Research Paper
Edge-enriched WS2 nanosheets on carbon nanofibers boosts NO2 detection at room temperature

https://doi.org/10.1016/j.jhazmat.2021.125120Get rights and content

Highlights

  • WS2 nanosheets are anchored on carbon nanofibers (CNFs) by hydrothermal method.

  • WS2@CNFs composites have abundant exposed WS2 edges.

  • Gas sensor based on WS2@CNFs exhibits remarkable NO2 sensitivity at room temperature.

  • Density functional theory (DFT) calculations verify strong NO2 adsorption on the WS2 edges.

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) hold great promise for room temperature (RT) NO2 sensors. However, the exposure of the edges of TMDs with high adsorption capability and electronic activity remains a great obstacle to achieve high sensor sensitivity. Herein, we demonstrate a high-performance RT NO2 gas sensor based on WS2 nanosheets/carbon nanofibers (CNFs) composite with abundant intentionally exposed WS2 edges. Few-layer WS2 nanosheets are anchored on CNFs through a hydrothermal process. The approach permits to achieve a coating presenting an optimized active surface area and accessibility of the sensing layers. The exposure of WS2 edges remarkably improves the sensing properties. Consequently, the WS2@CNFs composite exhibits excellent selectivity to NO2 at RT with improved response and much lower detection limit in comparison to the WS2 and CNFs counterparts. Density functional theory (DFT) calculations verify a surprisingly strong NO2 adsorption on WS2 edge sites (adsorption energy 3.40 eV) with a partial charge transfer of 0.394e, while a week adsorption on the basal surface of WS2 (adsorption energy 0.25 eV) with a partial charge transfer of 0.171e. The strategy proposed herein will be instructive to the design of efficient material structures for low-power NO2 sensors with optimized performances.

Introduction

With the emergence of the Internet of Things (IoT), the demand for various high-performance sensors is dramatically increasing (Swan, 2012, Yuan et al., 2019, Hancke et al., 2013, Chen et al., 2020). In particular, metal oxide semiconductor (MOS) gas sensors have attracted enormous attention for detection of toxic and explosive molecules because of their easy manufacture, low cost, high sensitivity, high stability, fast response speed and simple sensing mechanism (Walker et al., 2019, Xu et al., 2018, Cho et al., 2020). However, MOS sensors usually work at high temperature (200–450 °C) with high power consumption for heating (Xu et al., 2020a, Kumar et al., 2020, Ikram et al., 2019), which is not compatible with applications in IoT and sensor networks requiring low power consumption (Zhang et al., 2016, Kim et al., 2020, Majhi et al., 2021). Room temperature (RT) gas sensors have recently attracted enormous interests due to the zero-power consumption that does not require heating (Kumar et al., 2020, Zhang et al., 2016, Xu et al., 2020b), however the relatively low molecular activity on MOS under RT often leads to low sensor response (Wang et al., 2020).

Due to the versatile semiconducting properties and graphene-like structures (Zhang et al., 2019, Zhu et al., 2019, Wang et al., 2019), two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 and WS2 are widely studied in diverse fields, including gas sensor (Cha et al., 2017, Koo et al., 2018, Liu et al., 2017, Zheng et al., 2020). In particular, 2D TMDs are promising candidates as sensing layers for gas sensors due to their vulnerable change of carrier concentration on adsorption of gaseous molecules (Anichini et al., 2018). Among the many harmful gases, NO2 is the most common gas in chemical synthesis and industrial production, which may cause serious respiratory diseases when the NO2 concentration exceeds 1 ppm in atmosphere (Ikram et al., 2019). The United States Environmental Protection Agency (U.S. EPA) has formulated an exposure limit of 53 ppb (Zhou et al., 2019, Pham et al., 2019, U.S.E.P. Agency, Air Trends Summary Report, 〈https://www3.epa.gov/ttn/naaqs/standards/nox/s_nox_history.html#3〉.). In addition, NO2 gas can cause great damage to the environment, such as acid rain (Ou et al., 2015).

