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Solid State Sciences 13 (2011) 2011e2018

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Solid State Sciences

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Surfactant-assisted synthesis, characterizations, and room temperature ammoniasensing mechanism of nanocrystalline CuO

Iqbal Singha,*, R.K. Bedib,1

a P.G. Department of Physics, Khalsa College, Amritsar 143005, Punjab, IndiabMaterial Science Laboratory, Department of Physics, Guru Nanak Dev University, Amritsar 143005, Punjab, India

a r t i c l e i n f o

Article history:Received 7 July 2010Received in revised form3 September 2011Accepted 13 September 2011Available online 21 September 2011

Keywords:Sol-gel auto combustionSurfactantSensorElovich equation

* Corresponding author. Tel.: þ91 9872828203; faxE-mail addresses: iqbalsgh@yahoo.comand (I. Sin

rkbedi@rediffmail.com (R.K. Bedi).1 þ91 9814729284 (Mobile).

1293-2558/$ e see front matter � 2011 Elsevier Masdoi:10.1016/j.solidstatesciences.2011.09.003

a b s t r a c t

CuO nanocrystalline powder has been synthesized by a sol‒gel auto combustion route with cetyl-trimethylammonium bromide (CTAB) as cationic surfactant, and sodium dodecyl sulphate (SDS) asanionic surfactant. The powder samples are characterized by TGA/DTA, XRD, FESEM, and TEM tech-niques. Thermal analysis of the dried gel samples shows that addition of surfactant in the precursorincreases the heat of reaction, which is evolved in the decomposition of metal citrate complex. The CTABand SDS addition in the reaction mixture lowers the average crystallite size to few tens of nanometer.Surfactant doping in precursor causes a variation in lattice strain and changes to its type to compressive.CuO nanoparticles are bound together into facetselike weakly aggregated clusters, as indicated by FESEMimages. TEM micrographs indicate the porous, nearly spherical particles having crystallite size around 7and 18 nm for CTAB and SDS surfactant assisted CuO samples respectively. CuO nanoparticles assembledas thick film have been tested for their response to 100 ppm ammonia gas at room temperature. Cationicsurfactant assisted sample shows maximum response to ammonia as compared to anionic surfactant.The CTAB assisted sensor shows almost completes recovery in 500 s whereas SDS assisted sample shows75% recovery in the same time. The ammonia response of the films obeys the Elovich equation. Theresponse rate of sensor is found to be maximum for CTAB assisted CuO films as compared to othersamples. The kinetics of the response reaction shows that the ionic surfactants assisted CuO followssecond order reaction kinetics.

� 2011 Elsevier Masson SAS. All rights reserved.

1. Introduction

The shape and size are important factors in determining thestructural, physical, and chemical properties of metal oxide nano-particles. Different morphological nanostructures have differentelectronic, optical, and magnetic properties. The large surface tovolume ratio facilitates the use of metal oxide material in catalytic,solar cell and gas sensing, etc. important applications [1e3].

CuO is a p-type semiconductor with direct band gap of 1.36 eV[4], which makes it a promising material for solar cells [4,5], asa catalyst [6], and in gas sensors [7] etc. applications. Therefore, dueto versatile properties and wide applications of CuO, a variety ofstructures have been fabricated by using different synthesis tech-niques and reported in the literature [8e11]. Previous reports

: þ91 183 2255619.gh), rkbedi2008@gmail.com,

son SAS. All rights reserved.

suggest that higher temperature, longer reaction time, andsophisticated instruments were required for the growth of CuOnanostructures. There is a need to develop a simple, easy and costeffective method to synthesize the complex CuO nanostructures inlarge quantity.

