Design of Metal Oxide Hollow Structures Using Soft-templating Method for High-Performance Gas Sensors

http://dx.doi.org/10.5369/JSST.2016.25.3.178 pISSN 1225-5475/eISSN 2093-7563

Design of Metal Oxide Hollow Structures Using Soft-templating Method for High-Performance Gas Sensors

Young-Seok Shim

and Ho Won Jang

Abstract

Semiconductor gas sensors based on metal oxide are widely used in a number of applications, from health and safety to energy effi- ciency and emission control. Nanomaterials including nanowires, nanorods, and nanoparticles have dominated the research focus in this field owing to their large number of surface sites that facilitate surface reactions. Recently, metal oxide hollow structures using soft tem- plates have been developed owing to their high sensing properties with large-area uniformity. Here, we provide a brief overview of metal oxide hollow structures and their gas-sensing properties from the aspects of template size, morphology, and additives. In addition, a gas- sensing mechanism and perspectives are presented.

Keywords: Gas sensor, Soft-template method, Nanostructure, Hollow hemisphere, Metal decoration

1. INTRODUCTION

The functional convergence of the Internet with radio frequency identification, sensors, and smart objects led to the era of the Internet of Things (IoT), which supplies and accesses all real- world information [1]. Sensor technology is the most important factor in IoT because sensors offer enormous amounts of information for many types of environment. Accordingly, various sensors such as gas, pressure, illumination, tilt, temperature, and motion sensors have been heavily studied to more precisely measure and monitor changes in environments [2]. In particular, diverse applications of the gas sensor in broad fields including the detection of air pollutants and gaseous hazards, homeland security, medical diagnosis, and fuel combustion control have accelerated studies for high-performance gas sensors [3-7].

Gas sensors for applications in the IoT should meet special requirements such as low cost, miniaturized size, ease of integration with electronic circuits, and high sensing performance

[8]. Various gas sensors including optical, electrochemical, surface acoustic wave, and semiconductor metal oxide sensors have been extensively studied to fulfill the needs of the IoT [9]. In particular, owing to their simplicity in operation, low cost, flexibility in production, small size, and easy integration with electronic circuits, semiconductor gas sensors based on metal oxides are very promising as sensing elements for IoT [10-12].

Generally, the gas-sensing properties of metal oxide are determined by three basic factors: utility factor, transducer function, and receptor function [13]. The first factor is related to gas diffusion (porous structure), and the second factor is related to the mechanism of electron transport between adjacent crystals (neck control). The receptor function is related to the ability of the oxide surface to interact with target gases (metal additives and decoration). Over the past decade, metal oxide nanostructures such as nanoparticles, nanowires, nanotubes, and nanofibers with metal catalysts have significantly improved the three basic factors, leading to high-performance gas sensors [14]. However, since these nanostructures are commonly synthesized using wet chemical methods or transfer methods, developing versatile and reproducible fabrication processes has remained a challenge [15].

An alternative method for achieving highly sensitive metal oxide gas sensors is a soft-template method consisting of polymer microspheres, which is effective in fabricating long-range ordered submicron hollow structures of various metal oxides [16,17]. In spite of their potential advantages, no review focused on the gas- sensing properties of metal oxide nanostructures fabricated by the soft-template method has been published to the best of the

1

Center for Electronic Materials, Korea Institute of Science and Technology (KIST), 39-1, Hawolgok-Dong, Seongbuk-Gu, Seoul, 02791, Korea

2

Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University

Gwanangno, Gwanak-gu, Seoul 151-744, Korea

+

Corresponding author: [email protected], [email protected] (Received : May 21, 2016, Accepted : May 31, 2016)

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/

licenses/bync/3.0) which permits unrestricted non-commercial use, distribution,

and reproduction in any medium, provided the original work is properly cited.

author’s knowledge. In this paper, the authors present an overview of past developments to understand the current progress in gas- sensing using hollow structures by a soft-template method. This review focuses on the preparation and principal parameters to enhance the gas-sensing properties of hollow structures.

