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Sensors and Actuators B 136 (2009) 138–143

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l homepage: www.e lsev ier .com/ locate /snb

ighly sensitive and ultra-fast responding gas sensors using self-assembledierarchical SnO2 spheres

ae-Ryong Kima, Kwon-Il Choia, Jong-Heun Leea,∗, Sheikh A. Akbarb

Department of Materials Science and Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-713, Republic of KoreaCenter for Industrial Sensors and Measurements (CISM), Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA

r t i c l e i n f o

rticle history:eceived 30 June 2008eceived in revised form2 September 2008ccepted 14 November 2008

a b s t r a c t

The Sn3O4 hierarchical microspheres self-assembled from nanosheets were prepared by a hydrothermalreaction of aqueous solution containing SnCl2·2H2O, H2C2O4, HCl, and N2H4·H2O, which were success-fully transformed into nano-porous SnO2 hierarchical microspheres for gas sensor applications by heattreatment at 600 ◦C. The morphology of building blocks within the SnO2 hierarchical spheres could bemanipulated from 0-dimensional nanoparticles to 2-dimensional nanosheets by varying the amount of

vailable online 25 November 2008

eywords:ierarchical spheresnO2

as sensors

H2C2O4 and N2H4·H2O. The SnO2 hierarchical spheres showed both the ultra-fast response (∼1 s) and highsensitivity to 50 ppm C2H5OH. The dramatic improvement in gas sensing characteristics was explained bythe rapid diffusion of the target gas onto the entire sensing materials through the nano-porous networkof nanosheets.

© 2008 Elsevier B.V. All rights reserved.

ast responseydrothermal synthesis

. Introduction

Oxide semiconductors are promising materials for the facilend reliable detection of toxic and explosive gases. A highly resis-ive shell layer is established on the semiconducting surface byhe adsorption of oxygen with a negative charge (O− or O2−) at00–400 ◦C. The oxidative or reductive interaction between theharged surface oxygen and target gases causes a change in thelectrical conductivity in proportion to the gas concentration [1–3].he gas sensitivity increases rapidly when the dimensions of oxideensing materials become comparable or smaller than the typicalhickness of the electron depletion layer near the surface (severalm) [4]. Therefore, various nanostructures such as nanopowders5], nanowires [6–11], nanotubes [12–14], and nanobelts [15,16]ith a high surface area to volume ratio have been widely examined

o achieve ultra-high gas sensitivity. However, nano-scale materi-ls easily aggregate due to the strong and inevitable van der Waalsttraction and tend to irreversibly form hard secondary aggregates17,18]. When secondary aggregates are dense and large, the in-

iffusion of the sensing gas toward the sensing surface as well ashe counter-diffusion of the product gases to the ambient take aelatively long time, which slows the overall gas response reac-ion considerably. In this respect, the accomplishment of both high

∗ Corresponding author. Tel.: +82 2 3290 3282; fax: +82 2 928 3584.E-mail address: jongheun@korea.ac.kr (J.-H. Lee).

925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2008.11.016

sensitivity and rapid response remains a challenging issue in oxidesemiconductor gas sensors.

The hierarchical assembly of nanostructures such as nanowiresand nanosheets can provide a non- or less-agglomerated structurewithout sacrificing the high surface area/volume ratio. Therefore,rapid communication with the ambient gas and high sensi-tivity can be accomplished. To date, the preparation of SnO2hierarchical structures by two-step thermal evaporation [19],hydrothermal/solvothermal reaction [20,21], biotemplate-directedsol–gel method [22,23], and polypeptide mediated self-assembly[24,25] have been reported. Recently, Wang et al. [26] prepared theSnO2 urchin-like structures composed of short nanorods and SnO2hollow microspheres by hydrothermal reaction using SnCl2, H2O2,and methenamine. They reported that the gas responses of bothnanostructures are higher than that of SnO2 nanoparticles. This sug-gests the possibility of hierarchical nanostructures as a promisinggas sensing material.

In this paper, we suggest a new chemical route to prepare thenano-porous hierarchical and dense SnO2 microspheres through ahydrothermal reaction and subsequent heat treatment. The hierar-chical SnO2 spheres show a considerably faster response and highersensitivity than their denser counterparts.

