ZnO Nanowires-C Microfiber Hybrid Nanosensor for Liquefied Petroleum Gas Detection

July 6, 2017 | Autor: Salvatore Iannotta | Categoria: Engineering, Technology, CHEMICAL SCIENCES, Nanoscience and nanotechnology
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Article Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 14, 5088–5094, 2014 www.aspbs.com/jnn

ZnO Nanowires-C Microfiber Hybrid Nanosensor for Liquefied Petroleum Gas Detection D. T. T. Le1 , S. Iannotta2 , N. V. Hieu1 , C. Corradi3 , T. Q. Huy4 , M. Pola2 , and M. Tonezzer3 ∗ 1

International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), Hanoi, 10000, Viet Nam 2 IMEM CNR, Parco Area delle Science 37/A, I-43124 Parma, Italy 3 Nanoscience Laboratory—IMEM CNR, via Alla Cascata 56/C, I-38050 Trento, Italy 4 National Institute of Hygiene and Epidemiology, 1-Yersin Street, Hanoi, 10000, Viet Nam Zinc oxide nanowires are integrated onto carbon microfibers using a two-step approach which includes electrochemical deposition of zinc and its thermal oxidation. Such nano-on-micro hybrid architecture is then used as resistive gas sensor. Some properties like mechanical flexibility, low cost and large-area fabrication make this design appealing for different applications. The huge surfaceto-volume ratio of such structure comes from being structured at both microscale and nanoscale (ZnO nanowires and C microfiber) and leads to a strong and rapid response/recovery times when it is used as aDelivered gas sensor. fabrication process of to: the Sung ZnO-C device is very simple and doesn’t byThe Publishing Technology Kyun Kwan University On: Fri,show 28 Mar 2014 liquefied 17:06:22petroleum gas sensing involve any expensiveIP: lithographic step. The sensors excellent Copyright: Scientific properties, with very fast response onAmerican gas exposure (aboutPublishers 3 s) and very good reversibility (less than 2%). In addition, the carbon microfiber substrate allows the use of the ZnO-C sensor also in applications where flexibility is required (for example integrated in fabric).

Keywords: Zinc Oxide, Resistive Sensors, Gas Sensors, Nanowires, LPG.

1. INTRODUCTION In the recent years, the increased awareness of urban pollution1 and latent hazards in industrial or domestic environments, expanded the need for gases detection, particularly for toxic and potentially explosive gases. Liquefied petroleum gas (LPG) is the most used cooking fuel for household applications in urban areas.2 LPG (which is composed by hydrocarbons like CH4 , C3 H8 , C4 H10     Cn H2n+2 ) is potentially dangerous: unintentional leakage of liquefied petroleum gas even at low concentrations is a serious risk to human life and health as it is a flammable gas. The Lower Explosive Limit (LEL) as indicated by National Institute for Occupational Safety and Health (NIOSH) and Occupational Safety and Health Administration (OSHA) standards for chemical hazards is 2.1% by volume in air (21000 parts per million or ppm) for propane and 1.9% by volume in air (19000 ppm) for butane (which are largely the main components of LPG). ∗

Author to whom correspondence should be addressed.


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The Permissible Exposure Limit (PEL) for LPG as stated by NIOSH and OSHA standards is 1000 ppm. Accurate monitoring of LPG leakages even at low concentrations could prevent accidental explosions. As a consequence, there is an increased demand for simple, low-cost and reliable sensors enabling its detection at ppm (parts per million) level. Although a variety of techniques are available for gas detection, solid state metal oxides have been found to display the best sensitivity for LPG, making it a better choice over alternatives (even if very recently hybrid combinations of metal oxide and organic materials are being investigated).3 4 The main parameters of these materials, which largely influence their performance as gas sensors, primarily include the morphology of oxide material (grain size and effective surface area). The most investigated metal oxide materials as gas sensors are zinc, tin and indium oxides.5 Among them, zinc oxide (ZnO) is very attractive due to its chemical and thermal stability, its good biocompatibility, and, particularly, to its high response to toxic and combustible gases. Up to now, various forms of 1533-4880/2014/14/5088/007


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ZnO Nanowires-C Microfiber Hybrid Nanosensor for Liquefied Petroleum Gas Detection

