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Synthesis of CdS nanoparticles by a novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant
Materials Letters 57 (2003) 2755 – 2760 www.elsevier.com/locate/matlet
Synthesis of CdS nanoparticles by a novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant Wenzhong Wang a,b, Zhihui Liu c, Changlin Zheng a, Congkang Xu a, Yingkai Liu a, Guanghou Wang a,b,* a
National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China b Structural Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China c Department of Biological Science and Technology, Nanjing University, Nanjing 210093, People’s Republic of China Received 16 October 2002; accepted 26 October 2002
Abstract A novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant, C18H37O(CH2CH2O)10H (abbreviated as C18EO10), has been developed to synthesize uniform cubic-phase h-CdS nanoparticles with an average diameter of ca. 5 nm. The CdS nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV – VIS optical absorption spectrum and X-ray photoelectron spectrum (XPS). The roles of nonionic surfactant, C18EO10, in the formation of CdS nanoparticles were discussed in detail. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Solid-state synthesis; CdS; Nanoparticles
1. Introduction CdS is an important semiconductor owing to its unique electronic and optical properties, and its potential applications in solar energy conversion, nonlinear optical, photoelectrochemical cells and heterogeneous photocatalysis [1,2]. The synthesis of CdS nanopar-
* Corresponding author. National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China. Fax: +86-25-3595535. E-mail address: [email protected] (G. Wang).
ticles has been tried by various methods such as the direct reaction of metals with sulfur powders under high temperature [3], the thermal decomposition of molecular precursors containing M –S bonds [4,5] or the use of poisonous H2S as the S2 source at higher temperature [6] and chemical precipitation method involving the precipitation of metal ions with Na2S as the source of S2 ions [7,8], in which the inhomogeneity at an early stage results in a broadening of the product particle size distributions. Recently, Wang et al. [9] and Yin et al. [10] prepared CdS nanocrystallites using g-irradiation. However, either complex process control, reagents or long synthesis time may be required for this route.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01371-X
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W. Wang et al. / Materials Letters 57 (2003) 2755–2760
Herein, we report on the successful preparation of CdS nanoparticles by a novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant, C18H37O(CH2CH2O)10H (abbreviated as C18EO10). This novel route requires neither complex apparatus, reagents nor sophisticated techniques, and avoids the use of noxious H2S, and can be carried out in air at room temperature for the synthesis of CdS nanoparticles with almost uniform size and shapes in high yields.
3. Results and discussion 3.1. Formation of CdS nanoparticles The CdS nanoparticles most likely form via following solid-state reaction: CdCl2 2:5H2 OðsÞ room temperature
þ Na2 S9H2 OðsÞ ! CdSðsÞ grinding
þ2NaCl þ 11:5H2 O:
ð1Þ
2. Experimental All of the chemical reagents used were of analytical grade. The procedure employed for preparing CdS nanoparticles is as follows. In a typical synthesis, 5.841 g of CdCl22.5H2O and 6.144 g of Na2S9H2O were ground for 5 min each before mixing with 8 ml of C18EO10. After 30 min of grinding, the orange products were washed several times with distilled water in an ultrasonic bath (100 W), and subsequently washed with EtOH to remove C18EO10, NaCl and unreacted precursors completely. Finally, the products were dried for 8 h in an oven at 60 jC. The crystal structure and composition of the asprepared CdS nanoparticles were analyzed by powder X-ray diffraction (XRD) using a Rigaku Dmax g A Xray diffractometer with Cu-Ka radiation (k = 0.154178 nm). A scan rate of 0.02j s 1 was applied to record the powder patterns in the 2h range of 15 –70j. Powder morphology and size were characterized by transmission electron microscopy (TEM), on a JEM200 CX transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) image was carried out on a JEOL-2010 electron microscope, using an accelerating voltage of 200 kV. Absorption spectrum was recorded using a Shimadzu UV – VIS UV 265 spectrophotometer in the wavelength range of 200 –800 nm at room temperature. The X-ray photoelectron spectra (XPS) were collected on an ESCALAB M K II X-ray photoelectron spectrometer, using Mg-Ka X-ray as the excitation sources. The binding energies obtained from the XPS analysis were corrected with reference to C1s (284.6 eV).
