Significantly Enhanced Power Factors of p-Type Carbon Nanotube

May 30, 2019 - This work provides a basis for improving the TE performance of p-type .... Then BF3·O(Et)2 (1.2 mL of a 2.65 M stock solution, 2.64 mm...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11832−11840

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Significantly Enhanced Power Factors of p‑Type Carbon NanotubeBased Composite Films by Tailoring the Peripheral Substituents in Porphyrin Yan Zhou,†,‡,§ Xiaojun Yin,† Yijia Liu,† Xiaoyan Zhou,† Tao Wan,† Shichao Wang,† Chunmei Gao,*,§ and Lei Wang*,†,‡

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†

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, ‡Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, and §College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060, China S Supporting Information *

ABSTRACT: Considering that the development of p-type singlewalled carbon nanotube (SWCNT)/organic small molecule (OSM)-based thermoelectric (TE) materials with high performance is lagging behind and only a few structure−activity relationships of OSMs on SWCNT composites have been reported, new structures need to be developed for the sake of stimulating this scientific issue. Taking advantages of porphyrins (such as their aromatic systems, facile structures, high stability, and ease of processing), we first synthesized a series of free base porphyrins with different substituents as potent p-type SWCNTbased TE composites. Notably, SWCNT/Por-5F with electronwithdrawing groups exhibits the highest power factor (PF) of 279.3 μW m−1 K−2 at room temperature (two times higher than that of SWCNT/Por-NH2), which is among the highest values for SWCNT/OSM-based p-type TE materials reported up to date. The carrier transport behavior was demonstrated to obey the fluctuation-induced tunneling (FIT) model in SWCNT/ porphyrin composites, and we find that the differences in log P values of the substituents in porphyrins might contribute to the dramatic changes in the dispersion degree of OSMs in SWCNT networks, subsequently affecting the TE properties of our materials. This work provides a basis for improving the TE performance of p-type SWCNT/OSM-based TE materials. KEYWORDS: Thermoelectric, p-Type, Single-walled carbon nanotubes, Porphyrins

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INTRODUCTION Thermoelectric materials (TE), which can hold heat and turn it into electricity, have currently drawn surging interest due to their critical roles in waste−heat recovery and environmental protection.1−3 The performance of a TE material is welldefined by the figure of merit (ZT) given by, ZT = S2σT/κ. In addition, a high Seebeck coefficient S, along with a large electric conductivity σ and a low thermal conductivity κ will contribute to a good TE material. Alternatively, the power factor (PF = S2σ) of organic materials can serve as a TE performance parameter in cases for which the thermal conductivity is difficult to evaluate.4−6 The past years have witnessed rapid growth in the types of inorganic TE materials; one of the typical examples is bismuth telluride (Bi2Te3) with a relatively high ZT value (about 1).7,8 However, drawbacks, such as its toxicity, high cost, process brittleness, and high thermal conductivity κ, impose restrictions on its practical use in the TE field.9−11 Recently, organic TE materials have attracted increasing interest owing to their salient features of low cost, structural variation, facile processability, and low thermal conductivity.1,9,12 The greatest progress has been © 2019 American Chemical Society

achieved on the production of polymer-based organic materials, some of which manifest comparable or even higher PFs than traditional inorganic TE materials.13,14 Single-walled carbon nanotubes (SWCNTs) have been tremendously utilized as TE materials owing to their unique properties, such as sp2-planar frameworks, high conductivity, low weight density, and flexibility.15,16 An additional advantage is that they possess an extremely large surface area that allows for the efficient adsorption of substances, and greatly enhances the TE performance. This enhanced performance is evidenced by an efficient charge transfer and π−π interactions through carbon nanotubes and adsorbed substances.17,18 P-type SWCNT/polymer-based TE materials have been well studied and reported.19,20 For example, SWCNT/PEDOT:PSS-based hybrid films prepared by vacuum filtration21 or spin-casting22 method displayed PFs of 105 μW m−1 K−2 and 21.1 μW m−1 K−2, respectively. Grunlan and co-workers23 further increased Received: April 26, 2019 Revised: May 18, 2019 Published: May 30, 2019 11832

