Fabrication and Characterization of an Ammonia Gas ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 16, NO. 16, AUGUST 15, 2016

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Fabrication and Characterization of an Ammonia Gas Sensor Based on PEDOT-PSS With N-Doped Graphene Quantum Dots Dopant Mahdieh Hakimi, Alireza Salehi, and Farhad A. Boroumand

Abstract— This paper investigated a room temperature resistive ammonia gas sensor based on a conductive polymer and N-doped graphene quantum dots (N-GQDs) dopant made on a transparent substrate with electrodes. The sensor fabricated with conductive polymer showed a good sensing response that increases considerably with the addition of N-GQDs. The sensing response of the poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT-PSS) to NH3 increased from 30.13% to 212.32% at 1500 ppm with the addition of 50 wt% N-GQDs. The response time of the N-GQDs doped sensor decreased to 6.8 min when compared with the sensor without N-GQDs and the stability of the sensor having combined N-GQDs and PEDOT-PSS was higher than that of the PEDOT-PSS sensor. Meanwhile, the structure and morphology of the sensing film are characterized by Fourier transform infrared spectroscopy and field emissions scanning electron microscopy. Index Terms— Gas sensor, PEDOT-PSS, N-GQDs, ammonia.

I. I NTRODUCTION AS sensors are important for industrial applications such as fire detection [1], leak detection [2], car ventilation control [3], and pollution monitoring [4]. Detection of volatile organic compounds (VOCs) or odors generated by food has become increasingly important in the food industry [5]. Of the gases examined in the literature ammonia is hazardous to human body. The present study used this gas in different concentrations. Ammonia (NH3 ) is a colorless and corrosive gas that is commonly-used in fertilizers, refrigeration, water purification, and manufacturing for nitrogenous products [6]. This gas is dangerous for human health if it passes a threshold of 25 ppm in air [7]. Conducting polymers are widely-used as sensing layers in gas sensors. The advantages of the use of polymers as sensing layers are that they allow room temperature operation, and feature a high sensing response, short response time, and ease of device fabrication [8]. PEDOT (poly(3,4-ethylenedioxythiophene) exhibits relatively high conductivity compared to polymers such as polyaniline. It is known that PEDOT is an insoluble material and if

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Manuscript received April 4, 2016; revised June 16, 2016; accepted June 23, 2016. Date of publication June 28, 2016; date of current version July 18, 2016. The associate editor coordinating the review of this paper and approving it for publication was Prof. Tony Huang. The authors are with the Department of Electronics, Faculty of Electrical Engineering, K. N. Toosi University of Technology, Tehran 1631714191, Iran (e-mail: [email protected]; salehi@kntu. ac.ir; [email protected]). Digital Object Identifier 10.1109/JSEN.2016.2585461

synthesized in the presence of poly(4-styrenesulfonate) (PSS) an aqueous dispersion can be obtained. The electrical properties of PEDOT-PSS allow it to respond strongly to changes in the environment [9]. PEDOT-PSS has limited structural and chemical properties that remain major obstacles and inhibit its use in practical applications. Its combination with novel carbon nanostructures is a potential solution to these shortcomings [10]. Graphene has great potential for applications requiring a transparent, flexible, and conductive material. It is promising for microelectronic and optoelectronic devices [11]. Graphene is a single layer of carbon atoms in a honeycomb structure that has generated excitement because of its large surface area, high carrier transport mobility, superior mechanical flexibility, and excellent thermal/chemical stability [12]. Graphene quantum dots (GQDs) feature a single atomic layer of nano-sized graphite and offer the excellent features of graphene such as high surface area, large diameter, and better surface grafting using π conjugation and surface groups [13]. The diameter of the GQDs and their chemical derivatives ranges from 3 to 20 nm [12]. Moreover, GQDs have a wide range of applications including as electrochemical biosensors [14] and in bioimaging [15] and photovoltaic devices [12], [16]. Nitrogen substitution in the GQDs lattice produces more active sites which can be used in various fields including sensors [17]. II. E XPERIMENTAL P ROCEDURE PEDOT-PSS aqueous solution (1.3 wt% dispersed in H2 O, conductive grade) was purchased from Sigma-Aldrich. The chemical structure of the PEDOT-PSS is shown in Fig. 1. It can be seen that PEDOT has a positive charge and PSS has a negative charge which makes the combination a neutral material. The N-GQDs were synthesized by hydrothermal processing of citric acid and urea. The details of synthesis of N-doped GQDs has been explained in a recent report by the authors [18]. The PEDOT-PSS and N-GQDs were combined together and used as sensing films. A 1 cm × 1 cm polyethylene terephthalate (PET) substrate was used to fabricate the sensor. The substrate was precleaned with methanol, isopropanol, and acetone and dried with nitrogen gas. For better performance, aluminum interdigitated electrodes with a thickness of approximately 100 nm were deposited on the substrate by vacuum evaporation.

