Comparative Study of Proton Exchange in Tri- and Hexatitanates: Correlations between Stability and Electronic...

A hydrothermal method is considered to be convenient and is extensively used in preparing titanate architectures, but the intermediate and final products are complicated and variable. To date, it is accepted that intermediates are tri- and hexatitanates. Here, atomic structures, energetics, and correlations between stability and electronic properties of proton exchange in tri- and hexatitanates, i.e., Na_2-HTi₃O₇ and Na_2-HTi₆O₁₃, are investigated by first-principles calculations. We found that the bond length of Na-O bonds plays a significant role in determining the activity of tunnel oxygen atoms, while the proton substitution sites are closely related to the activity of tunnel O atom in titanates. As H⁺ concentration increases, the formation energy of Na_2-HTi₃O₇ and Na_2-HTi₆O₁₃ decreases first and then increases, suggesting that completely protonated titanates, i.e., H₂Ti₃O₇ and H₂Ti₆O₁₃, are unstable. However, we found that H⁺ substitution would take place even in an alkaline solution both for Na₂Ti₃O₇ and Na₂Ti₆O₁₃. With a decrease in the pH, the process of H⁺ exchange becomes more energetically favorable. Compared to Na₂Ti₃O₇, Na⁺ ions are more easily exchanged by H⁺ ions in Na₂Ti₆O₁₃ at the same pH value. We found that there is a strong correlation between stability and electronic properties during the Na⁺-H⁺ exchange process. Finally, hydrogen bonds are observed in H₂Ti₃O₇ and Na_2-HTi₆O₁₃ complexes, which make them more stable than Na_2-HTi₃O₇ complexes without H-bonds. All of these findings provide insight into understanding the geometry of possible intermediates in the preparation of titanates and suitable conditions for the synthesis of titanates.