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Current Applied Physics 12 (2012) 1355e1360

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Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Gas sensor based on nanoporous hematite nanoparticles: Effect of synthesispathways on morphology and gas sensing properties

Nguyen Duc Cuong a,b,c, Tran Thai Hoa c, Dinh Quang Khieu c, Nguyen Duc Hoa a,*, Nguyen Van Hieu a,*

a International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet, Hanoi, Vietnamb Faculty of Hospitality and Tourism, Hue University, 22 Lam Hoang, Vy Da Ward, Hue City, VietnamcCollege of Sciences, Hue University, 77 Nguyen Hue, Phu Nhuan Ward, Hue City, Vietnam

a r t i c l e i n f o

Article history:Received 12 January 2012Received in revised form27 February 2012Accepted 21 March 2012Available online 29 March 2012

Keywords:NanoporousFe2O3

Gas sensorSynthesis

* Corresponding authors. Tel.: þ84 4 38680787; faxE-mail addresses: ndhoa@itims.edu.vn (N.D. Hoa

Hieu).

1567-1739/$ e see front matter � 2012 Elsevier B.V.doi:10.1016/j.cap.2012.03.026

a b s t r a c t

The development of a low cost and scalable gas sensor for the detection of toxic and flammable gaseswith fast response and high sensitivity is extremely important for monitoring environmental pollution.In this work, we introduce two different synthesis pathways for the preparation of scalable Fe2O3

nanoparticles for gas sensor applications. One is co-precipitation and the other is hydrothermal method.The gas sensing properties of the a-Fe2O3 nanoparticles (NPs) fabricated by different synthesis pathwayswere studied and compared. The performance of the NPs in the detection of toxic and flammable gasessuch as carbon dioxide, ammonia, liquefied petroleum gas, ethanol, and hydrogen was evaluated. TheFe2O3 NP-based gas sensors exhibited high sensitivity and a response time of less than a minute toanalytic gases. However, the NPs fabricated by the one-step direct method exhibited higher sensitivitiesthan those generated by the a-Fe2O3 NPs obtained by co-precipitation synthesis possibly because of theirnanoporous structure. This performance is attributed to the large specific surface area of the NPs, whichresults in higher sensitivity.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Air pollution caused by toxic and flammable gases, such as CO,NH3, NO2, and H2, is one of the critical problems contributory toglobal warming, climate change, and damage to human health [1].Therefore, the development of gas sensors for the detection of suchgases has received considerable interest in recent years [2].Essentially, resistive gas sensors based on different forms of metaloxides, organic compounds, and polymers have been developed formonitoring air quality; the studies focused mainly on theenhancement of gas sensor performance, as well as the realizationof fast response and recovery time, low cost, low powerconsumption, and high sensitivity [3].

The working mechanism of a resistive metal oxide-based gassensor relies on variations in the electrical conductivity of a sensinglayer; these variations are caused by surface reactions, such asoxidation or reduction, upon exposure to different gases. Becausethese surface reactions depend on the active centers and defectsexisting on the surface layer of materials, sensor response is usuallydetermined by the grain size and surface-to-volume ratio of

: þ84 4 38692963.), hieu@itims.edu.vn (N. Van

All rights reserved.

materials [4]. Therefore, the nanostructures of metal oxides havebeen investigated for gas sensor applications [5], in which both n-and p-type semiconductors such as Fe2O3 [6], WO3 [7], SnO2 [8],and CuO [9] exhibit a significant resistance change upon exposureto trace concentrations of reducing or oxidizing gases. Of thesematerials, different iron oxides (Fe2O3) phases [i.e., a-Fe2O3(hematite), b-Fe2O3,g-Fe2O3 (maghemite), and e -Fe2O3] have alsobeen studied for different applications [10,11]. Hematite (a-Fe2O3)is one of the most stable phases with n-type semiconductingproperties (Eg: w2.1 eV); it has been widely used as a catalyst,pigment, gas sensor, and electrode material owing to its low cost,high resistance to corrosion, and environment-friendly properties[12,13]. Given the excellent physical and chemical properties of a-Fe2O3, considerable attention has been directed to its controlled-synthesis nanostructures, such as nanopropellers [14], nanorods,nanoporous, nanoleaflets [15], and nanospheres [16], as well asnanocubic [17] and hexagonal platelets [18]. Furthermore, becauseof its affordability, good stability, and reversibility, a-Fe2O3 hasbeen proven an important semiconducting material for gas sensorapplications. Gas sensors based on a-Fe2O3 nanostructures havebeen explored by many researchers. However, the conventionalmaterials have low specific surface areas and low sensitivity totarget gases, creating the need for developing a-Fe2O3 nano-structures with high surface areas.

