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Low temperature acetylene gas sensor based on Ag nanoparticles-loaded ZnO-reduced graphene oxide hybrid
Sensors and Actuators B 207 (2015) 362–369
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Low temperature acetylene gas sensor based on Ag nanoparticles-loaded ZnO-reduced graphene oxide hybrid A.S.M. Iftekhar Uddin, Duy-Thach Phan, Gwiy-Sang Chung ∗ School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 680-749, South Korea
a r t i c l e
i n f o
Article history: Received 20 August 2014 Received in revised form 23 September 2014 Accepted 20 October 2014 Available online 31 October 2014 Keywords: Acetylene sensor ZnO nanoparticles Reduced graphene oxide Silver Hybrid Chemical route
a b s t r a c t This paper scrutinizes the fabrication of a chemiresistive type of acetylene (C2 H2 ) gas sensor by synthesizing a silver (Ag)-loaded zinc oxide (ZnO)-reduced graphene oxide (Gr) hybrid via a facile chemical route. The as-synthesized hybrid was characterized in detail in terms of its structural, morphological and compositional properties. The physical properties of the hybrid exhibited a well-structured crystalline nature and mixed phases of Ag, Gr, and ZnO. The morphological characterization revealed that particlelike nanostructures of the ZnO and Ag mixer were well distributed and closely affixed onto the surface of thin-layer reduced graphene oxide sheets. At an optimum temperature of 150 ◦ C, the 3 wt% Ag-loaded ZnO–Gr hybrid showed preferential detection of acetylene gas with a response value of 21.2 for 100 ppm gas concentrations. The fabricated sensor showed a low detection limit of 1 ppm, fast response and recovery times of 25 s and 80 s, respectively, and good repeatability. Experimental results also showed that the synthesized hybrid had a negligible relative humidity (RH) effect up to 31% RH, and then deteriorated significantly with increasing RH concentrations. After detailed examination, we conclude that an Ag-loaded ZnO–Gr hybrid could be an effective means of fabricating high-performance practical C2 H2 sensors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Acetylene (C2 H2 ) is a colorless, flammable unsaturated hydrocarbon gas with a distinctive odor, widely used as a fuel in oxyacetylene welding and cutting of metals, and as a raw material in various industrial and consumer products, such as acetaldehyde, synthetic rubber, paints, fabric and floor coverings, dry-cleaning solvents, and insecticide sprays. Generally, acetylene is not toxic, but when generated from calcium carbide, can contain toxic impurities such as traces of phosphine and arsine. Notable hazards are associated with its intrinsic instability, especially when it is liquefied, pressurized, heated or mixed with air. Consequently, the combustion of acetylene can produce a large amount of heat (its highest flame temperature is about 3300 ◦ C), and it can explode with extreme violence if the absolute pressure of the gas exceeds 103 kPa (15 psi). Therefore, for environmental and safety purposes, the development of a highly effective C2 H2 sensor has become increasingly important to meet the demands of accurate environmental monitoring, for early leakage warning and for avoiding
∗ Corresponding author. Tel.: +82 52 259 1248; fax: +82 52 259 1686. E-mail address: [email protected] (G.-S. Chung). URL: http://home2.ulsan.ac.kr/user/gschung (G.-S. Chung). http://dx.doi.org/10.1016/j.snb.2014.10.091 0925-4005/© 2014 Elsevier B.V. All rights reserved.
incomplete combustion. However, information on C2 H2 sensors is still very limited in the literature. Most recently, Wang et al. [1] showed successful advancement in C2 H2 sensing with a response of 17 (for 2000 ppm) at 250 ◦ C by synthesizing nickel-doped zinc oxide (ZnO) nanofibers. Tamaekong et al. [2] reported a Pt/ZnO thick film-based C2 H2 sensor at a working temperature of 300 ◦ C with a sensor response of 43 (for 1000 ppm). Zhang et al. [3] hydrothermally synthesized a hierarchical nanoparticle (NP)-decorated ZnO microdisk-based C2 H2 sensor with a large detection range (1–4000 ppm) and very high response (7.9 for 1 ppm) at 420 ◦ C. Dong et al. [4] reported arc plasma-assisted Ag/ZnO composites for C2 H2 sensing, which had a maximum response of 42–5000 ppm C2 H2 at 120 ◦ C. In addition, SnO2 NPs [5], Pd–SnO2 [6], Sm2 O3 –SnO2 [7], Au/MWCNT [8], Ag/Pd–SiO2 [9], etc, have been studied for C2 H2 sensing. However, high working temperature, low sensitivity and synthesis-process complexity, are still a great challenge. In the last few years, among widely investigated metal oxide groups, ZnO has attracted considerable interest in sensing applications to detect volatile and toxic gases due to its high conductive electron mobility and good adsorption characteristics under the working conditions of the sensors [10–15]. However, drawbacks like low sensitivity, poor selectivity and high working temperature limit practical applications in the gas-sensing area (i.e., flammable
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and explosive environments). Therefore, to overcome the shortcomings and to enhance sensing characteristics, researchers have been focusing on metal oxide–metal oxide and metal–metal oxide compositions with numerous synthesis techniques [16–20]. Interestingly, inherent catalytic properties of metal aggregates, dispersed into a metal oxide matrix, modify the surface reactions and greatly enhance the charge carrier separation of the oxide matrix. This phenomenon helps to boost the sensing mechanism, and hence improve sensing performance [21]. Among numerous metals, Ag has attracted immense interest, due to its promising catalytic properties, in the development of photocatalysis [22,23], catalytic oxidation [24], chemical and gas sensing [25,26], etc. A number of Ag–ZnO nanostructure-based highperformance gas sensors have been reported [25,27–30]. Moreover, two-dimensional graphene has concerned much attention since its discovery due to its excellent electrical, chemical and optical properties. Most recently, Fowler et al. [31] discussed chemically derived graphene as a highly sensitive chemical sensor due to its extraordinary carrier mobility. Besides the synergistic interplay between graphene and metal–metal oxide composition, graphene as a template plays a vital role in enhancing the physical properties and sensing performance of the composite materials [19,32–34]. In this context, this paper is focused on the fabrication of a C2 H2 sensor based on an Ag-loaded ZnO-reduced graphene oxide (Gr) hybrid via a facile chemical route in order to enhance C2 H2 sensing performance at low operating temperatures. To the best of our knowledge, an Ag-loaded ZnO–Gr hybrid nanostructure-based acetylene gas sensor has not yet been reported in the literature. The fabricated sensor was evaluated systematically in terms of response, response/recovery times and selectivity toward C2 H2 . The humidity effect on the fabricated sensor was also studied in this work. The main aim of this work is to fabricate an effective, high-performance C2 H2 sensor that can operate at low temperature.