Surfactant based methods are usually considered suitable forlarge scale synthesis of variety of nanostructures. Surfactants areemployed as a stabilizer, capping agent and template which cancontrol the size, anisotropic growth and agglomeration. CTAB andSDS surfactants have been widely used as cationic and anionicsurfactants respectively, for synthesizing large number of inorganicmaterials with a controlled shape and size [12,13]. The addition ofsurfactant in precursor solution reduces the surface tension of thesolution, facilitates nucleation, and controls the formation of newphases. Surfactant molecules are composed of a hydrophilic headand a hydrophobic tail which results in the formation of reversemicelles in the precursor solution. Hydrocarbonic tail can play therole of fuel in the combustion process, whereas the formation ofreverse micelles acts as growth controller, and maintain thedistance between the particles, hence works as agglomeration

I. Singh, R.K. Bedi / Solid State Sciences 13 (2011) 2011e20182012

inhibiter. The supermolecular arrangement of surfactants whichact as template is utilized for synthesizing nanocrystal superlattice,nanotubes, nanorods, nanowires, spherical nanoparticles, meso-structure with different compositions and pore size. These mate-rials attract considerable attention because of their remarkablylarge surface area and narrow pore size distributions, which makethem ideal candidates for catalysts, molecular sieves, gas sensors,etc. Chen et al. [14] have discussed the synthesis of nickel nano-particle in an aqueous solution of CTAB. Wang et al. [15] havediscussed the synthesis of SnO2 nanoparticles with surfactantmediated method. Kang et al. [16] have discussed the effect of CTABconcentration on the synthesis of gold nanorods and found that itfacilitates the growth of nanorods in one direction. Mandel et al.[17] have elaborated in detail the growth of anisotropic silvernanoparticles in the presence of SDS surfactant. Zhang et al. [18]and Salleh et al. [19] have investigated the the effect of CTAB andSDS surfactant concentration on the synthesis of Cu2O and Fe2O3powder respectively The mechanism of micelle formation in SDSand CTAB surfactants have been discussed in detail by Topallaret al. [20].

Sol-gel auto-combustion is one of the cheapest methods withan advantage of a lower calcination temperature and ultra fineproduct particles having large surface to volume ratio [21e26].This route has an advantage over other studies involving sol-gelcombustion, as the surfactant based reaction keeps the sol parti-cles separated because of the electrostatic stabilization. In thepresent work, nanostructures of CuO particles are synthesized byusing CTAB and SDS surfactant in sol-gel auto combustion route.The effects of addition of different type of surfactant in the reac-tion mixture have been investigated. The addition of surfactantresults in the lowering of crystallite size, controlling of agglom-eration, and modifying the gas sensing properties of the CuOnanoparticles. The response of surfactant assisted CuO thick filmand kinetics have been explored for 100 ppm ammonia gas atroom temperature.

2. Experimental

2.1. Synthesis of nanosized CuO

Cu(NO3)2 3H2O and citric acid were used as starting materials.The precursor solution was prepared by taking metal nitrate tocitric acid (MN:CA) ratio 1:1. The pH of the solutionwas adjusted to7 by adding liquid ammonia. In the first step, the solution wascontinuously stirred using a magnetic stirrer at 80 �C for 4 h toform a homogeneous sol. An aqueous solution of CTAB and SDS wasprepared by dissolving required quantity of salt in deionized waterto give a final concentration of 0.5 M. In the second step, the 10 mLof aqueous surfactant solution was added drop wise to thehomogeneous sol. The solution was thermally dehydrated in anoven (80 � 5 �C), which transformed it into a viscous liquid. Theviscous liquid was heated on a preheated hot plate maintained attemperature of about 300 �C. The material underwent foaming,followed by the decomposition of metal-citrate complex with thegeneration of large volume of gases. The whole process took lessthan 5 min to complete. The voluminous and foamy combustionproduct could be easily crushed to get the decomposed gel. Thedetailed mechanism of auto combustion reaction with metalnitrate and citric acid (ratio 1:1) with different surfactantconcentration has already been discussed [27,28]. The decomposedgel samples were calcined at 400 �C for 4 h with heating rate of10 �C/min using a muffle furnace (Macro Scientific), whichcompletely transformed it into a pure CuO powder without anyimpurity.

2.2. Characterization

The thermal analysis of the dried gel samples with, and withoutthe addition of surfactant was carried out using Perkin Elmer (Pyrisdiamond) thermal analyzer. The weight of the samples taken forthermal analysis were approximately 10.36� 0.15 mg. Air was usedas purged gas in the analysis. The heating rate and air flow rate of10 �C/min and 50 mL/min, respectively, were taken with aluminapowder as a reference material. The samples were kept at thecalcination temperature for the desired period of time and werecooled down inside the furnace.

