Zinc oxide doped graphene oxide films for gas ...

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Zinc oxide doped graphene oxide films for gas sensing applications Chetna, Shani Kumar, A. Garg, A. Chowdhuri, V. Dhingra, S. Chaudhary, and A. Kapoor Citation: AIP Conference Proceedings 1728, 020579 (2016); doi: 10.1063/1.4946630 View online: http://dx.doi.org/10.1063/1.4946630 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Gas sensing performance of nano zinc oxide sensors AIP Conf. Proc. 1724, 020106 (2016); 10.1063/1.4945226 Gas sensing properties of zinc oxide thin films prepared by spray pyrolysis AIP Conf. Proc. 1451, 191 (2012); 10.1063/1.4732411 Effect of Sputtering Gas environments on the Properties of Aluminum—doped Zinc Oxide Thin Films for Photovoltaic Application AIP Conf. Proc. 1391, 235 (2011); 10.1063/1.3646837 Enhanced gas sensing of Au nanocluster-doped or -coated zinc oxide thin films J. Appl. Phys. 102, 083103 (2007); 10.1063/1.2798922 Physical characterization of hafnium oxide thin films and their application as gas sensing devices J. Vac. Sci. Technol. A 16, 3564 (1998); 10.1116/1.580999

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Zinc Oxide Doped Graphene Oxide Films for Gas Sensing Applications Chetna1,a), Shani Kumar1, A.Garg2, A.Chowdhuri2, V.Dhingra2, S.Chaudhary1 and A.Kapoor1 1

Department of Electronic Science, University of Delhi South Campus, Benito Juarez Road, New Delhi-110021, INDIA 2 Department of Electronic Science, Acharya Narendra Dev College, University of Delhi, Kalkaji, New Delhi110019, INDIA a)

Corresponding author: [email protected]

Abstract. Graphene Oxide (GO) is analogous to graphene, but presence of many functional groups makes its physical and chemical properties essentially different from those of graphene. GO is found to be a promising material for low cost fabrication of highly versatile and environment friendly gas sensors. Selectivity, reversibility and sensitivity of GO based gas sensor have been improved by hybridization with Zinc Oxide nanoparticles. The device is fabricated by spin coating of deionized water dispersed GO flakes (synthesized using traditional hummer’s method) doped with Zinc Oxide on standard glass substrate. Since GO is an insulator and functional groups on GO nanosheets play vital role in adsorbing gas molecules, it is being used as an adsorber. Additionally, on being exposed to certain gases the electric and optical characteristics of GO material exhibit an alteration in behavior. For the conductivity, we use Zinc Oxide, as it displays a high sensitivity towards conduction. The effects of the compositions, structural defects and morphologies of graphene based sensing layers and the configurations of sensing devices on the performances of gas sensors were investigated by Raman Spectroscopy, X-ray diffraction(XRD) and Keithley Sourcemeter.

INTRODUCTION Graphene, a single layer sheet of carbon atoms, has attracted great interest in recent years for its extraordinary mechanical, electrical, thermal and optical properties. Due to its unusual properties, Graphene holds a number of promising applications.1,2,3 Graphene oxide (GO) which is a precursor of graphene, is structurally decorated with many functional groups on its basal planes and edges, such as hydroxyl, epoxy and carboxylic acid groups, which make GO as an excellent candidate for sensor application. 4 The functional group attached on its surface aids in enhancing the sensitivity of the sensor towards the analyte. The sensor can be fabricated by the simple and cost effective solution based method which is possible due to these functional groups associated with GO. Gas sensors are of critical importance because of their application in various fields such as medical, security, defense etc. The 2-d structure of graphene and its derivative makes them a potential material for gas sensors due to its high surface to volume ratio.5 The basic principal behind the GO based gas sensor is the charge transfer mechanism in which charges are transferred from adsorbate molecules to graphene and thereby change its electrical conductivity.6,7 Whereas GO film itself is a good material for gas sensing especially for hydrogen, the oxygenous functional groups would interact with many gas species and hybridization with other material is advantageous for selective gas sensing.5 In this work, we reported an simple and reproducible method to improve the sensing performance of a graphene oxide gas sensor based on Zinc Oxide doped graphene oxide film and demonstrated it with various gases. Addition of Zinc Oxide is an effective way to make GO materials selective and sensitive to analyte gases. 5

EXPERIMENTAL In this study we used Zinc Oxide doped Graphene Oxide film of thickness 10nm grown by spin coating on standard glass substrate. First, the Graphene Oxide flakes were synthesized using Hummer’s method. In this method, International Conference on Condensed Matter and Applied Physics (ICC 2015) AIP Conf. Proc. 1728, 020579-1–020579-4; doi: 10.1063/1.4946630 Published by AIP Publishing. 978-0-7354-1375-7/$30.00

