- Email: [email protected]
Decoration of multiwall nanotubes with cadmium sulfide nanoparticles
Carbon 44 (2006) 2021–2026 www.elsevier.com/locate/carbon
Decoration of multiwall nanotubes with cadmium sulfide nanoparticles Chensha Li
b
a,b,* ,
Yaping Tang b, Kefu Yao a, Feng Zhou b, Qiang Ma b, Hao Lin c, Maosheng Tao b, Ji Liang a
a Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China New Material Division, Institute of Nuclear Energy Technology, Tsinghua University, Beijing 102201, China c Chemical Engineering Department, Xinjiang Petroleum Institute, Urumqi 830000, China
Received 9 June 2005; accepted 22 January 2006 Available online 20 March 2006
Abstract This study focuses on in situ synthesis of CdS nanoparticles on the surfaces of multiwalled carbon nanotubes. By chemical reaction of cadmium chloride and thioacetamide in the solution with carbon nanotubes, which were pretreated by air oxidization and acid modification, cadmium sulfide nanoparticles densely supported on carbon nanotubes with 10 nm size and homogeneous distribution were prepared. The composite material with the composite structure of CdS decorating the nanotube surfaces was characterized using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction and electron diffraction pattern. The CdS nanoparticles are of cubic crystal structure, show good adhesion to the nanotubes. This method can be extended to prepare other inorganic nanoparticle–carbon nanotube composites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical treatment; Electron microscopy; Crystal structure; Functional groups
1. Introduction Since their discovery, carbon nanotubes (CNTs) have been continuously attracting strong interest from many areas of science and technology due to their unique structure-dependent electronic, mechanical, and chemical properties [1]. High accessible specific area, cylindrically layered and hollow tubule nanostructures with their high thermal and chemical stabilities make it possible for CNTs to function as supports for preparing other nanophase materials. Because of their unique size-tunable chemical and physical properties, semiconductor nanoparticles have attracted much interest during the past decade [2]. These nanoparticles have shown potential applications in molecular electronics, nonlinear optics, catalysis, and pho-
*
Corresponding author. Address: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62771221. E-mail address: [email protected] (C. Li). 0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.01.033
toelectrochemical cells. They also represent ideal building blocks for the construction of nanostructured materials [3]. Several semiconductor nanoparticles such as tin oxide [4], cadmium selenide [5–7], zinc sulfide [6], titanium oxide [5,8], and silicon oxide [9,10] have been bound to the surfaces of CNTs. In this paper, we report that nanosized CdS particles could be bound to multiwalled carbon nanotubes (MWCNTs) through solution synthesis routes, and the relevant processing parameters are discussed. 2. Experimental procedure CNTs were produced catalytically with Ni particles as the catalyst [11,12]. Hydrofluoric acid treatment was employed to remove the catalyst particles, ultrasonic and hand grinding were employed to disperse the CNTs. Transmission electron microscopy (TEM) image (Fig. 1) shows that the as produced multiwalled CNTs had a diameter of 30–40 nm and a length of several microns to several tens of microns.
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hour, then purified by distilled water till pH value being neutrality and finally dried at 100 °C for 24 h [12]. These pretreated CNTs are called ‘‘acid-washed CNTs’’. For combination of air oxidization and acid modification, the CNTs were oxidized by air and then modified by concentrated nitric acid as specified above, then purified by distilled water until pH value readed neutrality and finally dried at 100 °C for 24 h to give ‘‘air/acid-treated CNTs’’. The nanosized CdS supported on CNTs (CdS/CNTs) was synthesized through the reaction of cadmium salt and the sulfocompound, as the source of sulfide ions, in a CNTs suspension solution. The relevant chemical reaction can be expressed as follows: CNT
Cd2þ þ S2 ! CdS Fig. 1. Transmission electron microscopy (TEM) of the as produced CNTs.
