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Reduced graphene oxide as ultra fast temperature sensor Satyaprakash Sahoo,*,1 Sujit K. Barik,1 G. L. Sharma,1 Geetika Khurana,1 J. F. Scott2 and Ram S.

Katiyar1

1Department of Physics, University of Puerto Rico, San Juan, USA

2Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, United Kingdom

We demonstrate the excellent temperature sensing property of a chemically synthesized reduced

graphene oxide (rGO). It is found that with increase in temperature from 80 to 375K, the

resistivity of reduced graphene oxide monotonically decreases. The ultra-fast temperature

sensing property is demonstrated by keeping and removing a block of ice under the rGO sensor,

which shows the resistance of rGO increases by 15% in 592 miliseconds and recovers in 8.92

seconds. The temperature sensing of rGO is compared with a standard platinum thermo sensor

(Pt 111) and found the sensitivity is much better in rGO.

* Corresponding Author: E-mail: satya504@gmail.com

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In recent years graphene has gained a tremendous research interest due to its unusual

physical properties such as high carrier mobility, quantum Hall effect, high electrical and thermal

conductivity etc.1-5 The charge carriers in graphene are mass less Dirac fermions. Graphene is the

basic building block of carbon nanotube, fullerene and graphite. On the other hand, graphene

oxide is a derivative of graphene in which most of the pi-bonds between carbon-carbon atoms

are shared either by oxygen or functional hydroxyl group.6 Thus both graphene and graphene

oxide share the same crystallographic atomic layered structure. However, the presence of oxygen

and hydroxyl groups make graphene oxide more resistive to electric field than that of graphene.

The reduction of graphene oxide can be achieved under harassing reducing environments using

hydrazin or high temperature treatment and the electrical conductivity can significantly be tuned.

The advantage of GO over graphene is mainly due to the fact that the former can be produced

easily and in large quantities. Secondly, GO is usually dispersed as single sheet in water and

hence a continuous film of GO can easily be prepared on a substrate.7 The reduced graphene

oxide (rGO) has shown many promising applications such as gas sensor, field effect transistor,

bio-sensor, etc.8-12 Being atomically thin and having a high surface-to-volume ratio, its surface

can absorb gas molecules very efficiently. Although there are many reports on the gas sensing

properties of GO, its temperature sensing properties have not been reported so far. Here we

report the temperature sensing properties of rGO over a wide temperature range (375 K – 80 K)

and demonstrate the ultra fast sensing properties using an ice cube.

Graphene oxide (GO) synthesis was performed using a modified Hummers method.13

Concentrated H2SO4 was added to highly oriented pyrolytic graphite (HOPG, 2g) in a at room

temperature followed by continuous stirring using magnetic stirrer. The flask was kept in an ice

bath to maintain a constant low reaction temperature. Potassium permanganate (KMnO4, 7g) was

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added very slowly to the solution. After that excess of distilled water was added slowly to the

solution. Hydrogen peroxide was added slowly while stirring until the gas evolution stopped.

The resultant mixture filtered using a vacuum glass filter and the precipitates obtained were dried

for 24 hours in a vacuum oven at room temperature. In the present study the exfoliation of the

GO sheets was performed in this manner by sonicating the graphite oxide in water for 2 hours.

The sensor device was fabricated by drop casting the GO solution directly on the platinum

interdigital electrodes (Pt-IDE) and was allowed to dry and then heated at about 400 OC for few

minutes then followed by hydrazin vapor treatment. It may be noted that the resistance of the GO

before annealing was about few mega-ohms but changes to several ohms after reduction. The

platinum electrodes were made out of Pt metal on a thin (1 mm) Al2O3 substrate using electron

beam lithography. A thinner substrate was used to ensure quick thermal equilibrium between the

sensor and the environment. The temperature dependent resistance was measured using a MMR

temperature controller (K-20) and Keithley 2401 meter. The sensing properties of rGO were

measured using a Keithley 2401 meter.

