- Email: [email protected]
Tuning the detection limit in hybrid organic-inorganic materials for improving electrical performance of sensing devices
Sensors and Actuators A 298 (2019) 111480
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Tuning the detection limit in hybrid organic-inorganic materials for improving electrical performance of sensing devices D.S. Calheiro, R.F. Bianchi Departamento de Física, Instituto de Ciências Exatas e Biológicas, Universidade Federal de Ouro Preto, Ouro Preto, MG, 35400-000, Brazil
a r t i c l e
i n f o
Article history: Received 14 March 2019 Received in revised form 7 June 2019 Accepted 2 July 2019 Available online 14 September 2019 Keywords: Organic electronics Flexible devices Device performance Hopping mechanism
a b s t r a c t Research in hybrid electronics has included advances in materials, devices and architectures. However, in practice, controversy still exists on some details which limit hybrid materials to high-performance applications, such as processing–structure–design–property relations. This paper describes a practical approach to enhancing the sensing performance of a prototype ammonia gas sensor based on electrical conductivity changes, percolation theory and current limitation to a semiconducting polymer-metal oxide medium. This device is based on fully-gravure printed polyaniline/indium - tin oxide nanocomposites, Pani100−x ITOx [0 ≤ x≤ 100% (wt/wt)], layers on a freestanding high-density polyethylene substrate. We find that the electrical current of the device decreases and tends to saturate as the gas concentration increases, and the value of this electrical current limit (IL ) depends on x: the higher the value of x, the smaller the IL , when the current that flows through the electronic device was dominated by the ITO-nanoparticle filled PAni, which increase the concentration of hopping carriers and contribute to the desired electrical response of a heterogeneous gas sensor. In this regime, we find a good linear relationship between x and ammonia concentration. These findings suggest new directions for future research on the development and investigation of organic-inorganic devices in which the electrical current variation is desired for enhanced sensitivity and stability of hybrid sensors. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Hybrid materials have attracted increasing interest with the recent developments in printed electronics technology [1–4], due to their potential application as low-cost and large-scale integrated devices on flexible, stretchable, and even patchable freestanding substrates [5–10]. Consequently, many printed devices have the potential to become commercially available, but it is still a challenging task to attend simultaneously their functionality, design ability, and manufacturability to yield a better fabrication cost and optical and/or electrical performance during operation [11–16]. These challenges are mainly due to practical stability, reversibility, and sensitivity from device-to-device, which also leads to measurement-to-measurement variations so that it is important to gain fundamental insights into the influence of these effects and problems and how they could be avoided to design more efficient devices [17–21]. For example, there are a surprising number of hybrid devices with outstanding gas sensing properties [22], but some authors continue to report low accuracy outputs, which have
E-mail address: [email protected] (R.F. Bianchi). https://doi.org/10.1016/j.sna.2019.07.005 0924-4247/© 2019 Elsevier B.V. All rights reserved.
made these sensors unsuitable for real-time and convenient detection [23–25]. More recent evidence highlights these limitations as a metrology challenge that has been leading to the continuous development of new measurement techniques [26]. In this paper, a practical approach is presented for solving the measurement-to-measurement problems of a flexible printed organic–inorganic gas sensor. This approach is based on tuning the ammonia gas sensing performance of polyaniline (PAni)-indium tin oxide (ITO) composites, which depends on the electrical conductivity charges of PAni under ammonia exposure, on the PAni-ITO mass ratio and finally on the low percolation regime and electrical current limitations of PAni-ITO composites. The dependence between the limiting ammonia detection of the sensor in the presence of gas for 0–35 ppm ammonia is obtained by simply adjusting the x value in PAni100−x ITOx printed thin film composites. 2. Experimental procedure 2.1. Gravure-printed PAni100−x ITOx thin film gas sensor on a flexible substrate Fig. 1 is a schematic illustration of both the design and the preparation process of our printed organic-inorganic gas sensor. This
2
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 1. Schematics illustrating the printing process for the fabrication of a flexible organic–inorganic hybrid sensor using gravure printed Pani100−x ITOx thi n film (0 ≤ x ≤ 50%) as an active layer and a pair of parallel screen-printable silver-filled conductive electrodes. (a) gravure printing process for the fabrication of a Pani100−x ITOx layer on HDPE substrate (thickness = 720 m using a lab-scale printer (IGT Printability Tester G1-5) and a global standard tester gravure printing cylinder (printing width = 50 mm, printing force = 10 N, printing length = 15 cm, screen ruling = 80 lines/cm, cell depth = 33 m, screen angle = 140◦ , printing speed = 1 m/s and blade pressure = 6 N). (b) mechanical cutting process for the fabrication of single sensors, area = (1.8 × 0.8) cm2 . (c) doping process of PAni in 1 mol/l HCl solution for 5 s, which increases the electrical conductivity of PAni from 4.5 × 10−8 S/m (blue emeraldine base) to 7.8 × 10−3 S/m (green emeraldine salt). (d) screen-printable silver-filled conductive ink (Cl- 1001/EMS product using 200 screen mesh count) with low electrical resistance properties (15 m) for (e) the fabrication of two parallel screen printed electrodes (length = 10 mm and width = 0.6 mm). Photo of a fully printed organic-inorganic hybrid sensor.
