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Polyaniline-Titanium dioxide composite as humidity sensor at room temperature
S. Kotresh1, Y.T. Ravikiran1'*, H. G. Raj Prakash2, S.C. Vijaya Kumari3
1 Department of PG Studies and Research in Physics, Government Science College,
Chitradurga 577501, India 2Department of Physics, JNN College of Engineering, Shivamogga 577204, India 3Department of Physics, SJM College of Arts, Science and Commerce, Chitradurga 577501, India
PACS 81.07.Pr DOI 10.17586/2220-8054-2016-7-4-732-739
In the present work, Polyaniline (PANI) and Polyaniline-Titanium dioxide (PANI-TiO2) composite have been synthesized separately by chemical polymerization of aniline without/with TiO2. Characteristic absorption bands of PANI and TiO2 in the composite were confirmed by FT-IR studies, indicating the interaction of PANI with TiO2. Comparative study of XRD patterns of PANI and the composite confirmed superficial deposition of PANI on TiO2 and the average size of the composite particle was found to be 25 nm. The aggregated granular porous morphology of the composite was confirmed by its SEM image. The composite's sensing response to humidity at room temperature was tested and was found to be 84.21 % in the range 25 % - 95 % RH. The response and recovery time of the composite at 95 % RH were measured to be 60s and 100s respectively and its sensing stability over a period of one month was also confirmed. Keywords: polyaniline composites, humidity sensor, sensitivity, stability. Received: 9 February 2016 Revised: 26 April 2016
1. Introduction
In recent years, nanocomposites have evolved as the most sought after materials in various fields such as sensors, super capacitors, catalysis, light emitting diodes and fuel cells because they offer flexibility in terms of tuning their electrical and mechanical properties for best synergetic effects [1-5]. Thus, reliable fabrication of devices using these materials by cost effective methods is a major concern of many researchers. One such method is the incorporation of metal/metal oxides into the conducting polymers via a chemical or electrochemical route [6-12].
With the advancement of moisture-sensitive technologies and the increasing awareness for quality of life issues, humidity sensing has gained in importance in various areas like medicine, agriculture and electronic industries [13]. With the rapid advances in technology, increasing efficiency, reducing cost, reliability and miniaturization for fabrication of sensors are the major concerns in any research. In recent years, conducting polymer based sensors have become more favorable over metal oxide based sensors in terms of increased sensitivity, simple processing and their room temperature operation [14]. Among all conducting polymers, PANI is the most preferred because of its unique characteristics, such as tunable electrical and chemical properties, easy processing and low costs [15]. Recently, PANI based metal oxide composites have been found to be more advantageous in terms of enhanced humidity sensitivity and such composites have been studied by many researchers. Electrospun PANI composite nanofibers have shown good sensitivity to humidity with a good recovery and response time as reported by Q. Lin et al. [16]. M.T.S. Chani et al. Studied the humidity sensitivity of an orange dye PANI composite film by impedance measurements and have showed a uniform change in impedance for humidity values ranging from 30 -90 % RH [17]. Room temperature humidity sensing of PANI-PVA composite has been studied by Y. Li and others, and they showed that the sensitivity of the composite increases with increased impedance from 10 - 90 % RH and has both quick response and recovery times [18]. K. C. Sajjan et al. have reported better humidity sensing response for a PANI-Cr2O3 composite over the range 20 - 95 % RH [19]. S.C. Nagaraju et al. have reported good humidity sensing response for PANI-Pr2O3 composites in the range 10 - 90 % RH [20]. In this work, we have attempted to fabricate a low cost humidity sensing device made of PANI-TiO2 composite which can be easily prepared and operable at room temperature. We preferred TiO2 with PANI for the composite because it is a white n-type transition metal oxide semiconductor with a wide band gap (3.0 - 3.2 eV) and has a large dielectric constant (^100) [21,22]. This compound's compatible physical properties were considered favorable for use in fiber optics, photovoltaics, gas sensors, biocompatible materials, electrolytic capacitors and dye sensitized solar cells [21].
In this work, PANI and PANI-TiO2 composite were synthesized by a cost effective, simple in situ chemical polymerization method and were structurally characterized using FTIR, XRD and SEM techniques. Based on these studies, humidity sensitivity of the composite's film prepared by spin coating technique was tested and analyzed. Its stability, recovery and response time have also been studied. Both processability and sensing studies were carried out at room temperature.
2. Experimental
2.1. Materials
Aniline (C6H5NH2) (99.5 %), ammonium persulfate [(NH4)2S2Og] (98 %), hydrochloric acid (HCl) (34.5%) and titanium dioxide (TiO2) (99.5 %), all of analytical grade reagents were purchased from s.d. Fine Chemicals, Mumbai, India. Aniline monomer was doubly distilled before use.
