Thermal Decomposition of Hydrated Graphite Oxide: A Computational Study

We study the behavior of hydrated graphite oxide (GO) at high temperatures using thermally-accelerated molecular dynamics simulations based on ab initio density-functional theory. Our results suggest that GO, a viable candidate for water treatment and desalination membranes, is more heat resilient than currently used organic materials. The system we consider to represent important aspects of thermal processes in highly disordered GO is a hydrated GO bilayer in a vacuum. Our study provides microscopic insight into reactions involving water and functional epoxy-$\mathrm{O}$ and $\mathrm{OH}$ groups bonded to graphene layers, and also describes the swelling of the structure by water vapor pressure at elevated temperatures. We find the system withstands simulation temperatures up to approximately $2500\mathrm{K}$ before the graphitic layers start decomposing, implying the possibility of cleaning biofouling residue from a GO-based membrane by heating it in an inert-gas atmosphere.

Figure 1

Ball-and-stick models of a GO bilayer containing ${\mathrm{H}}_2\mathrm{O}$ molecules in the interlayer region. (a) A $2\times 2$ GO bilayer supercell in perspective view. $d_{\mathrm{il}}$ is the interlayer distance. (b) The top layer in top view displaying $\mathrm{O}$ atoms forming 1,3-epoxy groups and $\mathrm{OH}$ groups chemisorbed on the graphene layer.

Figure 2

Temperature fluctuations during an NVE simulation of the hydrated-GO-bilayer unit cell containing 308 atoms. The temperatures at which each ensemble is initialized are indicated by the labels and are discussed in the main text.

Figure 3

Interlayer distance $d_{\mathrm{il}}$, averaged across the unit cell, at different simulation temperatures $T$ as a function of time $t$.

Figure 4

Average distribution of coordination numbers $Z$ within the $\mathrm{C}$ backbone of the bilayer as a function of the simulation temperature. For better comparison, the discrete distribution is convoluted by a Gaussian with a FWHM of 0.5.

Figure 5

Average $\mathrm{C}$-$\mathrm{C}$ pair correlation function $⟨g(r)⟩$ of the system as a function of the initial simulation temperature $T$, convoluted with a Gaussian with a FWHM of $0.1\text{Å}$.

Figure 6

(a) Average $\mathrm{C}$—$\mathrm{O}$ distance $⟨d(\text{C—O})⟩$ between oxygen atoms forming $\mathrm{OH}$ groups (dotted line) or epoxy bonds (dashed line) and their closest carbon neighbors as a function of the initial simulation temperature $T$. (b) Average $\mathrm{O}$—$\mathrm{H}$ distance $⟨d(\text{O—H})⟩$ between oxygen atoms forming $\mathrm{OH}$ groups (dotted line) or water molecules (dashed line) and their closest hydrogen neighbors as a function of $T$. Large error bars in the distribution indicate detachment of atoms or exchange of atoms with nearby molecules or radicals.