Enhancement of NO2-Sensing Performance at Room Temperature by

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Enhancement of NO2 Sensing Performance at Room Temperature by Graphene Modified Polythiophene Shouli Bai, Jun Guo, Jianhua Sun, Pinggui Tang, Aifan Chen, Ruixian Luo, and Dianqing Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00418 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 20, 2016

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Industrial & Engineering Chemistry Research

Enhancement of NO2 sensing performance at room temperature by graphene modified polythiophene Shouli Bai,a Jun Guo,a Jianhua Sun,a,bPinggui Tang,*a Aifan Chen,a Ruixian Luo,aand Dianqing Li*a a

State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemicals Analysis, Beijing University of Chemical Technology, Beijing 100029, China b

Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004 , China.

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ABSTRACT: Hybrids of ethylenediamine modified-reduced graphene oxide (RGO) and polythiophene (PTh) were synthesized successfully by in-situ chemical polymerization under room temperature for 2 h and loaded on a flexible PET film to construct a smart sensor. The structure and properties of the hybrids have been characterized by XRD, SEM, TG, UV-vis and FTIR analysis. The sensing performance of pure PTh and hybrids based sensors to NO2 were examined at room temperature, the results indicate that the hybrid film sensor with 5 wt% RGO not only exhibits high sensitivity to 10 ppm NO2 gas, which is nearly 4 times higher than that of pure PTh, and excellent selectivity, but also has flexible, low cost, portable and wearable characteristics. The mechanism of sensing performance enhanced by incorporating graphene into PTh also was discussed, which is attributed to large specific surface of the hybrid and synergetic effects between the components in hybrid. Keywords: Polythiophene; Graphene; NO2 sensor; Polyethylene terephthalate substrate.

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1. INTRODUCTION Nitrogen dioxide (NO2) is a toxic and irritating gas emitted from exhaust of industry and transportation, which may cause acid rain and photochemical smog and be serious harmful to environment.1, 2 A very low concentration of NO2 could cause damage on human’s nervous system and even lead people to lose consciousness. With the rapid development of portable electronics, there is an urgent demand to develop and fabricate portable sensors with small size, low cost and room temperature operating for online detection of low concentration NO2.3 Conducting polymers such as polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh) and their derivatives have received many attentions. As one of promising conducting polymers, polythiophene and its derivatives have characteristics of inherently porous structure, remarkable environmental stability and easy preparation.4, 5 Particularly, they have sensing responses to some toxic gases at low operation temperature, but they still exhibit some drawbacks, including low sensitivity to gas, low thermal stability and mechanical strength. Xu et al.5 synthesized PTh/SnO2 hybrid and reported the sensing response of 5.67 to 200 ppm NO2 at operating temperature of 90 °C. Guo et al.6 prepared the organic–inorganic PTh/WO3 hybrids and the response of sensor was up to 14.0, when exposed to 100 ppm NO2 gas at 70 °C. So, the problems of low sensitivity and relatively high operating temperature for sensors based on polythiophene are still not resolved now. In recent years, graphene has been intensively studied as a new generation solid state gas sensor.7 The graphene material has great potential applications in many fields, owing 3



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to excellent electron transport capacity, large specific surface area and outstanding mechanical strength.8,

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Hybridization of conducting polymers with graphene offers

further advantages in gas sensing performance. The sensing performance of novel hybrids will exceed their constituent counterparts.10 Nowadays, there are few reports about hybrids of graphene and polythiophene or its derivatives for gas detecting. Yang et al. 11 prepared

hybrids

of

the

reduced

graphene

oxide

(rGO)

and

porous

poly(3,4-ethylenediox-ythiophene) (PEDOT), and the response of the material to 20 ppm NO2 was 1.41 at room temperature. In this work, we combine the features of PTh and RGO to generate a novel gas sensing material by the in-situ chemical polymerization method. The novel hybrid exhibits more excellent sensing performance than that reported in literature and the details are listed in Table 1. Moreover, we use a PET thin film as a substrate and load the hybrid on substrate to structure a facile and smart sensor, which will be expected to open a new window to develop a flexible and wearable sensor operating at room temperature.

