Ultralong Cadmium Hydroxide Nanowires: Synthesis, Characterization

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Langmuir 2007, 23, 9064-9068

Ultralong Cadmium Hydroxide Nanowires: Synthesis, Characterization, and Transformation into CdO Nanostrands Mingfu Ye,†,‡ Haizheng Zhong,† Wenjun Zheng,‡ Rui Li,† and Yongfang Li*,† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Department of Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed January 15, 2007. In Final Form: April 29, 2007 Ultralong Cd(OH)2 nanowires were fabricated by a hydrothermal method from Cd(CH3COO)2‚2H2O (0.01 mol/L) and C6H12N4 (0.015 mol/L) aqueous solution at 95 °C for 16 h without using any templates and were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM). The length of the nanowires reached several micrometers, giving an aspect ratio of a few thousands. The formation mechanism of the nanowires is attributed to the oriented attachment of small particles. The growth method for the 1D nanostructure presented here offers an excellent tool for the design of other advanced materials with anisotropic properties. The Cd(OH)2 nanowires efficiently captured negatively charged dye, and the adsorbed dye molecules can be released after the addition of EDTA. The Cd(OH)2 nanowires as template compounds were further transformed into CdO semiconductor nanomaterials with similar morphology by calcination under 350 °C in air for 3 h.

1. Introduction One-dimensional (1D) nanostructures, such as nanowires, nanorods, nanotubes, and nanobelts, have received increasing interest in the field of nanoscience,1-5 due to their unique sizeand shape-dependent physical properties. In the past decade, remarkable advances have been made in the synthesis and characterization of such materials, which are expected to open the way for nanoelectronics, ultrasmall optoelectronic devices, biosensors, and so forth.6-9 The 1D nanomaterials have been synthesized through high-temperature processes such as vaporliquid-solid (VLS) growth and oxide-assisted growth,10,11 and by anisotropic growth of crystalline nanomaterials using proper capping agents.12 The other synthetic strategy is the use of hard templates such as a polycarbonate membrane and porous alumina,13 and soft templates like polymer14 and organic surfactants,15 which physically confine the growing nanomaterials within the 1D nanospace. However, the template method is limited by the template materials, and sometimes the post-removal of * Fax: 86-10-62559373; E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Nankai University. (1) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher. E.; Kadavanich, A.; Alivisatos, A. P. Nature (London) 2000, 404, 59-61. (2) Iijima, S. Nature (London) 1991, 354, 56-58. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947-1947. (4) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 12871289. (5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353-389. (6) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature (London) 2001, 409, 66-69. (7) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289-1292. (8) Yu, W. W.; Peng, X. Angew. Chem., Int. Ed. 2002, 41, 2368-2371. (9) Zhang, W. X.; Wen, X. G.; Yang, S. H. Inorg. Chem. 2003, 42, 50055014. (10) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004, 34, 83-122. (11) Yin, Y.; Zhang, G.; Xia, Y. AdV. Funct. Mater. 2002, 12, 293-298. (12) Guo, L.; Ji, Y. L.; Xu, H. J. Am. Chem. Soc. 2002, 124, 14864-14865. (13) Li, Y.; Xu, D. S.; Zhang, Q. M.; Chen, D. P.; Huang, F. Z.; Xu, Y. J.; Guo, G. L.; Gu, Z. N. Chem. Mater. 1999, 11, 3433-3435. (14) Yu, S. H.; Antonietti, M.; Colfen, H.; Hartmann. J. Nano Lett. 2003, 3, 379-382. (15) Chen, D.; Shen, G. Z.; Tang, K. B.; Liang, Z. H.; Zheng, H. G. J. Phys. Chem. B 2004, 108, 11280-11284.

