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Ultrasonic spray deposition as a new route to luminescent MOF film synthesis
Journal of Luminescence 212 (2019) 322–327
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Ultrasonic spray deposition as a new route to luminescent MOF film synthesis
T
J.U. Balderasa,∗, D. Navarrob, V. Vargasa, M.M. Tellez-Cruzc, S. Carmonad, C. Falconye a
Centro de Investigación y de Estudios Avanzados del IPN, Programa de Doctorado en Nanociencias y Nanotecnología, Av. IPN 2508, 07360, CDMX, Mexico Centro de Investigación en Química Aplicada, Departamento de Materiales Avanzados, Blvd. Enrique Reyna Hermosillo 140, 25294, Saltillo, Coahuila, Mexico c Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Química, Av. Instituto Politécnico Nacional 2508, San Pedro Zacatenco, Gustavo A. Madero CDMX, 07360, Mexico d CONACYT-Benemérita Universidad Autónoma de Puebla, Posgrado en Física Aplicada, Facultad de Ciencias Físico-Matemáticas, Av. San Claudio y Av. 18 sur, San Manuel Ciudad Universitaria, Puebla, Puebla, 72570, Mexico e Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Física, Av. Instituto Politécnico Nacional 2508, San Pedro Zacatenco, Gustavo A. Madero CDMX, 07360, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasonic spray deposition MOF films In-situ assembly Luminescence
The deposition of high-quality metal-organic framework (MOF) films is still challenging but offers potential applications in gas storage, catalysis, sensing, lighting, and solar energy harvesting. This work reports a singlestep, simple, fast and inexpensive method for in situ synthesis and deposition of crystalline luminescent Tb2(BDC)3 (Tb = terbium) (BDC = 1,4-benzenedicarboxylate) MOF films using ultrasonic spray deposition on top of several types of substrates including glass slides, metal oxides films, filter paper, and polyimide sheets. The glass supported films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), profilometry, infrared (IR), and luminescence spectroscopies. As expected, the emission spectra present highintensity characteristic emission bands of the sensitized terbium luminescence for all the films. Nevertheless, the temperature of the substrate, during deposition, plays a crucial role in the final structural and morphological characteristics of the MOFs, which in turn, dramatically modifies the excitation features. The MOF-film synthesis/deposition method here reported can be extended to other MOF structures and substrates.
1. Introduction Metal-organic frameworks (MOFs) are a relatively new class of hybrid materials constituted by metal ions inter-connected by organic bridging ligands to form crystal-like periodic structures. Thanks to their unique characteristics, they find potential applications in hydrogen storage [1], catalysis [2], sensing [3], electronics [4], photonics [5], luminescence [6], and so on. Since the first report of a MOF material, several different synthetic routes have been developed, nevertheless, most of them require long reaction times and/or high energy consumption. Moreover, those methods can only produce bulk particles that need to be assembled in a controlled way or embedded in another material, limiting their integration in functional devices by requiring additional processing steps (ex-situ MOF film assembly process) [7]. To overcome this problem, numerous creative approaches have been proposed to control the growth of MOF materials during thin films deposition (in-situ process), including automated epitaxy [8], liquid-phase epitaxy in combination
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with spin coating [9], liquid-liquid interphase, step-by-step spray [10] and many other methods reported elsewhere [11,12]. However, the scalability, purity, and price for industrial implementation are the main challenge for the emerging MOF-synthesis technologies [13]. As far as we know, the only report of the use of sonochemical means to assist the in-situ synthesis of MOF films for optical applications was published in 2015 by Z.-G. Gu et al. [14], using the dip coating technique. In this particular case, the optical characteristics of the MOF films were excellent, however, this deposition process was limited to the use of prefunctionalized substrates and a substantial amount of deposition cycles to get about 100 nm thickness films. The ultrasound-assisted spray deposition technique is a simple, large-area compatible, single-step, fast, and inexpensive method that has been widely used for film deposition of many inorganic [15–17] and organic [18–20] materials, and has never been used for the in-situ deposition of hybrid MOF films. Luminescent MOF materials have attracted considerable interest due to their excellent performance in analytical applications, catalysis, lighting, and solar energy harvesting. Although in 2012 the future
Corresponding author. E-mail address: [email protected] (J.U. Balderas).
https://doi.org/10.1016/j.jlumin.2019.04.051 Received 21 January 2019; Received in revised form 16 April 2019; Accepted 23 April 2019 Available online 26 April 2019 0022-2313/ © 2019 Published by Elsevier B.V.
