Nano gas sensors paper

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Sensors and Actuators B 155 (2011) 264–269

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s n b

ertically aligned ZnO nanorods and graphene hybrid architectures for igh-sensitive flexible gas sensors

aeseok Yi, Jung Min Lee, Won Il Park ∗

ivision of Materials Science Engineering, Hanyang University, Seoul 133-791, Republic of Korea

r t i c l e i n f o

rticle history: eceived 6 September 2010 eceived in revised form 5 December 2010 ccepted 18 December 2010 vailable online 25 December 2010

a b s t r a c t

We present the fabrication and characterization of new type of flexible gas sensors, composed mainly of a bottom ZnO conductive layer on metal foil, vertically aligned ZnO nanorod channel, and graphene-based top conductive electrode. Multiple cycling tests demonstrated the ZnO nanorods (NRs) and graphene (Gr) hybrid architectures accommodated the flexural deformation without mechanical or electrical failure for bending radius below 0.8 cm under the repeated bending and releasing up to 100 times. In addition, the hybrid architectures fabricated on glass substrate showed good optical transmittance larger than ∼70% for visible light, indicating potential application in transparent devices. Furthermore, our gas sensors demon-

eywords: as sensor nO nanorod raphene anorod-graphene hybrid architecture lexible device ransparent device

strated the ppm level detection of ethanol gas vapor with the sensitivity (resistance in air/resistance in target gas) as high as ∼9 for 10 ppm ethanol.

© 2010 Elsevier B.V. All rights reserved.

. Introduction

The demand for solid state gas sensors is rapidly growing for wide range of applications, including the detection of hazardous ases, environmental gas monitoring, humidity and air quality con- rol, and chemical process control. Semiconductor metal oxides xhibit chemical and thermal stability and high sensitivity to ombustible and toxic gases and thus can serve as the basis for olid-state gas sensors [1–4]. Up to now, considerable effort has een devoted to improve operating parameters such as sensitivity, electivity, and reliability of the sensors by introducing optimized tructure, doping, and chemical modification of the metal oxides 5–9].

The potency of one-dimensional (1D) semiconductor nanos- ructures, such as ZnO, SnO2, and In2O3 nanowires (NWs), for gas ensors has been explored since those nanostructures have several dvantages over polycrystalline thin films [10–15]. For example, he large surface area versus volume of the NWs, whose widths are

uch narrower than that of lithography-based thin film technol- gy, can increase the sensitivity of sensors beyond the limitations

f planar thin film devices [15–17]. In addition, high aspect ratio D nanostructures provide an excellent mechanical flexibility and ptical transparency, while their single crystalline structures hold

∗ Corresponding author. Tel.: +82 2 2220 0504; fax: +82 2 2220 0389. E-mail address: [email protected] (W.I. Park).

925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. oi:10.1016/j.snb.2010.12.033

an efficient pathway for charge carrier transport as the signal col- lections become more efficient. Indeed, these unique properties of NWs were exploited to fabricate ultrasensitive flexible sensors on plastic substrates [18–20]. However, in order to maximize the potential advantages of 1D nanostructures for such applications, complicated fabrication steps are required. Fabrication typically starts from the placement of nanomaterials onto desired substrates, followed by lithography and metallization steps to produce the metallic contacts to the ends of nanostructures [3,16,17]. The use of percolating NW networks [21] or laterally grown NW arrays [22] might simplify the process by eliminating the tedious NW assembly or registration steps, but it lacks reliability and control.

Vertically aligned nanorods (NRs), which are an alternative to these laterally deposited or networked NW sensors, exhibit addi- tional attractive properties. Since individual NRs or their bundles can be easily configured to vertical device platforms, they provide potential for device scaling and integration of the largest number density in mono-layered structures [23–25]. Indeed, metal oxide nanostructures, such as ZnO and TiO2 NRs, can be readily grown vertically via low temperature hydrothermal process that can be easily applicable to large area, amorphous, and chip substrates such as glass, plastics, or metal foils [26,27]. However, to utilize the key advantages of vertical NR array for gas sensors, it is essential to

construct the top contacts to the individual NR tips without filling the interspaces between the NRs with other supporting materials.