For surface oxidization, the tube of a resistance furnace, 60 mm in diameter and 1.0 m in length with the two ports open, was initially set to 600 °C. A quartz tube carrying the CNTs was placed in the center of the furnace tube. Temperature was maintained at 600 °C for 20 min, while the quartz tube was rotated at the rate of 25 r/min with a quartz rod so that the CNTs were mixed with air [11]. These pretreated CNTs are called ‘‘air-oxidized CNTs’’. For an acid modification, the CNTs were heated in concentrated nitric acid at the boiling point temperature for one
ð1Þ
In our experiment, the selected cadmium salt was cadmium chloride (CdCl2) and the selected sulfocompound were sodium sulfide (Na2S) and thioacetamide (CH3CSNH2). At room temperature, the CNTs (140 mg) were dispersed into CdCl2 aqueous solution (100 ml, 0.002 M) and agitated with a magnetic agitator for 30 min, and then the stoichiometric sulfocompound aqueous solution (10 ml, 0.02 M) was added drop by drop. After the process of adding stoichiometric sulfocompound was completed (under 30 min), the mixture solution was continuously agitated for 2 h. The deposition was repeatedly washed with distilled water by centrifugation, and then dried at 100 °C for 24 h. The samples synthesized by adding the CH3CSNH2 solution to CdCl2/air-oxidized
Fig. 2. The composite structure of CdS/CNTs when air oxidization and acid modification are used for CNT pretreatment. (a) TEM image; (b) electron diffraction pattern (ED) of CdS nanoparticles; (c) high resolution transmission electron microscopy (HRTEM) image, arrow1 indicates (1 1 1) crystal planes of cubic-phase CdS, arrow 2 indicates the interface between CdS and CNT; (d) scanning electric microscopy (SEM) image.
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CNTs solution, CdCl2/acid-washed CNTs solution and CdCl2/air/acid-treated CNTs solution were, respectively, called CdS/air-oxidized CNTs, CdS/acid-washed CNTs and CdS/air/acid-treated CNTs. The sample synthesized by adding the Na2S solution to CdCl2/air/acid-treated CNTs solution was called CdS/air/acid-treated CNTs(Na). The samples were characterized by transmission electron microscopy (TEM, JEOL-200CX), high-resolution transmission electron microscopy (HRTEM, H-9000NAR), field emission scanning electron microscopy (FE-SEM, AMRAY-1910), X-ray diffraction (XRD, Rigaku Dmaxc A X-ray diffractometer with Cu-Ka radiation, k = 0.154178 nm) and ‘‘PERKINELMER Spectrum GX FTIR System Jeol-200CX’’ infrared spectrometer. 3. Results and discussion The TEM image of CdS/air/acid-treated CNTs (Fig. 2(a)) shows CdS particles homogeneously and densely spread on the surfaces of CNTs. Fig. 2(b) shows the electron diffraction (ED) pattern of a selected area containing some nanoparticles. The ED pattern shows a set of spotty rings due to the random orientation of the nanoparticles, three rings correspond to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic CdS phase of zinc blende structure, respectively [13]. The HRTEM image is shown in Fig. 2(c), the lattice fringes and the interface between the CdS particles and the outermost shell of nanotube are clearly visible, as indicated by arrows. It indicates a well developed interface between the CdS particles and the nanotube and the orientations of crystal planes inside the particles are uniform. Moreover, the HRTEM image shows that the diameters of the CdS particles are about 10 nm. SEM image (Fig. 2(d)) shows that most CNTs are homogeneously and densely covered by CdS nanoparticles. The XRD pattern of CdS/air/acid-treated CNTs (Fig. 3) gave peaks which can be assigned to the (1 1 1), (2 2 0), and (3 1 1) phase of the cubic CdS crystal structure of zinc blende [13], though the peak of (0 0 2) of CNTs [14] is overlapped with that of (1 1 1) of cubic CdS and the
Fig. 3. XRD pattern of the CdS/CNTs when air oxidization and acid modification are used for CNT pretreatment.
Fig. 4. Poor dispersion of CdS when only air oxidation is used for CNT pretreatment: (a) TEM image; (b) SEM images.
Fig. 5. Poor dispersion of CdS when only acid modification is used for CNT pretreatment: (a) TEM image; (b) SEM images. Arrow indicates free CdS particles.
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residual Ni catalyst caused the weak peak at 77.48° [15]. These demonstrate that the cubic CdS crystal structure of zinc blende was formed on the surfaces of CNTs, which agrees well with the ED pattern discussed above. The TEM image of CdS/air-oxidized CNTs is shown in Fig. 4(a). It is observed that the nanotubes are coated by a few CdS particles with the diameters being larger than that of nanotube walls. Fig. 4(b), a SEM image, shows that many CdS particles exist in the CNT network, their diameters are generally larger than 200 nm. The TEM image of CdS/acid-washed CNTs, shown in Fig. 5(a), indicates that the nanotubes are not completely and uniformly covered by CdS particles. Some aggregations formed by free CdS particles, which were not fixed by CNTs’ surfaces, exist beyond the CNTs, as indicated by arrows. SEM image of this sample (Fig. 5(b)) also indicates a very incomplete covering of CdS particles on nanotubes, only sparse CdS nanoparticles are attached on CNTs’ surfaces, and many CNTs have mainly bare surfaces. Fig. 6(a), a TEM image of CdS/ air/acid-treated CNTs(Na), and Fig. 6(b), a SEM image of this sample, show that the CNTs’ surfaces were not compactly covered by CdS phase, many polydispersed small CdS particles only loosely stack on the surfaces of individ-
Transmission
2024
3450
1195
a
1398 1742 1582 1631
b 500
1000
1500
2000 2500 3000 Wavenumber (cm-1)
3500
4000
Fig. 7. Infrared transmission spectra: (a) as produced CNTs; (b) airoxidized CNTs; (c) acid-washed CNTs.
