Highly sensitive and selective LPG sensor based on α-Fe2O3 nanorods

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Sensors and Actuators B 152 (2011) 299–306

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

ighly sensitive and selective LPG sensor based on �-Fe2O3 nanorods

ewyani Patil a, Virendra Patil b, Pradip Patil a,∗

Department of Physics, North Maharashtra University, Jalgaon 425 001, Maharashtra, IndiaCenter for Materials Characterization, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, Maharashtra, India

r t i c l e i n f o

rticle history:eceived 1 June 2010eceived in revised form5 November 2010ccepted 15 December 2010vailable online 23 December 2010

eywords:

a b s t r a c t

The �-Fe2O3 nanorods were successfully synthesized without any templates by calcining the �-FeOOHprecursor in air at 300 ◦C for 2 h and their LPG sensing characteristics were investigated. The �-FeOOHprecursor was prepared through a simple and low cost wet chemical route at low temperature (40 ◦C)using FeSO4·7H2O and CH3COONa as starting materials. The formation of �-FeOOH precursor and itstopotactic transformation to �-Fe2O3 upon calcination was confirmed by X-ray diffraction measurement(XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM)and transmission electron microscopy (TEM) analysis. The �-Fe2O3 nanorods exhibited outstanding gas

◦

-Fe2O3 nanorodsPG sensoremiconductor gas sensorsESEMEMPSRD

sensing characteristics such as, higher gas response (∼1746–50 ppm LPG at 300 C), extremely rapidresponse (∼3–4 s), relatively slow recovery (∼8–9 min), excellent repeatability, good selectivity and loweroperating temperature (∼300 ◦C). Furthermore, the �-Fe2O3 nanorods are able to detect up to 5 ppm forLPG with reasonable response (∼15) at the operating temperature of 300 ◦C and they can be reliablyused to monitor the concentration of LPG over the range (5–60 ppm). The experimental results clearlydemonstrate the potential of using the �-Fe2O3 nanorods as sensing material in the fabrication of LPG

nsing

sensors. Plausible LP G se

. Introduction

Liquefied petroleum gas (LPG) is widely used as fuel for domes-ic heating and industrially to provide a clean source of energy forurning. It is a combustible gas which mainly consists of butane70–80%), propane (5–10%) and propylene, butylene, ethylene and

ethane (1–5%) [1]. It is potentially hazardous due to the high pos-ibility of explosion accidents caused by leakage or by human error.ence, it is crucial to detect it in its early stages to give alarm anderform effective suppression [2]. This has stimulated considerable

nterest for scientific research to develop reliable, efficient, simplend cost-effective chemical sensors to monitor LPG having goodensitivity and selectivity in recent years and many efforts, in thiseld, are today devoted to the synthesis of novel sensing materi-ls with enhanced performance. The metal oxide semiconductorsuch as undoped SnO2 [3], SnO2 doped with nobel metals such asu [4], Au [5], Pt [5] and Pd [5], CdO [6], WO3 [7], Al doped ZnO [8]nd BaTiO3 [9] have been investigated in the past decades as LPG

ensing materials because of their low cost and power consump-ion, simplicity of fabrication and use, versatility in detecting a wideange of toxic/flammable gases and stability in harsh environments.

∗ Corresponding author. Tel.: +91 2572257474.E-mail address: [email protected] (P. Patil).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2010.12.025

mechanism of the �-Fe2O3 nanorods is also discussed.© 2010 Elsevier B.V. All rights reserved.

Hematite (�-Fe2O3) has received considerable attention in thepast few years due to its application potential in many technologi-cal areas such catalysis, lithium rechargeable batteries, gas sensors,pigments and magnetic materials [10–14]. It is the most stable ironoxide under ambient conditions, which exhibits n-type semicon-ducting properties with an indirect band gap of 2.2 eV. The �-Fe2O3has long been used as sensing material in the fabrication of gas sen-sors for the detection of H2 [15], LPG [16], CO [17], NO2 [11], H2S[18], O2 [19] and ethanol [20].

