STUDY OF GAS SENSING PROPERTIES OF ZnO · PDF fileZnO gas sensors in various forms have been...

O C T O B E R 2 0 0 9 • I S S U E N O . 3 0 9 • 3 1 9

F ο υο υο υο υο υ n δ εδ εδ εδ εδ ε r ’ s δ α ψ δ α ψ δ α ψ δ α ψ δ α ψ I s s u e

α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξα β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εψ ζ α β χ δ ε φ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ εφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξφ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ ψ ζ α β χ δ ε φ γ η ι ϕ κ λ μ ν ο π θ ρ σ τ υ ϖ ω ξ

S T U D Y O F G A S S E N S I N G P R O P E R T I E S O F Z n ON A N O W I R E S E N S O R

T h i s p a p e r r e c e i v e d t h e B e s t P o s t e r A w a r d a t t h e ( N a t i o n a l

S e m i n a r o n P h y s i c s a n d Te c h n o l o g y o f S e n s o r s ) N S P T S - 1 3 h e l d a t

U n i v e r s i t y o f P u n e , f r o m M a r c h 3 - 5 , 2 0 0 8

A B S T R A C T

We have studied the gas sensing propert ies of thick f i lms prepared using ZnO nanowires.For th is purpose, a paste of nanowires was prepared in methanol , pa inted on Aluminasubstrate fol lowed by annealing at 500ºC. ZnO nanowires were prepared by carbothermalmethod and character ized by SEM, EDX and XRD. The performance of f i lms as a chemicalsensor was studied, as a function of temperature for several toxic gases such as H2S, NH3,NO, CO and CH4. These f i lms were found to be h ighly sens i t ive to H2S and NO gases. I twas observed, that at room temperature, the fi lm is more sensitive to H2S gas as comparedto NO gas, but recovery after exposure to NO gas is faster. With increase in temperature,there is not much change in recovery t ime after NO gas exposure, but on exposure to H2Sgas, recovery t ime decreases appreciably with increasing temperature.

Introduction

Semiconductor metal-oxide-based gas sensors arecommonly used for environmental and emissionmonitoring, automotive, domestic, industrial andmedical applications, due to their advantages such assmall dimensions, low cost and inertness. The gassensing mechanism of these sensors, involveschemisorption of oxygen on the oxide surface, followedby charge transfer during the reactions of oxygen withtarget gas molecules. The adsorbed oxygen extractselectrons from the semiconducting material, leadingto change in carrier density and conductivity, whichcorresponds to the gas concentration. Since thesensing mechanism is surface reaction which is directlyrelated to the granularity, porosity and surface-to-

volume ratio of grains, the performance of the sensorsimproves with reduction in the size of oxide particles.Among the semiconductor metal oxides, zinc oxide(ZnO) was one of the earliest to be discovered and isone of the most widely used gas sensing material.ZnO gas sensors in various forms have been reportedto sense H2, NO2, NH3, CH4, O2, CO and ethanol.Recent advances in synthesis and investigation ofphysical properties of nanostructures, provide anopportunity to greatly improve the performance ofthese materials, for gas sensing. Variety of ZnOnanostructures such as nanowires, nanobelts,nanospring, nanorod, nanocombs, nanotetrapod,nanotube, nanonail, nanosheet, nanohelics have beensynthesized and studied. Better gas sensing propertiesof nanostructured ZnO have been reported [1-4].

R . M o h a n

D e p a r t m e n t o f P h y s i c s , B a r k a t u l l a h U n i v e r s i t y , B h o p a l ,

M . B h a r t i

C h a u d h r y D e v i l a l U n i v e r s i t y , S i r s a , H a r y a n a

a n d

M. K a u r, K . G a n a p a t h i a n d S . K . G u p t a

Te c h n i c a l P h y s i c s & P r o t o t y p e E n g i n e e r i n g D i v i s i o n

3 2 0 • I S S U E N O . 3 0 9 • O C T O B E R 2 0 0 9


D R . H O M I B H A B H A C E N T E N A R Y Y E A R

We have synthesized ZnO nanowires by carbothermalmethod and studied their gas sensing properties, as afunction of concentration as well as temperature. It isobserved, that at room temperature, these films arereasonably sensitive towards H2S and NO gases.Recovery time as function of increasing concentration,significantly increases for H2S, but remains nearly samefor NO. With increase in temperature, recovery timefor H2S gas decreases, while it does not have muchvariation for NO. This suggests a difference in sensingmechanism of the two gases.

Experimental

Synthesis: A conventional horizontal tube furnacewas used, for the synthesis of ZnO nanowires. Mixtureof ZnO and graphite in 3:1 ratio (by weight) was usedas a source material. It was loaded on to a quartzboat and placed in the center of 1 m long quartztube. High purity gases were introduced through oneside of the furnace and other side of the quartz tubewas connected to a water bubbler. The flow of gaseswas controlled with rotameters. The material washeated to 1050°C under a constant flow of 500 sccmargon. On stabilizing the temperature, the gasatmosphere was switched to 98% argon and 2%oxygen at same flow rate. The furnace was maintainedunder these conditions for 30 mins and then cooledto room temperature at a rate of 6ºC / min. Whitespongy material was deposited all along the tube, ingas flow direction.