the template after the synthesis is complicated. Therefore, template-free approaches have been pursued in recent years. Now, the precise morphology control of nanomaterials and the following scale-up for industrial production remain a huge challenge for the research of nanosciences. In addition to typical template methods, the oriented attachment (OA) approach was also used to prepare the nanowires.16 The OA approach generates nanowires by attaching existing dotshaped nanocrystals along a given crystal orientation. Since it was first reported a few years ago, the OA-based growth was subsequently described in other materials, such as CdTe17 and PbSe nanowires18 and ZnSe nanorods.19 In this process, adjacent particles orient so as to share a common crystallographic orientation. Subsequent elimination of the interface between them results in the formation of a larger single crystal. Initially, the product retains an irregular shape that reflects formation from two individual particles. Microstructures are often the byproducts of OA-based crystal growth. In particular, dislocations and planar defects are often a direct consequence of OA and the region where initial particle-particle contact occurred.16 In order to control the crystal growth, many additives including both organic and inorganic species have been utilized in the liquid-phase synthesis. These additives can bind to certain crystal planes and reduce the surface energy of these planes, which allows the crystals to grow along those planes that have weak binding capabilities with the additives.20,21 The most widely used additives are organic surfactants such as CTAB, AOT, and organic amines. Advantages of the template-free approach could be that there is no need of posttreatment to get rid of the template materials in practical applications and it is feasible for scale-up of the industrial mass production. (16) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969-971. (17) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237-240. (18) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140-7147. (19) Cozzoli, P. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Chem. Mater. 2005, 17, 1296-1306. (20) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2154-2157. (21) Sun, X. P.; Dong, S. J.; Wang, E. K. Angew. Chem., Int. Ed. 2004, 43, 6360-6363.

10.1021/la070111c CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007

Ultralong Cadmium Hydroxide Nanowires

Hydrothermal synthesis has become a powerful tool for the fabrication of nanomaterials with some significant advantages, such as controllable particle size, low temperature, and costeffectiveness.22 Cadmium hydroxide, Cd(OH)2, is a wide band gap semiconductor with a wide range of possible applications including solar cells, photo transistors and diodes, transparent electrodes, sensors, cathode electrode materials of batteries, and so forth.23-25 These applications of cadmium hydroxide are based on its specific optical and electrical properties. For example, Cd(OH)2 films show a high transparency in the visible region of the solar spectrum, as well as high electrical conductivity. Cadmium hydroxide has been also proven to be an important precursor that could be converted into cadmium oxide through dehydration or into other functional materials (e.g., CdS, CdSe) by reaction with appropriate elements or compounds.26 In this paper, we report a facile hydrothermal method for the direct growth of ultralong Cd(OH)2 nanowires from Cd(CH3COO)2‚ 2H2O (0.01 mol/L) and C6H12N4 (0.015 mol/L) aqueous solution at 95 °C for 16 h without using any templates. To the best of our knowledge, this method has never been employed in growing Cd(OH)2.26-32 The Cd(OH)2 nanowires efficiently captured negatively charged dye, and the adsorbed dye molecules can be released after the addition of EDTA. The cadmium hydroxide nanostrands were transformed into CdO by calcinations. The nanomaterials of groups II-VI semiconductors including CdO are of particular interest for their important optical, optoelectronic, and catalytic properties. 2. Experimental Section 2.1. Materials. Cd(CH3COO)2‚2H2O (Beijing Shuanghuan Chemical Reagents Company) and hexamethylenetetramine (C6H12N4) (Tianjin Chemical Reagents Company) are analytical grade and were used as purchased without further purification. Deionized water was used as solvent in all the experiments. 2.2. Preparation. The typical synthesis of the Cd(OH)2 nanowires underwent the following steps. Cd(CH3COO)2‚2H2O (0.001 mol) and C6H12N4 (0.0015 mol) were dissolved in deionized water (100 mL), then the reaction solution was poured into a 15 mL Teflonlined stainless steel autoclave up to 80% of the total volume. The autoclaves were sealed and moved to a laboratory oven at 95 °C for 16 h without shaking or stirring. Then, the autoclaves were taken out from the oven and air-cooled to room temperature. After the completion of the hydrothermal process, the resulting white solid products were centrifuged, washed with deionized water and ethanol to remove the ions possibly remaining in the final products, and dried at room temperature in air. In addition, a similar synthetic procedure was also used to prepare other related morphologies. For surface charging of the Cd(OH)2 nanowires, the as-prepared Cd(OH)2 (0.001 mol) products were immersed into 0.25 mol.L-1 Congo Red solutions (100 mL) under vigorous stirring for 5 days, then were centrifuged and washed with deionized water. The (22) Xu, R. R.; Pang, W. Q. Inorganic Synthetic and PreparatiVe Chemistry; Higher Education Press (China): Beijing, 2001. (23) Zhang, H.; Ma, X. Y.; Ji, Y. J.; Xu, J.; Yang, D. R. Mater. Lett. 2005, 59, 56-58. (24) Ristic, M.; Popovic, S.; Music, S. Mater. Lett. 2004, 58, 2494-2499. (25) Motupally, S.; Jain, M.; Srinivasan, V.; Weidner, J. W. J. Electrochem. Soc. 1998, 145, 34-39. (26) Zhong, H. Z.; Li, Y. C.; Zhou, Y.; Yang, C. H.; Li, Y. F. Nanotechnology 2006, 17, 772-777. (27) Ichinose, I.; Kurashima, K.; Kunitake, T. J. Am. Chem. Soc. 2004, 126, 7162-7163. (28) Tang, B.; Zhou, L. H.; Ge, J. C.; Niu, J. Y.; Shi, Z. Q. Inorg. Chem. 2005, 44, 2568-2569. (29) Mane, R. S.; Han, S. H. Electrochem. Commun. 2005, 7, 205-208. (30) Li, X. M.; Chu, H. B.; Li, Y. J. Solid State Chem. 2006, 179, 96-102. (31) Shi, W. D.; Wang, C.; Wang, H. S.; Zhang, H. Cryst. Growth Des. 2006, 6, 915-918. (32) Miao, J. J.; Fu, R. L.; Zhu, J. M.; Xu, K.; Zhu, J. J.; Chen, H. Y. Chem. Commun. 2006, 28, 3013-3015

