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Available online at www.sciencedirect.com

Talanta 74 (2007) 235–240

Grain-size effects on gas response in nanostructured Gd0.9Ba0.1CoO3

Carlos R. Michel ∗, Edgar Lopez Mena, Alma H. MartınezDepartamento de Fısica, CUCEI, Universidad de Guadalajara, Blvd. Marcelino

Garcıa Barragan 1421, Guadalajara, Jalisco 44410, Mexico

Received 21 April 2007; received in revised form 30 May 2007; accepted 31 May 2007Available online 13 June 2007

bstract

The perovskite Gd0.9Ba0.1CoO3 was prepared via solution and solution-polymerization methods in aqueous media. The thermal decompositionf precursor powders produced well crystallized nanostructured materials possessing an extensive nanoporosity. The average particle size obtained

y the solution route was 150 nm, whereas by the solution-polymerization method was 61 nm. The characterization of gas response, performedn thick-films, revealed that the electrical resistance variation increases with reducing grain size. In oxygen, the films prepared with powder ofolution-polymerization registered an increment of 1000%, compared to the powder made by the solution route, whereas in carbon dioxide was100%.

2007 Elsevier B.V. All rights reserved.

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eywords: Cobalt oxides; Nanostructures; Electron microscopy; Chemical sen

. Introduction

In recent years, the global climate change has emerged as onef the most immediate challenges of the mankind; as it is wellnown, this phenomenon is mainly caused by the emission ofarbon dioxide to the atmosphere by combustion engines andndustrial processes. To determine the concentration of CO2,nd other polluting gases, intensive scientific research has beenade to develop solid state gas sensors, which are elaboratedith inorganic oxides such as SnO2, ZnO and WO3 [1–6]. Theasic principle of operation of this kind of sensors is their changef electrical conductance, caused by the exposition to differentoncentrations of gases.

Transition metal oxides with the perovskite-type structureABO3), are suitable compounds for environmental gas sensingpplications due to their notable gas sensitivity, wide range ofperating temperature, high reproducibility in gas response andrystal structure stability. In this area, much of the research haseen focused on iron perovskites with formula: Ln1−xMxFeO3

Ln = La, Sm, M = Ca, Sr); according to the results of the authors,hese materials exhibit a noticeable behavior in the detection ofO, CO2, O2 and NOx [7–10]. The application of these oxides

∗ Corresponding author. Tel.: +52 33 33 45 41 47; fax: +52 33 33 45 41 47.E-mail address: michelucr@yahoo.es (C.R. Michel).

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039-9140/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.talanta.2007.05.054

s gas sensors is not limited to the gases mentioned before; forxample, Suo et al. found that nanocrystalline La1−xSrxFeO3as good sensitivity to ethanol [11,12]. Besides, other authorsave observed that La1−xCaxFeO3 exhibit high sensitivity toiquefied petroleum gas (LPG), gasoline and acetone in [13,14].ccording to Fierro et al., the gas sensing properties of the ironerovskites are associated to the iron spin states, particularly tohe amount of high-spin state Fe3+ [15].

Even though gas sensors made with perovskites contain-ng cobalt seem to be less studied, Ishihara et al. reportedhat the introduction of a small amount of cobalt in La0.8Sr0.2a0.8Mg0.15O3 improves its sensitivity to O2 and C3H6 [16].everal authors have also explored the application of cobalterovskites as gas sensors with good results [17–21].

On the other hand, the gas response of a material can bemproved by reducing the particle size to the nanometer scale;anoporosity also plays an important role by increasing the spe-ific surface area. To prepare nanostructured materials, severalynthesis methods have been developed, of particular interest ishe solution-polymerization method, because this process givesutstanding results in the formation of nanoporous oxides; thisethod will be referred here after as s-p method [22–29]. In

his route, a polymer stabilizer such as polyvinyl alcohol (PVA),olyacrylic acid (PAA) or polyethylene glycol (PEG) may bedded to an aqueous solution containing the cations of interest, toromote their homogeneous distribution in the polymer network.

