ZnO-Reduced Graphene Oxide Composites Sensitized with Graphitic

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ZnO-Reduced Graphene Oxide Composites Sensitized with Graphitic Carbon Nitride Nanosheets for Ethanol Sensing Fanli Meng, Yuanlong Chang, Wenbo Qin, Zhenyu Yuan, Junpeng Zhao, Junjie Zhang, Erchou Han, Shangyu Wang, Minghui Yang, Yanbai Shen, and Methat Ibrahim ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00257 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Nano Materials

ZnO-Reduced Graphene Oxide Composites Sensitized with Graphitic Carbon Nitride Nanosheets for Ethanol Sensing Fanli Meng 1, Yuanlong Chang 1, Wenbo Qin 1, Zhenyu Yuan 1,*,Junpeng Zhao 1, Junjie Zhang 1, Erchou Han 1, Shangyu Wang 1, Minghui Yang 2,*, Yanbai Shen3, Medhat Ibrahim4,*

1College

of Information Science and Engineering, Northeastern University, Shenyang 110819,

China

2Ningbo

Institute of Materials Technology & Engineering, Chinese Academy of Sciences,

Ningbo 315201, China

3College

of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China

4Spectroscopy

Department, National Research Centre, Giza, 12622, Egypt

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In this work, we reported a ternary sensing material using graphitic carbon nitride (g-C3N4) nanosheets as sensitization modifier for zinc oxide (ZnO)/reduced graphene oxide (rGO), rather than widely reported noble metal nanoparticles. The nanostructures of 2D graphene oxide (GO)hybridized by g-C3N4 were synthesized in advance by combining ultrasonic dispersion and electrostatic self-assembly strategy. ZnO nanoparticles were coated on GO/g-C3N4 through a hydrothermal process, in which GO were totally reduced to rGO. Compared with pure ZnO and ZnO/rGO, the ZnO/rGO/g-C3N4 nanocomposites exhibited a remarkable enhancement on the responses to ethanol vapor. The ternary nanocomposites showed the highest response of 178 (Ra/Rg) to 100 ppm ethanol at 300 °C with a detect limitation lower than 500 ppb, which was about 2-folds and 9-folds higher than ZnO/rGO and pure ZnO samples, respectively. The enhancement was attributed to the sensitization of g-C3N4 and the ample interfacial contact between rGO and gC3N4. The ethanol sensing mechanism associated with ZnO nanoparticles and GO/g-C3N4 2D hybrid structure was also discussed in detail.

KEYWORDS: ZnO; RGO; G-C3N4; Nanocomposite; Gas sensor; Ethanol

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1. INTRODUCTION Metal oxide semiconductors (MOSs) have been extensively used as gas sensors because of their easy production, low cost and simple measurement 1-6. As a well-known n-type MOS, zinc oxide (ZnO, Eg = 3.37eV at 25 °C) has received substantial attention in the field of volatile organic compounds (VOCs) detection due to its easy preparation and significant response to most kinds of VOCs

7-14.

Xu et al. firstly reported on 1D ZnO for side-heated type MOS gas sensor in 20057.

Generally, sensing materials with a specific nanostructure have the possibility of improving their gas sensing properties. Thus, various ZnO based architectures such as nanorods, nanowires, flower-like, urchin-like, hollow-sphere have been reported, with their common advantages of high specific surface area and other individual characteristics8-15. In recent years, 2-dimensional (2D) graphene oxide (GO) and reduced graphene oxide (rGO) have been used to composite with ZnO nanostructures because of their high specific surface area and abundant functional groups16, 17. Yi et al. reported a kind of ZnO nanorods (NRs) and graphene based on CVD and hydrothermal method18. The response of ZnO NRs/Graphene sensor towards 50 ppm ethanol can reach to 90 at 300 °C, with the response time longer than 30 min. The long response time should be attributed to weak catalytic property. Noble metal (Pt, Au, Ag, etc.) is effective in many catalyst system. In order to further strengthen the sensitivity of the MOSs/rGO, noble metal nanoparticles were added into the composites forming a ternary nanocomposites19. However, noble metal nanoparticles (NPs) tend to aggregate due to big surface energy, which adverse to their sensitization performance, not to mention their 3



