Gas Sensors
Transparent Conducting Films of Hierarchically Nanostructured Polyaniline Networks on Flexible Substrates for High-Performance Gas Sensors Shouli Bai, Chaozheng Sun, Pengbo Wan,* Cheng Wang, Ruixian Luo, Yaping Li, Junfeng Liu, and Xiaoming Sun* Transparent conducting films (TCFs), essential components for numerous transparent electronic devices, have recently attracted substantial research attention because of their potential application in displays, touch screens, solar cells, and smart windows.[1] Various transparent electronic devices have been developed on flexible substrates using diverse TCF nanomaterials including carbon nanotubes,[2] graphene,[3] and metal nanowires.[4] For example, as reported by Cui and co-workers, transparent conducting electrodes[5] and transparent lithium-ion batteries[6] have been fabricated using silver nanowire (Ag NW) and grid-structured electrodes, respectively. Wearable devices could greatly change people’s daily life because they would be conveniently mobile and could be attached to clothes or arms, and they have therefore become the trend of research nowadays.[7] However, transparent chemical gas sensor devices, portable and smart sensors with the potential for being integrated into transparent electronics, smart windows, and other transparent or invisible devices, and also for offering real-time analysis of chemical vapor analytes with only low power consumption and low operation temperature, have rarely been demonstrated. Among the various nanomaterial TCFs, metallic nanowire films, particularly TCFs of Ag NW networks, exhibit comparable sheet resistance and optical transmittance to that of ITO films. However, the stability of these films is not high, which can be exploited. For example, Ag NWs can be etched by strong oxidants, such as ammonium persulfate (APS) and ferric chloride,[8] or could be broken and transferred to Ag2O at high temperature.[9] The limitation of metallic nanowire TCFs prompts the development of effective methods for fabricating TCFs with increased lifetime and stability. Due to the instability of Ag NWs,
Prof. S. L. Bai, C. Z. Sun, Prof. P. B. Wan, C. Wang, R. X. Luo, Y. P. Li, J. F. Liu, Prof. X. M. Sun State Key Laboratory of Chemical Resource Engineering College of Science PO Box 98, Beijing University of Chemical Technology Beijing 100029, PR China E-mail: [email protected]; [email protected] DOI: 10.1002/smll.201401865 small 2014, DOI: 10.1002/smll.201401865
they could be potentially employed as excellent sacrificial template candidates to fabricate hollow or hierarchical nanostructures in strong oxidant-containing reactions. Meanwhile, conducting polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) have been widely studied as promising candidates for transparent flexible organic conductors or semiconductors in developing flexible or wearable electronics, displays, sensors, and other devices,[10] due to their outstanding electrical performance, tunable morphology, scalable manufacture, flexibility, and low cost—especially for PANI, a “classic” conducting polymer that stands out for good environmental stability and electrical conductivity.[11] However, the gas sensing performance, optical transmittance, and electrical sheet resistance of PANI films still leave room for improvement, and traditional gas sensing materials are mostly solid powders and need ceramic tubes or inter-finger probes to support them, making the fabrication of gas sensors a challenge. Recent developments in nanotechnology allow for the fabrication of various conducting polymer nanomaterials[12] (nanotubes, nanorods, nanowires, nanofibers, etc.) and composites through synthesis methods such as solid-phase template synthesis, interfacial polymerization, electrochemical synthesis, and rapid mixing methods.[13] Nanostructured conducting polymers with unique electrical properties feature high surface-to-volume ratios and small dimensions, which facilitate enhanced interaction between sensing materials and analytes for high sensitivity and enable fast adsorption/ desorption kinetics for analytes on the materials for a rapid response and recovery. Such materials have been widely used to construct various sensor devices with enhanced sensing performance compared to their bulk counterparts. For example, sensors from conducting polymers in the forms of 1D or quasi-1D confined nanostructures have been demonstrated to have significantly enhanced performance due to their high surface-to-volume ratios and small dimensions.[14] With a combination of nanostructured conducting polymers from the sacrificial Ag NW template and network structures of TCFs, the transparent and conductive films of hierarchically nanostructured conducting polymers could be potentially fabricated on flexible polyethylene terephthalate (PET) substrates for high-performance portable gas sensing at room temperature.
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Scheme 1. Schematic illustration for fabrication of transparent conducting film of hierarchically nanostructured PANI networks on flexible PET substrate by firstly coating of Ag NWs, and subsequently in situ chemical oxidative polymerization of aniline nearby the sacrificial Ag NWs template for high-performance sensor devices.
Herein, we report the fabrication of transparent conducting films of hierarchically nanostructured PANI networks on PET substrates by first Meyer-rod coating Ag NWs, and subsequently in situ chemical oxidative polymerizing aniline near sacrificial Ag NWs template. By exploiting the advantage of a superior network structure from TCFs and the instability of Ag NW, the hierarchically nanostructured PANI-containing transparent film could be obtained near the sacrificial Ag NW template of the Ag NW network TCFs. The as-prepared transparent film could be assembled into transparent chemical gas sensor devices for high-performance portable gas sensing at room temperature because of the quasi-one-dimensional confined nanostructure in the network film and a large specific surface area of hierarchically nanostructured PANI (Scheme 1). The transparent chemical gas sensor provides enhanced sensing performance and good transparency at visible wavelengths, creating opportunities for application in transparent electronic circuitry and optoelectronic devices with avenues for further functional integration.