The phase identification of the decomposed gel and the calcinedpowder samples were performed by taking X-ray diffraction (XRD)pattern using X’Pert Panlytical diffractometer (Cu Ka radiation,l ¼ 1.5405 Ǻ, 30 mA, 40 kV) in 2q range from 30 to 80�. The surfacetopography of copper oxide powder sample was studied by fieldemission scanning electron micrographs, taken on JEOL JSM-6700Fsystem, with a beamvoltage of 30 kV. TEM images were taken usingtransmission electron microscope system (HRTEM, model FEITechnai 30) operated at 300 kV.

A known quantity of CuO powder was thoroughly mixed withfew drops of diethnolamine (used as organic binder), grinded inmortar and pestle to obtain a fine paste. The uniform layer of pastewas deposited onto the glass slides, which were rinsed in nitricacid, and then ultrasonically cleaned. The layer was dried at 400 �Cin the muffle furnace for 1 h to burn out the organic binder. Thethickness of the film was monitored using depth profiler (Dektek3030 XT) and was found to be 50 � 0.5 mm.

The thick films were characterized for their response toammonia gas in dc conductivity mode. The corresponding changein resistance was observed in the measuring cell (volume 500 cm3).The resistance in presence of air and ammonia gas atmosphere atroom temperature (300 K) has beenmeasured using Keithley 6517Aelectrometer having in built source of constant voltage powersupply. Gas response (S) of thick film sensor to ammonia is definedas the ratio of change in the resistance of the sample on exposure toammonia to the resistance in air and is given by the equation asfollows

S ¼ jRg � RajRa

(1)

whereRg andRa areCuO thickfilm resistance,measured in ammoniaand air atmosphere respectively. The time taken for the sensor toattain 90% of themaximum change in resistance on exposure to thetarget gas is the response time. The time taken by the sensor to getback 90% of the original resistance is the recovery time.

3. Results and discussion

3.1. Thermal analysis

Fig. 1. shows the decomposition of metalecitrate complex byauto combustion reaction. It has been observed that the rate andexothermicity of the reaction is influenced by the addition ofsurfactant to the precursor solution. The appearance of firstexothermic peak in the DTA curve of all samples is attributed toredox reaction. This peak gets broadened and its position shiftsfrom temperature of 201e206 �C with the addition of SDS, whileaddition of CTAB induces a shift to a temperature of 263 �C. Theenergy release in case of CTAB and SDS assisted samples iscomparatively higher. The increased exothermicity of reactionstrongly shows the emission of large quantity of gaseous productwhich keeps the powder loosely agglomerated. A secondexothermic peak of weaker intensity appears in DTA plot as shown

a

b

c

a

b

c

Fig. 1. The TG/DTA plots of the dried gel samples obtained (a) without surfactant, (b)with CTAB, and (c) with SDS. All the dried gel samples obtained from the precursorsolution having MN:CA ratio 1:1, at pH 7.

I. Singh, R.K. Bedi / Solid State Sciences 13 (2011) 2011e2018 2013

in the Fig. 1. This corresponds to the burning of the residual organicmatter that leave the system in the form of COx and NOx (x ¼ 1, 2),gases. This peak appears at temperature of 323 �C without surfac-tant while with the addition of CTAB it shifts to 332 �C. The SDSaddition shifts it to temperature of 341 �C. Investigations reveal thataddition of surfactant affects the decomposition behavior of themetal citrate complex and these are potential source of heat, playa pivotal role as a fuel besides citric acid.

A gradual weight loss of 10% in TG plots (Fig. 1.) occurs in thetemperature range of 100e150 �C, which is due to the removal ofresidual water in the gel. First step in the major weight loss of about87% appears in the temperature range of 190e220 �C. The CTABassisteddriedgel decomposition results in aweight loss of about 62%,whereas SDS addition results in the weight loss of 79% in thetemperature rangeof190e250 �C.Nosignificantweight losshasbeenobserved beyond 400 �C in all the three samples, this result indicatesthermal stabilization of the sample. Based upon these observationsthe optimum temperature of calcination has chosen to be 400 �C.

