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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Copper-Phenylacetylide Nanobelt/Single-Walled Carbon Nanotube Composites: Mechanochromic Luminescence Phenomenon and Thermoelectric Performance Ni Feng,‡,∥ Caiyan Gao,§,∥ Cun-Yue Guo,*,‡ and Guangming Chen*,†,§ †

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R. China ‡ School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: We report flexible films of organic−inorganic thermoelectric (TE) composites based on organometallic coordination compound [copper-phenylacetylide (PhC2Cu)] nanobelts and single-walled carbon nanotubes (SWCNTs). Interestingly, an unusual mechanochromic luminescence phenomenon from bright green to dark red is clearly observed after grinding the PhC2Cu crystalline nanobelts. The PhC2Cu/SWCNT composites display high mechanical flexibility and excellent TE performance. The maximum power factor at room temperature can reach as high as 200.2 ± 10.9 μW m−1 K−2. The present study opens an avenue to fabricate novel organic−inorganic TE composite materials using organometallic coordination compounds such as PhC2Cu. KEYWORDS: thermoelectric, mechanochromic luminescence, organometallic coordination compound, composites, flexible films contribute to the energy filtering effect, resulting in simultaneous improvements in the electrical conductivity and the Seebeck coefficient.23−27 We have synthesized a series of TE composite films of CNTs with poly-Schiff base,28 poly(3,4ethylenedioxythiophene) (PEDOT),29,30 and polypyrrole (PPy).31−33 In addition, n-type TE materials using CNTs modified by polymer34 and small organic molecules35−37 have also attracted much current interest. For example, aminosubstituted rylene diimide derivative/single-walled CNT (SWCNT) composites reach a high power factor (room temperature) of 135 ± 14 μW m−1 K−2 and an output power of 3.3 μW for the corresponding device at a temperature difference of 50 °C.36 Organometallic coordination compound is a promising candidate for TE applications because of its molecular structure versatility and morphology controllability.38,39 For instance, Zhu et al. reported the TE properties of copper 7,7,8,8tetracyano-p-quinodimethane (CuTCNQ) nanocrystals and CuTCNQ nanorod arrays, and the highest power factor was

1. INTRODUCTION Thermoelectric (TE) materials are promising green energy materials with versatile applications in harvesting waste or lowquality heat, local cooling, wearable electronics, and so forth.1−4 In the recent decades, organic and organic−inorganic composite TE materials have received increasing attention because of various advantages, such as light weight, flexibility, high adjustability in molecular structure, and environmentally benign nature.5−19 Because of the low thermal conductivities (κ usually between 0.1 and 0.5 W m−1 K−1) and the difficulty to quantitatively measure the corresponding values along the film in-plane direction, their TE performance is always evaluated by the power factor (S2σ) rather than the figure of merit (ZT = S2σT/κ), where S, σ, and T are the Seebeck coefficient or thermopower, electrical conductivity, and absolute temperature, respectively.20−22 Benefiting from the advantages of both components, organic−inorganic composites are an effective way to achieve enhanced TE properties. Carbon nanotubes (CNTs) with a typical one-dimensional nanostructure can serve as the template for in situ polymerization of conjugated polymers. The large interfacial surface areas and the strong interfacial π−π interactions between organic molecules or polymers and CNTs © XXXX American Chemical Society

Received: December 17, 2017 Accepted: January 24, 2018

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DOI: 10.1021/acsami.7b19167 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Morphological and structural characterizations for the PhC2Cu powder samples before (PhC2Cu-0) and after (PhC2Cu-1) mechanical grinding in a pestle. (A) Comparison of color changes of the solid-state mechanochromic luminescence phenomenon under (a) ambient light and (b) irradiation by UV light with a wavelength of 365 nm; (B) XRD patterns of PhC2Cu-0 and PhC2Cu-1; (C) FESEM images of (a) PhC2Cu-0 and (b) PhC2Cu-1; (D) UV−vis absorption spectra; (E) solid-state emission spectra with the excitation wavelength at 365 nm.

