Selective detection of carbon monoxide (CO) gas by reduced graphene oxide (rGO) at room temperature

Article abstract of DOI:10.1039/c6ra06058g

Graphene materials have been widely explored for fabrication of gas sensors because of their atom-thick two-dimensional conjugated structures, high conductivity and large specific surface area. Thin graphene layers with attached functional groups desirable for gas sensing applications were synthesized by a wet chemical route (modified Hummers' method). Sensing performances of reduced graphene oxide (rGO) against carbon monoxide (CO) were studied in terms of percent sensitivity (sensor response), response and recovery times and I/V characteristics at room temperature as well as at elevated temperatures. Sensitivity data indicate the highest activity (～71% sensitivity against 30 ppm CO) occurs at room temperature (RT), indicating that the sensor is best operated at RT. Sensor response is quick (within 30 s) even to a trace amount (0.001% or, 10 ppm) of CO gas. Selectivity of the sensor was demonstrated by using different n-type reducing gases (like carbon monoxide, ammonia, methane and hydrogen), at different concentrations, showing negligible sensitivity towards gases other than CO. The synthesized material proves to be good as a selective room temperature sensor for harmful and poisonous carbon-monoxide gas.

Full text of DOI:10.1039/c6ra06058g

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Selective Detection of Carbon Monoxide (CO) Gas by
Reduced Graphene Oxide (rGO) at Room Temperature
Dipankar Panda, Anupam Nandi, Swapan K. Datta, Hiranmay Saha and
Sanhita Majumdar
*
Centre of Excellence for Green Energy and Sensor Systems (CEGESS),
Indian Institute of Engineering Science and Technology (IIEST),
Shibpur, Howrah, West Bengal, INDIA
*Corresponding Author and Address for Correspondence
Dr. Sanhita Majumdar
Centre of Excellence for Green Energy and Sensor Systems (CEGESS)
Indian Institutes of Engineering Science and Technology (IIEST)
P.O. Botanic Garden, Shibpur, Howrah, West Bengal, India.
Pin code: 711103.
Eꢀmail: email2sm@gmail.com
Telephone: +91ꢀ33ꢀ6455ꢀ1644
Fax: +91ꢀ33ꢀ2668ꢀ4564

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Abstract
Graphene materials have been widely explored for fabrication of gas sensors because of their
atomꢀthick twoꢀdimensional conjugated structures, high conductivity and large specific surface
area. Thin graphene layers with attached functional groups desirable for gas sensing applications
were synthesized by wet chemical route (modified Hummers’ method). Sensing performances of
reduced graphene oxide (rGO) against carbon monoxide (CO) were studied in terms of percent
sensitivity (sensor response), response and recovery times and I/V characteristics at room
temperature as well as at elevated temperatures. Sensitivity data indicate highest activity (~71%
sensitivity against 30 ppm CO) at room temperature (RT), indicating that the sensor is best
operable at RT. Sensor response is quick (within 30 sec.) even to a trace amount (0.001% or, 10
ppm) of CO gas. Selectivity of the sensor was demonstrated by using different nꢀtype reducing
gases (like carbon monoxide, ammonia, methane and hydrogen), at different concentrations,
showing negligible sensitivity towards gases other than CO. The synthesized material proves to
be good as a selective room temperature sensor for harmful and poisonous carbonꢀmonoxide gas.
Keywords: Graphene, Gas sensor, Carbon monoxide, Room temperature sensitivity, Selectivity
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Introduction
The use of nanostructured materials in gas detection applications has attracted considerable
interest in developing sensors with tailorꢀmade performances. A 2D sheet of sp2 bonded carbon
atoms on a honeycomb lattice, called ‘Graphene’ has attracted researchers all over the world due
to its extra ordinary electronic, mechanical and optical properties since its discovery by Geim
and Novoselov in 2004.1 Graphene has extremely large specific surface area and low electrical
noise due to its high degree of crystallinity2 making it an ideal material for gas sensor
applications. An extremely small change in the resistance of a graphene sheet caused by gas
adsorption even down to the molecule level is detectable. The most common source of graphite
used for chemical reactions including oxidation, is flake graphite, which is a naturally occurring
mineral. Reduced graphene oxide (rGO) or graphene sheets synthesized from such graphite can
be processed or assembled into ultrathin sensing layers by a variety of wet techniques such as
coating (dropꢀcoat/spinꢀcoat), casting, inkjetꢀprinting, Langmuir–Blodgett technique and layerꢀ
byꢀlayer deposition, simplifying the procedures of fabricating gas sensors.3ꢀ4
The first experimental study of graphene for gas sensing was performed by Novoselov et al.1
They demonstrated the gasꢀsensing potential of graphene to detect water or ethanol vapors or
ammonia gas. However, it was Schedin et al.5 who demonstrated that graphene is an ideal
material for high sensitive gas detection. Of late, experimental and theoretical assessments of
graphene’s performance in gas and/or vapor sensing have been made in several reports.6ꢀ8
Sensing is a complex function that requires the integration of a number of properties from
interface accessibility to transduction efficiency, molecular sensitivity and mechanical or
electrical robustness. It appears that graphene fulfills many of these requirements and the
realization of highly sensitive graphene sensors could illustrate how the previously described
properties can be brought in synergy to this specific objective. Indeed, graphene is a pure
interface with almost all atoms exposed to the analyte of interest, since it is only few layers
thick.9 Moreover, local destruction of the sp2 lattice preserves its mechanical robustness and does
not jeopardize its 2D delocalized transport properties unlike carbon nanotubes. Electronic and
mechanical properties can be exploited to perform the transduction of the sensing signal.10
Since the last few decades, metal oxide based solid state gas sensors are being extensively used,
but they suffer from various major drawbacks like poor selectivity, high operating temperatures,
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lack of stability etc. High operating temperature restricts their use in remote places. Moreover,
high power consumption necessary to achieve high temperature for their operation may cause
danger for gases which are flammable/explosive. Poor selectivity makes these sensors unsuitable
in places where more than a gas species is available. The aim of the present study was to
overcome these problems by using graphene (rGO) as the working material for gas sensors.
