The effect of oxygen functional groups on the electrical transport of multi-layered graphene oxide

Abstract

Graphene oxide (GO) has emerged as one of the active research fields as well as graphene. In early days, GO had been focused on obtaining good quality graphene. Nowadays, however, GO itself has been much attention due to various oxygen functional groups which can provide a facile method for hybrid structure, and applicability for energy device electrode by reduction to electrical conduction. Especially, Pd-doped GO is expected for hydrogen fuel cell electrode by providing large surface area and trap sites for well dispersed Pd nanoparticles. Although many conduction mechanism of GO already reported, a comprehensive description on electrical conduction of multi-layered GO (MGO) was not enough. In this thesis, the electrical conduction mechanism of MGO and Pd-decorated MGO (PdGO) are investigated as a function of annealing temperature and H2 gas pressure.

To investigate the electronic transport mechanisms in MGO, the temperature-dependent electrical conductivity (σ(T)) has been measured as a function of annealing temperature (Ta). An individual MGO flake is gradually reduced as the thermal annealing temperatures Ta increases from 88 to 300 °C, reduction process of MGO flake upon heating is confirmed by X-ray photoelectron spectroscopy at each annealing stage. As Ta increases, σ(T) of the MGO increases. The σ(T) is well interpreted by variable-range hopping in disordered regions in series with activated conduction across small barriers. The charge localized states are formed for hopping with the oxygen functional groups in GO, and the small activation barriers with the domain boundaries between the clustered oxygen functional groups and graphitic region. Both the hopping and activation barrier resistances decrease systematically as the Ta increases.

Afterwards, MGO and PdGO are electrically evaluated for adsorptive hydrogen pressure at room temperature. PdGO is made by Suzuki-Miyaura coupling reaction using the MGO and Pd acetate, and dispersed Pd nanoparticles are confirmed by high resolution TEM. A pressure-dependent electrical conductance of MGO and PdGO has been measured in situ during hydrogen gas exposure (up to 20 bar) and release processes. As H2 pressure increases, the electrical conductance of MGO is increases while that of PdGO is decreased. Using atomic force microscopy, X-ray photoelectron spectroscopy, and Fourier transform-infrared spectroscopy analysis, it was found that MGO reduction due to H2 accounts for the increase in electrical conductance of the MGO. For PdGO, an increase of OH groups was observed, which shows that PdGO was oxidized when exposed to high H2 pressure. The PdGO oxidation can be explained by a hydrogen spillover effect resulting in the decrease of electrical conductance.