Hafnium, Titanium and Zirconium Intercalation

We demonstrate an intercalation chemistry for titanium, hafnium, and zirconium in the highly oxidized state (Zr⁴⁺, Hf⁴⁺, Ti⁴⁺) into 2D layered materials through formation of a soluble, molecular bis-tetrahydrofuran (THF) metal halide complex. This method overcomes challenges distinct to intercalation of highly oxyphilic group 4 metals through creation of a molecular species that is highly soluble in organic solvents and is kinetically competent in reactivity. Metal intercalation is demonstrated in several layered hosts including MoO_3,Bi₂Se₃, Si₂Te₃, and GeS. This strategy intercalates, on average, 3 atomic % or less of Hf, Ti, and Zr, limited by the high oxidation state and charge transfer with the host. MoO₃is chemochromically switched from transparent white to dark blue through Zr⁴⁺, Hf⁴⁺, Ti⁴⁺ intercalation. Contrast with methods for intercalation of zero-valent metals in 2D layered materials is demonstrated. We believe that this bis-THF chemistry offers a unique way to ionically switch properties of 2D layered materials and achieves a route to intercalate otherwise elusive guests.