device was used only for the evaluation of our electrical experimental proposed method, and consists of a polyaniline (PAni) – indium tin doped oxide (ITO) composite thin film, where PAni was chemically synthesized by oxidative polymerization of aniline, using the route described elsewhere [27,28], while ITO nanoparticles (In2 O3 /SnO2 , < 50 nm particles) was commercially obtained from Sigma-Aldrich (CAS 50926-11/Product No 544876). ITO was used as received, without further purification or cleaning, and it was mixed at a different concentration, (x) of 0, 10, 20, 30, 40, and 50 wt% on PAni100−x ITOx (0 ≤ x ≤ 50) thin film, after the solutions were stirred at room temperature for 1 h. For the development of the sensor, PAni100−x ITOx thin films were obtained from a gravure printed technique (IGT, model G1-5) [29]. A syringe was used to deposit different concentrations of PAni100−x ITOx solution onto a doctor blade (Fig. 1a). The solution was then printed on a high-density polyethylene (HDPE) fiber substrate, forming a PAniITO-coated fiber substrate, which was subjected to an ultrasonic cleaning process in a Milli-Q water bath. Next, freestanding PAniITO/HDPE films were dried for 30 min under vacuum at 50 ◦ C. The films were cut to obtain single devices (0.8 × 1.8 cm2 ) (Fig. 1b). The samples were then dipped in 1 M HCl for 5 s to enhance the electrical conductivity of PAni, resulting in the formation of a doped light green PAni-coated HDPE fiber film (Fig. 1c). After this stage, the samples were dried for 10 min under vacuum at 50 ◦ C to remove traces of HCl. To complete the manufacturing process, a pair of parallel screen-printed electrode lines were prepared on the top of
the PAni-ITO/HDPE films (Fig. 1d). The samples were dried under vacuum for 1 h at 35 ◦ C to remove traces of solvents from metallic ink, so the devices were ready for the electrical measurements (Fig. 1e). In this way, six films with different concentration, namely PAni100 ITO0 , PAni90 ITO10 , PAni80 ITO20 , PAni70 ITO30 , PAni60 ITO40 and PAni50 ITO50 , were obtained and here these films are discussed as ammonia gas sensors.
2.2. Electrical characterization and ammonia calibration method for the ammonia gas sensor DC measurements were carried out with an Electrometer High Resistance Meter (Keithley, Model 6517A) with an applied voltage of 1.0 V. All electrical measurements were carried out in a homemade gas chamber with a temperature between 20 - 25 ◦ C and relative humidity controlled between 50 and 55%. An ammoniaair mixture was injected into the chamber at a rate of 5 ppm every 2 min. A high-performance electrochemical diffusion sensor (Instrutherm, Model DG-200) was used as a reference device for monitoring ammonia gas levels in the chamber and to compare the PAni-ITO based sensor accuracy against a more exacting standard device. The DG-200 sensor has an ammonia detection range of 0–100 ppm, a detection limit of 1 ppm, and characteristic response time of approximately 30 s. Finally, Scanning Electron Microscope
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 2. Electrical current of PAni100−x ITOx for x = 0, 10, 20, 30, 40 and 50%, and Scanning Electron Microscopy (SEM) images for PAni100−x ITOx for x = 0, 30 and 50%. Inset, a schematic illustration of the morphology of the samples a function of x. The diameter of ITO nanoparticles is ∼50 nm. All measurements were carried out at room temperatures.
3
Fig. 3. Electrical response of the PAni100−x ITOx films as function of ammonia concentration and x. (a) the inverse normalized current variation, Ix ([NH3 =0])/Ix ([NH3 ]), of PAni100−x ITOx exposed to 0–35 ppm and (b) the electrical current limitation (IL ) for all samples.
3.1. Device characterization (SEM) images of the PAni-ITO/HDPE films were obtained using a VEGA3|TESCAN.
3. Results and discussion Fig. 2 shows the relationship between electrical current and the PAni-ITO mass ratio on PAni100−x ITOx films (0 ≤ x ≤ 50). As x increases, the electrical current increases from 9 × 10−4 to 9 × 10−3 A. This behavior is typically observed in the low percolation regime [30], i.e., much lower than conventional weakly doped PAni-ITO composite [31]. The inset contains SEM micrographs showing the dispersion of ITO nanoparticles. As can be seen, as x increase, the nanoparticles tend to link together to form conductive ITOagglomerates on and through HDPE fibers, from poor (x ≤ 30) to higher (x ≥ 30) dispersion in the PAni100−x ITOx film. From the analysis of the SEM images, the average ITO nanoparticle size and HDPE average fiber diameter are around 78 nm and 9 m, respectively. From the same images, the agglomerate surface area distribution of nanoparticles in PAni100−x ITOx films is obtained that varies from 0% (x = 0) to 26% (x = 30) and abruptly increases to a highly distributed surface area of 82% when x is equal to 50%. An illustration of this ITO dispersion evolution is also shown in Fig. 2, in which ITO nanoparticles are represented as dark grey, while doped PAni coated HDPE fiber is represented by green. The ITO dispersion dependence on x confirms the presence of conductive paths to improve the electrical current in PAni100−x ITOx films [30–33], thus improving the low percolation regime of the HCl-doped PAni-ITO composite. In this case, the electrical conductivity of the PAni100−x ITOx films is controlled by the: (i) PAni-HDPE, when x ≤ 10; (ii) PAni-ITO-HDPE, when 10 ≤ x ≤ 40; and (iii) ITO nanoparticles, when x≥ 40.