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ACS Sustainable Chemistry & Engineering

Figure 1. Chemical structures of designed porphyrin-based OSMs.

the PF value to 160 μW m−1 K−2 by adding PVAc (polyvinyl acetate) to the SWCNT/PEDOT:PSS-based composite. Chen’s group24 reported that SWCNT/PANI achieved a PF value of 176 μW m−1 K−2 (Table S1). Moreover, Jang et al.25 found that SWCNT/P3HT had a large σ value (2760 S cm−1) and a high PF value of 267 μW m−1 K−2, which is regarded as good p-type SWCNT/polymer-based TE materials. In stark contrast, only a limited number of TE materials involving ptype organic small molecule (OSM) scaffolds have been developed, including Te-doped SWCNT materials (PF: 36.4 μW m−1 K−2)26 and pyrene-core OSMs with different peripheral substituents (PFmax: 113.2 μW m−1 K−2),27 which lags far behind that of polymers. To date, very few SWCNT/ OSM hybrid films manifest a p-type behavior with PF values higher than 200 μW m−1 K−2 at any evaluated temperature. Thus, more investigations and efforts are needed to develop novel p-type high performance SWCNT/OSM-based TE materials. It is commonly known that porphyrins possess unique planer structures, having substituted aromatic macrocyclic rings, which have been reported to easily engage in π−π stacking with the π-electronic surface of graphite and CNTs.28 More valuable is that this tactic does not disorder the inherent electronic structure of CNTs and, subsequently, gives birth to structurally integralnevertheless functionalCNTs.28−30 Hence, it is suggested in this study that porphyrins are potential composite forward carbon nanomaterials. Additionally, a free base porphyrin core offers flexibility for the attachment of different substitutes, which will benefit the study of the relationship between material structures and TE properties. To date, porphyrins have been widely used in optoelectronic devices, photovoltaic cells, and even molecular rotors;31−35 however, porphyrins are rarely used in the TE field.19,36−38 Lambert’s group39−41 reported the TE performance of metalloporphyrins and their great potential in the field of TE devices after coordinating the porphyrins with gold electrodes and relating to DFT calculations. Until now, only the photoelectric properties of porphyrin functionalized SWCNTs were reported;42 however, no studies have focused on the TE properties of SWCNT/free base porphyrin-based composite films, although porphyrins have been indicated to produce significant advantages in SWCNT composites. In addition, a porphyrin scaffold, especially the peripheral substituents that contribute to the TE properties, has not been studied yet. Our group has investigated a series of SWCNT-based organic TE materials.27,43−46 Here, to further improve upon our previous work and provide new structures that can be applied to the TE field, we designed and synthesized five porphyrin-based OSMs (Figure 1) as promising p-type TE

materials. The effect of the peripheral substituents in porphyrins on the performance of the composites was studied and the carrier transport model was discussed. We found that the TE performance of composite films greatly depends on the dispersion degree of OMSs in SWCNTs. Finally, SWCNT/ Por-N manifests the highest PF value of 287.2 μW m−1 K−2 at 360 K, and this is among the highest values reported to date for p-type SWCNT/OSM composites at an appropriate temperature.

