Anne Lepre

Characterisation of the photocatalyst Pilkington Activ (TM): a reference film photocatalyst?

Pilkington Glass Activ™ represents a possible suitable successor to P25 TiO2, especially as a benchmark photocatalyst film for comparing other photocatalyst or PSH self-cleaning films. Activ™ is a glass product with a clear, colourless, effectively invisible, photocatalytic coating of titania that also exhibits PSH. Although not as active as a film of P25 TiO2, Activ™ vastly superior mechanical stability, very reproducible activity and widespread commercial availability makes it highly attractive as a reference photocatalytic film. The photocatalytic and photo-induced superhydrophilitic (PSH) properties of Activ™ are studied in some detail and the results reported. Thus, the kinetics of stearic acid destruction (a 104 electron process) are zero order over the stearic acid range 4-129 monolayers and exhibit formal quantum efficiencies (FQE) of 0.7×10−5 and 10.2×10−5 molecules per photon when irradiated with light of 365±20 and 254 nm, respectively; the latter appears also to be the quantum yield for Activ™ at 254 nm. The kinetics of stearic acid destruction exhibit Langmuir-Hinshelwood-like saturation type kinetics as a function of oxygen partial pressure, with no destruction occurring in the absence of oxygen and the rate of destruction appearing the same in air and oxygen atmospheres. Further kinetic work revealed a Langmuir adsorption type constant for oxygen of 0.45±0.16 kPa−1 and an activation energy of 19±1 kJ mol−1. A study of the PSH properties of Activ™ reveals a high water contact angle (67°) before ultra-bandgap irradiation reduced to 0° after prolonged irradiation. The kinetics of PSH are similar to those reported by others for sol-gel films using a low level of UV light. The kinetics of contact angle recovery in the dark appear monophasic and different to the biphasic kinetics reported recently by others for sol-gel films [J. Phys. Chem. B 107 (2003) 1028]. Overall, Activ™ appears a very suitable reference material for semiconductor film photocatalysis.

Photodecomposition of ozone sensitised by a film of titanium dioxide on glass

High levels of ozone (typically 850 ppm) are readily decomposed by semiconductor photocatalysis, using a thin film of the semiconductor titanium dioxide (Degussa P25 TiO2) cast on a glass tube, and UVA light, i.e. light of energy greater than that of the bandgap of the semiconductor (ultra-bandgap light); in the absence of this light the thermal decomposition of ozone is relatively slow. The semiconductor films show no evidence of chemical or photochemical wear with repeated use. At high levels of ozone, i.e. 100 ppm less than or equal to [O-3] less than or equal to 1400 ppm, the initial rate of ozone decomposition by semiconductor photocatalysis is independent of [O-3], whereas, at lower ozone concentrations, i.e. 5 ppm less than or equal to [O-3] less than or equal to 100 ppm, the initial rate of ozone photodestruction decreases in a smooth, but non-linear, manner with decreasing [O-3]. The kinetics of ozone photodecomposition fit a Langmuir-Hinshelwood type kinetic equation and the possible mechanistic implications of these results are briefly discussed.

Breath-by-breath measurement of carbon dioxide using a plastic film optical sensor

An optical sensor has been developed for breath-by-breath gaseous CO2 analysis. The detector is based on a general formulation described in previous work where a phase-transfer agent, tetraoctyl ammonium hydroxide, is used to incorporate a hydrophilic pH-sensitive dye into a hydrophobic plastic film to create an effectively solid-state colorimetric sensor. In this work the formulation has been modified to yield a sensor which is capable of responding to clinically important levels of CO2 (0.1–5%) in less than 200 ms. This is comparable with the response of commercially available capnometers based on infrared sensing of CO2, which are currently widely used for clinical analysis. These films have good stability under the hot humid conditions involved in respiratory gas monitoring (37°C gas temperature, 100% humidity) and, when stored in a freezer, show no deterioration in performance over an 80 day period.

Effect of plasticizer-polymer compatibility on the response characteristics of optical thin CO2 and O2 sensing films

Abstract A homologous family of dialkyl phthalates has been used to investigate the effect of plasticizer/polymer compatibility on the response characteristics of transparent, plastic, thin optical gas sensing films for CO 2 and oxygen. Plasticizer/polymer compatibilities were determined through the value of the difference in solubility parameter, i.e. Δ δ , for the plasticizer and polymer with a Δ δ value of zero indicating high compatibility. A strong correlation was found between plasticizer/polymer compatibility and sensitivity in phenol red/ethyl cellulose CO 2 -sensitive films and this relationship extended to CO 2 -sensitive films based on other polymers such as polystyrene and poly(methyl methacrylate). It extended also to optical O 2 -sensitive films implying that the relationship is general for thin-film optical sensors. Other results from the CO 2 -sensitive films in different polymers indicated that the film sensitivity is largely independent of the polymer matrix regardless of its inherent gas permeability, when a sufficient quantity of compatible plasticizer is present.

Use of luminescent gold compounds in the design of thin-film oxygen sensors

The use of two gold compounds incorporated into thin plastic films as luminescence quenching oxygen sensors is described. The films are sensitive both to gaseous oxygen and to oxygen dissolved in nonaqueous media such as ethanol. The luminescence quenching of these sensors by oxygen obeys the Stern−Volmer equation and Stern−Volmer constants of 5.35 × 10-3 and 0.9 × 10-3 Torr-1 are found, respectively, for the two dyes in a polystyrene polymer matrix. The sensitivity of the films is strongly influenced by the nature of the polymer matrix, and greatest sensitivity was found in systems based on the polymers polystyrene or cellulose acetate butyrate. Sensitivity was not found to be temperature dependent though raising the temperature from 15 to 50 °C did result in a slight decrease in emission intensity and a hypsochromic shift in the emission wavelength. The rate of response and recovery of the sensors can be increased either by decreasing film thickness or by increasing the operating temperature. The operat...

Development of novel thermochromic plastic films for optical temperature sensing

The preparation of plastic film optical ‘CO2-based’ temperature-sensing films that utilise the temperature-dependent acid–base equilibria of indicator dyes is described. In film formulations a suitable dye, such as phenolphthalein, in a hydrophobic base, tetraoctylammonium hydroxide, solubilised within a plasticised, hydrophobic polymer matrix, creates a system which is sensitive to ambient CO2 levels. The resultant solution, when cast on to glass supports, yields ‘CO2-based’ temperature-sensitive films which change colour in response to a highly temperature-dependent reaction between the deprotonated form of the dye and CO2 dissolved in the film. The absorbance characteristics of these films display a fully reversible response to temperature over a temperature range which is largely determined by the pKa of the dye and the ambient CO2 concentration. The magnitude of the response is dependent on the dye concentration. The response time towards changes in temperature is typically ⩽2.2 min and the films show good stability under operational conditions. A simple mechanism of the reaction is suggested and an associated working equation has been derived and fitted to data obtained for a typical sensor functioning over the range 278–333 K. A ‘CO2-based’ temperature-sensing film is used successfully alongside a standard CO2 sensing film. This combination not only provides temperature information but also ensures that the response of the CO2 sensor is corrected for any changes in temperature. In addition, both sensors use the same interrogating light and light intensity monitoring system because they contain the same phenolphthalein dye. The latter two features represent an improvement on the existing optical systems used to measure CO2 and temperature.

Luminescent Gold Compounds in Optical Oxygen Sensors

The use of luminescent gold(I) phosphine complexes as oxygen sensors is reported. Room temperature phosphorescent-based sensors have advantages over those based on fluorescence, and they have significant potential for development in clinical applications.