Graphene Composite for Detecting NO2 at

nanomaterials Article

Preparation of g-C3N4/Graphene Composite for Detecting NO2 at Room Temperature Shaolin Zhang 1 , Nguyen Thuy Hang 1 , Zhijun Zhang 1 , Hongyan Yue 2 and Woochul Yang 1, * 1 2

*

Department of Physics, Dongguk University, Seoul 04620, Korea; [email protected] (S.Z.); [email protected] (N.T.H.); [email protected] (Z.Z.) School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150040, China; [email protected] Correspondence: [email protected]; Tel.: +82-2-2260-3185

Academic Editors: Xianghui Hou and Fang Xu Received: 8 November 2016; Accepted: 4 January 2017; Published: 12 January 2017

Abstract: Graphitic carbon nitride (g-C3 N4 ) nanosheets were exfoliated from bulk g-C3 N4 and utilized to improve the sensing performance of a pure graphene sensor for the first time. The role of hydrochloric acid treatment on the exfoliation result was carefully examined. The exfoliated products were characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-Vis spectroscopy. The exfoliated g-C3 N4 nanosheets exhibited a uniform thickness of about 3–5 nm and a lateral size of about 1–2 µm. A g-C3 N4 /graphene nanocomposite was prepared via a self-assembly process and was demonstrated to be a promising sensing material for detecting nitrogen dioxide gas at room temperature. The nanocomposite sensor exhibited better recovery as well as two-times the response compared to pure graphene sensor. The detailed sensing mechanism was then proposed. Keywords: graphene; g-C3 N4 ; composite; gas sensor; room temperature

1. Introduction Gas sensors are devices that able to respond to specific gasses and they play an important role in industrial chemical processing, environmental monitoring, agriculture, medicine, public safety, and indoor air quality control [1]. Up to now, most commercial gas sensors have been based on metal oxide semiconductors due to their numerous advantages such as low cost, simplicity in measurements, high sensitivity towards various gases with ease of fabrication, and high compatibility with other processes [2–4]. However, these conventional sensors generally require high working temperatures (200 ◦ C to 500 ◦ C); this not only degrades the long-term sensing performance but also greatly limits their applications, e.g., wearable devices [5–7]. Recent studies have revealed that two-dimensional (2D) materials, especially graphene, have the potential to detect numerous gases at room temperature owing to their large surface area, high carrier mobility, and low noise. However, many reports have also demonstrated that pristine graphene has a poor gas sensing property owing to the absence of dangling bonds in the structure [8,9]. In order to solve this problem, a doping process and defect engineering have been conducted to improve the gas adsorption of graphene [10,11]. On the other hand, compositing methods such as decorating graphene with metal and metal oxide nanoparticles, as well as with polymers, have also been studied. Compared to pristine graphene, sensors based on graphene compositing with nanoparticles of metals or metal oxides have demonstrated highly sensitive and selective sensing behavior [12–15], which could be attributed to the excellent catalytic properties and synergistic effects of the partner materials [16–18]. On the other hand, graphitic carbon nitride (g-C3 N4 ), as another emerging component with a two-dimensional characteristic structure has attracted considerable attention recently, due to the Nanomaterials 2017, 7, 12; doi:10.3390/nano7010012

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Nanomaterials 2017, 7, 12 hand, On the other

2 of 11a graphitic carbon nitride (g-C3N4), as another emerging component with two-dimensional characteristic structure has attracted considerable attention recently, due to the tris-triazine units that areare connected by amino groups in each layerlayer and the van der forces tri-s-triazine units that connected by amino groups in each andweak the weak vanWaals der Waals between layers. The presence of nitrogen atoms in the graphene-like layered g-C 3N4 structure gives forces between layers. The presence of nitrogen atoms in the graphene-like layered g-C3 N4 structure g-C3N 4 its unique properties, such as semiconductor properties, solid alkalinity, and complexing gives g-C 3 N4 its unique properties, such as semiconductor properties, solid alkalinity, and complexing ability, thusendowing endowingititwith withbetter betterperformance performancethan thangraphene grapheneininsome someaspects aspects[19,20]. [19,20].g-C g-CN 3N4 has ability, thus 3 4 has been extensively used as a polymeric photo-catalyst for solar hydrogen production and been extensively used as a polymeric photo-catalyst for solar hydrogen production and environmental environmental purification, well as oxygen reduction and A evolution [21–23]. few recent studies purification, as well as oxygenas reduction and evolution [21–23]. few recent studiesAhave also reported have also reported that nanostructured g-C 3N4 exhibits excellent gas sensing properties owing to its that nanostructured g-C3 N4 exhibits excellent gas sensing properties owing to its excellent catalytic excellent[24,25]. catalytic property the [24,25]. Considering theassimilar as well as the property Considering similar 2D structures well as 2D the structures complementary properties complementary between g-C3N4 and graphene, a synergetic effect in sensing performance between g-C3 N4 properties and graphene, a synergetic effect in sensing performance could be expected if the could be expected if the two components are composited as a gas sensor material. two components are composited as a gas sensor material. thisstudy, study,we weinvestigated investigatedthe thegas gassensing sensingperformance performance the g-CN3N/graphene 4/graphene composite at InInthis ofof the g-C composite at 3 4 room temperature. g-C 3N4 nanosheets were prepared using an acid treatment enhanced liquid-phase room temperature. g-C3 N4 nanosheets were prepared using an acid treatment enhanced liquid-phase exfoliation process. process. The time onon exfoliation waswas carefully investigated. The exfoliation Theeffect effectofofacid acidtreatment treatment time exfoliation carefully investigated. obtained g-C3N 4 nanosheets were mixed with exfoliated graphene nanosheets to form a composite The obtained g-C 3 N4 nanosheets were mixed with exfoliated graphene nanosheets to form a which was then withtested NO2. The that compositing 3N4 with graphene composite whichtested was then withresults NO2 . demonstrated The results demonstrated that g-C compositing g-C3 N4 greatly improves the sensing performance of pure graphene. The sensing mechanism was carefully with graphene greatly improves the sensing performance of pure graphene. The sensing mechanism discussed. was carefully discussed.

