Work function of a free-standing or supported graphene monolayer can be calculated as done, e.g., in PHYSICAL REVIEW B 79, 195425 (2009). For accurate calculations, I would suggest using DFT-PlaneWaveCalculator with PseudoDojo pseudopotential and mixed Neumann-Dirichlet boundary conditions in the out-of-graphene-plane direction.

Note that you should make sure that your DFT-PW calculations are converged with respect to wave function cutoff and k-point sampling in the in-plane directions.

You may also find extra information on work function calculations at the following tutorial,

https://docs.quantumatk.com/tutorials/work_function_ag_100/work_function_ag_100.html. There is one important difference when using LCAO (tutorial) and PW (present suggestion) basis sets. When using PW basis set one does not need any ghost atoms, as often required for LCAO-based work function calculations.