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development of luminescent MOF films was expected to come out for a straightforward and instant application in devices [21], nowadays reports on luminescent MOF films are very scarce [22], perhaps because a practical in-situ deposition technique was not developed yet. Rareearth-based MOFs are highly promising materials for luminescence applications, thanks to their shielded nature of the f-electrons, most of the different lanthanide ions show diverse, narrow and predictable color emissions encompassing the ultraviolet, visible and infrared light energies. Lanthanide can present additional interesting characteristics when used as ionic nodes for MOF structures, for example, if the organic linker molecule is chosen carefully, an energy transfer mechanism known as “antenna effect” could take place, greatly enhancing the luminescence properties of the resulting hybrid material. Different spectral conversion mechanisms are responsible for lanthanide-based MOFs luminescence: a) Down-conversion, in which one high energy photon is converted into two lower energy photons; b) Down-shifting in which one high energy photon is transformed into one low energy photon and c) Up-conversion, in which two lower energy photons are converted into one high energy photon [23]. The unique properties of lanthanide ions among the great diversity of antennas available to be used as linkers in MOF arrangements and the different spectral conversion mechanisms opens infinite possibilities for the potential applications mentioned above. Here, the implementation of an ultrasound-assisted spray technique for in-situ deposition of luminescent MOF films is presented for the first time. The main novelty of this technique relies in the possibility of using a large variety of substrates without the need of previous surface functionalization, along with high deposition rates and potential scalability for large-area implementation. It was tested for the specific case of Tb2(BDC)3-framework films but it can be extended to other luminescent materials. BDC organic linker was chosen because, thanks to the excellent match between triplet (T1) energy level of BDC and the excited energy levels (5D4) of terbium, an efficient sensitization of Tb via “antenna effect” is obtained [24–26]. The photoluminescence excitation and emission characteristics obtained from these films are used to validate the ultrasound-assisted spray deposition method effectiveness. The characteristics of the obtained films and the flexibility of the technique to deposit MOF films onto many types of substrates are much better than those obtained by other in-situ deposition techniques of luminescent films with similar MOF architecture [27].
Fig. 1. Scheme of the ultrasonic spray deposition system.
evaporate, generating the matrix-free MOF films. 2.3. Solutions preparation and films deposition Two solutions were prepared for spray deposition. For the solution A, 1 mmol of BDC was dissolved in 80 mL of DMF, stirring magnetically at room temperature for 1 h. On the other hand, solution B was prepared dissolving 1 mmol of TbCl3·6H2O in 80 mL of DMF, stirring magnetically at room temperature for 1 h. DMF was chosen because it has been reported as an excellent solvent in similar ultrasound assisted spray technique systems and because BDC dissolves well in it. Each precursor solution was poured in a self-made glass flask (200 ml) and placed in an ultrasonic nebulizer (Yuehua Mod. WH-802) with a resonant frequency of 1.7 MHz working at 60 W. A compressed air flux was injected into each solution container at 10 L per minute to drag the produced mists to the mixing tube. The resulting mixed mist was finally deposited on a Corning glass substrate (30 mm × 30 mm, 1 mm thick) placed on top of a hot tin bath with adjustable temperature in the 120 to 220 °C range, fixing the deposition time to 2 min. 3. Results and discussion
2. Material and methods Fig. 2 shows the XRD patterns of the Tb3(BDC)3-framework films deposited onto a glass slide at different temperatures, along with the respective JCPDS card (00-157-1127). All films exhibited almost the same XRD patterns with diffraction lines close to those reported for the Tb2(BDC)3H2O particulate system [29]. The diffraction peaks showed high intensity at low deposition temperatures (Td), indicating high crystallinity for the MOF films; a gradual decrease in the peaks intensity (loss of crystallinity) was observed in films deposited at increasing temperatures (180–200 °C). Another insight into the effect of the deposition temperature on the structural characteristics was obtained by analyzing the FT-IR spectra of films and BDC ligand (Fig. 3). The FT-IR spectra of all deposited films showed the strong absorbing carboxylate bands characteristic of BDCbased compounds. Usually, the bands associated to the asymmetric stretching vibrations νasym(COO−) are observed between 1588 and 1616 cm−1, and the bands of the symmetric stretching vibrations νsym(COO−) appear in the range of 1397–1400 cm−1 [30]; the difference between these two frequencies Δ = νasym-νsym is often used as a spectroscopic proof of the carboxylate binding mode. The Δ values for films deposited at 120, 140 and 160 °C (150, 165 and 165 cm−1, respectively), are smaller than that of the 1,4-benzenedicarboxylic acid disodium salt (Na2BDC, Δ = 168 cm−1), indicating effective coordination in the form of bidentate bridging [31,32]. In contrast, when the deposition temperature was as high as 180 °C, an additional band
