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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
Patterned Growth of Luminescent Metal‐organic Framework Films: A Versatile Method for Electrochemical‐assisted Microwave Deposition Wei‐Jin Li,a,b Ji‐Fei Feng,a Zu‐Jin Lin,a Ying‐Long Yang,a Yan Yang,a Xu‐Sheng Wang,a Shui‐Ying Gao,a and Rong Caoa,*
Presented here is a facile method, electrochemical‐assisted microwave deposition technology, for the fabrication of luminescent metal‐organic frameworks (LMOFs) films. This method was further developed into a versatile method for preparing patterned LMOF films. The strategy based on this method can spatially locate microcrystals of MOFs on a surface, which provides great promise in application of anti‐counterfeiting barcode. Metal‐organic frameworks (MOFs) are a charming family of materials with permanent porosity, high surface area, and adjustable chemical functionalities.1,2 The specific qualities of MOFs materials have positioned them at the forefront of studies in gas storage3,4, catalysis5‐7 and separation8. Among the diverse metal‐organic frameworks, lanthanide based metal‐organic frameworks (LMOFs) have attracted great attention, not only because of their fascinating characteristic coordination and unique optical properties arising from 4f electrons, but also due to their potential applications in anti‐ counterfeiting barcode,9 luminescence sensors,10,11 and light‐ emitting devices (LEDs).12 The aforementioned qualities have also aroused many scientists’ interest in the use of LMOFs as smart membrane, lighting apparatus, sensor devices, and many other nano‐technological devices.12 However, the integration of LMOFs into devices for these applications demands materials to be deposited at specific locations on surface or even to be obtained as patterned films, which is of a great challenge since LMOFs are generally synthesized as bulky powders or single crystals.13‐15 Thus, exploring methods for both preparing and patterning LMOFs films is essential. Recently, to prepare metal‐organic frameworks films, a number of interesting methods have been employed, including
a.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Mater, Chinese Academy of Science, Fuzhou 350002, P. R. China Collaborative Innovation Center of Chemistry for Energy Materials (2011‐iChEM), Xiamen, 361005, P. R. China † Footnotes rela ng to the tle and/or authors should appear here. Electronic Supplementary Information (ESI) available: Instrument information data, cyclic voltammetry (CV) curve of synthesis of Ln(OH)3 layers, thickness of Ln(OH)3 and Ln‐MOF films.. See DOI: 10.1039/x0xx00000x b.
in‐situ growth,16,17 layer‐by‐layer growth,18 gel‐layer deposition,19,20 and dip‐coating21. Although these methods mostly produce dense and constant films, the precise pattern and quantitative control, which are crucial for integrated device fabrication, are far from reach. Current efficient methods for achieving patterned LMOFs films exhibit a technological breakthrough in the field of integrate applications of electronic devices.13‐15 However, most of the known methods, such as digital patterned deposition of precursors that is subsequently converted to LMOFs, lithographic techniques (soft lithography, photolithography, deep X‐ray lithography, and electron‐beam lithography), are typically time‐consuming and limited to comparatively small overall fields. Thus, the development of a facile and versatile method for patterning the films of LMOFs and controlling their performance of emission and colour is desirable. Electrochemical method has been turned out to be a powerful technique for the preparation of well‐closed MOFs films and patterned growth of MOF films.13, 22,23 However, most of MOF film prepared via electrochemical method are using the same metal support as metal source. For LMOFs, it is very difficult even idealistic to electrochemically deposit expensive rare earths and realize their practical applications.24 Although researchers have presented breakthrough technologies for depositing some special lanthanide MOFs on metallic plates, the substrate is limited at metallic plates and multiphase usually exists which probably affect the practical application of LMOFs.13 Inspired by free standing growth of MOFs films,25 we deposited lanthanide hydroxides as metal source on conductive glass and subsequently convert them into LMOFs films.26 Microwave synthesis has been reported as a simple and energy‐efficient technology to synthesis of LMOFs, which not only reduce the react time, but also enhance the product yield,27 and it may be an elegant platform to realize the conversion of lanthanide hydroxides precursor into LMOFs. Microwave synthesis with the assist of electrochemical plating method has not been used for the patterned fabrication of LMOFs films to date.
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In this work, we present a combination method of electrochemical preparation and microwave irradiation to deposit patterned LMOFs on conductive glass, which can play significance in decorative and commercial materials in real life. Two crucial steps are contained in our strategies for locating the spatial distribution of MOFs microcrystals. Firstly, patterned lanthanide hydroxide precursors were deposited on conductive glass through electrochemical deposition. Secondly, the patterned lanthanide hydroxide precursors were converted into MOFs in the aid of microwave synthesis (Fig. 1). The key issues to realize the specific deposition of patterned LMOFs are to control the writing pattern, pH of reactive solution, electrochemical deposition cycles and microwave reactive time, since lanthanide hydroxide precursors are probably dissolved in low pH solution and long reactive time of electrochemical deposition and microwave reaction would result precursors to exfoliate from the support. Herein, we focus on the patterned preparation of [Ln(TPO)2(HCOO)]·(Me2NH2)·(DMF)4·(H2O)6 (1) (H3TPO=tris‐4‐ carboxylphenyl)phosphineoxide); DMF=N,N‐ dimethylformamide) on conductive glass (fluorine‐doped tin oxide, FTO). Compound 1 is constructed by one crystallographically independent ion, one TPO3‐ ligand, and 0.5 formate anion. The two neighboring europium centres are bridged by one formate anions and two carboxylate groups from two TPO3‐ ligands, forming a binuclear europium cluster [Eu2O2(COO)6(HCOO)]. Furthermore, we also attempt to extend this facile method to deposit other LMOFs and build a platform for pattering LMOFs.
