The surface science of titanium dioxide

Photoinduced reactivity of titanium dioxide

The advantages in using nanostructured materials for electrochemical energy storage have largely focused on the benefits associated with short path lengths. In this paper, we consider another contribution, that of the capacitive effects, which become increasingly important at nanoscale dimensions. Nanocrystalline TiO2 (anatase) was studied over a dimensional regime where both capacitive and lithium intercalation processes contribute to the total stored charge. An analysis of the voltammetric sweep data was used to distinguish between the amount of charge stored by these two processes. At particle sizes below 10 nm, capacitive contributions became increasingly important, leading to greater amounts of total stored charge (gravimetrically normalized) with decreasing TiO2 particle size. The area normalized capacitance was determined to be well above 100 μF/cm2, confirming that the capacitive contribution was pseudocapacitive in nature. Moreover, reducing the particle size to the nanoscale regime led to faster...

Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO2 (Anatase) Nanoparticles

Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

7Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO2 and orthorhombic lithium titanate Li0.6TiO2. Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO2 to lithium titanate Li0.6TiO2. Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO2 phase containing only small Li amounts ≈ 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (Dmicr = 4.7 × 10-12 cm2 s-1 in anatase and Dmicr = 1.3 × 10-11 cm2 s-1 in lithium titanate). However, macroscopic intercalation data show activation energies of ∼ 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase bound...

Two Phase Morphology Limits Lithium Diffusion in TiO2 (Anatase): A 7Li MAS NMR Study

Recently, much attention has been focused on the use of nanostructured metal oxide semiconductors for various applications, such as electrochromic windows, photocatalytic devices, lithium-ion batteries, dielectrics in integrated circuits, and dye-sensitized TiO{sub 2} solar cells. Smooth nanometer-scale films of anatase TiO{sub 2} on indium-tin oxide substrates (ITO) are obtained by electron-beam evaporation of reduced TiO{sub 2} powder. Mott-Schottky analysis shows an abrupt change in slope when the depletion layer reaches the TiO{sub 2}/ITO interface. An electrostatic model is derived, which gives a quantitative description of the observed change in slope. From the potential at which the slope changes, the dielectric constant of anatase could be accurately determined. A value of 55 is found, which is significantly lower than those reported for anatase TiO{sub 2}.

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Mott-Schottky analysis of nanometer-scale thin-film anatase TiO2

Thin smooth anatase TiO2 films are obtained by electron beam evaporation of reduced TiO2. These films show a preferential (004) orientation when deposited on electron beam evaporated amorphous titanium. Electrochemical lithium insertion into these films is studied with several optical and electrochemical techniques. A coloration efficiency of 13 cm2 C-1 is found, which is twice as high as that reported for TiO2 films grown by chemical vapor deposition (CVD). Potential-dependent capacitance measurements show that after the extraction of lithium ions has taken place a small region at the surface of the electrode has a much higher dielectric constant than that of the bulk of the electrode. This is explained by the presence of irreversibly trapped lithium ions in the region where a (reversible) phase transformation from anatase TiO2 to anatase Li0.5TiO2 has occurred. The extent of this region depends strongly on the intercalation potential; values of 7 and 17 nm are found after 2.5 h of intercalation at −1.0 ...

Spatial Extent of Lithium Intercalation in Anatase TiO2

Electroceramics—the role of interfaces

In situ neutron reflectometry reveals the intercalation scheme of lithiated thin film TiO 2 anatase in terms of phase boundary movement. The Li-rich lithium titanate phase progressively moves inside the TiO 2 anatase electrode as a front parallel to the interface. In contrast to previous suggestions for this system, the phase front moves back during lithium extraction exactly in the way it came in. The electrochemical side reactions result in a ∼5.5 nm-thick film on top of the TiO 2 electrode extending into the organic electrolyte which is believed to passivate the Li-intercalation.

R. van de Krol

Papers

Two Phase Morphology Limits Lithium Diffusion in TiO2 (Anatase): A 7Li MAS NMR Study

Mott-Schottky analysis of nanometer-scale thin-film anatase TiO2

Spatial Extent of Lithium Intercalation in Anatase TiO2

Electroceramics—the role of interfaces

Nano-morphology of lithiated thin film TiO2 anatase probed with in situ neutron reflectometry