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Ceramics International 40 (2014) 5837–5842www.elsevier.com/locate/ceramint

Gas sensing performance of hydrothermally grown CeO2–ZnO composites

A.V. Rajgurea, N.L. Tarwalb, J.Y. Patila, L.P. Chikhalea, R.C. Pawarc, C.S. Leec,I.S. Mullad, S.S. Suryavanshia,n

aFerrite Materials Laboratory, School of Physical Sciences, Solapur University, Solapur 413255, IndiabResearch Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology (GIST), Gwangju 500712, Republic of Korea

cDepartment of Materials Engineering, Hanyang University, Ansan 426791, Republic of KoreadCentre for Materials for Electronic Technology (C-MET), Pune 411008, India

Received 26 June 2013; received in revised form 5 November 2013; accepted 5 November 2013Available online 12 November 2013

Abstract

The sensors based on cerium oxide–zinc oxide (CeO2–ZnO) composites were fabricated by using thick-film screen printing of hydrothermallygrown powders. The structural, morphological investigations were carried out by using XRD, FESEM and TEM and these studies revealed thatthe synthesized products were grown in high-density and possessed well-crystallinity. Furthermore, the gas responses were evaluated towards theethanol, acetone, liquid petroleum gas (LPG) and ammonia gases. The 2 wt% CeO2–ZnO composite exhibited excellent response of 94% at325 1C and better selectivity towards ethanol with low response and recovery time as compared to pure ZnO and can stand as reliable sensorelement for ethanol sensor related applications.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Composite; D. Zinc oxide; Gas sensor; Hydrothermal route; TEM

1. Introduction

With the advent of progress in the industrialization, there isa threat to human life and it is essential to detect, monitor thetoxic and inflammable gases. Among a variety of metal oxidesemiconductors such as ZnO, SnO2, Fe2O3, WO3, etc. [1–5],ZnO has proved to be one of the most attractive materialswith direct band gap of 3.37 eV and also finds significanttechnological applications in photocatalysts [6], solar cells [7],superhydrophobic coatings [8], transparent conductive oxidefilms, etc. [9]. It has proved to be an excellent candidate ingas sensing applications due to its good chemical andthermal stability, low cost and simplicity in sensor fabrication[10,11].

Various methods have been employed for the synthesisof ZnO thin/thick films, which include vapor phase routes suchas molecular beam epitaxy [12], r.f. sputtering method [13],pulsed laser deposition [14], thermal evaporation [15], spraypyrolysis [16], etc. These methods yield high quality products

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.11.025

g author. Tel.: þ91 217 2744770.ss: sssuryavanshi@rediffmail.com (S.S. Suryavanshi).

however they require high temperature, high vacuum, rigorousprocedures, expensive equipment which has limited their appli-cations, particularly where large scale production is required.Consequently, synthesis of ZnO with controlled morphologiesand sizes has gained strong importance in the development ofsensor devices. Among the various solution synthesis routes,the hydrothermal route is facile, eco-friendly and most effec-tive way of ZnO synthesis as no toxic gases are evolved duringthe synthesis. It has been proved to be a versatile approach forpreparation of ZnO morphologies as it provides narrow sizedistribution, sufficient crystallization and high-quality growthorientation [17].Significant efforts have been trialed out to improve sensor

performance which includes addition catalyst, addition of metalsor metal oxides as dopants, decreasing the grain size, control-ling the surface-to-volume ratio, to improve the sensitivityand selectivity of the sensor material [18]. In the present study,we have fabricated thick films by screen printing techniqueusing the hydrothermally grown pure ZnO and CeO2–ZnOcomposites. Effects of Ce addition on structural, morpho-logical and gas sensing properties of ZnO based sensor arereported herein.

ghts reserved.

θ

∗∗

∗

∗

∗

Fig. 1. XRD patterns of the pure ZnO (CZO) and CeO2–ZnO (CZ1, CZ2,CZ3, CZ4) composite samples.

