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Efficient gas sensitivity in mixed bismuth ferrite micro (cubes) and nano (plates) structures
Materials Research Bulletin 47 (2012) 4169–4173
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Efficient gas sensitivity in mixed bismuth ferrite micro (cubes) and nano (plates) structures Shivaji D. Waghmare a, Vijaykumar V. Jadhav a, Shaym K. Gore a, Seog-Joon Yoon b, Swapnil B. Ambade b, B.J. Lokhande c, Rajaram S. Mane a,*, Sung-Hwan Han b,* a b c
Center for Nanomaterials and Energy Devices, School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, Maharashtra, India Inorganic Nanomaterials Laboratory, Department of Chemistry, Hanyang University, Seoul 133-1791, Republic of Korea Department of Physics, Solapur University, Solapur, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 May 2012 Accepted 30 August 2012 Available online 18 September 2012
Mixed micro (cubes) and nano (plates) structures of bismuth ferrite (BFO) have been synthesized by a simple and cost-effective wet-chemical method. Structural, morphological and phase confirmation characteristics are measured using X-ray diffraction, field-emission scanning electron microscopy (FESEM) and energy dispersive X-ray analysis techniques. The digital FE-SEM photo-images of BFO sample confirmed an incubation of discrete micro-cubes into thin and regularly placed large number of nanoplates. The bismuth ferrite, with mixed structures, films show considerable performance when used in liquefied petroleum (LPG), carbon dioxide (CO2) and ammonium (NH3) gas sensors application. Different chemical entities in LPG have made it more efficient with higher sensitivity, recovery and response times compared to CO2 and NH3 gases. Furthermore, effect of palladium surface treatment on the gas sensitivity and the charge transfer resistances of BFO mixed structures is investigated and reported. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Chemical synthesis C. Surface properties C. Impedance spectroscopy
1. Introduction As there is an increasing demand for various gases in industries and the houses, it has become a major task to develop low temperature gas sensors. Based on the change of the resistance of a semiconductor in the sensors, the sensors can detect various gases in air due to the presence of a reducing gas. Semiconductor gas sensors, with advent of simplicity, high sensitivity and fast response, offer the potential for developing portable and inexpensive gas-detection instrumentation. Semiconductor gas sensors including SnO2, ZnO, Fe2O3, etc., have been fabricated by various means and well-studied in the literature to detect most of the reducing gases [1–12]. Nanostructures of various metal oxides and also chalcogenides have been considered to increase the specific surface area exposed to gas [13–15]. However, their commercialization is seriously limited on account of poor selectivity and low thermodynamic stability. Therefore, it is necessary to find alternative metal oxides, synthesized using wet chemical route, with competitive or greater gas performances than existing known metal oxides. Researchers are extensively involved in knowing and understanding numerous chemical and physical methods for
* Corresponding authors. Tel.: +91 9850331971; fax: +91 2462229517. E-mail addresses: [email protected] (R.S. Mane), [email protected] (S.-H. Han). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.08.078
synthesizing novel metal oxides such as Nb2O5, WO3, and SnO2, followed by the possible sensor mechanism. Recently, the ferrites, class of mixed metal oxides, have been demonstrated to be appropriate materials for semiconductor gas sensors [16–21]. Ferrites, prepared in the form of ceramic materials with very high particle density, usually preferred in catalytic [22,23], magnetic or electrical applications [24]. On the contrary, for gas sensor devices lower density and higher surface area are required. And there is real scarcity of efficient low temperature gas sensors of ferrite nanostructures in literature as on today [25]. With this intention, several methods including co-precipitation, micro-emulsion, citrate or hydrothermal, etc., [26–29] for the preparation of ferrite nanostructures are documented. Spray pyrolysis [30], sputtering [31,32] methods can also be used to synthesize ferrite planar devices. Logically, a semiconductor gas sensor presents the property of changing the conductivity of the sensing material when it is exposed to different gases which, in general, depends on the operating temperature range [33–40]. In this article, bismuth ferrite (Bi2Fe4O9, BFO) thick film composed of dual structures, i.e. micro (cubes) and nano (plates), are synthesized and further envisaged in a gas sensor application in various gas atmospheres including liquefied petroleum gas (LPG), carbon dioxide (CO2) and ammonia (NH3), etc. Structural elucidation and morphological evolution confirmation were investigated in the first step and the gas sensing performance measurements were operated in the second step. Effect of palladium surface
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3. Results and discussion
