Synthesis, Characterization, Low Temperature Magnetic Interactions and Catalytic Properties of Transition Metal-Oxide Nanostructures

Debnath, Bharati (2019) Synthesis, Characterization, Low Temperature Magnetic Interactions and Catalytic Properties of Transition Metal-Oxide Nanostructures. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Earth abundant transition metal oxides have been a subject of interest among researchers owing to their enormous potential as sensors, magnetic resonance imaging materials, catalysis and permanent magnets. Especially the nanostructured metal oxides with a variety of structural morphologies have a fascinating spectrum of magnetic and catalytic properties which can be intricately steered by their synthesis routes. The purpose of this thesis is to synthesize differently shaped metal oxide nanostructures to obtain improved size and morphology dependent structural, magnetic and catalytic properties. In chapter 1, introduction about the work is presented starting with a brief overview of nanoscience and nanotechnology. The chemical and physical properties of the nanomaterials along with different properties and applications of metal oxide nanostructures and nanocomposites are discussed. The magnetic properties such as saturation magnetization (Ms), coercivity, remanence magnetization (Mr), exchange bias and spin glass are briefly highlighted. A brief discussion of exchange bias in metal oxide nanostructures and the role of antiferromagnetic fractions are also presented. Besides, a detailed discussion on catalytic studies, the basics of water splitting and their essential guiding factors are also presented in this chapter. Detailed literature review on every topic is provided throughout. In chapter 2, different synthetic methodologies of metal oxide nanostructures such as co-precipitation method, and heating up methods are discussed. In particular a brief description about the characterization techniques used in this work for structural, morphological, magnetic and catalytic studies are presented. The procedure of Rietveld refinement to obtain crystal parameters is also discussed. Procedures of electrocatalytic measurements are also explained. Chapter 3 deals with the magnetic and electrocatalytic properties of CoFe₂O₄ nanoparticles (NPs). To prepare the CoFe₂O₄ NPs co-precipitation method is applied. As prepared NPs are then calcined at different temperatures (350, 550 and 650°C) under N2 atmosphere to prepare the calcined NPs. Rietveld refinement analyses show that the NPs crystallize in spinel structure with space group Fd3 m. The Transmission Electron Microscope (TEM) images show that the NPs are 8 nm (AP-CoFe2O4), 10 nm (CoF-1), 20 nm (CoF-2) and 55 nm (CoF-3) in diameter for as prepared, 350, 550 and 650°C calcined samples, respectively. Surface spin glass and an optimum size in the ferrimagnetic CoFe₂O₄ NPs with ~20 nm diameter can preserve very high magnetic anisotropy in terms of coercivity, Mr and squareness (Mr/Ms) in the sample. It is found that the spin-glass-ferrimagnetic interactions are responsible for exchange bias coupling. Along with the spin-glass-ferrimagnetic interactions, high anisotropy is generated in 20 nm NPs which exhibit the maximum energy product, BHmax of 2.3 MGOe at room temperature which is the highest among any reported CoFe₂O₄ system. The effect of particle size and crystallinity of the NPs with different calcination temperatures show a significant role in determining the electrocatalytic activity towards Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER). The optimized activity for HER and OER is exhibited by CoF-2 (η-10mAcm-2 = 218mV) and CoF-3 (η10mAcm-2 = 370mV), the best among those reported for CoFe₂O₄ system. Chapter 4 deals with the exchange bias interactions of an inverted core-shell system by incorporating antiferromagnetic NiO NPs into mesoporous ferrimagnetic CoFe2O4 matrix. Here too co-precipitation method is applied to synthesize porous CoFe₂O₄ NPs. NiO NPs are synthesized using one pot heating up method. Ligand exchange method is applied to trap 3.4 wt% of 9 nm NiO NPs inside the pores of 35 nm clustered CoFe₂O₄ NPs. The −NH₃⁺ groups of cysteamine on the NiO NP surface electrostatically bind to the −OSO₃− of sodium dodecyl sulfate attached to CoFe₂O₄ NPs. Rietveld refinement analyses confirm that CoFe2O4 NPs crystallize in the spinel structure with space group Fd3 m and NiO NPs crystallize in the face-centered cubic lattice with space group Fm3m. The NiO NP filled CoFe₂O₄ porous material is investigated for their structural and morphological characteristics by electron microscopy and elemental mapping. The organic functional groups attached to the NP surface are detected by infrared spectroscopy. Nitrogen adsorption/desorption isotherm measurements are performed to verify the mesoporous structure. The magnetic measurements confirm the presence of high anisotropy in terms of exchange bias and coercivity. The exchange bias of AP-CoFe₂O₄ is 394 Oe with a cooling field of 2 T. Although the hysteresis loop of the AP-CoFe₂O₄ is asymmetric in nature, after incorporation of NiO NPs, the asymmetric nature of the hysteresis loops of CoFe₂O₄ become symmetric and exchange