2 alanta 74 (2007) 235–240

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ttcmaGGwitothe perovskite-type structure (orthorhombic) remains withoutchange for such barium concentration. The identification ofGd2O3 was made by using the file no. 43-1014. On the otherhand, Fig. 2 shows a typical XRD pattern of a sample prepared

36 C.R. Michel et al. / T

hen this solution is heated, a strong oxidation reaction takeslace due to the contact between metal nitrates and organic mat-er, which produces nanostructured oxides with an extensiveanoporosity.

In this paper, the effect of the polymerization process onhe powder morphology of Gd0.9Ba0.1CoO3 was explored; theesults were compared to those obtained from a sample made byhe simple solution route. Besides, the sensitivity to oxygen andarbon dioxide, as well as their transient response to a changen gas composition was evaluated in both materials.

. Experimental

Nanocrystalline samples of Gd0.9Ba0.1CoO3 were preparedy solution and s-p methods; in both routes stoichiometricmounts of Gd(NO3)36H2O (Alfa Aesar), Ba(NO3)2 (Alfaesar) and Co(NO3)36H2O (J.T. Baker) were dissolved in00 ml of deionized water; this liquid will be named precur-or solution. By the solution method, 0.1 mol of anhydrousitric acid (Alfa Aesar) was added under stirring to the precur-or solution, producing a transparent liquid with pH 1; furthervaporation was made at 76 ◦C in a hot plate. The resultinglack powder was calcined in air using a muffle-type fur-ace with programmable temperature control. By the s-p route,.44 g polyvinyl alcohol (Av. Mol. Wt. 85,000, Sigma) and 3.2 gucrose (Chemical Products Monterrey) were dissolved undertrong stirring in the precursor solution at 60 ◦C. The pH of theesulting liquid was adjusted to 1 by adding citric acid. Furthervaporation and firing was made under the same conditions usedefore for the solution method.

Powders calcined at different temperatures were analyzed by-ray powder diffraction (XRD) using a Rigaku Miniflex appa-

atus (Cu K� radiation). The morphology of the oxides wasbserved by transmission electron microscopy (TEM) using aeol JEM-1010 microscope operated at 100 kV. Selected arealectron diffraction (SAED) was used during the observation ofamples by TEM to confirm sample crystallinity. To perform dclectrical characterization, thick-films of Gd0.9Ba0.1CoO3 wereeposited on alumina discs (Coors), by the screen printing tech-ique; further sintering was made at 500 ◦C for 3 h to improveonnectivity between grains. The microstructure of the filmsas observed by scanning electron microscopy (SEM) using a

eol JSM-5400LV microscope. The thickness of the films was.2 mm with 5 mm diameter; two high purity silver wires with.5 mm diameter were fixed in the perimeter forming and anglef 180◦ between them. The measurements were performed in dryynthetic air, O2 and CO2 inside a tubular furnace, using an Agi-ent 34401A digital voltmeter. The conductivity was calculatedccording to the classical relation:

= l

RS(1)

here R is the measured electrical resistance between the twolectrical contacts separated by a length l, and S is the sectionf the sample [30].

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ig. 1. X-ray powder diffraction patterns of Gd0.9Ba0.1CoO3 (solution method)alcined from 600 to 800 ◦C.

. Results and discussion

The XRD patterns of precursor powders calcined from 600o 800 ◦C (solution method) are shown in Fig. 1. The pat-ern corresponding to a sample fired at 600 ◦C exhibits tworystalline phases: Gd0.9Ba0.1CoO3 and Gd2O3, were the for-er is more abundant. The calcination at 700 ◦C reduced the

mount of unreacted Gd2O3, whereas at 800 ◦C, single phased0.9Ba0.1CoO3 perovskite was produced. The identification ofd0.9Ba0.1CoO3 was made by using the ICDD file no. 25-1057,hich indeed corresponds to GdCoO3; however, both oxides are

somorphic. The introduction of barium in GdCoO3 increasedhe unit cell volume in less than 3%, because the ionic radiusf Ba2+ (r = 1.36 A) is larger than Gd3+ (r = 0.94 A); however,

ig. 2. X-ray powder diffraction pattern of Gd0.9Ba0.1CoO3 prepared byolution-polymerization method (600 ◦C).