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Fig. 1. XRD patterns of the as-synthesized Fe3O4 (A) and a-Fe2O3 (B) NPs fabricated bytwo-step post-synthesis.

N.D. Cuong et al. / Current Applied Physics 12 (2012) 1355e13601356

Herein, we report the controlled-synthesis of scalable nano-porous a-Fe2O3 NPs for gas sensor applications. The nanoporous a-Fe2O3 NPs were synthesized by a one-step facile and scalablehydrothermal method. Their gas sensing properties were investi-gated and compared with those of condensed a-Fe2O3 NPs fabri-cated by post-synthesis, in which the fabrication included the co-precipitation of Fe3O4 and calcinations to generate a-Fe2O3 NPs.The effects of synthesis pathways on the morphologies and gassensing properties of the materials were investigated for thedetection of CO, NH3, and H2, among others. Our findings revealthat the nanoporous NPs exhibited higher sensitivity comparedwith that of the condensed counterpart. The developed sensorscould detect low concentrations of analytic gases at the ppm level.

2. Experimental

2.1. Co-precipitation synthesis of Fe2O3 nanoparticles

The Fe2O3 NPs were synthesized by a co-precipitation methodand subsequent calcinations at high temperature. First, the Fe3O4NPs were fabricated by a co-precipitation method using ferricchlorides (Fe3þ) and ferrous chlorides (Fe2þ) as precursors [19]. Inbrief, ferric chlorides (2 mmol) and ferrous chlorides (1 mmol)(molar ratio, 2:1) were dissolved in 100 ml HCl (pH ¼ 2). Thechemical precipitation was achieved by slowly adding a 0.1 Msolution of NaOH with vigorous stirring for 30 min at 80 �C. Theprecipitated Fe3O4 products were recovered by filtering, washing,and drying at 60 �C. The obtained Fe3O4 NPs were loaded in analumina boat and inserted into a tube furnace for calcinations at600 �C for about 5 h.

2.2. Hydrothermal synthesis of nanoporous Fe2O3 nanoparticles

Nanoporous Fe2O3 NPs were synthesized by a facile and scalablehydrothermal method. In a typical experiment, ferric nitrate(2 mmol), cetyltrimethylammonium bromide (1 g) and urea(15 mmol) were dissolved in 35 ml of distilled water by magneticstirring at room temperature for 2 h to obtain a slurry solution. Theslurry solution was then transferred into a 200 ml Teflon-linedautoclave for hydrothermal processing. The hydrothermal process-ing was carried out at 80 �C for 36 h and then naturally cooled toroom temperature. The solid products were collected and washedwith distilled water and ethanol several times by centrifugation toensure the total removal of the un-reacted inorganic ions andsurfactant. The collected powders were dried at 60 �C in air.

2.3. Material characterization

The microstructures and morphologies of the as-synthesizedFe3O4 and a-Fe2O3 nanostructures were characterized by X-raydiffraction (XRD, D8 Advance, Brucker, Germany), scanning elec-tron microscopy (SEM), and transmission electron microscopy(TEM). For SEM characterization, the powders of the synthesizedmaterials were attached onto an SEM holder by a carbon tape, andthen 10 nm Pt was coated onto it to prevent electrostatic chargegeneration. The SEM images were recorded at an accelerationvoltage of 15 kV. The TEM samples were prepared by dispersing thesynthesized materials in ethanol solution using an ultrasoniccleaner. Thereafter, the solution was dropped onto a carbon-coatedCu grid for TEM characterization.