Ag NW TCFs with flexibility, high transparency, and random network structures were fabricated by Meyer-rod coating of Ag NW inks on flexible PET substrates and drying at 120 °C in an oven, leaving a uniform, thin layer of Ag NWs on the substrate (Figure S1 and S2). The sheet resistance and optical transparency were 46 Ω/ⵧ and 85%, respectively. Ag NWs have high aspect ratios and could be etched by strong oxidants, such as APS. Therefore, Ag NW TCFs with random network structures could be potentially employed as excellent sacrificial template candidates to fabricate quasione-dimensional hierarchical nanostructures in strong oxidant-containing reactions, such as in situ polymerization of PANI chemically oxidized by APS. By dipping Ag NW TCF in aniline polymerizing solution with the concentration of aniline at 0.05 M and the ratio of [aniline]:[APS] = 1:1 for different times, different nanostructured PANI networks with variable transparency and conductivity could be achieved from the sacrificial template of Ag NW on the flexible PET substrate. Shown in Figure 1a–d are the scanning electron microscope (SEM) images of different nanostructured PANI
Figure 1. SEM images of different nanostructured PANI networks by dipping Ag NW TCF in aniline polymerizing solution with dipping time at a) 5 min, b) 10 min, c) 20 min and d) 30 min, respectively.
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small 2014, DOI: 10.1002/smll.201401865
Transparent Conducting Films of Hierarchically Nanostructured Polyaniline Networks
with dipping times of 5 min, 10 min, 20 min, and 30 min. The standard reduction potential of silver (+0.79 V) is lower than that of aniline (+1.02 V),[15] thus silver is more liable to be oxidized and etched by APS. Therefore, in the presence of APS, a distinct etching effect occurred and silver was transformed into silver ions. With the immersion time increased from 5 min to 30 min, Ag NWs of the TCFs were etched away and different nanostructured PANI were formed nearby the sacrificial Ag NW template. As shown in Figure 1a, the nanoparticle-like nanostructured PANI, with some of them connected in lines, was observed for the immersion time of 5 min. In the successive immersion of Ag NW TCF in aniline polymerizing solution, the different hierarchical nanostructures of PANI with network structures were formed (Figure 1c and d). For comparison, PET films without coatings of Ag NWs were dipped into aniline polymerizing solution for different times and particle-like PANI structures were obtained, as demonstrated in Figure S3a–d, implying that Ag NW TCFs with random network structures could be employed as excellent sacrificial templates to fabricate hierarchically nanostructured PANI networks. During the polymerization of aniline from the sacrificial template of unstable Ag NWs on the flexible PET substrate, the transparency and conductivity of the film changed simultaneously. The conductivity of the film varied from the sheet resistance of 46 Ω/ⵧ for Ag NW TCFs, to non-conducting states for 5 min and 10 min polymerization, to conducting states for 20 min (63 kΩ/ⵧ) and 30 min (25 kΩ/ⵧ) polymerization. At the same time, the optical transmittance of the corresponding films altered from 85% at 550 nm, to 69%, 67%, 65% and 69% for 5 min, 10 min, 20 min, and 30 min polymerization, respectively, as demonstrated in Figure 2. The nanostructured PANI network
Figure 2. a) Transmittance spectra of the Ag NW TCF and the various nanostructured PANI networks with immersion time of 5 min, 10 min, 20 min and 30 min, respectively. b) Photographs of transparent nanostructured PANI network films on PET substrates with the immersion time at 5 min, 10 min, 20 min and 30 min from left to right. The words ’’BUCT’’ are placed behind the films. small 2014, DOI: 10.1002/smll.201401865
TCF showed excellent transparency, ranging from 450 to 750 nm with a maximum transmittance of ∼65% (∼76% of transmittance compared to that of the original Ag NW TCF at 85%) (Figure 2a, 20 min sample). In the control experiment, different particle-like PANI with variable transparency and conductivity on PET substrates could be achieved by dipping PET in aniline polymerizing solution for 5 min, 10 min, 20 min, and 30 min (Figure S4). To confirm the formation of PANI, FTIR spectra were obtained and analyzed for particle-like pure PANI and the hierarchically nanostructured PANI networks from the sacrificial template of instable Ag NWs (Figure S5). FTIR spectra display the peak around 3000 cm−1 (C–H stretching) and the broad peak centering at 3400 cm−1 (N–H stretching) for both the particle-like pure PANI and the hierarchically nanostructured PANI, corresponding to the stretching vibrations of emeraldine PANI. The peaks at 1129 cm−1, 1298 cm−1, 1479 cm−1, 1561 cm−1 are assigned to C–H in-plane bending, C–N stretching vibration with aromatic conjugation, and C=C stretching vibration of quinoid and benzenoid rings, respectively. These results confirm the successful polymerization of PANI for both the particle-like pure PANI and the hierarchically nanostructured PANI. Due to a higher conjugation length of the hierarchically nanostructured PANI than that of particle-like pure PANI, the former has a higher integrated peak intensity of benzenoid rings against quinoid rings. UV-vis spectra for both the hierarchically nanostructured PANI network film and particle-like PANI film (20 min samples) show the characteristic bands of PANI at 354 nm for the π–π* transition of benzenoid rings and 813 nm for π–polaron transition (Figure S6). For Ag NW TCF, the peaks at 357 nm and 380 nm were observed. These results indicated the formation of PANI in both cases and the transformation of silver to silver salt in the synthesis of hierarchically nanostructured PANI networks, which is different from the reported Ag-PANI composites.[16] The characteristic peak for AgCl appears at 32.38° and characteristic peaks for Ag disappear at ∼38° and ∼44° from the X-ray diffraction (XRD) patterns of the hierarchically nanostructured PANI network film and Ag NW TCF (Figure S7a) respectively, indicating the transformation of silver to silver chloride in the synthesis of the hierarchically nanostructured PANI network film. Furthermore, the X-ray photoelectron spectroscopy (XPS) peaks of Ag 3d5/2 (368.20 eV) and Ag 3d3/2 (373.95 eV) clearly indicate the formation of Ag+ in the hierarchically nanostructured PANI network (Figure S8). We wondered if the prepared hierarchically nanostructured PANI-containing transparent films could be assembled and behave as high-performance portable gas sensors at room temperature because of the quasi-one-dimensional confined nanostructures in the network structure and the large specific surface area. In order to address this issue, the sensing property of the particle-like PANI film and the hierarchically nanostructured PANI-containing transparent film devices for ammonia (NH3) was tested. Detection of NH3 in air is of tremendous interest for environmental monitoring, workplace hazard monitoring, and homeland security because of its high toxicity, corrosivity, and volatility. It is a strong electrondonating species and has been widely investigated as a target
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Figure 3. a) The gas sensing of the hierarchically nanostructured PANI-containing transparent film and the particle-like PANI, both with polymerization times of 20 min, upon exposure to different concentrations of NH3 from 5 to 100 ppm at room temperature. b) Gas sensing selectivity test of the hierarchically nanostructured PANI-containing transparent film and the particle-like PANI film to various volatile organic gases. The gas concentration was 10 ppm. c) Sensing mechanism of NH3 interacted with acid-doped PANI.
