Accepted Manuscript Title: Thin Film Polypyrrole/SWCNTs Nanocomposites-Based NH3 Sensor Operated at Room Temperature Authors: Nguyen Van Hieu, Nguyen Quoc Dung, Tran Trung, Nguyen Duc Chien PII: DOI: Reference: S0925-4005(09)00393-1 doi:10.1016/j.snb.2009.04.061 SNB 11530 To appear in: Sensors and Actuators B Received date: Revised date: Accepted date: 13-1-2009 24-3-2009 23-4-2009 Please cite this article as: N Van Hieu, N.Q Dung, T Trung, N.D Chien, Thin Film Polypyrrole/SWCNTs Nanocomposites-Based NH3 Sensor Operated at Room Temperature, Sensors and Actuators B: Chemical (2008), doi:10.1016/j.snb.2009.04.061 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Manuscript Thin Film Polypyrrole/SWCNTs Nanocomposites-Based NH3 Sensor ip t Operated at Room Temperature Nguyen Van Hieu a,*, Nguyen Quoc Dung a, Tran Trung c, Nguyen Duc Chien b M an Faculty of Environment and Chemistry, Hung Yen University of Technology and Education, Vietnam pt ed *Corresponding author Nguyen Van Hieu, Ph.D International Training Institute for Materials Science (ITIMS), Hanoi University of Technology (HUT) No.1, Dai Co Viet Road, Hanoi, Vietnam 84 38680787 Phone: 84 38692963 Fax: hieu@itims.edu.vn/ E-mail: hieunv-itims@mail.hut.edu.vn No.1 Dai Co Viet, Hanoi, Vietnam Post address: Ac ce c) Institute of Engineering Physics (IEP), Hanoi University of Technology (HUT) us b) International Training Institute for Materials Science (ITIMS), HUT cr a) Page of 31 Abstract - A PPY/SWCNTs nanocomposite-based sensor with relatively high sensitivity and fast response-recovery was developed for detection of NH3 gas at room temperature The gas-sensitive composite thin film was prepared using chemical polymerization and spin-coating techniques, and characterized by Fourier transformed infrared spectra and Field- emission-scanning-electron microscopy The results reveal that the conjugated structure of the PPY layer was formed and the functionalized SWCNTs were well-embedded The effects of film thickness, annealing temperature, and SWCNTs content on gas-sensing properties of the composite thin film were investigated to optimize the gas-sensing performance The as-prepared thin film PPY/SWCNTs composite sensor with optimized process parameters had a response of 26 to 276% upon exposure to NH3 gas 10 concentration from 10 to 800 ppm, and their response and recovery times were around 22 and 38 s, 11 respectively 12 Keywords: Gas sensor; Carbon nanotubes; Polypyrrole; Nanocomposite cr us an M ed pt 14 Ac ce 13 ip t Page of 31 INTRODUCTION Conducting polymers such as polyaniline, polypyrrole (PPY), polythiophene, and their composites have been widely investigated as effective materials for chemical sensors [1-10] Among the conducting polymers, PPY and its composites have attracted considerable attention because they are easily synthesized, have relatively good environmental stability, and surface charge characteristics that can be easily modified by changing the dopant species into the materials during the synthesis It has been demonstrated that there are many approaches in the enhancement of the mechanical strength, chemical stability, and gas-sensing properties by combining PPY with organic 10 and inorganic materials to form composites [6] A large number of reports have been published on 11 chemical polymerization and electrochemical techniques to prepare polypyrrole/inorganic (TiO2, 12 Fe2O3, SnO2, WO3 …) and organic (poly[vinyl alcohol], carbon nanofibers, carbon 13 nanotubes)/polypyrrole nanocompiste sensors to detect a wide variety of gases such as NO2 [7], 14 humidity [11-13], H2S [14], VOCs (volatile organic compounds) [15,16], CO2 [17,18], and NH3 15 [19,20] ed M an us cr ip t The special geometry of