Shape-Controlled Synthesis of Single-Crystalline CdCO3 and Corresponding Porous CdO Nanostructures Zhiyong Jia, Yiwen Tang,* Lijuan Luo, and Bihui Li Institute of Nano-science and Technology, Central China Normal UniVersity, Wuhan 430079, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2116–2120
ReceiVed August 1, 2007; ReVised Manuscript ReceiVed April 8, 2008
ABSTRACT: Low-dimensional single-crystalline CdCO3 nanostructures such as nanowires, nanobelts, nanorolls, and one-dimensional (1D) hierarchical structures have been synthesized through a convenient, low-temperature hydrothermal method, and the products have been characterized by X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, and selected area electronic diffraction. The influences of ammonia concentration, reaction time, and temperature on the morphologies of CdCO3 have been investigated. In addition, these low-dimensional nanostructures can be transformed into oriented CdO nanoporous structures through heat treatment processing, and the corresponding shapes can be preserved completely.
1. Introduction One of the important goals of materials chemistry is to develop ways of tailoring the structure of materials on the nanometer length scale. Structurally well-defined building blocks are potentially useful in the synthesis of designed catalysts, photonic band gap materials, nanoscale electronic devices, and chemical separations media.1 The design of a generic method for the preparation of inorganic nanostructures with a broad range of well-defined and controllable morphologies is still needed in order to fully exploit their peculiar properties and unique applications.2 In spite of their fundamental and technological importance, the challenge to synthetically and systematically control the shape of inorganic nanostructures has been met with limited success. A general approach to the fabrication in a precisely controlled manner is not yet available, but it is widely accepted that organic ligands or surfactants play a key role in determining not only the size but also the shape of the products.3 Recently, a nonhydrolytic high-temperature thermal reaction method in the presence of organic surfactants has been developed and is often regarded as the mainstream synthetic chemistry in the field.4 Nevertheless, the method needs some special instruments and complex procedure, and usually the removal procedure of organic surfactants or solvent from asprepared products is difficult and tedious. In particular, for the use of expensive and environmental unfriendly organic surfactants or solvents such as 1-octadecene (ODE), trioctylphosphine (TOP), tri-n-octylphosphine (TOPO), oleic acid, stearic acid, hexadecylamine etc., therefore, it is difficult to scale up such a synthesis for making nanocrystals in large quantities (e.g., tens to hundreds of kilograms). On the other hand, industrial aqueous ammonia is an important inorganic chelating agent in the field of inorganic chemistry. This chelating agent can coordinate the majority of transition metal ions in water, but these soluble complex ions are rather unstable in water due to hydrolysis.5 If the hydrolysis or ligand exchange of these complex ions as precursors can be restricted, inorganic nanostructures containing these transition metal ions with a broad range of well-defined and controllable morphologies can be obtained. However, the effect of these complex ions as precursors on the nucleation and growth of nanomaterials has been less discussed. Research in this aspect * To whom correspondence should be addressed. E-mail: ywtang@ phy.ccnu.edu.cn; fax: +86-27-67861185.