We observed, during the experimental process, that the orange products were formed once upon mixing and grinding two reactants. This indicates that the chemical reaction rate of the reactive system is fast and the nucleation rate is far excess the growth rate of particle. Therefore, this reactive mechanism favors the formation of nanosized CdS particles. 3.2. Characterization of the as-prepared CdS nanoparticles Fig. 1 shows the XRD pattern of CdS nanoparticles prepared by one-step, solid-state reaction in the presence of a nonionic surfactant, C18EO10. It was compared with the data of the JCPDS file no. 10-454 and is in good agreement with that of pure cubic-
Fig. 1. XRD pattern of the as-prepared CdS nanoparticles.
W. Wang et al. / Materials Letters 57 (2003) 2755–2760
phase h-CdS, without signals from CdCl2, NaOH and other precursor compounds. The three peaks with 2h values of 26.514j, 43.762j and 52.0j correspond to the three crystal plans of (111), (220) and (311) of cubic-phase h-CdS, respectively. The broadness of the peaks indicates that the dimensions of the CdS nanoparticles are very small. The shoulders in the (111) and (311) diffraction peaks may be resulted from the X-ray irradiation on the sample. It is well known that the cubic CdS is a metastable phase [11]; when the X-ray irradiates on the CdS, it may induce phase transition from cubic phase to hexagonal phase CdS, resulting in the appearance of the shoulders in the (111) and (311) peaks. The TEM images of CdS nanoparticles are shown in Fig. 2a. It can be seen that nanoparticles have spherical morphology with an average diameter of ca. 5 nm. Because of the small dimensions and high surface energy of the particles, it is easy for them to aggregate as seen in Fig. 2a. We also can find from this figure that the morphology of the particles is almost homogeneous. Histogram revealing the size distribution of the CdS nanoparticles is shown in Fig. 2b. From Fig. 2b, we obtained that CdS nanoparticles range in size from 2 to 8 nm and that the largest percentage (60%) is in the range of 5– 6 nm. The average particle size is 5.5 nm calculated from Fig. 2b, which is in agreement
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with the result calculated for the half-width of diffraction peaks using the Scherrer’s formula, allowing for experimental error. There are several methods for the synthesis of CdS nanoparticles. For example, Yang et al. [12] synthesized size-quantized CdS crystals, which have diameters of 5 –15 nm, by using epitaxial growth. Xu et al. [13] prepared CdS nanoparticles with diameter of 100 nm via hydrothermal synthesis. Zhu et al. [14] produced CdS nanoparticles with 5 – 10 nm by using microwave irradiation method. Compared with the results obtained from these preparative routes, the present results demonstrate that this method can easily fabricate CdS nanoparticles with small dimensions (average diameter ca. 5.5 nm) and narrow size distribution (2– 8 nm). High-resolution transmission electron microscopy (HRTEM) image of the nanoparticles is shown in Fig. 3a. The lattice fringes clearly visible in the HRTEM image are indicative of crystallinity of the nanoparticles. The small dimensions of the nanoparticles do not allow the examination of a single nanoparticle by conventional selected area electron diffraction (SAED). Fig. 3b shows the SAED pattern of an area containing some nanoparticles. The SAED pattern shows a set of rings instead of spots due to the random orientation of the nanoparticles. Three rings correspond to the (111), (220) and (311) planes of the cubic CdS phase, respectively. This result is
Fig. 2. (a) TEM images of the CdS nanoparticles sample shown in Fig. 1. (b) Histogram showing the diameter distribution of the CdS nanoparticles.
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Fig. 3. (a) HRTEM image of the CdS nanoparticles. (b) SAED pattern of the CdS nanoparticles.
