Effect of Ammonium Hexafluorophosphate (NH₄PF₆) and Mixed Ammonium Hexafluorophosphate/tetrabutylammonium Hexafluorophosphate (NH₄PF₆/NBu₄PF₆) on the Morphological and Structural Evolution of TiO₂ Nanocatalyst

Abstract Titanium dioxide (TiO₂) nanocatalyst has received significant attention due to its superior photo-induced electron transfer properties, particularly in the metastable anatase phase, which underpins its application in advanced oxidation processes (AOPs). However, anatase TiO₂ crystals are predominantly dominated by the thermodynamically stable {101} facet, representing over 94% of the surface, whereas the highly reactive {001} facet diminishes rapidly under equilibrium growth, limiting photocatalytic efficiency. To address this limitation, this study evaluates the morphological and structural evolution of TiO₂ nanocatalysts synthesized via thermal decomposition of peroxotitanic acid in the presence of ammonium hexafluorophosphate (NH₄PF₆) and a mixed ammonium/tetrabutylammonium hexafluorophosphate system (NH₄PF₆/NBu₄PF₆). Field emission scanning electron microscopy (FE-SEM) revealed that fluorine incorporation effectively promoted anisotropic growth, producing rice grain-like nanocrystals with improved dispersion. X-ray diffraction (XRD) analysis demonstrated enhanced anatase phase stability in the co-doped NH₄PF₆/NBu₄PF₆–TiO₂ sample (85.81%) compared with NH₄PF₆–TiO₂ (59.68%) and undoped Peroxo–TiO₂ (57.12%), while Raman spectroscopy confirmed increased crystallinity and coherent lattice vibrations. Surface facet analysis indicated that {001} facet exposure was slightly higher in NH₄PF₆–TiO₂ (6.54%) than in the co-doped system (5.65%), reflecting the effect of dual-cation fluorination on crystal growth. Overall, the dual-cation strategy effectively suppresses anatase-to-rutile transformation, stabilizes the anatase phase, and regulates facet development, yielding TiO₂ nanocatalysts with improved structural integrity, controlled morphology, and tailored high-energy surfaces. These engineered materials present considerable potential for enhanced photocatalytic performance in sustainable energy conversion and environmental remediation applications.