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Microwave-assisted synthesis and humidity sensing of nanostructured α-Fe2O3
Materials Research Bulletin 44 (2009) 1179–1182
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Microwave-assisted synthesis and humidity sensing of nanostructured a-Fe2O3 Rupali G. Deshmukh, Satish S. Badadhe, Imtiaz S. Mulla * Physical and Materials Chemistry Division, National Chemical Laboratory, Homi Bhabha Road, Pashan, Pune 411008, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 June 2008 Received in revised form 23 August 2008 Accepted 28 September 2008 Available online 14 October 2008
Nanocrystalline a-Fe2O3 has been prepared on a large-scale by a facile microwave-assisted hydrothermal route from a solution of Fe(NO3)39H2O and pentaerythritol. A systematic study of the morphology, crystallinity and oxidation state of Fe using different characterization techniques, such as transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was performed. It reveals that nanostructured a-Fe2O3 comprises bundles of nanorods with a rhombohedral crystalline structure. The individual nanorod has 8–10 nm diameter and 50 nm length. The as-prepared nanostructured aFe2O3 (sensor) gives selective response towards humidity. The sensor shows high sensitivity, fast linear response to change in the humidity with almost 100% reproducibility. The sensor works at room temperature and rejuvenates without heat treatment. The as-prepared nanostructured a-Fe2O3 appears to be a promising humidity sensing material with the potential for commercialization. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis C. X-ray diffraction D. Electrical properties
1. Introduction
[6]. Hu and Yu [7] had demonstrated large-scale rapid synthesis of
a-Fe2O3 with controlled shapes by using microwave-assisted Recently, a great deal of work has been done on the synthesis of nanostructured materials with different size, morphology and porosity; owing to their fundamental scientific interest and technological applications [1]. One-dimensional nanostructured materials with controlled size are of great importance because of their size-dependent electrical, optical and magnetic properties [2]. Different approaches have been reported for the synthesis of one-dimensional nanostructures like vapor–liquid–solid technique (VLS) [3], vapor phase synthesis (VS) [4], and wet chemical methods like hydrothermal and solvothermal [5]. However, microwave-assisted synthesis offers great advantages over other conventional techniques, such as, prompting the rapid reaction due to the molecular level interaction of the microwave radiations with the reactant species. It is a clean, cheap, and convenient method of heating that often results in higher yields and shorter reaction time. It has acquired great attention as a promising method for the preparation of various nanostructured materials with different size and shape tailored by programming the time, energy and reactants. Moreover, it is environment-friendly, simple to operate, and energy efficient over the conventional heating processes. Hu et al. reported large-scale facile synthesis of dentritic a-Fe2O3 nanoarchitectures. Unlike conventional hydrothermal synthesis the microwave-enhanced hydrothermal process forms ‘hot spots’ by local superheating; thus creating massive product
hydrothermal route. They have observed microwave hydrothermal as an efficient and convenient system to get the tunable nanostructured a-Fe2O3 at the low cost. Zhang and Li [8] have presented a route of synthesizing nanorods of a-Fe2O3 by using microwave hydrothermal process. They have proposed growth mechanism of nanorod formation and reported magnetic property of 1D hematite. Morphology of a-Fe2O3 plays an important role in achieving desired properties, the one-dimensional oxides have been immensely studied due to their potential applications; they have also been reported exhibiting enhanced sensitivity towards humidity [9]. In view of these advantages, we have prepared aFe2O3 nanostructures by microwave hydrothermal process and studied their humidity sensing characteristics. Hematite (a-Fe2O3), an n-type semiconductor with a band gap of 2.1 eV possesses prospective applications in catalysis [10], photoelectrodes [11], pigments [12], and sensors [13]. The work described in this communication indicates an economical, simple and rapid route to prepare a-Fe2O3 nanorods of 50 nm length and 8 nm diameter in a microwave-assisted hydrothermal system. The as-synthesized a-Fe2O3 nanostructures exhibit promising humidity sensing properties. 2. Experimental 2.1. Synthesis
* Corresponding author. Tel.: +91 20 25902276; fax: +91 20 25902636. E-mail address: [email protected] (I.S. Mulla). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.09.044
In a typical procedure, 1.01 g of Fe(NO3)39H2O was dissolved in 50 ml of aqueous pentaerythritol (0.34 g); molar ratio 1:1,
