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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 23 (2014) 122–135

http://d1369-80

n CorrPhysica226025

E-m

journal homepage: www.elsevier.com/locate/mssp

Synthesis, characterization, magnetic measurements andliquefied petroleum gas sensing properties of nanostructuredcobalt ferrite and ferric oxide

Satyendra Singh a,b, Archana Singh a, B.C. Yadav a,c,n, Poonam Tandon a

a Department of Physics, University of Lucknow, Lucknow 226007, UP, Indiab Department of Physics, University of Allahabad, Allahabad 211002, UP, Indiac Department of Applied Physics, School for Physical Sciences, Babasaheb Bhimrao Ambedkar University, Lucknow 226025, UP, India

a r t i c l e i n f o

Available online 15 March 2014

Keywords:Nanostructured materialsChemical synthesisGas–solid reactionsMicrostructureX-ray diffractionMagnetic measurements

x.doi.org/10.1016/j.mssp.2014.02.04801 & 2014 Elsevier Ltd. All rights reserved.

espondnig author at: Department of Appliel Sciences, Babasaheb Bhimrao Ambedkar, UP, India. Mobile: þ91 9450094590.ail address: balchandra_yadav@rediffmail.co

a b s t r a c t

In the present paper, a series of nanostructured cobalt ferrite systems was synthesized indifferent compositions via chemical co-precipitation method. The X-ray diffractionanalysis of cobalt ferrite systems confirmed the formation of its nanoparticles havingminimum crystallite size 7 nm. The surface morphologies of the cobalt ferrite illustratethe distribution of partially agglomerated spherical nanoparticles having particle size�12 nm. The magnetic behaviors of the synthesized materials were characterized bymagnetic measurements. Liquefied petroleum gas sensing investigations of the fabricatedpellets illustrate that the cobalt ferrite synthesized in 1:1 M ratio possesses an improvedresponse in comparison to other compositions. The maximum sensitivity of cobalt ferritefilm sensor was 2.0 MΩ/s. The response and recovery times were �30 and 60 s,respectively. The sensor was 95% reproducible after three months of fabrication of thefilm, showing the stability of the fabricated sensor.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Liquefied petroleum gas is an inflammable gas, whichpresents many hazards to human being as well as envir-onment. Therefore, LPG sensor has become the recenttopic of research in view of industrial applications [1–5].We have been interested in carrying out our investigationswith a new material that possess good sensitivity for theLPG, with properties that are stable over time and thermalcycling after exposure to the various species likely to bepresent in the ambient. Metal-oxide nanoparticles havebeen the subject of much interest because of their unusual

d Physics, School forUniversity, Lucknow

m (B.C. Yadav).

optical, electronic and magnetic properties, which oftendiffer from the bulk due to higher surface to volume ratio[6–12]. In particular, ferric oxide is considered to be themost promising sensing materials of sensors due to thetemperature dependent surface morphology [13]. Fe2O3

does not require costly noble metal catalyst to perform as agood sensor. To achieve some specificity, the sensors canbe impregnated with dopants or the working temperaturecan be changed so that the sensor's resistance changeswhen specific gases react with the adsorbed oxygenmolecules [14–21]. Nowadays, the most popular strategiesemployed to enhance the gas sensor response are:(a) control of surface morphology in order to increasethe active surface area for the adsorption of gas and (b) useof additives which act as catalyst of the solid-gas reaction,by a chemical or electronic mechanism, thus, promotingthe improvement of the sensor properties [22–25].

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135 123

In order to enhance the sensitivity, ferric oxide waschemically modified and mixed oxides were formed.Inclusion of Co2þ (which is a transition metal ion) in ferricoxide change the properties of the base material. Theadditives formed the binary compounds which will influ-ence the size and surface morphology of the ferric oxideand hence modify the properties of that. Therefore, theyare able to change the sensitivity, selectivity and responseof ferric oxide based gas sensor. CoO can modify theintrinsic physical properties of Fe2O3, such as (a) theelectrical transport properties by introduction of newstates in the band structure of Fe2O3; (b) the surfacemorphology, which has a vital role in the chemical reac-tions between oxide and gas; and (c) the grain sizedistribution which contributes in determining the electri-cal resistance of the material.

Cobalt ferrite has an inverse spinel structure in which,in the ideal state, all Co2þ ions are in octahedral sites, andFe3þ ions are equally distributed between tetrahedral andoctahedral sites. Ferrites show very good surface reactivityand they have temperature dependent surface morphol-ogy. They are an alternative for inexpensive and robustdetection systems because of good chemical and thermalstability under operating conditions [22–24]. They alsoshow remarkable catalytic properties in oxidation reac-tions due to the high oxygen ion mobility at the filmsurface and thus are highly interesting for the develop-ment of sensors [22–24]. Therefore, in the present inves-tigation a special attention is focusing on CoFe2O4 systemin order to obtain reliable gas sensor operable at roomtemperature.

