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Hydrothermal synthesis and characterization of MnWO4 nanoplates and their ionic conductivity
Materials Chemistry and Physics 103 (2007) 433–436
Hydrothermal synthesis and characterization of MnWO4 nanoplates and their ionic conductivity Lei Zhang a,b , Canzhong Lu a,b,∗ , Yuansheng Wang a,b , Yao Cheng a,b a
The State Key Laboratory of Structural Chemistry, Fujian Institute of on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b Graduate School, Chinese Academy of Sciences, Beijing 100039, China
Received 17 November 2005; received in revised form 29 May 2006; accepted 18 February 2007
Abstract MnWO4 nanoplates with ionic conductivity were synthesized under hydrothermal condition. In particular, the ionic conductivity of MnWO4 nanoplates is based on the ionic conduction and hygroscopic character in moist atmosphere. When the water molecules are adsorbed, the conductance of MnWO4 will change. At room temperature, the electric capability of MnWO4 is very good (the conductance is 1.46 × 10−5 S cm−1 ). After being heated for 4 h at 100 ◦ C, the conductance of MnWO4 becomes so small that it is on the verge of nonconductor. The water molecules play an important role in the electric conduction of MnWO4 . Based on this character, MnWO4 can be used as humidity sensing material, and its possible humidity sensing mechanism is investigated in this paper. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; Nanoplates: Ionic conductivity; Humidity sensor
1. Instruction Tailoring the configuration and morphology of materials from the macroscale to nanoscale at all dimensions is the major challenging issue to material scientists. During past years, research on inorganic nanomaterials has attracted extensive attention because of their particular importance in investigating the sizes and shape-dependent properties as well as their novel properties and potential applications [1–3]. Increasing attention has been focused on some two-dimensional nanomaterials–nanoplates, not only for their basic scientific richness, but also for their potential applications in nanoscale electronic and optical devices. Nanoplates can be synthesized through several methods including template assisted hydrothermal method [4]. Among all these ways, hydrothermal method is the simplest one and has a strong point that the nanomaterials synthesized in this way are quite pure. MnWO4 nanoplates exhibit an interesting electrochemical property-ionic conductivity [5]. This kind of electrochemical ∗ Corresponding author at: The State Key Laboratory of Structural Chemistry, Fujian Institute of on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. Tel.: +86 591 83705794; fax: +86 591 83714946. E-mail address: [email protected] (C. Lu).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.060
property is very unusual in simple inorganic compounds. The MnWO4 nanoplates with ionic conductivity exhibit sensitive a high sensitivity to humidity changes: when the quantity of water molecules that MnWO4 adsorbs changes, the conductance of MnWO4 also changes accordingly. Based on this character, MnWO4 is a very promising material for humidity sensing [6]. Just because of the unusual electrical and electrochemical property, the synthesis and research on nanoionic conductor MnWO4 nanoplates can offer great help to the studies of humidity sensing materials [7]. Recently, tungstate materials have attracted much interest because of their luminescence behaviors and potential application [8–11]. However, the study on their electric capability has rather been unexplored. In this report, we demonstrate first a facile method to synthesize MnWO4 nanoplates with ironic conductivity and investigate the mechanism of the ionic conductivity of MnWO4 nanoplates. Compared with micron MnWO4 , MnWO4 nanoplates is more sensitive to humidity changes. When humidity increases from 30% to 60%, the resistance of micron MnWO4 decreases from 2.1 × 107 to 1.0 × 106 , but the resistance of MnWO4 nanoplates decreases from 3.9 × 106 to 5.8 × 104 . This phenomenon indicates that MnWO4 nanoplates have much excellent capability compared with micron MnWO4 and the research to MnWO4 nanoplates is quite significative.
