Morphology-Controllable Synthesis of Metal Organic Framework Cd3

Article pubs.acs.org/crystal

Morphology-Controllable Synthesis of Metal Organic Framework Cd3[Co(CN)6]2·nH2O Nanostructures for Hydrogen Storage Applications Lin Hu, Ping Zhang, Qianwang Chen,* Hao Zhong, Xianyi Hu, Xinrui Zheng, Yu Wang, and Nan Yan Hefei National Laboratory for Physical Sciences at Microscale and Department of Materials Science & Engineering, University of Science and Technology of China, Hefei, 230026, China S Supporting Information *

ABSTRACT: In this paper, a potential strategy for increasing the hydrogen sorption has been demonstrated by using the nanostructure of metal organic framework. Prussian Blue analogue (PBA) Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons were successfully obtained at room temperature in the presence of poly(vinylpyrrolidone) (PVP) and sodium dodecylbenzenesulfonate (SDBS), respectively. The as-prepared products were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and thermogravimetric analysis (TGA). Detailed proof indicated that the synthetic parameters such as surfactant, the ratio of different solvents (water and ethanol) play crucial roles in the morphology and size of the nanoparticles. The fine-detailed information about porous structures of the samples has also been studied using the Brunauer−Emmet−Teller isotherm. Most importantly, two kinds of nanostructures both display high adsorption on H2 and CO2, showing enhanced adsorption properties compared with the bulk materials. To our knowledge, this is the first report on the synthesis of Cd3[Co(CN)6]2 nanomaterials and their H2, CO2 adsorption applications at the nanoscale.

1. INTRODUCTION There has been considerable interest over the years in the series of transition metal cyanides, referred to as Prussian Blues (PBs) or their analogue due to the particular physical properties, including optical, electronic, and magnetic properties.1−5 Nevertheless, the gas sorption properties of PB coordination compounds were only investigated very recently.6−8 In particular, it has recently attracted intensive research efforts on hydrogen storage since Long and co-workers who published a series of papers using these porous solids for hydrogen storage at 77 K made the seminal work in this area in 2005.9 Meanwhile, Thallapally and co-workers accomplished abundance significant works on gas storage and separation (SO2, CO2, CH4) by using PBA with the M3II[MIII(CN)6]2·nH2O.10−12 Although great efforts have been paid to PBA, and many unique gas storage properties have been found, in these studies scientists mainly focus on the properties of bulk PBA. However, it has been found that the shape and size are both important factors to fine-tune the properties of the material. In this regard, nanometer-sized materials of PBA often exhibit remarkable size- and shape-dependent physical and chemical properties, which cannot be observed in their bulk analogues. For example, Co3[Co(CN)6]2 nanocomposites have been shown to exhibit improved CO2 storage properties over bulk Co3[Co(CN)6]2.11 Moreover, it is reported that the nanosized zeolitic imidazolate frameworks (ZIF-8) which is first synthesized by Nune and co-workers also displayed excellent gas (CO2, CH4, N2) sorption properties.13 In the other application areas, PB nanocomposites have been shown to display enhanced optical switching performance with excellent © 2012 American Chemical Society

contrast and switching speeds compare with bulk inorganic PB films.14 Therefore, precise control of size, shape, chemical composition and crystal structure allows one to not only observe unique properties of the PBA nanostructures but also to tune their chemical and physical properties as we desired. On the other hand, part of the PBA and other metal−organic frameworks (MOFs) with different shapes at the nanoscale have been synthesized (Table 1), such as Co3[Co(CN)6]2 polyhedra and nanorods by solvothermal synthesis,16 nanoparticles in reverse microemulsions,18,28 Zn3[Co(CN) 6]2 microspheres and micropolyhedrons under ultrasonic conditions.17 Especially, Lin et al. develop the microwave-assisted synthesis to obtain various morphologies of MOFs for biological applications.23−25 However, the hydrothermal methods, reverse microemulsion, microwave-assisted synthesis and ultrasonic conditions are relative complex processes, which are not low-cost idea. Our group has successfully fabricated Mn3[Co(CN)6]2 perfect nanocubes at room temperature with the help of poly(vinylpyrrolidone) (PVP) in water/ethanol system recently.29 With the example of our case, we can propose that the various nanostructures of other PBA could be also synthesized by a facile way at room temperature to investigate their H2 storage properties at the nanoscale. To the best of our knowledge, there are few reports on the H2 storage properties by using nanostructure of PBA to date. Received: November 11, 2011 Revised: March 22, 2012 Published: March 27, 2012 2257