ZnO-based gas sensors, such as thick films,6 thin films,7 8 nanoparticles9 10 and nanowires (NWs),11 12 have been investigated.13 The increased surface-to-volume ratio of quasi-1D ZnO nanowires provides them a much greater response compared to bulk ZnO and ZnO thin films.14 ZnO nanowires have been grown by physical15 16 (for example, using Zn foil or powder), and chemical17 18 methods confirming a high potential as sensing materials for different gases.19–21 In this work, monocrystalline zinc oxide nanowires were grown directly on highly conductive and flexible fabric made of carbon microfibers (C) with a diameter of 10 microns. As illustrated in previous studies,22 by integrating the microstructure of the fiber with the nanostructure of the nanowires, we supply a 3D structural design that attains mechanical flexibility, low cost and large-area fabrication. The higher surface area, due to the synergy between the microstructure and the nanostructure, gives to the ZnO-C sensor a strong response Figure 1. SEM images of samples. (a) carbon microfiber before zinc and rapid response/recovery time. In particular, we present deposition; (b) lower magnification image of the ZnO-C sensor; (c) carsystematic studies on LPG sensing characteristics of ZnObon microfiber with ZnO nanostructures sprawling radially; (d) zoom of C sensors. The results indicate that the ZnO nanowires the hierarchic nanostructures. exhibit good sensor response towards detection of LPG “grass” on the texture of the carbon microfiber is evident. in the operating temperature range 200 to 350  C, with Figure 1(c) shows a SEM picture of the decorated fibers, response and recovery times in the range of seconds, much covered with ZnO nanowires. shorter than values found in literature. The results illusThe nanowires are long about 2 microns, with a diametrated in this article demonstrate that the integration of Delivered by Publishing Technologyter to:distribution Sung Kyunwhich Kwanranges University between 50 and 150 nm. The nanowires with microfibers mayIP: pave the way to simple On: Fri, 28 Mar 2014 17:06:22 diameter of their “branches” is about 25 nm, as it can be and low cost sensor devices with excellent sensing perforCopyright: American Scientific Publishers noticed in Figure 1(d). mance, which could be used as a real-time detectors for Transmission electron microscopy (TEM) was perdangerous leakage of LPG in many environments. formed with a Tecnai G2 SuperTwin, operated at 200 kV, and can be seen in Figure 2(a). A high-resolution TEM 2. MATERIAL AND METHODS image is shown in Figure 2(b), confirming the good The characteristics of the commercial carbon microfiber crystallinity of the wires. (EletroChem. Inc.), having a diameter of about 10 micron Figure 2(c) shows the selected area electron diffraction is shown in Figure 1(a). (SAED) pattern which confirms that the nanowires are sinAs described previously,23 a two-steps was used in order gle crystalline wurtzite ZnO (a = b = 03249 nm, c = to grow ZnO nanowires on the carbon microfiber: 05205 nm) growing in the [110] direction. (1) Metallic zinc was deposited on the carbon microfibers These results are confirmed by the XRD spectrum by chemical electrodeposition: the microfiber was placed shown in Figure 3, in which only ZnO wurtzite reflections at a distance of 40 mm from a pure (99,99%) zinc elecare present. XRD spectra was collected in Bragg-Brentano trode inside a container with 40 ml of 0.75 molar solution geometry with a Panalitycal X’Pert Pro diffractometer. of ZnCl2 and a current of 40 mA was flowed through the A copper anode (wavelength of 1.5406 Å), a step size of solution for 20 minutes. 0.05 (2) and an average time of 60 s/step were used. (2) The carbon microfiber was dried and loaded on an aluThe sharp intense peaks in Figure 3 confirm the good crysmina vessel, then placed at the centre of a quartz tube that talline nature the nanostructures as wurtzite (hexagonal) was inserted in a horizontal furnace. The thermal oxidation ZnO, consistent with the standard values in the standard of the metallic zinc was obtained rising the temperature to data (JCPDS 36-1451 card). 500  C during 15 minutes, maintaining it for 2 hours and The inset in Figure 3 shows an EDS (energy dispersion then letting it cool down naturally.24 spectroscopy) spectrum which again confirm the good ZnO composition of the nanostructures. The small carbon peak The result of the thermal oxidation process on the comes probably from the carbon microfibers substrate, substrate is a very light grey layer, consisting of hiermore than from ambient pollution. archic ZnO nanowires radially rising from each carbon A home-built apparatus was used to test the gas sensing microfiber. The macroscopic result of the two-steps growth can be seen in Figure 1(b), in which the light grey properties of the ZnO-C system. It consists of a test J. Nanosci. Nanotechnol. 14, 5088–5094, 2014


ZnO Nanowires-C Microfiber Hybrid Nanosensor for Liquefied Petroleum Gas Detection

