Journal of Materials Chemistry...

Journal ofMaterials Chemistry A

REVIEW

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Graphene materi

KiEHRhHaIHIAA

supervision of Prof. Ki-Hyun Kim.quality management and the applmaterials towards environmental

aDepartment of Civil and Environmental

Wangsimni-Ro, Seoul 04763, Republic of KobNational Agri-Food Biotechnology Institut

India

Cite this: J. Mater. Chem. A, 2018, 6,22391

Received 7th August 2018Accepted 12th October 2018

DOI: 10.1039/c8ta07669c

rsc.li/materials-a

This journal is © The Royal Society of C

als as a superior platform foradvanced sensing strategies against gaseousammonia

Kumar Vikrant, a Vanish Kumar b and Ki-Hyun Kim *a

Ammonia (NH3) is an uncolored, toxic, corrosive, and reactive gas with a characteristic pungent stench. To

date, quantitative analysis of NH3 concentrations have been made using conventional techniques (e.g., ion

chromatography). In light of the complications involved in such applications, efforts have been made to

develop detection methods of NH3 that are more sensitive and selective. In this respect, graphene-based

gas sensors have attracted widespread attention because of graphene's distinctive electrical

characteristics (e.g., low electrical signal noise and great mobility) and large surface area. This review

article was designed to evaluate the potential usage of graphene-based gas sensors for effective

detection of NH3. We aim to understand the recent advances in this challenging area of research by

critically analyzing various experiments and comprehending their practical implications. This review

critically compares the performance of graphene-based NH3 sensors with those of other nanomaterials

for a broader understanding of the field. Also, we summarize the future prospects for advancement of

graphene technology for NH3 sensing.

1. Introduction

Current environmental and safety needs have resulted in greatdevelopments in the area of air quality management to effec-tively counteract the complications associated with soaring airpollution.1,2 The control and monitoring of unduly toxic and

umar Vikrant is a PhD studentn the Department of Civil andnvironmental Engineering,anyang University, Seoul,epublic of Korea. He completedis Bachelor of Technology withonors in Chemical Engineeringnd Technology at the Indiannstitute of Technology (Banarasindu University), Varanasi,ndia. At present, he works at their Quality and Materialspplication Lab under theHe is working in the area of airication of advanced functionalpollution control and analysis.

Engineering, Hanyang University, 222

rea. E-mail: [email protected]

e (NABI), S.A.S. Nagar, Punjab 140306,

hemistry 2018

odorous gases like ammonia (NH3, with a high vapor pressureof 1013.25 kPa at 25 �C) have been recognized as a vitalendeavor to conserve the quality of air (United States Environ-mental Protection Agency (USEPA)).3 In fact, ammonia is iden-tied as the most abundant inorganic pollutant that isdispersed globally. To further stress the signicance ofammonia in air quality management (AQM), its potent role inhampering visibility in the atmosphere and Earth's radiationbudget is well known through the genesis of secondary atmo-spheric particulates.4,5

Dr. Vanish Kumar is presentlyworking as INSPIRE-Faculty atthe National Agri-Food Biotech-nology Institute (NABI), Mohali,India. His current areas ofresearch include sensing of foodcontaminants, development ofnanosensing platforms,synthesis of advanced nano-structures, and analyticalapplications of nanomaterials.He has published more than 20research papers in reputedinternational journals.

J. Mater. Chem. A, 2018, 6, 22391–22410 | 22391

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http://orcid.org/0000-0001-7537-8540

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http://dx.doi.org/10.1039/c8ta07669c

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Journal of Materials Chemistry A Review

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In light of the signicance of ammonia in AQM, substantialattempts have been made to properly comprehend the delete-rious impacts of ammonia on animal/human health and pris-tine ecosystems over both the short and long term.6 NH3 hasbeen known to cause irritation of the eyes and respiratory tract.7

Extended exposure (e.g., 10 min) to large amounts of NH3 ($50ppm) may result in wet lung disease, blindness, blebs, burns,and even death.5 Also, at elevated concentrations, NH3 maycause acidication of water bodies or contribute to enhance-ment of nitrate levels in potable water.8

Advanced agricultural exercises, which typically involve largescale utilization of urea, have been acknowledged as theprimary source of NH3 globally.9 Although NH3 is produced innominal quantities from natural sources, it is primarilyproduced in very excessive amounts from anthropogenic sour-ces such as oil reneries, petrochemical plants, paper and pulpindustries, metallurgical operations, textile industries, watertreatment facilities, power plants, composting facilities, factoryfarms, livestock excreta, and exhaust from automobiles.5,10 Thecombined effects of operational setups for ammonia sourcesand their ventilation conditions primarily govern the NH3

concentrations in industrial effluents.8

Rapid and reliable sensing of gaseous NH3 is crucial tosafeguard human health and the environment. The detection ofgaseous NH3 has been conventionally achieved via ion chro-matography,11 colorimetry,12 and optical spectroscopy.13

However, the utilization of these techniques has oen beenrestricted due to many shortcomings, such as the requirementsof complex instrumentation, large operating costs, and time-consuming tendencies (sample pretreatment and prepara-tion).14,15 The drawbacks associated with conventional detectionstrategies have propelled the development of advanced sensorswith great sensitivity, high selectivity, rapid response, and gooddetection limits.16,17 In this respect, recent developments innanotechnology have enabled the fabrication of novel materialswith outstanding gas sensing capabilities.14,18 Graphene is

Prof. Ki-Hyun Kim was at FloridaState University (1984–1986) andthe University of South Floridafor a PhD, respectively. (1988–1992). He was a Research Asso-ciate at ORNL, USA (1992 to1994) and moved to Sang JiUniversity, Korea in 1995. In1999, he joined Sejong Univer-sity. In 2014 he moved toHanyang University. He was rec-ognised as one of the top 10National Star Faculties in Korea

in 2006. He is a serving editorial board member of several journals(e.g., Environmental Research, Atmospheric Pollution Research,and Sensors) and has published more than 570 articles in journalsincluding ‘Chemical Society Reviews’, ‘Progress in MaterialScience’, ‘Progress in Polymer Science’, ‘Coordination ChemistryReviews’, and ‘Trends in Analytical Chemistry’.

22392 | J. Mater. Chem. A, 2018, 6, 22391–22410

a zero band gap carbon allotrope consisting of carbon atomsarranged as a single layer in a hexagonal lattice.19 As summa-rized in Table 1, diverse forms of graphene-based sensingplatforms were introduced for efficient detection of NH3 basedon an electrochemical/conductometric approach in light of thesuperior characteristics of graphene (e.g., large surface area,high mechanical sturdiness, and good thermal and electricalconductivities).20,21 The vast potential of graphene-basedsensing platforms has been recognized further through theformation of composites with unique substrates (e.g., polyani-line (PANI) and polypyrrole (PPy); for more details, refer toSections 3.1 and 3.2).22,23 Also, as graphene can be readily con-verted into reduced graphene oxide (rGO, a multilayeredstructure obtained by reducing graphene oxide (GO)), all ofthese materials hold great promise for gaseous NH3 detection(for more details refer to Section 3.3).21 The graphene-basedNH3 sensing platforms have thus been constructed in a numberof congurations, e.g., pristine graphene, doped graphene,functionalized graphene, and as a composite with other syner-gistic materials.

This review is organized to highlight the detection principlesof graphene-based sensors, along with their practical implica-tions in gaseous NH3 sensing. We also dedicated a specicsection to assess the performance of graphene-based sensorswith other nanosensors for gaseous NH3 detection in terms ofquality assurance parameters (especially, limit of detection andresponse time) for a better understanding. Toward this end, weendeavor to pinpoint the present hindrances in graphenetechnology for gaseous NH3 sensing. Further, we also providefuture prospects for further advancement of this highly exigenteld of research. This review article aims to provide a detailedroadmap for further growth of graphene-based sensing tech-nologies by carefully surveying and detailing the recentadvancements in numerous case studies. Through this reviewarticle, we hope to stimulate the development of similar sensorsfor various other gaseous pollutants.

2. Sensing strategies for gaseous NH3

Until now, a range of sensing strategies have been employed tosense ammonia gas. These sensing strategies range fromcommon human perception to automated digital sensors. Overthe last several decades, the generation of color in sulfur smokein the presence of ammonia has been used to sense the leakageof ammonia gas.24 Observing the changing color of burningsulfur sticks is also a very common method used to check forammonia gas leakage in refrigeration plants. Likewise, the useof litmus paper has also been explored for determination ofammonia gas leakages.24 However, these signaling techniquesare not very reliable for quantication. Moreover, thesemethods are not adaptable if there is a considerably highammonia concentration.

