Graphene-Based Chemical Sensors - The Journal of Physical

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Graphene-Based Chemical Sensors Fazel Yavari† and Nikhil Koratkar*,†,‡ †

Department of Mechanical, Aerospace and Nuclear Engineering, and ‡Department of Materials Science and Engineering, Rensselaer Polytechnic Institute 110 Eighth Street, Troy, New York 12180, United States ABSTRACT: Pioneering research in 2004 by Geim and Novoselov (2010 Nobel Prize winners in Physics) of the University of Manchester led to the isolation of a monolayer graphene sheet. Graphene is a single-atom-thick sheet of sp2 hybridized carbon atoms that are packed in a hexagonal honeycomb crystalline structure. Graphene is the fundamental building block of all sp2 carbon materials including single-walled carbon nanotubes, mutliwalled carbon nanotubes, and graphite and is therefore interesting from the fundamental standpoint as well as for practical applications. One of the most promising applications of graphene that has emerged so far is its utilization as an ultrasensitive chemical or gas sensor. In this article, we review some of the significant work performed with graphene and its derivatives for gas detection and provide a perspective on the challenges that need to be overcome to enable commercially viable graphene chemical sensor technologies.

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The majority of graphene-based gas sensor work is based on changes in their electrical conductivity due to the adsorption of gas molecules on graphene’s surface. These gas molecules act as donors or acceptors on graphene, similar to other solid-state sensors.3 Graphene can be synthesized using a variety of techniques. Mechanical exfoliation of graphite was the initial method to obtain single-layer graphene sheets; however, this method has no control over the number of layers and is almost impossible to mass produce. Epitaxial growth of graphene and chemical vapor deposition (CVD) are two of the most promising techniques for bottom-up synthesis of high-quality graphene sheets.4,5 Top-down methods such as chemical exfoliation of graphite into graphene oxide6−9 and its thermal or chemical reduction to reduced graphene oxide (rGO) can lead to rGO films that can also be utilized to sensitively detect a variety of gas species. Schedin et al.10 were the first researchers who fabricated a microscopic sensor made from graphene that is capable of detecting individual gas molecules. They show that their sensor is able to respond as soon as a gas molecule attaches to or detaches from graphene’s surface. The adsorbed molecules change the local carrier concentration in graphene, which leads to step-like changes in resistance. This ultrahigh sensitivity stems from the fact that pristine graphene is an exceptionally low-noise material. The device shows concentration-dependent changes in electrical resistivity by adsorption of gases after which the sensor is regenerated by annealing at 150 °C under vacuum. Different gases have different effects on the resistivity. The magnitudes and the sign of the change (Figure 1) indicates whether the gas is an electron acceptor (e.g., NO2, H2O, and I2) or an electron donor (e.g., CO, ethanol, and NH3). Since

raphene, a single layer of carbon atoms formed in honeycomb lattice has emerged as a material that possesses exceptional thermal, mechanical, electronic and optical properties due to its unique two-dimensional sp2bonded structure.1 Electron transport experiments on graphene have shown, among other properties, unusual carrier-densitydependent electrical conductivity and exceptionally high electron mobilities.2 These outstanding electronic properties are due to the unique band structure of graphene, which exhibits conduction and valence bands with near-linear dispersion that touch at the Brillouin zone corners to form a zero band gap semiconductor. Graphene is highly sensitive to changes in its chemical environment for several reasons: First, suspended graphene has extremely high electron mobility at room temperature, and the electron transport in graphene remains ballistic up to 0.3 μm at 300 K. Second, every carbon atom in graphene is a surface atom providing the greatest possible surface area per unit volume so that electron transport through graphene is highly sensitive to adsorbed molecular species. Third, graphene has inherently low electrical noise due to the quality of its crystal lattice and its very high electrical conductivity. These properties make graphene an ideal candidate for the ultrahigh sensitivity detection of different gases existing in various environments. High levels of sensitivity in detection processes are important for different industrial, environmental, public safety and military applications.

