First-principles investigation of atomic structures and stability of proton-exchanged layered sodium titanate

Atomic structures and ionic substitutions in sodium titanate (Na2-xHxTi3O7), having Ti3O7 sheetlike layers with Na+ and H+ in between, were investigated by first-principles calculations. The formation energies via ion exchange of Na+ and H+ were analyzed by using the supercell total energies and ionic chemical potentials determined with the aid of experimental thermodynamic data. It was found that ionic substitutions of Na+ by H+ take place even in alkaline solution conditions as found in previous experiments. In addition, the bonding strength between the Ti3O7 layers tends to decrease with more Na+ substitutions by H+, which is related to pH effects on exfoliation events of Ti3O7 sheetlike layers from layered titanate prior to the nanotube formation.