A facile synthesis method for Ni(OH)₂ ultrathin nanosheets and their conversion to porous NiO nanosheets used for formaldehyde sensing

Single-crystalline β-Ni(OH)₂ ultrathin nanosheets were synthesized via a simple electrochemical reaction of Ni electrodes with a mixed solution of NaCl, NaOH, and NH₄Cl at room temperature. The average thickness of β-Ni(OH)₂ nanosheets is in the range 1-15 nm, which can be readily tuned by changing the concentration of NaCl. The phase structure, composition, morphology, and thickness of Ni(OH)₂ nanosheets were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The mechanism of the nanosheet formation is proposed as the selective adsorption of NH₃ molecules on the (001) crystal face of β-Ni(OH)₂ which suppresses growth in the [001] direction. Porous NiO ultrathin nanosheets were obtained by thermal decomposition of β-Ni(OH)₂ nanosheets in air at 400 °C for 2 h. Gas sensing properties of NiO ultrathin nanosheets were investigated, and the sensors exhibited high sensitivity, low detection limit, and wide dynamic range for detection of formaldehyde.