water containing NO3‐ ions (Eq. S1, Fig. 1a). The of Viewvoltage Article Online 10.1039/C6CC00519E cyclic voltammetry (CV) was set at the DOI: range of open circuit potential to ‐1.4 V. One couple of reduction and oxidation peaks were observed in the CV curves (Fig. S1). It was ascribed to the reduction of NO3‐ ions and generation of H2.27 This convenient approach of cathodic generation of OH‐ causes a pH gradient close to FTO surface. Further, lanthanide hydroxides layers were deposited on the surface of FTO with the coordination of lanthanide ions with OH‐ (Fig. 1a). The scanning electron microscopy (SEM) showed that a dense and homogeneous Eu(OH)3 and Tb(OH)3 layer have been deposited on FTO surface after 8 cycles electrochemical deposition (Fig. 1b, c). Subsequently, the hydroxide layers were converted into Ln‐MOFs under microwave irradiation (Fig. 2a, Fig. S2). The composition and morphology were well studied by powder X‐ ray diffraction (PXRD) and SEM. As shown in Fig. 2, well shape microcrystals were densely deposited on conductive FTO surface. PXRD was used to confirm the crystallinity and phase of as‐prepared MOF layer. It can be observed that the prepared Eu‐MOF and Tb‐MOF layer matched well with simulated curves from the single‐crystal data. (Fig. S3).
Fig. 2 (a) Schematic illustration of microwave conversion of lanthanide hydroxide layers to Ln‐MOF onto transparent FTO surface. (b) SEM images of Eu‐MOF layers on FTO surface. (c) SEM images of Tb‐MOF layers on FTO surface.
Fig. 1 (a) Schematic illustration of electrochemical lanthanide hydroxide layers on transparent FTO surface. (b) SEM images of Eu(OH)3 layers on FTO surface. (c) SEM images of Tb(OH)3 layers on FTO surface.
OH‐ ions were typically produced near the surface of inert fluorine‐doped tin oxide (FTO) by electrochemical reduction of
Considering the well spatial location of Ln‐MOFs layers, we try to develop them as luminescent devices. As a proof‐of concept experiment, we studied the thickness for Ln(OH)3 layer and luminescent emission for Ln‐MOF deposited in CV cycles. For this, Tb(OH)3 layers were deposited onto FTO surface after 2 cycles to 10 cycles of electrochemical reduction. As the number of CV cycles increases, the thickness of Tb(OH)3 layers grows gradually (Fig. S4). The results show that the thickness of Tb(OH)3 layers can be controlled by multiple cyclic voltammetry deposited cycles. However, as shown in Fig. S5, the intense emission of Tb‐MOF enhances with the number of
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Tb(OH)3 deposited cycles increases. Tb‐MOF converted by Tb(OH)3 layers, which was prepared at 6 cycles of electrochemical reduction, exhibits the strongest photoluminescence. However, when the deposited cycles of Tb(OH)3 were more than 6 cycles, the intensity of emission of Tb‐MOF decreased with the deposited cycles of Tb(OH)3 increases. The reason may be ascribed that the thicker of Tb(OH)3 layers, the easier of Tb(OH)3 fall off from FTO surface during microwave irradiation. Thus, rational control of microwave reaction time is needed for the conversion of Tb‐ hydroxides to Tb‐MOFs. DEKTAK XT step profile instrument was used to define the optimum microwave irradiation time. As can be seen in Fig. S6, the thickness of Tb‐MOF converted from 8 cycles of Tb(OH)3 increased from 0.2 μm to 2.2 μm when increasing the microwave reaction time from 10s to 90s. But it decreased to 1.2 μm under 120 s of microwave irradiation. The results confirm that the long microwave irradiation, the easier of Tb(OH)3 flake from FTO surface. Following the successful preparation of the Ln‐MOF films and thorough study of deposition parameters, their emission spectra were studied. The Eu‐MOF film shows the characteristic emission bands for f‐f transitions of europium ( Ⅲ ) ion when excited at 290 nm. The strongest photoluminescence peak located at 617 nm corresponds to the 5Do 7F2 transitions. The two main peaks at 592 and 700 nm are assigned to 5Do 7F1 and 5Do 7F1. The weak emission bands at 536 and 650 nm are attributed to 5Do 7Fo and 5Do 7F3. The results suggest that the spectrum is ruled by the strongest band of the 5Do 7F2 electron dipole transition, which is the claimed hypersensitive transition and is responsible for the brilliant red photoluminescence. As shown in Fig. S7, Tb‐MOF film shows characteristic terbium( Ⅲ ) emission bands, arising from the 5D4 7FJ(J=6,5,4,3, and 2) transitions. The strongest photoluminescence peak located at 544 nm corresponds to the 5D4 7F5 transition. The peaks at 490, 587 and 621 nm are attributed to 5D4 7F6, 5D4 7F4 and 5D4 7F3, respectively. The weak band at 650 nm is assigned to 5D4 7F2. The intense band of the 5D4 7F5 transition endows Tb‐MOF film with green photoluminescence. The photoluminescence behaviour of the prepared LMOF films match well with the results reported in the literature.28
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Fig. 3 (a) Schematic illustration of patterning growth of luminescent barcodes. (b) Presentation of anti‐counterfeiting barcode (Left: Eu‐MOF, Right: Tb‐MOF). (c) To demonstrate the applicability for large area patterning, “FJ” were spatially located onto FTO surface.
