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Hematite nanostructures: Morphology-mediated liquefied petroleum gas sensors
Sensors and Actuators B 188 (2013) 669–674
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hematite nanostructures: Morphology-mediated liquefied petroleum gas sensors Vijaykumar V. Jadhav a,b , Supriya A. Patil b , Dipak V. Shinde b , Shivaji D. Waghmare a , Manohar K. Zate a , Rajaram S. Mane a,b,∗ , Sung-Hwan Han b,∗∗ a b
Center for Nano-materials and Energy Devices, School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, MS, India Inorganic Nano-materials Laboratory, Department of Chemistry, Hanyang University, Seoul 133-1791, Republic of Korea
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Article history: Received 2 March 2013 Received in revised form 11 July 2013 Accepted 22 July 2013 Available online 30 July 2013 Keywords: ␣-Fe2 O3 Structures Morphology LPG sensors
a b s t r a c t Liquefied petroleum gas (LPG) sensors of hematite nanostructures viz. nanoparticles, nanoparticle-chains and nanorods, chemically synthesized from iron nitrate, sulfate and chloride precursors, respectively, in presence of urea as pH regulating agent are explored. These nanostructures were examined for their structures and morphologies. The Hematite morphology has an impact on crystal structure, Raman shift and charge transfer resistance value. On the basis of results presented herein, we proposed an importance of hematite nanostructures as a sensing material for the LPG sensors. Superior LPG sensor sensitivity of nanoparticles-chains (77.89%) over nanoparticles (68%) and nanorods (58.80%) at 1000 ppm LPG gas levels are imply that the hematite nanostructure (surface area) plays an important role in mediating charge transfer reaction. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The increasing concern for air-pollution and industrial safety establishes a need to monitor combustible and toxic gases for healthy and comfortable life. Tremendous efforts have been made to develop gas sensors at low temperatures [1,2]. Recently, researchers proved an importance of one-dimensional (1D) nanostructure semiconducting oxides over traditional thin and thick film sensors such as high surface-to-volume ratio, dimensions comparable to the extension of surface charge region, etc. [3–6]. The working mechanism of a gas sensor includes in the convection of electrical conductivity due to surface reactions such as oxidation or reduction caused by different gas exposures [7] which in fact, depends on the active centers and the defects existing on the surface layer of materials. Compared with conventional materials such as bulk or thin films applied to gas sensors, 1D nanostructures such as nanowires, nanorods (NRs), nanoribbons, and nanotubes, etc. have a relatively higher gas sensor response due to their ultrahigh surface-to-volume ratio. Reactions at grain boundaries
∗ Corresponding author at: Inorganic Nano-materials Laboratory, Department of Chemistry, Hanyang University, Seoul 133-1791, Republic of Korea. Tel.: +82 2292 5212; fax: +82 2299 0762. ∗∗ Corresponding author. E-mail addresses: rsmane [email protected] (R.S. Mane), [email protected] (S.-H. Han). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.07.072
could strongly modify the transport quality [8,9]. Because of low power consumption, low-cost, easy operation and high compatibility with microelectronic processing, etc., number of gas sensors of nanostructures of metal oxide semiconductors have widely been exploited to date [10]. Hematite, i.e., iron oxide (␣-Fe2 O3 ), generally an n-type semiconductor with energy gap of 2.1 eV, is the most thermodynamically stable phase among all available iron oxide phases under ambient conditions [9]. Owing to its environmental friendliness, high chemical and thermal stabilities, it has been previously envisaged in a variety of fields including gas sensors [10], lithium batteries [11], catalysts [12], pigments [13], magnetic devices [14], etc. Moreover, research on the intrinsic relationship between the morphology/size and the sensor property has engendered an urgent need for adjustable synthetic strategies, where the size and morphology of the hematite can be controlled with required functionalities. Accordingly, various physical and chemical methods so far have been adopted to synthesize hematite in diverse morphologies, such as nanoparticles (NPs) [15], NRs [16], nanotubes [17], hollow-spheres [18], nanobelts [19], nanoplates [20] and complex hierarchical structures [21,22], etc. Hydrothermal chemical method with structure-directing agent has proven its effectiveness in growing hematite nanostructures of various kinds [23,24]. For many decades, the hematite has been proved to be one of the good gas sensing candidates for the detection of gases including hydrogen [25], liquefied petroleum gas (LPG) [26], carbon monoxide [27], nitrogen dioxide [28], hydrogen sulfide [29], oxygen [30]
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and ethanol [31], etc. Of these, LPG is a combustible gas and it is widely used as fuel for domestic heating and industrial use. It is one of the extensively used gases in day-to-day activities. There is a need to detect the leakage in its early stages before explosion and perform active suppression [32]. In order to accomplish this, more attention has already been paid to the development of the gas sensors for the detection of LPG using several doped and undoped metal oxides [33]. In continuation with our hematite-based research [34], in this work the relationship between the hematite morphology and the LPG gas sensor performance is demonstrated. Different nanostructures of hematite viz. nanoparticle-chains (NPc), NPs, and NRs are chemically synthesized onto glass substrates and further used for their structure, morphology and electric properties. The experimental results clearly demonstrate the potential of hematite nanostructures as sensing material in the fabrication of LPG sensors. Charge transfer resistance, gas sensitivity, response time and recovery time are found to be dependent on the hematite morphology type. 2. Experimental details All reagents used in this experiment were of analytical grade, obtained from Sigma–Aldrich and were used without further purification. Distilled water was used throughout the experiment. Glass-substrates used for deposition were