Dielectric behaviour of adsorbed water. Part 1.—Measurement at room temperature on TiO2

Abstract

The dielectric behaviour in the Ti0,-H,O adsorption system has been investigated at frequencies from 0.1 Hz to 5 MHz at room temperature. A large dielectric dispersion has been found from 0.1 to lo4 Hz, which shifts to a higher frequency region when the coverage of physisorbed H,O increases. A simultaneous increase in conductance has been observed when the coverage increases. Thus, the dielectric relaxation observed at room temperature has been interpreted in terms of the interfacial polarization, which obeys the two-layer model and increases with increasing conductance. A mechanism is postulated for an enhancement of the conductance which takes place when H,O is adsorbed on TiO,.Recently, the properties of chemisorbed H,O, i.e. of surface hydroxyls, on metal oxides have been investigated as distinguished from those of physisorbed H20,1-4 and it has been clarified that the surface hydroxyls have significant effects on the surface properties of the s o l i d ~. ~-l ~ Dielectric studies provide important information on the structure of adsorbates and the strength of their binding on a solid surface.ll9 l 2 A number of investigators, using this technique, have dealt with different kinds of adsorbed H,Q, such as physisorbed H,O on solid s ~r f a c e s , ~~-l ~ capillary-condensed H,O in pores17-21 and interlayer H,O in crystalline minerals.,, The same technique was applied to the a-Fe,O,-H,O adsorption system at room temperature by McCafferty et u/.239 24 They observed a large dielectric dispersion in the low-frequency region and interpreted the phenomenon in terms of a Debye-type relaxation. Kurmakil7 also reported three kinds of relaxation for the adsorbed H,O on silica from dielectric measurements at 298 K. On the other hand, Kondo et al.25 observed three relaxation processes attributable to adsorbed H,O and surface hydroxyls on y-Al,03, from their measurements at temperatures from 77 to 390 K.The purpose of the present investigation is to clarify the dielectric behaviour of the Ti0,-H,O adsorption system and the effect of surface hydroxyls on the dielectric properties of the system. With TiO,, the properties of surface hydroxyls have been established by various techniques such as the adsorption isotherm,l? heat of immersion4~ 2 6 ~ 27 and i.r. spectra.' However, no investigation has been published on the dielectric properties of adsorbed H,O on TiO,, despite the practical importance of the substance not only for pigments but also for photocatalysts in H,O

Slow Debye-type peak observed in the dielectric response of polyalcohols

“…For all samples we measured the dielectric response with and without a Teflon film of 5 m thickness, placed between the sample and the upper electrode. The Teflon film was used in order to reduce the large contribution of conductivity and electrode polarization to the spectra at low frequencies ͑since this type of setup has been shown to reduce such effects 15 ͒. This method has to be used with caution 16 as this setup is a series circuit of two components, the sample and the insulating Teflon sheet.…”

The protein glass transition as measured by dielectric spectroscopy and differential scanning calorimetry

“…For the lower frequencies (10 − 2 -10 7 Hz) an Alfa-S High Resolution Dielectric Analyser was used, and the measurements in the higher frequency range (10 6 -10 9 Hz) were carried out using an Agilent 4291B RF Impedance Analyser. The samples were placed between two gold plated electrodes and a teflon film (thickness 100 μm), placed between the sample and the upper electrode, was used in order to reduce the large contribution of conductivity and electrode polarisation to the spectra at low frequencies (since this type of set-up has been shown to reduce such effects [30,31], even if care has to be taken when analysing the results [32]). The sample thickness was for all measurements 0.1 mm (determined by silica spacers), and the sample diameter was 20 mm and 10 mm for the low and high frequency measurements, respectively.…”

Reactive Ion Etching of Columnar Nanostructured ${\rm TiO}_{2}$ Thin Films for Modified Relative Humidity Sensor Response Time

“…The results presented here are for a 1 kHz probe signal, which imposes a fundamental limit on the measurement speed which will be possible. While the use of higher frequencies is possible, the dielectric constant of water decreases with frequency [23].…”