Fig. 2. XRD diffractogram of the decomposed gel samples obtained (a) withoutsurfactant (b) with CTAB, and (c) with SDS. The decomposed gel samples were ob-tained as product from auto combustion reaction of dried gel samples.

3.2. X-ray diffraction analysis

The X-ray diffractograms of the decomposed gel sampleswithout surfactant, with CTAB, and SDS surfactants are illustrated

in Fig. 2. The decomposed gel samples consists of mixed reflectionscoming from Cu2O and CuO phases of copper oxide. This fact isobviously supported by thermal analysis, which suggests thatinsufficient time makes the decomposition of metal-citratecomplex incomplete. Intense diffraction peaks corresponding to(002) and (111) atomic planes of CuO phase have been found for thedecomposed gel sample at 2q value 35.5� and 38.7� respectively.Among these prominent peaks, a peak having lower intensity alsoappears corresponding to (111) atomic plane of Cu2O at 2q value36.4�. A peak corresponding to (111) atomic plane of Cu at 2q valueof 43.1� can also be seen in the diffractogram.

The XRD diffractograms of the calcined samples withoutsurfactant, with CTAB, and SDS surfactants are shown in Fig. 3. Allthe reflections on the pattern can be indexed to monoclinic CuOphase, with peak positions corresponding to those reported byInternational Center for Diffraction Data (ICDD) card 41e254. Thedominant peaks located at 2q value 35.5� and 38.7� corresponds toreflection from (002) and (111) atomic planes respectively, indi-cates the existence of CuO phase. Interestingly, no peak corre-sponding to Cu2O phase has been noticed in the diffraction patternof calcined samples, this shows complete transformation to CuOphase. The presence of the Cu2O phase in the decomposed gel actslike catalyst site for the formation of CuO. The presence of Cu2Ophase has been confimed at different surfactant concentration andMN:CA ratios of the precursor reported earlier [27,28]. Theconversion of the Cu2O into CuO in the calcined sample results fromthe diffusion of oxygen atoms [20] and can be represented by thereaction

2Cu2Oþ O2 ¼ 4CuO (2)

The observed interplanar spacing, ‘d’ value of the calcinedsamples with andwithout surfactant shifts to lower and higher siderespectively as compared to the d value reported by ICDD card ofCuO. This suggests the accumulation of compressive strain in caseof surfactant assisted CuO sample. Although, tensile strain has beenobserved in case of the sample without surfactant. These obser-vations agree with those reported by Ramgir et al. [29]. The averagecrystallite size and the internal lattice strain of the decomposed gel,

a

b

c

Fig. 3. XRD diffractogram of the samples calcined at 400 �C (a) without surfactant, (b)with CTAB, and (c) with SDS. The diffractograms show that calcination results in theelimination of residual organic matter and improves the crystallinity of the CuOpowder.

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and calcined samples have been evaluated by the HalleWilliamsonequation expressed as [30]:

bcos ql

¼ 1Dþ εsin q

l(3)

where b is the FWHM of the powder, q the Bragg angle, l thewavelength of X-ray used, D the crystaliite size, and ε the internallattice strain. The D and ε values have been calculated from the leastsquare fit of b cos q/l vs sin q/l plots for all the prominent peakshaving comparatively higher intensity as shown in Fig. 4. Obser-vations reveal that introducing CTAB, and SDS surfactants to theprecursor sol results in drastic decrease of average crystallite size.

f

e

d

c

b

a

Fig. 4. Plot of Hall equation for the decomposed gel and the samples calcined at 400 �C(a); (b) without surfactant, (c); (d) with CTAB, and (e); (f) with SDS respectively. Theplot depicts the variation in the lattice strain and its type, as the slope of the plotchanges from positive to negative with the addition of surfactant.