2.5 μW m−1 K−2.39 Because of the low electrical conductivity, a few TE materials based on organometallic coordination compounds have been reported so far. In this work, we present the synthesis and TE performance of flexible films for novel composites of the organometallic coordination compound, copper-phenylacetylide (PhC2Cu), and SWCNTs. Interestingly, an unusual mechanochromic luminescent phenomenon is clearly observed for the PhC2Cu nanobelt, changing from bright yellow to light brown yellow after mechanical grinding.

nylon membrane (pore diameter: 0.22 μm) to attain the composite film. Finally, a flexible free-standing film was obtained after drying for 8 h under vacuum at 50 °C. 2.5. Structural and Morphological Characterizations. The element contents of carbon and hydrogen for the organometallic coordination compounds (PhC2Cu-0 and PhC2Cu-1) were determined using an elemental analyzer (FLASH EA1112). Ultraviolet− visible (UV−vis) spectra of PhC2Cu-0 and PhC2Cu-1 were recorded by a UV−vis spectrophotometer (UV-2600). Fluorescence emission spectra were recorded using a fluorescence spectrophotometer (F4500). The morphologies of the pure compound and the composite films were directly observed with a field-emission scanning electron microscope (Hitachi S-4800). Powder X-ray diffraction (XRD) measurements were carried out using a Rigaku D/mar 2400 diffractometer with Cu Kα radiation at a scanning rate of 5° min−1. 2.6. Thermoelectric Performance Measurements. The Seebeck coefficients for the film samples were measured using a commercialized Seebeck coefficient measuring system, thin-film thermoelectric parameter test system (MRS-3RT, Wuhan Joule Yacht Science & Technology Co., Ltd). During the measurements, a quasi-steady-state mode was adopted. The electrical conductivities for the composite films were measured by a Keithley 2000 multimeter (Keithley Instruments Inc., USA). At least five samples were measured, and the average value was used.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Copper(II) chloride (99%), ethanol (A. R. grade, >99.7%), methyl alcohol (A. R. grade, >99.5%), and triethylamine (Et3N, 99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Phenyl acetylene (97%) was purchased from Shanghai Aladdin Reagent Co. Ltd. The SWCNT (diameter: 85.0%) was provided by Shenzhen Nanotech Port Co. Ltd, China. All of the chemicals were used as received without further purification. In all of the experiments, deionized water was used. 2.2. Preparation of PhC2Cu Powder Samples before Grinding (PhC2Cu-0). First, 288 mg CuCl2 was dissolved in 40 mL of CH3OH, forming a 0.05 mol L−1 solution of CuCl2 in CH3OH. Then, 1.12 mL of Et3N was added into the solution as the alkali. A blue flocculated precipitate appeared during the addition of Et3N. Subsequently, 0.44 mL of phenyl acetylene (0.1 mol L−1) was added, and the blue flocculated precipitate immediately turned green. After that, the suspension was heated and maintained at 65 °C. Subsequently, the reaction was stopped when the flocculates turned yellow. Throughout the processes, the suspension was agitated vigorously. The PhC2Cu was obtained by centrifugation and washing with deionized water and ethanol several times. Finally, the asprepared precipitate was dried at 60 °C overnight. 2.3. Preparation of PhC2Cu Powder Samples after Grinding (PhC2Cu-1). The PhC2Cu-1 was obtained by grinding the as-prepared PhC2Cu-0 powder in a quartz mortar until the powder changed its color from light yellow to saffron yellow totally. 2.4. Preparation of PhC2Cu/SWCNT Composite Films. The PhC2Cu/SWCNT composites with different weight ratios were prepared via mechanical mixing of PhC2Cu (PhC2Cu-0 and PhC2Cu-1) and SWCNTs. The typical preparation procedure is as follows: 10 mg of SWCNTs was added into 30 mL of ethanol containing a desired amount of PhC2Cu and then ultrasonically treated for 30 min. Subsequently, the resulting mixture was stirred for 10 min at room temperature. After that, the mixture was vacuum-filtered by a

3. RESULTS AND DISCUSSION 3.1. Morphological and Structural Characterizations of the PhC2Cu Powder and Mechanochromic Luminescence. Figure 1 compares the morphological and structural characterizations of the synthesized PhC2Cu-0 and PhC2Cu-1 powder samples. In Figure 1A, the photographs clearly show that the as-prepared PhC2Cu-0 powder was bright yellow under ambient light (a), whereas after the mechanically grinding process, its color deepened and changed into light brown yellow (b) for PhC2Cu-1. Moreover, an obvious color change occurred from PhC2Cu-0 (bright green) to PhC2Cu-1 (dark red) after irradiation under a UV lamp with a wavelength of 365 nm. This means that a distinct mechanochromic luminescence took place. To characterize the synthesized product and elucidate the mechanism for the unusual mechanochromic luminescence phenomenon, elemental analysis, powder XRD, field-emission scanning electron microscopy (FESEM), UV−vis absorption, B