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EXPERIMENTAL SECTION

General Preparation of OSMs. Five porphyrin-based OSMs were prepared according to the method reported in the literature,47,48 and the synthetic route was shown in Figure S1. Using free base Por5F as an example, pyrrole (560 μL, 4.0 mmol) and pentafluorobenzaldehyde (1568 mg, 4.0 mmol) were dissolved in 820 mL of dry dichloromethane (DCM) under an argon atmosphere, and the mixture was stirred under an argon atmosphere for another 30 min. Then BF3·O(Et)2 (1.2 mL of a 2.65 M stock solution, 2.64 mmol) as a catalyst was added slowly with vigorous stirring, and the reaction was stirred at room temperature for 1 h. After that DDQ (1.36 g, 4.0 mmol) was added as an oxidizing agent, and after 1 h of stirring at room temperature, the solvent was removed in vacuo. The crude reaction mixture was passed through a short silica column with CH2Cl2 and the solvent was removed under reduced pressure to obtain the pure product. 5,10,15,20-Tetrakis(perfluorophenyl)porphyrin (Por-5F). Yield: 16%. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.92 (8 H, s), −2.92 (2 H, NH). 19F NMR (101 MHz, CDCl3) δ: −136.47, −136.49, −136.54, −136.56, −151.161, −151.22, −151.27, −161.40. 13 C NMR (101 MHz, CDCl3) δ: 147.87, 145.29, 143.63, 141.08, 138.90, 136.39, 131.15, 115.78, 115.74, 115.58, 115.55, 115.39, 115.35, 103.69. HRMS (ESI) calculated for C44H10F20N4 [M + H]+, 975.0586; found, 975.0668. 5,10,15,20-Tetra(pyridin-4-yl)porphyrin (Por-N). Yield: 18%. 1 H NMR (400 MHz, CDCl3) δ (ppm): 9.08 (8 H, d), 8.88 (8 H, s), 8.18 (8 H, d). 13C NMR (101 MHz, CDCl3) δ: 149.74, 148.42, 129.30, 117.80, 99.99. HRMS (ESI) calculated for C40H26N8 [M + H]+, 619.2280; found, 619.2354. 5,10,15,20-Tetraphenylporphyrin (Por-TPP). Yield: 28%, 1H NMR (400 MHz, CDCl3) δ (ppm): −2.75 (2 H, NH), 7.79 (12 H, m), 8.24 (8 H, d), 8.87 (8 H, s). 13C NMR (101 MHz, CDCl3) δ: 142.20, 134.55, 131.09, 127.70, 126.66, 120.14. HRMS (ESI) calculated for C44H30N4 [M + H]+, 615.2517; found, 615.2543. 5,10,15,20-Tetrakis(3,4,5-trimethoxyphenyl)porphyrin (Por-OMe). Yield: 24%. 1H NMR (400 MHz, CDCl3) δ (ppm): −2.78 (2 H, NH), 8.96 (8 H, s), 7.47 (8 H, s), 4.19 (12 H, s), 3.97 (24 H, s). 13C NMR (101 MHz, CDCl3) δ: 151.45, 145.44, 138.01, 137.54, 134.52, 129.28, 123.49, 120.06, 112.92, 61.32, 56.41. HRMS (ESI) calculated for C56H54N4O12 [M + H]+, 975.3738; found, 975.3805. 4,4′,4′′,4′′′-(Porphyrin-5,10,15,20-tetrayl)tetraaniline (PorNH2). Yield: 6%. 1H NMR (400 MHz, DMSO) δ (ppm): −2.72 (2 H, NH), 8.89 (8 H, s), 7.87 (8 H, d), 7.02 (8 H, d), 5.57 (8 H, s). 13C 11833

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ACS Sustainable Chemistry & Engineering Table 1. Thermoelectric Parameter Summary of The Studied Materials S [μV K−1]

σ [S cm−1]

PF [μW m−1 K−2]

films

298 K

max

298 K

max

298 K

max

SWCNT/Por-N SWCNT/Por-5F SWCNT/Por-TPP SWCNT/Por-OMe SWCNT/Por-NH2 SWCNT

50.5(±0.3) 53.3(±0.4) 50.2(±2.2) 57.7(±0.2) 49.9(±0.5) 38.2(±2.5)

55.6(±1.3) 55.7(±0.7) 55.9(±0.5) 59.3(±0.6) 56.5(±1.8) 45.2(±0.5)

1005.3(±50.8) 982.4(±20.4) 909.7(±2.9) 653.7(±33.6) 549.6(±48.4) 695.5(±71.9)

1005.3 (±50.8) 982.4(±20.4) 919.7(±1.9) 653.7(±33.6) 549.6(±48.4) 757.2(±4.9)

