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Cite this: RSC Adv., 2019, 9, 30226

Development of an acetone sensor using
nanostructured Co3O4 thin ﬁlms for exhaled breath
analysis†
Parthasarathy Srinivasan, a Arockia Jayalatha Kulandaisamy,aGanesh Kumar Mani, ab K. Jayanth Babu,a Kazuyoshi Tsuchiyab
and John Bosco Balaguru Rayappan *a
In recent times, the development of breath sensors for the detection of Diabetic Keto-Acidosis (DKA) has
been gaining prominent importance in the ﬁeld of health care and advanced diagnostics. Acetone is oneof the prominent biomarkers in the exhaled breath of persons aﬀected by DKA. In this background,nanostructured cobalt oxide sensing elements were fabricated using a spray pyrolysis technique at
diﬀerent deposition temperatures (473 to 773 K in steps of 100 K) towards the fabrication of an acetone
sensor. The inﬂuence of deposition temperature on the various properties of the nanostructured cobalt
oxide thin ﬁlms was investigated. Formation of cubic spinel phase cobalt oxide was conﬁrmed from thestructural analysis. The shifting of plane orientation from (3 1 1) to (2 2 0) at 773 K deposition
temperature revealed the migration of cobalt atoms to the highly favorable energy positions. Further, thedownshifted peak absorption wavelength and upshifted PL proﬁle at higher deposition temperature
conﬁrmed the migration of cobalt ions. The sensor fabricated at higher deposition temperature (773 K)

Received 5th June 2019
Accepted 16th September 2019

showed a sensing response of 235 at room temperature towards 50 ppm of acetone. Also, the fabricatedsensor showed a lower detection limit (LOD) of 1 ppm with the response–recovery times of 6 and 4 s,

DOI: 10.1039/c9ra04230j

respectively. The LOD reported here is lower than the minimum threshold level (1.71 ppm) signifying thersc.li/rsc-advancespresence of DKA.

1. Introduction
Acetone is the preferred chemical reagent in industries fortissue dehydration, paraﬃn purication, and plastic dissolution.1 According to the Occupational Safety and HealthAdministration (OSHA), the human permissible exposure leveltowards acetone is 1000 ppm as a general industrial standard.2
The National Institute for Occupational Safety and Health(NIOSH) reported the recommended exposure level of acetone
as 250 ppm as a time-weighted average.2 Irritation in eyes, skin,and the respiratory system are the general ill-eﬀects caused
when the concentration exceeds the permissible exposure
limit.2 Acetone is also one of the signicant biomarkers indicating the freshness levels of fruits and vegetables.3 Froma diagnostic perspective, the presence of trace level acetone (1.8ppm) in human exhaled breath indicates the presence ofaCentre for Nanotechnology & Advanced Biomaterials (CeNTAB), School of Electrical &Electronics Engineering (SEEE), SASTRA Deemed University, Thanjavur 613 401, Tamil
Nadu, India. E-mail: rjbosco@ece.sastra.edu; Fax: +91 4362 264120; Tel: +91 4362
264 101 ext. 2255b

Micro/Nano Technology Center, Tokai University, Japan† Electronic supplementary
10.1039/c9ra04230j

information

30226 | RSC Adv., 2019, 9, 30226–30239(ESI)

available.

See

DOI:

Diabetic Ketoacidosis (DKA).4 The concentration of a specicmarker present in the exhaled breath is directly related to themetabolic disorders in human beings. Since acetone acts asa fat metabolic nature, the concentration dependence on theexhaled breath is correlated to the fat loss.5 If the patient's
exhaled breath contains more than 1.71 ppm of acetone, it
indicates the presence of type-2 diabetic mellitus.5 On the other
hand, the threshold level of exhaled acetone indicating type-1
diabetes is 2.19 ppm.6
DKA is one of the lethal diseases among the diﬀerent age
groups having type-1 diabetes mellitus.7 It aﬀects the patients,
especially pregnant women having type-1, type-2, and gestational diabetes. Most common symptoms of DKA in pregnant
women are starvation, dehydration, nausea, and cortisol. Onset
of DKA is due to the insulin deciency in the pancreas, which
limits the glucose production from the body cells. In this case,instead of getting reacted with glucose, the cells breakdown thefats and results in the production of ketones (acetone) leads to
DKA.7
Blood sampling is the conventional clinical technique used
for the detection of DKA.7 However, due to its invasive nature, it
has not been considered as an eﬃcient diagnostic tool. In this
scenario, non-invasive detection of DKA employing exhaled
breath analysis has emerged as an eﬀective diagnostic

