Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric

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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 17957−17962

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Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films Travis G. Novak,† Hosun Shin,‡ Jungmo Kim,† Kisun Kim,† Ashraful Azam,† Chien Viet Nguyen,‡ Sun Hwa Park,‡ Jae Yong Song,*,‡ and Seokwoo Jeon*,† †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ‡ Korea Research Institute of Standards and Science (KRISS), Yuseong, 305-340 Daejeon, Korea S Supporting Information *

ABSTRACT: In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m−1 K−2) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m−1 K−2). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices. KEYWORDS: black phosphorus, exfoliation, organic thermoelectric, PEDOT:PSS, doping

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INTRODUCTION Black phosphorus (BP) has attracted much research attention in recent years because of many interesting properties, including a small direct band gap (0.3 eV in bulk form),1 high charge mobility (up to 1000 cm2 V−1 s−1),2,3 nonlinear optical properties,4 and excellent thermoelectric potential.5 Although the material was first discovered over 100 years ago,6 interest in BP has surged in recent years,7 as exfoliation techniques have allowed the production of two-dimensional (2D) flakes and dispersion into various solvents. Similar to other 2D materials, BP can be exfoliated using mechanical means (the scotch-tape method) or solvent exfoliation, with the latter being more practical for producing scalable quantities of BP. BP is typically synthesized by applying high temperatures and pressures to white phosphorus or red phosphorus (RP).8 Phosphorus itself is an abundant element0.1% of the earth’s crust9but it exists commonly as RP, an amorphous allotrope which has found use as a flame retardant in recent years because of its low cost and environmental friendliness.10 This traditional RP to BP conversion process makes high-quality BP prohibitively expensive, around 500 USD/g, 11 limiting applications which require large amounts. Therefore, finding alternative low-cost routes to synthesize BP is of great importance. © 2018 American Chemical Society

In this work, we demonstrate high-quality BP produced by simple ball-milling and separation of RP and its application to the first ever BP composite thermoelectric device. While other works have produced BP from RP,12,13 these exist as unexfoliated heterostructures, where a significant amount of unconverted RP remains, which may significantly decrease electrical conductivity. By contrast, our exfoliated flakes appear to be comparable to those produced from single-crystal BP, with no RP remaining and high crystallinity observed. Theoretical calculations have previously shown that BP has great potential as a thermoelectric material, with a predicted ZT approaching 4 for ideal doping conditions and crystal alignment at 500 K.5 Although experimental results for BP fall far short of theoretical performance, bulk BP crystals still display a high Seebeck coefficient, with Flores et al.14 finding a value of S = +335 ± 10 μV/K at room temperature. Saito et al.15 also found a high Seebeck value of S = +510 μV/K at 210 K using an iongated mechanically exfoliated single BP flake. However, to our knowledge, there have been no reports of thermoelectric experiments based on solvent-exfoliated BP, as well as no reports of BP in thermoelectric composites. Received: March 10, 2018 Accepted: May 9, 2018 Published: May 9, 2018 17957

DOI: 10.1021/acsami.8b03982 ACS Appl. Mater. Interfaces 2018, 10, 17957−17962

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic showing the conversion of RP to BP and the application to the PEDOT:PSS composite film (P + sBP). (b) XRD comparison of the RBP mixture to pure RP and pure BP showing the emergence of the characteristic BP peaks. (c) TGA analysis with oxidation onset points of pure BP and pure RP labeled.

Figure 2. (a) Evolution of the Raman spectra from raw RP to separated RP, eventually showing characteristic BP peaks. The location of the A1g peak is indicated. (b) SEM image of sBP flakes. (c) AFM image of sBP flakes with a line scan indicating the flake height. (d) TEM image with insets showing the measured lattice parameter and fast Fourier transform pattern.