The sensing properties of the PAni100−x ITOx devices were tested. Here the Figure-of-Merit (FoM) is the variation ratio between the DC electrical current value of the devices in the absence and presence of gas, Ix ([NH3 = 0])/(Ix ([NH3 ])). Fig. 3 shows Ix ([NH3 = 0])/Ix ([NH3 ]) of PAni100−x ITOx exposed to 0–35 ppm of ammonia gas. The maximum electrical current variation of the PAni-ITO films is inversely proportional to x (Fig. 3a) so that as x is higher, both the electrical current variation and the maximum ammonia concentration detection (insert in Fig. 3) are lower. In this case, the electrical current limitation is specified for the PAni100−x ITOx film when 5 ppm of ammonia gas corresponds to an electrical current variation lower than 10%. These results lead to a novel concept, multiple film sensing device in which the maximum limit of detection is tuned by the value of x. In addition, this electrical response is linear with the respect to gas concentration in the 5–25 ppm range showing a sensitivity equal to 0.5 ppm/x. This result indicates the control of electrical conductivity of polymer-metal oxide nanoparticles films in the low percolation regime. Fig. 4 shows the sensing performance of PAni100−x ITOx films for x equals 0, 10, 20, 30, 40, and 50. The measurements were evaluated in 0–35 ppm ammonia, for 8 applied cycles. After exposure to ammonia, the electrical current of the films decreases rapidly. In addition, the response and recovery time are around 2 min, irrespective of x value, while the sensitivity increases from 0.3 A/ppm to 17 mA/ppm, when x varies from 0 to 50% respectively, providing a good reproducibility of the electrical signal. These results show that a multi-film sensor (x = 0, 10, 20, 30, 40 and 50) has a good sensitivity to ammonia variation. For the study of the bulk conduction process of PAni100−x ITOx films, we assume the conductivity ( dc ) is directly proportional to
4
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 5. Concentration of hopping carriers Log dc = sim K1 [x]n as function of x1/3 for [x] n = 1/3 or 4/3, with x varying from 0 to 50% and the corresponding standard deviation (SD).
dent of [x]. In the first case, c ∝ 1/a3 , using Eqs. (1) and (2) we have a general dependence of dc on [x] of the type [28]: Fig. 4. The sensing performance of PAni100−x ITOx films for x equal to 0, 10, 20, 30, 40 and 50% exposed to NH3 concentration equal to 0 (no gas or “Gas OFF”) and 35 ppm (in the presence of gas, or “Gas ON”). The graph is an 8 cycles test of a PAni100−x ITOx film.
both electrical current and ITO concentration, [ITO], through an electronic hopping process described in refs. [27,28,34]. Therefore, we expected the following dependence of dc : dc ≈ 0 −1
c.e2 akT
(1)
where T is temperature, a is hopping distance, c is the density of carriers/atoms, v0 is jumping frequency, k is the Boltzmann constant and e is the electronic charge [28,34,35]. For electronic hopping we consider the following dependence:
0 ≈ e−a
(2)
in which the jumping frequency (0 ) is expected to be exponentially dependent on the hopping distance (a), with a constant value of [27]. Eqs. (1) and (2) suggest a large dependence of dc on the ITO concentration, i.e., the PAni-ITO ratio; this dependence can be explained by the addition of nanoparticles introducing new hopping centers for charge carriers, thereby decreasing the hopping distance [27,28,34]. In relation to the concentration dependence, two cases may be regarded: one in which the concentration of hopping carriers changes proportionally to [x] and the other in which the concentration of hopping carriers remains constant indepen-
Log
dc [x]4/3
≈ k1 [x]1/3
while in the second case, Log dc ≈ k2 [x]1/3 [x]1/3
(3)
(4)
In Eqs. (3) and (4) k1 and k2 are dimensionally different constants. Here, we used [x] = 0, 10, 20, 30, 40, and 50%, and found that Equation (3) provides a better fit, that is, with a smaller standard deviation (SD) than with Equation (4), a detailed description of this assumption is presented in ref [34]. Fig. 5 shows the result of the two cases presented through Eqs. (3) and (4). From the results shown in Fig. 5 it is concluded that the concentration of hopping carriers is essentially dependent the concentration of x used in the principle operation of the device. 4. Conclusion This paper has highlighted the importance of exploring the composition of materials in the development of the operating principle of a prototype device based on tuning the ammonia gas sensing performance of hybrid composites, which depends on the electrical conductivity changes of PAni under gas exposure, electrical conductivity of ITO and the relation x on PAni100−x ITOx . The results showed that the gas sensor has a good processingstructure-design-property and that relationships are observed in the electrical and cyclic results. In fact, the ITO nanoparticles were important and decisive for the electrical behavior of PAni100−x ITOx device in a low percolation regime. In this hybrid organic-inorganic composite, the transition from high to low conductivity and therefore the electric current limit is governed by both the value of x
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 6. Schematics of the hybrid sensor structure and working principle of the combined PANI-ITO materials in which the electrical current limit is desired for enhanced sensitivity and stability of sensing devices.