digital FE-SEM images show the presence of BFO dual-structures, i.e. micro (cubes) and nano (plates). The lower and higher resolution FE-SEM images, as seen in Fig. 2(a)–(d), were well distinguishable. Under close inspection discrete micro (cubes) structure was well-embedded into the uniformly distributed vertical nano (plates) indicating that both structures might have grown simultaneously. In a wide FE-SEM image, micron-sized regular cubes were inserted into spongy-type nanoplates. These cubes were limited in numbers compared to plates. We presumed that this dual-structure (cubes and plates) of the high surface area would be advantageous in gas sensors application. Micro (cube) structure, individual, was almost same in dimensions, i.e. length, height and width. Not a single cube was dangling or loosely connected to plates. On the contrary, nano (plates) structure was grown in one direction whereas micro (cubes) in 3D. All cubes were fixed well within the plates so that their inter-movement from one plate to another was completely restricted. Dimension of each micro-cube was different from one another. To confirm it, a close scan of single micro (cube) structure was carried out (Fig. 2(d)). All faces were smooth and polished however; at edges few scratches were seen. Interestingly, scratched region presented a large number of fine nanorods, well-connected to each other, within the limit of scan. Selected micro (cube) structure was about 10– 15 mm in length, width and height. There were considerable voids and spaces in-between both the structures suggesting that BFO film with mixed-structures would be an ideal material for gas sensing application as gas molecules can penetrate deep into the bottom of these structures by utilizing their complete surface area. Fig. 2(e) presents energy dispersive analysis X-ray (EDAX) spectrum of the BFO film. In consistent to XRD observation, EDAX was scanned separately for knowing elemental analysis on nano (plate) and micro (cube) structures. However, both structures were composed of the same chemical stoichiometry confirming that both forms were of BFO. The 6:25:69 ratios for Bi, Fe and O, supporting the formation of BFO, consistent to XRD results were noted.
3.1. Structural elucidation and morphological evolution
3.2. Gas sensing properties
Fig. 1 shows the XRD pattern of brown BFO thick film. The pattern exhibited Bi2Fe4O9 phase of BFO with rhombohedral perovskite structure as the reflection planes (1 2 0), (2 2 0), (2 0 1), (0 2 2), (2 4 0), (4 1 1) and (4 3 1) were belong to the Bi2Fe4O9 (JCPDF no. 25-0090). The film deposited onto a glass substrate, as discussed earlier, showed distinct and sharp XRD peaks. The BFO film proved to be a single phase, i.e. Bi2Fe4O9, was free from other impurities and motile mixed phases such as Bi2O2 or Fe2O3. The
These BFO structures would be very advantageous in gas sensing application on account of nano and micro structures as nanostructures have a large specific surface area. Measurement of the voltage across the reference resistance was followed by measurement of the sensor resistance in air and gases atmosphere as a function of temperature. The change in resistance of the sensor, due to the presence of these three gases, was noted as the gas response (S%).
treatment on a gas response of maximum sensitivity was also investigated. 2. Experimental details Commercially available bismuth nitrate pentahydrate [Bi(NO3)35H2O], ferric nitrate hydrate [Fe(NO3)39H2O] and citric acid (C6H8O7) were purchased and used as received without further purification. Both Bi(NO3)35H2O and Fe(NO3)39H2O precursors in a stoichiometric proportion were separately dissolved in the diluted nitric acid (20% HNO3) and then mixed together in a glass beaker of 100 ml capacity. The citric acid was considered as the chelating agent and for enhancing pH 10, 1 M ammonium hydroxide solution was preferred. Until a fluffy dried gel was obtained the resulting solution was stirred at 100 8C. The resultant dried gel was then transferred to a crucible, annealed at 500 8C for 1 h and got a brownish powder composed of BFO micro (cubes) and nano (plates) structures. The BFO powder containing micro (cubes) in nano (plates) was grinded well before making the thick film onto a glass substrate (layering single attachment of Schotch TM tape, 3 mm and 1 cm2 cell area). In the next step, BFO film was surface treated with palladium, one of the best catalytic materials. Both pristine and palladium treated BFO films were used in sensor application. The BFO film was structurally elucidated through the X-ray powder diffraction (XRD) obtained from Diffractometer (Rigaku D/max-g B X-ray diffractometer) with a Cu Ka radiation source (l = 0.15418 nm) operated at 40 kV and 80 mA and for the morphologically confirmation the digital photoimages recorded using the field-emission scanning electron microscopy (FE-SEM) at different magnifications were presented. Finally, BFO films were exposed to LPG, CO2 and NH3 gases for gas sensors application. Effect of LPG, CO2 and NH3 gases on the gas sensitivity of BFO film is studied. Palladium treatment was applied to BFO film for improving the gas sensitivity.
S ð%Þ ¼
Fig. 1. The X-ray diffraction pattern of BFO dual structure film.