For fabricating thermoelectric composite films with BP, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) was selected as a complementary material for several reasons. PEDOT:PSS has attracted a great deal of research attention in recent years as a thermoelectric material for many desirable attributes, including low-cost processing, high electrical conductivity, and low intrinsic thermal conductivity.16−22 In addition, its p-type character and dispersibility in organic solvents perfectly suit the combination with exfoliated BP flakes, and previous works have successfully demonstrated the compatibility of BP with organic semiconductors in composite films.23−25

The successful fabrication of these PEDOT:PSS composites with fully separated BP flakes resulted in a room-temperature power factor of 36.2 μW m−1 K−2, which is a 109% improvement over the power factor of the pristine PEDOT:PSS film prepared through the same method (17.3 μW m−1 K−2). In addition, the thermal conductivity of the composite film was found to be similar to the reference. The mechanism of enhancement was shown to be a simultaneous carrier mobility increase and carrier concentration decrease, which allowed for enhancement of both the Seebeck coefficient and electrical conductivity, a rare and desirable result in organic thermoelectric composites. This result shows not only the potential of this novel thermoelectric film but also the future 17958

DOI: 10.1021/acsami.8b03982 ACS Appl. Mater. Interfaces 2018, 10, 17957−17962

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Optical images showing an increase of sBP flakes. (b) SEM cross-section images of the composite film, with the inset showing energydispersive X-ray spectroscopy (EDS) maps of an isolated flake (scale bar = 3 μm). (c) Raman spectra of reference 2 wt % film showing mostly PEDOT:PSS peaks but also characteristic BP peaks (inset). (d) XPS data comparing surface and interior states of the composite film.

possibilities for these BP flakes to be used in other energy, electronic, or optical applications.

ball-milling, characteristic peaks corresponding to the BP A1g, B2g, and A2g modes appear but are convoluted with the RP spectra. In particular, the peak at 355.5 cm−1 is clearly a mixture of the characteristic BP and RP peaks. After sonication, the mixture shows no appreciable change, ruling out size effects on peak shifts, but after centrifugation, we see the BP peaks appear without convolution. Interestingly, the location of the A1g peak in both this survey average and a selected flake is equal to that of the exfoliated BP produced from a bulk crystal, as seen in Figure S1. Figure S2 shows the color change of the materials (RP, RBP, and sBP) in solution, with the sBP’s pale yellow color being similar to the previously reported dispersions made from pure BP flakes.26,27 Flake morphology was also found to be similar to previously reported solvent-exfoliated BP. The scanning electron microscopy (SEM) image in Figure 2b shows that most flakes are 100s of nm in lateral size and well-defined. A large flake ∼40 nm thick is seen using atomic force microscopy (AFM) (Figure 2c), with the flat basal plane indicating minimal oxidation.28 The transmission electron microscopy (TEM) image in Figure 2d shows the lattice spacing of 4.4 Å, corresponding to BP’s (010) plane.29 These smooth surfaces and the well-defined crystal structure of the flakes are further indication of the quality of this sBP. Characterization of sBP + PEDOT:PSS Composite Films. After successfully exfoliating the mixture into highquality BP flakes, we applied them to a PEDOT:PSS composite film in four different loading levels: 0.5, 1, 2, and 4 wt % sBP. The approximate wt % was confirmed by XPS in Figure S3. The morphology changes with the addition of sBP flakes can be seen clearly in the optical images of Figure 3a. Because PEDOT:PSS is highly transparent, it is mostly featureless in optical microscopy. However, after the sBP addition, the flakes can be clearly seen to be shiny and well-dispersed in the film. AFM measurements (Figure S4) indicate an increase in surface roughness with increasing sBP content, a result previously reported in PEDOT:PSS composites with nanofillers.30