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technique.7 Hence, it has been devised to develop an acetone
sensor with the following gure of merits such as selective
response, fast response & recovery characteristics and LOD in
the range of minimum threshold level indicating the onset of
DKA.
Recently, ZnO nanostructures decorated with platinum andniobium nanoparticles showed acetone sensing characteristics
with the maximum response of 188 and 224 towards 1000 ppm
respectively.8 The co-doping strategy has also been reported forthe enhanced acetone sensing signatures. Similarly, co-dopingof iron and carbon on WO3 nanostructure was reported forthe improved acetone sensing performances.9 Another interesting report from Kou et al.,10 depicts the Rh dopant inuenceon the electrospun SnO2 nanobers on the acetone sensing
characteristics. Rh doped SnO2 nanobers showed a sensing
response of 60.6, which was observed to be 9.6 times higher
than that of the undoped SnO2 nanobers.10 PrFeO3 hollow
nanobers showed noticeable acetone sensing signatures witha maximum response of 6 towards 10 ppm. Mixed potential typesensors based on CeO2 and MMnO3 (M: Sr, Ca, La and Sm) werereported recently, and the composite SrMnO3 exhibited fast
response and recovery times of 2 and 6 s respectively towards
20 ppm of acetone.11 But these sensing responses were achieved
only at the elevated operating temperatures. Though few room
temperature acetone sensors were reported,12–14 the sensing
response and transient proles not signicantly pronounced
and hence, the present investigation has aimed at the development of room temperature operated, highly selective &sensitive, cost-eﬀective and stable acetone sensor using Co3O4
sensing element with minimum dri against the variations inrelative humidity (% RH). In this context, cobalt oxide sensing
elements were fabricated using spray pyrolysis technique by
varying the deposition temperatures from 473 to 773 K in stepsof 100 K.

2.

Materials and methods

2.1

Deposition of sensing elements

Cobalt oxide thin lms were deposited onto the glass substrates(Blue Star, Mumbai) using automated spray pyrolysis unit (HOTH-04, HOLMARC, India).15 The precursor solution was
prepared by dissolving 0.1 M of cobalt acetate tetrahydrate(Co(CH3CO2)2$4H2O, purity 99.99%, Sigma Aldrich, USA) in
25 mL of deionized water and stirred continuously for 1 h.
Meanwhile, the glass substrates were cleaned with deionizedwater (Millipore, USA) followed by acetone and ultra-sonicated(Supersonic, Mumbai) for 20 min. The precursor solution was
loaded into the spray dispenser, and the deposition parameters
were optimized. The substrate to spray nozzle distance andsolution ow rate were maintained at 15 cm and 1 mL min1
respectively. The carrier gas (compressed air) with the pressureof 1 bar was maintained throughout the experiment at a solution spray rate of 15 cm min1.16,17 The solution was sprayedover the glass substrates by varying the deposition temperaturefrom 473 K to 773 K in terms of 100 K respectively. There was no
instance of Co3O4 formation was observed below 473 K, andthere was no physical stability of Co3O4 thin lm was foundThis journal is © The Royal Society of Chemistry 2019beyond 773 K (Fig. S1†). Hence, the deposition temperature
window was chosen between 473 K to 773 K.
2.2

Growth mechanism & reaction scheme

Formation of Co3O4 transforms from the direct decompositionof cobalt precursor into sequential stages starting from theformation of the octahedral phase of Co3+ to Co2+ ions.18 The
thermal decomposition reactions of cobalt acetate precursor tocobalt oxide are given in eqn (1)–(3).19
DCoðCH3 COOÞ2$ 4H2 O ! CoðOHÞðCH3 COOÞ þ 3H2 O
þ CH3 COOH

(1)yieldsCoðOHÞðCH3 COOÞ ! 0:5CoO þ 0:5CoCO3 þ 0:5H2 Oþ 0:5CH3 CoCH3(2)
yields

CoCO3 ! CoO þ CO2(3)

As an initial thermal decomposition reaction, the conversionof cobalt acetate to cobalt hydroxide took place by the production of acetic acid. Following the subsequent thermal
responses, the formation of CoO occurred, and it gets transformed to stable Co3O4 by the oxidation reaction when it was
subjected to annealing at 353 K as given in eqn (4).20
yields3CoO þ 0:5O2 ! Co3 O4

