- Email: [email protected]
Graphene as superior material for detection of volatile organic compounds
C H A P T E R
10 Graphene as superior material for detection of volatile organic compounds Shahid Pervez Ansari1, Mohammad Omaish Ansari2, Anish Khan3, 4 1
Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, India; 2Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia; 3Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia; 4Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia
1. Introduction Graphene is a much talked about and researched material at present, as in the past, and will continue to do so in future. The high place of graphene is because of its unique properties which could be utilized in most of the areas of scientific concern. In the coming discussion we will learn about the graphene and the reasons of its importance. Let us first introduce you to the word graphene itself. We all know about carbon and its various allotropes; graphite is an allotrope of carbon with a layered structure of sp2 hybrid carbon atoms, and these layers are separated by a specific distance by van der Waal forces. Due to these interlayer spaces, these also act as a solid lubricant. If these layers could be separated and used, they offer infinite possibilities. The name “graphene” was introduced in 1994 by Boehm, Setton, and Stumpp [1]. Graphene is a two-dimensional (2D) honeycomb lattice of packed sp2 hybrid carbon atoms with a carbonecarbon molecular bond length of 0.142 nm. The sp2 hybrid carbon atoms in this lattice arrangement provide p-conjugation (long range) in graphene due to which it exhibits extraordinary electrical, mechanical, and thermal properties [2]. Each carbon atom in the hexagonal lattice structure of graphene is connected to the neighboring three other carbon atoms by the overlap of sp2 orbitals to form s bond, and the fourth bond is formed by the overlap of pz orbitals which is a p bond oriented out of the plane in the z-direction. It fourth bond consists of a band of filled p orbitals, i.e., the valence band and
Graphene-Based Nanotechnologies for Energy and Environmental Applications https://doi.org/10.1016/B978-0-12-815811-1.00010-7
183
Copyright © 2019 Elsevier Inc. All rights reserved.
184
10. Graphene as superior material for detection of volatile organic compounds
a band of empty p* orbitals, i.e., the conduction band, and this band is responsible for the high conductivity to graphene [3]. The optical absorbance of graphene is 2.3% which makes it hard to directly visualize it and is thus practically transparent when present as a single sheet. The inherent specific properties of graphene, in contrast to the two-dimensional systems based on semiconductors which generally work at specific temperatures, remain same even at room temperatures and do not require high energy for activation. Graphene also shows similarity with the carbon nanotubes in relation to several important properties such as its high strength and is stronger than steel, its high stretchability and thus can act as a flexible conductor, and its exhibition of very high thermal conductivity which is even much higher than metallic silver [4]. The theoretical studies of graphene and its application for various calculations in solid state physics were presented in 1947 by P.R. Wallace. Thereafter he also predicted the electronic configuration of graphene and gave the linear dispersion relation [5]. Further work resulted in the wave equation for excitations in 1956 by J.W. McClure [6] which was later related to the Dirac equation by G.W. Semenoff in 1984 [7]. Graphene is typically defined as a material which consists of sp2 hybridized carbon atoms arranged in a sheet made of a monolayered arrangement of carbon atoms [8,9]. The IUPAC defines it as “a single carbon layer of the graphite structure, describing its nature by analogy to a polycyclic aromatic hydrocarbon of quasi infinite size” [10]. Chen et al. further suggested that apart from thickness, the linear dimensions are also important and if a sheet made of polyaromatic hydrocarbons reached beyond 100 nm in two dimensions, then it should be considered as graphene. From the abovementioned definitions, it can be seen that two important characteristics of graphene have not been incorporated in the definition, i.e., (1) its highly metallic nature owing to the absence of a band gap [1] and (2) its constituents, i.e., it should be composed of carbon and hydrogen only. But, up to 30% of oxygen content have been reported in the graphene prepared and therefore contradicts with the previous definition. Therefore, it can be said that there are a certain number of different views to define graphene, but we are far from its exact definition which includes all its properties (Fig. 10.1).
FIGURE 10.1
Suggested smallest graphene structure. [9].
III. Environment
1. Introduction
185
Several types of graphene-based materials or graphene with different functionality are known till date which are obtained during the synthesis of graphene from different precursors. These materials include graphite oxide, graphene oxide (GO), and reduced graphene oxide (rGO), etc. These may be collectively considered as graphene family and described by the presence of multiple/single layer and oxidation level of carbon atoms in these layers. These features of graphene members affect their important properties like conductivity or photoluminescence, etc. Simple classification of graphene family and its possible synthesis routes have been presented in Fig. 10.2. Top-down and bottom-up approaches are most commonly used for synthesis. The top-down approach starts with bulk precursors such as graphite or graphite oxide (Fig. 10.2). To finally get the graphene from bulk material, the bulk precursors are structurally decomposed stepwise, like GO is formed first which on reduction gives rGO and finally graphene. In a mechanical exfoliation route, graphite is decomposed by a layerby-layer method which results in graphene. While in the bottom-up approach, the materials are assembled layer by layer, like chemical vapor deposition of methane may be used to produce large areas of graphene. Geim and Novoselov (2004) made remarkable progress in the development of methods for graphene preparation. These scientists used sticky scotch tape to remove some layers/flakes from graphite and upon repeated experiments and optimization were finally able to get single-layered graphene [11]. They observed that this graphene possessed exceptionally high electrical [12], good mechanical [13], and optical properties [14], and for this later they were honored with the Nobel Prize in Physics ‘‘for groundbreaking experiments regarding the two-dimensional material graphene.’’ Hummers and Offeman (1958) [15] reported the oxidization of graphite into graphite oxide by using a combination of concentrated sulfuric acid (H2SO4), sodium nitrate (NaNO3), and potassium permanganate (KMnO4). Highly oxidative action of the above species creates anionic groups on the layers of graphite, which are carboxylates, epoxies, and hydroxylate groups, etc. The oxidation process resulted in the increase in the out-of-plane length of CeO covalent bonds which became w0.68 nm in final graphite oxide from its original length
FIGURE 10.2
Schematic representation of relation of graphene and family members.
III. Environment
186
10. Graphene as superior material for detection of volatile organic compounds
of 0.35 nm in graphite [16]. This exfoliation of the interlayer spacing and the presence of oxygen containing functional groups results in an anionic or polar behavior and thus the resulting GO becomes highly hydrophilic. This hydrophilic GO shows strong affinity to the water molecules which can easily penetrate between the graphene layers thereby giving very high dispersibility of graphite oxide in water. Due to the above reactions, the nature of some carbon atoms of graphene may change from sp2 to sp3 which disrupt the delocalization system in the graphene, thereby affecting its electrical conductivity very critically which may reach between 103 and 107Ucm depending on the oxygen concentration [17].
