ZnO hierarchical nanostructures grown at room temperature and their C2H5OH sensor applications

Sensors and Actuators B 155 (2011) 745–751

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

ZnO hierarchical nanostructures grown at room temperature and their C2 H5 OH sensor applications Kang-Min Kim, Hae-Ryong Kim, Kwon-Il Choi, Hyo-Joong Kim, Jong-Heun Lee ∗ Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea

a r t i c l e

i n f o

Article history: Received 2 October 2010 Received in revised form 12 January 2011 Accepted 23 January 2011 Available online 27 February 2011 Keywords: ZnO Gas sensors Hierarchical nanostructures C2 H5 OH sensors

a b s t r a c t Highly crystalline ZnO hierarchical nanostructures were prepared at room temperature through the alkaline hydrolysis of zinc salt by the forced mixing of two immiscible solutions: Zn-nitrate aqueous solution and oleic-acid-dissolved n-hexane solution. The oleic acid acted as a surfactant in the room-temperature formation of well-defined ZnO hierarchical nanostructures, which subsequently demonstrated a sensitive and selective detection of C2 H5 OH. The responses of these hierarchical nanostructures to 10–100 ppm C2 H5 OH ranged from 15.7 to 177.7, which were 7–9 times higher than those of the agglomerated nanoparticles. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The ZnO nanostructures have been widely used in various applications such as solar cells [1], light emission devices [2], gas sensors [3], piezoelectric devices [4], and biomaterials [5]. To enhance the functional properties of ZnO nanostructures, the size, shape, surface chemistry, defect, conductivity and dispersion should be designed carefully. In particular, for application as gas sensors, high surface area and nano-scale dimension are key factors to determine the gas response [6]. At 200–400 ◦ C, an electron depletion layer is formed near the surface of n-type ZnO semiconductors by the adsorption of oxygen with the negative charge [7]. Upon exposure to reducing gases such as CO and C2 H5 OH, the reducing gases are oxidized by the reaction with the negatively charged surface oxygen and release the electrons to the semiconducting core. This increases the conductivity in proportion to the concentration of reducing gases. Accordingly, the gas sensitivity can be best maximized with particles that are similarly sized or smaller than 2 times the electron depletion layer thickness [8]. Many studies have focused on enhancing the gas response by decreasing the dimensions of the following nanostructures: the size of nanoparticles [9], the diameter of nanowires [3,10], and the thickness of nanosheets [11]. However, as the van der Waals

∗ Corresponding author at: Department of Materials Science and Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-713, Korea. Tel.: +82 2 3290 3282; fax: +82 2 928 3584. E-mail addresses: [email protected], [email protected] (J.-H. Lee). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.01.040

attraction is inversely proportional to the particle size, significant agglomeration between nanostructures is nearly inevitable. When the diffusion of the analyte gas through the secondary aggregates becomes difficult, only the primary particles located in the outer region of the secondary particles contribute to the gas sensing whereas those at the inner part remain inactive, which eventually deteriorates the gas response. In this regards, well-defined and highly dispersed nanostructures with high surface area are very advantageous for the design of high performance gas sensors. Hierarchical structures are the higher dimension of microor nanostructures composed of many, low dimensional, nanobuilding blocks. A very high surface area/volume ratio provides abundant reaction sites for gas sensing and the well-defined, less-agglomerated configuration of the nanostructures facilitates the rapid and effective diffusion of the analyte gas toward the entire sensing surface [12]. Therefore, hierarchical structures are very promising material platforms in gas sensor applications. The hydrothermal reaction is one of the most popular chemical routes to prepare crystalline and hierarchical ZnO nanostructures [13–18]. For the more convenient preparation of hierarchical structures on a large scale, the alkaline hydrolysis of zinc salt in aqueous solution has been investigated [19–22]. The ambient-pressure process is the main advantage. This process can be simplified further into the room-temperature hydrolysis reaction using water-soluble capping or chelating agents [23,24]. The selection of surfactants during self-assembly reaction is very important in manipulating the morphology of hierarchical structures. In this regards, the use of both water-insoluble and water-soluble surfactants will open diverse possibilities in the design of hierarchical

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morphology. The emulsion-based approach is an attractive option to provide non-soluble surfactant in aqueous solution. In this contribution, we propose a new and facile chemical route to prepare highly crystalline ZnO hierarchical structures at room temperature by the gradual release of a water-insoluble surfactant (oleic acid) using an emulsion-based approach. The main focus of the study is directed at the effect of hierarchical structures on the enhancement of the gas response and selectivity to C2 H5 OH.