2. Experimental

Nano-porous hierarchical SnO2 spheres were prepared by thefollowing process. 1.1258 g of SnCl2·2H2O (GR grade, Junsei Chem-

H.-R. Kim et al. / Sensors and Actuators B 136 (2009) 138–143 139

F ense Sd dense

iCl(gcat(wla

ptticcCndars

mtmts(op

3

mHrHn(w

hierarchical spheres is Sn3O4 that can be converted to hierarchicalSnO2 spheres by a heat treatment. The flower-like Sn3O4 hierar-chical spheres were oxidized to pure SnO2 by a heat treatment at≥400 ◦C (Fig. 3).

ig. 1. SEM and TEM images of the as-prepared and heat-treated hierarchical and dense spheres; (e) hierarchical spheres after heat treatment at 600 ◦C for 2 h and (f)

cal Cl., Ltd., Japan) and 2 g of H2C2O4 (GR grade, Kanto Chemicalo., Ltd., Japan) were dissolved in distilled water (100 ml) fol-

owed by the addition of 35% HCl (0.2 ml). 2.16 g of 80% N2H4·H2OSamchun Chemical Co., Ltd., Korea) aqueous solution was addedradually under magnetic stirring until the stock solution becamelear. This was transferred to a Teflon-lined stainless-steel autoclavend reacted hydrothermally at 180 ◦C for 14 h. For the prepara-ion of dense SnO2 spheres, a stock solution containing SnCl2·2H2O1.1258 g), H2C2O4 (8 g), 35% HCl (0.2 ml), and 80% N2H4·H2O (9.15 g)as prepared and hydrothermally reacted at 180 ◦C for 14 h. The yel-

ow precipitate was collected, washed with distilled water, dried inir at 60 ◦C for 24 h and calcined at 600 ◦C for 2 h.

The hierarchical and dense SnO2 spheres were prepared in aaste form and applied to an alumina substrate with two Au elec-rodes. The sensor element was heat-treated to 550 ◦C for 30 mino decompose the organics in the paste. The sensor was installedn a quartz tube and the furnace temperature was stabilized at aonstant sensing temperature (400 ◦C). The gas concentration wasontrolled by changing the mixing ratio of the target gas (200 ppm2H5OH, in air balance) and dry synthetic air. A flow-through tech-ique was employed with a constant flow rate of 500 ml/min. Thec 2 probe resistance of the sensor was measured at 400 ◦C usingPicoammeter (Keithly 6487) interfaced with a computer. The gas

esponse (Ra/Rg) was calculated by comparing the resistance of theensor in high-purity air (Ra) to that in the target gas (Rg).

The specific surface area was measured using the BETethod (Micromeretics Inc., ASAP 2010, USA). Scanning elec-

ron microscopy (SEM, Hitachi S-4300) and transmission electronicroscopy (TEM, Technei-20, Phillips, Netherlands) were used

o observe the morphology of the hierarchical and dense SnO2pheres. The phase of the powders was analyzed by X-ray diffractionD/MAX-2500 V/PC, Cu K�, Rigaku Co., Ltd., Japan). The presencef residual Cl− ion in the sensor materials was checked by X-rayhotoelectron spectroscopy (XPS, Kratos AXIS).

. Results and discussion

Hierarchical and dense spheres were prepared by a hydrother-al reaction of an aqueous stock solution containing SnCl2·2H2O,2C2O4, HCl, and N2H4·H2O at 180 ◦C for 14 h. The hydrothermal

eaction of the stock solution containing low concentrations of2C2O4 and N2H4·H2O leads to the formation of flower-like andano-porous hierarchical structures assembled from nanosheetsFig. 1a and b) In contrast, dense spheres with a smooth surfaceere prepared from a stock solution with a higher H2C2O4 and

nO2 spheres: (a) and (b) as-prepared hierarchical spheres; (c) and (d) as-preparedspheres after heat treatment at 600 ◦C for 2 h.

N2H4·H2O concentration (Fig. 1c and d). The hierarchical and densemorphologies remained the same after a heat treatment at 600 ◦Cfor 2 h (Fig. 1a and e, c and f).

