Article pubs.acs.org/JPCC
Tunable Electrical-Sensing Performance of Random-Alternating Layered Graphene/Polyaniline Nanoarchitectures Min-Sik Kim,‡ Saerona Kim,‡ Hye Jeong Kong,‡ Oh Seok Kwon,§ and Hyeonseok Yoon*,†,‡ †
School of Polymer Science and Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea Department of Polymer Engineering, Graduate School, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, South Korea § BioNanotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), 125 Gwahak-ro, Yuseong-gu, Daejeon 34141, South Korea ‡
S Supporting Information *
ABSTRACT: Nanostructured materials feature a high surface-tovolume ratio and small dimensions, which are highly beneficial for sensor applications. In this work, graphite was physically exfoliated by a conducting polymer polyaniline (PANI), which resulted in the formation of random-alternating layered graphene/PANI (G-PANI) nanoarchitectures. Resistometric sensors were assembled using a GPANI nanoarchitecture film as the transducer electrode to examine the characteristics of the G-PANI nanoarchitectures in sensor applications. The sensing performance of the electrode depended on the type of dopant employed, and more importantly, the unique geometrical composition of the nanoarchitecture gave rise to anisotropic electrical properties. A series or parallel connection-like configuration of intercalated PANI nanolayers was formed when a voltage was applied perpendicular or parallel to the stacked graphene plane. Compared with the parallel connection-like configuration, the series connection-like configuration yielded a far better sensing performance, particularly in terms of sensitivity. For the series connection-like configuration, electrochemical impedance spectroscopy analysis confirmed that the nanoarchitecture film was comprised of numerous resistance circuit elements arranged in a chain.
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INTRODUCTION In materials’ applications, particularly sensors, rationally engineering the surface and interface is of key importance.1,2 A variety of functional materials has been used to develop sensors, in which unique surface/interface properties have been realized. One of the most promising material classes for sensor applications is conducting polymers,3 which exhibit near-roomtemperature reactivity and reversibly changeable energy bandgaps, along with the general advantages of polymers, such as facile synthesis, chemical/structural diversity, and mechanical flexibility. Moreover, reducing the size of conducting polymers to the nanoscale can ameliorate the main sensing-performance parameters, such as the sensitivity and response/recovery time.4−8 However, like other polymers, conducting polymers are unstable at the nanometer scale because of the nature of covalent bonds.3,9 Thus, it is difficult to fabricate nanostructured conducting polymers and apply them practically. Conducting polymers can be combined with other nanoscale materials to form stable hybrid nanostructures.10−12 Wellestablished carbon nanomaterials such as carbon nanotubes and graphene can be employed as nanoscale templates to construct nanohybrids with conducting polymers, which often results in synergistic effects for specific applications. There are many © XXXX American Chemical Society
remarkable research examples of the fabrication of conducting polymer nanohybrids containing nanocarbon elements.13,14 Recently, considerable effort has been directed toward the utilization of graphene, rather than carbon nanotubes. While chemical vapor deposition can yield high-quality, large-area graphene on a catalytic substrate,15−17 a top-down approach such as exfoliation allows the large-scale production of graphene sheets or flakes.18 Our group investigated the intercalation of functional materials into graphite without defects, which allowed us to exploit the inherent characteristics of individually separated graphene pieces.19 Importantly, conducting polymers can be used as an intercalant, which may lead to the formation of nanohybrids with unique electrical, structural, and mechanical properties.14 There have been many reports on the use of graphene/ conducting polymer composites in sensors.2,20−22 Most of these have focused only on detecting a target species of interest. In this study, we investigate the effect of an alternating layered graphene/polyaniline (PANI) structure on the performance of the resulting material-based electrical sensor. We fabricate Received: April 12, 2016 Revised: July 28, 2016
