Interaction of Magnesium Ions with Pristine Single-Layer and Defected Graphene/Water Interfaces Studied by Second Harmonic Generation

摘要

This work reports thermodynamic and electrostatic parameters for fused silica/water interfaces containing cm~2-sized graphene ranging from a single layer of pristine graphene to defected graphene. Second harmonic generation (SHG) measurements carried out at pH 7 indicate that the surface charge density of the fused silica/water interface containing the defected graphene (—0.009(3) to —0.010(3) C/m~2) is between that of defect-free single layer graphene (-0.0049(8) C/m~2) and bare fused silica (-0.013(6) C/m~2). The interfacial free energy of the fused silica/water interface calculated from the Lippmann equation is reduced by a factor of 7 in the presence of single-layer pristine graphene, while defected graphene reduces it only by a factor of at most 2. Subsequent SHG adsorption isotherm studies probing the Mg~(2+) adsorption at the fused silica/water interface result in fully reversible metal ion interactions and observed binding constants, K_(ads), of 4(1) — 5(1) × 10~3 M~(-1) for pristine graphene and 3(1) — 4(1) × 10~3 M~(-1) for defected graphene, corresponding to adsorption free energies, △G_(ads), referenced to the 55.5 molarity of water, of—30(1) to —31.1(7) kJ/mol for both interfaces, comparable to Mg~(2+) adsorption at the bare fused silica/water interface. Maximum Mg~(2+) ion densities are obtained from Gouy—Chapman model fits to the Langmuir adsorption isotherms and found to range from 1.1(5) — 1.5(4) × 10~(12) ions adsorbed per cm2 for pristine graphene and 2(1) — 3.1(5) × 10~(12) ions adsorbed per cm for defected graphene, slightly smaller than those of for Mg~(2+) adsorption at the bare fused silica/water interface ((2—4) × 10~(12) ions adsorbed per cm~2), assuming the magnesium ions are bound as divalent species. We conclude that the presence of defects in the graphene sheet, which we estimate here to be around 1.3 × 10~(11) cm~2, imparts only subtle changes in the thermodynamic and electrostatic parameters quantified here.