2.1. Materials and characterization All reagents and solvents were used as purchased without further purification for films deposition. For FTIR comparison and analysis purposes, Disodium terephthalate salt (Na2BDC) was prepared using the methodology reported elsewhere [28]. FTIR spectra were recorded on a 6700 FT-IR NICOLET. XRD patterns were obtained using a Bruker D8 Advance Eco diffractometer with Cu Kα radiation (λ = 1.54 Å) at 25 ± 3 °C. Morphology of films was determined by scanning electron microscopy (SEM) using a JEOL scanning electron microscope JSM6390. Film thickness estimations were made using a KLA Tencor Mod. D-600 Profilometer. Photoluminescence spectra were recorded using an Edinburgh Ins. M. 960 S spectrophotometer. 2.2. Ultrasonic spray deposition process description Crystalline Tb2(BDC)3-framework films were synthesized by ultrasonic spray deposition method. The experimental arrangement for this technique is depicted in Fig. 1 and consist of two precursor solutions placed each one in a separated ultrasonic nebulizer to produce an ultrafine mist of each precursor solution. These mists are then carried by a gas flux to a mixing zone, the resulting mixed mist is finally transported to the substrate surface (on top of a hot plate) where the solvents 323
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higher than 180 °C, the ν(C]O) band prevails over the νasym band, and the FTIR spectra of films are highly like that of the pure BDC ligand; this could be interpreted as an ineffective coordination mechanism, and thus, a very poor-quality MOF film, as previously suggested by XRD analysis. SEM images of films deposited at different temperatures are shown in Fig. 4. All of them presented excellent coverage of the glass substrate for 2 min of deposition, nevertheless, the morphology of films changes as a function of the deposition temperature. A highly rough surface composed of agglomerated semi-spherical particles of ∼ 500 nm diameter is evident for the film deposited at 120 °C. At 140 °C, the particle size decreases dramatically, and the morphology of the deposited film looks like an agglomeration of popcorn-like particles (∼ 50 nm). The film deposited at 160 °C shows a quasi-reticular morphology, this is likely to occur when nebulized droplets reach the hot substrate, evaporating the solvent at the edge of the droplet more quickly than at the center [34]. At 180 °C, the deposited film showed a smoother surface, and this can be related to the presence of less rigid monodentate coordination bonds, which results in a lack of structural rigidity [35]; the MOF film is more susceptible to collapse into a denser film. As expected, this densification effect is more evident in those films deposited at 200 and 220 °C, where the gradual loss of structural quality leads to smoothsurfaced and well-packed films. Fig. 5 shows the measured thickness values of the MOF films deposited at different temperatures for 2 min. The maximum thickness value seems to be leveled off around 600 nm for films deposited between 120 and 160 °C, although it must be considered that the surface roughness is different for each one, thus, the error bars in Fig. 5 depict the thickness uncertainty due to the film roughness. As expected, the film thickness (and the respective standard deviation) tends to decrease due to the progressive collapse of the MOF into more dense structures at higher temperatures, reaching its minimum (∼400 nm) at T ≥ 200 °C. Luminescence properties of the deposited films are expected to be highly influenced by the structural conditions of the MOF, thus, photoluminescence characterization can be used as an additional spectroscopic proof for the coordination system. The normalized emission spectra of films deposited at different temperatures (Fig. 6a) depict the characteristic peaks attributed to the 5D4→7F6, 5D4→7F5, 5D4→7F4, and 5 D4→7F3 transitions of Tb3+ with maxima at 487, 546, 582 and 622 nm, respectively. The 5D4→7F6,4,3 transitions have a moderate sensitivity to the ligand environment [36], thereby, the strongest emission arises from the so-called “hypersensitive” 5D4→7F5 transition. At temperatures from 120 to 200 °C, the stark-split doublet of this band is attributed to the effect of the BDC field. The overall emission properties hardly change with the increase in deposition temperature until 220 °C is reached, where the 5D4→7F5 stark splitting decreases to one band, which could be related to the ineffective sensitization of the Tb3+ ions for this temperature. Inset in Fig. 6a shows the intensity of the 5 D4→7F5 transition under a 306 nm excitation as a function of the deposition temperature. Clearly, the emission intensity slightly increases when the temperature increases from 120 to 160 °C. Upon further increase in temperature, a gradual drop in emission intensity was detected, showing its minimum for 220 °C. The decrease in emission intensity with the increase in temperature is related with changes in the excitation spectra and will be discussed below. The excitation spectra of the as-prepared films (Fig. 6b) were used to monitoring the 5D4→7F5 transition at 546 nm from the emission spectra. In general, these spectra are composed by multiple bands, which can be classified in two groups: the bands situated at shorter wavelengths (λ ≤ 280 nm) ascribed to π-π* intraligand transitions and the low-energy bands (λ ≥ 280 nm) associated to n-π* intraligand transitions [37]. At low Td, where the coordination of the ligand to the Tb+3 is mostly bidentate, the excitation spectra are composed by a balanced contribution from both π-π* and n-π* transitions. The gradual change to a monodentate coordination mode with the increase in Td (as discussed previously), implies that the n electrons located on the
Fig. 2. XRD patterns of Tb2(BDC)3 films deposited at different temperatures.