The crystallite size of around 18 nm has been observed in case ofthe CuO with CTAB, while SDS addition brings the average crys-tallite size to 15 nm, thus showing the formation of nanocrystallineCuO phase. Table 1 shows that negative slope of the plots observedin case of decomposed gel samples obtained with the addition ofsurfactant and confirms the presence of compressive strain in thecrystal lattice [31]. Interestingly, positive slope in case of decom-posed gel and its calcined sample without surfactant shows thepresence of tensile strain. The higher magnitude of the slope in theCuO sample prepared with CTAB in comparison to SDS surfactantsuggests the enhancement of the strain.

The lattice parameters (as bs c, a¼ g¼ 90�sb for monoclinicstructure) and the volume of unit cell have been calculated usingthe following relations

1

d2¼ 1

sin2b

h2

a2þ k2sin2b

b2þ l2

c2� 2hlcos b

ac

!(4)

V ¼ abcsin b (5)

where d is the interplanar spacing, h, k, l are Miller indices of thecrystal planes, a, b, c, b are the lattice parameters and V is thevolume of the unit cell. The values of the lattice parameters withestimated error are recorded in Table 1 and found to be in goodagreement with those contained in ICDD card. As seen from theTable 1 the CTAB and SDS addition causes a decrease in the crys-tallite size and an increase in the unit cell volume. The overall effectof surfactant addition is to generate a non-uniform strain in thenanocrystallites. Similar results have also been obtained by [31] forRu doping in the SnO2. CTAB addition generates maximum strain inthe CuO lattice and lowers the average crystallite size to 18 nm.

3.3. FESEM and TEM analysis

The FESEM images of the calcined sample are shown in Fig. 5.The porous structure of the agglomerates obtained in all thesamples may be attributed to the liberation of large quantity ofgaseous product during combustion. The particles appear to boundtogether into agglomerates of different shapes and sizes. Theagglomerates in the samplewithout surfactant show comparativelydense structure with distorted spherical shape. The shape of theparticles improves and approaches facets like morphology with theaddition of CTAB and SDS surfactants as evident in the Fig. 5. Theparticle size of the CTAB assisted samples lies in the broad range50e80 nm, whereas SDS addition have narrow particle sizedistribution in 20e30 nm and samples without surfactant showscomparatively large particle size around 125 nm.

The variation in the shape and particle size of the CuO nano-particles is in fact the consequence of the nature of synthesis ofpowder preparation. The particle size and the agglomeration in thematerial dependmainly on theway the combustion propagates andthe amount of disintegration occurs. In case of the reaction mixturewith MN:CA ratio 1:1, the reaction proceeds at higher propagationrate, and higher temperatures as compared to the fuel enrichedconditions. The higher reaction temperature appears to calcine theresultant particles. Consequently, the particle size grows as thereaction propagates even during the post thermal treatmentapplied to remove the organics. The surfactants which are source ofcarbon, and work as fuel besides citric acid, (as evident from thethermal analysis) may act as a space filling agent, which will leaveempty spaces as fuel burns during the reaction. This results in theexistence of huge porosity of the powder samples. Increasing thefuel content would further result in more gas liberation, whichhelps to disintegrate the agglomerates into nanoparticles, and their

Table 1Values of the lattice constant, a (Å), b (Å), c (Å), b (Degree), V cell volume (Å3), ε strain, D crystallite size calculated using Hall equation plot, crystallite size from TEM, response,a0 initial absorption rate, a potential barrier, first order, second order equation plot and k sensor response rate constant for the CuO samples calcined at 400 �C withoutsurfactant, with CTAB and SDS assisted samples.

Property Without surfactant CTAB SDS

a (Å) 4.682 (0.003) 4.698 (0.019) 4.708 (0.029)b (Å) 3.424 (0.007) 3.425 (0.006) 3.429 (0.002)c (Å) 5.114 0.022) 5.116 (0.020) 5.129 (0.007)b (Degree) 99.111 (0.151) 99.278 (0.016) 99.592 (0.330)V cell volume (Å3) 80.949 (0.390) 81.242 (0.478) 81.643 (0.523)Non uniform Strain (ε), (Standard Deviation)

(type of strain)0.0026 (0.0007)(tensile)

�0.0060(0.0010)(compressive)