2. Experimental 2.1. Synthesis and characterization All the chemicals used in the experiment were of analytical grade and obtained from Sigma Aldrich, Dongwoo Fine-Chem., and Dae Jung Chem. & Inds. Co. Ltd. Graphene oxide (GO) was prepared according to the process provided by Hummers and Offeman, and Phan et al. [35,36]. ZnO powder was prepared through a solvothermal method using 4 M of Zn(NO3 )2 ·6H2 O and 8 M of sodium hydroxide (NaOH) in ethanol, at 120 ◦ C for 8 h and dried at 60 ◦ C. The Ag-loaded ZnO–Gr hybrid was synthesized via a chemical route. In a typical process, an appropriate amount of silver nitrate (AgNO3 ) was added to the ZnO–GO at 2:1, followed by continuous stirring for 30 min. Hydrazine monohydrate was then used in the mixer as an agent to reduce GO to reduced graphene oxide (Gr) and Ag+ to Ag atoms at a temperature of 110 ◦ C for 8 h. Various samples were prepared by varying the weight percent (0–5 wt%) of AgNO3 to ZnO, referred to as ZnO–Gr/Ag%: ZG–Ag0, ZG–Ag1, ZG–Ag3, ZG–Ag5. Phase transition analysis was carried out with an X-ray diffractometer (XRD) (Rigaku Ultima IV) with Cu K␣ ( = 0.154056 nm) radiation with a 2 scanning range of 10–80◦ . The surface morphology and the compositional analysis of the as-prepared samples were examined with field emission scanning electron microscopy (FESEM) using a JEOL JSM-7600F (accelerating voltage: 10 kV). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and an energy-dispersive spectrometer (EDS) were carried out using a JEOL JEM-2010F.
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Fig. 1. Schematic diagram of the fabricated sensor device.
2.2. Gas sensing For electrical and gas sensing measurements, a resistivitytype sensor device was fabricated as follows: two electrodes (dimension: 2 mm × 4 mm, thickness: 100 nm) were fabricated by depositing platinum (Pt) on 6 mm × 12 mm Al2 O3 substrate by liftoff process. The spacing between two electrodes was 210 m. The sensing material was then well dissolved in ethanol (1 mg/mL) and deposited on the center of the patterned electrodes using a spraycoating process. The device was dried at 100 ◦ C on a hot-plate until on all solvent evaporates. This step was repeated for 3–4 times. Fig. 1 shows a schematic diagram of the fabricated sensor device. Finally, post-situ annealing in air was carried out with the sensor device at 400 ◦ C for 30 min before gas sensing tests. Gas sensing measurements were conducted at atmospheric pressure within a temperature range of 25–250 ◦ C for various C2 H2 concentrations in a sealed chamber using the flow-through technique. The fabricated sensor device was placed in an in-house gas sensor assembly (chamber), and a Keithley probe station (SCS4200) with a bias voltage fixed at 1 V was used for all measurements and data acquisition. A programmable heater integrated with the sensor holder in the chamber was used to adjust the temperature. A computerized mass flow controller (ATO-VAC, GMC 1200) system was used to vary the concentration of C2 H2 in synthetic air. The gas mixture was delivered to the chamber at a constant flow rate of 50 sccm (standard cubic centimeters per minute) with different C2 H2 concentrations. The gas chamber was purged with synthetic air between each C2 H2 pulse to allow the surface of the sensor to return to atmospheric conditions. The sensor response (S) was calculated using Ra /Rg , where Ra is the resistance of the sensor in the presence of synthetic air, and Rg is the resistance in the presence of C2 H2 at certain concentrations. The response time and recovery time of the sensor are defined as the time to reach 90% of total resistance change. 3. Results and discussion 3.1. Crystal structure and morphology The observed XRD patterns of pure ZnO, and 0–5 wt% Ag-loaded ZnO–Gr hybrids are displayed in Fig. 2. The characteristic diffraction peaks of the as-synthesized materials exhibited a well-structured crystalline nature and mixed phases of Gr and ZnO (Fig. 2(b)), and silver, Gr, and ZnO (Fig. 2(c–e)), respectively. Fig. 2a shows the diffraction peaks of pure ZnO. The diffraction peaks of ZnO (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), and (1 1 2) were endowed with a harmonious standard ZnO hexagonal wurtzite structure (JCPDS card No. 36-1451) and the peaks of Ag (1 1 1), (2 0 0) and (2 2 0)
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Fig. 2. XRD patterns of (a) pure ZnO and ZG–Ag hybrids; (b–e) unloaded, 1 wt%, 3 wt%, and 5 wt% Ag.
were indexed to the cubic phase silver (JCPDS card No. 04-0783). No spurious diffractions due to crystallographic impurities of ZnO, and no significant variations of ZnO peak positions were observed in the samples. However, the broadened peaks of ZnO (1 0 0), (0 0 2) and (1 0 1) in the hybrid samples indicated small but finite degradation in the crystalline structure of ZnO due to distortion of Ag and Gr sheets [19,37]. Conversely, a noticeable intensity increase in Ag peaks was observed with increasing amounts of Ag in the hybrid. The absence of any Ag2 O peaks suggested no oxidation of Ag and highlighted the thermal stability of Ag in the final product [27]. The characteristic reflection peak of Gr (0 0 2) appeared at 2 = 26.2◦ , suggesting chemical reduction of oxygen-containing functional groups of GO, reduced and restacked into an ordered crystalline structure of Gr [37]. The surface morphology of the synthesized pure ZnO, unloaded and 3 wt% Ag-loaded ZnO–Gr hybrids are presented in Fig. 3. ZnO nanoparticles with a mean diameter of 40 nm are shown in Fig. 3(a). Fig. 3(b) and (c) represent the unloaded and 3 wt% Ag-loaded ZnO–Gr hybrid, respectively, displaying ZnO and Ag nanoparticles closely affixed onto the Gr sheets with less aggregation, illustrating the excellent attachment between the Gr sheet and ZnO/Ag nanoparticles. It was evident that functional oxygen-containing groups, such as carboxylates of GO, played a vital role in the synthesis process due to its strong hydrophilic nature, and facilitated the firm attachment of ZnO and Ag nanoparticles onto the Gr sheets in the final product [38]. The elemental composition of the as-synthesized ZG–Ag hybrid was investigated by EDS and elemental mapping, and is shown in Fig. 4. A selected area of the EDS spectrum from Fig. 4(a) was taken, and the result is demonstrated in Fig. 4(b). The presence of various well-defined peaks of Zn, oxygen (O), carbon (C) and Ag confirmed the formation of a high purity ZG–Ag hybrid. To confirm the distribution of Zn, O, C and Ag ions onto the lattice surface, elemental mapping of the area shown in Fig. 4(a) was carried out, and the results are depicted in Fig. 4(c–f). The analysis shows that Zn, O, C and Ag are distributed homogeneously over the whole material, which further confirmed the formation of a well dispersed ZG–Ag hybrid. In order to obtain detailed information on the crystallinity and morphology of the 3 wt% Ag-loaded ZnO–Gr hybrid, TEM observations were carried out. TEM images of a ZG–Ag3 sample show a high yield of Ag/ZnO/Gr heterostructured nanocrystals consisting of metallic Ag nanoparticles, ZnO nanoparticles and very thin (transparent) Gr sheets as presented in Fig. 5(a)
Fig. 3. Plane-view FESEM micrographs of (a) pure ZnO; (b) unloaded and (c) 3 wt% Ag-loaded ZnO–Gr hybrids.