JCPDS #76-1778 Orthorhombic Zn(OH)2

(a)

Zn(OH)2 ZnO

2. Experimental

3. Results and discussion The powders prepared in the absence of oleic acid were identified as Zn(OH)2 according to XRD study (Fig. 1a). These were converted into pure ZnO phase by heat treatment at 500 ◦ C (Fig. 1b).

(b)

Intensity (Arb. Unit)

In 100 ml of deionized water was dissolved 1.19 g of Zn(NO3 )2 ·6H2 O (>99%, Kanto Chemical, Japan), after which 100 ml of n-hexane (C6 H14 , 99%, Acros organics, Belgium) and 0.45 g of oleic acid (C18 H34 O2 , >99%, Sigma Aldrich, USA) were added to the solution in sequence with continuous stirring. While n-hexane and oleic acid are miscible with each other, both of them are insoluble in aqueous solution. Thus, water-insoluble surfactant (oleic acid) was provided continuously and gradually by the forced stirring of immiscible mixture between the oleic-acid-dissolved n-hexane solution and the Zn-precursor-dissolved aqueous solution. After 3.2 g of 50% NaOH aqueous solution (Samchun Chemical Co., Korea) was instantaneously poured into the mixture, the resulting emulsion was stirred for 3 h for the reaction. The resultant products were collected by centrifugation, washed several times with deionized water and ethanol, and dried at room temperature. In order to study the role of oleic acid during the reaction, the Zn-precursors were also prepared in the absence of oleic acid. For this, the immiscible mixture between the Zn-precursor-dissolved aqueous solution and the n-hexane solution was stirred for 3 h after the NaOH solution was poured. The phase and crystallinity of the powders were analyzed by X-ray diffraction (XRD, Rigaku D/MAX-2500 V/PC). The morphology of the powders was investigated using field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Co. Ltd., Japan). To investigate the thermal decomposition of the precursors, thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) measurements (SDT Q600, TA Instruments) were carried out under air in the temperature range from room temperature to 700 ◦ C. The surface areas were measured by using the Brunauer–Emmett–Teller (BET) method (Tristar 3000, Micromeritics Co. Ltd.). The as-prepared precursors were prepared into a paste form and applied to an alumina substrate (size: 1.5 mm × 1.5 mm, thickness: 0. 25 mm) having two Au electrodes (electrode width: 1 mm, electrode spacing: 0.2 mm). The sensor element was heated to 500 ◦ C at 25 ◦ C/min and then treated at this temperature for 1 h for conversion into pure ZnO nanostructures and to decompose the organic content of the paste. The sensor was placed in a quartz tube and the temperature of the furnace was stabilized at 400 ◦ C. A flow-through technique with a constant flow rate of 500 cm3 /min was used and 4-way valve was employed to switch the gas atmospheres. The gas responses (S = Ra /Rg , Ra : resistance in dry air, Rg : resistance in gas) to 100 ppm C2 H5 OH, CO, H2 , and C3 H8 were measured at 400 ◦ C. The gas concentration was controlled by changing the mixing ratio of the parent gases (100 ppm C2 H5 OH, 200 ppm CO, 200 ppm H2 , and 100 ppm C3 H8 , all in dry air balance) and dry synthetic air. The dc 2probe resistance of the sensor was measured using an electrometer interfaced with a computer.

(c)

(d)

JCPDS #79-0207 Hexagonal ZnO

10

20

30

40

50

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70

80

2θ(deg, CuKα) Fig. 1. X-ray diffraction patterns of the as-prepared and heat-treated nanostructures: (a) Zn(OH)2 precipitates prepared in the absence of oleic acid, (b) ZnO powders after heat treatment of specimen (a) at 500 ◦ C for 1 h in air, (c) ZnO hierarchical nanostructures prepared in the presence of oleic acid, and (d) ZnO hierarchical nanostructures after heat treatment of specimen (c) at 500 ◦ C for 1 h in air.

