This paper is focused on the comparative LPG sensing performances of bulk titanium dioxide and the synthesized TiO2 through hydrolysis of TiCl3 . The mean value sensitivity for different volume percentages (1-5%) of LPG has been estimated. Sensor response as a function of exposure time and response time has also been calculated. The average sensitivity and sensor response of the sensing element made of Titania synthesized through hydrolysis of TiCl3 are found to be maximum 3.0 and 318 respectively for 5 vol.% of LPG. Response time of the sensor was 70 sec. SEM images show that synthesized TiO2 is more porous than the obtained from Qualigens before exposure to the LPG. The minimum crystallites size of the purchased TiO2 and synthesized TiO2 are found to be 65 nm and 18 nm respectively. XRD patterns divulge that TiO2 is tetragonal and crystalline in nature. The device response was found to be concentration dependent, slow at low concentration and fast at higher concentrations.
Keywords: LPG Sensor;TiO2 ; SEM; XRD; Sensitivity.
A device that can convert the concentration of an analyte gas into an electronic signal is known as a gas sensor. From few decades, there are several investigations on nanoscale semiconductor gas sensors [1-2]. Titanium dioxide (TiO2) is a significant n-type metallic semiconductor which has been pragmatic as a sensor for earlier few years for its surface interaction, electrical assets and charge conveyance. The size reduction of TiO2 sensors to nanometer shaped offers a virtuous opportunity to sensationally increase their sensing properties in contrast with their macroscale counterparts [3-6]. It is a flexible material wide utilized in trade, analysis and environmental cleansing. Physical properties of TiO2 have given numerous fields of investigation to researchers that create it appropriate for numerous applications [7-12]. The primary titania gas device was developed within the late 1970’s and early 1980’s and primarily accustomed observe the ratio air to fuel magnitude relation (A/F) .
It was additionally accustomed notice a large style of vaporish species like O2 [14-15], H2 , CO  and Roman deity . TiO2 powder will exist in either the anatase section with the foremost stable type being mineral to that the others can convert at sufficiently at high temperatures. The brookite section has associate degree orthorhombic crystal structure and though just like mineral in its mechanical properties however seldom used commercially. In distinction, the anatase and mineral forms square measure polygonal shape systems that have found wide applications . Although the energy band gap (3.239 eV) of the anatase section is wider than that of the mineral (3.02 eV) structure, recombination of electrons and holes happens abundant quicker on surface of mineral section. Since gassensing performance is generally restricted to the surface of the sensing material, so its surface area must be increased to maximize the gas sensing behavior [20-26]. Yadav et al have already reported bulk TiO2 for the sensing of LPG but sensitivity was not satisfactory . In the present paper, we have reported the synthesis of nanosized titania and made a comparative study of sensing with the bulk titania. It was observed that this is an advance stage of previous work and sensitivity of sensor is just about twice than the previously reported sensor
The beginning material was bulk TiO2 (Qualigens). Glass powder was used as a binder. These were created fine on grinding in mortar for 5-6 hrs. Nanosized TiO2 was synthesized by exploitation associate degree liquid TiCl3 resolution and heated at 648 K temperature. Chemical change is given as below:
A white colored titanium dioxide powder was obtained. The pellets, each the pellet having dimensions nine millimeters in diameter and 5 millimeters in thickness, are created by applying uniaxial pressure of 616 MPa at ambient temperature. This pellet was heat treated in an exceedingly cylindrical furnace (Ambassador, India) at 4500C for one unit of time. Once heating it had been exposed to LPG in specially designed instrument of execution at controlled conditions. Corresponding variations in resistance with the exposure time of LPG were recorded by Keithley Electrometer.
The morphology of the sensing pellets of bulk and synthesized TiO2 may be pictured from the scanning electron microscope (SEM, LEO-0430 Cambridge) before exposition of LPG as is shown in Figure 1 and 2. Micrograph show that particles of bulk titania are spherical in form and combined with adhesive glass particles to create clusters departure of additional areas as pores. Higher consistency will increase surface to volume quantitative relation of the materials, diffusion rate of gas and thus helps in obtaining smart sensitivity. SEMs show that the particles are clustered, of uniform size and equally distributed.
Figure 2 shows the SEM of synthesized titanium oxide at nanoscale having magnification 75.28 KX. From micrograph, it shows that the molecules of TiO2 contains spherical grains having diameter lying between 37-150 nm. Each grain is uniformly distributed over the surface. The typical pore size calculated by unitary methodology is 152 nm.
X-ray diffraction patterns obtained by X-Pert PRO XRD system (Netherland) reveals extent of crystallization of the TiO2 (Qualigens) ( Figure 3). Analysis shows that TiO2 exists at 2θ values 25.6, 34.9, 36.0, 38.0, 46.6, 54.2, 55.8, 61.9, 63.4, 69.2 and 71.30. These peaks have the ‘d’ values as 3.518, 2.643, 2.367, 2.379, 2.117, 1.764, 1.673, 1.593, 1.498, 1.439, 1.414, 1.355 and 1.3799 Å with corresponding planes (101), (103), (004), (112), (200), (100), (211), (213), (204), (110) and (220) respectively. 65 nm was the smallest particle size for bulk TiO2.
