Nanoscale View Article Online
Published on 17 September 2013. Downloaded by Syracuse University on 26/11/2013 21:52:25.
PAPER
View Journal | View Issue
Cite this: Nanoscale, 2013, 5, 12171
A hierarchically structured graphene foam and its potential as a large-scale strain-gauge sensor† Jun Kuang,ab Luqi Liu,*a Yun Gao,a Ding Zhou,a Zhuo Chen,a Baohang Hana and Zhong Zhang*a A hierarchically structured thermal-reduced graphene (ReG) foam with 0.5 S cm1 electrical conductivity is fabricated from a well-dispersed graphene oxide suspension via a directional freezing method followed by high-temperature thermal treatment. The as-prepared three-dimensional ReG foam has an ordered macroporous honeycomb-like structure with straight and parallel voids in the range of 30 mm to 75 mm separated by cell walls of several tens of nanometers thick. Despite its ultra-low density, the ReG foam has an excellent compression recovery along its in-plane direction. This property of the ReG foam can be attributed to its hierarchically porous structure, as demonstrated by the compression test. The excellent compression recovery and high conductivity provide the ReG foam with exceptional piezoresistive capabilities. The electrical resistance of the ReG foam shows a linearly decreasing trend with
Received 2nd July 2013 Accepted 12th September 2013
compressive strain increments of up to 60%, which cannot be observed in conventional rigid materialbased sensors and carbon nanotube-based polymer sensors. Such intriguing linear strain-responsive behavior, along with the fast response time and high thermal stability, makes the ReG foam a promising
DOI: 10.1039/c3nr03379a
candidate for strain sensing. We demonstrated that it could be used as a wearable device for real-time
www.rsc.org/nanoscale
monitoring of human health.
Introduction Graphene sheets are two-dimensional (2D) nanoscale structures with giant electron mobility, extremely high thermal conductivity, and extraordinary elasticity and stiffness.1 The production of macroscopic architectures with controlled microstructures using graphene sheets as building blocks is an important step toward practical applications. To this end, graphene-based macrostructures were fabricated in various forms, such as onedimensional graphene bers,2,3 2D graphene thin lms or sheets,4–6 and three-dimensional (3D) graphene macroscopic foams.7–10 These structures demonstrate potential applications in various areas, such as supercapacitor electrodes,8,11 transparent and exible electrodes,5,6,12 tissue scaffolds,13 catalysts,9,14,15 and gas sensors.16 Specically, 3D graphene foams have attracted considerable attention because of their large surface area, low density, porous structures, good electrical conductivity, and exibility. Over the past few years, several methods have been proposed for fabricating 3D graphene architectures based on individual graphene sheets or their chemical derivatives.7–9,14,17–19 For a
National Center for Nanoscience and Technology, Beijing 100190, P. R. China. E-mail: [email protected]; [email protected]; Fax: +86-10-6265-6765; Tel: +86-108254-5586; +86-10-8254-5587
b
University of Chinese Academy of Sciences, Beijing, 100049, China
† Electronic supplementary 10.1039/c3nr03379a
information
(ESI)
This journal is ª The Royal Society of Chemistry 2013
available.
See
DOI:
example, Chen7 utilized nickel foam as a template for growing graphene foam through chemical vapor deposition (CVD). Aer the sequent inltration by poly(dimethylsiloxane), the resulting composites had exible and stretchable properties with a high electrical conductivity of up to 10 S cm1. Unlike the templatedirected CVD method, a versatile method based on in situ chemical reduction of graphene oxide (GO) in the liquid phase was developed for preparing 3D graphene hydrogels.8,14,19 The as-prepared graphene hydrogel was electrically conductive, mechanically compressible, and exhibited a relatively high specic capacitance. However, aer further removal of water encapsulated inside the hydrogel, the resulting graphene aerogel did not retain appreciable compressibility and collapsed under compression. Alternatively, ice-template directional freezing is a generic, simple, and novel route for preparing 3D monolithic structures.10,17,20–22 For instance, 3D graphene-based polymer macroporous hybrid materials were obtained when an aqueous dispersion containing a homogeneous mixture of GO nanosheets and polymers was directionally frozen accompanied by subsequent freeze drying.10,17 In contrast to the fragile feature of the graphene aerogel prepared by the hydrothermal method, the as-prepared GO/polymer foams showed excellent exibility and robustness in the presence of a polymer phase. Given their high porosity, exibility, mechanical robustness, and electrical conductivity, synthetic macroscopic porous architectures based on carbon nanomaterials, such as carbon nanotube (CNT) aerogels,23–25 CNT forests,26 graphene foams,18 and
Nanoscale, 2013, 5, 12171–12177 | 12171
View Article Online
Published on 17 September 2013. Downloaded by Syracuse University on 26/11/2013 21:52:25.
Nanoscale carbon-based hybrid sponges,27,28 have recently shown potential as sensors and actuators in the smart material eld.23,24 For instance, owing to their mechanical robustness and good electrical conductivity, CNT sponges could act as compressible strain sensors. The resistivity of CNT sponges showed a nonlinear reversible change that decreased by approximately 20% under a compression strain of up to 60%.23 A similar resistivity change was also observed in the aligned CNT forests under compression.26 In addition, a novel hybrid sponge composed of silver nanowire–carbon based architectures also shows a similar behavior under compression.28 However, the resistivity change of such sensors occurs only at a relatively small strain level (i.e.,
Published on 17 September 2013. Downloaded by Syracuse University on 26/11/2013 21:52:25.