Fig. 3. IR spectra of Tb2(BDC)3 films deposited at different temperatures.
ascribed to the stretching vibration of carbonyl ν(C]O) appeared at 1681 cm−1, suggesting that a partial amount of the BDC linkers is now in a monodentate binding mode [33]. For deposition temperatures 324
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Fig. 4. SEM images of Tb2(BDC)3 films deposited onto a glass slide at 120 °C (a), 140 (b), 160 (c), 180 (d), 200 (e) and 220 °C (f).
reached. The excited singlet state is converted to a triplet state (T1) by inter-system crossing (ISC). From this point, an energy transfer (ET) to the5D4 energy levels of Tb is very likely to occur, according to Latva's empirical rule [38]. Finally, the terbium ion decays radiatively to its 7Fn ground levels, emitting in its characteristic wavelengths. To conclude this study, the versatility of the deposition technique was explored using additional materials as substrates (Fig. 7): a masked glass substrate (Fig. 7a), polyimide film (Fig. 7b), filter paper (Fig. 7c), alumina, titania and silica (not shown). For all these substrates, MOF films were successfully grown at 120 °C and all of them showed a bright green luminescence emission (observed with the naked eye) when exposed to a 254 nm UV irradiation from a Hg-lamp (Fig. 7d–f). All films showed good adhesion to the substrate. When hand-held, no scratches, particle detachment or changes in luminescence were observed under normal handling conditions. The scotch tape test, performed with films deposited on a glass slide, showed no appreciable film detachment, nevertheless, further investigations on the adhesion mechanisms need to be performed. It is therefore believed, that this technique can be readily extrapolated for the growth of other MOF architectures by the use of different metal ions, organic linkers or even, doped with particles [39,40]. The resulting films (luminescent or not) could find valuable applications in the fields of catalysis, sensing, lighting, solar energy harvesting, drug delivery, gas storage, and so on. Moreover, in the near future, this method for MOF films deposition could be further optimized and scaled for mass production of large-area devices in a wide variety of substrates and geometries.
Fig. 5. Average Film thickness of MOF films deposited at different temperatures during 2 min.
oxygen of the carbonyl group (n-π*) do not contribute to the energy transfer to the Tb3+, and thus, the charge-transfer mechanism (π-π*) from the BDC to the 5D4 orbitals of Tb3+ is the predominant excitation pathway. The scheme in Fig. 6c represents the most probable luminescence mechanism for the Tb2(BDC)3 films. First, the excitation energy is harvested by the BDC molecule, where an electron is promoted from the ground state (S0) to the first excited singlet state (S1) through π-π* and n-π* intramolecular transitions; then, the electron undergoes internal conversion (IC) process until lowest vibrational level of S1 is
Fig. 6. Normalized photoluminescence emission spectra (a) and excitation spectra (b) of Tb2(BDC)3 films deposited at different substrate temperatures. Scheme of the probable main luminescence mechanism in the Tb2(BDC)3 films. The inset in Fig. 6a shows the maximum intensity of the 5D4→7F5 transition under a 306 nm excitation as a function of the deposition temperature. 325
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Fig. 7. Photographs of the MOF films deposited on top of a masked glass (a,d), filter paper (b,e) and polyimide film (c,f) under ambient illumination (a–c) and 254 nm UV irradiation (d–f).
4. Conclusion
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In summary, the implementation of ultrasound-assisted spray coating as a new route for the fabrication of luminescent MOF films was presented, through the synthesis of Tb2(BDC)3-based films, which were successfully grown on top of a wide type of substrates by this simple, easily scalable and low-price in-situ method. Film synthesis/deposition takes only 2 min and no additional processing steps are required. Crystallinity, coordination modes, morphology and luminescence of the resulting films were greatly influenced by the temperature of the substrate during spray deposition as suggested by XRD, FTIR, SEM and photoluminescence analysis, respectively. In general, low temperatures (Td = 120–160 °C) yield highly crystalline MOF films showing highintensity emission bands (luminescence) thanks to the bidentate coordination mode between terbium ions and BDC ligand. Higher temperatures (Td = 180–200 °C) result in monodentate coordination mode, which in turn, causes the collapse of the MOF into films with smooth surfaces, lower structural quality and decreased luminescence properties. Results here reported lead to the belief that this MOF films in situ deposition technique could represent a breakthrough for the integration of MOF architectures in future functional devices. Acknowledgments We acknowledge the technical assistance of Z. Rivera and M. Guerrero from the Physics Department of CINVESTAV-IPN. Thanks are extended to Dr. J. Rodríguez-Hernández from CIQA- Saltillo for his support in XRD characterization. We are also thankful for the financial support from CONACYT. References [1] R. Balderas‐Xicohténcatl, P. Schmieder, D. Denysenko, D. Volkmer, M. Hirscher, High volumetric hydrogen storage capacity using interpenetrated metal–organic frameworks, Energy Technol. 6 (3) (2018) 510–512 https://doi.org/10.1002/ente. 201700608. [2] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev. 38 (5) (2009) 1450–1459
