2D-to-1D Conversion of a Layered Sodium Titanate via Rational Framework Splitting for Highly Efficient Cation Exchange.

Demand on high-performance ion exchangers is ever-increasing in energy and environment applications. Among many cation exchangers, layered alkali titanates generally show larger cation exchange capacity, but slower cation exchange rate due to their 2D micrometer-size particle morphologies, which limits their practical applications. Here, a rational conversion of a layered sodium titanate, Na₂Ti₃O₇, is reported to the corresponding 1D ultra-narrow nanowires via hydrothermal treatment under basic conditions. The formation of nanowires is thought to involve the partial exfoliation of Na₂Ti₃O₇ to form thin plate-like particles that subsequently split into nanowires along a crystallographically defined, chemically selective weakness in the Na₂Ti₃O₇ crystals. This process is similar to a recently burgeoning materials design using atomic-level weakness in solids, such as zeolites and metal-organic frameworks. The proposed formation scheme is further supported by comparative experiments performed on another layered alkali titanate, K_0.8Ti_1.73Li_0.27O₄, which possesses randomly distributed defects at the Ti sites. Thanks to the shortening of diffusion path lengths of the interlayer cations, the resulting Na₂Ti₃O₇ nanowires show an excellent cation exchange performance toward Cd²⁺ in aqueous solution, exceeding several existing cation exchangers such as zeolites and organic resins.