chemical for PANI gas detection due to a dramatic decrease in conductance from changing PANI’s emeraldine salt form into its emeraldine base form upon exposure to NH3. Figure 3a displays the real-time difference in resistance sensitivity from the hierarchically nanostructured PANIcontaining transparent film and the particle-like PANI film, both with polymerization times of 20 min, upon exposure to different concentrations of NH3 from 5 to 100 ppm at room temperature. The sensitivity S is defined as the normalized resistance change, S = (Rg-R0)/R0 = ΔR/R0, where Rg is the resistance of the functional film after exposure to gas analytes and R0 is the resistance in ambient air. It can be seen that the hierarchically nanostructured PANI-containing transparent film had a higher gas sensitivity than the particle-like PANI film. The sensitivity of the hierarchically nanostructured PANI-containing transparent film increased gradually with the increase of the concentration of NH3, showing a linear relationship (Figure S9a). Even though the NH3 concentration was as low as 5 ppm, the change in resistance of the hierarchically nanostructured PANI-containing transparent film could also be observed, indicating that hierarchically nanostructured PANI-containing transparent film synthesized by this method had superior gas sensing performance and could act as an “electronic nose” in chemical detection and recognition. The gas sensitivity decreased for the hierarchically nanostructured PANI-containing transparent film from polymerization times of 20 min to 30 min, while an increase occurred for the gas sensitivity of the pure PANI films from polymerization time of 20 min to 30 min (Figure S9b). This is likely due to the different growth of PANI, resulting in a larger size of PANI grains and a larger specific surface area for the particle-like PANI films, while leading to the hierarchically nanostructured PANI-containing transparent film to be relatively smooth and with a reduced specific surface area. Meanwhile, the hierarchically nanostructured PANIcontaining transparent film also had a specific response to NH3 in comparison with other volatile organic compounds, as shown in Figure 3b. The gas sensitivity of the hierarchically nanostructured PANI-containing transparent film to 10 ppm
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NH3 was 79 times higher than the sensitifiy to ethanol, 67 times higher than for acetone, and 74 times higher than for n-propanol. The hierarchically nanostructured PANI-containing transparent film exhibited higher selectivity to NH3 compared to the particle-like PANI film (Figure 3b). The distinct sensing selectivity could be probably attributed to the synergetic contributions from the quasi-one-dimensional confined nanostructure in the network structure film, the specific acid–base reaction, and large specific surface area of hierarchically nanostructured PANI networks. In conclusion, a transparent conducting film of hierarchically nanostructured PANI networks on a PET substrate was successfully prepared by a combination of Meyer-rod coating of Ag NWs and in situ chemical oxidative polymerization of aniline. By combining the advantages of superior network structures from TCFs and the instability of Ag NW, the hierarchically nanostructured PANI-containing transparent film could be obtained nearby the sacrificial Ag NW template of the Ag NW network TCFs. The as-prepared transparent film could be assembled into a transparent, portable chemical gas sensor device with enhanced gas sensing performance at room temperature in both sensitivity and selectivity to NH3 than pure PANI film because of the quasi-one-dimensional confined nanostructure in the network structure film and the large specific surface area of nanostructured PANI. It is anticipated that this line of research can be further extended to other conducting polymers for fabricating transparent chemical gas sensors, providing superior sensing performance and transparency within visible wavelengths, and creating opportunities for being integrated into transparent electronics, automobile windshields, and other transparent devices for real-world sensor applications.