High sensitivity gas detection using inexpensive sensor devices are important for different industrial, environmental, public safety and military applications.

© 2012 American Chemical Society

Received: March 22, 2012 Accepted: June 14, 2012 Published: June 14, 2012 1746

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the H2 molecule in the presence of the catalytic metal. Dissociated hydrogen atoms will accumulate at the surface of Pt and diffuse into the graphene/Pt boundary. Hydrogen atoms form covalent bonds with graphene, and this hydrogenated form of graphene will have an increased work function. The separation distance increase between graphene and Pt can also cause the Fermi-level shift to become larger. Therefore, the free carrier concentrations will increase, raising the conductance of the graphene/Pt device. Chen et al.12 also showed that the electrical resistivity of monolayer CVD-grown graphene exhibits significant changes upon exposure to oxygen (O2) at room temperature. They showed that O2 can be detected at concentrations of ∼1.25% in volume ratio (see Figure 3). When O2 molecules are attached

Figure 1. Changes in resistivity, ρ, of graphene by exposure to various gases diluted to 1 ppm. Region I: device in vacuum; II: exposure to diluted chemicals; III: evacuation of the experimental setup; and IV: annealing at 150 °C. Inset shows an optical micrograph of the graphene device. [Reprinted from ref 10 with permission.]

conductivity is proportional to the product of number of charge carriers and mobility, the change in conductivity must be due to changes in the number density or mobility of carriers, or both. Their Hall-effect measurements show that extra charge carriers were created during gas adsorption on their device. That means that gas adsorption can increase the number of holes if the gas is an acceptor or increase the number of electrons if the gas is a donor. This change in the carrier concentration is the basic mechanism that governs the operation of all electrical conductivity based graphene gas sensor devices. Chu et al.11 investigated the characteristics of hydrogen detection using epitaxial graphene covered with a thin layer of platinum as a catalyst. The multilayered graphene was grown by CVD on a Si-polar 4H-SiC substrate. Graphene covered with a thin film of Pt showed reduced resistance in response to exposure to 1% hydrogen at various temperatures. Drain current change as a function of time in response to hydrogen at a constant bias of 0.05 V and 175 °C background temperature is illustrated in Figure 2. This sensor works based on splitting of

Figure 3. Oxygen detection results using CVD-grown graphene. [Reprinted from ref 12 with permission.]

to the surface of graphene, they form epoxide and carboxylic groups that are electron-withdrawing and increase hole concentration of the conduction band that generate a significant decrease in resistance. In another approach, Dua et al.13 developed a flexible and rugged chemiresistor composed of a thin film of overlapped and reduced graphene oxide platelets (rGO film), which were printed onto flexible plastic surfaces such as poly(ethylene terephthalate) (PET) using inkjet techniques. Graphene oxide has numerous pendant oxygen functional groups that render it electrically insulating and preclude its use as a conductancebased sensor. By removal of oxygen and recovery of aromatic double-bonded carbons using hydrazine hydrate vapor as a reducing agent, the conductivity can be restored up to several orders of magnitude. Even so, this process does not lead to pure graphene, and some residual oxygen groups remain even after the reduction. Therefore, rGO is a material that has both high electrical conductivity and chemically active defect sites, making it a promising candidate for gas sensing. This sensor can reversibly and selectively detect chemically aggressive vapors such as NO2, Cl2, and so forth down to concentrations ranging from 100 ppm to 500 ppb. Inkjet printing of rGO platelets is obtained using aqueous surfactant-supported dispersions of rGO powder synthesized by the reduction of exfoliated graphite oxide (GO), by using ascorbic acid (vitamin C). Electron-withdrawing vapors such as NO2 increase the conductivity of the inkjet-printed rGO/PET films sharply,

Figure 2. Current versus time measurement of the graphene device showing response to 1% hydrogen at 0.05 V constant bias. [Reprinted from ref 11 with permission.] 1747

