*3.3. Ethylene-Sensing Characteristics*

Figure 4a shows the real-time sensing response versus time curves of four sensitive films (rGO, rGO/WSe2, rGO/Pd, and rGO/WSe2/Pd) when exposed to 10–100 ppm ethylene at room temperature. When exposed to ethylene, all resistance values decline rapidly and then approximately reach the saturation states after almost the same periods of time. After ethylene was purged by dry air, the resistance values gradually recovered to their initial states, which indicates the p-type semiconducting behaviors of all fabricated sensing films. The sensing response of all sensors increases with the increase of ethylene concentration ranging from 10 to 100 ppm, and it could be clearly seen that both of the rGO/WSe2 heterostructures and Pd NPs could greatly promote the ethylene sensing characteristics. Among them, pure rGO film shows the lowest sensing response, and the response to 100 ppm ethylene is nearly twice and four times larger after assembling with Pd and WSe2, respectively, and particularly, ten times for rGO/WSe2/Pd heterojunctions, compared with pure rGO based sensors. These results are consistent with the theoretical analysis results shown above, which further demonstrates the enhancement effects of rGO/WSe2 heterostructures and Pd NPs for ethylene sensing.

The repeatability of all kinds of sensitive films to 50 and 100 ppm ethylene was measured and shown in Figure 4b. It could be clearly seen that except for the case of rGO films in 50 ppm ethylene, almost similar response curves including response value, response/recovery time, and also stable baselines are obtained in five successive cycles, indicating the excellent repeatability properties of sensitive films. Moreover, to further prove its practical application in the agricultural environment, the selectivity properties of rGO/WSe2/Pd composite films were investigated. The typical interfering gas when used for fruit ripeness detection is CO2, which is produced by the respiration of fruits. The sensing response of the composite sensor to 50 ppm ethylene is significantly larger than that of 3000 ppm CO2 (Figure 4c), verifying the excellent selectivity of the rGO/WSe2/Pd sensitive films in fruit ripeness detection application scenarios. In addition, the response and recovery time (90% change in sensor resistance) were extracted and shown in Figure 4d,e. Among four kinds of sensitive films, the rGO/WSe2/Pd sensitive film exhibits the shortest response time (33 s) and recovery time (13 s) to 10 ppm ethylene. It could be attributed to the 2D structures of the rGO and WSe2 nanosheets, Se vacancies existing in the surface of WSe2 films, and also high activity of Pd NPs.

**Figure 4.** (**a**) Real-time sensing response of rGO-based self-assembled sensitive film to 10–100 ppm ethylene at room temperature, (**b**) repeatability, and (**c**) selectivity properties of self-assembled rGO/WSe2/Pd sensitive film when exposed to ethylene. (**d**) Response time and (**e**) recovery time of the ethylene sensor based on rGO/WSe2/Pd sensitive films.