C.R. Michel et al. / Talanta 74 (2007) 235–240 237

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Fig. 3. TEM images, SAED patterns and particle size distribution of Gd0.9

y s-p route, annealed at 600 ◦C; clearly this method producedell crystallized Gd0.9Ba0.1CoO3 at notable lower tempera-

ure.Fig. 3 shows the morphology, SAED patterns and particle

ize distributions of Gd0.9Ba0.1CoO3 produced by solution and-p methods. For Gd0.9Ba0.1CoO3 prepared by s-p method, anverage particle size of 61 nm was measured, whereas for a sam-le prepared by solution method was 150 nm. Thus, the s-p routeroduced a reduction of 60% in particle size, with a narrowerarticle size distribution, due to the calcination at lower temper-ture. Even though in both syntheses nanostructured materialsere produced, they have different particle shape; by the solutionethod, elongated particles were produced by particle coales-

ence; whereas by the s-p route, semispherical crystallites maye noticed. In order to verify the local crystallinity of samples,AED was made on a number of nanoparticles, the insets inEM images of Fig. 3 show the corresponding patterns; these

esults confirmed the crystallinity of the powders previously reg-stered in XRD patterns. The extensive nanoporosity, which is

ore abundant in the sample prepared by the s-p method, isssociated to the evolution of gases such as carbon dioxide and

ater vapor produced during the calcination of the precursorowders.

Previous to electrical characterization, the microstructure ofhick-films prepared with powders yielded by the solution and

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CoO3 prepared by (A) solution and (B) solution-polymerization methods.

-p methods was observed; Fig. 4 shows low and medium res-lution SEM micrographs of their top surfaces. The porosity ofhe films was evaluated from several images, on the basis of graycale differences between particles and porosity. The porosity oflms made with powder of solution method was approximately9%, whereas for films of s-p powder was 38%. Besides, SEMmages show a granular texture for both films, without presencef cracks.

The variation of electrical conductivity with temperature wasvaluated on the thick-films, the resulting Arrhenius-type plotsre shown in Fig. 5. Even though a similar semiconductor behav-or may be observed in these curves, the film made with powderf s-p method exhibits a conductivity of 2 orders of magnitudearger than the powder of solution route, with a less disper-ion of data. The difference in conductivity of the films maye explained as follows: if D ≥ 2L, where D is the grain sizeiameter and L is the space-charge layer (∼100 nm), the elec-ron transport takes place across each grain boundary. Since theowder produced by solution method has an average particleize of 150 nm, the electrical conduction in this material occursy this mode [1,31,32]. On the other hand if D < 2L, the space-

harge volume comprehends the whole grain, and the resistances mainly determined by the bulk resistance of each grain; dueo the average particle size obtained by s-p method was 61 nm,ts electrical conductivity is developed in this manner.

238 C.R. Michel et al. / Talanta 74 (2007) 235–240

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ig. 4. Low and medium magnification SEM images of the surface of Gd0.9

olymerization methods (C and D).

The activation energies of conduction (EA) were calculatedrom the Arrhenius-type graphs of Fig. 5. In each graph, twoemperature regions may be identified by their different slopes;or powder of s-p method, the first region comprehends from00 to 400 ◦C, and the second above 400 ◦C. For powder madey solution method, the change in slope is observed at approx-mately 450 ◦C; Table 1 shows the corresponding EA values.he most accepted model that explain the non-linear behav-

or of the Arrhenius graphs at low-temperature branches, isased in the small polaron hopping between Co2+ and Co3+

ons.The dynamic gas response of Gd0.9Ba0.1CoO3 thick-films

as evaluated as follows: dry synthetic air was supplied for0 min until the resistance reached a stable value, then a gas

ow (O2 or CO2) was delivered for 3 min. These experimentsere carried out at 385 ◦C, which corresponds to the temper-

ture where the larger sensitivity was registered in both gases.ig. 6 shows normalized plots obtained in: O2 (A) and CO2 (B);