ual CNTs (shown in Fig. 6(a)) or the surfaces of CNT bundles (shown in Fig. 6(b)). When the CNTs were oxidized by air, under these exact conditions, oxygen attacked the cylindrical walls of CNTs. Some outer graphitic layers have been etched off, giving rough CNT-surfaces, as shown in reference [11]. But acid modification cannot etch the nanotube surfaces [12]. The nature of the surface groups was characterized with infrared transmission spectroscopy (Fig. 7). In the IR spectra, the signature of C@O functional groups in carbonyls, carboxyls or carboxylic anhydrides is evident at about 1742 cm1 [16] and AOH functional groups appear at 3450 cm1. The peak at 1582 cm1 reflects the vibration of carbocyclic plane of CNTs. The signal at about 1631 cm1 is associated with C@O functional groups in quinones or ketones [17]. The signal at 1398 cm1 is associated with OAH bending deformation in carboxylic acids and phenolic groups [18]. The peak centered on 1195 cm1 is associated with CAO stretching in ethers, hydroxyls or carboxylic anhydrides [17]. It is evident that there are many more carbonyl groups, carboxylic groups and hydroxyl groups, etc., in acid modified CNTs than in as produced CNTs and only air oxidized CNTs. Our view of the mechanism of CdS deposition lies in the rate of chemical reaction. Thioacetamide is a weak electrolyte, S2 ions are furnished through a reaction [19]: CH3 CSNH2 ! CH3 CN þ 2Hþ þ S2 2
Fig. 6. (a) TEM image; (b) SEM image. The deposits pointed by arrows are CdS. (The sample was synthesized through the reaction of Na2S and CdCl2/CNTs).
c
ð2Þ
The liberated S ions of a very low concentration may immediately react with Cd2+ ions and the reaction (2) is continued by the consumption of S2 ions. Na2S is a strong electrolyte, and can completely dissociate to be Na2+ ions and S2 ions in aqueous solution, result in a high concentration of free S2 ions. The fact that nanosized CdS particles can be homogeneously and densely deposited on air/acid-treated CNTs
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might be due to the following factors: (1) The roughness of the CNTs’ surfaces increases the effective surface area and affords the graphene edges with a higher wettability for fixing CdS phase; (2) The abundant oxygen-containing functional groups can effectively adsorb Cd2+ ions in the aqueous solution due to their electronegativity [20], resulted in the Cd2+ ions being concentrated on CNTs’ surfaces, the rate of the formation of CdS nuclei on CNTs’ surfaces was much faster than that in solution. The CdS nuclei formed on the outermost shell of CNTs grew through the continuous surface reaction of Cd2+ ions with S2 ions on the surfaces to form CdS nanoparticles, thus enhancing CdS deposition specifically on the CNTs whilst minimizing random deposition in solution. Moreover, the interface between the CdS particles and the outermost shell of nanotube are compact and the orientations of crystal planes of CdS particles are uniform; (3) When thioacetamide was added into the solution of CdCl2/air/acid-treated CNTs, the nucleation mainly occurred through a reaction of S2 ions with the Cd2+ ions adsorbed on the surfaces of CNTs. The final result of uniform and dense CdS nanoparticles covering the surfaces of CNTs was achieved through the dissolution-controlled growth by the reaction of Cd2+ and S2 ions, which were supplied from thioacetamide. The sufficiently low steady concentration of S2 ions during the growth stage secured the formation of the uniform CdS particles because it prevented the nucleation during the growth stage. Though air-oxidized CNTs have rough surfaces and CdS/air-oxidized CNTs was synthesized by using thioacetamide. Air-oxidized CNTs have less oxygen-containing functional groups and thus have a weak adsorption capability for Cd2+ [19], resulted in only small amounts of CdS nuclei being formed on the CNTs’ surfaces. These sparse CdS nuclei could grow without mutual competition and restraint, finally resulted in the diameters of the CdS particles becoming larger than that of nanotube walls. Moreover, due to little Cd2+ being adsorbed on the CNTs’ surfaces, many CdS particles were generated in the solution during the process of adding CH3CSNH2. The size of these CdS particles could continuously increase for reason that they were not restricted by the confined surfaces of CNTs. Thus, it is observed (Fig. 5(b)) that many CdS particles generated in the solution with the diameters being generally larger than 200 nm