Resultsand andDiscussion Discussion 2.2.Results 2.1.Acid AcidTreatment TreatmentEnhanced EnhancedExfoliation Exfoliation 2.1. Uptotonow, now, lack a simple approach to producing 2D nanomaterials at a scale large has scale has Up thethe lack of aofsimple approach to producing 2D nanomaterials at a large been been theobstacle main obstacle to achieving wide application of 2D graphene-like materials. The feasibility the main to achieving wide application of 2D graphene-like materials. The feasibility of the of the developed recently ultrasound-assisted developed ultrasound-assisted liquid-phase exfoliation has been recently liquid-phase exfoliation method has beenmethod demonstrated for demonstrated for producing ultrathin nanosheets from bulk 2D material. The cavitation effect producing ultrathin nanosheets from bulk 2D material. The cavitation effect induced by the collapse by the collapsebubbles of ultrasonic-produced generates liquid jets with high temperature, ofinduced ultrasonic-produced generates liquidbubbles jets with high temperature, pressure, and speed, pressure, and speed, which would overcome the interlayer force structure and breaktoup the which would overcome the interlayer van der Waals force and van breakder upWaals the layered yield layered structure to yield individual nanosheets. However, this method is still robust enough individual nanosheets. However, this method is still not robust enough owing to not the low yield and owing to the tendency low yieldafter andexfoliation. re-aggregation tendency after exfoliation. address issues, a re-aggregation To address these issues, a possibleToroute is thethese optimization routeItisisthe optimization ofmaximization the solvent. Itof is exfoliation reported that the maximization exfoliation ofpossible the solvent. reported that the efficiency is related toofthe surface efficiency is related to The the surface energysurface of the solvent. The comparative surface the energy of the solvent. comparative energy between the solvent and energy the 2D between structured solvent and the 2D solute isXie favorable for the delamination. co-researchers solute is favorable forstructured the delamination. and her co-researchers revealedXie thatand theher surface energy of revealed that the energy of3water matches well with that of that g-C3the N4. high Theirdispersion works demonstrated water matches wellsurface with that of g-C N4 . Their works demonstrated of g-C3 N4 that the high dispersion of g-C3Nin4 individual canalternative be realizedstrategy in wateris[26]. Another alternative individual layers can be realized water [26].layers Another chemical modification, strategy is chemical which involves redox reactions, process. intercalation, or an as ion-exchange which involves redoxmodification, reactions, intercalation, or an ion-exchange Typically, shown in process. Typically, as shown in Figure 1, the into intercalation of foreign the cause interlayer Figure 1, the intercalation of foreign molecules the interlayer gallerymolecules of g-C3 N4into would the gallery of g-C 3N4 to would cause the interplanar space to swell, weakening the interlayer and interplanar space swell, weakening the interlayer reaction, and thus facilitating thereaction, exfoliation. thus facilitating the exfoliation. In several previous studies, sulphuric acid (H 2 SO 4 ) or nitric acid In several previous studies, sulphuric acid (H2 SO4 ) or nitric acid (HNO3 ) was adopted to intercalate (HNO wasinto adopted to intercalate into the by interlayer space of g-C3N4 followed by an the acid3)ions the interlayer spacethe of acid g-C3ions N4 followed an ultrasonic treatment, and ultrathin ultrasonic treatment, and ultrathin g-C 3 N 4 nanosheets were successfully synthesized [27,28]. g-C N nanosheets were successfully synthesized [27,28]. However, these strong oxidants might incur 3 4 However,damage these strong oxidants might incur structural damage acid to the products. non-oxidant structural to the products. Mild non-oxidant hydrochloric (HCl) wouldMild be more suitable hydrochloric (HCl) would be of more for preserving the intrinsic structure of g-C3N4. for preserving acid the intrinsic structure g-C3suitable N4 . Moreover, in these previous studies, the controlling Moreover, these previous studies, theacid controlling in therarely exfoliation process, such as acid factors in theinexfoliation process, such as treatmentfactors time, were considered. treatment time, were rarely considered.

