Graphene‐Coated ZnO and SiO₂ as Supports for CoO Nanoparticles with Enhanced Reducibility

Abstract

The insertion of a graphene layer between cobalt and a substrate modifies the morphology and the oxidation/reduction properties of supported cobalt particles. Co forms a relatively flat structure on ZnO and SiO , whereas individual Co nanoparticles are formed after graphene coating of these substrates. The graphene layer moderates the formation of cobalt oxide in 5×10 mbar O and promotes the reduction of oxidized Co in H at lower temperature. Angle-resolved XPS measurements indicate that this is mainly a consequence of the restricted interaction of cobalt with the oxide supports. After the low-pressure redox treatments, the graphene layer maintains a relatively high quality with a small number of defect sites.

A Brief Review of the Synthesis and Catalytic Applications of Graphene‐Coated Oxides

“…As shown by the Co 2p 3/2 XPS spectra in Figure , after annealing under UHV conditions cobalt supported on graphene‐coated ZnO could maintain the metallic state till 350 °C (Figure b). Contrarily, without the graphene layer Co was readily oxidized to CoO (Figure a) presumably through a solid state interface interaction with the ZnO support, as indicated later by angle‐resolved XPS spectra . In a more recent study, it was shown that graphene layer also weakens the interactions between Co and SiO 2 , even if SiO 2 is considered as a relatively inert substrate as compared to ZnO .…”

“…Contrarily, without the graphene layer Co was readily oxidized to CoO (Figure a) presumably through a solid state interface interaction with the ZnO support, as indicated later by angle‐resolved XPS spectra . In a more recent study, it was shown that graphene layer also weakens the interactions between Co and SiO 2 , even if SiO 2 is considered as a relatively inert substrate as compared to ZnO . Moreover, the interaction between Co and graphene dictates not only the oxidation state but also the particles size of cobalt.…”

“…Since after the dissolving process traces of PMMA might remain on the graphene, it is suggested to anneal for several hours under vacuum at temperatures around 300 °C for graphene surfaces with minimum contaminants. Through this method, graphene‐coated SiO 2 and ZnO samples among others, have been prepared and studied.…”

“…Apart from the metal‐support interface interaction, graphene can also influence the oxidation/reduction properties of cobalt nanoparticles supported on coated oxides (ZnO and SiO 2 ) under low (10 −7 mbar) and medium (7 mbar) pressure O 2 /H 2 environment . In particular, under low pressure O 2 the oxidation of Co supported on graphene‐coated ZnO and SiO 2 substrates was limited, as compared to cobalt directly attached on the oxide support. However, when the O 2 pressure increases to the mbar range the differences between cobalt supported on bare and graphene‐coated oxides fade.…”

“…The reduction of cobalt oxide particles under low and medium pressure H 2 conditions was dramatically promoted by the graphene layer, leading to cobalt reduction at significantly lower temperatures. Angle resolved XPS measurements indicate that this is mainly a consequence of the restriction of Co NPs interaction with the oxide supports . Moreover, at high temperature graphene protects Co from the formation of mixed Co x Zn 1− x O 2 compounds, which are more difficult to reduce than cobalt oxides (CoO or Co 3 O 4 ).…”