Here, we present chemical sensors based on ZnO NR and graphene (Gr) hybrid architectures, where Gr sheets coated

J. Yi et al. / Sensors and Actuators B 155 (2011) 264–269 265

r fabri

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Fig. 1. Schematic illustrating of the key steps fo

ith thin metal layers are employed as top electrodes for ZnO ertical-NR channels. Our approach to fabricate such devices xploits the fact that freestanding Gr sheets can be readily trans- erred onto arbitrary targets, including non-planar 3D structures 28,29]. In this study, we explore the potency of the strat- gy by integrating Gr/metal (Gr/M) sheets onto vertical nanorod rray structures. The resulting ZnO NRs–Gr/M hybrid architec- ures can maintain sufficient spaces between the monolithic NRs o allow for easy and fast gas transport [30]. Accordingly, the R surface area exposed to moving gas molecules can be max-

mized, thereby enabling highly sensitive gas sensor devices. In ddition, both ZnO NRs and Gr sheets exhibit unique mechan- cal deformability and good optical transmittance in the visible pectral range [28,31]. These mechanical and optical character- stics of the ZnO NRs–Gr/M hybrid architectures could enable ovel applications such as mechanically flexible or transparent ensors [18].

. Experimental

.1. Hydrothermal synthesis of ZnO NRs

ZnO NRs were synthesized using a wet hydrothermal process ainly on stainless steel (SUS) foils. Initially, 200-nm-thick ZnO

eeding layers were deposited by MOCVD using diethylzinc (DEZn) nd oxygen as the reactants and argon as a carrier gas. The detailed rocedure for ZnO seeding layer growth is described elsewhere 26,28]. Following seeding layer deposition, the hydrothermal syn- hesis of ZnO NRs was performed in an aqueous solution containing inc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma–Aldrich) and hex- methylenetetramine (C6H12N4, Sigma–Aldrich) in a 1:1 molar

atio (0.025 M). During the synthesis of ZnO NRs, the solution tem- erature was maintained in an oven at 85 ◦C for 6–8 h, which roduced vertically aligned ZnO NRs with an average diameter of 0–80 nm and a length between 0.5 and 1 �m.

cating the ZnO NRs–Gr/M hybrid architectures.

2.2. Graphene synthesis

Few-layer Gr films were synthesized on Si/SiO2 (200-nm-thick thermal oxide)/Ni (200-nm-thick) substrates by CVD in a cold- walled reactor, in which the substrate was self-heated to high growth temperature (up to 1,000 ◦C) by high current Joule heat- ing [28,32]. Gr synthesis was initiated by annealing the catalytic Ni layers at 900–1,000 ◦C for 10 min under a H2 (25%) and Ar (75%) atmosphere. After annealing the sample, 50 sccm methane (CH4) was used as a carbon precursor and introduced into the reactor for 1 min, followed by turning off the CH4 gas flow and rapid cooling of the samples to room temperature at the rate of 10 ◦C/s [32].

2.3. Flexible device fabrication and electromechanical property characterization

Fig. 1 schematically illustrates the steps for device fabrication. First, as-grown Gr films on Si/SiO2/Ni substrates were deposited with very thin Au/Ti (2 nm/2 nm) metal layers by thermal evapora- tion. Thin polymer support, such as a poly(methyl methacrylate) (PMMA) layer, was optionally spin-coated onto the Gr/M films, which improved the transfer process by minimizing the mechan- ical fractures. They were then separated from the substrates by chemical etching of the underlying SiO2 with a 10 vol.% hydrogen fluoride (HF) solution. The Gr/M film released from the substrate was floated on the solution due to their hydrophobic surfaces. Con- secutively, the film was carefully transferred onto an aqueous Ni etchant TFG to eliminate the remaining Ni layers, followed by rins- ing with deionized (DI) water. On the other hand, as-grown ZnO NRs on SUS foils were exposed to mild oxygen plasma (at the pres- sure of 100 mTorr and the discharge power of 30 W for 3 s) to make

their surfaces more hydrophilic. The plasma-treated specimen was soaked in DI water, just below the floating Gr/M film, and then slowly lifted to mount the Gr/M film onto the ZnO NRs. After nat- urally drying the sample, the polymer support was removed via

