creating vaccum (evaporation)

Dear all,

Hope this email finds you well.

I'm currently attempting to construct multiple layers of graphite (which serves as the Pt catalyst suppor with around 2000 C atoms per layer). The crystal structure is triclinic (or could be other forms) on which Platinum as the catalyst will be placed. This is for the simulation of a Polymer Electrode Membrane which requires me to add polymer and water (as the solvent) to the system. Except for the Pt / C the other components are in bulk phase and I will be looking at the impact of evaporation of solvent (water) on morphology of the system. 

I've now constructed multiple layers of graphite and was wondering to simulate the evaporation: I need to be utilizing. 1) Creating a huge supercell with an extended Z direction to create vacuum (please notice the attached image) ? or 2) Utilizing the module Build > Crystals > Build vacuum slab ?

Could you please shed some light on this particular system to create vacuum. Am I on the right track? 

Truly appreciate your feedback.

Yours respectfully,

Serene