The quantum yields of Eu‐MOF films and Tb‐MOF films are measured as 5.6% and 19.2%, respectively, which are smaller than their bulky Eu/Tb‐MOF powders’ quantum yields reported in the literature28. The reason for the difference of quantum yields between bulky MOFs powders and Eu/Tb‐MOF films is that only a small amount of Eu/Tb‐MOFs microcrystals are deposited on a surface forming a thin film, leading to the reduce of quantum yields from bulky MOF powders to Eu/Tb‐ MOF films. The decays of the Eu‐MOF films and Tb‐MOFs films are tested as 6.2 ms and 6.8 ms, respectively.(Fig. S8) The inherent characteristics of sharp emission spectra and various emission bands of the prepared Ln‐MOF film have paved the way to develop innovative platforms for building luminescent barcodes. For a broad range of potential applications, we introduce Eu‐MOF and Tb‐MOF, exploited as red and green emission sources, to build a barcode array system. As shown in Fig. 3a, we first coated poly(dimethylsiloxane) (PDMS) onto FTO surface. After sculpturing the patterned PDMS film, Ln(OH)3 layers were spatially located onto FTO surface without painting of PDMS by electrochemical deposition. Subsequently, the patterned Ln(OH)3 layers were converted into patterned Ln‐MOF films. As can be seen in Fig. 3b, 2D barcode array system of Eu‐MOF films and Tb‐MOF films were constructed. By varying the width, height, emission intensity, as well as the sequence of different lines, we can store a countless amount of cargo information. All of the above barcode information can only be detected under certain excitation (UV light), which may have potential application in the field of anti‐ counterfeiting. This facile method can also be used to pattern other dimension pictures (Fig. 3c).
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4 J. Park, D. Feng, H.‐C. Zhou, J. Am. Chem. Soc. 2015, 137, 1663‐ View Article Online 1672. DOI: 10.1039/C6CC00519E 5 M. T. Zhao, K. Deng, L. C. He, Y. Liu, G. D. Li, H. J. Zhao, Z. Y. Tang, J. Am. Chem. Soc. 2014, 136, 1738‐1741. 6 S. L. Zhao, H. J. Yin, L. Du, L. C. He, K. Zhao, L. Chang, G. Yin, H. Zhao, S. Q. Liu, Z. Y. Tang, ACS Nano, 2014, 8, 12660‐12668. 7 L. C. He, Y. Liu, J. Z. Liu, Y. S. Xiong, J. Z. Zheng, Y. L. Liu, Z. Y. Tang, Angew. Chem. Int. Ed., 2013, 52, 3741‐3745. 8 T. F. Liu, J. Lu, X. Lin, R. Cao, Chem. Commun. 2010, 46, 8439‐ 41. 9 Y. Lu, B. Yan, J. Mater. Chem. C 2014, 2, 7411‐7416. 10 Y. Wu, J. Han, P. Xue, R. Xu, Y. Kang, Nanoscale 2015, 7, 1753‐ 1759. 11 Y. T. Li, J. L. Tang, L. C. He, Y. Liu, Y. L. Liu, C. Y. Chen, Z. Y. Tang, Adv. Mater. 2015, 27, 4075‐4080. 12 E. F. de Melo, N. D. C. Santana, K. G. B. Alves, de Sa, G. F. D. Sá, C. P. de Melo, M. O. Rodrigues, S. A. Junior, J. Mater. Chem. C 2013, 1, 7574‐7581. 13 N. Campagnol, E. R. Souza, D. E. De Vos, K. Binnemans, J. Fransaer, Chem. Commun. 2014, 50, 12545‐7. 14 Z.‐G. Gu, Z. Chen, W.‐Q. Fu, F. Wang, J. Zhang, ACS Appl. Mat. Interfaces 2015, 7, 28585‐28590. 15 H. B. Zhang, M. Liu, X. Lei, T. Wen, J. Zhang, ACS Appl. Mat. Interfaces 2014, 6, 12594‐12599. 16 W. J. Li, S. Y. Gao, T. F. Liu, L. W. Han, Z. J. Lin, R. Cao, Langmuir 2013, 29, 8657‐8664. 17 W. Li, S. Zhou, S. Gao, S. Chen, M. Huang, R. Cao, Adv. Mater. Interfaces 2014, 2, 1400405‐1400410. 18 M. Tu, S. Wannapaiboon, R. A. Fischer, Inorg. Chem. Front. 2014, 1, 442‐463. 19 M. Tsotsalas, J. Liu, B. Tettmann, S. Grosjean, A. Shahnas, Z. Wang, C. Azucena, M. Addicoat, T. Heine, J. Lahann, J. Overhage, S. Brase, H. Gliemann, C. Wöll, J. Am. Chem. Soc. 2014, 136, 8‐11. 20 A. Schoedel, C. Scherb, T. Bein, Angew. Chem. Int. Ed. 2010, 49, 7225‐7228. 