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L. Zhang et al. / Materials Chemistry and Physics 103 (2007) 433–436 out in distilled water. In a typical experiment, 2 mmol MnSO4 ·H2 O and 2 mmol Na2 WO4 ·2H2 O were dissolved in 5 ml distilled water, respectively. The Na2 WO4 solution was added into the MnSO4 solution slowly under stirring, and result in a yellow precipitate. Then 3 ml DMF was dropped into the solution. After then vigorous stirring for 1 h was necessary to ensure that all the reagents were dispersed homogeneously in the solution at room temperature. The resultant was transferred into a teflon-lined reactor and heated at 160 ◦ C for 10 days. Then the precipitations were washed with distilled water for several times and dried at 100 ◦ C for 2 h. The X-ray diffraction (XRD) pattern of the product was recorded by step scan on a RIGAKU-DMAX2500 X-ray diffractometer with Cu K␣ radiation (λ = 0.1542 nm) at 40 kV and 100 mA. The scan range (2θ) was from 5◦ to 85◦ with the step of 0.05◦ and the resolution of 0.01◦ . The morphologies and microstructures of the as-synthesized sample were characterized with a JEOL2010 transmission electron microscope (TEM) equipped with energy dispersive spectroscopy (EDS) and operated at 200 kV. Electric capabilities were measured on a 4284A 20 HZ to 1 MHZ Precision LCR Meter.
Fig. 1. Typical XRD pattern of the as-prepared sample.
2. Experimental procedure MnSO4 ·H2 O and Na2 WO4 ·2H2 O of analytical grade purity were used as starting materials without further purification. All reactions were carried
3. Results and discussion 3.1. Morphological characterisation The XRD pattern of the as-prepared sample is shown in Fig. 1. All peaks can be indexed to a pure phase monoclinic
Fig. 2. TEM and HRTEM images of the as-synthesized sample and. (a) TEM micrograph of MnWO4 nanoplates, (b) HRTEM image and its FFT pattern taken from a portion of the nanoplate and (c) HRTEM image of MnWO4 nanoplate.
L. Zhang et al. / Materials Chemistry and Physics 103 (2007) 433–436
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MnWO4 phase with space group P12/C1 (PDF #720478). The calculated cell parameters are: a = 0.484 nm, b = 0.576 nm and c = 0.499 nm, in good agreement with the literature values. The strong diffraction peaks in Fig. 1 indicate that well-crystallized MnWO4 has been obtained under the given synthetic conditions. The transmission electron microscopy (TEM) image shown in Fig. 2a reveals the general morphology of MnWO4 nanoplates. The nanorods seen in the image actually are sideways nanoplates and the nanoparticles are shadows produced by superposed nanoplates. It can be seen that most of the MnWO4 nanoplates are in the size with lengths of 60–100 nm, widths of 50–80 nm and thicknesses of 15–30 nm. The high-resolution transmission electron microscopy (HRTEM) image taken from a portion of a nanoplate is shown in Fig. 2b, which confirms that the MnWO4 nanoplate is in single crystal from with uniform lattice structure, free of defects and dislocations. The corresponding fast Fourier transform (FFT) pattern (inset of Fig. 2b) is indexed to the monoclinic MnWO4 phase along the 0 0 1 zone axis. Fig. 2c shows the HRTEM image of an MnWO4 nanoplate viewing through the cross-section and the space distance of the crystal lattices is 0.245 nm, which corresponds to (1 2 0) plane. 3.2. Spectral properties The impedance plots of the MnWO4 nanoplates are shown in Fig. 3. Using Fig. 3a as an example, the measurement resulted in a typical behavior of an ionic conductor with a semicircle at high frequencies (from 150 Hz to 300 KHz) and a liner spoke at low frequencies (from 20 Hz to 150 Hz). The sample resistance from this plot is 58 k, giving a conductivity of 1.46 × 10−5 S cm−1 . Then MnWO4 nanoplates were heated up at 100 ◦ C, and the resistances were recorded at different time and different humidity. The resistances and conductivities of MnWO4 nanoplates at different time and different humidity are shown in Table 1. At room temperature (humidity 60%), the MnWO4 had a very good electric conduction capability. When heating MnWO4 in the oven at 100 ◦ C, the resistance increased and the conductivity decreased relevantly. Following the prolonging of heating, the resistance became bigger and the conductivity accordingly became smaller. It is sure something that influences the electric capability of MnWO4 was vanishing tardily when heating the MnWO4 . Obviously, it is water that is on the MnWO4 surface or inside the nanoplates. The MnWO4 Table 1 Resistances and conductivities of MnWO4 at different time and different humidity Time
Humidity (%)
R ()
K (S cm−1 )
Room temperature 1 h later 2 h later 3 h later 4 h later 5 h later
60 30 19 15 13 13
5.82 × 104 3.91 × 106 1.08 × 107 1.76 × 107 2.45 × 107 2.45 × 107
1.46 × 10−5 2.18 × 10−7 7.86 × 10−8 4.83 × 10−8 3.47 × 10−8 3.47 × 10−8
Fig. 3. The impedance spectroscopy measurements of electrical resistance of MnWO4 (a) at room temperature (humidity 60%), (b) was heated at 100 ◦ C for 1 h (humidity 30%), (c) was heated at 100 ◦ C for 2 h (humidity 19%), (d) was heated at 100 ◦ C for 3 h (humidity 15%), (e) was heated at 100 ◦ C for 4 h (humidity 13%) and (f) was heated at 100 ◦ C for 5 h (humidity 13%).