(or the concentration of hopping carriers) and the conductivity level of polyaniline. Finally, Fig. 6 shows the schematics of both device structure and working principle of the combined PANI-ITO materials. These results suggest new strategies and direction for future research on hybrid sensors based on a multiple film sensing device in which the maximum limit of detection is tune by the concentration of inorganic material. Acknowledgement The authors thanks CAPES, CNPq, Fapemig and INEO/CNpq. References [1] S. Agate, M. Joyce, L. Lucia, L. Pal, Cellulose and nanocellulose-based flexible-hybrid printed electronics and conductive composites-a review, Carbohydrate Polym. (2018). [2] S.K. Garlapati, M. Divya, B. Breitung, R. Kruk, H. Hahn, S. Dasgupta, Printed electronics based on inorganic semiconductors: From processes and materials to devices, Adv. Mater. (2018) 1707600. [3] D.B. Mitzi, Solution-processed inorganic semiconductors, J. Mater. Chem. 14 (2004) 2355–2365. [4] J.A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics, Science 327 (2010) 1603–1607. [5] J.-H. Kim, M.J. Han, S. Seo, Flexible, stretchable, and patchable organic devices integrated on freestanding polymeric substrates, J. Polymer Sci. Part B: Polym. Phys. 53 (2015) 453–460. [6] M. Koetse, E. Smits, E. Rubingh, P. Teunissen, R. Kusters, R. Abbel, J. Brand, Hybrid Printed Electronics, 2016. [7] M.-H. You, X.-X. Wang, X. Yan, J. Zhang, W.-Z. Song, M. Yu, Z.-Y. Fan, S. Ramakrishna, Y.-Z. Long, A self-powered flexible hybrid piezoelectric-pyroelectric nanogenerator based on non-woven nanofiber membranes, J. Mater. Chem. A 6 (2018) 3500–3509. [8] X. Li, H. Andersson, J. Sidén, T. Schön, Soldering surface mount components on screen-printed ag patterns on paper and polyimide substrates for hybrid printed electronics, Flexible Printed Electron. 3 (2018) 015003. [9] H. Wang, M. Wang, Y. Tang, A novel zinc-ion hybrid supercapacitor for long-life and low-cost energy storage applications, Energy Storage Mater. 13 (2018) 1–7. [10] Y.S. Rim, S.-H. Bae, H. Chen, N. De Marco, Y. Yang, Recent progress in materials and devices toward printable and flexible sensors, Adv. Mater. 28 (2016) 4415–4440. [11] A. Dey, Semiconductor metal oxide gas sensors: A review, Mater. Sci. Eng. B 229 (2018) 206–217. [12] Y. Lin, Y. Gao, F. Fang, Z. Fan, Recent progress on printable power supply devices and systems with nanomaterials, Nano Research, 2018, pp. 1–23. [13] H.S. Vogelbaum, G. Sauvé, Recently developed high-efficiency organic photoactive materials for printable photovoltaic cells: a mini review, Synthetic Metals 223 (2017) 107–121. [14] J.S. Chang, A.F. Facchetti, R. Reuss, A circuits and systems perspective of organic/printed electronics: Review, challenges, and contemporary and emerging design approaches, IEEE J. Emerging Select. Topics Circuits Syst. 7 (2017) 7–26. [15] S. Khan, L. Lorenzelli, R.S. Dahiya, Technologies for printing sensors and electronics over large flexible substrates: a review, IEEE Sensors J. 15 (2015) 3164–3185.
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[16] W. Wu, Inorganic nanomaterials for printed electronics: a review, Nanoscale 9 (2017) 7342–7372. [17] N.K. Noel, S.D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A.-A. Haghighirad, A. Sadhanala, G.E. Eperon, S.K. Pathak, M.B. Johnston, et al., Lead-free organic-inorganic tin halide perovskites for photovoltaic applications, Energy Environ. Sci. 7 (2014) 3061–3068. [18] W. Chen, J. Zhang, G. Xu, R. Xue, Y. Li, Y. Zhou, J. Hou, Y. Li, A semitransparent inorganic perovskite film for overcoming ultraviolet light instability of organic solar cells and achieving 14.03% efficiency, Adv. Mater. 30 (2018) 1800855. [19] D. Mitzi, Solution Processing of Inorganic Materials, John Wiley & Sons, 2008. [20] Q. Bao, S. Braun, C. Wang, X. Liu, M. Fahlman, Interfaces of (ultra) thin polymer films in organic electronics, Adv. Mater. Interfaces (2018) 1800897. [21] N.S. Ramgir, Electronic nose based on nanomaterials: Issues, challenges, and prospects, ISRN Nanomaterials 2013 (2013). [22] Z. Wang, T. Zhang, C. Zhao, T. Han, T. Fei, S. Liu, G. Lu, Rational synthesis of molybdenum disulfide nanoparticles decorated reduced graphene oxide hybrids and their application for high-performance NO2 sensing, Sens. Actuators B: Chem. 260 (2018) 508–518. [23] A. Goldoni, V. Alijani, L. Sangaletti, L. D’Arsiè, Advanced promising routes of carbon/metal oxides hybrids in sensors: A review, Electrochim. Acta 266 (2018) 139–150. [24] R.S. Andre, R.C. Sanfelice, A. Pavinatto, L.H. Mattoso, D.S. Correa, Hybrid nanomaterials designed for volatile organic compounds sensors: A review, Mater. Design (2018). [25] S. Bera, S. Kundu, H. Khan, S. Jana, Polyaniline coated graphene hybridized sno2 nanocomposite: Low temperature solution synthesis, structural property and room temperature ammonia gas sensing, J. Alloys Compd. 744 (2018) 260–270. [26] J. Wade, J.R. Hollis, S. Wood, Printed Electronics, IOP Publishing, 2018. [27] R. Bianchi, G. Leal Ferreira, C. Lepienski, R. Faria, Alternating electrical conductivity of polyaniline, J. Chem. Phys. 110 (1999) 4602–4607. [28] R. Bianchi, H. da Cunha, R. Faria, G.L. Ferreira, J.M.G. Neto, Electrical studies on the doping dependence and electrode effect of metal-pani-metal structures, J. Phys. D: Appl. Phys. 38 (2005) 1437. [29] G. Grau, J. Cen, H. Kang, R. Kitsomboonloha, W.J. Scheideler, V. Subramanian, Gravure-printed electronics: recent progress in tooling development, understanding of printing physics, and realization of printed devices, Flexible and Printed Electronics 1 (2016) 023002. [30] X. Wu, S. Qi, J. He, G. Duan, High conductivity and low percolation threshold in polyaniline/graphite nanosheets composites, J. Mater. Sci. 45 (2010) 483. [31] L. Cardoso, G. Gonc¸alves, D. Kanda, R. Bianchi, H. Nagashima, Charge transport in conjugated polymer-semiconductor nanoparticle composite near the percolation threshold, Appl. Phys. A 123 (2017) 799. [32] V. Roldughin, V. Vysotskii, Percolation properties of metal-filled polymer films, structure and mechanisms of conductivity, Progr. Org. Coat. 39 (2000) 81–100. [33] B. Shklovskii, A. Efros, Percolation theory and conductivity of disordered systems, Uspekhi Fiz. Nauk 117 (1975) 401. [34] J.M.G. Neto, G. Leal Ferreira, J.R. Santos Jr., H. Da Cunha, I. Dantas, R. Bianchi, Complex conductance of carnauba wax/polyaniline composites, Brazilian J. Phys. 33 (2003) 371–375. [35] J.C. Dyre, T.B. Schrøder, Universality of ac conduction in disordered solids, Rev. Modern Phys. 72 (2000) 873.