2.4. Sensor fabrication and gas sensing measurements

The gas sensors were fabricated by a thick film technique, inwhich the synthesized NPs were dispersed in ethanol solution, and

then drop-casted onto an interdigitated electrode substrate.Thereafter, the sensors were heat treated at high temperature toincrease the adhesion between the sensing materials andsubstrates. The gas sensing properties of the sensors were inves-tigated for the detection of different gases such as H2(25e500 ppm), CO (10e100 ppm), C2H5OH (50e500 ppm), and NH3(50e5000 ppm) at different temperatures (300e400 �C) usinga homemade setup system with a high speed switching gas flow(from/to air to/from balance gas). Balance gases (0.1% in air) werepurchased from Air Liquid Group (Singapore). The system employsa flow-through with a constant rate of 200 sccm, as reported in ref.[20]. During sensing measurement, the resistance of the nano-sensors was automatically recorded through Keithley controlled bya computer via a software program. The sensor response wasdefined as S ¼ R0/R, where R0 and R was the resistance of sensormeasured in air and in analytic gas, respectively.

3. Results and discussion

3.1. Material characteristics

The phase formation, and crystal structure of the as-synthesizedand calcinated NPs fabricated by the co-precipitation synthesispathway and investigated by XRD are shown in Fig.1 (A,B). The XRDpattern of the as-synthesized NPs exhibit a face-centered cubicprofile typical of the Fe3O4 crystal structure (JCPDS No. 65-3107).The main peaks can be indexed to the (220) (311) (400) (422) (422)(511), and (440) reflections [Fig. 1(A)]. However, after calcination at600 �C for 5 h [Fig. 1(B)], the cubic Fe3O4 was converted into a-Fe2O3 phase. The main peaks of a-Fe2O3 are indexed to a rhombo-hedral profile characteristics of the a-Fe2O3 crystal structure (JCPDSNo. 81e2810). No detectable peak of impurities and other phaseswas observed, indicating the formation of single-phase a-Fe2O3.The average crystalline sizes of the Fe3O4 and a-Fe2O3 NPs calcu-lated from the XRD data using the Scherrer equation (d ¼ 0.9l/(bcos q)) are about 15 and 20 nm, respectively [17].

The morphologies of the a-Fe2O3 NPs fabricated by co-precipitation post-synthesis were characterized by SEM and TEM(Fig. 2). The a-Fe2O3 NPs are irregularly shaped and they areaggregated because of the grain growth that occurred at a highcalcination temperature. The TEM image also confirms that the a-

Fig. 2. (A) SEM and (B) TEM images of the Fe2O3 NPs fabricated by two-step post-synthesis.

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Fig. 3. XRD patterns of the Fe2O3 NPs fabricated by the one-step direct hydrothermalpathway: (A) as-synthesized; (B) Calcinated NPs.

Fig. 4. (A) SEM and (B) TEM images of the nanoporous Fe2O3 NPs fabricated by theone-step direct hydrothermal synthesis pathway.

N.D. Cuong et al. / Current Applied Physics 12 (2012) 1355e1360 1357

Fe2O3 NPs have a condensed structure without any observation ofnanopores inside the NPs. The particle size estimated by TEM imagewas about 150 nm, which is larger than the value calculated fromthe XRD data. This result indicates that the NPs are not singlecrystals but are polycrystalline in nature.

In contrast to the NPs fabricated by the co-precipitationsynthesis pathway, the as-synthesized and calcinated NPsprepared by the one-step direct hydrothermal method havea rhombohedral a-Fe2O3 structure, as confirmed by the XRDpatterns (Fig. 3). No impurity peak in the calcinated NPs wereobserved, indicating that calcination does not change the crystalstructure of a-Fe2O3.

