A New Approach for the Calculation on the Entropic and Free Energy Contributions to Underpotential Deposition

In the present work we have studied the electrochemical nonpseudomorphic formation of Pb bidimensional structures on Au(111) surfaces. Based on the two-phase thermodynamic method, we have analyzed, from a computational perspective, entropic and energetic contributions to underpotential deposition. The occurrence of incommensurate nanophase formation, rotation of the adsorbate with respect to the substrate, and the effect of expansion and compression of the Pb monolayer were also analyzed. Our results show that the entropic contribution to monolayer underpotential deposition of Pb on Au(111) at room temperature is approximately 20% of the total free energy excess. For very small clusters, this contribution increases up to 45%. Simulations show that in the early stages of formation, Pb nanophases grow without rotation. When the size of the Pb islands is close 100 atoms, a rotation process takes places. At relatively high coverage, a strong lateral compression of Pb monolayer sets on, evidenced by a sharp decrease in the distance between nearest neighbors.