- Email: [email protected]
Preparation and characterization of ZnS nanoparticles synthesized from chitosan laurate micellar solution
Materials Letters 59 (2005) 989 – 993 www.elsevier.com/locate/matlet
Preparation and characterization of ZnS nanoparticles synthesized from chitosan laurate micellar solution P.S. Khiewa,*, S. Radimana, N.M. Huanga, Md. Soot Ahmada, K. Nadarajahb a
Nuclear Science Program, School of Applied Physics, Faculty Science and Technology, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor, Malaysia b SIRIM Berhad, Bioprocess and Technology Center, 1 Persiaran Mentri, Section 2, P.O. Box 7035, 40911 Shah Alam, Selangor, Malaysia Received 18 May 2004; accepted 8 November 2004 Available online 18 December 2004
Abstract Synthesis of ZnS nanostructured materials has been performed in the micellar solution system, containing chitosan laurate as the surfactant. The self-assembling of the surfactant molecules in water solution can form unique architecture that can be adopted as the reaction template for the formation of nanomaterials. The synthesized nanomaterials have been characterized by energy filter transmission electron microscopy (EFTEM), Energy Dispersive X-ray Analysis (EDAX), X-ray diffractometry (XRD) and UV–Visible absorption measurement in order to determine the size, morphology, composition, crystal structure and optical behavior of the products. The spectroscopic results showed that the synthesized nanoparticles exhibited strong quantum confinement effect as the optical band gap increased significantly as compared to the bulk materials. In addition, the size of the resulting nanoparticles is greatly affected by the surfactant concentration and range from 2 to 10 nm. It was found that the nanomaterials obtained existed in face-centered cubic structure and exhibited the characteristic line broadening feature in the XRD patterns. D 2004 Elsevier B.V. All rights reserved. PACS: 79.60. Jv; 87.64. Bx; 81.10. Dn; 61.82. Rx; 61.82. Fk Keywords: ZnS nanomaterials; Micellar solution; X-ray diffraction; Electron microscopy
1. Introduction Materials with nanoscopic dimensions such as quantum dots, nanorods, nanowires and nanotubes, have attracted a great deal of attention recently due to their intriguing properties that cannot be obtained from the conventional macroscopic (bulk) materials. These novel nanoscale materials are expected to have potential applications in areas such as optoelectronic device technology, photocatalyst fabrication and drug delivery system [1–3]. The unique characteristics of the nanomaterials are believed to have originated from the quantum confinement effects due to the reduction of band structure into discrete quantum levels as a result of the limited size of the particles. * Corresponding author. Tel.: +603 89214131; fax: +603 89213777. E-mail address: poisim_ [email protected] (P.S. Khiew). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.044
Surfactant (surface active agent) is amphiphilic molecule consisting of water insoluble (hydrophobic) unit, which is joined chemically to a water soluble (hydrophilic) fragment. Surfactants have been used for years as the cleaning and solubilization agents in the detergent formulations. However, the applications of surfactants have been extended to become as useful materials in the synthesis of nanostructured or mesostructured materials recently [4]. When surfactant is dissolved in water, its hydrophobic portion will try to minimize its interaction with water by selfassembling into different unique structure. Surfactants can form several types of well-organized aggregate structure and lyotropic liquid crystal phases in water or oil. The possible structures due to the self-assemble behavior of the surfactants in the mixture of water and oil include micelles aggregation, liquid crystal mesophases such as lamellar phase, hexagonal phase, cubic phase and non-equilibrium
990
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
system like myelin figures. The unique architectures that formed are suitable to be adopted as the templates for the formation of nanomaterials [5]. The development of soft chemistry technique in the preparation of nanoparticles seems to be particularly interesting and undergo rapid improvement in the last few years. The achievements in templating the nanomaterials within the surfactant system such as micellar solution, microemulsion and lyotropic condensed mesophases are quite promising. Han and co-workers [6] for example demonstrated that the conducting polyaniline nanoparticles can be templated from the micellar solution. In the previous study, we reported the successful preparation polyaniline-coated cadmium sulphide nanocomposites in the reverse microemulsion system [7]. The