Vertically aligned ZnO nanorods and graphene hybrid architectures for high-sensitive flexible gas sensors

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Abstract

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 demonstrated 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.

Introduction

The demand for solid state gas sensors is rapidly growing for a wide range of applications, including the detection of hazardous gases, environmental gas monitoring, humidity and air quality control, and chemical process control. Semiconductor metal oxides exhibit chemical and thermal stability and high sensitivity to combustible and toxic gases and thus can serve as the basis for solid-state gas sensors [1], [2], [3], [4]. Up to now, considerable effort has been devoted to improve operating parameters such as sensitivity, selectivity, and reliability of the sensors by introducing optimized structure, doping, and chemical modification of the metal oxides [5], [6], [7], [8], [9].

The potency of one-dimensional (1D) semiconductor nanostructures, such as ZnO, SnO2, and In2O3 nanowires (NWs), for gas sensors has been explored since those nanostructures have several advantages over polycrystalline thin films [10], [11], [12], [13], [14], [15]. For example, the large surface area versus volume of the NWs, whose widths are much narrower than that of lithography-based thin film technology, can increase the sensitivity of sensors beyond the limitations of planar thin film devices [15], [16], [17]. In addition, high aspect ratio 1D nanostructures provide an excellent mechanical flexibility and optical transparency, while their single crystalline structures hold an efficient pathway for charge carrier transport as the signal collections become more efficient. Indeed, these unique properties of NWs were exploited to fabricate ultrasensitive flexible sensors on plastic substrates [18], [19], [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 additional 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], [24], [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 with thin metal layers are employed as top electrodes for ZnO vertical-NR channels. Our approach to fabricate such devices exploits the fact that freestanding Gr sheets can be readily transferred onto arbitrary targets, including non-planar 3D structures [28], [29]. In this study, we explore the potency of the strategy by integrating Gr/metal (Gr/M) sheets onto vertical nanorod array structures. The resulting ZnO NRs–Gr/M hybrid architectures can maintain sufficient spaces between the monolithic NRs to allow for easy and fast gas transport [30]. Accordingly, the NR surface area exposed to moving gas molecules can be maximized, thereby enabling highly sensitive gas sensor devices. In addition, both ZnO NRs and Gr sheets exhibit unique mechanical deformability and good optical transmittance in the visible spectral range [28], [31]. These mechanical and optical characteristics of the ZnO NRs–Gr/M hybrid architectures could enable novel applications such as mechanically flexible or transparent sensors [18].

Section snippets

Hydrothermal synthesis of ZnO NRs

ZnO NRs were synthesized using a wet hydrothermal process mainly on stainless steel (SUS) foils. Initially, 200-nm-thick ZnO seeding layers were deposited by MOCVD using diethylzinc (DEZn) and oxygen as the reactants and argon as a carrier gas. The detailed procedure for ZnO seeding layer growth is described elsewhere [26], [28]. Following seeding layer deposition, the hydrothermal synthesis of ZnO NRs was performed in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2·6H2O,

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 deviation 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

Conclusions

We fabricated a new type of gas sensor using hybrid vertically grown ZnO NRs and free-standing Gr/M sheets. The ZnO NRs–Gr/M hybrid architectures on glass substrates exhibited good optical transmittance for visible light, while those on flexible metal foils could accommodate flexural deformation without mechanical or electrical failure under repeated bending–unbending up to 100 times. Furthermore, our gas sensors enabled ppm level detection for ethanol vapor with very high sensitivity. The

Acknowledgments

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0071357) and by Mid-career Researcher Program through NRF grant funded by the MEST (No. R01-2008-000-20778-0).

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

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

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

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