Solvothermal tert-butanol syntheses of electrochemically active multinary metal oxide nanomorphologies

This thesis deals with the synthesis of nanomaterials for electrochemical energy conversion and storage, with a special focus on materials for battery applications. Nanostructuring and nanoscaling are proven efficient means to enhance, and sometimes even to enable, the performance of electroactive materials. Processes involving transfer and transport of charges, which is the core of any electrochemical phenomena, greatly benefit from the increased surface areas and shortened diffusion pathways in nanomaterials. Our group has focused for several years on exploring the electrochemical properties of extremely small metal oxide nanocrystals with a particle size down to 1-3 nm. To achieve this size range, our group has developed a novel synthesis approach based on solvothermal reactions in tert-butanol acting both as a solvent and a reactant. The tert-butanol route has demonstrated unique possibilities compared to other synthesis approaches, including other types of solvothermal reactions. One of the specific features of this process is an extremely small particles size that can be achieved; crystallinity; the dispersibility of nanoparticles in different solvents without additional surface stabilization; and the formation of metastable and non-stoichiometric phases, which could be attributed to the kinetic control of the reaction process. These features made the nanoparticles obtained via the tert-butanol route promising building blocks for low-temperature bottom-up syntheses of porous nanomaterials via surfactant templated evaporation-induced self-assembly. Using the tert-butanol approach, numerous metal oxide compositions have been prepared so far, including binary oxides such as TiO2, NiO, SnO2 and FeOOH, doped oxides such as Nb-doped TiO2, Fe-doped NiO, Sb-doped SnO2 and doped FeOOH, and mixed and ternary oxides such as Co/NiO and Li4Ti5O12 (LTO). The nanoparticles obtained via the tert-butanol approach and the nanomorphologies assembled have demonstrated strongly enhanced performance in dye-sensitized solar cells and electrocatalytic and photoelectrochemical water splitting, and the nanostructured LTO has shown record charging rates when used as an anode in lithium ion batteries (LIBs). The suitability of the tert-butanol route for the fabrication of cathode materials in LIBs has been however so far not been investigated, so it was one of the motivations of this work. Further unexplored challenges to investigate were the possibility of fabricating more complex chemical compositions such as multinary oxides, which are of importance for the battery applications, and even more complex structures such as hybrid materials using the tert butanol approach. This thesis is mainly focussed on the extension of the solvothermal tert butanol syntheses route for metal oxide nanoparticles towards multinary functional materials up to pseudo quaternary oxides. Furthermore, the nanocrystals produced in this way are successfully assembled into nanocomposites and nanostructures, respectively, and embedded into devices with improved performance in photoelectrochemical water splitting and in particular as electrodes in LIBs. One special focus was on the synthesis of LIB cathode materials, which could be achieved for the first time with the tert butanol solvothermal synthesis. Chapter 1 introduces the principles of LIBs, the solvothermal nanoparticle synthesis and the relevant approaches towards nanostructures or compounds are depicted. Furthermore, it provides a short overview of the properties of the materials synthesized in this thesis. The basic principles of the techniques to characterize the morphology, the structure, the composition and the electrochemical behavior of the nanomaterials are described in Chapter 2. In Chapter 3 the solvothermal synthesis in tert butanol of crystalline, non agglomerated nanoparticles of the binary material Co3O4 is described. The properties of the Co3O4 nanocrystals within the size range of 3 7 nm are investigated, and they are later implemented in devices for photo driven water splitting. Due to the very small size of the nanoparticles and their high dispersibility, a homogenous deposition of the nanocrystals on mesoporous hematite layers is achieved. While the hematite acts as the photoactive absorber in the light induced water splitting reaction, the Co3O4 nanoparticles are applied as co catalysts. This surface treatment results in a distinct increase in photocurrent. The mechanisms involved are revealed by photoelectrochemical as well as transient absorption spectroscopy studies. The high performance is enabled as the Co3O4 nanoparticles help to suppress the surface electron hole recombination on time scales of milliseconds to seconds. Pseudo binary oxide nanoparticles are produced within the tin oxide system by implementation of an additional metal, here antimony. The tin oxide and antimony doped tin oxide (ATO) are synthesized in a microwave assisted synthesis in tert butanol in the presence of graphene oxide nanosheets to form nanocomposites where the ultrasmall metal oxide nanocrystals are anchored on the surface of reduced graphene oxide (rGO) sheets. These composite materials exhibit high electrical conductivity and a high structural stability during lithium incorporation which makes them, in particular the ATO, novel high capacity anode materials for LIBs. The greatest advantage of the ATO/rGO nanocomposite in comparison to tin oxide or ATO bulk material is the efficient buffering of the volume changes associated with the electrochemical processes during charging/discharging. Here, a reversible high capacity of 577 mAh g 1 charging/discharging within one minute could be achieved. The syntheses of the nanocomposites as well as the electrochemical testing are described in Chapter 4. In Chapters 5 and 6, the tert-butanol solvothermal synthesis of lithium containing multinary oxides as precursors for nanostructured cathode materials for LIBs is shown for the first time. Chapter 5 is focused on the syntheses of two different pseudo binary metal oxide nanoparticles of the lithium cobalt system. In a first step ultrasmall nanocrystals of cubic rock salt type Li0.15Co0.85O are produced in tert butanol. These nanoparticles themselves are not suitable as an effective cathode material in LIBs, but can be later transformed into high performance nanostructured LiCoO2 using block copolymers as surfactant. Due to the nanostructuring, over 50% of the theoretical specific capacity can still be achieved even at extremely shortcharge/discharge times of 72 s. In Chapter 6, the pseudo binary lithium cobalt system is expanded to the pseudo quaternary system of LiwNixCoyMnzO. Herein, for the first time four different metals could be homogeneously integrated in ultrasmall metal oxide nanoparticles with 1 4 nm in size by the tert butanol solvothermal synthesis route. Moreover, four different compositions in this system are deliberately synthesized, all adopting the cubic rock salt structure. In a second step, the biotemplate nanocrystalline cellulose is used to create desert rose structured Li(NixCoyMnz)O2 with fixed transition metal ratios. Although the synthesis approach via the rock salt type nanoparticles favors the cation mixing between lithium and nickel and therefore drastically reduces the specific capacities achievable, the desert rose structure shows promising stability for high power applications. In summary, the tert butanol route was significantly extended to produce different multinary metal oxide particles. These particles were then used either in compounds or together with surfactants to produce nanostructured materials for different applications, mainly as electrode materials in lithium ion batteries. These nanoparticles often showed significantly improved performance, especially at high rate operations as compared to the materials reported in the literature