2. EXPERIMENTAL SECTION 2.1 Preparation of Graphene Oxide (GO) All chemicals were analytical-grade reagents without further purification. Deionized water was used throughout the experiments. GO was synthesized by the chemical oxidation of natural graphite flakes using the Hummer’s method as described below. Under the condition of vigorously stirring and ice-water bath, 0.5 g graphite powder was 4


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added into a three-neck flask（500 ml）followed by 0.74 g NaNO3 and 34 mL H2SO4 (98%). Subsequently 5.0 g KMnO4 was put in, meanwhile the temperature should be confirmed less than 20 °C. After stirring at 35 °C for 3 h, 250 ml water and 4 mL H2O2 (30 wt %) needed to be further added into the mixture slowly. Then obtained bright yellow suspension was washed 5 times with HCl and water (1:10 v/v). The solid (GO) was finally dried at 50 oC overnight. 2.2 Preparation of RGO RGO was prepared according to the procedures reported in literatures.12, 13 GO were dispersed in water by ultrasonication for 1 h. Then 20 mL of ethylenediamine were added to 30 ml of 1 mg/ml GO water solution. Then the mixture was refluxed at 80 °C for 8 h in water bath. Finally the product was obtained and washed several times with ethanol and dried at 60 oC. 2.3 Preparation of RGO-PTh Hybrids RGO-PTh hybrids were prepared by an in-situ chemical polymerization of thiophene monomers in the presence of prepared RGO, using anhydrous FeCl3 as the oxidant. In a typical procedure, certain mass of RGO was dispersed in 25 ml chloroform under ultrasonication for 1 h. Then thiophene monomer (188 µL) was injected into the above solution. The mixture was stirred for 20 min. Subsequently, under vigorous stirring, a requisite amount of anhydrous FeCl3 with FeCl3/thiophene molar ratio (3:1) was added into the solution. With the colour of the mixture changing from yellow to deep black, the reaction further last for 2 h at room temperature. The product was washed with methanol 5



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several times and dried at 60 °C overnight. According to the mass ratio of RGO in the hybrid, a series of RGO-PTh hybrids with different added mass of RGO(3, 6, 10, 15 and 20 mg) were prepared and corresponding marked as 1.5%RGO-PTh, 3%RGO-PTh, 5%RGO-PTh, 8%RGO-PTh and 10%RGO-PTh, respectively. For comparison, pure PTh was synthesized in the absence of RGO by a similar procedure. The complete synthesis process was modeled in Figure 1.

Figure 1. Schematic diagram of preparation process for RGO-PTh hybrid 2.4 Characterization Crystallographic information of the samples was collected using powder X-ray diffraction (XRD, Rigaku D/MAX-2500 diffractometer, copper Kα radiation with λ= 0.154 nm). The product morphology was examined by field emission scanning electron microscopy (FESEM, Zeiss Supra 55, on 20.0 kV). Fourier transform infrared spectra of the samples were recorded between 500 and 3100 cm-1 wavenumber range using Nicolet 6700 at room temperature. The UV–Vis absorption spectra of the samples in ethanol solvent were recorded in the range of 245–700 nm with a Shimadzu UV-2550 type UV– 6


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Vis spectrophotometer (Japan). The thermo gravimetric (TG) analysis was carried out in nitrogen atmosphere with a heating rate of 10 °C min−1 using NETZSCH STA 449 F3 thermal analyzer. Adsorption/desorption N2 isotherms were measured on solid samples at 77 K on a Micromeritics Surface Area & Porosity Gemini VII 2390 system, from which the Brunauer–Emmett–Teller (BET) surface area was calculated using the multipoint BET method. 2.5 Measurements of Gas Sensing Performance The PET (1 cm×1 cm) films as the sensor substrates were pre-treated in 0.02 g/mL NaOH solution at 95 °C for 90 min and dried at 50 °C. Then, the PTh and RGO-PTh hybrids were drop-coated on the PET films. Subsequently the films were attached on the probes of the devices with silver paint. Gas sensing performance of films was measured using a JF02E gas sensor test system (Guiyan Jinfeng Technology Co., Kunming, China). The constructed sensors were installed into an 18 L air chamber. NO2 was used as the test gas to test the gas performance of PTh and RGO-PTh hybrids at room temperature. Test gases with different concentrations (1 ppm, 2 ppm, 4 ppm, 8 ppm and 10 ppm) were introduced into the air chamber via a syringe. After every test, the chamber should be opened to diffuse test gas away. The resistance of the sensor in air (Ra) and in the air–test gas mixture (Rg) was recorded, respectively. Measurement system for testing gas sensors is shown in Figure 2. The test loop mainly composes of three parts: measured voltage (Voutput), load resistance (Rreference) and sensor resistance (Rsensor). The sensor resistance can be calculated by Eq. (1): 7



