Photo-Assisted Chemical Sensors Based on Porphyrins Coated ZnO

Hybrid materials formed by a layer of dye molecules over a wide-band gap semiconductor are endowed by a number of properties making them appealing for optoelectronics and photovoltaic applications. However besides the conversion of visible photons into an excess of carriers, the combination of organic-inorganic materials offer other additional properties such as gas sensing.

ZnO and porphyrins are good examples of a metal oxide semiconductor and a dye molecule that give rise to a hybrid material with interesting properties. ZnO has been studied for gas sensors, optoelectronics, electronic devices, photo-anodes for dye-sensitized solar cells, and for mechanical energy harvesting. Porphyrins play fundamental roles in living systems such as the oxygen transport and charge transfer for catylitic processes in animals and the photosynthesis in plants. The rich chemistry makes them interesting for chemical sensing, indeed porphyrins are among the most versatile ligand platforms that can interact with airborne molecules by a wide spectrum of different mechanisms. The sensing properties of porphyrins are exploited for the development of artificial sensorial systems. In our group we have been interested to study the interplay of light and gas sensitivity in porphyrins-ZnO materials. We found that the conductivity of porphyrins-ZnO is both sensitive to the visible light and to donor-acceptor gases such as amines and alcohols. Furthermore, under visible light the sensitivity increases and, most interestingly, the selectivity towards electron donor species (e.g. amines) strongly increases.

In further studies we measured the surface photovoltage. This quantity that can be easily measured in the laboratory with a Kelvin probe is exploited in organic functionalized Field Effect Transistors sensors and it is also of great importance in the characterization of materials for photovoltaic applications. Kelvin probe studies, illustrates the interplay between the photosensitivity and the gas sensitivity in organic molecule functionalized semiconductor metal oxide nanostructure suggesting that volatile compounds may act as a doping to improve the photovoltaic properties of the dye-semiconductor hybrid.

The sensing properties of layers of porphyrins onto ZnO nanostructures offer a further degree of freedom for the design of sensor arrays extending the properties and the capabilities of porphyrins based sensor array in particular for medical diagnosis, the quality and control of foods and the detection of compounds signalling harmful or dangerous substances.