preparation of CdS nanoparticles with mesopores structure in bicontinuous cubic phase of lyotropic liquid crystals has been explored by Huang et al. [8] and they showed that the growth reaction in the cubic matrix did not disrupt the amphiphile structure. Zinc sulfide (ZnS) is a semiconductor material, which has a band gap of 3.70 eV [9] and is an ideal material for studies of variation in discrete gap energy due to the reduction of physical dimensions into nanoscale regime. Research over past several years revealed that ZnS particles are potential to be implemented as cathode-ray tube, luminescent materials in flat panel displays and IR windows [10,11]. There have been extensive reports in the past few years demonstrating the systematic exploration of growing ZnS nanoparticles with a narrow size distribution in the surfactant system. Tang et al. [12] for example studied the luminescence and photophysical properties of ZnS nanoparticles prepared by a reverse micelle method. They showed that ZnS nanocrystallites with an average size of 2.5 nm can be obtained in the system. In addition, they also reported that the surface state, shallow trap and deep trap existed in the band gap due to the surfactant passivation layers on the surface of the nanoparticles, can be distinguished from one another using the photoluminescence measurements. The study on the growth reaction of ZnS nanoparticles in the microemulsion system using UV– Vis absorption spectroscopy has been carried out by Calandra and co-workers [13]. They reported that the growing process of the ZnS nanoparticles can be well described by power laws. Zhang et al. [14] reported the successful preparation of ZnS nanorods in the lamellar liquid crystal template. They showed that the concentration of the surfactant and reactant has significant influence on the size and morphology of the nanomaterials obtained. Although surfactant templating technique is an effective pathway for the formation of nanomaterials, most of the commercial surfactant employed are hazardous chemical substances and bring bad effect to the environment. For practical applications, it is therefore necessary to obtain a more environmental friendly formulation for the fabrication of the nanoparticles [15].
Therefore, in the present study, we report for the first time, the preparation of ZnS nanoparticles in micellar solution, containing the chitosan laurate as the surfactant. Chitosan laurate is a novel glycolipid material which is biodegradable, non-toxic and can be adopted to form micellar system in aqueous solution.
2. Materials and method The food grade chitosan laurate were supplied by SIRIM Berhad, Malaysia. Zinc nitrate hexahydrate (98%) and sodium sulfide nonahydrate (99%) were both purchased from Sigma-Aldrich whereas ethanol (99%) was obtained from Fluka. Double distilled and deionized water (Purelab Prima Elga, having 18.2 MV electrical resistivity) was used throughout the sample preparations. All the chemicals, solvents and reagents were of analytical purity and were used as received without further purifications. Two types of micellar system consisting of surfactants and aqueous solutions (containing 0.10M Zn(NO3)2 and 0.10M Na2S, respectively) have been prepared. The clear micellar solution containing Na2S aqueous solution was added slowly into another micellar system consisting of a Zn(NO3)2 aqueous solution and the mixture was stirred for 2 min. The precipitated fine powders obtained from centrifugation were washed with distilled water and absolute ethanol in order to remove the surfactants, residual reactants and byproducts. The X-ray diffraction (XRD) measurements were performed by a Bruker X-ray diffractometer at a scanning rate of 18/min in the range of 208 to 808, using a monochromatized Cu Ka radiation (k=0.154 nm). The silicon standard peak (111) was used to evaluate the instrumental broadening. The EDAX measurements were performed on the Leo 1450 VP VPSEM instrument. The resulting nanoparticles were then redispersed in absolute ethanol using a vortex mixer followed by the sonication bath in order to obtain better particle dispersion. The size and morphology of the ZnS nanoparticles were investigated by Leo energy filter transmission electron microscope (EFTEM) with an accelerating voltage of 120 kV. Samples containing the products were deposited onto carbon film supported by copper grids and evaporated in air at room temperature. The particle size distribution was calculated by image analysis on more than 100 counts. The UV–Vis absorption spectra of the solutions were recorded on a Perkin-Elmer Lambda 35 spectrophotometer in the wavelength range 250–500 nm using a 10 mm cuvette. The measurement has been carried out at room temperature (~25 8C).