exist in the CNT network. Though CdS/acid-washed CNTs was synthesized by using thioacetamide, acid-washed CNTs can also have many oxygen-containing functional groups to adsorb Cd2+ and many CdS particles might grow on the nanotube surfaces. The surfaces of acid-washed CNTs are smooth and thus cannot effectively fix the CdS particles, resulted in many CdS particles falling off and the nanotube surfaces being mainly naked. The free CdS particles might aggregate together due to their high surface energy, as shown in Fig. 5(a). For the aqueous reaction system of Na2S and CdCl2/air/ acid-treated CNTs, though many Cd2+ could be adsorbed on CNTs’ surfaces, the relatively high supersaturation of
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free S2 ions in the solution led to a pronounced nucleation of CdS before the S2 ions diffused onto the surfaces of CNTs, and the growth of just generated nuclei concurrently occurred during the nucleation stage, led to the generation of polydispersed small CdS particles. The polydispersed small CdS particles aggregated to form the loose deposits. Though air/acid-treated CNTs have rough surfaces, they could not fix the CdS particles, which were not formed on their surfaces. These deposits could only stack on the surfaces of individual CNTs or the surfaces of CNT bundles, which are observed in the sample of CdS/air/acid-treated CNTs(Na) by electron microscopy. 4. Conclusion We have successfully prepared 10 nm CdS nanoparticles on carbon nanotube supports by a room-temperature solution chemical reaction method using thioacetamide and cadmium chloride. The surface state of CNTs is a critical factor to obtain nanosized CdS particles supported on CNTs. Air oxidization can etch the surfaces of CNTs, and acid modification can introduce functional groups on the surfaces of CNTs, both factors are beneficial to CdS/ CNT interaction. Keeping a low reaction rate between Cd2+ and S2 through selecting the appropriate reactants is also necessary to obtain a better covering of CdS nanoparticles on nanotubes. Acknowledgements This work was supported by: (a) the Major State Basic Research Development Program of China, Grant No. 10332020; (b) the Institute of Nuclear Energy Technology of Tsinghua University Project for Fundamental Research, Grant No.091131402. References [1] Rao CNR, Satishkumar BC, Govindaraj A, Nath M. Nanotubes. Chem Phys Chem 2001;2(2):78–105. [2] Alivisatos AP. Semiconductor clusters, nanocrystals, and quantum dots. Science 1996;271(5251):933–7. [3] Lin Y, Ska H, Emrick T, Dinsmore AD, Russell TP. Nanoparticle assembly and transport at liquid–liquid interfaces. Science 2003;299(5604):226–9. [4] Han WQ, Zettl A. Coating single-walled carbon nanotubes with tin oxide. Nano Lett 2003;3(5):681–3. [5] Banerjee S, Wong SS. Synthesis and characterization of carbon nanotube–nanocrystal heterostructures. Nano Lett 2002;2(3): 195–200. [6] Ravindran S, Chaudhary S, Colburn B, Ozkan M, Ozkan CS. Covalent coupling of quantum dots to multi-walled carbon nanotubes for electronic device applications. Nano Lett 2003;3(4):447–53. [7] Haremza JM, Hahn MA, Krauss TD, Chen S, Calcines J. Attachment of single CdSe nanocrystals to individual singlewalled carbon nanotubes. Nano Lett 2002;2(11):1253–8. [8] Lee SW, Sigmund WM. Formation of anatase TiO2 nanoparticles on carbon nanotubes. Chem Commun 2003(6):780–1. [9] Fu Q, Lu C, Liu J. Selective coating of single wall carbon nanotubes with thin SiO2 layer. Nano Lett 2002;2(4):329–32.
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[10] Seeger T, Kohler T, Frauenheim T, Grobert N, Ruhle M, Terrones M, et al. Nanotube composites: novel SiO2 coated carbon nanotubes. Chem Commun 2002(1):34–5. [11] Li CS, Wang DZ, Liang TX, Wang XF, Wu JJ, Hu XQ, et al. Oxidation of multi-walled carbon nanotubes by air: benefits for electric double layer capacitors. Powder Technol 2004;142(2–3): 175–9. [12] Li CS, Wang DZ, Liang TX, Wang XF, Liang J. A study of activated carbon nanotubes as double-layer capacitors electrode materials. Mater Lett 2004;58(29):3774–7. [13] Wang WZ, Liu ZH, Zheng CH, Xu CK, Liu YK, Wang GH. Synthesis of CdS nanoparticles by a novel and simple one-step, solidstate reaction in the presence of a nonionic surfactant. Mater Lett 2003;57(18):2755–60. [14] Perez-Cabero M, Rodriguez-Ramos I, Guerrero-Ruiz A. Characterization of carbon nanotubes and carbon nanofibers prepared by catalytic decomposition of acetylene in a fluidized bed reactor. J Catal 2003;215(2):305–16.
