The influence of post-growth heat treatments and etching on the nanostructure and properties of rutile TiO2 nanowires

Titanium(IV) oxide is one of the most promising materials for environmental and energy applications. Although it is already in application nowadays, still fundamental questions concerning the relation between synthesis conditions, nanostructure, defects and properties remain to be solved. This knowledge is the key to develop new Titanium(IV) oxide materials with tailored properties. This work approaches the problem by detailed investigation of hydrothermally grown rutile Titanium(IV) oxide nanowire arrays before, during and after certain post-growth treatments, namely heating and etching. The emphasis is set on the interplay between nanostructure and optical as well as electrical properties. Advanced transmission electron microscopy is used for a detailed characterization of the Titanium(IV) oxide nanowires and the properties are determined by ultra violet-visible spectroscopy and current-voltage measurements. For the post-growth heat treatment, mainly three heating conditions, which differ in temperature and environment, are conducted. Including the as-grown nanowires, which serve as a reference, four types of nanowires are investigated with respect to the influence of post-growth heat treatments: as-grown nanowires, nanowiresannealed in air at 500 °C, nanowires annealed in nitrogen at 500 °C and nanowires after a heat treatment in vacuum at 1050 °C. In addition, post-growth etching of the nanowire arrays is used, in order to obtain a variety of new one-dimensional morphologies with high surface areas. Due to the hydrothermal growth conditions, the as-grown nanowires are built by a nanofiber bundle and are full of defects, especially oxygen vacancies. Using a heat treatment at 500 °C in air leads to a transformation of the nanofiber bundle to a single crystalline nanowire. In addition, this heat treatment is capable to condense oxygen vacancies in voids, which intersperse the nanowire. The void formation can be observed in situ by transmission electron microscopy and the resulting voids are encapsulated by a Ti3+ rich material. As a result, the optical properties of nanowires after a heat treatment at 500 °C in oxygen improve as the band gap and defect related Urbach absorption are decreased. Furthermore, the removal of oxygen vacancies in the crystal structure converts the intrinsically n-type conducting nanowires to an insulator. Changing the heating environment to nitrogen does not affect the vacancy condensation. However, due to the slightly reducing atmosphere of nitrogen, the surface-near defects do not vanish and a core-shell nanowire, with a single-crystalline core that is full of voids and a Ti3+ rich shell, results. Due to the core-shell structure, the properties are completely changed and the nanowire arrays appear black instead of white and posses a metal-like conductivity. An increase of the annealing temperature to 1050 °C leads to void and defect free Titanium(IV) oxide nanowires. These nanowires show additional faceting at the tip, in order to compensate the free volume. The high temperature requires nanowire arrays grown on Silicon substrates and leads to diffusion of Silicon atoms. Consequently, this heat treatment results in the formation of a core-shell nanowire, but with an insulating, 4 nm thick Silicon(IV) oxide shell. Such a shell is promising for application as it suppresses undesired back-transfer of electrons. Thus, the last two heat treatments lead to nanowires with beneficially changed surfaces. This work is concluded with some synthesis strategies to derive new morphologies for solvothermally grown nanowires, which posses even larger surface areas. Using a combination of solvothermal growth, etching and heat treatment, the synthesis of nanostructures ranging from highly fibrous nanowires, over nanowires with tiny channels to rectangular nanotubes, is enabled

The role of vacancy condensation for the formation of voids in rutile TiO2 nanowires

Titanium dioxide nanowire (NW) arrays are incorporated in many devices for energy conversion, energy storage, and catalysis. A common approach to fabricate these NWs is based on hydrothermal synthesis strategies. A drawback of this low-temperature method is that the NWs have a high density of defects, such as stacking faults, dislocations, and oxygen vacancies. These defects compromise the performance of devices. Here, we report a postgrowth thermal annealing procedure to remove these lattice defects and propose a mechanism to explain the underlying changes in the structure of the NWs. A detailed transmission electron microscopy study including in situ observation at elevated temperatures reveals a two-stage process. Additional spectroscopic analyses and X-ray diffraction experiments clarify the underlying mechanisms. In an early, low-temperature stage, the as-grown mesocrystalline NW converts to a single crystal by the dehydration of surface-bound OH groups. At temperatures above 500 °C, condensation of oxygen vacancies takes place, which leads to the fabrication of NWs with internal voids. These voids are faceted and covered with Ti3+-rich amorphous TiOx

Tuning the Electronic Conductivity in Hydrothermally Grown Rutile TiO2 Nanowires: Effect of Heat Treatment in Different Environments