A.V. Rajgure et al. / Ceramics International 40 (2014) 5837–58425838

2. Experimental procedure

2.1. Synthesis of pure ZnO and CeO2–ZnO composite sensors

All the reagents were of analytical grade procured from s.d.fine chemicals and were used without further purification.An appropriate quantity of cerium nitrate hexahydrate (Ce(NO3)3 � 6H2O) was mixed into zinc acetate (Zn(CH3COO)2 �2H2O) solution to obtain various concentrations from 1 to 4 wt% and subsequently the initial and Ce added samples aredenoted as CZ0, CZ1, CZ2, CZ3 and CZ4, respectively. Thento this aqueous transparent solution (160 ml) an appropriatequantity of sodium hydroxide (NaOH) was added underconstant stirring to maintain at pH around 12. After 30 minstirring, the solution was transferred into teflon lined stainlesssteel autoclave of 250 ml capacity, subsequently sealed andheated at 100 1C for 10 h. After heating the autoclave wasallowed to cool naturally and the precipitate was washed andfurther dried in air at 60 1C for 2 h. Similar procedure wasadopted for synthesis of all the samples under investigation.The as-prepared powders were calcinated in air atmosphere at450 1C for 2 h and again sintered at 700 1C for 2 h. Thesesintered powders were used for structural and morphologicalinvestigations. Further the calcinated powders of pure ZnO andCeO2–ZnO composites were crushed using an agate mortarand pestle to achieve consistent and fine particles. Thick filmsof crushed powders were prepared by using screen printingtechnique. The fine powder was further mixed with mixture ofethyl cellulose, butyl carbitol acetate and terpineol as organicbinder, the ratio of inorganic to organic part was maintainedat 70:30 [19]. This thixotropic paste was used to depositthick films on ultrasonically cleaned alumina substrate with(2 cm� 1 cm in dimension) using screen with the nylon clothof 140 mesh counts. The resulting thick films were heated at700 1C for 2 h and utilized for gas sensing measurement.

2.2. Characterizations

The structural and morphological properties of the sinteredcomposite powders were investigated by using (Rigaku)D/max-Ra X-ray diffractometer with Cu Kα radiation (λ¼1.5406 Å) and Hitachi, S-4300 microscope operating at 5 kVinstrument, respectively. Elemental analysis of the compositefilms was carried out by using energy dispersive X-ray analysis(EDAX) spectroscopy coupled with the Hitachi, S-4300 micro-scope. The particle sizes and selected area electron diffraction(SAED) patterns of the sintered samples were observed usingtransmission electron microscopy (TEM) with PHILIPS CM-200 model.

3. Results and discussion

3.1. XRD

The phase identification and structural changes were inves-tigated with the help of X-ray diffraction (XRD) technique.Fig. 1 shows the typical XRD patterns recorded over 20–801

for the all synthesized samples. The high intense (1 0 1) peakalong with less intense peaks such as (100), (002), (102),(110), (021), (103), (200), (112), (201), (202), etc. areobserved in XRD pattern of CZO powder sample. All thesediffraction peaks are indexed to hexagonal wurtzite ZnOstructure (JCPDS card no. 36-1451). In addition to ZnO peaks,low intense CeO2 peaks like (111), (200), and (220) are alsoobserved for other samples. Since the ionic radii of the Ce4þ

ion (1.034 Å) are different from those of Zn2þ ion (0.74 Å), itis very difficult for Ce4þ to be incorporated into ZnO matricesand substitute for Zn2þ ion site. The CeO2–ZnO compositecould be simply formed by a chemical reaction; however, theformation of a single crystalline Ce-doped ZnO would involvea very complicated process. The crystallite sizes of pure ZnOand CeO2–ZnO composites were calculated by using Debye–Scherrer equation.

D¼ 0:9λβ cos θ

ð1Þ

where D is the crystallite size, λ is wavelength of radiationused, β is the full width at the half maximum peak at diffrac-tion angle 2θ. The estimated crystallite sizes are 79, 64, 51,72 and 94 nm for the CZ0, CZ1, CZ2, CZ3 and CZ4 powdersamples, respectively. It is seen that the calculated crystallitesizes of the CeO2–ZnO composite powders does not varysystematically with the Ce addition, which is attributed to thelattice disorder, produced in the powder samples at higherCe concentrations due to the difference in the ionic radii ofZn2þ and Ce4þ .

3.2. FESEM

The morphology and microstructure of the hydrothermallygrown pure ZnO and CeO2–ZnO composite powder samplessintered at 700 1C were characterized and analyzed byFESEM. The obtained FESEM photographs for all samplesare shown in Fig. 2, which indicates that the sintered productsare composed of almost uniform spherical grains. The averageparticle sizes calculated in proportion to the photos are about50–100 nm, which are in consistent with the results obtained

Fig. 2. FESEM micrograph of pure ZnO (CZO) and CeO2–ZnO (CZ1, CZ2, CZ3, CZ4) composite samples and EDAX spectrum of the CZ2 sample.

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from Debye–Scherrer equation. FESEM image for CZ0powder sample shows the granular morphology having theuniform grains of �80 nm. It is seen that on addition ofcerium the morphology varied systematically. For CZ1 powdersamples, we can see the non-uniform and bigger grainscompared to the CZ0 samples however the average grain sizeis found to be �68 nm. Interestingly, the uniform granularmorphology is observed with tiny grains having average grainsize of 52 nm for CZ2 sample. Excess addition of Ce into theZnO caused the agglomeration/clusters with grain sizeo500 nm due to the grain growth events. The bigger grainswith grain sizes between 70 and 100 nm are observed for CZ3and CZ4 samples. The observed smaller grain sizes for CZ2sample compared to all samples indicates that CZ2 sample isoptimum. Such uniformly distributed tiny grains of CeO2–ZnOcomposite are very useful for gas sensing applications [20].

