INTERNATIONAL JOURNAL OF PURE AND APPLIED RESEARCH IN ENGINEERING AND TECHNOLOGY

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1 INTERNATIONAL JOURNAL OF PURE AND APPLIED RESEARCH IN ENGINEERING AND TECHNOLOGY A PATH FOR HORIZING YOUR INNOVATIVE WORK DEVELOPMENT OF HYDROXYAPATITE NANOCEREMICS FOR METHANOL AND ETHANOL SENSOR RAJENDRA SHRIKRISHNA KHAIRNAR 1, VANJA KOKOL 2, SHAIKH RESHMA ANJUM 1 1. School of Physical Sciences, Swami Ramanand Teerth Marathwada University Nanded, Maharashtra, INDIA. 2. Institute of Engineering Materials and Design, Faculty of Mechanical Engineering, Maribor, University of Slovenia, SLOVENIA Accepted Date: 05/03/2015; Published Date: 01/05/2015 Abstract: Synthesized Hydroxyapatite (HAp) has been of substantial interest because of its unique properties such as bio-active, nontoxic, ostio-conductive, along with its salient features like porous structure, high specific surface area, and fine grain size. These salient features are infact more desirable and useful for making an efficient sensor device. In the present study sensor films are fabricated by screen printing technique on glass substrate by taking nano sized HAp synthesized in our lab. The HAp sample is characterized by means of XRD and AFM to identify microstructure, phase and grain size. The functional groups of HAp, based on Calcium, Phosphorus, and water molecules, are identified by FTIR. Methanol and Ethanol, at various ppm level, are injected into a specially designed sensor setup and electrical resistivity is calculated at different temperatures, to find the sensitivity of HAp substrate to these vapors. To use HAp as a sensing device, practical aspects like sensitivity, response time, regenerative capacity, uptake limit, etc. are studied in detail, which are more important parameter for commercial sensor device. The results reveal that the porous structure of HAp with nano grain size enable the sensor to detect low concentration (100ppm) of methanol and ethanol vapors at room temperature, with faster recovery time and higher uptake capacity. It is concluded that HAp nano \ ceramics can be developed commercially as methanol and ethanol sensors. Keywords: Ethanol, Gas sensors, Hydroxyapatite, Methanol, Thick films. Corresponding Author: PROF. RAJENDRA S KHAIRNAR Access Online On: How to Cite This Article: PAPER-QR CODE 79

2 INTRODUCTION Many industrial processes involve toxic gases and flammable vapors which burn when mixed with air. It can cause harmful health effect when encountered in higher concentration. To avoid such malicious outcomes, proper detection of these vapors is in great demand in industries as well as in solid waste management. Hence, development of sensors is essential for early detection and its controlling. Nanotechnology gives a new approach to the world of gas detection by introducing drastic changes in sensors design and performance. Recent research reports have confirmed the benefits of nano material in gas sensing applications [1,2,3]. The review of literature shows that nano materials have great potential to become noble sensing substrate for the development of electrochemical sensors. These materials consume less power as they have ability to operate at low functional temperature, hence safe to operate. Also in nano structured material the interaction between the detecting material and the sensing part is rapid because of higher specific surface area of the sensing material which makes the adsorption process easier and improves the sensitivity [4]. Calcium hydroxyapatite (HAp) is an inorganic compound composed of calcium, phosphorus, and hydroxyl ions (OH - ). The lattice of hydroxyl ions (OH - ) is located at the center of Ca 2+ triangles along with c-axis columns of a hexagonal unit cell. These (OH - ) ions play an important role during ionic conduction [4]. Recently these nano particles have increasingly been in demand and are used in a wide range of applications in tissue engineering, controlled drug delivery system, in treatment of bone disease etc. The synthesis of HAp has been a major subject for chemists and material scientists for many years since it is one of the most important and versatile biomaterials. Moreover, the dielectric properties and some salient features like porous structure, high specific surface area, and fine grain size of the synthesized HAp are attractive features for the researchers working in the field of chemical sensors [5, 6]. The aim of present study is to develop a gas sensor using nano sized material (HAp), which works with high stability and sensitivity at temperature as low as room temperature for the detection of flammable organic alcoholic vapors like methanol and ethanol. EXPERIMENTAL The method for synthesis of HAp, screen printing method for preparation of thick films as well as the sensing parameters of the sensor are already discussed according to the procedure described earlier [1,2]. The surface morphology of the sensing material is studied by Atomic Force Microscopy (AFM) in non-contact mode in order to observed nano grain size and surface nano 80

3 texture. X-ray powder diffraction (XRD), using a Rigaku Miniflex X-ray diffractometer, is studied for the determination of the crystallographic parameters and phase content. The functional groups are observed by Fourier Transform Infra-red Spectroscopy (FTIR). RESULTS AND DISSCUSSION Figure 1 Atomic Force Micrographs of nano crystalline HAp The surface morphology of nano HAp thick film is shown in figure 1. The images show that all the particles are uniformly spread throughout the surface. The surface also shows porous nature of the material with pore size ranging between 50 to 300 nm. The crystalline phase structure of HAp is determined with the help of XRD analysis. The profile in figure 2 shows the presence of characteristics peak corresponding to HAp. The peak occurring at 2θ value 31.73, shows 100% sensitivity with (1 1 2) plane. The other noticeable peaks are occurring at 2θ value of 32.17, 25.93, 49.39, and [7] indicating hexagonal phase of HAp. Debye Scherer s formula is used to calculate the average crystalline size for HAp material. The calculations confirm the development of nano crystalline HAp since the average size calculated is found to be 24 nm. Figure 2 XRD profile for HAp showing the intensity of diffracted X-rays from various planes as a function of 2θ value. 81

