CC BY
12UDC 620.3
SYNTHESIS OF THE CdS AND CuS NANOPARTICLES WITHIN THE MULTIWALL CARBON NANOTUBE - MALEIC ANHYDRIDE - 1-OCTENE MATRIX AND THEIR ELECTRICAL, OPTICAL AND STRUCTURAL CHARACTERIZATION
MALIKOV ELVIN YASHAR Doctor of Philosophy in Chemistry, Senior lecturer of the Chemistry faculty of Baku State
University, Baku, Azerbaijan
Abstract: CdS and CuS nanoparticles were separately synthesized by sonication from cadmium chloride, copper acetate dihydrate and thiourea using a multiwall carbon nanotube - maleic anhydride - 1-octene system as the matrix. The matrix was obtained by the "grafting from" approach from oxidized carbon nanotubes and maleic anhydride - 1-octene copolymer. Multiwall carbon nanotubes usedfor reinforcing the matrix were synthesized by Catalytic Chemical Vapor Deposition using Fe-Co/AhOs as the catalyst. The electrical, optical, and structural characterizations of the obtained nanostructures have been performed. The results showed that the obtained nanostructures can be used as the additives for preparation of various conductive or semiconductive nanocomposites.
Keywords: carbon nanotubes, polymers, terpolymers, nanoparticles, metal-chalcogenides, maleic anhydride
Introduction. Semiconductor nanoparticles have received considerable attention from the scientific community because of their unusual physical and chemical properties compared to their bulk phases. CdS and CuS semiconductors find applications in the production of phosphors, solar cells, sensors, light-emitting diodes, photocatalyst materials, and lasers [1-3].
In comparison to other methods, the preparation of nanoparticles with ultrasound is easy and more effective under mild conditions. Multiwall carbon nanotubes (MWCNTs) grafted with polymers are the most appropriate matrices for the synthesis of the CdS and CuS nanocrystals [4-6]. Their tendency to form entangled stacks generally makes MWCNTs unsuitable for manufacturing advanced plastic materials. However, grafting MWCNTs with maleic anhydride - 1-octene copolymer (MAO) via the "grafting from" approach gives us the opportunity to overcome this problem and to develop a new matrix for nanofabrication.
The novelty of this work is the use of the MWCNT-MAO matrix obtained via the "grafting from" approach for the synthesis of CdS and CuS nanocrystals under ultrasonic cavitation to prepare polymer nanocomposites and to investigate their electrical properties. The resulting polymer nanocomposites were characterized by XRD and UV-vis spectroscopy. Moreover, the electrophysical properties of the polymer nanocomposites were investigated. These techniques proved the completion of the synthesis and provided insight on certain mechanistic details of the method. The obtained advanced nanocomposite materials can be used e.g. as a precursor in the manufacturing of advanced plastic composite materials with electrical properties.
Experimental. MWCNTs were synthesized by the Catalytic Chemical Vapor Deposition method using a horizontal furnace, a fixed bed quartz tube reactor, and a gas delivery system. MWCNTs were grown on 1g alumina-supported Fe-Co catalyst from acetylene using nitrogen as the inert make up gas. The process lasted for 2 hours at 650°C. Remnant catalyst particles were removed by a combined acid-base purification protocol. In order to decorate the surface of the MWCNTs with -COOH functional groups they were subjected to 0.01 M aqueous KMnO4 solution at 80°C for 3 hours while stirring on a magnetic stirrer.
The MAO was synthesized via free radical terpolymerization in a butyl acetate (BA) solution in the presence of azobisisobutyronitrile (AIBN) as an initiator. 9.8 g of maleic anhydride (0.1 mol), 7.85 ml of 1-octene (0.05 mol), and 0.2 g of AIBN were thoroughly dissolved in 50 ml of BA, poured
into an ampule, and sealed off. This ampule was immersed in a glycerin bath and the temperature was raised to 80°C. Heating was maintained for 4 hours and then the copolymer was precipitated from the solution with isopropyl alcohol, washed several times, and dried in a vacuum.