consistent with that of XRD, and confirms that the as-synthesized CdS nanoparticles are composed of pure cubic-phase h-CdS. Fig. 4 shows the UV – VIS optical absorption spectrum of CdS nanoparticles We can estimate that an UV – VIS optical absorption excitonic peak of the nanoparticles is at ca. 460 nm , and shows a blue shift from that of bulk CdS (530 nm) crystals. This result is in agreement with the value of the reported literatures [15]. From the spectra, we also estimated the bandgap of CdS nanoparticles to be ca. 2.70 eV. Compared with that of bulk CdS (2.42 eV) [16], this clearly indicates the presence of quantum-size effects in the prepared CdS nanoparticles. Further evidence for the purity and composition of the as-prepared nanoparticles was obtained by X-ray photoelectron spectra (XPS). Fig. 5a shows the photoelectron spectrum of Cd3d. Peak at 405.25 eV corresponding to the binding energy of Cd3d5/2 is consistent with that observed in CdS [17]. Fig. 5b reveals the photoelectron spectrum of S2p of the sample. The
corresponding binding energy of S2p is 162.5 eV which is also in agreement with that obtained in CdS [17]. The two accompanying peaks of S2p binding energy at higher energy could be the photoelectron energy loss spectra. When photoelectron passes through the surface of CdS nanoparticles, it may take place inelastic collision between the photoelectron and atom or molecule resulting in photoelectron energy loss. This leads to the appearance of the discrete photoelectron energy loss spectra (accompanying peaks) in the XPS spectra [18]. The quantification of peaks of Cd3d and S2p give the ratio of Cd/S as 49.68:50.32, which is almost consistent with the stoichiometry of CdS. It should be noted that the absolute value of the surface composition can not be easily obtained because of the difficulty in discriminating the C signal from the organic amphiphile (in C18EO10) to those from the atmosphere contaminants. However, in our obtained XPS spectra, both Cd and S signals are stronger relative to the C signal, and Cd and S have approximate 1:1 stoichiometric ratio agreeing with the formation of CdS, indicating that the surfactant C18EO10 could be removed from the synthesized CdS nanoparticles. 3.3. Role of the surfactant C18EO10
Fig. 4. UV – VIS absorption spectrum of the CdS nanoparticles.
We speculated that the surfactant C18EO10 plays a critical role in the formation of CdS nanoparticles. It is known that the structures of products by the solidstate reaction depend on the rate of nucleation and growth of the reaction products. In the solid-state reaction between CdCl22.5H2O and Na2S9H2O to produce CdS nanoparticles, free water and NaCl are
W. Wang et al. / Materials Letters 57 (2003) 2755–2760
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Fig. 5. (a) X-ray photoelectron spectrum of Cd3d of the CdS nanoparticles. (b) X-ray photoelectron spectrum of S2p of the CdS nanoparticles.
produced. The surfactant C18EO10 solves in the free water to form a C18EO10 ‘‘shell’’ surrounding the CdS particles, preventing them from aggregating to larger particles, due to the following reasons [19 – 21]: (1) the C18EO10 ‘‘shell’’ separates the CdS particles, forming the physical and spacious obstacle between the particles; (2) the C18EO10 ‘‘shell’’ absorbs some of the collision energy during grinding, reducing the energy transferring into the reactants; and (3) the C18EO10 ‘‘shell’’ absorbs heat generated by the reaction, reducing the temperature during grinding. To further improve the understanding of the ‘‘shell’’ effects of the surfactant C18EO10, in the same C18EO10 solidstate reaction system, we have synthesized the semiconductors ZnS, PbS, CuS and PbO nanoparticles by only varying the reactants. The results further supported the proposed ‘‘shell’’ mechanism.
4. Conclusions A solid-state synthesis of CdS nanoparticles in the presence of a nonionic surfactant C18EO10 at ambient temperature has been developed. The average diameter of CdS nanoparticles is ca. 5 nm. The structural nature and the chemical composition of CdS nanoparticles have been analyzed by XRD, TEM, HRTEM, SAED, UV –VIS optical absorption and XPS. We speculated that the surfactant C18EO10 plays an important role in the formation of CdS nanoparticles. This easy and unique solid-state synthesis in the presence of a non-
ionic surfactant, C18EO10, may be extended to the synthesis of other materials of nanoparticles, such as semiconductors ZnS, CuS, NiS, FeS.
