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Structural and optical characterization of CdS nanorods synthesized by a PVA-assisted solvothermal method
Journal of Alloys and Compounds 461 (2008) 418–422
Structural and optical characterization of CdS nanorods synthesized by a PVA-assisted solvothermal method Hongmei Wang a,b,∗ , Pengfei Fang b , Zhe Chen b , Shaojie Wang b a
b
Jiaxing College, Jiaxing 314001, China Physics Department, Wuhan University, Wuhan 430072, China Received 16 June 2007; accepted 1 July 2007 Available online 7 July 2007
Abstract Cadmium sulphide (CdS) 1D nanocrystals were prepared using a simple poly(vinyl-alcohol) (PVA)-assisted solvothermal method which employed ethylenediamine (en) as solvent. The obtained nanorods were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible (UV–vis) absorption, and photoluminescence (PL) spectra. XRD results show that the nanorods are hexagonal phase. The TEM results indicate that the synthesized CdS nanorods with PVA-assisted showed larger aspect ratio and uniform faces compared with the sample prepared in the absence of PVA. The results of the photoluminescence and UV–vis spectroscopy measurements reveal that the as-prepared CdS nanorods show a quantum confinement effect. It is also found that the dosage of PVA is a vital factor in the morphology and optical properties of CdS nanorods. Moreover, when the best dosage of 3 g PVA/70 ml en was used, CdS nanorods with regular morphology and longer length were obtained. The probable mechanism for PVA-assisted solvothermal synthesis of CdS nanorods was also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: CdS nanorods; PVA; Solvothermal; Formation mechanism; Optical properties
1. Introduction One-dimensional (1D) semiconductor nanomaterials have attracted considerable research activities due to their great potential for fundamental studies of the roles of dimensionality and size in their physical properties as well as for application in optoelectronic nanodevices and functional materials [1–4]. As a direct band gap material with Eg of 2.42 eV at room temperature, cadmium sulfide (CdS) is one of the most the important groups II–VI semiconductors, which has potential application of lighting–emitting diodes, solar cell, or other electronic devices [5–7]. Various approaches, such as solvothermal route [8–10], a liquid crystal template [11], irradiation [12–15], electrochemical reactions [16], polymer controlled growth [17], and electrodeposition on a porous template [18], have been applied to achieve one-dimensional CdS nanocrystals. For instance, Qian et al. obtained CdS nanowires using ethylenediamine as a shape con-
∗
Corresponding author at: Jiaxing College, Jiaxing 314001, China. E-mail address: [email protected] (H. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.002
troller [17]. However, CdS nanowires with low aspect ratio could be obtained when only ethylenediame was used as the reaction medium. In order to improve the length and uniformity of diameter, some polymer-assisted synthesis such as PAA, PAN, PVP, and PVA have also been investigated [17,19–21]. The researches have indicated that these polymers changed either the surface chemistry of the crystals, or the concentration of soluble species that were available to grow the crystals. Both factors have a strong influence on the growth of CdS crystallites. Poly(vinyl-alcohol) (PVA), which is a water-soluble polymer, has an excellent effect on the growth of CdS nanowire [21]. In this paper, we prepare CdS nanorods based on using ethylenediamine as the reaction medium and PVA as the polymer-controller matrix by the solvothermal method. And we carry out a systematic investigation of the effect of PVA on the elongated growth of 1D CdS nanostructures. The CdS nanorods prepared with PVA-assisted show larger aspect ratios compared with the sample prepared without PVA. The present process provides a mild and effective route for the production of CdS nanorods which might be used in the fabrication of novel optical and electronic devices.