The gas sensing properties of the ferrites are dependenton its chemical composition and nanostructural character-istics, which can be controlled in the synthesis andfabrication processes [26–31]. In order to acquire materialswith the desired physical and chemical properties, thepreparation of cobalt ferrite nanoparticles through differ-ent routes has become an important area of research. Forthe gas sensing studies, there is a need for developingsynthesis and fabrication processes that are relativelysimple and yield controlled particle sizes. Cobalt ferritenanoparticles have been synthesized using various meth-ods, such as ball milling, co-precipitation, reverse micelles,hydrothermal methods, sol–gel, micro emulsions, laserablation, sonochemical approaches and aerosol method.Most of these method yielded nanoparticles of therequired sizes with relevant surface structures but theyare difficult to apply on larger scales. As these proceduresof synthesis require a lot of money, high reaction tem-perature, lengthy reaction period and their potential harmto the environment; we require to developing an easy andfacile synthesis procedure. Therefore, in the present inves-tigation, cobalt ferrite system was synthesized using anaqueous solution containing ferric chloride, cobaltousacetate, poly-ethylene glycol and deionized water using achemical co-precipitation method. This method does notrequire the addition of any other chemicals to the solution,and it has the advantages of simplicity, a low cost, a lack ofby-product effluents, and an environment friendlyoperation. This method produces nanoparticles that arespherical with narrow size distribution which is an

important parameter for their applications as sensors.In order to investigate the effect of microstructure ofcobalt ferrite system on its LPG sensing properties, aseries of cobalt ferrite materials was synthesized by vary-ing the ratio between the cobalt and ferric chloridesprecursors.

2. Experimental

2.1. Synthesis of materials

All reagents such as ferric chloride, cobaltous acetateand ammonium hydroxide were of analytical grade andused without further purification. The stoichiometricamount of starting materials, such as cobaltous acetateand ferric chloride were taken in 1:1, 1:2, 1:3 and 1:4 Mratios, respectively, and dissolved into required amount ofdistilled water to form 1 M solution. First of all, we havetaken cobaltous acetate and ferric chloride in 1:1 M ratiosand dissolved into respective required amount of distilledwater to form precursors. The above precursors wererefluxed at 60 1C for 4 h to get homogeneous solution.After that both were mixed with each other. The obtainedmixed solution was heated at 70–80 1C and magneticallystirred for 4 h. 10 ml of poly-ethylene glycol (PEG) wasadded to the mixed solution which acts as an encapsulat-ing agent. The resulting solution was precipitated byammonium hydroxide solution, which was added dropby drop to the above mixed solution and a black coloredprecipitate was obtained. The pH of the solution wasconstantly monitored as the NH4OH solution was addeduntil it reached to 12. Then the mixture was mechanicallystirred at room temperature for 6 h and the precipitatewas washed several times with distilled water until the pHof the filtrate became 7. This whole procedure wasrepeated for synthesis of CoFe2O4 in 1:2, 1:3 as well as1:4 M ratios, respectively. Here, ammonium hydroxide wasused for smooth liberation of the hydroxide ions in place ofstrong alkali (potassium hydroxide and sodium hydro-xide). Strong alkaline solutions are prone to result in theconversion of Fe3þ and Co2þ into CoFe2O4 immediately,usually leading to the formation of severely agglomeratednanoparticles with irregular shapes. Similar procedure wasused for the synthesis of nanostructured ferric oxide.

Initially in aqueous solution, metal ions exist asCoðH2OÞ2þ6 and FeðH2OÞ3þ6 . As pH was increased, the pre-dominant species existing in solution became CoðOHÞ2�x

6and FeðOHÞ3�y

y , respectively. At the solubility minima, thepredominant species in solution were Co(OH)2 and Fe(OH)3. Further, when the concentration of ammonia solu-tion was increased, cobalt and iron became more solubleas CoðOHÞ�3 and FeðOHÞ�4 species. The synthesis tempera-ture was maintained at 70–80 1C for 4–5 h under vigorousmagnetic stirring. The precipitate of CoFe2O4 was obtainedthrough centrifugal settling for 15 min at 3000 rpm. Theprecipitate was dried overnight at 100 1C. The driedmaterial was grinded into fine powder. The fine powderwas then annealed at 450 1C inside a tubular furnace for2 h with heating and cooling rate of 2 1C per minute.Sensing pellets of the synthesized powder were made by

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135124

hydraulic press (MB instruments, Delhi) under a uniaxialpressure of 616 MPa. Hereafter, the cobalt ferrite pellets in1:4, 1:3, 1:2 and 1:1 M ratios were named as P-1, P-2, P-3and P-4, respectively.

In our synthesis, we have used ammonium hydroxideas precipitating agent and carried out the precipitationreaction at pH 12. pH 12 was used in accordance with thepredictions where high production yields are expected.The advantage of this method is the controlled productionof ferrite particles; and its size and size distribution. Thereis no need of extra mechanical or microwave heat treat-ments. The selected stabilizer among a variety of surfac-tants and polymers with metal ion affinities shouldfunction well both in the control of the stability and thesurface morphology of the final products. For this purposewe have used poly-ethylene glycol (molecular weight 300)as an encapsulating agent which retards the nucleationand growth rate of the nanocrystals. This is much moreadvantageous for obtaining discrete products with a regularshape.