2. STRATEGY TO ENHANCE GAS-SENSING PROPERTIES OF HOLLOW STRUCTURES

2.1 Preparation of Soft Template for Metal Oxide Hollow Structures

Fig. 1 shows the general fabrication procedure for metal oxide hollow structures using a soft-template method. An aqueous solution containing poly(methylmethacrylate) (PMMA) and polystyrene beads is commonly used to prepare close-packed monolayer bead templates to fabricate hollow structures. Before spin-coating the suspension on a SiO

/Si substrate, a bare substrate was treated with O

plasma to make the surface of the substrate hydrophilic, which led to a bead template in a large area without pores, agglomerates, or multilayer regions. Then, metal oxide was deposited onto the soft templates. Finally, all samples were annealed to remove the templates. During the annealing, the metal oxide film crystallizes, resulting in metal oxide hollow structures [21].

These metal oxide hollow structures can be modified by changing the size and shape of the soft templates, resulting in enhancement of gas-sensing properties. Moon et al. prepared soft templates using different sizes of polymer spheres and made TiO

hollow hemispheres [18]. The use of soft templates with smaller spheres (300 nm in diameter) leads to close-packed TiO

hollow hemispheres with very large-scale uniformity and a more pronounced improvement in CO sensing compared with the use of

larger spheres (1 µm in diameter), as shown in Fig. 2. Further, when the soft-template-composed monolayer PS beads are exposed to O

plasma, the surface regions of the beads are etched by forming H

O, CO, and CO

gaseous by-products [19]. The etching rate was not isotropic over the beads. The top surface experienced the highest etch rate, and the bottom surface was not etched. The etch rate near the links was less than the etch rates on the perimeter side regions. Therefore, the links between the beads Fig. 2. Typical response curves of plain and nanostructured TiO

thin-film gas sensors to 50-ppm and 500-ppm CO gases at 300ºC. The resistance was measured at a dc bias of 1 V (adapted from [18]).

Fig. 3. Response curves and SEM images of embossed TiO

films to 500 ppm CO or C

H

OH as a function of O

plasma etching time (adapted from [19]).

Fig. 1. Schematic of the fabrication procedure for metal oxide hollow

structures.

became elongated with narrow nanolinks during 2 min of O

plasma etching.

Approximately 100 nm of TiO

film was deposited onto the plasma-treated polystyrene bead templates and then annealed.

Upon exposure to CO and C

H

OH, the nanolinked TiO

hollow hemispheres exhibited the highest response compared with other samples (Fig. 3). Moon et al. showed that the surface-to-volume ratio of an ensemble material composed of individual micro-/

nanostructures plays a critical role in modulating the sensing properties of the material.

2.2 Metal Decoration on Metal Oxide Hollow Structures

The present author and coworkers prepared WO

nanoigloos to improve the gas-sensing properties using a soft-template method and O

plasma treatment [19]. Ag, Pd, and Au nanoparticles are decorated on WO

nanoigloos by self-agglomeration during the annealing. The surface of the WO

nanoigloos, in which the utility factor and transducer function have been maximized by the porous thin-film structures, is decorated with Pd, Ag, and Au nanoparticles to enhance the receptor function. As previously mentioned, the O

plasma treatment causes modifications in the sphere size and the connections with neighbors. Fig. 4 shows a cross-sectional and high-magnification TEM image of the Pd-, Ag-, and Au-decorated WO

nanoigloos. When the nanoigloos were cut, some were cut in the middle and some were not, leading to different sizes in the cross-sectional views. Fig. 3(a–c) show that the inner sides of WO

nanoigloos are hollow, and polystyrene beads are completely removed by the calcination. The WO

poly-crystalline film was porous, which reveals that the WO

nanoigloos are crescent-shaped and that Pd, Ag, and Au film formed spherical nanoparticles.

Fig. 5 shows the response curves of the Pd, Ag, and Au nanoparticles on WO

nanoigloos for various gases. Although it is well known that WO

exhibits its highest response to NO

at low temperature (100–200 ºC), Pd-decorated WO

nanoigloos exhibit their highest response when H

is detected. For Ag- and Au- decorated WO

nanoigloos, the responses to NO

are much higher than those for bare WO

nanoigloos. However, metal nanoparticles are formed only on the outer (top) surface of metal oxide hollow structures. Therefore, both the outer and inner surface could not be utilized for metal decoration to further enhance the gas-sensing properties. Hence, a new method is necessary for enabling better gas-sensing properties.