256.1(±16.5) 279.3(±9.8) 229.3(±20.8) 217.6(±9.7) 137.1(±14.8) 100.6(±2.8)

287.2(±0.1) 285.2(±22.5) 252.1(±1.7) 225.2(±0.1) 151.5(±15.9) 154.4(±2.8)

Figure 2. (a) Energy levels of OSMs measured by CV and orbital distributions assumed by DFT calculations; the energy levels of SWCNTs were provided according to the literature.50 (b) Fitting of 1/ln σ to T according to the FIT model for the SWCNT/porphyrin composite films. NMR (101 MHz, CDCl3) δ: 145.93, 135.67, 132.78, 120.09, 113.45. HRMS (ESI) calculated for C44H34N8 [M + H]+, 675.2906; found, 675.2979. General Preparation of Composite Films. The SWCNTs (5 mg) were dispersed in anhydrous chlorobenzene (5 mL) by using a probe sonicator (JY99-IIDN, Scientz, China) in an ice−water bath for 30 min. Then five porphyrin-based p-type OSMs were added to the dispersion of SWCNTs (1 mg/mL). Finally, the mixtures were sonicated for 3 h. The composite ratios of the SWCNT/OSM composites were varied by adding different dosages of p-type OSMs. The glass substrates (15 × 15 mm) were cleaned using ultrasonication in acetone, chloroform, methanol, and ethanol, and then dried under a vacuum. Finally, the gel-like mixtures were dropped on clean glass substrates to prepare the composite films by volatilizing the solvent.27,43,44 General Information on the Instruments and Measurements. The UV−vis absorption spectra and the photoluminescence (PL) spectra of these OSMs were obtained using a Shimadzu UV2600 spectrophotometer and a Hitachi F-4600 fluorescence spectrophotometer, respectively. Cyclic voltammetry (CV) of these OSMs was measured on a CHI 660 in nitrogen-purged dichloromethane (DCM) solution of [Bu4N]PF6(0.1 M), and the electrochemical potentials were calibrated by the standard ferrocene (Fc/ Fc+). Moreover, the thermal gravimetric analysis (TGA) was evaluated on a TGA-Q55 (USA) at a heating rate of 10 °C/min under a nitrogen flow of 40 mL/min. The surface morphologies of the films were examined using scanning electron microscopy (SEM; SU 70). The Raman spectra of the composite films were recorded within the wavenumber range of 100−3000 cm−1 using a Raman spectrometer (RENIPHAW invia Raman Microscope) with an excitation wavelength of 514.5 nm. The electron binding energies were obtained from X-ray photoelectron spectroscopy (XPS; K-Alpha +, Thermo Fisher Scientific). The UV−vis diffuse reflectance spectra of porphyrins and SWCNT/porphyrin composite films was performed by UV/vis/NIR spectrophotometer (PerkinElmer lambda 950). The electrical conductivities and the Seebeck coefficients were measured in a vacuum environment using a TE parameter test system (MRS-3, JiaYiTong Company) in the temperature range of 298−420 K. The Seebeck coefficient of Ni at room temperature (16 ± 0.3 μV K−1 which complied with the literature value of 15 μV K−1) was

measured as the reference value. An average of these composite samples was tested to determine the TE performances for each SWCNT loading.27,44