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while applying a constant DC bias to the device (Figure 5B). The thermally reduced GO shows p-type semiconducting behavior in ambient conditions and is responsive to lowconcentration NO2 diluted in air at room temperature. The sensitivity is attributed to the electron transfer from the rGO to adsorbed NO2, which leads to enriched hole concentration and enhanced electrical conduction in the rGO sheet. In another novel approach, Li et al.15 have developed a graphene-based NO sensor fabricated by alternating current dielectrophoresis (ac-DEP) that can detect NO gas ranging from 2 to 420 ppb with response times of several hundred seconds at room temperature. This sensor device comprises sensitive channels of palladium-decorated reduced graphene oxide (Pd-rGO) connected across electrodes covered with CVD-grown graphene (Figure 6A). They observed a very high recovery time in the device and to overcome this problem, they used current annealing of the device. A moderate current of ∼1 mA for ∼1 min is applied to the device in order to accomplish this. Using this technique, a ∼1000 s recovery time is obtained at 2 ppb NO concentration (Figure 6B). Robinson et al.16 have developed a gas detection device fabricated from exfoliated GO platelets that are deposited on a substrate using high speed spin-casting to form a thin layer of large-area, wrinkle-free continuous film (Figure 7A). These networks of GO are tunably reduced to graphene by hydrazine hydrate vapor using different exposure times. The change in electrical conductivity of the networks upon exposure to trace levels of vapor is studied as a function of the chemical reduction. The level of reduction affects both the sensitivity and the level of 1/f noise. This sensor is able to detect three main classes of chemical-warfare agents and an explosive at ppb concentrations after 10 s from the exposure (Figure 7B). They demonstrated that the response curve of the device could be broken into two parts: the initial “rapid” (steep slope) and the successive “slow” (shallow slope) response. The rapid response is due to the adsorption of gas molecules onto low-energy binding sites, such as sp2-bonded carbon, and the slow response arises from molecular interactions with higher-energy binding sites, such as vacancies, structural defects, and oxygen functional groups. Adsorption on sp2-bonded carbon occurs through weak dispersive forces, while adsorption at a defect such as a carboxylic acid group, occurs through stronger singleand double-hydrogen bonding. With increasing reduction time of GO, the fast response increases while the slow response decreases. Also, they showed that the rapid response is recoverable, whereas the slow response is generally nonrecoverable without moderate heating.

which is consistent with an increase in the charge carrier density (Figure 4). Signal recovery is relatively slow when the

Figure 4. Plot of resistance versus time for inkjet-printed rGO/PET when exposed to (A) NO2 and (B) Cl2 vapor (Inset: plot for resistance versus vapor concentration). [Reprinted from ref 13 with permission.]

sensor is removed from the chamber, showing strong chemisorption of NO2 vapor on the graphene surface. However, the authors demonstrated that complete signal recovery is possible after the film is exposed to 254 nm UV light. Lu et al.14 developed a high-performance gas detection device using partially reduced graphene oxide (GO) sheets obtained through annealing at ∼300 °C in argon flow at atmospheric pressure. The device was fabricated by dispersing the GO suspension onto Au interdigitated electrodes with both finger width and interfinger spacing of about 1 μm. A few drops of the GO suspension were cast onto Au interdigitated electrodes, and a discrete network of GO sheets was left on the wafer after the solvents evaporated. A scanning electron microscopy (SEM) image of a single GO sheet bridging a pair of neighboring Au electrode fingers is shown in Figure 5A. Gases are detected by measuring the change in the current

Figure 5. (A) SEM image of a GO sheet bridging two Au fingers of an interdigitated electrode. (B) Room-temperature response of GO sensor to detection of NO2 in 25−100 ppm range. [Reprinted from ref 14 with permission.] 1748

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Figure 6. (A) Pd-rGO nitrogen oxide sensor. (B) The response of the sensor to different concentrations of NO. To decrease the recovery time, ultrapure Ar (down arrow) and current annealing are used to desorb the NO absorbed on the device. [Reprinted from ref 15 with permission.]

