- Email: [email protected]
Humidity effect on electrical properties of layered α-zirconium phosphate
Solid State Communications 134 (2005) 553–557 www.elsevier.com/locate/ssc
Humidity effect on electrical properties of layered a-zirconium phosphate Sukanta De, S.K. De* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 19 November 2004; received in revised form 17 February 2005; accepted 18 February 2005 by A.K. Sood Available online 19 March 2005
Abstract The electrical properties of alkali ions exchanged zirconium hydrogen phosphate Zr(HPO4)2$H2O (a-ZrP) are investigated by impedance spectroscopy technique. Cole–cole plots are used to describe the characteristic changes of electrical properties with relative humidity. The evaluated bulk resistance from the equivalent circuit shows exponential dependence of relative humidity. The surface protons of layered (a-ZrP) contribute to electrical conduction. The largest sensitivity is obtained for K ion exchanged systems. q 2005 Elsevier Ltd. All rights reserved. PACS: 92.60.Jq; 84.37.Cq; 66.30.Dn Keywords: D. Humidity; D. Electrical impedance measurement; D. Ionic conduction
1. Introduction The development of humidity sensors is very important for its wide applications in agriculture, meteorology and medicine. The dependence of conductance and capacitance of materials are generally used for the fabrication of humidity sensors. Ceramics [1] and polymers [2,3] based materials are employed for such devices. The main characteristic changes are the protonic conduction due to sorption of water molecule which primarily depends on the porosity of materials. The search for new materials with better sensitivity and stability are needed for sensor applications. The most stable crystalline zirconium phosphate Zr(HPO4)2$H2O conveniently known as a-zirconium phosphate (a-ZrP), may be suitable in humidity sensing device. Earlier it was used as molecular sieves, catalysts, ion and proton conductors [4–8]. High thermal and chemical stability, resistance to oxidation indicate a promising
material for sensors at different conditions. The most attractive feature of these classes of materials is large surface area due to its layered structure. Zirconium atoms in each layer lie almost in one plane and it is bridged through phosphate groups. Successive layers create zeolite type ˚ . Interlayer is cavities with narrow entrances of size 2.63 A formed by weak Van der Walls force. The presence of Bronsted and Lewis acid sites on the surfaces of tetravalent metal phosphates make it good ion exchangers. Ionic radii ˚ for Li, Na of monovalent alkali ions are 1.36, 1.96, 2.66 A and K ions, respectively. The restricted passage way to the cavity permit the alkali ions to occupy the lattice sites through exchangeable hydrogen [9]. The main purpose of the work is to increase interlayer spacing by intercalating alkali ions with higher ionic radii for better sensitivity.
2. Experimental 2.1. Materials
* Corresponding author. Tel.: C91 33 24734971; fax: C91 33 24732805. E-mail address: [email protected] (S.K. De).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.02.036
ZrOCl2, and conc. H3PO4 are of AR grade and procured from BDH Company and used without further purification.
554
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
2.2. Preparation of a-zirconium phosphate a-Zr(HPO4)2$H2O is prepared by direct precipitation method [10]. In a typical synthesis, 11 g of ZrOCl2$8H2O are dissolved in 160 ml of water; 92 ml of 85% H3PO4 and 8 ml of 40% hydrofluoric acid are then added while the solution is stirred. The total solution is now placed in a constant temperature (80 8C) for 7 days to evaporate hydrofluoric acid. When the fluoride ion concentration is lowered, zirconium phosphate begins to precipitate. 2.3. Preparation of exchanged phases a-Zirconium phosphate has an exchange capacity of 6.64 mequiv./g of exchanger [9]. Ion exchange phases are prepared by titration method as reported earlier [4,11–14]. To prepare ion exchanged phases 0.1 N MClC0.1 N MOH (MZ Li, Na, K) is added to a weighed amount of azirconium phosphate such that the ratio of MOH to azirconium phosphate is 6.64 mequiv./g. During reaction the pH of solutions is maintained between 9 and 10. The mixtures are stirred at room temperature for 2 days. The solids are separated by filtration and dried. For simplification Zr(HPO4)2$H2O and its ion exchange phases (Li, Na, K) are denoted by ZrP(HH), ZrP(LiLi), ZrP(NaNa), ZrP(KK), respectively. Samples are palletized using hydraulic pressure (applied pressure 5 ton). Pallets of 0.1 cm thickness and 0.7854 cm2 area are used for detail characterization. Capacitance (C) and loss factor (tan d) as a function of frequency (f) are measured by 4192A Agilent impedance analyzer in the frequency range 1 KHz to 2 MHz at different humidities. The electrical contacts are made by silver paint. To control humidity in the interval of relative humidity (RH) between 33 and 98%, a special chamber is designed. The humidity is decreased from 98% (humidity of air at room temperature) to 33% by passing dry and wet nitrogen through the sample chamber. All measurements for four samples are done at room temperature 27 8C. The real (Z1) and imaginary (Z2) components of complex impedance, Z is determined from the relations Z1 Z tan d=½uCð1C tan2 dÞ and Z2 Z 1=½uCð1C tan2 dÞ, where uZ2pf.