266 J. Yi et al. / Sensors and Actuators B 155 (2011) 264–269

Fig. 2. (a) SEM images of ZnO NRs–Gr/M hybrid architectures. (b) Optical transmittance spectra of the ZnO seed layer (blue triangles), ZnO NRs on the ZnO seed layers (red circles), and ZnO NRs–Gr/M on the ZnO seed layer (black squares) on a glass substrate. (Inset) A photo image of a ZnO NRs–Gr/M hybrid device on a glass substrate. (c) I–V c ares) R r/M s i to the

a d o b a w e t o 1 c b a N I m u a c (

2 h

a u t r s N t g

haracteristic curves of a ZnO NRs–Gr/M hybrid device measured before (black squ epresentative Raman spectra of the bare Gr film (blue) on glass substrate and the G

nterpretation of the references to color in this figure legend, the reader is referred

n oxygen plasma process (at the pressure of 100 mTorr and the ischarge power of 300 W for 150 s). Finally, the electrical contacts n the resulting ZnO NRs–Gr/M hybrid architectures were made y applying a silver paste to both the bottom ZnO seeding layer nd the top Gr/Ti/Au layer. One side of the substrate was covered ith SU-8 resist (MicroChem Corp.) that can isolate the top Gr/M

lectrode electrically from the bottom seeding layers and supports he Gr/M electrode to keep it from collapsing during installation f the electrical wiring. The device was annealed at 300 ◦C for 0 min under N2 ambient in order to obtain low-resistant ohmic ontacts. Electrical characterization of the devices was carried out y using a power supply as a voltage source and a picoammeter s a current meter. Custom programmed software (LabVIEW 6.0, ational Instrument) was used to control the units and collect data.

n addition, mechanical flexibility of the devices was evaluated by easuring the resistances under repeated bending and releasing

p to 100 times. The bending radius was carefully controlled by ttaching the ends of sample to the outside jaws of the vernier alliper. Typical radius of curvature in the bending state is 0.8 cm Fig. 3(a)).

.4. Optical transmittance characterization of ZnO NRs–Gr/M ybrid architectures

The hybrid architectures were fabricated on glass substrates nd then their optical transmittance was investigated by using an ltraviolet–visible–near infrared (UV–VIS–NIR) double beam spec- rophotometer at normal incidence in the 350–800 nm spectral ange. The transmittance spectra of the sample in each fabrication

teps (ZnO seed layer, ZnO NRs on the ZnO seed layer, and ZnO Rs–Gr/M on the ZnO seed layer) were measured sequentially. The

ransmission spectra of the samples were obtained by subtracting lass substrate spectrum from the sample spectra.

and after thermal annealing at 200 ◦ C (red circles) and 300 ◦ C (blue triangles). (d) heet (red) mounted on the ZnO NRs with the excitation wavelength of 514 nm. (For web version of the article.)

2.5. Characterization of gas sensors

The gas-sensing properties of ZnO NRs–Gr/M hybrid archi- tectures were measured at 300 ◦C inside quartz tube (3.81 cm diameter, 1 m length) placed in electronic furnace equipped with a gas delivery system and electrical feedthrough. After flushing the measuring apparatus with air at a flow rate of 1,000 sccm for 60 min, the sensor was exposed to ethanol vapor with different concentration in the range of 10–50 ppm.

3. Results and discussion

Fig. 2(a) shows the scanning electron microscopy (SEM) images of the suspended Gr/M sheet over a large area with support from a ZnO NR array. Bottom-up synthesis of ZnO NRs yielded some devi- ation in length and vertical alignment, and thus their top surface did not lie in a flat horizontal plane. Even in this extreme situation, exceptionally robust and flexible Gr/M sheets adhered to about half of ZnO NR tips by strong van der Waals forces as confirmed by cross-sectional SEM images.

One interesting aspect of these hybrid structures is that both ZnO and Gr sheets exhibit good optical transmittance in the visible spectral range. Light absorption/reflection by thin metal layers on Gr and light scattering by the array of ZnO NRs may cause a decrease in transmittance to a certain extent. We evaluated the optical trans- mittance of the ZnO NR–Gr hybrid architectures by fabricating the structures on glass substrates. Fig. 2(b) shows the optical transmit- tance spectra of the ZnO seed layer (blue triangles), ZnO NRs on the ZnO seed layers (red circles), and ZnO NRs–Gr/M on the ZnO seed layer (black squares) with the glass substrate absorption removed.

