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Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications
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CERAMICS INTERNATIONAL
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Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications Xueyan Lia, Ahmad Umarb, Zhuo Chena, Tong Tiana, Shiwei Wanga, Yao Wanga,n a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China b Department of Chemistry, Faculty of Science and Arts and Promising Centre for Sensors and Electronic Devices, Najran University, P.O. Box-1988, Najran 11001, Saudi Arabia Received 9 May 2015; received in revised form 3 June 2015; accepted 5 June 2015
Abstract This paper reports the supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide (rGO) nanocomposites and their applications towards NO2 gas sensors. To prepare the composite materials for sensing applications, rGO was used as a template which was readily prepared through the reduction of hydrazine hydrate. By using a facile and effective supramolecular assembly (SA) method, two nanocomposites, i.e. poly(allylamine hydrochloride)–rGO (PAH–rGO) and polystyrenesulfonate-rGO (PSS–rGO) were prepared and efficiently used as effective materials to fabricate highly sensitive and stable NO2 gas sensors. The prepared nanocomposites were examined using several techniques which confirmed the successful formation of PAH–rGO and PSS–rGO nanocomposites. By detailed sensing experiments, it was found that both PSS–rGO and PAH–rGO nanocomposites exhibited strong gas sensing response, good stability and favorable reversibility for the detection of NO2 gas. Finally, NO2 gas sensing mechanisms, based on the utilization of specific nanocomposites, were also discussed and presented in this paper. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Reduced graphene oxide (rGO); Poly(allylamine hydrochloride); Polystyrenesulfonate; Supramolecular assembly; Gas sensor
1. Introduction Recently, graphene, its derivatives and composites are considered as one of most important and functional materials which are used in variety of high-technological applications such as electronics, bio-medical, sensors and actuators, catalysis, photovoltaics and so on [1–9]. Structurally, the graphene is a single layer of carbon atoms densely packed in a honeycomb crystal lattice and due to its atom-thick two-dimensional conjugated structures, the graphene possess high conductivity and large specific surface area [8,9]. Thus, by exploring its fascinating properties such high conductivity and large surface area, recently, graphene and its composites are used to fabricate gas and chemical sensors [10–12]. The first graphene based n
Corresponding author. E-mail address: [email protected] (Y. Wang).
sensor was fabricated in 2007 by Schedin et al [13]. Afterwards, several attempts have been made to prepare graphene-based gas sensors and reported in the literaure [14–16]. The nitrogen dioxide (NO2) is considered as one of the most toxic gases which can cause photochemical smog and acid rain that threatens the environment and cause serious adverse effects to human beings and other living organisms [1,2]. Thus, because of solemn toxic effects, it is important to develop efficient, highly effective, sensitive and reproducible NO2 gas sensors [3–7]. Recently, a variety of graphene and its derivative materials have been explored for sensing of NO2 [17–20]. The choice of reduced graphene oxide (rGO) for sensing material is basically for two main reasons, i.e. (i) the low-cost and large quantity synthesis and (ii) availability of residual oxygenated groups which allows the surface modification of rGO for specific applications [21,22]. Even though, rGO is used for various sensing application, especially gas sensing, but it displayed poor sensing performances
http://dx.doi.org/10.1016/j.ceramint.2015.06.030 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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such as low sensitivity and irreversibility towards targeted gases [23,24]. Recently, Shi et al. reported a sort of NO2 sensor based on chemically modified graphene materials including sulfonated rGO and ethylenediamine-modified rGO, which exhibit favorable reversibility and selectivity for NO2 detection [25]. However, the original perfect atomic lattice of graphene may be affected by covalent bonds during the chemical modification together with the damage of intrinsic electrical conjugation of reduced graphene oxide (rGO) [26–28]. Thus, to retain the properties of rGO and enhance the performance of rGO based materials for gas sensing applications, non-covalent interactions to prepare polymer–graphene composites was adopted [12,17,28–32] This paper reports, a supramolecular assembly (SA) method to prepare polyelectrolytes-rGO composites and their effective utilization for the fabrication of highly sensitive, reproducible and stable NO2 gas sensors. Two poly-electrolytes, i.e. cationic poly(allylamine hydrochloride) (PAH) and anionic polystyrenesulfonate (PSS) were used to modify the rGO. Importantly, considering the fact that rGO is difficult to be dispersed in aqueous solutions; thus, to solve this problem, in this work, the GO was first supramolecularly assemble polyelectrolytes, following with the reduction by hydrazine hydrate [33,34], which leads the formation of PAH–rGO and PSS–rGO as shown in Scheme 1. Interestingly, after the modification, uniformly dispersed polyelectrolyte-modified rGO solutions were obtained which were further used to fabricate highly sensitive, stable and reproducible NO2 gas sensors. 2. Experimental details 2.1. Chemicals GO was purchased from XianFeng NANO Co., Ltd. Poly (sodium 4-styrenesulfonate) (PSS, 70,000 Mw), poly (allylamine hydrochloride) (PAH, 120,000–200000 Mw) and hydrazine hydrate were obtained from Alfa Aesar. All the chemicals used in this study were of analytical grade and used asreceived.