X-ray Diffraction of nanostructured TiO2 synthesized via reaction of TiCl3 shows less extent of crystallization and pattern is shown in Figure 4. It reveals that sensing material consists of titania. The height targeted at 2θ = 250 is high intense and allotted to polygon TiO2 with anatase phase. Alternative higher angle of reflections like (110) R, (200)A, (004)A were indicating polygon crystalline nature having rutile and anatase phases of TiO2 having ‘d’ spacing 3.25460 Å, 1.89237 Å, 2.38390 Å and FWHM 0.1417Å, 0.3464Å, 0.1102Å severally. 18 nm was the minimum constituent part for manufactured Titanium oxide, signifying its nanocrystalline nature.
TEM analysis of synthesized TiO2 sample (Figure 5), designates the nanocrystalline nature of particles, is almost orthorhombic or slightly stretched. Rather dark spots and less dark spot can be assumed to be the different orientation of titania nano crystallites respectively because the brightness and darkness of items in TEM image depends on its orientation. The minimum size of nanoparticle was 20 nm.
DSC curves of as synthesized powder is shown in Figure 6. In curve, it is clearly shown that two endothermic peaks of about 32.5 0C due to the evaporation of acetic acid and water. A sharp endothermic peak around 100 0C, followed by broad exothermic peaks may be seen. The broad peak changed to a plateau shape around 200 0C to 400 0C and continued until around 500 0C
Synthesized TiO2 nanomaterial was optically characterized by UV- visible spectroscope. From Figure 7(a), the photon energy of synthesized TiO2 lie between the range 1.112 to 6.19 eV (i.e. 300-1000 nm). Titanium dioxide nano particles show a strong change of their optical absorption when its size condenses to a few nanometers. Figure 7(b) is the Tauc plot for nano sized TiO2 and corresponding band gap was calculated as 3.48 eV
Pellets of bulk Titanium oxide and synthesized TiO2 were exposed to Liquefied Petroleum Gas. Corresponding change in resistance with change in leakage time (in sec.) was taken. LPG sensitivity is defined as bellow-
Where Rair is the resistance of sensor in air and Rg for the resistance of the sensor in the presence of LPG correspondingly.
Sensor response of a sensing material is defined as
In our experimental found out, before exposition of LPG, each the pellets i.e. bulk titania pellet and synthesized TiO2 might equilibrate within the gas chamber at surrounding temperature for 10 to 15 min and the stabilized resistance is taken as Ra. The stabilization of the sensing element in close air is very important because it ensures the stable zero level for gas sensing applications . With exposure of various vol. of LPG to bulk TiO2 pellet, the amendment in resistance are detected and shown in Figure 8. The variation of resistance with time for the synthesized pigment for various vol. % is shown in Figure 9. It discovered that the resistance of the sensing element increases fleetly as compared to earlier one. The response curve for bulk titania sensing is shown in Figure 10. Curves demonstrate that the response of the sensing element will increase with volume share of LPG. For bulk TiO2 component, the response time calculated and it is 70 sec. Sensor response curve for synthesized titania primarily based sensor is shown in Figure 11. Curve shows the amended gas response with exposure time. The utmost value of sensor response is found as 318.
The variation of gas sensitivity with LPG concentration is shown in Figure 12. It has average gas sensitivity 1.68 for 5% vol. of LPG, whereas for synthesized titania, it raises up to 3 for the same vol.% of LPG. Since as synthesized TiO2 has greater surface area, and hence the huge no. of active adsorption sites which results elevated average sensitivity.
The principle of operation of semiconductor gas device is predicated on the interaction of gas fragments with surface, that produces associate degree interchange or housing of free charge transporters . This sensing phenomenon point toward the surface of the sensor is amazingly vital from a basic persistence of view. 3 key factors are recognized to regulate the device response, i.e., the receptor function, the electrical device function and the utility factor. The receptor function is provided either by the surface of the grain or by an overseas material spread on them . The transducer function is expounded to the grain boundaries and penetrating once, then the grain size becomes smaller than the double the thickness of the area charge layer . The utility factor is the proportion of the grains nearby the target gas. Thus, receiving of small units would improve the detecting proficiency. In the case of TiO2, these parameters can be successfully controlled by hydrothermal action.
Metal compound based semiconductors are principally accustomed to discover small concentrations of reducing and ignitable gases in air. The detection mechanism of those gases entails oxygen within the atmosphere and is influenced by the presence of aquatic vapour. After we expose the LPG to the surface area of conductive chemical compound, reaction takes place between surface molecules and LPG . The interaction mechanism between the gas segment and the detecting solid includes primarily physisorption, chemisorptions, surface imperfections and bulk defects. The reactions taking place on the surface of TiO2 gas sensor can be concise as-
Where CnH2n+2 denote the numerous hydrocarbons.
Primary two reactions occur in air owing to that carrier concentration is low and consequently cumulative the resistance. Last reaction corresponds to decomposition of reducing carriers. This will increase the carrier concentration and henceforth decreases the resistance on exposing to reducing gases.
As the previous reports on LPG sensing of titania had not made on titania at room temperature, those were at higher temperature, that’s why our investigation is very important and useful. Our investigation demonstrates that synthesized nano sized Titanium oxide has quite potential for LPG sensing. From XRD, the minimum crystallites size of the purchased-bulk TiO2 and synthesized TiO2 were found to be 65 nm and 18 nm respectively. By sensitivity measurement it was found that the bulk TiO2 and synthesized TiO2 have average sensitivities 1.68 and 3.0 respectively. Synthesized TiO2 has good sensor response, higher sensitivity and stability as compared to bulk titanium oxide and therefore, it is promising material for LPG sensing in the industrial and environment monitoring.