Experimental Section Materials: Aniline (distilled before use) was obtained from J&K Chemical. Ammonium persulfate (APS) and concentrated hydrochloric acid were purchased from Beijing Chemical Reagent Factory
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small 2014, DOI: 10.1002/smll.201401865
Transparent Conducting Films of Hierarchically Nanostructured Polyaniline Networks
and were used without further purification. Polyethylene terephthalate (PET, 125 µm, with two protective layers on obverse side and reverse side) film was commercially available from Hui Zhixing Company (Hangzhou, China) and Ag NW ink (5 mg/mL, dispersed in methanol) was a product of ColdStones Tech. Company (Suzhou, China). Preparation of Ag NW TCFs: The obverse protective layer of PET was firstly removed and 120 µL Ag NW ink was dropped on the edge of a 10 cm by 10 cm PET substrate. Then, a Meyer rod (0.02 mm, Pushen) was pulled over the Ag NW ink, leaving a uniform wet film. Then the reverse protective layer of Ag NW film was removed. And the film was firstly dried at room temperature and then heated at 120 °C for 30 min. Preparation of Various PANI Films and Powders: In a typical procedure, aniline monomer (1.5 mmol) was added into 1 M HCl aqueous solution (15 mL) in a beaker, which was treated with ultrasonic wave and cooled at 5 °C. Then, APS (1.5 mmol) in precooled 1 M HCl solution (15 mL) was poured into the above monomer solution, which was shaken for 20 s and left at 5 °C. After that, blank PET substrates and Ag NW TCFs were immersed in the above solution for different time, respectively. After immersion for different time, the blank PET substrates and Ag NW TCFs with polymerized PANI were taken out and rinsed with ethanol, respectively. All the films were dried at 65 °C for 1 h. The precipitates in the above reaction solution were also collected for FTIR and XRD characterization. Gas Sensing Measurement: The gas sensing measurement was performed in a WS-30A measuring system (Zhengzhou Winsen Electronics Technology, China) equipped with an 18 L chamber and two fans. The functional films (8 mm by 8 mm) were attached on the probes of voltage-testing devices with silver paint. Calculated amount of ammonium hydroxide (weight percent of ammonia was 25%) was dropped onto a hot plate in the chamber to rapidly generate 5, 10, 50, and 100 ppm NH3 in the chamber. After exposure to a certain concentration of NH3, the functional films was exposed to air by removing the chamber. To test gas sensing selectivity of the functional films, it was also carried out with ethanol, acetone, and n-propanol by the same method. General Techniques: SEM images were obtained from a Zeiss Supra 55 instrument at a voltage of 20 kV. UV-vis transmittance and absorption spectra of the samples were recorded by a Shimadzu UV-3150 UV spectrophotometer. Fourier transform infrared (FTIR) spectra were collected on a Nicolet 6700 FTIR spectrometer. XRD patterns were performed on a Shimadzu XRD-6000 X-ray diffractometer using a Cu KR radiation source. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 spectrometer. The sheet resistance was measured by four-probe tester (RST-8).
Supporting Information
Acknowledgements This work was financially supported by National Natural Science Foundation of China, the 973 Program (2011CBA00503 and 2011CB932403), the Fundamental Research Funds for the Central Universities. [1] a) S. I. Park, Y. Xiong, R. H. Kim, P. Elvikis, M. Meitl, D. H. Kim, J. Wu, J. Yoon, C. J. Yu, Z. Liu, Science 2009, 325, 977–981; b) S. Bae, H. Kim, Y. Lee, X. Xu, J. S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Nat. Nanotechnol. 2010, 5, 574–578; c) Z. Yu, L. Li, Q. Zhang, W. Hu, Q. Pei, Adv. Mater. 2011, 23, 4453–4457; d) P. C. Hsu, S. Wang, H. Wu, V. K. Narasimhan, D. Kong, H. R. Lee, Y. Cui, Nat.Commun. 2013, 4, 2522–2528; e) Y. Wang, G. Zhang, W. Xu, P. B. Wan, Z. Lu, Y. Li, Xiaoming Sun, ChemElectroChem 2014, 1, 1138; f) J. X. Hou, Y. Kuang, H. Q. Shen, H. Cao, L. Luo, J. F. Liu, P. B. Wan, B. Q. Chen, X. M. Sun, T. W. Tan, RSC Adv. 2014, 4, 11136. [2] a) H. Gu, T. M. Swager, Adv. Mater. 2008, 20, 4433–4437; b) D. Zhang, K. Ryu, X. Liu, E. Polikarpov, J. Ly, M. E. Tompson, C. Zhou, Nano Lett. 2006, 6, 1880–1886. [3] a) J. Wang, M. Liang, Y. Fang, T. Qiu, J. Zhang, L. Zhi, Adv. Mater. 2012, 24, 2874–2878; b) P. B. Wan, S. Y. Yin, L. L. Liu, Y. G. Li, Y. J. Liu, X. T. Wang, W. R. Leow, B. Ma, X. D. Chen, Small 2014, 10, 647–652. [4] a) A. R. Rathmell, B. J. Wiley, Adv.Mater. 2011, 23, 4798–4803; b) X. M. Sun, Y. D. Li, Adv. Mater. 2005, 17, 2626–2630. [5] L. Hu, H. S. Kim, J. Y. Lee, P. Peumans, Y. Cui, ACS Nano 2010, 4, 2955–2963. [6] Y. Yang, S. Jeong, L. Hu, H. Wu, S. W. Lee, Y. Cui, Proc. Natl. Acad. Sci. USA 2011, 108, 13013–13018. [7] a) P. Bonato, Eng. Med. Biol., IEEE 2010, 29, 25–36; b) M. Segev-Bar, H. Haick, ACS Nano 2013, 7, 8366–8378; c) B. Liu, D. Tan, X. Wang, D. Chen, G. Shen, Small 2013, 9, 1998–2004. [8] a) W. Wang, W. Li, R. Zhang, J. Wang, Synth. Met. 2010, 160, 2255–2259; b) W. Wang, W. Li, J. Ye, R. Zhang, J. Wang, Synth. Met. 2010, 160, 2203–2207. [9] X. Chen, Z. Guo, W. H. Xu, H. B. Yao, M. Q. Li, J. H. Liu, X. J. Huang, S. H. Yu, Adv. Funct. Mater. 2011, 21, 2049–2056. [10] a) D. Li, J. Huang, R. B. Kaner, Acc. Chem. Res. 2008, 42, 135– 145; b) J. Janata, M. Josowicz, Nat. Mater. 2003, 2, 19–24. [11] a) J. Fei, Y. Cui, X. Yan, Y. Yang, K. Wang, J. Li, ACS Nano 2009, 3, 3714–3718; b) N. R. Chiou, A. J. Epstein, Adv. Mater. 2005, 17, 1679– 1683; c) J. Jang, J. Ha, J. Cho, Adv. Mater. 2007, 19, 1772–1775; d) S. Cho, O. S. Kwon, S. A. You, J. Jang, J. Mater. Chem. A 2013, 1, 5679–5688. [12] a) H. Yoon, Nanomaterials 2013, 3, 524–549; b) N. R. Chiou, C. Lu, J. Guan, L. J. Lee, A. J. Epstein, Nat. Nanotechnol. 2007, 2, 354–357; c) W. Zou, B. Quan, K. Wang, L. Xia, J. Yao, Z. Wei, Small 2011, 7, 3287–3291. [13] a) L. Pan, L. Pu, Y. Shi, S. Song, Z. Xu, R. Zhang, Y. Zheng, Adv. Mater. 