H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
419
2. Experimental 2.1. Preparation of CdS nanorods Some poly(vinyl-alcohol) (PVA) was dissolved into CdCl2 aqueous solution (1 mmol CdCl2 ·2.5H2 O and 100 ml distilled water). The solution was stirred at the room temperature for 24 h in order to achieve good dispersion of Cd2+ in PVA matrix. Then it was dehydrated at 70 ◦ C to get polymer gel and the gel was continued to be heated at 70 ◦ C until there was no obvious weight loss. The dried polymer gel doped with Cd2+ and 3 mmol thiourea was put into a Teflon liner autoclave of 100 ml capacity, which was filled with 70 ml ethylenediamine. The autoclave was sealed and maintained at 180 ◦ C for 24 h, and then cooled naturally to room temperature. Then, the precipitates were filtered and washed with distilled water and ethanol several times to remove the impurities. The products were dried in vacuum at 70 ◦ C for 8 h. Samples 1–4 were presented corresponding to the dosages of PVA 0 g, 1 g, 3 g, and 6 g/70 ml en, respectively.
2.2. Characterization of CdS nanorods The as-prepared power sample was characterized by X-ray power diffraction ˚ on a D8 Advance X-ray diffractometer with Cu K␣ irradiation at λ = 1.5406 A. Transmission electron microscopy (TEM) and energy diffraction X-ray analysis (EDAX) were performed on a JEM-2010 transmission electron microscope and a JEOL JEM-2010F high-resolution transmission electron microscope (HRTEM) at an accelerating voltage of 220 kV. TEM samples were deposited on thin amorphous carbon films supported by copper grids from ultrasonically processed ethanol solutions of products. The photoluminescence (PL) spectra were recorded at room temperature on a Hitachi Model F-4500 fluorescence spectrophotometer. The optical absorption spectra were measured by a Shimadzu UV–vis spectrophotometer (UV-2550).
3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of the samples 2–4, prepared in 70 ml ethylenediamine solvent at 180 ◦ C for 24 h using 1 g, 3 g and 6 g PVA, respectively. All the peaks can be indexed to hexag˚ and c = 6.72 A, ˚ onal CdS with lattice constants of a = 4.14 A which are consistent with the literature data of JCPDS 411049. Compared with the standard diffraction pattern, no peaks
Fig. 1. X-ray diffraction (XRD) patterns of CdS samples: (a) sample 2, (b) sample 3, and (c) sample 4.
Fig. 2. The TEM images of CdS samples: (a) sample 1 and (b) sample 3.
of impurities were detected, indicating the high purity of the products. 3.2. TEM analysis The morphologies of samples 1 and 3 characterized by TEM are shown in Fig. 2a and b. As Fig. 2a revealed, the sample 1 prepared without PVA was almost the short and irregular nanorods with different diameters. In a much clearer image of the nanorods (Fig. 3a), we can also see that the side faces of the products were not smooth. Nanorods with diameters of 20 nm and lengths up to 50–250 nm are observed in the TEM image (Fig. 3b). Moreover, the diameter keeps constant and the lateral surface is quite smooth. In Fig. 3b, we find straight traverse stripes which may illustrate the growth process of the crystals. The traverse stripes on the rods may be attributed to defects during the growth process. We notice that the rod-shaped crystal is preferentially grown along the c-axis. To determine the detail of the stripes, high-resolution transmission electron microscopy (HRTEM) was performed on a single nanorod of the sample 3, as shown in Fig. 4. The crystal plane space is calculated to be 0.342 nm, which is consistent with the (0 0 2) crystal planes of the wurtzite CdS. The HRTEM image reveals that the as-
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H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
Fig. 5. EDAX spectrum of sample 3.