In contrast, the powders prepared in the presence of oleic acid showed pure ZnO phase (Fig. 1c), which suggested that the oleic acid played an essential role in the self assembly of ZnO nanostructures and that crystalline ZnO powders can be prepared at room temperature. The crystal structure of ZnO was maintained after heat treatment at 500 ◦ C for 1 h (Fig. 1d). According to Scherrer’s equation, the average crystallite sizes of ZnO powders in Fig. 1b and d were calculated to be 22.7 and 17.5 nm, respectively. The SEM images of as-prepared Zn-precursors and heat-treated ZnO nanostructures were shown in Fig. 2. When the solution was reacted in the presence of oleic acid, ZnO nanostructures were hierarchically assembled from 2-dimensional (2-D) nanosheets (Fig. 2a). The assembled hierarchical structures were approximately 2 ␮m large. Closer inspection revealed the 2-D building blocks to be extremely thin (20–40 nm) (Fig. 2b). The less agglomerated configuration of hierarchical morphology remained similar upon heat treatment at 500 ◦ C for 1 h with rapid heating rate (25 ◦ C/min) (Fig. 2c), although the sharp and straight morphology of each nanosheet was changed into a rather blunt and slightly curved one (Fig. 2d). Hereinafter, in order to distinguish between as-prepared and heat-treated ZnO hierarchical structures, the ZnO hierarchical structures for sensor application that are heat-treated at 500 ◦ C for 1 h will be referred to as ‘ZnO-500-HN’ (ZnO-500 ◦ Chierarchical nanostructures). In contrast, the Zn(OH)2 precipitates

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Fig. 2. SEM images of the as-prepared and heat-treated nanostructures: (a and b) As-prepared ZnO hierarchical nanostructures prepared in the presence of oleic acid, (c and d) ZnO hierarchical nanostructures after heat treatment of specimen (a and b) at 500 ◦ C for 1 h (ZnO-500-HN), (e and f) as-prepared Zn(OH)2 precipitates prepared in the absence of oleic acid, and (g and h) ZnO agglomerated nanoparticles after heat treatment of specimen (e and f) at 500 ◦ C for 1 h (ZnO-500-AN).

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c

100 95 90 85

Heat flow (W/g)

90 85 80

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d

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0

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80

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100 95

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Weight (%)

a

0

-1 -2

-1 -2 -3

-3

0

100 200 300 400 500 600 700

Temperature (oC)

0

100 200 300 400 500 600 700

Temperature (oC)

Fig. 3. Thermo-gravimetric and differential scanning calorimetry curves of (a and b) as-prepared Zn(OH)2 precipitates prepared in the absence of oleic acid and (c and d) as-prepared ZnO hierarchical nanostructures prepared in the presence of oleic acid.

showed an elongated angular morphology (Fig. 2e) with a typical length of approximately 5 ␮m and each angular secondary particle consisted of small primary particles (Fig. 2f). This clearly demonstrated the important role played by oleic acid in the formation of the hierarchical nanostructures. Oleic acid is a representative surfactant that is widely used in the solution-based growth of oxide nanostructures [25,26]. It is also known to assemble vertically on various substrates and to form bilayer structure [27]. Accordingly, the ultra-thin morphology of the nanosheets (Fig 2a and b) was formed by the surfactant-mediated self assembly of ZnO nanocrystals in a 2-D manner and these nanosheets were eventually assembled into hierarchical structures in the latter stage of the reaction. The morphology of the ZnO agglomerated particles after heat treatment at 500 ◦ C for 1 h (Fig. 2g and h) was similar to that of the Zn(OH)2 precipitates (Fig. 2e and f) at the micrometer scale. However, the secondary particles consisted of many nanoscale primary particles and pores. Many pores were attributed to the degassing of water vapor during the dehydration procedure (Fig. 2g and h). Hereafter, for simplicity, the agglomerated configuration of nanoparticles in Fig. 2g and h will be referred to as ‘ZnO-500-AN’ (ZnO agglomerated nanoparticles). The results of TG and DSC analyses were shown in Fig. 3. The DSC curve of the Zn(OH)2 precipitates prepared in the absence of oleic acid showed a sharp endothermic peak at ∼122 ◦ C (Fig. 3b). At ∼110 ◦ C, the weight started to decrease abruptly and the weight loss from 110 ◦ C to 300 ◦ C was 18.9% (Fig. 3a), which agreed well with the weight loss by the dehydration of Zn(OH)2 into ZnO (18.9%). Thus, the abrupt weight decrease and the endothermic reaction between 110 and 130 ◦ C were attributed to the dehydration procedure from Zn(OH)2 to ZnO. In contrast, the ZnO hierarchical nanostructures prepared in the presence of oleic acid exhibited neither abrupt weight loss nor significant DSC peak (Fig. 3c and d), which further confirmed the crystalline structure of the as-prepared ZnO hierarchical nanostructures. The slight weight decrease upon heating to 400 ◦ C (∼4 wt%) may be explained by the dehydration of the physisorbed/chemisorbed OH radical and/or the gradual decomposition of the residual organic components attached on the surface of the ZnO hierarchical nanostructures during solvothermal self assembly. The gas responses of ZnO-500-HN and ZnO-500-AN sensors to 100 ppm C2 H5 OH, CO, H2 , and C3 H8 were measured at 400 ◦ C