Figure1.1.Schematic Schematicofofthe theacid acidtreatment treatmentenhanced enhancedliquid-phase liquid-phaseexfoliation exfoliationprocess processfrom from bulk g-C 3N4 Figure bulk g-C 3N 4 ultrathinnanosheets. nanosheets. totoultrathin

for some minor changes. No obvious difference was observed between the exfoliated products. All the samples presented a characteristic peak near 27.7°, which can be indexed to the (002) facet caused by the interlayer stacking reflection of conjugated aromatic systems. It was found that the (002) peaks of exfoliated g-C3N4 presented a slight left shift from 27.7° to 27.5°, suggesting loose packing in the Nanomaterials 2017, 7, 12 3 of 11 exfoliated g-C3N4 product. It is worth noting that the intensity of the (002) peak decreased significantly in the exfoliated sample compared with the bulk sample, demonstrating the successful Theofcrystal structures bulk g-C exfoliated different HClat acid 3 N4 and g-C 3N 4 products exfoliation bulk g-C 3N4. It of was also observed that the sensitivity of the with diffraction peak 12.74°, treatment times were examined by X-ray diffraction (XRD) patterns. As shown in Figure 2, after which represents the period tri-s-triazine group, was reduced in the exfoliated g-C3N4 compared with exfoliation, bulk g-C N and the resultant g-C3 N4 products remain substantially same except the bulk g-C3Nboth 4. This could3be4 attributed to the downgrade of the planar size afterthe exfoliation. for some minor changes. No obvious difference was observed between the exfoliated products. All the samples presented a characteristic peak near 27.7◦ , which can be indexed to the (002) facet caused by the interlayer stacking reflection of conjugated aromatic systems. It was found that the (002) peaks of exfoliated g-C3 N4 presented a slight left shift from 27.7◦ to 27.5◦ , suggesting loose packing in the exfoliated g-C3 N4 product. It is worth noting that the intensity of the (002) peak decreased significantly in the exfoliated sample compared with the bulk sample, demonstrating the successful exfoliation of bulk g-C3 N4 . It was also observed that the sensitivity of the diffraction peak at 12.74◦ , which represents the period tri-s-triazine group, was reduced in the exfoliated g-C3 N4 compared with the bulk g-C3 N4 . This could be attributed to the downgrade of the planar size after exfoliation.

Figure 2. X-ray diffraction (XRD) patterns of bulk g-C3N4 and g-C3N4 nanosheets exfoliated at different acid treatment times.

The light absorbance of the samples was studied by UV-Vis spectroscopy. As shown in Figure 3, g-C3N4 displayed photoabsorption from ultraviolet to visible light. The bulk g-C3N4 has a band edge at approximately 460 nm corresponding to 2.70 eV. For comparison, all the exfoliated g-C3N4 samples have a band edge at approximately 450 nm, corresponding to a bandgap of 2.74 eV, indicating a slight blue shift of 0.04 eV. The increase of bandgap after exfoliation can be attributed to the well-known quantum confinement effect. When the thickness of the g-C3N4 product after exfoliation becomes ultrathin, approaching the dimension of the de Broglie wavelength, the motion Figure 2. X-ray diffraction (XRD) patterns of bulk g-C3 N4 and g-C3 N4 nanosheets exfoliated at different of electrons confined, leading to the splitting of energy levels, and thus the conduction and valence acidis treatment times. bands shift in opposite directions, resulting in the increase of the bandgap energy. The The UV-Vis spectra of thewas samples also provide informationAsofshown the concentration light absorption absorbance of the samples studied by UV-Vis spectroscopy. in Figure 3, of the g-C to thefrom Lambert-Beer law of A/l = αC, where α is3 N absorption coefficient, g-C33N N44products. displayed According photoabsorption ultraviolet to visible light. The bulk g-C edge 4 has a band corresponding 2.70CeV. For comparison, all the exfoliated N samples A is at theapproximately absorbance, 460 l is nm light path length,toand is the concentration of solute, theg-C absorption of light 3 4 have a proportional band edge at approximately 450 nm, corresponding to a bandgap of 2.74 eV, indicating a slightin the is directly to both the concentration of the solute and the thickness of the medium blue shift of 0.04 eV. The increase of bandgap after exfoliation can be attributed to the well-known light path. Given that the light path was fixed throughout the measurement, the concentration of gWhen the thickness of the g-C3As N4 observed product after exfoliation C3N4quantum could beconfinement estimated effect. from the intensity of absorption. from Figure 3,becomes the g-C3N4 ultrathin, approaching the dimension of the de Broglie wavelength, the motion of electrons is confined, product with 0.5 h acid treatment exhibited the lowest concentration. The concentration increased leading to the splitting of energy levels, and thus the conduction and valence bands shift in opposite considerably as the acid treatment time extended to 1 h, and then slightly increased as the time further directions, resulting in the increase of the bandgap energy. increased to 2 h, suggesting the exfoliation approached saturation.

Figure 3. UV-Vis absorption spectra of diluted bulk g-C3 N4 and as prepared g-C3 N4 nanosheets exfoliated at different acid treatment times.