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RESULTS AND DISCUSSION Low-Cost BP Synthesis by Ball-Milling and Separation. Figure 1a shows the schematic of the process. BP is first produced from RP by a high-energy ball-milling process. The partial transformation can be seen in the X-ray diffraction (XRD) and thermogravimetric analysis (TGA) in Figure 1b,c. Because RP lacks a clear crystal structure, no distinct peaks appear in XRD, but after ball-milling, peaks corresponding to BP appear clearly. In TGA, the allotropes can be distinguished by their different oxidation onset points,12 where we find pure BP and RP to begin oxidative weight gain at 250 and 370 °C, respectively. As expected, the profile for the red/black phosphorus (RBP) mixture appears as a combination of these two phases, indicating that the sample has been transformed but still contains significant RP. After sonicating the mixture in N-methyl-2-pyrrolidone (NMP), we find that the remaining RP is removed by centrifugation. Because BP is a layered material with only weak van der Waals forces providing interlayer binding, it can be easily exfoliated in a variety of organic solvents.26,27 However, because of the amorphous crystal structure of RP, it is likely that it is not susceptible to shear exfoliation in the same way. This creates BP suspended in solution, while the remaining RP is mostly precipitated because of its molecule-like structure. The flakes produced by this method appear to be of similar size and quality to shear exfoliated flakes prepared from bulk BP crystals. The Raman spectra in Figure 2a show the evolution of the material from raw RP powder to separated BP (designated sBP) using averaged spectra over 100 points for each sample. Because RP and BP are composed of identical elements and bond types, it is impossible to distinguish the allotropes via characterization techniques such as X-ray fluorescence or X-ray photoelectron spectroscopy (XPS). However, we can find a difference in Raman spectra; for raw RP, only the peak at 343.5 cm−1 is clearly identifiable. After 17959

DOI: 10.1021/acsami.8b03982 ACS Appl. Mater. Interfaces 2018, 10, 17957−17962

Research Article

ACS Applied Materials & Interfaces An SEM cross-section image is seen in Figure 3b, with the overlapping P and S signals in the inset EDS map of an isolated particle indicating a uniform distribution of the sBP flakes in the PEDOT:PSS matrix. The Raman spectrum of the composite film in Figure 3c shows primarily the PEDOT:PSS peaks but also the characteristic BP peaks as well. These data are strong evidence for the uniform dispersion and structural preservation of the BP in the film. In addition, we note the appearance of a subtle shoulder peak at ∼1530 cm−1 and a shift of the peak at ∼1245 cm−1, as well as a subtle shift of the PEDOT peak in XPS (Figure S3), features that have been previously reported to indicate reduction of PEDOT:PSS through charge transfer in a Bi2Te3 composite.31 The oxidation state of the films was assessed through XPS in Figure 3d. It is known that pure BP flakes oxidize quickly upon exposure to atmospheric oxygen or oxygen dissolved in water through dissociative chemisorption of O2 on the flake surface, creating a rapid degradation of electrical properties.28,32 Interestingly, the surface of the film shows a significant single oxide peak, assigned to P2O5, but after etching for 60 s, the oxide peak is much smaller and appears to contain several intermediate states. These have been previously reported33 to be P−O−P and O−PO and are found at 131.4 and 132.8 eV, respectively. The finding of these intermediate states and the overall smaller oxide peak after etching indicates that the PEDOT:PSS may protect the sBP against oxidation. This is likely due to PEDOT:PSS effectively coating the surface of the sBP flakes and acting as a physical barrier to O2 diffusion. Previous reports on BP flakes have asserted similar oxidation protection through surface passivation when using materials such as hexagonal boron nitride,34 Al2O3,28 and perylene3,4,9,10-tetracarboxylic dianhydride.35 Enhancement of Thermoelectric Properties. The thermoelectric properties of the composite films were evaluated near room temperature, and the results are summarized in Figure 4a. We observe an increase in electrical conductivity in the sBP content range of 0.5−2 wt %, with the conductivity of the 2 wt % film (1446 S/cm) representing a 47% improvement over the reference (982 S/cm). We note that an increase of electrical conductivity in PEDOT:PSS films has been previously reported with the BP addition and attributed to an increase in hole mobility due to reduced electron−phonon coupling.23 Unlike many other high-Seebeck filler materials used in organic thermoelectric composites, BP has naturally high carrier mobility, particularly hole mobility,3 which is undoubtedly beneficial because of the p-type character of PEDOT:PSS. However, increasing the sBP content to 4 wt % results in a decrease of conductivity, a common result for PEDOT:PSS composites with high loading levels.18,30,36 In addition, it is observed that the Seebeck coefficient of the composite films continuously increases with increasing sBP content, from 13.3 μV/K in the reference film to 16.6 μV/K in the 4 wt % film, a result that is likely due to the high intrinsic Seebeck coefficient of BP and a potential energy-filtering effect that is commonly observed at interfaces.18,19,37 Because of the tradeoff between conductivity and the Seebeck coefficient, the best power factor was achieved with the 2 wt % sample, with the peak value of 36.2 μW m−1 K−2 representing a 109% increase over the reference film. This power factor is greater than many other reports of similar PEDOT and PEDOT:PSS composites near RT incorporating materials such as PbTe,38 Bi2S3,21 Bi2Te3,30 and ZnO,39 a remarkable result given that the films were not posttreated with