is not only of interest for theoretical purposes, for example, increasing the number of ligand species for controlling crystal growth and disclosing the intrinsical property of a ligand, but also of industrial potential for large-scale production. Recently, we provided a simple hydrothermal method for the synthesis of single-crystal nickel hydroxyl sulfate nanobelts.6 This simple hydrothermal method required neither expensive, environmentally unfriendly organic surfactants or solvents, nor special equipment. The low-cost inorganic small molecule ammonia is employed as a ligand to form the [Ni(NH3)6]2+ complex ion as precursor and ethanol is chosen as the solvent to restrict the hydrolysis of the [Ni(NH3)6]2+ complex ions. Herein, we demonstrate that low-dimensional CdCO3 nanostructures, such as nanowires, nanobelts, nanorolls, and onedimensional hierarchical structures have been synthesized through the same hydrothermal method. Furthermore, the use of sodium carbonate or carbon dioxide among the precursor materials, which are usually used for the synthesis of carbonate salts such as CaCO3, PbCO3, BaCO3, etc.,7 are not required in this approach. Moreover, cadmium carbonate may be directly converted from the cadmium carboxylate precursor through a self-catalytic process even at room temperature. The morphology diversification and the growth mechanism of CdCO3 nanocrystals involved in the growth habit from the kinetic to thermodynamic growth stage were studied carefully. As is known, CdO adopts the centrosymmetric rock salt structure (face-centered-cubic system) and exhibits interesting electronic and optical properties. Therefore, CdO has been thoroughly studied from a scientific perspective and for technological applications.8 Given the potential scientific interest that may arise with the growth of high-quality CdO nanowires, there are only a few studies on the synthesis of one-dimensional (1D) CdO nanostructures.9 However, the synthesis temperatures used in these studies were all relatively high, making each of these processes less desirable. Recently, it has been reported that an economically viable route for obtaining 1D cadmium oxide occurs through dehydration of Cd(OH)2 1D nanomaterials or thermal decomposition of 1D cadmium salts such as CdCO3. Yu et al. have reported that a novel solid-phase fabrication approach has been developed to prepare highly nanoporous cadmium oxide architectures via the one-step thermal decomposition of high-quality cadmium carbonate microcrystals.10 Herein, we demonstrate that the as-prepared CdCO3 nanocrystals with various morphologies could be fully transformed into
10.1021/cg7007254 CCC: $40.75 2008 American Chemical Society Published on Web 06/06/2008
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Figure 1. XRD pattern of the CdCO3 nanobelts.
porous CdO nanocrystals through the heat treatment process, and the corresponding shapes can be preserved completely.
2. Experimental Section In the synthesis of nanobelts, 0.69 g of Cd(CH3COO)2 · 2H2O (0.004 mol) was added into 8 mL of distilled water, and then 3 mL of ammonia (35% by v/v) was slowly added dropwise under magnetic stirring conditions. The resulting uniform and clear solution was added dropwise into 70 mL of ethanol. Then, the acquired solution was sealed into a 100 mL autoclave and heated at 180 °C for 24 h. After the sample was naturally cooled to room temperature, a flocculent precipitate was obtained. The precipitate was recovered from the solution by centrifugation, washed with distilled water three times, and dried at 45 °C in a vacuum oven. The whole process can be easily adjusted to prepare CdCO3 nanowires, 1D hierarchical structures, and nanorolls by simply changing the concentrations of ammonia (in our experiments, the concentrations of ammonia are represented by the added ammonia volume), temperature, and duration of the hydrothermal treatment while keeping other conditions unchanged. After being calcined at 450 °C for 0.5 h under the protection of Ar gas, the as-produced CdCO3 nanomaterials with various morphologies could be completely converted into CdO nanoporous materials. X-ray powder diffraction (XRD) patterns were recorded using a Bruker D8 X-ray diffractometer with Cu KR radiation, and the morphology and structure were determined using field emission scanning electron microscopy (FESEM, JSM-6700F) and transmission electron microscopy (TEM, JEOL 2010).
3. Results and Discussion 3.1. Structure and Morphology of Nanobelts. The XRD patterns of all the samples with various morphologies were found to be similar. A representative pattern is shown in Figure 1. All the peaks of the XRD pattern in Figure 1 could be well indexed to the pure hexagonal structure of CdCO3 (JCPDS. 721939) with lattice constants a ) 4.923 Å, c ) 16.28 Å. No peak for other types of cadmium carbonate and impurities were observed in the XRD pattern, demonstrating the high purity of the final products. The morphology and structure of the nanobelts were characterized by SEM and TEM, as shown in Figure 2. These images reveal that the sample consists of ultralong nanobelts of a few micrometers in length, and several tens of nanometers in width. The SAED pattern as shown in Figure 2c shows that the CdCO3 nanobelts are single crystals that are formed along the [104] direction, and are enclosed with (010) and (401j) as the top and side planes, respectively. Further detailed analysis of weaker diffractions is in progress.
Figure 2. SEM image (a), TEM image (b), and SAED pattern (c) of the as-prepared CdCO3 nanobelts.
