Systematic study of the correlation between surface chemistry, conductivity and electrocatalytic properties of graphene oxide nanosheets
Graphical abstract
Introduction
Graphene is one of the materials most studied nowadays for its numerous potential applications, including energy storage and sensing [1], [2], [3], [4], [5]. Outstanding performance of this nanosized material comes from its peculiar physico-chemical properties, such as high surface area (>2600 m2/g for single-layer graphene), excellent thermal and electric conductivity and high mechanical strength.
Large-scale production methods of graphene are currently used also at industrial level [6]. However, they usually yield a mixture of graphene multilayers, with a variable amount of monolayers. The only method to achieve quantitative exfoliation of graphite down to single layers is by chemical oxidation, which yields graphene oxide (GO) solutions mostly composed of monolayers of carbon nanosheets [7], [8]. The stability of GO suspensions is due to the high number of oxidized residues on the carbon surface; they favour the stability of GO in different solvents, water included, but render the material poorly conductive [9], [10]. Due to its highly defective nature, GO has long been the underdog of the graphene family. However, this prejudice is now being challenged and GO is used for many interesting potential applications.
GO can be reduced by thermal, chemical and electrochemical procedures, in order to partially remove the oxidized functional groups and to re-establish sp2 structure of the material. The material obtained is generally referred to as ‘reduced graphene oxide’ (RGO). Among the different methods used to such a purpose, the electrochemical approach allows fine tuning of the reduction level of the material, finally conditioning the surface chemistry and degree of interaction between adjacent graphene foils [11], [12], [13]. Correspondingly, the electrical conductivity of GO may be tuned from insulating to highly conductive [8], [14], and the surface chemistry from highly hydrophilic to hydrophobic character [15]. Electrochemical methods can be also used for controlled functionalization of graphene nanosheets [16] as well for production of graphene-based electrodes [16] or of composite foams [17]. In recent years, electrochemical procedures have also demonstrated to constitute suitable tools for mass production of graphene from graphite [7], [18] at industrial level.
Although it is well established that the elimination of oxidized functionalities improves the conductivity of the material [11], it was recently found that the residual oxidized groups, not removed by the reducing process, play a key role in the electrocatalytic properties of GO, as an example in favouring the electrochemical oxidation of gallic acid [13]. Electrochemical techniques are thus ideal to study both conductivity and electrocatalytic performance of GO at different degrees of reduction.
Even if several published works tried to study in detail the actual role played by the different functional groups on carbon nanosheets in the electrocatalytic performance of graphene, a clear explanation is still missing. The scenario is complicated by the lack of a definition of the chemical structure of the specific graphene used in many papers and by the possible presence of residual heavy metal ions coming from the synthetic procedure, that can themselves induce electrocatalytic processes [19].
In this work we deposited GO nanosheets on commercial screen-printed electrodes (SPE), typically used for electrochemistry. The GO was then reduced to various degrees by applying an electrochemical potential, in order to study the role of the different oxygen functional groups present on the nanosheets in the electrocatalytic properties of the material, at the basis of its actual application. For such a purpose, two particular chemical species of biological interest were used as the benchmarks: nicotinamide adenine dinucleotide (NADH) and ascorbic acid (AA), also known as vitamin C. As such, they constitute quite interesting benchmarks to test the electrocatalytic performance of the material, due to overvoltages affecting their electrochemical oxidation at bare surfaces [20], [21]. From an applicative point of view, they constitute two important chemical species that require to be detected in many biological frames by rapid and simple analytical systems. In particular, AA is one of the most important vitamins present in many biological and food matrices, whereas NADH/NAD+ redox couple constitutes the cofactor of many oxido-reductase enzymes. The possible occurrence of electrocatalytic process in charge of this latter species may lead to quite efficient amperometric biosensors for a wide number of chemical species constituting the substrate of a defined enzyme [20] and to enzymatic biofuel cells [22], [23], [24].
GO films have been used as such or after in situ electrochemical pre-treatment at fixed negative potentials. Such a pre-treatment leads to formation of RGO films with increasing electrical conductivity, related to a decreasing number of hydrophilic oxygen groups on its surface. The electrochemical and electrocatalytic performance of graphene films was correlated to the functional groups present at the material surface, measured by X-ray photoelectron spectroscopy (XPS). To understand the effect of oxygen-based functional groups on the performance of graphene, we also chemically modified the graphite electrode surface by electrochemical oxidation/activation of the surface [25] and subsequent deposition of hydroquinone (HQ) and catechol (CT) molecules.
The tunable electrochemical reduction of GO allows the obtainment of the right amount of defects to foster the electrocatalytic oxidation of both NADH and AA at low potential values.
The right surface chemistry and the good electrocatalytic performance can be achieved also skipping completely GO preparation-reduction steps, by direct use of electrochemically exfoliated graphene oxide (EGO). This material, directly obtained by electrochemical oxidation and exfoliation of bulk graphite, is more suitable than GO for large scale industrial applications because it can be obtained at low cost and high speed, without using the harsh chemical or solvents necessary for the production of GO or of graphene [7], [16], [17], [26].
Section snippets
Chemicals
All reagents were of analytical grade and supplied by Sigma-Aldrich.
GO was prepared from graphite flakes (Sigma Aldrich, 99% pure, <150 μm) using a modified Hummer's method, as described in Ref. [27]. The GO water suspension produced was then subjected to dialysis to remove residual metal ions and acids. The amount of residual Mn, which is the most abundant metal introduced in the synthetic process, resulted lower than 2.7 ppm, as determined by Inductively Coupled Plasma Mass Spectrometry
XPS, SEM and Raman characterization of GO and RGO
XPS analysis allowed us to monitor the changes in oxidation degree and surface chemistry of GO upon electrochemical reduction at different potentials. XPS analysis unambiguously identifies the chemical nature and amount of the different functional groups present on the surface of the electrode.
GO surface chemistry and oxidation degree depend strongly on the production method. Since the GO used in the present article is different from that reported in similar studies present in the literature
Conclusions
The results described in this work indicate that GO constitutes a versatile material possessing tuneable conductivity and surface chemistry that make it suitable for applications in which the activation of electrocatalytic processes is required. The combination of spectroscopic and electrochemical analyses allowed us to correlate the physico-chemical properties of the material with the oxygen-based functional groups present on graphene nanosheets such as C-OH, C=O and O-C=O. Similar species are
Acknowledgements
The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme (grant agreement n° 696656 Graphene Flagship) and the EC Marie-Curie ITN- iSwitch (GA no. 642196). Dr. Massimo Tonelli of the Centro Interdipartimentale Grandi Strumenti (CIGS - Università di Modena e Reggio Emilia) is acknowledged for the acquisition of the SEM images.
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