2.3

(4)

Characterization and sensor fabrication

Crystal structure of the deposited sensing elements was investigated using X-ray Diﬀractometer (Bruker, D8 Focus, Germany)with Cu Ka radiation of 1.5418 Å in the 2q range of 20 to 80 .
Morphology of the prepared nanostructures was characterizedusing Field Emission Scanning Electron Microscope (JEOL,
6701F, Japan). UV-vis spectrophotometer (PerkinElmer,
Lambda 25, USA) with the wavelength range of 200 to 800 nm
was employed to study the optical properties of the gas sensing
elements. Room temperature gas sensing characteristics werestudied using customized gas testing chamber integrated withhigh resistance electrometer (Keithley 6517B, USA).21 Gold
Interdigitated Electrode (IDE) pattern was fabricated over thegas sensing element using DC magnetron sputtering. The
detailed procedure for IDE fabrication and electrical contacts
establishment were reported in our previous work.22 From thefabricated IDE patterns, electrical contacts were establishedusing 25 mm thickness gold wire using wire bonder (TPT wire
bonder, HP 05, USA). The concentration of the test analytes wasstandardized using eqn (5),22CðppmÞ ¼

dVs RT
 106
MPb Vb(5)

where, C is the concentration of test vapor (ppm), d is thedensity of test vapor (g mL1), Vs is the volume of injectedvapour (mL), R is the universal gas constant (8.3145 J mol1 K1),
T is the absolute temperature (K), M is the molecular weight, Pb

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is the chamber pressure (atm) and Vb is the volume of thechamber (L).
The stable resistance of the sensing element measured at air
atmosphere was considered as the baseline resistance (Rair). Thechange in the resistance of sensor aer exposing to the analyte
was noted as (Ranalyte). The sensing response was calculatedusing the eqn (6),S¼

3.

Ranalyte
Rair(6)

Results and discussion

Fig. 1(a) shows the XRD patterns of cobalt oxide thin lms
deposited at diﬀerent deposition temperatures. Presence ofdiﬀraction planes (220), (311), (222), (400), (511) and (440) in theobserved spectra revealed the formation of polycrystalline
natured cobalt oxide thin lms with cubic spinel phase. The
observed XRD pattern is in good agreement with the standard
JCPDS card data [78-1970] with lattice parameter a ¼ 8.02 Å.
Increasing intensity of (311) planes with deposition temperature, indicated the enhanced crystallinity of the lms. However,at 773 K, the preferential plane orientation was shied from(311) to (220). This could be attributed to the eﬀect of higher
deposition temperature, where the atoms get diﬀused andmigrated towards energetically favorable positions by attaining
the permanent residency on the surface.23 Lowering of surface
free energy could be the reason for the alignment of crystallites
towards favourable positions.18,23 Also, the shiing of (311)plane towards a higher angle at higher deposition temperature
could be due to the signicant grain growth and the inuence of
surface energy (Fig. 1(b)). The same dri was observed from themorphological analysis, where the grains started to grow withan increase in substrate temperature.
The average crystallite size was calculated using the Debye–
Scherrer relation as stated in the eqn (7),D¼

Klb cos q(7)

where, K is the shape factor (0.94) and l is the wavelength of Xray source CuKa1 (1.5406 Å), b is the full width at half maxima(FWHM), and q is the angle of diﬀraction. The increase inaverage crystallite size from 15 to 35 nm with deposition
temperature revealed the elongated growth of the crystallites(Fig. 1(c)). It could be attributed to the movements of adatoms
on the surface at higher deposition temperature in-turn clustering of grains.24 Preferential plane orientation was estimatedusing the texture coeﬃcient relation, as stated in eqn (8),IðhklÞ
IoðhklÞ
#"TC ¼
 1  P IðhklÞ
N N IoðhklÞ