[15] Sun XC, Dong XL. Magnetic properties and microstructure of carbon encapsulated Ni nanoparticles and pure Ni nanoparticles coated with NiO layer. Mater Res Bull 2002;37(5):991–1004. [16] Bhabendra KP, Sandle NK. Effect of different oxidizing agent treatments on the surface properties of activated carbons. Carbon 1999;37(8):1323–32. [17] Figueiredo JL, Pereira MFR, Freitas MMA. Modification of the surface chemistry of activated carbons. Carbon 1999;37(9):1379– 89. [18] Shaffer MSP, Fan X, Windle AH. Dispersion and packing of carbon nanotubes. Carbon 1998;36(11):1603–12. [19] Sugimoto T, Dirige GE, Muramatsu A. Formation mechanism of uniform CdS particles from condensed Cd(OH)2 suspension. J Colloid Interf Sci 1995;176(2):442–53. [20] Li YH, Wang SG, Luan ZK, Ding J, Xu CL, Wu DH. Adsorption of cadmium (II) from aqueous solution by surface oxidized carbon nanotubes. Carbon 2003;41(5):1057–62.
Decoration of multiwall nanotubes with cadmium sulfide nanoparticles Chensha Li
b
a,b,* ,
Yaping Tang b, Kefu Yao a, Feng Zhou b, Qiang Ma b, Hao Lin c, Maosheng Tao b, Ji Liang a
a Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China New Material Division, Institute of Nuclear Energy Technology, Tsinghua University, Beijing 102201, China c Chemical Engineering Department, Xinjiang Petroleum Institute, Urumqi 830000, China
Received 9 June 2005; accepted 22 January 2006 Available online 20 March 2006
Abstract This study focuses on in situ synthesis of CdS nanoparticles on the surfaces of multiwalled carbon nanotubes. By chemical reaction of cadmium chloride and thioacetamide in the solution with carbon nanotubes, which were pretreated by air oxidization and acid modification, cadmium sulfide nanoparticles densely supported on carbon nanotubes with 10 nm size and homogeneous distribution were prepared. The composite material with the composite structure of CdS decorating the nanotube surfaces was characterized using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction and electron diffraction pattern. The CdS nanoparticles are of cubic crystal structure, show good adhesion to the nanotubes. This method can be extended to prepare other inorganic nanoparticle–carbon nanotube composites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical treatment; Electron microscopy; Crystal structure; Functional groups
1. Introduction Since their discovery, carbon nanotubes (CNTs) have been continuously attracting strong interest from many areas of science and technology due to their unique structure-dependent electronic, mechanical, and chemical properties [1]. High accessible specific area, cylindrically layered and hollow tubule nanostructures with their high thermal and chemical stabilities make it possible for CNTs to function as supports for preparing other nanophase materials. Because of their unique size-tunable chemical and physical properties, semiconductor nanoparticles have attracted much interest during the past decade [2]. These nanoparticles have shown potential applications in molecular electronics, nonlinear optics, catalysis, and pho-
*
Corresponding author. Address: Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62771221. E-mail address: [email protected] (C. Li). 0008-6223/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2006.01.033
toelectrochemical cells. They also represent ideal building blocks for the construction of nanostructured materials [3]. Several semiconductor nanoparticles such as tin oxide [4], cadmium selenide [5–7], zinc sulfide [6], titanium oxide [5,8], and silicon oxide [9,10] have been bound to the surfaces of CNTs. In this paper, we report that nanosized CdS particles could be bound to multiwalled carbon nanotubes (MWCNTs) through solution synthesis routes, and the relevant processing parameters are discussed. 2. Experimental procedure CNTs were produced catalytically with Ni particles as the catalyst [11,12]. Hydrofluoric acid treatment was employed to remove the catalyst particles, ultrasonic and hand grinding were employed to disperse the CNTs. Transmission electron microscopy (TEM) image (Fig. 1) shows that the as produced multiwalled CNTs had a diameter of 30–40 nm and a length of several microns to several tens of microns.
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hour, then purified by distilled water till pH value being neutrality and finally dried at 100 °C for 24 h [12]. These pretreated CNTs are called ‘‘acid-washed CNTs’’. For combination of air oxidization and acid modification, the CNTs were oxidized by air and then modified by concentrated nitric acid as specified above, then purified by distilled water until pH value readed neutrality and finally dried at 100 °C for 24 h to give ‘‘air/acid-treated CNTs’’. The nanosized CdS supported on CNTs (CdS/CNTs) was synthesized through the reaction of cadmium salt and the sulfocompound, as the source of sulfide ions, in a CNTs suspension solution. The relevant chemical reaction can be expressed as follows: CNT
Cd2þ þ S2 ! CdS Fig. 1. Transmission electron microscopy (TEM) of the as produced CNTs.
