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Cite this: Chem. Commun., 2014, 50, 4296 Received 16th December 2013, Accepted 10th February 2014
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Controlled incorporation of nanoparticles in metal–organic framework hybrid thin films† Weina Zhang,a Guang Lu,ab Shaozhou Li,a Yayuan Liu,a Hongbo Xu,a Chenlong Cui,a Weijin Yan,c Yanhui Yangc and Fengwei Huo*ad
DOI: 10.1039/c3cc49505a www.rsc.org/chemcomm
A facile encapsulation strategy was reported for preparing nanoparticles/metal–organic framework hybrid thin films which exhibit both the active (catalytic, magnetic, and optical) properties derived from the NPs and the size-selectivity originating from the welldefined microporous structure of the MOF thin films.
Metal–organic frameworks (MOFs)1 have attracted considerable attention over the past several decades due to their distinctive properties, such as large surface areas, ordered crystalline structures, highly regularized pores and structure functionalities. These properties make MOFs a promising new class of materials for diverse applications, such as gas storage,2 gas separation,3 heterogeneous catalysis,4 sensing,5 and drug delivery.6 While the recent interest is mainly focused on the synthesis and application of bulky MOFs,7,8 many potential applications in electrical, magnetic, and optical based nanotechnology devices require the use of porous MOF thin films.9 For example, in the past few years, MOF smart films10 as sensors,11 catalysts12 and other related nanodevices13 have provided enormous new opportunities to nanotechnology. It is well-known that the potential applications of MOF thin films can be further developed and extended by encapsulating various nanoparticles (NPs)14 within the framework matrix so that the functionalized MOF thin films can exhibit the novel chemical and physical properties endowed by the NPs. To date, several methods have been developed to fabricate the dense MOF thin films supported by porous materials, such as solvothermal15 or microwave16 synthesis. The critical disadvantages of these methods, including uncontrolled film thicknesses, poor a
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: [email protected]; Fax: +65-6790-9081; Tel: +65-6316-8921 b Institute of Functional Nano and Soft Materials, Soochow University, Suzhou, 215123, P.R. China c School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore d Institute of Advanced Materials, Nanjing University of Technology, Nanjing 211816, P.R. China † Electronic supplementary information (ESI) available: Experiment details, characterizations and materials. See DOI: 10.1039/c3cc49505a
4296 | Chem. Commun., 2014, 50, 4296--4298
morphologies, time-consuming fabrication processes and, especially, the rigorous processing conditions, make it difficult for NPs or other functional agents to be incorporated into the system. Moreover, even if MOF thin films are obtained under mild conditions (selfassembly,17 Langmuir Blodgett,18 chemical solution deposition19), they usually detach from the substrate too easily to be functionalized. Hence, developing a well-suited technique for the controlled encapsulation of NPs in MOF thin films remains a challenge, in spite of the urgent need and the application significance of functionalized MOF thin films. Herein, we report a general strategy to controllably encapsulate various kinds of NPs in zeolitic imidazolate framework (ZIF-8) thin films via direct growth of MOF thin films, followed by the spin coating of NPs. The controlled composition, shape, and size of NPs as well as the designed thickness of MOF thin films endow the obtained hybrid thin films with better functional flexibility and tunability. Correspondingly, the dense and continuous NP–ZIF-8 hybrid thin films with an adjustable sandwich structure displayed the active catalysis, sensing, and magnetic properties derived from the NPs and good selectivity due to the protection of microporous MOF thin films. The controlled incorporation of NPs into ZIF-8 thin films can be achieved by directed growth and spin coating. Various NPs were synthesized using established methods and their surfaces were functionalized with polyvinylpyrrolidone (PVP) either during or after synthesis. For the growth of thin films of the NP–ZIF-8 hybrid material, a glass substrate with hydroxyl groups was immersed into a methanolic solution of zinc nitrate (25 mM, 5 ml), and 2-methylimidazole (50 mM, 5 ml) for 40 min, followed by thorough washing with methanol and drying under nitrogen gas. The NPs were then spin-coated on the newly grown ZIF-8 thin film. By repeating the steps i and ii as illustrated in Scheme 1, the multilayer NP–ZIF-8 hybrid thin films were obtained. The method of spin coating not only enables MOF thin films to be functionalized by various functional species, but also eliminates the use of strong chemical solvents, protecting the MOF thin films from corrosion or peeling off from the substrate. To sufficiently investigate the fabrication procedure for NP–ZIF-8 hybrid thin films with a sandwich structure and controllable
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Scheme 1
ChemComm
The fabrication procedure for NP–ZIF-8 hybrid thin films.
