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ARTICLE IN PRESS+ModelTUSCI-275; No. of Pages 7

Journal of Taibah University for Science xxx (2016) xxx–xxx

Available online at www.sciencedirect.com

ScienceDirect

Al-Sn doped ZnO thin film nanosensor for monitoring NO2

concentration

.S. Hikku a, R. Krishna Sharma b,∗, R.V. William b, P. Thiruramanathan b, S. Nagaveena c

a Centre for Nano Science and Technology, Mepco Schlenk Engineering College, Sivakasi 626005, Indiab Department of Physics, Mepco Schlenk Engineering College, Sivakasi 626005, India

c Physics, Kalasalingam University, Krishnankovil, Tamil Nadu, India

Received 9 October 2015; received in revised form 29 December 2015; accepted 11 February 2016

bstract

The metal oxide semiconductor gas sensor technology is robust and has quick response times. In this work, aluminium and tin co-oped zinc oxide (ASZO) thin films were synthesized by a sol–gel dip-coating process as sensors for the greenhouse gas nitrogenioxide (NO2). The prepared ASZO thin films were characterized using such techniques as X-ray diffraction (XRD), scanninglectron microscopy (SEM), atomic force microscopy (AFM) and photoluminescence (PL) emission studies in order to analyze thelemental confirmation, particle size, surface roughness and optical emission properties, respectively. The XRD data reveals theexagonal structure of ASZO and that the preferential orientation is along 2θ = 36.19◦. SEM images of the ASZO thin film exhibit

od-like formations of ASZO on the substrate. The ASZO films show enhanced sensing behaviour, sensing NO2 gas even at 2 ppmt an operating temperature of 170 ◦C. The response and recovery times were determined to be 30 and 20 s, respectively.

2016 The Authors. Production and hosting by Elsevier B.V. on behalf of Taibah University. This is an open access article underhe CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

nsitivit

eywords: Greenhouse gas; Thin film; Gas sensor; Response time; Se

. Introduction

Trace gases play an important role in atmosphericomposition, radiative interaction, climate change andollution, despite their small concentrations. Increasen the atmospheric percentage of these trace gases dueo human activities, such as the burning of fossil fuels

Please cite this article in press as: G.S. Hikku, et al. Al-Sn doped ZJ. Taibah Univ. Sci. (2016), http://dx.doi.org/10.1016/j.jtusci.2016.

∗ Corresponding author. Tel.: +91 9894816237.E-mail address: krishsharma5555@gmail.com (R.K. Sharma).

eer review under responsibility of Taibah University.

ttp://dx.doi.org/10.1016/j.jtusci.2016.02.002658-3655 © 2016 The Authors. Production and hosting by Elsevier B.V. on

C BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

y

[1,2], causes major issues, including respiratory dis-eases, ozone depletion, global warming, and acid rain.Therefore, it is essential to monitor and mitigate toxicair pollutants, resulting in the need for gas sensors.NO2 is a very important trace gas because it is oneof the precursors which are highly poisonous, reactiveand potentially harmful to ozone [3–9]. Seven oxidesof nitrogen are found in ambient air. These includeNO, NO2, NO3, N2O, N2O3, N2O4 and N2O5. NOand NO2 and are collectively known as NOx becausethey are rapidly interconverted during the day. NOx

nO thin film nanosensor for monitoring NO2 concentration,02.002

behalf of Taibah University. This is an open access article under the

pollution is caused due to excessive emission fromautomobiles, trucks, industrial sources and various non-road vehicles (e.g., construction equipment and boats).Because NOx is an oxidizing agent, it reacts with volatile

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organic compounds (VOC) in the atmosphere to formcomplex compounds. NOx affects the mucosa of theeyes, nose, throat and respiratory tract, and also causesirritation. Continuous exposure to NOx gas leads tovarious respiratory conditions, such as decreased lungfunction, pulmonary diseases, acute or chronic bronchi-tis, pulmonary oedema, and diffuse lung injury [10–14].Thus, it is very essential to monitor and mitigate this pol-lutant. Currently, sensors for monitoring trace gases areimported from other countries and are highly costly. Asper current literature, ZnO thin film-based gas sensorsshow promising results as sensors for atmospheric gases[5,15–19].

