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Sensors and Actuators B 160 (2011) 1494– 1498

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

hort communication

hydrogen gas sensor employing vertically aligned TiO2 nanotube arraysrepared by template-assisted method

iwon Leea, Dai Hong Kimb, Seong-Hyeon Hongb, Jae Young Jhoa,∗

School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of KoreaDepartment of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 151-744, Republic of Korea

r t i c l e i n f o

rticle history:eceived 26 May 2011eceived in revised form 1 August 2011ccepted 2 August 2011vailable online 6 August 2011

a b s t r a c t

Vertically aligned TiO2 nanotube arrays for H2 gas sensor were fabricated by using atomic layer depositioncombined with anodic aluminum oxide template. Morphology and crystalline structure of the TiO2 nan-otubes were characterized, and gas sensing properties at low temperatures (<200 ◦C) were investigated.Uniform TiO2 nanotubes with anatase phase were synthesized to be vertically aligned on the substrateand electrically interconnected with each other. The TiO2 nanotube sensors were operated in air envi-

eywords:iO2 nanotube2 gastomic layer depositionAOas sensor

ronment, and the highest magnitude of the gas response (Rair/Rgas) was determined to be ∼100 toward1000 ppm H2/air at 100 ◦C. The sensor showed a very short response time (<1 s) and a high selectivity forH2 gas against several reducing gases including NH3, CO, and C2H5OH.

© 2011 Elsevier B.V. All rights reserved.

. Introduction

One of the potential alternatives for the fossil fuels is hydro-en gas in that it is renewable, clean, and abundant in air. Due to

high flammability and explosiveness of H2 gas in fuel cells, theetection of the hydrogen gas from the leakage is indispensableor safety. Therefore, a great deal of researches is being carried outor hydrogen gas sensors with high sensitivity, selectivity and fastesponse [1]. Also, a low operating temperature is important, since

high temperature environment containing explosive gas couldven cause explosion. As a H2 gas sensing material, titanium oxideTiO2) has been widely investigated owing to its high sensing abil-ties and selectivity [2–4]. Recently, a significant improvement ofiO2-based hydrogen gas sensor has been made through variousanostructures including nanoporous thin films [5–7], nanoparti-les [8], and nanotubes [9–11]. Among them, TiO2 nanotubes haveeen extensively studied as hydrogen sensors due to their highurface-to-volume ratio which play a very important role in the gasensing performance. Varghese et al. reported a H2 gas sensor using

ertically aligned TiO2 nanotube structures fabricated by anodiza-ion [12]. The method, however, appeared not to provide the facileontrol on the dimensional parameters of TiO2 nanotubes such as

∗ Corresponding author. Tel.: +82 2 880 8346; fax: +82 2 884 7355.E-mail address: jyjho@snu.ac.kr (J.Y. Jho).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.08.001

length, diameter, and wall thickness which strongly influence thegas sensing performance.

Template-assisted approaches coupled with atomic layer depo-sition (ALD) also can be a promising route to prepare the TiO2nanotubes. Anodic aluminum oxide (AAO) has been extensivelyused as a nano-porous template for various applications takingadvantage of very large specific surface area. In addition, depthand diameter of the nano-pores can be easily tuned by varyingthe condition of the anodization process. ALD can be an effec-tive technique for applying conformal coatings over AAO templatebecause it utilizes a series of self-limiting reactions of gas-phaseprecursors [13,14]. Specifically, ALD of TiO2 thin film onto theAAO provides TiO2 nanotubular structure inside the AAO tem-plate [15–17]. Although there have been several studies of TiO2nanotubes using AAO and ALD process, there is no report onthe H2 gas sensors using TiO2 nanotubes with vertically wellaligned structure after removing AAO, because it is difficult tohandle and construct the nanostructures during the fabricationprocess.

In this paper, we present a novel fabrication method of verti-cally aligned TiO2 nanotube arrays by ALD technique using AAOtemplate. The H2 gas sensors are prepared using the resulting

nanotube arrays equipped with Pt electrodes. The morpholog-ical features of the resulting nanotubes are characterized andthe gas sensing performance under air environment at low tem-peratures of below 200 ◦C is investigated. In particular, the TiO2

J. Lee et al. / Sensors and Actuators B 160 (2011) 1494– 1498 1495

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Fig. 1. Schematic of fabricatio

anotube sensor exhibited a fast response even in such a lowemperature range and a high selectivity against various reducingases.