More recently, to overcome the aforementioned limitations, the

present author and coworkers presented an effective strategy for utilizing metal decoration on both the inner and outer surfaces of hollow structures [22]. The fabrication procedure of a both-side Au-decorated close-packed SnO

nanodome array is shown in Fig.

6. The position of the Au decoration for SnO

nanodome arrays was controlled by changing the deposition sequence of the Au and SnO

films. For an outside Au-decorated SnO

nanodome array, a Fig. 4. Cross-sectional TEM image of (a) Au, (b) Pd, and (c) Ag-dec- orated WO

nanoigloos. High-magnification TEM image of (a) Au, (b) Pd, and (c) Ag nanoparticles on WO

nanoigloos (adapted from [20]).

Fig. 5. Response curves of (a) bare WO

nanoigloos, (b) WO

nanoi-

gloos with Ag, (c) WO

nanoigloos with Pd, and (d) WO

nanoigloos with Au to 50-ppm C

H

OH, CH

COCH

, H

,

C

H

, CO, CH

, and 5-ppm NO

at 200 ºC (adapted from

[20]).

SnO

film was first deposited onto the polystyrene beads, and then an Au film was deposited on the SnO

-coated polystyrene beads, as shown in Fig. 6(b).

Conversely, for the inside Au-decorated SnO

nanodome arrays, Au film was first deposited onto the polystyrene nanobeads, and a SnO

film was sequentially deposited, as shown in Fig. 6(c). The Au–SnO

adhesion is much stronger than the Au–polystyrene adhesion, which is the key to achieving Au decoration on the inner surface of SnO

nanodome arrays. When the polystyrene nanobeads were removed during the calcination, the Au film was attached to the inner surface of SnO

nanodomes and simultaneously agglomerated to Au nanoparticles. For a both-side decorated SnO

nanodome array, Au films were deposited before and after the deposition of the SnO

film, as shown in Fig. 6(d).

All samples exhibit different baseline resistances in ambient air, as shown in Fig. 7. In particular, both-side Au-decorated SnO

nanodomes exhibited the highest baseline resistance because large-barrier-height Schottky junctions were formed between the Au nanoparticles and SnO

. Upon exposure to various gases, the responses of the both-side Au-decorated SnO

nanodome arrays to 50-ppm C

H

OH, H

, CO, CH

, C

H

, and CH

COCH

at 300 ºC are 486.8, 13.45, 3.66, 1.55, 63.74, and 79.3, respectively, while responses of the bare SnO

nanodome arrays are 26.9, 1.46, 0.72, 0.25, 7.17, and 15.62, respectively.

2.3 Gas-Sensing Mechanism of Metal-Decorated Hollow Structures

It is apparent that Au decoration enhances the gas-sensing properties of SnO

nanodome arrays owing to two sensitizations such as electronic and chemical sensitizations [22]. The underlying mechanism for the enhancement of response by both- side Au decoration is described with schematics in Fig. 7. The modulation of barrier potentials in links between individual SnO

nanodomes upon exposure to a target gas determines the response of the sample. As illustrated in Fig. 8(a)–(d), the both-side Au- decorated sample shows the highest barrier potential. Upon exposure to ambient air, the adsorbed oxygen ions form an electron-depleted region on the surface of the SnO

. The effective depth of the enhanced electron-depleted region in the bare SnO

nanodome is shallower than in the inside, outside, and both-side Au-decorated SnO

nanodome arrays since Au–SnO

Schottky junctions thicken the depletion layers.

When the SnO

nanodome arrays were exposed to air, the outer surface was more electron-depleted owing to oxygen adsorption.

Fig. 6. Fabrication procedures for (a) monolayer of polystyrene beads, (b) outside, (c) inside, and (d) both-side Au-decorated SnO

nanodome arrays (adapted from [22]).

Fig. 7. Response curves of (a) bare SnO

nanodome arrays and (b) both-side Au-decorated SnO

nanodome arrays to 50-ppm C

H

OH, H

, CO, CH

, C

H

, and CH

COCH

. (c) Polar plot of response ratio (S

/ S

) between bare SnO

nanodome arrays and both-sides Au-decorated SnO

nanodome arrays.

(d) Responses and (e) response ratios (adapted from [22]).