R.G. Deshmukh et al. / Materials Research Bulletin 44 (2009) 1179–1182
1180
under magnetic stirring at room temperature until a clear solution was obtained. The solution was sealed in a Teflonlined double-walled digestion vessel. After treating at a controlled temperature of 130 8C for 15 min at 150 W using a microwave digestion system (Anton Paar 3000 SOLV), the vessel was naturally cooled down to room temperature. The product was collected after centrifugation for 10 min. Finally, it was washed with deionised water for several times and dried at 50 8C. The product thus formed was used for further characterization. 2.2. Characterization 2.2.1. Structural and morphological characterization X-ray photoelectron spectroscopy (XPS) PS measurements were carried out on a VG Micro Tech ESCA 3000 instrument at a pressure of >1 109 Torr (pass energy of 50 eV, electron take off angle 608, and overall resolution 1 eV). X-ray diffraction (XRD) data of powder sample was collected using an ‘X’ Pert Pro diffractometer, operating at a voltage of 20 kV (with Cu Ka radiation, l = 1.5401 A˚) in the range (2u) from 208 to 708. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and electron diffraction pattern were obtained on a Tecnai F30 FEG machine operated at 300 kV. Samples were prepared by placing a drop of aqueous suspension on the TEM copper grid followed by evaporation of the solvent. Brunauer– Emmett–Teller (BET) surface area was determined by nitrogen adsorption and desorption using a Quantachrome Autosorb Automated Gas Sorption System. 2.2.2. Sensing characterization The a-Fe2O3 powder thus prepared was drop-casted onto the Ag-patterned alumina substrate using isopropyl alcohol. The film was dried at room temperature and tested for humidity sensing activities. Relative humidity (% RH) of 92%, 84%, 75%, 62%, 51%, 33% and 11% were obtained by making 50 ml supersaturated solutions of KNO3, KCl, NaCl, NH4NO3, Ca(NO3)24H2O, MgCl26H2O and LiCl, respectively. Such supersaturated solutions were made in different air-tight chambers at room temperature and were allowed to attain the stable humidity by keeping for 24 h. Samples under investigation were switched rapidly between the chambers of different humidity to find the change in the current with respect to time and humidity, using a Keithley 485 Autoranging Picoammeter, Clevaenad, OH, U.S.A. and, an Aplab 7212 Regulated Power Supply, India.
The sensitivity (S) for different % RH was calculated as follows: I% RH S¼ ; I11% RH where I% RH and I11% RH are the conductivities of the sensor in different % RH and in 11% RH. 3. Results and discussion XPS spectrum revealed that the nanostructures are composed of Fe and O species. In the high-resolution Fe 2p spectrum (Fig. 1(A)), two peaks at the binding energies of 711.3 eV for Fe 2p3/2 and 724.6 eV for Fe 2p1/2 with a shake-up satellite at 719.5 eV were observed which is a characteristic of Fe(III) in Fe2O3 [14]. The powder X-ray diffractogram (Fig. 1(B)) of as-prepared iron oxide reveals single-phasic crystalline nature. All the diffraction peaks match well with the standard rhombohedral crystalline phase (JCPDS file no. 89-0597). The average crystallite size calculated from prominent peaks was found to be 30 nm. TEM was employed for further investigation of structural characteristics. The low magnification TEM image (Fig. 2(A)) of the as-synthesized a-Fe2O3 shows bundles of nanorods. To find the better insight of the morphology, the TEM image (Fig. 2(B)) of as-synthesized a-Fe2O3 was scanned at the higher magnification. It clearly shows 50 nm nanorods in the form of their bundles with uniform size and shape. The image indicates several bundles; each made-up of parallel nanorods of a-Fe2O3. The HRTEM image (Fig. 2(C)) depicts two parallel a-Fe2O3 nanorods with distinct lattice fringes of d-spacing 2.8 A˚, corresponding to the (1 1 0) planes of the rhombohedral hematite. The selected area electron diffraction (SAED) pattern (inset Fig. 2(C)) of a-Fe2O3 nanorod also reveals crystalline nature. The formation of rod-like nanostructures and their typical growth pattern into bundles might have occurred due to the pentaerythritol. It is worth noting that there is no reaction in the absence of pentaerythritol. The nitrogen adsorption and desorption analysis reveals that the Brunauer–Emmett–Teller surface area of the assynthesized a-Fe2O3 nanobundles is 69.89 m2/g (Fig. 3) with average pore diameter 3.5 nm. Such a high BET surface area is advantageous for chemical sensors. 3.1. Humidity sensing characteristics The humidity sensing characteristic of the sensor was carried out using a dynamic testing system. The sensitivity towards humidity from 11% to 92% RH is shown in Fig. 4(A). It indicates that the sensitivity (I% RH/I11% RH) towards humidity gradually increases
Fig. 1. (A) High-resolution XPS Fe 2p spectrum of as-synthesized a-Fe2O3. (B) XRD pattern of the as-synthesized a-Fe2O3.