Figure 1 shows the optical image of the rGO layer on the Pt-IDE. It shows that the rGO

evenly covers the electrodes except for few places, where some tearing and subsequent folding

of rGO is observed. Figure 2(a) is the FESEM image of the Pt-IDE on which few layers of GO

were deposited. Each of the Pt-IDE electrodes is 1000 and 20 µm in length and width,

respectively. The spacing between the consecutive electrode fingers is about 20 µm. FESEM

image of the several-layer rGO film on the Pt-IDE is shown in Fig. 2 (b), (c), and (d). The rGO

film is vey transparent, as can be seen from the high magnifying images [Fig. 2 (c) and (d)],

which indicates that our rGO sheets are very thin and may be consisting of only a few layers of

rGO. Figure 3 compares the Raman spectra of GO to that of rGO. Both GO and rGO have two

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distinct peaks: Those at 1350 and 1580 cm-1 are the so-called D and G Raman bands,

respectively.14 The peak height of D and G are almost the same in GO. However, the D band

intensity is larger than that of G in rGO, and this is due to the formation of large number of

defects during reduction process. The Raman spectra of GO and rGO are consistent with other

reported results.15 We fabricated many rGO sheets of different electrical resistance to study the

sensing properties.

Next, we will show the temperature sensing properties of two rGO sheets. Figure 4 shows

the current (I) verses voltage (V) plot of sample 1 at T = 375K and 80 K. It is found that the

current increases linearly with increasing voltage from -1 V to +1 V. The linear I-V characteristic

indicates the ohmic nature of rGO. Although the I-V plot shows ohmic nature for other

temperatures between 375 K and 80 K, we have not shown them here for clarity. Hence, we have

used the Ohm’s law to calculate the resistance (R = V/I) of the rGO sheets in the measured

temperature range. Samples of deferent resistances show similar behavior. It is worth to mention

here that a non-linear IV curve can be obtained, if the GO is not reduced enough.16

Figure 5 shows the temperature dependence of the resistance R(T) of rGO sensor. It is

found that the resistance decreases almost linearly with increase in temperature in the measured

temperature region 80 to 375 K, which shows the behavior of an intrinsic semiconductor. A

similar temperature dependent resistance behavior has been reported in metallic carbon nanotube

and monolayer and bilayer graphene.17,18 Uher et al.19 have reported such unusual temperature

dependent resistance in exfoliated graphite and according to them the negative coefficient of

resistivity is some form of activated behavior and not intrinsic to graphites rather related to high

density of defects. However, the exact mechanism is still not clear. It is important to know the

coefficient of resistance (α) of rGO temperature sensor for its sensor characteristic. Hence, we

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have calculated α for rGO from the above results using the expression, α=(1/R0)×(dR/dT), where

R0 is the resistance of the sample at 273 K and dR/dT is the slope of the R-T curve. We have

fitted the resistance behavior in the entire temperature region using a linear equation and the

slope is found to be -1.083 Ohm/K. As the value of R0 is 554 Ω at 273 K, the α is found to be

1.95×10-3 K-1 which is one order larger than that of reported carbon nanotube.17 This experiment

was performed several times and over several periods of times to ensure repeatability of the

result.

The temperature sensing performance test was conducted by keeping the rGO sensor

device on a block of ice. A constant voltage of 1V was applied across the two terminals of the

device and the change in current/resistance was monitored by periodically touching and

removing the ice block. The ice was in full physical contact with the back side of the device to

ensure proper thermal equilibrium between the ice and the sample. The thin substrate also helps

in quick thermal equilibrium. We have performed the temperature sensing in two rGO devices

with different room temperature resistances; sample 1 (240 ohm), sample 2 (520 ohm). Figure 6

shows the resistance verses time graph of sample 1 and sample 2. When the sample was just

touched to the ice block, the resistance increases abruptly and almost saturates after few

millisecond. Once the ice is removed the resistance drops exponentially and reaches room

temperature resistance. It is found that while the resistance increases by 15% in sample 2, it

increases by 12% in sample 1 upon touching the ice block with the sensor. As can be seen from

the graph that response time is much faster than the recovery time which a characteristic of

sensor. We calculate the recovery time for both the sensors by fitting the graphs using the

following simplified equation,

R(t)=R0+A exp[-(t-B)/ τ], (1)

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where R0 is the room temperature resistance, A is the amplitude, B is a constant and ζ is the

recovery/response time. The fitted curve is shown in Fig. 7 (a). The recovery times (τr) were

found to be around 6.35 and 8.19 seconds for sample 1 and sample 2, respectively. The response

times (τs) were also calculated from the plot and are found to be 0.58 and 0.59 seconds for

sample 1 and sample 2, respectively. The temperature sensitivity of rGO sensor was also

compared with that of standard platinum sensor (Pt 111) (see Fig. 7(b)) by using the similar

experimental set up. Both the response and recovery times in Pt 111 are found to be much slower

than that of the rGO sensor for same change in temperature (297 to 273K). The response and

recovery times of Pt 111 were calculated to be 8.66 and 15.51 seconds, respectively and the

change is resistance is also smaller (~ 6.8%). Note that, while the resistance of Pt 111 sensor

decreases with decreasing temperature, the resistance increases with decreasing temperature of

rGO.