Figure 7. (A) Optical image of electrically isolated GO device with interdigitated metal contacts. (B) Response of an rGO sensor to different concentration of hydrogen cyanide (HCN) as a chemical-warfare agent. [Reprinted from ref 16 with permission.]

Figure 8. (A) SEM image of CNWs. (B) Room-temperature sensing response for NO2 (100 ppm) and NH3 (1%). [Reprinted from ref 17 with permission.]

patterned synthesis of vertical graphene nanosheets using plasma-enhanced CVD to develop a field effect transistor (FET) sensor that is able to detect low-concentrations of gases. An atmospheric glow discharge method is used to grow vertically oriented few-layered graphene sheets with typical lateral dimensions of several micrometers (Figure 8A). The CNW also behaves like a p-type semiconductor, similar to graphene when exposed to an ambient environment. Upon exposure to 1% (10 000 ppm) of NH3, the sensor voltage (i.e., the resistance) increased, and upon the introduction of 100 ppm of NO2, the resistance of the sensor decreased, as shown in Figure 8B. However, similar to most other carbon nanotubeand graphene-based gas sensors, slow recovery was observed for CNW sensors. The thermal energy at room temperature is typically not enough to overcome the activation energy needed

Although individual graphene sheets are exquisitely sensitive to the chemical environment and offer ultrahigh sensitivity for gas-sensing, the fabrication and operation of devices that use individual graphene sheets for sensing can be complex, expensive, and can suffer from poor reliability due to contamination and large variability from sample-to-sample. Moreover, sensors based on rGO require the conversion of graphite into GO using strong oxidants, and then successive reduction of the dispersed GO into rGO using strong reducing agents (e.g., hydrazine), which can be time-consuming and expensive. Also, as mentioned earlier, the response time of rGO sensors specially during the desorption process is excessively long. In an attempt to overcome some of these limitations, Yu et al.17 have developed a senor based on vertically aligned graphene sheets or carbon nanowalls (CNWs). They used 1749

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sheets of graphene (Figure 10A,B). The graphene films in GF grow uniformly on the entire surface of the nickel foam scaffold and hence they are interconnected into each other and there is no interface or physical breaks in the network. This facilitates efficient electron transport though the GF. Moreover, since the GF is a free-standing network, there are no substrate effects that limit its performance. The sensor operates by measuring changes in electrical conductance of the foam due to gas adsorption/desorption. The device is highly sensitive since the walls of the foam are comprised of free-standing few-layer graphene sheets. The GF is capable of detecting ∼20 ppm of NH3 and NO2 at room-temperature (Figure 10C,D). Further, the foam is a mechanically robust and flexible macro-network that is easy to contact (without lithography) and manipulate. Moreover, Joule-heating expels chemi-sorbed NH3 and NO2 molecules from the foam’s surface, leading to fully reversible operation. In addition to graphene, GO, and GFs, there have been some recent reports22,23 on the use of graphene nanoribbons (GNRs) for gas sensing applications. GNRs are thin elongated strips of sp2 bonded carbon atoms that can be obtained by the oxidative unzipping of carbon nanotubes.24 In one such study,23 Pdfunctionalized GNR was shown to be highly effective at hydrogen detection. Graphene-based chemical sensors offer the possibility of ultrahigh sensitivity detection of a range of gas species in mixtures with air at room temperature and atmospheric pressure. Before the advent of graphene, there had been a large body of work on carbon nanotube-based chemical sensors.25−28 In general, graphene offers some important advantages compared to carbon nanotubes. First, a freestanding or suspended graphene sheet has both of its sides exposed to the chemical environment, thereby maximizing its sensitivity. For multiwalled nanotubes, the inner cylinders are shielded from the chemical environment. Even for single-walled nanotubes, the ends may be closed (e.g., for tubes grown by CVD), or the metal contact pads might cap the tubes and prevent the inside of the tube from participating in gas adsorption. Second, graphene exhibits inherently low electrical noise10 at room temperature, which arises from its unique twodimensional crystal lattice and high electron mobility. For these reasons, the sensitivity of graphene-based devices for molecular sensing is, in general, superior to that of carbon nanotubes; in fact Geim and co-workers have demonstrated that even the adsorption of single molecules10 could be detected using graphene. Graphene based sensors also have other practical advantages over their carbon nanotube counterparts. For instance, graphene is relatively easy to contact electrically and manipulate in comparison to carbon nanotubes due to its micrometer-scale in-plane dimensions. As a consequence, graphene devices may be more cost-effecttive and scalable from the point of view of mass production.