To obtain quantitative, sensitive, and specic signals forammonia gas, several sensing strategies have been exploited,such as solid state, electrochemical, colorimetric, uorescent,and surface plasmon resonance (SPR) methods, as well acousticgas sensors.25–35 To employ any of the above sensing strategies

This journal is © The Royal Society of Chemistry 2018


Table 1 Application of graphene-based sensors for gaseous NH3

Order Graphene-based material

Detection limit

Response time (s) ReferenceRaw information In ppb

1 CVD synthesized graphene 250 ppm 0.25 500 662 CVD synthesized graphene 500 ppb 500 — 393 NSM graphene 1 ppm 0.001 300 254 Ti/Gr 18 ppm 0.018 150 95 TiO2/PPy-GN 1 ppm 0.001 36 416 Graphene/mica 20 ppm 0.02 60 767 NO2-doped graphene 200 ppb 200 — 448 Graphene/Au NP 6 ppm 0.006 — 409 GMHW 0.3 ppm 0.0003 0.4 8110 P-GNS-400 1 ppm 0.001 134 7811 PANI/GO/PANI/ZnO LbL lm 23 ppm 0.023 30 2212 rGO-PANI hybrid loaded PET lm 100 ppm 0.1 20 9513 S,N-GQD/PANI 500 ppb 500 115 4314 rGO-PANI 50 ppm 0.05 1080 9615 Graphene/PANI 1 ppm 0.001 50 2816 aPcM-GO 800 ppb 800 — 10217 f-GO 100 ppm 0.1 — 4518 PEDOT-PSS/GO 1 ppm 0.001 95 10719 rGO 430 ppb 430 — 15120 rGO 1 ppb 1 1.4 11121 rGO/Ag — — 10 11322 NiPc/rGO 800 ppb 800 200 11623 ZnO-rGO 500 ppb 500 50 11724 PPy/rGO 1 ppm 0.001 — 2325 rGO/AgNWs 15 ppm 0.015 60 11426 rGO/P3HT 10 ppm 0.01 141 12327 ZnO/rGO 10 ppm 0.01 78 13328 PMMA/rGO 10 ppm 0.01 60 2629 Pd/SnO2/rGO 5 ppm 0.005 420 12530 Pd NP/TiO2 MR/RGO lm 2.4 ppm 0.0024 184 5731 ZnO NW-RGO 50 ppb 50 50 2532 rGO/TBPOMPc 0.3 ppm 0.0003 — 3733 RGO/3-CuPc 400 ppb 400 — 4234 Cu2O nanorod/rGO 100 ppm 0.1 28 11935 rGO/SnO2-nanorods 200 ppm 0.2 8 121

Review Journal of Materials Chemistry A

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for ammonia, the potential utility of diverse nanostructures(including graphene, metal nanoparticles, carbon nanotubes,polymers, semiconductor nanoparticles, and their composites)has been explored. Out of all tested materials, the superiority ofgraphene-based sensing has been recognized due to its widerange of properties and its applicability as an active material inmultiple sensing strategies. The sensing strategies adoptedusing graphene-based materials for development of ammoniagas sensing technologies are described in detail in this section.

2.1. Graphene-based electrochemical sensing strategy forammonia

The use of graphene-based materials for the sensing ofammonia gas was demonstrated due to its excellence amongmany (aforementioned) sensing strategies such as electro-chemical, SPR-based, and acoustic sensors. In particular, gra-phene-based materials were explored favorably due to theirsuperior electrochemical properties. Until now, graphene-basedmaterials have been tested for the sensing of diverse analytes viacyclic voltammetry (CV), amperometry, potentiometry,


impedance spectroscopy, and conductometry-based electro-chemical approaches.22,25,36–38 The graphene systems wereemployed for ammonia determination in a number of forms,e.g., pristine graphene, functionalized graphene, doped gra-phene, and as a composite with other potent structures.

For examination of different levels of ammonia gas, differentstackings of graphene (prepared by chemical vapor deposition(CVD)) were transferred onto interdigitated electrodes (IDEs).25

The sensor response was then determined by measuring theresistance change in the presence of ammonia gas. The sensorresponse change was quantied by calculating (Rg � R0)/R0 �100%, where Rg and R0 are the resistances of the graphene-based platform aer and before exposure to ammonia, respec-tively. Graphene prepared by CVD methods usually behaves likea p-type semiconductor. Hence, its exposure to ammonia (an n-type dopant for graphene) led to the decrease in the resistance,accompanied by a decrease in the concentration of holes. Theschematic representation of this sensor is shown in Fig. 1.

A variety of substrates (e.g., Si/SiO2, Cu, ITO, and Al2O3) wereexplored for graphene-based materials in designing sensitivesensing strategies for ammonia.39–42 The graphene-based

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Fig. 1 Schematic of an interdigitated electrode-based electro-chemical sensor for NH3.25


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structures were either deposited or transferred onto the desiredsubstrate for fabrication of the working electrode. Furthermodication in the developed layer could then be used forenhancement of sensing signals. To this end, four electrodes ofTi/Au were patterned (in a van der Pauw conguration) on thesurface of graphene (transferred onto a Si/SiO2 substrate froma Cu substrate) via e-beam evaporator deposition.39 A very thinlm of Ti was used to enhance the adhesion between Au and thegraphene surface. Moreover, the use of Au contacts resulted inp-doping of the graphene. The ammonia acts as a reducingagent and an n-type dopant to graphene, increasing the resis-tance. The Labview data acquisition system was used to calcu-late the horizontal and vertical resistances of graphene.Likewise, a TiO2 decorated polypyrrole–graphene nano-composite (TiO2/PPy-GN) was drop-coated onto an ITO surfacebetween two Cu strips.41 The size of the Cu strips was 0.5 cm� 4cm, while a 5 mm distance was maintained between the stripsfor drop-coating of the graphene composite.

The use of Au as a contact material for graphene-basedelectrodes is preferable for ammonia sensing because itincreases the concentration of holes in graphene.39 Interest-ingly, the doping of graphene with sulfur, nitrogen, uorine,and NO2 also enhanced the concentration of holes.43–45 n-Doping with ammonia in such structures causes a decrease inresistance due to decrease in the concentration of holes. It wasassumed that the larger the amount of holes present in gra-phene, the more sensitive the material is for ammoniaquantication.

Graphene-based sensing materials were also explored as partof interdigitated electrodes (IDEs) and micro-electromechanicalsystems (MEMS) for quantication of ammonia.23,42 In general,lithographic techniques are used to prepare these structures,and graphene-based materials were drop-coated onto thesefabricated structures. All the above mentioned electrochemicalsensing strategies work on the basic principle of analyte-based

22394 | J. Mater. Chem. A, 2018, 6, 22391–22410

alteration in the conductance of materials. In general, theexposure of graphene to ammonia gas resulted in an increase inresistance due to the decreased number of holes and change inthe Fermi level of graphene.

2.2. Graphene eld-effect transistors (GFETs)

Field-effect transistor (FET) based devices are promisingcandidates for the development of electronic gas sensors due totheir recognizable merits (e.g., rapid response, high sensitivity,and simple conguration).46 FET-based devices commonlyconsist of channel material, electrodes (source, drain, and gateelectrodes), and gate oxide. The changes in conductancebetween the presence and absence of analyte are used togenerate the sensing signals in FET-based devices. The use ofgraphene for the development of FET-based devices is veryattractive due to the excellent electronic properties, largeeffective surface area, high exibility, direct interaction with theanalyte, and easy functionalization of graphene. However, theuse of intrinsic graphene for FET conducting channels is alwayschallenging due to the zero band gap of graphene that tend toexhibit low on–off current ratios.47 Interestingly, as the elec-tronic properties of graphene can be modulated (e.g., by struc-tural modication, doping of metals and non-metals, moleculardoping, and generation of defects), they are highly suitable forefficient FET-based devices.