Amongst various gases, the detection of CO gas is especially important because it is toxic to
humans and other animals. Moreover, being a colorless, odorless and tasteless gas which is
slightly less dense than air, it is difficult to recognize in normal way. Carbon monoxide is highly
toxic to human beings; the maximum time weighted average (TWA) exposure value or
permissible exposure limit ascribed by the United States National Institute of Occupational
Safety and Health, is 35 ppm over an 8 h period.11 Inhaling CO over TWA can cause headache,
nausea, vomiting, dizziness and fatigue. CO combines irreversibly with hemoglobin (ironꢀcenter)
of blood and produces carboxyꢀhemoglobin, which prevents delivery of oxygen to tissues. Thus
carbon monoxide is constantly in the public eye, largely because the home is such a susceptible
place for carbon monoxide poisoning.
In this work the synthesis of graphene layers by inexpensive wet chemical route (through
modified Hummer’s method) has been reported with layer thickness of ~2 nm (detected by AFM
height profile measurement). The sensing properties of rGO (a nonꢀmetal oxide material) are
employed for detecting lowꢀconcentration of CO gas, under the most practical environment, viz.,
room temperature and atmospheric pressure, in presence of ambient humidity. Suitable sensing
sites have been established through prolonged refluxing. Sensors fabricated using synthesized
rGO show as high as 71% sensitivity at room temperature against 30 ppm carbon monoxide gas.
Significantly, sensitivity decreases with increasing operating temperature. Low cost fabrication
of the device prototype was, thus, feasible.
Experimental
Chemical used
Graphite powder [BurGOyne Urbidges]; H2SO4, Sulphuric acid [Merck, Emplura]; KMnO4,
Potassium permanganate [Merck]; H2O2, Hydrogen peroxide [Merck]; N2H4, Hydrazine hydrate
[Sigma Aldrich]. All the chemicals were of analytical reagent grade and used asꢀreceived,
without further purification.
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Wet chemical synthesis by modified Hummers’ method
Graphite powder (1g; particle size ~ 25µm) was slowly added with stirring to 98% H2SO4 (17.5
ml) and stirred vigorously for at least 2 hours. 3g of KMnO4 was then added as 0.15 (M)
solution, slowly with continuous stirring for minimum 30 min with temperature controlled
bellow 20° C, using ice. The stirring process was continued for another 30 min to arrest
temperature hike. Sulphuric acid adds functional groups on graphite surface (functionalization)
in the presence of reagent like KMNO4 (oxidizer). Thick brownish black colloidal solution was
obtained. 5 ml 30% H2O2 was added drop wise and stirred for another few minutes to convert
graphite to graphene oxide and to destroy the excess KMnO4. However, the graphene oxide
obtained contains traces of metal as impurity, which is eliminated by adding hydrochloric acid
followed by repeated (at least 5 times) washing with deꢀionized (DI) water (18 MOhm). The
blackish liquid turned into bright brownish yellow paste, which is pure graphene oxide (GO). To
obtain graphene, GO has to be reduced. First GO is diluted with DI water at 1:10 wt. ratio and
sonicated in an ultrasonic bath (Wensar WUC Seriesꢀ12L) for 30 min, followed by dropꢀwise
addition of hydrazine hydrate (20%) with constant stirring for 2 h. The liquid turned into black
Graphene (or, Reduced Graphene oxide, rGO).
In the Hummers method, a combination of potassium permanganate (KMnO4) and sulfuric acid
(H2SO4) is used. Though permanganate is commonly used as oxidant (e.g., deꢀhydroxylation),
the active species is, in fact, diꢀmanganese heptoxide (Mn2O7), generated in situ. The bimetallic
heptoxide is far more reactive than its monometallic tetraoxide counterpart.
Functionalization of the synthesized sample
The synthesized rGO liquid was finally refluxed at 100o C for ~18 h to attach desired oxide
species (like O2¯, O¯, O= etc.) on the surface. During prolonged reflux, condensation reactions
are responsible for introduction of the desired functional groups which ultimately facilitate the
gas sensing behavior by changing the resistivity/conductivity upon exposure to the analyte gas.
The sample was preserved as dispersed in DI water for future experiments and characterization.
Material characterizations
The phase identification of the synthesized GO was carried out by Xꢀray diffraction study (XRD;
RIGAKU Ultima IV; Cu Kα reactor of wavelength 1.54 Å.) using graphene thin film deposited
on 2.5 cm x 2.5 cm glass (silica) substrate by drop coating. Raman spectroscopy (Ranishaw
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inVia.) with laser excitation wavelengths of 515 nm (2.41 eV) equipped with a notch filter with
50 cmꢀ1 cutoff frequency was used to investigate the Gꢀband as a function of thickness. For the
measurement, graphene thin film was coated on polished glass substrate by spin coating. Fourier
transformed IR studies were carried out on FTIR instrument (Shimadzu, IRA Infinityꢀ1) using a
1.5 cm x 1.5 cm thin film, deposited on a polished silica wafer <100> (which are transparent
to IR). The aqueous suspension of exfoliated graphene sheets was dropped onto a polished silica
(SiO2) wafer substrate and characterized by an atomic force microscope (AFM; NTꢀMDT, Minus
K Technology) in semiꢀcontact mode to record the height profile and assess the layer thickness
of the synthesized material. FESEM (SIGMA ZEISS) was used to observe the surface
morphology and spreading of the drop coated thin film samples. For contact angle
measurements, the rGO, functionalized rGO, GO and graphite samples were drop coated on
cleaned glass substrates, dried at room temperature to make thin films and wettability was
measured against water through contact angle study using Drop Shape Analyzer (KRÜSS GmbH
–
DSA25) instrument. Optical transmittance (T) was measured by UVꢀVisible
Spectrophotometer (Perkin Elmer LAMBDA 25) as a function of wavelength in the range of
200–500 nm. Samples were also examined using a Tecnai G2 30ST, 300 kV high resolution
transmission electron microscope (HRTEM). For this, samples for measurement were prepared
by dropping homogeneous dispersion onto a carbonꢀcoated copper grid (200 mesh) at specific
intervals and dried in the ambient conditions. Currentꢀvoltage (I/V) measurement was carried out
using high resistance meter (HHV CMT 25, Keithley 6487 Voltage Source).