cleaned for 10 min in detergent water, acetone and isopropyl alcohol separately from an ultrasonic bath, dried in a stream of argon and stored for further use. In a typical synthesis of hematite nanostructures viz., NPc, NPs and NRs method, 25 ml stock solutions of 0.2 M Fe2 (SO4 )3 , 0.2 M Fe(NO3 )3 and 0.2 M FeCl3 , respectively, were mixed with 0.4 M urea (25 ml) separate solution and sealed in three falcon tubes with each of 50 ml capacity and maintained at 90 ◦ C for 12 h. Glass substrates were placed vertically in these tubes by maintaining inclined positions. After completion of the reactions, reddish iron oxide-hydroxide films obtained onto glass substrates were taken off, thoroughly washed with water, dried under a stream of argon and air-annealed at 500 ◦ C for 30 min before further measurements and LPG sensors. The crystallographic orientations of the deposited hematite films of said nanostructures were examined using X-ray diffraction (XRD) from 20◦ to 80◦ in 2 theta range with a step of 1◦ ˚ The morper min. The X-ray source was Cu K␣ ( = 1.5405 A). phologies were recorded using a field-emission scanning electron microscopy (FE-SEM, Hitachi S-4200) images. For electrochemical impedance spectroscopy (EIS) measurements, impedance analyzer (COMPACTSTATe: IVIUM Switzerland) was used. The sensor response was determined using S = (Ra − Rg /Ra ) × 100% relation, where Ra and Rg represent the resistances of the film in air and upon exposure to LPG, respectively. 3. Results and discussion 3.1. Structural elucidation and morphological evolution studies Fig. 1A–F presents the FE-SEM images of hematite NPc, NPs and NRs nanostructures at two different magnifications wherein, the growth of all nanostructures was uniform and crack-free. The NPc were nearly two micrometers in lengths and about 40 nanometers in widths. Physically, several NPc were connected to form corntype architecture. These corns were wide at centers and narrow at their ends (Fig. 1A and B). The ␣-Fe2 O3 NPs of irregular dimensions (close to 40 nm) were agglomerated and close to one another. Surfaces were smooth and polished (Fig. 1C and D). Growth of NRs was in the bundle form (Fig. 1E and F). Few NRs, with 100–200 nm
in lengths and 40–60 nm in diameters, were compiled together to form stick-type elongated architecture. Under close inspection it was found that these individual nanorods were bifurcated at some places along their lengths (Fig. 1E and F) which may be effective in LPG sensors application on account of their excessive surface areas. The XRD spectra of three different hematite nanostructures, i.e., NPc, NPs and NRs, deposited on glass substrates, are shown in Fig. 2A. The peaks appearing in the XRD are indexed using the JCPDS data (JCPDS 88-2346) with hexagonal unit cell. No impurity phases and peak shift were observed, suggesting a hematite (␣Fe2 O3 ) structures was same for all three forms. The (0 1 2), (1 1 0), (1 1 3), (0 2 4) and (0 1 8) reflection planes showed relatively higher intensitied for NPc as compared to NRs and NPs. Though all morphologies exhibit same structure, their degree of crystallinity was lower for NRs and NPs and higher for NPc. Effect of XRD peak intensity on electrical resistivity is recently investigated for ZnO thin films [35]. Due to fewer oxygen ions, and a lower ionic diffusion resistance, i.e., a higher charge transfer resistance, relatively smaller (0 0 2) peak intensity for ZnO NPs compared with other planes is reported [36]. In short, different crystallinites exist in hematite nanostructure contributed respective peak intensity in X-ray pattern and demonstrated different electrochemical properties (discussed below). Fig. 2B shows the Raman spectra of hematite nanostructures during irreducible vibrational modes wherein the acoustic A1u and the A2u modes are optically silent. The symmetrical modes are Raman active and the anti-symmetrical modes are infrared active [37]. Infra-red active modes were absent as hexagonal structure of hematite has an inversion center. These spectra revealed that all peaks correspond to ␣-Fe2 O3 , consistent to XRD results, and were free from any peak resembling with other phases of iron oxide. Peak positions at 229 cm−1 and 500 cm−1 were assigned to the A1g modes whereas, remaining five peak positions at 249, 295, 302, 414 and 615 cm−1 are due to the Eg modes [38]. 3.2. Charge transfer resistance kinetics The charge transfer kinetics in hematite nanostructured electrodes was studied in dark by applying an AC open circuit potential of −0.697 V amplitude in the frequency range of 1 mHz to 1 MHz using ESI spectra (see Ref. [34, Fig. 5] for more details). The impedance spectra for all nanostructures show one semicircle which can be attributed to charge transfer resistance at the fluorine-tin-oxide/Fe2 O3 /electrolyte interfaces in non-uniformtype photo-electrochemical cells. At high frequency region the crossover point of the highest frequency with the real part of the impedance is a combinational resistance of the electrolyte resistance, intrinsic resistance of substrate and contact resistance between the active material and the current collector. Nyquist plots were different for different hematite nanostructures suggesting that in spite of same structure, the charge transfer resistance and the series resistances were different. The lowest charge transfer resistance value was assigned to NRs (1512 ) compared to NPs (1730 ) and NPc (1870 ) forms. Series resistance of NRs electrode is higher than NPs and NPc electrodes which could have accounted for difference in their surface areas as surface area of NRs (13.66 m2 g−1 ) was smaller than NPs (15.99 m2 g−1 ) and NPc (65.62 m2 g−1 ), respectively. This confirmed that the electrode surface area and morphology had a direct impact on series resistance and charge transfer resistance [34]. To corroborate this observation DC two-point electrical conductivity measurement was operated for all nanostructures between ±10 V voltage span using Kiethely 2400 source meter at room temperature (Fig. 3A) [39]. Electrical conductivities for hematite NPc, NRs and NPs structures were 40.00 × 10−9 , 23.09 × 10−9 and 7.47 × 10−9 S/cm, respectively, and supporting that the electrical resistivity of hematite is morphology
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Fig. 1. FE-SEM images of NRs (A and B), NPs (C and D) and NPc (E and F) of hematite.
