Activated charcoal-mediated non-contact carbothermal reduction of TiO2 for controlled synthesis of Magnéli phase titanium suboxides

Abstract

Conventional Magnéli phase titanium suboxide syntheses often involve highly flammable H₂ gas, extensive precursor pretreatment and yield unwanted TiC or TiO_xC_y byproducts. To overcome these limitations, this work introduces a low-cost, safe and scalable carbothermal reduction synthesis method that eliminates the need for H₂ or pretreatment and does not produce carbon-based titanium byproducts. Magnéli phases were generated without physical contact between the bulk organic that acted as the reductant and TiO₂. Activated charcoal served as the reductant, reducing TiO₂ under an Ar flow, thus limiting O₂. The phase transition from anatase TiO₂ to Magnéli phases was studied as a function of reduction time. Critical assessment of surface and bulk defects in the series of Magnéli titanium suboxides synthesised revealed a possible redistribution of defects within the lattice with time while maintaining a constant total defect content. Optical property analysis indicated that increasing oxygen deficiency led to increased inter-bandgap absorbance and prolonged lifetime of the photogenerated charge carriers. Oxygen deficiencies exhibited a direct correlation with the water evaporation rate when these Magnéli phase titanium suboxides were applied in solar steam generation. This was attributed to reduced thermal conductivities with increasing oxygen vacancies due to increased phonon scattering by planar defects. The defect-rich Magnéli suboxide sample consisting mainly of Ti₆O₁₁ with a remarkably low thermal conductivity of <0.032 W m⁻¹ K⁻¹ at 25 °C showed a solar energy conversion efficiency of 56% when applied in an aerogel for solar steam generation. This method provides flexibility for fabricating materials tailored for diverse applications requiring specific defect concentrations or thermoelectric performance.