Synthesis And Photovoltaic Performance Of The TiO₂ Nano-Structured Thin Films

TiO2 has been found multiple applications in photocatalysis, hydrogen sensors, electrochromic devices, biomedicine, and solar energy conversion because of its remarkable physical and chemical properties. Especially, one-dimentional TiO2 nanotubes are currently under intense investigation, as they provide highly active surfaces with a large surface to volume ratio and a directional electrical channel compared to TiO2 nanoparticle-based films. Developing new synthetic strategies for these nanomaterials and exploring the effect of structure on properties are very important to both fundamental research and photovoltaic conversion application.In this dissertation, we focused on the one-dimentional TiO2 nanotube. By optimizing the reaction condition and anodization parameters, nanotubes with adjustable morphologies and structure can be achieved. And free-standing double-walled TiO2 nanotube arrays with bamboo structure were first time purposefully synthesized based on requirements of dye-sensitized solar cells. The main contents in our work were summarized as follows:Chapter 2, Sputtered Ti thin films were anodized in fluoride-ion-containing neutral electrolytes to form optically semitransparent nanoporous films, which transformed to be completely transparent after thermal annealing. The nanoporous films were studied at different stages, such as before and after anodization, as well as after thennal annealing using field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and UV-vis and Raman spectroscopy. It was observed that anodization at 20 V of high temperature deposited titanium films resulted in regular nanopore films with pore diameters of 30 nm. Structural investigations on the transparent nanopore arrays reveal the presence of anatase phase TiO2 even after annealing at 500℃, which was confirmed by XRD and Raman spectroscopy measurements. The solar-light induced photocatalytic decomposition of stearic acid and photoconductivity characteristics of these nanoporous thin films are also presented.Chapter 3, Ordered arrays of TiO2 nanotubes with double-walled structures and bamboo-like morphologies were prepared by a one-step titanium foil anodic oxidation using a solution of NH4F and ethylene glycol as electrolyte. FE-SEM, XRD and Raman spectroscopy were used to investigate the structure and morphology of the obtained TiO2 nanotubes. The as-prepared titania nanotubes combined the double-wall morphology and the bamboo structures. The current-versus-time curves were recorded to monitor the anodization process. By optimizing the electrochemical anodization conditions, TiO2 nanotubes with tunable structures can be reproducibly prepared.Chapter 4, Dye-sensitized solar cells (DSSCs) based on free-standing, double-walled TiO2 nanotubes with bamboo morphology prepared by a facile combination of alternating voltage anodization with two-step anodization treatment exhibited remarkable performance. The presence of additional double-walled and bamboo-type structures in vertically oriented nanotube arrays led to an increased dye loading. Subsequently, these obtained novel nanotubes were first time exploited to produce DSSCs in a frontside illumination mode, yielding a significantly high power conversion efficiency, of 3.46%, which was almost two times of the traditional single-walled TiO2 nanotube arrays with smooth surface. In addition, as for double-walled and bamboo-type TiO2 nanotubes based DSSCs, samples with higher density bamboo structure showed the higher conversion efficiency than that of lower density. The improved conversion efficiency can be derived from increased dye absorption quantity, which was verified by dye desorption test.Chapter 5, Highly ordered TiO2 nanotube arrays with different anodization time had been successfully prepared via electrochemical anodization of high purity Ti foils in non-aqueous electrolytes under constant voltage. Diffuse reflectance ultraviolet and visible (DRUV-vis) spectrums of annealed samples were investigated. All highly ordered TiO2 nanotube arrays showed visible-light absorption behaviors. From further FE-SEM observation, it was found that the photoresponse properties of the highly ordered TiO2 nanotube arrays have close relationship with their microstructure parameters. Since the as-prepared TiO2 nanotube arrays were amorphous and converted into anatase phase via high temperature annealing at 500℃; the crystalline transformation was confirmed by XRD and Raman spectroscopy measurements. In addition, the highly ordered TiO2 nanotube arrays used as photonic crystals materials were also proposed.Chapter 6, Well-crystallized TiO2 nanoparticles with average size of～20 nm were synthesized by hydrolysis of titania salt in aqueous medium. The effect of the optical properties of the obtained titania particles based thin films with different thickness on the photovoltaic performance of dye-sensitized solar cells were investigated. Differential thermal analysis/thermo-gravimetric analysis, FE-SEM, transmission electron microscopy and XRD were used to characterize the morphology, structure and crystal formation of the obtained samples. The optical properties such as reflectance and transmittance of the photoanodes with different thickness were systematically investigated. The reflectance property increased with increasing the film thickness, however, the transmittance property showed the opposite way. The improved scattering property with increasing the film thickness facilitated efficient utilization of solar spectrum, which was verified by incident photon-to-current conversion efficiency. The maximum energy conversion efficiency was achieved on photoelectrode film with 17.8μm.