exorbitant cost. Recently, graphitic carbon nitride (g-C3N4), as a developing 2D material, has been widely reported in catalytic field because of its optical catalysis feature and medium band gap energy structure20-22. In order to strengthen the catalytic property of g-C3N4, 2D g-C3N4 nanosheets have been fabricated via the exfoliation of bulk layered counterparts23-27. Zhang et al. reported a g-C3N4 nanosheets/graphene composite using for NO2 gas sensing, based on a self-assembly strategy28. With the presence of g-C3N4, the response of graphene towards NO2 was significantly strengthened suggesting that g-C3N4 nanosheets could be a potential substitution of noble metal nanoparticles in the practice of gas sensitivity enhancement. Besides, both graphene and g-C3N4 have the identical sp2-bonded C structure, which makes them the compossible materials to perform interaction29. However, the response and recovery times were long for the g-C3N4 nanosheets/graphene composite compared with MOS sensors. Cao et al. reported an ethanol gas sensor based on cocoon-like ZnO decorated g-C3N430. The inspiration was originated from sensors based on ZnO/rGO materials. However, poor electron conductivity of g-C3N4 leads to low response, which is 15.8 toward 100 ppm ethanol at 350 °C. In this work, g-C3N4 nanosheets, as a sensitization catalyst, was combined with GO in advance to address the aggregation problem of noble metal. Then ZnO nanoparticles were loaded on the 2D/2D hybrid nanostructure through hydrothermal process. During the hydrothermal reaction, GO was totally reduced to rGO by urea, providing a ternary composite of ZnO, rGO and g-C3N4. To our best knowledge, this is considered the first report on using g-C3N4 as substitution of noble metal nanoparticles in volatile organic gas (VOC) sensing sensitization. The as-fabricated 4


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ZnO/rGO/g-C3N4 nanocomposite demonstrated significantly improved ethanol sensing performance in comparison to pure ZnO and ZnO/rGO. The remarkably strengthened sensing property was attributed to the sensitization of g-C3N4 and the novel 2D/2D rGO/g-C3N4 structure, which successfully took advantage of the electron conductivity of rGO to facilitate the charge transfer together with the electron-hole separation of g-C3N4. On the whole, this work not only stresses the exploitation of GO/g-C3N4 as an ideal substrate for metal oxide semiconductor in gas sensing applications, but also emphasizes the specific catalytic use of g-C3N4 by a simple hybrid approach, rather than merely performing as a “normal” substrate for MOSs in sensing fields.

2. EXPERIMENTAL SECTION Graphite was purchased from J&K Chemical Ltd. Sodium nitrate was purchased from Avanti Polar. Potassium permanganate and urea were purchased from HyClone Inc. HCI (37.5%, AR) and zinc acetate dihydrate were purchased from Shandong West Asia Chemical Industry Co, Ltd. Deionized water (DI, >18.2 +K • cm-1) was used throughout the whole experiment. All other chemicals were of analytical grade and used without further purification. Synthesis of Bulk G-C3N4. Bulk g-C3N4 (BCN) was prepared by heating urea. Urea (10 g) was put into a crucible with a cover, then the whole alumina crucible was wrapped by a piece of Al foil. Then the sample was heated to 550 °C in a muffle furnace within 2 h and stayed for another 3 hours. The agglomerated product was grinded to obtain light yellow BCN powder.