R.G. Deshmukh et al. / Materials Research Bulletin 44 (2009) 1179–1182
1181
Fig. 2. (A) Low-magnification TEM image of the as-synthesized a-Fe2O3. (B) High-magnification TEM image of the as-synthesized a-Fe2O3. (C) HRTEM image of a-Fe2O3 with inset showing its corresponding electron diffraction pattern.
Fig. 3. Nitrogen adsorption–desorption isotherms of the as-synthesized a-Fe2O3.
up to 62% RH and then onward rapidly increases till 92% RH, signifying a strong influence of humidity on the conductivity of the sensor. The current, as recorded for the sensor, on exposure in the 11% RH chamber is 0.82 nA, which is 350 times lower than that obtained in the 92% RH (0.28 mA); for the fixed applied voltage (15 V). With increase in the humidity, the number of physisorbed water molecules get enhanced, which in turn increases flow of current in the material. Fig. 4(B) shows the change of current (log (A)) with respect to time by switching the sensor from dry air (11% RH) to the respective high % RH chambers at room temperature. It is seen that on exposure of the sensor to 92% RH, the current increases rapidly (<60 s) to about 95% of its final value, and then onward gradually attains a stable value. After attaining the stable current and retaining it for 2 min the sensor was re-introduced in the 11% RH chamber. The overall current change measuring nearly three orders of magnitude is observed on the exposure from 11% to 92% RH; moreover, it regains its original value within 3 min. This procedure was repeated for the other % RH. It is found that the sensitivity decreases with lowering of relative humidity. This indicates that the increase in current with
1182
R.G. Deshmukh et al. / Materials Research Bulletin 44 (2009) 1179–1182
Fig. 4. (A) Sensitivity versus % RH for a-Fe2O3. (B) Change in current with respect to time on switching the sensor from dry air to the respective % RH.
exposure to other gases like H2, LPG, H2S and ethanol thus exhibiting high humidity response without any interference from gas is an added advantage for the commercial application of this material. 4. Conclusions
Fig. 5. Current variations in the a-Fe2O3 as a function of time on switching from 11% to 92% RH at room temperature.
The simple microwave-assisted hydrothermal method was developed for the large-scale rapid synthesis of crystalline rhombohedral nanostructed a-Fe2O3. Pentaerythritol plays an important role in the formation of the typical morphology, which offers an advantage in the fast sensing of water molecules. The dynamic humidity testing confirms that a-Fe2O3 nanorods show high sensitivity, fast response, and nearly 100% recovery within 2– 3 min. The room temperature rejuvenation and the inertness towards any other test gas, indicates its potential as an ideal humidity sensor. Acknowledgements
increase in % RH is related to the number of the water molecules adsorbed on the sample. In view of this observation, it can be assumed that the displacement of the pre-adsorbed oxygen takes place by the adsorbed water molecules on the surface of a-Fe2O3. The unusual high humidity response with the excellent recovery in the presently reported sensor can be attributed to its typical morphology. This morphology imparts high surface area, which may be facilitating ease in the physisorption phenomenon. The stable OH group layers formed on the surface, upon physisorption of water, prevents drifts in resistance during exposure to wet environments [15]. In order to check the stability of the sensor, current variations as a function of time, at room temperature was measured (Fig. 5). The measurements were carried out by switching it from 11% to 92% RH and keeping it at 92% RH for 1 h. It was found that the current increases rapidly from 0.82 nA to 0.28 mA within 3 min and then remains stable. To check the reproducibility of the sensor, five samples were made and tested for the humidity sensing characteristics and it was found that all the samples show similar response. The samples did not show any change in the current on