21 P. Horcajada, C. Serre, D. Grosso, C. Boissiere, S. Perruchas, C. Sanchez, G. Ferey, Adv. Mater. 2009, 21, 1931‐1935. 22 A. Doménech, H. García, M. T. Doménech‐Carbó, F. Llabrés‐i‐ Xamena, Electrochem. Commun. 2006, 8, 1830‐1834. Conclusions 23 R. Ameloot, L. Stappers, J. Fransaer, L. Alaerts, B. F. Sels, D. E. De Vos, Chem. Mater. 2009, 21, 2580‐2582. We presented a versatile electrochemical‐assisted microwave preparation method for growing luminescent lanthanide MOF 24 L. R. Morss, Chem. Rev. 1976, 76, 827‐841. 25 Y. Mao, L. shi, H. Huang, W. Cao, J. Li, L. Sun, X. Jin, X. Peng, barcodes, and realized the control growth of microcrystals on Chem. Commun. 2013, 49, 5666‐5668. a desired location on a surface. This method is not only simple 26 H. Liu, H. Wang, T. Chu, M. Yu, Y. Yang, J. Mater. Chem. C 2014, 2, 8683‐8690. and facile, but also suitable to most lanthanide ions. Moreover, the films have strong luminescent properties and efficient 27 M. J. Siegfried, K. S. Choi, Adv. Mater. 2004, 16, 1743‐1746. Tb3+‐to‐Eu3+ energy transferability and hence they may 28 Z. J. Lin, Z. Yang, T. F. Liu, Y. B. Huang, R. Cao, Inorg. Chem. 2012, 51, 1813‐20. conceive to expand the scope of application in the field of 29 R. P. Sear, J. Phys. :Condens. Matter, 2007, 19, 033101‐ color display, luminescence sensors, anti‐counterfeiting 033129. barcode and structural probes. This research was supported by 973 Program (2014CB845605 and 2012CB821705), NSFC (21521061, 21331006, 51572260 and 21303205), and the Natural Science Foundation of Fujian Province of China (2014J05020).
Luminescence studies reveal the different lanthanide compositions could result in unique and discernible barcoded signals. After successfully developing Eu‐MOF films and Tb‐ MOF films as luminescent barcodes materials, we use the same method to fabricate Eu1‐xTbx‐MOF films with varying Tb3+ ion and Eu3+ ion stoichiometries (Tb/Eu: 0.4, 1.0, 1.5, 2.7, and 3.0) to yield different colour. The ratio of Tb and Eu element in the resulting Ln‐MOF films were also defined by ICP, and directly correlated to the feed ratio of experimental ratio (Table S1). As studied in Fig. S7b, the photoluminescence peak at 544 nm corresponds to 5D4 7F5 transition of the Tb3+ ions in the green region. The two peaks at ca. 592 and 617 nm are assigned to the 5Do 7F1 and 5Do 7F2 transitions of the Eu3+ ions, respectively. As can be seen in Fig. S9, the intensity of the peak at 544 nm are changed with Tb/Eu ratio. This behaviour suggests the existence of Tb3+‐to‐Eu3+ energy transfer. Moreover, the colors in the photograph of films are consistent with calculated results in CIE chromaticity diagram (Fig. S10). That means we can quantitatively control the luminescent intensities of the two emitting lanthanide ion by controlling the lanthanide composition. By achieving the various colors of films, various distinct barcodes can be obtained. Additionally, this technology can also be extended to prepare other Ln‐MOF films, such as Yb‐MOF films, Er‐MOF films, and so on (Fig. S11‐Fig. S14). As seen from the SEM images and PXRD curves, Ln‐MOFs films have been successfully deposited on conductive surface. The microstructures of the prepared Ln‐MOFs films are different. This phenomenon is ascribed to diverse nucleation rate aroused by different Ln3+ metal center.16,24,29 The results suggest that this technology is flexible and can be used widely.
Notes and references 1 L. ‐B. Sun, X. ‐Q. Liu, H. ‐C. Zhou, Chem. Soc. Rev. 2015, 44 , 5092‐5147. 2 Y.‐P. Chen, Y. Liu, D. Liu, M. Bosch, H.‐C. Zhou, J. Am. Chem. Soc. 2015, 137, 2919‐2930. 3 Q. ‐G. Meng, X. L. Xin, L. G. Zhang, F. D. Dai, R. M. Wang, D. F. Sun, J. Mater. Chem. A 2015, 3, 24016‐24021.