. Experimental

Schematic drawing for the fabrication of the TiO2 nanotube gasensor was illustrated in Fig. 1. AAO used was of hexagonally packedanopores with diameters of 70 nm. The AAO templates were intro-uced into the ALD chamber for TiO2 deposition. TiO2 ALD waserformed at 150 ◦C using TiCl4 and water as precursors. One cyclef ALD was composed of 0.2-s exposure of TiCl4, 3-s N2 purge, 0.3-sxposure of H2O, and 3-s N2 purge. The cycle was repeated for 300imes to deposit TiO2 layer with a thickness of 20 nm. After anneal-ng at 450 ◦C for 5 h, TiO2-coated AAO template was attached to alass substrate using thermo-curable Epoxy adhesives (Araldite®,untsman, Inc.). To achieve the TiO2 nanotubular structures, theAO template was to be selectively removed. Aluminum layer of the

emplate was dissolved by immersing in saturated HgCl2 solution.hen, the residual alumina layer of the AAO template was removedy treating in 0.1 M KOH solution for 1 h and subsequently rinsingith deionized water for several times. Finally, vertically aligned

iO2 nanotube arrays are left on the transparent glass substrate.o fabricate the H2 gas sensor, a pair of circular type Pt electrodesith a thickness of 40 nm and a gap of 1 mm was sputtered on the

ample and Au wires were bonded to the electrodes using Ag paste.The morphology of the nanostructures was characterized using

eld-emission scanning electron microscope (FESEM, SUPRA 55VP,arl Zeiss) and transmission electron microscope (TEM, LIBRA 120,arl Zeiss). The crystalline structure was analyzed by X-ray diffrac-ion (XRD) with CuK� radiation (� = 1.54 A) using a diffractometerD5005, Bruker).

The sensor devices were placed in a quartz tube located insiden electrical tube furnace with a gas inlet and outlet. A continu-us flow-through measurement system was used and the sampleases used were H2, ammonia (NH3), carbon monoxide (CO), and

thanol (C2H5OH) balanced with air. The gas sensing property waseasured by the changes of an electric resistance which was mea-

ured using a multimeter (2000 multimeter, Keithley, USA). In thistudy, the magnitude of gas response (S) was defined as the ratio

he TiO2 nanotube gas sensor.

(Rair/Rgas) of the resistance in air (Rair) to that in a sample gas (Rgas).The response time (t90%) was defined as the time required for thesensor to reach 90% of the final signal.

3. Result and discussion

Fig. 2(a) and (b) shows FESEM images of the AAO used as a tem-plate for ALD, which has highly ordered nanopores with the porediameter of 70 nm and depth of 250 nm. After removing the AAOtemplate, it is seen that the TiO2 nanotube arrays are successfullyformed. As shown in the top-view images (Fig. 2(c)), the nanotubesare highly ordered and crack-free with a tube-diameter of 70 nm.The cross-sectional image (Fig. 2(d)) depicts the vertically alignedTiO2 nanotubes with a tube-length of 250 nm. The dimension ofthe resulted TiO2 nanotubes accurately corresponds to that of thetemplate, indicating that ALD thoroughly formed the TiO2 thin filmon the entire surface of the nanopores of the AAO template. Thehigh order and vertical orientation of the nanotube arrays with-out cracks over the wide area is thought to be the result of strongadhesion of the bottom of the TiO2 nanotubes to the glass substrateand the complete removal of the AAO template. This novel fabrica-tion method can be readily applied to various substrates includinga plastic film for flexible devices.

To observe the individual TiO2 nanotubes, TEM analysis was car-ried out. The specimen for the TEM study was prepared by placinga drop of the ethanol suspension of the TiO2 nanotubes onto thecarbon-coated TEM grid. Fig. 3(a) shows the TEM image of the TiO2nanotube. All the nanotubes had the flat surface and uniform wallthickness which is approximately 20 nm. Also, it is visible that thebottom parts of the nanotubes are formed to be continued to theneighbor nanotubes, indicating that all the nanotubes are electri-cally interconnected. The crystal structure of the TiO2 nanotubeswas characterized by XRD analysis. Since it was hard to prepare theXRD specimens purely composed of TiO2 nanotube arrays, the AAOtemplates with the deposited TiO2 were ground into a powder. Forthe as-synthesized TiO2 nanotubes, only the amorphous halo was

observed as shown in Fig. 3(b). In order to induce the crystallinephase of the TiO2 nanotubes, annealing process was carried out.Since the anatase phase of TiO2 generally has higher and faster gasresponse toward H2 gas at low temperature than rutile phase of