2.2. Characterization techniques

The characterization of the prepared cobalt ferritesystem was done using various techniques to verify thecrystallite size and to explore other parameters of interest.X-ray diffraction analysis was carried out by X-ray dif-fractometer (X-Pert PRO PANalytical) to investigate thecrystalline structure using CuKα (0.154 nm) radiation. Mor-phological evolution was analyzed by scanning electronmicroscope. The elemental composition present in thesensing material with their respective weight percentagehas been studied by energy dispersive X-ray spectroscopy(Zeiss; Supra-40) at 20 kV accelerating voltage. Opticalcharacterization was done using UV–visible absorptionspectrophotometer (Varian, Carry-50 Bio) in UV and visibleregions. Thermal behavior of the as-synthesized cobaltferrite was examined by differential scanning calorimeter(Shimadzu), in a nitrogen flux (40 ml/min) with a heatingrate of 10 1C/min. Cobalt ferrite (P-4) was also investigatedby FTIR to determine the presence of hydroxyl groups andtheir reactivates. Spectrum was recorded at room tem-perature with a Bruker Tensor 27 FTIR spectrometer using250 signal averaged scans at a resolution of 4 cm�1.Magnetic properties were measured at room temperature(298 K) using vibrating sample magnetometer (VSM Lake-shore 7410).

2.3. Gas sensing measurements

For the LPG sensing measurements, the sensing mate-rial was placed within a specially designed gas chamberhaving a gas inlet knob for passing of LPG, and an outletknob for removing of stored LPG inside the chamber [32].Now this was exposed to LPG and the variations inelectrical resistance of the sensing material with the timefor the different vol.% of LPG were recorded using aKeithley electrometer (Model-6514). The sensitivity/response speed of the sensing material is defined as the

slope of the resistance-time curve and is given below [33]:

S¼ ΔRΔt

ð1Þ

where ΔR is the change in resistance of the sensingmaterial in time interval Δt.

Percentage sensor response for the sensing material isdefined as [34]

%S:R:¼ jRa�RgjRa

� 100 ð2Þ

where Ra and Rg are the resistance values of the sensor inair and gas–air mixture, respectively.

For the measurements of electrical properties in air, thesensing materials were put inside a tubular furnace withelectrical connections and variations in electrical resis-tance with temperature were recorded. The used heatingrate was 2 1C per minute.

3. Results and discussion

3.1. Surface morphological and energy-dispersiveX-ray analysis

Fig. 1(a)–(d) shows the surface morphologies of cobaltferrite system prepared in 1:4, 1:3, 1:2 and 1:1 M ratios,respectively. It is clearly seen that grains of the cobalt ferriteare at nanoscale and has a number of pores. Maximumnumber of pores (active sites) are observed in Fig. 1(d). Thepores serve as gas adsorption sites wherein the reaction ofLPG with adsorbed oxygen takes place. The surface of asolid pellet can be considered as the outermost layer ofatoms plus the region between 0.5 and 1.5 nm above andbelow it. This includes the bonding electrons on the insideand the dangling bonds on the outside. Therefore, surfacesgenerally have dangling bonds; they can be chemically veryreactive. Owing to this reactivity, they adsorb gases incontact with them. Adsorbed molecules are held in placeon the surface by a binding energy ΔHads called the heat ofadsorption. ΔHads decides whether the process is chemi-sorption or physiosorption. When chemical bonding (eithera covalent or an ionic type) takes place between surfaceatoms and adsorbed molecules, the phenomenon is calledchemisorption. If a much weaker van der Waals type ofbonding occurs, the phenomenon is referred as physiosorp-tion. These processes may lead to a rearrangement of theelectrons that constitute the surface layer.

For gas sensing point of view, Fig. 1(d) is more attrac-tive than the others. It shows approximately uniformdistribution of particles. Sphericals as well as elongatedparticles were found at the surface. Some moderatelyagglomerated particles were also present at the surface.It was found that these nanospheres were made up ofseveral oval-type elongated crystallites separated by fineand sharp grain boundaries. Quasi spherical CoFe2O4 NPsslowed down the nucleation and growth rate of spinelphase [28]. It is well known that faster growth rate usuallyfavors the formation of spherical particles as a result of lessselective crystallographic growth direction. Materials witha cubic crystal structure are prone to grow into a sphericalshape to minimize the surface tension.

Fig. 1. SEM images of: cobalt ferrite synthesized in (a) 1:4, (b) 1:3, (c) 1:2, and (d) 1:1 M ratios, (e) ferric oxide, and (f) EDAX of cobalt ferrite in 1:1 M ratio.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135 125

The SEM image of ferric oxide is shown in Fig. 1(e).Within the scanned surface area, aggregated-type uniformcoverage of particles was obtained. This figure showsreduced number of active sites in comparison to cobaltferrite system. Thus, ferric oxide has less porosity to that ofcobalt ferrite. A porous sensor, usually constructed bynanosized materials, possesses high surface area and exhibitsimproved sensitivity. The high active surface areas of sensorsare anticipated to increase the rate of the reaction with LPGand hence their sensing behavior, since the gas sensing

phenomenon is a kind of a surface catalyzed reaction [34].Therefore, porous surface morphology of cobalt ferrite film ispreferable for the fabrication of a gas sensor.

Porosities of the sensing pellets were calculated by therelation [21]

P ¼ 1� ddx

ð3Þ

where d¼m=V is the bulk density determined fromdimensions and mass of the pellet and dx is the X-ray

Fig. 2. Elemental mapping of cobalt ferrite: (a) mixed composition, (b) oxygen, (c) iron, and (d) cobalt.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135126

density of the material which has been calculated from thevalues of lattice parameters using the formula given asunder

dx ¼8MNa3

ð4Þ

where the factor ‘8’ represents the number of molecules ina unit cell of a spinel structure, M the molecular weight ofthe material, N the Avogadro number and a the latticeparameter of the material. Using the relations (3) and (4),the porosities of the sensing pellets of cobalt ferriteprepared in 1:4, 1:3, 1:2 and 1:1 M ratios were found 17,20, 25 and 31%, respectively. The specific surface area wasdetermined using equation [35]

A¼ 6dDm

ð5Þ

where d is the bulk density of cobalt ferrite pellet and Dm

is the average size. The number 6 is the shape factor. Therespective specific surface areas of cobalt ferrite preparedin 1:4, 1:3, 1:2 and 1:1 M ratios were 6.43, 6.57, 6.85 and7.04 m2/g, respectively.