carbon nanotubes (CNTs) and their amazing feature of being 17 all-surface reacting materials offer great potential applications such as in gas sensor devices working 18 at room temperature It has been reported that CNTs are very sensitive to the surrounding 19 environment The presence of O2, NH3, NO2 gases and many other molecules can either donate or 20 accept electrons resulting in an alteration of the overall conductivity [22,23] Such properties make 21 CNTs ideal for nano-scale gas sensing materials, and CNTs FETs (Field Effect Transistors) and 22 conductive-based devices have already been demonstrated as gas sensors [24,25] However, CNTs 23 still have certain limitations for gas sensor application such as their long recovery time, limited gas 24 detect, and weakness to humidity and other gases Therefore, the composites of PPY and CNTs have 25 received a great deal of attention for gas-sensing application PPY/SWCNTs and PPY/MWCNTs Ac ce pt 16 Page of 31 composites-based sensors have been already developed for the detection of ethanol and NH3, respectively [16,26] They have shown a higher sensitivity than both PPY- and CNTs-based sensors separately over a wide range of gas concentrations at room temperature However, the sensing properties of nanocomposite thin film have not yet been investigated in depth NH3 gas presents many hazards to both humans and the environment Due to its highly toxic characteristics, even low level concentrations (ppm) pose a serious threat NH3 sensors based on conventional materials such as SnO2 [27,28], TiO2 [29,30], In2O3 [31,32], WO3 [33,34] and ZnO [35,36] have been developed with good sensitivity and selectivity and fast response-recovery that can be used in detecting lower level ammonia gas presence However, most of the NH3 gas sensors 10 are fabricated using metal oxides which are only effectively operated at temperature ranges between 11 150-400°C, resulting in high power consumption and complexities in integration Thus, there is a 12 great need to develop a new class of materials for gas sensors that would have good performance 13 working at room temperature In this paper, we present the sensing-properties of the PPY/SWCNTs 14 nanocomposite thin film, prepared by simple and straightforward in situ chemical polymerization of 15 pyrrole mixed with SWCNTs, toward low concentration of NH3 (10 - 800 ppm) at room temperature 16 The influence of film thickness, SWCNTs content, and annealing temperature are studied to 17 optimize the gas-sensing properties of the composite thin film 19 20 cr us an M ed pt Ac ce 18 ip t EXPERIMENTAL Material synthesis and characterizations 21 All the chemicals used were of analytical grade The SWCNTs were produced by CVD 22 (chemical vapor deposition, Shenzhen Nanotech Port Co Ltd diameter 90%), and they exist as agglomerates Therefore, the SWCNTs were functionalized to 24 enhance their dispersion in the solvent The functionalization was carried out using a typical 25 procedure, as follows: 200 mg SWCNTs were suspended in 35 mL concentrated nitric acid (15 M) Page of 31 and refluxed for 12 h in a silicone oil bath maintained at 120oC to modify the SWCNTs surface, then rinsed with distilled H2O until the pH of the solution was neutral, and finally dried at 80oC in a vacuum oven (~10-2 torr) The formation of the nanocomposite of PPY and f-SWCNTs (functionalized SWCNTs) was carried out by in situ chemical polymerization of pyrrole monomer with f-SWCNTs 3.0 ml sodium dodecylbenzenesolfonate (DBSA) was dissolved in a solution of 20 ml water, then 0.1 g f-SWCNTs was added and stirred by ultrasonic for 30 to obtain a well-suspended SWCNTs The SWCNTs suspension was then transferred into a flask and allowed to stand