thickness, an Au–ZIF-8 hybrid thin film was taken as an example. It was found that Au NPs dispersed homogeneously on the ZIF-8 film at a spin coating rate of 1500 rpm (Fig. S1a, ESI†). As the spin coating rate increased, the quantity of Au NPs gradually decreased (Fig. S1b–d, ESI†). UV-vis spectra, in which the absorption peak of Au NPs (13 nm) is ca. 520 nm, and scanning electron microscopy (SEM) results were used to monitor the growth process of Au–ZIF-8 hybrid thin films. The signal of ZIF-8 films was very weak and the noise was high in UV-vis spectra, whereas the signal was substantially enhanced by introducing the Au NPs, which functionalized ZIF-8 films (Fig. 1a). The absorbance intensity of Au–ZIF-8 hybrid thin films increased linearly with the increasing number of the growth cycles (approximately 0.02 per growth cycle in Fig. 1b). The sandwich structure of Au–ZIF-8 hybrid thin films was further confirmed by the SEM image. A cross-sectional view obtained from SEM images showed a perfect hybrid multilayer structure (Fig. 1c). It is worth noting that the PVP modified-Au NPs facilitate the growth of the ZIF-8 films, which is proved by the flat film (Fig. S1a, ESI†) and the crosssectional (Fig. 1c) SEM image of an Au–ZIF-8 thin film. The effect could probably be attributed to the excellent wetting property of the PVP modified Au NPs since a thin hydrophilic layer can be easily formed by PVP due to its special branch structure and the hydrophilic functional group. As a result, during the repeated growth of ZIF-8 films, PVP not only facilitated the adsorption of NPs onto MOF films and their subsequent encapsulation, but also formed a fresh hydrophilic surface favorable for the growth of the ZIF-8 films.7
Fig. 1 (a) UV-vis absorption spectra indicating the growing process of a Au–ZIF-8 hybrid thin film. (b) The absorbance of Au in UV-vis spectra versus the number of growth cycle. (c) SEM image of the cross-section view of a Au–ZIF-8 hybrid thin film. (d) XRD spectra of a Au–ZIF-8 hybrid thin film and a ZIF-8 thin film.
This journal is © The Royal Society of Chemistry 2014
Fig. 2 Transmission electron microscopy (TEM) images of NP–ZIF-8 hybrid thin films. (a) Two layers of 13 nm Au NPs and one layer of 2.9 nm Pt NPs in between. (b) Three layers of mixed 13 nm Au NPs and 2.9 nm Pt NPs. (c) Two layers of 2.9 nm Pt NPs and one layer of 13 nm Au NPs in between. (d), (e), and (f) 5 layers of Fe3O4–ZIF-8, Pt–ZIF-8, and CdSe–ZIF-8 hybrid thin films, respectively. All scale bars are 100 nm.
We found that the hybrid multilayer thin films can be grown with other NPs such as Pt (2.9 nm), CdSe (4 nm) and Fe3O4 (8 nm). Different types of NPs imparted the ZIF-8 films with different colors: Au–ZIF-8 films appeared red, Fe3O4–ZIF-8 film appeared dark yellow, CdSe–ZIF-8 films appeared orange, and Pt–ZIF-8 films appeared gray (Fig. S3, ESI†). With the assistance of TEM analysis, the aforementioned NPs can be observed to disperse well within the ZIF-8 thin films and these NP–ZIF-8 hybrid thin films all possessed the welldefined multilayer structure (Fig. S2, ESI†). Interestingly, by the simple combination of repeating film growth and spin coating, different types of NPs can be encapsulated into the same layer as well as different layers (Fig. 2). The well-dispersed mixture of NPs imparts more flexible and tunable properties to the ZIF-8 thin films. It is mentioned that as revealed by the powder X-ray diffraction (XRD) measurements, all NP–ZIF-8 hybrid thin films exhibit diffraction patterns almost identical to those of ZIF-8 films without NPs (Fig. 1d). Therefore, the crystalline structure of the ZIF-8 host matrix nearly remains unchanged after loading NPs (Fig. S4, ESI†). Simultaneously, no obvious X-ray diffractions from NPs were observed, probably due to low concentrations and/or the small sizes of the embedded NPs. In addition, NPs were covered by ZIF-8 thin films with a thickness of more than 100 nm which may exceed the penetration depth of X-rays, resulting in the missing signals of the NPs in the spectra of XRD. Nevertheless, their existence was confirmed by TEM images (Fig. 2). Pt–ZIF-8 hybrid thin films as catalysts exhibited size-selectivity in hydrogenation catalysis of linear olefins and cyclo-olefins, due to the catalytic properties of Pt and the microporous structure of ZIF-8 (Fig. S5, ESI†). In order to examine the catalytic properties of Pt NPs and the molecular sieving capability of the ZIF-8 film matrix, the liquid-phase hydrogenation of n-hexene versus cis-cyclooctene was taken as an example to study selective catalysis. The reactions were carried out at room temperature with stirring at 700 rpm for 24 h. As show in Fig. 3a, hydrogenation of linear olefins catalyzed by the Pt–ZIF-8 hybrid thin films exhibited low percentage of conversion (14.9%) presumably owing to the small pore aperture (0.34 nm) of
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incorporated CdSe NPs (Fig. 3c). The CdSe–ZIF-8 hybrid thin films exhibited no fluorescence signal under the filament lamp, while exhibiting beautiful fluorescence under a UV lamp. Notably, the peak of CdSe–ZIF-8 (554 nm) is shifted slightly from that of free CdSe NPs (566 nm); this is consistent with the fact that the photoluminescence energies of these NPs are sensitive to their immediate environment. To assess the optical sensing capability offered by the surface-chemistry dependence of the CdSe emission and the specific size selectivity offered by ZIF-8, thiols of different molecular sizes were used to quench CdSe-based quantum dot emission by molecular adsorption (Fig. 3d). An interesting phenomenon was observed when the CdSe–ZIF-8 hybrid thin films were exposed to gaseous thiols of different molecular sizes: the linear 2-mercaptoethanol molecule can rapidly access the pore of ZIF-8 thin films and quench the emission of CdSe quantum dots encompassed in the thin film (Fig. 3e), while the cyclic cyclohexanethiol molecule was excluded by the small aperture of ZIF-8 thin films (Fig. 3f). In summary, NPs were incorporated into ZIF-8 thin films by means of spin coating without interrupting the crystalline structure of the host matrices. The successful formation of the NP–ZIF-8 hybrid thin films with a unique sandwich structure proved the capability of the proposed easy-to-implement and controllable approach to create new MOF film based materials and devices. The as-prepared NP–ZIF-8 hybrid thin films simultaneously exhibited the exceptional molecular sieving properties of MOFs and the functional behavior of NPs, enlarging the scope of applications of MOF thin films.
Fig. 3 Catalytic, magnetic, and photoluminescence properties of a NP–ZIF-8 hybrid thin film. (a) Catalytic properties of a Pt–ZIF-8 hybrid thin film in selective hydrogenation of linear hexene and cyclooctene. (b) Field-dependent magnetization curve of a Fe3O4–ZIF-8 hybrid thin film at room temperature. (c) Normalized photoluminescence spectra with excitation at 400 nm for CdSe NPs in methanol and the corresponding CdSe–ZIF-8 hybrid thin films. (d) Normalized photoluminescence intensity (emission at 550 nm and excitation at 350 nm) of CdSe–ZIF-8 hybrid thin films versus time after addition of thiol molecules. (e, f) Normalized photoluminescence spectra change with excitation at 350 nm for CdSe–ZIF-8 hybrid thin films after addition of thiol molecules, linear thiols (e) and cyclic thiols (f).