dependent, consistent to EIS measurement [34] and previously reported ZnO results [35]. 3.3. LPG gas sensors Fig. 3(B) show the sensitivity–temperature profiles of hematite sensors fabricated onto glass substrates toward 1000 ppm LPG. The sensitivity was decreased with rise in the temperature from 159 ◦ C to 171 ◦ C, a function of conductance change in hematite
nanostructures. The reducing gas generally reacts with oxygen adsorbed on the sensor surface. The reaction of adsorbed oxygen with reducing gas species releases electrons back to the hematite to maintain the neutrality, resulting in an increase of electron concentration consequently a decrease in the resistance. The surface activity of the material affects the oxygen adsorption property from the atmosphere. The interaction of these adsorbed oxygen species with reducing gas results in the change in conduction of the material. The gas response measurements were taken during cooling
Fig. 2. (A) XRD and (B) Raman spectra of hematite NRs, NPs, and NPc.
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Fig. 3. (A) Room temperature J–V, (B) gas sensitivity (%) vs. operating temperature under 1000 fixed ppm level of LPG, (C) dynamic response and recovery plots under 1000 fixed ppm level of LPG at 195 ◦ C, 171 ◦ C and 159 ◦ C, and (D) gas response (%) vs. LPG concentration for hematite nanostructures viz. NRs, NPs, and NPc.
after being heated to sufficiently high temperature for stability, thereby securing a good reproducibility of the response temperature characteristics. The equilibrium of the chemisorptions process results in stabilization of surface resistance and change in this gives rise to change in the resistance of the sensor element [40]. The hematite nanostructures are n-type semiconducting in behavior as there is a drop in resistance of the sensor when they are exposed to reducing gas. The LPG gas response was reached to its maximum at optimum temperature and then decreased at higher temperatures whereas; at low temperatures the gas response was limited by the rate of chemical reaction. However, at higher temperatures the rate of diffusion of gas molecule might prohibited the gas response. At the optimum temperature, the rates of chemical reaction and diffusion of gas molecule are same, attaining the equilibrium at that temperature and the sensor approves maximum response [41]. The response characteristics indicated that the sensor proved a high response toward LPG at an operating temperature of 159 ◦ C of 77.89% for NPc of hematite. When the LPG was introduced in the gas chamber, the gas response was initially increased with the time and remained unchanged. The NPc showed a better response than NRs and NPs due to spherical crystallites, which generally provides more effective surface area for interaction with LPG molecules [41] than NPs and NRs architectures. Response was decreased in the NPs at 171 ◦ C of 59% sensitivity. In case of
hematite NRs morphology operating temperature was 195 ◦ C with 68% sensitivity. At an operating temperature, in the absence of a target gas, oxygen gets adsorbed on the surface of the sensor and it extracts electrons from the conduction band of the sensor material [23]. Thus, the equilibration of the chemisorptions process results in stabilization of surface resistance. Any process that disturbs this equilibrium gives rise to changes in the conductance [23]. The reducing hydrogen species CH4 , C3 H8 , C4 H10 , etc., present in LPG bound to carbon atoms react with adsorbed oxygen through Cn H2n+2 + 2O− ↔ H2 O + Cn H2n O− + e− reaction. Here, Cn H2n+2 represent the CH4 , C3 H8 and C4 H10 . Fig. 3C shows the dynamic response and recovery spectra of hematite nanostructures exposed to 1000 ppm LPG gas. The responding ability of the sensor was smart for hematite NPc sensor due to its relatively high surface area as the surface area is one of the important factors that impact their responding ability. Moreover, the responding ability of the NPc sensor was increased upon exposure to 1000 ppm LPG, and then decreased rapidly and recovered to its initial value after LPG gas is released. The response time and recovery time were 3.03, 1.6 and 3.8 min and 4.3, 3.73 and 3.5 min for NPc, NPs and NRs sensors, respectively. Fig. 3D shows gas responses at various gas concentrations for hematite NPc, NPs and NRs sensors at 159 ◦ C, 171 ◦ C and 195 ◦ C operating temperatures, respectively. For NPc nanostructure the sensitivity of sensor was increased with LPG