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Figure 2. TEM images of a sample. (a) low resolution TEM image of a nanostructure branch; (b) HR-TEM picture of the same nanostructures; (c) FFT of the nanowire in (b), showing its growth direction.

chamber, a sensor holder, a Keithley 2410 multimeter, 3. RESULTS a Keithley 6517A electrometer, mass flow controllers 3.1. Sensor Response as a Function of and a data acquisition system developed with LabView Working Temperature of National Instruments. The samples have an area of The gas response of the ZnO-C sensors was first tested at about 11× cm2 , and were contacted with silver paste condifferent temperatures ranging from 200 to 350  C in order tacts (2 mm in diameter and 0.5 mm of distance) in order to optimize their working temperature. Figure 4 shows to be electrically measured. First, the I–V characteristics how the working temperature clearly affects the sensors were studied within ±10 V range, and the silver paste response to 500 ppm of LPG. electrical contacts showed a good ohmic behaviour. The In order to compare our results with actual literadevices were then conditioned at 350  C at 1 V in dry ture, in this paper we use the definition of sensor perair for 5 hours prior the measurements, in order to stabicentage response as SR = (RAIR − RLPG )/RAIR · 100 where lize their microstructure and guarantee that their electrical RAIR and RLPG are, respectively, the resistance of the properties would not change during gas tests. All the senexposed dry air without and with LPG. Delivered by Publishing Technology nanosensor to: Sung Kyun KwantoUniversity sors have a resistance in the range of115.145.183.222 1–12 M under dry All the sensor responses show a peak between 250 and IP: On: Fri, 28 Mar 2014 17:06:22 air atmosphere at 200–350  C. Copyright: American Scientific (depending on the gas concentration), with a 300  C Publishers The sensors were tested for LPG in the temperature maximum value of 68% for 1000 ppm LPG at 300  C. range of 200–350  C with a constant 1 V voltage between The bell-shape behaviour as a function of working temperthe metallic contacts. The sum of the gas flows was kept as ature in Figure 4 has already been found by other groups25 500 standard cubic centimetres per minute in dry air (79% and has been explained by gas-diffusion theory.26 The sennitrogen, 21% oxygen) by the use of mass flow controllers sor response starts raising around 100  C, but its response (MKS). The sensors operating temperature was controlled greatly increases over 200  C. For most of gas concentraby a feedback on the thermocouple inside the furnace. The tions, the optimum working temperature (the temperature resistance of the sensors in dry air or in test gases was meaat which the devices show the highest percentage response) sured by monitoring the output current across the them.

Figure 3. XRD spectrum of the nanostructured device, confirming it’s good crystallinity. The inset shows the EDS spectrum of the sensor, confirming its ZnO composition.


Figure 4. Response of the ZnO-C sensor to 500–1000 ppm of LPG as a function of the working temperature, showing a peak at 250–300  C.

J. Nanosci. Nanotechnol. 14, 5088–5094, 2014

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ZnO Nanowires-C Microfiber Hybrid Nanosensor for Liquefied Petroleum Gas Detection

is 250  C. Therefore 250  C has been chosen as the operating temperature for gas sensors characterization. 3.2. Sensor Response as a Function of Gas Concentration The sensor percentage response as a function of the LPG concentration is reported in Figure 5. Its increase is linear up to 1000 ppm, without any sign of saturation. The sensor response ranges from 33% to 49% for 500–1000 ppm LPG at 200  C, from 50% to 64% for 500–1000 ppm of LPG at 250  C, from 45% to 68% for 500–1000 ppm of LPG at 300  C, and from 7% to 27% for 500–1000 ppm of LPG at 350  C. These values clearly confirm how the ZnO-C devices best work at an intermediate temperature of 250–300  C. Figure 6.

