmission electron microscopy (SEM and TEM). Figure 2(a-e) shows SEM images of the un-doped and doped nanopar-ticles. As is evident from the SEM images, synthesized particles are near spherical in shape and involve aggregation of particles. Figure 3(a-e) shows TEM images of as synthesized un-doped and doped nanoparticles. The synthesized nano- Table 1. Evaluated parameters from XRPD data for undoped and doped CdS nanoparticles Samples Peak position 28 (in degrees) Particle size D (in nm) FWHM ß (in degrees) d-value (A) Observed Dislocation density p Calculated (in 1015 m/m3) Distortion parameter g Undoped CdS 26.804 53.62 0.1506 3.326 3.322 0.348 0.632 CdS:Cu2+ 26.816 32.18 0.2509 3.324 3.321 0.965 1.053 CdS:Cu2+Gd3+ 26.731 48.26 0.1673 3.335 3.331 0.429 0.704 CdS:Cu2+Er3+ 28.324 37.25 0.2175 3.151 3.147 0.721 0.862 CdS:Cu2+Tb3+ 28.446 26.92 0.3011 3.137 3.133 1.379 1.188 Khajuria et al.: Synthesis, Structural and Optical Characterization Acta Chim. Slov. 2016, 63, 104-112 107 (e) particles have nanoflake like shape. These nanoflakes then assemble to produce nearly spherical aggregates as observed in Figure 3. The particle size obtained from TEM and PXRD are in close agreement. The elemental mapping images shown in Figure 4, confirms the presence of dopants. 3. 3. Particle Size by Dynamic Light Scattering The particle size distribution of synthesized CdS na-noparticles was studied by dynamic light scattering technique. Figure 5 displays average particle size distribution Khajuria et al.: Synthesis, Structural and Optical Characterization ... 108 Acta Chim. Slov. 2016, 63, 104-112 Figure 3. TEM images for (a) undoped CdS (b) CdS:Cu2+ (c) CdS:Cu2+Gd3+ (d) CdS:Cu2+Er3+ (e) CdS:Cu2+Tb3+ of all the synthesized samples. Nanoparticles were dispersed uniformly in deionized water by mild sonication for 10 minutes before DLS analysis. As observed from Figure 5, approximate sizes of un-doped and doped CdS nano-particles are different. Due to surface solvation and agglomeration of particles in colloidal solution, the particle size analyzed from DLS method is usually larger in comparison to the particle sizes calculated from XRPD data. The particle sizes obtained from DLS are shown in Table 2. Table 2. Particle size obtained from DLS technique and TEM Samples Particle size by Particle size from DLS (in nm) TEM(in nm) Undoped CdS 80.6 28.5 CdS:Cu2+ 109 15 CdS:Cu2+Gd3+ 134 20 CdS:Cu2+Er3+ 172 21 CdS:Cu2+Tb3+ 164 42 Khajuria et al.: Synthesis, Structural and Optical Characterization Acta Chim. Slov. 2016, 63, 104-112 109 M » EE rc.,V ^Dr ■MCI M >■■ 11» 1SI1 W> Vlrr' JHHnL 1 i * J