Photosensitization Of TiO₂ Nanorods With CdS For Photovoltaic Devices

Titania nanocrystal film materials have very wide applications in photovoltaic conversion techniques and photocatalysis applications. Among all the TiO₂ films, one-dimensional single-crystalline TiO₂ array films, including nanotubes, nonorods and nanowires, have received considerable attention over polycrystalline ones for they can improve electron transport by providing direct electrical pathways for photogenerated electrons. However, it is well known that the large band gap of TiO₂ (3.2 eV) limits its absorption to the ultraviolet region, which takes only about 3–5% of the solar spectrum. Nanocrystalline CdS is a direct band-gap semiconductor material with a reasonable band-gap of 2.43 eV which matches the solar visible spectrum well. Sensibilization of TiO₂ film with CdS quantum dots can tune the response of the photoelectrode in the visible region. In addition, the conduction band position of CdS is higher than that of TiO₂, once incident photons are absorbed by the CdS QDs, photoexcited electrons in the conduction band of CdS will quickly transfer into that of the TiO₂ to decrease its energy level. and accelerate the charge separation and finally improve the photoelectric performance of TiO₂ film electrodes.In this thesis, the rutile TiO₂ nanorod array films were synthesized by a hydrothermal method. FESEM,XRD,UV-vis was used to characterize the morphologies, the crystal structure and the optical characterization of the of the as prepared films respectively. The entire surface of the FTO substrate is covered uniformly and densely with TiO₂ nanorods after the hydrothermal process. Our nanowires have lengths of 2.9μm and diameters between 60 and 200 nm, respectively, and planar densities of 8-12 wires/μm2. The XRD spectrum of TiO₂-NR deposited on FTO substrates indicates the structure of the nanorods to be of rutile type. We can found from the UV–vis absorption spectra that the peak maximum of the TiO₂-NR film occurs at around 380nm and has no significant absorbance for visible-light because of its large energy gap (3.2eV). The graph of characteristics of the photocurrent density versus measured potential (I–V curve) indicates that the maximum short circuit current is 0.14mA, when the time of hydrothermal treatment is 14h. In this case, the optimum energy conversion efficiency is 0.1%.CdS nanocrystals were deposited on the nanorods by a chemical bath deposition technique. The surface morphology, structure, optical and photoelectrochemical behaviors of the core–shell nanorod films are considered. After chemical bath deposition technique, the side of the TiO₂ nanorods is rather rough and possesses uniform CdS particles. The diameters of the CdS particles were more than 100nm. From the XRD pattern we can deduce that the phase of CdS belongs to hexagonal. The samples of CdS@TiO₂-NR showed significantly red-shift of the peak maxima at 550 nm, indicating the effective photoabsorption property for the ordered structure composites. The photoelectrochemical characteristics recorded under AM 1.5 illumination indicates that the as-fabricated CdS@TiO₂-NR core–shell nanorod arrays film electrode possesses higher photocurrent density than bare TiO₂ nanorods film electrode.The CdS QDs-sensitized TiO₂ nanorod arrays thin film electrodes are successfully prepared by hydrothermal method and sequential chemical bath deposition techniques subsequently. The surface morphology, structure, optical and photoelectrochemical behaviors of the as-prepared films are considered. The diameter of the CdS QDs is about 5～7 nm. HRTEM image also indicates that CdS QDs deposited on TiO₂ nanorod have crystalline hexagonal structure. and has no significant absorbance for visible-light because of its large energy gap (3.2eV). The samples of the nanorod arrays show significantly red-shift of the peak maxima at 550 nm after sensitized by CdS QDs, indicating the effective photoabsorption property for the ordered structure composites. The materials result in an energy conversion efficiency of 1.91% under AM 1.5 G illuminations, a large open-circuit photovoltage of 1.1 V, and a photogenerated photocurrent of 2.5 mA/cm2. These results clearly demonstrate that significant improvement on the energy conversion efficiency can be obtained via incorporating CdS QDs into the TiO₂ nanorod array films.