Structural Phase Transitions Of ?-Ni?OH?₂ Nanowires And NiO Nanoparticles Under High Pressure

Because of its distinctive structures and physical properties,nickel hydroxide has many practical applications in the fields of physics,chemistry and engineering.The initial research on nickel hydroxide was mainly in the electrode material and chemical corrosion in battery technology.Today,nickel-based batteries are ubiquitous,and the use of nickel hydroxide in modern battery technology is very mature.Nickel hydroxide is no longer limited to the study of batteries and chemical corrosion,they have many practical applications in modern technology.These applications include batteries,photocatalysis,electrocatalysis,supercapacitors,electrochromic devices,electrochemical sensors and more.High pressure is an effective means of adjusting the crystal structure to change the electronic and optical properties.Nickel hydroxide has been studied extensively under environmental conditions.The study of nickel hydroxide under high pressure is helpful to discover its new phenomena and properties.In this paper,paraotwayite type?-Ni?OH?₂ nanowires were synthesized by hydrothermal synthesis.They belong to a hydrous minerals.In geophysics and geochemistry,high-pressure studies of hydrous minerals may provide valuable information about the understanding of various geophysical phenomena and found more complex hydrous minerals abundantly in earth's mantle.However,there is no high-pressure research report on the complex?-Ni?OH?₂ up to now.The phase transition law of nanomaterials?-Ni?OH?₂ under high pressure is still unclear.The study of the hydrous minerals?-Ni?OH?₂ under high pressure will helps to understand the true response of such materials in the mantle.Compared with NiO of bulk materials,the behavior of NiO nanoparticles obtained by high-temperature calcination with?-Ni?OH?₂ nanowires as precursors has not been reported under high pressure.Compared with NiO of bulk materials,the behavior of NiO nanoparticles under high pressure has not been reported.In this paper,NiO nanoparticles obtained from?-Ni?OH?₂ nanowires by high-temperature calcination for high-pressure experimental study.The main work of this paper is to conduct high-pressure test and research on?-Ni?OH?₂ nanowires and NiO nanoparticles by means of high-pressure in-situ synchrotron radiation X-ray diffraction?XRD?,in-situ high-pressure Raman spectroscopy?Raman?,in-situ uv-visible spectroscopy?uv-visible?.The purpose of this work is to explore the structural phase transition and to understand the high-pressure phase transition mechanism of?-Ni?OH?₂ nanowires.Compared with previous studies on bulk material NiO under high pressure.Explore the difference between nanomaterials NiO and bulk materials NiO under high pressure,enriching people's understanding of NiO materials.The results of this paper are as follows:1.The experimental samples were synthesized by hydrothermal synthesis method and characterized by transmission electron microscopy?TEM?,X-ray diffractometry?XRD?,Raman spectroscopy,infrared spectroscopy.The experimental results show that we synthesized high quality monoclinic Paraotwayite type?-Ni?OH?₂ nanowires with a length of several micrometers and a diameter of 15-20nm.2.The high-pressure synchrotron XRD study of the monoclinic?-Ni?OH?₂nanowires was carried out for the first time.The abrupt slope changes of the lattice parameters varying with pressure are observed at⁹ GPa from our XRD results,while is no obvious crystal symmetry change until 21 GPa,which indicates?-Ni?OH?₂undergoes an isostructural phase transition.The bulk modulus of the low pressure phase and the high pressure phase were 41.2?4.2?GPa and 94.4?5.6?GPa,respectively.This is clearly different from the fact that?-Ni?OH?₂ has no phase transition under high pressure.3.The in situ high pressure Raman spectroscopy study of?-Ni?OH?₂ was carried out for the first time.The decrease in frequency and broadening of the OH stretching bands of?-Ni?OH?₂ with compression.Above 7.8 GPa,the vibrational modes of the hydroxyl group disappear,indicating the pressure-induced amorphization of H sublattice.These results are consistent with those of brucite-type hydroxides.All the Raman peaks recover when the pressure is released,manifesting the isostructural phase transition and the amorphization of the H sublattice are reversible.4.In situ high pressure Raman spectroscopy study of NiO nanoparticles was carried out for the first time.Studies have shown that the Raman signals of NiO nanoparticles and bulk materials are significantly different.No phase transition occurred within the pressure range of 30 GPa.This is consistent with the high pressure behavior of the NiO bulk materials.