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Applied Surface Science 283 (2013) 58 64

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Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com

Prepar cs osemico

C. Balama MEMS and Na al Unb Centre for Na

a r t i c

Article history:Received 20 MReceived in reAccepted 2 JunAvailable onlin

Keywords:LaNbO4NanopowderLPG sensorp-type semico

der TEM,rsor awithrious led t

lowe

1. Introdu

The research for newer materials which can be employed forhigh performance solid-state gas sensors has resulted in the devel-opment of various oxide materials. Semi-conducting metal oxidematerials like ZnO, TiO2, SnO2, In2O3, etc., have been exploited inthe past becmeasuremelent ceramiIt exhibits [1]. LaNbO4because of ithe eld of potential caThe gas senface, leadinin turn chation that leaon the matearea plays asized materand these aerties of semsensors, nanto the prep

CorresponE-mail add

ate, s[5,6]. Several shortcomings are encountered during these processesdue to heat treatments involved which results in higher tempera-tures that leads to high energy consumption. Also, limitations suchas difculty in controlling the component homogeneity, crystalsize, stoichiometric ratio, and formation of undesirable phase, all

0169-4332/$ http://dx.doi.oause of their simplicity, low cost and possible real timent. Among various metal oxides, LaNbO4 is an excel-c oxide having high thermal and mechanical stability.polymorphism viz. monoclinic and tetragonal phasesis used in solid oxide fuel cells, hydrogen sensors, etc.,ts enhanced physical and chemical properties [24]. Ingas-sensing materials, LaNbO4 could be considered asndidate for future developments of sensing materials.sing mechanism involves a redox reaction at the sur-g to a change in the depletion layer of the grains whichnges the electrical resistance. The most common reac-ds to change in conductance is the adsorption of gasesrial surface. Adsorption being a surface effect, surfacen important role in the sensing mechanism. In nano-ials large fraction of atoms are present at the surfacetoms are widely used to enhance the gas-sensing prop-

iconducting oxides. Hence, for the application of gasosized particles are desirable. Earlier work pertaining

aration of LaNbO4 powder relied on methods such as

ding author. Tel.: +82 062 530 1684.ress: [email protected] (D.-W. Lee).

of which to a great extent result in decrease of gas sensitivity fora given material. To eliminate the difculties in getting homoge-neous nanocrystalline stoichiometric compound, a new method ofsynthesis was tried. Due to the increased awareness of potentialhazards in both industrial and domestic environments, there is agrowing need to detect and monitor gases particularly those thatare toxic and potentially explosive. LPG that is widely used as acommon fuel for industrial and domestic purposes is potentiallyhazardous because of its explosive nature on account of leakage.NH3 is harmful and toxic in nature, is utilized extensively in manychemical industries, refrigeration systems, re power plants, etc.Therefore, it is necessary to develop good sensors for the detectionof LPG and NH3. Past two decades, considerable efforts have beendevoted to fabricate various semiconducting gas-sensing materialsbecause they can detect toxic gases, which are relevant for chemi-cal industries, environmental protection, public safety, and humanhealth.

In particular, high-temperature gas sensors are based on semi-conducting metal oxides as they are economical, small in size, easyto handle, and have fast response to changes in gas concentration.They also have additional advantages in terms of long operatinglife, high reliability leading to a low failure rate and effectivenessin a harsh environment.

see front matter 2013 Published by Elsevier B.V.rg/10.1016/j.apsusc.2013.06.013ation and LPG-gas sensing characteristinducting LaNbO4 ceramic material

urugana, D.-W. Leea,, A. Subramaniab

notechnology Laboratory, School of Mechanical Systems Engineering, Chonnam Nationnoscience and Technology, Pondicherry University, Puducherry 605014, India

l e i n f o

arch 2013vised form 22 May 2013e 2013e 13 June 2013

nducting gas sensor.

a b s t r a c t

Lanthanam niobate (LaNbO4) nanopowThermal analysis TG/DTA, XRD, SEM, were used to characterize the precuof LaNbO4 nanopowder was studied ethanol (C2H5OH), as a function of vaof the gas and response time. It reveaLPG (80%) and NH3 (59%) at relatively

ction solid st/ locate /apsusc

f p-type

iversity, Gwangju 500757, South Korea

was synthesized by a low temperature solution-based process. EDX, impedance analysis and Brunauer-Emmett-Teller (BET)nd the calcined LaNbO4 powders. The gas sensing behavior

gases like liquid petroleum gas (LPG), ammonia (NH3), andcontrolling factors like operating temperature, concentrationhat LaNbO4 is a very promising material for the detection ofr operating temperatures.

