7. MILD SYNTHESIS AND CHARACTERIZATION OF IRON OXIDE / CdS NANOPARTICLES. In the past decade, colloidal II VI semiconductor nanocrystals are often

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1 7. MILD SYNTHESIS AND CHARACTERIZATION OF IRON OXIDE / CdS NANOPARTICLES 7.1. Introduction In the past decade, colloidal II VI semiconductor nanocrystals are often referred as quantum dots and have attracted an intensive attention of scientists for its biomedical applications and fundamental studies due to their unique optical properties [1-3]. Nanoparticles are widely used as fluorescent labels in biology, light emitting diodes, photovoltaics, and biomolecular imaging [4]. Because of their strong band gap luminescence due to tunable particle size and sharp quantum confinement effect, QDs have marked advantages over traditional fluorescent dyes, they are more stable in harsh environment, photo-oxidation and organic dyes. QDs have in most cases emitting continuous excitation spectra together with a strong, narrow, and emission band peak. The photoluminescence peak may however vary with respect to added advantage of QDs on the particle size allow simultaneous excitation of many colors by a single excitation resource [5-10]. Magnetic nanoparticles, (e.g.) iron oxide, have additional importance in nanomaterials due to their interesting magnetic properties and employs in many advanced technology areas (e.g.) magnetic bead, labeled biomolecules have been conveniently and efficiently separated in an external magnetic field [11]. Magnetic nanoparticles can be functionalized with molecules to obtain new properties, such as drug molecules, fluorescent compounds and various hydrophobic and hydrophilic coatings. Superparamagnetic iron oxide nanoparticles (e.g. Fe 3 O 4, Fe 2 O 3, FePt) have been studied as contrast agents for clinical magnetic resonance imaging [12]. In the recent years, number of researchers have been reported the synthesis of such nanocomposites [12-15]. In this chapter, we have discussed the synthesis of 121

2 heterostructured nanomaterials by chemical route, expecting to obtain the materials with coherent fluorescence and magnetic properties Materials and Method All chemicals were purchased from Merck, India and used without further purification. The following chemicals were used in the experiments such as Sodium dodecyl sulfate, ferrous sulfate, ferric chloride, ammonia, sodium sulphide, cadmium acetate and PEG Synthesis of -Fe 2 O 3 / CdS Nanoparticles -Fe 2 O 3 /CdS hetero-nanostructure were synthesized via a two-step process and at two different experimental conditions. The schematic diagram was showed in Fig Experimental Section Synthesis of Iron oxide ( -Fe 2 O 3 ) Nanoparticles Sodium dodecyl sulfate (0.5, 0.25 g) was dissolved in 50 ml of water followed by the addition of 0.1 M of ferric chloride (FeCl 3 ) and 0.2 M ferrous sulphate (FeSO 4 ) added together and stirred for half an hr. Ammonia solution was added to adjust the ph range between 8-9 after that 0.1 M of cadmium acetate and 0.1 M of sodium sulphide were added into the solution of black precipitate. The whole reaction was performed under vigorous stirring and kept at room temperature for 4 hr. Again the solution was centrifuged at 6000 rpm using ethanol and water, finally the sample was dried in hot air oven at 60 C followed by annealing at 280 C for 2 hr in a muffle furnace. 122

3 FeCl 3 & FeSO 4 Iron oxide Surfactant (SDS) Reaction mixture -Fe 2 O 3 / CdS nanoparticles Cd aceate & Na 2 S CdS NPs Fig Schematic representation of iron oxide / CdS nanoparticles synthesis Synthesis -Fe 2 O 3 /CdS Nanoparticles The above synthesized iron oxide suspension was centrifuged at 6000 rpm using ethanol and water to remove excess amount of surfactant and the untreated precursors. The black precipitate was dissolved in 30 ml of PEG, 0.1 M cadmium acetate and 0.1 M of sodium sulphide was also added. The whole reaction was performed under vigorous stirring and kept at room temperature for 4 hr. Again the solution was centrifuged using ethanol and water at 6000 rpm and the sample was 123

4 dried in hot air oven at 60 C followed by annealing at 280 C for 2 hr in a muffle furnace. The above same experimental procedure was followed at different conditions, (i) Synthesis of -Fe 2 O 3 /CdS nanoparticles using 0.1 M cadmium acetate and 0.1 M of sodium sulphide at room temperature (ii) Synthesis of -Fe 2 O 3 /CdS nanoparticles using 0.1 M cadmium acetate and 0.1 M of sodium sulphide at 80 C. (iii) Synthesis of -Fe 2 O 3 /CdS nanoparticles using 30 ml of PEG, 0.1 M cadmium acetate and 0.1 M of sodium sulphide at room temperature (iv) Synthesis of -Fe 2 O 3 /CdS nanoparticles using 30 ml of PEG, 0.1 M cadmium acetate and 0.1 M of sodium sulphide at 80 C Results and Discussion UV-Visible Studies It has been well established that the constituent fluorescent nanoparticles showed UV Visible absorption for -Fe 2 O 3 /CdS nanoparticles. As expected, the absorption edge of the samples found in the range of nm. The samples were characterized using different solvent such as water and chloroform. In the different solvent medium, water and chloroform shows good absorptions peaks for the - Fe 2 O 3 /CdS at 340 and 350 nm shown in Fig