for molecular desorption, even for high-quality (defect-free) CNWs. Consequently, high-temperature desorption is necessary for full recovery, which increases the power consumption and device complexity. In another novel approach, Yi et al.18 used a ZnO nanorod /graphene hybrid architecture for 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. This device is composed mainly of a bottom ZnO conductive layer on metal foil, vertically aligned ZnO nanorod channel, and a graphene-based top conductive electrode (Figure 9). In yet

Figure 9. (A) Continuous changes in conductance of the ZnO nanorods/graphene hybrid sensor at different concentrations of ethanol gas at 300 °C. (B) Plot of sensitivity versus ethanol concentration. Schematic in the inset illustrates the ZnO nanorods/ graphene hybrid architecture. [Reprinted from ref 18 with permission.]

another approach, our group in collaboration with Prof. HuiMing Cheng’s group at the Shenyang National Laboratory for Materials Science in China has developed a gas sensor19 based on a macroscopic three-dimensional (3D) network of graphene sheets called graphene foam (GF). To fabricate the GF, a scaffold of porous nickel foam is used as a template for the deposition of graphene. Similar to the CVD process used in fabrication of single-layer graphene sheets on metal substrates, carbon atoms are deposited on the nickel foam using CH4 decomposition at ∼1000 °C under ambient pressure.20,21 The nickel scaffold is then removed using chemical etching. To maintain the integrity of the foam during the etching of the Ni and to prevent it from collapsing, a thin layer of poly(methyl methacrylate) (PMMA) is also deposited on the surface of the graphene formed on the nickel foam.21 In the final step, the PMMA layer is dissolved by hot acetone; what remains is a continuous 3D network of graphene formed as a free-standing macroscopic structure with extremely thin interconnected

Pristine graphene and its derivatives such as rGO have proven to be remarkably effective in sensing trace amounts of gas species in mixtures with air Graphene sensors also offer some distinct advantages with respect to commercially available sensor technologies29,30 such 1750

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Figure 10. (A) Photograph and (B) SEM image of the microporous GF structure showing a continuous network of 3D interconnected graphene sheets. (C,D) Normalized resistance change versus time for different concentrations of NH3 and NO2 in air. Joule heating to ∼400 K is used during desorption. [Reprinted from ref 19 with permission.]

concentrations to yield the same net change in conductance. For example, a mixture that contains an oxidizing agent such as NO2 and a reducing agent such as NH3 may not create any net change in the graphene’s conductivity. Future research needs to focus on

as catalytic sensors, metal oxide sensors, electro-chemical sensors, infrared and optical sensors. Catalytic sensors are inexpensive but lack sensitivity. Infrared and optical sensors show high sensitivity but are complex and expensive. Metal oxide and electro-chemical sensors are widely used in industry for chemical sensing. However, their sensitivity is moderate: typical values range from ∼30 to 50 ppm. Moreover they tend to be expensive, bulky, and heavy. Another major limitation of metal oxide sensors in particular is that they require elevated temperatures of 150 to 600 °C for adequate sensitivity.30 Since internal heating is necessary, this drives up the power consumption, complexity, and price of the sensor device. The graphene sensor on the other hand can provide ppm level detection capability for a range of gases at room temperature. Note that even for the graphene device, internal heating may be necessary for gas desorption (i.e., device cleaning) but not during the detection stage itself. Future Challenges. While pristine graphene and its derivatives such as rGO have proven to be remarkably effective in sensing trace amounts of various gas species in mixtures with air at room temperature and atmospheric pressure, there are a number of technical challenges that need to be overcome in order to enable practical applications: • Specificity: Graphene is exquisitely sensitive to the chemical environment and hence is easily affected by a range of different gas species and mixtures. Therefore, definitive identification of contaminants is challenging. For example, exposure to NO2 causes an increase in graphene conductance. However, a similar increase in conductance could also be generated by exposure to a different oxidizing agent such as O2. Moreover it is possible to combine different gases in different