Fig. 3. The TEM images of several nanorods: (a) sample 1 and (b) sample 3.
prepared CdS nanorod grows with a preferential direction of [0 0 1], c-axis. The EDAX spectrum (Fig. 5) for the sample 3 further verifies that the nanorods are mainly composed by cadmium and sulfur atoms, and carbon atoms come from carbon film on copper grid. Based on the above-mentioned results, it can be speculated that PVA plays an important role in 1D growth of CdS nanorods, and that the concentration of PVA in the system has a strong influence on the crystal growth behavior. The formation of CdS nanorods with larger aspect ratios can be explained as follows: at high temperature, thiourea decomposes in basic media to release S2− , which bonds with the complex ion [Cd·(en)2 ]2+ in the solution. Then, the CdS nanorods form after losing the volatile enthylenediamine molecules at certain temperature [22]. In the crystal growth, a reversible pathway between the solution phase and solid phase plays a critical role. Because ethylenediamine is a strongly bidentating solvent which readily chelated Cd2+ , an effectively reversible pathway between the solution phase and the solid phase is established (see in the following equation): (CdS)n + 2n en ⇔ n[Cd(en)2 ]2+ + nS2−
Fig. 4. HRTEM image of a single CdS nanorod in sample 3.
(1)
The solvent in the solution phase is inclined to transport the species from the smaller crystallites to the larger ones. From the thermodynamic viewpoint, the crystal growth can minimize the free energy of the system. The existence of ethylenediamine is in favor of the growth of CdS nanocrystallites along c-axis. Meanwhile, PVA has stronger interaction with the side faces of CdS nanorods than that with the ends of the c-axis direction. Therefore, PVA can inhibit the growth of the side faces by capping them heavily, in comparison, the faces less capped by PVA remained to be highly active to the continuous addition. As a result, the orientation growth along c-axis was advantaged. Without PVA, there is only effect of ethylenediamine presented above. When the dosage of PVA is small (sample 2), the PVA molecules separately absorb to the CdS particles, which is not enough for forming a continuous layer to completely passivate the side faces of nanorods, resulting in loose control over
H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
Fig. 6. PL spectra of CdS samples: (a) sample 2, (b) sample 3, and (c) sample 4.
the growth of CdS nanostructures in the lateral directions. As Fig. 3b shows, regular shaped nanorods with uniform diameter and smooth surfaces were obtained using 3 g PVA. In this case, the dosage of PVA molecules is appropriate for the preferential absorption to the lateral surfaces of the nanorods, preventing CdS from transmitting onto these faces, and leaving the [0 0 1] faces uncovered, which is favorable for the orientated growth along the c-axis directions. However, too much PVA is not favorable for the growth of particles. It may be due to that PVA might wrap the entire surface of CdS crystals which can inhibit the growth of all the directions. 3.3. Optical properties For the discussion of the optical properties of the nanorods synthesized by PVA-assisted solvothermal method, we also carried out some basal optical property examinations to evaluate the quality of the products. The room temperature photoluminescence (PL) spectra of samples 2–4 were shown in Fig. 6. They display two distinct emission bands at approximately 440 nm and 530 nm with 380 nm excitation, which were close to those in previous report for CdS nanorod [20]. The former emission band might be assigned to the electron–hole recombination of CdS, and the latter one might be associated with the surface trap induced emission owing to the PVA capping. When the dosage of PVA was 3 g (sample 3) in this system, the band emission is stronger and the surface emission is lower than those of the other two samples, which indicates that the dosage of 3 g PVA is appropriate. If two less or too much PVA was used, the surface emission of the products would become stronger because of the incomplete capping or surplus capping. Fig. 7 shows the UV–vis absorption spectra of samples 1 and 3. The absorption peaks of samples 1 and 3 are 430 nm and 472 nm, respectively. The blue shifts were found compared with the bandgap of the characteristic absorption of bulk CdS, which was probably ascribed to the size quantization. From Fig. 7, it can also be found that the blue
421
Fig. 7. Absorption spectra of CdS samples: (a) sample 1 and (b) sample 3.
