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.
The photocurrent response of dye-sensitized, porous nanocrystalline TiO2 cells was studied as a function of light intensity, in both the time domain (photocurrent transient measurements) and the frequency domain (intensity-modulated photocurrent spectroscopy). The photocurrent transients are characterized by a fast and a slow component. The rise time of the transients was in the range of milliseconds to seconds and exhibited a power law dependence on light intensity with an exponent of −0.6 to −0.8. The response to a modulated light intensity is characterized by a depressed semicircle in the complex plane. The time constant obtained from these spectra exhibits the same power law dependence on light intensity. The transient response of these cells is dominated by electron transport in the TiO2 film, and the results are shown to be consistent with a diffusion model where the diffusion coefficient for electrons in the particle network is a function of the light intensity.
The general concepts governing the electrochemical deposition of metal films onto semiconductors are discussed. Deposition onto semiconductor surfaces is complicated due to the band structure of the semiconductor, which affects both the thermodynamics and the kinetics of metal deposition processes. The influence of the potential distribution at the semiconductor/solution interface on the charge transfer mechanisms involved in deposition of metals is discussed. Models for electrochemical nucleation and growth are described and the influence of the unique physical properties of semiconductors is analysed. Finally, we present recent results for electrochemical deposition of gold, copper and platinum onto n-type silicon.
In this paper, we report on the preparation and characterization of two pseudohalogen redox couples for dye-sensitized TiO2 photoelectrochemical cells. The equilibrium potentials of the (SeCN)2/SeCN- and (SCN)2/SCN- couples are respectively 0.19 and 0.43 V more positive than for the I3 -/I- couple, providing the opportunity to determine the influence of the redox potential on the open circuit photovoltage. With the sensitizer cis-Ru(dcb)2(NCS)2 (N3), the incident photon-to-current conversion efficiency was 20% for the (SeCN)2/SeCN- couple and 4% for the (SCN)2/SCN- couple. Transient absorbance measurements showed that the quantum yield for electron injection is independent of the pseudohalogen redox couple and that the regeneration rates of the dye decrease in the order I- > SeCN- > SCN-. The effects of the redox potential on open circuit photovoltage were determined by independent measurement of the dependence of the sensitized TiO2 working electrode and the platinum counter electrode potentials on the cell voltage.
We report on the growth kinetics of TiO 2 nanoparticles synthesized from aqueous solution using titanium-(IV) isopropoxide as precursor. The radius of primary particles was found to be between 1.5 and 8 nm, and the average particle radius cubed is shown to increase linearly with time in agreement with the Lifshitz-Slyozov-Wagner model for coarsening. The rate constant for coarsening increases with temperature due to the temperature dependence of the viscosity of the solution and the equilibrium solubility of TiO 2 . At longer times and higher temperatures, secondary particles formed by epitaxial self-assembly of primary particles were observed with high-resolution transmission electron microscopy. The number of primary particles per secondary particle increases with time, and the percentage of primary particles present in the colloid decreases with increasing temperature.
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