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This article can be cited before page numbers have been issued, to do this please use: W. Li, J. Feng, Z. Lin, Y. Yang, Y. Yang, X. Wang, S. Gao and R. Cao, Chem. Commun., 2016, DOI: 10.1039/C6CC00519E.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Patterned Growth of Luminescent Metal‐organic Framework Films: A Versatile Method for Electrochemical‐assisted Microwave Deposition Wei‐Jin Li,a,b Ji‐Fei Feng,a Zu‐Jin Lin,a Ying‐Long Yang,a Yan Yang,a Xu‐Sheng Wang,a Shui‐Ying Gao,a and Rong Caoa,*
Presented here is a facile method, electrochemical‐assisted microwave deposition technology, for the fabrication of luminescent metal‐organic frameworks (LMOFs) films. This method was further developed into a versatile method for preparing patterned LMOF films. The strategy based on this method can spatially locate microcrystals of MOFs on a surface, which provides great promise in application of anti‐counterfeiting barcode. Metal‐organic frameworks (MOFs) are a charming family of materials with permanent porosity, high surface area, and adjustable chemical functionalities.1,2 The specific qualities of MOFs materials have positioned them at the forefront of studies in gas storage3,4, catalysis5‐7 and separation8. Among the diverse metal‐organic frameworks, lanthanide based metal‐organic frameworks (LMOFs) have attracted great attention, not only because of their fascinating characteristic coordination and unique optical properties arising from 4f electrons, but also due to their potential applications in anti‐ counterfeiting barcode,9 luminescence sensors,10,11 and light‐ emitting devices (LEDs).12 The aforementioned qualities have also aroused many scientists’ interest in the use of LMOFs as smart membrane, lighting apparatus, sensor devices, and many other nano‐technological devices.12 However, the integration of LMOFs into devices for these applications demands materials to be deposited at specific locations on surface or even to be obtained as patterned films, which is of a great challenge since LMOFs are generally synthesized as bulky powders or single crystals.13‐15 Thus, exploring methods for both preparing and patterning LMOFs films is essential. Recently, to prepare metal‐organic frameworks films, a number of interesting methods have been employed, including
a.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Mater, Chinese Academy of Science, Fuzhou 350002, P. R. China Collaborative Innovation Center of Chemistry for Energy Materials (2011‐iChEM), Xiamen, 361005, P. R. China † Footnotes rela ng to the tle and/or authors should appear here. Electronic Supplementary Information (ESI) available: Instrument information data, cyclic voltammetry (CV) curve of synthesis of Ln(OH)3 layers, thickness of Ln(OH)3 and Ln‐MOF films.. See DOI: 10.1039/x0xx00000x b.
in‐situ growth,16,17 layer‐by‐layer growth,18 gel‐layer deposition,19,20 and dip‐coating21. Although these methods mostly produce dense and constant films, the precise pattern and quantitative control, which are crucial for integrated device fabrication, are far from reach. Current efficient methods for achieving patterned LMOFs films exhibit a technological breakthrough in the field of integrate applications of electronic devices.13‐15 However, most of the known methods, such as digital patterned deposition of precursors that is subsequently converted to LMOFs, lithographic techniques (soft lithography, photolithography, deep X‐ray lithography, and electron‐beam lithography), are typically time‐consuming and limited to comparatively small overall fields. Thus, the development of a facile and versatile method for patterning the films of LMOFs and controlling their performance of emission and colour is desirable. Electrochemical method has been turned out to be a powerful technique for the preparation of well‐closed MOFs films and patterned growth of MOF films.13, 22,23 However, most of MOF film prepared via electrochemical method are using the same metal support as metal source. For LMOFs, it is very difficult even idealistic to electrochemically deposit expensive rare earths and realize their practical applications.24 Although researchers have presented breakthrough technologies for depositing some special lanthanide MOFs on metallic plates, the substrate is limited at metallic plates and multiphase usually exists which probably affect the practical application of LMOFs.13 Inspired by free standing growth of MOFs films,25 we deposited lanthanide hydroxides as metal source on conductive glass and subsequently convert them into LMOFs films.26 Microwave synthesis has been reported as a simple and energy‐efficient technology to synthesis of LMOFs, which not only reduce the react time, but also enhance the product yield,27 and it may be an elegant platform to realize the conversion of lanthanide hydroxides precursor into LMOFs. Microwave synthesis with the assist of electrochemical plating method has not been used for the patterned fabrication of LMOFs films to date.
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In this work, we present a combination method of electrochemical preparation and microwave irradiation to deposit patterned LMOFs on conductive glass, which can play significance in decorative and commercial materials in real life. Two crucial steps are contained in our strategies for locating the spatial distribution of MOFs microcrystals. Firstly, patterned lanthanide hydroxide precursors were deposited on conductive glass through electrochemical deposition. Secondly, the patterned lanthanide hydroxide precursors were converted into MOFs in the aid of microwave synthesis (Fig. 1). The key issues to realize the specific deposition of patterned LMOFs are to control the writing pattern, pH of reactive solution, electrochemical deposition cycles and microwave reactive time, since lanthanide hydroxide precursors are probably dissolved in low pH solution and long reactive time of electrochemical deposition and microwave reaction would result precursors to exfoliate from the support. Herein, we focus on the patterned preparation of [Ln(TPO)2(HCOO)]·(Me2NH2)·(DMF)4·(H2O)6 (1) (H3TPO=tris‐4‐ carboxylphenyl)phosphineoxide); DMF=N,N‐ dimethylformamide) on conductive glass (fluorine‐doped tin oxide, FTO). Compound 1 is constructed by one crystallographically independent ion, one TPO3‐ ligand, and 0.5 formate anion. The two neighboring europium centres are bridged by one formate anions and two carboxylate groups from two TPO3‐ ligands, forming a binuclear europium cluster [Eu2O2(COO)6(HCOO)]. Furthermore, we also attempt to extend this facile method to deposit other LMOFs and build a platform for pattering LMOFs.