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Figure 1. XRD pattern of the Cd(OH)2 nanowires prepared by the hydrothermal method from Cd(CH3COO)2‚2H2O (0.01 mol/L) and C6H12N4 (0.015 mol/L) aqueous solution at 95 °C for 16 h.

conversion of the Cd(OH)2 to CdO nanowires was carried out in an oven in air at 350 °C for 3 h. 2.3. Characterization. The size and morphology of the samples were examined by scanning electron microscopy (SEM, Hitachi S-4300). The crystal structure of the nanomaterials was identified by X-ray diffraction (XRD) analysis with a Rigaku D/max-2400 diffractometer operated at 40 kV voltage and a 120 mA current using Cu KR radiation, Samples for XRD measurements were solid powder prepared by drying the purified product under vacuum. Transmission electron microscopy (TEM) observations were performed with a JEO-2010 transmission electron microscope, accompanied by selected area electron diffraction (SAED) and highresolution transmission electron microscopy (HRTEM). The specimen for TEM imaging was prepared by suspending the powder sample in ethanol, then drop-dried on the copper grids at room temperature.

3. Results and Discussion 3.1. Synthesis and Characterization. Figure 1 shows the XRD pattern of the cadmium hydroxide sample prepared by the hydrothermal method from Cd(CH3COO)2‚2H2O (0.01 mol/L) and C6H12N4 (0.015 mol/L) aqueous solution at 95 °C for 16 h. All the peaks in this figure can be readily indexed to a pure hexagonal phase of Cd(OH)2 (space group P3hm1 (no 164)) with lattice constants a ) 3.4947 Å and c ) 4.7106 Å (JCPDS310228). The XRD results confirmed the composition of the Cd(OH)2 products. The morphology of the product, prepared by the hydrothermal method from Cd(CH3COO)2‚2H2O (0.01 mol/L) and C6H12N4 (0.015 mol/L) aqueous solution at 95 °C for 16 h, was visualized by TEM, as shown in Figure 2. The lower-magnification TEM image (Figure 2a) and SEM image (Figure 2e) reveal that nanowires of the Cd(OH)2 samples were formed, and the length of the individual Cd(OH)2 nanowire reached several micrometers. The higher magnification TEM image (Figure 2b) displays that the width of the nanowires is about 6-10 nm. The selected area electron diffraction (SAED) pattern inserted in Figure 2b performed on several Cd(OH)2 nanowires indicates that the Cd(OH)2 nanowires are crystals of hexagonal phase. High-resolution TEM (HR-TEM) images of the Cd(OH)2 nanowires are shown in Figure 2c,d. From the HR-TEM images, we can clearly observe the spacing of 0.260 nm (see Figure 2d), corresponding to the (101) lattice planes of hexagonal Cd(OH)2, and the nanowires were assembled from smaller Cd(OH)2 nanoparticles (Figure 2c). The lattice fringe details indicate that the interface between the particles involves rotation of one crystal relative to the other.