5 Fig Fe 2 O 3 /CdS nanoparticles taken at water and chloroform as a solvent Samples were again studied with and without annealing temperature condition. As prepared samples were showed broad peaks of absorption at nm for the above different experimental conditions are shown in Fig. 7.3 and the same samples were recorded UV-visible studies after the annealed sample at 280 C that peaks were shifted into nm and very sharp peaks were appeared due to formation of heterostructured and homogenous distribution of particles which are shown in Fig

6 Fig.7.3. Absorption spectra of a -Fe 2 O 3 /CdS nanoparticles -Fe 2 O 3 /CdS nanoparticles - Fe 2 O 3 /CdS nanoparticles synthesized using PEG as a surfactant at 80 C The Table 7.1 gives the optical data of as prepared and annealed samples. It indicates that the -Fe 2 O 3 /CdS nanoparticles have a little smaller band gap energy than that of CdS nanoparticles alone. Fe 2 O 3 nanoparticles could be due to the possibility of Fe 3+ ions adsorbed on the surface in the chemical process and modify the surface of the CdS nanoparticles and thus influencing absorption properties. This can be further considered on the basis of reported phenomena that a small amount of Fe 3+ ions due to its half-filled electronic configuration can act as a temporary photogenerated electron or hole in the trapping site and may inhibit the recombination of photo-generated charge carriers and extend their life-time [16]. 126

7 Table 7.1. Optical data -Fe 2 O 3 /CdS heterostructure nanoparticles taken from the UV- Visible Spectra Sample Preparation Wavelength (nm) condition As prepared samples Annealed samples at 280 C 1 RT RT C Photoluminescence Studies The Fig. 7.5 and 7.6 (a, b, & c) shows the fluorescence emission spectra which were recorded at different excitation wavelength of at 370 nm and 400 nm. The fluorescence emission peaks showed a huge red shift in its spectrum and are centered at around 720 nm and 760 nm that are gradually increased with increase in the corresponding excitation energy. The peak appeared at 720 nm can be assigned to derivative peak of 370 nm excitation energy and showed a trap state emission peaks (Fig.7.5 B) and their fluorescence data are given in the Table

8 Fig Fe 2 O 3 /CdS nanoparticles synthesized at room temperature and annealed at 280 -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature using PEG as a surfactant and annealed at Fe 2 O 3 /CdS nanoparticles synthesized using PEG as a surfactant at 80 C Table 7.2. Photoluminescence data of -Fe 2 O 3 /CdS heterostructure nanoparticles Emission Wavelength Trap state emission Sample Preparation (nm) 1 RT RT C No Emission 128

9 A B Fig Fe 2 O 3 /CdS nanoparticles taken at excitation wavelength of 370 nm [A] -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature b) -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature - Fe 2 O 3 /CdS nanoparticles synthesized using PEG as a surfactant at 80 C [B] The above corresponding trap state emission peaks at excitation wavelength of 370 nm Fig Fe 2 O 3 /CdS nanoparticles at excitation wavelength of -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature b) -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature using PEG -Fe 2 O 3 /CdS nanoparticles synthesized using PEG as a surfactant at 80 C 129

10 It is interesting to note that fresh iron oxide-cds nanocomposites showed very little fluorescence and the PL peak were centered near red shift. We have observed a decrease in PL intensity and this relates to a mechanism where surface ligands may be exposed to nanoparticle surface permitting non-radiative recombination and particle aggregation X-Ray Diffraction The Fig.7.7 shows the XRD pattern of iron oxide/cds nanocrystal heterostructures. Both the samples exhibited hexagonal phases and inverse spinal structure of iron oxide and wurtzite structure of CdS. In the heterostructure, lattice mismatch effect is often common and longer value of lattice mismatch system may be due to the random growth mechanism which leads to the non-core-shell heterostructures. The narrowing of XRD peaks is from the improved crystalline rather than increasing crystallite size. The sample that was prepared at higher temperature followed by annealing obviously showed sharper peaks but these are due to greater random lattice mismatch when compared with the sample that was prepared at the room temperature. The Fig.7.7 a and b are showed an unfavorable formation of heterostructures due room temperature based synthesis and their data are given in the Table

