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

Sodium hydroxo titanate nanowires with NaTi2O4(OH) formulation are fabricated by chimie-douce from TiO2 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 TiO2 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 (Cs2Ti6O12 type). The average structure of NaTi2O4(OH) nanowires consists of noncorrugated Ti2O3(OH) and NaO layers stacked along the [0 10] direction and an energy minimization was further conducted by theoretical methods. From TEM study, we demonstrated that NaTi2O4(OH) exhibits a modulated structure (q wave vector equals to 1/7 [001]*), which originates from the misfit between the NaO and Ti2O3(OH) sublattices. NaTi2O4(OH) structure has a monoclinic distortion (I2/m space group) with refined unit cell a = 0.37000(6) nm, b = 1.7816(4) nm, c = 2.2077(3) nm, beta = 92.70(1)degrees. Proton exchange followed by heat treatment convert the sodium hydroxo titanate into highly photoactive TiO2 nanowires. The activity is attributed to the presence of preferentially exposed (001) surface plane which is explained by the topotactic phase transformation of TiO2(B) into anatase.