Synthesis and characterization of nanostructured MnO2-CoO and its relevance as an opto-electronic humidity sensing device

Article abstract of DOI:10.1039/c8ra01975d

Metal carboxylates are widely used in science and technology and have been the subject of intense studies due to the practical importance of their products. The present paper reports the synthesis of MnO₂-CoO using metal carboxylates as precursors and the effect of humidity on the transmitted power through its thin film at room temperature. The refractive index of the material was found to be 1.445930 and the peak obtained from the photoluminescence spectra lies in the visible region. TEM reported a minimum grain size of ～5.7 nm and SAED confirmed the crystalline nature of the material, which was further confirmed by XRD. Fluorescence characteristics also confirmed the low dimensionality of the material. The film was then investigated using SEM which exhibited the porous morphology. Through UV-Vis spectroscopy, it was found that the absorption of the film takes place in the UV region and the optical band-gap was observed to be 3.849 eV from the Tauc plot. The film was employed as a transmission based opto-electronic humidity sensor. Average sensitivity was found to be ～2.225 μW/% RH with response and recovery times of 47 s and 59 s respectively. Experiments were repeated and the reproducibility of result was found to be ～89%.

Full text of DOI:10.1039/c8ra01975d

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PAPER
Synthesis and characterization of nanostructured
MnO2–CoO and its relevance as an opto-electronic
humidity sensing device†
Cite this: RSC Adv., 2018, 8, 20534
a
b
Samiksha Sikarwar,a B. C. Yadav,
G. I. Dzhardimalieva,
N. D. Golubevab
*
and Pankaj Srivastavaa
Metal carboxylates are widely used in science and technology and have been the subject of intense studies
due to the practical importance of their products. The present paper reports the synthesis of MnO2–CoO
using metal carboxylates as precursors and the effect of humidity on the transmitted power through its thin
film at room temperature. The refractive index of the material was found to be 1.445930 and the peak
obtained from the photoluminescence spectra lies in the visible region. TEM reported a minimum grain
size of ꢀ5.7 nm and SAED confirmed the crystalline nature of the material, which was further confirmed
by XRD. Fluorescence characteristics also confirmed the low dimensionality of the material. The film was
then investigated using SEM which exhibited the porous morphology. Through UV-Vis spectroscopy, it
was found that the absorption of the film takes place in the UV region and the optical band-gap was
observed to be 3.849 eV from the Tauc plot. The film was employed as a transmission based opto-
electronic humidity sensor. Average sensitivity was found to be ꢀ2.225 mW/% RH with response and
recovery times of 47 s and 59 s respectively. Experiments were repeated and the reproducibility of result
was found to be ꢀ89%.
Received 6th March 2018
Accepted 15th May 2018
DOI: 10.1039/c8ra01975d
rsc.li/rsc-advances
materials.11 Owing to the formation of ferromagnetic Fe3O4 and
CoFe2O4 nanoparticles, the thermolysis products of metal
Introduction
acrylates and their co-crystallizates behave as solid magnets
with a coercive force of 0.18 T and the residual magnetization of
Unique surface properties and high catalytic activity of metal
oxides make them very attractive for various sensor applica-
tions.1,2 Titanium, cerium, copper, iron, and manganese oxides
have been widely used as catalysts, photocatalysts and electro-
catalysts in a variety of reactions in organic synthesis.3–6 Tran-
sition metal oxides, such as Mn2O3, Co3O4, NiO, etc., are of
interest for optical waveguide gas sensors.7 V2O5 nanobelts
(V2O5$xH2O, V2O5$CTAB and V2O5 nanorods annealed at 400
ꢁC) revealed the properties of an ethanol sensor with high
selectivity and stability.8 The Cu–CuO nanocomposite was
successfully used to modify a glassy carbon electrode to detect
H2O2 and glucose.9
Metal carboxylates are widely used in science and tech-
nology. They have been the subject of intense studies due to the
practical importance of their products.10 In particular, the
controlled thermolysis of metal carboxylates can be considered
as an effective way for the synthesis of nanocomposite
15.5 mT at room temperature.12 The spinel ferrites Zn0.5Mn0.5
-
Fe2O4,13 CoFe2O4,14 and MnFe2O4 synthesized by pyrolysis of
the corresponding polyacrylate salts exhibit superparamagnetic
properties at room temperature.