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The UV-Vis absorption spectra of the samples also provide information of the concentration of the g-C3 N4 products. According to the Lambert-Beer law of A/l = αC, where α is absorption coefficient, A is the absorbance, l is light path length, and C is the concentration of solute, the absorption of light is directly proportional to both the concentration of the solute and the thickness of the medium in the light path. Given that the light path was fixed throughout the measurement, the concentration of g-C3 N4 could from the intensity of absorption. As observed from Figure 3, the g-C 3N Nanomaterials 2017,be 7, estimated 12 4 of 104 product with 0.5 h acid treatment exhibited the lowest concentration. The concentration increased considerably the acid treatment time extended 1 h, g-C and3Nthen slightly increased as4 the time further Figure 3.as UV-Vis absorption spectra of dilutedtobulk 4 and as prepared g-C3N nanosheets increased to 2 at h, different suggesting exfoliation exfoliated acidthe treatment times.approached saturation. The morphology of the exfoliated g-C3 N4 with 1 h HCl acid treatment was investigated using The morphology of(AFM). the exfoliated g-C 4 with 1 ah well-defined HCl acid treatment was using atomic force microscopy As shown in3N Figure 4a, sheet-like g-Cinvestigated 3 N4 nanostructure atomic force microscopy (AFM). As 3shown in Figure 4a, a well-defined g-C4b). 3N4 was observed. The thickness of the g-C N4 nanosheet was estimated to be aboutsheet-like 4 nm (Figure nanostructure was observed.thickness The thickness of the g-C 3N4N nanosheet was estimated to be about 4 nm Considering the theoretical of monolayer g-C is about 0.3–0.4 nm [20], this AFM result 3 4 (Figure the thickness of monolayer g-C3N4Aistotal about nm [20], this suggests4b). theConsidering obtained g-C comprises about 10 layers. of0.3–0.4 more than 50 AFM 3 Ntheoretical 4 nanosheet AFM result suggests the obtained g-Cstatistical 3N4 nanosheet comprises aboutis10 layers. total 4c. of more than samples have been examined and the thickness distribution shown in A Figure The figure 50 AFM samples have been examined and the statistical thickness distribution is shown in Figure 4c. shows that, for over 70% of the product, the thickness ranges from 3 to 5 nm. The figure shows that, for over 70% of the product, the thickness ranges from 3 to 5 nm.

Figure 4. atomic force microscopy (AFM) image; (b) corresponding thickness profileprofile along 4.(a)(a)Typical Typical atomic force microscopy (AFM) image; (b) corresponding thickness the yellow dasheddashed line inline (a),in and thickness distribution of theofexfoliated g-C3N 4 with 1 h acid along the yellow (a),(c) and (c) thickness distribution the exfoliated g-C N with 1h 3 4 treatment. acid treatment.

The the graphene graphene and and g-C g-C3N N4 nanosheets, and their composite were The microstructures microstructures of of the 3 4 nanosheets, and their composite were investigated through field emission scanning electron microscopy investigated through field emission scanning electron microscopy (FE-SEM). (FE-SEM). Figure Figure 5a,b 5a,b presents presents the the SEM images of exfoliated graphene and g-C 3N4, respectively. A large number of loosely stacked SEM images of exfoliated graphene and g-C3 N4 , respectively. A large number of loosely stacked graphene exfoliation. On On the the other hand, the graphene nanosheets nanosheets could couldbe beobserved, observed,suggesting suggestingsuccessful successful exfoliation. other hand, exfoliated g-C 3N4 nanosheets exhibited a severe restacking, whereby it is difficult to identify an the exfoliated g-C3 N4 nanosheets exhibited a severe restacking, whereby it is difficult to identify individual nanosheet. In In thethe case ofofthe 5c, porous porousg-C g-C3N N4/graphene an individual nanosheet. case thecomposite, composite,as asshown shown in in Figure Figure 5c, 3 4 /graphene film was observed. Remarkably, the restacking phenomenon of g-C 3N4 was greatly alleviated, which film was observed. Remarkably, the restacking phenomenon of g-C3 N4 was greatly alleviated, which could attributed to to the the self-assembly self-assembly process process between between g-C g-C3N N4 and graphene [23]. This finding could be be attributed 3 4 and graphene [23]. This finding reveals reveals that that compositing compositing 2D 2D materials materials would would be be favorable favorable in in preventing preventing the the restacking restacking of of the the components. It was noted that the evidence of compositing is difficult find even by high-resolution components. It was noted that the evidence of compositing is difficult find even by high-resolution SEM owing to to the the resemblance resemblance of of 2D 2D nanomaterials. nanomaterials. Other Other characterization characterization methods SEM (Figure (Figure S1), S1), owing methods would be required to further examine the g-C 3N4/graphene composite. would be required to further examine the g-C N /graphene composite. 3

4

Energy-dispersive X-ray spectroscopy (EDS) analysis and elemental mapping were also utilized to investigate the elemental composition and distribution of the g-C3 N4 /graphene composite, respectively. The elemental mapping analysis, as shown in Figure 6a–c, shows that the C and N elements are homogeneously dispersed, indicating the successful formation of the nanocomposite. According to the EDS spectrum in Figure 6d, besides the Si and O elements contributed by the oxidized silicon wafer, the main elements of the nanocomposite were N and C. The atomic ratio of both N and C was about 1:3, which is consistent with the starting ratio. Besides, it is noted that a neglectable amount Figure 5. Field emission scanning electron microscopy (FE-SEM) images of exfoliated (a) graphene; (b) g-C3N4; and (c) g-C3N4/graphene composite. The insets in (a,b) are SEM images of their bulk counterparts.