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Synthesis of G-C3N4 Nanosheets. The g-C3N4 nanosheets were prepared by sonication and protonation. Prepared BCN (0.6 g) was added into 200 mL HCI (0.5 mol L-1) and sonicated for 1h, followed by stirring for 4 h at room temperature. The result solution was repeatedly washed using DI until neutral. Finally, the g-C3N4 nanosheets was dried at 60 °C for 12 h and ground into powder for further utility. Synthesis of GO/G-C3N4 Composite. GO was prepared using an improved Hummers’ method as reported previously31. 0.5g g-C3N4 nanosheets were dispersed in 10mL 1 mg/mL GO and ultrasound-treated for 5 h. Then the mixture was vigorously stirred to achieve homogeneous suspension. Thus, GO/g-C3N4 composite was prepared. Preparation of ZnO/rGO/g-C3N4 Nanocomposite. 14 mmol urea was added to the GO/g-C3N4 composite, followed by stirring for 0.5 h. At the same time, 2 mmol Zn(Ac)2·2H2O was dissolved in 30 mL water. After these two solution were mixed and stirred for another 30 min, the result solution was put into an autoclave and heated at 100 °C for 6 h. During the hydrothermal process, GO was reduced to rGO31. The precipitate was washed with DI and ethanol for several times to eliminate the impurities. After drying in an oven at 70 °C for 24 h, the as-obtained precursors were heated to 350 °C at a rate of 5 °C /min and stayed for 3 h. After cooling to room temperature, the final product was saved for characterization and sensing test. As control groups, ZnO, ZnO/rGO and ZnO/g-C3N4 were prepared using the same method with no rGO/g-C3N4, or rGO added, respectively.

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Characterizations of Materials. Zeta potential measurements were obtained using the Zetasizer Nano ZS (Malvern Insturments). The surface morphologies of the GO/g-C3N4 and ZnO/GO/g-C3N4 nanocomposites were observed by field-emission scanning electron microscopy (FE-SEM) on a Hitachi S-4800. The internal constitution of ZnO/GO/g-C3N4 nanocomposites were analyzed using transmission electron microscopy (TEM) on a JEOL JEM-2010. X-ray diffraction spectra (XRD) were obtained on a PANalytical B.V. X pert PRO and a Cu GO radiation. X-ray photoelectron spectrometry (XPS) were obtained on an ESCALab-MK-II with an Al GO source gun. Fabrication of the Sensors and Gas Sensor Measurement System. The prepare method of gas sensor was presented in our previous report19. As the sensor structure diagram shown in Figure 1a, two gold rings were welded onto the alumina ceramic tube (1.2 mm in outside diameter, 0.8mm in inside diameter and 4mm in length) and two pairs of electrode wires (Pt) were welded to the gold ring as measurement electrodes of gas sensor. In the ceramic tube, a piece of Ni-Cr wire (~35 K5 was placed forming a micro heating resistor. To finish the gas sensor, 1 mg testing sample was ground in an agate mortar followed by adding 20 QH ethanol and sonicating to form homogeneous paste. The paste was dropped onto alumina ceramic tube and spread evenly under surface tension. After the evaporation of ethanol, thin sensing film with 1mm thickness was formed on the surface of alumina ceramic tube. For the purpose of stability, the prepared sensor was aged for 24 h at 300 °C for further test.

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The measurement system of gas sensing property is shown in Figure 1b. The gas sensing experiments were carried out in a transparent poly(methyl methacrylate) (PMMA) chamber, which was about 1 L. A Keysight B2902A Source/Measurement Unit (SMU) provides constant bias current for gas sensor and records the voltage change of the gas sensor 10 times per second. Given the voltage and the current, the resistance of gas sensor can be determined by Ohm’s Law. A RIGOL DP832A programmable linear DC power supply is used to heat Ni-Cr wire. By changing the output power of DC power supply, the work temperature can be conveniently changed. These two measurement instrument are both connected to computer to form a measurement system. The work temperature is controlled by a PID control program, which makes the temperature change fast and stable. The measurement process is also controlled by a program to make the data record easily. Above two programs are both developed on LabVIEW.

(a)

(b)

Figure 1. (a) Structure diagram of gas sensor. (b) Measurement system of gas sensing property.

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In the beginning of detecting process, saturated organic vapor was introduced into the chamber through an injecting needle. The measurement program records the voltage and current of every sampling point and calculates the resistance immediately. The records are stored in a twodimensional array and output as an Excel document after detection. When the gas-sensing detection is finished, the atmosphere in the testing chamber are refreshed by inputting pure air. In this work, the response of gas sensors was defined as Ra/Rg, where Ra was the resistance of the sensor measured in air and Rg was the resistance measured in testing gas, respectively. The response time is restricted to the time that the test sensor reaches 90% of the resistance change since the test gas is introduced. The recovery time is restricted to the time when the resistance reaches a 90% recovery since the fresh air blow in.