(8)

where, I(hkl) is the measured intensity, Io (hkl) is the standard
intensity observed from JCPDS data, and N is the number ofdiﬀraction peaks. At 673 K, the texture coeﬃcient was found to30228 | RSC Adv., 2019, 9, 30226–30239

be 1.9 for (311) plane, which indicated the preferential planeorientation (Fig. 1(d)). Interestingly, it was shied towards (220)plane with a maximum value of 2.10 at 773 K. This could be
due to the greater mobility of crystallites towards favorable
energy positions at higher deposition temperatures.23
Compact surface with smaller spherical nanograins wasobserved for the lm deposited at 473 K, as shown in Fig. 2.
Grains get elongated with an increase in deposition temperature. The lm deposited at 673 K exhibited clustered grains,which led to the formation of voids on the surface of Co3O4 thinlm. Further, at 773 K, grains get thermally agitated andmigrated towards lower stable energy position, which might
have facilitated the grain clustering and coalescence process.
The laser spectra analysis of the deposited cobalt oxide thinlms is shown in Fig. 3. They revealed the formation of roughsurface with an increase in deposition temperature. The lm
deposited at 673 K showed a maximum enhanced root meansquare (RMS) roughness factor in comparison with the otherlms. The 3D spectra provide strong information about the
distribution of particles on the surface. All the deposited thinlms showed a maximum distribution of particles around 1 to
2.5 mm. This revealed the impact of deposition temperature in
the migration of adatoms on the surface towards favorable
energy positions, thereby increasing the distribution of particles and the roughness prole.
XPS spectra of cobalt oxide thin lm deposited at 773 K isshown in Fig. 4. Survey spectra shown in Fig. 4(a) conrmed thepresence of Co, O, and C elements in cobalt oxide thin lm.
Existence of Co in 2p, 2s, 3s, and 3p levels was identied fromthe spectra.
Cobalt 2p core-level spectra comprised of two peaks corresponding to 2p3/2 and 2p1/2 at the binding energies of 780.68
and 796.61 respectively (Fig. 4(b)). The binding energy diﬀerence between these two levels was observed to be 15.9 eV and it
signies the splitting of spin–orbit doublet25,26 of Co2+. Deconvoluting the cobalt 2p spectra gives rise to the two small satellite
peaks at 785.1, and 801.6 eV corresponding to 2p3/2 and 2p1/2
respectively. Coexistence of tetrahedral Co2+ and octahedral
Co3+ was conrmed through the deconvoluted 2p spectra, and it
is consistent with the cobalt 2p spectra.27,28 This monovalentand divalent Co species revealed the existence of mixed oxidation states of 2+ and 3+. O 1s spectra (Fig. 4(c)) observed at thebinding energy of 530 eV was deconvoluted into two sub-levels
at 531.01, and 532.2 eV corresponding to Co–O in Co3O4 andchemisorbed oxygen ions respectively.28,29 Low intense C 1s
spectra around 284 eV (Fig. 4(d)) could be comprised of C]C,C–C, C–O, and O–C]O.29 Raman spectra of cobalt oxide thinlms are shown in Fig. 4(e). Presence of all Raman modes corresponding to the cubic spinel phase of Co3O4 was observed.
Peaks centered at 196, 475, 516, 612 and 682 cm1 correspondto F32g, Eg, F12g, F22g, and A1g modes respectively.30,31 The intensityof these Raman peaks was increased with an increase in deposition temperature. Absence of other Raman modes corresponding to diﬀerent cobalt oxide phases (CoO and Co2O3)
conrmed the existence of cobalt oxide in only Co3O4 phase.32
Due to lower surface energy, the intensity of A1g mode increased
drastically for the lm deposited at 573 K in comparison withThis journal is © The Royal Society of Chemistry 2019

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RSC AdvancesFig. 1(a) XRD patterns, (b) shift in 2q of 311 planes, (c) crystallite size & d-spacing, and (d) texture coeﬃcient, of deposited cobalt oxide thin ﬁlms.

Fig. 2

Scanning electron micrographs of cobalt oxide thin ﬁlms deposited at various deposition temperatures.