Recent studies reveal that the nanostructured metal oxideswith reduced dimensionality (i.e. in the form of nanoparticles,nanorods, nanotubes, nanowires and nanoribbons) are promis-ing sensing materials for highly sensitive chemical sensors due totheir small grain size and large surface-to-volume ratio [6,21–25].Consequently, different methods for synthesizing the �-Fe2O3with different morphologies and sizes such as nanoparticles [26],nanobelts [27], nanotubes [15], nanorods [28], nanowires [29] andurchin-like superstructures [30] have been reported. Nevertheless,it still remains a challenge to develop simple, low cost and versatileapproaches to synthesize 1-D nanostructures of �-Fe2O3.

The gas sensors based on �-Fe2O3 nanoparticles have been

widely investigated by many researchers in the past decades[31,32]. However, so far, there are only a few reports on the gas-sensing properties of 1-D nanostructures �-Fe2O3. Zheng et al. [33]prepared the �-Fe2O3 ceramic nanofibers by the electrospiningroute followed by calcinations at 800 ◦C for 6 h in air and stud-

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ed their ethanol sensing properties. These nanofibers exhibitedood sensitivity (∼4.25 for 1000 ppm ethanol at 300 ◦C), rapidesponse (∼3 s) and fast recovery (∼5 s to 500 ppm). ZhangYangt al. reported on controlled synthesis of hollow sea urchin-like �-e2O3 nanostructures via the hydrothermal approach followed bynnealing in air at 600 ◦C for 2 h. The sensors based on the �-Fe2O3anostructures showed high gas sensing responses, short responsend recovery time and long term stability in detecting ammonia,ormaldehyde, triethylamine, acetone and ethanol.

Within the present investigation, experiments have been car-ied out for the fabrication of sensitive and selective LPG sensorased on �-Fe2O3 nanorods. There is hardly any report on LPGensor based on �-Fe2O3 nanorods. The �-Fe2O3 nanorods wereynthesized without any templates by calcining the �-FeOOH pre-ursor in air at 300 ◦C for 2 h. This procedure is similar to thateported by Wang et al. [14], who have synthesized �-Fe2O3anorods without the use of any template or organic surfactant.hus, in this work, the �-FeOOH precursor was prepared throughsimple and low cost wet chemical route at low temperature

40 ◦C) using FeSO4·7H2O was used as the source of Fe2+ and theH3COONa was used as the precipitating agent to release hydroxyl

ons slowly during the reaction. Sensing characteristics of the �-e2O3 nanorods to LPG were systematically investigated.

. Experimental

.1. Materials

All chemicals were of analytical grade. The ferrous sulphateFeSO4·7H2O) and sodium acetate (CH3COONa) were purchasedrom E-Merck (India) and were used without further purification.

.2. Synthesis of the ˛-Fe2O3 nanorods

In this work, the �-Fe2O3 nanorods were synthesized withoutny templates by calcining the �-FeOOH precursor in air at 300 ◦Cor 2 h. The �-FeOOH precursor was prepared through a simplend low cost wet chemical route. The FeSO4·7H2O was used as theource of Fe2+ and the CH3COONa was used as the precipitatinggent to release hydroxyl ions slowly during the reaction. In a typi-al experiment, the aqueous solution containing 0.1 M FeSO4·7H2Ond 0.1 M CH3COONa was prepared in double distilled water andtirred continuously using a magnetic stirrer for 2 h at 40 ◦C tobtain a yellow colored precipitate. The resulting precipitate wasltered and washed with double distilled water and alcohol severalimes to remove impurities and by products present in the prod-ct. The precipitate, thus formed was dried at 40 ◦C under vacuumor 2 h and grinded into a powder, which is the �-FeOOH precur-or. The �-FeOOH precursor was calcined in air at 300 ◦C for 2 h tobtain the �-Fe2O3 nanorods. The color of the �-FeOOH precursoras changed from yellow to red during calcination.

