Ruisi Chen, Junhui Tang, Yajie Yan, and Ziqi Liang*

2000288 (1 of 7) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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CommuniCation

Solvent-Mediated n-Type Doping of SWCNTs to Achieve Superior Thermoelectric Power Factor

Ruisi Chen, Junhui Tang, Yajie Yan, and Ziqi Liang*

DOI: 10.1002/admt.202000288

either enabling efficient n-type doping or optimizing the property of the material. The commercialized SWCNTs usually feature p-type σ, which derives from the ambient oxygen doping during synthesis and storage as well as the hydroxyl and car-boxyl functional groups on the surface.[18] Considerable efforts have been dedicated to effectively and rapidly dope SWCNTs from p- to n-type while improving their air stability. For instance, the Yu group deco-rated SWCNTs with polyetherimide (PEI), which was physically adsorbed on the sur-face of SWCNTs via a thermally activated process.[19] Notably, the n-type doping process by employing PEI is time-con-suming, which required dozens of hours. Meanwhile, the σ decreased as a result of an inclusion of insulating PEI. Therefore, they further introduced a reducing agent, NaBH4, into the system, which simultane-ously raised both S and σ.[20] Similarly, the

Xie team dropped PEI dopant solution atop the SWCNT films and achieved a promising power factor (PF) of 1500 µW m−1 K−2 with excellent ambient stability.[21] In another work, Wu and co-workers treated SWCNTs with another reducing agent CaH2 after n-type small-molecule diethylenetriamine doping and obtained a maximum PF of 27.7 µW m−1 K−2.[22] Recently, the commonly used surfactant, cetyltrimethylammonium bromide (CTAB), was introduced to n-type dope SWCNTs, which deliv-ered a high PF of 185.7 µW m−1 K−2.[23] However, these studies failed to unravel the influences of solvents on the dispersion of SWCNTs as well as the molecular interaction between SWCNTs and dopants in solution, both of which are critical to determine the charge transfer and doping efficiency. Therefore, it remains as a crux to establish an effective n-doping approach to judi-cious selection of dispersion solvent and achieve superior TE properties in SWCNTs.

In this work, we for the first time exploit cost-effec-tive tetrabutylammonium bromide (TBAB) as an efficient n-type dopant for SWCNTs. Flexible n-doped SWCNT films are obtained from the facile conversion from p-type precur-sors, yielding intrinsically high σ. Importantly, it was found that the dispersion medium and dopant concentration strongly affect the TE performance of as-prepared thin films. There-fore, a set of comparative experiments were conducted among samples prepared in both inert and ambient atmosphere with different dispersion media and various dopant loadings to unveil the mechanism of n-type doping for SWCNTs. Based on the findings, the as-prepared optimal SWCNT films display a

Carbon nanotubes (CNTs) have been demonstrated as promising candidates for thermoelectrics (TEs) owing to their excellent electrical and mechanical properties. Here a new dopant to obtain flexible n-type CNT-based TE films from solution is reported. By doping single-walled carbon nanotubes (SWCNTs) with tetrabutylammonium bromide (TBAB), p-type nanotubes can be successfully converted to n-type form, which is confirmed by both Raman and X-ray photoemission spectroscopy (XPS) spectra. Furthermore, the influences of doping concentration and dispersion medium on TE properties are investigated. The optimal n-type SWCNT films fabricated in N,N-dimethylformamide (DMF) solvent exhibit an impressive power factor of 339 µW m−1 K−2 and excellent air stability by less than 10% variation of Seebeck coefficient during 192 h at room temperature without encapsulation, both of which are among the hitherto highest reported. This work sheds light on the interaction between TBAB molecules and SWCNTs in different dispersion media and offers a guideline for efficient n-doping of SWCNT based TE materials.

R. Chen, J. Tang, Y. Yan, Prof. Z. LiangDepartment of Materials ScienceFudan UniversityShanghai 200433, ChinaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.202000288.