11 Fig Fe 2 O 3 /CdS nanoparticles -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature b -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature using PEG as a surfactant c) -Fe 2 O 3 /CdS nanoparticles synthesized at using PEG as a surfactant at 80 -Fe 2 O 3 /CdS nanoparticles synthesized at 80 C Table Fe 2 O 3 /CdS heterostructure nanoparticles Theoretical data Observed data Position 2 ( deg) ( ) hkl Materials D spacing (Å ) Position 2 ( deg) D spacing (Å ) CdS Fe 2 O Fe 2 O Fe 2 O CdS CdS Fe 2 O

12 Raman Studies The Raman spectra were obtained at 532 nm, 40 mw laser of Raman spectrophotometer. The Fig.7.8 shows the -Fe 2 O 3 inverse spinal structure with three lines in Raman spectrum at 350, 500 and 700 cm -1 [17]. Raman spectrum of CdS shows two characteristic longitudinal optical phonon lines at about 300 cm -1 and 600 cm -1 [18]. The intensity peaks of CdS nanoparticles are much weaker than that of bulk CdS. This is expected owing to the small amount and it is seen that the first order longitudinal optical phonon Raman line is not only broadened, but also it shows asymmetric broadening towards the low frequency side is a well known and documented fact that confinement of phonons, optical as well as acoustic, influences the phonon therefore the grain size falls to a few nanometers [19]. Confinement of optical phonons causes an asymmetry in the line shape and shift towards the low frequency side compared to that of bulk CdS thus leading to the marked asymmetry of the Raman line towards the low frequency side and a shifted in the peak position again towards the low frequency side. 132

13 Fig.7.8. Raman spectra of -Fe 2 O 3 /CdS nanoparticles -Fe 2 O 3 /CdS -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature using PEG as a surfactant - Fe 2 O 3 /CdS nanoparticles synthesized using PEG as a surfactant at 80 -Fe 2 O 3 /CdS nanoparticles synthesized at 80 C Microscopy Studies The SEM and TEM images of -Fe 2 O 3 /CdS heterostructures are shown in the Fig. 7.9 (A-D). SEM picture showed that the particles are clustered, fluffy and lighter in appearance indicating that they will have high surface area. TEM image of -- Fe 2 O 3 /CdS heterostructures shows spherically distributed crystallites with an average particle size of about 12 nm. Increasing the amount of Cd/S reagents leads to the additional nucleation of CdS particles rather than the continued growth of existing particles because of this limitation. 133

14 Fig.7.9. Representation of low resolution SEM and TEM images of -Fe 2 O 3 /CdS nanoparticles (A & B) SEM images, (C & D) TEM images Magnetic Properties It is well known that CdS an iron oxide have an excellent optoelectronic and magnetic properties. Fig shows the magnetic characterization performed using a vibrating sample magnetometer. The M-H curves of iron oxides have weak ferromagnetic properties at room temperature, with a remanence of 5.73 emu g 1 and a saturation magnetization of emu g -1 in which are given Table 7.4. Magnetic studies are carried out for the sample c and d for the above given crystallographic pattern. The phenomena may be attributed to the presence of non-magnetic CdS nanoparticles in the heterostructures. 134

15 Fig Magnetization curves of a) -Fe 2 O 3 /CdS nanoparticles synthesized at room temperature b) -Fe 2 O 3 /CdS nanoparticles synthesized at 80 C Table 7.4. Magnetization data of -Fe 2 O 3 /CdS heterostructure nanoparticles Properties Sample A Sample B Coercivity G G Magnetization emu/g emu/g Retentivity emu/g emu/g It is found that the photoluminescence behaviors of the aqueous CdS nanoparticles which are dependent on the temperature during the measurement. PL emission spectra of the Fe 2 O 3 /CdS nanoparticles were recorded at reaction mixture 80 ºC (Fig. 7.5). It is shown that with increasing temperature, the emission peak shifted to the larger wavelength with decreased emission intensity and with the reduced FWHM. Hence, the emission intensity followed the same temperature dependent trend for all the experiments. 135

16 In the synthesis of the aqueous CdS nanoparticles, SDS is used as the capping molecule to disperse the nanoparticles in the suspension and to control the particle size. The sulphate group (SO - 4 ) of the SDS attached to the Cd cations on the particle surface and the tail part of alkyl group on the other end of the SDS was fully charged at reaction condition. As a result, the nanoparticles were stabilized and dispersed from each other due to the electrostatic repulsion. Using PEG and SDS polymer surfactant is the plays an effective role to controlling the particle size. In the comparison to the SDS, hydrophilic polymer PEG is played the key role for particle growth prevention Conclusion Magneto-fluorescent nanoparticles ( -Fe 2 O 3 /CdS) were synthesized and studied fluorescence and magnetic properties collectively at nanoscale by combining CdS and iron oxide nanoparticles with hydrophilic polymer of PEG. This opens the door to optimizing nanoparticles-based nanocarriers and fluorescent tags, which might serve as a multimodal platform incorporating other agents of interest, including plasmonic gold nanostructures. This approach showed the tremendous promise of CdS nanoparticles as a component of fluorescent probes for optoelectronic applications 136

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