Notes and references
the ZIF-8 film and lower load of Pt NPs. Nevertheless, Pt–ZIF-8 hybrid thin films exhibited no propensity for catalyzing the hydrogenation reaction of the sterically more demanding cyclooctene, which is consistent with the small pore size of ZIF-8 and also suggests the densification of the thin film as well as the preservation of the molecular size selectivity of Pt–ZIF-8 hybrid thin films. The reusability of the Pt–ZIF-8 hybrid thin film as a catalyst for the hydrogenation of n-hexene was demonstrated by similar conversion efficiencies in consecutive runs (14.9%, 14.7%, and 15.5%) (Fig. S6, ESI†). The field-dependent magnetization curve of the Fe3O4–ZIF-8 hybrid thin films at room temperature is shown in Fig. 3b. The magnetization is 3.65 emu g 1 in a magnetic field of 12 kOe. The variation of magnetization with an applied field, H (kOe), shows no hysteresis, that is, both the remanence and coercivity are zero. This indicates a superparamagnetic behavior, as expected from the nanoscale dimension of the particles. Therefore, ZIF-8 thin films play a good role in dispersing and stabilizing Fe3O4 NPs. The following demonstration of the CdSe–ZIF-8 hybrid thin films as the optical sensor further illustrates that both the MOFs and the functional species (CdSe NPs) can preserve their properties independently. Up to now, it has been hard to detect the fluorescence of a solid membrane of the CdSe. However, CdSe–ZIF-8 hybrid thin films exhibit good fluorescence properties due to the
1 O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705. 2 N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe and O. M. Yaghi, Science, 2003, 300, 1127. 3 J. R. Li, R. J. Kuppler and H. C. Zhou, Chem. Soc. Rev., 2009, 38, 1477. 4 J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450. 5 L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. V. Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105. 6 J. Della Rocca, D. Liu and W. Lin, Acc. Chem. Res., 2011, 44, 957. 7 G. Lu, S. Z. Li, Z. Guo, O. K. Farha, B. G. Hauser, X. Y. Qi, Y. Wang, X. Wang, S. Y. Han, X. G. Liu, J. S. DuChene, H. Zhang, Q. C. Zhang, X. D. Chen, J. Ma, S. C. J. Loo, W. D. Wei, Y. H. Yang, J. T. Hupp and F. W. Huo, Nat. Chem., 2012, 4, 310–316. 8 Q. L. Zhu, J. Li and Q. Xu, J. Am. Chem. Soc., 2013, 135, 10210. 9 G. Lu, O. K. Farha, W. Zhang, F. Huo and J. T. Hupp, Adv. Mater., 2012, 24, 3970. 10 C. Woll, O. Shekhah, J. Liu and R. A. Fischer, Chem. Soc. Rev., 2011, 40, 1081. 11 G. Lu and J. T. Hupp, J. Am. Chem. Soc., 2010, 132, 7832. 12 E. V. Ramos-Fernandez, M. Garcia-Domingos, J. Juan-Alcaniz, J. Ga-scon and F. Kapteijn, Appl. Catal., A, 2011, 391, 261. 13 M. D. Allendorf, A. Schwartzberg, V. Stavila and A. A. Talin, Chem.–Eur. J., 2011, 17, 11372. 14 B. L. Cushing, V. L. Kolesnichenko and C. J. O’Connor, Chem. Rev., 2004, 104, 3893. 15 S. R. Venna and M. A. Carreon, J. Am. Chem. Soc., 2010, 132, 76. 16 H. Bux, F. Y. Liang, Y. S. Li, J. Cravillon, M. Wiebcke and J. Caro, J. Am. Chem. Soc., 2009, 131, 16000. 17 E. Biemmi, C. Scherb and T. Bein, J. Am. Chem. Soc., 2007, 129, 8054. 18 R. Makiura, S. Motoyama, Y. Umemura, H. Yamanaka, O. Sakata and H. Kitagawa, Nat. Mater., 2010, 9, 565. 19 P. Horcajada, C. Serre, D. Grosso, C. Boissiere, S. Perruchas, C. Sanchez and G. Ferey, Adv. Mater., 2009, 21, 1931.
4298 | Chem. Commun., 2014, 50, 4296--4298
This journal is © The Royal Society of Chemistry 2014
Published on 17 February 2014. Downloaded by Temple University on 31/10/2014 09:16:36.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 4296 Received 16th December 2013, Accepted 10th February 2014
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Controlled incorporation of nanoparticles in metal–organic framework hybrid thin films† Weina Zhang,a Guang Lu,ab Shaozhou Li,a Yayuan Liu,a Hongbo Xu,a Chenlong Cui,a Weijin Yan,c Yanhui Yangc and Fengwei Huo*ad
DOI: 10.1039/c3cc49505a www.rsc.org/chemcomm
A facile encapsulation strategy was reported for preparing nanoparticles/metal–organic framework hybrid thin films which exhibit both the active (catalytic, magnetic, and optical) properties derived from the NPs and the size-selectivity originating from the welldefined microporous structure of the MOF thin films.
