Fabrication of nanocrystalline λ-Ti3O5 with tunable terahertz wave transmission properties across a temperature induced phase transitionElectronic supplementary information (ESI) available: Description of a heating cell used in the THz-TDS system. Morphology of the raw material of nano-TiO2 powder. XRD profiles of titanium oxides annealed from unmodified nano-TiO2 at temperatures ranging from 1000 °C to 1100 °C. FE-SEM and TEM morphologies of β-Ti3O5. Change in resistance of λ-Ti3O5 during the t

λ-Ti 3 O 5 was a newly discovered material with intriguing phase transition characteristics, which exhibits huge potential in the application of memory and tunable optoelectronic devices. However, the fabrication of λ-Ti 3 O 5 still presents a great challenge and its application needs further investigation. In this work, we developed a novel method to fabricate nanocrystalline λ-Ti 3 O 5 by carbothermal reduction of nano-TiO 2 , and explored its terahertz transmission properties through a temperature induced phase transition. A second phase was introduced to inhibit the grain growth of titanium oxide during the carbothermal reduction, by performing a surface modification of the precursor nano-TiO 2 particles with Al 2 O 3 . This process was proved to be critical for the formation of nanocrystalline λ-Ti 3 O 5 . An in situ XRD analysis combined with a first-principles calculation based on plane wave DFT indicated that the nanocrystalline λ-Ti 3 O 5 exhibited a semimetallic λ phase to metallic α phase transition across a large temperature range. The phase transition was accompanied by continuous, slow and reversible tuning of the terahertz transmission amplitude. This work provides considerable insights into the synthesis of λ-Ti 3 O 5 and opens up studies on the applications of λ-Ti 3 O 5 in the THz range, such as but not limited to sensors and smart windows. Nanocrystalline λ-Ti 3 O 5 was fabricated by carbothermal reduction of nano-TiO 2 . It exhibits a continuous and slow phase transition across a wide range of temperatures, which can lead to dynamic tuning of THz transmission.