Growth and in situ transformation of TiO₂ and HTiOF₃ crystals on chitosan-polyvinyl alcohol co-polymer substrates under vapor phase hydrothermal conditions

A chitosan-polyvinyl alcohol (CS/PVA) co-polymer substrate possessing a large number of amino and hydroxyl groups is used as a substrate to induce the direct growth and in situ sequential transformation of titanate crystals under HF vapor phase hydrothermal conditions. The process involves four distinct formation/transformation stages. HTiOF₃ crystals with well-defined hexagonal shapes are formed during stage I, and are subsequently transformed into {001} faceted anatase TiO₂ crystal nanosheets during stage II. Interestingly, the formed anatase TiO₂ crystals are further transformed into cross-shaped and hollow squareshaped HTiOF₃ crystals during stages III and IV, respectively. Although TiO₂ crystal phases and facet transformations under hydrothermal conditions have been previously reported, in situ crystal transformations between different titanate compounds have not been widely reported. Such crystal formation/transformations are likely due to the presence of large numbers of amino groups in the CS/PVA substrate. When celluloses possessing only hydroxyl groups are used as a substrate, the direct formation of {001} faceted TiO₂ nanocrystal sheets is observed (rather than any sequential crystal transformations). This substrate organic functional group-induced crystal formation/transformation approach could be applicable to other material systems.