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

Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.

https://science.sciencemag.org/content/sci/291/5510/1947.full.pdf

Nanobelts of Semiconducting Oxides

Size- and support-dependency in the catalysis of gold

Overview of membrane science and technology membrane transport theory membrane and modules concentration polarization reverse osmosis ultrafiltration microfiltration gas separation pervaporation ion exchange membrane processes - electrodialysis carrier facilitated transport medical applications of membranes other membranes processed.

Membrane Technology and Applications

Energy production and storage have become key issues concerning our welfare in daily life. Present challenges for batteries are twofold. In the first place, the increasing demand for powering systems of portable electronic devices and zero-emission vehicles stimulates research towards high energy and high voltage systems. In the second place, low cost batteries are required in order to advance towards smart electric grids that integrate discontinuous energy flow from renewable sources, optimizing the performance of clean energy sources. Na-ion batteries can be the key for the second point, because of the huge availability of sodium, its low price and the similarity of both Li and Na insertion chemistries. In spite of the lower energy density and voltage of Na-ion based technologies, they can be focused on applications where the weight and footprint requirement is less drastic, such as electrical grid storage. Much work has to be done in the field of Na-ion in order to catch up with Li-ion technology. Cathodic and anodic materials must be optimized, and new electrolytes will be the key point for Na-ion success. This review will gather the up-to-date knowledge about Na-ion battery materials, with the aim of providing a wide view of the systems that have already been explored and a starting point for the new research on this battery technology.

Na-ion batteries, recent advances and present challenges to become low cost energy storage systems

It is demonstrated that the sensing characteristics of a semiconductor gas sensor using SnO2 can be improved by controlling fundamental factors which affect its receptor and transducer functions. The transducer function is deeply related with the microstructure of the elements, i.e., the grain size of SnO2 (D) and the depth of the surface space-charge layer (L). The sensitivity is drastically promoted when D is made comparable to or less than 2L, either by control of D for pure SnO2 elements or by control of the Debye length for impurity-doped elements. On the other hand, the receptor function is drastically modified by the introduction of foreign receptors on the surface of SnO2. In the particular cases of Pd and Ag promoters, the oxides (PdO and Ag2O) formed in air interact with the SnO2 surface to produce an electron-deficient space-charge layer, and this contributes much to promoting the gas sensitivity. For a test gas having a specific reactivity, such specificity can be utilized for exploiting gas-selective receptors, as exemplified by CuOSnO2 and La2O3SnO2 elements, which detect H2S and ethanol gas respectively very sensitively.

New approaches for improving semiconductor gas sensors

Effects of grain size on gas sensitivity (S) are investigated by using porous sintered SnO2 elements fabricated with pure SnO2, foreign oxide-stabilized SnO2, or impurity-doped SnO2. When the SnO2 crystallite size (D) is controlled to a size in the range 5–32 nm, S for H2, CO and i-C4H10 is found to increase steeply as D decreases to be comparable with or less than 2L (≈ 6 nm) is both pure and stabilized elements, where L is the depth of the space-charge layer. However, S for ethyl alcohol gas is found to be also affected by surface acid-base properties, being greatly promoted by basic oxides. It is found that the control of L by doping impurities (Al3+ or Sb5+) into the SnO2 lattice results in great changes in S even when D is the same. Thus Al-doped SnO2 shows high sensitivity with increasing L even at D above 20 nm, while Sb-doped SnO2 is insensitive in the whole D region. A model for the grain-size effects is proposed, in which the transducer function is operated by a mechanism of grain control, neck control or grain-boundary control, depending on D.

Grain size effects on gas sensitivity of porous SnO2-based elements

Semiconductor gas sensors utilize porous polycrystalline resistors made of semiconducting oxides. The working principle involves the receptor function played by the surface of each oxide grain and the transducer function played by each grain boundary. In addition, the utility factor of the sensing body also takes part in determining the gas response. Therefore, the concepts of sensor design are determined by considering each of these three key factors. The requirements are selection of a base oxide with high mobility of conduction electrons and satisfactory stability (transducer function), selection of a foreign receptor which enhances surface reactions or adsorption of target gas (receptor function), and fabrication of a highly porous, thin sensing body (utility factor). Recent progress in sensor design based on these factors is described.

Oxide semiconductor gas sensors

Although gas sensors have been almost matured in some application fields, there are a variety of newly emerging markets and potential markets which will be substantiated when gas sensors are innovated sufficiently. The importance of materials design in innovating gas sensors are demonstrated by taking semiconductor gas sensors and solid electrolyte gas sensors as examples. In addition, attempts to make the sensor devices more intelligent and more quantitative are also important for further advancements of gas sensor technology.

https://www.uta.edu/rfmems/BMC/0720/0902_backup/Backup_0729/Background/Toward%20innovations%20of%20gas%20sensor%20technology.pdf

Toward innovations of gas sensor technology

The rate of oxygen permeation through La1−xSrxCo1−yFeyO3−δ was found to increase with an increase in Sr or Co content, showing that the permeability was mainly controlled by the amount of oxygen vacancies. The results obtained indicate that mixed conductive perovskite-type oxides are promising materials for oxygen permeation at elevated temperatures.

Noboru Yamazoe

Papers

New approaches for improving semiconductor gas sensors

Grain size effects on gas sensitivity of porous SnO2-based elements

Oxide semiconductor gas sensors

Toward innovations of gas sensor technology

Oxygen permeation through perovskite-type oxides