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GasSensingStudiesofPulsedLaserDepositionDepositedWO3NanorodBasedThinFilms

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A.Z.Sadek

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Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 8315–8319, 2013

Gas Sensing Studies of Pulsed Laser DepositionDeposited WO3 Nanorod Based Thin Films

Muhammad Z. Ahmad1�4, Joonhee Kang2�∗, Ahmad S. Zoolfakar1,Abu Z. Sadek1�3, and Wojtek Wlodarski1

1School of Electrical and Computer Engineering, RMIT University City Campus,GPO Box 2476V, Melbourne, Australia

2Department of Physics, University of Incheon, Incheon 406-772, Korea3School of Applied Sciences, RMIT University City Campus, Melbourne, VIC 3001, Australia

4M&A Research Center, MARDI HQ, 43400 Serdang, Selangor, Malaysia

WO3 nanorod based thin films were deposited via pulsed laser deposition onto quartz conducto-metric transducers with pre-patterned gold interdigitated transducers (IDT) employing the shortestwavelength (193 nm) ArF excimer laser. Micro-characterization techniques such as scanning elec-tron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM) wereemployed to study surface morphology and crystal structure. It was observed that the fabricatedfilms showed nanocolumnar features perpendicular to the surface. The measured sizes of thenanorods were found to be approximately ∼50 nm in diameter. The high resolution TEM (HRTEM)image of the nanorods based WO3 showed the WO3 lattice spacing of 3.79 Å corresponding to the(020) plane of monoclinic WO3. Gas sensing characterizations of the developed sensors were testedtowards hydrogen and ethanol at temperatures between room and 400 �C. The sensor exhibitedhigh response towards H2 and ethanol at operating temperatures of 170 and 400 �C, respectively.The excellent sensing characteristics of WO3 films towards ethanol and H2 at low concentrationsoffer great potential for low cost and stable gas sensing.

Keywords: Nanorods, Pulsed Laser Deposition, ARF Excimer Laser, Hydrogen Sensing.

1. INTRODUCTION

Tungsten trioxide (WO3� is a popular metal oxide withmany interesting electrical, optical, structural, and defectproperties in addition to being chemically stable andinexpensive.1–4 It is a transition metal oxide with a bandgap commonly observed in a range from 2.6 to 3.1 eV.2�5�6

It has been extensively used in electrochromic devices, gassensors, water splitting and batteries.2�5�7

Fabrication techniques such as Pulsed Laser Deposition(PLD), rf sputtering, sol–gel and chemical vapour depo-sition were employed to deposit high quality WO3 thinfilms.8–12 PLD is known to be one of the most reliableand favourable film deposition techniques.13 With PLD,wavelength, pulse energy and power of the laser beam areimportant parameters to tailor the properties of metal oxidethin films. Since high energy photons have higher kineticenergy and produce less thermal burst than other lasers,

∗Author to whom correspondence should be addressed.

argon fluoride (ArF) excimer laser was used to ablatesmaller clusters of target metal oxides for the improvementof film quality.Reliable gas sensors are required to monitor hydrogen

(H2� which is known to have a wide explosive concentra-tion range (>4%), low ignition energy (0.02 mJ) and largeflame propagation velocity.14–17 Aside from being colour-less, odourless and tasteless, H2 molecules have smallmolecular size making the gas difficult to contain.17 Accu-rate detection and monitoring of hydrogen concentrationsis very important. Alcohol sensors have numerous appli-cations especially in the areas of wine quality monitoring,breath analysis, food and biomedical industries, as well asmonitoring environmental pollution.16�18�19 Over the years,interest has grown in monitoring of these vapours to theppm level, and now, the demand for reliable and inexpen-sive H2 and ethanol sensors operating in wide concentra-tion range is extremely high. WO3 is a well known n-typesemiconductor which is highly sensitive towards both oxi-dising and reducing gases, making it a promising material

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Gas Sensing Studies of PLD Deposited WO3 Nanorod Based Thin Films Ahmad et al.

for gas sensing applications.10�20�21 Nanostructured WO3

is known to be a very attractive approach towards applica-tions in ethanol and H2 sensing.22–24