Future research needs to focus on functionalizing the graphene with capture agents that will enable the specific binding of target gases to the graphene surface. functionalizing the graphene with capture agents that will enable the specific binding of target gases to the graphene surface. Another approach may be to include multiple transduction mechanisms beyond simply electrical transduction. For example, optical, gravimetric (changes in graphene vibration frequencies), or ionization-based approaches (as have been successfully used with carbon nanotubes28) may provide additional information that can collectively be used to pinpoint the identity of the gas specie being detected and lower the rate of false alarms. • Reversibility: The thermal energy at room temperature is typically not enough to overcome the activation energy needed for molecular desorption for gases such as NO2 and NH3 on graphene surfaces. This necessitates hightemperature desorption in an inert Ar or vacuum environment to clean the graphene surface and expel the chemisorbed molecules. Such treatment precludes the reversible operation of graphene devices in the field 1751

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(2) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491− 495. (3) Moseley, P. T. Solid State Gas Sensors. Meas. Sci. Technol. 1997, 8, 223−237. (4) Srivastava, A; Galande, C; Ci, L.; Song, L.; Rai, C; Jariwala, D.; Kelly, K. F.; Ajayan, P. M. Novel Liquid Precursor-Based Facile Synthesis of Large-Area Continuous, Single, and Few-Layer Graphene Films. Chem. Mater. 2010, 22, 3457−3461. (5) Gilje, S.; Han, S.; Wang, M.; Wang, K. L.; Kaner, R. B. A Chemical Route to Graphene for Device Applications. Nano Lett. 2007, 7, 3394−3398. (6) Soldano, C.; Mahmood, A.; Dujardin, E. Production, Properties and Potential of Graphene. Carbon 2010, 48, 2127−2150. (7) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; et al. Single Sheet Functionalized Graphene by Oxidation and Thermal Expansion of Graphite. Chem. Mater. 2007, 19, 4396−4404. (8) Rafiee, M.; Rafiee, J.; Yu, Z.-Z.; Koratkar, N. Superhydrophobic to Superhydrophilic Wetting Control in Graphene Films. Adv. Mater. 2010, 22, 2151−2154. (9) Park, S.; Ruoff, R. S. Chemical Methods for the Production of Graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (10) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652−655. (11) Chu, B. H.; Lo, C. F.; Nicolosi, J.; Chang, C. Y.; Chen, V.; Strupinski, W.; Pearton, S. J.; Ren, F. Hydrogen Detection Using Platinum Coated Graphene Grown on SiC. Sens. Actuators B 2011, 157, 500−503. (12) Chen, C. W.; Hung, S. C.; Yang, M. D.; Yeh, C. W.; Wu, C. H.; Chi, G. C.; Ren, F.; Pearton, S. J. Oxygen Sensors Made by Monolayer Graphene under Room Temperature. Appl. Phys. Lett. 2011, 99, 243502. (13) Dua, V.; Surwade, S. P.; Ammu, S.; Agnihotra, S. R.; Jain, S.; Roberts, K. E.; Park, S.; Ruoff, R. S.; Manohar, S. K. All-Organic Vapor Sensor Using Inkjet-Printed Reduced Graphene Oxide. Angew. Chem., Int. Ed. 2010, 49, 2154−2157. (14) Lu, G.; Ocola, L. E.; Chen, J. Gas Detection Using LowTemperature Reduced Graphene Oxide Sheets. Appl. Phys. Lett. 2009, 94, 083111. (15) Li, W.; Geng, X.; Guo, Y.; Rong, J.; Gong, Y.; Wu, L.; Zhang, X.; Li, P.; Xu, J.; Cheng, G.; et al. Reduced Graphene Oxide Electrically Contacted Graphene Sensor for Highly Sensitive Nitric Oxide Detection. ACS Nano 2011, 5, 6955−6961. (16) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E. Reduced Graphene Oxide Molecular Sensors. Nano Lett. 2008, 8, 3137−3140. (17) Yu, K.; Wang, P.; Lu, G.; Chen, K.-H.; Bo, Z.; Chen, J. Patterning Vertically Oriented Graphene Sheets for Nanodevice Applications. J. Phys. Chem. Lett. 2011, 2, 537−542. (18) Yi, J.; Lee, J. M.; Il Park, W. Vertically Aligned ZnO Nanorods and Graphene Hybrid Architectures for Highly-Sensitive Flexible Gas Sensors. Sens. Actuators B 2011, 155, 264−269. (19) Yavari, F.; Chen, Z.; Thomas, A. V.; Ren, W.; Cheng, H.-M.; Koratkar, N. High Sensitivity Gas Detection Using a Macroscopic Three-Dimensional Graphene Foam Network. Sci. Rep. 2011, 1, 166. (20) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (21) Chen, Z. P.; Ren, W. C.; Gao, L. B.; Liu, B. L.; Pei, S. F.; Cheng, H. M. Three-Dimensional Flexible and Conductive Interconnected Graphene Networks Grown by Chemical Vapour Deposition. Nat. Mater. 2011, 10, 424−428. (22) Huang, B.; Li, Z.; Liu, Z.; Zhou, G.; Hao, S.; Wu, J.; Gu, B.-L.; Duan, W. Adsorption of Gas Molecules on Graphene Nanoribbons

