  To address the issue of electron correlation driven superconductivity in
graphene, we perform a systematic quantum Monte Carlo study of the pairing
correlation in the t-U-V Hubbard model on a honeycomb lattice. For V=0 and
close to half filling, we find that pairing with d+id symmetry dominates
pairing with extended-s symmetry. However, as the system size or the on-site
Coulomb interaction increases, the long-range part of the d+id pairing
correlation decreases and tends to vanish in the thermodynamic limit. An
inclusion of nearest-neighbor interaction V, either repulsive or attractive,
has a small effect on the extended-s pairing correlation, but strongly
suppresses the d+id pairing correlation.