(8 mg/mL) of PSS and PAH were prepared which were slowly added into already prepared 4 mL GO aqueous solution (1.0 mg/mL). Consequently, the obtained mixture solution was reduced by 10 mL hydrazine hydrate (1.12 μL/mL). The resultant solution was then heated at 100 oC for 1.5 h. Finally, the solution was purified by several time washing with DI water and further re-dissolved in DI water to receive the stable dispersion of PSS–rGO and PAH–rGO composites. 2.3. Characterizations The morphologies of PSS–rGO and PAH–rGO composites were observed by Transmission Electron Microscopy (TEM; JEOL JEM-2100F) and Scanning Electron Microscopy (SEM; HITACHI S-4800). The chemical composition, purity and chemical modifications of GO, rGO, PSS–rGO and PAH–rGO were examined by Fourier Transform Infrared (FTIR; Nicolet iN10MX) spectroscopy. Raman-scattering measurements of the prepared GO, rGO, PSS–rGO and PAH–rGO samples were examined by Raman-scattering spectroscopy (Lab RAM HR8000). The surface analysis and elemental compositions of the prepared materials were investigated through X-ray photoelectron spectroscopy (XPS; PHI 5000 Versa Probe). 2.4. Gas sense measurements To examine the gas sensing characteristics of the prepared materials, a gas sensing measurement system based on CGS1TP intelligent gas sense system was used. The gas sensors were fabricated and measured as reported in the literature [35]. Here, Ra and Rg can be defined as the electrical resistance values of the materials in the air and in the target gas, respectively. The sensibility of the gas sensors were assessed by the ratio of the initial and final resistances (Ra/Rg). Further, from the timeindependence of electrical resistance value curves, the gas sensing properties of the materials can be calculated. 3. Results and discussion
2.2. Synthesis of PSS–rGO and PAH–rGO composites For the synthesis, the GO was dispersed in deionized (DI) water by ultrasonication in order to form a homogeneous yellowish-brown solution. For the PSS and PAH attachments to the GO and prepare the PSS–rGO and PAH–rGO composites, in a typical reaction process, 10 mL aqueous solutions
3.1. Detailed characterizations of supramolecular prepared PSS–rGO and PAH–rGO nanocomposites Even though it is hard to dissolve rGO is water [36,37], but when the rGO was conjugated with polyelectrolyets such as PSS and PAH, the rGO composites exhibited great solubility
Scheme 1. Schematic illustration of two composite materials of PSS–rGO and PAH–rGO. Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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in water. Fig. 1 exhibits the physical pictures of water dispersions (1 mg mL 1) rGO, PSS–rGO and PAH–rGO. It is evident from the observed pictures that the aqueous solution of rGO is yellowish-brown color, whereas the aqueous solutions of PSS–rGO and PAH–rGO are stable, homogeneous and black in color, which confirmed that GO was successfully reduced by hydrazine hydrate [37]. Interestingly, it was observed that no precipitate exist in the aqueous solutions of PSS–rGO and PAH–rGO composites after placing over two weeks, indicating that it is feasible to modify rGO by the polyelectrolytes through supramolecular assembly. Thus, it can be confirmed that he PSS–rGO and PAH–rGO composites have favorable solubility in deionized water. The PSS is a typical anionic polyelectrolyte, including benzene rings offering the intrinsic driving force for π–π stacking with graphene and sulfophenyl groups compelling rGO to be hydrophilic [38,39]. Similarly, PAH is a typical cationic polyeletrolyte, anchors along the GO sheets with electrostatic interactions caused by the electropositivity of its protonated amine groups ð–NH3þ Þ and the electronegativity of carboxylic acid groups (–COO ) on GO [38]. In addition, the
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supramolecular modifications of PSS and PAH intensely augment the hydrophilicity of rGO after reduction of hydrazine hydrate. Interestingly, even after supramolecular modifications, the rGO keeps its regular structure and intrinsic electrical properties. The general morphologies of the prepared PSS–rGO and PAH–rGO composites were examined by TEM. Fig. 2(a) and (b) shows the typical TEM images of PSS–rGO and PAH–rGO composites, respectively. The observed TEM images obviously revealed that the polyelectrolytes–rGO via noncovalence retains favorably continuous and stable sheet structure with subtle wrinkles, demonstrating that it is a feasible way to modified graphene based on non-covalent interactions without damage in the morphology of graphene sheets. Fig. 2(c) exhibits the typical SEM image of PAH–rGO nanocomposite coated on the electrode surface which revealed that the nanocomposite is uniformly coated on the substrate surface. The surface modifications and chemical compositions of the prepared materials i.e. GO, rGO, PSS–rGO and PAH–rGO, were examined by FTIR spectroscopy. Fig. 3 exhibits the
Fig. 1. The physical pictures of water dispersion (1 mg mL 1) of: (a) rGO, (b) PSS–rGO and (c) PAH–rGO composites.