.3. Characterization

X-ray diffraction (XRD) analysis was performed with a Brukeriffractometer (D8, Advance, Bruker AXS model) with CuK� radi-tion (�=1.5406 nm) operating at 40 kV and 40 mA. The fieldmission scanning electron (FE-SEM) microscopy analysis was car-ied out with a Hitachi (S-4800, Hitachi, Japan) microscope. Theransmission electron microscopy (TEM) was used to determinehe particle size and the morphology of the nano-sized powder

ith a JEOL (1200 EX, Japan) microscope. The X-ray photoelec-

ron spectroscopy (XPS) measurements were performed using amicron energy analyser (EA 125) instrument with Al K� radiation

1486.6 eV) X-ray source. The pressure in the analyser chamber wasbout 10−10 torr during the XPS measurements. The survey scan

2 (degree)

Fig. 1. XRD patterns of �-FeOOH precursor.

spectra for all samples were recorded at 50 eV pass energy. Thecore level binding energies were corrected with the C 1s bindingenergy of 284.9 eV.

2.4. LPG sensing study

The �-Fe2O3 nanorods powder was pressed into pellets undera pressure of 15 MPa and the ohmic contacts were made with thehelp of silver paste to form the sensing element. The gas sensingstudies were carried out on these sensing elements in a static gaschamber to sense LPG in air ambient. The sensing element was keptdirectly on a heater in the gas chamber and the temperature wasvaried from 250 to 400 ◦C. The temperature of the sensing elementwas monitored by chromel-alumel thermocouple placed in contactwith the sensor. The known volume of the LPG was introduced intothe gas chamber pre-filled with air and it was maintained at atmo-spheric pressure. The electrical resistance of the sensing elementwas measured before and after exposure to LPG using a sensitivedigital multi meter (2000, Digital Multimeter, Keithley, U.S.A) con-trolled by a personal computer. The performance of the sensingelement is presented in terms of gas response (S), which is definedas:

S = Rair

Rgas(1)

where Rair and Rgas are the electrical resistance values of the sensorelement in air and in the presence of LPG gas, respectively.

3. Results and discussion

3.1. XRD results

The XRD pattern of the as-prepared precursor is shown in Fig. 1.The diffraction peak positions match well with the XRD pattern oforthorhombic �-FeOOH phase (JCPDS # 29-0713). This �-FeOOHprecursor was calcined at different temperatures in the rangebetween 300 and 600 ◦C in air for 2 h. The XRD patterns of the�-FeOOH precursor calcined at 300, 400, 500 and 600 ◦C in airfor 2 h are shown in Fig. 2. The diffraction peaks in all the plotsare in agreement with the standard XRD peaks, which confirmed

that the synthesized materials were iron (III) oxide (hematite, �-Fe2O3) (JCPDS # 33-0664) of rhombohedral geometry. We do notobserve the presence of any other peak due to hydroxide or impuri-ties, which confirmed that the �-FeOOH precursor has transformedcompletely into hematite on calcination in air for 2 h. It is clearly

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ig. 2. XRD patterns of �-FeOOH precursor calcined at (a) 300 ◦C, (b) 400 ◦C, (c)00 ◦C and (d) 600 ◦C in air for 2 h.

een that the diffraction peaks become sharper along with anncrease in the calcination temperature, indicating the enhance-

ent of crystallinity. The crystallite size (D) was calculated byebye–Scherrer formula given by

= 0.94�

B cos �(2)

here D is the average size of the crystallite, assuming that therains are spherical, � is the wavelength of X-ray radiation, B is theull width at half maxima of individual peak at 2� (where � is Braggngle). The variation of the crystallite size on the calcination tem-erature is shown in Fig. 3. The crystallite size increases with an

ncrease in the calcination temperature and it is found to be small-st (in the range of 5–10 nm) when the �-FeOOH precursor wasalcinate at 300 ◦C for 2 h. This observation may be attributed tohe particles growth and aggregation of small particles after beingalcined at higher temperatures. Hence, 300 ◦C was chosen as theptimum calcination temperature to prepare �-Fe2O3 nanorodsnd to investigate the LPG sensing properties.