The as-prepared hierarchical and dense spheres were identi-fied as Sn3O4 and SnO2, respectively (Fig. 2a and b). After the heattreatment at 600 ◦C for 2 h, the phases of both the powders wereSnO2 (Fig. 2c and d). According to JCPDS #16-0737 and JCPDS #41-1445, the main peaks of Sn3O4 [(1 1 1) peak, 2� = 27.079◦] and SnO2[(1 1 0) peak, 2� = 26.611◦] are very close. In Fig. 2a, the positionof the main peak (dotted line) is shifted by a 2� value of ∼0.29◦

compared to that of the SnO2 (1 1 1) peak (left solid line) shownin Fig. 2b–d, and the second highest peak of SnO2 (SnO2 (1 0 1)peak, arrowed position above the right solid line) is not observedin Fig. 2a. This suggests that the major phase of the as-prepared

Fig. 2. X-ray diffraction patterns of (a) as-prepared hierarchical spheres, (b) as-prepared dense spheres, (c) hierarchical spheres after heat treatment at 600 ◦C for2 h and (d) dense spheres after heat treatment at 600 ◦C for 2 h.

140 H.-R. Kim et al. / Sensors and Actu

Fig. 3. The phase evolution of Sn3O4 hierarchical spheres into SnO2 with increasingheat treatment temperature: X-ray diffraction patterns of (a) as-prepared hierarchi-cal Sn3O4 spheres and hierarchical spheres after heat treatment at (b) 200 ◦C, (c)300 ◦C, (d) 400 ◦C and (e) 500 ◦C for 2 h.

Fig. 4. Dynamic C2H5OH sensing characteristics at 400 ◦C: (a) gas response (Ra/Rg) of hichange in the resistance of the hierarchical spheres after exposure to 30 ppm C2H5OH; (d

ators B 136 (2009) 138–143

Two different gas sensors were fabricated using the nano-poroushierarchical and dense SnO2 spheres, and their dynamic sensingcharacteristics to C2H5OH at 400 ◦C were examined. Fig. 4a and bshows the change in the gas response (Ra/Rg, Ra: resistance in air,Rg: resistance in gas) to 10–30 ppm C2H5OH as a function of thesensing time. The Ra/Rg (referred to as sensitivity) values of thehierarchical spheres are ∼2 times higher than those of their densecounterpart. A more dramatic change was found in the responsespeed. The response curve of the hierarchical spheres is markedlysharper than that of their dense counterpart (Fig. 4a and b). In thedynamic variation of the resistance as a function of time (Fig. 4c andd), it was found that the 90% response time (t90%(air-to-gas)) of thehierarchical spheres to 30 ppm C2H5OH was extremely short (∼1 s)while that of the dense spheres was 90 s.

The Ra/Rg values of the hierarchical spheres to 10–50 ppmC2H5OH ranged from 7.7 to 33.1, while those of the dense spheresranged from 4.6 to 10.1 (Fig. 5a). The t90%(air-to-gas) values of thehierarchical spheres were considerably shorter (1–3 s) than thoseof the dense spheres (73–150 s) (Fig. 5b). This suggests that, whenexposed to 10–50 ppm C2H5OH, nano-porous hierarchical spheresshow a 50–90 times faster response speed and a 1.7–3.3 timeshigher sensitivity than the dense spheres.

In order to confirm that the hierarchical structures are advan-tageous to achieve both of high sensitivity and fast response, thegas sensing characteristics to H2 and C3H8 were also investigatedand the results were summarized in Fig. 6. The Ra/Rg values of thehierarchical spheres to 10–50 ppm H2 (6.2–11.9) were 2.1–2.8 timeshigher than those of the dense spheres (2.1–5.6) and the t90%(air-to-gas) values of the hierarchical spheres (2 s) were markedly shorterthan those of the dense spheres (7–29 s) (Fig. 6a). The Ra/Rg andt90%(air-to-gas) values of the hierarchical spheres to 10–50 ppmC3H8 (2.9–3.7 and 6 s) were ∼2.2 times higher and 2.3–8.0 timesshorter than those of the dense spheres (1.3–1.7 and 14–48 s),respectively (Fig. 6b). These results indicate that the enhanced gas

sensing performance of the hierarchical structures is not specific toa gas but general.