2007, 19, 461–464; b) S. H. Domingues, R. V. Salvatierra, M. M. Oliveira, A. J. Zarbin, Chem. Commun. 2011, 47, 2592–2594; c) C. Li, H. Bai, G. Shi, Chem. Soc. Rev. 2009, 38, 2397–2409; d) Z. Niu, P. Luan, Q. Shao, H. Dong, J. Li, J. Chen, D. Zhao, L. Cai, W. Zhou, X. Chen, Energy Environ. Sci. 2012, 5, 8726–8733; e) D. Li, R. B. Kaner, J. Am. Chem. Soc. 2006, 128, 968–975; f) G. Li, Y. Li, Y. Li, H. Peng, K. Chen, Macromolecules 2011, 44, 9319–9323. [14] a) O. S. Kwon, S. J. Park, J. S. Lee, E. Park, T. Kim, H. W. Park, S. A. You, H. Yoon, J. Jang, Nano lett. 2012, 12, 2797–2802; b) H. Yan, H. S. Choe, S. Nam, Y. Hu, S. Das, J. F. Klemic, J. C. Ellenbogen, C. M. Lieber, Nature 2011, 470, 240–244. [15] Y. Gao, D. Shan, F. Cao, J. Gong, X. Li, H. Ma, Z. Su, L. Qu, J. Phys. Chem. C 2009, 113, 15175–15181. [16] A. Drury, S. Chaure, M. Kröll, V. Nicolosi, N. Chaure, W. J. Blau, Chem. Mater. 2007, 19, 4252–4258.
Supporting Information is available from the Wiley Online Library or from the author.
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Transparent Conducting Films of Hierarchically Nanostructured Polyaniline Networks on Flexible Substrates for High-Performance Gas Sensors Shouli Bai, Chaozheng Sun, Pengbo Wan,* Cheng Wang, Ruixian Luo, Yaping Li, Junfeng Liu, and Xiaoming Sun* Transparent conducting films (TCFs), essential components for numerous transparent electronic devices, have recently attracted substantial research attention because of their potential application in displays, touch screens, solar cells, and smart windows.[1] Various transparent electronic devices have been developed on flexible substrates using diverse TCF nanomaterials including carbon nanotubes,[2] graphene,[3] and metal nanowires.[4] For example, as reported by Cui and co-workers, transparent conducting electrodes[5] and transparent lithium-ion batteries[6] have been fabricated using silver nanowire (Ag NW) and grid-structured electrodes, respectively. Wearable devices could greatly change people’s daily life because they would be conveniently mobile and could be attached to clothes or arms, and they have therefore become the trend of research nowadays.[7] However, transparent chemical gas sensor devices, portable and smart sensors with the potential for being integrated into transparent electronics, smart windows, and other transparent or invisible devices, and also for offering real-time analysis of chemical vapor analytes with only low power consumption and low operation temperature, have rarely been demonstrated. Among the various nanomaterial TCFs, metallic nanowire films, particularly TCFs of Ag NW networks, exhibit comparable sheet resistance and optical transmittance to that of ITO films. However, the stability of these films is not high, which can be exploited. For example, Ag NWs can be etched by strong oxidants, such as ammonium persulfate (APS) and ferric chloride,[8] or could be broken and transferred to Ag2O at high temperature.[9] The limitation of metallic nanowire TCFs prompts the development of effective methods for fabricating TCFs with increased lifetime and stability. Due to the instability of Ag NWs,
Prof. S. L. Bai, C. Z. Sun, Prof. P. B. Wan, C. Wang, R. X. Luo, Y. P. Li, J. F. Liu, Prof. X. M. Sun State Key Laboratory of Chemical Resource Engineering College of Science PO Box 98, Beijing University of Chemical Technology Beijing 100029, PR China E-mail: [email protected]; [email protected] DOI: 10.1002/smll.201401865 small 2014, DOI: 10.1002/smll.201401865
they could be potentially employed as excellent sacrificial template candidates to fabricate hollow or hierarchical nanostructures in strong oxidant-containing reactions. Meanwhile, conducting polymers, such as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline (PANI) have been widely studied as promising candidates for transparent flexible organic conductors or semiconductors in developing flexible or wearable electronics, displays, sensors, and other devices,[10] due to their outstanding electrical performance, tunable morphology, scalable manufacture, flexibility, and low cost—especially for PANI, a “classic” conducting polymer that stands out for good environmental stability and electrical conductivity.[11] However, the gas sensing performance, optical transmittance, and electrical sheet resistance of PANI films still leave room for improvement, and traditional gas sensing materials are mostly solid powders and need ceramic tubes or inter-finger probes to support them, making the fabrication of gas sensors a challenge. Recent developments in nanotechnology allow for the fabrication of various conducting polymer nanomaterials[12] (nanotubes, nanorods, nanowires, nanofibers, etc.) and composites through synthesis methods such as solid-phase template synthesis, interfacial polymerization, electrochemical synthesis, and rapid mixing methods.[13] Nanostructured conducting polymers with unique electrical properties feature high surface-to-volume ratios and small dimensions, which facilitate enhanced interaction between sensing materials and analytes for high sensitivity and enable fast adsorption/ desorption kinetics for analytes on the materials for a rapid response and recovery. Such materials have been widely used to construct various sensor devices with enhanced sensing performance compared to their bulk counterparts. For example, sensors from conducting polymers in the forms of 1D or quasi-1D confined nanostructures have been demonstrated to have significantly enhanced performance due to their high surface-to-volume ratios and small dimensions.[14] With a combination of nanostructured conducting polymers from the sacrificial Ag NW template and network structures of TCFs, the transparent and conductive films of hierarchically nanostructured conducting polymers could be potentially fabricated on flexible polyethylene terephthalate (PET) substrates for high-performance portable gas sensing at room temperature.
