Abstract
WS2 exfoliated by a combined ball milling and sonication technique to produce few-layer WS2 is characterized and assembled as chemo-resistive NO2, H2 and humidity sensors. Microstructural analyses reveal flakes with average dimensions of 110 nm, “aspect ratio” of lateral dimension to the thickness of 27. Due to spontaneous oxidation of exfoliated WS2 to amorphous WO3, films have been pre-annealed at 180 °C to stabilize WO3 content at ≈58%, as determined by X-ray Photoelectron Spectroscopy (XPS), Raman and grazing incidence X-ray Diffraction (XRD) techniques. Microstructural analysis repeated after one-year conditioning highlighted that amorphous WO3 concentration is stable, attesting the validity of the pre-annealing procedure. WS2 films were NO2, H2 and humidity tested at 150 °C operating Temperature (OT), exhibiting experimental detection limits of 200 ppb and 5 ppm to NO2 and H2 in dry air, respectively. Long-term stability of the electrical response recorded over one year of sustained conditions at 150 °C OT and different gases demonstrated good reproducibility of the electrical signal. The role played by WO3 and WS2 upon gas response has been addressed and a likely reaction gas-mechanism presented. Controlling the microstructure and surface oxidation of exfoliated Transition Metal Dichalcogenides (TMDs) represents a stepping-stone to assess the reproducibility and long-term response of TMDs monolayers in gas sensing applications.
Highlights
In recent years, layered materials such as two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a high level of interest due to their features, which make them appealing for potential applications in gas sensing [1,2], photo-electro-catalytic hydrogen evolution [3,4] optical and electronic devices [5,6] and energy storage [7,8].Few-layer 2D TMDs can be produced through different methodologies like mechanical and liquid phase exfoliation of bulk crystals, classified as top-down routes, or via direct bottom-up routes like chemical vapor deposition [9,10]
The two-step exfoliation followed by drop casting the centrifuged suspension leads to the deposition of thin films of well-packed and interconnected WS2 flakes with controlled and reproducible microstructure over large areas, representing a fast, simple and scalable method, compatible with standard microelectronic fabrication techniques
We found that a spontaneous oxidation of WS2 leading to the formation of amorphous WO3 on the surface of the exfoliated WS2 takes place, addressing the crucial drawback of surface oxidation of TMDs
Summary
In recent years, layered materials such as two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted a high level of interest due to their features, which make them appealing for potential applications in gas sensing [1,2], photo-electro-catalytic hydrogen evolution [3,4] optical and electronic devices [5,6] and energy storage [7,8].Few-layer 2D TMDs can be produced through different methodologies like mechanical and liquid phase exfoliation of bulk crystals, classified as top-down routes, or via direct bottom-up routes like chemical vapor deposition [9,10]. Low energy ball milling as a newly explored high-yield mechanical exfoliation method has been utilized for scalable production of mono and few-layer graphene [15,16] and TMDs nano-sheets [17,18]. We already presented the fabrication of chemo-resistive thin films gas sensor, by drop casting suspensions of few flakes graphene oxide [23], phosphorene [24] and more recently TMDs utilizing both liquid-exfoliated MoS2 [25] and commercially exfoliated suspensions of WS2 [26]. The aim of this work is to apply a low energy ball milling and sonication method to achieve a reproducible and high-yield exfoliation methodology and to test the gas sensing performances of the obtained material. We investigated the sensing responses to NO2, H2 and humidity of drop casted exfoliated WS2 chemo-resistive thin films, discussing the likely gas-response mechanism
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