1496 J. Lee et al. / Sensors and Actuators B 160 (2011) 1494– 1498

empla

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Fig. 2. Top and cross-sectional FESEM images of the AAO t

iO2 [4,18], annealing condition was set to be at 450 ◦C in air. Afternnealing, the obtained diffraction peaks of 2� = 25◦, 37◦, 48◦, 54◦,nd 55◦ were indexed for (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 1)f the TiO2 anatase phase (see Fig. 3(b)). It verifies that TiO2 nan-tubes successfully became the anatase crystalline phase by theeat treatment.

A typical response transient of the TiO2 nanotube sensors to H2as balanced with air measured at 100 ◦C is shown in Fig. 4. The con-entration of H2 was varied from 1000 to 100 ppm in discrete steps.ver the whole range of the concentration, the sensors exhibited a

apid decrease in the resistance upon injecting the target gas andhe subsequent recovery to an original level after removal of the H2as. The sensing signal was stable and repeatable for more than 10imes of cycles. The highest magnitude of gas response was mea-ured to be 100.5 toward 1000 ppm H2 at 100 ◦C and decreased to8.7 with decreasing the H2 concentration to 100 ppm. Importantly,ur sensor exhibited a reliable operation to H2 gas in air, while pre-ious reports usually showed H2 sensing in a N2 gas environment9–12]. In order to examine the temperature dependence, the gasensing property of the TiO2 nanotube sensor was investigated witharying temperature from 75 to 175 ◦C. It is clear from Fig. 5(a) thathe sensor was operated over the whole temperature range andhowed a maximum gas response at 100 ◦C regardless of the H2oncentration. Owing to the higher resistance of TiO2 nanotubeshan the equipment limitation (up to ∼109 �), the gas response atelow 75 ◦C could not be measured. The operating temperature inhis research, which is around 100 ◦C, is very low compared to thatf conventional semiconducting gas sensors, typically operating at00–500 ◦C [1]. According to the gas sensing mechanism proposedo far, in the presence of air, an electron depleted layer is inducedn the near-surface region of the metal oxide by the chemisorp-ion of oxygen immobilizing the conduction band electrons, then2 molecules abstract the oxygen thereby releasing the surface-

ound electrons, which consequently results in a change in theesistance of the material [19,20]. The oxygen has been reportedo be adsorbed on the metal oxide surface as various types of nega-ively charged species depending on the temperature [21,22]. From

te ((a) and (b)), and the TiO2 nanotube arrays ((c) and (d)).

the result in this research, it is suspected that certain type of oxy-gen species adsorb sufficiently on the TiO2 surface at around 100 ◦Cand then interact with the H2 gas that diffused efficiently throughthe vertically aligned TiO2 nanotubes. Such low operating tempera-ture of H2 sensors in ambient air is highly favorable for the practicalapplication with safe and low operating cost.

The response time (t90%) of the TiO2 nanotube sensors to1000 ppm H2 was also examined as a function of the operating tem-perature (see Fig. 5(b)). The response time at 75 ◦C is comparable tothat of conventional semiconducting gas sensors, whereas the sen-sor operated at above 100 ◦C showed a very short response timebelow 4 s. Furthermore, the response time is shortened to 0.6 s at175 ◦C, which is, to our best knowledge, unprecedented responsespeed in the field of H2 gas sensors. The response time generallyranges from several minutes in the thin film semiconductor H2 sen-sors to tens of seconds for sensors fabricated with nanostructures.Although a short response time has been reported for some of thesensors made of metal oxide nanostructures, they required a veryhigh operating temperature of above 300 ◦C [23]. It suggests thatthe extremely fast response in this work, which is below 1 s even inthe low operating temperature, was induced by distinguishable fea-tures of the TiO2 nanostructure in this work. The vertically alignedTiO2 nanotubes that are located apart from each other presumablyresulted in the rapid gas diffusion in-between the nanotubes, andconsequently allowed the H2 gas to quickly reach the entire surfaceof the nanotubes.