FET-based devices developed using graphene-based mate-rials are usually termed graphene FET (GFET). GFET-basedelectronic sensing devices have been developed and exploredfor a variety of sensing applications.44,48–50 Likewise, GFET-baseddevices are also explored for the determination of ammonia.44,51

As a typical example of GFET-based ammonia sensors, a back-gated GFET resistive sensor was fabricated (e.g., 10 mm channelwidth and 25 mm channel length) to provide four probe elec-trical connections using molecular doped graphene.44 The CVDgrown graphene was then transferred to a silicon substrate viaa poly(methyl methacrylate) (PMMA)-supported wet-transferprocess. The patterns and electric connections in graphenechannels were developed further using electron beam lithog-raphy, a low-power reactive-ion-etch process, deposition of Cr/Au (using e-beam evaporation), and a li-off process. Themolecular doping of graphene was achieved by exposure to NO2

gas (100 ppm concentration) at 500 torr for 50 min. The draincurrent (Id)-back-gate voltage (Vg) curves (commonly known asId–Vg curves) were used to monitor the extent of ammoniaexposure on GFETs along with the effect of NO2 doping. It wasobserved that NO2 doping signicantly enhanced sensitivitytoward ammonia (i.e., from 3.7% to 25.3% at 80 ppm ammoniaconcentration).

2.3. Graphene-based acoustic sensing strategy for ammonia

Other sensing strategies employed for graphene-basedammonia sensors use acoustic sensing. Acoustic sensingtechniques have been widely explored for sensitive determi-nation of ammonia using graphene materials. Graphene waseither used in the form of a composite or supporting materialfor development of quartz crystal microbalance (QCM)-based



Fig. 2 Schematic of a QCM-based sensor for determination of ammonia.28


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ammonia sensing devices.27–29 In the case of graphene/poly-aniline and GO/polystyrene, graphene was used as an activematerial for QCM-based sensing of ammonia, while in anotherstudy, graphene was used as isolation layer between thesensing lm (i.e., polyaniline) and QCM electrode. The gra-phene-based structures coated onto QCM exhibited adsorp-tion of ammonia, which led to a resonance frequency shi ofthe QCM (the schematic of this QCM sensor is provided inFig. 2). The shi in resonance frequency was used to measurethe mass of adsorbed ammonia using the Sauerbreyequation:28

Df ¼ �2:26� 10�6f02 � Dm

A(1)

Fig. 3 The experimental set-up for the SPR-based ammonia gas sensor


where Df and Dm are the shis in the resonance frequency ofthe QCM and the addition of mass on the QCM aer exposure toammonia, respectively. In contrast, f0 and A are the funda-mental resonance frequencies of the QCM and surface area ofthe electrode, respectively.

The use of graphene enhanced the adsorption of ammoniaand increased the resonance frequency shi of the QCM. On theother hand, the use of GO as an isolation material between thesensing material and QCM enhanced the stability and quality(Q) factor (note that a QCM with a lower Q value shows a largefrequency uctuation) of acoustic sensors.29 The use of a GOisolation layer conrmed enhancement in the Q factor of theQCM sensor by a factor of two relative to the QCM sensorwithout GO isolation.

.26

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2.4. Graphene-based surface plasmon resonance (SPR)sensing strategy for ammonia

An emerging eld in the quantication of gas pollutants usesefficient sensors based on surface plasmon resonance (SPR). Inaddition to several other nanostructures, graphene-basedstructures have demonstrated an excellent contribution toenhancement of SPR signals of a sensing probe. SPR-basedsensors generally work on the shiing of the resonance wave-length corresponding to an analyte. Using this principle, an SPRprobe for ammonia detection was prepared by depositing a Culm onto the clad portion of an optical ber.26 To enhance thesensitivity of the SPR-based ammonia sensor, the Cu lm wascoated with PMMA and PMMA/rGO. As shown in Fig. 3, thesensing chamber was rst evacuated with the aid of a rotarypump. The ammonia gas was then exposed to the sensingmaterial in an evacuated chamber via steel tubing from theammonia gas cylinder. A tungsten lamp was tted at the end ofthe optical ber, and the transmitted light was recorded usinga spectrophotometer situated at the other end of the opticalber. The reference signals and signals in the presence of theanalyte were used to obtain the nal SPR spectrum of ammoniagas. The graphene-based structure exhibited a larger red shi inthe SPR spectra than the Cu lm alone. The formation ofa complex between the graphene-based material and ammoniawas suspected for the shiing of the SPR curve toward a higherwavelength.26

The above discussed sensing strategies used for determina-tion of ammonia are based on the use of graphene either as theprimary sensing material or as a supporting material toenhance sensitivity. As such, the outstanding properties ofgraphenemake it applicable for development of highly sensitivesensors for ammonia using various methods. The presentsection is organized to describe the basic sensing strategiesemployed for ammonia using graphene-based materials. Thissection is mainly devoted to describing the types of instru-mentation required for sensing, general equations useful forsignal quantications, interaction of ammonia with the activesensing part (e.g., probe), and the role of graphene in thedevelopment of a sensing device using a particular strategy. Inthe next section, we discuss the contributions of graphene inthe development of ammonia sensors based on the aforemen-tioned sensing strategies. Moreover, we evaluate the basiccharacteristics of graphene-based sensing approaches by dis-cussing different sensing parameters, alterations in the sensingsignals upon interaction with ammonia, ways to improveammonia sensing using graphene-based structures, and thespecic sensing mechanisms associated with graphene-baseddevices.

3. Graphene-based sensing of NH3

As an alternative option to conventional sensing strategiesdeveloped for gaseous NH3, electrochemical nanosensors haveattracted a great deal of attention due to their fast response, lowdetection limits, great selectivity, good recovery, and stablesignal generation.52,53 Likewise, the electrochemical sensing

22396 | J. Mater. Chem. A, 2018, 6, 22391–22410

approach has been adopted as a useful tool for detection ofvarious pollutants, including gaseous NH3.54,55 Interestingly, thelimit of detection of electrochemical sensors primarily dependson the electrochemical characteristics of the transducer mate-rial (e.g., graphene).55 Also, the sensitivity of electrochemicalsensors is governed by the conductivity of the nanomaterial.15

As a consequence, the superior conductive characteristics ofgraphene have propelled it as an ideal candidate for fabricationof electrochemical gas sensors with enhanced performance.56

Recent years have witnessed a great rise in endeavors towardgraphene-based NH3 gas sensors owing to their numerousadvantages.25,57 The two-dimensional structure of grapheneensures a very large specic surface area, resulting in enhancedexposure to the target analyte (to induce an enhanced responsetoward the pollutant).58 Also, the high metallic conductivity ofgraphene can cause very low levels of Johnson–Nyquist noise.59

Interestingly, even if no charge carriers are present, a scantyamount of extra electrons can substantially change theconductance level of graphene.21 Graphene-based systems areknown to have negligible crystallographic defects, whichensures a low level of icker noise.20 Moreover, four-probequantication strategies can be successfully implementedusing graphene-based gas sensors (due to the highly planarstructure of graphene) to impart a very low contact resistance.60

In the detection of target analyte using conventional solidgas detectors, inherent noise is oen observed to be larger thanthe produced detection signal (resulting in poor resolution)owing to thermal movement of defects and charges.61,62 Inter-estingly, however, due to its unique structural framework, gra-phene can provide a great solution for gaseous NH3 sensing, asit possesses substantial potential to adsorb environmentalpollutants, which results in variation of its electrical conduc-tivity with a very small signal disturbance.21,63

3.1. Pristine graphene and its composites as gaseous NH3

sensors

The prospective application of graphene-based gaseous NH3

sensors depends on the number of layers in the graphenestructure and its purity level.21 Interestingly, controlled fabri-cation routes can be exploited to tailor the graphene frameworkdepending upon the desired application.64 At present, graphenecan be synthesized via various production routes, such aschemical vapor deposition (CVD), mechanical exfoliation, gra-phene oxide reduction, and silicon carbide annealing.65

However, most of these methods suffer from certain compli-cations, such as less efficient control of size, cost intensiveprocedures, complexities in mass production, and difficulty inthe formation of a large specic surface area.63 Also, the controlof the number of layers in the graphene layer is crucial insensing applications.64 Amongst these methodologies, CVD hasbeen proven to be the most promising owing to the productionof higher quality graphene with a large specic surface area ata relatively low cost compared to other methods.65

In this regard, cost-efficient copper and nickel catalysts andsubstrates have been used in a controlled CVD method tosynthesize large surface area bearing monolayer graphene



Fig. 4 Application of CVD grown graphene sheets for gaseous NH3 sensing. Panel (a): optical micrograph of a graphene film grown by CVD onCu and then transferred onto a Si/SiO2 substrate. Gold contact pads in the van der Pauw configuration were deposited on the film. Panel (b):percentage increase in graphene sheet resistance as a function of time for various NH3 concentrations under ambient conditions. After �360min, the samples were exposed to vacuum desorption conditions and heated to �200 �C using a hot plate, resulting in recovery of graphenesheet resistance; reproduced with permission from ref. 39.


