Elsevier

Applied Surface Science

Volume 529, 1 November 2020, 147110
Applied Surface Science

NO2 gas sensors based on CVD tungsten diselenide monolayer

https://doi.org/10.1016/j.apsusc.2020.147110Get rights and content

Highlights

  • Introduction of 2D Tungsten Diselenide (WSe2) monolayer for gas sensing.

  • Achieve fast response time of 18 s and recovery time of 38 s for 100 ppb of NO2.

  • Selective sensing towards NO2 within a wide range of concentrations from 0.1 to 5 ppm.

  • The sensor exhibits reliable long-term stability, reproducibility and reversibility.

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising for gas sensors owing to their large surface-to-volume ratios, but the long recovery times stemming from the slow molecular desorption have been an important limitation. In this work, we report on a sensor for nitrogen dioxide (NO2) made with a diselenide (WSe2) monolayer synthesized via chemical vapor deposition, which functions within a range of concentrations from 100 ppb to 5 ppm and at various temperatures, including room temperature. When operated at an optimized temperature of 250 °C, the WSe2 sensor exhibited the fast recovery, with the response (τres) and recovery (τrec) times of 18 s and 38 s, respectively, when exposed to 100 ppb NO2. The sensor also features high selectivity toward NO2 in the presence of other target gases, in addition to reversibility and long-term stability within 60 days. The overall characteristics of the WSe2 sensor make it suitable to monitor indoor/outdoor environments, mainly if further optimization is made with sensor design based on a model for the sensing mechanism we present in this paper.

Introduction

Human beings and animals are continually exposed to environmental hazards due to air pollution and emission from gas explosion accidents [1], [2], [3]. The exposure to toxic gases may lead to cancer and lung and heart diseases, whose early diagnosis requires gas sensing in health monitoring systems [4], [5]. Nitrogen dioxide (NO2) is one of the most hazardous polluting gases, coming mainly from exhaustion. Overexposure to NO2 has caused respiratory problems [6] as the NO2 gas molecules can easily cause inflammation of inner linings of lungs and reduce their immunity to infections, which is particularly harmful to people with asthma. Sensitive and selective NO2 gas sensors are therefore needed for monitoring indoor/outdoor environments to protect human health [7], which can, in principle, be exploited with the variety of materials employed in gas sensors. Selectivity and operation temperature are vital issues of the gas sensors. For example, metal oxides possess high sensitivity towards toxic gases, but their operating temperature and selectivity are still limited [8]. Graphene-based sensors, on the other hand, exhibit poor performance at low concentrations and low recovery speed [9]. Some strategies, including surface functionalization, designing heterostructures, fabrication of nanocomposite, and utilization of metal oxides, have been employed to improve the selectivity of a sensor [10], [11], [12].

Possible alternatives for efficient gas sensors are the two-dimensional (2D) monolayered materials, especially transition metal dichalcogenides (TMDs) with the general formula MX2 owing to their high surface area to volume ratios and susceptible large surfaces [13], [14]. These 2D TMDs have been synthesized directly on silicon or glass substrates by chemical vapor deposition (CVD) [15], [16], [17], [18], [19], [20], e.g., WS2, MoS2, MoSe2, and WSe2 were fabricated by CVD and atomic layer deposition (ALD) [21], [22], [23]. Recent works have proven that gas sensors made with 2D TMDs are sensitive to NO2 gas, including at room temperature [24], [25], [26], [27]. This has motivated us to institute a project and fabricate large-area and highly crystalline WSe2 monolayers using the vapor phase reaction of WO3 and Se carried by flowing gases in a tube furnace [28]. The rationale is to address the various factors that define the suitability of NO2 gas sensors, namely sensitivity, selectivity, response and recovery rate, and stability. We show that sensors made with WSe2 may be obtained with low detection limit, reproducibility, and reversibility in a wide range of NO2 concentrations with the fastest recovery for operation at an optimized temperature.

Section snippets

Synthesis

A single zone furnace shown in Fig. 1(a) was used for CVD growth of WSe2, with Se Powders (60 mg, 99.5%, Sigma-Aldrich) loaded outside the furnace at the upstream. When the furnace temperature reached 850 °C, the furnace was moved towards the upstream with a distance of 4 cm, where the temperature of Se powders reached 300 °C. WO3 powders (10 mg, 99.5%, Sigma-Aldrich) were loaded in the center of the furnace. The sapphire substrate was placed on top of the WO3 upside down. The sapphire

Results and discussion

Fig. 2(a) and (b) show the AFM image and height profile of WSe2 on a sapphire substrate, where the thickness is 0.73 nm, consistent with data for mechanically exfoliated monolayered WSe2 [28]. Fig. 2(c) shows the photoluminescence (PL) spectrum at room temperature taken with a micro-PL system (objective, 40X, NA 0.65) integrated with a 532 nm continuous wave (CW) laser, featuring a strong emission at ~ 760 nm assigned to the excitonic absorption. The Raman spectrum taken with excitation of a

Conclusion

In this work, we reported on a high-performance WSe2 gas sensor for NO2, which was synthesized with the CVD process and evaluated at room temperature and high temperatures. While the sensor was competitive with other chemiresistive sensors [13], [25], [49], [50] at room temperature, it exhibited the fastest recovery at an optimized temperature of 250 °C, also featuring fast recovery, reproducibility, and selectivity. The WSe2 sensor works within the detection range from 0.1 to 5 ppm, thus being

CRediT authorship contribution statement

Yichuan Wu: Writing - original draft, Methodology, Writing - review & editing. Nirav Joshi: Writing - original draft, Validation, Conceptualization, Writing - review & editing, Supervision, Investigation. Shilong Zhao: Methodology, Writing - original draft. Hu Long: Methodology, Software, Validation, Writing - review & editing. Liujiang Zhou: Writing - review & editing, Validation. Ge Ma: Writing - review & editing, Validation. Bei Peng: Writing - review & editing, Validation. Osvaldo N

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05-CH11231, within the sp2-Bonded Materials Program (KC2207), which provided for gas sensor response characterization. This work was also supported in part by the Berkeley Sensor and Actuator Center (BSAC), an Industry/University Research Cooperation Center. N.J was supported by the Sao Paulo

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