Energy-dispersive X-ray spectroscopy (EDS) analysis and elemental mapping were also utilized

graphene nanosheets could be observed, suggesting successful exfoliation. On the other hand, the exfoliated g-C3N4 nanosheets exhibited a severe restacking, whereby it is difficult to identify an individual nanosheet. In the case of the composite, as shown in Figure 5c, porous g-C3N4/graphene film was observed. Remarkably, the restacking phenomenon of g-C3N4 was greatly alleviated, which Nanomaterials 2017, 7, 12 5 of 11 could be attributed to the self-assembly process between g-C3N4 and graphene [23]. This finding reveals that compositing 2D materials would be favorable in preventing the restacking of the components. was noted that thecomposite. evidence of compositing is difficult findstrongly even bybonded high-resolution of Cl element It was observed in the This was originated from the Cl on the SEM (Figure S1), owing to the resemblance of 2D nanomaterials. Other characterization methods defective sites of g-C3 N4 [29]. would be required to further examine the g-C3N4/graphene composite.

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oxidized silicon wafer, the main elements of the nanocomposite were N and C. The atomic ratio of Figure 5. Field emission emission scanning scanning electron electron microscopy microscopy (FE-SEM) (FE-SEM) images images of of exfoliated exfoliated (a) (a) graphene; graphene; both (b) N and C4; was about 1:3, which is composite. consistentThe withinsets the starting ratio. Besides, itof istheir noted that a g-C33N and (c) g-C 3 N 4 /graphene in (a,b) are SEM images bulk N4 ; and (c) g-C3 N4 /graphene composite. The insets in (a,b) are SEM images of their neglectable amount of Cl element was observed in the composite. This was originated from the counterparts. bulk counterparts. strongly bonded Cl on the defective sites of g-C3N4 [29]. Energy-dispersive X-ray spectroscopy (EDS) analysis and elemental mapping were also utilized to investigate the elemental composition and distribution of the g-C3N4/graphene composite, respectively. The elemental mapping analysis, as shown in Figure 6a–c, shows that the C and N elements are homogeneously dispersed, indicating the successful formation of the nanocomposite. According to the EDS spectrum in Figure 6d, besides the Si and O elements contributed by the

Figure (a) Low-magnification Low-magnificationSEM SEMimage imageofofg-C g-C3N nanocomposite; Carbon 3N 4 /graphene Figure 6. 6. (a) 4/graphene nanocomposite; (b)(b) Carbon andand (c) (c) nitrogen elemental mapping captured in (a); (d) Energy-dispersive X-ray spectroscopy (EDS) pattern nitrogen elemental mapping captured in (a); (d) Energy-dispersive X-ray spectroscopy (EDS) pattern and and elemental elemental composition composition of of nanocomposite. nanocomposite.

X-ray photoelectron spectroscopy (XPS) measurement measurement was carried out to further study the compositionand and element binding energies of4 /graphene g-C3N4/graphene nanocomposite, as chemical composition element binding energies of g-C3 N nanocomposite, as displayed displayed in The Figure S2. The spectra S2a)indicated obviously the of co-existence of in Figure S2. survey XPSsurvey spectraXPS (Figure S2a)(Figure obviously theindicated co-existence the elements the elements C, N, and a small amount of O. No Cl component was observed, suggesting that most C, N, and a small amount of O. No Cl component was observed, suggesting that most of the Cl of thewere Cl ions were removed in the washing The emergence of element be attributed ions removed in the washing process. process. The emergence of element O may O bemay attributed to the to the adventitiously adsorbed contaminant, as surface adsorbed [30–32].The TheC1s C1s spectrum adventitiously adsorbed contaminant, suchsuch as surface adsorbed H2H O2O [30–32]. in Figure FigureS2b S2bcan canbebedeconvoluted deconvoluted into three peaks with binding energies of 288.2, displayed in into three peaks with binding energies of 288.2, 286, 286, and 2 C=C, respectively [24,29]. No C–O or C=O and 284.7 eV, which are assigned to N–C=N, N–C, and 284.7 eV, which are assigned to N–C=N, N–C, and sp2spC=C, respectively [24,29]. No C–O or C=O bond were observed revealing that the graphene preserved its structure and no oxidation occurred throughout the process. On the other hand, the N1s XPS spectra (Figure S2b) displayed two peaks at eV, which which are are assigned assigned to to C–N C–N and and C=N–C, C=N–C, respectively. respectively. The The C=N–C C=N–C represents the 400.3 and 398.7 eV, sp2 hybridized aromatic N bonded to carbon atoms (C=N–C) existed in g-C 3 N 4 hybridized carbon atoms (C=N–C) existed in g-C3 4, which which was known as N/C ratio the reactive pyridine N. Besides, the surface N/C ratio in in nanocomposite nanocomposite was was determined determined to to be be 0.27, 0.27, which is closed to that in EDS analysis. The above XPS results further demonstrated the formation of g-C33N N44/graphene /graphenenanocomposite. nanocomposite. 2.2. Gas Sensing Performance The performance performanceof ofthe theg-C g-C3N composite sensor examined gas. 4/graphene composite gasgas sensor waswas examined withwith NO2NO gas.2 The 3N 4 /graphene The graphene as a control also tested simultaneously. Figurethe 7a typical shows purepure graphene sensorsensor as a control samplesample was alsowas tested simultaneously. Figure 7a shows response curve of the sensors upon exposure to 5 ppm of NO2 at room temperature. As observed, upon exposure to NO2 gas, the resistance of both sensors decreased, suggesting a p-type behavior. It is known that NO2 as a typical electron withdrawer would increase the hole concentration of the sensor materials, and thus decrease (or increase) the resistance of p-type (or n-type) semiconductors. Remarkably, the composite sensor exhibited two-times the response of pure graphene, indicating that