3. Results and discussion The X-ray diffraction (XRD) patterns of ZrP(HH) and fully ion exchanged phases are shown in Fig. 1. All the lines of diffractograms are indexed with monoclinic unit cell. Two phases of Li substituted samples are represented by different symbols with proper (hkl) values. The first peak in Fig. 1 indicates (002) reflections. The inter-layer distance in the (002) direction is estimated from Bragg’s law, 2d sin qZ l. The first peak in Fig. 1 shifts to lower angle, which corresponds to the lattice expansion from 7.59 to ˚ with the exchange of alkali ions. Ion exchange 10.72 A
Fig. 1. X-ray diffraction pattern of (a) ZrP(HH), (b) ZrP(LiLi), (c) ZrP(NaNa), (d) ZrP(KK). Two different phases of ZrP(LiLi) represented by different symbols (B and *).
process in crystalline a-ZrP is complex. The degree of ion exchange mainly depends on pH value, for half exchange it is 2–3 and full exchange it is more than 6. All the samples are synthesized at PHZ9–10 of the solution for the complete ion exchange conversion. The compositions of fully exchanged are Zr(MPO4)2$nH2O, (MZLi, Na, K), nZ 2–4 for Li and 3 for Na and K. The small openings of the zeolite type cavities first allow to diffuse anhydrous ions to occupy the lattice sites. As the interlayers of parent a-ZrP are weakly bonded, the interlayer spacing adjusts easily depending on the stoichiometry of the samples. Subsequent rehydration of ions happen through the diffusion of water molecules into the crystal lattice. The fully Li ion exchanged phases dehydrate easily to produce a variety of partial hydrated phases at room temperature. The first two distinct peaks in Fig. 1(b) indicate the presence of two phases with ˚ (2–3H2O) and 10.07 A ˚ (4H2O) interlayer distance 8.8 A [12]. The water content of ZrP(NaNa) and ZrP(KK) are 3H2O. The XRD patterns of Na and K ions exchanged are isomorphous and the difference in d value is due to the difference in ionic radii of Na and K ions. Thus the spreading apart of layers are due to the ion exchanged of larger sizes and concomitantly intercalation of more water molecules compared to monohydrate ZrP(HH). Two immiscible phases are observed in Li due to smaller ionic radius and higher binding with water molecules [15] compared with Na and K ions. The intermediate phases of Na and K ions exchanged are very unstable due to larger interlayer distance. As a result of it a single stable phase in Na and K ions intercalated samples is formed as indicated in XRD spectra. The capacitance decreases with increase in frequency. The effect of humidity has been investigated at a minimum frequency for maximum variation of capacitance. The capacitance at room temperature for the lowest frequency of 100 Hz as a function of relative humidity is shown in Fig. 2. A significant large value of C is found for ZrP(KK) and a
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
Fig. 2. The dependence of the capacitance on the relative humidity measured at frequency 100 Hz at room temperature.
