Two-Dimensional Hybrid Nanomaterials

L. A. PÉREZ; F. FIORAVANTI; D. ARCINIEGAS JAIMES; N. BAJALES LUNA; G. I. LACCONI

Nanostructured Multifunctional Materials

CRC Press. Taylor and Francis Group

Año: 2021; p. 213 - 229

Graphene as a bidimensional material has received a lot of attention over the last decade because of its unique properties, such as its high mechanical strength, flexibility, transparency, impermeability to gases, among others. Likewise electrochemical processes with graphene based electrodes and energy storage using this carbon allotrope are particularly interesting due to its high surface area and conductivity, as well as the low interfacial capacitance. Although graphene and its derivatives show a singular behavior as an electrochemical interface because of their particular specific properties, the mechanisms by which charge transfer occur using these electrodes is not completely understood.Raman micro-spectroscopy is ideal to study the surfaces of graphene-based electrodes since it provides an appropriate tool to carry out a chemical and structural characterization. Thus, this technique allows to distinguish among single layer (1-LG), two-layers (2-LG) or multiple layer (m-LG) graphene, as well as graphite (HOPG) with and without structural defects (vacancies, grain boundaries, etc.), conductor and semiconductor carbon nanotubes, and non-IPR fullerenes from IPR ones. On the other hand, Raman spectroscopy is also very sensitive to the electronic structure of graphene materials. The characteristic bands observed in the Raman spectra are mainly the G mode and 2D mode, which are present in all carbon-related materials. Spectroscopic parameters of those bands, such as frequency, intensity and half-peak width, are influenced by many factors, being the number of layers of graphene, external/intentional or unintentional doping, surface stress and laser excitation energy, the most taken into account. In addition, when disorder in the matrix of graphene is present, a new band known as D mode arises and constitutes a clear fingerprint of defective graphene.Electrochemistry of graphene-related materials is of particular interest for diverse applications in electroanalytical and electrocatalysis processes. In fact, the Fermi level of graphene can be also analyzed by the Raman signals, obtaining for example, information on the doping of graphene and the molecular structure at graphene-electrolyte interface (atoms and adsorbed molecules). There are different methods that are coupled to the Raman spectroscopy for the analysis of graphene materials. Furthermore, a strategic combination of suitable techniques allows the achievement of interesting and integral characterization of graphene surfaces. Within this framework, electrochemical Raman spectroscopy of graphene is a valuable tool to obtain molecular in situ information of adsorbed species on graphene, but also on different surface sites of the graphene-related materials, which can promote dissimilar reactivity on the surface. This is due to the presence of surface functional groups, either on the edges of flakes or on the surface defects, plays a fundamental role in the reactivity of these materials.The special properties of graphene offer novel opportunities for applications. In addition to the important characteristics of graphene above mentioned, another outstanding attribute is the relatively simple modulation of its electronic structure, a feature that extends the utilities of graphene even beyond the current experience. In fact, direct injection of charge carriers into the conduction or valence bands, i.e. doping, represents a feasible route of displacing the Fermi level. In this sense, when higher levels of doping are desired and precise control of experimental conditions is required, then electrochemical doping should be the method of choice.In this chapter we introduce the fundamentals of Raman microscopy analysis of graphene-related materials, being the electrochemical processes on graphene the main focus of the descriptions. In situ configuration, i.e. in-situ Raman spectroelectrochemistry, is a very powerful and useful method to gain insight into molecular features, which take place simultaneously with the electrical potential application at the electrode/electrolyte interface. Moreover, this method not only allows monitoring the changes in the doping level, the increase of conductivity superficial and the activation of sites, but also provides information on eventual stress and increase of superficial disorder during the reaction, of main interest for the development of high impact nanotechnological devices.