The influence of film morphology and illumination conditions on the sensitivity of porphyrins-coated ZnO nanorods.

ZnO and porphyrins have complementary properties that make their combination attractive for diverse applications such as photovoltaic and chemical sensing. Among the other features, the organic layer morphology is supposed to influence both the chemical sensitivity and the charge transfer processes. In this paper, we studied the influence of the film morphology on the sensing properties by comparing porphyrins coated ZnO nanorods obtained with two different methods. In the first approach, each porphyrin unit is grafted onto preformed ZnO nanorods by a carboxylic group as linker. The second method is a one-pot procedure, where ZnO nanorods growth occurs in the presence of the water soluble tetrakis-(4-sulfonatophenyl)porphyrin. In both cases the macrocycles share the same Zn-tetraphenylporphyrin core structure, but decorated with different peripheral groups, necessary to comply with the material growth conditions. The adsorption of volatile organic molecules has been monitored measuring the contact potential difference between the sensitive surface and a gold electrode, by means of a Kelvin probe setup. Sensitive signals have been measured both in dark and under visible light. The results show that material preparation affects both the sensitivities to gases and light. A chemometric analysis of four sensors (first and second growth method, measured in dark and in light) shows two main evidences: (a) the interaction between volatile compounds and the sensing layer is largely dominated by non-specific dispersion interaction and (b) the signal of the four sensors becomes rather uncorrelated when the contribution of the dispersion interaction is removed. These results indicate that the differences due to film morphology are enough to differentiate the sensor behaviour, even when the same porphyrin nucleus is used as sensing element. This feature provides an additional degree of freedom for the development of gas sensor arrays.