Fe 2 O 3 -CNT nanocomposite for binary gas detection

1. Fe2O3 -CNT nanocomposite for binary gas detection Monika Joshi, R.P Singh & Vidur Raj Amity Institute of Nanotechnology, Amity University Uttar Pradesh, mjoshi@amity.edu ; rpsingh@aint.amity.edu; mevidurraj@gmail.com Abstract α-Fe2O3 is the most stable iron oxide with n-type semiconductor properties is extensively used as a gas sensor[1].By decorating MWNTs with Fe2O3, a new gas sensing devices with high sensitivity can be introduced. Modifications of CNTs with magnetic nanoparticles have changed the electronic properties of the sensing film. The electrical resistance of magnetic nanoparticles - carbon nanotubes (CNTs) networked films has been measured for the mixture of binary gases (one oxidizing and one reducing gas). It was found that composite film provides good response to low concentrations of gases and shows excellent selectivity in the presence of interfering gases like NH3 and ethanol at room temperature. Results and discussion Introduction Gas sensors are widely used in industry for environmental analysis, medical diagnostics and other various field applications. The main requirements of a good sensor are high sensitivity, fast response, low cost, high volume production, and high reliability. Several metal oxide semiconductor materials were reported to be usable as semiconductor gas sensors such as tin oxide, zinc oxide, titanium oxide, iron oxide and aluminum oxide. These materials have non-stoichiometric structures, so free electron originates from oxygen vacancies contributes to electronic conductivity [2].Fe2O3 have been widely chosen as sensing material because of their excellent characteristics such as low cost, high sensitivity, and fast recovery [3-4]. Problems encountered with Fe2O3 sensors are lack of flexibility, poor response times and operated at elevated temperature. These problems can be improved by surface fictionalization or incorporation of few additives into the oxide film [5]. Carbon nanotubes (CNTs) have generated great interest among researchers due to their unique electrical, physical, mechanical and chemical properties to develop high performance devices. The application of CNTs has the potential of revolutionizing the sensor industry due to their inherent properties such as small size, high strength, high electrical and thermal conductivity, and high specific surface area. However, gas sensors based on CNTs have certain limitations, such as sometimes low sensitivity to analytes for which they have low adsorption energy or low affinity, lack of selectivity, or irreversibility or long recovery time. To overcome these limitations, Fe2O3 is doped with CNTs differ to alter their chemical nature and enhance their sensing performance. In the present work we describe an approach for the preparation of magnetic-CNT nanocomposites. It has been investigated that Fe2O3 nanoparticles were uniformly assembled on the sidewalls of MWCNTs.The nanocomposite film exhibits excellent response towards different gases. Fig.5 reports the resistance of the magnetic-CNT composite film on the exposure of the gas mixture. The resistance of the film decrease on the exposure to oxidizing NO2 while increases upon the exposure to reducing gas ammonia. (b) Fig5The gas sensing behavior of composite film towards (a) NH3 (b) NO2 Experimental Exposure to the NH3 generates a decrease in the electrical resistance, thus a dominant character of the oxidizing properties is present in the mixture under test. This is attributed to the combined superposition of two simultaneous competitive effects: transfer of electrons from the reducing gas NH3 to the composite film, decreasing the hole major carriers density thus increasing the electrical resistance, shifting the Fermi level away from the valence band; and opposite transfer of electrons to the oxidizing gas (nitrous oxide)from the valence band of the composite film, increasing the density of holes, thereby decreasing the electrical resistance, shifting the Fermi level towards the valence band. The Raman technique is used