Ion-exchange between Na₂Ti₃O₇ and H ₂Ti₃O₇ nanosheets at different pH levels: An experimental and first-principles study

The models of Na_2-xH_xTi₃O₇ (x=0, 1, and 2) nanosheets were proposed to investigate the formation energies of ion-exchange using first-principles calculations. The calculated results demonstrated that sodium titanate nanosheet is energetically favorable for ion-exchange in a wide pH range, from acidic solution to even highly concentrated alkaline aqueous solution due to the negative formation energies. Therefore, the composition of sodium titanate nanosheet in alkaline solution should be Na_2-xH_xTi₃O₇ (0<x≤2) rather than Na;bsubesubbsubesubbsubesub. The formation energies of ion-exchange decrease with the pH decreasing. As a result, the thermodynamic driving force of ion-exchange is enhanced at low pH level. To further verify the calculated results, the ion-exchange properties of a series of titanate nanosheets in aqueous solutions at different pH levels were investigated. The experimental results are in good agreement with the theoretical deduction.