Metal–organic frameworks (MOFs)1 have attracted considerable attention over the past several decades due to their distinctive properties, such as large surface areas, ordered crystalline structures, highly regularized pores and structure functionalities. These properties make MOFs a promising new class of materials for diverse applications, such as gas storage,2 gas separation,3 heterogeneous catalysis,4 sensing,5 and drug delivery.6 While the recent interest is mainly focused on the synthesis and application of bulky MOFs,7,8 many potential applications in electrical, magnetic, and optical based nanotechnology devices require the use of porous MOF thin films.9 For example, in the past few years, MOF smart films10 as sensors,11 catalysts12 and other related nanodevices13 have provided enormous new opportunities to nanotechnology. It is well-known that the potential applications of MOF thin films can be further developed and extended by encapsulating various nanoparticles (NPs)14 within the framework matrix so that the functionalized MOF thin films can exhibit the novel chemical and physical properties endowed by the NPs. To date, several methods have been developed to fabricate the dense MOF thin films supported by porous materials, such as solvothermal15 or microwave16 synthesis. The critical disadvantages of these methods, including uncontrolled film thicknesses, poor a
School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: [email protected]; Fax: +65-6790-9081; Tel: +65-6316-8921 b Institute of Functional Nano and Soft Materials, Soochow University, Suzhou, 215123, P.R. China c School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore d Institute of Advanced Materials, Nanjing University of Technology, Nanjing 211816, P.R. China † Electronic supplementary information (ESI) available: Experiment details, characterizations and materials. See DOI: 10.1039/c3cc49505a
4296 | Chem. Commun., 2014, 50, 4296--4298
morphologies, time-consuming fabrication processes and, especially, the rigorous processing conditions, make it difficult for NPs or other functional agents to be incorporated into the system. Moreover, even if MOF thin films are obtained under mild conditions (selfassembly,17 Langmuir Blodgett,18 chemical solution deposition19), they usually detach from the substrate too easily to be functionalized. Hence, developing a well-suited technique for the controlled encapsulation of NPs in MOF thin films remains a challenge, in spite of the urgent need and the application significance of functionalized MOF thin films. Herein, we report a general strategy to controllably encapsulate various kinds of NPs in zeolitic imidazolate framework (ZIF-8) thin films via direct growth of MOF thin films, followed by the spin coating of NPs. The controlled composition, shape, and size of NPs as well as the designed thickness of MOF thin films endow the obtained hybrid thin films with better functional flexibility and tunability. Correspondingly, the dense and continuous NP–ZIF-8 hybrid thin films with an adjustable sandwich structure displayed the active catalysis, sensing, and magnetic properties derived from the NPs and good selectivity due to the protection of microporous MOF thin films. The controlled incorporation of NPs into ZIF-8 thin films can be achieved by directed growth and spin coating. Various NPs were synthesized using established methods and their surfaces were functionalized with polyvinylpyrrolidone (PVP) either during or after synthesis. For the growth of thin films of the NP–ZIF-8 hybrid material, a glass substrate with hydroxyl groups was immersed into a methanolic solution of zinc nitrate (25 mM, 5 ml), and 2-methylimidazole (50 mM, 5 ml) for 40 min, followed by thorough washing with methanol and drying under nitrogen gas. The NPs were then spin-coated on the newly grown ZIF-8 thin film. By repeating the steps i and ii as illustrated in Scheme 1, the multilayer NP–ZIF-8 hybrid thin films were obtained. The method of spin coating not only enables MOF thin films to be functionalized by various functional species, but also eliminates the use of strong chemical solvents, protecting the MOF thin films from corrosion or peeling off from the substrate. To sufficiently investigate the fabrication procedure for NP–ZIF-8 hybrid thin films with a sandwich structure and controllable
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Scheme 1
ChemComm
The fabrication procedure for NP–ZIF-8 hybrid thin films.
