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High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network

High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional SUBJECT AREAS: Graphene Foam Network SENSORS ELECTRONIC MATERIALS AND 1 2 1 2 2 1,3 DEVICES Fazel Yavari *, Zongping Chen *, Abhay V. Thomas , Wencai Ren , Hui-Ming Cheng & Nikhil Koratkar SURFACE CHEMISTRY 1 th APPLIED PHYSICS Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8 Street, Troy, New York 12180-3590, USA, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of 3 th Sciences, Shenyang 110016, China, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8 Received Street, Troy, New York 12180-3590, USA. 30 September 2011 Accepted Nanostructures are known to be exquisitely sensitive to the chemical environment and offer ultra-high 4 November 2011 sensitivity for gas-sensing. However, the fabrication and operation of devices that use individual nanostructures for sensing is complex, expensive and suffers from poor reliability due to contamination and Published large variability from sample-to-sample. By contrast, conventional solid-state and conducting-polymer 23 November 2011 sensors offer excellent reliability but suffer from reduced sensitivity at room-temperature. Here we report a macro graphene foam-like three-dimensional network which combines the best of both worlds. The walls of the foam are comprised of few-layer graphene sheets resulting in high sensitivity; we demonstrate parts-per-million level detection of NH and NO in air at room-temperature. Further, the foam is a 3 2 Correspondence and mechanically robust and flexible macro-scale network that is easy to contact (without Lithography) and can requests for materials rival the durability and affordability of traditional sensors. Moreover, Joule-heating expels chemisorbed should be addressed to molecules from the foam’s surface leading to fully-reversible and low-power operation. H.M.C. (cheng@imr. ac.cn) or N.K. 1–3 4–5 anoscale structures such as carbon nanotubes and graphene have been considered as candidates for ([email protected]) chemical sensing. Such sensors are attractive for ‘‘on-site’’ monitoring and can be used by emergency N rescue crews or first responders for monitoring gas purity, air quality, and sensing potentially dangerous 1–2 leaks. The Dai and Zettl groups first showed that the electrical conductance of semi-conducting single-walled * These authors carbon nanotubes (SWNTs) changes sensitively at room temperature on exposure to several gases due to charge contributed equally to transfer between adsorbed gas molecules and nanotubes. For example, SWNTs are found to exhibit large increase this work. in conductivity on exposure to oxygen . Similarly hole doped (p-type) SWNTs exhibit three orders of magnitude increase in conductance on exposure to 200 parts-per-million (ppm) of NO . On exposure to 1% (10,000 ppm) NH flow, the same nanotubes exhibited a two orders of magnitude decrease in conductance . Since O and NO 3 2 2 are strong oxidizing agents, they attract electrons from the p-type SWNT, thereby increasing the number of conducting holes. This hole (or p-type doping) shifts the Fermi level closer to the valence band causing increase in conductance. On the other hand, reducing agents such as NH that inject electrons into p-type SWNT reduce the number of holes, leading to reduced conductance. 4–5 Individual graphene sheets can also be used to construct ultra-sensitive gas sensor devices. Graphene exhibits an exceptional band structure which contains conduction and valence bands with quasi-linear dispersion that touch at the Brillouin zone corners to form a zero gap semiconductor. Due to these properties as well as ultra-high surface area provided by graphene, its electronic properties show strong dependence to surface absorbents, including gas molecules, that can alter the carrier density of graphene. It has been shown that graphene and reduced graphene oxide respond sensitively to the adsorption of trace amounts of different gases including NH , 6–12 NO ,H O, Cl and CO . The very high sensitivity of graphene is also due in-part to its inherently low electrical 2 2 2 4–5 noise at room temperature which arises from its unique two-dimensional crystal lattice. Despite the outstand- ing potential of graphene and carbon nanotubes as gas sensors, the path of these devices towards commercializa- tion is hindered by the complexity and cost of the fabrication process, durability/robustness of the sensor devices and large variability from sample-to-sample. For example, devices utilizing individual SWNT or graphene require 6–7, 12–16 patterning and subsequent electrode attachment using lithography which increases the cost. Since indi- vidual SWNT or graphene elements are exquisitely sensitive to the chemical environment, they are easily susceptible to extraneous factors. This can affect the repeatability and reliability of the device performance. SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 1 www.nature.com/scientificreports Besides, such devices can be fragile and difficult to repair and main- sample used in our study was ,0.5 mm. Raman spectroscopy ana- tain in the field. Here we report a mechanically robust and flexible 3D lysis of the foam (Figure 1c) indicates that the walls of the GF are macro graphene foam (GF) network that offers the durability and comprised of monolayer to few-layer graphene sheets. This is con- affordability of macro sensors coupled with the very high sensitivity sistent with the specific surface area of ,850 m /g measured for the of a nanosensor device. We recently reported the development of GF. The Raman spectra (Figure 1c) also show a strongly-suppressed such a free-standing continuous graphene-based network structure defect-related D band, which confirms the overall high quality of the for flexible electronics applications. Here we show that such an inex- graphene in the foam. pensive and simple GF device can also be utilized for chemical sens- In GF charge carriers can move rapidly with a small resistance ing with high sensitivity and reversibility. through the continuous and interconnected