Ammonia gas sensors based on chemically reduced graphene oxide sheets self-assembled on Au electrodes
© Wang et al.; licensee Springer. 2014
Received: 26 March 2014
Accepted: 4 May 2014
Published: 21 May 2014
We present a useful ammonia gas sensor based on chemically reduced graphene oxide (rGO) sheets by self-assembly technique to create conductive networks between parallel Au electrodes. Negative graphene oxide (GO) sheets with large sizes (>10 μm) can be easily electrostatically attracted onto positive Au electrodes modified with cysteamine hydrochloride in aqueous solution. The assembled GO sheets on Au electrodes can be directly reduced into rGO sheets by hydrazine or pyrrole vapor and consequently provide the sensing devices based on self-assembled rGO sheets. Preliminary results, which have been presented on the detection of ammonia (NH3) gas using this facile and scalable fabrication method for practical devices, suggest that pyrrole-vapor-reduced rGO exhibits much better (more than 2.7 times with the concentration of NH3 at 50 ppm) response to NH3 than that of rGO reduced from hydrazine vapor. Furthermore, this novel gas sensor based on rGO reduced from pyrrole shows excellent responsive repeatability to NH3. Overall, the facile electrostatic self-assembly technique in aqueous solution facilitates device fabrication, the resultant self-assembled rGO-based sensing devices, with miniature, low-cost portable characteristics and outstanding sensing performances, which can ensure potential application in gas sensing fields.
Chemiresistive sensors have aroused much attention in environment monitoring, industry and agriculture production, medical diagnosis, military, and public safety, etc. nowadays[1–5]. In order to meet the requirements of industry and other fields' demands, semi-conducting metal oxide, organic semiconductors, and carbon materials, etc., which have high aspect ratio and large specific surface area, have been widely used as sensing materials and the excellent performances of the resultant devices have been achieved[6–8].
Graphene, as a new member of carbon family, has emerged as a promising candidate for sensing because of its unique electronic, excellent mechanical, chemical, and thermal properties[9–18]. Excellent sensing performance of graphene towards different kinds of gases, including NO2, NH3, H2O, CO, trimethylamine, I2, ethanol, HCN, dimethyl methylphosphonate (DMMP), and DNT, have been reported[19–26]. Generally, there are three main methods to prepare graphene materials: micromechanical exfoliation of graphite, chemical vapor deposition, and reduction of graphene oxide (GO). The resultant graphene materials can be considered as excellent candidates for gas sensing, especially for chemically reduced graphene oxide (rGO). The rGO sheets have great potential for using as chemiresistors[29–32] due to their scalable production, easy processability in solution, large available surface area, etc. Hydrazine and ascorbic acid have been reported as excellent reducing agents for the reduction of GO, and the resultant rGO sheets show excellent responses to different vapors[20, 33]. Although many reports have been reported on the rGO sensing devices, it is still a great challenge to develop chemiresistive sensors based on rGO with miniature, low-cost, and portable characteristics.
In order to fabricate chemiresistive sensors based on nanomaterial, there are generally two main methods. One is to deposit nanomaterial on substrates followed by patterning electrodes on top of sensing materials. However, the process is complicated and requires exquisite skills. The other fascinating method is to drop-cast nanomaterial solution onto the pre-patterned electrode surfaces[29, 35]. This technique is facile, less expensive with higher yields, since it can be operated in solution, which benefits for the large-scale fabrication of the sensing devices. However, drop-casting method is very hard to ensure the reproducibility of the fabricated devices, which needs to be improved and applied in the realistic detection fields.
Herein, we report a facile and controllable self-assembly technique to fabricate rGO sensors, which could be used as an excellent NH3 gas sensing device. Negative GO sheets with large sizes (>10 μm) can be easily electrostatically attracted onto positive Au electrodes modified with cysteamine hydrochloride in aqueous solution. The assembled GO sheets on Au electrodes can be directly reduced into rGO sheets by hadrazine or pyrrole vapor and consequently provide the sensing devices based on self-assembled rGO sheets. In addition, pyrrole-vapor-reduced rGO-based sensor exhibits excellent response to NH3. We expect the easy, reproducible, green, and scalable fabrication of the sensors based on rGO reduced by pyrrole, with excellent performance, miniature, low-cost, and portable characteristics, can pave a new avenue for the application of assembled rGO devices in gas sensing field.
The natural graphite (32 meshes) used in this study was obtained from Qingdao Jinrilai Co. Ltd, Qingdao, China. Pyrrole was obtained from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and purified by distillation. Pre-determined NH3 gas (1 ppm) mixed with air was purchased from Beijing Beiyang Special Gases Institute Co. Ltd. (Beijing, China). Concentrated ammonia solution (25 wt.%) and all of other chemicals (analytical reagent grade) were purchased from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China) and were used without further purification. All of organic solvents were purified by distillation.