.2. XPS results

To further ascertain the formation of �-Fe2O3, the XPS analysisf the calcined �-FeOOH precursor was performed. The XPS sur-

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ig. 3. Variation of crystallite size of �-Fe2O3 nanorods on calcinations temperature.

ors B 152 (2011) 299–306 301

vey spectrum of the calcined �-FeOOH precursor [Fig. 4(a)] showsthe presence of Fe 2p (56%) and O 1s (31%). The deconvoluted Fe2p spectrum [Fig. 4(b)] is comprised of two peaks at 711.40 and724.80 eV, which corresponds to the Fe 2p3/2 and Fe 2p1/2, respec-tively. This is consistent with the previously reported values 710.8and 724.8 eV for the bulk �-Fe2O3. The energy difference betweenFe 2p3/2 and Fe 2p1/2 peaks is 13.4 eV. This value is characteristic ofFe3+ state indicating the formation of the �-Fe2O3 by the experi-mental methodology used [34–37]. Furthermore, Fe3+ satellite peakis observable in the spectrum at 718.5 eV, above the Fe 2p3/2 peak.The deconvoluted O 1s spectrum [Fig. 4(c)] is deconvoluted intotwo peaks at 530.41 and 532.11 eV, which are in good agreementwith the literature values of �-Fe2O3 [35,36]. The dominant peaklocated at 530.41 eV corresponds to the oxygen bonded as �-Fe2O3.The second peak at 532.11 eV is probably due to a hydroxide [35,36].Thus, the Fe 2p and O 1s spectra indicate that the valence states ofelements Fe and O are +3 and −2, respectively. The XPS results inconjunction with XRD and FTIR data confirm the formation of pure�-Fe2O3 when the �-FeOOH precursor was calcined at 300 ◦C in airfor 2 h.

3.3. Morphological analysis

The surface morphologies of the as-prepared �-FeOOH precur-sor and �-Fe2O3 nanorods were characterized by FESEM and TEM.The FESEM image of the as-prepared �-FeOOH precursor [Fig. 5(a)]shows uniform rodlike structure, which is characteristic crystallineshape of the �-FeOOH. As can be seen from Fig. 5(b), the surfacemorphology of the synthesized �-Fe2O3 nanorods is similar to thatof the �-FeOOH precursor. The nanorods have an aspect ratio (ratiobetween the diameter ∼40 nm and length 300 nm of the sample) of∼7 as evidenced from the FESEM. Thus, the FESEM results reveal thetopotactic transformation of �-FeOOH precursor to �-Fe2O3 pre-serving the shape of the starting material during the calcinationsprocess. The TEM image [Fig. 5(c)] of the as-prepared �-FeOOH pre-cursor exhibits a smooth and rodlike morphology. The selected areaelectron diffraction (SAED) pattern shown in the inset of Fig. 3(c)exhibits the pattern which correlated well with the XRD results. Itshows the spot type pattern which is indicative of the presence ofsingle crystallite particles. The TEM image of the �-FeOOH precur-sor after calcination at 300 ◦C in air for 2 h is shown in Fig. 5(d). It isclearly seen that the rodlike morphology is preserved even after thecalcinations of the �-FeOOH precursor. This is in agreement withthe FESEM result which indicates the topotactic transformation of�-FeOOH precursor to �-Fe2O3 during calcinations at 300 ◦C in air.However, it is significant to note that the �-Fe2O3 nanorods havemany pores along its axial direction. The formation of the poresmay be attributed to the removal of H2O from the �-FeOOH pre-cursor during the calcinations process. The SAED pattern shown inthe inset of Fig. 5(d) exhibits the spot type pattern which is indica-tive of the presence of single crystallite particles and no evidencewas found for more than one pattern, suggesting the single phasenature of the material. Thus, the calcination of the �-FeOOH pre-cursor at 300 ◦C in air results into the formation of porous �-Fe2O3nanorods.