3. Results and discussion The X-ray diffraction (XRD) patterns of the ZnS nanoparticles are depicted in Fig. 1. The spectrum presents
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
991
sequently increase the size of the particles due to the enhancement of exchange rate for the reactants. In contrast, this reaction is mild when the surfactant content in the system is high. The presence of bfreeQ and bboundQ water in the surfactant system has been well described by Garti and co-workers [17]. They showed that with the
Fig. 1. The powder XRD patterns of ZnS nanoparticles prepared in micellar solution containing (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.% of chitosan laurate.
three broaden peaks at about 28.58, 47.78 and 56.68 in 2u, which correspond to the Miller index of the reflecting planes for (111), (220) and (311). It is clear that all the diffraction peaks in the spectrum are analogous to the literature pattern of face-centered cubic phase ZnS [16], indicating a very high purity of the powder in all cases. The broadening of the diffraction peaks suggests that the dimensions of the nanoparticles are very small. The average size of the as-prepared nanocrystals can be calculated from the full width half maximum (FWHM) of the diffraction peaks by using the Debye–Scherrer formula. The reflecting peak at the crystal planes of (111) for all the samples is chosen to estimate the average size of the nanocrystals. The average crystallite size of samples prepared in 0.5 wt.% surfactant is estimated to be ca. 7.74 nm. This value decreased to 6.27 nm and 4.04 nm, after the surfactant content in the micellar system was further increased to 1.0 wt.% and 2.0 wt.%, respectively, as calculated using the Scherrer equation. The sizes and morphologies of the as-prepared nanoparticles were studied by EFTEM and the images are depicted in Fig. 2. From the micrographs, it is clear that the resulting particles are fairly monodispersed and most of them present spherical morphologies. The sizes of the products are affected by the surfactant concentration in the micellar system. We found that the size of the ZnS nanoparticles decrease as the surfactant content increase. When 2.0 wt.% surfactants are added into the system, the size of the nanoparticles obtained is in the range of 2.0– 6.0 nm. However, after the surfactant content was decreased to 0.5 wt.%, the size distribution of the particles is broadened to the range of 5.0–10.0 nm. The reason may be that at lower surfactant concentration, there are more bfreeQ water molecules in the system and the interfacial rigidity is lower as compared with that at higher surfactant content. This would lead to the rapid formation and aggregation of primary nuclei and sub-
Fig. 2. The TEM micrographs of ZnS nanoparticles prepared with different surfactant concentration: (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.% of chitosan laurate.
992
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
decrease of the total water content in the system, the presence of bboundQ water became more pronounced. Thus, in high surfactant content micelle system, the existence of bboundQ water is predominant. This will enhance the interfacial rigidity of the system and inhibits the crystal growth reaction, leading to the formation of particles smaller in size. The composition and the purity of the nanoparticles have been determined using the energy dispersive X-ray analysis (EDAX) and the patterns obtained are shown in Fig. 3. As can be seen from the figure, it shows the presence of C, O, Zn and S peaks. The existence of C and O impurities is believed to have originated from the surface contamination in the atmosphere and also the residual surfactants absorbed on the nanoparticles. The average atomic ratio of Zn/S, calculated from the quantification of the peaks (excluded C and O elements), gives the values of 54.5:45.5. These results indicate that the surface of the samples is rich in metal. The deviation of atomic ratio Zn:S to the expected 1:1 is believed to have originated from the absorption of excessive zinc ions on the surface of the nanoparticles. Wang et al. [18] also observed similar results on the nickel sulfide nanoparticles prepared by the sonochemical method. They suggested that the amount of metal ions absorbed on the surface of the nanoparticles is much larger as compared to the bulk materials. UV–Vis absorption spectroscopy is a useful technique to monitor the optical properties of the quantum-sized particles. Generally, the wavelength at the maximum exciton absorption (k max) decreases as the size of the nanoparticles decreases, as a consequence of quantum confinements of the photogenerated electron-hole carriers [13]. Fig. 4 shows the UV–Vis absorption spectra of the ZnS nanoparticles prepared in the micellar solution. The spectra of all the samples consist of a long wavelength tail and an absorption maximum around 300 nm. The corresponding band gap
Fig. 4. The UV–Vis absorption spectra of ZnS nanoparticles synthesized in micellar solution containing (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.%, as measured at room temperature (~25 8C).
