Sensors and Actuators B 115 (2006) 434–438

Sensing properties of rare earth oxide doped In2O3 by a sol–gel method

Xinshu Niu, Haoxiang Zhong ∗, Xinjun Wang, Kai JiangCollege of Chemistry and Environmental Sciences, Henan Key Laboratory for Environmental Pollution Control,

Henan Normal University, Xinxiang, Henan 453007, China

Received 5 June 2005; received in revised form 7 October 2005; accepted 7 October 2005Available online 15 November 2005

Abstract

The powders of pure In2O3 and Eu3+, Gd3+, Ho3+-doped In2O3 with the structure of bixbyite-type were synthesized by a sol–gel method inthe citric acid system. The structure and crystal phase of the powders were characterized by using an X-ray diffractometer. The gas sensingcharacteristics were obtained by measuring the response magnitude as a function of various controlling factors like dopant, operating temperature,concentrations of the test gases and the response time. The results showed that pure In2O3 had poor selectivity, while the In2O3 sensors dopedwith 5 wt.% Eu2O3, 5 wt.% Gd2O3, and 5 wt.% Ho2O3 increased the gas-sensing performance towards H2S as a reducing gas, but decreased gastowards Cl2 as an oxidizing gas at 195 ◦C. Namely, doping of the rare earth oxides remarkably improved the selectivity for H2S. In addition, itwr©

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as found that among the dopants, 5 wt.% Ho2O3 gave the highest the response value, selectivity, stability and the most excellent response andecovery characteristics to H2S at 195 ◦C. Thus, the Ho2O3-doped In2O3 is a promising candidate for detection of H2S in environments.

2005 Published by Elsevier B.V.

eywords: H2S sensor; In2O3; Rare earth; Sol–gel; Bixbyite-type oxide

. Introduction

Hydrogen sulfide is a toxic gas, often produced in coal, coalil or natural gas manufacturing. The threshold limited valueTLV) for hydrogen sulfide is 10 ppm. When the concentrationf H2S is higher than 250 ppm, it is dangerous to human bodynd may cause death. With these properties, H2S has becomerecent target of rather extensive research. So far H2S sen-

ors have mostly been based on semiconducting oxides, suchs WO3 [1–4], ZnO [5], SnO2 [6–10]. Although these sensorseportedly show fairly good H2S sensing properties, they havetill difficulties in sensitivity and/or response rate to very dilute2S. Apart from these, there have been attempts to develop2S sensors by using the solid electrolytes of K2SO4 [11] and2O3-stabilized zirconia (YSZ) tube with a sensing oxide layerf WO3 [12]. The resulting devices can detect H2S in air, but atigh temperature such as 820 ◦C and with longer response andecovery times (20 min). So, reliable and stable H2S sensors withigh response value, selectivity, fast response and low energy

consumption are in high demand for environmental safety andindustrial control purposes. It is well known that In2O3 has beenused extensively as a new semiconductor-type metal oxide foroxidizing gas detection [13,14], including toxic and pollutiongases (Cl2, O3, NO2). However, it also responds to reducinggases like H2S, presenting its poor selectivity. Rare earth ele-ments could be introduced into the body of In2O3 accordingto the structure characteristics, which may lead to overcomingthese drawbacks.

In this paper, In2O3 specimens doped with rare earth oxidesEu2O3, Gd2O3, and Ho2O3 were prepared by a sol–gel methodin the system of citric acid. Their gas sensing properties to H2S,H2, CO, Cl2, and NO2 were also discussed. The results showedthat In2O3 doped with 5 wt.% Ho2O3 was very promising indetecting H2S due to its high response and selectivity.

2. Experimental

2.1. Preparation of In2O3 doped with Eu3+, Gd3+, andHo3+

3+ 3+

∗ Corresponding author. Tel.: +86 373 3326336; fax: +86 373 3326336.E-mail addresses: haoxiangzhong@hotmail.com,

honghaoxiang@126.com (H. Zhong).

