Polyaniline/ WO 3 Nanocomposite-Based SAW Ammonia Sensor

Transcription

1 Polyaniline/ WO 3 Nanocomposite-Based SAW Ammonia Sensor Shih-Han Wang 1, Chi-Yen Shen 2*, Ruen-Tze Tsai 2, and Cheng-Bin Yang 1 Department of Chemical Engineering, I-Shou University, Kaohsiung Taiwan Department of Electrical Engineering, I-Shou University, Kaohsiung Taiwan Abstract Polyaniline/WO 3 nanocompoiste-based surface acoustic wave (SAW) gas sensor has been developed for ammonia sensing. The PANI/WO 3 nanocomposite thin film was coated onto STX Quartz SAW device. The nanocomposite membrane was characterized by using SEM and FTIR. The PANI/WO 3 and PANI SAW sensors were applied for ammonia sensing at ambient temperature. The nanocomposite-based SAW sensor showed the superior performance, and it performed fast response, high repeatability, and stable baseline. The sensing mechanism on the thin film was studied by XPS analysis. 1. Introduction It is important to detect ammonia level for the environmental and safety analysis, such as the emission level in the air for pollution from chemical and fertilizer industries. For biological and clinical purpose, it is also a significant marker for the quality of animal housing and breath analysis for noninvasive diagnosis. Hence, it is crucial to develop a reliable ammonia sensor for the sustainable life. The conjugated polymers, especially conducting polymers, such as polyaniline [1-3], polypyrrol [4] and polythiophene [], have been widely applied as gas sensing materials due to their unique electric properties. Among those conducting polymers, polyaniline has been frequently investigated due to its unique optoelectronic properties, and it performs relatively better environmental and chemical stability, reversible response and shorter response time [1-3]. Nevertheless, the polymer properties lack of the processing ability, mechanical strength and long term chemical stability. Metal oxides have superior environmental and chemical stability, but most of them should be operated in elevated temperature for gas sensing. Various metal oxides were used for ammonia gas sensing, such as TiO 2 [], ZnO [6], WO 3 [7] and etc.. Among those oxides, it has shown that tungsten oxides is one of the most promising materials for NH 3, NO and NO 2 gas sensing. The selectivity of those gases could be eliminated by introducing some other dopant. In order to incorporate and complement the extreme properties of pure polymeric and inorganic materials a lot of effort has been contributed to investigate the polymer/ metal oxide nanocomposites for gas sensing. In resent studies, many researchers exploited the properties of the nanocomposites of conducting polymer incorporated with various metal oxides, such as PANI/TiO 2, PANI/SnO 2, PANI/In 2 O 3 and PANI/WO 3 [8, 9]etc., for gas sensing. In this study, the promising materials, PANI and WO 3 were used as the sensing materials. The PANI/WO 3 nanocomposite structure was fabricated from the aniline monomer and tungsten oxide gel to enhance the uniformity of the thin film. In order to enhance the sensitivity, the synthesized nanocomposite thin film was coated onto the highly sensitive SAW device as the ammonia sensor. The sensing mechanism was investigated using the surface characterization of XPS. 2. Experimental 2.1 Materials and reagents All chemicals used were analytical reagent grade. Aniline monomer obtained from ACROS was distilled prior to use. Ammonium persulphate (NH 4 ) 2 S 2 O 8 (Showa, Japan), tungsten chloride hydrogen chloride (Union Chemical Works LTD., Taiwan) propanol (TEDIA, USA) and ammonium hydroxide (TEDIA, USA) were used without further treatment. 2.2 Synthesis of polyaniline/wo 3 nanocomposite

