Spontaneous Formation of a ZnO Monolayer by the Redox Reaction of Zn on Graphene Oxide

Abstract

Graphene-based two-dimensional heterostructures are of substantial interest both for fundamental studies and their various potential applications. Particularly interesting are atomically thin semiconducting oxides on graphene, which uniquely combine a wide band gap and optical transparency. Here, we report the atomic-scale investigation of a novel self-formation of a ZnO monolayer from the Zn metal on a graphene oxide substrate. The spontaneous oxidation of the ultrathin Zn metal occurs by a reaction with oxygen supplied from the graphene oxide substrate, and graphene oxide is deoxygenated by a transfer of oxygen from O-containing functional groups to the zinc metal. The ZnO monolayer formed by this spontaneous redox reaction shows a graphene-like structure and a band gap of about 4 eV. This study demonstrates a unique and straightforward synthetic route to atomically thin two-dimensional heterostructures made from a two-dimensional metal oxide and graphene, formed by the spontaneous redox reaction of a very thin metal layer directly deposited on graphene oxide.

Atomic Arrangements of Graphene-like ZnO

“…We focused on the atomic arrangement and merging dynamics occurring from edge configurations of the ZnO nanosheets and nano-flakes. According to previous calculation and experimental research [ 7 , 8 , 9 , 16 , 17 ], wurtzite ZnO films can transform into a graphene-like structure, which is chemically stable. The wurtzite ZnO is schematically explained in Figure 1 b.…”

“…Figure 2 shows ACTEM images of edge configurations such as armchair (AC) and zigzag (ZZ) edge configurations nominally of the g-ZnO sheets. Previous research [ 8 , 9 ] shows that formation energies of O- and Zn-terminated ZZ edge configuration gradually decrease as the lateral size of the grown ZnO sheet increases at a lateral growth at the edge of the monolayer g-ZnO sheet. Furthermore, atomically extended ZZ edge configuration are observed (as shown in Figure 2 a).…”

“…It is also used in the flexible fabrication of electronic devices [ 2 , 3 , 4 , 5 ] and can transform into a variety of nanostructures [ 6 ]. When thinned down to a ZnO with atomic layers, planer ZnO [ 7 , 8 , 9 , 10 ], which resembles a graphene structure, can also form by expanding lattice parameter by 1.6% (a = 3.303 Å) [ 11 ] rather than the wurtzite ZnO. Since bulk ZnO is not a layered material, there is a limit to obtain atomic-thick nanosheets by physical and chemical exfoliation methods, such as methods related to graphene [ 12 , 13 ] and transition metal dichalcogenides [ 13 , 14 , 15 ].…”

“…Since bulk ZnO is not a layered material, there is a limit to obtain atomic-thick nanosheets by physical and chemical exfoliation methods, such as methods related to graphene [ 12 , 13 ] and transition metal dichalcogenides [ 13 , 14 , 15 ]. However, it has been reported that ZnO nanosheets and graphene-like ZnO (g-ZnO) [ 7 , 8 , 9 , 10 , 16 ], which have trigonal planar coordination and are composed of Zn and O atoms alternately, can be synthesized by various deposition methods [ 7 , 8 , 9 ] with electron beam irradiation and a hydrothermal synthesis [ 10 ].…”

Rational Design of Self-Supported CuOx-Decorated Composite Films as an Efficient and Easy-Recycling Catalyst for Styrene Oxidation

“…Owing to their two-dimensional sp 2 hybrid carbon structures, large surface area, good mechanical strength, and versatile functional groups, graphene and its derivatives can serve as a catalyst support promoting interactions with multiple catalytic species. − By now, graphene-based catalysts have been frequently employed as heterogeneous catalysts in many reactions, including water splitting, oxygen reduction reaction, Fischer–Tropsch synthesis, selective hydrogenation, catalytic purification of VOCs, oxidation, and so on. − Compared with the pristine graphene derived from the micromechanical cleavage or chemical vapor deposition (CVD) methods, − graphene oxide (GO) is more suitable as a catalyst support because of its intrinsic oxygen-doped surface. , In particular, the oxygen functional groups of GO allow for strong metal-support interactions, which could improve the stability of catalytically active species. − Usually, the fabrication of graphene-based catalysts is achieved via a wide range of approaches, including thermal, chemical, and hydro-/solvothermal treatments, during which the reduction of GO and deposition of catalytically active species on the basal plane of GO simultaneously occur. − Nevertheless, due to the presence of the π–π interaction, the reduced graphene oxide (rGO) sheets are prone to aggregation during the reduction of GO . According to recent reports, the aggregation of graphene-based materials usually leads to a decrease in the availability of catalytically active sites resulting in limitations in the applications of these materials in catalysis. , …”