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Sensors and Actuators B 158 (2011) 246– 251

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

j o ur nal homep a ge: www.elsev ier .com/ locate /snb

ydrocarbon gas sensing of nano-crystalline perovskite oxides LnFeO3

Ln = La, Nd and Sm)

o Truong Giang ∗, Ha Thai Duy, Pham Quang Ngan, Giang Hong Thai, Do Thi Anh Thu,o Thi Thu, Nguyen Ngoc Toan

nstitute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam

r t i c l e i n f o

rticle history:eceived 10 March 2011eceived in revised form 31 May 2011

a b s t r a c t

In this paper, we report the properties of the nano-crystalline oxides LnFeO3 (Ln = La, Nd and Sm) synthe-sized by sol–gel citrate technique. The crystalline structure of the oxides was analyzed by X-ray diffraction(XRD). The surface morphologies of the LnFeO3 thick-films on Al2O3 substrates were measured by scan-

ccepted 2 June 2011vailable online 13 June 2011

eywords:ano-perovskite oxidesemiconductor gas sensorsydrocarbon gases

ning electron microscope (SEM). The gas-sensing characteristics of the oxides LnFeO3 were investigatedin methane (CH4), propane (C3H8) and n-hexane (C6H14) gases. All LnFeO3-based sensors exhibited goodHC sensing characteristics. The very high n-hexane sensitivities of NdFeO3 and SmFeO3-based sensorswere discussed in terms of an important role of rare earth elements.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

Semiconductor gas sensors based on metal oxides have beenidely used to detect and monitor flammable, explosive, poisonous

nd harmful gases. Remarkable advantages of gas-sensing applica-ions of metal oxides, like SnO2 [1,2], ZnO [3,4], In2O3 [4,5], TiO26], are low cost, high sensitivity and stability.

It is well known that nano-crystalline perovskite oxides ABO3A = rare earth metals, B = 3d transition metals and O = oxygen)ave been studied on gas sensor applications. Among these per-vskite oxides, LnFeO3 (Ln = La [7–9], Nd [10] and Sm [11–13])ere reported to have good gas-sensing properties for reduc-

ng/oxidizing gases. For examples, gas sensors based on oxidesaFeO3, NdFeO3, and SmFeO3 exhibited high sensitivities to carbononoxide (CO) [7,13,14], nitrogen oxides NOx [12,15,16] and alco-

ol (C2H5OH) [17–21]. However, these oxides exhibited fairly lowensitivities to hydrocarbon gases (HCs) [7–9,22] and were usuallytudied on applications in hydrocarbon conversion due to their highatalytic activity [23–25]. Nevertheless, perovskite oxides contin-ously attract a great deal of interests for possible applications inas sensors. In this paper, the results of our study on the responsesf nano-oxides LnFeO3 (Ln = La, Nd, and Sm) based semiconduc-

or gas sensors to several hydrocarbon gases are reported andiscussed.

∗ Corresponding author. Tel.: +84 4 37569318.E-mail address: gianght@ims.vast.ac.vn (H.T. Giang).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.06.013

2. Experimental

The powder oxides LnFeO3 (Ln = La, Nd, and Sm) were synthe-sized by sol–gel citrate method. Ln(NO3)3·6H2O, Fe(NO3)3·9H2O(analytical reagents, Merck-Germany) and citric acid were dis-solved in solutions of ethylene glycol at molar ratios of {Ln3+,Fe3+}:{citric acid}:{ethylene glycol} = 1:1:4. The solutions werestirred at 80 ◦C for 4 h to get homogeneous mixtures and then themixtures were dried at 150 ◦C to obtain forms of gel precursors.After that, the gels were ground to fine powders. Finally, the pow-ders were calcined at 700 ◦C for 3 h to obtain LnFeO3 powders usedfor subsequent analyses.

To fabricate the sensors, the powders LnFeO3 were mixedwith the organic binder (mixture of �-terpineol, antaroxand ethyl-cellulose at weight percent ratio of 95:2:3, respec-tively) and then screen-printed on Al2O3 substrates (dimen-sion: 6.3 mm × 6.3 mm × 0.25 mm, AdValue Technology-USA) withcomb-type platinum electrodes to create thick-films. The operatingtemperature of the sensors was applied by a mini heater installedon other side of the Al2O3 substrates. The films were calcined at500 ◦C for 1 h to get LnFeO3-based semiconductor gas sensors.

The crystalline structure of synthesized powders LnFeO3 wascharacterized by X-ray diffraction (Siemens D5000) using CuK�radiation. The surface morphologies and thicknesses of the LnFeO3thick-films on Al2O3 substrates were measured by scanning elec-

tron microscope (SEM, HITACHI S-4800).