The EDAX spectrum of P-4 is shown in Fig. 1(f). Itreveals the presence of Fe, Co, O, Si and Mg elements with18.43, 19.36, 58.50, 2.39 and 1.32 atomic weight percen-tages, respectively. This analysis did not indicate anysignificant presence of chloride and ammonium ions, thus,clearly confirms that the final material was free fromsurfactant contamination. The elemental mappings ofcobalt ferrite are shown by Fig. 2(a)–(d), in which Fig. 2(a)shows the homogeneous distribution of various elements

such as cobalt, iron and oxygen while Fig. 2(b)–(d) showsindependent distribution for oxygen, iron and cobalt,respectively.

3.2. X-ray diffraction analysis

The XRD pattern of cobalt ferrite (P-4) is shown in Fig. 3(a).It shows that the final product is CoFe2O4 with an inversespinel structure. The peaks are indexed to (111), (220), (311)and (400) diffraction planes, which confirms the presence ofsingle cobalt ferrite phase instead of mixed CoO and Fe2O3

phases. The peak intensity of (311) was relatively higher thanothers. This higher intensity can be attributed to an annealingeffect that boosts the crystallinity and specific orientation ofthe crystallites. No other phase/impurity was detected. Thebroadness in the observed diffraction peaks is a characteristicof nanosized particles. The size was calculated by defining fullwidth at half maximum (FWHM) of diffraction peaks andapplying Debye–Scherrer's equation [34]. The average crystal-lite size of CoFe2O4 was found to be13 nm.

The XRD pattern of ferric oxide is shown in Fig. 3(b).The presences of sharp diffraction peaks in the materialindicate the crystalline nature of the synthesized materialhaving α-Fe2O3 phase. The average crystallite size of Fe2O3

was found 18 nm.

3.3. UV–visible absorption analysis

Fig. 4(a) shows the variation in optical absorption ofcobalt ferrite film (P-4) with the wavelength. This data wasfurther used for analyzing optical energy band gap (Eg)

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135 127

using the formula for optical absorption of a semiconduc-tor [36]

α¼ Kðhv�EgÞn=2hv

ð6Þ

where α is the absorption coefficient, K is a constant, Eg theoptical energy band gap and n is an integer equal to 1 for adirect band gap and 4 for an indirect band gap. The plot of(αhν)2 versus energy (eV) was used for estimating thevalue of direct band gap energy of CoFe2O4 by extrapolat-ing curve to zero absorption. Fig. 4(b) is the plot of (αhν)2

with photon energy (hν) to determine the optical energy

0

200

400

600

800

1000

1200

1400

1600

220

111

311

400

Inte

nsity

(a.u

.)

Diffraction angle (degree)20 25 30 35 40 45 50 55 60 65

10 20 30 40 50 60 700

25

50

75

100

125

150

175

440

300

018

113

104

116

214

024

110

012

Inte

nsity

(a.u

.)

Diffraction angle (degree)

Fig. 3. XRD pattern of (a) cobalt ferrite in 1:1 M ratio and (b) ferric oxide.

0

2

4

6

8

10

Abs

orpt

ion

(a.u

.)

Wavelength (nm)300 400 500 600 700 800

Fig. 4. (a) UV–visible absorption spectrum of cob

gap of CoFe2O4. The band gap was found 3.85 eV. Thus, it isevident that CoFe2O4 shows significant blue shift of theabsorption peak relative to the bulk absorption. This blueshift is useful for the gas sensing applications. The blueshift of the absorption peak may be related to the quantumsize effect which arises due to very small size of thenanoparticles [37].

3.4. Differential scanning calorimetry analysis

DSC curve of the synthesized CoFe2O4 powder is shownin Fig. 5. The curve shows exothermic peak at 92 1C whichmay be due to the evaporation of chemical impurities andwater. The next broad exothermic peak starting from190 1C is corresponding to the formation of CoFe2O4.

3.5. Fourier transforms infrared spectroscopy

For the recording of FTIR spectra, the sample wasprepared by pressing the CoFe2O4 powder mixed withKBr in a weight ratio 1:100 by hydraulic press into a pellet.Fig. 6(a) shows the IR spectra of cobalt ferrite (P-4).A broad absorption peak near 3350 cm�1 indicates thepresence of O–H stretching vibration due to atmosphericmoisture and a peak at 2350 cm�1 indicates the presenceof O¼C¼O residues probably due to atmospheric CO2.

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0

400

800

1200

1600

2000

(α

hν) 2 ∗

106 c

m-2

eV-2

Energy (eV)

alt ferrite in 1:1 M ratio and (b) Tauc plot.

0 40 80 120 160 200 240 280-5

-4

-3

-2

-1

0

Hea

t Flo

w (a

.u.)