3. RESULTS AND DISCUSSION 3.1 Synthesis approach. The preparation process is mainly divided into two steps, which was pictorially shown in Figure 2. At first, the graphene oxide is combined with the g-C3N4 nanosheets via sonication in the light of electrostatic self-assembly. G-C3N4 can be easily protonated by HCI due to the abundant -C-Nmotif in atomic skeleton30. During the sonication and protonation, the bulk g-C3N4 was exfoliated to nanosheets and modified to a positively charged surface. The zeta potential value of g-C3N4 nanosheets was measured to be +15.0 mV, comparing with -17.7 mV for pristine g-C3N4, as it was shown in Figure 3. In contrast, the surface of GO was proved to be a significantly negatively 9



charged (-41.6 mV) (Figure 3). Consequently, the natural self-assembly between g-C3N4 nanosheets and GO was realized by the static electricity and U U stacking interactions.

Figure 2. Schematic diagram for the synthesis process of ZnO/rGO/g-C3N4 samples. 20

15.0

10 Zeta Potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0 -10 -20

-17.7

-30 -40 -41.6

-50 GO

pure g-C3N4

pCN

Figure 3. Zeta potential of the GO, pure g-C3N4 and pCN dispersed in DI water.

Then ZnO nanoparticles are grown on the surface of the GO/g-C3N4 hybrids in the hydrothermal environment containing urea, while GO is reduced to rGO. Zn2+ are adsorbed to the oxygencontaining functional groups (C=O, -COO- et.) on the surface of GO forming a growth seed. Under hydrothermal condition, the ZnO nanoparticles will grow up and cover on the surface of GO/g10


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C3N4. The GO was reduced by urea under hydrothermal condition, according to Li et al.32. Thus, the ZnO/rGO/g-C3N4 nanocomposites was prepared. 3.2 Characterization of ZnO, ZnO/rGO, ZnO/g-C3N4 and ZnO/rGO/g-C3N4. SEM image of prepared bulk g-C3N4 (BCN) is shown in Figure 4a. The surface of BCN is full of pleat formulated by corrugating and scrolling g-C3N4 sheet, which is similar to GO shown in Figure 4b. The inset of Figure 4a shows the exfoliated g-C3N4. After a long time protonation and sonication, the BCN was exfoliated into smaller and thinner nanosheets. The composite of g-C3N4 nanosheets and GO is shown in Figure 4c-d. Comparing with pure g-C3N4 or GO, the composite looks smoother, and the pleats are uniformly distributed on the surface forming regular wrinkles. As can be seen in Figure 4d, g-C3N4 nanosheets are tightly attached on GO. (a)

(b) 200 nm

1 m

10 m (d)

(c)

1 m

10 m

Figure 4. SEM of (a) g-C3N4 (the inset is exfoliated g-C3N4), (b) GO, and (c-d) the composite of g-C3N4 nanosheets and GO.

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The SEM image of obtained ZnO/rGO/g-C3N4 nanocomposite is shown in Figure 5a, and its elemental distributions are presented in Figure 5b-d. The middle part in Figure 5a indicates the presence of GO, which can be confirmed by the carbon distribution image shown in Figure 5c. At the same time, the nitrogen distribution (Figure 5d) concludes that g-C3N4 nanosheets are evenly distributed onto GO. Otherwise, the zinc distribution image shown in Figure 5b confirms that most area of GO/g-C3N4 composite was coated with ZnO nanoparticles. Figure 5e-f gives out the TEM image of ZnO/rGO/g-C3N4 nanocomposite. It is believed that the red arrow in Figure 5e points to a small piece of independent g-C3N4 nanosheet. From the perspective of morphology, both the SEM and TEM images indicate that the as-synthesized GO samples are flat or linearly wrinkled and the as-synthesized g-C3N4 samples are bulging or circularly wrinkled. Besides, the g-C3N4 samples are smaller than GO. The size of about 100 nm can double confirm that it is a g-C3N4 nanosheet. As it was shown in Figure 5f, the lattice fringes with interplanar spacing of 0.26 and 0.45 nm can be assigned to the (1 0 2) and (2 0 3) planes of the ZnO nanoparticles, respectively33.