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ࡾ࢙ࢋ࢔࢙࢕࢘ ൌ

ࢂ࢝࢕࢘࢑࢏࢔ࢍ ିࢂ࢕࢛࢚࢖࢛࢚ ࢂ࢕࢛࢚࢖࢛࢚

﹒ࡾ࢘ࢋࢌࢋ࢘ࢋ࢔ࢉࢋ (1)

According to the resistance value of the sensor, the appropriate load resistance card is selected and the working voltage (Vworking) in the circuit output voltage is set as 8 V.

Figure 2. (A) Photo of the sensor configuration for gas-sensing measurement system, (B) electric circuit 3. RESULTS AND DISCUSSION 3.1 Structure and Morphology of RGO-PTh Hybrids FESEM was used to investigate the surface morphology of the synthesized RGO-PTh hybrid. Figure 3a shows the representative FESEM image of RGO with overlap at the edges. The randomly fiber-like morphology of PTh is obviously seen (in Figure 3b and 3c), which identifies an porous structure. The FESEM image of RGO-PTh hybrid (Figure 3d) shows the fiber-like of PTh polymerized on the wrinkled surface of RGO, which suggests the successful combination between PTh and RGO. Such a porous and interconnected morphology is useful for gas sensing application. The X-ray diﬀraction (XRD) patterns of RGO, PTh and RGO-PTh hybrid are displayed in Figure 4, respectively. One broad reflection peak centered at 25° was

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observed in the XRD pattern of RGO. The characteristic diffraction peak of PTh is at 22.8°, indicating the basically amorphous nature of the polymer. After incorporating RGO into PTh, the diffraction peaks of the RGO-PTh hybrid became broad and diffused in the region of 2θ = 20–24°.

Figure 3. FESEM images of (a) RGO, (b, c) PTh and (d) RGO-PTh hybrid

Figure 4. XRD patterns of (a) RGO, (b) PTh and (c) RGO-PTh hybrid In Figure 5a, the FTIR spectrum of GO is in good agreement with previous work. The characteristic peaks of oxygen group are located at 1224 cm-1, 1063 cm-1 and 1725 9



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cm-1, which are attributed to the C–OH, C–O stretching vibration, and stretching vibration of the C=O bond of carbonyl or carboxyl groups, respectively.14, 15 The FTIR spectra of RGO (Figure 5b) is markedly different from that of GO. The characteristic bands of oxygen-containing functional groups almost disappear, leaving only a small bump at 1746 cm-1 in RGO attributed to the stretching vibration of C=O in lactones. Furthermore, there is a new broad band at 1535 cm-1 which is associated with the vibration of N–H groups.12, 16 The characteristics peak of PTh (Figure 5c) observed at 685 cm−1 is associated with the C–S stretching vibration in the thiophene ring. The peak at approximately 780 cm−1 is due to the C–H out-of-plane vibration of the 2, 5-substituted thiophene ring created by the polymerization of thiophene monomers. The band at 1025 cm−1 is attributed to C–H out of plane deformation vibration, and C–H in plane bending vibration is at 1109 cm−1. The peaks observed at 1639 cm−1 corresponds to C=C symmetric vibration of thiophene ring.1, 17, 18 The FTIR spectrum of RGO-PTh hybrid (Figure 5d) shows almost the same bands as that of PTh, but all main characteristic peaks are slight shift, which indicates that incorporation of RGO causes the change in chemical environment of PTh. Comparing to the spectrum of PTh, the C=C peak of RGO-PTh hybrid (1632 cm-1) is shifted by ~7 cm-1 to low wavenumber, which may be attributed to the π-π interaction between RGO and PTh. Figure 6 shows the UV-vis spectra of RGO, PTh and 5% RGO-PTh hybrid. The absorption peak of RGO is 266 nm, which suggests that π-conjugated network of RGO