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one gram of graphite powder was added to a 500ml round bottom flask in a bath ice containing 0.50g sodium nitrate (NaNo3) and 23ml concentrated sulfuric acid (H2SO4) kept at 5˚C and was stirred for 5min.Then 3g potassium permanganate (KMnO4) was added pinch by pinch to the mixture while maintaining the reaction mixture at 5˚C under continuous stirring for 2h.To oxidize the graphite, the mixture was slowly heated to 35˚C and kept under 30 min with vigorous stirring. Afterward, 46ml of deionized water was added to the mixture and due to the hydration heat the temperature raised to 98˚C. Stirring of the bath went on for 30 min at same temperature. With the addition of 140ml deionized water and 10ml hydrogen peroxide (10%v/v), the reaction was terminated. The resultant brown/yellowish color mixture was washed several times with diluted HCL solution (5%) and deionized water to obtain the desired GO solution of pH level 6. Later, GO solution was doped with 25 wt. % Zinc Oxide powder with the help of a cyclomixer for five minutes. After mixing, thin films were immediately synthesized by coating the easily available glass substrate with this solution using the spin coating unit. Finally, for gas sensing tests, copper (Cu) wires were attached to the electrode of graphene sensor using silver (Ag) paste, and the fabricated sensor was placed in the gas chamber with a gas inlet and outlet. Resistance measurements were performed on as-grown samples using Keithley (model 6487). Structure and defects of the film were estimated using X-ray diffraction (XRD) measurement (Rigaku Ultima IV) and Raman Spectroscopy (Renishaw Raman Microscope) respectively.

RESULTS AND DISCUSSION Figure 1(a) shows the XRD pattern of synthesized pristine graphene oxide films using well-known Hummers method. X-ray diffraction (XRD) is a useful technique not only for the phase identification but also for the identification of the lattice structure.8 The presence of diffraction peaks at 11˚ in (001) plane, which is mainly due to the oxidation of graphite. Figure 1(b) shows the XRD pattern of synthesized zinc oxide doped graphene oxide films and its diffraction peaks lies at 31˚in (100)plane, 34˚ in(002)plane, 36˚ in (101)plane and 47˚ in (102) plane respectively. It is observed that doping of zinc oxide not only increases conductivity of the film but also, intensity of GO peak also raised.

(a)

(b)

FIGURE 1. XRD Pattern of (a) Pristine Graphene Oxide film and (b) Zinc Oxide doped Graphene Oxide film.

Figure 2 (a) and (b) are the Raman spectra for synthesized graphene oxide films in pristine form and Zinc Oxide doped film. Raman spectroscopy is a powerful nondestructive tool to characterize carbonaceous materials, particularly for distinguishing ordered and disordered crystal structures of carbon.9,10 Figure 2 (a) shows a strong G band at 1587cm-1 which confirms the presence of carbon material in this sample and D band at 1347cm -1 indicating the sp2 hybridization of atoms of material. Figure 2(b) is the Raman spectra of synthesized zinc oxide doped graphene oxide films. An E2 band displayed at 434cm-1 made sure the presence of Zinc Oxide material in this sample.

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(a)

(b)

FIGURE 2. Raman spectra for synthesized graphene oxide films in (a) pristine form and (b) Zinc Oxide doped film.

We investigated the sensing performance of the sensor for various gases in terms of percentage response. The percentage response is defined as,  െ ᩞ ‡”…‡–ƒ‰‡”‡•’‘•‡ ൌ ‫ͲͲͳ כ‬Ψ  Where R is the resistance when there is no target gas flow and Ro is the resistance with gas flow respectively.11 Figure 3(a) shows the response of the sensor exposed to hydrogen gas. As the Zinc Oxide particles have high conductivity, so it may be responsible for decreasing the resistance of the sensor. Also, it should be noted that with repetitive bursts of hydrogen gas, recovery time is also increasing Since hydrogen is an electron donor, and on its exposure, the resistance of the device was found to be decreased, so it can be implied that ZnO doped GO forms an N-type semiconductor.

H2 Sensing

H2S Sensing Resistance(KΩ)

Resistance(KΩ)

200 150 100 50 0 0

100 200 Time(seconds)

14 H2S1

9

H2S2 4 0

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50

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Time(seconds)

(a)

(b)

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Resistance(KΩ)

60

SO2 Sensing

SO2 in

40 20 0 0

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(c) FIGURE 3. Sensing response of ZnO doped GO sensor when exposed to (a) H2 gas (b) H2S gas and (c) SO2 gas.

Figure 3(b) and 3(c) shows the resistance changes of graphene oxide sensor exposed to hydrogen sulfide gas and sulfur dioxide gas respectively. It is observed that ZnO doped GO sensor exhibits relatively poor response for H2S gas and no response for SO2 gas. It was also observed that for H2S exposure, the response of GO-ZnO sensor reduced with successive bursts of gas and both percentage response and range of resistance kept reducing.

CONCLUSION In this work, we successfully fabricated a Zinc Oxide doped Graphene Oxide gas sensor by using a simple and cost-effective method. Morphology and structural defects of graphene based sensing layers were inspected by using, XRD and Raman measurements. The gas sensing properties of the sensor were investigated by exposing to various gases. Although there was a significant decrease in the resistance of the device due to highly conductive nature of the Zinc oxide particles, rise in recovery time was also reported. In conclusion, we have fabricated a ZnO doped GO sensor that exhibited high percentage response to Hydrogen gas with fairly good response time.

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