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Sensors and Actuators B 166– 167 (2012) 678– 684

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

j o ur nal homep a ge: www.elsev ier .com/ locate /snb

ulse-like highly selective gas sensors based on ZnO nanostructures synthesizedy a chemical route: Effect of in doping and Pd loading

uneet Singh, V.N. Singh, Kiran Jain, T.D. Senguttuvan ∗

hysics of Energy Harvesting, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India

r t i c l e i n f o

rticle history:eceived 18 October 2011eceived in revised form 27 February 2012ccepted 13 March 2012vailable online 20 March 2012

a b s t r a c t

A low-temperature chemical route was used for the synthesis of hexagonal ZnO nanorods having an aspectratio of ∼10. The sensing properties of the thick films made from these nanorods were investigated. Itwas observed that the surface irregularities, calcination temperature and operating temperature alteredthe structure and thus the gas sensing properties. An effort was made to create surface misfits by dopingindium into the zinc oxide and study the changes in the sensor performance. Effect of palladium loadingon the selectivity was also investigated. It was observed that indium doping favors the ethanol sensing and

eywords:nO nanorodsetal–oxideas-sensors

ndium–palladiumPG

palladium loading enhances the sensor response toward LPG. The quick response, high sensor responseand selectivity are the main features of this work.

© 2012 Elsevier B.V. All rights reserved.

thanol

. Introduction

Semiconductor metal oxides as a gas sensing materials havettracted great attention due to some advantageous features, suchs; high sensor response, selectivity, low cost, simplicity in fabri-ation, fast response and recovery, non-toxicity and suitability toifferent doping [1]. ZnO has found to display good sensor responseo many gases like; C2H5OH, NO2, CO, etc. One-dimensional nanos-ructures, like nanorods and nanotubes have a high aspect ratio,nd therefore, they are very attractive candidates for designing theext-generation gas sensors. The gas-sensing property of any mate-ial is related to the surface state and the morphology. Materialsn bulk form have limited sensor response because they have aelatively low surface-to-volume ratio [2]. Yamazoe has demon-trated that reduction in crystal size would significantly increasehe sensor performance because of increased surface area availableor gas molecules. When the particle size is of the order of Debyeength, whole grain becomes depleted of the charge carrier (as they

et trapped in surface states), and they exhibit a poor conductiv-ty in the ambient air. The target gas activates these carriers fromrapped states in the conduction band resulting in change in dras-ic change in conductance [3]. There is a great demand for a highly

∗ Corresponding author. Tel.: +91 11 45609461; fax: +91 11 45609310.E-mail addresses: tdsen@mail.nplindia.org, tdsen@mail.nplindia.ernet.in

T.D. Senguttuvan).

925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2012.03.039

selective gas sensor with good response, recovery time and lowoperating cost. Therefore, several metal oxide nanomaterials arebeing investigated.

Zinc oxide (ZnO) is an n-type direct band semiconductor havinga wide-band gap (3.4 eV). It is known that by changing the sto-ichiometry and by doping; the conductivity can be altered. ZnOin the doped and undoped form are being studied intensively forgas sensing applications. In the presence of oxygen, a potentialbarrier (Schottky type) is formed at the inter grain boundaries ofthe film, which dominates the conductivity of the film. Depend-ing upon the type of gas i.e. oxidizing or reducing, the potentialbarrier height changes, resulting in increase or decrease in theconductivity. It is well known that the sensing property dependshighly on the grain size and porosity. Although, there are numer-ous reports on sensing properties of ZnO [4–7], there are fewreports on the effect of indium doping and palladium loading in ZnOnanorods. Indium is chosen to decrease the resistance of the pureZnO nanorods. Loading of noble metals increases the cost of the sen-sor but such modifications are necessary to monitor some explosiveand toxic gases from a mixture of gases [8]. These additives actas catalysts for the reaction between gas molecules and filmsurface.

This paper describes a low-temperature chemical route for thesynthesis of ZnO nanorods. The purpose of this investigation is to

evaluate the sensing properties of thick-film gas sensors based onZnO nanorods and to study the effect of indium doping and pal-ladium loading on the sensor response and selectivity of the ZnOsensor.