15
Humidity measurement is one of the most required param-
eters for various applications not only for improving the quality
of life but also for enhancing the industrial process.16,17
Advancements in sensor manufacturing technologies lead the
researchers to know the degree of efficiency with predictions
based on simulation techniques and design aids to improve
sensitivity and sensor quality along with time-saving.18 The
humidity plays very important role and has acquired such
incredible signicance due to the fact that these vapours consist
of highly reactive dipolar molecules which get condensed on or
evaporate from the surface even with small variations in
temperature of the environment. Optical humidity sensors are
better than their electronic counterparts due to their small size,
low weight, immune towards electromagnetic disturbances,
possibility of multiplexing information and working on high
pressure and ammable environments.19 Complex oxides con-
taining cobalt and manganese show different oxidation states
(Co2+, Co3+, Mn2+, Mn3+, and Mn4+)20 and hence attractive
distinct properties like enhanced tunable surface, specic
aNanomaterials and Sensors Research Laboratory, Department of Physics, Babasaheb
Bhimrao Ambedkar University, Lucknow-226025, U.P., India. E-mail:
balchandra_yadav@rediffmail.com; Tel: +919450094590
bLaboratory of Metallopolymers, Institute of Problems of Chemical Physics, Russian
Academy of Sciences, Chernogolovka, Moscow Region, 142432, Russia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra01975d
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desirable pseudocapacitance, energy density. Also they are easy synthesis of manganese ethylhexanoate, 46.5 g of manganese(II)
to fabricate, have low cost, efficient structural properties, nitrate hydrate was dissolved in 400 ml of H2O, then the solu-
abundant in nature and environment friendly21 and hence can tion of metal nitrate was mixed with 205 ml of ethylhexanoic
be proved as an excellent material for humidity sensing. Also acid at pH 5.6 by adding NH4OH. The mixture was stirred for 1 h
recently their multicomponent electrocatalysts have been at room temperature, then aer 12 h water phase was isolated in
fabricated, like Mn3O4–CoO/CNT, Co3O4/Co2MnO4 and gra- a separation funnel. The contents of cobalt and manganese in
phene/CNT/Co3O4 which showed excellent performance the extracts were 64 and 47.2 g kgꢂ1, respectively obtained aer
towards ORR (Oxidation Reduction Reaction) catalysis.22 A few elemental analysis of the products on the atomic absorption
works has been done on manganese oxide and cobalt oxide spectrometer AAS-3. Then the mixture of metal hexanoate's
based humidity sensor. Kato et al. in 1997 investigated the sol– extracts in the required ratio (1 : 2 by weight) was stirred for
gel processed electrolytic manganese dioxide (EMD) as 20 min at 105–108 ꢁC followed by vacuum distillation at 150 ꢁC
humidity sensor which showed good sensitivity between 50– for 10 h. The Co : Mn : O atomic ratio in the product obtained
80% RH.23 In another work, K. Miyazaki et al. in the same year was equal to 1.5 : 1.5 : 4 determined by quantitative chemical
developed a potential-type humidity sensor based on M1/MnOx/ analysis.
M2 galvanic cell system, where MnOx is an EMD–clay mineral
mixture, M1 is Pt, and M2 is Cu or Al. Clay mineral samples
included kaolinite, bentonite, activated clay, zeolite, and
muscovite. Silica sand was used as a reference material. The
EMD–kaolinite mixture responded in a wider humidity region
of 20–90% RH.24 Similarly Yang et al. in 2011 studied a micro
humidity sensor which consisted of interdigitated electrodes
and cobalt oxide nanosheet sensing lm (prepared by precipi-
tation–oxidation method). The capacitance of the sensor
changes with the adsorption/desorption of water vapor.25 In
another work, Yadav et al. in 2013 investigated the multilayered
sensing lms prepared by sol–gel spin coating process using
cobalt oxide precursor annealed at 450 ꢁC which showed linear
behavior towards entire range of % RH with average sensitivity
of 0.76 mW/% RH.26
Previously our group has worked on yttria stabilized zirconia
system and its opto-electronic humidity sensing27 but sensitivity
was found lesser. As MnO2–CoO is a hygroscopic material and
synthesized rst time in our laboratory at low dimension with
high aspect ratio, therefore in order to improve the sensitivity,
nanostructured MnO2–CoO has been investigated as an opto-
electronic humidity sensor.