3.4. LPG sensing characteristics

The LPG gas sensing experiments were performed at differenttemperatures in order to find out the optimum operating temper-ature for LPG gas detection. Before exposing to the LPG gas, the

sensing element was allowed to equilibrate inside the gas chamberat an operating temperature for 1 h. A number of experiments havebeen carried out to measure the gas response as a function of theoperating temperature. All the time the gas response of the sensorelement has approximately constant values, indicating the repeata-

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0 200 400 600 800 1000 1200

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Fe 2p1/2

Fe 2p3/2

O 1s

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530.41 eVc

nd (c)

bnawmia3i3ta

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Binding Energy (eV)700 710 720 730

Fig. 4. (a) XPS survey spectrum and XPS spectra, (b) Fe 2p a

ility of the sensor. The gas response of the synthesized �-Fe2O3anorods (calcined at 300 ◦C) to 50 ppm LPG as a function of oper-ting temperature is shown in Fig. 6(a). The gas response increasesith an increase in the operating temperature and attains a maxi-um at ∼300 ◦C (S ∼1746), followed by a decrease with a further

ncrease of the operating temeprature. Thus, the optimum oper-ting temperature for the �-Fe2O3 nanorods to detect LPG was at00 ◦C, which is the modest from the viewpoint of semiconduct-

ng oxide gas sensors. Hence, the optimum operating temperature00 ◦C was chosen in order to investigate the LPG sensing proper-ies such as response and recovery characteristics, reproducibilitynd selectivity.

When the commercially available �-Fe2O3 powder was used,he maximum gas response [Fig. 6(b)] to 50 ppm of LPG (S ∼170)ccurs at 350 ◦C and thereafter it decreases with an increase in theperating temperature. Thus, in the case of �-Fe2O3 nanorods, theperating temperature was reduced by 50 ◦C with improvement ofas response. The gas response of the �-Fe O nanorods is about ten

2 3imes greater than that of commercial �-Fe2O3 powder, indicatinghe improved sensivity of the �-Fe2O3 nanorods. These observa-ions reveal that the LPG sensing ability of �-Fe2O3 is significantlynhanced when it is in the form of nanorods. The enhanced LPG gas

Binding Energy (eV)522 524 526 528 530 532 534 536 538 540

O 1s of calcined �-FeOOH precursor at 300 ◦C in air for 2 h.

sensing performance of �-Fe2O3 nanorods over that of commer-cially available �-Fe2O3 powder may be attributed to their smallersize and higher specific surface area of the rod shaped morphology.

A number of experiments have been carried out to measure theLPG response as a function of operating temperature and the resultsof these repeated measurements are shown in Fig. 7. All the timethe LPG response of the sensor element has approximately constantvalues, indicating the repeatability of the sensor.

The dependence of the LPG response of the �-Fe2O3 nanorods to50 ppm of LPG at the optimum operating temperature of 300 ◦C onthe calcination temperature is shown in Fig. 8. The LPG response isfound to be maximum (∼1746) when the calcination temperaturewas 300 ◦C and it decreases with further increase in the calcinationtemperature. Thus, the optimum calcination temperature for the �-Fe2O3 nanorods to detect LPG with maximum response was 300 ◦C.

Besides the gas response, the response and recovery times arealso important parameters for evaluating the performance of gassensors. The response and recovery times are defined as the time

required for the sensor-resistance to change by 90% of the finalresistance. The response and recovery characteristics of the �-Fe2O3 nanorods to 50 ppm LPG gas at the operating temperature300 ◦C is shown in Fig. 9. Five samples were tested from each batch

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F OOH p

aswdreapp

c

Fa

300 ◦C is shown in Fig. 10. The �-Fe2O3 nanorods show good repro-

ig. 5. FESEM and TEM images of (a, c) �-FeOOH precursor and (b, d) calcined �-Fe

nd each sample was tested three times. It is evident that expo-ure to the LPG decreases the resistance of the sensing elementhich is consistent with the sensing behavior of n-type semicon-uctor oxide. As can be seen from Fig. 9, the sensor responds veryapidly after introduction of LPG and recovers slowly when it isxposed to air. The �-Fe2O3 nanorods have response time of ∼3–4 s

nd the recovery time of ∼8–9 min. The �-Fe2O3 nanorods may beorous and consequently, desorption of the LPG molecules is a slowrocess, which results into the slow recovery.