Fig. 2. Typical TEM images of (a) PSS–rGO and (b) PAH–rGO composites and (c) SEM image of PAH–rGO composite coated on the surface of an electrode substrate. Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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Fig. 3. Typical FTIR spectra of GO, rGO, PSS–rGO and PAH–rGO composites.
typical FTIR spectra of prepared materials. As evident from the observed FTIR spectra, the FTIR spectrum of GO exhibits a broad peak in the frequency range of around 3400 cm 1 which is attributed to the O–H stretching vibration. The characteristic band (CQO stretching vibration) of the carboxyl group in GO appears at 1730 cm 1; however, the sharp peak appearing at 1045 cm 1 can be assigned as C–O stretching vibration. The peak positioned at 1623 cm 1 is corresponding to the conjugated stretching vibration of CQC, indicating the structure of GO. As we can see, the characteristic band, associated to the oxygen-containing group of pristine GO almost disappears in the rGO spectrum which suggests the successful reduction of GO into rGO. Interestingly, it is seen in the observed FTIR spectra that after the PSS modification, three peaks appear at 1570 cm 1, 1450 cm 1 and 1410 cm 1 which are assigned to the stretching vibration of CQC conjugated with S in benzene rings. Moreover, the peak at 1127 cm 1 is due to the stretching vibration of C–C in PSS. The peaks appeared at 1126 cm 1, 1038 cm 1 and 1006 cm 1 are belonging to the characteristic peaks of SO-3, confirming the desired assembly of rGO with PSS via hydrophobic interaction. The FTIR spectrum of PAH–rGO shows broad peaks at 3360 cm 1 and 2920 cm 1 indicating the stretching vibration of N–H and –NH3þ , respectively. The C–H bending vibration was seen at 1457 cm 1 and 1384 cm 1. Finally, the characteristic peaks in PAH–rGO confirmed the favorable fabrication of PAH–rGO through electrostatic interaction. The prepared materials were further characterized by Raman-scattering spectroscopy as it is the most commonly used technique to characterize the carbon based materials. Fig. 4 exhibits the typical Raman-scattering spectra of prepared GO, rGO, PSS–rGO and PAH–rGO materials. As shown in Fig. 4, all the Raman spectra of GO, rGO, PSS–rGO and PAH–rGO show a D-band at 1340 cm 1 and a G-band at 1590 cm 1. The G-band is associated with the first-order scattering of the E2g mode and the D-band is attributed to the
Fig. 4. Typical Raman-scattering spectra of GO, rGO, PSS–rGO and PAH– rGO composites.
Fig. 5. Typical XPS survey scan spectra of rGO, PSS–rGO and PAH–rGO composites.
structural defects caused by the attachments of functional groups on the disordered structures of graphitic domains [40,41]. The ID/IG value of rGO (1.35) is larger than that of GO (1.15), revealing the formation of new sp2 clusters after reduction. The ID/IG value of PSS–rGO (1.33) or PAH–rGO (1.36) is close to that of rGO because of the modification of functional groups and the Raman shift of rGO, PSS–rGO, PAH–rGO is less than GO, which indirectly proves the successful chemical reduction by hydrazine hydrate [42]. In order to examine the elemental contents and structures, rGO, PSS–rGO and PAH–rGO were characterized by X-ray photoelectron spectroscopy (XPS) and results are demonstrated in Fig. 5. As shown in Fig. 5, the characteristic peaks of S 2p at 168 eV and Na 1s at 1071.55 eV appearing in the spectrum of PSS–rGO confirming the successful modification of rGO with PSS. The characteristic peaks at 401.65 and 197.9 eV, corresponding to N 1s and Cl 2p, respectively, representing the favorable assembly of rGO and PAH in the spectrum of PAH–rGO.
Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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3.2. NO2 gas sensing performances of PSS–rGO and PAH– rGO nanocomposites For application point of view, the PSS–rGO and PAH–rGO nanocomposites were used as potential materials to fabricate NO2 gas sensors. The properties and performances of the fabricated gas sensors can be estimated by recording their resistance values changed along with the absorption of gas molecules. Fig. 6 shows the sensitivity and reversibility curves toward representative concentrations of NO2. Fig. 6(a) and (b) illustrates the NO2 gas sensing responses of PSS–rGO and PAH–rGO based gas sensors towards various concentrations of NO2, respectively. Interestingly, excellent gas sensing response of PSS–rGO was observed after exposure to NO2 ranged from 1 ppm to 10 ppm of NO2 and its sensitivity was linearly increases from 1.30 to 2.12 times. The sensor based PAH–rGO shows a positive linear detection range to NO2 with different concentrations from 1 ppm to 15 ppm and the sensitivity measured from 1.31 to 5.61 times. It should be noted that sensors based on PSS–rGO and PAH–rGO exhibited a saturated response with 10 ppm of NO2 and 15 ppm of NO2, respectively. Furthermore, the materials enjoyed a good
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reversibility upon changing the atmosphere from NO2 to air; the reversibility of each concentration is analogous to the Fig. 6(c) and (d). These results demonstrated that the gas sensing performances of the rGO sensors based on SA modification are outstanding, conquering the poor properties of rGO for sensing gases. rGO is a kind of p-type semiconductor [43] and the modification of rGO with PSS, containing an electronwithdrawing sulfophenyl group, increase more hole-doped to graphene sheets. Further, the NO2 has ability to anchor itself to electron-rich sites such as the lone-pair electrons of S or O atoms in –So3 based on the fact that it is an electronwithdrawing molecule so that the adsorption of NO2 is supposed to strengthen the electron-withdrawing ability of the sulfophenyl group, further raising the doping level of the graphene sheets. Therefore, the conductance of the PSS–rGO layer increased significantly. Compared with the sulfophenyl group of PSS, –NH3þ of PAH, an electron-donating group with lone-pair electrons on nitrogen atoms, decreases the hole carrier density of rGO oppositely after electrostatic assembly, corresponding to the much larger resistance value of PAH– rGO [25]. The electron-rich N atoms of –NH3þ in PAH tend to
Fig. 6. The responses of the gas sensors towards various concentrations of NO2: (a) PSS–rGO in the range of 1–10 ppm and (b) PAH–rGO in the range of 1– 15 ppm. The sensitivity and reversibility of the gas sensors toward representative concentration of NO2: (c) PSS–rGO to 8 ppm and (d) PAH–rGO to 10 ppm. Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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Acknowledgments This work was supported by the Program for the New Century Excellent Talents in University (NCET-10-0035), the National Natural Science Foundation of China (Grant no. 51373005 and 21204087), the National Key Basic Research Program of China (2014CB931800) and the Fundamental Research Funds for the Central Universities Central Universities (YWF-14-HXXY-020 and YWF-15-HHXY-016).
References
Fig. 7. The stability curve of PSS–rGO and PAH–rGO composite based NO2 gas sensors after exposing them in air for 50 days. The error bars for each measurement lie within the symbols themselves.
be absorbed by NO2 when the PAH–rGO material is exposed to the atmosphere of NO2, which is able to weak the donating capacity of –NH3þ . Consequently, the hole carrier density of graphene sheets obtain partly restored so that the conductance of the sensor is increased. Furthermore, the response range of PAH–rGO is much wider. The NO2 molecules can also be desorbed from the materials after exposure in air atmosphere, which is attributed to the weak combination of NO2 molecules and functional groups of PSS–rGO and PAH–rGO. To verify the stability of the fabricated gas sensors, the PSS–rGO and PAH–rGO composites based gas sensors were exposed to 10 ppm NO2 once every 10 days for detecting the sensitivity, respectively. Fig. 7 exhibits the stability curves of PSS–rGO and PAH–rGO after exposing them in air for 50 days. As shown in Fig. 7, the relative sensitivity (the ratio of initial sensitivity and real-time sensitivity) of PSS–rGO decreased little from 100 to 96.69. By contrast, the sensitivity of PAH–rGO is much weaker than that of PSS–rGO, which is possibly resulting from the relatively fewer carboxylic acid groups remained after reduction for electrostatic interactions with PAH and much stronger π–π stacking of GO with PAH under the same experimental conditions [44].
4. Conclusions In summary, a facile and effective supramolecular assembly method was presented to fabricate polyelectrolytes-rGO composites for the fabrication of NO2 gas sensors. The fabricated NO2 gas sensors based on PSS–rGO and PAH–rGO composites exhibited high sensitivity, favorable reversibility and stronger responses of 2.12 and 5.62 times to 10 ppm NO2 and 15 ppm, respectively. The presented research indicates that the supramolecular assembly method is an easy and promising approach to prepare rGO-polyelectrolytes composites which can efficiently be used for the fabrication of highly sensitive, stable and reproducible gas-sensors.