large change of C with humidity is found. The capacitance decreases with decreasing humidity. The sharp decrease in C is found up to 70% of RH in case of K ions intercalated systems. Variation of capacitance and its magnitude is very low below 50% humidity for all samples. The adsorbed water molecules are very few at lower humidity. The capacitance is directly proportion to the dielectric constant. The dielectric constant of water is higher than air. As a result of it the capacitance becomes very low at low humidity and vice versa. Cole–cole plot of complex impedance Z, i.e. real (Z1) vs. imaginary (Z2) at different relative humidities for all the systems are shown in Fig. 3. A circular arc of higher radius is found for the lowest RH of 50% as depicted in Fig. 3(a). As the RH increases to 70%, the radius diminishes which yield a semicircle along with an incomplete arc. With further increase in RH the diameter of semi-circle at higher frequency becomes smaller and smaller. A spur as a part of second semicircle is observed in low frequency regime and it distorts the semicircle of higher frequency. Plots of Z1 vs. Z2 in Fig. 3 indicate the same characteristic features in all the samples. The basic difference is that the separation into two semicircles for four samples depends on relative humidity. The spur appeared at low frequency region dominates at the high humidity levels. The impedance data are simulated by the equivalent circuit as shown in Fig. 4. This circuit consists of two parallel combinations of resistance and capacitance corresponding to two semicircles in cole–cole plot. In the modelled circuit R1 and R2 are frequency independent Fig. 3. Impedance spectra, in complex plane representation of (a) ZrP(HH), (b) ZrP(LiLi), (c) ZrP(NaNa), (d) ZrP(KK). The solid lines are modelled impedance data.
555
556
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
resistances. The constant phase elements (CPE) capacitors C(u)ZA(iu)nK1 are assumed to describe the more flattened semicircles [16,17]. The parameter A is constant for a given set of experimental data. The exponent n varies between 0 and 1. CPE behaves as ideal capacitor for nZ1 and ideal resistor for nZ0. The experimental data are best fitted employing complex nonlinear curve fitting LEVM program developed by Macdonald [19]. The solid lines in Fig. 3 represent the best fitted calculated values. The parallel RC combination at low frequency is (R1, C1) and (R2, C2) at high frequency. The capacitor CN is introduced in the circuit to take into account of the non zero value of capacitance at high frequency. The parameter A2 increases with increase in humidity. The exponent n2 lies between 0.3 and 0.7 depending on humidity and samples. The layers of a-ZrP are stacked in such a way as to produce six sided cavities, which can accommodate water molecules. Water molecules residing in the cavities form hydrogen bonding with the phosphate group. The other water hydrogen atom does not participate in hydrogen bonding. There are no hydrogen bonds between the layers and only Van der Waals force exist to keep the layers apart. The protons of phosphate form hydronium ions with the lattice water molecules H2OCHCZH3OC. The high acidity of a-ZrP leads to hydronium ions instead of water inside the cavities. The diffusion of ions occurs through the openings of cavities parallel to the layer. The rotation of hydronium ions in the interior of crystals is difficult due to the strong hydrogen bonding with layered phosphate group. The free water molecules on the surface of crystals can easily rotate and contribute to ionic conduction. In Grotthus mechanism [18] hydronium ions reorient in proper position for protons conduction. The water molecules adsorbed on the surface of materials are involved in Grotthus type conduction [13]. The mobility of surface ions is about 104 times larger the interior hydronium ions [20]. Thus the surface conductivity is considerably higher than the interior and hence the surface ions dominate the total conduction. The systematic variation of impedance as evidence from cole–cole plots in Fig. 3 imply that electrical conduction depends on nature of alkali ions. The substituted alkali ions form coordination with both oxygens of layered phosphate group and surrounded water molecules. The distance between the oxygen atoms of the layer and alkali ions is large [21]. The charge carriers of the alkali ions substituted
Fig. 4. Predicted equivalent circuit.
samples are the hydrated alkali ions. The conductivity depends on the crystal structure, i.e. strength of interaction between mobile ions and the atoms forming the layers. The higher ionic radius in going from Li to K ions reduce the correlation with layered atoms and consequently conductivity is enhanced. Electrical conduction in the substituted samples mainly arises from the motion of hydrated alkali ions similar to observed in the layered cadmium thiosulphate systems [22,23]. The semicircle at high frequency in Figs. 3 represent protonic conduction originating from hydronium ions. The grain boundary and electrode-sample interface effects are given by the spur at low frequency region. These two regions are designated as bulk and grain boundary as shown in Fig. 4. The large capacitance observed at 100 Hz reflects grain boundary effect. Thus the interfacial polarization is responsible for the maximum capacitance found in K ions intercalated samples. The room temperature variation of resistance (R2) with relative humidity for all samples are shown in Fig. 5. The addition of larger ions induce lower resistance due to more open structures. The samples loose water with lowering of RH. As a consequence the interlayer separation decreases which narrow the entrance path to cavities and also reduce concentration of protons. This leads to enhancement of resistance with decreasing RH. The highest change of resistance is about three orders of magnitude in the measured humidity. The logarithm of resistance follows a straight line with humidity and can be expressed as R2 ðHÞ Z R2 ð0ÞexpðKmHÞ
(1)
where H is relative humidity and m is the slope of ln R2 against RH curve. The values of m for ZrP(HH), ZrP(NaNa) and ZrP(KK) are almost same except ZrP(LiLi) system.