�0.0033(0.0009)(compressive)

Average Crystallite size (nm) XRD(Standard Deviation)

100 (28) 18 (11) 15 (8)

Average Crystallite size (nm) TEM 60 7 18Response �0.82 �0.98 �0.99a’ (x 10�8) 6.0 10.7 2.6a (MU) 0.50 0.33. 0.25Ln(CeeCt) vs t equation �0.011x�12.305(R2 ¼ 0.9273) �0.01xe12.406(R2 ¼ 0.7025) �0.019xe10.317(R2 ¼ 0.5524)Ct vs (Ct e C0)/t equation 270.23x e 1E-07 (R2 ¼ 0.8073) 244.78x e 9E-08 (R2 ¼ 0.9869) 236.84x e 4E-07 (R2 ¼ 0.9236)k, rate constant 0.011 s�1 2895 U�1s�1 1080 U�1s�1

Minus sign with the response means that the sensor’s resistance decreases during the gas hit.

I. Singh, R.K. Bedi / Solid State Sciences 13 (2011) 2011e2018 2015

subsequent growth. Similar results have been discussed during thesynthesis of LiMn2O4 and Ni-YSZ cerment electrodes by combustionsynthesis respectively by Daniela et al. [32] and Marjan et al. [33].

TEM images of the sample with CTAB and SDS also show theporous nature, and minimum agglomeration with nearly sphericalshaped particles as evident in Fig. 5. TEM images also reveal broadercrystallite size distribution for sample without surfactant around60 nm, whereas CTAB and SDS addition reduces the crystallite sizeto 7 and 18 nm respectively. The spherical nature of the CuO crys-tallite appeared inTEM images, preparedwith CTAB and SDSmay beattributed to the formation of spherical micelles in the gel. In thisstudy, the concentration of cationic and anionic surfactants in thereaction mixture has been chosen to be higher than Critical MicelleConcentration (CMC). Above CMC, the surfactant molecules aggre-gate to form micelles. The micelles in the gel control the growth ofcrystallites as well as their agglomeration. The shape of micelledepends on the surfactant concentration, and the surroundingmedium of surfactant. In these TEM images, we assume thatmicelles are in nearly spherical shape. Micelles not only providefavourable site for the growth of the particulate assemblies, but alsoinfluence the processes including nucleation, growth and coagula-tion. The ionic surface of micelle is called as Stern layer. This issurrounded by counter ions and an oriented water molecule iscalled Gouy-Chapman layer or diffuse layer. The Stern and Gouy-Chapman layers are collectively known as electrical double layer[34]. This double layer serves as a diffusion barrier for the growthspecies. The diffusion limited growth would reduce the size distri-bution of the initial nuclei leading to monosized nanoparticles. Thesurfactant-assisted method is a cost effective and helps to synthe-size nanocrystals of controllable size which is simple and conve-nient way. The crystallite size as calculated from the XRD and TEMmeasurements are nearly in agreement with each other.

3.4. Ammonia sensing and its kinetics

The response of CuO thick films to 100 ppm ammonia gas atroom temperature is shown in the Fig. 6. A decrease in surfaceresistance of the film has been observed with the introduction ofammonia in the testing cell. This may be attributed to the redoxreaction of ammonia with the adsorbed oxygen species O2

�.Observations reveal that CuO thick film sensors have good sensingperformance combined with high sensitivity and a short responsetime. The response of the sensor without surfactant is found to be0.82, it increases to 0.98 and 0.99 for the samples prepared with the

addition of CTAB and SDS surfactants respectively. The higherresponse in case of the surfactant assisted sensor may be due to thesmall crystallite size and huge porosity of the material as suggestedby Jimenez et al. [35]. The response time for the sensor withoutsurfactant is 120 s. The CTAB and SDS addition decreases theresponse time to 70 s sand 30 s respectively. The sensor withoutsurfactant recovers only 58% in 500 s. Interestingly, CTAB and SDSassisted sensors recovers efficiently by 92% and 75% respectively inthe same time. Thus sensing performance is completely recoverableonly in case of CTAB doped CuO sensor. But the SDS surfactantassisted and without surfactant assisted samples does not recoverat all. The sensor recovery in case of CTAB assisted sensor is foundto be comparatively of longer duration. To increase the recovery ofthe sensor, it can be heated to temperature of about 100 �C. Raisingthe temperature helps to desorbs product of the activatedadsorption of ammonia.

