Single sheet iron oxides: synthesis and reactivity

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

Zhou Yin

Two-dimensional (2D) single sheets structure draws much attention due to their novel physicochemical properties distinct from bulk or even nano-sized materials, with opportunities to form highly reactive sorbents, catalysts and to make composites between different single-layer materials. The synthesis of thin sheets of layered double hydroxides (LDHs), or nanosheets, as 2D material, opens a new chapter for exploration of the materials chemistry and applications of metal hydroxides with a brucite-type structure. These nanosheets, which are made of single or a few stacked layers derived from LDH, exhibit specific chemical and physical properties, including superior ionic or electron mobility and high catalytic activity. In this thesis, the synthesis and applications of these LDH nanosheets have been summarized. The synthesis of LDH single sheets can be classified into two approaches: “top-down” and “bottom-up”. The “top-down” approach is the more widely developed method, which is based on delamination of already synthesized LDHs. The “bottom-up” method comprises direct synthesis of the nanosheet materials from solution by creating a physical/chemical micro-environment for oriented growth of single-layered LDH nanosheets. Key factors that control the success of delamination are the electrostatic attraction between the LDH metal hydroxide layers and interlayers, the nature of the interlayer, and properties of the solvent that is used for delamination. By intercalation of surfactants, amino acids or long-chain carboxylates, the electrostatic attractionof the metal hydroxide layer and the interlayer is weakened, and subsequent delamination of LDHs is facilitated. After ion exchange to form propper interlayers, delamination in formamide, butanol, water and other reagents have been reported as a result of osmotic swelling of LDHs caused by the interaction between the interlayer anions and the solvents. Among all the solvents that have been used for delamination, formamide and butanol are the most used solvents. The local order of metal hydroxide layer from the LDHs is inherited directly by the single sheet materials with the random disorder. The application of these LDH single sheets depends on the chemistry and properties of the parent LDHs. Because of their ultrathin thickness and that the surface active sites are fully exposed, some LDH single sheets exhibit superior electron mobility and high catalytic activity. LDH single sheets have been reported to be effective sorbents, catalysts in electrochemical and photochemical reactions, and building thin films together with other nanomaterials for designing new functionalities. Here we focus on the delamination of FeII-FeIII LDHs into single sheet iron oxide (SSI), to study its structure, sorption and electrochemical properties. The present Ph.D. project comprises: (1) Synthesis of the SSI with new insight into its structure and composition. We have developed a method for synthesis of SSIs from the FeII-FeIII LDH (green rust, GR) using surfactant intercalation of preformed GR, followed by aerial oxidation and delamination. The average height of the SSIs from AFM is 1 nm and the lateral dimensions vary from 20 nm to 100 nm. SAED shows a hexagonal pattern with d ≈ 0.28 nm as expected for the diffraction in GR and other Fe-oxides. Chemical analysis, TGA and XPS lead to the SSI composition FeO0.82(OH)1.38· 0.7H2O. For the oxidised, surfactant-intercalated GR and the SSI, pair distribution functions (PDF) from high energy X-ray scattering data demonstrates that there are two distinct interatomic distances for first neighbour Fe-Fe pairs, somewhat similar to the patterns for ferrihydrite and goethite, but in contrast to the single Fe-Fe nearest neighbour distance observed for the unoxidized GR. A modeled structural rearrangement with displacements of Fe atoms vertical to the plane of the original hydroxide layer to form a trilayer structure with Fe polyhedra that are linked via both corner- and edge-sharing, produced a PDF in agreement with the main features of the measured PDF pattern (figure on the couverture). Mössbauer spectroscopy showed that the SSI orders magnetically at 130 K and that the hyperfine parameters are distinct for SSI and different from other Fe-oxides such as ferrihydrite and feroxyhyte. Mössbauer is in general agreement with the model from X-ray scattering. (2) Surface complexation modeling of AsV adsorption onto the SSI. In this work, the sorption of AsV on SSI has been studied as a function of time, loading and pH. The kinetics of arsenate adsorption onto SSI was rapid compared to other iron oxides, reaching equilibrium within 60 minutes. Arsenic sorption and acid-base titration data could be successfully described with a 1pk Basic Stern Model (BSM). The point of zero charge was around 8. The intrinsic surface complexation equilibrium constants (log K) for protonation, deprotonation and interaction with the singly, doubly, and triply coordinated groups of SSI surface were obtained by fitting surface charge density in three electrolyte concentrations (1 mM, 10 mM and 100 mM) using BSM model. For SSI sorption with AsV, the sorption isotherm and sorption envelope were well described by postulating the presence of two bidentate species (non-protonated and protonated), with log K1 27.05 and log K2 31.61, respectively. Distribution of arsenate species on the SSI, the dominant species within pH 3.5-9.5 are bidentate complexes, and the amount of protonated bidentate became abundant at low pH. (3) Electrochemical reduction of chlorinated compounds using an SSI modified electrode. Here, the electrochemical reactivity of SSIs coated on indium tin oxide coated glass electrodes was investigated. Iron on the SSI modified electrode showed a typical Cyclic Voltammetry profile with reversible reduction and oxidation, suggesting the formation of FeII-OH/O-FeIII clusters as that in GRs were formed on the ITO electrode (< 0 V). Trichloroacetic acid (TCAA), perchloroethylene (PCE), trichloroethylene (TCE), tetrachloride (CT) and 4-chlorophenol are used to test the electrocatalytic reactivity of SSI. For TCAA, 27 % could be reduced to dichloroacetic acid within 6 h at a potential of -0.5V (versus Ag/AgCl). However, no measurable reduction of CT, TCE, PCE or 4-CP was observed which may be attributed to the low amount of FeII that is produced in the SSI electrode insufficient to degrade chlorinated compounds, and/or the hindered electron transfer from the electrode to the substrates due to weak or no sorption to the electrode.By giving an insight into the novel structure of SSI, its sorption and electrochemistry properties will be further revealed. The results from this Ph.D. project will give a better understanding for the future application and research of these ultrathin nanosheets.
ForlagDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
StatusUdgivet - 2018

ID: 193511944