Response transient of the ZnO-C sensor under exposure to

3.3. Dynamic Response different LPG concentrations (500–1000 ppm of LPG in dry air) at 200– 350  C. The dynamic resistance of the ZnO-C sensor under exposure to various levels of LPG concentrations (500– 1000 ppm of LPG in pure nitrogen) at different temperadefinition of percentage recovery %R was used, as %R = tures (200–350  C) is shown in Figure 6. The exposure of R − I/I × 100, in which I is the response Intensity and the sensor to the LPG/dry air mixture as opposed to pure R is the Recovery, defined as the difference between the dry air, lead to a rapid decrease of the sensor’s resistance signal during analyte exposure and that after recovery. to a relatively stable value. The percentage reversibility of the sensors is low on Switching the sensor to dry air again, its resistance average, as shown in Figure 7(a). It is clear from this was abruptly decreasing and rapidly reaching its previgraph that the sensor drift present at 200  C makes its ous value. The responseDelivered increases by clearly with increasing Publishing Technologyresponse to: Sungless Kyun Kwan University reversible, but at higher temperature, the IP: On: Fri,average 28 Marpercentage 2014 17:06:22 concentration of LPG. reversibility value is always equal or Copyright: American Publishers It is apparent from Figure 6 that a drift is present when Scientific lower than 2%. the sensor works at 200  C. The drift almost disappears Figure 7(b) shows the response of the sensors over a when the working temperature is increased to 250  C or period of two months, which is very stable and repeatable. more. In these cases (250–350  C) the fairly small devi3.4. Response and Recovery Times ations between the values before and after each LPG In this work we use the definition of response time as the injection demonstrate that the sensors have good reversibiltime taken for the sensor to reach 90% of the equilibrium ity. To give a quantitative evaluation of such reversibility value after it has been exposed to LPG, as can be seen in of the response of the sensors, in the present paper the Figure 8. In the same way, the recovery time is defined as the time the sensor needs to recover 90% of the initial signal after the removal of LPG.27 The best response time of the sensors was found at a working temperature of 350  C for a LPG concentration of 500 ppm, and was about 3 s, while the recovery time was about 10 s. The ZnO-C device sensing quickness responding to different concentrations of LPG can be inferred like in Figure 8 as a function of LPG concentration and working temperature, and will be now investigated quantitatively. Figure 9(a) shows how quickly the ZnO-C gas sensors responded to the injection and evacuation of LPG gas. Qualitatively, it is clear that both response and recovery times were affected by working temperature. Figure 8(a) shows the response time as a function of LPG concentration for different working temperatures, while the recovery time in the same conditions are shown in Figure 9(b). The response times ranged from 22 s to 31 s for 500– Figure 5. Percentage response as a function of LPG concentration 1000 ppm LPG at 200  C, and from 3.5 s to 7 s for (500–1000 ppm) at different working temperatures (200–350  C). J. Nanosci. Nanotechnol. 14, 5088–5094, 2014


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Figure 7. (a) Average reversibility values at different working temperatures (200–350  C); (b) percentage response as a function of time along a two months period. The sensor was tested in dry air with 500 ppm LPG at 300  C.

500–1000 ppm LPG at 300  C. The recovery times were These times are much shorter if compared with those found by Prajapati and co-workers9 with ZnO nanopartiquite longer, ranging from 46 s to 49 s for 500–1000 ppm  cles (response recovery time of 350 and 60 seconds 16.5 s to by 33 Publishing s for 500–1000 ppm LPG at 200 C, and from Delivered Technology to: Sung Kyunand Kwan University On: Fri,to2810000 Mar 2014 ppm 17:06:22 LPG, respectively), by Shinde and coto 12.5 s for 500–1000 LPG at 300  C, and from 10 sIP: Copyright: American Scientific workers28Publishers with ZnO nanoparticles (response and recovery ppm LPG at 350  C (not shown). time of 85 and 90 seconds to 2000 ppm LPG, respecIt is clear from Figure 9 that both response and recovtively) and by Gurav and co-workers29 with ZnO vertiery times are affected by the LPG concentration: a higher cally aligned nanorods (response and recovery time of gas concentration implies a longer time for the sensor to 510 and 130 seconds to 2600 ppm LPG, respectively) or respond to it. The speed of the sensing device is even more even30 (response and recovery time of 80 and 60 sechighly influenced by the working temperature: as seen in onds to 2600 ppm LPG, respectively, with Pd-doped ZnO Figure 9, the response time is much shorter at 350  C or nanorods). The short response and recovery times can be higher for all the LPG concentrations range. The recovexplained with two characteristics of the sensor: the large ery time is much shorter along the whole concentration surface-to-volume ratio due to the nano-on-micro strucrange when the working temperature is at least 300  C. ture, and the very thin nanowires at the very surface of the nanostructured ZnO, that also improve its sensor response. Their fast responses and recoveries make these kinds of sensing structures the ideal choices for real-time sensing in many fields and applications.


Figure 8. Calculation of response and recovery times from dynamic response graphs in Figure 5. The curve is relative to a concentration of 600 ppm LPG.