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concentration from 200 ppm to 1000 ppm and then remained unchanged with sensitivity (∼90%). Different operational times were taken to reach the maximum sensitivities for different nanostructures. The sensitivity was dropped rapidly in all cases when the gas was removed off from the testing atmosphere indicating that all designed sensors were exhibiting good recovery time values. Thus, the maximum sensitivity of 77.89% is obtain for hematite NPc at 159 ◦ C upon exposure to 1000 ppm LPG gas which in fact was higher than NPs (58.80%) and NRs (68%). 4. Conclusions In nutshell, this work demonstrates the synthesis of ␣-Fe2 O3 , corroborated from the X-ray diffraction and Raman shift analysis studies, in nanoparticle-chains, nanoparticles and nanorods structures, identified from the high and low resolution scanning electron microscopy images, using a simply wet chemical route at an ambient temperature. Different charge transfer resistance and series resistance values are obtained on account of their different surface areas caused by morphologies, suggesting that resistance of ␣-Fe2 O3 hematite is a morphology dependent. All morphologies confirm good sensitivity toward LPG gas at all concentrations. Due to least series resistance and higher surface area hematite nanoparticle-chain morphology has proved 77.89% LPG gas sensitivity which is higher than 68% and 58.80%, obtained for nanorods and nanoparticles morphologies at 1000 ppm level. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education(2013009768). Thanks to University Grant Commission, New Delhi, India for providing financial assistance through a research project: 41-844/2012(SR). References [1] D. Wang, X.F. Chu, M.L. Gong, Gas-sensing properties of sensors based on single crystalline SnO2 nanorods prepared by a simple molten-salt method, Sensors & Actuators B: Chemical 117 (2006) 183–187. [2] C.S. Rout, K. Ganesh, A. Govindaraj, C.N.R. Rao, Sensors for the nitrogen oxides, NO2 , NO and N2 O, based on In2 O3 and WO3 nanowires, Applied Physics A 85 (2006) 241–246. [3] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [4] A. Modi, N. Koratkar, E. Lass, B.Q. Wei, P.M. Ajayan, Miniaturized gas ionization sensors using carbon nanotubes, Nature 424 (2003) 171–174. [5] E.S. Snow, F.K. Perkins, E.J. Houser, S.C. Badescu, T.L. Reinecke, Chemical detection with a single-walled carbon nanotube capacitor, Science 307 (2005) 1942–1945. [6] A. Kolmakov, X.Y. Zhang, G.S. Cheng, M. Moskovits, Detection of CO and O2 using tin oxide nanowire sensors, Advanced Materials 15 (2003) 997–1000. [7] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N. Shen, X.Y. Ding, Z.L. Wang, Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks, Applied Physics Letters 88 (2006) 203101. [8] J. Ming, Y. Wu, L.Y. Wang, Y. Yu, F. Zhao, CO2 -assisted template synthesis of porous hollow bi-phase ␥-/␣-Fe2 O3 nanoparticles with high sensor property, Journal of Materials Chemistry 21 (2011) 17776–17782. [9] H. Huang, Q.K. Tan, Y.C. Lee, T.D. Tran, M.S. Tse, Semiconductor gas sensor based on tin oxide nanorods prepared by plasma-enhanced chemical vapor deposition with postplasma treatment, Applied Physics Letters 87 (2005) 163123–163125. [10] X.H. Liu, J. Zhang, X.Z. Guo, S.H. Wu, S.R. Wang, Porous, ␣-Fe2 O3 decorated by Au nanoparticles and their enhanced sensor performance, Nanotechnology 21 (2010) 095501–095508. [11] P. Zhang, Z.P. Guo, H.K. Liu, Submicron-sized cube-like ␣-Fe2 O3 agglomerates as an anode material for Li-ion batteries, Electrochimica Acta 55 (2010) 8521–8526. [12] L.L. Li, Y. Chu, Y. Liu, L.H. Dong, Template-free synthesis and photocatalytic properties of novel Fe2 O3 hollow spheres, Journal of Physical Chemistry C 111 (2007) 2123–2127. [13] F. Bondioli, A.M. Ferrari, C. Leonelli, T. Manfredini, Syntheses of Fe2 O3 /silica red inorganic inclusion pigments for ceramic applications, Materials Research Bulletin 33 (1998) 723–729.
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Shivaji D. Waghamare received his master degree in physics from SRTM University, Nanded, India in 2009. Now he is a PhD student at school of physical sciences, SRTM University Nanded, India. His research interest is making nano-materials for different applications include gas sensor, solar cell, super-capacitor and photo-active.
Biographies
Manohar K. Zate received a master degree in Physics from SRTM University, Nanded, India in 2009. Now he is a PhD student at School of Physical Sciences, SRTM University Nanded, India. His research interest is in structure of materials and their optoelectronic and electrochemical properties.
Vijaykumar V. Jadhav received a master degree in physics from SRTM University, Nanded, India in 2009. Now he is a PhD student at school of physical sciences, SRTM University Nanded, India. His research interest is in structure of materials and their optoelectronic and electrochemical properties, under the supervision of Prof. R.S. Mane. Supriya A. Patil is now a PhD student Hanyang University Seoul, South Korea. She is working under the supervision of Prof. Sung-Hwan Han. Dipak V. Shinde is now a PhD student Hanyang University Seoul, South Korea. He is working under the supervision of Prof. Sung-Hwan Han.
