Energy is very important in our life. Materials are essential to the realization of energy generation, conversion and storage. My thesis research is directed at the development of new nanostructured materials for effectively harvesting solar energy. It involves primarily chemistry-based bottom-up approaches, which are particularly attractive for the nanostructure synthesis due to their scalability, low cost and controllability. In particular, the low temperature hydro(solvo)thermal technique coupled with morphology conserved transformation routes was adopted for preparing many of the nanomaterials in my work since it is a facile, low cost, and environmentally benign technique amenable to large-scale production

The thesis is divided into nine chapters. Chapter 1 surveys the cu...[ Read more ]

Energy is very important in our life. Materials are essential to the realization of energy generation, conversion and storage. My thesis research is directed at the development of new nanostructured materials for effectively harvesting solar energy. It involves primarily chemistry-based bottom-up approaches, which are particularly attractive for the nanostructure synthesis due to their scalability, low cost and controllability. In particular, the low temperature hydro(solvo)thermal technique coupled with morphology conserved transformation routes was adopted for preparing many of the nanomaterials in my work since it is a facile, low cost, and environmentally benign technique amenable to large-scale production

The thesis is divided into nine chapters. Chapter 1 surveys the current literature and outlines motivation and scope of the work. Chapter 2 introduces experimanet techniques. Major findings of the study are discussed in Chapters 3 through 8, with conclusions and outlooks summarized in Chapter 9.

In chapter 1, I give a general introduction of this thesis on i) the significance of energy generation, mainly emphasizing on dye-sensitized solar cells (DSSCs); ii) advanced functional electrode materials for DSSCs; iii) developing strategies for preparing useful 1D and hierarchically porous nanomaterials, and most importantly, iv) the objectives of the thesis. Chapter 2 introduces experimental techniques used in my work.

In chapter 3 a superstructure of meso/micro-porous single-crystalline ZnO nanoplates is created by controlled thermal decomposition of a nanoplate precursor prepared from a hydrothermal process. This unique porous nanoplate structure has proved to be an excellent candidate for constructing photoanodes of low-cost and high-performance dye-sensitized solar cells (DSSC).

In Chapter 4 I report on the use of different sized TiO₂ nanospindles to construct a double-layered photoanode for dye-sensitized solar cell (DSSC). One layer made of larger nanospindles serves to enhance light scattering and the other consisting of smaller nanospindles acts to increase the roughness factor for efficient dye-adsorption. The two-layer structure with size-varied, single-crystalline TiO₂ nanospindles has demonstrated over 8.3% energy conversion efficiency. This was made possible by our successful large-scale synthesis of size-tunable, single-crystalline anatase TiO₂ nanospindles.

In Chapter 5 the work embarks upon three levels of undertaking ranging from nanomaterials synthesis to assembly and functionalization. Firstly, I have prepared size-tunable anatase TiO₂ nanospindles via a hydrothermal process by using tubular titanates as self-sacrificing precursors. Secondly, I have densely dispersed the TiO₂ nanospindles onto functional graphene oxides (GO) via a spontaneous self-assembly process. After annealing of the TiO₂/GO hybrid nanocomposite in an NH₃ gas flow, the TiO₂ surface was effectively nitridated and the GO was reduced to graphene sheets (GS) in order to further fortify the electronic functionality of the nanocomposite. Thirdly, the anatase@oxynitride/titanium nitride-GS (TiO₂@TiO_xN_y/TiN-GS) hybrid nanocomposite was studied as an anode material for lithium-ion batteries (LIBs), showing excellent rate capability and cycling performance compared to the pure TiO₂ nanospindles. Our systematic studies have revealed that the TiO₂@TiO_xN_y/TiN-GS nanocomposite with graphene nanosheets covered with the TiO₂@TiO_xN_y/TiN nanospindles on both sides provide a promising solution to the problems of poor electron transport and severe aggregation of TiO₂ nanoparticles by enhancing both electron transport through the conductive matrix and Li-ion accessibility to the active material from the liquid electrolyte. More generally, the size-tunable TiO₂ nanospindles with their unique (101) outer surface planes provide an archetype for the in depth investigation of their surface-specific and size-dependent physicochemical properties.