Zinc oxide is an inorganic semiconductor compoundwith the formula ZnO. In materials science, ZnO isclassified as a wide bandgap semiconductor from theII–VI group. ZnO thin films can act as effective sen-sors due to their various properties such as high electronmobility, transparent conducting behaviour, etc. [20–25].The electrical characteristics of semiconducting metaloxide materials change when there is a variation inthe composition of the surrounding gas. The develop-ment of gas sensors based on this effect, using thinfilms of n-type zinc oxide is in progress. However, thistype of gas sensor shows a lack of selectivity due toa non-specific gas detection mechanism, resulting inthe detection of many types of reducing gases. There-fore, to fabricate selective sensors, different impuritiessuch as Sn, Fe, Cu, Al, etc. are added as dopants tothe ZnO thin films. ZnO thin films have been exten-sively studied by many researchers for applications inefficient gas sensing. To meet with demands for severalapplications, such impurities as F, B, Al, Ga, In, and Snare added to the ZnO nanoparticles in order to increasetheir conductivity. ZnO nanorod-based H2 and O3 sen-sors have been prepared by molecular beam epitaxy andexhibit sensitivity towards ozone (O3) at room temper-ature and towards H2 at 112 ◦C [26]. When the ZnOnanorods are sputter-coated with Pd or Pt, the sensitivityis increased by a factor of 5, and these sensors can detectH2 down to concentrations of 10 ppm at 25 ◦C [27]. Thetransparent, conductive nature of the ZnO thin films,essential for gas sensor fabrication, is greatly enhancedby doping it with Al [28]. Therefore, carrier mobilityand concentration can be drastically increased by theaddition of Al as a dopant [29]. Additionally, Sn-dopedZnO thin films show enhanced sensitivity to NO2 gas,and the sensitivity is better than that of undoped ZnO

Please cite this article in press as: G.S. Hikku, et al. Al-Sn doped ZJ. Taibah Univ. Sci. (2016), http://dx.doi.org/10.1016/j.jtusci.2016.

[5]. The literature indicated that there was only lim-ited research on the topic of Al and Sn co-doped ZnOthin films as gas sensors. Therefore, this work showsthe preparation of Al and Sn co-doped ZnO thin films

PRESSsity for Science xxx (2016) xxx–xxx

as NO2 gas sensors and also investigates their sensingproperties.

2. Experimental

2.1. Preparation of ASZO

ASZO thin films were prepared using a conven-tional dip-coating method. The sol was prepared usingzinc acetate dihydrate, diethanolamine (DEA), and iso-propanol as precursors. Equimolar quantities of zincacetate (1 M) and DEA (1 M) were maintained in the pre-cursor solution. Tin was added as a dopant by introducing4 ml of SnCl2·2H2O dissolved in ethanol (0.2 M) intothe above mixture. The solution was then heat-refluxedfor 1 h at 70 ◦C. To dope with aluminium, a solutioncontaining 0.2 M aluminium nitrate dissolved in ethanolwas added to the previous mixture. A clear and homoge-neous solution was obtained by stirring at 70 ◦C for 2 h.The prepared ASZO solution was then aged for 24 h tomake it suitable for coating.

2.2. Preparation of thin films

The glass slide to be coated was sonicated in acetonefor 20 min to obtain a dust-free substrate. The dip-coatingprocess was carried out for with an average dip timeof 30 min, and dry time was maintained at 60 and 30 s,respectively. The prepared sample was kept in a hot-airoven at 100 ◦C for an hour followed by calcination of thethin film at 500 ◦C for 1 h to ensure crystallization andto remove unreacted precursors and impurities.

3. Results and discussion

3.1. X-ray diffraction

The diffraction pattern of the thin film calcinated at500 ◦C exhibited eight Bragg peaks, among which thepeak at 2θ = 36.19◦ was prominent, as shown in Fig. 1.Therefore, the preferential orientation was along (101)with the d-spacing of 0.248 nm. The 2θ values of all thesamples were well-matched with JCPDS No. 89-1397[30]. Because the doping concentrations of Al and Snwere very low in comparison to ZnO, the diffractionpeaks for Al and Sn were much reduced in the XRDpattern which showed dominant ZnO peaks. The XRD

nO thin film nanosensor for monitoring NO2 concentration,02.002

peaks for SnO2 and Al2O3 were in good agreementwith JCPDS Nos. 88-0287 and 81-1667, respectively.The prepared ASZO thin films showed monophasic andpolycrystalline hexagonal structure.

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s

C

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S

ldvtcd

D

T

TS

Cs

4

thickness of the film, RMS roughness (Rq), and aver-age roughness (Ra) of the thin film was interpreted andtheir values are 227.739 nm, 66.215 nm, and 50.229 nm,respectively. Roughness skewness (Rsk) is used to study

Fig. 1. X-ray diffraction pattern of ASZO film.