water containing NO3‐ ions (Eq. S1, Fig. 1a). The of Viewvoltage Article Online 10.1039/C6CC00519E cyclic voltammetry (CV) was set at the DOI: range of open circuit potential to ‐1.4 V. One couple of reduction and oxidation peaks were observed in the CV curves (Fig. S1). It was ascribed to the reduction of NO3‐ ions and generation of H2.27 This convenient approach of cathodic generation of OH‐ causes a pH gradient close to FTO surface. Further, lanthanide hydroxides layers were deposited on the surface of FTO with the coordination of lanthanide ions with OH‐ (Fig. 1a). The scanning electron microscopy (SEM) showed that a dense and homogeneous Eu(OH)3 and Tb(OH)3 layer have been deposited on FTO surface after 8 cycles electrochemical deposition (Fig. 1b, c). Subsequently, the hydroxide layers were converted into Ln‐MOFs under microwave irradiation (Fig. 2a, Fig. S2). The composition and morphology were well studied by powder X‐ ray diffraction (PXRD) and SEM. As shown in Fig. 2, well shape microcrystals were densely deposited on conductive FTO surface. PXRD was used to confirm the crystallinity and phase of as‐prepared MOF layer. It can be observed that the prepared Eu‐MOF and Tb‐MOF layer matched well with simulated curves from the single‐crystal data. (Fig. S3).
Fig. 2 (a) Schematic illustration of microwave conversion of lanthanide hydroxide layers to Ln‐MOF onto transparent FTO surface. (b) SEM images of Eu‐MOF layers on FTO surface. (c) SEM images of Tb‐MOF layers on FTO surface.
Fig. 1 (a) Schematic illustration of electrochemical lanthanide hydroxide layers on transparent FTO surface. (b) SEM images of Eu(OH)3 layers on FTO surface. (c) SEM images of Tb(OH)3 layers on FTO surface.
OH‐ ions were typically produced near the surface of inert fluorine‐doped tin oxide (FTO) by electrochemical reduction of
Considering the well spatial location of Ln‐MOFs layers, we try to develop them as luminescent devices. As a proof‐of concept experiment, we studied the thickness for Ln(OH)3 layer and luminescent emission for Ln‐MOF deposited in CV cycles. For this, Tb(OH)3 layers were deposited onto FTO surface after 2 cycles to 10 cycles of electrochemical reduction. As the number of CV cycles increases, the thickness of Tb(OH)3 layers grows gradually (Fig. S4). The results show that the thickness of Tb(OH)3 layers can be controlled by multiple cyclic voltammetry deposited cycles. However, as shown in Fig. S5, the intense emission of Tb‐MOF enhances with the number of
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Tb(OH)3 deposited cycles increases. Tb‐MOF converted by Tb(OH)3 layers, which was prepared at 6 cycles of electrochemical reduction, exhibits the strongest photoluminescence. However, when the deposited cycles of Tb(OH)3 were more than 6 cycles, the intensity of emission of Tb‐MOF decreased with the deposited cycles of Tb(OH)3 increases. The reason may be ascribed that the thicker of Tb(OH)3 layers, the easier of Tb(OH)3 fall off from FTO surface during microwave irradiation. Thus, rational control of microwave reaction time is needed for the conversion of Tb‐ hydroxides to Tb‐MOFs. DEKTAK XT step profile instrument was used to define the optimum microwave irradiation time. As can be seen in Fig. S6, the thickness of Tb‐MOF converted from 8 cycles of Tb(OH)3 increased from 0.2 μm to 2.2 μm when increasing the microwave reaction time from 10s to 90s. But it decreased to 1.2 μm under 120 s of microwave irradiation. The results confirm that the long microwave irradiation, the easier of Tb(OH)3 flake from FTO surface. Following the successful preparation of the Ln‐MOF films and thorough study of deposition parameters, their emission spectra were studied. The Eu‐MOF film shows the characteristic emission bands for f‐f transitions of europium ( Ⅲ ) ion when excited at 290 nm. The strongest photoluminescence peak located at 617 nm corresponds to the 5Do 7F2 transitions. The two main peaks at 592 and 700 nm are assigned to 5Do 7F1 and 5Do 7F1. The weak emission bands at 536 and 650 nm are attributed to 5Do 7Fo and 5Do 7F3. The results suggest that the spectrum is ruled by the strongest band of the 5Do 7F2 electron dipole transition, which is the claimed hypersensitive transition and is responsible for the brilliant red photoluminescence. As shown in Fig. S7, Tb‐MOF film shows characteristic terbium( Ⅲ ) emission bands, arising from the 5D4 7FJ(J=6,5,4,3, and 2) transitions. The strongest photoluminescence peak located at 544 nm corresponds to the 5D4 7F5 transition. The peaks at 490, 587 and 621 nm are attributed to 5D4 7F6, 5D4 7F4 and 5D4 7F3, respectively. The weak band at 650 nm is assigned to 5D4 7F2. The intense band of the 5D4 7F5 transition endows Tb‐MOF film with green photoluminescence. The photoluminescence behaviour of the prepared LMOF films match well with the results reported in the literature.28
View Article Online
DOI: 10.1039/C6CC00519E
Fig. 3 (a) Schematic illustration of patterning growth of luminescent barcodes. (b) Presentation of anti‐counterfeiting barcode (Left: Eu‐MOF, Right: Tb‐MOF). (c) To demonstrate the applicability for large area patterning, “FJ” were spatially located onto FTO surface.