energy of absorption maximum for all the samples has been determined using the following equation, Eg ¼
hc kmax
ð1Þ
where E g is the optical band gap energy, h is the Plank constant, c is the velocity of light in vacuum and k max is the wavelength at the maximum absorption. The value of the optical band gap energy for the ZnS nanoparticles synthesized with 0.5 wt.%, 1.0 wt.% and 2.0 wt.% surfactant content have been estimated to be ca. 4.21 eV, 4.24 eV and 4.36 eV, respectively, as calculated using Eq. (1). It is found that the band gap energy of the resulting nanoparticles shows marked increment as compared with that of ZnS bulk materials (3.70 eV). The blue shift of the absorption spectra is attributed to the quantum confinement of charge carriers in the nanoparticles, which is consistent with previous theoretical arguments by Brus [19].
4. Conclusion The micellar system containing the chitosan laurate has been developed as a template for the fabrication ZnS nanoparticles. The decrement of particles size with surfactant content is believed to be due to the increment of interfacial rigidity in the system at high surfactant content. It was also found that the surface of the nanoparticles was rich in metal, due to the absorption of excessive zinc ion in the precursor solution.
Acknowledgement
Fig. 3. The EDAX scanning pattern of the zinc sulfide nanoparticles.
This work was financially supported by IRPA research grant (Project no: 09-02-02-0032-SR0004/04-04). The author (P.S. Khiew) also wishes to acknowledge the partial
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
financial support from the National Science Fellowship (NSF), MOSTI, Malaysia.
References [1] Honma, S. Hirakawa, K. Yamada, J.M. Bae, Solid State Ionics 118 (1999) 29. [2] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 23 (2003) 285. [3] T.S. Phely-Bobin, R.J. Muisener, J.T. Koberstein, F. Papadimitrakopoulos, Synth. Met. 116 (2001) 439. [4] M. Antonietti, Curr. Opin. Colloid Interface Sci. 6 (2001) 244. [5] S.G. Dixit, A.R. Mahadeshwar, S.K. Haram, Colloids Surf., A Physicochem. Eng. Asp. 133 (1998) 69. [6] M.G. Han, S.K. Cho, S.G. Oh, S.S. Im, Synth. Met. 126 (2002) 53. [7] P.S. Khiew, N.M. Huang, S. Radiman, Md. Soot Ahmad, Mater. Lett. 58 (2003) 516. [8] N.M. Huang, C.S. Kan, S. Radiman, Appl. Phys., A Mater. Sci. Process. 115 (2002) 53.
993
[9] D. Denzler, M. Olschewski, K. Sattler, J. Appl. Phys. 84 (1998) 2841. [10] B. Bhattacharjee, D. Ganguli, K. Iakoubovskii, A. Stesmans, S. Chaudhuri, Bull. Mater. Sci. 25 (2002) 175. [11] S. Wageh, S.L. Zhao, X.R. Xu, J. Cryst. Growth 255 (2003) 332. [12] H. Tang, G.Y. Xu, L.Q. Weng, L.J. Pan, L. Wang, Acta Mater. 52 (2004) 1489. [13] P. Calandra, M. Goffredi, V.T. Liveri, Colloids Surf., A Physicochem. Eng. Asp. 160 (1999) 9. [14] D.B. Zhang, L.M. Qi, H.M. Cheng, J.M. Ma, J. Colloid Interface Sci. 246 (2002) 413 – 416. [15] P.S. Khiew, S. Radiman, N.M. Huang, Md. Soot Ahmad, J. Cryst. Growth 254 (2003) 235 – 243. [16] Joint Committee on Powder Diffraction Standards (JCPDS), File No. 77-2100. [17] N. Garti, A. Aserin, I. Tiunova, M. Fanun, Colloids Surf., A Physicochem. Eng. Asp. 170 (2000) 1. [18] H. Wang, J.R. Zhang, X.N. Zhao, S. Xu, J.J. Zhu, Mater. Lett. 55 (2002) 253 – 258. [19] L.E. Brus, J. Chem. Phys. 80 (1984) 4403.