at a temperature of 0-5oC for 30 The pyrrole monomer (1 ml) was afterwards slowly added drop-wise to the SWCNTs 10 suspension with constant magnetic stirring for 30 at the temperature of 0-5oC, then 0.345 g 11 (NH4)2S2O8 (APS) was added to start the polymerization process lasting 12 h After the 12 polymerization was finished, the composite powder formed was filtered and rinsed with DI water, 13 methanol, acetone, and ethanol until the filtrate became colorless The as-prepared composite 14 powder was then dried in vacuum at room temperature 17 18 cr us an M ed microscope (FE-SEM, 4800 Hitachi, Japan) were used to characterize the as-synthesized materials pt 16 Fourier transformed infrared (FTIR, Niconet 6700) and field-emission scanning electron Ac ce 15 ip t Sensor preparation and characterizations 19 A certain amount of the nanocomposite powder was suspended into ml CHCl3 (Chloroform, 20 containing 0.5ml DBSA) by ultrasonic stirring for 30 The composite suspension with different 21 contents (0.005, 0.01, 0.03, 0.1 g/ml) was prepared for variation of the film thickness by 22 spin-coating at 3000 rpm A silicon substrate with interdigitated- electrode on the top was used for 23 the gas-sensor fabrication The interdigitated-electrode was fabricated using a conventional 24 photolithographic method with a finger width of 100 µm and a gap size of 140 µm The fingers of 25 interdigitated-electrode were fabricated by sputtering 10 nm Ti and 200 nm Pt on a layer of silicon Page of 31 dioxide (SiO2) with the thickness of about 100 nm thermally grown on top of the silicon wafer The coating layers were dried in vacuum and then immersed in methanol to remove DBSA Finally, the samples were annealed at different temperatures (25, 200, 300, 400oC) for 40 The gas sensors were tested with NH3 using the injection technique More details about this testing system can be found elsewhere [37] The electrical resistance response during testing was monitored by the Precision Semiconductor Parameter Analyzer (HP4156A) The sensor response (S) for a given measurement was calculated as follows: S = Rg/Ra, where Rg and Ra are electrical resistances of the sensor in tested gas and in air, respectively us cr ip t an RESULTS AND DISSCUSION 10 Material characterizations 12 Fig presents the FT-IR spectra of PPY and PPY/SWCNTs composite It is clear that PPY and the 13 composite show very similar spectra The peaks at 2917 and 2854 cm−1 are associated with five 14 membered ring C–H stretching [21] The stretching and bending motion of N–H in PPY appear at 15 3361 and 1645 cm−1, respectively [21,38] The peaks at 1249 cm-1 and 1086 cm-1 are due to C-N 16 stretching and C-H deformation vibrations of PPY [21,26,40] Additionally, the peaks at 991 and 17 853 cm-1 related to the in-plane and out-of-plane vibration modes of N-H and C-H [38,39] It can be 18 recognized that the PPY and SWCNT/PPY composites show nearly identical numbers and positions 19 of the main IR bands, and that the characteristic peaks of MWNTs are hardly seen The intensity of 20 the transmission light to the SWCNTs is very low, and the corresponding reflective or scattering 21 light has to transfer the matrix layers of the conducting polymers However, the matrix layer of the 22 PPY has absorbed much of this light, suggesting that the SWCNTs are well-embedded within the 23 matrix of the PPY [41,42] Ac ce pt ed M 11 24 Fig shows the FE-SEM images of f-SWCNTs (Fig 2a), PPY (Fig 2b), and the 25 f-SWCNTs/PPY composite (Fig 2c) The typical morphology of PPY indicates that the particle size Page of 31 of PPY is lower than 100 nm with spherical morphology The obtained morphology of the f-SWCNTs shows that many nanotubes are loosely entangled without