Rajaram S. Mane is a professor at School of Physical Sciences, Swami Ramanand Teerth Marathawada University, Nanded. His research interests include solid and liquid state DSSCs, supercapacitors, gas sensors, etc. Sung-Hwan Han is a professor and Head of Chemistry Department, Hanyang University, Korea. His research topics include but not limited to developing new type of semiconductors beyond silicon with a focus on hybrid nano-structured materials with metaloxides/chalcogenides, conducting polymers, and organometallic compounds, which can be applied to fabricate hybrid solar cells, thin-film transistors and supercapacitors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Hematite nanostructures: Morphology-mediated liquefied petroleum gas sensors Vijaykumar V. Jadhav a,b , Supriya A. Patil b , Dipak V. Shinde b , Shivaji D. Waghmare a , Manohar K. Zate a , Rajaram S. Mane a,b,∗ , Sung-Hwan Han b,∗∗ a b
Center for Nano-materials and Energy Devices, School of Physical Sciences, Swami Ramanand Teerth Marathwada University, Nanded 431606, MS, India Inorganic Nano-materials Laboratory, Department of Chemistry, Hanyang University, Seoul 133-1791, Republic of Korea
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Article history: Received 2 March 2013 Received in revised form 11 July 2013 Accepted 22 July 2013 Available online 30 July 2013 Keywords: ␣-Fe2 O3 Structures Morphology LPG sensors
a b s t r a c t Liquefied petroleum gas (LPG) sensors of hematite nanostructures viz. nanoparticles, nanoparticle-chains and nanorods, chemically synthesized from iron nitrate, sulfate and chloride precursors, respectively, in presence of urea as pH regulating agent are explored. These nanostructures were examined for their structures and morphologies. The Hematite morphology has an impact on crystal structure, Raman shift and charge transfer resistance value. On the basis of results presented herein, we proposed an importance of hematite nanostructures as a sensing material for the LPG sensors. Superior LPG sensor sensitivity of nanoparticles-chains (77.89%) over nanoparticles (68%) and nanorods (58.80%) at 1000 ppm LPG gas levels are imply that the hematite nanostructure (surface area) plays an important role in mediating charge transfer reaction. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The increasing concern for air-pollution and industrial safety establishes a need to monitor combustible and toxic gases for healthy and comfortable life. Tremendous efforts have been made to develop gas sensors at low temperatures [1,2]. Recently, researchers proved an importance of one-dimensional (1D) nanostructure semiconducting oxides over traditional thin and thick film sensors such as high surface-to-volume ratio, dimensions comparable to the extension of surface charge region, etc. [3–6]. The working mechanism of a gas sensor includes in the convection of electrical conductivity due to surface reactions such as oxidation or reduction caused by different gas exposures [7] which in fact, depends on the active centers and the defects existing on the surface layer of materials. Compared with conventional materials such as bulk or thin films applied to gas sensors, 1D nanostructures such as nanowires, nanorods (NRs), nanoribbons, and nanotubes, etc. have a relatively higher gas sensor response due to their ultrahigh surface-to-volume ratio. Reactions at grain boundaries
∗ Corresponding author at: Inorganic Nano-materials Laboratory, Department of Chemistry, Hanyang University, Seoul 133-1791, Republic of Korea. Tel.: +82 2292 5212; fax: +82 2299 0762. ∗∗ Corresponding author. E-mail addresses: rsmane [email protected] (R.S. Mane), [email protected] (S.-H. Han). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.07.072
could strongly modify the transport quality [8,9]. Because of low power consumption, low-cost, easy operation and high compatibility with microelectronic processing, etc., number of gas sensors of nanostructures of metal oxide semiconductors have widely been exploited to date [10]. Hematite, i.e., iron oxide (␣-Fe2 O3 ), generally an n-type semiconductor with energy gap of 2.1 eV, is the most thermodynamically stable phase among all available iron oxide phases under ambient conditions [9]. Owing to its environmental friendliness, high chemical and thermal stabilities, it has been previously envisaged in a variety of fields including gas sensors [10], lithium batteries [11], catalysts [12], pigments [13], magnetic devices [14], etc. Moreover, research on the intrinsic relationship between the morphology/size and the sensor property has engendered an urgent need for adjustable synthetic strategies, where the size and morphology of the hematite can be controlled with required functionalities. Accordingly, various physical and chemical methods so far have been adopted to synthesize hematite in diverse morphologies, such as nanoparticles (NPs) [15], NRs [16], nanotubes [17], hollow-spheres [18], nanobelts [19], nanoplates [20] and complex hierarchical structures [21,22], etc. Hydrothermal chemical method with structure-directing agent has proven its effectiveness in growing hematite nanostructures of various kinds [23,24]. For many decades, the hematite has been proved to be one of the good gas sensing candidates for the detection of gases including hydrogen [25], liquefied petroleum gas (LPG) [26], carbon monoxide [27], nitrogen dioxide [28], hydrogen sulfide [29], oxygen [30]
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and ethanol [31], etc. Of these, LPG is a combustible gas and it is widely used as fuel for domestic heating and industrial use. It is one of the extensively used gases in day-to-day activities. There is a need to detect the leakage in its early stages before explosion and perform active suppression [32]. In order to accomplish this, more attention has already been paid to the development of the gas sensors for the detection of LPG using several doped and undoped metal oxides [33]. In continuation with our hematite-based research [34], in this work the relationship between the hematite morphology and the LPG gas sensor performance is demonstrated. Different nanostructures of hematite viz. nanoparticle-chains (NPc), NPs, and NRs are chemically synthesized onto glass substrates and further used for their structure, morphology and electric properties. The experimental results clearly demonstrate the potential of hematite nanostructures as sensing material in the fabrication of LPG sensors. Charge transfer resistance, gas sensitivity, response time and recovery time are found to be dependent on the hematite morphology type. 2. Experimental