Zixue Su

Formation, morphology control and applications of anodic TiO2 nanotube arrays

Anodic titanium dioxide films, especially anodic TiO2 nanotube arrays, have attracted extensive interest in the past decade. A number of electrolytes, either aqueous or non-aqueous, fluoride containing or fluoride free, have been chosen to grow anodic titanium oxide films. With great improvements in the morphology control on porosity, pore size, nanotube length and pore ordering, anodic titanium oxide films have been widely applied in photochemical water splitting, hydrogen sensing, dye-sensitized solar cells, templating for low dimensional nanomaterials and biomedical research. This article presents a brief review of the progress to date in the formation mechanism, morphology control and some applications of these smart materials.

Investigation of the pore formation in anodic aluminium oxide

The mechanism accounting for the self-organization of nanoscale pores during anodic oxidation of aluminium is studied. Microstructural studies support the equifield strength model, which can be used to explain the formation of the hemispherical electrolyte/oxide and oxide/metal interfaces, uniform thickness of the oxide layer, as well as self-adjustment of the pore size and pore ordering. The fundamentals of the model are the electric field enhanced oxide dissolution rate and oxygen anion migration rate. The most important factor for determining the porosity of anodic alumina films with both ordered and disordered pores is the relative dissociation rate of water. The relationships between the porosity and anodization conditions, such as voltage, current density, and electric field strength, are also estimated.

Formation, microstructures and crystallization of anodic titanium oxide tubular arrays

Formation of highly ordered TiO2 nanotubular arrays during anodization of titanium can be elucidated by using the equifield strength model and a double-layer structure. The two characteristic microstructural features of anodic titanium oxide (ATO) in comparison with anodic aluminium oxide (AAO), a thin titanium hydroxide layer and an O-ring like surface pattern, were investigated using scanning electron microscopy and high resolution transmission electron microscopy (HRTEM). Field-enhanced dissociation of water is extremely important in the formation of the nanotubes with a double-layer wall and an O-ring-like pattern, and in the determination of porosity. The relations between porosity of the ATO films and the anodization conditions, such as current density and electric field strength, have been established. Crystallization of the anodic TiO2 nanotubular arrays was also achieved and the microstructures were studied by using HRTEM.

Pore diameter control in anodic titanium and aluminium oxides

A nonlinear relation between the pore diameter and anodising voltage is established for nanoporous anodic titanium oxide (ATO) and anodic aluminium oxide (AAO). The pore diameters of both ATO and AAO have been found to increase with the anodising voltage and drop down when the voltage exceeds a critical value. The origin for the existence of this maximum value of pore diameter in AAO and ATO is discussed.

One‐Step Synthesis of Bismuth Telluride Nanosheets of a Few Quintuple Layers in Thickness

Strictly two dimensional (2D) crystals were previously believed to be non-existent due to inherent thermodynamical instability. Thin films of quasi 2D crystals had been limited until recently to epitaxial growth on single-crystal substrates with lattice matching achieved by routes such as molecular beam epitaxy (MBE). Graphene, 2D monolayers of graphite, was first observed experimentally in 2004 to exist as single layer (or 2–3 layer) freestanding films. It was explained that 2D crystals become intrinsically stable by gentle “crumpling” in the third dimension. Some inorganic, quasi-2D compounds, such as dichalcogenides (WS2 and MoS2), [5] boron nitride (BN) 7] and some complex oxides have been prepared by mechanical cleavage or chemical intercalation/ exfoliation methods. Bismuth telluride, Bi2Te3, as one of the best known thermoelectric materials, possesses the highest figure of merit and power factor at room temperature. 10] Calculations have shown that 2D Bi2Te3 layers in a quantum well structure have the potential to increase the figure of merit by a factor of 13 over the bulk telluride. Very recently, mechanical exfoliation techniques similar to those used in the preparation of graphene were employed by Teweldebrhan et al. to prepare the first 2D Bi2Te3 crystals. [12] Experiments indicate that these 2D Bi2Te3 crystals possess a high electrical conductivity and low thermal conductivity and Bi2Te3 2D thin films present a new class of topological insulators. 15] Also, very recently, rigid triangular and hexagonal Bi2Q3 (Q = Se,Te) nanoplates of 6 nm or less in thickness were deposited from the respective bulk chalcogenide powders. Here, we report the facile, one-step synthesis and growth of 2D Bi2Te3 nanosheets using chemical vapor transport (CVT) methods in a sealed tube. The surface-assisted CVT (SACVT) technique used here is a low-cost and uncomplicated synthesis method that can be used for producing a relatively high yield of nanomaterials. Performing the reaction in a closed container means that departures from the target stoichiometry of the products can be minimized. The folded edges of the nanometer scale sheets in the reactions described here provide a clear high-resolution transmission electron microscopy (HRTEM) signature for the exact thickness of the 2D Bi2Te3 crystals. Figure 1 shows a (negative) SEM image of the products deposited on the as-received Si wafer after the reaction. From the SEM image we can see that most of the particles have a thin sheet-like morphology and many edges of the particles

