Sc-doped NiO nanoflowers sensor with rich oxygen vacancy defects for enhancing VOCs sensing performances

Synthesis of NiO and Sc-doped NiO nanoflowers: NiO and Scdoped NiO nanoflowers were synthesized by a wet chemical process, the detailed procedures can be found in the previous report. Briefly, 1.5 ml of 0.5 M Ni(CH3COO)2$xH2O, 0.5 ml ethylenediamine and 10 ml of 7 M NaOH were throughly mixed in a glass bottle (20 ml). After heating the mixed precursor in oil bath (at100℃) for 20 min under vigorous magnetic stirring, light green precipitate was formed and subsequently washed by deionized water and absolute alcohol in sequence for several times, and finally dried in oven at 70℃ for 24 h. Further annealing the above products at 500℃ (a ramping rate of 1℃/min) in air flow for 120 min in a horizontal quartz-tube furnace results in the formation of NiO nanoflowers. The synthesis process of Sc-doped NiO nanoflowers was similar to the above process except for adding various amounts of Sc(NO3)3$H2O ([Sc]/([Sc]+[Ni]) ¼ 1%, 3%, 5%, 7%, 9% and 11%) in the precursor solution. The atomic ratios (at.%) of Sc in NiO samples were 1.7%, 4.4%, 6.6% 7.4%, 11.8% and 14.6%, respectively, as examined by energy dispersive spectroscopy (EDS). Characterization: The crystal structures of NiO and Sc-doped NiO nanoflowers were analyzed on X-ray diffraction (XRD, Rigaku Smartlab). XRD Rietveld refinements were performed by FULLPROF software. The morphology, structure, element composition and chemical states of the products were investigated by field-emission scanning electron microscope (FESEM, ZEISS SUPRA 55VP), transmission electron microscope (TEM, JEM-2011 200 kV), energy dispersive spectroscopy (EDS) and electron energy loss spectros copy (EELS, Tecnai G2 F30 300 kV), respectively. Thermogravimetric analyses (TGA) were carried out NETZSCH STA449F3 from room temperature to 800 C in air atmosphere with a heating rate of 10 ℃/min. The X-ray photoelectron spectroscopy (XPS) was examined by Esca Lab 250Xi spectrometer (Thermo Scientific) equipped with an Ar ion gun. The Brunauer-Emmett-Teller (BET) was carried out on Micromeritics Tristar II 3020 M with nitrogen adsorption-desorption apparatus. Raman spectrum was characterized by LabRAM HR Evolution(HORIBA) using Arþ laser beam. The excitation wavelength was 532 nm and the laser power was 5 mW. Hydrogen temperature programmed reduction (H2-TPR) tests were recorded on FINESORB 3010C instrument (20 mg NiO or 7.4 at.% Scdoped NiO samples were heated to 900℃with a ramp of 10 ℃/min under 30 sccm 5% H2/Ar, thermal conductivity detector was employed to monitor the amount of H2 consumption). Electron spin resonance (ESR) was measured on EOL JES-FA200 at room temperature. The magnetic properties were examined by superconducting quantum interference device (MPMS-XL5, Quantum Design, 2≤T≤400 K, 0≤H≤50 kOe) at room temperature