Colin L. Honeybourne

Novel composite organic–inorganic semiconductor sensors for the quantitative detection of target organic vapours

Composites of tin dioxide (an n-type semiconductor) and derivatives of the conducting polymer polypyrrole (a p-type semiconductor) gave reversible changes in electrical resistance at room temperature when exposed to a range of organic vapours. The optimum amount of polymer giving highest sensitivity was found by experiment to be 2.5% by mass for the polypyrrole chloride-tin dioxide composite. Composites containing 2.5% polymer by mass but differing in polymer derivative, were fabricated and exposed to low concentrations of ethanol, methanol, acetone, methyl acetate and ethyl acetate. All were found to give significant and reversible decreases in electrical resistance. Direct comparison with sensors constructed solely of tin dioxide or polypyrrole at room temperature showed the composites to be more sensitive. The gas sensitivity of the composite materials depended on the type of polymer derivative incorporated and the dopant anion associated with the polymer. The composites were simple to fabricate and gave differing response profiles to a range of organic vapours.

Solid thin films of extended π -systems: Deposition, characterisation and application

Abstract This review covers the deposition, characterisation and utilisation of thin films of highly conjugated organic polymers and macrocyclic ligands. The coverage is deliberately general rather than specialised, and key references are used instead of citing an exhaustive bibliography. The literature coverage is mainly confined to 1982–1985.

Use of thin films of conjugated organic macrocycles as the active elements in toxic-gas sensors operating at room temperature

A study of the piezoelectric and direct-current conductive properties of thin films of conjugated macrocyclic organic materials in the presence of industrial-pollutant gases is described. Particular attention is paid to the search for a reversible sensor for dinitrogen tetroxide that can operate in the range 10–1000 ppm at or near room temperature in an industrial atmosphere. Recommendations are given for a reversible system for detecting HCl and for a non-reversible system for the rapid detection of chlorine in the presence of electron-donor gases.

Oxidation state of the surface of palladium in the catalytic combustion of hydrogen

Abstract The activity of supported palladium catalysts for the oxidation of hydrogen (80-500°C, atmospheric pressure) has been investigated using a microcalorimetric technique and X-ray photoelectron spectroscopy. The reaction involved a surface layer of oxygen, and was inhibited by adsorbed water. Palladium (II) oxide was formed on the surface of metallic palladium under oxygen at 450°C. Heating in hydrogen at 250°C reduced this surface layer and produced deactivated catalysts. Activity was subsequently restored by exposure to oxygen at this temperature.

Semiconducting Tetrapyrrole Pigment Gas Sensors

A process of molecular tuning of the physicochemical properties of organic semiconductors is described, with special reference being made to natural tetrapyrrole pigments. The use of such organic semiconductors, in the form of thin films, is covered, with emphasis being placed upon relative performance characteristics. The view is expressed that chemical discrimination is possible.

Substituent and metal-ion complexation effects upon the gas-surface interactions of thin organic films

Abstract The dark d.c. surface currents are reported for thin films of organic semiconducting compounds when exposed to dinitrogen tetroxide, hydrogen chloride and chlorine. The magnitudes of the dark currents are a function of the π-electron bonding of the organic framework, and can also be varied (i) by altering the peripheral substituents or (ii) by metal-complex formation or (iii) by changing the physical characteristics of the films. The possibilities for gas-discriminatory analytical devices are discussed.

Charge distortion by sparkles can explain strong SHG by centrosymmetric squaraine dyes

Ashwell and his co-workers have published a report describing second harmonic generation (SHG) from a laser beam (Nd:YAG, 1.064 µm) by a Langmuir-Blodgett (LB) monolayer of a symmetrically substituted diaryl squaraine dye. The molecular geometry of the squaraine, and its unit cell is known to be centrosymmetric in the crystalline state. It has been stated that interfacial effects (arising from the twin-step function in the refractive index) cannot account for the magnitude of the observed SHG. It has also been assumed that molecular centrosymmetry has been retained in the LB film, with the observed SHG being attributed to the presence of non-centrosymmetric ensembles of these centrosymmetric molecules. We show that an electronically distorted (and therefore non-centrosymmetric) squaraine molecule can exhibit large SHG coefficients, these distortions being induced by the proximity of the highly polar species that constitute the molecular environment. In our model of a T-shaped dimer, both monomer components become non-centrosymmetric, the mutual interaction between the two monomers being modelled by using a negative or a positive sparkle. We attribute the noteworthy SHG output from LB films of Ashwells centrosymmetric squaraines to intramolecular charge transfer within the distorted monomers in the T-dimer.

The chemical degradation of gas-sensing meso-tetra-arylporphin thin films by high levels of dinitrogen tetroxide

Abstract Thin solid films of free base meso-tetra-arylporphins and their zinc, copper, platinum and palladium complexes have been exposed to high levels of dinitrogen tetroxide (NOX). The optical spectra (300–850 nm) were recorded in transmission before and after exposure to NOX. On the basis of the foregoing spectroscopic evidence, films resistant to permanent chemical damage by high levels of NOX (> 10,000 ppm) are identified. Reaction schemes are presented for those molecules that can be regenerated after chemical attack by NOX. The relevance of these observations to the selection of solid organic films suitable as industrial gas sensors is highlighted.

Langmuir–Blodgett multilayers of six compact porphyrin amphiphiles

By a modification of the Jackson–MacDonald condensation between two symmetrically substituted dipyrrylmethanes, we have synthesised two porphyrin free-base amphiphiles and four metalloporphyrin amphiphiles all of which serve as models for the naturally derived product mesoporphyrin IX dimethyl ester. These compounds have different substituents along their hydrophobic edge to those found in mesoporphyrin IX, which has the sequence Me, Et, Me, Et. Our materials have the sequences (Me)4 or (Et)4 thus making the hydrophobic edge slightly larger, or slightly smaller than that contained in mesoporphyrin IX.Upon an aqueous subphase, Langmuir films were obtained at pressures in the range 14–50 mN dm3 mol–1, with, at full compression, molecular areas of the order of 65 A2 and angles-of-tilt of the macrocyclic plane to the subphase of some 80°. LAXRD data were taken on Langmuir–Blodgett multilayer films of the (Et)4 compounds which proved to have the threshold minimum magnitude of hydrophobic edge for multilayer formation, with Y-type bilayers being given by the zinc and silver complexes.

Novel chromium(III)/(VI) adducts of XPS-determined mixed valence, from electroreduced chromium(VI)

SummaryConditions of chromium(VI) and acidity have been found at which a novel brown deposit from electroreduction. Its 1∶1 chromium(III):chromium(VI) composition, and the 2∶1 ratio of the alkali-leached product, were established by XPS. The composition bounds for electrodeposition of the 1∶1 solid and the competing insulative chromium(III) film were established electrochemically. Earlier reported chromium(III) and chromium(VI) solid or gels, some deposited from alkali, differ in colour and composition properties.