The analytical gas sources of the Singapore Oxygen Air LiquidePte Ltd. (SOXAL) were used for analyzing the gas-sensing charac-teristics of these sensors.

H.T. Giang et al. / Sensors and Actuators B 158 (2011) 246– 251 247

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ig. 1. XRD patterns of the powders LnFeO3 synthesized by sol–gel citrate technique.

All LnFeO3-based sensors were simultaneously investigated forhe gas-sensing characteristics in a chamber with 1000 mL in vol-me. The total flow rate of gases through the chamber was fixed00 mL/min.

. Results and discussion

X-ray diffraction patterns of the synthesized powder LnFeO3Ln = La, Nd, and Sm) were shown in Fig. 1. It is obvious that allatterns express the structures of the samples being orthorhombicerovskite type. The lattice constants of the oxides were calculatedrom the XRD patterns. The crystalline particle sizes (D) of thesexides were derived from the XRD peaks according to Scherrer’squation:

= 0.89�

cos �(1)

here � is the wavelength of X-ray, � is the diffraction angle and ishe true half-peak width. The Goldschmidt’s tolerance factor (t) [26]as used to evaluate the distortion of ABO3 crystallite structure, asefined in the equation:

= rA + rO√2(rB + rO)

(2)

here rA is the ionic radius of A, rB is the ionic radius of B, and rOs the ionic radius of oxygen. The tolerance factors of the oxidesnFeO3 were derived by replacing the values of ionic radii of La3+

0.136 A), Nd3+ (0.127 A), Sm3+ (0.124 A), Fe3+ (0.645 A) and O2−

0.140 A) [27] into Eq. (2). The lattice constants, the average crys-alline particle sizes and the tolerance factors of the oxides LnFeO3ere listed in Table 1. The results show that the structural distor-

ion of the oxides LnFeO3 increases with decreasing the ion radii of

are earth elements. At the same synthesis conditions of materials,he distortion of structures could restrict the growth of crystal-ite, which caused the decrease of the crystalline size of the oxidesn sequence LaFeO3, NdFeO3 and SmFeO3 as shown in Table 1.

able 1he lattice constants and tolerance factors of the powders LnFeO3.

Sample Latticeconstant (nm)

Crystallite size (nm) Tolerancefactor (t)

a b c

LaFeO3 0.5557 0.7837 0.5548 15 0.9543NdFeO3 0.5489 0.7701 0.5412 12 0.9232SmFeO3 0.5347 0.7643 0.5487 10 0.9128

Fig. 2. The surface morphologies of thick-films LaFeO3 (a), NdFeO3 (b) and SmFeO3

(c) on Al2O3 substrates.

The surface morphologies of the LnFeO3 thick-films were shownin Fig. 2a–c. These results show that the nano-particles of LnFeO3oxides associate and uniformly distribute on the Al2O3 substrates.

However, it is found that the particle-size decrease in order fromLaFeO3, NdFeO3 to SmFeO3, in agreement with crystalline-size cal-culated from the XRD patterns. Cross-sectional view of the LaFeO3

248 H.T. Giang et al. / Sensors and Actuators B 158 (2011) 246– 251

fifi

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Fig. 3. The cross-sectional view of thick-film LaFeO3 on Al2O3 substrate.

lm on Al2O3 substrate was shown in Fig. 3. The thickness of thelm was defined to be about 40 �m.

Electrical resistances of the LnFeO3 based sensors at differentemperatures in pure air were shown in Fig. 4. It is observed thathe resistances of all the sensors decrease with increasing the tem-erature from 180 to 280 ◦C, which is the intrinsic characteristic ofemiconductor.

The responses (S) of the LnFeO3-based sensors to HCs wereefined as following equation:

(%) = RG − RA

RA× 100 (3)

here RG and RA are the resistances of the sensor in air contain-ng HCs and in pure air at the same temperature, respectively.t is well known that the sensitivity of semiconductor gas sen-or depends on operating temperature. Thus, the LnFeO3-basedensors were investigated in air containing some hydrocarbonases at different temperatures. The responses of these sensorso 200 ppm of methane, propane, n-hexane at different operat-ng temperatures were indicated in Fig. 5a, b and c, respectively.or each gas, the position of maximum response peaks of LnFeO3-ased sensors reveals a tendency to shift towards low temperatureange in the sequence La < Nd < Sm. The maximum responses of the

aFeO3, NdFeO3, and SmFeO3 based sensors to 200 ppm n-hexanere 110, 344, and 434%, respectively, while the values to 200 ppmropane are 30, 98, and 86%; and to 200 ppm methane are only 8,0 and 22%. It was clear that the LnFeO3 based sensors exhibited

ig. 4. The temperature dependence of resistance of the LnFeO3-based sensors inir.