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Ultrasonic spray deposition as a new route to luminescent MOF film synthesis
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J.U. Balderasa,∗, D. Navarrob, V. Vargasa, M.M. Tellez-Cruzc, S. Carmonad, C. Falconye a
Centro de Investigación y de Estudios Avanzados del IPN, Programa de Doctorado en Nanociencias y Nanotecnología, Av. IPN 2508, 07360, CDMX, Mexico Centro de Investigación en Química Aplicada, Departamento de Materiales Avanzados, Blvd. Enrique Reyna Hermosillo 140, 25294, Saltillo, Coahuila, Mexico c Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Química, Av. Instituto Politécnico Nacional 2508, San Pedro Zacatenco, Gustavo A. Madero CDMX, 07360, Mexico d CONACYT-Benemérita Universidad Autónoma de Puebla, Posgrado en Física Aplicada, Facultad de Ciencias Físico-Matemáticas, Av. San Claudio y Av. 18 sur, San Manuel Ciudad Universitaria, Puebla, Puebla, 72570, Mexico e Centro de Investigación y de Estudios Avanzados del IPN, Departamento de Física, Av. Instituto Politécnico Nacional 2508, San Pedro Zacatenco, Gustavo A. Madero CDMX, 07360, Mexico b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ultrasonic spray deposition MOF films In-situ assembly Luminescence
The deposition of high-quality metal-organic framework (MOF) films is still challenging but offers potential applications in gas storage, catalysis, sensing, lighting, and solar energy harvesting. This work reports a singlestep, simple, fast and inexpensive method for in situ synthesis and deposition of crystalline luminescent Tb2(BDC)3 (Tb = terbium) (BDC = 1,4-benzenedicarboxylate) MOF films using ultrasonic spray deposition on top of several types of substrates including glass slides, metal oxides films, filter paper, and polyimide sheets. The glass supported films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), profilometry, infrared (IR), and luminescence spectroscopies. As expected, the emission spectra present highintensity characteristic emission bands of the sensitized terbium luminescence for all the films. Nevertheless, the temperature of the substrate, during deposition, plays a crucial role in the final structural and morphological characteristics of the MOFs, which in turn, dramatically modifies the excitation features. The MOF-film synthesis/deposition method here reported can be extended to other MOF structures and substrates.
1. Introduction Metal-organic frameworks (MOFs) are a relatively new class of hybrid materials constituted by metal ions inter-connected by organic bridging ligands to form crystal-like periodic structures. Thanks to their unique characteristics, they find potential applications in hydrogen storage [1], catalysis [2], sensing [3], electronics [4], photonics [5], luminescence [6], and so on. Since the first report of a MOF material, several different synthetic routes have been developed, nevertheless, most of them require long reaction times and/or high energy consumption. Moreover, those methods can only produce bulk particles that need to be assembled in a controlled way or embedded in another material, limiting their integration in functional devices by requiring additional processing steps (ex-situ MOF film assembly process) [7]. To overcome this problem, numerous creative approaches have been proposed to control the growth of MOF materials during thin films deposition (in-situ process), including automated epitaxy [8], liquid-phase epitaxy in combination
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with spin coating [9], liquid-liquid interphase, step-by-step spray [10] and many other methods reported elsewhere [11,12]. However, the scalability, purity, and price for industrial implementation are the main challenge for the emerging MOF-synthesis technologies [13]. As far as we know, the only report of the use of sonochemical means to assist the in-situ synthesis of MOF films for optical applications was published in 2015 by Z.-G. Gu et al. [14], using the dip coating technique. In this particular case, the optical characteristics of the MOF films were excellent, however, this deposition process was limited to the use of prefunctionalized substrates and a substantial amount of deposition cycles to get about 100 nm thickness films. The ultrasound-assisted spray deposition technique is a simple, large-area compatible, single-step, fast, and inexpensive method that has been widely used for film deposition of many inorganic [15–17] and organic [18–20] materials, and has never been used for the in-situ deposition of hybrid MOF films. Luminescent MOF materials have attracted considerable interest due to their excellent performance in analytical applications, catalysis, lighting, and solar energy harvesting. Although in 2012 the future
Corresponding author. E-mail address: [email protected] (J.U. Balderas).
https://doi.org/10.1016/j.jlumin.2019.04.051 Received 21 January 2019; Received in revised form 16 April 2019; Accepted 23 April 2019 Available online 26 April 2019 0022-2313/ © 2019 Published by Elsevier B.V.