(Fig. 4). The response of ZnO-500-HN sensor was very high (177.1) to 100 ppm C2 H5 OH but negligible to CO, H2 , and C3 H8 (1.6–3.1) (Fig. 4a). The selective detection of C2 H5 OH with the negligible cross sensitivities to CO, H2 , and C3 H8 can be applied for screening of intoxicated drivers on the road because it can minimize the interferences with automotive emissions such as CO and hydrocarbons from gasoline engine [28]. Although the ZnO-500-AN sensor also exhibited selective detection to C2 H5 OH, the response to 100 ppm C2 H5 OH (24.7) was significantly smaller than that of ZnO500-HN sensor (Fig. 4b). These results demonstrated the capability of ZnO-500-HN sensor to detect C2 H5 OH in a selective manner with very high gas response. The gas sensing transients to 10–100 ppm C2 H5 OH are shown in Fig. 5. The resistance in air (Ra ) was stable after the repetitive exposure to various concentrations of C2 H5 OH. The times to reach 90% variation in resistance upon exposure to C2 H5 OH and air were defined as 90% response ( res ) and 90% recovery times ( recov ), respectively. The  res and  recov values upon the exposures to 50 ppm C2 H5 OH and air were 4 s and 357 s for ZnO-500-HN sensor, while those were 26 s and 75 s for ZnO-500-AN sensor, respectively.

Fig. 4. Gas responses of (a) ZnO-500-HN sensor and (b) ZnO-500-AN sensor to 100 ppm of C2 H5 OH, CO, H2 , and C3 H8 at 400 ◦ C.

K.-M. Kim et al. / Sensors and Actuators B 155 (2011) 745–751

Fig. 5. C2 H5 OH sensing transients of (a) ZnO-500-HN sensor and (b) ZnO-500-AN sensor at 400 ◦ C.

When a reducing gas is detected using n-type oxide semiconductors, the  res value is usually shorter than the  recov value [28,29]. This reflects that the in-diffusion of reducing gas to the sensor surface and its oxidation with negatively charged surface oxygen during the gas sensing reaction are faster than the serial surface reactions regarding the adsorption, dissociation, and ionization of oxygen during the recovery reaction. Accordingly, the shorter  res value (4 s) in ZnO-500-HN sensor in comparison to that in ZnO500-AN sensor (26 s) can be attributed to the faster diffusion of analyte gases through the less agglomerated hierarchical structures. And this is consistent with the literature data that the gas