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the carrier gas into the test chamber. The presence of water molecules in air ambient increases the electrical resistance of the sensor. Water molecules can affect the sensing response; therefore, a humidity sensor (HIH4000) was used to maintain the humidity inside the chamber at 10%RH. III. R ESULTS AND D ISCUSSION A. Mechanism of Gas Detection

Fig. 1.

Structure of PEDOT (top) and PSS (bottom).

Fig. 2. Schematic picture of the gas sensor structure and polymer deposition.

It was found that sensor resistance increased upon exposure to NH3 at different concentrations. The response characteristic of the gas sensor was analyzed as the percentage of change in sensor resistance as follows: (R g − R0 ) × 100% (1) Sensing response = R0 where R0 and Rg are the sensor resistance in pure air and in the test gas, respectively [10]. A previous study by the authors has reported [18] that VOCs are not reactive at room temperature, making it hard to detect VOCs by chemical reactions in conducting polymers. Typical sensing mechanisms proposed for detection of VOCs include the absorbance and swelling of conducting polymer systems [18]. The reasons for the increase in sensor resistance are: I. Oxygen molecules in the air are physically adsorbed onto the surface and capture electrons from the surface to form chemically-adsorbed oxygen ions [19] Different oxygen species form on the surface of the film at different operating temperatures according to the following reactions [20]: ⎫ O2 (g) → O2 (ads), ⎬ ; for T < 100°C, O2 (ads) + e− → O− (2) 2 ⎭ − → 2O− ; + e for T > 100°C. O− 2 It is further shown in (3), that when the film is exposed to ammonia gas at room temperature, it reacts with the oxygen and the electrons return to the surface [20] as: − 4NH3 + 5O− 2 → 4NO + 6H2 O + 5e

Fig. 3.

Schematic picture of the measurement system.

Silver conductive paste was used for contact with the external circuit. The sensitive film was deposited using the drop-cast method. The distance between the interdigitated electrodes was kept to about 400 μm to obtain a high response to the NH3 gas. A schematic of the fabricated gas sensor is shown in Fig. 2. To allow measurement, the sensor was placed inside a chamber (Fig. 3) and ammonia gas was introduced through a mass flow controller (Alicat scientific, Tucson). The sensor is a resistive device; thus a LCR meter (Pintek-LCR900) was used for resistance measurements. N2 gas was used as

(3)

The transfer of electrons to the surface of the film makes it less p-type and increases the sensor resistance [20]. Because the reaction shown in (2), is low at room temperature, this mechanism is not very effective for the present purposes. The sensor must operate at room temperature to avoid destroying the polymers at high temperature. II. The NH3 molecules are absorbed onto the surface of the film by physisorption and there is charge transfer from the NH3 molecules to the film [21]; hence, the holes on the surface interact with the electron-donating ammonia analyte to increase the resistance. III. The N-GQDs/PEDOT-PSS compound may form a p-n heterojunction [22]. It is believed that as CeO2 /PANI is exposed to electron-donating NH3 , the number of holes decreases and increases the depletion layer of the PANI side [22]. IV. For PEDOT-PSS, more electron hopping occurs between the shorter PEDOT chains than between the longer PSS chains; however, N-GQD is a conductive pathway and swelling causes separation of conductive pathways

HAKIMI et al.: FABRICATION AND CHARACTERIZATION OF AN AMMONIA GAS SENSOR

Fig. 4.

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Schematic picture of (50 wt%) N-GQDs/PEDOT-PSS sensor (a) before and (b) after exposure to NH3 .