any particle-like impurities The morphology of the as-synthesized PPY/SWCNTs composite (see Fig 3c) shows that the SWCNTs are well-embedded within the matrix of the PPY The FT-IR spectra and FE-SEM characterizations confirm that the as-synthesized SWCNTs/PPY nanocomposite prepared in the present work are similar to the carbon nanotubes/PPY composites prepared by previous reports such as through chemical polymerization [41,42] vapor phase polymerization [26], and electrochemical polymerization [43,44] cr ip t Gas-sensing properties an 10 us Fig shows a typical response curve of the thin film SWCNTs/PPY composite gas sensors 12 during gas-sensing at room temperature The response curve indicates that the resistance signal 13 varies with time over the two of cyclic tests Before each cyclic test, the sensor was exposed to the 14 air and the measured resistance of the sensor was equal to Ra At the beginning of each cyclic test, a 15 desired NH3 gas was injected into the chamber (4L) The measured resistance changed gradually 16 After a certain time, the resistance changed very slowly, almost reaching a stable value, Rg, 17 corresponding to the response of the sensor to NH3 gas The glass chamber was then removed from 18 the sensor to expose the sensor to the air again The measured resistance was restored to its original 19 value, Ra The 90% response time for gas exposure (t90%(air-to-gas)) and that for recovery (t90%(gas-to-air)) 20 were calculated from the resistance–time data shown in Fig The t90%(air-to-gas) value is around 22 s, 21 while the t90%(gas-to-air) value is around 38 s As can be seen, these values are lower than those of both 22 the PPY- and the CNTs-based NH3 gas sensors reported in the literature [45,46] The 23 response-recovery time of the SWCNTs/PPY composite is even shorter than that of the single wire 24 and tube gas sensor devices made from PPY nanowires and SWCNTs [48,49] The 25 response-recovery time comparison data are indicated in table I Ac ce pt ed M 11 Page of 31 Fig shows that the resistance of the sensor increases when it is exposed to NH3 gas (electron-donor) This behavior is similar to that of the PPY- and SWCNTs-based sensors [4,7,10,17,22, 24, 27, 46-49], in which both PPY and SWCNTs behave as p-type semiconductors This suggests that the PPY/SWCNTs composite also behaves as a p-type semiconductor, and is consistent with the case of the PPY/MWCNTs composite sensor prepared by vapor phase polymerization [26] This also implies that the adsorption of NH3 on the PPY/SWCNTs composite results in reducing the number of holes in PPY and SWCNTs because NH3 is an electron-donating gas Similar to PPY- and SWCNTs-based sensors, the electron charge transfer is the main mechanism in changing the resistance of the PPY/SWCNTs on adsorption of NH3 gas However, 10 upon NH3 adsorption, electron charge transfer is likely to only occur between NH3 and PPY because 11 the SWCNTs are well-embedded within the PPY an us cr ip t The fast response-recovery of the PPY/SWCNTs composite sensor could be explained as 13 follows It is well-known that the adsorption process of NH3 gas on CNTs is attributed to 14 physisorption and chemisorption, whereas chemisorption is due to site-defect on the sidewall of 15 CNTs [50-52], and the defects of CNTs are unavoidable during the synthesis and purification 16 processes Thus, NH3 chemically adsorbed on the CNTs are hardly removed upon air exposure, 17 resulting in slow response and recovery of CNTs-based sensors For PPY/SWCNTs-based sensors, 18 the adsorption process of NH3 gas can only be by physisorption due to the fact that the SWCNTs are 19 well-embedded in the PPY, and the site-defects on the sidewall of SWCNTs are functionalized