The effect of ambient humidity on the electrical properties of graphene oxide films
© Yao et al.; licensee Springer. 2012
Received: 14 June 2012
Accepted: 15 June 2012
Published: 2 July 2012
We investigate the effect of water adsorption on the electrical properties of graphene oxide (GO) films using the direct current (DC) measurement and alternating current (AC) complex impedance spectroscopy. GO suspension synthesized by a modified Hummer's method is deposited on Au interdigitated electrodes. The strong electrical interaction of water molecules with GO films was observed through electrical characterizations. The DC measurement results show that the electrical properties of GO films are humidity- and applied voltage amplitude-dependent. The AC complex impedance spectroscopy method is used to analyze the mechanism of electrical interaction between water molecules and GO films in detail. At low humidity, GO films exhibit poor conductivity and can be seen as an insulator. However, at high humidity, the conductivity of GO films increases due to the enhancement of ion conduction. Our systematic research on this effect provides the fundamental supports for the development of graphene devices originating from solution-processed graphene oxide.
KeywordsGraphene oxide Humidity sensing Complex impedance spectroscopy Nano device
Graphene oxide (GO), a single thin sheet of graphite oxide that conventionally serves as a precursor material for preparing graphene [1–3], has received increasing attentions in the application of optoelectronic [4, 5] and sensor devices  due to its inherent electrical and mechanical properties. As an oxidation product of graphene, GO can be viewed as a two-dimension network of sp2 and sp3-bonded hybridized carbon atoms arranged in a dense honeycomb crystal structure. Many oxygen-containing groups, including hydroxyl, epoxy and carboxylic acid, were bonded to the two-dimension network. The presence of sp3-bonded hybridized carbon atoms weakens the conductivity and enhances the hydrophilic property of GO. Recently, exploring the feasibility of integrating GO into graphene-based electronic devices has motivated immense studies on the intrinsic electrical [7, 8] and mechanical [9, 10] properties of GO. It is well known that the electrical properties of GO would be influenced by some external stimulations, including reducibility reagent , electric field [12–14], temperature [15, 16], light , etc. Due to the tunable electrical property, GO is considered as a potential electrical material candidate for graphene-based electronic devices.
In earlier works [7, 8], several groups have fabricated pristine thin-film-GO-based field effect transistors and resistive switching memory devices. In these applications, GO acts as a charge transport layer. These works indicate that the direct current (DC) electrical transport properties of GO films is temperature-dependent, and GO film exhibits p-type semi-conducting characteristic at room temperature in ambient. Since GO contains sp3-bonded hybridized carbon atoms, it is worth noting that GO can capture water vapor from external environment easily, owing to its notable hydrophilicity [18–20]. Hence, studies on the effect of atmosphere relative humidity (RH) on the electrical and mechanical properties of GO are beneficial to the application of practical GO-based electrical device. Previous studies have noted that the water adsorption of GO can affect its structural and mechanical properties [18, 19]. The uptake of water molecules increases the interlayer distance of GO sheets and forms hydrogen-bonding networks. In addition, an interesting phenomenon about the interaction of water vapor with GO films has been recently reported by Geim and his colleagues . They have found that water molecules can readily permeate through GO films without blockage; however, other molecules including ethanol, hexane, acetone, decane and propanol don't show this characteristic. Until now, the effect of water adsorptions on electrical properties of GO is still undefined in physics. Thereby, the investigation of this effect is essential in the development of graphene electronics, especially graphene device originating from solution-processed GO. In this paper, we use both DC measurement and alternating current (AC) impedance spectroscopy methods to elucidate the effect of ambient humidity on the electrical properties of GO films.
Graphite oxide was synthesized via the oxidative treatment of natural graphite using the modified Hummer's method . Then, graphite oxide was exfoliated to single-layered GO sheets by ultrasonicating graphite oxide suspension for 1 h. The obtained brown suspension was used as coating solution. Atomic force microscope (CSPM5500, Benyuan, China) was used to characterize the apparent heights of the obtained GO sheets. Fourier-transform infrared (FT-IR) spectrometer (5700, Nicolet, USA) was used to characterize the FT-IR spectra of the GO film.
