Graphene-Based Gas Sensor Loaded with Lead-Free Perovskite Nanocrystals

Introduction Semiconducting devices based on graphene show outstanding potential due to their mechanical, electronic and chemical properties. However, the development of commercial graphene-based gas sensors is still challenging due to significant drawbacks that should be overcome. For instance, pristine graphene shows poor sensitivity and selectivity towards gas molecules, which makes necessary its modification/functionalisation. Sensing performance can be improved using different strategies such as the decoration of graphene with metal oxide nanoparticles. However, the use of metal oxides generally involves operating the sensor well above room temperature, thus increasing power consumption and reducing the lifetime of the sensor due to aging. Therefore, new research efforts have been focused towards the development of graphene loaded with alternative nanomaterials. Recently, perovskite nanocrystals have been identified as a promising option due to their superior sensitivity and selectivity, combined with their capability to work under room temperature conditions. However, the use of perovskites in gas sensing has been limited due to their degradation when exposed to ambient moisture, hindering its potential application under real conditions. Nevertheless, we demonstrated for the first time a stable gas sensor (over 6 months) based on graphene loaded with a lead halide perovskite (MAPbBr 3 ), even when operated under atmospheres with a significant level of moisture [1]. The reason is that the hydrophobic character of the graphene protects the perovskite nanocrystals against their degradation in humid environments. More recently, we studied the role of the anions and cations in the gas sensing mechanism towards volatile organic compounds (VOCs) [2]. Considering the perovskite formula (ABX 3 ), we synthesized several nanocrystals by changing the cation A (MA, FA and Cs) and the anion X (Br, I and Cl). However, the use of lead in these perovskites induces potential risks to human health and the environment. For that reason, graphene loaded with lead-free perovskites has been developed here to reduce the potential risks associated with the manipulation of lead and to assess the new sensing properties by replacing the cation B. Method Lead-free perovskite nanocrystals (Cs 3 Cu 2 Br 5 ) have been synthesized through a hot injection method. Afterwards, the graphene nanoflakes have been loaded with Cs 3 Cu 2 Br 5 via impregnation technique. Once a homogeneous perovskite distribution was achieved, the nanocomposite was deposited via spray pyrolysis onto alumina substrates with platinum screen-printed electrodes. The gas sensors were placed in an airtight testing chamber connected to an automated gas mixture and delivery system. The gas sensing properties were studied under dry and humid conditions. Characterization Several characterisation techniques were employed to analyse the nanomaterials synthesised. The crystalline structure of lead-free perovskite was evaluated through X-Ray Diffraction (XRD), while X-Ray Photoelectron Microscopy (XPS) was performed to graphene nanoflakes to analyse the carbon configuration (sp 2 and sp 3 ratio) and to study the presence of different oxygen functional groups. High-Resolution Transmission Electron Microscopy (HR-TEM) was used to assess size and interplanar distances in perovskite nanocrystals. Additionally, the spatial distribution of perovskites over the graphene nanoflakes was evaluated via a Field Emission Scanning Electron Microscope (FESEM). Finally, the sensing properties of the sensitive hybrid film have been assessed. Results and Conclusions The as-synthesized Cs 3 Cu 2 Br 5 perovskite nanocrystals show high crystallinity and an average size of a few nanometres. Furthermore, NO 2 detection at room temperature has been demonstrated to be sensitive and reproducible ( Figure 1 ). Preliminary measurements using this lead-free perovskite show responses to NO 2 about 3-fold higher than those registered for graphene loaded with MAPbBr 3 nanocrystals reported in [1]. This nanomaterial has been demonstrated as a feasible option to work at room temperature. Room temperature operation translates into low-power consumption and enhanced sensor lifetime, since high operating temperatures usually involve non-reversible changes in the crystalline structure of nanomaterials. Besides, the well-known instability of perovskites in the presence of relative humidity is overcome thanks to the protective character of graphene-based due to its high hydrophobicity. The combination of these two nanomaterials in the same composite allows taking advantage of the outstanding properties of both, graphene nanoflakes and perovskite nanocrystals. Therefore, gases can be detected at trace levels in a few minutes by using low-cost and low-power sensors. However, the use and manipulation of lead presents additional dangerousness, e.g. at the synthesis of perovskites or at disposal of sensors, which might put at risk human health and the environment. For that reason, the development of a graphene-based gas sensor loaded with lead-free perovskites constitutes a step forward to achieve more sustainable chemoresistors, linked to the concept of green chemistry. Furthermore, once the effect of the cation A and the anion X was assessed, this novel approach will provide a deep understanding about the role of cation B in the sensing mechanisms. References [1] 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. [2] J. Casanova-Cháfer, R. García-Aboal, P. Atienzar, E. Llobet, The role of anions and cations in the gas sensing mechanisms of graphene decorated with lead halide perovskite nanocrystals , Chem. Commun. 56 ( 2020 ) 8956-8959. DOI: 10.1039/D0CC02984J. Figure 1. Example of the resistance changes (black line, left-Y) registered when detecting NO 2 at ppb level (red line, right-Y) for a graphene sensor loaded with Cs 3 Cu 2 Br 5 perovskite nanocrystals. Sensor is operated at room temperature. Figure 1