nanoplates are quite sensitive to water vapor and can be used as humidity sensing material. The reason MnWO4 has humidity sensitive property is that it can make responses fast to the change of water molecules concentration. This commendably explains the phenomena that appeared in the electric capability experiment [12–14]. Based on prevenient researches [6], the sensing mechanism of humidity sensor MnWO4 contains two parts: the water molecules adsorption on the surface and the ionic conductivity of MnWO4 . The adsorption can be divided into two stages: chemical adsorption and physical adsorption. Because of the sizes of MnWO4 nanoplates, the surface area is very large and the MnWO4 molecules on the surface are quite active. Both physical adsorption and chemical adsorption take place easily when water molecules are adsorbed. At a low humidity, chemical adsorption plays the primary role. The adsorbed water molecules are decomposed into hydronium ions and hydroxyls on the surface of MnWO4 nanoplates. Because ionic conductor itself dose not have electric capability, the ionic electric capability of MnWO4 depends on those hydronium ions and hydroxyls [15–17,7]. While humidity changes, both the quantity
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L. Zhang et al. / Materials Chemistry and Physics 103 (2007) 433–436
of absorbed-water molecules and the concentrations of hydronium ions and hydroxyls change accordingly. The change of the concentrations of hydronium ions and hydroxyls can directly influence the resistance of MnWO4 . At higher humidity, subsequent layers of water molecules happen physical adsorption on the chemical absorbed layer. The Grottuss proton transfer occurs among the hydroniumn ions and results in an increase in protonic conductibility. Following the further increase of humidity, the multi-layers of physical-absorbed water molecules tend to concentrate in capillary pores. The radius of the capillary pores are defined as rk = 2γM/ρRT ln (Ps /P), there in M is the molecular mass of water, γ the surface tension of water, ρ the density of water, Ps the vapour pressure at saturation and P is the vapour pressure. The ionic conduction in condensed water also occurs in the physical absorbed layers [6]. And the quantity of physical absorbed water influences alike on the resistance of MnWO4 . By the mechanism, the change of humidity is represented by the change of the resistance of MnWO4 . Just about the ionic electric capability makes MnWO4 nanoplates a kind of humidity sensing material. 4. Conclusion In summary, MnWO4 nanoplates with ionic conductivity have been synthesized under hydrothermal condition, of which the sizes were controlled within 100 nm. From the electric capability experiment, it is assured that MnWO4 nanoplates are of ionic conductor properties. The ionic conductivity theory successfully explains the mechanism of humidity sensor MnWO4 . Based on its ionic conductivity, MnWO4 is quite sensitive to water molecules, and is a promising humidity sensing material.