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development of luminescent MOF films was expected to come out for a straightforward and instant application in devices [21], nowadays reports on luminescent MOF films are very scarce [22], perhaps because a practical in-situ deposition technique was not developed yet. Rareearth-based MOFs are highly promising materials for luminescence applications, thanks to their shielded nature of the f-electrons, most of the different lanthanide ions show diverse, narrow and predictable color emissions encompassing the ultraviolet, visible and infrared light energies. Lanthanide can present additional interesting characteristics when used as ionic nodes for MOF structures, for example, if the organic linker molecule is chosen carefully, an energy transfer mechanism known as “antenna effect” could take place, greatly enhancing the luminescence properties of the resulting hybrid material. Different spectral conversion mechanisms are responsible for lanthanide-based MOFs luminescence: a) Down-conversion, in which one high energy photon is converted into two lower energy photons; b) Down-shifting in which one high energy photon is transformed into one low energy photon and c) Up-conversion, in which two lower energy photons are converted into one high energy photon [23]. The unique properties of lanthanide ions among the great diversity of antennas available to be used as linkers in MOF arrangements and the different spectral conversion mechanisms opens infinite possibilities for the potential applications mentioned above. Here, the implementation of an ultrasound-assisted spray technique for in-situ deposition of luminescent MOF films is presented for the first time. The main novelty of this technique relies in the possibility of using a large variety of substrates without the need of previous surface functionalization, along with high deposition rates and potential scalability for large-area implementation. It was tested for the specific case of Tb2(BDC)3-framework films but it can be extended to other luminescent materials. BDC organic linker was chosen because, thanks to the excellent match between triplet (T1) energy level of BDC and the excited energy levels (5D4) of terbium, an efficient sensitization of Tb via “antenna effect” is obtained [24–26]. The photoluminescence excitation and emission characteristics obtained from these films are used to validate the ultrasound-assisted spray deposition method effectiveness. The characteristics of the obtained films and the flexibility of the technique to deposit MOF films onto many types of substrates are much better than those obtained by other in-situ deposition techniques of luminescent films with similar MOF architecture [27].
Fig. 1. Scheme of the ultrasonic spray deposition system.
evaporate, generating the matrix-free MOF films. 2.3. Solutions preparation and films deposition Two solutions were prepared for spray deposition. For the solution A, 1 mmol of BDC was dissolved in 80 mL of DMF, stirring magnetically at room temperature for 1 h. On the other hand, solution B was prepared dissolving 1 mmol of TbCl3·6H2O in 80 mL of DMF, stirring magnetically at room temperature for 1 h. DMF was chosen because it has been reported as an excellent solvent in similar ultrasound assisted spray technique systems and because BDC dissolves well in it. Each precursor solution was poured in a self-made glass flask (200 ml) and placed in an ultrasonic nebulizer (Yuehua Mod. WH-802) with a resonant frequency of 1.7 MHz working at 60 W. A compressed air flux was injected into each solution container at 10 L per minute to drag the produced mists to the mixing tube. The resulting mixed mist was finally deposited on a Corning glass substrate (30 mm × 30 mm, 1 mm thick) placed on top of a hot tin bath with adjustable temperature in the 120 to 220 °C range, fixing the deposition time to 2 min. 3. Results and discussion
2. Material and methods Fig. 2 shows the XRD patterns of the Tb3(BDC)3-framework films deposited onto a glass slide at different temperatures, along with the respective JCPDS card (00-157-1127). All films exhibited almost the same XRD patterns with diffraction lines close to those reported for the Tb2(BDC)3H2O particulate system [29]. The diffraction peaks showed high intensity at low deposition temperatures (Td), indicating high crystallinity for the MOF films; a gradual decrease in the peaks intensity (loss of crystallinity) was observed in films deposited at increasing temperatures (180–200 °C). Another insight into the effect of the deposition temperature on the structural characteristics was obtained by analyzing the FT-IR spectra of films and BDC ligand (Fig. 3). The FT-IR spectra of all deposited films showed the strong absorbing carboxylate bands characteristic of BDCbased compounds. Usually, the bands associated to the asymmetric stretching vibrations νasym(COO−) are observed between 1588 and 1616 cm−1, and the bands of the symmetric stretching vibrations νsym(COO−) appear in the range of 1397–1400 cm−1 [30]; the difference between these two frequencies Δ = νasym-νsym is often used as a spectroscopic proof of the carboxylate binding mode. The Δ values for films deposited at 120, 140 and 160 °C (150, 165 and 165 cm−1, respectively), are smaller than that of the 1,4-benzenedicarboxylic acid disodium salt (Na2BDC, Δ = 168 cm−1), indicating effective coordination in the form of bidentate bridging [31,32]. In contrast, when the deposition temperature was as high as 180 °C, an additional band