Nanometer powders of In2O3 doped with Eu , Gd , andHo3+ were prepared by the sol–gel method. InCl3·4H2O, Eu2O3,Gd2O3, and Ho2O3 were used as the starting materials. They are

925-4005/$ – see front matter © 2005 Published by Elsevier B.V.

oi:10.1016/j.snb.2005.10.004

X. Niu et al. / Sensors and Actuators B 115 (2006) 434–438 435

all analytical reagent grade. A required amount of InCl3·4H2Owas dissolved in distilled water (0.4 mol dm−3), and an ammoniasolution (5 mol dm−3) was slowly dropped into it. The final pH ofthe solution was about 8.5. The hydrated precipitate formed wasseparated centrifugally, and washed several times with distilledwater. The precipitation was dissolved in an aqueous nitric acidsolution (1:1 volume fraction), and Eu2O3 in another nitric acidsolution (1:1 volume fraction). A given amount of the In3+andEu3+ solutions were thoroughly mixed. Then a certain amount ofan aqueous citric acid solution (0.5 mol dm−3) was introducedinto the above mixture. The final the mole ratio of In3+ plus Eu3+

to citric acid was 1:2. The mixture was stirred to obtain a homo-geneous and stable sol. The sol was evaporated on a water bathat 80 ◦C, then dried at 110 ◦C in a baking oven until a xerogelformed. The xerogel was ground into powders in an agate mor-tar. The powders were slowly heated to 400 ◦C and pre-calcinedfor 1 h to make the organic matter decompose completely, thencalcined at 600 ◦C for 2 h in a muffle furnace. The product wascooled to room temperature. Gd3+- and Ho3+-doped In2O3 pow-ders were synthesized in a similar manner.

The structure and crystal state of the products were charac-terized by using an X-ray diffractometer (Bluker D8-Advance)with Cu K� radiation (λ = 1.5406 A) operating at 20 mA and40 kV. The data were collected by a step scanning method for20◦ ≤ 2θ ≤ 70◦, with a step width of 0.05◦ and a step time of 1 s.

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Fig. 1. XRD patterns of (a) pure In2O3, (b) Eu3+-In2O3, (c) Gd3+-In2O3, and(d) Ho3+-In2O3.

peaks. The mean grain size was calculated by Debye–Scherrerequation: D = 0.89λ/(β cos θ), where D is the mean grain size, λthe wavelength, β the full width at half maximum, and θ is thescanning angle. The results are listed in Table 1.

It is clear from Table 1 that Eu3+-In2O3 has the smallest meangrain size (D) and the same lattice constant as pure In2O3, whileD of Gd3+-In2O3 is the largest among them. Though D of Ho3+-In2O3 is not the smallest, its lattice constant is the biggest amongthe samples, denoting that Ho3+-In2O3 possibly has the largestlattice distortion. In addition, the average grain size estimatedfrom XRD data has been confirmed by direct observation witha transmission electron microscope.

Because the smaller grain size and the larger lattice distortionare beneficial for contacts between gases and material surfaces,the Ho3+-In2O3 material can adsorb a larger number of gasesand therefore can increase the response to gases.

3.2. Sensor performance test

3.2.1. Response magnitude testFig. 2 illustrates the response profiles to 10 ppm H2S of pure,

Eu3+-, Gd3+-, and Ho3+-doped In2O3 sensors as a function ofoperating temperature. It is observed that when exposed to H2S,the responses of all the samples increase with the operatingtemperature up to 130 ◦C. When the operating temperature fur-t 3+

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.2. Sensor design and structure

A certain amount of �-terpineol was added to the powder,hen the mixture was then ground to form paste. The pastebtained was coated onto ceramic tubes, on which two plat-num wires had been installed at each end. The paste on theeramic tubes was calcined at 300 ◦C for 1 h, then 600 ◦C forh in air and aged at 320 ◦C for 240 h. A small Ni–Cr alloy coilas placed through the tube as a microheater. Electrical contactsere made with two platinum wires attached to the electrodes.he sensors obtained were set up in a glass test chamber with aolume of 0.18 m3 and kept under a continuous flow of fresh airor 30 min before measurement. When testing, a given amountf gases were injected into the above chamber. To have a uni-orm concentration distributed, the chamber was equipped withsmall electrical fan. The measuring principle of gas sensing

roperties is described elsewhere [15]. The response to gases,, is defined as S = Rg/Ra, where Rg and Ra are the resistance ofsensor in a target oxidizing gas/air mixture and in air, respec-

ively. On the other hand, if a target is a reducing gas, S = Ra/Rg16].