Fig. 5. Variation of the resistance R2 of all samples with relative humidity.
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
However, the sensitivity of ZrP(KK) is the highest for more open structure. The anomalous behavior in resistance with humidity is due to mixed phases in Li ion exchanged materials as evident from XRD in Fig. 1(b).
4. Conclusion The electrical conduction of layered a-ZrP arises from hydronium ions on the surface of crystals. The intercalated alkali ions are mobile and contribute to electrical conduction through diffusion within the layer of the substituted zirconium phosphate. The capacitance and resistance are strongly humidity dependent. The monovalent alkali ion exchanged have reasonably good sensitivity in the humidity range of 50–96%. These compounds may be used as materials for humidity sensors.
Acknowledgements This work is funded by the Department of Atomic Energy, Government of India (Project sanction No: 2001/37/4/BRNS). Sukanta De is thankful to Council of Scientific and Industrial Research, Government of India for providing fellowship.
References [1] O.K. Vaxghese, L.K. Malhotra, J. Appl. Phys. 87 (2000) 7457. [2] A. Bearzotti, I. Fratoddi, L. Palummo, S. Petrocco, A. Furlani, C. Lo Sterzo, M.V. Russo, Sens. Actuators, B 76 (2001) 316. [3] K.G. Ong, C.A. Grimes, C.L. Robbins, R.S. Singh, Sens. Actuators, A 93 (2001) 33.
557
[4] A. Clearfield, Chem. Rev. 88 (1988) 125. [5] A. Clearfield (Ed.), Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982. [6] G. Alberti, U. Costantino, in: M.S. Whitinghan, A.J. Jacobson (Eds.), Intercalation Chemistry, Academic Press, New York, NY, 1982. [7] A. Clearfield, Design of New Materials, Plenum Press, New York, NY, 1987. [8] G. Alberti, U. Costantino, in: J.L. Atwood, J.E.D. Davis, D.D. MacNicol (Eds.), Inclusion Compounds In-organic and Physical Aspects of Inclusion vol. 5, Oxford University Press, Oxford, 1991. [9] A. Clearfield, J.A. Stynes, J. Inorg. Nucl. Chem. 26 (1964) 117. [10] G. Alberti, E. Torracca, J. Inorg. Nucl. Chem. 30 (1968) 317. [11] A. Clearfield, S.P. Pack, J.M. Troup, J. Inorg. Nucl. Chem. 39 (1977) 1437. [12] A. Clearfield, J. Troup, J. Phys. Chem. 74 (1970) 314. [13] A. Clearfield, U. Costantino, Compr. Supramol. Chem. 7 (1996) 107. [14] A. Clearfield, J.M. Garces, J. Inorg. Nucl. Chem. 41 (1979) 879. [15] G. Alberti, U. Costantino, S. Allulli, M.A. Massucci, M. Pelliccioni, J. Inorg. Nucl. Chem. 35 (1973) 1347. [16] J.R. Macdonald, Impedance Spetroscopy, Wiley, New York, 1987. [17] A.K. Jonscher, Dielectric Relaxation in Solids, Chesla Dielectric Group, UK, 1983. [18] J.H. Anderson, G.A. Parks, J. Phys. Chem. 72 (1968) 3662. [19] http://www.solartonanalytical.com/downloads/downloads. html. [20] G. Alberti, M. Casciola, U. Costantino, G. Levi, G. Ricciardi, J. Inorg. Nucl. Chem. 40 (1978) 533. [21] D.M. Poojary, A. Clearfield, Inorg. Chem. 33 (1994) 3685. [22] P. Jeevanandam, S. Vasudevan, Solid State Ionics 104 (1997) 45. [23] N. Arun, P. Jeevanandam, S. Vasudevan, K.V. Ramanathan, J. Chem. Phys. 111 (1999) 1231.