As known, the electrical conductance of ZnO (and of all metal oxides) nanowires changes when they are immersed in an oxidizing or reducing gas (LPG in our case) due to a two-step process occurring on its surface.31 In the first step reaction, atmospheric oxygen molecules are first physisorbed on the surface sites, and then ionized, by capturing electrons from the conduction band,32 thus being ionosorbed on the surface of the nanowire. This reaction consumes the electrons inside the nanowire, therefore clarifying why the ZnO resistance J. Nanosci. Nanotechnol. 14, 5088–5094, 2014

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ZnO Nanowires-C Microfiber Hybrid Nanosensor for Liquefied Petroleum Gas Detection

Figure 9. Sensor performance. (a) Response time and (b) recovery time as a function of the LPG gas concentration at different working temperatures. Table I. Comparison of the present data with recent literature (the third increases after the oxygen exposure. Just the opposite haplast row, denoted with an asterisk, relates to ZnO nanorods sensitized pens in the case of a reducing gas: it reacts with the oxygen with Pd, while the second last row, denoted with two asterisks, relates to at the nanostructure surface, thus releasing an electron into ZnO nanorods sensitized with Ag). the conduction band with an increase in the semiconductor LPG Working Percentage Response Recovery conductance. concentration temperature response time time In the case of LPG, the hydrocarbons namely propane [%] [sec] [sec] Reference [ppm] [ C]Kwan University Delivered by Publishing Technology to: Sung Kyun (C3 H8 ), and butane (C4 H10 ) present in it interact with IP: On: Fri,10000 28 Mar 201432517:06:2249 350 60 [9]—2011 the adsorbed oxygen ions present onCopyright: the surfaceAmerican of the Scientific Publishers 2000 400 31 84 90 [28]—2007 nanowire. The hydrocarbons are thus converted to CO2 and 2600 400 12 [29]—2012 H2 O due to this interaction. The overall reaction of LPG 24 300 38 511 130 molecules with adsorbed oxygen species can be elucidated 2600 300 37 [30]—2011 8 with the following reaction: ∗ − Cn H2n+2 +3n+1O− ads → nCO2 +n+1H2 O+3n+1e

where Cn H2n+2 denote C3 H8 , C4 H10 , etc. This reaction produces CO2 and H2 O releasing the trapped electrons back to the conduction band of the sensing material. As a result the resistance of the sensing material decreases upon exposure to LPG. Following these facts we will now examine the mechanism that rules the ZnO-C sensors. For n-type zinc oxide single-crystals, the intrinsic carrier concentration is mainly established by deviations from stoichiometric balance. They are usually present as interstitial zinc and oxygen vacancies, which are primarily atomic defects acting as electron donors.33 The electrons in the conduction band resultant from the point defects play a key role in gas sensing properties of various materials. Therefore, the electrical conductivity of nanocrystalline ZnO robustly depends on the surface states formed by molecular adsorption. The thickness Ld of nanosized ZnO at a temperature around 300  C can be found in literature as approximately 10 nm.34 35 The small nanowires at the surface of our sensors have a diameter of about 30 nm, thus the effect of J. Nanosci. Nanotechnol. 14, 5088–5094, 2014

2600 100∗∗ 1000

225 332 250

60 30 64





[35]—2006 Present work

the gas adsorption is strong because it affects most of the nanostructure body, giving rise to a high gas response. The nano-on-micro architecture allows these sensors to reach very good performance in LPG detection, compared with recent literature, as can be seen in Table I:  28–30 36 As can be deduced by Table I, the present ZnO-C sensor has good sensing performance in LPG detection at relatively low working temperature. The percentage response is high compared with other recent sensors based on ZnO nanostructures, and comparable with metal sensitized nanostructures. Response and recovery times are much shorter than the ones found in literature, making this architecture ideal for real-time sensing in dangerous environments applications to detect LPG.

5. CONCLUSIONS Resuming, we have produced a hierarchic micro-on-nano structure constituted of ZnO nanowires obtained on a 5093

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carbon microfiber and working as a promising LPG sensor. This micro-on-nano configuration leads to a severely high surface and flexibility of the device. Gas sensing performance is very good at 250–300  C, with a sensor percentage response of 50% for 500 ppm and 64% for 1000 ppm of LPG at 250  C. Response and recovery time are fast (< 4 and 10 seconds at 350  C for 500 ppm LPG) and the device shows good reversibility (≤ 2% for 250  C or higher). The sensor selectivity is being investigated for a future work. In addition, the absence of any lithographic step makes the devices very easy and cheap to manufacture. Furthermore, the procedure developed here can also be extended to other nanowires materials paving a way to develop other types of simple and low-cost gas sensors.

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Received: 8 May 2013. Accepted: 1 August 2013.


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