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sheets. These graphene sheets were further transferred ontoa silica insulator to showcase a good NH3 sensing capability(detection limit (DL) of 0.25 ppb with 500 s response time)66

(Table 1). In a similar approach, controlled CVD-grown gra-phene sheets showcased a DL of 500 ppb for gaseous NH3 underambient conditions with good reversibility by desorbing theadsorbed species at 200 �C under vacuum via a hot plate(Fig. 4).39 Graphene showcases a p-type character under ambientconditions owing to the electron extracting nature of oxygenand water functionalities.67 Essentially, the adsorption of NH3

molecules onto graphene sheets is observed to elevate theresistance of the lm through donation of electrons by NH3 toincrease sensitivity.63

In a recent study, monolayered controlled CVD-grown gra-phene sheets were stacked on top of each other to assess theeffect of stacking in reference to non-stacked monolayer (NSM)graphene.25 Interestingly, NSM graphene showcased bettersensitivity and performance (DL of 0.001 ppb with 300 sresponse time) compared to double layer graphene and morestacked, few-layer types of graphene sensors (Table 1) (Fig. 5).

Fig. 5 Application of monolayered and stacked graphene toward gaseouto gaseous NH3 at room temperature. Panel (b): repeatability test of sensomore stacking few layer sensor); reproduced with permission from ref. 2


The ratio of change for integral electrons in the total density ofstates was observed to be 26.65% for NSM graphene upon NH3

exposure, compared to 8.29% for triple layered graphene, whichresulted in better performance of NSM graphene.25 Notably, theeffective sensing area is the same for monolayered and stackedgraphene. However, in stacked systems, the multiple graphenelayers act as barriers for distribution of the doping charge(generated by the adsorbed NH3 molecules), resulting in poorperformance.63

As explained above, NSM graphene showcased betterperformance than double layered graphene. Nonetheless, moreextensive exploration is desirable for three dimensional (3D)graphene-based NH3 sensors due to their potential advantages,especially more surface active sites.68 Research on such systemswas not conducted preferably as the fabrication of most 3Dgraphene-based systems involves uneconomical 3Dtemplates.69,70 To overcome such limitations, a 3D rGO hydrogel(RGOH) was synthesized to allow one-step self-assembly ofsensors through a hydrothermal approach (i.e., bypassing theneed for expensive templates).70 The RGOH was capable of

s NH3 sensing. Panel (a): exposure of three different graphene samplesrs (black: NSM sensor, red: less stacking double layer sensor, and blue:5.

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sensing NH3 at room temperature with a DL of 20 ppm.70 Onsimilar lines, a NaHSO3 functionalized 3D RGOH was designedusing an economic one-step fabrication which was capable ofdetecting NH3 concentrations as low as 1.48 ppm.69 Also, thisRGOH was tted with a microheater to boost the selectivity ofthe sensor towards NH3 through temperature-based rGOmodulation.69 In another study, rGO nanosheets were self-assembled to form 3D networks with enhanced NH3 sensingcapability (e.g., relative to two-dimensional rGO-basedsensors).68 The potential of such 3D sensors should be ascrib-able to increased interactions in the cross-section of rGO.

Although pristine graphene showcases great potential forgaseous NH3 sensing, the lack of surface functionalities results insubpar sensitivity under ambient conditions.71 The incorporationof metal and their oxides into the pristine electronic frameworkof graphene has been proven to elevate its sensing capabilities.72

A Ti-modied graphene-based gaseous NH3 sensor (Ti/Gr) wasdesigned to operate at room temperature, while illuminatedunder visible light (Fig. 6).9 Ti/Gr showcased good performance(DL of 0.018 ppb with 150 s response time) ascribed to thecatalytic synergistic effect between graphene and naturallyproduced TiOx

72 (Table 1). Interestingly, the sensing performanceof metal modied graphene gas sensors can be upgraded if thecombination of electron–hole pairs (e�–h+) is facilitated by nar-rowing the band gap through the illumination with visible light(Fig. 6).73 Along similar lines, pristine graphene, synthesized viathe CVD method using Cu substrates, was transferred onto SiO2/Si supports and decorated with Au nanoparticles (resulting inimproved graphene conductivity) to enhance the sensitivity andperformance in quantifying gaseous NH3 (Table 1).40 Interest-ingly, Au nanoparticles, which bond with graphene structures viaweak van der Waals forces, are useful for preserving the elec-tronic structure of a graphene system.74 Also, the higher workfunction of Au relative to graphene is useful to increase the p-typecharacteristics of graphene due to electron transfer from gra-phene to Au (Fig. 7).75 Interestingly, upon exposure of the gra-phene–Au nanocomposite to gaseous NH3, the Au nanoparticlescan dissociate NH3 molecules to reduce the work function of thenanoparticles.40 As a consequence, the Fermi level is equilibratedthrough the transfer of electrons from Au to graphene, resultingin enhanced sensitivity (Fig. 7).76

Fig. 6 Application of Ti/Gr toward gaseous NH3 sensing. Panel (a): responrelative resistance changes of Ti/Gr devices under visible light illuminatio

22398 | J. Mater. Chem. A, 2018, 6, 22391–22410

In a similar approach, a mica-supported graphene-based-detector was observed to perform well (DL of 0.02 ppb with 60 sresponse time) for gaseous NH3 sensing with an enhancedsensitivity, which can be attributed to the higher p-dopinginduced in the graphene framework by mica (Table 1).72,76

Essentially, the sorbed NH3 molecules on the graphene surfaceelevate the Fermi level. This may heighten the resistance of thegraphene layer, which can be exploited for sensing applica-tions.77 Pristine graphene was doped with NO2 to increase theamount of holes in the graphene structure, leading to bettersensitivity and performance in sensing gaseous NH3 (Table 1).44

Phosphorus-modied graphene sheets (P-GNS-400) weresynthesized at elevated temperatures via annealing of grapheneoxide and by combining with triphenylphosphine for thesensing of gaseous NH3.78 The P-GNS-400 displayed goodperformance (DL of 0.001 ppb with 134 s response time) towardNH3 sensing, which may be attributed to the presence of elec-tron rich phosphorus species (Table 1).78

A recent trend in graphene-based gas sensors is fabricationof passive optical sensors for highly specic operations to effi-ciently remove or suppress electromagnetic interferences.79

Such sensors exploit the optical intensity attenuation effect ofgraphene to sensitively detect hazardous gases like NH3.80 Inthis regard, a graphene/microber hybrid waveguide (GMHW)was fabricated as an optical gas sensor for NH3. The GMHWshowcased excellent performance (a DL of 0.0003 ppb with a 0.4s response time) to support the great potential of such a sensingmethod (Table 1).81 Essentially, sorption of NH3 moleculesaffects the conductivity of graphene to alter the refractive indexof the GMHW.79 The resulting shi in wavelength is spectrallyderegulated via a microber-based interferometer.80

Interestingly, advanced forms of graphene composites withfunctional materials (e.g., metal–organic frameworks (MOFs))have also been investigated for various sensing applications ofgaseous targets owing to their highly target specic and largeadsorption capacities.63,82 In fact, MOF/graphene sensors havealready been successfully applied for the detection of manyother hazardous gaseous volatile organics such as formalde-hyde, acetone, hydrogen sulde, and nitrobenzene.14,63,82 Theuse of such composite forms for NH3 sensing application is notyet found, although they can theoretically afford the synergistic

se of Ti/Gr devices to NH3 gas under visible light illumination. Panel (b):n at room temperature; reproduced with permission from ref. 9.



Fig. 7 Application of graphene/Au nanoparticles toward gaseous NH3 sensing. Panel (a): theoretical energy band diagram of graphene/Aunanoparticles contacts. Panel (b): response of the device for 58 ppm of NH3 in dry air at room temperature along with fitted data based on theFreundlich isotherm model; reproduced with permission from ref. 40.