network of high quality CVD-grown graphene building blocks, resulting in high electrical conductivity (,10 S/cm). The operational mechanism of the GF Results sensors presented in this work is based on changes in their electrical To fabricate the GF, a scaffold of porous nickel foam is used as a conductivity, due to charge transfer induced by the adsorption of gas template for the deposition of graphene. Using chemical vapor 4–5 18 molecules acting as donors or acceptors on to the foam’s surface. depositon (CVD) , carbon atoms are deposited on the nickel foam To measure the change in conductivity of GF by adsorption of gases, using CH4 decomposition at,1000u C under ambient pressure. The a four-probe resistance measurement technique was used (see nickel scaffold is then removed using chemical etching by a hot HCl Methods). Note that no lithographic patterning or manipulation of (or FeCl ) solution. To maintain the integrity of the foam during the the sample is necessary; the flexible and robust macro-scale GF can etching of the Ni and to prevent it from collapsing, a thin layer of be used directly in the as-produced condition. Proof-of-concept gas poly(methyl methacrylate) (PMMA) is also deposited on the surface sensing experiments are performed using trace amounts of NH and of the graphene formed on the nickel foam. In the final step the NO in mixtures with air at room temperature and atmospheric PMMA layer is dissolved by hot acetone. What remains is a continu- pressure. The temperature dependence of desorption of gas species ous 3D network of graphene formed as a free-standing macroscopic is also studied using a hot plate with a temperature sensor. Heating structure with extremely thin interconnected sheets of graphene increases the reversibility of the device and also accelerates the re- (Figure 1a–b). More details of the fabrication process of GF can be 17 sponse rates. Another advantage of the robust GF device is that Joule found in our previous work. The GF resembles the interconnected heating can be conveniently used to locally heat the foam thereby 3D scaffold structure of the initial nickel foam template. The gra- achieving excellent reversibility of the sensor response. All of the data phene films in GF grow on the entire surface of the nickel foam presented in this paper are obtained using one device, but similar scaffold and hence they are interconnected into each other and there response was reproduced in three other devices. is no interface or physical breaks in the network. Importantly, gra- phene films formed on the different surfaces of the nickel foam are well separated from each other, providing more adsorption sites for Discussion gas molecules. The GF is also very porous, flexible and light-weight To demonstrate the proof-of-concept, we chose ammonia (NH )asa and has a very high specific surface area of,850 m /g determined by test gas. Ammonium nitrate present in explosives is known to gradu- standard BET nitrogen cryosorption characterization. The GF fab- ally decompose and release trace amounts of NH which if detected ricated using a CH concentration of 0.7 vol% which was used in the would indicate presence of an explosive. NH a toxic gas, is also 4 3, present study has an extremely low density of ,5 mg/cm , corres- used in a variety of industrial and medical processes and therefore ponding to a very high porosity of,99.7%. The thickness of the GF it becomes important to monitor for NH leaks. The GF device was Figure 1 | Characterization of the GF structure. (a) Photograph and (b) scanning electron micrograph of the microporous GF structure showing a continuous network of 3D interconnected graphene sheets that comprise the walls of the foam-like structure. The robust and flexible GF strips can be easily handled and manipulated. Electrical contacts to the macro-scale GF can be established without the need for lithography. (c) Typical Raman spectra obtained at various locations on the GF indicating that the foam walls are comprised of mono to few-layer graphene. SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 2 www.nature.com/scientificreports placed in an environmental chamber with electrical feed-through, and air was pumped out of the chamber to establish a high vacuum (,10 torr). Next NH gas pre-mixed with air at the appropriate concentration was released into the chamber. We exposed the GF device for ,800 seconds to ,1000 parts-per-million (ppm) of NH diluted in air at atmospheric pressure and room temperature (,300 K). The time evolution of the change in resistance due to adsorption of gas molecules is shown in Figure 2a. NH appears to cause a decrease in the number of charge carriers in the foam result- ing in an increase in resistance. Graphene under ambient conditions has been observed to display p-type behavior due to the electron withdrawing nature of adsorbed water or oxygen containing moeties which induce a holelike carrier concentration. NH is a reducing agent with a lone electron pair that can be donated to the p-type GF thereby reducing the conductance. This change in resistance normalized by the initial resistance (DR/R) is defined as the sensitiv- ity of the GF and it is,30% for the case of,1000 ppm of ammonia. The resistance of the GF reaches ,90% of its steady state value after ,500 seconds. Steady state is defined as the condition when the resistance value shows,1% change over a,60 second time interval. To study the reversibility of the device, the gas input is closed and the NH -dosed device was exposed to vacuum desorption conditions for another ,800 seconds. As Figure 2a shows the resistance of the sample does not return to the initial value even after prolonged exposure to vacuum and ,13.8% of the response is not recoverable. This shows that the desorption process is much slower than the adsorp- tion process, likely due to chemisorption of NH to the graphene surface. Purging the chamber with pure air for ,800 seconds also did not lead to full reversibility of the sensor response. To stimulate the desorption process in GF, a heater was placed under the GF device and a temperature sensor was used to measure the foam temperature. Figure 2b shows