Temperature (°C)

Fig. 5. DSC curve for as-synthesized cobalt ferrite.

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008 CoFe2O4

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)500 1000 1500 2000 2500 3000 3500 500 1000 1500 2000 2500 3000 3500 4000

-0.015

-0.010

-0.005

0.000

Tran

smitt

ance

(a.u

.)

Wavenumber (cm-1)

Fe2O3

Fig. 6. Infrared spectra of (a) cobalt ferrite and (b) ferric oxide.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135128

The appearance of the band near 1400–1630 cm�1 areassigned to the C¼O stretching. The absorption bandsarising at lower wave number represent the bondingbetween Co–O–Fe and can be attributed to the presenceof cobalt ferrite phase [28].

Ferric oxide was also investigated using FTIR and thespectrum is shown in Fig. 6(b). The peaks near 3380 and2350 cm�1 indicate the presence of –OH and O¼C¼Oresidues probably due to atmospheric moisture and CO2,respectively. The appearance of the band near 1400–1630 cm�1 are assigned to the C¼O stretching. The peakat 551 cm–1 indicates the presence of α-Fe2O3 in thematerial. The characteristic absorption at 925 cm–1 maybe due to the possibility of α-FeOOH.

3.6. LPG sensing properties

Sensing pellets of CoFe2O4 systemwere made by hydrau-lic press machine under an uniaxial pressure of 616 MPa.These pellets were placed between the Ag-pellet-Agconfiguration of the resistance measuring holder. This con-figuration was well fitted in the glass chamber having gasinlet knob associated with the concentration measuringsystem. Before passing the LPG in the chamber, the gaschamber with resistance measuring holder was stabilizedfor 10–15 min. The stabilized resistance of the pellet wastaken as stabilized resistance in presence of air (Ra). Here-after, a certain amount of gas (e.g., 1, 2, 3 vol.%) was allowedto pass inside the gas chamber and corresponding variationsin resistance of the pellet were observed with time. Fig. 7(a)–(d) shows the variations in resistance of CoFe2O4 systemprepared in different molar ratios with time for differentconcentrations of LPG; in which Fig. 7(a) shows the varia-tions in electrical resistance of P-1 with time. Each curve ofthe figure shows that as time increases the resistance ofpellet increases drastically in the beginning and later on itincreases slowly. Finally, when we opened the outlet of thegas chamber, the resistance approaches to their initial valueof stabilized resistance in air (Ra) for further range of time.The variations in resistance for P-2 are shown in Fig. 7(b)and were found more in comparison to P-1. Maximumvariations in resistance were observed for P-4 which isshown in Fig. 7(d). Therefore, this pellet is most sensitiveamong all.

Sensitivity curves of P-1, P-2, P-3 and P-4 sensors as afunction of LPG concentrations are plotted in Fig. 8(a)–(d),respectively. We observed that as the concentration of LPGincreases, the sensitivity increases rapidly in the beginningand later it becomes saturated. The linear relationshipbetween sensitivity and gas concentration may be attrib-uted to the availability of sufficient number of sensing siteson the pellet surface. The small LPG concentration impliesa small surface coverage of gas molecules, resulting in asmall surface reaction between the surface adsorbed oxy-gen species and the gas molecules. The increase in the LPGconcentration increases the surface reaction due to a largesurface coverage. Further on increasing the LPG concen-tration, the surface reaction does not increase and even-tually saturation takes place. The maximum values of thesensitivities of P-1, P-2, P-3, and P-4 sensors were �1.1, 1.5,1.6, and 1.7 MΩ/min, respectively. The maximum value ofsensitivity for cobalt ferrite (P-4) synthesized in molarratio 1:1 may be attributed to its surface morphologicalstructure and porosity of the material. The reproducibilitycurve for P-4 was also plotted which elucidated thatresults are found reproducible up to 92% after threemonths of fabrication of pellet sensor.

The porosity, size and density of the pores are thesimple parameters to assess the effect of different compo-sitions of cobalt ferrite since it controls the diffusionkinetics of the gases into the pellets. However, graingrowth will reveal a monotonic decrease in sensitivity ofthe sensor, as P-4 shows maximum response and theresponse continuously decreases as grain growth increases.The largest grain size was observed for P-1 and its responseto LPG was least. Comparative sensitivity data of cobaltferrite system is summarized in Table 1. If another veryprobable cause, the density of cobalt vacancy, is to be thedominant parameter, the density of cobalt vacancy shouldbe the highest for P-4. Thus, we tentatively conclude thatthe change in response of CoFe2O4 system was mainlycaused by the combined effect of the grain size and porositywhile the other details need to be further explored.

Response and recovery times are very important para-meters among the other parameters of gas sensors. Thetime taken by the sensing material to reach 90% of themaximum resistance is known as response time and isfound �1 min for 1 vol.% of LPG. The time taken by the

Fig. 7. Dynamic response of cobalt ferrite with time for (a) P-1, (b) P-2, (c) P-3, and (d) P-4.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135 129

sensor to retrieve its previous state is known as recoverytime and is found larger �4 min. The poor recoveryobserved at room temperature is due to the bulky natureof sensing pellet. When the CoFe2O4 pellet is exposed tothe LPG, it goes deeper into the pellet and comes out veryslowly. This results into a longer recovery time.

