Synthesis and Characterization of TiO2 Nanocrystals: Application in Dye-Sensitized Solar Cells
Date Issued
2016
Date
2016
Author(s)
PENG, JIA-DE
Abstract
This dissertation aimed to develop TiO2 nanocrystals with facile synthesis process and used as the materials of photoanodes for dye-sensitized solar cells (DSSCs) with high cell efficiencies (η`s). In recent years, literatures mentioned that (001)-facets TiO2 can further increase the dye loading as compared to (101)-facets TiO2, so we synthesized and characterized (001)-facets TiO2 nanosheets, and their applied in DSSCs. The results indicate that the synthesized TiO2 nanosheets have a smaller surface area and yet with a higher dye-loading capacity (dye molecules per gram and per surface area of TiO2). An optimal η of 8.77% is obtained for the DSSC with the TiO2 nanosheets film, as compared to that of the cell with the P25 film (6.92%) (Chapter 3). Besides, we used phosphotungstic acid (PWA) to modify TiO2 nanosheets (PWA-TiO2-NS). The results indicate that PWA modified TiO2 nanosheets have 73.4 mV higher open–circuit voltage as compared to the unmodified one, resulting in a 14.8% higher η for the corresponding DSSC (9.32%). Moreover, by blending gold particles with the optimized content of PWA-TiO2-NS, the η of the pertinent DSSC could further be enhanced to 10.03%, owing to the plasmonic effect of the gold particles (Chapter 4). However, because of the vulnerability of nanosheets to overlap on each other, in other words, due to the relatively large thickness in the [001] direction, these structures are not expected to show a high surface area. Reduction of thickness of the nanosheets in [001] direction and increase of 2D lateral size of (001) planes, while simultaneously allowing the sheets to self-assemble into a hierarchical 3D architecture is one of the optimal solutions for preventing this aggregation. In this way, not only the surface area of the sheet–like TiO2, but also the percentage of its exposed (001)-facets can be largely increased. However, ordered structures of this type are yet to be obtained for TiO2. In Chapter 5, the mono–dispersed TiO2 microspheres with highly exposed (001)-facets (ca. 82%), high surface area (112.2 m2/g), good scattering ability, self–ordered 3D porous network and suitable mesopores in microspheres had been successfully synthesized by an in situ facet–controlling approach and employed as the photoanode material for the DSSCs. The pertinent DSSC exhibits a η of 11.13%, which represents a 37.2% improvement over that obtained for a DSSC containing the common Ref–TiO2 (η = 8.11%). In Chapter 6, owing to the presence of large pores among the TiO2 microspheres, formed a freeway to facilitate the penetration of the electrolyte solution, it successfully coped with the mass transport problem of a cobalt-based electrolyte. However, the enhancement in mass diffusion due to large voids among the microspheres is expected to be accompanied by a large decrease in the dye loading, on account of the associated high porosity of the spheres-based TiO2 film. Surprisingly, in our case, the film of S84-TiO2 shows an unique property, i.e., its porosity is very high (εfilm = 0.73) for facilitating the diffusion of the cobalt electrolyte, while the associated dye loading is also very high (1.06×1010 mol/(cm2μm)). This unique property makes our S84-TiO2 film be distinguishable from iother TiO2 films reported in the literature for use in cobalt-based DSSCs. An efficiency of 11.43% was obtaned for the DSSC with S84-TiO2; this value is 29.7% higher than that obtained for the DSSC with the Ref–TiO2 (8.81%). In Chapter 7, we newly synthesized (101)-facets TiO2 microspheres (MSs) and focused on its application in room-temperature ionic liquids (RTILs)-based DSSCs. Presence of macropores in the MS film, as confirmed by high resolution scanning electron microscopy (HR–SEM) and mercury porosimeter, facilitates the penetration of IL-based electrolytes into the thin film, whereas its high surface area (108.1 m2/g) helps for high dye loading. Further, the large particle size increases the scattering ability of incident light leading to an excellent increment in the number of photons. Moreover, a new bi–ionic liquid (bi–IL) containing a mixture of 1–propyl–3–methylimidazolium iodide (PMII) and triethylmethylammonium methyl sulfate (TEMAMS) (65:35 = v/v) is prepared, which possesses unique thermal plastic characteristics and shows gel–state at room temperature.The results demonstrated that DSSCs containing the photoanode made of a MS structure show a superior η (6.18%) than that of the commercial transparent layer based (5.24%) and commercial scattering based TiO2 films (4.99%). The DSSCs with PMII/TEMAMS shows the extraordinary durability and unfailing stability of the cells for 1,200 h.
Subjects
Dye-sensitized solar cells
TiO2
photoanodes
nanosheets
ionic liquids
electrolytes
Type
thesis