P. Singh et al. / Sensors and Actuators B 166– 167 (2012) 678– 684 679

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Fig. 1. (a) UV–vis spectra, (b) PL spectra,

. Experimental details

.1. Synthesis of ZnO seeds

Precursor solutions of Zn(CH3COO)2·2H2O and NaOH wereade separately by dispersing them in 2-propanol under constant

tirring at 50 ◦C. After mixing; solutions were cooled below 4 ◦C. Inhe next step, hydroxide solution was added drop wise to the zinccetate solution under vigorous stirring. A transparent colloidalolution of ZnO nanoparticles was obtained when the mixture wasged at 60 ◦C for 2.5 h [9]. The seeds were used for synthesizing ZnOanorods.

.2. Synthesis of ZnO nanorods

For the synthesis of ZnO nanorods, 2.5 ml of 0.5 M hexam-thylenetetramine (HMTA) was added to 32.5 ml water and heatedo 70 ◦C. 5 ml seed solution was added to it [10]. The temperatureas raised to 90 ◦C and 5 ml PEG (Mw = 10,000, 1 g dissolved in 10 mlater) and 5 ml zinc nitrate hydrate (0.25 M) solutions were addedrop-wise into the flask simultaneously. The mixture was heated at0 ◦C for 2 h while stirring. The solution was washed with hot dis-

illed water several times in order to remove excess PEG. Powderas obtained by drying the solution at 80 ◦C.

For indium doping, 2 wt% of indium nitrate was added to theolution of zinc nitrate in water. 1 wt% Pd was loaded on the thick

) TEM images of synthesized ZnO seeds.

film sensors by dipping the film into the solution of PdCl2 and thencalcining it at 600 ◦C for an hour.

2.3. Fabrication of sensors

ZnO nanorods were mixed with ethyl cellulose (10:1 ratio) tomake the paste and butyl-carbitol and terpineol were mixed tomake the paste viscous. The paste was applied on the aluminasubstrates. The deposited film was dried at 100 ◦C for an hour andcalcined at 600 ◦C for an hour. Ramp rate for calcinations was keptat 3 ◦C min−1 in order to avoid cracks in the deposited film. Sen-sor response characteristics were recorded using a Keithley Digitalmultimeter 6487 interfaced with a personal computer.

2.4. Sample characterization

The crystalline phase and crystallite orientation of the sampleswere characterized using Bruker D8-advance X-ray diffractome-ter with CuK� (� = 1.5418 A) incident radiation. The morphologicaland structural characteristics of ZnO nanorods were investigated byscanning electron microscope (SEM, LEO 440), and high-resolutiontransmission electron microscope (HRTEM, Technai G20-stwin).

Chemical bonding of the sample was analyzed using X-ray photo-electron spectroscopy (XPS, Perkin-Elmer 1257) with a 279.4 mmdiameter high resolution electron energy analyzer and Al K� X-raysource.

680 P. Singh et al. / Sensors and Actuators B 166– 167 (2012) 678– 684

Fig. 2. (a) UV–vis spectra of ZnO powder, (b) XRD pattern showing the comparison of as prepared and calcined ZnO powder, (c) SEM image of ZnO powder, (d) TEM imageshowing ZnO nanorods, (e) typical HRTEM image of a single ZnO nanorod, (f) EDX spectrum of Pd loaded nanorods coated over Si substrate and (g) TEM micrograph of Pdloaded nanorods.

P. Singh et al. / Sensors and Actuators B 166– 167 (2012) 678– 684 681

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Fig. 3. XPS spectra of the synthesized ZnO nanorods

. Results and discussion

.1. Characterization of seeds

Optical absorption spectrum of dispersed ZnO seeds is shown in

ig. 1a. An absorption peak centered at 364 nm was observed, whichs slightly blue-shifted in comparison to the bulk value. PL spec-rum consisted of 2 peaks centered at 381 nm and 541 nm (Fig. 1b).article radius and size distribution were obtained from the TEM

ig. 4. (a) SEM image of the sensor film calcined at 600 ◦C, (b) resistance versus temperaifferent operating temperatures.

ull-range survey spectra and (b) indium 3d spectra.

images. TEM image showed that the colloidal solution containedalmost spherical particles having narrow size distribution and amean diameter of 5 nm (Fig. 1c).