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combination between the high selectivity of MOFs and greatsensitivity of graphene.82 The possibility of developing advancedforms of sensing systems can be backed up by a number ofstudies. For instance, an MOF (Cu3(HITP)2) has displayed goodsensing capability for NH3 with a detection limit of 0.5 ppm.83

Hence, MOF-based systems are expected to be integratedsuccessfully with graphene-based detectors for the constructionof effective sensing systems for NH3.

3.2. Conducting polymer-based graphene sensors

Conducting polymers (e.g., polyaniline (PANI) and polypyrrole(PPy)) have been utilized extensively in modern gas sensorsowing to their manifold advantages such as good electricalconductivity, great stability under ambient conditions, uniqueredox properties, and easy synthesis.84,85 Interestingly, thespecic surface area and morphology of these conductingpolymers can be easily tailored to suit specic applications viasuitable selection of dopants and their concentrations duringsynthesis in the solution phase.85,86 These unique characteris-tics of conducting polymers have been exploited in modern gassensors to improve the limit of detection, sensitivity, andresponse time.87 The synergistic combination of graphene andconducting polymers has been proposed as a unique option toimprove the selectivity and sensitivity of graphene-based plat-forms in sensing gaseous NH3.88 Also, the utilization of hybridsensor systems was demonstrated to be efficient in overcomingthe drawbacks of pristine conducting polymers, which havea low sensitivity in sensing gaseous NH3 and poor thermalstability.87

A TiO2-decorated nanocomposite of PPy and graphene (TiO2/PPy-GN) displayed good performance for detection of gaseousNH3 (a DL of 1 ppt with a 36 s response time) (Table 1).41

Essentially, TiO2 possesses great gas sensing capabilities withthe added advantage of a highly inert surface.89 TiO2 is an n-typesemiconductor with a 3.3 eV band gap and is capable ofproviding a practical sensing range for gaseous NH3.90 However,as TiO2 is largely ineffective at ambient temperature, it needs tobe used in the form of specialized composites for meaningful


operation.89 In the case of TiO2/PPy-GN, a p–n junction formsbetween TiO2 (n-type characteristics) and PPy-GN (p-type char-acteristics).41 This heterojunction results in the formation ofa depletion layer (positively charged) on the TiO2 surface,leading to enhancement of sensitivity through a reduction insorption enthalpy and the activation barrier for NH3 mole-cules.88 A graphene/PANI nanocomposite was synthesized foreffective sensing of gaseous NH3 (DL of 0.001 ppb with 50 sresponse time) (Table 1).28 As the graphene/PANI sensor comesin contact with gaseous NH3, protons from the –NH-function-alities are transferred to the NH3 gas molecules. These trans-ferred protons contribute to conversion of NH3 into NH4

+ andconverting PANI into a base form through a reversible interac-tion.91 This chemical interaction helped to increase the elec-trical resistance of the sensor by localizing polarons in thegraphene/PANI framework.92

It is well-known that pristine graphene showcases a substan-dard dispersion in commonly used solvents. Hence, the use ofpure graphene is generally avoided in the formation of hybridcomposites with conducting polymers due to the very complexand expensive synthesis procedures.93 In contrast, GO is knownto be highly hydrophilic (due to the presence of numerous epoxyand hydroxyl functionalities) with great dispersibility in mostsolvents, with several growth and nucleation sites for polymers.94

In light of these factors, GO, ZnO, and PANI were synergisticallycombined to obtain hybrid layer-by-layer lms (PANI/GO/PANI/ZnO LbL) for effective sensing of gaseous NH3 (DL of 0.023 ppbwith 30 s response time) (Table 1).22 Interestingly, it was observedthat the net electrical resistance of the sensor system was low-ered with an increase in the amount of tetralayer graphene,which reects augmentation by PANI (Fig. 8).22

Pristine GO displays poor electrical conductivity due to manydefects in its structure, and it is less suitable for direct gassensing applications.63 In this regard, GO is oen reducedchemically to rGO to restore the conjugation (sp3 hybridizedcarbon atoms are converted to the sp2 state), which results inimproved electrical conductivity.21 An inexpensive and highlyexible polymer polyethylene terephthalate (PET) was used as

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Fig. 8 Application of PANI/GO/PANI/ZnO LbL films toward gaseous NH3 sensing. Panel (a): sensor responses at different concentrations of NH3.Panel (b): change in electrical resistance as a function of time under NH3 exposure; reproduced with permission from ref. 22.


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a substrate to synthesize an rGO-PANI loaded hybrid gaseousNH3 sensor (a DL of 0.1 ppb with a 20 s response time) (Table1).95 Essentially, upon exposure to NH3 gas, the transfer ofelectrons may take place between the rGO and conjugated PANIvia p–p interactions to acquire improved sensitivity.92 This rGO-based PANI sensor also displayed great selectivity toward NH3 inthe presence of other common volatile gaseous pollutants suchas acetone, ethanol, methanol, formaldehyde, and ethylbenzene.95 The intriguing sensitivity of the rGO-PANI hybridloaded PET lm can be ascribed to the acid–base deprotonationactivity of PANI upon exposure to gaseous NH3.63 Along similarlines, an rGO-PANI-based gaseous NH3 sensor was effectivelyfabricated by anchoring PANI particles onto rGO nanosheets.The rGO-PANI hybrid particles were utilized as an oxidant aswell as a template during polymerization of aniline monomericunits.96 The developed system demonstrated a fairly averageperformance compared to other sensors (a DL of 0.05 ppb witha 1080 s response time) (Table 1).

For sensing applications, PET was used as a substrate tofabricate graphene quantum dots (tied edge graphene sheetswith a lateral size #100 nm) doped with N and S (S,N-GQD/PANI; a DL of 500 ppb with a 115 s response time). The resultingsensor presented great selectivity toward NH3 in the presence ofother volatile gaseous contaminates such as acetone, toluene,methanol, ethanol, propanol, and chlorobenzene (Fig. 9).43 Theperformance of this sensor was ascribed to the proliferation ofcharge carriers similar to holes with increased involvement of pelectrons.43 Graphene quantum dots have attracted substantialattention for fabrication of gas sensors owing to their uniquecharacteristics governed by edge effects and quantum conne-ment, such as great photostability, unique optical and elec-tronic properties, low toxicity, high chemical inertness, andgood uorescence.97,98

3.3. Graphene oxide (GO)- and reduced graphene oxide(rGO)-based gaseous NH3 sensors

Graphene oxide (GO) has been extensively applied in gassensing technologies owing to its large surface area, unique two-dimensional framework, surface functionalities rich in oxygen,

22400 | J. Mater. Chem. A, 2018, 6, 22391–22410

less noise in electrical signals, and great mobility of elec-trons.20,21 GO is oen combined with suitable substrates (e.g.,polymers, metal oxides, and transition metals) to compensatefor the demerits of pristine GO, e.g., poor selectivity, easyrecovery, and low sensitivity toward gaseous NH3.99,100 In thisregard, metal phthalocyanines (PcMs) have attracted greatattention as they possess a distinctive 18p-conjugated frame-work with a diversity of central metallic centers with tunablesurface functionalities.101 Cobalt (Co(II) ion) bearing tetra-b-aminephthalocyanine, coupled covalently with GO, was appliedfor sensing gaseous NH3 with a DL of 800 ppb (Table 1)(Fig. 10).102 Interestingly, the sensitivity of such sensors hasbeen observed to increase as PcMs increased the number ofavailable surface active sites for NH3 molecules (Fig. 10(a)).103

Also, the synergistic effect between PcMs and GO was exploitedby such sensors owing to the excellent electrical properties ofgraphene (Fig. 10(b)).102

The uorination method was utilized to insert uorineatoms onto the GO surface to form f-GO sensors for gaseousNH3 (DL of 0.1 ppb) (Table 1).45 As uorine is highly electro-negative, it leads to elevation of the surface polarity of GO,which in turn diminishes the Fermi level accompanied by anincreased amount of holes.104 Heterojunctions based onSchottky diodes have also attracted great attention as prospec-tive candidates for detection of hazardous gases like ammoniaat a low concentration (ppb) level.105 As gaseous NH3 moleculescome into contact with diode layers, barriers, and interfaces, thechemical make-up of the diode facilitates generation ofa current–voltage (I–V) response.106

A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT-PSS) layer was deposited on an n-type GO sample.107

The PEDOT-PSS GO sensors displayed relatively enhancedsensitivity but with a rather slow response (a DL of 0.001 ppbwith a 95 s response time) in the presence of common volatilegaseous pollutants such as ethanol, methanol, propanol,acetone, and chlorobenzene107 (Table 1). Essentially, the oxygenfunctionalities present on the GO surface should help PEDOTadhere to the sensor surface, such that PEDOT can be separatedfrom the PSSs. Also, the oxygen functionalities can electrostat-ically induce repulsions between positively charged PEDOT



Fig. 10 Application of aPcM-GO toward gaseous NH3 sensing. Panel (a): a schematic illustration of the gas sensing mechanism of aPcM-GOsensors upon interaction with NH3. Panel (b): response of GO and aPcM-GO hybrid sensors to different concentrations of NH3 gas at 28 �C;reproduced with permission from ref. 102.