shift is more obvious in the presence of PVA. We can conclude that the prepared CdS nanorods have smaller diameters with the dosage of PVA, which is agreement with the above TEM results. 4. Conclusions In summary, the CdS nanorods were prepared by a simple polymer-controlled technique using poly(vinyl-alcohol) (PVA) as matrix. The effect of PVA in this reaction system was investigated, and the dosage of PVA is deemed as a critical parameter in tailing the shape and size of CdS nanostructures. The TEM results verified that the CdS nanorods with uniform and larger length could be synthesized by using appropriate dosage of 3 g PVA/70 ml en. In its absence, only short and irregular CdS nanorods were obtained. The optical properties of the products are influenced by the morphology and the polymeric capping of the crystals. The details of the mechanism need further study in future. Acknowledgements The work is supported by the Chinese National Foundation of Natural Science Research (nos. 10475062 and 20304009). References [1] X.F. Duan, Y. Huang, R. Agawal, C.M. Lieber, Nature 421 (2003) 241. [2] M.S. Fubrer, J. Nygard, L. Shih, M. Forero, Y.-G. Yoon, M.S.C. Mazzoni, H.J. Choi, Science 288 (2000) 494. [3] Z. Peng, G. Lian, Langmuir 19 (2003) 208. [4] H. Zhang, D.R. Yang, X.Y. Ma, D.L. Que, Mater. Lett. 59 (2005) 3037. [5] L.P. Deshmukh, S.G. Holikatli, P.P. Hankare, J. Am. Chem. Soc. 126 (2004) 1950. [6] P.K. Khanna, V.V.V.S. Subbarao, Mater. Lett. 58 (2004) 2801. [7] X.C. Wu, Y.R. Tao, J. Cryst. Growth 242 (2002) 309–312. [8] J. Yang, J. Zeng, S. Yu, L. Yang, G. Zhou, Y. Qian, Chem. Mater. 12 (2000) 3259. [9] J. Wu, Y. Jiang, Q. Li, X. Liu, Y. Qian, J. Cryst. Growth 235 (2002) 421. [10] M. Chen, Y. Xie, J. Liu, Y. Xiong, S. Zhang, Y. Qian, X. Liu, J. Mater. Chem. 12 (2002) 748.
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[11] Y. Li, J. Wan, Z. Gu, Mater. Sci. Eng. A 286 (2000) 106. [12] S.I. Nikirenko, Y. Koltypin, Y. Mastai, M. Koltypin, A. Gedanken, J. Mater. Chem. 12 (2002) 1450. [13] C.Y. Wang, X. Mo, Y. Zhou, Y.R. Zhu, H.T. Liu, Z.Y. Chen, J. Mater. Chem. 10 (2000) 607. [14] X. Mo, C. Wang, M. You, Y. Zhu, Z. Chen, Y. Hu, Mater. Res. Bull. 36 (2001) 2277. [15] S.I. Nikitenko, Y. Koltypin, Y. Mastai, M. Koltypin, A. Gedanken, J. Mater. Chem. 12 (2002) 1450. [16] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100 (1996) 14037.
[17] J.H. Zhan, X.G. Yang, D.W. Wang, S.D. Li, Y.T. Xie, Y.N. Xia, Y.T. Qian, Adv. Mater. 12 (2000) 1348. [18] D. Xu, Y. Xu, D. Chen, G. Guo, L. Gui, Y. Tang, Chem. Phys. Lett. 325 (2000) 340. [19] M. Chen, Y. Xie, H.Y. Chen, Z.P. Qiao, Y.J. Zhu, Y.T. Qian, J. Colloid Interf. Sci. 229 (2000) 217. [20] Q.Q. Wang, G.L. Zhao, G.R. Han, Mater. Lett. 59 (2005) 2625. [21] J.X. Yao, G.L. Zhao, D. Wang, G.R. Han, Mater. Lett. 59 (2005) 3652. [22] J. Yang, J.H. Zeng, S.H. Yu, L. Yang, G.E. Zhou, Y.T. Qian, Chem. Mater. 12 (2000) 3259.
