Oxidation Resistance of Fe and Cu Foils Coated with Reduced Graphene Oxide Sheets

Brief Introduction of Graphene

History of Carbon Allotropes

New Member of Carbon Allotropes: Graphene

Electronic Properties of graphene

Overview of Quantum Hall Effect (QHE)
Half Integer Quantum Hall Effect in Single-layer Graphene
Approaches to enhance Carrier Mobility of Graphene Device

Mechanical Properties of graphene
Optical Properties of graphene
Thermal Properties of graphene

Isolation or Synthesis Methods of Grpahene

Mechanical Exfoliated Graphene from Graphite
Chemical Exfoliation of Graphene in liquid
Chemically Converted Graphene : Reduced Graphene Oxide (RGO)
Growth of Graphene by Chemical Vapor Deposition (CVD)
Epitaxial Growth of Graphene

Control of Size and Physical Properties of Graphene Oxide by Changing Oxidation

Introduction

The physical properties of graphene oxide, such as electrical properties, band gap energy, transparency, optical properties, and surface charge can be determined by the nature and amount of functional groups on graphene oxide sheets. Also, the size of graphene oxide sheets is affected by the degree of oxidation of graphite: Functional groups and defect sites, which are increased by a high degree of oxidation, can lead to the breakdown of the graphene oxide sheets during the exfoliation process. Furthermore, functional groups such as epoxy and hydroxyl groups on the ground plane of graphene oxide can act as nucleation sites for the growth of inorganic materials such as TiO2, silica and nanocrystals of Ni, Co and Fe, which show different sizes and shapes according to the degree of oxidation of the graphene oxide substrate.18-20 Therefore, control of graphite oxidation is necessary in various applications of graphene oxide sheets.

Recently, it has been reported that graphene oxide sheets with different oxygen content can be easily fabricated by changing the starting graphite, oxidation time and amount of oxidant, causing variation in the size distribution, electrical conductivity and energy bandgap.21-24 So far, no correlation has been reported between the properties or size of graphene oxide sheets and the oxidation temperature. Herein, we synthesized graphene oxide sheets at different oxidation temperatures using the modified Hummers method, and investigated their C/O ratios, size distributions, surface charges, and optical properties. This means that at higher temperatures, more functional groups, including oxygen, are introduced into the graphene oxide sheet.

Furthermore, we have shown that the surface charge and optical properties of graphene oxide sheets are related to the degree of oxidation.

Experimental

Materials
Preparation of graphite oxide (GO)
Measurements and characterization

For measuring the oxidation state of graphite oxide (GO), the elemental content was calculated for carbon, hydrogen, oxygen, nitrogen and sulfur by an elemental analyzer, a Flash 2000 (Thermo Scientific, The Netherlands). UV/Vis absorption spectra of the graphene oxide solution samples were measured with a Cary 5000 (Varian, USA) in a range of 200 – 800 nm, and surface charges (zeta potential) of the samples were measured with a Nano ZS (Malvern, UK ) in a range at pH 2.5 - 10.

Results and Discussion

Degree of oxidation of graphite
Size distribution of graphene oxide sheets
Comparison of surface charge via zeta-potential measurement

Average size of GO (mm). d) Average size of graphene oxide sheets at different oxidation temperatures. UV-Vis absorption spectra of graphene oxide suspension in water obtained at different oxidation temperatures (concentration of each sample is 0.075 g/ml). The reactivity of nano-sized materials is closely related to their size, shape and surface properties.

In the case of graphene oxide, surface properties such as surface charge can depend on the amount and type of functional groups on the surface of graphene oxide. During the oxidation process, functional groups (mainly epoxy group on the basal plane and hydroxyl and carboxyl groups on the edge side) are introduced into the graphene oxide sheets. Functional groups at the edges of graphene oxide sheets can weakly develop negative charges in solution due to deprotonation, resulting in a hydrophilic nature.

The graphene oxide suspension sample obtained at 35 oC showed the highest negative value at all pH levels. Based on the results, it is estimated that the graphene oxide sheets obtained at 35 oC have several functional groups with a negative charge. This result is consistent with the average size and C/O ratio results of the present GO samples.

Table 1. Element analysis of GO samples obtained at different oxidation temperature

Conclusion

Materials
Preparation of Grraphene Oxide (GO)
Preparation of Posivively Charged GO
Fabrication of GO Thin Films Using Layer-by-Layer (LbL) Assembly
Reduction and Transfer of GO Thin Film

Vapor Reduction of GO Thin Film
Thermal Reduction of GO Thin Film
Transfer of RGO Thin Film onto Metal Surface

Oxidation of RGO Thin Film Coated Fe and Cu
Measurements and Characterization
Characterization of GO and Positively Charged GO
Charaterization of Layer-by-Layer Assembled GO Thin Film
Charaterization of RGO Thin Film via Vapor Reduction Method (vRGO)
Characterization of RGO Thin Film via Thermal Reduction Method (tRGO)
The Ability of Oxidation Resistance of vRGO thin film coated Fe
Reduction Temperature Dependant Ability of Oxidation Resistance of tRGO thin film coated
Thickness Dependant Ability of Oxidation Resistance of tRGO thin film coated Fe
The Ability of Oxidation Resistance of tRGO thin film coated Cu

Although unique sp hybridized structures and impermeability were partially destroyed in the case of graphene oxide during oxidation of graphite, it is expected that multilayer stacked RGO thin film can play a role as an alternative with restored impermeability by complimenting their defects themselves. Also RGO thin film shows the ability of oxidation resistance with good thermal stability after recovering their own structures. Although RGO thin film could be fabricated using reduced graphene oxide solution, it is still the case.

Therefore, in this study, we have successfully fabricated a uniform RGO thin film via vapor or thermal reduction method after making a GO LbL thin film on a 300nm SiO2/Si substrate. As mentioned above, there are several types of reducing agents such as hydrazine, hydrazine monohydrate, sodium borohydride (NaBH4), hydrogen iodine (HI) for the reduction of graphene oxide thin film. The fabricated RGO thin film on 300 nm SiO2 / Si substrate should be transferred to Cu or Fe foils to test their oxidation resistance ability.

The detached PMMA/RGO thin film was washed more than three times to eliminate the remaining HF acid molecules in their inner layer and transferred to Cu or Fe foil. The biggest advantage of Layer-by-Layer (LbL) assembly is that it is possible to precisely control the thickness of thin layers. Therefore, the thermal reduction of GO is considered as a suitable method to make an RGO thin film with oxidation resistance.

XPS of (a) GO and (b) vapor-reduced RGO (vRGO), (c) thickness changes of GO and vRGO thin films. This trend indicated that the obvious decrease in the thickness of RGO thin film was proportional to the increase in the reduction temperature, and high temperature could lead to thermal damage on GO surfaces. Vapor reduction of RGO thin film was insufficient for protection and oxidation resistance of the metal surface due to residual hydrazine molecules and defect sites.

Thus, RGO thin film fabrication using the thermal reduction method was used instead of steam reduction to protect the metal surface from oxidizing circumstances. Our results suggest that the thick RGO thin film affects the appearance of the Fe sheet and is not beneficial for oxidation resistance, the uniformly fabricated 5BL RGO thin film is an optimal condition. From previous experimental data, 5BL RGO thin film reduced at 1,100 oC was established as an optimal condition and applied to Cu.

The ability to achieve oxidation resistance with solution-processed RGO thin films should provide an alternative way to circumvent existing techniques. Fabrication and evaluation of reduced solution-processed graphene oxide electrodes for bottom-contact p- and n-channel organic thin-film transistors.