In this study, WO3 nanorods based thin films weredeposited via PLD onto a cleaned 8× 12 mm quartztransducers with pre-patterned gold interdigitated trans-ducers (IDT) employing the shortest wavelength (193 nm)ArF excimer laser. Micro-characterization techniques suchas scanning electron microscopy (SEM), X-ray diffrac-tion (XRD) and transmission electron microscopy (TEM)were employed to study surface morphology and crys-tal structure. It was observed that the fabricated filmsshowed nanorods features perpendicular to the surface.The developed sensors were tested towards H2 and ethanol(C2H5OH) at temperatures between room and 400 �C. Thesensor exhibited high response towards H2 and ethanolat operating temperatures of 170 and 400 �C, respec-tively. In this experiment, we report the sensing behaviourof nanostructured WO3 thin films. The films provideenhanced surface-to-volume ratios which consequentlyincreases interaction of analytes with the surface of thenanostructures.

2. EXPERIMENT SETUP

ArF excimer laser has the wavelength of 193 nm which isthe shortest wavelength used for the fabrication of semi-conductor oxides thin films. By optimizing the depositionparameters, the desired surface morphology and crys-tallinity structures of thin films were achieved. The laserbeam path in air and target-to-substrate distance was mini-mized to 67.5 cm and 35 mm due to significant energy lossexperienced in air. The optimum incident laser power was80 mJ/pulse when the laser pulse rate was fixed at 5 Hz.The high vacuum cryo-pump in PLD system was able toachieve the base pressure of 10−7 torr. During the depo-sition, the oxygen pressure was set at 200 mtorr and thetarget was rotated at an angular speed of 9 rpm to obtainuniform films. To enhance the uniformity of the film thesubstrate was also rotated at an angular speed of 5 rpm.The samples were annealed for 4 hours at 400 �C with

a ramp up/down of 2 �C/min. Annealing was performedto enhance the crystal structure of WO3 nanoparticles andto eliminate possible contamination. Once completed, goldwires were attached to the samples with silver epoxy andleft to dry on hot plate at 100 �C for 15 mins. The sen-sor was then mounted into a custom made gas chamberset-up connected to computer controlled mass flow con-trollers (MFCs) and a data acquisition system (Fig. 1). Thesensors were operated at every 50 �C interval from roomtemperature to 400 �C. The sensors were exposed towardsH2 of 0.05% for every 50 �C increment. Once the opti-mized sensor operating temperature for H2 was obtained,a dynamic response was performed. Consequently, thesesteps were repeated to obtain the sensing performance

Fig. 1. Gas sensing measurement setup.

towards C2H5OH (12.5 ppm). Optimization and dynamicresponse of the sensor was performed in a chamber thatwas maintained using an external heater and a thermocou-ple was used to monitor the operating temperature in-situ.Film resistance was measured with a Keithley 2001 multi-meter connected to a data acquisition system for real timedata logging.

3. RESULTS AND DISCUSSION

3.1. Structural Micro-Characterization

Figure 2 shows the SEM micrograph of the depositedWO3 thin films. The low magnification SEM image inFigure 2(a) reveals the uniform and homogeneous growthof WO3 nanostructured films on the quartz surface (onleft of image) and on the Au IDT (on right of image).From the high magnification SEM image in Figure 2(b)the size of the nanorods was estimated to be approximately50 nm in diameter. SEM images as well as the TEM imageof developed WO3 thin films which show clear nanorodsstructures can be obtained elsewhere.9 XRD analysis wasemployed to determine the crystallographic structure of thegrown films, and the results are shown in Figure 3. TheXRD peaks corresponded to the monoclinic WO3 [ICDD83-0947]. The study indicates good crystalline formationof the thin film. Further imaging towards the nanostruc-tured thin films employing TEM microscope reveals theapproximate WO3 nanorod size of 50 nm as shown inFigure 4(a). The high resolution TEM (HRTEM) image ofthe nanorods based WO3 (shown in Fig. 4(b)) reveals theWO3 lattice spacing of 3.79 Å corresponding to the (020)plane of monoclinic WO3. The nanostructured tungstenoxide film thickness was determined to be approximately100 nm using a Tencor P-16 profilometer.

