Self-organized TiO2 nanotube layers

Introduction

TiO2 is essential for various applications including photocatalysis1, solar energy conversion into electricity2, sensing3, and is also frequently used in biomedical applications4 due to its biocompatibility. For most of these applications, a high surface area is of advantage. Therefore, it is desirable to use TiO2 nanostructures possessing such surface area. Among the TiO2's available nanoscale structures, nanotubes belong to one of the most promising morphologies. Different synthesis approaches, e.g., electrospinning5, template assisted methods6, or hydrothermal synthesis7 are frequently used to prepare TiO2 nanotubes. Using the abovementioned methods, nanotubes are produced in powder form, i.e., with a random ordering of the tubes. A continuous layer of TiO2 nanotubes (TNT layers), which are vertically oriented, interconnected to each other, and strongly connected to the underlying substrate can be prepared via electrochemical anodic oxidation. 

Pioneering Efforts & Self-Organization Phenomena 

Due to the particular growth during electrochemical anodic oxidation, TNT layers are highly self-ordered. Electrochemical anodic oxidation, as such, is a century old process widely used in industry to create compact or porous oxide layers on the surface of a metal substrate. There are four main components for this technique and the typical setup involves the voltage source, the anode, the cathode, and the electrolyte. Synthesis of TNT layers via this method was first reported in 19848 by Andrew's group. However, a significant attention to this material was received after pioneering efforts by Albu, Macak, and Schmuki more than two decades later9,10. Indeed, throughout the last 15 years, different electrolytes based on aqueous HF, NaF, NH4F, or fluoride containing organics have been used for direct growth of TNT layers on Ti surfaces. The influence of anodization conditions, substrates, and electrolytes on the resulting TNT layers (their dimensions, spacing, and ordering) was also sufficiently explored11. 

There are three main steps in the formation of the nanotubular structure (Eq. 1-3.). First, the oxidation of Ti into Ti4+ (Eq. 1). Second, the formation of an oxide layer (Eq. 2). Finally, dissolution of the oxide layer to form the nanotubular structure (Eq. 3).

                                         Oxidation of Ti:                                                           Ti -> Ti4+ + 4 e-                          (Eq. 1)      

                                        Formation of oxide layer:                         Ti4+ + 2 H2O -> TiO2 + 4 H+         (Eq. 2)                             

                                        Dissolution of oxide layer:                TiO2 + 6 F- + 4H+ -> [TiF6]2- + 2 H2O               (Eq. 3)

Using an optimized set of experimental parameters (e.g., electrolyte composition and pH, anodization time, applied voltage, etc.), self-organizing conditions are established and the nanotubular structure is grown. The key to the formation of the nanotubular structure is the presence of F- ions (or other highly reactive anions) in the electrolyte since it establishes a steady-state equilibrium between oxide formation and dissolution which is required for the self-organization.   

Physico-Chemical Properties & Applications

TNT layers present a favorable morphology for photochemical and photoelectrochemical applications as they offer (i) a high surface area and enhanced incident light absorption due to their one-dimensional ordered structure12; (ii) a direct path for the photogenerated charge carriers towards the underlying Ti subtrate13,14; and (iii) an easily tunable dimensions, in particular diameter and wall thickness15. On the other hand, TNT layers are recognized as excellent biocompatible material owing to their low cytotoxicity, high stability, and antibacterial properties. Another advantage of TNT layers is that they can be directly grown, with controllable geometry, on Ti substrates. 

Due to the above mentionned characteristics of TNT layers (i.e., excellent biocompatibility, controllable dimensions, high surface area, large surface-to-volume ratio, etc.), they have been shown to be a superior platform for local drug delivery16,17,18. Indeed, by adjusting the morphology of TNT layers, the release kinetics of specific drugs can be tailored to achieve stable and sustained release16. Since TNT layers possess also good hemocompatibility and anticoagulation characteristics, they are promising for vascular implants and biomedical applications due to increased osteoblast cell adhesion and proliferation19,20, increased growth of hydroxyapatite19,20, enhanced protein adsorption21, and improved cellular behavior and tissue integration22.

Future Perspectives

Although TNT layers are extensively studied during the last decade, there are still progresses to make in this field, and it includes the following non-exhaustive list: (i) preparation of nanotubes on large scale (≥50 cm2); (ii) preparation of well-separated nanotubes (i.e., single tubes); (iii) preparation of single-wall TNT layers; (iv) preparation of high-aspect ratio membranes; (v) preparation of nanotubes with solely TiO2 rutile phase; (vi) improving the physico-chemical properties with secondary materials.

In our group, we provide deep studies in the field of TNT layers independently but also in collaboration with Dr. Jan M. Macak from Center of Materials and Nanotechnologies (CEMNAT, University of Pardubice) whom is a pioneer in the field of self-organized TiO2 nanotube layers and the recipient of the prestigious ERC Starting Grant. 

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