(low magnification) and 5(b) (high magnification). The HRTEM image (Fig. 5(c)) from a selected area of Fig. 5(b) shows a distinguished interface and the continuity of lattice fringes of the ZnO nanoparticle, Gr and the metallic Ag nanoparticle. The corresponding selected area electron diffraction (SAED), shown in the inset of Fig. 5(c) reveals the polycrystalline nature of the hybrid sample. The polycrystalline nature may be attributed due to the random orientation of Gr sheets on the ZnO surface and covered by the Ag. The measured spacing between adjacent lattice fringes are 0.28 nm and 0.24 nm, corresponding to the (1 0 0) and (1 0 1) planes of the hexagonal ZnO. The lattice fringes with interplanar spacing of 0.235 nm corresponding to the (1 1 1) planes of fcc Ag are observed. The lattice distance of 0.34 nm can be ascribed to the (0 0 2) plane of Gr.
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Fig. 4. The typical EDS spectrum and elemental mapping of the 3 wt% Ag-loaded ZnO–Gr hybrid.
3.2. Acetylene sensing characteristics Generally, a chemiresistive semiconductor sensor is based on the resistance change of the sensing material due to the chemical and electronic interaction between the adsorbed gas and the sensing material. The sensing mechanism can be described in terms of oxygen adsorption reaction on the sensing material surface, which strongly depends on operating temperature, and the stable oxygen ions are O2 − , O− and O2− that operate below 100 ◦ C, within 100–300 ◦ C, and above 300 ◦ C, respectively [39]. Hence, the oxygen adsorption reaction can be represented as follows: O2 (gas) + e− ↔ O2 − (adsorb)(low temperature) −
−
−
2−
O2 (gas) + e ↔ Oads (moderate temperature) O2 (gas) + e ↔ Oads
(high temperature)
(1) (2) (3)
The sensor responses of the as-synthesized pure ZnO, ZnO–Gr and Ag-loaded ZnO–Gr to 100 ppm acetylene are plotted in Fig. 6(a) as a function of operating temperature. It indicates that sensors based on two or more components mixed together are more sensitive to target gases than their individual counterparts alone, suggesting a synergistic effect among the interplaying components [40]. We can see that all the hybrid samples exhibited higher
sensor response than the ZG–Ag0 sample. With ZG–Ag0, the sensor response was about 16 at 250 ◦ C, whereas the responses of the ZG–Ag1 to ZG–Ag5 samples were 10.7, 21.2 and 12.8, respectively, at 150 ◦ C. Pure ZnO showed negligible response within the temperature range of 25–250 ◦ C. Generally, an acetylene sensor based on a pure ZnO nanostructure works at high operating temperatures (about 420 ◦ C) [3]. This behavior is due to the potential barrier formed by the chemisorbed oxygen ions on the ZnO surface, which prevents the C2 H2 molecules from reacting at low temperature. Ag has better oxygen dissociation ability than ZnO and catalytically activates the dissociation of molecular oxygen on the ZG–Ag hybrid surface, which creates higher acetylene sensing active sites on the surface of ZG–Ag1 to ZG–Ag5 hybrids than a pure ZnO or ZnO–Gr composite. The formation of depletion zones around the ZnO particles due to the addition of Ag is attributed to the modulation of the nano-Schottky barriers and hence improves the surface reactivity at low temperature [41]. At the same time, the oxygen ions (Oads − ) on the sensing material surface become more active with the acetylene molecules, which leave the electrons from the surface on the conduction band of the sensing material and thus, the chemical reaction between acetylene gas molecules and oxygen ions can be represented as:
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Fig. 5. (a) TEM, and (b–c) HRTEM images of the ZG–Ag3 hybrid sample. Inset: the corresponding SAED pattern of the ZG–Ag3 sample.
KEth
C2 H2 + 5O− −→2CO2 + H2 O + 5e− ads
(4)
This reaction causes an increase in conductivity due to the free electrons on the sensing material surface, and thus decreases the resistance of the sensor. From Eq. (4), the rate equation of electron density can be written as: dn = KEth (T ) [O− ] [C2 H2 ] ads dt or, n = KEth (T ) [O− ] [C2 H2 ]t + no ads
(5)
(6)
where, KEth (T) is the reaction rate constant or coefficient, n is the electron density under an acetylene atmosphere and no is the electron density under an air atmosphere. Usually, the reaction rate coefficient and electron density have a strong relationship to operating temperature, and magnitude increases exponentially with rising temperatures. However, sensor response (S = Ra /Rg ) is directly proportional to the reaction rate coefficient and is inversely proportional to electron density [42]. For acetylene gas sensing, these two parameters compete with each other and result in maximum sensor response of the ZG–Ag hybrid at an optimum operating temperature of 150 ◦ C which reveals the superiority of our sensor device. In Table 1, we summarized the previously reported results regarding C2 H2 sensors and compared with our work to show the advancement of our sensor device. Several researchers have reported that the sensor response is highly affected by the presence of an efficient and appropriate catalyst that enhances the surface reactivity toward the target gaseous molecules [43,44]. After Ag loading on ZnO–Gr, additional active
sites were produced on the surface of the ZG–Ag hybrid due to the spillover effect, and more C2 H2 molecules could be adsorbed on the surface. Thus, the presence of appropriate Ag could enhance sensor response. Among the sensors tested, 3 wt% Ag-loaded ZnO–Gr was confirmed to exhibit the highest C2 H2 response value at 150 ◦ C; that is, the optimum loading amount of Ag. However, beyond 3 wt% Ag loading, the surface may dissociate more oxygen molecules exceeding the percolation threshold causing spillover zone overlap [45]. This situation may hinder an effective oxygen delivery system and reduce the probability of C2 H2 adsorption, leading to a decline in sensor response. Representative sensor response variations of ZG–Ag3 toward 1–1000 ppm C2 H2 at a working temperature of 150 ◦ C are plotted in Fig. 6(b). A maximum sensor response of 34 was recorded at a 1000 ppm gas concentration at 150 ◦ C. In the hybrid, Gr acted as a matrix or template that helped to connect and to form an additional conducting pathway between Ag and ZnO nanoparticles, leading to a high signal-to-noise ratio. It is clearly observed from Fig. 6(b) that the ZG–Ag hybrid could reliably detect C2 H2 at a 1 ppm concentration, but the response curve trend did not follow good linearity throughout the 1–1000 ppm gas concentration. With increasing gas concentration, the sensor response increased steeply from 1 ppm to 50 ppm and then increased with a relatively gentle slope from 50 ppm to 1000 ppm. This phenomenon might be attributed to the power law dependence on concentration, which arose from receptor and transducer functions of C2 H2 with the sensor surface and the change of surface potential [46]. The dynamic response of the 3 wt% Ag-loaded ZnO–Gr hybrid sensor to periodic changes between 100 ppm C2 H2 and synthetic
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Table 1 Summary of reported works on C2 H2 gas sensors. Ref.