The Fe2O3 NPs fabricated by hydrothermal synthesis have veryhomogenous morphologies with nearly spherical shapes, as shownin Fig. 4. The synthesized NPs are well separated but not asaggregated as the products obtained by co-precipitation synthesis.The Fe2O3 NPs fabricated by hydrothermal synthesis have anaverage particle size of about 100 nm [Fig. 4(A)]. The TEM imageindicates that the Fe2O3 NPs have nanopores of less than about10 nm distributed randomly inside the NPs [Fig. 4(B)].

3.2. Gas sensing properties

The CO gas sensing properties (measured at different tempera-tures) of the sensor based on the Fe2O3 NPs synthesized by co-precipitation synthesis are shown in Fig. 5 (A). The sensor could

detect CO gas at low ppm concentrations at all measured temper-atures. However, the sensor exhibited the highest sensitivity at350 �C, in which the sensor resistance decreased rapidly uponexposure to CO gas and reached saturation within a minute. After

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Fig. 5. CO sensing properties of the Fe2O3 NPs fabricated by two different synthesispathways measured at different temperatures: (A) Two-step post-synthesis; (B) One-step direct hydrothermal method.

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Fig. 6. H2 sensing properties of the Fe2O3 NPs fabricated by two different synthesispathways measured at different temperatures: (A) Two-step post-synthesis; (B) One-step direct hydrothermal method.

N.D. Cuong et al. / Current Applied Physics 12 (2012) 1355e13601358

switching from CO to air, the sensor resistance recovered to theinitial resistance, indicating the reversible interaction between COmolecules and the surface of the Fe2O3 sensing layer. The sensorresponse (Ro/R) increased from 1.3 to 2.45 when CO concentrationincreased from 10 to 100 ppm.

The gas sensing properties of the nanoporous Fe2O3 NPssynthesized by hydrothermal synthesis pathway are shown inFig. 5(B). The nanoporous Fe2O3 NP-based sensor exhibited sensingproperties similar to those of the NPs fabricated by co-precipitationsynthesis (the condensed one). The sensor response (Ro/R) to10 ppm CO was approximately 5, which is 3.8-fold higher than theresponse of the condensed Fe2O3 NPs. The sensitivities of thenanoporous Fe2O3 NPs fabricated by direct synthesis were muchhigher than those of the NPs prepared by the co-precipitationmethod.

The H2 sensing properties of synthesized nanosensors areshown in Fig. 6. Both nanosensors exhibited a similar trend ofresponse to that with CO gas, where the sensor resistancedecreased rapidly upon exposure to H2 gas. The nanosensors coulddetect H2 gas at a very low concentration of ppm level, which ismuch lower than the “lower explosive limit (w4%)”, suggestinga possibility of using for practical application in monitoring of

hydrogen leak in air. The sensor response of the porous Fe2O3 NPswas higher than that of the condensed once, whereas the sensi-tivity to 500 ppm H2 concentration measured at 300 �C was about27 and 4.5 for the porous and condensed nanosensors, respectively.The higher sensitivity of the porous Fe2O3 nanosensor was possiblydue to the porous structure of Fe2O3 NPs which provided largersensing sites for gas adsorption and resulted in higher sensitivity.

The other gas sensing properties of Fe3O4 NPs prepared by thetwo different synthesis pathways at different concentrations of H2,NH3, and PLG gases are shown in Fig. 7. The sensitivities of thenanoporous Fe2O3 NPs to different gases were higher than those ofthe condensed counterpart. In particular, the sensitivity of thenanoporous Fe2O3 NPs to H2 was the highest compared with thevalues exhibited by the condenses NPs and other gases. This resultcan be explained by the difference in the diffusion of H2 and othergases into the nanopores of nanoporous Fe2O3. Because H2 mole-cules have smaller size, thus they could diffuse much faster in thenanopores than other larger gas molecules, and resulted in highersensitivity. This result is consistent with other reports about the gassensing properties of porous materials, in which the response to H2was higher than others such as CO, CH4 etc [21,22]. Furthermore,the sensor response (R0/R) of the nanoporous Fe2O3 NPs increasedlinearly with H2 concentration under the measured ranges(25e500 ppm). This result indicates that the nanoporous Fe2O3 NPs

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Fig. 7. Gas sensing properties of the Fe2O3 NPs prepared by two-step post-synthesis (A, C, E), and one-step direct hydrothermal method (B, D, F).

