Two-dimensional (2D) transition metal dichalcogenides (TMDs) and metal chalcogenides (MCs), despite their excellent gas sensing properties, are subjected to spontaneous oxidation in ambient air, negatively affecting the sensor’s signal reproducibility in the long run. Taking advantage of spontaneous oxidation, we synthesized fully amorphous a -SnO 2 2D flakes (≈30 nm thick) by annealing in air 2D SnSe 2 for two weeks at temperatures below the crystallization temperature of SnO 2 ( T < 280 °C). These engineered a -SnO 2 interfaces, preserving all the precursor’s 2D surface-to-volume features, are stable in dry/wet air up to 250 °C, with excellent baseline and sensor’s signal reproducibility to H 2 S (400 ppb to 1.5 ppm) and humidity (10–80% relative humidity (RH)) at 100 °C for one year. Specifically, by combined density functional theory and ab initio molecular dynamics, we demonstrated that H 2 S and H 2 O compete by dissociative chemisorption over the same a -SnO 2 adsorption sites, disclosing the humidity cross-response to H 2 S sensing. Tests confirmed that humidity decreases the baseline resistance, hampers the H 2 S sensor’s signal (i.e., relative response (RR) = R a / R g ), and increases the limit of detection (LOD). At 1 ppm, the H 2 S sensor’s signal decreases from an RR of 2.4 ± 0.1 at 0% RH to 1.9 ± 0.1 at 80% RH, while the LOD increases from 210 to 380 ppb. Utilizing a suitable thermal treatment, here, we report an amorphization procedure that can be easily extended to a large variety of TMDs and MCs, opening extraordinary applications for 2D layered amorphous metal oxide gas sensors.

Spontaneous degradation of 2D transition-metal dichalcogenides/chalcogenides (TMDs/MCs) gas sensors in dry/wet air represents one of the most significant drawback of these interfaces, hampering the reproducibility of the baseline resistance and sensor’s signal stability (i.e., sensor’s creep). Herein, we report a simple protection strategy stimulating the formation of a self-assembled oxide (a-MO x ) over TMDs/MCs, which promotes effective passivation of the underlying surface and excellent gas sensing response. Liquid-phase-exfoliated few-layers 2D-In2Se3 have been annealed in air at 180 °C for 24 h to yield an a-In2O3/In2Se3 heterostructure comprising a self-assembled a-In2O3 amorphous skin (5–10 nm) over 2D-crystalline In2Se3 (5–30 nm). The isomorphic conversion of In2Se3 into a-In2O3 specifically enables the layered shape of the precursor 2D-In2Se3 to be preserved after annealing, therefore providing all the surface-to-volume advantages of 2D interfaces. The excellent baseline and sensor’s signal reproducibility to H2 (5–100 ppm) and NO2 (400 ppb–1 ppm) after 1 year of delivery at 100 °C operating temperature demonstrated that the oxide skin effectively passivates the underlying 2D-In2Se3 from further oxidation. Significantly, the a-In2O3/In2Se3 heterostructure shows better H2 sensing response with respect to 2D TMDs/MCs sensors, with experimental detection limits as low as 5 ppm H2 and 400 ppb NO2, with associated RR (R a/R g) = 2.1 to 100 ppm H2 and RR (R g/R a) = 2.3 to 1 ppm NO2 in dry air. A charge carrier mechanism between the a-In2O3/In2Se3 heterostructure and H2, NO2, and H2O molecules is presented to discuss the humidity cross response to H2 and NO2. The passivation strategy here proposed can be extended to a large variety of TMDs/MCs, opening new perspectives for the effective exploitation of layered amorphous gas-sensing interfaces.