Material
LOD (ppm)
Maxm gas concentration (ppm)
Response (S = Ra /Rg )
Operating temp. (◦ C)
Resp./recov. time (s)
Selectivity
[1] [2] [3] [5] [7] This work
5 at% Ni doped ZnO 2 at% Pt loaded ZnO ZnO SnO2 6 wt% Sm2 O3 doped SnO2 Ag/ZnO–Gr
100 50 1 200 10 1
2000 10,000 4000 10,000 4000 1000
17 (2000 ppm) 43 (1000 ppm) 52 (200 ppm) 6.3 (10,000 ppm) 63.8 (1000 ppm) 21.2 (100 ppm)
250 300 420 300 180 150
5/10 6/65 15/19 34/10 min 3/17 25/80
– Good Excellent Poor Good Excellent
air at 150 ◦ C is illustrated in Fig. 7(a). We can see that the resistance of the sensor quickly lessened when the C2 H2 was injected and returned to its initial baseline value when the gas was stopped, which suggested good reversibility of the as-synthesized sensing material. Fig. 7(a) also shows good repeatability of the sensor with a negligible drift (∼1–2%) within a cycle-to-cycle response. Moreover, the sensor exhibited a fast response time (∼25 s) and a relatively slow recovery time (∼80 s) as shown in Fig. 7(b). The sensor response of the unloaded and 3 wt% Ag-loaded ZnO–Gr hybrids were also compared with other test gases, and the results from exposure to 100 ppm of different gases are plotted in Fig. 8. It is clearly observed that the 3 wt% Ag-loaded ZnO–Gr hybrid has a high response toward C2 H2 and a negligible response to H2 ,
CO, CO2 , NO2 , and O2 compared to the unloaded one. This situation may be attributed to the high selective absorption of the Ag catalyst with C2 H2 molecules. Relative humidity interference is an important consideration in sensor function. It is believed that water molecules could affect the gas response of ZnO-based sensors. The reaction between chemisorbed oxygen ions on the sensing material surface and the water molecules results in a decrease in baseline resistance, and hence reduces the response of the sensor [47]. In the present work, C2 H2 sensing behaviors of the Ag-loaded ZnO–Gr hybrid were also investigated in the air atmosphere in the presence of water vapors, and similar consequences were observed in the hybrid where ZnO acted as the core material for sensing features. Fig. 9 shows the C2 H2 sensing behaviors of the 3 wt% Ag-loaded ZnO–Gr hybrid at an optimum temperature of 150 ◦ C for different RH concentrations. The humidity concentration of the synthetic air that we used in our gas
Fig. 6. (a) Gas sensing properties of pure ZnO, and the unloaded-5 wt% Ag-loaded ZnO–Gr hybrids to 100 ppm C2 H2 at different operating temperatures; (b) response variation of the ZG–Ag3 sample to different C2 H2 concentrations at 150 ◦ C.
Fig. 7. (a) Resistance change during cyclic tests (C2 H2 and synthetic air), (b) a magnified response and recovery characteristic cycle of the ZG–Ag3 sample to 100 ppm C2 H2 at 150 ◦ C.
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C2 H2 adsorption, and thus deteriorates sensor performance [48,49]. Variations in response magnitude and response/recovery time with respect to different RH concentrations are plotted in Fig. 9(b). It shows that the response magnitude of the sensor was progressively reduced and response/recovery time increased with increasing RH.
4. Conclusions
Fig. 8. Selectivity histogram of the unloaded and 3 wt% Ag-loaded ZnO–Gr hybrid to 100 ppm C2 H2 , H2 , CO, CO2 , NO2 and O2 gases at 150 ◦ C.
chamber was about 11%. Fig. 9(a) shows that the baseline resistance of the ZG–Ag3 sample shifted to a lower level at 43% RH without any significant change in response/recovery time. We did not find any shift of baseline resistance or response/recovery time due to increments in humidity up to 31%. Afterward, with increasing RH concentration (61% and 74%), the baseline resistance was lowered more, with a noteworthy increase in response/recovery time. An explanation can be presented as follows. In a moist environment, water molecules tend to lessen the chemisorbed oxygen species due to the gradual formation of chemisorbed OH− on the hybrid surface, which reduces the surface area and acts as a barrier against
In summary, acetylene gas sensing characteristics of an Ag nanoparticle-embedded ZnO–Gr hybrid were studied. The characterizations of the as-synthesized sensing material indicated a homogeneously distributed and closely affixed Ag–ZnO mixer on the surface of reduced graphene oxide. At optimum conditions (150 ◦ C, 3 wt% Ag-loaded ZnO–Gr), the as-synthesized hybrid had an enhanced C2 H2 sensing property compared with individual counterparts. The hybrid showed a high response of 21.2 (100 ppm C2 H2 ) at 150 ◦ C, an acceptable detection range of 1–1000 ppm, excellent repeatability, fast response/recovery times of 25/80 s, good selectivity and performance stability up to 31% RH concentration. Overall, these features of the adopted approach to our ZG–Ag hybrid offer tremendous potential for developing lowcost, simple, highly sensitive C2 H2 sensors, which may provide challenges as well as more opportunities in the field of C2 H2 sensing.
Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by 2014 the Ministry of Science, ICT and future Planning (NRF2014R1A2A2A01002668).
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Fig. 9. (a) Resistance variation and (b) response magnitude and response/recovery time variation versus the humidity concentration curve of the 3 wt% Ag-loaded ZnO–Gr hybrid at 150 ◦ C.