3.2. Characterization of nanorods

A peak at ∼300 nm was observed in the UV–vis spectrum(Fig. 2a) of the nanorod solution. A typical powder XRD patternobtained is shown in Fig. 2b. All the diffraction peaks were in

ture curves, and (c) variation of gas sensor response of undoped ZnO nanorods at

682 P. Singh et al. / Sensors and Actuators B 166– 167 (2012) 678– 684

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ig. 5. Plot showing (a) sensor response of In doped ZnO nanorods at different operat different operating temperatures, (c) the sensor response of indium doped ZnO nime of indium doped ZnO for ethanol at 550 ◦C and (e) response and recovery time

ccordance with those from the known wurtzite-structure (hexag-nal) ZnO with lattice constants, a = 3.249 A and c = 5.206 A (JCPDS6-1451) [11,12]. SEM and TEM micrograph showed that the length

nd diameter of nanorods were about 500–800 nm and 30–70 nm,espectively. Sample consisted of large quantities of straight andmooth solid rods (Fig. 2c). Fig. 2d and e shows the TEM andRTEM micrograph. EDX studied was carried out on the dispersed

mperatures, (b) change in sensor response with change in concentration of ethanolds to various test gases with good selectivity for ethanol, (d) response and recoveryium doped ZnO for LPG at 550 ◦C.

nanorods coated over a silicon substrate. EDX spectrum (Fig. 2f)of nanorods showed the presence of Zn, In, Pd and O. TEM micro-graph of Pd loaded nanorods is shown in Fig. 2g. Pd nanoparticles

are observed on the surface of the nanorods. The composition ofZnO nanowires was determined by the XPS. The two strong peakslocated at 530.9 and 1022 eV (Fig. 3a) are due to the O (1s) and Zn(2p3/2), respectively [13]. Presence of In 3p and 3d (2 peaks) peaks

P. Singh et al. / Sensors and Actuators B 166– 167 (2012) 678– 684 683

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ig. 6. (a) Sensor response of Pd loaded in doped ZnO nanorods at different operatt 550 ◦C, and (c) response of Pd loaded film for different concentrations of LPG at 5

t 665, 452.4 and 445 eV revealed the presence of indium in theample (Fig. 3b).

.3. Characterization of sensor film

The morphology of the grains did not alter after heating at00 ◦C. The powder XRD pattern did not show any significanthange in the hexagonal structure, and the crystallinity of the filmsred at 600 ◦C (Fig. 2b). SEM images further confirmed this (Fig. 4a).here was no significant change in the morphology due to indiumoping as well. The diameters of rods were in the range 30–70 nm.

.4. Gas sensing studies

Response was defined as; R = Ra/Rg, where Ra is the resistancef the sensing film in the air, and Rg is the resistance of the filmn the target gas. At each temperature, the sensor was first stabi-ized in the air to obtain a stable resistance (Ra) value. The responsend recovery time were defined as the time at which the sensorchieved 90% of the total resistance change, during adsorption and

esorption, respectively. A continuous decrease in the Ra value with

ncreasing temperature confirmed the semiconducting behavior ofhe film (Fig. 4b). The response for ethanol and LPG were tested. Theffects of indium doping and palladium loading were also studied.

mperatures, (b) plot showing highly selective Pd loaded In doped ZnO film for LPG

3.4.1. Response of undoped ZnO nanorodsThe response of the sensor for ethanol (400 ppm) and LPG

(4000 ppm) was tested in the temperature range 300–550 ◦C.The response decreased with a decrease in temperature. Highestresponse of 13 and 17 was observed for LPG and ethanol, respec-tively at 550 ◦C (Fig. 4c). The inability to detect a particular gas fromthe mixture of different gases is the main drawback of pure ZnOfilms. Therefore, dopants were added to enhance sensor responseand selectivity.

3.4.2. Response of indium doped ZnO nanorodsUpon doping with indium, the resistance decreased consider-

ably and the sensor response of the doped film increased for boththe testing gases (Fig. 5a). Indium is a third group element whichwhen doped in ZnO, acts as a donor by replacing Zn2+ ion. Highelectron concentration due to doping increased the conductivity ofthe film [13].