. Results and discussion

.1. Structural characteristics of In2O3 doped with Eu3+,d3+, and Ho3+

X-ray diffraction (XRD) patterns of pure, Eu3+-, Gd3+-, ando3+-In2O3 powders are shown in Fig. 1. It was found that all

he powders were cubic phases of bixbyite type structure. Theattice parameters of the materials were calculated from the XRD

her increases, the responses for pure and Eu -doped In2O3ecrease. But for Gd3+- and Ho3+-doped In2O3, the optimumemperature is about 195 ◦C, then the responses fall with theising temperature. We selected the temperature of 195 ◦C foromparison of the sensing properties of the four sensors. Theesponses values of pure, Eu3+-, Gd3+-, and Ho3+-doped In2O3

able 1attice constant of powders

ample Lattice constant (A) a = b = c Mean grain size (nm)

u3+-In2O3 11.1195 19d3+-In2O3 11.0813 25o3+-In2O3 11.1219 23ure In2O3 11.1195 20

436 X. Niu et al. / Sensors and Actuators B 115 (2006) 434–438

Fig. 2. Gas responses to 10 ppm H2S as a function of operating temperature.

are 50.2, 106.0, 52.3, 1061.4 at 195 ◦C, respectively. It is obvi-ous that Ho3+-doped In2O3 has a markedly increased responsecompared with that of pure In2O3. Since In2O3 is an n-typesemiconducting oxide and the gas sensing mechanism belongsto the surface-controlled type [17], the gas response is intimatelyrelated to grain size, surface state, oxygen adsorption and lat-tice distortion. The highest response of Ho3+-doped In2O3 isprobably attributed to its largest lattice distortion and highestsurface activity, which result in stronger interaction betweenH2S molecules and the surface active sites. About the mecha-nism of the temperature dependence of the sensors is still underinvestigation.

3.2.2. Selectivity testSelectivity is the ability that a gas sensor distinguishes

between different kinds of gases. In general, the sensitivityincreases as the concentration of a test gas increases. Theresponses of pure, Eu3+-, Gd3+-, and Ho3+-doped In2O3 sen-sors to different gases of H2S, H2, CO, Cl2, and NO2 are shownas a function of the gas concentration in Figs. 3–6, respectively.As can be seen from the figures, it is obvious that pure In2O3is quite sensitive to both an oxidizing gas Cl2 and a reducing

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Fig. 4. Gas responses of Eu3+-In2O3 as a function of gas concentration at 195 ◦C.

Fig. 5. Gas responses of Gd3+-In2O3 as a function of gas concentration at195 ◦C.

Fig. 6. Gas responses of Ho3+-In2O3 as a function of gas concentration at195 ◦C.

ig. 3. Gas responses of pure In2O3 as a function of gas concentration at 195 ◦C.

X. Niu et al. / Sensors and Actuators B 115 (2006) 434–438 437

Table 2Gas responses of rare earth doped In2O3 at 195 ◦C

Sample Response magnitude

H2S (10 ppm) H2 (100 ppm) CO (100 ppm) Cl2 (100 ppm) NO2 (100 ppm)

Eu3+-In2O3 106.0 1.5 1.4 15.4 12.4Gd3+-In2O3 52.3 2.0 29.0 129.2 79.0Ho3+-In2O3 1061.4 4.5 47.1 10.7 35.8

gas H2S, indicating that the sensitivity of pure In2O3 is poor.However, with doping a rare earth oxide the selectivity for H2Shas been largely improved, since responses to Cl2 and otheroxidizing gases markedly decreased. The improved selectivityis possibly explained as follows. When O2, is adsorbed on theIn2O3 surface, it traps electron(s) from the body of the n-typesemiconductive In2O3 due to the strong electronegativity of theoxygen atom to produce negatively charged chemisorbed oxy-gen such as O2

−, O−, and O2−. As the result, the concentrationof electrons in the n-type In2O3 decreases and hence the resis-tance of the material increases.

O2 + e → O2−(ad)

When exposed to a reducing gas, the interaction with thesurface chemisorbed oxygen can take place in different ways ofsurface reactions. For example [18]:

H2S + (3/2)O2(ad) → H2O(g) + SO2(g) + (3/2)e

As can be seen, the electrons released return into the surfaceof the oxide and the resistance of the material decreases. Fromthe point of the catalytic chemistry, the surface acid–base prop-erties of an oxide could be advantageously utilized to favor aparticular reaction on the surface. It is possible that a rare earthoxide provides the basic surface for In2O3 and makes it easilyadsorb acidic H2S gas. In addition, the gas response is stronglydedsdCtfo1rtHd

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As is shown in Table 3 and Fig. 7, the responses to H2S ofpure, Eu3+-, Gd3+-, and Ho3+-In2O3 at 195 ◦C were all veryfast, but the recovery times were 78, 74, 91, and 34 s, respec-tively, indicating that Ho3+-doped In2O3 was of remarkablyimproved recovery characteristics. It is clear that pure, Eu3+-,and Gd3+-In2O3 have poor recovery characteristics, but they arenot permanent changes. After a long time, they recovered theoriginal state, as can be proved from the following stability test.