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the typical response curve of the sensors upon exposure to 5 ppm of NO2 at room temperature. As observed, upon exposure to NO2 gas, the resistance of both sensors decreased, suggesting a p-type behavior. It is known that NO2 as a typical electron withdrawer would increase the hole concentration of the sensor materials, and thus decrease (or increase) the resistance of p-type Nanomaterials 2017, 7, 12 6 of 10 (or n-type) semiconductors. Remarkably, the composite sensor exhibited two-times the response of pure graphene, indicating that the incorporation of g-C3 N4 would benefit the sensing performance of NO2 gas molecules. Our results indicate that the incorporation of specific semiconductors can achieve the graphene sensor.

full recovery of the graphene sensor without the need for a complicated heating or vacuum process.

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NO2 gas molecules. Our results indicate that the incorporation of specific semiconductors can achieve full recovery of the graphene sensor without the need for a complicated heating or vacuum process.

Figure Typicalresponse responseofofpure puregraphene graphene and and g-C3N44/graphene /graphene composite composite sensors toward Figure 7. 7.(a)(a) Typical toward 5 5 ppm of NO gas at room temperature; (b) Dynamic response of pure graphene and g-C N /graphene 2 at room temperature; (b) Dynamic response of pure graphene and g-C 3 34N4/graphene ppm of NO2 gas composite sensorstoward towardvarious variousconcentrations concentrations of composite sensors of NO NO22 gas gas at atroom roomtemperature. temperature.

It is well-known thatdynamic intrinsic response grapheneofshows recovery, due to thecomposite strong Figure 7b shows the pure poor graphene and partially g-C3N4/graphene adsorption energy the gas from molecule. Thus, heating an elevated temperature or vacuum treatments sensors toward NO2ofranging 5 to 200 ppm. Theatsensors responded well with the target gas. The are required to clean the graphene sensor [33–35]. As observed in Figure 7a, it isNo noted that the pure responses increased monodirectionally as the concentration of NO 2 increased. obvious saturation graphene sensor barely showed recovery when the NO gas was removed, while about 20% recovery 2 state was observed in the composite sensor, suggesting a broad detection range. Remarkably, the was observed in the composite sensormore within 200 s. This is the possibly attributed the thermal excitation composite sensor two-times response than sensors totopure graphene in all the Figure 7. (a)showed Typical response of pure graphene and g-C3Nother 4/graphene composite sensors toward 5 of g-C N that introduced active carriers and facilitated the desorption process of NO gas molecules. 3 4 2 tests. The property our nanocomposite sensor wasofalso Figure 8a. ppmstability of NO2 gas at roomof temperature; (b) Dynamic response pureexamined graphene as andshown g-C3N4in /graphene Our results indicate that the incorporation of specific semiconductors cantoachieve full recovery of the The nanocomposite sensor responded repeatedly upon cycled exposure 20 ppm of NO 2 , suggesting composite sensors toward various concentrations of NO2 gas at room temperature. graphene sensor without the need for a complicated heating or vacuum process. an excellent repeatability. Figure 7b shows the dynamic response of pure graphene and g-C N4 /graphene composite sensors Figure 7b shows the dynamic response of pure graphene3 and g-C3N4/graphene composite toward NO2 ranging from 5 to 200 ppm. The sensors responded well with the target gas. The responses sensors toward NO2 ranging from 5 to 200 ppm. The sensors responded well with the target gas. The increased monodirectionally as the concentration of NO2 increased. No obvious saturation state was responses increased monodirectionally as the concentration of NO2 increased. No obvious saturation observed in the composite sensor, suggesting a broad detection range. Remarkably, the composite state was observed in the composite sensor, suggesting a broad detection range. Remarkably, the sensor showed two-times more response than the other sensors to pure graphene in all the tests. composite sensor showed two-times more response than the other sensors to pure graphene in all the The stability property of our nanocomposite sensor was also examined as shown in Figure 8a. The tests. The stability property of ourrepeatedly nanocomposite sensor was also in Figure 8a. nanocomposite sensor responded upon cycled exposure to examined 20 ppm of as NOshown 2 , suggesting an The nanocomposite sensor responded repeatedly upon cycled exposure to 20 ppm of NO2, suggesting excellent repeatability. an excellent repeatability. Figure 8. (a) Cyclic response of g-C3N4/graphene composite sensor toward 20 ppm of NO2; (b) Proposed sensing mechanism of g-C3N4/graphene composite sensor.