Rg Ra DR 100 ¼ 100 Ra Ra
(1)
wherein Ra is the stabilized (baseline) resistance of BFO in air atmosphere and Rg is the resistance in the presence of target gas. The two-probe dc measurement technique was used to measure the electrical resistance in presence of these gases and air atmosphere. For electrical measurements, silver paste contacts were applied at the edges of the BFO film separated by 1 cm, as top electrodes whose ohmic nature was tested within 5 V. Due O2 + 2e ! 2Oads, when BFO structures film exposed to air, oxygen molecules adsorb on respective surfaces, and oxygen molecules attract electrons, causing the carrier concentration and electron mobility to decrease and the resistance to increase [35–38]. The electrical and gas sensing characteristics were monitored using a home-built static gas-sensing system. In Fig. 3(a) the variation of gas sensitivity (%) with respect to operating temperature at fixed 3500 ppm level of LPG, CO2, and NH3 gases was monitored. The
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Fig. 2. The FE-SEM images of BFO film at different magnifications and corresponding elemental analysis through EDAX. Micro-cubes in nano-plates were clearly visualized.
Fig. 3. (a) Gas sensitivity (%) vs. operating temperature variations of BFO films at 3500 ppm of LPG, CO2, and NH3 gases. (b) Dynamic response and recovery plots at 3500 ppm of LPG, CO2, and NH3 at 160, 175 and 180 8C, respectively. (c) Variation of gas response (%) with respect to different concentrations of LPG, CO2, and NH3 gases. (d) Resistance change of pristine and Pd-treated BFO films.
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(2)
up to 6000 ppm, as with a further increase in the gas concentration did not produce any significant change in gas response, indicating that above certain limit adsorption of gas molecules over surface could be prohibited.
(3)
3.3. Effect of Pd surface treatment
adsorption kinematics can be explained by the following ways: CO2 CO2
ðgasÞ þ e ! CO2 ðadsÞ þ O
ðadsÞ
ðadsÞ þ 2e ! CO
ðgasÞ þ 2O2
ðadsÞ
The adsorption of O was very interesting step in BFO gas sensors, because the O ions assisted the adsorbed oxidizing ions to draw the electrons from the metal oxide surface. The electron concentration on the surface of BFO structures might have increased on account of following reaction. 2NH3 þ 3O
ðadsÞ ! N2 þ 3H2 O þ 3e
(4)
At the operating temperature of BFO film atmospheric oxygen atoms are adsorbed in the term of O ions by capturing electrons from the conduction band. When the BFO film sensor is exposed to LPG (reducing gaseous species), LPG molecules remove adsorbed oxygen ions from the surfaces and produces water molecules along with electrons [41] as: Cn H2n þ 2 þ ð3n þ 1ÞOðadsÞ ! nCO2 þ ðn þ 1ÞH2 OðgÞ þ ð3n þ 1Þe (5) Here CnH2n + 2 represents a mixture of hydrocarbons like propane (C3H8; n = 3) and butane (C4H10; n = 4), the main components of LPG. This reaction produces more electrons and thus reduces the resistivity of BFO upon exposure to LPG. It is well-known that the gas response of the metal-oxide semiconductor sensors is mainly determined by the surface interactions of the target gases with the sensing material. Therefore, it is certain that for the greater surface areas of the material, the interactions between the adsorbed gases and the sensor surfaces are stronger, i.e. the gas response is higher. The response time and recovery time of BFO film in different gases were tested and presented in Fig. 3(b). The LPG sensing mechanism of BFO structures sensors involves the chemisorption of oxygen present on the surface, followed by charge transfer during the reaction of oxygen with LPG molecules, which may cause a resistance change on the surface of sensing elements, nano (plates) and micro (cubes), in the present case [40]. The adsorption of gas on the surface of BFO structures might be responsible for the significant change in the electrical resistance. An increase in the gas concentration is resulted in the entire nano and micro surfaces being covered, restricting further surface adsorption of CO2 molecules. The excess CO2 gas molecules that did not reach to the surface active sites of the sensor could be then expected to persist in the surrounding region. This maintained a constant gas response even at higher gas concentration levels. Therefore, from this observation, it was concluded that the BFO structures can be used to monitor the concentration of CO2 gas over the range 10– 6000 ppm. When BFO film was exposed to a reducing gas such as NH3, the adsorbed oxygen molecules react with the physisorbed NH3 and release electrons into the BFO, thereby increasing the conductivity. The resistance was fully reversible to its original value in a short time after