Structural and optical characterization of CdS nanorods synthesized by a PVA-assisted solvothermal method Hongmei Wang a,b,∗ , Pengfei Fang b , Zhe Chen b , Shaojie Wang b a
b
Jiaxing College, Jiaxing 314001, China Physics Department, Wuhan University, Wuhan 430072, China Received 16 June 2007; accepted 1 July 2007 Available online 7 July 2007
Abstract Cadmium sulphide (CdS) 1D nanocrystals were prepared using a simple poly(vinyl-alcohol) (PVA)-assisted solvothermal method which employed ethylenediamine (en) as solvent. The obtained nanorods were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible (UV–vis) absorption, and photoluminescence (PL) spectra. XRD results show that the nanorods are hexagonal phase. The TEM results indicate that the synthesized CdS nanorods with PVA-assisted showed larger aspect ratio and uniform faces compared with the sample prepared in the absence of PVA. The results of the photoluminescence and UV–vis spectroscopy measurements reveal that the as-prepared CdS nanorods show a quantum confinement effect. It is also found that the dosage of PVA is a vital factor in the morphology and optical properties of CdS nanorods. Moreover, when the best dosage of 3 g PVA/70 ml en was used, CdS nanorods with regular morphology and longer length were obtained. The probable mechanism for PVA-assisted solvothermal synthesis of CdS nanorods was also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: CdS nanorods; PVA; Solvothermal; Formation mechanism; Optical properties
1. Introduction One-dimensional (1D) semiconductor nanomaterials have attracted considerable research activities due to their great potential for fundamental studies of the roles of dimensionality and size in their physical properties as well as for application in optoelectronic nanodevices and functional materials [1–4]. As a direct band gap material with Eg of 2.42 eV at room temperature, cadmium sulfide (CdS) is one of the most the important groups II–VI semiconductors, which has potential application of lighting–emitting diodes, solar cell, or other electronic devices [5–7]. Various approaches, such as solvothermal route [8–10], a liquid crystal template [11], irradiation [12–15], electrochemical reactions [16], polymer controlled growth [17], and electrodeposition on a porous template [18], have been applied to achieve one-dimensional CdS nanocrystals. For instance, Qian et al. obtained CdS nanowires using ethylenediamine as a shape con-
∗
Corresponding author at: Jiaxing College, Jiaxing 314001, China. E-mail address: [email protected] (H. Wang).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.07.002
troller [17]. However, CdS nanowires with low aspect ratio could be obtained when only ethylenediame was used as the reaction medium. In order to improve the length and uniformity of diameter, some polymer-assisted synthesis such as PAA, PAN, PVP, and PVA have also been investigated [17,19–21]. The researches have indicated that these polymers changed either the surface chemistry of the crystals, or the concentration of soluble species that were available to grow the crystals. Both factors have a strong influence on the growth of CdS crystallites. Poly(vinyl-alcohol) (PVA), which is a water-soluble polymer, has an excellent effect on the growth of CdS nanowire [21]. In this paper, we prepare CdS nanorods based on using ethylenediamine as the reaction medium and PVA as the polymer-controller matrix by the solvothermal method. And we carry out a systematic investigation of the effect of PVA on the elongated growth of 1D CdS nanostructures. The CdS nanorods prepared with PVA-assisted show larger aspect ratios compared with the sample prepared without PVA. The present process provides a mild and effective route for the production of CdS nanorods which might be used in the fabrication of novel optical and electronic devices.
