Synthesis and optical properties of monodispersed Ni2+-doped ZnS nanoparticles
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Nickel-doped zinc sulfide nanoparticles were successfully synthesized in air atmosphere through chemical precipitation method using surfactants in aqueous medium. The product was characterized by different techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared spectrometer (FT-IR), UV–visible absorption and photoluminescence (PL). Crystal structure, size and morphology of the ZnS:Ni2+ nanoparticles were investigated by XRD and TEM. In the PL emission, a couple of new peaks were observed instead of a single peak by changing the precursor solution. In addition, an enhanced PL emission was observed using surfactants. Phase changes were also observed at different temperatures.
KeywordsNanostructures Semiconductors Chemical synthesis Oxidation
There has been much interest recently in synthesizing nanometer-sized semiconductor particles because such particles exhibit size-dependent optical and electrical properties. The nanocrystalline materials are good candidates for application in optoelectronic devices due to their reduced size, enabling a reduction in the size of the electronic circuit. Moreover, it is also possible to tune their properties to suit a specific application by merely changing their size (Sampra and Sarma 2004). When compared to their bulk counterparts, semiconductors in the nanoparticle range also exhibit different behavior. Decreasing the particle size gives rise to quantum confinement effect (Brus 1986), wherein an increase in the energy gap as well as splitting of the conduction and valence bands into discrete energy levels becomes evident. These particles are expected to have higher quantum efficiencies in applications such as light emitters.
Doping with optically active luminescent materials manipulates the band structure of the nanoparticles and show intense emissions in a wide range of wavelength depending on the impurity type, concentration and crystal dimensions, and also play key roles in luminescence efficiency and the positions of emission bands, thus influencing their practical applications. The doped luminescent nanoparticles are of strong interest for possible use in optoelectronics such as LEDs and lasers or as novel phosphors because of their interesting magnetic (Kennedy et al. 1995; Counio et al. 1996; Igarashi et al. 1997; Feltin et al. 1999) and electro-optical properties (Murugadoss et al. 2010; Pohl and Gumlich 1989; Devisschere et al. 1995; Yu et al. 1998). Recently, the doped nanomaterials have been largely studied due to their widespread applications in various devices such as sensors, solar cells, lasers, photocatalysts, photodetectors, IR detectors, optical communication, color television, flat panel display, phosphors, light emitting diodes, etc. (Ong and Chang 2001; Fang et al. 2009; Kar and Biswas 2008; Becker and Bard 1983; Klimov et al. 2007; Wang et al. 2005; Green and Hersam 2008; Toyama et al. 2009; Brus 1991; Hoffman et al. 1992).
Among the various semiconductor compounds, the nanocrystals belonging to II–VI groups were the most studied since they exhibit interesting size-tunable optical properties due to the strong quantum confinement effect (Alivisatos 1996). Especially, zinc sulfide is an important II–VI group semiconductor compound with direct wide band gap energy of 3.67 eV at room temperature. Therefore, in this work the ZnS has been chosen as a host material to synthesize doping nanoparticles for enhanced optical properties. In doped ZnS nanocrystals, impure ion occupies the Zn lattice site and behaves as a trap site for electron and holes. The electrons are excited from the ZnS valence band to conduction band by absorbing the energy equal or greater than their band gap energy. Subsequent relaxation of these photo-excited electrons to some surface states or levels is followed by radiative decay enabling the luminescence in visible region.
Several techniques have been used to synthesize doped ZnS nanoparticles such as sol–gel, chemical precipitation, solid state reaction, auto-combustion, etc. In this paper, the ZnS nanoparticles with varying initial doping concentrations (0.5–5 %) of Ni2+ were synthesized through a chemical precipitation method, as it is simple, inexpensive and more productive. The luminescence properties of the ZnS:Ni2+ nanoparticles were investigated by photoluminescence (PL) spectroscopy. The optical study was adopted to determine the actual doping concentrations. To promote a confined and stable growth of ZnS:Ni2+ nanoparticles, the capping molecules PMMA, PEG, PVP and CTAB were used in the reaction process. It is indicated that the addition of capping molecule could greatly influence the PL emission and size confinement of ZnS:Ni2+ nanostructures. The objective of the present study is to synthesize luminescent ZnS doped with transition metal Ni2+ and create bio-organic interface with bio-compatible inorganic/organic material for possible use as nanoscale fluorescent probes for potential pharmaceutical, biological and medical applications such as targeted drug delivery and labeling of biological cells.
To synthesize ZnS and ZnS:Ni2+, the following materials were used. All the glassware used in this experimental work were acid washed. The chemicals used were analytical reagent grade without further purification. Ultrapure water was used for all dilution and sample preparation. Zinc acetate dehydrate [Zn(CH3COO)2·2H2O], nickel acetate [Ni(CH3COO)2·4H2O] and sodium sulfide (Na2S·xH2O) obtained from Nice Chemical company were used as precursors. Poly methyl methacrylate (PMMA-1,20,000) and polyvinylpyrolidone (PVP-40,000) were obtained from Aldrich and Otto Chemika, respectively. Polyethylene glycol (PEG-6,000–7,000) and cetyltrimethylammonium bromide (CTAB-364.46) were obtained from S. D fine Chem. Ltd and Spectrochem. Pvt. Ltd, respectively. All the chemicals are above 98 % purity.