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ACS Applied Materials & Interfaces and solid-state emission spectroscopies were employed. First, the elemental analysis data illustrated in Table S1 show that the compositions of both PhC2Cu power samples are consistent with the expected value, very well demonstrating that no chemical reaction occurred related to removal or adsorption of atmospheric molecules during the grinding process. Moreover, the crystalline structure and the sample purity are studied by XRD patterns, as shown in Figure 1B. Both curves are similar in shape, that is, with a series of sharp peaks characteristic of wellcrystallization. In other words, both PhC2Cu powder samples before and after grinding display high crystallinity. Indeed, all of the peaks are almost consistent with the crystallographic data recorded in the Cambridge Crystallographic Data Center (CCDC-242490),40 and no additional diffraction peaks resulting from the impurities can be found. These confirm that PhC2Cu crystals with high purity have been synthesized. Furthermore, most of the main crystalline plane distances of the PhC2Cu crystals increase after grinding (Figure S1 and Table S2), suggesting that the mechanochromic luminescence phenomenon may result from the local distortions in crystal packing.41 Figure 1C shows that PhC2Cu-0 (a) displays a nanobelt morphology with an average width of 30 nm and length of 10 μm. By contrast, PhC2Cu-1 reveals scalelike aggregates (b), possibly induced by the morphology damage or being crushed during grinding. Note that PhC2Cu-1 still remains with a high degree of crystallinity, suggested by the XRD pattern in Figure 1B. Finally, UV−vis absorption (Figure 1D) and solid-state emission (Figure 1E) spectra were collected to study the mechanical grinding-induced deviance of the electronic structures for the mechanochromic luminescence phenomenon. In Figure 1D, both PhC2Cu-0 and PhC2Cu-1 show a similar absorption band with the peak at around 453 nm, which may be ascribed to the metal-to-ligand charge transfer of [d(Cu) → π*(CCPh)] overlapping with the π → π* transition.42 The main deviance lies in the absorption intensity, that is, PhC2Cu-0 < PhC2Cu-1. On the other hand, the solid-state emission spectra shown in Figure 1E are distinctly different at the excited wavelength of 365 nm. PhC2Cu-0 shows a single broad emission band at λmax = 510 nm, whereas two broad emission bands at 519 and 557 nm occur for PhC2Cu-1. The band at 519 nm may be the red shift of the band at 510 nm, and the band at 557 nm is new. As confirmed by the above XRD patterns (Figure 1B), the reason may be the local distortions in crystal packing, originating from the mechanical grinding. In addition, the morphology changes significantly from nanobelt to agglomerate, as displayed in Figure 1C. A similar mechanochromic luminescence has already been found because of the changes of the crystalline structure.41 Therefore, we conclude that crystalline nanobelts of PhC2Cu with high purity have been achieved, and an unusual mechanochromic luminescence phenomenon (bright green → dark red) occurred because of the crystal distortions and crystal morphological change. 3.2. Morphological and Structural Characterizations of the PhC2Cu/SWCNT Composites. Then, the composites of PhC2Cu-0/SWCNT and PhC2Cu-1/SWCNT were prepared by a convenient solution mixing procedure, aided by ultrasonication. Figure 2A shows that both composite films are dark green and highly flexible and can be easily bent to a high degree or rolled up without any breakage or damage. The mechanical feature is much superior to the conventional fragile inorganic TE materials, enabling their versatile applications in complex environments. Indeed, there is a slight difference in color, that

Figure 2. Morphological and structural characterizations of the PhC2Cu/SWCNT composites at the PhC2Cu content of 50 wt %. (A) Flexible films of (a) PhC2Cu-0/SWCNT and (b) PhC2Cu-1/ SWCNT composites; (B) XRD patterns of PhC2Cu-0/SWCNT and PhC2Cu-1/SWCNT composites; (C) FESEM images of (a) PhC2Cu0/SWCNT and (b) PhC2Cu-1/SWCNT composite films.