The sensor showed a good sensor response of around 35 forethanol and around 20 for LPG at 550 ◦C. The sensor response versusconcentration plot is shown in Fig. 5b. At a certain concentrationof the gas, the response saturated, as the whole surface of the film

became covered with the gas molecule [14]. The film showed goodselectivity for ethanol. The experiments were also carried out tostudy the response to other interfering gases like CO, NH3, acetone,etc., but it was observed that the sensor response of these gases

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as very low compared to ethanol (Fig. 5c). The reason for the highensor response only to ethanol may be the optimum porosity of thelm and indium misfits (which adsorb oxygen from air and oxidizeshe target gas) [14]. The oxygen molecules from air get adsorbedn the film surface and capture the electrons from the conductionand of the film which results in the decrease of conductivity of thelm.

2(air) + 4e− → 2O2−(filmsurface) (1)

As discussed earlier, the EDAX spectrum showed that the sam-le is oxygen deficient. By adsorbing some O2 from the atmosphere,his deficiency reduces to some extent. This decreases the elec-ronic conductivity of the film. When ethanol is exposed to the film,t gets oxidized by the adsorbed oxygen ions releasing electronsnd CO2 and H2O are formed as the final products (after a series ofntermediate steps).

2H5OH(gas) + 6O(filsurface)2− → 2CO2(gas) + 3H2O(gas)

+ 12 e(conductionband)− (2)

Electrically neutral oxygen atoms evolve leaving behind thelectrons in the trapped states. The decomposition of each ethanoleleased some energy, which is enough to release these trappedlectrons to the conduction band thereby increasing the conduc-ivity of the ZnO film [14].

Due to faster oxidation of the gas, the response time is low.he recovery time was about 40 s for 50 ppm of ethanol at 550 ◦CFig. 5d). For LPG, the film showed poor recovery even for low con-entrations (Fig. 5e).

.4.3. Effect of palladium loadingBy loading 1 wt% palladium on the indium doped film; the sensor

esponse decreased compared to the unloaded film but the sen-or became selective for LPG, with a maximum sensor response ofbout 16 (Fig. 6a). This was much higher as compared to a sensoresponse of ∼2 for ethanol at 550 ◦C. Various other gases were alsoested at 550 ◦C, but LPG have the highest sensor response (Fig. 6b).ig. 6c shows the sensor response for different concentration of LPGt 550 ◦C.

LPG is a highly flammable gas and consists of butane C4H1070–80%), propane C6H8 (5–10%), propylene (C3H6) and butylene4H8 in small percentages [15]. Therefore, LPG sensing mechanism

s considered complex and not many explanations are availablebout the intermediate steps of the reaction [16]. An electron iseleased as soon as LPG reacts with the chemisorbed oxygen on thelm surface. This increases the conductivity of the film [16]. Theeaction is as follows:

nH2n + 2 + 2O− → H2O + CnH2nO− + e− (3)

The enhanced interaction of reducing gas molecules (in the pres-nce of Pd) with the adsorbed oxygen available on the ZnO filmurface leads to a significant reduction in the value of Rg therebyiving maximum sensor response [15]. Oxygen molecules from thetmosphere are first adsorbed on Pd surface and spill-over to thelm. In the same manner, LPG is first adsorbed on to the surface ofd particles and then reacts with the adsorbed oxygen moleculess shown in the above reaction, thereby increasing the surface con-uctivity [16].

. Conclusion

In summary, we have synthesized ZnO nanorods with an aspectatio of over 10, which are good for enhancing gas sensing. The

rs B 166– 167 (2012) 678– 684

synthesis was performed at low temperatures. Indium doping(2 wt%) successfully enhanced the sensor response for the test-ing gases and made the sensor selective for ethanol at almost allthe operating temperatures. The film showed the response time ofless than 2 s and a recovery time of about 50 s. Enhanced responsefor LPG was observed with 1 wt% Pd loading. Not much responsewas found toward other gases like ethanol, acetone, CO, NH3 andpropanol indicating high selectivity toward LPG.

References

[1] C. Xiangfeng, J. Dongli, A.B. Djuris, Y.H. Leung, Gas-sensing properties ofthick film based on ZnO nano-tetrapods, Chem. Phys. Lett. 401 (2005)426–429.

[2] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Review metal oxide gas sensors:sensitivity and influencing factors, Sensors 10 (2010) 2088–2106.

[3] J. Huang, Q. Wan, Review gas sensors based on semiconducting metal oxideone-dimensional nanostructures, Sensors 9 (2009) 9903–9924.