Fig. 5. The responses of the LnFeO3-based sensors to 200 ppm of CH4 (a), C3H8 (b),and C6H14 (c) at different temperatures.

high sensitivities to n-hexane, especially in NdFeO3 and SmFeO3sensors, fairly good sensitivities to propane and low sensitivitiesto methane. The gas-sensing of the metal semiconductor oxidesdepends on numerous parameters such as crystalline structures,

electronic configurations, catalytic properties and absorbent prop-erties. By XPS investigation of perovskite-type LnFeO3 (Ln = La, Nd,Sm, Gd, and Dy), Aono et al. [16] indicated that the adsorbed oxy-

H.T. Giang et al. / Sensors and Actuators B 158 (2011) 246– 251 249

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ery times of LnFeO3 sensors decrease with increasing distortion

ig. 6. The resistance of LnFeO3-based sensors to different n-hexane concentrationst 200 ◦C.

en concentration on particle surface is the highest in SmFeO3 oxidend increases in sequence of LaFeO3, NdFeO3 and SmFeO3 oxides.eanwhile, the gas sensitivity of the metal oxide sensor strongly

epends on the adsorbed oxygen concentration. Furthermore, thetrength of carbon–hydrogen bonds in each molecule decreases inrder from methane, propane to n-hexane, hence n-hexane andropane easily react with the adsorbed oxygen (Oad) in compar-

son with methane. These analyses could be used to account foresponse characteristics of the LnFeO3 (Ln = La, Nd, and Sm) sen-ors to methane, propane and n-hexane as the results representedn Fig. 5.

Fig. 6 illustrates the resistance responses to different n-hexaneoncentrations (0, 100, 200, 300, and 400 ppm) of the LnFeO3-ased sensors at 200 ◦C. The resistance of the sensors increases with

ncreasing gas concentrations. The results demonstrate that thexides LnFeO3 have gas-sensing characteristics of p-type semicon-uctor. The responses (S) of the LnFeO3-based sensors to differentoncentrations of methane, propane and n-hexane were indicatedn Fig. 7a, b and c. In all the sensors, the responses increase with HConcentrations, as similar characteristics of CO-sensing of NdFeO310] or ethanol-sensing of La1−xPbxFeO3 [17] and LaMgxFe1−xO318]. However, the responses of these sensors reach the satura-ion values at methane concentration above 200 ppm, while theseharacteristics of the sensors to propane and n-hexane gases areot observed in range 0–400 ppm.

The response and recovery times of the LnFeO3-based sensorsere defined as the time needed to reach 90% of the resistance

aturation value in air containing n-hexane and the time neededo recover 10% of the original resistance value, respectively. Inemiconductor gas sensor, at certain operating temperature, theecovery time is usually longer than response time. Fig. 8 showshe response of resistances of the LnFeO3 based sensors to 200 ppm-hexane at 230 ◦C. The dependences of response and recoveryimes of the LnFeO3-based sensors on the operating temperaturen 200 ppm n-hexane were shown in Fig. 9. It is obvious thathe response and recovery times decrease with increasing theperating temperature. These are the common characteristics ofemiconductor gas sensor based on metal oxides. Especially, Fig. 9lso shows that the response and recovery times decrease withequence of LaFeO3, NdFeO3, and SmFeO3 based sensors. The radiif lanthanoid ions decrease with sequence La3+, Nd3+, and Sm3+,hich increase the distortion of orthorhombic structures of LnFeO3.

or a perfect crystalline structure, the lattice and adsorbed oxygenpecies may be difficult to separate or associate at the surface of thexides. Thus, it requires more energy to initiate the chemical reac-

Fig. 7. The responses of the LnFeO3-based sensors to gas concentration of methane(a), propane (b), and n-hexane (c).

tion between the adsorbed oxygen and reducing/oxidizing gases.At this point, it could be concluded that the response and recov-

of crystallite structure or decreasing the ionic radii of lanthanide.This relation is used for interpreting that the response and recov-ery times increase in the sequence of SmFeO3, NdFeO3 and LaFeO3

250 H.T. Giang et al. / Sensors and Actu

Fig. 8. The responses of the LnFeO3 sensors to 200 ppm n-hexane at 230 ◦C.