3.2. Synthesis of CdCO3 Nanowires and Nanobelts. The detailed examination revealed that the morphologies of the CdCO3 nanocrystals could be facilely controlled through adjusting the experimental parameters. The experiments of different ammonia concentrations and reaction temperatures were conducted. Representative SEM patterns of the products at different ammonia concentrations under 180 °C reaction temperature are shown in Figure 3. The images show that the samples display different shapes nanocrystals, including uniform nanowires and nanobelts of lengths up to tens of micrometers and diameter of 20-200 nm, as well as nanorolls and spherical nanoparticles. Figure 3a shows a typical image of CdCO3 nanowires with diameters of 10-20 nm. Figure 3b shows small flat nanowires which are possibly intermediate shapes between thin nanowires and wide nanobelts. Figure 3c,d shows typical nanobelts, but the latter is wider and thicker. The nanorolls and nanoparticles are shown in Figures 3e,f, respectively. The evident tendency of morphology shift could be found: as the concentration of ammonia increased, the diameter or width of the CdCO3
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Figure 5. SEM images of the CdCO3 architectures: (a) low-magnification and (b) higher-magnification.
Figure 3. SEM images of the CdCO3 prepared at 180 °C under different concentrations of ammonia: (a) 2 mL; (b) 2.5 mL; (c) 3 mL; (d) 4 mL; (e) 5 mL; and (f) 6 mL.
Figure 4. SEM images of the nanowires: (a) low-magnification and (b) higher-magnification.
nanocrystals increased and the morphology shifted from 1D nanowires and nanobelts to 2D nanorolls, and finally to 3D spherical nanoparticles. CdCO3 nanowires could be directly obtained when the amount of aqueous ammonia added to the starting materials at room temperature (about 20 °C) was about 1 mL. Figure 4 shows a typical SEM image of CdCO3 nanowires with diameters of 5-8 nm. This result also indicates that the disintegration rate of the carboxyl anions is much faster, even at room temperature. If only the reaction temperature is increased to 180 °C while keeping other conditions unchanged, the diameter of the CdCO3 nanowires can be increased to 15-20 nm. Although the diameter of the nanowires can be increased with reaction temperature, nanobelts have never been obtained under the condition of 1-2 mL of ammonia. Indeed, only by increasing the ammonia concentration to 3 mL and treating the reactants at 180 °C, CdCO3 nanobelts can be obtained. 3.3. Synthesis of CdCO3 Hierarchical Structures. When the reaction temperature was increased to 220 °C at 4 mL of ammonia and the reaction time was decreased to 6 h, the CdCO3 hierarchical structures were obtained. The low-magnification SEM image (Figure 5a) clearly reveals that the sample consists of two kinds of morphology: the nanobelts with diameter of
about 200 nm and the nanowires with diameter of about 15 nm. From the high-magnification SEM image (Figure 5b), we can observe that the thin nanowires are coiled around the wide nanobelts leading to the formation of the hierarchical structures. The high flexibility of the nanowires can be explained by their small cross-section,11 as well as by the construction of the cadmium carbonate. On the other hand, when the reaction time was kept at 220 °C for 12 h while keeping other conditions unchanged, only thicker and wider nanobelts were obtained instead of a 1D hierarchical structure. Although the direct synthesis of complex nanostructure architecture is desirable, it still remains a challenge in areas of materials science. However, based on the above results, we can find that the 1D CdCO3 hierarchical structures were easily obtained in our system. 3.4. Formation Mechanism of CdCO3 Nanocrystals. In general, the synthesis of carbonates via solution methods needs sodium carbonate or carbon dioxide among the starting precursor,7 but in our system, these materials were not requisite as starting materials for the successful fabrication of cadmium carbonate nanostructures. It is probable that the carboxyl anion in solution can be decomposed in base solution due to the selfcatalysis effect of cadmium cations on carboxyl anions, which is called the Kolbe-Schmitt Reaction.12 But the catalyst mechanism behind transformation of carboxyl anions into carbonate anions requires further investigation. The following chemical reactions illustrate a few of the many possible reaction pathways that could be at play for such a complex system:
Cd2+ + 4NH3 h [Cd(NH3)4]2+
(1)
[Cd(NH3)4]2+ h Cd2+ + 4NH3
(2)
CH3COO- + OH- f CH4 v + CO23
(3)
Cd2+ + CO23 h CdCO3 V
(4)