N.D. Cuong et al. / Current Applied Physics 12 (2012) 1355e1360 1359

N.D. Cuong et al. / Current Applied Physics 12 (2012) 1355e13601360

are highly promising materials for the detection of hydrogen at lowconcentrations.

The sensingmechanism of the Fe2O3 NP-based gas sensors can beexplained by the depletion region. During the gas sensing measure-ment, the oxygen in the air captured the electrons from the Fe2O3crystal and ion-adsorbed (O�

2 ;O� and O2�) on the surface of the

sensing layer; this phenomenon resulted in the formation of theelectron-depletion region [23]. Upon exposure to reducing gases,such as CO, the CO molecules interacted with the pre-adsorbedoxygen and released electrons, according to the equa-tionCOþ O�ðadsÞ4CO2 þ e�. The release of free electrons increasedthe total carrier and reduced the electron-depletion region, resultingin a decrease in sensor resistance. After the analytic gas flow wasdiscontinued, the adsorption of oxygenmolecules onto the surface ofthe sensing layer returned the sensor resistance to the initial value.

4. Conclusion

For thepractical application of gas sensors in the detection of toxicand/orflammable gases, the development of suitable sensing deviceswith lowcost, fast response, and high sensitivity is very important. Inthis work, we introduced two different synthesis pathways for thescalable synthesis ofa-Fe2O3NPs for effective gas sensor applications.The effects of synthesis pathways on the morphologies and gassensing properties of NPs were investigated and discussed. Ourexperiments demonstrated that the nanoporous a-Fe2O3 NPs fabri-cated by the one-step direct hydrothermal method are highlypromising materials for the detection of toxic and flammable gases,despite the selectivity of sensors needed to be improved.

Acknowledgments

The current work was financially supported by the Application-oriented Basic Research Program (2009e2012, Code: 05/09/HÐ-DTÐL). N.V. Hieu also would like to acknowledge the financialsupported by National Key Research Program on Science andTechnology under Contract No. 10/2011/HÐ-ÐTTN-KC02/11-15.

References

[1] N.D. Hoa, N.V. Quy, Y. Cho, D. Kim, Porous single-wall carbon nanotube filmsformed by in-situ arc-discharge deposition for gas sensors application, Sens.Actuators B 135 (2009) 656e663.

[2] D. Yang, I. Kamienchick, D.Y. Youn, A. Rothschild, I.D. Kim, Ultrasensitive andhighly selective gas sensors based on electrospun SnO2 nanofibers modifiedby Pd loading, Adv. Funct. Mater. 20 (2010) 4258e4264.

[3] J.H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures:Overview, Sens. Actuators B 140 (2009) 319e336.

[4] Y. Shimizu, A. Jono, T. Hyodo, M. Egashira, Preparation of large mesoporousSnO2 powder for gas sensor application, Sens. Actuators B 108 (2005) 56e61.

[5] R.L. VanderWal, G.W. Hunterb, J.C. Xu, M.J. Kulis, G.M. Berger, T.M. Ticich,Metal-oxide nanostructure and gas-sensing performance, Sens. Actuators B138 (2009) 113e119.

[6] B. Sun, J. Horvat, H.S. Kim, W.S. Kim, J. Ahn, G. Wang, Synthesis of mesoporousa-Fe2O3 nanostructures for highly sensitive gas sensors and high CapacityAnode materials in Lithium ion Batteries, J. Phys. Chem. C 114 (2010)18753e18761.