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packed with PANI interlayers. In particular, the highmagnification SEM images reveal the rugged surface of the exfoliated graphene pieces, indicating that the graphene surface was covered by PANI. Interestingly, a nanofibrillar morphology was observed on the surface, which is a characteristic of PANI at the nanoscale.24 Elemental mapping of the sample confirmed that the nanoarchitecture consisted of carbon, nitrogen, and sulfur, which originated from graphene, PANI, and DBSA/ CSA, respectively (Figures 1c−1e). Thus, it can be considered that the G-PANI nanoarchitecture had an alternating layered graphene and PANI structure, as illustrated in Figure 1f. Raman spectroscopy indicated that the graphene was exfoliated without serious degradation (no D peak; see Figure S2 in Supporting Information). Thus, the graphene preserved its good electrical properties, such as outstanding conductivity, and served as a metal-like electrode. Because PANI was located between stacked graphene layers, it is reasonable to postulate that the G-PANI nanoarchitecture yielded a series connection-like configuration of PANI nanolayers when a voltage was applied perpendicularly to the graphene x−y plane. PANI has been widely integrated into sensor devices as a sensing material, for which several different transduction mechanisms have been used, including conductometric/ resistometric, amperometric, potentiometric, colorimetric, and gravimetric modes.25−29 The unique anisotropic structure of the G-PANI nanoarchitecture film might affect the electricalsignal transduction mechanism. To verify this hypothesis, a systematic investigation was conducted for the resistometric gas-phase sensing behavior of the nanoarchitecture film. First, a gas sensor was designed with an electrical feedthrough for the resistance measurement, as illustrated in Figure 2a and Figure 2b. The G-PANI film was inserted into the electrode holder, where the orientation of the film between the electrodes determined the series or parallel connection of the intercalated PANI nanolayers (Figure 2c). The colorless, toxic, and corrosive gas ammonia was chosen as a target analyte to examine the sensing performance of the G-PANI film. First, the potential of the G-PANI film as an electrical transducer was examined. Figures 3a and 3b show the typical electrical responses of a G-PANI electrode, where PANI was treated with a ternary dopant (CSA/DBSA/SBT) and the stacked layers of the G-PANI film were inserted parallel to the electrode faces. The real-time resistance change (ΔR = R − R0) was normalized by dividing it by the initial resistance R0. Upon cyclic exposure to 1 ppm ammonia and nitrogen streams, the resistance of the G-PANI increased with a response time of ∼40 s when the film was exposed to 1 ppm ammonia gas (Figure 3a). In addition, the intensity of the resistance change increased with the gas concentration (Figure 3b). Thus, the GPANI film demonstrated its basic ability to provide reversible, reproducible, and concentration-dependent electrical responses to the target gas. The main properties of conducting polymers are greatly affected by the type of dopant introduced into them. The overall morphology of the G-PANI film was almost independent of the dopant (see Figure S4 in Supporting Information). However, the electrical response of the G-PANI film depended on the type of dopant. Figure 3c and Figure 3d show the electrical responses of G-PANI films treated with different dopants. The combinatorial doping of PANI with two or three selected compounds yielded different response profiles in terms of the response/recovery time and sensitivity. The sorption of the target moleculeammonia in G-PANI films is a major factor determining the response profile, in which
random-alternating layered graphene/PANI (G-PANI) nanoarchitecture films using a facile physical exfoliation approach. While graphene is a two-dimensional (2D) metal-like conductor, PANI is a semiconductor. Thus, the alternating layered nanoarchitecture films are composed of numerous interfacial contacts between the 2D conductor and semiconductor, which can result in unique anisotropic properties. We provide in-depth insight into the effects of the geometrical composition of the hybridized nanomaterials on the electricalsensing performance.
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RESULTS AND DISCUSSION G-PANI nanoarchitectures were fabricated in a large quantity using a physical top-down approach, although the precise control of their geometrical composition was limited.14 Soluble PANI (conductivity ∼0.7 S cm−1) was first synthesized with different dopants, such as camphorsulfonic acid (CSA), dodecylbenzenesulfonic acid (DBSA), and D-sorbitol (SBT).23 Then, the PANI was intercalated into graphite in an organic solvent with the aid of ultrasonication. The resulting black colloidal dispersion was densely assembled on a porous membrane using vacuum filtration, and random-alternating layered G-PANI nanoarchitecture films were obtained (see Figure S1 in Supporting Information). In Figure 1a and Figure 1b scanning electron microscopy (SEM) images of G-PANI nanoarchitectures show that graphene pieces were densely
Figure 1. (a, b) SEM images of a G-PANI film treated with a ternary dopant (CSA, DBSA, and SBT). (c−e) Elemental-mapping images of the G-PANI film: (c) carbon, (d) nitrogen, and (e) sulfur. (f) Scheme describing the alternating layered structure of the G-PANI film. B
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Figure 2. (a, b) Schemes showing the resistometric sensor setup for measuring the electrical response of the G-PANI films (electrode, blue; PANI, green; graphene, black). Schemes describing (c) the series connection and parallel connection-like structures formed by the different orientations of the G-PANI film between the electrodes.
