Fabrication of nanobeads structured perovskite type neodymium iron oxide film: Its structural, optical, electrical and LPG sensing investigations
Introduction
Liquefied petroleum gas is one of the most harmful gases due to its inflammable and explosive nature. Therefore the development of portable LPG sensors having small sized, long lifetimes, quick in response and have sufficient sensitivity for the detection of LPG in the ambient environment are necessary and demanding in order to prevent the explosion accidents in homes and industries for safety requirements. Solid-state LPG sensors using metal oxides are the most promising for the detection of LPG because of their compact structure, high selectivity, low cost, and the ability of continuous monitoring [1], [2], [3], [4], [5]. The need for reliable, cheap and user-friendly gas sensors for the detection of LPG is industrially important and has led to a considerable expansion in the field of sensor research and development. For this reason efforts are made nowadays by scientific research communities in leading laboratories all over the world to focus on the investigation of novel LPG sensitive materials suited for solid-state gas sensors [6], [7], [8], [9]. Consequently, their performances have to be improved dramatically by adopting preparation conditions and by controlling deposition processing [10]. Several types of LPG sensors such as chemical sensors, the resistive and conductive type sensors using semiconductors and sensors based on metal polymer complexes have been investigated by different research groups in various parts of the world [11], [12], [13], [14], [15]. Therefore, a great attention has been recently paid to the development of new material “architectures” at the nano-scale. Rare earth oxides are well known to display a high surface basicity, fast oxygen ion mobility and interesting catalytic properties [16], [17], [18], [19]. These are the thoughts to be important in chemical sensing. As a well-known functional rare earth material, neodymium has been widely applied in fields such as catalyst, humidity sensing and optics due to its unique properties [20]. Our research is focused on enhancing the material active surface area while keeping the high crystalline degree of the nanostructures. Since the detection of a gas involves adsorption phenomena at the material surface, it is expected that the higher the surface, the greater the sensor response because of availability of a number of gas adsorption sites [21], [22]. With this endeavor, porous semiconductor metal oxides are promising material. Here we adopted the sonochemical assisted pathway for synthesizing nanocrystalline material with enhanced surface-to-volume ratio, i.e.,which is an elegant strategy.
In the present script the results concerning the gas-sensing characteristics of LPG sensor fabricated using pellet and thin film of NdFeO3 are presented. The change in electrical resistance of the sensor in the presence of LPG can be used for gas sensing measurements. The performance and reliability of the fabricated sensor shows its potential application for the detection of LPG. The regular channels of a bead structured nanosized materials are used for accommodating a number of sensing sites for adsorption of LPG. The strategies followed to maximize the sensor response involve the use of catalytic additives. The additives are chosen on the basis of their ability for enhancing the interactions between the gas modulus and the sensing surface [23], [24], [25]. There are few studies concerning the use of additives for increasing the gas sensor response. However, no clear information about the chemical state and effects of the additives is shown and thus, their role in the sensing mechanism is not yet clarified. From a fundamental point of view, it is interesting to achieve a better understanding of the interaction mechanism between the sensing material and the target-gas. This knowledge will provide information regarding the conditions under which better sensor responses may be obtained and thus, it would facilitate further improvements in the design and fabrication of sensors.
One-dimensional nanostructures (e.g., nanotubes, nanowires, nanorods, nanospindles, nanocombs and nanobelts) have been intensively synthesized because they are expected to play an important role as fabricating electronic and optoelectronic devices with nanoscale dimensions [26], [27], [28], [29], [30], [31]. Particularly, porous tubular nanostructures have attracted considerable research interest because of their improved performance in gas sensors as compared with their solid wire counterpart [32], [33], [34], [35], [36], [37]. Nanobead structure may be useful as efficient adsorbents for LPG molecules because of their large surface to volume ratio and well-defined pores in comparison to spherical nanomaterials. Also a single nanobead is advantageous as sensing material for the development of nanoscale devices. On inspiring with this idea, we synthesized porous hollow nanobeads and fabricated a thin film on an alumina substrate as a new approach to the development of LPG sensor operable at room temperature.
As mentioned above, NdFeO3 can provide many opportunities for improved sensing behavior. Few research works have been carried out on the gas sensing properties of nanotubes, nanorods, nanocombs, nanoflowers etc. [38], [39], [40], [41], [42], [43]. In our previous paper we fabricated Fe2O3 nanonails using screen printing technique and studied their sensing properties [44]. However, the achievement of rapid sensor response and recovery characteristics for this nanostructured material is still a challenge. Here we have reported fast responding and recovering sensing behaviors of NdFeO3 hollow nanobeads. The doping concept (doping of Nd in Fe2O3) may provide an alternative route for improving the gas sensing properties [45], [46]. In the present work, we prepared neodymium iron oxide hollow nanobeads by sonication assisted precipitation technique. To the best of our knowledge, this is the first report of neodymium iron oxide hollow nanobeads. More importantly, the neodymium iron oxide nanobeads have high sensitivity to LPG due to its special surface morphological structure. Since the hollow nanobeads are sensitive to adsorbed gas molecules and have large variations in resistance when adsorbing the LPG, this structure is suitable to gas-sensing applications. Actually, the theory for the operation of such sensors involves adsorption/desorption phenomena and reactions at the surface, therefore, the “surface accessibility” is crucial to maintaining the high sensitivity of nanostructured materials [47], [48], [49], [50], [51], [52]. For the design of sensing materials, both size effects and porous microstructures are very important. Thus, the hollow beads synthesized by sonication assisted precipitation route are extremely suitable to LPG sensing applications. The present work is to investigate whether the electrical properties of the rare earth oxides and their catalytic species are suitable to be used as gas sensors. The synthesized material was characterized using various techniques to explore the parameters of interest. Further we have prepared the pellet and thin film of synthesized material and these were investigated with exposure to LPG at room temperature (25 °C).