The quantum yields of Eu‐MOF films and Tb‐MOF films are measured as 5.6% and 19.2%, respectively, which are smaller than their bulky Eu/Tb‐MOF powders’ quantum yields reported in the literature28. The reason for the difference of quantum yields between bulky MOFs powders and Eu/Tb‐MOF films is that only a small amount of Eu/Tb‐MOFs microcrystals are deposited on a surface forming a thin film, leading to the reduce of quantum yields from bulky MOF powders to Eu/Tb‐ MOF films. The decays of the Eu‐MOF films and Tb‐MOFs films are tested as 6.2 ms and 6.8 ms, respectively.(Fig. S8) The inherent characteristics of sharp emission spectra and various emission bands of the prepared Ln‐MOF film have paved the way to develop innovative platforms for building luminescent barcodes. For a broad range of potential applications, we introduce Eu‐MOF and Tb‐MOF, exploited as red and green emission sources, to build a barcode array system. As shown in Fig. 3a, we first coated poly(dimethylsiloxane) (PDMS) onto FTO surface. After sculpturing the patterned PDMS film, Ln(OH)3 layers were spatially located onto FTO surface without painting of PDMS by electrochemical deposition. Subsequently, the patterned Ln(OH)3 layers were converted into patterned Ln‐MOF films. As can be seen in Fig. 3b, 2D barcode array system of Eu‐MOF films and Tb‐MOF films were constructed. By varying the width, height, emission intensity, as well as the sequence of different lines, we can store a countless amount of cargo information. All of the above barcode information can only be detected under certain excitation (UV light), which may have potential application in the field of anti‐ counterfeiting. This facile method can also be used to pattern other dimension pictures (Fig. 3c).
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4 J. Park, D. Feng, H.‐C. Zhou, J. Am. Chem. Soc. 2015, 137, 1663‐ View Article Online 1672. DOI: 10.1039/C6CC00519E 5 M. T. Zhao, K. Deng, L. C. He, Y. Liu, G. D. Li, H. J. Zhao, Z. Y. Tang, J. Am. Chem. Soc. 2014, 136, 1738‐1741. 6 S. L. Zhao, H. J. Yin, L. Du, L. C. He, K. Zhao, L. Chang, G. Yin, H. Zhao, S. Q. Liu, Z. Y. Tang, ACS Nano, 2014, 8, 12660‐12668. 7 L. C. He, Y. Liu, J. Z. Liu, Y. S. Xiong, J. Z. Zheng, Y. L. Liu, Z. Y. Tang, Angew. Chem. Int. Ed., 2013, 52, 3741‐3745. 8 T. F. Liu, J. Lu, X. Lin, R. Cao, Chem. Commun. 2010, 46, 8439‐ 41. 9 Y. Lu, B. Yan, J. Mater. Chem. C 2014, 2, 7411‐7416. 10 Y. Wu, J. Han, P. Xue, R. Xu, Y. Kang, Nanoscale 2015, 7, 1753‐ 1759. 11 Y. T. Li, J. L. Tang, L. C. He, Y. Liu, Y. L. Liu, C. Y. Chen, Z. Y. Tang, Adv. Mater. 2015, 27, 4075‐4080. 12 E. F. de Melo, N. D. C. Santana, K. G. B. Alves, de Sa, G. F. D. Sá, C. P. de Melo, M. O. Rodrigues, S. A. Junior, J. Mater. Chem. C 2013, 1, 7574‐7581. 13 N. Campagnol, E. R. Souza, D. E. De Vos, K. Binnemans, J. Fransaer, Chem. Commun. 2014, 50, 12545‐7. 14 Z.‐G. Gu, Z. Chen, W.‐Q. Fu, F. Wang, J. Zhang, ACS Appl. Mat. Interfaces 2015, 7, 28585‐28590. 15 H. B. Zhang, M. Liu, X. Lei, T. Wen, J. Zhang, ACS Appl. Mat. Interfaces 2014, 6, 12594‐12599. 16 W. J. Li, S. Y. Gao, T. F. Liu, L. W. Han, Z. J. Lin, R. Cao, Langmuir 2013, 29, 8657‐8664. 17 W. Li, S. Zhou, S. Gao, S. Chen, M. Huang, R. Cao, Adv. Mater. Interfaces 2014, 2, 1400405‐1400410. 18 M. Tu, S. Wannapaiboon, R. A. Fischer, Inorg. Chem. Front. 2014, 1, 442‐463. 19 M. Tsotsalas, J. Liu, B. Tettmann, S. Grosjean, A. Shahnas, Z. Wang, C. Azucena, M. Addicoat, T. Heine, J. Lahann, J. Overhage, S. Brase, H. Gliemann, C. Wöll, J. Am. Chem. Soc. 2014, 136, 8‐11. 20 A. Schoedel, C. Scherb, T. Bein, Angew. Chem. Int. Ed. 2010, 49, 7225‐7228. 21 P. Horcajada, C. Serre, D. Grosso, C. Boissiere, S. Perruchas, C. Sanchez, G. Ferey, Adv. Mater. 