Humidity effect on electrical properties of layered a-zirconium phosphate Sukanta De, S.K. De* Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received 19 November 2004; received in revised form 17 February 2005; accepted 18 February 2005 by A.K. Sood Available online 19 March 2005
Abstract The electrical properties of alkali ions exchanged zirconium hydrogen phosphate Zr(HPO4)2$H2O (a-ZrP) are investigated by impedance spectroscopy technique. Cole–cole plots are used to describe the characteristic changes of electrical properties with relative humidity. The evaluated bulk resistance from the equivalent circuit shows exponential dependence of relative humidity. The surface protons of layered (a-ZrP) contribute to electrical conduction. The largest sensitivity is obtained for K ion exchanged systems. q 2005 Elsevier Ltd. All rights reserved. PACS: 92.60.Jq; 84.37.Cq; 66.30.Dn Keywords: D. Humidity; D. Electrical impedance measurement; D. Ionic conduction
1. Introduction The development of humidity sensors is very important for its wide applications in agriculture, meteorology and medicine. The dependence of conductance and capacitance of materials are generally used for the fabrication of humidity sensors. Ceramics [1] and polymers [2,3] based materials are employed for such devices. The main characteristic changes are the protonic conduction due to sorption of water molecule which primarily depends on the porosity of materials. The search for new materials with better sensitivity and stability are needed for sensor applications. The most stable crystalline zirconium phosphate Zr(HPO4)2$H2O conveniently known as a-zirconium phosphate (a-ZrP), may be suitable in humidity sensing device. Earlier it was used as molecular sieves, catalysts, ion and proton conductors [4–8]. High thermal and chemical stability, resistance to oxidation indicate a promising
material for sensors at different conditions. The most attractive feature of these classes of materials is large surface area due to its layered structure. Zirconium atoms in each layer lie almost in one plane and it is bridged through phosphate groups. Successive layers create zeolite type ˚ . Interlayer is cavities with narrow entrances of size 2.63 A formed by weak Van der Walls force. The presence of Bronsted and Lewis acid sites on the surfaces of tetravalent metal phosphates make it good ion exchangers. Ionic radii ˚ for Li, Na of monovalent alkali ions are 1.36, 1.96, 2.66 A and K ions, respectively. The restricted passage way to the cavity permit the alkali ions to occupy the lattice sites through exchangeable hydrogen [9]. The main purpose of the work is to increase interlayer spacing by intercalating alkali ions with higher ionic radii for better sensitivity.
2. Experimental 2.1. Materials
* Corresponding author. Tel.: C91 33 24734971; fax: C91 33 24732805. E-mail address: [email protected] (S.K. De).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.02.036
ZrOCl2, and conc. H3PO4 are of AR grade and procured from BDH Company and used without further purification.
554
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
2.2. Preparation of a-zirconium phosphate a-Zr(HPO4)2$H2O is prepared by direct precipitation method [10]. In a typical synthesis, 11 g of ZrOCl2$8H2O are dissolved in 160 ml of water; 92 ml of 85% H3PO4 and 8 ml of 40% hydrofluoric acid are then added while the solution is stirred. The total solution is now placed in a constant temperature (80 8C) for 7 days to evaporate hydrofluoric acid. When the fluoride ion concentration is lowered, zirconium phosphate begins to precipitate. 2.3. Preparation of exchanged phases a-Zirconium phosphate has an exchange capacity of 6.64 mequiv./g of exchanger [9]. Ion exchange phases are prepared by titration method as reported earlier [4,11–14]. To prepare ion exchanged phases 0.1 N MClC0.1 N MOH (MZ Li, Na, K) is added to a weighed amount of azirconium phosphate such that the ratio of MOH to azirconium phosphate is 6.64 mequiv./g. During reaction the pH of solutions is maintained between 9 and 10. The mixtures are stirred at room temperature for 2 days. The solids are separated by filtration and dried. For simplification Zr(HPO4)2$H2O and its ion exchange phases (Li, Na, K) are denoted by ZrP(HH), ZrP(LiLi), ZrP(NaNa), ZrP(KK), respectively. Samples are palletized using hydraulic pressure (applied pressure 5 ton). Pallets of 0.1 cm thickness and 0.7854 cm2 area are used for detail characterization. Capacitance (C) and loss factor (tan d) as a function of frequency (f) are measured by 4192A Agilent impedance analyzer in the frequency range 1 KHz to 2 MHz at different humidities. The electrical contacts are made by silver paint. To control humidity in the interval of relative humidity (RH) between 33 and 98%, a special chamber is designed. The humidity is decreased from 98% (humidity of air at room temperature) to 33% by passing dry and wet nitrogen through the sample chamber. All measurements for four samples are done at room temperature 27 8C. The real (Z1) and imaginary (Z2) components of complex impedance, Z is determined from the relations Z1 Z tan d=½uCð1C tan2 dÞ and Z2 Z 1=½uCð1C tan2 dÞ, where uZ2pf.
