A systematic study of the controlled generation of crystalline iron oxide nanoparticles on graphene using a chemical etching process

Abstract

Chemical vapor deposition (CVD) of carbon precursors employing a metal catalyst is a well-established method for synthesizing high-quality single-layer graphene. Yet the main challenge of the CVD process is the required transfer of a graphene layer from the substrate surface onto a chosen target substrate. This process is delicate and can severely degrade the quality of the transferred graphene. The protective polymer coatings typically used generate residues and contamination on the ultrathin graphene layer. In this work, we have developed a graphene transfer process which works without a coating and allows the transfer of graphene onto arbitrary substrates without the need for any additional post-processing. During the course of our transfer studies, we found that the etching process that is usually employed can lead to contamination of the graphene layer with the Faradaic etchant component FeCl₃, resulting in the deposition of iron oxide Fe _x O _y nanoparticles on the graphene surface. We systematically analyzed the removal of the copper substrate layer and verified that crystalline iron oxide nanoparticles could be generated in controllable density on the graphene surface when this process is optimized. It was further confirmed that the Fe _x O _y particles on graphene are active in the catalytic growth of carbon nanotubes when employing a water-assisted CVD process.

Keywords: carbon nanotubes; chemical vapor deposition; graphene; iron oxide; nanoparticles.

Figure 1

Characterization of graphene obtained by the modified etching process with an aqueous solution of ammonium persulfate. a) Representative Raman spectrum of graphene normalized to the intensity of the G’ peak. The spectral location of the G and G’ peak, the intensity ratio of G’/G and the full width at half maximum of the G’ peak indicate the synthesis of single to few layer graphene. b) and c) High-resolution TEM images of as-synthesized graphene.

Figure 2

Characterization of CVD graphene transferred onto a TEM grid and a SiO₂/Si wafer by the modified etching process. The etchant was 1 M iron(III) chloride in 10% hydrochloric acid solution. a–d) TEM micrographs of graphene with deposited crystalline iron oxide nanoparticles. e) SAED of cubic iron oxide Fe_0.942O nanoparticles on graphene. f) Raman spectra of graphene after the transfer process with an aqueous solution of ammonium persulfate (black trace) and iron(III) chloride (red trace). Spectra are normalized to the intensity of the G’ peak and show similar peak positions, shapes and intensities.

Figure 3

Schematic of the proposed mechanism for the formation of iron oxide nanoparticles on graphene during the modified transfer of graphene by chemical etching. (A) Etching of copper using a hydrochloric solution of iron(III) chloride. (B) Formation of iron oxide nanoparticles. (C) Adsorption of nanoparticles on CVD graphene. (D) Cleaning of graphene and removal of free nanoparticles by dilution with water. (E) Transfer of the as-functionalized CVD graphene onto a target substrate.

Figure 4

TEM micrographs of graphene functionalized with crystalline iron oxide nanoparticles using an increased ratio of copper to iron oxide etchant. Magnification increases from a) to c). Graphene on copper foil was etched with a solution of 1 M iron(III) chloride in 10% hydrochloric acid.

Figure 5

TEM micrographs of iron oxide nanoparticles on graphene; a,b) before and c,d) after thermal annealing at 450 °C for 24 h in a hydrogen atmosphere. The nanoparticles are highly mobile and tend to agglomerate during the heat treatment procedure.

Figure 6

Iron oxide decorated graphene layer on a SiO₂/Si wafer for CNT growth. The nanoparticles are located between the substrate and the CVD graphene. For a schematic of the complete growth process see Figure 8.

Figure 7

Characterization of CNTs grown on iron oxide nanoparticles/graphene obtained by a modified copper etching process using a solution of 1 M iron(III) chloride in 10% hydrochloric acid and an increased ratio of copper to FeCl₃ etchant. a) SEM images of substrate areas where randomly oriented growth of CNTs was observed (inset: higher magnification image). b) SEM image of an area with VACNTs. c) TEM and d) Raman spectroscopy of VACNTs confirms the synthesis of multiwalled carbon nanotubes with an average number of 4–7 walls.

Figure 8

Proposed mechanism for the synthesis of CNTs on a metal oxide decorated graphene surface. (A) As the temperature increases during the CVD process, the iron oxide nanoparticles tend to agglomerate. The graphene can either remain intact (A₁) or may rupture during this process (A₂). (B) Graphene on the nanoparticle surface may rupture and/or decompose on the iron oxide nanoparticle surface at higher temperatures. (C) Synthesis of CNTs is initiated by exposure of the iron oxide nanoparticle surface to the carbon precursor. (D) Growth of CNTs on top of the graphene. Note that the metal oxide particles are found underneath the graphene layer as the etchant solution agitates from below.