Fig. 9. The response and recovery times of the LnFeO3 sensors at different temper-a

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[25] B.P. Barbero, J.A. Gamboa, L.E. Cadus, Synthesis and characterisation ofLa1−xCaxFeO3 perovskite-type oxide catalysts for total oxidation of volatileorganic compounds, Applied Catalysis B 65 (2006) 21–30.

tures.

ensors, as shown in Fig. 9. However, dramatic increase of the dis-ortion of crystalline structure may decrease the stability of the gasensors. From Fig. 9, it is also found that the recovery time of theaFeO3-based sensor is very high at low temperature range. This is

disadvantage of the LaFeO3 oxide for applications in gas sensors.

. Conclusions

In summary, nano-crystalline oxides LnFeO3 (Ln = La, Nd, andm) with orthorhombic structure were synthesized by sol–gel cit-ate technique. The LnFeO3-based sensors were investigated for theas sensing characteristics in air containing methane, propane and-hexane gas. It is found that rare earth elements play the importantoles to gas sensitivities of nano-crystalline oxides LnFeO3. The oxy-en adsorption characteristics governed by the magnitude of theelative structural distortions of the LnFeO3 lattices as well as thetrength of carbon–hydrogen bonds in n-hexane compound maye the parameters to explain the extreme sensitivities to n-hexanef the NdFeO3 and SmFeO3 sensors. The decrease of response andecovery times in sequence of LaFeO3, NdFeO3 and SmFeO3 sensors

ould be accounted over the distortion of perovskite structure.

[

ators B 158 (2011) 246– 251

Acknowledgments

This work is supported in part by the grant-in-aid for scien-tific research from National Foundation for Science and TechnologyDevelopment (NAFOSTED), Department of Science and Technologyof Ha Noi (DOST); and Vietnam Academy of Science and Technology(VAST).

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iographies

o Truong Giang received his BSc degree in materials science from College of Nat-ral Science, Hanoi, Vietnam in 2001. In 2003, he graduated from Internationalraining Institute for Materials Science (ITIMS), Hanoi University of TechnologyHUT), Vietnam with the MSc degree in materials science. Since 2007, he has beenorking as PhD student at Laboratory of Gas Sensors and Monitors, Institute of Mate-

ials Science, Vietnam Academy of Science and Technology. He is now the leader ofhis Laboratory. His current research interests are gas-sensing materials, fabricationf sensing devices and solid-state gas sensors.

a Thai Duy graduated from the Hanoi University of Technology (HUT), Vietnamith the BEng degree in physics in 2010. He is now the research assistant at Labora-

ory of Gas Sensors and Monitors, Institute of Materials Science, Vietnam Academyf Science and Technology. His current research interests are gas-sensing materialsnd its application for gas sensors.

ham Quang Ngan received MSc degree in radio-electronics from the College ofatural Science, Hanoi, Vietnam in 2007. He is now the researcher at Laboratoryf Gas Sensors and Monitors, Institute of Materials Science, Vietnam Academy ofcience and Technology. His current research interests are the electronics techniqueor application of gas sensor.

ators B 158 (2011) 246– 251 251

Giang Hong Thai received his BSc degree in materials science from the Collegeof Natural Science, Hanoi, Vietnam in 2003. In 2010, he graduated from Collegeof Technology, Vietnam National University, Hanoi, Vietnam with the MSc degreein materials science. He is now the researcher of the Laboratory of Gas Sensorsand Monitors, Institute of Materials Science, Vietnam Academy of Science andTechnology. His current research interests are semiconductor gas sensors and imple-mentation of gas application.

Do Thi Anh Thu received her BSc degree in 1998; MSc degree in 2002 in chemistryfrom College of Natural Science, Hanoi. She is now the researcher of the Laboratoryof Gas Sensors and Monitors, Institute of Materials Science, Vietnam Academy ofScience and Technology. Her current research interests are nanocrystalline materialssynthesis for gas sensor application.

Do Thi Thu received her BSc degree in physics from College of Education, Hanoi,Vietnam in 2010. She is now the research assistant at the Laboratory of Gas Sensorsand Monitors, Institute of Materials Science, Vietnam Academy of Science and Tech-nology. Her current research interests are nanocrystalline materials synthesis andgas sensor.

Nguyen Ngoc Toan received BEng degree in physics in 1975 from Lvov PolytechnicState University, Ukraine; PhD degree in 1985 from Institute of Radio-Engineering

and Electronics of Czechoslovak Academy of Sciences, Prague. She is now the deputy-professor of Laboratory of Gas Sensors and Monitors, Institute of Materials Science,Vietnam Academy of Science and Technology. Her current research interests arenanotechnology, nano-materials, gas-sensing metal oxide and solid-state gas sen-sors.