Acknowledgements This work was supported by grants from the National Natural Science Foundation of China (20521101, 20425313, 90206040, 20333070 and 20303021) and the Natural Science Foundation of Fujian Province (2005HZ01-1). Thanks to the helps of the crew of Xintao Wu group in Fujian Institute of Research on the Structure of Matter CAS. References [1] H. Yang, N. Coombs, G.A. Ozin, Nature 386 (1997) 692. [2] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. [3] G.G. Zhou, M.K. Lv, F. Gu, D. Xu, D.R. Yuan, J. Cryst. Growth 276 (2005) 577–582. [4] Y.H. Zheng, Y. Cheng, Y.S. Wang, J. Cryst. Growth 280 (2005) 569–574. [5] M.A.K.L. Dissanayake, P.A.R.D. Jayathilaka, R.S.P. Bokalawela, Electrochim. Acta (2005). [6] W.M. Qu, W. Wlodarksi, Sens. Actuators B 64 (2000) 76–82. [7] J. Maier, Solid State Ionics 175 (2004) 7–12. [8] S.H. Yu, M. Antonietti, H. C¨olfen, M. Giersig, Angew Chem. Int. Ed. 41 (2002) 2356. [9] N. Saito, N. Sonoyama, T. Sakata, Bull. Chem. Soc. Jpn. 69 (1996) 2191. [10] S.J. Chen, J.H. Zhou, X.T. Chen, J. Li, L.H. Li, J.M. Hong, Z.L. Xue, X.Z. You, Chem. Phys. Lett. 375 (2003) 185. [11] X.L. Hu, Y.J. Zhou, Langmuir 20 (2004) 1521. [12] P.G. Su, C.L. Uen, Talanta 66 (2005) 1247–1253. [13] R. Nohria, R.K. Khillan, Y. Su, R. Dikshit, Y. Lvov, K. Varahramyan, Sens. Actuators. [14] Y.Y. Xu, X.J. Li, J.T. He, X. Hu, H.Y. Wang, Sens. Actuators B 105 (2005) 219–222. [15] P.G. Su, W.Y. Tsai, Sens. Actuators B 100 (2004) 417–422. [16] S.K. Shukla, G.K. Parashar, A.P. Mishra, P Misra, B.C. Yadav, R.K. Shuklad, L.M. Balid, G.C. Dubey, Sens. Actuators B 98 (2004) 5–11. [17] J. Wang, Q.H. Lin, R.Q. Zhou, B.K. Xu, Sens. Actuators B 81 (2002) 248–253.
Hydrothermal synthesis and characterization of MnWO4 nanoplates and their ionic conductivity Lei Zhang a,b , Canzhong Lu a,b,∗ , Yuansheng Wang a,b , Yao Cheng a,b a
The State Key Laboratory of Structural Chemistry, Fujian Institute of on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China b Graduate School, Chinese Academy of Sciences, Beijing 100039, China
Received 17 November 2005; received in revised form 29 May 2006; accepted 18 February 2007
Abstract MnWO4 nanoplates with ionic conductivity were synthesized under hydrothermal condition. In particular, the ionic conductivity of MnWO4 nanoplates is based on the ionic conduction and hygroscopic character in moist atmosphere. When the water molecules are adsorbed, the conductance of MnWO4 will change. At room temperature, the electric capability of MnWO4 is very good (the conductance is 1.46 × 10−5 S cm−1 ). After being heated for 4 h at 100 ◦ C, the conductance of MnWO4 becomes so small that it is on the verge of nonconductor. The water molecules play an important role in the electric conduction of MnWO4 . Based on this character, MnWO4 can be used as humidity sensing material, and its possible humidity sensing mechanism is investigated in this paper. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydrothermal; Nanoplates: Ionic conductivity; Humidity sensor
1. Instruction Tailoring the configuration and morphology of materials from the macroscale to nanoscale at all dimensions is the major challenging issue to material scientists. During past years, research on inorganic nanomaterials has attracted extensive attention because of their particular importance in investigating the sizes and shape-dependent properties as well as their novel properties and potential applications [1–3]. Increasing attention has been focused on some two-dimensional nanomaterials–nanoplates, not only for their basic scientific richness, but also for their potential applications in nanoscale electronic and optical devices. Nanoplates can be synthesized through several methods including template assisted hydrothermal method [4]. Among all these ways, hydrothermal method is the simplest one and has a strong point that the nanomaterials synthesized in this way are quite pure. MnWO4 nanoplates exhibit an interesting electrochemical property-ionic conductivity [5]. This kind of electrochemical ∗ Corresponding author at: The State Key Laboratory of Structural Chemistry, Fujian Institute of on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China. Tel.: +86 591 83705794; fax: +86 591 83714946. E-mail address: [email protected] (C. Lu).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.060
