Layered Amorphous a-SnO2 gas sensors by controlled oxidation of 2D-SnSe2

ABSTRACT 2D-layered amorphous a-SnO2 is utilized for the first time to detect NO2, H2 and NH3 gases at 100°C operating temperature opening new perspectives for the exploitation of amorphous metal-oxides interfaces for gas sensing applications. Liquid-phase exfoliated 2D-SnSe2 flakes have been subjected to controlled oxidation in air (from 1 h to 170 h) at temperatures below the crystallization temperature of SnO2 (200°C) yielding template self-assembled, amorphous a-SnO2 thin-layers (10 nm to 40 nm thick) grown over 2D-SnSe2. A suitable oxidation process has been optimized by “in operando” monitoring the base line resistance (BLR) while changing the annealing conditions, demonstrating that at least 48 h at 200°C are required to stabilize BLR, corresponding to the formation of a self-terminating a-SnO2 oxide with excellent BLR and sensor’s signal reproducibility over one year. Humidity cross-response on gas sensing demonstrated that increasing the relative humidity (RH@25°C) from dry air to 80%, (Rg/Ra) to 1 ppm NO2 increases from 2.2 to 5.7 while (Ra/Rg) to 100 ppm H2 and NH3 decreases from 2.7 to 1.7 and from 1.8 to 1.6 (σ ± 0.2). A preliminary ab-initio DFT modeling of water adsorption over amorphous a-SnO2 highlighted that dissociative adsorption is thermodynamically favored. The thin film sensor’s conduction model, comprising stacked layers of a-SnO2/SnSe2 flakes, is explained by the formation of Schottky barriers between the flakes. The seamless texture of amorphous a-SnO2 skin with no grain-boundaries, no crystal-planes orientations, greatly simplify gas sensing mechanisms assumptions, paving the way for a new class of “layered amorphous metal-oxides gas-sensors” (LAMOS).