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RSC AdvancesFig. 3

Laser micrographs of cobalt oxide thin ﬁlms deposited at various deposition temperatures.

other lms. All the peak positions were shied towards lower
wave numbers by 15–25 cm1, which could be assigned tophonon vector uncertainty in optical phonon connementeﬀect.33
The optical absorbance of the deposited cobalt oxide thinlms was observed in the range of 300 to 700 nm (Fig. 5(a)). All
the lms showed high absorbance in the visible region, and thepeak absorption wavelengths were found to be 440, 460, 468 and
420 nm respectively. The lms deposited at 573, and 673 K
showed larger cut-oﬀ wavelength in-turn decreased optical

30230 | RSC Adv., 2019, 9, 30226–30239

bandgap. The lower peak absorption wavelength of 420 nm forthe lm deposited at 773 K, indicated the higher energy states ofthe valence electrons and resulted in the larger optical
bandgap.23,34 The optical band gap of the cobalt oxide thin lms
was estimated using Tauc's relation as expressed in eqn (9),35
ahn1/n ¼ B(hn  Eg)(9)where, Eg is optical band gap, hn is photon energy, and B isconstant. Where n ¼ 1/2, 2, 3/2 and 3 equivalents to allowed

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Fig. 4 (b) XPS spectra of cobalt oxide thin ﬁlms prepared at 773 K (a) survey spectra, (b) Co 2p, (c) O 1s, and (d) C 1s spectra, and (e) Raman spectraof cobalt oxide thin ﬁlms prepared at four diﬀerent deposition temperatures.

indirect, direct, forbidden indirect and forbidden direct bandgaps respectively. The bandgap was estimated by extrapolating
the linear portion of the curve to the x-axis, as shown in
Fig. 5(b). Optical bandgap of cobalt oxide thin lm comprises of
two energy bands corresponding to direct and indirect transitions namely higher and lower energy bands due to the two-folddegeneracy at the valence band.23,36 Optical bandgap was only
observed in the range of 1.76 to 2.17 eV corresponding to higher
energy bands. The observed high energy band values are inThis journal is © The Royal Society of Chemistry 2019

good agreement with the results obtained by Louardi et al.37
Though the lm deposited at 773 K showed increased crystallite
size, the larger optical band gap might be due to the Moss–
Burstein shi.38 This shi of DE in the optical band gap occurs
when the carrier concentration exceeds the conduction band
density of states.38 Thus, the increased bandgap of Co3O4 could
be assigned to the Moss–Burstein shi and characteristic defectcenters of Co3O4. Extinction coeﬃcient was determined using
the following relation eqn (10),RSC Adv., 2019, 9, 30226–30239 | 30231

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Paper(a) Absorption spectra, (b) optical bandgap, (c) extinction coeﬃcient, and (d) PL spectra, of cobalt oxide thin ﬁlms deposited at diﬀerent
temperatures.

Fig. 5

k¼

al
4p

(10)

where, a is the absorption coeﬃcient, and l is wavelength.
An increase in the extinction coeﬃcient as a function ofdeposition temperature was observed (Fig. 5(c)). However, thelm deposited at 773 K showed a drastic decrease in theextinction coeﬃcient conrmed the higher energy migration &occupation of Co2+ atoms at elevated temperature.39 Presence ofcharacteristic defect centers in Co3O4 sensing elements wasconrmed through room temperature photoluminescence
spectra at an excitation wavelength of 440 nm as shown in
Fig. 5(d). The sharp peak in the energy region of 2.5 eV (496 nm)
could be due to the excitation of charge carriers from thevalence band to conduction band defect states.40 In other words,
the presence of this defect mode could be assigned to theTable 1transition of electrons from OII–CoII (ref. 40) and is in good
agreement with the relative intensity of Alg Raman mode. Films
deposited at 673 and 773 K showed upshied peak towards
higher energy (2.54 eV and 2.64 eV respectively) indicated themigration of surface Co atoms.41
To extensively investigate the electron transport properties of
the cobalt oxide in the grain regime, electrical properties suchas carrier concentration, mobility, grain & grain boundary
resistances, and activation energy were calculated and listed inTable 1. p-Type behavior of the deposited cobalt oxide thin lms
was conrmed from the positive values of hall coeﬃcient (RH).
Higher carrier concentration and mobility of the lm deposited
at 773 K could be attributed to the elongated grain prole. Since
grain regime decides the transport properties, the boundaries
between grains are limited as the grain size increased withdeposition temperature. Hence, grain & grain boundary

Electrical parameters of the cobalt oxide thin ﬁlm deposited at various temperatures

Samples (K)Carrier concentration
 1010 (cm3)

Mobility (cm2 V1 s1)Grain resistance(MU)

Grain boundary
resistance (MU)

