Tio2 Nanostructures And Their Applications

Published Date: 02 Nov 2017

Table of Contents

Type chapter title (level 1) 1

Type chapter title (level 2) 2

Type chapter title (level 3) 3

Type chapter title (level 1) 4

Type chapter title (level 2) 5

Type chapter title (level 3) 6

1. Introduction

2. TiO2 nanostructures

2.1 Nanowires

2.2 Nanotubes

2.3 Quantum dots

1. Introduction

Titania (TiO2) is certainly one of the most studied transition metal oxides in materials science (over 40000 publications in last decade). This material has attracted enormous interest due to its interesting properties and found applications in dye-sensitized solar cells, photoelectrolytic cells, photocatalysts and biomedicine. Bulk titania is known for its non-toxicity, eco-friendliness, corrosion resistance. The most eminent functional features are exceptional biocompatibility (biomedical applications), unique ionic and electronic properties of TiO2. Titania in all of its crystal forms is a wide band gap semiconductor (ca. 3 eV) with appropriate band-edges that make it utile in solar cells, photoelectrolytic cells and photocatalytic reactions. Photogenerated excitons (electron–hole pairs) can be exploited for splitting water into oxygen (O2) and hydrogen (H2), or can be used for the degradation of hazardous wastes such as organic pollutants in ground waters or toxic air pollutants.[19–21] The first milestones laid in titania research area were indisputably the reports by Fujishima and Honda in early 70s on photoelectrolytic water splitting on a TiO2 anode, [22] and the works by Gerischer and Tributsch, [23] Dare-Edwards et al. ,[24] and Gratzel and O’Regan[25] that introduced the use of titanium dioxide for solar energy conversion in the 80s and 90s. [26] Over the past two decades, the spectrum of potential applications has become only wider towards devices with increasingly sophisticated photovoltaic, electrochromic, self-cleaning or antifogging properties, biomedical coatings, sensors, and smart-surface coatings.[27–39]

2. TiO2 nanostructures

Titania along with its various crystalline forms (anatase, rutile and brookite) also presents interest based on architecture of its nanostructures, namely nanowires, nanotubes and quantum dots.

2.1 TiO2 Nanotubes

From the moment of discovery of carbon nanotubes,1 the combination of intriguing molecular geometry and exclusive properties inspired the field of nanotechnology and triggered huge efforts in chemistry, physics and materials science. One dimensional (1-D) nanostructures possess exciting electronic properties, such as high electron mobility and quantum confinement effects, high specific surface area, and also demonstrate high mechanical strength.2-4 Despite that carbon still holds the name of the most explored nanotubular material, a wide spectrum of other materials (e.g. transition metal oxides and sulfides) have been synthesized in a 1-D or quasi-1-D geometry (nanowires, nanofibers, nanorods and nanotubes) and have also exhibited novel properties and characteristics.

Fabrication of 1-D titania nanostructures is achieved by various routes comprising sol–gel methods, template-directed methods, solvothermal approaches and by electrochemical anodization.5-9[8,10, 14, 15, 36,37, 44–58] One of the first endeavors to obtain titania nanotubes is most likely the study reported by Hoyer,[10] who utilized an electrochemical deposition technique into an ordered Al2O3 template. Later methods primarily included other template-directed methods, sol–gel techniques, and solvothermal methods with or without templates [14, 44, 47–50, 52,56–59] and atomic layer deposition (ALD) into the template. [60–63] Many of the processes are acid-catalysed hydrolysis reactions of titanium alkoxide precursors (e.g. titanium isopropoxide) followed by the condensation reactions. For instance, TiO2 sol was filled into the pores of the Al2O3 template, then a thermal processing was applied, and finally the Al2O3 template was selectively washed out by dissolution. [52] TiO2 nanotubes are also prepared by using fibrous templates of suitable amphiphilic surfactants above their critical micelle concentration; these amphiphilic surfactant molecules assemble in a solvent to give a worm-like micelles, which are consequently utilized as templates for titania preparation. In described approach, nanotube formation is carried out on the surface of fibrous templates (micelles or reverse micelles) with a cylindrical exterior surface. The titanium precursors can successfully react on the surface of a template, and after removal of the template (calcination), a nanotube structure is achieved. [8] Usually TiCl4 or Ti[O-iPr]4 solutions are used as the titanium precursor. Moreover, by varying water to micelle ratios [45, 46] crystallite size can be controlled.