removing the testing gas from the system by purging with air. The sensors response to NH3 for many times without obvious change in signal intensity, was an Indication of high repeatability, good stability and long lifetime. The BFO structures film put in the home-built sensor unit and measured sensitivity under said gases and found that LPG exhibited the maximum sensitivity in all. In Fig. 3(a), the variation in gas response within the concentration range of 10–6000 ppm levels of LPG, CO2, and NH3 gases was plotted. Principally, three regions namely: (a) a sharp initial rise in gas response (high response region), (b) a nearly linear intermediate region, and (c) a region in which the sensor completely saturates (saturation region), were observed. For BFO film, an active region or gas uptake capacity was
The BFO films were treated in palladium chloride solution at 70 8C for 1 h, air-annealed at 300 K, used as Pd-treated and finally, compared with the pristine BFO film. Pd-thermal treatment creates oxygen species in the form of O, O2 and O2 on the ferrite surface which plays an important role in the gas sensing phenomena [42]. The results obtained during the gas-sensing characteristics of Pdtreated BFO showed more gas sensitivity in it than pristine BFO film. The gas sensing behaviour of Pd-treated BFO film for LPG gas was nearly about 750% and its operating temperature shifted towards the lower temperature nearly about 130 8C (Fig. 3(a)). Also, Pd-treatment enabled BFO structures to decrease the resistance (Fig. 3(d)). This shift in response towards the lower operating temperatures on Pd incorporation could be due to the well-known catalytic activity of palladium. Being the surfacecontrolled phenomena, the grain size, surface states, oxygen adsorption and the lattice defects play important role during sensing operation. Normally, the smaller the grain size, the higher is its gas sensitivity. This is due to the enhanced surface-to-volume ratio. Addition of palladium markedly improved the selectivity of BFO towards LPG gas (Fig. 3(a)–(c)). The interaction of reducing or combustible gases, with the surface chemisorbed oxygen ions exists in various forms such as O, O2 and O2 can take place in different ways. R þ O ! RO þ e
(6)
The gas-sensing phenomenon is intimately associated with the occurrence of surface catalysed combustion. Based on this approach, combustion of LPG (hydrocarbon containing CH4, C3H8 and C4H10 with reducing hydrogen species bound to carbon atoms) might be taking place for the formation of CO, CO2 and H2O with the release of electrons for the conduction. The enhanced sensitivity to Pd-treated BFO film is attributed to the formation of highly reactive species according to the reaction. The Pd atoms could be weakly bonded with the oxygen gas (O2 + 2Pd $ 2Pd: O) and the resulting complex might be readily dissociated at relatively low temperatures, and thereby, the oxygen atoms could be produced as the created atoms moves along the surface of the grains. This migration was induced by the catalyst atoms. The oxygen atoms capture electrons from the surface layer and acceptor surface states are formed. On the exposure of LPG on Pd-treated BFO film, the LPG molecules could have reacted with adsorbed oxygen. The product CnH2n–O might have not been desorbed and would have passivated the surface for further adsorption of oxygen. On the other hand, at higher operating temperature oxygen cannot physically adsorb on the surface. Thus, for Pd-treated BFO film the optimum operating temperature was 130 8C. As compared with the functional oxides, the response time and recovery times reported for the ferrites were higher, mainly due to their high processing temperature (<400 8C) [42]. In addition, further understanding electrochemical behaviour of Pd treatment, electrochemical impedance spectroscopy (EIS) measurements of pristine and Pd-treated BFO films were conducted (Fig. 4(a) and (b)). In Bode plot, peak from higher frequency indicates interfacial layer of BFO/substrate and electrolyte/BFO, respectively [43,44]. Peaks of Pd-treated BFO film were at higher frequencies than pristine BFO film case indicating that the Pd-treatment shortened the electron lifetimes within BFO. Based on correlation between Bode plot and Nyquist plot, shortening
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Fig. 4. (a) Bode and (b) Nyquist plots of pristine and Pd-treated BFO films.
electron lifetimes means decreased interfacial resistance [45]. Pdtreatment was responsible for decrease in interfacial resistance between other contact regions. Therefore, Pd-treatment helped to BFO structures to decrease an internal resistance and thereby, interfacial electron transfer resistance for fast charge injection rate.