4 The FTIR spectra of the sample synthesized by wet chemical process, recoded at room temperature with wave number range 4000 to 500 cm -1 is shown in figure 3. The presence of hydroxyl stretching mode is observed at 3756 cm -1, while hydroxyl libration mode is observed to be at 1636 cm -1. The peaks in the range 1030 to 1090 cm -1 ascribed to asymmetric stretching mode of ν3 vibration. The bands appearing at 963 cm 1 and 900 cm -1 is due to ν 1 fundamental mode of (PO 4 ) 3 ; whereas ν 2 mode of vibration is observed in the range 568 cm -1 to 603 cm -1[1,2,3]. Figure 3 Typical FTIR Spectra of HAp showing the presence of hydroxyl (OH) and (PO 4 ) 3 mode of vibrations Operating Temperature The alcoholic vapors response to HAp sensing material is measured at elevated temperatures for various concentrations of the vapors ranging from 100 ppm to 500 ppm. Figure 4 represents the plot showing the response of methanol and ethanol at selected interval of temperature. The nature of plot indicates that for lower temperature range, the interaction between HAp and alcoholic vapors is more showing rapid increment in the sensitivity factor. The dramatic improvement in the sensitivity factor in presence of methanol and ethanol vapors occurs at room temperature. Thus it indicates that the sensor device is workable at room temperature for the detection of ethanol and methanol. 82

5 Response and Recovery Time Figure 4 Temperature v/s Sensitivity curve The response time of the sensor is defined as the time needed for the output to reach a steady state. The response time of HAp for the detection of various concentrations of methanol and ethanol ranging from 100 ppm to 700 ppm is measured by keeping the temperature constant (27 0 C). Figure 5 indicates the measurement profile of response and recovery performance of the sensing device, after exposing the sensor material to methanol and ethanol vapors and atmospheric air respectively. It can be seen that in presence of alcoholic vapors, the sensitivity of the device for vapors increase abruptly, this is attributed to the adsorption of active organic molecules on the active site of porous HAp surface. The recovery parameter describes the sensor s ability to return to its initial state after it has been exposed to atmospheric air. The reversibility performance of the sensor is recorded with respect to time. The recovery time profile of HAp sensor suggest that the process of desorption of methanol vapors from the surface of HAp is faster as compared to desorption processes of ethanol, since the vapor pressure of methanol is greater, with weak intermolecular forces. 83

6 Figure 5 Measurement profile of response and recovery time for various concentrations of methanol and ethanol. Maximum Concentration limit The maximum concentration detection limit (vapor uptake capacity) of sensing material is measured by holding the sensor at its working temperature of 27 0 C. The concentrationresponse relationship exhibits linear and saturated behavior, i.e. approximately linear for low concentrations and saturated for higher concentrations of detecting vapors as shown in figure 6. Since the approximate molecular size of methanol (0.37nm) is less than ethanol molecule (0.43nm), a large number of methanol molecules are adsorbed on the porous surface of HAp. Hence the sensor shows higher uptake of methanol up to 900 ppm while maximum of 600 ppm uptake is observed for ethanol. 84

7 Figure 6 Concentration response plot exhibiting the maximum concentration detection limit of the sensing material at operating temperature of 27 0 C. For low concentration of vapors, there is enough active site for the vapors to be adsorbed, on the material surface. However; as the concentration increases the lack of active site changes the behavior of the plot and hence the nature of curve shows saturated region. CONCLUSION The characteristics peaks appearing in XRD diffractograms confirms the hexagonal crystalline nano structure of the material, supported by AFM micrographs. The sensing parameter study shows that the resistance value of the sensor decreases in presence of the detecting vapors; and attains the maximum at room temperature. This highest value of sensitivity factor for detecting vapors increases with increasing concentration of detecting vapors. The comparative study concludes that the sensitivity of the sensor is much larger in presence of ethanol vapors than methanol. HAp is useful sensor device, operating at room temperature at low ppm level and hence can be used for early detection of vapors. REFERENCES 1. M. Mahabole, R. Mane, R. khairnar, Gas sensing and dielectric study on Cobalt doped Hydroxyapatite thick films, Adv. Mat. Lett., vol. 4 No. 1, 2013,pp R. Mene, M. Mahabole, R. Aiyer, R. Khairnar, Hydroxyapatite nanoceramic thick films: An efficient CO gas sensor,open Appl. Phys. J., vol. 3,2010, pp

8 3. M. Mahabole, R. Aiyer, C. Ramakrishna, S. Sreedhar, R. Khairnar, Synthesis, characterization and gas sensing property of hydroxyapatite ceramic, Bull. Mater. Sci,vol. 28 No. 5, 2005, pp J. Giselle, R. Jordi, F. Xevier, Gas sensors based on nano structured materials, Analyst, vol. 132, 2007, pp M. Ferraz, F. Manteriro, C. Manuel, Hydroxyapatite nano particles; A review of preparation methodologies, J. Appl. Biomate. Biomech., vol 2, 2004, pp J. Gittings, C. Bowen, A. Dent, I. Turner, F. Baxter, S. Cartmell, J. Chudhuri, Influence of porosity on polarization and electrical properties of hydroxyapatite based ceramics, Ferroelectrics, vol. 390, 2009, pp R. Ramli, R. Adnan, M. Abu-Bakar, S. Masudi, Synthesis and characterization of pure nano porous Hydroxyapatite, J. Phy. Sci., vol. 22, No.1, 2011, pp