[7] N.V. Hieu, V.V. Quang, N.D. Hoa, D.J. Kim, Preparing large-scale WO3nanowire-like structure for high sensitivity NH3 gas sensor through a simpleroute, Curr. App. Phys. 11 (2011) 657e661.

[8] B. Wang, L.F. Zhu, Y.H. Yang, N.S. Xu, G.W. Yang, Fabrication of a SnO2nanowire gas sensor and sensor performance for hydrogen, J. Phys. Chem. C112 (2008) 6643e6647.

[9] N.D. Hoa, N.V. Quy, H. Jung, D. Kim, H. Kim, S.K. Hong, Synthesis of porous CuOnanowires and its application to hydrogen detection, Sens. Actuators B 146(2010) 266e272.

[10] R.M. Cornell, U. Schwertmann, The Iron Oxide: Structure, Properties, Reaction,Occurrences and Uses, Wiley-VCH, Germany, 2003, ISBN 978-3-527-60644-3,29e32.

[11] Z. Jing, Y. Wang, S. Wu, Preparation and gas sensing properties of pure anddoped g-Fe2O3 by an anhydrous solvent method, Sens. Actuators B 113 (2006)177e181.

[12] C. Wu, P. Yin, X. Zhu, C. OuYang, Y. Xie, Synthesis of hematite (a-Fe2O3)Nanorods: Diameter-size and shape effects on their applications in Magne-tism, Lithium ion Battery, and gas sensors, J. Phys. Chem. B 110 (2006)17806e17812.

[13] F. Zhang, H. Yang, X. Xie, L. Li, L. Zhang, J. Yu, H. Zhao, B. Liu, Controlledsynthesis and gas-sensing properties of hollow sea urchin-like a-Fe2O3nanostructures and a-Fe2O3 nanocubes, Sens. Actuators B 141 (2009)381e389.

[14] W.H. Yang, C.F. Lee, H.Y. Tang, D.B. Shieh, C.S. Yeh, Iron oxide nanopropellersprepared by a low-temperature solution approach, J. Phys. Chem. B 110(2006) 14087e14091.

[15] Z. Zhang, Md. F. Hossain, T. Takahashi, Fabrication of shape-controlled a-Fe2O3nanostructures by sonoelectrochemical anodization for visible light photo-catalytic application, Mater. Lett. 64 (2010) 435e438.

[16] L. Song, S. Zhang, Formation of a-Fe2O3/FeOOH nanostructures with variousmorphologies by a hydrothermal route and their photocatalytic properties,Coll. Surf. A 348 (2009) 217e220.

[17] Y. Wang, H. Yang, Synthesis of iron oxide nanorods and nanocubes in animidazolium ionic liquid, Chem. Eng. J. 147 (2009) 71e78.

[18] D. Peng, S. Beysen, Q. Li, Y. Sun, Linyu Yang, Hydrothermal synthesis ofmonodisperse a-Fe2O3 hexagonal platelets, Particuology 8 (2010) 386e389.

[19] A.H. Lu, E.L. Salabas, F. Schuth, Magnetic nanoparticles: synthesis, Protection,Functionalization, and application, Angew. Chem. Int. Ed. 46 (2007)1222e1244.

[20] L.V. Thong, L.T.N. Loan, N.V. Hieu, Comparative study of gas sensor perfor-mance of SnO2 nanowires and their hierarchical nanostructures, Sens. Actu-ators B 150 (2010) 112e119.

[21] J. Yang, K. Hidajat, S. Kawi, Synthesis, characterization and sensing propertiesof nano-SnO2 supported on SBA-15 as highly sensitive semiconductor gassensors, J. Mater. Chem. 19 (2009) 292e298.

[22] G. Li, S. Kawi, SMCM-41 modified SnO2 gas sensors: sensitivity and selectivityproperties, Sens. Actuators B 59 (1999) 1e8.

[23] D. Patil, V. Patil, P. Patil, Highly sensitive and selective LPG sensor based ona-Fe2O3 nanorods, Sens. Actuators B 152 (2011) 299e306.