Analysis of the electro-optical response of graphene oxide dispersions under alternating fields
Graphical abstract
Introduction
Graphene oxide (GO) consists of a single layer of graphene, oxidised in different degrees with oxygen-containing functional groups, which strongly affect the electric and mechanical properties of the material [1,2]. While GO has been mainly studied as a precursor for large-scale graphene fabrication by reduction processes [3], in the recent years, it is gathering much interest because of its potential use in applications like photocatalysis, drug delivery, separation membranes, transparent conductive films, or optical devices [[4], [5], [6], [7], [8], [9]]. Among other suitable properties, GO particles are dispersable in polar solvents and easily functionalised [4]. Moreover, their mechanical and electric properties can be tuned by modifying the degree of oxidation, providing high versatility [[10], [11], [12]].
Controlled orientation of the GO flakes, essential for many of these applications, is being targeted by different methods, such as spray deposition, vacuum filtration, the Langmuir-Blodgett technique, or magnetic orientation [[13], [14], [15], [16]]. In particular, the use of electric fields, widespread for many materials, presents difficulties in the case of graphene oxide [17]. Thus, only recently, high-frequency AC electric fields have been successfully used to orient weakly-interacting GO platelets in suspension [[18], [19], [20], [21]]. Hence, much work is still needed to understand the behaviour of GO flakes under the effect of electric fields, necessary for future implementation in applications.
In this work, we analyse the orientation ordering of diluted suspensions of GO particles under the application of oscillating electric fields, as a function of the field frequency and amplitude. Experimentally, this electro-orientation is monitored via the analysis of the macroscopic linear dichroism that arises in the suspension when the particles are oriented by the external field.
Section snippets
Theoretical background
Under the application of an external electric field, non-spherical particles in suspension tend to orient with their major axis parallel to the field direction, and the absorbance A of the sample becomes anisotropic [22]. To quantify this effect, the linear dichroism (LD) is defined aswhere is the absorption of the sample along the direction perpendicular (parallel) to the applied field. The absorbance anisotropy is proportional to the state of orientation of the system,
Experimental section
Graphene oxide particles were purchased from Graphenea, Spain, and dispersed in aqueous suspension with the help of a sonicator. A microscope image of these particles is shown in Fig. 1, where it can be observed that the sample is very polydisperse. From the measurement of 150 particles, an average major axis length of 2.5±2.2 μm was obtained for the graphene flakes, where the error represents the standard deviation of the distribution.
The GO platelets present a small negative charge, being the
General features
Fig. 3 shows a complete linear dichroism signal of a graphene oxide dispersion. Here, it can be observed that LD is negative, as expected for planar particles. Therefore, we can conclude that the orientation of the GO layers is normal, with their major axis along the field direction. When the field is turned on, the dichroism signal grows in a short time, which depends on the value of the field strength, and reaches a stationary value. When the field is turned off, the dichroism decays during
Conclusions
The electro-optical behaviour of graphene oxide flakes provides much information on their interaction with external electric fields, essential for their possible use in applications in which controlled orientation is required. In this work, the electro-orientation of GO was determined in the absence of interparticle interactions, by means of the measurement of linear dichroism. The results were analysed to characterise the geometrical and optical properties, and to determine the polarization
Acknowledgement
Financial support of this investigation by Junta de Andalucía, Spain (grant No. PE2012-FQM0694) and University of Granada (Program ‘Proyectos de investigación precompetitivos) is gratefully acknowledged. One of the authors (P. A.) thanks MEC, Spain for her FPU grant (FPU13/02364). Thanks are due to Prof. A. Ramos, University of Sevilla, Spain for fruitful discussions.
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