Thermodynamically, all of the nanocrystals will grow toward the shape having the lowest energy at equilibrium, which is governed by classic theory. However, the formation dynamics can affect the shape of the formed nanocrystals. Indeed, the formation of nanocrystals is found to be a highly kinetics-driven process. In principle, before the reaction reaches the equilibrium stage, any metastable nanocrystaline shape can be arrested by tuning the reaction conditions.13 How far these transformations proceed along a series of increasingly stable intermediates depends on the solubilities of the minerals and on the free energies of activation of their interconversions, all of which are strongly influenced by additives.14 In our system, the various metastable nanocrystaline shapes such as nanowires, nanobelts, nanorolls, and 1D hierarchical structures can be obtained simply by adjusting the temperature, the ammonia concentration, and the reaction time. The morphologies and sizes of the CdCO3 nanocrystals are summarized in Table 1. Under the conditions
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Table 1. Summary of Various Sizes and Morphologies of CdCO3 Nanocrystals Prepared at Different Reaction Conditions ammonia concentration (mL)
reaction temperature (°C)
reaction time (h)
diameter width (nm)
morphology
1 1-2 2.5 3-4 5 6 4
20 180 180 180 180 180 220
24 24 24 24 24 24 6
5–10 10–20 20–25 30–300 500–1000 10–25 100–200
nanowires nanowires nanowires nanobelts nanorolls nanoparticles architectures
of 1 mL ammonia concentration and room temperature, the [Cd(NH3)4]2+ complex ions are very unstable and easily decomposed. In such a case, the system is highly kinetically driven by an extremely high monomer concentration and the high anisotropic growth along the [104] direction would be favored, such that the thin CdCO3 nanowires can be obtained, as shown in Figure 4. If only the reaction temperature was increased while keeping 1 mL ammonia unchanged, eqs 2, 3, and 4 would all favor the right-hand side of the equilibrium. So the monomer concentration would be quickly exhausted due to the quickened nucleation and growth rate under the high temperature condition, where the growth quickly reaches the thermodynamic equilibrium, and a ripening process occurs,15 so the diameter of the CdCO3 nanowires would become thicker. Our experimental results also showed that with increased temperature (such as 60 and 90 °C), but keeping all other conditions unchanged, the morphology of the CdCO3 nanowires remains the same, but the diameter may increase slightly. The result also indicates that increasing temperature only quickens the growth to achieve thermodynamic equilibrium and does not have any evident effect on the morphology shift. When the ammonia concentration was increased from 2.5 to 5 mL and the reaction temperature was kept at 180 °C, a series of increasingly stable intermediates emerged owing to the increased solubility constant (Ksp) of cadmium carbonate in the bulk solution, as given by eq 1, and subsequently the monomer concentration remains at a rather high level. Then it is possible that the growth rate along the [104] direction may still be the fastest growth direction, while the other growth directions may also be favored. So the metastable shapes, such as the flat nanowires, the nanobelts, and nanorolls with increased width and lowered energy crystal faces, may be arrested in the metastable growth process with the rather high monomer concentration, as shown in Figure 3b-e. As the ammonia concentration was increased to 6 mL, the growth process directly reached the thermodynamic growth stage because of the very high solubility of cadmium carbonate in the bulk solution leading to a very low monomer concentration and growth rate. The final shape is spherical nanoparticles, which are thermodynamically favorable,16 as shown in Figure 3f. When the reaction temperature was increased to 220 °C and the ammonia concentration was still 4 mL, the early growth process is still in the metastable growth stage similar to that under the conditions of 180 °C and 4 mL ammonia concentration, such that the thicker and wider nanobelts would be produced. At the same time, early cooling due to a shortened reaction time causes the growth to be shifted to a highly kinetic 1D growth stage because of the lower solubility and the extremely high monomer concentration still remaining in the bulk solution, leading to the formation of a large number of nanowires. Thus, hierarchical structures can be obtained, as shown in Figure 5. When the reaction time was increased to 12 h, the monomer concentration was depleted for the growth
Figure 6. XRD pattern of the porous CdO nanobelts after 0.5 h treatment of as-synthesized CdCO3 at 450 °C.
