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

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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, K0.8Ti1.73Li0.27O4, 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.