Fabrication of sensing element
Porosity and regularity of the deposited lm can be restricted by
monitoring the sample preparation environment like gel
density/viscosity, velocity of the spinner and heating/annealing
temperature. Spin coating technique was used to deposit
uniform sol–gel precursor MnO2–CoO thin lm with nanoscale
on at borosilicate substrates28 having dimensions 1.5 ꢃ 1.5
cm2 using a photoresist spinner (METREX RC100, India). Prior
to spin coating, the at borosilicate substrates were rinsed
thoroughly with deionized water and isopropyl alcohol followed
by acetone ultrasonically for 15 min each in order to remove
microscopic contaminations.29 The cleaned substrates were
dried in an electric oven (METREX) at 100 ꢁC for half an hour to
remove the organic precursors and moisture completely from
them. A small amount of gel was dropped on the centre of the
substrate and then rotated at high speed in order to extend the
gel by centrifugal force on the whole substrate. The rotation was
sustained for 60 s at 3000 rpm so that uid spins off the edges of
the substrate and desired thickness of the lm could be ach-
ieved.30 Aer thin lm formation, they were kept for drying on
a hot plate at 60 ꢁC for 15 min and then annealed at 650 ꢁC for
3 h in a tubular electric furnace (METREX) in 78% nitrogen.
Further, these lms were used as sensing elements to investi-
gate them as opto-electronic humidity sensing based on
transmission.
Experimental
Synthesis
Reagents. Ethylhexanoic acid (95%), Belgium, ACROS;
Co(NO3)2$6H2O (98%) (PANREAC reagent), Mn(NO3)2$4H2O
(PANREAC reagent) are used as received. Metal hexanoates were
synthesized by the extraction method in the two-phase system
of metal nitrate solution-organic acid by the following reaction
given in eqn (1):
Description of sensing set-up
The experimental set-up used for opto-electronic humidity
sensing is shown in Fig. 1. It consists of 2 mW He–Ne LASER
(Research Electro Optics Inc. 30 989) with 633 nm wavelength
as input light source. A diverging lens of 10ꢃ magnication is
used as beam expander for splitting the laser light beam so
that it may incident and cover the whole surface of the
sensing lm. A glass chamber with humidity controlled
system consisting of a hygrometer (HTC-1) of the range
(Mn+
)
aqua + CH3(CH2)3CH(C2H5)COOH /
M[CH3(CH2)3CH(C2H5)COO]n + nH+.
(1)
(Mn+ – metal ion, n ¼ 2, 3, 4).
For the synthesis of cobalt(II) ethylhexanoate, 45.5 g of 10–90% RH along with a dish is kept inside for holding the
cobalt(II) nitrate hydrate was dissolved in 160 ml of H2O, and humidier/dehumidier solution. The saturated solution of
then the solution of metal nitrate was mixed with 80 ml of K2SO4/KOH (Fisher Scientic) in distilled water is used as
ethylhexanoic acid at pH 5 by adding NH4OH. The resulting humidier/dehumidier. The sensing element is xed on
mixture was stirred for 1 h at room temperature, then aer 12 h one wall of the chamber such that it is perpendicular to the
water phase was isolated in a separation funnel. For the incident beam and also the beam covers the maximum
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Fig. 1 Schematic of transmission based opto-electronic humidity sensing set-up.
surface area of the lm. A beam condenser is used for
Results and discussion
Refractometer analysis
converging the laser light beam to obtain the maximum
intensity at a xed point. An optical power meter (Newport
1916-R) is employed for detecting the modulated power of the
transmitted beam by the sensing element. The detector part
is kept perpendicular to the beam for obtaining the
maximum output power and the whole experiment was per-
formed on the Vibration Free Table (M-RS2000-48-8) at room
temperature of 300 K.
