Non-equilibrium crystallisation of aqueous sol-gel-derived Bi₄Ti₃O₁₂ thin films using concentrated solar energy

Abstract Controlling the crystallisation of sol-gel-derived oxide thin films remains one of the most critical challenges in chemical solution deposition, as the post-deposition thermal treatment largely determines phase formation, densification and microstructural development. This issue is particularly relevant for layered ferroelectric oxides such as Bi₄Ti₃O₁₂ (BiT), whose crystallisation under conventional thermal conditions often promotes exaggerated anisotropic grain growth and the formation of secondary phases associated with bismuth volatilisation. In this work, Bi₄Ti₃O₁₂ thin films were prepared by an aqueous sol-gel route combined with spin coating and subsequently crystallised using concentrated solar energy (CSE) as an alternative radiative annealing strategy. The use of an aqueous precursor chemistry avoids organic solvents, while the high radiative fluxes delivered by a Fresnel-lens-based solar concentrator provide access to thermal conditions far from equilibrium. The crystallisation behaviour and microstructural evolution of the films were systematically compared with those obtained by conventional hot-plate annealing. The results demonstrate that concentrated solar irradiation fundamentally modifies the crystallisation pathway of the sol-gel-derived films. Whereas excessive radiative input accelerates anisotropic grain growth and promotes pyrochlore formation, a narrow processing window centred around 400 °C and short dwell times enables complete phase formation, good densification and microstructural homogeneity while suppressing the equilibrium growth of plate-like grains. The study further reveals that the combination of rapid heating rates and strong thermal gradients allows the balance between crystallisation, densification and grain growth to be finely controlled. These findings demonstrate that concentrated solar energy represents a powerful processing tool for tailoring the crystallisation of sol-gel-derived oxide thin films under non-equilibrium conditions. Together with the significant reduction in processing temperature and thermal treatment time, this approach offers a sustainable and energy-efficient alternative for the thermal processing of functional oxide coatings.