Fig. 8. Schematic drawings of major current path between Pt elec-

trodes for (a) bare SnO

nanodomes, (b) inside, (c) outside

and (d) both-side Au-decorated SnO

nanodomes. Arrows

indicate current flow, and size of each arrow denotes mag-

nitude of current. Conduction band edges correspond to

potential barriers in air (adapted from [22]).

Gas diffusion to the inner surface of the nanodomes hardly occurs;

this was already shown in our previous study [21]. However, the insides of the SnO

nanodomes could be more electron depleted by Au decoration for the inside Au-decorated SnO

nanodome arrays. Although the outside Au-decorated sample has a thicker depletion layer on the outer surface, the total thickness of the depletion layers on both the outer and inner surfaces would be larger in the inside Au-decorated sample, resulting in a higher response. In this respect, the total thickness of the depletion layers on both the outer and inner surfaces should be the largest in the both-side decorated sample, undoubtedly leading to an ultrahigh response to various gases. In other words, the both-side decoration leads to the maximized electronic sensitization effect in the given geometric configuration.

3. CONCLUSIONS AND PERSPECTIVES

Although gas sensors based on metal oxide nanostructures have been widely used in many fields owing to their large surface-to- volume ratio, there are still problems with uniformity and compatibility in well-established production processes [23,24]. In this article, metal oxide hollow structures using a soft template, which is a facile and effective method to form highly ordered nanostructures, were reviewed.

The gas-sensing properties of a metal oxide hollow structure can be easily enhanced by changing the size and shape of the soft templates. In addition, surface modifications on both the outer and inner sides of metal oxide hollow structures maximize the three basic factors (receptor function, transducer function, and utility factor).

ACKNOWLEDGMENT

This work was financially supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT

& Future Planning as a Global Frontier Project.

REFERENCES

[1] M. Swan, “Sensor Mania! The Internet of things, wearable computing, objective metrics, and the quantified self 2.0”, J.

Sens. Actuator Netw., Vol. 1, pp. 217-253, 2012.

[2] D. Miorandi, S. Sicari, F. D. Pellegrini, and I. Chlamtac,

“Internet of things: Vision, applications and research chal-

lenges”, Ad Hoc Netw., Vol. 10, pp. 1497-1516, 2012.

[3] L. Atzori, A. Iera, and G. Morabito, “The Internet of Things:

A survey”, Comput. Netw., Vol. 54, pp. 2787-2805, 2010.

[4] N. Barsan, D. Koziej, and U. Weimar, “Metal oxide-based gas sensor research: How to?”, Sens. Actuators B, Vol. 121, pp. 18–35, 2007.

[5] H. Nanto, H. Sokooshi, and T. Kawai, “Aluminum-doped ZnO thin film gas sensor capable of detecting freshness of sea foods”, Sens. Actuators B, Vol. 14, pp. 715-717, 1993.

[6] N. Yamazoe, “Toward innovations of gas sensor technol- ogy”, Sens. Actuators B, Vol. 108, pp. 2-14, 2005.

[7] R. Moos, R. Muller, C. Plog, A. Knezevic, H. Leye, E.

Irion, T. Braun, K. J. Marquardt, and K. Binder, “Selective ammonia exhaust gas sensor for automotive applications”, Sens. Actuators B, Vol. 83, pp. 181-189, 2002.

[8] J. Gubbi, R. Buyya, S. Marusic, and M. Palaniswami, Future gener., Internet of Things (IoT): A vision, archi- tectural elements, and future directions”, Comput. Syst., Vol.

29, pp. 1645-1660, 2013.

[9] X. Bevenot, A. Trouillet, C. Veillas, H. Gagnaire, and M.

Clement, “Hydrogen leak detection using an optical fibre sensor for aerospace applications”, Sens. Actuators B, Vol.

67, pp. 57-67, 2000.

[10] J. M. Jeon, Y. S. Shim, S. D. Han, D. H. Kim, Y. H. Kim, C. Y. Kang, J. S. Kim, M. Kim, and H. W. Jang, “Vertically ordered SnO

nanobamboos for substantially improved detection of volatile reducing gases”, J. Mater. Chem. A, Vol. 3, pp. 17939-17945, 2015.