a

responding speed of well-ordered and less-agglomerated configuration of hierarchical structures is significantly faster than that of agglomerated nanoparticles [30,31]. However, the reason why the  recov value is shorter in ZnO-500-AN sensor remains unclear. Considering relatively fast gas diffusion, the out-diffusion of reaction product gases (CO2 and H2 O) and in-diffusion of oxygen (O2 ) can be excluded from the rate determining steps in the recovery reaction of ZnO-500-HN sensor. In the XRD patterns (Fig. 1), the ratios between the intensities of (0 0 2) plane to (1 0 1) plane in ZnO-500-AN (Fig. 1b) and ZnO-500-HN (Fig. 1d) specimens are 0.593 and 0.487, respectively. This indicates the possibility that two sensor specimens with different preferred orientations may show the different oxygen adsorption behaviors during recovery although the further investigation is needed to confirm this. The gas responses of ZnO-500-HN sensor to 1–100 ppm C2 H5 OH ranged from 2.3 to 177.1, which were 7–9 times higher than those of ZnO-500-AN sensor (Fig. 6a). The C2 H5 OH responses of various ZnO nanostructures such as nanoparticles, nanowires, nanorods, hierarchical structures, and nanofibers in the literature were summarized in Table 1 [20,22,32–44]. The C2 H5 OH response values of ZnO-500-HN sensor in the present study were among the highest values reported in the literature for ZnO sensors. The C2 H5 OH detection limits of ZnO-500-HN and ZnO-500-AN sensors were estimated to be approximately <0.7 and <7.4 ppm (Fig. 6b), respectively when the criterion for gas detection was set to Ra /Rg > 1.2. This result demonstrated the capability of ZnO-500-HN sensor to detect even a sub-ppm-level concentration of C2 H5 OH. Hierarchical nanostructures with less agglomerated configuration have been reported to show very high gas responses due to their high surface area and nano-porous structures [30,31,45–47]. To confirm this, the pore volume and surface area were measured by nitrogen adsorption-desorption isotherm (Fig. 7). The surface areas of ZnO-500-HN and ZnO-500-AN after heat treatment at 500 ◦ C for 1 h were 14.0 and 6.9 m2 /g, respectively. Over the entire range of pore sizes (2–70 nm), the pore volumes of ZnO-500-HN were significantly higher than those of ZnO-500-AN. Both the high surface area and high pore volume result from the well-defined hierarchical nanostructures that were self-assembled from ultrathin (20–40 nm) building blocks (ZnO nanosheets). Accordingly, the enhanced gas response of the hierarchical nanostructures in Fig. 6 was attributed to the higher surface area for gas sensing reaction and the more effective diffusion of analyte gas through the nanoand meso-porous architecture.

b

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ZnO-500-HN ZnO-500-AN

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Gas response (Ra/Rg )

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Fig. 6. Gas responses of ZnO-500-HN sensor and ZnO-500-AN sensor to 1−100 ppm C2 H5 OH at 400 ◦ C: (a) linear and (b) log–log plots.

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Table 1 Gas responses to C2 H5 OH of the ZnO sensors in the present study and those reported in the literature [20,22,32–44]. Sensing materials (preparation)

[C2 H5 OH] (ppm)

Ra /Rg

Tsens a (◦ C)

Reference

ZnO hierarchical nanostructures (emulsion-based solution growth) ZnO nanowires (thermal evaporation) ZnO nanowires (thermal evaporation) ZnO tetrapod nanowires (thermal evaporation) ZnO tetrapod nanowires (thermal evaporation) ZnO nanoparticles (solution growth) ZnO hierarchical nanostructures (solution growth) ZnO hierarchical nanostructures (solution growth) ZnO hierarchical structures (hydrothermal method) ZnO hierarchical nanostructures (hydrothermal method) ZnO nut-like structures (hydrothermal method) ZnO nanofibers (electrospinning) ZnO hierarchical nanostructures (microwave solution growth) ZnO nanoparticles (flame spray pyrolysis) ZnO nanorods (solid state reaction) Al-doped ZnO nanostructures (thermal evaporation + milling)

100 100 100 100 500 100 100 100 100 50 100 160 100 200 100 1000

177.7 ∼1.7 ∼33 ∼50 5.3 12 25.4 32 ∼14.6 23 ∼28 ∼4 9 18.2 10.8 95

400 300 300 300 300 400 320 330 300 240 250 220 350 300 330 280

Present study [32] [33] [34] [35] [36] [20] [37] [38] [39] [40] [41] [22] [42] [43] [44]

a

Tsens : sensing temperature.

References

10-1

3

.

Pore volume (cm /gnm)

ZnO-500-HN ZnO-500-AN

10-2

10-3

1

10

100

Pore diameter (nm) Fig. 7. Pore size distributions of ZnO-500-HN and ZnO-500-AN specimens determined from the nitrogen adsorption–desorption isotherm.