the change in resistance as a function of time when a heater is used. The GF temperature is also measured and illustrated in the graph as a function of time. As seen in this graph, the heating significantly accelerates the desorption pro- cess, by exciting the molecules vibrationally so that they enter a repulsive state . After the heater is turned off, the DR/R of the GF equilibriates close to the original value, showing good reversibility. Even though the use of an external heater helps to achieve complete desorption in a shorter time, it will make the device bulky and com- plicated. A much more elegant approach is to use the Joule heating effect to generate the heat locally within the sample. For calibration purpose, the GF sample was heated in the 300–450 K temperature range using the external heater under vacuum (,10 torr) and the reduction in resistance was measured as a function of temperature (see Supplementary Fig. S1). The GF displays a semi-conducting behavior with a negative coefficient of thermal resistivity of about ,0.00125 V/K. Next, a controlled Joule heating current (I) was applied to resistively heat the GF sample. Increasing the current also decreases the values of resistance (Supplementary Fig. S2), confirm- ing that there is a Joule heating effect. From Figures S1 and S2 (Supplementary Section), one can associate each value of current Figure 2 | GF sensor response. Change in the normalized resistance of the (I) to a temperature (T), for example, to achieve ,400 K in the GF as a function of time during adsorption of ,1000 ppm of NH and GF, a current of ,125 mA is required. In figure 2c, we show the desorption using (a) room temperature degassing and (b) high results of an experiment performed with Joule heating (I5125 mA) temperature degassing. The color bar at the top of the graph in (b) shows used to accelerate the desorption process. As seen in this graph, the the change in temperature as a function of time. (c) Corresponding adsorption process is fully reversible and the desorption time is adsorption and desorption response obtained by using Joule heating of the almost as short as the adsorption time. Note that the electrical cur- sample during the desorption step. The control current supplied to the GF rent range for sensing signals is ,1 mA, while the Joule heating to heat it to ,400 K is ,125 mA. current as previously discussed is of the order of ,100 mA. Figure 3a shows the reversibility of the sensor over several cycles with Joule heating during the desorption step. Figure 3b shows the The DR/R of the GF device decreases from ,30% for 1000 ppm to normalized change in GF resistance (DR/R) as function of time for ,5% for 20 ppm of NH which is well above the background noise different concentrations of NH ranging from 1000 ppm (0.1%) to level. Typical NH gas sensors operate in the 1 to 0.1% range of NH 3 3 3 20 ppm (0.002%). All tests are performed at atmospheric pressure concentrations in air. For example in Ref. 2, the individual SWNT and room temperature with Joule heating used during desorption. device was used to detect ,0.1% (i.e. 1000 ppm) of NH in air. Here SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 3 www.nature.com/scientificreports temperature. By contrast, the GF sensor undergoes ,30% resistance change at room temperature at an order of magnitude lower NH concentration of ,1000 ppm (Fig. 3b). Other viable NH sensor technologies such as metal oxide sensors require high temperature operation (between 300–500u C) for adequate sensitivity . These devices suffer from high power consumption due to their high working temperatures. Therefore the ability to detect gas species at room temperatures with high sensitivity and low production cost is of great practical relevance. In addition to NH we also explored the effectiveness of the GF sensor for NO detection at room temperature and atmospheric pressure. Explosives such as nitrocellulose gradually degrade and a byproduct is known to be NO gas, which provides a fingerprint for its detection. Monitoring environmental pollution effects related to combustion or automotive emissions also requires NO detection in real-time. Figure 3c shows the normalized change in the GF resist- ance (DR/R) as a function of time for different concentrations of NO ranging from 200 ppm down to 20 ppm. NO appears to act as a p-type dopant for the GF resulting in a decrease in the resistance. The DR/R of the GF device decreases from ,16% for 200 ppm to ,4% for 20 ppm of NO . As before Joule heating of the sample to,400 K using 125 mA control current was effective in achieving full revers- ibility of the sensor response. Compared to this commercial poly- pyrole conducting polymer sensors can detect,0.1% (1000 ppm) of NO by an ,10% resistance change in ,5–10 minutes at room temperature . The GF sensor shows a comparable resistance change at room temperature at an order of magnitude lower NO concen- tration (Fig. 3c). Other commercial NO detectors (e.g. metal oxide sensors such as Cd-doped SnO ) are limited by high power con- sumption requirements since they require elevated temperature 22–23 operation (.200u C) for adequate sensitivity . We have demonstrated the effectiveness of macroscopic and con- tinuous graphene foam-like 3D networks for the high sensitivity detection of NH and NO at room temperature and atmospheric 3 2 pressure. Large sheets (several tens of inches in dimensions) of GF can be synthesized using thermal CVD and then cut into tens or even hundreds of smaller foams which can be directly used in the as- produced state. The cost and complexity of sensor fabrication, main- tenance and operation are therefore likely to be lower as compared to individually deposited or cleaved graphene sheets that must be pat- terned lithographically to establish electrical contacts. By contrast, the GF device is mechanically robust, flexible and can be handled easily. Contacts to the macroscale foams can be established by simply attaching wires using silver epoxy. Importantly, the GF structures also offer high sensitivity of gas detection in the ppm range. Since the walls of the foam are comprised on average of only few-layer