Challenge toward Complex g-C ₃ N ₄ /Titanates/Titania Nanoarchitectures: Top-Down versus Bottom-Up Approach

A comprehensive investigation of CN–TiO2 and CN–TNT/TiO2 heterostructures was made, taking into account, in particular, the role of the synthesis methods, i.e., ex situ (top-down) and in situ (bottom-up), on the structural, morphological, and optical properties. It was shown that the samples, obtained from different synthesis routes, although with similar elemental composition and functional/terminal groups, have peculiar differences in terms of structure, morphology, and surface and optical properties. Concerning CN–TiO2 samples (from an ex situ approach), CN-functionalized anatase TiO2 particles forming aggregates with a lower BET surface area are obtained, whereas CN–TNT/TiO2 (from an in situ approach) shows more uniform composition and higher surface area due to the presence of nitrogen-rich groups, which prevents the titanate nanotube conversion into TiO2 particles. FTIR and X-ray Photoelectron Spectroscopy (XPS) also confirm the formation of C–Ti bonds at the particle surface for both samples. The optical properties show a progressive band gap narrowing from pristine anatase TiO2 nanoparticles to CN–TiO2 and CN–TNT/TiO2. Band alignment analysis confirms the formation of type-II heterojunctions in both composites, thus supporting effective charge separation and enhanced visible light absorption. These results suggest that CN–TiO2 and CN–TNT/TiO2 heterostructures, with their tailored electronic and optical properties, show significant potential for photocatalytic applications in the environmental remediation and energy conversion fields. The TiO2-based materials, doped with carbon and nitrogen, were investigated using a multitechnique approach, including microscopies (field emission scanning electron microscopy, high-resolution transmission electron microscopy and atomic force microscopy), EDS mapping, spectroscopies (FTIR, XPS, and UV–vis), X-ray diffraction, and porosity analyses.