Influence of pH on the sonochemical formation and phase evolution of sodium titanates

Abstract

This work reports, for the first time, a systematic study of the sonochemical synthesis of sodium titanates under controlled pH conditions, elucidating how the reaction medium governs phase selectivity, morphology, and electrical behavior. Sodium titanate powders were obtained via ultrasonic irradiation with pH values ranging from 2 to 14 and characterized by X-ray diffraction, FTIR, UV–vis spectroscopy, scanning electron microscopy, and impedance spectroscopy. Rietveld refinement demonstrated that the crystalline phases evolve markedly with pH: acidic media (pH 2–6) produce multiphase mixtures of Na₂Ti₆O₁₃ + TiO₂ + NaCl, moderately alkaline conditions (pH 8–10) yield biphasic Na₂Ti₆O₁₃/Na₂Ti₃O₇ with a maximum Na₂Ti₆O₁₃ content of 67 wt% at pH 10, and strongly alkaline synthesis (pH 14) stabilizes Na-rich Na₁₆Ti₁₀O₂₈ (79 wt%). UV–vis spectra revealed optical band gaps from 2.62 to 3.78 eV, and SEM analysis showed a transition from aggregated nanograins to well-defined microrods at pH 10. Impedance spectroscopy confirmed that the sample obtained at pH 10 exhibits the highest ionic conductivity (σ_DC = 9.52 × 10⁻⁴ S cm⁻¹), attributed to the predominance of tunnel-type Na₂Ti₆O₁₃. These results demonstrate that pH control within a sonochemical route represents a new and effective strategy to direct the formation of layered, tunnel, and Na-rich titanates, providing a fast, low-cost, and reproducible pathway for tailoring the structural and functional properties of sodium-based materials for energy applications.