The Studies Of Structure And Photoelectrochemistry Performance On Cobalt-based Two-dimension Nanomaterials

Due to the global energy shortage and environment problem, it became more and more important to develop renewable energy materials to meet the demand of energy. Since hydrogen has the characteristics of green, easy storage and high calorific value, and the solar energy is a kind of limitless reserve and clean energy, solar water splitting as a desirable approach of hydrogen generation attracts the wide attention. The 3d transition metal (Fe-, Co-, Ni-) oxides (TMOs) are cheap, stable and active, which have been widely used in photoelectric catalysts. However, the recombination of photogenerated electron-hole pairs and the large overpotential problems, causing by the shorter diffusion length of charge carrier and the slower rate of electron transfer, limit the conversion efficiency from solar to hydrogen. Ultrathin 2-dimension (2D) nanomaterials (including graphene) not only have high specific surface and large exposed surface active sites, but also possess unique electron structure and fast carrier transfer property that can bringing the high photoelectrocatalytic activity. Therefore, the formation of ultrathin 2D TMOs nanosheet is an important and effective mean to modulate the electronic structure and then to enhance the photoelectrochemical properties.Based on the above ideas and designs, we used ultrathin CoOOH nanosheet as model to detect the structure and photoelectrochemical properties. In this thesis, we used wet-chemical method to synthesize an ultrathin γ-CoOOH nanosheet with half-metallic characteristic; the peculiar surface active sites lower the water oxidation overpotential and exhibit an extraordinary large mass activities and long-term stability. An ultrathin layer β-CoOOH semiconductor was well designed and successfully synthesized, the thickness of this nanosheet is shorter than the carrier diffusion length, which can effectively suppress the electron-hole recombination and thus enhance the hydrogen production rate and the quantum efficiency at visible light region. Based on synchrotron radiation X-ray absorption fine structure (XAFS) technology and electrochemical analysis method, the local structure of CoOOH nanosheet was detected and then was confirmed to be possessed of a surface distorted structure. These results give us an in-depth understanding for structure-performance relationship, In addition, transmission electron microscope (TEM), atomic force microscope (AFM) and X-ray diffraction (XRD) were used to observe the morphology and structure characteristic. UV-Vis spectrophotometer, gas chromatograph (GC), electrochemical workstation and first principle calculation were adopted to detect the material properties.The main content in this dissertation is as following:1, Study of oxygen evolution reaction (OER) performance and structure of ultrathin y-CoOOH nanosheetEndowing transition-metal oxide electrocatalysts with high water oxidation activity is greatly desired for production of clean and sustainable chemical fuels. Here, we present an atomically thin cobalt oxyhydroxide (y-CoOOH) nanosheet as an efficient electrocatalyst for water oxidation. The y-CoOOH nanosheet electrocatalyst with thickness of 1.4 nm can effectively oxidize water with extraordinarily large mass activities of 66.6 A·g-1,20 times higher than that of y-CoOOH bulk and 2.4 times higher than that of the benchmarking IrO2 electrocatalyst. Experimental characterization and first-principles calculations provide solid evidence to the half-metallic nature of the as-prepared nanosheet that exhibit local structure distortion of the surface CoO6-x octahedron. This greatly enhances the electrophilicity of H2O and facilitates the interfacial electron transfer between Co ions and adsorbed -OOH species to form O2, resulting in the high electrocatalytic activity of layered CoOOH for water oxidation.2, Study of hydrogen evolution reaction (HER) performance and structure of ultrathin p-CoOOH nanosheetThe easy electron-hole pair recombination in earth-abundant transition metal oxide (TMO) is a major limitation for developing high-efficiency hydrogen evolution photocatalysts. Here, via the metal-hydroxyl-contained layered P-CoOOH semiconductor being thinned into atomically two-dimensional nanostructure, we successfully overcome the bulk carrier recombination in TMOs. The ultrafast transient absorption spectroscopy reveals that the electron-hole recombination in the as-prepared P-CoOOH nanosheet with thickness of 1.3 nm is almost suppressed. This leads to prominent electron-hole separation efficiencies of 60-90% in 350-450 nm, 10 times higher than that of bulk counterpart. X-ray absorption spectroscopy and first-principles calculations demonstrate that the [HO-CoO6-x] on nanosheet surface promotes the H+ adsorption and H2 desorption on the catalytic sites. The P-CoOOH nanosheet suspension in methanol aqueous solution exhibits high hydrogen production rate of 160 μmol·g-1·h-1, and hydrogen production rate in aqueous Na2SO3 solution reach about 1200 μmol·g-1·h-1.3, XAFS study on structure-activity correlations of a-Co(OH)2 nanosheet water oxidation catalystsUnderstanding the structure-performance of the metal hydroxide materials is critical to the design of an efficient oxygen evolution reaction (OER) catalyst for water oxidation. However, a challenge is to identify and collect surface active site relative to bulk when we use x-ray absorption fine structure (XAFS) technology to probe the surface local structure of bulk catalysts. Here, we have prepared an ultrathin a-Co(OH)2 nanosheet with large exposed surface Co site as a high-efficiency O2 evolution catalyst. Using XAFS technique, we have investigated the oxidation state and local structural evolutions of the a-Co(OH)2 nanosheets catalyst before and after OER process. A coordination-miss Co sites (CoO6-X) with oxidation state of+3.3 on the nanosheet surface is revealed during the OER procedure, indicating the in situ formation of y-CoOOH nanosheet is a key factor leading to an enhanced water oxidation performance.