Another notable feature of the TiO2 nanotube array sensor ofthis study is the excellent selectivity for H2 gas against the otherreducing gases. Fig. 6 reveals the sensitivities of the TiO2 nanotubesensor exposed to three kinds of 1000 ppm reducing gases (NH3, CO,and C2H5OH) at 100 ◦C. Although ethanol gas showed the highestgas response, the sensitivities for those gases are below 2.5, whichwere almost negligible compared to that of H2 gas. The high selec-

tivity for H2 against other gases can be explained by the anatasephase of TiO2 as mentioned above and the catalytic action of the Ptelectrode [24]. Especially, it is confirmed that the Pt catalyst selec-tively oxidize the H2 rather than other gases at low temperature

J. Lee et al. / Sensors and Actuators B 160 (2011) 1494– 1498 1497

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Fig. 5. (a) Magnitude of gas response at various hydrogen concentrations and (b)response time of TiO2 nanotube sensor to 1000 ppm hydrogen as a function ofsensing temperature.

ig. 3. (a) TEM image of the TiO2 nanotubes and (b) X-ray diffraction pattern ofristine and annealed TiO2 nanotubes.

25]. As the parameter of the nanotube sensor would be dependentn the dimension of the nanotube, further works are being focusedn the effect of the dimensional variation of nanotube such as wall

hickness, length, and diameter of TiO2 nanotube to improve the2 sensing performance.

ig. 4. Response transient of the TiO2 nanotube sensor to hydrogen gas at 100 ◦C.

Fig. 6. Magnitude of gas response of TiO2 nanotube sensor to 1000 ppm of variousgases at 100 ◦C.

4. Conclusion

Vertically aligned TiO2 nanotube arrays for H2 gas sensors weresuccessfully developed by a novel fabrication method using ALD onAAO template. The nanotubes were synthesized to be electricallyinterconnected each other and formed to be attached vertically

on the substrate after removing the template. The H2 gas sensorusing the nanotubes operated at low temperature ranges of around100 ◦C in air environment with a maximum gas response of 100.5to the 1000 ppm H2 exposure and exhibited the great stability and

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epeatability. The response time was extremely short (below 1 s)ven in the low temperature range, which is thought to be theesult of well aligned and controlled TiO2 nanotubes in this work.n addition, the TiO2 nanotube sensor showed high selectivity for

2 against several reducing gases including NH3, CO, and C2H5OH.rom such results, it is potentially expected that TiO2 nanotubesabricated in this work would be a better candidate for the H2 sen-ors for a practical application when compared with other metalxide H2 sensors.

cknowledgements

This research was supported by the Public welfare & Safetyesearch program through the National Research Foundation oforea (NRF) funded by the Ministry of Education, Science and Tech-ology (2010-0020455). The authors also would like to thank Yohanho and Kyusoon Shin in Seoul National University for providinghe AAO template.

eferences

[1] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for developmentof integrated gas sensors—A comprehensive review, Crit. Rev. Solid State Mater.Sci. 29 (2004) 111–188.

[2] N. Yamazoe, G. Sakai, K. Shimanoe, Oxide semiconductor gas sensors, Catal.Surv. Asia 7 (2003) 63–75.

[3] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A new detector for gaseouscomponents using semiconductive thin films, Anal. Chem. 34 (1962) 1502–1503.

[4] H. Tang, K. Prasad, R. Sanjinés, F. Lévy, TiO2 anatase thin films as gas sensors,Sens. Actuators B 26–27 (1995) 71–75.

[5] Y.-K. Jun, H.-S. Kim, J.-H. Lee, S.-H. Hong, High H2 sensing behavior of TiO2 filmsformed by thermal oxidation, Sens. Actuators B 107 (2005) 264–270.

[6] Y. Shimizu, T. Hyodo, M. Egashira, H2 sensing performance of anodically oxi-dized TiO2 thin films equipped with Pd electrode, Sens. Actuators B 121 (2007)219–230.

[7] A.Z. Sadek, J.G. Partridge, D.G. McCulloch, Y.X. Li, X.F. Yu, W. Wlodarski, K.Kalantar-zadeh, Nanoporous TiO2 thin film based conductometric H2 sensor,Thin Sold Films 518 (2009) 1294–1298.

[8] M.E. Franke, T.J. Koplin, U. Simon, Metal, Metal oxide nanoparticles in chemire-sistors: does the nanoscale matter? Small 2 (2006) 36–50.