It is well known that reduction or oxidation reaction ofammonia on themetal oxide surface results in the production of N2,NO or NO2 gases and water. NO is quickly oxidizes to NO2 in thepresence of air and CuO is sensitive to this gas. The large recoverytime of CuO based sensor might be due to the formation of NOx

species which are hardly to desorb at room temperatures hencecauses slow recovery. The detailed sensing mechanism has beendiscussed earlier [28].

The activated chemical adsorption of gases on a solid decreasesthe reaction rate due to an increase in surface coverage [36]. One ofthe most useful equation describing such kind of adsorption wasgiven by Elovich [37] and is given as

dqdt

¼ a0e�aq (6)

where q is the quantity of gas adsorbed during time t, a and a0 arethe constants during any one experiment. The kinetic law ofchemisorption was established by the work of Zedowitsch [38]. Tosimplify Elovich’s equation, Chien and Clayton [39] assumed thata0at[1 and applying the boundary conditions of q ¼ 0 at t ¼ 0 andq ¼ q at t ¼ t, then following equation

q ¼ 1alnðt þ t0Þ �

1alnðt0Þ (7)

reduces to

q ¼ 1aln�a’a�þ 1alnðtÞ (8)

Fig. 5. Shows the FESEM, and TEM images of the CuO sample (a) and (d) without surfactant, (b) and (e) with CTAB, and (c) and (f) with SDS respectively. The porous nature andmorphological variations with the addition of surfactant can be clearly observed in the FESEM and TEM images.

I. Singh, R.K. Bedi / Solid State Sciences 13 (2011) 2011e20182016

The constants can be obtained from the slope and the interceptof the line plot of q against ln(t). In the Elovich equation constanta0 is regarded as the initial adsorption rate and it depends on theactivation energy [39]. Constant a is related to a measure of theextent to which the surface has been screened by potential barrierfor successive adsorption. Ammonia adsorption leads to a chargetransfer between film and gas molecules. However, this process islimited by a potential barrier formed by the dipole layer, which iscreated by the redistribution of charge between the NH3 moleculeand CuO film. The potential barrier increases with successive

adsorption, which induces surface heterogeneity. The adsorbate-eadsorbate interactions are known to make the heat of adsorptionand desorption, dependent on the surface coverage [40]. Thismakes adsorption rate to decrease exponentially with time due toan increase in surface coverage.

In present study, the change in conductance (ΔC) of CuO films isproportional to the amount of the adsorbed NH3 molecules (q). Theplot of ΔC vs. ln(t) is linear after a time of 100e110 s for differentthick film samples as evident in Fig. 7, indicating that the adsorp-tion of ammonia on the surface of CuO films is in accordance with

a

b

c

Fig. 6. Response of CuO thick films for 100 ppm ammonia at room temperature (a)without surfactant, (b) with CTAB, and (c) with SDS. The plot shows the variation in theresponse of the sensor with time for thick film samples.

I. Singh, R.K. Bedi / Solid State Sciences 13 (2011) 2011e2018 2017

the Elovich equation. The values of the constants a0 and a have beenobtained from the plot and are recorded in the Table 1. The initialadsorption rate has been shown maxima for the films preparedwith CTAB assisted sample of CuO, whereas it is minimum for filmsmade from the sample with SDS surfactant. The value of a, whichrepresents the barrier for the successive adsorption has been foundto be larger (0.5 MU) in case of CuO films without surfactant and itdecreases to 0.33 and 0.25 MU respectively for CTAB and SDSassisted CuO samples. Higher response, maximum recovery, lowpotential barrier, and high absorption rate in case of CTAB assistedsensor shows maximum advantage in comparison to othersamples.

a

b

c

Fig. 7. Variation of change in conductance (ΔC) vs. ln(t) for CuO thick films (a) withoutsurfactant, (b) with CTAB, and (c) with SDS. The plot verifies that the sensor responsefor ammonia gas at room temperature is in accordance with the Elovich equation.

