Surface segregation of hydrogen in free-standing Pd-H alloy nanofilms

The interaction between mechanics and chemistry plays an essential and critical role in the behaviors and properties of materials, especially in nanoscale alloys. Based on the classical Gibbs and McLean adsorption isotherms, the present study takes the freestanding nanometer thick films of Pd-H solid solutions as a typic example to investigate surface segregation of hydrogen. The surface eigenstress model is further developed here to give analytic formulas, which have the capability to quantitatively predict the size-dependent surface segregation. Molecular dynamics (MD) simulations are conducted on free-standing Pd-H nanofilms. The MD simulations verify the theoretical analytic results and determine the values of parameters involved in the theoretical analysis. The integrated theoretical and numerical study exhibits that both surface excess H concentration and apparent biaxial Young’s modulus of Pd-H thin films depend on the nominal H concentration and the film thickness. The MD simulations determine the values of three parameters involved in the theoretical analysis. Especially, the parameter of the differentiation in reference chemical potential behaves like the molar free energy of segregation in the McLean adsorption isotherm.