thickness, an Au–ZIF-8 hybrid thin film was taken as an example. It was found that Au NPs dispersed homogeneously on the ZIF-8 film at a spin coating rate of 1500 rpm (Fig. S1a, ESI†). As the spin coating rate increased, the quantity of Au NPs gradually decreased (Fig. S1b–d, ESI†). UV-vis spectra, in which the absorption peak of Au NPs (13 nm) is ca. 520 nm, and scanning electron microscopy (SEM) results were used to monitor the growth process of Au–ZIF-8 hybrid thin films. The signal of ZIF-8 films was very weak and the noise was high in UV-vis spectra, whereas the signal was substantially enhanced by introducing the Au NPs, which functionalized ZIF-8 films (Fig. 1a). The absorbance intensity of Au–ZIF-8 hybrid thin films increased linearly with the increasing number of the growth cycles (approximately 0.02 per growth cycle in Fig. 1b). The sandwich structure of Au–ZIF-8 hybrid thin films was further confirmed by the SEM image. A cross-sectional view obtained from SEM images showed a perfect hybrid multilayer structure (Fig. 1c). It is worth noting that the PVP modified-Au NPs facilitate the growth of the ZIF-8 films, which is proved by the flat film (Fig. S1a, ESI†) and the crosssectional (Fig. 1c) SEM image of an Au–ZIF-8 thin film. The effect could probably be attributed to the excellent wetting property of the PVP modified Au NPs since a thin hydrophilic layer can be easily formed by PVP due to its special branch structure and the hydrophilic functional group. As a result, during the repeated growth of ZIF-8 films, PVP not only facilitated the adsorption of NPs onto MOF films and their subsequent encapsulation, but also formed a fresh hydrophilic surface favorable for the growth of the ZIF-8 films.7
Fig. 1 (a) UV-vis absorption spectra indicating the growing process of a Au–ZIF-8 hybrid thin film. (b) The absorbance of Au in UV-vis spectra versus the number of growth cycle. (c) SEM image of the cross-section view of a Au–ZIF-8 hybrid thin film. (d) XRD spectra of a Au–ZIF-8 hybrid thin film and a ZIF-8 thin film.
This journal is © The Royal Society of Chemistry 2014
Fig. 2 Transmission electron microscopy (TEM) images of NP–ZIF-8 hybrid thin films. (a) Two layers of 13 nm Au NPs and one layer of 2.9 nm Pt NPs in between. (b) Three layers of mixed 13 nm Au NPs and 2.9 nm Pt NPs. (c) Two layers of 2.9 nm Pt NPs and one layer of 13 nm Au NPs in between. (d), (e), and (f) 5 layers of Fe3O4–ZIF-8, Pt–ZIF-8, and CdSe–ZIF-8 hybrid thin films, respectively. All scale bars are 100 nm.
We found that the hybrid multilayer thin films can be grown with other NPs such as Pt (2.9 nm), CdSe (4 nm) and Fe3O4 (8 nm). Different types of NPs imparted the ZIF-8 films with different colors: Au–ZIF-8 films appeared red, Fe3O4–ZIF-8 film appeared dark yellow, CdSe–ZIF-8 films appeared orange, and Pt–ZIF-8 films appeared gray (Fig. S3, ESI†). With the assistance of TEM analysis, the aforementioned NPs can be observed to disperse well within the ZIF-8 thin films and these NP–ZIF-8 hybrid thin films all possessed the welldefined multilayer structure (Fig. S2, ESI†). Interestingly, by the simple combination of repeating film growth and spin coating, different types of NPs can be encapsulated into the same layer as well as different layers (Fig. 2). The well-dispersed mixture of NPs imparts more flexible and tunable properties to the ZIF-8 thin films. It is mentioned that as revealed by the powder X-ray diffraction (XRD) measurements, all NP–ZIF-8 hybrid thin films exhibit diffraction patterns almost identical to those of ZIF-8 films without NPs (Fig. 1d). Therefore, the crystalline structure of the ZIF-8 host matrix nearly remains unchanged after loading NPs (Fig. S4, ESI†). Simultaneously, no obvious X-ray diffractions from NPs were observed, probably due to low concentrations and/or the small sizes of the embedded NPs. In addition, NPs were covered by ZIF-8 thin films with a thickness of more than 100 nm which may exceed the penetration depth of X-rays, resulting in the missing signals of the NPs in the spectra of XRD. Nevertheless, their existence was confirmed by TEM images (Fig. 2). Pt–ZIF-8 hybrid thin films as catalysts exhibited size-selectivity in hydrogenation catalysis of linear olefins and cyclo-olefins, due to the catalytic properties of Pt and the microporous structure of ZIF-8 (Fig. S5, ESI†). In order to examine the catalytic properties of Pt NPs and the molecular sieving capability of the ZIF-8 film matrix, the liquid-phase hydrogenation of n-hexene versus cis-cyclooctene was taken as an example to study selective catalysis. The reactions were carried out at room temperature with stirring at 700 rpm for 24 h. As show in Fig. 3a, hydrogenation of linear olefins catalyzed by the Pt–ZIF-8 hybrid thin films exhibited low percentage of conversion (14.9%) presumably owing to the small pore aperture (0.34 nm) of
