J Mater Sci (2014) 49:685–690 DOI 10.1007/s10853-013-7749-z
Ammonia sensing properties of (SnO2–ZnO)/polypyrrole coaxial nanocables Hamed Akbari Khorami • Aryan Eghbali • Mansoor Keyanpour-Rad • Mohammad Reza Vaezi Mahmoud Kazemzad
•
Received: 9 July 2013 / Accepted: 17 September 2013 / Published online: 26 September 2013 Ó Springer Science+Business Media New York 2013
Abstract In this work, (SnO2–ZnO)/polypyrrole (PPy) coaxial nanocables have been synthesized through simple chemical routes. The SnO2–ZnO composite nanofibers with narrow distribution of diameter size and an average of 75 nm were synthesized via the electrospinning method. In this experiment, we were able to polymerize a shell of PPy, as a typical conducting polymer, on surface of SnO2–ZnO nanofibers using the vapor-phase polymerization of Pyrrole monomer. The prepared nanomaterial exhibits a linear response to Ammonia (NH3) concentrations at room temperature. The obtained results make NH3 detection and determination of its concentration feasible. The superior features of this nanomaterial include simple synthesis method, high sensitivity, and quick response and recovery times. The aforementioned characteristics of this nanomaterial indicate the potential of industrial applications.
Introduction Nowadays, we face with toxic, volatile and combustible gases in the environment including domestic, laboratorial, and industrial places. Detecting these harmful gases is vital in order to control air pollution, prevent human life, and protect nature from being damaged. NH3 is one of these gases which is widely used in industrial processes [1–3] and medical diagnoses [4, 5]. Hence, many researchers are working to develop reliable and affordable methods for detecting NH3 as well as other gases [6–9].
H. A. Khorami (&) A. Eghbali M. Keyanpour-Rad M. R. Vaezi M. Kazemzad Division of Nanotechnology and Advanced Materials, Materials and Energy Research Center (MERC), 31787-316 Karaj, Iran e-mail:
[email protected]
NH3 sensors based on conducting polymers have shown better sensing responses among various sensors based on different materials [6]. Conducting polymer-based sensors can work at room temperature unlike the metal-oxidebased sensors which require high operating temperature to activate the absorption and desorption of NH3 for its detection [6]. Low operating temperature leads to an increase in sensor life time and a decrease in power consumption. It also makes sensor operation easier [6, 10]. Conducting polymers have various notable features, such as controllable electrical conductivity, low energy optical transitions, low ionization potential, and high electron affinity, which make them suitable candidates for sensing applications [10, 11]. Polypyrrole (PPy) is one of the most stable conducting polymers under ambient conditions. It has attracted more attention as an NH3 sensor because of its unique conductometric response to NH3 [11]. Both one-dimensional and thin film of PPy have been synthesized either by electrochemical or chemical polymerization of pyrrole (Py) monomer [12–16]. The sensing mechanism in PPy is based on chemical reactions which take place on its surface. Therefore, it is more accurate when its surface to volume ratio is higher, because there are more potential places for reactions to occur. The porous one-dimensional PPy has higher surface to volume ratio than its thin film and it can improve sensitivity of sensor. There are numerous methods for preparation of onedimensional PPy, such as template-based [17, 18], selfassembly [19], and electrospinning [20–22]. Among these techniques, chemical polymerization of Py on porous templates is found to be simple, inexpensive, and effective process for synthesizing ultrathin PPy layers in a highly reproducible manner. Vapor-phase polymerization briefly
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consists of two steps: preparation of an oxidant substrate and exposure to monomer vapor. The monomer diffuses into the substrate and polymerizes on its surface. This technique is considered as a very useful method for preparing a pure conducting polymer [23]. Coupled metal-oxides have significant electrical properties and strong surface interactions between the closely packed nanoparticles in the coupled oxide system [10, 24, 25]. Nanofibrous membrane has easy fabrication process and highly porous structure that leads to a high surface to volume ratio of nanofibers. As a result, the coupled metaloxide nanofibrous membranes are envisioned as an excellent candidate for a porous template [10, 25]. In brief, electrospinning process is a method in which an electrical field is applied between a collector plate and a viscous polymeric or metal/polymeric solution inside a syringe. Afterward, the process continues by ejection of the solution from the needle to the target [26]. Finally, the collected fibers on the target could be calcined to obtain the polycrystalline oxide nanofibers. The present work reports the synthesis of core–shell-like structure of (SnO2–ZnO)/PPy nanocables in two steps: (1) synthesis of SnO2–ZnO composite nanofibers as the core via the electrospinning method [27]; (2) vapor-phase polymerization of Py on the surface of SnO2–ZnO nanofibers. Transmission electron microscopy (TEM) was used to display the core–sheath structure of prepared nanocables. Multiple sensing tests were done at room temperature. The prepared nanomaterial could detect NH3 with fast response and recovery times and it shows a linear response to NH3 concentrations. This linear behavior makes determination of NH3 concentrations feasible.