We acknowledge Mr. R.K. Jha for BET analysis and Dr. K.R. Patil for the XPS characterization. The work is supported by the Department of Science Technology, New Delhi, India. References [1] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025. [2] D.W. Kim, I.S. Hwang, S.J. Kwon, H.Y. Kang, K.S. Park, Y.J. Choi, K.J. Choi, J.G. Park, Nano Lett. 7 (2007) 3041. [3] G. Shen, Y. Bando, C. Tang, D. Golberg, J. Phys. Chem. B 110 (2006) 7199. [4] Y. Huang, K. Yu, Z. Zhu, Curr. Appl. Phys. 7 (2007) 702. [5] F. Zhou, X. Zhao, C. Yuan, L. Li, Cryst. Growth Des. 8 (2008) 723. [6] X. Hu, J.C. Yu, J. Gong, J. Phys. Chem. C 111 (2007) 11180. [7] X. Hu, Jimmy C. Yu, Adv. Funct. Mater. 18 (2008) 880. [8] X. Zhang, Q. Li, Mater. Lett. 62 (2008) 988. [9] G. Wang, Q. Wang, W. Lu, J. Li, J. Phys. Chem. B 110 (2006) 22029. [10] A. Kay, I. Cesar, M. Gra¨tzel, J. Am. Chem. Soc. 128 (2006) 15714. [11] J.A. Glasscock, P.R. Barnes, F.I.C. Plumb, N. Savvides, J. Phys. Chem. C 111 (2007) 6477. [12] C. Lin, Y. Li, M. Yu, P. Yang, J. Lin, Adv. Funct. Mater. 17 (2007) 1459. [13] C. Wu, P. Yin, X. Zhu, C.O. Yang, Y. Xie, Phys. Chem. B 110 (2006) 17806. [14] G.J. Scilla, G.H. Morrison, Anal. Chem. 49 (1977) 1521. [15] Y. Qiu, S. Yang, Adv. Funct. Mater. 17 (2007) 1345.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Microwave-assisted synthesis and humidity sensing of nanostructured a-Fe2O3 Rupali G. Deshmukh, Satish S. Badadhe, Imtiaz S. Mulla * Physical and Materials Chemistry Division, National Chemical Laboratory, Homi Bhabha Road, Pashan, Pune 411008, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 June 2008 Received in revised form 23 August 2008 Accepted 28 September 2008 Available online 14 October 2008
Nanocrystalline a-Fe2O3 has been prepared on a large-scale by a facile microwave-assisted hydrothermal route from a solution of Fe(NO3)39H2O and pentaerythritol. A systematic study of the morphology, crystallinity and oxidation state of Fe using different characterization techniques, such as transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy was performed. It reveals that nanostructured a-Fe2O3 comprises bundles of nanorods with a rhombohedral crystalline structure. The individual nanorod has 8–10 nm diameter and 50 nm length. The as-prepared nanostructured aFe2O3 (sensor) gives selective response towards humidity. The sensor shows high sensitivity, fast linear response to change in the humidity with almost 100% reproducibility. The sensor works at room temperature and rejuvenates without heat treatment. The as-prepared nanostructured a-Fe2O3 appears to be a promising humidity sensing material with the potential for commercialization. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis C. X-ray diffraction D. Electrical properties
1. Introduction
[6]. Hu and Yu [7] had demonstrated large-scale rapid synthesis of
a-Fe2O3 with controlled shapes by using microwave-assisted Recently, a great deal of work has been done on the synthesis of nanostructured materials with different size, morphology and porosity; owing to their fundamental scientific interest and technological applications [1]. One-dimensional nanostructured materials with controlled size are of great importance because of their size-dependent electrical, optical and magnetic properties [2]. Different approaches have been reported for the synthesis of one-dimensional nanostructures like vapor–liquid–solid technique (VLS) [3], vapor phase synthesis (VS) [4], and wet chemical methods like hydrothermal and solvothermal [5]. However, microwave-assisted synthesis offers great advantages over other conventional techniques, such as, prompting the rapid reaction due to the molecular level interaction of the microwave radiations with the reactant species. It is a clean, cheap, and convenient method of heating that often results in higher yields and shorter reaction time. It has acquired great attention as a promising method for the preparation of various nanostructured materials with different size and shape tailored by programming the time, energy and reactants. Moreover, it is environment-friendly, simple to operate, and energy efficient over the conventional heating processes. Hu et al. reported large-scale facile synthesis of dentritic a-Fe2O3 nanoarchitectures. Unlike conventional hydrothermal synthesis the microwave-enhanced hydrothermal process forms ‘hot spots’ by local superheating; thus creating massive product