Anodic formation of nanoporous and nanotubular metal oxides

A localized dielectric breakdown model with good universality is introduced to explain the pore initiation, separation and growth processes of nanoporous and nanotubular anodic metal oxides. It is suggested that the degree of localized dielectric breakdown, which is mainly determined by the dielectric strength and energy band gap of the anodic oxide, electrolyte used, anodizing field and also temperature during anodization, has a significant effect on the pore formation. Continuous nanoporous films tend to grow under low degree of localized dielectric breakdown of the anodic oxides, and the growth in number and size of voids induced by high degree of localized dielectric breakdown at the inter-pore areas leads to the separation of neighbouring pores and, therefore, formation of nanotubular structures. Specially, anodic TiO2 nanotubes are believed to grow by continuous localized dielectric breakdown and self-healing processes at the base of main pores. Alternating dielectric breakdown and oxidation processes at the inter-pores areas lead to the formation of commonly observed O-ring like ridges.

Crystal growth of Si nanowires and formation of longitudinal planar defects

Si nanowires were fabricated using Au nanoparticles as catalyst, either templated by porous anodic aluminium oxide films or on a smooth substrate of Si(100). The growth orientation of the nanowires and longitudinal planar defects such as twin defects and stacking faults were investigated using HRTEM. It was proposed that the nanowire growth was thermodynamically controlled with a slow growth rate and the growth orientation was normally the [111] zone axis of the cubic Si. When the growth rate was fast, the nanowire growth was kinetically controlled, leading to a growth orientation along the [112] zone axis. The formation mechanisms of various defects, such as twin defects, stacking faults and antiphase boundaries, are discussed.

Porous anodic metal oxides

Porous anodic aluminium oxide (AAO) and anodic titanium oxide (ATO) attracted an increased attention in the recent years due to their high potentials of application in nanotechnology. This article presents a brief review of some important developments of these smart materials including anodization methods, formation mechanisms of the pores, self-ordering processes and applications. Anodization of other metals are also highlighted.

Cathodoluminescence investigation of silicon nanowires fabricated by thermal evaporation of SiO

Silicon nanowire samples fabricated by thermal evaporation of SiO powder were investigated by Cathodoluminescence. Three main bands were found at low temperatures, namely, peak 1 at about 620–650 nm (2.0–1.91 eV), peak 2 at 920 nm (1.35 eV), and peak 3 at 1280 nm (0.97 eV). An additional broad band (peak 4) in the infrared region with its maximum at ∼1570 nm (0.79 eV) appears at room temperature. The origins of the emission bands are discussed.

Ultrafast Elemental and Oxidation-State Mapping of Hematite by 4D Electron Microscopy

We describe a new methodology that sheds light on the fundamental electronic processes that occur at the subsurface regions of inorganic solid photocatalysts. Three distinct kinds of microscopic imaging are used that yield spatial, temporal, and energy-resolved information. We also carefully consider the effect of photon-induced near-field electron microscopy (PINEM), first reported by Zewail et al. in 2009. The value of this methodology is illustrated by studying afresh a popular and viable photocatalyst, hematite, α-Fe2O3 that exhibits most of the properties required in a practical application. By employing high-energy electron-loss signals (of several hundred eV), coupled to femtosecond temporal resolution as well as ultrafast energy-filtered transmission electron microscopy in 4D, we have, inter alia, identified Fe4+ ions that have a lifetime of a few picoseconds, as well as associated photoinduced electronic transitions and charge transfer processes.