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

Abstract

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 [0 10] 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 (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 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.