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CERAMICS INTERNATIONAL
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Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications Xueyan Lia, Ahmad Umarb, Zhuo Chena, Tong Tiana, Shiwei Wanga, Yao Wanga,n a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China b Department of Chemistry, Faculty of Science and Arts and Promising Centre for Sensors and Electronic Devices, Najran University, P.O. Box-1988, Najran 11001, Saudi Arabia Received 9 May 2015; received in revised form 3 June 2015; accepted 5 June 2015
Abstract This paper reports the supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide (rGO) nanocomposites and their applications towards NO2 gas sensors. To prepare the composite materials for sensing applications, rGO was used as a template which was readily prepared through the reduction of hydrazine hydrate. By using a facile and effective supramolecular assembly (SA) method, two nanocomposites, i.e. poly(allylamine hydrochloride)–rGO (PAH–rGO) and polystyrenesulfonate-rGO (PSS–rGO) were prepared and efficiently used as effective materials to fabricate highly sensitive and stable NO2 gas sensors. The prepared nanocomposites were examined using several techniques which confirmed the successful formation of PAH–rGO and PSS–rGO nanocomposites. By detailed sensing experiments, it was found that both PSS–rGO and PAH–rGO nanocomposites exhibited strong gas sensing response, good stability and favorable reversibility for the detection of NO2 gas. Finally, NO2 gas sensing mechanisms, based on the utilization of specific nanocomposites, were also discussed and presented in this paper. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Reduced graphene oxide (rGO); Poly(allylamine hydrochloride); Polystyrenesulfonate; Supramolecular assembly; Gas sensor
1. Introduction Recently, graphene, its derivatives and composites are considered as one of most important and functional materials which are used in variety of high-technological applications such as electronics, bio-medical, sensors and actuators, catalysis, photovoltaics and so on [1–9]. Structurally, the graphene is a single layer of carbon atoms densely packed in a honeycomb crystal lattice and due to its atom-thick two-dimensional conjugated structures, the graphene possess high conductivity and large specific surface area [8,9]. Thus, by exploring its fascinating properties such high conductivity and large surface area, recently, graphene and its composites are used to fabricate gas and chemical sensors [10–12]. The first graphene based n
Corresponding author. E-mail address: [email protected] (Y. Wang).
sensor was fabricated in 2007 by Schedin et al [13]. Afterwards, several attempts have been made to prepare graphene-based gas sensors and reported in the literaure [14–16]. The nitrogen dioxide (NO2) is considered as one of the most toxic gases which can cause photochemical smog and acid rain that threatens the environment and cause serious adverse effects to human beings and other living organisms [1,2]. Thus, because of solemn toxic effects, it is important to develop efficient, highly effective, sensitive and reproducible NO2 gas sensors [3–7]. Recently, a variety of graphene and its derivative materials have been explored for sensing of NO2 [17–20]. The choice of reduced graphene oxide (rGO) for sensing material is basically for two main reasons, i.e. (i) the low-cost and large quantity synthesis and (ii) availability of residual oxygenated groups which allows the surface modification of rGO for specific applications [21,22]. Even though, rGO is used for various sensing application, especially gas sensing, but it displayed poor sensing performances
http://dx.doi.org/10.1016/j.ceramint.2015.06.030 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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such as low sensitivity and irreversibility towards targeted gases [23,24]. Recently, Shi et al. reported a sort of NO2 sensor based on chemically modified graphene materials including sulfonated rGO and ethylenediamine-modified rGO, which exhibit favorable reversibility and selectivity for NO2 detection [25]. However, the original perfect atomic lattice of graphene may be affected by covalent bonds during the chemical modification together with the damage of intrinsic electrical conjugation of reduced graphene oxide (rGO) [26–28]. Thus, to retain the properties of rGO and enhance the performance of rGO based materials for gas sensing applications, non-covalent interactions to prepare polymer–graphene composites was adopted [12,17,28–32] This paper reports, a supramolecular assembly (SA) method to prepare polyelectrolytes-rGO composites and their effective utilization for the fabrication of highly sensitive, reproducible and stable NO2 gas sensors. Two poly-electrolytes, i.e. cationic poly(allylamine hydrochloride) (PAH) and anionic polystyrenesulfonate (PSS) were used to modify the rGO. Importantly, considering the fact that rGO is difficult to be dispersed in aqueous solutions; thus, to solve this problem, in this work, the GO was first supramolecularly assemble polyelectrolytes, following with the reduction by hydrazine hydrate [33,34], which leads the formation of PAH–rGO and PSS–rGO as shown in Scheme 1. Interestingly, after the modification, uniformly dispersed polyelectrolyte-modified rGO solutions were obtained which were further used to fabricate highly sensitive, stable and reproducible NO2 gas sensors. 2. Experimental details 2.1. Chemicals GO was purchased from XianFeng NANO Co., Ltd. Poly (sodium 4-styrenesulfonate) (PSS, 70,000 Mw), poly (allylamine hydrochloride) (PAH, 120,000–200000 Mw) and hydrazine hydrate were obtained from Alfa Aesar. All the chemicals used in this study were of analytical grade and used asreceived.