2. Graphene as sensor Detection of volatile organic compounds (VOCs) may be a new and quick growing frontier in speedy, extremely selective, sensitive, noninvasive analysis and in diagnosis for detection of human diseases [18]. VOC sensing based diagnostic technologies should be enforced worldwide [19] to enable monitoring in populations at high risk, besides also enabling early detection of the disease which boosts the effectiveness of medical aid [20]. Numerous VOCs are generated within the body because of the alteration in metabolic pathways, principally associated with the carbohydrate metabolism, hemoprotein P450, liver enzymes, and aerobic stress [21]. The self-generated VOCs by the body may offer reliable and extremely valuable information related to human health. There are a few completely different biologically generated VOC molecules which are discharged from exhaled breath and may function as VOC biomarkers for detection. The VOC family consists of various hydrocarbons and their derivatives like saturated/unsaturated hydrocarbons, aromatic compounds, alcohols, aldehydes, ketones, and nitriles, which are generated via numerous physicalechemicalebiological pathways [22e24]. An illness in a person could also be recognized on the idea of specific pattern of VOCs, with low probabilities of interference by alternative diseases [25,26]. Aromatic, open-chain, and chlorinated hydrocarbons, etc. are listed within the family of VOCs. These enter in our atmosphere primarily from human activities that embody industrial production of materials of human needs like adhesives, paints, building materials, printing materials, and artificial chemicals, etc. [27,28]. In recent years, VOCs are known collectively for their extraordinarily harmful category of pollutants to human health and can cause skin diseases, headache, nausea, raw throat, carcinoma, etc. [29]. Because of the intense health threats caused by the presence of those VOCs within the atmosphere, these have attracted the sincere attention of the scientific community for their economical removal and management. For this many physicalechemical technologies together with sorption, condensation, combustion, chemical process combustion, and thermal oxidization are utilized for their removal of VOCs from gaseous streams. Among these different techniques, the adsorption technique has been widely used as it is economical and can be easily applied at a massive scale [30]. As mentioned earlier, graphene is a two-dimensional single-layered structure. When it is exposed to an appropriate surface, extremely high sensitivity to external factors is observed owing to the exposure and contact of most of the atoms to the external surrounding environment. Therefore, it is often aforesaid that the exposure of the graphene atoms to the external surroundings affects its electrical characteristics even at slight changes in the external surroundings, thereby rendering it sensitive toward external chemical and electrical changes.
III. Environment
2. Graphene as sensor
187
These properties of graphene are significantly utilized to develop different types of gas sensors. Novoselov et al. [31] were the first to study the gas-sensing properties of graphene. However, there was no systematic analysis report out there on the gas-sensing characteristics of graphene till a few years later. Therein in that report, graphene was reported to exhibit the capability of a highly efficient individual gas molecule attributable to its low electronic noise [32]. However, it is very tough to succeed to a perfection level in real detection conditions despite the distinctive characteristics of graphene [33]. Exfoliated GO was used by Robinson et al. [34] to prepare an ultrathin continuous network, which was later reduced by exposure to hydrazine hydrate vapor. Thus prepared ultrathin rGO network was used to fabricate gas sensor. The main advantages of rGO for gas sensing are (1) the presence of large number of defects which act as a site for interaction, (2) some functional groups of GO, which are inevitably left in rGO even after reduction, and (3) the rGO is electron rich due to the presence of sp2 bonded structure, and this offers efficient charge transport in comparison to nonconductive GO. The rGO-based sensors can show extremely high sensitivity and selectivity and can detect explosives, chemicals, etc. in very low dosages, i.e., at parts per billion (ppb) levels. They have also been utilized to detect different types of VOCs and NO2 [35]. Graphene-based VOC sensors have attracted enormous interest in the diagnosis of diseases after the work of Schedin et al [36]. The graphene-based sensing devices show excellent response due to the high density of electrons which offer high mobility of charge carriers, low consumption of power, and compatibility with present-day and future electronics [37]. The atomic-level thickness of graphene and high surface-to-volume ratio can attain high porosity as per the method of synthesis, controllable functionalization with different reagents, and zero bandgap which are the properties that make it a favorable candidate for the fabrication of gas sensors for diverse analyte molecules. The mobility of electrons in graphene is 2.5 105 cm2 V1s1, transport is w0.3 mm at 300 K, and it also has low electrical noise which allows it to be readily used for the fabrication of highly efficient sensors with good sensitivity and selectivity. The interaction of analyte with graphene (either with defects or functional groups) results in the change in the mobility of charge transport thereby affecting conductivity, and this gives graphene high detection sensitivity in comparison to its counterparts [38]. Wang et al. [39] used composite of nonconducting polydiacetylene (PDA) with graphene to develop a handy litmus-type chemosensor for effective detection of hazardous VOCs. The highly ordered PDA monolayer in the composite was supported efficiently on the strong graphene layer. The resultant sensor responded efficiently to VOCs (w0.01%) even at minute quantities and was successfully tested for chloroform (CHCl3), dimethylformamide (DMF), methanol (CH3OH), and tetrahydrofuran (THF). The change in the color on exposure to varied concentrations of VOCs can be easily perceived by the naked eye and a logarithmic response was observed between the concentration of VOCs (from w0.01%e to 10%) and the chromatic response. The morphological changes in the PDA polymer were also observed by the scanning tunneling microscopy which suggests intrinsic mechanism of the chromatic variety at the molecular level. Ahn et al. [40] used GO for the fabrication of VOC sensors for the selective detection of formaldehyde, benzene, and toluene in order to monitor the quality of indoor air. The sensors were fabricated by using a silicon dioxide sensor with platinum bottom electrodes on which the sensing layer of GO was deposited. The sensor was then subjected to thermal treatment to
III. Environment
188
10. Graphene as superior material for detection of volatile organic compounds
get the desired reduction of GO in rGO. By varying the temperatures different degrees of rGO were obtained which differed in their functional groups in terms of type of functionality and number. The results further revealed that the incomplete reduction of GO also help in the selective adsorption of gaseous analytes which improves sensitivity. Dua et al. [41] showed that the reduced graphene could be prepared by exfoliating graphite oxide in aqueous solution of vitamin C (green reducing agent). They also prepared rGO-based flexible chemiresistor using an inkjet printing technique. These chemiresistors exhibited good reversibility and selectivity toward sensing the harmful gases/vapors. Gautam and Jayatissa [42] used SiO2/Si substrate for the anchoring of graphene by chemical deposition method and later functionalized its surface with the gold and platinum nanoparticles to improve its gas sensing characteristics. The gold and platinum functionalized graphene sensors showed good recovery, reliability, repeatability, and response like any other sensing device for gas sensing applications. These sensors were mainly studied for acetic acid, acetone, and ethanol and exhibited good sensitivity toward said gases. The average response rate of these sensors of organic vapors was reported to be w0.5% per ppm which suggests high sensitivity of these functionalized graphene-based sensors. Al2O3 and graphene nanocomposites from GO solution under supercritical CO2 atmosphere for VOC sensing were reported by Jiang et al. [43]. A very high chemiluminescence and high selectivity for ethanol was exhibited by this nanocomposite. These two properties were further exploited to fabricate low-cost sensing devices for the detection of ethanol vapor. Liu et al. [44] prepared grapheneeZnFe2O4 composites by solvothermal method and studied their gas sensing properties. Their studies revealed that the amount of ZnFe2O4 in the nanocomposite is related to the operating working temperature of the sensor and an increase in the amount of ZnFe2O4 resulted in a decrease in the operating temperature. Sensors based on the polymer optical fibers and graphene for the sensing of acetone was reported for the first time by Zhang et al. [45]. The sensing property was evaluated in terms of the reflected light, on the exposure of acetone to sensors. A significant sensitivity (44e352 ppm) with POF having graphene coating than those without graphene coating was observed, which proves that the graphene was an integral part of the sensing process. Yavari et al. [46] reported commercially superior sensors for the detection of NO2 and NH3. These workers suggested that CVD-synthesized graphene has an outstanding property for detection of NO2 and NH3 at room temperature and could detect these compounds at ppb level.