Fig. 9 Selectivity of the flexible pure PANI and S,N-GQD/PANI hybrid gas sensor to the vapors of various volatile pollutants at 100 ppm at 25 �C at57% relative humidity; reproduced with permission from ref. 43.


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chains to generate surface active sites for capturing NH3

molecules (Fig. 11(a)).108 Interestingly, as the amount of GOpresent in the sensor increased, the number of oxygen func-tionalities increased the electrostatic bonding between GO andPEDOT. As a consequence, a reduction in electrostatic repulsionis observed amongst the positively charged PEDOT chainsthrough the formation of a coiled structure (Fig. 11(b)).107

In recent times, the great potential of rGO as a prospectivecandidate for gas sensors has been recognized owing to itscapability for large-scale production and processability insolutions along with its large specic surface area.109,110 GO canbe reduced chemically via a pyrrole to make rGO-based sensorswith good performance and selectivity toward gaseous NH3 (aDL of 1 ppb with a 1.4 s response time) (Table 1).111

Noble metals such as Au, Ag, and Pt are oen used to formcomposites as a means to enhance their sensing capabilities


toward gaseous pollutants.112 In this respect, Ag nanoparticlesdeposited on an rGO surface via a mini-arc plasma-based vapordeposition approach (rGO/Ag) were tested to detect gaseousNH3 (Fig. 12).113 In a similar approach, rGO nanosheets werecoated onto Ag nanowires (rGO/AgNWs) to fabricate sensors foreffective detection of gaseous NH3 with good stability andrecovery (a DL of 0.015 ppb with a 60 s response time) (Table1).114 The AgNWs provided a great pathway for transfer of chargecarriers.115 However, the amount of AgNWs combined with rGOneeds to be controlled, as an excess concentration of AgNWsmay lead to their agglomeration to form undesirable pits on therGO surface.114 The presence of pits is thought to reduce theresponse through a reduction of the sorption capacity.115

Tetra-a-iso-pentyloxyphthalocyanine nickel (NiPc) wassynergistically combined with rGO to fabricate sensors forgaseous NH3 detection with a DL of 800 ppb (a response time of

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Fig. 11 Conformations of a PEDOT:PSS : GO sensing film after adding GO to PEDOT:PSS. Panel (a) 0.04 wt% GO. Panel (b) 0.08 wt% GO;reproduced with permission from ref. 107.

Fig. 12 Application of rGO/Ag toward gaseous NH3 sensing. Panel (a): schematic illustration of the process to fabricate rGO/Ag hybrid sensordevices and the subsequent sensing measurements. Panel (b): dynamic sensing response evolution of rGO with different loadings of Agnanoparticles; reproduced with permission from ref. 113.


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200 s) (Table 1).116 Essentially, anchoring of NiPc onto the rGOsurface (e.g., via p–p stacking) efficiently ameliorated electrontransfer and electrical conductivity.116 ZnO nanowires were alsocombined with rGO for detection of gaseous NH3 (a DL of 500ppb with a 50 s response time) (Table 1).117 The synergisticcombination of the n-type properties of ZnO and the intrinsiccharacteristics of rGO led to increases in the amount of activesites and in the electrical conductivity.118 Likewise, Cu2Onanorods were synergistically combined with rGO to effectivelydetect gaseous NH3 with good sensitivity at room temperature (aDL of 0.1 ppb with a 28 s response time)119 (Table 1). Interest-ingly, when Cu2O nanorods combined with rGO are exposed toatmospheric air laden with NH3, the oxygen species was seen tochemisorb ammonia by capturing free electrons from theconduction band, which was accompanied by reduced resis-tance and an elevated number of vacancies.120 The observedeffects of chemisorption should be ascribable to the return ofelectrons to the conduction band.119 Similarly, SnO2 nanorodswere synergistically combined with rGO to effectively detectgaseous NH3 (a DL of 0.2 ppb with an 8 s response time) at roomtemperature with good sensitivity121 (Table 1). The good

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performance of this sensor was attributed to the high number ofsurface active sites, coupled with formation of ohmic junctionsat the semiconducting SnO2–rGO interface.120

Polypyrrole (PPy) was anchored on the surface of rGOnanosheets to fabricate a PPy/rGO hybrid composite sensor forgaseous NH3 detection (a DL of 0.001 ppb) (Table 1).23 Thesuperior performance of PPy/rGO was ascribed to the formationof hydrogen bonding and p–p stacking between rGO and PPy,which resulted in a faster transport of charge carriers and a highavailability of active surface sites for NH3 molecules122 (Fig. 13).Poly (3-hexylthiophene) was coupled with rGO to form nano-composite (rGO/P3HT) sensors for gaseous NH3 detection (a DLof 0.01 ppb with a 141 s response time) with appreciableselectivity in the presence of interfering substances like CO2,CO, SO2, and NO2 (ref. 123) (Table 1). The performance of rGO/P3HT was also attributed to the p–p interactions between P3HTand rGO, which led to formation of a p–n junction structure.124

A ternary nanocomposite lm sensor, synergistically combiningPd, SnO2, and rGO (Pd/SnO2/rGO), was developed for effectivedetection of gaseous NH3 (a DL of 0.005 ppb with a 420 sresponse time) (Table 1).125 As the NH3 molecules are adsorbed




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onto this ternary sensor, the resistance is reduced due to releaseof electrons into the conduction band, which are transferred viaefficient pathways provided by rGO.21 In a similar approach,a ternary nanocomposite based on Pd nanoparticles, TiO2

micro-rods, and rGO (Pd NPs/TiO2 MRs/rGO) was synthesizedvia a one pot polyol method with subsequent annealing.57 Thisternary sensor showed sensing performance for gaseous NH3 (a

Fig. 13 Schematic of interaction of NH3 with a PPy/rGO hybrid;reproduced with permission from ref. 23.

Fig. 14 Application of rGO/3-CuPc toward gaseous NH3 sensing. Panel (ahybrids. Panel (b): response of the rGO/3-CuPc hybrid sensor to 3200 pCH4, and CO gas; reproduced with permission from ref. 42.


DL of 0.0024 ppb with a 184 s response time) at room temper-ature with great stability, sensitivity, and selectivity (Table 1).57

Essentially, Pd species tend to preferably combine with oxygenfunctionalities of the rGO, such that new pathways are createdfor easy conduction of electrical signals along with an increasednumber of surface active sites for gaseous NH3 molecules.126

The PcM molecules oen tend to agglomerate during theformation of the lm. If such agglomeration occurs, it candrastically hamper the gas sorption capacities and/or chargetransfer characteristics of the PcMs.101 To overcome this draw-back, PcMs are oen combined with some other novel materialsto form nanocomposites for optimal detection of hazardousgases like ammonia.127 Specically, rGO was functionalized withPb-based 1,8,15,22-tetra-(4-tert-butylphenoxyl)-metal-lophthalocyanine (rGO/TBPOMPc) through a self-assemblysolution based method involving p–p stacking.37 This nano-composite sensor displayed good performance for NH3 sensing(a DL of 50 ppb with a 50 s response time) at room temperaturewith good recovery (Table 1).37 TBPOMPc is known to containrigid phenoxyl functionalities that can efficiently prevent itsagglomeration. Also, these phenoxyl groups are helpful inreducing the charge transfer resistance in the nanocompositesensor while increasing the number of available active surfacesites for gaseous NH3 adsorption.101,127 In a similar approach,rGO was functionalized with 1,8,15,22-tetra-iso-pentyloxyph-thalocyanine copper (rGO/3-CuPc) through chemical tech-niques involving p–p stacking (Fig. 14(a)).42 This sensor showedaverage detection capability for gaseous NH3 (a DL of 400 ppb)with good reversibility and selectivity in the presence of H2,CH4, CO, and CO2 (Fig. 14(b)) (Table 1).42

