First-principles study of hydrogen adsorption on titanium-decorated single-layer and bilayer graphenes

The adsorption of hydrogen molecules on titanium-decorated (Ti-decorated) single-layer and bilayer graphenes is studied using density functional theory (DFT) with the relativistic effect. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) are used for obtaining the region of the adsorption energy of H₂ molecules on Ti-decorated graphene. We find that a graphene layer with titanium (Ti) atoms adsorbed on both sides can store hydrogen up to 9.51 wt% with average adsorption energy in a range from -0.170 eV to -0.518 eV. Based on the adsorption energy criterion, we find that chemisorption is predominant for H₂ molecules when the concentration of H₂ molecules absorbed is low while physisorption is predominant when the concentration is high. The computation results for the bilayer graphene decorated with Ti atoms show that the lower carbon layer has no contribution to hydrogen adsorption.