Improved recovery of NO2 sensors using heterojunctions between transition metal dichalcogenides and ZnO nanoparticles

The stable recovery of gas sensors is an important indicator for evaluating their performance. Hitherto, the use of external light sources and/or an increase in the operating temperature has been effective in improving the recovery rate of gas sensors. Herein, heterojunctions were formed between the two-dimensional transition metal dichalcogenide nanosheets and zero-dimensional ZnO nanoparticles to improve the recovery rate of a NO2 sensor. Scanning electron microscopy and Raman spectroscopy suggested a successful deposition of ZnO nanoparticles onto the MoS2 and WSe2 nanosheets. The sensing response to 10 ppm NO2 gas at 100 °C indicated that the heterojunction formed by ZnO and MoS2 or WSe2 successfully improved the recovery rate of the sensor by 11.87% and 19.44%, respectively, whereas the sensitivity remained constant. The proposed approach contributes to improving the performance of gas sensors.

Carbon-based materials, such as graphene and carbon nanotubes (CNT), have been widely studied owing to their excellent electrical conductivity, stability, high surface-to-volume ratios, and low toxicity [13][14][15].Carbon-based NO 2 gas sensors exhibit excellent responses to ppb-level NO 2 at room temperature.However, these sensors often require prolonged recovery times because of the strong bonding between NO 2 molecules and the sp 2 -carbon in carbon-based materials [16,17], which significantly affects the overall performance of the carbonbased gas sensors.
Chemical doping [42], exposed edge positions [43], photoactivation [44], and noble metal/other nanomaterial functionalization [45][46][47] have been employed to enhance the sensitivity of sensors.Furthermore, recovery rate, which is one of the critical performance indicators for sensors, needs to be improved.Heterojunctions, external light assistance, and high operating temperatures have been reported to improve sensor recovery rate [15,44,[48][49][50].Among them, heterojunctions have received significant attention because of their energysaving capabilities.
In this study, heterojunctions were fabricated to improve the recovery of TMDC-based gas sensors.The recovery rate of the sensors could be improved by approximately 5.72-11.87%and 8.10-19.44%via spraycoating of a small amount of n-type ZnO nanoparticles onto the p-type MoS 2 and WSe 2 nanosheets, respectively.