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Scheme 1. Schematic illustration for fabrication of transparent conducting film of hierarchically nanostructured PANI networks on flexible PET substrate by firstly coating of Ag NWs, and subsequently in situ chemical oxidative polymerization of aniline nearby the sacrificial Ag NWs template for high-performance sensor devices.
Herein, we report the fabrication of transparent conducting films of hierarchically nanostructured PANI networks on PET substrates by first Meyer-rod coating Ag NWs, and subsequently in situ chemical oxidative polymerizing aniline near sacrificial Ag NWs template. By exploiting the advantage of a superior network structure from TCFs and the instability of Ag NW, the hierarchically nanostructured PANI-containing transparent film could be obtained near the sacrificial Ag NW template of the Ag NW network TCFs. The as-prepared transparent film could be assembled into transparent chemical gas sensor devices for high-performance portable gas sensing at room temperature because of the quasi-one-dimensional confined nanostructure in the network film and a large specific surface area of hierarchically nanostructured PANI (Scheme 1). The transparent chemical gas sensor provides enhanced sensing performance and good transparency at visible wavelengths, creating opportunities for application in transparent electronic circuitry and optoelectronic devices with avenues for further functional integration.
Ag NW TCFs with flexibility, high transparency, and random network structures were fabricated by Meyer-rod coating of Ag NW inks on flexible PET substrates and drying at 120 °C in an oven, leaving a uniform, thin layer of Ag NWs on the substrate (Figure S1 and S2). The sheet resistance and optical transparency were 46 Ω/ⵧ and 85%, respectively. Ag NWs have high aspect ratios and could be etched by strong oxidants, such as APS. Therefore, Ag NW TCFs with random network structures could be potentially employed as excellent sacrificial template candidates to fabricate quasione-dimensional hierarchical nanostructures in strong oxidant-containing reactions, such as in situ polymerization of PANI chemically oxidized by APS. By dipping Ag NW TCF in aniline polymerizing solution with the concentration of aniline at 0.05 M and the ratio of [aniline]:[APS] = 1:1 for different times, different nanostructured PANI networks with variable transparency and conductivity could be achieved from the sacrificial template of Ag NW on the flexible PET substrate. Shown in Figure 1a–d are the scanning electron microscope (SEM) images of different nanostructured PANI
Figure 1. SEM images of different nanostructured PANI networks by dipping Ag NW TCF in aniline polymerizing solution with dipping time at a) 5 min, b) 10 min, c) 20 min and d) 30 min, respectively.