bias of 233 Oe is obtained at 5 K with a cooling field of 2 T. Also the values of Ms and coercivity become double after incorporation of NiO NPs, thereby enhancing the overall magnetic properties. In chapter 5, both morphology and surfactant-dependent control of surface oxidation is demonstrated. Here, MnO is considered as the representative oxide system because manganese has multiple oxidation states and can easily transform from +2 to its higher oxidation states. By varying the nature and amount of surfactant, accelerating agent, surfactant precursor ratio, and reaction rates, MnO nanostructures are designed in the form of solid nanooctahedra as well as the octahedral nanostructures with clustered NPs having MnO core and Mn₃O₄ shell. The TEM images show that the side lengths of the octahedra are ~80 nm for M-1 (prepared with trioctylphosphine oxide), ~160 nm for M-2 (prepared with oleic acid and trioctylamine), and ~80 nm for M-3 (prepared with trioctylamine), respectively. Rietveld refinement shows the fitted Mn3O4 phases are 3, 5, and 3.7% in M-1, M-2, and M-3, respectively. The surface oxidation of these nanostructures is also examined by X-ray Photoelectron Spectroscopy and further characterized by magnetic exchange coupling. The solid nano-octahedra are least oxidized compared to that of the NP-clustered octahedral nanostructures. The ferrimagnetic Mn3O4 surface layers contribute greater magnetic moment over the antiferromagnetic MnO core which increases the overall magnetic moment. The interfacial spin coupling between MnO core and the Mn₃O₄ shell also creates high exchange anisotropy. In chapter 6, multifunctionality in terms of enhanced magnetic exchange coupling and OER activity of self-assembled Mn₃O₄ NPs are discussed. Monodisperse Mn₃O₄ NPs are synthesized via one pot thermal decomposition of Mn-acetate in 1, 2-dichlorobenzene. Mn₃O₄ NPs are then self-assembled in situ into spheres, hierarchical flakes and cubes by regulating the surfactant-metal precursor molar ratio, reaction atmosphere and time. The secondary phase of Mn₂O₃ is incorporated differently, depending on the type of self-assembly. Mn₃O₄ phase crystallize in tetragonal crystal structure with I4₁/amd space group and Mn₂O₃ phase crystallize in orthorhombic phase with Pcab space group. Rietveld refinement shows that the wt% of Mn₂O₃ is 2, 3.5, and 6.5 in the flakes, spheres, and cubes, respectively whereby the highest percentage of Mn₂O₃ in the cubes enhances its multifunctionality. TEM images show that the self-assembled ordered nanostructures are composed of 9−14 nm monodisperse NPs. A series of control experiments are also performed by varying the reaction conditions in order to elucidate the formation of flakes and cubes from the nucleation stage. Magnetic measurements show that the hysteresis loop shift corresponding to coupling between antiferromagnetic Mn₂O₃ and ferrimagnetic Mn₃O₄ is 3813 Oe for the cubes, which is record high for any reported Mn₃O₄−Mn₂O₃ system. The presence of an eg¹ electron due to a higher Mn₂O₃ fraction in the cubes facilitated high structural flexibility for optimum strength of interaction between the catalyst and intermediate ions during OER. Likewise, a current density 10 mA cm⁻² is attained at overpotential of 0.946 ± 0.02 V for the cubic nanostructure, which is superior to those of spheres and flakes. In chapter 7, different types of manganese oxide (Mn-O) NPs namely MnO, α-Mn₂O₃, Mn₃O₄ and δ-MnO₂ are synthesized and used as highly recyclable heterogeneous Fenton catalysts for organic dye degradation in wastewater treatment. MnO, α-Mn₂O₃, Mn₃O₄, and δ-MnO₂ are synthesized by heating up, autoclave, heating up, and co-precipitation methods, respectively. Cubic MnO and α-Mn₂O₃, tetragonal Mn3O4 and monoclinic δ-MnO₂ crystallize with space group Fm3m, I2₁3, I4₁/amd and C2/m, respectively. The average NP-diameter of 32, 26, 20 and 35 nm for MnO, α-Mn₂O₃, Mn₃O₄ and δ- MnO₂, respectively are confirmed by the electron microscopic studies. The Mn₃O₄ NPs are used as the model system to understand the role of methylene blue and H₂O₂ concentrations, catalyst content and reaction temperature, as well as the kinetics, reaction activation energy and the degradation pathway of methylene blue. The catalytic activity of Mn₃O₄ is better than α-Mn₂O₃, and δ-MnO₂ due to the presence of both Mn³⁺ and Mn²⁺ ions. The availability of active sites proportionate with the specific surface area could not interpret the relative performance since the specific surface area of MnO, α-Mn₂O₃, Mn₃O₄ and δ-MnO₂ NPs are 71, 16, 63 and 165 m²/g, respectively. In fact, the presence of unpaired eg electrons in both Mn²⁺ and Mn³⁺ ions in Mn-O is found to be crucial for effective electron transfer to H₂O₂ for the generation of intermediate radicals. The best methylene blue degradation activity of 99.3% is observed with 15–20 nm Mn₃O₄ particles at 80°C within 60 min. The mechanism of the degradation process follows first-order kinetics and is dependent on the generation of •OH radicals at the initial stage and ¹O₂ radicals after 30 min. In chapter 8, the summary of the present work and the future prospects are discussed.

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