The crystallite size of the ASZO formed on the glassubstrate can be calculated using the formula,

rystallite size (D) = 0.9λ

βcos θ(1)

here λ is the wavelength of the X-ray (0.15406 nm), β isull-width half maximum (FWHM) of the correspondingiffraction peak (hkl) and θ is the Bragg diffraction angle.he crystallite size calculated from the XRD data was

arger in value. This is because of the high calcinationemperature, at which the crystallites merge with eachther resulting in larger crystallites. The strain associatedith the ASZO thin film can be calculated using the

xpression below,

train (ε) = βcos θ

4(2)

From Table 1, the calculated strain value was veryow due to the release in strain associated with ASZO,uring annealing. The number of dislocations in a unitolume of crystalline material can be determined byhe dislocation density. The dislocation density, whichan be defined as the number of defects in the film wasetermined by the formula,

1

Please cite this article in press as: G.S. Hikku, et al. Al-Sn doped ZJ. Taibah Univ. Sci. (2016), http://dx.doi.org/10.1016/j.jtusci.2016.

islocation density (δ) =D2 (3)

The calculated structural parameters are tabulated inable 1.

able 1tructural parameters of ASZO thin film.

rystalliteize (D) (nm)

Strain(ε)

Dislocation density × 1014

(lines/m−2)

4.55 0.00079 5.04

Fig. 2. SEM micrograph of ASZO thin film.

3.2. SEM analysis

Fig. 2 shows the SEM image of the prepared ASZOthin film. The SEM micrograph was taken with a mag-nification of 8000× at an operating voltage of 10 kV.The ASZO particles, which could be visualized from theSEM micrograph, were distributed evenly over the sub-strate. The particle size of ASZO particles on the glasssubstrate varied from 100 to 200 nm.

3.3. AFM studies

Fig. 3 shows the topographical image of ASZO thinfilm, as obtained by AFM. From the figure, the averageparticle size distribution in the ASZO film was found tobe approximately 200 nm. Surface roughness, skewnessand kurtosis of the ASZO thin film were calculated usingthe obtained AFM data and are tabulated in Table 2. The

nO thin film nanosensor for monitoring NO2 concentration,02.002

Fig. 3. AFM studies of ASZO thin film.

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Table 2Results obtained from AFM studies.

Thickness 227.739 nmRMS roughness 66.215 nmAverage roughness 50.229 nm

Skewness −1.230Kurtosis 4.686

the symmetrical behaviour of surface variations about amean line. Rsk is more sensitive to deep valleys and highpeaks. The negative skewness obtained for the ASZOthin film was due to the presence of more valleys thanpeaks. Roughness kurtosis (Rku) is the expression todetermine the distribution of spikes about a mean line.For spiky surfaces, Rku is greater than 3; for rough sur-faces, Rku is less than 3; and for random surfaces, Rku is3. The Rku obtained for the ASZO thin film shows a valuegreater than 3, indicating the surface is spiky which canbe well understood from the AFM image.

3.4. Photoluminescence (PL) spectra

The PL spectrum of the prepared thin films was ana-lyzed with an excitation wavelength of 325 nm, in therange from 350 to 600 nm. From Fig. 4, it is seen thatthe ASZO thin film showed four peaks; a major peak at364 nm and three minor peaks at 381, 412 and 431 nm,respectively. The UV band showed two peaks at 364 nmand 381 nm which were attributed to near-band edgeemission [31]. The emission at 364 nm was due to theelectron transition from excitons bound to donors, andthe emission at 381 nm was attributed to donor-acceptor

Please cite this article in press as: G.S. Hikku, et al. Al-Sn doped ZJ. Taibah Univ. Sci. (2016), http://dx.doi.org/10.1016/j.jtusci.2016.

pair recombination. The two low-intensity peaks foundin the violet-blue region at 412 and 431 nm, respectively,were attributed to deep-level emissions. The reason for

Fig. 4. PL emission spectra of ASZO film.

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the blue emissions is not known conclusively and debatesare ongoing, considering various aspects such as oxygenvacancy (VO), zinc vacancy (VZn), oxygen interstitial(Oi), zinc interstitial (Zni), oxygen anti-site (OZn) andzinc anti-site (ZnO) [32]. All of these phenomena con-tribute to the deep-level emissions.

