VOCs Detection Employing Graphene Loaded with Perovskite Nanocrystals
Authors: Casanova-Chafer Juan , Garcia-Aboal Rocio , Atienzar Pedro , Llobet Eduard 
Published: May 2020

Introduction

Despite graphene shows outstanding electronic and physicochemical properties to be employed as a gas sensor, its implementation in commercial devices still faces many issues. The main drawbacks that are limiting real applications are the low sensitivity and selectivity towards gas molecules of graphene in its pristine form. In consequence, further modifications are needed to enhance these essential gas-sensing parameters. One of the easiest and most effective strategies is the decoration of graphene (or other carbon nanomaterials) with metal or metal-oxide nanoparticles [1], an approach that has been extensively studied by many researchers.
In contrast, perovskites are promising materials that despite showing rich surface chemistry have attracted limited research interest for gas sensing until now. Probably because of their inherent problems, such as high degradation in contact with ambient moisture or their stability problems even at moderate working temperatures. Nevertheless, we recently reported the first use of graphene loaded with perovskite nanocrystals [2] to detect NO2 and NH3. Our approach has been demonstrated as a novel option to employ perovskites in dry and humid air, instead of their use in inert atmospheres by employing nitrogen without ambient moisture [3]. Here we deepen our study by analyzing, for the first time, a wide range of perovskite-graphene hybrids for gas sensing.

Methods

Chemical-sensitive films composed by graphene nanolayers loaded with different perovskite nanocrystals are developed. In particular, five different perovskite nanocrystals are synthesized and studied. Considering the general formula of perovskites (ABX3
), we started with the perovskite nanocrystal already employed as a gas sensor in our previous work, which is MAPbBr3 (1). Thus, as our main objective is to analyze the effect on the gas sensing properties of the different elements in perovskites, first we change the cation A, obtaining two different perovskites with caesium (CsPbBr3) (2) and formamidinium (FAPbBr3) (3). After that, two new nanocrystals are obtained by changing the anion X, replacing the bromide by iodine (MAPbI3) (4) and chlorine (MAPbCl3) (5). Different synthesis processes are performed to obtain the five nanocrystals, but all of them show diameters below to 10 nm, which are found suitable for the decoration of graphene.

Characterization

All the nanomaterials employed in this work are characterized in depth employing techniques such as X-Ray Photoelectron Microscopy (XPS) to obtain detailed information about the graphene functional groups; High-Resolution Transmission Electron Microscopy (HR-TEM) to analyze the nanocrystal size and its interplanar distances; Field Emission Scanning Electron Microscope (FESEM) to evaluate the spatial nanocrystal distribution over the graphene nanolayers; and finally, an X-Ray Diffraction (XRD) analysis to study the crystalline structure of perovskites. Afterwards, the films developed are integrated in gas sensing devices, which are placed in an airtight Teflon chamber connected to calibrated bottles of nitrogen dioxide, ammonia, benzene and toluene. Consecutive gas dilutions are performed in order to obtain different analyte concentrations. With these results, the effect of the cation and anion in the detection of pollutant gases and vapors was studied at low concentrations. Full details will be given at the conference.

Results and Conclusions

The resulting hybrid nanomaterials, i.e. graphene loaded with perovskite nanocrystals have been demonstrated as great options for room-temperature operated gas sensing, entailing to low-power consumption devices. Additionally, the high instability of perovskites over time is clearly ameliorated here, thanks to the highly hydrophobic character of graphene, which protects the nanocrystals from the effect of ambient moisture. As a result, we can take advantage of the outstanding perovskite properties by increasing the stability and the sensor shelf-life. Also, sensitive, reproducible, reversible and remarkably stable responses to nitrogen dioxide, ammonia and VOCs can be obtained (see Figure 1) for sensors operated room temperature. Even, higher sensitivity can be obtained to both, electron donor and electron-withdrawing gases, thanks the presence of perovskite nanoparticles in comparison to their bare graphene counterparts. The reason probably is because perovskites can act as ambipolar charge transporters. These results will allow a better understanding about the role in the sensing mechanism of the different perovskite components, and constitutes a novel approach to employ perovskites in gas sensors, showing a high potential to improve the performance of other usual graphene decorations. For instance, despite the good results reported by loading graphene with metal oxides, usually these are associated with high cost (derived from the use of noble metals) and high working temperatures. Conversely, the decoration of graphene with perovskite nanocrystals opens a new low-cost production concept linked to low-power consumption devices.

References

[1] J. Casanova-Cháfer, E. Navarrete, X. Noirfalise, P. Umek, C. Bittencourt, E. Llobet, Gas Sensing with Iridium Oxide Nanoparticle Decorated Carbon Nanotubes, Sensors. 19 (2019) 113. doi:10.3390/s19010113.
[2] J. Casanova-Cháfer, R. García-Aboal, P. Atienzar, E. Llobet, Gas Sensing Properties of Perovskite Decorated Graphene at Room Temperature, Sensors. 19 (2019) 4563. doi:10.3390/s19204563.
[3] C. Bao, J. Yang, W. Zhu, X. Zhou, H. Gao, F. Li, G. Fu, T. Yu, Z. Zou, A resistance change effect in perovskite CH3NH3PbI3 films induced by ammonia, Chem. Commun. 51 (2015) 15426–15429. doi:10.1039/c5cc06060e.

Figure 1