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(a)

(b)

Zn

(d)

N

1 m

C

(c)

(f)

c (e)

d = 0.45 nm ZnO (2 0 3)

69

d = 0.26 nm ZnO (1 0 2)

5 nm

50 nm

Figure 5. (a) SEM images of ZnO/rGO/g-C3N4 nanocomposite. (b-d) Distributions of zinc, carbon and nitrogen. (e) TEM image of ZnO/rGO/g-C3N4 nanocomposite (the red arrow point to a piece of independent g-C3N4 nanosheet). (f) TEM image of a single ZnO nanoparticle in ZnO/rGO/gC3N4 nanocomposite.

The crystal structures of samples were characterized with XRD, as shown in Figure 6. All peaks in the pattern can be suitably indexed to ZnO phase (PDF No.36-1451). No characteristic peaks of GO and g-C3N4 can be observed because of their little amount. The sharp diffraction peaks also implied the high crystallinity of ZnO.

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ZnO/rGO/g-C3N4

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ZnO/rGO ZnO/g-C3N4 ZnO ZnO (PDF#36-1451)

20

40 2

60

80

degree

Figure 6. XRD pattern of ZnO, ZnO/rGO, ZnO/g-C3N4, ZnO/rGO/g-C3N4.

So as to study the surface chemical states of the species in the ZnO/rGO/g-C3N4 nanocomposite, XPS analyzation was conducted in the binding energy range of 0-1100eV. Figure 7a shows the XPS survey spectrum of the ZnO/rGO/g-C3N4, in which all peaks are assigned to the ternary composite. The presence of ZnO can be confirmed by the Zn 2p spectrum (Figure 7b). The two peaks are assigned to Zn 2p3/2 (1021.4 eV) and Zn 2p1/2 (1044.5 eV), respectively28. Although gC3N4 is really less in amount, the N1s XPS peak (Figure 7c) still find out referred peaks, which are C=N-C species in 398.5 eV, N-[C]3 species in 399.8 eV and C-NHx in 400.7 eV29. Since there are only small amounts of hydrogen atoms at the edges and defects, the peak area of C-NHx is small. The reality that peak area of C=N-C is slightly larger than that of N-[C]3 is also in line with the actual situation. Figure 7d reveals the C 1s high-resolution spectra, which is suitably in keeping 14


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with the C 1s spectra of rGO in composite situation. In detail, the C 1s spectrum of the ZnO/rGO/gC3N4 nanocomposite consists of nonoxy-genated ring C (284.8 eV), C-O species (286.2 eV), C=O

800 600 400 200 Binding Energy (eV)

0

Zn 2p3/2 Zn 2p1/2

1050

1040 1030 Binding Energy (eV)

(d) C 1s

N-[C]3

Intensity (a.u.)

N 1s

C=N-C

C-NHx

402

Zn 2p

Intensity (a.u.)

Zn 3s Zn 3p Zn 3d

C 1s

O KLL Zn LMM3 O1s Zn KLL Zn LMM1

Zn 2p3/2

1000

(c)

(b)

O KLL

Intensity (a.u.)

(a)

Zn 2p1/2

species (287.8 eV) and C(O)OH species (289.0 eV)31.

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 398 Binding Energy (eV)

396

1020

C=C

C=O O-C=O

290

C-O

288 286 284 Binding Energy (eV)

282

Figure 7. (a) XPS survey of the ZnO/GO/g-C3N4 nanocomposite. (b) Zn 2p (c) N 1s and (d) C 1s XPS spectra of the ZnO/GO/g-C3N4 nanocomposite.