of the initial nanobelts, and only thicker and wider nanobelts were obtained. On the basis of the above discussion, controlling the temperature, ammonia concentration, and reaction time are convenient factors for adjusting the crystals growth habits from the extremely high monomer concentration 1D metastable growth stage (1D nanowires) to the rather high monomer concentration 2D metastable growth stage (1D nanobelts and 2D nanorolls), finally to the low monomer concentration thermodynamic 3D growth stage (nanoparticles). Moreover, this can be reversed such as from the 2D metastable growth stage to the highly anisotropic 1D metastable growth stage by changing the reaction time, so that the 1D hierarchical structure can be obtained. As a result, the morphology shift may be easily accomplished through changing the ammonia concentration, temperature, and reaction time. On the other hand, this work has also provided a general, simple, and effective method to control the shape of other transition-metal carbonates. 3.5. Formation of CdO Porous Nanobelts through Thermal Decomposition. Figure 6 shows the XRD patterns of the as-decomposed products of the cadmium carbonate nanobelts obtained after 0.5 h treatment at 450 °C. The XRD peaks of cadmium carbonate completely disappeared and only the peaks of cadmium oxide were observed. All of the peaks in this pattern can be indexed as the pure cubic phase of CdO with a measured lattice constant of a ) 4.695 Å (JCPDS 75-0594). After complete decomposition at 450 °C, the well-faceted cadmium carbonate nanobelts were fully transformed into nanoporous cadmium oxide nanobelts with no significant changes in the overall morphology, as shown in Figure 7a,b. Figure 7c display the corresponding selected area electron diffraction (SAED) patterns. A survey of the brightest diffraction spots indicates that the nanoporous CdO nanobelts are consistently oriented crystals with the rock salt structure. The other morphologies of CdCO3 such as nanorolls and nanowires can also be transformed into CdO crystals at the same thermal treatment. Figure 8a shows that the CdCO3 nanorolls have been transformed into porous nanoplates, but the CdCO3 nanowires have changed into CdO rough nanowires, and it is obvious that the rough nanowires are actually composed of single nanoparticles attached to each other, as shown in Figure 8a. The formation of these porous CdO nanomaterials might be ascribed to the
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Figure 8. (a) SEM image of the porous nanoplates and (b) TEM image of the rough nanowires under the condition of 0.5 h treatment of the corresponding CdCO3 morphologies at 450 °C.
Various metastable morphologies of nanostructures such as nanobelts, nanowires, nanorolls, and 1D hierarchical structures have been selectively prepared by controlling the temperature, the concentration of ammonia, and the reaction time. A possible morphology shift and growth mechanism involved in the growth habits from the kinetic to thermodynamic growth stage has been discussed. Moreover, the CdCO3 nanoparticles with various morphologies are fully transformed into porous CdO nanocrystals through heat treatment processing, and the corresponding shapes can be preserved completely. This work also provides a general, simple, and effective method to control the shape of other transition-metal carbonates, and to synthesize porous nanomaterials with various shapes. Acknowledgment. This work is supported by the COSTIND and Ministry of Education of China under grant No. A1420060185 and the Key Project of Chinese Ministry of Education (No. 108097).
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Figure 7. SEM image (a), TEM image (b), and SAED pattern (c) of the porous CdO nanobelts after 0.5 h treatment of as-synthesized CdCO3 at 450 °C.
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release of carbon dioxide from the original CdO nanomaterials due to the thermal treatment.
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4. Conclusions
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In summary, we report a simple hydrothermal approach to realize the shape-controlled synthesis of CdCO3 nanocrystals.
CG7007254
