Chimie Douce Route to Sodium Hydroxo Titanate Nanowires with Modulated Structure and Conversion to Highly Photoactive Titanium Dioxides

Sodium hydroxo titanate nanowires with NaTi 2 O 4 (OH) formulation are fabricated by chimie-douce from TiO 2 powders in a hot concentrated base solution. In the present work, we investigate, in detail, the structure and morphology of the titanate, acid-exchanged materials, and phases formed during dehydration of these compounds; we then discuss the relationship between the initial titanate structure and TiO 2 formation. XRD, ED, HRTEM, EDS, density measurements, and TG/MS results suggest that the titanate and acid-exchanged structures were closely related to the pseudolepidocrocite arrangement (“Cs 2 Ti 6 O 12 ” type). The average structure of NaTi 2 O 4 (OH) nanowires consists of noncorrugated Ti 2 O 3 (OH) and NaO layers stacked along the [010] direction and an energy minimization was further conducted by theoretical methods. From TEM study, we demonstrated that NaTi 2 O 4 (OH) exhibits a modulated structure (q wave vector equals to 1/7 [001]*), which originates from the misfit between the NaO and Ti 2 O 3 (OH) sublattices. NaTi 2 O 4 (OH) structure has a monoclinic distortion ( I 2/ m space group) with refined unit cell a = 0.37000(6) nm, b = 1.7816(4) nm, c = 2.2077(3) nm, β = 92.70(1)°. Proton exchange followed by heat treatment convert the sodium hydroxo titanate into highly photoactive TiO 2 nanowires. The activity is attributed to the presence of preferentially exposed (001) surface plane which is explained by the topotactic phase transformation of TiO 2 (B) into anatase.