Biographies
Daniel Silva Calheiro is Graduate student of Federal University of Ouro Preto in Materials Engineering. She received his M.S. (2018) in Materials Engineering from Federal University of Ouro Preto. His research interests are in organic electronic and electrical properties of semiconducting polymers films and devices.
Rodrigo Fernando Bianchi is a professor in Department of Physics, Federal University of Ouro Preto, MG Brazil. He received his PhD in Materials Science and Engineering from University of São Paulo in 2002. He served as a postdoctoral research associate at University of São Paulo (2002-2006) and University of California (2011-2013) His current research interests include organic electronic and medical devices.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Tuning the detection limit in hybrid organic-inorganic materials for improving electrical performance of sensing devices D.S. Calheiro, R.F. Bianchi Departamento de Física, Instituto de Ciências Exatas e Biológicas, Universidade Federal de Ouro Preto, Ouro Preto, MG, 35400-000, Brazil
a r t i c l e
i n f o
Article history: Received 14 March 2019 Received in revised form 7 June 2019 Accepted 2 July 2019 Available online 14 September 2019 Keywords: Organic electronics Flexible devices Device performance Hopping mechanism
a b s t r a c t Research in hybrid electronics has included advances in materials, devices and architectures. However, in practice, controversy still exists on some details which limit hybrid materials to high-performance applications, such as processing–structure–design–property relations. This paper describes a practical approach to enhancing the sensing performance of a prototype ammonia gas sensor based on electrical conductivity changes, percolation theory and current limitation to a semiconducting polymer-metal oxide medium. This device is based on fully-gravure printed polyaniline/indium - tin oxide nanocomposites, Pani100−x ITOx [0 ≤ x≤ 100% (wt/wt)], layers on a freestanding high-density polyethylene substrate. We find that the electrical current of the device decreases and tends to saturate as the gas concentration increases, and the value of this electrical current limit (IL ) depends on x: the higher the value of x, the smaller the IL , when the current that flows through the electronic device was dominated by the ITO-nanoparticle filled PAni, which increase the concentration of hopping carriers and contribute to the desired electrical response of a heterogeneous gas sensor. In this regime, we find a good linear relationship between x and ammonia concentration. These findings suggest new directions for future research on the development and investigation of organic-inorganic devices in which the electrical current variation is desired for enhanced sensitivity and stability of hybrid sensors. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Hybrid materials have attracted increasing interest with the recent developments in printed electronics technology [1–4], due to their potential application as low-cost and large-scale integrated devices on flexible, stretchable, and even patchable freestanding substrates [5–10]. Consequently, many printed devices have the potential to become commercially available, but it is still a challenging task to attend simultaneously their functionality, design ability, and manufacturability to yield a better fabrication cost and optical and/or electrical performance during operation [11–16]. These challenges are mainly due to practical stability, reversibility, and sensitivity from device-to-device, which also leads to measurement-to-measurement variations so that it is important to gain fundamental insights into the influence of these effects and problems and how they could be avoided to design more efficient devices [17–21]. For example, there are a surprising number of hybrid devices with outstanding gas sensing properties [22], but some authors continue to report low accuracy outputs, which have
E-mail address: [email protected] (R.F. Bianchi). https://doi.org/10.1016/j.sna.2019.07.005 0924-4247/© 2019 Elsevier B.V. All rights reserved.
