In-situ study of the carbon gasification reaction of highly oriented pyrolytic graphite promoted by cobalt oxides and the novel nanostructures appeared after reaction
Graphical abstract
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.
Section snippets
Experimental details
The HOPG substrates (ZYB grade, supplied by Bruker) were cleaved using scotch tape, and then cleaned by annealing at 400 °C under ultra-high vacuum conditions (below 2 × 10−8 mbar). The experiments started with the deposition of ∼2 equivalent monolayers (Eq-ML) of CoO Cobalt oxide by reactive thermal evaporation of metallic cobalt rods (HMW Hauner, 99.99+ % purity) under oxygen atmosphere at 2 × 10−5 mbar. The substrates were maintained at room temperature during the evaporation process. The
The role of cobalt oxide in the creation of defects on HOPG
It is well known that oxygen molecules do not absorb on HOPG surfaces at room temperature [30]. Nevertheless, deposition of CoO has been found to promote the creation of defects at the graphite surface and, afterwards, the oxidation of graphite, as reported by our group [14]. The C 1s XPS spectra depicted in Fig. 1a show the effects of 2 Eq- ML of CoO on HOPG after being exposed to different O2 pressures during 1 h at room temperature. For low pressures (10−3 mbar) (see inset in Fig. 1a), a
Conclusions
In summary, we have studied the carbon gasification reaction driven by CoO on HOPG by means of in-situ NAP-XPS. At room temperature, CoO acts as an “oxygen pump” into the graphite lattice, weakening the σ bonds of the carbon and creating defects after both the oxidation and the reduction of the graphite. This reduces the temperature at which the carbon gasification reaction occurs comparing to metallic Cobalt clusters, leading to nano-channeling at only 400 °C. During the re-oxidation process
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This investigation has been funded by the MINECO of Spain through the FIS2015-67367-C2-1-P P and MAT2017-85089-C2-1-R projects and by the Comunidad de Madrid through the NANOMAGCOST-CM Ref: P2018/NMT4321 project. The experiments were performed at CIRCE/NAPP beamline at ALBA Synchrotron with the collaboration of ALBA staff. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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