calculations. Table 1 presents the calculated interaction energies (Eint), along with the absolute energies (Eab) of the individual components. The sorption kinetics of ammonia in the PANI/dopant were estimated according to the Eint values. DFT and HF calculations both revealed that the PANI/dopant become stabilized in the order of CSA < SBT < DBSA in the presence of ammonia. The binary dopant, i.e., CSA/DBSAtreated PANI, yielded the most stable energy state in the presence of ammonia, which increased the irreversibility (a rise in the baseline) in the sensing response. On the other hand, CSA/SBT-treated PANI was estimated to have a relatively unstable energy state when coupled with ammonia, which resulted in a slow response, i.e., a continuous increase in the resistance. Lastly, the Eint of the ternary dopant, i.e., CSA/ DBSA/SBT-treated PANI (t-PANI) with ammonia, was positioned between those of the two binary dopant-treated PANI samples with ammonia, yielding the most desirable response profile with three clear response/saturation/recovery stages, as shown in Figure 3a and Figure 3b. Next, the effect of the stacked orientation of the graphene/tPANI (G-t-PANI) layers on the sensing performance was examined. Figure 4a shows the electrical-response profile of the sensing electrodes, and Figure 4b plots the R0 and ΔR values of each electrode before the normalization of the response. Two G-t-PANI electrodes whose stacked layers were parallel (selectrode) and perpendicular (p-electrode) to the electrode, respectively, yielded different electrical-response profiles in terms of the response/recovery time and sensitivity. Particularly, the response intensity of the former (electrode i) was 1 order of magnitude larger than that of the latter (electrode ii). The dependence of the response profile on the orientation of the stacked layers demonstrates the anisotropic properties of the G-PANI nanoarchitecture film. The electrical responses of two half-thickness G-t-PANI films connected to an s-electrode (iii) and a bulk t-PANI film electrode (iv), respectively, as controls, were also examined. The series connection of the two
Figure 3. Electrical responses of G-PANI films upon periodic exposure to ammonia, in which PANI was treated with different dopants: (a, b) CSA/DBSA/SBT, (c) CSA/DBSA, and (d) CSA/SBT.
noncovalent interactions between the PANI/dopant and ammonia play an important role, including π−π stacking and van der Waals forces. Density functional theory (DFT) and the Hartree−Fock (HF) method were employed to estimate the interaction of PANI/dopants with ammonia.30 The pentamer structure of PANI was employed with the molecular structures of other dopant compounds as miniature models for the Table 1. Interaction Energies (Eint) of NH3 and PANI/Dopants
Eab (hartree) method b
HF
DFTc
ensemble
pentamer/dopant
NH3
pentamer/dopant/NH3
Einta (hartree)
PANI/CSA/NH3 PANI/DBSA/NH3 PANI/SBT/NH3 PANI/CSA/NH3 PANI/DBSA/NH3 PANI/SBT/NH3
−2562.7080 −2799.8457 −2163.1591 −2577.2246 −2816.0845 −2176.5694
−55.4369 −55.4369 −55.4369 −56.5598 −56.5598 −56.5598
−2618.7219 −2855.9865 −2219.2061 −2633.6395 −2872.6238 −2233.0201
−0.5769 −0.7038 −0.6099 0.1449 0.0205 0.1091
Eint = Eab(pentamer/dopant/NH3) − Eab(pentamer/dopant) − Eab(NH3). b6-31G++(d,p) basis set was used. cB3LYP functional/6-31G++(d,p) basis set was used. a
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Figure 4. (a) Representative electrical responses of G-PANI electrodes upon periodic exposure to 1 ppm ammonia, in which the orientation of the stacked layers of the G-PANI film inserted between the electrodes was adjusted: (i) a G-t-PANI s-electrode, (ii) a G-t-PANI p-electrode; two control electrodes; (iii) two half-thickness G-t-PANI films connected to an s-electrode and (iv) a bulk t-PANI film were tested under the same conditions. (b) Initial resistance (R0), response intensity (ΔR), and (c) signal-to-noise ratio calculated according to the electrical response of the electrodes.