dx.doi.org/10.1021/cg2014896 | Cryst. Growth Des. 2012, 12, 2257−2264

Crystal Growth & Design

Article

Table 1. Summary of Crystal Growth of MOFs Nanostructure MOFs Cd3[Co(CN)6]2·nH2O Co3[Co(CN)6]2 Zn3[Co(CN)6]2·nH2O Co3[Co(CN)6]2 Fe4[Fe(CN)6]3 ZIF-8 ZIF-8 IEMOF1 MIL-101 Mn3(BTC)2(H2O)6 [Gd2(bhc)(H2O)6] MIL-53 et al. Fe3(μ3-O)Cl(H2O)2(BDC)3 [Cu3(btc)2]n [In(OH)(bdc)]n

nanotechnology

morphology control methods

solvothermal method ultrasonic reflux etching precipitate precipitate heating microwave methods microwave synthesis microwave synthesis solvothermal synthesis microwave heating microwave irradiation heat and drying

surfactant CTAB surfactant PVP surfactant CTAB surfactant CTAB chemical functionalities surfactant polymer surfactant CTAB reaction time surfactant CTAB surfactant CTAB surfactant CTAB lauric acid solvent

wt% H2

ref

1.24

15 16 17 18 19 20 13 21 22 23 24 22b 25 26 27

could complement the blank space of current research area, and an effective strategy of increasing the hydrogen sorption by using the nanostructure of metal organic framework has been proposed at the same time.

Although hydrogen is being considered to be an alternative to fossil fuel derivatives as the secondary energy bearer for mobile technologies which play an important role in human life, many facts in real word explain the urgency in the hydrogen technology development, particularly for its storage and then its use through highly efficient and economically viable.30−32 In this regards, the availability of a suitable storage method is also the main challenge. Cd3[Co(CN)6]2·nH2O, is a typical PBA with the chemical formula M3II[MIII(CN)6]2·nH2O (Figure 1),

2. EXPERIMENTAL SECTION All of the reagents used were of analytical purity and used without further purification. 2.1. Synthesis of Cd3[Co(CN)6]2·nH2O Nanocubes. Solution A: 0.0114 g of CdCl2·nH2O and 0.2 g of PVP (K-30) were dissolved in 20 mL distilled water system under agitated stirring to get a transparent solution. Solution B: 0.0166 g of K3[Co(CN)6]2 was dissolved in 10 mL of distilled system. Solution B was added into solution A drop by drop slowly using a syringe. The whole reaction process was kept at room temperature with agitated stirring. After 10 min, the reaction was aged at room temperature without any interruption for 24 h. The resulting white precipitation was filtered and washed several times with absolute ethanol and finally dried under oven at 60 °C. 2.2. Synthesis of Cd3[Co(CN)6]2·nH2O Octahedron. Solution A: 0.0114 g of CdCl2·nH2O and 0.2 g of SDBS were dissolved in 10 mL distilled water system under agitated stirring to get a transparent solution. Solution B: 0.0166 g of K3[Co(CN)6]2 was dissolved in 10 mL of distilled system. Solution B was added into solution A slowly and regularly using a syringe. The whole reaction process was kept at room temperature with agitated stirring. After 10 min, the reaction was aged at room temperature without any interruption for 24 h. The resulting white precipitation was filtered and washed several times with absolute ethanol and finally dried under oven at 60 °C. Characterization. The powder X-ray diffraction (XRD) patterns were collected on a Japan Rigaku D/MAX-cAX-ray diffractometer equipped with Cu Ka radiation over the 2θ range of 10−70°. Field emission scanning electron microscopy (FE-SEM) images were performed on a JEOL JSM-6700 M scanning electron microscope. Thermogravimetric analysis (TGA) was carried out using a Shimadzu50 thermoanalyser under air gas flow at 10 °C min−1 in the temperature range 30−800 °C. Specific surface areas were computed from the results of N2 physisorption at 77 K (Micromeritics ASAP 2020) by using the Brunauer−Emmet−Teller (BET) and Barrett− Joyner−Halenda (BJH).