H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
419
2. Experimental 2.1. Preparation of CdS nanorods Some poly(vinyl-alcohol) (PVA) was dissolved into CdCl2 aqueous solution (1 mmol CdCl2 ·2.5H2 O and 100 ml distilled water). The solution was stirred at the room temperature for 24 h in order to achieve good dispersion of Cd2+ in PVA matrix. Then it was dehydrated at 70 ◦ C to get polymer gel and the gel was continued to be heated at 70 ◦ C until there was no obvious weight loss. The dried polymer gel doped with Cd2+ and 3 mmol thiourea was put into a Teflon liner autoclave of 100 ml capacity, which was filled with 70 ml ethylenediamine. The autoclave was sealed and maintained at 180 ◦ C for 24 h, and then cooled naturally to room temperature. Then, the precipitates were filtered and washed with distilled water and ethanol several times to remove the impurities. The products were dried in vacuum at 70 ◦ C for 8 h. Samples 1–4 were presented corresponding to the dosages of PVA 0 g, 1 g, 3 g, and 6 g/70 ml en, respectively.
2.2. Characterization of CdS nanorods The as-prepared power sample was characterized by X-ray power diffraction ˚ on a D8 Advance X-ray diffractometer with Cu K␣ irradiation at λ = 1.5406 A. Transmission electron microscopy (TEM) and energy diffraction X-ray analysis (EDAX) were performed on a JEM-2010 transmission electron microscope and a JEOL JEM-2010F high-resolution transmission electron microscope (HRTEM) at an accelerating voltage of 220 kV. TEM samples were deposited on thin amorphous carbon films supported by copper grids from ultrasonically processed ethanol solutions of products. The photoluminescence (PL) spectra were recorded at room temperature on a Hitachi Model F-4500 fluorescence spectrophotometer. The optical absorption spectra were measured by a Shimadzu UV–vis spectrophotometer (UV-2550).
3. Results and discussion 3.1. XRD analysis Fig. 1 shows the XRD patterns of the samples 2–4, prepared in 70 ml ethylenediamine solvent at 180 ◦ C for 24 h using 1 g, 3 g and 6 g PVA, respectively. All the peaks can be indexed to hexag˚ and c = 6.72 A, ˚ onal CdS with lattice constants of a = 4.14 A which are consistent with the literature data of JCPDS 411049. Compared with the standard diffraction pattern, no peaks
Fig. 1. X-ray diffraction (XRD) patterns of CdS samples: (a) sample 2, (b) sample 3, and (c) sample 4.
Fig. 2. The TEM images of CdS samples: (a) sample 1 and (b) sample 3.
of impurities were detected, indicating the high purity of the products. 3.2. TEM analysis The morphologies of samples 1 and 3 characterized by TEM are shown in Fig. 2a and b. As Fig. 2a revealed, the sample 1 prepared without PVA was almost the short and irregular nanorods with different diameters. In a much clearer image of the nanorods (Fig. 3a), we can also see that the side faces of the products were not smooth. Nanorods with diameters of 20 nm and lengths up to 50–250 nm are observed in the TEM image (Fig. 3b). Moreover, the diameter keeps constant and the lateral surface is quite smooth. In Fig. 3b, we find straight traverse stripes which may illustrate the growth process of the crystals. The traverse stripes on the rods may be attributed to defects during the growth process. We notice that the rod-shaped crystal is preferentially grown along the c-axis. To determine the detail of the stripes, high-resolution transmission electron microscopy (HRTEM) was performed on a single nanorod of the sample 3, as shown in Fig. 4. The crystal plane space is calculated to be 0.342 nm, which is consistent with the (0 0 2) crystal planes of the wurtzite CdS. The HRTEM image reveals that the as-
420
H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
Fig. 5. EDAX spectrum of sample 3.
Fig. 3. The TEM images of several nanorods: (a) sample 1 and (b) sample 3.