Characterization: Surface morphology of thesamples was studied using a Scanning ElectronMicroscope (SEM) VEGA MV2300T/40 (TS 5130 MM,TESCAN). Chemical composition of the nanowires wasconfirmed, by recording Energy Dispersive X-rayanalysis spectrum (Oxford Instruments Inca energy 250system). The phases present and structure of nanowireswere identified by X-ray diffraction (XRD), carried outusing Cu-Kα radiation.

Gas sensingGas sensingGas sensingGas sensingGas sensing::::: Thick films were made out of

nanowires by making a paste of nanowires in

methanol, painting them on Alumina substrate,

followed by annealing at 500ºC. Two gold contacts

were thermally evaporated on these films. Silver wires

were attached to these gold film contact pads, to

measure the resistance change, on exposure to the

gases.

For measurement of the response to gases, the filmswere loaded in a housing having a volume of250 cm3 as described earlier [5]. Measured quantityof desired gas (having 1000 ppm gas concentration)was introduced in the housing, using a micro-syringe,so as to yield desired concentration and resistance ofthe film was measured, as function of time. Aftersteady state was achieved, recovery of sensors wasstudied by opening the housing to the atmosphere.

Results

Structural Characterization: The Scanning ElectronMicroscopy images (SEM) in Fig. 1 show, that asgrown samples consist of nanowires, with typicallengths in the range of several micrometers anddiameter of 50-150 nm. Energy Dispersive X-rayspectrum confirms, that the elements present in thepowdered sample are zinc and oxygen (Fig. 2). X-raydiffraction pattern of the nanowires is given in Fig. 3.The diffractogram indicates a standard hexagonalstructure of ZnO (JCPDS 80.0075) with latticeparameters a = b = 0.324 nm and c = 0.520 nm.Traces of metallic Zn is also indicated in the diffractionpattern.

Electrical Characterization

Room temperatureRoom temperatureRoom temperatureRoom temperatureRoom temperature ::::: ZnO nanowires wereinvestigated, for their use as gas sensors for severaltoxic gases (H2S, NH3, NO, CO and CH4). The filmswere found to exhibit good sensitivity towards H2Sand NO gases (Figs. 4 (a) and (b)) and very little or noresponse to other gases. The sensor response (definedas S = Ra/Rg for reducing gas and Rg/Ra for oxidizinggases where Ra and Rg are resistances in air and gasrespectively) as a function of gas concentration forH2S and NO gases is shown in Figs. 5 (a) and (b)respectively. Sensor response for 2 and 60 ppm of

O C T O B E R 2 0 0 9 • I S S U E N O . 3 0 9 • 3 2 1



Fig.1: SEM micrographs of ZnO nanowires at lower and highermagnifications

Fig. 2: EDAX spectrum from a bunch of ZnO nanowires

Fig. 3: XRD pattern from a bunch of ZnO nanowires

H2S gas is ~3 and 16 respectively, as compared to~1.5 and 8 for NO gas at same concentrations. It isalso observed that at higher concentrations (20 ppmin Fig. 4 (a)) recovery is very slow for H2S gas(~6 hrs). However for NO gas, recovery time is lessthan 10 minutes even for concentrations as high as60 ppm.

Elevated temperaturesElevated temperaturesElevated temperaturesElevated temperaturesElevated temperatures

The dynamic properties of the sensors suchas sensitivity, response and recovery timesare known to be temperature dependent.Generally the base resistance, response andrecovery times of sensors, decrease withincrease of temperature. Thereforeresponse-recovery curves were recorded asa function of temperature for both the gases(Figs. 6 (a) and (b)). It is observed, that for10 ppm of NO gas, response and recoverytimes decrease from 5 mins to 10 secs and10 min to 2 min respectively, whentemperature is increased from 30 to 300°C.In contrast, for 10 ppm of H2S gas,response and recovery times decrease from5 mins to 25 secs and 3 hrs to 3 mins onincreasing temperature to 300°C.

Discussion

It can be seen from Fig. 4, that recoverytime of the sensor on exposure to H2Sincreases with increase in gasconcentration, while for NO gas it is smallat all concentrations. Even as a functionof temperature,there is a huge change inrecovery time for H2S gas (3 hrs to 3 mins),as that compared to NO gas (10 min to2 min). The different response transientsof the sensor to these two gases, mightbe due to different sensing mechanismsfor the two gases.

Oxygen vacancies present in ZnO, acts aselectron donors to make it an n-typesemiconductor. Oxygen molecules fromthe ambient are adsorbed at the grainboundaries, which in turn capture electronsfrom the conduction band, formingadsorbed oxygen ion. This causes adecrease in carrier concentration andincrease in resistance of the sample. Whenthe sensors are exposed to a reducing gas,the gas reacts with the adsorbed oxygen,

3 2 2 • I S S U E N O . 3 0 9 • O C T O B E R 2 0 0 9


D R . H O M I B H A B H A C E N T E N A R Y Y E A R

Generally, sensing mechanism of allgases by any metal oxides is explainedby this mechanism. As the oxygenadsorption-desorption mechanism isnearly same for two gases, differencein recovery time characteristics,indicates a different mechanism forone of the gases.