Compared with bare ZnO films that showed an optical transparency of ∼95% in the spectral range of 400–800 nm, the growth of ZnO NRs led to lowering the transmittance by ∼9%. Even after the transfer of Gr/M, the optical transmittance still exceeds ∼70% for wave-

tuators B 155 (2011) 264–269 267

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c d ( a d c t c o G a G t t n s o i s 1 i t w

t N T i h s a b o d s ( s u a r 1 i a

i s r r w b t i s o c t

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Fig. 3. (a) Photo images of a flexible ZnO NRs–Gr/M hybrid device on a SUS foil in the release and bending states. (b) Variation in resistance changes of the device accompanying the repeated bending–unbending cyclic test. (Inset) I–V curves for the device in the release (red/dotted line) and bending (black/solid line) states. (c) Schematic geometries of the ZnO NRs–Gr/M hybrid architecture in the release and bending states. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

J. Yi et al. / Sensors and Ac

engths longer than 400 nm, thereby demonstrating opportunities s invisible sensors.

To obtain high reliability in device operation, it is essential to ontrol the electrical contact properties. The ZnO NRs–Gr/M hybrid evices exhibited nonlinear current–voltage (I–V) characteristic black squares in Fig. 2(c)), which represent large potential barriers t the top and bottom electrical contacts. Thermal annealing of the evice at 200 ◦C for 10 min resulted in a roughly 20-fold increase in urrent, but considerable barriers still existed at the junctions. As he annealing temperature approached 300 ◦C, the nonlinear I–V haracteristics became linear with an additional current increase f ∼25 times. In case of the top contact between ZnO NRs and r/M sheet, the improved contact property indicates that thermal nnealing activates the stepwise diffusion of Ti atoms from the top r surface to the ZnO NRs tips, where the interfacial reaction lowers

he Schottky barrier height [33]. Although atomic diffusion across he Gr layer is less favorable than in-plane diffusion, one mecha- ism involves atomic migration via defects including vacant lattice ites [34,35]. Raman spectroscopy was used to investigate the effect f metal deposition and incorporation on the structural character- stics of Gr sheets (Fig. 2(d)). Compared with bare Gr films, the Gr/M heet exhibited a broad background signal, especially in the range of ,300–1,600 cm−1 [36]. However, there were no significant changes

n the 2D band peak position or the G to 2D band intensity ratio of he Gr/M sheet, indicating that no major degradation of the Gr films as induced, even after thermal annealing.

The small-area junctions between NR tops/Gr and NR bot- oms/seed layer plus the air gaps between the NRs enable the Rs to be less affected by flexural (bending) deformation [28]. his unique layout, combined with outstanding mechanical flex- bility and strength of the Gr/M sheets, could allow the ZnO NR–Gr ybrid architectures to accommodate certain levels of bending and tretching. These devices showed good mechanical deformability, s they can be stable without mechanical or electrical failure for ending radius less than ∼0.8 cm, corresponding to a tensile strain f ∼1.3% (Fig. 3(a)). Specifically, a comparison of I–V curves for the evices in the release state versus those in the bend state shows a mall change (8.18 mA versus 8.22 mA at 3 V) in I–V characteristic Fig. 3(b), inset). Repeated bending–unbending cyclic tests demon- trate that the resistance of the device changed by ∼0.2 ± 0.01% pon bending the substrate to radius of curvature less than 0.8 cm, nd it was fully recovered to the original value when the substrate eturned to the release state. This characteristic was stable up to 00 of cycles and reproducible with other devices, demonstrat-

ng the robust nature of ZnO NR–Gr hybrid architectures and their pplication in flexible devices.

One peculiar feature of this device is the decrease in bending- nduced resistance, which is distinct from a previous study that howed the resistance in Gr increases with decreasing bending adius [28]. In our device, total resistance was the sum of all the esistances in series, including those of Gr/M sheet and ZnO NRs as ell as their junctions. Stretching of the Gr/M sheet accompanied

y mechanical deformation may lead to an increase in its resis- ance to a certain degree; on the contrary, mechanical bending may ncrease the number of NRs electrically connected to the top Gr/M heet (Fig. 3(c)), and thus decreasing the channel resistance. Based n this model, it was concluded that the contribution of resistance hange by the NR channels accompanying the bending prevails over hose by the Gr/M sheet, thereby decreasing the total resistance.

To evaluate the gas-sensing ability of the ZnO NRs–Gr/M hybrid rchitectures, we placed the sample inside a measuring apparatus onsisting of horizontal tube furnace with a vacuum sealed 3-in.

iameter quartz tube connected to gas-delivery system. Fig. 4(a) hows the conductance responses of the ZnO NRs–Gr/M hybrid ensor device to air containing various concentrations of ethanol apor with various concentrations recorded at 300 ◦C. Several key

Fig. 4. (a) Continuous changes in conductance of the ZnO NRs–Gr/M hybrid sensor at different concentrations of ethanol gas at 300 ◦ C. (b) Plot of sensitivity versus ethanol concentration.