is, PhC2Cu-1 is darker than PhC2Cu-0. The XRD patterns shown in Figure 2B are almost the same, that is, a series of sharp peaks characteristic of the crystalline PhC2Cu can be observed. This suggests that PhC2Cu particles dispersed in the PhC2Cu/SWCNT composites preserve their high crystalline behaviors. Moreover, the dispersion morphology is observed by FESEM images (Figure 2C). The two constituents including the PhC2Cu-0 nanobelts and the SWCNTs are dispersed very well in the composite (a). By contrast, large agglomerates of PhC2Cu-1 can be observed in the PhC2Cu-1/SWCNT composite (b). Thus, the PhC2Cu-0 nanobelts are much more easily dispersed in SWCNTs than PhC2Cu-1, and thus the interfacial area in the PhC2Cu-0/SWCNT composite is much higher than that in the PhC2Cu-1/SWCNT composite. 3.3. TE Performance at Room Temperature for the Flexible Films of PhC2Cu/SWCNT Composites. The TE performance at room temperature is shown in Figure 3. In this work, the TE performance was measured for the composites with the PhC2Cu contents from 10 to 90 wt % because the Seebeck coefficients for the pure PhC2Cu samples are difficult to obtain by common measurements because of the low electrical conductivities. In Figure 3A, the electrical conductivities of both composite films decrease monotonously with the increased PhC2Cu content. According to the seriesC


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connected two-component mixture model, the electrical conductivities of the PhC2Cu/SWCNT composite can be calculated as follows: σ−1 = X1σ1−1 + X2σ2−1, where X is the volume fraction of the component.43 Because the inherent electrical conductivity of PhC2Cu-0 or PhC2Cu-1 is extremely low, the composite electrical conductivities are determined by the contribution of the pure SWCNTs, with the electrical conductivity of 802.6 ± 29.2 S cm−1. As a consequence, with increased PhC2Cu content, both composites display reduced electrical conductivities. On the other hand, the electrical conductivities of the PhC2Cu-1/SWCNT composites are higher than those of PhC2Cu-0/SWCNT for the same SWCNT contents. The main reason may be the morphology difference as revealed in Figure 2C, where the interfacial contact surface area for the PhC2Cu-0/SWCNT composite is distinctly higher than that of the PhC 2Cu-1/SWCNT composite. Because PhC2Cu is electrically insulating, a high interfacial surface area inevitably hinders the carrier transport, thus reducing the composite electrical conductivity. For PhC2Cu contents higher than 70 wt %, the difference between the electrical conductivities of the two composites becomes little, possibly because the PhC2Cu/SWCNT interfacial areas are high enough that the difference can be almost ignored. As a whole, at the PhC2Cu mass content between 10 and 90 wt %, the electrical conductivities of the PhC2Cu-0/SWCNT composites reduce from 616.7 ± 12.0 to 38.0 ± 2.4 S cm−1, whereas the PhC2Cu-1/SWCNT composites reveal the conductivities in the range of 666.2 ± 6.7 to 45.3 ± 4.0 S cm−1. Figure 3B displays the composite Seebeck coefficients. On the one hand, the composites exhibit enhanced Seebeck coefficients (∼55 μV K−1) compared with the pure SWCNTs, 44.3 ± 0.2 μV K−1. The reason may be explained by the energy filtering effect. The interfacial energy barrier that preferentially filters the low-energy carriers at the interfaces between PhC2Cu and SWCNTs, and only high-energy carriers can transfer, thereby resulting in the increased Seebeck coefficients.44,45 On the other hand, the Seebeck coefficients of all of the PhC2Cu-0/ SWCNT and PhC2Cu-1/SWCNT composite films are almost the same, being ∼55 μV K−1. In other words, the composite Seebeck coefficients are essentially independent of the PhC2Cu morphology, dispersion state, and mass content. One reason may be that both the molecular structure and the crystalline nature are similar for PhC2Cu-0 and PhC2Cu-1 in the two composites. In-depth investigations are needed, although similar results have also been reported.46 Finally, based on the above results, the power factors for the composites are shown in Figure 3C. Owing to the nearly constant Seebeck

Figure 3. (A) Electrical conductivities, (B) Seebeck coefficients, and (C) power factors at room temperature for the flexible films of PhC2Cu-0/SWCNT and PhC2Cu-1/SWCNT composites in the PhC2Cu content range of 10 to 90 wt %.

