Reduced graphene oxide as ultra fast temperature sensor

Transcription

1 Reduced graphene oxide as ultra fast temperature sensor Satyaprakash Sahoo, *,1 Sujit K. Barik, 1 G. L. Sharma, 1 Geetika Khurana, 1 J. F. Scott 2 and Ram S. Katiyar 1 1Department of Physics, University of Puerto Rico, San Juan, USA 2 Department 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: satya504@gmail.com 1

2 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 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 H 2 SO 4 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 (KMnO 4, 7g) was 2

3 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 O C 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) Al 2 O 3 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 3

4 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 4

5 have calculated α for rgo from the above results using the expression, α=(1/r 0 ) (dr/dt), where R 0 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 Ohm/K. As the value of R 0 is 554 Ω at 273 K, the α is found to be 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)=R 0 +A exp[-(t-b)/ τ], (1) 5

6 where R 0 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 (τ r ) were found to be around 6.35 and 8.19 seconds for sample 1 and sample 2, respectively. The response times (τ s ) 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 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-ER

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9 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. 9

10 Fig. 1. Sahoo. et al. 10

11 (a) (b) (c) (d) Fig. 2. Sahoo et al. 11

12 D band G band Intensity (arb. units) rgo GO Raman shift (cm -1 ) Fig. 3. Sahoo et al. 12

13 K 375 K 0.4 I (ma) V (Volt) Fig. 4. Sahoo et al. 13

14 R (Ohm) T (K) Fig. 5. Sahoo et al. 14

15 273 K 600 Sample K 500 OFF (Remove Ice) 450 Sample ON (Insert Ice) R (Ohm) 300 R (Ohm) 650 (a) Time (Sec) R(T) = *exp(-(t-68.78)/7.72) = sec Sample 1 =6 270 t1 = 0.58 sec R(T) = *exp(-(t-64.48)/6.35) Time (Sec) Fig. 6. Sahoo et al. 15 R (Ohm) sec 550 t2 R (Ohm) t2 Sample t1 = sec (b) 330

16 R (Ohm) R/R sample 1 = 12.1% R/R Sample 1 Pt 111 = 6.8% τ 2 = 6.35 sec τ r = sec 110 τ s = 0.58 sec τ s = 8.66 sec Pt Time (Sec) R (Ohm) Fig. 7. Sahoo et al. 16