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Biographies A.S.M. Iftekhar Uddin received his B.Sc. Eng. from the Faculty of Engineering, International Islamic University Chittagong, Chittagong, Bangladesh, in 2005. He joined as lecturer in Sylhet International University, Sylhet, Bangladesh, in 2006 and promoted as Assistant professor in 2010. He is now working as a Ph.D. candidature in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include ZnO and graphene based nanosensors and flexible nanosensors. Duy-Thach Phan received his B.E. from the School of Electrical Engineering, Hochiminh University of Technology, Ho Chi Minh, Vietnam, in 2008, and M.E. from the School of Electrical Engineering, Ulsan University, Ulsan, South Korea, in 2010. He is now working as a Ph.D. candidate in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include SiC, ZnO, AlN based on SAW sensors, graphene-based sensors, and FEM and atomistic scale modeling. Gwiy-Sang Chung received his B.E. and M.E. Degrees in Electronic Engineering from Yeungman University, Kyongsan, South Korea, in 1983 and 1985, respectively, and his Ph.D. Degree from Toyohashi University of Technology, Toyohashi, Japan, in 1992. He joined the Electronics and Telecommunications Research Institute (ETRI), Daejon, South Korea, in 1992, where he worked on Si-on-insulator materials and devices. Moreover, he also worked as a visiting scholar at UC Berkeley and Stanford University, CA, USA, in 2004 and 2009, respectively. He is now a professor in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include Si, SiC, ZnO, AlN-M/NEMS, flexible self-powered wireless sensors nodes, energy harvesting, and graphene-based composites. He is the author or co-author of more than 125 scientific and technical SCI international journal papers.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Low temperature acetylene gas sensor based on Ag nanoparticles-loaded ZnO-reduced graphene oxide hybrid A.S.M. Iftekhar Uddin, Duy-Thach Phan, Gwiy-Sang Chung ∗ School of Electrical Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 680-749, South Korea
a r t i c l e
i n f o
Article history: Received 20 August 2014 Received in revised form 23 September 2014 Accepted 20 October 2014 Available online 31 October 2014 Keywords: Acetylene sensor ZnO nanoparticles Reduced graphene oxide Silver Hybrid Chemical route
a b s t r a c t This paper scrutinizes the fabrication of a chemiresistive type of acetylene (C2 H2 ) gas sensor by synthesizing a silver (Ag)-loaded zinc oxide (ZnO)-reduced graphene oxide (Gr) hybrid via a facile chemical route. The as-synthesized hybrid was characterized in detail in terms of its structural, morphological and compositional properties. The physical properties of the hybrid exhibited a well-structured crystalline nature and mixed phases of Ag, Gr, and ZnO. The morphological characterization revealed that particlelike nanostructures of the ZnO and Ag mixer were well distributed and closely affixed onto the surface of thin-layer reduced graphene oxide sheets. At an optimum temperature of 150 ◦ C, the 3 wt% Ag-loaded ZnO–Gr hybrid showed preferential detection of acetylene gas with a response value of 21.2 for 100 ppm gas concentrations. The fabricated sensor showed a low detection limit of 1 ppm, fast response and recovery times of 25 s and 80 s, respectively, and good repeatability. Experimental results also showed that the synthesized hybrid had a negligible relative humidity (RH) effect up to 31% RH, and then deteriorated significantly with increasing RH concentrations. After detailed examination, we conclude that an Ag-loaded ZnO–Gr hybrid could be an effective means of fabricating high-performance practical C2 H2 sensors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Acetylene (C2 H2 ) is a colorless, flammable unsaturated hydrocarbon gas with a distinctive odor, widely used as a fuel in oxyacetylene welding and cutting of metals, and as a raw material in various industrial and consumer products, such as acetaldehyde, synthetic rubber, paints, fabric and floor coverings, dry-cleaning solvents, and insecticide sprays. Generally, acetylene is not toxic, but when generated from calcium carbide, can contain toxic impurities such as traces of phosphine and arsine. Notable hazards are associated with its intrinsic instability, especially when it is liquefied, pressurized, heated or mixed with air. Consequently, the combustion of acetylene can produce a large amount of heat (its highest flame temperature is about 3300 ◦ C), and it can explode with extreme violence if the absolute pressure of the gas exceeds 103 kPa (15 psi). Therefore, for environmental and safety purposes, the development of a highly effective C2 H2 sensor has become increasingly important to meet the demands of accurate environmental monitoring, for early leakage warning and for avoiding
∗ Corresponding author. Tel.: +82 52 259 1248; fax: +82 52 259 1686. E-mail address: [email protected] (G.-S. Chung). URL: http://home2.ulsan.ac.kr/user/gschung (G.-S. Chung). http://dx.doi.org/10.1016/j.snb.2014.10.091 0925-4005/© 2014 Elsevier B.V. All rights reserved.
incomplete combustion. However, information on C2 H2 sensors is still very limited in the literature. Most recently, Wang et al. [1] showed successful advancement in C2 H2 sensing with a response of 17 (for 2000 ppm) at 250 ◦ C by synthesizing nickel-doped zinc oxide (ZnO) nanofibers. Tamaekong et al. [2] reported a Pt/ZnO thick film-based C2 H2 sensor at a working temperature of 300 ◦ C with a sensor response of 43 (for 1000 ppm). Zhang et al. [3] hydrothermally synthesized a hierarchical nanoparticle (NP)-decorated ZnO microdisk-based C2 H2 sensor with a large detection range (1–4000 ppm) and very high response (7.9 for 1 ppm) at 420 ◦ C. Dong et al. [4] reported arc plasma-assisted Ag/ZnO composites for C2 H2 sensing, which had a maximum response of 42–5000 ppm C2 H2 at 120 ◦ C. In addition, SnO2 NPs [5], Pd–SnO2 [6], Sm2 O3 –SnO2 [7], Au/MWCNT [8], Ag/Pd–SiO2 [9], etc, have been studied for C2 H2 sensing. However, high working temperature, low sensitivity and synthesis-process complexity, are still a great challenge. In the last few years, among widely investigated metal oxide groups, ZnO has attracted considerable interest in sensing applications to detect volatile and toxic gases due to its high conductive electron mobility and good adsorption characteristics under the working conditions of the sensors [10–15]. However, drawbacks like low sensitivity, poor selectivity and high working temperature limit practical applications in the gas-sensing area (i.e., flammable
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and explosive environments). Therefore, to overcome the shortcomings and to enhance sensing characteristics, researchers have been focusing on metal oxide–metal oxide and metal–metal oxide compositions with numerous synthesis techniques [16–20]. Interestingly, inherent catalytic properties of metal aggregates, dispersed into a metal oxide matrix, modify the surface reactions and greatly enhance the charge carrier separation of the oxide matrix. This phenomenon helps to boost the sensing mechanism, and hence improve sensing performance [21]. Among numerous metals, Ag has attracted immense interest, due to its promising catalytic properties, in the development of photocatalysis [22,23], catalytic oxidation [24], chemical and gas sensing [25,26], etc. A number of Ag–ZnO nanostructure-based highperformance gas sensors have been reported [25,27–30]. Moreover, two-dimensional graphene has concerned much attention since its discovery due to its excellent electrical, chemical and optical properties. Most recently, Fowler et al. [31] discussed chemically derived graphene as a highly sensitive chemical sensor due to its extraordinary carrier mobility. Besides the synergistic interplay between graphene and metal–metal oxide composition, graphene as a template plays a vital role in enhancing the physical properties and sensing performance of the composite materials [19,32–34]. In this context, this paper is focused on the fabrication of a C2 H2 sensor based on an Ag-loaded ZnO-reduced graphene oxide (Gr) hybrid via a facile chemical route in order to enhance C2 H2 sensing performance at low operating temperatures. To the best of our knowledge, an Ag-loaded ZnO–Gr hybrid nanostructure-based acetylene gas sensor has not yet been reported in the literature. The fabricated sensor was evaluated systematically in terms of response, response/recovery times and selectivity toward C2 H2 . The humidity effect on the fabricated sensor was also studied in this work. The main aim of this work is to fabricate an effective, high-performance C2 H2 sensor that can operate at low temperature.
