Elsevier

Applied Surface Science

Volume 509, 15 April 2020, 145118
Applied Surface Science

Full Length Article
Controlled ultra-thin oxidation of graphite promoted by cobalt oxides: Influence of the initial 2D CoO wetting layer

https://doi.org/10.1016/j.apsusc.2019.145118Get rights and content

Highlights

  • Formation of a 2D CoO wetting layer with (D4h) instead of the conventional 3D character(Oh) of bulk CoO.

  • This wetting layer breaks the Carbon σ bonds of the graphite surface.

  • It allows the injection of oxygen into the graphite substrate.

  • The thickness of the oxidised graphite can be controlled via deposition of CoO.

  • These findings could be of great interest in applications using carbon-based materials.

Abstract

The interaction of CoO with highly oriented pyrolytic graphite (HOPG) was studied using a set of complementary techniques. The morphology of the CoO thin film was determined using atomic force microscopy (AFM), whereas the electronic structure was investigated using X-ray absorption (XAS) and photoemission (PES) spectroscopies. The experimental spectra were analyzed using a configuration interaction CoO6 cluster model calculation. The early stages of growth are characterized by the formation of a CoO wetting layer at the CoO/HOPG interface. The electronic structure of the CoO wetting layer presents a clear 2D character, which is closer to the 2D HOPG substrate than to the 3D CoO bulk. This character of the wetting layer explains the posterior formation of CoO islands and excludes the alternative layer by layer growth mode. Further, the interaction between the CoO wetting layer and the outermost graphite layer favors the oxidation of the HOPG substrate which can be controlled by the thickness of the deposited CoO overlayer.

Introduction

The combination of cobalt oxides with carbon-based materials gives rise to important properties [1], leading to excellent performances in applications such as catalysts, biochemical sensors and cathodes for Li-ion batteries [2]. For instance, nano-sized cobalt oxides have demonstrated excellent behavior as negative electrode for Li-ion batteries [3], however the integrity of the electrode after many discharge and recharge cycles is not guaranteed. To overcome this problem, nanocomposites of cobalt oxides with graphene have been proposed as electrodes [4]. Also, cobalt nanoparticles are known to catalyze the formation of carbon nanotubes indicating a strong interaction of these materials [5]. Other interesting experiment involving cobalt oxides and graphite is the possibility of nano-patterning on highly oriented pyrolytic graphite (HOPG) with cobalt nanoparticles, via carbon gasification, at relatively low temperatures (400 °C), whereas other related works reported similar results at much higher temperatures (>800 °C). This was explained as due to the presence of a previously grown CoO ultra-thin layer, prior to its reduction to metallic Co nanoparticles, which produces a large number of defects at the HOPG surface [6], [7]. In spite of the numerous applications of cobalt oxides and carbon-based materials, few fundamental studies have been done on these systems, and the mechanisms involved in the interaction of these materials remains unclear.

The growth of cobalt oxides on HOPG has already been studied. It was found that for low coverages, less than 50 equivalent monolayers, the main oxide is CoO with Co2+ chemical species, whereas for larger coverages, the main component is Co3O4 which contains Co2+/Co3+ ions [8]. The growth mode was found to be of the Stranski–Krastanov mode, i.e. the formation of an initial CoO wetting layer followed by the growth of CoO islands. Other studies involving CoO/oxide interfaces have also been reported [9], [10], [11], [12]. In general, cobalt oxides grow on other oxides in the form of CoO (Co2+) using reactive thermal evaporation as the growth method. On the other hand, the growth in the form of the spinel Co3O4 (Co2+/Co3+) involves a more energetic growth method, such as oxygen plasma deposition [9]. Regarding the growth mode of CoO on other oxides, it depends mainly on the lattice mismatch of the oxide substrate with respect the CoO lattice. For instance, CoO grows in a Frank van der Merwe form, i.e. layer by layer mode, on Al2O3 and MgO, whereas it grows in a Volmer-Webber manner, i.e, islands mode, on SiO2 [11].

The purpose of this paper is to study the interaction of ultra-thin-films of cobalt oxides deposited on a HOPG substrate at room temperature (RT). It is paid special attention to the physical/chemical interactions at the interface during the early stages of growth. The study involves both, the electronic structure of the CoO 2D overlayer as well as its effect on the HOPG substrate. To this end, we have grown cobalt oxides layers by reactive thermal evaporation of metallic cobalt in an oxygen atmosphere (2 × 10−5 mbar) at room temperature. We have used various x-ray spectroscopy techniques, with different probing depths, to characterize and analyze the chemical changes at the interface. In particular, the early stages of growth are characterized by the formation of a 2D-CoO wetting layer, which influences the island growth mode of the CoO overlayers. We also show that the formation of the cobalt oxide wetting layer promotes the posterior oxidation of the HOPG substrate.

The results presented in this paper are divided in three sections. First, we make a brief summary of the growth of CoO on HOPG. Then, we present an analysis of the electronic structure of the CoO overlayers. Finally, we study the oxidation of the HOPG substrate after the CoO deposition.

Section snippets

Experimental and methods

The in situ experiments and measurements presented in this work have been performed in three different vacuum chambers. One located in our laboratory for X-ray photoelectron spectroscopy (XPS) measurements, other located at the PM4 beamline at the BESSY II synchrotron from Helmholtz-Zentrum Berlin for X-ray absorption spectroscopy (XAS) measurements and other located at the CIRCE beamline (NAPP) from the ALBA synchrotron in Barcelona for near ambient pressure X-ray photoelectron spectroscopy

Growth of CoO on HOPG

As mentioned in the previous section, CoO grows on HOPG following a Stranski–Krastanov growth mode. This leads to the formation of an initial CoO wetting layer followed by the growth of CoO islands, as it can be seen in Fig. 1a–c. The initial wetting layer, from the results in Fig. 1a and 1, has a thickness slightly larger than 4 Å, in agreement with the lattice constant of bulk CoO determined at 305 K (a = 4.2614 Å) [21]. Finally, only when the CoO wetting layer has grown on HOPG, dendritic

Conclusions

We studied the electronic structure of the wetting layer formed at the CoO/HOPG interface, as well as the effect of the CoO deposition on the HOPG substrate. The growth of CoO on HOPG begins by the deposition of a CoO wetting layer (this can be concluded from the analysis of AFM height profiles). This wetting layer forms a bridge between further CoO layers and the HOPG substrate; but its nature is closer to the 2D character of HOPG than to the 3D order of bulk CoO (this can be observed in the

CRediT authorship contribution statement

C. Morales: Data Curation, Formal analysis. D. Díaz-Fernández: Data Curation, Formal analysis. R.J.O. Mossanek: Methodology, Formal Analysis, Software. M. Abbate: Methodology, Formal Analysis, Software. J. Méndez: Data curation, Software, Formal analysis. V. Pérez-Dieste: Resources, Supervision. C. Escudero: Resources, Supervision. J. Rubio-Zuazo: Resources, Supervision, Formal analysis. P. Prieto: Data Curation, Formal analysis. L. Soriano: Conceptualization, Funding Adquisition, Supervision,

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.

Acknowledgements

This investigation has been funded by the MINECO of Spain through the FIS2015-67367-C2-1-P project and by the Comunidad de Madrid through the NANOMAGCOSTCM-P2018/NMT4321 project. Authors R.J.O.M and M.A. thank the financial support of CNPq-Brazil. We thank HZB for the allocation of synchrotron radiation beamtime. Funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement n.°312284 is acknowledged. We thank the ALBA synchrotron staff for the

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