Fabrication And Characterization Of Porous Nanocrystalline TiO₂ Films By Template-directed Assembly And Performance In DSSC Application

Porous nanocrystalline TiO₂ (nc-TiO₂) film is an important semiconductor material. Recently, its photoelectrochemical properties have attracted much attention for research works, which are desired for many applications, such as solar energy converters, optoelectronic devices and photocatalysts.In this paper, two methods (dip-drawing and in-situ solvent evaporation) were employed for preparing PS colloidal crystal array. In self-assembly of PS colloidal crystal by dip-drawing method, the elevated emulsion temperature could promote ordered arrangement of PS spheres during the dynamic drawing process. At proper temperature, the array thickness and orderliness can be enhanced by increasing the PS volume fraction of emulsion, meanwhile which obviously reduced the fracture lines in array. In in-situ evaporation process for preparing PS colloidal crystal, self-assembly behaves a multi-nucleus site evolution, and undergoes the transformation from square to close packing arrangement. Technique of decreasing atmospheric pressure was adopted for accelerating evaporation so as for a more stable arrangement and wider range of pressure for ordered colloidal crystal.The mesoporous TiO₂ precusor sol was prepared using tetrabutyl orthotitanate (Ti(OBu)4) as titanium source, P123 as template and HCl as hydrolysis inhibitor. With film fabrication by dip-drawing and then calcining at 450℃for 3h, mesoporous anatase nc-TiO₂ film was obtained with grain size of ¹0nm. The Brunauer-Emmett-Teller surface area (SBET), total pore volume and average mesoporous size is 89.6m2/g, 0.18cm3/g and 5.7nm, respectively. Through using different hydrolysis inhibitors, the microstructure of mesoporous TiO₂ was researched. Results show that under effect of inhibitor HCl or acetylacetone-HCl, higher SBET and porosity of 176.3m2/g, 139.2m2/g and 60%, 57% respectively, were obtained. Because the additive HCl possesses strong inhibiting effect for hydrolysis, which results in small size of Ti oligomers that favors connection with PEO moieties for formation of more ordered mesoporous structure of TiO₂ film.Using PS colloidal crystal/P123 as bitemplate through dip-drawing/sol-gel for film fabrication, ordered macro-mesoporous anatase TiO₂ film was prepared after calcining at 450℃, with grain size of 7.4 nm and band gap of 3.00eV. The perfection of inverse opal structure mainly depends on the precursor sol concentration. Under low sol concentrations, worse wetting ability of sol to PS spheres led to incomplete TiO₂ inorganic wall. In concentration range of 0.13-0.15M, more ordered macroprous structure were fabricated, resulting from big surface tension and small contact angle that both favor the permeation of sol into PS template. Along with increase of sol concentration, fluidity of sol could not match the spread in gaps among PS spheres well, leading to worse orderliness of macroporous structure.Mesoporous nc-TiO₂ films were doped by metal element of Nb,Sb,Ge,Zn and Sn, and its photovoltaic performance was studied as n-electrode of dye-sensitized solar cells (DSSCs). The results show that when using Ge doped TiO₂ film as anode, open-circuit photovoltage of DSSC increases comparing with pure TiO₂. The short-circuit photocurrent of DSSCs based on Nb or Sb doped anode rise, since Nb and Sb are both donor for Ti that favors transfer of electron received from dye in the TiO₂ electrode. Larger SBET of Sb doped TiO₂ film raises the light absorptivity. And Ge doped DSSC possesses the highest conversion efficiency of 2.95%.