[7] [8] [9] [10] [11]
4. Conclusions
[13] [14] [15]
In summary, pristine bismuth ferrite (Bi2Fe4O9, BFO) films composed of micro (cubes) and nano (plates) structures were synthesized by a simple and cost-effective wet-chemical route using citric acid as chelating agent followed by air-calcination at a relatively high temperature. These films were further used to sense the LPG, CO2 and NH3 gases. We achieved operating temperatures nearly about 160, 175, and 180 8C and gas sensitivities nearly about 650, 500, and 200% for LPG, CO2 and NH3 gasses, respectively. The fast response and recovery times range between 0–200 and 400– 600 s, respectively, proved the feasibility of BFO mixed structures as a gas sensor. The maximum gas uptake capacity (saturation region) was 6000 ppm. The BFO micro (cubes) structure was inserted into the nano (plates) structure. Functional characterization towards LPG, CO2, and NH3 gases sensors revealed a high sensitivity LPG gas detection down to 3500 ppm in dry air with good reversibility. Palladium-treated BFO film showed a better gas sensitivity (700%) and decreased operating temperature (130 8C) compared to pristine BFO film due to decrease in interfacial resistance as evidenced from the Nyquist plots. Acknowledgement This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2009-0063373). References [1] [2] [3] [4]
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Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Efficient gas sensitivity in mixed bismuth ferrite micro (cubes) and nano (plates) structures Shivaji D. Waghmare a, Vijaykumar V. Jadhav a, Shaym K. Gore a, Seog-Joon Yoon b, Swapnil B. Ambade b, B.J. Lokhande c, Rajaram S. Mane a,*, Sung-Hwan Han b,* a b c
Center for Nanomaterials and Energy Devices, School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, Maharashtra, India Inorganic Nanomaterials Laboratory, Department of Chemistry, Hanyang University, Seoul 133-1791, Republic of Korea Department of Physics, Solapur University, Solapur, India
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 May 2012 Accepted 30 August 2012 Available online 18 September 2012
Mixed micro (cubes) and nano (plates) structures of bismuth ferrite (BFO) have been synthesized by a simple and cost-effective wet-chemical method. Structural, morphological and phase confirmation characteristics are measured using X-ray diffraction, field-emission scanning electron microscopy (FESEM) and energy dispersive X-ray analysis techniques. The digital FE-SEM photo-images of BFO sample confirmed an incubation of discrete micro-cubes into thin and regularly placed large number of nanoplates. The bismuth ferrite, with mixed structures, films show considerable performance when used in liquefied petroleum (LPG), carbon dioxide (CO2) and ammonium (NH3) gas sensors application. Different chemical entities in LPG have made it more efficient with higher sensitivity, recovery and response times compared to CO2 and NH3 gases. Furthermore, effect of palladium surface treatment on the gas sensitivity and the charge transfer resistances of BFO mixed structures is investigated and reported. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Chemical synthesis C. Surface properties C. Impedance spectroscopy
1. Introduction As there is an increasing demand for various gases in industries and the houses, it has become a major task to develop low temperature gas sensors. Based on the change of the resistance of a semiconductor in the sensors, the sensors can detect various gases in air due to the presence of a reducing gas. Semiconductor gas sensors, with advent of simplicity, high sensitivity and fast response, offer the potential for developing portable and inexpensive gas-detection instrumentation. Semiconductor gas sensors including SnO2, ZnO, Fe2O3, etc., have been fabricated by various means and well-studied in the literature to detect most of the reducing gases [1–12]. Nanostructures of various metal oxides and also chalcogenides have been considered to increase the specific surface area exposed to gas [13–15]. However, their commercialization is seriously limited on account of poor selectivity and low thermodynamic stability. Therefore, it is necessary to find alternative metal oxides, synthesized using wet chemical route, with competitive or greater gas performances than existing known metal oxides. Researchers are extensively involved in knowing and understanding numerous chemical and physical methods for
* Corresponding authors. Tel.: +91 9850331971; fax: +91 2462229517. E-mail addresses: [email protected] (R.S. Mane), [email protected] (S.-H. Han). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.08.078
synthesizing novel metal oxides such as Nb2O5, WO3, and SnO2, followed by the possible sensor mechanism. Recently, the ferrites, class of mixed metal oxides, have been demonstrated to be appropriate materials for semiconductor gas sensors [16–21]. Ferrites, prepared in the form of ceramic materials with very high particle density, usually preferred in catalytic [22,23], magnetic or electrical applications [24]. On the contrary, for gas sensor devices lower density and higher surface area are required. And there is real scarcity of efficient low temperature gas sensors of ferrite nanostructures in literature as on today [25]. With this intention, several methods including co-precipitation, micro-emulsion, citrate or hydrothermal, etc., [26–29] for the preparation of ferrite nanostructures are documented. Spray pyrolysis [30], sputtering [31,32] methods can also be used to synthesize ferrite planar devices. Logically, a semiconductor gas sensor presents the property of changing the conductivity of the sensing material when it is exposed to different gases which, in general, depends on the operating temperature range [33–40]. In this article, bismuth ferrite (Bi2Fe4O9, BFO) thick film composed of dual structures, i.e. micro (cubes) and nano (plates), are synthesized and further envisaged in a gas sensor application in various gas atmospheres including liquefied petroleum gas (LPG), carbon dioxide (CO2) and ammonia (NH3), etc. Structural elucidation and morphological evolution confirmation were investigated in the first step and the gas sensing performance measurements were operated in the second step. Effect of palladium surface
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3. Results and discussion
digital FE-SEM images show the presence of BFO dual-structures, i.e. micro (cubes) and nano (plates). The lower and higher resolution FE-SEM images, as seen in Fig. 2(a)–(d), were well distinguishable. Under close inspection discrete micro (cubes) structure was well-embedded into the uniformly distributed vertical nano (plates) indicating that both structures might have grown simultaneously. In a wide FE-SEM image, micron-sized regular cubes were inserted into spongy-type nanoplates. These cubes were limited in numbers compared to plates. We presumed that this dual-structure (cubes and plates) of the high surface area would be advantageous in gas sensors application. Micro (cube) structure, individual, was almost same in dimensions, i.e. length, height and width. Not a single cube was dangling or loosely connected to plates. On the contrary, nano (plates) structure was grown in one direction whereas micro (cubes) in 3D. All cubes were fixed well within the plates so that their inter-movement from one plate to another was completely restricted. Dimension of each micro-cube was different from one another. To confirm it, a close scan of single micro (cube) structure was carried out (Fig. 2(d)). All faces were smooth and polished however; at edges few scratches were seen. Interestingly, scratched region presented a large number of fine nanorods, well-connected to each other, within the limit of scan. Selected micro (cube) structure was about 10– 15 mm in length, width and height. There were considerable voids and spaces in-between both the structures suggesting that BFO film with mixed-structures would be an ideal material for gas sensing application as gas molecules can penetrate deep into the bottom of these structures by utilizing their complete surface area. Fig. 2(e) presents energy dispersive analysis X-ray (EDAX) spectrum of the BFO film. In consistent to XRD observation, EDAX was scanned separately for knowing elemental analysis on nano (plate) and micro (cube) structures. However, both structures were composed of the same chemical stoichiometry confirming that both forms were of BFO. The 6:25:69 ratios for Bi, Fe and O, supporting the formation of BFO, consistent to XRD results were noted.