3.3 Gas Sensing Property of ZnO, ZnO/rGO, ZnO/g-C3N4 and ZnO/rGO/g-C3N4. In order that we can find the optimum operating temperature of prepared sensors, the response of ZnO/rGO/g-C3N4 sensor has been investigated in different operating temperatures. Using 50 ppm ethanol vapor as targeted gas, Figure 8a illustrates the dependence between response and 15



operating temperature of the prepared sensors. Tests had been made at 5 levels of temperature ranging from 250°C to 350°C. As shown in Figure 8a, the sensor responses significantly depend on their operating temperature. For example, the response of the sensor based on ZnO/rGO/g-C3N4 increases rapidly when the temperature is lower than 300 °C, and reaches a maximum value of 61 at 300 °C, then decreases directly with further increase of temperature. This phenomenon illustrates that the optimum operating temperature of the ZnO/rGO/g-C3N4-based sensor is 300 °C. Equally, for ZnO, ZnO/g-C3N4 and ZnO/rGO based sensors, the maximum responses are 14, 9 and 47, respectively, at the same temperature of 300 °C. Obviously, the ZnO/rGO/g-C3N4-based sensor demonstrates the highest response value in these four sensors. Otherwise, it can also be discovered that the optical operating temperature is not changed by composition with other materials. In fact, many studies have shown that compounding with GO can reduce the operating temperature of the sensor. The contradiction between this work and other studies can be attributed to the short amount of GO added. The responses of ZnO/rGO/g-C3N4 nanocomposites to sequential concentration of ethanol gas from 500 ppb to 100 ppm at 300 °C are presented in Figure 8b. The response value of ZnO/rGO/gC3N4 nanocomposites to 100 ppm of ethanol gas is 178, and the detection limit is lower than 500 ppb at the optimum operating temperature of 300 °C. The trend is fitting by a logistic function appropriately, as it is shown in Fig. 8c. At less than 5 ppm and large than 200 ppm, the response changes weakly with the increase of ethanol vapor concentration. In the middle concentration part, the resistance of gas sensor shows a sensitive change following the change of ethanol vapor 16


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concentration. This phenomenon is consistent with the specific situation in the surface reaction of the gas sensor. When the gas concentration is in a low level (< 5 ppm), the utilization of active sites is insufficient, which means only a small amount of ethanol molecules can react on the surface of gas sensing material. On the contrast, the active sites tend to saturate while the gas concentration is in a high level (> 200 ppm), and the excess ethanol molecules cannot react adequately. Therefore, the response of the sensor changes slowly at low and high concentration ranges. Considering the importance of selectivity, thirteen different gases including acetone, butyl acetate, actinide, ammonia, formaldehyde, chlorobenzene, acetic acid, formic acid, isopropanol, ethanol, methanol, methylbenzene and benzene with equal concentration (100 ppm) have been exposed to the ZnO/rGO/g-C3N4 nanocomposites at 300 °C and the relevant responses are shown in Fig. 8d. In comparison with the other gases, the sensor exhibits superior selectivity to ethanol vapor. The excellent selectivity of the obtained ZnO/rGO/g-C3N4 sensor here is likely to be attributed to the presence of g-C3N4. Firstly, the N atoms in g-C3N4 would combine with the ZnO and lead to electron-coupling effect, reducing the energy required for arousing ZnO from its ground state to excited state, thus promoting the ethanol oxidation34-36. Secondly, g-C3N4, as a catalyst promoter, could improve O species concentration on the surface of ZnO in the ethanol oxidation reaction, which is beneficial to the transformation of alcohol to corresponding aldehyde37. The beneficial effect by adding catalyst promoter into catalyst system is the wellknown “bifunctional mechanism”38. Therefore, the ZnO/rGO/g-C3N4 could acts a selective response to ethanol. 17



Figure 8. (a) Response values of the sensors based on ZnO, ZnO/rGO, ZnO/g-C3N4 and ZnO/rGO/g-C3N4 to 50 ppm ethanol at different operating temperature. (b) the response of ZnO/rGO/g-C3N4 sensor to different concentrations of ethanol operated at 300 °C. (c) the fitting curve of (b). (d) Selective test based on ZnO/rGO/g-C3N4 sensor to 100 ppm different gas at 300 °C.