details All reagents used in this experiment were of analytical grade, obtained from Sigma–Aldrich and were used without further purification. Distilled water was used throughout the experiment. Glass-substrates used for deposition were cleaned for 10 min in detergent water, acetone and isopropyl alcohol separately from an ultrasonic bath, dried in a stream of argon and stored for further use. In a typical synthesis of hematite nanostructures viz., NPc, NPs and NRs method, 25 ml stock solutions of 0.2 M Fe2 (SO4 )3 , 0.2 M Fe(NO3 )3 and 0.2 M FeCl3 , respectively, were mixed with 0.4 M urea (25 ml) separate solution and sealed in three falcon tubes with each of 50 ml capacity and maintained at 90 ◦ C for 12 h. Glass substrates were placed vertically in these tubes by maintaining inclined positions. After completion of the reactions, reddish iron oxide-hydroxide films obtained onto glass substrates were taken off, thoroughly washed with water, dried under a stream of argon and air-annealed at 500 ◦ C for 30 min before further measurements and LPG sensors. The crystallographic orientations of the deposited hematite films of said nanostructures were examined using X-ray diffraction (XRD) from 20◦ to 80◦ in 2 theta range with a step of 1◦ ˚ The morper min. The X-ray source was Cu K␣ ( = 1.5405 A). phologies were recorded using a field-emission scanning electron microscopy (FE-SEM, Hitachi S-4200) images. For electrochemical impedance spectroscopy (EIS) measurements, impedance analyzer (COMPACTSTATe: IVIUM Switzerland) was used. The sensor response was determined using S = (Ra − Rg /Ra ) × 100% relation, where Ra and Rg represent the resistances of the film in air and upon exposure to LPG, respectively. 3. Results and discussion 3.1. Structural elucidation and morphological evolution studies Fig. 1A–F presents the FE-SEM images of hematite NPc, NPs and NRs nanostructures at two different magnifications wherein, the growth of all nanostructures was uniform and crack-free. The NPc were nearly two micrometers in lengths and about 40 nanometers in widths. Physically, several NPc were connected to form corntype architecture. These corns were wide at centers and narrow at their ends (Fig. 1A and B). The ␣-Fe2 O3 NPs of irregular dimensions (close to 40 nm) were agglomerated and close to one another. Surfaces were smooth and polished (Fig. 1C and D). Growth of NRs was in the bundle form (Fig. 1E and F). Few NRs, with 100–200 nm
in lengths and 40–60 nm in diameters, were compiled together to form stick-type elongated architecture. Under close inspection it was found that these individual nanorods were bifurcated at some places along their lengths (Fig. 1E and F) which may be effective in LPG sensors application on account of their excessive surface areas. The XRD spectra of three different hematite nanostructures, i.e., NPc, NPs and NRs, deposited on glass substrates, are shown in Fig. 2A. The peaks appearing in the XRD are indexed using the JCPDS data (JCPDS 88-2346) with hexagonal unit cell. No impurity phases and peak shift were observed, suggesting a hematite (␣Fe2 O3 ) structures was same for all three forms. The (0 1 2), (1 1 0), (1 1 3), (0 2 4) and (0 1 8) reflection planes showed relatively higher intensitied for NPc as compared to NRs and NPs. Though all morphologies exhibit same structure, their degree of crystallinity was lower for NRs and NPs and higher for NPc. Effect of XRD peak intensity on electrical resistivity is recently investigated for ZnO thin films [35]. Due to fewer oxygen ions, and a lower ionic diffusion resistance, i.e., a higher charge transfer resistance, relatively smaller (0 0 2) peak intensity for ZnO NPs compared with other planes is reported [36]. In short, different crystallinites exist in hematite nanostructure contributed respective peak intensity in X-ray pattern and demonstrated different electrochemical properties (discussed below). Fig. 2B shows the Raman spectra of hematite nanostructures during irreducible vibrational modes wherein the acoustic A1u and the A2u modes are optically silent. The symmetrical modes are Raman active and the anti-symmetrical modes are infrared active [37]. Infra-red active modes were absent as hexagonal structure of hematite has an inversion center. These spectra revealed that all peaks correspond to ␣-Fe2 O3 , consistent to XRD results, and were free from any peak resembling with other phases of iron oxide. Peak positions at 229 cm−1 and 500 cm−1 were assigned to the A1g modes whereas, remaining five peak positions at 249, 295, 302, 414 and 615 cm−1 are due to the Eg modes [38]. 3.2. Charge transfer resistance kinetics The charge transfer kinetics in hematite nanostructured electrodes was studied in dark by applying an AC open circuit potential of −0.697 V amplitude in the frequency range of 1 mHz to 1 MHz using ESI spectra (see Ref. [34, Fig. 5] for more details). The impedance spectra for all nanostructures show one semicircle which can be attributed to charge transfer resistance at the fluorine-tin-oxide/Fe2 O3 /electrolyte interfaces in non-uniformtype photo-electrochemical cells. At high frequency region the crossover point of the highest frequency with the real part of the impedance is a combinational resistance of the electrolyte resistance, intrinsic resistance of substrate and contact resistance between the active material and the current collector. Nyquist plots were different for different hematite nanostructures suggesting that in spite of same structure, the charge transfer resistance and the series resistances were different. The lowest charge transfer resistance value was assigned to NRs (1512 ) compared to NPs (1730 ) and NPc (1870 ) forms. Series resistance of NRs electrode is higher than NPs and NPc electrodes which could have accounted for difference in their surface areas as surface area of NRs (13.66 m2 g−1 ) was smaller than NPs (15.99 m2 g−1 ) and NPc (65.62 m2 g−1 ), respectively. This confirmed that the electrode surface area and morphology had a direct impact on series resistance and charge transfer resistance [34]. To corroborate this observation DC two-point electrical conductivity measurement was operated for all nanostructures between ±10 V voltage span using Kiethely 2400 source meter at room temperature (Fig. 3A) [39]. Electrical conductivities for hematite NPc, NRs and NPs structures were 40.00 × 10−9 , 23.09 × 10−9 and 7.47 × 10−9 S/cm, respectively, and supporting that the electrical resistivity of hematite is morphology
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Fig. 1. FE-SEM images of NRs (A and B), NPs (C and D) and NPc (E and F) of hematite.