Thermoelectric (TE) technology has been considered as an effective, silent, and clean means for energy harvesting by converting dissipated heat from human bodies or other power consuming systems into electrical energy, wherein flexible and light-weight TE materials are of significant prospects.[1–4] One can improve the performance of TE materials by optimizing their mutually coupled characteristics of electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ), which collectively contribute to the dimensionless figure of merit (zT) in the form of S2σT/κ, where T is the absolute temperature.[5–11] Recent years have seen the tremendous development of organic and hybrid TE materials because of their cost-effectiveness, low-toxicity, and light weight in comparison to their inorganic counterparts, thereby holding broad prospect for wearable TE devices.[12–15] Among them, single-walled carbon nanotubes (SWCNTs) stand out owing to their excellent electrical and mechanical properties.[16,17] Given the lack of high-performance and air-stable n-type SWCNTs that need to be compatible with the existing p-type analogs, further enhancement may focus on

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maximum PF of 339 µW m−1 K−2 coupled with excellent sta-bility by less than 10% variation of Seebeck coefficient during the test period of 192 h in the air without encapsulation.

TBAB is an inexpensive and ambient stable cationic sur-factant with certain solubility in all kinds of polar solvents. Figure 1a displays the distribution of TBAB molecules and SWCNTs, suggesting the doping reaction mechanism of the two components. The structure of TBAB is also shown Figure 1a, in which the relatively short alkyl chains may pro-mote carrier transport when compared to the commonly adopted polymers such as PEI.[26] According to the previous studies, small molecules with similar structure were also used to dope SWCNTs or fullerenes, which proposed that the n-type doping is attributed to the charge transfer from the halogen anions to SWCNTs.[23–25] For TBAB, the Br anions react with the SWCNTs under ultrasonication by donating electrons to the nanotubes, generating Br2, which are subsequently

rinsed through vacuum filtration or evaporate during the drying process, while the remaining quaternary ammonium cations are adsorbed on the SWCNTs for charge neutraliza-tion. The TBAB-doped SWCNT films were fabricated by mixing SWCNTs with TBAB in N,N-dimethylformamide (DMF), eth-anol (EtOH), and deionized water (DI-H2O), respectively, fol-lowed by a sequence of ultrasonication, vacuum filtration and drying. In this study, we prepared a variety of samples with dif-ferent mass ratios of SWCNTs and TBAB ranging from 10:3, 10:5, 10:7, 10:10, 10:20 to 10:50. The as-prepared freestanding TBAB-doped SWCNT film was cut into a rectangular shape as shown in Figure 1b, which could be easily folded. The bending radius was measured as less than 1.5 mm by rolling the thin film with a length of 3.5 cm onto tweezers, indicative of its outstanding mechanical flexibility. In short, through a simple solution mixing procedure, the freestanding and flexible TBAB-doped n-type SWCNT films are acquired.

To characterize the topographical features of the SWCNT thin films, field-emission scanning electron microscopy (FE-SEM) imaging was conducted for the samples prepared in DMF, EtOH, and DI-H2O, which are denoted as D-CNTs, E-CNTs, and W-CNTs, respectively. The results are shown in Figure 2a–c, indicating that the structures of nanotube bundles in all the samples remain intact, which facilitate charge trans-port within SWCNTs. Nevertheless, the D-CNTs and E-CNTs exhibit more homogeneous distribution without agglomera-tion in comparison to W-CNTs, which can be attributed to the undesirable dispersion of SWCNTs in water. In order to inves-tigate the influence of solvents on the dispersion of SWCNTs as well as the final TE performance, the relevant parameters of the three media are summarized in Table S1 in the Supporting Information. Given the hydrophobic nature of SWCNTs, which

Figure 1. a) A proposed scheme illustrative of the distribution of TBAB in SWCNTs. b) The flexibility of the TBAB-doped SWCNT thin film. The inset shows the measurement of the bending radius (<1.5 mm).