3. Sensing mechanism Graphene is an intrinsically inert and nonselective material. Due to high density of electrons it shows extremely high efficiency toward electric current conductance and also possesses special features for ballistic charge transport. These are the two important factors that suggest that this 2D material could be considered as an ideal material to serve as a platform/support material, wherein we can evaluate other specific functions by composite preparation or by doping of graphene. Most of the composites of graphene show characteristics of semiconductors in normal conditions, i.e., their conductivity could be determined by charge carrier concentration. In chemiresistors, response to external conditions is sensed by the change in conductivity, which in turn depends upon the concentration of electron and
III. Environment
3. Sensing mechanism
189
hole. Porous materials (even in bulk phase) owing to their extremely high surface area can easily adsorb gas molecules. The adsorbed gas molecules can interact in different ways with the various types of groups present on the graphene surface. The graphene adsorbed gas molecules system thereafter may either capture or donate electrons, i.e., from the gas molecules to graphene, and this process causes changes in the concentration of the carrier in the semiconductor system. On the basis of different types of doping possibilities, a wide variety of graphene-based sensors (p-type or n-type) may be obtained. In p-type semiconductors, the concentration of holes exceeds the concentration of electrons or simply in other words, the holes are in the majority carriers and electrons are the minority carriers. As mentioned above, the graphene on exposure to different types of gases or different atmospheres will result in different types of interactions and thus the graphene-based system will behave differently. As, for example, the exposure of doped graphene to the reducing atmosphere, like NH3, would result in the withdrawing of the electrons from the gas, which would consequently result in an increase in the concentration of electron and as a result of this a decrease in the resistance of graphene is observed which is overall characteristic of n-type semiconductors. In contrast, when exposed to an oxidative atmosphere such as NO2, the electrons donated to the gaseous molecules. Thus there is an increase in the concentration of holes and consequently overall resistance of graphene increases. This is a typical characteristic of p-type semiconductors. This general process of gas sensing as described above is illustrated by Fig. 10.3. This universal and old theory is called as ‘‘oxygen anion barrier model”and is used for the understanding of the gas sensing mechanism based on metal-oxide semiconductors [47e50]. Zhou et al. [49] showed that the initial electrical resistance of sensors to the target gases can be regulated by controlling the total flow rate of gases and this affects sensing properties considerably. Apart from this, the amount of graphene was also found to an important parameter, and a minimum threshold quantity of deposited rGO was found to be critical for the proper functioning of the sensors, and poor sensitivity and selectivity was observed on any amount lower than the threshold limit. On the basis of these observations, these workers proposed their sensing mechanism for rGO-based chemiresistors at room temperatures. Zhu et al. [51] further showed that the oxygen-rich functional groups on the surface of rGO can interact with the target gas, and this interaction may be selective for different gases. There might be the possibilities of two types of interactions owing to the presence of oxygencontaining functional groups, i.e., the selective strong or weak interactions such as electrostatic, dipoleedipole, van der Waals with the molecules of the target gas, and secondly the impendence to charge interaction between the sp2-hybridized electron-rich carbon areas in the molecules and rGO gas. Apart from these, the p-type graphene and n-type graphene have been observed to transform into each other on thermal annealing. Wang et al. [52]on studying the pen electricity in stacked films of rGO showed that the penep double transformation of graphene films can be controlled by varying the temperature from 100 to 400 C. The p-type reduced graphene showed high sensitivity toward ethanol (response of 58% to 1 ppm ethanol) with high detection limits in ppm. However, on annealing, the interconversion of p-type to n-type was observed and the n-type stacked films of rGO showed comparatively much lower response (response of 0.5%e50 ppm ethanol). Thus the control of different types of carriers by varying the temperature can be an effective way of fabrication of p-n-p switchable sensor.
III. Environment
190
10. Graphene as superior material for detection of volatile organic compounds
FIGURE 10.3 Schematic representation of the electronic and catalytic mechanism in graphene-based gas sensors with metal oxide additives. R is the reducing agent [57].
The basic sensing mechanism of graphene-based gas sensors was recently reviewed by Yuan and Shi [53]. These workers presented a detailed description of different types of gas sensors, i.e., from the common chemiresistor and field effect transistor to other sensors that work on measuring the changes in surface work function or the frequency of surface acoustic waves, etc. On the exposure of sensor toward the target gas, initial physical chemisorptions both then chemical interactions with the functional groups occur. Pure graphene generally also has a lot of defects due to the corrosive chemicals and the exfoliation process, and these defects apart may act as a site for interaction due to the absence of any functional groups. The theoretical first principles simulation also showed that the defects in graphene interact strongly with CO, NO, or NO2, while weak interaction was observed for NH3. SalehKhojin et al. [54] showed that line defects and edges seem to play a dominant role in the case of graphene in contrast to carbon nanotubes where point defects play a dominant role as reported. The importance of defects for sensing was also highlighted in their work on sensors fabricated from the CVD-grown graphene nanoribbons. The CVD-grown graphene nanoribbons with defective sites showed much higher sensing response than defectless
III. Environment
References
191
graphene produced by sticky scotch tape method. The adsorption of gases leads to an exchange of charge between the adsorbate layer and the material, resulting in a variation of the concentration of free electrons in the different regions. Thus there exist two different models for the understanding of the gas sensing mechanism [55e57]. These proposed models are based on the following: (1) oxygen ionosorption and (2) oxygen vacancies. The generally accepted mechanism for the sensor response on exposure to external conditions like air is presented in Fig. 10.3, and the adsorption of oxygen may be given in terms of following equations: O2ðgasÞ þ e %O 2ðadsÞ O 2ðadsÞ þ e %O2 2ðadsÞ %2O ðadsÞ On exposure to external gas, like CO, the reactions at the surface may be given as CO þ O ðadsÞ /CO2 þ e 2CO þ O 2ðadsÞ /CO2 þ e
4. Conclusions and future prospects As we have discussed the importance of graphene and sensors based on it in detail, it can be said that without any doubt graphene-based sensing devices are far better than their counterparts made out of other materials. As there are a number of graphene and its family members have been utilized for their specific sensing properties, it also opens a large number of opportunities. The members showed high selectivity toward different gases when treated in different environments. Therefore, the area of graphene-based sensors especially for VOCs still needs careful research and economic production of graphene and its family members.
References [1] Boehm HP, Setton R, Stumpp E. Pure Appl Chem 1994;66:1893. [2] Matthew JA, Vincent CT, Richard BK. Honeycomb carbon: a review of graphene. Chem Rev 2010;110:132e45. [3] Konstantinos S, Rudolf P. In: Vasilios G, editor. An introduction to graphene, functionalization of graphene. Wiley-VCH Verlag GmbH & Co. KGaA; 2014. [4] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, Geim AK. Proc Natl Acad Sci U S A 2005;102:10451. [5] Wallace PR. Phys Rev 1947;71:476. [6] McClure JW. Phys Rev 1956;104:666. [7] Semenoff GW. Phys Rev Lett 1984;53:2449. [8] Goh MS, Pumera M. Single-, few-, and multilayer graphene not exhibiting significant advantages over graphite microparticles in electroanalysis. Anal Chem 2010;82:8367e70. [9] S. Kochmann, Graphene as a sensor material, PhD Dissertation, University of Resenburg, Germany, 2013. [10] Fitzer E, Kochling K-H, Boehm HP, Marsh H. Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995). Pure Appl Chem 1995;67:473e506.