PAPER
View Journal | View Issue
Cite this: Nanoscale, 2013, 5, 12171
A hierarchically structured graphene foam and its potential as a large-scale strain-gauge sensor† Jun Kuang,ab Luqi Liu,*a Yun Gao,a Ding Zhou,a Zhuo Chen,a Baohang Hana and Zhong Zhang*a A hierarchically structured thermal-reduced graphene (ReG) foam with 0.5 S cm1 electrical conductivity is fabricated from a well-dispersed graphene oxide suspension via a directional freezing method followed by high-temperature thermal treatment. The as-prepared three-dimensional ReG foam has an ordered macroporous honeycomb-like structure with straight and parallel voids in the range of 30 mm to 75 mm separated by cell walls of several tens of nanometers thick. Despite its ultra-low density, the ReG foam has an excellent compression recovery along its in-plane direction. This property of the ReG foam can be attributed to its hierarchically porous structure, as demonstrated by the compression test. The excellent compression recovery and high conductivity provide the ReG foam with exceptional piezoresistive capabilities. The electrical resistance of the ReG foam shows a linearly decreasing trend with
Received 2nd July 2013 Accepted 12th September 2013
compressive strain increments of up to 60%, which cannot be observed in conventional rigid materialbased sensors and carbon nanotube-based polymer sensors. Such intriguing linear strain-responsive behavior, along with the fast response time and high thermal stability, makes the ReG foam a promising
DOI: 10.1039/c3nr03379a
candidate for strain sensing. We demonstrated that it could be used as a wearable device for real-time
www.rsc.org/nanoscale
monitoring of human health.
Introduction Graphene sheets are two-dimensional (2D) nanoscale structures with giant electron mobility, extremely high thermal conductivity, and extraordinary elasticity and stiffness.1 The production of macroscopic architectures with controlled microstructures using graphene sheets as building blocks is an important step toward practical applications. To this end, graphene-based macrostructures were fabricated in various forms, such as onedimensional graphene bers,2,3 2D graphene thin lms or sheets,4–6 and three-dimensional (3D) graphene macroscopic foams.7–10 These structures demonstrate potential applications in various areas, such as supercapacitor electrodes,8,11 transparent and exible electrodes,5,6,12 tissue scaffolds,13 catalysts,9,14,15 and gas sensors.16 Specically, 3D graphene foams have attracted considerable attention because of their large surface area, low density, porous structures, good electrical conductivity, and exibility. Over the past few years, several methods have been proposed for fabricating 3D graphene architectures based on individual graphene sheets or their chemical derivatives.7–9,14,17–19 For a
National Center for Nanoscience and Technology, Beijing 100190, P. R. China. E-mail: [email protected]; [email protected]; Fax: +86-10-6265-6765; Tel: +86-108254-5586; +86-10-8254-5587
b
University of Chinese Academy of Sciences, Beijing, 100049, China
† Electronic supplementary 10.1039/c3nr03379a
information
(ESI)
This journal is ª The Royal Society of Chemistry 2013
available.
See
DOI:
example, Chen7 utilized nickel foam as a template for growing graphene foam through chemical vapor deposition (CVD). Aer the sequent inltration by poly(dimethylsiloxane), the resulting composites had exible and stretchable properties with a high electrical conductivity of up to 10 S cm1. Unlike the templatedirected CVD method, a versatile method based on in situ chemical reduction of graphene oxide (GO) in the liquid phase was developed for preparing 3D graphene hydrogels.8,14,19 The as-prepared graphene hydrogel was electrically conductive, mechanically compressible, and exhibited a relatively high specic capacitance. However, aer further removal of water encapsulated inside the hydrogel, the resulting graphene aerogel did not retain appreciable compressibility and collapsed under compression. Alternatively, ice-template directional freezing is a generic, simple, and novel route for preparing 3D monolithic structures.10,17,20–22 For instance, 3D graphene-based polymer macroporous hybrid materials were obtained when an aqueous dispersion containing a homogeneous mixture of GO nanosheets and polymers was directionally frozen accompanied by subsequent freeze drying.10,17 In contrast to the fragile feature of the graphene aerogel prepared by the hydrothermal method, the as-prepared GO/polymer foams showed excellent exibility and robustness in the presence of a polymer phase. Given their high porosity, exibility, mechanical robustness, and electrical conductivity, synthetic macroscopic porous architectures based on carbon nanomaterials, such as carbon nanotube (CNT) aerogels,23–25 CNT forests,26 graphene foams,18 and
Nanoscale, 2013, 5, 12171–12177 | 12171
View Article Online
Published on 17 September 2013. Downloaded by Syracuse University on 26/11/2013 21:52:25.
Nanoscale carbon-based hybrid sponges,27,28 have recently shown potential as sensors and actuators in the smart material eld.23,24 For instance, owing to their mechanical robustness and good electrical conductivity, CNT sponges could act as compressible strain sensors. The resistivity of CNT sponges showed a nonlinear reversible change that decreased by approximately 20% under a compression strain of up to 60%.23 A similar resistivity change was also observed in the aligned CNT forests under compression.26 In addition, a novel hybrid sponge composed of silver nanowire–carbon based architectures also shows a similar behavior under compression.28 However, the resistivity change of such sensors occurs only at a relatively small strain level (i.e.,