Preparation and characterization of ZnS nanoparticles synthesized from chitosan laurate micellar solution P.S. Khiewa,*, S. Radimana, N.M. Huanga, Md. Soot Ahmada, K. Nadarajahb a
Nuclear Science Program, School of Applied Physics, Faculty Science and Technology, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor, Malaysia b SIRIM Berhad, Bioprocess and Technology Center, 1 Persiaran Mentri, Section 2, P.O. Box 7035, 40911 Shah Alam, Selangor, Malaysia Received 18 May 2004; accepted 8 November 2004 Available online 18 December 2004
Abstract Synthesis of ZnS nanostructured materials has been performed in the micellar solution system, containing chitosan laurate as the surfactant. The self-assembling of the surfactant molecules in water solution can form unique architecture that can be adopted as the reaction template for the formation of nanomaterials. The synthesized nanomaterials have been characterized by energy filter transmission electron microscopy (EFTEM), Energy Dispersive X-ray Analysis (EDAX), X-ray diffractometry (XRD) and UV–Visible absorption measurement in order to determine the size, morphology, composition, crystal structure and optical behavior of the products. The spectroscopic results showed that the synthesized nanoparticles exhibited strong quantum confinement effect as the optical band gap increased significantly as compared to the bulk materials. In addition, the size of the resulting nanoparticles is greatly affected by the surfactant concentration and range from 2 to 10 nm. It was found that the nanomaterials obtained existed in face-centered cubic structure and exhibited the characteristic line broadening feature in the XRD patterns. D 2004 Elsevier B.V. All rights reserved. PACS: 79.60. Jv; 87.64. Bx; 81.10. Dn; 61.82. Rx; 61.82. Fk Keywords: ZnS nanomaterials; Micellar solution; X-ray diffraction; Electron microscopy
1. Introduction Materials with nanoscopic dimensions such as quantum dots, nanorods, nanowires and nanotubes, have attracted a great deal of attention recently due to their intriguing properties that cannot be obtained from the conventional macroscopic (bulk) materials. These novel nanoscale materials are expected to have potential applications in areas such as optoelectronic device technology, photocatalyst fabrication and drug delivery system [1–3]. The unique characteristics of the nanomaterials are believed to have originated from the quantum confinement effects due to the reduction of band structure into discrete quantum levels as a result of the limited size of the particles. * Corresponding author. Tel.: +603 89214131; fax: +603 89213777. E-mail address: poisim_ [email protected] (P.S. Khiew). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.044
Surfactant (surface active agent) is amphiphilic molecule consisting of water insoluble (hydrophobic) unit, which is joined chemically to a water soluble (hydrophilic) fragment. Surfactants have been used for years as the cleaning and solubilization agents in the detergent formulations. However, the applications of surfactants have been extended to become as useful materials in the synthesis of nanostructured or mesostructured materials recently [4]. When surfactant is dissolved in water, its hydrophobic portion will try to minimize its interaction with water by selfassembling into different unique structure. Surfactants can form several types of well-organized aggregate structure and lyotropic liquid crystal phases in water or oil. The possible structures due to the self-assemble behavior of the surfactants in the mixture of water and oil include micelles aggregation, liquid crystal mesophases such as lamellar phase, hexagonal phase, cubic phase and non-equilibrium
990
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
system like myelin figures. The unique architectures that formed are suitable to be adopted as the templates for the formation of nanomaterials [5]. The development of soft chemistry technique in the preparation of nanoparticles seems to be particularly interesting and undergo rapid improvement in the last few years. The achievements in templating the nanomaterials within the surfactant system such as micellar solution, microemulsion and lyotropic condensed mesophases are quite promising. Han and co-workers [6] for example demonstrated that the conducting polyaniline nanoparticles can be templated from the micellar solution. In the previous study, we reported the successful preparation polyaniline-coated cadmium sulphide nanocomposites in the reverse microemulsion system [7]. The preparation of CdS nanoparticles with mesopores structure in bicontinuous cubic phase of lyotropic