The sensing mechanism of the g-C3N4/graphene composite sensor is proposed, as schematically shown Figure 8b. On the one hand, the triazine structure of g-C3N4 nanosheet behaves similarly to a base that is ready to interact with the oxidized NO2 gas molecules. This characteristic of g-C3N4 nanosheet as well as its excellent catalytic property endows it with a promising capacity for the adsorption of NO2 molecules. On the other hand, graphene with superior carrier mobility would act similarly to a 8. signal highway in theof sensing Upon adsorption, oxidized NO2ofgas molecule Figure /graphene composite sensor the toward 20 ppm NO 2; (b) Figure 8. (a) (a)Cyclic Cyclicresponse response of g-C g-C33N N4process. 4 /graphene composite sensor toward 20 ppm of NO2 ; with strong electron affinity deposits a hole in the g-C 3 N 4 nanosheet. These hole carriers are then Proposed sensing mechanism of of g-C 3N34N /graphene composite sensor. (b) Proposed sensing mechanism g-C composite sensor. 4 /graphene rapidly conducted by the graphene. In short, the complementary natures of g-C3N4 and graphene in the sensing processmechanism introduce aofsynergetic that contributes the excellent sensing property of The sensing the g-C3Neffect 4/graphene compositeto sensor is proposed, as schematically the nanocomposite addition, considering the different roles of both materials insimilarly the sensing shown Figure 8b. sensor. On the In one hand, the triazine structure of g-C 3N4 nanosheet behaves to a process, a tradeoff onto theinteract sensingwith performance would be2 expected if the composition of the composite base that is ready the oxidized NO gas molecules. This characteristic of g-C3N4 isnanosheet changed. Through systematic investigation, an optimized ratio g-Ca3Npromising 4 and graphene could as well as its excellent catalytic property endows it of with capacity for be the

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The sensing mechanism of the g-C3 N4 /graphene composite sensor is proposed, as schematically shown Figure 8b. On the one hand, the triazine structure of g-C3 N4 nanosheet behaves similarly to a base that is ready to interact with the oxidized NO2 gas molecules. This characteristic of g-C3 N4 nanosheet as well as its excellent catalytic property endows it with a promising capacity for the adsorption of NO2 molecules. On the other hand, graphene with superior carrier mobility would act similarly to a signal highway in the sensing process. Upon adsorption, the oxidized NO2 gas molecule with strong electron affinity deposits a hole in the g-C3 N4 nanosheet. These hole carriers are then rapidly conducted by the graphene. In short, the complementary natures of g-C3 N4 and graphene in the sensing process introduce a synergetic effect that contributes to the excellent sensing property of the nanocomposite sensor. In addition, considering the different roles of both materials in the sensing process, a tradeoff on the sensing performance would be expected if the composition of the composite is changed. Through systematic investigation, an optimized ratio of g-C3 N4 and graphene could be achieved to obtain a high-performance sensor with excellent sensitivity and promising recovery property. This is the expected direction in the future research. 3. Materials and Methods 3.1. Preparation of Bulk g-C3 N4 Raw melamine purchased from Sigma-Aldrich Korea Ltd. (Yongin, Korea) was used as received. The fabrication process was modified from the reported methods [36–38]. Typically, a certain amount of melamine was placed into a crucible with a cover and heated to 600 ◦ C with a heating rate of 1 ◦ C/min in air, and then maintained for 4 h. After natural cooling to ambient temperature, a yellow product was obtained. The obtained yellow product was subsequently ground thoroughly for further processing and characterization. 3.2. Acid Treatment Enhanced Exfoliation of g-C3 N4 In a typical synthesis, 0.5 g of yellow bulk g-C3 N4 powder was added to 25 mL hydrochloric acid (HCl, 36.46%, Deajung Korea Ltd., Siheum, Korea) and stirred for 1 h. The obtained transparent yellow dispersion was then filtrated and washed repeatedly with water until the pH value became neutral, followed by a drying process at 100 ◦ C for 6 h. This acid-treated powder was re-dispersed in 200 mL deionized (DI) water, followed by sonication treatment for 2 h in an ultrasonic bath (200 W, NXPC-2010, Sonics & Materials, Inc., Newtown, CT, USA). Subsequently, the mixture was centrifuged at 8000 rpm for 10 min to remove the residual un-exfoliated g-C3 N4 particles. Finally, a light white suspension was obtained. In order to determine the concentration of the product, 100 mL of g-C3 N4 dispersion was transferred to a pre-weighted vial and dried in an oven at 100 ◦ C for 24 h. The concentration of g-C3 N4 in dispersion after 1 h acid treatment was determined to be about 50 µg/mL. 3.3. Preparation of Graphene Graphene was prepared following the process developed in our previous work [39]. Briefly, 4 g of graphite powder (Sigma-Aldrich Korea Ltd., Yongin, Korea, 20 µm) was dispersed in 200 mL of an aqueous mixture containing 20 vol % acetone and 68 vol % tetrahydrofuran. The dispersion was then ultrasonically treated for 1 h using a horn probe sonic tip (Sonic VCX 750, Sonics & Materials, Inc., Newtown, CT, USA). A cooling water system was used to keep the processing temperature below 5 ◦ C throughout. The ultrasonic power was set to 600 W with a pulse for 20 s on and 10 s off. After the ultrasonic treatment, the black dispersion was centrifuged at 1500 rpm for 30 min. The supernatant was then carefully decanted and retained for the next process. The concentration of the graphene was estimated to be around 250 µg/mL.