III. Environment
192
10. Graphene as superior material for detection of volatile organic compounds
[11] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA. Science 2004;306:666. [12] Castro N, Guinea F, Peres NMR, Novoselov KS, Geim AK. Rev Mod Phys 2009;81:109. [13] Frank IW, Tanenbaum DM, van der Zande AM, McEuen PLJ. Vac. Sci. Technol. 2007;25:2558e61. [14] Meye JC, Geim AK, Katsnelson MI, Novoselov KS, Booth TJ, Roth S. Nature 2007;446:60e3. [15] Hummers WS, Offeman RE. J Am Chem Soc 1958;80:1339. [16] Bourlinos AB, Gournis D, Petridis D, Szabo T, Szeri A, Dekany I. Langmuir 2003;9:6050. [17] Allen MJ, Tung VC, Kaner RB. Chem Rev 2010;110:132. [18] Broza YY, Haick H. Nanomaterial-based sensors for detection of disease by volatile organic compounds. Nanomedicine 2013;8:785e806. [19] Lewis NS. Comparisons between mammalian and artificial olfaction based on arrays of carbon blackpolymer composite vapor detectors. Acc Chem Res 2004;37:663e72. [20] Hanney S, Buxton M, Green C, Coulson D, Raftery J. An assessment of the impact of NHS health technology assessment programme. Health Technol Assess 2007;11:200. [21] Broza YY, Zuri L, Haick H. Combined volatolomics for monitoring of human body chemistry. Sci Rep 2014;4:4611e6. [22] Miekisch W, Schubert JK, Noeldge-Schomburg GFE. Diagnostic potential of breath analysis-focus on volatile organic compounds. Clin Chim Acta 2004;347:25e39. [23] Broza YY, Mochalski P, Ruzsanyi V, Amann A, Haick H. Hybrid volatolomics and disease detection. Angew Chem Int Ed 2015;54:11036e48. [24] Konvalina G, Haick H. Sensors for breath testing: from nanomaterials to comprehensive disease detection. Acc Chem Res 2014;47:66e76. [25] Nakhleh MK, Jeries R, Gharra Al, Binder A, Broza YY, Pascoe M, et al. Detecting active pulmonary tuberculosis with a breath test using nanomaterial-based sensors. Eur Respir J 2014;43:1522e5. [26] Peng G, Hakim M, Broza YY, Billan S, Abdah-Bortnyak R, Kuten A, et al. Detection of lung, breast, colorectal, and prostate cancers from exhaled breath using a single array of nanosensors. Br J Canc 2010;103:542e51. [27] Li SD, Tian SH, Du CM, He C, Cen CP, Xiong Y. Vaseline-loaded expanded graphite as a new adsorbent for toluene. Chem Eng J 2010;162:546e51. [28] Sakai N, Yamamoto S, Matsui Y, Khan MF, Latif MT, Mohd MA, Yoneda M. Characterization and source profiling of volatile organic compounds in indoor air of private residences in Selangor State. Malaysia. Sci. Total Environ. 2017;586:1279e86. [29] Dai HX, Jing SG, Wang HL, Ma YG, Li L, Song WM, Kan HD. VOC characteristics and inhalation health risks in newly renovated residences in Shanghai, China. Sci Total Environ 2017;577:73e83. [30] Yu L, Wang L, Xu W, Chen L, Fu M, Wu J, Ye D. Adsorption of VOCs on reduced graphene oxide. J. Environ Sci 2018;67:171e8. [31] Meng FL, Guo Z, Huang X-J. Graphene-based hybrids for chemiresistive gas sensors. Trends in Analytical Chemistry 2015;68:37e47. [32] Schedin F, Geim AK, Morozov SV, Hill EW, Blake P, Katsnelson MI, et al. Detection of individual gas molecules adsorbed on graphene. Nat Mater 2007;6:652e5. [33] Fowler JD, Allen MJ, Tung VC, Yang Y, Kaner RB, Weiller BH. Practical chemical sensors from chemically derived graphene. ACS Nano 2009;3:301e6. [34] Robinson JT, Perkins FK, Snow ES, Wei ZQ, Sheehan PE. Reduced graphene oxide molecular sensors. Nano Lett 2008;8:3137e40. [35] Su PG, Shieh HC. Flexible NO2 sensors fabricated by layer-by-layer covalent anchoring and in situ reduction of graphene oxide. Sensor Actuator B Chem 2014;190:865e72. [36] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [37] Lu W, Lieber CM. Nanoelectronics from the bottom up. Nat Mater 2007;6:841e50. [38] Yavari F, Koratkar N. Graphene-based chemical sensors. J Phys Chem Lett 2012;3:1746e53. [39] Wang X, Sun X, Hu PA, Zhang J, Wang L, Feng W, Lei S, Yang B, Cao W. Colorimetric sensor based on selfassembled polydiacetylene/graphene-stacked composite film for vapor-phase volatile organic compounds. Adv Funct Mater 2013:1e7. [40] Ahn H, Jung B, Park JR, Joo JC. Graphene oxide based selective VOCs sensor for indoor air quality monitoring. Mater Sci Forum 2015;804:39e42.
III. Environment
References
193
[41] Dua V, Surwade SP, Ammu S, Agnihotra SR, Jain S, Roberts KE, Park S, Ruoff RS, Manohar SK. All-organic vapor sensor using inkjet-printed reduced graphene oxide. Angew Chem Int Ed 2010;49:2154e7. [42] Gautam M, Jayatissaa Ahalapitiya H. Detection of organic vapors by graphene films functionalized with metallic Nanoparticles. J Appl Phys 2012;112:114326e8. [43] Jiang Z, Wang J, Meng L, Huang Y, Liu L. A highly efficient chemical sensor material for ethanol: Al2O3/Graphene nanocomposites fabricated from graphene oxide. Chem Commun 2011;47:6350e2. [44] Liu F, Chu X, Dong Y, Zhang W, Sun W, Shen L. Acetone gas sensors based on graphene-ZnFe2O4 composite prepared by solvothermal method. Sensor Actuator B 2013;188:469e74. [45] Zhang H, Kulkarni A, Kim H, Woo D, Kim YJ, Hong BH, Choi JB, Kim T. Detection of acetone vapor using graphene on polymer optical fiber. J Nanosci Nanotechnol 2011;11:5939e43. [46] Yavari F, Castillo E, Gullapalli H, Ajayan PM, Koratkar N. High sensitivity detection of NO2 and NH3 in air using chemical vapor deposition grown graphene. Appl Phys Lett 2012;100:203120. [47] Dayan NJ, Sainkar SR, Karekar RN, Aiyer RC. Formulation and characterization of ZnO: Sb thick-film gas sensors. Thin Solid Films 1998;325:254e8. [48] Yamazoe N, Sakai G, Shimanoe K. Oxide semiconductor gas sensors. Catal Surv Asia 2003;7:63e75. [49] Zhou Y, Jiang YD, Xie T, Tai HL, Xie GZ. A novel sensing mechanism for resistive gas sensors based on layered reduced graphene oxide thin films at room temperature. Sens Actuators B 2014;203:135e42. [50] Wang T, Huang D, Yang Z, Xu S, He G, Li X, Hu N, Yin G, He D, Zhang L. A review on graphene-based gas/ vapor sensors with unique properties and potential applications. Nano-Micro Lett 2016;8:95e119. [51] Zhu M, Li X, Chung S, Zhao L, Li X, Zang X, Wang K, Wei J, Zhong M, Zhou K, Xie D, Zhu H. Photo-induced selective gas detection based on reduced graphene oxide/Si Schottky diode. Carbon 2015;84:138e45. [52] Wang R-C, Chang Y-M. Switch of p-n electricity of reduced-graphene-oxide-flake stacked films enabling room-temperature gas sensing from ultrasensitive to insensitive. Carbon 2015;91:416e22. [53] Yuan W, Shi G. Graphene-based gas sensors. J Mater Chem A 2013;1:10078e91. [54] Salehi-Khojin A, Estrada D, Lin KY, Bae M-H, Xiong F, Pop E, Masel RI. Polycrystalline graphene ribbons as chemiresistors. Adv Mater 2012;24:53e7. [55] Wang C, Yin L, Zhang L, Xiang D, Gao R. Metal oxide gas sensors:Sensitivity and influencing factors. Sensors 2010;10:2088e106. [56] Barsan N, Weimar U. Understanding the fundamental principles of metaloxide based gas sensors; the example of CO sensing with SnO2sensors inthe presence of humidity. J Phys Condens Matter 2003;15:R813e39. [57] Chatterjee SG, Chatterjee S, Ray AK, Chakraborty AK. Grapheneemetal oxide nanohybrids for toxic gas sensor: a review. Sensor Actuator B 2015;221:1170e81.