3. Results and discussion The X-ray diffraction (XRD) patterns of ZrP(HH) and fully ion exchanged phases are shown in Fig. 1. All the lines of diffractograms are indexed with monoclinic unit cell. Two phases of Li substituted samples are represented by different symbols with proper (hkl) values. The first peak in Fig. 1 indicates (002) reflections. The inter-layer distance in the (002) direction is estimated from Bragg’s law, 2d sin qZ l. The first peak in Fig. 1 shifts to lower angle, which corresponds to the lattice expansion from 7.59 to ˚ with the exchange of alkali ions. Ion exchange 10.72 A
Fig. 1. X-ray diffraction pattern of (a) ZrP(HH), (b) ZrP(LiLi), (c) ZrP(NaNa), (d) ZrP(KK). Two different phases of ZrP(LiLi) represented by different symbols (B and *).
process in crystalline a-ZrP is complex. The degree of ion exchange mainly depends on pH value, for half exchange it is 2–3 and full exchange it is more than 6. All the samples are synthesized at PHZ9–10 of the solution for the complete ion exchange conversion. The compositions of fully exchanged are Zr(MPO4)2$nH2O, (MZLi, Na, K), nZ 2–4 for Li and 3 for Na and K. The small openings of the zeolite type cavities first allow to diffuse anhydrous ions to occupy the lattice sites. As the interlayers of parent a-ZrP are weakly bonded, the interlayer spacing adjusts easily depending on the stoichiometry of the samples. Subsequent rehydration of ions happen through the diffusion of water molecules into the crystal lattice. The fully Li ion exchanged phases dehydrate easily to produce a variety of partial hydrated phases at room temperature. The first two distinct peaks in Fig. 1(b) indicate the presence of two phases with ˚ (2–3H2O) and 10.07 A ˚ (4H2O) interlayer distance 8.8 A [12]. The water content of ZrP(NaNa) and ZrP(KK) are 3H2O. The XRD patterns of Na and K ions exchanged are isomorphous and the difference in d value is due to the difference in ionic radii of Na and K ions. Thus the spreading apart of layers are due to the ion exchanged of larger sizes and concomitantly intercalation of more water molecules compared to monohydrate ZrP(HH). Two immiscible phases are observed in Li due to smaller ionic radius and higher binding with water molecules [15] compared with Na and K ions. The intermediate phases of Na and K ions exchanged are very unstable due to larger interlayer distance. As a result of it a single stable phase in Na and K ions intercalated samples is formed as indicated in XRD spectra. The capacitance decreases with increase in frequency. The effect of humidity has been investigated at a minimum frequency for maximum variation of capacitance. The capacitance at room temperature for the lowest frequency of 100 Hz as a function of relative humidity is shown in Fig. 2. A significant large value of C is found for ZrP(KK) and a
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
Fig. 2. The dependence of the capacitance on the relative humidity measured at frequency 100 Hz at room temperature.
