Synthesis of TiO2 nanorods by hydrothermal method and their opto-electronic device applications

Titanium dioxide (TiO2) is one of the most significant semiconductor materials due to its excellent physical, chemical properties which allows to be employed for many applications. Its superior features and suitability for various applications stems from its wide band gap, crystalline phase structure, dimension, surface feature, chemical stability, morphology and low manufacturing cost. Recently, therefore, TiO2 nanorods (NRs) have attracted wide range of attention as one-dimensional material for a wide range of device applications including supercapacitors, dye sensitized solar cells, superhydrophobic/superhydrophilic surfaces, data storage devices, medical engineering, lithium batteries, and gas sensors. Although there are various synthesis techniques, hydrothermal method come into prominence for the synthesis of TiO2 NRs since it is a relatively simple, cost effective and controllable technique. In this thesis, thus, it has been mainly focused on the synthesis of TiO2 NRs via hydrothermal method under various growth parameters. It is a well-known fact that to employ hydrothermally grown TiO2 NRs in many opto-electronic device applications, the understanding of synthesis mechanism and the control over their morphological features would play a significant role. In this regard, each growth parameter such as acidity, precursor concentration, growth temperature and growth time were systematically studied in detail. For fabricated TiO2 nanorods, the scanning electron microscopy (SEM) images have clearly revealed that the length, diameter, density and distribution of TiO2 NRs on the substrate can be tuned by changing growth parameters. X-ray diffraction (XRD) analysis has shown the presence of a rutile crystal phase of fabricated TiO2 NRs which is deduced by comparing the observed reflection peaks with the ICDD (international center of diffraction data) cards in database. For the investigation of optical characteristic of synthesized TiO2 NRs, transmittance and reflectance measurements have been conducted in the 300-1000 nm wavelength range. The parameters of samples having less transmittance and less reflectance have determined, which is essential features for opto-electronic devices converted of light energy to electrical energy. Taken together the results, the optimum parameters for synthesis TiO2 NRs were determined, which can be employed for the fabrication of high performance opto-electronic devices in a cost effective way. Moreover, the hydrothermal growth parameters have efects on formation of very special structures such as nanotube and urchin-like structure as well as morphological properties of nanostructures. In this study, it was also shown that nanotubes and the spherical hollow TiO2 structure based on urchin-like structure could be produced by using hydrothermal method. Nanotubes were fabricated at two different steps including growth and etching of nanorods. The spherical hollow TiO2 structures were constructed by coalescence of urchin-like nanorods via hydrolysis reactions at high temperature. The morphological features of these different fabricated structures was also investigated by SEM images. The fabrication of such different nanostructures offer promising potential to construct high-performance opto-electronic devices. Following the synthesis process, a few device structures based on the optimized TiO2 NRs were constructed to test their performance for different opto-electronic applications. It is a well-known fact that TiO2 NRs are especially suitable for the construction of high performance visible/infrared blind ultra violet (UV) photodetectors (PDs) due to their unique properties such as large surface to volume ratio, high electron-hole separation efficiency, hole trapping by O2 adsorption and having a large band gap. As an application of the optimized TiO2 NRs, a Schottky-type UV PD was fabricated by thermally evaporating silver (Ag) metallic top contact onto TiO2 NRs. The current density- voltage (J-V) characteristics of Ag/TiO2 NRs based Schottky type PD were investigated under illuminated and dark conditions. Under dark condition, the J-V characteristic has confirmed the formation of a Schottky barrier between Ag and TiO2, deduced from the rectifying behavior of the device measured under different voltage polarities. In addition, the spectral photoresponse and the photoresponse vs. time characteristics of the constructed Schottky type- UV PD have been studied in detail, exhibited a high gain under a illumination beam consisted of photons matching the band gap of the TiO2. The synthesized TiO2 NRs were also employed for the fabrication of a core-shell FTO/TiO2-NRs/p-CdTe structured heterojunction solar cell. The results have shown that the constructed solar cell exhibits photovoltaic behavior with a power conversion efficiency of 0.42% (under AM 1.5G), which is the record efficiency reported so far for the same materails combination and device architecture.