liquid crystals has been explored by Huang et al. [8] and they showed that the growth reaction in the cubic matrix did not disrupt the amphiphile structure. Zinc sulfide (ZnS) is a semiconductor material, which has a band gap of 3.70 eV [9] and is an ideal material for studies of variation in discrete gap energy due to the reduction of physical dimensions into nanoscale regime. Research over past several years revealed that ZnS particles are potential to be implemented as cathode-ray tube, luminescent materials in flat panel displays and IR windows [10,11]. There have been extensive reports in the past few years demonstrating the systematic exploration of growing ZnS nanoparticles with a narrow size distribution in the surfactant system. Tang et al. [12] for example studied the luminescence and photophysical properties of ZnS nanoparticles prepared by a reverse micelle method. They showed that ZnS nanocrystallites with an average size of 2.5 nm can be obtained in the system. In addition, they also reported that the surface state, shallow trap and deep trap existed in the band gap due to the surfactant passivation layers on the surface of the nanoparticles, can be distinguished from one another using the photoluminescence measurements. The study on the growth reaction of ZnS nanoparticles in the microemulsion system using UV– Vis absorption spectroscopy has been carried out by Calandra and co-workers [13]. They reported that the growing process of the ZnS nanoparticles can be well described by power laws. Zhang et al. [14] reported the successful preparation of ZnS nanorods in the lamellar liquid crystal template. They showed that the concentration of the surfactant and reactant has significant influence on the size and morphology of the nanomaterials obtained. Although surfactant templating technique is an effective pathway for the formation of nanomaterials, most of the commercial surfactant employed are hazardous chemical substances and bring bad effect to the environment. For practical applications, it is therefore necessary to obtain a more environmental friendly formulation for the fabrication of the nanoparticles [15].
Therefore, in the present study, we report for the first time, the preparation of ZnS nanoparticles in micellar solution, containing the chitosan laurate as the surfactant. Chitosan laurate is a novel glycolipid material which is biodegradable, non-toxic and can be adopted to form micellar system in aqueous solution.
2. Materials and method The food grade chitosan laurate were supplied by SIRIM Berhad, Malaysia. Zinc nitrate hexahydrate (98%) and sodium sulfide nonahydrate (99%) were both purchased from Sigma-Aldrich whereas ethanol (99%) was obtained from Fluka. Double distilled and deionized water (Purelab Prima Elga, having 18.2 MV electrical resistivity) was used throughout the sample preparations. All the chemicals, solvents and reagents were of analytical purity and were used as received without further purifications. Two types of micellar system consisting of surfactants and aqueous solutions (containing 0.10M Zn(NO3)2 and 0.10M Na2S, respectively) have been prepared. The clear micellar solution containing Na2S aqueous solution was added slowly into another micellar system consisting of a Zn(NO3)2 aqueous solution and the mixture was stirred for 2 min. The precipitated fine powders obtained from centrifugation were washed with distilled water and absolute ethanol in order to remove the surfactants, residual reactants and byproducts. The X-ray diffraction (XRD) measurements were performed by a Bruker X-ray diffractometer at a scanning rate of 18/min in the range of 208 to 808, using a monochromatized Cu Ka radiation (k=0.154 nm). The silicon standard peak (111) was used to evaluate the instrumental broadening. The EDAX measurements were performed on the Leo 1450 VP VPSEM instrument. The resulting nanoparticles were then redispersed in absolute ethanol using a vortex mixer followed by the sonication bath in order to obtain better particle dispersion. The size and morphology of the ZnS nanoparticles were investigated by Leo energy filter transmission electron microscope (EFTEM) with an accelerating voltage of 120 kV. Samples containing the products were deposited onto carbon film supported by copper grids and evaporated in air at room temperature. The particle size distribution was calculated by image analysis on more than 100 counts. The UV–Vis absorption spectra of the solutions were recorded on a Perkin-Elmer Lambda 35 spectrophotometer in the wavelength range 250–500 nm using a 10 mm cuvette. The measurement has been carried out at room temperature (~25 8C).