Recent advancements in gas sensor technology have resultedin increasing interest in sensors based on surface plasmon

): synthetic process and schematic interaction process of rGO/3-CuPcpm NH3 compared with those of the same concentrations of CO2, H2,

J. Mater. Chem. A, 2018, 6, 22391–22410 | 22403



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resonance (SPR) owing to their manifold practical applicationsin biomedical and physicochemical sciences.128,129 A surfaceplasmon is essentially an electromagnetic wave that has beenpolarized trans-magnetically via p-polarized light, typically ata dielectric–metal interface.130 Exponential decay is observed forthe electric eld of the surface plasmon, which most oenrequires optical bers, prisms, or metamaterial-based wavevector matching for excitation.128 Fiber optic sensors based onSPRs utilize the wavelength interrogation (calibrated on thebasis of the refractive index of the sensing medium) techniqueto detect gaseous pollutants.129 In this regard, poly(methylmethacrylate) was coupled with rGO to form nanocomposites(PMMA/rGO) for effective detection of gaseous NH3 based onSPR technology (a DL of 0.01 ppb with a 60 s response time)26

(Table 1). Upon interaction of NH3 molecules with the over-layerof the sensor, the refractive index of the detection layerundergoes an alteration, which can be exploited for successfuldetection of ammonia.130

4. Performance comparison betweengraphene-based gaseous NH3 sensorsand other nanomaterials

The effective utilization of graphene-based systems is a steptoward redressing the various shortcomings of previousconventional NH3 gas sensing methodologies. The advanta-geous properties of graphene-based materials (such as largespecic surface area, great mechanical strength, and goodelectrical/thermal conductivity) make them ideal candidates foran electrochemical sensing system for NH3. The usage of gra-phene-based gas sensors for NH3 thus holds great potential,which is oen functionally similar (the presence of comparableoxygen-rich functional groups on the surface of materials withsemiconducting properties) to other prospective candidates(e.g., metal oxides and carbon nanotubes) (Table 2).

This section is dedicated to specically comparing the perfor-mance of graphene-based NH3 sensing platforms in relation to

Table 2 Application of nanomaterials for gaseous NH3 sensing

Order Nanomaterial

Detectio

Raw info

1 ZnO–PANI 10 ppm2 PANI-CNT 4 ppm3 Pd–PANI 10 ppm4 SWCNT–pyrene 0.5 ppm5 Mn-doped ZnO nanosphere 20 ppm6 Al–ZnO/CuO 100 ppm7 Ag/ZnO 10 ppm8 HCI doped-PANI-nanober/WS2

nanosheet50 ppm

9 PQT-12/CdSe QD 20 ppm10 InHCF-NP 3 ppb11 WS2/TiO2 QD 0.8 ppm12 Silica modied-CeO2 0.5 ppm13 ZnFe2O4 100 ppm14 Pt/NiO thin lm 10 ppb

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other nanomaterial-based sensing systems developed recently(e.g., metal oxides, quantum dots, and carbon nanotubes) forfurther improvement. As can be seen from Tables 1 and 2, the NH3

sensing capability of graphene-based systems is superior to othersystems based on advanced nanostructures in terms of detect-ability. Nevertheless, in some instances, the advanced nano-materials outperformed graphene-based systems in terms ofdetection limit and/or corresponding response times. The lowestdetection limit of 0.0003 ppb was reported amongst the graphene-based systems using the GMHW and rGO/TBPOMPc37,81 (Table 1).However, an extremely low response time of 0.4 s makes theGMHW themost promising candidate among the graphene-basedsensors (Table 1). For other advanced nanostructures, the lowestreported DL was 0.0005 ppb for a single wall carbon nanotube–pyrene (SWCNT–pyrene) composite and silica doped CeO2 (ref.131 and 132) (Table 2). However, SWCNT–pyrene displayed a fastresponse time of 5 s over the silica modied CeO2 (760 s), con-rming the superior performance of the former over the latter(Table 2). An analysis of Tables 1 and 2 clearly indicates thatgraphene-based systems show better results not only in responsetimes, but also in sensitivity compared to other nanostructures.

The major ascendancy of graphene-based sensing is attrib-uted to its capacity to adsorb NH3 molecules to alter theinherent conductance level of the graphene framework withrelatively less noise, while interacting directly with theammonia molecules.20,21 In many other nanomaterial-basedNH3 sensors, chemisorption of oxygen onto the nanomaterial tocapture conduction electrons for formation of O2

�, O2�, and O�

plays a prominent role in generation of a detection signal.133

However, these sensors may have a relatively poor selectivitytoward NH3, with insignicant noise levels compared to gra-phene-based systems.132 Also, temperature should be adjustedto a certain level for regulation of chemisorbed oxygen speciesupon NH3 exposure. Interestingly, the high selectivity of gra-phene-based systems for NH3 at room temperature has beendemonstrated, while this is not the case for other gaseouspollutants (e.g., hydrogen sulde) under similar temperatureconditions.14

n limit

Response time (s) Referencermation In ppb

0.01 21 1520.004 18 1530.01 100 1540.0005 5 1310.02 4 1340.1 14 1550.01 — 1560.05 260 54

0.02 50 1573 360 1580.0008 — 1590.0005 760 1320.1 95 16010 15 106




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Various modication schemes have been developed forsynthesis of advanced nanocomposites as a means to overcomethe intrinsic shortcomings of pristine frameworks in thesensing of gaseous pollutants.63 As discussed earlier, conduct-ing polymers such as PANI have oen been combined withpristine frameworks to ameliorate their electrical properties forsensing applications (for more details refer to Section 3.2).22 Inthis regard, PANI nanobers were synergistically combined withWS2 nanosheets to maximize the sensitivity toward gaseousNH3 (DL of 0.05 ppb with 260 s response time)54 (Table 2). Ina manner similar to graphene/PANI nanocomposites, PANI-nanober/WS2 nanosheets detected gaseous NH3 throughformation of p–n heterojunctions between p-type PANI and n-type WS2. However, PANI-nanober/WS2 nanosheets displayedpoor performance under similar conditions compared to gra-phene/PANI composites (a DL of 0.001 ppb with a 50 s responsetime) (Tables 1 and 2). On a similar note, cheap and biocom-patible ZnO nanospheres with favorable electrical propertieswere doped with Mn (to increase the amount of surface defects)for effective sensing of gaseous NH3 (a DL of 0.02 ppb with a 4 sresponse time).134 The detection principal of an Mn-doped ZnOnanosphere is based on the chemisorbed oxygen mechanism,as explained earlier. The Mn-doped ZnO nanosphere out-performed ZnO-based graphene NH3 sensors (e.g., PANI/GO/PANI/ZnO LbL lms, ZnO-rGO, and ZnO NW-RGO) both interms of detection limit and response time. This may beaccounted for by the better capture capacity for conductingelectrons by ZnO nanospheres relative to ZnO/graphenesensors134 (Tables 1 and 2).

5. Challenges in graphenetechnology for NH3 sensing

The potential of graphene-based sensors for effective detectionof gaseous pollutants (e.g., NH3) has been well documented inthe literature. Nevertheless, future implications of graphenetechnology for practical sensing applications are not yet fullyrecognized. In this regard, the major hindrances include theinability of present synthesis routes to produce uniformly highquality graphene in large quantities, lack of theoreticalmodeling and simulation studies for better understanding ofprevailing mechanisms, poor recovery of graphene-based gassensors post application, and poor electrical stability.21,135 In thesubsequent subsections, we briey explore these challengingissues that serve as limiting factors for progress of graphene-based gas sensors (e.g., transition from laboratory experimentsto efficient commercial applications). We also provide a newperspective on strategies to overcome these barriers.

