Adsorption of para-hydrogen on fullerenes

Adsorption of para-hydrogen on the outer surface of a single fullerene is studied theoretically, by means of ground state quantum Monte Carlo simulations. We compute energetics and radial density profiles of para-hydrogen for various coverages on a variety of small fullerenes. The equilibrium adsorbed solid monolayer is commensurate with the surface of the fullerene; as the chemical potential is increased, a discontinuous change is generally observed, to an incommensurate, compressible layer. Quantum exchanges of hydrogen molecules are absent in these systems.

Figure 1

(Color online) Energy per $p\text{−}{\mathrm{H}}_2$ molecule $e\left(N\right)$ computed by VPI, as a function of the number of molecules adsorbed on a ${\mathrm{C}}_{20}$ fullerene. Filled squares: results obtained with the angularly averaged potential (2). Filled circles: results obtained by explicitly modeling all carbon atoms in the fullerene. Solid lines are polynomial fits to the VPI data. Inset shows the number of particles $N$ plotted as a function of the chemical potential $\mu$, for the fully corrugated model. The chemical potential of bulk solid $p\text{−}{\mathrm{H}}_2$ is $-88\mathrm{K}$ (from Ref. 33), and that is where the $N\left(\mu \right)$ curve ends.

Figure 2

(Color online) Same as Fig. 1, but for a ${\mathrm{C}}_{36}$ fullerene. The commensurate layer has $N_C=20$ $p\text{−}{\mathrm{H}}_2$ molecules and is compressible, as evidenced by the finite slope of the curve $N\left(\mu \right)$ for $20⩽N⩽28$.

Figure 3

(Color online) Same as Fig. 1, but for a ${\mathrm{C}}_{60}$ fullerene.

Figure 4

Same as Fig. 1, but for a ${\mathrm{C}}_{80}$ fullerene.

Figure 5

(Color online) Radial density profiles of a $p\text{−}{\mathrm{H}}_2$ layer adsorbed on ${\mathrm{C}}_l$. Solid lines: profiles obtained for the smooth fullerene model. Dotted lines: profiles of commensurate layers on corrugated model. Dashed lines: profiles of incommensurate layers on corrugated models. Density is given in ${\mathrm{\AA }}^{-3}$, whereas the distance $r$ from the center of the fullerene is given in Å.