Figure 2. SEM images of TBAB-doped SWCNT films as prepared in a) DMF, b) EtOH, and c) DI-H2O, respectively. d) A proposed schematic illustration of the doping mechanism of TBAB for SWCNTs in different solvents.





results from the small polarity of carbon structure and lack of hydrogen bond formation, the dispersity deteriorates as the solvent polarity increases from EtOH (4.3), DMF (6.4) to DI-H2O (10.2). Upon the introduction of TBAB surfactant, a large amount of foam was produced in water during the soni-cating process and the stronger polarity in DI-H2O renders a more orientation alignment of surfactant molecules, which stabilizes and leaves the foam around the SWCNT clusters. Consequently, the SWCNTs tend to aggregate and float on the surface of water, thus weakening the effect of ultrasonication, displaying undesirable morphology in Figure 2c. In contrast, SWCNTs can be homogeneously and spontaneously dispersed in DMF and EtOH, as shown in Figure S1a–c (Supporting Information). Additionally, thin film obtained from DMF has fewer pores, suggesting higher density and better carrier trans-port. Such solvent-dependent dispersion remarkably influences the doping process as well, which is illustrated in Figure 2d. By comparing the physical properties of the three media in Table S1 (Supporting Information), it can be concluded that the doping efficiency is mainly attributed to the polarity and surface tension. The effect of polarity is discussed above as the resultant aggregation of CNT cluster leads to inadequate contact with dopants. Moreover, the relatively larger surface tension of H2O significantly hinders the access to CNTs as a result of the poor wettability, thus attenuating the doping effect. Moreover, the generated Br2 is detected by the faded pH-indicator paper after ultrasonicating, which confirms the reaction mechanism between dopants and SWCNTs (Figure S1d, Supporting Information).

To explore the n-doping mechanism of TBAB, the samples were further characterized by Raman measurements. Figure 3 shows Raman spectra of undoped SWCNTs and TBAB-doped SWCNT films prepared in DMF, EtOH, and DI-H2O, respec-tively. The D bands are negligible compared to the G bands, suggesting that the doping process does not introduce excessive defects to SWCNTs in all three solvents, which consequently yields intrinsic high σ of the n-type films. Meanwhile, the TBAB doping shifts the G bands to higher wavenumbers, which

implies an enhancement of bond energy for sp2 hybridized C atoms. As discussed above, it can be assumed that the electrons provided by the Br ion enter the π-conjugated structure of the carbon rings, thus increasing the electron cloud density on the C−C bonds, which directly verifies the successful n-type doping by TBAB.

Furthermore, X-ray photoelectron spectroscopy (XPS) characterization was employed to elucidate the interaction between TBAB molecules and SWCNTs. As discussed above, the tetrabutylamine cations tend to residue on the surface of SWCNTs to maintain the electric neutrality after the doping process, thus the N atom contents correlate positively with the doping levels to some extent. Figure 4a shows the comparison between the XPS spectra of undoped CNTs and TBAB-doped CNT samples using the above-mentioned three different solvents. Both W-CNTs and undoped SWCNTs show neg-ligible N content in contrast to those prepared in DMF and EtOH solvents. This results from the fact that SWCNTs are not well-dispersed in water as indicated in Figure S1a (Sup-porting Information), which leads to inadequate n-doping. In addition, there is almost no Br element shown in the spectra (Figure S2a, Supporting Information) at low dopant contents, thus confirming the molecule interaction mechanism of CNTs and TBAB. The detailed element scanning is also given in Figure 4a, from which the chemical environment of C atoms can be inferred after peaks fitting and identification process. Since the peaks of −CH2−CH2− and C (CNT) are undistin-guishable, other peaks that represent the oxidation state of C are analyzed in Figure 4b. The spectral peaks of C1s for dif-ferent terminated groups present downward shifts upon the TBAB doping. The notable shifts originate from the fact that some C atoms in CNTs obtain electrons from Br− in TBAB. The outer electron density of C atoms is therefore increased, thus enhancing the shielding effect over the nucleus and decreasing the binding energy (Eb) of the inner electrons. Moreover, the successive decline of all carbon related peaks towards lower Eb from DI-H2O, EtOH to DMF implies increasing doping level, which agrees well with the results

Figure 3. Raman spectra of n-type CNT films prepared in DMF, EtOH, and DI-H2O, respectively. The spectra on the left and right panel show the D band and G band, respectively.





in Raman analysis. The full survey XPS spectra for different solvents treated sample are provided in Figure S3 (Supporting Information).