3.1. Structural elucidation and morphological evolution
3.2. Gas sensing properties
Fig. 1 shows the XRD pattern of brown BFO thick film. The pattern exhibited Bi2Fe4O9 phase of BFO with rhombohedral perovskite structure as the reflection planes (1 2 0), (2 2 0), (2 0 1), (0 2 2), (2 4 0), (4 1 1) and (4 3 1) were belong to the Bi2Fe4O9 (JCPDF no. 25-0090). The film deposited onto a glass substrate, as discussed earlier, showed distinct and sharp XRD peaks. The BFO film proved to be a single phase, i.e. Bi2Fe4O9, was free from other impurities and motile mixed phases such as Bi2O2 or Fe2O3. The
These BFO structures would be very advantageous in gas sensing application on account of nano and micro structures as nanostructures have a large specific surface area. Measurement of the voltage across the reference resistance was followed by measurement of the sensor resistance in air and gases atmosphere as a function of temperature. The change in resistance of the sensor, due to the presence of these three gases, was noted as the gas response (S%).
treatment on a gas response of maximum sensitivity was also investigated. 2. Experimental details Commercially available bismuth nitrate pentahydrate [Bi(NO3)35H2O], ferric nitrate hydrate [Fe(NO3)39H2O] and citric acid (C6H8O7) were purchased and used as received without further purification. Both Bi(NO3)35H2O and Fe(NO3)39H2O precursors in a stoichiometric proportion were separately dissolved in the diluted nitric acid (20% HNO3) and then mixed together in a glass beaker of 100 ml capacity. The citric acid was considered as the chelating agent and for enhancing pH 10, 1 M ammonium hydroxide solution was preferred. Until a fluffy dried gel was obtained the resulting solution was stirred at 100 8C. The resultant dried gel was then transferred to a crucible, annealed at 500 8C for 1 h and got a brownish powder composed of BFO micro (cubes) and nano (plates) structures. The BFO powder containing micro (cubes) in nano (plates) was grinded well before making the thick film onto a glass substrate (layering single attachment of Schotch TM tape, 3 mm and 1 cm2 cell area). In the next step, BFO film was surface treated with palladium, one of the best catalytic materials. Both pristine and palladium treated BFO films were used in sensor application. The BFO film was structurally elucidated through the X-ray powder diffraction (XRD) obtained from Diffractometer (Rigaku D/max-g B X-ray diffractometer) with a Cu Ka radiation source (l = 0.15418 nm) operated at 40 kV and 80 mA and for the morphologically confirmation the digital photoimages recorded using the field-emission scanning electron microscopy (FE-SEM) at different magnifications were presented. Finally, BFO films were exposed to LPG, CO2 and NH3 gases for gas sensors application. Effect of LPG, CO2 and NH3 gases on the gas sensitivity of BFO film is studied. Palladium treatment was applied to BFO film for improving the gas sensitivity.
S ð%Þ ¼
Fig. 1. The X-ray diffraction pattern of BFO dual structure film.