Slow response to H2S at high

concentration may arise, due to

interaction of ZnO with H2S, forming

ZnS [6-8]. On exposure to low

concentration of H2S, (as discussed

earlier) recovery is complete, since the

amount of ZnS formed, is small. As

we go to higher concentrations, the

amount of ZnS formed is more and

the desulfurization rate is same, hence

there is an increase in recovery time.

At room temperature, desulfurization

rate is slow resulting in slow recovery,

with increase in temperature this rate

increases, which helps the sample to

recover faster.

The difference in behaviour of recoverytime in the case of two gases withincreasing concentration as well astemperature, could be attributed todifference in sensing mechanism. Incase of NO gas, sensing takes placeby adsorption-desorption process andin case of H2S gas, ZnO reacts withthe gas to give ZnS.

Conclusion

A conductometric gas sensor has been fabricated,based on ZnO nanowires synthesized by carbothermalmethod. Gas sensing properties of this sensor revealthat it is sensitive to both H2S and NO gases. Theresponse transients of these two gases have been

Fig. 4: Response and recovery curves of ZnO nanowires thick film with(a) different concentrations of H2S gas and (b) different concentrationsof NO gas

resulting in the release of the trapped electrons backinto the conduction band. This leads to an increase incarrier concentration and decrease in resistance of thesensor. However on exposure to oxidizing gas, reactiontake place directly with the oxide surface, ratherthan with the oxygen chemisorbed at the surface.

Fig. 5: Sensor response as a function of concentration at room temperaturefor (a) H2S gas and (b) NO gas

Fig. 6: Response and recovery curves of ZnO nanowire thick film for(a) 10 ppm NO gas as a function of temperature (b) 10 ppm H2S gas at150°C, 200°C and 300°C.

O C T O B E R 2 0 0 9 • I S S U E N O . 3 0 9 • 3 2 3



A B O U T T H E A U T H O R S

studied, as a function of temperature as well asconcentration. The difference in behaviour of recoverytime as a function of temperature and concentrationhas been explained in terms of different interactionmechanisms of the two gases.

References

1. Ge C., Bai Z., Hu M., Zeng D., Cai S., Xie C.,Preparation and gas-sensing property of ZnOnanorod-bundle thin films, Mater. Lett. 2007.

2. Bie Li-Jian, Yan Xiao-Na, Duan Yue-Qin, YuanZhi-Hao, Nanopillar ZnO gas sensor for hydrogenand ethanol, Sens. Actuat. B, 126, 2007,604-608.

3. Wen X., Fang Y., Pang Q., Yang C., Wang J.,Ge W., Wong K. S. and Yang S., J. Phys. Chem.B, 109, 2005, 15303-15308.

4. Hsueh Ting-Jen, Chang Shoou-Jinn, HsuCheng-Liang, Lin Yan-Ru, Chen I.-Cherng, Highlysensitive ZnO nanowire ethanol sensor with Pdadsorption, Appl. Phys. Lett., 91, 2007,531111-3.

5. Kaur M., Gupta S.K., Betty C.A., Saxena V., KattiV.R., Gadkari S.C. and Yakhmi J.V., Sens. Actuat.B, 107, 2005, 360.

6. Wang D., Chu X. and Gong M.,Nanotechnology 18, 2007, 185601.

7. Li Y., Guo H., Li C. and Zhang S., Ind. Eng.Chem. Res. 36, 1997, 3982-3987.

8. Hayter C. E., Evans J., Corker J. M., Oldman R.J. and Williams B. P., J. Mater. Chem. 12, 2002,3172-3177.

Dr. S. K. Gupta joined BARC in 1975 and is presently Head of Thin Films Devices Sectionin TPPED. Over the years, he has worked on space quality silicon solar cells, high temperaturesuperconductor thin films and single crystals, gas sensors and thermoelectric materials. Hehas carried out extensive studies on vortex dynamics in superconductors. He is a memberof the National Academy of Sciences, India.

Dr. Manmeet Kaur received her Ph.D. from Devi Ahilya Vishwavidyalaya, Indore in 1998.Her thesis work involved effect of heavy ion irradiation on high temperature superconductors.She joined BARC, Mumbai in 1999 as Dr. K.S. Krishnan Research associate and is presentlyworking as Scientific Officer. Her present interests include development of metal oxide thinfilms and nanomaterials for sensing toxic gases.

Ms. Kailasa Ganapathi joined BARC in the year 2006. She is working on developmentof H2S gas sensors and growth of nanowires of different materials. She has supplied manyH2S sensors to Heavy Water plants at Manuguru and Kota.

STUDY OF GAS SENSING PROPERTIES OF ZnO · PDF fileZnO gas sensors in various forms have been...

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Transcript of STUDY OF GAS SENSING PROPERTIES OF ZnO · PDF fileZnO gas sensors in various forms have been...