Figure 9a gives the responses and recovery curves of ZnO, ZnO/rGO, ZnO/rGO/g-C3N4 to 100 ppm of ethanol at 300 °C. It can be seen from the picture that sensors based on ZnO/rGO and ZnO/rGO/g-C3N4 have the same response time (76 s) and recovery time (6 s), but ZnO/rGO/gC3N4 sensor exhibits better response. Apparently, the addition of g-C3N4 can increase the response 18


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of gas sensor without extending response time. The increased response can be attributed to the combination of g-C3N4 and rGO. Moreover, Figure 9b illustrates the repeatability of the sensor based on ZnO/rGO/g-C3N4 nanocomposites, where the responses of the three detection cycles are almost the same. Considering the high operating temperature at 300 °C, the long-time stability (14 days) of ZnO/rGO/g-C3N4 sensor and ZnO sensor was also researched and depicted in Figure 9c. The slightly fluctuating curve above 150 indicated that the ZnO/rGO/g-C3N4 sensor had good stability. In fact, g-C3N4 is synthesized under 550 °C, which means a high temperature tolerance. At the same time, rGO was proved to be stable at 300 °C by our past report18 and ZnO sensor also had great stability at 300 °C, as can be seen in Figure 9c. Thus the long-time stability of ZnO/rGO/g-C3N4 sensor is foreseeable. To evaluate the gas-sensing performances of ZnO/rGO/gC3N4 nanocomposites, the comparison between this work and other literature is summarized in Table 1. As can be observed, the ZnO/rGO/g-C3N4 nanocomposites exhibits great competitive ability.

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(a)

200

Response (Ra/Rg)

76 s

Gas out

150

6s Gas out

100 50

Gas out

ZnO ZnO/rGO ZnO/rGO/g-C3N4

Gas in

0

0

200

400

600

Time (s)

(b) Response (Ra/Rg)

200 150 100 50 0

(c)

0

250

500 Time (s)

200

750

1000

ZnO/rGO/g-C3N4 ZnO

Response (Ra/Rg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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150 100 50 0

0

2

4

6 8 10 Time (day)

12

14

Figure 9. (a) Response-recovery curve of ZnO, ZnO/rGO and ZnO/rGO/g-C3N4 sensor to 100 ppm ethanol at 300 °C. (b) Repeatability curve of ZnO/rGO/g-C3N4 sensor to 100 ppm ethanol at 300 °C. (c) Long time stability of ZnO/rGO/g-C3N4 sensor to 100 ppm ethanol at 300 °C.

Table 1 Comparison of sensing characteristics for ethanol sensors based on ZnO, conducting polymers and noble metal reported in the literature. 20


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Materials

Concentration (ppm)

Operating temperature

Response(Ra/Rg)

(°C)

Response/Recovery time (s/s)

ZnO (NRs)7

400

300

~6.7

~150/~150

ZnO (NRs)9

50

330

8.5

8/10

ZnO(nanoflower)39

400

350

30

10/4

100

260

~70

~15/~15

50

250

66

8/5

50

260

27.5

~20/~30

ZnO/MoS2

40

Pt/ZnO/g-C3N4

41

ZnO/rGO42 SnO2/g-C3N4

43

100

300

~100

~60/~70

Ag/Fe2O3 44

100

250

6.3

5.5/16

In2O3 NWs45

100

370

2

10/20

100

150

14.2

15/20

100

210

13

93/87

100

350

15.8

25/45

100

200

2.5

-/-

100

300

178

76/6

TiO2/MoS2 46 Co3O4/g-C3N4 ZnO/g-C3N4 Au/SnO2

47

23

/rGO19

this work

3.4 Sensing Mechanism of the Nanocomposites of ZnO/rGO/g-C3N4. The ZnO/rGO/g-C3N4 nanocomposites exhibited increased gas sensing properties to ethanol because of small size effect, enhanced electron conductivity and the catalysis activity of g-C3N4 nanosheets. The sensing progress of the ZnO/rGO/g-C3N4 nanocomposite was represents in Figure 10 in detail. On the one hand, the small size effect of ZnO plays a part in the sensitization. The response strongly increases as the morphology of ZnO changed from giant sheets (pure ZnO) to nanoparticles less than 20 nm in diagram (Figure 5d). Since the depletion layer covers entire particle and can be fully depleted during the gas sensing procedure, the sensing film will perform a large change in conductivity, thus performing a sharp change in resistance. On the other hand, the brilliant electron conductivity of rGO also contributes to the high response. The electron 21