(8 mg/mL) of PSS and PAH were prepared which were slowly added into already prepared 4 mL GO aqueous solution (1.0 mg/mL). Consequently, the obtained mixture solution was reduced by 10 mL hydrazine hydrate (1.12 μL/mL). The resultant solution was then heated at 100 oC for 1.5 h. Finally, the solution was purified by several time washing with DI water and further re-dissolved in DI water to receive the stable dispersion of PSS–rGO and PAH–rGO composites. 2.3. Characterizations The morphologies of PSS–rGO and PAH–rGO composites were observed by Transmission Electron Microscopy (TEM; JEOL JEM-2100F) and Scanning Electron Microscopy (SEM; HITACHI S-4800). The chemical composition, purity and chemical modifications of GO, rGO, PSS–rGO and PAH–rGO were examined by Fourier Transform Infrared (FTIR; Nicolet iN10MX) spectroscopy. Raman-scattering measurements of the prepared GO, rGO, PSS–rGO and PAH–rGO samples were examined by Raman-scattering spectroscopy (Lab RAM HR8000). The surface analysis and elemental compositions of the prepared materials were investigated through X-ray photoelectron spectroscopy (XPS; PHI 5000 Versa Probe). 2.4. Gas sense measurements To examine the gas sensing characteristics of the prepared materials, a gas sensing measurement system based on CGS1TP intelligent gas sense system was used. The gas sensors were fabricated and measured as reported in the literature [35]. Here, Ra and Rg can be defined as the electrical resistance values of the materials in the air and in the target gas, respectively. The sensibility of the gas sensors were assessed by the ratio of the initial and final resistances (Ra/Rg). Further, from the timeindependence of electrical resistance value curves, the gas sensing properties of the materials can be calculated. 3. Results and discussion
2.2. Synthesis of PSS–rGO and PAH–rGO composites For the synthesis, the GO was dispersed in deionized (DI) water by ultrasonication in order to form a homogeneous yellowish-brown solution. For the PSS and PAH attachments to the GO and prepare the PSS–rGO and PAH–rGO composites, in a typical reaction process, 10 mL aqueous solutions
3.1. Detailed characterizations of supramolecular prepared PSS–rGO and PAH–rGO nanocomposites Even though it is hard to dissolve rGO is water [36,37], but when the rGO was conjugated with polyelectrolyets such as PSS and PAH, the rGO composites exhibited great solubility
Scheme 1. Schematic illustration of two composite materials of PSS–rGO and PAH–rGO. Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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in water. Fig. 1 exhibits the physical pictures of water dispersions (1 mg mL 1) rGO, PSS–rGO and PAH–rGO. It is evident from the observed pictures that the aqueous solution of rGO is yellowish-brown color, whereas the aqueous solutions of PSS–rGO and PAH–rGO are stable, homogeneous and black in color, which confirmed that GO was successfully reduced by hydrazine hydrate [37]. Interestingly, it was observed that no precipitate exist in the aqueous solutions of PSS–rGO and PAH–rGO composites after placing over two weeks, indicating that it is feasible to modify rGO by the polyelectrolytes through supramolecular assembly. Thus, it can be confirmed that he PSS–rGO and PAH–rGO composites have favorable solubility in deionized water. The PSS is a typical anionic polyelectrolyte, including benzene rings offering the intrinsic driving force for π–π stacking with graphene and sulfophenyl groups compelling rGO to be hydrophilic [38,39]. Similarly, PAH is a typical cationic polyeletrolyte, anchors along the GO sheets with electrostatic interactions caused by the electropositivity of its protonated amine groups ð–NH3þ Þ and the electronegativity of carboxylic acid groups (–COO ) on GO [38]. In addition, the
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supramolecular modifications of PSS and PAH intensely augment the hydrophilicity of rGO after reduction of hydrazine hydrate. Interestingly, even after supramolecular modifications, the rGO keeps its regular structure and intrinsic electrical properties. The general morphologies of the prepared PSS–rGO and PAH–rGO composites were examined by TEM. Fig. 2(a) and (b) shows the typical TEM images of PSS–rGO and PAH–rGO composites, respectively. The observed TEM images obviously revealed that the polyelectrolytes–rGO via noncovalence retains favorably continuous and stable sheet structure with subtle wrinkles, demonstrating that it is a feasible way to modified graphene based on non-covalent interactions without damage in the morphology of graphene sheets. Fig. 2(c) exhibits the typical SEM image of PAH–rGO nanocomposite coated on the electrode surface which revealed that the nanocomposite is uniformly coated on the substrate surface. The surface modifications and chemical compositions of the prepared materials i.e. GO, rGO, PSS–rGO and PAH–rGO, were examined by FTIR spectroscopy. Fig. 3 exhibits the
Fig. 1. The physical pictures of water dispersion (1 mg mL 1) of: (a) rGO, (b) PSS–rGO and (c) PAH–rGO composites.