Experimental Synthesis of (SnO2–ZnO)/PPy nanocables As reported in our previous work [27], SnO2–ZnO composite nanofibers were synthesized via the electrospinning method. The electrospinning solution was prepared by dissolving 2 g polyvinyl alcohol (PVA) (Art. No. 821038, Merck) in 18 ml distilled water, followed by adding 1 g Zinc Acetate (Art. No. A769302639, Merck) and 1 g Stannous Chloride (Art. No. 3378443916, Merck). The prepared mix was kept at 60 °C under continuous stirring at 400 rpm for 4 h. The electrospinning process was done with a constant feeding rate of 0.2 ml/h and 6 cm distance between the tip of the syringe and the Al plate collector while the electric potential was maintained at 16 kV. Afterward, the resulting electrospun nanofibers were calcined at 650 °C and an ambient atmosphere for 2 h using a
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tube furnace leading to the formation of SnO2–ZnO composite nanofibers. The prepared SnO2–ZnO composite nanofibers were used as a template in the next step and the Py monomers in a vapor-phase were polymerized on it through the following steps. The nanofibrous template was soaked in 0.1 molarity ethanolic FeCl3 solution for 30 min and then left in the air for 10 min. FeCl3 is used as an oxidant for the polymerization process. Later on, the template was exposed to saturated Py vapor for 3 h. During the polymerization, the iron (III) ions absorbed on the surface of the nanofibers and functioned as an initiator of the polymerization process. An iron (III) ion oxidizes a Py monomer (1). Two of oxidized Py monomers will bond together and a Py dimer and two hydrogen ions will be liberated (2). The affording Py dimer will be oxidized using an iron (III) ion through reaction (3) and then bond with another oxidized Py (4), results a Py trimer. Continuously PPy will be produced by repeating this oxidation procedure (5). Py þ Fe3þ ! Pyþ + Fe2þ
ð1Þ
2Pyþ ! Py Py + 2Hþ
ð2Þ
Py Py þ Fe3þ ! Py Pyþ + Fe2þ þ
þ
Py Py þ Py ! Py Py Py + 2H
ð3Þ þ
ð4Þ
: : : : Py Pyþ þ Pyþ ! Py Py Py + 2Hþ ð5Þ A vessel containing Py placed on a stirrer was used for polymerization of Py on the template, while the template was hanging up from the vessel. The Py was stirred and heated for 3 h. During the polymerization process, an electric motor was connected to the vessel which helped with the evaporation process of Py and guided the Py vapor from bottom of the vessel to the template. Through the polymerization process the gradual production of blackcolored precipitates on the template serves as an indication of PPy production. The surface morphology of the electrospun nanofibers and PPy-coated nanofibers were studied using a Philips XL30 Scanning Electron Microscope (SEM). TEM was used for investigation of PPy coating. Sensor fabrication and measurements Preparing sensing samples were done according to the following procedure. A 10 mm 9 10 mm 9 1 mm alumina plate as substrate was mounted on Al plate in the
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electrospinning procedure and nanofibers were electrospun onto it. Although the alumina substrate is not conductive, the broad surrounding Al plate causes the electrospinning jets are ejected from the needle of the syringe and flight to the substrate. After calcination of the electrospun nanofibers at 650 °C for 2 h, the SnO2–ZnO nanofibrous membrane was resulted. The nanofibrous membrane was coated by PPy resulting (SnO2–ZnO)/PPy membrane. As reported elsewhere [28], sensor fabrication and related measurements were done according to the following procedure. Thin Pt wires with a gap of about 1 mm were cemented to the sample area by the silver paste. A sensor probe was formed by mounting the sample on a thick-walled borosilicate glass tubing. Two insulated connection cables were guided to the impedance measurement device and then sensing behavior of (SnO2–ZnO)/PPy toward NH3 gas was studied at room temperature. The NH3 concentrations ranged from 10.2 to 85 ppm during the sensing test. In this study, the sensor response is defined as Rg/Ra, where Rg and Ra are the steady state resistances of the sensor in gas contaminated air and pure air, respectively. According to this definition, Ra and Rg should both be measured at the operating temperature of the device. The time taken by the sensor for achieving 90 % of total resistance change is defined as the response time in case of absorption or the recovery time in case of desorption.