2.1. Materials and characterization All reagents and solvents were used as purchased without further purification for films deposition. For FTIR comparison and analysis purposes, Disodium terephthalate salt (Na2BDC) was prepared using the methodology reported elsewhere [28]. FTIR spectra were recorded on a 6700 FT-IR NICOLET. XRD patterns were obtained using a Bruker D8 Advance Eco diffractometer with Cu Kα radiation (λ = 1.54 Å) at 25 ± 3 °C. Morphology of films was determined by scanning electron microscopy (SEM) using a JEOL scanning electron microscope JSM6390. Film thickness estimations were made using a KLA Tencor Mod. D-600 Profilometer. Photoluminescence spectra were recorded using an Edinburgh Ins. M. 960 S spectrophotometer. 2.2. Ultrasonic spray deposition process description Crystalline Tb2(BDC)3-framework films were synthesized by ultrasonic spray deposition method. The experimental arrangement for this technique is depicted in Fig. 1 and consist of two precursor solutions placed each one in a separated ultrasonic nebulizer to produce an ultrafine mist of each precursor solution. These mists are then carried by a gas flux to a mixing zone, the resulting mixed mist is finally transported to the substrate surface (on top of a hot plate) where the solvents 323
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higher than 180 °C, the ν(C]O) band prevails over the νasym band, and the FTIR spectra of films are highly like that of the pure BDC ligand; this could be interpreted as an ineffective coordination mechanism, and thus, a very poor-quality MOF film, as previously suggested by XRD analysis. SEM images of films deposited at different temperatures are shown in Fig. 4. All of them presented excellent coverage of the glass substrate for 2 min of deposition, nevertheless, the morphology of films changes as a function of the deposition temperature. A highly rough surface composed of agglomerated semi-spherical particles of ∼ 500 nm diameter is evident for the film deposited at 120 °C. At 140 °C, the particle size decreases dramatically, and the morphology of the deposited film looks like an agglomeration of popcorn-like particles (∼ 50 nm). The film deposited at 160 °C shows a quasi-reticular morphology, this is likely to occur when nebulized droplets reach the hot substrate, evaporating the solvent at the edge of the droplet more quickly than at the center [34]. At 180 °C, the deposited film showed a smoother surface, and this can be related to the presence of less rigid monodentate coordination bonds, which results in a lack of structural rigidity [35]; the MOF film is more susceptible to collapse into a denser film. As expected, this densification effect is more evident in those films deposited at 200 and 220 °C, where the gradual loss of structural quality leads to smoothsurfaced and well-packed films. Fig. 5 shows the measured thickness values of the MOF films deposited at different temperatures for 2 min. The maximum thickness value seems to be leveled off around 600 nm for films deposited between 120 and 160 °C, although it must be considered that the surface roughness is different for each one, thus, the error bars in Fig. 5 depict the thickness uncertainty due to the film roughness. As expected, the film thickness (and the respective standard deviation) tends to decrease due to the progressive collapse of the MOF into more dense structures at higher temperatures, reaching its minimum (∼400 nm) at T ≥ 200 °C. Luminescence properties of the deposited films are expected to be highly influenced by the structural conditions of the MOF, thus, photoluminescence characterization can be used as an additional spectroscopic proof for the coordination system. The normalized emission spectra of films deposited at different temperatures (Fig. 6a) depict the characteristic peaks attributed to the 5D4→7F6, 5D4→7F5, 5D4→7F4, and 5 D4→7F3 transitions of Tb3+ with maxima at 487, 546, 582 and 622 nm, respectively. The 5D4→7F6,4,3 transitions have a moderate sensitivity to the ligand environment [36], thereby, the strongest emission arises from the so-called “hypersensitive” 5D4→7F5 transition. At temperatures from 120 to 200 °C, the stark-split doublet of this band is attributed to the effect of the BDC field. The overall emission properties hardly change with the increase in deposition temperature until 220 °C is reached, where the 5D4→7F5 stark splitting decreases to one band, which could be related to the ineffective sensitization of the Tb3+ ions for this temperature. Inset in Fig. 6a shows the intensity of the 5 D4→7F5 transition under a 306 nm excitation as a function of the deposition temperature. Clearly, the emission intensity slightly increases when the temperature increases from 120 to 160 °C. Upon further increase in temperature, a gradual drop in emission intensity was detected, showing its minimum for 220 °C. The decrease in emission intensity with the increase in temperature is related with changes in the excitation spectra and will be discussed below. The excitation spectra of the as-prepared films (Fig. 6b) were used to monitoring the 5D4→7F5 transition at 546 nm from the emission spectra. In general, these spectra are composed by multiple bands, which can be classified in two groups: the bands situated at shorter wavelengths (λ ≤ 280 nm) ascribed to π-π* intraligand transitions and the low-energy bands (λ ≥ 280 nm) associated to n-π* intraligand transitions [37]. At low Td, where the coordination of the ligand to the Tb+3 is mostly bidentate, the excitation spectra are composed by a balanced contribution from both π-π* and n-π* transitions. The gradual change to a monodentate coordination mode with the increase in Td (as discussed previously), implies that the n electrons located on the
Fig. 2. XRD patterns of Tb2(BDC)3 films deposited at different temperatures.