prepared CdS nanorod grows with a preferential direction of [0 0 1], c-axis. The EDAX spectrum (Fig. 5) for the sample 3 further verifies that the nanorods are mainly composed by cadmium and sulfur atoms, and carbon atoms come from carbon film on copper grid. Based on the above-mentioned results, it can be speculated that PVA plays an important role in 1D growth of CdS nanorods, and that the concentration of PVA in the system has a strong influence on the crystal growth behavior. The formation of CdS nanorods with larger aspect ratios can be explained as follows: at high temperature, thiourea decomposes in basic media to release S2− , which bonds with the complex ion [Cd·(en)2 ]2+ in the solution. Then, the CdS nanorods form after losing the volatile enthylenediamine molecules at certain temperature [22]. In the crystal growth, a reversible pathway between the solution phase and solid phase plays a critical role. Because ethylenediamine is a strongly bidentating solvent which readily chelated Cd2+ , an effectively reversible pathway between the solution phase and the solid phase is established (see in the following equation): (CdS)n + 2n en ⇔ n[Cd(en)2 ]2+ + nS2−
Fig. 4. HRTEM image of a single CdS nanorod in sample 3.
(1)
The solvent in the solution phase is inclined to transport the species from the smaller crystallites to the larger ones. From the thermodynamic viewpoint, the crystal growth can minimize the free energy of the system. The existence of ethylenediamine is in favor of the growth of CdS nanocrystallites along c-axis. Meanwhile, PVA has stronger interaction with the side faces of CdS nanorods than that with the ends of the c-axis direction. Therefore, PVA can inhibit the growth of the side faces by capping them heavily, in comparison, the faces less capped by PVA remained to be highly active to the continuous addition. As a result, the orientation growth along c-axis was advantaged. Without PVA, there is only effect of ethylenediamine presented above. When the dosage of PVA is small (sample 2), the PVA molecules separately absorb to the CdS particles, which is not enough for forming a continuous layer to completely passivate the side faces of nanorods, resulting in loose control over
H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
Fig. 6. PL spectra of CdS samples: (a) sample 2, (b) sample 3, and (c) sample 4.
the growth of CdS nanostructures in the lateral directions. As Fig. 3b shows, regular shaped nanorods with uniform diameter and smooth surfaces were obtained using 3 g PVA. In this case, the dosage of PVA molecules is appropriate for the preferential absorption to the lateral surfaces of the nanorods, preventing CdS from transmitting onto these faces, and leaving the [0 0 1] faces uncovered, which is favorable for the orientated growth along the c-axis directions. However, too much PVA is not favorable for the growth of particles. It may be due to that PVA might wrap the entire surface of CdS crystals which can inhibit the growth of all the directions. 3.3. Optical properties For the discussion of the optical properties of the nanorods synthesized by PVA-assisted solvothermal method, we also carried out some basal optical property examinations to evaluate the quality of the products. The room temperature photoluminescence (PL) spectra of samples 2–4 were shown in Fig. 6. They display two distinct emission bands at approximately 440 nm and 530 nm with 380 nm excitation, which were close to those in previous report for CdS nanorod [20]. The former emission band might be assigned to the electron–hole recombination of CdS, and the latter one might be associated with the surface trap induced emission owing to the PVA capping. When the dosage of PVA was 3 g (sample 3) in this system, the band emission is stronger and the surface emission is lower than those of the other two samples, which indicates that the dosage of 3 g PVA is appropriate. If two less or too much PVA was used, the surface emission of the products would become stronger because of the incomplete capping or surplus capping. Fig. 7 shows the UV–vis absorption spectra of samples 1 and 3. The absorption peaks of samples 1 and 3 are 430 nm and 472 nm, respectively. The blue shifts were found compared with the bandgap of the characteristic absorption of bulk CdS, which was probably ascribed to the size quantization. From Fig. 7, it can also be found that the blue
421
Fig. 7. Absorption spectra of CdS samples: (a) sample 1 and (b) sample 3.