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incorporated CdSe NPs (Fig. 3c). The CdSe–ZIF-8 hybrid thin films exhibited no fluorescence signal under the filament lamp, while exhibiting beautiful fluorescence under a UV lamp. Notably, the peak of CdSe–ZIF-8 (554 nm) is shifted slightly from that of free CdSe NPs (566 nm); this is consistent with the fact that the photoluminescence energies of these NPs are sensitive to their immediate environment. To assess the optical sensing capability offered by the surface-chemistry dependence of the CdSe emission and the specific size selectivity offered by ZIF-8, thiols of different molecular sizes were used to quench CdSe-based quantum dot emission by molecular adsorption (Fig. 3d). An interesting phenomenon was observed when the CdSe–ZIF-8 hybrid thin films were exposed to gaseous thiols of different molecular sizes: the linear 2-mercaptoethanol molecule can rapidly access the pore of ZIF-8 thin films and quench the emission of CdSe quantum dots encompassed in the thin film (Fig. 3e), while the cyclic cyclohexanethiol molecule was excluded by the small aperture of ZIF-8 thin films (Fig. 3f). In summary, NPs were incorporated into ZIF-8 thin films by means of spin coating without interrupting the crystalline structure of the host matrices. The successful formation of the NP–ZIF-8 hybrid thin films with a unique sandwich structure proved the capability of the proposed easy-to-implement and controllable approach to create new MOF film based materials and devices. The as-prepared NP–ZIF-8 hybrid thin films simultaneously exhibited the exceptional molecular sieving properties of MOFs and the functional behavior of NPs, enlarging the scope of applications of MOF thin films.
Fig. 3 Catalytic, magnetic, and photoluminescence properties of a NP–ZIF-8 hybrid thin film. (a) Catalytic properties of a Pt–ZIF-8 hybrid thin film in selective hydrogenation of linear hexene and cyclooctene. (b) Field-dependent magnetization curve of a Fe3O4–ZIF-8 hybrid thin film at room temperature. (c) Normalized photoluminescence spectra with excitation at 400 nm for CdSe NPs in methanol and the corresponding CdSe–ZIF-8 hybrid thin films. (d) Normalized photoluminescence intensity (emission at 550 nm and excitation at 350 nm) of CdSe–ZIF-8 hybrid thin films versus time after addition of thiol molecules. (e, f) Normalized photoluminescence spectra change with excitation at 350 nm for CdSe–ZIF-8 hybrid thin films after addition of thiol molecules, linear thiols (e) and cyclic thiols (f).
Notes and references
the ZIF-8 film and lower load of Pt NPs. Nevertheless, Pt–ZIF-8 hybrid thin films exhibited no propensity for catalyzing the hydrogenation reaction of the sterically more demanding cyclooctene, which is consistent with the small pore size of ZIF-8 and also suggests the densification of the thin film as well as the preservation of the molecular size selectivity of Pt–ZIF-8 hybrid thin films. The reusability of the Pt–ZIF-8 hybrid thin film as a catalyst for the hydrogenation of n-hexene was demonstrated by similar conversion efficiencies in consecutive runs (14.9%, 14.7%, and 15.5%) (Fig. S6, ESI†). The field-dependent magnetization curve of the Fe3O4–ZIF-8 hybrid thin films at room temperature is shown in Fig. 3b. The magnetization is 3.65 emu g 1 in a magnetic field of 12 kOe. The variation of magnetization with an applied field, H (kOe), shows no hysteresis, that is, both the remanence and coercivity are zero. This indicates a superparamagnetic behavior, as expected from the nanoscale dimension of the particles. Therefore, ZIF-8 thin films play a good role in dispersing and stabilizing Fe3O4 NPs. The following demonstration of the CdSe–ZIF-8 hybrid thin films as the optical sensor further illustrates that both the MOFs and the functional species (CdSe NPs) can preserve their properties independently. Up to now, it has been hard to detect the fluorescence of a solid membrane of the CdSe. However, CdSe–ZIF-8 hybrid thin films exhibit good fluorescence properties due to the
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4298 | Chem. Commun., 2014, 50, 4296--4298
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