4. Conclusions Highly crystalline ZnO hierarchical nanostructures were prepared at room temperature using an emulsion-based solution reaction. Two immiscible solutions, zinc–nitrate aqueous solution and oleic-acid-dissolved n-hexane, were mixed in the presence of NaOH. The controlled release of the water-insoluble surfactant (oleic acid) to the alkaline hydrolysis of zinc salt in an aqueous environment via the emulsion-based approach was the key for the formation of well-defined and crystalline ZnO hierarchical nanostructures. The responses of the hierarchical nanostructures to 1–100 pm C2 H5 OH were among the highest values reported in the literature [20,22,32–44] and were 7–9 times higher than those of the agglomerated nanoparticles. This was explained by the large surface area and the effective diffusion of the analyte gas to the sensor surface as a result of the well aligned hierarchical assembly of ultra-thin nanosheets with less-agglomerated configuration. Acknowledgements This work was supported by KOSEF NRL program grant funded by the Korean government (MEST) (No.R0A-2008-000-20032-0) and the Fundamental R&D program for Core Technology of Materials (M2008010013) funded by Ministry of Knowledge Economy.

[1] Q. Zhang, C.S. Dandeneau, X. Zhou, G. Cao, ZnO nanostructures for dyesensitized solar cells, Adv. Mater. 21 (2009) 4087–4108. [2] O. Lupan, T. Pauport‘e, B. Viana, Low-temperature growth of ZnO nanowire arrays on p-silicon (1 1 1) for visible-light-emitting diode fabrication, J. Phys. Chem. C 114 (2010) 14781–14785. [3] O. Lupan, V. Ursaki, G. Chai, L. Chow, G.A. Emelchenko, I.M. Tiginyanu, A.N. Gruzintsev, A.N. Redkin, Selective hydrogen gas nanosensor using individual ZnO nanowire with fast response at room temperature, Sens. Actuators B 144 (2010) 56–66. [4] X. Wang, J. Song, J. Liu, Z.L. Wang, Direct-current nanogenerator driven by ultrasonic waves, Science 316 (2007) 102–105. [5] J. Lee, B.S. Kang, B. Hicks, T.F. Chancellor, B.H. Chu, H.-T. Wang, B.G. Keselowsky, F. Ren, T.P. Lele, The control of cell adhesion and viability by zinc oxide nanorods, Biomaterials 39 (2008) 3743–3749. [6] Z.L. Wang, Functional oxide nanobelts: materials, properties and potential applications in nanosystems and biotechnology, Annu. Rev. Phys. Chem. 55 (2004) 159–196. [7] A. Gurlo, Interplay between O2 and SnO2 : Oxygen ionosorption and spectroscopic evidence for adsorbed oxygen, Chem. Phys. Chem. 7 (2006) 2041–2052. [8] C.N. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2 -based elements, Sens. & Actuators B 3 (1991) 147–155. [9] T. Kida, T. Doi, K. Shimanoe, Synthesis of monodispersed SnO2 nanocrystals and their remarkably high sensitivity to volatile organic compounds, Chem. Mater. 8 (2010) 2662–2667. [10] Z. Fan, D. Wang, P.-C. Chang, W.-Y. Tseng, J.G. Lu, ZnO nanowire field-effect transistor and oxygen sensing property, Appl. Phys. Lett. 85 (2004) 5923. [11] C.-S. Moon, H.-R. Kim, G. Auchterlonie, J. Drennan, J.-H. Lee, Highly sensitive and fast responding CO sensor using SnO2 nanosheets, Sens. Actuators B 131 (2008) 556–564. [12] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336. [13] B. Liu, H.C. Zeng, Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm, J. Am. Chem. Soc. 125 (2004) 4430–4431. [14] P.-S. Cho, K.-W. Kim, J.-H. Lee, NO2 sensing characteristics of ZnO nanorods prepared by hydrothermal method, J. Electroceram. 17 (2005) 975–978. [15] A. Zuo, P. Hu, L. Bai, F. Yuan, Synthesis of tunable 3D ZnO architectures assembled with nanoplates, Cryst. Res. Technol. 44 (2009) 613–618. [16] R. Yi, N. Zhang, H. Zhou, R. Shi, G. Qui, Selective synthesis and characterization of flower-like ZnO microstructures via a facile hydrothermal route, Mater. Sci. & Eng. B 153 (2008) 25–30. [17] L. Jiang, G. Li, Q. Ji, H. Peng, Morphological control of flower-like ZnO nanostructures, Mater. Lett. 61 (2007) 1964–1967. [18] Y. Zhang, J. Mu, Controllable synthesis of flower- and rod-like ZnO nanostructures by simply tuning the ratio of sodium hydroxide to zinc acetate, Nanotechnology 18 (2007) 075606. [19] U. Pal, P. Santiago, Controlling the morphology of ZnO nanostructures in a lowtemperature hydrothermal Process, J. Phys. Chem. B 109 (2005) 15317–15321. [20] J. Huang, Y. Wu, C. Gu, M. Zhai, K. Yu, M. Yang, J. Liu, Large-scale synthesis of flowerlike ZnO nanostructures by a simple chemical solution route and its gas-sensing property, Sens. Actuators B 146 (2010) 206–212. [21] S. Yamabi, H. Imai, Growth conditions for wurzite zinc oxide films in aqueous solutions, J. Mater. Chem. 12 (2002) 3773–3778. [22] Z. Zing, J. Zhan, Fabrication and gas-sensing properties of porous ZnO nanoplates, Adv. Mater. 20 (2008) 4547–4551. [23] B. Liu, H.C. Zeng, Room temperature solution synthesis of monodispersed single-crystalline ZnO nanorods and derived hierarchical nanostructures, Langmuir 20 (2004) 4196–4204. [24] W. Zhao, X. Song, Z. Yin, C. Fan, G. Chen, S. Sun, Self-assembly of ZnO nanosheets into nanoflowers at room temperature, Mater. Res. Bull. 43 (2008) 3171–3176.