Page 22 of 34

generated from the sensing process can be transformed rapidly through rGO. Otherwise, p-n heterojunctions are created between ZnO nanoparticles and rGO48, and the barrier allows the ZnO nanoparticles to lose more electrons through a rapidly transform during the formation of the depletion layer. As a consequence, the total resistance of the nanocomposites will be remarkably changed, which can be confirmed by comparing the response curve of pure ZnO and ZnO/rGO in Figure 9a.

ZnO g-C3N4

rGO ZnO/rGO/g-C3N4 nanocomposites RO + H2O RO + H2O O

O2

Larger depletion layer

Activation

Depletion layer

ROH

O O2

O2

ZnO

O2

O2

O2

H bond adsorb

ZnO e

O2

O

O2

O

ZnO nanoparticle

e

e

rGO

e

Figure 10. Schematic representation of the interaction mechanism between ZnO/rGO/g-C3N4 nanocomposites and ethanol.

22


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Besides, the sensitivity is strengthened by electronic sensitization mechanism and uniformly deposition of g-C3N4. The presence of N atoms provides abundant electrons for g-C3N4, which may increase the electron density of the sensing film. Since the wide band gap of g-C3N4 improved the separation of electron-hole pairs, under high temperature excitation, the electrons from conduction band overflow and activate oxygen molecular to charged oxygen series (O2-, O- and O2-). With the help of the 2D/2D heterojunction interface of rGO and g-C3N4, these electrons can easily flow into ZnO nanoparticles. The increased electron amount does not decrease the resistance of material immediately. In contrast, the extra electrons are captured by oxygen molecular, help forming a larger depletion layer and more charged oxygen series, as it is depicted in Figure 10. During the sensing procedure, a larger depletion layer will lead to more significant change in sensor resistance, thus providing a higher response. Besides, the uniform distribution of g-C3N4, which can be seen in Figure 4c, address the aggregation problem of noble metal particles in the noble metal sensitization. Literally, the agglomeration of noble metal NPs could decrease the specific surface area and the response of the sensor49. However, the uniform manner of g-C3N4 on GO make the enhancement property to its best. Consequently, the ZnO/rGO/g-C3N4 nanocomposite reaches higher response (178 to 100 ppm ethanol at 300 °C, Figure 9a) during the same response time, compared with ZnO/rGO nanocomposite (86 to 100 ppm ethanol at 300 °C, Figure 9a).

4. CONCLUSION 23



In summary, ZnO/rGO/g-C3N4 nanocomposites were successfully synthesized by sonication process and hydrothermal synthesis. ZnO nanoparticles grew tightly on the surface of rGO/g-C3N4 composite and were confined at little size, which was confirmed by SEM, TEM and XPS. The gas sensing properties of original ZnO and ZnO/rGO/g-C3N4 nanocomposites were compared. It was found that ZnO/rGO/g-C3N4 nanocomposites had improved response performance and selective detection for ethanol. The response value of ZnO/rGO/g-C3N4 nanocomposites to 100 ppm of ethanol gas is 178, and the detection limit is lower than 500 ppb at the optimum operating temperature of 300 °C. The ZnO/rGO/g-C3N4 nanocomposites will provide an avenue to fabricate ethanol gas sensors for practical application.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z.Y. Yuan); [email protected] (M.H. Yang); [email protected] (M. Ibrahim) ORCID Fanli Meng: 0000-0002-2477-1542 Zhenyu Yuan: 0000-0003-2988-2214 Minghui Yang: 0000-0003-1071-1327 Author Contributions All authors have contributed and approved the final version of the manuscript. 24


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61833006, 61673367 and 61504023), the Fundamental Research Funds for the Central Universities in China (N180408018, N170405001, N180102032 and N170407005) and the Liaoning Province Natural Science Foundation (20180550483 and 20170540324).

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ZnO-Reduced Graphene Oxide Composites Sensitized with Graphitic

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