Sensor fabrication and measurements
For sensor fabrication, a small amount of liquid sample (1 ml) was diluted with DI water (5 ml)
and ultrasonicated for minimum of ~30 min. to get a perfectly homogeneous dispersion. Simple
Doctors’ glass slide (2.5 cm x 2.5 cm) cleaned with boiled DI water followed by acetone to avoid
contaminations from impurities, was used as the substrate. Gold electrode (thickness ~100 nm)
was deposited on the substrate by thermal evaporator (HHV AUTO 500) using a stainless steel
mask. Two copper wires were attached by soldering with the gold electrodes, to measure the
current output. Well dispersed graphene was drop coated on the substrate [displayed in Fig. 1
(a)], dried at room temperature to make it a conducting film for gas measurement studies.
Substrates were placed on a heater (oneꢀdegree resolution of heating) and was closed with a
Borosil glass beaker (volume ~250 ml) having two small holes at the top and side wall, to inject
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the target gas and collect the exhausts, respectively [schematically represented in Fig. 1 (b)]. The
substrate was heated to the desired temperature by the help of conduction heating method
(temperature modulation has been calibrated using thermocouple) and after achieving stability,
was exposed to the desired sensing gas and the change in resistance was recorded using Agilent
Multimeter.
Fig. 1: (a) Real image of graphene sensor sample, drop coated on a glass substrate with
attached Cuꢀwires. (b) Schematic diagram of gas measurement system.
Results and discussion
Characterization of graphite, graphene oxide (GO), graphene sheets (rGO) and
functionalized rGO
Fig. 2 represents the xꢀray diffraction patterns of graphite powder, graphene oxide and graphene
sheets. Fig. 2(a) depicts the xꢀray diffractogram of graphite (JCPDF, file no.: 75ꢀ2078) and Fig.
2(b) is the same of the procured graphite. The procured one is showing peak at 26.6o, which
matches well with the JCPDF impressions and corresponds with <111> plane having interlayer
spacing of 3.3 Å. Fig. 2(c) is the XRD profile of graphene oxide (JCPDF, file no.: 82ꢀ2261)
whereas, Fig. 2(d) is the same of the synthesized one. Two broad peaks at 9.7° and 12.4° of Fig.
2(d) indicate the nanoꢀmetric size and correspond with d111 (9.1 Å) and d200 (7.1 Å) planes,
respectively. One much broader hump at 2θ value ranging between appx. 17.8ꢀ30.6° [Fig. 2(d)]
may be due to the tendency of reduction of GO to graphene while heating for drying. The dꢀ
spacing of graphene oxide increased to 7.1 Å from 3.3 Å of graphite powder, which is ascribed
to the oxideꢀinduced Oꢀcontaining functional groups and inserted H2O molecules9 that can be
confirmed by further studies. These results suggest that the graphite powder has been completely
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oxidized to GO. Fig. 2(e) represents the XRD pattern of synthesized reduced graphene oxide or,
graphene. The Fig. shows one clear peak at 24.6°, the most prominent characteristic peak of
graphene, indicating that the graphene oxide has been completely reduced to rGO. Peak at 24.6°
corresponds to interlayer spacing of 3.6 Å of d002 plane. Lattice strain was calculated using
WilliamsonꢀHall analysis12 and found to be 0.26. The same for GO and graphite are 0.03 and
0.01, respectively (shown in Table I). The gradual escalation of lattice strain in the order rGO >
GO > Graphite, indicates the introduction of defects within the lattice structure probably due to
uneven epitaxial growth during synthesis, which ultimately benefits its application based on
sensitivity.
Fig. 2: Xꢀray diffractograms of (a) Graphite (JCPDF, file no.: 75ꢀ2078) (b) Procured
graphite (BurGOyne Urbidges) (c) Graphene oxide (JCPDF, file no.:82ꢀ2261) (d)
Synthesized graphene oxide and (e) Synthesized reduced graphene oxide or, graphene.
Fig. 3 shows the Raman spectra of GO, rGO and functionalized (refluxed) rGO taken at 515 nm
LASER excitation (green LASER) along with the reference of bare glass (substrate) in the range
of 500–2000 cm−1. It is a powerful nonꢀdestructive tool to distinguish ordered and disordered
crystal structures of carbon. Each of the deꢀconvoluted Raman spectra shows two peaks;
disorderꢀinduced (D) band and tangential (G) band. G band is usually assigned to the E2g phonon
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of C sp2 atoms, while D band is a breathing mode of κꢀpoint phonons of A1g symmetry.13 Two
distinct peaks at ca. 1360 and 1608 cmꢀ1 correspond with D and G band of GO whereas, the same
at ca. 1345 and 1600 cmꢀ1 correspond with those of rGO. On the other hand, the D and G band of
functionalized rGO appear at ca. 1375 and 1615 cmꢀ1, respectively. There are some minor peak
shifts of GO and functionalized rGO with respect to rGO probably caused by stress.14 However,
the peak shifting for GO and functionalized rGO with respect to rGO are in the same direction
indicating the presence of oxygen containing groups for both the species with respect to rGO.
Intensity of both the bands of GO increases substantially, indicating the decrease in size of the
inꢀplane sp2 domains, possibly due to the extensive oxidation and ultrasonic exfoliation as, when
the exfoliated GO is chemically reduced, the intensity of the D and G band increases further.15
This can be due to defects introduced during preparation procedure.14 The intense ‘D’ peak
indicates that the graphene sheets have structural disorder. Increased intensity ratio of D and G
band (ID/IG) in rGO (1.04) compared with GO (0.91) suggests a decrease in the average size of
sp2 domain due to the reduction process.