made these sensors unsuitable for real-time and convenient detection [23–25]. More recent evidence highlights these limitations as a metrology challenge that has been leading to the continuous development of new measurement techniques [26]. In this paper, a practical approach is presented for solving the measurement-to-measurement problems of a flexible printed organic–inorganic gas sensor. This approach is based on tuning the ammonia gas sensing performance of polyaniline (PAni)-indium tin oxide (ITO) composites, which depends on the electrical conductivity charges of PAni under ammonia exposure, on the PAni-ITO mass ratio and finally on the low percolation regime and electrical current limitations of PAni-ITO composites. The dependence between the limiting ammonia detection of the sensor in the presence of gas for 0–35 ppm ammonia is obtained by simply adjusting the x value in PAni100−x ITOx printed thin film composites. 2. Experimental procedure 2.1. Gravure-printed PAni100−x ITOx thin film gas sensor on a flexible substrate Fig. 1 is a schematic illustration of both the design and the preparation process of our printed organic-inorganic gas sensor. This
2
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 1. Schematics illustrating the printing process for the fabrication of a flexible organic–inorganic hybrid sensor using gravure printed Pani100−x ITOx thi n film (0 ≤ x ≤ 50%) as an active layer and a pair of parallel screen-printable silver-filled conductive electrodes. (a) gravure printing process for the fabrication of a Pani100−x ITOx layer on HDPE substrate (thickness = 720 m using a lab-scale printer (IGT Printability Tester G1-5) and a global standard tester gravure printing cylinder (printing width = 50 mm, printing force = 10 N, printing length = 15 cm, screen ruling = 80 lines/cm, cell depth = 33 m, screen angle = 140◦ , printing speed = 1 m/s and blade pressure = 6 N). (b) mechanical cutting process for the fabrication of single sensors, area = (1.8 × 0.8) cm2 . (c) doping process of PAni in 1 mol/l HCl solution for 5 s, which increases the electrical conductivity of PAni from 4.5 × 10−8 S/m (blue emeraldine base) to 7.8 × 10−3 S/m (green emeraldine salt). (d) screen-printable silver-filled conductive ink (Cl- 1001/EMS product using 200 screen mesh count) with low electrical resistance properties (15 m) for (e) the fabrication of two parallel screen printed electrodes (length = 10 mm and width = 0.6 mm). Photo of a fully printed organic-inorganic hybrid sensor.
device was used only for the evaluation of our electrical experimental proposed method, and consists of a polyaniline (PAni) – indium tin doped oxide (ITO) composite thin film, where PAni was chemically synthesized by oxidative polymerization of aniline, using the route described elsewhere [27,28], while ITO nanoparticles (In2 O3 /SnO2 , < 50 nm particles) was commercially obtained from Sigma-Aldrich (CAS 50926-11/Product No 544876). ITO was used as received, without further purification or cleaning, and it was mixed at a different concentration, (x) of 0, 10, 20, 30, 40, and 50 wt% on PAni100−x ITOx (0 ≤ x ≤ 50) thin film, after the solutions were stirred at room temperature for 1 h. For the development of the sensor, PAni100−x ITOx thin films were obtained from a gravure printed technique (IGT, model G1-5) [29]. A syringe was used to deposit different concentrations of PAni100−x ITOx solution onto a doctor blade (Fig. 1a). The solution was then printed on a high-density polyethylene (HDPE) fiber substrate, forming a PAniITO-coated fiber substrate, which was subjected to an ultrasonic cleaning process in a Milli-Q water bath. Next, freestanding PAniITO/HDPE films were dried for 30 min under vacuum at 50 ◦ C. The films were cut to obtain single devices (0.8 × 1.8 cm2 ) (Fig. 1b). The samples were then dipped in 1 M HCl for 5 s to enhance the electrical conductivity of PAni, resulting in the formation of a doped light green PAni-coated HDPE fiber film (Fig. 1c). After this stage, the samples were dried for 10 min under vacuum at 50 ◦ C to remove traces of HCl. To complete the manufacturing process, a pair of parallel screen-printed electrode lines were prepared on the top of
the PAni-ITO/HDPE films (Fig. 1d). The samples were dried under vacuum for 1 h at 35 ◦ C to remove traces of solvents from metallic ink, so the devices were ready for the electrical measurements (Fig. 1e). In this way, six films with different concentration, namely PAni100 ITO0 , PAni90 ITO10 , PAni80 ITO20 , PAni70 ITO30 , PAni60 ITO40 and PAni50 ITO50 , were obtained and here these films are discussed as ammonia gas sensors.
2.2. Electrical characterization and ammonia calibration method for the ammonia gas sensor DC measurements were carried out with an Electrometer High Resistance Meter (Keithley, Model 6517A) with an applied voltage of 1.0 V. All electrical measurements were carried out in a homemade gas chamber with a temperature between 20 - 25 ◦ C and relative humidity controlled between 50 and 55%. An ammoniaair mixture was injected into the chamber at a rate of 5 ppm every 2 min. A high-performance electrochemical diffusion sensor (Instrutherm, Model DG-200) was used as a reference device for monitoring ammonia gas levels in the chamber and to compare the PAni-ITO based sensor accuracy against a more exacting standard device. The DG-200 sensor has an ammonia detection range of 0–100 ppm, a detection limit of 1 ppm, and characteristic response time of approximately 30 s. Finally, Scanning Electron Microscope
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 2. Electrical current of PAni100−x ITOx for x = 0, 10, 20, 30, 40 and 50%, and Scanning Electron Microscopy (SEM) images for PAni100−x ITOx for x = 0, 30 and 50%. Inset, a schematic illustration of the morphology of the samples a function of x. The diameter of ITO nanoparticles is ∼50 nm. All measurements were carried out at room temperatures.
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Fig. 3. Electrical response of the PAni100−x ITOx films as function of ammonia concentration and x. (a) the inverse normalized current variation, Ix ([NH3 =0])/Ix ([NH3 ]), of PAni100−x ITOx exposed to 0–35 ppm and (b) the electrical current limitation (IL ) for all samples.