3. Results and discussion The X-ray diffraction (XRD) patterns of the ZnS nanoparticles are depicted in Fig. 1. The spectrum presents
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
991
sequently increase the size of the particles due to the enhancement of exchange rate for the reactants. In contrast, this reaction is mild when the surfactant content in the system is high. The presence of bfreeQ and bboundQ water in the surfactant system has been well described by Garti and co-workers [17]. They showed that with the
Fig. 1. The powder XRD patterns of ZnS nanoparticles prepared in micellar solution containing (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.% of chitosan laurate.
three broaden peaks at about 28.58, 47.78 and 56.68 in 2u, which correspond to the Miller index of the reflecting planes for (111), (220) and (311). It is clear that all the diffraction peaks in the spectrum are analogous to the literature pattern of face-centered cubic phase ZnS [16], indicating a very high purity of the powder in all cases. The broadening of the diffraction peaks suggests that the dimensions of the nanoparticles are very small. The average size of the as-prepared nanocrystals can be calculated from the full width half maximum (FWHM) of the diffraction peaks by using the Debye–Scherrer formula. The reflecting peak at the crystal planes of (111) for all the samples is chosen to estimate the average size of the nanocrystals. The average crystallite size of samples prepared in 0.5 wt.% surfactant is estimated to be ca. 7.74 nm. This value decreased to 6.27 nm and 4.04 nm, after the surfactant content in the micellar system was further increased to 1.0 wt.% and 2.0 wt.%, respectively, as calculated using the Scherrer equation. The sizes and morphologies of the as-prepared nanoparticles were studied by EFTEM and the images are depicted in Fig. 2. From the micrographs, it is clear that the resulting particles are fairly monodispersed and most of them present spherical morphologies. The sizes of the products are affected by the surfactant concentration in the micellar system. We found that the size of the ZnS nanoparticles decrease as the surfactant content increase. When 2.0 wt.% surfactants are added into the system, the size of the nanoparticles obtained is in the range of 2.0– 6.0 nm. However, after the surfactant content was decreased to 0.5 wt.%, the size distribution of the particles is broadened to the range of 5.0–10.0 nm. The reason may be that at lower surfactant concentration, there are more bfreeQ water molecules in the system and the interfacial rigidity is lower as compared with that at higher surfactant content. This would lead to the rapid formation and aggregation of primary nuclei and sub-
Fig. 2. The TEM micrographs of ZnS nanoparticles prepared with different surfactant concentration: (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.% of chitosan laurate.
992
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
decrease of the total water content in the system, the presence of bboundQ water became more pronounced. Thus, in high surfactant content micelle system, the existence of bboundQ water is predominant. This will enhance the interfacial rigidity of the system and inhibits the crystal growth reaction, leading to the formation of particles smaller in size. The composition and the purity of the nanoparticles have been determined using the energy dispersive X-ray analysis (EDAX) and the patterns obtained are shown in Fig. 3. As can be seen from the figure, it shows the presence of C, O, Zn and S peaks. The existence of C and O impurities is believed to have originated from the surface contamination in the atmosphere and also the residual surfactants absorbed on the nanoparticles. The average atomic ratio of Zn/S, calculated from the quantification of the peaks (excluded C and O elements), gives the values of 54.5:45.5. These results indicate that the surface of the samples is rich in metal. The deviation of atomic ratio Zn:S to the expected 1:1 is believed to have originated from the absorption of excessive zinc ions on the surface of the nanoparticles. Wang et al. [18] also observed similar results on the nickel sulfide nanoparticles prepared by the sonochemical method. They suggested that the amount of metal ions absorbed on the surface of the nanoparticles is much larger as compared to the bulk materials. UV–Vis absorption spectroscopy is a useful technique to monitor the optical properties of the quantum-sized particles. Generally, the wavelength at the maximum exciton absorption (k max) decreases as the size of the nanoparticles decreases, as a consequence of quantum confinements of the photogenerated electron-hole carriers [13]. Fig. 4 shows the UV–Vis absorption spectra of the ZnS nanoparticles prepared in the micellar solution. The spectra of all the samples consist of a long wavelength tail and an absorption maximum around 300 nm. The corresponding band gap
Fig. 4. The UV–Vis absorption spectra of ZnS nanoparticles synthesized in micellar solution containing (a) 0.5 wt.%, (b) 1.0 wt.%, (c) 2.0 wt.%, as measured at room temperature (~25 8C).