The presence of Cl− ion in the sensor material can decrease thegas sensitivity [27]. To check the possibility of residual Cl− ion, thehierarchical and dense spheres after heat treatment at 600 ◦C for

erarchical SnO2 spheres; (b) gas response (Ra/Rg) of hierarchical SnO2 spheres; (c)) change in the resistance of the dense spheres after exposure to 30 ppm C2H5OH.

H.-R. Kim et al. / Sensors and Actuators B 136 (2009) 138–143 141

se tim

2oSsrec

F

Fig. 5. (a) Gas response (Ra/Rg) to 10–50 ppm C2H5OH and (b) 90% respon

h were analyzed using XPS. In both of specimens, the positionsf Sn 3d5/2 peaks (486.6 and 486.4 eV) agree well with that of bulk

nO2 (486.6 eV) and no peak is found in the Cl 2p3/2 region (nothown). This indicates that Sn is in the tetravalent state and theesidual Cl− ion is absent or very small in amount. Accordingly, thenhancement of gas sensing behavior in the hierarchical spheresan be attributed to the structural advantages.

ig. 6. Gas response (Ra/Rg) and 90% response time (t90%(air-to-gas)) to (a) 10–50 ppm H2

e (t90%(air-to-gas)) of the hierarchical and dense SnO2 spheres at 400 ◦C.

The t90%(air-to-gas) values of most oxide semiconductor gas sen-sors reported in the literature range from 30 to 500 s [28–31], and

it is rare to find a sensor with a t90%(air-to-gas) <10 s. To our knowl-edge, the response time of 1 s in the sensing of C2H5OH of this studyis one of the shortest values ever reported.

The gas response time should be understood in the frameworkof gas diffusion toward the SnO2 surface and its reaction with the

and (b) 10–50 ppm C3H8 of the hierarchical and dense SnO2 spheres at 400 ◦C.

142 H.-R. Kim et al. / Sensors and Actu

Fhi

atnasgttoialgr

anscistdh2ct

aicdstrit

s3hh(sb

ig. 7. Pore-size distributions of the hierarchical and dense SnO2 spheres aftereat treatment at 600 ◦C for 2 h (determined from nitrogen adsorption–desorption

sotherm).

dsorbed oxygen. At a given temperature and target gas for detec-ion, the surface reaction between a gas and adsorbed oxygen doesot vary significantly from sample to sample. Accordingly, in thebsence of a catalyst that promotes a surface reaction, gas diffusionhould be considered to be a primary factor that determines theas response kinetics. For example, Rossinyol et al. [32] reportedhat the response time of ordered mesoporous WO3 nanostruc-ures is significantly shorter than that of the agglomerated formf WO3, which shows that the pore structure of sensing materialss a key factor for determining the response kinetics. The presentuthors also reported that nano-porous SnO2 whiskers [33] andess-agglomerated SnO2 nanosheets [34] can achieve a more rapidas response than dense whiskers and aggregated SnO2 particles,espectively.

Next to the heat treatment of the hierarchical and dense spherest 600 ◦C for 2 h, the pore-size distribution was examined using theitrogen adsorption–desorption isotherm (Fig. 7). Both the powdershowed a bimodal distribution of pores. The hierarchical spheresontained a large volume of mesopores and sub-micro-pores rang-ng in size from 4.5 to 20 nm and 33 to 100 nm, respectively. Theizes of the mesopores decreased to 3–10 nm, and the volume ofhe sub-micro-pores in the range of 30–100 nm in the dense spheresecreased considerably. The thickness of the sensor films using theierarchical and dense spheres were approximately the same as0 �m. Therefore, the ultra-fast gas response kinetics in the hierar-hical spheres can be explained by the enhanced gas diffusion dueo the larger mesopores and the increased pore volume.

Although all the responses to C2H5OH, H2 and C3H8 in the hier-rchical spheres are significantly fast, there are slight differencesn the response time. The shorter t90%(air-to-gas) value to H2 (2 s)ompared with that to C3H8 (6 s) might be explained by the fasteriffusion of the smaller molecule (H2). However, the comparable orhorter t90%(air-to-gas) value to C2H5OH (1–3 s) compared with thato H2 (2 s) cannot be explained simply by the different gas diffusionate. This implies that the kinetics of oxidation and/or adsorptionn addition to the gas diffusion can play the minor role to changehe gas response speed.

