Polyaniline- (PANI) praseodymium Oxide (Pr2O3) composites have been synthesized by in situ polymerization method with different weight percentages. The synthesized composites have been characterized by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The temperature dependent conductivity shows that the conductivity is due to the hopping of polarons and bipolarons. These composites show negative thermal coefficient (
Conducting polymer has achieved more attention towards the humidity sensing application in present era due to its low weight, easy processing, and high absorption capability because of its porous nature [
The proper control of the surface composition of polyaniline-metal oxide humidity sensors is essential for obtaining rapid, reliable, and selective response to the adsorption of water vapor [
In this work, PANI-Pr2O3 composites have been prepared by in situ polymerization method with various weight percentages. The structural property and surface morphology have been analyzed by using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscope (SEM). Further the DC conductivity and humidity sensing behavior of these composites have been studied by using Kelvin two-probe techniques.
All chemicals used for synthesis are of analytical reagent (AR) grade. Aniline, ammonium peroxydisulphate (NH4)2S2O8; (APS), hydrochloric acid (HCl), and Pr2O3 have been purchased from Sigma-Aldrich, India. Aniline monomer is doubly distilled before use. The composite samples so synthesized are processed into circular pellets of 10 mm diameter and 2 mm thickness by applying pressure of 10 ton in a UTM-40 machine (40 Ton Universal Testing Machine) using a hydraulic press. For conductivity measurements, the pellets are coated with silver paste on either side of the surfaces to provide good electrical contacts.
Aniline (0.1 mol) is dissolved in 1 M hydrochloric acid for 20 min to form aniline hydrochloride. The above solution is refluxed with 1 mL of camphor sulphonic acid (CAS) which acts as a surfactant for 3 hours at 0°C to 5°C. Further, 0.1 M of ammonium persulfate [(NH4)2S2O8] is added to the refluxed solution which acts as the oxidant and is added drop-wise with continuous stirring for 6 hours at ice temperature and is kept 24 hours without stirring for complete polymerization. The green precipitate powder is recovered using vacuum filter and washed with 0.1 HCl in order to remove the excess chlorine ions followed by water and acetone to remove the excess ammonium persulfate and unreacted monomers from the composites. Finally the resultant precipitate is dried under dynamic oven for 24 hours to achieve a constant weight [
0.1 mol aniline is dissolved in 1 M hydrochloric acid and to this known amount of Pr2O3 (10, 20, 30, 40, and 50 wt%) is added and stirred for 20 min to form metal oxide suspended aniline hydrochloride. The above solution is refluxed for 3 hours at 0°C to 5°C with 1 mL of camphor sulphonic acid (CAS) which acts as a surfactant. Further, 0.1 M of ammonium persulfate [(NH4)2S2O8] is added to the refluxed solution which acts as an oxidant and added drop-wise with continuous stirring for 6 hours at ice temperature and is kept 24 hours without stirring to complete the polymerization. The green precipitate powder of PANI-Pr2O3 composite is recovered by using vacuum filter and washed with 0.1 HCl in order to remove the excess chlorine ions followed by water and acetone to remove the excess ammonium persulfate and unreacted monomers from the composites. Finally the resultant precipitate is dried under dynamic oven for 24 hours to achieve a constant weight.
The above synthesized PANI-Pr2O3 composites are structurally and surface morphologically characterized by using different techniques like FTIR, X-ray diffraction (XRD), and scanning electron microscopy.
The FTIR spectra of polyaniline and its composites are recorded on Perkin Elmer (model 783) IR spectrometer in KBr medium at room temperature. For recording FTIR spectra, sample powders are mixed with KBr in the ratio of 1 : 25 by weight and grounded to ensure the uniform dispersion of samples in KBr pellets. The mixed powders are pressed in a cylindrical dye to obtain clean discs of approximately 1 mm thickness [
The X-ray diffraction patterns of the prepared samples are obtained by employing Philips X-ray diffractometer using
The surface morphology of polyaniline and its composites are studied by using Phillips XL30 ESEM scanning electron microscope (SEM). The powder samples are dispersed on the surface of carbon tape mounted on aluminum tab and conducting gold is sputtered on the sample to avoid charging at the sample surfaces and hence selected areas are photographed.