2013 Published by Elsevier B.V.

pray pyrolysis and solgel process were used to prepare

C. Balamurugan et al. / Applied Surface Science 283 (2013) 58 64 59

In this paper, we report for rst time the LPG, NH3 and ethanolsensing properties of nanocrystalline LaNbO4 prepared by thesolution-based method. The characteristics of LaNbO4-based sen-sor have been studied by measuring resistance of the sensormaterial in observed threducing gaethanol (C2ing future ftime in the

2. Experim

Niobiumhydroxide, peroxide wwere of reagthe variousniobium pesource becamoisture su

LaNbO4solution-baNb2O5 fromacid with aNb2O5 wasplex. To thioxalate waheated at 6was then adhydrous niooxide was by centrifuaddition of chain assocization procassayed at 1(Nb2O5) pre

The secamount of citric acid soThe mixturethe reactanacid C(OH)(three carboions which low color soby adding aevolution ocous gel wa200 C. Furtblack precu2 h, to get acharacteriz

The as-pneous diffeanalysis (T10 C/min tion and cosample. X-rpowders wCu K radiat 2 = 204 s. Scherertallite size

investigated by scanning electron microscopy (Model: Hitachi, SN-3400N). The particle size and compositional analyses were carriedout using transmission electron microscopy (Model: Philips CM-200), and energy dispersive spectroscopy attached with TEM. For

vestigation, Sample was prepared by deposition and evap- of an ethanolic LaNbO4 dispersion on a lacey-lm Cu grid.erEmmettTeller (BET) analysis of prepared powder sam-s carried out with a (Micrometrisics, ASAP 2020), applyingdsorbate. Impedance measurement of the powder was car-t in the temperature range 30175 C in air environment. Aiostat/Galvanostat (Model: Micro auto lab type III) analyzered for the impedance measurement in the frequency range500 kHz.

sensor studies, LaNbO4 nanopowder was mixed with 2%inyl alcohol as a binder to form a paste which was coated ontoic tube substrate provided with platinum wire electrodes

ctrical contacts. The tube was about 8 mm in length, 2 mmrnal diameter and 1.6 mm in internal diameter. The sensort was then heat-treated at 500 C for an hour to removeidual polymer, which helped to make it rigid and impart its

nature. A small nichrome alloy coil was placed inside thes a m. Chrratur

gas anc3) m

ses wd th

valuir an

gas tages ands andvoltation al fo].

(Rg R

s theraturgas-fties 0 to 3

1. Thair and then in a reducing gas environment. It has beenat there is an increase in gas response when exposed toses like liquid petroleum gas (LPG) ammonia (NH3) andH5OH) at 250 C. LaNbO4-based sensor has very promis-or LPG detection in the low ppm range with a responseseconds range.

ental details

pentoxide, lathanam nitrate, citric acid, ammoniumammonium oxalate, hydrouoric acid and hydrogenere purchased from Merck and CDH. All the chemicalsent grade and used without further purication. Among

niobium compound (niobium alkoxides and NbCl5),ntoxide was selected as a starting material of niobiumuse it is not only inexpensive but also easy to controlsceptibility.nanoparticles were synthesized by simple two stepsed method. The rst step is the preparation of hydrated

Nb2O5. In which Nb2O5 was dissolved in hydrouoric constant stirring for about 6 h at 60 C temperature,

dissolved in HF to form [NbOF5]2 or [NbF7]2 com-s, a freshly prepared aqueous solution of ammoniums added in excess, with rapid stirring to the solution5 C. The required amount of the aqueous ammoniaded drop-by-drop, after terminating the reaction, to getbium oxide (Nb2O5nH2O) as precipitate. This hydrousltered and washed with 10% aqueous ammonia solutiongation to eliminate uoride free hydrated Nb2O5. Theammonium oxalate helped to truncate the polynucleariated with hydrous niobium ions during the neutral-ess [7]. The prepared hydrous niobium oxide was then000 C for 3 h to estimate the amount of niobium oxidesent in the hydrous oxide.