For the investigation of the refractive index of the lm, the lm
was kept on the prism of the digital refractometer (Anton Par
Abbemat 550) at room temperature and the piston of the
instrument was pressed. Then screen showed the refractive
index of the material as 1.445930.
Scanning electron microscopy (SEM)
The SEM images of the lm at 10 mm, 5 mm and 1 mm are shown
in Fig. 2(a–c) respectively. Surface morphology of the MnO2–
CoO thin lms exhibits highly porous nature. The dimension of
Characterization techniques
The prepared lm was investigated through a digital Refrac- these pores is of micron order (as depicted in Fig. 2(c)); hence
tometer (Anton Par Abbemat 550) in its original state at room the lm is macroporous in nature. This porosity enhances the
temperature (300 K). The instrument is fast and non-destructive surface area of the material and hence provides many dangling
with a high accuracy of 0.000001. It is widely used for calibrating bonds suitable for adsorption/desorption of water molecule.
the official standards. TEM analysis of the material was carried These active centres serve as humidity adsorption sites and the
out using Transmission Electron Microscope, JEOL JEM 200 CX.
MnO2–CoO precursor in diluted form was loaded over carbon
coated copper grid with mesh size 200 at the scale of 50 nm. The
morphological study of the lm has been carried out at various
scales and resolutions using Scanning Electron Microscope,
JEOL JSM 840 A. The structural information of the lm was
studied by X-ray diffractometer X'Pert Pro recording system
sensitivity of the sensor depends on the availability of these
dangling bonds.
Transmission electron microscopy (TEM)
Fig. 3 shows the TEM-SAED image of nanostructured MnO2–
CoO dilute precursor solution. Fig. 4(a) shows the SAED pattern
of the material at 10 nmꢂ1 scale. Here the concentric rings
conrm the polycrystalline structure of nanocomposite. The
indexing has been done for the observed rings of MnO2
(tetragonal) and CoO (simple cubic) separately. Fig. 3(b) corre-
sponds to the TEM image at 50 nm scale. It can be observed
from the gure that there is uniform distribution of spherical
particles of nano dimension. The minimum grain diameter as
obtained from the micrograph is ꢀ5.7 nm which is not yet re-
ported in open literature.
˚
(PANalytical, Netherlands) using Cu Ka (l ¼ 1.5406 A) radiation.
The instrument was operated at 45 kV, 40 mA with scanning
parameters of 0ꢁ to 70ꢁ; typical 2q step size: 0.008ꢁ. Optical
characterizations were done by obtaining Fluorescence, Photo-
luminescence and UV-Vis absorption spectrum. The Fluores-
cence and Photoluminescence spectrum of dilute precursor
were obtained using Perkin-Elmer LS55 Luminescence spec-
trophotometer operated with a xenon discharge lamp (20 kW
power) as the excitation source at room temperature in the
visible range of the electromagnetic spectrum. UV-Vis absorp-
tion spectrum of the lm was recorded in the wavelength
X-ray diffraction (XRD)
regime of 190–1100 nm using UV-Vis Spectrophotometer Fig. 4(a) shows the XRD pattern of the lm of nanostructured
(Evolution 201) with Hg lamp as the excitation source.
composite of MnO2–CoO annealed at 650 ꢁC. The asterisk (*)
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Fig. 2 SEM micrographs of nanostructured MnO2–CoO thin film at (a) 10 mm, (b) 5 mm and (c) 1 mm, showing the porous nature and hence
enhanced surface area and availability of more active sites suitable for adsorption/desorption of moisture.
peaks denote the MnO2 crystals whereas hatch (#) peaks the uorescence spectrum is obtained in the wavelength regime
represent CoO crystals. The diffraction pattern shows good of 400–750 nm. The spectrum is shown in Fig. 4(b) where the
agreement with the diffraction data from JCPDS 240735 and peak in the lower wavelength region shows the blue shiing
750 419 for MnO2 crystals and CoO crystals respectively. The with respect to that of the reported literature of the constituent
minimum crystallite size of the material was estimated as 4 nm materials31,32 and hence conrms the low dimensionality of the
using Debye–Scherrer Formula from (110) peak which is the nanostructured composite of MnO2–CoO.