In order to improve the sensitivity, response andrecovery characteristics of the pellet sensor, we preparedthe thick films using screen printing technology. Further,the prepared films were annealed at 450 1C, which con-verts the films as sensing materials. The thicknesses ofthese films were �4 μm measured by Accurion variableangle spectroscopic ellipsometer (Nanofilm EP3 Imaging)over the wavelength range of 300–1000 nm. These filmswere investigated for the LPG sensing behaviors. Theresponse characteristic of the CoFe2O4 film (fabricatedusing precursor P-4) in presence of the LPG is shown inFig. 9(a). Sensitivity of the film as a function of LPGconcentration is illustrated in Fig. 9(b). When the sensingfilm is exposed to the gas, the change in resistance ismainly due to the reaction between the LPG and theoxygen species adsorbed on its surface. As gas concentra-tion was increased, the sensitivity of the sensor wasgradually increased and after that it became saturated.Maximum sensitivity of CoFe2O4 film to the LPG was2.0 MΩ/s, achieved for 5 vol.% of LPG. Thus, the sensitivityof LPG sensor has increased by a factor �75 in comparison

to that of solid state LPG sensor made up of CoFe2O4 pellet(P-4). Sensor response for cobalt ferrite film sensor wasalso estimated. A maximum value of sensor response�3000 was observed. Fig. 9(c) shows the reproducibilitycurve of CoFe2O4 film after three months of fabrication ofthe film sensor. It was found that after three months, itperforms 95% of its initial performance, showing thestability of the film.

For a comparative assessment, the LPG sensing proper-ties of ferric oxide thick film sensor and its characteristichave also been investigated (shown in Fig. 9(d)). The studyof the temporal variation of resistance for different con-centration of the LPG show that the resistance increasessharply at initial stage of exposure. Later it increasesslowly and when outlet is opened then resistance sud-denly decreases up to its initial value of stabilized resis-tance (Ra). The variation in sensitivity with concentrationof the LPG is shown in Fig. 9(e). Maximum value of thesensitivity is 1.6 MΩ/s for 5 vol.% of LPG. Sensor responsecurves for this sensor are shown in Fig. 9(f). A maximumvalue of sensor response �2700 was observed for 5 vol.%of LPG. The response of the nanostructured materials isdirectly related to exposed surface volume. Therefore, thedifference in response for sensing films might be attrib-uted to adsorption of LPG and reaction between LPG andthe adsorbed oxygen species. The amount of adsorbedoxygen species is quite important for providing enough

Table 1Comparative data of cobalt ferrite system.

Cobalt ferrite systemsynthesized in

Porosity(%)

Specific surfacearea (m2/g)

Sensitivity(MΩ/min)

1:4 M ratio 17 6.43 1.11:3 M ratio 20 6.57 1.51:2 M ratio 25 6.85 1.61:1 M ratio 31 7.04 1.7

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Sens

itivi

ty (M

Ω/m

in.)

1 2 3 4 5

Concentration of LPG (vol.%) Concentration of LPG (vol.%)

Fig. 8. Sensitivity of cobalt ferrite pellet sensor (a) P-1, (b) P-2, (c) P-3, and (d) P-4.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135130

reactants for the reaction. As the cobalt ferrite surfaceshows more adsorption sites and exposed surface area,therefore, it shows better response than ferric oxide.

The gas sensing mechanism of the LPG sensor is asurface controlled phenomenon i.e., it is based on thesurface area of the film at which the LPG is adsorbed andreacts with pre-adsorbed oxygen molecules [38]. Theoxygen chemisorptions centers viz., oxygen vacancies,localized donor and acceptor states and other defects areformed on the surface during synthesis. These centers arefilled by adsorbing oxygen from air. When the sensingmaterial is put inside the gas sensing setup then aftersome time an equilibrium state between oxygen of sensingmaterial and atmospheric oxygen is achieved through thechemisorptions process at room temperature. The stabi-lized resistance at present state is known as resistance inpresence of air (Ra). The reaction kinematics may beexplained by the following reactions:

O2 ðgasÞ2O2 ðadsÞ ð7Þ

O2 ðadsÞþe�-O�2 ð8Þ

The electron transfer from the conduction band to thechemisorbed oxygen results in the decrease in the electronconcentration at the film surface. As a consequence, anincrease in the resistance of the film is observed. In theLPG, the reducing hydrogen species are bound to thecarbon, therefore, LPG dissociates into the reactive redu-cing components on the film surface. When the film isexposed to reducing gas like LPG, it reacts with the

chemisorbed oxygen and a surface charge layer would beformed. When the LPG reacts with the surface oxygen ionsthen the combustion products such as water depart and apotential barrier to charge transport would be developedi.e., this mechanism involves the displacement of adsorbedoxygen species by formation of water. The overall reactionof the LPG with the chemisorbed oxygen may be takenplace as [39]

CnH2nþ2þO�2 -CnH2nOþH2Oþe� ð9Þ

where CnH2nþ2 represents the various hydrocarbons.When the flow of LPG is stopped for recovery, the oxygenmolecules in air will be absorbed by the surface of CoFe2O4,and the capture of electrons through the processes indi-cated in equations, will reduce the sensor resistancetowards the initial stable surface state of CoFe2O4. At thefirst cycle of exposure, LPG molecules interact with the pre-adsorbed oxygen on the surface of CoFe2O4, and the sensorresistance increases. The pre-adsorbed oxygen again forms

0

50

100

150

200

250

300

350

Time (seconds)