Results and discussion Characterization The quantities of the electrospinning solution and the electrospinning process parameters such as PVA concentration, feeding rate, electric potential, and distance between the needle tip and the collector were selected from our previous work [29]. The SnO2–ZnO nanofibers have a smooth surface, uniform morphology, low average diameter size, and narrow distribution of diameter size for the selected parameters [29]. The microstructures of electrospun nanofibers before and after calcination are illustrated in Fig. 1a, b, respectively. As depicted, nanofibers have smooth and uniform edges in both micrographs and are randomly distributed. The highly porous structure of prepared nanofibrous membrane causes ease of diffusion of NH3 molecules into the inner layers of membrane. Beside nanoscale dimension of the fibers, its porous structure provides higher accessible surface area which causes higher amount and rate of reaction. By comparing Fig. 1a, b, it is revealed that the diameter sizes of the nanofibers are reduced. This is due to removal of volatile species and decomposition of the
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precursor to metal-oxides during calcination process. Figure 1b shows the texture of nanofibrous membrane is kept unchanged even after calcination. Figure 1c, d depicts the distribution of nanofiber’s diameter size before and after calcination, respectively. The average diameter size of nanofibers before calcination process was 147 nm. On the other hand, the majority of calcined nanofiber’s diameter size falls into the range of 60–80 nm with an average of 75 nm. Figure 2 shows the SEM micrograph of PPy-coated nanofibers which its texture is similar to those electrospun nanofibers before and after calcination. Despite showing the morphology of PPy-coated nanofibers in the SEM image, the PPy layer is not revealed (see Fig. 2); therefore, the TEM analysis was done to examine the PPy layer. Figure 3 illustrates the TEM image of a PPy-coated nanofiber. As shown in Fig. 3, the PPy-coated nanofiber has a core–shell structure and the ultrathin PPy layer could be seen clearly. Also, the core of nanocable is about 70–80 nm, confirming the nanofiber’s diameter size measurements which was derived from SEM images. As seen in Fig. 3, the PPy-coated nanofiber is shorter than those which are shown in the SEM images (see Figs. 1 and 2). In sample preparation for TEM analysis, the prepared nanocables peeled off from the substrate, dispersed in ethanol, and ultrasonicated for 20 s. This caused the nanocables break down to the shorter length. Furthermore, gradual production of black-colored precipitates on the template during the polymerization is another indication for formation of ultrathin PPy layer. Even though, the original SnO2–ZnO nanofiber has a white color, it becomes a yellowish compound when it is immersed into an ethanolic solution of FeCl3. When the sample was exposed to pyrrole vapor the color changes from yellow to black. This is an indication of PPy formation on the surface of the nanofibers. Gas sensing behavior The response of (SnO2–ZnO)/PPy nanocable toward sensing NH3 was tested at room temperature and different concentrations of NH3, ranging from 10.2 to 85 ppm. The (SnO2–ZnO)/PPy nanocable is a resistor-type sensor. The sensing mechanism of this sensor mainly relies on its electrical conductivity (or resistivity) changes. The change of conductivity is contributed by interactions between positive charge along the PPy chains and surrounding environment. When PPy is exposed to electron donating gases such as NH3, a redox reaction occurs and its effective number of charge carrier decreases, thus reducing its conductance [30–32]. Working temperature is one of the most important parameter for gas sensors which affects sensor life time,
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Fig. 1 SEM images of SnO2–ZnO nanofibers a before calcination, b after calcination and distribution of nanofiber’s diameter size c before calcination and d after calcination [27]
Fig. 2 SEM image of (SnO2–ZnO)/PPy nanocables Fig. 3 TEM image of (SnO2–ZnO)/PPy nanocables
power consumption, and ease of operation. Our proposed sensor works at room temperature, while the conventional NH3 gas sensors based on metal-oxide thin films operate at temperature region of 250–450 °C [33, 34].
