| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 97 === In this study, the influences of Ti4+ concentration and reaction temperature on TiO2 phases (anatase, brookite and rutile) formed from hydrolysis of TiCl4 in 5 M HNO3(aq) were investigated. It is found that at the temperature of 100 oC, in the [Ti4+] range between 0.3 and 0.6 M, only brookite/rutile mixed phases form. While at the temperature of 70 oC, as [Ti4+] equals to 0.3, 0.6 and 0.8 M, respectively, brookite/rutile, anatase/brookite/rutile and anatase/rutile mixed phases are obtained. Separation of mixed phases into individual pure anatase, brookite and rutile was achieved via mixing as-synthesized samples with C2H5OH followed by centrifugation. Among them, brookite exhibits the best photocatalytic activity in the photobleaching of methylene blue under 300 nm UV illumination.
Pure anatase, brookite and rutile samples were characterized via various instruments after calcination at 450 oC for 30 min (condition the same as the fabrication of electrode for dye-sensitized solar cell and water splitting). As revealed by HRTEM, the three phases exhibit their distinctive morphologies: nanoparticle for anatase, nanoplate for brookite and nanorod for rutile. The BET surface areas are 91, 76 and 32 m2/g, respectively. UV-vis spectra showed that the scattering abilities are in the order of rutile, brookite and anatase.
Photovoltaic performance of dye-sensitized solar cells made up of anatase, brookite and rutile with the same thickness were measured under AM 1.5 (100 mW/cm2). Due to the superior capability for N719 dye adsorption (N719ads = 0.060 μmol cm-2), anatase-based cell exhibits the highest photoconversion efficiency (η = 4.26%), as compared to those of brookite- (2.50%, N719ads = 0.041 μmol cm-2) and rutile-based (1.55%, N719ads = 0.016 μmol cm-2) cells.
Anatase nanoparticles prepared via sol-gel method (SG) were utilized as active layer and brookite/rutile, due to their better scattering abilities, as scattering layer for DSSC photoanodes. Photoconversion efficiencies were increased from 7.09% without scattering layers to 8.44% and 9.10% with rutile/brookite as scattering layers, respectively. Brookite can not only adsorb more dye but scatter incident light.
Photocatalytic water splitting reaction indicated photoconversion efficiencies of pure anatase-, brookite- and rutile-based working electrodes are 0.43%, 0.87% and 0.80%, respectively. Brookite and rutile exhibit better light-harvesting efficiencies than anatase in incident photon-to-current efficiency (IPCE) measurements. Transient times of anatase, brookite and rutile calculated by photocurrent relaxation with time are 0.20, 0.71 and 0.42 s, respectively, which revealed brookite electrode has the longest electron lifetime than the other two polymorphs.
The effect of H2SO4 concentration on TiO2 phases formed in TiCl4/HNO3(aq) system was studied via in-situ XRD, NSRRC. It is showed that addition of H2SO4 enables the suppression of brookite/rutile and promotion of anatase in this system, and formation orders of anatase and rutile can be well-controlled by adjusting [H2SO4]. The case that anatase emerges before rutile was chosen to fabricate TiO2/TCO electrode. Such bilayer microstructure is adopted to utilize smaller anatase to adsorb more dye and larger rutile particles to scatter incident light. The photoconversion efficiency is 4.11%, and the result demonstrates that this facile process is very promising to fabricate TiO2/TCO electrode for efficient photovoltaic devices.
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