Results and discussion
The TMDC and diluted ZnO dispersions are successively spray-coated onto a SiO 2 wafer with an interdigitated Au electrode to form a heterojunction (Fig. 1).We employed tip sonication to exfoliate bulk TMDC, transforming it into a few-layered structure to increase the surface area.Initially, 300 mg of bulk TMDC powder purchased from Alfa Aesar was placed into 100 ml of isopropyl alcohol (IPA) and subjected to tip sonication for 4 h at 150 W power.The TMDC-containing beaker remained immersed in an ice and water mixture throughout the exfoliation process.Subsequently, the exfoliated TMDC solution underwent centrifugation at 2000 rpm for 1 h, yielding a supernatant enriched with few-layered TMDC.The resulting concentrations of the MoS 2 and WSe 2 dispersions were 0.06 and 0.075 mg/ml, respectively.The ZnO solution purchased from Sigma Aldrich was diluted to 0.06 mg/ml.The TMDC/ZnO heterojunction was confined to the dimensions of 4.2 mm × 1.85 mm using a shadow mask and annealed at 250 °C for 2 h in an Ar atmosphere to remove impurities.The finally prepared TMDC/ZnO heterojunction sample is shown in Fig. 1c.
Figure 2a, c demonstrate the TMDC nanosheets exfoliated through ultrasonic treatment with sizes predominantly ranging from 100 to 300 nm.These nanosheets are uniformly dispersed on a SiO 2 wafer, forming a compact and conductive network.The interstitial spaces between the TMDC nanosheets facilitate the penetration of NO 2 gas molecules, thereby leading to an increased number of adsorption sites for NO 2 gas. Figure 2b, d  in the inset of Fig. 2b, d.Aggregation of ZnO nanoparticles occurred due to its high concentration.On the other hand, we diluted and re-dispersed ZnO nanoparticles for sensor fabrication in order to minimize the aggregation of nanoparticles.
As shown in Fig. 3, the Raman spectrum of the TMDCbased gas sensor excited by a 532 nm laser exhibits prominent peaks of MoS 2 located at 407.41 and 383.4 cm −1 .These two peaks correspond to the out-of-plane (A 1g ) and in-plane (E 1 2g ) modes of MoS 2 .The difference in the positions of these peaks is ~ 24.01 cm −1 , indicating the presence of a few-layered structure of the MoS 2 [51].A single prominent Raman peak of WSe 2 appears at 254.3 cm −1 , which can be assigned to the overlapping of E 1 2g and A 1g , and indicates a few-layered structure of the WSe 2 [52].Furthermore, the typical E 2 (high) peak of ZnO is observed at 438.2 cm −1 in MoS 2 /ZnO and WSe 2 /ZnO spectra [53], indicating the successful deposition of ZnO nanoparticles onto the TMDC nanosheets.
In this work, we used a custom-built gas chamber for conducting gas sensing experiments.This chamber is seamlessly integrated with both a gas mass flow controller and a temperature controller.Throughout the experimentation process, temperature was precisely maintained as the sample stage within the gas chamber was consistently maintained at 100 °C.To facilitate the experiments, the regulated flow of NO 2 gas and custom dry air was introduced through a mass flow controller.This ensured a steady total flow rate of 500 sccm.The I-V curves and resistance measurements of the gas sensor were accurately recorded using a Keithley 2634B sourcemeter.The TMDC/ZnO-based sensor exhibits a significantly higher resistance than the TMDC-based sensor, which can be attributed to the formation of heterojunctions (Fig. 4a, b).Furthermore, the addition of ZnO nanoparticles significantly reduces the signal drift of the TMDC-based sensor, whereas the response of the sensor remains unchanged (Fig. 4c).We studied the recovery rate of the sensors which can be defined as [54,55]:  The electrons of ZnO near the ZnO/TMDC interface tend to diffuse into TMDC, whereas the holes in TMDC tend to diffuse into ZnO before the Fermi levels of the p-type TMDC and n-type ZnO reach equilibrium (Fig. 5a).This leads to the formation of a depletion layer on the ZnO surface that gradually generates an internal electric field in the ZnO/TMDC interface region The internal electric field hinders further diffusion of carriers (Fig. 5b).When the sensor is exposed to NO 2 gas, the NO 2 molecules first capture electrons from ZnO with large adsorption energy and increase the width of the built-in electric field, resulting in a decrease in the sensor conductivity at the initial stage of NO 2 gas influx (the parts inside the blue dotted line in Fig. 4c).As the influx of NO 2 gas continues, NO 2 molecules start capturing electrons from the p-type TMDC surface with many active sites.The balance of the built-in electric field is broken, the holes accumulated at the ZnO side return to TMDC, and the width of the built-in electric field decreases (Fig. 5c).Furthermore, the desorption of NO 2 molecules on ZnO with large adsorption energy is slow and incomplete when air is introduced.This significantly improves the recovery rate of TMDC/ZnO-based gas sensors.Nonetheless, incompletely desorbed NO 2 molecules are still present on TMDC.The recovery rate could be further improved by employing external light sources [56,57].

Conclusion
In this study, heterojunctions were fabricated via spraycoating a small amount of n-type ZnO nanoparticles onto the p-type TMDC nanosheets.SEM images and Raman spectra confirmed the successful deposition of small ZnO nanoparticles onto the large TMDC nanosheets.Furthermore, the fabricated heterojunction successfully improved the recovery rate of MoS 2 /ZnO and WSe 2 / ZnO sensors by ~ 5.72-11.87%and ~ 8.10-19.44%,respectively.This study is expected to contribute to the development of high-performance NO 2 gas sensors.

Fig. 2 Fig. 3
Fig. 2 Scanning electron microscopic images of a MoS 2 -, b MoS 2 /ZnO-, c WSe 2 -, d WSe 2 /ZnO-based gas sensors.The red arrow represents the ZnO nanoparticles (NP).The insets in b and d show SEM image of undiluted ZnO nanoparticles.The scale bars in the insets are 250 nm

Fig. 4 I-V characteristics of a MoS 2 -,
Fig. 4 I-V characteristics of a MoS 2 -, MoS 2 /ZnO -, b WSe 2 -and WSe 2 /ZnO-based gas sensor in air environment at 100 °C.c Gas sensing response of transition metal dichalcogenides (TMDC)-based gas sensor to 10 ppm NO 2 gas at 100 °C.The part where the conductivity decreased at the early stage of NO 2 exposure is outlined by the blue dotted line.d Recovery rate per cycle of MoS 2 -and MoS 2 /ZnO-based gas sensor.e Recovery rate per cycle of WSe 2 -and WSe 2 /ZnO-based gas sensor

Fig. 5
Fig. 5 Band diagrams of transition metal dichalcogenides (TMDC)/ZnO heterojunction a before equilibrium, b after equilibrium in air and c NO 2 gas atmosphere