property is very unusual in simple inorganic compounds. The MnWO4 nanoplates with ionic conductivity exhibit sensitive a high sensitivity to humidity changes: when the quantity of water molecules that MnWO4 adsorbs changes, the conductance of MnWO4 also changes accordingly. Based on this character, MnWO4 is a very promising material for humidity sensing [6]. Just because of the unusual electrical and electrochemical property, the synthesis and research on nanoionic conductor MnWO4 nanoplates can offer great help to the studies of humidity sensing materials [7]. Recently, tungstate materials have attracted much interest because of their luminescence behaviors and potential application [8–11]. However, the study on their electric capability has rather been unexplored. In this report, we demonstrate first a facile method to synthesize MnWO4 nanoplates with ironic conductivity and investigate the mechanism of the ionic conductivity of MnWO4 nanoplates. Compared with micron MnWO4 , MnWO4 nanoplates is more sensitive to humidity changes. When humidity increases from 30% to 60%, the resistance of micron MnWO4 decreases from 2.1 × 107 to 1.0 × 106 , but the resistance of MnWO4 nanoplates decreases from 3.9 × 106 to 5.8 × 104 . This phenomenon indicates that MnWO4 nanoplates have much excellent capability compared with micron MnWO4 and the research to MnWO4 nanoplates is quite significative.
434
L. Zhang et al. / Materials Chemistry and Physics 103 (2007) 433–436 out in distilled water. In a typical experiment, 2 mmol MnSO4 ·H2 O and 2 mmol Na2 WO4 ·2H2 O were dissolved in 5 ml distilled water, respectively. The Na2 WO4 solution was added into the MnSO4 solution slowly under stirring, and result in a yellow precipitate. Then 3 ml DMF was dropped into the solution. After then vigorous stirring for 1 h was necessary to ensure that all the reagents were dispersed homogeneously in the solution at room temperature. The resultant was transferred into a teflon-lined reactor and heated at 160 ◦ C for 10 days. Then the precipitations were washed with distilled water for several times and dried at 100 ◦ C for 2 h. The X-ray diffraction (XRD) pattern of the product was recorded by step scan on a RIGAKU-DMAX2500 X-ray diffractometer with Cu K␣ radiation (λ = 0.1542 nm) at 40 kV and 100 mA. The scan range (2θ) was from 5◦ to 85◦ with the step of 0.05◦ and the resolution of 0.01◦ . The morphologies and microstructures of the as-synthesized sample were characterized with a JEOL2010 transmission electron microscope (TEM) equipped with energy dispersive spectroscopy (EDS) and operated at 200 kV. Electric capabilities were measured on a 4284A 20 HZ to 1 MHZ Precision LCR Meter.
Fig. 1. Typical XRD pattern of the as-prepared sample.
2. Experimental procedure MnSO4 ·H2 O and Na2 WO4 ·2H2 O of analytical grade purity were used as starting materials without further purification. All reactions were carried
3. Results and discussion 3.1. Morphological characterisation The XRD pattern of the as-prepared sample is shown in Fig. 1. All peaks can be indexed to a pure phase monoclinic
Fig. 2. TEM and HRTEM images of the as-synthesized sample and. (a) TEM micrograph of MnWO4 nanoplates, (b) HRTEM image and its FFT pattern taken from a portion of the nanoplate and (c) HRTEM image of MnWO4 nanoplate.