[4] C.Y. Lin, Y. Chia, J.G. Chen, W.-Y. Feng, C.-W. Lin, J.W. Huang, J.J. Tunney, K.C. Ho,Using a TiO2/ZnO double-layer film for improving the sensing performance ofZnO based NO gas sensor, Sens. Actuators B 157 (2011) 361–367.

[5] M. Chen, Z.H. Wang, D.M. Han, F.B. Gu, G.S. Guo, High-sensitivity NO2 gas sen-sors based on flower-like and tube-like ZnO nanomaterials, Sens. Actuators B157 (2011) 565–574.

[6] D. Zhang, S.K. Lee, S. Chava, C.A. Berven, V. Katkanant, Investigation of electricaland optoelectronic properties of zinc oxide nanowires, Phys. B-Cond. Matt. 406(2011) 3768–3772.

[7] S.C. Navale, V. Ravi, I.S. Mulla, S.W. Gosavi, S.K. Kulkarni, Low temperaturesynthesis and NOx sensing properties of nanostructured Al-doped ZnO, Sens.Actuators B 126 (2007) 382–386.

[8] S. Basu, P.K. Basu, Nanocrystalline metal oxides for methane sensors: role ofnoble metals, J. Sens. 2009 (2009) 1–20.

[9] P.E. Strizhak, O.Z. Didenko, G.R. Kosmambetov, Synthesis and characterizationof ZnO/MgO solids prepared by deposition of preformed colloidal ZnO nanopar-ticles, Mater. Lett. 62 (2008) 4094–4096.

10] X.M. Liu, Y.C. Zhou, Seed-mediated synthesis of uniform ZnO nanorodsin the presence of polyethylene glycol, J. Cryst. Growth 270 (2004)527–534.

11] H. Zhang, D. Yang, X. Ma, N. Du, J. Wu, D. Que, Straight and thin ZnO nanorods:hectogram-sacle synthesis at low-temperature and cathodoluminescence, J.Phys. Chem. B 110 (2006) 827–830.

12] J.X. Wang, X.N. Sun, Y. Yang, N-P transition sensing behaviors of ZnO nanotubesexposed to NO2 gas, Nanotechnology 20 (2009) 465501.

13] B. Joseph, P.K. Manoj, V.K. Vaidyan, Studies on preparation and characterizationof indium doped zinc oxide films by chemical spray deposition, Bull. Mater. Sci.28 (2005) 487–493.

14] D.R. Patil, L.A. Patil, D.P. Amalnerkar, Ethanol gas sensing properties of Al2O3-doped ZnO thick film resistors, Bull. Mater. Sci. 30 (2007) 553–559.

15] R.S. Khadayate, J.V. Sali, S.B. Rane, P.P. Patil, Preparation and characterizationof WO3-based liquid petroleum gas sensor, Mater. Manuf. Processes 22 (2007)277–280.

16] D. Haridas, A. Chowdhuri, K. Sreenivas, V. Gupta, Enhanced LPG response char-acteristics of SnO2 thin film based sensors loaded with Pt clusters, Int. J. SmartSens. Intell. Sys. 2 (2009) 503–514.

Biographies

Punnet Singh is doing her M. Tech. project at National Physical Laboratory, NewDelhi. He has done his M. Tech. in Nanotechnology from Amity University, Noida,India. Her current research interests are Nanostructure materials and gas sensors.

Dr. V.N. Singh obtained his M.Sc. from Banaras Hindu University with specializa-tion in Solid State Physics and M. Tech. in solid-state materials from IIT Delhi. Heobtained his PhD from IIT Delhi in the area of nanocomposite based gas sensors. Heis working as a scientist at National Physical Laboratory, New Delhi. His researchinterest includes gas sensors, solar cells, nanomaterials and HRTEM.

Dr. Kiran Jain did her PhD in the field of high Tc superconductivity from DelhiUniversity. She is working as a scientist in National Physical Laboratory, New Delhifrom the past 30 years on the diverse area of research material science such as high Tcsuperconductors, ceramics, nanocrystalline semiconductors, thin film photovoltaicsand metal oxide gas sensors.

Dr. T.D. Senguttuvan obtained his M.E., from REC Trichy in the field of MaterialsScience with specialization in Ceramics and PhD, in sol–gel processing from IIT Delhi.He is working as a scientist in National Physical Laboratory, New Delhi from thepast 14 years. His research interest includes ceramic structures, Powder processing,Microwave Sintering and metal oxide gas sensors.