The reproducibility and stability are important parameters to beonsidered when evaluating the performance of a sensor. It is useful

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ig. 6. Effect of operating temperature on the gas response of (a) �-Fe2O3 nanorodsnd (b) commercially available �-Fe2O3 powder to 50 ppm LPG.

recursor. The corresponding SAED patterns are shown in insets of (c) and (d).

to have both a stable base line resistance and a reproducible signalchange to a given analyte concentration. The reproducibility andstability of the �-Fe2O3 nanorods were measured by repeating thetest four times. The gas response of the �-Fe2O3 nanorods uponperiodic exposure to 50 ppm LPG at an operating temperature of

ducibility and reversibility upon repeated exposure and removalof LPG under same conditions. Thus, the �-Fe2O3 nanorods exhibitgood stability as well as an excellent repeatability of the response.

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I II III

Fig. 7. Replicates of the measurements for the LPG response of �-Fe2O3 nanorodsto 50 ppm LPG as a function of operating temperature.

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300 400 500 6000

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Fig. 8. Dependence of LPG response on the calcination temperature of �-Fe2O3

nanorods to 50 ppm LPG at the operating temperature of 300 ◦C.

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Fig. 9. Response of �-Fe2O3 nanorods to 50 ppm LPG at the operating temperatureof 300 ◦C.

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Fig. 10. Repetitive response of �-Fe2O3 nanorods to 50 ppm LPG at the operatingtemperature of 300 ◦C.

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Fig. 11. Response of �-Fe2O3 nanorods upon sequential exposure to LPG with con-centrations varying from 5 to 60 ppm at the operating temperature of 300 ◦C.

This suggests that the �-Fe2O3 nanorods can be used as a reusablesensing material for the detection of LPG.

Fig. 11 represents the gas response of the �-Fe2O3 nanorods atthe operating temperature of 300 ◦C to LPG with concentrationsvarying from 5 to 60 ppm. It is observed that the gas responseincreased with an increase in the LPG concentration. Furthermore,a base line remains stable and it has good reversibility. The depen-dence of the gas response of the �-Fe2O3 nanorods on the LPGconcentration at the operating temperature 300 ◦C is shown inFig. 12. The gas response increases approximately linearly as theLPG concentration increases from 5 to 60 ppm. The linearity of thegas response suggests that the �-Fe2O3 nanorods can be reliablyused to monitor the concentration of LPG over this range. The �-Fe2O3 nanorods are able to detect up to 5 ppm for LPG with goodresponse (S ∼15) at the operating temperature 300 ◦C.

Selectivity is an important parameter of gas sensors and the

gas response towards a specific gas needs to be markedly higherthan those to other gases for selective gas detection. To study theselective behavior of the �-Fe2O3 nanorods to LPG at an operat-ing temperature of 300 ◦C, the gas response towards H2, CO, CO2

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Fig. 12. Relationship between gas response of �-Fe2O3 nanorods and LPG concen-tration.