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was formed.19-21 The absorption spectrum of PTh exhibits absorption peak at around 307 nm which is due to π–π* inter-band-transition of PTh rings.22 PTh is also observed in the spectrum of hybrid. The absorption band at 445 nm in the spectrum of the RGO-PTh corresponds to π−π* electronic transition within highly π-conjugated PTh backbone.23, 24

Figure 5. FTIR spectra of (a) GO, (b) RGO, (c) PTh and (d) RGO-PTh hybrid

Figure 6. UV-vis spectra of (a) RGO, (b) PTh and (c) 5% RGO-PTh In general, the specific surface area is one of the most important parameters closely

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related to the gas sensing performance. N2 adsorption/desorption analysis, shown in Figure 7, was carried out to measure the specific surface area of PTh and 5 wt% RGO-PTh. The BET surface area of PTh and 5 wt% RGO-PTh are determined to be 22.0833 m2g-1 and 37.6855 m2g-1, respectively. The large specific surface area of hybrid is beneficial to absorb more test gas resulting in improvement of sensing performance.

Figure 7. N2 adsorption/desorption isotherms of (A) PTh;(B) 5 wt% RGO-PTh hybrid. Thermal stability of the hybrid was investigated by TG analysis as shown in Figure 8. For pure PTh, a continuous and unsteady mass loss of about 75% in sum was observed at temperatures up to 800 oC, which may be attributed to the decomposition of the PTh with a rapid decrease in mass from 400 oC. Different from pure PTh, the 5%RGO-PTh hybrid is still stable at the temperature below 180 °C, which exceeds 80 °C higher than the pure PTh. As seen in the TG curve of the hybrid, the mass loss of the 5%RGO-PTh hybrid is about 17% at 700 °C, which is much less than that of PTh. Thus from the TG analysis, it is evident that both the decomposition temperature and the residual mass increase. Hence, the thermal stability of PTh can be significantly improved by incorporating RGO.5, 6, 25

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Figure 8. TG curves of prepared (a) PTh and (b) 5%RGO-PTh hybrid 3.2 Gas Sensing Performance of RGO-PTh Hybrids The semiconductor sensing materials are usually classified as n-type and p-type according to the variation of resistance upon exposure to reducing or oxidizing gases. It was found from experiment that the PTh based sensor shows a sudden decrease in the resistance upon exposure to oxidizing gas of NO2, which exhibits the p-type semiconductor behavior, therefore, the response for oxidizing gas NO2 is defined as the ratio of Ra/Rg, while that for reducing gas is defined as the ratio of Rg /Ra. Figure 9A shows the sensing responses of pure PTh (blue curves) and 5 wt% RGO– PTh hybrid (red curves) to different concentrations of NO2 (1-10 ppm) at room temperature (25 °C). The response of 5%RGO hybrid based sensor to 10 ppm NO2 reaches 26.36, which is nearly 4 times higher than that of pure PTh to corresponding concentrations of NO2. Figure 9B shows sensing properties of sensors based on RGO-PTh hybrids with different RGO mass ratio (1.5%, 3%, 5%, 8% and 10%) to 1-10

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ppm NO2 at room temperature, and the highest response to 10 ppm NO2 corresponding to different RGO mass ratio are 7.2, 17.1, 26.36, 16.8 and 13.4, respectively. It can be observed that the responses of sensors increase then decrease with the increasing in the content of RGO, and the optimum mass fraction of RGO component is 5% in hybrid. Although the RGO plays an important role in the gas sensing performance, the excess RGO leads to the decline of the gas sensing performance. Besides, the linear tendency are observed for both PTh and 5%RGO-PTh from Figure 9C, the slope of straight line based on 5%RGO-PTh is larger than that based on PTh. When the detection limit is defined as 3 times of the standard deviation of noise, from Figure 9D the detection limit is estimated to be approximately 0.90 ppm and 0.52 ppm for the sensors based on pure PTh and 5wt%RGO composite, respectively. From the viewpoint of practical application, a sensor should present rather high selectivity. Because a high response can usually enhance the detection limit, while a better selectivity usually enables the gas sensor to exclusively response to a certain target gas, so, selectivity is one of important gas sensing performance for sensing materials. To determine the selectivity, the sensing performance of PTh and RGO-PTh ﬁlms operated at room temperature to various gases such as NH3, Cl2, NO2, C2H5OH and CH2O (each with a fixed concentration of 10 ppm) were tested and the corresponding results are shown in Figure 10. It is obvious that the 5%RGO-PTh sensor not only has much higher response to NO2 but also exhibits higher selectivity to NO2 than above mentioned gases.

