Two-Step Exfoliation of WS2 for NO2, H2 and Humidity Sensing Applications

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.

Keywords: 2D-materials; H2; NO2; WS2; cross sensitivity; exfoliation; gas sensors.

Figure 1

Comparison of the effect of long-time ball milling on WS₂ exfoliation: (a) Comparison of the particle size distribution of the starting WS₂ commercial powder (blue), 72 h ball milled (red) and 72 h ball milled and 90 min sonicated (green), (b) AFM picture of 72 h ball milled and 90 min sonicated and associated thickness profile along the white line, (c) TEM picture of the 72 h ball milled and 90 min sonicated WS₂, (d) Comparison of the particle size distribution in case of 2 h ball milling, (e) AFM picture of 2 h ball milled and 90 min sonicated and associated thickness profile, (f) TEM picture of the flakes obtained by 2 h ball milling and 90 min sonication of WS₂.

Figure 2

(a) AFM picture of the 2 h ball milled and 90 min sonicated WS₂, (b) thickness profile of the stacked flake along the white line of Figure (a), (c) low-resolution TEM picture of an exfoliated flake.

Figure 3

(a,c) TEM of 2 h ball milled and 90 min sonicated WS₂, (b) HRTEM corresponding to the circled area shown in figure (a) with highlighted the interlayer distance (0.63 nm) and lattice spacing (0.27 nm and 0.25 nm), corresponding to (100) and (101) planes of WS₂ respectively, (d) HRTEM of the edge of the flake corresponding to the circled area shown in figure (c) with highlighted the 7 nm thick edge corresponding to 11 layers. The inset shows the Selected Area Electron Diffraction (SAED) of the flake.

Figure 4

(a–d) AFM images of WS₂ exfoliated corresponding to four different samples prepared under the same conditions (i.e., 2 h ball milling and 90 min sonication). Statistical analysis corresponding to thickness distribution (e), Lateral dimensions (f) and surface area coverage (g).

Figure 5

X-Ray Photoemission Spectroscopy (XPS) spectra of W 4f core level acquired respectively on (a) pristine WS₂ commercial powder (WS₂ PWD), (b) Exfoliated WS₂ by ball milling and sonication at 25 °C, (c) WS₂ exfoliated and post-annealed at 180 °C. All the components and their relative atomic percentages are labelled in the figure.

Figure 6

Raman spectra of WS₂ bulk powder, WS₂ as-exfoliated and WS₂ flakes post-annealed at 180 °C.

Figure 7

SEM image of sensor obtained by drop casting exfoliated WS₂ and annealing at 180 °C on Si₃N₄ substrate provided with Pt finger-type electrodes (30 microns apart).

Figure 8

Electrical responses of the exfoliated WS₂ post-annealed at 180 °C, at 150 °C operating temperature in dry air. (a) Comparison of the normalized dynamic response to NO₂ (100 ppb–5 ppm) and H₂ (1–250 ppm), (b) NO₂ cross-sensitivity to H₂: first panel, the response to 120 ppm H₂ in dry air, second panel, response to 120 ppm H₂ with 600 ppb NO₂, third panel, response to 120 ppm H₂ (as to first panel) for comparison, (c) Reproducibility and baseline recovery by exposing the film to both pulse and cumulative H₂ concentrations in the range 40–100 ppm. H₂ concentrations are highlighted in the figure by grey shadowed rectangular plots.

Figure 9

WS₂ exfoliated and post-annealed at 180 °C. (a) Long-term stability properties of the electrical resistances of the baseline (lower curve) and 800 ppb NO₂ over a period of 12 months (equivalent to approximately 5 months of continuous operation at 150 °C operating temperature). Average resistance values with associated standard deviations are calculated over a set of 5 consecutive measurements. (b) Comparison of the XPS signals of the as-exfoliated WS₂ annealed at 180 °C (lower curve) and the same sample after one-year conditioning to various gases and 150 °C operating temperature.

Figure 10

Electrical responses of the exfoliated WS₂ post-annealed at 180 °C (150 °C operating temperature) to different Relative Humidity (RH) conditions. (a) Normalized dynamic response to humidity (10–80% RH). The inset depicts the corresponding sensitivity plot. (b) Dynamic response to increasing NO₂ concentrations in air with 40% RH, (c) Dynamic response to increasing H₂ concentrations in in air with 40% RH, (d) Comparison of the sensitivity plots to NO₂ and H₂ in dry air and 40% RH, respectively.

Figure 11

HRTEM images of the WS₂ film pre-annealed at 180 °C. (b) Magnification of the yellow area of Figure 11 (a) displaying the presence of ordered structures (i.e., inside the red square) referred to crystalline WS₂ and the presence of disordered ones (i.e., inside the green square) attributed to amorphous WO₃. Related Selected Area Electron Diffraction (SAED) patterns are shown in (c) and (d), highlighting the occurrence of sharper reflections (d) associated to crystalline WS₂.

Figure 12

Chemical composition, crystalline structure and microstructural features of a fully oxidized WO₃ thin film. (a) W 4f core level XPS spectra, (b) XRD grazing incidence spectra. Top right inset shows the close up of the 2θ region characteristic of crystalline WO₃ (corresponding peaks of crystalline WO₃, according to ICDS 98-001-7003, are highlighted by dashed green lines), (c) electrical response of the fully oxidized WO₃ amorphous film to NO₂ and different OTs.