2. Experimental 2.1. Synthesis and characterization All the chemicals used in the experiment were of analytical grade and obtained from Sigma Aldrich, Dongwoo Fine-Chem., and Dae Jung Chem. & Inds. Co. Ltd. Graphene oxide (GO) was prepared according to the process provided by Hummers and Offeman, and Phan et al. [35,36]. ZnO powder was prepared through a solvothermal method using 4 M of Zn(NO3 )2 ·6H2 O and 8 M of sodium hydroxide (NaOH) in ethanol, at 120 ◦ C for 8 h and dried at 60 ◦ C. The Ag-loaded ZnO–Gr hybrid was synthesized via a chemical route. In a typical process, an appropriate amount of silver nitrate (AgNO3 ) was added to the ZnO–GO at 2:1, followed by continuous stirring for 30 min. Hydrazine monohydrate was then used in the mixer as an agent to reduce GO to reduced graphene oxide (Gr) and Ag+ to Ag atoms at a temperature of 110 ◦ C for 8 h. Various samples were prepared by varying the weight percent (0–5 wt%) of AgNO3 to ZnO, referred to as ZnO–Gr/Ag%: ZG–Ag0, ZG–Ag1, ZG–Ag3, ZG–Ag5. Phase transition analysis was carried out with an X-ray diffractometer (XRD) (Rigaku Ultima IV) with Cu K␣ ( = 0.154056 nm) radiation with a 2 scanning range of 10–80◦ . The surface morphology and the compositional analysis of the as-prepared samples were examined with field emission scanning electron microscopy (FESEM) using a JEOL JSM-7600F (accelerating voltage: 10 kV). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and an energy-dispersive spectrometer (EDS) were carried out using a JEOL JEM-2010F.
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Fig. 1. Schematic diagram of the fabricated sensor device.
2.2. Gas sensing For electrical and gas sensing measurements, a resistivitytype sensor device was fabricated as follows: two electrodes (dimension: 2 mm × 4 mm, thickness: 100 nm) were fabricated by depositing platinum (Pt) on 6 mm × 12 mm Al2 O3 substrate by liftoff process. The spacing between two electrodes was 210 m. The sensing material was then well dissolved in ethanol (1 mg/mL) and deposited on the center of the patterned electrodes using a spraycoating process. The device was dried at 100 ◦ C on a hot-plate until on all solvent evaporates. This step was repeated for 3–4 times. Fig. 1 shows a schematic diagram of the fabricated sensor device. Finally, post-situ annealing in air was carried out with the sensor device at 400 ◦ C for 30 min before gas sensing tests. Gas sensing measurements were conducted at atmospheric pressure within a temperature range of 25–250 ◦ C for various C2 H2 concentrations in a sealed chamber using the flow-through technique. The fabricated sensor device was placed in an in-house gas sensor assembly (chamber), and a Keithley probe station (SCS4200) with a bias voltage fixed at 1 V was used for all measurements and data acquisition. A programmable heater integrated with the sensor holder in the chamber was used to adjust the temperature. A computerized mass flow controller (ATO-VAC, GMC 1200) system was used to vary the concentration of C2 H2 in synthetic air. The gas mixture was delivered to the chamber at a constant flow rate of 50 sccm (standard cubic centimeters per minute) with different C2 H2 concentrations. The gas chamber was purged with synthetic air between each C2 H2 pulse to allow the surface of the sensor to return to atmospheric conditions. The sensor response (S) was calculated using Ra /Rg , where Ra is the resistance of the sensor in the presence of synthetic air, and Rg is the resistance in the presence of C2 H2 at certain concentrations. The response time and recovery time of the sensor are defined as the time to reach 90% of total resistance change. 3. Results and discussion 3.1. Crystal structure and morphology The observed XRD patterns of pure ZnO, and 0–5 wt% Ag-loaded ZnO–Gr hybrids are displayed in Fig. 2. The characteristic diffraction peaks of the as-synthesized materials exhibited a well-structured crystalline nature and mixed phases of Gr and ZnO (Fig. 2(b)), and silver, Gr, and ZnO (Fig. 2(c–e)), respectively. Fig. 2a shows the diffraction peaks of pure ZnO. The diffraction peaks of ZnO (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), and (1 1 2) were endowed with a harmonious standard ZnO hexagonal wurtzite structure (JCPDS card No. 36-1451) and the peaks of Ag (1 1 1), (2 0 0) and (2 2 0)
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Fig. 2. XRD patterns of (a) pure ZnO and ZG–Ag hybrids; (b–e) unloaded, 1 wt%, 3 wt%, and 5 wt% Ag.
were indexed to the cubic phase silver (JCPDS card No. 04-0783). No spurious diffractions due to crystallographic impurities of ZnO, and no significant variations of ZnO peak positions were observed in the samples. However, the broadened peaks of ZnO (1 0 0), (0 0 2) and (1 0 1) in the hybrid samples indicated small but finite degradation in the crystalline structure of ZnO due to distortion of Ag and Gr sheets [19,37]. Conversely, a noticeable intensity increase in Ag peaks was observed with increasing amounts of Ag in the hybrid. The absence of any Ag2 O peaks suggested no oxidation of Ag and highlighted the thermal stability of Ag in the final product [27]. The characteristic reflection peak of Gr (0 0 2) appeared at 2 = 26.2◦ , suggesting chemical reduction of oxygen-containing functional groups of GO, reduced and restacked into an ordered crystalline structure of Gr [37]. The surface morphology of the synthesized pure ZnO, unloaded and 3 wt% Ag-loaded ZnO–Gr hybrids are presented in Fig. 3. ZnO nanoparticles with a mean diameter of 40 nm are shown in Fig. 3(a). Fig. 3(b) and (c) represent the unloaded and 3 wt% Ag-loaded ZnO–Gr hybrid, respectively, displaying ZnO and Ag nanoparticles closely affixed onto the Gr sheets with less aggregation, illustrating the excellent attachment between the Gr sheet and ZnO/Ag nanoparticles. It was evident that functional oxygen-containing groups, such as carboxylates of GO, played a vital role in the synthesis process due to its strong hydrophilic nature, and facilitated the firm attachment of ZnO and Ag nanoparticles onto the Gr sheets in the final product [38]. The elemental composition of the as-synthesized ZG–Ag hybrid was investigated by EDS and elemental mapping, and is shown in Fig. 4. A selected area of the EDS spectrum from Fig. 4(a) was taken, and the result is demonstrated in Fig. 4(b). The presence of various well-defined peaks of Zn, oxygen (O), carbon (C) and Ag confirmed the formation of a high purity ZG–Ag hybrid. To confirm the distribution of Zn, O, C and Ag ions onto the lattice surface, elemental mapping of the area shown in Fig. 4(a) was carried out, and the results are depicted in Fig. 4(c–f). The analysis shows that Zn, O, C and Ag are distributed homogeneously over the whole material, which further confirmed the formation of a well dispersed ZG–Ag hybrid. In order to obtain detailed information on the crystallinity and morphology of the 3 wt% Ag-loaded ZnO–Gr hybrid, TEM observations were carried out. TEM images of a ZG–Ag3 sample show a high yield of Ag/ZnO/Gr heterostructured nanocrystals consisting of metallic Ag nanoparticles, ZnO nanoparticles and very thin (transparent) Gr sheets as presented in Fig. 5(a)
Fig. 3. Plane-view FESEM micrographs of (a) pure ZnO; (b) unloaded and (c) 3 wt% Ag-loaded ZnO–Gr hybrids.