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RESULTS AND DISCUSSION Thermoelectric Properties. With the aim to evaluate the TE performance of prepared SWCNT/porphyrin composites, in the first set of experiments, we measured the S and σ values and then calculated PFs of these samples as a function of SWCNT content at room temperature. As shown in Table 1, the S values of all the hybrid films were positive, indicating that p-type hole transport was dominant. When the content of carbon nanotubes increased from 1:10 to 1:2, the σ values of these composites increased rapidly (Figure S2). However, further subjoining the SWCNT content to 1:1 led to a decrease in the σ values, indicating that OSMs played a curial role in improving the σ values. When the content of small molecules was reduced, the effect of the π−π interactions between OSMs and SWCNTs was weakened (as shown by Raman spectroscopy discussed below), and this resulted in attenuation in hole transport. The highest σ was obtained by SWCNT/Por-N with a value of 1005.3 S cm−1, followed by SWCNT/Por-5F (982.4 S cm−1) and SWCNT/Por-TPP (909.7 S cm−1). However, both SWCNT/Por-OMe and SWCNT/Por-NH2 manifested unexpected low σs with values of 653.7 and 549.6 S cm−1, respectively (Table 1). Compared to σ, the S values of all the composites showed little change, therefore the power factor (PF) trend was in accordance with σ, reaching a maximum at 1:2. Obviously, SWCNT/Por-5F displayed the supreme PF value of 279.3 μW m−1 K−2 at room temperature, two times over SWCNT/Por-NH2 (137.1 μW m−1 K−2), which is an advantage of most p-type carbon nanotube-based organic hybrid films containing OSMs. In addition, the TE performance of pure SWCNTs was also measured to clarify the role OSMs played in the composites, as displayed in Table 1, and all of our materials manifest higher 11834

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ACS Sustainable Chemistry & Engineering Table 2. log P Values of the Peripheral Substituents of OSMsa

a

Note: Calculated by ACD/Percepta 14.0.0 (Build 2726).

Figure 3. Schematic representation of the interactions between the SWCNTs and the surfactants.

PFs than the pure SWCNTs with the value of 100.6 μW m−1 K−2 at room temperature, indicating that OSMs made a crucial contribution in enhancing TE performance of these hybrid films. To better understand these materials, the temperaturedependence of the TE performance of these films with the optimized composite ratio (1:2) was investigated (Figure S2d−f). As displayed in Figure S2e, all the materials manifested a slightly enhanced Ss with an increase in the temperature (RT to 340 K). However, the σ values varied in the opposite direction, which resulted from the restricted relationship between them. Therefore, the PFs first ascended and then offered a descending tendency. Overall, the topmost PF (287.2 μW m−1 K−2) was achieved by the SWCNT/Por-N composite, followed by that achieved by SWCNT/Por-5F (285.2 μW m−1 K−2), SWCNT/Por-TPP (252.1 μW m−1 K−2), SWCNT/Por-OMe (225.2 μW m−1 K−2), and SWCNT/ Por-NH2 (151.5 μW m−1 K−2). To provide insight into the temperature dependence and the carrier transport mechanism in SWCNT/porphyrin networks, we fitted data of 1/ln σ to T according to previous studies.49 The well linear fitting of 1/ln σ to T in Figure 2b indicated that the carrier transport behavior of our materials obeys the FIT (fluctuation induced tunneling) model.49 Compared to that of the pure SWCNTs, the conductivities of OSMs is too low to evaluate. As a result, most of the carriers pass through the SWCNTs by thermally assisted electrons tunneling at the intertube junctions, which is the dominant section for conductivity. In addition, the carrier transport behavior was discussed with a comparison of energy levels between pure SWCNTs and OSMs. The energy levels of these porphyrinbased OSMs were investigated using density functional theory (DFT) calculations on the basis of B3LYP with 6-31G* (Table S2). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of these OSM semiconductors are evenly distributed on the central core of the porphyrin rings. As shown in Figure 2a, the HOMO energy level of pure SWCNT is −5.05 eV (determined by

