We report on the synthesis of phase-pure TiO(2) nanoparticles in anatase, rutile and brookite structures, using amorphous titania as a common starting material. Phase formation was achieved by hydrothermal treatment at elevated temperatures with the appropriate reactants. Anatase nanoparticles were obtained using acetic acid, while phase-pure rutile and brookite nanoparticles were obtained with hydrochloric acid at a different concentration. The nanomaterials were characterized using x-ray diffraction, UV-visible reflectance spectroscopy, dynamic light scattering, and transmission electron microscopy. We propose that anatase formation is dominated by surface energy effects, and that rutile and brookite formation follows a dissolution-precipitation mechanism, where chains of sixfold-coordinated titanium complexes arrange into different crystal structures depending on the reactant chemistry. The particle growth kinetics under hydrothermal conditions are determined by coarsening and aggregation-recrystallization processes, allowing control over the average nanoparticle size.

Titanium dioxide nanoparticles have been prepared by solution-phase methods in the three phases that occur naturally, anatase, rutile, and brookite. The amorphous titania starting material was prepared from titanium(IV) iso-propoxide using iso-propanol as solvent and a small quantity of water. The resulting material was treated hydrothermally in an acid digestion vessel at temperatures between 175 ˚C and 230 ˚C with different reactants to obtain the three phases or controlled mixtures of two phases. The nanomaterials were characterized by a variety of techniques, including X-ray diffraction, Raman spectroscopy, electron microscopy, dynamic light scattering, and UV-Vis absorbance spectrophotometry. The results illustrate the relation between the properties of the nanoparticles in the colloid, in the powder, and in nanostructured thin films prepared with the materials. A thorough understanding of synthesis methods is essential for the preparation of nanomaterials with tailored structural, morphological, and ultimately, physical properties.

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The transport and recombination properties of dye-sensitized solar cells based on phase-pure anatase and brookite nanomaterials are compared as a function of the surface chemistry and morphology. Phase-pure brookite has been synthesized from amorphous TiO2 using two different solutions at low and high pH, resulting in different size and morphology of brookite nanoparticles. The smaller short-circuit current density (J SC = 6.6 mA cm–2) for acidic brookite compared to anatase (9.8 mA cm–2) was related to the light harvesting efficiency because of the lower amount of dye adsorbed. However, a larger open-circuit voltage for acidic brookite indicates the promise of the material. The basic brookite-based solar cells gave a very low J SC (0.10 mA cm–2), which increased dramatically by a factor of about 30 after an acid treatment of the films, illustrating the effect of surface chemistry. A combination of experiments shows that the improvement is related to an increase in injection efficiency. Electrochemical impedance and intensity-modulated photocurrent and photovoltage spectroscopies show that electron transport is faster in the acid-treated basic brookite nanomaterial, related to the larger feature sizes. However, the recombination kinetics is also significantly faster, with as net result a smaller diffusion length and hence smaller collection efficiency.

Dye-sensitized solar cells can be considered as a future candidate to complement current photovoltaic systems; however, scaling-up the dye-sensitized solar cell is a complicated issue. One of the challenges is the module resistance, which includes the resistance of the transparent conducting oxide substrate. By increasing the substrate area, the internal module series resistance increases, resulting in a decrease of the fill factor; hence, charge collectors must be introduced in the module. In this work, silver lines are incorporated in a mini-module design and the dependence of distance between the two silver current collectors, on the working and counter electrode sides, respectively, and the module series resistance is illustrated. A module of 7 cells with 0.60 mm silver lines, and 23.8 cm2 of active surface area reached an efficiency of 4.8%.