[11] Y. H. Kim, S. J. Kim, Y. J. Kim, Y. S. Shim, S. Y. Kim, B.

H. Hong, and H. W. Jang, “Self-activated transparent all graphene gas sensor with endurance to humidity and mechanical bending”, ACS Nano, Vol. 9, pp. 10453-10460, 2015.

[12] Y. H. Kim, K. Y. Kim, Y. R. Choi, Y.-S. Shim, J.-M. Jeon, J.-H. Lee, S. Y. Kim, S. Han, and H. W. Jang, “Ultra- sensitive reversible oxygen sensing in liquid-exfoliated MoS2 nanoparticles”, J. Mater. Chem. A, Vol. 4, pp. 6070- 6076, 2016.

[13] C. X. Wang, L. W. Yin, L. Y. Zhang, D. Xiang, and R. Gao,

“Metal oxide gas sensors: sensitivity and influencing fac- tors”, Sensors, Vol. 10, pp. 2088-2106, 2010.

[14] J. H. Lee, “Gas sensors using hierarchical and hollow oxide nanostructures:overview”, Sens. Actuators B, Vol. 140, pp.

319–336, 2009.

[15] K. J. Choi and H. W. Jang, “One-dimensional oxide nano- structures as gas-sensing materials: Review and issues”

Sensors, Vol. 10, pp. 4083-4099, 2010.

[16] X. W. Lou, L. A. Archer, and Z. C. Yang, “Hollow micro- /nanostructures: synthesis and applications”, Adv. Mater., Vol. 20, p. 3987, 2008.

[17] Y. Wan and D. Y. Zhao, “On the controllable soft-tem- plating approach to mesoporous silicates”, Chem. Rev., Vol.

107, pp. 2821-2860, 2007.

[18] H. G. Moon, H. W. Jang, J. S. Kim, H. H. Park, and S. J.

Yoon, “Size effects in the CO sensing properties of nano-

structures TiO

thin films fabricated by colloidal tem-

plaitng”, Electron. Mater. Lett., Vol. 6, No. 1, pp. 31-34,

2010.

[19] H. G. Moon, Y. S. Shim, D. S, H. H. Park, S. J. Yoon, and H. W. Jang, “Embossed TiO

thin films with tailored links between hollow hemispheres: Synthesis and gas-sensing properties”, J. Phys. Chem. C, Vol. 115, pp. 9993-9999, 2011.

[20] Y. S. Shim, L. Zhang, D. H. Kim, Y. H. Kim, Y. R. Choi, S. H. Nahm, C. Y. Kang, W. Lee, and H. W. Jang, “Highly sensitive and selective H

and NO

gas sensors based on surface-decorated WO

nanoigloos”, Sens. Actuators B, Vol.

198, pp. 294-301, 2014.

[21] Y. S. Shim, H. G. Moon, D. H. Kim, L. Zhang, S. J. Yoon, Y. S. Yoon, C. Y. Kang, and H. W. Jang, “Au-decoated WO

cross-linked nanodomes for ultrahigh sensitive and selective sensing of NO

and C

H

OH”, RSC Adv., Vol. 3, pp. 10452- 10459, 2013

[22] Y. S. Shim, D. H. Kim, H. Y. Jeong, Y. H. Kim, S. H.

Nahm, C. Y. Kang, J. S. Kim, W. Lee, and H. W. Jang, “Uti- lization of both-side metal decoration in close-packed SnO

nanodome arrays for ultrasensitive gas sensing”, Sens. Actu- ators B, Vol. 213, pp. 314-321, 2015.

[23] E. Comini, G. Faglia, G. Sberveglieri, Z. W. Pan, and Z. L.

Wang, “Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts”, Appl. Phys. Lett., Vol. 81, pp. 1869-1871, 2002.

[24] H. G. Moon, Y. S. Shim, H. Y. Jeong, M. H. Jeong, J. Y.

Jung, S. M. Han, J. K. Kim, J. S. Kim,H. H. Park, J. H. Lee,

H. L. Tuller, S. J. Yoon, and H. W. Jang, “Self-activated

ultrahigh chemosensitivity of oxide thin film nanostruc-

turesfor transparent sensors”, Sci. Rep., Vol 2, pp. 588,

2012.