2.2. Preparation of PANI-TiO2 composite
The method of preparation of pure PANI and the PANI-TiO2 composite at room temperature is as follows; firstly, a solution of 6.7 ml of aniline monomer in 60 ml 1M HCl was prepared. It was added to 180 ml of deionized water in which 0.67 g of TiO2 powder was previously dispersed. Then adsorption of aniline on TiO2 was facilitated by sonication of the above mixture. Then 7.5 g of (NH4)2S2O8 in 60 ml deionized water was added dropwise into the above mixture with continuous stirring for 30 min and then allowed to polymerize for 8 -10 hrs. The reaction product was collected by centrifugation and washed successively with deionized water and acetone until a clear precipitate was obtained which was dried in vacuum until it turned dark green.
2.3. Characterization of the samples
The FTIR spectra of pristine PANI, PANI-TiO2 composite and TiO2 were recorded using Nicolet 750 FTIR spectrometer in KBr medium at room temperature. The X-ray diffraction pattern of each sample was obtained by employing Siemens D-5000 powder X-Ray diffractometer with CuKa source radiation of wavelength 1.54x 10-10 m. SEM images of PANI and the composite samples were recorded using a Hitachi S-520 scanning electron microscope.
2.4. Humidity sensing measurements
The experimental set up used for humidity sensing measurements has been described previously in our literature [23]. A film of the composite was prepared by dissolving the composite powder in m-cresol and then coating it on a glass plate using spin coating unit (Make: Delta Scientific Pvt. Ltd, India, Model: Delta Spin I). Then, interdigitated silver electrodes were printed on it and it was then placed in a specially designed glass chamber well equipped for maintaining required relative humidity. The chamber was tightly closed at the top with a cork. One end of these electrodes were connected to the film by passing through the cork while the other ends were connected to a digital LCR meter (Make: Hioki, Japan, Model: 3532-50) which recorded the impedance changes before and after exposing the sample to humidity at a selected frequency of 100 Hz. The relative humidity in the chamber was varied using saturated salt solutions and monitored using humidity meter (Mextech-DT-615).
3. Results and discussion
3.1. Fourier transform infrared spectroscopy
FT-IR analysis was used to examine the composite structure after polymerization. The FT-IR spectra of TiO2, PANI and PANI-TiO2 composite are shown in Fig. 1.
The FT-IR spectrum of pure TiO2(Fig. 1(c)) shows strong absorption bands at 648 cm-1 and 419 cm-1 which are the characteristic vibrations of TiO2. These are respectively assigned to the Ti-O-Ti stretching vibration and for Ti-O and Ti-O-Ti combined stretching vibrations and all these results agreed with the published literature [24].
In the FT-IR spectrum of pristine PANI (Fig. 1(b)), the absorption bands at 3419 cm-1, 1600 cm-1, 1498 cm-1, 1299 cm-1, 1147 cm-1 and 880 cm-1 are attributed to the PANI chain. These are respectively assigned to O-H stretching of absorbed water, the C=C stretching of quinoid ring, the C=C stretching of benzenoid ring, the C-N stretching of imine and secondary amine of PANI backbone, the C-H stretching in-plane bending vibration and the 1,4 coupling of aromatic ring and all these have agreed well with the pattern published in the literature [25,26].
The FT-IR spectrum of the composite sample (Fig. 1(a)), exhibited characteristic absorption bands at 3425 cm-1, 1630 cm-1, 1497 cm-1, 1299 cm-1, 1136 cm-1, 896 cm-1, 611 cm-1 and 419 cm-1, which are associated with the characteristic vibrations of both PANI and TiO2 but with a small shift are due to the interaction of oxygen ions of TiO2 and nitrogen atoms of PANI macromolecules present in the composite [27].
4000 3500 3000 2500 2000 1500 10U0 500
Wavcnumbcr (cm1)
Fig. 1. Fourier transform infrared spectra of (a) PANI-TiO2 composite (b) Pristine PANI and (c) TiO2
3.2. X-ray diffraction
The XRD patterns of PANI, PANI-TiO2 composite and that of TiO2 with assigned hkl values are shown in Fig. 2. The XRD pattern of pristine PANI (Fig. 2(a)), reveals some degree of crystallinity with the appearance of broad peaks corresponding to (100) and (110) planes [JCPDS No. 53-1718] in the region 29 = 15 - 25 ° similar to those reported in our earlier literature [?, 28] The XRD pattern of TiO2 (Fig. 2(c)) clearly shows peaks corresponding to the planes (110) (101) (200) (111) (210) (211) (220) (002) (310) and (112) in the range 29 = 20 - 70 ° and the peaks when compared with JCPDS No. 870710 were confirmed to be of rutile TiO2. The same pattern for TiO2 has also been confirmed in previous literatures [29,30].
Comparison of XRD patterns of PANI, TiO2 and the PANI-TiO2 composite (Fig. 2(b)) indicates predominance of TiO2 in the composite, the rutile phase (110) of TiO2 has remained while the broad weak peaks of PANI have disappeared. All these suggest that there is interaction between PANI and TiO2 and that TiO2 has restricted the growth of PANI chains hampering its crystalline behavior. So it can be concluded that PANI has just deposited on TiO2 and has no effect on the crystalline behavior of TiO2. Similar study has been reported in earlier literature [30].