Fig. 3: Raman spectra of (a) Bare glass (used as the substrate of the samples) (b)
Reduced graphene oxide (rGO), (c) Functionalized (refluxed) rGO and (d) Graphene
oxide (GO).
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The mean crystallite size (DRAMAN) of GO, functionalized rGO and rGO layers were calculated
using the following relation16
DRAMAN = (2.4x10ꢀ10)λl4(ID / IG)ꢀ1…………………………….………………... (1)
Where, λl is the LASER excitation wavelength (515 nm) and ID/IG is the intensity ratio of D and
G bands. The mean crystallite size of rGO, functionalized rGO and GO calculated from the
Raman peaks are given in Table I.
rGO, GO, functionalized (refluxed) rGO and graphite thin films were also used to test their
wettability by measuring their static contact angle with water (shown in Fig. 4). It was found that
the static contact angle between graphite film and water is 115o (TableꢀI). Because graphite
consists of carbon atoms without any polarity, it demonstrates hydrophobic properties. For GO
film and water, measured static contact angle is 46.5o, much less than 90o. Hence, GO shows a
good hydrophilic property, as GO surface was presumably grafted with hydroxyl and epoxy
groups. These polar groups change the graphite properties from hydrophobic to hydrophilic.
After chemical reduction of GO by hydrazine hydrate, hydroxyl and epoxy groups are mostly
eliminated, which is exhibited through the measured static contact angle of 138o which is larger
than 90o.
Fig. 4: Static contact angle measurement with water and (a) graphene oxide, (b)
functionalized (refluxed) reduced graphene oxide (c) graphite and (d) reduced graphene
oxide.
It is interesting to note that this contact angle is larger than that of graphite film and water. Even
though isolated graphene also shows hydrophobic property, its degree of aqueous wettability is
less than that of graphite. This can be understood easily as nonꢀpolarity of rGO is comparatively
greater than graphite due to preferential reduction involving deꢀoxygenation and/or, deꢀ
hydroxylation during reduction by hydrazine hydrate. For this reason, graphite preserves more
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oxygen containing functional groups than rGO. This can be further experimentally evidenced by
FTIR analyses (provided later on). On the other hand, contact angle between functionalized rGO
and water is 98.2o, which is also hydrophobic in nature but the hydrophobic tendency is much
lower than rGO and graphite. Hence, from the experimental data we may conclude that
wettability trend of these Cꢀforms follow the order: GO > functionalized (refluxed) rGO >
Graphite > rGO.
Reduced graphene oxide was characterized by FTIR in the midꢀinfrared range, and the results are
shown in Fig. 5. The sharp peaks appearing at ca. 1050 and ca. 1389 cmꢀ1 were characteristic
peaks of υCꢀO and the δCꢀOH stretching vibrations, respectively.17 The peak at ca. 984 and 3441
cm−1 can be attributed to O–H stretching vibrations of adsorbed water molecules or interlayer
water molecules and structural ꢀOH groups. This particular peak is almost absent in rGO as, it is
preferentially deꢀoxygenated or, deꢀhydroxylated during the reduction process by hydrazine
hydrate. On the contrary, graphite shows this peak (at 3441 cm−1) though, the magnitude is much
less than that of GO or functionalized rGO. This deꢀoxygenation or, deꢀhydroxylation process of
rGO is also confirmed by the wettability trend observed by the contact angle studies discussed
earlier. In GO sample, the peak at ca. 1685 cm−1 can be attributed to O–H bending vibrations18ꢀ19
while, peak at ca. 1582 cm−1 reflects the skeletal vibration of GO.20 The presence of oxygen or
−OH indicates that during the oxidation process of the graphite powder with KMnO4 in
concentrated sulfuric acid medium, the original extended conjugated πꢀorbital system of graphite
was destroyed and oxygenꢀcontaining functional groups were inserted into carbon skeleton ie.,
the presence of these vibrations is indicative of the attachment of epoxide and hydroxyl groups.
After reduction to rGO by hydrazine hydrate, these functional groups derived from the intensive
oxidation have been eliminated (evidenced from Fig. 5). Disappearance of these two peaks (1582
and 1685 cm−1) in rGO sample indicate that the graphene oxide (GO) have been successfully
exfoliated to graphene sheets (rGO). However, some peaks especially those originating from ꢀ
OH functional group (like, peaks at ca. 984 and/or, 3441 cm−1) reꢀappear in the skeleton of
functionalized rGO (by prolonged refluxing) indicating the presence of structural deformities,
which ultimately facilitate the sensing performances.
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Fig. 5: FTIR spectra of graphite, graphene oxide (GO), reduced graphene oxide
(rGO) and functionalized rGO (refluxed rGO).
A few representative TEM, HRTEM and related images of rGO thin films prepared by
modified Hummers’ method are presented in Fig. 6 (aꢀd). Fig. 6(a) and (b) depict the bright field
images showing uniform thin layers in nanoꢀmetric dimension. Electron diffraction pattern is
depicted in a selected area (SAED) suggesting the crystalline nature of the nanostructure [Fig.
6(c)]. The Fig. is indexed with <002> diffraction plane which corroborate well with Braggs’
plane obtained from the XRD patterns and satisfy the reciprocal relationship between Miller
indices (h, k, l planes) and SAED rings diameter.21 However, among the two techniques, the
SAED technique is more authentic than Xꢀray diffraction because the dimensions of areas
concerned are as small as several hundred nanometers whereas, Xꢀray diffraction deals with
areas typically the samples’ area, the dimensions of which is to be measured in several
centimeters. The highꢀresolution transmission electron microscopic (HRꢀTEM) image of rGO is
presented in Fig. 6(d) in which, those lattice planes (fringes) are visible that possess interꢀplanar
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spacing greater than the lateral spatial resolution limits of the instrument. Here, the HR image
displays a dꢀspacing value of 3.6 Å corresponding to the <002> crystal plane.