III. Environment
10 Graphene as superior material for detection of volatile organic compounds Shahid Pervez Ansari1, Mohammad Omaish Ansari2, Anish Khan3, 4 1
Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, India; 2Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia; 3Chemistry Department, King Abdulaziz University, Jeddah, Saudi Arabia; 4Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia
1. Introduction Graphene is a much talked about and researched material at present, as in the past, and will continue to do so in future. The high place of graphene is because of its unique properties which could be utilized in most of the areas of scientific concern. In the coming discussion we will learn about the graphene and the reasons of its importance. Let us first introduce you to the word graphene itself. We all know about carbon and its various allotropes; graphite is an allotrope of carbon with a layered structure of sp2 hybrid carbon atoms, and these layers are separated by a specific distance by van der Waal forces. Due to these interlayer spaces, these also act as a solid lubricant. If these layers could be separated and used, they offer infinite possibilities. The name “graphene” was introduced in 1994 by Boehm, Setton, and Stumpp [1]. Graphene is a two-dimensional (2D) honeycomb lattice of packed sp2 hybrid carbon atoms with a carbonecarbon molecular bond length of 0.142 nm. The sp2 hybrid carbon atoms in this lattice arrangement provide p-conjugation (long range) in graphene due to which it exhibits extraordinary electrical, mechanical, and thermal properties [2]. Each carbon atom in the hexagonal lattice structure of graphene is connected to the neighboring three other carbon atoms by the overlap of sp2 orbitals to form s bond, and the fourth bond is formed by the overlap of pz orbitals which is a p bond oriented out of the plane in the z-direction. It fourth bond consists of a band of filled p orbitals, i.e., the valence band and
Graphene-Based Nanotechnologies for Energy and Environmental Applications https://doi.org/10.1016/B978-0-12-815811-1.00010-7
183
Copyright © 2019 Elsevier Inc. All rights reserved.
184
10. Graphene as superior material for detection of volatile organic compounds
a band of empty p* orbitals, i.e., the conduction band, and this band is responsible for the high conductivity to graphene [3]. The optical absorbance of graphene is 2.3% which makes it hard to directly visualize it and is thus practically transparent when present as a single sheet. The inherent specific properties of graphene, in contrast to the two-dimensional systems based on semiconductors which generally work at specific temperatures, remain same even at room temperatures and do not require high energy for activation. Graphene also shows similarity with the carbon nanotubes in relation to several important properties such as its high strength and is stronger than steel, its high stretchability and thus can act as a flexible conductor, and its exhibition of very high thermal conductivity which is even much higher than metallic silver [4]. The theoretical studies of graphene and its application for various calculations in solid state physics were presented in 1947 by P.R. Wallace. Thereafter he also predicted the electronic configuration of graphene and gave the linear dispersion relation [5]. Further work resulted in the wave equation for excitations in 1956 by J.W. McClure [6] which was later related to the Dirac equation by G.W. Semenoff in 1984 [7]. Graphene is typically defined as a material which consists of sp2 hybridized carbon atoms arranged in a sheet made of a monolayered arrangement of carbon atoms [8,9]. The IUPAC defines it as “a single carbon layer of the graphite structure, describing its nature by analogy to a polycyclic aromatic hydrocarbon of quasi infinite size” [10]. Chen et al. further suggested that apart from thickness, the linear dimensions are also important and if a sheet made of polyaromatic hydrocarbons reached beyond 100 nm in two dimensions, then it should be considered as graphene. From the abovementioned definitions, it can be seen that two important characteristics of graphene have not been incorporated in the definition, i.e., (1) its highly metallic nature owing to the absence of a band gap [1] and (2) its constituents, i.e., it should be composed of carbon and hydrogen only. But, up to 30% of oxygen content have been reported in the graphene prepared and therefore contradicts with the previous definition. Therefore, it can be said that there are a certain number of different views to define graphene, but we are far from its exact definition which includes all its properties (Fig. 10.1).
FIGURE 10.1
Suggested smallest graphene structure. [9].
III. Environment
1. Introduction
185
Several types of graphene-based materials or graphene with different functionality are known till date which are obtained during the synthesis of graphene from different precursors. These materials include graphite oxide, graphene oxide (GO), and reduced graphene oxide (rGO), etc. These may be collectively considered as graphene family and described by the presence of multiple/single layer and oxidation level of carbon atoms in these layers. These features of graphene members affect their important properties like conductivity or photoluminescence, etc. Simple classification of graphene family and its possible synthesis routes have been presented in Fig. 10.2. Top-down and bottom-up approaches are most commonly used for synthesis. The top-down approach starts with bulk precursors such as graphite or graphite oxide (Fig. 10.2). To finally get the graphene from bulk material, the bulk precursors are structurally decomposed stepwise, like GO is formed first which on reduction gives rGO and finally graphene. In a mechanical exfoliation route, graphite is decomposed by a layerby-layer method which results in graphene. While in the bottom-up approach, the materials are assembled layer by layer, like chemical vapor deposition of methane may be used to produce large areas of graphene. Geim and Novoselov (2004) made remarkable progress in the development of methods for graphene preparation. These scientists used sticky scotch tape to remove some layers/flakes from graphite and upon repeated experiments and optimization were finally able to get single-layered graphene [11]. They observed that this graphene possessed exceptionally high electrical [12], good mechanical [13], and optical properties [14], and for this later they were honored with the Nobel Prize in Physics ‘‘for groundbreaking experiments regarding the two-dimensional material graphene.’’ Hummers and Offeman (1958) [15] reported the oxidization of graphite into graphite oxide by using a combination of concentrated sulfuric acid (H2SO4), sodium nitrate (NaNO3), and potassium permanganate (KMnO4). Highly oxidative action of the above species creates anionic groups on the layers of graphite, which are carboxylates, epoxies, and hydroxylate groups, etc. The oxidation process resulted in the increase in the out-of-plane length of CeO covalent bonds which became w0.68 nm in final graphite oxide from its original length
FIGURE 10.2
Schematic representation of relation of graphene and family members.
III. Environment
186
10. Graphene as superior material for detection of volatile organic compounds
of 0.35 nm in graphite [16]. This exfoliation of the interlayer spacing and the presence of oxygen containing functional groups results in an anionic or polar behavior and thus the resulting GO becomes highly hydrophilic. This hydrophilic GO shows strong affinity to the water molecules which can easily penetrate between the graphene layers thereby giving very high dispersibility of graphite oxide in water. Due to the above reactions, the nature of some carbon atoms of graphene may change from sp2 to sp3 which disrupt the delocalization system in the graphene, thereby affecting its electrical conductivity very critically which may reach between 103 and 107Ucm depending on the oxygen concentration [17].