least value of crystallite size reported in open literature. The
crystallite size plays an important parameter in sensing as it
increases the aspect ratio and hence provides more active
Photoluminescence spectroscopy
The PL spectrum of the dilute nanostructured MnO2–CoO
precursor was obtained at room temperature in the wavelength
range of 350–750 nm. The spectrum is shown in Fig. 5(a). The
intensity and spectral content of the emitted photo-
centres for the adsorption/desorption of water molecules.
Fluorescence spectroscopy
Fluorescence spectroscopy is a contactless, non-destructive luminescence is a direct measure of numerous important
technique to probe the electronic structure of materials. Here, material properties, as well as band gap determination,
Fig. 3 Images showing (a) SAED pattern of the material where the concentric rings show the polycrystalline nature of MnO2–CoO nano-
composite and (b) TEM image at 50 nm scale showing all the particles of nano dimension with minimum grain diameter of ꢀ5.7 nm.
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Fig. 4 Curves showing (a) XRD pattern of nanostructured MnO2–CoO composite from which minimum crystallite size was estimated as 4 nm
and (b) fluorescence spectrum of dilute MnO2–CoO precursor in the visible regime of the electromagnetic spectrum where the blue shifted peak
confirms its low dimension.
dirtiness levels, imperfection detection and recombination where ‘h’ is Planck's constant, ‘n’ is the transition frequency and
mechanisms. The shiing of the emission peak may be due to the exponent ‘n’ characterizes the nature of band transition (n ¼
the non-uniform size of the particles (as conrmed from TEM) 1/2 and 3/2, 2 and 3 corresponds to direct allowed and directly
in the precursor sol.33 The emission peaks at 380 nm and forbidden, indirect allowed and indirect forbidden transitions,
400 nm corresponds to the violet emission, which may provide respectively). Here the transition being the directly allowed type,
a new scope in the eld of luminescent display materials.
therefore n is taken as 1/2 in the above equation. The optical
energy band-gap Eg was obtained by extrapolating the straight
line portion of Tauc curve to a ¼ 0. The estimated band-gap of
the thin lm was found to be 3.849 eV, which is quite high and
hence it can also be used in various optical devices.
UV-visible (UV-Vis) absorption analysis
The optical absorbance spectrum of the MnO2–CoO thin lm
deposited on the at borosilicate substrate was recorded in the
wavelength range 190–1100 nm and is shown in Fig. 5(b). It is
observed from data that lm exhibits absorption in the ultra-
violet range of the electromagnetic spectrum. Tauc curve (inset
of Fig. 5(b)) was plotted to show the variation of the square of
the optical absorption coefficient (a2) with photon energy hn
from the absorption data. The optical energy band-gap Eg of the
lm can be estimated from the eqn (2):
Humidity sensing
The optical humidity sensing characteristics for MnO2–CoO
thin lm annealed at 650 ꢁC has been investigated and plotted
in Fig. 6(a). The curve shows an almost linear decrement in
transmitted power with a corresponding increase in % RH in all
the lower (10–30% RH), intermediate (30–70% RH) and higher
regions of % RH (70–90% RH), maintaining a constant slope in
(2) the entire range of % RH. In our case, sensitivity is the most
Eg ¼ hn ꢂ (ahn)1/n
.
Fig. 5 Curves depicting at room temperature (a) photoluminescence spectrum in the wavelength regime of 350–750 nm where the peaks
correspond to violet emission (b) absorption spectrum of thin film and inset display the Tauc plot for optical energy band-gap calculation
showing the band-gap of 3.849 eV.