A typical experimental setup used for the measurement of dc conductivity consists of a sample holder securely locating the sample coated with silver paste in order to get better contact inserted from top aperture of the heating furnace. A potential difference is applied across the sample. The electrodes and thermocouple leads are taken out from top aperture and connected to Keithely 2100 electrometer to measure the change in current of the samples and temperature indicator, respectively. A heater placed at the bottom of the furnace is capable of giving a variable but linear rate of temperature increases over a broad range of temperatures. The DC conductivity of all the samples is obtained by measuring current flowing through a piece of the material and using the sample dimensions σ can be calculated using
Humidity sensor chamber is made up of side glass plates of size 250 mm × 250 mm in dimension and 5 mm thickness provided with top and bottom glass plates. The chamber is made airtight by rubber beading [
Showing the schematic diagram of humidity sensor setup.
Figures
Showing the XRD spectra of (a) Pr2O3 and (b) PANI-Pr2O3 composites (30 wt%).
Figure
Showing the FTIR spectra of (a) PANI, (b) Pr2O3, and (c) PANI-Pr2O3 composites (30 wt%).
Figures
Showing the SEM image of (a) PANI, (b) Pr2O3, and (c) PANI-Pr2O3 composites (30 wt%).
Figure
Showing the DC conductivity of PANI and PANI-Pr2O3 composites for various wt%.
Thermal coefficient of PANI and PANI-Pr2O3 composites for various wt%.
Figure
Change in resistance as function of relative humidity of PANI and PANI-Pr2O3 composites.
Schematic illustration of humidity absorption in polyaniline composites.
Finally, by forming more layers, a large amount of water molecules is physisorbed on the necks and flat surfaces, hence singly bonded water vapor molecules become mobile and able to form continuous dipoles and electrolyte layers between the electrodes, resulting in an increased dielectric constant and bulk conductivity. Therefore, the slight variations of conductivity with humidity adsorption can be due to a water protonation and polarons conduction mechanism on the composite surface.
The sensitivity of detecting the humidity is defined by the percentage change in resistance per 10% change in the value of relative humidity [
Variation of sensitivity against relative humidity for PANI and its composites.
The vapor is removed from the humidity chamber by using vacuum pump but we cannot remove it completely from the samples. Therefore once the humidity of chamber attends 40% RH, heating should be switched on till the electrical resistance comes to base level in order to start next cycle. Figure
Showing the response and recovery time against time of PANI and PANI-Pr2O3 composites.
In this paper, we have reported humidity sensing properties of polyaniline-Pr2O3 composites prepared by in situ polymerization. The SEM micrograph of the composite shows the encapsulation of Pr2O3 in PANI matrix and reveals the presence of capillary pores which facilitate the sensing behavior. From the XRD spectrum of PANI-Pr2O3 composite, it is evident that with the Pr2O3 content in PANI, the peak intensity corresponding to both PANI and Pr2O3 is modified which confirms coexistence of PANI and Pr2O3 phase in the composite. The FTIR spectrum reveals the formation of composite with strong interaction between PANI chains and Pr2O3 particles. The humidity sensing mechanism is investigated using homemade two-probe method. The studies on temperature dependent conductivity of the composite at various humidity levels have shown the increase in sensitivity for the composite with increase in %RH. Among all composites, 30 wt% of PANI-Pr2O3 composite shows high sensitivity and exhibits the maximum humidity sensing response and shorter response-recovery time. Hence our studies suggest that PANI-Pr2O3 composite can be a promising material for high performance humidity sensing applications.
The authors declare that there is no conflict of interests regarding the publication of this paper.