3.3. EDAX

To assess the elemental composition of CZ2 sample, theEDAX analysis was done and the results are mentioned inFig. 2. In EDAX spectrum, numerous well-defined peaks wereevident related to Zn, O and Ce which clearly support that thesynthesized products are made of Zn, O and Ce with valuesas per the initial precursor concentrations. No other peakrelated to impurities was detected in the spectrum whichfurther confirms that the synthesized products are CeO2–ZnOcomposites.

3.4. TEM

The typical TEM image for CZ2 sample is shown in Fig. 3,which clearly confirms the formation of CeO2–ZnO composite.

Fig. 3. TEM and SAED micrographs for CZ2 sample.

Fig. 4. The plot of ethanol gas response as a function of operating temperaturefor pure ZnO and CeO2–ZnO composite samples at 2000 ppm concentration.

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The average particle size is found to be 57 nm which is in goodagreement with calculated crystallite size from XRD analysis.To differentiate these different particles of CeO2 and ZnO fromthe TEM image, the SAED pattern is helpful. From SAEDpattern (Fig. 3), it is clear that the synthesized material iscomposed of significant amount of CeO2 material leading todiffuse rings at center, and well crystalline ZnO materialleading to well defined spot pattern around diffuse rings.Similar results for CeO2–ZnO composite have been reportedby the Zhang et al. [21].

3.5. Gas sensing properties

The gas response measurements were performed as explainedin our earlier report [16]. The gas response (S%) was calculatedusing the following equation:

Sð%Þ ¼ ðRa�RgÞRa

� 100 ð2Þ

where Ra is the resistance of the film in air and Rg is the resis-tance of the film after the gas exposure. The high performance ofthe sensor was confirmed through the repeatability and reprodu-cibility experiments. For repeatability, two to three cycles ofthe gas-sensing characteristics were performed on each material.For reproducibility, the gas-sensing performances of at least twoto three samples of each type were tested.

Fig. 4 shows the relationship between operating temperatureand gas response of pure and CeO2–ZnO composite sensorsat 2000 ppm ethanol vapor. For each sample the responsetowards ethanol increased with operating temperature, reachingits maximum and then decreased rapidly with the increase inoperating temperature. Also it is evident that the response wasinfluenced by Ce addition. This behavior is mainly due to theinfluence of operating temperature on the amount of absorbedoxygen species on the surface of ZnO film [22]. At lowtemperature the amount of absorbed oxygen species is low sothe sensor response exhibited is consequently small whileat very high temperature the progressive desorption of thepreviously adsorbed oxygen species occurs and hence thesensor response decreases. The CZ2 sample showed maximumresponse of 94% at moderate operating temperature of 325 1C,

which is highest among all the other samples CZ0 (62.70% at350 1C), CZ1 (72.70% at 350 1C), CZ3 (80.98% at 350 1C)and CZ4 (29.15% at 350 1C), respectively. Smaller crystallitesize of CZ2 sample provides larger specific surface area andhigher surface activity for oxygen adsorption. The fasterreaction of adsorbed oxygen with ethanol gives a large changein the electrical conductivity of the sensor and eventually ahigher sensor response. When the Ce concentration is lessthan the optimum value the distribution is more discrete andthis amount may not be sufficient to promote the reactioneffectively whereas in excess Ce case there is almost agglom-eration of these particles which hinders the reactions. At theoptimum concentration (2 wt% Ce) there is uniform distribu-tion of these particles as a result of which not only is the initialresistance of the sensor high but this amount can promote thereaction most effectively leading to enhanced response.The transient response characteristics of all the samples at

2000 ppm ethanol are shown in Fig. 5. These measurementswere performed by injecting ethanol gas into the chamber firstand sensor's resistance was measured in air and in the presenceof ethanol. All the samples respond rapidly as soon as ethanolgas was injected into the chamber. The response and recoverytimes for pure ZnO and CeO2–ZnO composite samples aregiven in Table 1. The fast response in case of CZ2 sample i.e.response time of 8 s and recovery time of 35 s was attributed to

Fig. 5. Plot of dynamic response for pure ZnO and CeO2–ZnO compositesamples.

Table 1The observed response and recovery time for pure ZnO and CeO2–ZnOcomposite samples.

Sample code CZ0 CZ1 CZ2 CZ3 CZ4

tresponse (s) 30 11 8 10 40trecovery (s) 57 68 35 50 78

°

°

°

°

Fig. 6. Plot of gas response verses test gas concentration for CZ2 sample at325 1C.

Fig. 7. Plot of stability graph for CZ2 sample.

