Preparation Of Layered Alkali Metal Cobalt Oxide Nanoflakes And Their Applications

Layered alkali cobalt oxides（LCOs）have attracted wide attention and research interest in the fields of thermoelectricity,energy,catalysis,superconductivity due to its diverse crystal structure,excellent electrical properties and unique physical and chemical properties.Recent researches have found that layered cobalt oxide nanomaterials have different properties from bulk LCOs.When the size and thickness of LCOs are reduced to a single layer or a few layers,significant changes in optical,electron transport,diffraction,and vibration properties can be observed.However,it is still challenge to develop the simple preparation method of LCOs nanomaterials and explore their applications in various fields.In this thesis,we carried out the research on the preparation of LCO nanoflakes.And their gas sensing properties and electrocatalysis activity for hydrogen evolution were investigated.The main research contents are as follows:1.Na_xCoO₂ nanoflakes with thickness less than 30 nm were efficiently prepared via aqueous ultrasonic exfoliation of Na_xCoO₂ single crystals and their gas sensing performance was invesitgated.Na_xCoO₂ single crystals were firstly prepared by molten salt method.Na_xCoO₂ was then exfoliated using water as solvent to obtain Na_xCoO₂nanoflakes.Although the gas sensing properties of cobalt oxides,such as CoO,Co₃O₄,and spinel type MCoO₂（M=Zn,Ni,Fe,Mn,Cu）,have been widely studied,the research on the gas sensing properties of LCOs is still unreported.In this reseach,the sensor based on Na_xCoO₂ nanoflakes was fabricated and their gas sensing performance was investigated to find that the sensor exhibits good selectivity to n-butanol.Owning to the high electric conductivity of Na_xCoO₂ layers,the sensor presents a very low noise and a high signal-to-noise ratio value with the detection limit at ppb level.And the sensor exhibits good moisture resistance probably due to the presence of the lattice water in Na_xCoO₂ nanoflakes.2.Na_xCoO₂ nanoflakes were further modified using transition metal ions to find that Fe³⁺doping can efficiently change the sensitive characteristics of Na_xCoO₂nanoflakes with the special selectivity to triethylamine.Firstly,Na_xCoO₂ powder was synthesized using citric acid method,and Na_xCoO₂ nanoflakes were obtained by ball milling method.After soaking Na_xCoO₂ nanoflakes in Fe³⁺solution,Fe³⁺doped Na_xCoO₂ nanoflakes were obtianed.The gas sensor based on Fe³⁺doped Na_xCoO₂nanoflakes had the highest response to triethylamine with good selectivity,which the response to 50 ppm triethylamine can be up to 15.1 with 5 times higher than the undoped Na_xCoO₂ nanoflakes.3.Pd²⁺doped Na_xCoO₂ nanoflakes were prepared by liquid phase immersion method,and their electrocatalytic activites were investigated to find they present excellent catalytic activity for hydrogen evolution reaction.Na_xCoO₂ powder was firstly synthesized by citric acid method,and then Na_xCoO₂ nanoflakes were prepared by ball milling method.Sequently,Na_xCoO₂ nanoflakes with different doping ratios of Pd²⁺were prepared by solution immersion method.And they were used as electrocatalysis for hydrogen evolution reactions to find that the sample with the ratio of 1 to 0.5 for Na_xCoO₂ to Pd Cl₂ exhibits the best catalytic activity for the hydrogen evolution in acid reaction condition.The overpotential is only 31.3 m V at a current density of 10 m A cm^-2 with good electrochemical cycle stability.