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A Fluorene-Based Two-Dimensional Covalent Organic Framework with Thermoelectric Properties through Doping Liangying Wang, Bin Dong, Rile Ge, Fengxing Jiang, and Jingkun Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14916 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 14, 2017
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ACS Applied Materials & Interfaces
A Fluorene-Based Two-Dimensional Covalent Organic Framework with Thermoelectric Properties through Doping Liangying Wang,†,‡ Bin Dong,†,§,‡,* Rile Ge,† Fengxing Jiang,# Jingkun Xu#,* †
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China #
College of Pharmacy, Jiangxi Science and Technology Normal University, Nanchang
330013, China §
College of Chemical Engineering, China University of Mining and Technology,
Xuzhou 221116, China ‡
L. Wang and B. Dong contributed equally to this work.
ABSTRACT:
Organic
semiconductors
have
great
potentials
as
flexible
thermoelectric materials. A fluorene-based covalent organic framework (FL-COF-1) was designed with the aim of creating an enhanced π–π interactions among the crystalline backbones. By the introduction of fluorene units into the frameworks, the FL-COF-1 had high thermal stability with a BET surface area over 1300 m2 g−1. The open frameworks were favorable for doping with iodine and followed with the improved charge carrier mobility. The compressed pellet of I2@FL-COF-1 exhibited a high Seebeck coefficient of 2450 µV K−1 and power factor of 0.063 µW m−1 K−2 under room temperature, giving the first example of COFs’ potential application as thermoelectric materials.
Keywords: Covalent Organic Frameworks, Fluorene units, Iodine Doping, Electrical Conductivity, Thermoelectric Properties.
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1. INTRODUCTION Covalent organic frameworks (COFs) are a unique type of porous crystalline materials by integrating organic subunits into predictable structures following the rules of reticular chemistry.1−3 These materials are made from light elements (e.g., B, C, N, O, Si) linked by covalent bonds and exhibit low density, exceptional high surface area, and uniform pore size distribution. The predesigned skeletons and properties of COFs can be adjusted via cautiously choosing the building units and reaction conditions to enable framework crystallization. Since the first report in 2005,4 COFs have been explored on their promising applications in gas adsorption and separation,5,6 catalysis,7−10 and optoelectronics.11−14 Two-dimensional (2D) COFs are prone to adopt nearly eclipsed stacked structures of the aromatic subunits into oriented columnar alignments, ideally suited for transporting excitons or charge carriers over the whole framework. By engineering the π-electron rich or deficient moieties into well-defined 2D COFs, they are expected to emerge as electroactive organic materials.2 Some functional moieties have been successfully incorporated into COFs such as porphyrin,11,14,15 thiophene,13,16 tetrathiafulvalene,17−19 and benzothiadiazole.12,20 Although the intrinsic electrical conductivity of COFs still remains low, that is remedied through the formation of charge-transfer complexes by chemical doping methods.17−19 Organic semiconductors have emerged as flexible thermoelectric materials due to the low thermal conductivity and material abundance, which have more superiority to the usually brittle and toxic inorganic thermoelectric materials.21,22 A variety of highly conducting charge-transfer complexes
based
on conducting polymers and
small-molecule organic conductors have been investigated.23−27 Fluorene-based conjugated polymers have attracted tremendous attention in the development of organic semiconductors owing to their excellent electronic charge-transport properties.28,29 Accordingly, we designed a fluorene-based COF with the aim of creating an enhanced π–π interactions among the crystalline backbones and followed with the improved charge carrier concentration and mobility by iodine doping. The fluorene-based COF (FL-COF-1, Figure 1a) was synthesized by Schiff-base
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condensation via a solvothermal method. After doping with iodine, the compressed pellet of I2@FL-COF-1 exhibited an improved conductivity and displayed a high Seebeck coefficient, which was promising for thermoelectric applications.
2. EXPERIMENTAL SECTION 2.1. Materials and methods. 1,3,5-Triformylbenzene (TFB) was synthesized following the reported procedures.30 Its 1H NMR and
13
C NMR spectra were in
accordance with those reported previously. 2,7-Diaminofluorene (DAFL) was purchased from TCI. o-Dichlorobenzene (o-DCB), dimethylacetamide (DMAC), acetone, tetrahydrofuran (THF), acetic acid (AcOH), and iodine were obtained from Shanghai Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received. The NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometer, where the chemical shift (δ in ppm) was determined with tetramethylsilane as an internal standard. Powder X-ray diffraction data was collected on a PANalytical X’Pert Pro Multipurpose Diffractometer using Cu Kα radiation at 40 mA and 40 kV (2θ = 2.5−40°, step size: 0.017o). Fourier transform infrared spectra were recorded on a Bruker TENSOR 27 spectrometer in the wavenumber region of 4000−400 cm−1. Elemental analysis was performed on an Elementar Vario EL III elemental analyzer. Scanning electron microscopy (SEM) images were obtained on a FEI Quanta 200 F with an accelerating voltage of 20 kV. Transmission electron microscope (TEM) morphology was observed by Tecnai G2 F30 (FEI Company) operating at 120 kV. Thermogravimetric analyses were done on a Netzsch STA 449 F3 thermal analyzer in a nitrogen atmosphere (heating rate: 10 oC/min). Raman spectra were carried out on a Senterra Raman microscope spectrometer equipped with a 532 nm diode laser. Nitrogen sorption isotherms were collected at 77 K using a Quantachrome Autosorb-iQ2 analyzer. Before the TE measurements, the samples were compressed into pellets and cut into rectangular bars (0.8×0.8×0.05 cm3). The electrical conductivity was obtained by a four-point probe apparatus with a Keithley 2700 Multimeter (Cleveland, OH) and the Labview (National Instruments, Austin, TX) after the four metal lines were painted by silver. The Seebeck coefficient was