large change of C with humidity is found. The capacitance decreases with decreasing humidity. The sharp decrease in C is found up to 70% of RH in case of K ions intercalated systems. Variation of capacitance and its magnitude is very low below 50% humidity for all samples. The adsorbed water molecules are very few at lower humidity. The capacitance is directly proportion to the dielectric constant. The dielectric constant of water is higher than air. As a result of it the capacitance becomes very low at low humidity and vice versa. Cole–cole plot of complex impedance Z, i.e. real (Z1) vs. imaginary (Z2) at different relative humidities for all the systems are shown in Fig. 3. A circular arc of higher radius is found for the lowest RH of 50% as depicted in Fig. 3(a). As the RH increases to 70%, the radius diminishes which yield a semicircle along with an incomplete arc. With further increase in RH the diameter of semi-circle at higher frequency becomes smaller and smaller. A spur as a part of second semicircle is observed in low frequency regime and it distorts the semicircle of higher frequency. Plots of Z1 vs. Z2 in Fig. 3 indicate the same characteristic features in all the samples. The basic difference is that the separation into two semicircles for four samples depends on relative humidity. The spur appeared at low frequency region dominates at the high humidity levels. The impedance data are simulated by the equivalent circuit as shown in Fig. 4. This circuit consists of two parallel combinations of resistance and capacitance corresponding to two semicircles in cole–cole plot. In the modelled circuit R1 and R2 are frequency independent Fig. 3. Impedance spectra, in complex plane representation of (a) ZrP(HH), (b) ZrP(LiLi), (c) ZrP(NaNa), (d) ZrP(KK). The solid lines are modelled impedance data.
555
556
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
resistances. The constant phase elements (CPE) capacitors C(u)ZA(iu)nK1 are assumed to describe the more flattened semicircles [16,17]. The parameter A is constant for a given set of experimental data. The exponent n varies between 0 and 1. CPE behaves as ideal capacitor for nZ1 and ideal resistor for nZ0. The experimental data are best fitted employing complex nonlinear curve fitting LEVM program developed by Macdonald [19]. The solid lines in Fig. 3 represent the best fitted calculated values. The parallel RC combination at low frequency is (R1, C1) and (R2, C2) at high frequency. The capacitor CN is introduced in the circuit to take into account of the non zero value of capacitance at high frequency. The parameter A2 increases with increase in humidity. The exponent n2 lies between 0.3 and 0.7 depending on humidity and samples. The layers of a-ZrP are stacked in such a way as to produce six sided cavities, which can accommodate water molecules. Water molecules residing in the cavities form hydrogen bonding with the phosphate group. The other water hydrogen atom does not participate in hydrogen bonding. There are no hydrogen bonds between the layers and only Van der Waals force exist to keep the layers apart. The protons of phosphate form hydronium ions with the lattice water molecules H2OCHCZH3OC. The high acidity of a-ZrP leads to hydronium ions instead of water inside the cavities. The diffusion of ions occurs through the openings of cavities parallel to the layer. The rotation of hydronium ions in the interior of crystals is difficult due to the strong hydrogen bonding with layered phosphate group. The free water molecules on the surface of crystals can easily rotate and contribute to ionic conduction. In Grotthus mechanism [18] hydronium ions reorient in proper position for protons conduction. The water molecules adsorbed on the surface of materials are involved in Grotthus type conduction [13]. The mobility of surface ions is about 104 times larger the interior hydronium ions [20]. Thus the surface conductivity is considerably higher than the interior and hence the surface ions dominate the total conduction. The systematic variation of impedance as evidence from cole–cole plots in Fig. 3 imply that electrical conduction depends on nature of alkali ions. The substituted alkali ions form coordination with both oxygens of layered phosphate group and surrounded water molecules. The distance between the oxygen atoms of the layer and alkali ions is large [21]. The charge carriers of the alkali ions substituted
Fig. 4. Predicted equivalent circuit.
samples are the hydrated alkali ions. The conductivity depends on the crystal structure, i.e. strength of interaction between mobile ions and the atoms forming the layers. The higher ionic radius in going from Li to K ions reduce the correlation with layered atoms and consequently conductivity is enhanced. Electrical conduction in the substituted samples mainly arises from the motion of hydrated alkali ions similar to observed in the layered cadmium thiosulphate systems [22,23]. The semicircle at high frequency in Figs. 3 represent protonic conduction originating from hydronium ions. The grain boundary and electrode-sample interface effects are given by the spur at low frequency region. These two regions are designated as bulk and grain boundary as shown in Fig. 4. The large capacitance observed at 100 Hz reflects grain boundary effect. Thus the interfacial polarization is responsible for the maximum capacitance found in K ions intercalated samples. The room temperature variation of resistance (R2) with relative humidity for all samples are shown in Fig. 5. The addition of larger ions induce lower resistance due to more open structures. The samples loose water with lowering of RH. As a consequence the interlayer separation decreases which narrow the entrance path to cavities and also reduce concentration of protons. This leads to enhancement of resistance with decreasing RH. The highest change of resistance is about three orders of magnitude in the measured humidity. The logarithm of resistance follows a straight line with humidity and can be expressed as R2 ðHÞ Z R2 ð0ÞexpðKmHÞ
(1)
where H is relative humidity and m is the slope of ln R2 against RH curve. The values of m for ZrP(HH), ZrP(NaNa) and ZrP(KK) are almost same except ZrP(LiLi) system.