Figure 1. Porous framework of Prussian blue analogues Cd3[Co(CN)6]2·nH2O.

and H2 storage properties of its bulk materials have been simply reported by Cameron J. Kepert,15 which showed highest adsorption capacity of 1.24 wt % loading at 77 K and 1.01× 105 Pa among other PBA M3II[MIII(CN)6]2·nH2O (MII = Mn, Fe, Co, Ni, Cu, Zn). However, the nanostructure of PBA Cd3[Co(CN)6]2·nH2O and its CO2, H2 storage properties at the nanoscale have not been reported. In this study, for the first time large-scale Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons were simply achieved at room temperature in the presence of surfactants. The solvent ethanol and surfactants were employed to control the morphology and size of the product.27 Most importantly, the as-prepared Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons exhibit excellent CO 2 , H 2 storage properties. Especially, the Cd 3 [Co(CN)6]2·nH2O nanocubes with the size of 0.8−1 μm display 1.3 H2 wt% loading at 77 K and 1.01× 105 Pa, indicating an enhanced storage property compared with the bulk materials. Moreover, the CO 2 adsorption property of Cd 3 [Co(CN)6]2·nH2O nanocubes is higher than that of Co3[Co(CN) 6] 2·nH2 O irregular nanoparticles under the same condition.11 Therefore, we believe that researching the gas storage properties of metal organic framework at the nanoscale

3. RESULTS AND DISCUSSION The chemical compositions and phases of nanocubes and octahedrons were determined by X-ray diffraction. Figure 2 shows the X-ray diffraction pattern of the as-prepared product obtained under PVP. All the reflections can be readily indexed as a pure face-centered cubic (fcc) phase of Cd3[Co(CN)6]2·nH2O [space group: F43m (No. 216)] with lattice 2258



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Figure 2. The XRD pattern of Cd3[Co(CN)6]2·nH2O nanocubes.

constant α = 10.55 Å, which is in good agreement with the standard values for bulk cubic Cd3[Co(CN)6]2·nH2O (JCPDS No. 89-3740). The sharp peaks indicate that the as-prepared product has a good crystallinity. As shown in Figure 3, the peak

Figure 4. (a) The XRD pattern of mesocrystals obtained in 30 mL of distilled water. (b) The XRD pattern of bulk material.

significant function which is the control of the growth rate. The viscosity of ethanol is stronger than that of water. To some extent, higher viscosity can slow down the growth rate and be more advantageous for the isotropic growth. Therefore, when the ethanol was employed in the reaction system, the Cd2+ could move slowly during the nucleation and growth process to form uniform nanocubes. Recently, it is reported a novel solvent-mediated process that combines polymorph transformation with high shear could be employed to produce small crystal.33 Changing the PVP to SDBS did not result in the formation of nanocubes, and octahedrons with rough surface were obtained in the presence of 0.2 g of SDBS. Figure 6a,b shows the images of octahedrons obtained in 20 mL of distilled water, which display the typical octahedron morphology with the size of 600 nm. More interestingly, when the concentrations of reactants were reduced by adding additional distilled water (10 mL) into reaction system, the octahedrons composed of nanocrystallines were obtained (Figure 6c,d). The octahedron is rather rough and presents a hierarchical structure by assembling of nanocrytals, which may be explained by the nonclassical crystallization that the octahedrons should be formed by the oriented attachment of Cd3[Co(CN)6]2·nH2O nanocrystals.34,35 This result may be also consistent with the Prussian Blue (PB) mesocrystal which fabricated by a hydroythermal method.36 Other experiments, such as polarized optical microscopy and dynamic light scattering, will be require to further confirm the Cd3[Co(CN)6]2·nH2O mesocrystal formation.35 The octahedron obtained in the 30 mL distilled water and bulk materials were also characterized by the XRD, as shown in Figure 4. All the peaks of XRD pattern of octahedron (Figure 4a) display broader width and weaker intensity, compared to that of bulk material (Figure 4b) and nanocubes (Figure 2), which show that the crystals are nanosize. Moreover, SDBS possess high viscosity, which may provide microemulsion reaction condition at room temperature. The XRD pattern of the product obtained in microemulsion system at low temperature often display the peaks with broader width and weaker intensity.18 In the case of fcc nanocrystals, their final morphology is often determined by the consequence of competitive growth of (111) and (100) faces. The preferred growth of (100) faces results in the formation of octahedrons, while the preferred growth of (111) faces leads to the formation of cubes. When a particle

Figure 3. The XRD pattern of Cd3[Co(CN)6]2·nH2O octahedron in 20 mL of distilled water.