K.-M. Kim et al. / Sensors and Actuators B 155 (2011) 745–751 [25] P.D. Cozzoli, A. Kornowski, H. Weller, Low-temperature synthesis of soluble and processable organic-capped anatase TiO2 nanorods, J. Am. Chem. Soc. 125 (2003) 14539–14548. [26] J. Du, M. Yang, S.N. Cha, D. Rhen, M. Kang, D.J. Kang, Indium hydroxide and indium oxide nanosphere, nanoflowers, microcubes, and nanorods: synthesis and optical properties, Cryst. Growth Des. 8 (2008) 2312–2317. [27] C. Schliehe, B.H. Juarez, M. Pelletier, S. Jander, D. Gresnykh, M. Nagel, A. Meyer, S. Foerster, A. Kornowski, C. Klinke, H. Weller, Ultrathin PbS sheets by twodimensional oriented attachment, Science 329 (2010) 550–553. [28] K.-W. Kim, P.-S. Cho, S.-J. Kim, J.-H. Lee, C.-Y. Kang, J.-S. Kim, S.-J. Yoon, The selective detection of C2 H5 OH using SnO2 -ZnO thin films gas sensors prepared by combinatorial solution deposition, Sens. Actuators B 123 (2007) 318–324. [29] J.-H. Park, J.-H. Lee, Gas sensing characteristics of polycrystalline SnO2 nanowires prepared by “polyol method”, Sens. Actuators B 136 (2009) 151–157. [30] H.-R. Kim, K.-I. Choi, J.-H. Lee, S.A. Akbar, Highly sensitive and ultra-fast responding gas sensors using self-assembled hierarchical SnO2 spheres, Sens. Actuators B 136 (2009) 138–143. [31] K.-I. Choi, H.-R. Kim, J.-H. Lee, Enhanced CO sensing characteristics of hierarchical and hollow In2 O3 microspheres, Sens. Actuators B 137 (2009) 497–503. [32] T.-J. Hsueh, C.-L. Hsu, S.-J. Chang, I.-C. Chen, Laterally grown ZnO nanowire ethanol sensors, Sens. Actuators B 126 (2007) 473–477. [33] Q. Wan, Q.H. Li, Y.J. Chen, T.H. Wang, X.L. He, J.P. Li, C.L. Lin, Fabrication and ethanol sensing characteristics of ZnO nanowire gas sensors, Appl. Phys. Lett. 84 (2004) 3654–3656. [34] D. Calestani, M. Zha, R. Mosca, A. Zappettini, M.C. Carotta, V.D. Natale, L. Zanotti, Growth of ZnO tetrapods for nanostructure-based gas sensors, Sens. Actuators B 133 (2010) 472–478. [35] N.V. Hieu, N.D. Chien, Low-temperature growth and ethanol-sensing characteristics of quasi-one-dimensional ZnO nanostructures, Sens. Actuators B 403 (2008) 50–56. [36] R.C. Singha, O. Singha, M.P. Singh, P.S. Chandic, Synthesis of zinc oxide nanorods and nanoparticles by chemical route and their comparative study as ethanol sensors, Sens. Actuators B 135 (2008) 352–357. [37] X. Jia, H. Fan, Room temperature solid-state synthesis and ethanol sensing properties of sea-urchin-like ZnO nanostructures, Mater. Lett. 64 (2010) 1574–1576. [38] P. Feng, Q. Wan, T.H. Wang, Contact-controlled sensing properties of flowerlike ZnO nanostructures, Appl. Phys. Lett. 87 (2005) 213111. [39] W.-D. Zhang, W.-H. Zhang, X.-Y. Ma, Tunable ZnO nanostructures for ethanol sensing, J. Mater. Sci. 44 (2009) 4677–4682. [40] Y. Zeng, T. Zhang, L. Qiao, Preparation and gas sensing properties of the nutlike ZnO microcrystals via a simple hydrothermal route, Mater. Lett. 63 (2009) 843–846. [41] W.-Y. Wu, J.-M. Ting, P.-J. Huang, Electrospun ZnO nanowires as gas sensors for ethanol detection, Nanoscale. Res. Lett. 4 (2009) 513–517. [42] N. Tamaekong, C. Liewhiran, A. Wisitsoraat, S. Phanichphant, Flame-spraymade undoped zinc oxide films for gas sensing applications, Sensors 10 (2010) 7863–7873.