[9] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, C.A. Grimes, Hydrogen sensingusing titania nanotubes, Sens. Actuators B 93 (2003) 338–344.

10] M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, K.G. Ong, Unprecedentedultra-high hydrogen gas sensitivity in undoped titania nanotubes, Nanotech-nology 17 (2006) 298–402.

11] E. S ennik, Z. C olak, N. Kılınc , Z.Z. Öztürk, Synthesis of highly-ordered TiO2 nan-otubes for a hydrogen sensor, Int. J. Hydrogen Energy 35 (2010) 4420–4427.

12] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Grimes, Extremechanges in the electrical resistance of titania nanotubes with hydrogen expo-sure, Adv. Mater. 15 (2003) 624–627.

s B 160 (2011) 1494– 1498

13] J.W. Elam, D. Routkevitch, P.P. Mardilovich, S.M. George, Conformal coating onultrahigh-aspect-ratio nanopores of anodic alumina by atomic layer deposition,Chem. Mater. 15 (2003) 3507–3517.

14] C. Bae, Y. Yoon, W.S. Yoon, H. Yoo, D. Han, J. Cho, B.H. Lee, M.M. Sung, M.G. Lee,J. Kim, H. Shin, Controlled fabrication of multiwall anatase TiO2 nanotubulararchitectures, Chem. Mater. 21 (2009) 2574–2576.

15] J. Lee, H. Ju, J.K. Lee, H.S. Kim, J. Lee, Atomic layer deposition of TiO2 nanotubesand its improved electrostatic capacitance, Electrochem. Commun. 12 (2010)210–212.

16] C. Bae, H. Yoo, S. Kim, K. Lee, J. Kim, M.M. Sung, H. Shin, Template-directedsynthesis of oxide nanotubes fabrication, characterization, and applications,Chem. Mater. 20 (2008) 756–767.

17] Y.-H. Chang, C.-M. Liu, Y.-C. Tseng, C. Chen, C.-C. Chen, H.-E. Cheng, Direct probeof heterojunction effects upon photoconductive properties of TiO2 nanotubesfabricated by atomic layer deposition, Nanotechnology 21 (2010) 225602.

18] Y. Shimizu, N. Kuwano, T. Hyodo, M. Egashira, High H2 sensing performance ofanodically oxidized TiO2 film contacted with Pd, Sens. Actuators B 83 (2002)195–201.

19] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: howto? Sens. Actuators B 121 (2007) 18–35.

20] Michael Tiemann, Porous metal oxides as gas sensors, Chem. Eur. J. 13 (2007)8376–8388.

21] N. Yamazoe, J. Fuchigami, M. Kishikawa, T. Seiyama, Interactions of tin oxidesurface with O2, H2O and H2, Surf. Sci. 86 (1979) 335–344.

22] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electro-ceram. 7 (2001) 143–167.

23] S. Joo, I. Muto, N. Hara, Hydrogen gas sensor using Pt- and Pd-added anodicTiO2 nanotube films, J. Electrochem. Soc. 157 (2010) J221–J226.

24] U. Roland, T. Braunschweig, F. Roessner, On the nature of spilt-over hydrogen,J. Mol. Catal. A: Chem. 127 (1997) 61–84.

25] W. Shin, M. Matsumiya, N. Izu, N. Murayama, Hydrogen-selective thermoelec-tric gas sensor, Sens. Actuators B 93 (2003) 304–308.

Biographies

Jiwon Lee studied chemical and biological engineering and received his BSdegree in 2004 from Seoul National University in Korea. He is currently study-ing for a PhD degree at Seoul National University. His research activities arefocused on semiconductor nanostructures for gas sensors and photovoltaiccells.

Dai Hong Kim studied materials science and engineering and received his MS degreein 2008 at Seoul National University. He is currently studying for a PhD degree atSeoul National University. His research interests are semiconducting gas sensorsand thin film deposition.

Seong-Hyeon Hong has been an associate professor at Seoul National Universitysince 1998. He received his MS degree in 1990 from Seoul National Univer-sity and PhD degree in 1996 from Pennsylvania State University. His currentresearch interests include the development of nano-structured materials for sensorapplications.

Jae Young Jho received his BS degree and MS degree from Seoul National Universityin 1979 and 1981, respectively, and PhD degree in 1990 from University of Michigan.He has been a professor at Seoul National University since 1993. His current researchinterest includes polymeric actuator and sensor materials.