For surface oxidization, the tube of a resistance furnace, 60 mm in diameter and 1.0 m in length with the two ports open, was initially set to 600 °C. A quartz tube carrying the CNTs was placed in the center of the furnace tube. Temperature was maintained at 600 °C for 20 min, while the quartz tube was rotated at the rate of 25 r/min with a quartz rod so that the CNTs were mixed with air [11]. These pretreated CNTs are called ‘‘air-oxidized CNTs’’. For an acid modification, the CNTs were heated in concentrated nitric acid at the boiling point temperature for one
ð1Þ
In our experiment, the selected cadmium salt was cadmium chloride (CdCl2) and the selected sulfocompound were sodium sulfide (Na2S) and thioacetamide (CH3CSNH2). At room temperature, the CNTs (140 mg) were dispersed into CdCl2 aqueous solution (100 ml, 0.002 M) and agitated with a magnetic agitator for 30 min, and then the stoichiometric sulfocompound aqueous solution (10 ml, 0.02 M) was added drop by drop. After the process of adding stoichiometric sulfocompound was completed (under 30 min), the mixture solution was continuously agitated for 2 h. The deposition was repeatedly washed with distilled water by centrifugation, and then dried at 100 °C for 24 h. The samples synthesized by adding the CH3CSNH2 solution to CdCl2/air-oxidized
Fig. 2. The composite structure of CdS/CNTs when air oxidization and acid modification are used for CNT pretreatment. (a) TEM image; (b) electron diffraction pattern (ED) of CdS nanoparticles; (c) high resolution transmission electron microscopy (HRTEM) image, arrow1 indicates (1 1 1) crystal planes of cubic-phase CdS, arrow 2 indicates the interface between CdS and CNT; (d) scanning electric microscopy (SEM) image.
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CNTs solution, CdCl2/acid-washed CNTs solution and CdCl2/air/acid-treated CNTs solution were, respectively, called CdS/air-oxidized CNTs, CdS/acid-washed CNTs and CdS/air/acid-treated CNTs. The sample synthesized by adding the Na2S solution to CdCl2/air/acid-treated CNTs solution was called CdS/air/acid-treated CNTs(Na). The samples were characterized by transmission electron microscopy (TEM, JEOL-200CX), high-resolution transmission electron microscopy (HRTEM, H-9000NAR), field emission scanning electron microscopy (FE-SEM, AMRAY-1910), X-ray diffraction (XRD, Rigaku Dmaxc A X-ray diffractometer with Cu-Ka radiation, k = 0.154178 nm) and ‘‘PERKINELMER Spectrum GX FTIR System Jeol-200CX’’ infrared spectrometer. 3. Results and discussion The TEM image of CdS/air/acid-treated CNTs (Fig. 2(a)) shows CdS particles homogeneously and densely spread on the surfaces of CNTs. Fig. 2(b) shows the electron diffraction (ED) pattern of a selected area containing some nanoparticles. The ED pattern shows a set of spotty rings due to the random orientation of the nanoparticles, three rings correspond to the (1 1 1), (2 2 0) and (3 1 1) planes of the cubic CdS phase of zinc blende structure, respectively [13]. The HRTEM image is shown in Fig. 2(c), the lattice fringes and the interface between the CdS particles and the outermost shell of nanotube are clearly visible, as indicated by arrows. It indicates a well developed interface between the CdS particles and the nanotube and the orientations of crystal planes inside the particles are uniform. Moreover, the HRTEM image shows that the diameters of the CdS particles are about 10 nm. SEM image (Fig. 2(d)) shows that most CNTs are homogeneously and densely covered by CdS nanoparticles. The XRD pattern of CdS/air/acid-treated CNTs (Fig. 3) gave peaks which can be assigned to the (1 1 1), (2 2 0), and (3 1 1) phase of the cubic CdS crystal structure of zinc blende [13], though the peak of (0 0 2) of CNTs [14] is overlapped with that of (1 1 1) of cubic CdS and the
Fig. 3. XRD pattern of the CdS/CNTs when air oxidization and acid modification are used for CNT pretreatment.
Fig. 4. Poor dispersion of CdS when only air oxidation is used for CNT pretreatment: (a) TEM image; (b) SEM images.
Fig. 5. Poor dispersion of CdS when only acid modification is used for CNT pretreatment: (a) TEM image; (b) SEM images. Arrow indicates free CdS particles.