Acknowledgements This work was supported by the National Natural Science Foundation of P.R. China (nos. 29890210, 10023001, 10074024).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
K. Hu, M. Brust, A. Bard, Chem. Mater. 10 (1998) 1160. H. Weller, Angew. Chem., Int. Ed. Engl. 32 (1993) 41. C. Kaito, Y. Saito, K. Fujita, J. Cryst. Growth 94 (1989) 967. D.M. Milhemy, E. Matijevic, J. Chem. Soc., Faraday Trans. (1984) 563. B. Ludoph, M.A. Malik, Chem. Commun. (1998) 1849. N.L. Pickeet, D.F. Foster, D.J. Cole Hamilton, J. Mater. Chem. 6 (1996) 507. N. Herron, Y. Wang, J. Am. Chem. Soc. 112 (1990) 1322. W. Chen, Z. Liu, Z. Wang, Solid State Commun. 100 (1996) 101. C.Y. Wang, X. Mo, Y. Zhou, Y.R. Zhu, H.T. Liu, Z.Y. Chen, J. Mater. Chem. 10 (2000) 607. Y.D. Yin, X.L. Xu, X.W. Ge, C.J. Xia, Z.C. Zhang, Chem. Commun. (1998) 1641. O. Zelaya-Angel, F.L. de Castillo-Alvarado, J. AvendanoLopez, A. Escamilla-Esquivel, G. Contreras-Puente, R. Lozada-Morales, G. Torres-Delgado, Solid State Commun. 104 (1997) 161.
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[12] J.P. Yang, F.C. Meldrum, J.H. Fendler, J. Phys. Chem. 99 (1995) 5500. [13] G.Q. Xu, B. Li, S.J. Xu, C.H. Chew, S.J. Chua, L.M. Gana, J. Phys. Chem. Solids 61 (2000) 829. [14] J.J. Zhu, M.G. Zhou, J.Z. Xu, X.H. Liao, Mater. Lett. 47 (2001) 25. [15] A. Henglein, Chem. Rev. 89 (1989) 1861. [16] B. Ludolph, M.A. Malik, P. O’Brien, N. Revaprasadu, Chem. Commun. (1998) 1849. [17] C.D. Wanger, W.M. Riggs, L.E. Davis, J.E. Moulder,
[18] [19] [20] [21]
G.E. Muilenber, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer, Physical Electronics Division, USA, 1979. D. Briggs, Handbook of X-ray and Ultraviolet Photoelectron Spectroscopy, Heyden, London, 1997. C. Liu, X.M. Jang, B.Z. Li, W.F. Zhang, Applications of Surfactant, Beijing Polytechnic Univ. Press, Beijing, 1992. X.T. Wang, L.M. Hu, D. Gu, Y. Hu, Huaxue Tongbao 5 (1995) 13. R.E. Johnson, W.H. Morrison, Adv. Ceram. 21 (1987) 323.
Synthesis of CdS nanoparticles by a novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant Wenzhong Wang a,b, Zhihui Liu c, Changlin Zheng a, Congkang Xu a, Yingkai Liu a, Guanghou Wang a,b,* a
National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China b Structural Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China c Department of Biological Science and Technology, Nanjing University, Nanjing 210093, People’s Republic of China Received 16 October 2002; accepted 26 October 2002
Abstract A novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant, C18H37O(CH2CH2O)10H (abbreviated as C18EO10), has been developed to synthesize uniform cubic-phase h-CdS nanoparticles with an average diameter of ca. 5 nm. The CdS nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV – VIS optical absorption spectrum and X-ray photoelectron spectrum (XPS). The roles of nonionic surfactant, C18EO10, in the formation of CdS nanoparticles were discussed in detail. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Solid-state synthesis; CdS; Nanoparticles
1. Introduction CdS is an important semiconductor owing to its unique electronic and optical properties, and its potential applications in solar energy conversion, nonlinear optical, photoelectrochemical cells and heterogeneous photocatalysis [1,2]. The synthesis of CdS nanopar-
* Corresponding author. National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China. Fax: +86-25-3595535. E-mail address: [email protected] (G. Wang).
ticles has been tried by various methods such as the direct reaction of metals with sulfur powders under high temperature [3], the thermal decomposition of molecular precursors containing M –S bonds [4,5] or the use of poisonous H2S as the S2 source at higher temperature [6] and chemical precipitation method involving the precipitation of metal ions with Na2S as the source of S2 ions [7,8], in which the inhomogeneity at an early stage results in a broadening of the product particle size distributions. Recently, Wang et al. [9] and Yin et al. [10] prepared CdS nanocrystallites using g-irradiation. However, either complex process control, reagents or long synthesis time may be required for this route.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01371-X
2756
W. Wang et al. / Materials Letters 57 (2003) 2755–2760
Herein, we report on the successful preparation of CdS nanoparticles by a novel and simple one-step, solid-state reaction in the presence of a nonionic surfactant, C18H37O(CH2CH2O)10H (abbreviated as C18EO10). This novel route requires neither complex apparatus, reagents nor sophisticated techniques, and avoids the use of noxious H2S, and can be carried out in air at room temperature for the synthesis of CdS nanoparticles with almost uniform size and shapes in high yields.