positions of the X-ray powder diffraction patterns of product obtained under SDBS in 20 mL of distilled water are identical with those of Figure 2, which are also in good agreement with the standard values. However, the intensities of all peaks are weaker than that in Figure 2, indicating the weak crystallinity. The FE-SEM was used to investigate the size and morphology of the product. Figure 5 shows the typical images of the sample obtained under PVP. As to what can be observed from Figure 5a, the nanocubes turn out to be large scaled and perfect. The high-magnification FE-SEM (Figure 5b) reveals that the surfaces of the nanocubes are extremely smooth with a size about 0.8−1 μm. It is noteworthy that the nanocubes have a tendency to change into truncated nanocubes as shown by the arrow in Figure 5d when 5 mL ethanol was employed in the reaction system as solvent (Figure 5c,d). It should be mentioned that the ratio of C2H5OH/H2O in the reaction system was 1:5 (5 mL: 25 mL). Moreover, the nanocubes also turn out to be large scaled and with excellent uniformity in size and shape. On the basis of a statistical evaluation of at least 20 nanocubes, a mean particle diameter was deduced to be 200 nm. And this diameter is much smaller than that (0.8−1 μm) of the product obtained without ethanol. The different properties of the used organic solvents, such as polarity and viscosity, may play important roles in the shape and size control of the product. The crystallite sizes of nanometer crystals obtained from ethanol indicate that the viscosity of the solvent has a 2259



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Figure 5. FE-SEM images of the Cd3[Co(CN)6]2·nH2O nanocubes obtained in the water system under PVP (a) at a lower magnification and (b) at a higher magnification. FE-SEM images of the Cd3[Co(CN)6]2·nH2O nanocubes obtained in the ethanol/water system under PVP (c) at a lower magnification and (d) at a higher magnification.

Figure 6. FE-SEM images of the Cd3[Co(CN)6]2·nH2O octahedron obtained in the 20 mL water under SDBS (a) at a lower magnification and (b) at a higher magnification. FE-SEM images of the Cd3[Co(CN)6]2·nH2O octahedron obtained in 30 mL water system (c) at a lower magnification and (d) at a higher magnification.

difficult to control the morphology and size of the products.5,37 It can be reasonably inferred that to achieve the goal mentioned above, especially for those with small Ksp values, a slow reaction process is of great importance. During nucleation and growth processes of Cd3[Co(CN)6]2·nH2O nanoparticles, the PVP and SDBS may be weakly coordinated to Cd2+ ions, consequently, surfactant provided steric stabilization to the formation of the surfactant-protected nanoparticles without aggregations. In our experiment, dropwise add with a syringe and low concentrations of reactants provide a suitable condition for slow

grows, facets tend to form on the low-index planes to minimize the surface energy. During the crystals growth process, PVP and SDBS selectively adsorb on some specific crystal surfaces of the crystal nuclei, and then could effectively lower the energy of particles and induce particles epitaxy and assembly into nanocubes and octahedron, respectively. On the other hand, It is well-known that PBA Cd3[Co(CN)6]2·nH2O has a small solubility-product constant indicating that upon direct mixing, Cd2+ and [Co(CN)6]3− immediately react to form Cd3[Co(CN)6]2·nH2O. For this kind of rapid reaction, it is quite 2260



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(CN)6]2·nH2O nanocubes and octahedrons become a blue color when trapped water molecules are removed by heating at 200 °C, whereas they turn white upon exposure to air quickly (one minute) due to the charge transfer interactions between the host and water molecules (Figure 8). This phenomenon was also observed in other PBA materials.7

reaction process, therefore the product with relative uniform size and shape could be obtained. In some case, other experimental parameters such as the stirring rate, size of the vessel and even solvent−solute interactions can also affect the size and shape of the crystalline particles.38 We also have investigated how the above factors affect the size and shape of the crystalline particles (the other experimental parameters follow the product which shown in Figure 5c,d). However, compared with Figure 5c,d, the size and shape of the crystalline particles have negligible changes (Figure S1, Supporting Information). Therefore, the stirring rate, size of the vessel and solvent−solute interactions may be less important factors to affect the size and shape of the product in our case. The growth mechanism for the formation of Cd3[Co(CN)6]2 nanocubes and octahedrons has been simply illustrated in Scheme 1.

Figure 8. Color changing properties of Cd3[Co(CN)6]2·nH2O after nanoparticles activated at 200 °C.