2 The synthesis procedure of tungsten oxide sol was similar to that reported by Wang et.al. [] 3 g tungsten (VI) hexachloride was mixed with ml of propanol which was carried out in iced water for min. The hydrolysis process was accomplished by adding ml of. M ammonia hydroxide solution in room temperature overnight, and then the chloride ion was removed by de-ionized water washing. The precipitate was peptized using.1n ammonium hydroxide slowly and refluxed for three days..1 mol of distilled aniline was dissolved in 1M HCl and mixed with tungsten oxide gel with vigorous stirring in order to well disperse in aniline monomer solution..1 M ammonium persulphate was added into the well mixed solution dropwise and the polymerization process carried out for h. Subsequently, the obtained PANI/WO 3 precipitated was cleaned using de-ionized water and washed with 1 N HCl solution, and diluted in propanol for thin film preparation. 2.3 Fabrication of the SAW sensor The SAW sensor was a two-port resonator based on STX quartz in this study. Each of the input and output interdigital transducers (IDT) of Al has 4.finger pairs, the period p is 16μm, each of shorted grating reflector has Al strip gratings, aperture of IDT is 96μm, the thickness of Al is nm, the metallization ratio M/p is., and the center-tocenter distance between the IDTs is 6μm. The operating frequency of the resonator is MHz. The PANI/WO 3 nanocomposite thin film was coated on the space between input and output IDTs of the two-port resonator. This study made use of a dualdevice configuration to reduce interference from the environment. A non-coated resonator was used as a reference, and a coated resonator was used as a sensing device. The SAW resonator was connected to an RF electronic oscillator circuit to generate RF signals. 2.4 Characterization of the sensing membrane The surface morphology of the deposited nanocomposite thin film was observed by the field-emission scanning electron microscopy (Hitachi 4). The X- ray photoelectron spectroscopy (XPS, PHI Quantera SXM) with a monochromatic Al Kα radiation source was employed to determine the chemical structure of the PANI/WO 3 nanocomposite thin film. The chemical structure of the deposited thin films was analyzed by Furious Transform Infrared Spectrometer (FT-IR) using the reflection mode through the IR-microscopy (NICOLET 7). 2. Sensing Apparatuses Dry nitrogen was the carrier gas. Mass flow controllers (Sierra, USA) were used to control and monitor the flow. The gaseous ambience was maintained at a flow rate of 1 ml/min. To sense for ammonia, the dry nitrogen and ammonia were premixed at a desired NH 3 concentration before enter the reaction. The dual-device configuration was put into a Teflon sealed cm 3 sensing chamber. All the sensing experiment was carried out at ambient temperature ( o C). A frequency counter connected to a computer system via a RS-232 interface board monitored the frequency differences between the reference device and the sensing device. 3. Results and Discussion 3.1 Membrane characterization The surface morphology of as-synthesized sensing membrane was analyzed using FE-SEM as shown in Fig.1. The PANI/WO 3 showed the tungsten oxide was embedded in the network of PANI which constructed a porous and homogeneous thin film. As long as the increasing of tungsten oxide additive, the significant phase separation appeared, hence, the ratio of PANI/WO 3 was 1:2 hereafter. Fig. 1. SEM image of the surface morphology of PANI/WO 3 nanocomposite thin film. Chemical structure of PANI and PANI/WO 3 nanocomposite thin films were characterized by the reflective FTIR scanning in the range from cm -1 to 4 cm -1 as shown in Fig. 2. The spectra showed the bands correspond to PANI in the range from 8 cm -1 to 16 cm -1, but the position was shifted caused by the interaction between tungsten oxide nanoparticles and PANI matrix. The tungsten oxide characteristic

3 peak appeared at 7 cm -1 and the tungsten oxide inplan bending vibration of O-H at 1414 cm -1. repeatability of the PANI/WO 3 nanocomposite thin film for NH 3 sensing was good as shown in the response no matter at 1.37 ppm or ppm. PANI-WO3 PANI (PANI ) 1.37 ppm % Reflectance f/f(ppm) NH 3 On NH 3 Off Time(sec) Wavenumber/ cm -1 (PANI /WO 3 ) 1.37 ppm Fig. 2. Reflective FTIR spectra of PANI and PANI/WO 3 nanocomposite thin films. f/f /ppm NH 3 On NH 3 Off 3.2 Sensing performance of PANI and PANI/WO 3 Membranes Time/ sec The sensing performance of PANI and PANI/WO 3 nanocomposite membranes for ammonia gas were carried out in the ammonia gas mixing with dry N 2 in the desired concentrations. There are three mechanisms might contribute to the sensing response, which are mass loading, acustoelectric effect and elastic effect. Considering the perturbation mechanism, the propagation characteristics of the acoustic wave are influenced as follows: Δf Δ v 4μ λ + μ = = cmfδ ρs + cefhδ 2 fo v v λ + 2μ (1) 2 2 K σ s Δ σ s + vc s where c m and c e are sensitivity coefficients, ρ s is the coating density, λand μ are the bulk and shear module of the coating, K 2 is the electromechanical coupling coefficient, σs is the sheet conductivity of the film, and C s is the capacitance per substrate length. The three terms on the right of Eq. (1) indicate the contributions of change in mass loading, elastic effect, and acoustoelectric interaction, respectively, to total change in the SAW phase velocity. The transient responses of ammonia sensing indicated that the frequency increased while the sensing membrane exposed in the ammonia atmosphere. The experimental result suggested the sensing mechanism was elastic effect dominated. The sensitivity of ammonia sensing using the PANI/WO 3 nanocomposite thin film was relatively higher than that using PANI thin film at 1.37 ppm as shown in Fig.3 The f/f - (PANI /WO 3 ) ppm N Time/ sec Fig. 3. Transient response of PANI and PANI/WO 3 thin film for ammonia sensing. The sensitivity of the ammonia gas sensing was defined as ( f/f)/ C, and the sensitivity of this PANI/WO 3 nanocomposite thin film for ammonia gas sensing is around.17 at room temperature. Through the sensing range from 7 to ppm, the response time was less than sec which could meet the requirement for safety in industrial. 3.3 Sensing Mechanism N 3 XPS was used to analyze polyaniline and PANI/WO 3 nanocomposite thin film surface wherein the ammonia sensing reaction taking place. The surface adsorption phenomenon of ammonia on the coated thin film was investigated by XPS measurement of N 1s spectra as shown in Fig. 4. The photoelectron spectra were collected from three conditions: (i) the asmade PANI/WO 3 nanocomposite or PANI thin film; (ii) the thin film exposed in the 23.4 ppm NH 3 atmosphere for min at room temperature; (iii) the sample in

