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

Surface modification of inorganic nanoparticles for development of organic–inorganic nanocomposites—A review

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

This review paper encompasses a detailed study of semiconductor metal oxide (SMO) gas sensors. It provides for a detailed comparison of SMO gas sensors with other gas sensors, especially for ammonia gas sensing. Different parameters which affect the performance (sensitivity, selectivity and stability) of SMO gas sensors are discussed here under. This paper also gives an insight about the dopant or impurity induced variations in the SMO materials used for gas sensing. It is concluded that dopants enhance the properties of SMOs for gas sensing applications by changing their microstructure and morphology, activation energy, electronic structure or band gap of the metal oxides. In some cases, dopants create defects in SMOs by generating oxygen vacancy or by forming solid solutions. These defects enhance the gas sensing properties. Different nanostructures (nanowires, nanotubes, heterojunctions), other than nanopowders have also been studied in this review. At the end, examples of SMOs are given to illustrate the potential use of different SMO materials for gas sensing.

Semiconductor metal oxide gas sensors: A review

Titanium Dioxide Nanomaterials for Sensor Applications

Nanostructured anatase TiO2 was produced by flame spray pyrolysis (FSP) and tested for sensing of volatile organic compounds and CO at 500 °C. The as-prepared powders were characterized by transmission/scanning electron microscopy, X-ray diffraction and nitrogen adsorption. Titania films about 30 μm thick on alumina substrates interdigitated with gold electrodes were prepared by drop-coating a heptanol suspension of these powders. The films showed a high signal of n-type sensor to isoprene, acetone and ethanol at concentrations ranging from 1 to 75 ppm in dry N2/O2 at 500 °C. The response (within seconds) and recovery (within minutes) times were very fast. Heat-treatment at 900 °C caused a nearly complete anatase to rutile transformation and a transition to p-type sensing behavior. That resulted in a poor sensor signal to all hydrocarbons tested and considerably longer recovery times than that of the anatase sensor. That rutile sensor could detect CO that the original, anatase sensor could not. For ethanol the sensor response changed back to n-type.

/pdf/sensing-of-organic-vapors-by-flame-made-tio2-nanoparticles-u6159j0yqe.pdf

Sensing of Organic Vapors by Flame-Made TiO2 Nanoparticles

The detection and monitoring of gases in exhaled human breath up to date has been limited by the lack of appropriate materials and technologies which could rapidly and selectively identify the presence and monitor the concentration of trace levels of specific analytes-biomarkers. We present a metal oxide-based nanosensor that is highly specific to ammonia gas in breath-simulating environments at low part-per-billion concentrations. The design of a handheld breath analyzer for gas detection in exhaled human breath is described. Semiconducting ceramics are presented as suitable sensor materials for easy and affordable noninvasive diagnostics.

/pdf/nanosensor-and-breath-analyzer-for-ammonia-detection-in-pb9eh6bnrz.pdf

Nanosensor and Breath Analyzer for Ammonia Detection in Exhaled Human Breath

Nanosensor Device for Breath Acetone Detection L. Wang1, K. Kalyanasundaram2, M. Stanacevic3, and P. Gouma4 ∗ 1Department of Electrical and Computer Engineering, The University of British Columbia, Vancouver, BC V6T1Z4, Canada 2Sensitron Semiconductor, Deer Park, NY 11729, USA 3Department of Electrical and Computer Engineering, SUNY Stony Brook, Stony Brook, NY 11794, USA 4Center for Nanomaterials and Sensor Development, SUNY Stony Brook, Stony Brook, NY 11794-2275, USA

/pdf/nanosensor-device-for-breath-acetone-detection-2nk3y6h7n3.pdf

Nanosensor Device for Breath Acetone Detection

Measurement of NO gas in exhaled human breath may be used to monitor oxidative stress and pulmonary diseases. Until now, only bulk, expensive, chemiluminescence-based NO monitors have been available to medicine. A nanosensing probe based on WO3 selectively detecting minute nitric oxide gas concentrations in the presence of interfering volatile compounds is presented. This is possible due to the chemical affinity of rhenium trioxide based phases to oxidizing gases. The NO nanoprobe is expected to lead to portable and affordable, noninvasive, single breath sampling, NO diagnostics.

A selective nanosensing probe for nitric oxide

Single-crystal MoO3 nanowires were produced using the electrospinning technique. High-resolution transmission electron microscopy revealed that the one-dimensional nanostructures are 10–50 nm in diameter, on the order of 1–2 μm in length, and have the orthorhombic MoO3 structure. The structure, crystallinity, and sensoric character of these electrostatically processed nanowires are discussed. It has been demonstrated that the nonwoven network of MoO3 nanowires exhibits an order of magnitude higher sensitivity compared with that of a sol-gel based sensor. This is promising for use of the nanowire sensors in nanomedicine.

Krithika Kalyanasundaram

Papers

Sensing of Organic Vapors by Flame-Made TiO2 Nanoparticles

Nanosensor and Breath Analyzer for Ammonia Detection in Exhaled Human Breath

Nanosensor Device for Breath Acetone Detection

A selective nanosensing probe for nitric oxide

Electrospun single-crystal MoO3 nanowires for biochemistry sensing probes