In order to evaluate the effect of doping level on the ther-moelectric parameters, we measured the electrical proper-ties of these D-CNTs, E-CNTs, and W-CNTs, respectively, with various TBAB:SWCNT mass ratios ranging from 3:10 to 50:10 as shown in Figure 5a–c. The σ of both D-CNTs and E-CNTs exhibit a sequence of positive and then negative cor-relation as the TBAB concentration increases, which reach a maximum value of 1105 S cm−1 for D-CNTs. An increase of doping level raises the carrier concentration, which aids in charge transport within the SWCNTs. However, exces-sive dopants cannot be ionized and instead will function as insulating impurities which remarkably hinder charge transport. Figure S2a (Supporting Information) indicates the increase of Br content in the samples, suggestive of the remaining TBAB molecules at a high dopant concentration. In contrast, the W-CNTs show relatively lower σ with an almost increasing dependence on the doping content. Such phenomena can be interpreted as follows. First, the infe-rior film morphology shown in Figure 1c strongly retards charge transport in W-CNTs. Second, TBAB surfactants tend to promote bubbles in water and the enrichment of TBAB molecules on the interface between gas/liquid phases dilutes the active dopants thus deteriorating the doping level. How-ever, more TBAB molecules aid in the dispersion of CNTs in water, which improves σ at a higher doping concentration. In addition, the overall differences in σ arise from the disper-

sion morphology among the three media as reflected from the SEM images in Figure 2.

Besides, all the TBAB-doped thin films show negative S values, indicative of successful n-type doping. In general, as the doping concentration increases, the S values of the samples show a decreasing trend in DMF and EtOH due to the higher doping level as supported by XPS analysis in Figure S2b,c (Supporting Information), while that of W-CNTs remain unchanged. As discussed above, the doping process in DI-H2O is suppressed by the undesirable morphology and the foam produced. The S of D-CNTs reaches an optimal value of −58.7 µV K−1, which partly results from the electrons offered by lone pairs among a large number of N atoms in DMF.[25] This is further confirmed by samples without the addition of TBAB dopant as presented in Table S2 (Supporting Informa-tion), in which DMF-treated SWCNT film indicates negative S. Notably, the doping process of E-CNTs has to be isolated from oxygen since they could not transfer to n-type when following the same preparation procedures as the other samples, which derives from the polarity-dependent oxygen dissolution in dif-ferent solvent.[34] This argument is also validated by testing in other solvents such as ethyl acetate, tetrahydrofuran, acetic acid, and acetone with similar low polarity, in which SWCNTs still preserve p-type characteristics after ultrasonication. In sum, the PF follows a sequence of D-CNT > E-CNT > W-CNT in which the highest PF reaches 338.9 µW m−1 K−2 for DMF mediated n-type SWCNTs at a ratio of SWCNT:TBAB = 10:5 wt%, which is among the highest values for the n-type doped SWCNTs as indicated in Figure 5d.

Figure 4. a) XPS spectra of n-type CNT films prepared in DMF, EtOH, and DI-H2O, respectively. b) Specific analysis of peaks that represent the oxidation state of C.





Ambient stability is another vital evaluating parameter of thermoelectric materials for practical applications. Thus, the time-dependent S and σ variations were tested to eval-uate the durability of as-prepared TBAB doped SWCNT films under room temperature and a relative humidity of 30 ± 3% without encapsulation as shown in Figure 6. Even after 190 h, the S and σ retain ≈90% and ≈70% of the initial value, respectively. Notably, the σ of the samples show an increase of ≈10% after being stored back into N2 atmosphere from air for dozens of hours, which indicates the self-healing ability of the TBAB-doped SWCNT films. Since n-type TE materials are vulnerable to the oxygen and moisture,[18] the decrease of conductivity can be partly ascribed to the H2O molecules in air, which is detrimental to charge transport by intruding into the films and staying between the SWCNTs. Hence, removing the moisture by storing the films back into a dry and inert atmosphere can recover the electrical property. For practical applications in TE generators, the resistance against oxygen and moisture can be improved via encapsulation with polyimide or polymethyl methacrylate. Our previous efforts also showed the potential of enhanced ambient stability by introducing hydrophobic organic components and anti-

oxidants to protect CNTs while further ameliorating the TE performance.