3. Results and discussion 3.1. Formation of CdS nanoparticles The CdS nanoparticles most likely form via following solid-state reaction: CdCl2 2:5H2 OðsÞ room temperature
þ Na2 S9H2 OðsÞ ! CdSðsÞ grinding
þ2NaCl þ 11:5H2 O:
ð1Þ
2. Experimental All of the chemical reagents used were of analytical grade. The procedure employed for preparing CdS nanoparticles is as follows. In a typical synthesis, 5.841 g of CdCl22.5H2O and 6.144 g of Na2S9H2O were ground for 5 min each before mixing with 8 ml of C18EO10. After 30 min of grinding, the orange products were washed several times with distilled water in an ultrasonic bath (100 W), and subsequently washed with EtOH to remove C18EO10, NaCl and unreacted precursors completely. Finally, the products were dried for 8 h in an oven at 60 jC. The crystal structure and composition of the asprepared CdS nanoparticles were analyzed by powder X-ray diffraction (XRD) using a Rigaku Dmax g A Xray diffractometer with Cu-Ka radiation (k = 0.154178 nm). A scan rate of 0.02j s 1 was applied to record the powder patterns in the 2h range of 15 –70j. Powder morphology and size were characterized by transmission electron microscopy (TEM), on a JEM200 CX transmission electron microscope, using an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) image was carried out on a JEOL-2010 electron microscope, using an accelerating voltage of 200 kV. Absorption spectrum was recorded using a Shimadzu UV – VIS UV 265 spectrophotometer in the wavelength range of 200 –800 nm at room temperature. The X-ray photoelectron spectra (XPS) were collected on an ESCALAB M K II X-ray photoelectron spectrometer, using Mg-Ka X-ray as the excitation sources. The binding energies obtained from the XPS analysis were corrected with reference to C1s (284.6 eV).
We observed, during the experimental process, that the orange products were formed once upon mixing and grinding two reactants. This indicates that the chemical reaction rate of the reactive system is fast and the nucleation rate is far excess the growth rate of particle. Therefore, this reactive mechanism favors the formation of nanosized CdS particles. 3.2. Characterization of the as-prepared CdS nanoparticles Fig. 1 shows the XRD pattern of CdS nanoparticles prepared by one-step, solid-state reaction in the presence of a nonionic surfactant, C18EO10. It was compared with the data of the JCPDS file no. 10-454 and is in good agreement with that of pure cubic-
Fig. 1. XRD pattern of the as-prepared CdS nanoparticles.