Fig. 6: (a) and (b) TEM micrograph of rGO (c) SAED patterns with indexed plane and
(d) indexed fringes of HRꢀTEM image.
DC currentꢀvoltage (I/V) characteristics of asꢀfabricated graphene sensors at bias voltage ranging
between ꢀ5 to +5 V at thirteen different operating temperatures between 30o C (room
temperature) and 150o C (at 10o C intervals) were recorded in air with a high resistance meter
and the I/V characteristic plots obtained, presented in Fig. 7. The I/V characteristic curves
demonstrate a good ohmic behaviour, indicating that the metalꢀcarbon junction between gold
(Au electrode) and graphene and the carbonꢀcarbon junction between graphene layers are both
ohmic in nature. This is of prime significance as sensitivity of gas sensors can be maximized
when these junctions are ohmic or have negligible junction resistance. Fig. 7 also shows that at a
particular voltage, the current between the electrodes increases continuously with increase in
temperature, which is the characteristic property of semiconductors and is very common in
metalꢀsemiconductor contact systems. The semiconducting behaviour of graphene is thought to
originate from the polarization of adsorbed molecules (e.g., water and oxygen sp.) and/or to
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defects introduced into the graphene sheets during the preparation22 or even to the influence of
the supporting substrate.23
Fig. 7: Current Vs. Voltage (IꢀV) characteristic plots of graphene sensor films measured at
temperatures between room temperature (30 o C) and 150o C (at 10o C intervals) in air.
In presence of oxidising or reducing gases, adsorbed species on the surface of graphene sheets
acting as electron acceptor or donor, respectively. Thus, like many semiconducting gas sensors,
sensitivity of gas sensors made of sheets of graphene, is also a surface phenomenon and charge
transportation through adsorbed species is likely to be a function of sheet thickness and
orientation with respect to ‘dirac point’ which is comparable with crystallite size24 in polyꢀ
crystallites. However further explorations to elucidate the transport mechanism in graphene film,
have not been made in this study.
Sensitivity measurements of rGO film
Fig. 8 [(a), (b) & (c)] depicts the sensing characteristics of 18 h refluxed functionalized rGO
films. Fig. 8 (a) exhibits the sensor response changes with respect to temperature (from room
temperature upto 120o C, at 30o C intervals), in a 3D bar diagram format against different
concentrations (10, 20 and 30 ppm) of carbon nomoxide (CO) gas. It can be seen that with the
increment of temperature, sensor response drops. Room temperature (29.6o C) exposure of 30
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ppm CO displays the maximum sensitivity of ~71%, which indicates that the sensor is best suited
at room temperature. Without refluxing rGO film neither shows any room temperature sensitivity
nor sensitivities at 60o and 90o C. It only shows 12%, 18% and 32% sensitivity against 10 ppm,
20 ppm and 30 ppm CO gas respectively, at 120o C temperature. Fig. 8 (b) and (c) demonstrate
the dynamic responseꢀrecovery curves of functionalized graphene (rGO) sensor films against
different concentrations of CO gas, measured at room temperature and elevated (120o C)
temperature, respectively. The sensor signal showing pꢀtype gas sensing behavior with 71%,
52% and 44% sensitivity at room temperature and 51%, 43% and 23% sensitivity at 120o C
temperature, against 30 ppm, 20 ppm and 10 ppm CO gas exposure, respectively. Sensitivity at
60o and 90o C follow the intermediate trend (Table II). Fig. 8(b) and (c) also show that the sensor
response increases with increasing concentration of the gas. After reaching a saturation plateau
the dynamic curves switched from adsorption mode to desorption mode; these steady states
(plateau) indicate saturation of gas molecules at the depletion regions as well as the response
time of the sensors. The percent sensitivity or sensor response (S) of such pꢀtype sensors was
calculated using the following relation.25
S = [(Rgas – Rair) / Rair] x 100 …………………………………………….. (2)
Where, ‘Rgas’ is the resistance in presence of target gas and ‘Rair’ is the same in air.
The response is quick (within 30 sec.) even to a trace amount (0.001% or, 10 ppm) of CO gas,
however, the recovery is not so swift (>15 min) probably because of slow oxidation of gas
molecules on the surface of the sensor films.
It has been observed that, the sensors are not able to recover 100% in normal way. However, the
sensors may recover almost abundantly and come to their initial baseline value if exposed to UVꢀ
light, after removal of the target gas [Fig. 8(c)]. For UV exposure, the LED UV flashlight (385
nm) is exposed (LEDꢀ120 Inspection Light; Groz Engineering) during the recovery period for 1
min, when the CO gas is turned off; although the UV light does not speed up the recovery
process. In this study, a number of experiments have been carried out to measure the sensitivity
as a function of operating temperature and time. In all the cases, sensitivity of the sensor element
showed approximately constant values, indicating the repeatability of the sensor.
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Fig. 8: (a) Sensor response of functionalized rGO film at different operating
temperatures (from room temp. upto 120o C) against different concentrations of CO.
(b) Dynamic responseꢀrecovery curves of functionalized rGO measured at room
temperature against different concentrations of CO and (c) Dynamic responseꢀrecovery
curves of functionalized rGO measured at elevated (120o C) temperature against CO
gas of similar concentrations as of (b) (along with the responseꢀrecovery curve
recovered by the help of short UV exposure).
Humidity effects on rGO film
Most of the gas sensors perform in humid atmospheric condition, hence in order to elucidate the
role of humidity in the gas sensing properties of the rGO sensors, its responses towards water
vapor were investigated at room temperature (30o C) as well as at elevated temperature (120o C).