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small 2014, DOI: 10.1002/smll.201401865
Transparent Conducting Films of Hierarchically Nanostructured Polyaniline Networks
with dipping times of 5 min, 10 min, 20 min, and 30 min. The standard reduction potential of silver (+0.79 V) is lower than that of aniline (+1.02 V),[15] thus silver is more liable to be oxidized and etched by APS. Therefore, in the presence of APS, a distinct etching effect occurred and silver was transformed into silver ions. With the immersion time increased from 5 min to 30 min, Ag NWs of the TCFs were etched away and different nanostructured PANI were formed nearby the sacrificial Ag NW template. As shown in Figure 1a, the nanoparticle-like nanostructured PANI, with some of them connected in lines, was observed for the immersion time of 5 min. In the successive immersion of Ag NW TCF in aniline polymerizing solution, the different hierarchical nanostructures of PANI with network structures were formed (Figure 1c and d). For comparison, PET films without coatings of Ag NWs were dipped into aniline polymerizing solution for different times and particle-like PANI structures were obtained, as demonstrated in Figure S3a–d, implying that Ag NW TCFs with random network structures could be employed as excellent sacrificial templates to fabricate hierarchically nanostructured PANI networks. During the polymerization of aniline from the sacrificial template of unstable Ag NWs on the flexible PET substrate, the transparency and conductivity of the film changed simultaneously. The conductivity of the film varied from the sheet resistance of 46 Ω/ⵧ for Ag NW TCFs, to non-conducting states for 5 min and 10 min polymerization, to conducting states for 20 min (63 kΩ/ⵧ) and 30 min (25 kΩ/ⵧ) polymerization. At the same time, the optical transmittance of the corresponding films altered from 85% at 550 nm, to 69%, 67%, 65% and 69% for 5 min, 10 min, 20 min, and 30 min polymerization, respectively, as demonstrated in Figure 2. The nanostructured PANI network
Figure 2. a) Transmittance spectra of the Ag NW TCF and the various nanostructured PANI networks with immersion time of 5 min, 10 min, 20 min and 30 min, respectively. b) Photographs of transparent nanostructured PANI network films on PET substrates with the immersion time at 5 min, 10 min, 20 min and 30 min from left to right. The words ’’BUCT’’ are placed behind the films. small 2014, DOI: 10.1002/smll.201401865
TCF showed excellent transparency, ranging from 450 to 750 nm with a maximum transmittance of ∼65% (∼76% of transmittance compared to that of the original Ag NW TCF at 85%) (Figure 2a, 20 min sample). In the control experiment, different particle-like PANI with variable transparency and conductivity on PET substrates could be achieved by dipping PET in aniline polymerizing solution for 5 min, 10 min, 20 min, and 30 min (Figure S4). To confirm the formation of PANI, FTIR spectra were obtained and analyzed for particle-like pure PANI and the hierarchically nanostructured PANI networks from the sacrificial template of instable Ag NWs (Figure S5). FTIR spectra display the peak around 3000 cm−1 (C–H stretching) and the broad peak centering at 3400 cm−1 (N–H stretching) for both the particle-like pure PANI and the hierarchically nanostructured PANI, corresponding to the stretching vibrations of emeraldine PANI. The peaks at 1129 cm−1, 1298 cm−1, 1479 cm−1, 1561 cm−1 are assigned to C–H in-plane bending, C–N stretching vibration with aromatic conjugation, and C=C stretching vibration of quinoid and benzenoid rings, respectively. These results confirm the successful polymerization of PANI for both the particle-like pure PANI and the hierarchically nanostructured PANI. Due to a higher conjugation length of the hierarchically nanostructured PANI than that of particle-like pure PANI, the former has a higher integrated peak intensity of benzenoid rings against quinoid rings. UV-vis spectra for both the hierarchically nanostructured PANI network film and particle-like PANI film (20 min samples) show the characteristic bands of PANI at 354 nm for the π–π* transition of benzenoid rings and 813 nm for π–polaron transition (Figure S6). For Ag NW TCF, the peaks at 357 nm and 380 nm were observed. These results indicated the formation of PANI in both cases and the transformation of silver to silver salt in the synthesis of hierarchically nanostructured PANI networks, which is different from the reported Ag-PANI composites.[16] The characteristic peak for AgCl appears at 32.38° and characteristic peaks for Ag disappear at ∼38° and ∼44° from the X-ray diffraction (XRD) patterns of the hierarchically nanostructured PANI network film and Ag NW TCF (Figure S7a) respectively, indicating the transformation of silver to silver chloride in the synthesis of the hierarchically nanostructured PANI network film. Furthermore, the X-ray photoelectron spectroscopy (XPS) peaks of Ag 3d5/2 (368.20 eV) and Ag 3d3/2 (373.95 eV) clearly indicate the formation of Ag+ in the hierarchically nanostructured PANI network (Figure S8). We wondered if the prepared hierarchically nanostructured PANI-containing transparent films could be assembled and behave as high-performance portable gas sensors at room temperature because of the quasi-one-dimensional confined nanostructures in the network structure and the large specific surface area. In order to address this issue, the sensing property of the particle-like PANI film and the hierarchically nanostructured PANI-containing transparent film devices for ammonia (NH3) was tested. Detection of NH3 in air is of tremendous interest for environmental monitoring, workplace hazard monitoring, and homeland security because of its high toxicity, corrosivity, and volatility. It is a strong electrondonating species and has been widely investigated as a target
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Figure 3. a) The gas sensing of the hierarchically nanostructured PANI-containing transparent film and the particle-like PANI, both with polymerization times of 20 min, upon exposure to different concentrations of NH3 from 5 to 100 ppm at room temperature. b) Gas sensing selectivity test of the hierarchically nanostructured PANI-containing transparent film and the particle-like PANI film to various volatile organic gases. The gas concentration was 10 ppm. c) Sensing mechanism of NH3 interacted with acid-doped PANI.
