Metalloporphyrin-decorated semiconducting oxides for gas sensing applications

Abstract

The air quality and environmental problems have attracted more and more researchers’ attention in the past few decades. Since Taguchi released the first practical semiconductor gas sensor applied to the market, more and more researchers have invested in the research on the improvement and application of semiconductor gas sensors and a large number of modified tin oxide gas sensors with various gas sensing properties have been reported in the past few decades. This project has created new solid-state materials of metalloporphyrin-decorated metal oxides for gas sensing applications, directed at the measurement of nitrogen dioxide at low concentration in air. The hypothesis was that metalloporphyrins grafted onto the surface of tin dioxide could act as electronic surface states, such that changes in the energy or electron occupancy of the highest occupied molecular orbital of the porphyrin would be reflected in changes of the conductivity of the tin dioxide. Adsorption of nitrogen dioxide as an axial ligand of the porphyrin would then be signalled by a change in electrical conductivity. Tin dioxide doped with In(III) was successfully synthesised. This was a metastable material since In(III) is known to be insoluble in SnO2. Successful substitution of indium was signalled by a significantly decreased electrical conductivity , such that the effect of adsorption of surface donor states could be observed, and confirmed by X-ray photoelectron spectroscopy (XPS). Ruthenium(II)-carbonyl, platinum and palladium porphyrins having peripheral carboxylic acid groups were synthesised: the peripheral carboxylate being presumed, from literature studies, important to secure ‘flat-on’ adsorption of the porphyrin onto the oxide. All porphyrins and metalloporphyrins were fully characterized by 1H NMR, UV-Vis, IR and MS and the metal oxides fully characterized by powder XRD, SEM, EDS, XPS and BET adsorption. The increase in electrical conductivity of the oxide consequent on adsorption of the porphyrin confirmed the first hypothesis. Surface states within the band gap of the oxide, associated with the porphyrin, were detected by XPS. The Ru-CO porphyrin-decorated oxide was explored as a sensor for NO2 in air, at concentrations relevant to the study of urban air quality – up to a mixing ratio of 400 parts-per-billion (109) by volume. There was a significant signal to the presence of NO2 with the sensor heated at temperatures up to 150oC, confirming the second hypothesis. However, the porphyrin was rapidly and irreversibly decomposed at temperatures above 125oC, observable by the loss of the distinctive orange-red colour of the porphyrin and by changes in the XP valence band spectrum. Although the temperature instability would limit the practical application of this particular formulation, the successful confirmation of the hypotheses underpinning this work shows that the idea of engineering surface states of metal oxides in order to create specific gas sensors is workable and worthy of further exploration.