Fig. 6. FE-SEM images of (a) the N-GQDs and (b) (50 wt%) N-GQDs/PEDOT-PSS composite were drop casted on the PET.

Fig. 5.

FTIR spectra of N-GQDs.

and increases the PEDOT distance in the film. Electron hopping then becomes more difficult [10] and conductivity decreases upon exposure to ammonia (Fig. 4). The most probable process for the increase in sensor resistance is swelling. B. Measurements Fourier transform infrared spectroscopy (FT-IR Bruker (Germany) VERTEX 70) of the N-GQDs in the range of 4000 ∼ 400 cm−1 is shown in Fig. 5. The bands at 3435.77, 1711.43, 1622.4, and 1424.24 cm−1 are attributed to the –OH group, C=O, aromatic C=C, and symmetric stretching vibration of the COO− , respectively. The bands at 1245.69 and 1032.78 cm−1 are attributed to the C-N stretching mode for the benzenoid unit and C-O stretching mixed with C-OH bending. The peaks for C=C indicate the existence of delocalized π-electrons in the N-GQD molecules. C-N confirmed the doping of N in the sample [21], [23]–[27]. Fig. 6 shows the Field Emissions Scanning Electron Microscopy (FE-SEM) images of pure N-GQDs and N-GQDs/ PEDOT-PSS composite. It can be seen that the N-GQDs

Fig. 7. Sensing response of N-GQDs/PEDOT-PSS sensor to 1500 ppm NH3 .

are relatively uniform. In addition, Fig. 6(b) shows that the composite without obvious aggregation can be because of the oxygen-related functionalities on the N-GQD surface [18], [28]. Different percentages of N-GQD were added to determine its influence on the sensing response. The results are shown in Fig. 7. The sensing response increased strongly after the addition of N-GQDs. It is evident that the addition of 50 wt% N-GQDs increased the sensing response to 212.32%.

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TABLE I R ISE -T IME VS P ERCENTAGE OF N-GQDs IN PEDOT-PSS

Fig. 9. Sensing response of (50 wt%) N-GQDs/PEDOT-PSS and PEDOT-PSS sensors to NH3 at room temperature.

Fig. 8.

Sensing response of sensor for 50 wt% N-GQDs in PEDOT-PSS.

Because N-GQD acts as a conductive pathway, movement of the carriers increased as the percentage of N-GQD content increased. It should be noted that the best results were found for 50 wt% N-GQDs and so, we demonstrate its results here. NH3 gas was applied for 15 min to reach saturation and determine the stability of the sensor. The response time was defined as the time required to reach 90% of the final equilibrium after the gas was injected [7]. It was found that when the N-GQD content in PEDOT-PSS increased to 50%, the rise-time decreased from 12.5 to 6.8 min (Table. I). The good sensing performance of the sensors with 50% N-GQDs in PEDOT-PSS can be attributed to the increase in specific surface area by the N-GQDs which enhanced interactions between the sensing film and NH3 molecules via the π electron network [10]. Fig. 8 shows the dynamic response of the sensor at 1500 ppm NH3 at room temperature. It is shown in the figure that the sensing response increased upon exposure to 1500 ppm NH3 . Furthermore, the sensing response recovered to its original value in the absence of NH3 gas when it was replaced with air. The recovery time for the sensor was measured for 9 min. Fig. 9 compares the results of the sensing responses of the N-GQDs/PEDOT-PSS and PEDOT-PSS sensors. As seen, the response of the N-GQDs/PEDOT-PSS sensor increased strongly from 37.5% at exposure to 30 ppm NH3 to 212.32% at 1500 ppm NH3 . The PEDOT-PSS sensor showed no discernable increase as the NH3 concentration increased to 1500 ppm. The N-GQDs/PEDOT-PSS sensor response rate was higher [21] and is consistent with Table. I.

Fig. 10. Selectivity of (50 wt%) N-GQDs/PEDOT-PSS sensor to various vapors of 1500 ppm at room temperature.

