In-situ study of the carbon gasification reaction of highly oriented pyrolytic graphite promoted by cobalt oxides and the novel nanostructures appeared after reaction

Abstract

Cobalt interaction and its effects on carbon-based systems at the nanoscale have recently attracted much attention in different fields, such as catalysis of carbon nanotubes or graphene and graphite nano-patterning taking advantage of its ferromagnetic behavior. Experiments performed in our laboratories show how the re-oxidation process of two equivalent monolayers of CoO deposited on highly oriented pyrolytic graphite at 400 °C leads to the formation of nanochannels at lower temperature than using other methods. Here we present the in-situ characterization of the carbon gasification reaction that drives this process by means of near ambient pressure X-ray photoelectron spectroscopy performed at the ALBA synchrotron facility. The reason why this reaction takes place at such low temperature compared to other methods is due to the weakening of the carbon σ bonds by the initial CoO wetting layer formed at the early stages of growth on the graphite surface. Besides nanochannels, ex-situ atomic force microscopy measurements also show the appearance of two more kinds of nanostructures: nano-strips and nano-rings. The appearance of these nanostructures reveals the impressive modification of the surface after the re-oxidation process mediated by the cobalt oxide.

Introduction

Nowadays cobalt is an essential catalytic element used in a wide variety of industrial processes, highlighting Fischer−Tropsch synthesis. Although cobalt behavior at macroscopic scale has been thoroughly characterized in the last decades, atomic scale details of the interactions between cobalt and different substrates and reactants remain under discussion [1]. In particular, its interaction and effects on carbon-based systems at the nanoscale have recently attracted much attention. For instance, it is well known that metallic cobalt catalyzes the formation of carbon nanotubes (CNTs) [2]. On the other hand, since the discovery of graphene, the fabrication of nanostructures by patterning on graphene and graphite surfaces has become a hot research topic due to multiple technological applications in a wide range of fields such as electronics [3], sensors [4], light processing [5] or energy storage [6], among others. In this regard, different works have been done also with Ni, Ag, Co and Fe nanoparticles [[7], [8], [9], [10]]. Moreover, graphene and graphite nano-channelling by Co nanoparticles at about 550 °C in air can be controlled by taking advantage of their ferromagnetic behavior [11]. However, the atomic scale mechanisms behind these processes are still mostly unknown, which hinders the control and applications of these novel nanostructures.

Previous experiments presented by us showed a novel method of nano-patterning in highly oriented pyrolytic graphite (HOPG) via carbon gasification under oxygen atmosphere at 400 °C, starting with cobalt oxide (CoO) as catalyst material instead of typical metallic cobalt nanoparticles [12]. Compared to similar studies using metallic nanoparticles (Ni, Ag, Co and Fe), our channeling method significantly reduces the reaction temperature, leading to an improvement of the reaction efficiency. A qualitative model of the process based on the initial weakening of the graphite σ bonds was suggested as explanation, although no direct evidence of the graphite lattice modification by the CoO during the reaction could be shown. Compared to Co nanoparticles, initial CoO deposition tends to create defects and oxidize the uppermost layers of graphite, which may facilitate the subsequent carbon gasification reaction. A complete characterization of the early stages of growth of CoO on HOPG and the initial interaction between both materials at room temperature can be found elsewhere [13,14]. However, the inability to follow the reaction process in-situ by conventional X-Ray photoelectron spectroscopy (XPS) has hindered its description and the comprehension of the role of each actor during the intermediate steps of heating and oxygen exposure.

As a consequence of the same carbon gasification reaction, through atomic force (AFM) and Kelvin probe force (KPFM) microscopies characterization we report the appearance of two new types of nanostructures: nano-strips and nano-rings. As it has been previously reported in the literature, both carbon and cobalt based systems, have the ability to promote and lead to novel and exotic nanostructures. For example, similar features have been found at the early stages of growth of carbon nanotubes (CNTs) [15], metal nano-rings promoted by carbon-based surfaces [16], Co superparamagnetic nano-rings [17] or Co₃O₄ nano-rings [18]. Due to their low dimensionality, all these structures have attracted much attention because of their novel properties compared to bulk materials. Furthermore, the interaction between different types of liquid solutions with solid surfaces have also attracted attention in the last decades due to its importance on many physical, chemical and industrial processes. In this way, self-assemblies of organic and inorganic molecules forming nano-strips on graphitic samples have been the focus of a great number of publications in the last decades [[19], [20], [21]].

In this work, we present in-situ near ambient pressure X-Ray photoelectron spectroscopy (NAP-XPS) measurements performed at the ALBA synchrotron facility. We first describe the effects of CoO deposition and further oxygen exposure on HOPG. After that, we focus on the characterization of the kinetics and transient chemical states of the carbon and Co atoms during the complete carbon gasification process, which implies initial reduction of the CoO to metallic Co nanoparticles and subsequent oxidation due to oxygen exposure, leading to the carbon gasification reaction and a very defective HOPG surface. Finally, we discuss ex-situ AFM/KPFM measurements of the re-oxidized surface, focusing on the appearance of new novel nanostructures such as nano-rings and nano-strips.