(low magnification) and 5(b) (high magnification). The HRTEM image (Fig. 5(c)) from a selected area of Fig. 5(b) shows a distinguished interface and the continuity of lattice fringes of the ZnO nanoparticle, Gr and the metallic Ag nanoparticle. The corresponding selected area electron diffraction (SAED), shown in the inset of Fig. 5(c) reveals the polycrystalline nature of the hybrid sample. The polycrystalline nature may be attributed due to the random orientation of Gr sheets on the ZnO surface and covered by the Ag. The measured spacing between adjacent lattice fringes are 0.28 nm and 0.24 nm, corresponding to the (1 0 0) and (1 0 1) planes of the hexagonal ZnO. The lattice fringes with interplanar spacing of 0.235 nm corresponding to the (1 1 1) planes of fcc Ag are observed. The lattice distance of 0.34 nm can be ascribed to the (0 0 2) plane of Gr.
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Fig. 4. The typical EDS spectrum and elemental mapping of the 3 wt% Ag-loaded ZnO–Gr hybrid.
3.2. Acetylene sensing characteristics Generally, a chemiresistive semiconductor sensor is based on the resistance change of the sensing material due to the chemical and electronic interaction between the adsorbed gas and the sensing material. The sensing mechanism can be described in terms of oxygen adsorption reaction on the sensing material surface, which strongly depends on operating temperature, and the stable oxygen ions are O2 − , O− and O2− that operate below 100 ◦ C, within 100–300 ◦ C, and above 300 ◦ C, respectively [39]. Hence, the oxygen adsorption reaction can be represented as follows: O2 (gas) + e− ↔ O2 − (adsorb)(low temperature) −
−
−
2−
O2 (gas) + e ↔ Oads (moderate temperature) O2 (gas) + e ↔ Oads
(high temperature)
(1) (2) (3)
The sensor responses of the as-synthesized pure ZnO, ZnO–Gr and Ag-loaded ZnO–Gr to 100 ppm acetylene are plotted in Fig. 6(a) as a function of operating temperature. It indicates that sensors based on two or more components mixed together are more sensitive to target gases than their individual counterparts alone, suggesting a synergistic effect among the interplaying components [40]. We can see that all the hybrid samples exhibited higher
sensor response than the ZG–Ag0 sample. With ZG–Ag0, the sensor response was about 16 at 250 ◦ C, whereas the responses of the ZG–Ag1 to ZG–Ag5 samples were 10.7, 21.2 and 12.8, respectively, at 150 ◦ C. Pure ZnO showed negligible response within the temperature range of 25–250 ◦ C. Generally, an acetylene sensor based on a pure ZnO nanostructure works at high operating temperatures (about 420 ◦ C) [3]. This behavior is due to the potential barrier formed by the chemisorbed oxygen ions on the ZnO surface, which prevents the C2 H2 molecules from reacting at low temperature. Ag has better oxygen dissociation ability than ZnO and catalytically activates the dissociation of molecular oxygen on the ZG–Ag hybrid surface, which creates higher acetylene sensing active sites on the surface of ZG–Ag1 to ZG–Ag5 hybrids than a pure ZnO or ZnO–Gr composite. The formation of depletion zones around the ZnO particles due to the addition of Ag is attributed to the modulation of the nano-Schottky barriers and hence improves the surface reactivity at low temperature [41]. At the same time, the oxygen ions (Oads − ) on the sensing material surface become more active with the acetylene molecules, which leave the electrons from the surface on the conduction band of the sensing material and thus, the chemical reaction between acetylene gas molecules and oxygen ions can be represented as:
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Fig. 5. (a) TEM, and (b–c) HRTEM images of the ZG–Ag3 hybrid sample. Inset: the corresponding SAED pattern of the ZG–Ag3 sample.
KEth
C2 H2 + 5O− −→2CO2 + H2 O + 5e− ads
(4)
This reaction causes an increase in conductivity due to the free electrons on the sensing material surface, and thus decreases the resistance of the sensor. From Eq. (4), the rate equation of electron density can be written as: dn = KEth (T ) [O− ] [C2 H2 ] ads dt or, n = KEth (T ) [O− ] [C2 H2 ]t + no ads
(5)
(6)
where, KEth (T) is the reaction rate constant or coefficient, n is the electron density under an acetylene atmosphere and no is the electron density under an air atmosphere. Usually, the reaction rate coefficient and electron density have a strong relationship to operating temperature, and magnitude increases exponentially with rising temperatures. However, sensor response (S = Ra /Rg ) is directly proportional to the reaction rate coefficient and is inversely proportional to electron density [42]. For acetylene gas sensing, these two parameters compete with each other and result in maximum sensor response of the ZG–Ag hybrid at an optimum operating temperature of 150 ◦ C which reveals the superiority of our sensor device. In Table 1, we summarized the previously reported results regarding C2 H2 sensors and compared with our work to show the advancement of our sensor device. Several researchers have reported that the sensor response is highly affected by the presence of an efficient and appropriate catalyst that enhances the surface reactivity toward the target gaseous molecules [43,44]. After Ag loading on ZnO–Gr, additional active
sites were produced on the surface of the ZG–Ag hybrid due to the spillover effect, and more C2 H2 molecules could be adsorbed on the surface. Thus, the presence of appropriate Ag could enhance sensor response. Among the sensors tested, 3 wt% Ag-loaded ZnO–Gr was confirmed to exhibit the highest C2 H2 response value at 150 ◦ C; that is, the optimum loading amount of Ag. However, beyond 3 wt% Ag loading, the surface may dissociate more oxygen molecules exceeding the percolation threshold causing spillover zone overlap [45]. This situation may hinder an effective oxygen delivery system and reduce the probability of C2 H2 adsorption, leading to a decline in sensor response. Representative sensor response variations of ZG–Ag3 toward 1–1000 ppm C2 H2 at a working temperature of 150 ◦ C are plotted in Fig. 6(b). A maximum sensor response of 34 was recorded at a 1000 ppm gas concentration at 150 ◦ C. In the hybrid, Gr acted as a matrix or template that helped to connect and to form an additional conducting pathway between Ag and ZnO nanoparticles, leading to a high signal-to-noise ratio. It is clearly observed from Fig. 6(b) that the ZG–Ag hybrid could reliably detect C2 H2 at a 1 ppm concentration, but the response curve trend did not follow good linearity throughout the 1–1000 ppm gas concentration. With increasing gas concentration, the sensor response increased steeply from 1 ppm to 50 ppm and then increased with a relatively gentle slope from 50 ppm to 1000 ppm. This phenomenon might be attributed to the power law dependence on concentration, which arose from receptor and transducer functions of C2 H2 with the sensor surface and the change of surface potential [46]. The dynamic response of the 3 wt% Ag-loaded ZnO–Gr hybrid sensor to periodic changes between 100 ppm C2 H2 and synthetic
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Table 1 Summary of reported works on C2 H2 gas sensors. Ref.