Figure 4. (a) Thermoelectric parameters of the films with different loading levels of sBP. (b) Carrier concentration and mobility data, with band diagram showing the hole transfer from PEDOT:PSS to sBP.

any organic solvents or salts, which are known to significantly improve the conductivity of PEDOT:PSS.16,17,22 Although the power factor of the samples fall short of reported values for PEDOT:PSS composites with WS2,36 Te nanotubes,18 and carbon nanotubes,40 we note that these designs incorporated an extremely high wt % of high aspect ratio material (50% or more), which undoubtedly increases in the in-plane thermal conductivity. By contrast, our sBP flakes achieve a peak power factor at only 2 wt %, and time-dependent thermoreflectance (TDTR) results showed very little change in through-plane thermal conductivity between the composite and reference samples (Figure S6). Given the low loading level of the flakes, we expect that the thermal conductivity is mostly dominated by the continuous PEDOT:PSS matrix rather than the nanofillers, and the preservation of the intrinsically low thermal conductivity of PEDOT:PSS may be attributable to the presence of phonon scattering at interfaces, as has been previously asserted in other PEDOT:PSS composites.18,30 On the basis of TDTR results, we calculated a peak ZT value of 0.042 at room temperature; however, because of the thermal conductivity of PEODT:PSS being several times higher inplane,40 it is likely that this overestimates the actual in-plane ZT value. We also note that the films were stable in the measurement range of 40−120 °C. The complete thermoelectric data as a function of temperature can be seen in Figure S7. Mechanism of Enhancement. To elucidate the effects of sBP flakes in the PEDOT:PSS matrix, we performed Hall measurements to evaluate the carrier concentration and mobility (Figure 4b). It is known that the Seebeck coefficient of thermoelectric materials is generally inversely proportional to the carrier concentration, while electrical conductivity is proportional, making a simultaneous increase of both thermoelectric parameters rare. In our composite films, we observe an increase of charge mobility with a decrease in the carrier concentration as sBP content increases (Figure 4b), a trend 17960

DOI: 10.1021/acsami.8b03982 ACS Appl. Mater. Interfaces 2018, 10, 17957−17962

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ACS Applied Materials & Interfaces

solution (0.5 mL) in DMSO and 0.25 mL of PEDOT:PSS in water were stirred briefly before being drop-cast on a 1 in. air-plasma-treated glass substrate and annealed in air for 15 min at 150 °C. The pristine PEDOT:PSS was prepared in the same way but with 0.5 mL of pure DMSO instead of the sBP solution. Thermoelectric Measurement. The Seebeck coefficient, electrical conductivity, and thermoelectric power factor were measured with a Linseis LSR-3 with the sample kept in a helium atmosphere. The heating rate was set to 1 °C/min with a 3.5 °C gradient across the sample, which measured 1.5 cm long and 0.5 cm wide. Four measurements were taken for each temperature point and averaged, with the error bar representing the standard deviation among these measurements. The through-plane thermal conductivity of the film was measured using a TDTR method (Linseis TF-LFA) in the temperature range of 313−393 K and evaluated using heat conduction and fitting methodology as described in the literature.41,42 The carrier concentration and mobility measurements were performed using a Hall measurement system (Ecopia HMS-3000) at room temperature.