Rg Ra DR 100 ¼ 100 Ra Ra
(1)
wherein Ra is the stabilized (baseline) resistance of BFO in air atmosphere and Rg is the resistance in the presence of target gas. The two-probe dc measurement technique was used to measure the electrical resistance in presence of these gases and air atmosphere. For electrical measurements, silver paste contacts were applied at the edges of the BFO film separated by 1 cm, as top electrodes whose ohmic nature was tested within 5 V. Due O2 + 2e ! 2Oads, when BFO structures film exposed to air, oxygen molecules adsorb on respective surfaces, and oxygen molecules attract electrons, causing the carrier concentration and electron mobility to decrease and the resistance to increase [35–38]. The electrical and gas sensing characteristics were monitored using a home-built static gas-sensing system. In Fig. 3(a) the variation of gas sensitivity (%) with respect to operating temperature at fixed 3500 ppm level of LPG, CO2, and NH3 gases was monitored. The
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Fig. 2. The FE-SEM images of BFO film at different magnifications and corresponding elemental analysis through EDAX. Micro-cubes in nano-plates were clearly visualized.
Fig. 3. (a) Gas sensitivity (%) vs. operating temperature variations of BFO films at 3500 ppm of LPG, CO2, and NH3 gases. (b) Dynamic response and recovery plots at 3500 ppm of LPG, CO2, and NH3 at 160, 175 and 180 8C, respectively. (c) Variation of gas response (%) with respect to different concentrations of LPG, CO2, and NH3 gases. (d) Resistance change of pristine and Pd-treated BFO films.
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(2)
up to 6000 ppm, as with a further increase in the gas concentration did not produce any significant change in gas response, indicating that above certain limit adsorption of gas molecules over surface could be prohibited.
(3)
3.3. Effect of Pd surface treatment
adsorption kinematics can be explained by the following ways: CO2 CO2
ðgasÞ þ e ! CO2 ðadsÞ þ O
ðadsÞ
ðadsÞ þ 2e ! CO
ðgasÞ þ 2O2
ðadsÞ
The adsorption of O was very interesting step in BFO gas sensors, because the O ions assisted the adsorbed oxidizing ions to draw the electrons from the metal oxide surface. The electron concentration on the surface of BFO structures might have increased on account of following reaction. 2NH3 þ 3O
ðadsÞ ! N2 þ 3H2 O þ 3e
(4)
At the operating temperature of BFO film atmospheric oxygen atoms are adsorbed in the term of O ions by capturing electrons from the conduction band. When the BFO film sensor is exposed to LPG (reducing gaseous species), LPG molecules remove adsorbed oxygen ions from the surfaces and produces water molecules along with electrons [41] as: Cn H2n þ 2 þ ð3n þ 1ÞOðadsÞ ! nCO2 þ ðn þ 1ÞH2 OðgÞ þ ð3n þ 1Þe (5) Here CnH2n + 2 represents a mixture of hydrocarbons like propane (C3H8; n = 3) and butane (C4H10; n = 4), the main components of LPG. This reaction produces more electrons and thus reduces the resistivity of BFO upon exposure to LPG. It is well-known that the gas response of the metal-oxide semiconductor sensors is mainly determined by the surface interactions of the target gases with the sensing material. Therefore, it is certain that for the greater surface areas of the material, the interactions between the adsorbed gases and the sensor surfaces are stronger, i.e. the gas response is higher. The response time and recovery time of BFO film in different gases were tested and presented in Fig. 3(b). The LPG sensing mechanism of BFO structures sensors involves the chemisorption of oxygen present on the surface, followed by charge transfer during the reaction of oxygen with LPG molecules, which may cause a resistance change on the surface of sensing elements, nano (plates) and micro (cubes), in the present case [40]. The adsorption of gas on the surface of BFO structures might be responsible for the significant change in the electrical resistance. An increase in the gas concentration is resulted in the entire nano and micro surfaces being covered, restricting further surface adsorption of CO2 molecules. The excess CO2 gas molecules that did not reach to the surface active sites of the sensor could be then expected to persist in the surrounding region. This maintained a constant gas response even at higher gas concentration levels. Therefore, from this observation, it was concluded that the BFO structures can be used to monitor the concentration of CO2 gas over the range 10– 6000 ppm. When BFO film was exposed to a reducing gas such as NH3, the adsorbed oxygen molecules react with the physisorbed NH3 and release electrons into the BFO, thereby increasing the conductivity. The resistance was fully reversible to its original value in a short time after removing the testing gas from the system by purging with air. The sensors response to NH3 for many times without obvious change in signal intensity, was an Indication of high repeatability, good stability and long lifetime. The BFO structures film put in the home-built sensor unit and measured sensitivity under said gases and found that LPG exhibited the maximum sensitivity in all. In Fig. 3(a), the variation in gas response within the concentration range of 10–6000 ppm levels of LPG, CO2, and NH3 gases was plotted. Principally, three regions namely: (a) a sharp initial rise in gas response (high response region), (b) a nearly linear intermediate region, and (c) a region in which the sensor completely saturates (saturation region), were observed. For BFO film, an active region or gas uptake capacity was
