Electrochemical anodic oxidation (anodization) has long been regarded as one of the most effective methods for producing TiO 2 nanostructures with broad applications. Traditionally, majority of TiO 2 nanostructures have been synthesized using aqueous or organic electrolytes containing fluoride ions (F - ), based on the widely accepted view that F - plays a critical role in enabling the self-organized growth process. In recent years, however, increasing attention has been given to developing fluoride-free anodization routes. Aqueous and organic systems incorporating chloride, bromide, nitrate, sulfate, phosphate, and other anions have shown promise in forming structurally diverse and functionally novel TiO 2 nanostructures. These approaches address both environmental concerns and the need for more versatile surface structures. While much of the foundational literature in this area stems from early studies, recent publications have begun to revisit and refine these alternative systems. High temperature, strictly dehydrated phosphate or phosphate glycerol electrolytes yield long robust nanotubes and mesoporous frameworks, while chloride and nitrate media produce high aspect ratio tubes, sponge like layers and microflower morphologies. Organic acid and silicate electrolytes further give doped or composite oxides with promising photocatalytic, electrochemical and bioactive performance. Against this background, this review provides a comprehensive summary of the morphologies, growth conditions, mechanisms and applications of TiO 2 nanostructures formed in fluoride free electrolytes. Particular emphasis is placed on common mechanistic motifs across different systems, and a conceptual map is proposed that links electrolyte classes to characteristic current behavior and nanostructure types, and highlights promising directions for environmentally conscious nanomaterial design.
Zeng et al. (Sun,) studied this question.