3.1. Device characterization (SEM) images of the PAni-ITO/HDPE films were obtained using a VEGA3|TESCAN.
3. Results and discussion Fig. 2 shows the relationship between electrical current and the PAni-ITO mass ratio on PAni100−x ITOx films (0 ≤ x ≤ 50). As x increases, the electrical current increases from 9 × 10−4 to 9 × 10−3 A. This behavior is typically observed in the low percolation regime [30], i.e., much lower than conventional weakly doped PAni-ITO composite [31]. The inset contains SEM micrographs showing the dispersion of ITO nanoparticles. As can be seen, as x increase, the nanoparticles tend to link together to form conductive ITOagglomerates on and through HDPE fibers, from poor (x ≤ 30) to higher (x ≥ 30) dispersion in the PAni100−x ITOx film. From the analysis of the SEM images, the average ITO nanoparticle size and HDPE average fiber diameter are around 78 nm and 9 m, respectively. From the same images, the agglomerate surface area distribution of nanoparticles in PAni100−x ITOx films is obtained that varies from 0% (x = 0) to 26% (x = 30) and abruptly increases to a highly distributed surface area of 82% when x is equal to 50%. An illustration of this ITO dispersion evolution is also shown in Fig. 2, in which ITO nanoparticles are represented as dark grey, while doped PAni coated HDPE fiber is represented by green. The ITO dispersion dependence on x confirms the presence of conductive paths to improve the electrical current in PAni100−x ITOx films [30–33], thus improving the low percolation regime of the HCl-doped PAni-ITO composite. In this case, the electrical conductivity of the PAni100−x ITOx films is controlled by the: (i) PAni-HDPE, when x ≤ 10; (ii) PAni-ITO-HDPE, when 10 ≤ x ≤ 40; and (iii) ITO nanoparticles, when x≥ 40.
The sensing properties of the PAni100−x ITOx devices were tested. Here the Figure-of-Merit (FoM) is the variation ratio between the DC electrical current value of the devices in the absence and presence of gas, Ix ([NH3 = 0])/(Ix ([NH3 ])). Fig. 3 shows Ix ([NH3 = 0])/Ix ([NH3 ]) of PAni100−x ITOx exposed to 0–35 ppm of ammonia gas. The maximum electrical current variation of the PAni-ITO films is inversely proportional to x (Fig. 3a) so that as x is higher, both the electrical current variation and the maximum ammonia concentration detection (insert in Fig. 3) are lower. In this case, the electrical current limitation is specified for the PAni100−x ITOx film when 5 ppm of ammonia gas corresponds to an electrical current variation lower than 10%. These results lead to a novel concept, multiple film sensing device in which the maximum limit of detection is tuned by the value of x. In addition, this electrical response is linear with the respect to gas concentration in the 5–25 ppm range showing a sensitivity equal to 0.5 ppm/x. This result indicates the control of electrical conductivity of polymer-metal oxide nanoparticles films in the low percolation regime. Fig. 4 shows the sensing performance of PAni100−x ITOx films for x equals 0, 10, 20, 30, 40, and 50. The measurements were evaluated in 0–35 ppm ammonia, for 8 applied cycles. After exposure to ammonia, the electrical current of the films decreases rapidly. In addition, the response and recovery time are around 2 min, irrespective of x value, while the sensitivity increases from 0.3 A/ppm to 17 mA/ppm, when x varies from 0 to 50% respectively, providing a good reproducibility of the electrical signal. These results show that a multi-film sensor (x = 0, 10, 20, 30, 40 and 50) has a good sensitivity to ammonia variation. For the study of the bulk conduction process of PAni100−x ITOx films, we assume the conductivity ( dc ) is directly proportional to
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Fig. 5. Concentration of hopping carriers Log dc = sim K1 [x]n as function of x1/3 for [x] n = 1/3 or 4/3, with x varying from 0 to 50% and the corresponding standard deviation (SD).
dent of [x]. In the first case, c ∝ 1/a3 , using Eqs. (1) and (2) we have a general dependence of dc on [x] of the type [28]: Fig. 4. The sensing performance of PAni100−x ITOx films for x equal to 0, 10, 20, 30, 40 and 50% exposed to NH3 concentration equal to 0 (no gas or “Gas OFF”) and 35 ppm (in the presence of gas, or “Gas ON”). The graph is an 8 cycles test of a PAni100−x ITOx film.