Interdigitated electrodes (IDEs) were fabricated on an n-type silicon wafer with a top layer of SiO2 (300 nm) formed by thermal oxidization. Ti/Au layers with the thickness of 100:400 nm were deposited on SiO2 layer using magnetron sputtering. The Au electrodes with a 20-μm-wide gap were formed through photolithography followed by wet etching. Before being functionalized by GO films, the IDE was rinsed with distilled water and ethanol and dried in vacuum overnight. The device was fabricated by dispersing the GO suspension onto Au IDE. A few drops (4 μl) of the GO suspension were cast onto Au IDE by a micro-syringe. After drying at room temperature for 6 h, a discrete network of GO sheets was left on the Au IDE.
Results and discussion
Characterizations of material and device
DC electrical property of GO films at various humidity levels
The DC electrical property, i.e., current–voltage (I-V) characteristics of the GO-film-functionalized IDE, is measured with a voltage sweeping mode at various humidity points. In this configuration, one electrode of the IDE is loaded with the sweeping voltage bias, and the other electrode is grounded. Recent works have noted that the DC electrical property of GO films can be influenced by the amplitude of sweeping voltage [13, 14]. Thereby, we investigate the I-V characteristics of GO films with low (−1 to 1 V) and high (−4 to 4 V) sweeping voltages at various humidity levels, respectively.
AC complex impedance spectroscopy of GO films at various humidity points
As the humidity increased stepwise above 54% RH, straight line-type impedance appears at the low-frequency region, and semicircle-type impedance appears at the high-frequency region shown in Figure 6c,d,e,f. In this case, water vapor concentration reaches a higher value. More and more water molecules are absorbed onto the surface of GO films, resulting in an increase of hydronium ions through the hopping mechanism mentioned above. As a result, the bulk conductivity of GO films increases with increasing RH, resulting in a decrease of the diameter of the semicircle. Meanwhile, partial hydronium ions are hydrated into water molecules and H+ afresh, leading to a formation of a liquid layer around the interlayer of GO sheets by two-dimensional capillary or swelling effect. This process increases the interlayer distance of GO sheets largely, which can be sufficient to accommodate a monolayer of water. The formation of a liquid layer provides a conduction path across, between GO films and electrodes as illustrated in Figure 8c, increasing the mobility of the diffusion ions (including hydronium ions and H+). As a result, ion conduction, i.e., Warburg impedance Zw, appears. Therefore, Warburg impedance Zw is added in the equivalent circuit shown in Figure 7b. When RH increased above 80% RH, the Warburg impedance became dominant. Thus, we can consider that the major conduction process is attributed to adsorbed-water-induced ion conduction at high RH rather than the intrinsic conductivity of GO films. Based on the discussion above, we summarized the interaction mechanism of GO films with different amounts of water molecules, which was illustrated in Figure 8.
The dependence of the exciting frequency on impedance versus humidity response of GO films
We used the DC measurement and AC complex impedance spectroscopy methods to investigate the effect of ambient humidity on the electrical properties of GO films. The strong interaction of water molecules with GO films was observed through electrical characterizations. The DC measurement results show that the electrical properties of GO films were affected by ambient humidity and the amplitude of applied voltage. The electrical sensing mechanism of GO films was discussed by analyzing the characteristics of AC complex impedance spectroscopy. At low RH (<54%), GO films exhibited poor conduction property due to the presence of sp3-bonded hybridized carbon atoms. As RH increased stepwise above 54%, the conductivity of GO films increased sharply due to strong water-adsorption-induced ion conduction. The results are beneficial to the development of graphene-based electronics, especially graphene device arising from solution-processed GO. Finally, the exciting-frequency-dependent impedance of the GO films versus humidity was discussed, and this result suggested a potential application of GO films in impedance-type humidity sensor.
Supports for this work from the National Scientific Foundation of China (no. 61171050) and the Opening Project of State Key Laboratory of Electronic Thin Films and Integrated Devices (no. KFJJ201015) are acknowledged.
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