Table 1. Comparison of the Maximum Thermoelectric Performance (Power Factors and ZT) and the Corresponding Electrical Conductivity, Seebeck Coefficient, and Thermal Conductivity for Copper-Phenylacetylide/CNT and Some Typical Conducting Polymer/CNT Composites sample

electrical conductivity (S cm−1)

PhC2Cu PEDOT/SWCNT PEDOT/SWCNT PANI/SWCNT PANI/SWCNT P3HT/SWCNT PTh/MWCNT PPy/SWCNT PPy/MWCNT

666.2 ± 6.7 ∼1.35 × 103// ∼4000 769 125 ∼1000 ∼22.5 399 ± 14 39.4

Seebeck coefficient (μV K−1)

power factor (μW m−1 K−2)

∼41// 14−26 65 40 ∼29 ∼26.8 22.2 ± 0.1 24.4

200.2 ± 10.9 160// ∼140 176 20 95 ± 12 ∼1.62 19.7 ± 0.8 2.2 D

thermal conductivity (W m−1 K−1)

0.4−0.7 0.43 ∼1.5 0.19 ± 0.05 ∼0.76

ZT

refs

∼0.03 0.12 4 × 10−3 >0.01 8.71 × 10−4

this work 47 48 43 49 50 14 31 51



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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21402209, 51573190) for the financial support. G.C. acknowledges the Youth Innovation Promotion Association, Chinese Academy of Sciences (no. 2012024).

coefficients, the power factors behave very similar to the electrical conductivities in Figure 3A. The power factors decrease with the PhC2Cu mass content for the two composites, and at a low content of less than 70 wt %, the power factors of the PhC2Cu-1/SWCNT composites are higher than those of the PhC2Cu-0/SWCNT composites at the same contents. The highest power factors for the PhC2Cu-0/ SWCNT and PhC2Cu-1/SWCNT composite films reached as high as 182.9 ± 5.6 and 200.2 ± 10.9 μW m−1 K−2, respectively. For comparison, some typical examples of conducting polymer/CNT TE composites reported in the literature are illustrated in Table 1. The main constituents and the TE performance are included.

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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/acsami.7b19167. Elemental analysis data and XRD spectra and data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.-Y.G.). *E-mail: [email protected] (G.C.). ORCID

Guangming Chen: 0000-0002-9848-9101 Author Contributions ∥

REFERENCES

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4. CONCLUSIONS In summary, we report flexible films of novel organic−inorganic TE composites based on the organometallic coordination compound of PhC2Cu and SWCNTs. An unusual interesting mechanchromic luminescence phenomenon from bright green to dark red was clearly observed for PhC2Cu under a UV lamp irradiation (365 nm), possibly due to the local distortions in crystal packing and the crystal morphological change. Flexible composite films were prepared by a convenient solution mixing process. PhC2Cu nanobelts were homogeneously dispersed in the PhC2Cu-0/SWCNT composites, whereas obvious aggregates of PhC2Cu could be observed in the PhC2Cu-1/SWCNT composites. The electrical conductivities of both the composites reduce monotonically with the increased PhC2Cu content, and the maximum is 666.2 ± 6.7 S cm−1. In addition, the PhC2Cu-1/SWCNT composites afford higher electrical conductivities than the corresponding PhC2Cu-0/SWCNT composites for the same PhC2Cu contents. In sharp contrast, the Seebeck coefficients of the PhC2Cu/SWCNT composites are nearly independent of the composite morphology, dispersion state, and mass content, being approximately 55 μV K−1. As a consequence, the composite power factors behave similar to the electrical conductivities. The maximum power factor of the PhC2Cu/SWCNT composites reaches as high as 200.2 ± 10.9 μW m−1 K−2. Therefore, we conclude that composites based on organometallic coordination compounds such as PhC2Cu are promising in the future exploration of novel materials with high TE performance. The high flexibility of the composite films may greatly widen their applications in complex environments such as wearable electronics and e-skins.

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N.F. and C.G. contributed equally.

Notes

The authors declare no competing financial interest. E


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