In Chapter 6 a photoanode based on ZnO nanotetrapods, which feature good vectorial electron transport and network forming ability, has been developed for efficient photoelectrochemical water splitting. Two strategies have been validated in significantly enhancing light harvesting. The first was demonstrated through a newly developed branch-growth method to achieve secondary and even higher generation branching of the nanotetrapods. Nitrogen-doping represents the second strategy. The pristine ZnO nanotetrapod anode yielded a photocurrent density higher than those of the corresponding nanowire devices reported so far. This photocurrent density was significantly increased for the new photoanode architecture based on the secondary branched ZnO nanotetrapods. After N-doping, the photocurrent density enjoyed an even more dramatic enhancement to 0.99 mA/cm² at +0.31 V vs. Ag/AgCl. The photocurrent enhancement is attributed to the greatly increased roughness factor for boosting light harvesting associated with the ZnO nanotetrapod branching, and the increased visible light absorption due to the N-doping induced band-gap narrowing of ZnO.

In chapter 7 we first reported a unique photoanode architecture made of the branched anatase TiO₂ nanotetrapods, which can be achieved by dissolution and nucleation processes with the assistance of ZnO nanotetrapods template, has been developed for efficient photoelectrochemical water splitting. Encapsulation of shell-isolated Au into the branched TiO₂ nanotetrapods is important to enhance absorption at ultraviolet and visible frequencies. This photocurrent density was significantly increased for the photoanode based on the branched (Au@SiO₂)@TiO₂ nanotetrapods. The enhancement can be attributed to the unique TiO₂ structure consisting of thin nanowires, as well as the increased optical absorption originating from both surface plasmon resonances.

Chapter 8 is devoted to a number of case studies in nanomaterials synthesis by morphology conserved transformation. Firstly, we extended this method to prepare porous ZnCo₂O₄ nanoflakes by a pyrolysis-induced transformation of novel hexagonal shaped, highly ordered, and inorganic-organic-inorganic layered hybrid nanodisks. It is shown that the hexagonal hybrid nanodisks are constructed from organic molecule (ethylene glycol) directed assembly of inorganic bilayers. The porous ZnCo₂O₄ nanoflakes have been tested as a lithium ion battery electrode, showing high capacity and high cyclability. Secondly, Sn@C nanowires (UTP@CW,~21 wt % carbon and ~77 wt % tin) with a high encapsulation density of ultrafine tin nanoparticles in mesoporous carbon nanowires are the first prepared by adopting a morphology-conserved solution approach. The straightforward hydrothermal process and the low cost reagents involved portend the potential for large scale production and wide applications. Moreover, the material exhibit excellent reversible capacities and cycling performance for lithium ion batteries, especially at high current rates. Thirdly, we have synthesized hierarchically structured Mn₂O₃ nanomaterials with different morphologies and pore structures by means of morphology-conserved transformation. The key step of this method consists in the formation of a precursor containing the target materials interlaced with the judiciously chosen polyol-based organic molecules, which are subsequently knocked out to generate the final nanomaterials. In this work, two kinds of precursor morphologies, oval-shaped and straw-sheaf-shaped, have been selectively prepared by hydrothermal treatment of different functional polyol molecules (oval-shape with fructose and straw-sheaf-shape with β-cyclodextrin) and potassium permanganate. Thermal decomposition of the precursors resulted in the formation of mesoporous Mn₂O₃ maintaining the original morphologies. These novel hierarchical nanostructures with different pore sizes/structures prompted us to examine their potential as anode materials for lithium ion batteries (LIBs). The electrochemical results with reference to LIBs show that both of our mesoporous Mn₂O₃ nanomaterials deliver high reversible capacities and excellent cycling stabilities at a current density of 200 mA g^-1 compared to the commercial Mn₂O₃ nanoparticles. Moreover, the straw-sheaf-shaped Mn₂O₃ exhibits a higher specific capacity and a better cycling performance than the oval-shaped one, due to the relatively higher surface area and the peculiar nanostrip structure resulting in the reduced length for lithium ion diffusion.

Finally, Chapter 9 presents a summary with conclusions and outlook of my thesis research.