The selectivity of the N-GQDs/PEDOT-PSS sensor to 1500 ppm of carbon dioxide, ethanol, acetone, and toluene was compared with the response to NH3 . Fig. 10 demonstrates the excellent selectivity of the sensor to ammonia gas. Because the sensing mechanism for VOCs is to swell [29], this suggests that hydrogen caused separation and the sensing response was better for materials containing hydrogen. It is believed that the molecular size of ammonia is smaller than of the other alcohols shown in Fig. 10. It can be concluded that this sensor has excellent sensing response to ammonia. Polymer sensors, used for environmental control suffer from the major drawback of a sensing response to ultra-violet (UV) radiation and the presence of oxidizing gases. The longterm and thermal stability of these materials is low and gas sensors on the base of such materials have short lifetimes, especially when operating in normal atmosphere containing water and active gases. These devices are often not suitable for industrial fabrication [30]. It was found that PEDOT-PSS has a short lifetime. The measurements showed that after 35 days, the sensing response of the PEDOT-PSS sensor decreased from 30.13% to 5.54%, while the sensing response for a combination of N-GQDs and PEDOT-PSS decreased

HAKIMI et al.: FABRICATION AND CHARACTERIZATION OF AN AMMONIA GAS SENSOR

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TABLE II S ENSING P ROPERTIES OF S ENSOR VS O THER A MMONIA S ENSORS

to 1500 ppm NH3 gas. The sensor showed good selectivity to ammonia and high sensing response. ACKNOWLEDGEMENT The authors would like to thank Dr. F.A. Taromi and J.N. Gavgani. R EFERENCES

Fig. 11. The effect of the operating temperature on the sensing response of (50 wt%) N-GQDs/PEDOT-PSS sensor.

from 212.32% to 117.09%. The stability of the sensor having combined N-GQDs and PEDOT-PSS was higher than that of the PEDOT-PSS sensor. Effect of temperature on sensing response is shown in Fig. 11. As seen, the sensing response decreased as the temperature increased. The best operating temperature for the sensor is room temperature. An increase in temperature from 25 to 80 °C expanded the matrix, increasing resistance and carrier movement decreased. This decreased the sensing response. The sensing response, response and recovery time of several sensor materials are listed in Table. II. The results of the present study are comparable to those of other ammonia sensors. Sensing film thickness was calculated to be about 3 μm. It is anticipated that response time decreased as the sensing film thickness decreased. IV. C ONCLUSIONS This study deposited sensing film onto interdigitated Al electrodes. Gas sensors were fabricated using a conductive polymer and N-GQDs dopant on a transparent substrate at room temperature. The properties of the sensor improved with the addition of N-GQDs dopant in PEDOT-PSS. It was found that the optimum amount of dopant added was 50 wt% N-GQDs. It showed a response of 212.32% upon exposure

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Mahdieh Hakimi received the B.Sc. degree in electrical engineering from Shariaty University in 2013, and the M.Sc. degree in electrical engineering from the K. N. Toosi University of Technology in 2015. Her research interests are organic electronics and semiconductor technology.

Alireza Salehi received the B.Sc. degree from the Kiel University of Applied Sciences, Germany, the M.Sc. degree from the University of Bremen, Germany, and the Ph.D. degree from Cardiff University, all in electronics. He joined the Faculty of Electrical Engineering, K. N. Toosi University of Technology, as an Assistant Professor in 1996, where he has been a Professor in Nanoelectronics since 2006. His research and teaching interests are fabrication and analysis of optoelectronic devices using semiconductor and organic materials. His research presently focuses on gas sensors based on several semiconductors and organic materials.

Farhad A. Boroumand received the B.E. degree in electronics from the Ferdowsi University of Mashhad, Iran, in 1988, the M.Tech. degree from the Indian Institute of Technology Delhi, New Delhi, India, in 1992, and the Ph.D. degree in integrated electronics from King’s College London, London, U.K., in 2000, with a focus on interactions between isolated GaAs-based MESFET’s. In 2000 and 2006, he worked on four post-doctoral research projects at Sheffield University, U.K., and Surrey University, U.K., concerning nano and organic electronics and photonic devices. He is currently an Assistant Professor with the K. N. Toosi University of Technology, Tehran, Iran, teaching courses, such as nanotechnology, organic electronics, semiconductor devices, and modern physics. He has published over 70 journal and conference papers. His research interests are in the area of nanoorganic electronics, detectors, solid state physics, and optoelectronic devices.