4 (ii)was purged by N 2 for min. In the N 1s spectra, the as-synthesized PANI, two peaked was included in the spectra from different N form, C-N and N=N. After exposed in the ammonia atmosphere, the peak around and 42 ev increased, which was contributed from NH 3 and NH 4 +, respectively. It seems difficult to desorpted the ammonia molecular from the PANI surface. On the PANI/WO 3 surface, ammonia adsorpted as the formula of NH 3 and the oxidized form and it desorpted from PANI/WO 3 surface completely after purged. Two different kinds of reactions took place on those different sensing membrane. CPS Binding Energy/ ev PANI PANI-NH3 PANI-de Fig., the response time prolonged while the ammonia concentration decreased. This phenomena was different to the behavior that Shen et. al. reported [3]. The experimental results suggested that the rate determine step should be the chemical reaction step and the second order reaction took place on the sensing membrane. The second order chemical reaction might involve in the sensing process. Based on the hypothesis proposed by Kukla et. al.[2], the sensing mechanism of the ammonia gas on PANI membrane was a deprotonation-reprotonation process and the sensing response showed exponential change along the concentration. However, the ammonia sensing response showed a linear function proportional to the ammonia concentration, it might cause by the interaction of the adsorpted ammonia and the intra structure interaction between PANI and tungsten oxide. The p-n junction was formed and it lowered the energy gap of ammonia adsorption and desorption process. Consequently, the adsoprted ammonia molecular on PANI/WO 3 nanocomposite film was almost desorpted entirely. The deprotonated process was assisted by the electron transfer from tungsten oxide surface. There was no tungsten oxide appeared in PANI thin film, and the desorption did not proceed completely as shown in the N1s of PANI after N 2 purged for min as shown in Fig. 4. (a) 4 PANI/WO3 PANI/WO3-NH3 PANI/WO3-de CPS Fig.. Response time corresponding to different ammonia concentration Binding Energy/ ev (b) Fig. 4. N 1s level XPS spectrum of nitrogen atoms in (a) PANI thin film and in (b) PANI/WO 3 nanocomposite film. The response time defined as the time required to reach 9% of the plateau frequency in the transient response. According to the relationship between response time and ammonia concentration as shown in 4. Conclusions The PANI/WO 3 nano-composited thin film was produced from oxidation of the mixtures of tungsten oxide gel and the aniline monomer. The PANI/WO 3 nanocomposite-based STX Quartz SAW showed high sensitivity and repeatability for NH 3 sensing in the range of ppm.

5 . References [1] Y.S. Lee, K.D. Song, J.S. Huh, W.Y. Chung and D.D. Lee, Fabrication of clinical gas sensor using MEMS process, Sens. Actuators B: Chem. 8 () [2] A.L. Kukla, Y.M. Shirshow, S.A. Piletsky, Ammonia sensors based on sensitive polyaniline films. Sens. Actuat. B 37 (1996) [3] C.Y. Shen and S.Y. Liou, Surface acoustic wave gas monitor for ppm ammonia detection, Sens. Actuators B: Chem. 131 (8) [4] S.A. Krutovertsev, S.I. Sorokin, A.V. Zorin, Y.A. Letuchy and O.Y. Antonova, Polymer film-based sensors for ammonia detection,, Sens. Actuators B: Chem. 7(1992) [] Y. Shimizu, T. Okamoto, Y. Takao and M. Egashira, Desorption behavior of ammonia from TiO2-based specimens-ammonia sensing mechanism of double-layer sensors with TiO2-based catalyste layer, J. Mol. Catal. A: Chem. () [6] J.B.K. Law and J.T.L. Thong, Improving the NH 3 gas sensitivity of ZnO nanowire sensors by reducing the carrier concentration, Nanotech. 19 (8) 2 [7] V. Srivastava and K. Jain, Highly sensitive NH 3 sensor using Pt catalyzed silica coating over WO 3 thick films, Sens. Actuators B: Chem. 133 (8) [8] N. Parvatikar, S. Jain, S. Khasim, M. Revansiddappa, S.V. Bhoraskar and M.V.N. Ambika Prasad, Electrical and humidity sensing properties of polyaniline/wo 3 composites, Sens. Actuators B: Chem. 114 (6) [9] A polyaniline/wo 3 nanofiber composite-based ZnO/64 o YX LiNbO 3 SAW hydrogen gas sensor, Synth. Met. 8(8) [] S.H. Wang, T.C. Chou, and C.C. Liu, Nano-crystalline tungsten oxide NO 2 sensor, Sens. Actuators B Chem. 94 (3) Acknowledgment The support I-Shou University (ISU ) is gratefully appreciated.