Fig. 5. Variation of the resistance R2 of all samples with relative humidity.
S. De, S.K. De / Solid State Communications 134 (2005) 553–557
However, the sensitivity of ZrP(KK) is the highest for more open structure. The anomalous behavior in resistance with humidity is due to mixed phases in Li ion exchanged materials as evident from XRD in Fig. 1(b).
4. Conclusion The electrical conduction of layered a-ZrP arises from hydronium ions on the surface of crystals. The intercalated alkali ions are mobile and contribute to electrical conduction through diffusion within the layer of the substituted zirconium phosphate. The capacitance and resistance are strongly humidity dependent. The monovalent alkali ion exchanged have reasonably good sensitivity in the humidity range of 50–96%. These compounds may be used as materials for humidity sensors.
Acknowledgements This work is funded by the Department of Atomic Energy, Government of India (Project sanction No: 2001/37/4/BRNS). Sukanta De is thankful to Council of Scientific and Industrial Research, Government of India for providing fellowship.
References [1] O.K. Vaxghese, L.K. Malhotra, J. Appl. Phys. 87 (2000) 7457. [2] A. Bearzotti, I. Fratoddi, L. Palummo, S. Petrocco, A. Furlani, C. Lo Sterzo, M.V. Russo, Sens. Actuators, B 76 (2001) 316. [3] K.G. Ong, C.A. Grimes, C.L. Robbins, R.S. Singh, Sens. Actuators, A 93 (2001) 33.
557
[4] A. Clearfield, Chem. Rev. 88 (1988) 125. [5] A. Clearfield (Ed.), Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982. [6] G. Alberti, U. Costantino, in: M.S. Whitinghan, A.J. Jacobson (Eds.), Intercalation Chemistry, Academic Press, New York, NY, 1982. [7] A. Clearfield, Design of New Materials, Plenum Press, New York, NY, 1987. [8] G. Alberti, U. Costantino, in: J.L. Atwood, J.E.D. Davis, D.D. MacNicol (Eds.), Inclusion Compounds In-organic and Physical Aspects of Inclusion vol. 5, Oxford University Press, Oxford, 1991. [9] A. Clearfield, J.A. Stynes, J. Inorg. Nucl. Chem. 26 (1964) 117. [10] G. Alberti, E. Torracca, J. Inorg. Nucl. Chem. 30 (1968) 317. [11] A. Clearfield, S.P. Pack, J.M. Troup, J. Inorg. Nucl. Chem. 39 (1977) 1437. [12] A. Clearfield, J. Troup, J. Phys. Chem. 74 (1970) 314. [13] A. Clearfield, U. Costantino, Compr. Supramol. Chem. 7 (1996) 107. [14] A. Clearfield, J.M. Garces, J. Inorg. Nucl. Chem. 41 (1979) 879. [15] G. Alberti, U. Costantino, S. Allulli, M.A. Massucci, M. Pelliccioni, J. Inorg. Nucl. Chem. 35 (1973) 1347. [16] J.R. Macdonald, Impedance Spetroscopy, Wiley, New York, 1987. [17] A.K. Jonscher, Dielectric Relaxation in Solids, Chesla Dielectric Group, UK, 1983. [18] J.H. Anderson, G.A. Parks, J. Phys. Chem. 72 (1968) 3662. [19] http://www.solartonanalytical.com/downloads/downloads. html. [20] G. Alberti, M. Casciola, U. Costantino, G. Levi, G. Ricciardi, J. Inorg. Nucl. Chem. 40 (1978) 533. [21] D.M. Poojary, A. Clearfield, Inorg. Chem. 33 (1994) 3685. [22] P. Jeevanandam, S. Vasudevan, Solid State Ionics 104 (1997) 45. [23] N. Arun, P. Jeevanandam, S. Vasudevan, K.V. Ramanathan, J. Chem. Phys. 111 (1999) 1231.