Fig. 2. Typical TEM images of (a) PSS–rGO and (b) PAH–rGO composites and (c) SEM image of PAH–rGO composite coated on the surface of an electrode substrate. Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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Fig. 3. Typical FTIR spectra of GO, rGO, PSS–rGO and PAH–rGO composites.
typical FTIR spectra of prepared materials. As evident from the observed FTIR spectra, the FTIR spectrum of GO exhibits a broad peak in the frequency range of around 3400 cm 1 which is attributed to the O–H stretching vibration. The characteristic band (CQO stretching vibration) of the carboxyl group in GO appears at 1730 cm 1; however, the sharp peak appearing at 1045 cm 1 can be assigned as C–O stretching vibration. The peak positioned at 1623 cm 1 is corresponding to the conjugated stretching vibration of CQC, indicating the structure of GO. As we can see, the characteristic band, associated to the oxygen-containing group of pristine GO almost disappears in the rGO spectrum which suggests the successful reduction of GO into rGO. Interestingly, it is seen in the observed FTIR spectra that after the PSS modification, three peaks appear at 1570 cm 1, 1450 cm 1 and 1410 cm 1 which are assigned to the stretching vibration of CQC conjugated with S in benzene rings. Moreover, the peak at 1127 cm 1 is due to the stretching vibration of C–C in PSS. The peaks appeared at 1126 cm 1, 1038 cm 1 and 1006 cm 1 are belonging to the characteristic peaks of SO-3, confirming the desired assembly of rGO with PSS via hydrophobic interaction. The FTIR spectrum of PAH–rGO shows broad peaks at 3360 cm 1 and 2920 cm 1 indicating the stretching vibration of N–H and –NH3þ , respectively. The C–H bending vibration was seen at 1457 cm 1 and 1384 cm 1. Finally, the characteristic peaks in PAH–rGO confirmed the favorable fabrication of PAH–rGO through electrostatic interaction. The prepared materials were further characterized by Raman-scattering spectroscopy as it is the most commonly used technique to characterize the carbon based materials. Fig. 4 exhibits the typical Raman-scattering spectra of prepared GO, rGO, PSS–rGO and PAH–rGO materials. As shown in Fig. 4, all the Raman spectra of GO, rGO, PSS–rGO and PAH–rGO show a D-band at 1340 cm 1 and a G-band at 1590 cm 1. The G-band is associated with the first-order scattering of the E2g mode and the D-band is attributed to the
Fig. 4. Typical Raman-scattering spectra of GO, rGO, PSS–rGO and PAH– rGO composites.
Fig. 5. Typical XPS survey scan spectra of rGO, PSS–rGO and PAH–rGO composites.
structural defects caused by the attachments of functional groups on the disordered structures of graphitic domains [40,41]. The ID/IG value of rGO (1.35) is larger than that of GO (1.15), revealing the formation of new sp2 clusters after reduction. The ID/IG value of PSS–rGO (1.33) or PAH–rGO (1.36) is close to that of rGO because of the modification of functional groups and the Raman shift of rGO, PSS–rGO, PAH–rGO is less than GO, which indirectly proves the successful chemical reduction by hydrazine hydrate [42]. In order to examine the elemental contents and structures, rGO, PSS–rGO and PAH–rGO were characterized by X-ray photoelectron spectroscopy (XPS) and results are demonstrated in Fig. 5. As shown in Fig. 5, the characteristic peaks of S 2p at 168 eV and Na 1s at 1071.55 eV appearing in the spectrum of PSS–rGO confirming the successful modification of rGO with PSS. The characteristic peaks at 401.65 and 197.9 eV, corresponding to N 1s and Cl 2p, respectively, representing the favorable assembly of rGO and PAH in the spectrum of PAH–rGO.
Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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3.2. NO2 gas sensing performances of PSS–rGO and PAH– rGO nanocomposites For application point of view, the PSS–rGO and PAH–rGO nanocomposites were used as potential materials to fabricate NO2 gas sensors. The properties and performances of the fabricated gas sensors can be estimated by recording their resistance values changed along with the absorption of gas molecules. Fig. 6 shows the sensitivity and reversibility curves toward representative concentrations of NO2. Fig. 6(a) and (b) illustrates the NO2 gas sensing responses of PSS–rGO and PAH–rGO based gas sensors towards various concentrations of NO2, respectively. Interestingly, excellent gas sensing response of PSS–rGO was observed after exposure to NO2 ranged from 1 ppm to 10 ppm of NO2 and its sensitivity was linearly increases from 1.30 to 2.12 times. The sensor based PAH–rGO shows a positive linear detection range to NO2 with different concentrations from 1 ppm to 15 ppm and the sensitivity measured from 1.31 to 5.61 times. It should be noted that sensors based on PSS–rGO and PAH–rGO exhibited a saturated response with 10 ppm of NO2 and 15 ppm of NO2, respectively. Furthermore, the materials enjoyed a good
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reversibility upon changing the atmosphere from NO2 to air; the reversibility of each concentration is analogous to the Fig. 6(c) and (d). These results demonstrated that the gas sensing performances of the rGO sensors based on SA modification are outstanding, conquering the poor properties of rGO for sensing gases. rGO is a kind of p-type semiconductor [43] and the modification of rGO with PSS, containing an electronwithdrawing sulfophenyl group, increase more hole-doped to graphene sheets. Further, the NO2 has ability to anchor itself to electron-rich sites such as the lone-pair electrons of S or O atoms in –So3 based on the fact that it is an electronwithdrawing molecule so that the adsorption of NO2 is supposed to strengthen the electron-withdrawing ability of the sulfophenyl group, further raising the doping level of the graphene sheets. Therefore, the conductance of the PSS–rGO layer increased significantly. Compared with the sulfophenyl group of PSS, –NH3þ of PAH, an electron-donating group with lone-pair electrons on nitrogen atoms, decreases the hole carrier density of rGO oppositely after electrostatic assembly, corresponding to the much larger resistance value of PAH– rGO [25]. The electron-rich N atoms of –NH3þ in PAH tend to
Fig. 6. The responses of the gas sensors towards various concentrations of NO2: (a) PSS–rGO in the range of 1–10 ppm and (b) PAH–rGO in the range of 1– 15 ppm. The sensitivity and reversibility of the gas sensors toward representative concentration of NO2: (c) PSS–rGO to 8 ppm and (d) PAH–rGO to 10 ppm. Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030
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Acknowledgments This work was supported by the Program for the New Century Excellent Talents in University (NCET-10-0035), the National Natural Science Foundation of China (Grant no. 51373005 and 21204087), the National Key Basic Research Program of China (2014CB931800) and the Fundamental Research Funds for the Central Universities Central Universities (YWF-14-HXXY-020 and YWF-15-HHXY-016).
References
Fig. 7. The stability curve of PSS–rGO and PAH–rGO composite based NO2 gas sensors after exposing them in air for 50 days. The error bars for each measurement lie within the symbols themselves.
be absorbed by NO2 when the PAH–rGO material is exposed to the atmosphere of NO2, which is able to weak the donating capacity of –NH3þ . Consequently, the hole carrier density of graphene sheets obtain partly restored so that the conductance of the sensor is increased. Furthermore, the response range of PAH–rGO is much wider. The NO2 molecules can also be desorbed from the materials after exposure in air atmosphere, which is attributed to the weak combination of NO2 molecules and functional groups of PSS–rGO and PAH–rGO. To verify the stability of the fabricated gas sensors, the PSS–rGO and PAH–rGO composites based gas sensors were exposed to 10 ppm NO2 once every 10 days for detecting the sensitivity, respectively. Fig. 7 exhibits the stability curves of PSS–rGO and PAH–rGO after exposing them in air for 50 days. As shown in Fig. 7, the relative sensitivity (the ratio of initial sensitivity and real-time sensitivity) of PSS–rGO decreased little from 100 to 96.69. By contrast, the sensitivity of PAH–rGO is much weaker than that of PSS–rGO, which is possibly resulting from the relatively fewer carboxylic acid groups remained after reduction for electrostatic interactions with PAH and much stronger π–π stacking of GO with PAH under the same experimental conditions [44].
4. Conclusions In summary, a facile and effective supramolecular assembly method was presented to fabricate polyelectrolytes-rGO composites for the fabrication of NO2 gas sensors. The fabricated NO2 gas sensors based on PSS–rGO and PAH–rGO composites exhibited high sensitivity, favorable reversibility and stronger responses of 2.12 and 5.62 times to 10 ppm NO2 and 15 ppm, respectively. The presented research indicates that the supramolecular assembly method is an easy and promising approach to prepare rGO-polyelectrolytes composites which can efficiently be used for the fabrication of highly sensitive, stable and reproducible gas-sensors.
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Please cite this article as: X. Li, et al., Supramolecular fabrication of polyelectrolyte-modified reduced graphene oxide for NO2 sensing applications, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.06.030