Boosting the Oxide-based Chemoresistor Sensing Performances : the Role of Graphene Oxide and Porphyrins

The sensing of gas molecules is of fundamental importance for environmental monitoring, control of chemical processes, and so on [1]. Furthermore, recent success in non-invasive medical diagnostics, based on human’s breath analysis, is pushing forward the development of extremely sensitive gas sensors for ppb detection of specific analytes (e.g. acetone) [1,2]. In recent years, graphene-based gas sensors have attracted much attention and different structures have been developed showing high sensing performances [2]. However, they still suffer from several problems, which could be overcome by covering the graphene surface with metal oxides (MOS). Besides, thanks to the high chemical versatility, promising results could be also obtained by coupling porphyrin-based macrocycles to MOS nanoparticles. As such, boosted potentialities, especially in terms of tuned selectivity and low water interference, may be obtained. Therefore, the present work is aimed at evaluating and comparing the sensing performances at both mild temperatures (also exploiting the UV light) of SnO2 matrix coupled with two different porphyrins and graphene oxide (GO, in a SnO2/GO weight ratio of 32:1 [4]) materials towards the sensing of acetone molecules. Specifically, two zinc porphyrins have been adopted, namely zinc tetraphenylporphyrin (ZnTPP, inset of Fig. 1a) and a perfluorinated derivative of ZnTPP, possessing in β position a conjugated electron withdrawing linkers terminating in a cyanoacrylic group (ZnTPPF20-β-BDT-CN, namely ZnTPPF20, see inset of Fig. 1b). Figure 1 shows the sensing responses of the three hybrids obtained at 150 °C with and without the aim of UV irradiations. Notably, the sensor responses of ZnTPPF20 are about ten times more intense than those of ZnTPP@Sn and Sn32@GO, whose intensities are similar. The LOD is the same for all the samples, i.e. 600 ppb. By computing the response and recovery times, it can be stated that the former for the three hybrids is comparable, whereas the recovery time of SnO2–porphyrins are significantly longer. Switching the UV lamp on, the samples ability to sense acetone drastically changed. First, the LOD reached the 200 ppb for all the materials. Regarding the response intensities, ZnTPPF20 can guarantee the more intense sensor response, although all the composites showed similar values. As known by the numerous studies on dye sensitized solar cells, porphyrins can transfer photoelectrons towards the n-type MOS increasing the conductivity and decreasing the surface band bending [5]. The same mechanism can also be activated by an absorbed electron donor molecule. Under UV light, the highest occupied molecular orbital (HOMO) level of porphyrins is almost depleted of electrons and then their transfer from the absorbed molecule is expected to be more efficient.