2009, 21, 1931‐1935. 22 A. Doménech, H. García, M. T. Doménech‐Carbó, F. Llabrés‐i‐ Xamena, Electrochem. Commun. 2006, 8, 1830‐1834. Conclusions 23 R. Ameloot, L. Stappers, J. Fransaer, L. Alaerts, B. F. Sels, D. E. De Vos, Chem. Mater. 2009, 21, 2580‐2582. We presented a versatile electrochemical‐assisted microwave preparation method for growing luminescent lanthanide MOF 24 L. R. Morss, Chem. Rev. 1976, 76, 827‐841. 25 Y. Mao, L. shi, H. Huang, W. Cao, J. Li, L. Sun, X. Jin, X. Peng, barcodes, and realized the control growth of microcrystals on Chem. Commun. 2013, 49, 5666‐5668. a desired location on a surface. This method is not only simple 26 H. Liu, H. Wang, T. Chu, M. Yu, Y. Yang, J. Mater. Chem. C 2014, 2, 8683‐8690. and facile, but also suitable to most lanthanide ions. Moreover, the films have strong luminescent properties and efficient 27 M. J. Siegfried, K. S. Choi, Adv. Mater. 2004, 16, 1743‐1746. Tb3+‐to‐Eu3+ energy transferability and hence they may 28 Z. J. Lin, Z. Yang, T. F. Liu, Y. B. Huang, R. Cao, Inorg. Chem. 2012, 51, 1813‐20. conceive to expand the scope of application in the field of 29 R. P. Sear, J. Phys. :Condens. Matter, 2007, 19, 033101‐ color display, luminescence sensors, anti‐counterfeiting 033129. barcode and structural probes. This research was supported by 973 Program (2014CB845605 and 2012CB821705), NSFC (21521061, 21331006, 51572260 and 21303205), and the Natural Science Foundation of Fujian Province of China (2014J05020).
Luminescence studies reveal the different lanthanide compositions could result in unique and discernible barcoded signals. After successfully developing Eu‐MOF films and Tb‐ MOF films as luminescent barcodes materials, we use the same method to fabricate Eu1‐xTbx‐MOF films with varying Tb3+ ion and Eu3+ ion stoichiometries (Tb/Eu: 0.4, 1.0, 1.5, 2.7, and 3.0) to yield different colour. The ratio of Tb and Eu element in the resulting Ln‐MOF films were also defined by ICP, and directly correlated to the feed ratio of experimental ratio (Table S1). As studied in Fig. S7b, the photoluminescence peak at 544 nm corresponds to 5D4 7F5 transition of the Tb3+ ions in the green region. The two peaks at ca. 592 and 617 nm are assigned to the 5Do 7F1 and 5Do 7F2 transitions of the Eu3+ ions, respectively. As can be seen in Fig. S9, the intensity of the peak at 544 nm are changed with Tb/Eu ratio. This behaviour suggests the existence of Tb3+‐to‐Eu3+ energy transfer. Moreover, the colors in the photograph of films are consistent with calculated results in CIE chromaticity diagram (Fig. S10). That means we can quantitatively control the luminescent intensities of the two emitting lanthanide ion by controlling the lanthanide composition. By achieving the various colors of films, various distinct barcodes can be obtained. Additionally, this technology can also be extended to prepare other Ln‐MOF films, such as Yb‐MOF films, Er‐MOF films, and so on (Fig. S11‐Fig. S14). As seen from the SEM images and PXRD curves, Ln‐MOFs films have been successfully deposited on conductive surface. The microstructures of the prepared Ln‐MOFs films are different. This phenomenon is ascribed to diverse nucleation rate aroused by different Ln3+ metal center.16,24,29 The results suggest that this technology is flexible and can be used widely.
Notes and references 1 L. ‐B. Sun, X. ‐Q. Liu, H. ‐C. Zhou, Chem. Soc. Rev. 2015, 44 , 5092‐5147. 2 Y.‐P. Chen, Y. Liu, D. Liu, M. Bosch, H.‐C. Zhou, J. Am. Chem. Soc. 2015, 137, 2919‐2930. 3 Q. ‐G. Meng, X. L. Xin, L. G. Zhang, F. D. Dai, R. M. Wang, D. F. Sun, J. Mater. Chem. A 2015, 3, 24016‐24021.
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