A novel ammonia gas sensors based on p-type delafossite AgAlO2

AgAlO2 nanoparticles were prepared by hydrothermal method. All the reagents were analytical grade and used without further purifications. First, 15 mmol Al(NO3)3·9H2O and 15 mmol AgNO3 were dissolved in 70 mL deionized water at room temperature to get a clear solution. Then, 2.4 g NaOH was added to the above solution and stirred for 3 h. The obtained suspension solutions were poured into 100 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 493 K for 60 h. Then, the obtained precipitates were washed with diluted ammonia (1 mol/L), diluted nitric acid (1 mol/L) in sequence in order to dissolve and wash off any possible Ag-related secondary phase. Finally, the precipitates were washed with deionized water for several times.Alumina substrates with predefined Pt heating electrode and interdigital electrodes were purchased from Hua Chuang Rui Ke, China. AgAlO2 paste was prepared by mixing 0.2 g as-synthesized AgAlO2 crystals, 0.03 g ethyl cellulose M9 and 0.2 g terpineol in 3 mL ethanol. The sensor was fabricated by spreading the paste onto the interdigital electrodes and then sintering in air at 400 °C for 2 h.A Rigaku SmartLab X-ray diffractometer (XRD) with Cu-Ka source operated at 40 kV and 150 mA in the 2q region of 10-80° at a scan rate of 0.25°/s was used to identify the crystalline phases. A Hitachi SU8000 model field-emission scanning electron microscope (FE-SEM) with electron dispersion spectrum (EDS) accessory was used to check the morphology and chemical composition of the samples. The accelerating voltage is 15 kV and the acquisition time for elemental mapping is 120 s. UVeViseNIR Diffuse reflectance spectra (DRS) were recorded using a spectrophotometer (CARY-5E) with BaSO4 as the background between 240 and 2500 nm. The Brunauer-Emmett-Teller (BET) surface areas were determined by nitrogen adsorption-desorption isotherm measurements at 77 K ona Micromeritics ASAP2460 system. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB250Xi XPS system with a monochromatic Al Ka source and a charge neutralizer. All the binding energies were referred to the C 1s peak at 284.8 eV of the surface adventitious carbon.Mott-Schottky measurements were operated on an electrochemical work-station (Zahner Company, Germany). The AgAlO2 pellet was used as the work electrode. A platinum sheet and Ag/AgCl (saturation KCl solution) were used as counter electrode and reference electrode, respectively. The measurement was carried out in 1 M NaOH solutions (pH 13.5) with frequency of 5000 Hz.The gas sensing experiments were carried out in a static test system (Wuhan Huachuang Ruike Science & Technology Co. Ltd.), including a 20 L gas chamber and a gas measurement system based on IEEE1451.2 standard. The detailed description of the sensor module resistance test was mentioned in Ref. [21]. The measurement range of resistance was 100 U ~1 GU with an error less than 5% and 1 GU ~10 GU with an error less than 15%. The test gases were prepared by drawing out their headspace vapors into an injector at room temperature, respectively. The concentration of the test gas can be calculated by the saturated vapor pressure of the organic vapor under a standard atmospheric pressure. If no otherwise specified, the gas sensing experiments were operated at 293 K 35% RH. The gas response to ethanol is defined as Rg/Ra, where Rg and Ra are the resistance of the gas sensor in the target gas and in air, respectively. The gas response to ammonia is defined as Ra/Rg. The response time is the time for the resistance changes to reach 90% of the total change from air to the test gas. The recovery time of the sensor was recorded as a duration in which the resistance changes to reach 90% of the total change when the air was introduced into the gas chamber.