The specific surface areas of the hierarchical and dense SnO2pheres after the heat treatment at 600 ◦C for 2 h were 46.4 and4.7 m2/g, respectively. The 1.34 times higher surface area in the

ierarchical spheres is feasible considering the less-agglomeratedierarchical structure. However, it is lower than the gas responseRa/Rg) ratio between the hierarchical and dense SnO2 sphereshown in Figs. 5 and 6 (1.7–3.3). This discrepancy can be explainedy the difference in aggregation. In hierarchical spheres, the tar-

ators B 136 (2009) 138–143

get gas diffuses rapidly toward the entire sensing surface throughthe non-agglomerated assembled structure. In contrast, the in-diffusion of target gas in the dense agglomerated spheres occursslowly and gradually. Moreover, gas diffusion to the central partof the secondary spheres becomes difficult even after a substan-tial gas sensing time. Only the primary particles located near thesurface of the secondary spheres become active. In this case, thedecrease in the resistance will not be high because conduction doesnot occur through the entire secondary spheres but through the thinand circumventing path along the surface.

Finally, the different gas diffusion from the surface of thick filmto the interior region near a pair of electrodes might change the gasresponse because the higher conductivity change will occur whenthe gas molecules diffuse further close to the electrodes [35,36].The size and packing configuration of the secondary hierarchicaland dense spheres in the gas sensing films were similar to eachother. Nevertheless the enhancement of gas response by the effec-tive gas diffusion toward the electrodes through the nano-poroushierarchical structures should be also taken into account althoughthe detailed reaction requires further study.

4. Conclusion

This study suggests a facile chemical route for preparing Sn3O4and SnO2 hierarchical spheres by a hydrothermal reaction of aque-ous solution containing SnCl2·2H2O, H2C2O4, HCl, and N2H4·H2Ofollowed by oxidation at 600 ◦C. Hierarchical SnO2 microspheresassembled from nanosheets show a considerably faster (50–90times) response speed and higher (1.7–3.3 times) gas response(Ra/Rg) to 10–50 ppm C2H5OH than dense SnO2 counterparts andthis tendency was also valid for the sensing of H2 and C3H8. Theaccomplishment of both the rapid response and high sensitivity,which is a challenging task in oxide semiconductors, is attributedto the rapid and effective gas diffusion to the sensing materialthrough the nano-porous assembly of the nanosheets. Ultra-fastdetection of gases reported in this study is believed to be a keystep in realizing the rapid detection of toxic/dangerous gases, insitu gas monitoring, and fast responding artificial olfaction usingsensor arrays.

Acknowledgement

This work was supported by the Korea Science and EngineeringFoundation (KOSEF) NRL program grant funded by the Korean gov-ernment (MEST) (No. R0A-2008-000-20032-0) and a grant fromthe Fundamental R&D program for Core Technology of Materials(M2008010013) funded by the Ministry of Knowledge Economy,Republic of Korea.

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Biographies

Hae-Ryong Kim studied materials science and engineering and received his BS andMS degrees in 2005 and 2007, respectively, at Korea University in Korea. He is cur-rently studying for PhD degree at Korea University. His research interest is oxidenanostructures for chemical sensor applications.

Kwon-Il Choi studied materials science and engineering and received his BS degreefrom Korea University in 2008. He is currently a master course student at Korea Uni-versity. His research topic is oxide nanostructures for chemical sensor applications.

Jong-Heun Lee has been a Professor at Korea University since 2008. He receivedhis BS, MS, and PhD degree from Seoul National University in 1987, 1989, and 1993,respectively. He then spent 7 years developing automotive air-fuel-ratio sensors atSamsung Advanced Institute of Technology. His current research interests includechemical sensors, functional nanostructures, and solid oxide electrolytes.

Sheikh A. Akbar is a Professor of materials science and engineering andFounder of the NSF Center for Industrial Sensors and Measurements (CISM). Heobtained his PhD from Purdue University in 1985. His current research deals withsynthesis–microstructure–property relations of ceramic bulk, thin-film and nanos-tructures for chemical sensing and catalysis.