with 20 the PPY during the polymerization process It has been also found that the PPY/SWCNTs-based 21 sensor in the present work exhibits shorter response-recovery time than that of the PPY-based 22 sensors reported in previous works [45,48] This could be attributed to the fact that our sensors are 23 made of the thin-film type deposited by spin-coating of the PPY/SWCNTs composite suspension 24 Another reason could be that the composite thin film forms permanent nano-channels due to the 25 hallow-core of SWCNTs, enabling the ammonia molecules to be diffused in and out of the Ac ce pt ed M 12 Page of 31 PPY/SWCNTs composite faster than that of the pristine PPY This also means that the PPY/SWCNTs composite is more porous than the PPY, as has been already claimed by previous reports [16,26] The thin film and the porous film could be the reasons explaining why the PPY/SWCNTs composite-based sensor exhibits a higher response compared to that of PPY-based sensors Additionally, SWCNTs are known to be strong adsorbers of NH3 gas molecules [53] which may help the PPY to interact more easily with them Therefore, this could be another contribution of SWCNTs in the response enhancement of the PPY/SWCNTs composite sensor cr ip t a) The effect of film thickness us It is well known that the thickness of the sensitive layer has a great influence on the 11 gas-sensing performance of thin film sensors, which have provided a much better platform for 12 producing high performance gas sensors However, the influence of the film thickness of PPY as 13 well as PPY/carbon nanotubes composite on their gas-sensing performance has been lacking much 14 thus far As such, in the present work, we try to explore the effect of the thickness of the 15 composite-sensing layer on the sensitivity, to find the optimal thickness for the composite gas sensor 16 As indicated in the previous section, different the PPY/SWCNTs composite suspensions in 17 chloroform were prepared for the gas sensor fabrication with variation in the film thickness We 18 have not yet successfully characterized the film thickness, but we believe that the film thickness 19 decreases with the decrease of the content of the composite suspension Fig 4a shows the sensor 20 response of all as-fabricated sensors up to 150 ppm NH3 at room temperature It can be seen that the 21 sensor response increases with the decrease of PPY/SWCNTs suspension content This can be 22 attributed to the decrease of the film thickness Fig 4b indicates that the electrical resistance of the 23 composite film increases with decreased content of the suspension, and is confirmed by the decrease 24 of the film thickness The mechanism of the film thickness on the response of the PPY/SWCNTs 25 composite sensor has not been yet understood thus far However, a plausible explanation could be Ac ce pt ed M an 10 Page of 31 10 11 12 13 14 15 16 ip t Au-loaded In2O3 ceramics, Sens Actuators B 56 (1999) 31-36 33 X Wang, N Miura, N Yamazoe, Study of WO3-based sensing materials for NH3 and NO cr 32 V Romanovskaya, M Ivanovskaya, P Bogdanov, A study of sensing properties of Pt- and detection, Sens Actuators B 66 (2000) 74-76 34 V Srivastava, K Jain, Highly sensitive NH3 sensor using Pt catalyzed silica coating over WO3 us gas sensing, Mater Sci Eng C, 26 (2006) 500-504 thick films, Sens Actuators B 133 (2008) 46-52 an 31 H Mbarek, M Saadoun, B Bessais, Screen-printed tin-doped indium oxide (ITO) films for NH3 35 D.R Patil, L.A Patil, P.P Patil, Cr2O3-activated ZnO thick film resistors for ammonia gas sensing operable at room temperature, Sens 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Actuators B 80 (2001) 125-131 57 X Zhang, J Zhang, R Wang, T Zhu, and Z Liu, Surfactant-directed