Activationenergy (eV)C3H6O
response

473
573
673
773

3.41
4.19
4.08
8.26

36
52
93
128

160.3
104.2
4.4
6.6

947.6
891.7
16.1
12.9

0.457
0.316
0.252
0.218

19.4
132
200
235

30232 | RSC Adv., 2019, 9, 30226–30239

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Fig. 6

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Nyquist impedance plot of cobalt oxide thin ﬁlms deposited at (a) 473 & 573 K, and (b) 673 & 773 K.

resistances were estimated using AC impedance analysis, as
shown in Fig. 6.
The observed data were tted to the Randel's equivalentmodel, and the circuit parameters were calculated. The lms
deposited at 473 and 573 K showed larger grain and grainboundary resistances in comparison to the other lms, and it
could be due to the lesser movement of atoms towards favorable

energy positions because of decit thermal energy. On the other
hand, a drastic decrease in the grain and grain boundary
resistances were observed for the lms deposited at higher
deposition temperatures (673 and 773 K). Also, it conrmed thelimited grain boundaries between the elongated grains andscattering eﬀects.

Fig. 7 Arrhenius plots of cobalt oxide thin ﬁlms deposited at diﬀerent temperatures.

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Fig. 8 (a) Selectivity of the deposited cobalt oxide sensing elements towards 50 ppm of various analytes, and (b) comparative C3H6O sensing
response of the cobalt oxide sensing elements at diﬀerent deposition temperatures.

The activation energy of the deposited sensing elements wascalculated using the Arrhenius equation (eqn (11)),42,43


DEa
(11)
G ¼ GΟ exp kTwhere, DEa denotes activation energy, G represents conductance, GO is temperature independent factor, k is the Boltzmann's constant and T implies absolute temperature.
Activation energy can be calculated from the slope of ln(R)against 1/T as shown in Fig. 7. A decreasing trend of activationenergy with an increase in deposition temperature wasobserved. This trend signies the role of deposition temperature on the formation of active surface for the acetone–Co3O4

Fig. 9interaction. Since, the lm deposited at 773 K required relatively lower activation energy of 0.218 eV, the electrons crossed
over the potential barrier easily and resulted in the enhanced
sensing response.
Selectivity of the sensor is a key parameter in discriminating
the specic reactive analyte towards the sensor surface, whenthe mixture of analytes is presented. In this context, the selectivity of the Co3O4 sensing element were studied in the presence
biomarkers (for exhaled breath diseases) such as acetone(C3H6O), ammonia (NH3), ethanol (C2H5OH), xylene (C8H10),
toluene (C7H8) and acetaldehyde (C2H4O) and shown in
Fig. 8(a). All the sensing elements were highly selective towards
C3H6O. The detailed sensing investigations were carried out for

C3H6O sensing mechanism at the cobalt oxide surface represents the sequential interaction stages.

30234 | RSC Adv., 2019, 9, 30226–30239

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RSC Advances(a) Transient response characteristics of the ﬁlm deposited at 773 K, (b) log concentration (vs.) log(S  1) (inset ﬁgure is the response trend
at diﬀerent concentration of C3H6O for the ﬁlm deposited at 773 K), (c) response–recovery times towards 50 ppm, and (d) response–recovery
times towards diﬀerent concentration of C3H6O.
Fig. 10the lm deposited at 773 K alone, since it showed a maximum
sensing response of 235 towards 50 ppm at room temperature.
The sensing response was found to be increased from 19.5 to235 for the lms deposited at four diﬀerent substrate temperatures (Fig. 8(b)). This trend could be attributed to the nature of
the sensing element and the properties of the test analytes. As
far as the properties of gas sensing elements are concerned, theTable 2morphology, defect state, crystallinity, and surface roughness
decide the selective detection of target analytes.21,42
Enhanced crystallinity with crystal reorientation, shi in thedefect states, elongated growth of aggregated nanograins withimproved roughness prole of the lm deposited at 773 K,
could have enhanced the surface with more reactive oxygensites in-turn resulted in the increased sensing response. On the

Comparative analysis of the present work with C3H6O sensors reported in the literature

S. No Material & preparation technique

Operating temperature(K)
Response (S)LOD (ppm) Ref.