Material
LOD (ppm)
Maxm gas concentration (ppm)
Response (S = Ra /Rg )
Operating temp. (◦ C)
Resp./recov. time (s)
Selectivity
[1] [2] [3] [5] [7] This work
5 at% Ni doped ZnO 2 at% Pt loaded ZnO ZnO SnO2 6 wt% Sm2 O3 doped SnO2 Ag/ZnO–Gr
100 50 1 200 10 1
2000 10,000 4000 10,000 4000 1000
17 (2000 ppm) 43 (1000 ppm) 52 (200 ppm) 6.3 (10,000 ppm) 63.8 (1000 ppm) 21.2 (100 ppm)
250 300 420 300 180 150
5/10 6/65 15/19 34/10 min 3/17 25/80
– Good Excellent Poor Good Excellent
air at 150 ◦ C is illustrated in Fig. 7(a). We can see that the resistance of the sensor quickly lessened when the C2 H2 was injected and returned to its initial baseline value when the gas was stopped, which suggested good reversibility of the as-synthesized sensing material. Fig. 7(a) also shows good repeatability of the sensor with a negligible drift (∼1–2%) within a cycle-to-cycle response. Moreover, the sensor exhibited a fast response time (∼25 s) and a relatively slow recovery time (∼80 s) as shown in Fig. 7(b). The sensor response of the unloaded and 3 wt% Ag-loaded ZnO–Gr hybrids were also compared with other test gases, and the results from exposure to 100 ppm of different gases are plotted in Fig. 8. It is clearly observed that the 3 wt% Ag-loaded ZnO–Gr hybrid has a high response toward C2 H2 and a negligible response to H2 ,
CO, CO2 , NO2 , and O2 compared to the unloaded one. This situation may be attributed to the high selective absorption of the Ag catalyst with C2 H2 molecules. Relative humidity interference is an important consideration in sensor function. It is believed that water molecules could affect the gas response of ZnO-based sensors. The reaction between chemisorbed oxygen ions on the sensing material surface and the water molecules results in a decrease in baseline resistance, and hence reduces the response of the sensor [47]. In the present work, C2 H2 sensing behaviors of the Ag-loaded ZnO–Gr hybrid were also investigated in the air atmosphere in the presence of water vapors, and similar consequences were observed in the hybrid where ZnO acted as the core material for sensing features. Fig. 9 shows the C2 H2 sensing behaviors of the 3 wt% Ag-loaded ZnO–Gr hybrid at an optimum temperature of 150 ◦ C for different RH concentrations. The humidity concentration of the synthetic air that we used in our gas
Fig. 6. (a) Gas sensing properties of pure ZnO, and the unloaded-5 wt% Ag-loaded ZnO–Gr hybrids to 100 ppm C2 H2 at different operating temperatures; (b) response variation of the ZG–Ag3 sample to different C2 H2 concentrations at 150 ◦ C.
Fig. 7. (a) Resistance change during cyclic tests (C2 H2 and synthetic air), (b) a magnified response and recovery characteristic cycle of the ZG–Ag3 sample to 100 ppm C2 H2 at 150 ◦ C.
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C2 H2 adsorption, and thus deteriorates sensor performance [48,49]. Variations in response magnitude and response/recovery time with respect to different RH concentrations are plotted in Fig. 9(b). It shows that the response magnitude of the sensor was progressively reduced and response/recovery time increased with increasing RH.
4. Conclusions
Fig. 8. Selectivity histogram of the unloaded and 3 wt% Ag-loaded ZnO–Gr hybrid to 100 ppm C2 H2 , H2 , CO, CO2 , NO2 and O2 gases at 150 ◦ C.
chamber was about 11%. Fig. 9(a) shows that the baseline resistance of the ZG–Ag3 sample shifted to a lower level at 43% RH without any significant change in response/recovery time. We did not find any shift of baseline resistance or response/recovery time due to increments in humidity up to 31%. Afterward, with increasing RH concentration (61% and 74%), the baseline resistance was lowered more, with a noteworthy increase in response/recovery time. An explanation can be presented as follows. In a moist environment, water molecules tend to lessen the chemisorbed oxygen species due to the gradual formation of chemisorbed OH− on the hybrid surface, which reduces the surface area and acts as a barrier against
In summary, acetylene gas sensing characteristics of an Ag nanoparticle-embedded ZnO–Gr hybrid were studied. The characterizations of the as-synthesized sensing material indicated a homogeneously distributed and closely affixed Ag–ZnO mixer on the surface of reduced graphene oxide. At optimum conditions (150 ◦ C, 3 wt% Ag-loaded ZnO–Gr), the as-synthesized hybrid had an enhanced C2 H2 sensing property compared with individual counterparts. The hybrid showed a high response of 21.2 (100 ppm C2 H2 ) at 150 ◦ C, an acceptable detection range of 1–1000 ppm, excellent repeatability, fast response/recovery times of 25/80 s, good selectivity and performance stability up to 31% RH concentration. Overall, these features of the adopted approach to our ZG–Ag hybrid offer tremendous potential for developing lowcost, simple, highly sensitive C2 H2 sensors, which may provide challenges as well as more opportunities in the field of C2 H2 sensing.
Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by 2014 the Ministry of Science, ICT and future Planning (NRF2014R1A2A2A01002668).
References
Fig. 9. (a) Resistance variation and (b) response magnitude and response/recovery time variation versus the humidity concentration curve of the 3 wt% Ag-loaded ZnO–Gr hybrid at 150 ◦ C.
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Biographies A.S.M. Iftekhar Uddin received his B.Sc. Eng. from the Faculty of Engineering, International Islamic University Chittagong, Chittagong, Bangladesh, in 2005. He joined as lecturer in Sylhet International University, Sylhet, Bangladesh, in 2006 and promoted as Assistant professor in 2010. He is now working as a Ph.D. candidature in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include ZnO and graphene based nanosensors and flexible nanosensors. Duy-Thach Phan received his B.E. from the School of Electrical Engineering, Hochiminh University of Technology, Ho Chi Minh, Vietnam, in 2008, and M.E. from the School of Electrical Engineering, Ulsan University, Ulsan, South Korea, in 2010. He is now working as a Ph.D. candidate in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include SiC, ZnO, AlN based on SAW sensors, graphene-based sensors, and FEM and atomistic scale modeling. Gwiy-Sang Chung received his B.E. and M.E. Degrees in Electronic Engineering from Yeungman University, Kyongsan, South Korea, in 1983 and 1985, respectively, and his Ph.D. Degree from Toyohashi University of Technology, Toyohashi, Japan, in 1992. He joined the Electronics and Telecommunications Research Institute (ETRI), Daejon, South Korea, in 1992, where he worked on Si-on-insulator materials and devices. Moreover, he also worked as a visiting scholar at UC Berkeley and Stanford University, CA, USA, in 2004 and 2009, respectively. He is now a professor in the School of Electrical Engineering, University of Ulsan, Ulsan, South Korea. His research interests include Si, SiC, ZnO, AlN-M/NEMS, flexible self-powered wireless sensors nodes, energy harvesting, and graphene-based composites. He is the author or co-author of more than 125 scientific and technical SCI international journal papers.