polypyrrole/CNT 15 nanocables: synthesis, characterization, and enhanced electrical properties, Chem Phys Chem 16 (2004) 998-1002 18 19 pt 58 N.G Sahoo, Y.C Jung, H.H So, J.W Cho, Polypyrrole coated carbon nanotubes: Synthesis, Ac ce 17 ed 14 characterization, and enhanced electrical properties, Synth Metals 157 (2007) 374-379 19 Page 19 of 31 List of figure captions Fig FT-IR spectra of (a) PPY and (b) PPY/SWCNTs (1 wt %) nanocomposite Fig FE-SEM image of (a) f-SWCNTs, (b) PPY, and (c)PPY/SWCNTs composite Fig Response curve of SWCNTs/PPY composite sensor to NH3 at room temperature Fig The NH3 gas sensing characteristics of PPY/SWCNTs composite at different film thickness; (a) transient responses of the sensor to 150 ppm NH3; (b) the sensor response as a function of the film resistance Fig The NH3 gas sensing characteristics of PPY/SWCNTs composite at different 10 SWCNTs contents; (a) transient responses of the sensor to 150 ppm NH3; (b) the sensor 11 response as a function of the SWCNTs content 12 Fig The NH3 gas sensing characteristics of PPY/SWCNTs composite annealing at 13 different temperatures; (a) transient responses of the sensors to 150 ppm NH3; (b) the sensor 14 response as a function of the annealing temperature 15 Fig The NH3 gas sensing characteristics of PPY/SWCNTs composite at different 16 operating temperature;(a) transient responses of the sensors to 150 ppm NH3;(b) the sensor 17 response as a function of operating temperature 18 Fig Step wise increase in electrical resistance obtained with increasing NH3 gas 19 concentration from air to 800 ppm NH3 in air (a); the sensor response as a function of the 20 NH3 gas concentration (b) 21 Table I: Response-recovery times from this work and the literatures Ac ce pt ed M an us cr ip t 22 20 Page 20 of 31 Biographies Nguyen Van Hieu received his MSc degree from the International Training Institute for Material Science (ITIMS), Hanoi University of Technology (HUT) in 1997 and PhD degree from the Department of Electrical Engineering, University of Twente, Netherlands in 2004 Since 2004, he has been a research lecturer at the ITIMS In 2007, he worked as a post-doctoral fellow, Korea University His current research interests include nanomaterials nanofabrications, characterizations and applications to electronic devices, gas sensors and biosensors Contact hieu@itims.edu.vn or hieunv-itims@mail.hut.edu.vn us cr ip t an 10 Nguyen Quoc Dung received the BS degree in Chemistry at Hanoi University of Education in 2004, 12 and MSc degree in materials science from the International Training Institute of Material Science 13 (ITIMS), Hanoi University of Technology (HUT), in 2006 Her research interest is the development 14 of conducting polymer and conducting polymer/carbon nanotubes composites gas sensing 15 applications pt 16 ed M 11 Tran Trung received MSc degree in 1994 and PhD degree in 1998 from Department of 18 Electrochemistry, Hanoi University of Technology During 2000 and 2001 he worked as a 19 post-doctoral fellow in Pusan National University, Korea At present he has been working as 20 Associate Professor at Faculty of Environment and Chemistry, Hung-Yen University of Technology 21 and Education His research activities are related with the design, fabrication and characterization of 22 organic-inorganic hybrids and nanomaterials for application to electronic devices and battery 23 systems Ac ce 17 24 25 21 Page 21 of 31 Nguyen Duc Chien received the engineering degree in electronic engineering at Leningrad Electrotechnical University, Russian, in 1976, and the MSc and PhD in microelectronics at Grenoble Polytechnique University, France, in 1985 and 1988, respectively He has worked as associated professor