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Fig. 6 Variation of transmitted power with % RH for nanostructured MnO2–CoO thin film to investigate the [a] sensing response (sensitivity
observed was 2.225 mW/% RH) [b] reproducibility characteristics (ꢀ89% reproducible with 1.969 mW/% RH in this case) where inset figure displays
the error bars [c] aging effect (after one week, two weeks, one month and three months); and [d] response (47 s) and recovery (59 s) times.
concerned parameter, calculated by taking the slope of the sensitivity, in this case, is found as ꢀ1.875 mW/% RH showing
sensing response curve. Also, the sensitivity is dened as the 15.73% hysteresis.
ratio of the change in output signal to that of input signal, as
shown by eqn (3):
Reproducibility is another most concerned parameter for an
efficient sensor. It is the distribution of output when measure-
ments are taken under a similar environmental state of affairs
as earlier.34 Here the experiment for observing the reproduc-
ibility of sensor was performed and plotted in Fig. 6(b). The
inset displays the error bars of the experiment. From the error
bars, it can be observed that at 20% RH, there is maximum error
whereas it is minimum at 55 and 65% RH. The sensitivity
observed over the entire range of % RH, while observing the
reproducibility, was 1.969 mW/% RH and results were found
ꢀ89% reproducible. This high reproducibility depicts the quite
stable nature of the sensing element.
Ageing effect is also an important sensor parameter. It is the
distribution of sensor outputs when performing consecutive
readings under similar conditions aer a long time. The
consecutive measurements were taken aer one week, two
weeks, one month and two months showing as green, red, blue
Sensitivity ¼ DIt/D% RH mW/% RH.
(3)
where DIt is the change in transmitted power of laser light ob-
tained at the detector and D% RH is the relative change in %
RH, measured from hygrometer. So, sensitivity has been
calculated by taking the ratio of the difference in the initial and
nal readings of transmitted power to the corresponding
difference in % RH. The sensitivity in the range 10–30% RH is
2.85 mW/% RH, 30–70% RH is 1.825 mW/% RH and 70–90% RH
is 2 mW/% RH. The average sensitivity of the sensing element is
taken as the average of all the values and is found ꢀ2.225 mW/%
RH as calculated from the blue-lined curve of Fig. 6(a). The red
line in the curve of Fig. 6(a) shows the hysteresis while dehu-
midifying the environment inside the chamber. The average
Table 1 Sensing parameters for nanostructured MnO2–CoO thin film based sensor
Sensor parameters
Values
Average sensitivity
2.225 mW/% RH
Hysteresis
15.73%
Reproducibility
ꢀ89%
Response time
47 s
Recovery time
59 s
Aging effect
Insignicant
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Table 2 A brief literature review showing the humidity sensing capability of recently reported materials
S. no.
Material
Sensitivity
Ref
1
2
3
4
5
6
7
8
9
PMMA microber loop resonator (PMLR) coated with ZnO
0.1746 dBm/% RH
0.193 dB/% RH
0.838 mW/% RH
0.88 mW/% RH
1.831 mW/% RH
1.84 mW/% RH
1.86 mW/% RH
1.937 mW/% RH
2.1 mW/% RH
36
37
18
38
39
26
34
27
Graphene oxide/PVA
Cu(NO3)2$(AAm)4$2H2O
Zinc oxide
Zn(NO3)2$(AAm)4$2H2O
Cobalt titanate
MgTiO2
Y2O3–ZrO2
Sc(CH2]CH–COO)3
MnO2–CoO
40
10
2.225 mW/% RH
Present work
and brown coloured curves respectively in Fig. 6(c). The almost and the akes formation with enhanced surface morphology;
perfect overlapping among the curves indicates that nano- wide optical energy band-gap is also an important factor
structured MnO2–CoO is a quite stable material having insig- responsible for high sensitivity of the sensing element. The low
nicant effect of ageing and the sensor is quite stable in the dimension of the material is also a very important factor
entire range of % RH.
responsible for good sensing property. As the grain size
Response and recovery times were also measured for the decreases, the aspect (surface to volume) ratio increases. By
reported sensor. Response time is the time requisite to attain increasing aspect ratio more active sites for adsorption/
10% of the nal sensor output following stepwise ramping of % desorption can be generated. It was observed that as the
RH and recovery time is the time taken by the sensor signal to humidity inside the chamber increases, the transmitted power
come back to its original value aer a step concentration vari- observed at the detector decreases. This decrease in output
ation from a denite value to zero.35 Here both the response and power with the corresponding increase in % RH is the respon-
recovery times were calculated from Fig. 6(d) and it was sible characteristic for its application as an opto-electronic
observed that for sensing lm, the response time was 47 s and humidity sensor.
the recovery time was 59 s. All the sensor parameters have been
shown in Table 1.
