Titanium dioxide sensors

Figure 2. AFM measurement of an area (20 lm by 20 Lim) coated with titanium dioxide (sensor type A). Next to, but not onto, one of the electrode fingers. 

With type-B sensors, even small amounts of humidity (about 1 g(H20)/m (N2)) also caused a response (Fig. 7). This humidity effect on titanium dioxide sensors was to be expected and has been described in the literature [2]. 

Titanium dioxide exists in nature as three different polymorphs rutile, anatase and brookite. This material has been extensively studied during the last few decades due to its interesting physical properties and numerous technological applications. Rutile and anatase (a popular white pigment) are widely used in photocataly s and as sensors. Both of them have had new structural and electronic applications suggested recently (see for a review). 

Titanium dioxide supported gold catalysts exhibit excellent activity for CO oxidation even at temperatures as low as 90 K [1]. The key is the high dispersion of the nanostructured gold particles over the semiconducting Ti02 support. The potential applications of ambient temperature CO oxidation catalysts include air purifier, gas sensor and fuel cell [2]. This work investigates the effects of ozone pretreatment on the performance of Au/Ti02 for CO oxidation. 

In the near future, UV photodiodes made from polycrystalline wide band-gap semiconductors may fill the gap in the market. Although they have a lower sensitivity (photocurrent per area) they promise to have a better merit-rating in terms of photocurrent per sensor costs. The other major drawback of polycrystalline photodiodes, the risetime of micro- to milliseconds, is not relevant for household applications. Fuji Xerox Laboratories in Japan are developing visible-blind UV photodiodes made from polycrystalline GaN [12], while twlux AG in Berlin, Germany is developing visible-blind UV photodiodes made from polycrystalline titanium dioxide [13]. A prototype is shown in Fig. 5.45. 

This is a preliminary approach to the use of a new generation of solid-state sensors based on the capacity of the sensor element to catalyze the photodegradation of various kinds of organic compounds and to recognize their structure on the basis of the type of process catalyzed. The electron holes present in the Ti02 structure are able to promote the oxidative process of substances present in the environment, in particular the ones easily adsorbed on it. Titanium dioxide is a well-known photocatalyst [5-13]. Less famous are its characteristics as sensor material [14-18] of the ability of the organic molecules to be completely degraded, that is mineralized. 

Ti02 is an important oxide with a broad range of applications in catalysis (as a catalyst or a support) (6), photocatalysis (35, 36), and sensor technology it is also used as a pigment. Of the three titanium dioxide polymorphs (rutile, anatase, and brookite), rutile and anatase have been most widely investigated they are the only ones reviewed here. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Savage, N. O. Akbar, S. A. Dutta, P. K., Titanium dioxide based high temperature carbon monoxide selective sensor, Sens. Actuators B 2001, 72, 239-248... 

Other, less specific potentiometric immunoprobes are based on the antigen-induced potential shift of chemically modified semiconductor electrodes (Fig. 125) (Yamamoto et al., 1983). The surface of a titanium dioxide electrode is covered by a BrCN-activated polymer membrane and inserted into an antibody-containing solution. The antibody binds covalently to the activated electrode surface. The antigen to be determined is added when the potential difference between the sensor and a reference electrode is stable. As a result of the immunological reaction a new... 

Yamamoto et al. (1978) proposed the use of an antibody-coated titanium dioxide electrode to detect HCG in the urine of pregnant women. However, the measured potential of such sensors also depends on buffer composition, pH, and the ionic strength of the sample. Moreover, nonspecific adsorption of proteins at the transducer surface falsifies the measurement. This means that the accuracy of the analysis depends on the ratio of specific and nonspecific interactions. 

Zakrzewska found that titanium dioxide doped with Nb and Cr should be considered as a bulk sensor. Its performance was governed by the diffusion of point defects, i.e. very slow diffusion of Ti vacancies for Ti02. 9.5 at% of Nb and fast diffusion of oxygen vacancies in the case of HOt. 2.5 at% Cr sensor. The corresponding response times were 55 min for TiOT. 9.5 at% of Nb and 20 s for HOt. 2.5 at% Cr [292]. 

Taking into account its possible appHcations such as gas sensors, dielectric ceramics, and photocatalysts, titanium dioxide (Ti02) has been extensively studied, and it has been shown that titanium oxide as anatase phase can be synthesized at room temperature without use of any previous or further thermal or hydrothermal treatment, performing hydrolysis and polycondensation processes in saturated metal chloride solutions. 

Castaneda, L. (2007) Effects of palladium coatings on oxygen sensors of titanium dioxide thin films . Materials Science and Engineering B-Solid State Materials for Advanced Technology, 139(2-3), 149-154. 

Kunitake and coworkers have explored molecularly imprinted ultrathin titanium dioxide films. Employing a procedure of sequential chemisorption and activation onto quartz crystal microbalance (QCM) resonators, they have been able to produce stable films selective for azobenzene carboxylic acids [26] and protected amino acids [27]. In collaboration with Willner and co-workers, they have also used their synthesis techniques to functionalize ion-sensitive field-effect transistors (ISFET) with imprinted Ti02 films [28[. The resulting sensors displayed impressive selectivity in distinguishing two chloroaromatic acids. Additionally, Willner and coworkers [29] have used similar ISFET devices to generate enantioselective and enantiospecific sensors for 2-methylferrocene carboxylic acid, 2-phenylbutanoic acid, and 2-propanoic acid. 

Impedimetric titanium dioxide thin-film gas sensors... 

ABSTRACT Titanium dioxide, or TiO, is a well-known sensor material. We have investigated the impedance characteristics and surface roughness of two different types of devices involving TiOj-coated planar interdigital electrodes. One of them has a silicon dioxide (SiOj) insulation layer, the other one is built up without this layer. In this contribution, the response to selected hydrocarbons and humidity (at a temperature near 300 °C) and the influence of the substrate material will be discussed in some detail. 

Impedance spectroscopy titanium dioxide gas sensor humidity hydrocarbons ... 

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