that has previously been observed in Bi2Te3/PEDOT:PSS composites at similarly low loading levels of Bi2Te3 and attributed to the energy-filtering effect.30 In this effect, low energy carriers are effectively scattered at the matrix/nanofiller interfaces, which increases the average thermal energy of the remaining carriers and as a result increases the Seebeck coefficient.18,19,37 Because of the initial misalignment of the BP bands with the PEDOT:PSS Fermi level, it is known that holes can transfer to the BP flakes,23 which may explain the decrease in the carrier concentration. In addition, we note that while the carrier concentration appears to continuously decrease with increasing sBP wt %, mobility tends to saturate at higher loading levels because of stronger carrier scattering offsetting the high intrinsic mobility of the filler material.30,36 This explains why the 4 wt % sample shows reduced electrical conductivity.

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CONCLUSIONS In conclusion, we have synthesized thermoelectric composite films based on PEDOT:PSS and low-cost exfoliated BP. The novel BP/RP separation strategy produced BP that was comparable to that prepared from bulk crystals, with high crystallinity and minimal oxidation observed. The thermoelectric composite film with 2 wt % sBP flakes showed a power factor of 36.2 μW m−1 K−2, a 109% increase over the pure PEDOT:PSS reference film. We attributed the increase to a simultaneous mobility increase and a carrier concentration decrease in the film, which allowed for both the electrical conductivity and Seebeck coefficient to be increased, a rare combination in organic thermoelectric composites. There are many ways in which this system could be further optimized, such as tuning the sBP flake size, thickness, or doping level, as well as further optimization of the PEDOT:PSS. These results show the potential for this sBP material in low-cost, nontoxic, and lightweight thermoelectric applications.

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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.8b03982. Additional figures showing Raman spectra, optical images of solutions, XPS data, AFM characterization, backscattered-electron images, and additional thermoelectric measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.J.). *E-mail: [email protected] (J.Y.S.). ORCID

Seokwoo Jeon: 0000-0002-5338-0671 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017M3D1A1039558) and supported by the Low-dimensional Materials Genome Development funded by the Korea Research Institute of Standards and Science (KRISS-201717011082).

METHODS

Synthesis of RBP. The RBP powder was synthesized through a high-energy ball-milling approach with a Spex 8000D Mixer/Mill machine. RP (8 g, 99.99%, Sigma-Aldrich) was put in a tungsten carbide vial with two tungsten carbide balls (11.2 mm in diameter). The weight ratio of balls to powder was 3:1, and the vial filling volume was about 9%. To avoid formation of white phosphorus due to the high-energy milling, the RP powder was milled for 2 h with the breaks of 10 min after each 30 min milling. As the ball-milling progressed, the color of the RP powder gradually changed from red to black, indicating the phase transformation of RP to BP. All processes were carried out in an argon-filled glovebox to minimize the degradation of RBP by oxygen and water. Exfoliation and Separation. The RBP powder (150 mg) was dispersed in 150 mL of anhydrous NMP (Aldrich) and sonicated in an ice bath for 4 h using a Sonics Vibra-cell VCX 750 tip sonicator set to 70% amplitude with a 55/5 s on/off pulse pattern. The solution was then centrifuged at 2000 rpm for 3 h to precipitate the unexfoliated BP and RP, leaving the well-dispersed supernatant sBP to be collected. sBP Characterization. Raman spectra for Figure 2a were collected by measuring 100 evenly spaced points on a 10 × 10 μm grid drop-cast on an SiO2 substrate and averaging the resulting spectra. SEM images of the flakes were obtained by vacuum filtration of the sBP solution over a 0.2 μm anodisc. For TEM images, the sBP solutions were dried on a carbon grid. Fabrication of PEDOT:PSS Composites. Prior to application in the composite film, the sBP flakes were centrifuged at 10 000 rpm, and the precipitate was redispersed in various amounts of dimethyl sulfoxide (DMSO) to create different concentrations of solution. sBP

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REFERENCES

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DOI: 10.1021/acsami.8b03982 ACS Appl. Mater. Interfaces 2018, 10, 17957−17962