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Figure 4 shows the typical response/recovery features of nanocables versus time for different concentrations of NH3. The sensing test setup uses a constant current in its circuit
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Fig. 4 Sensing behavior of (SnO2–ZnO)/PPy nanocables versus time with different concentrations of NH3 at room temperature
Fig. 6 Sensor response of (SnO2–ZnO)/PPy nanocables toward 50 ppm concentrations of NH3, C2H5OH, and CH3OH
and raises its resistance (sensor response). The sensor exhibits a linear response to NH3 concentrations with the following equation:
Fig. 5 Sensor response of (SnO2–ZnO)/PPy nanocables versus NH3 concentrations at room temperature
which includes the sensing probe. While the resistance of a sensing probe is varying, in a course of exposing to NH3 concentration, the corresponding changes of the voltage are measured and recorded. As shown in Fig. 4, the voltage of a sensor starts to increase while it is exposed to a specific concentration of NH3. After a while, the voltage reaches to a constant amount and it does not change further which indicates that the senor is saturated. After saturation, the sensing probe is exposed to pure air and recovery happens. Therefore, the voltage decreases to the initial state. By changing the concentration of NH3, the saturation level will change. This is used to evaluate the sensor response toward different concentrations of NH3. Multiple sensing tests were done and the sensor response, as defined in the experimental section, was calculated for them. The average of sensor responses is plotted versus NH3 concentrations in Fig. 5. The estimates standard deviations of sensor responses are less than 9 % which shows the reliability of sensor response. The amount of reaction occurring between NH3 and PPy increases by increasing NH3 concentrations. The higher amount of reaction involves more free electrons along the PPy chains
Y ¼ 11:467 þ 1:116X ð6Þ Using the above equation, it is possible to determine the unknown amount of NH3. The response and recovery times of our proposed sensor when exposed to various concentrations of NH3 are calculated and are always less than a minute. The surface to volume ratio of one-dimensional nanomaterial is high. Also the nanofibrous membrane has highly porous structure. These characteristics bring broad potential locations for reaction between NH3 and the positive charges along PPy chains. Furthermore, the highly porous structure of membrane causes easier diffusion of target gas to the membrane which decreases the response and recovery times of sensor. To investigate selectivity of the sensor, sensing tests were also carried out at room temperature for 50 ppm concentrations of NH3, C2H5OH, and CH3OH. The response values for NH3, C2H5OH, and CH3OH are 48.18, 2.87, and 5.12, respectively. Figure 6 shows the bar chart for selectivity of the sensor. It is observed that the sensor response for NH3 is better than C2H5OH and CH3OH by a factor of *10. Higher response toward NH3 than C2H5OH and CH3OH can be explained on the basis of different interactions between the prepared nanomaterial and adsorbed gas. The high response for NH3 indicates that the prepared nanomaterial is selective for this gas.
Conclusion SnO2–ZnO electrospun nanofibrous membrane was synthesized and used as a porous template for polymerizing ultrathin PPy layer via the vapor-phase polymerization.
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Multiple sensing tests were done with prepared nanomaterial to detect NH3 gas at room temperate. The estimates standard deviations of sensor responses are less than 9 %. The prepared (SnO2–ZnO)/PPy nanocable shows a linear and reliable response to detect small concentrations of NH3 with fast response and recovery times. The linear response of sensor allows determination of the unknown amount of NH3. The microstructural features of prepared nanomaterials lead to high sensitivity of sensor at low concentrations of NH3. This also includes a fast response and recovery times. This desirable sensing behavior besides the low operating temperature and simple synthesis method indicates the potential of industrial applications for this nanomaterial.
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