Self-assembly of GO sheets on Au electrodes
GO sheets with large sizes were prepared similar to the method reported by Zhao et al.. Large-size GO aqueous solution with the concentration at 2.5 mg/mL was prepared by mild sonication (80 W for 5 min) and stored for the further self-assembly process.
The standard microfabrication procedures were exploited to obtain the Au electrodes according to the method illustrated by us before. The parallel electrodes with the gap distance of 1 μm were formed by sputtering 10 nm Cr and 180 nm Au onto a patterned photoresist mold. A lift-off process was further carried out to remove the photoresist. The resultant electrodes were sonicated in ethanol, washed with deionized water thoroughly, and finally dried by nitrogen flow.
In order to obtain positively charged Au electrodes, the electrodes were immersed in 1 mM of cysteamine hydrochloride aqueous solution for 24 h, followed by washing with water and ethanol successively, each for three times. The resultant positive electrodes were further immersed in GO aqueous solution with different concentrations (1, 0.5, and 0.25 mg/mL) for 24 h. After washing with water and ethanol, each for three times, the electrodes were dried by purging air. Consequently, GO sheets bridged between Au electrodes were fabricated.
Chemical reduction of assembled GO sheets on Au electrodes
The GO sheets on the electrodes were easily reduced by hydrazine or pyrrole vapor. Typically, the electrodes with GO sheets were put in a vessel, and 3 drops of hydrazine were added besides the electrode. Then the vessel was sealed and put into the oven with the temperature at 90°C for 12 h. The resultant rGO sheets on the electrodes, denoted as Hy-rGO, were washed with distilled water and ethanol (each for three times) and dried by purging air.
For the purpose of the comparison, the rGO reduced by 3 drops of pyrrole, denoted as Py-rGO, was also fabricated according to the method mentioned above.
Atomic force microscope (AFM) was performed using a Dimension Icon instrument (Veeco, Plainview, New York, USA). The morphologies of the samples on the electrodes were observed by field emission scanning electron microscopy (FE-SEM; Carl Zeiss Ultra 55, Carl Zeiss AG, Oberkochen, Germany). Raman scattering was performed on a Renishaw inVia Reflex Raman spectrometer (Renishaw, Zhabei District, Shanghai, China) using a 514-nm laser source.
The sensing tests were carried out on a homemade gas handling system as illustrated in our previous report. The NH3 environments with the concentrations at parts per billion and parts per million levels were easily produced by diluting the NH3 gas with dry air. The humidity inside the test chamber was monitored by a Honeywell HIH-4000 humidity sensor (Honeywell Inc., Shanghai, China) and less than 5%. All of the sensing tests were carried out using a precision semiconductor parameter analyzer (Agilent 4156C; Agilent Technologies, Beijing, China) at room temperature. The flow rate of the balance gas (dry air) was controlled to be at 1 L/min. The sensor response was evaluated by the resistance change at a sampling voltage of 50 mV.
Results and discussion
Self-assembly technique for the fabrication of devices based on rGO sheets
where La is the size of the crystalline domains within CRG, λlaser is the excitation wavelength of the Raman spectra, and is the D/G intensity ratio. A D/G ratio of 1.4 and 0.9 with the excitation wavelength at 514 nm for Hy-rGO and Py-rGO respectively in our work (Figure 3c) suggested that crystalline domains with the size of ca. 12 and ca. 18.7 nm respectively had been formed in within the resultant Hy-rGO and Py-rGO flakes.
Evaluation of sensing devices based on assembled rGO sheets
where R0 is the resistance of rGO device before the exposure to NH3 gas, and Rgas is the resistance of rGO device in the NH3/air mixed gas.