3.5. Gas sensing analysis

In order to characterize sensor performance, a set ofparameters was used, including sensitivity, recovery andresponse times, and working temperature [33]. The gas-sensing mechanism in oxide-based devices is governedby the reactions with gas molecules, occurring at the sur-face of the metal oxide. The mechanism initiates withthe chemisorption of oxygen molecules over the surfaceof the ASZO thin film, followed by the charge trans-fer phenomenon between the test gas and the adsorbedoxygen molecules [34]. When there is a charge trans-fer phenomenon, the resistance of the thin film changes.The presence of oxidizing gases increases the film resis-tance while the presence of reducing gases reduces theresistance. In order to measure the sensor resistance in adesired concentration of the analyte gas, a known con-centration of gas in ppm was injected into the housingusing a micro–syringe. The volume of the test cham-ber was 10 l. Therefore, to measure the response of thesensor for 1 ppm, 10 �l of the test gas was injected intothe test chamber. The resistance of the sensor as a func-tion of time was measured. The recovery of the sensorwas studied by analysing the variation in resistance afterremoving the sensor and exposing it to air. The sen-sor responses for oxidizing and reducing gases can becalculated using the following expressions:

So = Rg − Ra

Ra× 100

C(reducing gas) (4)

Sr = Ra − Rg

Rg× 100

C(oxidizing gases) (5)

where Rg and Ra are the resistance of the sensing elementinfluenced by gas concentration and by air, respectively,and C is the gas concentration in ppm. The operatingtemperature is one of the most important properties ofthe as-prepared gas sensor. The optimal operating tem-perature is closely related not only to the propertiesof the material itself but also to the gas-sensing pro-

nO thin film nanosensor for monitoring NO2 concentration,02.002

cess of the gas towards the surface of materials. Forthe prepared ASZO thin films, the operating tempera-ture was determined to be 170 ◦C and is represented inFig. 5(a).

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F(a

iiT

Acknowledgements

We thank the management and the principal of Mepco

ig. 5. (a) Temperature dependent sensitivity of the ASZO thin film.b) Response behaviour of ASZO thin film. (c) Sensitivity of Sensorgainst gas concentration.

It is evident from the figure that the sensitivity

Please cite this article in press as: G.S. Hikku, et al. Al-Sn doped ZJ. Taibah Univ. Sci. (2016), http://dx.doi.org/10.1016/j.jtusci.2016.

ncreased with increased temperature, reached a max-mum at 170 ◦C, and gradually declined thereafter.his finding is observed due to the loss of adsorbed

PRESSsity for Science xxx (2016) xxx–xxx 5

oxygen on the surface of the thin film with the increasein temperature. This results in further increase in elec-tron concentration on the thin film surface, improves thefilm conductivity, and thus increases the sensitivity. Thisphenomenon results in decreased electron conductionat low temperatures and enhanced conduction at highertemperatures. Response time and the recovery time arethe other major parameters to be considered. Responsetime is defined as the time required to reach 90% ofthe saturation response and the recovery time can becalculated as the time taken by the sensor to recover90% of the response when the gas flow is switched offand subsequently filled with air. The response time andrecovery time of the sensor were approximately 30 and20 s, respectively, as shown in Fig. 5(b). Desorption is anirreversible process whereby the resistance is stabilizedfor few seconds, which results in a delay in recoverytime. Fig. 5(c) shows the increase in sensitivity of theprepared thin film with increasing test gas concentration.From the figure, it is seen that the sensitivity drasticallyincreased with increasing concentration of the test gasand the sensing behaviour started at NO2 gas concen-trations of 2 ppm. The fabricated sensor showed goodsensitivity and a fast response time (30 s) to the NO2 gas.Additionally, the operating temperature and the powerconsumptions were greatly decreased compared to pre-vious reports of un-doped ZnO [35] and Ga-doped ZnOfilm [36].

4. Conclusions

The Al-Sn co-doped ZnO thin film sensor was fab-ricated using a sol–gel process. ASZO thin films werecharacterized using XRD, SEM, AFM, and PL emis-sion spectra. Sensing characteristics of the ASZO thinfilm sensor were also studied. The ASZO thin filmsshowed an average thickness of 227 nm (derived fromAFM data) and exhibited good sensitivity towardsNO2. The ASZO thin film sensors showed better per-formance, sensing NO2 gas even at 2 ppm under anoperating temperature of 170 ◦C. The response timeand recovery time were measured to be 30 and 20 s,respectively.

nO thin film nanosensor for monitoring NO2 concentration,02.002

Schlenk Engineering College for providing facilities andconstant encouragement for the successful completion ofthe project.

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