751

[43] Z.-P. Sun, L. Liu, L. Zhang, D.-Z. Jia, Rapid synthesis of ZnO nano-rods by onestep, room-temperature, solid-state reaction and their gas-sensing properties, Nanotechnology 17 (2006) 2266–2270. [44] Z Yang, Y. Huang, G. Chen, Z. Guo, S. Cheng, S. Huang, Ethanol gas sensor based on Al-doped ZnO nanomaterial with many gas diffusing channels, Sens. Actuators 140 (2009) 549–556. [45] J. Huang, K. Yu, C. Gu, Y. Wu, M. Yang, J. Liu, Preparation of porous flowershaped SnO2 nanostructures and their gas sensing properties, Sens. Actuators B 147 (2010) 467–474. [46] C. Wang, L. Yin, L. Zhang, Y. Qi, N. Lun, N. Liu, Large scale synthesis and gas-sensing properties of anatase TiO2 three-dimensional hierarchical nanostructures, Langmuir 26 (2010) 12841–12848. [47] X. Gou, G. Wang, X. Kong, D. Wexler, J. Horvat, J. Yang, J. Park, Flutelike porous hematite nanorods and branched nanostructures: synthesis, characterization and application for gas-sensing, Chem. Eur. J. 14 (2008) 5996–6002.

Biographies Kang-Min Kim studied materials science and engineering and received his BS degree from Kangwon National University, Korea, in 2004. In 2006, he received his MS degree from Korea University. He has worked for Samsung Corning and Samsung Corning Precision Glass Companies for 3 years. He is currently studying for a Ph.D. at Korea University. His research interests are oxide nanostructure-based gas sensors and solar cells. Hae-Ryong Kim studied materials science and engineering and received his BS and MS degrees in 2005 and 2007, respectively, from Korea University. He is currently studying for a Ph.D. at Korea University. His research interest is oxide nanostructures for chemical sensor applications. Kwon-Il Choi studied materials science and engineering and received his BS and MS degrees from Korea University in 2008 and 2010, respectively. He is currently studying for a Ph.D. at Korea University. His research topic is the use of oxide nanostructures for chemical sensor applications. Hyo-Joong Kim studied materials science and engineering and received his BS degree in 2008. He is currently a master course student at Korea University. His research topic is oxide semiconductor gas sensors. Jong-Heun Lee joined the Department of Materials Science and Engineering at Korea University as an associate professor in 2003, where he is currently a professor. He received his BS, MS, and Ph.D. degrees from Seoul National University in 1987, 1989, and 1993, respectively. Between 1993 and 1999, he developed automotive air-fuelratio sensors at the Samsung Advanced Institute of Technology. He was a Science and Technology Agency of Japan (STA) fellow at the National Institute for Research in Inorganic Materials (currently NIMS, Japan) from 1999 to 2000 and a research professor at Seoul National University from 2000 to 2003. His current research interests include chemical sensors, functional nanostructures, and solid oxide electrolytes.