gra- phene films, charge carrier transport through the foam is highly sensitive to the adsorption/desorption of gas species. The very large porosity of the foam also makes it possible for gas molecules to infiltrate uniformly into the entire structure resulting in high sensi- tivity and stable operation of the device. Moreover, Joule-heating can be used to expel chemisorbed gas molecules on the GF leading to fully Figure 3 | Reversibility and sensitivity of the GF sensor for detection of reversible operation. The simple, low-cost sensors described here NH and NO . (a) Normalized change in electrical resistance of the GF as a 3 2 could be deployed for a variety of applications, such as envir- function of time for detection of,1000 ppm of NH in three cycles. With onmental monitoring, sensing in chemical processing plants, and Joule heating during the desorption step, the GF sensor exhibits a fully gas detection for counter-terrorism. reversible response. (b) Normalized resistance change vs. time for different concentrations of NH in air. The adsorption step is performed at room temperature, while Joule heating to,400 K is used during desorption. (c) Methods Corresponding normalized resistance change vs. time for different Graphene Foam (GF) Synthesis: To fabricate the GF, a scaffold of porous nickel foam is used as a template for the deposition of graphene. Chemical vapor deposition concentrations of NO in air. (CVD) is used to deposit carbon atoms on the nickel foam using CH decomposition at ,1000uC under ambient pressure. The nickel scaffold is then removed using we detect NH concentrations that are nearly two orders of mag- chemical etching by a hot HCl (or FeCl ) solution. To maintain the integrity of the foam during the etching of the Ni and to prevent it from collapsing, a thin layer of nitude lower (,20 ppm) using a robust and inexpensive GF device poly(methyl methacrylate) (PMMA) is also deposited on the surface of the graphene that is also very easy to handle and use. Commercially available formed on the nickel foam. In the final step the PMMA layer is dissolved by hot conducting polymer sensors undergo ,30% resistance change in acetone resulting in a free-standing three-dimensional graphene network structure. 5–10 minutes when exposed to ,1% (10,000 ppm) of NH at room Additional details regarding the process are provided in Reference 17. SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 4 www.nature.com/scientificreports Electrical Conductivity Measurements: The foam is cut into small pieces of 16. Pati, R., Zhang, Y., Nayak, S., Ajayan, P. M. Effect of H O adsorption on electron ,2 mm in width and ,10 mm in length and then connected to a chip carrier using transport in a carbon nanotube. Appl. Phys. Lett. 81, 2638 (2002). thin copper wires at four different points. The contacts to the wires are simply made 17. Chen, Z. P., Ren, W. C., Gao, L. B., Liu, B. L., Pei, S. F., Cheng, H. M. Three- using conductive silver epoxy (EJ-2189 two part kit from EPOXY Technology) on the dimensional flexible and conductive interconnected graphene networks grown by foam side, and wire bonding on the chip carrier side. The four leads of the chip carrier chemical vapour deposition. Nature Materials 10, 424–428 (2011). are then connected to a current source and voltmeter to conduct the four-probe 18. Li, X. S. et al. Large-area synthesis of high-quality and uniform graphene films on measurement test using LabView. The four-probe method is important in that it copper foils. Science 324, 1312–1314 (2009). eliminates the effect of contact resistances. In such a configuration, the sensor res- 19. J. Moser, A. Verdaguer, D. Jimenez, A. Barreiro, A. Bachtold, The environment of ponse depends only on the changes in resistance of the graphene sheet and spurious graphene probed by electrostatic force microscopy. Appl. Phys. Lett. 92 (2008). effects related to the role of the electrical contacts are removed. 20. Yang, F., Taggart, D. K., Penner, R. M. Joule heating a palladium nanowire sensor for accelerated response and recovery to hydrogen gas. Small 6, 1422–1429 (2010). 21. J. Miasik, A. Hooper, B. Tofield, Conducting polymer gas sensors J. Chem. Soc. 1. Collins, P., Bradley, K., Ishigami, M. & Zettl, A. Extreme oxygen sensitivity of Faraday Trans. 1 82, 1117–1126 (1986). electronic properties of carbon nanotubes. Science 287, 1801–1804 (2000). 22. Y. Shimizu, M. Egashira, Basic aspects and challenges of semiconductor gas 2. Kong, J. et al. Nanotube molecular wires as chemical sensors. Science 287, 622–625 sensors. MRS Bull. 24, 18–24 (1999). (2000). 23. G. Sberveglieri, S. Groppelli, P. Nelli, Highly sensitive and selective NO and NO x 2 3. Ong, K.-G., Zeng, K., Grimes, C. A wireless, passive carbon nanotube-based gas sensor based on Cd-doped SnO thin films. Sens. Actuators B 4, 457–461 (1991). sensor. IEEE Sensors 2, 82–88 (2002). 4. Schedin, F. et al. Detection of individual gas molecules adsorbed on graphene. Nature Materials 6, 652–655 (2007). 5. Leenaerts, O., Partoens, B., Peeters, F. M. Adsorption of H O, NH , CO, NO ,and 2 3 2 Acknowledgements NO on graphene: A first-principles study. Physical Review B 77, 125416 (2008). 6. Yavari, F. et al. Tunable bandgap in graphene by the controlled adsorption of N.K. acknowledges funding support from the Advanced Energy Consortium (AEC). H.M.C. acknowledges support from the National Science Foundation of China (Nos. water molecules. Small 6, 2535–2538 (2010). 7. Dan, Y. P., Lu, Y., Kybert, N. J., Luo, Z. T. & Johnson, A. T. C. Intrinsic response 50921004, 50872136 and 50972147) and the Chinese Academy of Sciences (No. of graphene vapor sensors. Nano Letters 9, 1472–1475 (2009). KJCX2-YW-231). 8. Wu, W. et al. Wafer-scale synthesis of graphene by chemical vapor deposition and its application in gas sensing. Sensors and Actuators B: Chemical 150, 296–300 Author contributions (2010). NK and HMC planned the experiments. ZC and WR fabricated the graphene foam. FY, 9. Fowler, J. D., Allen, M. J., Tung, V. C., Yang, Y., Kaner, R. B., Weiller, B. H. AVT and NK performed the gas sensing experiments and processing of the data. All authors Practical chemical sensors from chemically derived graphene. ACS Nano 3, contributed to the discussion of the results. HMC and NK wrote the paper. 