ond step involved the dissolution of stoichiometrichydrous niobium pentoxide and lanthanum nitrate inlution (2 mol/mol of metal ion) in the presence of H2O2.

was stirred on a hot plate with a magnetic stirrer untilts were dissolved fully. We utilize in the midst of citricCOOH)(CH2COOH)2H2O in this work. Citric acid hasxylic and one hydroxyl group for coordinating metalhelps in homogenous mixing to get the transparent yel-lution. The pH of solution was maintained at around 7queous ammonia to it. Ammonia addition led to rapidf O2 and NH3, and the solution turned colorless. Vis-s formed by dehydrating the solution on a hot plate ather heating led to charring of the gel and formation ofrsor powder. This dried mass was calcined at 900 C for

novel LaNbO4 nanopowder. The resultant powder wased and evaluated for gas sensor applications.repared precursor powder was subjected to simulta-rential thermal analysis (DTA) and thermogravimetricGA) (Model: Pyris Diamond) at a heating rate offrom 30 to 900 C to determine the phase forma-mplete crystallization temperature of the precursoray diffraction measurements of the calcined LaNbO4ere carried out using X-ray diffraction analyzer withation at room temperature. XRD data were obtained80, with a step size of 0.02 and a count time ofs equation was used to calculate the average crys-of the powder. The morphology of the samples was

TEM inorationBrunauple waN2 as aried ouPotentwas usof 1 Hz

Forpoly-va ceramfor elein exteelementhe resporoustube aaturestempefor theperform(300 cmtest gapart ansteadyfresh afor theing volsensorsensorsignal centramaterition [8

S(%) =

Rg itempeunder properfrom 5

Fig.icro heater to provide the required operating temper-omel-alumel thermocouple (TC) was used to sense thee of the sensor. The schematic of sensor element usedsensitivity studies is shown in Fig. 1. The gas sensinge of the sensor was studied in a sealed test chamberade up of aluminum with a gas inlet and an outlet. Theere injected into the test chamber through an injectione resistance was measured as a function of time untiles were attained. The chamber was then purged withd the experiments were repeated. The electric circuitresponse measurement is shown in Fig. 2. The operat-

(Vh) was supplied to either of the coils for heating the the circuit voltage (VC = 10 V) was supplied across the

the load resistor (RL = 1 M) connected in series. Thege across the load, which changed with minute con-of gas, was measured. For the p-type semiconductingllowing formula is used for sensor response (S) calcula-

Ra)

a 100 (1)

resistance under a given gas concentration at constante and, Ra is the base resistance at constant temperatureree atmosphere (dry air). In this study, the gas sensingof LaNbO4 was investigated for LPG, NH3 and ethanol50 C. At the optimal operating temperature, the sensor

e schematic of sensor element used for the gas response studies.

60 C. Balamurugan et al. / Applied Surface Science 283 (2013) 58 64

Fig. 2. Electrical circuit of the gas response test system (Vh: heating voltage, Vc:circuit voltage, Vc: output signal voltage and RL: load resistance).

response was measured as a function of various concentrations oftest gases a

3. Results

The therat temperatas shown intered at 12(19%), whicchemically mic peak oat 363 C, rand decomeration of Cof heat facimic peak ophase transco-existencNo more w738 C.

Fig. 3. SimultLaNbO4.

-ray diffraction patterns of LaNbO4 precursor calcined at (a) 600 C, (b)c) 800 C and (d) 900 C.

4 shows the XRD pattern of LaNbO4 powder calcined at00 C for 2 h. The carbonaceous precursor of LaNbO4, whend below 700 C was amorphous. The diffraction patternss well as response time.

and discussion

mal decomposition of LaNbO4 precursor was evaluatedures ranging from 30 to 900 C using TG/DTA analysis

Fig. 3. The endothermic peak on the DTA curves cen-8 C, together with drastic weight loss in the TG curveh corresponds to the evaporation of residual water andbound water in the precursor sample. The rst exother-ccurred with a huge mass loss of 30% and centeredepresenting the reaction of nitrates with citric acidposition of the residual organic matter with the lib-O2 and N2. The generation of an enormous amountlitated formation of LaNbO4. The last sharp exother-ccurred at 738 C with mass loss. This is due to theition of tetragonal LaNbO4 to monoclinic phase or thee of this phase together and crystallization of LaNbO4.eight loss was found after temperature higher than