1g MAO was dissolved in 25 ml N,N-dimethylformamide (DMF) to form a polymer solution at room temperature. 0.35 g AIBN and 1.75 g oxidized MWCNTs were added to this polymer solution in a beaker and the mixture was diluted with DMF to 50 ml. The mixture was then sonicated at 23 kHz using a sonics vibro cell equipment operating at 80°C. The sonication process was allowed to run for 3 hours. A black dispersed mixture was obtained as a result of this "grafting from" process.
0.48 g of CdCl2 • 2.5 H2O (2.1 • 10-3 mol) and 0.76 g of thiourea (0.01 mol) were added to the MWCNT-MAO nanostructure and sonicated under the same conditions for three different time periods like 2 hours, 4 hours and 6 hours to obtain three different mixture. The resulting mixtures were precipitated, washed with deionized water, filtered with 0.2 mkm polycarbonate membrane, and dried. The same process was also carried out for 0.457 g Cu(CH3COO)2 • 2H2O (2.1 • 10-3 mol) under the same conditions for 2 hours of cavitation.
Results and Discussion. Powder XRD patterns were recorded on a Rigaku MiniFlex Desktop X-ray diffractometer using CuKa radiation (1.5418 A). The results show the peaks at 24.70°, 26.48°, 28.02°, 43.54°, 47.78°, and 51.78° which can be assigned to the (100), (002), (l01), (110), (103), and (112) reflections of hexagonal CdS (JPDS no.: 6-314). Peaks at 22.36°, 27.32°, 29.44°, 31.80°, 48.10°, 52.86°, 59.18°, and 73.98° can be assigned to the (004), (102), (103), (110), (114), (116), and (208) reflections of the hexagonal phase of CuS (covellite) (PDF No: 00-001-1281).
The results for average coherence length estimated with Scherrers formula using (002) Bragg peak [7] were 10.5 nm (pristine-MWCNT), 6.2 nm (oxidized-MWCNT), and 8.3 nm (MWCNT-MAO). The results show that the value decreases from pristine MWCNT to oxidized one. The reason behind this is that the oxidation results in the partial loss of the outermost graphitic layers and the introduction of defects, which reduce the symmetry of the plane. Binding the MAO to the defective regions increases the value. This happens because the polymer layers cover the surface of the MWCNTs and restore the smoothness and orderliness on the surface to some extent by securing exfoliated wall parts back to their previous positions. The formation of the CdS and CuS nanocrystals reduces the order and therefore, the value decreases again. The mean diameters of CdS nanoparticles obtained with 2 hours, 4 hours, and 6 hours of cavitation were calculated with Scherrer s formula using (100) and (101) peaks and the results were as 7.9 nm, 4.2 nm, and 3.9 nm, respectively. The mean diameter calculated for CuS nanocrystals was 3.18 nm. The sonication time affects the size of the obtained CdS nanoparticles. Thus, by increasing the time period we can easily reduce the size of the nanoparticles to some extent.
The optical properties and the band gap of the synthesized materials were characterized by UV-vis spectroscopy using SPECORD 250 PLUS UV-vis spectrometer. The absorbance intensities of the samples uniformly decrease with increasing of the wavelength. The peak observed at 273 nm in the absorbance spectrum of the oxidized MWCNT is the result of the plasmonic effect [8].
The Kubelka-Munk approach is applied in order to characterize the optical properties using the result from optical reflectance [9].
The band gap for MWCNT-MAO/CdS(2h), MWCNT-MAO/CdS (4h), MWCNT-MAO/CdS(6h), and MWCNT-MAO/CuS samples obtained from the dependence of the [F(R)hv]1/2 on hv were 3.94 eV, 4.1 eV, 4.35 eV, and 2.28 eV respectively. These results are higher than that of bulk CdS (2.42 eV) and CuS (2 eV) and show the blue shifting due to the crystalline size [1,10]. Such shifting of the band gap is the result of quantum size effects in nanoparticles and it is typical for low-dimensional systems. These rather broad band gap values suggest that the nanocomposites could find applications in optical devices such as optical insulators and optical harvesting parts of photovoltaic devices.