301–306 (2009). 10. Lu, G. H. et al. Toward practical gas sensing with highly reduced graphene oxide: a new signal processing method to circumvent run-to-run and device-to-device Additional information variations. ACS Nano 5, 1154–1164 (2011). Supplementary information accompanies this paper at http://www.nature.com/ 11. Lu, G. H., Ocola, L. E., Chen, J. H. Gas detection using low-temperature reduced scientificreports graphene oxide sheets. Applied Physics Letters 94, 3, 08311 (2009). 12. Robinson, J. T., Perkins, F. K., Snow, E. S., Wei, Z. Q., Sheehan, P. E. Reduced Competing financial interests: The authors declare no competing financial interests. graphene oxide molecular sensors. Nano Letters 8, 3137–3140 (2008). License: This work is licensed under a Creative Commons 13. Zhang, Y. B., Tan, Y. W., Stormer, H. L., Kim, P. Experimental observation of the Attribution-NonCommercial-NoDerivative Works 3.0 Unported License. To view a copy quantum Hall effect and Berry’s phase in graphene. Nature 438, 201–204 (2005). of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 14. Du, X., Skachko, I., Barker, A., Andrei, E. Y. Approaching ballistic transport in How to cite this article: Yavari, F. et al. High Sensitivity Gas Detection Using a Macroscopic suspended graphene. Nature Nanotechnology 3, 491–495 (2008). Three-Dimensional Graphene Foam Network. Sci. Rep. 1, 166; DOI:10.1038/srep00166 15. Wei, B., Vajtai, R., Ajayan, P. M. Reliability and current carrying capacity of (2011). carbon nanotubes. Appl. Phys. Lett. 79, 1172 (2001). SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 5 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Scientific Reports Springer Journals

High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network

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Abstract

High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional SUBJECT AREAS: Graphene Foam Network SENSORS ELECTRONIC MATERIALS AND 1 2 1 2 2 1,3 DEVICES Fazel Yavari *, Zongping Chen *, Abhay V. Thomas , Wencai Ren , Hui-Ming Cheng & Nikhil Koratkar SURFACE CHEMISTRY 1 th APPLIED PHYSICS Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8 Street, Troy, New York 12180-3590, USA, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of 3 th Sciences, Shenyang 110016, China, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8 Received Street, Troy, New York 12180-3590, USA. 30 September 2011 Accepted Nanostructures are known to be exquisitely sensitive to the chemical environment and offer ultra-high 4 November 2011 sensitivity for gas-sensing. However, the fabrication and operation of devices that use individual nanostructures for sensing is complex, expensive and suffers from poor reliability due to contamination and Published large variability from sample-to-sample. By contrast, conventional solid-state and conducting-polymer 23 November 2011 sensors offer excellent reliability but suffer from reduced sensitivity at room-temperature. Here we report a macro graphene foam-like three-dimensional network which combines the best of both worlds. The walls of the foam are comprised of few-layer graphene sheets resulting in high sensitivity; we demonstrate parts-per-million level detection of NH and NO in air at room-temperature. Further, the foam is a 3 2 Correspondence and mechanically robust and flexible macro-scale network that is easy to contact (without Lithography) and can requests for materials rival the durability and affordability of traditional sensors. Moreover, Joule-heating expels chemisorbed should be addressed to molecules from the foam’s surface leading to fully-reversible and low-power operation. H.M.C. (cheng@imr. ac.cn) or N.K. 1–3 4–5 anoscale structures such as carbon nanotubes and graphene have been considered as candidates for ([email protected]) chemical sensing. Such sensors are attractive for ‘‘on-site’’ monitoring and can be used by emergency N rescue crews or first responders for monitoring gas purity, air quality, and sensing potentially dangerous 1–2 leaks. The Dai and Zettl groups first showed that the electrical conductance of semi-conducting single-walled * These authors carbon nanotubes (SWNTs) changes sensitively at room temperature on exposure to several gases due to charge contributed equally to transfer between adsorbed gas molecules and nanotubes. For example, SWNTs are found to exhibit large increase this work. in conductivity on exposure to oxygen . Similarly hole doped (p-type) SWNTs exhibit three orders of magnitude increase in conductance on exposure to 200 parts-per-million (ppm) of NO . On exposure to 1% (10,000 ppm) NH flow, the same nanotubes exhibited a two orders of magnitude decrease in conductance . Since O and NO 3 2 2 are strong oxidizing agents, they attract electrons from the p-type SWNT, thereby increasing the number of conducting holes. This hole (or p-type doping) shifts the Fermi level closer to the valence band causing increase in conductance. On the other hand, reducing agents such as NH that inject electrons into p-type SWNT reduce the number of holes, leading to reduced conductance. 4–5 Individual graphene sheets can also be used to construct ultra-sensitive gas sensor devices. Graphene exhibits an exceptional band structure which contains conduction and valence bands with quasi-linear dispersion that touch at the Brillouin zone corners to form a zero gap semiconductor. Due to these properties as well as ultra-high surface area provided by graphene, its electronic properties show strong dependence to surface absorbents, including gas molecules, that can alter the carrier density of graphene. It has been shown that graphene and reduced graphene oxide respond sensitively to the adsorption of trace amounts of different gases including NH , 6–12 NO ,H O, Cl and CO . The very high sensitivity of graphene is also due in-part to its inherently low electrical 2 2 2 4–5 noise at room temperature which arises from its unique two-dimensional crystal lattice. Despite the outstand- ing potential of graphene and carbon nanotubes as gas