This study reports synthesis and characterization of nanoparticles for fabricating the TiO2 nanocrystalline films used in dye-sensitized solar cells: phase-pure anatase nanoparticles from a titanate-directed route, and brookite (27%) and rutile (1.2%)-containing anatase nanoparticles from a sol−gel route. After nanoparticle-necking into films, X-ray diffraction pattern simulation shows that the defect density of the anatase (AN) films is less than that of the brookite/rutile-containing anatase (AN-br) films. The defect states in the AN-br films lower the short circuit current and conversion efficiency of the resulting solar cells. Intensity-modulated photocurrent/photovoltage spectroscopic analysis demonstrates electron transport in trap-free and trap-limited diffusion modes and shows that the defects serve as electron trap state to retard electron transport for collection and increase the traveling time prior to recombination. Impedance analysis shows that the trap-free mode extends the electron diffusion length in TiO2 films and its contribution magnitude governs the electron collecting efficiency.
WO 3 mesoporous films were prepared using a sol-gel method to serve as the photoanode for water cleavage in a 1 M HClO 4 solution with Pt as the counter electrode. Post-treatment of the as-synthesized WO 3 film with methanol and ethanol vapors, especially with methanol, improves the photocurrent during photoelectrolysis of water. Water and hexane vapors have a deleterious influence on the WO 3 film. Scanning electron microscopy and X-ray diffraction analyses showed that treatment with methanol vapors did not alter the configuration of the nanocrystalline framework or the WO 3 mesoscopic structure. Optical absorption and W L 3 -edge X-ray absorption near-edge structural analyses also revealed that the W-ion electronic and oxidation states remained unchanged after methanol treatment. However, extended X-ray absorption fine structure analysis of the W L 3 -edge showed that the coordination number of W 6+ sites in the WO 3 film significantly increased with the methanol treatment, indicating a corresponding decrease in the defect-state density of the film. The observed increase in the coordination number resulted in a 25% increase in the electron transit rate of the WO 3 film and enhanced solar energy conversion by 32% for the photoelectrolysis of water. We conclude that post-treatment with methanol vapor remedies the defect region in nanocrystalline WO 3 films, and improves the film's electron transport performance in a photoelectrochemical cell.Scheme 1 Processes of charge transport, back transfer, and chemical interaction in a photoelectrochemical cell for water cleavage. The n-type semiconducting photoanode is positively biased for generation of O 2 gas, with corresponding H 2 gas generation on the counter electrode. V CB , V VB , V FB , and E F represent the conduction-band, valence-band, flatband, and Fermi-energy levels of the electrodes.
Nanocrystalline titania (TiO 2 ) anatase films are widely utilized as substrates for electron conduction in photoelectrochemical devices. In this paper, we subjected the lattice disorder of TiO 2 anatase nanoparticles and the resulting nanocrystalline films to analysis with X-ray absorption fine structure spectroscopy. The TiO 2 nanoparticles were synthesized from dehydration of a titanate and from a conventional sol-gel method. Although both specimens had similar first shell Ti 41 coordination numbers (CNs) of ca. 5.7, the titanate-derivative TiO 2 was shown to be phase-pure anatase and the sol-gel TiO 2 contained a minute amount of brookite impurity. After nanoparticle necking into films, the former TiO 2 exhibited a negligible decrease in the CN, whereas the latter showed a significant decrease to a value of ca. 4.9. As a result, the titanate-derivative film is more efficient than the sol-gel one in transmitting electrons injected from a photoexcited dye. Significant lattice distortion near the grain boundaries of films are believed to occur during necking of the nanoparticles containing impurities. We have demonstrated that the synthesis of phase-pure nanoparticles is essentially important in fabricating films with a minimal degree of lattice disorder.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.