5.1. Large scale production of graphene

Graphene-based detectors will not reach the status of cheapcommercial gas sensors unless they can be produced on a largescale with desired quality and uniformity. Conventionally, top–down (graphite oxidation by Hummer's oxidation followed bysubsequent reduction) and bottom–up (thermal desorption onmetal substrates or Si and chemical vapor deposition)


approaches have been used to synthesize graphene.136,137 Thebottom–up approach produces graphene with good quality,relatively few defects, and good characteristics for electronicapplications.65 However, as its yield is very low, it is only suitablefor laboratory applications.138 Although the top–down approachcan produce large amounts of graphene, the samples aregenerally of poor quality for sensing applications due to struc-tural defects.65 Also, top–down methods typically demandexpensive chemicals (e.g., sulfuric acid) in large quantitiesaccompanied by hazardous manufacturing conditions (e.g.,a high thermal reduction temperature). As such, top–downmethods are not yet sufficiently attractive.137

New and innovative bulk production approaches are beingdeveloped and studied for possible production of high qualitygraphene with desired properties. In this regard, the non-elec-tried electrochemical exfoliation technique has recently beendemonstrated to provide a good yield ($80%) of high qualitynanosheets of graphene with great electrical conductivity forelectronic purposes.139 Also, a non-dispersion exfoliationapproach was recently proposed for large-scale production ofhigh-quality graphene sufficiently suitable for sensing applica-tions.140 Moreover, as this new strategy is water-based, it is alsobenecial to avoid utilization of toxic and expensive solventsand chemicals.140

5.2. Modeling and simulation

A proper understanding of the underlying mechanismsinvolved in the interaction of graphene-based sensors withgaseous NH3 molecules is imperative for designing advancedsensors with improved performance. Graphene-based sensorshave been applied for sensing of various gaseous pollutants.135

However, a single universal sensor cannot be designed to detecta wide-range of gases because not all the adsorbed gas mole-cules may produce detectable signals upon interaction with theparticular material used for fabrication.21 Although graphenematerials are found to have excellent sensitivity, they are oencomposited with other advanced functional materials or poly-mers to enhance their selectivity towards NH3 molecules. Theselection of a suitable material to form graphene compositesremains as a big practical challenge along with the solution andrenement of their selectivity towards NH3 under real-worldconditions. Moreover, as discussed in the previous section,large variations in the performance of different graphene-basedNH3 sensing devices have been observed. As a result, a suitabledesign of the detector structure with enhanced sensing perfor-mance is an essential element to further upgrade the detectioncapacities of graphene-based sensing systems.

Theoretical modeling and simulations have been demon-strated to be highly useful in predicting the output signals whengaseous pollutants interact with sensors.141 As a result, sensorperformance can be optimized and enhanced by simulatingdifferent novel materials against the target analyte. Suchmodeling should thus help develop a suitable detector designfor a gas sensing application, which can then be veriedthrough experimental analysis for real world applications.142

Although many recent studies exist on the modeling and

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simulation of graphene-based gas sensors, none specicallyfocused on NH3. As such, it remains a wide-open eld ofresearch.143,144 Recently, SIESTA and COMSOL programs wereapplied to model SnO2/rGO gas sensors for detecting NO2.145

Similar modeling studies are thus imperative to maximize theefficiency of graphene-based gas sensors for NH3.

5.3. Slow recovery and electrical stability

Graphene-based gas sensors have shown great potential towardthe sensing of NH3. However, the pristine forms of rGO, GO,and graphene oen show slow recovery and poor electricalstability during the sensing operation.146 As a consequence, theyare oen synergistically combined with different materials toboost their sensing performance (Table 1). For instance, inorder to invigorate the stability of electrical signals for gra-phene-based sensors, they are oen coupled with conductingpolymers such as PANI and PPy.147 Such a synergistic combi-nation was proven to greatly improve the electrical conductivity,response time, sensitivity, and detection limit (for more detailsrefer to Section 3.2).84,85 Also, pristine GO and rGO have oenbeen combined with metal oxides and other materials toameliorate their NH3 sensing performance (Table 1) (for moredetails refer to Section 3.2). Such possibilities for new syner-gistic combinations have been tested extensively. Developmentof practical hybrid graphene-based sensors that are economicaland sufficient to exhibit enhanced performance is imperative topropel graphene-based gaseous NH3 sensors as practical real-world detectors.

5.4. Requirement of high working temperature and lowpressure/vacuum

Graphene-based gaseous NH3 sensors oen require optimizedhigher working temperatures ($100 �C) and relatively lowpressures to work efficiently.21,148 Essentially, heated evacuatedchambers are oen used for graphene-based sensing applica-tions to activate the detection material by desorbing the sorbedgases on its surface.149 In addition to cleaning/activationpurposes, these conditions ensure elevated mobility of elec-trons or defects in the graphene framework, resulting inelevated detection capabilities.150 Essentially, as the adsorbedgases (e.g., oxygen) attract free electrons, the resulting potentialbarriers with poor ow of electrons should be regulated throughtemperature/pressure optimization.20,25 To make the graphenetechnology more attractive in a practical sense, more attentionneeds to be shied toward fabrication of hybrid composites thatcan operate under ambient conditions to avoid high mainte-nance/operation costs.

6. Conclusions and future prospects

This review article accentuates diverse graphene-based sensingapproaches for coherent sensing of gaseous NH3. The super-cilious electrical properties and framework attributes of gra-phene (e.g., large specic surface area, tunable functional sites,great mechanical strength, and good electrical and thermalconductivities) make it an attractive platform for the sensing of

22406 | J. Mater. Chem. A, 2018, 6, 22391–22410

NH3. The NH3 sensing potential of electrochemical graphenesensors has been elucidated in terms of detection limits andresponse times (Table 1). The ascendancy of graphene-basedgas sensors is attributed to the fact that they can detectpollutant molecules at very low concentrations (e.g., less thanppb).

Essentially, the conductivity of graphene is substantiallyaltered upon interaction with the target analyte, with the help ofmeagre amounts of electrons, as elucidated in Section 3.1.Moreover, in order to ameliorate the stability of the generatedelectrical signals, graphene is oen synergistically combinedwith conducting polymers such as PANI. The graphene-con-ducting polymer hybrid sensors were demonstrated to haveimproved selectivity and sensitivity toward NH3. Also, thesynergistic combination of GO and rGO with other advancedmaterials (e.g., metal oxides) led to considerable enhancementin selectivity, recovery, and sensitivity toward gaseous NH3.

In Section 4, a comparative analysis was performed to assessthe performance between graphene-based sensors and otheradvanced nanomaterial-based sensors (Table 2) to offer a prac-tical knowledge on the scope of graphene technology for NH3

sensing. Graphene-based sensors were observed to showcasebetter performance when juxtaposed with other advancedsensors, both in terms of sensitivity and selectivity. Criticalhindrances in advancement of graphene technology as pro-cient gas sensors nonetheless exist and are explained as (i)uneconomical and cumbersome large-scale production tech-niques of graphene, (ii) a lack of modeling and simulationstudies, and (iii) poor electrical recovery and response of pris-tine graphene-based systems. These drawbacks can be effec-tively minimized through a number of routes: utilizing some ofthe advanced synthesis routes introduced recently (e.g., non-electried electrochemical exfoliation), investigating the gra-phene–NH3 interactions through advanced simulation tech-niques, and appropriately selecting other advanced materials tobe synergistically combined with graphene-based systems toproduce more efficient gas sensors.

A vigorous collaboration among various scientic areas mayeffectively vanquish the present hindrances while enhancingthe present understanding of graphene-based NH3 detectors interms of functionalities, properties, and sensitivity. Newresearch on graphene sensors should be extended with the aimof designing graphene-based NH3 sensors with advancedfunctionalities that are sensitive, swi, mobile, robust,economical, and practical in nature. Such efforts will assist inthe progress of graphene-based systems to ultimately makesignicant contributions toward the advancement of air qualitymanagement strategies.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

This research acknowledges the support provided by the R&DCenter for Green Patrol Technologies through the R&D for




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Global Top Environmental Technologies funded by the Ministryof Environment (MOE) and by a grant from the NationalResearch Foundation of Korea (NRF) funded by the Ministry ofScience, ICT, & Future Planning (Grant No.2016R1E1A1A01940995). KHK also acknowledges support bythe Korea Ministry of Environment (MOE) (2015001950001) aspart of “The Chemical Accident Prevention Technology Devel-opment Project”. VK acknowledges the support from theDepartment of Science and Technology, New Delhi, India forthe INSPIRE Faculty Award.

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