half-thickness films yielded a response profile that was similar to that of the original film. In the response profile, the response intensitygiven as the normalized resistance change (ΔR/ R0)was related to the unique interior structure of the film. The half-thickness film had approximately half (∼1/2Riii0) of the initial resistance (Riii0) of the original film if it had a series connection-like interior configuration. Each of the two halfthickness films connected in series had approximately a half of the resistance change of the original film (1/2ΔRiii), which might lead to a similar response intensity. The R0 and ΔR values of electrode i were measured and determined to be similar to those of electrode iii. Another controlthe bulk filmproduced a nonideal response profile with a high initial resistance (Riv0), confirming the structural advantage of the GPANI film for sensing. Figure 4c shows the signal-to-noise ratio of the electrical response of the electrodes. It is difficult to directly compare the response intensities of the electrodes because the response profiles of electrodes ii and iv deviated significantly from the ideal response profile. However, at the same exposure time, electrodes i and iii exhibited larger signalto-noise ratios than electrodes ii and iv. These results indicate that the G-PANI s-electrode had a series connection-like microstructure of PANI nanolayers embedded between graphene layers. Further tests were conducted to inspect the unique interior structure of the G-PANI nanoarchitecture films. The thickness of the film was increased 2-, 3-, and 4-fold in order to compare the electrical properties (Figure 5). The response intensity increased up to the 3-fold thickness and then decreased abruptly at the 4-fold thickness. The decreased response intensity for the 4-fold thickness G-PANI s-electrode was due to the resistance change (ΔR = R − R0) becoming saturated while the initial resistance (R0) increased in proportion to the electrode thickness. The signal-to-noise exhibited a similar trend to the resistance change. Importantly, the initial resistance increased linearly with the thickness of the film. The linear dependence of the initial resistance on the thickness, i.e., the number of stacked layers, indicates a series connection,
Figure 5. Effect of the number of stacked layers in the G-t-PANI selectrode on the electrical response, in which the thickness (T) of the G-PANI film was increased 2-, 3-, and 4-fold: (a) initial resistance (R0), response intensity (ΔR), and (b) signal-to-noise ratio calculated according to the electrical response of the electrodes (see Figure S7 in Supporting Information).
confirming that the G-PANI s-electrode was composed of alternating stacked layers of graphene and PANI parallel to the electrode. Lastly, the G-t-PANI s-electrode was characterized using electrochemical impedance spectroscopy (EIS) (Figure 6). The Nyquist plots of all the G-PANI s-electrodes had almost the same shape: an arc depressed below the x-axis (ZIm = 0) at high frequencies by inductance. As illustrated in Figure 6a, the Nyquist plot was fitted to an equivalent circuit composed of resistance (R), inductance (L), and capacitance (C).31 Specifically, the G-PANI s-electrode was modeled using an electrical circuit, in which (L + (R/C)) circuit elements were D
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PANI-based resistometric sensors. In particular, it appeared that the response intensity of the G-t-PANI s-electrode was 1 order of magnitude larger than that of the G-t-PANI pelectrode. The G-PANI nanoarchitecture was easily prepared via the physical exfoliation of graphite with the conducting polymer PANI. Therefore, this type of combination is applicable to various kinds of functional materials, which may yield unique, unprecedented properties.
