Synthesis and application of graphene–silver nanowires composite for ammonia gas sensing

2.1.1. Synthesis of rGO.

2.1.2. Synthesis of silver nanowires (Ag NWs).

The gas sensor reported herein was fabricated on a quartz wafer with thermal evaporation and two silver electrodes (figure 1). The GO solution was spin-coated directly onto a clean quartz wafer. Subsequently, these dry GO films were exposured to hydrazine and heated to 800 °C in the high vacuum of ∼10⁻⁶ Torr to reduce to rGO films. After that, two silver planar electrode arrays were deposited on the rGO films using thermal evaporation method with 6 mm distance between them. Finally, we used spray-coating method to disperse Ag NWs arranged between two electrodes to complete our gas-sensing devices.

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The as-fabricated rGO–Ag NWs composite devices with two planar electrodes were sensitive to gaseous species without any further treatment and were used for gas detection by simply measuring the resistance change of the device upon exposure to various gases.

Besides, to obviously estimate and compare the gas sensitivity of rGO–Ag NWs composite in our subject, the gas sensing devices based on intrinsic rGO material (without Ag NWs) were also fabricated for a reference sample. Thus, for the reader's convenience we refer to two kinds of sensing sample: gas sensing rGO device and gas sensing rGO–Ag NWs device (figure 1) in the following presentation.

Relying on the popular gas sensor testing systems, that were designed by equipment manufacturers and laboratories, the homemade NH₃ gas testing equipment was set up to measure the sensing properties of the as-fabricated devices (such as: sensitivity, stability, response time and recovery time of the devices). Our measurement system included an Ar gas tank, gas hoses and solenoids system, two flowmeters, a bubbler with NH₃ solution and an airtight test chamber connected with collect-store data DAQ component. The Ar gas played a role as carrier gas, dilution gas and purge gas [9].

The device was mounted in the test chamber with electrical feedthroughs. The measurement included two processes: absorption and desorption. In absorption process, the flowing NH₃ target gas (consisting of Ar carrier gas) from the bubbler into the test chamber for the period time and the change in resistance of sensor were recorded in that time. Moreover, we let the Ar purge gas flow through the test chamber to re-establish the rGO initial resistance. That step will give information about desorption ability of sensor in desorption process. All of the above procedures were controlled by computer.

UV–Vis spectroscopy was used to monitor the combination of rGO and Ag NWs into a composite. The spectra were characterized by two absorbance peaks at 270 and 375 nm (figure 2) which were characteristic peaks of rGO and Ag NWs material, respectively [1,7,8].

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Besides, AFM images (figure 3) revealed that rGO synthesized by chemical methods had small sheet dimensions (called an island). The Ag NWs were used to play the role of bridges connecting together many rGO islands to improve electrical properties of rGO–Ag NWs composite. The results were much higher conductance (∼ 3.5 kΩ for rGO sheets and ∼0.5 kΩ for rGO–Ag NWs composite) but their transparency in both of cases is unchanged (∼ 80%).

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In sensor devices, the contact resistance normally existing between the sensing material and the metal electrode further distorted the conventional sensitivity formulation. Figure 4 shows linear and symmetric I–V curves. This indicates that contact areas in our devices, including rGO sheets—silver electrodes and rGO sheets—Ag NWs contacts, are possibly Ohmic contacts. Therefore, we believe that the electrical contacts do not play a significant role in our devices and the data are most consistent with simple charge transfer.

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Electronic measurements of the gas sensing devices revealed that NH₃ gas sensitivity of rGO material was improved by spray-coating of Ag NWs on its surface. This conclusion was proved by combining several facts that were confirmed under experimental conditions (i.e. room temperature and atmospheric pressure) from the as-fabricated devices [6] as follows:

Firstly, the sensitivity ability of sensing rGO devices (figure 5(a)) was investigated. The experiment consisted of the introduction of the (NH₃/Ar) target gas for 600 s, and (Ar) purging gas for 300 s. The result indicated that intrinsic sensing of gas sensing rGO devices was low electrical response upon exposure to NH₃ gas (sensitivity 3–4%) and this material could not be completely restored at room temperature. The flow of testing gas (NH₃ 0,1%/Ar) was held the same in all experiments.

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Analysis of the gas sensing curves in figure 5 can be divided into two parts: the 'rapid' (steep slope) and 'slow' (shallow slope) response. The rapid response arises from molecular adsorption onto low-energy binding sites, such as sp²-bonded carbon, and the slow response arises from molecular interactions with higher energy binding sites, such as vacancies, structural defects, and oxygen functional groups [3,4].

For the next step, the sensitivity ability of rGO–Ag NWs composite was studied and the whole above experiment process was repeated. The data in figure 5(b) shows that the presence of Ag NWs can improve the sensing properties of rGO sheets, namely the sensitivity increases from ∼3.5% (rGO) to ∼28% (rGO–Ag NWs), the response time decreases from 600 s (rGO) to ∼200 s (rGO–Ag NWs) and the recovery time goes down from 300 s (rGO) to ∼60 s (rGO–Ag NWs). In particular, complete NH₃ molecular desorption on the surface of rGO–Ag NWs composite took place at room temperature. Through suggestions, we can guess that connecting together individual rGO sheets by Ag NWs caused the increase of the mobility of carriers in rGO sheets and led to higher gas sensing properties of rGO. In addition, due to the appearance of Ag NWs bridges, the number of the sites with high binding energies in rGO sheets decreases while the number of those with low binding energies increases. Because the molecular gas was mainly absorbed at the sites with low binding energies, the appearance of Ag NWs briges led to the complete desorption ability of rGO–Ag NWs composite.

Finally, we measured gas sensitivity of rGO–Ag NWs composite with the change of the Ag NWs content corresponding to the volume of solution spray-coated onto the same rGO surface. From the results, the greater the volume of Ag NWs solution used, the higher the sensitivity of devices that was reached, while the response and recovery times decreased (figure 6(a)), meaning that Ag NWs continue to improve the sensing properties of rGO. However, if we continuously increased the volume of Ag NWs solution, the sensing properties of rGO could decrease. To explain this phenomenon we guess that the greater the volume of Ag NWs solution used, the larger the area of the rGO sheets that was covered. This cover prevented the touch between molecular gas and rGO sheets and led to the decrease of sensing properties of devices.

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Additionally, from figure 6(b) the excellent repeatable response of devices to NH₃ gas with ten cycles was measured. This showed sensor stability and it offered an opportunity to explore the cycle-to-cycle variation for sensors. The room temperature sensing performance of the rGO–Ag NWs devices under atmospheric pressure was very encouraging for practical applications when considering the simplicity and low cost of fabricating these devices, and the potential opportunities for optimization.