2. Graphene as sensor Detection of volatile organic compounds (VOCs) may be a new and quick growing frontier in speedy, extremely selective, sensitive, noninvasive analysis and in diagnosis for detection of human diseases [18]. VOC sensing based diagnostic technologies should be enforced worldwide [19] to enable monitoring in populations at high risk, besides also enabling early detection of the disease which boosts the effectiveness of medical aid [20]. Numerous VOCs are generated within the body because of the alteration in metabolic pathways, principally associated with the carbohydrate metabolism, hemoprotein P450, liver enzymes, and aerobic stress [21]. The self-generated VOCs by the body may offer reliable and extremely valuable information related to human health. There are a few completely different biologically generated VOC molecules which are discharged from exhaled breath and may function as VOC biomarkers for detection. The VOC family consists of various hydrocarbons and their derivatives like saturated/unsaturated hydrocarbons, aromatic compounds, alcohols, aldehydes, ketones, and nitriles, which are generated via numerous physicalechemicalebiological pathways [22e24]. An illness in a person could also be recognized on the idea of specific pattern of VOCs, with low probabilities of interference by alternative diseases [25,26]. Aromatic, open-chain, and chlorinated hydrocarbons, etc. are listed within the family of VOCs. These enter in our atmosphere primarily from human activities that embody industrial production of materials of human needs like adhesives, paints, building materials, printing materials, and artificial chemicals, etc. [27,28]. In recent years, VOCs are known collectively for their extraordinarily harmful category of pollutants to human health and can cause skin diseases, headache, nausea, raw throat, carcinoma, etc. [29]. Because of the intense health threats caused by the presence of those VOCs within the atmosphere, these have attracted the sincere attention of the scientific community for their economical removal and management. For this many physicalechemical technologies together with sorption, condensation, combustion, chemical process combustion, and thermal oxidization are utilized for their removal of VOCs from gaseous streams. Among these different techniques, the adsorption technique has been widely used as it is economical and can be easily applied at a massive scale [30]. As mentioned earlier, graphene is a two-dimensional single-layered structure. When it is exposed to an appropriate surface, extremely high sensitivity to external factors is observed owing to the exposure and contact of most of the atoms to the external surrounding environment. Therefore, it is often aforesaid that the exposure of the graphene atoms to the external surroundings affects its electrical characteristics even at slight changes in the external surroundings, thereby rendering it sensitive toward external chemical and electrical changes.
III. Environment
2. Graphene as sensor
187
These properties of graphene are significantly utilized to develop different types of gas sensors. Novoselov et al. [31] were the first to study the gas-sensing properties of graphene. However, there was no systematic analysis report out there on the gas-sensing characteristics of graphene till a few years later. Therein in that report, graphene was reported to exhibit the capability of a highly efficient individual gas molecule attributable to its low electronic noise [32]. However, it is very tough to succeed to a perfection level in real detection conditions despite the distinctive characteristics of graphene [33]. Exfoliated GO was used by Robinson et al. [34] to prepare an ultrathin continuous network, which was later reduced by exposure to hydrazine hydrate vapor. Thus prepared ultrathin rGO network was used to fabricate gas sensor. The main advantages of rGO for gas sensing are (1) the presence of large number of defects which act as a site for interaction, (2) some functional groups of GO, which are inevitably left in rGO even after reduction, and (3) the rGO is electron rich due to the presence of sp2 bonded structure, and this offers efficient charge transport in comparison to nonconductive GO. The rGO-based sensors can show extremely high sensitivity and selectivity and can detect explosives, chemicals, etc. in very low dosages, i.e., at parts per billion (ppb) levels. They have also been utilized to detect different types of VOCs and NO2 [35]. Graphene-based VOC sensors have attracted enormous interest in the diagnosis of diseases after the work of Schedin et al [36]. The graphene-based sensing devices show excellent response due to the high density of electrons which offer high mobility of charge carriers, low consumption of power, and compatibility with present-day and future electronics [37]. The atomic-level thickness of graphene and high surface-to-volume ratio can attain high porosity as per the method of synthesis, controllable functionalization with different reagents, and zero bandgap which are the properties that make it a favorable candidate for the fabrication of gas sensors for diverse analyte molecules. The mobility of electrons in graphene is 2.5 105 cm2 V1s1, transport is w0.3 mm at 300 K, and it also has low electrical noise which allows it to be readily used for the fabrication of highly efficient sensors with good sensitivity and selectivity. The interaction of analyte with graphene (either with defects or functional groups) results in the change in the mobility of charge transport thereby affecting conductivity, and this gives graphene high detection sensitivity in comparison to its counterparts [38]. Wang et al. [39] used composite of nonconducting polydiacetylene (PDA) with graphene to develop a handy litmus-type chemosensor for effective detection of hazardous VOCs. The highly ordered PDA monolayer in the composite was supported efficiently on the strong graphene layer. The resultant sensor responded efficiently to VOCs (w0.01%) even at minute quantities and was successfully tested for chloroform (CHCl3), dimethylformamide (DMF), methanol (CH3OH), and tetrahydrofuran (THF). The change in the color on exposure to varied concentrations of VOCs can be easily perceived by the naked eye and a logarithmic response was observed between the concentration of VOCs (from w0.01%e to 10%) and the chromatic response. The morphological changes in the PDA polymer were also observed by the scanning tunneling microscopy which suggests intrinsic mechanism of the chromatic variety at the molecular level. Ahn et al. [40] used GO for the fabrication of VOC sensors for the selective detection of formaldehyde, benzene, and toluene in order to monitor the quality of indoor air. The sensors were fabricated by using a silicon dioxide sensor with platinum bottom electrodes on which the sensing layer of GO was deposited. The sensor was then subjected to thermal treatment to
III. Environment
188
10. Graphene as superior material for detection of volatile organic compounds
get the desired reduction of GO in rGO. By varying the temperatures different degrees of rGO were obtained which differed in their functional groups in terms of type of functionality and number. The results further revealed that the incomplete reduction of GO also help in the selective adsorption of gaseous analytes which improves sensitivity. Dua et al. [41] showed that the reduced graphene could be prepared by exfoliating graphite oxide in aqueous solution of vitamin C (green reducing agent). They also prepared rGO-based flexible chemiresistor using an inkjet printing technique. These chemiresistors exhibited good reversibility and selectivity toward sensing the harmful gases/vapors. Gautam and Jayatissa [42] used SiO2/Si substrate for the anchoring of graphene by chemical deposition method and later functionalized its surface with the gold and platinum nanoparticles to improve its gas sensing characteristics. The gold and platinum functionalized graphene sensors showed good recovery, reliability, repeatability, and response like any other sensing device for gas sensing applications. These sensors were mainly studied for acetic acid, acetone, and ethanol and exhibited good sensitivity toward said gases. The average response rate of these sensors of organic vapors was reported to be w0.5% per ppm which suggests high sensitivity of these functionalized graphene-based sensors. Al2O3 and graphene nanocomposites from GO solution under supercritical CO2 atmosphere for VOC sensing were reported by Jiang et al. [43]. A very high chemiluminescence and high selectivity for ethanol was exhibited by this nanocomposite. These two properties were further exploited to fabricate low-cost sensing devices for the detection of ethanol vapor. Liu et al. [44] prepared grapheneeZnFe2O4 composites by solvothermal method and studied their gas sensing properties. Their studies revealed that the amount of ZnFe2O4 in the nanocomposite is related to the operating working temperature of the sensor and an increase in the amount of ZnFe2O4 resulted in a decrease in the operating temperature. Sensors based on the polymer optical fibers and graphene for the sensing of acetone was reported for the first time by Zhang et al. [45]. The sensing property was evaluated in terms of the reflected light, on the exposure of acetone to sensors. A significant sensitivity (44e352 ppm) with POF having graphene coating than those without graphene coating was observed, which proves that the graphene was an integral part of the sensing process. Yavari et al. [46] reported commercially superior sensors for the detection of NO2 and NH3. These workers suggested that CVD-synthesized graphene has an outstanding property for detection of NO2 and NH3 at room temperature and could detect these compounds at ppb level.