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3.4. Fabrication of g-C3 N4 /Graphene Composite Sensor Two as-prepared dispersions containing equivalent weights of g-C3 N4 and graphene were mixed with ultrasonic treatment for 1 h to achieve complete mixing. After mixing, the mixture was evaporated in an oven for 24 h at a moderate temperature (~60 ◦ C) to improve the concentration of solute without introducing severe aggregation. Alumina substrates (4 × 4 mm) with interdigital Pt electrodes were carefully cleaned and placed on a hot-plate with a temperature of about 100 ◦ C. Concentrated solution was then drop-casted onto the sensor substrates using micro-pipettes. The thickness of the thin films was about 1 µm and was controlled by the volume of the solution in the micro-pipettes. After coating, the alumina substrates were heated at 100 ◦ C in an oven for 1 h to eliminate the remaining solvent and then sintered at 200 ◦ C in Ar gas for 1 h to improve the adhesion and contact. Pure graphene-based sensor as the control sample was also prepared using the same method. 3.5. Measurement of g-C3 N4 /Graphene Composite Sensor The sensors were placed in a stainless chamber having a total volume of 10 cm3 . Nitrogen gas was used as the carrier gas. NO2 gas with a starting concentration of 500 ± 1 ppm in nitrogen was used as the target gas. The accurate concentration control of the target gas was achieved by using a mixing system equipped with mass flow controllers (MFC, Tylan 2900, Mykrolis Corporation, Billerica, MA, USA) and mass flow meters. All the measurements were conducted under ambient condition. The total gas flow of the carrier and target gases was kept at 250 sccm throughout the measurement process. The electrical conductance signal of the sensors was collected and recorded by data acquisition (Agilent 34970A, Keysight Technologies, Santa Rosa, CA, USA) through a customized clamp and wire connector. The sensing response was defined as: S(%) =

RG − R N × 100 RN

where RN and RG represented the resistance of the sensors upon exposure to nitrogen and target gas, respectively. 3.6. Material Characterization For material characterization, samples were prepared by drop-casting the solution containing g-C3 N4 or g-C3 N4 /graphene nanocomposite onto an oxidized silicon wafer, followed by a drying process at 80 ◦ C for 12 h. X-ray diffraction (XRD) was performed using a Rigaku RINT2200 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with monochromatized Cu-Kα radiation. Atomic force microscopy (AFM) measurements were carried out on a Bruker MutiMode-8 system (Bruker Corporation, Billerica, MA, USA). The microstructure, morphology, and elemental distribution of the samples were systematically investigated by Raman spectroscopy (Horiba LabRAM HR Evolution systems, Horiba, Ltd., Kyoto, Japan, operating at a wavelength of 785 nm), Scanning Electron Microscopy (SEM, JSM-7100FA, Hitachi, Ltd., Tokyo, Japan), and Energy-dispersive X-ray spectroscopy (EDS) mapping (co-equipped with SEM) (Hitachi, Ltd., Tokyo, Japan), respectively. Chemical compositions and element binding energies were analyzed using X-ray photoelectron spectroscopy (XPS) on an Ulvac-phi Veresprobe II system (Ulvac-Phi, Inc., Chigasaki, Japan) with monochromatic Al Kα as an excitation source. The absorption spectroscopy measurements were performed using a Varian Cary 6000i and a 1 cm cuvette. 4. Conclusions In summary, we successfully prepared g-C3 N4 nanosheets from bulk g-C3 N4 using a facile HCl acid treatment followed by an ultrasonic process. The effect of acid treatment on the exfoliation result was carefully examined. The exfoliated g-C3 N4 nanosheets exhibited a uniform thickness of about 3–5 nm and a lateral size of about 1–2 µm. g-C3 N4 /graphene nanocomposite was prepared via

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a self-assembly process. Remarkably, the restacking phenomenon of g-C3 N4 was greatly alleviated after compositing with graphene. A promising sensing performance of g-C3 N4 /graphene nanocomposite toward NO2 gas at room temperature was demonstrated. The nanocomposite sensor exhibited better recovery as well as two-times the response compared to the pure graphene sensor. The promising performance of the nanocomposite sensor was attributed to a synergetic effect in which the graphene with superior carrier mobility acts as the signal pathway, while the g-C3 N4 nanosheet, with an active surface, plays the role of analyte acceptor. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/7/1/12/s1. Acknowledgments: This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education (MOE) (Nos. 2015R11A1A01058991 and 2014R1A1A2059913). Author Contributions: Shaolin Zhang and Woochul Yang conceived and designed the experiments; Shaolin Zhang and Nguyen Thuy Hang performed the experiments; Shaolin Zhang, Zhijun Zhang, and Hongyan Yue analyzed the data; and Shaolin Zhang and Nguyen Thuy Hang wrote the paper. Conflicts of Interest: The authors declare no conflicts of interest.

Abbreviations The following abbreviations are used in this manuscript: g-C3 N4 HCl DI Water NO2 2D Material H2 SO4 HNO3 EDS XRD FE-SEM AFM UV-Vis spectroscopy MFC

Graphitic Carbon Nitride Hydrochloric Acid Deionized Water Nitrogen Dioxide Two Dimensional Material Sulphuric Acid Nitric Acid Energy-dispersive X-ray Spectroscopy X-ray Diffraction Field Emission Scanning Electron Microscopy Atomic Force Microscopy Ultraviolet-visible Spectroscopy Mass Flow Controllers

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