sensors, the path of these devices towards commercializa- tion is hindered by the complexity and cost of the fabrication process, durability/robustness of the sensor devices and large variability from sample-to-sample. For example, devices utilizing individual SWNT or graphene require 6–7, 12–16 patterning and subsequent electrode attachment using lithography which increases the cost. Since indi- vidual SWNT or graphene elements are exquisitely sensitive to the chemical environment, they are easily susceptible to extraneous factors. This can affect the repeatability and reliability of the device performance. SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 1 www.nature.com/scientificreports Besides, such devices can be fragile and difficult to repair and main- sample used in our study was ,0.5 mm. Raman spectroscopy ana- tain in the field. Here we report a mechanically robust and flexible 3D lysis of the foam (Figure 1c) indicates that the walls of the GF are macro graphene foam (GF) network that offers the durability and comprised of monolayer to few-layer graphene sheets. This is con- affordability of macro sensors coupled with the very high sensitivity sistent with the specific surface area of ,850 m /g measured for the of a nanosensor device. We recently reported the development of GF. The Raman spectra (Figure 1c) also show a strongly-suppressed such a free-standing continuous graphene-based network structure defect-related D band, which confirms the overall high quality of the for flexible electronics applications. Here we show that such an inex- graphene in the foam. pensive and simple GF device can also be utilized for chemical sens- In GF charge carriers can move rapidly with a small resistance ing with high sensitivity and reversibility. through the continuous and interconnected network of high quality CVD-grown graphene building blocks, resulting in high electrical conductivity (,10 S/cm). The operational mechanism of the GF Results sensors presented in this work is based on changes in their electrical To fabricate the GF, a scaffold of porous nickel foam is used as a conductivity, due to charge transfer induced by the adsorption of gas template for the deposition of graphene. Using chemical vapor 4–5 18 molecules acting as donors or acceptors on to the foam’s surface. depositon (CVD) , carbon atoms are deposited on the nickel foam To measure the change in conductivity of GF by adsorption of gases, using CH4 decomposition at,1000u C under ambient pressure. The a four-probe resistance measurement technique was used (see nickel scaffold is then removed using chemical etching by a hot HCl Methods). Note that no lithographic patterning or manipulation of (or FeCl ) solution. To maintain the integrity of the foam during the the sample is necessary; the flexible and robust macro-scale GF can etching of the Ni and to prevent it from collapsing, a thin layer of be used directly in the as-produced condition. Proof-of-concept gas poly(methyl methacrylate) (PMMA) is also deposited on the surface sensing experiments are performed using trace amounts of NH and of the graphene formed on the nickel foam. In the final step the NO in mixtures with air at room temperature and atmospheric PMMA layer is dissolved by hot acetone. What remains is a continu- pressure. The temperature dependence of desorption of gas species ous 3D network of graphene formed as a free-standing macroscopic is also studied using a hot plate with a temperature sensor. Heating structure with extremely thin interconnected sheets of graphene increases the reversibility of the device and also accelerates the re- (Figure 1a–b). More details of the fabrication process of GF can be 17 sponse rates. Another advantage of the robust GF device is that Joule found in our previous work. The GF resembles the interconnected heating can be conveniently used to locally heat the foam thereby 3D scaffold structure of the initial nickel foam template. The gra- achieving excellent reversibility of the sensor response. All of the data phene films in GF grow on the entire surface of the nickel foam presented in this paper are obtained using one device, but similar scaffold and hence they are interconnected into each other and there response was reproduced in three other devices. is no interface or physical breaks in the network. Importantly, gra- phene films formed on the different surfaces of the nickel foam are well separated from each other, providing more adsorption sites for Discussion gas molecules. The GF is also very porous, flexible and light-weight To demonstrate the proof-of-concept, we chose ammonia (NH )asa and has a very high specific surface area of,850 m /g determined by test gas. Ammonium nitrate present in explosives is known to gradu- standard BET nitrogen cryosorption characterization. The GF fab- ally decompose and release trace amounts of NH which if detected ricated using a CH concentration of 0.7 vol% which was used in the would indicate presence of an explosive. NH a toxic gas, is also 4 3, present study has an extremely low density of ,5 mg/cm , corres- used in a variety of industrial and medical processes and therefore ponding to a very high porosity of,99.7%. The thickness of the GF it becomes important to monitor for NH leaks. The GF device was Figure 1 | Characterization of the GF structure. (a) Photograph and (b) scanning electron micrograph of the microporous GF structure showing a continuous network of 3D interconnected graphene sheets that comprise the walls of the foam-like structure. The robust and flexible GF strips can be easily handled and manipulated. Electrical contacts to the macro-scale GF can be established without the need for lithography. (c) Typical Raman spectra obtained at various locations on the GF indicating that the foam walls are comprised of mono to few-layer graphene. SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 2 www.nature.com/scientificreports placed in an environmental chamber with electrical feed-through, and air was pumped out of the chamber to establish a high vacuum (,10 torr). Next NH gas pre-mixed with air at the appropriate concentration was released into the chamber. We