3. Sensing mechanism Graphene is an intrinsically inert and nonselective material. Due to high density of electrons it shows extremely high efficiency toward electric current conductance and also possesses special features for ballistic charge transport. These are the two important factors that suggest that this 2D material could be considered as an ideal material to serve as a platform/support material, wherein we can evaluate other specific functions by composite preparation or by doping of graphene. Most of the composites of graphene show characteristics of semiconductors in normal conditions, i.e., their conductivity could be determined by charge carrier concentration. In chemiresistors, response to external conditions is sensed by the change in conductivity, which in turn depends upon the concentration of electron and
III. Environment
3. Sensing mechanism
189
hole. Porous materials (even in bulk phase) owing to their extremely high surface area can easily adsorb gas molecules. The adsorbed gas molecules can interact in different ways with the various types of groups present on the graphene surface. The graphene adsorbed gas molecules system thereafter may either capture or donate electrons, i.e., from the gas molecules to graphene, and this process causes changes in the concentration of the carrier in the semiconductor system. On the basis of different types of doping possibilities, a wide variety of graphene-based sensors (p-type or n-type) may be obtained. In p-type semiconductors, the concentration of holes exceeds the concentration of electrons or simply in other words, the holes are in the majority carriers and electrons are the minority carriers. As mentioned above, the graphene on exposure to different types of gases or different atmospheres will result in different types of interactions and thus the graphene-based system will behave differently. As, for example, the exposure of doped graphene to the reducing atmosphere, like NH3, would result in the withdrawing of the electrons from the gas, which would consequently result in an increase in the concentration of electron and as a result of this a decrease in the resistance of graphene is observed which is overall characteristic of n-type semiconductors. In contrast, when exposed to an oxidative atmosphere such as NO2, the electrons donated to the gaseous molecules. Thus there is an increase in the concentration of holes and consequently overall resistance of graphene increases. This is a typical characteristic of p-type semiconductors. This general process of gas sensing as described above is illustrated by Fig. 10.3. This universal and old theory is called as ‘‘oxygen anion barrier model”and is used for the understanding of the gas sensing mechanism based on metal-oxide semiconductors [47e50]. Zhou et al. [49] showed that the initial electrical resistance of sensors to the target gases can be regulated by controlling the total flow rate of gases and this affects sensing properties considerably. Apart from this, the amount of graphene was also found to an important parameter, and a minimum threshold quantity of deposited rGO was found to be critical for the proper functioning of the sensors, and poor sensitivity and selectivity was observed on any amount lower than the threshold limit. On the basis of these observations, these workers proposed their sensing mechanism for rGO-based chemiresistors at room temperatures. Zhu et al. [51] further showed that the oxygen-rich functional groups on the surface of rGO can interact with the target gas, and this interaction may be selective for different gases. There might be the possibilities of two types of interactions owing to the presence of oxygencontaining functional groups, i.e., the selective strong or weak interactions such as electrostatic, dipoleedipole, van der Waals with the molecules of the target gas, and secondly the impendence to charge interaction between the sp2-hybridized electron-rich carbon areas in the molecules and rGO gas. Apart from these, the p-type graphene and n-type graphene have been observed to transform into each other on thermal annealing. Wang et al. [52]on studying the pen electricity in stacked films of rGO showed that the penep double transformation of graphene films can be controlled by varying the temperature from 100 to 400 C. The p-type reduced graphene showed high sensitivity toward ethanol (response of 58% to 1 ppm ethanol) with high detection limits in ppm. However, on annealing, the interconversion of p-type to n-type was observed and the n-type stacked films of rGO showed comparatively much lower response (response of 0.5%e50 ppm ethanol). Thus the control of different types of carriers by varying the temperature can be an effective way of fabrication of p-n-p switchable sensor.
III. Environment
190
10. Graphene as superior material for detection of volatile organic compounds
FIGURE 10.3 Schematic representation of the electronic and catalytic mechanism in graphene-based gas sensors with metal oxide additives. R is the reducing agent [57].
The basic sensing mechanism of graphene-based gas sensors was recently reviewed by Yuan and Shi [53]. These workers presented a detailed description of different types of gas sensors, i.e., from the common chemiresistor and field effect transistor to other sensors that work on measuring the changes in surface work function or the frequency of surface acoustic waves, etc. On the exposure of sensor toward the target gas, initial physical chemisorptions both then chemical interactions with the functional groups occur. Pure graphene generally also has a lot of defects due to the corrosive chemicals and the exfoliation process, and these defects apart may act as a site for interaction due to the absence of any functional groups. The theoretical first principles simulation also showed that the defects in graphene interact strongly with CO, NO, or NO2, while weak interaction was observed for NH3. SalehKhojin et al. [54] showed that line defects and edges seem to play a dominant role in the case of graphene in contrast to carbon nanotubes where point defects play a dominant role as reported. The importance of defects for sensing was also highlighted in their work on sensors fabricated from the CVD-grown graphene nanoribbons. The CVD-grown graphene nanoribbons with defective sites showed much higher sensing response than defectless
III. Environment
References
191
graphene produced by sticky scotch tape method. The adsorption of gases leads to an exchange of charge between the adsorbate layer and the material, resulting in a variation of the concentration of free electrons in the different regions. Thus there exist two different models for the understanding of the gas sensing mechanism [55e57]. These proposed models are based on the following: (1) oxygen ionosorption and (2) oxygen vacancies. The generally accepted mechanism for the sensor response on exposure to external conditions like air is presented in Fig. 10.3, and the adsorption of oxygen may be given in terms of following equations: O2ðgasÞ þ e %O 2ðadsÞ O 2ðadsÞ þ e %O2 2ðadsÞ %2O ðadsÞ On exposure to external gas, like CO, the reactions at the surface may be given as CO þ O ðadsÞ /CO2 þ e 2CO þ O 2ðadsÞ /CO2 þ e
4. Conclusions and future prospects As we have discussed the importance of graphene and sensors based on it in detail, it can be said that without any doubt graphene-based sensing devices are far better than their counterparts made out of other materials. As there are a number of graphene and its family members have been utilized for their specific sensing properties, it also opens a large number of opportunities. The members showed high selectivity toward different gases when treated in different environments. Therefore, the area of graphene-based sensors especially for VOCs still needs careful research and economic production of graphene and its family members.
References [1] Boehm HP, Setton R, Stumpp E. Pure Appl Chem 1994;66:1893. [2] Matthew JA, Vincent CT, Richard BK. Honeycomb carbon: a review of graphene. Chem Rev 2010;110:132e45. [3] Konstantinos S, Rudolf P. In: Vasilios G, editor. An introduction to graphene, functionalization of graphene. Wiley-VCH Verlag GmbH & Co. KGaA; 2014. [4] Novoselov KS, Jiang D, Schedin F, Booth TJ, Khotkevich VV, Morozov SV, Geim AK. Proc Natl Acad Sci U S A 2005;102:10451. [5] Wallace PR. Phys Rev 1947;71:476. [6] McClure JW. Phys Rev 1956;104:666. [7] Semenoff GW. Phys Rev Lett 1984;53:2449. [8] Goh MS, Pumera M. Single-, few-, and multilayer graphene not exhibiting significant advantages over graphite microparticles in electroanalysis. Anal Chem 2010;82:8367e70. [9] S. Kochmann, Graphene as a sensor material, PhD Dissertation, University of Resenburg, Germany, 2013. [10] Fitzer E, Kochling K-H, Boehm HP, Marsh H. Recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995). Pure Appl Chem 1995;67:473e506.