hydrothermal route. They have observed microwave hydrothermal as an efficient and convenient system to get the tunable nanostructured a-Fe2O3 at the low cost. Zhang and Li [8] have presented a route of synthesizing nanorods of a-Fe2O3 by using microwave hydrothermal process. They have proposed growth mechanism of nanorod formation and reported magnetic property of 1D hematite. Morphology of a-Fe2O3 plays an important role in achieving desired properties, the one-dimensional oxides have been immensely studied due to their potential applications; they have also been reported exhibiting enhanced sensitivity towards humidity [9]. In view of these advantages, we have prepared aFe2O3 nanostructures by microwave hydrothermal process and studied their humidity sensing characteristics. Hematite (a-Fe2O3), an n-type semiconductor with a band gap of 2.1 eV possesses prospective applications in catalysis [10], photoelectrodes [11], pigments [12], and sensors [13]. The work described in this communication indicates an economical, simple and rapid route to prepare a-Fe2O3 nanorods of 50 nm length and 8 nm diameter in a microwave-assisted hydrothermal system. The as-synthesized a-Fe2O3 nanostructures exhibit promising humidity sensing properties. 2. Experimental 2.1. Synthesis
* Corresponding author. Tel.: +91 20 25902276; fax: +91 20 25902636. E-mail address: [email protected] (I.S. Mulla). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.09.044
In a typical procedure, 1.01 g of Fe(NO3)39H2O was dissolved in 50 ml of aqueous pentaerythritol (0.34 g); molar ratio 1:1,
R.G. Deshmukh et al. / Materials Research Bulletin 44 (2009) 1179–1182
1180
under magnetic stirring at room temperature until a clear solution was obtained. The solution was sealed in a Teflonlined double-walled digestion vessel. After treating at a controlled temperature of 130 8C for 15 min at 150 W using a microwave digestion system (Anton Paar 3000 SOLV), the vessel was naturally cooled down to room temperature. The product was collected after centrifugation for 10 min. Finally, it was washed with deionised water for several times and dried at 50 8C. The product thus formed was used for further characterization. 2.2. Characterization 2.2.1. Structural and morphological characterization X-ray photoelectron spectroscopy (XPS) PS measurements were carried out on a VG Micro Tech ESCA 3000 instrument at a pressure of >1 109 Torr (pass energy of 50 eV, electron take off angle 608, and overall resolution 1 eV). X-ray diffraction (XRD) data of powder sample was collected using an ‘X’ Pert Pro diffractometer, operating at a voltage of 20 kV (with Cu Ka radiation, l = 1.5401 A˚) in the range (2u) from 208 to 708. The transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and electron diffraction pattern were obtained on a Tecnai F30 FEG machine operated at 300 kV. Samples were prepared by placing a drop of aqueous suspension on the TEM copper grid followed by evaporation of the solvent. Brunauer– Emmett–Teller (BET) surface area was determined by nitrogen adsorption and desorption using a Quantachrome Autosorb Automated Gas Sorption System. 2.2.2. Sensing characterization The a-Fe2O3 powder thus prepared was drop-casted onto the Ag-patterned alumina substrate using isopropyl alcohol. The film was dried at room temperature and tested for humidity sensing activities. Relative humidity (% RH) of 92%, 84%, 75%, 62%, 51%, 33% and 11% were obtained by making 50 ml supersaturated solutions of KNO3, KCl, NaCl, NH4NO3, Ca(NO3)24H2O, MgCl26H2O and LiCl, respectively. Such supersaturated solutions were made in different air-tight chambers at room temperature and were allowed to attain the stable humidity by keeping for 24 h. Samples under investigation were switched rapidly between the chambers of different humidity to find the change in the current with respect to time and humidity, using a Keithley 485 Autoranging Picoammeter, Clevaenad, OH, U.S.A. and, an Aplab 7212 Regulated Power Supply, India.