Figure 7c, d displays the dynamic response of the resultant Hy-rGO- and Py-rGO-based sensing devices toward NH3 gas under the concentration of 50 ppm. In order to determine the optimal condition for the fabrication of sensing devices based on assembled rGO, the response of different sensing devices fabricated under different assembly concentration of GO solution were studied, and the exposure time of 12 min was defined here as the effective response time. From Figure 7c,d, we can observe that the resistance of the devices increases significantly when NH3 was introduced into the chamber. As the assembly concentration of GO solution decreases, the response of the resultant Hy-rGO-based sensors increased from 1.6% to 5.3%, suggesting that fewer rGO sheets bridged in between the gaps of electrodes benefited for the final sensing performance of the sensing devices. Two main reasons may account for the decrease of sensing performance as the increase of GO concentration: (1) the large size of graphene sheets, which is different from the sheets reported before; the interconnecting point is much less and not good for the penetration of gas molecules, which causes the little variation of the resistance of the interior sheets; (2) the stacking structure of the graphene sheets with a dense structure can prevent the gas molecules from rapidly penetrating into the inner space of the films, which is different from the situation of graphene films with the porous or three-dimensional structure. This was also the case for Py-rGO-based sensors. When the assembly concentrations of GO solution was high (1 mg/mL), much more Py-rGO sheets were deposited on the surfaces of Au electrodes; as a result, it is hard for NH3 gas to penetrate into the rGO flakes and the complete interaction between NH3 and rGO sheets could not be ensured. Hence, a lower response value of 9.8% was obtained. When the assembly concentration of GO solution decreased to 0.5 mg/mL, the response of the resultant Py-rGO device increased to 14.2%, which was much higher than that of Py-rGO device fabricated with GO concentration at 1 mg/mL. However, further decrease of GO concentration did not increase the response of the resultant rGO sensing device. Instead, a much lower response value of 5.5% was obtained. This might be due to the crack of rGO sheets as mentioned above. The majority of rGO sheets were cracked between the electrode gaps, resulting in a rapid change of resistance of the resultant device and consequently leading to a lower response value. Most importantly, it was noticed that all of the responses of Py-rGO devices were higher than those of sensing devices based on Hy-rGO (as shown in Figure 7c,d), suggesting that Py-rGO-based sensing devices could be used as better sensors for the detection of NH3 gas. Since 0.5 mg/mL was the optimal parameter for the fabrication of the Py-rGO sensors, which exhibited the best sensing performance during the NH3 detection, further studies would focus on Py-rGO device fabricated under assembly concentration of GO solution at 0.5 mg/mL, with the purpose of demonstrating the potential utility as well as probing the sensing properties of the resultant assembled Py-rGO sensors.
Furthermore, the sensor response exhibits an excellent recovery characteristic (as shown in Figure 8a). As illuminated with IR lamp together with flushed with dry air over the periods ranging from 134 to 310 s, the resistance of the device decreased and essentially recovered to the initial values. Since the Py-rGO sensors can be easily recovered, long-term practical work of the devices can be promised.
It is suggested that the excellent sensing properties of Py-rGO-based sensors are governed by the intrinsic properties of rGO as well as adsorbed PPy molecules. On one hand, rGO sheets still have parts of oxygen-based moieties and structure defects after chemical reduction process, which can generally lead to the p-type semiconducting behavior of the resultant rGO. NH3, as a reducing agent, has a lone electron pair that can be easily donated to the p-type rGO sheets, leading to the increase of the resistance of the rGO devices. Since the rGO-based sensing devices studied in our work are fabricated by self-assembly technique, NH3 gas can interact with rGO sheets completely and result in excellent sensing performance of the devices during the testing process. On the other hand, PPy molecules, as conducting polymers, can be generally considered as excellent NH3 gas sensing materials. Hence, the PPy molecules, which are attached on the surfaces of rGO sheets, play important roles in the enhancement of the sensing performance of the rGO devices and consequently show a better sensing performance than that of Hy-rGO devices.
In this work, a useful ammonia gas sensor based on chemically reduced graphene oxide (rGO) sheets using self-assembly technique has been successfully fabricated and studied for the first time. Negative GO sheets with large sizes (>10 μm) can be easily electrostatically attracted onto positive Au electrodes modified with cysteamine hydrochloride in aqueous solution. The assembled GO sheets on Au electrodes can be directly reduced into rGO sheets by hydrazine or pyrrole vapor and consequently provides the sensing devices based on self-assembled rGO sheets. The NH3 gas sensing performance of the devices based on rGO reduced from pyrrole (Py-rGO) have been investigated and compared with that of sensors based on rGO reduced from hydrazine (Hy-rGO). It is found that assembled Py-rGO exhibits much better (more than 2.7 times with the concentration of NH3 at 50 ppm) response to NH3 than that of assembled Hy-rGO. Furthermore, this novel gas sensor based on assembled Py-rGO showed excellent responsive repeatability to NH3. Since this technique can be incorporated with standard microfabrication process, we suggest that the work reported here is a significant step toward the real-world application of gas sensors based on self-assembled rGO.
The authors gratefully acknowledge financial supports by the Natural Science Foundation of Jiangsu Province (no. BK2012184), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (no. 13KJB430018), the National Natural Science Foundation of China (no. 51302179 and no. 51102164), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Key Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (no. 10KJA140048), the International Cooperation Project (no. 2013DFG12210) by MOST, Medical-Engineering (Science) cross-Research Fund of Shanghai Jiao Tong University (no. YG2012MS37 and no. YG2013MS20).
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