Although 2D TMDs gas sensors have been applied to detect NH3, NO2, as well as H2S (Tang et al., 2019, Li et al., 2017, Mayorga-Martinez et al., 2015, Kim et al., 2018), the role played by the basal surface and edges of TMDs is still of great concern in gas sensing. Cho and co-workers (Cho et al., 2015) revealed that the vertically aligned MoS2 exhibited better NO2 sensing properties than horizontally aligned MoS2 nanosheets, due to the low molecular adsorption energy on TMDs edge sites (Choi et al., 2017, Ko et al., 2016, Kim et al., 2019, Shim et al., 2018). In an attempt to increase the exposure of edges of TMDs to improve the gas sensing properties, Cha et al. (2017) reported WS2 nanoflakes functionalized multi-channel carbon nanofibers (MTCNFs) to detect NO2. The WS2@MTCNFs based sensors exhibited notable response of 15% to 1 ppm NO2 at RT with the response/recovery time of 5.7/3.73 min. Later, Koo et al. (2018) synthesized few-layered WS2 nanosheets decorated hollow carbon nanocages for RT NO2 gas sensor, which showed a response of 48.2% to 5 ppm NO2 and a detection limit of 100 ppb. Although the introduction of carbon materials into WS2 improved the conductivity and the sensing properties, the sensor performances are still to be optimized.

Herein, we propose a sheet-on-tube structure in order to maximize the exposure of TMDs edges. As a proof-of-concept, 2D WS2 nanosheets are grown on CNFs via a facile hydrothermal route, which offers abundant highly reactive sites for NO2 adsorption. The structure-property relationships of the WS2@CNFs composite for NO2 detection at RT have been investigated by various characterizations. Experiments reveal that the WS2@CNFs sensor delivers excellent gas sensing properties in terms of outstanding selectivity, fast response/recovery times and very low detection limit, as well as repeatability. The improved sensing properties are attributed to the synergistic interaction of edge-rich WS2 nanosheets and high conductivity CNFs. In addition, density functional theory (DFT) simulations also indicate that the edge sites are more active than the basal surface of WS2 nanosheets, which leads to strong NO2 adsorption and a more effective charge transfer.

Section snippets

Materials

Tungsten hexachloride (WCl6, 99.5%), methyl orange (C14H14N3SO3Na, A.R.), pyrrole (C4H5N, ≥ 98%), thioacetamide (C2H5NS, ≥ 99%) and ferric chloride (FeCl3, ≥ 97%) were purchased Sinopham Chemical Reagent Co. Ltd. All chemicals were used without further purification.

Synthesis of CNFs

The CNFs were synthesized using a procedure similar to that described in Ref. (Ma et al., 2019). 0.327 g of methyl orange (MO) was dissolved in 200 ml of distilled water under stirring, and then added 1.622 g of FeCl3 and 0.7 ml of

Structural and morphological characteristics

Fig. 1a illustrates the synthesis process of the sensing materials and final sensors. First, Ppy tubes were generated by the polymerization of Py, and then further annealed to obtain CNFs. Next, the prepared CNFs and the precursors of WS2 were hydrothermally reacted together, and the resulting products were annealed to prepare ultrathin WS2 sheets functionalized CNFs. In addition, the sensor used for the test is obtained by drop-coating ultrasonically dispersed sensing materials onto the

Conclusion

In conclusion, a highly sensitive and selective NO2 gas sensor is successfully developed based on WS2@CNFs nanocomposites with abundant intentionally exposed WS2 edge sites. The WS2@CNFs sensor delivers remarkable sensing response, excellent repeatability and selectivity, as well as ultralow LOD towards NO2 detection, making our material suitable for practical application. The improved response and selectivity is due to the synergistic contribution from WS2 edge-rich structure and high

CRediT authorship contribution statement

Yongshan Xu and Jiayue Xie conducted material synthesis, analyzed the data and wrote the manuscript. Yunfan Zhang and Fenghui Tian contributed to DFT calculation and analysis. Chen Yang and Wei Zheng contributed to the gas sensor test. Xianghong Liu, Jun Zhang and Nicola Pinna reviewed and revised the manuscript. The corresponding authors Xianghong Liu and Jun Zhang ensure that the descriptions are accurate and submission of this manuscript was agreed by all authors.

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.

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (No. 61971252 and 51972182), the Shandong Provincial Natural Science Foundation (ZR2020JQ27 and ZR2019BF008), the Youth Innovation Team Project of Shandong Provincial Education Department (2020KJN015) and Qingdao Applied Fundamental Research Project (19-6-2-71-cg).

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    Y.X. and J.X. contributed equally to this work.

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