IVESTIGATIONS OF METAL ASSISTED TITANIUM DIOXIDE (TiO2) NANOCRYSTALS

Abstract

7.1 Summary The research topic of the thesis entitled “Investigations of Metal Assisted Titanium Dioxide (TiO2) Nanocrystals” disclosed the structural, morphological, compositional and optical properties of TiO2 nanoparticles (NPs) and discussed their utility for photocatalytic applications. In the current thesis, TiO2 NPs were prepared using the sol-gel method and comprehensively explored their properties. The crystalline structural and optical characteristics of TiO2 NPs (like X-ray diffraction, absorption, photoluminescence, and time-resolved photoluminescence) along with photocatalytic applications have also been discussed in detail in the previous chapter. 7.2 Important Findings of Research Work Nanocrystals of anatase, mixed and rutile phases of TiO2 and metal-doped TiO2 have been synthesized via the sol-gel method. The prepared samples were characterized by various analytical tools. For the structural and surface morphology analysis, essential tools such as XRD, SEM and TEM were used. EDX analysis has been carried out for elemental identification present in prepared nanomaterials. TGA, FTIR spectroscopy, Raman spectroscopy, UV-visible & PL spectroscopy, time-resolved photoluminescence (TRPL) spectroscopy at varying temperatures were used to understand the structural and transitions following photoexcitation. TiO2 NPs were prepared by the sol-gel method with titanium isopropoxide as a precursor at different annealing temperatures. The analyzed XRD patterns, Raman and Fourier transform infrared spectra demonstrated the structural transformation from 160 amorphous to anatase and further to rutile phase while increasing annealing temperature. In addition, a mixed-phase of TiO2 NPs is formed, which consists of both phases. The absorption and photoluminescence (PL) spectra of mixed and rutile phases are shifted towards a longer wavelength region. The indirect band gap structure changed into the direct band gap during the structural transformation. Both absorption and PL spectra shifted towards lower energy regions, which might be due to the increase in size or the induced oxygen vacancies produced at a higher temperature. Furthermore, the photocatalytic activity of all the three different structural TiO2 NPs was examined. Furthermore, the photocatalytic performance of the different types of TiO2 NPs was examined through the degradation of a dye, rhodamine B (RhB), under UV radiation and measuring changes in absorption and PL spectra. The anatase phase structure shows higher photocatalytic activity than the rutile phase. However, the mix phase has the highest photocatalytic activity among all the structures, which degraded RhB entirely at a faster rate. On the other hand, the rutile phase is unable to take part in this process. Thus, the mix phase of TiO2 NPs is beneficial for industrial and environmental applications. The transition metal ions (Ag+, Cu2+, and Ni2+) doped and undoped TiO2 NPs have been synthesized using a cost-effective sol-gel method with a 1.0 wt% dopant concentration. The microstructure and chemical compositions of these NPs were examined using various techniques such as x-ray diffractometric, field emission scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared and absorption and photoluminescence (PL) spectroscopy. The absorption and photoluminescence (PL)-excitation spectra of metal-doped TiO2 NPs are shifted to the longer wavelength region, which indicates a reduced bandgap than the bare TiO2 NPs. The absorption and PL spectra of methylene blue (MB) in the presence of undoped and metal ions doped TiO2 NPs show dramatic changes upon UV-irradiation. The absolute absorption 161 intensity reduced entirely and the solution of MB became colorless in the presence of UV irradiation. The PL of the degraded dye exhibits a new band in the shorter wavelength region, which has a multi-exponential decay function and an increased average PL lifetime. The dye degradation rate is higher for metal ions doped catalyst and highest for Cu2+ doped TiO2 NPs. Thus, Cu ions-doped TiO2 shows the highest photocatalytic activity. The order of catalytic degradation rate under UV irradiation was found to be Cu– TiO2 > Ni– TiO2 >Ag– TiO2 >anatase TiO2. The analysis of the PL spectra and PL-decays reveals the formation of smaller species that emits at a shorter wavelength region, thus helps in understanding the degradation of dye molecules. TiO2 NPs synthesized by employing the sol-gel routes and annealed at a different temperature from 400 to 900 °C. Three different nanostructures were formed, namely anatase, mixed (anatase/rutile) and rutile phases. The structure and morphology of as-synthesized NPs were confirmed using XRD and FESEM analysis. The XRD analysis of TiO2 NPs was carried out in the 290 K to 77 K temperature range and found no significant change in XRD patterns that means thermally stable TiO2 NPs. The PL spectra and contour maps of TiO2 NPs show that the anatase phase falls in the visible region. However, mixed and rutile phases fall in both visible and NIR regions as the temperature decreases from 290 K to 77 K. The visible PL band is ascribed to donor-acceptor recombination. In contrast, oxygen vacancies serve as donors and hydroxyl groups function as accepter sites. NIR PL band attributed to the trapped electrons in rutile TiO2, which recombine with free holes and intrinsic defects. The fast component of the decay processes was aided by the immediate formation of trapped electrons in luminescence sites. The indirect trap processes were responsible for the power-law component in the rutile phase, which was the recombination of trapped electrons formed via a deep trap state. It was observed that the electron-hole pairs thermally separated, preventing the formation of STEs directly. The PL 162 and PL decay studies under weak excitation conditions prove to be a more valuable and appropriate method to evaluate the trap states distribution and their carrier dynamic effects, which were vibrant to understand the photocatalytic processes better. Two-dimensional (2D) layered MoS2 nanosheets (NSs), which possess a vast range of unique properties and hold great potential for various applications. MoS2 NSs were synthesized by a hydrothermal method and the obtained NSs bear crystalline and layered structure. Absorption and electroabsorption (E-A) spectra of MoS2 doped in a PMMA thin film were measured at different temperatures (290-40 K). The E-A spectra observed at the second harmonic of the modulation frequency of the applied electric field (1.0 kHz) were analyzed with an integral method by assuming the Stark effect as a dominant feature. The absorption spectra consist of multiple transitions, among which five transitions are contributed to the E-A spectra. The changes in electric dipole moment (Δμ) and polarizability (Δα) of each transition were determined at different temperatures. Two electronic resonance states were identified for two excitonic bands of MoS2 NSs, which showed a strong E-A signal.