In conclusion, we have successfully demonstrated the TBAB doping for SWCNTs in solution to obtain the flexible

Figure 5. a) Seebeck coefficient, b) electrical conductivity, and c) power factor of SWCNT/TBAB films as a function of the mass ratio of SWCNT:TBAB at room temperature. d) Literature comparison of PFs for CNT- based n-type films, measured at in-plane direction at room temperature. The stars, circles, squares, and triangles symbolize vacuum filtration, drop casting, drop immersion, and other methods, respectively.[21–23,25–33]

Figure 6. Air stability for the n-type SWCNT films prepared in DMF without encapsulation.





and freestanding n-type SWCNTs thin films. The influ-ences of both dispersion medium and doping concentration on their thermoelectric properties are investigated. Upon gradual addition of TBAB, the PF first increases and subse-quently decreases; on the other hand, the TE performance of the TBAB-doped SWCNTs follows the solvent sequence of DMF > EtOH > DI-H2O. The n-type films obtained from DMF solvent exhibit the PF up to 339 µW m−1 K−2 at a ratio of SWCNT:TBAB = 10:5 wt% and display excellent air stability without encapsulation, for instance, by less than 10% vari-ation of Seebeck coefficient during the test period of 192 h, both of which are among the hitherto highest reported. This work unravels how molecular interaction between TBAB and SWCNTs are influenced by dispersion media whereby a guideline is offered for the successful n-type doping of SWCNT based TE materials.

Experimental SectionChemicals and Materials: All the chemicals used in the experiments

were purchased and used without further purification, including SWCNTs (NTP8022, Shenzhen Nanotech Port Co., Ltd), TBAB (Shanghai Reagent Aladdin Co. Ltd), EtOH (Sinopharm Chemical Reagent Co., Ltd), and DMF (J&K Scientific, Ltd.).

Fabrication of TBAB/CNT Films: First, CNTs and TBAB with mass ratios of 10:3, 10:5, 10:7, 10:10, 10:20, or 10:50 mg mg−1 were added into 10 mL dispersion medium of H2O, DMF, or EtOH. Subsequently, the mixture was treated by ultrasonic cleaner and cell disruptor at room temperature for 1 h and 15 min, respectively. After vacuum filtration using qualitative filter paper and drying at 50 °C in vacuum for 2 h, the freestanding and flexible n-type TBAB-doped CNT films were obtained.

Characterization and Measurements: Electrical properties of thin films were measured at room temperature by using a custom-built apparatus according to our previous report.[35] A four-probe technique was used to measure electrical conductivity on a multimeter (Keithley 2010) and a source meter (Keithley 2400). The Seebeck coefficient was measured by heating one resistor block while simultaneously measuring the generated temperature gradient (ΔT) and thermoelectric voltage (ΔV). Field-emission scanning electron microscopy images were acquired on JEOL JSM-6701F at an accelerating voltage of up to 30 kV. XPS analysis was conducted with an X-ray photoemission spectroscope (PHI5300), all the peaks were calibrated by C1s. And the Raman spectra were recorded using RM2000 (Renishaw).

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was sponsored by National Natural Science Foundation of China (NSFC) under Grant No. 51673044, 52011530126 and International Cooperation Project of Ministry of Science and Technology (MOST) under Grant No. 2017YFE0107800.

Conflict of InterestThe authors declare no conflict of interest.

Keywordselectrical conductivity, hybrid thermoelectrics, n-type doping, Seebeck coefficient, single-walled carbon nanotubes

Received: March 27, 2020Revised: June 17, 2020

Published online: July 23, 2020

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Ruisi Chen, Junhui Tang, Yajie Yan, and Ziqi Liang*

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