Fig. 9(a) and (b) exhibit the relative humidity (RH) effects on the prolonged refluxed rGO and
without refluxing rGO films, respectively. It was found that the resistance changes (in terms of K
Ohm) of the refluxed rGO film was only 15.75% at 120o C temperature [Fig. 9(a)], even in
presence of excess humidity (as high as ~98% RH) and it became almost stable after a certain
period of time which indicate that it is almost free from the interference of humidity. However,
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the effect was slightly high (25.94%) at room temperature, which was fairly natural as, with
temperature increment surface adsorbates (like H2O) start disappearing. Influence of RH on
without refluxed rGO was also investigated to compare the stability or shelf life period of both
the films as, the presence of relative humidity is responsible for surface perturbation leading to
adsorptionꢀdesorptionꢀtransformation of various species of oxygen including water molecules at
the surface and interface regions. Fig. 9(b) shows that as the relative humidity gradually
enhanced from 30% to 98% at room temperature, resistance of without refluxing rGO film (in M
Ohm range) goes down continuously and there was no saturation plateau in the similar range of
time interval as of Fig. 9(a), indicating that without refluxed rGO considerably affected by
ambient moisture though, it does not show any room temperature sensitivity against gases.
However, literature shows5 graphene film may sense water molecules but the effect reduces
pretty well by introducing the reflux method, followed by a good shelf life period of such
sensors.
Fig. 9: Relative humidity effects (RH) on (a) the prolonged refluxed functionalized rGO
[at room temperature and at elevated (120o C) temperature] (b) and without refluxing rGO
Selectivity tests of rGO film
To illuminate the selectivity, rGO films were also exposed against some other reducing gases
like, methane (CH4) and hydrogen (H2) and ammonia (NH3). The selectivity coefficient26 β =
SCO/Sgas (i.e., the sensor response ratio between the target gas, CO and other gas) exhibits that β =
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13.07 for CH4, 8.23 for H2 and 1.21 for NH3 at the same operating temperature (120o C). The
results (shown in Table II) confirm that the synthesized sensor is selective to CO gas with
negligible cross sensitivity against very high concentrations (1000 ppm of CH4 and H2 each and
100 ppm of NH3) compared to CO (maximum conc. of CO is only 30 ppm). Moreover, the
sensor does not show any methane and/or hydrogen sensitivity at room temperature. The
minimum temperature requirement to exhibit any sensor signal of such sensors is 100o C.
However, the sensor shows room temperature sensitivity against 100 ppm ammonia, though the
value is negligible (only 7% sensitivity) compared to the room temperature sensitivity against
CO gas (71%). A comparison of CO sensing characteristics and device performances of different
sensing materials already reported is presented in a tabular format (Table III)27ꢀ35 to compare and
show the advancement and significance of the fabricated device made by functionalized rGO
(presented at the end of Table III).
Sensing mechanism
The sensing mechanism involves surface conduction modulated by adsorbed gas molecules;
electrical conductivity depends strongly on surface states produced by molecular adsorption,
which result in spaceꢀcharge layer changes and band modulation. On the surface of the sensor
materials, two types of adsorption can occur: physisorption, the first step of the association of the
gas species with the sensor surface, and chemisorption, which involves exchange of electrons
between the adsorbed species and the material surface. The major difference between these two
processes is that physisorption is exothermic while chemisorption is endothermic.36 The sensing
characteristics are widely considered to be related with chemisorbed oxygen species, water
and/or hydroxyl ions, which can act as intermediates, catalyzing the charge transfer processes
between gas species and the film and is responsible for the study of gas sensing mechanisms
¯
(Fig. 10). The adsorbed oxygen on the surface can be of several forms: O2, O2 , O– and O2–. Of
these species, O2 is quite inactive because its activation energy is high; its concentration is also
very low. If reducing agent (like, CO) is introduced, the O– disappears very quickly relative to
¯
¯
O2 , indicating higher activity of O¯ than O2 . The lattice oxygen, O2– can also be reactive with
the incoming reducing agent. It has been proposed that adsorbed oxygen changes to various
oxygen anionic species, transferring an electron form rGO to the chemisorbed oxygen according
to the following processes37:
¯
O2 (gas) ↔ O2 (ad) ↔ O2 (ad) ↔ O¯ (ad) ↔ O2– (ad) ↔ O2– (lattice) ................. (3)
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Where, CO acts as a donor molecule (nꢀtype donor) (Fig. 10) with charge transfer capacity of
0.012 eV)38 and makes rGO to behave as pꢀtype; oxygen groups in defected rGO are responsible
for reactions with C−O bond of CO gas molecules.
In this study, The prolonged reflux method helped to attach hydroxyl groups ( ̶ OH) into the
reduced graphene oxide skeleton which ultimately increases the number of electrons in the
conductance band leading to an augmented depletion layer. The attached OH¯ is responsible for
generation of oxygen ion (O¯) which remains chemisorbed on the surface of the material and
gradually transforms into some other forms of anionic oxygen (like, O=) by extracting free
electrons from the conduction band, resulting in an electron depleted surface layer and a
consequent rise in resistance. The relevant chemical species exist in equilibrium39ꢀ41 as follows:
2OH¯ ꢀ H2O + O¯ (chemisorbed) + e¯ …………………………..…….…..…(4)
O¯ + e¯ ꢀ O= (chemisorbed) …………………………………..……………….(5)
When the sensor film surface is exposed to the analyte gas (like CO), it gets physisorbed:
CO (g) ꢀ CO (physisorbed) ……………………………………………………(6)
and the probable ways of decomposition of such analytes on the sensor surface may be
considered as42ꢀ43
:
CO (physisorbed) + O¯ ꢀ CO2 + e¯ ……………………………….…………..(7)
CO (physisorbed) + O= ꢀ CO2 + 2e¯ ……………………….………………….(8)
or, CO (physisorbed) + OH¯ ꢀ CO2 + H• …………………….…………….….(9)
OH¯ + H• ꢀ H2O …………………………………………………….………...(10)
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The processes indicate the inꢀsitu generation of atomic/ionic oxygen, which makes the process
irreversible in nature. These chemisorbed charged oxygen species at the grain boundaries are
responsible for the space charge appearance and band modulation. Therefore, the variation of the
chemisorbed molecule density is supposed to be the main factor responsible for the electrical
response whereas the rGO phase remains chemically stable. For this reason, such sensors can be
used repeatedly and has prolonged shelfꢀlife (unless humidity or other external interfering agents
affect the sensor surface).
