Elsevier

Chemical Physics Letters

Volume 648, 16 March 2016, Pages 87-90
Chemical Physics Letters

Electric behavior of interlayer water in graphene oxide films

https://doi.org/10.1016/j.cplett.2016.01.072Get rights and content

Highlights

  • Current–voltage characteristics of graphene oxide (GO) films.

  • The three distinctly different states of GO films.

  • A decrease in current with time at a constant voltage.

  • Elimination of water in the absence of electrochemical processes.

  • The theoretical model of water diffusivity in the process of electroconductivity.

Abstract

Current–voltage characteristics of graphene oxide (GO) films were explored in water vapor as a function of film water content. It has been found that the films under study may contain free and bound water that can be completely eliminated in vacuum. A new phenomenon – a decrease in current with time at a constant voltage – was revealed. This was associated with elimination of water in the absence of electrochemical processes. The theoretical model was suggested to rationalize the behavior of water diffusivity in the process of electroconductivity.

Introduction

Attention to graphene and graphene materials has abruptly increased after isolation of graphene nano sheets in 2004. Initially, graphene oxide (GO) was regarded just as a starting material for preparation of graphene, but nowadays GO is suggested as a candidate for per se use in a variety of materials and devices, along with several processes for fabrication of GO films [1], [2], [3], [4], [5]. In [6], the GO films 6–40 nm thick were prepared by chemical (with hydrazine) and thermal (1100 °C) reduction. Thus prepared GO films were proposed for use as conductors in microelectronic devices [7]. It should be noted that the properties of GO can be expected to markedly depend on the morphology of starting films, especially if the latter ones comprise more than ten nano sheets (n > 10). This is because the GO nano sheets are randomly distributed and not planar. Basically, the film morphology is defined by the presence of interlayer water whose amount and status depend on ambient humidity and a type of drying procedure.

The behavior of interlayer water was studied in detail for graphite oxide [8], [9], [10]. By using neutron scattering, it has been shown [8] that, in the interlayer space, water may exist at high humidity in two forms: capable or incapable of translational motion. The interlayer distance in graphite oxide was found [9] to abruptly change in the vicinity of freezing point. Talyzin et al. [9] have also established the presence of two types of water in the interlayer space of graphite oxide. The same conclusion was also reached by Zhu et al. [10] who investigated the thermal expansion of graphene layers and explained the negative thermal effect by reversible diffusion of free water in the layered assembly of GO: adsorption of water at low temperatures and its desorption at higher temperatures. GO is also known to contain the bound water that is diffusion-silent. Graphite oxide and graphene oxide may have different structures: one is formed by ordered graphite layers while another, by randomly arranged GO nano sheets. But the literature data suggest that water in both interlayer spaces behaves similarly.

Several phenomena affecting the electrical properties of the films were disclosed in our previous studies [11], [12] on the photoconductivity of GO films in water vapor. At the initial stages of exposure to water vapor, the polarization current was found to grow due to the presence of free water in the film. Then, during experimental run at constant applied voltage and temperature, the current gradually decreased, which was attributed to changes in the film morphology.

In this work, we investigated the effect of ambient humidity on the electric properties of 100–500 nm GO films, with special emphasis on the influence of preliminary drying.

Section snippets

Experimental

Synthesis of GO and preparation of its aqueous suspension was carried out as described elsewhere [11], [12]. The spectral and elemental characteristics of GO samples are given in our previous publication [13]. Upon GO synthesis, the element composition of the product corresponded to the formula C8O4.6H1.8(H2O)0.58. X-ray diffraction studies demonstrated that the interlayer distance in our sample was equal to 0.81 nm. The characteristics of GO are presented detail in Supporting Information. A

Results and discussion

The GO films deposited onto the electrodes and dried at room temperature exhibited protonic electroconductivity in water vapor [11], [12]. Figure 1 shows the behavior of current i through a GO film upon variation in relative ambient humidity (RH). The protonic conductivity linearizes in the log i–RH coordinates (see the insert to Figure 1).

Figure 2 shows the behavior of current i upon stepwise variation in applied voltage U. As is seen in Figure 2, this dependence is linear, at low U and low

Conclusions

GO films may exist in three distinctly different states: (a) without interlayer water, (b) with interlayer bound water, and (c) with both bound and free interlayer water. Upon drying of GO films at normal conditions, free water is retained in the film, thus giving rise to polarization current at RH 100% for U = 0.1–10 V. Upon drying of GO films at RH 7%, the bound water is retained and gives rise to proton conductivity. In the presence of bound water in GO films, the diffusivity of water in the

Acknowledgments

This work was financially supported by the Russian Foundation for Basic Research (project no. 13-03-00696).

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