1

303 (RT)

235 towards 50 ppm

1

Present work

673

50

45

543

224 towards 1000 ppm for Nb decorated
ZnO nanowire
2193 towards 300 ppm

0.5

46

503

1.72 towards 0.1 ppm

0.1

47

453

141.3 towards 200 ppm

10

48

593
393
673

8.91 towards 20 ppm
300 towards 50 ppm
90 towards 500 ppm

0.5
50
15

49
50
51

2
3
4
5
6
7
8

Co3O4 nanostructures using spraypyrolysis technique
ZnO-wire decorated with Pt & Nb using
sputtering technique
SnO2 decorated SiO2 using
microemulsion-ultrasonic precipitation
Ytterbium ferrites using sol–gel
technique
PrFeO3 hollow nanobers using
electrospinning technique
WO3–Cr2O3 using sol–gel technique
NdFeO3 using sol–gel technique
Cr doped ZnO lms using co-sputtering
techniqueThis journal is © The Royal Society of Chemistry 2019

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Fig. 12 (a) Repeatability of the sensor for ﬁve diﬀerent cycles, and (b)stability of the sensor over 60 days, towards 50 ppm of C3H6O.

Fig. 11 (a) Variation in acetone sensing response, and (b) change inbaseline resistance of the sensing element deposited at 773 K, at
diﬀerent RH levels.

other hand, the properties of the test analyte such as kinetic
diameter, ionization & bond dissociation energy, dipole
moments, and HOMO–LUMO energy levels might have inuenced the sensing response.42 Since, C3H6O has lower ionization energy, moderate bond dissociation energy and relatively
higher kinetic diameter as 9.69 eV, 393 kJ mol1 and 0.42 nm
respectively in comparison with other test analytes, it was highly
selective towards the cobalt oxide surface.44 Also, the HOMO–
LUMO energy levels of C3H6O might have matched with thebandgap energy of cobalt oxide, which in-turn leads to better
sensing response.44
C3H6O interaction with the p-type Co3O4 sensing surface
follows sequential reactions at the ambient conditions. The
sensing behavior of Co3O4 towards C3H6O is schematically presented in Fig. 9. The room temperature sensing behavior of cobalt
oxide depends on the surface adsorption and desorption of C3H6O
molecules on the available adsorption sites.44 In the ambientenvironment, the gaseous oxygen molecules get chemisorbed on
the p-type Co3O4 surface by liberating the holes within the Debye
length as expressed in eqn (12). As a result, Co3O4 surface getsenriched with majority carriers (holes). As such, the carrier
concentration gets increased by reducing the depletion width.44
O2(gas) 4 O2(ads) + h+30236 | RSC Adv., 2019, 9, 30226–30239(12)

Due to the relative humidity in the ambient, the liberated
holes get interacted with the water molecules inside thechamber resulted in the evolution of hydrogen and hydroxylions on the surface as depicted in Fig. 9 (step 2) and given in eqn(13),
H2O + h+ 4 OH(ads) + H+

(13)

Upon C3H6O interaction with the chemisorbed oxygen
species, the formation of an intermediate unstable hydroxymethyl compound CH3–CO2–CH3 gets facilitated as given ineqn (14). As a result of the strong C3H6O–Co3O4 interaction,
more oxygen adsorption sites are consumed in-turn decrease
the holes concentration.44
CH3  CO  CH3 þ O2ðadsÞ þ hþ /Co3 O4 surface CH3  CO2  CH3ðadsÞ

(14)

Further, this intermediate unstable compound gets dissociated into methyl group and acetate group. The dissociatedacetate group reacts with the hydrogen ion due to its electronegative nature and results in the formation of acetic acid. The
remaining OH ions can further dissociate into water and oxygen
molecules. Further, chemisorption of these oxygen molecules
resulted in the modulation of depletion width. Hence, thesensor resistance gets increased from the initial baseline value
and attained stable resistance (Ranalyte). Upon sensor recovery,
the interacted C3H6O molecules get desorbed from the surface,This journal is © The Royal Society of Chemistry 2019

