Nanonails structured ferric oxide thick film as room temperature liquefied petroleum gas (LPG) sensor
Introduction
In recent years on account of the attractive scientific and industrial applications of Fe2O3 nanoparticles, novel methods for their synthesis and new approaches for their characterizations have been reported [1], [2], [3], [4], [5], [6], [7], [8], [9]. It has been reported that different techniques of preparation lead to different phases or mixtures of phases of ferric oxide and different degrees of size control [10], [11], [12], [13]. A number of investigations have been reported on various ferric oxides such as nanostructured α-Fe2O3 and γ-Fe2O3. These deal mostly with magnetic and optical properties of ferric oxide [14], [15], [16], [17], [18]. However, in recent year ferric oxides have received increasing attention in the field of gas sensing, catalysis and anticorrosive agents [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]. Ferrites show very good surface reactivity and they have temperature dependent surface morphology [11]. They also show remarkable catalytic properties in oxidation reactions due to the high oxygen ion mobility at the film surface and thus are highly interesting for the development of sensors [12]. Basic requirement for the sensor is its change in electrical conductivity with exposure of LPG to semiconducting oxides which depends on their band gaps, surface morphology, size, diffusion rate of gas and specific surface area [30]. The semi-conductive properties of metal oxides represent the basis for their use as gas sensors, since the number of free charge carriers within the metal oxide and thus its electrical conductivity reversibly depends on the interactions with the ambient gas atmosphere [31]. For sensor application of nanostructured materials the charge transfer either results from adsorption or chemisorptions of gas molecules at the sensor surface, or from diffusion of the gas into the bulk of the sensor material [32]. Since the sensing mechanism is based on the chemisorptions reaction that takes place at the surface of the metal oxide, so increasing specific surface area of the sensitive film leads to more sites for adsorption of surrounding gases. Some recent researches have been concentrated on improving gas sensitivity as well as reducing operating temperature by introducing dopants or decreasing Fe2O3 particle size [21], [22], [23], [24], [25], [26]. It is well known that the properties of the materials e.g., magnetic, optic, electric, adsorption and catalytic properties are dependent not only on the composition, but also on the particle size and morphological structure. In recent years, considerable efforts have been devoted to the synthesis of nanostructures showing novel one-dimensional morphologies like rods, tubes and fibers. Among them, rod shaped metal oxides have exhibited numerous interesting physical and chemical properties that make them useful for a wide range of technological applications such as gas sensing, pigment and catalysis [11], [33], [34], [35].
Authors have already investigated nanostructured α-Fe2O3 as solid state LPG sensor but the sensitivity of the sensor was considerably small [36]. Therefore, we planned to replace solid state pellet with a screen printed thick film of α-Fe2O3 having larger specific surface area. The main goal of our present investigation is to design and fabricate a LPG sensor which would be robust, cost effective and more sensitive than previous sensor [36]. These investigations may be considered as the next step from our previous work in view of enhancement of sensitivity of LPG sensor [37], [38], [39], [40].
To the best of our knowledge, the synthesized Fe2O3 nanonails are reported for the first time here. Therefore, the present work provides not only the new synthesis of uniform nanonails of different nail edges and lengths by screen printing technology, but also the fascinating LPG sensing characteristics involved.
Section snippets
Synthesis of material
Nanocrystalline ferric oxide thick film was prepared on borosilicate glass substrate by screen printing method. Ferric chloride solution was made by dissolving required amount of ferric chloride in de-ionized water to form 1.0 M solution. Then, 10 ml of ethylene glycol was also added, which works as a capping agent. The paste used for the preparation of film was prepared by the precipitation of ferric chloride (FeCl3) solution by a drop wise addition of base agent, e.g., ammonium hydroxide (NH4
Surface morphology
The surface morphology of the film before exposition of LPG can be visualized from the scanning electron microscope (SEM, LEO-0430 Cambridge) as shown in Fig. 1(a) and (b). These figures show that the surface of the film is porous and has homogeneous distribution of nanonails-like structure with nail edge, ∼64–214 nm and of unspecified length, which provides larger specific surface area for the adsorption of LPG. The surface morphology can be well tuned by controlling various parameters during
Results and discussion
Fig. 7 shows the variation of resistance with time for different concentrations of LPG. Curves exhibit that the resistance increases sharply at initial stage, then becomes constant and finally reaches to its initial value (Ra). Average sensitivity was plotted as a function of concentration of LPG (vol.%) as shown in Fig. 8. It has maximum average sensitivity 18 for 2 vol.% of LPG. We observed that as the concentration of LPG increases, the average sensitivity increases linearly in the beginning
Conclusions
We have successfully synthesized nanonails structured α-Fe2O3 via hydroxide precipitation method and prepared thick film by the screen printing technique. The film was investigated for LPG sensing characteristics. The screen printing method has certain advantages such as low cost, simple in construction and having excellent sensing properties. SEM micrographs show nanonails and porous structure before exposing LPG. XRD analysis shows all the peaks correspond to rhombohedral α-phase of Fe2O3.
Acknowledgement
Mr. Satyendra Singh is thankful to CSIR, India, for junior research fellowship vides Award No. 09/107(0331)2008-EMR.
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