Simultaneous tuning of particle size and phase composition of TiO2-δ nanoparticles by a simple liquid immiscibility strategy

•Simultaneous tuning of particle size and phase composition of TiO2-δ nanoparticles was achieved by controllable solidification.•A quantitative model between the particle size and the fraction of rutile phase was established, and the nucleation mechanism of the dual-phase particle was discussed.•TiO2-δ nanoparticles showed enhanced electrocatalytic activity towards nitrate reduction reaction.