for the quantitative and qualitative analysis of CNT(fig.2). The FT-IR spectra of oxidized CNTs were taken(fig.3).The peak observed at frequency around 3400 cm-1 is refers to the O-H stretch of the hydroxyl group which can be ascribed to the oscillation of carboxyl groups. Carboxyl groups on the surfaces of as-received MWCNTs could be due to the partial oxidation of the surfaces of MWCNTs during purification by HNO3 . Fig. 4 shows the XRD pattern of the Fe2O3–CNT composites. The diffraction peak at 2y=26.3 is ascribed to the reflection of graphite, while other diffraction peaks in the range of 30°< θ <80° match well with the reflection of the reflection phase of the magnetic nanoparticles. The peak broadening of the pattern indicates that the as-prepared magnetic nanoparticles are small in size. Synthesis of Fe2O3 nanoparticles 0.1M of ferric nitrate Fe(NO3)3 solution and 0.2 ml of citric acid (Aldrich 98%) solution was prepared in distilled water as the solvent. Then, ferric nitrate solution was drop wise added to the citric acid solution with a vigorous stirring at 70 °C for 2 hrs. The solution was then refluxed at 60 oC for 8 hrs. The solution was then heated to a temperature of 70oC with vigorous stirring until the gel was formed .The dried gel was annealed at 400oC and a dark red color powder was obtained indicates the formation of Fe2O3 nanoparticles. Conclusion Synthesis of MWCNT The sensing characteristics of Fe2O3–CNT composite sensors towards gases of different chemical nature are found to be very promising. By incorporating Fe2O3 in CNTs the structure exhibited a dramatic improvement in gas sensing properties including response time and selectivity.The gas sensing characteristics of magnetic-CNT sensor towards NH3 and nitrous oxide are found to be excellent. MWNT were synthesized by using Chemical Vapor deposition technique (CVD).In this process Acetylene was used as a precursor and Ni nanoparticles was used as a catalyst. MWNT were grown on Ni nanoparticles at 800 °C. The nanotubes were purified by Conc. HNO3 at 90 0C for 60 minutes in an oil bath. Then the solution was vibrantly ultrasonicated for 30 minutes. References Synthesis of CNT-Fe2O3 nanocomposite 5 mg of Fe2O3 nanoparticles were dissolved in 3ml of benzene. Then, 2.5 mg of MWCNT was dispersed into the above solution under sonication. The final solution was autoclaved at 110°C for 3 hrs. The black precipitate was washed 3-4 times with ethanol. Finally, the magnetic-CNT composite were dried in a vacuum oven. [1] W. Chang, D. Lee, Characteristics of a-Fe2O3 thick film gas Sensors, Thin Solid Films 200 (1991) ,pp. 329–339. [2] O.K. Tan, W. Zhu, Q. Yan, L.B. Kong, Size effect and gas sensing Characteristics of nanocrystalline xSnO2-(1−x)_-Fe2O3 ethanol sensors, Sens. Actuators B 65(2009), pp. 361–365. [3] M. Ivanovskaya, D. Kotikov, G. Faglia, P. Nelli, Influence of chemical composition and structure of factor of Fe2O3/In2O3 sensors on their selectivity and sensitivity to ethanol, Sens. Actuators B 96 (2003), pp.503–598. [4] S.F. Si, C.H. Li, X. Wang, Q. Peng, Y.D. Li, Fe2O3/ZnO core-shell nanorods for gas sensors, Sens. Actuators B 119 (2006) .pp.52–56. [5] J.F. Liu, X. Wang, Q. Peng, Y.D. Li, Preparation and gas sensing properties of vanadium oxide nanobelts coated with semiconductor oxides, Sens. Actuators B 115 (2006) ,pp. 481–48 Fig.1 XRD image of Fe2O3 nanoparticles Fig.2 Raman spectra of MWCNT Characterization The phases of the Fe2O3 samples were characterized by XRD(fig.1).The sample showed the major characteristic peaks for as grown crystalline metallic iron at values of 32.2(200), 35.7(311), 43.5(202), 57.6(511) and 62.5(400) degrees. These indicate that the as grown iron oxide nanoparticles is well crystalline Fe2O3 in nature. Acknowledgement Fig3 FTIR spectroscopy of MWCNT Fig.4 XRD image of Fe2O3-CNT composite film The authors are thankful to Dr. Ashok.K. Chauhan, Founder President RBEF and President, Amity Institute of Nanotechnology, Amity University Uttar Pradesh for his continued guidance and motivation during the tenure of this work.