The sensitivity (S) for different % RH was calculated as follows: I% RH S¼ ; I11% RH where I% RH and I11% RH are the conductivities of the sensor in different % RH and in 11% RH. 3. Results and discussion XPS spectrum revealed that the nanostructures are composed of Fe and O species. In the high-resolution Fe 2p spectrum (Fig. 1(A)), two peaks at the binding energies of 711.3 eV for Fe 2p3/2 and 724.6 eV for Fe 2p1/2 with a shake-up satellite at 719.5 eV were observed which is a characteristic of Fe(III) in Fe2O3 [14]. The powder X-ray diffractogram (Fig. 1(B)) of as-prepared iron oxide reveals single-phasic crystalline nature. All the diffraction peaks match well with the standard rhombohedral crystalline phase (JCPDS file no. 89-0597). The average crystallite size calculated from prominent peaks was found to be 30 nm. TEM was employed for further investigation of structural characteristics. The low magnification TEM image (Fig. 2(A)) of the as-synthesized a-Fe2O3 shows bundles of nanorods. To find the better insight of the morphology, the TEM image (Fig. 2(B)) of as-synthesized a-Fe2O3 was scanned at the higher magnification. It clearly shows 50 nm nanorods in the form of their bundles with uniform size and shape. The image indicates several bundles; each made-up of parallel nanorods of a-Fe2O3. The HRTEM image (Fig. 2(C)) depicts two parallel a-Fe2O3 nanorods with distinct lattice fringes of d-spacing 2.8 A˚, corresponding to the (1 1 0) planes of the rhombohedral hematite. The selected area electron diffraction (SAED) pattern (inset Fig. 2(C)) of a-Fe2O3 nanorod also reveals crystalline nature. The formation of rod-like nanostructures and their typical growth pattern into bundles might have occurred due to the pentaerythritol. It is worth noting that there is no reaction in the absence of pentaerythritol. The nitrogen adsorption and desorption analysis reveals that the Brunauer–Emmett–Teller surface area of the assynthesized a-Fe2O3 nanobundles is 69.89 m2/g (Fig. 3) with average pore diameter 3.5 nm. Such a high BET surface area is advantageous for chemical sensors. 3.1. Humidity sensing characteristics The humidity sensing characteristic of the sensor was carried out using a dynamic testing system. The sensitivity towards humidity from 11% to 92% RH is shown in Fig. 4(A). It indicates that the sensitivity (I% RH/I11% RH) towards humidity gradually increases
Fig. 1. (A) High-resolution XPS Fe 2p spectrum of as-synthesized a-Fe2O3. (B) XRD pattern of the as-synthesized a-Fe2O3.
R.G. Deshmukh et al. / Materials Research Bulletin 44 (2009) 1179–1182
1181
Fig. 2. (A) Low-magnification TEM image of the as-synthesized a-Fe2O3. (B) High-magnification TEM image of the as-synthesized a-Fe2O3. (C) HRTEM image of a-Fe2O3 with inset showing its corresponding electron diffraction pattern.
Fig. 3. Nitrogen adsorption–desorption isotherms of the as-synthesized a-Fe2O3.
up to 62% RH and then onward rapidly increases till 92% RH, signifying a strong influence of humidity on the conductivity of the sensor. The current, as recorded for the sensor, on exposure in the 11% RH chamber is 0.82 nA, which is 350 times lower than that obtained in the 92% RH (0.28 mA); for the fixed applied voltage (15 V). With increase in the humidity, the number of physisorbed water molecules get enhanced, which in turn increases flow of current in the material. Fig. 4(B) shows the change of current (log (A)) with respect to time by switching the sensor from dry air (11% RH) to the respective high % RH chambers at room temperature. It is seen that on exposure of the sensor to 92% RH, the current increases rapidly (<60 s) to about 95% of its final value, and then onward gradually attains a stable value. After attaining the stable current and retaining it for 2 min the sensor was re-introduced in the 11% RH chamber. The overall current change measuring nearly three orders of magnitude is observed on the exposure from 11% to 92% RH; moreover, it regains its original value within 3 min. This procedure was repeated for the other % RH. It is found that the sensitivity decreases with lowering of relative humidity. This indicates that the increase in current with
1182
R.G. Deshmukh et al. / Materials Research Bulletin 44 (2009) 1179–1182
Fig. 4. (A) Sensitivity versus % RH for a-Fe2O3. (B) Change in current with respect to time on switching the sensor from dry air to the respective % RH.