The average diameters of the nanocrystals in MWCNT-MAO/CdS(2h), MWCNT-MAO/CdS(4h), MWCNT-MAO/CdS(6h), and MWCNT-MAO/CuS nanocomposites were
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calculated as 6.2 nm, 5.9 nm, 5.5 nm, and 6.7 nm respectively. These values obtained from optical results show that the sonication time affects the size of the obtained CdS nanoparticles. Thus, by increasing the sonication time we can easily reduce the size of the nanoparticles. These results correspond with the results obtained from the other methods.
The electrical properties of the obtained nanocomposites were investigated by an Immittance meter MNIPI E7-20 device under an alternating electric field, in 200 Hz-1 MHz frequency diapason. The specimens were prepared by mixing MWCNT-based nanocomposites with the aqueous polyvinyl alcohol solution and the formation of thin films of this mixture at room temperature. The specimens were placed between two electrodes in a "sandwich" manner. Both sides of the thin films were provided by adhesive copper tape contact materials to ensure proper contact between electrodes and the thin film. The thicknesses of the thin films were defined by the mechanical micrometer. The electrical capacitances and resistances of the specimens were measured under different frequencies. The following equations were used to characterize the real and imaginary parts of permittivity [11]:
s'=Cd/s0S s"=d/Rs0Sro
where s' is the real part of the permittivity, s" is the imaginary part of the permittivity, s0 is the dielectric constant in vacuum (8.854 • 10-12 F/m), d is the thickness of the thin film, S is the area of the electrodes, C is the parallel plate capacitance, R is resistance, ro is frequency.
The embedded semiconductor nanoparticles within the polymer matrix change the electrical properties of the system. The reason is the change in the concentration of polymer macromolecules in the polymer nanocomposite system due to the insertion of the nanoparticles into the polymer matrix. Moreover, since the nanocomposites are two-phase systems, an interaction between nanoparticles and polymer matrix can increase the polarization of the system under the external electrical field and that can increase the dielectric permittivity.
It was defined that, in all investigated specimens the electrical capacitances and resistances are inversely proportional to frequency. Thus, the electrical conductivity increases with increasing frequency. Since the addition of the obtained nanostructures to the non-conductive polymer increases its conductivity, the obtained nanostructures can be used as the additives for preparation of various conductive or semi conductive nanocomposites.
Figure 1. Dependence of resistance of the pristine-PVA (a), PVA/MWCNT-MAO/CdS(2h) (b), PVA/MWCNT-MAO/CdS(4h) (c), PVA/MWCNT-MAO/CdS(6h) (d) and PVA/MWCNT-MAO/CuS(e) samples on the logarithmic frequency.
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Figure 2. Dependences of the real part (1) and the imaginary part (2) of permittivity of the pristine-PVA (a), PVA/MWCNT-MAO/CdS(2h) (b), PVA/MW CNT-MAO/CdS(4h) (c) and PVA/MWCNT-MAO/CdS(6h) (d) and PVA/MWCNT-MAO/CuS (e) samples on the logarithmic frequency.
Conclusion. The matrix for the synthesis of the metal-chalcogenide nanoparticles was obtained by the "grafting from" approach using oxidized carbon nanotubes and maleic anhydride -1-octene copolymer. The facile sonochemical approach was used in order to separately synthesize the CdS and CuS nanoparticles within the obtained matrix. The resulting polymer nanocomposites were characterized by XRD and UV-vis spectroscopy. Moreover, the electrophysical properties of the polymer nanocomposites were investigated. These techniques proved the completion of the synthesis and provided insight on certain mechanistic details of the method. The UV-vis. spectroscopy results showed the higher band gap for the obtained samples in comparison to bulk CdS and CuS and revealed the blue shifting due to the crystalline size which is the result of quantum size effects in nanoparticles and it is typical for low-dimensional systems. The calculations using optical results showed that the sonication time affects the size of the obtained CdS nanoparticles. Thus, by increasing the sonication time we can easily reduce the size of the nanoparticles. It was defined that, in all investigated specimens the electrical capacitances and resistances are inversely proportional to frequency. Thus, the electrical conductivity increases with increasing frequency. The results prove that the obtained advanced nanocomposite materials can be used e.g. as a precursor in the manufacturing of advanced plastic composite materials with electrical properties.
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