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residual Ni catalyst caused the weak peak at 77.48° [15]. These demonstrate that the cubic CdS crystal structure of zinc blende was formed on the surfaces of CNTs, which agrees well with the ED pattern discussed above. The TEM image of CdS/air-oxidized CNTs is shown in Fig. 4(a). It is observed that the nanotubes are coated by a few CdS particles with the diameters being larger than that of nanotube walls. Fig. 4(b), a SEM image, shows that many CdS particles exist in the CNT network, their diameters are generally larger than 200 nm. The TEM image of CdS/acid-washed CNTs, shown in Fig. 5(a), indicates that the nanotubes are not completely and uniformly covered by CdS particles. Some aggregations formed by free CdS particles, which were not fixed by CNTs’ surfaces, exist beyond the CNTs, as indicated by arrows. SEM image of this sample (Fig. 5(b)) also indicates a very incomplete covering of CdS particles on nanotubes, only sparse CdS nanoparticles are attached on CNTs’ surfaces, and many CNTs have mainly bare surfaces. Fig. 6(a), a TEM image of CdS/ air/acid-treated CNTs(Na), and Fig. 6(b), a SEM image of this sample, show that the CNTs’ surfaces were not compactly covered by CdS phase, many polydispersed small CdS particles only loosely stack on the surfaces of individ-
Transmission
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3450
1195
a
1398 1742 1582 1631
b 500
1000
1500
2000 2500 3000 Wavenumber (cm-1)
3500
4000
Fig. 7. Infrared transmission spectra: (a) as produced CNTs; (b) airoxidized CNTs; (c) acid-washed CNTs.
ual CNTs (shown in Fig. 6(a)) or the surfaces of CNT bundles (shown in Fig. 6(b)). When the CNTs were oxidized by air, under these exact conditions, oxygen attacked the cylindrical walls of CNTs. Some outer graphitic layers have been etched off, giving rough CNT-surfaces, as shown in reference [11]. But acid modification cannot etch the nanotube surfaces [12]. The nature of the surface groups was characterized with infrared transmission spectroscopy (Fig. 7). In the IR spectra, the signature of C@O functional groups in carbonyls, carboxyls or carboxylic anhydrides is evident at about 1742 cm1 [16] and AOH functional groups appear at 3450 cm1. The peak at 1582 cm1 reflects the vibration of carbocyclic plane of CNTs. The signal at about 1631 cm1 is associated with C@O functional groups in quinones or ketones [17]. The signal at 1398 cm1 is associated with OAH bending deformation in carboxylic acids and phenolic groups [18]. The peak centered on 1195 cm1 is associated with CAO stretching in ethers, hydroxyls or carboxylic anhydrides [17]. It is evident that there are many more carbonyl groups, carboxylic groups and hydroxyl groups, etc., in acid modified CNTs than in as produced CNTs and only air oxidized CNTs. Our view of the mechanism of CdS deposition lies in the rate of chemical reaction. Thioacetamide is a weak electrolyte, S2 ions are furnished through a reaction [19]: CH3 CSNH2 ! CH3 CN þ 2Hþ þ S2 2
Fig. 6. (a) TEM image; (b) SEM image. The deposits pointed by arrows are CdS. (The sample was synthesized through the reaction of Na2S and CdCl2/CNTs).
c
ð2Þ
The liberated S ions of a very low concentration may immediately react with Cd2+ ions and the reaction (2) is continued by the consumption of S2 ions. Na2S is a strong electrolyte, and can completely dissociate to be Na2+ ions and S2 ions in aqueous solution, result in a high concentration of free S2 ions. The fact that nanosized CdS particles can be homogeneously and densely deposited on air/acid-treated CNTs
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might be due to the following factors: (1) The roughness of the CNTs’ surfaces increases the effective surface area and affords the graphene edges with a higher wettability for fixing CdS phase; (2) The abundant oxygen-containing functional groups can effectively adsorb Cd2+ ions in the aqueous solution due to their electronegativity [20], resulted in the Cd2+ ions being concentrated on CNTs’ surfaces, the rate of the formation of CdS nuclei on CNTs’ surfaces was much faster than that in solution. The CdS nuclei formed on the outermost shell of CNTs grew through the continuous surface reaction of Cd2+ ions with S2 ions on the surfaces to form CdS nanoparticles, thus enhancing CdS deposition specifically on the CNTs whilst minimizing random deposition in solution. Moreover, the interface between the CdS particles and the outermost shell of nanotube are compact and the orientations of crystal planes of CdS particles are uniform; (3) When thioacetamide was added into the solution of CdCl2/air/acid-treated CNTs, the nucleation mainly occurred through a reaction of S2 ions with the Cd2+ ions adsorbed on the surfaces of CNTs. The final result of uniform and dense CdS