cyclic voltammetry)50 and the HOMOs of these OSMs were measured to be Por-N (−5.78 eV), Por-5F (−5.52 eV), PorTPP (−4.90 eV), Por-OMe (−5.10 eV), and Por-NH2 (−4.91 eV) by cyclic voltammetry (CV). Different peripheral substituents in porphyrin modulate the HOMO energy levels, from which electro-donating porphyrins exhibit higher HOMOs and narrower band gaps related to electro-withdrawing porphyrins. On the basis of a previous study, the organic semiconductors with HOMO energy levels closer to the Fermi level (EF) or HOMO of SWCNTs are favorable for hole transport.51 The HOMOs of electro-donating porphyrins approach the Fermi level (EF) and the HOMOs of SWCNTs. However, the σ values of their hybrid films are much lower than that of electro-withdrawing porphyrin-based composite films. This result implies that the energy levels of OSMs do not play a key role in the variation of σ values. TE properties of SWCNT-based materials are strongly affected by the type of coated dispersants and the degree of dispersion of carbon nanotubes, which play a key role in exploring the variation of the TE performance of these hybrid films. In detail, we believe that the hydrophobic constants52 (log P) of peripheral substituents in porphyrin except for PorN might contribute to the different σ values, which are shown in Table 2. Carbon nanotubes are hydrophobic functional materials as well.53,54 On the basis of the principle of “like dissolves like”, Por-5F with pentafluorobenzene groups (log P = 2.46) has superior hydrophobic interactions toward carbon nanotubes, and contributes to the well dispersed surface of carbon nanotubes.55 These characteristics in turn strengthen the π−π interactions between Por-5F and SWCNTs and result in the outstanding σ. Conversely, as illustrated in Figure 3, PorNH2 with aminobenzene groups containing the lowest LogP value (1.25) displays the least binding ability with the carbon nanotubes, which might contribute to its lower σ value. However, even though pyridinium porphyrin (Por-N) is relatively hydrophilic, the pyridine-like N (double-bonded N) has been reported to promote the formation of closely interlinked morphologies in carbon nanotubes,56,57 and it was 11835

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(766.91 cm2/(V s)) were about 1.5 times more than that of SWCNT/Por-OMe (508.32 cm2/(V s)) and two times over that of SWCNT/Por-NH2(324.63 cm2/(V s)), which exceedingly coincides with their corresponding high σ values. Our investigations revealed that the introduction of hydrophobic groups in porphyrin backbone had a dramatic impact on the carrier mobilites of SWCNT-based TE materials, which is essential to electric conductivity. In addition, the hydrophobic porphyrins strengthened the π−π interactions between OSMs and SWCNTs, thereby promoting the transport of holes from SWCNTs to the OSMs.58,61

also thought that pyridine-like N could form hydrogen bonding with SWCNT networks (approved by XPS spectra), which might benefit the superhigh σ of SWCNT/Por-N-based composites. The excellent TE performance of SWCNT/Por5F is not only due to the hydrophobic properties of Por-5F mentioned above but also attributed to the fluorine-containing substances, which have been reported to work as a strong acceptor to modify the graphene surface. No defects and clustering were observed58 which made a contribution to the outstanding TE performance of the SWCNT/Por-5F. In a word, the OSMs here affect the dispersion of carbon nanotubes, which will lead to different numbers of tube− tube junctions and contribute to the formation of closely interlinked morphologies (confirmed by the SEM discussed below). Further systematic study of the carrier concentration and mobility was conducted through the Hall Effect to elucidate the thermoelectric data above. According to eq 1,59 the Seebeck coefficient (S) of the material is strongly dependent on the carrier concentration (n), the effective mass of the carrier (m*) and the temperature (T). Therefore, the Seebeck coefficient can be improved by reducing the carrier concentration. On the contrary, according to eq 2, a high carrier concentration (n) and mobility (μ) are prerequisites for high conductivity (σ).60 However, the carrier concentration and mobility of the material do not always vary synchronously. Our materials manifest similar carrier concentrations (Table 3), and thus lead to similar Ss. Yet, the carrier mobility values

S=

film name SWCNT/Por-N SWCNT/Por-5F SWCNT/Por-TPP SWCNT/Por-OCH3 SWCNT/Por-NH2

carrier mobility (cm2/(V s))

electric conductivity (S cm−1)

× × × × ×

752.13 766.91 654.37 508.32 324.63

1005.3 982.4 909.7 653.7 549.6

3.12 2.75 3.02 2.83 3.16

1019 1019 1019 1019 1019

2

(1)

σ = neμ

(2)