Synthesis of ZnS:Ni2+ nanoparticles
The ZnS nanoparticles doped with different Ni2+ concentrations were synthesized in de-ionized water at air atmosphere. In a typical experiment, 5.48 g (0.5 M) of Zn(CH3COO)2·2H2O in 50 ml aqueous and Ni(CH3COO)2·4H2O in 25 ml aqueous with different concentrations (0.5, 1.0, 1.5, 2, 3, 4 and 5 %) were mixed drop by drop. The concentration of Ni2+ was adjusted by controlling the quantity of nickel acetate in the above mixture. The mixture was stirred magnetically at 80 °C until a homogeneous solution was obtained. Then, 2.75 g (0.5 M) of 50 ml Na2S was added drop by drop to the above mixture. After the Na2S injection a white, voluminous precipitate appears. It slowly dissolves under the formation of ZnS:Ni2+ nanoparticles during incubation under stirring for 30 min at 80 °C. The obtained dispersions are transparent and were purified by dialysis against de-ionized water and ethanol several times to remove impurities. The products were dried in hot air oven at 80 °C for 2 h. The undoped ZnS nanoparticle was synthesized following the same procedure in the absence of the doping material.
Synthesis of surfactants capped ZnS:Ni2+ nanoparticles
The X-ray diffraction (XRD) patterns of the powdered samples were recorded using X’PERT PRO diffractometer with a Cu-Kα radiation (λ = 1.5406 Ǻ). The crystallite size was estimated using the Scherrer equation of the major XRD peak. The morphology and size of the nanoparticles were analyzed through TEM (PHILIPS-CM200; 20–200 kV) microscopy. In addition, the particle size distribution of the capped particles was obtained by particles size analyzer (Nanotrac Specifications Model: Nanotrac NPA 150). The FT-IR spectra were obtained on an AVATOR 360 spectrometer using KBr pellet technique. The optical absorption spectra of all the samples in de-ionized water were recorded using UV-1650PC SHIMADZU spectrometer. Fluorescence measurements were performed on a RF-5301PC spectrophotometer. Emission (350–600 nm) spectra were recorded under the different excitation wavelengths at room temperature.
Results and discussion
Crystal structure and morphology
The peak observed at 1,636 indicates nitrogen–oxygen interaction of PVP (Fig. 4d). Furthermore, the intensity of the IR peaks for PVP capped particles (Fig. 4d) is higher than that of the uncapped ZnS:Ni2+ nanoparticles, which indicates the homogeneous formation of particles. The peaks at 1,512, 1,332 and 1,340 cm−1 in Fig. 4e represent the formation of PEG on the surface of the ZnS:Ni2+ nanoparticles (Silverstein et al. 1981). The strong band of hydroxyl group in Fig. 4e shifted towards lower wavenumber side with the decrease in absorption from Fig. 4a indicates hydrogen bond formed in the ZnS:Ni2+/PEG interface (Santhiya et al. 1999). The spectrum of CTAB is presented in Fig. 4f. The strong peaks observed at 1,610 and 1,402 cm−1 are shifted to long wavenumber side from the uncapped particles (Fig. 4a), indicating the presence of CTAB on the ZnS:Ni2+ surface. The broad peaks for all the samples (Fig. 4a–e) in the range of 3,410–3,465 cm−1 correspond to –OH group. The presence of this band can be clearly attributed to the adsorption of same atmosphere water during FT-IR measurements. The bands at 1,500–1,650 cm−1 and at 2,370 cm−1 are due to the C=O stretching mode arising from the absorption of atmospheric CO2 on the surface of the nanoparticles (Qadri et al. 1999).
The PL position of the ZnS was tuned by adding impurity (Ni) as shown in Fig. 7. All the PL spectra of Ni2+-doped ZnS nanoparticles are red shifted from the undoped ZnS. This implies that the dopant is created a trap state between the valance and conduction band. Hence, a least quantum of energy was provided to the nearest neighbor atom or lattice when the electron transition from the conduction band to trap state.As a result, the emission energy is reduced. It is worth noting that with the increase of the doping concentration, the peak position of this emission band shifts from blue to green region. Qu et al. observed the similar red shift for ZnS:Eu nanoparticles with increasing doping levels (Qu et al. 2002). In addition, relative intensity of the Ni-doped ZnS nanoparticles was reduced by increase in the Ni concentrations. The PL quenching is may be due to increase in the radiationless transition by the higher concentration of the nickel ions.
ZnS:Ni2+ nanoparticles with very narrow size distribution were prepared from chemical precipitation method in aqueous medium at air atmosphere. The TEM images of all the surfactant capped ZnS:Ni2+ nanoparticles show that the particles are almost spherical in shape with homogeneous distribution.
ZnS:Ni2+ nanoparticles were successfully capped and the surface is modified by bio-compatible organic materials (PMMA, PVP, PEG and CTAB). Among the different capping agents, PMMA is more effective than others. The capped ZnS:Ni2+ nanoparticles show better dispersion as they were adsorbed on the surface of nanoparticles so as to fulfill the steric hindrance between nanoparticles and prevent agglomeration. Successful surface modification and bioinorganic interface make the ZnS:Ni2+ nanoparticles suitable for use in nanoscale fluorescent probes for biological and medicinal applications such as targeted drug delivery, ultra-sensitive disease detection and labeling of biological cells. UV light is absorbed effectively by ZnS:Ni2+ nanoparticles, so that they can be used in sunscreens.
The author would like to acknowledge the service rendered by scientific officers of IIT Bombay, Mumbai, CECRI, Karaikudi, STIC, Cochin and CISL, Annamalai University, for recording TEM, XRD, and FT-IR spectra. The author also likes to thank Dr. N. Rajendran, Department of Chemistry, Annamalai University, India, for providing UV–visible and PL facility.
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Open AccessThis article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.