Fig. 4. X700 C, (

Fig.6009calcineaneously recorded TGA/DTA cures of the carbonaceous precursor of

obtained afpeaks of bocard No. 86peaks at 2and 29.05, reection shperature thwhereas dithe temperobtained, a(card No. 7observed. Tcrystalline. monoclinicThe calculaof the sampter values. Asharper witthat the grater calcinations at 700 and 800 C showed reectionth tetragonal and monoclinic phases of LaNbO4 (JCPDS-0911and 78-0158) [9]. It can be observed that the

values of 29.05, 31.21, 34.74, 53.30 and 59.68 at 700 C,31.21, 40.04, 40.43, 46.46, 60.21 and 63.63 at 800 Cow monoclinic phase crystallites. With increasing tem-e diffraction peaks of tetragonal gradually decreaseffraction peaks of monoclinic become stronger. Whenature reaches 900 C monoclinic phase of LaNbO4 isll the peaks were well consistent with standard JCPDS8-0158) and no characteristic peak of impurities washis conrmed the sample as pure monoclinic LaNbO4This is considerably a lower temperature to obtain the

phase of LaNbO4 compared with all previous report.ted lattice parameters [a = 5.52, b = 5.15 and c = 11.47]le matched closely with the standard lattice parame-t higher calcination temperatures, XRD peaks becamehout obvious extension in width. It can be concludedin size and crystallinity increased with the calcination


Fig. 5. SEM image of the LaNbO4 nanopowder calcined at 900 C.

temperature in the range of 700900 C. The crystalline size D, wasdetermined by using the Scherrer equation;

D = 0.9(B cos

where is tfull width aand is theof the grain900 C, resp

Fig. 5 shoder particleparticles. Tnature. TEM900 C for 2shape of theeter of LaNpattern of thtern showeplan conrand (2 0 0) monoclinicdiffraction a

The atomand were 1which was

Fig. 6

Fig. 7. SAED analysis of the LaNbO4 nanopowder calcined at 900 C.

Fig. 8.

mplex impedance plot of the LaNbO4 nanopowder at 30 Cn in Fig. 9.The high frequency semi-circle absence indicates

e material conductivity depends on the grain boundaries. Thence R of the sample was obtained from the intercept on theof the complex impedance plot. The conductivity of LaNbO4lated by using the following equation;

(3))(2)

he corrected wavelength of the X-ray radiation, B is thet half maximum corrected for instrumental broadening,

Bragg angle of the diffraction peak. The average sizes were calculated as 35, 42 and 62 nm for 700, 800 andectively.ws a SEM micrograph of the calcined powder. The pow-s are found to be agglomerates of nano-sized primaryhe particles were spherical-like structure with porous

image of the prepared LaNbO4 powder calcinated at h is shown in Fig. 6. The image conrms the spherical

particles, with uniform distribution. The average diam-bO4 particles was found to be about 60 nm. The SAEDe calcined sample is shown in Fig. 7. The diffraction pat-

d that the powder is well crystalline, and the observedmed the presence of crystal lattice of (1 1 2), (0 1 3)orientation. The observed diffraction plan revealed the

phase of the sample. This is consistent with the X-raynalysis.ic % for La, Nb and O were found out from EDX (Fig. 8),

7.60, 17.50 and 64.90, respectively, (La1.06Nb1.05O3.89),very close to the stoichiometric ratio of pure LaNbO4.

The cois showthat thresistax-axis is calcu

= LRA. TEM image of the LaNbO4 nanopowder calcined at 900 C. Fig. 9 EDX analysis of the LaNbO4 nanopowder calcined at 900 C.. Nyquist plot of the LaNbO4 nanopowder obtained at 30 C.


Fig. 10. Arrhenius plot of log conductivity versus reciprocal temperature forLaNbO4.

where L and A are the thickness and known area of the sample. Elec-trical conductivity varied linearly as a function of temperature. It isnoted from the Arrhenius plot that increased temperature increasesthe conductivity of the sample (Fig. 10). The activation energy ofconduction process was calculated from the slope of log versus1/T plot. Inenergy of Lperature deobserved ina semicondface area ofby Brunaue24.36 m2/g.