exposed the GF device for ,800 seconds to ,1000 parts-per-million (ppm) of NH diluted in air at atmospheric pressure and room temperature (,300 K). The time evolution of the change in resistance due to adsorption of gas molecules is shown in Figure 2a. NH appears to cause a decrease in the number of charge carriers in the foam result- ing in an increase in resistance. Graphene under ambient conditions has been observed to display p-type behavior due to the electron withdrawing nature of adsorbed water or oxygen containing moeties which induce a holelike carrier concentration. NH is a reducing agent with a lone electron pair that can be donated to the p-type GF thereby reducing the conductance. This change in resistance normalized by the initial resistance (DR/R) is defined as the sensitiv- ity of the GF and it is,30% for the case of,1000 ppm of ammonia. The resistance of the GF reaches ,90% of its steady state value after ,500 seconds. Steady state is defined as the condition when the resistance value shows,1% change over a,60 second time interval. To study the reversibility of the device, the gas input is closed and the NH -dosed device was exposed to vacuum desorption conditions for another ,800 seconds. As Figure 2a shows the resistance of the sample does not return to the initial value even after prolonged exposure to vacuum and ,13.8% of the response is not recoverable. This shows that the desorption process is much slower than the adsorp- tion process, likely due to chemisorption of NH to the graphene surface. Purging the chamber with pure air for ,800 seconds also did not lead to full reversibility of the sensor response. To stimulate the desorption process in GF, a heater was placed under the GF device and a temperature sensor was used to measure the foam temperature. Figure 2b shows the change in resistance as a function of time when a heater is used. The GF temperature is also measured and illustrated in the graph as a function of time. As seen in this graph, the heating significantly accelerates the desorption pro- cess, by exciting the molecules vibrationally so that they enter a repulsive state . After the heater is turned off, the DR/R of the GF equilibriates close to the original value, showing good reversibility. Even though the use of an external heater helps to achieve complete desorption in a shorter time, it will make the device bulky and com- plicated. A much more elegant approach is to use the Joule heating effect to generate the heat locally within the sample. For calibration purpose, the GF sample was heated in the 300–450 K temperature range using the external heater under vacuum (,10 torr) and the reduction in resistance was measured as a function of temperature (see Supplementary Fig. S1). The GF displays a semi-conducting behavior with a negative coefficient of thermal resistivity of about ,0.00125 V/K. Next, a controlled Joule heating current (I) was applied to resistively heat the GF sample. Increasing the current also decreases the values of resistance (Supplementary Fig. S2), confirm- ing that there is a Joule heating effect. From Figures S1 and S2 (Supplementary Section), one can associate each value of current Figure 2 | GF sensor response. Change in the normalized resistance of the (I) to a temperature (T), for example, to achieve ,400 K in the GF as a function of time during adsorption of ,1000 ppm of NH and GF, a current of ,125 mA is required. In figure 2c, we show the desorption using (a) room temperature degassing and (b) high results of an experiment performed with Joule heating (I5125 mA) temperature degassing. The color bar at the top of the graph in (b) shows used to accelerate the desorption process. As seen in this graph, the the change in temperature as a function of time. (c) Corresponding adsorption process is fully reversible and the desorption time is adsorption and desorption response obtained by using Joule heating of the almost as short as the adsorption time. Note that the electrical cur- sample during the desorption step. The control current supplied to the GF rent range for sensing signals is ,1 mA, while the Joule heating to heat it to ,400 K is ,125 mA. current as previously discussed is of the order of ,100 mA. Figure 3a shows the reversibility of the sensor over several cycles with Joule heating during the desorption step. Figure 3b shows the The DR/R of the GF device decreases from ,30% for 1000 ppm to normalized change in GF resistance (DR/R) as function of time for ,5% for 20 ppm of NH which is well above the background noise different concentrations of NH ranging from 1000 ppm (0.1%) to level. Typical NH gas sensors operate in the 1 to 0.1% range of NH 3 3 3 20 ppm (0.002%). All tests are performed at atmospheric pressure concentrations in air. For example in Ref. 2, the individual SWNT and room temperature with Joule heating used during desorption. device was used to detect ,0.1% (i.e. 1000 ppm) of NH in air. Here SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 3 www.nature.com/scientificreports temperature. By contrast, the GF sensor undergoes ,30% resistance change at room temperature at an order of magnitude lower NH concentration of ,1000 ppm (Fig. 3b). Other viable NH sensor technologies such as metal oxide sensors require high temperature operation (between 300–500u C) for adequate sensitivity . These devices suffer from high power consumption due to their high working temperatures. Therefore the ability to detect gas species at room temperatures with high sensitivity and low production cost is of great practical relevance. In addition to NH we also explored the effectiveness of the GF sensor for NO detection at room temperature and atmospheric pressure. Explosives such as nitrocellulose gradually degrade and a byproduct is known to be NO gas, which provides a fingerprint for its detection. Monitoring environmental pollution effects related to combustion or automotive emissions also requires NO detection in real-time. Figure 3c shows the normalized change in the GF resist- ance (DR/R) as a function of time for different concentrations of NO ranging from 200 ppm down to 20 ppm. NO appears to act as a p-type