Photophysical Properties and Raman Spectroscopy. The UV−vis diffuse reflectance spectra of these films were investigated and they were shown in Figure S4. In comparison with the individual spectra of SWCNT, the characteristic peaks of porphyrins in corresponding composite films (with hybrid ratio of 1:2) are absent, suggesting that OSMs successfully loaded onto SWCNTs. In addition, as shown in Figure S5, changes can be observed after the treatment with SWCNTs in contrast to the UV−vis diffuse reflectance spectra of OMSs, revealing the π−π interactions between OSMs and carbon nanotubes. To elucidate the intermolecular interaction of porphyrin OSMs with SWCNTs, Raman spectra of these films were obtained. In Figure 4, the peaks at approximately 1347 and 1590 cm−1 are characteristic D and G bands of the SWCNTs, respectively.62 Importantly, it was noted that some obvious G band red shifts can be clearly distinguished in the SWCNT/ porphyrin-based composites compared to in the pure SWCNTs. These results are due to the π−π conjugated interactions between porphyrin molecules and carbon nanotubes, which is well in accord with reported p-type SWCNT/ OSM hybrid films.27 In addition, more pronounced redshifts were observed for the SWCNT/porphyrin-based composites with a mass ratio of 1:2 than a mass ratio of 1:1 (Figure S6). Therefore, it can be concluded that a much stronger π−π interaction existed in the film with a composite ratio of 1:2. As a result, all the films manifested the best TE performance at a

Table 3. Carrier Concentrations and Carrier Mobilities of the SWCNT/OSM Composite Films with an Optimized Composite Ratio of 1:2 carrier concentration (cm−3)

i π y3 m*T jjj zzz 2 3eh k 3n {

8π 2KB2

of these hybrid films are quite different. The carrier mobilities of SWCNT/Por-N (752.13 cm2/(V s)) and SWCNT/Por-5F

Figure 4. (a) Raman spectra and (b) shifted G bands of the SWCNT/porphyrin composite films with the optimized ratio of 1:2. 11836

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ACS Sustainable Chemistry & Engineering ratio of 1:2. Furthermore, the D/G ratios of all the films were similar to pure SWCNTs, indicating no structural defects were induced by OSMs in the SWCNTs.27,43 X-ray Photoelectron Spectroscopy (XPS) Analysis. As shown in Figure 5, the XPS patterns of pure SWCNTs and

pyridine group in Por-N and SWCNTs, and thus promote to the formation of uniformly and randomly distributed surface morphology in SWCNT/Por-N networks. All the results are consistent with their excellent TE properties. SEM Studies. The morphologies of the SWCNT/ porphyrin composites were directly demonstrated by SEM. The carbon nanotubes dispersed with Por-5F (Figure 6d) and Por-N (Figure 6e) have smaller diameter bundles and lower porosity than those dispersed with Por-TPP (Figure 6c), PorOMe (Figure 6b), and Por-NH2(Figure 6a), resulting in a large number of tube−tube junctions and a small contact resistance, which is one of the main reasons for their high electrical conductivities.64 In addition, for the SWCNT/Por-5F and SWCNT/Por-N composites, a well dispersed surface and uniform coating of the OSMs toward the carbon nanotubes can be clearly observed with an optimized mass ratio of 1:2, benefiting the formation of continually conductive networks. Such networks build a bridge for strong π−π conjugated interactions between OSMs and SWCNTs, acting as channels for carrier transport, thus leading to the greatly enhanced σ values of these films.27 However, for SWCNT/Por-TPP, SWCNT/Por-OMe, and SWCNT/Por-NH2 hybrid films (especially for SWCNT/Por-NH2), less homogeneous surfaces were investigated (Figure 6), which resulted in unexpected σ values. Furthermore, with increasing content of OSMs (mass ratios of SWCNT/OSM 1:5 and 1:10), serious aggregation behaviors were observed for the SWCNT/Por-5F and SWCNT/Por-N hybrid films (Figures S10 and S11), which is believed to be related to their poor TE performance at these composite ratios. Thermogravimetry Analysis (TGA). To investigate the thermal stability of these hybrid films, TGA measurements were conducted. As displayed in Figure 7, the TGA curves showed that the Td (defined as the 5% weight loss) values of SWCNT/Por-5F, SWCNT/Por-N, and SWCNT/Por-N were 453, 415, and 395 °C, respectively, which were higher than those of SWCNT/Por-OMe (267 °C) and SWCNT/Por-NH2 (201 °C). Two electron-withdrawing porphyrins-based SWCNT composites revealed excellent thermal stability with