3.2.4. Stability testStability is the consistence of the output signal vibration of a

sensing element under continuous testing. The responses of thethree rare earth doped sensors to 10 ppm H2S were measured at3rd, 7th, 15th, 30th days after the first measurement. The resultsare shown in Fig. 8. It can be seen that all the sensors havethe nearly constant responses to10 ppm H2S, indicating goodstability of the sensors. The good stability may be because thematerials were prepared under higher temperature calcinationand were aged for 10 days. By the heat processing and aging,the surface energy state and the internal stress of the materi-als were reduced, and their composition and structure becamehomogeneous and equilibrated, contributing to the long termstability. In addition, the surface structures of the sensors wouldbe possibly stabilized by the doping with rare earth oxides withlarger ionic radii.

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ependent on the chemical composition of the sensor, differ-nt rare earth elements with their own structural characteristicsifferently improve the selectivity to various extents. Table 2hows that the values of responses of three rare earth oxidesoped In2O3 sensors for 10 ppm H2S, 100 ppm H2, 100 ppmO, 100 ppm Cl2, and 100 ppm NO2 at 195 ◦C. It is observed

hat the responses of Ho3+- and Eu3+-In2O3 for 10 ppm H2S arear larger than those to other gases. For example, the responsesf Ho3+-In2O3 to 10 ppm H2S is 20 times larger than that to00 ppm CO which is the strongest interfering gas. Besides, theesponses to 10 ppm H2S of Ho3+-In2O3 is 10 times larger thanhat of Eu3+-In2O3, which means that the H2S selectivity ofo3+-doped In2O3 is the best among the three rare earth oxideoped In2O3.

.2.3. Response and recovery timeThe response and recovery times are used to characterize the

ensor performance. The response time is defined as the periodn which the sensor output change reaches 90% of the steadyalue. The recovery time is the time for the resistance to recover0% of the total variation when the testing gas is removed. In thenvestigation, a period of 5 min was selected in a whole course.

able 3omparisons of response time and recovery time of In2O3-based sensors

o10 ppm H2S at 195 ◦C

ample Response time (s) Recovery time (s)

n2O3 13 78u3+-In2O3 26 74d3+-In2O3 12 91o3+-In2O3 10 34

ig. 7. Response transients of (a) pure In2O3, (b) Eu3+-In2O3, (c) Gd3+-In2O3,nd (d) Ho3+-In2O3 to 10 ppm H2S at 195 ◦C.

438 X. Niu et al. / Sensors and Actuators B 115 (2006) 434–438

Fig. 8. Long-term stability of rare earth oxide doped In2O3 sensors.

4. Conclusions

Pure, Eu3+-, Gd3+-, and Ho3+-doped In2O3 gas sensor werefabricated by a sol–gel method. It was found that 5 wt.% Ho3+-In2O3 sensor exhibited the highest response value, excellentselectivity and quick response behavior to H2S gas, which wasmainly attributed to its smaller grain size and larger lattice dis-tortion. The sensor is very promising for H2S gas detection in therange from 1 to 100 ppm with a response time in second range.The effect of Ho2O3 doping amount into In2O3 on the gas char-acteristics should be further researched, when more enlargementof the sensor signals is required for easier applications.

Acknowledgement

This work was supported by Nature Science Foundation ofHenan Province (No. 0424270073)

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iographies

inshu Niu is a professor in nanometer ceramic materials at Collegef Chemistry and Environmental Science, Henan Normal University. Hisesearch interests include the synthesis of nanometer materials and their appli-ation in sensors, catalysts and their application in selectivity increase of gasensors.

aoxiang Zhong is a postgraduate in inorganic chemistry at Collegef Chemistry and Environmental Science, Henan Normal University. Heresearch interests include the synthesis of rare earth mixed oxide nanometeraterials and their gas sensing properties.

injun Wang received the PhD degree from university of science and tech-ology of China in 2003. His research interests include the synthesis ofnorganic nanometer materials and their application in sensors, catalysts.

ai Jiang received the PhD degree from Changchun Institute of Appliedhemistry Chinese Academy of Sciences in 1997. His research interests

nclude the synthesis of nanometer ceramic and electrode materials.