Fig. 11 shLaNbO4 basgas sensitiv(59%) and eLaNbO4 basperature of

Fig. 11. Gas regases as a func

Fig. 12. Variation in gas response for LPG, NH3 and ethanol gas as a function of gasconcentrations at 250 C.

the variation in gas response for LPG, NH3 and ethanol gas as afunction of gas concentrations at 250 C. The sensitivity increasedlinearly with gas concentrations up to 500 ppm. The gas displayedthe tendency to get saturated at concentrations above 500 ppm.The response of 500 ppm LPG was higher than that of 500 ppm NH3

hanoe ressign o

axiced

raturack

s theLaNbs waximumovo resan 10 the temperature range of 30-175 C, the activationaNbO4 was found to be 0.29 eV. The observed tem-pendence on electrical conductivity is similar to those

semiconductor oxide systems. LaNbO4 may behave asuctor due to its oxygen deciency [10]. The specic sur-

LaNbO4 nanopowder calcined at 900 C was estimatedr-Emmett-Teller (BET) method and it was found to be

ows the gas response versus operating temperature fored sensor for different test gases. At 250 C, the highestity was observed for LPG (80%), followed by ammoniathanol (22%). This demonstrates that the response ofed sensor to ammonia and ethanol at an operating tem-250 C is very low compared to that of LPG. Fig. 12 shows

and etgas. Ththe dereach mintrodutempecome bdepictgas of LPG gathe mawas retime tless thsponse characteristics of LaNbO4 nanopowder for various reducingtion of operating temperature.

Fig. 13. Respooptimum operl, implying that the sensor was highly selective for LPGponse and recovery times are important parameters inf sensors. The response is dened as the time taken to

mum response when the required amount of test gas is into the chamber and keeping the sensor at operatinge. The recovery time is the time taken by the sensor toto its initial value after the gas was removed. Fig. 13

response and recovery curves to 500 ppm LPG and NH3O4 sensor at an operating temperature 250 C. Whens introduced, the response of the sensor increased andm response time was less than 25 s. After the LPG gas

ed, the response gradually decreased and the recoverytore original value from the maximum response was0 s. The response and recovery time for NH3 was 10 snse and recovery of LaNbO4 sensor as a function of time at theirating temperatures for LPG and NH3 gases.


and 80 s, respectively. It is well known that LPG consists of CH4,C3H8, and C4H10, etc. and in these molecules the reducing hydro-gen species are bound to carbon therefore LPG dissociates less easilyinto reactive reducing components on the LaNbO4 Surface. Sensormaterials arthey show they are exLaNbO4 nanvity when tsensor.

Metal oxconcentratiabsence of tmaterial anating a depthe intergraprocess resinteractionsthe nature bulk defectsensor surfmaterial ansensor remdecreases whas a strongtivity. In chesurface atombate and thea more impconductivit

= o exp(

where is k is Boltzmis the partof the poilattice.

When a on the surfaband and prsuch as O2

holes in valdecrease duform of sucdepends oning temperasensitivity tand O2 areesting procreactive anence of redgas molecuand effectivsensor mate

The surfcan be desc

CnH2n+2 + 2

CnH2n: O +

Here, Cnrepresents surface.

The reaction between NH3 and the chemisorbed oxygen O cantake place as described below [14]:

2NH3 + 3O(ads) N2 + 3H2O + 3e (7)

trone conf thes ancreasctor.

avaith

4 shre ofnt a

reacmpexygempetratir theratur

clus

ovel per

LaNb andre deius rrespoC thless t

y

ocryas

wled

s wom, thunde1A2A

nces

okkermal ional

Hsiao lumiuminaugs

hotanauigsed La

Tolch LaNbamic okkeperatay pyr. Das,m nio4.e classied as n-type or p-type, depending on whetheran increase or decrease in electrical resistance, whenposed to reducing test gases. In the present study, theopowder based sensor offered an increase in resisti-