1 vol.%2 vol.% 3 vol.%4 vol.%5 vol.%

0

50

100

150

200

250

300

350

5 vol.% Rep. (5 vol.%)

R (M

Ω)

R (M

Ω)

Time (seconds)

0 500 1000 1500 2000 2500

0 500 1000 1500 2000 2500

1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Sens

itivi

ty (M

Ω/s

econ

ds)

LPG concentration (vol.%)

Fig. 9. (a) Variations in resistance of cobalt ferrite film with time for different vol.% of LPG, (b) sensitivities of sensor, (c) reproducibility curve after threemonths, (d) variations in resistance with time for ferric oxide film, (e) sensitivities of sensor, and (f) sensor response curves.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135 131

H2O molecules and is removed from CoFe2O4. However, inthis case, no oxygen molecules are supplied during theexposure to the CoFe2O4 surfaces from the LPG. Thissudden total exhaust in surface oxygen resulted in theinitial dramatic increase in resistance at the first cycle.Since the sensing mechanism of these devices is basedon the chemisorptions that take place at the surface ofthe metal oxide, so increasing specific surface area of thesensitive materials leads to more sites for adsorption ofsurrounding gases.

Here in the present case, it may be seen from the SEMimages that cobalt ferrite offers more gas adsorption sitesfor interacting the gas molecules in comparison to ferricoxide. As a result, cobalt ferrite film (P-4) shows enhancedsensitivity by a factor of �20% in comparison to the ferricoxide. The enhanced sensing performance of the cobaltferrite film may be attributed to its porous structure andhigher active surface area. The pores can act as channelsfor diffusion of the LPG, and thus provides more activesites. This improves the reaction of the LPG with surface

0

50

100

150

200

250

300

350

400

Temperature (°C)

Temperature (°C)

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

20.0

ln R

1000/T (K-1)

1000/T (K-1)

0

50

100

150

200

250

300

R (M

Ω)

R (M

Ω)

100 120 140 160 180 200 220 240 260 280 300 320 1.6 1.8 2.0 2.2 2.4 2.6

100 150 200 250 300 350 1.6 1.8 2.0 2.2 2.4 2.6

16.0

16.5

17.0

17.5

18.0

18.5

19.0

19.5

ln R

Fig. 10. (a) Variations in resistance of cobalt ferrite with temperature, (b) Arrhenius plot for cobalt ferrite, (c) variations in resistance with temperature forferric oxide, and (d) Arrhenius plot for ferric oxide.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135132

adsorbed oxygen. These features make the cobalt ferritefilm better for gas sensing applications.

3.7. Electrical properties in air

Electrical resistance behavior in air against the tem-perature of the CoFe2O4 (P-4) is shown in Fig. 10(a). Thisreveals the semiconducting nature of the material. Thedecrease in resistance with the temperature is due tothermally activated mobility of the carriers. Fig. 10(b)shows ln R versus 1000/T Arrhenius plot for CoFe2O4.The logarithm of resistance of CoFe2O4 had an almostlinear relationship with reciprocal temperature (1/T) asexpected for a typical semiconducting material. Arrheniusplot has a slope (Ea/2K) according to equation

ln R¼ ln R0þEa=2KT ð10Þwhere Ea, K and T are the activation energy, Boltzmannconstant and absolute temperature of the material, respec-tively. By measuring the slope, we can calculate theactivation energy of the material. The activation energyof cobalt ferrite was found �0.65 eV. On interaction of gaswith the material, the small activation energy providessufficient energy to electrons to reach the conduction bandand yields sufficient changes in conductivity. A largevariation in resistance indicates the greater sensitivity ofthe sensing film. Thus, this film gives sufficient responseon interaction with the LPG. It can be noted that a change

in temperature will alter the resistance because both thecharge of the surface oxygen species (O2, O2

�,O� or O2�) as

well as their coverage can be altered in this process.Fig. 10(c) shows the variations in resistance of Fe2O3

with temperature. In temperature range 110–160 1C, theresistance of pellet decreases rapidly and after that itbecomes constant, suggesting the semiconducting natureof the film. In the lower temperature range the film hashigh resistance. But, with increasing temperature; itsresistance became small and smaller. Fig. 10(d) showsthe Arrhenius plot for Fe2O3. By measuring the slope ofArrhenius plot of a linear zone, the activation energy hasbeen calculated and found to be 0.79 eV.

3.8. Magnetic properties

Magnetic characterization of CoFe2O4 (P-4) is shown inFig. 11(a). It exhibited behavior characteristic of ferromag-netic ordering. The ferromagnetic hysteresis M–H loopshowing the effect of the magnetic field on magnetization.The dipole alignment leads to saturation magnetization, aremanent magnetization, and a coercive field. The magne-tization loop shows significant hysteresis, and the values ofsaturation magnetization (Ms), remanent magnetization(Mr), and coercivity were 26.1 emu/g, 6.74 emu/g, and367 Oe, respectively. Less hysteresis would be expectedto show the weak ferromagnetism. The value of saturationmagnetization (26.1 emu/g) is much lower than that

Fig. 11. Magnetization measurements of (a) cobalt ferrite, (b) bulk ferric oxide, and (c) nanostructured ferric oxide.