Scheme 1. Schematic Illustration of the Growth Mechanism for the Formation of Cd3[Co(CN)6]2 Nanocubes and Octahedrons

Full nitrogen sorption isotherms were measured to obtain information about the specific surface area and pores size of such Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons at 77 K. Before measurement, the samples were heated at 200 °C for 10 h under a vacuum to dehydrate completely according to the result of thermogravimetric analysis. The adsorption/desorption results were shown in Figures 9 and 10. The typical Type I

Thermogravimetric analysis of Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons was shown in Figure 7a. The nanocubes display a sharp weight loss of 23.4% between room temperature and 200 °C illustrate the loss of water molecules from the porous structure, while the octahedrons exhibit a weight loss 17.9% of water molecules. This result indicates that a large number of surfactants remain in the pores (framework) of octahedrons, which is also proved by the enhanced second weight loss of 24.69%, compared to that of nanocubes. The second weight loss of both nanostructures was observed between 300 and 400 °C due to the oxidation of the cyanide and residual surfactants. The TGA scan for bulk Cd3[Co(CN)6]2·nH2O which is almost consistent with that of nanoparticle has been shown in Figure 7b. However, the decomposition temperature of second steps is slightly higher than that of nanoparticles. This result may attribute to surface effect of nanomaterials which can drop the decomposition temperature of materials. More interesting, the Cd3[Co-

Figure 9. Nitrogen adsorption/desorption isotherms and pore size distribution (inset) of Cd3[Co(CN)6]2·nH2O nanocubes obtained under 0.2 g of PVP.

isotherms were observed for both products. The BET specific surface area of Cd3[Co(CN)6]2·nH2O obtained under 0.2 g of

Figure 7. (a) The TGA scans for octahedrons and nanotubes. (b) The TGA scans for bulk Cd3[Co(CN)6]2·nH2O. 2261



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Figure 12. Hydrogen sorption isotherms for the Cd 3 [Co(CN)6]2·nH2O octahedrons obtained under 0.2 g of SDBS.

Figure 10. Nitrogen adsorption/desorption isotherms and pore size distribution (the inset) of Cd3[Co(CN)6]2·nH2O octahedrons obtained under 0.2 g of SDBS. 2

worth mentioning that the bulk materials of Cd3[Co(CN)6]2·nH2O showed 1.24 wt % loading at 77 K and 1.01× 105 Pa,15 which is lower than that of Cd3[Co(CN)6]2·nH2O nanocubes, and about 5 wt %% was increased. Although the surface area of Cd3[Co(CN)6]2 bulk materials (about 667 m2 g−1 according to Supporting Information of the literature)15 is higher than that of S1 (572 m2 g−1), better H2 adsorption performance is shown by using S1. This result proves that the H2 adsorption properties of Cd3[Co(CN)6]2 highly dependent on not only the surface area but also the size and morphology of nanoparticles, and the materials at the nanoscale are more favorable to adsorption applications. The gas storage capacities of PBA have relationship with not only the porous framework but also the surface adsorption. Indeed, the porous framework, or in another word, the internal surface, is not easy to be changed. However, downsizing MOFs to the nanometer regime will lead to more possibilities of enhancing their gas storage capacity for the following reasons at the same time. As we all know, chemical and physical phenomena are usually strongly affected when material becomes nanometer-sized. With decreased particle size, the proportion of the atoms lies on the surface will increase as a result. Furthermore, compared with the bulk material, the nanosized PCPs possess higher surface energy. The binding energy between the surface atoms is higher than that of the internal atomic. Surface atom, lacking adjacent atom, display unsaturated properties and are easy to combine with other atoms, which will improve their surface adsorption as well. Moreover, if the size of the particles decreases, gas molecule will be more likely to have access to the internal holes or channels due to the increased specific surface area. When it comes to the bulk material, on the contrary, gas molecules tend to stay in the entrance of the hole instead of getting further. The S2 exhibits weaker adsorption capacity of H2 because of the lower surface area (376 m2 g−1), compared to that of S1. On the other hand, the different nanostructure may also attribute to the distinct adsorption capacity. Nanocubes and octahedron show different sizes, which will result in so distinct surface areas and total surface numbers that two important factors to determine adsorption capacity. Moreover, different exposed lattice planes show distinct energy. In other words, the crystal facets (such as (111), (100)) can effect the interactions between H2 and adsorbents due to different surface energies. Therefore, we deduce that nanostructure with various morphologies may be another factor to determine the adsorption capacity of metal organic framework materials besides the surface area. Of course, the detailed relationships