W. Wang et al. / Materials Letters 57 (2003) 2755–2760
phase h-CdS, without signals from CdCl2, NaOH and other precursor compounds. The three peaks with 2h values of 26.514j, 43.762j and 52.0j correspond to the three crystal plans of (111), (220) and (311) of cubic-phase h-CdS, respectively. The broadness of the peaks indicates that the dimensions of the CdS nanoparticles are very small. The shoulders in the (111) and (311) diffraction peaks may be resulted from the X-ray irradiation on the sample. It is well known that the cubic CdS is a metastable phase [11]; when the X-ray irradiates on the CdS, it may induce phase transition from cubic phase to hexagonal phase CdS, resulting in the appearance of the shoulders in the (111) and (311) peaks. The TEM images of CdS nanoparticles are shown in Fig. 2a. It can be seen that nanoparticles have spherical morphology with an average diameter of ca. 5 nm. Because of the small dimensions and high surface energy of the particles, it is easy for them to aggregate as seen in Fig. 2a. We also can find from this figure that the morphology of the particles is almost homogeneous. Histogram revealing the size distribution of the CdS nanoparticles is shown in Fig. 2b. From Fig. 2b, we obtained that CdS nanoparticles range in size from 2 to 8 nm and that the largest percentage (60%) is in the range of 5– 6 nm. The average particle size is 5.5 nm calculated from Fig. 2b, which is in agreement
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with the result calculated for the half-width of diffraction peaks using the Scherrer’s formula, allowing for experimental error. There are several methods for the synthesis of CdS nanoparticles. For example, Yang et al. [12] synthesized size-quantized CdS crystals, which have diameters of 5 –15 nm, by using epitaxial growth. Xu et al. [13] prepared CdS nanoparticles with diameter of 100 nm via hydrothermal synthesis. Zhu et al. [14] produced CdS nanoparticles with 5 – 10 nm by using microwave irradiation method. Compared with the results obtained from these preparative routes, the present results demonstrate that this method can easily fabricate CdS nanoparticles with small dimensions (average diameter ca. 5.5 nm) and narrow size distribution (2– 8 nm). High-resolution transmission electron microscopy (HRTEM) image of the nanoparticles is shown in Fig. 3a. The lattice fringes clearly visible in the HRTEM image are indicative of crystallinity of the nanoparticles. The small dimensions of the nanoparticles do not allow the examination of a single nanoparticle by conventional selected area electron diffraction (SAED). Fig. 3b shows the SAED pattern of an area containing some nanoparticles. The SAED pattern shows a set of rings instead of spots due to the random orientation of the nanoparticles. Three rings correspond to the (111), (220) and (311) planes of the cubic CdS phase, respectively. This result is
Fig. 2. (a) TEM images of the CdS nanoparticles sample shown in Fig. 1. (b) Histogram showing the diameter distribution of the CdS nanoparticles.
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Fig. 3. (a) HRTEM image of the CdS nanoparticles. (b) SAED pattern of the CdS nanoparticles.
consistent with that of XRD, and confirms that the as-synthesized CdS nanoparticles are composed of pure cubic-phase h-CdS. Fig. 4 shows the UV – VIS optical absorption spectrum of CdS nanoparticles We can estimate that an UV – VIS optical absorption excitonic peak of the nanoparticles is at ca. 460 nm , and shows a blue shift from that of bulk CdS (530 nm) crystals. This result is in agreement with the value of the reported literatures [15]. From the spectra, we also estimated the bandgap of CdS nanoparticles to be ca. 2.70 eV. Compared with that of bulk CdS (2.42 eV) [16], this clearly indicates the presence of quantum-size effects in the prepared CdS nanoparticles. Further evidence for the purity and composition of the as-prepared nanoparticles was obtained by X-ray photoelectron spectra (XPS). Fig. 5a shows the photoelectron spectrum of Cd3d. Peak at 405.25 eV corresponding to the binding energy of Cd3d5/2 is consistent with that observed in CdS [17]. Fig. 5b reveals the photoelectron spectrum of S2p of the sample. The
corresponding binding energy of S2p is 162.5 eV which is also in agreement with that obtained in CdS [17]. The two accompanying peaks of S2p binding energy at higher energy could be the photoelectron energy loss spectra. When photoelectron passes through the surface of CdS nanoparticles, it may take place inelastic collision between the photoelectron and atom or molecule resulting in photoelectron energy loss. This leads to the appearance of the discrete photoelectron energy loss spectra (accompanying peaks) in the XPS spectra [18]. The quantification of peaks of Cd3d and S2p give the ratio of Cd/S as 49.68:50.32, which is almost consistent with the stoichiometry of CdS. It should be noted that the absolute value of the surface composition can not be easily obtained because of the difficulty in discriminating the C signal from the organic amphiphile (in C18EO10) to those from the atmosphere contaminants. However, in our obtained XPS spectra, both Cd and S signals are stronger relative to the C signal, and Cd and S have approximate 1:1 stoichiometric ratio agreeing with the formation of CdS, indicating that the surfactant C18EO10 could be removed from the synthesized CdS nanoparticles. 3.3. Role of the surfactant C18EO10
Fig. 4. UV – VIS absorption spectrum of the CdS nanoparticles.