est gases were introduced. Hence it behaved as a p-type

ide semiconductors are mainly used to detect smallons of reducing and combustible gases in air. In theest gas, oxygen gets chemisorbed on the surface of oxided it extracts electrons from the conduction band, cre-letion layer at the surface of the individual grains andnular region. Thus, equilibrium of the chemisorptionsults in the stabilization of the surface resistance. The

between the test gas and sensing surface depend onof physisorption, chemisorptions, surface defects ands. Physisorption is the weakest form of adsorption toace; no electrons are exchanged between the sensord the gas molecules. Hence, the conductivity of theains unchanged. Usually, physisorption monotonicallyith increasing operating temperature. Chemisorptioner interaction that shows a better sensitivity and selec-misorptions, adsorbate forms chemical bonds with thes and thus the electronic structure of both the adsor-

surface are modied. Hence, defects on the surface playortant role than bulk defects. The change in electricaly of a material is given by;

EakT

)P(O2)

1/n (4)

the electrical conductivity, Ea is the activation energy,anns constant, T is the absolute temperature, p(O2)ial pressure of O2, and n depends on the naturent defects arising when O2 is removed from the

sensor is exposed to air, oxygen molecules are adsorbedce of the LaNbO4. It traps electrons from the conductionoduces negatively charged chemisorbed oxygen species, O and O2, respectively. Hence, the concentration ofence band increase and the resistance of the materiale to increased concentration of available carrier. Theh chemisorbed oxygen species at the surface strongly

the operating temperature. At relatively lower operat-ture, the surface adsorbs more O2 and this causes theo become low. As the temperature increases, both O

chemisorbed. The adsorption of O was the most inter-ess in sensors, because these oxygen ions were mored thus made the material more sensitive to the pres-ucing gases. When LaNbO4 is exposed to reducing gas,les react with adsorbed oxygen species on the surfacee electron transfer occurs from the gas molecules torial.ace reactions between the LPG and the oxygen speciesribed as follows [1113]:

O H2O + CnH2n: O + e (5)

O CO2 + H2O + e (6)

H2n+2 represents CH4, C3H8, C4H10, etc., while CnH2n:Opartially oxidized intermediates on the LaNbO4

Elecinto thsome ocarrieri.e., inconduon thereact wLaNbOperatusufciewhichular teof all oThe teconcensons fotempe

4. Con

A nlow temture ofby XRDperatuArrhenbetter of 250about

Novelt

Nancess. Gtime.

Ackno

Thiprogragrant, f2012R

Refere

[1] T. MThenat

[2] Y.J.andof L

[3] R. Hort

[4] R. Hdop

[5] J.R.ingCer

[6] T. Mtemspr

[7] R.Nsiu71s released from the above reaction are injected backduction band of sensor material. These recombine with

holes, thus reducing the concentration of the charged decreasing the conduction of the sensor material,ing electrical resistance of the p-type LaNbO4 semi-The maximum response for various gases dependsilability of ionic species on the sensor surface whichgas molecules. Compared to ammonia and ethanol,

owed highest response for LPG at an operating tem- 250 C. This may be attributed to the availability ofdsorbed ionic species of oxygen on the sensor surfacet most effectively with LPG molecules at this partic-rature. Beyond the optimum temperature, desorptionn ionic species occurs causing a decrease in response.rature plays an important role in the charge carrieron and the Debye length. This may be one of the rea-

decrease in response of the LaNbO4 sensor at highere.

ion

nanocrystalline LaNbO4 powder was prepared by usingature solution-based process. Typical monoclinic struc-O4 was obtained at 900 C for 2 h and it was conrmed

electron diffraction pattern of TEM analysis. The tem-pendent conductivity plots of the sample obeyed theelation. Sensitivity analysis showed that LaNbO4 has anse to LPG and NH3 gases at an operating temperaturean C2H5OH. The response times of LPG and NH3 werehan 25 and 10 s, respectively.

stalline LaNbO4 prepared by low temperature pro-sensing behaviour of LaNbO4 reported at the rst

gement

rk was supported by the WCU (World Class University)rough a National Research Foundation of Korea (NRF)d by the Korea government (MEST) (No. R32-20087 and2A01014711).

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Preparation and LPG-gas sensing characteristics of p-type semiconducting LaNbO4 ceramic material1 Introduction2 Experimental details3 Results and discussion4 ConclusionNoveltyAcknowledgementReferences

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