The BFO films were treated in palladium chloride solution at 70 8C for 1 h, air-annealed at 300 K, used as Pd-treated and finally, compared with the pristine BFO film. Pd-thermal treatment creates oxygen species in the form of O, O2 and O2 on the ferrite surface which plays an important role in the gas sensing phenomena [42]. The results obtained during the gas-sensing characteristics of Pdtreated BFO showed more gas sensitivity in it than pristine BFO film. The gas sensing behaviour of Pd-treated BFO film for LPG gas was nearly about 750% and its operating temperature shifted towards the lower temperature nearly about 130 8C (Fig. 3(a)). Also, Pd-treatment enabled BFO structures to decrease the resistance (Fig. 3(d)). This shift in response towards the lower operating temperatures on Pd incorporation could be due to the well-known catalytic activity of palladium. Being the surfacecontrolled phenomena, the grain size, surface states, oxygen adsorption and the lattice defects play important role during sensing operation. Normally, the smaller the grain size, the higher is its gas sensitivity. This is due to the enhanced surface-to-volume ratio. Addition of palladium markedly improved the selectivity of BFO towards LPG gas (Fig. 3(a)–(c)). The interaction of reducing or combustible gases, with the surface chemisorbed oxygen ions exists in various forms such as O, O2 and O2 can take place in different ways. R þ O ! RO þ e
(6)
The gas-sensing phenomenon is intimately associated with the occurrence of surface catalysed combustion. Based on this approach, combustion of LPG (hydrocarbon containing CH4, C3H8 and C4H10 with reducing hydrogen species bound to carbon atoms) might be taking place for the formation of CO, CO2 and H2O with the release of electrons for the conduction. The enhanced sensitivity to Pd-treated BFO film is attributed to the formation of highly reactive species according to the reaction. The Pd atoms could be weakly bonded with the oxygen gas (O2 + 2Pd $ 2Pd: O) and the resulting complex might be readily dissociated at relatively low temperatures, and thereby, the oxygen atoms could be produced as the created atoms moves along the surface of the grains. This migration was induced by the catalyst atoms. The oxygen atoms capture electrons from the surface layer and acceptor surface states are formed. On the exposure of LPG on Pd-treated BFO film, the LPG molecules could have reacted with adsorbed oxygen. The product CnH2n–O might have not been desorbed and would have passivated the surface for further adsorption of oxygen. On the other hand, at higher operating temperature oxygen cannot physically adsorb on the surface. Thus, for Pd-treated BFO film the optimum operating temperature was 130 8C. As compared with the functional oxides, the response time and recovery times reported for the ferrites were higher, mainly due to their high processing temperature (<400 8C) [42]. In addition, further understanding electrochemical behaviour of Pd treatment, electrochemical impedance spectroscopy (EIS) measurements of pristine and Pd-treated BFO films were conducted (Fig. 4(a) and (b)). In Bode plot, peak from higher frequency indicates interfacial layer of BFO/substrate and electrolyte/BFO, respectively [43,44]. Peaks of Pd-treated BFO film were at higher frequencies than pristine BFO film case indicating that the Pd-treatment shortened the electron lifetimes within BFO. Based on correlation between Bode plot and Nyquist plot, shortening
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Fig. 4. (a) Bode and (b) Nyquist plots of pristine and Pd-treated BFO films.
electron lifetimes means decreased interfacial resistance [45]. Pdtreatment was responsible for decrease in interfacial resistance between other contact regions. Therefore, Pd-treatment helped to BFO structures to decrease an internal resistance and thereby, interfacial electron transfer resistance for fast charge injection rate.
[7] [8] [9] [10] [11]
4. Conclusions
[13] [14] [15]
In summary, pristine bismuth ferrite (Bi2Fe4O9, BFO) films composed of micro (cubes) and nano (plates) structures were synthesized by a simple and cost-effective wet-chemical route using citric acid as chelating agent followed by air-calcination at a relatively high temperature. These films were further used to sense the LPG, CO2 and NH3 gases. We achieved operating temperatures nearly about 160, 175, and 180 8C and gas sensitivities nearly about 650, 500, and 200% for LPG, CO2 and NH3 gasses, respectively. The fast response and recovery times range between 0–200 and 400– 600 s, respectively, proved the feasibility of BFO mixed structures as a gas sensor. The maximum gas uptake capacity (saturation region) was 6000 ppm. The BFO micro (cubes) structure was inserted into the nano (plates) structure. Functional characterization towards LPG, CO2, and NH3 gases sensors revealed a high sensitivity LPG gas detection down to 3500 ppm in dry air with good reversibility. Palladium-treated BFO film showed a better gas sensitivity (700%) and decreased operating temperature (130 8C) compared to pristine BFO film due to decrease in interfacial resistance as evidenced from the Nyquist plots. Acknowledgement This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-sensitized Solar Cells (No. 2009-0063373). References [1] [2] [3] [4]
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