We speculated that the surfactant C18EO10 plays a critical role in the formation of CdS nanoparticles. It is known that the structures of products by the solidstate reaction depend on the rate of nucleation and growth of the reaction products. In the solid-state reaction between CdCl22.5H2O and Na2S9H2O to produce CdS nanoparticles, free water and NaCl are
W. Wang et al. / Materials Letters 57 (2003) 2755–2760
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Fig. 5. (a) X-ray photoelectron spectrum of Cd3d of the CdS nanoparticles. (b) X-ray photoelectron spectrum of S2p of the CdS nanoparticles.
produced. The surfactant C18EO10 solves in the free water to form a C18EO10 ‘‘shell’’ surrounding the CdS particles, preventing them from aggregating to larger particles, due to the following reasons [19 – 21]: (1) the C18EO10 ‘‘shell’’ separates the CdS particles, forming the physical and spacious obstacle between the particles; (2) the C18EO10 ‘‘shell’’ absorbs some of the collision energy during grinding, reducing the energy transferring into the reactants; and (3) the C18EO10 ‘‘shell’’ absorbs heat generated by the reaction, reducing the temperature during grinding. To further improve the understanding of the ‘‘shell’’ effects of the surfactant C18EO10, in the same C18EO10 solidstate reaction system, we have synthesized the semiconductors ZnS, PbS, CuS and PbO nanoparticles by only varying the reactants. The results further supported the proposed ‘‘shell’’ mechanism.
4. Conclusions A solid-state synthesis of CdS nanoparticles in the presence of a nonionic surfactant C18EO10 at ambient temperature has been developed. The average diameter of CdS nanoparticles is ca. 5 nm. The structural nature and the chemical composition of CdS nanoparticles have been analyzed by XRD, TEM, HRTEM, SAED, UV –VIS optical absorption and XPS. We speculated that the surfactant C18EO10 plays an important role in the formation of CdS nanoparticles. This easy and unique solid-state synthesis in the presence of a non-
ionic surfactant, C18EO10, may be extended to the synthesis of other materials of nanoparticles, such as semiconductors ZnS, CuS, NiS, FeS.
Acknowledgements This work was supported by the National Natural Science Foundation of P.R. China (nos. 29890210, 10023001, 10074024).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
K. Hu, M. Brust, A. Bard, Chem. Mater. 10 (1998) 1160. H. Weller, Angew. Chem., Int. Ed. Engl. 32 (1993) 41. C. Kaito, Y. Saito, K. Fujita, J. Cryst. Growth 94 (1989) 967. D.M. Milhemy, E. Matijevic, J. Chem. Soc., Faraday Trans. (1984) 563. B. Ludoph, M.A. Malik, Chem. Commun. (1998) 1849. N.L. Pickeet, D.F. Foster, D.J. Cole Hamilton, J. Mater. Chem. 6 (1996) 507. N. Herron, Y. Wang, J. Am. Chem. Soc. 112 (1990) 1322. W. Chen, Z. Liu, Z. Wang, Solid State Commun. 100 (1996) 101. C.Y. Wang, X. Mo, Y. Zhou, Y.R. Zhu, H.T. Liu, Z.Y. Chen, J. Mater. Chem. 10 (2000) 607. Y.D. Yin, X.L. Xu, X.W. Ge, C.J. Xia, Z.C. Zhang, Chem. Commun. (1998) 1641. O. Zelaya-Angel, F.L. de Castillo-Alvarado, J. AvendanoLopez, A. Escamilla-Esquivel, G. Contreras-Puente, R. Lozada-Morales, G. Torres-Delgado, Solid State Commun. 104 (1997) 161.
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[12] J.P. Yang, F.C. Meldrum, J.H. Fendler, J. Phys. Chem. 99 (1995) 5500. [13] G.Q. Xu, B. Li, S.J. Xu, C.H. Chew, S.J. Chua, L.M. Gana, J. Phys. Chem. Solids 61 (2000) 829. [14] J.J. Zhu, M.G. Zhou, J.Z. Xu, X.H. Liao, Mater. Lett. 47 (2001) 25. [15] A. Henglein, Chem. Rev. 89 (1989) 1861. [16] B. Ludolph, M.A. Malik, P. O’Brien, N. Revaprasadu, Chem. Commun. (1998) 1849. [17] C.D. Wanger, W.M. Riggs, L.E. Davis, J.E. Moulder,
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