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measured by Keithley 2700 and 4200 systems as shown in Scheme S1. An ohmic resistance was employed for a temperature gradient (5 K) along the long edge of the rectangle-shaped blocks. The temperature difference was detected by depositing two Pt100 thermocouples with the silver paste. Seebeck coefficient (S), as an inherent property of the materials at relevant temperature, was calculated by the equation S = ∆V/∆T, where ∆V was given from electrical potential differences (Vcold – Vhot) and ∆T was the temperature gradient along the voltage drop (Thot – Tcold).31 2.2. Synthetic procedures. 2,7-Diaminofluorene (DAFL, 31 mg, 0.158 mmol) and 1,3,5-triformylbenzene (TFB, 17.1 mg, 0.105 mmol) were suspended in o-DCB (0.75 mL) and DMAC (0.75 mL) with 3 M AcOH (0.25 mL) in a Pyrex tube of 5 mL. After degassed via three freeze-pump-thaw cycles, the tube was flame-sealed and then put into an oven at 120 ºC for 3 days to yield a yellow precipitate, which was collected by filtration. After washed with THF and acetone for 3 times, the yellow powder was dried in vacuum at 120 °C for 12 h to afford FL-COF-1 of 39 mg (92% yield). Elemental analysis calcd (%) for (C19H12N2)n: C 85.05, H 4.51, N 10.44; found: C 80.79, H 4.35, N 9.93. To prepare the I2@FL-COF-1, a small vial (5 mL) filled with 60 mg of the FL-COF-1 powder was placed into a brown vial (30 mL) charged with 400 mg of iodine, which was then capped tightly and kept in an oven at 75 oC for over 24 h until no obvious weight change.32 The doping material was dried in vacuum at 75 °C for 6 h before further experiments.
3. RESULTS AND DISCUSSION The condensation of 2,7-diaminofluorene (DAFL) and 1,3,5-triformylbenzene (TFB)
was
carried
out
in
a
mixture
of
o-dichlorobenzene
(o-DCB),
dimethylacetamide (DMAC), and 3 M aqueous acetic acid (3:3:1, v/v). The reaction mixture was left at 120 oC for 3 days to afford a yellow precipitate, which was filtrated, washed with tetrahydrofuran and acetone, and finally dried under vacuum to give yellow powders (FL-COF-1, Figure 1a) in 92% yield. The FL-COF-1 cannot dissolve in water and conventional organic solvents. The successful preparation of the
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imine-linked FL-COF-1 was identified by Fourier transform infrared (FTIR) spectroscopy (Figure S1), which exhibited a C=N stretching band at 1624 cm–1. The composition of FL-COF-1 was verified by elemental analysis and the experimental data (C 80.79, H 4.35, N 9.93) were very close to the calculated values (C 85.05, H 4.51, N 10.44). From the scanning electron microscopy (SEM) image, the FL-COF-1 comprised a large amount of spherical particles with diameters of nearly 1 µm (Figure 1b). The TEM images of FL-COF-1 also reflected the spherical morphology (Figure S2). The self-assembly process of the reversible polymerization for COFs was dependent on the low energy principle. Since the extent of π-π stacking efficiency among the adjacent COF layers, an inside-out Ostwald ripening mechanism was accounted for their unique morphological diversity.33,34 The crystallinity of FL-COF-1 was examined by powder X-ray diffraction (PXRD) analysis (Figure 2). The diffraction peaks with 2θ of 3.61, 6.18, 7.19, 9.58, and 25.78° were assigned as (100), (110), (200), (210), and (001) facets, providing the formation of a hexagonal columnar structure. The presence of (100) facet at 3.61° suggested that the FL-COF-1 owned porous cavities with a diameter of 24.45 Å, while the (001) facet at 25.78° corresponded to an interlayer interval of 3.45 Å. To examine the detailed structure of FL-COF-1, the geometries of the molecular building blocks were optimized with the Materials Studio (ver. 4.4) suite of programs.35 The unit cell was created with a P1 space group of α = β = 90°, γ = 120° and a = b = 28.4382 Å, c = 3.4852 Å. The calculated PXRD pattern based on the eclipsed AA stacking structure was consistentd with the experimental one in peak positions and intensities. Compared with the experimental one, the Pawley refinement provied the negligible difference with RWP of 6.67% and RP of 5.06% (Figure 2). Moreover, the PXRD pattern based on an alternatively staggered AB model was incompatible with the experimental one (Figure 2). Thermogravimetric analysis (TGA) revealed that the FL-COF-1 owned the high thermal stability with the most drastic weight loss over 460 oC (Figure S3). In order to check the chemical stability, the FL-COF-1 was immersed in hexane, methanol, water, or aqueous NaOH solutions (1 M). After left at room temperature for one day, the
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sample was filtrated, dried, and measured by PXRD (Figure 3a). The diffraction peaks were retained, suggesting the durability of crystalline FL-COF-1 because of the chemically robust imine bond. Even in boiling water for one day, the FL-COF-1 still retained good crystallinity. As shown in Figure 3b, the FTIR spectra revealed that the imine bond (1624 cm–1) were retained in all the treated samples. Nevertheless, almost no solid remained after the FL-COF-1 was submerged in aqueous HCl solutions (1 M), probably resulting from the hydrolysis of the imine bond. Nitrogen sorption isotherm was measured at 77 K to examine the porosity of FL-COF-1. As shown in Figure 4a, the sorption curve of FL-COF-1 was reversible, showing
a
significant
uptake
at
low
pressure
(P/Po