Fig. 10: Schematic representation of gas sensing mechanism of CO adsorption on
graphene surface (carbon atom in black, oxygen in red and hydrogen atom in white).
In the decomposition reactions of analyte gases in presence of surface adsorbates, the resultant
products are carbon dioxide (CO2) and water (H2O) though the reaction proceeds through several
intermediates. These reactions decrease device conductivity as a function of time44 and hence
increase the resistance (pꢀtype behavior) of the device prototype. As conductivity is proportional
to the product of chargeꢀcarrier density and mobility, it is evident that changes in the number
density or mobility of carriers, or both, must be responsible for the change in conductivity.45
Graphene’s linear band structure around the Dirac points (the six
κ
points in the Brillouin zone)
is responsible for the gas adsorption, which possibly increase the number of electrons as the gas
is a donor one. Closedꢀshell adsorbates such as CO do not directly change the band structure of
graphene, but rather alter the distribution of charges within graphene.46. Adsorbed water is a
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prevalent surface “impurity”, especially when located between the graphene and the substrate,
which can shift the impurity bands of the substrate in the vicinity of graphene’s Fermi level and
so cause indirect doping of the graphene. Indeed, even if not initially present between the
graphene and substrate, water molecules might diffuse into this region upon exposure to humid
air.47 Recent theoretical studies also support this conclusion by demonstrating that gas molecules
adsorb only weakly on pristine graphene, but adsorb more strongly on defective graphene (i.e.,
graphene that contains a vacancy due to point defect or incomplete removal of epoxy/oxygenated
functional groups, that may generate during the reduction process or attachment of desired
functional groups by prolonged condensation reaction through reflux method) surface.48,9 These
defects and foreign functional groups remarkably affect the electronic conductivity of graphene
based devices like gas sensors. The relative position of HOMO and LUMO of the adsorbate CO
molecule with respect to Dirac point in graphene is presented in Fig. 11, which determines the
direction of charge transfer.
Fig. 11: The relative position of HOMO and LUMO of the adsorbate CO molecule with
respect to Dirac point in graphene (C atom in black and O atom in red).
Usually CO molecule acts as a donor but the sites of the charge transfer only depends on the
orientation of the molecules with respect to the surface. The differences in charge transfer are
due to differences in orbital overlap between the HOMO (5σ; weakly antibonding) of the CO
molecule and graphene. The LUMO (2π; strongly antibonding symmetry) seems to play no
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important role in the electron doping process although it is closer to the Dirac point than the
HOMO. In this case, HOMO is thus the more important orbital and confers transferability of
charge to graphene. Because the HOMO is mainly located on the C atom, the charge transfer is
largest when the C atom is closest to the surface, smallest when the O atom is closer to the
surface, and intermediate when both atoms are at (almost) equal distance from the surface.
Graphene also exhibits dangling edgeꢀbonds during CO adsorption.49 However, such edge
effects, though providing energetically favourable sites for gas absorption, contribute only to a
small extent (~2%) of the total electrical response of a typical microscale graphene device.50
Stronger adsorption arises from either defects or doping with suitable components.48
Conclusion
Graphene or reduced graphene oxide can be synthesized with added functional groups through
wet chemical bottom up approach (modified Hummers’ method), with graphene oxide as the
intermediate product. This material (graphene) possesses high conductivities that can undergo
measurable changes in terms of conductivity/resistivity when exposed to CO gas and produces
an electrical signal with a magnitude proportional to the concentration of the analyte gas. Gas
sensing experiments exhibit about 71% sensitivity at room temperature against 30 ppm CO with
negligible cross sensitivity against other reducing gases like NH3, CH4 and H2. Graphene is thus
a highly potential material for development of CO gas sensor with high degree of sensitivity,
selectivity and reliability proving it selective to CO gas at room temperature within permissible
exposure limits.
Supporting Information
AFM image of a multilayer graphene flake with height profile; FESEM micrograph of RGO;
UVꢀVisible spectra.
Acknowledgement
Authors acknowledge the financial support from Ministry of New and Renewable Energy
(MNRE) and Department of Science & Technology (DST), Govt. of India. Authors also
acknowledges Prof. Partha Chaudhuri, IACS, Kolkata, for assistance in recording the RAMAN
spectra. DP is indebted to DSTꢀCSIR Sensor Hub (Grant no. GAP 0332).
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Table captions
Table I: Some physical properties of different carbon components (Graphene, Graphene oxide
and Graphite).
Table II: Sensor response of functionalized rGO film at different temperatures against different
gases at diverse concentrations.
Table III: Comparison of CO sensing characteristics of different sensing materials.
Fig. captions
Fig. 1: (a) Real image of graphene sensor sample, drop coated on a glass substrate with attached
Cuꢀwires. (b) Schematic diagram of gas measurement system.
Fig. 2: Xꢀray diffractograms of (a) Graphite (JCPDF, file no.: 75ꢀ2078) (b) Procured graphite
(BurGOyne Urbidges) (c) Graphene oxide (JCPDF, file no.:82ꢀ2261) (d) Synthesized graphene
oxide and (e) Synthesized reduced graphene oxide or, graphene.
Fig. 3: Raman spectra of (a) Bare glass (used as the substrate of the samples) (b) Reduced
graphene oxide (rGO), (c) Functionalized (refluxed) rGO and (d) Graphene oxide (GO).
Fig. 4: Static contact angle measurement with water and (a) graphene oxide, (b) functionalized
(refluxed) reduced graphene oxide (c) graphite and (d) reduced graphene oxide.
