Synthesis of SrTiO3 and Al-doped SrTiO3via the deep eutectic solvent route

Abstract

SrTiO₃ and aluminum-doped SrTiO₃ are synthesized by calcination of metal salts dissolved in a deep eutectic solvent (DES) without any post-synthesis treatment. The DES used is the eutectic mixture of choline chloride (hydrogen bond acceptor) and malonic acid (hydrogen bond donor). Titanium(IV) oxide bis(2,4-pentanedionate) is utilized as the non-volatile, easy-to-handle, DES-soluble titanium precursor. The ammonia gas evolved during the calcination process provides a reducing atmosphere, resulting in the formation of Ti³⁺ and oxygen vacancies within the SrTiO₃ matrix. According to UV-Vis spectroscopy and X-ray photoelectron spectroscopy, the amount of Ti³⁺ species and oxygen vacancies (V_O) in the synthesized perovskite can be tuned by varying the duration of the calcination process and by adding Al³⁺ dopants. Solid state ²⁷Al NMR spectroscopy and powder X-ray diffraction confirm the doping of aluminum into the octahedral site of the perovskite structure. Surface photovoltage spectroscopy confirms that Al³⁺ dopants can eliminate Ti³⁺ defects in Al-doped SrTiO₃. Ultraviolet illumination experiments in water and aqueous methanol show that SrTiO₃ and aluminum-doped SrTiO₃, after modification with Rh_xCr_2−xO₃ or Pt co-catalysts, evolve small amounts of H₂ (EQE of 0.0113–0.0173% at 375 nm) with only traces of O₂ detected. The lack of photocatalytic activity is attributed to rapid electron-hole recombination in the oxygen vacancy-rich materials and to the lack of crystal facets that could aid charge separation.