exposure to other gases like H2, LPG, H2S and ethanol thus exhibiting high humidity response without any interference from gas is an added advantage for the commercial application of this material. 4. Conclusions
Fig. 5. Current variations in the a-Fe2O3 as a function of time on switching from 11% to 92% RH at room temperature.
The simple microwave-assisted hydrothermal method was developed for the large-scale rapid synthesis of crystalline rhombohedral nanostructed a-Fe2O3. Pentaerythritol plays an important role in the formation of the typical morphology, which offers an advantage in the fast sensing of water molecules. The dynamic humidity testing confirms that a-Fe2O3 nanorods show high sensitivity, fast response, and nearly 100% recovery within 2– 3 min. The room temperature rejuvenation and the inertness towards any other test gas, indicates its potential as an ideal humidity sensor. Acknowledgements
increase in % RH is related to the number of the water molecules adsorbed on the sample. In view of this observation, it can be assumed that the displacement of the pre-adsorbed oxygen takes place by the adsorbed water molecules on the surface of a-Fe2O3. The unusual high humidity response with the excellent recovery in the presently reported sensor can be attributed to its typical morphology. This morphology imparts high surface area, which may be facilitating ease in the physisorption phenomenon. The stable OH group layers formed on the surface, upon physisorption of water, prevents drifts in resistance during exposure to wet environments [15]. In order to check the stability of the sensor, current variations as a function of time, at room temperature was measured (Fig. 5). The measurements were carried out by switching it from 11% to 92% RH and keeping it at 92% RH for 1 h. It was found that the current increases rapidly from 0.82 nA to 0.28 mA within 3 min and then remains stable. To check the reproducibility of the sensor, five samples were made and tested for the humidity sensing characteristics and it was found that all the samples show similar response. The samples did not show any change in the current on
We acknowledge Mr. R.K. Jha for BET analysis and Dr. K.R. Patil for the XPS characterization. The work is supported by the Department of Science Technology, New Delhi, India. References [1] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025. [2] D.W. Kim, I.S. Hwang, S.J. Kwon, H.Y. Kang, K.S. Park, Y.J. Choi, K.J. Choi, J.G. Park, Nano Lett. 7 (2007) 3041. [3] G. Shen, Y. Bando, C. Tang, D. Golberg, J. Phys. Chem. B 110 (2006) 7199. [4] Y. Huang, K. Yu, Z. Zhu, Curr. Appl. Phys. 7 (2007) 702. [5] F. Zhou, X. Zhao, C. Yuan, L. Li, Cryst. Growth Des. 8 (2008) 723. [6] X. Hu, J.C. Yu, J. Gong, J. Phys. Chem. C 111 (2007) 11180. [7] X. Hu, Jimmy C. Yu, Adv. Funct. Mater. 18 (2008) 880. [8] X. Zhang, Q. Li, Mater. Lett. 62 (2008) 988. [9] G. Wang, Q. Wang, W. Lu, J. Li, J. Phys. Chem. B 110 (2006) 22029. [10] A. Kay, I. Cesar, M. Gra¨tzel, J. Am. Chem. Soc. 128 (2006) 15714. [11] J.A. Glasscock, P.R. Barnes, F.I.C. Plumb, N. Savvides, J. Phys. Chem. C 111 (2007) 6477. [12] C. Lin, Y. Li, M. Yu, P. Yang, J. Lin, Adv. Funct. Mater. 17 (2007) 1459. [13] C. Wu, P. Yin, X. Zhu, C.O. Yang, Y. Xie, Phys. Chem. B 110 (2006) 17806. [14] G.J. Scilla, G.H. Morrison, Anal. Chem. 49 (1977) 1521. [15] Y. Qiu, S. Yang, Adv. Funct. Mater. 17 (2007) 1345.