energy of absorption maximum for all the samples has been determined using the following equation, Eg ¼
hc kmax
ð1Þ
where E g is the optical band gap energy, h is the Plank constant, c is the velocity of light in vacuum and k max is the wavelength at the maximum absorption. The value of the optical band gap energy for the ZnS nanoparticles synthesized with 0.5 wt.%, 1.0 wt.% and 2.0 wt.% surfactant content have been estimated to be ca. 4.21 eV, 4.24 eV and 4.36 eV, respectively, as calculated using Eq. (1). It is found that the band gap energy of the resulting nanoparticles shows marked increment as compared with that of ZnS bulk materials (3.70 eV). The blue shift of the absorption spectra is attributed to the quantum confinement of charge carriers in the nanoparticles, which is consistent with previous theoretical arguments by Brus [19].
4. Conclusion The micellar system containing the chitosan laurate has been developed as a template for the fabrication ZnS nanoparticles. The decrement of particles size with surfactant content is believed to be due to the increment of interfacial rigidity in the system at high surfactant content. It was also found that the surface of the nanoparticles was rich in metal, due to the absorption of excessive zinc ion in the precursor solution.
Acknowledgement
Fig. 3. The EDAX scanning pattern of the zinc sulfide nanoparticles.
This work was financially supported by IRPA research grant (Project no: 09-02-02-0032-SR0004/04-04). The author (P.S. Khiew) also wishes to acknowledge the partial
P.S. Khiew et al. / Materials Letters 59 (2005) 989–993
financial support from the National Science Fellowship (NSF), MOSTI, Malaysia.
References [1] Honma, S. Hirakawa, K. Yamada, J.M. Bae, Solid State Ionics 118 (1999) 29. [2] M.L. Curri, R. Comparelli, P.D. Cozzoli, G. Mascolo, A. Agostiano, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 23 (2003) 285. [3] T.S. Phely-Bobin, R.J. Muisener, J.T. Koberstein, F. Papadimitrakopoulos, Synth. Met. 116 (2001) 439. [4] M. Antonietti, Curr. Opin. Colloid Interface Sci. 6 (2001) 244. [5] S.G. Dixit, A.R. Mahadeshwar, S.K. Haram, Colloids Surf., A Physicochem. Eng. Asp. 133 (1998) 69. [6] M.G. Han, S.K. Cho, S.G. Oh, S.S. Im, Synth. Met. 126 (2002) 53. [7] P.S. Khiew, N.M. Huang, S. Radiman, Md. Soot Ahmad, Mater. Lett. 58 (2003) 516. [8] N.M. Huang, C.S. Kan, S. Radiman, Appl. Phys., A Mater. Sci. Process. 115 (2002) 53.
993
[9] D. Denzler, M. Olschewski, K. Sattler, J. Appl. Phys. 84 (1998) 2841. [10] B. Bhattacharjee, D. Ganguli, K. Iakoubovskii, A. Stesmans, S. Chaudhuri, Bull. Mater. Sci. 25 (2002) 175. [11] S. Wageh, S.L. Zhao, X.R. Xu, J. Cryst. Growth 255 (2003) 332. [12] H. Tang, G.Y. Xu, L.Q. Weng, L.J. Pan, L. Wang, Acta Mater. 52 (2004) 1489. [13] P. Calandra, M. Goffredi, V.T. Liveri, Colloids Surf., A Physicochem. Eng. Asp. 160 (1999) 9. [14] D.B. Zhang, L.M. Qi, H.M. Cheng, J.M. Ma, J. Colloid Interface Sci. 246 (2002) 413 – 416. [15] P.S. Khiew, S. Radiman, N.M. Huang, Md. Soot Ahmad, J. Cryst. Growth 254 (2003) 235 – 243. [16] Joint Committee on Powder Diffraction Standards (JCPDS), File No. 77-2100. [17] N. Garti, A. Aserin, I. Tiunova, M. Fanun, Colloids Surf., A Physicochem. Eng. Asp. 170 (2000) 1. [18] H. Wang, J.R. Zhang, X.N. Zhao, S. Xu, J.J. Zhu, Mater. Lett. 55 (2002) 253 – 258. [19] L.E. Brus, J. Chem. Phys. 80 (1984) 4403.