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the presence of optimum CeO2 content, which catalyzes thereaction promoting the rapid electron transfer between theadsorbate and the adsorbent [23]. The sensor resistance rejuve-nates to its initial value after purging the ethanol away whichindicates the surface of ZnO regains the original microstructureafter refreshing with carrier gas (air).

Fig. 6 shows the observed gas response of CZ2 sampletowards various test gas concentrations. The response wasfound to be increased with increase in test gas concentration,however beyond a certain concentration the response getssaturated. At higher concentration the increase in surfacereaction will be gradual, where the saturation point on thecoverage of molecules is reached and we observe constantresponse above a certain concentration. For all test gases,beyond 1000 ppm the slope of the graphs is found to decreasewith concentration indicating the saturation of gas response. Infact, the response of a metal oxide semiconductor is usuallydepicted as S¼A[C]NþB, where A and B are constants and [C]is the concentration of the target gas or vapor. N usually hasa value between 0.5 and 1.0, depending on the change ofthe surface species and the stoichiometry of the elementaryreactions on the surface [24]. As shown in Fig. 6 the curvi-linear relationship between response and ethanol concentrationwas observed, indicating that N¼0.5 for all test gases.

To confirm the selectivity of CZ2 sample towards ethanol,the response towards other gases like acetone, LPG andammonia was also measured at optimum operating tempera-ture. The CZ2 sensor exhibits response of 47%, 35%, 20% and7% towards 100 ppm concentration at 325 1C of ethanol,acetone, LPG, and ammonia, respectively. It is seen that theCZ2 sensor exhibits high selectivity and remarkable responsetowards ethanol with response time of 7 s and recovery timeof 27 s.

The sensor reliability is strongly dependent on the stabilityexhibited by the sensor material. It is well known that oxidebased sensor possess common drawback of decrease in res-ponse due to ageing induced effects. Stability is the consis-tence of the response of a sensor under continuous testing.The responses of the CZ2 towards ethanol were measuredon 10th, 20th, 30th, 40th, 50th and 60th days after the firstmeasurement and shown in Fig. 7. It was found that after 2months, the material performs 96% of its earlier performanceconfirming the stability of the sensor material for commercialapplication.

3.6. Gas sensing mechanism

ZnO is a typical n-type semiconductor and its gas sensingmechanism belongs to the surface-controlled type. It isgenerally thought that absorbed oxygen, which traps electronsfrom the conductance band, plays the most important role ingas sensitivity. In ZnO gas sensors, the changes of resistanceare mainly induced by the adsorption and desorption of oxygenmolecules from the surfaces of the grains.The mechanism of the ethanol detection on the surface of

ZnO may be represented as follows [25,16]:

CH3CH2OHadsþ6Oads� -2CO2þ3H2Oþ6e� (3)

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The surface of the sensor material is covered with chemisorbedoxygen ions. The proportion of these chemisorbed oxygen ionsdepended on the temperature of sample. When ethanol reacts withadsorbed oxygen species on surface of sensor, the electrons returnto the metal oxide leading to a decrease in resistance [26].

The sensitivities of the sensor samples (CZ1, CZ2 and CZ3) arehigher than that of CZ0 and CZ4 sensor samples, which can beascribed to the more active adsorption center produced by the Ceaddition. In addition, CeO2 phases congregating on the surface ofthe ZnO particles have dual effect on gas-sensing properties,namely, positive effect and negative effect. Ge et al. have discussedmore details about the Ce doping effects on the sensitivity of theZnO. They proposed model which shows the appropriate dopingbenefits to the improvement of gas-sensing properties, but super-abundant doping worsens the gas-sensing properties [27]. There-fore, the enhanced sensitivity for CZ2 sample is due to theappropriate Ce addition and temperature.

4. Conclusion

Pure ZnO and CeO2–ZnO composite film sensors weresynthesized by using hydrothermal and screen printing routes.XRD studies confirmed the formation of CeO2–ZnO compositematerials having crystallite size of 51–94 nm. The morpholo-gical investigations showed the sample surface is fully coveredby the spherical grains with average grain size of 50–100 nm.EDAX analysis corroborated the presence of the Ce, Zn and Oin the CZ2 sample. The gas sensing results showed that 2 wt%CeO2–ZnO composite exhibits excellent response (94%) toethanol at 325 1C with fast response (8 s) and recovery time(35 s). The enhanced response is attributed to the smallercrystallite size, which helps in greater oxygen adsorption onthe film surface. From transient response and stability study weconcluded that 2 wt% CeO2–ZnO composite can be utilized asa reliable sensor element in ethanol sensor related applications.

Acknowledgments

One of the authors J.Y. Patil acknowledges DAE-BRNSfor the grant of JRF. All the authors gratefully acknowledgeDAE-BRNS, India for the financial support.

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