Insight into the Humidity Dependent Pseudo-n-Type Response of p‑CuScO2 toward Ammonia

Material Preparation. CuScO2 was prepared by a hydrothermal method based on previous literature. Reagents of analytical grade were used without purifification. First, 5 mmol of Sc(NO3)3·H2O and 5 mmol of Cu(NO3)2·3H2O were dissolved in 10 mL of deionized water and 3 mL of ethylene glycol at RT. Then, KOH was added to adjust the pH of the solution to 3.2. The obtained solution was sealed in a 25 mL Teflflon-lined autoclave and maintained at 220 °C for 60 h.The obtained precipitates were washed with diluted ammonia anddeionized water 3−5 times, respectively. Finally, the precipitates were dried at 60 °C in an oven.The sensor substrates were purchased from Huachuang Ruike Science and Technology Wuhan Co., Ltd., on which interdigital electrodes and a heating electrode have been prefabricated. CuScO2 slurry was prepared by dispersing 0.2 g of CuScO2 powders in 2 mL of ethanol. CuScO2 sensors were fabricated by brushing the slurry on the interdigital electrodes and then annealing at 200 °C for 3 h (labeled as S-200), 250 °C for 3 h (labeled as S-250), and 350 °C for 1 h (labeled as S-350) in air, respectively.Material Characterization. Powder X-ray diffffraction (XRD) measurement was carried out on an XPert Pro X-ray diffffractometer. A VEGA3 TESCAN scanning electron microscope (SEM) was used to characterize the morphology. Transmission electron microscope (TEM) images, selected-area diffffraction patterns, high-resolution TEM (HRTEM) images, and energy-dispersive X-ray spectroscopy (EDS) analysis were obtained with a JEM-2011.Thermogravimetricanalysis (TGA, NETZSCH STA449F3) was performed from 25 to 800 °C at a heating rate of 10 °C/min under air atmosphere. A Thermo Scientifific ESCALAB250Xi system was used for X-ray photoelectron spectroscopy (XPS) measurements. The Mott−Schottky measurement was obtained on an electrochemical workstation (Zahner Company, Germany) at a frequency of 2000 Hz. The electrolyte was 1 M NaOH solution, and the counter electrode was a platinum sheet. The reference electrode was Ag/AgCl (saturated KClsolution), and the work electrode was CuScO2. In order to improve the intergranular contact, the CuScO2 electrode was annealed at 400 °C for 1 h in a vacuum of about 1.5 Pa.Sensor Response Tests. The gas sensing experiments were performed in a four channel gas sensing measurement device (SD101) produced by Huachuang Ruike Science and Technology Wuhan Co. Ltd (www.whhcrk.com).The gas response properties of the sensors at RT at 40% RH were tested using a static system. The headspace saturated vapors of the volatile organic solutions were used as test gases, and their concentrations were calculated by the saturated vapor pressure under standard atmospheric pressure. For the convenience of description, the sensor resistances in the test gases and in air were represented as Rgas and Rair, respectively. The response time is defifined as the time for 90% resistance changes upon exposure to the test gas, while the recovery time is the time required for 90%resistance changes upon exposure to air.