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METHODS Materials. Graphite flakes, aniline (≥99.5%), ammonium persulfate (≥98.0%), sulfuric acid (H2SO4, 95.0−98.0%), 1methyl-2-pyrrolindone (NMP, 99.5%), (1S)-(+)-10-CSA, 4DBSA, and SBT (≥98%) were purchased from Sigma-Aldrich. A hydrochloric acid (fuming, 37%) solution was obtained from Merck KGaA, and an ammonium hydroxide (NH4OH) solution (25%) was purchased from OCI Co. Ltd. G-PANI Nanoarchitectures. G-PANI nanoarchitectures were prepared through a sonication-mediated physical exfoliation route with careful modification of the previous methods. First, PANIs as the intercalant was synthesized as follows. Aniline was dissolved in 75 mL of a 1 M HCl solution, and ammonium persulfate (APS) was dissolved in 50 mL of the 1 M HCl solution. The APS/HCl solution was added to the aniline/HCl solution, and then the mixture was stirred for 1.5 h at 3 °C to induce oxidative polymerization. The resulting PANI was retrieved via a filtration process using excess water and then resuspended in 125 mL of 0.1 M NH4OH. Then, the PANI underwent the same retrieval process and was dried in vacuum at room temperature. Postsynthetic doping was performed by blending the PANI (3.3 mol %, repeat unit Mw 91.13 g mol−1) with CSA (1.3 mol %) and/or DBSA (0.4 mol %) in NMP for 15 min. The blended solution was subject to sonication, followed by centrifugation. As a result, a stable PANI solution was obtained, to which SBT (0.7 mol %) was optionally added for further doping. Next, G-PANI nanoarchitectures were prepared as follows. First, 50 mg of graphite were ultrasonicated in 50 mL of NMP for 30 min. Subsequently, 3 g of the asprepared PANI solution were added to the NMP solution, and the mixture was ultrasonicated again for 30 min. Finally, the GPANI solution was filtered to remove the solvent and extra impurities, during which G-PANI layers were sequentially stacked to form a film. Resistometric Sensors. A homemade sensor setup was devised to conduct resistometric sensing of ammonia gas, as illustrated in Figure 2. A G-PANI film with dimensions of 5 mm × 5 mm × 0.5 mm was fixed with a nickel electrode-embedded framework (made by three-dimensional printing) in a glass tube (diameter 8 mm). Nitrogen was used as a carrier gas, and gas flows were supplied to the electrode at a flow rate of 3 L min−1. Mass-flow controllers were used to control the concentration of the gases. The real-time resistance change of the transducer electrode was monitored at room temperature using a Keithley 2636A sourcemeter, in which the applied voltage was 0.1 V. The response time was defined as the time taken for the normalized resistance change to reach 90% of the total resistance change. Characterization. SEM was conducted using a JEOL JSM7500F microscope equipped with an energy dispersive X-ray spectrometer. EIS measurements were performed using Metrohm Autolab B.V. PGSTA101. The frequency range was 100 mHz to 1 MHz, with an applied alternating-current potential of 0.01 V.
Figure 6. EIS analysis of the G-t-PANI s-electrode with respect to the thickness (T) of the G-PANI film: (a) Nyquist plots (inset: scheme of the electrode, describing the distributed impedance components with the equivalent circuit) and component values calculated by fitting the Nyquist plot to the circuit model; (b) Bode modulus plots. The dots are data points and the lines are fitting curves.
connected in series. The best-fit component values are given in the table in Figure 6a. The resistance exceeded a few tens of ohms, whereas the inductance and capacitance were as small as a few microhenrys and nanofarads, respectively. Thus, it can be considered that the resistance was the most significant component in the equivalent circuit. Importantly, the resistance was linearly proportional to the thickness of the film, where the slope was the resistance at the thickness T. In Figure 6b, the Bode-modulus plots show the dependence of the resistance on the thickness of the film over a wide range of frequencies.32 At frequencies less than 104 Hz, the resistance in the Bode plot was commensurate with that in the table of Figure 6a. These EIS data indicate that increasing the thickness of the film resulted in an increase in the stacked number of graphene and PANI layers, i.e., the number of (L + (R/C)) circuit elements arranged in a chain.
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CONCLUSIONS Random-alternating layered G-PANI nanoarchitecture films exhibited unique anisotropic electrical properties. The films consisted of a series combination of numerous resistors along the z-axis of stacked graphene planes. Importantly, the anisotropic properties modulated the performance of GE
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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/acs.jpcc.6b03705. Overall preparation scheme, Raman spectroscopy data, SEM images and ultraviolet−visible spectra of G-PANIs treated with different dopants, HRTEM image, and electrical response curves (PDF)
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AUTHOR INFORMATION
Corresponding Author
*(H.Y.) E-mail:
[email protected]. Fax: +82-62-530-1779. Telephone: +82-62-530-1778. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (NRF-2015R1A2A2A01007166) and by the KRIBB Initiative Research Program.
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