Fig. 5: FTIR spectra of graphite, graphene oxide (GO), reduced graphene oxide (rGO) and
functionalized rGO (refluxed rGO).
Fig. 6: (a) and (b) TEM micrograph of rGO (c) SAED patterns with indexed plane and (d)
indexed fringes of HRꢀTEM image.
Fig. 7: Current Vs. Voltage (IꢀV) characteristic plots of graphene sensor films measured at
temperatures between room temperature (30 o C) and 150o C (at 10o C intervals) in air.
Fig. 8: (a) Sensor response of functionalized rGO film at different operating temperatures (from
room temp. upto 120o C) against different concentrations of CO. (b) Dynamic responseꢀrecovery
curves of functionalized rGO measured at room temperature against different concentrations of
CO and (c) Dynamic responseꢀrecovery curves of functionalized rGO measured at elevated
(120o C) temperature against CO gas of similar concentrations as of (b) (along with the
responseꢀrecovery curve recovered by the help of short UV exposure.
Fig. 9: Relative humidity effects (RH) on (a) the prolonged refluxed functionalized rGO [at
room temperature and at elevated (120o C) temperature] (b) and without refluxing rGO films.
Fig. 10: Schematic representation of gas sensing mechanism of CO adsorption on graphene
surface (carbon atom in black, oxygen in red and hydrogen atom in white).
Fig. 11: The relative position of HOMO and LUMO of the adsorbate CO molecule with respect
to Dirac point in graphene (C atom in black and O atom in red).
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Table I: Some Physical Properties of Different Carbon Components (Graphene, Graphene oxide
and Graphite)
Samples
Contact
angle
Crystallite size from
RAMAN
Lattice strain calculated
from XRD
(degree)
DRAMAN
(nm)
Graphene or,
Reduced
138
16
0.26
Graphene
Oxide (rGO)
Graphene Oxide
(GO)
46.5
115
18
0.03
Graphite
_
0.01
Functionalized
(refluxed) rGO
98.2
19
̶
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Table II: Sensor response of functionalized rGO Film at Different
Temperatures Against Different Gases at Diverse Concentrations
Temperature
Sensor Response (%S)
30
20
10 ppm
CO
1000
ppm
CH4
1000
ppm
H2
100
ppm
NH3
ppm ppm
CO
CO
Room
Temperature
(RT)
71
52
44
57
40
35
23
7
*ND
*ND
*ND
3.1
*ND
*ND
*ND
5.6
60o C
90o C
68
64
60
51
65
48
45
43
15
28
33
42
100o C
120o C
3.9
6.2
*ND: Not Detectable
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Table III. Comparison of CO Sensing Characteristics of Different Sensing Materials
Sl.
Material(s) sensing CO gas
Sensing parameter(s)
Selectivity to CO
Ref.
No.
1. Nanoꢀcrystalline LaFeO3 and
LaFe0.8Co0.2O3 particles
Sensitivity measured against 100
ppm CO.
Selective to CO at 100o C 27
operating temperature
2. Pd doped SnO2
Sensitivity measured in presence of Selectivity tests not
28
humidity in the temperature range
performed
between 200 and 400o C.
[Pd(η2ꢀol)(8ꢀ(diꢀtertꢀ
butylphosphinooxy)quinol
ine)] is selective to CO,
even in presence of
humidity.
3. Nanogravimetric sensors made Sensitivity measured against 125
29
by [Pd(η2ꢀol)(8ꢀ(diꢀtertꢀ
and 250 ppm CO.
butylphosphinooxy)quinoline)]
and [Pd(η2ꢀol)(8ꢀ(diꢀtertꢀ
butylphosphinooxy)ꢀ2ꢀ
methylquinoline)]
4. Rb2CO3 promoted In203
Responds well to 200ꢀ4000 ppm
CO in wet air at 300° C and
exhibits low cross sensitivities to
H2, CH4, C3H6, NO and CO2.
Selective to CO at
elevated temperature
(200ꢀ300O C)
30
31
32
5. Al doped graphene
CO sensitivity enhancement shown Selectivity tests not
via introducing large amount of
shallow acceptor states by forming
Al–CO bond.
performed
6. Alꢀdoped ZnO films deposited Maximum sensitivity of 61.6%
Selectivity tests not
performed
onto SiO2/Si substrates
reported for 65 nm ZnO:Al film
thickness at the operating
temperature of 400 o C. Sensitivity
and response time improved by
increasing the operating
temperature.
7. Auꢀ and Ptꢀdoped SnO2
nanocomposites
̶
In2O3 Showed superb sensitivity for both Not selective to CO
reducing and oxidizing gases like
33
34
CO, NO and NO2..
8. CuOꢀ and ZnOꢀdoped pelletꢀ
type SnO2
Sensitivity measured for 200 ppm
CO and 200 ppm H2 in the
Showed high selectivity
to CO at elevated
(~160°C) operating
temperature.
temperature range 80 to 450 o C.
9. 25% NiFe2O4–75%
La0.8Pb0.2Fe0.8Co0.2O3
Exhibits excellent sensitivity
toward CO at operating
Selective to 250 ppm CO
35
at ~175 o C operating
composite nanoꢀpowders
temperatures between 125 and 175 temperature in presence
o C.
of other gases like, 500
ppm C4H10, 500 ppm H2
and 5 ppm NO2.
10 Functionalized (18 h refluxed
at 100 OC) reduced graphene
oxide (rGO)
Showed the highest sensitivity
(~71%) against 30 ppm CO at
room temperature. Sensitivity
decreases with increasing
operating temperature.
Selective to CO. Other nꢀ
type reducing gases (like,
100 ppm ammonia, 1000
ppm methane and 1000
ppm hydrogen) showed
negligible sensitivity
Present
Study
compared to CO.
28

Page 29 of 29
RSC Advances
View Article Online
DOI: 10.1039/C6RA06058G
Graphical Abstract
254x190mm (96 x 96 DPI)