O

O

able 1alculated activation energies of conduction (EA) for Gd0.9Ba0.1CoO3 prepared by s

Solution-polymerization

EA (eV), 100 ◦C < T < 400 ◦C EA (eV), 400 ◦C < T

ir 0.40 0.51O2 0.30 0.61

2 0.36 0.59

CoO3 thick-films made with powders of: solution (A and B), and solution-

n O2 these graphs were constructed according to the equation:(Rair − RO2 )/Rair) × 100%; whereas in CO2 the relation usedas: ((RCO2 − Rair)/Rair) × 100%; where Rair, RO2 and RCO2

re the resistances in such gases. From Fig. 6(A), the normal-zed value of the powder made by s-p method (O2) was 1000%arger than the powder of solution method; in CO2, the incre-

ent was 1100%. Since both materials have the same chemicalomposition, the improvement in gas response is straightforwardssociated to the reduction in particle size.

The most accepted mechanism for oxygen sensing in p-ype semiconductor materials, such as Gd0.9Ba0.1CoO3 involveshe adsorption of oxygen species on its surface. The electron-ransfer occurred during adsorption takes place by one of threeossible ways:

2(gas) + e− → O2(ads)− (2)

2(gas) + 2e− → 2O(ads)− (3)

olution-polymerization and solution methods

Solution

EA (eV), 100 ◦C < T < 450 ◦C EA (eV), 450 ◦C < T

0.42 0.570.37 0.610.34 0.13

C.R. Michel et al. / Talanta 74 (2007) 235–240 239

Fig. 5. Arrhenius plots of Gd0.9Ba0.1CoO3 thick-films made with powders of:(

O

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Fig. 6. Normalized dynamic gas response of Gd0.9Ba0.1CoO3 thick-films in (A)O2 and (B) CO2.

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A) solution and (B) solution-polymerization routes.

2(gas) + 4e− → 2O(ads)2− (4)

here (ads) means the adsorbed species [33]. Then, for a p-ype semiconductor, oxygen adsorption produces an increase inhe number of holes in the valence band, which increases itsonductivity. In the opposite case, in a reducing or electron-onating gas such as CO or CO2, electrons are injected intohe conduction band and then coupled with holes in the valenceand, producing an increase in the resistance.

Finally, the reproducibility of gas response of a filmade with powder of s-p method was tested at 385 ◦C, by

upplying carbon dioxide and air during 3 cycles; later oxy-en and air were delivered for the same number of times;ig. 7 shows the resulting graph. Clearly the ability ofesponse of this material was not altered by delivering differentases.

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ig. 7. Consecutive gas response in CO2 and O2 of Gd0.9Ba0.1CoO3 made byolution-polymerization route (385 ◦C).

. Conclusion

The solution-polymerization process in aqueous media issuitable method for the preparation of many nanostructured

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40 C.R. Michel et al. / T

norganic materials. This route has the advantage over the sim-le solution method that the resulting materials are obtainedt a lower calcination temperature, with similar crystallinity.esides, due to polyvinyl alcohol and sucrose are cheap com-ounds, this method is a low-cost process to obtain materialsith large values of specific surface area, which can be applied

n fields like gas sensors, membranes, electrodes for solid oxideuel cells and catalysts. In gas sensor applications, the reductionn grain size, promoted by this method, notably improves thelectrical response, as it was demonstrated in this work.

cknowledgments

Financial support from the Coordinacion General Academicaf the Universidad de Guadalajara, in the framework of PROAEProgram is greatly acknowledged. E.L.M. and A.H.M. thank toONACYT for their doctorate scholarships.

eferences

[1] N. Yamazoe, Sens. Actuators B 5 (1991) 7.[2] Y. Liu, E. Koep, M. Liu, Chem. Mater. 17 (2005) 3997.[3] A.S. Zuruzi, A. Kolmakov, N.C. MacDonald, M. Moskovits, Appl. Phys.