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CO2

CO Ethanol H2

LPG10

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asnsec

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ig. 13. Bar chart showing the gas response of �-Fe2O3 nanorods for different gases.he gas concentration and operating temperature in all cases were 50 ppm and00 ◦C, respectively.

nd ethanol with concentration of 50 ppm each were also mea-ured. The corresponding results are shown in Fig. 13. The �-Fe2O3anorods exhibit higher response to LPG (S ∼1750), whereas ithows a considerably lower response (< 7.62) to H2, CO, CO2 andthanol. In order to quantify the selectivity to LPG, the selectivityoefficient (K) of LPG to another gas, which is defined as [38]:

= SLPG

Sgas(3)

here SLPG and Sgas are the responses of sensors in LPG and anotheras, respectively was calculated. The selectivity coefficients for the-Fe2O3 nanorods were 229.34 to H2, 171.10 to CO2, 440.39 toO and 257.77 to ethanol. Higher K values imply the more selec-ive detection to LPG in the presence of other gases. For example,= 229.34 for H2 indicates that the gas response to LPG is 229.34

imes higher than that to H2. Thus, the experimental results indi-ate that the �-Fe2O3 nanorods based sensor has a good selectivityo LPG. Based on the observed results, it can be concluded that theormation of �-Fe2O3 nanorods is not only effective in enhancinghe LPG response but also in making it selective for the detectionf LPG.

Thus, the results of gas sensing experiments reveal that the �-e2O3 nanorods have good LPG sensing properties such as higheras response, good selectivity, short response time, good recovery,xcellent repeatability and lower operating temperature.

.5. LPG sensing mechanism

The LPG sensing mechanism of �-Fe2O3 nanorods is a surfaceontrolled process. It is based on the changes in the resistance of �-e2O3 nanorods, which is controlled by the LPG species and amountf the chemisorbed oxygen on the surface [39–46]. The adsorp-ion of the test gases, which depends on both the type of the testas and the sensor material, affects both the response character-stics and response time. However, the LPG sensing mechanism iscomplex process and it is believed that it proceeds through sev-

ral intermediate steps which are not yet understood. �-Fe2O3 isn n-type semiconductor, in which electrons are the majority car-

iers. When �-Fe2O3 nanorods are exposed to air, a certain amountf oxygen from air adsorb on its surface. The �-Fe2O3 nanorodsnteract with the oxygen, by transferring the electrons from theonduction band to adsorbed oxygen atoms, resulting into the for-ation of ionic species such as O2

− or O−. The reaction kinematics

ors B 152 (2011) 299–306 305

may be explained by the following reactions [39–46]

O2(gas) ↔ O2(absorbed) (4)

O2(absorbed) + e− ↔ O2− (5)

O2− + e− ↔ 2O− (6)

The adsorbed oxygen species on �-Fe2O3 nanorods act as elec-tron acceptors that generate a surface depletion layer and thus,resistance of �-Fe2O3 nanorods increases.

It is well known that the LPG consists of CH4, C3H8 and C4H10 etc.and in these molecules the reducing hydrogen species are boundto carbon therefore the LPG dissociates less easily into the reac-tive reducing components on the �-Fe2O3 nanorods surface. Whenexposed to reducing gas like LPG, the gas molecules are chemi-adsorbed at the active sites on the surface of the �-Fe2O3 nanorods.These molecules react with the adsorbed oxygen species and latticeoxygen (O2

−) of �-Fe2O3 nanorods at the operating temperature,thereby releasing the trapped electrons back to the conductionband which decreases the depletion layer width and the resistanceof the �-Fe2O3 nanorods [39–46]. The overall reaction of ionic oxy-gen species with LPG may be described by

CnH2n+1 + 2O− → H2O + CnH2nO− + e− (7)

When �-Fe2O3 nanorods are heated at a temperature of 50–250 ◦C,the reaction products do not desorb from its surface. Nevertheless,they cover the sensing sites on the surface of �-Fe2O3 nanorods,which prevents the further reaction of the LPG with chemisorbedoxygen. Subsequently, no appreciable change in the resistance ofthe �-Fe2O3 nanorods is observed.

At temperature 300 ◦C, the reaction products may get des-orbed immediately after their formation providing the opportunityfor new gas species to react with the sensing sites on the sur-face of �-Fe2O3 nanorods. Thus, the LPG reacts most effectivelywith chemisorbed oxygen at such particular temperature, whichresults in the significant decrease in the resistance of the �-Fe2O3nanorods. Therefore, the maximum sensitivity of the �-Fe2O3nanorods towards LPG is expected at such a particular temperature.