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Figure 3. TEM images of Cd(OH)2 samples prepared in the aqueous solution containing 0.01 mol L-1 cadmium acetate and 0.015 mol L-1 hexamethylenetetramine with different reaction time and temperature: (a) at 150 °C for 16 h; (b) at 95 °C for 2 h; and (c) at 95 °C for 5 h. (d) TEM images of Cd(OH)2 samples prepared by hydrothermal method from cadmium acetate (0.01 mol L-1) and ammonia (0.06 mol L-1) solution at 95 °C for 16 h. Figure 2. TEM (a,b), HRTEM (c,d), and SEM (e) images of Cd(OH)2 nanowires prepared by the hydrothermal method from Cd(CH3COO)2‚2H2O (0.01 mol/L) and C6H12N4 (0.015 mol/L) aqueous solution at 95 °C for 16 h. The inset in Figure 2b is the SAED pattern recorded from the same nanowire.

3.2. Formation Mechanism of the Nanostrctures. The apparent reaction for the formation of the cadmium hydroxide is as follows:

2Cd(CH3COO)2‚2H2O + C6H12N4+ 6H2O f 2Cd(OH)2+ 6CH2O + 4NH3‚CH3COOH

The Cd(OH)2 nanowires were grown through a hydrothermal solution process. Solutions containing cadmium ions give white precipitates upon meeting sufficient OH- ions in solution, as long as [Cd2+][OH-]2 is greater than the Ksp value of Cd(OH)2 at certain temperatures. However, Cd(OH)2 is not always the final product during the hydrothermal courses, since hydrated cadmium ion complexes (n[Cd(H2O)p]2+) were transferred to cadmium hydroxide ([Cdn(OH)2n]) through the stepwise replacement of water molecules in the hydrated cadmium ion complexes by hydroxide groups. This process is usually referred to as “olation” and offers the intermediate cadmium complex [Cdn(OH)m(H2O)np-m](2n-m)+. It can be described as follows:

C6H12N4 + 10H2O f 6CH2O + 4NH3‚H2O NH3‚H2O h NH4+ + OHn[Cd(H2O)p]2+ + mOH- h [Cdn(OH)m(H2O)np-m](2n-m)+ f [Cdn(OH)2n](s) + (np-m)H+ It is believed that self-assembly of the colloidal particles into crystallographic orientation of the aggregated particles resulted

in the formation of the nanowires. Our experimental results revealed that the growth of the nanowires depends on the solution temperature and reaction time; in particular, temperature plays a more important role. For the same reaction solution of 0.01 mol/L Cd(CH3COO)2‚2H2O and 0.015 mol/L C6H12N4 aqueous solution, when the reaction temperature was fixed at 60 °C with a reaction time of 16 h, there was no white solid product formed; while as the reaction temperature was raised to 150 °C with a reaction time of 16 h, only Cd(OH)2 nanodots were obtained (see Figure 3a). When the reaction temperature was set at 95 °C, a gradual aggregation growth of Cd(OH)2 nanoparticles was observed; for example, after reaction for 2 h, the quasi-spherical aggregations of the Cd(OH)2 nanoparticles were formed (Figure 3b), with the increase in the reaction time, the aggregation turned to 1D growth. The pearl-chain-like nanostructures were observed after reaction for 5 h (Figure 3c), and the XRD test shows that they are purely the hexagonal phase of Cd(OH)2. After reaction for 16 h, the nanowires were formed (see Figure 2). In classic colloidal models, crystal growth in a solution can be categorized as either kinetically or thermodynamically controlled and is subject to Ostwald ripening during the synthetic courses. Usually, it is the Ostwald ripening that determines the morphology of the final products, since it is believed that the initially formed nuclei are defect-free. During the Ostwald ripening process, larger particles tend to grow even larger at the expense of the smaller particles, which dissolve into the solution progressively. Specific morphology might evolve from this process, provided that the synthetic conditions favor this particle evolution. The anisotropic growth of the nanocrystals in a template-free method is generally related to the different surface energy of different crystal planes of the nanocrystals. Those planes with high surface energy have a strong tendency to capture monomers from the reaction solution in order to reduce their surface energy. This leads to growth along those planes and

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Figure 4. Schematic diagram illustrating the morphological change with temperature and reaction time: (top) Ostwald ripening (at high temperature), and (bottom) growth by oriented attachment (at low temperature). Scheme 1. Molecular Structure of Congo Red