−1

PVP were calculated to be 573 m g , which is lower than that of bulk M3II[MIII(CN)6]2·nH2O, and it therefore appears that some residual species is indeed present in the pores and the possibility of porous Cd3[Co(CN)6]2·nH2O as a reservoir for various gases. Moreover, it has been indicate that the size of the pore distribute in a narrow range, which is below 6 nm. It could also prove that the nanoparticles that formed the product are porous structure. The BET specific surface area of Cd3[Co(CN)6]2·nH2O obtained under 0.2 g of SDBS were calculated to be 376 m2 g−1 with a wide porous volume distributions of 2− 50 nm. The low specific surface area is resulted from a large number of residual SDBS appears in the porous framework of Cd3[Co(CN)6]2·nH2O. This result is similar with our previous report that the surface area of Mn3[Co(CN)6]2·nH2O nanocubes is lower than that of its bulk material.39 Considering that the Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons which possess porous nanostructures and high surface area may apparently contribute to gas storage properties, we attempted to measure the gas sorption properties of Cd3 [Co(CN)6]2·nH2 O nanoparticles. The samples obtained in the presence of 0.2 g of PVP (nanocubes) and 0.2 g of SDBS (octahedrons) were denoted as S1 and S2, resepectively. S1 and S2 were activated at 200 °C for 10 h to remove the adsorbed water molecules from the pores of the structure. Figures 11 and 12 show the hydrogen adsorption isotherm for S1 and S2, where the calculated H2 wt% at 77 K and 1 bar pressure were 1.3% and 1.05%, respectively. It is

Figure 11. Hydrogen sorption isotherms for the Cd 3 [Co(CN)6]2·nH2O nanocubes obtained under 0.2 g of PVP. 2262



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results prove that the nanostructures of metal organic framework are more suitable for gas storage applications. Studies pertaining to the gas adsorption of the Cd3[Co(CN)6]2·nH2O nanoparticles at high pressure are in progress.

between the nanostructure and adsorption−desorption properties are complicated, which requires further study. In addition to H2 adsorption properties, the adsorption of CO2 is equally important since CO2 is responsible for the global warming and of the related climate changes. S1 and S2 exhibit 53 cm3 g−1 and 38.5 cm3 g−1 under the pressure of 1 bar (Figure 13). These

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1: (a) SEM image of the product obtained at a high stirring rate (b) SEM image of the product obtained in 50 ml beaker (c) SEM image of the product obtained by using cadmium acetate as reactant instead of cadmium chloride. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81 551 3607292. Tel: +81 551 3607292. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (NSFC, 21071137).

Figure 13. CO2 uptake of S1 (black) and S2 (red) at different pressures.

high capacity are compared favorably with Co3[Co(CN)6]2 irregular nanoparticles.11 Especially, the CO2 wt% of S1 about 10% is much higher than that of Co3[Co(CN)6]2 irregular nanoparticles (7.1−8.5%).11 The adsorption curves indicate, furthermore, the adsorption quantity rising with the increase of pressure, which indicated stronger adsorption ability of our product at high pressures. To our knowledge, so far, no information about CO2 adsorption application of Cd3[Co(CN)6]2 has been mentioned in the literature. We believe Cd3[Co(CN)6]2 nanoparticles can be employed as excellent candidates for CO2 adsorption applications based on above results.

REFERENCES

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4. CONCLUSIONS In conclusion, Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons were fabricated by a surfactant-assisted (PVP and SDBS) process through a simple precipitation method at room temperature. The results demonstrated that it is possible to control the morphology and size of Cd3[Co(CN)6]2·nH2O by adjusting process parameters such as surfactant and solvent. Surfactant PVP and SDBS played crucial roles in the formation of uniform Cd3[Co(CN)6]2·nH2O nanostructures, which derive from selectively adsorbed on some specific crystal surfaces of crystal nuclei. The detailed formation mechanism has been proposed, and it is a typical example to understand the effect of surfactants and solvent in the process of nucleation and crystal growth at room temperature, and then on the formation of different morphologies and sizes of products. N2 adsorption showed that the Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons possessed lower surface area, compared to that of the bulk M3II[MIII(CN)6]2·nH2O due to some residual species in the pores. Most interestingly, the Cd3[Co(CN)6]2·nH2O nanocubes and octahedrons dispalyed excellent CO2, H2 adsorption, although the surface area is relatively low. In particular, the Cd3[Co(CN)6]2·nH2O nanocubes display 1.3 H2 wt% loading at 77 K and 1.01 × 105 Pa, showing an enhanced adsorption capacity compared to the bulk Cd3[Co(CN)6]2·nH2O (1.24% under the same conditions). These 2263



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Morphology-Controllable Synthesis of Metal Organic Framework Cd3

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