L. Zhang et al. / Materials Chemistry and Physics 103 (2007) 433–436
435
MnWO4 phase with space group P12/C1 (PDF #720478). The calculated cell parameters are: a = 0.484 nm, b = 0.576 nm and c = 0.499 nm, in good agreement with the literature values. The strong diffraction peaks in Fig. 1 indicate that well-crystallized MnWO4 has been obtained under the given synthetic conditions. The transmission electron microscopy (TEM) image shown in Fig. 2a reveals the general morphology of MnWO4 nanoplates. The nanorods seen in the image actually are sideways nanoplates and the nanoparticles are shadows produced by superposed nanoplates. It can be seen that most of the MnWO4 nanoplates are in the size with lengths of 60–100 nm, widths of 50–80 nm and thicknesses of 15–30 nm. The high-resolution transmission electron microscopy (HRTEM) image taken from a portion of a nanoplate is shown in Fig. 2b, which confirms that the MnWO4 nanoplate is in single crystal from with uniform lattice structure, free of defects and dislocations. The corresponding fast Fourier transform (FFT) pattern (inset of Fig. 2b) is indexed to the monoclinic MnWO4 phase along the 0 0 1 zone axis. Fig. 2c shows the HRTEM image of an MnWO4 nanoplate viewing through the cross-section and the space distance of the crystal lattices is 0.245 nm, which corresponds to (1 2 0) plane. 3.2. Spectral properties The impedance plots of the MnWO4 nanoplates are shown in Fig. 3. Using Fig. 3a as an example, the measurement resulted in a typical behavior of an ionic conductor with a semicircle at high frequencies (from 150 Hz to 300 KHz) and a liner spoke at low frequencies (from 20 Hz to 150 Hz). The sample resistance from this plot is 58 k, giving a conductivity of 1.46 × 10−5 S cm−1 . Then MnWO4 nanoplates were heated up at 100 ◦ C, and the resistances were recorded at different time and different humidity. The resistances and conductivities of MnWO4 nanoplates at different time and different humidity are shown in Table 1. At room temperature (humidity 60%), the MnWO4 had a very good electric conduction capability. When heating MnWO4 in the oven at 100 ◦ C, the resistance increased and the conductivity decreased relevantly. Following the prolonging of heating, the resistance became bigger and the conductivity accordingly became smaller. It is sure something that influences the electric capability of MnWO4 was vanishing tardily when heating the MnWO4 . Obviously, it is water that is on the MnWO4 surface or inside the nanoplates. The MnWO4 Table 1 Resistances and conductivities of MnWO4 at different time and different humidity Time
Humidity (%)
R ()
K (S cm−1 )
Room temperature 1 h later 2 h later 3 h later 4 h later 5 h later
60 30 19 15 13 13
5.82 × 104 3.91 × 106 1.08 × 107 1.76 × 107 2.45 × 107 2.45 × 107
1.46 × 10−5 2.18 × 10−7 7.86 × 10−8 4.83 × 10−8 3.47 × 10−8 3.47 × 10−8
Fig. 3. The impedance spectroscopy measurements of electrical resistance of MnWO4 (a) at room temperature (humidity 60%), (b) was heated at 100 ◦ C for 1 h (humidity 30%), (c) was heated at 100 ◦ C for 2 h (humidity 19%), (d) was heated at 100 ◦ C for 3 h (humidity 15%), (e) was heated at 100 ◦ C for 4 h (humidity 13%) and (f) was heated at 100 ◦ C for 5 h (humidity 13%).
