Role of Water in the Transformation of Protonated Titanate Nanoribbons to Anatase Nanoribbons

Transformation of layered protonated titanate (H2Ti3O7) nanoribbons to anatase nanoribbons under hydrothermal conditions is a catalyzed process where topotactic transformation and dissolution-recrystallization compete. In the present work, the transformation was achieved with sufficiently high thermal input, calcination, or catalysis when performed under hydrothermal conditions. X-ray diffraction and scanning and transmission electron microscopy combined with crystallography of product materials revealed that the success of catalysis depended on the suspension's pH value as well on the counterions present in the reaction mixture. The process of the transformation from protonated titanate to anatase under hydrothermal conditions is explained for the reaction medium pH range of 1-13.7 with the aid of the zeta-potential of protonated titanate nanoribbons. Protonated titanate nanoribbons were stable under basic conditions, and their transformation to anatase nanoribbons depended on the counterions present in the reaction mixture and their concnetration as well. With decreasing the pH of the reaction mixture, both processes-topotactic transformation and dissolution-recrystallization-were accelerated. Upon the hydrothermal transformation, the nanoribbons served as the substrate for the heterogeneous nucleation of anatase nanocrystals. Sodium titanate nanoribbons were found to be suitable for the transformation to anatase nanoribbons under hydrothermal conditions as well. Presence of water in the reaction mixture was revealed to be crucial for the transformation progress under hydrothermal/solvothermal conditions since used organic solvents served only as a thermal reaction medium and did not promote the catalytic transformation.