Another well-established solution-based method to synthesize 1-D titania nanostructures is the solvothermal method, which was first reported by Kasuga et al.[14, 47] In this method, bulk titania powder is treated with sodium hydroxide solution in a teflon-coated autoclave at 100–150°C for several hours, followed by hydrochloric acid treatment. [14, 47,48] The general formation of the tubular architecture is based on exfoliation of titania crystal planes in the alkali medium and stabilization in the form of Ti-O-Na+ ionic species. This step is succeeded by a nanolayer sheets rolling into tubes during cooling or HCl treatment processes. The cause for the rolling-up is still under debate, because several experimental factors are vital to achieve a material with tube geometry. These tubular structures are made of multiple shells, with an inner diameter of ca. 5 nm, a shell spacing of less than 1 nm, and an average tube diameter of ca. 10 nm. Amount, length and size distribution of fabricated nanotubes are dependent on the specific reaction conditions, which introduces certain control over mentioned parameters. [48]

Atomic layer deposition technique makes it possible to conformably coat surfaces of templates (e.g. porous Al2O3) with consequent atomic layers, one after the other, by using alternating cycles of exposure to a titania precursor followed by purge process and hydrolysis. [60–63] Al2O3-template approaches allow nanotube fabrication with vertical alignment to the substrate, but at the same time in certain cases they have critical template removal step.

All these titanium precursor solution- or template-based processes result in single tubes or loose agglomerates of the same or bundles that are dispersed in a solution, and often a wide distribution of tube lengths is obtained. Implementation of the nanotubular structures in electrically contacted devices is usually achieved by compact placement of the nanotubes into layers on an electrode surface. However, this process results in a random orientation of the nanotubes on the surface of an electrode, which seriously impedes many advantages of the 1-D directionality.

Figure 1. The electrochemical anodization and anodic morphologies: a) I) metal electropolishing, II) compact anodic oxide formation, III) self-ordered oxides, IV) fast oxide nanotube formation, V) ordered nanoporous layer formation. Examples of morphologies of obtained structures: b) highly ordered nanoporous Al2O3 (taken with permission from Ref. [116]), c) highly ordered TiO2 nanotubes d) disordered TiO2 nanotubes grown in bundles.

In contrast to aforementioned techniques, the electrochemical anodization method leads to TiO2 nanotube arrays aligned perpendicular to the substrate surface (Figure 1) with well-defined and controllable well-defined nanotube length. The nanotubes are attached to the metal surface and, therefore are already electrically connected and easy to work with. The use of an electrochemical anodization method allows almost any shape of titanium surfaces to be coated with a dense and defined nanotube layer and is, thus a very easy to scale up structuring process. Fabrication of titania nanotube in the form of the arrays by electrochemical anodization can be summarized in four synthesis generations. The first generation employs aqueous HF-based electrlolytes such as HF, HNO3/HF, H2SO4/HF, H2Cr2O7/HF, CH3COOH/NH4F, H2SO4/NH4F, H3PO4/HF and H3PO4/NH4F. The second generation uses buffered electrolytes which include NaF or KF in buffered environment. The third generation utilizes polar organic electrolytes such as formamide, dimethyl formamide, dimethylsulfoxide ethylene glycol, diethylene glycol, glycerol and methanol/H2O solutions of HF or its salts. The fourth generation comprises fluoride-free electrolytes such HCl, aqueous H2O2 and aqueous HCl/H2O2 based electrolytes.

2.2 TiO2 nanowires

TiO2 nanowires are also prepared by using micelle templates of appropriate surfactants above their critical micelle concentration (the surfactant molecules aggregate and disperse in a liquid to give so-called spherical or rod-like micelles, which are used as template for TiO2 preparation).