Nanostructured porous films as LPG sensing elements have many unique advantages in comparison to solid state pellet based gas sensors, such as well defined porosity, homogeneous film thickness, controllable pore sizes etc. This paper gives the recent progresses in the fabrication of LPG sensors using nanostructured porous film. By controlling the microstructure or the chemical composition of the porous film, nanostructured materials based sensors with both high sensitivity, and fast response and recovery times can be realized. More importantly, from the dependence of sensing parameters on film microstructures, we can easily fabricate the controlled structure of the porous films with desired sensing parameters as per practical requirement. Beads nanostructures prepared by sol–gel spin coating process showed very-fast sensing behavior due to the rapid diffusion of the LPG. However, the reliable gas-sensing characteristics are hampered by the sluggish recovery behavior of NdFeO3 based sensor.
Section snippets
Experimental
Neodymium and iron oxides were synthesized separately using chemical precipitation process. Further, they were mixed in 1:1 molar ratio through a solvent under constant stirring of 6 h at room temperature. The solution was sonicated in high power ultrasound (Ultrasonic processor, Sonics, 600 W, 20 kHz, probe length 25 cm, diameter 15 mm, titanium alloy) for 15 min. Precipitate of neodymium iron oxide was obtained through sonication assisted technique. Currently, sonochemical technique has proven a
Surface morphological investigations
The surface morphology of the sensing film (NdFeO3) was analyzed using scanning electron microscope (SEM, LEO-0430, Cambridge) and shown in Fig. 1(a) which exhibits the uniform distribution of nanostructured porous beads throughout the whole surface. The nanobeads are well represented in inset of Fig. 1(a). This type of surface morphology drastically enhanced the sensitivity of the sensor by maintaining a thermally stable high surface area and sensing channels inside the nanobeads. The
Conclusions
Sonication assisted precipitation technique has been used for the synthesis of porous neodymium iron oxide nanobeads which were applied as LPG sensing material and has been demonstrated as an evidence of hollow structure effects on the sensing characteristics. Moreover, the gas sensing characteristics, including the gas response, the response and recovery times, electrical properties of NdFeO3 were also investigated. Based on sensing analyses, hollow beads played a key role as a strong gas
Acknowledgements
Mr. Satyendra Singh is thankful to Council of Scientific and Industrial Research (CSIR), India for senior research fellowship vide award no. 09/107(0331)/2008-EMR. Corresponding author is grateful to Department of Science and Technology, Government of India for financial support in the form of project (SERC-FAST TRACK, SR/FTP/PS-21/2009).
Satyendra Singh has received his B.Sc. and M.Sc. degrees in 2005 and 2007, respectively from University of Lucknow, Lucknow, Uttar Pradesh, India. Currently he is a senior research fellow (CSIR) in Department of Physics, University of Lucknow, Lucknow and doing his Ph.D. under the supervision of Dr. B.C. Yadav, of the same university. His research topic is “Synthesis and characterization of nanostructured semiconducting oxides and their applications in gas sensor”.
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Satyendra Singh has received his B.Sc. and M.Sc. degrees in 2005 and 2007, respectively from University of Lucknow, Lucknow, Uttar Pradesh, India. Currently he is a senior research fellow (CSIR) in Department of Physics, University of Lucknow, Lucknow and doing his Ph.D. under the supervision of Dr. B.C. Yadav, of the same university. His research topic is “Synthesis and characterization of nanostructured semiconducting oxides and their applications in gas sensor”.
Archana Singh has received her B.Sc. and M.Sc. degrees in 2008 and 2010, respectively from University of Lucknow, Lucknow, Uttar Pradesh, India. Currently she is doing Ph.D. in Department of Physics, University of Lucknow, Lucknow. Her area of interest is “Synthesis and characterization of polymer assisted nanostructured semiconducting oxides as gas sensors”.
B.C. Yadav has received his B.Sc. and M.Sc. degrees from Dr. R. M. L. Avadh University, Faizabad, Uttar Pradesh, India in 1991 and 1993, respectively. He obtained his Ph.D. degree in 2001 from Department of Physics, University of Lucknow, Uttar Pradesh, India. Currently he is an Associate Professor in the Department of Applied Physics, School for Physical Sciences in the Babasaheb Bhimrao Ambedkar University, Lucknow. He is recipient of prestigious Young Scientist Award-2005 instituted by the State Council of Science and Technology. Also Dr. Yadav was selected in 2010 for Brain Pool International Fellowship of South Korea. He has published more than sixty five research/review papers in reputed international journals. His current interests of research includes the synthesis of metal oxides nanoparticles, metallopolymers, nanocomposites, carbon nanotubes etc., characterizations and their applications as sensors, thin and thick film sensors, pressure sensor, glucose sensors etc.
Prabhat K. Dwivedi is an experimental physicist. He received his Ph.D. from the Harcourt Butler Technological Institute, Kanpur. He has been a Visiting Researcher at the University of Alberta/TR Labs, Edmonton, Canada before joining DST unit on Nanosciences at IIT Kanpur. His main research interest lies in optical properties of photonic materials, micro- and nano-mechanical systems and development of micro/nanofabrication infrastructure. Currently, he is working on fabrication of 3D structures on different materials using gray scale lithography.