III. Environment
192
10. Graphene as superior material for detection of volatile organic compounds
[11] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA. Science 2004;306:666. [12] Castro N, Guinea F, Peres NMR, Novoselov KS, Geim AK. Rev Mod Phys 2009;81:109. [13] Frank IW, Tanenbaum DM, van der Zande AM, McEuen PLJ. Vac. Sci. Technol. 2007;25:2558e61. [14] Meye JC, Geim AK, Katsnelson MI, Novoselov KS, Booth TJ, Roth S. Nature 2007;446:60e3. [15] Hummers WS, Offeman RE. J Am Chem Soc 1958;80:1339. [16] Bourlinos AB, Gournis D, Petridis D, Szabo T, Szeri A, Dekany I. Langmuir 2003;9:6050. [17] Allen MJ, Tung VC, Kaner RB. Chem Rev 2010;110:132. [18] Broza YY, Haick H. Nanomaterial-based sensors for detection of disease by volatile organic compounds. Nanomedicine 2013;8:785e806. [19] Lewis NS. Comparisons between mammalian and artificial olfaction based on arrays of carbon blackpolymer composite vapor detectors. Acc Chem Res 2004;37:663e72. [20] Hanney S, Buxton M, Green C, Coulson D, Raftery J. An assessment of the impact of NHS health technology assessment programme. Health Technol Assess 2007;11:200. [21] Broza YY, Zuri L, Haick H. Combined volatolomics for monitoring of human body chemistry. Sci Rep 2014;4:4611e6. [22] Miekisch W, Schubert JK, Noeldge-Schomburg GFE. Diagnostic potential of breath analysis-focus on volatile organic compounds. Clin Chim Acta 2004;347:25e39. [23] Broza YY, Mochalski P, Ruzsanyi V, Amann A, Haick H. Hybrid volatolomics and disease detection. Angew Chem Int Ed 2015;54:11036e48. [24] Konvalina G, Haick H. Sensors for breath testing: from nanomaterials to comprehensive disease detection. Acc Chem Res 2014;47:66e76. [25] Nakhleh MK, Jeries R, Gharra Al, Binder A, Broza YY, Pascoe M, et al. Detecting active pulmonary tuberculosis with a breath test using nanomaterial-based sensors. Eur Respir J 2014;43:1522e5. [26] Peng G, Hakim M, Broza YY, Billan S, Abdah-Bortnyak R, Kuten A, et al. Detection of lung, breast, colorectal, and prostate cancers from exhaled breath using a single array of nanosensors. Br J Canc 2010;103:542e51. [27] Li SD, Tian SH, Du CM, He C, Cen CP, Xiong Y. Vaseline-loaded expanded graphite as a new adsorbent for toluene. Chem Eng J 2010;162:546e51. [28] Sakai N, Yamamoto S, Matsui Y, Khan MF, Latif MT, Mohd MA, Yoneda M. Characterization and source profiling of volatile organic compounds in indoor air of private residences in Selangor State. Malaysia. Sci. Total Environ. 2017;586:1279e86. [29] Dai HX, Jing SG, Wang HL, Ma YG, Li L, Song WM, Kan HD. VOC characteristics and inhalation health risks in newly renovated residences in Shanghai, China. Sci Total Environ 2017;577:73e83. [30] Yu L, Wang L, Xu W, Chen L, Fu M, Wu J, Ye D. Adsorption of VOCs on reduced graphene oxide. J. Environ Sci 2018;67:171e8. [31] Meng FL, Guo Z, Huang X-J. Graphene-based hybrids for chemiresistive gas sensors. Trends in Analytical Chemistry 2015;68:37e47. [32] Schedin F, Geim AK, Morozov SV, Hill EW, Blake P, Katsnelson MI, et al. Detection of individual gas molecules adsorbed on graphene. Nat Mater 2007;6:652e5. [33] Fowler JD, Allen MJ, Tung VC, Yang Y, Kaner RB, Weiller BH. Practical chemical sensors from chemically derived graphene. ACS Nano 2009;3:301e6. [34] Robinson JT, Perkins FK, Snow ES, Wei ZQ, Sheehan PE. Reduced graphene oxide molecular sensors. Nano Lett 2008;8:3137e40. [35] Su PG, Shieh HC. Flexible NO2 sensors fabricated by layer-by-layer covalent anchoring and in situ reduction of graphene oxide. Sensor Actuator B Chem 2014;190:865e72. [36] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [37] Lu W, Lieber CM. Nanoelectronics from the bottom up. Nat Mater 2007;6:841e50. [38] Yavari F, Koratkar N. Graphene-based chemical sensors. J Phys Chem Lett 2012;3:1746e53. [39] Wang X, Sun X, Hu PA, Zhang J, Wang L, Feng W, Lei S, Yang B, Cao W. Colorimetric sensor based on selfassembled polydiacetylene/graphene-stacked composite film for vapor-phase volatile organic compounds. Adv Funct Mater 2013:1e7. [40] Ahn H, Jung B, Park JR, Joo JC. Graphene oxide based selective VOCs sensor for indoor air quality monitoring. Mater Sci Forum 2015;804:39e42.
III. Environment
References
193
[41] Dua V, Surwade SP, Ammu S, Agnihotra SR, Jain S, Roberts KE, Park S, Ruoff RS, Manohar SK. All-organic vapor sensor using inkjet-printed reduced graphene oxide. Angew Chem Int Ed 2010;49:2154e7. [42] Gautam M, Jayatissaa Ahalapitiya H. Detection of organic vapors by graphene films functionalized with metallic Nanoparticles. J Appl Phys 2012;112:114326e8. [43] Jiang Z, Wang J, Meng L, Huang Y, Liu L. A highly efficient chemical sensor material for ethanol: Al2O3/Graphene nanocomposites fabricated from graphene oxide. Chem Commun 2011;47:6350e2. [44] Liu F, Chu X, Dong Y, Zhang W, Sun W, Shen L. Acetone gas sensors based on graphene-ZnFe2O4 composite prepared by solvothermal method. Sensor Actuator B 2013;188:469e74. [45] Zhang H, Kulkarni A, Kim H, Woo D, Kim YJ, Hong BH, Choi JB, Kim T. Detection of acetone vapor using graphene on polymer optical fiber. J Nanosci Nanotechnol 2011;11:5939e43. [46] Yavari F, Castillo E, Gullapalli H, Ajayan PM, Koratkar N. High sensitivity detection of NO2 and NH3 in air using chemical vapor deposition grown graphene. Appl Phys Lett 2012;100:203120. [47] Dayan NJ, Sainkar SR, Karekar RN, Aiyer RC. Formulation and characterization of ZnO: Sb thick-film gas sensors. Thin Solid Films 1998;325:254e8. [48] Yamazoe N, Sakai G, Shimanoe K. Oxide semiconductor gas sensors. Catal Surv Asia 2003;7:63e75. [49] Zhou Y, Jiang YD, Xie T, Tai HL, Xie GZ. A novel sensing mechanism for resistive gas sensors based on layered reduced graphene oxide thin films at room temperature. Sens Actuators B 2014;203:135e42. [50] Wang T, Huang D, Yang Z, Xu S, He G, Li X, Hu N, Yin G, He D, Zhang L. A review on graphene-based gas/ vapor sensors with unique properties and potential applications. Nano-Micro Lett 2016;8:95e119. [51] Zhu M, Li X, Chung S, Zhao L, Li X, Zang X, Wang K, Wei J, Zhong M, Zhou K, Xie D, Zhu H. Photo-induced selective gas detection based on reduced graphene oxide/Si Schottky diode. Carbon 2015;84:138e45. [52] Wang R-C, Chang Y-M. Switch of p-n electricity of reduced-graphene-oxide-flake stacked films enabling room-temperature gas sensing from ultrasensitive to insensitive. Carbon 2015;91:416e22. [53] Yuan W, Shi G. Graphene-based gas sensors. J Mater Chem A 2013;1:10078e91. [54] Salehi-Khojin A, Estrada D, Lin KY, Bae M-H, Xiong F, Pop E, Masel RI. Polycrystalline graphene ribbons as chemiresistors. Adv Mater 2012;24:53e7. [55] Wang C, Yin L, Zhang L, Xiang D, Gao R. Metal oxide gas sensors:Sensitivity and influencing factors. Sensors 2010;10:2088e106. [56] Barsan N, Weimar U. Understanding the fundamental principles of metaloxide based gas sensors; the example of CO sensing with SnO2sensors inthe presence of humidity. J Phys Condens Matter 2003;15:R813e39. [57] Chatterjee SG, Chatterjee S, Ray AK, Chakraborty AK. Grapheneemetal oxide nanohybrids for toxic gas sensor: a review. Sensor Actuator B 2015;221:1170e81.
III. Environment