In summary the temperature sensing effect in film of reduced graphene oxide a few

layers thick has been studied. The linearity in resistance as a function of temperature is verified

over a wide range of temperature (from 80 to 375K). The temperature sensitivity is found to be

much faster than the standard platinum thermometer. The response and recovery times of rGO

are 8 and 3 times faster than that of PT-111 sensor in the temperature range of 297 to 273K. Thus

rGO could be a potential candidate as a fast temperature sensor.

Acknowledgements: The authors acknowledge partial financial support from DoE through Grant

No. DE-FG02-ER46526.

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Figure Captions

Fig. 1. Optical microscopy image of rGO film on Pt-IDE.

Fig. 2. (a) FESEM image of a Pt-IDE. (b), (c) and (d) FESEM image of few layer reduced

graphene oxide with different magnifications.

Fig. 3. Comparison of Raman spectra of graphene oxide and reduced graphene oxide.

Fig. 4. Current verses voltage graph of rGO thin film deposited on Pt-IDE measured at 397 K.

Fig. 5. Temperature dependent resistance of the rGO temperature sensor.

Fig. 6. (a) Temperature sensing behavior of two rGO devices of different resistances; top and

bottom graph represents sample 1 and 2, respectively. The on and off-state in the graph represent

the contact and removal of a block of ice to the device. The corresponding resistances scales are

indicated by arrows. (b) Temperature sensing behavior for sample 1 and sample 2 are compared

for one period of time. The recovery time is calculated using equation (1).

7. A comparison of temperature sensing behavior of rGO device with that of a standard platinum

thermometer (PT-111). In both cases the recovery times were calculated by fitting Eq. 1. to the

graph which is shown as solid line. The corresponding resistances and time scales are indicated

by arrows.

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Fig. 1. Sahoo. et al.

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Fig. 2. Sahoo et al.

(a) (b)

(c) (d)

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Fig. 3. Sahoo et al.

1200 1350 1500 1650 1800

G band

GO

Inte

nsity

(arb

. uni

ts)

Raman shift (cm-1)

rGO

D band

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-1.0 -0.5 0.0 0.5 1.0-1.6

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

I (

mA

)

V (Volt)

80 K 375 K

Fig. 4. Sahoo et al.

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50 100 150 200 250 300 350 400

450

500

550

600

650

700

750

800

R

(Ohm

)

T (K)

Fig. 5. Sahoo et al.

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Fig. 6. Sahoo et al.

0 100 200 300

240

270

300

330Sa

mpl

e 2

Sam

ple

1R (O

hm)

Time (Sec)

(a)

350

400

450

500

550

600

650

OFF (Remove Ice)

273 K

R (O

hm)296 K

ON (Insert Ice)

0 30 60 90 120

240

270

300

330

Sample 2

τ 1 = 0

.592

sec τ

2 = 7.72 sec

R(T) = 515.5 + 95.86*exp(-(t-68.78)/7.72)

τ 1 = 0

.58

secR (O

hm)

Time (Sec)

τ2 = 6.35 sec

R(T) = 241.83 + 40.83*exp(-(t-64.48)/6.35)

Sample 1

(b)

350

400

450

500

550

600

650R

(Ohm

)

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Fig. 7. Sahoo et al.

200

220

240

260

280

30 60 90 120

τ2 = 6.35 sec

τ s = 0

.58

sec

τs = 8.66 sec

R (O

hm)

τr = 15.51 sec

Pt 111

Sample 1

∆R/Rsample 1 = 12.1%∆R/RPt 111 = 6.8%

0 50 100 150 200 250 300

105

110

115

120

125

130

Time (Sec)

R (O

hm)