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135 133

reported for the multi domain, bulk cobalt ferrite(74.08 emu/g) [40]. The decrease in saturation magnetiza-tion, compared to that of bulk material, depends ondifferent parameters. In the thermal treatment method,the heating rate of annealing is one of the most importantparameters that can effectively increase or decrease thesaturation magnetization. In fact, since the nanoparticlesare the mixed spinel structure type rather than the inversespinel structure type (bulk) because of the presence ofCo3þ ions and also a cation distribution with cobalt ionson the tetrahedral site [41], the saturation magnetization isreduced [42]. The existence or absence of the differenttypes of inter grain group exchanges is determined by thevalue of remanance ratio (R¼Mr/Ms) that varies from 0 to 1[28]. It has been reported that Ro0.5 is for the particleinteract by magneto-static interaction, while R¼0.5 is forrandomly oriented non-interacting particles that undergocoherent rotations [43]. Finally, 0.5oRo1 confirms theexistence of exchange-coupling particles. For cobalt ferriteR was found 0.26 which illustrates that the particlesinteract by magneto-static interaction.

Fig. 11(b) shows a typical magnetization variation (M)versus the applied magnetic field (H) for bulk ferric oxide(Merck) of AR grade. The values of saturation magnetiza-tion (Ms), remanent magnetization (Mr), and coercivitywere 1.71 emu/g, 0.59 emu/g, and 314 Oe, respectively.

Magnetic measurement of nanostructured ferric oxide asprepared (crystallite size �18 nm) is shown in Fig. 11(c)which is quite different from the nature of the curveshown by Fig. 11(b) for bulk Fe2O3. As seen in above figure,magnetization decreasing to zero when the applied field isremoved which illustrates super paramagnetic behavior.The magnetic measurements data of all these materials areshown in Table 2.

Nanostructured superparamagnetics can act as single-domains and are therefore small enough to overcomeenergy barriers that would prevent alignment in thedirection of the field at room temperature. As a result,SPMs have an increased susceptibility and do not requirevery high magnetic fields or low temperatures to reachsaturation. Because these materials require the presence ofa field to align their magnetic moments and magnetize,they lose most or all of their magnetization once the fieldis removed. The nanosized ferric oxide nanoparticles withaverage crystallite size �18 nm have almost no remnantmagnetization at zero magnetic field strength, which is anindication of super paramagnetism. This implies that themagnetic domain size of these particles is approximatelyor perhaps slightly larger than 18 nm. This behavior is animportant property for magnetic targeting carriers. In fact,the difference between ferromagnetism and super para-magnetism fabricates the particle size. Literature data

Table 2Magnetic properties of the materials.

Magneticmaterial

(Specific) saturationmagnetization (emu/g)

(Specific) residualmagnetization (emu/g)

Coercivity(Oe)

(Specific) magneticsusceptibility at 298 K

CoFe2O4 26.1 6.74 367 –

Fe2O3 – – 224 þ4.6�10�6 cm3/gFe2O3 bulk 1.71 0.59 314 –

S. Singh et al. / Materials Science in Semiconductor Processing 23 (2014) 122–135134

imply that below a critical size, the particles show thecharacter of super paramagnetism. When decreasing thesize of magnetic particles, they change from multi domainto single domain. If the single-domain particles becomesmall enough, the magnetic moment in the domainfluctuates in direction, due to thermal agitation whichleads to super paramagnetism.

When a ferromagnetic material decomposes into nano-sized grains, the result is a collection of single-domainparticles that orient along an applied magnetic field,a phenomenon called super paramagnetism. Single-domain particles can transfer in the super paramagneticstate below a critical particle size, which depends on thematerial type. Super paramagnetism is characterized bylack of hysteresis loop and by saturation magnetizationclose to the value of a ferromagnetic material. Superparamagnetism enables to avoid agglomeration of theparticles when the external magnetic field is removed.However, the super paramagnetic nanoparticles mayagglomerate tending to decrease their surface energy,which may result in the loss of their unique magneticproperties.

4. Conclusion

In this paper, we have described the synthesis of cobaltferrite system and characterized them for structural,optical, magnetic and surface morphological properties.The XRD of the material demonstrate spinel ferrite havingcubic symmetry. The effect of the composition of CoFe2O4

on the surface morphologies and LPG sensing propertieswere investigated. CoFe2O4 in 1:1 M ratios (P-4) showedimproved sensing response in comparison to other com-positions. The maximum sensitivity of cobalt ferrite filmsensor was 2.0 MΩ/s, which is �75 times greater than thatof a solid state pellet sensor. The response and recoverytimes of thick film sensor (�30 and 60 s, respectively) areless than that of pellet sensor. The results were foundreproducible up to 95% after three months of observations,showing the stability of the sensor. Thus, this studyexplored the relevancy and possibility of utilizing nanos-tructured CoFe2O4 thick film for the detection of LPG atroom temperature.

Prime novelty statement

A series of cobalt ferrite materials was synthesized byvarying the ratio of the cobalt and ferric chloride precursorswhich exhibits significant variations in LPG sensing proper-ties. These cobalt ferrite systems show advancement instructural, optical and surface morphologies. The effect of

these properties on sensor response was studied and thepossible sensing mechanism has been discussed.

Acknowledgment

Satyendra Singh acknowledges the financial supportprovided by the University Grants Commission, Indiaunder the U.G.C.-Dr. D.S. Kothari Postdoctoral Fellowship.Corresponding author is grateful to DST-BRNS for provid-ing the grant vide sanction number 2013/34/27/BRNS/2693.

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