nanoparticles covering the surfaces of CNTs was achieved through the dissolution-controlled growth by the reaction of Cd2+ and S2 ions, which were supplied from thioacetamide. The sufficiently low steady concentration of S2 ions during the growth stage secured the formation of the uniform CdS particles because it prevented the nucleation during the growth stage. Though air-oxidized CNTs have rough surfaces and CdS/air-oxidized CNTs was synthesized by using thioacetamide. Air-oxidized CNTs have less oxygen-containing functional groups and thus have a weak adsorption capability for Cd2+ [19], resulted in only small amounts of CdS nuclei being formed on the CNTs’ surfaces. These sparse CdS nuclei could grow without mutual competition and restraint, finally resulted in the diameters of the CdS particles becoming larger than that of nanotube walls. Moreover, due to little Cd2+ being adsorbed on the CNTs’ surfaces, many CdS particles were generated in the solution during the process of adding CH3CSNH2. The size of these CdS particles could continuously increase for reason that they were not restricted by the confined surfaces of CNTs. Thus, it is observed (Fig. 5(b)) that many CdS particles generated in the solution with the diameters being generally larger than 200 nm exist in the CNT network. Though CdS/acid-washed CNTs was synthesized by using thioacetamide, acid-washed CNTs can also have many oxygen-containing functional groups to adsorb Cd2+ and many CdS particles might grow on the nanotube surfaces. The surfaces of acid-washed CNTs are smooth and thus cannot effectively fix the CdS particles, resulted in many CdS particles falling off and the nanotube surfaces being mainly naked. The free CdS particles might aggregate together due to their high surface energy, as shown in Fig. 5(a). For the aqueous reaction system of Na2S and CdCl2/air/ acid-treated CNTs, though many Cd2+ could be adsorbed on CNTs’ surfaces, the relatively high supersaturation of
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free S2 ions in the solution led to a pronounced nucleation of CdS before the S2 ions diffused onto the surfaces of CNTs, and the growth of just generated nuclei concurrently occurred during the nucleation stage, led to the generation of polydispersed small CdS particles. The polydispersed small CdS particles aggregated to form the loose deposits. Though air/acid-treated CNTs have rough surfaces, they could not fix the CdS particles, which were not formed on their surfaces. These deposits could only stack on the surfaces of individual CNTs or the surfaces of CNT bundles, which are observed in the sample of CdS/air/acid-treated CNTs(Na) by electron microscopy. 4. Conclusion We have successfully prepared 10 nm CdS nanoparticles on carbon nanotube supports by a room-temperature solution chemical reaction method using thioacetamide and cadmium chloride. The surface state of CNTs is a critical factor to obtain nanosized CdS particles supported on CNTs. Air oxidization can etch the surfaces of CNTs, and acid modification can introduce functional groups on the surfaces of CNTs, both factors are beneficial to CdS/ CNT interaction. Keeping a low reaction rate between Cd2+ and S2 through selecting the appropriate reactants is also necessary to obtain a better covering of CdS nanoparticles on nanotubes. Acknowledgements This work was supported by: (a) the Major State Basic Research Development Program of China, Grant No. 10332020; (b) the Institute of Nuclear Energy Technology of Tsinghua University Project for Fundamental Research, Grant No.091131402. References [1] Rao CNR, Satishkumar BC, Govindaraj A, Nath M. Nanotubes. Chem Phys Chem 2001;2(2):78–105. [2] Alivisatos AP. Semiconductor clusters, nanocrystals, and quantum dots. Science 1996;271(5251):933–7. [3] Lin Y, Ska H, Emrick T, Dinsmore AD, Russell TP. Nanoparticle assembly and transport at liquid–liquid interfaces. Science 2003;299(5604):226–9. [4] Han WQ, Zettl A. Coating single-walled carbon nanotubes with tin oxide. Nano Lett 2003;3(5):681–3. [5] Banerjee S, Wong SS. Synthesis and characterization of carbon nanotube–nanocrystal heterostructures. Nano Lett 2002;2(3): 195–200. [6] Ravindran S, Chaudhary S, Colburn B, Ozkan M, Ozkan CS. Covalent coupling of quantum dots to multi-walled carbon nanotubes for electronic device applications. Nano Lett 2003;3(4):447–53. [7] Haremza JM, Hahn MA, Krauss TD, Chen S, Calcines J. Attachment of single CdSe nanocrystals to individual singlewalled carbon nanotubes. Nano Lett 2002;2(11):1253–8. [8] Lee SW, Sigmund WM. Formation of anatase TiO2 nanoparticles on carbon nanotubes. Chem Commun 2003(6):780–1. [9] Fu Q, Lu C, Liu J. Selective coating of single wall carbon nanotubes with thin SiO2 layer. Nano Lett 2002;2(4):329–32.
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