Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

https://link.springer.com/content/pdf/10.1007%2Fs10853-010-5113-0.pdf

Review of the anatase to rutile phase transformation

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces.

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

The effect of grain size on the sensitivity of chemoresistive nanocrystalline metal-oxide gas sensors was evaluated by calculating the effective carrier concentration as a function of the surface state density for a typical sensing material, SnO2, with different grain sizes between 5 and 80 nm. This involved numerical computation of the charge balance equation (the electroneutrality condition) using approximated analytical solutions of Poisson’s equation for small spherical crystallites. The calculations demonstrate a steep decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely, when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with ambient gas molecules, these calculations enable us to simulate the response curves of nanocrystalline gas sensors. The simulations show that the conductivity increases line...

The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors

The sensing behavior of thin TiO2 films exposed to air pulses was studied by means of electrical conductivity measurements. The TiO2 rutile films, with a grain size of ∼20–30 nm, were deposited on oxidized Si substrates by means of reactive sputtering and subsequently vacuum annealed (reduced) at 400°C. The films were exposed to air pulses at temperatures between 100°C and 325°C and their electrical conductivity was measured with embedded interdigital Au electrodes. When air was introduced into the system, the conductivity decreased and vice-versa. The conductivity changes during identical pulses were the same and reproducible, though the conductivity underwent some drift.

The kinetics of the conductivity response during a pulse of air followed a logarithmic law, typical of Elovich–Roginsky chemisorption kinetics, during the first stage of exposure and a modified parabolic law, typical of oxidation kinetics, afterwards. A two-stage model is proposed to describe the response kinetics which involves first surface processes where charge transfer to oxygen adsorbates dominates followed by oxidation of the pre-reduced nonstoichiometric TiO2−x film by diffusion of oxygen.

Sensing behavior of TiO2 thin films exposed to air at low temperatures

A computational method for numerical calculations of adsorption isotherms for both non-dissociative and dissociative chemisorption of gases on semiconductors is presented. The method enables calculating the equilibrium coverage of chemisorbed species and the chemisorption-induced potential barrier as a function of the ambient gas pressure, temperature, doping level, and the characteristic properties of the semiconductor/gas interaction. The computational method is applied for simulating the depletive chemisorption of oxygen on n-type SnO 2 . For both non-dissociative and dissociative chemisorption it is found that the chemisorption-induced potential barrier is proportional to the logarithm of the ambient oxygen pressure. This logarithmic relationship is important for device modeling of SnO 2 -based oxygen sensors since the sensor response, i.e. the change in the electrical conductivity, is related to the chemisorption-induced surface or intergranular potential barrier. The origin of this logarithmic relationship is attributed to the equilibration of the electrochemical potentials of chemisorbed oxygen adions and free oxygen molecules in the ambient gas phase.

Numerical computation of chemisorption isotherms for device modeling of semiconductor gas sensors

Electronic properties of reduced (vacuum-annealed) and oxidized (air-annealed) TiO2 films were investigated by in situ conductivity and current–voltage measurements as a function of the ambient oxygen pressure and temperature, and by ex situ surface photovoltage spectroscopy. The films were quite conductive in the reduced state but their resistance drastically increased upon exposure to air at 350 °C. In addition, the surface potential barrier was found to be much larger for the oxidized versus the reduced films. This behavior may be attributed to the formation of surface and grain boundary barriers due to electron trapping at interface states associated with chemisorbed oxygen species.

Y. Komem

Papers

The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors

Sensing behavior of TiO2 thin films exposed to air at low temperatures

Numerical computation of chemisorption isotherms for device modeling of semiconductor gas sensors

Electronic and transport properties of reduced and oxidized nanocrystalline TiO2 films

Surface photovoltage spectroscopy study of reduced and oxidized nanocrystalline TiO2 films