At higher temperatures (>300 ◦C), the amount of the adsorbedoxygen is less and therefore, a lesser amount of ionic species areformed. Therefore, in the presence of the LPG, the probability ofthe reduction reaction of the gas with chemisorbed oxygen is less,which results into a very small change in resistance of the �-Fe2O3nanorods at higher temperatures. Therefore, the �-Fe2O3 operatesas a sensing element to the LPG only within a specific temperaturewindow. In the present case, the optimum operating temperaturefor the �-Fe2O3 nanorods was 300 ◦C at which the gas responseattains its maximum value. The enhanced LPG gas sensing perfor-mance of �-Fe2O3 nanorods over that of commercially available�-Fe2O3 powder may be attributed to their smaller size and higherspecific surface area of the rod shaped morphology, which can pro-vide more adsorption-desorption sites for LPG molecules.

4. Conclusions

The �-Fe2O3 nanorods were successfully synthesized withoutany templates by calcining the �-FeOOH precursor in air at 300 ◦Cfor 2 h and their LPG sensing characteristics were investigated. The�-FeOOH precursor was prepared through a simple and low costwet chemical route at low temperature (40 ◦C) using FeSO4·7H2Owas used as the source of Fe2+ and the CH3COONa was used as

the precipitating agent to release hydroxyl ions slowly during thereaction. The formation of �-FeOOH precursor and its topotactictransformation to �-Fe2O3 upon calcination was confirmed by XRD,FTIR, TGA, XPS, SEM and TEM analysis. The gas response of the �-Fe2O3 nanorods to 50 ppm of LPG is maximum at the operating

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Maharashtra University, Jalgaon, India. He received his master’s degree (M.Sc.) in

06 D. Patil et al. / Sensors and

emperature 300 ◦C and it is found to be ∼1746. The response timeas nearly 3–4 s and the recovery time was found to be ∼8–9 min.

he synthesized �-Fe2O3 nanorods are able to detect up to 5 ppmor LPG with reasonable response (∼15) at the operating tempera-ure 300 ◦C. Further, it was shown that the �-Fe2O3 nanorods cane reliably used to monitor the concentration of LPG over the range5–60 ppm). Due to the fact that they have excellent LPG sensingharacteristics and can be synthesized easily, the �-Fe2O3 nanorodsould be an ideal candidate for application in LPG sensors.

cknowledgements

The financial support from University Grants CommissionUGC), New Delhi, India through the major research project no. F4-3/2008 (SR) is gratefully acknowledged. We also thank Mr. T.ai Kamaraju and Mr. B. V. Cholkar, Forevision Instruments (India)vt. Ltd. for FESEM analysis of the samples.

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Biographies

Dewyani Patil is presently working as post-doctoral fellow in Department of Mate-rials Science and Engineering, Chungnam National University, Daejeon, South Korea.She received her master’s degree (M.Sc.) in Electronics in 2004 and Ph.D. in Physicsin 2010 from North Maharashtra University, Jalgaon, India. Her research interestsinclude conducting polymers and conducting polymer composites for chemical andbiological sensors.

Virendra Patil is working as project assistant in National Chemical Laboratory, Pune,India. He received his master’s degree (M.Sc.) in Physical Chemistry in 2009 fromUniversity of Pune, Pune, India. His research interests include preparation of 1-Dnanostructured semiconducting metal oxides for the fabrication of gas sensors.

Pradip Patil is working as professor in Physics at Department of Physics, North

Physics in 1983 and Ph.D. degree in 1988 from University of Pune, Pune, India. Hejoined North Maharashtra University, Jalgaon, India as the Head, Department ofPhysics since its inception. His main interests include development of conduct-ing polymers and conducting polymer nanocomposites for their applications ascorrosive protective coatings, chemical and biological sensors.