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So, the 5% RGO-PTh hybrid is a promising sensing material for efficient and selective detection of NO2.

Figure 9. (A) Transient responses of sensors based on PTh and 5%RGO-PTh hybrid to different NO2 concentrations at room temperature; (B) Response of sensors based on RGO-PTh hybrids with different mass fractions of RGO to 1-10 ppm NO2 at room temperature; (C) Response linear fitting curve of the sensing response of PTh and 5%RGO-PTh to different concentration NO2 at room temperature; (D) Relationship between the sensor response and NO2 concentration dilogarithm curve

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Figure 10. Response of sensors based on PTh and 5%RGO-PTh to different tested gases at room temperature The delocalization of charge along the chain is responsible for the conductivity of the conducting polymer.26 During the typical sensing process, the analyte directly adsorbs on surface of conducting polymers and then react with them. When the PTh is exposed to NO2 (electron-accepting), NO2 extracts the electrons from PTh, contributing to the increase of holes density and decrease of resistance. The sensor response to NO2 was significantly enhanced by PTh hybridizing with modified graphene. There are three plausible reasons responsible for the excellent sensing performance of RGO-PTh hybrid: (1) the sensing process of such sensor involves adsorption/desorption on surface of material. A larger surface area of hybrid is can be achieved by incorporating RGO, which is beneficial to improve the sensing performance.27-29 (2)PTh can interact with RGO through π−π interaction. The FTIR analysis confirms that π−π interaction exists between the PTh and RGO. This π−π

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interaction also can cause a charge carriers increase and the electron transfer may occur between the conjugated PTh and RGO sheets, which results in the resistance decrease of hybrids. Furthermore, the introducing of RGO could also improve the electron-transfer rate during the process of NO2 sensing. (3) According to FTIR analysis, the characteristic peak located at 1639 cm-1 of PTh is shifted to the 1632 cm-1, which means that the required energy of electrons to delocalize along the PTh chain comparatively decreases. This suggests that NO2 will capture electrons from hybrids more easily. Therefore, the resistance of hybrid in NO2 decreases, consequently increasing the sensing performance of the resultant hybrid.11, 26, 30 Table 1 Response of PTh and graphene based sensors to NO2 reported in literatures materials

concentration

temperature

response

Ref.

(ppm)

(°C)

PTh

100

Room temperature (RT)

1.49

1

SnO2 /PTh

200

90

5.67

5

WO3/PTh

100

70

14.8

6

rGO

10

RT

3.3

12

rGO/porous PEDOT

20

RT

1.41

11

This work

10

RT

26.36

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4. CONCLUSIONS In summary, the RGO-PTh hybrids have been successfully synthesized through in-situ chemical oxidation polymerization. The RGO/PTh hybrids show enhanced gas sensing performance compared with their constituent counterparts due to the complementary and synergistic effects between both. The response of the hybrid with 5wt% RGO achieves nearly 4 times higher than that of pure PTh at room temperature. The hybrid is loaded on a flexible PET thin film to structure a smart sensor. The sensor not only exhibits high sensitivity, good selectivity to NO2, but also has flexible, simple and inexpensive characteristics, which will open a new window to develop a kind of portable and wearable electronic devices to detect hazardous gases in environment. ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Fax and Tel: + 86-10-64436992. E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

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This work was supported by the National Natural Science Foundation of China (Grant No. 51372013), the Fundamental Research Funds for the Central Universities (YS1406) and Beijing Engineering Center for Hierarchical Catalysts.

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Enhancement of NO2-Sensing Performance at Room Temperature by

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