nanoplates are quite sensitive to water vapor and can be used as humidity sensing material. The reason MnWO4 has humidity sensitive property is that it can make responses fast to the change of water molecules concentration. This commendably explains the phenomena that appeared in the electric capability experiment [12–14]. Based on prevenient researches [6], the sensing mechanism of humidity sensor MnWO4 contains two parts: the water molecules adsorption on the surface and the ionic conductivity of MnWO4 . The adsorption can be divided into two stages: chemical adsorption and physical adsorption. Because of the sizes of MnWO4 nanoplates, the surface area is very large and the MnWO4 molecules on the surface are quite active. Both physical adsorption and chemical adsorption take place easily when water molecules are adsorbed. At a low humidity, chemical adsorption plays the primary role. The adsorbed water molecules are decomposed into hydronium ions and hydroxyls on the surface of MnWO4 nanoplates. Because ionic conductor itself dose not have electric capability, the ionic electric capability of MnWO4 depends on those hydronium ions and hydroxyls [15–17,7]. While humidity changes, both the quantity
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L. Zhang et al. / Materials Chemistry and Physics 103 (2007) 433–436
of absorbed-water molecules and the concentrations of hydronium ions and hydroxyls change accordingly. The change of the concentrations of hydronium ions and hydroxyls can directly influence the resistance of MnWO4 . At higher humidity, subsequent layers of water molecules happen physical adsorption on the chemical absorbed layer. The Grottuss proton transfer occurs among the hydroniumn ions and results in an increase in protonic conductibility. Following the further increase of humidity, the multi-layers of physical-absorbed water molecules tend to concentrate in capillary pores. The radius of the capillary pores are defined as rk = 2γM/ρRT ln (Ps /P), there in M is the molecular mass of water, γ the surface tension of water, ρ the density of water, Ps the vapour pressure at saturation and P is the vapour pressure. The ionic conduction in condensed water also occurs in the physical absorbed layers [6]. And the quantity of physical absorbed water influences alike on the resistance of MnWO4 . By the mechanism, the change of humidity is represented by the change of the resistance of MnWO4 . Just about the ionic electric capability makes MnWO4 nanoplates a kind of humidity sensing material. 4. Conclusion In summary, MnWO4 nanoplates with ionic conductivity have been synthesized under hydrothermal condition, of which the sizes were controlled within 100 nm. From the electric capability experiment, it is assured that MnWO4 nanoplates are of ionic conductor properties. The ionic conductivity theory successfully explains the mechanism of humidity sensor MnWO4 . Based on its ionic conductivity, MnWO4 is quite sensitive to water molecules, and is a promising humidity sensing material.
Acknowledgements This work was supported by grants from the National Natural Science Foundation of China (20521101, 20425313, 90206040, 20333070 and 20303021) and the Natural Science Foundation of Fujian Province (2005HZ01-1). Thanks to the helps of the crew of Xintao Wu group in Fujian Institute of Research on the Structure of Matter CAS. References [1] H. Yang, N. Coombs, G.A. Ozin, Nature 386 (1997) 692. [2] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176. [3] G.G. Zhou, M.K. Lv, F. Gu, D. Xu, D.R. Yuan, J. Cryst. Growth 276 (2005) 577–582. [4] Y.H. Zheng, Y. Cheng, Y.S. Wang, J. Cryst. Growth 280 (2005) 569–574. [5] M.A.K.L. Dissanayake, P.A.R.D. Jayathilaka, R.S.P. Bokalawela, Electrochim. Acta (2005). [6] W.M. Qu, W. Wlodarksi, Sens. Actuators B 64 (2000) 76–82. [7] J. Maier, Solid State Ionics 175 (2004) 7–12. [8] S.H. Yu, M. Antonietti, H. C¨olfen, M. Giersig, Angew Chem. Int. Ed. 41 (2002) 2356. [9] N. Saito, N. Sonoyama, T. Sakata, Bull. Chem. Soc. Jpn. 69 (1996) 2191. [10] S.J. Chen, J.H. Zhou, X.T. Chen, J. Li, L.H. Li, J.M. Hong, Z.L. Xue, X.Z. You, Chem. Phys. Lett. 375 (2003) 185. [11] X.L. Hu, Y.J. Zhou, Langmuir 20 (2004) 1521. [12] P.G. Su, C.L. Uen, Talanta 66 (2005) 1247–1253. [13] R. Nohria, R.K. Khillan, Y. Su, R. Dikshit, Y. Lvov, K. Varahramyan, Sens. Actuators. [14] Y.Y. Xu, X.J. Li, J.T. He, X. Hu, H.Y. Wang, Sens. Actuators B 105 (2005) 219–222. [15] P.G. Su, W.Y. Tsai, Sens. Actuators B 100 (2004) 417–422. [16] S.K. Shukla, G.K. Parashar, A.P. Mishra, P Misra, B.C. Yadav, R.K. Shuklad, L.M. Balid, G.C. Dubey, Sens. Actuators B 98 (2004) 5–11. [17] J. Wang, Q.H. Lin, R.Q. Zhou, B.K. Xu, Sens. Actuators B 81 (2002) 248–253.