Lett. 88 (2006) 102904.[4] J.T. McCue, J.Y. Ying, Chem. Mater. 19 (2007) 1009.[5] S. Choudhury, C.A. Betty, K.G. Girija, S.K. Kulshreshtha, Appl. Phys. Lett.

89 (2006) 071914.[6] L.G. Teoh, I.M. Hung, J. Shieh, W.H. Lai, M.G. Hon, Electrochem. Solid

State Lett. 6 (2003) G108.[7] M.L. Post, J.J. Tunney, D. Yang, X. Du, D.L. Singleton, Sens. Actuators B

59 (1999) 190.[8] J.J. Tunney, M.L. Post, X. Du, D. Yang, J. Electrochem. Soc. 149 (2002)

H113.[9] J.W. Yoon, M.L. Grilli, E. Di Bartolomeo, R. Pollini, Sens. Actuators B 76

(2001) 483.

[

[[

74 (2007) 235–240

10] M.C. Carotta, G. Martinelli, Y. Sadaoka, P. Nunziante, Sens. Actuators B48 (1998) 270.

11] H. Suo, F. Wu, Q. Wang, G. Liu, F. Qiu, B. Xu, M. Zhao, Sens. ActuatorsB 45 (1997) 245.

12] H. Suo, J. Wang, F. Wu, G. Liu, B. Xu, M. Zhao, J. Solid State Chem. 130(1997) 152.

13] L.B. Kong, Y.S. Shen, Sens. Actuators B 30 (1996) 217.14] Y. Wang, J. Chen, X. Wu, Mater. Lett. 49 (2001) 361.15] M.A. Pena, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981.16] T. Ishihara, M. Fukuyama, A. Dutta, K. Kabemura, H. Nishiguchi, Y. Takita,

J. Electrochem. Soc. 150 (2003) H241.17] R. Mochinaga, T. Yamasaki, T. Arakawa, Sens. Actuators B 52 (1998)

96.18] E.L. Brosha, R. Mukundan, D.R. Brown, F.H. Garzon, J.H. Visser, M.

Zanini, Z. Zhou, E.M. Logothetis, Sens. Actuators B 69 (2000) 171.19] H.D. Wiemhofer, H.G. Bremes, U. Nigge, W. Zipprich, Solid State Ionics

150 (2002) 63.20] U. Nigge, H.D. Wiemhofer, E.W.J. Romer, H.J.M. Bouwmeester, T.R.

Schulte, Solid State Ionics 146 (2002) 163.21] F. Garzon, I. Raistrick, E. Brosha, R. Houlton, B.W. Chung, Sens. Actuators

B 50 (1998) 125.22] S.J. Lee, E.A. Benson, W.M. Kriven, J. Am. Ceram. Soc. 82 (1999)

2049.23] M.A. Gulgun, M.H. Nguyen, W.M. Kriven, J. Am. Ceram. Soc. 82 (1999)

556.24] Y. Shimizu, T. Murata, J. Am. Ceram. Soc. 80 (1997) 2702.25] L.W. Tai, H.U. Anderson, J. Am. Ceram. Soc. 75 (1992) 3490.26] M.A. Gulgun, O.O. Popoola, W.M. Kriven, J. Am. Ceram. Soc. 77 (1994)

531.27] S.J. Lee, W.M. Kriven, J. Am. Ceram. Soc. 81 (1998) 2605.28] R.N. Das, Mater. Lett. 47 (2001) 344.29] R.N. Das, A. Bandyopadhyay, S. Bose, J. Am. Ceram. Soc. 84 (2001) 2421.30] C.R. Paul, S.A. Nasar, Introduction to Electromagnetic Fields, McGraw

31] J. Tamaki, Z. Zhang, K. Fujimori, M. Akiyama, T. Harada, N. Miura, N.Yamazoe, J. Electrochem. Soc. 141 (1994) 2207.

32] V.E. Bochenkov, G.B. Sergeev, Adv. Colloid Interface Sci. 116 (2005) 245.33] M.I. Baraton, L. Merhari, Rev. Adv. Mater. Sci. 4 (2004) 15.