Applying the Scherrer formula [31] using Eq. (1) to the most prominent sharp peak at 28 °, we obtained the size of the crystallite t to be 25 nm:
kA
t = ' (1)
where b is the breadth in radians, 9 is the Bragg angle and A is the wavelength of radiation. The coefficient k normally takes a value close to 0.9.
3.3. Scanning electron microscopy
The morphology of TiO2, PANI and PANI-TiO2 composite are shown in Fig. 3. The morphology of pristine PANI shown in Fig. 3(a) is an agglomerated granular structure with uniform surface. The morphology of the composite (Fig. 3(b)) shows predominance of TiO2 particles, their surface being covered by PANI, which agrees very well with the results obtained by XRD studies.
The Scanning electron micrograph of TiO2 shown in Fig. 3(c) reveals aggregated granular morphology and agrees well with the SEM images in previously reported studies [32].
The average grain size DAvr of the composite material calculated using Mendelson's formula [33] given by Eq. (2) is 25 nm:
_ 1.56 x CL
°Avr = M'N ' (2)
Fig. 2. X-ray diffraction spectra of (a) Pristine PANI (b) PANI-TiO2 composite and (c) TiO2
where CL is the total length of the test line used, N is the number of intercepts and M' is the magnification of the photograph.
3.4. Humidity sensing response studies
The sensing response of the composite for various % RH are measured at 100 Hz using fractional base line manipulation method [34] using Eq. (3):
z0 - zrh .. „„
Sh =--- x 100 (3)
where Z0 is the impedance of the sample before exposure to moisture (dry air with 25 % RH) and ZRH is the impedance at various % RH.
The sensing response characteristic curve of the composite thus obtained by the above method is shown in Fig. 4. It can be seen that the composite is sensitive and its response is linear in the 25 - 95 % RH range with a maximum sensing response of 84.21 %. This linear response of the composite in this range with increasing humidity may be attributed to the following probabilities: (i) At low RH mobility of TiO2 ions in the composite is restricted due to curling up of polymer chains. As humidity increases, polymer chain uncurls and becomes aligned by absorbing water molecules paving way for faster hopping of charge carriers, resulting in increased sensing response of the composite [21,35]. (ii) Porosity of the polymers as revealed by SEM studies may facilitate absorption of water molecules as RH increases causing a decrease in the impedance of the composite [13].
Efficiency of the composite as a sensing device was studied by measuring its response and recovery times [36]. Humidity sensing response and recovery characteristic curve of the composite at 100 Hz is shown in Fig. 5. For the purposes of our study, two chambers, one with 25 % RH and another with 95 % RH were maintained separately. The response time of 60 s was recorded when the sample was moved from 25 % RH to 95 % RH and a recovery
Fig. 3. Scanning electron micrographs of (a) Pristine PANI (b) PANI-TiO2 composite and (c) TiO2
Fig. 4. Variation of sensing response and Impedance with relative humidity at 100 Hz
time of 100 s was recorded when the sample was moved from 95 % RH to 25 % RH, switching time being 1s in both the cases, establishing that the composite can become a good humidity sensing device.
To examine the practical viability of the composite for use as a sensor, variation of sensing response at 55 % RH and at 95 % RH at a frequency of 100 Hz after every one week for one month was studied [37]. The results presented in the graph shown in Fig. 6 suggest stable sensing ability both during adsorption and desorption with a negligible degradation of 3 % in sensing response. This finding suggests that the composite, apart from being a good sensor, is also stable and so it can be practically feasible to utilize it in a sensor device.
Fig. 5. Response and recovery characteristic curves of PANI-TiO2 composite at 100 Hz
Fig. 6. Stability of PANI-TiO2 composite at 100 Hz
4. Conclusions
The PANI-TiO2 composite synthesized by simple chemical polymerization method showed a room temperature sensing response of 84.21 % in the range 25 % - 95 % RH. The sensing response of the composite film prepared by cost effective spin coating method was measured using LCR meter at a selected frequency of 100 Hz. The efficiency of the composite as a sensor was also confirmed with a recorded response time of 60s and a recovery time of 100s. The composite has shown stable sensing ability over a period of one month. All these results show that the composite has the potential to become an efficient humidity sensing device operable at room temperature besides being easily prepared and cost effective.
Acknowledgement
The author is thankful to the University Grants Commission, New Delhi, for their financial support provided under major research project (41-917/2012 (SR) dated: 23/07/2012), to carry out this work. The Author wishes to acknowledge SAIF, Cochin, for providing facility for characterization of the samples. The author is also thankful to Commissioner, Department of Collegiate Education, Government of Karnataka and Principal Prof. C.M. Vivekananda, Government Science College, Chitradurga for their encouragement and support to carry out this research work.
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