Fabrication of iron titanium oxide thin film and its application as opto-electronic humidity and liquefied petroleum gas sensors
Introduction
The preparation of nanocomposites based on semiconducting oxides is an important goal for improved functional performance in advanced fields such as opto-electronics, sensing and catalysis. Recently mixed metal oxides or binary semiconducting systems have attracted more and more attention for the fabrication of materials showing novel technological applications, which are often superior to those of the component characteristics.
In recent years, humidity sensing is becoming more important, mainly in control systems for industrial processes and human comfort. An extensive diversity of metal oxides and their nanocomposites, ceramics and polymers have been investigated as humidity sensors [1], [2], [3], [4], [5], [6], [7]. These sensors are mainly based on the variation in electrical parameters such as impedance, resistance and capacitance [8], [9], [10]. Additionally, another category of the sensors is also available whose principle is based on variations in optical parameters such as refractive index (r.i.), intensity modulation, frequency shift and wavelength modulation [11], [12], [13], [14]; these are frequently called optical/opto-electronic sensors. Optical sensors have tremendous advantages over their electrical counterpart as they can operate without any interface from nearby electric or magnetic fields.
Titanium dioxide has been extensively used as an environmentally harmonious and clean photo catalyst because of its various merits, such as optical and electronic properties, low cost, high photocatalytic activity, chemical stability and non-toxicity. Titania is an attractive photocatalytic material in the control of toxic pollutants by means of gas–solid or liquid–solid photo catalyzed reactions [15], [16]. However, the main crystallographic form of titania is anatase, which can only absorb light wavelengths shorter than 400 nm and generates the energy band gap of 3.2 eV. In the literature few reports on titania as gas sensor were found but the sensitivity of such kind of sensors is minute. Fe2O3 is another well documented material for the fabrication of sensors [17], [18]. It has a relatively narrow band gap (2.2 eV) with reasonable stability. However, its energy conversion efficiency is quite low, which has been attributed to low optical absorption coefficient and low electron mobility. Nanocomposites of Fe2O3 and TiO2 could be a motivation to develop the better sensors. As it is a well-known material for photo electrodes with visible light response and photo electrochemical stability. Therefore, such type of reports on this system has been observed. Morosin et al. [19] and Ginley and Butler [20] studied single-crystal and polycrystalline FeTiO3, Fe2TiO4 and Fe2TiO5 as the anodes for photo electrolysis of water.
Chemical sensors play a very important role in both environmental protection and human health. The fabrication of metal oxide semiconducting nanomaterials having large surface to volume ratio for gas sensing applications is currently a major hub of nanoscience and nanotechnology. Recently, some composite oxides such as spinel AB2O4 and perovskite ABO3 were found to be more attractive than single-metal oxides for their better selectivity and/or sensitivity to certain gases. In particular, the perovskite iron titanium oxide (FeTiO3), in which the transition metal Fe+++ incorporated into the lattice of the parent structure may be proved a promising material in detecting liquefied petroleum gas. Here we have investigated the opto-electronic humidity and LPG sensing properties of nanostructured iron titanium oxide.
To the best of our knowledge, the liquefied petroleum gas sensing and opto-electronic humidity sensing properties of iron titanium oxide have not been reported as yet. Therefore, the synthesis of iron titanium oxide via sol–gel technology was carried out. The sol–gel technique [21], [22], [23], [24] enables us to obtain oxides in a relatively simple way and at low temperature. The composition and the microstructure may be finely controlled and tailored for specific applications in the sol–gel process [25], [26], [27], [28]. In the present investigation, thin films were easily fabricated through the sol–gel spin coating technique and found that the sensitivity of the fabricated opto-electronic humidity sensor was enhanced by many folds to the earlier reported works [29], [30], [31], [32], [33] and is depicted in Table 1. The comparison of the fabricated LPG sensor with earlier reported LPG sensor may be seen in Table 2 along with the properties of the fabricated sensors which shows that in the present investigation the sensor response was enhanced by many folds [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]. Thus, present investigation is an advance step towards the fabrication of a robust and trustworthy opto-electronic humidity and LPG sensing device.
Section snippets
Material preparation
All chemical reagents used for the preparation of α-Fe2O3 and TiO2 were of AR grade. Nanostructured TiO2 and α-Fe2O3 were synthesized separately using chemical routes. Titania precursor was prepared by sol–gel method using titanium tetrachloride. Twenty-five milliliters of TiCl4 solution was added drop wise into 125 ml of ice-water under vigorous stirring to obtain a diluted TiCl4 solution. Further 20 g of citric acid was added into the above solution which produces a clear transparent solution.
Surface morphological analysis
Surface morphology of the sensing film was investigated using scanning electron microscope (SEM, LEO-0430, Cambridge). Fig. 3(a) shows a SEM image of the as prepared composite film at 300 nm scale and 23 KX magnifications which exhibits that the grains are almost homogeneously distributed. SEM image of the film annealed at 450 °C shown by Fig. 3(b) reveals that the film is more porous in comparison to Fig. 3(a). The surface consists of a number of active sites which are responsible for the
Optical humidity sensing properties
Fig. 7(a) shows the variations in the reflected intensity Ir (µW) of iron titanium oxide thin film with %RH for different angles of incidence θi=47, 50, 53 and 56°, respectively. Curve I for θi=47° exhibits that the sensor is most sensitive in low humidity region i.e. 5–40%RH. In initial state of the absorption process of the water vapor, the intensity decreases rapidly and sensor showed quick response to the detection of moisture, and the average sensitivity was found 5 µW/%RH for θi=47°. In
Conclusions
Iron titanium oxide was successfully synthesized by sol–gel method and the thin film was fabricated using spin coating process. The synthesized material was characterized for their structural, optical, thermal and surface morphological properties. From SEM micrographs we observed that the film is porous having uniform distribution of grains. XRD analysis reveals a rhombohedral crystal structure with the minimum crystallite size as 9 nm. The film was investigated with the exposition of liquefied
Acknowledgments
Corresponding author is thankful to Department of Science and Technology, India for financial support in the form of FAST-TRACK Project (SR/FTP/PS-21/2009).
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