dopant for the GF resulting in a decrease in the resistance. The DR/R of the GF device decreases from ,16% for 200 ppm to ,4% for 20 ppm of NO . As before Joule heating of the sample to,400 K using 125 mA control current was effective in achieving full revers- ibility of the sensor response. Compared to this commercial poly- pyrole conducting polymer sensors can detect,0.1% (1000 ppm) of NO by an ,10% resistance change in ,5–10 minutes at room temperature . The GF sensor shows a comparable resistance change at room temperature at an order of magnitude lower NO concen- tration (Fig. 3c). Other commercial NO detectors (e.g. metal oxide sensors such as Cd-doped SnO ) are limited by high power con- sumption requirements since they require elevated temperature 22–23 operation (.200u C) for adequate sensitivity . We have demonstrated the effectiveness of macroscopic and con- tinuous graphene foam-like 3D networks for the high sensitivity detection of NH and NO at room temperature and atmospheric 3 2 pressure. Large sheets (several tens of inches in dimensions) of GF can be synthesized using thermal CVD and then cut into tens or even hundreds of smaller foams which can be directly used in the as- produced state. The cost and complexity of sensor fabrication, main- tenance and operation are therefore likely to be lower as compared to individually deposited or cleaved graphene sheets that must be pat- terned lithographically to establish electrical contacts. By contrast, the GF device is mechanically robust, flexible and can be handled easily. Contacts to the macroscale foams can be established by simply attaching wires using silver epoxy. Importantly, the GF structures also offer high sensitivity of gas detection in the ppm range. Since the walls of the foam are comprised on average of only few-layer gra- phene films, charge carrier transport through the foam is highly sensitive to the adsorption/desorption of gas species. The very large porosity of the foam also makes it possible for gas molecules to infiltrate uniformly into the entire structure resulting in high sensi- tivity and stable operation of the device. Moreover, Joule-heating can be used to expel chemisorbed gas molecules on the GF leading to fully Figure 3 | Reversibility and sensitivity of the GF sensor for detection of reversible operation. The simple, low-cost sensors described here NH and NO . (a) Normalized change in electrical resistance of the GF as a 3 2 could be deployed for a variety of applications, such as envir- function of time for detection of,1000 ppm of NH in three cycles. With onmental monitoring, sensing in chemical processing plants, and Joule heating during the desorption step, the GF sensor exhibits a fully gas detection for counter-terrorism. reversible response. (b) Normalized resistance change vs. time for different concentrations of NH in air. The adsorption step is performed at room temperature, while Joule heating to,400 K is used during desorption. (c) Methods Corresponding normalized resistance change vs. time for different Graphene Foam (GF) Synthesis: To fabricate the GF, a scaffold of porous nickel foam is used as a template for the deposition of graphene. Chemical vapor deposition concentrations of NO in air. (CVD) is used to deposit carbon atoms on the nickel foam using CH decomposition at ,1000uC under ambient pressure. The nickel scaffold is then removed using we detect NH concentrations that are nearly two orders of mag- chemical etching by a hot HCl (or FeCl ) solution. To maintain the integrity of the foam during the etching of the Ni and to prevent it from collapsing, a thin layer of nitude lower (,20 ppm) using a robust and inexpensive GF device poly(methyl methacrylate) (PMMA) is also deposited on the surface of the graphene that is also very easy to handle and use. Commercially available formed on the nickel foam. In the final step the PMMA layer is dissolved by hot conducting polymer sensors undergo ,30% resistance change in acetone resulting in a free-standing three-dimensional graphene network structure. 5–10 minutes when exposed to ,1% (10,000 ppm) of NH at room Additional details regarding the process are provided in Reference 17. SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 4 www.nature.com/scientificreports Electrical Conductivity Measurements: The foam is cut into small pieces of 16. Pati, R., Zhang, Y., Nayak, S., Ajayan, P. M. Effect of H O adsorption on electron ,2 mm in width and ,10 mm in length and then connected to a chip carrier using transport in a carbon nanotube. Appl. Phys. Lett. 81, 2638 (2002). thin copper wires at four different points. The contacts to the wires are simply made 17. Chen, Z. P., Ren, W. C., Gao, L. B., Liu, B. L., Pei, S. F., Cheng, H. M. 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Gas detection using low-temperature reduced scientificreports graphene oxide sheets. Applied Physics Letters 94, 3, 08311 (2009). 12. Robinson, J. T., Perkins, F. K., Snow, E. S., Wei, Z. Q., Sheehan, P. E. Reduced Competing financial interests: The authors declare no competing financial interests. graphene oxide molecular sensors. Nano Letters 8, 3137–3140 (2008). License: This work is licensed under a Creative Commons 13. Zhang, Y. B., Tan, Y. W., Stormer, H. L., Kim, P. Experimental observation of the Attribution-NonCommercial-NoDerivative Works 3.0 Unported License. To view a copy quantum Hall effect and Berry’s phase in graphene. Nature 438, 201–204 (2005). of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 14. Du, X., Skachko, I., Barker, A., Andrei, E. Y. Approaching ballistic transport in How to cite this article: Yavari, F. et al. High Sensitivity Gas Detection Using a Macroscopic suspended graphene. Nature Nanotechnology 3, 491–495 (2008). Three-Dimensional Graphene Foam Network. Sci. Rep. 1, 166; DOI:10.1038/srep00166 15. Wei, B., Vajtai, R., Ajayan, P. M. Reliability and current carrying capacity of (2011). carbon nanotubes. Appl. Phys. Lett. 79, 1172 (2001). SCIENTIFIC REPORTS | 1 : 166 | DOI: 10.1038/srep00166 5

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Published: Nov 23, 2011

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