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and hence the sensor resistance decreased further from thestable resistance and attained baseline air resistance.
C3H6O sensing response towards diﬀerent concentrations(1–50 ppm) of the lm deposited at 773 K is shown in Fig. 10(a).
The linear detection range was observed as 1 to 50 ppm (inset of
Fig. 10(b)). A more signicant change in the sensor resistances
was observed with an increase in C3H6O concentration. To
conrm the linearity between the C3H6O sensing response andC3H6O concentration, the relation between log(concentration)
against log(S  1) was plotted and shown in Fig. 10(b). The
observed plot was tted to the linear expression y ¼ 2.45728x 
1.8693 with tting parameter adjacent R2 value as 0.97 indicated
the good linearity between the concentration and sensing
response.
Response and recovery times of the lm deposited at 773 K
were found to be 28 and 34 s respectively towards 50 ppm of
C3H6O at room temperature (Fig. 10(c)). At lower concentrationof C3H6O, few C3H6O molecules make interaction with thesensor surface, which resulted in the slow response. At higher
concentrations of C3H6O, a greater number of C3H6O molecules
get interacted, which leads to a faster response and recovery
transients (Fig. 10(d)).
To emphasis the salient features of this sensor, its performance has been compared with the literature and presented inTable 2. The LOD of 1 ppm with fast response–recovery, switransient characteristics, room temperature operation, good
repeatability, and long-term stability are the incremental
features of the present work.
Since the sensing response is highly reliant on the humid
environment, the inuence of relative humidity (% RH) on thesensing performance was investigated at various % RH levels. %RH was measured using the Digital Humidity and Temperature(DHT11) sensor integrated to the sensing chamber.22 Diﬀerent
RH levels were maintained by keeping the corresponding
saturated solutions inside the sensing chamber.52,53 Change inthe sensor baseline resistance at diﬀerent RH levels is shown in
Fig. 11(b). C3H6O sensing responses were decreased by 7.65 and16.59% at higher humidity levels of 78 and 89% RH, respectively. This could be attributed to the contribution of hydroxylions on the sensor surface. The hindrance eﬀect of hydroxylions on the chemisorbed oxygen ions limited the C3H6O–CO3O4
interaction which leads to Grotthuss physisorption mechanism.
In contrast, sensing response was increased by 3.40% at lower
humidity level (32%) due to the hopping charge transportmechanism.52,54–56
Repeatability of the sensor was studied for diﬀerent cycles,
as shown in Fig. 12(a). The sensor showed good repeatability,
and the results were reproduced aer 4 to 5 response cycles.
Stability of the sensor was investigated over 60 days in theinterval of 10 days as shown in Fig. 12(b) in which the sensor
showed good stability towards 50 ppm of C3H6O even aer 60
days. The lowest detection limit of the sensor was observed to be
1 ppm, which is lower than the threshold level of 1.71 and2.19 ppm. Hence, this sensor can be used to detect the presenceof type-2 diabetic mellitus and type-1 diabetes, respectively.5,6
Since the developed sensor showed enhanced acetone
sensing characteristics at room temperature, the sensing

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performance of the developed sensor was compared with thesensing performance of commercially purchased Co3O4 nanoparticles (ESI (S1)†). Better sensing response of the Co3O4 thin
lm than the commercially purchased Co3O4 nanoparticles
revealed the signicant role of microstructural properties,22,57–62
morphology63–69 and synthesis methods70–73 in deciding theoverall sensing performance.

4. Conclusion
Spray deposited nanostructured cobalt oxide thin lms wereprepared at various deposition temperatures. Microstructuralproperties of the prepared nanostructures revealed the formation of polycrystalline natured Co3O4 lm with cubic spinel
phase. The shi in the preferential plane orientation and theformation of aggregated morphology at 773 K deposition
temperature revealed the occupancy of Co atoms towards
energetically favorable positions. Electrical and optical properties of the prepared nanostructures were also studied asa function of deposition temperature. Room temperature
acetone sensing properties of the deposited sensing elements
were investigated. The Co3O4 thin lm deposited at 773 K
showed a maximum sensing response of 235 towards 50 ppm ofacetone. Also, the sensor showed the LOD of 1 ppm, which islower than the threshold level signifying the presence of DKA.
Hence, the fabricated sensor can be utilized as a non-invasive
diagnosis tool for the detection of trace-level acetone inexhaled breath.

Conﬂicts of interest
There is no conict of interest to declare.

Acknowledgements
Authors wish to express their sincere thanks to the Departmentof Science & Technology, New Delhi, India, for their nancial
support SR/FST/ET-II/2018/221 and ECR/2016/001805. One of
the authors Mr Parthasarathy Srinivasan, wish to expresses his
sincere thanks to the Council of Scientic and Industrial
Research (HRDG-09/1095/0016/2016-EMR-I). Authors also
thank Nano Mission, DST (SR/NM/PH-16/2007 and SR/NM/PH04/2015) for their nancial support. They also acknowledge
SASTRA Deemed University, Thanjavur for extending infrastructure support to carry out this work.

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