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68 J. Yi et al. / Sensors and A

eatures were noted. First, the initial conductance of the device in ir decreased significantly with increasing operating temperature, ecreasing to near zero (below 0.1 �S) at 300 ◦C. The mechanism or lowering conductance is based on the depletion of ZnO NR hannels by oxygen ionosorption on their surface by either O−

r O2− depending on the operating temperature [21]. Second, the evice exhibited a rapid conductance increase with the delivery f ethanol vapor and subsequent restoration to a non-conducting tate after the chamber was refilled with air that did not contain thanol vapor. When the ZnO NRs are exposed to a reducing gas uch as ethanol, the gas substitutes surface-bound oxygen, thereby eleasing electrons back into the ZnO crystals and increasing their lectrical conductivity. The gas sensitivity of the sensor is defined as = Ra/Rg, where Ra and Rg are the resistances of the sensor recorded

n air and in the presence of ethanol gas, respectively. Specifically, he sensitivity of our device was as high as ∼9–10 ppm ethanol and 90 for 50 ppm, which is significantly larger than those of previ- us ethanol gas sensors based on 1D ZnO nanostructures [3,21,22] nd comparable to the best data achieved by Chen et al. [37]. Pos- ible reasons to explain this result are that there exist sufficient ore spaces between the monolithic NRs, thereby maximizing the pecific surface area; on the other hand, the single-crystal NR chan- el provides a more stable and efficient conductance path than do he percolating NRs/nanoparticles. Lastly, the conductance change f the ZnO NRs–Gr/M hybrid sensor was proportional to the con- entration of ethanol vapor in air. Indeed, a plot summarizing the ensitivity versus ethanol concentration (Fig. 4(b)) exhibits a linear esponse characteristic at low concentrations (from 10 to 40 ppm). his result suggests the applicability of our device for quantita- ive detection of reducing gas molecules at very low concentration. lthough the sensor structure can essentially be fabricated on the ubstrate that is flexible and transparent at the same time, it cannot e endurable at high operating temperature (300 ◦C) for gas sens-

ng due to low glass transition temperature of the plastic materials. urther investigation to avoid the above limitation by lowering the perating temperature is needed.

. Conclusions

We fabricated a new type of gas sensor using hybrid vertically rown ZnO NRs and free-standing Gr/M sheets. The ZnO NRs–Gr/M ybrid architectures on glass substrates exhibited good optical ransmittance for visible light, while those on flexible metal foils ould accommodate flexural deformation without mechanical or lectrical failure under repeated bending–unbending up to 100 imes. Furthermore, our gas sensors enabled ppm level detection or ethanol vapor with very high sensitivity. The result suggests that he combination of 1D nanocrystals and 2D Gr improves the per- ormances of the sensors and also imposes additional mechanical r optical functions to the devices.

cknowledgments

This work was supported by Basic Science Research Program hrough the National Research Foundation of Korea (NRF) funded y the Ministry of Education, Science and Technology (2009- 071357) and by Mid-career Researcher Program through NRF rant funded by the MEST (No. R01-2008-000-20778-0).

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iographies

aeseok Yi received his BS in materials science and engineering from Hanyang Uni- ersity. He is currently working on his PhD under the supervision of professor Won l Park at Hanyang University. His current research projects include the fundamental tudy of 1D nanomaterials and their application in high-performance sensors and hotovoltaics.

rs B 155 (2011) 264–269 269

Jung Min Lee received a BS degree in materials science and engineering from Hanyang University (Seoul, Korea) in 2008. Now he is working on his PhD under the supervision of professor Won Il Park. His research interests include nanomaterial synthesis, hybrid nanoarchitectures, and next-generation flexible devices.

Won Il Park received his BS degree in materials science and engineering from Yonsei University (Seoul, Korea) in 2000, and his PhD degree in materials science and engineering (Electronic Materials Program) from Pohang Univer- sity of Science and Technology (POSTECH) in 2005. He then spent 2 years as a postdoctoral fellow in Prof. Charles M. Lieber’s group at Harvard Univer-

sity. He has been an assistant professor in division of materials science and engineering at Hanyang University since 2007. His research interest includes syn- thesis and properties of semiconductor nanostructures (nanowires, nanorods, and graphene), and novel applications for high-sensitive sensors and next-generation optoelectronics.