*2.3. Instruments and Measurements*

The surface morphology and microstructure of sensing materials and films were observed by scanning emission microscopy (SEM, Zeiss Gemini, Oberkochen, Germany) and transmission electron microscope (TEM, FEI G2F20, Hillsboro, OR, USA). The chemical compositions and surface states of the samples were examined by X-ray photoelectron spectroscopy (XPS, Thermo Sacalab 250Xi, Waltham, MA, USA). The Raman spectra were recorded with He-Ne laser excitation at 532 nm using a Raman spectrometer (Renishaw inVia, Wotton-under-Edge, UK).

The ethylene-sensing properties of the sensors were measured at room temperature by a homemade dynamic test system (Figure S1) [26]. The ethylene standard gas (10–100 ppm in dry air), main interference gas (CO2, 30,000 ppm in dry air), and carrier gas dry air were supplied by Chengdu Xuyuan Chemical Co., Ltd., Chengdu, China. The concentration of tested ethylene and CO2 was controlled by the mass flow control (MFC300, Wuxi Aitoly Electronics Co., Ltd., Wuxi, China) with the dry air as carrier gas. The fabricated ethylene sensors were put into the test chamber and their resistances were recorded by a realtime multimeter resistance acquisition system (Keithley 2700, Cleveland, OH, USA). The sensing response of the sensor was defined as (Rg − R0)/R0, where Rg and R0 represent the steady-state resistance value of the sensor in the tested gas atmosphere and dry air, respectively. The response/recovery time was defined as the time required for 90% change of the resistance during the adsorption/desorption process.