where such high-temperature annealing treatment may not be feasible. Overcoming this challenge will require innovative methods such as functionalizing graphene surfaces to control the binding energy of target molecules to the graphene surface. • Reliability: Electrical conductivity of graphene is exquisitely sensitive to changes in the environmental conditions such as moisture, temperature, residual charge build-up, or contamination, and this creates additional difficulties for reliable and repeatable sensing. One possible solution to this issue is to use arrays/films of graphene sheets or macro-GFs that may be less susceptible to extraneous factors. • Cost: To be competitive with commercial sensors, graphene-based devices must be mass producible at low cost. Unlike carbon nanotubes, which do not exist in nature, graphene sheets are already present in graphite. Therefore top-down methods such as exfoliation of GO could be used to mass produce graphene nanosheets at low cost. Such top-down options do not exist for most other categories of nanofillers. Moreover CVD synthesis of macro-GFs and roll-to-roll deposition of graphene on large area substrates by CVD could substantially lower the cost of graphene-based chemical sensors.

To be competitive with commercial sensor technologies, graphene-based sensors must be mass producible at low cost.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Fazel Yavari is a Ph.D. candidate in the Department of Mechanical, Aerospace and Nuclear Engineering at Rensselaer Polytechnic Institute in Troy, New York. His research interests are in graphene synthesis/ characterization and in understanding the interaction of various molecular species with graphene and functionalized graphene surfaces. Nikhil Koratkar is the John A. Clark and Edward T. Crossan Professor of Engineering at the Rensselaer Polytechnic Institute. He holds joint appointments in the Departments of Mechanical, Aerospace and Nuclear Engineering and Materials Science and Engineering at Rensselaer. Koratkar’s research work has focused on the synthesis, characterization, and application of nanoscale material systems. This includes graphene, graphene oxide, carbon nanotubes, fullerenes, as well as metal and silicon nanostructures produced by a variety of techniques such as exfoliation of graphite, chemical vapor deposition, and oblique angle sputter and e-beam deposition. Additional information regarding Koratkar’s research can be accessed at his homepage: http://www.rpi.edu/∼koratn.

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