Fig. 3. IR spectra of Tb2(BDC)3 films deposited at different temperatures.
ascribed to the stretching vibration of carbonyl ν(C]O) appeared at 1681 cm−1, suggesting that a partial amount of the BDC linkers is now in a monodentate binding mode [33]. For deposition temperatures 324
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Fig. 4. SEM images of Tb2(BDC)3 films deposited onto a glass slide at 120 °C (a), 140 (b), 160 (c), 180 (d), 200 (e) and 220 °C (f).
reached. The excited singlet state is converted to a triplet state (T1) by inter-system crossing (ISC). From this point, an energy transfer (ET) to the5D4 energy levels of Tb is very likely to occur, according to Latva's empirical rule [38]. Finally, the terbium ion decays radiatively to its 7Fn ground levels, emitting in its characteristic wavelengths. To conclude this study, the versatility of the deposition technique was explored using additional materials as substrates (Fig. 7): a masked glass substrate (Fig. 7a), polyimide film (Fig. 7b), filter paper (Fig. 7c), alumina, titania and silica (not shown). For all these substrates, MOF films were successfully grown at 120 °C and all of them showed a bright green luminescence emission (observed with the naked eye) when exposed to a 254 nm UV irradiation from a Hg-lamp (Fig. 7d–f). All films showed good adhesion to the substrate. When hand-held, no scratches, particle detachment or changes in luminescence were observed under normal handling conditions. The scotch tape test, performed with films deposited on a glass slide, showed no appreciable film detachment, nevertheless, further investigations on the adhesion mechanisms need to be performed. It is therefore believed, that this technique can be readily extrapolated for the growth of other MOF architectures by the use of different metal ions, organic linkers or even, doped with particles [39,40]. The resulting films (luminescent or not) could find valuable applications in the fields of catalysis, sensing, lighting, solar energy harvesting, drug delivery, gas storage, and so on. Moreover, in the near future, this method for MOF films deposition could be further optimized and scaled for mass production of large-area devices in a wide variety of substrates and geometries.
Fig. 5. Average Film thickness of MOF films deposited at different temperatures during 2 min.
oxygen of the carbonyl group (n-π*) do not contribute to the energy transfer to the Tb3+, and thus, the charge-transfer mechanism (π-π*) from the BDC to the 5D4 orbitals of Tb3+ is the predominant excitation pathway. The scheme in Fig. 6c represents the most probable luminescence mechanism for the Tb2(BDC)3 films. First, the excitation energy is harvested by the BDC molecule, where an electron is promoted from the ground state (S0) to the first excited singlet state (S1) through π-π* and n-π* intramolecular transitions; then, the electron undergoes internal conversion (IC) process until lowest vibrational level of S1 is
Fig. 6. Normalized photoluminescence emission spectra (a) and excitation spectra (b) of Tb2(BDC)3 films deposited at different substrate temperatures. Scheme of the probable main luminescence mechanism in the Tb2(BDC)3 films. The inset in Fig. 6a shows the maximum intensity of the 5D4→7F5 transition under a 306 nm excitation as a function of the deposition temperature. 325
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Fig. 7. Photographs of the MOF films deposited on top of a masked glass (a,d), filter paper (b,e) and polyimide film (c,f) under ambient illumination (a–c) and 254 nm UV irradiation (d–f).
4. Conclusion
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In summary, the implementation of ultrasound-assisted spray coating as a new route for the fabrication of luminescent MOF films was presented, through the synthesis of Tb2(BDC)3-based films, which were successfully grown on top of a wide type of substrates by this simple, easily scalable and low-price in-situ method. Film synthesis/deposition takes only 2 min and no additional processing steps are required. Crystallinity, coordination modes, morphology and luminescence of the resulting films were greatly influenced by the temperature of the substrate during spray deposition as suggested by XRD, FTIR, SEM and photoluminescence analysis, respectively. In general, low temperatures (Td = 120–160 °C) yield highly crystalline MOF films showing highintensity emission bands (luminescence) thanks to the bidentate coordination mode between terbium ions and BDC ligand. Higher temperatures (Td = 180–200 °C) result in monodentate coordination mode, which in turn, causes the collapse of the MOF into films with smooth surfaces, lower structural quality and decreased luminescence properties. Results here reported lead to the belief that this MOF films in situ deposition technique could represent a breakthrough for the integration of MOF architectures in future functional devices. Acknowledgments We acknowledge the technical assistance of Z. Rivera and M. Guerrero from the Physics Department of CINVESTAV-IPN. Thanks are extended to Dr. J. Rodríguez-Hernández from CIQA- Saltillo for his support in XRD characterization. We are also thankful for the financial support from CONACYT. References [1] R. Balderas‐Xicohténcatl, P. Schmieder, D. Denysenko, D. Volkmer, M. Hirscher, High volumetric hydrogen storage capacity using interpenetrated metal–organic frameworks, Energy Technol. 6 (3) (2018) 510–512 https://doi.org/10.1002/ente. 201700608. [2] J. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev. 38 (5) (2009) 1450–1459
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