chemical for PANI gas detection due to a dramatic decrease in conductance from changing PANI’s emeraldine salt form into its emeraldine base form upon exposure to NH3. Figure 3a displays the real-time difference in resistance sensitivity from the hierarchically nanostructured PANIcontaining transparent film and the particle-like PANI film, both with polymerization times of 20 min, upon exposure to different concentrations of NH3 from 5 to 100 ppm at room temperature. The sensitivity S is defined as the normalized resistance change, S = (Rg-R0)/R0 = ΔR/R0, where Rg is the resistance of the functional film after exposure to gas analytes and R0 is the resistance in ambient air. It can be seen that the hierarchically nanostructured PANI-containing transparent film had a higher gas sensitivity than the particle-like PANI film. The sensitivity of the hierarchically nanostructured PANI-containing transparent film increased gradually with the increase of the concentration of NH3, showing a linear relationship (Figure S9a). Even though the NH3 concentration was as low as 5 ppm, the change in resistance of the hierarchically nanostructured PANI-containing transparent film could also be observed, indicating that hierarchically nanostructured PANI-containing transparent film synthesized by this method had superior gas sensing performance and could act as an “electronic nose” in chemical detection and recognition. The gas sensitivity decreased for the hierarchically nanostructured PANI-containing transparent film from polymerization times of 20 min to 30 min, while an increase occurred for the gas sensitivity of the pure PANI films from polymerization time of 20 min to 30 min (Figure S9b). This is likely due to the different growth of PANI, resulting in a larger size of PANI grains and a larger specific surface area for the particle-like PANI films, while leading to the hierarchically nanostructured PANI-containing transparent film to be relatively smooth and with a reduced specific surface area. Meanwhile, the hierarchically nanostructured PANIcontaining transparent film also had a specific response to NH3 in comparison with other volatile organic compounds, as shown in Figure 3b. The gas sensitivity of the hierarchically nanostructured PANI-containing transparent film to 10 ppm
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NH3 was 79 times higher than the sensitifiy to ethanol, 67 times higher than for acetone, and 74 times higher than for n-propanol. The hierarchically nanostructured PANI-containing transparent film exhibited higher selectivity to NH3 compared to the particle-like PANI film (Figure 3b). The distinct sensing selectivity could be probably attributed to the synergetic contributions from the quasi-one-dimensional confined nanostructure in the network structure film, the specific acid–base reaction, and large specific surface area of hierarchically nanostructured PANI networks. In conclusion, a transparent conducting film of hierarchically nanostructured PANI networks on a PET substrate was successfully prepared by a combination of Meyer-rod coating of Ag NWs and in situ chemical oxidative polymerization of aniline. By combining the advantages of superior network structures from TCFs and the instability of Ag NW, the hierarchically nanostructured PANI-containing transparent film could be obtained nearby the sacrificial Ag NW template of the Ag NW network TCFs. The as-prepared transparent film could be assembled into a transparent, portable chemical gas sensor device with enhanced gas sensing performance at room temperature in both sensitivity and selectivity to NH3 than pure PANI film because of the quasi-one-dimensional confined nanostructure in the network structure film and the large specific surface area of nanostructured PANI. It is anticipated that this line of research can be further extended to other conducting polymers for fabricating transparent chemical gas sensors, providing superior sensing performance and transparency within visible wavelengths, and creating opportunities for being integrated into transparent electronics, automobile windshields, and other transparent devices for real-world sensor applications.
Experimental Section Materials: Aniline (distilled before use) was obtained from J&K Chemical. Ammonium persulfate (APS) and concentrated hydrochloric acid were purchased from Beijing Chemical Reagent Factory
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small 2014, DOI: 10.1002/smll.201401865
Transparent Conducting Films of Hierarchically Nanostructured Polyaniline Networks
and were used without further purification. Polyethylene terephthalate (PET, 125 µm, with two protective layers on obverse side and reverse side) film was commercially available from Hui Zhixing Company (Hangzhou, China) and Ag NW ink (5 mg/mL, dispersed in methanol) was a product of ColdStones Tech. Company (Suzhou, China). Preparation of Ag NW TCFs: The obverse protective layer of PET was firstly removed and 120 µL Ag NW ink was dropped on the edge of a 10 cm by 10 cm PET substrate. Then, a Meyer rod (0.02 mm, Pushen) was pulled over the Ag NW ink, leaving a uniform wet film. Then the reverse protective layer of Ag NW film was removed. And the film was firstly dried at room temperature and then heated at 120 °C for 30 min. Preparation of Various PANI Films and Powders: In a typical procedure, aniline monomer (1.5 mmol) was added into 1 M HCl aqueous solution (15 mL) in a beaker, which was treated with ultrasonic wave and cooled at 5 °C. Then, APS (1.5 mmol) in precooled 1 M HCl solution (15 mL) was poured into the above monomer solution, which was shaken for 20 s and left at 5 °C. After that, blank PET substrates and Ag NW TCFs were immersed in the above solution for different time, respectively. After immersion for different time, the blank PET substrates and Ag NW TCFs with polymerized PANI were taken out and rinsed with ethanol, respectively. All the films were dried at 65 °C for 1 h. The precipitates in the above reaction solution were also collected for FTIR and XRD characterization. Gas Sensing Measurement: The gas sensing measurement was performed in a WS-30A measuring system (Zhengzhou Winsen Electronics Technology, China) equipped with an 18 L chamber and two fans. The functional films (8 mm by 8 mm) were attached on the probes of voltage-testing devices with silver paint. Calculated amount of ammonium hydroxide (weight percent of ammonia was 25%) was dropped onto a hot plate in the chamber to rapidly generate 5, 10, 50, and 100 ppm NH3 in the chamber. After exposure to a certain concentration of NH3, the functional films was exposed to air by removing the chamber. To test gas sensing selectivity of the functional films, it was also carried out with ethanol, acetone, and n-propanol by the same method. General Techniques: SEM images were obtained from a Zeiss Supra 55 instrument at a voltage of 20 kV. UV-vis transmittance and absorption spectra of the samples were recorded by a Shimadzu UV-3150 UV spectrophotometer. Fourier transform infrared (FTIR) spectra were collected on a Nicolet 6700 FTIR spectrometer. XRD patterns were performed on a Shimadzu XRD-6000 X-ray diffractometer using a Cu KR radiation source. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALAB 250 spectrometer. The sheet resistance was measured by four-probe tester (RST-8).
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Supporting Information is available from the Wiley Online Library or from the author.
small 2014, DOI: 10.1002/smll.201401865
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: June 26, 2014 Revised: July 28, 2014 Published online:
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