shift is more obvious in the presence of PVA. We can conclude that the prepared CdS nanorods have smaller diameters with the dosage of PVA, which is agreement with the above TEM results. 4. Conclusions In summary, the CdS nanorods were prepared by a simple polymer-controlled technique using poly(vinyl-alcohol) (PVA) as matrix. The effect of PVA in this reaction system was investigated, and the dosage of PVA is deemed as a critical parameter in tailing the shape and size of CdS nanostructures. The TEM results verified that the CdS nanorods with uniform and larger length could be synthesized by using appropriate dosage of 3 g PVA/70 ml en. In its absence, only short and irregular CdS nanorods were obtained. The optical properties of the products are influenced by the morphology and the polymeric capping of the crystals. The details of the mechanism need further study in future. Acknowledgements The work is supported by the Chinese National Foundation of Natural Science Research (nos. 10475062 and 20304009). References [1] X.F. Duan, Y. Huang, R. Agawal, C.M. Lieber, Nature 421 (2003) 241. [2] M.S. Fubrer, J. Nygard, L. Shih, M. Forero, Y.-G. Yoon, M.S.C. Mazzoni, H.J. Choi, Science 288 (2000) 494. [3] Z. Peng, G. Lian, Langmuir 19 (2003) 208. [4] H. Zhang, D.R. Yang, X.Y. Ma, D.L. Que, Mater. Lett. 59 (2005) 3037. [5] L.P. Deshmukh, S.G. Holikatli, P.P. Hankare, J. Am. Chem. Soc. 126 (2004) 1950. [6] P.K. Khanna, V.V.V.S. Subbarao, Mater. Lett. 58 (2004) 2801. [7] X.C. Wu, Y.R. Tao, J. Cryst. Growth 242 (2002) 309–312. [8] J. Yang, J. Zeng, S. Yu, L. Yang, G. Zhou, Y. Qian, Chem. Mater. 12 (2000) 3259. [9] J. Wu, Y. Jiang, Q. Li, X. Liu, Y. Qian, J. Cryst. Growth 235 (2002) 421. [10] M. Chen, Y. Xie, J. Liu, Y. Xiong, S. Zhang, Y. Qian, X. Liu, J. Mater. Chem. 12 (2002) 748.
422
H. Wang et al. / Journal of Alloys and Compounds 461 (2008) 418–422
[11] Y. Li, J. Wan, Z. Gu, Mater. Sci. Eng. A 286 (2000) 106. [12] S.I. Nikirenko, Y. Koltypin, Y. Mastai, M. Koltypin, A. Gedanken, J. Mater. Chem. 12 (2002) 1450. [13] C.Y. Wang, X. Mo, Y. Zhou, Y.R. Zhu, H.T. Liu, Z.Y. Chen, J. Mater. Chem. 10 (2000) 607. [14] X. Mo, C. Wang, M. You, Y. Zhu, Z. Chen, Y. Hu, Mater. Res. Bull. 36 (2001) 2277. [15] S.I. Nikitenko, Y. Koltypin, Y. Mastai, M. Koltypin, A. Gedanken, J. Mater. Chem. 12 (2002) 1450. [16] D. Routkevitch, T. Bigioni, M. Moskovits, J.M. Xu, J. Phys. Chem. 100 (1996) 14037.
[17] J.H. Zhan, X.G. Yang, D.W. Wang, S.D. Li, Y.T. Xie, Y.N. Xia, Y.T. Qian, Adv. Mater. 12 (2000) 1348. [18] D. Xu, Y. Xu, D. Chen, G. Guo, L. Gui, Y. Tang, Chem. Phys. Lett. 325 (2000) 340. [19] M. Chen, Y. Xie, H.Y. Chen, Z.P. Qiao, Y.J. Zhu, Y.T. Qian, J. Colloid Interf. Sci. 229 (2000) 217. [20] Q.Q. Wang, G.L. Zhao, G.R. Han, Mater. Lett. 59 (2005) 2625. [21] J.X. Yao, G.L. Zhao, D. Wang, G.R. Han, Mater. Lett. 59 (2005) 3652. [22] J. Yang, J.H. Zeng, S.H. Yu, L. Yang, G.E. Zhou, Y.T. Qian, Chem. Mater. 12 (2000) 3259.