both electrical current and ITO concentration, [ITO], through an electronic hopping process described in refs. [27,28,34]. Therefore, we expected the following dependence of dc : dc ≈ 0 −1
c.e2 akT
(1)
where T is temperature, a is hopping distance, c is the density of carriers/atoms, v0 is jumping frequency, k is the Boltzmann constant and e is the electronic charge [28,34,35]. For electronic hopping we consider the following dependence:
0 ≈ e−a
(2)
in which the jumping frequency (0 ) is expected to be exponentially dependent on the hopping distance (a), with a constant value of [27]. Eqs. (1) and (2) suggest a large dependence of dc on the ITO concentration, i.e., the PAni-ITO ratio; this dependence can be explained by the addition of nanoparticles introducing new hopping centers for charge carriers, thereby decreasing the hopping distance [27,28,34]. In relation to the concentration dependence, two cases may be regarded: one in which the concentration of hopping carriers changes proportionally to [x] and the other in which the concentration of hopping carriers remains constant indepen-
Log
dc [x]4/3
≈ k1 [x]1/3
while in the second case, Log dc ≈ k2 [x]1/3 [x]1/3
(3)
(4)
In Eqs. (3) and (4) k1 and k2 are dimensionally different constants. Here, we used [x] = 0, 10, 20, 30, 40, and 50%, and found that Equation (3) provides a better fit, that is, with a smaller standard deviation (SD) than with Equation (4), a detailed description of this assumption is presented in ref [34]. Fig. 5 shows the result of the two cases presented through Eqs. (3) and (4). From the results shown in Fig. 5 it is concluded that the concentration of hopping carriers is essentially dependent the concentration of x used in the principle operation of the device. 4. Conclusion This paper has highlighted the importance of exploring the composition of materials in the development of the operating principle of a prototype device based on tuning the ammonia gas sensing performance of hybrid composites, which depends on the electrical conductivity changes of PAni under gas exposure, electrical conductivity of ITO and the relation x on PAni100−x ITOx . The results showed that the gas sensor has a good processingstructure-design-property and that relationships are observed in the electrical and cyclic results. In fact, the ITO nanoparticles were important and decisive for the electrical behavior of PAni100−x ITOx device in a low percolation regime. In this hybrid organic-inorganic composite, the transition from high to low conductivity and therefore the electric current limit is governed by both the value of x
D.S. Calheiro and R.F. Bianchi / Sensors and Actuators A 298 (2019) 111480
Fig. 6. Schematics of the hybrid sensor structure and working principle of the combined PANI-ITO materials in which the electrical current limit is desired for enhanced sensitivity and stability of sensing devices.
(or the concentration of hopping carriers) and the conductivity level of polyaniline. Finally, Fig. 6 shows the schematics of both device structure and working principle of the combined PANI-ITO materials. These results suggest new strategies and direction for future research on hybrid sensors based on a multiple film sensing device in which the maximum limit of detection is tune by the concentration of inorganic material. Acknowledgement The authors thanks CAPES, CNPq, Fapemig and INEO/CNpq. References [1] S. Agate, M. Joyce, L. Lucia, L. Pal, Cellulose and nanocellulose-based flexible-hybrid printed electronics and conductive composites-a review, Carbohydrate Polym. (2018). [2] S.K. Garlapati, M. Divya, B. Breitung, R. Kruk, H. Hahn, S. Dasgupta, Printed electronics based on inorganic semiconductors: From processes and materials to devices, Adv. Mater. (2018) 1707600. [3] D.B. Mitzi, Solution-processed inorganic semiconductors, J. Mater. Chem. 14 (2004) 2355–2365. [4] J.A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics, Science 327 (2010) 1603–1607. [5] J.-H. Kim, M.J. Han, S. Seo, Flexible, stretchable, and patchable organic devices integrated on freestanding polymeric substrates, J. Polymer Sci. Part B: Polym. Phys. 53 (2015) 453–460. [6] M. Koetse, E. Smits, E. Rubingh, P. Teunissen, R. Kusters, R. Abbel, J. Brand, Hybrid Printed Electronics, 2016. [7] M.-H. You, X.-X. Wang, X. Yan, J. Zhang, W.-Z. Song, M. Yu, Z.-Y. Fan, S. Ramakrishna, Y.-Z. Long, A self-powered flexible hybrid piezoelectric-pyroelectric nanogenerator based on non-woven nanofiber membranes, J. Mater. Chem. A 6 (2018) 3500–3509. [8] X. Li, H. Andersson, J. Sidén, T. Schön, Soldering surface mount components on screen-printed ag patterns on paper and polyimide substrates for hybrid printed electronics, Flexible Printed Electron. 3 (2018) 015003. [9] H. Wang, M. Wang, Y. Tang, A novel zinc-ion hybrid supercapacitor for long-life and low-cost energy storage applications, Energy Storage Mater. 13 (2018) 1–7. [10] Y.S. Rim, S.-H. Bae, H. Chen, N. De Marco, Y. Yang, Recent progress in materials and devices toward printable and flexible sensors, Adv. Mater. 28 (2016) 4415–4440. [11] A. Dey, Semiconductor metal oxide gas sensors: A review, Mater. Sci. Eng. B 229 (2018) 206–217. [12] Y. Lin, Y. Gao, F. Fang, Z. Fan, Recent progress on printable power supply devices and systems with nanomaterials, Nano Research, 2018, pp. 1–23. [13] H.S. Vogelbaum, G. Sauvé, Recently developed high-efficiency organic photoactive materials for printable photovoltaic cells: a mini review, Synthetic Metals 223 (2017) 107–121. [14] J.S. Chang, A.F. Facchetti, R. Reuss, A circuits and systems perspective of organic/printed electronics: Review, challenges, and contemporary and emerging design approaches, IEEE J. Emerging Select. Topics Circuits Syst. 7 (2017) 7–26. [15] S. Khan, L. Lorenzelli, R.S. Dahiya, Technologies for printing sensors and electronics over large flexible substrates: a review, IEEE Sensors J. 15 (2015) 3164–3185.
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Biographies
Daniel Silva Calheiro is Graduate student of Federal University of Ouro Preto in Materials Engineering. She received his M.S. (2018) in Materials Engineering from Federal University of Ouro Preto. His research interests are in organic electronic and electrical properties of semiconducting polymers films and devices.
Rodrigo Fernando Bianchi is a professor in Department of Physics, Federal University of Ouro Preto, MG Brazil. He received his PhD in Materials Science and Engineering from University of São Paulo in 2002. He served as a postdoctoral research associate at University of São Paulo (2002-2006) and University of California (2011-2013) His current research interests include organic electronic and medical devices.