The change in the transmitted power with respect to varia-
tion of % RH can be explained on the basis of adsorption and
desorption mechanism of the water molecule in the capillaries
formed on the surface of nanostructured bimetallic oxide thin
lm. The sensing mechanism, dependent on the surface
morphology is based on the chemisorption and physisorption
of a water molecule. In the beginning, the nanostructured
bimetallic oxide thin lm was free from water molecules with
only dehydrated air in its pores. When humidity was created
inside the chamber using a humidier, then at the low humid
environment, adsorption of water vapour takes place rapidly on
Sensing mechanism
It may be noted here that the lm shows encouraging sensitivity
due to rapid adsorption and desorption rate. A brief literature
review has been shown in Table 2. Nanostructured MnO2–CoO
gel having high refractive index 1.448829, has a greater angle of
transmittance. The laser light intensity aer being transmitted
from the lm was collected on a photodetector through a lens
combination. Besides the beautiful spherical shaped clusters
Fig. 7 (a) Ray diagram of sensing mechanism showing the decrease in output power with exposure of % RH showing (i) surface chemisorption at
lower humidity (10–30% RH), (ii) chemisorption on capillary walls at intermediate humidity (30–70% RH) and (iii) physisorption with full capillary
condensation at higher humidity (70–90% RH) (b) Theoretical results confirming the experimental results.
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the surface of the lm. The metal oxide surface being highly fellowship grant no. DST/INSPIRE Fellowship/2014/IF140317,
electrovalent has sufficient electrostatic force to break one of India.
the O–H bonds of H2O and form a strong chemical bond
between M+ (metal ion) and OHꢂ and forms the chemisorbed
layer. With the increase in % RH inside the chamber, adsorp-
tion on the surface of the lm increases and the refractive index
References
of the material decreases and tends to the bending of trans-
mitted laser light beam. In the mid humidity range (30–70%
RH), adsorption of water vapour in the form of OHꢂ ion not only
takes place on the surface but also on the walls of capillaries of
the lm. This leads to the increase in the thickness of moisture
layer which tends to a lateral shi of the transmitted light from
the sensing element. So the number of light rays reaching the
detector gradually decreases and therefore less power is ob-
tained at the output. In high humid region (70–90% RH), the
condensation of water vapour takes place through the capil-
laries and forms a meniscus in the capillaries of the lm in the
form of a physisorbed layer. In this range, moisture layer
acquires the maximum thickness and thus the lateral shi of
transmitted light takes place to its maximum level. Hence the
highest reduction in transmitted power is obtained, as most of
the power could not reach the detector and is scattered in the
surroundings.41–43 The whole sensing mechanism has been
explained by ray diagram in Fig. 7(a). The number of rays
reaching the detector has been calculated on the above
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Nanostructured MnO2–CoO was successfully synthesized using
metal carboxylates as precursors and thin lms were fabricated
via spin coating technique and annealed at 650 ꢁC. TEM
investigated the minimum grain diameter of ꢀ5.7 nm and
SAED conrmed the polycrystalline nature. The minimum
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
19 J. Ascorbe, J. M. Corres, I. R. Matias and F. J. Arregui, Sens.
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Authors gratefully acknowledge the Department of Science and
Technology (DST), Government of India (Ref no. INT/RUS/ 20 B. V. Golovkin and G. V. Bazuev, Russ. J. Gen. Chem., 2010,
RFBR/P-148) and Russian Foundation for Basic Researches 80(2), 213–218.
(RFBR) for nancial support in the form of the Indo-Russian 21 C. Li, J. Balamurugan, T. D. Thanh, N. H. Kim and J. H. Lee, J.
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