Figure 5. From the top to the bottom, the XPS spectra of pure SWCNT, SWCNT/Por-TPP, SWCNT/Por-N, SWCNT/Por-5F, SWCNT/Por-OMe, and SWCNT/Por-NH2.

SWCNT/porphyrin composites were investigated. Different from pure SWCNTs, the SWCNT/porphyrin composites showed peaks at about 400 eV, and these were assigned to the N (1s) in porphyrin molecules, confirming that the composite process was successful. An obvious upshifted F (1s) peak at approximately 690 eV was found for the SWCNT/Por5F film relative to the Por-5F, which indicated strong π−π interactions occurred between Por-5F and SWCNTs (Figure S7). Moreover, compared with the nitrogen peak of Por-N and its composite SWCNT/Por-N, the N(1s) core apparently shifted to higher energy (Figure S8a), suggesting that there may exist a strong hydrogen bonding interaction63 through the

Figure 6. SEM images of (a) SWCNT/Por-NH2, (b) SWCNT/Por-OMe, (c) SWCNT/Por-TPP, (d) SWCNT/Por-5F, and (e) SWCNT/Por-N composite films with an optimized mass ratio of 1:2. 11837

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ACS Sustainable Chemistry & Engineering

*Tel.: 0755-26985047. E-mail: [email protected]. ORCID

Lei Wang: 0000-0002-2313-2095 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project No51773118), Shenzhen Science and Technology Research Grant (No. JCYJ20170818093417096, and JCYJ20180305125204446), and Shenzhen University (No. 2018040 and No. 2016003). We gratefully acknowledge support from the Instrumental Analysis Center of Shenzhen University (Xili Campus).

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Figure 7. TGA curves of the SWCNT/porphyrin composites.

about 50% of the weight left even at a high temperature of 600 °C. Overall, all of our materials manifested high thermal stability, which indicates that they would be promising candidates for further applications in the TE device field.

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CONCLUSION In summary, a novel strategy that tailors the structure−TE property relationships by varying the peripheral substituents in porphyrin is developed. Hydrophobic porphyrin-based hybrid films were found to manifest great enhancement in both σ and PF values, of which SWCNT/Por-5F manifested the highest PF value of 279.3 μW m−1 K−2 at room temperature, and this is one of the highest values that have been reported for SWCNT/OSM-based p-type TE materials. The carrier transport behavior of these materials follows the FIT model. The corresponding reasons for the remarkable enhanced σ values of SWCNT/Por-5F and SWCNT/Por-N composites other than SWCNT/Por-OMe and SWCNT/Por-NH2 were discussed based on the hydrophobic effect and morphologies (larger number of junctions and lower porosity). Moreover, the TGA results showed that our materials had high thermal stability. Our investigations revealed that the introduction of hydrophobic groups to porphyrin backbones dramatically influence the carrier mobility by affecting the dispersion of carbon nanotubes, which is critical for the improvement of σ values. The insights provided by this study open the door for the promotion of organic semiconductors with high σ values to the TE field and serve to accelerate the development of organic TE materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02337. Synthetic scheme, TE performance, UV−vis diffuse reflectance spectra, Raman spectra, SEM images, 1H NMR, 13C NMR, and MS spectra of these OSMs (PDF)

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DOI: 10.1021/acssuschemeng.9b02337 ACS Sustainable Chem. Eng. 2019, 7, 11832−11840