This paper reviews the state of the art of anodized titanium dioxide nanotubes (TiO 2 NTs), with an emphasis on the growth mechanism leading to their formation and the effect of heat treatment on their structure and properties. The discussion is primarily focused on TiO 2 NTs grown in fluoride containing electrolytes, although the mechanism of formation of NTs in fluoride free solutions via Rapid Breakdown Anodization (RBA) is briefly covered. After an initial overview of progress made on the synthesis of anodized TiO 2 NTs the review provides an analysis of the factors affecting the anodizing process (fluoride concentration, electrolyte type, applied potential and anodizing time). Details of the current-time transient, the chemistry of the process and the chemical composition of the anodic films are described which provide key information to unveil the nanotube growth mechanism. The main debate is whether NTs growth in fluoride containing solutions occurs via field-assisted plastic flow (i.e. a constant upward displacement of the oxide to form the NTs) combined with field-assisted ejection of the Ti 4+ ions (i.e. ions are ejected into the electrolyte without oxide formation) or via field-assisted dissolution (i.e. preferential dissolution at the pore base where the field is stronger) or whether both processes play a role. Whenever anodization takes place in organic solutions the experimental evidence supports the plastic flow model, whereas in aqueous media field-assisted (and chemical) dissolution occur. The mechanism of rib formation on the walls of the NTs is also reviewed, and it clearly emerges that the applied potential and water content in the electrolyte are key factors in determining whether the NTs are ribbed or smooth. There also appears to be a relationship between the presence of ribs and the evolution of oxygen bubbles at the anode. The impact of thermal treatment on the properties of the NTs is also described. A variety of crystalline structures are present in the NTs (i.e. anatase or rutile), depending on the heat treatment temperature and atmosphere and the resulting electrical properties can be varied from dielectric to semi-metallic. A heat treatment temperature limit ranging from 500 to 800°C exists, depending on preparation history, above which sintering of nanoscale titania particles occurs leading to collapse of the NTs structure. Future work should aim at using annealing not just to influence the resulting crystalline phase, but also for generating defects to be exploited in specific applications (i.e. photocatalysis, water splitting and photovoltaics).

Titanium oxide (TiO 2 ) nanotubes prepared by electrolytic anodisation of a titanium electrode have been systematically heat treated to control the conversion of the as-prepared amorphous structure to nanocrystalline anatase and rutile. Raman spectroscopy revealed that the temperature of calcination is critical in determining the structure and crystallinity of the titania. X-ray Photoelectron Spectroscopy analysis shows the as-prepared film to consist mainly of oxide, although a small amount of fluoride contamination remains from the electrolyte. Organic components from post-anodising cleaning treatments were also present. Fluorine ions are gradually ejected from the anodic layer during annealing and the fluorine concentration is negligible in samples that are heat treated above 400• C. Choosing the appropriate annealing temperature allows the structure to be made up of defined proportions of anatase and rutile with a reduced contamination of species from the electrolyte or organic solvents.

This paper investigates the formation mechanism of ribs on the outer wall of anodized TiO 2 nanotubes (NTs) prepared in a NaF/Glycerol electrolyte containing 2wt% of water. The effect of potential and time on the morphology of the NTs is evaluated along with growth efficiency (% of total charge at the electrode used to form the oxide) and X-ray Photoelectron Spectroscopy (XPS) measurements, providing an insight into the mechanism of formation of ribs. XPS analysis confirms the presence of fluorine, as TiF 6 2-, and carbon as impurities in the anodic film. The growth efficiency of the process decreases from ~70% at 10V to 55-58% at 20-30V and 14% at 40V. Similarly, the anodic growth factor (migration of ions expressed as the maximum radius of the "oxide cell" per applied potential, nm V-1) decreases at higher potentials, due to oxygen bubbles evolving at the anode at 20-40V and disrupting the anodizing process. The formation of gas bubbles also affects the morphology of the NTs; while NTs are smooth at 10V, oxide rings appear over the range 20-40V. Partial dissolution of the oxide rings due to fluorine ions eventually reshapes the NTs forming ribs, whereas excessive dissolution over extended anodizing times tends to smoothen the NTs and eventually leads to collapse of the NTs. On the basis of these observations, we suggest oxygen evolution (requiring a minimum amount of water in the electrolyte and a sufficiently high potential 20-40V) plays a primary role on the formation of ribs on anodized TiO 2 NTs. Ribs are also observed on NTs grown in aqueous electrolytes, although since dissolution is more difficult to control the resulting structure is more irregular than in organic media.

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