Hydrothermally grown rutile TiO2 nanowires are intrinsically full of lattice defects, especially oxygen vacancies. These vacancies have a significant influence on the structural and electronic properties of the nanowires. In this study, we report a post-growth heat treatment in different environments that allows control of the distribution of these defects inside the nanowire, and thus gives direct access to tuning of the properties of rutile TiO2 nanowires. A detailed transmission electron microscopy study is used to analyze the structural changes inside the nanowires which are correlated to the measured optical and electrical properties. The highly defective as-grown nanowire arrays have a white appearance and show typical semiconducting properties with n-type conductivity, which is related to the high density of oxygen vacancies. Heat treatment in air atmosphere leads to a vacancy condensation and results in nanowires which possess insulating properties, whereas heat treatment in N-2 atmosphere leads to nanowire arrays that appear black and show almost metal-like conductivity. We link this high conductivity to a TiO2-x shell which forms during the annealing process due to the slightly reducing N-2 environment

Fabrication and characterization of abrupt TiO 2 -SiO x core-shell nanowires by a simple heat treatment

Three dimensional hierarchical metal oxide nanostructures, like TiO2 nanowire arrays, have attracted great attention for electrochemical energy conversion and storage applications. The functionality of such devices can be further enhanced by adding a nanowire shell with a different stoichiometry or composition compared to the core. Here, we report an approach with a facile heat treatment at 1050 °C, which allows the fabrication of rutile TiO2–SiOx core-shell nanowire arrays on silicon substrates. Our detailed electron microscopic investigation shows that this method is able to cover hydrothermally grown rutile TiO2 nanowires with a uniform shell of several nanometers in thickness. Moreover, the treatment improves the quality of the rutile TiO2 core by removing lattice defects, introduced from the hydrothermal growth. Electron energy loss spectroscopy reveals that the homogeneous shell around the TiO2 core consists of amorphous SiOx and does not form any intermediate phase with TiO2 at the interface. Thus, the properties of the TiO2 core are not affected by the shell, while the shell suppresses undesired electron back transfer. Latter leads to performance losses in many applications, e.g., dye sensitized solar cells, and is the main reason for a fast degradation of devices incorporating organic materials and TiO2

Role of Vacancy Condensation in the Formation of Voids in Rutile TiO₂ Nanowires

Titanium dioxide nanowire (NW) arrays are incorporated in many devices for energy conversion, energy storage, and catalysis. A common approach to fabricate these NWs is based on hydrothermal synthesis strategies. A drawback of this low-temperature method is that the NWs have a high density of defects, such as stacking faults, dislocations, and oxygen vacancies. These defects compromise the performance of devices. Here, we report a postgrowth thermal annealing procedure to remove these lattice defects and propose a mechanism to explain the underlying changes in the structure of the NWs. A detailed transmission electron microscopy study including in situ observation at elevated temperatures reveals a two-stage process. Additional spectroscopic analyses and X-ray diffraction experiments clarify the underlying mechanisms. In an early, low-temperature stage, the as-grown mesocrystalline NW converts to a single crystal by the dehydration of surface-bound OH groups. At temperatures above 500 °C, condensation of oxygen vacancies takes place, which leads to the fabrication of NWs with internal voids. These voids are faceted and covered with Ti³⁺-rich amorphous TiO_<i>x</i>

Non-agglomerated iron oxyhydroxide akaganeite nanocrystals incorporating extraordinary high amounts of different dopants

Dispersible nonagglomerated akaganeite (β-FeOOH) nanocrystals doped with various elements in different oxidation states such as Co(II), Ni(II), V(III), Ti(IV), Sn(IV), Si(IV), and Nb(V) were prepared using a microwave-assisted solvothermal synthesis in tert-butanol. The doping elements could be incorporated in very high concentrations of up to 20 at. %, which is attributed to the kinetic control of the phase formation during the solvothermal reaction, together with the extremely small crystal size, which can stabilize the unusual structural compositions. The particle morphology is mostly anisotropic consisting of nanorods ∼4 nm in width and varying length. Depending on the doping element, the length ranges from ∼4 nm, resulting in an almost-spherical shape, to 90 nm, giving the highest aspect ratio. The particles are perfectly dispersible in water, giving stable colloidal dispersions that can be deposited on different substrates to produce thin films 35–250 nm thick. In addition, films up to 30 μm thick, consisting of interconnected mesoporous spheres, can be prepared in situ during the reactions. The nanostructures assembled from akaganeite nanocrystals are stable up to high temperatures of >400 °C. They can be transformed to hematite (α-Fe2O3) by heating between 480 °C and 600 °C without losing the morphology, which can be used for the fabrication of doped hematite nanostructures. The tunable properties of the doped akaganeite nanoparticles make them excellent candidates for a wide range of applications, as well as versatile building blocks for the fabrication of doped hematite nanomorphologies