at the Institute of Engineering Physics (IEP), Hanoi University of Technology (HUT) From 1989 to 1990 he worked as a visiting professor at the Grenoble University, France From 1992 to 2006 he was a vice director of the International Training Institute for Materials Science (ITIMS), HUT, where he established a Laboratory of Microelectronics and Sensors Since 2003 he has been the Director of the IEP, HUT His research interests include: characterizations and modeling of MOS devices, nanomaterials for chemical sensor, biosensor, optoelectronic materials 10 and devices, and MEMS devices He has been a leader of many national research projects related to 11 microelectronic devices and functional nanomaterials Dr Nguyen Duc Chien is a member of 12 Physics Society of Vietnam and Vietnamese Materials Research Society Ac ce pt ed M an us cr ip t 22 Page 22 of 31 Figure(s) Figure Ac ce pt (b) ed M an us cr ip t (a) Page 23 of 31 ip t Figure us cr (a) an 80 nm 200 nm pt ed M (b) Ac ce (c) 60 nm Page 24 of 31 ip t Figure cr 0.75 Response ~ 22 s 0.70 us Air 0.60 an R(MW) 0.65 0.55 Recovery ~ 38 s NH3, 150 ppm M 0.50 100 200 300 400 Ac ce pt ed Time (s) Page 25 of 31 ip t us cr Figure 1.7 PPy/SWCNTs: Chloroform 0.1 g/ml 0.03 g/ml 0.01 g/ml 0.005 g/ml an 150 ppm NH3 1.5 1.4 M (a) 1.3 1.2 1.1 ed Response (Rg/Ra) 1.6 1.0 0.9 100 pt 200 300 400 500 600 700 Time (s) Ac ce 1.5 Decrease of film thickness Response (Rg/Ra) 1.4 (b) 1.3 1.2 1.1 PPy-SWCNTs/Clorofom = g/ml 1.0 0.1 0.03 0.01 0.005 100k 200k 300k 400k 500k 600k 700k 800k Resistance ( ) Page 26 of 31 Figure 2.4 ip t 2.2 2.0 cr 1.8 SWCNTs content 0.0% 0.5% 1.0% 3.0% 5.0% 1.6 1.4 us Response (Rg/Ra) 150 ppm NH3 (a) 1.2 0.8 300 600 an 1.0 900 1200 1500 1800 2100 2.2 (b) M Time (s) 150 ppm NH3 ed 1.8 1.6 pt Response (Rg/Ra) 2.0 Ac ce 1.4 1.2 0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 SWCNTs content (%) Page 27 of 31 Figure 2.4 2.0 1.8 ip t 2.2 150 ppm NH3 cr 1.6 1.4 1.2 1.0 0.8 200 400 600 800 1000 an us Response (Rg/Ra) (a) Annealing temp o HT @25 C o HT @ 200 C o HT @ 300 C o HT @ 400 C 1200 Time (s) 2.0 ed 1.8 pt 1.6 1.4 1.2 Ac ce Response (Rg/Ra) (b) M 2.2 50 100 150 200 250 300 350 400 450 o Heat-treated temperature ( C) Page 28 of 31 Figure Operating temperature o @ 25 C o @ 40 C o @ 50 C 1.6 (a) 150 ppm NH3 1.5 cr 1.4 1.3 1.2 us Response (Rg/Ra) 1.7 ip t 1.8 1.1 0.9 200 400 600 800 Time (s) 1.70 M ed 1.55 1.50 Ac ce pt Response (Ra/Ra) 1200 (b) 1.60 1.40 1000 150 ppm NH3 1.65 1.45 an 1.0 1.35 24 27 30 33 36 39 42 45 48 51 o Operating temp ( C) Page 29 of 31 (a) 22 400ppm 20 NH3 220ppm 140ppm 16 80ppm 14 an R(M) 18 300ppm 40ppm 10ppm 12 200 400 Air M 10 Air cr 800ppm 500ppm us 24 ip t Figure 600 800 1000 1200 1400 1600 1800 3.0 2.8 (b) 2.4 2.2 Ac ce Response(Rg/Ra) pt 2.6 ed Time (s) 2.0 1.8 1.6 1.4 1.2 100 200 300 400 500 600 700 800 NH3 concentration (ppm) Page 30 of 31 Table(s) Table I: Response-recovery time from this work and the literatures Response time (sec) 240 60 840 6000 References This work [45] [46] [48] [49] 202 120 1800 43200 Ac ce pt ed M an us cr PPY/SWCNT PPY thin film SWCNTs film Single wire of PPY Single wire of SWCNTs Recovery time (sec) ip t Sensor type Page 31 of 31 ...Manuscript Thin Film Polypyrrole/ SWCNTs Nanocomposites-Based NH3 Sensor ip t Operated at Room Temperature Nguyen Van Hieu a,*, Nguyen... The thin film and the porous film could be the reasons explaining why the PPY /SWCNTs composite-based sensor exhibits a higher response compared to that of PPY-based sensors Additionally, SWCNTs. .. the PPY /SWCNTs composite sensor depends on the reactivity and diffusion of NH3 gas molecules inside the composite-sensing thin film Therefore, when the thickness of the film decreases, the sensor