dependent, consistent to EIS measurement [34] and previously reported ZnO results [35]. 3.3. LPG gas sensors Fig. 3(B) show the sensitivity–temperature profiles of hematite sensors fabricated onto glass substrates toward 1000 ppm LPG. The sensitivity was decreased with rise in the temperature from 159 ◦ C to 171 ◦ C, a function of conductance change in hematite
nanostructures. The reducing gas generally reacts with oxygen adsorbed on the sensor surface. The reaction of adsorbed oxygen with reducing gas species releases electrons back to the hematite to maintain the neutrality, resulting in an increase of electron concentration consequently a decrease in the resistance. The surface activity of the material affects the oxygen adsorption property from the atmosphere. The interaction of these adsorbed oxygen species with reducing gas results in the change in conduction of the material. The gas response measurements were taken during cooling
Fig. 2. (A) XRD and (B) Raman spectra of hematite NRs, NPs, and NPc.
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Fig. 3. (A) Room temperature J–V, (B) gas sensitivity (%) vs. operating temperature under 1000 fixed ppm level of LPG, (C) dynamic response and recovery plots under 1000 fixed ppm level of LPG at 195 ◦ C, 171 ◦ C and 159 ◦ C, and (D) gas response (%) vs. LPG concentration for hematite nanostructures viz. NRs, NPs, and NPc.
after being heated to sufficiently high temperature for stability, thereby securing a good reproducibility of the response temperature characteristics. The equilibrium of the chemisorptions process results in stabilization of surface resistance and change in this gives rise to change in the resistance of the sensor element [40]. The hematite nanostructures are n-type semiconducting in behavior as there is a drop in resistance of the sensor when they are exposed to reducing gas. The LPG gas response was reached to its maximum at optimum temperature and then decreased at higher temperatures whereas; at low temperatures the gas response was limited by the rate of chemical reaction. However, at higher temperatures the rate of diffusion of gas molecule might prohibited the gas response. At the optimum temperature, the rates of chemical reaction and diffusion of gas molecule are same, attaining the equilibrium at that temperature and the sensor approves maximum response [41]. The response characteristics indicated that the sensor proved a high response toward LPG at an operating temperature of 159 ◦ C of 77.89% for NPc of hematite. When the LPG was introduced in the gas chamber, the gas response was initially increased with the time and remained unchanged. The NPc showed a better response than NRs and NPs due to spherical crystallites, which generally provides more effective surface area for interaction with LPG molecules [41] than NPs and NRs architectures. Response was decreased in the NPs at 171 ◦ C of 59% sensitivity. In case of
hematite NRs morphology operating temperature was 195 ◦ C with 68% sensitivity. At an operating temperature, in the absence of a target gas, oxygen gets adsorbed on the surface of the sensor and it extracts electrons from the conduction band of the sensor material [23]. Thus, the equilibration of the chemisorptions process results in stabilization of surface resistance. Any process that disturbs this equilibrium gives rise to changes in the conductance [23]. The reducing hydrogen species CH4 , C3 H8 , C4 H10 , etc., present in LPG bound to carbon atoms react with adsorbed oxygen through Cn H2n+2 + 2O− ↔ H2 O + Cn H2n O− + e− reaction. Here, Cn H2n+2 represent the CH4 , C3 H8 and C4 H10 . Fig. 3C shows the dynamic response and recovery spectra of hematite nanostructures exposed to 1000 ppm LPG gas. The responding ability of the sensor was smart for hematite NPc sensor due to its relatively high surface area as the surface area is one of the important factors that impact their responding ability. Moreover, the responding ability of the NPc sensor was increased upon exposure to 1000 ppm LPG, and then decreased rapidly and recovered to its initial value after LPG gas is released. The response time and recovery time were 3.03, 1.6 and 3.8 min and 4.3, 3.73 and 3.5 min for NPc, NPs and NRs sensors, respectively. Fig. 3D shows gas responses at various gas concentrations for hematite NPc, NPs and NRs sensors at 159 ◦ C, 171 ◦ C and 195 ◦ C operating temperatures, respectively. For NPc nanostructure the sensitivity of sensor was increased with LPG