eventually producing crystals with anisotropic morphology. For the formation mechanism of the nanowire morphology (in low temperature), two steps of (1) nucleation and growth and (2) self-assembly by oriented attachment should be involved, and the assembly of the nanoparticles should be the key step for the formation of the nanowires. We observed that the Cd(OH)2 nanoparticles with the size of a few nanometers can coalesce under hydrothermal conditions in a way we call oriented attachment. In such formed aggregates, the crystalline lattice planes may be almost perfectly aligned. The impetus for the aggregation of the particles in this case should still be the reduction of the total surface energy through elimination of the higher surface energy of the lattice faces by the aggregation. This growth mechanism involves the irreversible and specifically oriented self-assembly of primary nanocrystals and results in the formation of the nanostructures. This mechanism was also proposed by other groups.33-37 At high temperature (such as 150 °C), it is the Ostwald ripening that forms larger particles, while at a suitable medium temperature such as 95 °C, the gradually oriented aggregation took place. Figure 4 illustrates the morphological change with temperature and reaction time. In this work, C6H12N4 was used as the OH- source through gradual thermohydrolysis (as shown in the reaction equation mentioned above); the slow release of the OH- ions is very important for controlling the nucleation rate and for preparing the Cd(OH)2 nanowires. If we directly use ammonia as the OHsource, the Cd(OH)2 nanowires could not be obtained. To check the effect of the OH- source, we added ammonia (3 mL, 2.0 mol/L) to the Cd(CH3COO)2‚2H2O (100 mL, 0.01 mol/L) solution under vigorous stirring, and a white precipitate appeared immediately. Then, the hydrothermal treatment of the solution at 95 °C for 16 h resulted in the Cd(OH)2 hexagonal plates with holes in them and few nanowires (see Figure 3d). We prolonged the reaction time to 32 h, and a similar morphology was observed. 3.3. Surface Charge and Transformation to CdO Nanostrands. The nanowires mentioned above were positively charged ([Cdn(OH)m(H2O)np-m](2n-m)+), and so they could adsorb nega(33) Claudia, P.; Andreas, K.; Horst, W. Angew. Chem., Int. Ed. 2002, 41, 1188-1191. (34) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707-12712. (35) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430-4431. (36) Zhang, D. F.; Sun, L. D.; Yin, J. L.; Yan, C. H. AdV. Mater. 2003, 15, 1022-1025. (37) Huang, F.; Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2003, 107, 1047010475.

Figure 5. TEM images of fibrous nanocomposites of cadmium hydroxide nanowires and Conge Red.

Figure 6. UV-vis absorption spectra of (a) Congo Red solution with a concentration of 0.025 mM and (b) the Congo Red solution after the electrostatic combination with Cd(OH)2 nanowires with a molar ratio of Cd2+/CR equal to 12:1 and filtering out the electrostatic composites from the solution with a polycarbonate membrane filter.

tively charged molecules.27,38 Here, we studied the electrostatic combination of Conge Red dye molecules (see Scheme 1) on the Cd(OH)2 nanowires and got gelled precipitates after the combination. When the amount of nanowires was much lower than that of dye molecules, the gelled precipitates were not always formed. However, their electrostatic composites were readily filtered from the solution. To evaluate the trapping ability, the as-prepared white Cd(OH)2 nanowires (10-3 mol) were added into 0.25 mol L-1 Congo Red solutions (100 mL) under vigorous stirring for 5 days to ensure adsorbing saturation. Then, the final red products were centrifuged, washed with deionized water, and measured by means of inductively coupled plasma atomic emission (ICP) using a standard HCl/HNO3 digestion. Through calculation, we obtained the molar ratio of cadmium ions against Congo Red molecules of about 12:1. The morphologies of the electrostatic composites are visualized by TEM, as shown in Figure 5. Considering the diameter of the nanowire (10 nm), it is clear that the dye molecules are inserted between the inorganic nanowires. We also monitored the electrostatic combination of Conge Red dye molecules on the Cd(OH)2 nanowires by measuring UV-vis absorption spectra of the Congo Red solutions before and after the electrostatic combination. As shown in Figure 6, (38) Luo, Y. H.; Huang, J. G.; Ichinose, I. J. Am. Chem. Soc. 2005, 127, 8296-8297.

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filtered out together with the nanowires. These properties could be applied to the effective separation of other matter from the dilute solutions.27 Interestingly, when these electrostatic composites were added into an excessive amount of ethylenediaminetetraacetic acid trisodium salt solution (EDTA-3Na, pH 7.7), the colorless solution reddened. EDTA is known to give a very stable metal complex with a divalent cadmium ion (the EDTA-Cd2+ complex stability constant reaches about 1013 at pH 7.039). The Congo Red adsorbed on the nanowires could also be released when the electrostatic composites were put into an aqueous HCl solution, because the Cd(OH)2 nanowires can be dissolved in the aqueous solution with pH of