3.2. Electrical Characterization

Fabricated sensors were placed into a measuring chamberand exposed towards H2 with various concentrations. Oncecompleted, the sensors were tested towards ethanol with

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(a)

(b)

Fig. 2. SEM images of deposited WO3 thin film at (a) low magnifica-tion (b) high magnification.

Fig. 3. XRD pattern of deposited WO3 films.

(a) (b)

Fig. 4. (a) TEM image of deposited WO3 thin film; (b) lattice fringesof the WO3 on (020) plane.

Fig. 5. (a) Dynamic response of the WO3 based sensor towards H2 ofvarious concentrations (b) measured sensor response (S).

multiple concentrations. The target gas/vapour utilized wascertified H2 (1.03% H2� and ethanol (500 ppm) in a bal-ance of synthetic air, which was diluted with syntheticair via a computerized mass flow controller to main-tain a constant flow rate of 200 sccm throughout testing.The dynamic performances of the sensor at their respec-tive optimized operating temperatures are presented inFigures 5 (for H2� and 6 (for ethanol). The optimized sen-sor operating temperatures (Fig. 7) were 170 and 400 �Ctowards H2 and ethanol, respectively. It was found that

Fig. 6. (a) Dynamic response of the WO3 based sensor towardsC2H5OH of various concentrations (b) measured sensor response (S).

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Fig. 7. Response of developed WO3 based sensor towards (a) H2

(0.05%) and (b) C2H5OH (12.5 ppm) with increasing operatingtemperatures.

the sensor resistivity decreases upon exposure to bothH2 and ethanol. The result suggest that the WO3 basedfilms is n-type, which is in good agreement with othertungsten oxide gas sensors.24–26 Redox reactions and sur-face interactions for both analytes (H2 and ethanol) withn-type materials have been previously described in detailby Xiang et al.27 Kwak and Yong,28 and Chen et al.16

The sensor responses (S) of the prepared sensor attheir optimized operating temperatures are presented inFigures 5(b) and 6(b). The sensor response toward H2

and ethanol was calculated according to the equation, S =Rair/Rgas, where Rair and Rgas are the sensor resistance inair and in the presence of the gas and vapour, respectively.The measured sensor response towards the lowest concen-tration of ethanol (12.5 ppm) and H2 (0.05%) are 1.5 and1.2, respectively.The response time (response t90� is defined as the time

for reaching 90% of the full response change of the sensorafter testing gas is in and the recovery time (recovery t90�is defined as the time for falling to 10% of its maximumresponse after testing gas is out. The measured responset90 and recovery t90 for 0.05% H2 at the optimized operat-ing temperature in a balance of synthetic air were 28 and64 s, respectively. Similarly, response t90 and recovery t90for ethanol concentrations of 12.5 ppm were 16 and 24 s,respectively. These results clearly show that the fabricatednanostructured WO3 thin film based sensor is suitable forthe detection of H2 and ethanol in low concentrations atdifferent operating temperatures. The sensor is also suit-able for the detection of a wide range of H2 and ethanolconcentrations.

4. CONCLUSIONS

The H2 and ethanol sensitive WO3 nanorods based filmswere successfully deposited onto a pre-patterned IDT onquartz substrate via pulsed laser deposition with 193 nmArF excimer laser. The measured sizes of the nanorodsdeposited on both quartz substrate and Au IDTs werefound to be approximately ∼50 nm in diameter. Thedeveloped sensors were tested towards H2 and ethanol bal-anced in synthetic air at the temperature range of room to

400 �C. Overall, the developed WO3 film based sensorsshowed good performance towards H2 and ethanol at 170and 400 �C, respectively. The sensors were observed to bemore sensitive, to have faster response and good short termrepeatability, and to have a stable baseline towards ethanolvapour compared to H2. The excellent sensing characteris-tics of WO3 films towards ethanol and H2 at low concen-trations offer great potential for low cost and stable gassensing.

Acknowledgments: The authors acknowledge the facil-ities, the scientific and technical assistance, of theAustralian Microscopy and Microanalysis ResearchFacility (AMMRF) and the RMIT Microscopy andMicroanalysis Facility (RMMF) at RMIT University.Muhammad Z. Ahmad gratefully acknowledges theMalaysian Agricultural Research and Development Insti-tute (MARDI) for financial assistance.

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Received: 31 July 2012. Accepted: 26 December 2012.

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