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concentration from 200 ppm to 1000 ppm and then remained unchanged with sensitivity (∼90%). Different operational times were taken to reach the maximum sensitivities for different nanostructures. The sensitivity was dropped rapidly in all cases when the gas was removed off from the testing atmosphere indicating that all designed sensors were exhibiting good recovery time values. Thus, the maximum sensitivity of 77.89% is obtain for hematite NPc at 159 ◦ C upon exposure to 1000 ppm LPG gas which in fact was higher than NPs (58.80%) and NRs (68%). 4. Conclusions In nutshell, this work demonstrates the synthesis of ␣-Fe2 O3 , corroborated from the X-ray diffraction and Raman shift analysis studies, in nanoparticle-chains, nanoparticles and nanorods structures, identified from the high and low resolution scanning electron microscopy images, using a simply wet chemical route at an ambient temperature. Different charge transfer resistance and series resistance values are obtained on account of their different surface areas caused by morphologies, suggesting that resistance of ␣-Fe2 O3 hematite is a morphology dependent. All morphologies confirm good sensitivity toward LPG gas at all concentrations. Due to least series resistance and higher surface area hematite nanoparticle-chain morphology has proved 77.89% LPG gas sensitivity which is higher than 68% and 58.80%, obtained for nanorods and nanoparticles morphologies at 1000 ppm level. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education(2013009768). Thanks to University Grant Commission, New Delhi, India for providing financial assistance through a research project: 41-844/2012(SR). References [1] D. Wang, X.F. Chu, M.L. Gong, Gas-sensing properties of sensors based on single crystalline SnO2 nanorods prepared by a simple molten-salt method, Sensors & Actuators B: Chemical 117 (2006) 183–187. [2] C.S. Rout, K. Ganesh, A. Govindaraj, C.N.R. Rao, Sensors for the nitrogen oxides, NO2 , NO and N2 O, based on In2 O3 and WO3 nanowires, Applied Physics A 85 (2006) 241–246. [3] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [4] A. Modi, N. Koratkar, E. Lass, B.Q. Wei, P.M. Ajayan, Miniaturized gas ionization sensors using carbon nanotubes, Nature 424 (2003) 171–174. [5] E.S. Snow, F.K. Perkins, E.J. Houser, S.C. Badescu, T.L. Reinecke, Chemical detection with a single-walled carbon nanotube capacitor, Science 307 (2005) 1942–1945. [6] A. Kolmakov, X.Y. Zhang, G.S. Cheng, M. Moskovits, Detection of CO and O2 using tin oxide nanowire sensors, Advanced Materials 15 (2003) 997–1000. [7] A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N. Shen, X.Y. Ding, Z.L. Wang, Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks, Applied Physics Letters 88 (2006) 203101. [8] J. Ming, Y. Wu, L.Y. Wang, Y. Yu, F. Zhao, CO2 -assisted template synthesis of porous hollow bi-phase ␥-/␣-Fe2 O3 nanoparticles with high sensor property, Journal of Materials Chemistry 21 (2011) 17776–17782. [9] H. Huang, Q.K. Tan, Y.C. Lee, T.D. Tran, M.S. Tse, Semiconductor gas sensor based on tin oxide nanorods prepared by plasma-enhanced chemical vapor deposition with postplasma treatment, Applied Physics Letters 87 (2005) 163123–163125. [10] X.H. Liu, J. Zhang, X.Z. Guo, S.H. Wu, S.R. Wang, Porous, ␣-Fe2 O3 decorated by Au nanoparticles and their enhanced sensor performance, Nanotechnology 21 (2010) 095501–095508. [11] P. Zhang, Z.P. Guo, H.K. Liu, Submicron-sized cube-like ␣-Fe2 O3 agglomerates as an anode material for Li-ion batteries, Electrochimica Acta 55 (2010) 8521–8526. [12] L.L. Li, Y. Chu, Y. Liu, L.H. Dong, Template-free synthesis and photocatalytic properties of novel Fe2 O3 hollow spheres, Journal of Physical Chemistry C 111 (2007) 2123–2127. [13] F. Bondioli, A.M. Ferrari, C. Leonelli, T. Manfredini, Syntheses of Fe2 O3 /silica red inorganic inclusion pigments for ceramic applications, Materials Research Bulletin 33 (1998) 723–729.
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Shivaji D. Waghamare received his master degree in physics from SRTM University, Nanded, India in 2009. Now he is a PhD student at school of physical sciences, SRTM University Nanded, India. His research interest is making nano-materials for different applications include gas sensor, solar cell, super-capacitor and photo-active.
Biographies
Manohar K. Zate received a master degree in Physics from SRTM University, Nanded, India in 2009. Now he is a PhD student at School of Physical Sciences, SRTM University Nanded, India. His research interest is in structure of materials and their optoelectronic and electrochemical properties.
Vijaykumar V. Jadhav received a master degree in physics from SRTM University, Nanded, India in 2009. Now he is a PhD student at school of physical sciences, SRTM University Nanded, India. His research interest is in structure of materials and their optoelectronic and electrochemical properties, under the supervision of Prof. R.S. Mane. Supriya A. Patil is now a PhD student Hanyang University Seoul, South Korea. She is working under the supervision of Prof. Sung-Hwan Han. Dipak V. Shinde is now a PhD student Hanyang University Seoul, South Korea. He is working under the supervision of Prof. Sung-Hwan Han.
Rajaram S. Mane is a professor at School of Physical Sciences, Swami Ramanand Teerth Marathawada University, Nanded. His research interests include solid and liquid state DSSCs, supercapacitors, gas sensors, etc. Sung-Hwan Han is a professor and Head of Chemistry Department, Hanyang University, Korea. His research topics include but not limited to developing new type of semiconductors beyond silicon with a focus on hybrid nano-structured materials with metaloxides/chalcogenides, conducting polymers, and organometallic compounds, which can be applied to fabricate hybrid solar cells, thin-film transistors and supercapacitors.