The kinetics of the reaction between adsorbed ammonia andavailable oxygen species on the film surface has been determinedfrom the plot of ln(Ce � Ct) vs t and Ct vs Ct � C0/t respectively forfirst and second order reaction mechanism respectively, using themethod adopted by Tongpool et al. [36] and Kamalpreet et al. [41].The Ce and Ct are the electrical conductance of the sensor at satu-ration point and at any time t respectively. C0 is the electricalconduction of the sensor in air without any gas in the testing cell.The conformity between the experimental data and the modelpredicted values has been expressed by correlation coefficient (R2

values close or equal to 1). A relatively high R2 value indicates thatthe model successfully describes the kinetics of the adsorption. Theregression equations and R2 values for the first and second orderequations for the samples are shown in Table 1. The highest R2 valuefor the first order in case of the sample without surfactant said togive good correlation as evident in Fig. 8. This indicates that thereaction between the ammonia and adsorbed oxygen species onthe surface of CuO films preparedwithout the addition of surfactantis of first order. The large values of R2 indicate linear variation forthe plot of Ct vs Ct� C0/t for CuO films as shown in Fig. 9. This showsthat the surfactant assisted sensors follows the second orderreaction kinetics. The rate constant ‘k’ of the response has beendetermined from the slope of the linear plot, and is found to be0.011 s�1 for the first order reaction. While for CTAB and SDSassisted sensors its value is 2895 and 1080 U�1sec�1 respectively.The rate constant for the CTAB sensor has been found to be highestamong the three, thus indicating the fastest reaction betweenammonia and oxygen species. The CTAB assisted CuO nanoparticlesare excellent amongst the three different sensors tested forammonia at room temperature.

Though the gas sensor based on CuO thick film showscomparatively better response toward NH3 at room temperature ascompared to the CuO sensor tested by Alexy et al. [42]. It is inter-esting to reveal that the ammonia gas sensor based upon CuOshowsmuch better response at room temperature in comparison ton-type materials like ZnO and SnO2 at 100 �C [3]. Several sensors,which were fabricated and tested under identical conditions,

a

b

c

Fig. 8. Variation of ln(Ce � Ct) vs. t for CuO thick films (a) without surfactant, (b) withCTAB, and (c) with SDS. The linear nature of the plot for the sample prepared withoutsurfactant shows the first order reaction whereas surfactant assisted samples fails toverify the plot.

a

b

c

Fig. 9. Plot of Ct vs. (Ct � C0)/t for the CuO thick films (a) without surfactant, (b) withCTAB, and (c) with SDS. The linear nature of the plot shows that the reaction is of 2ndorder for surfactant assisted samples.

I. Singh, R.K. Bedi / Solid State Sciences 13 (2011) 2011e20182018

exhibited similar gas response, but have comparatively longerrecovery time due to slow rate of desorption. More detailedinvestigations have to be performed in this regard. In future oursearch for finding the materials to be doped in CuO to improve itsrecovery at room temperature will be continued.

4. Conclusions

A nanocrystalline CuO powder is produced by a surfactantassisted sol-gel auto combustion method. The results show that theaddition of CTAB and SDS surfactant reduces the grain size drasti-cally to the value 18 and 15 nm respectively. The XRD result revealsthat maximum compressive strain has been introduced in thecrystal lattice with the addition of surfactant. The addition ofsurfactant generates nearly spherical CuO nanoparticles withminimum agglomeration. In response to 100 ppm ammonia gas atroom temperature the surfactant assisted samples show excellent

sensitivity as compared to sample without surfactant, tested underidentical conditions. The kinetics of the reaction show that additionof surfactant changes the order of the reaction from first to secondorder and the sensor preparedwith CTAB exhibits maximum sensorresponse rate constant.

Acknowledgement

The authors thank Director IIT Roorkee, STIC, Kochi and RSIC,Punjab University Chandigarh for providing FESEM, EDAX and XRDfacilities.

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